EXPRESSION OF INK4 INHIBITORS OF CYCLIN D-DEPENDENT KINASES DURING MOUSE-BRAIN DEVELOPMENT

In situ hybridization of mouse embryo sections demonstrated expression of mRNAs encoding two polypeptide inhibitors (p18(INK4c) and p19(INK4 d)) of cyclin D-dependent kinase (CDK) 4 and CDK6 in the central nervo us system. No expression of two other INK4 members, p16(INK4a) and p15 (INK4b), was observed. The p19(INK4d) and p18(INK4c) proteins formed c omplexes with either CDK4 or CDK6 in a temporal pattern consistent wit h the results of in site hybridization. Expression of INK4c was observ ed at embryonic day 13.5 in neuroepithelial zones of the developing br ain, being restricted to dividing neuroblasts but absent from differen tiating postmitotic neurons. In the neocortex, p18(INK4c) was expresse d precisely at those developmental stages when neuroblasts switch from a symmetric to an asymmetric pattern of cell division with concomitan t increases in their G(1) interval. INK4d RNA was detected from embryo nic day 11.5 onward, at higher levels than INK4c and with a distinctly different spatial and temporal pattern. Marked INK4d expression was s een in dorsal root ganglia, spinal cord, and focally throughout the br ain, but primarily in postmitotic neurons, Neural expression of INK4d continued postnatally into adulthood in postmitotic cells of the denta te gyrus, the pyramidal layer of the hippocampus, and in discrete regi ons of the cerebral cortex, cerebellum, thalamus, and brainstem. Downr egulation of p18(INK4d) in the dentate gyrus after kainic acid-induced seizures indicated that its expression could also be modified in nond ividing cells by excitotoxic stress. Therefore, p19(INK4d) may contrib ute to maintaining the quiescent state, acting as a buffer to prevent reactivation of cyclin D-dependent kinases in terminally differentiate d cells.