TREM2 modulates differential deposition of modified and non-modified Aß species in extracellular plaques and intraneuronal deposits.

Progressive accumulation of Amyloid-ß (Aß) deposits in the brain is a characteristic neuropathological hallmark of Alzheimer's disease (AD). During disease progression, extracellular Aß plaques undergo specific changes in their composition by the sequential deposition of different modified Aß species. Microglia are implicated in the restriction of amyloid deposits and play a major role in internalization and degradation of Aß. Recent studies showed that rare variants of the Triggering Receptor Expressed on Myeloid cells 2 (TREM2) are associated with an increased risk for AD. Post-translational modifications of Aß could modulate the interaction with TREM2, and the uptake by microglia. Here, we demonstrate that genetic deletion of TREM2 or expression of a disease associated TREM2 variant in mice lead to differential accumulation of modified and non-modified Aß species in extracellular plaques and intraneuronal deposits. Human brains with rare TREM2 AD risk variants also showed altered deposition of modified Aß species in the different brain lesions as compared to cases with the common variant of TREM2. These findings indicate that TREM2 plays a critical role in the development and the composition of Aß deposits, not only in extracellular plaques, but also intraneuronally, that both could contribute to the pathogenesis of AD.

Differential interaction with TREM2 modulates microglial uptake of modified Aß species.

Rare coding variants of the microglial triggering receptor expressed on myeloid cells 2 (TREM2) confer an increased risk for Alzheimer's disease (AD) characterized by the progressive accumulation of aggregated forms of amyloid ß peptides (Aß). Aß peptides are generated by proteolytic processing of the amyloid precursor protein (APP). Heterogeneity in proteolytic cleavages and additional post-translational modifications result in the production of several distinct Aß variants that could differ in their aggregation behavior and toxic properties. Here, we sought to assess whether post-translational modifications of Aß affect the interaction with TREM2. Biophysical and biochemical methods revealed that TREM2 preferentially interacts with oligomeric Aß, and that phosphorylation of Aß increases this interaction. Phosphorylation of Aß also affected the TREM2 dependent interaction and phagocytosis by primary microglia and in APP transgenic mouse models. Thus, TREM2 function is important for sensing phosphorylated Aß variants in distinct aggregation states and reduces the accumulation and deposition of these toxic Aß species in preclinical models of Alzheimer's disease.

Novel Phosphorylation-State Specific Antibodies Reveal Differential Deposition of Ser26 Phosphorylated Aß Species in a Mouse Model of Alzheimer's Disease.

Aggregation and deposition of amyloid-ß (Aß) peptides in extracellular plaques and in the cerebral vasculature are prominent neuropathological features of Alzheimer's disease (AD) and closely associated with the pathogenesis of AD. Amyloid plaques in the brains of most AD patients and transgenic mouse models exhibit heterogeneity in the composition of Aß deposits, due to the occurrence of elongated, truncated, and post-translationally modified Aß peptides. Importantly, changes in the deposition of these different Aß variants are associated with the clinical disease progression and considered to mark sequential phases of plaque and cerebral amyloid angiopathy (CAA) maturation at distinct stages of AD. We recently showed that Aß phosphorylated at serine residue 26 (pSer26Aß) has peculiar characteristics in aggregation, deposition, and neurotoxicity. In the current study, we developed and thoroughly validated novel monoclonal and polyclonal antibodies that recognize Aß depending on the phosphorylation-state of Ser26. Our results demonstrate that selected phosphorylation state-specific antibodies were able to recognize Ser26 phosphorylated and non-phosphorylated Aß with high specificity in enzyme-linked immunosorbent assay (ELISA) and Western Blotting (WB) assays. Furthermore, immunofluorescence analyses with these antibodies demonstrated the occurrence of pSer26Aß in transgenic mouse brains that show differential deposition as compared to non-phosphorylated Aß (npAß) or other modified Aß species. Notably, pSer26Aß species were faintly detected in extracellular Aß plaques but most prominently found intraneuronally and in cerebral blood vessels. In conclusion, we developed new antibodies to specifically differentiate Aß peptides depending on the phosphorylation state of Ser26, which are applicable in ELISA, WB, and immunofluorescence staining of mouse brain tissues. These site- and phosphorylation state-specific Aß antibodies represent novel tools to examine phosphorylated Aß species to further understand and dissect the complexity in the age-related and spatio-temporal deposition of different Aß variants in transgenic mouse models and human AD brains.