Abstract
Synaptic vesicle glycoprotein 2A (SV2A) is a prototype synaptic vesicle protein regulating action potential-dependent neurotransmitters release. SV2A also serves as a specific binding site for certain antiepileptics and is implicated in the treatment of epilepsy. Here, to elucidate the role of SV2A in modulating epileptogenesis, we generated a novel rat model (Sv2aL174Q rat) carrying a Sv2a-targeted missense mutation (L174Q) and analyzed its susceptibilities to kindling development. Although animals homozygous for the Sv2aL174Q mutation exhibited normal appearance and development, they are susceptible to pentylenetetrazole (PTZ) seizures. In addition, development of kindling associated with repeated PTZ treatments or focal stimulation of the amygdala was markedly facilitated by the Sv2aL174Q mutation. Neurochemical studies revealed that the Sv2aL174Q mutation specifically reduced depolarization-induced GABA, but not glutamate, release in the hippocampus without affecting basal release or the SV2A expression level in GABAergic neurons. In addition, the Sv2aL174Q mutation selectively reduced the synaptotagmin1 (Syt1) level among the exocytosis-related proteins examined. The present results demonstrate that dysfunction of SV2A due to the Sv2aL174Q mutation impairs the synaptic GABA release by reducing the Syt1 level and facilitates the kindling development, illustrating the crucial role of SV2A-GABA system in modulating kindling epileptogenesis.
Highlights
Previous studies demonstrated that animals lacking Synaptic vesicle glycoprotein 2A (SV2A) failed to grow, exhibited severe seizures and died within 3 weeks[3,4]
A previous study using knockout-rescue techniques showed that the neighboring amino acid sequence (D179 and E182) in the 1st transmembrane region is essential for the normal structure and function of SV2A7
Since the L174Q mutation of Sv2a interrupted the depolarization-induced GABA release, we evaluated the changes in expression of exocytosis modulator proteins, syntaxin binding protein 1 (Stxbp1), N-ethylmaleimide-sensitive factor (Nsf ), N-ethylmaleimide-sensitive factor alpha (Napa) and Syt[1] in the hippocampus (Fig. 7A)
Summary
Previous studies demonstrated that animals lacking SV2A failed to grow, exhibited severe seizures and died within 3 weeks[3,4]. A recent study showed that a homozygous missense mutation (R383Q) in the SV2A gene resulted in intractable epilepsy, involuntary movements, microcephaly and developmental retardation[20]. All these findings suggest that SV2A is implicated in the pathogenesis and treatment of epileptic disorders, but detailed functions and mechanisms (e.g., neurotransmitter specificity) of SV2A in epileptogenesis remain unknown. In order to clarify the function and mechanism of SV2A in modulating epileptogenesis, we generated Sv2a-targeted rats carrying a missense mutation (L174Q) in the 1st transmembrane spanning region of SV2A, using gene-driven N-ethyl-N-nitrosourea (ENU) mutagenesis/MuT-POWER techniques[21]. While gross behaviors of the Sv2aL174Q rats were normal, these animals exhibited a markedly high susceptibility to the development of kindling, an experimental model of epileptogenesis, and caused disrupted GABA release in the hippocampus, illustrating the crucial role of SV2A-GABA system in modulating epileptogenesis
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