Trem2 Splicing and Expression are Preserved in a Human Aß-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 Trem2R47H KI rat was created. Human A ss has higher propensity to form toxic A ss species, which are considered the main pathogenic entity in AD, as compared to rodent A ss, the rat Amyloid Precursor Protein (App) gene was mutated to produce human A ss. Trem2 splicing and expression was measured in Trem2R47H 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 A ss and soluble APP, and Western analysis, for full length APP, ss CTF and aCTF. Trem2 expression and Trem2 levels are unchanged in Trem2R47H KI rats. The artifactual splicing seen in KI mouse models is not present; additionally, two novel isoforms of rat Trem2 are described. Trem2R47H rat brains have lower human A ss 38, sAPPa and sAPP ss levels. Thus, Trem2R47H KI rats may prove valuable to define pathogenic mechanisms triggered by the Trem2 R47H variant, including those mediated by toxic species of human A ss peptides.