The roles of microglia in response to pathological stimuli in the brain

As the principal immune cell of the brain, microglia cells are responsible for monitoring the activity of other cells in the CNS, and are able to respond to harmful stimuli with a myriad of supportive and defensive mechanisms. With these capacities, microglia are involved in numerous diseases of the CNS, ranging from acute damage and infection to chronic neurodegeneration. In chapter 2 we examine the motile functions of microglia, with particular focus on the response of microglia to damage. Microglia cells exhibit two forms of motility, constant movement of filopodia probing surrounding brain tissue, and outgrowth of larger processes in response to nearby damage. The mechanisms and functions of these motile processes are not well characterized. Using two photon microscopy we investigated microglia motility, and explored the relationship between process outgrowth and filopodia movement. We found that fiolopodia sensing and rapid process outgrowth activities of microglia are mediated by distinct mechanisms, but both require actin polymerization. We also showed that rapid outgrowth of microglia processes contacted the damaged area and resulted in a decrease in lesion volume, whereas inhibition of process outgrowth allowed lesion volume to increase and spread into the surrounding tissue. In chapter 3 we examine the response of microglia to immune stimuli and use specific interfering peptides to modulate this response. We used peptides blocking LPS-induced activation of the innate immune Toll-Like Receptor 4 (TLR4) to prevent downstream signaling in the brain, and thereby suppress sickness behavior. Interfering peptides blocked TLR4 signalling and prevented second messenger activation and cytokine production normally induced by LPS treatment. These peptides also blocked morphological changes in microglia induced by LPS. Further, injections of interfering peptides prevented LPS-induced sickness behavior, as assessed in novel homecage behavior and with the intracranial self-stimulation paradigm. Taken together our studies demonstrate distinct responses of microglia to different types of pathological insults: acute damage, and immune stimuli. Further, these studies provide means of regulating the dynamic responses of microglia to different stimuli. Ultimately, this research reveals pathways by which microglia can be manipulated, and potentially provides therapeutic targets to enhance the recovery of the brain from acute injury or infection.Medicine, Faculty ofGraduat