Regulation of the p35/cdk5 kinase in neuronal development and neurodegenerative disease

Cyclin-dependent kinase 5 (cdk5) was originally isolated by its close homology to the human cdc2 gene which is a key regulator of cell cycle progression. However, unlike other cdks, cdk5's activity is required in postmitotic neurons. The neuronal specific p35 protein, which shares no homology to cyclins, was identified by virtue of its association and activation of cdk5. Gene targeting studies in mice have shown that the p35/cdk5 kinase is required for the proper neuronal migration and development of the mammalian cortex.; In order to further understand cdk5 function, I have focused on understanding the regulation of the p35/cdk5 kinase. This thesis therefore describes the various modes of regulation for the cdk5 kinase. The rate-limiting factor for cdk5 activity is its association with p35, a non-cyclin positive regulatory subunit. During neuronal development cdk5 is activated by p35. I demonstrate here the phosphorylation-stimulated, ubiquitin-mediated degradation of p35, which provides a rapid negative feedback loop for cdk5 activity. As the p35/cdk5 kinase is involved in the regulation of cytoskeletal dynamics, an event central to neurite outgrowth and neuronal migration, it is conceivable that dynamic kinase activation and deactivation is required in order to accommodate these processes. Additionally, I describe the relationship between cdk5 and neurodegenerative disease. I have found that p25, an N-terminal truncation fragment of p35, which selectively accumulates in the brains of patients with Alzheimer's disease and other neurodegenerative diseases exhibiting neurofibrillary tangles. p25 is detected in neurons with neurofibrillary tangles and its accumulation is accompanied by elevation in cdk5 kinase activity. Although cdk5 is activated by both p35 and p25, the two kinase complexes have different properties. p25 is a more stable protein than p35, and it is not targeted to the membrane. Therefore, conversion of p35 to p25 causes prolonged activation of the cdk5 kinase as well as altering cdk5 subcellular distribution. In cultured primary cortical neurons, ectopic expression of p25/cdk5 kinase produces extensive hyperphosphorylation of tau which is often accompanied by the appearance of aggregated silver-positive material in the cell soma and neurite degeneration. Furthermore, p25/cdk5 expressing cells exhibit profound nuclear fragmentation and cell death. These observations suggest a causal relationship between p25 accumulation and a subsequent deregulation of cdk5 activity, tau hyperphosphorylation and neurodegeneration.