Regulation of Wallerian degeneration and nerve growth factor withdrawal-induced pruning of axons of sympathetic neurons by the proteasome and the MEK/Erk pathway

Treatment of transected distal axons of rat sympathetic neurons in compartmented cultures with MG132 (5 μM) and other inhibitors of proteasome activity, preserved axonal mitochondrial function, assessed by Mitotracker-Orange and MTT staining, for at least 24 h. MG132 similarly protected axons from undergoing branch elimination (pruning) in response to local NGF deprivation. Axons protected by MG132 displayed persistent phosphorylation of Erk1/2, and pharmacological inhibition of MEK activity with U0126 (50 μM) restored rapid axonal degeneration. Therefore, the proteasome does not appear to be necessary as a general effecter of protein degradation during axonal degeneration. Rather, the proteasome functions in the regulation of signaling pathways that control axonal survival and degeneration. Specifically, the down-regulation of the MEK/Erk pathway by the proteasome plays roles in Wallerian degeneration of severed axons and axonal pruning in response to local NGF deprivation. Identification of the pathways that regulate axonal survival and degeneration will provide possible target sites for pharmacological treatments of neurodegenerative diseases and traumatic injury.