Apoptosis in caspase-inhibited neurons

Background: There is growing evidence of apoptosis in neurodegenerative dis ease. However, it is still unclear whether the pathological manifestations observed in slow neurodegenerative diseases are due to neuronal loss or whe ther they are related to independent degenerative events in the axodendriti c network. It also remains elusive whether a single, caspase-based executin g system involving caspases is responsible for neuronal loss by apoptosis. Materials and Methods: Long-term exposure to the microtubule-disassembling agent, colchicine, was used to disrupt the axodendritic network and eventua lly trigger caspase-3-mediated apoptosis in cultures of cerebellar granule cells. For this model, we investigated the role of Bcl-2 and caspases in ne urite degeneration and death of neuronal somata. Results: Early degeneration of the axodendritic network occurred by a Bcl-2 and caspase-independent mechanism. Conversely, apoptosis of the cell body was delayed by Bcl-2 and initially blocked by caspase inhibition. However, when caspase activity was entirely blocked by zVAD-fmk, colchicine-exposed neurons still underwent delayed cell death characterized by cytochrome rele ase, chromatin condensation to irregularly shaped clumps, DNA-fragmentation , and exposure of phatidylserine. Inhibitors of the proteasome reduced thes e caspase-independent apoptotic-like features of the neuronal soma. Conclusion: Our data suggest that Bcl-2-dependent and caspase-mediated deat h programs account only partially for neurodegenerative changes in injured neurons. Blockage of the caspase execution machinery may only temporarily r escue damaged neurons and classical apoptotic features can still appear in caspase-inhibited neurons.