Severe deficiency of voltage-gated sodium channel NaV1.2 elevates neuronal excitability in adult mice

Scn2a encodes voltage-gated sodium channel NaV1.2, a main mediator of neuronal action potential firing. The current paradigm suggests that NaV1.2 gain-of-function variants enhance neuronal excitability resulting in epilepsy, whereas NaV1.2 deficiency impairs excitability contributing to autism. This paradigm, however, does not explain why 20~30% of patients with NaV1.2 deficiency still develop seizures. Here we report a counterintuitive finding that severe NaV1.2 deficiency results in increased neuronal excitability. Using a unique NaV1.2-deficient mouse model, we found enhanced intrinsic excitabilities of principal neurons in the prefrontal cortex and striatum, brain regions known to be involved in Scn2a-related seizures. This increased excitability is autonomous, and is reversible by the genetic restoration of Scn2a expression in adult mice. RNA-sequencing revealed that the downregulation of multiple potassium channels including KV1.1, and KV channel openers alleviated hyperexcitability of NaV1.2-deficient neurons. This unexpected neuronal hyperexcitability may serve as a cellular basis underlying NaV1.2 deficiency-related seizures. Since Scn2a null (100% knockout) is lethal, we thus generated a novel NaV1.2-deficient mouse model via a gene-trap strategy . Using this unique mouse model, we investigated how severe NaV1.2 deficiency affects neuronal excitabilities in principal neurons of the cortico-striatal circuit.Scn2a编码电压门控钠通道NaV1.2，后者是神经元动作电位发放的主要介导因子。当前主流研究范式认为，NaV1.2功能获得性变异会增强神经元兴奋性，进而引发癫痫；而NaV1.2功能缺失会削弱神经元兴奋性，从而导致自闭症。然而，这一研究范式无法解释为何20%~30%的NaV1.2功能缺失患者仍会出现癫痫发作。本研究报道了一项反直觉的发现：严重的NaV1.2功能缺失反而会导致神经元兴奋性升高。本研究利用一种独特的NaV1.2功能缺失小鼠模型，发现前额叶皮层与纹状体中的主神经元内在兴奋性增强——这两个脑区已被证实与Scn2a相关的癫痫发作有关。这种兴奋性升高具有自主性，且可通过在成年小鼠体内恢复Scn2a的基因表达来逆转。RNA测序（RNA-sequencing）结果显示，包括KV1.1在内的多种钾通道表达下调，而钾通道开放剂可缓解NaV1.2功能缺失神经元的过度兴奋性。这种意料之外的神经元过度兴奋性，可能是NaV1.2功能缺失相关癫痫发作的细胞机制基础。由于Scn2a纯合敲除（100%基因敲除）具有致死性，本研究通过基因陷阱（gene-trap）策略构建了一种新型的NaV1.2功能缺失小鼠模型。本研究利用这一独特的小鼠模型，探究了严重的NaV1.2功能缺失如何影响皮层-纹状体环路中主神经元的兴奋性。