The expression of Cdk5, p35, p39, and Cdk5 kinase activity in developing, adult, and aged rat brains.

The expression of cyclin-dependent kinase 5 (Cdk5) and its regulatory subunits, p35 and p39, was investigated in rat brain from embryonic day 12 (E12) to postnatal 18 months (18M). The Cdk5 protein levels increased from E12 to postnatal day 7 (P7) and remained at this level until 18M. The Cdk5 kinase activity and the levels of both p35 mRNA and protein were low at E12, became prominent at E18-P14 but then decreased in the adult and aged rat brains of 3M to 18M. In comparison, the expression pattern of p39 appeared to have an inverse relationship to that of Cdk5 and p35. In regional distribution studies, p35 protein levels and Cdk5 kinase activity were significantly higher in the cerebral cortex and hippocampus, but lower in the cerebellum and striatum. These results suggested that Cdk5, p35 and p39 might have region-specific and developmental stage-specific functions in rat brain.