UC Riverside

ABSTRACT OF THE DISSERTATION

Neuropathology Following Repeated Traumatic Brain Injury and the Influence of Inflammatory Status on Injury Outcome

by

Virginia Dorothy-Marie Donovan

Doctor of Philosophy, Graduate Program in Cell, Molecular, and Developmental Biology Program

University of California, Riverside, August 2014

Dr. Andy Obenaus, Co-Chairperson

Dr. Monica J. Carson, Co-Chairperson

Traumatic brain injury (TBI) is an increasing public health concern, accounting for approximately 30% of injury related deaths. Clinical and experimental studies have demonstrated that the brain remains vulnerable following TBI and subsequent injuries may worsen tissue damage. Following injury, microglia, the brain macrophage, have demonstrated both cytotoxic and cytoprotective functions. However, the role of microglial activation in injury progression and resolution is not understood. This dissertation examines the progression of tissue damage following repeated mild TBI (rmTBI) and investigates whether inflammatory status at the time of a moderate-to-severe injury influences outcome.

Using a novel rmTBI model, we found acute 2-fold increases in tissue damage on T2-weighted magnetic resonance imaging (MRI) when injuries were induced 7 (7D), but not 3 days (3D) apart. Furthermore, injuries 7D apart resulted in 2-fold more blood deposition, while those 3D apart showed more edema. Though sub-acute grey matter pathology was transient. We therefore assessed white matter, not directly underneath the impact, using diffusion tensor MRI and electron microscopy at sub-acute (14-day) and long-term (60-day) timepoints. At sub-acute times, we found that rmTBI 7D apart resulted in white matter changes localized to regions immediately adjacent to the impact sites. However, long-term assessment revealed that changes in white matter integrity within rmTBI animals became widespread.

The effect of inflammatory status on TBI induced hemispheric swelling and lesion pathology was tested 7D post injury. We compared MRI pathology with inflammatory molecule expression from naïve, wildtype (TBI; no pre-treatment), wildtype with LPS-challenge 24-hrs before TBI and TREM2KO animals. MRI showed an 80% reduction in hemispheric swelling within TREM2KO and LPS-challenged animals compared to wildtype. Reduced swelling coincided with a 2-fold increase in microglial expression of inflammatory genes compared to naïve animals. However, LPS-challenged and TREM2KO mice showed a reduced inflammatory response from wildtype. Furthermore, comparison of TREM2KO microglia with cortical tissue revealed an opposing number of expressed genes.

Overall, these studies demonstrate that (1) brain vulnerability is dependent on the time interval between and location of injuries, (2) rmTBI results in early white matter disruption that progresses long-term and (3) reduced swelling occurs during limited microglial activation.