Abstract: Our research focuses on microglia, a type of brain cell crucial to understanding and treating several central nervous system diseases. In our first funding cycle (2014-2019), we developed a technique to remove and replace microglia using a drug that targets a specific receptor (CSF1R), which has since become a standard method in neuroscience. This approach has significantly advanced our understanding of microglia and has shown therapeutic potential in various disease and injury models. In our second funding cycle (2019-2024), we further explored microglia replacement and discovered several key findings, including a new method of reprogramming microglia and a novel route for bone marrow-derived cells to enter the brain allowing us to fully replace the microglial tissue with bone-marrow derived monocytes/myeloid cells. Building on these findings, we propose three specific aims for our next research phase. First, we will investigate a new method of microglia recovery that avoids the need for complete microglia removal. This involves temporarily inhibiting CSF1R, leading to the reprogramming and proliferation of microglia and other brain cells. Second, we will explore the potential of replacing dysfunctional microglia with functional bone marrow-derived cells as a treatment for genetic disorders associated with microglia dysfunction. Lastly, we will study the role of peripheral myeloid cells entering the brain in the development of Alzheimer's disease. Our research aims to deepen our understanding of microglia biology and explore its therapeutic potential, moving towards more clinically relevant applications.