Expression of cyclin-dependent kinase 6, but not cyclin-dependent kinase 4, alters morphology of cultured mouse astrocytes.

Disruption of the pRb pathway is a common mechanism in tumor formation. The D-cyclin-associated kinases, cyclin-dependent kinase (cdk) 4 and cdk6, are important regulators of the G(1)-S phase transition and are elevated in several types of cancers, including gliomas. To investigate potential functional differences in these kinases, mouse astrocytes were taken from chimeric mice and propagated in tissue culture. These multipolar tissue-culture astrocytes were infected with viruses expressing either cdk4 or cdk6. Interestingly, expression of cdk6 resulted in a distinct and rapid morphology change from multipolar to bipolar. This change was not observed in control astrocytes or in astroyctes infected with cdk4. Several other differences in cdk4- and cdk6-infected cells were noted, including differential binding to a subset of cell-cycle inhibitor proteins and a distinct pattern of subcellular localization of these kinases. Immunoblot and immunofluorescence analyses revealed that cdk6-infected astrocytes had an altered expression profile of known markers of glial differentiation. Together, these data indicate several important differences between cdk4 and cdk6 that highlight unique functional roles for these cyclin-dependent kinases.

Changes in motility, gene expression and actin dynamics: Cdk6-induced cytoskeletal changes associated with differentiation in mouse astrocytes.

Cyclin dependent kinase (cdk) 4 and cdk6 have historically been understood to be D-cyclin kinases that phosphorylate pRb in the nucleus to regulate G1 phase of the cell cycle. In conflict with this understood redundancy are several studies that have demonstrated a novel role for cdk6 in differentiation. Cdk6 expression must be reduced to allow proper osteoblast and osteoclast differentiation, enforced cdk6 expression blocked differentiation of mouse embryo fibroblasts, and cdk6 expression in primary astrocytes favored the expression of progenitor cell markers (Ericson et al. [2003] Mol Cancer Res 1:654-664; Matushansky et al. [2003] Oncogene 22:4143-4149; Ogasawara et al. [2004a] J Bone Miner Res 19:1128-1136; Ogasawara et al. [2004b] Mol Cell Biol 24:6560-6568). Experiments shown here investigate novel cytoplasmic and nuclear functions of cdk6. These data demonstrate that cdk6 expression in mouse astrocytes results in changes in patterns of gene expression, changes in the actin cytoskeleton including loss of stress fibers, and enhanced motility. These changes in cdk6-infected cells are associated with the process of cellular differentiation.