FAT3

FAT3 is a large transmembrane atypical cadherin that functions as a cell-surface receptor coordinating polarized neuronal morphology, circuit assembly, and progenitor proliferation during development (PMID:21903076, PMID:11811999). Its cytoplasmic domain acts as a signaling and adaptor hub: discrete intracellular motifs separately control amacrine cell migration versus neurite retraction, with the retraction function executed through direct binding and localization of Ena/VASP actin regulators, such that mislocalizing Ena/VASP alone reproduces the Fat3 mutant phenotype (PMID:27122175, PMID:35108541). Through its intracellular domain FAT3 also engages the synaptic phosphatase receptor PTPσ to localize the glutamate receptor GRIK1 at OFF-cone bipolar cell synapses, a step required for normal high-frequency visual responses, and this synapse-localization role is genetically separable from its control of neurite outgrowth (PMID:35108541, PMID:39903280). Beyond morphogenesis, FAT3 acts upstream of the Hippo pathway, restraining LATS1/2 phosphorylation to stabilize active Yap and maintain proliferating neural progenitors (PMID:36042367), and modulates β-catenin to support canonical Wnt signaling during cranial neural crest induction and craniofacial development (PMID:41933378). FAT3 is broadly required for neuronal integrity, as its loss causes motor neuron branching defects, axonal degeneration, and perinatal lethality in animal models (PMID:41937739).

1. 2002 Medium

   Establishing the molecular identity of FAT3 was the prerequisite for any mechanistic study; cloning defined it as a giant cadherin-superfamily transmembrane protein with developmental expression patterns pointing to a neural role.

   Evidence cDNA cloning, domain analysis, Northern blot, and immunostaining of developing rat brain and spinal cord

   PMID:11811999

   Open questions at the time

   • No functional assay linking the architecture to a cellular activity
   • Ligand and binding partners undefined
   • Inferred axon fasciculation role not directly tested
2. 2011 High

   The first loss-of-function phenotype answered what FAT3 does in vivo, showing it is required for amacrine cells to retract trailing neurites and achieve unipolar morphology during migration.

   Evidence Genetic knockout mice with retinal histology and amacrine cell morphology analysis

   PMID:21903076

   Open questions at the time

   • Molecular effectors of retraction not identified
   • Whether FAT3 acts cell-autonomously not yet resolved
   • Signaling downstream of the receptor unknown
3. 2016 High

   Two studies defined the mechanistic basis of FAT3 action: cell-autonomous receptor signaling through an intracellular domain that recruits Ena/VASP actin regulators, and a Kif5-ID motif that links FAT3 to kinesin-mediated transport with developmental switching via alternative splicing.

   Evidence Time-lapse imaging, cell-autonomous rescue, ICD–Ena/VASP and ICD–Kif5B Co-IP, mutagenesis, and Ena/VASP redistribution phenocopy in mice and MDCK cells

   PMID:27122175 PMID:27788242

   Open questions at the time

   • Structural basis of ICD–Ena/VASP and ICD–Kif5B binding not resolved
   • Whether transport and actin-regulatory functions are coupled in the same cells unclear
   • Extracellular ligand driving receptor activation unknown
4. 2022 High

   Domain dissection separated FAT3's distinct downstream functions, showing migration and neurite retraction are controlled by different ICD motifs and that neurite outgrowth is regulated independently of synapse formation.

   Evidence ICD-deletion and domain-mutant mice, pulldown assays, and synaptic marker immunostaining

   PMID:35108541

   Open questions at the time

   • Identity of all motif-specific binding partners incomplete
   • How separate motifs are spatially coordinated unknown
5. 2022 Medium

   FAT3 was placed upstream of the Hippo pathway, answering how it controls progenitor number: it suppresses LATS1/2 phosphorylation to keep Yap active and sustain neural progenitor proliferation.

   Evidence Fat3 knockdown in chick neural tube and knockout mice, with western blots for phospho-LATS1/2 and Yap

   PMID:36042367

   Open questions at the time

   • Mechanism by which FAT3 represses LATS1/2 not defined
   • Single lab
   • Link between receptor engagement and kinase regulation unmapped
6. 2025 High

   The synaptic role of FAT3 was given molecular form: its ICD binds PTPσ to localize the glutamate receptor GRIK1 at OFF-cone bipolar cell synapses, with loss impairing visual physiology and perception.

   Evidence FAT3–PTPσ Co-IP, GRIK1 immunostaining, ERG recordings, and behavioral assays in Fat3 mutant mice (peer-reviewed; preprint #6)

   PMID:37961274 PMID:39903280

   Open questions at the time

   • Whether FAT3–PTPσ acts in cis or trans at the synapse unclear
   • Direct interaction between FAT3 and GRIK1 not established
   • Generalizability beyond OFF-cone bipolar synapses untested
7. 2026 Medium

   Two 2026 studies broadened FAT3's developmental reach, linking it to Wnt/β-catenin signaling in cranial neural crest and to motor neuron integrity and axonal maintenance across organisms.

   Evidence fat3 knockdown in zebrafish and Drosophila and Fat3 mouse knockout/knockin, with β-catenin western blots, CNCC marker staining, nerve histology, and motor assays

   PMID:41933378 PMID:41937739

   Open questions at the time

   • Mechanism connecting FAT3 to β-catenin levels undefined
   • Single lab per finding
   • Whether axonal degeneration reflects a developmental or maintenance defect unresolved

• The extracellular ligand that activates FAT3 and the structural mechanism coupling receptor engagement to its multiple intracellular outputs (Ena/VASP, Kif5B, PTPσ, Hippo, Wnt) remain unknown.
• No identified extracellular binding partner/ligand
• No structural model of the ICD or its multivalent interactions
• How a single receptor partitions among migration, retraction, transport, synaptic, and signaling functions is unresolved