MyoD and myogenesis in C. elegans

One of the goals in developmental biology is the identification of key regulatory genes that govern the transition of embryonic cells from a pluripotent potential to a specific, committed cell fate. During vertebrate skeletal myogenesis, this transition is regulated by the MyoD family of genes. C. elegans has muscle analogous to vertebrate skeletal muscle and has a gene(hlh-1) related to the MyoD family. The molecular and genetic characterization of hlh-1 shows that it is very similar to the vertebrate MyoD family in many respects, including its expression pattern and DNA binding activity. The hlh-1 product is required for proper myogenesis, but it is not required for myogenic commitment during embryogenesis in the nematode. The role of this MyoD-related gene in nematode myogenesis is discussed and compared to those of the vertebrate MyoD family.     

The MyoD family of transcription factors and skeletal myogenesis

Gene targeting has allowed the dissection of complex biological processes at the genetic level. Our understanding of the nuances of skeletal muscle development has been greatly increased by the analysis of mice carrying targeted null mutations in the Myf-5, MyoD and myogenin genes, encoding members of the myogenic regulatory factor (MRF) family. These experiments have elucidated the hierarchical relationships existing between the MRFs, and established that functional redundancy is a feature of the MRF regulatory network. Either MyoD or Myf-5 is sufficient for the formation or survival of skeletal myoblasts. Myogenin acts later in development and plays an essential in vivo role in the terminal differentiation of myotubes.     

Transformation by activated ras or fos prevents myogenesis by inhibiting expression of MyoD1

Transformation of myoblasts by activated ras inhibits myogenic differentiation. We demonstrate that this oncogene inhibits expression of the muscle regulatory factors MyoD1 and myogenin. Expression of retroviral-encoded MyoD1 in ras-transformed myoblasts leads to the re-expression of both terminal differentiation markers and lineage markers expressed in proliferating myoblasts (including endogenous MyoD1 and myogenin), suggesting that ras inhibits myogenic differentiation in a manner dependent on the loss of MyoD1 expression. In addition, we show that fos transformation of myoblasts inhibits muscle differentiation by a similar mechanism.     

Identification of a new hybrid serum response factor and myocyte enhancer factor 2-binding element in MyoD enhancer required for MyoD expression during myogenesis

MyoD is a critical myogenic factor induced rapidly upon activation of quiescent satellite cells, and required for their differentiation during muscle regeneration. One of the two enhancers of MyoD, the distal regulatory region, is essential for MyoD expression in postnatal muscle. This enhancer contains a functional divergent serum response factor (SRF)-binding CArG element required for MyoD expression during myoblast growth and muscle regeneration in vivo. Electrophoretic mobility shift assay, chromatin immunoprecipitation, and microinjection analyses show this element is a hybrid SRF- and MEF2 Binding (SMB) sequence where myocyte enhancer factor 2 (MEF2) complexes can compete out binding of SRF at the onset of differentiation. As cells differentiate into postmitotic myotubes, MyoD expression no longer requires SRF but instead MEF2 binding to this dual-specificity element. As such, the MyoD enhancer SMB element is the site for a molecular relay where MyoD expression is first initiated in activated satellite cells in an SRF-dependent manner and then increased and maintained by MEF2 binding in differentiated myotubes. Therefore, SMB is a DNA element with dual and stage-specific binding activity, which modulates the effects of regulatory proteins critical in controlling the balance between proliferation and differentiation.     

Coupling of the cell cycle and myogenesis through the cyclin D1-dependent interaction of MyoD with cdk4

Proliferating myoblasts express the muscle determination factor, MyoD, throughout the cell cycle in the absence of differentiation. Here we show that a mitogen-sensitive mechanism, involving the direct interaction between MyoD and cdk4, restricts myoblast differentiation to cells that have entered into the G0 phase of the cell cycle under mitogen withdrawal. Interaction between MyoD and cdk4 disrupts MyoD DNA-binding, muscle-specific gene activation and myogenic conversion of 10T1/2 cells independently of cyclin D1 and the CAK activation of cdk4. Forced induction of cyclin D1 in myotubes results in the cytoplasmic to nuclear translocation of cdk4. The specific MyoD-cdk4 interaction in dividing myoblasts, coupled with the cyclin D1-dependent nuclear targeting of cdk4, suggests a mitogen-sensitive mechanism whereby cyclin D1 can regulate MyoD function and the onset of myogenesis by controlling the cellular location of cdk4 rather than the phosphorylation status of MyoD.