CADM4

CADM4 (Necl-4/SynCAM4) is an immunoglobulin superfamily cell adhesion molecule that functions as a central mediator of axon-glia interactions during myelination and as a modulator of receptor tyrosine kinase signaling in epithelia and endothelia. In the PNS, CADM4 is expressed by myelinating Schwann cells at internodes where it binds axonal CADM3 in trans, recruits the polarity protein Par-3 via its cytoplasmic PDZ-binding domain, and requires ZDHHC3-mediated palmitoylation at C347 for stable plasma membrane retention; loss of Cadm4 or its palmitoylation causes focal hypermyelination, myelin outfoldings, and ion channel redistribution in the PNS, while in the CNS excess Cadm4-mediated adhesion blocks myelin sheet elongation and C347A knock-in mice exhibit severe CNS dysmyelination (PMID:17558405, PMID:23825401, PMID:30551998, PMID:30585357, PMID:39327467). CADM4 also cis-interacts with VEGFR2, ErbB3, and integrin α6β4, recruiting the phosphatase PTPN13 to inhibit ErbB2/ErbB3 and VEGFR2 signaling and independently regulating the PLCγ–Raf–MEK–ERK cascade downstream of VEGF, thereby contributing to contact inhibition of growth and tumor suppression (PMID:23611113, PMID:25893857, PMID:28601637, PMID:16261159). In GABAergic neurons, CADM4 restrains ErbB4-dependent synapse formation; its loss increases GABAergic synapse density, enhances inhibitory synaptic currents, and triggers excitotoxic neuronal death (PMID:40728771).

1. 2001 Medium

   Identification of CADM4 (TSLL2) as a new immunoglobulin superfamily member structurally related to CADM1/TSLC1 established it as part of a discrete adhesion molecule subfamily, raising the question of whether it shares tumor-suppressor and adhesion functions.

   Evidence Molecular cloning and sequence analysis with Northern blot expression profiling

   PMID:11536053

   Open questions at the time

   • No functional data beyond structural prediction
   • Expression pattern not resolved at protein level
   • No binding partners identified
2. 2006 Medium

   Demonstration that CADM4 mediates homophilic cell aggregation and suppresses tumor growth in xenografts established it as both an adhesion molecule and a candidate tumor suppressor, but left the molecular mechanism of tumor suppression unresolved.

   Evidence Cell aggregation assay in MDCK cells and subcutaneous xenograft of prostate cancer cells in nude mice

   PMID:16261159

   Open questions at the time

   • Tumor suppression mechanism not defined
   • Homophilic vs. heterophilic binding hierarchy unclear
   • Endogenous relevance in non-cancer tissues not tested
3. 2007 High

   Identification of CADM4 as the principal Necl on myelinating Schwann cells and its heterophilic trans-binding to axonal CADM3 established CADM4 as the key glial adhesion molecule at the axon-Schwann cell interface, explaining how myelination initiation depends on axon-glial recognition.

   Evidence Co-immunoprecipitation, dominant-negative overexpression, soluble domain competition, and in vitro myelination assays

   PMID:17558405

   Open questions at the time

   • Intracellular signaling downstream of CADM4 in Schwann cells not identified
   • In vivo genetic validation lacking at this point
   • Whether CADM4 functions similarly in CNS oligodendrocytes unknown
4. 2011 Medium

   Association of CADM4 with the actin-binding protein 4.1B and tumor suppression in renal cell carcinoma linked CADM4's cytoplasmic domain to cytoskeletal scaffolding, but the mechanistic basis of growth inhibition remained open.

   Evidence Co-immunoprecipitation from kidney tissue and nude mouse xenograft tumor suppression assay

   PMID:21544807

   Open questions at the time

   • Direct vs. indirect nature of 4.1B interaction not resolved
   • No signaling pathway analysis downstream of 4.1B
   • Relevance to endogenous kidney physiology untested in knockout
5. 2013 High

   Two advances resolved CADM4's in vivo myelination role and its receptor tyrosine kinase regulatory mechanism: Schwann cell-specific Cadm4 knockout produced focal hypermyelination with myelin outfoldings and ion channel mislocalization, proving CADM4 constrains myelin growth via its cytoplasmic domain; concurrently, identification of cis-interactions with ErbB3 and integrin α6β4 plus recruitment of PTPN13 to inhibit ErbB2/ErbB3 signaling provided the first defined signaling pathway for CADM4-mediated contact inhibition.

   Evidence Conditional knockout and dominant-negative transgenic mice with EM, electrophysiology, and behavioral assays; Co-IP with signaling phosphorylation readouts and hemidesmosome assays in epithelial cells

   PMID:23611113 PMID:23825401

   Open questions at the time

   • How cytoplasmic domain restrains myelin growth mechanistically unresolved
   • Whether PTPN13-ErbB3 axis operates in Schwann cells unknown
   • Direct structural basis of cis-interaction with RTKs not determined
6. 2014 Medium

   Zebrafish cadm4 was shown to restrict cardiac outflow tract size by limiting second heart field progenitor production, revealing a developmental role outside the nervous system.

   Evidence Morpholino knockdown and mRNA overexpression with confocal live imaging and lineage tracing in zebrafish

   PMID:24813897

   Open questions at the time

   • Mechanism by which cadm4 limits progenitor production not defined
   • Not validated in mammalian cardiac development
   • Morpholino-based; genetic mutant confirmation lacking
7. 2015 Medium

   Discovery that CADM4 cis-interacts with VEGFR2 and context-dependently either inhibits (confluent cells via PTPN13) or promotes (sparse cells via Rac1) VEGF signaling explained how a single adhesion molecule can exert opposing effects on endothelial behavior depending on cell density.

   Evidence Co-IP, siRNA knockdown, live imaging, phosphorylation and Rac1 activation assays in endothelial cells

   PMID:25893857

   Open questions at the time

   • In vivo vascular relevance not tested
   • Switch mechanism between inhibitory and activating modes not molecularly defined
   • Whether CADM4 directly bridges VEGFR2 and PTPN13 or acts indirectly unclear
8. 2017 Medium

   Epistasis experiments showed CADM4 is required for VEGF-induced PLCγ–Raf–MEK–ERK signaling independently of PTPN13 and ROCK, establishing a second, phosphatase-independent signaling arm through which CADM4 regulates endothelial proliferation.

   Evidence siRNA knockdown with phosphorylation cascade analysis and VEGFR2 internalization assay

   PMID:28601637

   Open questions at the time

   • Direct molecular target linking CADM4 to PLCγ not identified
   • Functional consequence for angiogenesis in vivo untested
   • Single lab observation
9. 2018 High

   Two studies resolved CADM4's CNS myelination role and its mechanism for organizing Schwann cell polarity: excess Cadm4 extracellular domain in oligodendrocytes created abundant but non-elongating contacts causing hypomyelination, showing that regulated adhesion-to-wrapping transition is essential; separately, CADM4's cytoplasmic tail was found to bind Par-3's first PDZ domain, recruiting it to the adaxonal Schwann cell membrane at sites of CADM3 engagement.

   Evidence Transgenic mouse and oligodendrocyte-neuron co-cultures; unbiased PDZ-domain proteomic screen with Co-IP, co-localization, and siRNA knockdown in myelinating co-cultures

   PMID:30551998 PMID:30585357

   Open questions at the time

   • Whether Par-3 recruitment mediates the myelin-restraining function remains untested genetically
   • Axonal receptors required for oligodendrocyte Cadm4 signaling only partially identified
   • Downstream effectors of Par-3 at the adaxonal membrane not defined
10. 2022 Medium

    Proteomic identification of a CADM4–Mpp6–4.1G–Lin7 complex in Schmidt-Lanterman incisures placed CADM4 within a defined membrane skeleton at non-compact myelin domains, suggesting a structural scaffolding role in myelin maintenance.

    Evidence Mass spectrometry of purified PNS myelin with electron microscopy quantification of SLIs in FA2H-deficient mice

    PMID:35445918

    Open questions at the time

    • Direct binary interactions within the complex not validated
    • Functional consequence of disrupting this complex at SLIs unknown
    • Observed enrichment in disease model; normal-state stoichiometry not established
11. 2024 High

    Identification of ZDHHC3-mediated palmitoylation at C347 as essential for CADM4 plasma membrane retention resolved how post-translational modification controls CADM4 surface expression; C347A knock-in and ZDHHC3 knockout mice both exhibit severe CNS dysmyelination and impaired oligodendrocyte differentiation via aberrant WNT–β-catenin signaling.

    Evidence Site-directed mutagenesis, C347A knock-in mouse, ZDHHC3 knockout mouse, palmitoylation assay, membrane fractionation, WNT pathway analysis, electrophysiology, behavioral testing

    PMID:39327467

    Open questions at the time

    • Whether palmitoylation similarly controls PNS myelination functions not fully explored
    • How loss of membrane CADM4 activates WNT–β-catenin mechanistically unresolved
    • Other potential palmitoylation sites and additional palmitoyl transferases not excluded
12. 2025 Medium

    Discovery that CADM4 at GABAergic synapses restrains ErbB4-dependent synapse formation on inhibitory neurons revealed a synaptic function beyond myelination: Cadm4 knockout increases GABAergic synapse density, enhances inhibitory currents, and causes excitotoxic neuronal death.

    Evidence Knockout mouse with immunofluorescence, mIPSC electrophysiology, ErbB4 pathway analysis, and hippocampal neuron culture

    PMID:40728771

    Open questions at the time

    • Whether CADM4 directly binds ErbB4 or acts via intermediate partners not determined
    • Cell-type-specific rescue experiments not performed
    • Behavioral consequences of synaptic phenotype not fully characterized

• Key unresolved questions include the structural basis of CADM4's cis-interactions with multiple receptor tyrosine kinases, how its cytoplasmic domain coordinates Par-3 recruitment with myelin growth restraint, and whether its myelination and tumor-suppressive functions share a common signaling logic.
• No crystal or cryo-EM structure of CADM4 or its complexes
• Genetic epistasis between Par-3 and CADM4 in myelination not tested in vivo
• Unified signaling model across myelination, vascular, and tumor-suppressor contexts lacking