title: Constitutive expression of functionally active cyclin-dependent kinases and their binding partners suggests noncanonical functions of cell cycle regulators in differentiated neurons.
abstract: 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 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.
authors.affiliation: Paul Flechsig Institute for Brain Research, Department of Neuroanatomy, University of Leipzig, 04109 Leipzig, Germany.
authors.forename: Stefanie
authors.initials: S
authors.lastname: Schmetsdorf
authors.forename: Ulrich
authors.initials: U
authors.lastname: Gärtner
authors.forename: Thomas
authors.initials: T
authors.lastname: Arendt
mesh_terms: D000818:Animals; D015153:Blotting, Western; D018797:Cell Cycle Proteins / Q000096:biosynthesis / Q000502:physiology*; D002454:Cell Differentiation / Q000502:physiology; D018844:Cyclin-Dependent Kinases / Q000037:antagonists & inhibitors / Q000502:physiology*; D016213:Cyclins / Q000378:metabolism; D004591:Electrophoresis, Polyacrylamide Gel; D004791:Enzyme Inhibitors / Q000494:pharmacology; D007150:Immunohistochemistry; D047468:Immunoprecipitation; D051379:Mice; D042282:Microdissection; D009474:Neurons / Q000187:drug effects / Q000502:physiology*; D012333:RNA, Messenger / Q000096:biosynthesis; D020133:Reverse Transcriptase Polymerase Chain Reaction
chemical_list: D018797:Cell Cycle Proteins; D016213:Cyclins; D004791:Enzyme Inhibitors; D012333:RNA, Messenger; D018844:Cyclin-Dependent Kinases