LGI1 mutations in autosomal dominant and sporadic lateral temporal epilepsy

Biological characterization of ADAM22 variants reveals the importance of a disintegrin domain sequence in cell surface expression

ADAM metallopeptidase domain 22 (ADAM22) is a neuronal membrane-spanning protein that is a potential receptor for leucine-rich, glioma-inactivated 1 (LGI1), and leucine-rich repeat LGI family, member 4 (LGI4). Several lines of study have shown a direct interaction between ADAM22 and LGI1, a mutation which is responsible for inherited epilepsy in humans. Both ADAM22-deficient mice and claw paw mice, congenitally deficient in LGI4, show hypomyelination in the peripheral nerves, suggesting that these molecules are involved in myelination processes. These findings mark ADAM22 as a potential target molecule for epilepsy or demyelination diseases. To investigate the relationship between ADAM22 mutation and its biological character, we designed and examined several ADAM22 variants. We discovered that the ADAM22 P81R variant, the most common polymorphic variation, works as well as the wild-type ADAM22. We also showed that mutations in the disintegrin domain cause a marked decrease in the processing of ADAM22 preproteins, and result in reduced LGI4-binding abilities. Our findings provide valuable information for mutation screening of the ADAM22 gene in patients suffering from epilepsy or demyelinating diseases.

Adam22 is a major neuronal receptor for Lgi4-mediated Schwann cell signaling

The segregation and myelination of axons in the developing PNS, results from a complex series of cellular and molecular interactions between Schwann cells and axons. Previously we identified the Lgi4 gene (leucine-rich glioma-inactivated4) as an important regulator of myelination in the PNS, and its dysfunction results in arthrogryposis as observed in claw paw mice. Lgi4 is a secreted protein and a member of a small family of proteins that are predominantly expressed in the nervous system. Their mechanism of action is unknown but may involve binding to members of the Adam (A disintegrin and metalloprotease) family of transmembrane proteins, in particular Adam22. We found that Lgi4 and Adam22 are both expressed in Schwann cells as well as in sensory neurons and that Lgi4 binds directly to Adam22 without a requirement for additional membrane associated factors. To determine whether Lgi4-Adam22 function involves a paracrine and/or an autocrine mechanism of action we performed heterotypic Schwann cell sensory neuron cultures and cell type-specific ablation of Lgi4 and Adam22 in mice. We show that Schwann cells are the principal cellular source of Lgi4 in the developing nerve and that Adam22 is required on axons. Our results thus reveal a novel paracrine signaling axis in peripheral nerve myelination in which Schwann cell secreted Lgi4 functions through binding of axonal Adam22 to drive the differentiation of Schwann cells.

Lgi1 null mutant mice exhibit myoclonic seizures and CA1 neuronal hyperexcitability

LGI1 in humans is responsible for a predisposition to autosomal dominant partial epilepsy with auditory features (ADPEAF). However, mechanisms of how LGI1 mutations cause epilepsy remain unclear. We have used a mouse chromosome engineering strategy to create a null mutation for the gene ortholog encoding LGI1. The Lgi1 null mutant mice show no gross overall developmental abnormalities from routine histopathological analysis. After 12-18 days of age, the homozygous mutant mice all exhibit myoclonic seizures accompanied by rapid jumping and running and die shortly thereafter. The heterozygous mutant mice do not develop seizures. Electrophysiological analysis demonstrates an enhanced excitatory synaptic transmission by increasing the release of the excitatory neurotransmitter glutamate, suggesting a basis for the seizure phenotype. This mouse model, therefore, provides novel insights into the mechanism behind ADPEAF and offers a new opportunity to study the mechanism behind the role of LGI1 in susceptibility to myoclonic seizures.

Disruption of LGI1-linked synaptic complex causes abnormal synaptic transmission and epilepsy

Epilepsy is a devastating and poorly understood disease. Mutations in a secreted neuronal protein, leucine-rich glioma inactivated 1 (LGI1), were reported in patients with an inherited form of human epilepsy, autosomal dominant partial epilepsy with auditory features (ADPEAF). Here, we report an essential role of LGI1 as an antiepileptogenic ligand. We find that loss of LGI1 in mice (LGI1(-/-)) causes lethal epilepsy, which is specifically rescued by the neuronal expression of LGI1 transgene, but not LGI3. Moreover, heterozygous mice for the LGI1 mutation (LGI1(+/-)) show lowered seizure thresholds. Extracellularly secreted LGI1 links two epilepsy-related receptors, ADAM22 and ADAM23, in the brain and organizes a transsynaptic protein complex that includes presynaptic potassium channels and postsynaptic AMPA receptor scaffolds. A lack of LGI1 disrupts this synaptic protein connection and selectively reduces AMPA receptor-mediated synaptic transmission in the hippocampus. Thus, LGI1 may serve as a major determinant of brain excitation, and the LGI1 gene-targeted mouse provides a good model for human epilepsy.

Advances on the genetics of mendelian idiopathic epilepsies

Genetic factors play an increasingly recognized role in idiopathic epilepsies. Since 1995, positional cloning strategies in multi-generational families with autosomal dominant transmission have revealed 11 genes (KCNQ2, KCNQ3, CHRNA4, CHRNA2, CHRNB2, SCN1B, SCN1A, SCN2A, GABRG2, GABRA1, and LGI1) and numerous loci for febrile seizures and epilepsies. To date, all genes with the exception of LGI1 (leucine-rich glioma inactivated 1), encode neuronal ion channel or neurotransmitter receptor subunits. Molecular approaches have revealed great genetic heterogeneity, with the vast majority of genes remaining to be identified. One of the major challenges is now to understand phenotype-genotype correlations. This review focuses on the current knowledge on the molecular basis of these rare Mendelian autosomal dominant forms of idiopathic epilepsies.

LGI1-associated epilepsy through altered ADAM23-dependent neuronal morphology

Most epilepsy genes encode ion channels, but the LGI1 gene responsible for autosomal dominant partial epilepsy with auditory features produces a secreted protein. LGI1 is suggested to regulate PSD-95 via ADAM22. However, no unbiased screen of LGI1 action has been conducted. Here, we searched for brain genes supporting high affinity LGI-1 binding. ADAM23 was the only LGI1 interactor identified. The related proteins, ADAM22 and ADAM11, but not ADAM12, bind LGI1. Neither ADAM23 nor ADAM11, nor some forms of ADAM22, contain PDZ-interacting sequences, suggesting PSD-95-independent mechanisms in ADPEAF. Because ADAMs modulate integrins, we examined LGI1 effect on neurite outgrowth. LGI1 increases outgrowth from wild-type but not ADAM23-/- neurons. Furthermore, CA1 pyramidal neurons of ADAM23-/- hippocampi have reduced dendritic arborization. ADAM23-/- mice exhibit spontaneous seizures, while ADAM23+/- mice have decreased seizure thresholds. Thus, LGI1 binding to ADAM23 is necessary to correctly pattern neuronal morphology and altered anatomical patterning contributes to ADPEAF.