FRMD6

FRMD6 (Willin) is a FERM-domain scaffolding protein that functions as a multivalent upstream regulator of Hippo signaling, epithelial polarity, receptor tyrosine kinase activity, and mTOR signaling. FRMD6 directly binds and activates MST1/2 kinase through its N-terminal FERM domain, promoting LATS1 activation and inhibitory YAP/TAZ phosphorylation, thereby suppressing proliferation and inducing senescence; this pathway is antagonized by ezrin and regulated upstream by p53, SMADs, and TGF-β (PMID:21666719, PMID:38926528). Independent of core Hippo kinases, FRMD6 recruits aPKC/Par6 to apical junctional complexes to suppress ROCK-dependent actomyosin contractility, localizes to junctions through direct nectin binding, inhibits RTK (c-Met/PDGFR) signaling in glioblastoma, and scaffolds the mTOR–S6K interaction to activate mTOR signaling in lung cancer cells (PMID:21685893, PMID:22512338, PMID:27661120, PMID:37060526). In neuronal cells, FRMD6 modulates ERK1/2 signaling, promotes retinoic acid–induced differentiation, and maintains mitochondrial homeostasis, rescuing Aβ-induced mitochondrial dysfunction (PMID:33088261, PMID:36231104).

1. 2005 Medium

   Establishing FRMD6 as a FERM-domain-containing, phospholipid-binding protein that localizes to the plasma membrane independently of actin, answering the basic question of its domain architecture and subcellular targeting.

   Evidence Subcellular fractionation, co-localization imaging, cytochalasin D and EGF stimulation in cultured cells

   PMID:16137681

   Open questions at the time

   • No functional consequence of membrane localization was demonstrated
   • Phospholipid binding specificity not mapped to specific lipid species
   • Single lab characterization without independent replication
2. 2011 High

   Two studies established FRMD6 as a dual regulator of epithelial cell biology: one showed it activates the Hippo kinase cascade (MST→LATS→pYAP) via its FERM domain, while the other demonstrated it recruits aPKC/Par6 to apical junctions to suppress ROCK-mediated apical constriction, answering how FRMD6 controls both proliferation and cell shape.

   Evidence Phospho-Western blotting of MST/LATS/YAP, FERM domain deletion, shRNA-induced EMT (Hippo); siRNA double knockdown, ROCK inhibitor rescue, co-IP (aPKC/Par6 pathway)

   PMID:21666719 PMID:21685893

   Open questions at the time

   • Whether Hippo activation and aPKC recruitment are mechanistically linked or independent functions
   • No structural basis for FERM domain–MST interaction
   • Endogenous stoichiometry of FRMD6 at junctions not determined
3. 2012 Medium

   Identifying nectins as direct binding partners that recruit FRMD6 to apical junctional complexes answered how FRMD6 is positioned at cell–cell contacts to exert its polarity and signaling functions.

   Evidence Co-immunoprecipitation of FRMD6–nectin, afadin knockdown abolishing junctional FRMD6 localization

   PMID:22512338

   Open questions at the time

   • Binding interface between nectin and FRMD6 FERM domain not mapped
   • Single Co-IP study without reciprocal pull-down validation reported
   • Whether nectin binding is required for Hippo activation not tested
4. 2013 Medium

   Demonstrating FRMD6-mediated YAP phosphorylation and nuclear exclusion in sciatic nerve fibroblasts extended Hippo activation beyond epithelial cells and linked it to migration and growth control in mesenchymal contexts.

   Evidence Overexpression with nuclear/cytoplasmic fractionation, scratch wound and proliferation assays in sciatic nerve fibroblasts

   PMID:23593160

   Open questions at the time

   • Loss-of-function data in these cells not provided
   • Mechanism linking Hippo activation to directional migration not defined
5. 2016 Medium

   Revealing that FRMD6 suppresses glioblastoma growth by inhibiting c-Met and PDGFR activation rather than through canonical Hippo kinases established a Hippo-independent tumor-suppressive mechanism and broadened FRMD6's signaling repertoire.

   Evidence Overexpression/knockdown in GBM cells, xenograft models, constitutively active TPR-Met rescue reversing FRMD6 anti-tumor effects

   PMID:27661120

   Open questions at the time

   • Direct physical interaction between FRMD6 and RTKs not shown
   • How FRMD6 inhibits RTK activation mechanistically remains undefined
   • Single lab without independent replication in other GBM models
6. 2019 Medium

   Placing FRMD6 downstream of BDNF/TrkB-T1 signaling in aged cardiac endothelial cells identified a receptor-level input that activates FRMD6 to promote Hippo-dependent migration, answering how FRMD6 is engaged by extracellular signals.

   Evidence Co-immunoprecipitation of TrkB-T1–FRMD6, siRNA knockdown, BDNF stimulation and migration assays in aged cardiac microvascular endothelial cells

   PMID:30667167

   Open questions at the time

   • Direct versus adaptor-mediated interaction with TrkB-T1 not distinguished
   • Relevance to in vivo cardiac aging not validated genetically
7. 2020 High

   Two studies expanded FRMD6's roles in cancer suppression and neuronal biology: CRISPR knockout in prostate cancer with in vivo double-KO mouse models confirmed tumor-suppressive function through Hippo/YAP and c-MYC, while neuronal studies showed FRMD6 modulates cell mechanics, ERK signaling, and NeuroD1-dependent differentiation.

   Evidence CRISPR KO, xenografts, Frmd6/Pten double-KO mouse prostate model, phospho-proteomics (prostate cancer); overexpression/knockdown with ERISM biophysics, pERK/NeuroD1 Western blots, retinoic acid differentiation (neuronal cells)

   PMID:33088261 PMID:33249427

   Open questions at the time

   • Whether ERK regulation in neurons proceeds through RTK inhibition as in GBM not tested
   • Mechanistic link between FRMD6 and c-MYC signaling not delineated
   • In vivo neuronal phenotype of Frmd6 loss not assessed
8. 2022 Medium

   Demonstrating that FRMD6 maintains mitochondrial morphology and function in neurons and rescues Aβ-induced mitochondrial fragmentation established a novel organelle-level function, linking FRMD6 to neurodegeneration-relevant biology.

   Evidence siRNA knockdown and overexpression in HT-22 and primary neurons, mitochondrial morphology imaging, oxygen consumption assays, Aβ peptide rescue

   PMID:36231104

   Open questions at the time

   • Whether mitochondrial regulation is mediated through ERK, Hippo, or an independent pathway not resolved
   • No in vivo neuronal model tested
   • Single lab report
9. 2023 High

   Two studies completed the upstream-to-downstream wiring of FRMD6: one demonstrated direct FRMD6–MST co-immunoprecipitation and placed FRMD6 downstream of p53/SMAD/TGF-β to drive senescence via YAP-regulated CCN3; the other showed FRMD6 scaffolds mTOR–S6K to activate mTOR signaling, revealing a Hippo-independent pro-growth output.

   Evidence Co-IP of FRMD6–MST, in vivo lung senescence model, TGF-β induction, CCN3 rescue (senescence); Co-IP of FRMD6–mTOR and FRMD6–S6K, KO MEFs and mice, phospho-S6K/S6 Western blots (mTOR)

   PMID:37060526 PMID:38926528

   Open questions at the time

   • How FRMD6 simultaneously activates Hippo (anti-growth) and mTOR (pro-growth) in the same cell type is paradoxical and unresolved
   • Structural basis of FRMD6–MST and FRMD6–mTOR interactions unknown
   • Context-dependent switching between Hippo and mTOR arms not defined
10. 2026 Medium

    Identification of FRMD6 as an upstream regulator of VE-cadherin phosphorylation and adherens junction stability in endothelial cells extended its junctional regulatory role to vascular biology and angiogenesis.

    Evidence siRNA knockdown in endothelial cells, transcriptome sequencing, VE-cadherin phosphorylation blots, in vivo rat calvarial defect model

    PMID:41916139

    Open questions at the time

    • Direct mechanism linking FRMD6 to VE-cadherin phosphorylation not identified
    • Whether this proceeds through Hippo, aPKC, or a distinct pathway not distinguished

• The central unresolved question is how FRMD6 coordinates its opposing downstream outputs — Hippo-mediated growth suppression versus mTOR-mediated growth promotion — in a context-dependent manner, and what structural features or post-translational modifications govern pathway selection.
• No structural model of FRMD6 or its complexes with MST, mTOR, or nectins
• Post-translational modifications of FRMD6 that regulate pathway choice not characterized
• In vivo genetic models for neuronal and vascular functions of FRMD6 lacking