{"gene":"FAT4","run_date":"2026-04-28T17:46:03","timeline":{"discoveries":[{"year":2008,"finding":"Fat4 is required for planar cell polarity (PCP) signaling and oriented cell divisions during kidney tubule elongation; loss of Fat4 disrupts PCP and leads to cystic kidney disease. Fat4 genetically interacts with PCP genes Vangl2 and Fjx1, and Fat4 represses Fjx1 expression, indicating conservation of Fat signaling.","method":"Gene-targeted knockout mouse, genetic epistasis (Fat4;Vangl2, Fat4;Fjx1 double mutants), oriented cell division analysis","journal":"Nature genetics","confidence":"High","confidence_rationale":"Tier 2 — clean KO with defined cellular phenotype plus genetic epistasis, replicated across multiple loci","pmids":["18604206"],"is_preprint":false},{"year":2011,"finding":"Dchs1 and Fat4 function as a ligand-receptor pair during murine development. Mutation of either gene increases protein staining for the other, and single/double mutants display similar phenotypes across multiple organs (ear, kidney, skeleton, intestine, heart, lung), indicating they act in the same pathway.","method":"Gene-targeted knockout mouse (Dchs1 mutant), phenotypic comparison with Fat4 mutants, immunostaining","journal":"Development (Cambridge, England)","confidence":"High","confidence_rationale":"Tier 2 — reciprocal genetic interaction and protein stabilization shown across multiple organs, replicated","pmids":["21303848"],"is_preprint":false},{"year":2013,"finding":"FAT4 and DCHS1 (receptor-ligand cadherin pair) regulate cerebral cortical neurogenesis; loss of either increases neural progenitor numbers and reduces differentiation. These effects were countered by concurrent knockdown of Yap, placing Dchs1 and Fat4 upstream of Yap in the Hippo signaling pathway during neurogenesis.","method":"Mouse embryonic neuroepithelium knockdown, genetic epistasis with Yap knockdown, cell counting assays","journal":"Nature genetics","confidence":"High","confidence_rationale":"Tier 2 — loss-of-function with defined cellular phenotype plus genetic epistasis with Yap, replicated in human mutations","pmids":["24056717"],"is_preprint":false},{"year":2014,"finding":"Fat4 and Dchs1 are expressed in complementary gradients and are required for collective tangential migration of facial branchiomotor (FBM) neurons and their planar cell polarity. Fat4 and Dchs1 act intrinsically within FBM neurons and extrinsically in the neuroepithelium. Fat-PCP and Frizzled-PCP regulate FBM migration along orthogonal axes.","method":"Conditional knockout mouse, mosaic inactivation, cell polarity analysis, genetic interaction studies","journal":"Current biology : CB","confidence":"High","confidence_rationale":"Tier 2 — tissue-specific knockouts with defined cellular phenotype and orthogonal axis demonstration","pmids":["24998526"],"is_preprint":false},{"year":2015,"finding":"FAT4 acts non-autonomously in the renal stroma to control nephron progenitor (cap mesenchyme) self-renewal. Loss of Yap from cap mesenchyme in Fat4-null mice does not rescue the expanded progenitor pool, demonstrating FAT4 regulates cap mesenchyme independently of YAP. Excess progenitors in Fat4 mutants depend on Six2 (genetic epistasis). Dchs1 and its paralogue Dchs2 function in the cap mesenchyme to restrict progenitor numbers.","method":"Tissue-specific conditional knockout, Fat4/Six2 double mutants, electron microscopy, gene expression analysis","journal":"Development (Cambridge, England)","confidence":"High","confidence_rationale":"Tier 2 — tissue-specific KO with cell-autonomous/non-autonomous dissection, plus genetic epistasis","pmids":["26116661"],"is_preprint":false},{"year":2015,"finding":"Dchs1 protein localizes in a polarized manner within cap mesenchyme cells, accumulating at the interface with stromal cells, indicating direct interaction with a stromal protein (Fat4). Dchs1 mutation reduces ureteric bud branching and impairs nephron morphogenesis and is required within cap mesenchyme cells.","method":"Antibody staining of genetic mosaics, conditional knockout, polarization analysis","journal":"Development (Cambridge, England)","confidence":"High","confidence_rationale":"Tier 2 — polarized localization shown in genetic mosaics, tissue-specific knockouts define cell-autonomous requirement","pmids":["26116666"],"is_preprint":false},{"year":2015,"finding":"Fat1 and Fat4 interact genetically to regulate cranial neural tube closure, cortical precursor proliferation, and apical constriction. Proteomic analysis reveals Fat1 and Fat4 bind different sets of actin-regulating and junctional proteins. In vitro data show Fat1 and Fat4 form cis-heterodimers, providing a mechanism for coordinating diverse interactors at apical junctions.","method":"Mouse knockout genetics, in utero electroporation knockdown, proteomic analysis, co-IP/pulldown for cis-heterodimer","journal":"Development (Cambridge, England)","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods including proteomics, genetic interaction, and in vitro heterodimer demonstration","pmids":["26209645"],"is_preprint":false},{"year":2016,"finding":"Dchs1-Fat4 PCP pathway controls cell orientation within the early skeletal condensation to define shape and dimensions of the mouse sternum, driving cell intercalation. This is the first demonstration that Fat4 and Dchs1 establish polarized cell behavior intrinsically within mesenchyme.","method":"Knockout mouse analysis, cell orientation measurements, live imaging of mesenchymal cell polarity","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 2 — KO with defined cellular polarity phenotype plus first demonstration of intrinsic mesenchymal PCP","pmids":["27145737"],"is_preprint":false},{"year":2016,"finding":"Fat4-Dchs1 regulates vertebral development by controlling cell proliferation in the early sclerotome independently of Yap and Taz. Genetic analysis of Fat4;Yap and Fat4;Taz double mutants and expression of transcriptional target Ctgf indicates Fat4-Dchs1 signaling uses a Hippo-independent mechanism for vertebral proliferation.","method":"Fat4/Dchs1 knockout mice, Fat4;Yap and Fat4;Taz double mutants, Ctgf expression analysis","journal":"Development (Cambridge, England)","confidence":"High","confidence_rationale":"Tier 2 — double mutant epistasis demonstrates Yap/Taz-independent mechanism","pmids":["27381226"],"is_preprint":false},{"year":2017,"finding":"In the mouse heart, Fat4 modulates Hippo signaling to restrict cardiomyocyte growth and proliferation. Fat4 is not required for canonical Hippo kinase activation but sequesters Amotl1 out of the nucleus. Nuclear translocation of Amotl1 accompanies Yap1 to promote cardiomyocyte proliferation, identifying Amotl1 as a mammalian-specific intermediate for non-canonical Hippo signaling downstream of Fat4.","method":"Fat4 knockout mouse, cardiomyocyte size/proliferation assays, subcellular fractionation of Amotl1/Yap1, Hippo kinase activity assays","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 2 — KO with defined phenotype, nuclear sequestration mechanism with multiple orthogonal methods","pmids":["28239148"],"is_preprint":false},{"year":2017,"finding":"Fat4-Ds1 (Dchs1) complexes accumulate at cell boundaries in a threshold-like manner and exhibit dramatically slower dynamics than unbound Fat4 and Ds1, indicating a localized feedback mechanism based on enhanced stability of Fat4-Ds1 complexes. Co-expression of Fat4 and Ds1 in the same cells is sufficient to induce polarization of Fat4-Ds1 complexes.","method":"Synthetic biology platform with mammalian cells expressing human Fat4 and Ds1, quantitative live imaging, FRAP","journal":"eLife","confidence":"High","confidence_rationale":"Tier 1–2 — reconstituted system with quantitative live imaging and biophysical measurements","pmids":["28826487"],"is_preprint":false},{"year":2017,"finding":"Fat4 and Dachsous1 are specifically required for lymphatic valve morphogenesis. Valve endothelial cells are disoriented and fail to form proper valve leaflets in Fat4 and Dachsous1 knockout mice. Dachsous1 is polarized to membrane protrusions and cellular junctions of valve endothelial cells.","method":"Fat4 and Dachsous1 knockout mouse, Lifeact-GFP live imaging, immunostaining of polarized Dchs1 localization","journal":"Arteriosclerosis, thrombosis, and vascular biology","confidence":"High","confidence_rationale":"Tier 2 — KO with defined cellular phenotype plus polarized localization demonstrated in vivo and in vitro","pmids":["28705793"],"is_preprint":false},{"year":2019,"finding":"FAT4 directly interacts with RET (tyrosine kinase receptor) through extracellular cadherin repeats. FAT4 expression perturbs assembly of the RET-GFRA1-GDNF complex, reducing RET signaling. Removal of one copy of Gdnf rescues Fat4−/− kidney development, supporting that FAT4 fine-tunes RET signaling via a juxtacrine mechanism.","method":"Co-immunoprecipitation, conditional knockout analysis, Gdnf heterozygous rescue genetics, RET signaling assays","journal":"Developmental cell","confidence":"High","confidence_rationale":"Tier 2 — direct binding shown by Co-IP, functional rescue by genetic epistasis, multiple orthogonal methods","pmids":["30853441"],"is_preprint":false},{"year":2019,"finding":"Dchs1-Fat4 signaling is essential for osteoblast differentiation. Loss of Dchs1-Fat4 signaling leads to increased Yap-Tead activity and increased osteoprogenitor proliferation. Yap and Taz differentially regulate Runx2 transcriptional activity; Yap-Runx2 and Taz-Runx2 complex activities are altered in Dchs1/Fat4 mutant osteoblasts.","method":"Dchs1/Fat4 knockout mice, osteoblast differentiation assays, co-immunoprecipitation of Yap/Taz-Runx2 complexes, luciferase reporter assays","journal":"Development (Cambridge, England)","confidence":"High","confidence_rationale":"Tier 2 — KO with defined differentiation phenotype plus Co-IP and reporter assays identifying mechanism","pmids":["31358536"],"is_preprint":false},{"year":2020,"finding":"FAT4 functions in a lymphatic endothelial cell-autonomous manner to control cell polarity in response to flow and is required for lymphatic vessel morphogenesis. FAT4 is identified as a target gene of GATA2, a key transcriptional regulator of lymphatic vascular development.","method":"Conditional knockout (lymphatic endothelial cell-specific), flow-induced polarity assays, ChIP/reporter assays for GATA2 regulation","journal":"The Journal of clinical investigation","confidence":"High","confidence_rationale":"Tier 2 — cell-autonomous conditional KO with polarity phenotype, replicated in multiple developmental contexts","pmids":["32182215"],"is_preprint":false},{"year":2020,"finding":"Hedgehog signaling transcriptionally activates Fat4 and Dchs1 (via GLI2). Fat4 and Dchs1 are required for mesenchymal cell clustering and villus formation in gut development. The Fat4-Dchs1 axis acts in parallel to the core-Vangl2 PCP axis to control mesenchymal cell clustering and WNT5A-guided oriented cell migration.","method":"Fat4 and Dchs1 knockout mice, GLI2 ChIP/transcriptomics, genetic interaction with Vangl2, live light-sheet fluorescence microscopy of PDGFRα+ cells","journal":"Developmental cell","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods including KO, ChIP, live imaging, and genetic interactions","pmids":["32155439"],"is_preprint":false},{"year":2023,"finding":"The co-crystal structure of human Fat4 and Dachsous1 (Dchs1) reveals that their binding domains form an extended interface along extracellular cadherin (EC) domains 1-4 of each protein. Fat4-Dchs1 affinity is among the highest reported for cadherin superfamily members, attributed to an extensive salt bridge network. Extracellular phosphorylation modifications are predicted to directly modulate Fat-Dachsous binding.","method":"X-ray crystallography (co-crystal structure), biophysical affinity measurements, structural modeling of phosphorylation effects","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1 — crystal structure with biophysical validation","pmids":["36797229"],"is_preprint":false},{"year":2016,"finding":"Fat4 suppression in gastric cancer cells leads to increased phosphorylated Yap and nuclear accumulation of Yap, promoting proliferation and migration. Re-expression of full-length Fat4 decreases phosphorylated Yap and inhibits cell cycle progression. Fat4 reduction also leads to cytoplasmic accumulation of β-catenin.","method":"shRNA knockdown, Fat4 overexpression rescue, western blotting, nuclear fractionation","journal":"Cancer biology & therapy","confidence":"Medium","confidence_rationale":"Tier 3 — single lab, KD/OE with pathway readout but no direct binding evidence","pmids":["26575609"],"is_preprint":false},{"year":2019,"finding":"FAT4 regulates EMT and autophagy in colorectal cancer cells partially via the PI3K-AKT signaling pathway, specifically through PI3K/AKT/mTOR and PI3K/AKT/GSK-3β signaling axes.","method":"Transwell invasion assays, MTT assays, western blotting, tumor xenograft model","journal":"Journal of experimental & clinical cancer research : CR","confidence":"Medium","confidence_rationale":"Tier 3 — single lab, KD/OE with pathway readouts but no direct binding or epistasis","pmids":["30832706"],"is_preprint":false},{"year":2015,"finding":"FAT4 functions as a tumor suppressor in gastric cancer by modulating Wnt/β-catenin signaling; knockdown of FAT4 activates Wnt/β-catenin signaling and induces EMT.","method":"shRNA knockdown, western blotting for β-catenin pathway components, xenograft model","journal":"British journal of cancer","confidence":"Medium","confidence_rationale":"Tier 3 — single lab, KD with pathway readout","pmids":["26633557"],"is_preprint":false},{"year":2019,"finding":"FAT4 interacts with the deubiquitinating enzyme USP51; USP51 directly binds FAT4 and controls its protein level. Ablating USP51 decreases FAT4 protein level while overexpression of USP51 increases FAT4 protein level. This interaction is essential for FAT4's tumor suppressor function in endometrial cancer.","method":"Co-immunoprecipitation, shRNA knockdown, overexpression, Hippo pathway readouts","journal":"American journal of translational research","confidence":"Medium","confidence_rationale":"Tier 3 — single lab, Co-IP with functional follow-up","pmids":["31217854"],"is_preprint":false},{"year":2023,"finding":"FAT4 binds to β-catenin and antagonizes its nuclear localization, promotes phosphorylation and degradation of β-catenin by the destruction complex (AXIN1, APC, GSK3β, CK1). FAT4 overexpression decreases PD-L1 mRNA expression transcriptionally and causes aberrant PD-L1 glycosylation via STT3A, leading to PD-L1 degradation — all in a β-catenin-dependent manner.","method":"Co-immunoprecipitation (FAT4-β-catenin), functional/mechanistic experiments in vivo and in vitro, immunofluorescence, xenograft models","journal":"Journal of experimental & clinical cancer research : CR","confidence":"Medium","confidence_rationale":"Tier 3 — single lab, Co-IP with multiple pathway readouts","pmids":["37658376"],"is_preprint":false},{"year":2025,"finding":"UBE4B (ubiquitin factor E4B) directly binds to FAT4 and mediates its ubiquitination and proteasomal degradation, thereby inhibiting FAT4-dependent autophagy and promoting gastric cancer progression.","method":"Co-immunoprecipitation, quantitative TMT proteomics, ubiquitination assay, western blotting, xenograft model","journal":"Cell death & disease","confidence":"Medium","confidence_rationale":"Tier 2–3 — Co-IP with proteomic identification of substrate, ubiquitination assay, but single lab","pmids":["40701960"],"is_preprint":false},{"year":2024,"finding":"IL-32 interacts with FAT4 and MST1/2 proteins (identified by immunoprecipitation and mass spectrometry), promoting MST1/2 phosphorylation and activating the Hippo/YAP signaling pathway, causing matrix metabolism disorder in nucleus pulposus cells.","method":"Immunoprecipitation and mass spectrometry, western blotting, lentiviral knockdown, in vivo rat model","journal":"International immunopharmacology","confidence":"Medium","confidence_rationale":"Tier 3 — Co-IP/MS identifies interaction, functional follow-up in single lab","pmids":["39178518"],"is_preprint":false},{"year":2025,"finding":"The intracellular domain (ICD) of Fat4 controls trans-endocytosis of Dchs1 into Fat4 cells and boundary accumulation of Fat4/Dchs1 complexes. Removing the Fat4 ICD reduces both trans-endocytosis and boundary accumulation but does not affect complex diffusion at the boundary. Actin polymerization is required for boundary accumulation of Fat4/Dchs1 complexes.","method":"Quantitative live imaging, ICD deletion mutants, actin polymerization inhibition, FRAP","journal":"Biophysical journal","confidence":"High","confidence_rationale":"Tier 1–2 — reconstituted system, quantitative live imaging with domain-deletion mutants","pmids":["39955614"],"is_preprint":false},{"year":2026,"finding":"FAT4 directly interacts with YAP via co-immunoprecipitation. This interaction retains YAP in the cytoplasm to block its nuclear translocation, independently of canonical Hippo phosphorylation cascade, suppressing proliferation and migration in multiple myeloma cells.","method":"Co-immunoprecipitation, FAT4 knockdown in vitro and in vivo (zebrafish and mouse), nuclear/cytoplasmic fractionation","journal":"Cancer science","confidence":"Medium","confidence_rationale":"Tier 3 — single lab, Co-IP with functional follow-up in multiple models","pmids":["42023818"],"is_preprint":false}],"current_model":"FAT4 is a large transmembrane atypical cadherin that forms high-affinity heterophilic complexes with its ligand DCHS1 through EC domains 1–4 (co-crystal structure resolved); these stable boundary complexes, whose dynamics are controlled by the Fat4 intracellular domain via trans-endocytosis, regulate planar cell polarity across multiple tissues (kidney, brain, heart, lymphatics, skeleton). Mechanistically, FAT4 acts upstream of Hippo signaling by sequestering Amotl1 and YAP in the cytoplasm (non-canonically, independent of Hippo kinases in some contexts), interacts directly with RET receptor through its extracellular cadherin repeats to restrict RET-GFRA1-GDNF complex assembly, suppresses Wnt/β-catenin by binding β-catenin and promoting its destruction complex-mediated degradation, and its own protein levels are regulated by the ubiquitin E4 ligase UBE4B-mediated proteasomal degradation and stabilized by the deubiquitinase USP51."},"narrative":{"teleology":[{"year":2008,"claim":"Establishing that mammalian Fat4 functions in planar cell polarity answered whether Drosophila Fat-PCP signaling is conserved in vertebrates and identified Fat4 as essential for oriented cell division during kidney tubule elongation.","evidence":"Fat4 knockout mice with genetic epistasis against Vangl2 and Fjx1","pmids":["18604206"],"confidence":"High","gaps":["Direct binding partner (Dachsous orthologue) not yet identified in mammals","Mechanism by which Fat4 controls cell orientation unknown","Relationship to Hippo pathway not tested"]},{"year":2011,"claim":"Demonstrating that Dchs1 and Fat4 function as a ligand–receptor pair across multiple organs established the core signaling module for vertebrate Fat-PCP and resolved the identity of the mammalian Dachsous orthologue partnering Fat4.","evidence":"Phenotypic comparison of Dchs1 and Fat4 knockout mice across ear, kidney, skeleton, intestine, heart, and lung","pmids":["21303848"],"confidence":"High","gaps":["Direct physical interaction not yet demonstrated biochemically","Downstream intracellular effectors uncharacterized","Whether Fat4-Dchs1 signals through Hippo pathway untested"]},{"year":2013,"claim":"Placing Fat4-Dchs1 upstream of YAP in cortical neurogenesis linked Fat-PCP signaling to the Hippo pathway for the first time in mammals, revealing that Fat4 loss increases neural progenitors via YAP activation.","evidence":"Mouse neuroepithelium knockdown with genetic epistasis using concurrent Yap knockdown","pmids":["24056717"],"confidence":"High","gaps":["Whether Fat4 acts on YAP through canonical Hippo kinases or via an alternative mechanism unknown","Direct protein–protein interactions between Fat4 and Hippo components not shown"]},{"year":2014,"claim":"Demonstrating that Fat4-Dchs1 controls tangential migration of facial branchiomotor neurons along one axis while Frizzled-PCP controls the orthogonal axis established that Fat-PCP and core-PCP operate as independent, axis-specific polarity systems.","evidence":"Conditional knockout mice with mosaic inactivation and cell polarity analysis","pmids":["24998526"],"confidence":"High","gaps":["Molecular mechanism distinguishing Fat-PCP from Frizzled-PCP axis specification not resolved","Whether Fat4-Dchs1 gradients are instructive or permissive for migration unclear"]},{"year":2015,"claim":"Tissue-specific dissection of Fat4 in kidney development revealed that Fat4 acts non-autonomously from the stroma to regulate nephron progenitor self-renewal and, critically, that this function is YAP-independent, demonstrating context-dependent Hippo pathway engagement.","evidence":"Tissue-specific conditional knockouts, Fat4/Yap and Fat4/Six2 double-mutant epistasis","pmids":["26116661","26116666"],"confidence":"High","gaps":["Alternative downstream effector in stroma not identified","How stromal Fat4 communicates non-autonomously to cap mesenchyme at the molecular level unknown"]},{"year":2015,"claim":"Identifying that Fat1 and Fat4 form cis-heterodimers and recruit distinct sets of actin-regulating proteins revealed a mechanism for integrating diverse intracellular signals at apical junctions during neural tube closure.","evidence":"Co-immunoprecipitation for cis-heterodimer, proteomics of interactors, mouse knockout genetics","pmids":["26209645"],"confidence":"High","gaps":["Stoichiometry and structural basis of Fat1-Fat4 heterodimer not resolved","Whether cis-heterodimer formation is constitutive or regulated unknown"]},{"year":2016,"claim":"Showing that Fat4-Dchs1 establishes polarized cell behavior intrinsically within mesenchyme (sternum) and independently of Yap/Taz (vertebral column) demonstrated that Fat4-PCP operates via Hippo-independent effectors in skeletal development.","evidence":"Fat4/Dchs1 knockout mice with cell orientation analysis; Fat4/Yap and Fat4/Taz double-mutant epistasis with Ctgf readout","pmids":["27145737","27381226"],"confidence":"High","gaps":["Identity of Hippo-independent effectors in skeletal mesenchyme unknown","Whether Fat4 directly transduces mechanical signals in mesenchyme not tested"]},{"year":2017,"claim":"Identifying Amotl1 as the intermediary that Fat4 sequesters out of the nucleus to prevent YAP co-activation in cardiomyocytes resolved how Fat4 modulates Hippo signaling non-canonically without affecting Hippo kinase activity.","evidence":"Fat4 knockout mouse hearts, subcellular fractionation of Amotl1/Yap1, Hippo kinase activity assays","pmids":["28239148"],"confidence":"High","gaps":["Whether Amotl1 sequestration mechanism operates in tissues beyond the heart untested","Direct binding between Fat4 intracellular domain and Amotl1 not demonstrated"]},{"year":2017,"claim":"Reconstitution of Fat4-Dchs1 complex dynamics showed that boundary complexes exhibit dramatically reduced mobility and threshold-like accumulation, establishing a biophysical model for how Fat4-Dchs1 generates polarized signaling at cell interfaces.","evidence":"Synthetic biology platform with human Fat4/Ds1, quantitative live imaging, FRAP","pmids":["28826487"],"confidence":"High","gaps":["Molecular identity of the feedback mechanism stabilizing boundary complexes not determined","Role of intracellular domain in complex dynamics not yet dissected"]},{"year":2019,"claim":"Demonstrating that FAT4 directly binds RET receptor and restricts RET-GFRA1-GDNF complex assembly revealed a juxtacrine signaling function for FAT4 distinct from its Hippo and PCP roles, explaining kidney phenotypes via titration of growth factor signaling.","evidence":"Co-immunoprecipitation, Gdnf heterozygous rescue of Fat4−/− kidney, RET signaling assays","pmids":["30853441"],"confidence":"High","gaps":["Which specific cadherin repeats mediate RET interaction not mapped","Whether Fat4-RET interaction occurs in non-renal tissues unknown"]},{"year":2019,"claim":"Identifying that USP51 deubiquitinates and stabilizes FAT4 protein established the first post-translational regulatory axis controlling FAT4 abundance.","evidence":"Co-immunoprecipitation of USP51-FAT4, knockdown/overexpression affecting FAT4 levels","pmids":["31217854"],"confidence":"Medium","gaps":["Ubiquitination sites on FAT4 not mapped","E3 ligase counterpart not identified in this study","Single-lab finding without independent replication"]},{"year":2020,"claim":"Establishing that Fat4 is a GATA2 target gene required cell-autonomously for flow-induced polarity in lymphatic endothelium placed Fat4 within the transcriptional hierarchy controlling lymphatic valve morphogenesis.","evidence":"Lymphatic endothelial cell-specific conditional knockout, flow polarity assays, GATA2 ChIP/reporter","pmids":["32182215"],"confidence":"High","gaps":["Mechanosensory mechanism linking flow to Fat4-dependent polarity not identified","Whether Dchs1 is the relevant ligand in lymphatic valves not confirmed"]},{"year":2020,"claim":"Demonstrating that Hedgehog/GLI2 transcriptionally activates Fat4 and Dchs1 for mesenchymal cell clustering in gut villus formation connected Fat4-PCP to upstream morphogen regulation and showed it operates in parallel with core Vangl2-PCP.","evidence":"Fat4/Dchs1 knockout mice, GLI2 ChIP, genetic interaction with Vangl2, live light-sheet imaging","pmids":["32155439"],"confidence":"High","gaps":["Whether Hedgehog regulation of Fat4 is conserved outside gut development unknown","Intracellular effectors mediating Fat4-driven mesenchymal clustering not identified"]},{"year":2023,"claim":"Solving the co-crystal structure of Fat4-Dchs1 EC1–4 revealed an extensive salt-bridge network underlying one of the highest-affinity cadherin interactions known and predicted that extracellular phosphorylation directly modulates binding affinity, providing a structural basis for graded signaling.","evidence":"X-ray crystallography, biophysical affinity measurements, structural modeling","pmids":["36797229"],"confidence":"High","gaps":["Functional validation of phosphorylation-dependent affinity modulation in vivo not performed","Whether the kinase responsible for extracellular phosphorylation (Four-jointed orthologue) acts on mammalian Fat4-Dchs1 not confirmed"]},{"year":2023,"claim":"Showing that FAT4 directly binds β-catenin and promotes its destruction-complex-mediated degradation mechanistically explained how FAT4 suppresses Wnt/β-catenin signaling, with downstream consequences for PD-L1 transcription and glycosylation.","evidence":"Co-immunoprecipitation of FAT4–β-catenin, functional assays in vitro and in vivo, xenograft models","pmids":["37658376"],"confidence":"Medium","gaps":["Domain on FAT4 mediating β-catenin binding not mapped","Single-lab finding; independent validation of FAT4–β-catenin direct interaction needed","Whether this mechanism operates in normal development or only cancer contexts unknown"]},{"year":2025,"claim":"Demonstrating that the Fat4 intracellular domain controls trans-endocytosis of Dchs1 and actin-dependent boundary accumulation resolved how Fat4-Dchs1 complex dynamics are regulated at cell interfaces, completing the biophysical model initiated in 2017.","evidence":"ICD deletion mutants, quantitative live imaging, FRAP, actin polymerization inhibition","pmids":["39955614"],"confidence":"High","gaps":["Specific ICD motifs and binding partners mediating trans-endocytosis not identified","Whether trans-endocytosis is required for downstream PCP signaling not tested in vivo"]},{"year":2025,"claim":"Identifying UBE4B as the E4 ubiquitin ligase that ubiquitinates FAT4 for proteasomal degradation completed the ubiquitin-mediated regulatory circuit (with USP51 as the opposing deubiquitinase) governing FAT4 protein turnover.","evidence":"TMT proteomics, co-immunoprecipitation, ubiquitination assays, xenograft model","pmids":["40701960"],"confidence":"Medium","gaps":["Specific ubiquitination sites on FAT4 not mapped","Whether UBE4B and USP51 compete for the same FAT4 pool not tested","Single-lab finding"]},{"year":null,"claim":"Key unresolved questions include: the identity of intracellular effectors mediating Fat4's Hippo-independent, YAP-independent functions in kidney and skeleton; the structural basis for Fat4 cis-heterodimer formation with Fat1; whether extracellular phosphorylation modulates Fat4-Dchs1 affinity in vivo; and the specific ICD motifs that recruit endocytic and actin-regulatory machinery to drive trans-endocytosis.","evidence":"","pmids":[],"confidence":"Low","gaps":["No in vivo validation of phosphorylation-dependent affinity tuning","Hippo-independent downstream effectors remain molecularly unidentified","Structural basis of Fat1-Fat4 cis-heterodimer unknown"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0098631","term_label":"cell adhesion mediator activity","supporting_discovery_ids":[0,1,3,7,11,14,16]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[2,9,12,13,21,25]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[6,9]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[5,10,11,16,24]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[2,9,12,13,17,21,25]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[0,1,3,4,7,8,11,14,15]},{"term_id":"R-HSA-1500931","term_label":"Cell-Cell communication","supporting_discovery_ids":[0,3,5,7,10,14,16,24]}],"complexes":["Fat4-Dchs1 trans-heterodimer","Fat1-Fat4 cis-heterodimer"],"partners":["DCHS1","FAT1","RET","AMOTL1","YAP1","CTNNB1","USP51","UBE4B"],"other_free_text":[]},"mechanistic_narrative":"FAT4 is a large atypical cadherin that functions as a core component of the non-canonical planar cell polarity (PCP) pathway, regulating oriented cell division, collective cell migration, and tissue morphogenesis across multiple organ systems including kidney, brain, heart, skeleton, gut, and lymphatic vasculature. FAT4 forms a high-affinity heterophilic trans-complex with its ligand DCHS1 through extracellular cadherin domains 1–4, and these boundary complexes are stabilized by a localized feedback mechanism controlled by the FAT4 intracellular domain via trans-endocytosis and actin polymerization [PMID:36797229, PMID:28826487, PMID:39955614]. FAT4 modulates Hippo signaling non-canonically by sequestering Amotl1 and YAP in the cytoplasm independently of canonical Hippo kinases, directly interacts with the RET receptor to restrict RET-GFRA1-GDNF complex assembly during kidney development, and suppresses Wnt/β-catenin signaling by binding β-catenin and promoting its destruction-complex-mediated degradation [PMID:28239148, PMID:42023818, PMID:30853441, PMID:37658376]. FAT4 protein levels are regulated by UBE4B-mediated ubiquitination and proteasomal degradation and stabilized by the deubiquitinase USP51 [PMID:40701960, PMID:31217854]."},"prefetch_data":{"uniprot":{"accession":"Q6V0I7","full_name":"Protocadherin Fat 4","aliases":["Cadherin family member 14","FAT tumor suppressor homolog 4","Fat-like cadherin protein FAT-J"],"length_aa":4981,"mass_kda":542.7,"function":"Cadherins are calcium-dependent cell adhesion proteins. FAT4 plays a role in the maintenance of planar cell polarity as well as in inhibition of YAP1-mediated neuroprogenitor cell proliferation and differentiation (By similarity)","subcellular_location":"Membrane","url":"https://www.uniprot.org/uniprotkb/Q6V0I7/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/FAT4","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/FAT4","total_profiled":1310},"omim":[{"mim_id":"616843","title":"LYMPHATIC MALFORMATION 6; LMPHM6","url":"https://www.omim.org/entry/616843"},{"mim_id":"616006","title":"HENNEKAM LYMPHANGIECTASIA-LYMPHEDEMA SYNDROME 2; HKLLS2","url":"https://www.omim.org/entry/616006"},{"mim_id":"615546","title":"VAN MALDERGEM SYNDROME 2; VMLDS2","url":"https://www.omim.org/entry/615546"},{"mim_id":"612483","title":"FAT ATYPICAL CADHERIN 3; FAT3","url":"https://www.omim.org/entry/612483"},{"mim_id":"612411","title":"FAT ATYPICAL CADHERIN 4; FAT4","url":"https://www.omim.org/entry/612411"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Cytosol","reliability":"Approved"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in 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Fat4 genetically interacts with PCP genes Vangl2 and Fjx1, and Fat4 represses Fjx1 expression, indicating conservation of Fat signaling.\",\n      \"method\": \"Gene-targeted knockout mouse, genetic epistasis (Fat4;Vangl2, Fat4;Fjx1 double mutants), oriented cell division analysis\",\n      \"journal\": \"Nature genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean KO with defined cellular phenotype plus genetic epistasis, replicated across multiple loci\",\n      \"pmids\": [\"18604206\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Dchs1 and Fat4 function as a ligand-receptor pair during murine development. Mutation of either gene increases protein staining for the other, and single/double mutants display similar phenotypes across multiple organs (ear, kidney, skeleton, intestine, heart, lung), indicating they act in the same pathway.\",\n      \"method\": \"Gene-targeted knockout mouse (Dchs1 mutant), phenotypic comparison with Fat4 mutants, immunostaining\",\n      \"journal\": \"Development (Cambridge, England)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal genetic interaction and protein stabilization shown across multiple organs, replicated\",\n      \"pmids\": [\"21303848\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"FAT4 and DCHS1 (receptor-ligand cadherin pair) regulate cerebral cortical neurogenesis; loss of either increases neural progenitor numbers and reduces differentiation. These effects were countered by concurrent knockdown of Yap, placing Dchs1 and Fat4 upstream of Yap in the Hippo signaling pathway during neurogenesis.\",\n      \"method\": \"Mouse embryonic neuroepithelium knockdown, genetic epistasis with Yap knockdown, cell counting assays\",\n      \"journal\": \"Nature genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — loss-of-function with defined cellular phenotype plus genetic epistasis with Yap, replicated in human mutations\",\n      \"pmids\": [\"24056717\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Fat4 and Dchs1 are expressed in complementary gradients and are required for collective tangential migration of facial branchiomotor (FBM) neurons and their planar cell polarity. Fat4 and Dchs1 act intrinsically within FBM neurons and extrinsically in the neuroepithelium. Fat-PCP and Frizzled-PCP regulate FBM migration along orthogonal axes.\",\n      \"method\": \"Conditional knockout mouse, mosaic inactivation, cell polarity analysis, genetic interaction studies\",\n      \"journal\": \"Current biology : CB\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — tissue-specific knockouts with defined cellular phenotype and orthogonal axis demonstration\",\n      \"pmids\": [\"24998526\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"FAT4 acts non-autonomously in the renal stroma to control nephron progenitor (cap mesenchyme) self-renewal. Loss of Yap from cap mesenchyme in Fat4-null mice does not rescue the expanded progenitor pool, demonstrating FAT4 regulates cap mesenchyme independently of YAP. Excess progenitors in Fat4 mutants depend on Six2 (genetic epistasis). Dchs1 and its paralogue Dchs2 function in the cap mesenchyme to restrict progenitor numbers.\",\n      \"method\": \"Tissue-specific conditional knockout, Fat4/Six2 double mutants, electron microscopy, gene expression analysis\",\n      \"journal\": \"Development (Cambridge, England)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — tissue-specific KO with cell-autonomous/non-autonomous dissection, plus genetic epistasis\",\n      \"pmids\": [\"26116661\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Dchs1 protein localizes in a polarized manner within cap mesenchyme cells, accumulating at the interface with stromal cells, indicating direct interaction with a stromal protein (Fat4). Dchs1 mutation reduces ureteric bud branching and impairs nephron morphogenesis and is required within cap mesenchyme cells.\",\n      \"method\": \"Antibody staining of genetic mosaics, conditional knockout, polarization analysis\",\n      \"journal\": \"Development (Cambridge, England)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — polarized localization shown in genetic mosaics, tissue-specific knockouts define cell-autonomous requirement\",\n      \"pmids\": [\"26116666\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Fat1 and Fat4 interact genetically to regulate cranial neural tube closure, cortical precursor proliferation, and apical constriction. Proteomic analysis reveals Fat1 and Fat4 bind different sets of actin-regulating and junctional proteins. In vitro data show Fat1 and Fat4 form cis-heterodimers, providing a mechanism for coordinating diverse interactors at apical junctions.\",\n      \"method\": \"Mouse knockout genetics, in utero electroporation knockdown, proteomic analysis, co-IP/pulldown for cis-heterodimer\",\n      \"journal\": \"Development (Cambridge, England)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods including proteomics, genetic interaction, and in vitro heterodimer demonstration\",\n      \"pmids\": [\"26209645\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Dchs1-Fat4 PCP pathway controls cell orientation within the early skeletal condensation to define shape and dimensions of the mouse sternum, driving cell intercalation. This is the first demonstration that Fat4 and Dchs1 establish polarized cell behavior intrinsically within mesenchyme.\",\n      \"method\": \"Knockout mouse analysis, cell orientation measurements, live imaging of mesenchymal cell polarity\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — KO with defined cellular polarity phenotype plus first demonstration of intrinsic mesenchymal PCP\",\n      \"pmids\": [\"27145737\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Fat4-Dchs1 regulates vertebral development by controlling cell proliferation in the early sclerotome independently of Yap and Taz. Genetic analysis of Fat4;Yap and Fat4;Taz double mutants and expression of transcriptional target Ctgf indicates Fat4-Dchs1 signaling uses a Hippo-independent mechanism for vertebral proliferation.\",\n      \"method\": \"Fat4/Dchs1 knockout mice, Fat4;Yap and Fat4;Taz double mutants, Ctgf expression analysis\",\n      \"journal\": \"Development (Cambridge, England)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — double mutant epistasis demonstrates Yap/Taz-independent mechanism\",\n      \"pmids\": [\"27381226\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"In the mouse heart, Fat4 modulates Hippo signaling to restrict cardiomyocyte growth and proliferation. Fat4 is not required for canonical Hippo kinase activation but sequesters Amotl1 out of the nucleus. Nuclear translocation of Amotl1 accompanies Yap1 to promote cardiomyocyte proliferation, identifying Amotl1 as a mammalian-specific intermediate for non-canonical Hippo signaling downstream of Fat4.\",\n      \"method\": \"Fat4 knockout mouse, cardiomyocyte size/proliferation assays, subcellular fractionation of Amotl1/Yap1, Hippo kinase activity assays\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — KO with defined phenotype, nuclear sequestration mechanism with multiple orthogonal methods\",\n      \"pmids\": [\"28239148\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Fat4-Ds1 (Dchs1) complexes accumulate at cell boundaries in a threshold-like manner and exhibit dramatically slower dynamics than unbound Fat4 and Ds1, indicating a localized feedback mechanism based on enhanced stability of Fat4-Ds1 complexes. Co-expression of Fat4 and Ds1 in the same cells is sufficient to induce polarization of Fat4-Ds1 complexes.\",\n      \"method\": \"Synthetic biology platform with mammalian cells expressing human Fat4 and Ds1, quantitative live imaging, FRAP\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — reconstituted system with quantitative live imaging and biophysical measurements\",\n      \"pmids\": [\"28826487\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Fat4 and Dachsous1 are specifically required for lymphatic valve morphogenesis. Valve endothelial cells are disoriented and fail to form proper valve leaflets in Fat4 and Dachsous1 knockout mice. Dachsous1 is polarized to membrane protrusions and cellular junctions of valve endothelial cells.\",\n      \"method\": \"Fat4 and Dachsous1 knockout mouse, Lifeact-GFP live imaging, immunostaining of polarized Dchs1 localization\",\n      \"journal\": \"Arteriosclerosis, thrombosis, and vascular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — KO with defined cellular phenotype plus polarized localization demonstrated in vivo and in vitro\",\n      \"pmids\": [\"28705793\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"FAT4 directly interacts with RET (tyrosine kinase receptor) through extracellular cadherin repeats. FAT4 expression perturbs assembly of the RET-GFRA1-GDNF complex, reducing RET signaling. Removal of one copy of Gdnf rescues Fat4−/− kidney development, supporting that FAT4 fine-tunes RET signaling via a juxtacrine mechanism.\",\n      \"method\": \"Co-immunoprecipitation, conditional knockout analysis, Gdnf heterozygous rescue genetics, RET signaling assays\",\n      \"journal\": \"Developmental cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — direct binding shown by Co-IP, functional rescue by genetic epistasis, multiple orthogonal methods\",\n      \"pmids\": [\"30853441\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Dchs1-Fat4 signaling is essential for osteoblast differentiation. Loss of Dchs1-Fat4 signaling leads to increased Yap-Tead activity and increased osteoprogenitor proliferation. Yap and Taz differentially regulate Runx2 transcriptional activity; Yap-Runx2 and Taz-Runx2 complex activities are altered in Dchs1/Fat4 mutant osteoblasts.\",\n      \"method\": \"Dchs1/Fat4 knockout mice, osteoblast differentiation assays, co-immunoprecipitation of Yap/Taz-Runx2 complexes, luciferase reporter assays\",\n      \"journal\": \"Development (Cambridge, England)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — KO with defined differentiation phenotype plus Co-IP and reporter assays identifying mechanism\",\n      \"pmids\": [\"31358536\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"FAT4 functions in a lymphatic endothelial cell-autonomous manner to control cell polarity in response to flow and is required for lymphatic vessel morphogenesis. FAT4 is identified as a target gene of GATA2, a key transcriptional regulator of lymphatic vascular development.\",\n      \"method\": \"Conditional knockout (lymphatic endothelial cell-specific), flow-induced polarity assays, ChIP/reporter assays for GATA2 regulation\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — cell-autonomous conditional KO with polarity phenotype, replicated in multiple developmental contexts\",\n      \"pmids\": [\"32182215\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Hedgehog signaling transcriptionally activates Fat4 and Dchs1 (via GLI2). Fat4 and Dchs1 are required for mesenchymal cell clustering and villus formation in gut development. The Fat4-Dchs1 axis acts in parallel to the core-Vangl2 PCP axis to control mesenchymal cell clustering and WNT5A-guided oriented cell migration.\",\n      \"method\": \"Fat4 and Dchs1 knockout mice, GLI2 ChIP/transcriptomics, genetic interaction with Vangl2, live light-sheet fluorescence microscopy of PDGFRα+ cells\",\n      \"journal\": \"Developmental cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods including KO, ChIP, live imaging, and genetic interactions\",\n      \"pmids\": [\"32155439\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"The co-crystal structure of human Fat4 and Dachsous1 (Dchs1) reveals that their binding domains form an extended interface along extracellular cadherin (EC) domains 1-4 of each protein. Fat4-Dchs1 affinity is among the highest reported for cadherin superfamily members, attributed to an extensive salt bridge network. Extracellular phosphorylation modifications are predicted to directly modulate Fat-Dachsous binding.\",\n      \"method\": \"X-ray crystallography (co-crystal structure), biophysical affinity measurements, structural modeling of phosphorylation effects\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structure with biophysical validation\",\n      \"pmids\": [\"36797229\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Fat4 suppression in gastric cancer cells leads to increased phosphorylated Yap and nuclear accumulation of Yap, promoting proliferation and migration. Re-expression of full-length Fat4 decreases phosphorylated Yap and inhibits cell cycle progression. Fat4 reduction also leads to cytoplasmic accumulation of β-catenin.\",\n      \"method\": \"shRNA knockdown, Fat4 overexpression rescue, western blotting, nuclear fractionation\",\n      \"journal\": \"Cancer biology & therapy\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — single lab, KD/OE with pathway readout but no direct binding evidence\",\n      \"pmids\": [\"26575609\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"FAT4 regulates EMT and autophagy in colorectal cancer cells partially via the PI3K-AKT signaling pathway, specifically through PI3K/AKT/mTOR and PI3K/AKT/GSK-3β signaling axes.\",\n      \"method\": \"Transwell invasion assays, MTT assays, western blotting, tumor xenograft model\",\n      \"journal\": \"Journal of experimental & clinical cancer research : CR\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — single lab, KD/OE with pathway readouts but no direct binding or epistasis\",\n      \"pmids\": [\"30832706\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"FAT4 functions as a tumor suppressor in gastric cancer by modulating Wnt/β-catenin signaling; knockdown of FAT4 activates Wnt/β-catenin signaling and induces EMT.\",\n      \"method\": \"shRNA knockdown, western blotting for β-catenin pathway components, xenograft model\",\n      \"journal\": \"British journal of cancer\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — single lab, KD with pathway readout\",\n      \"pmids\": [\"26633557\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"FAT4 interacts with the deubiquitinating enzyme USP51; USP51 directly binds FAT4 and controls its protein level. Ablating USP51 decreases FAT4 protein level while overexpression of USP51 increases FAT4 protein level. This interaction is essential for FAT4's tumor suppressor function in endometrial cancer.\",\n      \"method\": \"Co-immunoprecipitation, shRNA knockdown, overexpression, Hippo pathway readouts\",\n      \"journal\": \"American journal of translational research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — single lab, Co-IP with functional follow-up\",\n      \"pmids\": [\"31217854\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"FAT4 binds to β-catenin and antagonizes its nuclear localization, promotes phosphorylation and degradation of β-catenin by the destruction complex (AXIN1, APC, GSK3β, CK1). FAT4 overexpression decreases PD-L1 mRNA expression transcriptionally and causes aberrant PD-L1 glycosylation via STT3A, leading to PD-L1 degradation — all in a β-catenin-dependent manner.\",\n      \"method\": \"Co-immunoprecipitation (FAT4-β-catenin), functional/mechanistic experiments in vivo and in vitro, immunofluorescence, xenograft models\",\n      \"journal\": \"Journal of experimental & clinical cancer research : CR\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — single lab, Co-IP with multiple pathway readouts\",\n      \"pmids\": [\"37658376\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"UBE4B (ubiquitin factor E4B) directly binds to FAT4 and mediates its ubiquitination and proteasomal degradation, thereby inhibiting FAT4-dependent autophagy and promoting gastric cancer progression.\",\n      \"method\": \"Co-immunoprecipitation, quantitative TMT proteomics, ubiquitination assay, western blotting, xenograft model\",\n      \"journal\": \"Cell death & disease\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — Co-IP with proteomic identification of substrate, ubiquitination assay, but single lab\",\n      \"pmids\": [\"40701960\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"IL-32 interacts with FAT4 and MST1/2 proteins (identified by immunoprecipitation and mass spectrometry), promoting MST1/2 phosphorylation and activating the Hippo/YAP signaling pathway, causing matrix metabolism disorder in nucleus pulposus cells.\",\n      \"method\": \"Immunoprecipitation and mass spectrometry, western blotting, lentiviral knockdown, in vivo rat model\",\n      \"journal\": \"International immunopharmacology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — Co-IP/MS identifies interaction, functional follow-up in single lab\",\n      \"pmids\": [\"39178518\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"The intracellular domain (ICD) of Fat4 controls trans-endocytosis of Dchs1 into Fat4 cells and boundary accumulation of Fat4/Dchs1 complexes. Removing the Fat4 ICD reduces both trans-endocytosis and boundary accumulation but does not affect complex diffusion at the boundary. Actin polymerization is required for boundary accumulation of Fat4/Dchs1 complexes.\",\n      \"method\": \"Quantitative live imaging, ICD deletion mutants, actin polymerization inhibition, FRAP\",\n      \"journal\": \"Biophysical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — reconstituted system, quantitative live imaging with domain-deletion mutants\",\n      \"pmids\": [\"39955614\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"FAT4 directly interacts with YAP via co-immunoprecipitation. This interaction retains YAP in the cytoplasm to block its nuclear translocation, independently of canonical Hippo phosphorylation cascade, suppressing proliferation and migration in multiple myeloma cells.\",\n      \"method\": \"Co-immunoprecipitation, FAT4 knockdown in vitro and in vivo (zebrafish and mouse), nuclear/cytoplasmic fractionation\",\n      \"journal\": \"Cancer science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — single lab, Co-IP with functional follow-up in multiple models\",\n      \"pmids\": [\"42023818\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"FAT4 is a large transmembrane atypical cadherin that forms high-affinity heterophilic complexes with its ligand DCHS1 through EC domains 1–4 (co-crystal structure resolved); these stable boundary complexes, whose dynamics are controlled by the Fat4 intracellular domain via trans-endocytosis, regulate planar cell polarity across multiple tissues (kidney, brain, heart, lymphatics, skeleton). Mechanistically, FAT4 acts upstream of Hippo signaling by sequestering Amotl1 and YAP in the cytoplasm (non-canonically, independent of Hippo kinases in some contexts), interacts directly with RET receptor through its extracellular cadherin repeats to restrict RET-GFRA1-GDNF complex assembly, suppresses Wnt/β-catenin by binding β-catenin and promoting its destruction complex-mediated degradation, and its own protein levels are regulated by the ubiquitin E4 ligase UBE4B-mediated proteasomal degradation and stabilized by the deubiquitinase USP51.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"FAT4 is a large atypical cadherin that functions as a core component of the non-canonical planar cell polarity (PCP) pathway, regulating oriented cell division, collective cell migration, and tissue morphogenesis across multiple organ systems including kidney, brain, heart, skeleton, gut, and lymphatic vasculature. FAT4 forms a high-affinity heterophilic trans-complex with its ligand DCHS1 through extracellular cadherin domains 1–4, and these boundary complexes are stabilized by a localized feedback mechanism controlled by the FAT4 intracellular domain via trans-endocytosis and actin polymerization [PMID:36797229, PMID:28826487, PMID:39955614]. FAT4 modulates Hippo signaling non-canonically by sequestering Amotl1 and YAP in the cytoplasm independently of canonical Hippo kinases, directly interacts with the RET receptor to restrict RET-GFRA1-GDNF complex assembly during kidney development, and suppresses Wnt/β-catenin signaling by binding β-catenin and promoting its destruction-complex-mediated degradation [PMID:28239148, PMID:42023818, PMID:30853441, PMID:37658376]. FAT4 protein levels are regulated by UBE4B-mediated ubiquitination and proteasomal degradation and stabilized by the deubiquitinase USP51 [PMID:40701960, PMID:31217854].\",\n  \"teleology\": [\n    {\n      \"year\": 2008,\n      \"claim\": \"Establishing that mammalian Fat4 functions in planar cell polarity answered whether Drosophila Fat-PCP signaling is conserved in vertebrates and identified Fat4 as essential for oriented cell division during kidney tubule elongation.\",\n      \"evidence\": \"Fat4 knockout mice with genetic epistasis against Vangl2 and Fjx1\",\n      \"pmids\": [\"18604206\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct binding partner (Dachsous orthologue) not yet identified in mammals\", \"Mechanism by which Fat4 controls cell orientation unknown\", \"Relationship to Hippo pathway not tested\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Demonstrating that Dchs1 and Fat4 function as a ligand–receptor pair across multiple organs established the core signaling module for vertebrate Fat-PCP and resolved the identity of the mammalian Dachsous orthologue partnering Fat4.\",\n      \"evidence\": \"Phenotypic comparison of Dchs1 and Fat4 knockout mice across ear, kidney, skeleton, intestine, heart, and lung\",\n      \"pmids\": [\"21303848\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct physical interaction not yet demonstrated biochemically\", \"Downstream intracellular effectors uncharacterized\", \"Whether Fat4-Dchs1 signals through Hippo pathway untested\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Placing Fat4-Dchs1 upstream of YAP in cortical neurogenesis linked Fat-PCP signaling to the Hippo pathway for the first time in mammals, revealing that Fat4 loss increases neural progenitors via YAP activation.\",\n      \"evidence\": \"Mouse neuroepithelium knockdown with genetic epistasis using concurrent Yap knockdown\",\n      \"pmids\": [\"24056717\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether Fat4 acts on YAP through canonical Hippo kinases or via an alternative mechanism unknown\", \"Direct protein–protein interactions between Fat4 and Hippo components not shown\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Demonstrating that Fat4-Dchs1 controls tangential migration of facial branchiomotor neurons along one axis while Frizzled-PCP controls the orthogonal axis established that Fat-PCP and core-PCP operate as independent, axis-specific polarity systems.\",\n      \"evidence\": \"Conditional knockout mice with mosaic inactivation and cell polarity analysis\",\n      \"pmids\": [\"24998526\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular mechanism distinguishing Fat-PCP from Frizzled-PCP axis specification not resolved\", \"Whether Fat4-Dchs1 gradients are instructive or permissive for migration unclear\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Tissue-specific dissection of Fat4 in kidney development revealed that Fat4 acts non-autonomously from the stroma to regulate nephron progenitor self-renewal and, critically, that this function is YAP-independent, demonstrating context-dependent Hippo pathway engagement.\",\n      \"evidence\": \"Tissue-specific conditional knockouts, Fat4/Yap and Fat4/Six2 double-mutant epistasis\",\n      \"pmids\": [\"26116661\", \"26116666\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Alternative downstream effector in stroma not identified\", \"How stromal Fat4 communicates non-autonomously to cap mesenchyme at the molecular level unknown\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Identifying that Fat1 and Fat4 form cis-heterodimers and recruit distinct sets of actin-regulating proteins revealed a mechanism for integrating diverse intracellular signals at apical junctions during neural tube closure.\",\n      \"evidence\": \"Co-immunoprecipitation for cis-heterodimer, proteomics of interactors, mouse knockout genetics\",\n      \"pmids\": [\"26209645\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Stoichiometry and structural basis of Fat1-Fat4 heterodimer not resolved\", \"Whether cis-heterodimer formation is constitutive or regulated unknown\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Showing that Fat4-Dchs1 establishes polarized cell behavior intrinsically within mesenchyme (sternum) and independently of Yap/Taz (vertebral column) demonstrated that Fat4-PCP operates via Hippo-independent effectors in skeletal development.\",\n      \"evidence\": \"Fat4/Dchs1 knockout mice with cell orientation analysis; Fat4/Yap and Fat4/Taz double-mutant epistasis with Ctgf readout\",\n      \"pmids\": [\"27145737\", \"27381226\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of Hippo-independent effectors in skeletal mesenchyme unknown\", \"Whether Fat4 directly transduces mechanical signals in mesenchyme not tested\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Identifying Amotl1 as the intermediary that Fat4 sequesters out of the nucleus to prevent YAP co-activation in cardiomyocytes resolved how Fat4 modulates Hippo signaling non-canonically without affecting Hippo kinase activity.\",\n      \"evidence\": \"Fat4 knockout mouse hearts, subcellular fractionation of Amotl1/Yap1, Hippo kinase activity assays\",\n      \"pmids\": [\"28239148\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether Amotl1 sequestration mechanism operates in tissues beyond the heart untested\", \"Direct binding between Fat4 intracellular domain and Amotl1 not demonstrated\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Reconstitution of Fat4-Dchs1 complex dynamics showed that boundary complexes exhibit dramatically reduced mobility and threshold-like accumulation, establishing a biophysical model for how Fat4-Dchs1 generates polarized signaling at cell interfaces.\",\n      \"evidence\": \"Synthetic biology platform with human Fat4/Ds1, quantitative live imaging, FRAP\",\n      \"pmids\": [\"28826487\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular identity of the feedback mechanism stabilizing boundary complexes not determined\", \"Role of intracellular domain in complex dynamics not yet dissected\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Demonstrating that FAT4 directly binds RET receptor and restricts RET-GFRA1-GDNF complex assembly revealed a juxtacrine signaling function for FAT4 distinct from its Hippo and PCP roles, explaining kidney phenotypes via titration of growth factor signaling.\",\n      \"evidence\": \"Co-immunoprecipitation, Gdnf heterozygous rescue of Fat4−/− kidney, RET signaling assays\",\n      \"pmids\": [\"30853441\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Which specific cadherin repeats mediate RET interaction not mapped\", \"Whether Fat4-RET interaction occurs in non-renal tissues unknown\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Identifying that USP51 deubiquitinates and stabilizes FAT4 protein established the first post-translational regulatory axis controlling FAT4 abundance.\",\n      \"evidence\": \"Co-immunoprecipitation of USP51-FAT4, knockdown/overexpression affecting FAT4 levels\",\n      \"pmids\": [\"31217854\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Ubiquitination sites on FAT4 not mapped\", \"E3 ligase counterpart not identified in this study\", \"Single-lab finding without independent replication\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Establishing that Fat4 is a GATA2 target gene required cell-autonomously for flow-induced polarity in lymphatic endothelium placed Fat4 within the transcriptional hierarchy controlling lymphatic valve morphogenesis.\",\n      \"evidence\": \"Lymphatic endothelial cell-specific conditional knockout, flow polarity assays, GATA2 ChIP/reporter\",\n      \"pmids\": [\"32182215\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanosensory mechanism linking flow to Fat4-dependent polarity not identified\", \"Whether Dchs1 is the relevant ligand in lymphatic valves not confirmed\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Demonstrating that Hedgehog/GLI2 transcriptionally activates Fat4 and Dchs1 for mesenchymal cell clustering in gut villus formation connected Fat4-PCP to upstream morphogen regulation and showed it operates in parallel with core Vangl2-PCP.\",\n      \"evidence\": \"Fat4/Dchs1 knockout mice, GLI2 ChIP, genetic interaction with Vangl2, live light-sheet imaging\",\n      \"pmids\": [\"32155439\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether Hedgehog regulation of Fat4 is conserved outside gut development unknown\", \"Intracellular effectors mediating Fat4-driven mesenchymal clustering not identified\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Solving the co-crystal structure of Fat4-Dchs1 EC1–4 revealed an extensive salt-bridge network underlying one of the highest-affinity cadherin interactions known and predicted that extracellular phosphorylation directly modulates binding affinity, providing a structural basis for graded signaling.\",\n      \"evidence\": \"X-ray crystallography, biophysical affinity measurements, structural modeling\",\n      \"pmids\": [\"36797229\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Functional validation of phosphorylation-dependent affinity modulation in vivo not performed\", \"Whether the kinase responsible for extracellular phosphorylation (Four-jointed orthologue) acts on mammalian Fat4-Dchs1 not confirmed\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Showing that FAT4 directly binds β-catenin and promotes its destruction-complex-mediated degradation mechanistically explained how FAT4 suppresses Wnt/β-catenin signaling, with downstream consequences for PD-L1 transcription and glycosylation.\",\n      \"evidence\": \"Co-immunoprecipitation of FAT4–β-catenin, functional assays in vitro and in vivo, xenograft models\",\n      \"pmids\": [\"37658376\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Domain on FAT4 mediating β-catenin binding not mapped\", \"Single-lab finding; independent validation of FAT4–β-catenin direct interaction needed\", \"Whether this mechanism operates in normal development or only cancer contexts unknown\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Demonstrating that the Fat4 intracellular domain controls trans-endocytosis of Dchs1 and actin-dependent boundary accumulation resolved how Fat4-Dchs1 complex dynamics are regulated at cell interfaces, completing the biophysical model initiated in 2017.\",\n      \"evidence\": \"ICD deletion mutants, quantitative live imaging, FRAP, actin polymerization inhibition\",\n      \"pmids\": [\"39955614\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Specific ICD motifs and binding partners mediating trans-endocytosis not identified\", \"Whether trans-endocytosis is required for downstream PCP signaling not tested in vivo\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Identifying UBE4B as the E4 ubiquitin ligase that ubiquitinates FAT4 for proteasomal degradation completed the ubiquitin-mediated regulatory circuit (with USP51 as the opposing deubiquitinase) governing FAT4 protein turnover.\",\n      \"evidence\": \"TMT proteomics, co-immunoprecipitation, ubiquitination assays, xenograft model\",\n      \"pmids\": [\"40701960\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Specific ubiquitination sites on FAT4 not mapped\", \"Whether UBE4B and USP51 compete for the same FAT4 pool not tested\", \"Single-lab finding\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include: the identity of intracellular effectors mediating Fat4's Hippo-independent, YAP-independent functions in kidney and skeleton; the structural basis for Fat4 cis-heterodimer formation with Fat1; whether extracellular phosphorylation modulates Fat4-Dchs1 affinity in vivo; and the specific ICD motifs that recruit endocytic and actin-regulatory machinery to drive trans-endocytosis.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No in vivo validation of phosphorylation-dependent affinity tuning\", \"Hippo-independent downstream effectors remain molecularly unidentified\", \"Structural basis of Fat1-Fat4 cis-heterodimer unknown\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0098631\", \"supporting_discovery_ids\": [0, 1, 3, 7, 11, 14, 16]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [2, 9, 12, 13, 21, 25]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [6, 9]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [5, 10, 11, 16, 24]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [2, 9, 12, 13, 17, 21, 25]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [0, 1, 3, 4, 7, 8, 11, 14, 15]},\n      {\"term_id\": \"R-HSA-1500931\", \"supporting_discovery_ids\": [0, 3, 5, 7, 10, 14, 16, 24]}\n    ],\n    \"complexes\": [\n      \"Fat4-Dchs1 trans-heterodimer\",\n      \"Fat1-Fat4 cis-heterodimer\"\n    ],\n    \"partners\": [\n      \"DCHS1\",\n      \"FAT1\",\n      \"RET\",\n      \"AMOTL1\",\n      \"YAP1\",\n      \"CTNNB1\",\n      \"USP51\",\n      \"UBE4B\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}