{"gene":"STRA6","run_date":"2026-04-28T21:42:57","timeline":{"discoveries":[{"year":1997,"finding":"STRA6 encodes a highly hydrophobic integral membrane protein of a novel type with no similarity to previously characterized integral membrane proteins; it is strongly expressed at blood-organ barriers and its expression in testis Sertoli cells is spermatogenic cycle-dependent and lost in RAR-alpha null mutants, suggesting a role in retinoid transport machinery.","method":"Subtractive hybridization cDNA cloning, in situ hybridization, expression analysis in RAR-alpha null mutants","journal":"Mechanisms of development","confidence":"Medium","confidence_rationale":"Tier 2 — foundational identification with expression and genetic evidence, single lab","pmids":["9203140"],"is_preprint":false},{"year":2007,"finding":"STRA6 functions as the high-affinity cell-surface receptor for plasma retinol-binding protein (RBP) and mediates cellular uptake of retinol (vitamin A) from holo-RBP; STRA6 couples with LRAT to facilitate retinol transfer into cells.","method":"Biochemical retinol uptake assays in transfected cells, RBP binding studies","journal":"Cell metabolism","confidence":"High","confidence_rationale":"Tier 1 — in vitro reconstitution of retinol uptake activity, foundational paper >100 citations","pmids":["17339024"],"is_preprint":false},{"year":2007,"finding":"Loss-of-function mutations in STRA6 cause pleiotropic developmental malformations (anophthalmia, heart defects, diaphragmatic hernia); missense mutations alter the geometry of transmembrane helix-connecting loops, and C-terminal mutations disrupt an SH2-binding motif (T644M) and a phosphorylation site (R655C), indicating these functional sites are essential for STRA6 activity.","method":"Homozygosity mapping, sequencing, immunoblot of patient fibroblasts, structural modeling","journal":"American journal of human genetics","confidence":"High","confidence_rationale":"Tier 2 — human loss-of-function with protein absence confirmed, multiple orthogonal approaches, >200 citations","pmids":["17273977"],"is_preprint":false},{"year":2008,"finding":"STRA6-dependent retinol transfer from RBP4 into NIH 3T3 fibroblasts is enhanced by LRAT and is bidirectional, indicating STRA6 acts as a retinol channel/transporter; in zebrafish, stra6 loss-of-function causes vitamin A deprivation of developing eyes, and in stra6-deficient embryos, holo-Rbp4 provokes nonspecific vitamin A excess in other tissues impairing RA receptor signaling.","method":"In vitro retinol transfer assays, zebrafish morpholino knockdown, pharmacological rescue","journal":"Cell metabolism","confidence":"High","confidence_rationale":"Tier 1 — in vitro reconstitution plus in vivo genetic model with multiple orthogonal methods, >150 citations","pmids":["18316031"],"is_preprint":false},{"year":2012,"finding":"STRA6 functions as a cytokine receptor that activates JAK/STAT signaling upon binding of holo-RBP; receptor phosphorylation is triggered not simply by extracellular ligand binding, but by STRA6-mediated translocation of retinol from serum RBP to the intracellular acceptor CRBP-I, and STRA6 phosphorylation is required for retinol uptake to proceed, demonstrating that transport and signaling are critically interdependent.","method":"Phosphorylation assays, retinol uptake assays, CRBP-I interaction studies, dominant-negative mutants","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 1 — multiple orthogonal biochemical methods establishing mechanistic coupling, replicated by subsequent work","pmids":["22665496"],"is_preprint":false},{"year":2012,"finding":"Transthyretin (TTR) blocks the ability of holo-RBP to associate with STRA6, thereby suppressing both STRA6-mediated retinol uptake and STRA6-initiated JAK/STAT signaling; TTR protects mice from RBP-induced insulin resistance.","method":"In vitro binding assays, cell-based retinol uptake and signaling assays, mouse glucose tolerance tests","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 1 — reconstituted binding blockade in vitro confirmed with in vivo functional consequence","pmids":["22826435"],"is_preprint":false},{"year":2012,"finding":"STRA6 catalyzes bidirectional retinol transport: retinol influx from holo-RBP (coupled to CRBP-I and LRAT) and retinol efflux into apo-RBP (strongly coupled to both CRBP-I and CRBP-II); STRA6 also catalyzes retinol exchange between intracellular CRBP-I and extracellular RBP.","method":"In vitro retinol influx, efflux, and exchange assays with purified components","journal":"The Journal of membrane biology","confidence":"High","confidence_rationale":"Tier 1 — reconstituted in vitro assays with defined components demonstrating multiple catalytic activities","pmids":["22815070"],"is_preprint":false},{"year":2012,"finding":"STRA6 deficiency in mice (stra6-/- null) results in severe reduction of retinoid content in the RPE and retina, impaired retinol uptake, shortened rod outer/inner segments, reduced cone photoreceptor number, reduced scotopic and photopic ERG responses, and persistent primary hyperplastic vitreous; however, STRA6 is not the only pathway for RPE retinol uptake.","method":"Stra6 knockout mouse, HPLC retinoid quantification, ERG, histology, immunocytochemistry","journal":"Investigative ophthalmology & visual science","confidence":"High","confidence_rationale":"Tier 2 — clean KO mouse with multiple defined functional readouts","pmids":["22467576"],"is_preprint":false},{"year":2013,"finding":"STRA6 ablation in mice effectively protects from RBP-induced suppression of insulin signaling (JAK/STAT pathway), demonstrating that STRA6 couples circulating holo-RBP levels to cell signaling and insulin resistance; contribution of STRA6 to retinol uptake by non-ocular tissues in vivo is small, and only the eye is critically dependent on STRA6 for retinoid homeostasis.","method":"Stra6 knockout mouse, insulin signaling assays, retinoid quantification, glucose tolerance tests","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — clean KO with multiple physiological readouts, confirms prior mechanistic findings","pmids":["23839944"],"is_preprint":false},{"year":2013,"finding":"LRAT (lecithin:retinol acyl transferase), by esterifying retinol, is necessary for activation of the STRA6/JAK2/STAT5 cascade by holo-RBP; LRAT-null mice are protected from holo-RBP-induced suppression of insulin responses, demonstrating that STRA6 signaling requires STRA6-mediated retinol transport supported by LRAT-catalyzed retinol metabolism.","method":"LRAT-null mice, cell-based JAK2/STAT5 signaling assays, insulin response measurements","journal":"FASEB journal","confidence":"High","confidence_rationale":"Tier 2 — genetic null mouse plus cell-based mechanistic assays with multiple readouts","pmids":["24036882"],"is_preprint":false},{"year":2013,"finding":"STRA6 is expressed in adipocyte precursor cells and mediates bidirectional retinol transport dictated by apo- and holo-RBP4 isoforms; holo-RBP4 blocks adipocyte differentiation via RARα activation, while apo-RBP4 triggers retinol efflux reducing cellular retinoids and RARα activity, enhancing adipogenesis synergistically with ectopic STRA6.","method":"Cell-based retinol transport assays, adipocyte differentiation assays, RARα reporter assays, in vivo retinoid mobilization","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 1-2 — multiple orthogonal in vitro and in vivo methods demonstrating bidirectional transport with functional consequences","pmids":["23959802"],"is_preprint":false},{"year":2013,"finding":"STRA6 upregulation by DNA damage occurs in a p53-dependent manner; STRA6 expression induces apoptosis and influences p53-mediated cell fate decisions by converting arrest to cell death, induces mitochondria depolarization and ROS accumulation, and is present not only at the cell membrane but also in the cytosol; these functions are independent of downstream RA signaling.","method":"p53 overexpression and knockdown, flow cytometry (apoptosis, ROS, mitochondrial potential), subcellular fractionation, STRA6 knockdown","journal":"Cell death and differentiation","confidence":"Medium","confidence_rationale":"Tier 2 — multiple cell-based assays in single lab, novel finding","pmids":["23449393"],"is_preprint":false},{"year":2014,"finding":"STRA6 is a bona fide vitamin A transporter critical for ocular vitamin A uptake; Stra6 knockout mice on vitamin A-sufficient diets show markedly reduced ocular retinoids, choroid/RPE malformations, early cone photoreceptor cell death, and reduced rod outer segment length; pharmacological doses of vitamin A restore transport across blood-retinal and blood-CSF barriers and rescue vision.","method":"Stra6 knockout mouse, HPLC retinoid quantification, ophthalmic imaging, histology, pharmacological rescue","journal":"Human molecular genetics","confidence":"High","confidence_rationale":"Tier 2 — clean KO with multiple phenotypic readouts and pharmacological rescue, replicated by other KO studies","pmids":["24852372"],"is_preprint":false},{"year":2014,"finding":"Holo-RBP/STRA6 signaling activates a JAK2-STAT3/STAT5 cascade that promotes oncogenic transformation; expression of RBP and STRA6 is markedly upregulated in human breast and colon tumors; STRA6 expression is critical for tumor formation by colon carcinoma cells in vivo, and holo-RBP/STRA6 signaling efficiently induces fibroblast oncogenic transformation.","method":"STRA6 knockdown/overexpression, xenograft mouse models, JAK2/STAT signaling assays, tumor formation assays","journal":"Cancer research","confidence":"High","confidence_rationale":"Tier 2 — in vitro and in vivo models with defined signaling pathway, multiple cell types","pmids":["25237067"],"is_preprint":false},{"year":2014,"finding":"An alternative Stra6 promoter driven by a half-site retinoic acid response element (RARE) is used in embryonic stem cells and some tissues; both RARγ and RXRα occupy the STRA6 RARE; RA increases p300 binding and H3K27 acetylation and decreases Suz12/H3K27me3 at both promoters; in vitamin A-deficient kidneys, differential promoter usage occurs, with the longer transcript (Stra6L) greatly increased while the shorter (Stra6S) decreases.","method":"CRISPR-Cas9 RARE deletion, luciferase reporter assays, ChIP, mouse models of vitamin A deficiency, RARγ null mice","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — CRISPR validation plus multiple chromatin and expression assays, mechanistic depth","pmids":["25544292"],"is_preprint":false},{"year":2015,"finding":"G304K mutant STRA6 protein is mislocalized and has severely reduced vitamin A uptake activity; inhibition of retinoic acid synthesis in zebrafish reproduces the colobomatous microanophthalmia phenotype, indicating diminished RA levels account for eye malformations in STRA6 p.G304K patients.","method":"Functional vitamin A uptake assays, localization studies in transfected cells, zebrafish RA synthesis inhibition model","journal":"Human mutation","confidence":"High","confidence_rationale":"Tier 2 — direct functional assay of mutant protein + in vivo zebrafish model","pmids":["21901792"],"is_preprint":false},{"year":2016,"finding":"Cryo-EM structure of zebrafish STRA6 at 3.9 Å resolution reveals: (1) a novel topology with one intramembrane and nine transmembrane helices forming an intricate homodimer; (2) calmodulin tightly bound in a noncanonical arrangement; (3) an arch-like extracellular structure covering a deep lipophilic cleft that is open to the membrane, suggesting retinol internalization by direct diffusion into the lipid bilayer after release from RBP.","method":"Single-particle cryo-electron microscopy, structural analysis, mutagenesis of RBP-binding residues","journal":"Science","confidence":"High","confidence_rationale":"Tier 1 — high-resolution cryo-EM structure with functional validation, highly cited","pmids":["27563101"],"is_preprint":false},{"year":2016,"finding":"STRA6 is critical for transport of vitamin A across blood-tissue barriers in eyes, brain, and testis; fat and lung rely instead on dietary vitamin A; in testis and brain, Stra6 expression is regulated by vitamin A levels, which reduces vitamin A consumption under dietary limitation to sequester it for the eye.","method":"Stra6 knockout mice on vitamin A-sufficient and -deficient diets, tissue retinoid quantification, retinoid-dependent process analysis","journal":"FASEB journal","confidence":"High","confidence_rationale":"Tier 2 — clean KO with multiple tissues and dietary conditions, confirms and extends prior KO studies","pmids":["27189978"],"is_preprint":false},{"year":2017,"finding":"STRA6 undergoes circadian patterning in adipose tissue driven in part by the nuclear transcription factor REV-ERBα; STRA6 is necessary for diurnal rhythmicity of insulin action and JAK/STAT signaling in adipose tissue, establishing holo-RBP/STRA6 as a regulator of diurnal insulin responses.","method":"Circadian expression analysis, Stra6 knockout mice, adipose tissue insulin signaling assays","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 — KO mouse with defined circadian and insulin readouts, single lab","pmids":["28733465"],"is_preprint":false},{"year":2017,"finding":"RBP4 activation of STRA6 transduces a JAK2-STAT3 signaling cascade that promotes colon cancer stem cell maintenance and sphere/tumor initiation frequency; downregulation of STRA6 or RBP4 decreases cancer stem cell fraction and tumor growth in vivo.","method":"STRA6/RBP4 knockdown, cancer stem cell assays, sphere formation, xenograft mouse model","journal":"Stem cell reports","confidence":"High","confidence_rationale":"Tier 2 — in vitro and in vivo evidence with defined signaling pathway","pmids":["28689994"],"is_preprint":false},{"year":2019,"finding":"miR-873 directly targets STRA6, downregulating its expression; STRA6 activates the Wnt/β-catenin signaling pathway in gastric cancer cells, promoting proliferation and metastasis.","method":"Dual-luciferase reporter assay, rescue experiments, STRA6 knockdown/overexpression, in vitro and in vivo tumor assays","journal":"Journal of experimental & clinical cancer research","confidence":"Medium","confidence_rationale":"Tier 3 — single lab, luciferase reporter plus KD/KO functional assays","pmids":["31694721"],"is_preprint":false},{"year":2019,"finding":"O-GlcNAcylation directly modifies STRA6 in diabetic kidneys, suppressing RBP4 binding activity and disrupting downstream retinol signaling (CRBP1, LRAT, ADH, retinoic acid, RARs); inhibition of O-GlcNAcylation rescues STRA6-retinol signals and reverses increases in TGFβ1 and collagen in high-glucose-treated cells.","method":"Immunoprecipitation, proximity ligation assay, OGT/OGA knockdown and transfection, HPLC retinoid quantification, Western blot","journal":"Biochimica et biophysica acta. General subjects","confidence":"Medium","confidence_rationale":"Tier 2 — multiple orthogonal methods in a single lab identifying O-GlcNAcylation as a PTM on STRA6 with functional consequence","pmids":["30905621"],"is_preprint":false},{"year":2020,"finding":"STRA6 expressed in pancreatic β-cells mediates RBP4's inhibitory effect on insulin synthesis through a JAK2/STAT1/ISL-1 signaling pathway; RBP4 directly suppresses glucose-stimulated insulin secretion via STRA6 in a dose- and time-dependent manner.","method":"Primary islet isolation, INS-1E cell assays, siRNA knockdown of STRA6, JAK2/STAT1 inhibitors, RBP4 transgenic mice","journal":"Diabetes","confidence":"High","confidence_rationale":"Tier 2 — multiple in vitro and in vivo approaches identifying STRA6/JAK2/STAT1/ISL-1 as the mechanistic axis","pmids":["33199363"],"is_preprint":false},{"year":2021,"finding":"Calmodulin binding to STRA6 via the BP2 region is Ca2+-dependent: at resting intracellular Ca2+ (<100 nM), BP2 interacts with the C-lobe of CaM; as Ca2+ rises to ~1000 nM (signaling event level), BP2 interacts with both N- and C-lobes of Ca2+-loaded CaM; BP2 binding increases CaM's Ca2+-binding affinity and slows Ca2+ dissociation, suggesting Ca2+-dependent regulation of vitamin A transport by STRA6.","method":"NMR chemical shift perturbations, thermodynamic Ca2+ binding studies, kinetic koff measurements","journal":"Journal of molecular biology","confidence":"High","confidence_rationale":"Tier 1 — NMR structural/biophysical characterization with quantitative thermodynamic and kinetic data","pmids":["34592217"],"is_preprint":false},{"year":2021,"finding":"Retinol from hepatic stellate cells is transferred to hepatocytes via STRA6-mediated transport; TIF1γ suppression in HSCs upregulates STRA6 and promotes retinol release, and STRA6 knockdown in HSCs blocks retinol transfer to hepatocytes, reducing lipogenesis-related gene upregulation and triglyceride accumulation; in vivo STRA6 blocking reduces liver fibrosis.","method":"siRNA knockdown of STRA6, HSC isolation from transgenic mice, HepG2 cell retinol uptake assays, in vivo fibrosis model","journal":"Cell & bioscience","confidence":"Medium","confidence_rationale":"Tier 2 — in vitro and in vivo evidence but single lab","pmids":["33407858"],"is_preprint":false},{"year":2022,"finding":"STRA6 is critical to adjust the stoichiometry of chromophore and opsins in rod and cone photoreceptors; STRA6-deficient mice show decreased chromophore throughout life, downregulation of opsin genes, mislocalized cone opsins, and significant amounts of chromophore-free opsins in rods; pharmacological vitamin A ameliorates the rod phenotype but does not restore cone visual pigment synthesis.","method":"Stra6 knockout mice, HPLC retinoid quantification, histology, ERG, immunohistochemistry, pharmacological rescue","journal":"Human molecular genetics","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods in KO model with specific mechanistic readouts","pmids":["34508587"],"is_preprint":false},{"year":2023,"finding":"STRA6 is required for proper induction of vascular smooth muscle cell (SMC) lineages in human cardiac outflow tract development; STRA6-knockout human embryonic stem cells can differentiate into cardiomyocytes but cannot differentiate into mesodermal or neural crest-derived SMCs; an interaction between RA nuclear receptors RARα/RXRα and TBX1 (OFT-specific transcription factor) was identified as downstream of STRA6-mediated RA signaling.","method":"STRA6-knockout hESCs, single-cell RNA-seq comparison, population RNA-seq, in vitro differentiation assays, co-immunoprecipitation (RARα/RXRα-TBX1)","journal":"Cardiovascular research","confidence":"High","confidence_rationale":"Tier 2 — hESC KO with multiple differentiation readouts and novel protein-protein interaction identification","pmids":["36635482"],"is_preprint":false},{"year":2023,"finding":"STRA6 interacts with integrin-linked kinase (ILK) and activates downstream AKT/mTOR signaling in thyroid carcinoma cells; STRA6 also activates STAT3-mediated SREBP1 expression to reprogram lipid metabolism, promoting tumor metastasis.","method":"Co-immunoprecipitation, RNA sequencing, STRA6 knockdown, in vitro and in vivo tumor assays, siRNA-LNP delivery","journal":"Endocrinology","confidence":"Medium","confidence_rationale":"Tier 2-3 — Co-IP plus functional KD assays in single lab; novel interaction finding","pmids":["36592123"],"is_preprint":false},{"year":2024,"finding":"Holo-RBP4 activates STRA6 to promote phosphorylation of downstream JAK2 and STAT3, causing muscle atrophy; inhibition of STRA6/JAK2/STAT3 pathway by siRNA or specific inhibitors reduces atrophy marker expression (Atrogin-1, MuRF1) and rescues myogenesis markers (MyoD, MyoG) in C2C12 myotubes treated with holo-RBP4.","method":"siRNA knockdown, pathway inhibitors, denervation mouse model, RBP4 knockout mice, C2C12 myotube assays","journal":"Journal of cachexia, sarcopenia and muscle","confidence":"High","confidence_rationale":"Tier 2 — in vitro and in vivo models with defined STRA6/JAK2/STAT3 pathway placement and multiple readouts","pmids":["39031684"],"is_preprint":false},{"year":2024,"finding":"VIRMA-mediated m6A modification of STRA6 mRNA (at the 3' UTR) stabilizes STRA6 mRNA via the m6A reader IGF2BP2, upregulating STRA6 expression; STRA6 then activates STAT3/HIF-1α signaling to promote glycolysis in pancreatic ductal adenocarcinoma.","method":"RNA-seq, m6A-seq, RIP assay, luciferase assay, mRNA stability assay, STRA6/VIRMA knockdown in vitro and in vivo","journal":"Cancer letters","confidence":"Medium","confidence_rationale":"Tier 2 — multiple sequencing and functional assays establishing m6A regulation of STRA6, single lab","pmids":["38604311"],"is_preprint":false},{"year":2024,"finding":"STRA6 knockdown in NSCLC cells suppresses proliferation, migration, and invasion by reducing lipid synthesis; mechanistically, STRA6 activates JAK2/STAT3 signaling, which induces SREBP-1 expression to promote adipogenesis and provide energy for tumor growth.","method":"STRA6 knockdown, in vitro and in vivo tumor assays, JAK2/STAT3 pathway analysis, lipid metabolism assays","journal":"Molecular and cellular biochemistry","confidence":"Medium","confidence_rationale":"Tier 2 — in vitro and in vivo KD with defined signaling readouts, single lab","pmids":["39168951"],"is_preprint":false},{"year":2015,"finding":"Purified STRA6 protein interacts with RBP in a retinol-independent manner with a stoichiometry consistent with 1 mole RBP per mole STRA6 monomer, as shown by surface plasmon resonance; STRA6-GFP correctly localizes to the cell surface and retains RBP binding.","method":"Surface plasmon resonance, GFP fusion expression and purification, cell surface localization assay","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 1 — direct binding measurement with purified protein, but single lab, preliminary characterization","pmids":["25816144"],"is_preprint":false},{"year":2020,"finding":"Reconstitution of purified zebrafish STRA6 in liposomes demonstrates STRA6-mediated retinol uptake activity in a defined in vitro proteoliposome assay; the cryo-EM structure shows STRA6 in lipid-filled nanodiscs maintaining its homodimeric topology.","method":"Cryo-EM, proteoliposome reconstitution, in vitro retinol uptake assay","journal":"Methods in enzymology","confidence":"High","confidence_rationale":"Tier 1 — reconstituted in vitro transport assay with structurally validated protein","pmids":["32359662"],"is_preprint":false}],"current_model":"STRA6 is a multitransmembrane homodimeric membrane protein (9 TM helices per protomer, with calmodulin bound intracellularly in a Ca2+-dependent manner) that serves as the high-affinity cell-surface receptor for plasma retinol-binding protein (RBP4/RBP), mediating bidirectional vitamin A (retinol) transport across the membrane by releasing retinol from holo-RBP into the cell (facilitated by LRAT-mediated esterification and CRBP-I) or loading retinol into apo-RBP for efflux; in addition to this transport function, STRA6 acts as a cytokine signaling receptor whose phosphorylation—triggered by retinol translocation to CRBP-I—activates a JAK2/STAT3/STAT5 cascade that regulates insulin responses, adipogenesis, cancer stem cell maintenance, diurnal insulin rhythmicity, and other cellular processes, while transthyretin blocks holo-RBP access to STRA6, and O-GlcNAcylation of STRA6 disrupts its RBP4-binding and downstream signaling in diabetes."},"narrative":{"teleology":[{"year":1997,"claim":"Before STRA6's function was known, its identification as a retinoic acid-responsive gene encoding a novel integral membrane protein at blood–organ barriers established the molecular starting point and predicted a role in retinoid transport.","evidence":"Subtractive hybridization cloning and in situ hybridization in wild-type and RARα-null mice","pmids":["9203140"],"confidence":"Medium","gaps":["No biochemical activity assigned","Membrane topology unknown","No binding partner identified"]},{"year":2007,"claim":"The central question of STRA6's biochemical function was resolved by demonstrating it is the high-affinity cell-surface receptor for plasma RBP that mediates cellular retinol uptake, and simultaneously, human loss-of-function mutations established its physiological necessity for eye and heart development.","evidence":"Transfected-cell retinol uptake assays with RBP binding (Cell Metabolism); homozygosity mapping and patient fibroblast analysis showing protein absence (Am J Hum Genet)","pmids":["17339024","17273977"],"confidence":"High","gaps":["Directionality of transport not yet tested","Structural basis unknown","Signaling capacity not suspected"]},{"year":2008,"claim":"The transport mechanism was shown to be bidirectional and LRAT-coupled, and zebrafish loss-of-function confirmed that STRA6 gates vitamin A specifically to the developing eye rather than acting as a general cellular uptake channel.","evidence":"In vitro retinol influx/efflux assays plus zebrafish morpholino knockdown with pharmacological rescue","pmids":["18316031"],"confidence":"High","gaps":["Efflux coupling partners not fully defined","Structural mechanism of bidirectionality unknown"]},{"year":2012,"claim":"A paradigm shift occurred when STRA6 was revealed to function simultaneously as a cytokine receptor: retinol translocation to CRBP-I triggers STRA6 phosphorylation and JAK2/STAT3-5 activation, transthyretin blocks holo-RBP access to suppress both transport and signaling, and STRA6 catalyzes retinol exchange between intracellular CRBPs and extracellular RBP.","evidence":"Phosphorylation/retinol uptake assays with dominant-negative mutants; TTR binding competition in vitro and insulin resistance protection in mice; purified-component exchange assays","pmids":["22665496","22826435","22815070"],"confidence":"High","gaps":["Phosphorylation site(s) on STRA6 not mapped biochemically","How TTR sterically excludes holo-RBP structurally unknown","Calmodulin's role in signaling vs transport not dissected"]},{"year":2012,"claim":"Stra6-knockout mice established that STRA6 is essential for ocular retinoid homeostasis and photoreceptor maintenance but dispensable for retinol uptake in most non-ocular tissues, delineating tissue-specific dependence.","evidence":"Stra6-null mice with HPLC retinoid quantification, ERG, and histology across tissues","pmids":["22467576","23839944"],"confidence":"High","gaps":["Alternative retinol uptake pathways in non-ocular tissues not identified","Whether STRA6 loss affects brain and testis long-term not fully addressed"]},{"year":2013,"claim":"LRAT was shown to be required for STRA6/JAK2/STAT5 activation, demonstrating that retinol metabolism is mechanistically coupled to the signaling output, and bidirectional transport through STRA6 was shown to regulate adipogenesis by controlling cellular retinoid levels and RARα activity.","evidence":"LRAT-null mice protected from holo-RBP-induced insulin suppression; adipocyte differentiation assays with apo- vs holo-RBP4","pmids":["24036882","23959802"],"confidence":"High","gaps":["Whether LRAT and CRBP-I compete or cooperate at STRA6 unknown","Relative contribution of transport vs signaling to adipogenesis unclear"]},{"year":2014,"claim":"STRA6 transcription was shown to be regulated by a dual-promoter system driven by a half-site RARE occupied by RARγ/RXRα, with vitamin A status controlling promoter switching, and STRA6/JAK2/STAT3 signaling was linked to oncogenic transformation in breast and colon tumors.","evidence":"CRISPR-Cas9 RARE deletion and ChIP in mouse tissues; STRA6 knockdown in xenograft tumor models","pmids":["25544292","25237067"],"confidence":"High","gaps":["Whether the two promoter-derived isoforms have distinct signaling properties unknown","Oncogenic role observed in limited tumor types"]},{"year":2016,"claim":"The cryo-EM structure at 3.9 Å resolution revealed STRA6's unprecedented homodimeric architecture with a deep lipophilic cleft open to the membrane, explaining how retinol is released from RBP and diffuses into the bilayer, and constitutively bound calmodulin was identified intracellularly.","evidence":"Single-particle cryo-EM of zebrafish STRA6 with mutagenesis of RBP-binding residues","pmids":["27563101"],"confidence":"High","gaps":["No structure of STRA6–RBP complex available","How retinol traverses from the cleft to intracellular acceptors unresolved","Calmodulin's functional role in transport not demonstrated structurally"]},{"year":2017,"claim":"STRA6 was placed in the circadian regulatory network, with REV-ERBα driving its diurnal expression in adipose tissue, and STRA6/JAK2/STAT3 signaling was shown to maintain cancer stem cell self-renewal in colon carcinoma.","evidence":"Circadian expression analysis in Stra6-KO mice; cancer stem cell sphere-formation and xenograft assays with STRA6 knockdown","pmids":["28733465","28689994"],"confidence":"Medium","gaps":["Whether circadian STRA6 oscillation directly modulates retinol flux or only signaling is unclear","Cancer stem cell mechanism may be tissue-specific"]},{"year":2019,"claim":"O-GlcNAcylation was identified as a post-translational modification of STRA6 in diabetic kidneys that suppresses RBP4 binding and disrupts downstream retinol signaling, providing a molecular link between hyperglycemia and impaired vitamin A homeostasis.","evidence":"Immunoprecipitation, proximity ligation assay, OGT/OGA manipulation in high-glucose-treated cells","pmids":["30905621"],"confidence":"Medium","gaps":["Specific O-GlcNAcylation sites on STRA6 not mapped","Whether this modification affects STRA6 signaling independently of transport unknown","Single-lab finding awaits independent confirmation"]},{"year":2021,"claim":"NMR studies defined the Ca²⁺-dependent calmodulin–STRA6 interaction in molecular detail: at resting Ca²⁺, only the CaM C-lobe engages the BP2 peptide, while signaling-level Ca²⁺ recruits both lobes, suggesting Ca²⁺ transients regulate STRA6 transport activity.","evidence":"NMR chemical shift perturbation, thermodynamic Ca²⁺-binding and kinetic dissociation measurements","pmids":["34592217"],"confidence":"High","gaps":["Functional effect of CaM engagement on retinol transport rate not measured in cells","No structure of full-length CaM–STRA6 complex at atomic resolution"]},{"year":2022,"claim":"STRA6 was shown to be essential for maintaining chromophore–opsin stoichiometry throughout life; its absence causes persistent chromophore deficit, opsin downregulation, and cone opsin mislocalization that pharmacological vitamin A only partially rescues.","evidence":"Stra6-KO mice with lifelong retinoid quantification, ERG, immunohistochemistry, and vitamin A supplementation","pmids":["34508587"],"confidence":"High","gaps":["Why cones are more sensitive than rods to STRA6 loss not mechanistically explained","Alternative chromophore delivery routes to cones not identified"]},{"year":2023,"claim":"STRA6 was linked to cardiac outflow tract development through its role in supplying retinol for retinoic acid signaling to RAR/RXR–TBX1 complexes, and independently shown to interact with ILK to activate AKT/mTOR signaling in thyroid cancer.","evidence":"STRA6-KO hESC differentiation with scRNA-seq and Co-IP of RARα/RXRα–TBX1; Co-IP of STRA6–ILK in thyroid carcinoma cells with knockdown and xenograft assays","pmids":["36635482","36592123"],"confidence":"Medium","gaps":["STRA6–ILK interaction awaits reciprocal validation and structural characterization","Whether the cardiac SMC phenotype reflects retinol transport or STRA6 signaling is undetermined"]},{"year":2024,"claim":"STRA6/JAK2/STAT3 signaling was extended to muscle atrophy (via Atrogin-1/MuRF1 induction) and pancreatic cancer glycolysis (via STAT3/HIF-1α), and STRA6 mRNA was shown to be stabilized by VIRMA-mediated m⁶A modification read by IGF2BP2, adding epitranscriptomic regulation.","evidence":"Denervation and RBP4-KO mouse models for atrophy; m⁶A-seq and RIP assays for epitranscriptomic control; knockdown and xenograft assays for pancreatic cancer","pmids":["39031684","38604311","39168951"],"confidence":"Medium","gaps":["Whether m⁶A regulation of STRA6 operates in non-cancer contexts unknown","Relative contribution of STRA6 transport vs signaling to muscle atrophy not dissected"]},{"year":null,"claim":"Key unresolved questions include the atomic-resolution structure of the STRA6–holo-RBP complex, the identity of specific phosphorylation and O-GlcNAcylation sites, how calmodulin binding functionally modulates retinol transport in cells, the mechanism by which retinol traverses from the lipophilic cleft to intracellular acceptors, and the molecular basis for tissue-specific dependence on STRA6 versus alternative retinol uptake pathways.","evidence":"","pmids":[],"confidence":"Low","gaps":["No atomic STRA6–RBP co-structure","Phosphorylation sites not mapped by mass spectrometry","Alternative retinol uptake pathways in non-ocular tissues molecularly unidentified"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0005215","term_label":"transporter activity","supporting_discovery_ids":[1,3,6,32]},{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[4,13,22,28]},{"term_id":"GO:0038024","term_label":"cargo receptor activity","supporting_discovery_ids":[1,31]},{"term_id":"GO:0008289","term_label":"lipid binding","supporting_discovery_ids":[6,16]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[1,15,16,31]}],"pathway":[{"term_id":"R-HSA-382551","term_label":"Transport of small molecules","supporting_discovery_ids":[1,3,6,10,17,32]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[2,7,12,26]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[13,19,27,29,30]}],"complexes":["STRA6 homodimer","STRA6–calmodulin complex"],"partners":["RBP4","LRAT","CRBP1","CAM","TTR","JAK2","ILK"],"other_free_text":[]},"mechanistic_narrative":"STRA6 is a multitransmembrane cell-surface receptor and bidirectional retinol transporter that couples vitamin A (retinol) delivery from plasma retinol-binding protein (RBP4) to intracellular JAK/STAT signaling. Structurally, STRA6 forms a homodimer with nine transmembrane helices per protomer and a deep lipophilic cleft open to the membrane through which retinol enters the bilayer after release from holo-RBP; calmodulin is constitutively bound intracellularly in a Ca²⁺-dependent manner [PMID:27563101, PMID:34592217]. STRA6 mediates retinol influx coupled to LRAT-catalyzed esterification and CRBP-I, and retinol efflux into apo-RBP; retinol translocation to CRBP-I triggers STRA6 phosphorylation and activation of JAK2/STAT3/STAT5 cascades that regulate insulin signaling, adipogenesis, cancer stem cell maintenance, and muscle homeostasis, while transthyretin blocks holo-RBP access to suppress both transport and signaling [PMID:22665496, PMID:22826435, PMID:22815070, PMID:28689994, PMID:39031684]. Loss-of-function mutations in STRA6 cause Matthew-Wood syndrome, characterized by anophthalmia and congenital heart defects, and STRA6 knockout in mice confirms its essential role in ocular retinoid homeostasis and photoreceptor chromophore supply [PMID:17273977, PMID:22467576, PMID:34508587]."},"prefetch_data":{"uniprot":{"accession":"Q9BX79","full_name":"Receptor for retinol uptake STRA6","aliases":["Retinol-binding protein receptor STRA6","Stimulated by retinoic acid gene 6 protein homolog"],"length_aa":667,"mass_kda":73.5,"function":"Functions as a retinol transporter. Accepts all-trans retinol from the extracellular retinol-binding protein RBP4, facilitates retinol transport across the cell membrane, and then transfers retinol to the cytoplasmic retinol-binding protein RBP1 (PubMed:18316031, PubMed:22665496, PubMed:9452451). Retinol uptake is enhanced by LRAT, an enzyme that converts retinol to all-trans retinyl esters, the storage forms of vitamin A (PubMed:18316031, PubMed:22665496). Contributes to the activation of a signaling cascade that depends on retinol transport and LRAT-dependent generation of retinol metabolites that then trigger activation of JAK2 and its target STAT5, and ultimately increase the expression of SOCS3 and inhibit cellular responses to insulin (PubMed:21368206, PubMed:22665496). Important for the homeostasis of vitamin A and its derivatives, such as retinoic acid (PubMed:18316031). STRA6-mediated transport is particularly important in the eye, and under conditions of dietary vitamin A deficiency (Probable). Does not transport retinoic acid (PubMed:18316031)","subcellular_location":"Cell membrane","url":"https://www.uniprot.org/uniprotkb/Q9BX79/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/STRA6","classification":"Not Classified","n_dependent_lines":1,"n_total_lines":1208,"dependency_fraction":0.0008278145695364238},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/STRA6","total_profiled":1310},"omim":[{"mim_id":"616428","title":"MICROPHTHALMIA/COLOBOMA 10; MCOPCB10","url":"https://www.omim.org/entry/616428"},{"mim_id":"615524","title":"MICROPHTHALMIA, SYNDROMIC 12; MCOPS12","url":"https://www.omim.org/entry/615524"},{"mim_id":"610745","title":"STIMULATED BY RETINOIC ACID 6; STRA6","url":"https://www.omim.org/entry/610745"},{"mim_id":"601186","title":"MICROPHTHALMIA, SYNDROMIC 9; MCOPS9","url":"https://www.omim.org/entry/601186"},{"mim_id":"309800","title":"MICROPHTHALMIA, SYNDROMIC 1; MCOPS1","url":"https://www.omim.org/entry/309800"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in many","driving_tissues":[{"tissue":"cervix","ntpm":22.3}],"url":"https://www.proteinatlas.org/search/STRA6"},"hgnc":{"alias_symbol":["SLC69A1","FLJ12541"],"prev_symbol":[]},"alphafold":{"accession":"Q9BX79","domains":[{"cath_id":"-","chopping":"49-130_144-234_241-252","consensus_level":"medium","plddt":81.7081,"start":49,"end":252},{"cath_id":"-","chopping":"293-594","consensus_level":"medium","plddt":88.0308,"start":293,"end":594}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9BX79","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9BX79-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9BX79-F1-predicted_aligned_error_v6.png","plddt_mean":77.75},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=STRA6","jax_strain_url":"https://www.jax.org/strain/search?query=STRA6"},"sequence":{"accession":"Q9BX79","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9BX79.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9BX79/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9BX79"}},"corpus_meta":[{"pmid":"17273977","id":"PMC_17273977","title":"Mutations 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of STRA6.","date":"2024","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/38395306","citation_count":4,"is_preprint":false},{"pmid":"34945773","id":"PMC_34945773","title":"STRA6 and Placental Retinoid Metabolism in Gestational Diabetes Mellitus.","date":"2021","source":"Journal of personalized medicine","url":"https://pubmed.ncbi.nlm.nih.gov/34945773","citation_count":4,"is_preprint":false},{"pmid":"32359662","id":"PMC_32359662","title":"Sample preparation for structural and functional analyses of the STRA6 receptor for retinol-binding protein.","date":"2020","source":"Methods in enzymology","url":"https://pubmed.ncbi.nlm.nih.gov/32359662","citation_count":4,"is_preprint":false},{"pmid":"34584880","id":"PMC_34584880","title":"miR-363-3p inhibits rat lung alveolar type II cell proliferation by downregulating STRA6 expression and induces cell apoptosis via cellular oxidative stress and G1-phase cell cycle arrest.","date":"2021","source":"Translational 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\"Subtractive hybridization cDNA cloning, in situ hybridization, expression analysis in RAR-alpha null mutants\",\n      \"journal\": \"Mechanisms of development\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — foundational identification with expression and genetic evidence, single lab\",\n      \"pmids\": [\"9203140\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"STRA6 functions as the high-affinity cell-surface receptor for plasma retinol-binding protein (RBP) and mediates cellular uptake of retinol (vitamin A) from holo-RBP; STRA6 couples with LRAT to facilitate retinol transfer into cells.\",\n      \"method\": \"Biochemical retinol uptake assays in transfected cells, RBP binding studies\",\n      \"journal\": \"Cell metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution of retinol uptake activity, foundational paper >100 citations\",\n      \"pmids\": [\"17339024\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Loss-of-function mutations in STRA6 cause pleiotropic developmental malformations (anophthalmia, heart defects, diaphragmatic hernia); missense mutations alter the geometry of transmembrane helix-connecting loops, and C-terminal mutations disrupt an SH2-binding motif (T644M) and a phosphorylation site (R655C), indicating these functional sites are essential for STRA6 activity.\",\n      \"method\": \"Homozygosity mapping, sequencing, immunoblot of patient fibroblasts, structural modeling\",\n      \"journal\": \"American journal of human genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — human loss-of-function with protein absence confirmed, multiple orthogonal approaches, >200 citations\",\n      \"pmids\": [\"17273977\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"STRA6-dependent retinol transfer from RBP4 into NIH 3T3 fibroblasts is enhanced by LRAT and is bidirectional, indicating STRA6 acts as a retinol channel/transporter; in zebrafish, stra6 loss-of-function causes vitamin A deprivation of developing eyes, and in stra6-deficient embryos, holo-Rbp4 provokes nonspecific vitamin A excess in other tissues impairing RA receptor signaling.\",\n      \"method\": \"In vitro retinol transfer assays, zebrafish morpholino knockdown, pharmacological rescue\",\n      \"journal\": \"Cell metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution plus in vivo genetic model with multiple orthogonal methods, >150 citations\",\n      \"pmids\": [\"18316031\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"STRA6 functions as a cytokine receptor that activates JAK/STAT signaling upon binding of holo-RBP; receptor phosphorylation is triggered not simply by extracellular ligand binding, but by STRA6-mediated translocation of retinol from serum RBP to the intracellular acceptor CRBP-I, and STRA6 phosphorylation is required for retinol uptake to proceed, demonstrating that transport and signaling are critically interdependent.\",\n      \"method\": \"Phosphorylation assays, retinol uptake assays, CRBP-I interaction studies, dominant-negative mutants\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — multiple orthogonal biochemical methods establishing mechanistic coupling, replicated by subsequent work\",\n      \"pmids\": [\"22665496\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Transthyretin (TTR) blocks the ability of holo-RBP to associate with STRA6, thereby suppressing both STRA6-mediated retinol uptake and STRA6-initiated JAK/STAT signaling; TTR protects mice from RBP-induced insulin resistance.\",\n      \"method\": \"In vitro binding assays, cell-based retinol uptake and signaling assays, mouse glucose tolerance tests\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstituted binding blockade in vitro confirmed with in vivo functional consequence\",\n      \"pmids\": [\"22826435\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"STRA6 catalyzes bidirectional retinol transport: retinol influx from holo-RBP (coupled to CRBP-I and LRAT) and retinol efflux into apo-RBP (strongly coupled to both CRBP-I and CRBP-II); STRA6 also catalyzes retinol exchange between intracellular CRBP-I and extracellular RBP.\",\n      \"method\": \"In vitro retinol influx, efflux, and exchange assays with purified components\",\n      \"journal\": \"The Journal of membrane biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstituted in vitro assays with defined components demonstrating multiple catalytic activities\",\n      \"pmids\": [\"22815070\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"STRA6 deficiency in mice (stra6-/- null) results in severe reduction of retinoid content in the RPE and retina, impaired retinol uptake, shortened rod outer/inner segments, reduced cone photoreceptor number, reduced scotopic and photopic ERG responses, and persistent primary hyperplastic vitreous; however, STRA6 is not the only pathway for RPE retinol uptake.\",\n      \"method\": \"Stra6 knockout mouse, HPLC retinoid quantification, ERG, histology, immunocytochemistry\",\n      \"journal\": \"Investigative ophthalmology & visual science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean KO mouse with multiple defined functional readouts\",\n      \"pmids\": [\"22467576\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"STRA6 ablation in mice effectively protects from RBP-induced suppression of insulin signaling (JAK/STAT pathway), demonstrating that STRA6 couples circulating holo-RBP levels to cell signaling and insulin resistance; contribution of STRA6 to retinol uptake by non-ocular tissues in vivo is small, and only the eye is critically dependent on STRA6 for retinoid homeostasis.\",\n      \"method\": \"Stra6 knockout mouse, insulin signaling assays, retinoid quantification, glucose tolerance tests\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean KO with multiple physiological readouts, confirms prior mechanistic findings\",\n      \"pmids\": [\"23839944\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"LRAT (lecithin:retinol acyl transferase), by esterifying retinol, is necessary for activation of the STRA6/JAK2/STAT5 cascade by holo-RBP; LRAT-null mice are protected from holo-RBP-induced suppression of insulin responses, demonstrating that STRA6 signaling requires STRA6-mediated retinol transport supported by LRAT-catalyzed retinol metabolism.\",\n      \"method\": \"LRAT-null mice, cell-based JAK2/STAT5 signaling assays, insulin response measurements\",\n      \"journal\": \"FASEB journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic null mouse plus cell-based mechanistic assays with multiple readouts\",\n      \"pmids\": [\"24036882\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"STRA6 is expressed in adipocyte precursor cells and mediates bidirectional retinol transport dictated by apo- and holo-RBP4 isoforms; holo-RBP4 blocks adipocyte differentiation via RARα activation, while apo-RBP4 triggers retinol efflux reducing cellular retinoids and RARα activity, enhancing adipogenesis synergistically with ectopic STRA6.\",\n      \"method\": \"Cell-based retinol transport assays, adipocyte differentiation assays, RARα reporter assays, in vivo retinoid mobilization\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — multiple orthogonal in vitro and in vivo methods demonstrating bidirectional transport with functional consequences\",\n      \"pmids\": [\"23959802\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"STRA6 upregulation by DNA damage occurs in a p53-dependent manner; STRA6 expression induces apoptosis and influences p53-mediated cell fate decisions by converting arrest to cell death, induces mitochondria depolarization and ROS accumulation, and is present not only at the cell membrane but also in the cytosol; these functions are independent of downstream RA signaling.\",\n      \"method\": \"p53 overexpression and knockdown, flow cytometry (apoptosis, ROS, mitochondrial potential), subcellular fractionation, STRA6 knockdown\",\n      \"journal\": \"Cell death and differentiation\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple cell-based assays in single lab, novel finding\",\n      \"pmids\": [\"23449393\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"STRA6 is a bona fide vitamin A transporter critical for ocular vitamin A uptake; Stra6 knockout mice on vitamin A-sufficient diets show markedly reduced ocular retinoids, choroid/RPE malformations, early cone photoreceptor cell death, and reduced rod outer segment length; pharmacological doses of vitamin A restore transport across blood-retinal and blood-CSF barriers and rescue vision.\",\n      \"method\": \"Stra6 knockout mouse, HPLC retinoid quantification, ophthalmic imaging, histology, pharmacological rescue\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean KO with multiple phenotypic readouts and pharmacological rescue, replicated by other KO studies\",\n      \"pmids\": [\"24852372\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Holo-RBP/STRA6 signaling activates a JAK2-STAT3/STAT5 cascade that promotes oncogenic transformation; expression of RBP and STRA6 is markedly upregulated in human breast and colon tumors; STRA6 expression is critical for tumor formation by colon carcinoma cells in vivo, and holo-RBP/STRA6 signaling efficiently induces fibroblast oncogenic transformation.\",\n      \"method\": \"STRA6 knockdown/overexpression, xenograft mouse models, JAK2/STAT signaling assays, tumor formation assays\",\n      \"journal\": \"Cancer research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — in vitro and in vivo models with defined signaling pathway, multiple cell types\",\n      \"pmids\": [\"25237067\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"An alternative Stra6 promoter driven by a half-site retinoic acid response element (RARE) is used in embryonic stem cells and some tissues; both RARγ and RXRα occupy the STRA6 RARE; RA increases p300 binding and H3K27 acetylation and decreases Suz12/H3K27me3 at both promoters; in vitamin A-deficient kidneys, differential promoter usage occurs, with the longer transcript (Stra6L) greatly increased while the shorter (Stra6S) decreases.\",\n      \"method\": \"CRISPR-Cas9 RARE deletion, luciferase reporter assays, ChIP, mouse models of vitamin A deficiency, RARγ null mice\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — CRISPR validation plus multiple chromatin and expression assays, mechanistic depth\",\n      \"pmids\": [\"25544292\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"G304K mutant STRA6 protein is mislocalized and has severely reduced vitamin A uptake activity; inhibition of retinoic acid synthesis in zebrafish reproduces the colobomatous microanophthalmia phenotype, indicating diminished RA levels account for eye malformations in STRA6 p.G304K patients.\",\n      \"method\": \"Functional vitamin A uptake assays, localization studies in transfected cells, zebrafish RA synthesis inhibition model\",\n      \"journal\": \"Human mutation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — direct functional assay of mutant protein + in vivo zebrafish model\",\n      \"pmids\": [\"21901792\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Cryo-EM structure of zebrafish STRA6 at 3.9 Å resolution reveals: (1) a novel topology with one intramembrane and nine transmembrane helices forming an intricate homodimer; (2) calmodulin tightly bound in a noncanonical arrangement; (3) an arch-like extracellular structure covering a deep lipophilic cleft that is open to the membrane, suggesting retinol internalization by direct diffusion into the lipid bilayer after release from RBP.\",\n      \"method\": \"Single-particle cryo-electron microscopy, structural analysis, mutagenesis of RBP-binding residues\",\n      \"journal\": \"Science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — high-resolution cryo-EM structure with functional validation, highly cited\",\n      \"pmids\": [\"27563101\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"STRA6 is critical for transport of vitamin A across blood-tissue barriers in eyes, brain, and testis; fat and lung rely instead on dietary vitamin A; in testis and brain, Stra6 expression is regulated by vitamin A levels, which reduces vitamin A consumption under dietary limitation to sequester it for the eye.\",\n      \"method\": \"Stra6 knockout mice on vitamin A-sufficient and -deficient diets, tissue retinoid quantification, retinoid-dependent process analysis\",\n      \"journal\": \"FASEB journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean KO with multiple tissues and dietary conditions, confirms and extends prior KO studies\",\n      \"pmids\": [\"27189978\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"STRA6 undergoes circadian patterning in adipose tissue driven in part by the nuclear transcription factor REV-ERBα; STRA6 is necessary for diurnal rhythmicity of insulin action and JAK/STAT signaling in adipose tissue, establishing holo-RBP/STRA6 as a regulator of diurnal insulin responses.\",\n      \"method\": \"Circadian expression analysis, Stra6 knockout mice, adipose tissue insulin signaling assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — KO mouse with defined circadian and insulin readouts, single lab\",\n      \"pmids\": [\"28733465\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"RBP4 activation of STRA6 transduces a JAK2-STAT3 signaling cascade that promotes colon cancer stem cell maintenance and sphere/tumor initiation frequency; downregulation of STRA6 or RBP4 decreases cancer stem cell fraction and tumor growth in vivo.\",\n      \"method\": \"STRA6/RBP4 knockdown, cancer stem cell assays, sphere formation, xenograft mouse model\",\n      \"journal\": \"Stem cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — in vitro and in vivo evidence with defined signaling pathway\",\n      \"pmids\": [\"28689994\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"miR-873 directly targets STRA6, downregulating its expression; STRA6 activates the Wnt/β-catenin signaling pathway in gastric cancer cells, promoting proliferation and metastasis.\",\n      \"method\": \"Dual-luciferase reporter assay, rescue experiments, STRA6 knockdown/overexpression, in vitro and in vivo tumor assays\",\n      \"journal\": \"Journal of experimental & clinical cancer research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — single lab, luciferase reporter plus KD/KO functional assays\",\n      \"pmids\": [\"31694721\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"O-GlcNAcylation directly modifies STRA6 in diabetic kidneys, suppressing RBP4 binding activity and disrupting downstream retinol signaling (CRBP1, LRAT, ADH, retinoic acid, RARs); inhibition of O-GlcNAcylation rescues STRA6-retinol signals and reverses increases in TGFβ1 and collagen in high-glucose-treated cells.\",\n      \"method\": \"Immunoprecipitation, proximity ligation assay, OGT/OGA knockdown and transfection, HPLC retinoid quantification, Western blot\",\n      \"journal\": \"Biochimica et biophysica acta. General subjects\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods in a single lab identifying O-GlcNAcylation as a PTM on STRA6 with functional consequence\",\n      \"pmids\": [\"30905621\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"STRA6 expressed in pancreatic β-cells mediates RBP4's inhibitory effect on insulin synthesis through a JAK2/STAT1/ISL-1 signaling pathway; RBP4 directly suppresses glucose-stimulated insulin secretion via STRA6 in a dose- and time-dependent manner.\",\n      \"method\": \"Primary islet isolation, INS-1E cell assays, siRNA knockdown of STRA6, JAK2/STAT1 inhibitors, RBP4 transgenic mice\",\n      \"journal\": \"Diabetes\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple in vitro and in vivo approaches identifying STRA6/JAK2/STAT1/ISL-1 as the mechanistic axis\",\n      \"pmids\": [\"33199363\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Calmodulin binding to STRA6 via the BP2 region is Ca2+-dependent: at resting intracellular Ca2+ (<100 nM), BP2 interacts with the C-lobe of CaM; as Ca2+ rises to ~1000 nM (signaling event level), BP2 interacts with both N- and C-lobes of Ca2+-loaded CaM; BP2 binding increases CaM's Ca2+-binding affinity and slows Ca2+ dissociation, suggesting Ca2+-dependent regulation of vitamin A transport by STRA6.\",\n      \"method\": \"NMR chemical shift perturbations, thermodynamic Ca2+ binding studies, kinetic koff measurements\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — NMR structural/biophysical characterization with quantitative thermodynamic and kinetic data\",\n      \"pmids\": [\"34592217\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Retinol from hepatic stellate cells is transferred to hepatocytes via STRA6-mediated transport; TIF1γ suppression in HSCs upregulates STRA6 and promotes retinol release, and STRA6 knockdown in HSCs blocks retinol transfer to hepatocytes, reducing lipogenesis-related gene upregulation and triglyceride accumulation; in vivo STRA6 blocking reduces liver fibrosis.\",\n      \"method\": \"siRNA knockdown of STRA6, HSC isolation from transgenic mice, HepG2 cell retinol uptake assays, in vivo fibrosis model\",\n      \"journal\": \"Cell & bioscience\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — in vitro and in vivo evidence but single lab\",\n      \"pmids\": [\"33407858\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"STRA6 is critical to adjust the stoichiometry of chromophore and opsins in rod and cone photoreceptors; STRA6-deficient mice show decreased chromophore throughout life, downregulation of opsin genes, mislocalized cone opsins, and significant amounts of chromophore-free opsins in rods; pharmacological vitamin A ameliorates the rod phenotype but does not restore cone visual pigment synthesis.\",\n      \"method\": \"Stra6 knockout mice, HPLC retinoid quantification, histology, ERG, immunohistochemistry, pharmacological rescue\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods in KO model with specific mechanistic readouts\",\n      \"pmids\": [\"34508587\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"STRA6 is required for proper induction of vascular smooth muscle cell (SMC) lineages in human cardiac outflow tract development; STRA6-knockout human embryonic stem cells can differentiate into cardiomyocytes but cannot differentiate into mesodermal or neural crest-derived SMCs; an interaction between RA nuclear receptors RARα/RXRα and TBX1 (OFT-specific transcription factor) was identified as downstream of STRA6-mediated RA signaling.\",\n      \"method\": \"STRA6-knockout hESCs, single-cell RNA-seq comparison, population RNA-seq, in vitro differentiation assays, co-immunoprecipitation (RARα/RXRα-TBX1)\",\n      \"journal\": \"Cardiovascular research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — hESC KO with multiple differentiation readouts and novel protein-protein interaction identification\",\n      \"pmids\": [\"36635482\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"STRA6 interacts with integrin-linked kinase (ILK) and activates downstream AKT/mTOR signaling in thyroid carcinoma cells; STRA6 also activates STAT3-mediated SREBP1 expression to reprogram lipid metabolism, promoting tumor metastasis.\",\n      \"method\": \"Co-immunoprecipitation, RNA sequencing, STRA6 knockdown, in vitro and in vivo tumor assays, siRNA-LNP delivery\",\n      \"journal\": \"Endocrinology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — Co-IP plus functional KD assays in single lab; novel interaction finding\",\n      \"pmids\": [\"36592123\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Holo-RBP4 activates STRA6 to promote phosphorylation of downstream JAK2 and STAT3, causing muscle atrophy; inhibition of STRA6/JAK2/STAT3 pathway by siRNA or specific inhibitors reduces atrophy marker expression (Atrogin-1, MuRF1) and rescues myogenesis markers (MyoD, MyoG) in C2C12 myotubes treated with holo-RBP4.\",\n      \"method\": \"siRNA knockdown, pathway inhibitors, denervation mouse model, RBP4 knockout mice, C2C12 myotube assays\",\n      \"journal\": \"Journal of cachexia, sarcopenia and muscle\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — in vitro and in vivo models with defined STRA6/JAK2/STAT3 pathway placement and multiple readouts\",\n      \"pmids\": [\"39031684\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"VIRMA-mediated m6A modification of STRA6 mRNA (at the 3' UTR) stabilizes STRA6 mRNA via the m6A reader IGF2BP2, upregulating STRA6 expression; STRA6 then activates STAT3/HIF-1α signaling to promote glycolysis in pancreatic ductal adenocarcinoma.\",\n      \"method\": \"RNA-seq, m6A-seq, RIP assay, luciferase assay, mRNA stability assay, STRA6/VIRMA knockdown in vitro and in vivo\",\n      \"journal\": \"Cancer letters\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple sequencing and functional assays establishing m6A regulation of STRA6, single lab\",\n      \"pmids\": [\"38604311\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"STRA6 knockdown in NSCLC cells suppresses proliferation, migration, and invasion by reducing lipid synthesis; mechanistically, STRA6 activates JAK2/STAT3 signaling, which induces SREBP-1 expression to promote adipogenesis and provide energy for tumor growth.\",\n      \"method\": \"STRA6 knockdown, in vitro and in vivo tumor assays, JAK2/STAT3 pathway analysis, lipid metabolism assays\",\n      \"journal\": \"Molecular and cellular biochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — in vitro and in vivo KD with defined signaling readouts, single lab\",\n      \"pmids\": [\"39168951\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Purified STRA6 protein interacts with RBP in a retinol-independent manner with a stoichiometry consistent with 1 mole RBP per mole STRA6 monomer, as shown by surface plasmon resonance; STRA6-GFP correctly localizes to the cell surface and retains RBP binding.\",\n      \"method\": \"Surface plasmon resonance, GFP fusion expression and purification, cell surface localization assay\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 — direct binding measurement with purified protein, but single lab, preliminary characterization\",\n      \"pmids\": [\"25816144\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Reconstitution of purified zebrafish STRA6 in liposomes demonstrates STRA6-mediated retinol uptake activity in a defined in vitro proteoliposome assay; the cryo-EM structure shows STRA6 in lipid-filled nanodiscs maintaining its homodimeric topology.\",\n      \"method\": \"Cryo-EM, proteoliposome reconstitution, in vitro retinol uptake assay\",\n      \"journal\": \"Methods in enzymology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstituted in vitro transport assay with structurally validated protein\",\n      \"pmids\": [\"32359662\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"STRA6 is a multitransmembrane homodimeric membrane protein (9 TM helices per protomer, with calmodulin bound intracellularly in a Ca2+-dependent manner) that serves as the high-affinity cell-surface receptor for plasma retinol-binding protein (RBP4/RBP), mediating bidirectional vitamin A (retinol) transport across the membrane by releasing retinol from holo-RBP into the cell (facilitated by LRAT-mediated esterification and CRBP-I) or loading retinol into apo-RBP for efflux; in addition to this transport function, STRA6 acts as a cytokine signaling receptor whose phosphorylation—triggered by retinol translocation to CRBP-I—activates a JAK2/STAT3/STAT5 cascade that regulates insulin responses, adipogenesis, cancer stem cell maintenance, diurnal insulin rhythmicity, and other cellular processes, while transthyretin blocks holo-RBP access to STRA6, and O-GlcNAcylation of STRA6 disrupts its RBP4-binding and downstream signaling in diabetes.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"STRA6 is a multitransmembrane cell-surface receptor and bidirectional retinol transporter that couples vitamin A (retinol) delivery from plasma retinol-binding protein (RBP4) to intracellular JAK/STAT signaling. Structurally, STRA6 forms a homodimer with nine transmembrane helices per protomer and a deep lipophilic cleft open to the membrane through which retinol enters the bilayer after release from holo-RBP; calmodulin is constitutively bound intracellularly in a Ca²⁺-dependent manner [PMID:27563101, PMID:34592217]. STRA6 mediates retinol influx coupled to LRAT-catalyzed esterification and CRBP-I, and retinol efflux into apo-RBP; retinol translocation to CRBP-I triggers STRA6 phosphorylation and activation of JAK2/STAT3/STAT5 cascades that regulate insulin signaling, adipogenesis, cancer stem cell maintenance, and muscle homeostasis, while transthyretin blocks holo-RBP access to suppress both transport and signaling [PMID:22665496, PMID:22826435, PMID:22815070, PMID:28689994, PMID:39031684]. Loss-of-function mutations in STRA6 cause Matthew-Wood syndrome, characterized by anophthalmia and congenital heart defects, and STRA6 knockout in mice confirms its essential role in ocular retinoid homeostasis and photoreceptor chromophore supply [PMID:17273977, PMID:22467576, PMID:34508587].\",\n  \"teleology\": [\n    {\n      \"year\": 1997,\n      \"claim\": \"Before STRA6's function was known, its identification as a retinoic acid-responsive gene encoding a novel integral membrane protein at blood–organ barriers established the molecular starting point and predicted a role in retinoid transport.\",\n      \"evidence\": \"Subtractive hybridization cloning and in situ hybridization in wild-type and RARα-null mice\",\n      \"pmids\": [\"9203140\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No biochemical activity assigned\", \"Membrane topology unknown\", \"No binding partner identified\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"The central question of STRA6's biochemical function was resolved by demonstrating it is the high-affinity cell-surface receptor for plasma RBP that mediates cellular retinol uptake, and simultaneously, human loss-of-function mutations established its physiological necessity for eye and heart development.\",\n      \"evidence\": \"Transfected-cell retinol uptake assays with RBP binding (Cell Metabolism); homozygosity mapping and patient fibroblast analysis showing protein absence (Am J Hum Genet)\",\n      \"pmids\": [\"17339024\", \"17273977\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Directionality of transport not yet tested\", \"Structural basis unknown\", \"Signaling capacity not suspected\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"The transport mechanism was shown to be bidirectional and LRAT-coupled, and zebrafish loss-of-function confirmed that STRA6 gates vitamin A specifically to the developing eye rather than acting as a general cellular uptake channel.\",\n      \"evidence\": \"In vitro retinol influx/efflux assays plus zebrafish morpholino knockdown with pharmacological rescue\",\n      \"pmids\": [\"18316031\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Efflux coupling partners not fully defined\", \"Structural mechanism of bidirectionality unknown\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"A paradigm shift occurred when STRA6 was revealed to function simultaneously as a cytokine receptor: retinol translocation to CRBP-I triggers STRA6 phosphorylation and JAK2/STAT3-5 activation, transthyretin blocks holo-RBP access to suppress both transport and signaling, and STRA6 catalyzes retinol exchange between intracellular CRBPs and extracellular RBP.\",\n      \"evidence\": \"Phosphorylation/retinol uptake assays with dominant-negative mutants; TTR binding competition in vitro and insulin resistance protection in mice; purified-component exchange assays\",\n      \"pmids\": [\"22665496\", \"22826435\", \"22815070\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Phosphorylation site(s) on STRA6 not mapped biochemically\", \"How TTR sterically excludes holo-RBP structurally unknown\", \"Calmodulin's role in signaling vs transport not dissected\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Stra6-knockout mice established that STRA6 is essential for ocular retinoid homeostasis and photoreceptor maintenance but dispensable for retinol uptake in most non-ocular tissues, delineating tissue-specific dependence.\",\n      \"evidence\": \"Stra6-null mice with HPLC retinoid quantification, ERG, and histology across tissues\",\n      \"pmids\": [\"22467576\", \"23839944\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Alternative retinol uptake pathways in non-ocular tissues not identified\", \"Whether STRA6 loss affects brain and testis long-term not fully addressed\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"LRAT was shown to be required for STRA6/JAK2/STAT5 activation, demonstrating that retinol metabolism is mechanistically coupled to the signaling output, and bidirectional transport through STRA6 was shown to regulate adipogenesis by controlling cellular retinoid levels and RARα activity.\",\n      \"evidence\": \"LRAT-null mice protected from holo-RBP-induced insulin suppression; adipocyte differentiation assays with apo- vs holo-RBP4\",\n      \"pmids\": [\"24036882\", \"23959802\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether LRAT and CRBP-I compete or cooperate at STRA6 unknown\", \"Relative contribution of transport vs signaling to adipogenesis unclear\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"STRA6 transcription was shown to be regulated by a dual-promoter system driven by a half-site RARE occupied by RARγ/RXRα, with vitamin A status controlling promoter switching, and STRA6/JAK2/STAT3 signaling was linked to oncogenic transformation in breast and colon tumors.\",\n      \"evidence\": \"CRISPR-Cas9 RARE deletion and ChIP in mouse tissues; STRA6 knockdown in xenograft tumor models\",\n      \"pmids\": [\"25544292\", \"25237067\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether the two promoter-derived isoforms have distinct signaling properties unknown\", \"Oncogenic role observed in limited tumor types\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"The cryo-EM structure at 3.9 Å resolution revealed STRA6's unprecedented homodimeric architecture with a deep lipophilic cleft open to the membrane, explaining how retinol is released from RBP and diffuses into the bilayer, and constitutively bound calmodulin was identified intracellularly.\",\n      \"evidence\": \"Single-particle cryo-EM of zebrafish STRA6 with mutagenesis of RBP-binding residues\",\n      \"pmids\": [\"27563101\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No structure of STRA6–RBP complex available\", \"How retinol traverses from the cleft to intracellular acceptors unresolved\", \"Calmodulin's functional role in transport not demonstrated structurally\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"STRA6 was placed in the circadian regulatory network, with REV-ERBα driving its diurnal expression in adipose tissue, and STRA6/JAK2/STAT3 signaling was shown to maintain cancer stem cell self-renewal in colon carcinoma.\",\n      \"evidence\": \"Circadian expression analysis in Stra6-KO mice; cancer stem cell sphere-formation and xenograft assays with STRA6 knockdown\",\n      \"pmids\": [\"28733465\", \"28689994\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether circadian STRA6 oscillation directly modulates retinol flux or only signaling is unclear\", \"Cancer stem cell mechanism may be tissue-specific\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"O-GlcNAcylation was identified as a post-translational modification of STRA6 in diabetic kidneys that suppresses RBP4 binding and disrupts downstream retinol signaling, providing a molecular link between hyperglycemia and impaired vitamin A homeostasis.\",\n      \"evidence\": \"Immunoprecipitation, proximity ligation assay, OGT/OGA manipulation in high-glucose-treated cells\",\n      \"pmids\": [\"30905621\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Specific O-GlcNAcylation sites on STRA6 not mapped\", \"Whether this modification affects STRA6 signaling independently of transport unknown\", \"Single-lab finding awaits independent confirmation\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"NMR studies defined the Ca²⁺-dependent calmodulin–STRA6 interaction in molecular detail: at resting Ca²⁺, only the CaM C-lobe engages the BP2 peptide, while signaling-level Ca²⁺ recruits both lobes, suggesting Ca²⁺ transients regulate STRA6 transport activity.\",\n      \"evidence\": \"NMR chemical shift perturbation, thermodynamic Ca²⁺-binding and kinetic dissociation measurements\",\n      \"pmids\": [\"34592217\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Functional effect of CaM engagement on retinol transport rate not measured in cells\", \"No structure of full-length CaM–STRA6 complex at atomic resolution\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"STRA6 was shown to be essential for maintaining chromophore–opsin stoichiometry throughout life; its absence causes persistent chromophore deficit, opsin downregulation, and cone opsin mislocalization that pharmacological vitamin A only partially rescues.\",\n      \"evidence\": \"Stra6-KO mice with lifelong retinoid quantification, ERG, immunohistochemistry, and vitamin A supplementation\",\n      \"pmids\": [\"34508587\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Why cones are more sensitive than rods to STRA6 loss not mechanistically explained\", \"Alternative chromophore delivery routes to cones not identified\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"STRA6 was linked to cardiac outflow tract development through its role in supplying retinol for retinoic acid signaling to RAR/RXR–TBX1 complexes, and independently shown to interact with ILK to activate AKT/mTOR signaling in thyroid cancer.\",\n      \"evidence\": \"STRA6-KO hESC differentiation with scRNA-seq and Co-IP of RARα/RXRα–TBX1; Co-IP of STRA6–ILK in thyroid carcinoma cells with knockdown and xenograft assays\",\n      \"pmids\": [\"36635482\", \"36592123\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"STRA6–ILK interaction awaits reciprocal validation and structural characterization\", \"Whether the cardiac SMC phenotype reflects retinol transport or STRA6 signaling is undetermined\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"STRA6/JAK2/STAT3 signaling was extended to muscle atrophy (via Atrogin-1/MuRF1 induction) and pancreatic cancer glycolysis (via STAT3/HIF-1α), and STRA6 mRNA was shown to be stabilized by VIRMA-mediated m⁶A modification read by IGF2BP2, adding epitranscriptomic regulation.\",\n      \"evidence\": \"Denervation and RBP4-KO mouse models for atrophy; m⁶A-seq and RIP assays for epitranscriptomic control; knockdown and xenograft assays for pancreatic cancer\",\n      \"pmids\": [\"39031684\", \"38604311\", \"39168951\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether m⁶A regulation of STRA6 operates in non-cancer contexts unknown\", \"Relative contribution of STRA6 transport vs signaling to muscle atrophy not dissected\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the atomic-resolution structure of the STRA6–holo-RBP complex, the identity of specific phosphorylation and O-GlcNAcylation sites, how calmodulin binding functionally modulates retinol transport in cells, the mechanism by which retinol traverses from the lipophilic cleft to intracellular acceptors, and the molecular basis for tissue-specific dependence on STRA6 versus alternative retinol uptake pathways.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No atomic STRA6–RBP co-structure\", \"Phosphorylation sites not mapped by mass spectrometry\", \"Alternative retinol uptake pathways in non-ocular tissues molecularly unidentified\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0005215\", \"supporting_discovery_ids\": [1, 3, 6, 32]},\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [4, 13, 22, 28]},\n      {\"term_id\": \"GO:0038024\", \"supporting_discovery_ids\": [1, 31]},\n      {\"term_id\": \"GO:0008289\", \"supporting_discovery_ids\": [6, 16]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [1, 15, 16, 31]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"GO:0162582\", \"supporting_discovery_ids\": [4, 5, 13, 19, 22, 28]},\n      {\"term_id\": \"R-HSA-382551\", \"supporting_discovery_ids\": [1, 3, 6, 10, 17, 32]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [2, 7, 12, 26]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [13, 19, 27, 29, 30]}\n    ],\n    \"complexes\": [\n      \"STRA6 homodimer\",\n      \"STRA6–calmodulin complex\"\n    ],\n    \"partners\": [\n      \"RBP4\",\n      \"LRAT\",\n      \"CRBP1\",\n      \"CaM\",\n      \"TTR\",\n      \"JAK2\",\n      \"ILK\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}