Emerging Role of Tripartite Synaptic Transmission in the Pathomechanism of Autosomal-Dominant Sleep-Related Hypermotor Epilepsy

TL;DR

This paper explores how a genetic mutation in CHRNA4 contributes to a rare form of epilepsy by affecting brain communication during sleep.

Contribution

The paper proposes a new hypothetical pathomechanism for ADSHE involving tripartite synaptic transmission and astroglial hemichannels.

Findings

Loss-of-function of S284L-mutant nAChRs leads to GABAergic disinhibition and enhanced ripple-burst oscillations during sleep.

Upregulation of astroglial hemichannels under GABAergic disinhibition contributes to epileptogenic fast-ripple oscillations.

ADSHE-mutant nAChRs alone are insufficient for ictogenesis but initiate epileptogenesis during neurodevelopment.

Abstract

Autosomal-dominant sleep-related hypermotor epilepsy (ADSHE) was the first distinct genetic epilepsy proven to be caused by mutation of the CHRNA4 gene, originally reported in 1994. In the past three decades, pathomechanisms of ADSHE associated with mutant nicotinic acetylcholine receptors (nAChRs) have been explored via various studies, including in vitro experiments and genetic rodent models. However, findings emphasize that functional abnormalities of ADSHE-mutant nAChRs alone cannot generate ictogenesis; rather, development of abnormalities in various other transmission systems induced by ADSHE-mutant nAChRs during the neurodevelopmental process before the ADSHE onset is involved in development of epileptogenesis/ictogenesis. Intra-thalamic GABAergic disinhibition induced by loss-of-function of S284L-mutant nAChRs (S286L-mutant nAChRs in rat ADSHE models) contributes to enhancing propagation of physiological ripple-burst high-frequency oscillation (HFO) and Erk signaling during sleep, leading to enhancement of the trafficking of pannexin1, connexin43, and P2X7 purinergic receptor to the astroglial plasma membrane. The combination of activation of physiological ripple-HFO and upregulation of astroglial hemichannels under the GABAergic disinhibition plays an important role in generation of epileptogenic fast-ripple-HFO during sleep. Therefore, loss-of-function of the S284L-mutation alone cannot drive ictogenesis but contributes to the development of epileptogenesis as an initial abnormality. Based on these recent findings using genetic rat ADSHE models, harboring the rat S286L-mutant Chrna4 corresponding to the human S284L-mutant CHRNA4, this report proposes hypothetical pathomechanisms of ADSHE.

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