Holzherr Boris, Lehmann-Horn Frank, Kuzmenkina Elza, Fan Chunxiang, Jurkat-Rott Karin
Division of Neurophysiology in the Center of Rare Diseases at Ulm University, Ulm, Germany.
Acta Myol. 2014 May;33(1):22-33.
We studied the consequences of the Nav1.4 mutation R1448H that is situated in the fourth voltage sensor of the channel and causes paramyotonia, a cold-induced myotonia followed by weakness. Previous work showed that the mutation uncouples inactivation from activation. We measured whole-cell Na(+) currents at 10, 15, 20, and 25°C using HEK293 cells stably transfected with wildtype (WT) and R1448H Na(+) channels. A Markov model was developed the parameters of which reproduced the data measured on WT and R1448H channels in the whole voltage and temperature range. It required an additional transient inactivated state and an additional closed-state inactivation transition not previously described. The model was used to predict single-channel properties, free energy barriers and temperature dependence of rates. It allowed us to draw the following conclusions: i) open-state inactivation results from a two-step process; ii) the channel re-openings that cause paramyotonia originate from enhanced deactivation/reactivation and not from destabilized inactivation; iii) the closed-state inactivation of R1448H is strikingly enhanced. We assume that latter explains the episodic weakness following cold-induced myotonia.
我们研究了位于通道第四电压感受器的Nav1.4突变R1448H的后果，该突变导致发作性肌强直，即一种冷诱导的肌强直并伴有肌无力。先前的研究表明，该突变使失活与激活解偶联。我们使用稳定转染野生型（WT）和R1448H钠通道的HEK293细胞，在10、15、20和25°C下测量全细胞钠电流。建立了一个马尔可夫模型，其参数在整个电压和温度范围内重现了在WT和R1448H通道上测得的数据。该模型需要一个额外的瞬时失活状态和一个先前未描述的额外的关闭状态失活转变。该模型用于预测单通道特性、自由能垒和速率的温度依赖性。由此我们得出以下结论：i）开放状态失活是一个两步过程；ii）导致发作性肌强直的通道重新开放源于增强的失活/再激活，而非失活的不稳定；iii）R1448H的关闭状态失活显著增强。我们推测，后者解释了冷诱导肌强直后的发作性肌无力。
