Microglia are the immune competent cells of the central nervous system (CNS). During development, microglia play critical roles in pruning synapses and refining neuronal connectivity. In the adult brain, microglia constantly survey the parenchyma for cellular damage or invading pathogens. Upon detection of such events, microglia become activated and shift to a phagocytic phenotype, secreting pro-inflammatory molecules and adopting an amoeboid morphology. As part of the resolution/repair process, microglia return to a surveillant state and produce anti-inflammatory molecules. Unfortunately, with severe insults, such as traumatic brain injury or chronic neurodegeneration, microglia remain activated and contribute to an inflammatory process that is never, or poorly, resolved. In this way, we hypothesize that microglia contribute deleteriously to functional outcomes.
The goal of my dissertation is to determine the contributions of microglia to neuronal health and cognition in both the healthy and injured brain. The direct assessment of microglia-specific contributions is possible due to the discovery by our lab that microglia are dependent upon signaling through the colony-stimulating factor 1 receptor (CSF1R) for their survival. Treatment with a small-molecule CSF1R inhibitor eliminates >99% of microglia from the adult mouse brain. Critically, microglia fully repopulate the CNS upon withdrawal of the CSF1R inhibitor, effectively renewing this cellular compartment.
Using a genetic model of inducible neuronal loss, I have determined that the elimination of microglia during a lesion is detrimental to cellular health, while the elimination of microglia following a lesion results in the reversal of many lesion-induced deficits. Importantly, this research suggests that the microglia-mediated immune response is beneficial during insult or injury, but deleterious after such an event. Moreover, repopulation of the brain with new microglia following neuronal lesioning largely resets the inflammatory milieu and confers functional benefits.
Finally, long-term elimination of microglia was employed in order to determine if these cells shape the synaptic landscape in the healthy adult brain, as they do during development. Indeed, I found that microglial elimination in healthy adult mice results in brain-wide and robust increases in dendritic spine numbers and excitatory neuronal connectivity, indicating that microglia modulate synaptic function throughout the course of the lifetime.