LGI2

LGI2 is a secreted, glycosylated neuronal protein that organizes axonal and synaptic connectivity in the nervous system by acting extracellularly on metalloproteinase-lacking ADAM family receptors (PMID:21829378, PMID:36828548). It is secreted into the extracellular medium in glycosylated form, and its secretion depends on correct folding of its leucine-rich repeat and EAR propeller domains; non-synonymous mutations affecting the signal peptide, LRR, or EAR domain render the protein secretion-deficient and cause it to accumulate in the endoplasmic reticulum, with structurally homologous EAR mutations producing the same defect in LGI1 (PMID:15857855, PMID:20863412). Once secreted, LGI2 binds the metalloproteinase-lacking ADAM receptors ADAM22/ADAM23, and a protein-truncating mutation that blocks secretion also abolishes this ADAM interaction, placing LGI2 in the same receptor pathway as LGI1 (PMID:21829378). Through its interaction with axonal ADAM23, LGI2 (with LGI3) controls the accumulation and stability of juxtaparanodal Kv1 (Shaker-type) potassium channel complexes in myelinated axons, thereby regulating the action-potential refractory period and high-frequency burst firing (PMID:36828548). LGI2 is expressed predominantly in GABAergic interneurons, and its ablation in mice provokes ictal-like activity in hippocampal slices and impairs spatial reversal learning and cognitive flexibility (PMID:41177215); it is also expressed presynaptically by inferior olivary neurons, where it acts together with C1QL1 and CRTAC1 to specify the mature climbing fiber synaptic connectivity onto cerebellar Purkinje cells (PMID:39658623).

1. 2005 Medium

   Establishing whether LGI2 is secreted defined the basic mode of action of the protein, showing it acts in the extracellular space like other LGI family members.

   Evidence Transfection of 293T cells and detection of LGI2 (and splice form LGI2B) in culture medium

   PMID:15857855

   Open questions at the time

   • No receptor or extracellular partner identified at this stage
   • Native neuronal secretion not yet demonstrated
2. 2010 Medium

   Mapping which domains are required for secretion clarified how disease-associated mutations disrupt LGI2 function at the level of trafficking rather than activity.

   Evidence Transfection-based secretion and subcellular localization assays with mutagenesis of signal peptide, LRR, and EAR domains, plus 3D modeling of the EAR domain

   PMID:20863412

   Open questions at the time

   • Does not establish the extracellular binding partner
   • Structural model not experimentally resolved
3. 2011 High

   Identifying the ADAM22/ADAM23 interaction and a secretion-blocking truncating mutation placed LGI2 in a defined receptor pathway shared with LGI1 and linked it to neurological disease.

   Evidence GWAS mapping and identification of a truncating mutation in a canine (Lagotto Romagnolo) model, with secretion and ADAM interaction assays

   PMID:21829378

   Open questions at the time

   • Downstream effect of ADAM binding not yet defined
   • Functional consequence at axons/synapses not established
4. 2023 High

   Linking LGI2-ADAM23 signaling to juxtaparanodal Kv1 channel clustering defined the cellular consequence of the interaction and its impact on axonal excitability.

   Evidence ADAM23 genetic knockout, peripheral nerve immunohistochemistry, and electrophysiological refractory-period measurements

   PMID:36828548

   Open questions at the time

   • Relative contributions of LGI2 versus LGI3 not resolved
   • Mechanism of Kv1 recruitment downstream of ADAM23 unknown
5. 2024 Medium

   Demonstrating presynaptic LGI2 expression in inferior olivary neurons and combinatorial action with C1QL1/CRTAC1 extended its role from axonal channel clustering to circuit-level synapse specification.

   Evidence Mouse postnatal developmental expression analysis and genetic dissection of presynaptic secreted protein contributions to climbing fiber-Purkinje cell connectivity

   PMID:39658623

   Open questions at the time

   • Receptor mediating the synapse-specification function not defined
   • Molecular interplay with C1QL1/CRTAC1 not mechanistically resolved
6. 2025 Medium

   Cell-type-resolved expression and knockout phenotyping established LGI2 as a GABAergic-interneuron protein whose loss causes epileptiform activity and cognitive deficits, connecting molecular function to organismal phenotype.

   Evidence Expression analysis, Lgi2 knockout mice, hippocampal slice electrophysiology, and behavioral testing

   PMID:41177215

   Open questions at the time

   • Causal link between ictal activity and the specific ADAM/Kv1 pathway not directly established
   • Circuit mechanism for cognitive flexibility deficits unknown

• How LGI2's two roles — juxtaparanodal Kv1 channel stabilization and presynaptic synapse-identity specification — are coordinated at the molecular level, and which receptors mediate each, remains unresolved.
• No structural data on the LGI2-ADAM complex
• Mechanism coupling ADAM binding to Kv1 clustering undefined
• Receptor for the synapse-specification function not identified