Triggering Receptor Expressed on Myeloid cells 2 (TREM2), which is expressed on myeloid cells including microglia in the CNS, hasrecently been identified as a risk factor for Alzheimer's disease (AD). TREM2 transmits intracellular signals through its transmembrane binding partner DNAX-activating protein 12 (DAP12). Homozygous mutations inactivating TREM2 or DAP12 lead to Nasu-Hakola disease; however, how AD risk-conferring variants increase AD risk is not clear. To elucidate the signaling pathways underlying reduced TREM2 expression or loss of function in microglia, we respectively knocked down and knocked out the expression of TREM2 in in vitro and in vivo models. We found that TREM2 deficiency reduced the viability and proliferation of primary microglia, reduced microgliosis in Trem2 Ϫ / Ϫ mouse brains, induced cell cycle arrest at the G 1 /S checkpoint, and decreased the stability of ␤-catenin, a key component of the canonical Wnt signaling pathway responsible for maintaining many biological processes, including cell survival. TREM2 stabilized ␤-catenin by inhibiting its degradation via the Akt/GSK3␤ signaling pathway. More importantly, treatment with Wnt3a, LiCl, or TDZD-8, which activates the ␤-catenin-mediated Wnt signaling pathway, rescued microglia survival and microgliosis in Trem2 Ϫ / Ϫ microglia and/or in Trem2 Ϫ / Ϫ mouse brain. Together, our studies demonstrate a critical role of TREM2-mediated Wnt/␤-catenin pathway in microglial viability and suggest that modulating this pathway therapeutically may help to combat the impaired microglial survival and microgliosis associated with AD.

BackgroundNeuroinflammation is characterized by microglial activation and the increased levels of cytokines and chemokines in the central nervous system (CNS). Recent evidence has implicated both beneficial and toxic roles of microglia when over-activated upon nerve injury or in neurodegenerative diseases, including Alzheimer’s disease (AD). The low-density lipoprotein receptor-related protein 1 (LRP1) is a major receptor for apolipoprotein E (apoE) and amyloid-β (Aβ), which play critical roles in AD pathogenesis. LRP1 regulates inflammatory responses in peripheral tissues by modulating the release of inflammatory cytokines and phagocytosis. However, the roles of LRP1 in brain innate immunity and neuroinflammation remain unclear.MethodsIn this study, we determined whether LRP1 modulates microglial activation by knocking down Lrp1 in mouse primary microglia. LRP1-related functions in microglia were also assessed in the presence of LRP1 antagonist, the receptor-associated protein (RAP). The effects on the production of inflammatory cytokines were measured by quantitative real-time PCR (qRT-PCR) and enzyme-linked immunosorbent assay (ELISA). Potential involvement of specific signaling pathways in LRP1-regulated functions including mitogen-activated protein kinases (MAPKs) and nuclear factor-κB (NF-κB) were assessed using specific inhibitors.ResultsWe found that knocking down of Lrp1 in mouse primary microglia led to the activation of both c-Jun N-terminal kinase (JNK) and NF-κB pathways with corresponding enhanced sensitivity to lipopolysaccharide (LPS) in the production of pro-inflammatory cytokines. Similar effects were observed when microglia were treated with LRP1 antagonist RAP. In addition, treatment with pro-inflammatory stimuli suppressed Lrp1 expression in microglia. Interestingly, NF-κB inhibitor not only suppressed the production of cytokines induced by the knockdown of Lrp1 but also restored the down-regulated expression of Lrp1 by LPS.ConclusionsOur study uncovers that LRP1 suppresses microglial activation by modulating JNK and NF-κB signaling pathways. Given that dysregulation of LRP1 has been associated with AD pathogenesis, our work reveals a critical regulatory mechanism of microglial activation by LRP1 that could be associated with other AD-related pathways thus further nominating LRP1 as a potential disease-modifying target for the treatment of AD.

Objective Apolipoprotein E (apoE), a major cholesterol carrier in the brain, is associated with a strong risk for Alzheimer’s disease. Compared to the risky APOE4 gene allele, the effects of the protective APOE2 gene allele are vastly understudied, and thus, need to be further clarified. Methods We reviewed National Alzheimer’s Coordinating Center (NACC) clinical records and performed preclinical experiments using human apoE-targeted replacement (apoE-TR) mice, which do not show amyloid pathology. Results Clinically, the APOE2 allele was associated with less cognitive decline during aging. This effect was also seen in subjects with little amyloid pathology, or after adjusting for Alzheimer’s disease-related pathologies. In animal studies, aged apoE2-TR mice also exhibited preserved memory function in the water maze tests. Regardless, apoE2-TR mice showed similar or greater age-related changes in synaptic loss, neuroinflammation and oxidative stress compared to apoE3- or apoE4-TR mice. Interestingly, apoE concentrations in the cortex, hippocampus, plasma and cerebrospinal fluid (CSF) were positively correlated with memory performance across apoE isoforms, where apoE2-TR mice had higher apoE levels. Moreover, apoE2-TR mice exhibited the lowest levels of cholesterol in the cortex, in spite of higher levels in CSF and plasma. These cholesterol levels were associated with apoE levels and memory performance across apoE isoforms. Interpretation APOE2 is associated with less cognitive decline during aging. This can occur independently of age-related synaptic/neuroinflammatory changes and amyloid accumulation. Higher levels of apoE and associated cholesterol metabolism in APOE2 carriers might contribute to this protective effect.

Mutations in TREM2, which has been proposed to regulate the inflammatory responses and the clearance of apoptotic neurons and/or amyloid-β (Aβ), are genetically linked to increased risk for late-onset Alzheimer’s disease (AD). Interestingly, a missense variant in TREM-like transcript 2 (TREML2), a structurally similar protein encoded by the same gene cluster with TREM2 on chromosome 6, has been shown to protect against AD. However, the molecular mechanisms by which TREM2 and TREML2 regulate the pathogenesis of AD, and their functional relationship, if any, remain unclear. Here, we show that lipopolysaccharide (LPS) stimulation significantly suppressed TREM2 but increased TREML2 expression in mouse brain. Consistent with this in vivo result, LPS or oligomeric Aβ treatment down-regulated TREM2 but up-regulated TREML2 expression in primary microglia. Importantly, modulation of TREM2 or TREML2 levels had opposing effects on inflammatory responses with enhancement or suppression of LPS induced pro-inflammatory cytokine gene expression observed upon TREM2 or TREML2 down-regulation, respectively. In addition, the proliferation of primary microglia was significantly decreased when TREM2 was down-regulated, whereas it was increased upon TREML2 knockdown. Together, our results suggest that several microglial functions are strictly regulated by TREM2 and TREML2, whose dysfunctions likely contribute to AD pathogenesis by impairing brain innate immunity. Our findings provide novel mechanistic insights into the functions of TREM2 and TREML2 in microglia and have implications on designing new therapeutic strategies to treat AD.

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