TREM2 Protein — Triggering Receptor Expressed on Myeloid Cells 2

Overview

TREM2 (Triggering Receptor Expressed on Myeloid Cells 2) is a type I transmembrane receptor expressed primarily on microglia in the central nervous system. It serves as a critical sensor of lipid-containing ligands and mediates microglial phagocytosis, metabolic adaptation, and the transition to disease-associated microglia (DAM) in Alzheimer’s disease^{1The mechanistic basis of Alzheimer's disease as an age-related neurodegenerative disorder2020 · Cell · DOI 10.1016/j.cell.2020.09.016Open reference}. TREM2 is one of the strongest genetic risk factors for late-onset Alzheimer’s disease—rare loss-of-function variants increase AD risk approximately 2-3 fold, comparable to the effect of a single APOE4 allele^{2Rare coding variants in PLCG2, ABI3, and TREM2 implicate microglial innate immunity in Alzheimer's disease2017 · Nat Neurosci · DOI 10.1038/s41593-017-0021-8Open reference}.

The discovery that TREM2 variants confer significant AD risk sparked intense research into microglial biology and neuroinflammation as therapeutic targets. TREM2’s primary effect is modulating microglial responses to amyloid pathology, positioning it as a key target for immunotherapy^{3TREM2 deficiency eliminates the neuroprotective function of resolving microglia2015 · Cell Rep · DOI 10.1016/j.cell.2015.12.057Open reference}.

Protein Structure

Domain Architecture

Structural Features

• Ig-like V-type domain: Classic immunoglobulin beta-sandwich fold with a hydrophobic binding groove enriched in aromatic residues. The pocket accommodates lipid ligands, apolipoproteins, and amyloid-beta oligomers^{4Structural basis for CD33 dysfunction in Alzheimer's disease2016 · J Biol Chem · DOI 10.1074/jbc.M116.714741Open reference}.

• Transmembrane helix: Contains a charged aspartate residue (D175) that pairs with a lysine in DAP12’s transmembrane domain, forming a stable receptor-adaptor complex.

• Proteolytic cleavage site: ADAM10/17 cleavage at position H157/S158 releases soluble TREM2 (sTREM2) constitutively. This shedding is upregulated under inflammatory conditions^{5An Alzheimer-associated TREM2 mutation impairs proteolytic processing of the receptor2020 · Neuron · DOI 10.1016/j.neuron.2020.07.023Open reference}.

Crystal Structure Insights

Structural studies reveal:

• Ligand-binding pocket: A hydrophobic groove accommodating phosphatidylserine, lipidated ApoE, and Aβ oligomers

• Dimerization interface: TREM2 forms side-by-side dimers, which may be important for high-affinity ligand binding and signaling

• Antibody epitopes: Therapeutic antibodies bind the Ig-like domain, often near the ligand-binding region

Post-Translational Modifications

Soluble TREM2 (sTREM2)

Proteolytic Processing

TREM2 undergoes constitutive and induced proteolytic cleavage:

• Shedases: ADAM10 and ADAM17 mediate cleavage at the stem domain (H157)

• sTREM2: Released extracellular fragment (~20 kDa) detectable in cerebrospinal fluid

• γ-secretase: Following shedding, the remaining membrane stub is cleaved by γ-secretase

sTREM2 as Biomarker

The ratio of sTREM2 to full-length TREM2 may serve as a biomarker for microglial activation status in AD^{6Increased soluble TREM2 in cerebrospinal fluid predicts amyloid and tau pathology2020 · EMBO Mol Med · DOI 10.1126/scitranslmed.abb3457Open reference}.

TREM2 Variants and Alzheimer’s Risk

Risk Variants

Mechanism of Risk

TREM2 AD risk variants exhibit:

• Reduced ligand binding: Impaired recognition of lipidated ApoE and Aβ

• Defective signaling: Reduced microglial activation in response to ligands

• Altered sTREM2: Some variants affect shedding and sTREM2 levels

• DAM deficiency: Failure to transition to disease-associated microglia

Disease-Associated Microglia (DAM)

TREM2 is essential for the transition from homeostatic microglia to disease-associated microglia (DAM)^{7A unique microglia type associated with restricting development of Alzheimer's disease2017 · Cell · DOI 10.1016/j.cell.2017.05.018Open reference}:

DAM Transition Pathway

The transition requires TREM2 signaling and is arrested in TREM2-deficient states^{8TREM2 deficiency impairs transition from homeostatic to DAM microglia2017 · EMBO Mol Med · DOI 10.15252/emmm.201707673Open reference}:

flowchart TD
    A["Homeostatic<br/>Microglia"] -->|"Abeta/Lipid<br/>Exposure"| B["TREM2-dependent<br/>Early DAM"]
    B -->|"Continued<br/>Activation"| C["TREM2-independent<br/>Late DAM"]

    A -->|"P2RY12+<br/>TMEM119+<br/>CX3CR1+"| A_label["Homeostatic<br/>Markers"]
    B -->|"APOE+<br/>CST7+<br/>LPL+<br/>TYROBP+"| B_label["Early DAM<br/>Markers"]
    C -->|"ITGAX+<br/>APOC1+<br/>SPP1+"| C_label["Late DAM<br/>Markers"]

    style A fill:#0a1929,stroke:#333
    style B fill:#3a3000,stroke:#333
    style C fill:#3b1114,stroke:#333

Functional consequences of arrest:

• Impaired Abeta phagocytosis and clearance

• Reduced lipid droplet accumulation for phagocytic energy

• Altered inflammatory response to amyloid

• Increased neuronal death around plaques

TREM2 and Lipid Metabolism

Microglial lipid metabolism is tightly regulated by TREM2 signaling^{9TREM2 regulates microglial metabolic adaptation to amyloid pathology2020 · Nat Neurosci · DOI 10.1038/s41593-020-0597-0Open reference}:

Key metabolic changes upon TREM2 activation:

• Glycolysis upregulation: Increased glucose metabolism to fuel phagocytosis

• Lipid droplet formation: Accumulation of lipid droplets for energy storage

• Cholesterol efflux: Regulation of intracellular cholesterol levels

• Phospholipid remodeling: Membrane composition changes for phagocytosis

ApoE as central lipid shuttle^{10TREM2 interacts with ApoE in lipid binding and enhances ApoE-mediated microglial activation2020 · J Neurosci · DOI 10.1523/JNEUROSCI.1555-19.2020Open reference}:

• TREM2 recognizes lipidated ApoE particles as primary ligands

• ApoE4 shows reduced TREM2 binding (Kd ~ 300 nM vs ~ 100 nM for ApoE3)

• This partially explains ApoE4’s increased AD risk through impaired microglial activation

Cross-Talk with Other Microglial Receptors

TREM2-CD33 Balance

TREM2 (activating) and CD33 (inhibitory) represent a balance regulating microglial function:

TREM2-CX3CR1 Interaction

Signaling Pathways

Core Signaling Cascade

1. Ligand binding → TREM2 dimerization/clustering

2. DAP12 ITAM phosphorylation by SRC family kinases

3. SYK recruitment and activation

4. Downstream pathways:

   • PI3K/AKT → cell survival, metabolism

   • MAPK/ERK → gene transcription, proliferation

   • NF-κB → inflammatory gene expression

   • mTOR → metabolic adaptation

Transcriptional Targets

Therapeutic Targeting

Agonist Antibodies in Clinical Development

Mechanism of Antibody Therapy

Agonistic antibodies work by:

• Binding the TREM2 extracellular domain

• Inducing receptor clustering (mimicking ligand binding)

• Activating downstream signaling in the absence of ligand

• Promoting microglial DAM transition

Summary

TREM2 represents a pivotal microglial receptor linking neuroinflammation to neurodegeneration. Its role as an AD risk factor, combined with its tractability as a cell surface target and active clinical trials, makes it one of the most promising therapeutic targets in Alzheimer’s disease. The ongoing AL002 Phase 2 trials will test whether microglial activation via TREM2 agonism can modify disease progression.

See Also

• TREM2 Gene — Gene page with genetics and biology

• CD33 Protein — Complementary microglial receptor

• DAP12 Protein — Signaling adaptor

• Microglia — Cell type where TREM2 is expressed

• Alzheimer’s Disease — Primary disease association

• Microglial Phagocytosis — Process TREM2 enhances