Neuroinflammation and Microglia Pathway in Alzheimer's Disease

Overview

Neuroinflammation is a central pathological feature of Alzheimer’s disease (AD), involving chronic activation of microglia, astrocytes, and the complement system ^{1Neuroinflammation in Alzheimer's disease2015 · Lancet Neurology · DOI 10.1016/S1474-4422(15)70016-5Open reference}. While neuroinflammation was originally considered a secondary response to amyloid and tau pathology, increasing evidence suggests it plays a primary role in disease progression. This page focuses on microglial-mediated neuroinflammation in AD ^{2TREM2 lipid sensing sustains the microglial response in an Alzheimer's disease model2015 · Cell · DOI 10.1016/j.cell.2015.12.056Open reference}.

Microglia in AD

Normal Microglial Function

Microglia are the resident immune cells of the CNS ^{3Microglia in Alzheimer's disease2018 · Journal of Cell Biology · DOI 10.1083/jcb.201712069Open reference}:

Microglial States

Microglia adopt different activation states in AD ^{4A unique microglia type associated with Alzheimer's disease2017 · Cell · DOI 10.1016/j.cell.2017.07.018Open reference}:

1. Homeostatic: Resting, surveilling

2. Disease-associated microglia (DAM): TREM2-dependent

3. Activated microglia: Pro-inflammatory (M1-like)

4. Alternative activation: Anti-inflammatory (M2-like)

TREM2-Dependent Microglial Response

TREM2 Biology

TREM2 (Triggering Receptor Expressed on Myeloid Cells 2) is a microglial receptor critical for AD:

• Ligands: Aβ oligomers, lipoproteins, apoptotic cells

• Signaling: DAP12 adaptor protein

• Function: Phagocytosis, survival, cytokine production

DAM (Disease-Associated Microglia)

flowchart TD
    A["Early AD"] --> B["Homeostatic microglia"]
    B --> C["TREM2-independent DAM"]
    C --> D["TREM2 activation"]
    D --> E["TREM2-dependent DAM"]

    E --> E1["Phagocytic activation"]
    E --> E2["Lipid metabolism changes"]
    E --> E3["Inflammatory response"]

    E1 --> F["Abeta clearance"]
    E2 --> G["Metabolic reprogramming"]
    E3 --> H["Chronic inflammation"]

    style A fill:#0a1929,stroke:#333
    style E fill:#0e2e10,stroke:#333
    style H fill:#3b1114,stroke:#333

TREM2 Variants

Common AD risk variants in TREM2 ^{5TREM2 variants in Alzheimer's disease2013 · New England Journal of Medicine · DOI 10.1056/NEJMoa1211851Open reference}:

• R47H: Increased AD risk (~3x)

• R62H: Modest risk increase

• D87N: Rare variant

• Y38C: Rare pathogenic

Complement System in AD

Complement Activation

The complement system is highly activated in AD:

Classical pathway:

• C1q binds to Aβ plaques

• Initiates cascade

• Generates C3a, C5a (anaphylatoxins)

Consequences:

• Microglial activation

• Synaptic elimination

• Inflammation amplification

C1q and Synaptic Pruning

C1q marks synapses for elimination:

1. C1q binds to vulnerable synapses

2. C3 activation recruits microglia

3. Complement receptor 3 (CR3) mediates phagocytosis

4. Synaptic loss in early AD

Inflammatory Cytokines in AD

Key Cytokines

Cytokine Signaling Pathways

Aβ plaques → Microglial activation → NF-κB activation → Pro-inflammatory cytokine release → Neuronal dysfunction

Astrocyte Involvement

Reactive Astrocytes

Astrocytes become reactive in AD:

• A1 astrocytes: Neurotoxic, induced by IL-1α, TNF, C1q

• Loss of function: Impaired glutamate uptake, potassium buffering

• Gain of function: Pro-inflammatory cytokine release

Astrocyte-Microglial Interactions

Cross-talk between astrocytes and microglia:

1. Astrocyte-derived cytokines activate microglia

2. Microglial factors modulate astrocyte reactivity

3. Feedback loops amplify inflammation

Neuroinflammation-Tau Interaction

Bidirectional Relationship

Tau pathology and neuroinflammation interact:

1. Inflammation drives tau: Cytokines promote tau phosphorylation

2. Tau drives inflammation: Extracellular tau activates microglia

3. Spread: Inflammation facilitates tau propagation

Microglial Phagocytosis of Tau

• Impaired clearance in AD

• Tau aggregates overwhelm microglia

• Failed degradation leads to inflammation

Therapeutic Approaches

Anti-Inflammatory Strategies

Microglial Modulation

Clinical Trials

Biomarkers

Inflammatory Markers

See Also

• TREM2 pathway

• Complement system

• Neuroinflammation overview

• Alzheimer’s disease

• TREM2 gene