Molecular basis for the loss-of-function effects of the Alzheimer's disease–associated R47H variant of the immune receptor TREM2

Abstract

Triggering receptor expressed on myeloid cells 2 (TREM2) is an immune receptor expressed on the surface of microglia, macrophages, dendritic cells, and osteoclasts. The R47H TREM2 variant is a significant risk factor for late-onset Alzheimer's disease (AD), and the molecular basis of R47H TREM2 loss of function is an emerging area of TREM2 biology. Here, we report three high-resolution structures of the extracellular ligand-binding domains (ECDs) of R47H TREM2, apo-WT, and phosphatidylserine (PS)-bound WT TREM2 at 1.8, 2.2, and 2.2 Å, respectively. The structures reveal that Arg plays a critical role in maintaining the structural features of the complementarity-determining region 2 (CDR2) loop and the putative positive ligand-interacting surface (PLIS), stabilizing conformations capable of ligand interaction. This is exemplified in the PS-bound structure, in which the CDR2 loop and PLIS drive critical interactions with PS via surfaces that are disrupted in the variant. Together with and characterization, our structural findings elucidate the molecular mechanism underlying loss of ligand binding, putative oligomerization, and functional activity of R47H TREM2. They also help unravel how decreased and stability of TREM2 contribute to loss of function in disease.

Functional insights from biophysical study of TREM2 interactions with ApoE and Aβ1-42

“…2.A). Part of this region comprises a phospholipid binding surface identified in a recent co-crystal structure [16], and computational analyses have suggested that the stability of this binding surface may be disrupted by variants associated with neurodegenerative diseases, including AD risk variants [35]. Remarkably, we found that point mutations to this surface (M41D, W44D, L69D, W70D, and F74D) decreased binding affinity for ApoE4 by 5-200-fold ( Fig.…”

“…We found that point mutations to a distal hydrophobic surface composed of the CDR1-3 loops on TREM2 reduced binding to ApoE by about 10-1000-fold. It is worth noting that a recent crystal structure of the TREM2 R47H variant [16] and results from molecular dynamics simulations of TREM2 R47H, R62H, and T96K variants [35] show that these AD risk variants induce structural instability in the CDR2 loop, which comprises a central portion of this hydrophobic patch. Thus, these structurally-induced impacts on the hydrophobic patch could explain why TREM2 basic patch mutations display moderate decreases in binding to ApoE.…”

“…R47H, R62H [14], etc.) do not grossly impact protein structure or stability but instead likely disrupt interactions with important ligands [15,16]. Several proteins and biological compounds have been demonstrated to bind TREM2 [17], and AD-relevant ligands such as apoptotic cells [18], phospholipids [18,19], low-and high-density lipoprotein [20,21], apolipoprotein E (ApoE) [22], and oligomeric Aβ [23][24][25] have been confirmed to induce TREM2-mediated signaling or phagocytosis.…”

IPSC-derived neuronal cultures expressing the Alzheimer’s disease associated rare TREM2 R47H variant enables the construction of an Aβ-induced gene regulatory network

“…Moreover, we did not observe profound difference in the differentiation and maturation propensity between iPSCs derived from LOAD patients and healthy donors-in agreement with previous reports (Flamier et al, 2018;Hossini et al, 2015;Israel et al, 2012;Ochalek et al, 2017). Cheng-Hathaway et al, and Sudom et al, reported that Trem2 R47H knock-in mice showed reduced Trem2 mRNA and protein expression in the brain as well as reduced soluble fragments of Trem2 (sTrem2) in plasma (Cheng-Hathaway et al, 2018;Sudom et al, 2018). More recently, Xiang et al…”

Microglial inflammation and phagocytosis in Alzheimer's disease: Potential therapeutic targets

“…Reduced solubility, increased turnover, altered glycosylation, and reduced sTREM2 shedding of the R47H variant had previously been reported in cell culture and in vivo. Thus, the new structural data can shed some light on how these are connected (Park et al, 2015;Park, Ji, Kim, An, & Yoon, 2017;Sudom et al, 2018).…”