{"gene":"SYTL4","run_date":"2026-06-10T10:51:54","timeline":{"discoveries":[{"year":2002,"finding":"Slp4-a forms complexes with Rab27A and Rab8 (but not Rab3A) on dense-core vesicles in PC12 cells, and expression of Slp4-a (but not a Rab27A-binding-deficient mutant) inhibits KCl-dependent NPY secretion, establishing Slp4-a as a negative regulator of dense-core vesicle exocytosis that acts specifically through Rab27A.","method":"Immunocytochemistry, subcellular fractionation, co-immunoprecipitation in intact cells, NPY secretion assay with Rab27A-binding-deficient mutant","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal co-IP in intact cells plus loss-of-function mutagenesis with defined secretory readout, replicated across multiple constructs","pmids":["12176990"],"is_preprint":false},{"year":2003,"finding":"Slp4-a uniquely binds both the GDP-bound (T23N) and GTP-bound (Q78L) forms of Rab27A via distinct domains (SHD2 TGDWFY sequence for GDP-form; SHD1 for GTP-form), and interaction with GDP-bound Rab27A is the primary mechanism by which Slp4-a inhibits dense-core vesicle exocytosis in PC12 cells. Munc18-1 directly interacts with the C-terminal domain of Slp4-a independently of Rab27A.","method":"Deletion and point mutation analyses, immunoprecipitation, cotransfection assays, NPY secretion assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple domain-mapping mutations with functional secretion readout in the same study, multiple orthogonal methods","pmids":["12590134"],"is_preprint":false},{"year":2005,"finding":"The linker domain of Slp4-a (residues 144–354) interacts with syntaxin-2/3 in a Munc18-2-dependent manner in parotid acinar cells, and anti-Slp4-a linker domain antibody introduced into permeabilized parotid acinar cells attenuates isoproterenol-stimulated amylase release, demonstrating that Slp4-a modulates exocytosis through syntaxin-2/3 interaction in exocrine cells.","method":"Co-immunoprecipitation in COS-7 cells, deletion analysis, endogenous complex detection in rat parotid gland, antibody inhibition in streptolysin O-permeabilized acinar cells","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Strong — endogenous complex validated, antibody perturbation experiment with functional readout, multiple orthogonal methods in single study","pmids":["16186111"],"is_preprint":false},{"year":2006,"finding":"The linker domain of Slp4-a directly binds Munc18-1, and this interaction promotes docking of dense-core vesicles to the plasma membrane in PC12 cells. Despite increasing docked vesicle number, Slp4-a strongly inhibits KCl-induced exocytosis, and this inhibitory effect is abolished when the Slp4-a linker domain is replaced by the corresponding Slp5 linker (which cannot bind Munc18-1). Slp4-a thus simultaneously bridges Rab27A on dense-core vesicles and the Munc18-1·syntaxin-1a complex at the plasma membrane.","method":"Deletion and chimeric domain analyses, co-immunoprecipitation, morphological docking assay (electron microscopy or imaging implied), NPY secretion assay with domain-swap mutants in PC12 cells","journal":"Molecular biology of the cell","confidence":"High","confidence_rationale":"Tier 2 / Strong — domain-swap mutagenesis with reciprocal co-IP and quantitative secretion readout, multiple orthogonal methods","pmids":["16481396"],"is_preprint":false},{"year":2006,"finding":"MicroRNA-9 (mir-9) overexpression in insulin-secreting cells reduces glucose- and KCl-evoked exocytosis by decreasing expression of the transcription factor Onecut-2, which in turn elevates Granuphilin/Slp4 levels. Onecut-2 binds the granuphilin promoter and represses its transcription; siRNA silencing of Onecut-2 increases Granuphilin expression and mimics the inhibitory effect of mir-9 on secretion.","method":"Electrophoretic mobility shift assay, chromatin immunoprecipitation, transfection/overexpression, RNA interference, secretion assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Strong — ChIP plus EMSA plus RNAi with functional secretion readout, multiple orthogonal methods in single study","pmids":["16831872"],"is_preprint":false},{"year":2012,"finding":"Slp4-a (granuphilin) is an endogenous component of Weibel-Palade bodies (WPBs) in endothelial cells, localizes to WPBs in a Rab27A/Rab3B/Rab3D-dependent manner, and acts as a positive regulator of hormone-evoked WPB exocytosis (VWF secretion). siRNA knockdown of Slp4-a reduces WPB exocytosis, whereas overexpression of EGFP-Slp4-a increases it, in contrast to MyRIP which is a negative regulator. The probability of WPB release is proposed to depend on the fractional occupancy of WPB-Rab27A by Slp4-a versus MyRIP.","method":"siRNA knockdown, EGFP-fusion overexpression, VWF secretion assay, immunofluorescence localization, subcellular fractionation","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal gain- and loss-of-function with quantitative secretion readout, localization experiments, multiple orthogonal methods","pmids":["22898601"],"is_preprint":false},{"year":2014,"finding":"STXBP1 (syntaxin-binding protein 1) and syntaxins-2 and -3 are endogenous binding partners of Slp4-a in endothelial cells identified by unbiased proteomics. STXBP1 interacts with syntaxin-2 and -3 but not syntaxin-4. siRNA silencing of STXBP1 impairs histamine- and forskolin-induced VWF secretion. STXBP1-haploinsufficient patient-derived endothelial cells show impaired stimulus-evoked VWF secretion despite normal WPB morphology, indicating that the Rab27A–Slp4-a–STXBP1 complex promotes WPB exocytosis.","method":"Unbiased proteomic screen, co-immunoprecipitation, siRNA knockdown, patient-derived blood outgrowth endothelial cells (EIEE4 mutation), VWF secretion assay","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 / Strong — unbiased proteomics plus co-IP plus patient-derived functional cell model with secretion readout, multiple orthogonal methods","pmids":["24700782"],"is_preprint":false},{"year":2009,"finding":"In rat parotid acinar cells under resting conditions, Slp4-a is present on both apical plasma membrane and secretory granule fractions. Following isoproterenol stimulation, intracellular localization and expression levels of Slp4-a remain unchanged, in contrast to Slac2-c/MyRIP which redistributes and is degraded.","method":"Subcellular fractionation, immunoblotting of parotid acinar cells following isoproterenol stimulation","journal":"Archives of oral biology","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — subcellular fractionation with stimulation time-course, single lab but clear negative result for Slp4-a redistribution versus positive result for Slac2-c","pmids":["19185850"],"is_preprint":false},{"year":2020,"finding":"SYTL4 colocalizes with microtubules in TNBC cells and interacts with microtubules through its middle region (linker and C2A domain). SYTL4 directly binds microtubules and inhibits in vitro microtubule polymerization; downregulation of SYTL4 stabilizes the microtubule network and slows microtubule growth rate, conferring paclitaxel sensitivity.","method":"Colocalization imaging, biochemical pulldown/interaction assay with microtubules, in vitro microtubule polymerization assay, siRNA knockdown with microtubule dynamics readout, mouse model and patient-derived organoids","journal":"Theranostics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vitro microtubule polymerization assay plus co-localization plus domain mapping, single lab with multiple orthogonal methods but novel and unreplicated finding","pmids":["33042263"],"is_preprint":false},{"year":2025,"finding":"SYTL4-mediated exocytosis is required for CXC motif chemokine ligand 8 (CXCL8) secretion in pancreatic cancer cells; SLC6A14-mediated glutamine metabolism drives this process via mTOR/NF-κB signaling, and SYTL4 knockdown reduces CXCL8 secretion and its paracrine activation of cancer-associated fibroblasts.","method":"Knockdown experiments, secretion assay for CXCL8, in vivo tumor models","journal":"Experimental & molecular medicine","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single lab, SYTL4 role inferred from knockdown with secretion readout but limited mechanistic detail on how SYTL4 mediates CXCL8 exocytosis specifically","pmids":["41444422"],"is_preprint":false}],"current_model":"SYTL4/Slp4-a is a Rab27A effector that localizes to secretory granules and Weibel-Palade bodies via its N-terminal Slp homology domain (SHD), where it simultaneously engages Rab27A (in both GTP- and GDP-bound states through distinct SHD sub-domains) and, through its linker domain, the Munc18-1·syntaxin-1a complex (or Munc18-2·syntaxin-2/3 in exocrine cells) to dock dense-core vesicles at the plasma membrane while ultimately inhibiting Ca²⁺-triggered fusion; in endothelial cells it acts as a positive regulator of WPB exocytosis by recruiting STXBP1 and syntaxins-2/3; additionally, SYTL4 directly binds microtubules through its linker-C2A region and inhibits microtubule polymerization, a function linked to paclitaxel resistance in breast cancer."},"narrative":{"mechanistic_narrative":"SYTL4 (Slp4-a/granuphilin) is a Rab27A effector that controls the docking and Ca²⁺-triggered fusion of dense-core secretory granules and Weibel-Palade bodies (WPBs) by bridging vesicular Rab GTPases to the plasma-membrane SNARE-priming machinery [PMID:12176990, PMID:16481396]. Through its N-terminal Slp homology domain it engages Rab27A in both nucleotide states, using distinct sub-domains (the SHD2 TGDWFY motif for GDP-bound Rab27A and SHD1 for the GTP-bound form), with the GDP-bound interaction being the dominant route by which it inhibits exocytosis [PMID:12590134]. Its central linker domain directly binds the Munc18-1·syntaxin-1a complex—and Munc18-2·syntaxin-2/3 in exocrine acinar cells—to dock granules at the plasma membrane while ultimately suppressing KCl-evoked secretion, since increasing docked-vesicle number is uncoupled from fusion and the linker is required for the inhibitory effect [PMID:16186111, PMID:16481396]. In endothelial cells SYTL4 instead acts as a positive regulator of hormone-evoked WPB exocytosis and von Willebrand factor release, recruiting STXBP1 together with syntaxin-2/3, an activity opposed by MyRIP at the same Rab27A docking site [PMID:22898601, PMID:24700782]. Independent of its secretory role, SYTL4 directly binds microtubules through its linker–C2A region and inhibits microtubule polymerization, a function that promotes paclitaxel resistance in triple-negative breast cancer [PMID:33042263].","teleology":[{"year":2002,"claim":"Established Slp4-a as a Rab27A-specific effector on dense-core vesicles and a negative regulator of exocytosis, defining its first molecular partner and functional output.","evidence":"Co-IP, subcellular fractionation, and NPY secretion assay with a Rab27A-binding-deficient mutant in PC12 cells","pmids":["12176990"],"confidence":"High","gaps":["Did not resolve which downstream plasma-membrane machinery executes the inhibition","Rab8 interaction left functionally uncharacterized"]},{"year":2003,"claim":"Defined the dual nucleotide-state recognition of Rab27A by distinct SHD sub-domains and identified GDP-bound binding as the mechanism of secretion inhibition, while linking Slp4-a to Munc18-1.","evidence":"Domain deletion/point mutagenesis, immunoprecipitation, and NPY secretion assays in PC12 cells","pmids":["12590134"],"confidence":"High","gaps":["Structural basis of simultaneous GDP/GTP-form binding not resolved","Functional consequence of Munc18-1 binding not yet established at this stage"]},{"year":2005,"claim":"Extended the mechanism to exocrine cells by showing the linker domain engages syntaxin-2/3 via Munc18-2 and is required for stimulated secretion, establishing tissue-specific SNARE partner usage.","evidence":"Co-IP, deletion mapping, endogenous complex detection, and antibody-inhibition amylase release in permeabilized parotid acinar cells","pmids":["16186111"],"confidence":"High","gaps":["Whether syntaxin-2/3 engagement is activating or inhibitory in this context not fully disentangled","Direct versus Munc18-bridged contact not separated"]},{"year":2006,"claim":"Resolved the bridging model—linker-domain Munc18-1 binding drives vesicle docking yet exocytosis is still inhibited—demonstrating that docking and fusion are mechanistically uncoupled by Slp4-a.","evidence":"Chimeric Slp4-a/Slp5 domain swaps, co-IP, morphological docking assay, and NPY secretion in PC12 cells","pmids":["16481396"],"confidence":"High","gaps":["Molecular step at which fusion is blocked downstream of docking not defined","Whether the GDP-Rab27A interaction and Munc18-1 bridging act independently unclear"]},{"year":2006,"claim":"Placed granuphilin/Slp4 expression under transcriptional and microRNA control, showing the mir-9→Onecut-2→granuphilin axis tunes insulin-secreting cell exocytosis.","evidence":"EMSA, ChIP, RNAi, and secretion assays in insulin-secreting cells","pmids":["16831872"],"confidence":"High","gaps":["Did not test SYTL4 protein-level mechanism, only its regulation","Direct effect of granuphilin dose on fusion not measured here"]},{"year":2009,"claim":"Distinguished Slp4-a from MyRIP by showing it is stably retained on apical membrane and granules without stimulus-evoked redistribution or degradation, indicating a constitutive scaffolding role.","evidence":"Subcellular fractionation and immunoblotting after isoproterenol stimulation in rat parotid acinar cells","pmids":["19185850"],"confidence":"Medium","gaps":["Single-lab fractionation without live imaging","Functional meaning of stable localization not directly tested"]},{"year":2012,"claim":"Revealed a context-dependent reversal of polarity—in endothelial WPBs Slp4-a is a positive regulator of exocytosis, with release probability set by competition with MyRIP for Rab27A.","evidence":"siRNA knockdown, EGFP-Slp4-a overexpression, VWF secretion, and localization in endothelial cells","pmids":["22898601"],"confidence":"High","gaps":["Molecular basis for opposite directionality versus PC12 cells not defined","Stoichiometry of Slp4-a vs MyRIP occupancy not quantified in vivo"]},{"year":2014,"claim":"Identified STXBP1 and syntaxin-2/3 as endogenous SYTL4 partners by unbiased proteomics and linked the Rab27A–Slp4-a–STXBP1 complex to VWF secretion, including a human STXBP1-haploinsufficient phenotype.","evidence":"Proteomic screen, co-IP, siRNA, and patient-derived blood outgrowth endothelial cells with VWF secretion readout","pmids":["24700782"],"confidence":"High","gaps":["Whether STXBP1 recruitment is direct or via syntaxin not resolved","Mechanism converting docking into productive fusion still incomplete"]},{"year":2020,"claim":"Uncovered a secretion-independent function: SYTL4 directly binds and destabilizes microtubules via its linker–C2A region, linking it to paclitaxel resistance in TNBC.","evidence":"Colocalization, microtubule pulldown, in vitro polymerization assay, siRNA with dynamics readout, mouse and organoid models","pmids":["33042263"],"confidence":"Medium","gaps":["Single-lab, unreplicated finding","Relationship between microtubule and secretory functions not integrated","No structural detail of the microtubule-binding interface"]},{"year":2025,"claim":"Implicated SYTL4-mediated exocytosis in CXCL8 secretion in pancreatic cancer downstream of glutamine metabolism and mTOR/NF-κB signaling.","evidence":"Knockdown, CXCL8 secretion assay, and in vivo tumor models in pancreatic cancer cells","pmids":["41444422"],"confidence":"Low","gaps":["Limited mechanistic detail on how SYTL4 specifically mediates CXCL8 exocytosis","Single lab, not independently confirmed","Rab27A/SNARE dependence in this context not tested"]},{"year":null,"claim":"How SYTL4 switches between negative regulation (dense-core granules) and positive regulation (WPBs), and how its microtubule-destabilizing activity relates to its secretory scaffolding role, remain unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No structure of the Rab27A–Slp4-a–Munc18/syntaxin assembly","Determinants of cell-type-specific directionality unknown","Whether microtubule and exocytic functions are mutually exclusive untested"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[0,3,6]},{"term_id":"GO:0008092","term_label":"cytoskeletal protein binding","supporting_discovery_ids":[8]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[0,3,5]}],"localization":[{"term_id":"GO:0031410","term_label":"cytoplasmic vesicle","supporting_discovery_ids":[0,7]},{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[3,7]},{"term_id":"GO:0005856","term_label":"cytoskeleton","supporting_discovery_ids":[8]}],"pathway":[{"term_id":"R-HSA-5653656","term_label":"Vesicle-mediated transport","supporting_discovery_ids":[0,3,5,6]},{"term_id":"R-HSA-109582","term_label":"Hemostasis","supporting_discovery_ids":[5,6]}],"complexes":["Rab27A-Slp4-a-Munc18-1-syntaxin-1a docking complex","Rab27A-Slp4-a-STXBP1-syntaxin-2/3 complex"],"partners":["RAB27A","RAB8A","STX1A","STX2","STX3","STXBP1","MYRIP"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q96C24","full_name":"Synaptotagmin-like protein 4","aliases":["Exophilin-2","Granuphilin"],"length_aa":671,"mass_kda":76.0,"function":"Modulates exocytosis of dense-core granules and secretion of hormones in the pancreas and the pituitary. Interacts with vesicles containing negatively charged phospholipids in a Ca(2+)-independent manner (By similarity)","subcellular_location":"Membrane; Cell membrane; Cytoplasmic vesicle, secretory vesicle membrane","url":"https://www.uniprot.org/uniprotkb/Q96C24/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/SYTL4","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/SYTL4","total_profiled":1310},"omim":[{"mim_id":"611186","title":"MICRO RNA 9-1; MIR9-1","url":"https://www.omim.org/entry/611186"},{"mim_id":"604894","title":"ONE CUT HOMEOBOX 2; ONECUT2","url":"https://www.omim.org/entry/604894"},{"mim_id":"301126","title":"SYNAPTOTAGMIN-LIKE 5; SYTL5","url":"https://www.omim.org/entry/301126"},{"mim_id":"300723","title":"SYNAPTOTAGMIN-LIKE 4; SYTL4","url":"https://www.omim.org/entry/300723"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Vesicles","reliability":"Approved"}],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in all","driving_tissues":[{"tissue":"endometrium 1","ntpm":40.0},{"tissue":"ovary","ntpm":49.2}],"url":"https://www.proteinatlas.org/search/SYTL4"},"hgnc":{"alias_symbol":["Slp4"],"prev_symbol":[]},"alphafold":{"accession":"Q96C24","domains":[{"cath_id":"3.30.40.10","chopping":"61-118","consensus_level":"medium","plddt":83.2419,"start":61,"end":118},{"cath_id":"2.60.40.150","chopping":"358-478","consensus_level":"high","plddt":86.081,"start":358,"end":478},{"cath_id":"2.60.40.150","chopping":"496-508_524-659","consensus_level":"high","plddt":88.9757,"start":496,"end":659}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q96C24","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q96C24-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q96C24-F1-predicted_aligned_error_v6.png","plddt_mean":67.81},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=SYTL4","jax_strain_url":"https://www.jax.org/strain/search?query=SYTL4"},"sequence":{"accession":"Q96C24","fasta_url":"https://rest.uniprot.org/uniprotkb/Q96C24.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q96C24/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q96C24"}},"corpus_meta":[{"pmid":"16831872","id":"PMC_16831872","title":"MicroRNA-9 controls the expression of Granuphilin/Slp4 and the secretory response of insulin-producing cells.","date":"2006","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/16831872","citation_count":288,"is_preprint":false},{"pmid":"22898601","id":"PMC_22898601","title":"The interplay between the Rab27A effectors Slp4-a and MyRIP controls hormone-evoked Weibel-Palade body exocytosis.","date":"2012","source":"Blood","url":"https://pubmed.ncbi.nlm.nih.gov/22898601","citation_count":83,"is_preprint":false},{"pmid":"12176990","id":"PMC_12176990","title":"Slp4-a/granuphilin-a regulates dense-core vesicle exocytosis in PC12 cells.","date":"2002","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/12176990","citation_count":71,"is_preprint":false},{"pmid":"16481396","id":"PMC_16481396","title":"The Slp4-a linker domain controls exocytosis through interaction with Munc18-1.syntaxin-1a complex.","date":"2006","source":"Molecular biology of the cell","url":"https://pubmed.ncbi.nlm.nih.gov/16481396","citation_count":59,"is_preprint":false},{"pmid":"12590134","id":"PMC_12590134","title":"Slp4-a/granuphilin-a inhibits dense-core vesicle exocytosis through interaction with the GDP-bound form of Rab27A in PC12 cells.","date":"2003","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/12590134","citation_count":58,"is_preprint":false},{"pmid":"24700782","id":"PMC_24700782","title":"STXBP1 promotes Weibel-Palade body exocytosis through its interaction with the Rab27A effector Slp4-a.","date":"2014","source":"Blood","url":"https://pubmed.ncbi.nlm.nih.gov/24700782","citation_count":49,"is_preprint":false},{"pmid":"16186111","id":"PMC_16186111","title":"Slp4-a/granuphilin-a interacts with syntaxin-2/3 in a Munc18-2-dependent manner.","date":"2005","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/16186111","citation_count":44,"is_preprint":false},{"pmid":"33042263","id":"PMC_33042263","title":"SYTL4 downregulates microtubule stability and confers paclitaxel resistance in triple-negative breast cancer.","date":"2020","source":"Theranostics","url":"https://pubmed.ncbi.nlm.nih.gov/33042263","citation_count":42,"is_preprint":false},{"pmid":"16473610","id":"PMC_16473610","title":"Analysis of the role of Rab27 effector Slp4-a/Granuphilin-a in dense-core vesicle exocytosis.","date":"2005","source":"Methods in enzymology","url":"https://pubmed.ncbi.nlm.nih.gov/16473610","citation_count":39,"is_preprint":false},{"pmid":"31323913","id":"PMC_31323913","title":"High Functioning Autism with Missense Mutations in Synaptotagmin-Like Protein 4 (SYTL4) and Transmembrane Protein 187 (TMEM187) Genes: SYTL4- Protein Modeling, Protein-Protein Interaction, Expression Profiling and MicroRNA Studies.","date":"2019","source":"International journal of molecular sciences","url":"https://pubmed.ncbi.nlm.nih.gov/31323913","citation_count":17,"is_preprint":false},{"pmid":"19185850","id":"PMC_19185850","title":"Redistribution of Rab27-specific effector Slac2-c, but not Slp4-a, after isoproterenol-stimulation in rat parotid acinar cells.","date":"2009","source":"Archives of oral biology","url":"https://pubmed.ncbi.nlm.nih.gov/19185850","citation_count":8,"is_preprint":false},{"pmid":"41444422","id":"PMC_41444422","title":"SLC6A14-mediated glutamine promotes SYTL4-CXCL8 axis activation to drive gemcitabine resistance and immune evasion in pancreatic cancer.","date":"2025","source":"Experimental & molecular medicine","url":"https://pubmed.ncbi.nlm.nih.gov/41444422","citation_count":5,"is_preprint":false},{"pmid":"35838328","id":"PMC_35838328","title":"Effect of Slp4-a on Membrane Bending During Prefusion of Vesicles in Blood-Brain Barrier.","date":"2023","source":"Journal of biomechanical engineering","url":"https://pubmed.ncbi.nlm.nih.gov/35838328","citation_count":1,"is_preprint":false},{"pmid":"42196512","id":"PMC_42196512","title":"SYTL4 May Serve as a New Predictive Biomarker for Survival and Trastuzumab Treatment Responsiveness in HER2-Positive Breast Cancer.","date":"2026","source":"International journal of molecular sciences","url":"https://pubmed.ncbi.nlm.nih.gov/42196512","citation_count":0,"is_preprint":false},{"pmid":null,"id":"bio_10.1101_2025.10.11.25337587","title":"Machine Learning for Predicting and Maximizing the Response of Breast Cancer Patients to Neoadjuvant Therapy","date":"2025-10-14","source":"bioRxiv","url":"https://doi.org/10.1101/2025.10.11.25337587","citation_count":0,"is_preprint":true}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":11055,"output_tokens":3153,"usd":0.04023,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":10370,"output_tokens":3640,"usd":0.071425,"stage2_stop_reason":"end_turn"},"total_usd":0.111655,"stage1_batch_id":"msgbatch_017qqVz33Ga63SCDwzb35XPb","stage2_batch_id":"msgbatch_01Scr2Ei9x4HJMiRGxwrmvLK","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2002,\n      \"finding\": \"Slp4-a forms complexes with Rab27A and Rab8 (but not Rab3A) on dense-core vesicles in PC12 cells, and expression of Slp4-a (but not a Rab27A-binding-deficient mutant) inhibits KCl-dependent NPY secretion, establishing Slp4-a as a negative regulator of dense-core vesicle exocytosis that acts specifically through Rab27A.\",\n      \"method\": \"Immunocytochemistry, subcellular fractionation, co-immunoprecipitation in intact cells, NPY secretion assay with Rab27A-binding-deficient mutant\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal co-IP in intact cells plus loss-of-function mutagenesis with defined secretory readout, replicated across multiple constructs\",\n      \"pmids\": [\"12176990\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Slp4-a uniquely binds both the GDP-bound (T23N) and GTP-bound (Q78L) forms of Rab27A via distinct domains (SHD2 TGDWFY sequence for GDP-form; SHD1 for GTP-form), and interaction with GDP-bound Rab27A is the primary mechanism by which Slp4-a inhibits dense-core vesicle exocytosis in PC12 cells. Munc18-1 directly interacts with the C-terminal domain of Slp4-a independently of Rab27A.\",\n      \"method\": \"Deletion and point mutation analyses, immunoprecipitation, cotransfection assays, NPY secretion assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple domain-mapping mutations with functional secretion readout in the same study, multiple orthogonal methods\",\n      \"pmids\": [\"12590134\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"The linker domain of Slp4-a (residues 144–354) interacts with syntaxin-2/3 in a Munc18-2-dependent manner in parotid acinar cells, and anti-Slp4-a linker domain antibody introduced into permeabilized parotid acinar cells attenuates isoproterenol-stimulated amylase release, demonstrating that Slp4-a modulates exocytosis through syntaxin-2/3 interaction in exocrine cells.\",\n      \"method\": \"Co-immunoprecipitation in COS-7 cells, deletion analysis, endogenous complex detection in rat parotid gland, antibody inhibition in streptolysin O-permeabilized acinar cells\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — endogenous complex validated, antibody perturbation experiment with functional readout, multiple orthogonal methods in single study\",\n      \"pmids\": [\"16186111\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"The linker domain of Slp4-a directly binds Munc18-1, and this interaction promotes docking of dense-core vesicles to the plasma membrane in PC12 cells. Despite increasing docked vesicle number, Slp4-a strongly inhibits KCl-induced exocytosis, and this inhibitory effect is abolished when the Slp4-a linker domain is replaced by the corresponding Slp5 linker (which cannot bind Munc18-1). Slp4-a thus simultaneously bridges Rab27A on dense-core vesicles and the Munc18-1·syntaxin-1a complex at the plasma membrane.\",\n      \"method\": \"Deletion and chimeric domain analyses, co-immunoprecipitation, morphological docking assay (electron microscopy or imaging implied), NPY secretion assay with domain-swap mutants in PC12 cells\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — domain-swap mutagenesis with reciprocal co-IP and quantitative secretion readout, multiple orthogonal methods\",\n      \"pmids\": [\"16481396\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"MicroRNA-9 (mir-9) overexpression in insulin-secreting cells reduces glucose- and KCl-evoked exocytosis by decreasing expression of the transcription factor Onecut-2, which in turn elevates Granuphilin/Slp4 levels. Onecut-2 binds the granuphilin promoter and represses its transcription; siRNA silencing of Onecut-2 increases Granuphilin expression and mimics the inhibitory effect of mir-9 on secretion.\",\n      \"method\": \"Electrophoretic mobility shift assay, chromatin immunoprecipitation, transfection/overexpression, RNA interference, secretion assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — ChIP plus EMSA plus RNAi with functional secretion readout, multiple orthogonal methods in single study\",\n      \"pmids\": [\"16831872\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Slp4-a (granuphilin) is an endogenous component of Weibel-Palade bodies (WPBs) in endothelial cells, localizes to WPBs in a Rab27A/Rab3B/Rab3D-dependent manner, and acts as a positive regulator of hormone-evoked WPB exocytosis (VWF secretion). siRNA knockdown of Slp4-a reduces WPB exocytosis, whereas overexpression of EGFP-Slp4-a increases it, in contrast to MyRIP which is a negative regulator. The probability of WPB release is proposed to depend on the fractional occupancy of WPB-Rab27A by Slp4-a versus MyRIP.\",\n      \"method\": \"siRNA knockdown, EGFP-fusion overexpression, VWF secretion assay, immunofluorescence localization, subcellular fractionation\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal gain- and loss-of-function with quantitative secretion readout, localization experiments, multiple orthogonal methods\",\n      \"pmids\": [\"22898601\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"STXBP1 (syntaxin-binding protein 1) and syntaxins-2 and -3 are endogenous binding partners of Slp4-a in endothelial cells identified by unbiased proteomics. STXBP1 interacts with syntaxin-2 and -3 but not syntaxin-4. siRNA silencing of STXBP1 impairs histamine- and forskolin-induced VWF secretion. STXBP1-haploinsufficient patient-derived endothelial cells show impaired stimulus-evoked VWF secretion despite normal WPB morphology, indicating that the Rab27A–Slp4-a–STXBP1 complex promotes WPB exocytosis.\",\n      \"method\": \"Unbiased proteomic screen, co-immunoprecipitation, siRNA knockdown, patient-derived blood outgrowth endothelial cells (EIEE4 mutation), VWF secretion assay\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — unbiased proteomics plus co-IP plus patient-derived functional cell model with secretion readout, multiple orthogonal methods\",\n      \"pmids\": [\"24700782\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"In rat parotid acinar cells under resting conditions, Slp4-a is present on both apical plasma membrane and secretory granule fractions. Following isoproterenol stimulation, intracellular localization and expression levels of Slp4-a remain unchanged, in contrast to Slac2-c/MyRIP which redistributes and is degraded.\",\n      \"method\": \"Subcellular fractionation, immunoblotting of parotid acinar cells following isoproterenol stimulation\",\n      \"journal\": \"Archives of oral biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — subcellular fractionation with stimulation time-course, single lab but clear negative result for Slp4-a redistribution versus positive result for Slac2-c\",\n      \"pmids\": [\"19185850\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"SYTL4 colocalizes with microtubules in TNBC cells and interacts with microtubules through its middle region (linker and C2A domain). SYTL4 directly binds microtubules and inhibits in vitro microtubule polymerization; downregulation of SYTL4 stabilizes the microtubule network and slows microtubule growth rate, conferring paclitaxel sensitivity.\",\n      \"method\": \"Colocalization imaging, biochemical pulldown/interaction assay with microtubules, in vitro microtubule polymerization assay, siRNA knockdown with microtubule dynamics readout, mouse model and patient-derived organoids\",\n      \"journal\": \"Theranostics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vitro microtubule polymerization assay plus co-localization plus domain mapping, single lab with multiple orthogonal methods but novel and unreplicated finding\",\n      \"pmids\": [\"33042263\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"SYTL4-mediated exocytosis is required for CXC motif chemokine ligand 8 (CXCL8) secretion in pancreatic cancer cells; SLC6A14-mediated glutamine metabolism drives this process via mTOR/NF-κB signaling, and SYTL4 knockdown reduces CXCL8 secretion and its paracrine activation of cancer-associated fibroblasts.\",\n      \"method\": \"Knockdown experiments, secretion assay for CXCL8, in vivo tumor models\",\n      \"journal\": \"Experimental & molecular medicine\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single lab, SYTL4 role inferred from knockdown with secretion readout but limited mechanistic detail on how SYTL4 mediates CXCL8 exocytosis specifically\",\n      \"pmids\": [\"41444422\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"SYTL4/Slp4-a is a Rab27A effector that localizes to secretory granules and Weibel-Palade bodies via its N-terminal Slp homology domain (SHD), where it simultaneously engages Rab27A (in both GTP- and GDP-bound states through distinct SHD sub-domains) and, through its linker domain, the Munc18-1·syntaxin-1a complex (or Munc18-2·syntaxin-2/3 in exocrine cells) to dock dense-core vesicles at the plasma membrane while ultimately inhibiting Ca²⁺-triggered fusion; in endothelial cells it acts as a positive regulator of WPB exocytosis by recruiting STXBP1 and syntaxins-2/3; additionally, SYTL4 directly binds microtubules through its linker-C2A region and inhibits microtubule polymerization, a function linked to paclitaxel resistance in breast cancer.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"SYTL4 (Slp4-a/granuphilin) is a Rab27A effector that controls the docking and Ca²⁺-triggered fusion of dense-core secretory granules and Weibel-Palade bodies (WPBs) by bridging vesicular Rab GTPases to the plasma-membrane SNARE-priming machinery [#0, #3]. Through its N-terminal Slp homology domain it engages Rab27A in both nucleotide states, using distinct sub-domains (the SHD2 TGDWFY motif for GDP-bound Rab27A and SHD1 for the GTP-bound form), with the GDP-bound interaction being the dominant route by which it inhibits exocytosis [#1]. Its central linker domain directly binds the Munc18-1·syntaxin-1a complex—and Munc18-2·syntaxin-2/3 in exocrine acinar cells—to dock granules at the plasma membrane while ultimately suppressing KCl-evoked secretion, since increasing docked-vesicle number is uncoupled from fusion and the linker is required for the inhibitory effect [#2, #3]. In endothelial cells SYTL4 instead acts as a positive regulator of hormone-evoked WPB exocytosis and von Willebrand factor release, recruiting STXBP1 together with syntaxin-2/3, an activity opposed by MyRIP at the same Rab27A docking site [#5, #6]. Independent of its secretory role, SYTL4 directly binds microtubules through its linker–C2A region and inhibits microtubule polymerization, a function that promotes paclitaxel resistance in triple-negative breast cancer [#8].\",\n  \"teleology\": [\n    {\n      \"year\": 2002,\n      \"claim\": \"Established Slp4-a as a Rab27A-specific effector on dense-core vesicles and a negative regulator of exocytosis, defining its first molecular partner and functional output.\",\n      \"evidence\": \"Co-IP, subcellular fractionation, and NPY secretion assay with a Rab27A-binding-deficient mutant in PC12 cells\",\n      \"pmids\": [\"12176990\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not resolve which downstream plasma-membrane machinery executes the inhibition\", \"Rab8 interaction left functionally uncharacterized\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Defined the dual nucleotide-state recognition of Rab27A by distinct SHD sub-domains and identified GDP-bound binding as the mechanism of secretion inhibition, while linking Slp4-a to Munc18-1.\",\n      \"evidence\": \"Domain deletion/point mutagenesis, immunoprecipitation, and NPY secretion assays in PC12 cells\",\n      \"pmids\": [\"12590134\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of simultaneous GDP/GTP-form binding not resolved\", \"Functional consequence of Munc18-1 binding not yet established at this stage\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Extended the mechanism to exocrine cells by showing the linker domain engages syntaxin-2/3 via Munc18-2 and is required for stimulated secretion, establishing tissue-specific SNARE partner usage.\",\n      \"evidence\": \"Co-IP, deletion mapping, endogenous complex detection, and antibody-inhibition amylase release in permeabilized parotid acinar cells\",\n      \"pmids\": [\"16186111\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether syntaxin-2/3 engagement is activating or inhibitory in this context not fully disentangled\", \"Direct versus Munc18-bridged contact not separated\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Resolved the bridging model—linker-domain Munc18-1 binding drives vesicle docking yet exocytosis is still inhibited—demonstrating that docking and fusion are mechanistically uncoupled by Slp4-a.\",\n      \"evidence\": \"Chimeric Slp4-a/Slp5 domain swaps, co-IP, morphological docking assay, and NPY secretion in PC12 cells\",\n      \"pmids\": [\"16481396\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular step at which fusion is blocked downstream of docking not defined\", \"Whether the GDP-Rab27A interaction and Munc18-1 bridging act independently unclear\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Placed granuphilin/Slp4 expression under transcriptional and microRNA control, showing the mir-9→Onecut-2→granuphilin axis tunes insulin-secreting cell exocytosis.\",\n      \"evidence\": \"EMSA, ChIP, RNAi, and secretion assays in insulin-secreting cells\",\n      \"pmids\": [\"16831872\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not test SYTL4 protein-level mechanism, only its regulation\", \"Direct effect of granuphilin dose on fusion not measured here\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Distinguished Slp4-a from MyRIP by showing it is stably retained on apical membrane and granules without stimulus-evoked redistribution or degradation, indicating a constitutive scaffolding role.\",\n      \"evidence\": \"Subcellular fractionation and immunoblotting after isoproterenol stimulation in rat parotid acinar cells\",\n      \"pmids\": [\"19185850\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab fractionation without live imaging\", \"Functional meaning of stable localization not directly tested\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Revealed a context-dependent reversal of polarity—in endothelial WPBs Slp4-a is a positive regulator of exocytosis, with release probability set by competition with MyRIP for Rab27A.\",\n      \"evidence\": \"siRNA knockdown, EGFP-Slp4-a overexpression, VWF secretion, and localization in endothelial cells\",\n      \"pmids\": [\"22898601\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular basis for opposite directionality versus PC12 cells not defined\", \"Stoichiometry of Slp4-a vs MyRIP occupancy not quantified in vivo\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Identified STXBP1 and syntaxin-2/3 as endogenous SYTL4 partners by unbiased proteomics and linked the Rab27A–Slp4-a–STXBP1 complex to VWF secretion, including a human STXBP1-haploinsufficient phenotype.\",\n      \"evidence\": \"Proteomic screen, co-IP, siRNA, and patient-derived blood outgrowth endothelial cells with VWF secretion readout\",\n      \"pmids\": [\"24700782\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether STXBP1 recruitment is direct or via syntaxin not resolved\", \"Mechanism converting docking into productive fusion still incomplete\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Uncovered a secretion-independent function: SYTL4 directly binds and destabilizes microtubules via its linker–C2A region, linking it to paclitaxel resistance in TNBC.\",\n      \"evidence\": \"Colocalization, microtubule pulldown, in vitro polymerization assay, siRNA with dynamics readout, mouse and organoid models\",\n      \"pmids\": [\"33042263\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab, unreplicated finding\", \"Relationship between microtubule and secretory functions not integrated\", \"No structural detail of the microtubule-binding interface\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Implicated SYTL4-mediated exocytosis in CXCL8 secretion in pancreatic cancer downstream of glutamine metabolism and mTOR/NF-κB signaling.\",\n      \"evidence\": \"Knockdown, CXCL8 secretion assay, and in vivo tumor models in pancreatic cancer cells\",\n      \"pmids\": [\"41444422\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"Limited mechanistic detail on how SYTL4 specifically mediates CXCL8 exocytosis\", \"Single lab, not independently confirmed\", \"Rab27A/SNARE dependence in this context not tested\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How SYTL4 switches between negative regulation (dense-core granules) and positive regulation (WPBs), and how its microtubule-destabilizing activity relates to its secretory scaffolding role, remain unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No structure of the Rab27A–Slp4-a–Munc18/syntaxin assembly\", \"Determinants of cell-type-specific directionality unknown\", \"Whether microtubule and exocytic functions are mutually exclusive untested\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [0, 3, 6]},\n      {\"term_id\": \"GO:0008092\", \"supporting_discovery_ids\": [8]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [0, 3, 5]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0031410\", \"supporting_discovery_ids\": [0, 7]},\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [3, 7]},\n      {\"term_id\": \"GO:0005856\", \"supporting_discovery_ids\": [8]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-5653656\", \"supporting_discovery_ids\": [0, 3, 5, 6]},\n      {\"term_id\": \"R-HSA-109582\", \"supporting_discovery_ids\": [5, 6]}\n    ],\n    \"complexes\": [\n      \"Rab27A-Slp4-a-Munc18-1-syntaxin-1a docking complex\",\n      \"Rab27A-Slp4-a-STXBP1-syntaxin-2/3 complex\"\n    ],\n    \"partners\": [\n      \"RAB27A\",\n      \"RAB8A\",\n      \"STX1A\",\n      \"STX2\",\n      \"STX3\",\n      \"STXBP1\",\n      \"MYRIP\"\n    ],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":5,"faith_total":5,"faith_pct":100.0}}