An Investigation on the Pathogenic Mechanism of Mutant LGI1-Induced Autosomal Dominant Lateral Temporal Lobe Epilepsy.

Autosomal dominant lateral temporal lobe epilepsy (ADLTE) is a seizure disorder with the epileptic focus found in or near the auditory center of temporal lobe cortex. Epileptic seizures observed in ADLTE patients result in auditory hallucinations with or without generalized convulsive response and unconsciousness. Molecular genetics studies identified numerous missense or truncating mutations of LGI1 (leucine-rich glioma inactivated gene 1) as the cause of ADLTE. LGI1 mutation is also associated with sporadic partial epilepsy with auditory features. Thus, LGI1 mutations are involved in the pathogenesis of both hereditary and sporadic epileptic disorders. Up to now, little is known about the pathogenic mechanism of ADLTE and physiological function of LGI1 in the brain. LGI1 protein and mRNA are widely expressed in brain tissues, including cerebellum, hippocampus, putamen and cortical areas of the frontal, parietal, occipital and temporal lobes. However, clinical seizure manifestation and EEG abnormality clearly indicate that mutant LGI1 only causes neuronal hyperexcitability and epileptic seizures of temporal lobe cortex, suggesting that a cell-specific mechanism is involved in the pathogenesis of ADLTE and that neocortical neurons of the temporal lobe, which are vulnerable to mutant LGI1-induced epileptogenesis, should be used to study the epileptogenic mechanism of mutant LGI1. In order to gain an insight into the pathogenic mechanism of ADLTE and physiological function of LGI1 in the brain, we will investigate cellular and molecular mechanisms underlying ADLTE mutant LGI1-induced epileptogenesis by performing the following experiments using in vitro cellular and in vivo transgenic mouse models of ADLTE: (1) An upregulated glutamatergic transmission and/or a downregulated GABAergic neurotransmission is expected to cause the neuronal hyperexcitability, which leads to epileptiform activity of affected neurons and subsequent synchronized discharges of a population of neurons and an epileptic seizure. LGI1 is the member of leucine-rich repeat protein family, which has been shown to regulate the outgrowth of neuronal processes and the formation, maturation and plasticity of glutamatergic and GABAergic synapses in the brain. Therefore, it is hypothesized that wild-type LGI1 plays an important role in regulating glutamatergic and/or GABAergic neurotransmission in the temporal cortex and that ADLTE mutant LGI1 causes a dysregulation of glutamatergic and/or GABAergic synaptic activities, which results in the genesis and spread of epileptic seizures. To test this hypothesis, we will study the effect of wild-type or ADLTE mutant LGI1 on glutamatergic and GABAergic 4 neurotransmissions in temporal lobe pyramidal neurons. (2) ADLTE mutant LGI1 could alter glutamatergic and/or GABAergic synaptic transmission in temporal lobe pyramidal neurons by affecting glutamate/GABA release from nerve terminals or the density or efficacy of postsynaptic glutamatergic or GABAergic receptors. It is also possible that ADLTE mutant LGI1 causes the hyperexcitability and epileptiform discharges of temporal pyramidal neurons by increasing the number of excitatory glutamatergic synapses and/or reducing the number of inhibitory GABAergic synapses. In the present study, a series of experiments are performed to investigate the action site and molecular mechanism of ADLTE mutant LGI1-induced alteration of glutamatergic and/or GABAergic neurotransmission in temporal pyramidal neurons. (3) Initial study reported that LGI1 protein contains a transmembrane segment. In contrast to the original finding, subsequent studies showed that when expressed in cell lines, LGI1 protein was secreted into the medium and that ADLTE LGI1 mutants were not secreted. Thus, it has been proposed that LGI1 is a secreted protein and that loss-of-function ADLTE mutations cause an impairment of LGI1 secretion, which results in the epileptogenesis of ADLTE. However, it is still unknown whether LGI1 functions as a secreted protein or transmembrane protein in temporal cortical neurons. To better understand the molecular mechanism underlying LGI1-induced biological effects and the involvement of impaired LGI1 secretion in the pathogenesis of ADLTE, we will study the functional importance of extracellular secretion in wild-type LGI1-induced biological effects and the effect of ADLTE mutation on LGI1 secretion in cultured temporal cortical neurons. (4) LGI1 is likely to regulate glutamatergic and/or GABAergic synaptic activities in the temporal cortex by interacting with unknown proteins. An altered interaction between ADLTE mutant LGI1 and protein interactor(s) may result in the epileptogenesis. Identification of proteins that interact with LGI1 is required to understand the physiological function of LGI1 in the temporal cortex and pathogenic mechanism of ADLTE. In the present study, proteomic analysis is performed to identify protein interactor(s) of LGI1 in the temporal cortical neurons. Our study should shed light on the pathogenic mechanism of ADLTE and physiological function of LGI1. The results obtained from the present investigation could also lead to the development of possible therapeutic strategies for ADLTE and other epileptic disorders.

Project ID:PC9801-2486
External Project ID:NSC97-2320-B182-021-MY3