Mitochondrial response to axonal injury

Abstract

The failure of axonal regeneration is due to myriad reasons both cell intrinsic and extrinsic. In this thesis, I sought to investigate an intrinsic reason for regeneration failure in the CNS. Specifically, I investigated the role of axonal mitochondria in the axonal response to injury. A viral vector (AAV) containing a mitochondrially targeted fluorescent protein (mitoDsRed) as well as fluorescently tagged LC3 (GFP-LC3), an autophagosomal marker, was injected into primary motor cortex, to label the corticospinal tract (CST), of adult rats. The axons of the CST were then injured by dorsal column lesion at C4-C5. We found that mitochondria in injured CST axons near the injury site are fragmented and fragmentation of mitochondria persists for two weeks before returning to pre-injury lengths. Fragmented mitochondria have consistently been shown to be dysfunctional and detrimental to cellular health. Interestingly, transection of axons within the sciatic nerve resulted in mitochondrial fission but did not result in the fragmentation of mitochondria. Inhibition of Drp1, the GTPase responsible for mitochondrial fission, using a specific pharmacological inhibitor (mDivi-1) blocked fragmentation. Additionally, it was determined that there is increased mitophagy in CST axons following spinal cord injury based on increased colocalization of mitochondria and LC3. In vitro models revealed that mitochondrial calcium uptake is necessary for injury induced mitochondrial fission, as inhibiting mitochondrial calcium uptake using RU360, a mitochondrial calcium uniporter inhibitor, prevented injury induced fission. This phenomenon was also observed in vivo. These studies indicate that following injury, both in vivo and in vitro, axonal mitochondria undergo increased fission, which may result in an ATP deficit that contributes to the lack of regeneration seen in CNS neurons.