Skeletal muscle differentiation entails the coordination of muscle-specific gene expression and terminal withdrawal from the cell cycle. This cell cycle arrest in the G0 phase requires the retinoblastoma tumor suppressor protein (Rb). The function of Rb is negatively regulated by cyclin-dependent kinases (Cdks), which are controlled by Cdk inhibitors. Expression of MyoD, a skeletal muscle-specific transcriptional regulator, activated the expression of the Cdk inhibitor p21 during differentiation of murine myocytes and in nonmyogenic cells. MyoD-mediated induction of p21 did not require the tumor suppressor protein p53 and correlated with cell cycle withdrawal. Thus, MyoD may induce terminal cell cycle arrest during skeletal muscle differentiation by increasing the expression of p21.

The generation of neurons from stem cells involves the activity of proneural basic helix-loop-helix (bHLH) proteins, but the mechanism by which these proteins irreversibly commit stem cells to neuronal differentiation is not known. Here we report that expression of the transcription factors Sox1, Sox2 and Sox3 (Sox1-3) is a critical determinant of neurogenesis. Using chick in ovo electroporation, we found that Sox1-3 transcription factors keep neural cells undifferentiated by counteracting the activity of proneural proteins. Conversely, the capacity of proneural bHLH proteins to direct neuronal differentiation critically depends on their ability to suppress Sox1-3 expression in CNS progenitors. These data suggest that the generation of neurons from stem cells depends on the inhibition of Sox1-3 expression by proneural proteins.

Within the developing vertebrate nervous system, the mechanisms that coordinate neuronal subtype identity with generic features of neuronal differentiation are poorly defined. We show here that a bHLH protein, Olig2, is expressed selectively by motor neuron progenitors and has a key role in specifying the subtype identity and pan-neuronal properties of developing motor neurons. The role of Olig2 in the specification of motor neuron subtype identity depends on regulatory interactions with progenitor homeodomain proteins, whereas its role in promoting pan-neuronal properties is associated with expression of another bHLH protein, Ngn2. Both aspects of Olig2 function appear to depend on its activity as a transcriptional repressor. Together, these studies show that Olig2 has a critical role in integrating diverse features of motor neuron differentiation in the developing spinal cord.