Glycogen synthase kinase-3 is involved in the regulation of the cell cycle in cerebellar granule cells

Recent studies have demonstrated that neuronal reentry in the cell cycle and specifically the expression of the transcription factor E2F-1, constitutes a pathway that may be involved in neuronal apoptosis after serum and potassium withdrawal. Other enzymes such as glycogen synthase kinase-3beta (GSK-3beta) are also involved in this apoptotic stimulus, and thus in the process of neuronal cell death. Primary cerebellar granule cells (CGNs) were used in this study to determine whether pharmacological inhibition of GSK-3beta is involved in neuronal modulation of the cell cycle, and specifically in the regulation of E2F-1 and retinoblastoma protein (Rb). CGNs showed a dramatic increase in GSK-3beta activity after 2h of serum and potassium deprivation. Immunoblot and activity assays revealed that lithium and SB415286 inhibit fully the activation of GSK-3beta and attenuate the expression of cyclin D, cyclin E, pRb phosphorylation and the transcription factor E2F-1. These data were confirmed using AR-014418, a selective GSK-3beta inhibitor that prevents the expression of cell-cycle proteins. Our data indicate that GSK-3beta inhibition regulates, in part, the cell cycle in CGNs by inhibiting Rb phosphorylation and thus inhibiting E2F-1 activity. However, the selective inhibition of GSK-3beta with AR-A014418 had not effect on cell viability or apoptosis mediated by S/K withdrawal. Furthermore, our results suggest that selective GSK-3beta inhibition is not sufficient to protect against apoptosis in this S/K withdrawal model, indicating that Li(+) and SB415286 neuroprotective effects are mediated by the inhibition of additional targets to GSK3beta. Therefore, there is a connection between cell cycle and GSK-3beta activation and that these, along with other mechanisms, are involved in the molecular paths leading to the apoptotic process of rat CGNs triggered by S/K withdrawal.

Inhibition of phosphatidylcholine synthesis is associated with excitotoxic cell death in cerebellar granule cell cultures

Glucose deprivation (GD) enhances the sensitivity of cerebellar granule cells to die by excitotoxicity. Neither 70 min of GD, a treatment that depletes cell energy resources, nor exposure to 20 microM glutamate (GLU) for 30 min, induce significant cell death in cultures of cerebellar granule cells. However, the combined treatment with GLU and GD induces choline (Cho) release before excitotoxic cell death. We investigated whether the neurotoxic effect of this treatment is related with inhibition of phosphatidylcholine (PC) synthesis. We found that exposure to GLU for 30 min, to GD for 70 min, and to the combination of both, inhibited PC synthesis at the end of treatment by 71%, 92% and 91%, respectively. The inhibition of PC synthesis was accompanied by a decrease in the incorporation of [(3)H]Cho into phosphocholine and by an increase of the intracellular content of free [(3)H]Cho, indicating that these treatments inhibit the synthesis of PC by inhibiting choline kinase activity. However, only the combined treatment with GLU and GD induced a prolonged inhibition of PC synthesis that extended after the end of treatment. These results show that excitotoxic death is associated with sustained inhibition of PC synthesis and suggest that this effect of the combined treatment with GLU and GD on PC synthesis is produced by an action on an enzymatic step downstream of choline kinase activity.