Biophysical mapping of TREM2-ligand interactions reveals shared surfaces for engagement of multiple Alzheimer’s disease ligands

TL;DR

This study identifies specific regions on the TREM2 receptor involved in binding various Alzheimer's disease-related proteins, suggesting potential drug targets.

Contribution

The study maps TREM2-ligand interactions using TREM2 variants and biolayer interferometry, revealing shared and distinct binding sites.

Findings

Mutations in the hydrophobic site of TREM2 disrupt binding to apoE4 and TDP-43.

apoE4 and oAβ42 share overlapping binding sites on TREM2, while C1q binding is affected by mutations in the basic site.

TREM2 uses the hydrophobic site for multiple ligands and distinct basic sites for others like IL-34.

Abstract

TREM2 is a signaling receptor expressed on microglia that has emerged as an important drug target for Alzheimer’s disease and other neurodegenerative diseases. While a number of TREM2 ligands have been identified, little is known regarding the structural details of how they engage. To better understand this, we created a protein library of 28 different TREM2 variants that could be used to map interactions with various ligands using biolayer interferometry. The variants are located in previously identified putative binding surfaces on TREM2 called the hydrophobic site, basic site, and site 2. We found that mutations to the hydrophobic site ablated binding to apoE4 and TDP-43. Competition binding experiments indicated that apoE4 and oAβ42 share overlapping binding sites on TREM2. In contrast, binding to C1q was disrupted most strongly by mutations to the basic site, including R46, with some mutations to the hydrophobic site also attenuating binding, thus suggesting a broader mediation of binding across the two sites. Supporting this, competition experiments indicated that C1q binding could be blocked by both apoE and oAβ42. TREM2 binding to IL-34 was mediated by the basic site at a surface centering on R76. Competition binding experiments validated the unique site for IL-34, showing little to no competition with either oAβ42 or apoE4. However, competition experiments between C1q and IL34 suggest that the ligands compete for binding at the basic site. Altogether, our results suggest that TREM2 utilizes the hydrophobic site (consisting of CDR1, CDR2, and CDR3) as a common site to engage multiple ligands, and uses distinct basic sites to engage others. Our findings imply that pharmaceutical strategies targeting these surfaces might be effective to modulate TREM2 functions.

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