GATING MECHANISMS OF HERG AND RELATED CHANNELS

心脏兴奋性是由包括IKr在内的几种膜电流的活动决定的，IKr负责心室动作电位的终端复极化。当IKr被HERG基因突变破坏或被药物阻断时，就会导致危及生命的心律失常。HERG在青蛙卵母细胞中的表达产生具有IKr生物物理和药理学特性的电流，表明HERG亚基是天然通道的关键成分。HERG通道表现出快速失活，在去极化过程中抑制电流；在复极化过程中，通道从失活状态中恢复过来，在关闭之前通过打开状态。由于失活是缓慢的，HERG电流在心脏动作电位复极时达到最大，并有助于确保复极化的完成。这项工作的长期目标是了解特异性HERG在心脏中的生理作用的门控机制。最近发现，N端可以调节失活率，而自然存在的可选N端导致HERG和小鼠HERG同源物Merg1编码的通道之间的功能多样性。N端失活的调制源于一个与促进c型失活的第二结构域在空间上可分离的结构域。这两种功能都被S4-S5环的修饰所破坏，这表现了N末端的缺失。本研究的目的是表征N端调控失活的机制，利用酵母双杂交系统鉴定门控修饰因子，并表征HERG亚家族中两个新发现的基因HERG2和HERG3编码的通道中心的电生理特性和区域分布。如果这一建议的目的得以实现，一个更详细的HERG门控图像将随着对心脏中HERG相关通道的生理学的更深入的理解而出现。

Cardiac excitability is determined by the activity of several membrane currents including IKr, which is responsible for the terminal repolarization of the ventricular action potential. When IKr is disrupted by mutations in the HERG gene or blocked by drugs, life-threatening arrhythmias can result. Expression of HERG in frog oocytes produces currents with the biophysical and pharmacological properties of IKr, suggesting that HERG subunits are key components of the native channel. HERG channels exhibit a rapid inactivation that suppresses the current during depolarization; during repolarization the channels recover from inactivation and pass through the open state prior to closing. Because deactivation is slow, HERG current reaches its maximum during repolarization of the cardiac action potential and helps to ensure that repolarization is complete. The long-range goals of this work are to understand the gating mechanisms that specialize HERG for its physiological role in the heart. The N terminus has recently been found to regulate deactivation rate, and naturally-occuring alternative N termini lead to functional diversity among channels encoded by HERG and Merg1, the HERG ortholog in mouse. The modulation of deactivation by the N terminus arises from a domain that is spatially separable from a second domain that promotes C-type inactivation. Both of these functions are disrupted by modifications of the S4-S5 loop, which phenocopies a deletion of the N terminus. The aims of this proposal are to characterize the mechanism by which the N terminus regulates deactivation, to identify modifiers of gating using the yeast two-hybrid system and to characterize the electrophysiological properties and regional distribution within the heart of channels encoded by HERG2 and HERG3, two newly-identified genes within the HERG subfamily. If the aims of this proposal are achieved a more detailed picture of HERG gating will emerge along with a greater understanding of the physiology of HERG-related channels in the heart.