Trem2R47H NSS; 5xFAD mice display age/disease progression‐dependent changes in plaques and plaque‐associated microglia, and increased plasma neurofilament light chain

Abstract

AbstractBackgroundGenome‐Wide Association Studies revealed Triggering receptor expressed on myeloid cells 2 (TREM2) R47H mutation as one of the strongest genetic risk factors for late‐onset Alzheimer’s Disease (AD). In the brain, TREM2 is a transmembrane receptor expressed exclusively by myeloid cells. Many current TREM2*R47H mouse models have observed evidence of cryptic splicing products of the mutant allele, resulting in artefactually reduced protein product. Model Organism Development & Evaluation for Late‐Onset Alzheimer’s Disease (MODEL‐AD) consortium at University of California ‐ Irvine has developed the Trem2 R47H NSS (Normal Splice Site) mouse model where the Trem2 allele is expressed at a level similar to the wild‐type Trem2 allele, with no evidence of cryptic splicing products from the mutant allele.MethodWe generated four groups – WT (C57BL6/J), Trem2 R47H homozygous (HO), 5xFAD, and 5xFAD/ Trem2 R47H HO. Coronal brain slices of 4 and 12‐month‐old mice (n=5/sex/genotype/age) were immunolabeled with Thioflavin‐S for dense‐core plaques, ionized calcium binding adaptor molecule 1 (IBA1) for microglia and lysosome‐associated membrane glycoprotein 1 (LAMP1) for neuritic dystrophy. We also measured plasma neurofilament light chain (NfL) as a surrogate measure of brain damage via Meso Scale Discovery (MSD) technology.ResultSex‐related differences in dense‐core Aß plaque burden and size are found in 4‐month 5xFAD/ Trem2 R47H, where female 5xFAD/ Trem2 R47H mice exhibit increased plaque load compared to 5xFAD mice. In males, stark reductions in plaque load are observed. This sex‐difference is not observed at 12‐month. Examination of IBA1+ microglia at 4‐month reveals significant reductions in microglia volume in both Trem2 R47H and 5xFAD/ Trem2 R47H mice compared to their controls. Notably, there is a significant impairment of plaque‐microglia interaction in 5xFAD/ Trem2 R47H at 4‐month that is rescued at 12‐month. Moreover, we find a significant increase in plaque‐associated neuritic damage in 5xFAD/ Trem2 R47H when immunolabeled with LAMP1 at 4‐month that was also absent at 12‐month. Similarly, plasma NfL levels revealed significant increases in brain damage of 5xFAD/ Trem2 R47H compared to 5xFAD at 4‐month and a trending increase at 12‐month.ConclusionCollectively, these results show the effects of the Trem2 R47H variant on plaque development and downstream pathology, highlighting sex differences as well as age/ disease progression‐dependent changes.