TREM2 in Alzheimer's Disease: From Mechanisms to Therapeutic Strategies

This review explores how microglial receptor TREM2 contributes to late-onset Alzheimer’s disease (LOAD) risk and neuropathology. LOAD has substantial heritability (≈60–80%). APOE ε4 is the strongest common genetic risk factor (≈3–4× risk for one ε4 allele; up to ≈12× for two), while recent GWAS and sequencing studies have identified rare TREM2 variants that increase risk ≈2–4×, comparable to one APOE ε4 allele. The review asks how TREM2 structure, expression, signaling, and ligand interactions (notably with ApoE and Aβ) influence microglial functions—including phagocytosis, inflammatory signaling, proliferation, survival—and how TREM2 and its variants modulate amyloid and tau pathologies across disease stages. Clarifying these mechanisms is critical because TREM2 is being considered as a therapeutic target in AD.

The review synthesizes recent findings on: (1) TREM2 structure and expression—TREM2 is a 230-aa transmembrane receptor of the TREM family, expressed on microglia in the brain; its expression varies by CNS region, is modulated by inflammatory context, increases with aging, and is upregulated in neurodegenerative conditions including AD. (2) Signaling and ligands—TREM2 signals via DAP12 (and DAP10) through ITAM-mediated recruitment of Syk, PI3K/Akt, MAPK, elevating intracellular Ca2+. Ligands include microbial products (LPS/LTA), various lipids and lipoproteins (HDL, LDL), apolipoproteins (ApoE, ApoA1, ApoA2, ApoB, clusterin), and Aβ (with stronger binding to oligomers). In vivo data support a functional pathway link between ApoE and TREM2 in microglial responses to plaques. (3) Functions—TREM2 enhances phagocytosis (apoptotic neurons, debris, bacteria), modulates inflammatory signaling (often anti-inflammatory but context-dependent), and promotes myeloid cell proliferation and survival (e.g., via Wnt/β-catenin and ERK), with loss-of-function associated with Nasu-Hakola disease. (4) Variants—Rare variants (R47H, R62H, D87N, T96K, L211P, R136Q) alter ligand binding and signaling to varying degrees; R47H is the most studied, linked to earlier onset and faster decline, increased CSF tau but not Aβ42, and partial loss-of-function phenotypes in vitro and in vivo models. (5) Amyloid pathology—TREM2 deficiency/haploinsufficiency consistently reduces plaque-associated microgliosis, impairs plaque compaction, reduces microglial proliferation, and increases neuritic dystrophy; effects on Aβ burden are age/burden-dependent and model-specific. Human TREM2 expression in mice enhances microglial disease-associated gene programs and reduces amyloid metrics. (6) Tau pathology—Evidence links TREM2 to tau via CSF sTREM2 correlation with total/phospho-tau and microglial involvement in tau spread. In pure tauopathy models, complete TREM2 loss often reduces neuroinflammation and neurodegeneration without consistently changing tau phosphorylation/insolubility (PS19), while in hTau mice TREM2 deletion can exacerbate tau phosphorylation/insolubility, indicating model- and stage-dependent effects. (7) TREM2–ApoE axis—Multi-omic and genetic data indicate that TREM2 and ApoE cooperate to drive a neurodegeneration-associated microglial (MGnD) phenotype; deletion of TREM2 or ApoE prevents the MGnD switch and can reduce tau-mediated neurodegeneration; ApoE is required for full microglial responses to plaques similarly to TREM2. (8) Therapeutic implications—Given the biphasic, context-dependent roles of TREM2, therapeutic strategies may need to activate TREM2 signaling early (amyloid phase) but carry risks at later stages; antibody agonists and expression modulation are being explored, recognizing off-target and cell type considerations.

Narrative literature review synthesizing data from human genetics, in vitro biochemical and cellular assays, and in vivo studies in multiple AD-related mouse (and Drosophila) models. No experimental methodology specific to this article was performed.

• Genetic risk: Rare TREM2 variants increase LOAD risk ≈2–4×; APOE ε4 increases risk ≈3–4× (one allele) and up to ≈12× (two alleles). The ε2 allele decreases risk and delays onset.
• Ligands and signaling: TREM2 binds lipids/lipoproteins and apolipoproteins including ApoE and clusterin; Aβ (especially oligomers) binds and activates TREM2–DAP12 signaling, triggering downstream Syk–PI3K/Akt–MAPK pathways.
• Microglial functions: TREM2 promotes phagocytosis, modulates inflammatory tone (often anti-inflammatory but context dependent), and supports proliferation/survival (e.g., via Wnt/β-catenin, ERK). TREM2 deficiency leads to reduced ATP, elevated stress/autophagy and impaired mTOR signaling in plaque-associated microglia.
• Amyloid pathology: Across APPPS1-21 and 5xFAD models, TREM2 deficiency/haploinsufficiency reduces plaque-associated microgliosis and compaction, increases neuritic dystrophy, and alters Aβ burden in an age/burden-dependent manner (e.g., decreased Aβ at 2 months but increased at 8 months in APPPS1-21 with TREM2 KO; increased insoluble Aβ42/plaques in 5xFAD hippocampus at 8.5 months with TREM2 deletion). Human TREM2 BAC expression in 5xFAD reduces soluble/insoluble Aβ42, plaque area, and neuritic dystrophy, and reprograms microglial gene expression toward disease-associated profiles.
• Variant biology: R47H and R62H reduce binding to lipoprotein ligands and decrease signaling; T96K can increase ligand binding/signaling; D87N shows reduced binding but increased signaling—highlighting complex structure–function relationships. R47H impairs TREM2 maturation and shedding; R62H carriers show reduced uptake of Aβ–lipoprotein complexes by monocyte-derived macrophages.
• In vivo impacts of R47H: Humanized or knock-in models show impaired microglial clustering/activation at plaques, more diffuse plaques, and increased neuritic dystrophy; some CRISPR models exhibit reduced Trem2 RNA due to a cryptic splice site (mouse-specific), cautioning interpretation.
• Tau pathology: sTREM2 correlates with CSF total and pTau181, not Aβ42. In PS19 mice, complete TREM2 loss reduces neuroinflammation and neurodegeneration without altering tau phosphorylation/insolubility, whereas TREM2 haploinsufficiency can exacerbate tau pathology and atrophy at late stages. In hTau mice, TREM2 deletion exacerbates tau phosphorylation/insolubility. Overall, TREM2 effects on tau are model- and stage-dependent.
• TREM2–ApoE pathway: ApoE is a TREM2 ligand; ApoE and TREM2 act in the same pathway for microglial responses to plaques; TREM2/ApoE drive microglial transition from homeostatic to MGnD states, influencing neurodegeneration.
• Therapeutics: Targeting TREM2 (e.g., activating antibodies, expression modulation) may be beneficial early, but timing and safety are critical due to biphasic effects and expression outside microglia.

The compiled evidence indicates that TREM2 functions as a lipid- and damage-associated receptor that sustains microglial responses to amyloid and potentially tau-related insults. TREM2’s ability to promote microglial clustering at plaques, compact amyloid, and maintain metabolic fitness connects its signaling to neuroprotection during the amyloid-deposition phase. Conversely, in later or tau-dominant stages, partial TREM2 function can contribute to neuroinflammation and injury, while complete loss can blunt harmful microglial activation and reduce neurodegeneration in some models. Rare AD-associated TREM2 variants, especially R47H, confer partial loss-of-function, diminishing microglial plaque engagement and leading to diffuse plaques and neuritic dystrophy, aligning with increased clinical risk and CSF tau alterations. The TREM2–ApoE axis emerges as a key regulator of the microglial transcriptional switch to a neurodegeneration-associated phenotype (MGnD), positioning TREM2 both upstream and/or downstream of ApoE in a feedback loop critical to AD progression. These insights support therapeutic strategies that enhance TREM2 early to bolster beneficial microglial functions, while cautioning that later-stage or tau-driven disease may require different modulation to avoid exacerbating injury.

TREM2 plays multifaceted, stage-dependent roles in AD pathogenesis. It promotes microglial phagocytosis, clustering, and plaque compaction during amyloid deposition, and its rare variants confer partial loss-of-function linked to increased AD risk, CSF tau changes, and impaired microglial responses. TREM2 also intersects with tau pathology and, together with ApoE, drives a microglial shift to a neurodegeneration-associated state. While TREM2 is a promising therapeutic target, optimal timing, modality (activation vs inhibition), disease stage, and off-target effects must be carefully defined. Future work should clarify the precise mechanisms by which human TREM2 variants alter signaling and microglial phenotypes in vivo, delineate TREM2’s roles across amyloid and tau phases, and map the directionality within the TREM2–ApoE pathway to inform safe and effective interventions.

• Heterogeneity across models: Effects of TREM2 on Aβ burden and tau pathology vary by mouse model, brain region, age, and disease burden, limiting generalizability.
• In vitro vs in vivo discrepancies: Ligand interactions (e.g., Aβ binding) shown in vitro require further in vivo confirmation and may depend on co-factors such as ApoE.
• Variant modeling caveats: Some CRISPR knock-in models activate mouse-specific cryptic splice sites (e.g., R47H), causing Trem2 haploinsufficiency not reflective of human biology.
• Stage dependence: Biphasic effects complicate interpretation and therapeutic translation; timing of TREM2 modulation is critical.
• Population frequency: AD-risk TREM2 variants are rare (<1%), raising questions about therapeutic benefit in non-carriers.
• Review nature: As a narrative review, no systematic methodology was applied; potential publication bias and incomplete coverage of literature are possible.