I.12 Modulating muscle stem cells to enhance regeneration to ameliorate DMD disease progression

Abstract

Duchenne muscular dystrophy (DMD) is a lethal X-linked muscle-wasting disease. Dystrophin functions as an essential component of the large oligomeric dystrophin-glycoprotein complex (DGC). The DGC acts to connect the actin cytoskeleton of the myofiber to the surrounding extracellular matrix through the sarcolemma. In the absence of dystrophin, DGC assembly is impaired which weakens the muscle fibers rendering them highly susceptible to injury, causing accumulation of inflammation and fibrosis, and progressive loss of muscle mass and function. We discovered that dystrophin is highly expressed in activated muscle stem cells and associates with the serine-threonine kinase Mark2/Par1b, a regulator of cell polarity. In the absence of dystrophin, Mark2 is downregulated, resulting in the inability to localize the cell polarity regulator Pard3 to the opposite side of the cell. Consequently, the number of asymmetric divisions and hence the generation of myogenic progenitors is significantly reduced. Our findings indicate that muscle wasting in DMD not only is caused by myofiber fragility, but also is exacerbated by impaired regeneration owing to intrinsic satellite cell dysfunction. Through a small-molecule screen, we made the discovery of a novel signal transduction pathway involving epidermal growth factor receptor (EGFR) and Aurora kinase A (Aurka) as key regulators of asymmetric satellite cell divisions. Inhibiting EGFR causes a substantial shift from asymmetric to symmetric division modes, whereas EGF treatment increases asymmetric divisions. EGFR activation acts through Aurka to orient mitotic centrosomes, and inhibiting Aurka blocks EGF stimulation-induced asymmetric division. In vivo EGF treatment markedly activates asymmetric divisions of dystrophin-deficient satellite cells in mdx mice, increasing progenitor numbers, enhancing regeneration, and restoring muscle strength. Therefore, activating an EGFR-dependent polarity pathway promotes functional rescue of dystrophin-deficient satellite cells and enhances muscle force generation. In collaboration with Satellos Bioscience, we have been developing small molecule drugs that stimulate asymmetric division by modulating the EGFR/Aurka and other pathways. Our goal is to develop small molecule modulators of muscle stem cell asymmetric division as a treatment for DMD.