Project Summary The goal of our study is to better understand the pathogenic role of human microglia in Alzheimer’s disease (AD) in Down syndrome (DS) and develop new therapeutic avenues for the treatment of AD in DS as well as AD in general population. Our studies are in line with the goals of RFA-OD-20-005 because we will focus on evaluating the genetic factors associated with trisomy 21 and their impacts on neurodegeneration using human tissue and a novel human-mouse chimeric brain model and we will also use gene editing to remove triplicated genes. The foundation of our studies is that recent genome-wide association studies have shown that many AD risk genes are highly and sometimes exclusively expressed by the brain-resident macrophage, microglia. Recent transcriptomic studies have also clearly demonstrated that human vs. mouse microglia exhibit distinct gene expression profiles, and more importantly, they age differently under both normal and diseased conditions. These findings argue for the utilization of species-specific research tools to investigate microglial functions in human brain aging and degeneration. We propose to use a novel human induced pluripotent stem cell (hiPSC)-based microglial chimeric mouse model that can recapitulate features of adult and aging human microglia to investigate the role of human microglia in AD in DS. While the aggregation of amyloid-beta (Ab) precedes that of tau, tau protein pathology commences in humans much sooner than was previously thought. Contrary to the marked microglial activation reported in amyloidogenic transgenic mouse models, in human brain tissue derived from AD and DS patients, brain regions particularly relevant in AD development, such as the hippocampal formation, exhibit low and late Ab pathology, whereas hyperphosphorylated tau (p-tau) accumulates starting in the early stages of the disease. The preferential accumulation of p-tau over Ab plaques could induce a totally different microglial response. Here we put forward a new tau/microglial senescence hypothesis that human microglial senescence and functional changes, induced by soluble p-tau, likely occur prior to neurodegeneration and is causatively linked to the AD progression and cognitive decline in DS. We have created control and DS microglial mouse chimeras by engrafting control and DS hiPSC-derived microglia into mouse brains. We will characterize the dynamic responses of DS and control hiPSC-derived microglia to pathological soluble p-tau in human microglial chimeric mouse brains, by using newly invented robotic four-dimensional long-term imaging technology. We will determine the changes in synaptic functions by electrophysiological recordings and behavioral performance of DS microglial chimeras after exposure to pathological soluble p-tau, as compared to control microglial chimeras. Moreover, single-cell RNA-sequencing analysis of chimeric mouse brains and CRISPR/Cas9-mediated removal of triplicated genes will be performed to ...