ROLE OF CYCLINS IN NEURONAL DIFFERENTIATION OF IMMORTALIZED HIPPOCAMPAL CELLS

The proto-oncogene cyclin D1 and the neuron-specific cyclins p35 and p 39 are expressed during brain maturation. To investigate the role of t hese cyclins in neuronal differentiation, we used a conditionally immo rtalized rat hippocampal cell line, H19-7, that expresses cyclin-depen dent kinases 4 and 5 (cdk4 and -5). Cyclin D1, which activates cdk4 an d binds but does not activate cdk5, was increased upon differentiation of the H19-7 cells. However, microinjection of either sense or antise nse cyclin D1 cDNA or anti-cyclin D1 antibodies had no effect on morph ological differentiation of the cells. On the other hand, neurite outg rowth was stimulated by expression of p35 or p39, both of which activa te cdk5. A dominant-negative mutant of cdk5 blocked both p35- and p39- induced neurite extension as well as basic fibroblast growth factor (b FGF)-induced neuronal differentiation. However, of these cyclins, only antisense p39 prevented bFGF-induced neurite outgrowth. These studies indicate that cyclin D1 is neither necessary nor sufficient for morph ological differentiation, that p35 is sufficient but not required, and that p39 is both necessary and sufficient for neurite outgrowth in th e hippocampal cells. Taken together, these results represent the first demonstration of a specific role for p39 in neuronal differentiation, implicate the cyclin-activated kinase cdk5 in this process, and indic ate that p39 is able to mediate neurite outgrowth in the presence or a bsence of cyclin D1.