Anti Cyclin-Dependent Kinase 5 Activator 1, p35 - phospho Ser8 pAb (Rabbit, Affinity Purified)

Cyclin-dependent kinase 5 (Cdk5) is a unique member of the Cdk family. Its activity in postmitotic neurons is completely dependent upon association with one of two neuronal specific activators, p35 or p39. Cdk5/p35 is involved in a panoply of processes critical to central nervous system function both during development and throughout maturity including neuronal migration during corticogenesis, neurite outgrowth, regulation of the synaptic vesicle cycle, neurotransmitter release, and postsynaptic neurotransmitter receptor regulation and signaling. The mechanisms by which Cdk5 activity is normally regulated remains to be fully delineated. Furthermore, because aberrant Cdk5 activity has been implicated in the etiology of neurodegenerative diseases, identifying the biochemical mechanisms contributing to deregulation of Cdk5 is of substantial biomedical relevance.

Deregulation of Cdk5 results from removal of the first 98 amino acids of p35 by the Ca2-dependent cysteine protease, calpain, leaving Cdk5 associated with the N-terminal truncated form p25. Cleavage of p35 to p25 changes the subcellular distribution of active Cdk5 from membranes to the cytosolic fraction, thereby altering substrate specificity. p25 accumulates in neurons undergoing various types of cell death. Expression of Cdk5/p25 in cultured cells results in increased phospho-Tau levels in comparison to cells expressing Cdk5/p35. Furthermore, exogenous overexpression of p25 in transgenic mice results in a neurodegenerative phenotype including the formation of paired helical filaments, Tau aggregation, and neuronal loss similar to that observed in Alzheimer disease.

Ca2-dependent activation of the cytoplasmic protease calpain is involved in apoptotic and necrotic cell death. Calpain generally recognizes motifs between conformational domains and cleaves substrate proteins in a limited manner, although the physiological function of calpain activity remains unclear. In some cases calpain cleavage is suspected to be a signaling process. Calpain-mediated cleavage of many proteins including neurofilament proteins, alpha II-spectrin (20), NR2 subunits of N-methyl-D-aspartic acid receptors, and ezrin is suppressed through phosphorylation. However, how such signaling works and the mechanisms of phosphorylation-dependent inhibition are unknown.

Previously we demonstrated that Cdk5 phosphorylated p35, that p35 occurred as a phosphoprotein in neurons, and that the phosphorylation state of p35 affected its susceptibility to calpain cleavage. Phospho-p35 predominates in fetal rat brain and is resistant to the cleavage by calpain, whereas unphosphorylated p35 present during adulthood is more vulnerable to calpain-dependent cleavage. Here we report that Ser8 and Thr138 of p35 serve as the sites of Cdk5-dependent phosphorylation. Furthermore, phosphorylation at these sites reduces the susceptibility of p35 to calpain cleavage. Moreover, specific dephosphorylation of Thr138 increases the susceptibility of p35 to cleavage by calpain in adult rat brains, suggesting that phosphorylation of this site is a particularly critical determinant of Cdk5-dependent neuronal cell death in neurodegenerative diseases.

[from: Kamei, H., Saito, T., Ozawa, M., Fujita, Y., Asada, A., Bibb, J.A., Saido, T.C., Sorimachi, H. and Hisanaga, S. Suppression of Calpain-dependent Cleavage of the CDK5 Activator p35 to p25 by Site-specific Phosphorylation J. Biol. Chem. 282(3): 1687-1694 (2007)]