Fellowship: Chris Fry Regulation of skeletal muscle extracellular matrix remodeling by satellite cells

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 preservation 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 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 quality and function. Satellite cells may mediate skeletal muscle ECM production through modulation of transforming growth factor â (TGF-â) signaling. TGF-â promotes increased production of ECM in muscle fibroblasts, and satellite cells may prevent TGF-â downstream signaling, thereby influencing fibroblast activity and ECM production. It is hypothesized that in the absence of satellite cells, production of ECM by muscle fibroblasts will increase via activation of TGF-â signaling, resulting in muscle fibrosis and attenuated muscle growth and function. The Pax7-DTA mouse allows for the conditional ablation of satellite cells in adult muscle. The use of this model will define the role of satellite cells in the maintenance of the muscle environment and ECM production. Regulation of fibroblast ECM production via satellite cells will also be assessed during muscle overload to study hypertrophy of the plantaris muscle. Aim 1 will assess TGF-â signaling activity, fibroblast proliferation, expression of ECM components and functional consequences of muscle fibrosis in skeletal muscle following satellite cell ablation. Immunohistochemistry, qRT-PCR, western blotting and whole muscle function assays will be used to quantify the activity of TGF-â signaling, fibroblast number, expression of ECM components and specific force of the plantaris following satellite cell depletion in control and overloaded muscle. Increased fibroblast proliferation, expression of ECM components and depressed force production in satellite cell-ablated muscle will demonstrate a regulatory role for satellite cells in mediating fibroblast ECM production. Aim 2 will utilize an in vitro co-culture system pairing primary myoblasts and fibroblasts isolated from mouse muscle to better define satellite cell regulation of ECM production by fibroblasts. Activity of TGF-â signaling and gene/protein expression of ECM components will be quantified. Decreased expression of ECM components in fibroblasts co-cultured with myoblasts will demonstrate myoblast-mediated control of fibroblast ECM production. Manipulation of the TGF-â pathway will determine if it is the primary mediator of the myoblast-fibroblast interaction. These studies may establish a new role for satellite cells in adult skeletal muscle adaptability that may prove equally as important as their well-characterized role in muscle fiber regeneration.