Department of Physiology, Institute for Cardiovascular Research, VU University Medical Center Amsterdam, De Boelelaan 1118, 1081, BT Amsterdam, The Netherlands.
INMR, The Children's Hospital at Westmead and Discipline of Paediatrics & Child Health, University of Sydney, Cnr Hawkesbury Road & Hainsworth Street, Sydney, Australia.
Skelet Muscle. 2015 Apr 28;5:12. doi: 10.1186/s13395-015-0037-7. eCollection 2015.
Nemaline myopathy (NM), the most common non-dystrophic congenital myopathy, is characterized by generalized skeletal muscle weakness, often from birth. To date, no therapy exists that enhances the contractile strength of muscles of NM patients. Mutations in NEB, encoding the giant protein nebulin, are the most common cause of NM. The pathophysiology of muscle weakness in NM patients with NEB mutations (NEB-NM) includes a lower calcium-sensitivity of force generation. We propose that the lower calcium-sensitivity of force generation in NEB-NM offers a therapeutic target. Levosimendan is a calcium sensitizer that is approved for use in humans and has been developed to target cardiac muscle fibers. It exerts its effect through binding to slow skeletal/cardiac troponin C. As slow skeletal/cardiac troponin C is also the dominant troponin C isoform in slow-twitch skeletal muscle fibers, we hypothesized that levosimendan improves slow-twitch muscle fiber strength at submaximal levels of activation in patients with NEB-NM.
To test whether levosimendan affects force production, permeabilized slow-twitch muscle fibers isolated from biopsies of NEB-NM patients and controls were exposed to levosimendan and the force response was measured.
No effect of levosimendan on muscle fiber force in NEB-NM and control skeletal muscle fibers was found, both at a submaximal calcium level using incremental levosimendan concentrations, and at incremental calcium concentrations in the presence of levosimendan. In contrast, levosimendan did significantly increase the calcium-sensitivity of force in human single cardiomyocytes. Protein analysis confirmed that the slow skeletal/cardiac troponin C isoform was present in the skeletal muscle fibers tested.
These findings indicate that levosimendan does not improve the contractility in human skeletal muscle fibers, and do not provide rationale for using levosimendan as a therapeutic to restore muscle weakness in NEB-NM patients. We stress the importance of searching for compounds that improve the calcium-sensitivity of force generation of slow-twitch muscle fibers. Such compounds provide an appealing approach to restore muscle force in patients with NEB-NM, and also in patients with other neuromuscular disorders.
先天性肌营养不良症（Nemaline myopathy，NM）是最常见的非营养不良性先天性肌肉疾病，其特征为全身骨骼肌无力，常起病于出生时。迄今为止，尚无增强 NM 患者肌肉收缩力的疗法。编码巨大蛋白nebulin 的基因（NEB）突变是 NM 的最常见病因。伴有 NEB 突变的 NM 患者（NEB-NM）肌肉无力的病理生理学包括肌力产生的钙离子敏感性降低。我们提出肌力产生的钙离子敏感性降低是一个治疗靶点。左西孟旦是一种钙离子增敏剂，已被批准用于人类，其开发目的是针对心肌纤维。它通过与慢型骨骼肌/心肌肌钙蛋白 C 结合发挥作用。由于慢型骨骼肌/心肌肌钙蛋白 C 也是慢型肌纤维中主要的肌钙蛋白 C 同工型，我们假设左西孟旦可改善 NEB-NM 患者亚最大激活水平下的慢型肌纤维强度。
为了检测左西孟旦是否影响肌力产生，我们用递增浓度的左西孟旦处理取自 NEB-NM 患者和对照者活检的慢型肌纤维，以渗透化肌纤维的方式进行，同时测量肌力反应。
无论在亚最大钙离子浓度下使用递增浓度的左西孟旦，还是在存在左西孟旦时使用递增钙离子浓度，均未发现左西孟旦对 NEB-NM 和对照骨骼肌纤维的肌力产生影响。相比之下，左西孟旦确实显著增加了人心肌细胞的钙离子敏感性。蛋白分析证实，所测试的骨骼肌纤维中存在慢型骨骼肌/心肌肌钙蛋白 C 同工型。
这些发现表明，左西孟旦不能改善人类骨骼肌纤维的收缩性，也不能为将左西孟旦用作治疗 NEB-NM 患者肌肉无力的方法提供依据。我们强调寻找可提高慢型肌纤维肌力产生钙离子敏感性的化合物的重要性。此类化合物为恢复 NEB-NM 患者以及其他神经肌肉疾病患者的肌肉力量提供了一种有吸引力的方法。
