It is clear that the role of Cdk5 in neuronal cell death is much more complex than previously believed. Regarding Bruce Lamb's hypothesis about the role of p39, p25, and p10, I'd like to point out respectfully that these subunits may not block cell cycle reentry since they do not form any complex with Cdk5 and E2F1. An alternate hypothesis may be a facilitation of cell cycle reentry, since p25/Cdk5 is able to phosphorylate Rb within the nucleus (see ARF related news story).

References:
Hamdane M, Bretteville A, Sambo AV, Schindowski K, Bégard S, Delacourte A, Bertrand P, Buée L. p25/Cdk5-mediated retinoblastoma phosphorylation is an early event in neuronal cell death. J Cell Sci. 2005 Mar 15;118(Pt 6):1291-8. Abstract.

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