Role of Calpains in Axonal Injury and Degeneration

Axonal injury and degeneration, whether primary or secondary, contribute to the morbidity and mortality seen in many acquired and inherited central nervous system (CNS) and peripheral nervous system (PNS) disorders, such as traumatic brain injury (TBI), spinal cord injury, cerebral ischemia, neurodegenerative diseases, and demyelinating and hereditary neuropathies. The calpain family of proteases has been causally linked to many of these acute and chronic disorders. In particular, previous studies have suggested that pathologic calpain activity may be responsible for axonal dysfunction and degeneration. Even though the immediate cause of increased intra-axonal calpain activity varies, the downstream effects of calpains may be similar. To address the role of calpains in axonal injury, I have relied on two animal models: nerve transection and optic nerve stretch. First, I demonstrated that overexpression of the endogenous calpain inhibitor, calpastatin, in transgenic mice provided biochemical and morphological protection to transected PNS axons, and biochemical protection to CNS axons. I then studied the role of calpains in synaptic dysfunction in transected PNS axons. There was no detectable protection of synaptic function with calpastatin overexpression. Next, I developed and characterized a new in vivo model of optic nerve stretch that isolates mechanical strain primarily to axons of retinal ganglion cells in the rat. This model of optic nerve stretch recapitulates the signature pathology seen in axons after human TBI, which is transport disruption. Within my study of optic nerve stretch, I used both the pharmacologic calpain inhibitor MDL-28170 and adeno-associated viral vector-mediated overexpression of calpastatin to examine the role of calpains in impairment of retrograde transport after mechanical stretch of axons. Calpastatin overexpression, but not short-duration MDL-28170, protected retrograde axonal transport. Taken together, my dissertation work extends our understanding of the pathologic role of calpains in axonal injury and degeneration