Microglial mTOR Activation Upregulates Trem2 and Enhances b-Amyloid Plaque Clearance in the 5XFAD Alzheimer’s Disease Model

Abstract

The mechanistic target of rapamycin (mTOR) signaling pathway plays a major role in key cellular processes including metabolism and differentiation; however, the role of mTOR in microglia and its importance in Alzheimer’s disease (AD) have remained largely uncharacterized. We report that selective loss of Tsc1, a negative regulator of mTOR, in microglia in mice of both sexes, caused mTOR activation and upregulation of Trem2 with enhanced b-Amyloid (Ab) clearance, reduced spine loss, and improved cognitive function in the 5XFAD AD mouse model. Combined loss of Tsc1 and Trem2 in microglia led to reduced Ab clearance and increased Ab plaque burden revealing that Trem2 functions downstream of mTOR. Tsc1 mutant microglia showed increased phagocytosis with upregulation of CD68 and Lamp1 lysosomal proteins. In vitro studies using Tsc1-deficient microglia revealed enhanced endocytosis of the lysosomal tracker indicator Green DND-26 suggesting increased lysosomal activity. Incubation of Tsc1-deficient microglia with fluorescent-labeled Ab revealed enhanced Ab uptake and clearance, which was blunted by rapamycin, an mTOR inhibitor. In vivo treatment of mice of relevant genotypes in the 5XFAD background with rapamycin, affected microglial activity, decreased Trem2 expression and reduced Ab clearance causing an increase in Ab plaque burden. Prolonged treatment with rapamycin caused even further reduction of mTOR activity, reduction in Trem2 expression, and increase in Ab levels. Together, our findings reveal that mTOR signaling in microglia is critically linked to Trem2 regulation and lysosomal biogenesis, and that the upregulation of Trem2 in microglia through mTOR activation could be exploited toward better therapeutic avenues to Ab-related AD pathologies.