TREM2 (Triggering Receptor Expressed on Myeloid Cells 2) is a cell surface receptor primarily expressed on microglia in the central nervous system. While extensively validated in Alzheimer's Disease, TREM2's role in Amyotrophic Lateral Sclerosis (ALS) represents a promising yet under-explored therapeutic target. This mechanism page explores the biological rationale for targeting TREM2 in ALS and identifies research gaps.
¶ TREM2 Biology and Microglial Function
TREM2 is a transmembrane receptor belonging to the immunoglobulin superfamily, expressed predominantly on microglia and peripheral macrophages. Its ligand binding triggers signaling through the adaptor protein TYROBP (DAP12), leading to activation of downstream pathways including SYK, PI3K, and MAPK cascades.
Key microglial functions mediated by TREM2:
- Phagocytosis: TREM2 signaling enhances clearance of debris, apoptotic cells, and protein aggregates
- Cytokine regulation: Modulates production of inflammatory cytokines including IL-6, TNF-α, and IL-1β
- Metabolic adaptation: Supports microglial lipid metabolism and mitochondrial function
- Cell survival: Provides pro-survival signaling through AKT activation
¶ ALS Pathophysiology and Microglial Involvement
ALS is characterized by cytoplasmic inclusions of TDP-43 protein in motor neurons and glial cells. TDP-43 pathology is present in >95% of ALS cases and is a hallmark of the disease. Unlike Alzheimer's Disease which features amyloid-beta and tau aggregates, ALS involves TDP-43 mislocalization and aggregation.
Post-mortem studies and animal models demonstrate robust microglial activation in ALS:
- Progressive activation: Microglial activation increases throughout disease progression
- Spatial correlation: Activated microglia surround motor neurons and correlate with neuronal loss
- Diverse phenotypes: Both pro-inflammatory (M1) and neuroprotective (M2-like) microglial states are observed
- TREM2 expression: Microglial TREM2 expression is altered in ALS, though the pattern differs from AD
The C9orf72 gene (most common ALS genetic cause, accounting for ~40% of familial cases) regulates lysosomal function, potentially linking to TREM2-mediated phagocytosis. C9orf72 knockout mice show impaired microglial clearance of protein aggregates, suggesting that enhancing TREM2 signaling could compensate for this deficit.
- TREM2 variants: While TREM2 R47H is strongly associated with AD risk, evidence for ALS is weaker
- TYROBP: Genetic variants in the TREM2 signaling adaptor show nominal association with ALS
- Gene expression: Single-cell RNA-seq studies show altered TREM2 expression in ALS microglia
The TREM2 signaling adaptor TYROBP (also known as DAP12) has shown stronger genetic associations with ALS than TREM2 itself. TYROBP variants may alter microglial signaling capacity, affecting phagocytosis and inflammatory responses. This suggests that even without direct TREM2 variants, the TREM2-TYROBP pathway could be therapeutically modulated in ALS.
| Feature |
Alzheimer's Disease |
ALS |
| Primary proteinopathy |
Amyloid-β, Tau |
TDP-43 |
| TREM2 validation |
Strong (genetic, therapeutic) |
Emerging |
| Microglial role |
Central to progression |
Significant but understudied |
| Therapeutic attempts |
Anti-amyloid + anti-TREM2 |
Limited |
¶ TREM2 and Neuroinflammation
The role of TREM2 in ALS-associated neuroinflammation is complex and context-dependent. In early disease stages, TREM2 activation may promote beneficial microglial phenotypes that support neuronal survival through enhanced phagocytosis of debris and protein aggregates. However, chronic TREM2 signaling can contribute to a pro-inflammatory environment that accelerates motor neuron damage.
Key observations from recent research:
- Stage-dependent effects: TREM2 expression patterns differ between early and late-stage ALS
- Microglial heterogeneity: Single-cell studies reveal diverse microglial states in ALS, with TREM2+ microglia showing distinct transcriptional profiles
- Therapeutic window: Timing of TREM2 modulation appears critical — early intervention may be more beneficial
¶ TREM2 Polymorphisms and ALS Risk
While TREM2 genetic variants (particularly R47H) are strongly associated with Alzheimer's disease risk, the relationship between TREM2 variants and ALS is less clear:
- Some studies suggest weak associations between TREM2 variants and ALS susceptibility
- The TYROBP (DAP12) adaptor protein shows more consistent genetic links to ALS
- Rare TREM2 variants may modify disease progression rather than onset
- Microglial dysfunction: ALS involves microglial activation that fails to clear pathological protein aggregates
- Cross-disease mechanism: TREM2's role in phagocytosis could help clear TDP-43 inclusions
- Modulation approach: Unlike amyloid-targeted therapies, TREM2 modulation may enhance beneficial microglial functions
- Combination potential: Could synergize with antisense oligonucleotides targeting SOD1 or C9orf72
- TREM2 agonistic antibodies: Enhance microglial phagocytosis and metabolic function
- Small molecule TREM2 activators: Oral compounds targeting the TREM2 pathway
- TYROBP/DAP12 modulators: Target the downstream signaling adaptor
- Gene therapy: Viral vector delivery of functional TREM2
- Timing: Optimal intervention window (presymptomatic vs. symptomatic) unclear
- Biomarker need: No validated biomarkers for TREM2 pathway activity in ALS patients
- Safety concerns: Over-activation could exacerbate neuroinflammation
- Patient selection: Which ALS subtypes would benefit most from TREM2-targeted therapy
¶ Preclinical Models and Therapeutic Development
Several preclinical models have been developed to study TREM2 in ALS:
- SOD1^G93A mice: TREM2 expression is upregulated in symptomatic mice; Trem2 knockout accelerates disease
- C9orf72 models: TREM2-mediated phagocytosis may be impaired due to lysosomal dysfunction
- TREM2 knockout: Conflicting results — some studies show accelerated disease, others show minimal effect
Multiple pharmaceutical companies are developing TREM2-activating antibodies:
- AL002 (Alector/AbbVie): Fc-engineered TREM2 antibody in clinical trials for AD; potential for ALS translation
- AL003 (Alector): Alternative TREM2-targeting approach
- BT-2111 (Biogen): TREM2 modulator in development
- Polyhydroxyfullerenes: Shown to activate TREM2 signaling in vitro
- TYROBP agonists: Target the downstream adaptor protein
- Lipid-based agonists: Target TREM2's lipid-binding pocket
No validated biomarkers exist for TREM2 pathway activity in ALS, representing a major gap:
- CSF sTREM2: Measurable but not validated for therapeutic monitoring in ALS
- PET ligands: In development for imaging microglial activation
- Blood biomarkers: Under investigation
¶ Clinical Trials and Future Directions
¶ Current Clinical Landscape
No TREM2-targeted therapies are currently in ALS clinical trials. However:
- Alzheimer's disease trials (AL002, others) will provide proof-of-concept
- Lessons learned from AD trials can inform ALS trial design
- Biomarker development in AD may translate to ALS
Given ALS complexity, TREM2-targeted therapy may be most effective as part of combination approaches:
- With antisense oligonucleotides: Target SOD1, C9orf72, or FUS
- With anti-inflammatory agents: Modulate neuroinflammation
- With neurotrophic factors: Support neuronal survival
flowchart TD
A["TDP-43 / SOD1 Aggregates"] --> B["TREM2 Receptor Activation"]
B --> C["TYROBP (DAP12) ITAM Phosphorylation"]
C --> D["SYK Kinase Activation"]
D --> E["PI3K/AKT Pathway"]
D --> F["MAPK/ERK Pathway"]
D --> G["NF-κB Pathway"]
E --> H["Microglial Survival<br/>Metabolic Support"]
F --> I["Proliferation<br/>Cytokine Expression"]
G --> J["Inflammatory Gene<br/>Transcription"]
H --> K["Enhanced Phagocytosis"]
K --> L["TDP-43 Aggregate Clearance"]
L --> M["Motor Neuron Protection"]
I --> N["Pro-inflammatory Response"]
N --> O["Potential Neuronal Damage"]
style A fill:#e1f5fe,stroke:#333
style M fill:#c8e6c9,stroke:#333
style O fill:#ffcdd2,stroke:#333
| Pathway |
Effect in ALS Microglia |
Therapeutic Angle |
| PI3K/AKT |
Metabolic support, survival signaling |
Agonist enhancement |
| MAPK/ERK |
Proliferation, cytokine expression |
Context-dependent |
| NF-κB |
Inflammatory gene transcription |
Modulation |
| SYK |
Actin remodeling, phagocytosis |
Agonist enhancement |
TREM2 represents one of the few molecular targets with evidence of relevance across multiple neurodegenerative diseases:
Multiple approaches to TREM2 modulation are in development across diseases:
Antibody-Based Approaches:
- AL002 (Alector/AbbVie): Fc-engineered TREM2 agonist, Phase 2 for AD
- AL003 (Alector): Alternative antibody approach
- BT-2111 (Biogen): TREM2 modulator
Small Molecule Approaches:
- TREM2 agonists targeting lipid-binding domain
- TYROBP signaling pathway modulators
- SYK inhibitors for pathway modulation
¶ Research Gaps and Future Directions
¶ Critical Questions Remaining
- Does TREM2 expression correlate with ALS disease severity and progression?
- Are TREM2 genetic variants modifiers of ALS phenotype?
- Can TREM2 agonism enhance clearance of TDP-43 aggregates in vivo?
- What is the optimal therapeutic window for intervention?
- Are there biomarkers to predict TREM2 therapy response in ALS patients?
- Single-cell RNA-seq analysis of TREM2+ microglia in ALS post-mortem tissue
- Development of TREM2 PET ligands for patient stratification
- Preclinical studies of TREM2-modulating compounds in ALS mouse models
- Analysis of TREM2 expression in patient-derived iPSC microglia
¶ Summary and Key Takeaways
- Biological Rationale: TREM2-mediated microglial phagocytosis could enhance clearance of TDP-43 aggregates in ALS
- Genetic Evidence: Weaker than AD but TYROBP associations suggest pathway relevance
- Preclinical Data: Limited but supports altered TREM2 responses in ALS models
- Therapeutic Challenge: No ALS-specific TREM2 trials; depends on AD trial results
- Cross-Disease Potential: Shared TDP-43 pathology with FTD provides additional mechanistic justification
- Basic Science: Characterize TREM2+ microglia in ALS post-mortem tissue using single-cell approaches
- Biomarker Development: Establish CSF sTREM2 as pathway activity marker in ALS
- Preclinical: Test TREM2 modulators in ALS animal models
- Clinical: Prepare for ALS trials contingent on AD proof-of-concept
- Patient Selection: Develop criteria for identifying patients most likely to respond