Non-apoptotic neurite degeneration in apoptotic neuronal death: pivotal role of mitochondrial function in neurites.

The length and thinness of neurites render them greatly susceptible to a variety of insults. Accumulating evidence suggests that neurite degeneration is not a passive, but an active and causative, event in some neurodegenerative diseases. Nonetheless, the mechanisms underlying neurite degeneration remain largely unknown. To elucidate the relevant mechanisms, we employed a mutant C57BL/Wld mouse with a unique phenotype of resistance to Wallerian degeneration, and separately analyzed the destruction of cell soma and neurites following treatment with vinblastine, a microtubule-disrupting agent, in superior cervical ganglion neurons. Vinblastine induced macromolecular synthesis-dependent cell death, which was indistinguishable between the wild-type and mutant mice. Evidence for a loss of mitochondrial cytochrome c, caspase activation, and nuclear fragmentation, has indicated that this type of cell death is entirely apoptotic. Consistent with this, the ATP level in the cell soma was well maintained and indistinguishable between wild-type and mutant mice. In neurites of wild-type neurons, vinblastine induced an early loss of mitochondrial membrane potential (MMP) and ATP depletion preceding caspase-independent degeneration, suggesting that this type of neurite degeneration is principally non-apoptotic. In contrast, neurites of mutant neurons were markedly resistant to vinblastine-induced degeneration, and both the MMP and the ATP content in the neurites were well maintained. Exposure of mutant neurons to carbonyl cyanide m-chlorophenyl-hydrazone, an uncoupler, caused extreme neurite degeneration following rapid MMP loss. Collectively, our findings suggest that: 1) neurite degeneration is regulated through a non-apoptotic process achieved by mitochondrial dysfunction in neurites; 2) the mitochondrial functional status is controlled separately in neurites and in the neuronal soma.