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

Abstract

The myogenic basic helix-loop--helix (bHLH) proteins MyoD, myf5, myogenin, and MRF4 can initiate myogenesis when expressed in nonmuscle cells. During embryogenesis, each of the myogenic bHLH genes is expressed in a unique temporospatial pattern within the skeletal muscle lineage, suggesting that they play distinct roles in muscle development. Gene targeting has shown that MyoD and myf5 play partially redundant roles in the genesis of myoblasts, whereas myogenin is required for terminal differentiation. MRF4 is expressed transiently in the somite myotome during embryogenesis and then becomes up-regulated during late fetal development to eventually become the predominant myogenic bHLH factor expressed in adult skeletal muscle. On the basis of its expression pattern, it has been proposed that MRF4 may regulate skeletal muscle maturation and aspects of adult myogenesis. To determine the function of MRF4, we generated mice carrying a homozygous germ-line mutation in the MRF4 gene. These mice showed only a subtle reduction in expression of a subset of muscle-specific genes but showed a dramatic increase in expression of myogenin, suggesting that it may compensate for the absence of MRF4 and demonstrating that MRF4 is required for the down-regulation of myogenin expression that normally occurs in postnatal skeletal muscle. Paradoxically, MRF4--null mice exhibited multiple fib anomalies, including extensive bifurcations, fusions, and supernumerary processes. These results demonstrate an unanticipated regulatory relationship between myogenin and MRF4 and suggest that MRF4 influences fib outgrowth through an indirect mechanism.

The epigenetic network regulating muscle development and regeneration

“…Enforced activation of any of the four muscle bHLH factors (Myf5, MyoD, myogenin, or MRF4) converts 10T1/2 fibroblasts to a muscle phenotype (Davis et al, 1987;Braun et al, 1989;Rhodes and Konieczny, 1989;Wright et al, 1989;Miner and Wold, 1990), making them the ideal candidates to start the nuclear reprogramming necessary to specify the myogenic lineage. In cultured myoblasts induced to differentiate, the expression of the early muscle MRFs, MyoD and/or Myf5, preceeds the expression of myogenin and MRF4 and is therefore likely to be the first step in the commitment of multipotent precursors to muscle Bober et al, 1991;Ott et al, 1991;Zhang et al, 1995). However, the mechanisms leading to MyoD (or Myf 5) induction in muscle precursors are still largely unknown.…”

Myogenic regulatory factors: Redundant or specific functions? Lessons from Xenopus

“…The opposite interpretation of the knockout results consists in considering that a threshold level of myogenic regulatory factor, whatever its identity, is solely required for activating muscle target genes. Although the single knockout mice are close to phenotypically normal Braun et al, 1992, Patapoutian et al, 1995, Zhang et al, 1995a, the double and, to a greater extent, triple knock-out mice display variable but severe defects in muscle development. Quite remarkably, in double knock-out mice, the most dramatic effects on myogenesis are obtained by targeting two simultaneously expressed MRFs.…”

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).…”