More than 30 mutations in LGI1, a secreted neuronal
protein, have been reported with
autosomal dominant lateral temporal lobe epilepsy (ADLTE). Although LGI1 haploinsufficiency is thought to cause ADLTE, the underlying molecular mechanism that results in abnormal brain excitability remains mysterious. Here, we focused on a mode of action of
LGI1 autoantibodies associated with
limbic encephalitis (LE), which is one of acquired epileptic disorders characterized by subacute onset of
amnesia and
seizures. We comprehensively screened human sera from patients with immune-mediated
neurological disorders for
LGI1 autoantibodies, which also uncovered novel
autoantibodies against six
cell surface antigens including DCC, DPP10, and ADAM23. Our developed ELISA arrays revealed a specific role for
LGI1 antibodies in LE and concomitant involvement of multiple
antibodies, including
LGI1 antibodies in
neuromyotonia, a peripheral nerve disorder.
LGI1 antibodies associated with LE specifically inhibited the
ligand-receptor interaction between LGI1 and ADAM22/23 by targeting the
EPTP repeat domain of LGI1 and reversibly reduced synaptic
AMPA receptor clusters in rat hippocampal neurons. Furthermore, we found that disruption of LGI1-ADAM22 interaction by soluble extracellular domain of ADAM22 was sufficient to reduce synaptic
AMPA receptors in rat hippocampal neurons and that levels of
AMPA receptor were greatly reduced in the hippocampal dentate gyrus in the epileptic LGI1 knock-out mouse. Therefore, either genetic or acquired loss of the LGI1-ADAM22 interaction reduces the
AMPA receptor function, causing epileptic disorders. These results suggest that by finely regulating the synaptic
AMPA receptors, the LGI1-ADAM22 interaction maintains physiological brain excitability throughout life.