Background: Muscle stem cells (MuSCs) are requisite for skeletal muscle regeneration and homeostasis. Proper functioning of MuSCs, including activation, proliferation, and fate decision, is determined by an orchestrated series of events and communication between MuSCs and their niche consisting of the host myofiber and neighbouring cells. A multitude of biochemical stimuli are known to regulate fate and function of MuSCs. However, in addition to biochemical factors, it is conceivable that MuSCs residing between basal lamina and sarcolemma of myofibers are subjected to mechanical forces during muscle stretch-shortening cycles due to myofascial connections between MuSCs and myofibers. MuSCs have been shown to respond to mechanical forces in vitro but it remains to be proven whether physical forces are also exerted on MuSCs in their native niche and whether they contribute to the functioning and fate of MuSCs. Methods: MuSCs deformation in their native niche resulting from mechanical loading of ex vivo myofiber bundles were visualized utilizing mT/mG double-fluorescent Cre-reporter mouse and multiphoton microscopy. MuSCs were subjected to 1 hour pulsating fluid shear stress with a peak shear stress rate of 8.8 Pa/s. After treatment, nitric oxide and mRNA expression levels of genes involved in regulation of MuSC proliferation and differentiation were determined. Results: Ex vivo stretching of extensor digitorum longus and soleus myofiber bundles caused compression as well as tensile and shear deformation of MuSCs in their niche. MuSCs responded to pulsating fluid shear stress in vitro with increased nitric oxide production and an upward trend in iNOS mRNA levels, while nNOS expression was unaltered. Pulsating fluid shear stress enhanced gene expression of c-Fos, Cdk4, and IL-6, while expression of Wnt1, MyoD, Myog, Wnt5a, COX2, Rspo1, Vangl2, Wnt10b, and MGF remained unchanged. Conclusions: We conclude that MuSCs in their native niche are subjected to force-induced deformations due to myofiber stretch-shortening. Moreover, MuSCs are mechanoresponsive as evident by pulsating fluid shear stress-mediated expression of factors by MuSCs known to promote proliferation.

中文翻译：

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肌纤维的拉伸在其天然生态位中诱导肌肉干细胞的拉伸和剪切变形

背景：肌肉干细胞（MuSCs）是骨骼肌再生和体内平衡所必需的。MuSC的正常功能，包括激活，增殖和命运决定，是由一系列事件和MuSC及其由宿主肌纤维和邻近细胞组成的利基之间的交流决定的。已知多种生化刺激物可调节MuSC的命运和功能。但是，除了生化因素之外，还可以想到，由于MuSC与肌纤维之间的肌筋连接，位于肌纤维的基底层和肌膜之间的MuSC会在肌肉拉伸缩短周期中受到机械力的作用。已证明MuSC在体外对机械力有反应但是，尚有待证明物理力是否也在其自然环境中施加在MuSC上，以及它们是否对MuSC的功能和命运有所贡献。方法：利用mT / mG双荧光Cre-reporter小鼠和多光子显微镜观察由离体肌纤维束的机械负荷引起的MuSCs固有位的变形。MuSC经受1小时的脉动流体剪切应力，峰值剪切应力率为8.8 Pa / s。处理后，测定参与MuSC增殖和分化调控的基因的一氧化氮和mRNA表达水平。结果：离体指趾长伸肌和比目鱼肌纤维束的拉伸引起其适当位置的MuSCs压缩以及拉伸和剪切变形。MuSCs在体外对搏动的流体剪切应力作出反应，增加了一氧化氮的产生和iNOS mRNA水平的上升趋势，而nNOS的表达未改变。脉动切变应力增强c-Fos，Cdk4和IL-6的基因表达，而Wnt1，MyoD，Myog，Wnt5a，COX2，Rspo1，Vangl2，Wnt10b和MGF的表达增强。保持不变。结论：我们的结论是，由于肌纤维的拉伸缩短，MuSCs在其自身的生态位中会受到力引起的变形。此外，MuSCs具有机械响应性，这是通过已知促进增殖的MuSCs脉动剪切应力介导的因子表达来证明的。