Myogenin is required for late but not early aspects of myogenesis during mouse development.

Abstract

Abstract. Mice with a targeted mutation in the myogenic basic helix-loop-helix regulatory protein myogenin have severe muscle defects resulting in perinatal death. In this report, the effect of myogenin's absence on embryonic and fetal development is investigated. The initial events of somite differentiation occurred normally in the myogenin-mutant embryos. During primary myogenesis, muscle masses in mutant embryos developed simultaneously with control siblings, although muscle differentiation within the mutant muscle masses was delayed. More dramatic effects were observed when secondary myofibers form. During this time, very little muscle formation took place in the mutants, suggesting that the absence of myogenin affected secondary myogenesis more severely than primary myogenesis. Monitoring mutant neonates with fiber type-specific myosin isoforms indicated that different fiber types were present in the residual muscle. No evidence was found to indicate that myogenin was required for the formation of muscle in one region of the embryo and not another. The expression patterns of a MyoD-lacZ transgene in myogenin-mutant embryos demonstrated that myogenin was not essential for the activation of the MyoD gene. Together, these results indicate that late stages of embryogenesis are more dependent on myogenin than early stages, and that myogenin is not required for the initial aspects of myogenesis, including myotome formation and the appearance of myoblasts. SKELETAL muscle in vertebrates originates from somitic mesoderm as pluripotent mesodermal cells become committed to a myogenic fate. Committed myoblasts populate areas throughout the developing embryo, ultimately differentiating into bundles of multinucleate myofibers. Four key players in myogenic events are the basic helix-loop-helix (bHLH) ~ regulatory proteins: MyoD, Myf5, myogenin, and MRF4 (for recent reviews see Emerson, 1993;Weintraub, 1993;Olson and Klein, 1994). These muscle-specific transcription factors are individually able to initiate the entire muscle differentiation program when introduced into tissue culture cells of nonmuscle origin. Using gene-knockout technology, several laboratories have created mice lacking functional myogenic bHLH factors and are now providing useful models for studying skeletal muscle development (Braun et al

p21CIP1 and p57KIP2 control muscle differentiation at the myogenin step

“…Furthermore, expression of MEF2C, a myogenic transcription factor thought to be downstream of myogenin (Cserjesi et al 1994;Edmondson et al 1994;Molkentin et al 1995), is reduced in p21 −/− p57 −m/+ animals, consistent with a defect in myogenin activity or an earlier step. The involvement of myogenin in the Rbdependent step is supported by Tedesco et al (1995), who have shown that SV40 T antigen blocks myogenesis after myogenin has been expressed, and Novitch et al (1996), who demonstrated that in the absence of Rb, MyoD induces the expression of both p21 and myogenin, but not later differentiation markers such as MHC, which depend on the function of myogenin (Hasty et al 1993;Nabeshima et al 1993;Venuti et al 1995). Whereas myogenin has been shown to interact physically with Rb in vitro, our studies do not address whether this connection is direct.…”

Inactivation of the myogenic bHLH gene MRF4 results in up-regulation of myogenin and rib anomalies.

“…Mice lacking myogenin possess normal numbers of skeletal myoblasts at the time of birth, but they show a severe reduction of skeletal muscle fibers (Hasty et al 1993;Nabeshima et al 1993;Venuti et al 1995). The undifferentiated myogenic cells that populate the presumptive muscle-forming regions of myogeninnull mice express MyoD and myfS, indicating that these myogenic factors act at an earlier point than myogenin in the myogenic pathway and suggesting that they cannot direct the formation of normal skeletal muscle in the absence of myogenin.…”

“…cRNA transcripts were labeled with [3sS]UTP using an in vitro transcription kit (Stratagene) according to the manufacturer's instructions {Venuti et al 1995).…”

MyoD is required for myogenic stem cell function in adult skeletal muscle.

“…Mice lacking myogenin die at birth because of a virtual absence of myofibers; however, normal numbers of MyoD-expressing myoblasts are present (Hasty et al 1993;Nabeshima et al 1993;Venuti et al 1995). In contrast, mice lacking MRF4 are viable with seemingly normal skeletal muscle but display a four-fold increase in myogenin (Patapoutian et al 1995;Zhang et al 1995).…”