FLRT3

FLRT3 is a type I transmembrane leucine-rich repeat protein that functions as a cell adhesion molecule and signaling co-receptor governing tissue morphogenesis, axon guidance, neuronal migration, and vascular integrity (PMID:10644439, PMID:19056886, PMID:21673655). Its shed ectodomain acts as a chemorepellent ligand for the Netrin receptors Unc5B and Unc5D, which it binds with high affinity; this FLRT-Unc5 repulsion controls cortical neuron and interneuron migration in vivo (PMID:19492039, PMID:21673655, PMID:34301831). At the cell surface, FLRT3 serves as a co-receptor for Robo1 that converts thalamic axons to Netrin-1 attraction by upregulating surface DCC through PKA activation (PMID:24560577), and binds the latrophilin LPHN3 through a defined interface in which the LPHN3 olfactomedin β-propeller docks onto the concave face of the FLRT3 LRR domain to drive glutamatergic synapse development (PMID:26235031). FLRT3 calibrates cadherin-based adhesion via its cytoplasmic tail, which recruits the small GTPase RND1 to modulate C-cadherin adhesion, an activity restrained by complex formation with PAPC (PMID:20027292); genetic ablation in mouse disrupts basement membrane integrity and triggers EMT-like behavior, causing headfold, endoderm migration, and ventral body wall fusion defects independently of FGF or Nodal/Wnt signaling (PMID:19056886, PMID:18448090). In endothelium, FLRT3 is transcriptionally induced by VEGF/VEGFR2 and supports survival and capillary formation (PMID:30930791), and protects the endothelial barrier against ischemia-reperfusion injury by interacting with RND3 to block RhoA-mediated cytoskeletal disruption (PMID:40047936). FLRT3 also acts as an EMT suppressor whose expression is directly repressed by TGF-β/SMAD4 signaling in colorectal cancer cells (PMID:35560224), and inhibits T cell activity through UNC5B such that antibody blockade restores anti-tumor killing (PMID:38427724).

1. 1999 Medium

   Established the molecular architecture of FLRT3, defining it as a glycosylated type I transmembrane LRR protein and setting the structural framework for its later ligand and adhesion roles.

   Evidence Heterologous expression and biochemical characterization in SF9 and COS-1 cells

   PMID:10644439

   Open questions at the time

   • No function assigned at this stage
   • Localization and binding partners undefined
2. 2004 Medium

   Showed FLRT3 is a functional cell-surface molecule that promotes neurite outgrowth without homophilic binding, implying it acts through a heterophilic partner rather than self-association.

   Evidence CHO cell co-culture neurite outgrowth assays and homophilic binding tests, replicated across two labs

   PMID:14706654 PMID:15485775

   Open questions at the time

   • Receptor mediating neurite outgrowth not identified
   • Signaling mechanism unknown
3. 2008 High

   In vivo knockouts established FLRT3 as a morphogenetic boundary and adhesion factor whose loss disrupts basement membrane integrity and provokes EMT-like ingression, independent of FGF/Nodal/Wnt patterning.

   Evidence Two independent mouse null alleles with developmental phenotyping, in situ hybridization, and pathway readouts

   PMID:18448090 PMID:19056886

   Open questions at the time

   • Molecular partner mediating adhesion control not yet defined
   • Mechanism linking FLRT3 to basement membrane integrity unresolved
4. 2009 Medium

   Identified Unc5B/Unc5D as high-affinity FLRT3 partners and linked the FLRT3 cytoplasmic tail to RND1-dependent adhesion control, providing the receptor and GTPase modules for FLRT3 signaling.

   Evidence Expression screen, co-immunoprecipitation, morpholino knockdown, and Xenopus cell deadhesion assays

   PMID:19492039 PMID:20027292

   Open questions at the time

   • Single organism (Xenopus) for adhesion mechanism
   • PAPC regulation tested in embryonic context only
5. 2011 High

   Demonstrated that shed FLRT2/FLRT3 ectodomains act as repulsive guidance cues for Unc5-positive neurons, defining a paracrine repulsion mechanism that controls cortical neuron migration timing.

   Evidence Mouse genetic knockouts and overexpression, ectodomain shedding assays, and in vitro/in vivo migration analysis

   PMID:21673655

   Open questions at the time

   • Protease responsible for shedding not defined here
   • FLRT3-specific Unc5 partner specificity vs FLRT2 incompletely resolved
6. 2014 High

   Revealed a distinct cell-surface role: FLRT3 partners with Robo1 to switch thalamic axons toward Netrin-1 attraction via PKA-driven surface DCC upregulation.

   Evidence In vitro and in vivo axon guidance assays in conditional knockouts, co-immunoprecipitation, and PKA activity assays

   PMID:24560577

   Open questions at the time

   • How Slit1/Robo1 engagement is coupled to PKA not fully mapped
   • Generalizability beyond thalamic axons untested
7. 2015 High

   Provided the atomic-resolution basis for FLRT3 synaptic function by solving the LPHN3 OLF–FLRT3 LRR complex structure, defining a high-affinity heterophilic interface driving glutamatergic synapse development.

   Evidence X-ray crystallography, isothermal titration calorimetry, and domain-deletion mapping

   PMID:26235031

   Open questions at the time

   • Cellular signaling downstream of the FLRT3–LPHN3 complex not detailed
   • Interplay with Unc5 binding on the same LRR face unresolved
8. 2019 Medium

   Extended FLRT3 function to vasculature, showing it is VEGF/VEGFR2-inducible via transcriptional elongation and bifunctionally regulates endothelial survival, tube formation, and migration.

   Evidence siRNA knockdown, nascent RNA measurement, and endothelial tube/migration/viability assays

   PMID:30930791

   Open questions at the time

   • Receptor/effector mediating endothelial effects not identified here
   • Single endothelial cell system
9. 2021 High

   Confirmed FLRT–Unc5 repulsion operates non-cell-autonomously on cortical interneurons, with epistasis to Unc5B/Unc5D establishing the receptor specificity of the migration phenotype.

   Evidence FLRT2/FLRT3 and Unc5B/Unc5D double-knockout mice, in vitro chemorepulsion, and in vivo interneuron mapping

   PMID:34301831

   Open questions at the time

   • Relative contribution of FLRT3 vs FLRT2 to interneuron guidance not separated
   • Layer-specific cues remaining unidentified
10. 2022 Medium

    Positioned FLRT3 as an EMT suppressor in cancer that is transcriptionally silenced by stromal TGF-β/SMAD4 signaling, linking its adhesion role to tumor invasion control.

    Evidence Gain- and loss-of-function with migration/invasion assays, xenografts, and TGF-β/SMAD4 pathway epistasis

    PMID:35560224

    Open questions at the time

    • Direct vs indirect SMAD4 regulation of FLRT3 not fully resolved here
    • Single tumor type
11. 2024 Medium

    Identified FLRT3 as an immune checkpoint-like ligand that suppresses T cell activity via UNC5B, providing a therapeutically actionable interaction in tumor immune evasion.

    Evidence Gain-of-function genetic screen, T cell killing assays, humanized cancer models, and monoclonal antibody blockade

    PMID:38427724

    Open questions at the time

    • Signaling within T cells downstream of UNC5B not detailed
    • Single-lab finding
12. 2024 Medium

    Documented direct SMAD4 regulation of FLRT3 and a FLRT3-dependent program controlling cardiomyocyte apoptosis, autophagy, and ion channel expression, broadening its tissue context.

    Evidence Dual-luciferase reporter, ChIP-PCR, knockdown, and apoptosis/ion-channel readouts in Ang II-stimulated H9C2 cardiomyocytes

    PMID:38509727

    Open questions at the time

    • Mechanism linking FLRT3 to ion channel genes unknown
    • Cell-line model only
13. 2025 Medium

    Defined a vascular protective mechanism in which FLRT3 binds RND3 to suppress RhoA-mediated cytoskeletal disruption and maintain endothelial barrier integrity, with FLRT3 turnover by autophagic-lysosomal degradation under stress.

    Evidence In vivo lentiviral gain/loss-of-function, Evans blue extravasation, electron microscopy, co-immunoprecipitation, and RhoA pathway analysis

    PMID:40047936

    Open questions at the time

    • Direct vs indirect FLRT3–RND3 binding not crystallographically defined
    • Single-lab finding

• How FLRT3 integrates its multiple competing partners (Unc5, Robo1, LPHN3, cadherins, Rnd-family GTPases) at a single surface to produce context-specific adhesion, repulsion, or attraction outputs remains unresolved.
• No unified model of partner selection across tissues
• Regulation of ectodomain shedding vs cell-surface retention not defined
• Cytoplasmic signaling output remains poorly mapped