Rachel E. Bennett - US grants

DESCRIPTION (provided by applicant): There are an estimated 1.6-3.8 million sports-related concussions each year. Human concussions do not usually cause macroscopic lesions visible by CT or MRI, but multiple concussions can lead to axonal injury, long-term cognitive impairments, and neurological disease. These long- term changes are best described in professional athletes, termed dementia pugilistica in boxers and chronic traumatic encephalopathy in other athletes. Studies in these individuals have shown that activation of the brain's resident immune cells, microglia, occurs after traumatic brain injury in areas of axonal injury. Microglial activation in white matter has also been noted after concussion in post- mortem-samples. Whether this response contributes to ongoing axonal injury, protects against further damage, or is neutral in concussion is not known. To fill this knowledge gap, our lab has developed a reproducible model of repetitive concussive injury in mouse similar to Longhi and colleagues (Neurosurgery 2005). In this model, two closed-skull impacts delivered 24 hours apart result in a consistent pattern of axon degeneration and microglial activation without neuronal cell loss. The central hypothesis of this proposal is that persistent microglial activatio following repetitive concussive injury in mouse results in axon degeneration and electrophysiological compromise. To address this hypothesis, the phenotype of microglia within white matter will be compared across time points. A microglial-specific toxin will be administered to determine how elimination of microglia effects axonal degeneration in both the acute and chronic injury phase. If successful, these experiments will greatly increase our knowledge of the role of activated microglia in axonal injury in this mouse model of repetitive concussive trauma. These results may deepen our understanding of the pathological processes that cause cognitive impairments in concussed individuals and could have important implications for therapeutics. For these experiments the applicant will be trained in multiple techniques including small animal surgery, immunohistochemistry, stereology, flow cytometry, qPCR, extracellular brain slice electrophysiology, and statistical analysis of quantitative data. PUBLIC HEALTH RELEVANCE: There are an estimated 1.6-3.8 million sports-related concussions each year and mounting evidence that multiple concussions result in long-term cognitive impairments and increased risk for Alzheimer's disease. After brain injury microglia, the immune cells of the brain, invade injured regions and may produce toxic compounds that result in tissue damage. This proposal is aimed at determining how microglia contribute to concussive injury, which could ultimately lead to therapeutics that reduce lasting impairments in concussed individuals.

Project Summary/Abstract Vascular changes are increasingly common with age and a risk factor for some forms of cognitive impairment including Alzheimer's disease (AD). In particular, alterations to cerebral perfusion have been widely reported, with decreased blood flow observed in cortical and limbic regions associated with the accumulation of amyloid beta plaques and tau-containing neurofibrillary tangles. Though not expressed by cells that make up cerebral vasculature, the neuronal protein tau has been observed inside of the vascular endothelium in post mortem tissue sections highlighting the intriguing possibility that it may directly affect endothelial cells. In mice, overexpression of tau appears to alter vascular density, reduce average vessel diameter particularly within the capillary bed, and increase the incidence of vessels blocked by leukocytes. These findings could explain, in part, altered cerebral perfusion changes in Alzheimer's disease patients, which may be a key contributor to cognitive decline. The overall goal of this project is to determine how these tau-induced vascular alterations observed in mice affect cerebral perfusion and, ultimately, neurodegeneration. Experiments will be carried out in aging tau overexpressing mice carrying the human P301L mutation (Tg4510 line) and advanced in vivo imaging methods. To assess cerebral perfusion, key measurements will be made in awake mice by two-photon microscopy including red blood cell flow, cerebral oxygenation, and of the hemodynamic response using a visual stimulation paradigm. If tau induces early vascular dysfunction, it will be evident by a reduction in hemodynamic response and poor tissue oxygenation, which will lead to subsequent neuron loss. Further, reduced perfusion could be partially explained by the observation of vessels block by leukocytes. In a second set of experiments, a novel cranial window port method will be used to administer labeled tau and protein directly into the parenchyma to determine if tau is sufficient to increase expression of cell adhesion molecules (CAMs) and induce blood vessel blockages by leukocytes. These investigations will also make use a doxycycline repressible promoter to turn off tau expression in mice and determine if changes in endothelial CAM expression and leukocyte blockage is reversible, which is an important consideration for the development of targeted therapeutics. Findings from these studies will further our understanding of tau biology in non- neuronal subtypes as well as the impact of vascular alterations on brain health more generally, which has broad implications beyond Alzheimer's disease. Finally, by utilizing magnetic resonance imaging methods developed for assessing tumor microvasculature in vivo, we have a unique opportunity to validate the use of these techniques in a neurodegenerative disease model such that we can directly translate the transgenic mouse work to human AD research.! !