Investigating the Role of Microglia and Extracellular Vesicles in Spinal Cord Injury-Induced Brain Dysfunction

Abstract

Spinal cord injury (SCI) causes brain neurodegeneration leading to cognitive and affective changes, including memory loss and mood alterations. Using SCI models, we and others have demonstrated that progressive neurodegeneration is accompanied by neuroinflammation, including sustained microglial activation. The primary goal of this dissertation was to test the hypothesis that SCI triggers brain microglia-mediated neuroinflammation and secondary neurological dysfunction and to study the underpinning mechanisms including changes in systemic and central extracellular vesicles (EVs). First, we probed the mechanisms responsible for microglia activation and examined the effect of pharmacological depletion of microglia on posttraumatic neuropathology and cognitive/depressive-like behavior in a mouse SCI model. Microglial depletion significantly improved neuronal cell loss in key brain regions and associated cognitive/depressive-like behavioral outcomes after SCI. The transcriptomes of the spinal cord and brain were also substantially altered, supporting our hypothesis that microglia significantly contribute to changes related to inflammation, neurotransmission, and apoptosis after SCI. Second, we studied changes in circulating EVs after SCI. EVs are biological nanoparticles released from cells that contribute to intercellular communication and can become altered with disease. We found a significant increase in plasma tetraspanin CD81+ EVs after SCI at 1d post-injury. Surface CD81 was decreased on astrocytes at the injury site, suggesting that these cells may release CD81+ EVs into circulation. Total plasma EV microRNA content was also significantly modified, similar to the profile previously described in inflammatory astrocyte EVs. Notably, when injected into the cerebroventricular system, plasma EVs from SCI mice increased brain expression of several inflammatory genes, including markers of astrocyte reactivity. Finally, we examined the brain transcriptional profile and EV changes 19 months post-SCI in male and female mice. While we observed strong sex-dependent differences in the overall brain transcriptome after SCI, the homeostatic microglial phenotype was reduced in both sexes. Chronic SCI increased EV count in the brain and modified their microRNA content, which may explain the observed transcriptional changes. Plasma EV markers were also elevated late after injury, especially in males. Collectively, these experiments are the first to characterize EV dynamics after SCI and suggest that EVs may be involved in posttraumatic brain inflammation.