A Missense Mutation of the Gene Encoding Synaptic Vesicle Glycoprotein 2A (SV2A) Confers Seizure Susceptibility by Disrupting Amygdalar Synaptic GABA Release

Kentaro Tokudome,Tomoji Mashimo,Ryo Terada,Tadao Serikawa,Masashi Sasa,Yukihiro Ohno,Takahiro Okumura,Saki Shimizu,Naofumi Kunisawa

doi:10.3389/fphar.2016.00210

Abstract

Synaptic vesicle glycoprotein 2A (SV2A) is specifically expressed in the membranes of synaptic vesicles and modulates action potential-dependent neurotransmitter release. To explore the role of SV2A in the pathogenesis of epileptic disorders, we recently generated a novel rat model (Sv2aL174Q rat) carrying a missense mutation of the Sv2a gene and showed that the Sv2aL174Q rats were hypersensitive to kindling development (Tokudome et al., 2016). Here, we further conducted behavioral and neurochemical studies to clarify the pathophysiological mechanisms underlying the seizure vulnerability in Sv2aL174Q rats. Sv2aL174Q rats were highly susceptible to pentylenetetrazole (PTZ)-induced seizures, yielding a significantly higher seizure scores and seizure incidence than the control animals. Brain mapping analysis of Fos expression, a biological marker of neural excitation, revealed that the seizure threshold level of PTZ region-specifically elevated Fos expression in the amygdala in Sv2aL174Q rats. In vivo microdialysis study showed that the Sv2aL174Q mutation preferentially reduced high K+ (depolarization)-evoked GABA release, but not glutamate release, in the amygdala. In addition, specific control of GABA release by SV2A was supported by its predominant expression in GABAergic neurons, which were co-stained with antibodies against SV2A and glutamate decarboxylase 1. The present results suggest that dysfunction of SV2A by the missense mutation elevates seizure susceptibility in rats by preferentially disrupting synaptic GABA release in the amygdala, illustrating the crucial role of amygdalar SV2A-GABAergic system in epileptogenesis.

Highlights

Synaptic vesicle glycoprotein 2A (SV2A) is highly expressed in the brain including the cerebral cortex, limbic regions, and cerebellum, where it modulates action potential-dependent neurotransmitter release (Crowder et al, 1999; Janz et al, 1999; Xu and Bajjalieh, 2001; Custer et al, 2006; Chang and Südhof, 2009)
In order to confirm seizure susceptibility of Sv2aL174Q rats, we evaluated the responses of Sv2aL174Q and F344 rats to PTZ injections (30–50 mg/kg, i.p.)
The present study confirmed that Sv2aL174Q rats carrying a missense mutation, L174Q, in the Sv2a gene were highly sensitive to PTZ-induced seizures, supporting the notion that SV2A plays the crucial role in controlling seizure susceptibility

Summary

Introduction

Synaptic vesicle glycoprotein 2A (SV2A) is highly expressed in the brain including the cerebral cortex, limbic regions, and cerebellum, where it modulates action potential-dependent neurotransmitter release (Crowder et al, 1999; Janz et al, 1999; Xu and Bajjalieh, 2001; Custer et al, 2006; Chang and Südhof, 2009). Previous studies suggest that SV2A plays an important role in the pathogenesis and treatment of epileptic disorders This is because (1) SV2A-knockout mice exhibited severe seizures (Crowder et al, 1999; Janz et al, 1999), (2) SV2A serves as a specific binding site for certain antiepileptics (e.g., levetiracetam and its analogs; Lynch et al, 2004; Pollard, 2008; Kaminski et al, 2009; Correa-Basurto et al, 2015; Klitgaard et al, 2016) and (3) the expressional levels of SV2A are reported to be altered in various epileptic conditions both in animals (e.g., chemicallyand electrically induced kindling) and humans (e.g., intractable temporal lobe epilepsy and focal cortical dysplasia; Matveeva et al, 2007; Feng et al, 2009; Ohno et al, 2009a, 2012b; Toering et al, 2009; van Vliet et al, 2009; Crèvecoeur et al, 2014; Serajee and Huq, 2015). The detailed mechanisms underlying the regulation of seizure susceptibility by SV2A remain to be clarified

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Conclusion