Mitochondrial Transport and Function in Axon Degeneration

Axon degeneration plays a critical but ill-defined role in Parkinson disease (PD), and is actively regulated by pathways that are not well understood. Mitochondria orchestrate many of the processes that underlie axonal homeostasis, and mitochondrial dysfunction is implicated in PD pathogenesis. A better understanding of axonal mitochondria may therefore identify the cell biological processes that mediate degeneration and protection of the axon. Recent studies have suggested that mitochondrial transport is critical to axon protection after injury. I studied mitochondrial morphology, transport, and function in living zebrafish embryos. Using time-lapse confocal imaging of peripheral sensory neurons, I investigated the effect of injury on mitochondrial transport. I used two-photon laser axotomy to induce Wallerian degeneration (WD) of the distal axon. I found that acute transport arrest occurs in the distal axon, and motility does not recover before fragmentation. Although transport arrest was preserved in the proximal axon, which does not degenerate, increased mitochondrial trafficking after injury did not always correlate with axon protection. To determine whether mitochondrial ROS production is relevant to WD, I expressed the redox-sensitive biosensor roGFP2 in the mitochondrial matrix. After injury, roGFP2 was rapidly and persistently oxidized in the distal, but not the proximal, axon. The axon-protective protein WldS, which had only a mild and temporary effect on transport arrest after axotomy, robustly inhibited roGFP2 oxidation and degeneration. To further investigate the importance of mitochondrial ROS production after injury, I expressed the transcriptional co-activator PGC-1alpha, which has roles in mitochondrial biogenesis and ROS detoxification. I saw that overexpression of this protein delays roGFP2 oxidation after injury, and delays WD. Mitochondrial ROS production is therefore a better predictor of axonal vulnerability than mitochondrial transport, and ROS detoxification may be a relevant therapeutic target to prevent axon degeneration. I then studied degeneration induced by alpha-synuclein (aSyn), a protein that is associated with PD. I expressed human wild-type aSyn in peripheral sensory neurons and saw that axon pathology precedes cell death in these cells. Early changes in mitochondrial morphology were consistent with increased fragmentation, and axonal varicosities were filled with swollen mitochondria. I also saw reduced mitochondrial trafficking in aSyn-expressing cells. Motile mitochondria favored retrograde transport towards the cell body. I hypothesized that protection of mitochondria might prevent degeneration in this model, as it had after axotomy. I therefore expressed PGC-1alpha in cells expressing aSyn, and saw robust protection against axon degeneration and cell death. These results suggest that axon degeneration pathways converge on mitochondrial dysfunction. Protection of mitochondria may therefore be a promising therapeutic target in the prevention and treatment of neurodegenerative disease