Regulation of skeletal muscle extracellular matrix remodeling by satellite cells

描述（由申请人提供）：骨骼肌质量的维持，以及在负荷下增加质量的能力，对全身功能和生活质量至关重要。卫星细胞或肌肉干细胞在肌肉质量调节中的作用尚不明确。有效的肌肉肥大可以独立于卫星细胞发生；然而，在没有卫星细胞的情况下，长期的肌肉适应尚未被探索。初步数据表明，在缺少卫星细胞的情况下，肌肉变成纤维化和肥大的高原。这提出了激活的卫星细胞在调节肌纤维生态位中发挥旁分泌作用的可能性。过量的细胞外基质（ECM）产生和肌肉纤维化，在肌肉营养不良和衰老中观察到，对肌肉的可塑性和功能产生负面影响。激活的卫星细胞可能通过调节转化生长因子（TGF-）信号介导骨骼肌ECM的产生。TGF-¿促进肌肉成纤维细胞中ECM的产生，而卫星细胞可能阻止TGF-¿的下游信号传导，从而影响成纤维细胞的活性和ECM的产生。假设在没有激活的卫星细胞的情况下，肌肉成纤维细胞通过激活TGF-¿信号增加ECM的产生，导致肌肉纤维化和肌肉生长和功能减弱。此外，我们提出验证新的假设，即肌肉纤维化，而不是肌核结构域上限，限制了卫星细胞耗尽肌肉的肌肉肥大。在两个新的小鼠品系中，跖肌的功能过载将决定激活的卫星细胞的作用

DESCRIPTION (provided by applicant): The maintenance of skeletal muscle mass, and the ability to increase mass in response to load, are crucial for whole body function and quality of life. The role of satellite cells, or muscle stem cells, in muscle mass regulation is equivocal. Effective muscle hypertrophy can occur independently of satellite cells; however, long- term muscle adaptation in the absence of satellite cells has not been explored. Preliminary data suggest that in the absence of satellite cells, muscle becomes fibrotic and hypertrophy plateaus. This raises the possibility that activated satellite cells play a paracrine role in regulating the muscle fiber niche. Excess extracellular matrix (ECM) production and muscle fibrosis, observed in muscular dystrophies and in aging, negatively impact muscle plasticity and function. Activated satellite cells may mediate skeletal muscle ECM production through modulation of transforming growth factor ¿ (TGF-¿) signaling. TGF-¿ promotes increased production of ECM in muscle fibroblasts, and satellite cells may prevent TGF-¿ downstream signaling, thereby influencing fibroblast activity and ECM production. It is hypothesized that in the absence of activated satellite cells, production of ECM by muscle fibroblasts increases via activation of TGF-¿ signaling, resulting in muscle fibrosis and attenuated muscle growth and function. Further, we propose to test the novel hypothesis that muscle fibrosis, rather than a myonuclear domain ceiling, limits muscle hypertrophy in satellite cell-depleted muscle. Functional overload of plantaris muscle in two novel mouse strains will determine the role of activated satellite cells in modulating the extracellular environment to enable hypertrophy. The Pax7-DTA strain enables specific depletion of satellite cells, and the Pax7-N-WASp strain enables satellite cell activation and proliferation, but not fusion. Aim 1 will assess TGF-¿ signaling activity, fibroblast proliferation, expression of ECM components, functional consequences of fibrosis, and the hypertrophic response in skeletal muscle following satellite cell depletion in the Pax7-DTA mouse in response to overload. Immunohistochemistry, fluorescent activated cell sorting, qRT-PCR, western blotting, biochemical assays and whole muscle function assessment will be performed. Increased fibroblast proliferation, expression of ECM components and depressed force production associated with reduced growth in satellite cell-depleted muscle will demonstrate a regulatory role for activated satellite cells in controlling the fiber extracellular environment. Aim 2 will utilize the Pax7-N-WASp mouse to determine the role of activated satellite cells in the hypertrophic response irrespective of their ability to contribute new nucleito growing fibers. Activated satellite cell regulation of fibroblast proliferation and ECM production will be assessed during muscle overload as in Aim 1. Restoration of hypertrophy in Pax7-N-WASp mice will demonstrate a novel role for activated satellite cells in adult skeletal muscle adaptability that may prove equally as important as their well-characterized role in muscle fiber regeneration.