The genetic hierarchy that controls muscle development and repair
In the developing muscle, a pool of myogenic progenitor cells is formed and maintained. These resident progenitors provide a source of undifferentiated cells for muscle growth in development and generate satellite cells in the perinatal period.
Recently, we could demonstrate that the transcription factor RBP-J, the major mediator of Notch signaling, is essential in keeping myogenic progenitor cells in an undifferentiated state. In the absence of RBP-J, progenitor cells undergo uncontrolled myogenic differentiation, resulting in a lack of muscle growth in development and severe muscle hypotrophy (Vasyutina 2007).
Satellite cells are not generated in RBP-J mutant mice. Satellite cells, the stem cells of the adult muscle, were identified by Pax7 expression. In control animals, Pax7+ satellite cells (shown in red) are positioned under the basal lamina, which outlines muscle fibers (laminin, green). In RBP-J mutants, the population of satellite cells is not present.
Skeletal muscle fibers are syncytia that arise by the fusion of myogenic cells. Mononucleated myogenic cells, the myoblasts, fuse with each other to form multinucleated myotubes. During development and in the adult, myoblast fusion allows generation, growth and repair of muscle fibers. Rac1 and Cdc42 are small G-proteins that regulate actin dynamics. We analyzed the function of Rac1 and Cdc42 in myogenesis using conditional mutagenesis in mice. We showed that in the absence of Rac1 and Cdc42, myoblast fusion is severely compromised in vivo and in vitro. The deficit in fusion of Rac1 or Cdc42 mutant myoblasts correlated with a deficit in the recruitment of actin fibers and vinculin to myoblast contact sites. Moreover, we demonstrated that Rac1 and Cdc42 are required in both fusion partners. Thus, our analysis demonstrated that the function of Rac1 is evolutionarily conserved from insects to mammals, and that Cdc42, a molecule hitherto not implicated in myoblast fusion, is essential for the fusion of murine myoblasts (Vasyutina 2009).
Myoblast fusion in mice requires Rac1 and Cdc42. In the muscle of a control animal, single myoblasts (visualized by MyoD in red) fuse to generate multinucleated myotube. In conditional Rac1 or Cdc42 mutant muscle, myoblasts do not fuse and myofibers remain short and thin.
Skeletal muscle growth and regeneration rely on myogenic progenitor and satellite cells, the stem cells of postnatal muscle. Elimination of Notch signals during mouse development results in premature differentiation of myogenic progenitors and formation of very small muscle groups. We could show that this drastic effect is rescued by mutation of the muscle differentiation factor MyoD. However, rescued myogenic progenitors do not assume a satellite cell position and contribute poorly to myofiber growth. The disrupted homing is due to a deficit in basal lamina assembly around emerging satellite cells and to their impaired adhesion to myofibers. On a molecular level, emerging satellite cells deregulate the expression of basal lamina components and adhesion molecules like integrin a7, collagen XVIIIa1, Megf10, and Mcam. We concluded that Notch signals control homing of satellite cells, stimulating them to contribute to their own microenvironment and to adhere to myofibers (Bröhl 2012).
Muscle stem cells require Notch signaling to settle in their niche. Schematic diagram showing the anatomical localization of the stem cells of the muscle that are also called satellite cells (SC, red) wedged between the basal lamina (BL) and the myofiber (MF) plasma membrane (PM). Satellite cells nuclei express Pax3/Pax7 and are shown in red, nuclei of the myofiber (myonuclei, MN) are shown in yellow. (B,C) Analysis of emerging satellite cells at E17.5 using antibodies against Pax3 (red) and laminin (green). Shown are sections of back muscle from control and Rbpj; MyoD mutant mice. Arrows point towards satellite cells located below the basal lamina, and arrowheads point towards the Pax3+ cells located in the interstitial space of Rbpj/MyoD double mutants.