TREM2 expression level is critical for microglial state, metabolic capacity and efficacy of TREM2 agonism.

Triggering receptor expressed on myeloid cells 2 (TREM2) is a central regulator of microglial activity and sequence variants are major risk factors for late onset Alzheimer's disease (LOAD). To better understand the molecular and functional changes associated with TREM2 signalling, we generated a TREM2 reporter mouse model and observed a gradual upregulation of reporter expression with increasing plaque proximity. Isolated microglia were sorted based on reporter expression and their transcriptomic profiles acquired in both wildtype and APP transgenic animals, allowing us to disentangle TREM2 versus pathology-specific effects. Bulk RNA36 sequencing highlighted TREM2 level-dependent changes in major immunometabolic pathways, with enrichment of genes in oxidative phosphorylation and cholesterol metabolism in microglia with increased TREM2 expression. To confirm these findings, we next analysed uptake of fluorodeoxyglucose (FDG) and examined metabolomic and lipidomic profiles. Again, independent of Aß pathology, TREM2 expression correlated with uptake of FDG as well as increased cellular redox, energetics, and cholesterol homeostasis. Finally, we performed chronic treatment with a brain penetrant TREM2 agonist and identified a window of TREM2 expression where microglia are most responsive. Thus, our data provide novel insights into TREM2-mediated regulation of microglial metabolic function and informs current efforts to bring TREM2 agonists into clinical application. Overall design: A C57Bl/6J based mouse model with mKate2 fluorescence reporter for the expression of TREM2 was created. The experimental mice expressed the reporter heterozygous while AppSAA and the hTfR knock in were homozygous. 8-month-old Trem2-mKate2KI/wt APPSAA/SAA.hTfRKI/KI mice were injected with 1 mg/kg Trem2 agonist (4D9) coupeled with an antibody transport vehicle (ATV:4D9), monthly until 11 months. At 12 months, mice were injected with 30 mg/kg FDG, 641 perfused 1 hour later and the extracted microglia were then sorted into low, mid and high mKate2 expressing populations with FACS. The cells were stored in Trizol until RNA-Seq. The samples include longtime treated ATV:4D9 microglia and control treated miicroglia sorted into mKate2(Trem2) low, mid and high expressing cells.