Phosphorylation of retinoblastoma protein in rat brain after transient middle cerebral artery occlusion

Although mature neurones do not replicate genomic DNA, some cell cycle-rela ted kinases are aberrantly activated in neurones after ischaemia. As hyperp hosphorylation of retinoblastoma (Rb) protein is the common pathway in mito tic signal cascade, this study investigated the phosphorylation state of th e Rb protein as well as its mRNA level in rat brain after transient middle cerebral artery (MCA) occlusion. Immunohistochemical analysis revealed that neurones in the sham-operated brain expressed Rb protein without the hyper phosphorylated form. Immunoreactivity for the hyperphosphorylated form of R b protein progressively increased from 1 h to 3 days after ischaemia in neu rones in the MCA territory. Western blot analysis demonstrated similar chan ge. However, reverse transcription-polymerase chain reaction study revealed that Rb showed no definite change at the mRNA level. These results suggest that Rb protein is progressively hyperphosphorylated in the brain after is chaemia, which may activate apoptotic mechanisms in neuronal cells of the b rain after ischaemia.