Monocyte/macrophage- and neutrophil-mediated inflammatory responses can be stimulated through a variety of receptors, including G protein-linked 7-transmembrane receptors (e.g., FPR1), Fc receptors, CD14 and Toll-like receptors (e.g., TLR4), and cytokine receptors (e.g., IFNGR1). Engagement of these receptors can also prime myeloid cells to respond to other stimuli. Myeloid cells express receptors belonging to the Immunoglobulin (Ig) superfamily, such as TREM2, or to the C-type lectin superfamily. Depending on their transmembrane and cytoplasmic sequence structure, these receptors have either activating (e.g., KIR2DS1) or inhibitory functions (e.g., KIR2DL1).
Upon stimulation, TREM2 engages DAP12, causing the two tyrosines on its immunoreceptor tyrosine-based activation motif (ITAM) to become phosphorylated. Spleen tyrosine kinase (Syk) then docks to these phosphorylation sites and activates the phosphatidylinositol-3 kinase (PI3K) cascade which promotes several cellular functions such as cell survival, phagocytosis, pro-inflammatory cytokine production, and cytoskeletal rearrangement via various transcription factors including AP1, NF-κB, and NFAT.
A rare missense mutation (rs75932628-T) in the gene encoding TREM2, (predicted to result in an R47H substitution), confers a significant risk of Alzheimer's disease. Given the reported antiinflammatory role of TREM2 in the brain, it is suspected of interfering with the brain’s ability to prevent the buildup of plaque. TREM2 mutations increase the risk of neurodegenerative conditions such as Alzheimer's disease, amyotrophic lateral sclerosis, and Parkinson's disease. TREM2 interacts with DAP12 in microglia to trigger phagocytosis of amyloid beta peptide and apoptotic neurons without inflammation. Mutations in TREM2 impair the normal proteolytic maturation of the protein which in turn interferes with phagocytosis and may therefore contribute to the pathogenesis of Alzheimer's disease.
Soluble TREM2 has been detected in human cerebrospinal fluid (CSF), where it was found to be elevated in CSF of patients with multiple sclerosis and other inflammatory neurological conditions in comparison to patients without inflammatory neurologic disorders.
A recent study from Cruchaga lab identified MS4A4A as a major regulator of soluble TREM2 levels.Cruchaga and his team also demonstrated that TREM2 is implicated on disease in general and not only in those individuals that carry TREM2 risk variants. Using Mendelian randomization, they also demonstrate that highly soluble TREM2 levels are protective. These results provide a mechanistic explanation of one the AD risk GWAS loci, MS4A4A: this gene modified risk for AD by modulating TREM2 levels.[clarification needed]
TREM2 transcript levels are upregulated in the lung parenchyma of smokers.
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