As part of an international collaboration, we provided strong evidence implicating TREM2 in LOAD by showing that TREM2 R47H is significantly associated with LOAD. In our combined case-control series of over 11,000 subjects, R47H has an odds ratio of 4.0 (P=6.6x10-9) similar to that of the well-known APOE ?4 allele. In this series, a second TREM2 missense variant, D87N, also shows significant association. Remarkably, a recent study in which we participated shows that the risk associated with R47H extends beyond AD to FTD and PD, indicating that TREM2 may be a general neurodegenerative disease risk gene. TREM2 is a transmembrane signaling receptor in microglia known to function with its adaptor protein TYROBP (also known as DAP12) to effect non-inflammatory phagocytosis of apoptotic neurons. The disease-associated TREM2 variants may well act by altering the normal functioning of this transmembrane receptor. It is known, however, that soluble forms of TREM2 (sTREM2) are detectable in CSF, and are increased in patients with multiple sclerosis and CNS inflammation. The R47H and D87N variants are located in the extracellular domain of TREM2 and will, therefore, be present in sTREM2. Here, we explore the hypothesis that the disease-associated R47H and D87N variants may exert some or all of their disease-modifying effects on sTREM2 rather than on full-length TREM2 (FL-TREM2). sTREM2 could be generated by two mechanisms that are not mutually exclusive: proteolytic cleavage of FL-TREM2 and alternative splicing of exon 4, which contains the membrane-spanning domain. Our preliminary data and a recently published study show that, in cultured cells, sTREM2 is produced by proteolytic cleavage. R47H and D87N could act by altering this proteolytic cleavage thereby altering the relative amount of soluble and FL-TREM2. If sTREM2 functions as a decoy receptor for ligands that trigger signaling by FL-TREM2, then R47H and D87N may act additionally by altering the interaction of sTREM2 with these signaling ligands. To evaluate these mechanisms in vitro, we will quantitate sTREM2 and full-length TREM2 by sandwich ELISA in cell lines and in primary mouse microglia cultures expressing human WT, R47H, and D87N TREM2. To evaluate functional effects of sTREM2, we will generate soluble forms of WT, R47H, and D87N TREM2, and test their effects in functional assays analyzing direct binding to and phagocytosis of apoptotic neurons. In complementary in vivo studies, we will employ samples from the Mayo Clinic Study of Aging to evaluate associations (i) between sTREM2 and the R47H variant and (ii) between sTREM2 and conversion to MCI or AD. In brain samples, we will evaluate the association of R47H with sTREM2, FL- TREM2, the sTREM2/FL-TREM2 ratio, Braak stage, biochemical measures of A?, and immunohistopathological measures of amyloid and tau pathology. We will also analyze sTREM2, FL-TREM2 and the sTREM2/FL-TREM2 ratio in AD as compared to non-AD brains.
Alzheimer's disease, Parkinson's disease, and frontotemporal dementia are neurodegenerative disorders for which there are no good treatments that significantly alter disease progression. We have recently discovered that the risk of all three diseases is substantially increased by genetic variants that change an immune system protein known as TREM2. In this application we propose experiments to understand how TREM2 variants alter the metabolism and function of TREM2, with the ultimate goal of discovering novel therapeutic targets for neurodegenerative diseases.