The long-term goals of the proposed research are to elucidate the mechanisms that regulate apoptosis of neurons in the developing mammalian brain. Growing evidence suggests that components of the cell cycle are reactivated in dying neurons in the brain. However, the role and mechanisms by which the cell cycle machinery control apoptosis of neurons remain largely to be elucidated. We have found that the mitotic kinase Cdc2 induces apoptosis in primary rat cerebellar granule neurons. These findings raise two major questions: how does Cdc2 trigger apoptosis of postmitotic neurons, and what is the role of Cdc2 function in neuronal apoptosis in the intact developing cerebellar cortex? To address these questions we propose the following specific aims: (1) Elucidate the mechanisms by which Cdc2 induces neuronal apoptosis. Our preliminary studies suggest the hypothesis that Cdc2 triggers apoptosis by phosphorylating protein substrates of the major pro-survival kinase Akt with the net effect of opposing Akt-promotion of neuronal survival. We will take a multi-pronged approach to identify novel substrates of Cdc2 that contribute to neuronal apoptosis. (2) Determine Cdc2 function in the intact cerebellar cortex. We will characterize the expression and activity of Cdc2 in granule neurons in the rat cerebellar cortex in both rat cerebellar slices and in vivo, and we will determine the effect of inhibition of Cdc2 on neuronal apoptosis in the cerebellar cortex. Together, the proposed research should provide fundamental and novel insights into the role and mechanisms of cell cycle reactivation in apoptosis of postmitotic neurons. Since cell cycle reactivation is thought to play an important role in neurodegenerative diseases, our studies should also lead to novel insights into the pathogenesis of these devastating diseases of brain.