TREM2-expressing microglia represent a specialized population of brain immune cells that play a critical role in Alzheimer's disease and other neurodegenerative conditions. These cells express the Triggering Receptor Expressed on Myeloid Cells 2 (TREM2), which is essential for microglial survival, phagocytosis, and inflammatory responses.
TREM2-expressing microglia are a subset of microglia that express the TREM2 receptor on their surface. TREM2 is a cell surface receptor belonging to the immunoglobulin superfamily that is expressed exclusively on myeloid cells including microglia in the brain.
The TREM2 pathway has become one of the most important therapeutic targets in Alzheimer's disease:
- TREM2 variants increase AD risk by 3-4 fold
- TREM2 is required for amyloid plaque clearance
- TREM2 dysfunction leads to neuroinflammation
- TREM2 agonists are in clinical development
¶ Morphology and Markers
- Cell Type: Activated microglia
- Marker Genes: TREM2, CD33, CX3CR1, IBA1 (AIF1)
- Neurotransmitter: Not applicable (immune cells)
- Morphology: Amoeboid shape with multiple branching processes, enlarged soma in disease states
- Location: Throughout brain parenchyma, enriched around amyloid plaques
TREM2-expressing microglia perform critical homeostatic functions:
- Amyloid clearance: TREM2 mediates uptake of amyloid-beta
- Apoptotic cell clearance: Engulfment of dead neurons
- Synaptic pruning: Developmental synapse elimination
- Lipid metabolism: Processing of myelin debris
¶ Survival and Proliferation
- Energy metabolism: TREM2 supports microglial metabolic fitness
- Cell survival: Prevents apoptosis under stress
- Proliferation: Supports microglial expansion in disease
- Cytokine production: Modulates inflammatory mediator release
- NF-κB signaling: TREM2 regulates inflammatory pathways
- Complement activation: Involved in complement cascade
¶ Ligands
- Amyloid-beta: Direct binding to TREM2
- Lipids: Apolipoproteins, especially APOE
- Phosphatidylserine: On apoptotic cells
- Heat shock proteins: Released from stressed cells
- DAP12: TREM2 signals through TYROBP/DAP12 adaptor protein
- PI3K/Akt: Survival signaling
- MAPK pathways: Inflammatory response
- mTOR: Metabolic regulation
TREM2 microglia show disease-specific changes:
- Risk variants: TREM2 R47H, R62H increase AD risk 3-4 fold
- Plaque association: TREM2+ microglia surround amyloid plaques ("disease-associated microglia" or DAM)
- Impaired clearance: TREM2 variants reduce amyloid phagocytosis
- Neuroinflammation: Dysregulated inflammatory response
- Synucleinopathy: TREM2 may affect alpha-synuclein clearance
- Neuroinflammation: TREM2 modulates microglial response
- Genetic interaction: TREM2 variants may modify PD risk
- Motor neuron disease: TREM2 is upregulated in ALS microglia
- Inflammatory response: TREM2 modulates neuroinflammation
- Genetic variants: TREM2 associations with ALS risk
- Rare disorder: TREM2 or TYROBP mutations cause this disease
- Premature dementia: Early-onset frontal lobe dementia
- Bone cysts: Polycystic lipomembranous osteodysplasia
TREM2-expressing microglia transition to a DAM state in neurodegeneration:
- Triggered by: TREM2 activation
- Features: Upregulated phagocytic genes
- Function: Attempted amyloid clearance
- Triggered by: Prolonged disease stress
- Features: Pro-inflammatory phenotype
- Function: May become dysfunctional
- Reduced phagocytosis: Impaired amyloid clearance
- Enhanced inflammation: Excessive cytokine release
- Metabolic stress: Energy depletion
- Neurotoxicity: May contribute to neuronal loss
- Purpose: Enhance microglial function
- Examples: AL002, AZD8329
- Status: Clinical trials in AD
- Purpose: Reduce excessive inflammation
- Potential: May help if overactivated
- Rationale: APOE is a TREM2 ligand
- Approach: APOE-targeting therapies
- TREM2 variants: R47H, R62H increase risk
- Genetic counseling: For at-risk individuals
- sTREM2: Soluble TREM2 in CSF
- Levels: Changes in AD progression
- PET: TSPO shows microglial activation
- MRI: Structural changes associated with inflammation
- TREM2 knockouts: Mouse models for study
- iPSC models: Human microglia from patients
- Single-cell RNAseq: Profiling TREM2+ cells
- CRISPR: Gene editing of TREM2
- Antibodies: TREM2-targeting therapeutics
- Flow cytometry: Isolating TREM2+ cells
The study of Substantia Nigra Pars Compacta In Parkinson'S Disease has evolved significantly over the past decades. Research in this area has revealed important insights into the underlying mechanisms of neurodegeneration and continues to drive therapeutic development.
Historical context and key discoveries in this field have shaped our current understanding and will continue to guide future research directions.
- Guerreiro et al., TREM2 variants in Alzheimer's disease (2013)
- Jonsson et al., TREM2 and Alzheimer's disease (2013)
- Keren-Shaul et al., A unique microglia type associated with AD (2017)
- Ulgen et al., TREM2 in neurodegenerative disease (2020)
- Song et al., TREM2 biology and therapeutic targeting (2023)