A TREM1 variant alters the accumulation of Alzheimer-related amyloid pathology

TREM1 facilitates microglial phagocytosis of amyloid beta

As the most common type of neurodegenerative disease, Alzheimer's disease (AD) is characterized by the accumulation of amyloid-β peptide (Aβ) within the brain. Triggering receptor expressed on myeloid cells (TREM) 1 is an immune receptor expressed by mononuclear phagocytes including monocytes and microglia, coupling with TYRO protein tyrosine kinase binding protein to regulate immune reactions. Emerging evidence indicates that rs6910730^G, an intronic variant of TREM1, is associated with an increased Aβ neuropathology in the brains of elderly subjects, but the underlying mechanisms remain unclear. Here, using two independent cohorts of healthy individuals, we provided evidence that rs6910730^G reduced the ability of human monocytes for Aβ phagocytosis, and this reduction was likely attributed to a decreased monocytic TREM1 expression. By knockdown and overexpression of Trem1 in mouse primary microglia, we showed that TREM1 facilitated microglial phagocytosis of Aβ. In support of this finding, knockdown of Trem1 in the brains of APP/PSEN1 mice increased Aβ_1-42 levels and total amyloid burden, whereas selective overexpression of Trem1 on microglia or activation of Trem1 signaling by an agonistic antibody ameliorated Aβ neuropathology and rescued AD-related spatial cognitive impairments. Altogether, these findings uncover the role of TREM1 in microglial Aβ clearance, and establish TREM1 as a potential therapeutic target for AD.

TREMs in Alzheimer's disease: Genetic and clinical investigations

Triggering receptor expressed on myeloid cells (TREMs) receptors constitute a family modulators in human innate immunity system that encode by a gene cluster. Rare variants in TREM2 were reported to be associated with significant Alzheimer's disease (AD) risk. However, inconsistent results were also reported in some studies of Non-European descents. Recently, the other TREM family members are also considered to involve in AD and cerebrospinal fluid (CSF) soluble form of TREM2 (sTREM2) levels has also been associated with respond to progression of disease. In this review, we converged the data of genetic and clinical investigations to identify the clearer role of TREMs in AD. Here, comprehensively analyze of multidisciplinary fields highlights the contribution of TREMs locus to AD development.

Increased DNA methylation near TREM2 is consistently seen in the superior temporal gyrus in Alzheimer's disease brain

Although mutations within the TREM2 gene have been robustly associated with Alzheimer's disease, it is not known whether alterations in the regulation of this gene are also involved in pathogenesis. Here, we present data demonstrating increased DNA methylation in the superior temporal gyrus in Alzheimer's disease brain at a CpG site located 289 bp upstream of the transcription start site of the TREM2 gene in 3 independent study cohorts using 2 different technologies (Illumina Infinium 450K methylation beadchip and pyrosequencing). A meta-analysis across all 3 cohorts reveals consistent AD-associated hypermethylation (p = 3.47E-08). This study highlights that extending genetic studies of TREM2 in AD to investigate epigenetic changes may nominate additional mechanisms by which disruption to this gene increases risk.

Genome research in pre-dementia stages of Alzheimer's disease

Genetic characterization of individuals at risk of Alzheimer's disease (AD), i.e. people having amyloid deposits in the brain without symptoms, people suffering from subjective cognitive decline (SCD) or mild cognitive impairment (MCI), has spurred the interests of researchers. However, their pre-dementia genetic profile remains mostly unexplored. In this study, we reviewed the loci related to phenotypes of AD, MCI and SCD from literature and performed the first meta-analyses evaluating the role of apolipoprotein E (APOE) in the risk of conversion from a healthy status to MCI and SCD. For AD dementia risk, an increased number of loci have been identified; to date, 28 genes have been associated with Late Onset AD. In MCI syndrome, APOE is confirmed as a pheno-conversion factor leading from MCI to AD, and clusterin is a promising candidate. Additionally, our meta-analyses revealed APOE as genetic risk factor to convert from a healthy status to MCI [OR = 1.849 (1.587-2.153); P = 2.80  × 10-15] and to a lesser extent from healthy status to SCD [OR = 1.151 (1.015-1.304); P = 0.028]. Thus, we believe that genetic studies in longitudinal SCD and MCI series may provide new therapeutic targets and improve the existing knowledge of AD. This type of studies must be completed on healthy subjects to better understand the natural disease resistance to brain insults and neurodegeneration.

The roles of inflammation and immune mechanisms in Alzheimer's disease

The Alzheimer's Association's Research roundtable met in April 2015 to explore the role of neuroinflammatory mechanisms in the progression of Alzheimer's disease (AD). The ability of innate immune cells, particularly microglia and astrocytes, to mediate neuroinflammation in AD has been implicated as a significant contributor to disease pathogenesis. Adaptive immunity, which plays an important role in responding to injury and some diseases of the central nervous system, may contribute to neuroinflammation in AD as well. Communication between the central and peripheral immune systems may also be important in AD. An increased understanding of the physiology of the innate immune system may aid the identification of new therapeutic targets or mechanisms. The development of predictive animal models and translatable neuroinflammation biomarkers for AD would also facilitate the advancement of novel treatments for innate immunity. Important challenges impeding the advancement of new therapeutic agents and strategies to overcome them were discussed.

TREM2 Function in Alzheimer's Disease and Neurodegeneration

Alzheimer's disease (AD), the most common cause of dementia in the elderly, is a complex neurodegenerative disease marked by the appearance of amyloid-β (Aβ) plaques and hyperphosphorylated tau tangles. Alzheimer's disease has a strong genetic component, and recent advances in genome technology have unearthed novel variants in several genes, which could provide insight into the pathogenic mechanisms that contribute to AD. Particularly interesting are variants in the microglial-expressed receptor TREM2 which are associated with a 2-4-fold increased risk of developing AD. Since the discovery of a link between TREM2 and AD, multiple studies have emerged testing whether partial or complete loss of TREM2 function contributed to Aβ deposition or Aβ-associated microgliosis. Although some confounding conflicting data have emerged from these studies regarding the role of TREM2 in regulating Aβ deposition within the hippocampus, the most consistent and striking observation is a strong decrease in microgliosis surrounding Aβ plaques in TREM2 haploinsufficient and TREM2 deficient mice. Interestingly, a similar impairment in microgliosis has been reported in mouse models of prion disease, stroke, and multiple sclerosis, suggesting a critical role for TREM2 in supporting microgliosis in response to pathology in the central nervous system. In this Review, we summarize recent reports on the role of TREM2 in AD pathology and hypothesized mechanisms by which TREM2 function could influence AD-induced microgliosis.

TREM2 variants: new keys to decipher Alzheimer disease pathogenesis

Genome-wide association studies have identified rare variants of the gene that encodes triggering receptor expressed on myeloid cells 2 (TREM2) - an immune receptor that is found in brain microglia - as risk factors for non-familial Alzheimer disease (AD). Furthermore, animal studies have indicated that microglia have an important role in the brain response to amyloid-β (Aβ) plaques and that TREM2 variants may have an impact on such a function. We discuss how TREM2 may control the microglial response to Aβ and its impact on microglial senescence, as well as the interaction of TREM2 with other molecules that are encoded by gene variants associated with AD and the hypothetical consequences of the cleavage of TREM2 from the cell surface.

Angiopoietin-like protein 4 serum levels and gene polymorphisms are associated with large artery atherosclerotic stroke

BACKGROUND

Angiopoietin-like protein 4 (ANGPTL4) is a central player in lipid metabolism and atherosclerosis and may thus be involved in ischaemic stroke. However, no study in humans has investigated the association of ANGPTL4 gene polymorphisms or serum levels with ischaemic stroke.

METHODS

We investigated the influence of the tagged single nucleotide polymorphisms (tSNPs) rs4076317 (c.207C>G) and rs1044250 (c.797C>T; T266M) of the ANGPTL4 gene on ischaemic stroke risk in a large group of 712 large artery atherosclerotic (LAA) stroke patients and 828 controls. In addition, we examined the association of the serum ANGPTL4 levels with lipid metabolism, LAA stroke severity and ischaemic volume in a sample of 302 LAA stroke patients and 307 controls.

RESULTS

The findings reveal that rs4076317 exerts a co-dominant effect on lower serum TG levels compared with common homozygotes. Fewer stroke cases were homozygous for variants of rs4076317 compared with the controls (7.0% vs. 10.9%). The serum ANGPTL4 levels in patients were significantly higher than those in the controls in a univariate manner (P=0.001) and after adjustment for other risk factors (1.463 [1.215-1.835]; P<0.001). Consistently, the ANGPTL4 levels were statistically correlated with higher NIHSS scores (r=0.172, P=0.003) and larger lesion volumes (r=0.124, P=0.031).

CONCLUSION

We concluded that the tagged SNPs and high serum levels of ANGPTL4 are associated with LAA stroke and the lipid characteristics.

Microglial malfunction: the third rail in the development of Alzheimer's disease

Studies of Alzheimer's disease (AD) have predominantly focused on two major pathologies: amyloid-β (Aβ) and hyperphosphorylated tau. These misfolded proteins can accumulate asymptomatically in distinct regions over decades. However, significant Aβ accumulation can be seen in individuals who do not develop dementia, and tau pathology limited to the transentorhinal cortex, which can appear early in adulthood, is usually clinically silent. Thus, an interaction between these pathologies appears to be necessary to initiate and propel disease forward to widespread circuits. Recent multidisciplinary findings strongly suggest that the third factor required for disease progression is an aberrant microglial immune response. This response may initially be beneficial; however, a maladaptive microglial response eventually develops, fueling a feed-forward spread of tau and Aβ pathology.

CD33 modulates TREM2: convergence of Alzheimer loci

Here, we report results from a protein quantitative trait analysis in monocytes from 226 individuals to evaluate cross-talk between Alzheimer loci. We find that the NME8 locus influences PTK2B and that the CD33 risk allele leads to greater TREM2 expression. Further, we observe (1) a decreased TREM1/TREM2 ratio with a TREM1 risk allele, (2) decreased TREM2 expression with CD33 suppression, and (3) elevated cortical TREM2 mRNA expression with amyloid pathology.