The role of FGF-mediated fast inactivation of Nav channels in cell excitability

摘要 电压依赖性Na+(Nav)通道是大多数神经元最显著的特征，以及其他可兴奋的， 它们产生动作电位(AP)的能力。在几个经典案例中，导航频道的主要作用是 确保AP的故障安全可靠性，无论是在轴突中的远距离AP传播中，还是在 AP在骨骼肌或心肌中的重复性。然而，人们越来越认识到，导航频道 在许多神经元和神经内分泌细胞中，神经元和神经内分泌细胞在调节细胞放电行为方面发挥着中心作用， 有助于调节重复发射时的AP高度或持续时间的变化。这些 对触发的影响，部分原因是由特定定义的NAV当前可用性的使用依赖变化 不同导航通道的失活特性。已经描述了多种类型的NAV快速灭活 频道。一种是典型的快速失活，这是NAV形成孔洞的α亚单位固有的。另一项涉及 通过N端片段的特异性细胞内成纤维细胞生长因子同源因子(IFGFs)阻断毛孔。 当两种形式的快速失活同时存在时，它们就会以一种竞争性的方式活动。这个 从两种形式的失活中恢复的差异则关键地定义了NAV当前的可用性。这个 IFGFs调节不同NAV通道的特性的程度才刚刚开始被理解。一些人 挑战是在许多细胞和多个FGF中存在多种NAV变体，一些是失活的，一些是 非停用，可以竞争与Navs的关联。在这个项目中，我们将利用一个相对简单的 肾上腺髓质的嗜铬细胞(CCS)，它们具有神经元的兴奋性特性，但提供 梳理iFGFs作用的优势。使用电生理学和遗传学相结合的方法 从老鼠身上删除特定亚基的操作，这个项目将定义Nav失活的属性 在CCS中的电流，梳理在这样的细胞中观察到的双途径快速失活行为，确定 识别CCS中发现的特定NAV电流亚型，并评估iFGFs在产生 不寻常的失活行为。该项目有望为灭活的属性提供新的见解 在成纤维细胞生长因子的介导下，正常快速失活和i成纤维细胞生长因子介导的失活的区别 两者都存在，在不同的iFGFs和不同的NAV亚型的作用中具有潜在的特异性 天然细胞，以及胞内iFGF介导的失活对细胞兴奋性的影响。作为潜在的疾病 导致iFGFs或其相关NAV通道的突变被揭示，该项目的结果将 对评估影响和潜在的未来治疗策略很重要。

Abstract Voltage-dependent Na+ (Nav) channels typify the most distinctive feature of most neurons and other excitable, their capacity to generate action potentials (AP). In several classic cases, a primary role of Nav channels is to ensure failsafe reliability of the AP, whether in long distance AP propagation in an axon or in contributing to reproducibility of APs in skeletal or cardiac muscle. However, there is a growing realization that Nav channels in many neurons and neuroendocrine cells play central roles in the regulation of cell firing behavior, contributing to accommodation during repetitive firing with associated changes in AP height or duration. These effects on firing arise, in part, from use-dependent changes in Nav current availability defined by the specific inactivation properties of different Nav channels. Multiple kinds of fast inactivation have been described for Nav channels. One is classic fast inactivation that is intrinsic to the Nav pore-forming α subunit. Another involves pore block by N-terminal segments of specific intracellular fibroblast growth factor homologous factors (iFGFs). When two forms of fast inactivation are present at the same time, they act in a competitive fashon. The differences in recovery from inactivation of the two forms then critically define Nav current availability. The extent to which iFGFs regulate properties of different Nav channels is only beginning to be understood. Some challenges are that there are multiple Nav variants in many cells and multiple FGFs, some inactivating, some noninactivating, that can compete for association with Navs. In this project, we will utilize a relatively simple cell, chromaffin cells (CCs) of the adrenal medulla, which have the excitability properties of neurons, but offer advantages for teasing apart the role of iFGFs. Using methods of electrophysiology coupled with genetic manipulations that delete specific subunits from mice, this project will define properties of inactivation of Nav current in CCs, tease apart the dual-pathway fast inactivation behavior observed in such cells, determine the identity of specific subtypes of Nav currents found in CCs, and assess the role of iFGFs in producing the unusual inactivation behavior. This project is expected to provide new insight into the properties of inactivation mediated by FGF's, the distinctions between normal fast inactivation and iFGF-mediated inactivation when both are present, potential specificity in the roles of different iFGFs and different Nav isoforms in regards to native cells, and the impact of cytosolic iFGF-mediated inactivation on cell excitability. As potential disease causing mutations in iFGF's or their associated Nav channels become revealed, the results of this project will be important to assessing the impact and potential future therapeutic strategies.