Changes in the expression of novel Cdk5 activator messenger RNA (p39(nck5ai) mRNA) during rat brain development

We previously reported that a neuron-specific Cdk5 activator, p35(nck5a), was most prominent in the newborn rat brain. In the adult brain, the expression decreased in most regions except hippocampus and primary olfactory cortex. A novel neuron-specific Cdk5 activator, p39(nck5ai), has been recently cloned. To clarify whether two activators were differentially distributed throughout brain development, in this study, we examined the spatial and temporal expression of p39(nck5ai) in the development rat brain. Northern blot analysis showed that p39(nck5ai) expression was low in 15-day old fetuses and newborn, and was most prominent in the 1-3 week-old rat brains. In the adult rat brain, expression declined to the same level as in newborn rat brain. In situ hybridization showed that p39(nck5ai) mRNA was weakly expressed in all neurons of all regions in the newborn rat brain and the transcriptional level was highest in all regions in the 3 week-old rat brain. In the adult, expression was decreased in most neurons except Purkinje and granule cells in the cerebellum which retained high levels. These results suggest that p35(nck5a) and p39(nck5ai) may have different functional roles in distinct brain regions during different states of the rat brain development.