LGI4

LGI4 is a secreted, glycosylated leucine-rich repeat protein produced by Schwann cells that functions as a paracrine ligand controlling peripheral nerve myelination and glial development (PMID:16341215, PMID:20220021). Schwann cell-secreted LGI4 binds directly to axonal ADAM22 — and also to ADAM23 and ADAM11 — without requiring additional membrane cofactors, defining a glia-to-axon signaling axis that drives Schwann cell differentiation and myelination (PMID:18974846, PMID:20220021); cell type-specific knockouts place LGI4 on the Schwann cell side and ADAM22 on the axon (PMID:20220021). Beyond myelinating Schwann cells, LGI4 promotes proliferation and differentiation of glial-restricted progenitors across sensory, sympathetic, and enteric ganglia, an activity that is genetically epistatic to ADAM22 (PMID:21068328). The myelination function is genetically validated in disease: biallelic loss-of-function mutations in humans abolish peripheral nerve myelin (PMID:28318499), and arthrogryposis-associated missense mutations act predominantly by impairing trafficking and secretion of LGI4 through the endomembrane system rather than by disrupting ADAM22 binding or intrinsic myelinating activity (PMID:34288120). The same LGI4-ADAM22 axis is targeted by autoimmunity, as IgG4 autoantibodies against LGI4 in patients reduce expression of the myelination transcription factor Krox20 and produce nodal/paranodal damage in vivo (PMID:36631269, PMID:41092257). LGI4 has additionally been linked to a paracrine role in tumor proliferation and invasion (PMID:24662834), to suppression of MEK1/2-ERK1/2 inflammatory signaling in intestinal epithelium downstream of IGF2BP2-mediated mRNA stabilization (PMID:40663812), and to modulation of cardiac KV1.5 channel function [PMID:bio_10.1101_2024.10.03.616587].

1. 2005 High

   Established that LGI4 is a Schwann cell-derived secreted LRR protein functionally required for peripheral myelination, answering whether the claw paw phenotype reflected an autonomous glial signaling defect.

   Evidence Positional cloning of the claw paw mutation plus siRNA knockdown and exogenous protein rescue in neuron-Schwann cell cocultures

   PMID:16341215

   Open questions at the time

   • Did not identify the receptor or binding partner
   • Mechanism of how secreted LGI4 acts on Schwann cells was undefined
2. 2008 Medium

   Identified the receptor specificity of LGI4 by showing direct binding to ADAM22, ADAM23, and ADAM11, establishing that ADAM22 is not the sole receptor.

   Evidence Immunoprecipitation, mass spectrometry, and quantitative cell-ELISA binding assays

   PMID:18974846

   Open questions at the time

   • Functional relevance of ADAM11/ADAM23 binding to myelination not resolved
   • Single lab
3. 2010 High

   Defined the paracrine architecture of the signaling axis — LGI4 secreted by Schwann cells acts on axonal ADAM22 — resolving which cell contributes ligand and which contributes receptor.

   Evidence Direct cell-based binding assay and cell type-specific conditional knockout mice (Schwann cell Lgi4 ablation, axonal Adam22 ablation)

   PMID:20220021

   Open questions at the time

   • Downstream Schwann cell signaling events not mapped
   • Structural basis of LGI4-ADAM22 binding not determined
4. 2010 High

   Extended LGI4 function beyond myelinating Schwann cells to glial progenitor proliferation/differentiation across PNS ganglia, acting through ADAM22.

   Evidence Gene-targeted knockout mice, expression profiling, and compound-mutant epistasis with Adam22

   PMID:21068328

   Open questions at the time

   • Signaling effectors downstream of ADAM22 in progenitors not identified
5. 2014 Medium

   Showed LGI4 can mediate paracrine signaling in a non-neural context, with ADAM23-negative tumor cells secreting LGI4 to promote proliferation/invasion of neighboring ADAM23-positive cells.

   Evidence siRNA ablation with proliferation and invasion assays in heterotypic tumor cell co-cultures

   PMID:24662834

   Open questions at the time

   • In vivo tumor relevance not established here
   • Receptor engaged on responding cells not directly confirmed
6. 2017 High

   Confirmed in humans that biallelic LGI4 loss-of-function causes absence of peripheral myelin, linking the secretion-dependent paracrine mechanism to disease.

   Evidence Whole-exome sequencing, iPSC-based secretion assay, and transmission electron microscopy of patient sciatic nerve

   PMID:28318499

   Open questions at the time

   • Genotype-phenotype range across mutation classes not fully defined
7. 2021 Medium

   Resolved the pathogenic mechanism of disease-associated missense alleles as a trafficking/secretion defect rather than loss of ADAM22 binding or intrinsic activity.

   Evidence In vitro expression, protein trafficking analysis, ADAM22 binding assays, and coculture myelination assays

   PMID:34288120

   Open questions at the time

   • Specific ER/secretory checkpoint that retains mutant protein not identified
   • Single lab
8. 2023 Medium

   Demonstrated that anti-LGI4 IgG4 autoantibodies functionally impair the LGI4-ADAM22 myelination axis, implicating it in CIDP.

   Evidence Cell-based binding assay with LGI4/ADAM22 co-transfection and Krox20 RT-PCR in Schwann cell cultures exposed to patient serum

   PMID:36631269

   Open questions at the time

   • Causal in vivo demonstration not yet provided in this study
   • Single lab
9. 2025 Medium

   Provided in vivo evidence that anti-LGI4 IgG causes nodal/paranodal pathology, localizing the autoimmune lesion to the node.

   Evidence Intraneural injection of LGI4-IgG4/IgG2 into mouse sciatic nerve with immunohistochemistry and electron microscopy, plus BrdU proliferation and Krox20 RT-PCR

   PMID:41092257

   Open questions at the time

   • Molecular events linking antibody binding to nodal disruption not detailed
   • Single lab
10. 2025 Medium

    Identified a non-neural intestinal role in which IGF2BP2 stabilizes LGI4 mRNA via m6A and LGI4 suppresses MEK-ERK driven inflammation.

    Evidence meRIP-qPCR, RNA pulldown/RIP, mRNA stability assays, IGF2BP2 knockout mice, and AAV9 LGI4 rescue in DSS colitis

    PMID:40663812

    Open questions at the time

    • Receptor mediating LGI4 effect on epithelial MEK-ERK signaling not identified
    • Single lab
11. 2024 Medium

    Linked LGI4 to cardiac electrophysiology through modulation of KV1.5 channel association, surface expression, and IKur.

    Evidence Co-immunoprecipitation, patch clamping, and AAV cardiac-specific overexpression in mice with ECG (preprint)

    PMID:bio_10.1101_2024.10.03.616587

    Open questions at the time

    • Preprint, not yet peer-reviewed
    • Endogenous cardiac role versus overexpression effect not separated
12. 2025 Low

    Reported an intracellular tumor-promoting role in glioma via p53 binding and NF-kB feedback, distinct from the canonical secreted paracrine model.

    Evidence Co-immunoprecipitation, nuclear fractionation, reporter assays, and knockdown/overexpression

    PMID:41130018

    Open questions at the time

    • Single lab, mechanistic claims not independently replicated
    • Reconciliation of an intracellular p53-binding role with the secreted ligand model unresolved

• How LGI4-ADAM22 engagement is transduced into intracellular Schwann cell differentiation signaling, and how the diverse non-neural activities (epithelial, cardiac, tumor) mechanistically relate to the canonical secreted paracrine axis, remain unresolved.
• No structural model of the LGI4-ADAM complex
• Downstream effector pathway in Schwann cells unmapped
• Unclear whether intracellular roles reflect a separate function or contamination of context