Cytochrome P450 46A1 (CYP46A1 or cholesterol 24-hydroxylase) controls cholesterol elimination from the brain and plays a role in higher order brain functions. Genetically enhanced CYP46A1 expression in mouse models of Alzheimer's disease mitigates the manifestations of this disease. We enhanced CYP46A1 activity pharmacologically by treating 5XFAD mice, a model of rapid amyloidogenesis, with a low dose of the anti-HIV medication efavirenz. Efavirenz was administered from 1 to 9 months of age, and mice were evaluated at specific time points. At one month of age, cholesterol homeostasis was already disturbed in the brain of 5XFAD mice. Nevertheless, efavirenz activated CYP46A1 and mouse cerebral cholesterol turnover during the first four months of administration. This treatment time also reduced amyloid burden and microglia activation in the cortex and subiculum of 5XFAD mice as well as protein levels of amyloid precursor protein and the expression of several genes involved in inflammatory response. However, mouse short-term memory and long-term spatial memory were impaired, whereas learning in the context-dependent fear test was improved. Additional four months of drug administration (a total of eight months of treatment) improved long-term spatial memory in the treated as compared to the untreated mice, further decreased amyloid-β content in 5XFAD brain, and also decreased the mortality rate among male mice. We propose a mechanistic model unifying the observed efavirenz effects. We suggest that CYP46A1 activation by efavirenz could be a new anti-Alzheimer's disease treatment and a tool to study and identify normal and pathological brain processes affected by cholesterol maintenance.

The P522R variant of PLCG2, expressed by microglia, is associated with reduced risk of Alzheimer's disease (AD). Yet, the impact of this protective mutation on microglial responses to AD pathology remains unknown. Chimeric AD and wild-type mice were generated by transplanting PLCG2-P522R or isogenic wild-type human induced pluripotent stem cell microglia. At 7 months of age, single-cell and bulk RNA sequencing, and histological analyses were performed. The PLCG2-P522R variant induced a significant increase in microglial human leukocyte antigen (HLA) expression and the induction of antigen presentation, chemokine signaling, and T cell proliferation pathways. Examination of immune-intact AD mice further demonstrated that the PLCG2-P522R variant promotes the recruitment of CD8⁺ T cells to the brain. These data provide the first evidence that the PLCG2-P522R variant increases the capacity of microglia to recruit T cells and present antigens, promoting a microglial transcriptional state that has recently been shown to be reduced in AD patient brains.

Increasing evidence suggests that Alzheimer's disease pathogenesis is not restricted to the neuronal compartment, but includes strong interactions with immunological mechanisms in the brain. Misfolded and aggregated proteins bind to pattern recognition receptors on microglia and astroglia, and trigger an innate immune response characterised by release of inflammatory mediators, which contribute to disease progression and severity. Genome-wide analysis suggests that several genes that increase the risk for sporadic Alzheimer's disease encode factors that regulate glial clearance of misfolded proteins and the inflammatory reaction. External factors, including systemic inflammation and obesity, are likely to interfere with immunological processes of the brain and further promote disease progression. Modulation of risk factors and targeting of these immune mechanisms could lead to future therapeutic or preventive strategies for Alzheimer's disease.