Orienting Muscle Stem Cells for Regeneration in Homeostasis, Aging, and Disease

Abstract

Muscle stem cells, or satellite cells, are required for skeletal muscle maintenance, growth, and repair. Following satellite cell activation, several factors drive asymmetric cell division to generate a stem cell and a proliferative progenitor that forms new muscle. The balance between symmetric self-renewal and asymmetric division significantly impacts the efficiency of regeneration. In this Review, we discuss the relationship of satellite cell heterogeneity and the establishment of polarity to asymmetric division, as well as how these processes are impacted in homeostasis, aging, and disease. We also highlight therapeutic opportunities for targeting satellite cell polarity and self-renewal to stimulate muscle regeneration.

Keywords: Duchenne muscular dystrophy; PAR complex; aging; asymmetric division; cell polarity; muscle stem cell; regeneration; satellite cell; skeletal muscle; therapy.

Figure 1.. The Satellite Cell Niche Establishes Polarity

Quiescent satellite cells (white) sit below the basal lamina enriched in high-molecular weight proteins such as laminin (red) and fibronectin, and atop the muscle fiber sarcolemma expressing dystrophin (green), as depicted on the immunofluorescence micrograph. This anatomical location within the niche establishes satellite cell polarity. The apical interface between the satellite cell and the myofiber is enriched for CD34, Notch, Sdc-3, NCAM, and M-Cadherin, while the basal interface is enriched for Tie2, c-MET, Integrin-α7/β1, and Sdc-4, and Fzd7 receptors. Satellite cells are characterized by Pax7 expression. Satellite cell quiescence is maintained by Notch signaling through NCID/RBPJκ activity that promotes Pax7 expression while non-methylated Pax7 limits Myf5 transcription. FGF signaling is mitigated by Spry1 and Integrin-β1, which repress both p38α/β activation, and in turn, MyoD expression. Accumulation of p27^kip1 inhibits Cyclin Dependent Kinases, maintaining satellite cells in a quiescent state. During aging, accumulation of p16^INK4a inhibits Cyclin D, causing cells to be non-refractory to cell cycle entry, leading to cell senescence. Cytoplasmic Myf5 mRNA bound by miR-31 is sequestered to repress its translation. MyoD mRNA is degraded by TTP and Staufen1, while Dek is repressed by miR-489 to inhibit cell cycle entry. Phosphorylated eIF2α selectively blocks translation to further repress cell cycle entry and myogenic commitment.

Figure 2.. The PAR Complex Regulates Asymmetric Division

In healthy satellite cells, dystrophin act as a scaffolding protein during mitosis to bind Par1b, leading to asymmetric segregation of Pard3 and the PAR complex and apical-basal orientation of the centrosomes prior to mitotic division. In DMD, satellite cells lacking dystrophin have impaired Par1b polarization, and in turn, non-restricted Pard3, leading to impaired cell polarity. Consequently, satellite cells in DMD exhibit a marked reduction in rate of asymmetric division due to impaired apical-basal centrosome localization.

Figure 3.. The Molecular Regulation of Asymmetric Muscle Stem Cell Division

Following an asymmetric division in vivo, unequal localization of Numb, p38α/β, G-CSFR, and Pard3 to the apical daughter cell leads to myogenic commitment. Carm1 methylation of Pax7 allows histone methyltransferase complex (HMT) binding to the Myf5 locus, while p38α/β recruitment of the SWI-SNF complex to the MyoD promoter allows Myf5 and MyoD expression in the committed daughter cell. Basal localization of Spry1, miR-489 and p-Carm1 maintain stem cell fate in the basal daughter cell. Satellite cells in DMD lacking the dystrophin scaffold have impaired polarity and impaired myogenic commitment in part through loss of both Carm1-p38γ/β1-syntrophin interaction and Carm1 nuclear localization. Aged satellite cells are characterized by pervasive STAT3 and p38α/β signaling leading to myogenic commitment. Accumulation of p16^ink leads to cell cycle exit and senescence in aged satellite cells.

Figure 4.. The Balance between Symmetric and Asymmetric Division Significantly Impacts the Efficiency of Muscle Regeneration

Schematic depicting changes in self-renewal and myogenic commitment in youthful muscle, Wnt7a stimulated muscle, and aged and dystrophic muscle. Following injury, satellite cells balance self-renewal and commitment through symmetric expansion of the satellite cell pool and asymmetric commitment, forming progenitors that repair the myofibers and stem cells to facilitate progenitor formation. Increased self-renewal leads to augmented repair process by increasing the number of satellite cells available to contribute to progenitor formation. Intrinsic and extrinsic changes in satellite cell signaling with age promote cell commitment at the expense of satellite cell self-renewal. Myopathy such as Duchenne muscular dystrophy (DMD) significantly impairs the regenerative process due to reduced progenitor formation from mdx satellite cells. Yields are hypothetical based upon 4 initial stem cell divisions.