Unravelling the molecular and cellular mechanisms contributing to secondary axonal degeneration following traumatic brain injury

Traumatic brain injury (TBI) is currently a leading cause of death and disability in individuals under the age of 45 years in developed nations. Accordingly, the incidence of , TBI is high in people during the most productive years of their life and often results in prolonged or life-long impairments in cognition and personality. In this regard, diffuse axonal injury (DAI) is proposed to be a major cause of on-going disability following mild to severe forms of brain injury. As DAI represents an evolving form of axonopathy, taking hours to days to develop, there is a substantial 'therapeutic window' for possible interventions. This thesis introduces a novel in vitro method for causing mild-moderate transient axonal shear strain that leads. over 2-3 days, to secondary DAI-like axotomy. This injury method provides important mechanistic insights into the underlying cause of DAI, presenting also an ideal platform for investigating new therapeutic approaches. Altered membrane permeability, particularly in regard to disrupted calcium homeostasis, and the gradual degradation of the cytoskeleton resulting in axon swellings, are important pathological features of DAI. It has long been proposed that altered membrane permeability, due to mechanical strain, is an initiator for secondary axotomy. This thesis demonstrates, using cell-impermeant tracers, that altered membrane permeability is not an immediate consequence of injury, rather it is associated with the delayed degradation of the axon cytoskeleton. Conversely, this thesis demonstrates, using calcium imaging techniques, immediate increases in intracellular calcium, principally due to release of calcium from intracellular stores. Immunocytochemical analysis further demonstrated the calcium-modulated release of pro-apoptotic proteins, which are also associated with the degradation of the cytoskeleton, at later time-points. The pharmacological inhibition of the calcium-activated phosphatase, calcineurin, attenuated cytoskeletal damage and delayed secondary axotomy. These results suggest the release of calcium from intracellular stores, at the time of injury, initiates the post-injury sequelae, which results in cytoskeletal degradation and secondary axotomy due to calcium activated proteases and phosphatases. Altered ubiquitin proteasome system (UPS) activity is proposed to play an important role in the secondary degeneration of neurons following injury and in many neurodegenerative disorders. Accordingly, the pharmacological inhibition of the UPS is proposed to attenuate axonal degeneration. This thesis illustrates that UPS activity plays an important role in delaying secondary axotomy, indicated by an accelerated progression to secondary axotomy following inhibition of the UPS. Furthermore, mild to moderate mechanical injury induces neurite sprouting. These highly dynamic neurites contained growth associated proteins, however, lacked growth guidance structures, which may contribute to undirected and abnormal synaptic connections that underlie epileptiform activity following human cases of DA.