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Oligodendrocyte differentiation is a complex process believed to be controlled by an intrinsic mechanism associated with cell-cycle arrest. Recently, the cell-cycle inhibitor protein p27 Kip1 has been proposed as a key element in causing growth arrest of oligodendrocyte precursor cells. To investigate the effects of p27 upon oligodendrocyte cell development, we have introduced the p27 cDNA in oligodendrocyte progenitor cells using an adenovirus vector. Progenitor cells normally express low levels of p27. After adenoviral infection and p27 overexpression, progenitor cells were able to undergo cell-cycle arrest, even in the presence of strong mitogens. The effects of p27 were shown to be directly upon cyclin-dependent kinase-2 (CDK2), the protein kinase complex responsible for G1/S transition, as immunodepletion of oligodendrocyte extracts of p27 protein resulted in the activation of CDK2 activity. However, cells that became growth arrested owing to infection with p27 adenovirus did not display conventional oligodendrocyte differentiation markers, such as O4 or O1. Taken together, these data provide mechanistic evidence indicating that p27 is primarily involved in oligodendroglial progenitor proliferation by inhibiting CDK2 activity and inducing oligodendrocyte cell-cycle arrest.

Schwann cell proliferation is regulated by multiple growth factors and axonal signals. However, the molecules that control growth arrest of Schwann cells are not well defined. Here we describe regulation of the cyclin-dependent kinase-2 (CDK2) protein, an enzyme that is necessary for the transition from G1 to S phase. Levels of CDK2 protein were elevated in proliferating Schwann cells cultured in serum and forskolin. However, when cells were grown with either serum-free media or at high densities, CDK2 levels declined to low levels. The decrease in CDK2 levels was associated with growth arrest of Schwann cells. The modulation of CDK2 appears to be regulated at the transcriptional level, because CDK2 mRNA levels and its promoter activity both decline during cell cycle arrest. Furthermore, analysis of the CDK2 promoter suggests that Sp1 DNA binding sites are essential for maximal activation in Schwann cells. Together, these data suggest that CDK2 may represent a significant target of developmental signals that regulate Schwann cell proliferation and that this regulation is mediated, in part, through regulation of Sp1 transcriptional activity.

The generation of different glial cell types in the central nervous system depends upon a wide variety of proliferative and differentiative signals. Here we report that changes in the levels of cyclin-dependent kinase 2 (CDK2) and the cell cycle inhibitor p27 kip1 accompany the differentiation of central glia-4 (CG-4) progenitor cells to an astrocytic cell phenotype in the presence of fetal calf serum. Although a decrease in CDK2 levels was observed in both oligodendrocyte and astrocyte cells derived from CG-4 cells, a striking increase in the levels of p27 was observed during the differentiation of astrocyte cells. In astrocyte cell extracts, inhibition of CDK2 activity could be overcome with exogenously added cyclin E. Furthermore, depletion of p27 from astrocyte extracts lowered the amount of cyclin E required for CDK2 activation. Taken together, these results suggest that the inhibitory action of p27 upon cyclin E-CDK2 may prevent entry of cells into the S phase and regulate the progression of CG-4 cells toward an astrocytic lineage.

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