Microglia play both protective and pathogenic roles in neurodegenerative disease but mechanisms that specify
or restrict these opposing activities remain poorly understood. Molecular, cellular, and genetic studies implicate
defects in phagocytosis, lysosomal function and lipid metabolism as contributing to pathological phenotypes. In
line with these findings, the accumulation of lipid droplets is a hallmark of microglia in aging and
neurodegenerative disease states. Due to the essential roles of LXRs and SREBPs in the regulation of cellular
cholesterol and fatty acid metabolism, we hypothesize that the formation of lipid droplets is a manifestation of
general defects in the normal homeostatic functions of LXRs and SREBPs that are broadly linked to pathological
microglia phenotypes. Although conventional LXR ligands have been shown to exert protective effects in mouse
models of neurodegeneration, their mechanisms of action have not been established and they cannot be used
clinically because they induce hypertriglyceridemia. Here, we will build on our prior work demonstrating that the
endogenous LXR agonist desmosterol and synthetic desmosterol mimetics mimic normal physiology by
coordinately regulating LXR and SREBP and do not induce hypertriglyceridemia. Three Specific Aims are
proposed. In Specific Aim 1, we will test the hypothesis that age and inflammation-dependent formation of lipid
droplets is regulated by the LXR and SREBP pathways by selectively knocking out LXRs or SREBPs in microglia
and pharmacologically by treating animals with either the conventional LXR agonist GW3965 that selectively
activates LXRs or the desmosterol mimetics that simultaneously activate LXRs and suppress SREBPs. In
Specific Aim 2, we will test the hypothesis that coordinate regulation of LXRs and SREBPs in the brain by
desmosterol mimetics reduce the number and/or activity of pathogenic microglia and confer similar or greater
protection against neurodegeneration in the Tau/APOE4 model than the conventional LXR agonist GW3965
without causing hypertriglyceridemia. In Specific Aim 3 we will test the hypothesis that LXR and SREBP have
conserved roles in regulating lipid metabolism in human iPSC-derived microglia in vitro and following engraftment
into the brains of immune-deficient humanized mice. Effects of GW3965 and desmosterol mimetics will be
assessed in vitro and in engrafted hosts under homeostatic conditions during aging and in response to LPS
activation. In each Aim, we will investigate the molecular functions of LXRs and SREBPs by determining the
effects of GW3965 and desmosterol mimetics on microglia transcriptomes and epigenetic landscapes and the
genome wide binding patterns of LXRs and SREBPs. Collectively, these studies will advance understanding of
the linkage of lipid droplets with neurodegeneration, provide insights into mechanisms underlying protective
effects of LXR agonists, and potentially open a path towards clinical development of safe and effective LXR
agonists for prevention and treatment of neurodegenerative disease.