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

Abstract

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.A ffecting 1-2% of the population, epilepsy is one of the most common neurological disorders. Epilepsy is characterized by recurrent unprovoked seizures and is caused by disturbances in the delicate balance between excitation and inhibition in neural circuits (1, 2). Recent human genetic studies have established the channelopathy concept for idiopathic (inherited) epilepsies: Many of the genes whose mutations cause human epilepsy encode ion channel subunits (1, 2). Examples include voltagegated ion channels (K + , Na + , Ca 2+ , and Cl -channels) and ligandgated ion channels (nicotinic acetylcholine and GABA A receptors), which regulate neuronal excitability.Leucine-rich glioma inactivated 1 (LGI1) is a unique human epilepsy-related gene in that it does not encode an ion channel subunit (3-5), but is a neuronal secreted protein (6). Mutations in LGI1 are linked to autosomal dominant partial epilepsy with auditory features (ADPEAF, also known as autosomal dominant lateral temporal lobe epilepsy [ADLTE]) (3-5), which is an inherited epileptic syndrome characterized by partial seizures with acoustic or visual hallucinations. So far, 25 LGI1 mutations have been described in familial ADPEAF patients and sporadic cases (7). Interestingly, at least six tested ADPEAF mutations all abolish LGI1 secretion (6,8).Recent proteomic analysis identified LGI1 as a subunit of presynaptic Kv 1 (shaker type)-voltage gated potassium channels (9). It was shown that LGI1 selectively prevents inactivation of the Kv 1 channel mediated by a cytoplasmic regulatory protein, Kvβ. Because LGI1 is a secreted protein, it remains unclear how LGI1 modulates a cytosolic potassium channel mechanism. LGI1 was also isolated from the brain as a component of a protein complex mediated by PSD-95, a representative postsynaptic scaffolding protein.LGI1 functions as a ligand for the epilepsy-related ADAM22 transmembrane protein, which is anchored by PS...

LGI1 tunes intrinsic excitability by regulating the density of axonal Kv1 channels

“…LGI1 depletion might induce loss of AMPA receptor function in inhibitory interneurons, thus increasing overall excitability in the hippocampus (6). However, inhibitory synaptic transmission is not affected in LGI1 mutant mice (7,13) and conditional LGI1 knock out in inhibitory parvalbumin interneurons does not confer an epileptic phenotype (8).…”

“…This subset was initially identified by the presence of serum antibodies that immunoprecipitate a VGKC complex labeled with 125 I-α dendrotoxin (16,17), a ligand which binds to channels containing Kv1.1, Kv1.2, or Kv1.6 subunits (18). Immunoprecipitation of the VGKC by LE sera is mainly due to an anti-LGI1 antibody specificity (14,15), consistent with association of LGI1 with Kv1 channels in a multiprotein complex (19), containing ADAM22 and -23, MAGUKs, Caspr2, and contactin2 (6,14,19,20).…”

“…Analysis of glutamatergic neurotransmission in hippocampal slices from Lgi1 −/− mice has yielded conflicting evidence, as both enhanced excitatory transmission (7,21) and reduced AMPA receptor function (6,22,23) have been reported.…”

“…Most ADEAF mutations impair secretion of the protein (3,4). Animal models of LGI1 depletion, including Lgi1 −/− mice (5)(6)(7)(8), mutant rats (9), and knockdown in zebrafish (10), all display spontaneous seizures. However, the mechanisms by which LGI1 deficiency leads to epilepsy are unknown.…”

Heterozygous Reelin Mutations Cause Autosomal-Dominant Lateral Temporal Epilepsy

“…For LGI1, we used the antibody ab30868, whose specificity has previously been demonstrated. 33 The temporal cortex and hippocampus of immature (3 weeks old) and mature (2 months old) rats were examined. As shown in Figure 5, a subset of neurons in both brain areas were immunoreactive for both Reelin and LGI1.…”