Microglia, the immune cells of the central nervous system, are long lived. In mice, microglia have an average lifespan of 15.5 months. When microglia are experimentally depleted from the mouse brain, microglia populations quickly return to steady state levels. The mechanisms of this observed repopulation are unclear. More importantly, the mechanisms of microglia replenishment in the healthy brain are not well understood. The literature supports two competing hypotheses. One is that microglia proliferate simply by dividing. Another possibility is that pools of microglia progenitor cells within the central nervous system divide and differentiate into microglia. I hypothesize that microglia proliferate primarily through the differentiation of progenitor cells. Available data to date suggest CD133 as a potential marker for microglia progenitor cells. In order to study these putative progenitor populations, I used a genetic reporter mouse line in which administration of tamoxifen induces TdTomato expression specifically in CD133-expressing cells. TdTomato, a red fluorescent protein, allows these cells to be visualized under a fluorescence microscope. Importantly, all the progeny of these cells also express TdTomato, allowing us to determine whether CD133 cells generate new microglia over time. After tamoxifen treatment at the age of 10 weeks, mice were sacrificed at three and nine months of age. Brains were fixed, sectioned, and labeled with antibodies to a microglia specific protein and to TdTomato. Daughter microglia that differentiated from CD133-expressing cells express both markers. Using a fluorescence microscope, I identified several microglia daughter cells of CD133-expressing cells. This suggests that microglia populations replenish in the healthy brain at least in part through the division of CD133-expressing cells. We can apply this new knowledge about how new microglia are generated in the healthy mouse brain to further our understanding of how microglia population dynamics are affected in both health and disease.

Microglia are innate immune cells in the CNS that exhibit a sustained pro-inflammatory response in the Alzheimer's disease (AD) brain. Sustained pro-inflammatory responses by microglia can promote excessive synaptic pruning and neuronal death, exacerbating neurodegeneration. MicroRNAs can regulate microglia inflammatory behaviors by modifying gene expression at the post-transcriptional level by suppressing expression of target genes. MiR-155 is a microRNA that targets suppressors of inflammation and is dysregulated in neurodegenerative disorders. Additionally, miR-155 deletion has been reported to be neuroprotective in several models of neural injury and degeneration. The impact of microglia specific miR-155 regulation on the neuroinflammatory response or behavioral outcomes of AD models has yet to be elucidated. We hypothesize that miR-155 deletion in microglia decreases neuroinflammatory response to AD, thus improving memory impairments typically observed in AD. We use a mouse model expressing a transgene of associated mutant forms of human amyloid precursor protein and presenilin 1 (APP/PS1). We crossed APP/PS1 mice with a tamoxifen-inducible Cre model or a constitutive Cre model to conditionally or constitutively delete miR-155 in microglia. We use open field chambers and T-maze to assess general behavior and spatial memory at 6, 9, and 12 months. When miR-155 was deleted specifically in microglia, no difference was seen in the spatial memory as measured through T-maze tests, compared to APP/PS1 mice. However, increased locomotor activity was seen in open field tests at 6 and 9 months. Similarly, when miR-155 was deleted in microglia and peripheral myeloid cells, there were no significant differences in spatial memory, though increases in locomotor activity at 6 and 9 months and potential decreases in anxiety at 6 months were also seen in open field. These results suggest that miR-155 may play a more complex role in the regulatory response of neuroinflammation during AD.