contributed equally to this work Neurodegeneration in Alzheimer's disease and various experimental lesion paradigms are associated with an unscheduled upregulation of cell cycle--related proteins, indicating a link between cell cycle reactivation and neuronal death. Recent evidence, however, suggests that at least some of the canonical cell cycle regulators are constitutively expressed in differentiated neurons of the adult brain. Systematic investigations on the constitutive expression of cell

more »

... cycle regulators in differentiated neurons in vivo, providing the basis for further insights into their potential role under pathological conditions, however, have not been carried out. Here, we demonstrate a constitutive neuronal expression of Cdks 1, 2, and 4; their activators cyclins D, A, B, and E; and their inhibitors p15 Ink4b , p16 Ink4a , p18 Ink4c , p19 Ink4d , p21 Waf1/Cip1 , p27 Kip1 , and p57 Kip2 within the neocortex of adult mice by western blot and immunocytochemistry. Expression was verified by single-cell reverse transcriptase--polymerase chain reaction applied to individual microscopically identified neurons captured with laser dissection. Immunoprecipitation and in vitro kinase assays revealed that Cdks 1, 2, and 4 are properly complexed to cyclins and exhibit kinase activity. This physiological expression of positive cell cycle regulators in adult neurons is clearly not related to neuronal proliferation. Taken together, our findings demonstrate a constitutive expression of functionally active cyclin-dependent kinases and their regulators in differentiated neurons suggesting a noncanonical role of cell cycle regulators potentially linked to neuronal plasticity and/or stability.