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The Alzheimer's Disease-Associated R47H Variant of TREM2 Has an Altered Glycosylation Pattern and Protein Stability.

ABSTRACT: The R47H coding variant of the triggering receptor expressed on myeloid cells-2 (TREM2) increases the risk of Alzheimer's disease (AD) similar to apolipoprotein E4. TREM2 R47H has recently been shown to have impaired binding to damage-associated lipid or apolipoprotein ligands. However, it is not known how this R47H variant affects the biochemical characteristics of TREM2 and alters the pathogenesis of AD. We previously reported that TREM2-R47H has a slightly different glycosylation pattern from wild-type. A more detailed characterization in our present study confirms that TREM2 R47H has an altered glycosylation pattern and reduced stability. TREM2 R47H shows different glycosylation profiles from analysis using monensin or kifunensine treatment which were confirmed by mass spectrometry. The solubility of TREM2 R47H and its cleaved products such as intracellular domain (ICD) is also decreased, increasing its proteasomal and lysosomal degradation. The different biochemical characteristics of TREM2 R47H, including glycosylation, solubility and processing, may offer insights into a future therapeutic strategy for AD.

SUBMITTER: Park JS

PROVIDER: S-EPMC5241589 | biostudies-literature | 2016

REPOSITORIES: biostudies-literature

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The Alzheimer's Disease-Associated R47H Variant of TREM2 Has an Altered Glycosylation Pattern and Protein Stability.

Park Ji-Seon JS Ji In Jung IJ Kim Dong-Hou DH An Hyun Joo HJ Yoon Seung-Yong SY

Frontiers in neuroscience 20170118

The R47H coding variant of the triggering receptor expressed on myeloid cells-2 (TREM2) increases the risk of Alzheimer's disease (AD) similar to apolipoprotein E4. TREM2 R47H has recently been shown to have impaired binding to damage-associated lipid or apolipoprotein ligands. However, it is not known how this R47H variant affects the biochemical characteristics of TREM2 and alters the pathogenesis of AD. We previously reported that TREM2-R47H has a slightly different glycosylation pattern from ...[more]

PMID: 28149270

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Project description:BackgroundMolecular characterization of late-onset Alzheimer's disease (LOAD), the leading cause of age-related dementia, has revealed transcripts, proteins, and pathway alterations associated with disease. Assessing these postmortem signatures of LOAD in experimental model systems can further elucidate their relevance to disease origins and progression. Model organisms engineered with human genetic factors further link these signatures to disease-associated variants, especially when studies are designed to leverage homology across species. Here we assess differential gene splicing patterns in aging mouse models carrying humanized APOE4 and/or the Trem2*R47H variant on a C57BL/6J background. We performed a differential expression of gene (DEG) and differential splicing analyses on whole brain transcriptomes at multiple ages. To better understand the difference between differentially expressed and differentially spliced genes, we evaluated enrichment of KEGG pathways and cell-type specific gene signatures of the adult brain from each alteration type. To determine LOAD relevance, we compared differential splicing results from mouse models with multiple human AD splicing studies.ResultsWe found that differentially expressed genes in Trem2*R47H mice were significantly enriched in multiple AD-related pathways, including immune response, osteoclast differentiation, and metabolism, whereas differentially spliced genes were enriched for neuronal related functions, including GABAergic synapse and glutamatergic synapse. These results were reinforced by the enrichment of microglial genes in DEGs and neuronal genes in differentially spliced genes in Trem2*R47H mice. We observed significant overlap between differentially spliced genes in Trem2*R47H mice and brains from human AD subjects. These effects were absent in APOE4 mice and suppressed in APOE4.Trem2*R47H double mutant mice relative to Trem2*R47H mice.ConclusionsThe cross-species observation that alternative splicing observed in LOAD are present in Trem2*R47H mouse models suggests a novel link between this candidate risk gene and molecular signatures of LOAD in neurons and demonstrates how deep molecular analysis of new genetic models links molecular disease outcomes to a human candidate gene.

Project description:BackgroundTREM2 is a microglial cell surface receptor, with risk mutations linked to Alzheimer's disease (AD), including R47H. TREM2 signalling via SYK aids phagocytosis, chemotaxis, survival, and changes to microglial activation state. In AD mouse models, knockout (KO) of TREM2 impairs microglial clustering around amyloid and prevents microglial activation. The R47H mutation is proposed to reduce TREM2 ligand binding. We investigated cell phenotypes of the R47H mutant and TREM2 KO in a model of human microglia, and compared their transcriptional signatures, to determine the mechanism by which R47H TREM2 disrupts function.MethodsWe generated human microglia-like iPSC-macrophages (pMac) from isogenic induced pluripotent stem cell (iPSC) lines, with homozygous R47H mutation or TREM2 knockout (KO). We firstly validated the effect of the R47H mutant on TREM2 surface and subcellular localization in pMac. To assess microglial phenotypic function, we measured phagocytosis of dead neurons, cell morphology, directed migration, survival, and LPS-induced inflammation. We performed bulk RNA-seq, comparing significant differentially expressed genes (DEGs; p < 0.05) between the R47H and KO versus WT, and bioinformatically predicted potential upstream regulators of TREM2-mediated gene expression.ResultsR47H modified surface expression and shedding of TREM2, but did not impair TREM2-mediated signalling, or gross phenotypes that were dysregulated in the TREM2 KO (phagocytosis, motility, survival). However, altered gene expression in the R47H TREM2 pMac overlapped by 90% with the TREM2 KO and was characterised by dysregulation of genes involved with immunity, proliferation, activation, chemotaxis, and adhesion. Downregulated mediators of ECM adhesion included the vitronectin receptor αVβ3, and consequently, R47H TREM2 pMac adhered weakly to vitronectin compared with WT pMac. To counteract these transcriptional defects, we investigated TGFβ1, as a candidate upstream regulator. TGFβ1 failed to rescue vitronectin adhesion of pMac, although it improved αVβ3 expression.ConclusionsThe R47H mutation is not sufficient to cause gross phenotypic defects of human pMac under standard culture conditions. However, overlapping transcriptional defects with TREM2 KO supports the hypothesised partial loss-of-function effects of the R47H mutation. Furthermore, transcriptomics can guide us to more subtle phenotypic defects in the R47H cells, such as reduced cell adhesion, and can be used to predict targets for therapeutic intervention.

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The Alzheimer's Disease-Associated R47H Variant of TREM2 Has an Altered Glycosylation Pattern and Protein Stability.

Publications

The Alzheimer's Disease-Associated R47H Variant of TREM2 Has an Altered Glycosylation Pattern and Protein Stability.

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