{"gene":"FCHSD2","run_date":"2026-06-09T23:54:43","timeline":{"discoveries":[{"year":2013,"finding":"FCHSD2 interacts with WASP and N-WASP (mammalian orthologs of Drosophila Wsp) and stimulates F-actin assembly in vitro via the Arp2/3 pathway.","method":"Co-immunoprecipitation/pulldown and in vitro F-actin polymerization assay","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct binding and functional in vitro actin assembly assay, single lab, two orthogonal methods","pmids":["23437151"],"is_preprint":false},{"year":2013,"finding":"FCHSD2 localizes along cochlear hair cell stereocilia in a punctate pattern, as determined by immunofluorescence in mouse cochlear tissue.","method":"Immunofluorescence localization in mouse cochlear hair cells","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — direct localization experiment in native tissue, single lab, replicated in follow-up studies","pmids":["23437151"],"is_preprint":false},{"year":2018,"finding":"ERK1/2 phosphorylates FCHSD2 to activate it, and this phosphorylation is required for ERK1/2-dependent regulation of clathrin-coated pit (CCP) initiation and clathrin-mediated endocytosis (CME) specifically in cancer cells; loss of FCHSD2 in NSCLC cells increases cell-surface EGFR expression and enhances cell proliferation and migration.","method":"Kinase inhibitor studies, siRNA/shRNA knockdown, live-cell imaging of CME dynamics, identification of FCHSD2 as ERK1/2 substrate","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 / Strong — ERK1/2 substrate identification combined with functional CME assays and phenotypic rescue, multiple orthogonal methods","pmids":["30249660"],"is_preprint":false},{"year":2020,"finding":"FCHSD2 controls recycling of EGFR and MET receptor tyrosine kinases; loss of FCHSD2 shunts these receptors into late endosomes/lysosomes for degradation and triggers nuclear translocation of active ERK1/2, leading to transcriptional upregulation of EGFR and MET. Rab7 is essential for the FCHSD2 depletion-induced trafficking effects.","method":"siRNA knockdown, live-cell imaging of endosomal trafficking, subcellular fractionation, epistasis with Rab7 co-depletion","journal":"PLoS biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (knockdown, trafficking assays, nuclear fractionation, genetic epistasis), single lab with rigorous controls","pmids":["32678845"],"is_preprint":false},{"year":2021,"finding":"FCHSD2 promotes apical and lateral cell protrusion (microvilli/filopodia) formation. Apical protrusion formation requires cooperation with CDC42 and N-WASP (a trimeric complex), whereas lateral protrusion formation is driven by the F-BAR domain of FCHSD2 independently of N-WASP. Both activities require FCHSD2's plasma membrane-binding ability.","method":"Overexpression and domain-deletion constructs in cultured cells, co-immunoprecipitation of FCHSD2-CDC42-N-WASP complex, morphological protrusion assays","journal":"Biochimica et biophysica acta. Molecular cell research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal Co-IP for complex formation plus functional protrusion assay, single lab, two orthogonal methods","pmids":["34520816"],"is_preprint":false},{"year":2021,"finding":"CRISPR-Cas9-mediated deletion of FCHSD2 in human EndoC-βH1 pancreatic β-cells impairs glucose-stimulated insulin secretion, identifying FCHSD2 as a regulator of insulin secretion in β-cells.","method":"CRISPR-Cas9 knockout in human EndoC-βH1 cells, glucose-stimulated insulin secretion assay","journal":"Cell reports","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — clean loss-of-function with defined secretory phenotype, single lab, single method","pmids":["33535042"],"is_preprint":false},{"year":2022,"finding":"FCHSD2 and CDC42 cooperatively maintain cochlear hair cell stereocilia; Fchsd2 knockout mice develop progressive hearing loss with severe stereocilia degeneration, and Fchsd2/Cdc42 double knockout mice show more severe deficits, establishing a cooperative genetic interaction.","method":"Fchsd2 knockout mouse generation, auditory brainstem response (ABR) measurements, scanning electron microscopy of stereocilia, Fchsd2/Cdc42 double knockout epistasis","journal":"Journal of cell science","confidence":"High","confidence_rationale":"Tier 2 / Strong — in vivo knockout with defined auditory phenotype, double-mutant epistasis, multiple readouts","pmids":["35892293"],"is_preprint":false},{"year":2022,"finding":"The C-terminal PDZ-binding motif of FCHSD2 directly binds the PDZ3 domain of deafness-related PDZD7, as confirmed by crystal structure at 2.0 Å resolution showing the FCHSD2 tail threading through the αB/βB groove of PDZD7 PDZ3.","method":"Yeast two-hybrid screening, Co-IP in COS-7 cells, X-ray crystallography (2.0 Å crystal structure)","journal":"The Biochemical journal","confidence":"High","confidence_rationale":"Tier 1 / Strong — atomic-resolution crystal structure with biochemical validation by orthogonal methods","pmids":["35695292"],"is_preprint":false},{"year":2024,"finding":"The endosomal scaffolding protein MICAL-L1 directly recruits FCHSD2 to the endosomal membrane, where FCHSD2 is required for ARP2/3-mediated branched actin generation, endosome fission, and receptor recycling to the plasma membrane. MICAL-L1 subsequently recruits EHD1 for nucleotide hydrolysis-driven fission, placing FCHSD2 upstream of EHD1 in the fission pathway.","method":"Co-immunoprecipitation, live-cell imaging, siRNA knockdown, endosome fission and receptor recycling assays","journal":"Molecular biology of the cell","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal Co-IP, pathway epistasis, functional endosomal fission and recycling assays, replicated in peer-reviewed publication matching preprint","pmids":["39382837"],"is_preprint":false}],"current_model":"FCHSD2 is an F-BAR/SH3 domain protein that is activated by ERK1/2 phosphorylation and functions at multiple membrane compartments: at the plasma membrane it promotes clathrin-coated pit initiation and clathrin-mediated endocytosis of EGFR and MET, cooperates with CDC42 and N-WASP to drive actin-dependent cell protrusion formation, and maintains cochlear hair cell stereocilia; at endosomes it is recruited by MICAL-L1 to drive ARP2/3-mediated branched actin assembly required for endosome fission and receptor recycling; its C-terminal PDZ-binding motif physically engages the PDZD7 PDZ3 domain; and in pancreatic β-cells it is required for glucose-stimulated insulin secretion."},"narrative":{"mechanistic_narrative":"FCHSD2 is an F-BAR/SH3 domain membrane-remodeling protein that couples membrane curvature sensing to Arp2/3-dependent branched actin assembly across multiple compartments [PMID:23437151, PMID:34520816]. It directly binds WASP/N-WASP and stimulates F-actin polymerization through the Arp2/3 pathway in vitro [PMID:23437151]. At the plasma membrane, FCHSD2 is activated by ERK1/2 phosphorylation to promote clathrin-coated pit initiation and clathrin-mediated endocytosis in cancer cells, and its loss raises cell-surface EGFR and enhances proliferation and migration [PMID:30249660]. Beyond internalization, FCHSD2 governs the post-endocytic fate of EGFR and MET: it directs their recycling, and its depletion shunts these receptors into Rab7-dependent late endosomal/lysosomal degradation while driving nuclear ERK1/2 translocation and transcriptional upregulation of both receptors [PMID:32678845]. At the endosome, FCHSD2 is directly recruited by the scaffold MICAL-L1 to generate Arp2/3-mediated branched actin required for endosome fission and receptor recycling, acting upstream of EHD1 in the fission pathway [PMID:39382837]. FCHSD2 also builds cell-surface protrusions: apical microvilli/filopodia formation requires a trimeric complex with CDC42 and N-WASP, while lateral protrusions depend on its F-BAR domain independently of N-WASP, and both require its plasma-membrane binding [PMID:34520816]. In vivo, FCHSD2 localizes along cochlear hair cell stereocilia and cooperates genetically with CDC42 to maintain them, with Fchsd2 knockout mice developing progressive hearing loss and stereocilia degeneration [PMID:23437151, PMID:35892293]. Its C-terminal PDZ-binding motif directly engages the PDZ3 domain of the deafness-related protein PDZD7, resolved at atomic resolution [PMID:35695292]. FCHSD2 is additionally required for glucose-stimulated insulin secretion in human pancreatic β-cells [PMID:33535042].","teleology":[{"year":2013,"claim":"Established FCHSD2 as an actin-nucleation effector by showing it binds the WASP/N-WASP machinery and stimulates Arp2/3-dependent F-actin assembly, defining its core biochemical activity.","evidence":"Co-IP/pulldown and in vitro F-actin polymerization assay; immunofluorescence in mouse cochlear hair cells","pmids":["23437151"],"confidence":"Medium","gaps":["Did not define which membrane compartments host this activity in vivo","No domain mapping of the FCHSD2-N-WASP interaction"]},{"year":2018,"claim":"Resolved how FCHSD2 activity is controlled and where it acts at the cell surface, identifying it as an ERK1/2 substrate whose phosphorylation gates clathrin-coated pit initiation and CME in cancer cells.","evidence":"Kinase inhibitor studies, siRNA/shRNA knockdown, live-cell CME imaging, ERK1/2 substrate identification in NSCLC cells","pmids":["30249660"],"confidence":"High","gaps":["Phosphosite(s) and how phosphorylation changes FCHSD2 conformation/activity not defined","Cancer-cell specificity of the ERK1/2-FCHSD2 axis not mechanistically explained"]},{"year":2020,"claim":"Extended FCHSD2 function beyond uptake to receptor recycling, showing it prevents EGFR/MET degradation and that its loss triggers a Rab7-dependent rerouting to lysosomes plus a nuclear ERK1/2 feedback loop upregulating the receptors.","evidence":"siRNA knockdown, live-cell endosomal trafficking imaging, subcellular fractionation, Rab7 co-depletion epistasis","pmids":["32678845"],"confidence":"High","gaps":["Molecular link between FCHSD2 loss and ERK1/2 nuclear translocation not resolved","Did not identify the endosomal machinery FCHSD2 acts with"]},{"year":2021,"claim":"Dissected FCHSD2's role in cell-surface morphogenesis, separating an N-WASP/CDC42-dependent apical protrusion activity from an F-BAR-domain-driven, N-WASP-independent lateral protrusion activity.","evidence":"Domain-deletion/overexpression constructs, reciprocal Co-IP of FCHSD2-CDC42-N-WASP complex, protrusion morphology assays","pmids":["34520816"],"confidence":"Medium","gaps":["Performed largely by overexpression; endogenous contribution unquantified","Stoichiometry and assembly order of the trimeric complex unknown"]},{"year":2021,"claim":"Identified a physiological role in metabolic secretion, showing FCHSD2 is required for glucose-stimulated insulin secretion in human β-cells.","evidence":"CRISPR-Cas9 knockout in EndoC-βH1 cells with glucose-stimulated insulin secretion assay","pmids":["33535042"],"confidence":"Medium","gaps":["Single method; the membrane-trafficking step FCHSD2 controls in β-cells not defined","Not linked to its endocytic or actin functions mechanistically"]},{"year":2022,"claim":"Provided in vivo proof that FCHSD2 maintains sensory hair cell structure, with knockout mice showing progressive hearing loss and stereocilia degeneration and a cooperative genetic interaction with CDC42.","evidence":"Fchsd2 knockout mice, ABR measurements, scanning EM of stereocilia, Fchsd2/Cdc42 double-knockout epistasis","pmids":["35892293"],"confidence":"High","gaps":["Molecular mechanism by which FCHSD2 sustains stereocilia actin not defined","Whether the cochlear role uses the same N-WASP/Arp2/3 pathway as in cultured cells unclear"]},{"year":2022,"claim":"Defined an atomic-resolution interaction linking FCHSD2 to deafness machinery, showing its C-terminal PDZ-binding motif threads through the PDZ3 groove of PDZD7.","evidence":"Yeast two-hybrid, Co-IP in COS-7 cells, 2.0 Å X-ray crystal structure","pmids":["35695292"],"confidence":"High","gaps":["Functional consequence of the FCHSD2-PDZD7 interaction in vivo not tested","Whether PDZD7 localizes FCHSD2 to stereocilia not established"]},{"year":2024,"claim":"Placed FCHSD2 within the endosomal fission pathway, showing MICAL-L1 directly recruits it to drive Arp2/3 branched actin for endosome fission and recycling, acting upstream of EHD1.","evidence":"Reciprocal Co-IP, live-cell imaging, siRNA knockdown, endosome fission and receptor recycling assays","pmids":["39382837"],"confidence":"High","gaps":["How branched actin mechanically couples to EHD1-driven scission not resolved","Whether ERK1/2 phosphorylation regulates the endosomal pool of FCHSD2 unknown"]},{"year":null,"claim":"How a single F-BAR/SH3 protein is partitioned between plasma-membrane CME, protrusion formation, and endosomal fission, and whether ERK1/2 phosphorylation coordinates these pools, remains unresolved.","evidence":"","pmids":[],"confidence":"High","gaps":["No unified model of FCHSD2 compartment targeting","Regulatory inputs beyond ERK1/2 unidentified","Phosphosite-level control across compartments undefined"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0008092","term_label":"cytoskeletal protein binding","supporting_discovery_ids":[0,8]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[0,4,8]},{"term_id":"GO:0008289","term_label":"lipid binding","supporting_discovery_ids":[4]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[2,4]},{"term_id":"GO:0005768","term_label":"endosome","supporting_discovery_ids":[3,8]},{"term_id":"GO:0005856","term_label":"cytoskeleton","supporting_discovery_ids":[1,6]}],"pathway":[{"term_id":"R-HSA-5653656","term_label":"Vesicle-mediated transport","supporting_discovery_ids":[2,8]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[2,3]},{"term_id":"R-HSA-9709957","term_label":"Sensory Perception","supporting_discovery_ids":[6,7]}],"complexes":["FCHSD2-CDC42-N-WASP trimeric complex"],"partners":["WASL","WAS","CDC42","MICALL1","PDZD7"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"O94868","full_name":"F-BAR and double SH3 domains protein 2","aliases":["Carom","Protein nervous wreck 1","NWK1","SH3 multiple domains protein 3"],"length_aa":740,"mass_kda":84.3,"function":"Adapter protein that plays a role in endocytosis via clathrin-coated pits. Contributes to the internalization of cell surface receptors, such as integrin ITGB1 and transferrin receptor (PubMed:29887380). Promotes endocytosis of EGFR in cancer cells, and thereby contributes to the down-regulation of EGFR signaling (PubMed:30249660). Recruited to clathrin-coated pits during a mid-to-late stage of assembly, where it is required for normal progress from U-shaped intermediate stage pits to terminal, omega-shaped pits (PubMed:29887380). Binds to membranes enriched in phosphatidylinositol 3,4-bisphosphate or phosphatidylinositol 3,4,5-trisphosphate (PubMed:29887380). When bound to membranes, promotes actin polymerization via its interaction with WAS and/or WASL which leads to the activation of the Arp2/3 complex. Does not promote actin polymerisation in the absence of membranes (PubMed:29887380)","subcellular_location":"Cytoplasm; Cell junction; Membrane, clathrin-coated pit; Cell membrane; Cell projection, stereocilium","url":"https://www.uniprot.org/uniprotkb/O94868/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/FCHSD2","classification":"Not Classified","n_dependent_lines":2,"n_total_lines":1208,"dependency_fraction":0.0016556291390728477},"opencell":{"profiled":true,"resolved_as":"","ensg_id":"ENSG00000137478","cell_line_id":"CID000664","localizations":[{"compartment":"cytoplasmic","grade":3},{"compartment":"membrane","grade":2},{"compartment":"cell_contact","grade":1}],"interactors":[],"url":"https://opencell.sf.czbiohub.org/target/CID000664","total_profiled":1310},"omim":[{"mim_id":"619714","title":"CONGENITAL DISORDER OF GLYCOSYLATION, TYPE Iw, AUTOSOMAL DOMINANT; CDG1WAD","url":"https://www.omim.org/entry/619714"},{"mim_id":"617556","title":"FCH AND DOUBLE SH3 DOMAINS PROTEIN 2; FCHSD2","url":"https://www.omim.org/entry/617556"},{"mim_id":"617555","title":"FCH AND DOUBLE SH3 DOMAINS PROTEIN 1; FCHSD1","url":"https://www.omim.org/entry/617555"},{"mim_id":"615596","title":"CONGENITAL DISORDER OF GLYCOSYLATION, TYPE Iw, AUTOSOMAL RECESSIVE; CDG1WAR","url":"https://www.omim.org/entry/615596"},{"mim_id":"611565","title":"BRIDGE-LIKE LIPID TRANSFER PROTEIN FAMILY, MEMBER 1; BLTP1","url":"https://www.omim.org/entry/611565"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Plasma membrane","reliability":"Approved"},{"location":"Actin filaments","reliability":"Approved"},{"location":"Nuclear speckles","reliability":"Additional"},{"location":"Focal adhesion sites","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/FCHSD2"},"hgnc":{"alias_symbol":["KIAA0769"],"prev_symbol":["SH3MD3"]},"alphafold":{"accession":"O94868","domains":[{"cath_id":"1.20.1270.60","chopping":"18-268","consensus_level":"medium","plddt":92.8315,"start":18,"end":268},{"cath_id":"2.30.30.40","chopping":"471-529","consensus_level":"high","plddt":86.1854,"start":471,"end":529},{"cath_id":"2.30.30.40","chopping":"570-626","consensus_level":"high","plddt":83.0344,"start":570,"end":626},{"cath_id":"1.20.58","chopping":"318-422","consensus_level":"high","plddt":87.5899,"start":318,"end":422}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/O94868","model_url":"https://alphafold.ebi.ac.uk/files/AF-O94868-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-O94868-F1-predicted_aligned_error_v6.png","plddt_mean":74.94},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=FCHSD2","jax_strain_url":"https://www.jax.org/strain/search?query=FCHSD2"},"sequence":{"accession":"O94868","fasta_url":"https://rest.uniprot.org/uniprotkb/O94868.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/O94868/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/O94868"}},"corpus_meta":[{"pmid":"23437151","id":"PMC_23437151","title":"FCHSD1 and FCHSD2 are expressed in hair cell stereocilia and cuticular plate and regulate actin polymerization in vitro.","date":"2013","source":"PloS one","url":"https://pubmed.ncbi.nlm.nih.gov/23437151","citation_count":35,"is_preprint":false},{"pmid":"30249660","id":"PMC_30249660","title":"Role for ERK1/2-dependent activation of FCHSD2 in cancer cell-selective regulation of clathrin-mediated endocytosis.","date":"2018","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/30249660","citation_count":34,"is_preprint":false},{"pmid":"15067381","id":"PMC_15067381","title":"Identification and characterization of human FCHSD1 and FCHSD2 genes in silico.","date":"2004","source":"International journal of molecular medicine","url":"https://pubmed.ncbi.nlm.nih.gov/15067381","citation_count":33,"is_preprint":false},{"pmid":"32678845","id":"PMC_32678845","title":"FCHSD2 controls oncogenic ERK1/2 signaling outcome by regulating endocytic trafficking.","date":"2020","source":"PLoS biology","url":"https://pubmed.ncbi.nlm.nih.gov/32678845","citation_count":17,"is_preprint":false},{"pmid":"22902056","id":"PMC_22902056","title":"FCHSD2 predicts response to chemotherapy in acute myeloid leukemia patients.","date":"2012","source":"Leukemia research","url":"https://pubmed.ncbi.nlm.nih.gov/22902056","citation_count":9,"is_preprint":false},{"pmid":"33535042","id":"PMC_33535042","title":"Chromatin 3D interaction analysis of the STARD10 locus unveils FCHSD2 as a regulator of insulin secretion.","date":"2021","source":"Cell reports","url":"https://pubmed.ncbi.nlm.nih.gov/33535042","citation_count":8,"is_preprint":false},{"pmid":"34520816","id":"PMC_34520816","title":"FCHSD2 cooperates with CDC42 and N-WASP to regulate cell protrusion formation.","date":"2021","source":"Biochimica et biophysica acta. Molecular cell research","url":"https://pubmed.ncbi.nlm.nih.gov/34520816","citation_count":7,"is_preprint":false},{"pmid":"39382837","id":"PMC_39382837","title":"Endosomal actin branching, fission, and receptor recycling require FCHSD2 recruitment by MICAL-L1.","date":"2024","source":"Molecular biology of the cell","url":"https://pubmed.ncbi.nlm.nih.gov/39382837","citation_count":4,"is_preprint":false},{"pmid":"35892293","id":"PMC_35892293","title":"FCHSD2 is required for stereocilia maintenance in mouse cochlear hair cells.","date":"2022","source":"Journal of cell science","url":"https://pubmed.ncbi.nlm.nih.gov/35892293","citation_count":4,"is_preprint":false},{"pmid":"35695292","id":"PMC_35695292","title":"Deafness-related protein PDZD7 forms complex with the C-terminal tail of FCHSD2.","date":"2022","source":"The Biochemical journal","url":"https://pubmed.ncbi.nlm.nih.gov/35695292","citation_count":2,"is_preprint":false},{"pmid":"38979241","id":"PMC_38979241","title":"Endosomal actin branching, fission and receptor recycling require FCHSD2 recruitment by MICAL-L1.","date":"2024","source":"bioRxiv : the preprint server for biology","url":"https://pubmed.ncbi.nlm.nih.gov/38979241","citation_count":0,"is_preprint":false},{"pmid":"37224049","id":"PMC_37224049","title":"IRF3-mediated lncRNA FTX promotes cell proliferation, migration, invasion and suppresses cell apoptosis in oral squamous cell carcinoma by up-regulating FCHSD2 via miR-708-5p.","date":"2023","source":"Cellular and molecular biology (Noisy-le-Grand, France)","url":"https://pubmed.ncbi.nlm.nih.gov/37224049","citation_count":0,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":8267,"output_tokens":2251,"usd":0.029283,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":9314,"output_tokens":3536,"usd":0.067485,"stage2_stop_reason":"end_turn"},"total_usd":0.096768,"stage1_batch_id":"msgbatch_01AsR7g8dGTEiEw9U83bJeWn","stage2_batch_id":"msgbatch_01YV5ZyxtFNUZEjxRcpd9jzb","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2013,\n      \"finding\": \"FCHSD2 interacts with WASP and N-WASP (mammalian orthologs of Drosophila Wsp) and stimulates F-actin assembly in vitro via the Arp2/3 pathway.\",\n      \"method\": \"Co-immunoprecipitation/pulldown and in vitro F-actin polymerization assay\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct binding and functional in vitro actin assembly assay, single lab, two orthogonal methods\",\n      \"pmids\": [\"23437151\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"FCHSD2 localizes along cochlear hair cell stereocilia in a punctate pattern, as determined by immunofluorescence in mouse cochlear tissue.\",\n      \"method\": \"Immunofluorescence localization in mouse cochlear hair cells\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — direct localization experiment in native tissue, single lab, replicated in follow-up studies\",\n      \"pmids\": [\"23437151\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"ERK1/2 phosphorylates FCHSD2 to activate it, and this phosphorylation is required for ERK1/2-dependent regulation of clathrin-coated pit (CCP) initiation and clathrin-mediated endocytosis (CME) specifically in cancer cells; loss of FCHSD2 in NSCLC cells increases cell-surface EGFR expression and enhances cell proliferation and migration.\",\n      \"method\": \"Kinase inhibitor studies, siRNA/shRNA knockdown, live-cell imaging of CME dynamics, identification of FCHSD2 as ERK1/2 substrate\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — ERK1/2 substrate identification combined with functional CME assays and phenotypic rescue, multiple orthogonal methods\",\n      \"pmids\": [\"30249660\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"FCHSD2 controls recycling of EGFR and MET receptor tyrosine kinases; loss of FCHSD2 shunts these receptors into late endosomes/lysosomes for degradation and triggers nuclear translocation of active ERK1/2, leading to transcriptional upregulation of EGFR and MET. Rab7 is essential for the FCHSD2 depletion-induced trafficking effects.\",\n      \"method\": \"siRNA knockdown, live-cell imaging of endosomal trafficking, subcellular fractionation, epistasis with Rab7 co-depletion\",\n      \"journal\": \"PLoS biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (knockdown, trafficking assays, nuclear fractionation, genetic epistasis), single lab with rigorous controls\",\n      \"pmids\": [\"32678845\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"FCHSD2 promotes apical and lateral cell protrusion (microvilli/filopodia) formation. Apical protrusion formation requires cooperation with CDC42 and N-WASP (a trimeric complex), whereas lateral protrusion formation is driven by the F-BAR domain of FCHSD2 independently of N-WASP. Both activities require FCHSD2's plasma membrane-binding ability.\",\n      \"method\": \"Overexpression and domain-deletion constructs in cultured cells, co-immunoprecipitation of FCHSD2-CDC42-N-WASP complex, morphological protrusion assays\",\n      \"journal\": \"Biochimica et biophysica acta. Molecular cell research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal Co-IP for complex formation plus functional protrusion assay, single lab, two orthogonal methods\",\n      \"pmids\": [\"34520816\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"CRISPR-Cas9-mediated deletion of FCHSD2 in human EndoC-βH1 pancreatic β-cells impairs glucose-stimulated insulin secretion, identifying FCHSD2 as a regulator of insulin secretion in β-cells.\",\n      \"method\": \"CRISPR-Cas9 knockout in human EndoC-βH1 cells, glucose-stimulated insulin secretion assay\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — clean loss-of-function with defined secretory phenotype, single lab, single method\",\n      \"pmids\": [\"33535042\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"FCHSD2 and CDC42 cooperatively maintain cochlear hair cell stereocilia; Fchsd2 knockout mice develop progressive hearing loss with severe stereocilia degeneration, and Fchsd2/Cdc42 double knockout mice show more severe deficits, establishing a cooperative genetic interaction.\",\n      \"method\": \"Fchsd2 knockout mouse generation, auditory brainstem response (ABR) measurements, scanning electron microscopy of stereocilia, Fchsd2/Cdc42 double knockout epistasis\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — in vivo knockout with defined auditory phenotype, double-mutant epistasis, multiple readouts\",\n      \"pmids\": [\"35892293\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"The C-terminal PDZ-binding motif of FCHSD2 directly binds the PDZ3 domain of deafness-related PDZD7, as confirmed by crystal structure at 2.0 Å resolution showing the FCHSD2 tail threading through the αB/βB groove of PDZD7 PDZ3.\",\n      \"method\": \"Yeast two-hybrid screening, Co-IP in COS-7 cells, X-ray crystallography (2.0 Å crystal structure)\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — atomic-resolution crystal structure with biochemical validation by orthogonal methods\",\n      \"pmids\": [\"35695292\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"The endosomal scaffolding protein MICAL-L1 directly recruits FCHSD2 to the endosomal membrane, where FCHSD2 is required for ARP2/3-mediated branched actin generation, endosome fission, and receptor recycling to the plasma membrane. MICAL-L1 subsequently recruits EHD1 for nucleotide hydrolysis-driven fission, placing FCHSD2 upstream of EHD1 in the fission pathway.\",\n      \"method\": \"Co-immunoprecipitation, live-cell imaging, siRNA knockdown, endosome fission and receptor recycling assays\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal Co-IP, pathway epistasis, functional endosomal fission and recycling assays, replicated in peer-reviewed publication matching preprint\",\n      \"pmids\": [\"39382837\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"FCHSD2 is an F-BAR/SH3 domain protein that is activated by ERK1/2 phosphorylation and functions at multiple membrane compartments: at the plasma membrane it promotes clathrin-coated pit initiation and clathrin-mediated endocytosis of EGFR and MET, cooperates with CDC42 and N-WASP to drive actin-dependent cell protrusion formation, and maintains cochlear hair cell stereocilia; at endosomes it is recruited by MICAL-L1 to drive ARP2/3-mediated branched actin assembly required for endosome fission and receptor recycling; its C-terminal PDZ-binding motif physically engages the PDZD7 PDZ3 domain; and in pancreatic β-cells it is required for glucose-stimulated insulin secretion.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"FCHSD2 is an F-BAR/SH3 domain membrane-remodeling protein that couples membrane curvature sensing to Arp2/3-dependent branched actin assembly across multiple compartments [#0, #4]. It directly binds WASP/N-WASP and stimulates F-actin polymerization through the Arp2/3 pathway in vitro [#0]. At the plasma membrane, FCHSD2 is activated by ERK1/2 phosphorylation to promote clathrin-coated pit initiation and clathrin-mediated endocytosis in cancer cells, and its loss raises cell-surface EGFR and enhances proliferation and migration [#2]. Beyond internalization, FCHSD2 governs the post-endocytic fate of EGFR and MET: it directs their recycling, and its depletion shunts these receptors into Rab7-dependent late endosomal/lysosomal degradation while driving nuclear ERK1/2 translocation and transcriptional upregulation of both receptors [#3]. At the endosome, FCHSD2 is directly recruited by the scaffold MICAL-L1 to generate Arp2/3-mediated branched actin required for endosome fission and receptor recycling, acting upstream of EHD1 in the fission pathway [#8]. FCHSD2 also builds cell-surface protrusions: apical microvilli/filopodia formation requires a trimeric complex with CDC42 and N-WASP, while lateral protrusions depend on its F-BAR domain independently of N-WASP, and both require its plasma-membrane binding [#4]. In vivo, FCHSD2 localizes along cochlear hair cell stereocilia and cooperates genetically with CDC42 to maintain them, with Fchsd2 knockout mice developing progressive hearing loss and stereocilia degeneration [#1, #6]. Its C-terminal PDZ-binding motif directly engages the PDZ3 domain of the deafness-related protein PDZD7, resolved at atomic resolution [#7]. FCHSD2 is additionally required for glucose-stimulated insulin secretion in human pancreatic \\u03b2-cells [#5].\",\n  \"teleology\": [\n    {\n      \"year\": 2013,\n      \"claim\": \"Established FCHSD2 as an actin-nucleation effector by showing it binds the WASP/N-WASP machinery and stimulates Arp2/3-dependent F-actin assembly, defining its core biochemical activity.\",\n      \"evidence\": \"Co-IP/pulldown and in vitro F-actin polymerization assay; immunofluorescence in mouse cochlear hair cells\",\n      \"pmids\": [\"23437151\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Did not define which membrane compartments host this activity in vivo\", \"No domain mapping of the FCHSD2-N-WASP interaction\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Resolved how FCHSD2 activity is controlled and where it acts at the cell surface, identifying it as an ERK1/2 substrate whose phosphorylation gates clathrin-coated pit initiation and CME in cancer cells.\",\n      \"evidence\": \"Kinase inhibitor studies, siRNA/shRNA knockdown, live-cell CME imaging, ERK1/2 substrate identification in NSCLC cells\",\n      \"pmids\": [\"30249660\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Phosphosite(s) and how phosphorylation changes FCHSD2 conformation/activity not defined\", \"Cancer-cell specificity of the ERK1/2-FCHSD2 axis not mechanistically explained\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Extended FCHSD2 function beyond uptake to receptor recycling, showing it prevents EGFR/MET degradation and that its loss triggers a Rab7-dependent rerouting to lysosomes plus a nuclear ERK1/2 feedback loop upregulating the receptors.\",\n      \"evidence\": \"siRNA knockdown, live-cell endosomal trafficking imaging, subcellular fractionation, Rab7 co-depletion epistasis\",\n      \"pmids\": [\"32678845\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Molecular link between FCHSD2 loss and ERK1/2 nuclear translocation not resolved\", \"Did not identify the endosomal machinery FCHSD2 acts with\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Dissected FCHSD2's role in cell-surface morphogenesis, separating an N-WASP/CDC42-dependent apical protrusion activity from an F-BAR-domain-driven, N-WASP-independent lateral protrusion activity.\",\n      \"evidence\": \"Domain-deletion/overexpression constructs, reciprocal Co-IP of FCHSD2-CDC42-N-WASP complex, protrusion morphology assays\",\n      \"pmids\": [\"34520816\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Performed largely by overexpression; endogenous contribution unquantified\", \"Stoichiometry and assembly order of the trimeric complex unknown\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Identified a physiological role in metabolic secretion, showing FCHSD2 is required for glucose-stimulated insulin secretion in human \\u03b2-cells.\",\n      \"evidence\": \"CRISPR-Cas9 knockout in EndoC-\\u03b2H1 cells with glucose-stimulated insulin secretion assay\",\n      \"pmids\": [\"33535042\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Single method; the membrane-trafficking step FCHSD2 controls in \\u03b2-cells not defined\", \"Not linked to its endocytic or actin functions mechanistically\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Provided in vivo proof that FCHSD2 maintains sensory hair cell structure, with knockout mice showing progressive hearing loss and stereocilia degeneration and a cooperative genetic interaction with CDC42.\",\n      \"evidence\": \"Fchsd2 knockout mice, ABR measurements, scanning EM of stereocilia, Fchsd2/Cdc42 double-knockout epistasis\",\n      \"pmids\": [\"35892293\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Molecular mechanism by which FCHSD2 sustains stereocilia actin not defined\", \"Whether the cochlear role uses the same N-WASP/Arp2/3 pathway as in cultured cells unclear\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Defined an atomic-resolution interaction linking FCHSD2 to deafness machinery, showing its C-terminal PDZ-binding motif threads through the PDZ3 groove of PDZD7.\",\n      \"evidence\": \"Yeast two-hybrid, Co-IP in COS-7 cells, 2.0 \\u00c5 X-ray crystal structure\",\n      \"pmids\": [\"35695292\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Functional consequence of the FCHSD2-PDZD7 interaction in vivo not tested\", \"Whether PDZD7 localizes FCHSD2 to stereocilia not established\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Placed FCHSD2 within the endosomal fission pathway, showing MICAL-L1 directly recruits it to drive Arp2/3 branched actin for endosome fission and recycling, acting upstream of EHD1.\",\n      \"evidence\": \"Reciprocal Co-IP, live-cell imaging, siRNA knockdown, endosome fission and receptor recycling assays\",\n      \"pmids\": [\"39382837\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"How branched actin mechanically couples to EHD1-driven scission not resolved\", \"Whether ERK1/2 phosphorylation regulates the endosomal pool of FCHSD2 unknown\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How a single F-BAR/SH3 protein is partitioned between plasma-membrane CME, protrusion formation, and endosomal fission, and whether ERK1/2 phosphorylation coordinates these pools, remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"No unified model of FCHSD2 compartment targeting\", \"Regulatory inputs beyond ERK1/2 unidentified\", \"Phosphosite-level control across compartments undefined\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0008092\", \"supporting_discovery_ids\": [0, 8]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [0, 4, 8]},\n      {\"term_id\": \"GO:0008289\", \"supporting_discovery_ids\": [4]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [2, 4]},\n      {\"term_id\": \"GO:0005768\", \"supporting_discovery_ids\": [3, 8]},\n      {\"term_id\": \"GO:0005856\", \"supporting_discovery_ids\": [1, 6]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-5653656\", \"supporting_discovery_ids\": [2, 8]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [2, 3]},\n      {\"term_id\": \"R-HSA-9709957\", \"supporting_discovery_ids\": [6, 7]}\n    ],\n    \"complexes\": [\"FCHSD2-CDC42-N-WASP trimeric complex\"],\n    \"partners\": [\"WASL\", \"WAS\", \"CDC42\", \"MICALL1\", \"PDZD7\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":9,"faith_total":9,"faith_pct":100.0}}