Genetic variations in the myeloid immune receptor TREM2 are linked to several neurodegenerative diseases. To determine how TREM2 variants contribute to these diseases, we performed structural and functional studies of wild-type and variant proteins. Our 3.1 Å TREM2 crystal structure revealed that mutations found in Nasu-Hakola disease are buried whereas Alzheimer’s disease risk variants are found on the surface, suggesting that these mutations have distinct effects on TREM2 function. Biophysical and cellular methods indicate that Nasu-Hakola mutations impact protein stability and decrease folded TREM2 surface expression, whereas Alzheimer’s risk variants impact binding to a TREM2 ligand. Additionally, the Alzheimer’s risk variants appear to epitope map a functional surface on TREM2 that is unique within the larger TREM family. These findings provide a guide to structural and functional differences among genetic variants of TREM2, indicating that therapies targeting the TREM2 pathway should be tailored to these genetic and functional differences with patient-specific medicine approaches for neurodegenerative disorders.DOI: http://dx.doi.org/10.7554/eLife.20391.001

Background:The function of MAGP2 was studied by inactivating its gene (Mfap5 Ϫ/Ϫ ) in mice.

INTRODUCTION:TREM2 is an innate immune receptor expressed on myeloid cells including microglia in the brain. How TREM2 engages different ligands remains poorly understood. METHODS: We used comprehensive BLI analysis to investigate the TREM2 interactions with ApoE and monomeric amyloid beta (mAβ42). RESULTS: TREM2 binding did not depend on ApoE lipidation, and there were only slight differences in affinity observed between ApoE isoforms (E4 > E3 > E2). Surprisingly, diseaselinked TREM2 variants within a "basic patch" minimally impact ApoE binding. Instead, TREM2 has a unique hydrophobic surface that can bind to ApoE. This direct engagement requires the hinge region of ApoE. TREM2 directly binds mAβ42 and can potently inhibit Aβ42 polymerization, suggesting a potential mechanism for soluble TREM2 (sTREM2) in preventing AD pathogenesis. DISCUSSION: These findings demonstrate that TREM2 has at least two separate surfaces to engage ligands and uncovers a potential function for sTREM2 in directly inhibiting Aβ polymerization.

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