Jani Vivek P, Song Taejeong, Gao Chengqian, Gong Henry, Sadayappan Sakthivel, Kass David A, Irving Thomas C, Ma Weikang
Department of Biomedical Engineering, The Johns Hopkins School of Medicine, Baltimore, MD 21205, USA.
Division of Cardiology, Department of Medicine, Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA.
PNAS Nexus. 2024 Jan 30;3(2):pgae039. doi: 10.1093/pnasnexus/pgae039. eCollection 2024 Feb.
There is a growing awareness that both thick-filament and classical thin-filament regulations play central roles in modulating muscle contraction. Myosin ATPase assays have demonstrated that under relaxed conditions, myosin may reside either in a high-energy-consuming disordered-relaxed (DRX) state available for binding actin to generate force or in an energy-sparing super-relaxed (SRX) state unavailable for actin binding. X-ray diffraction studies have shown that the majority of myosin heads are in a quasi-helically ordered OFF state in a resting muscle and that this helical ordering is lost when myosin heads are turned ON for contraction. It has been assumed that myosin heads in SRX and DRX states are equivalent to the OFF and ON states, respectively, and the terms have been used interchangeably. In this study, we use X-ray diffraction and ATP turnover assays to track the structural and biochemical transitions of myosin heads, respectively, induced with either omecamtiv mecarbil (OM) or piperine in relaxed porcine myocardium. We find that while OM and piperine induce dramatic shifts of myosin heads from the OFF to the ON state, there are no appreciable changes in the population of myosin heads in the SRX and DRX states in both unloaded and loaded preparations. Our results show that biochemically defined SRX and DRX can be decoupled from structurally defined OFF and ON states. In summary, while SRX/DRX and OFF/ON transitions can be correlated in some cases, these two phenomena are measured using different approaches, reflect different properties of the thick filament, and should be investigated and interpreted separately.
人们越来越意识到，粗肌丝和经典细肌丝调节在调节肌肉收缩中都起着核心作用。肌球蛋白ATP酶分析表明，在松弛状态下，肌球蛋白可能处于高能量消耗的无序松弛（DRX）状态，可与肌动蛋白结合以产生力，也可能处于节能的超松弛（SRX）状态，无法与肌动蛋白结合。X射线衍射研究表明，在静息肌肉中，大多数肌球蛋白头部处于准螺旋有序的关闭状态，而当肌球蛋白头部开启进行收缩时，这种螺旋有序性就会丧失。人们一直认为，处于SRX和DRX状态的肌球蛋白头部分别等同于关闭和开启状态，这两个术语也被交替使用。在本研究中，我们分别使用X射线衍射和ATP周转分析来追踪在松弛的猪心肌中用奥米卡替麦卡比（OM）或胡椒碱诱导的肌球蛋白头部的结构和生化转变。我们发现，虽然OM和胡椒碱会使肌球蛋白头部从关闭状态急剧转变为开启状态，但在空载和负载制剂中，处于SRX和DRX状态的肌球蛋白头部数量没有明显变化。我们的结果表明，生化定义的SRX和DRX可以与结构定义的关闭和开启状态解耦。总之，虽然SRX/DRX和关闭/开启转变在某些情况下可能相关，但这两种现象是使用不同方法测量的，反映了粗肌丝的不同特性，应该分别进行研究和解释。
