THE CDK5 P35 KINASE IS ESSENTIAL FOR NEURITE OUTGROWTH DURING NEURONAL DIFFERENTIATION/

Cyclin-dependent kinase 5 (cdk5) is highly homologous to other members of the cdk family that are known to function in proliferating cells. Despite the structural similarity, cdk5-associated histone H1 kinase a ctivity is only detectable in postmitotic neurons of the central nervo us system (CNS). p35 is a neuronal specific cdk5 regulator that activa tes cdk5 kinase activity upon association. The cdk5/p35 kinase activit y increases during the progression of CNS neurogenesis, suggesting a f unction of cdk5 in neuronal differentiation. Here we show that both cd k5 and p35 proteins are present in the growth cones of developing neur ons. The staining pattern of cdk5 in the growth cones is similar to th at of actin filaments but not microtubules. To address the functional significance of the cdk5/p35 kinase in neurogenesis, we ectopically ex pressed wild-type or mutant kinases in cortical cultures. Expression o f dominant-negative mutants of cdk5 (Cdk5N(144) and cdk5T(33)) inhibit ed neurite outgrowth, which was rescued by coexpression of the wild-ty pe proteins. A similar extent of neurite outgrowth inhibition was obta ined by transfection of an antisense p35 construct, which in turn was only rescued by p35 but not cdk5 coexpression. In contrast, longer neu rites were elaborated in neurons that coexpressed exogenous cdk5 and p 35. These observations suggest that the cdk5/p35 kinase plays a critic al role in neurite outgrowth during neuronal differentiation.