Developmental expression and co-localization of cyclin G1 and the B ' subunits of protein phosphatase 2a in neurons

Cyclin G1 is a recently cloned transcriptional target of p53, it is located in neurons and ventricular ependymal cells and is elevated in neurons afte r axotomy and cerebral ischemia. The biological function for cyclin G1 in d ifferentiated neurons has thus far not been elucidated. Recently, cyclin G1 has been shown to interact with the B' subunits of serine/threonine protei n phosphatase 2A (PP2A) in a rat fibroblast cell line [K. Okamoto, C., Kami bayashi, M. Serrano, C. Prives, M.C. Mumby, D. Beach, p53-dependent associa tion between cyclin G and the B' subunit of protein phosphatase 2A, Mel. Ce ll. Biol. 16 (1996) 6593-6602]. To further explore whether a similar intera ction between cyclin G1 and PP2A B' subunits exists in the central nervous system, the present study compared the regional and developmental expressio n pattern, subcellular distribution and complex formation between cyclin G1 and the PP2A B' regulatory subunits in the rat brain. In situ hybridizatio n of cyclin G1 and the B'alpha and B'beta subunits of PP2A showed an overla pping distribution in neurons of the cerebral cortex, hippocampus and thala mus at embryonic and early postnatal ages, but their developmental regulati on differed. Whereas mRNA and protein levels of PP2A B' subunits were high in the cortical plate, subiculum, hippocampal areas and thalamus at E20 and decreased with age, those of cyclin G1 increased with age and were maximal in the adult cortex and hippocampus. In rat 14-day-old embryonic cortical cultures, cyclin G1 and PP2A B'alpha protein co-localized in nuclear and pe rinuclear areas of neurons, and both proteins were highly expressed in nucl ei of cortical and hippocampal pyramidal cells and the mitral cell layer of the neonatal olfactory bulb. Both cyclin G1 and the PP2A regulatory B'alph a subunits were specifically expressed in neurons and not in glial cells. A ntibodies raised against the B'alpha subunits of PP2A immunoprecipitated cy clin G1 in adult cortical lysates, indicating the presence of a complex inv olving cyclin G1 and the B'alpha subunits of PP2A. This study shows that th e regional and subcellular localization;di PP2A B' regulatory subunits and cyclin G1 are very similar at early postnatal stages. We discuss the possib le functions of a cyclin G1-PP2A B'alpha complex in neurons. (C) 1999 Elsev ier Science B.V. All rights reserved.