Involvement of cyclin-dependent kinase 5/p35^nck5a in the synaptic reorganization of rat hippocampus during kindling progression

To test the hypothesis that a complex of cyclin-dependent kinase 5 (Cdk5) and p35^nck5a plays an important role in sprouting in the kindling rat hippocampus, we studied the changes in kinase activity, expression level and subcellular localization during kindling progression. The kinase activity in kindling rats was significantly higher than that in normal rats. The changes in kinase activity coincided with those of the p35^nck5a expression in kindling rats. In contrast, the expression of Cdk5 was constant at all stages of kindling. Subcellular localization of Cdk5, however, changed markedly in the hippocampal neurons during kindling progression Cdk5 translocated from axon to soma when the kinase activity was high. The phosphorylation level of tau protein was in good agreement with the Cdk5 kinase activity. In contrast, MAP kinase activity was not correlated with tau phosphorylation during kindling progression. These findings suggest that Cdk5/p35^nck5a plays an important role in synaptic reorganization, and the translocation of Cdk5 to soma from axons is a crucial regulatory mechanism of kinase activity.