Defining the transcriptional signature of skeletal muscle stem cells1,2

Abstract

Satellite cells, the main source of myoblasts in postnatal muscle, are located beneath the myofiber basal lamina. The myogenic potential of satellite cells was initially documented based on their capacity to produce progeny that fused into myotubes. More recently, molecular markers of resident satellite cells were identified, further contributing to defining these cells as myogenic stem cells that produce differentiating progeny and self-renew. Herein, we discuss aspects of the satellite cell transcriptional milieu that have been intensively investigated in our research. We elaborate on the expression patterns of the paired box (Pax) transcription factors Pax3 and Pax7, and on the myogenic regulatory factors myogenic factor 5 (Myf5), myogenic determination factor 1 (MyoD), and myogenin. We also introduce original data on MyoD upregulation in newly activated satellite cells, which precedes the first round of cell proliferation. Such MyoD upregulation occurred even when parent myofibers with their associated satellite cells were exposed to pharmacological inhibitors of hepatocyte growth factor and fibroblast growth factor receptors, which are typically involved in promoting satellite cell proliferation. These observations support the hypothesis that most satellite cells in adult muscle are committed to rapidly entering myogenesis. We also detected expression of serum response factor in resident satellite cells prior to MyoD expression, which may facilitate the rapid upregulation of MyoD. Aspects of satellite cell self-renewal based on the reemergence of cells expressing Pax7, but not MyoD, in myogenic cultures are discussed further herein. We conclude by describing our recent studies using transgenic mice in which satellite cells are traced and isolated based on their expression of green fluorescence protein driven by regulatory elements of the nestin promoter (nestin-green fluorescence protein). This feature provides us with a novel means of studying satellite cell transcriptional signatures, heterogeneity among muscle groups, and the role of the myogenic niche in directing satellite cell self-renewal.

Are Human and Mouse Satellite Cells Really the Same?

“…On activation, satellite cells promptly enter the myogenic program (Cornelison and Wold 1997;Yablonka-Reuveni et al 2008) and may not therefore retain the capabilities of quiescent satellite cells.…”

All muscle satellite cells are equal, but are some more equal than others?

“…Quiescent satellite cells express several markers, which include Pax7, M-cadherin, CD34 and -in Myf5 nlacZ/+ mice -β-galactosidase from the targeted Myf5 allele (Beauchamp et al, 2000; Irintchev et al, 1994;Seale et al, 2000). MyoD is rapidly induced during activation in satellite cells in vivo and, in vitro, virtually all satellite cells express MyoD after ~24 hours of culture (Grounds et al, 1992;Yablonka-Reuveni et al, 2008; YablonkaReuveni and Rivera, 1994;Zammit et al, 2004). After proliferating as Pax7-positive and MyoD-positive satellite-cell-derived myoblasts, most cells then downregulate Pax7, maintain MyoD expression and commit to myogenic differentiation.…”

G-CSF supports long-term muscle regeneration in mouse models of muscular dystrophy

“…We then examined the direct role of G-CSF on SCs from wild-type mice. At 72 h, the subpopulation of activated SCs had lost MYOD expression in vivo and on cultured myofibres [21][22][23] . Surprisingly, G-CSF significantly increased the population of SCs throughout the differentiation stages, including self-renewal SCs, as shown by the PAX7 þ /MYOD À cells at 72 h in wild-type mice; in addition, csf3r À / À mice showed significantly decreased numbers of SCs throughout the differentiation stages ( Fig.…”

“…3a,b). At 24 h, most PAX7 þ cells express MYOD, are activated and thereafter proliferate and commit to differentiation [21][22][23] . In the csf3r À / À mice, the number of MYOD þ /PAX7 þ cells was significantly decreased in PAX7 þ cells activated for 24 h (Fig.…”