Defining the transcriptional signature of skeletal muscle stem cells

¹ Department of Biological Structure, University of Washington School of Medicine, Seattle, WA, 98195
² Department of Cell and Developmental Biology, Sackler Faculty of Medicine, Tel Aviv University, Tel Aviv 69978, Israel

^* To whom correspondence should be addressed. E-mail: reuveni{at}u.washington.edu.

	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 and further contributed to defining these cells as myogenic stem cells that produce differentiating progeny and self-renew. Herein, we discuss aspects of satellite cell transcriptional milieu that are 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 about 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 typically involved in initiating satellite cell proliferation. These observations support the hypothesis that most satellite cells in adult muscle are committed to rapidly enter 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 further discussed herein. We conclude with describing our recent studies using transgenic mice in which satellite cells are traced and isolated based on their expression of green fluorescence protein (GFP) driven by regulatory elements of the nestin promoter (nestin-GFP). This feature provides us with a novel means to study satellite cell transcriptional signatures, heterogeneity between muscle groups, and the role of the myogenic niche in directing satellite cell self-renewal.

Key Words: Green fluorescence protein (GFP), Myogenic determination factor 1 (MyoD), myogenic factor 5 (Myf5), nestin, paired box gene 3 (Pax3), paired box gene 7 (Pax7), satellite cells, self-renewal, serum response factor (SRF), sex-determining region Y (sry)-box gene (Sox)