Department of Bioengineering, University of Washington, Seattle, WA, USA.
J Physiol. 2013 Jun 15;591(12):3049-61. doi: 10.1113/jphysiol.2013.252650. Epub 2013 Apr 29.
Little is known about the contraction and relaxation properties of fetal skeletal muscle, and measurements thus far have been made with non-human mammalian muscle. Data on human fetal skeletal muscle contraction are lacking, and there are no published reports on the kinetics of either fetal or adult human skeletal muscle myofibrils. Understanding the contractile properties of human fetal muscle would be valuable in understanding muscle development and a variety of muscle diseases that are associated with mutations in fetal muscle sarcomere proteins. Therefore, we characterised the contractile properties of developing human fetal skeletal muscle and compared them to adult human skeletal muscle and rabbit psoas muscle. Electron micrographs showed human fetal muscle sarcomeres are not fully formed but myofibril formation is visible. Isolated myofibril mechanical measurements revealed much lower specific force, and slower rates of isometric force development, slow phase relaxation, and fast phase relaxation in human fetal when compared to human adult skeletal muscle. The duration of slow phase relaxation was also significantly longer compared to both adult groups, but was similarly affected by elevated ADP. F-actin sliding on human fetal skeletal myosin coated surfaces in in vitro motility (IVM) assays was much slower compared with adult rabbit skeletal myosin, though the Km(app) (apparent (fitted) Michaelis-Menten constant) of F-actin speed with ATP titration suggests a greater affinity of human fetal myosin for nucleotide binding. Replacing ATP with 2 deoxy-ATP (dATP) increased F-actin speed for both groups by a similar amount. Titrations of ADP into IVM assays produced a similar inhibitory affect for both groups, suggesting ADP binding may be similar, at least under low load. Together, our results suggest slower but similar mechanisms of myosin chemomechanical transduction for human fetal muscle that may also be limited by immature myofilament structure.
关于胎儿骨骼肌肉的收缩和松弛特性，人们知之甚少，迄今为止的测量都是用人以外的哺乳动物肌肉进行的。缺乏关于人类胎儿骨骼肌肉收缩的数据，也没有关于胎儿或成人骨骼肌肉肌球蛋白纤维动力学的发表报告。了解人类胎儿肌肉的收缩特性对于理解肌肉发育以及与胎儿肌肉肌节蛋白突变相关的各种肌肉疾病是有价值的。因此，我们对发育中的人类胎儿骨骼肌肉的收缩特性进行了描述，并将其与成人骨骼肌肉和兔腰大肌进行了比较。电子显微镜照片显示，人类胎儿肌肉肌节尚未完全形成，但可以看到肌原纤维的形成。分离的肌球蛋白纤维机械测量显示，与成人骨骼肌肉相比，人类胎儿的比特定力更低，等长力发展速度更慢，慢相松弛速度更慢，快相松弛速度也更慢。与成人组相比，慢相松弛的持续时间也显著延长，但同样受到升高的 ADP 的影响。在体外运动（IVM）测定中，与成人兔骨骼肌球蛋白相比，人类胎儿骨骼肌球蛋白上 F-肌动蛋白的滑行速度要慢得多，尽管通过 ATP 滴定测定 F-肌动蛋白速度的 Km(app)（表观（拟合）米氏常数）表明人类胎儿肌球蛋白对核苷酸结合的亲和力更大。用 2 脱氧-ATP（dATP）代替 ATP 会使两组的 F-肌动蛋白速度增加相似的量。向 IVM 测定中滴定 ADP 会对两组产生相似的抑制作用，这表明 ADP 结合可能相似，至少在低负荷下是这样。总的来说，我们的结果表明，人类胎儿肌肉的肌球蛋白化学机械转导速度较慢，但机制相似，这也可能受到不成熟的肌丝结构的限制。
