{"gene":"SYT4","run_date":"2026-06-10T10:51:54","timeline":{"discoveries":[{"year":2010,"finding":"Rat SYT4 inhibits SNARE-catalyzed membrane fusion in both the absence and presence of Ca2+, functioning as a negative/inhibitory isoform. This is due to a conserved Asp-to-Ser substitution in the C2A domain that abolishes Ca2+ binding; reverting this substitution restores Ca2+-stimulated fusion. In contrast, Drosophila SYT4 stimulates SNARE-mediated membrane fusion in response to Ca2+, with its C2B domain sensing Ca2+ and being sufficient to stimulate fusion.","method":"In vitro SNARE-catalyzed membrane fusion assay, C2 domain chimera analysis, point mutagenesis of Ca2+-coordinating residues","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — reconstituted in vitro fusion assay with mutagenesis and chimeric domain analysis in a single rigorous study","pmids":["20688915"],"is_preprint":false},{"year":2013,"finding":"Presynaptic cells deliver Synaptotagmin 4 (Syt4) to the postsynaptic cell via anterograde exosome release, thereby enabling Ca2+-dependent retrograde signaling at the Drosophila NMJ. Thus, the presynaptic cell supplies an essential component of postsynaptic retrograde signaling machinery.","method":"Exosome isolation and characterization, live imaging, genetic loss-of-function at Drosophila NMJ","journal":"Neuron","confidence":"High","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal methods (exosome fractionation, imaging, genetics) in a single focused study with clear mechanistic model","pmids":["23522040"],"is_preprint":false},{"year":2016,"finding":"The postsynaptic t-SNARE Syntaxin 4 (Syx4) controls membrane levels of Syt4 and the transsynaptic adhesion protein Neuroligin 1 (Nlg1) at Drosophila NMJs, regulating retrograde signaling, synaptic bouton number, and activity-dependent plasticity. Genetic interaction experiments placed Syx4, Syt4, and Nlg1 in overlapping and parallel pathways controlling synaptic growth.","method":"pHluorin-tagged Syt4 trafficking screen, genetic epistasis (double mutant analysis), live imaging of Syt4 membrane levels at NMJ","journal":"eLife","confidence":"High","confidence_rationale":"Tier 2 / Moderate — reciprocal genetic epistasis and live fluorescence imaging with multiple loss-of-function alleles in a single focused study","pmids":["27223326"],"is_preprint":false},{"year":2017,"finding":"JNK phosphorylates Syt4 at serine 135, which destabilizes the Syt4–Kif1A interaction and shifts DCV (dense core vesicle) trafficking from microtubule-dependent long-range transport to actin-based capture at presynaptic boutons. Neuronal activity increases DCV capture via this JNK-S135 phosphorylation mechanism.","method":"Phospho-site mutagenesis (S135A/S135E), co-immunoprecipitation (Syt4-Kif1A interaction), live imaging of DCV trafficking in hippocampal neurons, activity-dependent capture assays","journal":"Cell reports","confidence":"High","confidence_rationale":"Tier 1–2 / Moderate — phospho-site mutagenesis combined with Co-IP and live imaging, multiple orthogonal methods in a single study","pmids":["29166604"],"is_preprint":false},{"year":2011,"finding":"Syt4 and Syt7 are expressed in dopaminergic neuron dendrites and their knockdown severely reduces somatodendritic (STD) dopamine release, whereas terminal release requires Syt1. Ca2+ influx through N- and P/Q-type voltage-gated channels (not intracellular Ca2+ stores) is required to trigger STD DA release through Syt4/Syt7.","method":"siRNA knockdown of Syt isoforms in cultured dopaminergic neurons, amperometric/electrochemical measurement of dopamine release, pharmacological block of Ca2+ channel subtypes","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — clean KD with defined functional readout and pharmacological dissection, single lab","pmids":["21576241"],"is_preprint":false},{"year":2018,"finding":"Syt4 is a non-Ca2+-binding paralog of Syt7 that localizes on insulin granules in pancreatic β cells. Syt4 levels increase ~8-fold during β cell maturation and inversely correlate with the number of readily releasable vesicles. Syt4 ablation increases basal insulin secretion and impairs GSIS; precocious expression represses basal secretion but impairs islet morphogenesis. Myt transcription factors repress Syt4 transcription. Human SYT4 similarly regulates GSIS in EndoC-βH1 cells.","method":"Syt4 knockout and transgenic overexpression in mice, immunolocalization on insulin granules, capacitance/patch-clamp measurement of readily releasable pool, siRNA knockdown in human β cell line, ChIP for Myt binding","journal":"Developmental cell","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (KO, overexpression, human cell line KD, granule localization, vesicle pool measurements) replicated across species","pmids":["29656931"],"is_preprint":false},{"year":2012,"finding":"Silencing SYT4 in INS1-832/13 β cells reduces glucose-stimulated insulin secretion (GSIS). SYT4 and STX1A protein levels are correspondingly decreased in human type 2 diabetes islets.","method":"siRNA knockdown of SYT4 in INS1-832/13 cells, insulin secretion assay (GSIS), Western blotting of human T2D islets, microarray expression profiling","journal":"Molecular and cellular endocrinology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — clean KD with defined secretory phenotype, corroborated by protein-level analysis in human tissue, single lab","pmids":["22939844"],"is_preprint":false},{"year":2023,"finding":"ARMS (ankyrin repeat-rich membrane spanning protein) directly interacts with Syt4 through its N-terminal ankyrin repeats 1–8; both the C2A and C2B domains of Syt4 are required for binding. Residues E15 and W72 of ARMS are essential for complex formation. ARMS does not interact with Syt1 or Syt3, indicating specificity for Syt4. This interaction was previously shown to negatively regulate BDNF secretion.","method":"Co-immunoprecipitation, GST pull-down, point mutagenesis guided by AlphaFold2 structural predictions, domain deletion mapping","journal":"International journal of molecular sciences","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal pull-down and Co-IP with mutagenesis, single lab, structure prediction used but validated biochemically","pmids":["38069318"],"is_preprint":false},{"year":2024,"finding":"SYT4 promotes vesicle efflux by binding to SNAP25, contributing to exosomal secretion and enzalutamide resistance in prostate cancer cells. BRD4 mediates transcriptional upregulation of SYT4 in enzalutamide-resistant cells.","method":"Co-immunoprecipitation (SYT4-SNAP25 binding), siRNA knockdown of SYT4 combined with enzalutamide treatment, BRD4 inhibitor experiments, antisense oligonucleotide (ASO) targeting SYT4","journal":"Cancer science","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — single Co-IP plus functional KD assay, single lab, mechanism partially characterized","pmids":["38889208"],"is_preprint":false},{"year":2024,"finding":"Syt4 overexpression in the medial prefrontal cortex (mPFC) promotes stress susceptibility (anhedonia), while Syt4 knockdown promotes resilience. The pro-susceptible effects of Syt4 are mediated through reduction in BDNF–TrkB signaling in the mPFC.","method":"Viral overexpression and shRNA knockdown of Syt4 in mPFC, FosTRAP/optogenetics for circuit identification, RNA-seq with WGCNA, sucrose preference and social reward behavioral assays","journal":"Experimental & molecular medicine","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — clean gain- and loss-of-function with defined behavioral readout and BDNF-TrkB pathway measurement, single lab","pmids":["38297157"],"is_preprint":false},{"year":2024,"finding":"SYT4 directly interacts with PSMC6 via its C2B domain (amino acids 288–423), stabilizes PSMC6 protein, and thereby activates Wnt/β-catenin signaling to drive gastric cancer proliferation and suppress apoptosis.","method":"Immunoprecipitation-mass spectrometry (IP-MS), Co-IP, GST pull-down, TOP/FOP luciferase reporter assay for Wnt/β-catenin activity, in vitro and in vivo (xenograft) functional assays, domain deletion mapping","journal":"International journal of biological sciences","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal methods (IP-MS, Co-IP, pull-down, reporter assay, in vivo model) in a single study, single lab","pmids":["41281742"],"is_preprint":false},{"year":2024,"finding":"SYT4 promotes melanogenesis and dendrite extension in melanocytes by interacting with ERK to decrease p-ERK activity, which upregulates CREB. CREB upregulation leads to increased MITF and melanogenic enzymes (TYR, TYRP1, DCT) and TRPM1. SYT4 regulates Ca2+ influx via TRPM1 channels; intracellular Ca2+ activates CAMK4, which phosphorylates CREB to further drive MITF transcription.","method":"SYT4 overexpression in alpaca melanocytes and B16-F10 cells, Western blotting for ERK/p-ERK/CREB/MITF/melanogenic enzymes, intracellular Ca2+ imaging, tyrosinase activity assay","journal":"Cell biochemistry and function","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — overexpression with multiple downstream pathway readouts, single lab, no direct interaction assay for SYT4-ERK","pmids":["31743468"],"is_preprint":false},{"year":2020,"finding":"Glucose excess inhibits activity-dependent CREB phosphorylation and CREB-mediated transcription of SYT4 in hippocampal neurons. This reduces SYT4 protein expression and impairs miniature excitatory postsynaptic current frequency and NMDA receptor-mediated currents.","method":"ChIP for CREB binding at SYT4 promoter, Western blotting, electrophysiology (mEPSC and NMDAR currents) in autaptic hippocampal neurons, diabetic mouse model","journal":"Frontiers in cell and developmental biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — ChIP at SYT4 promoter plus electrophysiological readout, single lab","pmids":["32974347"],"is_preprint":false},{"year":2021,"finding":"Loss of retromer complex leads to accumulation of Syt4 (and other EV cargoes) at Drosophila presynaptic terminals and increased release of Syt4 in extracellular vesicles. Rab11 suppresses Syt4 cargo accumulation in retromer mutants, indicating that EV traffic at synapses reflects a balance between Rab4/Rab11 recycling and retromer-dependent removal from EV precursor compartments.","method":"Drosophila genetics (retromer and rab mutants), immunofluorescence of Syt4 at presynaptic terminals, EV isolation and cargo quantification, genetic epistasis (rab11 suppression of retromer mutant)","journal":"The Journal of cell biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic epistasis with multiple alleles and EV cargo quantification, single lab","pmids":["34019080"],"is_preprint":false},{"year":2022,"finding":"Endocytic machinery (including Nwk/nervous wreck, dynamin/shibire, and AP-2) promotes local maintenance of a release-competent pool of EV cargoes including Syt4 at Drosophila presynaptic terminals. Loss of nwk phenocopies synaptic plasticity defects caused by loss of Syt4, and nwk acts upstream of retromer-dependent removal and retrograde axonal transport of EV cargoes.","method":"Drosophila endocytic mutants, fluorescence imaging of Syt4 cargo levels, genetic epistasis (nwk-syt4 double mutant, nwk-retromer epistasis), synaptic plasticity assays (bouton formation)","journal":"The Journal of cell biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic epistasis with multiple alleles and functional plasticity readout, single lab","pmids":["35320349"],"is_preprint":false},{"year":2024,"finding":"Loss of ESCRT machinery disrupts release of Syt4 in extracellular vesicles from Drosophila motor neurons, but ESCRT depletion does not affect the signaling activities of Syt4. EVs containing Syt4 are phagocytosed by glia and muscles. These data suggest Syt4 may not require EV-mediated intercellular transfer for its signaling function and that synaptic EV release may serve primarily as a proteostatic mechanism for Syt4.","method":"ESCRT component depletion in Drosophila motor neurons, EV cargo quantification, signaling readout assays, phagocytosis imaging","journal":"The Journal of cell biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — clean genetic depletion with multiple functional readouts, single lab; finding is mechanistically informative as a negative (signaling persists without EVs)","pmids":["38842573"],"is_preprint":false},{"year":2024,"finding":"Syt4 is required for axon elongation and spontaneous axonal sprouting after spinal cord injury. Silencing Syt4 in primary cortical neurons inhibits neurite elongation and alters expression of genes in neuronal development signaling pathways. In vivo, Syt4 inhibition in cortical neurons prevents corticospinal tract axonal sprouting and impairs neurological recovery after spinal cord injury.","method":"Loss-of-function genetic screen in cortical neurons, siRNA/shRNA-mediated Syt4 silencing, neurite elongation assay, RNA-seq of gene expression changes, spinal cord injury model with anterograde tracing of corticospinal tract, behavioral recovery assessment","journal":"The Journal of neuroscience","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vitro and in vivo loss-of-function with defined morphological and behavioral readouts, single lab","pmids":["39266302"],"is_preprint":false},{"year":2024,"finding":"In diabetic retinopathy, SYT4 overexpression manipulates Ca2+ influx to induce GLUT1 fusion with the plasma membrane, promotes abnormal GLUT1 expression and excessive glucose uptake, and induces ARPE-19 cell apoptosis. Parkin deficiency inhibits proteasomal degradation of SYT4, causing SYT4 accumulation and enhanced GLUT1 membrane fusion. Myt1 transcription factor dysregulation further activates SYT4-mediated stimulus-secretion coupling. Parkin overexpression or Myt1 overexpression blocked these effects.","method":"SYT4 overexpression/knockdown in ARPE-19 cells, Ca2+ imaging, flow cytometry for GLUT1 membrane localization and apoptosis, Parkin overexpression rescue, Western blotting, streptozotocin mouse model","journal":"World journal of diabetes","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple functional assays with genetic rescue experiments, single lab","pmids":["38766439"],"is_preprint":false},{"year":2020,"finding":"Exocytosis in Type II vestibular hair cells displays a high-order Ca2+ dependence that is independent of Syt4; the Ca2+ dependence and release kinetics of the readily releasable pool (RRP) are not affected by Syt4 knockout. However, Syt4 may play a role in regulating the secondary releasable pool (SRP) in these cells.","method":"Patch-clamp capacitance measurements of exocytosis in control vs. Syt4 knockout mouse Type II vestibular hair cells, analysis of RRP and SRP kinetics","journal":"Physiological reports","confidence":"Medium","confidence_rationale":"Tier 1 / Weak — direct electrophysiological measurement in KO tissue, single lab, limited by single method; RRP finding is a confirmed negative, SRP role is weak positive","pmids":["32691536"],"is_preprint":false},{"year":2011,"finding":"Astrocytes endogenously express Syt4 (not Syt1). Exogenous expression of Syt1 in astrocytes (which express endogenous Syt4) increased the proportion of transient fusion events upon bradykinin stimulation and reduced fusion pore dwell time, indicating that Syt4 and Syt1 have distinct roles in regulating fusion pore dynamics in glial exocytosis.","method":"TIRFM imaging of synapto-pHluorin fusion events in cultured astrocytes, exogenous Syt1 expression, pharmacological stimulation (bradykinin, mechanical)","journal":"The Journal of physiology","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single lab, indirect inference about Syt4 function from Syt1 overexpression in Syt4-expressing astrocytes; Syt4's specific contribution not directly tested","pmids":["21746780"],"is_preprint":false}],"current_model":"SYT4 is a synaptic vesicle/dense core vesicle-associated C2-domain protein that acts primarily as an inhibitory or modulatory Ca2+ sensor: in mammals, its C2A domain carries a Asp-to-Ser substitution that abolishes Ca2+ binding, causing it to inhibit SNARE-mediated membrane fusion rather than stimulate it; it localizes on insulin granules and dense core vesicles (DCVs) to suppress basal secretion and set Ca2+ sensitivity during β-cell maturation; it is captured at presynaptic boutons via JNK-dependent phosphorylation at S135 that uncouples it from the kinesin Kif1A and redirects DCVs from microtubule to actin-based retention; in the postsynaptic compartment it mediates Ca2+-dependent retrograde signaling at Drosophila NMJs, where its membrane levels are controlled by the t-SNARE Syntaxin 4, retromer, and endocytic machinery; it binds ARMS through its C2A/C2B domains and SNAP25 to regulate vesicle efflux; it supports somatodendritic dopamine release together with Syt7; and it influences BDNF–TrkB signaling in the mPFC to regulate stress-induced anhedonia, as well as axon sprouting after spinal cord injury."},"narrative":{"mechanistic_narrative":"SYT4 is a vesicle-associated C2-domain protein that functions as a non-canonical, largely Ca2+-insensitive synaptotagmin paralog modulating regulated secretion and intercellular signaling [PMID:20688915, PMID:29656931]. In mammals its C2A domain carries a conserved Asp-to-Ser substitution that abolishes Ca2+ binding, converting it into an inhibitor of SNARE-catalyzed membrane fusion; reverting this substitution restores Ca2+-stimulated fusion, whereas the Drosophila ortholog instead stimulates fusion through a Ca2+-sensing C2B domain [PMID:20688915]. In pancreatic β cells SYT4 localizes to insulin granules, rises ~8-fold during β-cell maturation under repression by Myt transcription factors, and inversely scales with the readily releasable pool to suppress basal secretion and tune glucose-stimulated insulin secretion [PMID:29656931, PMID:22939844]. SYT4 trafficking is actively regulated: JNK phosphorylation at serine 135 uncouples it from the kinesin Kif1A, shifting dense-core-vesicle transport from microtubule-based long-range delivery to activity-dependent actin-based capture at presynaptic boutons [PMID:29166604]. At the Drosophila NMJ SYT4 mediates Ca2+-dependent retrograde signaling, with its synaptic membrane levels set by the t-SNARE Syntaxin 4 alongside Neuroligin 1, by retromer- and Rab11-balanced sorting, and by endocytic machinery (Nwk, dynamin, AP-2) that maintains a release-competent pool [PMID:27223326, PMID:34019080, PMID:35320349]. SYT4 binds partners including ARMS through its C2A/C2B domains and SNAP25, links that connect it to control of BDNF secretion and vesicle efflux [PMID:38069318, PMID:38889208]. Functionally it supports somatodendritic dopamine release together with Syt7 [PMID:21576241] and is required for axonal sprouting and recovery after spinal cord injury [PMID:39266302]. Across non-neuronal contexts SYT4 has been linked to BDNF–TrkB-dependent stress susceptibility in the mPFC, Wnt/β-catenin signaling in gastric cancer, melanogenesis, and GLUT1-mediated glucose uptake in diabetic retinopathy [PMID:38297157, PMID:41281742, PMID:31743468, PMID:38766439].","teleology":[{"year":2010,"claim":"Established the core biochemical identity of mammalian SYT4 as an inhibitory fusion clamp rather than a stimulatory Ca2+ sensor, resolving why this isoform diverges functionally from canonical synaptotagmins.","evidence":"Reconstituted in vitro SNARE-catalyzed fusion assay with C2-domain chimeras and point mutagenesis of Ca2+-coordinating residues","pmids":["20688915"],"confidence":"High","gaps":["Does not establish the in vivo fusion target or vesicle context in mammalian cells","Divergent stimulatory behavior of Drosophila SYT4 leaves the conserved physiological role ambiguous"]},{"year":2011,"claim":"Showed SYT4 has compartment-specific secretory roles, supporting somatodendritic dopamine release in concert with Syt7 distinct from terminal release driven by Syt1.","evidence":"siRNA knockdown of Syt isoforms in cultured dopaminergic neurons with amperometric dopamine measurement and Ca2+-channel pharmacology","pmids":["21576241"],"confidence":"Medium","gaps":["Does not define whether SYT4 acts as a direct Ca2+ sensor or modulator in this context","Knockdown specificity and compensation by other Syt isoforms not fully excluded"]},{"year":2012,"claim":"Linked SYT4 to insulin secretion and human disease relevance by showing knockdown reduces glucose-stimulated insulin secretion and that SYT4 protein is decreased in type 2 diabetes islets.","evidence":"siRNA knockdown in INS1-832/13 β cells with GSIS assay and Western blotting of human T2D islets","pmids":["22939844"],"confidence":"Medium","gaps":["Correlative protein-level association in human tissue does not establish causality","Mechanism of how SYT4 sets secretory competence not resolved here"]},{"year":2013,"claim":"Revealed that SYT4 is transferred between cells via anterograde exosome release, redefining how a postsynaptic retrograde signaling component is supplied by the presynaptic neuron.","evidence":"Exosome isolation, live imaging, and genetic loss-of-function at the Drosophila NMJ","pmids":["23522040"],"confidence":"High","gaps":["Whether mammalian SYT4 uses analogous intercellular transfer not addressed","Molecular machinery sorting SYT4 into exosomes not defined here"]},{"year":2016,"claim":"Identified the t-SNARE Syntaxin 4 as a controller of synaptic SYT4 membrane levels, placing SYT4, Syx4, and Neuroligin 1 in convergent pathways governing retrograde signaling and synaptic growth.","evidence":"pHluorin-tagged Syt4 trafficking screen and genetic epistasis with live NMJ imaging in Drosophila","pmids":["27223326"],"confidence":"High","gaps":["Direct physical interaction between Syx4 and Syt4 not demonstrated","Conservation in mammalian synapses untested"]},{"year":2017,"claim":"Defined an activity-dependent switch governing SYT4 vesicle trafficking, where JNK phosphorylation at S135 uncouples SYT4 from Kif1A and redirects dense-core vesicles to actin-based boutonic capture.","evidence":"S135A/S135E phospho-site mutagenesis, Syt4-Kif1A co-immunoprecipitation, and live DCV imaging in hippocampal neurons","pmids":["29166604"],"confidence":"High","gaps":["Identity of the actin-based capture machinery not defined","Whether captured DCVs preferentially undergo fusion or proteostatic turnover unresolved"]},{"year":2018,"claim":"Established SYT4 as a developmentally regulated brake on basal secretion, with Myt-repressed expression rising during β-cell maturation to suppress basal insulin release and set Ca2+ sensitivity, conserved in human cells.","evidence":"Syt4 knockout/overexpression mice, insulin-granule immunolocalization, RRP capacitance measurements, human β-cell line knockdown, and Myt ChIP","pmids":["29656931"],"confidence":"High","gaps":["Mechanism by which non-Ca2+-binding SYT4 sets vesicle pool size not fully resolved","Relationship between maturation role and acute secretory clamp unclear"]},{"year":2020,"claim":"Connected SYT4 expression to activity- and metabolism-dependent transcription, showing CREB drives SYT4 transcription and that glucose excess suppresses this to impair synaptic transmission.","evidence":"ChIP for CREB at the SYT4 promoter with electrophysiology (mEPSC, NMDAR currents) in autaptic hippocampal neurons and a diabetic mouse model","pmids":["32974347"],"confidence":"Medium","gaps":["Whether reduced SYT4 is causal for the electrophysiological deficits not isolated","Direct postsynaptic vs presynaptic locus of SYT4 action unresolved"]},{"year":2020,"claim":"Tested the necessity of SYT4 for fast exocytosis in vestibular hair cells, establishing it is dispensable for readily releasable pool kinetics but may shape the secondary releasable pool.","evidence":"Patch-clamp capacitance measurements in control vs Syt4-knockout mouse Type II vestibular hair cells","pmids":["32691536"],"confidence":"Medium","gaps":["SRP role is a weak positive requiring confirmation","Limited to a single electrophysiological method"]},{"year":2022,"claim":"Resolved the trafficking logic that maintains synaptic SYT4, showing endocytic machinery (Nwk, dynamin, AP-2) sustains a release-competent cargo pool upstream of retromer-dependent removal.","evidence":"Drosophila endocytic mutants with Syt4 cargo imaging, nwk-syt4 and nwk-retromer epistasis, and synaptic plasticity assays","pmids":["35320349"],"confidence":"Medium","gaps":["Direct cargo-recognition step for SYT4 endocytosis not identified","Mammalian relevance untested"]},{"year":2021,"claim":"Defined retromer and Rab11 as opposing regulators of SYT4 sorting into extracellular vesicles, framing synaptic EV traffic as a recycling-versus-removal balance.","evidence":"Drosophila retromer and rab mutants with Syt4 immunofluorescence, EV cargo quantification, and rab11 suppression epistasis","pmids":["34019080"],"confidence":"Medium","gaps":["Whether EV-released SYT4 has signaling significance left open","Direct binding of SYT4 to sorting machinery not shown"]},{"year":2023,"claim":"Identified ARMS as a specific direct partner of SYT4 engaged through both C2A and C2B domains, providing a molecular link to BDNF secretion control.","evidence":"Co-IP, GST pull-down, AlphaFold2-guided point mutagenesis, and domain-deletion mapping","pmids":["38069318"],"confidence":"Medium","gaps":["Functional consequence of the SYT4-ARMS interaction for fusion not directly measured","Single-lab biochemistry without cellular validation of the mapped residues"]},{"year":2024,"claim":"Dissociated SYT4's signaling activity from its EV-mediated transfer, showing ESCRT-dependent EV release is dispensable for SYT4 signaling and may instead serve proteostasis.","evidence":"ESCRT depletion in Drosophila motor neurons with EV cargo quantification, signaling readouts, and phagocytosis imaging","pmids":["38842573"],"confidence":"Medium","gaps":["Reconciliation with earlier exosome-transfer model not fully resolved","Proteostatic interpretation inferred rather than directly demonstrated"]},{"year":2024,"claim":"Extended SYT4 function into disease and developmental contexts, implicating it in mPFC BDNF-TrkB-dependent stress susceptibility, gastric cancer Wnt/β-catenin signaling, prostate cancer vesicle efflux, melanogenesis, GLUT1-driven glucose uptake in retinopathy, and axonal sprouting after spinal cord injury.","evidence":"Gain/loss-of-function in mPFC with behavioral assays; Co-IP/IP-MS, GST pull-down and reporter assays for PSMC6 and SNAP25; overexpression with pathway readouts in melanocytes and ARPE-19 cells; loss-of-function with SCI tracing and behavior","pmids":["38297157","41281742","38889208","31743468","38766439","39266302"],"confidence":"Medium","gaps":["Many contexts rely on single-lab overexpression or knockdown without reconstitution","Whether these effects share a common SYT4 fusion/trafficking mechanism is unclear","Direct SYT4-ERK and SYT4-GLUT1 interactions not established"]},{"year":null,"claim":"It remains unresolved how the conserved non-Ca2+-binding biochemistry of mammalian SYT4 maps onto its diverse cellular roles, and whether a single fusion-clamp mechanism unifies its actions across neurons, β cells, and cancer.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No structural model of SYT4 engaged with its native SNARE complex","No unifying account linking inhibitory fusion activity to its disease and developmental phenotypes","Mammalian counterpart of the Drosophila EV/retrograde-signaling program untested"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[0,5]},{"term_id":"GO:0140313","term_label":"molecular sequestering activity","supporting_discovery_ids":[0]}],"localization":[{"term_id":"GO:0031410","term_label":"cytoplasmic vesicle","supporting_discovery_ids":[3,5]},{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[2,17]},{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[2]}],"pathway":[{"term_id":"R-HSA-9609507","term_label":"Protein localization","supporting_discovery_ids":[3,13,14]},{"term_id":"R-HSA-5653656","term_label":"Vesicle-mediated transport","supporting_discovery_ids":[1,13,15]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[9,10,11]},{"term_id":"R-HSA-112316","term_label":"Neuronal System","supporting_discovery_ids":[2,4,16]}],"complexes":[],"partners":["KIF1A","SNAP25","STX4","NLG1","ARMS/KIDINS220","PSMC6","SYT7"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q9H2B2","full_name":"Synaptotagmin-4","aliases":["Synaptotagmin IV","SytIV"],"length_aa":425,"mass_kda":48.0,"function":"Synaptotagmin family member which does not bind Ca(2+) (By similarity) (PubMed:23999003). Involved in neuronal dense core vesicles (DCVs) mobility through its interaction with KIF1A. Upon increased neuronal activity, phosphorylation by MAPK8/JNK1 destabilizes the interaction with KIF1A and captures DCVs to synapses (By similarity). Plays a role in dendrite formation by melanocytes (PubMed:23999003)","subcellular_location":"Cytoplasmic vesicle, secretory vesicle, neuronal dense core vesicle membrane","url":"https://www.uniprot.org/uniprotkb/Q9H2B2/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/SYT4","classification":"Not Classified","n_dependent_lines":0,"n_total_lines":1208,"dependency_fraction":0.0},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/SYT4","total_profiled":1310},"omim":[{"mim_id":"608741","title":"SYNAPTOTAGMIN 11; SYT11","url":"https://www.omim.org/entry/608741"},{"mim_id":"600103","title":"SYNAPTOTAGMIN 4; SYT4","url":"https://www.omim.org/entry/600103"},{"mim_id":"185605","title":"SYNAPTOTAGMIN 1; SYT1","url":"https://www.omim.org/entry/185605"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Vesicles","reliability":"Approved"},{"location":"Plasma membrane","reliability":"Approved"}],"tissue_specificity":"Tissue enriched","tissue_distribution":"Detected in many","driving_tissues":[{"tissue":"brain","ntpm":140.6}],"url":"https://www.proteinatlas.org/search/SYT4"},"hgnc":{"alias_symbol":["KIAA1342","HsT1192"],"prev_symbol":[]},"alphafold":{"accession":"Q9H2B2","domains":[{"cath_id":"2.60.40.150","chopping":"154-278","consensus_level":"high","plddt":91.3503,"start":154,"end":278},{"cath_id":"2.60.40.150","chopping":"288-421","consensus_level":"high","plddt":89.1304,"start":288,"end":421}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9H2B2","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9H2B2-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9H2B2-F1-predicted_aligned_error_v6.png","plddt_mean":72.94},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=SYT4","jax_strain_url":"https://www.jax.org/strain/search?query=SYT4"},"sequence":{"accession":"Q9H2B2","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9H2B2.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9H2B2/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9H2B2"}},"corpus_meta":[{"pmid":"23522040","id":"PMC_23522040","title":"Regulation of postsynaptic retrograde signaling by presynaptic exosome release.","date":"2013","source":"Neuron","url":"https://pubmed.ncbi.nlm.nih.gov/23522040","citation_count":205,"is_preprint":false},{"pmid":"32982666","id":"PMC_32982666","title":"Environment and Gene Association With Obesity and Their Impact on Neurodegenerative and Neurodevelopmental Diseases.","date":"2020","source":"Frontiers in neuroscience","url":"https://pubmed.ncbi.nlm.nih.gov/32982666","citation_count":112,"is_preprint":false},{"pmid":"22939844","id":"PMC_22939844","title":"Reduced insulin secretion correlates with decreased expression of exocytotic genes in pancreatic islets from patients with type 2 diabetes.","date":"2012","source":"Molecular and cellular endocrinology","url":"https://pubmed.ncbi.nlm.nih.gov/22939844","citation_count":107,"is_preprint":false},{"pmid":"29656931","id":"PMC_29656931","title":"Synaptotagmin 4 Regulates Pancreatic β Cell Maturation by Modulating the Ca2+ Sensitivity of Insulin Secretion Vesicles.","date":"2018","source":"Developmental cell","url":"https://pubmed.ncbi.nlm.nih.gov/29656931","citation_count":75,"is_preprint":false},{"pmid":"17222959","id":"PMC_17222959","title":"Brain regions and genes affecting postural control.","date":"2007","source":"Progress in neurobiology","url":"https://pubmed.ncbi.nlm.nih.gov/17222959","citation_count":63,"is_preprint":false},{"pmid":"21746780","id":"PMC_21746780","title":"Temporal characteristics of vesicular fusion in astrocytes: examination of synaptobrevin 2-laden vesicles at single vesicle resolution.","date":"2011","source":"The Journal of physiology","url":"https://pubmed.ncbi.nlm.nih.gov/21746780","citation_count":60,"is_preprint":false},{"pmid":"21576241","id":"PMC_21576241","title":"Somatodendritic dopamine release requires synaptotagmin 4 and 7 and the participation of voltage-gated calcium channels.","date":"2011","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/21576241","citation_count":58,"is_preprint":false},{"pmid":"8872307","id":"PMC_8872307","title":"Two synaptotagmin genes, Syt1 and Syt4, are differentially regulated in adult brain and during postnatal development following kainic acid-induced seizures.","date":"1996","source":"Brain research. 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This is due to a conserved Asp-to-Ser substitution in the C2A domain that abolishes Ca2+ binding; reverting this substitution restores Ca2+-stimulated fusion. In contrast, Drosophila SYT4 stimulates SNARE-mediated membrane fusion in response to Ca2+, with its C2B domain sensing Ca2+ and being sufficient to stimulate fusion.\",\n      \"method\": \"In vitro SNARE-catalyzed membrane fusion assay, C2 domain chimera analysis, point mutagenesis of Ca2+-coordinating residues\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — reconstituted in vitro fusion assay with mutagenesis and chimeric domain analysis in a single rigorous study\",\n      \"pmids\": [\"20688915\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Presynaptic cells deliver Synaptotagmin 4 (Syt4) to the postsynaptic cell via anterograde exosome release, thereby enabling Ca2+-dependent retrograde signaling at the Drosophila NMJ. Thus, the presynaptic cell supplies an essential component of postsynaptic retrograde signaling machinery.\",\n      \"method\": \"Exosome isolation and characterization, live imaging, genetic loss-of-function at Drosophila NMJ\",\n      \"journal\": \"Neuron\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal methods (exosome fractionation, imaging, genetics) in a single focused study with clear mechanistic model\",\n      \"pmids\": [\"23522040\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"The postsynaptic t-SNARE Syntaxin 4 (Syx4) controls membrane levels of Syt4 and the transsynaptic adhesion protein Neuroligin 1 (Nlg1) at Drosophila NMJs, regulating retrograde signaling, synaptic bouton number, and activity-dependent plasticity. Genetic interaction experiments placed Syx4, Syt4, and Nlg1 in overlapping and parallel pathways controlling synaptic growth.\",\n      \"method\": \"pHluorin-tagged Syt4 trafficking screen, genetic epistasis (double mutant analysis), live imaging of Syt4 membrane levels at NMJ\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal genetic epistasis and live fluorescence imaging with multiple loss-of-function alleles in a single focused study\",\n      \"pmids\": [\"27223326\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"JNK phosphorylates Syt4 at serine 135, which destabilizes the Syt4–Kif1A interaction and shifts DCV (dense core vesicle) trafficking from microtubule-dependent long-range transport to actin-based capture at presynaptic boutons. Neuronal activity increases DCV capture via this JNK-S135 phosphorylation mechanism.\",\n      \"method\": \"Phospho-site mutagenesis (S135A/S135E), co-immunoprecipitation (Syt4-Kif1A interaction), live imaging of DCV trafficking in hippocampal neurons, activity-dependent capture assays\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Moderate — phospho-site mutagenesis combined with Co-IP and live imaging, multiple orthogonal methods in a single study\",\n      \"pmids\": [\"29166604\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Syt4 and Syt7 are expressed in dopaminergic neuron dendrites and their knockdown severely reduces somatodendritic (STD) dopamine release, whereas terminal release requires Syt1. Ca2+ influx through N- and P/Q-type voltage-gated channels (not intracellular Ca2+ stores) is required to trigger STD DA release through Syt4/Syt7.\",\n      \"method\": \"siRNA knockdown of Syt isoforms in cultured dopaminergic neurons, amperometric/electrochemical measurement of dopamine release, pharmacological block of Ca2+ channel subtypes\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — clean KD with defined functional readout and pharmacological dissection, single lab\",\n      \"pmids\": [\"21576241\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Syt4 is a non-Ca2+-binding paralog of Syt7 that localizes on insulin granules in pancreatic β cells. Syt4 levels increase ~8-fold during β cell maturation and inversely correlate with the number of readily releasable vesicles. Syt4 ablation increases basal insulin secretion and impairs GSIS; precocious expression represses basal secretion but impairs islet morphogenesis. Myt transcription factors repress Syt4 transcription. Human SYT4 similarly regulates GSIS in EndoC-βH1 cells.\",\n      \"method\": \"Syt4 knockout and transgenic overexpression in mice, immunolocalization on insulin granules, capacitance/patch-clamp measurement of readily releasable pool, siRNA knockdown in human β cell line, ChIP for Myt binding\",\n      \"journal\": \"Developmental cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (KO, overexpression, human cell line KD, granule localization, vesicle pool measurements) replicated across species\",\n      \"pmids\": [\"29656931\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Silencing SYT4 in INS1-832/13 β cells reduces glucose-stimulated insulin secretion (GSIS). SYT4 and STX1A protein levels are correspondingly decreased in human type 2 diabetes islets.\",\n      \"method\": \"siRNA knockdown of SYT4 in INS1-832/13 cells, insulin secretion assay (GSIS), Western blotting of human T2D islets, microarray expression profiling\",\n      \"journal\": \"Molecular and cellular endocrinology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — clean KD with defined secretory phenotype, corroborated by protein-level analysis in human tissue, single lab\",\n      \"pmids\": [\"22939844\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"ARMS (ankyrin repeat-rich membrane spanning protein) directly interacts with Syt4 through its N-terminal ankyrin repeats 1–8; both the C2A and C2B domains of Syt4 are required for binding. Residues E15 and W72 of ARMS are essential for complex formation. ARMS does not interact with Syt1 or Syt3, indicating specificity for Syt4. This interaction was previously shown to negatively regulate BDNF secretion.\",\n      \"method\": \"Co-immunoprecipitation, GST pull-down, point mutagenesis guided by AlphaFold2 structural predictions, domain deletion mapping\",\n      \"journal\": \"International journal of molecular sciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal pull-down and Co-IP with mutagenesis, single lab, structure prediction used but validated biochemically\",\n      \"pmids\": [\"38069318\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"SYT4 promotes vesicle efflux by binding to SNAP25, contributing to exosomal secretion and enzalutamide resistance in prostate cancer cells. BRD4 mediates transcriptional upregulation of SYT4 in enzalutamide-resistant cells.\",\n      \"method\": \"Co-immunoprecipitation (SYT4-SNAP25 binding), siRNA knockdown of SYT4 combined with enzalutamide treatment, BRD4 inhibitor experiments, antisense oligonucleotide (ASO) targeting SYT4\",\n      \"journal\": \"Cancer science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — single Co-IP plus functional KD assay, single lab, mechanism partially characterized\",\n      \"pmids\": [\"38889208\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Syt4 overexpression in the medial prefrontal cortex (mPFC) promotes stress susceptibility (anhedonia), while Syt4 knockdown promotes resilience. The pro-susceptible effects of Syt4 are mediated through reduction in BDNF–TrkB signaling in the mPFC.\",\n      \"method\": \"Viral overexpression and shRNA knockdown of Syt4 in mPFC, FosTRAP/optogenetics for circuit identification, RNA-seq with WGCNA, sucrose preference and social reward behavioral assays\",\n      \"journal\": \"Experimental & molecular medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — clean gain- and loss-of-function with defined behavioral readout and BDNF-TrkB pathway measurement, single lab\",\n      \"pmids\": [\"38297157\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"SYT4 directly interacts with PSMC6 via its C2B domain (amino acids 288–423), stabilizes PSMC6 protein, and thereby activates Wnt/β-catenin signaling to drive gastric cancer proliferation and suppress apoptosis.\",\n      \"method\": \"Immunoprecipitation-mass spectrometry (IP-MS), Co-IP, GST pull-down, TOP/FOP luciferase reporter assay for Wnt/β-catenin activity, in vitro and in vivo (xenograft) functional assays, domain deletion mapping\",\n      \"journal\": \"International journal of biological sciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal methods (IP-MS, Co-IP, pull-down, reporter assay, in vivo model) in a single study, single lab\",\n      \"pmids\": [\"41281742\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"SYT4 promotes melanogenesis and dendrite extension in melanocytes by interacting with ERK to decrease p-ERK activity, which upregulates CREB. CREB upregulation leads to increased MITF and melanogenic enzymes (TYR, TYRP1, DCT) and TRPM1. SYT4 regulates Ca2+ influx via TRPM1 channels; intracellular Ca2+ activates CAMK4, which phosphorylates CREB to further drive MITF transcription.\",\n      \"method\": \"SYT4 overexpression in alpaca melanocytes and B16-F10 cells, Western blotting for ERK/p-ERK/CREB/MITF/melanogenic enzymes, intracellular Ca2+ imaging, tyrosinase activity assay\",\n      \"journal\": \"Cell biochemistry and function\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — overexpression with multiple downstream pathway readouts, single lab, no direct interaction assay for SYT4-ERK\",\n      \"pmids\": [\"31743468\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Glucose excess inhibits activity-dependent CREB phosphorylation and CREB-mediated transcription of SYT4 in hippocampal neurons. This reduces SYT4 protein expression and impairs miniature excitatory postsynaptic current frequency and NMDA receptor-mediated currents.\",\n      \"method\": \"ChIP for CREB binding at SYT4 promoter, Western blotting, electrophysiology (mEPSC and NMDAR currents) in autaptic hippocampal neurons, diabetic mouse model\",\n      \"journal\": \"Frontiers in cell and developmental biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — ChIP at SYT4 promoter plus electrophysiological readout, single lab\",\n      \"pmids\": [\"32974347\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Loss of retromer complex leads to accumulation of Syt4 (and other EV cargoes) at Drosophila presynaptic terminals and increased release of Syt4 in extracellular vesicles. Rab11 suppresses Syt4 cargo accumulation in retromer mutants, indicating that EV traffic at synapses reflects a balance between Rab4/Rab11 recycling and retromer-dependent removal from EV precursor compartments.\",\n      \"method\": \"Drosophila genetics (retromer and rab mutants), immunofluorescence of Syt4 at presynaptic terminals, EV isolation and cargo quantification, genetic epistasis (rab11 suppression of retromer mutant)\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic epistasis with multiple alleles and EV cargo quantification, single lab\",\n      \"pmids\": [\"34019080\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Endocytic machinery (including Nwk/nervous wreck, dynamin/shibire, and AP-2) promotes local maintenance of a release-competent pool of EV cargoes including Syt4 at Drosophila presynaptic terminals. Loss of nwk phenocopies synaptic plasticity defects caused by loss of Syt4, and nwk acts upstream of retromer-dependent removal and retrograde axonal transport of EV cargoes.\",\n      \"method\": \"Drosophila endocytic mutants, fluorescence imaging of Syt4 cargo levels, genetic epistasis (nwk-syt4 double mutant, nwk-retromer epistasis), synaptic plasticity assays (bouton formation)\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic epistasis with multiple alleles and functional plasticity readout, single lab\",\n      \"pmids\": [\"35320349\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Loss of ESCRT machinery disrupts release of Syt4 in extracellular vesicles from Drosophila motor neurons, but ESCRT depletion does not affect the signaling activities of Syt4. EVs containing Syt4 are phagocytosed by glia and muscles. These data suggest Syt4 may not require EV-mediated intercellular transfer for its signaling function and that synaptic EV release may serve primarily as a proteostatic mechanism for Syt4.\",\n      \"method\": \"ESCRT component depletion in Drosophila motor neurons, EV cargo quantification, signaling readout assays, phagocytosis imaging\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — clean genetic depletion with multiple functional readouts, single lab; finding is mechanistically informative as a negative (signaling persists without EVs)\",\n      \"pmids\": [\"38842573\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Syt4 is required for axon elongation and spontaneous axonal sprouting after spinal cord injury. Silencing Syt4 in primary cortical neurons inhibits neurite elongation and alters expression of genes in neuronal development signaling pathways. In vivo, Syt4 inhibition in cortical neurons prevents corticospinal tract axonal sprouting and impairs neurological recovery after spinal cord injury.\",\n      \"method\": \"Loss-of-function genetic screen in cortical neurons, siRNA/shRNA-mediated Syt4 silencing, neurite elongation assay, RNA-seq of gene expression changes, spinal cord injury model with anterograde tracing of corticospinal tract, behavioral recovery assessment\",\n      \"journal\": \"The Journal of neuroscience\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vitro and in vivo loss-of-function with defined morphological and behavioral readouts, single lab\",\n      \"pmids\": [\"39266302\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"In diabetic retinopathy, SYT4 overexpression manipulates Ca2+ influx to induce GLUT1 fusion with the plasma membrane, promotes abnormal GLUT1 expression and excessive glucose uptake, and induces ARPE-19 cell apoptosis. Parkin deficiency inhibits proteasomal degradation of SYT4, causing SYT4 accumulation and enhanced GLUT1 membrane fusion. Myt1 transcription factor dysregulation further activates SYT4-mediated stimulus-secretion coupling. Parkin overexpression or Myt1 overexpression blocked these effects.\",\n      \"method\": \"SYT4 overexpression/knockdown in ARPE-19 cells, Ca2+ imaging, flow cytometry for GLUT1 membrane localization and apoptosis, Parkin overexpression rescue, Western blotting, streptozotocin mouse model\",\n      \"journal\": \"World journal of diabetes\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple functional assays with genetic rescue experiments, single lab\",\n      \"pmids\": [\"38766439\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Exocytosis in Type II vestibular hair cells displays a high-order Ca2+ dependence that is independent of Syt4; the Ca2+ dependence and release kinetics of the readily releasable pool (RRP) are not affected by Syt4 knockout. However, Syt4 may play a role in regulating the secondary releasable pool (SRP) in these cells.\",\n      \"method\": \"Patch-clamp capacitance measurements of exocytosis in control vs. Syt4 knockout mouse Type II vestibular hair cells, analysis of RRP and SRP kinetics\",\n      \"journal\": \"Physiological reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Weak — direct electrophysiological measurement in KO tissue, single lab, limited by single method; RRP finding is a confirmed negative, SRP role is weak positive\",\n      \"pmids\": [\"32691536\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Astrocytes endogenously express Syt4 (not Syt1). Exogenous expression of Syt1 in astrocytes (which express endogenous Syt4) increased the proportion of transient fusion events upon bradykinin stimulation and reduced fusion pore dwell time, indicating that Syt4 and Syt1 have distinct roles in regulating fusion pore dynamics in glial exocytosis.\",\n      \"method\": \"TIRFM imaging of synapto-pHluorin fusion events in cultured astrocytes, exogenous Syt1 expression, pharmacological stimulation (bradykinin, mechanical)\",\n      \"journal\": \"The Journal of physiology\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single lab, indirect inference about Syt4 function from Syt1 overexpression in Syt4-expressing astrocytes; Syt4's specific contribution not directly tested\",\n      \"pmids\": [\"21746780\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"SYT4 is a synaptic vesicle/dense core vesicle-associated C2-domain protein that acts primarily as an inhibitory or modulatory Ca2+ sensor: in mammals, its C2A domain carries a Asp-to-Ser substitution that abolishes Ca2+ binding, causing it to inhibit SNARE-mediated membrane fusion rather than stimulate it; it localizes on insulin granules and dense core vesicles (DCVs) to suppress basal secretion and set Ca2+ sensitivity during β-cell maturation; it is captured at presynaptic boutons via JNK-dependent phosphorylation at S135 that uncouples it from the kinesin Kif1A and redirects DCVs from microtubule to actin-based retention; in the postsynaptic compartment it mediates Ca2+-dependent retrograde signaling at Drosophila NMJs, where its membrane levels are controlled by the t-SNARE Syntaxin 4, retromer, and endocytic machinery; it binds ARMS through its C2A/C2B domains and SNAP25 to regulate vesicle efflux; it supports somatodendritic dopamine release together with Syt7; and it influences BDNF–TrkB signaling in the mPFC to regulate stress-induced anhedonia, as well as axon sprouting after spinal cord injury.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"SYT4 is a vesicle-associated C2-domain protein that functions as a non-canonical, largely Ca2+-insensitive synaptotagmin paralog modulating regulated secretion and intercellular signaling [#0, #5]. In mammals its C2A domain carries a conserved Asp-to-Ser substitution that abolishes Ca2+ binding, converting it into an inhibitor of SNARE-catalyzed membrane fusion; reverting this substitution restores Ca2+-stimulated fusion, whereas the Drosophila ortholog instead stimulates fusion through a Ca2+-sensing C2B domain [#0]. In pancreatic \\u03b2 cells SYT4 localizes to insulin granules, rises ~8-fold during \\u03b2-cell maturation under repression by Myt transcription factors, and inversely scales with the readily releasable pool to suppress basal secretion and tune glucose-stimulated insulin secretion [#5, #6]. SYT4 trafficking is actively regulated: JNK phosphorylation at serine 135 uncouples it from the kinesin Kif1A, shifting dense-core-vesicle transport from microtubule-based long-range delivery to activity-dependent actin-based capture at presynaptic boutons [#3]. At the Drosophila NMJ SYT4 mediates Ca2+-dependent retrograde signaling, with its synaptic membrane levels set by the t-SNARE Syntaxin 4 alongside Neuroligin 1, by retromer- and Rab11-balanced sorting, and by endocytic machinery (Nwk, dynamin, AP-2) that maintains a release-competent pool [#2, #13, #14]. SYT4 binds partners including ARMS through its C2A/C2B domains and SNAP25, links that connect it to control of BDNF secretion and vesicle efflux [#7, #8]. Functionally it supports somatodendritic dopamine release together with Syt7 [#4] and is required for axonal sprouting and recovery after spinal cord injury [#16]. Across non-neuronal contexts SYT4 has been linked to BDNF\\u2013TrkB-dependent stress susceptibility in the mPFC, Wnt/\\u03b2-catenin signaling in gastric cancer, melanogenesis, and GLUT1-mediated glucose uptake in diabetic retinopathy [#9, #10, #11, #17].\",\n  \"teleology\": [\n    {\n      \"year\": 2010,\n      \"claim\": \"Established the core biochemical identity of mammalian SYT4 as an inhibitory fusion clamp rather than a stimulatory Ca2+ sensor, resolving why this isoform diverges functionally from canonical synaptotagmins.\",\n      \"evidence\": \"Reconstituted in vitro SNARE-catalyzed fusion assay with C2-domain chimeras and point mutagenesis of Ca2+-coordinating residues\",\n      \"pmids\": [\"20688915\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Does not establish the in vivo fusion target or vesicle context in mammalian cells\", \"Divergent stimulatory behavior of Drosophila SYT4 leaves the conserved physiological role ambiguous\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Showed SYT4 has compartment-specific secretory roles, supporting somatodendritic dopamine release in concert with Syt7 distinct from terminal release driven by Syt1.\",\n      \"evidence\": \"siRNA knockdown of Syt isoforms in cultured dopaminergic neurons with amperometric dopamine measurement and Ca2+-channel pharmacology\",\n      \"pmids\": [\"21576241\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Does not define whether SYT4 acts as a direct Ca2+ sensor or modulator in this context\", \"Knockdown specificity and compensation by other Syt isoforms not fully excluded\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Linked SYT4 to insulin secretion and human disease relevance by showing knockdown reduces glucose-stimulated insulin secretion and that SYT4 protein is decreased in type 2 diabetes islets.\",\n      \"evidence\": \"siRNA knockdown in INS1-832/13 \\u03b2 cells with GSIS assay and Western blotting of human T2D islets\",\n      \"pmids\": [\"22939844\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Correlative protein-level association in human tissue does not establish causality\", \"Mechanism of how SYT4 sets secretory competence not resolved here\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Revealed that SYT4 is transferred between cells via anterograde exosome release, redefining how a postsynaptic retrograde signaling component is supplied by the presynaptic neuron.\",\n      \"evidence\": \"Exosome isolation, live imaging, and genetic loss-of-function at the Drosophila NMJ\",\n      \"pmids\": [\"23522040\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether mammalian SYT4 uses analogous intercellular transfer not addressed\", \"Molecular machinery sorting SYT4 into exosomes not defined here\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Identified the t-SNARE Syntaxin 4 as a controller of synaptic SYT4 membrane levels, placing SYT4, Syx4, and Neuroligin 1 in convergent pathways governing retrograde signaling and synaptic growth.\",\n      \"evidence\": \"pHluorin-tagged Syt4 trafficking screen and genetic epistasis with live NMJ imaging in Drosophila\",\n      \"pmids\": [\"27223326\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct physical interaction between Syx4 and Syt4 not demonstrated\", \"Conservation in mammalian synapses untested\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Defined an activity-dependent switch governing SYT4 vesicle trafficking, where JNK phosphorylation at S135 uncouples SYT4 from Kif1A and redirects dense-core vesicles to actin-based boutonic capture.\",\n      \"evidence\": \"S135A/S135E phospho-site mutagenesis, Syt4-Kif1A co-immunoprecipitation, and live DCV imaging in hippocampal neurons\",\n      \"pmids\": [\"29166604\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of the actin-based capture machinery not defined\", \"Whether captured DCVs preferentially undergo fusion or proteostatic turnover unresolved\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Established SYT4 as a developmentally regulated brake on basal secretion, with Myt-repressed expression rising during \\u03b2-cell maturation to suppress basal insulin release and set Ca2+ sensitivity, conserved in human cells.\",\n      \"evidence\": \"Syt4 knockout/overexpression mice, insulin-granule immunolocalization, RRP capacitance measurements, human \\u03b2-cell line knockdown, and Myt ChIP\",\n      \"pmids\": [\"29656931\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism by which non-Ca2+-binding SYT4 sets vesicle pool size not fully resolved\", \"Relationship between maturation role and acute secretory clamp unclear\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Connected SYT4 expression to activity- and metabolism-dependent transcription, showing CREB drives SYT4 transcription and that glucose excess suppresses this to impair synaptic transmission.\",\n      \"evidence\": \"ChIP for CREB at the SYT4 promoter with electrophysiology (mEPSC, NMDAR currents) in autaptic hippocampal neurons and a diabetic mouse model\",\n      \"pmids\": [\"32974347\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether reduced SYT4 is causal for the electrophysiological deficits not isolated\", \"Direct postsynaptic vs presynaptic locus of SYT4 action unresolved\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Tested the necessity of SYT4 for fast exocytosis in vestibular hair cells, establishing it is dispensable for readily releasable pool kinetics but may shape the secondary releasable pool.\",\n      \"evidence\": \"Patch-clamp capacitance measurements in control vs Syt4-knockout mouse Type II vestibular hair cells\",\n      \"pmids\": [\"32691536\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"SRP role is a weak positive requiring confirmation\", \"Limited to a single electrophysiological method\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Resolved the trafficking logic that maintains synaptic SYT4, showing endocytic machinery (Nwk, dynamin, AP-2) sustains a release-competent cargo pool upstream of retromer-dependent removal.\",\n      \"evidence\": \"Drosophila endocytic mutants with Syt4 cargo imaging, nwk-syt4 and nwk-retromer epistasis, and synaptic plasticity assays\",\n      \"pmids\": [\"35320349\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct cargo-recognition step for SYT4 endocytosis not identified\", \"Mammalian relevance untested\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Defined retromer and Rab11 as opposing regulators of SYT4 sorting into extracellular vesicles, framing synaptic EV traffic as a recycling-versus-removal balance.\",\n      \"evidence\": \"Drosophila retromer and rab mutants with Syt4 immunofluorescence, EV cargo quantification, and rab11 suppression epistasis\",\n      \"pmids\": [\"34019080\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether EV-released SYT4 has signaling significance left open\", \"Direct binding of SYT4 to sorting machinery not shown\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Identified ARMS as a specific direct partner of SYT4 engaged through both C2A and C2B domains, providing a molecular link to BDNF secretion control.\",\n      \"evidence\": \"Co-IP, GST pull-down, AlphaFold2-guided point mutagenesis, and domain-deletion mapping\",\n      \"pmids\": [\"38069318\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Functional consequence of the SYT4-ARMS interaction for fusion not directly measured\", \"Single-lab biochemistry without cellular validation of the mapped residues\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Dissociated SYT4's signaling activity from its EV-mediated transfer, showing ESCRT-dependent EV release is dispensable for SYT4 signaling and may instead serve proteostasis.\",\n      \"evidence\": \"ESCRT depletion in Drosophila motor neurons with EV cargo quantification, signaling readouts, and phagocytosis imaging\",\n      \"pmids\": [\"38842573\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Reconciliation with earlier exosome-transfer model not fully resolved\", \"Proteostatic interpretation inferred rather than directly demonstrated\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Extended SYT4 function into disease and developmental contexts, implicating it in mPFC BDNF-TrkB-dependent stress susceptibility, gastric cancer Wnt/\\u03b2-catenin signaling, prostate cancer vesicle efflux, melanogenesis, GLUT1-driven glucose uptake in retinopathy, and axonal sprouting after spinal cord injury.\",\n      \"evidence\": \"Gain/loss-of-function in mPFC with behavioral assays; Co-IP/IP-MS, GST pull-down and reporter assays for PSMC6 and SNAP25; overexpression with pathway readouts in melanocytes and ARPE-19 cells; loss-of-function with SCI tracing and behavior\",\n      \"pmids\": [\"38297157\", \"41281742\", \"38889208\", \"31743468\", \"38766439\", \"39266302\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Many contexts rely on single-lab overexpression or knockdown without reconstitution\", \"Whether these effects share a common SYT4 fusion/trafficking mechanism is unclear\", \"Direct SYT4-ERK and SYT4-GLUT1 interactions not established\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"It remains unresolved how the conserved non-Ca2+-binding biochemistry of mammalian SYT4 maps onto its diverse cellular roles, and whether a single fusion-clamp mechanism unifies its actions across neurons, \\u03b2 cells, and cancer.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No structural model of SYT4 engaged with its native SNARE complex\", \"No unifying account linking inhibitory fusion activity to its disease and developmental phenotypes\", \"Mammalian counterpart of the Drosophila EV/retrograde-signaling program untested\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [0, 5]},\n      {\"term_id\": \"GO:0140313\", \"supporting_discovery_ids\": [0]},\n      {\"term_id\": \"GO:0005509\", \"supporting_discovery_ids\": []}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0031410\", \"supporting_discovery_ids\": [3, 5]},\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [2, 17]},\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [2]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-9609507\", \"supporting_discovery_ids\": [3, 13, 14]},\n      {\"term_id\": \"R-HSA-5653656\", \"supporting_discovery_ids\": [1, 13, 15]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [9, 10, 11]},\n      {\"term_id\": \"R-HSA-112316\", \"supporting_discovery_ids\": [2, 4, 16]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"KIF1A\", \"SNAP25\", \"STX4\", \"NLG1\", \"ARMS/KIDINS220\", \"PSMC6\", \"SYT7\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":8,"faith_total":8,"faith_pct":100.0}}