Trem2 Splicing and Expression are Preserved in a Human Aβ-producing, Rat Knock-in Model of Trem2-R47H Alzheimer's Risk Variant.

The R47H variant of the Triggering-Receptor-Expressed on Myeloid cells 2 (TREM2) increases the risk of Alzheimer's disease (AD). Mutagenesis of exon 2 in Knock-in (KI) mouse models of the R47H variant introduced a cryptic splice site, leading to nonsense mediated decay. Since haploinsufficiency does not model Trem2-R47H function, a new rat KI model, the Trem2 KI rat was created. Human Aβ has higher propensity to form toxic Aβ species, which are considered the main pathogenic entity in AD, as compared to rodent Aβ, the rat Amyloid Precursor Protein (App) gene was mutated to produce human Aβ. Trem2 splicing and expression was measured in Trem2 KI rat brains and microglia by qualitative and quantitative RT-PCR. Trem2 levels and Trem2 processing was assessed by Western analysis. APP metabolite levels were determined by enzyme-linked immunosorbent assay (ELISA), for Human Aβ and soluble APP, and Western analysis, for full length APP, βCTF and αCTF. Trem2 expression and Trem2 levels are unchanged in Trem2 KI rats. The artifactual splicing seen in KI mouse models is not present; additionally, two novel isoforms of rat Trem2 are described. Trem2 rat brains have lower human Aβ38, sAPPα and sAPPβ levels. Thus, Trem2 KI rats may prove valuable to define pathogenic mechanisms triggered by the Trem2 R47H variant, including those mediated by toxic species of human Aβ peptides.