New insights into the role of TREM2 in Alzheimer's disease

Abstract

Alzheimer's disease (AD) is the leading cause of dementia. The two histopathological markers of AD are amyloid plaques composed of the amyloid-β (Aβ) peptide, and neurofibrillary tangles of aggregated, abnormally hyperphosphorylated tau protein. The majority of AD cases are late-onset, after the age of 65, where a clear cause is still unknown. However, there are likely different multifactorial contributors including age, enviornment, biology and genetics which can increase risk for the disease. Genetic predisposition is considerable, with heritability estimates of 60-80%. Genetic factors such as rare variants of TREM2 (triggering receptor expressed on myeloid cells-2) strongly increase the risk of developing AD, confirming the role of microglia in AD pathogenesis. In the last 5 years, several studies have dissected the mechanisms by which TREM2, as well as its rare variants affect amyloid and tau pathologies and their consequences in both animal models and in human studies. In this review, we summarize increases in our understanding of the involvement of TREM2 and microglia in AD development that may open new therapeutic strategies targeting the immune system to influence AD pathogenesis.

Keywords: Alzheimer’s disease; ApoE; Gliosis; Microglia; Neurodegeneration; TREM2.

Fig. 1

TREM2 ligands, signaling and functions. Ligands binding to TREM2 induce the association of TREM2 to DAP12 through an electrostatic interaction between a conserved positively-charged lysine in TREM2 (aa186) and a negatively- charged aspartic acid residue in DAP12, generating tyrosine phosphorylation of DAP12 within its immunoreceptor tyrosine-based activation motifs (ITAMS) by Src family kinases

Fig. 2

Schematic summary of the role of TREM2 and its variants in AD. a. Functional TREM2 has been suggested to allow microglia activation (by amyloid and NFTs for example), promote microglia clustering around plaques, amyloid uptake (early stage of the disease) and plaque compaction through binding to plaque-associated ApoE or directly to oligomeric Aβ. b. AD-associated TREM2 variants resulting in TREM2 partial loss-of-function abolished microglia clustering around plaque and phagocytic activity. These changes could be caused by a blockage of microglia in homeostatic stages because of less plaque-associated ApoE or other reasons. The consequences are filamentous plaques associate with increased dystrophic neurites and a possible increase of tau pathology (in early stages)