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

摘要 电压依赖性Na+（Nav）通道代表了大多数神经元和其他可兴奋， 产生动作电位（AP）。在一些典型的情况下，导航通道的主要作用是 确保AP的故障安全可靠性，无论是在轴突中的长距离AP传播中还是在促进 AP在骨骼肌或心肌中的再现性。然而，越来越多的人意识到， 在许多神经元和神经内分泌细胞中，在细胞放电行为的调节中起着中心作用， 有助于在重复放电期间的调节，并伴随AP高度或持续时间的相关变化。这些 对发射的影响，部分地是由于导航电流可用性的使用依赖性变化引起的，导航电流可用性由具体的 不同Nav通道的失活特性。已经描述了Nav的多种快速灭活 渠道一种是典型的快速失活，这是Nav孔形成α亚基固有的。另一个涉及 孔被特异性细胞内成纤维细胞生长因子同源因子（iFGF）的N末端区段阻断。 当两种形式的快速失活同时存在时，它们以竞争方式起作用。的 两种形式的灭活回收率差异则决定性地定义了Nav电流可用性。的 iFGF调节不同Nav通道性质的程度才刚刚开始被理解。一些 挑战在于，在许多细胞和多个FGF中存在多种Nav变体，一些失活。 非失活，可以与Navs竞争关联。在这个项目中，我们将利用一个相对简单的 细胞，肾上腺髓质的嗜铬细胞（CC），具有神经元的兴奋性，但提供 这是将iFGF的作用分开的优势。利用电生理学和遗传学的方法 从小鼠中删除特定亚基的操作，该项目将定义Nav失活的特性， 电流的CC，梳理除了双途径快速失活行为观察到的细胞，确定 识别CC中发现的特定Nav电流亚型，并评估iFGF在产生 不寻常的失活行为。该项目有望为失活的性质提供新的见解 通过FGF介导的，正常的快速失活和iFGF介导的失活之间的区别， 两者都存在，不同iFGF和不同Nav亚型在以下方面的作用具有潜在特异性： 天然细胞，以及胞质iFGF介导的失活对细胞兴奋性的影响。作为潜在疾病 导致iFGF或其相关的Nav通道突变，该项目的结果将 重要的是评估的影响和潜在的未来治疗策略。

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.