Cyclin-Dependent Kinase 5 Is Essential for Neuronal Cell Cycle Arrest and Differentiation

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Abstract
Cyclin-dependent kinase 5 (Cdk5) is a serine/threonine kinase with significant homology to cell cycle-related Cdks but is not believed to be active in a typical cell cycle. In Cdk5-deficient embryos and Cdk5 chimeras, migration and survival of postmitotic neurons is compromised in a cell-autonomous manner. In the present study, we show that loss of Cdk5 leads to both failure of neuronal differentiation and loss of cell cycle control. Using specific cytoskeletal proteins as indices of neuronal differentiation, we find that Cdk5-deficient neurons are significantly arrested or delayed in their developmental program bothin vivoandin vitro. For example, immunocytochemistry of embryonic day 16 (E16) cortex reveals that the expression of microtubule-associated protein 2c (Map-2c), a marker of mature neurons, is nearly absent inCdk5-/-cells that have migrated to the cortical plate while these same cells continue to express nestin. Similarly,in vitro, Map-2-positive cells are rare in cultures from E16Cdk5-/-embryos. Cell cycle control is also deficient inCdk5-/-cells.In vivo, neurons engaged in cell cycle activities are found in the cortical plate, and,in vitro, class III β-tubulin-positive cells continue to label with bromodeoxyuridine even after 5 d of incubation. Transfection of a wild-type Cdk5 construct reveals that cell cycle control can be regained inCdk5-/-cells by overexpression of Cdk5. These data indicate that Cdk5 is necessary for both neuronal differentiation and cell cycle inhibition.