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TREM-2 （Triggering Receptor Expressed on Myeloid cells-2） is a 35 kDa type I transmembrane protein belonging to the TREM family and the immunoglobulin （Ig） superfamily. Mature human TREM-2 comprises a 156 amino acid （aa） extracellular domain （ECD） with one V-type Ig-like domain, a 21 aa transmembrane （TM） domain, and a 35 aa cytoplasmic tail. Soluble forms of the TREM-2 ECD are generated by alternative splicing or proteolytic cleavage, and the cytoplasmic domain can be liberated by gamma-secretase-mediated intramembrane cleavage. A positively charged lysine within the transmembrane segment allows TREM-2 to associate with the signal adapter protein DAP12, which inhibits macrophage activation.
TREM-2 is expressed on macrophages, immature myeloid dendritic cells, osteoclasts, microglia, and adipocytes. It promotes the differentiation and function of osteoclasts, the production of inflammatory cytokines by adipocytes, insulin resistance, and the phagocytic clearance of bacteria. In the central nervous system （CNS）, TREM-2 binds to ApoE, ApoA1, and ApoB, mediating the clearance of apoptotic neurons, amyloid plaques, and cell debris following demyelination. TREM-2 also interacts with and modifies signaling through Plexin A1 on dendritic cells and osteoclasts.
Mutations in TREM-2 or DAP12 are associated with the development of Alzheimer's disease and Nasu-Hakola disease （NHD/PLOSL）, characterized by presenile dementia and bone cysts. Soluble TREM-2 is elevated in the cerebrospinal fluid of patients with active multiple sclerosis （MS）, and TREM-2 blockade exacerbates disease symptoms in the experimental autoimmune encephalomyelitis （EAE） model of MS.