NOMIS Researcher(s)
Published in Neuron
TREM2 variants increase the risk for Alzheimer’s disease. In this issue of Neuron, Lee et al. demonstrate that TREM2-dependent microglial functions prevent accumulation and spreading of tau, but only in the presence of amyloid pathology. This provides additional fuel for the amyloid cascade hypothesis and supports a protective function of microglia.
Research field(s)
Health Sciences, Clinical Medicine, Neurology & Neurosurgery
NOMIS Researcher(s)
Published in Journal of Neuroinflammation
Background: P301S tau transgenic mice show age-dependent accumulation of neurofibrillary tangles in the brainstem, hippocampus, and neocortex, leading to neuronal loss and cognitive deterioration. However, there is hitherto only sparse documentation of the role of neuroinflammation in tau mouse models. Thus, we analyzed longitudinal microglial activation by small animal 18 kDa translocator protein positron-emission-tomography (TSPO μPET) imaging in vivo, in conjunction with terminal assessment of tau pathology, spatial learning, and cerebral glucose metabolism. Methods: Transgenic P301S (n = 33) and wild-type (n = 18) female mice were imaged by 18F-GE-180 TSPO μPET at the ages of 1.9, 3.9, and 6.4 months. We conducted behavioral testing in the Morris water maze, 18F-fluordesoxyglucose (18F-FDG) μPET, and AT8 tau immunohistochemistry at 6.3-6.7 months. Terminal microglial immunohistochemistry served for validation of TSPO μPET results in vivo, applying target regions in the brainstem, cortex, cerebellum, and hippocampus. We compared the results with our historical data in amyloid-β mouse models. Results: TSPO expression in all target regions of P301S mice increased exponentially from 1.9 to 6.4 months, leading to significant differences in the contrasts with wild-type mice at 6.4 months (+ 11-23%, all p < 0.001), but the apparent microgliosis proceeded more slowly than in our experience in amyloid-β mouse models. Spatial learning and glucose metabolism of AT8-positive P301S mice were significantly impaired at 6.3-6.5 months compared to the wild-type group. Longitudinal increases in TSPO expression predicted greater tau accumulation and lesser spatial learning performance at 6.3-6.7 months. Conclusions: Monitoring of TSPO expression as a surrogate of microglial activation in P301S tau transgenic mice by μPET indicates a delayed time course when compared to amyloid-β mouse models. Detrimental associations of microglial activation with outcome parameters are opposite to earlier data in amyloid-β mouse models. The contribution of microglial response to pathology accompanying amyloid-β and tau over-expression merits further investigation.
Research field(s)
Health Sciences, Clinical Medicine, Neurology & Neurosurgery
NOMIS Researcher(s)
Published in Neuron
Alzheimer’s disease (AD) is currently untreatable, and therapeutic strategies aimed to slow cognitive decline have not yet been successful. Many of these approaches have targeted the amyloid cascade, indicating that novel treatment strategies are required. Recent genome-wide association studies (GWASs) have identified a number of risk factors in genes expressed in microglia, underscoring their therapeutic potential in neurodegeneration. In this review, we discuss how the recently defined functions of these AD risk genes can be targeted therapeutically to modulate microglial cell state and slow the progression of AD. Antibody-mediated stimulation of the triggering receptor of myeloid cells 2 (TREM2) is on the forefront of these candidate therapeutic approaches based on a combination of compelling human genetics and emerging preclinical data. This and other approaches to modify microglial function are a topic of intensive study and provide an opportunity for innovative AD treatments, which may be applied alone or potentially in combination with classical anti-amyloid therapies.
Research field(s)
Health Sciences, Clinical Medicine, Neurology & Neurosurgery
NOMIS Researcher(s)
Published in Science Translational Medicine
Soluble triggering receptor expressed on myeloid cells 2 (sTREM2) in cerebrospinal fluid (CSF) has been associated with Alzheimer’s disease (AD). TREM2 plays a critical role in microglial activation, survival, and phagocytosis; however, the pathophysiological role of sTREM2 in AD is not well understood. Understanding the role of sTREM2 in AD may reveal new pathological mechanisms and lead to the identification of therapeutic targets. We performed a genome-wide association study (GWAS) to identify genetic modifiers of CSF sTREM2 obtained from the Alzheimer’s Disease Neuroimaging Initiative. Common variants in the membrane-spanning 4-domains subfamily A (MS4A) gene region were associated with CSF sTREM2 concentrations (rs1582763; P = 1.15 × 10-15); this was replicated in independent datasets. The variants associated with increased CSF sTREM2 concentrations were associated with reduced AD risk and delayed age at onset of disease. The single-nucleotide polymorphism rs1582763 modified expression of the MS4A4A and MS4A6A genes in multiple tissues, suggesting that one or both of these genes are important for modulating sTREM2 production. Using human macrophages as a proxy for microglia, we found that MS4A4A and TREM2 colocalized on lipid rafts at the plasma membrane, that sTREM2 increased with MS4A4A overexpression, and that silencing of MS4A4A reduced sTREM2 production. These genetic, molecular, and cellular findings suggest that MS4A4A modulates sTREM2. These findings also provide a mechanistic explanation for the original GWAS signal in the MS4A locus for AD risk and indicate that TREM2 may be involved in AD pathogenesis not only in TREM2 risk-variant carriers but also in those with sporadic disease.
Research field(s)
Health Sciences, Biomedical Research, Developmental Biology
NOMIS Researcher(s)
Published in EMBO Molecular Medicine
Microglia adopt numerous fates with homeostatic microglia (HM) and a microglial neurodegenerative phenotype (MGnD) representing two opposite ends. A number of variants in genes selectively expressed in microglia are associated with an increased risk for neurodegenerative diseases such as Alzheimer’s disease (AD) and frontotemporal lobar degeneration (FTLD). Among these genes are progranulin (GRN) and the triggering receptor expressed on myeloid cells 2 (TREM2). Both cause neurodegeneration by mechanisms involving loss of function. We have now isolated microglia from Grn−/− mice and compared their transcriptomes to those of Trem2−/− mice. Surprisingly, while loss of Trem2 enhances the expression of genes associated with a homeostatic state, microglia derived from Grn−/− mice showed a reciprocal activation of the MGnD molecular signature and suppression of gene characteristic for HM. The opposite mRNA expression profiles are associated with divergent functional phenotypes. Although loss of TREM2 and progranulin resulted in opposite activation states and functional phenotypes of microglia, FDG (fluoro-2-deoxy-d-glucose)-μPET of brain revealed reduced glucose metabolism in both conditions, suggesting that opposite microglial phenotypes result in similar wide spread brain dysfunction.
Research field(s)
Health Sciences, Biomedical Research, Developmental Biology
