{"gene":"FKBP1B","run_date":"2026-04-28T17:46:03","timeline":{"discoveries":[{"year":1995,"finding":"FKBP12.6 (encoded by FKBP1B) was cloned from human tissue; the recombinant protein exhibits peptidyl-prolyl cis-trans isomerase (PPIase) activity and selectively associates with the cardiac ryanodine receptor isoform RyR2 (not RyR1) in cardiac muscle sarcoplasmic reticulum. Upon binding rapamycin, FKBP12.6 forms a complex with mTOR, and in transfected Jurkat cells it mediates calcineurin inhibition by FK506.","method":"cDNA cloning, recombinant protein expression, co-immunoprecipitation with native cardiac SR, calcineurin inhibition assay, mTOR binding assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1–2 — original cloning paper with multiple orthogonal biochemical assays; foundational study replicated by many subsequent labs","pmids":["7592869"],"is_preprint":false},{"year":1996,"finding":"FKBP12.6 selectively binds to cardiac RyR2 but not skeletal RyR1; only FKBP12.6 (not FKBP12) can exchange with endogenously bound FKBP12.6 on cardiac SR, explaining why the cardiac CRC is isolated as a complex with FKBP12.6 whereas skeletal muscle CRC associates with FKBP12. FK506 dissociates FKBP12.6 from cardiac SR.","method":"35S-labeled FKBP isoform binding/competition assays with native cardiac and skeletal SR vesicles; cosedimentation","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1–2 — direct in vitro binding assay with radiolabeled probes; replicated across multiple species","pmids":["8702774"],"is_preprint":false},{"year":1997,"finding":"Cyclic ADP-ribose (cADPR) binds directly to FKBP12.6 on the ryanodine receptor in pancreatic islet microsomes; this binding dissociates FKBP12.6 from the RyR, thereby releasing Ca2+ from the ER. Microsomes depleted of FKBP12.6 by cADPR treatment no longer respond to cADPR for Ca2+ release, establishing FKBP12.6 as the cADPR receptor on the islet RyR.","method":"Radioligand binding ([3H]FK506, [3H]cADPR), Ca2+ flux assay from islet microsomes, immunoprecipitation of FKBP12.6 after cADPR treatment","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1–2 — direct binding and functional reconstitution in native microsomes with loss-of-function verification","pmids":["9013543"],"is_preprint":false},{"year":2000,"finding":"PKA phosphorylation of RyR2 dissociates FKBP12.6 from the channel and increases channel open probability. A macromolecular complex on the SR comprising RyR2, FKBP12.6, PKA, protein phosphatases PP1 and PP2A, and the anchoring protein mAKAP was defined. In failing human hearts, RyR2 is PKA-hyperphosphorylated, causing defective channel regulation due to FKBP12.6 dissociation.","method":"Cosedimentation, co-immunoprecipitation, single-channel lipid bilayer recordings, phosphorylation assays on human failing heart tissue","journal":"Cell","confidence":"High","confidence_rationale":"Tier 1–2 — multiple orthogonal methods (co-IP, cosedimentation, single-channel recording) in both recombinant and native systems; highly cited foundational paper","pmids":["10830164"],"is_preprint":false},{"year":2000,"finding":"In pacing-induced canine heart failure, the stoichiometry of FKBP12.6 bound per RyR2 monomer decreases from ~1:1 to ~0.4:1; this partial loss of RyR2-bound FKBP12.6 correlates with conformational changes in RyR2 and a prominent spontaneous Ca2+ leak from the SR. FKBP12.6 protein expression is significantly reduced in failing SR.","method":"[3H]dihydro-FK506 and [3H]ryanodine binding assays, stopped-flow Ca2+ release measurements, Western blotting of SR fractions","journal":"Circulation","confidence":"High","confidence_rationale":"Tier 1–2 — quantitative binding stoichiometry combined with functional Ca2+ flux measurements in native SR","pmids":["11044432"],"is_preprint":false},{"year":2000,"finding":"Crystal structure of FKBP12.6 in complex with rapamycin determined at 2.0 Å resolution. The structures of FKBP12.6 and FKBP12 are nearly identical except for a displacement in the helical region of FKBP12.6 toward the hydrophobic pocket, a feature not predicted by homology modelling and which likely underlies RyR2-binding specificity.","method":"X-ray crystallography at 2.0 Å resolution","journal":"Acta crystallographica. Section D, Biological crystallography","confidence":"High","confidence_rationale":"Tier 1 — crystal structure with functional interpretation of RyR2-selectivity-conferring residues","pmids":["10713512"],"is_preprint":false},{"year":2001,"finding":"Adenovirus-mediated overexpression of FKBP12.6 in adult rabbit cardiomyocytes reduces Ca2+ leak through RyR2 by 53%, increases SR Ca2+ load, and increases fractional shortening by 21%, demonstrating that FKBP12.6 stabilizes the closed conformation of RyR2 and enhances excitation-contraction coupling.","method":"Adenoviral gene transfer, Fura-2 Ca2+ imaging, SR Ca2+ uptake/leak assay in permeabilized myocytes, caffeine contracture measurements","journal":"Circulation research","confidence":"High","confidence_rationale":"Tier 2 — clean gain-of-function with specific mechanistic readouts (RyR2 leak, SR load, contractility) replicated in subsequent studies","pmids":["11157671"],"is_preprint":false},{"year":2001,"finding":"FKBP12.6 binding characteristics of cardiac microsomes are widely conserved across vertebrates; most species (human, rabbit, rat, mouse, chicken, frog, fish) contain both FKBP12 and FKBP12.6 associated with RyR2, with dog being the exception (only FKBP12.6).","method":"[3H]FK506 and [3H]dihydro-FK506 binding and exchange assays on isolated cardiac microsomes from eight vertebrate species","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 2 — systematic comparative binding study across species with quantitative radioligand assays","pmids":["11237759"],"is_preprint":false},{"year":2002,"finding":"Disruption of the FKBP12.6 gene in male mice causes cardiac hypertrophy with dysregulated Ca2+ sparks (increased amplitude and duration) and increased Ca2+-induced Ca2+ release gain. Female knockout mice develop the same Ca2+ dysregulation but are protected from hypertrophy by oestrogen; tamoxifen treatment of female knockouts induces hypertrophy, placing FKBP12.6-mediated Ca2+ regulation upstream of an oestrogen-sensitive hypertrophic pathway.","method":"Gene knockout mouse model, confocal Ca2+ spark imaging, echocardiography, tamoxifen pharmacological intervention","journal":"Nature","confidence":"High","confidence_rationale":"Tier 2 — in vivo loss-of-function with mechanistic dissection by hormonal rescue; published in Nature","pmids":["11907581"],"is_preprint":false},{"year":2002,"finding":"FKBP12.6 associates with RyR2 (not RyR1, RyR3, or IP3Rs) in tracheal smooth muscle; cADPR-induced Ca2+ release and spontaneous Ca2+ release in tracheal myocytes are mediated through FKBP12.6, as they are blocked by excess recombinant FKBP12.6 and absent in FKBP12.6-knockout myocytes. Force development is impaired in FKBP12.6-null tracheal smooth muscle.","method":"Co-immunoprecipitation, intracellular dialysis of cADPR, Ca2+ imaging, isometric force measurements in FKBP12.6-knockout mouse tissue","journal":"American journal of physiology. Cell physiology","confidence":"High","confidence_rationale":"Tier 2 — loss-of-function KO combined with pharmacological and reconstitution experiments in multiple readouts","pmids":["14592808"],"is_preprint":false},{"year":2002,"finding":"The FKBP12.6-binding site on RyR2 is located within the N-terminal region (residues 305–1937) rather than the previously proposed central domain isoleucine-proline motif. The first 1937 N-terminal residues are sufficient for GST-FKBP12.6 binding, and binding is conformation-dependent; co-expression of overlapping fragments that restore channel function does not restore FKBP12.6 binding.","method":"GST-FKBP12.6 pulldown assays with systematic deletion/point mutants of RyR2 expressed in HEK293 cells","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1–2 — systematic mutagenesis and deletion analysis with quantitative pulldown; multiple mutants tested","pmids":["12446682"],"is_preprint":false},{"year":2002,"finding":"FKBP12.6 cADPR-mediated activation of RyR2 Ca2+ release channels in arterial smooth muscle (bovine coronary artery) requires intact FKBP12.6 on the receptor: FK506 removal of FKBP12.6 blocks cADPR-induced channel activation, and anti-FKBP12 antibody abolishes both FK506- and cADPR-induced RyR activation in planar lipid bilayer recordings.","method":"Planar lipid bilayer single-channel recording, FK506 treatment, anti-FKBP12 antibody blockade, gradient centrifugation depletion","journal":"American journal of physiology. Heart and circulatory physiology","confidence":"High","confidence_rationale":"Tier 1 — single-channel electrophysiology with pharmacological and antibody interventions showing FKBP12.6 necessity for cADPR-mediated activation","pmids":["11893565"],"is_preprint":false},{"year":2003,"finding":"FKBP12.6 knockout mice consistently exhibit exercise-induced ventricular arrhythmias causing sudden cardiac death. RyR2 mutations linked to CPVT reduce FKBP12.6 affinity for RyR2 and increase single-channel open probability under exercise-simulating conditions, establishing that FKBP12.6 dissociation from RyR2 is arrhythmogenic.","method":"FKBP12.6-/- mouse model, treadmill exercise, single-channel bilayer recordings, coimmunoprecipitation, [3H]ryanodine binding","journal":"Cell","confidence":"High","confidence_rationale":"Tier 1–2 — in vivo KO phenotype plus single-channel reconstitution and mechanistic mutation analysis; highly cited","pmids":["12837242"],"is_preprint":false},{"year":2003,"finding":"FKBP12.6 expressed in CHO cells co-localizes with RyR2 at ER membranes (sequestered from cytoplasm as RyR2 levels increase), suppresses agonist-induced Ca2+ release, and causes ER Ca2+ superfilling, demonstrating in situ that FKBP12.6 (but not FKBP12) specifically modulates hRyR2 channel functionality. Rapamycin reverses the FKBP12.6 effect.","method":"Stable CHO cell lines expressing graded hRyR2 levels, confocal microscopy, Ca2+ flux measurements, rapamycin pharmacology","journal":"The Biochemical journal","confidence":"High","confidence_rationale":"Tier 2 — dose-dependent in situ reconstitution with specific pharmacological reversal; isoform specificity demonstrated","pmids":["12443530"],"is_preprint":false},{"year":2003,"finding":"Co-expression of FKBP12.6 (but not FKBP12) with dysregulated RyR2 in CHO cells suppresses intracellular Ca2+ flux, restores normal cell viability and proliferation, demonstrating that FKBP12.6 prevents RyR2-mediated cellular toxicity through specific RyR2 channel stabilization.","method":"Stable CHO(hRyR2) cell lines, Ca2+ flux measurements, cell viability/proliferation assays, co-expression of FKBP isoforms","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 — cell-based gain-of-function with functional readouts; single lab but multiple complementary assays","pmids":["12754204"],"is_preprint":false},{"year":2004,"finding":"PKA phosphorylation at serine-2808 of RyR2 does not dissociate FKBP12.6 from RyR2; site-specific phospho-antibodies show FKBP12.6 binds equally to both phosphorylated and non-phosphorylated Ser-2808 forms, and the phosphomimetic S2808D mutant retains FKBP12.6 binding, challenging the PKA-dissociation model.","method":"Site-directed mutagenesis, site-specific phospho-antibodies, co-immunoprecipitation of recombinant and native RyR2, exogenous PKA phosphorylation","journal":"Circulation research","confidence":"High","confidence_rationale":"Tier 1–2 — rigorous mutagenesis combined with phospho-specific antibodies and multiple binding assays; challenges prior model","pmids":["14715536"],"is_preprint":false},{"year":2004,"finding":"FKBP12.6 overexpression in rat cardiac myocytes decreases the occurrence, amplitude, duration, and width of spontaneous Ca2+ sparks but increases global [Ca2+]i transient amplitude and SR Ca2+ load, demonstrating that FKBP12.6 reduces local stochastic RyR2 openings while enhancing coordinated Ca2+ release during excitation-contraction coupling.","method":"Adenoviral overexpression, confocal Ca2+ spark imaging (Rhod-2), field stimulation, caffeine application in rat cardiomyocytes","journal":"American journal of physiology. Heart and circulatory physiology","confidence":"High","confidence_rationale":"Tier 2 — direct gain-of-function with quantitative spark and transient measurements; confirmed by multiple subsequent studies","pmids":["15271664"],"is_preprint":false},{"year":2004,"finding":"A novel C-terminal FKBP12.6-binding site exists on RyR2 proximal to the pore-forming transmembrane domains; a large C-terminal RyR2 construct shows rapamycin-sensitive binding specifically to FKBP12.6 but not FKBP12, whereas short C-terminal fragments can displace FKBP12.6 from native RyR2 in competition assays.","method":"Competition binding assays with native RyR2, mammalian cell expression of C-terminal constructs, GST-pulldown, rapamycin competition","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 — complementary binding approaches identifying a novel binding site; single lab","pmids":["15591045"],"is_preprint":false},{"year":2004,"finding":"Hypoxia- and norepinephrine-induced Ca2+ release and pulmonary artery vasoconstriction are significantly enhanced in FKBP12.6-knockout pulmonary artery smooth muscle cells, showing that FKBP12.6 suppresses hypoxic and neurotransmitter-driven RyR2 activation in pulmonary vascular smooth muscle.","method":"FKBP12.6 knockout mouse PASMCs, Ca2+ imaging, Cl- and K+ current recordings, isometric force measurements, FK506/rapamycin pharmacology","journal":"Cell calcium","confidence":"High","confidence_rationale":"Tier 2 — KO loss-of-function with multiple functional readouts in native smooth muscle","pmids":["15036951"],"is_preprint":false},{"year":2005,"finding":"Cryo-EM 3-D reconstruction localizes FKBP12.6 binding on open-state RyR2 to the sides of the cytoplasmic region adjacent to domain 9 (part of the clamp structures). The conformation of FKBP12.6-bound RyR2 differs substantially from FKBP12.6-depleted RyR2 especially in the transmembrane region and clamp structures, providing structural basis for FKBP12.6-mediated channel stabilization.","method":"Cryoelectron microscopy, 3-D reconstruction, quantitative difference mapping, X-ray structure docking","journal":"Biophysical journal","confidence":"High","confidence_rationale":"Tier 1 — structural determination with functional correlation; direct visualization of FKBP12.6 on RyR2","pmids":["16214874"],"is_preprint":false},{"year":2005,"finding":"FKBP12.6 central domain of RyR2 does not support FKBP12/12.6 interaction in yeast two-hybrid or immunoprecipitation assays; a distinct alternatively spliced variant of FKBP12.6 cannot interact with RyR. An interaction of FKBP12.6 with the cytoplasmic domain of TGF-β receptor type I was confirmed as a positive control, supporting specificity.","method":"Yeast two-hybrid, in vitro immunoprecipitation with overlapping RyR2 fragments, alternative splice variant analysis","journal":"Cell biochemistry and biophysics","confidence":"Medium","confidence_rationale":"Tier 2 — systematic domain interaction mapping; single lab","pmids":["16049346"],"is_preprint":false},{"year":2005,"finding":"FKBP12.6 (calstabin2) Asp-37 is a key negatively charged residue involved in RyR2 binding; a D37S mutant calstabin2 with neutralized charge binds to constitutively PKA-phosphorylated RyR2-S2808D. Restoring calstabin2 stoichiometry at RyR2 by JTV519 treatment or genetic manipulation rescued cardiac function in a mouse myocardial infarction model, and the rescue was absent in calstabin2-/- mice.","method":"Site-directed mutagenesis of calstabin2, co-immunoprecipitation, murine MI model with JTV519 treatment, echocardiography","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1–2 — mutagenesis identifying critical binding residue combined with in vivo rescue; calstabin2-/- controls confirm mechanism","pmids":["15972811","16481613"],"is_preprint":false},{"year":2007,"finding":"Removal of FKBP12.6 (by FK506 treatment or in FKBP12.6-null mice) does not alter conductance, Ca2+- or caffeine-activation properties of RyR2 in lipid bilayer recordings, does not change store-overload-induced Ca2+ release propensity in HEK293 cells, and FKBP12.6-null mice do not exhibit enhanced stress-induced ventricular arrhythmias, in contrast to earlier reports.","method":"Single-channel lipid bilayer recordings, [3H]ryanodine binding, HEK293 Ca2+ imaging, FKBP12.6-null mouse stress arrhythmia protocol","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 1–2 — rigorous single-channel electrophysiology in multiple systems; single lab but systematic","pmids":["17921453"],"is_preprint":false},{"year":2007,"finding":"Oxidizing agents (H2O2, diamide) reduce FKBP12.6 binding to RyR2 through cysteine residues on the ryanodine receptor (not on FKBP12.6 itself, as a cysteine-null FKBP12.6 mutant retains redox-sensitive interaction). H2O2 effect is state-dependent (requires open state), whereas diamide is state-independent, establishing redox regulation of the RyR2-FKBP12.6 interaction.","method":"Co-immunoprecipitation and cosedimentation of [35S]FKBP12.6 with native cardiac SR under oxidizing/reducing conditions, cysteine-null FKBP12.6 mutant","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1–2 — systematic redox manipulation with mutagenesis and state-dependent experiments; mechanistically precise","pmids":["17200109"],"is_preprint":false},{"year":2008,"finding":"FKBP12.6-deficient mice have increased susceptibility to atrial fibrillation (AF), inducible in 81% vs. 7% of wild-type mice. SR Ca2+ leak in FKBP12.6-/- atrial myocytes is 53% larger and spontaneous SR Ca2+ release events are increased; both AF and spontaneous releases are blocked by the RyR2 antagonist tetracaine, establishing that FKBP12.6-dependent SR Ca2+ leak drives AF initiation.","method":"FKBP12.6-/- mouse electrophysiology, intracardiac stimulation, Ca2+ imaging in atrial myocytes, tetracaine pharmacology","journal":"Heart rhythm","confidence":"High","confidence_rationale":"Tier 2 — KO loss-of-function with electrophysiological and Ca2+ imaging endpoints plus pharmacological rescue","pmids":["18598963"],"is_preprint":false},{"year":2008,"finding":"FKBP12.6 disruption in pancreatic beta-cells impairs glucose-induced insulin secretion downstream of ATP production independently of KATP channels. FKBP12.6-/- mice show glucose intolerance and insufficient insulin secretion after glucose challenge, while sulfonylurea- or KCl-induced secretion is unaffected, placing FKBP12.6-mediated cADPR/RyR Ca2+ signalling in the pathway of glucose-stimulated insulin secretion.","method":"FKBP12.6-/- mouse model (homologous recombination), glucose tolerance tests, perifusion insulin secretion assays, islet Ca2+ measurements","journal":"Biochemical and biophysical research communications","confidence":"High","confidence_rationale":"Tier 2 — genetic KO with specific functional dissection identifying pathway position for glucose-stimulated insulin secretion","pmids":["18466757"],"is_preprint":false},{"year":2008,"finding":"Cardiac-specific conditional FKBP12.6 overexpression in mice prevents isoproterenol-triggered ventricular tachycardia; Ca2+ spark frequency is reduced 50% (persisting under isoproterenol), SR Ca2+ load is unchanged, L-type Ca2+ current density decreases 15%, and Na+/Ca2+ exchanger protein is reduced 18%, demonstrating that enhanced FKBP12.6-RyR2 binding prevents diastolic SR Ca2+ leak and triggered arrhythmias.","method":"Conditional cardiac-specific transgenic mouse, burst pacing arrhythmia protocol, confocal Ca2+ spark imaging, voltage clamp, Western blotting","journal":"Circulation","confidence":"High","confidence_rationale":"Tier 2 — cardiac-specific conditional gain-of-function with comprehensive electrophysiology and Ca2+ imaging","pmids":["18378612"],"is_preprint":false},{"year":2009,"finding":"In cardiac myocytes, FKBP12.6 binds RyR2 with very high affinity (Kd ~0.7 nM) and only FKBP12.6 (not FKBP12) inhibits resting RyR2 activity. PKA-dependent phosphorylation of RyR2 does not alter binding kinetics or affinity of either FKBP isoform. Quantitative immunoblots show endogenous [FKBP12.6] is ≤150 nM and virtually all FKBP12.6 is RyR2-bound, occupying ~10-20% of RyR2 channels.","method":"Fluorescently labeled FKBP in permeabilized myocytes, FRAP kinetics, Ca2+ spark imaging, quantitative immunoblots","journal":"Circulation research","confidence":"High","confidence_rationale":"Tier 1 — direct in-cell affinity measurement by FRAP/fluorescence plus quantitative proteomics; multiple orthogonal methods","pmids":["20431056"],"is_preprint":false},{"year":2009,"finding":"FKBP12.6 dissociation from RyR2 does not significantly contribute to beta-adrenergic-stimulated Ca2+ release in cardiomyocytes (ISO increases Ca2+ sparks equally in WT and FKBP12.6-KO), but DOES mediate cADPR-induced Ca2+ spark increases (cADPR effect is absent in FKBP12.6-KO myocytes). Twitch force is not significantly different between WT and KO papillary muscles.","method":"FKBP12.6-KO mouse cardiomyocytes, Ca2+ spark imaging, isoproterenol and cADPR pharmacology, papillary muscle force measurements, Western blotting","journal":"Cardiovascular research","confidence":"High","confidence_rationale":"Tier 2 — clean KO model with paired pharmacological dissection of two signalling pathways","pmids":["19578067"],"is_preprint":false},{"year":2009,"finding":"FKBP12.6-knockout mice show fed hyperinsulinemia, enhanced glucose-stimulated insulin secretion (GSIS) and islet Ca2+ elevation, and resistance to high-fat diet-induced hyperglycaemia, demonstrating that FKBP12.6 normally restrains RyR-mediated Ca2+ release during glucose stimulation to limit insulin secretion.","method":"FKBP12.6-/- mouse model, glucose and insulin tolerance tests, in vivo and in vitro GSIS, islet Ca2+ imaging, high-fat diet challenge","journal":"FASEB journal","confidence":"High","confidence_rationale":"Tier 2 — in vivo and in vitro loss-of-function with multiple metabolic and Ca2+ readouts","pmids":["19805579"],"is_preprint":false},{"year":2011,"finding":"CaMKII phosphorylation of RyR2 at Ser-2814 (not Ser-2808) is the downstream target responsible for SR Ca2+ leak, delayed afterdepolarizations, and AF in FKBP12.6-/- mice; S2814A knock-in into FKBP12.6-/- background reduces Ca2+ spark frequency, SR Ca2+ leak, and AF susceptibility, while S2808A does not protect.","method":"Double-mutant mouse models (FKBP12.6-/- × S2814A or S2808A RyR2 knock-in), pacing-induced AF, Ca2+ spark imaging in atrial myocytes, DAD recordings","journal":"Circulation research","confidence":"High","confidence_rationale":"Tier 1–2 — genetic epistasis with double knock-in/KO, identifying specific phosphorylation site downstream of FKBP12.6","pmids":["22158709"],"is_preprint":false},{"year":2012,"finding":"FKBP12 is a high-affinity activator of RyR2 (sensitises channel to cytosolic Ca2+), whereas FKBP12.6 has very low intrinsic efficacy but antagonises FKBP12-mediated RyR2 activation. Physiological FKBP12 concentrations (3 µM) increase Ca2+ wave frequency and decrease SR Ca2+ content; FKBP12.6 opposes these effects, establishing a dual regulatory model of RyR2 by competing FKBP isoforms.","method":"Single-channel bilayer recordings with purified FKBP isoforms, mathematical modelling, Ca2+ wave imaging in permeabilized cardiac cells","journal":"PloS one","confidence":"High","confidence_rationale":"Tier 1 — single-channel electrophysiology with purified proteins plus quantitative modelling and cell Ca2+ imaging","pmids":["22363773"],"is_preprint":false},{"year":2013,"finding":"Sirolimus (rapamycin)-FKBP12.6 complex impairs endothelial barrier function by activating protein kinase C-α, which disrupts the p120-VE-cadherin interaction; siRNA knockdown of FKBP12.6 phenocopies sirolimus, and ryanodine pretreatment prevents the sirolimus-induced Ca2+ increase and barrier disruption.","method":"Transendothelial electrical resistance, siRNA knockdown, Ca2+ imaging, PKC-α phosphorylation assays, Evans blue permeability in vivo, p120-VE-cadherin co-immunostaining","journal":"Arteriosclerosis, thrombosis, and vascular biology","confidence":"Medium","confidence_rationale":"Tier 2 — multiple cell-based and in vivo readouts; mechanistic pathway defined; single lab","pmids":["23887639"],"is_preprint":false},{"year":2015,"finding":"miR-34a directly targets the 3'-UTR of FKBP1B mRNA; FKBP1B expression decreases during adipogenesis in parallel with miR-34a increase; FKBP1B overexpression attenuates MDI-induced adipogenesis and suppresses PPARγ/C/EBPα expression, identifying FKBP1B as a negative regulator of adipogenic differentiation downstream of miR-34a.","method":"3'-UTR luciferase reporter assay, miR-34a inhibitor/mimic transfection, FKBP1B overexpression in 3T3-L1 preadipocytes, adipogenesis assays","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 2 — direct 3'-UTR binding validated with functional gain-of-function; single lab","pmids":["26471303"],"is_preprint":false},{"year":2016,"finding":"Total chemical synthesis and refolding of calstabin 2 (FKBP12.6) yields a catalytically active PPIase enzyme whose crystal structure confirms correct fold; N-terminal exotic amino acid substitutions do not alter catalytic activity, establishing structure-function relationships in the isomerase domain.","method":"Native chemical ligation synthesis, protein refolding, PPIase activity assay, X-ray crystallography","journal":"Protein science","confidence":"High","confidence_rationale":"Tier 1 — total synthesis with enzymatic validation and crystal structure; rigorous chemical biology approach","pmids":["27670942"],"is_preprint":false},{"year":2017,"finding":"Cryo-EM structure of rabbit RyR2 in complex with FKBP12.6 in the closed state at 11.8 Å reveals two phosphorylation-related RyR2 conformations; FKBP12.6 binding rigidifies the HD2 domain of RyR2, stabilising the closed state. The more flexible conformation likely corresponds to a phosphorylated P2 domain that requires less energy to open.","method":"Single-particle cryo-EM, atomic model building, heterogeneity analysis of conformational states","journal":"Science signaling","confidence":"High","confidence_rationale":"Tier 1 — cryo-EM structure with atomic model; directly links FKBP12.6 binding to HD2 rigidity and closed-state stabilisation","pmids":["28536302"],"is_preprint":false},{"year":2017,"finding":"Absence or pharmacological removal of FKBP12.6 from cardiomyocytes increases LCC-RyR2 coupling fidelity and accelerates LCC-to-spark signalling kinetics without changing L-type Ca2+ channel open probability; synergistic destabilisation by FKBP12.6 dysfunction and catecholaminergic signalling produces chaotic Ca2+ waves and ventricular arrhythmias.","method":"Whole-cell patch clamp combined with confocal Ca2+ imaging in FKBP12.6-KO and FK506/rapamycin-treated myocytes, loose-seal patch-clamp LCC sparklet-spark coupling","journal":"Cardiovascular research","confidence":"High","confidence_rationale":"Tier 1–2 — direct single-channel and Ca2+ imaging readouts; KO and pharmacological corroboration; mechanistically precise","pmids":["28077437"],"is_preprint":false},{"year":2018,"finding":"FKBP12.6 protects against angiotensin II-induced cardiac hypertrophy in vivo; FKBP12.6-/- mice show aggravated AngII-induced hypertrophy while cardiac-specific FKBP12.6-TG mice are protected. The mechanism involves FKBP12.6 reducing intracellular [Ca2+]i and thereby inhibiting calcineurin/NFATc4, CaMKII/MEF-2, AKT/GSK3β/NFATc4, and AKT/mTOR signalling pathways.","method":"FKBP12.6-/- and cardiac-specific FKBP12.6 TG mouse models, AngII osmotic pump infusion, echocardiography, Ca2+ imaging in H9c2 cells, Western blotting of signalling pathways","journal":"Journal of cellular and molecular medicine","confidence":"Medium","confidence_rationale":"Tier 2 — dual KO/OE in vivo models with pathway analysis; single lab","pmids":["29682889"],"is_preprint":false},{"year":2020,"finding":"Rieske iron-sulfur protein (RISP)-dependent ROS generation in pulmonary artery smooth muscle cells dissociates FKBP12.6 from RyR2, increasing RyR channel activity and Ca2+ release, which activates NF-κB/cyclin D1 signalling to promote PASMC proliferation and pulmonary hypertension. FKBP12.6 KO or FK506 exacerbates hypoxia-induced PH, while the RyR2/FKBP12.6 stabiliser S107 is protective.","method":"SMC-specific RyR2 KO and RISP knockdown mice, FKBP12.6 KO mice, S107 drug treatment, chronic hypoxia model, Ca2+ imaging, NF-κB/cyclin D1 pathway analysis, cell proliferation assays","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 2 — multiple genetic models (KO, knockdown, drug stabilisation) with in vivo PH endpoint and mechanistic pathway definition","pmids":["32669538"],"is_preprint":false},{"year":1994,"finding":"Molecular cloning of human FKBP1B (designated OTK4): the protein shares 88% amino acid identity with FKBP12 and recombinant OTK4 expressed in E. coli exhibits peptidyl-prolyl cis-trans isomerase (PPIase) activity. Two alternatively spliced transcripts are ubiquitously expressed in human tissues.","method":"cDNA library screening, recombinant protein expression, PPIase enzymatic activity assay, RT-PCR tissue expression","journal":"Biochemical and biophysical research communications","confidence":"High","confidence_rationale":"Tier 1 — original cloning paper with direct enzymatic activity measurement","pmids":["7513996"],"is_preprint":false},{"year":2005,"finding":"Genomic structure of human FKBP1B (FKBP12.6 gene) spans ~16 kb with 4 exons and 3 introns on chromosome 2p21-23. Reporter gene and EMSA analyses identify that Sp3 transcription factor drives FKBP12.6 promoter activity via a consensus Sp-family element at -58 to -24.","method":"Fluorescence in situ hybridisation, reporter gene assays, electrophoretic mobility shift assays, promoter deletion analysis","journal":"Gene","confidence":"Medium","confidence_rationale":"Tier 2 — direct promoter functional analysis with EMSA confirming Sp3 binding; single lab","pmids":["16122887"],"is_preprint":false},{"year":2010,"finding":"FKBP12.6 bound to RyR1 and RyR2 adopts the same orientation as determined by FRET between site-specifically labelled FKBP12.6 and RyR-bound calmodulin. Fluorescent labelling at position 41 of FKBP12.6 reduces RyR1 affinity 10-fold; position 32 reduces maximal inhibition of [3H]ryanodine binding by half, delineating surface residues important for RyR binding versus inhibition.","method":"Site-directed single-cysteine labelling of FKBP12.6, FRET measurements, [3H]ryanodine binding inhibition assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — FRET-based structural mapping with mutagenesis defining orientation and binding-critical residues","pmids":["20404344"],"is_preprint":false},{"year":2016,"finding":"FKBP12.6 and FKBP12 facilitate termination of store overload-induced Ca2+ release (SOICR) in wild-type RyR2 by raising the SOICR termination threshold without changing the activation threshold; an arrhythmogenic RyR2 CPVT mutant retains FKBP association but FKBPs are unable to regulate the mutant channel, representing a novel mechanism of CPVT arrhythmia.","method":"Single-cell Ca2+ imaging in HEK293 cells expressing WT or CPVT mutant RyR2, FKBP overexpression, SOICR threshold measurements","journal":"The Biochemical journal","confidence":"Medium","confidence_rationale":"Tier 2 — functional dissection of SOICR termination vs. activation; CPVT mutant comparison; single lab","pmids":["27154203"],"is_preprint":false}],"current_model":"FKBP1B encodes calstabin 2 (FKBP12.6), a 12.6-kDa cis-trans peptidyl-prolyl isomerase that selectively associates (at ~0.7 nM affinity) with the cardiac ryanodine receptor RyR2 as part of a macromolecular complex including PKA, PP1, PP2A, and mAKAP on the sarcoplasmic reticulum; bound FKBP12.6 rigidifies the RyR2 HD2 clamp domain, stabilises the channel closed state, reduces spontaneous Ca2+ spark frequency, and facilitates SOICR termination, while its dissociation—triggered by PKA/CaMKII hyperphosphorylation, oxidative stress (via RyR2 cysteines), or cADPR binding—increases RyR2 open probability and Ca2+ leak, leading to arrhythmias, cardiac hypertrophy, pulmonary hypertension, and impaired glucose-stimulated insulin secretion."},"narrative":{"teleology":[{"year":1994,"claim":"Identification of FKBP1B as a novel FKBP12-related gene with intrinsic PPIase activity established that mammals encode a second immunophilin closely related to FKBP12 but with distinct tissue expression.","evidence":"cDNA cloning from human tissue, recombinant PPIase assay, RT-PCR tissue survey","pmids":["7513996"],"confidence":"High","gaps":["No binding partner or physiological substrate identified","Functional distinction from FKBP12 unknown"]},{"year":1995,"claim":"Demonstration that FKBP12.6 selectively associates with cardiac RyR2 (not RyR1) and mediates calcineurin inhibition via FK506 resolved the question of isoform specificity and linked FKBP12.6 to cardiac Ca²⁺ regulation.","evidence":"Co-immunoprecipitation with native cardiac SR, rapamycin-mTOR binding assay, calcineurin inhibition in Jurkat cells","pmids":["7592869"],"confidence":"High","gaps":["Functional consequence of RyR2 binding on channel gating not yet determined","In vivo relevance unestablished"]},{"year":1996,"claim":"Selective FKBP12.6-RyR2 binding was confirmed by isoform exchange experiments showing only FKBP12.6 can compete for the cardiac RyR binding site, establishing the molecular basis for tissue-specific FKBP-RyR pairing.","evidence":"Radiolabeled FKBP12/12.6 competition assays with cardiac and skeletal SR vesicles","pmids":["8702774"],"confidence":"High","gaps":["Structural basis for selectivity unknown","Stoichiometry of binding not quantified"]},{"year":1997,"claim":"Discovery that cADPR binds FKBP12.6 on the RyR and dissociates it to trigger Ca²⁺ release from islet microsomes revealed FKBP12.6 as the intracellular cADPR-responsive element in non-cardiac tissues.","evidence":"Radioligand binding with [³H]cADPR, Ca²⁺ flux from pancreatic islet microsomes, immunoprecipitation after cADPR treatment","pmids":["9013543"],"confidence":"High","gaps":["Whether cADPR binds FKBP12.6 directly or acts through RyR conformational change debated","In vivo metabolic consequences unknown"]},{"year":2000,"claim":"Three concurrent advances defined FKBP12.6's structural and functional role: the crystal structure revealed a displaced helix conferring RyR2 selectivity; a macromolecular SR complex (RyR2–FKBP12.6–PKA–PP1–PP2A–mAKAP) was delineated; and PKA hyperphosphorylation in failing human hearts was shown to dissociate FKBP12.6, causing Ca²⁺ leak.","evidence":"X-ray crystallography at 2.0 Å; co-IP/cosedimentation and single-channel bilayer recordings on human failing heart tissue; stoichiometric binding assays in canine HF model","pmids":["10713512","10830164","11044432"],"confidence":"High","gaps":["Whether PKA phosphorylation at Ser-2808 is the sole dissociation trigger contested","Cryo-EM localization of FKBP12.6 on RyR2 not yet available"]},{"year":2001,"claim":"Adenoviral FKBP12.6 overexpression in cardiomyocytes provided the first direct gain-of-function evidence that FKBP12.6 stabilizes RyR2 closure, reduces Ca²⁺ leak, and enhances excitation-contraction coupling.","evidence":"Adenoviral gene transfer in rabbit cardiomyocytes, Fura-2 Ca²⁺ imaging, SR leak assay, caffeine contracture","pmids":["11157671"],"confidence":"High","gaps":["In vivo cardiac-specific overexpression not yet tested","Effect on arrhythmia susceptibility unknown"]},{"year":2002,"claim":"Generation of FKBP12.6-knockout mice established in vivo causality: male knockouts develop cardiac hypertrophy with dysregulated Ca²⁺ sparks, female knockouts are protected by estrogen, and tracheal smooth muscle loses cADPR-induced Ca²⁺ release, demonstrating tissue-wide dependence on FKBP12.6 for RyR2 regulation.","evidence":"FKBP12.6⁻/⁻ mouse model, confocal Ca²⁺ spark imaging, echocardiography, tamoxifen intervention, isometric force measurements in tracheal smooth muscle","pmids":["11907581","14592808"],"confidence":"High","gaps":["Mechanism of estrogen-dependent protection unclear","Whether phenotype is exclusively RyR2-dependent not formally excluded"]},{"year":2002,"claim":"Mapping the FKBP12.6-binding site to the RyR2 N-terminal region (residues 305–1937) overturned the prior model of a central isoleucine-proline motif and showed binding is conformation-dependent.","evidence":"GST-FKBP12.6 pulldown with systematic RyR2 deletion/point mutants in HEK293 cells","pmids":["12446682"],"confidence":"High","gaps":["Atomic contacts not resolved","Additional C-terminal binding site later proposed (PMID:15591045) but relationship between sites unclear"]},{"year":2003,"claim":"FKBP12.6-knockout mice exhibit exercise-induced ventricular arrhythmias and sudden cardiac death, and CPVT-linked RyR2 mutations reduce FKBP12.6 affinity, directly linking FKBP12.6 dissociation to arrhythmogenesis.","evidence":"Treadmill exercise in FKBP12.6⁻/⁻ mice, single-channel bilayer recordings, co-IP with CPVT mutant RyR2","pmids":["12837242"],"confidence":"High","gaps":["Later studies (PMID:17921453) failed to reproduce stress-induced arrhythmias in an independent FKBP12.6-null line, creating unresolved controversy","Mechanism by which CPVT mutations alter FKBP12.6 affinity not structurally defined"]},{"year":2004,"claim":"The PKA-dissociation model was challenged by the finding that phosphorylation at Ser-2808 does not dissociate FKBP12.6, while separate work showed FKBP12.6 overexpression reduces Ca²⁺ spark frequency and increases coordinated Ca²⁺ transients, refining understanding of FKBP12.6's role in E-C coupling.","evidence":"Site-directed mutagenesis with phospho-specific antibodies and co-IP (PMID:14715536); adenoviral overexpression with confocal spark imaging in rat cardiomyocytes (PMID:15271664)","pmids":["14715536","15271664"],"confidence":"High","gaps":["Whether CaMKII phosphorylation at Ser-2814 (rather than PKA at Ser-2808) is the relevant dissociation trigger not yet tested","Ser-2808 controversy unresolved between labs"]},{"year":2005,"claim":"Cryo-EM localized FKBP12.6 to the cytoplasmic clamp region of RyR2, and mutagenesis identified Asp-37 as a critical FKBP12.6 residue for RyR2 binding; a charge-neutralized D37S mutant can bind phosphorylated RyR2, and the drug JTV519 rescues cardiac function only in the presence of FKBP12.6.","evidence":"Cryo-EM 3-D reconstruction with difference mapping; D37S mutagenesis, co-IP, murine MI model with JTV519","pmids":["16214874","15972811","16481613"],"confidence":"High","gaps":["Resolution insufficient for atomic contact mapping","JTV519 mechanism of action on FKBP12.6–RyR2 not fully defined"]},{"year":2007,"claim":"Oxidative stress was shown to reduce FKBP12.6 binding through RyR2 cysteine residues (not FKBP12.6 cysteines), establishing redox regulation as a distinct dissociation mechanism, though a contemporaneous study found no effect of FKBP12.6 removal on RyR2 gating, highlighting ongoing controversy.","evidence":"Co-IP/cosedimentation of [³⁵S]FKBP12.6 under oxidizing/reducing conditions with cysteine-null mutant (PMID:17200109); single-channel bilayer recordings and stress arrhythmia protocol in independent FKBP12.6⁻/⁻ line (PMID:17921453)","pmids":["17200109","17921453"],"confidence":"High","gaps":["Identity of specific RyR2 cysteines mediating FKBP12.6 dissociation unknown","Reason for discrepant arrhythmia phenotypes across FKBP12.6-null lines unresolved"]},{"year":2008,"claim":"FKBP12.6 was connected to two new pathophysiological contexts: atrial fibrillation (81% AF inducibility in knockouts via SR Ca²⁺ leak) and glucose-stimulated insulin secretion (impaired GSIS in knockout islets), broadening the gene's functional scope beyond ventricular arrhythmia.","evidence":"Intracardiac stimulation and Ca²⁺ imaging in FKBP12.6⁻/⁻ atrial myocytes; glucose tolerance tests and perifusion insulin secretion in FKBP12.6⁻/⁻ mice","pmids":["18598963","18466757"],"confidence":"High","gaps":["Whether FKBP12.6 loss phenocopies human AF genetics unknown","Relative contributions of cADPR-RyR2 vs. other pathways in β-cell Ca²⁺ signaling not fully delineated"]},{"year":2009,"claim":"Quantitative in-cell measurements established FKBP12.6's binding affinity for RyR2 at ~0.7 nM with ~10–20% occupancy of RyR2 channels, and demonstrated that cADPR (but not β-adrenergic stimulation) requires FKBP12.6 for Ca²⁺ spark augmentation, while a separate study showed FKBP12.6 knockout enhances rather than impairs GSIS.","evidence":"FRAP kinetics of fluorescent FKBP12.6 in permeabilized myocytes, quantitative immunoblots (PMID:20431056); cADPR/ISO pharmacology in KO myocytes (PMID:19578067); in vivo/in vitro GSIS in KO mice (PMID:19805579)","pmids":["20431056","19578067","19805579"],"confidence":"High","gaps":["Conflicting reports on GSIS direction (impaired vs. enhanced) in FKBP12.6 KO mice unresolved","PKA-independence of FKBP12.6 dissociation contradicts earlier Marks lab findings"]},{"year":2011,"claim":"Genetic epistasis in double-mutant mice resolved the phosphorylation controversy for AF: CaMKII phosphorylation at RyR2-Ser2814 (not PKA at Ser-2808) is the downstream event causing SR Ca²⁺ leak and AF in FKBP12.6-deficient hearts.","evidence":"FKBP12.6⁻/⁻ × S2814A or S2808A RyR2 knock-in mice, pacing-induced AF, Ca²⁺ spark imaging, DAD recordings","pmids":["22158709"],"confidence":"High","gaps":["Whether this applies to ventricular arrhythmias and heart failure models not tested in same genetic framework","Upstream mechanism by which FKBP12.6 loss activates CaMKII unclear"]},{"year":2012,"claim":"A dual-regulation model was established: FKBP12 activates RyR2 by sensitizing it to Ca²⁺, while FKBP12.6 antagonizes this activation through competitive binding, revealing that the balance of FKBP isoforms tunes RyR2 gating.","evidence":"Single-channel bilayer recordings with purified FKBP isoforms, mathematical modelling, Ca²⁺ wave imaging in permeabilized cells","pmids":["22363773"],"confidence":"High","gaps":["In vivo relevance of isoform competition not validated","Relative abundance of FKBP12 vs. FKBP12.6 at RyR2 in different tissues poorly quantified"]},{"year":2016,"claim":"FKBP12.6 was shown to facilitate SOICR termination by raising the termination threshold, and CPVT mutant RyR2 channels retain FKBP binding but lose FKBP-mediated regulation, identifying a novel disease mechanism for catecholaminergic polymorphic VT.","evidence":"Single-cell Ca²⁺ imaging in HEK293 cells expressing WT or CPVT mutant RyR2, SOICR threshold measurements","pmids":["27154203"],"confidence":"Medium","gaps":["Mechanism by which CPVT mutations uncouple FKBP12.6 binding from channel regulation structurally undefined","Single heterologous cell system"]},{"year":2017,"claim":"High-resolution cryo-EM of the RyR2–FKBP12.6 complex revealed that FKBP12.6 rigidifies the HD2 clamp domain of RyR2 in the closed state, providing the structural mechanism for channel stabilization, while concurrent electrophysiology showed FKBP12.6 loss increases LCC-RyR2 coupling fidelity.","evidence":"Single-particle cryo-EM at 11.8 Å with conformational heterogeneity analysis (PMID:28536302); whole-cell patch clamp combined with confocal Ca²⁺ imaging in FKBP12.6-KO myocytes (PMID:28077437)","pmids":["28536302","28077437"],"confidence":"High","gaps":["Atomic-resolution structure of the FKBP12.6–RyR2 interface not achieved","Mechanism linking HD2 rigidity to pore closure at atomic level unresolved"]},{"year":2020,"claim":"RISP-dependent mitochondrial ROS was identified as the physiological trigger for FKBP12.6 dissociation from RyR2 in pulmonary artery smooth muscle, activating NF-κB/cyclin D1-driven proliferation and pulmonary hypertension, while the FKBP12.6-RyR2 stabilizer S107 was protective.","evidence":"SMC-specific RyR2 KO, RISP knockdown, FKBP12.6 KO mice, S107 treatment, chronic hypoxia PH model, Ca²⁺ imaging, pathway analysis","pmids":["32669538"],"confidence":"High","gaps":["Specific RyR2 cysteines targeted by RISP-ROS not identified","Whether S107 has clinical utility in pulmonary hypertension untested in humans"]},{"year":null,"claim":"Key unresolved questions include: the atomic-resolution structure of the FKBP12.6–RyR2 interface; reconciliation of contradictory FKBP12.6-null arrhythmia phenotypes across independent knockout lines; the precise identity of RyR2 cysteines mediating redox-dependent FKBP12.6 dissociation; and whether pharmacological stabilization of the FKBP12.6–RyR2 complex is therapeutically viable in arrhythmia, heart failure, and pulmonary hypertension.","evidence":"","pmids":[],"confidence":"Medium","gaps":["Atomic-resolution interface structure lacking","Contradictory KO arrhythmia phenotypes unresolved","Human genetic evidence directly linking FKBP1B variants to disease absent"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[0,1,3,6,12,16,26,31,36]},{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[0,34,39]}],"localization":[{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[0,1,3,4,13]}],"pathway":[{"term_id":"R-HSA-382551","term_label":"Transport of small molecules","supporting_discovery_ids":[2,6,9,11,16,25,27,28,29]}],"complexes":["RyR2–FKBP12.6–PKA–PP1–PP2A–mAKAP macromolecular complex"],"partners":["RYR2","MTOR","PPP3CA","TGFBR1","PRKAR2A"],"other_free_text":[]},"mechanistic_narrative":"FKBP1B encodes calstabin 2 (FKBP12.6), a peptidyl-prolyl cis-trans isomerase that functions as a critical stabilizer of the cardiac ryanodine receptor RyR2 in a macromolecular sarcoplasmic reticulum complex containing PKA, PP1, PP2A, and mAKAP [PMID:7592869, PMID:10830164]. Bound FKBP12.6 rigidifies the RyR2 HD2 clamp domain, stabilizes the channel closed state, reduces spontaneous Ca²⁺ spark frequency, and facilitates termination of store-overload-induced Ca²⁺ release; its dissociation—triggered by oxidative modification of RyR2 cysteines, CaMKII phosphorylation at Ser-2814, or cADPR binding—increases RyR2 open probability and diastolic SR Ca²⁺ leak [PMID:28536302, PMID:22158709, PMID:17200109, PMID:9013543]. Loss of FKBP12.6 in mice causes exercise-induced ventricular tachycardia, atrial fibrillation, sex-dependent cardiac hypertrophy, enhanced hypoxia-driven pulmonary vasoconstriction and pulmonary hypertension, and impaired glucose-stimulated insulin secretion in pancreatic β-cells [PMID:12837242, PMID:18598963, PMID:11907581, PMID:32669538, PMID:18466757]. Beyond the heart, FKBP12.6 mediates cADPR-dependent Ca²⁺ release through RyR2 in airway and vascular smooth muscle, and when complexed with rapamycin it can engage mTOR or disrupt endothelial barrier function via PKCα-dependent VE-cadherin uncoupling [PMID:14592808, PMID:11893565, PMID:23887639]."},"prefetch_data":{"uniprot":{"accession":"P68106","full_name":"Peptidyl-prolyl cis-trans isomerase FKBP1B","aliases":["12.6 kDa FK506-binding protein","12.6 kDa FKBP","FKBP-12.6","FK506-binding protein 1B","FKBP-1B","Immunophilin FKBP12.6","Rotamase","h-FKBP-12"],"length_aa":108,"mass_kda":11.8,"function":"Has the potential to contribute to the immunosuppressive and toxic effects of FK506 and rapamycin. PPIases accelerate the folding of proteins. It catalyzes the cis-trans isomerization of proline imidic peptide bonds in oligopeptides","subcellular_location":"Cytoplasm; Sarcoplasmic reticulum","url":"https://www.uniprot.org/uniprotkb/P68106/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/FKBP1B","classification":"Not Classified","n_dependent_lines":0,"n_total_lines":1208,"dependency_fraction":0.0},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/FKBP1B","total_profiled":1310},"omim":[{"mim_id":"600620","title":"FK506-BINDING PROTEIN 1B; FKBP1B","url":"https://www.omim.org/entry/600620"},{"mim_id":"180902","title":"RYANODINE RECEPTOR 2; RYR2","url":"https://www.omim.org/entry/180902"},{"mim_id":"114251","title":"CALSEQUESTRIN 2; CASQ2","url":"https://www.omim.org/entry/114251"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Golgi apparatus","reliability":"Approved"},{"location":"Vesicles","reliability":"Approved"},{"location":"Primary cilium","reliability":"Additional"},{"location":"Basal body","reliability":"Additional"},{"location":"Cytosol","reliability":"Additional"}],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in all","driving_tissues":[{"tissue":"brain","ntpm":73.7}],"url":"https://www.proteinatlas.org/search/FKBP1B"},"hgnc":{"alias_symbol":["OTK4","FKBP12.6","PPIase","FKBP9"],"prev_symbol":["FKBP1L"]},"alphafold":{"accession":"P68106","domains":[{"cath_id":"3.10.50.40","chopping":"2-106","consensus_level":"high","plddt":95.4051,"start":2,"end":106}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/P68106","model_url":"https://alphafold.ebi.ac.uk/files/AF-P68106-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-P68106-F1-predicted_aligned_error_v6.png","plddt_mean":94.88},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=FKBP1B","jax_strain_url":"https://www.jax.org/strain/search?query=FKBP1B"},"sequence":{"accession":"P68106","fasta_url":"https://rest.uniprot.org/uniprotkb/P68106.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/P68106/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/P68106"}},"corpus_meta":[{"pmid":"10830164","id":"PMC_10830164","title":"PKA 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FK506/FK590 dissociates FKBP12.6 from cardiac SR.\",\n      \"method\": \"35S-labeled FKBP probe binding/exchange assays, cosedimentation with stripped SR vesicles\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — direct biochemical reconstitution with radiolabeled probes, replicated across isoforms and muscle types\",\n      \"pmids\": [\"8702774\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"PKA phosphorylation of RyR2 dissociates FKBP12.6 (calstabin2) from the channel and increases channel open probability. In failing human hearts, RyR2 is PKA hyperphosphorylated, resulting in FKBP12.6 dissociation and defective channel function. A macromolecular complex comprising RyR2, FKBP12.6, PKA, PP1, PP2A, and the anchoring protein mAKAP was defined by cosedimentation and coimmunoprecipitation.\",\n      \"method\": \"Cosedimentation, coimmunoprecipitation, single-channel recordings, PKA phosphorylation assay, failing human heart tissue\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — multiple orthogonal methods (Co-IP, cosedimentation, single-channel electrophysiology) in one highly cited study\",\n      \"pmids\": [\"10830164\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"In pacing-induced heart failure, the stoichiometric ratio of FKBP12.6 per RyR (normally ~1:1) is markedly decreased, accompanied by loss of RyR conformational regulation and an abnormal diastolic Ca2+ leak through RyR that is not further increased by FK506. Protein expression of FKBP12.6 was significantly decreased in failing SR.\",\n      \"method\": \"[3H]dihydro-FK506 and [3H]ryanodine binding assays, SR vesicle Ca2+ leak measurement, Western blot\",\n      \"journal\": \"Circulation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple biochemical assays in a canine heart failure model, replicated by independent group\",\n      \"pmids\": [\"11044432\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"FKBP12.6 gene disruption in mice causes cardiac hypertrophy in males but not females; both sexes show dysregulated Ca2+ release (increased Ca2+ spark amplitude and duration, increased CICR gain). Females treated with the oestrogen receptor antagonist tamoxifen develop hypertrophy, indicating oestrogen protects against the hypertrophic response to Ca2+ dysregulation caused by FKBP12.6 loss.\",\n      \"method\": \"FKBP12.6 knockout mice, Ca2+ spark imaging, echocardiography, tamoxifen treatment\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean KO with defined cardiac phenotype and functional Ca2+ measurements, replicated with pharmacological intervention\",\n      \"pmids\": [\"11907581\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"FKBP12.6 binds to and regulates type 2 RyRs (RyR2) but not RyR1, RyR3, or IP3 receptors in tracheal smooth muscle. FKBP12.6 mediates cADPR-induced Ca2+ release through RyR2 in tracheal myocytes; recombinant excess FKBP12.6 and absence of FKBP12.6 (KO mice) abolish cADPR effects on Ca2+ release and smooth muscle force development.\",\n      \"method\": \"Co-immunoprecipitation, Ca2+ imaging in myocytes, FKBP12.6 knockout mice, pharmacological dissection\",\n      \"journal\": \"American journal of physiology. Cell physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — KO mice + recombinant protein competition + functional assays across multiple cell types\",\n      \"pmids\": [\"14592808\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"FKBP12.6 binding site on RyR2 is located within the NH2-terminal domain (residues 305–1937); an NH2-terminal fragment containing the first 1937 residues is sufficient for GST-FKBP12.6 binding. The isoleucine-proline motif at residues 2427–2428 is not the core binding site. Binding is likely conformation-dependent.\",\n      \"method\": \"Deletion and point mutagenesis of RyR2 expressed in HEK293 cells, GST-FKBP12.6 pulldown\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — systematic deletion analysis with multiple constructs and pulldown validation\",\n      \"pmids\": [\"12446682\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"FKBP12.6 deficiency in mice leads to exercise-induced RyR2 PKA phosphorylation, partial FKBP12.6 dissociation, increased Ca2+ release, ventricular arrhythmias, and sudden cardiac death. CPVT-linked RyR2 mutations reduce FKBP12.6 affinity for RyR2 and increase single-channel activity under exercise-simulating conditions.\",\n      \"method\": \"FKBP12.6 knockout mice, exercise testing, single-channel electrophysiology, binding affinity assays, CPVT patient mutations\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — KO mice with defined arrhythmia phenotype, single-channel recordings, and disease-mutation validation; highly cited\",\n      \"pmids\": [\"12837242\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Adenoviral overexpression of FKBP12.6 in adult rabbit cardiomyocytes reduces SR Ca2+ leak through RyR2 by ~53% and increases SR Ca2+ load and myocyte contractility, indicating FKBP12.6 stabilizes the closed conformation of RyR2.\",\n      \"method\": \"Adenoviral gene transfer, SR Ca2+ uptake in permeabilized myocytes with ruthenium red, caffeine-induced contractures, fractional shortening measurements\",\n      \"journal\": \"Circulation research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — gain-of-function with multiple functional Ca2+ readouts in primary cardiomyocytes\",\n      \"pmids\": [\"11157671\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"In CHO cells stably expressing human RyR2, FKBP12.6 (but not FKBP12) is sequestered from the cytoplasm to ER membranes proportionally to RyR2 expression levels, and co-expression of FKBP12.6 markedly decreases agonist-induced Ca2+ release and promotes superfilling of ER Ca2+ stores. These effects are antagonized by rapamycin.\",\n      \"method\": \"CHO cell stable expression, confocal microscopy, Ca2+ release assays, pharmacological (rapamycin) dissection\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — direct subcellular redistribution demonstrated with functional Ca2+ consequence, rapamycin control\",\n      \"pmids\": [\"12443530\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"PKA phosphorylation at Ser-2808 of RyR2 does not dissociate FKBP12.6 from RyR2; FKBP12.6 binds both phosphorylated and non-phosphorylated forms of RyR2, and the phosphomimetic S2808D mutant retains FKBP12.6 binding. Complete exogenous PKA phosphorylation does not disrupt the native FKBP12.6-RyR2 complex.\",\n      \"method\": \"Site-specific phospho-antibodies, recombinant RyR2 expression in HEK293 cells, native FKBP12.6-RyR2 complex disruption assays, GST pulldown\",\n      \"journal\": \"Circulation research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — rigorous site-directed mutagenesis plus phospho-specific antibodies plus native complex experiments\",\n      \"pmids\": [\"14715536\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"Crystal structure of FKBP12.6 in complex with rapamycin determined at 2.0 Å resolution. FKBP12.6 and FKBP12 structures are nearly identical except for a displacement in the helical region of FKBP12.6 toward the hydrophobic pocket, which was not predicted by homology modeling and likely confers RyR2-binding specificity.\",\n      \"method\": \"X-ray crystallography at 2.0 Å resolution\",\n      \"journal\": \"Acta crystallographica. Section D, Biological crystallography\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structure with functional implication for binding specificity\",\n      \"pmids\": [\"10713512\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Cryo-electron microscopy 3D reconstruction places FKBP12.6 binding along the sides of the cytoplasmic region of open RyR2, adjacent to domain 9 (clamp structures). FKBP12.6 binding induces conformational changes in the transmembrane region and clamp structures, and the X-ray structure of FKBP12.6 was docked to identify key interface residues (Gln-31, Asn-32, Phe-59).\",\n      \"method\": \"Cryo-electron microscopy and 3D reconstruction, structural docking of FKBP12.6 X-ray structure\",\n      \"journal\": \"Biophysical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — structural method directly localizing FKBP12.6 on RyR2\",\n      \"pmids\": [\"16214874\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"FKBP12.6 C-terminal domain of RyR2 (proximal to the pore-forming transmembrane domains) contains a novel FKBP12.6-binding site; a large RyR2 C-terminal construct expressed in mammalian cells shows rapamycin-sensitive binding specifically to FKBP12.6 but not FKBP12.\",\n      \"method\": \"Competition binding assays, mammalian cell expression, rapamycin-sensitivity, immunoprecipitation\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple binding assays but binding site not reconstituted in isolation; single lab\",\n      \"pmids\": [\"15591045\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Asp-37 of FKBP12.6 (calstabin2) is a key residue involved in RyR2 binding; a D37S mutant calstabin2 binds to constitutively PKA-phosphorylated RyR2-S2808D. Manipulating stoichiometry between calstabin2 and RyR2 (via viral delivery in mice) restores normal cardiac function after myocardial infarction.\",\n      \"method\": \"Site-directed mutagenesis of calstabin2, binding assays with RyR2-S2808D, murine myocardial infarction model with viral FKBP12.6 delivery\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — mutagenesis identifying binding residue combined with in vivo rescue experiment\",\n      \"pmids\": [\"16481613\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"FKBP12.6 plays a critical role in mediating cADPR-induced activation of RyR2 Ca2+ release channels in bovine coronary arterial smooth muscle; removal of FKBP12.6 by FK506 + gradient centrifugation abolishes cADPR-induced increase in RyR open probability in planar lipid bilayer recordings.\",\n      \"method\": \"Planar lipid bilayer single-channel recordings, immunodepletion of FKBP12.6, anti-FKBP12 antibody blockade\",\n      \"journal\": \"American journal of physiology. Heart and circulatory physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — reconstituted single-channel recordings with direct protein removal and antibody blockade\",\n      \"pmids\": [\"11893565\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"FKBP12.6 expressed in pulmonary artery smooth muscle cells (PASMCs) associates specifically with RyR2 (not RyR1, RyR3, or IP3Rs); FKBP12.6 KO mice show enhanced hypoxic Ca2+ response and hypoxic vasoconstriction, as well as enhanced norepinephrine-induced Ca2+ release and force generation, demonstrating that FKBP12.6 restrains RyR2-mediated Ca2+ release in response to hypoxia and neurotransmitters.\",\n      \"method\": \"Immunoprecipitation, Ca2+ imaging, pharmacological dissection (FK506, rapamycin, ruthenium red), FKBP12.6 KO mice, isolated trachealis tension assay\",\n      \"journal\": \"Cell calcium\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — KO mice + pharmacology + binding specificity across RyR isoforms\",\n      \"pmids\": [\"15036951\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"In permeabilized cardiomyocytes, FKBP12.6 binds RyR2 with very high affinity (Kd ≈ 0.7 nM) versus FKBP12 (Kd ≈ 206 nM), with the difference mediated by koff. Virtually all myocyte FKBP12.6 is RyR2-bound (only 10-20% of RyR2s have FKBP12.6). FKBP12.6 but not FKBP12 inhibits basal RyR2 activity. PKA-dependent RyR2 phosphorylation does not alter FKBP12.6/12 binding affinity or kinetics.\",\n      \"method\": \"Fluorescent-labeled FKBP binding in situ, FRAP, Ca2+ spark imaging, quantitative immunoblots in rat and mouse ventricular myocytes\",\n      \"journal\": \"Circulation research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — direct fluorescence binding kinetics in intact myocytes with FRAP, multiple orthogonal methods\",\n      \"pmids\": [\"20431056\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"FKBP12 (not FKBP12.6) is a high-affinity activator of RyR2, sensitizing it to cytosolic Ca2+, whereas FKBP12.6 has very low efficacy but antagonizes FKBP12 effects. Mathematical modeling and single-channel recordings demonstrate that the relative concentrations of FKBP12 and FKBP12.6 determine RyR2 activity.\",\n      \"method\": \"Single sheep RyR2 channels in planar phospholipid bilayers, Ca2+ wave measurements in permeabilized rat cardiac cells, mathematical modeling\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstituted single-channel recordings with both isoforms plus cellular functional assays\",\n      \"pmids\": [\"22363773\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"FKBP12.6 deficiency increases SR Ca2+ leak in atrial myocytes (53% larger than WT), promotes spontaneous Ca2+ release events, and markedly increases susceptibility to pacing-induced atrial fibrillation; RyR2 antagonist tetracaine blocks both events, linking SR Ca2+ leak through RyR2 to AF in FKBP12.6-/- mice.\",\n      \"method\": \"FKBP12.6 knockout mice, intracardiac electrophysiology, intracellular Ca2+ transient recording, tetracaine pharmacology\",\n      \"journal\": \"Heart rhythm\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — KO mice with defined arrhythmia and Ca2+ phenotype plus pharmacological rescue\",\n      \"pmids\": [\"18598963\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Inhibition of CaMKII phosphorylation at RyR2-S2814 (S2814A knock-in) reduces Ca2+ spark frequency, SR Ca2+ leak, and delayed afterdepolarizations in atrial myocytes from FKBP12.6-/- mice and prevents atrial fibrillation induction, while S2808A mutation does not protect. This places CaMKII-mediated S2814 phosphorylation downstream of FKBP12.6 loss in the AF pathway.\",\n      \"method\": \"Genetic epistasis: FKBP12.6-/- crossed with RyR2-S2814A or S2808A knock-in mice; Ca2+ spark imaging; intracardiac electrophysiology\",\n      \"journal\": \"Circulation research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — epistasis in double-mutant mice with defined molecular and electrophysiological readouts\",\n      \"pmids\": [\"22158709\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Conditional cardiac-specific overexpression of FKBP12.6 in mice increases FKBP12.6 binding to RyR2 at baseline and after isoproterenol, reduces Ca2+ spark frequency by 50%, decreases ICa,L density, reduces Ca2+ transient peak amplitude, and prevents triggered ventricular tachycardia induced by isoproterenol + burst pacing.\",\n      \"method\": \"Conditional transgenic mice, isoproterenol treatment, Ca2+ spark confocal imaging, patch-clamp, in vivo electrophysiology\",\n      \"journal\": \"Circulation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — gain-of-function transgenic with multiple mechanistic Ca2+ readouts and in vivo arrhythmia endpoint\",\n      \"pmids\": [\"18378612\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Cryo-EM structure of rabbit RyR2 complexed with FKBP12.6 in the closed state at 11.8 Å reveals that FKBP12.6 binding rigidifies the HD2 domain (part of the Clamp domain), stabilizing RyR2 in the closed state. Two RyR2 conformations in the dataset were proposed to correspond to different phosphorylation states of the P2 domain, with phosphorylation predisposing RyR2 to open.\",\n      \"method\": \"Cryo-electron microscopy, 3D reconstruction, atomic model building\",\n      \"journal\": \"Science signaling\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct structural determination of RyR2-FKBP12.6 complex\",\n      \"pmids\": [\"28536302\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Co-expression of FKBP12.6 (but not FKBP12) with dysregulated RyR2 in CHO cells suppresses elevated intracellular Ca2+ flux, restores normal cell viability and proliferation, independently of resting [Ca2+] or ER Ca2+ load. Dysregulated RyR2 enables functional cross-talk with IP3Rs to trigger apoptosis.\",\n      \"method\": \"CHO stable expression of recombinant human RyR2, co-expression of FKBP12.6 or FKBP12, cell viability assays, Ca2+ imaging, pharmacological dissection\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — defined cellular phenotype with molecular specificity (FKBP12.6 vs FKBP12), single lab\",\n      \"pmids\": [\"12754204\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"VTSIP-associated RyR2 mutations increase binding to FKBP12.6, while ARVD2-associated mutations decrease binding, as measured by quantitative yeast two-hybrid. This bidirectional effect on FKBP12.6-RyR2 interaction correlates with distinct clinical phenotypes (increased versus unchanged cytosolic Ca2+).\",\n      \"method\": \"Quantitative yeast two-hybrid system with RyR2 disease mutations\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — yeast two-hybrid, single lab, consistent with clinical phenotype correlation\",\n      \"pmids\": [\"12459180\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"FKBP12.6 deficiency in pancreatic beta-cells impairs glucose-induced insulin secretion; FKBP12.6-/- mice show glucose intolerance and impaired glucose-stimulated Ca2+ elevation and insulin secretion from islets, while sulfonylurea- or KCl-induced insulin secretion is unaffected, indicating FKBP12.6 controls RyR-mediated Ca2+ release required for glucose-stimulated insulin secretion.\",\n      \"method\": \"FKBP12.6 knockout mice, glucose tolerance tests, islet insulin secretion assays, glucose oxidation measurements\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — KO mice with defined glucose-stimulated phenotype, single lab\",\n      \"pmids\": [\"18466757\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Dissociation of FKBP12.6 from RyR2 does not significantly contribute to beta-adrenergic-stimulated Ca2+ release in cardiac cells; isoproterenol increases Ca2+ spark frequency in both WT and FKBP12.6 KO myocytes. In contrast, cADPR stimulation increases Ca2+ spark frequency only in WT (not KO) cells, implicating FKBP12.6-RyR2 dissociation specifically in cADPR-mediated Ca2+ release.\",\n      \"method\": \"FKBP12.6 knockout mice, Ca2+ spark imaging, papillary muscle force measurements, voltage-clamp, thapsigargin/phospholamban antibody controls\",\n      \"journal\": \"Cardiovascular research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — KO mice with multiple orthogonal functional readouts, single lab\",\n      \"pmids\": [\"19578067\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Sirolimus-FKBP12.6 complex impairs endothelial barrier function by activating protein kinase C-α and disrupting the p120-VE cadherin interaction; siRNA knockdown of FKBP12.6 phenocopies sirolimus, and pretreatment with ryanodine (stabilizing RyR2) prevents sirolimus-induced intracellular Ca2+ rise and barrier dysfunction.\",\n      \"method\": \"HAEC cell culture, transendothelial electrical resistance, siRNA knockdown, PKCα inhibition, Ca2+ imaging, Evans blue in vivo vascular permeability\",\n      \"journal\": \"Arteriosclerosis, thrombosis, and vascular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — siRNA KD phenocopies drug, pharmacological rescue with ryanodine, in vivo confirmation\",\n      \"pmids\": [\"23887639\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"FKBP12.6 protects against AngII-induced cardiac hypertrophy by reducing intracellular Ca2+ concentration and inhibiting calcineurin/NFATc4, CaMKII/MEF-2, AKT/GSK3β/NFATc4, and AKT/mTOR signalling pathways; FKBP12.6 KO aggravates hypertrophy while cardiac-specific overexpression prevents it.\",\n      \"method\": \"FKBP12.6 KO and cardiac-specific TG mice with AngII infusion, Ca2+ imaging, Western blot of signaling pathway proteins, H9c2 overexpression\",\n      \"journal\": \"Journal of cellular and molecular medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — both KO and gain-of-function with mechanistic pathway analysis, single lab\",\n      \"pmids\": [\"29682889\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Total chemical synthesis and refolding of calstabin 2 (FKBP12.6) yields a fully active PPIase enzyme; its crystal structure was determined by X-ray diffraction and confirmed catalytic activity comparable to recombinant wild-type protein. N-terminal exotic amino acid incorporation did not alter catalytic activity.\",\n      \"method\": \"Native chemical ligation, protein refolding, PPIase activity assay, X-ray crystallography\",\n      \"journal\": \"Protein science : a publication of the Protein Society\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — chemical synthesis + crystallography + enzymatic activity validation\",\n      \"pmids\": [\"27670942\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"In pulmonary arterial smooth muscle cells under chronic hypoxia, Rieske iron-sulfur protein (RISP)-dependent ROS generation causes dissociation of FKBP12.6 from RyR2, leading to increased RyR2 channel activity and Ca2+ release, NF-κB/cyclin D1 activation, cell proliferation, and pulmonary hypertension. S107 (stabilizer of RyR2/FKBP12.6 complex) prevents these effects; FKBP12.6 KO enhances PH.\",\n      \"method\": \"SMC-specific RyR2 KO, RISP knockdown, FKBP12.6 KO mice, FK506, S107 treatment, Ca2+ imaging, NF-κB/cyclin D1 pathway analysis, mouse chronic hypoxia model\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple genetic models (KO, knockdown) with mechanistic pathway and pharmacological intervention\",\n      \"pmids\": [\"32669538\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"miR-34a directly targets the 3'-UTR of FKBP1B mRNA to suppress FKBP1B expression during adipogenesis; miR-34a inhibitor blocks adipogenesis and recovers FKBP1B expression, while FKBP1B overexpression attenuates MDI-induced adipogenesis and PPARγ/C/EBPα expression. Shikonin inhibits adipogenesis by suppressing miR-34a and thereby recovering FKBP1B.\",\n      \"method\": \"miRNA inhibitor/mimic transfection, 3'-UTR luciferase reporter assay, FKBP1B overexpression in 3T3-L1 preadipocytes, RT-PCR, Western blot\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct 3'-UTR binding confirmed, overexpression phenotype, single lab\",\n      \"pmids\": [\"26471303\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"CHF1/Hey2 transcription factor regulates FKBP12.6 expression in the myocardium; conditional deletion of CHF1/Hey2 increases FKBP12.6 expression, and treatment of knockout myocytes with FK506 (which dissociates FKBP12.6 from RyR2) restores contractile function, placing CHF1/Hey2 upstream of FKBP12.6-RyR2 regulation of Ca2+ handling.\",\n      \"method\": \"Cardiac conditional KO mice, aortic banding, FK506 rescue, gene expression analysis, Ca2+ transient and myocyte contractility measurements\",\n      \"journal\": \"American journal of physiology. Heart and circulatory physiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis via conditional KO + pharmacological rescue, single lab\",\n      \"pmids\": [\"22408025\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"A novel RyR2 mutation R2401H in the FKBP12.6 binding region was identified in a CPVT patient; this region of RyR2 is implicated as critical for FKBP12.6-mediated Ca2+ channel gating regulation.\",\n      \"method\": \"PCR-SSCP and DNA sequencing in CPVT patient, genetic screening of 190 controls\",\n      \"journal\": \"International journal of cardiology\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — genetic identification only, no direct functional validation of FKBP12.6 interaction with mutant\",\n      \"pmids\": [\"15749201\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"FKBP12.6 (calstabin2/FKBP1B) is a 12.6-kDa cis-trans peptidyl-prolyl isomerase that selectively binds as four copies to the tetrameric cardiac ryanodine receptor (RyR2) at a conformationally sensitive site spanning RyR2 residues 305–1937 and the C-terminal domain, stabilizing RyR2 in the closed state to prevent diastolic SR Ca2+ leak; its dissociation—triggered by PKA or CaMKII phosphorylation of RyR2, oxidative stress, or cADPR signaling—increases RyR2 open probability and SR Ca2+ release, predisposing to arrhythmias, cardiac hypertrophy, and heart failure, while its forced re-association restores normal Ca2+ homeostasis and contractile function.\"\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1995,\n      \"finding\": \"FKBP12.6 (encoded by FKBP1B) was cloned from human tissue; the recombinant protein exhibits peptidyl-prolyl cis-trans isomerase (PPIase) activity and selectively associates with the cardiac ryanodine receptor isoform RyR2 (not RyR1) in cardiac muscle sarcoplasmic reticulum. Upon binding rapamycin, FKBP12.6 forms a complex with mTOR, and in transfected Jurkat cells it mediates calcineurin inhibition by FK506.\",\n      \"method\": \"cDNA cloning, recombinant protein expression, co-immunoprecipitation with native cardiac SR, calcineurin inhibition assay, mTOR binding assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — original cloning paper with multiple orthogonal biochemical assays; foundational study replicated by many subsequent labs\",\n      \"pmids\": [\"7592869\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"FKBP12.6 selectively binds to cardiac RyR2 but not skeletal RyR1; only FKBP12.6 (not FKBP12) can exchange with endogenously bound FKBP12.6 on cardiac SR, explaining why the cardiac CRC is isolated as a complex with FKBP12.6 whereas skeletal muscle CRC associates with FKBP12. FK506 dissociates FKBP12.6 from cardiac SR.\",\n      \"method\": \"35S-labeled FKBP isoform binding/competition assays with native cardiac and skeletal SR vesicles; cosedimentation\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — direct in vitro binding assay with radiolabeled probes; replicated across multiple species\",\n      \"pmids\": [\"8702774\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"Cyclic ADP-ribose (cADPR) binds directly to FKBP12.6 on the ryanodine receptor in pancreatic islet microsomes; this binding dissociates FKBP12.6 from the RyR, thereby releasing Ca2+ from the ER. Microsomes depleted of FKBP12.6 by cADPR treatment no longer respond to cADPR for Ca2+ release, establishing FKBP12.6 as the cADPR receptor on the islet RyR.\",\n      \"method\": \"Radioligand binding ([3H]FK506, [3H]cADPR), Ca2+ flux assay from islet microsomes, immunoprecipitation of FKBP12.6 after cADPR treatment\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — direct binding and functional reconstitution in native microsomes with loss-of-function verification\",\n      \"pmids\": [\"9013543\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"PKA phosphorylation of RyR2 dissociates FKBP12.6 from the channel and increases channel open probability. A macromolecular complex on the SR comprising RyR2, FKBP12.6, PKA, protein phosphatases PP1 and PP2A, and the anchoring protein mAKAP was defined. In failing human hearts, RyR2 is PKA-hyperphosphorylated, causing defective channel regulation due to FKBP12.6 dissociation.\",\n      \"method\": \"Cosedimentation, co-immunoprecipitation, single-channel lipid bilayer recordings, phosphorylation assays on human failing heart tissue\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — multiple orthogonal methods (co-IP, cosedimentation, single-channel recording) in both recombinant and native systems; highly cited foundational paper\",\n      \"pmids\": [\"10830164\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"In pacing-induced canine heart failure, the stoichiometry of FKBP12.6 bound per RyR2 monomer decreases from ~1:1 to ~0.4:1; this partial loss of RyR2-bound FKBP12.6 correlates with conformational changes in RyR2 and a prominent spontaneous Ca2+ leak from the SR. FKBP12.6 protein expression is significantly reduced in failing SR.\",\n      \"method\": \"[3H]dihydro-FK506 and [3H]ryanodine binding assays, stopped-flow Ca2+ release measurements, Western blotting of SR fractions\",\n      \"journal\": \"Circulation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — quantitative binding stoichiometry combined with functional Ca2+ flux measurements in native SR\",\n      \"pmids\": [\"11044432\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"Crystal structure of FKBP12.6 in complex with rapamycin determined at 2.0 Å resolution. The structures of FKBP12.6 and FKBP12 are nearly identical except for a displacement in the helical region of FKBP12.6 toward the hydrophobic pocket, a feature not predicted by homology modelling and which likely underlies RyR2-binding specificity.\",\n      \"method\": \"X-ray crystallography at 2.0 Å resolution\",\n      \"journal\": \"Acta crystallographica. Section D, Biological crystallography\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structure with functional interpretation of RyR2-selectivity-conferring residues\",\n      \"pmids\": [\"10713512\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Adenovirus-mediated overexpression of FKBP12.6 in adult rabbit cardiomyocytes reduces Ca2+ leak through RyR2 by 53%, increases SR Ca2+ load, and increases fractional shortening by 21%, demonstrating that FKBP12.6 stabilizes the closed conformation of RyR2 and enhances excitation-contraction coupling.\",\n      \"method\": \"Adenoviral gene transfer, Fura-2 Ca2+ imaging, SR Ca2+ uptake/leak assay in permeabilized myocytes, caffeine contracture measurements\",\n      \"journal\": \"Circulation research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean gain-of-function with specific mechanistic readouts (RyR2 leak, SR load, contractility) replicated in subsequent studies\",\n      \"pmids\": [\"11157671\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"FKBP12.6 binding characteristics of cardiac microsomes are widely conserved across vertebrates; most species (human, rabbit, rat, mouse, chicken, frog, fish) contain both FKBP12 and FKBP12.6 associated with RyR2, with dog being the exception (only FKBP12.6).\",\n      \"method\": \"[3H]FK506 and [3H]dihydro-FK506 binding and exchange assays on isolated cardiac microsomes from eight vertebrate species\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — systematic comparative binding study across species with quantitative radioligand assays\",\n      \"pmids\": [\"11237759\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Disruption of the FKBP12.6 gene in male mice causes cardiac hypertrophy with dysregulated Ca2+ sparks (increased amplitude and duration) and increased Ca2+-induced Ca2+ release gain. Female knockout mice develop the same Ca2+ dysregulation but are protected from hypertrophy by oestrogen; tamoxifen treatment of female knockouts induces hypertrophy, placing FKBP12.6-mediated Ca2+ regulation upstream of an oestrogen-sensitive hypertrophic pathway.\",\n      \"method\": \"Gene knockout mouse model, confocal Ca2+ spark imaging, echocardiography, tamoxifen pharmacological intervention\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — in vivo loss-of-function with mechanistic dissection by hormonal rescue; published in Nature\",\n      \"pmids\": [\"11907581\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"FKBP12.6 associates with RyR2 (not RyR1, RyR3, or IP3Rs) in tracheal smooth muscle; cADPR-induced Ca2+ release and spontaneous Ca2+ release in tracheal myocytes are mediated through FKBP12.6, as they are blocked by excess recombinant FKBP12.6 and absent in FKBP12.6-knockout myocytes. Force development is impaired in FKBP12.6-null tracheal smooth muscle.\",\n      \"method\": \"Co-immunoprecipitation, intracellular dialysis of cADPR, Ca2+ imaging, isometric force measurements in FKBP12.6-knockout mouse tissue\",\n      \"journal\": \"American journal of physiology. Cell physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — loss-of-function KO combined with pharmacological and reconstitution experiments in multiple readouts\",\n      \"pmids\": [\"14592808\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"The FKBP12.6-binding site on RyR2 is located within the N-terminal region (residues 305–1937) rather than the previously proposed central domain isoleucine-proline motif. The first 1937 N-terminal residues are sufficient for GST-FKBP12.6 binding, and binding is conformation-dependent; co-expression of overlapping fragments that restore channel function does not restore FKBP12.6 binding.\",\n      \"method\": \"GST-FKBP12.6 pulldown assays with systematic deletion/point mutants of RyR2 expressed in HEK293 cells\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — systematic mutagenesis and deletion analysis with quantitative pulldown; multiple mutants tested\",\n      \"pmids\": [\"12446682\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"FKBP12.6 cADPR-mediated activation of RyR2 Ca2+ release channels in arterial smooth muscle (bovine coronary artery) requires intact FKBP12.6 on the receptor: FK506 removal of FKBP12.6 blocks cADPR-induced channel activation, and anti-FKBP12 antibody abolishes both FK506- and cADPR-induced RyR activation in planar lipid bilayer recordings.\",\n      \"method\": \"Planar lipid bilayer single-channel recording, FK506 treatment, anti-FKBP12 antibody blockade, gradient centrifugation depletion\",\n      \"journal\": \"American journal of physiology. Heart and circulatory physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — single-channel electrophysiology with pharmacological and antibody interventions showing FKBP12.6 necessity for cADPR-mediated activation\",\n      \"pmids\": [\"11893565\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"FKBP12.6 knockout mice consistently exhibit exercise-induced ventricular arrhythmias causing sudden cardiac death. RyR2 mutations linked to CPVT reduce FKBP12.6 affinity for RyR2 and increase single-channel open probability under exercise-simulating conditions, establishing that FKBP12.6 dissociation from RyR2 is arrhythmogenic.\",\n      \"method\": \"FKBP12.6-/- mouse model, treadmill exercise, single-channel bilayer recordings, coimmunoprecipitation, [3H]ryanodine binding\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — in vivo KO phenotype plus single-channel reconstitution and mechanistic mutation analysis; highly cited\",\n      \"pmids\": [\"12837242\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"FKBP12.6 expressed in CHO cells co-localizes with RyR2 at ER membranes (sequestered from cytoplasm as RyR2 levels increase), suppresses agonist-induced Ca2+ release, and causes ER Ca2+ superfilling, demonstrating in situ that FKBP12.6 (but not FKBP12) specifically modulates hRyR2 channel functionality. Rapamycin reverses the FKBP12.6 effect.\",\n      \"method\": \"Stable CHO cell lines expressing graded hRyR2 levels, confocal microscopy, Ca2+ flux measurements, rapamycin pharmacology\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — dose-dependent in situ reconstitution with specific pharmacological reversal; isoform specificity demonstrated\",\n      \"pmids\": [\"12443530\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Co-expression of FKBP12.6 (but not FKBP12) with dysregulated RyR2 in CHO cells suppresses intracellular Ca2+ flux, restores normal cell viability and proliferation, demonstrating that FKBP12.6 prevents RyR2-mediated cellular toxicity through specific RyR2 channel stabilization.\",\n      \"method\": \"Stable CHO(hRyR2) cell lines, Ca2+ flux measurements, cell viability/proliferation assays, co-expression of FKBP isoforms\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — cell-based gain-of-function with functional readouts; single lab but multiple complementary assays\",\n      \"pmids\": [\"12754204\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"PKA phosphorylation at serine-2808 of RyR2 does not dissociate FKBP12.6 from RyR2; site-specific phospho-antibodies show FKBP12.6 binds equally to both phosphorylated and non-phosphorylated Ser-2808 forms, and the phosphomimetic S2808D mutant retains FKBP12.6 binding, challenging the PKA-dissociation model.\",\n      \"method\": \"Site-directed mutagenesis, site-specific phospho-antibodies, co-immunoprecipitation of recombinant and native RyR2, exogenous PKA phosphorylation\",\n      \"journal\": \"Circulation research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — rigorous mutagenesis combined with phospho-specific antibodies and multiple binding assays; challenges prior model\",\n      \"pmids\": [\"14715536\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"FKBP12.6 overexpression in rat cardiac myocytes decreases the occurrence, amplitude, duration, and width of spontaneous Ca2+ sparks but increases global [Ca2+]i transient amplitude and SR Ca2+ load, demonstrating that FKBP12.6 reduces local stochastic RyR2 openings while enhancing coordinated Ca2+ release during excitation-contraction coupling.\",\n      \"method\": \"Adenoviral overexpression, confocal Ca2+ spark imaging (Rhod-2), field stimulation, caffeine application in rat cardiomyocytes\",\n      \"journal\": \"American journal of physiology. Heart and circulatory physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — direct gain-of-function with quantitative spark and transient measurements; confirmed by multiple subsequent studies\",\n      \"pmids\": [\"15271664\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"A novel C-terminal FKBP12.6-binding site exists on RyR2 proximal to the pore-forming transmembrane domains; a large C-terminal RyR2 construct shows rapamycin-sensitive binding specifically to FKBP12.6 but not FKBP12, whereas short C-terminal fragments can displace FKBP12.6 from native RyR2 in competition assays.\",\n      \"method\": \"Competition binding assays with native RyR2, mammalian cell expression of C-terminal constructs, GST-pulldown, rapamycin competition\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — complementary binding approaches identifying a novel binding site; single lab\",\n      \"pmids\": [\"15591045\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Hypoxia- and norepinephrine-induced Ca2+ release and pulmonary artery vasoconstriction are significantly enhanced in FKBP12.6-knockout pulmonary artery smooth muscle cells, showing that FKBP12.6 suppresses hypoxic and neurotransmitter-driven RyR2 activation in pulmonary vascular smooth muscle.\",\n      \"method\": \"FKBP12.6 knockout mouse PASMCs, Ca2+ imaging, Cl- and K+ current recordings, isometric force measurements, FK506/rapamycin pharmacology\",\n      \"journal\": \"Cell calcium\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — KO loss-of-function with multiple functional readouts in native smooth muscle\",\n      \"pmids\": [\"15036951\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Cryo-EM 3-D reconstruction localizes FKBP12.6 binding on open-state RyR2 to the sides of the cytoplasmic region adjacent to domain 9 (part of the clamp structures). The conformation of FKBP12.6-bound RyR2 differs substantially from FKBP12.6-depleted RyR2 especially in the transmembrane region and clamp structures, providing structural basis for FKBP12.6-mediated channel stabilization.\",\n      \"method\": \"Cryoelectron microscopy, 3-D reconstruction, quantitative difference mapping, X-ray structure docking\",\n      \"journal\": \"Biophysical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — structural determination with functional correlation; direct visualization of FKBP12.6 on RyR2\",\n      \"pmids\": [\"16214874\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"FKBP12.6 central domain of RyR2 does not support FKBP12/12.6 interaction in yeast two-hybrid or immunoprecipitation assays; a distinct alternatively spliced variant of FKBP12.6 cannot interact with RyR. An interaction of FKBP12.6 with the cytoplasmic domain of TGF-β receptor type I was confirmed as a positive control, supporting specificity.\",\n      \"method\": \"Yeast two-hybrid, in vitro immunoprecipitation with overlapping RyR2 fragments, alternative splice variant analysis\",\n      \"journal\": \"Cell biochemistry and biophysics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — systematic domain interaction mapping; single lab\",\n      \"pmids\": [\"16049346\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"FKBP12.6 (calstabin2) Asp-37 is a key negatively charged residue involved in RyR2 binding; a D37S mutant calstabin2 with neutralized charge binds to constitutively PKA-phosphorylated RyR2-S2808D. Restoring calstabin2 stoichiometry at RyR2 by JTV519 treatment or genetic manipulation rescued cardiac function in a mouse myocardial infarction model, and the rescue was absent in calstabin2-/- mice.\",\n      \"method\": \"Site-directed mutagenesis of calstabin2, co-immunoprecipitation, murine MI model with JTV519 treatment, echocardiography\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — mutagenesis identifying critical binding residue combined with in vivo rescue; calstabin2-/- controls confirm mechanism\",\n      \"pmids\": [\"15972811\", \"16481613\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Removal of FKBP12.6 (by FK506 treatment or in FKBP12.6-null mice) does not alter conductance, Ca2+- or caffeine-activation properties of RyR2 in lipid bilayer recordings, does not change store-overload-induced Ca2+ release propensity in HEK293 cells, and FKBP12.6-null mice do not exhibit enhanced stress-induced ventricular arrhythmias, in contrast to earlier reports.\",\n      \"method\": \"Single-channel lipid bilayer recordings, [3H]ryanodine binding, HEK293 Ca2+ imaging, FKBP12.6-null mouse stress arrhythmia protocol\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1–2 — rigorous single-channel electrophysiology in multiple systems; single lab but systematic\",\n      \"pmids\": [\"17921453\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Oxidizing agents (H2O2, diamide) reduce FKBP12.6 binding to RyR2 through cysteine residues on the ryanodine receptor (not on FKBP12.6 itself, as a cysteine-null FKBP12.6 mutant retains redox-sensitive interaction). H2O2 effect is state-dependent (requires open state), whereas diamide is state-independent, establishing redox regulation of the RyR2-FKBP12.6 interaction.\",\n      \"method\": \"Co-immunoprecipitation and cosedimentation of [35S]FKBP12.6 with native cardiac SR under oxidizing/reducing conditions, cysteine-null FKBP12.6 mutant\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — systematic redox manipulation with mutagenesis and state-dependent experiments; mechanistically precise\",\n      \"pmids\": [\"17200109\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"FKBP12.6-deficient mice have increased susceptibility to atrial fibrillation (AF), inducible in 81% vs. 7% of wild-type mice. SR Ca2+ leak in FKBP12.6-/- atrial myocytes is 53% larger and spontaneous SR Ca2+ release events are increased; both AF and spontaneous releases are blocked by the RyR2 antagonist tetracaine, establishing that FKBP12.6-dependent SR Ca2+ leak drives AF initiation.\",\n      \"method\": \"FKBP12.6-/- mouse electrophysiology, intracardiac stimulation, Ca2+ imaging in atrial myocytes, tetracaine pharmacology\",\n      \"journal\": \"Heart rhythm\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — KO loss-of-function with electrophysiological and Ca2+ imaging endpoints plus pharmacological rescue\",\n      \"pmids\": [\"18598963\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"FKBP12.6 disruption in pancreatic beta-cells impairs glucose-induced insulin secretion downstream of ATP production independently of KATP channels. FKBP12.6-/- mice show glucose intolerance and insufficient insulin secretion after glucose challenge, while sulfonylurea- or KCl-induced secretion is unaffected, placing FKBP12.6-mediated cADPR/RyR Ca2+ signalling in the pathway of glucose-stimulated insulin secretion.\",\n      \"method\": \"FKBP12.6-/- mouse model (homologous recombination), glucose tolerance tests, perifusion insulin secretion assays, islet Ca2+ measurements\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO with specific functional dissection identifying pathway position for glucose-stimulated insulin secretion\",\n      \"pmids\": [\"18466757\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Cardiac-specific conditional FKBP12.6 overexpression in mice prevents isoproterenol-triggered ventricular tachycardia; Ca2+ spark frequency is reduced 50% (persisting under isoproterenol), SR Ca2+ load is unchanged, L-type Ca2+ current density decreases 15%, and Na+/Ca2+ exchanger protein is reduced 18%, demonstrating that enhanced FKBP12.6-RyR2 binding prevents diastolic SR Ca2+ leak and triggered arrhythmias.\",\n      \"method\": \"Conditional cardiac-specific transgenic mouse, burst pacing arrhythmia protocol, confocal Ca2+ spark imaging, voltage clamp, Western blotting\",\n      \"journal\": \"Circulation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — cardiac-specific conditional gain-of-function with comprehensive electrophysiology and Ca2+ imaging\",\n      \"pmids\": [\"18378612\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"In cardiac myocytes, FKBP12.6 binds RyR2 with very high affinity (Kd ~0.7 nM) and only FKBP12.6 (not FKBP12) inhibits resting RyR2 activity. PKA-dependent phosphorylation of RyR2 does not alter binding kinetics or affinity of either FKBP isoform. Quantitative immunoblots show endogenous [FKBP12.6] is ≤150 nM and virtually all FKBP12.6 is RyR2-bound, occupying ~10-20% of RyR2 channels.\",\n      \"method\": \"Fluorescently labeled FKBP in permeabilized myocytes, FRAP kinetics, Ca2+ spark imaging, quantitative immunoblots\",\n      \"journal\": \"Circulation research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct in-cell affinity measurement by FRAP/fluorescence plus quantitative proteomics; multiple orthogonal methods\",\n      \"pmids\": [\"20431056\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"FKBP12.6 dissociation from RyR2 does not significantly contribute to beta-adrenergic-stimulated Ca2+ release in cardiomyocytes (ISO increases Ca2+ sparks equally in WT and FKBP12.6-KO), but DOES mediate cADPR-induced Ca2+ spark increases (cADPR effect is absent in FKBP12.6-KO myocytes). Twitch force is not significantly different between WT and KO papillary muscles.\",\n      \"method\": \"FKBP12.6-KO mouse cardiomyocytes, Ca2+ spark imaging, isoproterenol and cADPR pharmacology, papillary muscle force measurements, Western blotting\",\n      \"journal\": \"Cardiovascular research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean KO model with paired pharmacological dissection of two signalling pathways\",\n      \"pmids\": [\"19578067\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"FKBP12.6-knockout mice show fed hyperinsulinemia, enhanced glucose-stimulated insulin secretion (GSIS) and islet Ca2+ elevation, and resistance to high-fat diet-induced hyperglycaemia, demonstrating that FKBP12.6 normally restrains RyR-mediated Ca2+ release during glucose stimulation to limit insulin secretion.\",\n      \"method\": \"FKBP12.6-/- mouse model, glucose and insulin tolerance tests, in vivo and in vitro GSIS, islet Ca2+ imaging, high-fat diet challenge\",\n      \"journal\": \"FASEB journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — in vivo and in vitro loss-of-function with multiple metabolic and Ca2+ readouts\",\n      \"pmids\": [\"19805579\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"CaMKII phosphorylation of RyR2 at Ser-2814 (not Ser-2808) is the downstream target responsible for SR Ca2+ leak, delayed afterdepolarizations, and AF in FKBP12.6-/- mice; S2814A knock-in into FKBP12.6-/- background reduces Ca2+ spark frequency, SR Ca2+ leak, and AF susceptibility, while S2808A does not protect.\",\n      \"method\": \"Double-mutant mouse models (FKBP12.6-/- × S2814A or S2808A RyR2 knock-in), pacing-induced AF, Ca2+ spark imaging in atrial myocytes, DAD recordings\",\n      \"journal\": \"Circulation research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — genetic epistasis with double knock-in/KO, identifying specific phosphorylation site downstream of FKBP12.6\",\n      \"pmids\": [\"22158709\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"FKBP12 is a high-affinity activator of RyR2 (sensitises channel to cytosolic Ca2+), whereas FKBP12.6 has very low intrinsic efficacy but antagonises FKBP12-mediated RyR2 activation. Physiological FKBP12 concentrations (3 µM) increase Ca2+ wave frequency and decrease SR Ca2+ content; FKBP12.6 opposes these effects, establishing a dual regulatory model of RyR2 by competing FKBP isoforms.\",\n      \"method\": \"Single-channel bilayer recordings with purified FKBP isoforms, mathematical modelling, Ca2+ wave imaging in permeabilized cardiac cells\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — single-channel electrophysiology with purified proteins plus quantitative modelling and cell Ca2+ imaging\",\n      \"pmids\": [\"22363773\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Sirolimus (rapamycin)-FKBP12.6 complex impairs endothelial barrier function by activating protein kinase C-α, which disrupts the p120-VE-cadherin interaction; siRNA knockdown of FKBP12.6 phenocopies sirolimus, and ryanodine pretreatment prevents the sirolimus-induced Ca2+ increase and barrier disruption.\",\n      \"method\": \"Transendothelial electrical resistance, siRNA knockdown, Ca2+ imaging, PKC-α phosphorylation assays, Evans blue permeability in vivo, p120-VE-cadherin co-immunostaining\",\n      \"journal\": \"Arteriosclerosis, thrombosis, and vascular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple cell-based and in vivo readouts; mechanistic pathway defined; single lab\",\n      \"pmids\": [\"23887639\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"miR-34a directly targets the 3'-UTR of FKBP1B mRNA; FKBP1B expression decreases during adipogenesis in parallel with miR-34a increase; FKBP1B overexpression attenuates MDI-induced adipogenesis and suppresses PPARγ/C/EBPα expression, identifying FKBP1B as a negative regulator of adipogenic differentiation downstream of miR-34a.\",\n      \"method\": \"3'-UTR luciferase reporter assay, miR-34a inhibitor/mimic transfection, FKBP1B overexpression in 3T3-L1 preadipocytes, adipogenesis assays\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct 3'-UTR binding validated with functional gain-of-function; single lab\",\n      \"pmids\": [\"26471303\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Total chemical synthesis and refolding of calstabin 2 (FKBP12.6) yields a catalytically active PPIase enzyme whose crystal structure confirms correct fold; N-terminal exotic amino acid substitutions do not alter catalytic activity, establishing structure-function relationships in the isomerase domain.\",\n      \"method\": \"Native chemical ligation synthesis, protein refolding, PPIase activity assay, X-ray crystallography\",\n      \"journal\": \"Protein science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — total synthesis with enzymatic validation and crystal structure; rigorous chemical biology approach\",\n      \"pmids\": [\"27670942\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Cryo-EM structure of rabbit RyR2 in complex with FKBP12.6 in the closed state at 11.8 Å reveals two phosphorylation-related RyR2 conformations; FKBP12.6 binding rigidifies the HD2 domain of RyR2, stabilising the closed state. The more flexible conformation likely corresponds to a phosphorylated P2 domain that requires less energy to open.\",\n      \"method\": \"Single-particle cryo-EM, atomic model building, heterogeneity analysis of conformational states\",\n      \"journal\": \"Science signaling\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — cryo-EM structure with atomic model; directly links FKBP12.6 binding to HD2 rigidity and closed-state stabilisation\",\n      \"pmids\": [\"28536302\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Absence or pharmacological removal of FKBP12.6 from cardiomyocytes increases LCC-RyR2 coupling fidelity and accelerates LCC-to-spark signalling kinetics without changing L-type Ca2+ channel open probability; synergistic destabilisation by FKBP12.6 dysfunction and catecholaminergic signalling produces chaotic Ca2+ waves and ventricular arrhythmias.\",\n      \"method\": \"Whole-cell patch clamp combined with confocal Ca2+ imaging in FKBP12.6-KO and FK506/rapamycin-treated myocytes, loose-seal patch-clamp LCC sparklet-spark coupling\",\n      \"journal\": \"Cardiovascular research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — direct single-channel and Ca2+ imaging readouts; KO and pharmacological corroboration; mechanistically precise\",\n      \"pmids\": [\"28077437\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"FKBP12.6 protects against angiotensin II-induced cardiac hypertrophy in vivo; FKBP12.6-/- mice show aggravated AngII-induced hypertrophy while cardiac-specific FKBP12.6-TG mice are protected. The mechanism involves FKBP12.6 reducing intracellular [Ca2+]i and thereby inhibiting calcineurin/NFATc4, CaMKII/MEF-2, AKT/GSK3β/NFATc4, and AKT/mTOR signalling pathways.\",\n      \"method\": \"FKBP12.6-/- and cardiac-specific FKBP12.6 TG mouse models, AngII osmotic pump infusion, echocardiography, Ca2+ imaging in H9c2 cells, Western blotting of signalling pathways\",\n      \"journal\": \"Journal of cellular and molecular medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — dual KO/OE in vivo models with pathway analysis; single lab\",\n      \"pmids\": [\"29682889\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Rieske iron-sulfur protein (RISP)-dependent ROS generation in pulmonary artery smooth muscle cells dissociates FKBP12.6 from RyR2, increasing RyR channel activity and Ca2+ release, which activates NF-κB/cyclin D1 signalling to promote PASMC proliferation and pulmonary hypertension. FKBP12.6 KO or FK506 exacerbates hypoxia-induced PH, while the RyR2/FKBP12.6 stabiliser S107 is protective.\",\n      \"method\": \"SMC-specific RyR2 KO and RISP knockdown mice, FKBP12.6 KO mice, S107 drug treatment, chronic hypoxia model, Ca2+ imaging, NF-κB/cyclin D1 pathway analysis, cell proliferation assays\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple genetic models (KO, knockdown, drug stabilisation) with in vivo PH endpoint and mechanistic pathway definition\",\n      \"pmids\": [\"32669538\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1994,\n      \"finding\": \"Molecular cloning of human FKBP1B (designated OTK4): the protein shares 88% amino acid identity with FKBP12 and recombinant OTK4 expressed in E. coli exhibits peptidyl-prolyl cis-trans isomerase (PPIase) activity. Two alternatively spliced transcripts are ubiquitously expressed in human tissues.\",\n      \"method\": \"cDNA library screening, recombinant protein expression, PPIase enzymatic activity assay, RT-PCR tissue expression\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — original cloning paper with direct enzymatic activity measurement\",\n      \"pmids\": [\"7513996\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Genomic structure of human FKBP1B (FKBP12.6 gene) spans ~16 kb with 4 exons and 3 introns on chromosome 2p21-23. Reporter gene and EMSA analyses identify that Sp3 transcription factor drives FKBP12.6 promoter activity via a consensus Sp-family element at -58 to -24.\",\n      \"method\": \"Fluorescence in situ hybridisation, reporter gene assays, electrophoretic mobility shift assays, promoter deletion analysis\",\n      \"journal\": \"Gene\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct promoter functional analysis with EMSA confirming Sp3 binding; single lab\",\n      \"pmids\": [\"16122887\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"FKBP12.6 bound to RyR1 and RyR2 adopts the same orientation as determined by FRET between site-specifically labelled FKBP12.6 and RyR-bound calmodulin. Fluorescent labelling at position 41 of FKBP12.6 reduces RyR1 affinity 10-fold; position 32 reduces maximal inhibition of [3H]ryanodine binding by half, delineating surface residues important for RyR binding versus inhibition.\",\n      \"method\": \"Site-directed single-cysteine labelling of FKBP12.6, FRET measurements, [3H]ryanodine binding inhibition assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — FRET-based structural mapping with mutagenesis defining orientation and binding-critical residues\",\n      \"pmids\": [\"20404344\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"FKBP12.6 and FKBP12 facilitate termination of store overload-induced Ca2+ release (SOICR) in wild-type RyR2 by raising the SOICR termination threshold without changing the activation threshold; an arrhythmogenic RyR2 CPVT mutant retains FKBP association but FKBPs are unable to regulate the mutant channel, representing a novel mechanism of CPVT arrhythmia.\",\n      \"method\": \"Single-cell Ca2+ imaging in HEK293 cells expressing WT or CPVT mutant RyR2, FKBP overexpression, SOICR threshold measurements\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — functional dissection of SOICR termination vs. activation; CPVT mutant comparison; single lab\",\n      \"pmids\": [\"27154203\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"FKBP1B encodes calstabin 2 (FKBP12.6), a 12.6-kDa cis-trans peptidyl-prolyl isomerase that selectively associates (at ~0.7 nM affinity) with the cardiac ryanodine receptor RyR2 as part of a macromolecular complex including PKA, PP1, PP2A, and mAKAP on the sarcoplasmic reticulum; bound FKBP12.6 rigidifies the RyR2 HD2 clamp domain, stabilises the channel closed state, reduces spontaneous Ca2+ spark frequency, and facilitates SOICR termination, while its dissociation—triggered by PKA/CaMKII hyperphosphorylation, oxidative stress (via RyR2 cysteines), or cADPR binding—increases RyR2 open probability and Ca2+ leak, leading to arrhythmias, cardiac hypertrophy, pulmonary hypertension, and impaired glucose-stimulated insulin secretion.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"FKBP12.6 (calstabin2/FKBP1B) is a peptidyl-prolyl cis-trans isomerase that functions as the principal stabilizer of the cardiac ryanodine receptor (RyR2) in its closed state, thereby preventing diastolic sarcoplasmic reticulum Ca²⁺ leak in cardiomyocytes, smooth muscle, and pancreatic beta cells [PMID:8702774, PMID:11157671, PMID:18466757]. Four copies of FKBP12.6 bind each RyR2 tetramer at a conformation-dependent interface spanning N-terminal residues 305–1937 and a C-terminal site near the pore domain, rigidifying the HD2/clamp region to lock the channel shut; dissociation—triggered by oxidative stress, cADPR signaling, or CaMKII-mediated RyR2-S2814 phosphorylation—increases channel open probability and Ca²⁺ release [PMID:12446682, PMID:15591045, PMID:28536302, PMID:11893565, PMID:22158709]. Genetic ablation of FKBP12.6 in mice causes sex-dependent cardiac hypertrophy, exercise-induced ventricular arrhythmias and sudden death resembling catecholaminergic polymorphic ventricular tachycardia (CPVT), pacing-induced atrial fibrillation, enhanced hypoxic pulmonary vasoconstriction, and impaired glucose-stimulated insulin secretion, while cardiac-specific overexpression or pharmacological re-stabilization of the FKBP12.6–RyR2 complex rescues Ca²⁺ homeostasis, contractility, and arrhythmia susceptibility [PMID:11907581, PMID:12837242, PMID:18598963, PMID:18378612, PMID:32669538, PMID:18466757]. CPVT-linked RyR2 mutations reduce FKBP12.6 binding affinity, directly implicating disruption of this interaction in heritable arrhythmia syndromes [PMID:12837242].\",\n  \"teleology\": [\n    {\n      \"year\": 1996,\n      \"claim\": \"The discovery that FKBP12.6 selectively associates with cardiac RyR2—not skeletal RyR1—established it as the isoform-specific modulatory subunit of the cardiac Ca²⁺ release channel, resolving why cardiac SR copurifies with FKBP12.6 rather than FKBP12.\",\n      \"evidence\": \"Radiolabeled FKBP12.6 binding/exchange assays and cosedimentation with stripped cardiac SR vesicles\",\n      \"pmids\": [\"8702774\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis for isoform selectivity unknown\", \"Stoichiometry of FKBP12.6 per RyR2 tetramer not yet measured\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Structural determination of FKBP12.6 revealed a subtle helical displacement toward the hydrophobic pocket compared to FKBP12, providing the first structural rationale for its differential RyR2-binding specificity.\",\n      \"evidence\": \"X-ray crystallography of FKBP12.6–rapamycin complex at 2.0 Å resolution\",\n      \"pmids\": [\"10713512\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No structure of FKBP12.6 bound to RyR2 peptide or domain\", \"Specific residues mediating selectivity not yet identified\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Demonstration that PKA phosphorylation of RyR2 dissociates FKBP12.6 and that failing human hearts show RyR2 hyperphosphorylation with FKBP12.6 depletion linked the FKBP12.6–RyR2 interaction to a pathological mechanism in heart failure.\",\n      \"evidence\": \"Co-IP, cosedimentation, single-channel recordings, PKA phosphorylation assays in failing human heart tissue\",\n      \"pmids\": [\"10830164\", \"11044432\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Specific phosphorylation site(s) responsible for dissociation not identified\", \"Causality versus correlation in human tissue not resolved\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Gain-of-function overexpression of FKBP12.6 in cardiomyocytes proved it is sufficient to reduce SR Ca²⁺ leak and enhance contractility, establishing its role as a stabilizer of RyR2's closed state.\",\n      \"evidence\": \"Adenoviral FKBP12.6 overexpression in rabbit cardiomyocytes with SR Ca²⁺ uptake, caffeine contracture, and fractional shortening assays\",\n      \"pmids\": [\"11157671\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Supraphysiological expression levels may not reflect endogenous regulation\", \"Effect on arrhythmia susceptibility not tested\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"FKBP12.6 knockout mice revealed that loss of FKBP12.6 causes sex-dependent cardiac hypertrophy and dysregulated Ca²⁺ sparks, providing the first in vivo genetic evidence for its cardioprotective role and uncovering an estrogen-dependent modifier.\",\n      \"evidence\": \"FKBP12.6 KO mice with echocardiography, Ca²⁺ spark imaging, and tamoxifen reversal\",\n      \"pmids\": [\"11907581\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism of estrogen protection not defined at the molecular level\", \"Compensatory changes in FKBP12 not fully assessed\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Mapping the FKBP12.6 binding site to RyR2 residues 305–1937 and demonstrating conformation-dependence resolved the binding domain question and ruled out a previously proposed Ile-Pro motif at 2427–2428.\",\n      \"evidence\": \"Systematic deletion and point mutagenesis of RyR2 in HEK293 cells with GST-FKBP12.6 pulldown\",\n      \"pmids\": [\"12446682\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Minimal binding epitope within the large 305–1937 region not defined\", \"C-terminal binding site not yet identified\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Reconstitution experiments in smooth muscle and planar lipid bilayers established that FKBP12.6 mediates cADPR-induced Ca²⁺ release through RyR2, extending its role beyond cardiac muscle to vascular biology.\",\n      \"evidence\": \"Single-channel recordings with FKBP12.6 immunodepletion in coronary artery SR, and Ca²⁺ imaging in tracheal myocytes from FKBP12.6 KO mice\",\n      \"pmids\": [\"11893565\", \"14592808\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular mechanism by which cADPR modulates FKBP12.6–RyR2 interaction unknown\", \"Whether cADPR displaces or allosterically modifies the complex not resolved\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Exercise-induced sudden cardiac death in FKBP12.6 KO mice and reduced FKBP12.6 affinity of CPVT-linked RyR2 mutants directly implicated FKBP12.6 dissociation in heritable catecholaminergic ventricular tachycardia.\",\n      \"evidence\": \"FKBP12.6 KO mice with exercise-induced arrhythmia testing, single-channel recordings, and binding assays with CPVT patient RyR2 mutations\",\n      \"pmids\": [\"12837242\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Not all CPVT mutations tested for FKBP12.6 affinity\", \"Rescue by FKBP12.6 re-expression not yet demonstrated in CPVT mutation carriers\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"A direct challenge to the PKA-Ser2808 dissociation model showed that PKA phosphorylation at Ser-2808 does not dissociate FKBP12.6, indicating the mechanism linking adrenergic stimulation to FKBP12.6 displacement is more complex than initially proposed.\",\n      \"evidence\": \"Phospho-specific antibodies, S2808D phosphomimetic mutagenesis, GST pulldown, and native complex disruption assays in HEK293 cells\",\n      \"pmids\": [\"14715536\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Alternative phosphorylation site(s) mediating dissociation not identified\", \"Discrepancy with earlier PKA dissociation data not fully reconciled\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Identification of a second FKBP12.6-binding site in the RyR2 C-terminal domain (near the pore) suggested the interaction is multisite, potentially explaining conformation-dependent binding.\",\n      \"evidence\": \"Competition binding assays, mammalian cell expression of large RyR2 C-terminal fragment, rapamycin-sensitivity\",\n      \"pmids\": [\"15591045\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"C-terminal binding site not reconstituted in isolation\", \"Not independently confirmed by a second laboratory\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Cryo-EM localization of FKBP12.6 on RyR2 placed it along the cytoplasmic clamp domain near domain 9, showing it induces conformational changes propagating to the transmembrane pore region.\",\n      \"evidence\": \"Cryo-EM 3D reconstruction with docking of FKBP12.6 X-ray structure\",\n      \"pmids\": [\"16214874\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Limited resolution prevents atomic-level interface characterization\", \"Only open-state complex visualized\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Identification of Asp-37 as a key RyR2-contact residue on FKBP12.6, and demonstration that a D37S mutant can bind phosphorylated RyR2 and rescue cardiac function post-myocardial infarction, provided proof-of-concept for engineered FKBP12.6 variants as therapeutics.\",\n      \"evidence\": \"Site-directed mutagenesis, binding assays with RyR2-S2808D, viral FKBP12.6 delivery in murine MI model\",\n      \"pmids\": [\"16481613\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Long-term safety and efficacy of viral FKBP12.6 delivery not assessed\", \"Binding interface beyond Asp-37 not fully mapped\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"FKBP12.6 KO mice proved susceptible to pacing-induced atrial fibrillation with enhanced atrial SR Ca²⁺ leak, extending the arrhythmia phenotype from ventricle to atrium and establishing FKBP12.6–RyR2 disruption as a mechanism for AF.\",\n      \"evidence\": \"FKBP12.6 KO mice, intracardiac electrophysiology, Ca²⁺ transient recording, tetracaine rescue\",\n      \"pmids\": [\"18598963\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether FKBP12.6 re-expression can reverse established AF not tested\", \"Contribution of structural remodeling not excluded\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Cardiac-specific FKBP12.6 overexpression in mice reduced Ca²⁺ spark frequency by 50% and prevented catecholamine-induced ventricular tachycardia, providing the strongest in vivo gain-of-function evidence for its anti-arrhythmic role.\",\n      \"evidence\": \"Conditional transgenic mice, isoproterenol challenge, confocal Ca²⁺ imaging, patch-clamp, in vivo electrophysiology\",\n      \"pmids\": [\"18378612\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Effect of chronic overexpression on cardiac remodeling not fully characterized\", \"Mechanism of ICa,L density reduction not explained\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Quantitative in situ binding measurements determined FKBP12.6 binds RyR2 with sub-nanomolar affinity (Kd ~0.7 nM) and that PKA phosphorylation does not alter this affinity, reinforcing that the PKA–dissociation model requires revision.\",\n      \"evidence\": \"Fluorescent FKBP binding in permeabilized rat/mouse myocytes, FRAP, Ca²⁺ spark imaging, quantitative immunoblots\",\n      \"pmids\": [\"20431056\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Only ~10–20% of RyR2 sites occupied by FKBP12.6 despite high affinity — reason unexplained\", \"CaMKII-mediated phosphorylation effect on binding not tested here\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Genetic epistasis showed CaMKII phosphorylation at RyR2-S2814 (not PKA at S2808) is the critical downstream event linking FKBP12.6 loss to atrial fibrillation, resolving which kinase pathway mediates the arrhythmogenic phenotype.\",\n      \"evidence\": \"FKBP12.6 KO crossed with RyR2-S2814A or S2808A knock-in mice; Ca²⁺ spark imaging; intracardiac EP\",\n      \"pmids\": [\"22158709\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether CaMKII directly phosphorylates RyR2 after FKBP12.6 loss or acts indirectly not distinguished\", \"Ventricular arrhythmia epistasis not tested\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Single-channel reconstitution revealed that FKBP12 activates RyR2 while FKBP12.6 antagonizes this effect, establishing a competitive balance model in which the ratio of the two isoforms tunes channel activity.\",\n      \"evidence\": \"Single RyR2 channels in planar bilayers with purified FKBP12 and FKBP12.6, Ca²⁺ wave measurements in permeabilized cardiomyocytes\",\n      \"pmids\": [\"22363773\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo relevance of the competitive balance not validated genetically\", \"Structural basis for opposing effects of the two isoforms unknown\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"A cryo-EM structure of the closed-state RyR2–FKBP12.6 complex revealed that FKBP12.6 rigidifies the HD2 domain of the clamp, providing the first structural explanation for how FKBP12.6 stabilizes channel closure.\",\n      \"evidence\": \"Cryo-EM 3D reconstruction of rabbit RyR2–FKBP12.6 at 11.8 Å with atomic model building\",\n      \"pmids\": [\"28536302\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Resolution insufficient for side-chain contacts\", \"Open-state RyR2 complex with FKBP12.6 not captured\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Identification of RISP-dependent mitochondrial ROS as the upstream trigger for FKBP12.6 dissociation from RyR2 in hypoxic pulmonary artery smooth muscle connected FKBP12.6 loss to NF-κB/cyclin D1–driven vascular remodeling and pulmonary hypertension.\",\n      \"evidence\": \"SMC-specific RyR2 KO, RISP knockdown, FKBP12.6 KO mice, S107 stabilizer, chronic hypoxia model\",\n      \"pmids\": [\"32669538\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct oxidative modification site on RyR2 or FKBP12.6 not identified\", \"Relevance to human pulmonary arterial hypertension not established\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the atomic-resolution interface between FKBP12.6 and RyR2, the precise post-translational modification(s) that dissociate FKBP12.6 in vivo, the reason only ~10–20% of RyR2 sites are FKBP12.6-occupied despite sub-nanomolar affinity, and whether engineered FKBP12.6 variants can serve as viable anti-arrhythmic therapeutics in humans.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Atomic-resolution structure of FKBP12.6–RyR2 interface not available\", \"Mechanism linking oxidative stress or CaMKII to FKBP12.6 dissociation at molecular level unknown\", \"Sub-stoichiometric occupancy of RyR2 by FKBP12.6 in native cardiomyocytes unexplained\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [28]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [0, 1, 7, 8, 16, 17, 20]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [8]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [8, 16]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-382551\", \"supporting_discovery_ids\": [0, 1, 7, 8, 14, 16, 17, 20]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [1, 19, 27, 29]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [1, 2, 6, 18]}\n    ],\n    \"complexes\": [\n      \"RyR2 macromolecular complex (RyR2/FKBP12.6/PKA/PP1/PP2A/mAKAP)\"\n    ],\n    \"partners\": [\n      \"RYR2\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"mechanistic_narrative\": \"FKBP1B encodes calstabin 2 (FKBP12.6), a peptidyl-prolyl cis-trans isomerase that functions as a critical stabilizer of the cardiac ryanodine receptor RyR2 in a macromolecular sarcoplasmic reticulum complex containing PKA, PP1, PP2A, and mAKAP [PMID:7592869, PMID:10830164]. Bound FKBP12.6 rigidifies the RyR2 HD2 clamp domain, stabilizes the channel closed state, reduces spontaneous Ca²⁺ spark frequency, and facilitates termination of store-overload-induced Ca²⁺ release; its dissociation—triggered by oxidative modification of RyR2 cysteines, CaMKII phosphorylation at Ser-2814, or cADPR binding—increases RyR2 open probability and diastolic SR Ca²⁺ leak [PMID:28536302, PMID:22158709, PMID:17200109, PMID:9013543]. Loss of FKBP12.6 in mice causes exercise-induced ventricular tachycardia, atrial fibrillation, sex-dependent cardiac hypertrophy, enhanced hypoxia-driven pulmonary vasoconstriction and pulmonary hypertension, and impaired glucose-stimulated insulin secretion in pancreatic β-cells [PMID:12837242, PMID:18598963, PMID:11907581, PMID:32669538, PMID:18466757]. Beyond the heart, FKBP12.6 mediates cADPR-dependent Ca²⁺ release through RyR2 in airway and vascular smooth muscle, and when complexed with rapamycin it can engage mTOR or disrupt endothelial barrier function via PKCα-dependent VE-cadherin uncoupling [PMID:14592808, PMID:11893565, PMID:23887639].\",\n  \"teleology\": [\n    {\n      \"year\": 1994,\n      \"claim\": \"Identification of FKBP1B as a novel FKBP12-related gene with intrinsic PPIase activity established that mammals encode a second immunophilin closely related to FKBP12 but with distinct tissue expression.\",\n      \"evidence\": \"cDNA cloning from human tissue, recombinant PPIase assay, RT-PCR tissue survey\",\n      \"pmids\": [\"7513996\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No binding partner or physiological substrate identified\", \"Functional distinction from FKBP12 unknown\"]\n    },\n    {\n      \"year\": 1995,\n      \"claim\": \"Demonstration that FKBP12.6 selectively associates with cardiac RyR2 (not RyR1) and mediates calcineurin inhibition via FK506 resolved the question of isoform specificity and linked FKBP12.6 to cardiac Ca²⁺ regulation.\",\n      \"evidence\": \"Co-immunoprecipitation with native cardiac SR, rapamycin-mTOR binding assay, calcineurin inhibition in Jurkat cells\",\n      \"pmids\": [\"7592869\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Functional consequence of RyR2 binding on channel gating not yet determined\", \"In vivo relevance unestablished\"]\n    },\n    {\n      \"year\": 1996,\n      \"claim\": \"Selective FKBP12.6-RyR2 binding was confirmed by isoform exchange experiments showing only FKBP12.6 can compete for the cardiac RyR binding site, establishing the molecular basis for tissue-specific FKBP-RyR pairing.\",\n      \"evidence\": \"Radiolabeled FKBP12/12.6 competition assays with cardiac and skeletal SR vesicles\",\n      \"pmids\": [\"8702774\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis for selectivity unknown\", \"Stoichiometry of binding not quantified\"]\n    },\n    {\n      \"year\": 1997,\n      \"claim\": \"Discovery that cADPR binds FKBP12.6 on the RyR and dissociates it to trigger Ca²⁺ release from islet microsomes revealed FKBP12.6 as the intracellular cADPR-responsive element in non-cardiac tissues.\",\n      \"evidence\": \"Radioligand binding with [³H]cADPR, Ca²⁺ flux from pancreatic islet microsomes, immunoprecipitation after cADPR treatment\",\n      \"pmids\": [\"9013543\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether cADPR binds FKBP12.6 directly or acts through RyR conformational change debated\", \"In vivo metabolic consequences unknown\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Three concurrent advances defined FKBP12.6's structural and functional role: the crystal structure revealed a displaced helix conferring RyR2 selectivity; a macromolecular SR complex (RyR2–FKBP12.6–PKA–PP1–PP2A–mAKAP) was delineated; and PKA hyperphosphorylation in failing human hearts was shown to dissociate FKBP12.6, causing Ca²⁺ leak.\",\n      \"evidence\": \"X-ray crystallography at 2.0 Å; co-IP/cosedimentation and single-channel bilayer recordings on human failing heart tissue; stoichiometric binding assays in canine HF model\",\n      \"pmids\": [\"10713512\", \"10830164\", \"11044432\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether PKA phosphorylation at Ser-2808 is the sole dissociation trigger contested\", \"Cryo-EM localization of FKBP12.6 on RyR2 not yet available\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Adenoviral FKBP12.6 overexpression in cardiomyocytes provided the first direct gain-of-function evidence that FKBP12.6 stabilizes RyR2 closure, reduces Ca²⁺ leak, and enhances excitation-contraction coupling.\",\n      \"evidence\": \"Adenoviral gene transfer in rabbit cardiomyocytes, Fura-2 Ca²⁺ imaging, SR leak assay, caffeine contracture\",\n      \"pmids\": [\"11157671\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo cardiac-specific overexpression not yet tested\", \"Effect on arrhythmia susceptibility unknown\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Generation of FKBP12.6-knockout mice established in vivo causality: male knockouts develop cardiac hypertrophy with dysregulated Ca²⁺ sparks, female knockouts are protected by estrogen, and tracheal smooth muscle loses cADPR-induced Ca²⁺ release, demonstrating tissue-wide dependence on FKBP12.6 for RyR2 regulation.\",\n      \"evidence\": \"FKBP12.6⁻/⁻ mouse model, confocal Ca²⁺ spark imaging, echocardiography, tamoxifen intervention, isometric force measurements in tracheal smooth muscle\",\n      \"pmids\": [\"11907581\", \"14592808\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism of estrogen-dependent protection unclear\", \"Whether phenotype is exclusively RyR2-dependent not formally excluded\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Mapping the FKBP12.6-binding site to the RyR2 N-terminal region (residues 305–1937) overturned the prior model of a central isoleucine-proline motif and showed binding is conformation-dependent.\",\n      \"evidence\": \"GST-FKBP12.6 pulldown with systematic RyR2 deletion/point mutants in HEK293 cells\",\n      \"pmids\": [\"12446682\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Atomic contacts not resolved\", \"Additional C-terminal binding site later proposed (PMID:15591045) but relationship between sites unclear\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"FKBP12.6-knockout mice exhibit exercise-induced ventricular arrhythmias and sudden cardiac death, and CPVT-linked RyR2 mutations reduce FKBP12.6 affinity, directly linking FKBP12.6 dissociation to arrhythmogenesis.\",\n      \"evidence\": \"Treadmill exercise in FKBP12.6⁻/⁻ mice, single-channel bilayer recordings, co-IP with CPVT mutant RyR2\",\n      \"pmids\": [\"12837242\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Later studies (PMID:17921453) failed to reproduce stress-induced arrhythmias in an independent FKBP12.6-null line, creating unresolved controversy\", \"Mechanism by which CPVT mutations alter FKBP12.6 affinity not structurally defined\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"The PKA-dissociation model was challenged by the finding that phosphorylation at Ser-2808 does not dissociate FKBP12.6, while separate work showed FKBP12.6 overexpression reduces Ca²⁺ spark frequency and increases coordinated Ca²⁺ transients, refining understanding of FKBP12.6's role in E-C coupling.\",\n      \"evidence\": \"Site-directed mutagenesis with phospho-specific antibodies and co-IP (PMID:14715536); adenoviral overexpression with confocal spark imaging in rat cardiomyocytes (PMID:15271664)\",\n      \"pmids\": [\"14715536\", \"15271664\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether CaMKII phosphorylation at Ser-2814 (rather than PKA at Ser-2808) is the relevant dissociation trigger not yet tested\", \"Ser-2808 controversy unresolved between labs\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Cryo-EM localized FKBP12.6 to the cytoplasmic clamp region of RyR2, and mutagenesis identified Asp-37 as a critical FKBP12.6 residue for RyR2 binding; a charge-neutralized D37S mutant can bind phosphorylated RyR2, and the drug JTV519 rescues cardiac function only in the presence of FKBP12.6.\",\n      \"evidence\": \"Cryo-EM 3-D reconstruction with difference mapping; D37S mutagenesis, co-IP, murine MI model with JTV519\",\n      \"pmids\": [\"16214874\", \"15972811\", \"16481613\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Resolution insufficient for atomic contact mapping\", \"JTV519 mechanism of action on FKBP12.6–RyR2 not fully defined\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Oxidative stress was shown to reduce FKBP12.6 binding through RyR2 cysteine residues (not FKBP12.6 cysteines), establishing redox regulation as a distinct dissociation mechanism, though a contemporaneous study found no effect of FKBP12.6 removal on RyR2 gating, highlighting ongoing controversy.\",\n      \"evidence\": \"Co-IP/cosedimentation of [³⁵S]FKBP12.6 under oxidizing/reducing conditions with cysteine-null mutant (PMID:17200109); single-channel bilayer recordings and stress arrhythmia protocol in independent FKBP12.6⁻/⁻ line (PMID:17921453)\",\n      \"pmids\": [\"17200109\", \"17921453\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of specific RyR2 cysteines mediating FKBP12.6 dissociation unknown\", \"Reason for discrepant arrhythmia phenotypes across FKBP12.6-null lines unresolved\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"FKBP12.6 was connected to two new pathophysiological contexts: atrial fibrillation (81% AF inducibility in knockouts via SR Ca²⁺ leak) and glucose-stimulated insulin secretion (impaired GSIS in knockout islets), broadening the gene's functional scope beyond ventricular arrhythmia.\",\n      \"evidence\": \"Intracardiac stimulation and Ca²⁺ imaging in FKBP12.6⁻/⁻ atrial myocytes; glucose tolerance tests and perifusion insulin secretion in FKBP12.6⁻/⁻ mice\",\n      \"pmids\": [\"18598963\", \"18466757\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether FKBP12.6 loss phenocopies human AF genetics unknown\", \"Relative contributions of cADPR-RyR2 vs. other pathways in β-cell Ca²⁺ signaling not fully delineated\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Quantitative in-cell measurements established FKBP12.6's binding affinity for RyR2 at ~0.7 nM with ~10–20% occupancy of RyR2 channels, and demonstrated that cADPR (but not β-adrenergic stimulation) requires FKBP12.6 for Ca²⁺ spark augmentation, while a separate study showed FKBP12.6 knockout enhances rather than impairs GSIS.\",\n      \"evidence\": \"FRAP kinetics of fluorescent FKBP12.6 in permeabilized myocytes, quantitative immunoblots (PMID:20431056); cADPR/ISO pharmacology in KO myocytes (PMID:19578067); in vivo/in vitro GSIS in KO mice (PMID:19805579)\",\n      \"pmids\": [\"20431056\", \"19578067\", \"19805579\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Conflicting reports on GSIS direction (impaired vs. enhanced) in FKBP12.6 KO mice unresolved\", \"PKA-independence of FKBP12.6 dissociation contradicts earlier Marks lab findings\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Genetic epistasis in double-mutant mice resolved the phosphorylation controversy for AF: CaMKII phosphorylation at RyR2-Ser2814 (not PKA at Ser-2808) is the downstream event causing SR Ca²⁺ leak and AF in FKBP12.6-deficient hearts.\",\n      \"evidence\": \"FKBP12.6⁻/⁻ × S2814A or S2808A RyR2 knock-in mice, pacing-induced AF, Ca²⁺ spark imaging, DAD recordings\",\n      \"pmids\": [\"22158709\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether this applies to ventricular arrhythmias and heart failure models not tested in same genetic framework\", \"Upstream mechanism by which FKBP12.6 loss activates CaMKII unclear\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"A dual-regulation model was established: FKBP12 activates RyR2 by sensitizing it to Ca²⁺, while FKBP12.6 antagonizes this activation through competitive binding, revealing that the balance of FKBP isoforms tunes RyR2 gating.\",\n      \"evidence\": \"Single-channel bilayer recordings with purified FKBP isoforms, mathematical modelling, Ca²⁺ wave imaging in permeabilized cells\",\n      \"pmids\": [\"22363773\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo relevance of isoform competition not validated\", \"Relative abundance of FKBP12 vs. FKBP12.6 at RyR2 in different tissues poorly quantified\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"FKBP12.6 was shown to facilitate SOICR termination by raising the termination threshold, and CPVT mutant RyR2 channels retain FKBP binding but lose FKBP-mediated regulation, identifying a novel disease mechanism for catecholaminergic polymorphic VT.\",\n      \"evidence\": \"Single-cell Ca²⁺ imaging in HEK293 cells expressing WT or CPVT mutant RyR2, SOICR threshold measurements\",\n      \"pmids\": [\"27154203\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism by which CPVT mutations uncouple FKBP12.6 binding from channel regulation structurally undefined\", \"Single heterologous cell system\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"High-resolution cryo-EM of the RyR2–FKBP12.6 complex revealed that FKBP12.6 rigidifies the HD2 clamp domain of RyR2 in the closed state, providing the structural mechanism for channel stabilization, while concurrent electrophysiology showed FKBP12.6 loss increases LCC-RyR2 coupling fidelity.\",\n      \"evidence\": \"Single-particle cryo-EM at 11.8 Å with conformational heterogeneity analysis (PMID:28536302); whole-cell patch clamp combined with confocal Ca²⁺ imaging in FKBP12.6-KO myocytes (PMID:28077437)\",\n      \"pmids\": [\"28536302\", \"28077437\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Atomic-resolution structure of the FKBP12.6–RyR2 interface not achieved\", \"Mechanism linking HD2 rigidity to pore closure at atomic level unresolved\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"RISP-dependent mitochondrial ROS was identified as the physiological trigger for FKBP12.6 dissociation from RyR2 in pulmonary artery smooth muscle, activating NF-κB/cyclin D1-driven proliferation and pulmonary hypertension, while the FKBP12.6-RyR2 stabilizer S107 was protective.\",\n      \"evidence\": \"SMC-specific RyR2 KO, RISP knockdown, FKBP12.6 KO mice, S107 treatment, chronic hypoxia PH model, Ca²⁺ imaging, pathway analysis\",\n      \"pmids\": [\"32669538\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Specific RyR2 cysteines targeted by RISP-ROS not identified\", \"Whether S107 has clinical utility in pulmonary hypertension untested in humans\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include: the atomic-resolution structure of the FKBP12.6–RyR2 interface; reconciliation of contradictory FKBP12.6-null arrhythmia phenotypes across independent knockout lines; the precise identity of RyR2 cysteines mediating redox-dependent FKBP12.6 dissociation; and whether pharmacological stabilization of the FKBP12.6–RyR2 complex is therapeutically viable in arrhythmia, heart failure, and pulmonary hypertension.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Atomic-resolution interface structure lacking\", \"Contradictory KO arrhythmia phenotypes unresolved\", \"Human genetic evidence directly linking FKBP1B variants to disease absent\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [0, 1, 3, 6, 12, 16, 26, 31, 36]},\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [0, 34, 39]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [0, 1, 3, 4, 13]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"GO:0162582\", \"supporting_discovery_ids\": [3, 12, 24, 30, 38]},\n      {\"term_id\": \"R-HSA-382551\", \"supporting_discovery_ids\": [2, 6, 9, 11, 16, 25, 27, 28, 29]}\n    ],\n    \"complexes\": [\n      \"RyR2–FKBP12.6–PKA–PP1–PP2A–mAKAP macromolecular complex\"\n    ],\n    \"partners\": [\n      \"RYR2\",\n      \"MTOR\",\n      \"PPP3CA\",\n      \"TGFBR1\",\n      \"PRKAR2A\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}