Marco Colonna Department of Pathology and Immunology, Washington University in St Louis, MO, USA
Alzheimer's disease (AD) is a neurodegenerative disease that causes late-onset dementia. The R47H variant of the microglial receptor TREM2 triples AD risk in genome-wide association studies. In mouse AD models, TREM2-deficient microglia fail to proliferate and cluster around the amyloid-β plaques characteristic of AD. In vitro, the common variant (CV) of TREM2 binds anionic lipids, whereas R47H mutation impairs binding. We addressed two major questions: a) Why TREM2 is essential for microglia proliferation and survival? By combined metabolomics and RNA sequencing (RNA-seq) we linked TREM2-deficiency to defective mammalian target of rapamycin (mTOR) signaling, which affects ATP levels and biosynthetic pathways. Metabolic derailment and autophagy were offset in vitro through Dectin-1, a receptor that elicits TREM2-like intracellular signals, and cyclocreatine, a creatine analog that can supply ATP. Dietary cyclocreatine tempered autophagy, restored microglial clustering around plaques, and decreased plaque-adjacent neuronal dystrophy in TREM2-deficient mice with amyloid-β pathology. Thus, TREM2 enables microglial responses during AD by sustaining cellular energetic and biosynthetic metabolism. b) Which are the TREM2 ligands and which are the effects of the R47H variant in vivo? For this purpose we generated transgenic mice expressing human CV or R47H TREM2 and lacking endogenous TREM2 in the 5XFAD AD model. Only the CV transgene restored amyloid-β-induced microgliosis and microglial activation, indicating that R47H impairs TREM2 function in vivo. Moreover, our results demonstrate that TREM2 interacts with neurons and plaques during amyloid-β accumulation and R47H impairs this interaction.
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