MyoD: beyond the call of duty

The basic helix-loop-helix transcription factor, MyoD, induces differentiation of myoblasts into myotubes by transcriptional activation of downstream target genes. However, MyoD is expressed much prior to the onset of differentiation in early myogenic precursors and cultured myoblasts, and is upregulated in muscle stem cells (satellite cells) soon after injury. Previously, no role has been attributed to MyoD in dividing cells. Most models presume it is transcriptionally inactive in dividing cells, but present solely for priming the transition into differentiated state when conditions are permissive. Now, in a simple and elegant study, Konieczny and colleagues not only show that MyoD protein is active in dividing cells, but also identify two direct transcriptional targets of MyoD [1xIdentification of novel MyoD gene targets in proliferating myogenic stem cells. Wyzykowski, J.C. et al. Mol. Cell. Biol. 2002; 22: 6199–6208Crossref | PubMed | Scopus (35)See all References][1].Previously, activation of a MyoD-Estrogen Receptor (MyoD-ER) fusion protein, induced by β-estradiol, in C3H10T1/2 cells has been used to identify MyoD targets during early differentiation. These fibroblasts, become myogenic and differentiate into myotubes, but only in the presence of both differentiation medium and β-estradiol.Using this system, the authors performed a PCR-select subtractive hybridization between cells maintained in growth medium in the presence and absence of β-estradiol, thus selecting for genes upregulated by the transcriptional activity of MyoD in dividing cells. As MyoD is known to be able to activate its own transcription, upregulation of endogenous MyoD RNA in the presence of β-estradiol serves as proof that MyoD-ER is transcriptionally active in dividing cells. They identified two of the clones isolated from this selection as Inhibitor of differentiation (Id3) and Neuronal Pentraxin 1 (NP1). The genes encoding Id3 and NP1 are upregulated even in the absence of protein synthesis, indicating that they are direct targets of MyoD. The authors observed NP1 and Id3 expression in several commonly used myoblast cell lines, thus confirming that the results are not an artifact of the system. Finally they show that expression of a constitutively active G protein, H-Ras, which is known to block MyoD-induced terminal differentiation of myoblasts, has no effect on the ability of MyoD to induce Id3 and NP1 expression. This indicates that MyoD has distinct transcriptional activities in growing and differentiating myoblasts, and is probably responsible for targeting a distinct set of genes in each case.Although not the first report of the presence of an active MyoD protein in non-differentiating cells, this paper is still of great significance as it illustrates a method for identifying downstream target genes of MyoD. Analysis of the function of these gene products should lead us closer to understanding the difference between a committed myoblast (expressing MyoD) and a multipotential precursor and/or stem cell (not expressing MyoD) in the context of both development and regeneration.