Pathological and cognitive alterations in mouse models of traumatic brain injury and hypoperfusion
Spain, Aisling Mary
Intact white matter is critical for normal cognitive function. In traumatic brain injury (TBI), chronic cerebral hypoperfusion and Alzheimer’s disease (AD) damage to white matter is associated with cognitive impairment. However, these conditions are associated with grey matter damage or with other pathological states and the contribution of white matter damage in isolation to their pathogenesis is not known. Furthermore, TBI is a risk factor for AD and cerebral hypoperfusion is an early feature of AD. It is hypothesised that white matter damage following TBI or chronic cerebral hypoperfusion will be associated with cognitive deficits and that white matter changes after injury contribute to AD pathogenesis. To investigate this, this thesis examined the contribution of white matter damage to cognitive deficits after TBI and chronic cerebral hypoperfusion and furthermore, investigated the role of white matter damage in the relationship between TBI and AD. Three studies addressed these aims. In the first, mild TBI was induced in wild-type mice and the effects on axons, myelin and neuronal cell bodies examined at time points from 4 hours to 6 weeks after injury. Spatial reference learning and memory was tested at 3 and 6 weeks after injury. Injured mice showed axonal damage in the cingulum, close to the injury site in the hours after injury and at 6 weeks, damage in the thalamus and external capsule were apparent. Injured and sham animals had comparable levels of neuronal damage and no change was observed in myelin. Injured animals showed impaired spatial reference learning at 3 weeks after injury, demonstrating that selective axonal damage is sufficient to impair cognition. In the second study mild TBI was induced in a transgenic mouse model of AD and the effects on white matter pathology and AD-related proteins examined 24 hours after injury. There was a significant increase in axonal damage in the cingulum and external capsule and parallel accumulations of amyloid were observed in these regions. There were no changes in tau or in overall levels of AD-related proteins. This suggests that axonal damage may have a role in mediating the link between TBI and AD. The third study used a model of chronic cerebral hypoperfusion in wild type mice and investigated white matter changes after one and two months of hypoperfusion as well as a comprehensive assessment of learning and memory. Chronic cerebral hypoperfusion resulted in diffuse myelin damage in the absence of ischaemic neuronal damage at both 1 and 2 months after induction of hypoperfusion. Hypoperfused animals also showed minimal axonal damage and microglial activation. Cognitive testing revealed a selective impairment in spatial working memory but not spatial reference or episodic memory in hypoperfused animals, showing that modest reductions in blood flow have effects on white matter sufficient to cause cognitive impairment. These results demonstrate that selective damage to white matter components can have a long-term impact on cognitive function as well as on the development of AD. This suggests that minimisation of axonal damage after TBI is a target for reducing subsequent risk of AD and that repair or prevention of white matter damage is a promising strategy for rescuing cognitive function in individuals who have experienced mild TBI or chronic cerebral hypoperfusion.
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