Department of Physiology and Biophysics, University of Washington, Seattle, Washington.
Department of Physiology and Membrane Biology, University of California, Davis, California.
Am J Physiol Cell Physiol. 2020 Mar 1;318(3):C598-C604. doi: 10.1152/ajpcell.00573.2019. Epub 2020 Jan 22.
Excitation-contraction (EC) coupling is the coordinated process by which an action potential triggers cardiac myocyte contraction. EC coupling is initiated in dyads where the junctional sarcoplasmic reticulum (jSR) is in tight proximity to the sarcolemma of cardiac myocytes. Existing models of EC coupling critically depend on dyad stability to ensure the fidelity and strength of EC coupling, where even small variations in ryanodine receptor channel and voltage-gated calcium channel-α 1.2 subunit separation dramatically alter EC coupling. However, dyadic motility has never been studied. Here, we developed a novel strategy to track specific jSR units in dissociated adult ventricular myocytes using photoactivatable fluorescent proteins. We found that the jSR is not static. Instead, we observed dynamic formation and dissolution of multiple dyadic junctions regulated by the microtubule-associated molecular motors kinesin-1 and dynein. Our data support a model where reproducibility of EC coupling results from the activation of a temporally averaged number of SR Ca release units forming and dissolving SR-sarcolemmal junctions. These findings challenge the long-held view that the jSR is an immobile structure and provide insights into the mechanisms underlying its motility.
兴奋-收缩（EC）偶联是动作电位引发心肌细胞收缩的协调过程。EC 偶联在连接的肌浆网（jSR）与心肌细胞的肌膜紧密相邻的二联体中启动。现有的 EC 偶联模型严重依赖二联体的稳定性，以确保 EC 偶联的保真度和强度，其中即使是 Ryanodine 受体通道和电压门控钙通道-α 1.2 亚基分离的微小变化也会极大地改变 EC 偶联。然而，二联体的运动性从未被研究过。在这里，我们开发了一种使用光活化荧光蛋白追踪分离的成年心室肌中二联体特定 jSR 单位的新策略。我们发现 jSR 不是静态的。相反，我们观察到多个二联体连接的动态形成和溶解，这受微管相关分子马达驱动蛋白-1 和动力蛋白的调节。我们的数据支持这样一种模型，即 EC 偶联的重现性源自形成和溶解 SR-肌膜连接的暂时平均数量的 SR Ca 释放单位的激活。这些发现挑战了 jSR 是一种不可移动结构的长期观点，并为其运动性的机制提供了见解。
