Expression and processing analyses of wild type and p.R47H TREM2 variant in Alzheimer's disease brains

Background: Genetic analyses showed that the triggering receptor expressed in myeloid cells 2 (TREM2) p.R47H variant increases the risk for Alzheimer's disease (AD). The question of whether the p.R47H mutation affects expression or function of the receptor remains unanswered. To address this question we quantified mRNA and analyzed protein profiles of WT and p.R47H TREM2 in human brains. Methods: Quantitative real-time PCR (qPCR) was performed using 2 sets of primers one that detects all TREM2 mRNA isoforms and one specific for the alternative spliced isoform (TREM2alt) that encodes for the extracellular domain (soluble TREM2). Because in the brain TREM2 is expressed primarily in microglial cells, we also assessed the levels of IBA1 to control for microglial variability across samples. For TREM2 protein quantitation and N-glycosylation processing, RIPA brain extracts were analyzed by Western blot before and after EndoH and PNGaseF treatments. Results: We identified statistically significant increased levels of TREM2 transcripts in the temporal cortex of AD subjects when compared with controls; TREM2alt was likewise higher in AD cases, but was not significant after adjustment for covariates. Quantitative analysis of TREM2 protein confirmed qPCR results that showed higher levels in AD than in control brains. Among AD subjects, we observed a trend towards higher mRNA and protein TREM2 levels in carriers of the p. R47H risk allele. Analysis of individual TREM2 species found no difference in the relative amounts of mature and immature species, and carboxyl terminal fragments between non carriers and p. R47H samples. Furthermore, TREM2 species from either non carriers or p. R47H brains were equally susceptible to EndoH and PNGaseF treatments. Conclusions: Our results suggest that TREM2 expression is increased in AD. Furthermore, we provide evidence indicating that p. R47H mutation does not affect the levels of TREM2 either directly by altering expression or indirectly by affecting processing of the protein. Our data support previous findings that suggest that p. R47H variant affects TREM2 function by altering binding properties of the receptor rather than expression.