Molecular Physiology of Myotonia and Periodic Paralysis

描述（由申请人提供）：肌强直和周期性麻痹是骨骼肌的遗传性疾病，其中电压门控离子通道的突变改变了纤维的电兴奋性。该项目的长期目标是描述这些疾病中突变通道的功能缺陷，并确定异常通道行为如何在受影响的个体中产生症状。在这些疾病中，肌肉功能障碍是由纤维的电兴奋性的间歇性紊乱引起的，其可能是病理性增强或抑制的。肌强直是一种兴奋性增强的疾病，其中单个刺激激发动作电位的高频爆发，其产生持续数秒的不自主持续肌肉收缩。相反，周期性麻痹是由去极化引起的肌肉兴奋性丧失引起的。成人骨骼肌钠通道（NaV1.4）中的错义突变可能导致肌强直、周期性麻痹或同一个体中两者的组合。这种临床表型变异的病理生理学基础，都是由共同钠通道基因突变引起的，是本提案研究的主要重点。 我们的实验方法是通过测量离子电流来识别突变通道行为的改变，然后使用计算机或动物模型来探索通道功能的特定改变如何引起肌强直或周期性麻痹。目的1是表征NaV1.4通道的门控行为，新的重点是表征在其天然骨骼肌环境中表达的通道的这些特性。在我们实验室中产生的两种小鼠品系在NaV1.4（M1592V和R669H）中具有敲入点突变，这为表征肌肉中发生的突变通道行为提供了独特的机会。我们对NaV1.4的疾病相关突变的门控的研究还将探索令人兴奋的新发现，即S4电压传感器结构域内的精氨酸的错义突变可能通过不同于中心孔的替代渗透途径引起门控孔电流。动作电位向横管系统（TTS）的传播和其中K+的活动依赖性积累是肌强直易感性的关键决定因素。目的2将提供更好的理解，通过使用最先进的光学方法来测量TTS电压瞬变和分析模型来估计K+积累在正常哺乳动物肌肉和小鼠模型的肌强直和周期性麻痹的肌肉兴奋性的这一重要特征。目的3是对携带M1592V或R669H突变的小鼠的临床表型和肌肉电生理特性进行比较分析，作为进一步了解在人类中观察到的这些NaV1.4等位基因疾病的不同表型的机制基础的模型（高钾性周期性麻痹伴肌强直与低钾性周期性麻痹不伴肌强直形成对比）。公共卫生相关性：拟议的研究旨在为一组人类神经肌肉疾病的病理生理基础提供更完整的理解：从基因缺陷到临床症状。这些研究也将进一步我们的知识钠通道功能在分子水平上，将导致一个更好的理解肌肉兴奋性的决定因素，将提供一个更合理的设计治疗策略的机制的见解，并将作为一个模型系统，了解其他疾病的兴奋性，如癫痫，偏头痛，或心律失常。

DESCRIPTION (provided by applicant): The myotonias and periodic paralyses are heritable diseases of skeletal muscle in which mutations of voltage-gated ion channels alter the electrical excitability of the fiber. The long-term goals of this project are to characterize the functional defects of mutant channels in these disorders and to determine how abnormal channel behavior produces symptoms in affected individuals. In these disorders, muscle dysfunction is caused by intermittent derangements in the electrical excitability of the fiber, which may be pathologically enhanced or depressed. Myotonia is a disorder of enhanced excitability wherein a single stimulus elicits a high- frequency burst of action potentials that produces involuntary persistent muscle contraction lasting seconds. Conversely, periodic paralysis results from a depolarization -induced loss of muscle excitability. Missense mutations in the adult skeletal muscle sodium channel (NaV1.4) may cause myotonia, periodic paralysis, or a combination of both in the same individual. The pathophysiological basis for this variation in clinical phenotype, all arising from mutations in a common sodium channel gene is a major focus of the studies in this proposal. Our experimental approach is to identify alterations in the behavior of mutant channels by measuring ionic current, and then use computer or animal-based models to explore how specific alterations in channel function give rise to myotonia or periodic paralysis. Aim 1 is to characterize the gating behavior of NaV1.4 channels, with a new focus on characterizing these properties for channels expressed in their native skeletal muscle environment. The availability of two mouse lines generated in our lab with knock-in point mutations in NaV1.4 (M1592V and R669H) offers a unique opportunity to characterize mutant channel behavior as occurs in muscle. Our studies on gating of disease- associated mutations of NaV1.4 will also explore the exciting new finding that missense mutations of arginines within S4 voltage-sensor domains may give rise to gating pore currents through an alternative permeation pathway different from the central pore. The propagation of action potentials into the transverse tubular system (TTS) and the activity-dependent accumulation of K+ therein are critical determinants of susceptibility to myotonia. Aim 2 will provide greater understanding for this important feature of muscle excitability by using state-of-the-art optical methods to measure TTS voltage transients and analytical models to estimate K+ accumulation both in normal mammalian muscle and for mouse models of myotonia and periodic paralysis. Aim 3 is a comparative analysis of the clinical phenotypes and electrophysiological properties of muscle from mice harboring either the M1592V or R669H mutations, as a model for gaining further insight on the mechanistic basis for the divergent phenotypes observed in humans for these allelic disorders of NaV1.4 (hyperkalemic periodic paralysis with myotonia contrasted by hypokalemic periodic paralysis without myotonia). PUBLIC HEALTH RELEVANCE: The proposed studies are designed to provide a more complete understanding of the pathophysiologic basis for a group of human neuromuscular diseases: from gene defect to clinical symptoms. These studies will also further our knowledge of Na channel function at the molecular level, will lead to an improved understanding of the determinants of muscle excitability, will provide mechanistic insights for a more rational design of therapeutic strategies, and will serve as a model system for understanding other disorders of excitability such as epilepsy, migraine, or cardiac arrhythmia.