MyoD is required for myoblast proliferation
Myf5 and MyoD are the two muscle regulatory factors of the muscle basic-helix-loop helix family expressed from the myoblast stage to maintain the identity and to promote the subsequent differentiation of muscle precursor cells. In order to get insight on their role we have studied the capacity to proliferate and to differentiate of Myf5 an MyoD null myoblasts in primary cultures and in the subsequent passages. Our results indicate that Myf5 null myoblasts differ from wt myoblasts in that they undergo precocious differentiation : they become myogenin and troponin T positive and fail to incorporate BrdU under culture conditions and at a time when wt cells are not yet differentiated and continue to proliferate. In primary cultures of MyoD null cells, up to 60% of the cells were scored as myoblasts on the basis of the expression of Myf5. These MyoD deficient myoblasts, unlike MyoD expressing cells, were poorly differentiating and displayed a severe growth defect which led to their elimination from the cultures : within a few passages myoblasts were absent from MyoD deficient cultures which mostly consisted of senescent cells. That a null mutation in either gene reduces the proliferative potential of cultured myoblasts raises the possibility that Myf5 and MyoD serve proliferation of muscle precursor cells. Muscle regeneration is impaired in MyoD null mice. The severity of the phenotype of MyoD deficient myoblasts argues in favor of a critical role for MyoD in the expansion of the pool of muscle precursor cells required for muscle repair.
Mitotic instability of Myf5
Transcription factors Myf5 and MyoD play critical roles in controlling myoblast identity and differentiation.
In the myogenic cell line, C2, we have found that Myf5 expression , unlike that of MyoD, is restricted to cycling cells and regulated by proteolysis at mitosis. In the present study , we have examined Myf5 proteolysis through stable transfection of myogenically convertible U2OS cells with Myf5 derivatives under the control of tetracycline-sensitive promoter. A motif within the basic helix-loop-helix domain of Myf5 ( R 93 to Q 101) resembles the destruction box characteristic of substrates of mitotic proteolysis and thought to be recognized by the anaphase-promoting complex or cyclosome (APC). Mutation of this motif in Myf5 stabilises the protein at mitosis but does not affect its constitutive turnover. Conversely, mutation of a serine residue (SER 158) stabillises Myf5 in non synchronized cultures but not at mitosis. Thus, at least two proteolytic pathways control Myf5 in cycling cells. The mitotic proteolysis of Myf5 is unlike that which has been described for other destruction box dependent substrates: down-regulation of Myf5 at mitosis appears to precede that of known targets of the APC and is not affected by a dominant negative version of the ubiquitin carrier protein UbcH10, implicated in the APC-mediated pathway. Finally, we find that induction of Myf5 perturbs the passage of cells through mitosis, suggesting that regulation of Myf5 levelsat mitosis may influence cell cycle progression of Myf5 expressing muscle precursor cells