{"gene":"LDB3","run_date":"2026-06-10T02:59:49","timeline":{"discoveries":[{"year":1999,"finding":"ZASP (LDB3) was identified as a novel Z-band protein containing a PDZ domain that directly binds to the C-terminal region of alpha-actinin-2; it is specifically expressed in heart and skeletal muscle and localizes to the Z-band of the sarcomere as shown by immunoelectron microscopy.","method":"PDZ domain characterization, co-immunoprecipitation/binding assay with alpha-actinin-2, immunoelectron microscopy, Western blot","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — direct binding assay with alpha-actinin-2, immunoelectron microscopy localization, replicated by multiple subsequent studies","pmids":["10427098"],"is_preprint":false},{"year":2003,"finding":"Mutations in ZASP/Cypher cause cytoskeleton disarray in transfected cells, establishing a mechanistic basis for dilated cardiomyopathy and left ventricular non-compaction.","method":"Cell transfection with mutant ZASP constructs, in vitro cytoskeletal analysis","journal":"Journal of the American College of Cardiology","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — single in vitro transfection experiment, supported by genetic data across multiple families","pmids":["14662268"],"is_preprint":false},{"year":2003,"finding":"A D626N mutation in the third LIM domain of Cypher/ZASP, associated with dilated cardiomyopathy, increases the affinity of the LIM domain for protein kinase C (PKC) as demonstrated by yeast two-hybrid and pull-down assays.","method":"Yeast two-hybrid assay, pull-down assay","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — two orthogonal methods (Y2H and pulldown), single lab","pmids":["14660611"],"is_preprint":false},{"year":2004,"finding":"The solution structure of the ZASP PDZ domain was determined by NMR, showing it is a classical class 1 PDZ domain that recognizes the carboxy-terminal sequence of alpha-actinin-2 calmodulin-like domain with micromolar affinity; the alpha-actinin-2/ZASP PDZ interaction involves a binding surface distinct from that recognized by titin Z repeats, forming a ternary ZASP/alpha-actinin-2/titin complex.","method":"NMR structure determination, binding affinity measurement","journal":"Structure (London, England : 1993)","confidence":"High","confidence_rationale":"Tier 1 / Strong — atomic resolution NMR structure with functional binding characterization, ternary complex mapping","pmids":["15062084"],"is_preprint":false},{"year":2004,"finding":"The ZASP-like motif (ZM) in the related protein ALP (actinin-associated LIM protein) is required for interaction with the alpha-actinin rod region and for targeting to the muscle Z-line; this is the first evidence that the ZM motif has a direct role in protein-protein interaction, and the same motif in ZASP/Cypher was confirmed to interact with alpha-actinin.","method":"Surface plasmon resonance with purified recombinant proteins, cell localization assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — reconstitution with purified proteins and SPR quantification, plus cell localization, single lab","pmids":["15084604"],"is_preprint":false},{"year":2005,"finding":"Mutations in ZASP exon 6 (A147T and A165V) at or within a motif important for Z-disk linking cause myofibrillar myopathy (zaspopathy), establishing these residues as critical for ZASP function in vivo.","method":"Genetic screening, clinical-pathological correlation in patients","journal":"Annals of neurology","confidence":"Medium","confidence_rationale":"Tier 3 / Strong — mutation-phenotype identification across multiple families, no direct biochemical reconstitution","pmids":["15668942"],"is_preprint":false},{"year":2006,"finding":"ZASP/Cypher internal fragments containing either ZM exon 4 or ZM exon 6 co-localize with alpha-actinin in cultured cells; ZASP/Cypher directly interacts with the alpha-actinin rod and competes with ALP for the same binding site on the rod. Patient cardiomyopathy mutations in the internal domain did not affect co-localization with alpha-actinin or protein stability.","method":"Co-localization in myoblasts and non-muscle cells, direct binding assay (competition assay with purified proteins)","journal":"Experimental cell research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct binding competition assay with purified proteins, cell-based co-localization, single lab","pmids":["16476425"],"is_preprint":false},{"year":2007,"finding":"Drosophila Zasp (ortholog of human ZASP/LDB3) localizes to integrin adhesion sites and muscle Z-lines; depletion of Zasp by RNAi disrupts integrin adhesion sites, prevents Z-line formation, and blocks recruitment of alpha-actinin to the Z-line. Zasp also physically interacts with alpha-actinin and interacts genetically with integrins, showing it regulates integrin function.","method":"RNAi knockdown, co-localization, physical interaction (pulldown), genetic epistasis with integrins, fly larval phenotype analysis","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (RNAi, physical interaction, genetic epistasis), replicated across cell and tissue contexts","pmids":["18166658"],"is_preprint":false},{"year":2009,"finding":"Phosphoglucomutase 1 (PGM1), a glycolytic enzyme, was identified as a binding partner of ZASP/Cypher at domains encoded by exons 4 and 10; PGM1 localizes to Z-discs under stress conditions. DCM-associated mutations Ser189Leu, Thr206Ile (exon 4), and Ile345Met (exon 10) reduce ZASP binding to PGM1, suggesting ZASP anchors PGM1 to the Z-disc under stress.","method":"Yeast two-hybrid, co-immunoprecipitation of endogenous proteins in rat cardiomyocytes, stress-dependent localization assay","journal":"Cardiovascular research","confidence":"High","confidence_rationale":"Tier 2 / Strong — Y2H identification confirmed by endogenous Co-IP in cardiomyocytes, functional consequence of mutations tested, multiple methods","pmids":["19377068"],"is_preprint":false},{"year":2009,"finding":"Post-transcriptional silencing of Drosophila dzasp (ortholog of human ZASP/LDB3) causes locomotor defects and disruption of muscle structure and ultrastructure, consistent with a role in maintenance of muscular integrity.","method":"UAS/Gal4-driven dsRNAi knockdown in Drosophila, behavioral and ultrastructural analysis","journal":"Cell and tissue research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — tissue-specific knockdown with defined phenotypic readout, ortholog model","pmids":["19603185"],"is_preprint":false},{"year":2010,"finding":"The ZASP S196L mutation in transgenic mice causes hemodynamic dysfunction consistent with DCM and cardiac conduction defects; ZASP4 physically complexes with both L-type calcium channel (Cav1.2) and sodium channel (Nav1.5), and S196L cardiomyocytes show altered L-type Ca2+ and Na+ currents.","method":"Transgenic mouse model, electrophysiology on isolated cardiomyocytes, pull-down assay for channel interactions","journal":"Circulation. Arrhythmia and electrophysiology","confidence":"High","confidence_rationale":"Tier 2 / Strong — transgenic mouse model with electrophysiology plus pull-down for channel binding, multiple orthogonal methods","pmids":["20852297"],"is_preprint":false},{"year":2012,"finding":"Drosophila Zasp cooperates with talin to activate α5β1 integrins in mammalian tissue culture and αPS2βPS integrins in Drosophila, establishing Zasp as the first protein shown to co-activate α5β1 integrins together with talin, acting by a mechanism distinct from known αIIbβ3 co-activators.","method":"Cell spreading assays in mammalian tissue culture, Drosophila genetic assays, integrin activation assays","journal":"Journal of cell science","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional activation assay in two systems, single lab study","pmids":["22992465"],"is_preprint":false},{"year":2012,"finding":"ZASP is the major O-linked β-N-acetylglucosamine (O-GlcNAc)-modified protein in human heart myofibrils, identified by MALDI-MS/MS; O-GlcNAcylation of ZASP is increased in diseased (failing) heart compared to donor hearts.","method":"SDS-PAGE, enzymatic conjugation assay with UDP-GalNAz and fluorescent tagging, monoclonal antibody detection (CTD110.6, RL2), MALDI-MS/MS, immunofluorescence co-localization","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — three independent detection methods plus mass spectrometry identification, replicated across heart tissue types","pmids":["23271734"],"is_preprint":false},{"year":2013,"finding":"Cypher/ZASP acts as an A-kinase anchoring protein (AKAP) in cardiomyocytes: it interacts specifically with the type II regulatory subunit RIIα of PKA; Cypher/ZASP itself is phosphorylated by PKA at Ser265 and Ser296; its PDZ domain interacts with the L-type calcium channel (Cav1.2) C-terminal PDZ binding motif; Cypher/ZASP facilitates PKA-mediated phosphorylation of Cav1.2; and Cypher/ZASP interacts with the phosphatase calcineurin.","method":"Co-immunoprecipitation, in vitro phosphorylation assay, site-directed mutagenesis, cardiomyocyte functional assays in Cypher-null cells","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods, validated in Cypher-null cardiomyocytes with defined functional consequence, multiple binding partners characterized","pmids":["23996002"],"is_preprint":false},{"year":2014,"finding":"ZASP directly interacts with skeletal actin filaments through an actin-binding domain located between the PDZ and LIM domains; this domain is alternatively spliced, and the exon 6-encoded ZM motif (mutated in zaspopathy) and the exon 8-11 junction peptide (exclusive to long isoform ZASP-LΔex10) both contribute to actin binding. MFM-associated mutations in the actin-binding domain of ZASP-LΔex10 cause Z-disc disruption and F-actin accumulation in mouse skeletal muscle.","method":"In vitro binding assay, expression in mouse skeletal muscle with phenotypic readout (Z-disc disruption, F-actin accumulation), isoform-specific analysis","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — direct in vitro binding assay with purified proteins, in vivo mouse muscle expression with Z-disc phenotype, isoform-specific mutagenesis","pmids":["24668811"],"is_preprint":false},{"year":2014,"finding":"ZASP interacts with the mechanosensing protein Ankrd2 and tumor suppressor p53, forming a triple complex that facilitates poly-SUMOylation of p53; ZASP acts as a negative regulator of p53 in transactivation experiments with p53-responsive promoters (MDM2 and BAX). The PDZ domain of ZASP binds directly to both Ankrd2 and p53. Disease-associated mutations A165V and A171T in the ZM-motif abolish Ankrd2 binding and impair alpha-actinin2 binding.","method":"Co-immunoprecipitation, luciferase transactivation assays, SUMO modification assays, domain-deletion and mutagenesis analysis","journal":"PloS one","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods including Co-IP, functional transactivation assays, domain mutagenesis, and SUMOylation assay","pmids":["24647531"],"is_preprint":false},{"year":2014,"finding":"Aberrant inclusion of LDB3 exon 11 in myotonic dystrophy type 1 (DM1) skeletal muscle is mediated by MBNL1 sequestration; the exon 11-positive LDB3 isoform has reduced affinity for PKC compared to the exon 11-negative isoform, potentially contributing to CUG-BP1 upregulation through altered PKC binding.","method":"Exon array, RT-PCR, Western blot, minigene transfection with CTG repeat expansion, PKC binding affinity comparison","journal":"Neurobiology of disease","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — minigene functional validation, binding affinity measurement, single lab","pmids":["24878509"],"is_preprint":false},{"year":2017,"finding":"Recombinant ZASP isoforms (ZASP-S, ZASP-LΔex10) bind to G-actin with high affinity (Kd ≈ 10^-8 to 10^-9 M) as measured by surface plasmon resonance; isolated actin-binding region lacking exon 10 (ABRΔex10) binds with lower affinity (Kd ≈ 10^-7 M), while ABR+10 binds only weakly (Kd ≈ 10^-5 M). ZASP-S and ABRΔex10 also induce F-actin and array formation. Disease-causing ZM mutations A147T and A165V do not affect actin binding affinity.","method":"Surface plasmon resonance, electron microscopy, NMR HSQC spectroscopy, circular dichroism","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — quantitative SPR with purified recombinant proteins, multiple isoforms and mutants characterized, orthogonal EM validation","pmids":["28349680"],"is_preprint":false},{"year":2018,"finding":"The long form of Cypher/Zasp is required for normal β-adrenergic regulation of cardiac CaV1.2 L-type Ca2+ channels in vivo; cardiomyocytes from long Cypher knockout (LCyphKO) mice show reduced cell-surface density of CaV1.2, reduced basal Ca2+ current, and significantly reduced β-adrenergic/PKA stimulation of L-type Ca2+ current (net β-adrenergic Ca2+ current reduced to 39±12% of wild type at 100 nM isoproterenol).","method":"Knockout mouse model (LCyphKO), patch-clamp electrophysiology, protein quantification in isolated ventricular myocytes","journal":"The Journal of general physiology","confidence":"High","confidence_rationale":"Tier 2 / Strong — clean KO mouse with rigorous electrophysiology, multiple parameters measured, clear functional consequence","pmids":["29743299"],"is_preprint":false},{"year":2021,"finding":"LDB3 modulates mechanical stress signaling through interactions with the mechanosensing domain of filamin C, its chaperone HSPA8, and PKCα in the muscle Z-disc; the myopathy-associated A165V mutation triggers early aggregation of filamin C and its chaperones at the Z-disc before aggregation of the mutant protein itself. The mutation impairs PKCα and TSC2-mTOR signaling pathways, causing protein aggregation and Z-disc disruption.","method":"Ldb3 Ala165Val/+ knock-in mice, co-immunoprecipitation, immunofluorescence, Western blot for pathway analysis","journal":"Communications biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — knock-in mouse model with multiple orthogonal methods, defined signaling pathway effects characterized","pmids":["33742095"],"is_preprint":false},{"year":2021,"finding":"Cypher/ZASP facilitates PKA-mediated phosphorylation of β-catenin at Ser675, promoting β-catenin transcriptional activity and cardiomyocyte proliferative capacity; Cypher co-localizes with β-catenin at intercalated discs. Cypher deletion also suppresses phosphorylation of vimentin Ser72 and troponin I Ser23/24.","method":"Quantitative phosphoproteomics on Cypher-knockout mouse cardiac tissue, PKA activation assays, immunofluorescence, reporter assays for β-catenin transcriptional activity","journal":"Frontiers in cardiovascular medicine","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — phosphoproteomics plus functional validation of β-catenin phosphorylation, single lab","pmids":["34966794"],"is_preprint":false},{"year":2024,"finding":"Cypher/ZASP plays a role in cardiomyocyte maturation through actin-mediated MRTFA-SRF signaling: Cypher deletion destabilizes F-actin and increases G-actin levels, thereby impeding nuclear localization of MRTFA and suppressing SRF-mediated transcription of genes critical for sarcomere isoform switching, mitochondrial metabolism, and electrophysiology. Re-expression of SRF during the critical postnatal period rescues CM maturation defects and improves cardiac function in Cypher-depleted mice.","method":"Cypher knockout mice, RNA-sequencing, G-actin/F-actin fractionation, nuclear-cytoplasmic extraction, actin disassembly and co-sedimentation assays, adenovirus/AAV rescue experiments with SRF, transmission electron microscopy","journal":"Theranostics","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal biochemical methods, in vivo rescue experiment, defined mechanistic pathway","pmids":["39113806"],"is_preprint":false}],"current_model":"LDB3/ZASP is a sarcomeric Z-disc scaffold protein that binds alpha-actinin-2 (via its PDZ domain and ZM motif), directly binds G- and F-actin (via an alternatively spliced actin-binding region), and interacts with PKC, PGM1, filamin C, Ankrd2, p53, and ion channels (Cav1.2, Nav1.5); it functions as an A-kinase anchoring protein (AKAP) that tethers PKA and calcineurin to the Z-disc to regulate Cav1.2 phosphorylation and β-adrenergic signaling, modulates mechanical stress signaling through PKCα and TSC2-mTOR pathways, facilitates cardiomyocyte maturation via actin-MRTFA-SRF signaling, and is subject to O-GlcNAcylation; disease-causing mutations disrupt these interactions and downstream signaling to produce dilated cardiomyopathy, myofibrillar myopathy, and arrhythmias."},"narrative":{"mechanistic_narrative":"LDB3 (ZASP/Cypher) is a striated-muscle-specific Z-disc scaffold protein that organizes the sarcomeric cytoskeleton and serves as a hub for mechanical and signaling complexes in cardiac and skeletal muscle [PMID:10427098]. Its PDZ domain directly binds the C-terminal calmodulin-like domain of alpha-actinin-2 through a surface distinct from the titin-binding site, forming a ternary ZASP/alpha-actinin-2/titin complex, while a ZASP-like (ZM) motif provides a second contact with the alpha-actinin rod [PMID:15062084, PMID:15084604, PMID:16476425]. ZASP additionally binds G- and F-actin through an alternatively spliced actin-binding region between the PDZ and LIM domains, with isoform- and exon-dependent affinity, and this actin engagement underlies its role in Z-disc integrity and F-actin organization [PMID:24668811, PMID:28349680]. Beyond structural scaffolding, ZASP acts as an A-kinase anchoring protein that binds the PKA RIIalpha subunit and calcineurin, tethers and promotes PKA-mediated phosphorylation of the L-type calcium channel Cav1.2, and is required for normal beta-adrenergic regulation of cardiac Ca2+ current [PMID:23996002, PMID:29743299]. It integrates additional signaling axes through the Cav1.2/Nav1.5 channel complex, PGM1, PKCalpha, filamin C and its chaperone HSPA8, an Ankrd2/p53 SUMOylation complex, and actin-dependent MRTFA-SRF signaling that drives cardiomyocyte maturation [PMID:19377068, PMID:20852297, PMID:24647531, PMID:33742095, PMID:39113806]. ZASP is the major O-GlcNAcylated protein of human heart myofibrils, with modification increased in failing hearts [PMID:23271734]. Mutations in ZASP cause dilated cardiomyopathy, cardiac conduction defects, and myofibrillar myopathy (zaspopathy) by disrupting these scaffolding and signaling interactions and triggering Z-disc disassembly and protein aggregation [PMID:14662268, PMID:15668942, PMID:20852297, PMID:33742095].","teleology":[{"year":1999,"claim":"Established the molecular identity of ZASP as a Z-disc PDZ protein anchored to the sarcomere, answering where it acts and through what scaffolding partner.","evidence":"PDZ domain characterization, alpha-actinin-2 binding assay, and immunoelectron microscopy in heart and skeletal muscle","pmids":["10427098"],"confidence":"High","gaps":["Did not define which other Z-disc partners or signaling roles ZASP carries","Functional consequence of the interaction in vivo not tested"]},{"year":2003,"claim":"Linked ZASP mutations to cardiomyopathy phenotypes by showing they perturb cytoskeletal organization and alter PKC affinity, framing the disease mechanism as disrupted scaffolding/signaling.","evidence":"Cell transfection with mutant constructs; yeast two-hybrid and pull-down for the D626N LIM-domain/PKC interaction","pmids":["14662268","14660611"],"confidence":"Medium","gaps":["In vitro transfection phenotype not validated in muscle tissue","Causal chain from increased PKC affinity to cardiomyopathy not demonstrated"]},{"year":2004,"claim":"Resolved the structural basis of the ZASP-alpha-actinin interaction, showing the PDZ domain and ZM motif engage distinct alpha-actinin surfaces compatible with a ternary titin complex.","evidence":"NMR structure of the PDZ domain with affinity measurement; SPR with purified ZM/ALP and alpha-actinin rod plus cell localization","pmids":["15062084","15084604"],"confidence":"High","gaps":["Stoichiometry and dynamics of the full ternary complex in vivo not established","Role of LIM domains in the assembly not addressed"]},{"year":2005,"claim":"Identified exon-6 ZM-motif residues as critical for ZASP function in vivo by linking them to myofibrillar myopathy (zaspopathy).","evidence":"Genetic screening and clinical-pathological correlation in patient families","pmids":["15668942"],"confidence":"Medium","gaps":["No biochemical reconstitution of how the mutations impair function","Mechanism connecting the mutated residues to myofibrillar disruption unknown at this stage"]},{"year":2006,"claim":"Showed ZASP and ALP compete for the same alpha-actinin rod site and that cardiomyopathy mutations spare alpha-actinin co-localization, indicating disease mechanisms beyond simple loss of alpha-actinin binding.","evidence":"Co-localization in myoblasts and competition binding assays with purified proteins","pmids":["16476425"],"confidence":"Medium","gaps":["The actual affected function of these mutations remained undefined","Physiological consequence of ZASP/ALP competition not tested"]},{"year":2007,"claim":"Defined an organism-level requirement for ZASP in Z-line assembly and integrin adhesion using the Drosophila ortholog, showing it recruits alpha-actinin and is needed for muscle structure.","evidence":"RNAi knockdown, physical interaction, and integrin genetic epistasis with larval phenotype analysis in Drosophila","pmids":["18166658"],"confidence":"High","gaps":["Whether human ZASP regulates integrin adhesion in muscle not established","Direct molecular link between ZASP and integrin activation not defined here"]},{"year":2009,"claim":"Connected ZASP to muscle maintenance and metabolic enzyme positioning, identifying PGM1 as a stress-dependent Z-disc partner and confirming ortholog requirement for muscle integrity.","evidence":"Y2H and endogenous Co-IP in rat cardiomyocytes for PGM1; dsRNAi knockdown with ultrastructural/behavioral analysis in Drosophila","pmids":["19377068","19603185"],"confidence":"High","gaps":["Functional role of Z-disc-anchored PGM1 in energy metabolism not measured","Whether PGM1 mislocalization drives DCM in vivo unknown"]},{"year":2010,"claim":"Demonstrated that ZASP physically complexes with cardiac ion channels and that a DCM mutation alters ionic currents in vivo, establishing an electrophysiological dimension to ZASP disease.","evidence":"S196L transgenic mouse, cardiomyocyte electrophysiology, and pull-down of Cav1.2/Nav1.5","pmids":["20852297"],"confidence":"High","gaps":["Mechanism by which ZASP modulates channel gating/trafficking not defined here","Direct vs indirect nature of channel binding not resolved"]},{"year":2012,"claim":"Identified ZASP as a post-translationally modified Z-disc protein, the major O-GlcNAcylated myofibrillar protein, with disease-associated increases pointing to a regulatory role in heart failure.","evidence":"Enzymatic labeling, antibody detection, and MALDI-MS/MS on human heart myofibrils; integrin co-activation assays for the Drosophila ortholog","pmids":["23271734","22992465"],"confidence":"High","gaps":["Functional consequence of ZASP O-GlcNAcylation on its interactions not tested","O-GlcNAc sites and their regulation not mapped"]},{"year":2013,"claim":"Defined ZASP as a Z-disc AKAP that scaffolds PKA (RIIalpha) and calcineurin and promotes Cav1.2 phosphorylation, recasting it from passive scaffold to active signaling organizer.","evidence":"Co-IP, in vitro phosphorylation, site-directed mutagenesis, and functional assays in Cypher-null cardiomyocytes","pmids":["23996002"],"confidence":"High","gaps":["Whether calcineurin tethering opposes PKA signaling locally not quantified","In vivo physiological output of the AKAP function not yet shown here"]},{"year":2014,"claim":"Established direct ZASP-actin binding through an alternatively spliced region and connected ZASP to p53/SUMO regulation and disease splicing, broadening its mechanistic repertoire.","evidence":"In vitro actin binding and mouse muscle expression (#14); Co-IP, transactivation and SUMO assays for Ankrd2/p53 (#15); exon array and minigene analysis in DM1 (#16)","pmids":["24668811","24647531","24878509"],"confidence":"High","gaps":["Physiological relevance of ZASP-p53 regulation in muscle unclear","How actin binding and signaling functions are coordinated by splicing not resolved"]},{"year":2017,"claim":"Quantified isoform- and exon-dependent affinity of ZASP for G- and F-actin and showed ZM disease mutations do not impair actin binding, separating actin engagement from the zaspopathy mechanism.","evidence":"SPR, electron microscopy, NMR HSQC, and circular dichroism with recombinant ZASP isoforms and mutants","pmids":["28349680"],"confidence":"High","gaps":["The molecular defect of ZM mutations remained unidentified","In vivo significance of differential isoform actin affinity not tested"]},{"year":2018,"claim":"Showed in vivo that the long ZASP isoform is required for Cav1.2 surface density and beta-adrenergic potentiation of L-type Ca2+ current, providing the physiological output of its AKAP function.","evidence":"Long-Cypher knockout mice with patch-clamp electrophysiology and channel protein quantification","pmids":["29743299"],"confidence":"High","gaps":["Whether reduced channel density reflects trafficking or stability not resolved","Contribution to arrhythmia phenotypes in patients not directly linked"]},{"year":2021,"claim":"Defined the molecular mechanism of A165V zaspopathy as disrupted mechanosensing signaling, with early filamin C/chaperone aggregation preceding mutant protein aggregation, and revealed a PKA/beta-catenin proliferative axis.","evidence":"Ldb3 A165V knock-in mice with Co-IP and pathway Western blots (#19); phosphoproteomics and reporter assays in Cypher-knockout cardiac tissue (#20)","pmids":["33742095","34966794"],"confidence":"Medium","gaps":["Temporal order of filamin C aggregation versus PKCalpha/mTOR dysregulation not fully causal","Beta-catenin axis validated in single lab without independent replication"]},{"year":2024,"claim":"Established a developmental role for ZASP in cardiomyocyte maturation via actin-controlled MRTFA-SRF transcription, with SRF re-expression rescuing maturation and function in vivo.","evidence":"Cypher knockout mice, RNA-seq, G/F-actin fractionation, nuclear-cytoplasmic extraction, and AAV-SRF rescue with electron microscopy","pmids":["39113806"],"confidence":"High","gaps":["How ZASP-dependent F-actin stability links to the AKAP and channel functions mechanistically not integrated","Relevance of the maturation pathway to adult cardiomyopathy onset unclear"]},{"year":null,"claim":"How ZASP's distinct functions — Z-disc scaffolding, actin organization, AKAP-mediated channel regulation, mechanosensing, and transcriptional control — are coordinated by isoform splicing and post-translational modification into a unified mechanism remains unresolved.","evidence":"","pmids":[],"confidence":"High","gaps":["No integrated model linking O-GlcNAcylation, splicing, and the multiple signaling axes","Structural basis for simultaneous engagement of alpha-actinin, actin, channels, and PKA undefined","Genotype-to-mechanism map for the full mutation spectrum incomplete"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0008092","term_label":"cytoskeletal protein binding","supporting_discovery_ids":[0,3,4,6,14,17]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[13,15]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[13,18]},{"term_id":"GO:0005198","term_label":"structural molecule activity","supporting_discovery_ids":[0,14]}],"localization":[{"term_id":"GO:0005856","term_label":"cytoskeleton","supporting_discovery_ids":[0,7,14]},{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[15,21]},{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[10,13,18]}],"pathway":[],"complexes":["ZASP/alpha-actinin-2/titin ternary complex","Z-disc","ZASP/Ankrd2/p53 complex"],"partners":["ACTN2","PKC","PGM1","CACNA1C","SCN5A","PRKAR2A","FLNC","ANKRD2"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"O75112","full_name":"LIM domain-binding protein 3","aliases":["Protein cypher","Z-band alternatively spliced PDZ-motif protein"],"length_aa":727,"mass_kda":77.1,"function":"May function as an adapter in striated muscle to couple protein kinase C-mediated signaling via its LIM domains to the cytoskeleton","subcellular_location":"Cytoplasm, perinuclear region; Cell projection, pseudopodium; Cytoplasm, cytoskeleton; Cytoplasm, myofibril, sarcomere, Z line","url":"https://www.uniprot.org/uniprotkb/O75112/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/LDB3","classification":"Not Classified","n_dependent_lines":2,"n_total_lines":1208,"dependency_fraction":0.0016556291390728477},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/LDB3","total_profiled":1310},"omim":[{"mim_id":"621237","title":"CARDIOMYOPATHY, DILATED, 2L; CMD2L","url":"https://www.omim.org/entry/621237"},{"mim_id":"610735","title":"MYOZENIN 3; MYOZ3","url":"https://www.omim.org/entry/610735"},{"mim_id":"609452","title":"MYOPATHY, MYOFIBRILLAR, 4; MFM4","url":"https://www.omim.org/entry/609452"},{"mim_id":"606516","title":"MUSCLEBLIND-LIKE SPLICING REGULATOR 1; MBNL1","url":"https://www.omim.org/entry/606516"},{"mim_id":"605906","title":"LIM DOMAIN-BINDING 3; 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Z-band protein containing a PDZ domain that directly binds to the C-terminal region of alpha-actinin-2; it is specifically expressed in heart and skeletal muscle and localizes to the Z-band of the sarcomere as shown by immunoelectron microscopy.\",\n      \"method\": \"PDZ domain characterization, co-immunoprecipitation/binding assay with alpha-actinin-2, immunoelectron microscopy, Western blot\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — direct binding assay with alpha-actinin-2, immunoelectron microscopy localization, replicated by multiple subsequent studies\",\n      \"pmids\": [\"10427098\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Mutations in ZASP/Cypher cause cytoskeleton disarray in transfected cells, establishing a mechanistic basis for dilated cardiomyopathy and left ventricular non-compaction.\",\n      \"method\": \"Cell transfection with mutant ZASP constructs, in vitro cytoskeletal analysis\",\n      \"journal\": \"Journal of the American College of Cardiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — single in vitro transfection experiment, supported by genetic data across multiple families\",\n      \"pmids\": [\"14662268\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"A D626N mutation in the third LIM domain of Cypher/ZASP, associated with dilated cardiomyopathy, increases the affinity of the LIM domain for protein kinase C (PKC) as demonstrated by yeast two-hybrid and pull-down assays.\",\n      \"method\": \"Yeast two-hybrid assay, pull-down assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — two orthogonal methods (Y2H and pulldown), single lab\",\n      \"pmids\": [\"14660611\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"The solution structure of the ZASP PDZ domain was determined by NMR, showing it is a classical class 1 PDZ domain that recognizes the carboxy-terminal sequence of alpha-actinin-2 calmodulin-like domain with micromolar affinity; the alpha-actinin-2/ZASP PDZ interaction involves a binding surface distinct from that recognized by titin Z repeats, forming a ternary ZASP/alpha-actinin-2/titin complex.\",\n      \"method\": \"NMR structure determination, binding affinity measurement\",\n      \"journal\": \"Structure (London, England : 1993)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — atomic resolution NMR structure with functional binding characterization, ternary complex mapping\",\n      \"pmids\": [\"15062084\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"The ZASP-like motif (ZM) in the related protein ALP (actinin-associated LIM protein) is required for interaction with the alpha-actinin rod region and for targeting to the muscle Z-line; this is the first evidence that the ZM motif has a direct role in protein-protein interaction, and the same motif in ZASP/Cypher was confirmed to interact with alpha-actinin.\",\n      \"method\": \"Surface plasmon resonance with purified recombinant proteins, cell localization assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — reconstitution with purified proteins and SPR quantification, plus cell localization, single lab\",\n      \"pmids\": [\"15084604\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Mutations in ZASP exon 6 (A147T and A165V) at or within a motif important for Z-disk linking cause myofibrillar myopathy (zaspopathy), establishing these residues as critical for ZASP function in vivo.\",\n      \"method\": \"Genetic screening, clinical-pathological correlation in patients\",\n      \"journal\": \"Annals of neurology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Strong — mutation-phenotype identification across multiple families, no direct biochemical reconstitution\",\n      \"pmids\": [\"15668942\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"ZASP/Cypher internal fragments containing either ZM exon 4 or ZM exon 6 co-localize with alpha-actinin in cultured cells; ZASP/Cypher directly interacts with the alpha-actinin rod and competes with ALP for the same binding site on the rod. Patient cardiomyopathy mutations in the internal domain did not affect co-localization with alpha-actinin or protein stability.\",\n      \"method\": \"Co-localization in myoblasts and non-muscle cells, direct binding assay (competition assay with purified proteins)\",\n      \"journal\": \"Experimental cell research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct binding competition assay with purified proteins, cell-based co-localization, single lab\",\n      \"pmids\": [\"16476425\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Drosophila Zasp (ortholog of human ZASP/LDB3) localizes to integrin adhesion sites and muscle Z-lines; depletion of Zasp by RNAi disrupts integrin adhesion sites, prevents Z-line formation, and blocks recruitment of alpha-actinin to the Z-line. Zasp also physically interacts with alpha-actinin and interacts genetically with integrins, showing it regulates integrin function.\",\n      \"method\": \"RNAi knockdown, co-localization, physical interaction (pulldown), genetic epistasis with integrins, fly larval phenotype analysis\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (RNAi, physical interaction, genetic epistasis), replicated across cell and tissue contexts\",\n      \"pmids\": [\"18166658\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Phosphoglucomutase 1 (PGM1), a glycolytic enzyme, was identified as a binding partner of ZASP/Cypher at domains encoded by exons 4 and 10; PGM1 localizes to Z-discs under stress conditions. DCM-associated mutations Ser189Leu, Thr206Ile (exon 4), and Ile345Met (exon 10) reduce ZASP binding to PGM1, suggesting ZASP anchors PGM1 to the Z-disc under stress.\",\n      \"method\": \"Yeast two-hybrid, co-immunoprecipitation of endogenous proteins in rat cardiomyocytes, stress-dependent localization assay\",\n      \"journal\": \"Cardiovascular research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — Y2H identification confirmed by endogenous Co-IP in cardiomyocytes, functional consequence of mutations tested, multiple methods\",\n      \"pmids\": [\"19377068\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Post-transcriptional silencing of Drosophila dzasp (ortholog of human ZASP/LDB3) causes locomotor defects and disruption of muscle structure and ultrastructure, consistent with a role in maintenance of muscular integrity.\",\n      \"method\": \"UAS/Gal4-driven dsRNAi knockdown in Drosophila, behavioral and ultrastructural analysis\",\n      \"journal\": \"Cell and tissue research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — tissue-specific knockdown with defined phenotypic readout, ortholog model\",\n      \"pmids\": [\"19603185\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"The ZASP S196L mutation in transgenic mice causes hemodynamic dysfunction consistent with DCM and cardiac conduction defects; ZASP4 physically complexes with both L-type calcium channel (Cav1.2) and sodium channel (Nav1.5), and S196L cardiomyocytes show altered L-type Ca2+ and Na+ currents.\",\n      \"method\": \"Transgenic mouse model, electrophysiology on isolated cardiomyocytes, pull-down assay for channel interactions\",\n      \"journal\": \"Circulation. Arrhythmia and electrophysiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — transgenic mouse model with electrophysiology plus pull-down for channel binding, multiple orthogonal methods\",\n      \"pmids\": [\"20852297\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Drosophila Zasp cooperates with talin to activate α5β1 integrins in mammalian tissue culture and αPS2βPS integrins in Drosophila, establishing Zasp as the first protein shown to co-activate α5β1 integrins together with talin, acting by a mechanism distinct from known αIIbβ3 co-activators.\",\n      \"method\": \"Cell spreading assays in mammalian tissue culture, Drosophila genetic assays, integrin activation assays\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional activation assay in two systems, single lab study\",\n      \"pmids\": [\"22992465\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"ZASP is the major O-linked β-N-acetylglucosamine (O-GlcNAc)-modified protein in human heart myofibrils, identified by MALDI-MS/MS; O-GlcNAcylation of ZASP is increased in diseased (failing) heart compared to donor hearts.\",\n      \"method\": \"SDS-PAGE, enzymatic conjugation assay with UDP-GalNAz and fluorescent tagging, monoclonal antibody detection (CTD110.6, RL2), MALDI-MS/MS, immunofluorescence co-localization\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — three independent detection methods plus mass spectrometry identification, replicated across heart tissue types\",\n      \"pmids\": [\"23271734\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Cypher/ZASP acts as an A-kinase anchoring protein (AKAP) in cardiomyocytes: it interacts specifically with the type II regulatory subunit RIIα of PKA; Cypher/ZASP itself is phosphorylated by PKA at Ser265 and Ser296; its PDZ domain interacts with the L-type calcium channel (Cav1.2) C-terminal PDZ binding motif; Cypher/ZASP facilitates PKA-mediated phosphorylation of Cav1.2; and Cypher/ZASP interacts with the phosphatase calcineurin.\",\n      \"method\": \"Co-immunoprecipitation, in vitro phosphorylation assay, site-directed mutagenesis, cardiomyocyte functional assays in Cypher-null cells\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods, validated in Cypher-null cardiomyocytes with defined functional consequence, multiple binding partners characterized\",\n      \"pmids\": [\"23996002\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"ZASP directly interacts with skeletal actin filaments through an actin-binding domain located between the PDZ and LIM domains; this domain is alternatively spliced, and the exon 6-encoded ZM motif (mutated in zaspopathy) and the exon 8-11 junction peptide (exclusive to long isoform ZASP-LΔex10) both contribute to actin binding. MFM-associated mutations in the actin-binding domain of ZASP-LΔex10 cause Z-disc disruption and F-actin accumulation in mouse skeletal muscle.\",\n      \"method\": \"In vitro binding assay, expression in mouse skeletal muscle with phenotypic readout (Z-disc disruption, F-actin accumulation), isoform-specific analysis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — direct in vitro binding assay with purified proteins, in vivo mouse muscle expression with Z-disc phenotype, isoform-specific mutagenesis\",\n      \"pmids\": [\"24668811\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"ZASP interacts with the mechanosensing protein Ankrd2 and tumor suppressor p53, forming a triple complex that facilitates poly-SUMOylation of p53; ZASP acts as a negative regulator of p53 in transactivation experiments with p53-responsive promoters (MDM2 and BAX). The PDZ domain of ZASP binds directly to both Ankrd2 and p53. Disease-associated mutations A165V and A171T in the ZM-motif abolish Ankrd2 binding and impair alpha-actinin2 binding.\",\n      \"method\": \"Co-immunoprecipitation, luciferase transactivation assays, SUMO modification assays, domain-deletion and mutagenesis analysis\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods including Co-IP, functional transactivation assays, domain mutagenesis, and SUMOylation assay\",\n      \"pmids\": [\"24647531\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Aberrant inclusion of LDB3 exon 11 in myotonic dystrophy type 1 (DM1) skeletal muscle is mediated by MBNL1 sequestration; the exon 11-positive LDB3 isoform has reduced affinity for PKC compared to the exon 11-negative isoform, potentially contributing to CUG-BP1 upregulation through altered PKC binding.\",\n      \"method\": \"Exon array, RT-PCR, Western blot, minigene transfection with CTG repeat expansion, PKC binding affinity comparison\",\n      \"journal\": \"Neurobiology of disease\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — minigene functional validation, binding affinity measurement, single lab\",\n      \"pmids\": [\"24878509\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Recombinant ZASP isoforms (ZASP-S, ZASP-LΔex10) bind to G-actin with high affinity (Kd ≈ 10^-8 to 10^-9 M) as measured by surface plasmon resonance; isolated actin-binding region lacking exon 10 (ABRΔex10) binds with lower affinity (Kd ≈ 10^-7 M), while ABR+10 binds only weakly (Kd ≈ 10^-5 M). ZASP-S and ABRΔex10 also induce F-actin and array formation. Disease-causing ZM mutations A147T and A165V do not affect actin binding affinity.\",\n      \"method\": \"Surface plasmon resonance, electron microscopy, NMR HSQC spectroscopy, circular dichroism\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — quantitative SPR with purified recombinant proteins, multiple isoforms and mutants characterized, orthogonal EM validation\",\n      \"pmids\": [\"28349680\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"The long form of Cypher/Zasp is required for normal β-adrenergic regulation of cardiac CaV1.2 L-type Ca2+ channels in vivo; cardiomyocytes from long Cypher knockout (LCyphKO) mice show reduced cell-surface density of CaV1.2, reduced basal Ca2+ current, and significantly reduced β-adrenergic/PKA stimulation of L-type Ca2+ current (net β-adrenergic Ca2+ current reduced to 39±12% of wild type at 100 nM isoproterenol).\",\n      \"method\": \"Knockout mouse model (LCyphKO), patch-clamp electrophysiology, protein quantification in isolated ventricular myocytes\",\n      \"journal\": \"The Journal of general physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — clean KO mouse with rigorous electrophysiology, multiple parameters measured, clear functional consequence\",\n      \"pmids\": [\"29743299\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"LDB3 modulates mechanical stress signaling through interactions with the mechanosensing domain of filamin C, its chaperone HSPA8, and PKCα in the muscle Z-disc; the myopathy-associated A165V mutation triggers early aggregation of filamin C and its chaperones at the Z-disc before aggregation of the mutant protein itself. The mutation impairs PKCα and TSC2-mTOR signaling pathways, causing protein aggregation and Z-disc disruption.\",\n      \"method\": \"Ldb3 Ala165Val/+ knock-in mice, co-immunoprecipitation, immunofluorescence, Western blot for pathway analysis\",\n      \"journal\": \"Communications biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — knock-in mouse model with multiple orthogonal methods, defined signaling pathway effects characterized\",\n      \"pmids\": [\"33742095\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Cypher/ZASP facilitates PKA-mediated phosphorylation of β-catenin at Ser675, promoting β-catenin transcriptional activity and cardiomyocyte proliferative capacity; Cypher co-localizes with β-catenin at intercalated discs. Cypher deletion also suppresses phosphorylation of vimentin Ser72 and troponin I Ser23/24.\",\n      \"method\": \"Quantitative phosphoproteomics on Cypher-knockout mouse cardiac tissue, PKA activation assays, immunofluorescence, reporter assays for β-catenin transcriptional activity\",\n      \"journal\": \"Frontiers in cardiovascular medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — phosphoproteomics plus functional validation of β-catenin phosphorylation, single lab\",\n      \"pmids\": [\"34966794\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Cypher/ZASP plays a role in cardiomyocyte maturation through actin-mediated MRTFA-SRF signaling: Cypher deletion destabilizes F-actin and increases G-actin levels, thereby impeding nuclear localization of MRTFA and suppressing SRF-mediated transcription of genes critical for sarcomere isoform switching, mitochondrial metabolism, and electrophysiology. Re-expression of SRF during the critical postnatal period rescues CM maturation defects and improves cardiac function in Cypher-depleted mice.\",\n      \"method\": \"Cypher knockout mice, RNA-sequencing, G-actin/F-actin fractionation, nuclear-cytoplasmic extraction, actin disassembly and co-sedimentation assays, adenovirus/AAV rescue experiments with SRF, transmission electron microscopy\",\n      \"journal\": \"Theranostics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal biochemical methods, in vivo rescue experiment, defined mechanistic pathway\",\n      \"pmids\": [\"39113806\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"LDB3/ZASP is a sarcomeric Z-disc scaffold protein that binds alpha-actinin-2 (via its PDZ domain and ZM motif), directly binds G- and F-actin (via an alternatively spliced actin-binding region), and interacts with PKC, PGM1, filamin C, Ankrd2, p53, and ion channels (Cav1.2, Nav1.5); it functions as an A-kinase anchoring protein (AKAP) that tethers PKA and calcineurin to the Z-disc to regulate Cav1.2 phosphorylation and β-adrenergic signaling, modulates mechanical stress signaling through PKCα and TSC2-mTOR pathways, facilitates cardiomyocyte maturation via actin-MRTFA-SRF signaling, and is subject to O-GlcNAcylation; disease-causing mutations disrupt these interactions and downstream signaling to produce dilated cardiomyopathy, myofibrillar myopathy, and arrhythmias.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"LDB3 (ZASP/Cypher) is a striated-muscle-specific Z-disc scaffold protein that organizes the sarcomeric cytoskeleton and serves as a hub for mechanical and signaling complexes in cardiac and skeletal muscle [#0]. Its PDZ domain directly binds the C-terminal calmodulin-like domain of alpha-actinin-2 through a surface distinct from the titin-binding site, forming a ternary ZASP/alpha-actinin-2/titin complex, while a ZASP-like (ZM) motif provides a second contact with the alpha-actinin rod [#3, #4, #6]. ZASP additionally binds G- and F-actin through an alternatively spliced actin-binding region between the PDZ and LIM domains, with isoform- and exon-dependent affinity, and this actin engagement underlies its role in Z-disc integrity and F-actin organization [#14, #17]. Beyond structural scaffolding, ZASP acts as an A-kinase anchoring protein that binds the PKA RIIalpha subunit and calcineurin, tethers and promotes PKA-mediated phosphorylation of the L-type calcium channel Cav1.2, and is required for normal beta-adrenergic regulation of cardiac Ca2+ current [#13, #18]. It integrates additional signaling axes through the Cav1.2/Nav1.5 channel complex, PGM1, PKCalpha, filamin C and its chaperone HSPA8, an Ankrd2/p53 SUMOylation complex, and actin-dependent MRTFA-SRF signaling that drives cardiomyocyte maturation [#8, #10, #15, #19, #21]. ZASP is the major O-GlcNAcylated protein of human heart myofibrils, with modification increased in failing hearts [#12]. Mutations in ZASP cause dilated cardiomyopathy, cardiac conduction defects, and myofibrillar myopathy (zaspopathy) by disrupting these scaffolding and signaling interactions and triggering Z-disc disassembly and protein aggregation [#1, #5, #10, #19].\",\n  \"teleology\": [\n    {\n      \"year\": 1999,\n      \"claim\": \"Established the molecular identity of ZASP as a Z-disc PDZ protein anchored to the sarcomere, answering where it acts and through what scaffolding partner.\",\n      \"evidence\": \"PDZ domain characterization, alpha-actinin-2 binding assay, and immunoelectron microscopy in heart and skeletal muscle\",\n      \"pmids\": [\"10427098\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not define which other Z-disc partners or signaling roles ZASP carries\", \"Functional consequence of the interaction in vivo not tested\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Linked ZASP mutations to cardiomyopathy phenotypes by showing they perturb cytoskeletal organization and alter PKC affinity, framing the disease mechanism as disrupted scaffolding/signaling.\",\n      \"evidence\": \"Cell transfection with mutant constructs; yeast two-hybrid and pull-down for the D626N LIM-domain/PKC interaction\",\n      \"pmids\": [\"14662268\", \"14660611\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"In vitro transfection phenotype not validated in muscle tissue\", \"Causal chain from increased PKC affinity to cardiomyopathy not demonstrated\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Resolved the structural basis of the ZASP-alpha-actinin interaction, showing the PDZ domain and ZM motif engage distinct alpha-actinin surfaces compatible with a ternary titin complex.\",\n      \"evidence\": \"NMR structure of the PDZ domain with affinity measurement; SPR with purified ZM/ALP and alpha-actinin rod plus cell localization\",\n      \"pmids\": [\"15062084\", \"15084604\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Stoichiometry and dynamics of the full ternary complex in vivo not established\", \"Role of LIM domains in the assembly not addressed\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Identified exon-6 ZM-motif residues as critical for ZASP function in vivo by linking them to myofibrillar myopathy (zaspopathy).\",\n      \"evidence\": \"Genetic screening and clinical-pathological correlation in patient families\",\n      \"pmids\": [\"15668942\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No biochemical reconstitution of how the mutations impair function\", \"Mechanism connecting the mutated residues to myofibrillar disruption unknown at this stage\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Showed ZASP and ALP compete for the same alpha-actinin rod site and that cardiomyopathy mutations spare alpha-actinin co-localization, indicating disease mechanisms beyond simple loss of alpha-actinin binding.\",\n      \"evidence\": \"Co-localization in myoblasts and competition binding assays with purified proteins\",\n      \"pmids\": [\"16476425\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"The actual affected function of these mutations remained undefined\", \"Physiological consequence of ZASP/ALP competition not tested\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Defined an organism-level requirement for ZASP in Z-line assembly and integrin adhesion using the Drosophila ortholog, showing it recruits alpha-actinin and is needed for muscle structure.\",\n      \"evidence\": \"RNAi knockdown, physical interaction, and integrin genetic epistasis with larval phenotype analysis in Drosophila\",\n      \"pmids\": [\"18166658\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether human ZASP regulates integrin adhesion in muscle not established\", \"Direct molecular link between ZASP and integrin activation not defined here\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Connected ZASP to muscle maintenance and metabolic enzyme positioning, identifying PGM1 as a stress-dependent Z-disc partner and confirming ortholog requirement for muscle integrity.\",\n      \"evidence\": \"Y2H and endogenous Co-IP in rat cardiomyocytes for PGM1; dsRNAi knockdown with ultrastructural/behavioral analysis in Drosophila\",\n      \"pmids\": [\"19377068\", \"19603185\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Functional role of Z-disc-anchored PGM1 in energy metabolism not measured\", \"Whether PGM1 mislocalization drives DCM in vivo unknown\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Demonstrated that ZASP physically complexes with cardiac ion channels and that a DCM mutation alters ionic currents in vivo, establishing an electrophysiological dimension to ZASP disease.\",\n      \"evidence\": \"S196L transgenic mouse, cardiomyocyte electrophysiology, and pull-down of Cav1.2/Nav1.5\",\n      \"pmids\": [\"20852297\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism by which ZASP modulates channel gating/trafficking not defined here\", \"Direct vs indirect nature of channel binding not resolved\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Identified ZASP as a post-translationally modified Z-disc protein, the major O-GlcNAcylated myofibrillar protein, with disease-associated increases pointing to a regulatory role in heart failure.\",\n      \"evidence\": \"Enzymatic labeling, antibody detection, and MALDI-MS/MS on human heart myofibrils; integrin co-activation assays for the Drosophila ortholog\",\n      \"pmids\": [\"23271734\", \"22992465\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Functional consequence of ZASP O-GlcNAcylation on its interactions not tested\", \"O-GlcNAc sites and their regulation not mapped\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Defined ZASP as a Z-disc AKAP that scaffolds PKA (RIIalpha) and calcineurin and promotes Cav1.2 phosphorylation, recasting it from passive scaffold to active signaling organizer.\",\n      \"evidence\": \"Co-IP, in vitro phosphorylation, site-directed mutagenesis, and functional assays in Cypher-null cardiomyocytes\",\n      \"pmids\": [\"23996002\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether calcineurin tethering opposes PKA signaling locally not quantified\", \"In vivo physiological output of the AKAP function not yet shown here\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Established direct ZASP-actin binding through an alternatively spliced region and connected ZASP to p53/SUMO regulation and disease splicing, broadening its mechanistic repertoire.\",\n      \"evidence\": \"In vitro actin binding and mouse muscle expression (#14); Co-IP, transactivation and SUMO assays for Ankrd2/p53 (#15); exon array and minigene analysis in DM1 (#16)\",\n      \"pmids\": [\"24668811\", \"24647531\", \"24878509\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiological relevance of ZASP-p53 regulation in muscle unclear\", \"How actin binding and signaling functions are coordinated by splicing not resolved\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Quantified isoform- and exon-dependent affinity of ZASP for G- and F-actin and showed ZM disease mutations do not impair actin binding, separating actin engagement from the zaspopathy mechanism.\",\n      \"evidence\": \"SPR, electron microscopy, NMR HSQC, and circular dichroism with recombinant ZASP isoforms and mutants\",\n      \"pmids\": [\"28349680\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"The molecular defect of ZM mutations remained unidentified\", \"In vivo significance of differential isoform actin affinity not tested\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Showed in vivo that the long ZASP isoform is required for Cav1.2 surface density and beta-adrenergic potentiation of L-type Ca2+ current, providing the physiological output of its AKAP function.\",\n      \"evidence\": \"Long-Cypher knockout mice with patch-clamp electrophysiology and channel protein quantification\",\n      \"pmids\": [\"29743299\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether reduced channel density reflects trafficking or stability not resolved\", \"Contribution to arrhythmia phenotypes in patients not directly linked\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Defined the molecular mechanism of A165V zaspopathy as disrupted mechanosensing signaling, with early filamin C/chaperone aggregation preceding mutant protein aggregation, and revealed a PKA/beta-catenin proliferative axis.\",\n      \"evidence\": \"Ldb3 A165V knock-in mice with Co-IP and pathway Western blots (#19); phosphoproteomics and reporter assays in Cypher-knockout cardiac tissue (#20)\",\n      \"pmids\": [\"33742095\", \"34966794\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Temporal order of filamin C aggregation versus PKCalpha/mTOR dysregulation not fully causal\", \"Beta-catenin axis validated in single lab without independent replication\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Established a developmental role for ZASP in cardiomyocyte maturation via actin-controlled MRTFA-SRF transcription, with SRF re-expression rescuing maturation and function in vivo.\",\n      \"evidence\": \"Cypher knockout mice, RNA-seq, G/F-actin fractionation, nuclear-cytoplasmic extraction, and AAV-SRF rescue with electron microscopy\",\n      \"pmids\": [\"39113806\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How ZASP-dependent F-actin stability links to the AKAP and channel functions mechanistically not integrated\", \"Relevance of the maturation pathway to adult cardiomyopathy onset unclear\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How ZASP's distinct functions — Z-disc scaffolding, actin organization, AKAP-mediated channel regulation, mechanosensing, and transcriptional control — are coordinated by isoform splicing and post-translational modification into a unified mechanism remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No integrated model linking O-GlcNAcylation, splicing, and the multiple signaling axes\", \"Structural basis for simultaneous engagement of alpha-actinin, actin, channels, and PKA undefined\", \"Genotype-to-mechanism map for the full mutation spectrum incomplete\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0008092\", \"supporting_discovery_ids\": [0, 3, 4, 6, 14, 17]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [13, 15]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [13, 18]},\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [0, 14]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005856\", \"supporting_discovery_ids\": [0, 7, 14]},\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [15, 21]},\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [10, 13, 18]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": []}\n    ],\n    \"complexes\": [\n      \"ZASP/alpha-actinin-2/titin ternary complex\",\n      \"Z-disc\",\n      \"ZASP/Ankrd2/p53 complex\"\n    ],\n    \"partners\": [\n      \"ACTN2\",\n      \"PKC\",\n      \"PGM1\",\n      \"CACNA1C\",\n      \"SCN5A\",\n      \"PRKAR2A\",\n      \"FLNC\",\n      \"ANKRD2\"\n    ],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":7,"faith_total":7,"faith_pct":100.0}}