LGI2

LGI2 is a neuronally secreted glycoprotein that functions as an extracellular ligand for ADAM22/ADAM23 receptors to organize synaptic and axonal architecture during postnatal brain development. LGI2 is secreted in glycosylated form and binds ADAM22/ADAM23; disease-causing truncations prevent both secretion and receptor interaction, causing retention in the endoplasmic reticulum (PMID:21829378, PMID:20863412). Through its interaction with axonal ADAM23, LGI2 (together with LGI3) is required for the clustering and stability of juxtaparanodal Kv1 potassium channel complexes in myelinated axons, thereby regulating the refractory period and enabling high-frequency burst firing (PMID:36828548). Expressed predominantly in GABAergic interneurons with peak expression during juvenile development, LGI2 suppresses epileptiform network activity; its loss causes ictal-like discharges in hippocampal circuits and deficits in cognitive flexibility, and a truncating mutation in canine LGI2 causes remitting focal epilepsy (PMID:41177215, PMID:21829378).

1. 2005 Medium

   Establishing that LGI2 is a secreted protein resolved whether LGI family members beyond LGI1 function extracellularly, positioning LGI2 as a potential signaling ligand rather than an intracellular factor.

   Evidence Secretion assay after transfection of LGI2B splice form into 293T cells

   PMID:15857855

   Open questions at the time

   • Secretion shown only in heterologous 293T cells, not in neurons
   • Receptor or binding partner unknown
   • Functional consequence of secretion not tested
2. 2010 Medium

   Demonstrating that EAR-domain mutations homologous to epilepsy-causing LGI1 variants block LGI2 secretion and cause ER retention established a shared pathogenic mechanism across the LGI family.

   Evidence Secretion assays and subcellular localization of multiple LGI2 mutants in transfected cells, 3D structural modeling

   PMID:20863412

   Open questions at the time

   • No disease-causing LGI2 mutations in humans confirmed at this point
   • ADAM receptor interaction not yet tested for LGI2
   • Neuronal relevance of secretion failure not assessed in vivo
3. 2011 High

   Identifying a truncating LGI2 mutation that abolishes both secretion and ADAM22/ADAM23 interaction as the cause of remitting focal epilepsy in dogs established LGI2 as a functional ligand for ADAM receptors and directly linked its loss to epileptogenesis.

   Evidence Genome-wide association in Lagotto Romagnolo dogs, truncating mutation identified, secretion and ADAM interaction assays with wild-type vs. mutant protein

   PMID:21829378

   Open questions at the time

   • No human LGI2-epilepsy mutations reported
   • Downstream signaling events at ADAM receptors not characterized
   • Neural circuit mechanism of epilepsy not defined
4. 2023 High

   Showing that LGI2 interaction with axonal ADAM23 is required for juxtaparanodal Kv1 channel clustering and high-frequency firing defined a specific cell-biological function — axonal ion channel organization — beyond synaptic remodeling.

   Evidence LGI2 knockout mice, immunohistochemistry for Kv1 channel mislocalization, electrophysiology measuring refractory period and burst firing, co-immunoprecipitation

   PMID:36828548

   Open questions at the time

   • Relative contributions of LGI2 vs. LGI3 to Kv1 clustering not fully separable
   • Structural basis of LGI2-ADAM23 interaction unresolved
   • Whether Kv1 mislocalization contributes to epilepsy phenotype not tested
5. 2024 Medium

   Identifying LGI2 as a presynaptic secreted factor from inferior olivary neurons that specifies excitatory synapse identity on Purkinje cells expanded its role from axonal organization to synapse-type specification in cerebellar circuits.

   Evidence Mouse developmental expression analysis and in vivo cerebellar circuit dissection, molecular epistasis with C1QL1 and CRTAC1

   PMID:39658623

   Open questions at the time

   • Single study; independent replication needed
   • ADAM receptor involvement at climbing fiber synapses not confirmed
   • Whether LGI2 cerebellar function relates to epilepsy phenotype unknown
6. 2025 Medium

   Demonstrating that LGI2 is expressed in hippocampal GABAergic interneurons with a juvenile peak and that its ablation causes epileptiform discharges and cognitive inflexibility identified the cell type and developmental window through which LGI2 suppresses network hyperexcitability.

   Evidence In situ hybridization, LGI2-deficient mouse hippocampal slice electrophysiology, behavioral testing for spatial reversal learning

   PMID:41177215

   Open questions at the time

   • Mechanism by which interneuron-secreted LGI2 suppresses excitability not defined
   • Whether cognitive deficits are secondary to developmental epileptiform activity or reflect an independent function
   • Single lab; human relevance not established

• The downstream signaling events triggered by LGI2-ADAM22/ADAM23 binding that mediate channel clustering, synapse specification, and epilepsy suppression remain undefined, and no human Mendelian epilepsy mutations in LGI2 have been reported.
• No structural model of LGI2-ADAM complex
• Intracellular signaling cascade downstream of ADAM engagement unknown
• Human genetic validation lacking