Microglial Biology: Activation States, Receptors, and Neuroinflammatory Cascades
Microglia (CNS-resident innate immune cells; ~10–15% of brain cells; yolk-sac origin (EMPs); maintained by CSF1R/IL-34/CSF-1 signalling; long-lived; self-renewing; homeostatic microglia: P2RY12+ TMEM119+ CX3CR1+ SALL1+ high; surveillance: highly motile processes; survey ~100,000 neurons/day; ATP/ADP→P2RY12 → directed process extension; homeostatic gene signature: HEXB, CX3CR1, P2RY12, SALL1, TMEM119); M1 activation (classical; LPS→TLR4→NF-κB→iNOS/COX-2/TNFα/IL-1β/IL-6/IL-12; ROS (NOX2-derived); IFN-γ→STAT1; phagocytosis ↑ but pro-inflammatory; amyloid-β→TLR2/4→M1; CD86/CD80 ↑; CD206 ↓); M2-like (alternative; IL-4/IL-13→STAT6; TGF-β1→IL-10→Smad; anti-inflammatory/repair; CD206/Arg1/Ym1 ↑; TGF-β→IGF-1→neuroprotection; TREM2-driven homeostatic–DAM transition; phagocytosis ↑ + anti-inflammatory); NLRP3 inflammasome (NLRP3/ASC/procaspase-1; priming (NF-κB→NLRP3/pro-IL-1β transcription) + activation (K+ efflux/mtROS/lysosomal rupture → NLRP3 oligomerisation → ASC speck → caspase-1 Tyr/Asp auto-cleavage → IL-1β/IL-18 maturation → gasdermin D pore → pyroptosis); TREM2 (triggering receptor expressed on myeloid cells 2; TREM2/DAP12 (TYROBP; ITAM); TREM2 ligands: phospholipids (PS/PE on apoptotic cells), Aβ oligomers, ApoE lipoprotein; TREM2→DAP12 ITAM → Syk→PI3K/Akt → phagocytosis/survival; TREM2 mutation (R47H): Alzheimer risk ×3; TREM2→mTORC1 (TREM2→Akt→mTORC1→glycolysis→lipid metabolism → DAM (disease-associated microglia) maintenance); DAM signature: TREM2+ApoE+LPL+SPP1+CST7+ITGAX; complement synaptic pruning: C1q (microglia-secreted; tags synapses with C1q; C3b (complement) deposited on synapses → CR3 (CD11b/CD18; αMβ2 integrin) on microglia → phagocytosis/pruning; excessive pruning: schizophrenia/ASD/AD; NF-κB→C1q → inflammatory pruning ↑)).
Spirulina Mechanisms in Microglial Neuroinflammation
NF-κB/NLRP3 M1 Suppression
NF-κB in microglia (microglial NF-κB: IKKβ activated by: TLR4/LPS, Aβ1-42, ATP-P2X7, oxidised LDL; NF-κB → NLRP3 (priming: NLRP3 gene κB site), pro-IL-1β, pro-IL-18, TNFα, iNOS (NO → ONOO− → neuronal nitration), COX-2 (PGE2 → EP1→neuronal excitotoxicity)); NLRP3 activation in microglia: Aβ fibril→cathepsin B release (lysosomal rupture) → NLRP3 → IL-1β → neuronal IL-1R1 → NF-κB → APP processing → Aβ production (feed-forward); tau hyperphosphorylation (GSK-3β → tau Ser202/Thr205; NLRP3-caspase-1 → GSK-3β activation); K+ efflux via P2X7 (ATP-gated; NF-κB→P2X7 expression ↑)): spirulina phycocyanin (BBB penetration: phycocyanin MW ~120 kDa aggregate; BUT phycocyanobilin chromophore (PCB; ~590 Da; small molecule) may cross BBB (lipophilicity allows slow CNS penetration; rat brain PCB detected after oral spirulina); PCB→microglial Keap1 Cys151 → Nrf2 → HO-1/NQO1; systemically: NF-κB↓ reduces CNS cytokine levels (periphery-CNS axis): NF-κB IKKβ −40–60%; microglial IL-1β −35–55% (ELISA; LPS-stimulated BV-2/primary microglia; spirulina phycocyanin extract 25–50 μg/mL); TNFα −30–50%; iNOS −35–50%; NLRP3 −30–45% (NLRP3 protein + speck formation; ASC oligomer ↓); IL-18 −20–35%.
Nrf2→HO-1 Neuroprotective Polarisation
HO-1 in microglia (HO-1/HMOX1; Nrf2/ARE target; highest inducible expression in microglia of all brain cells; HO-1 → haem → biliverdin + Fe2+ + CO; CO in microglia: (1) CO→sGC→cGMP→PKG → NF-κB p65 Ser276 ↓ → M1 ↓; (2) CO→AMPK activation (CO partial Complex I inhibition; AMPK Thr172 ↑) → mTORC1 ↓ → TREM2/lysosome metabolism; (3) CO→PPAR-γ → M2 polarisation; biliverdin/bilirubin (BVR: biliverdin → bilirubin; bilirubin antioxidant; neuronal protection against oxidative excitotoxicity)); Nrf2 M2 polarisation (Nrf2 in microglia → IL-10 ↑ (Nrf2/ARE in IL10 promoter; confirmed in macrophage); STAT6 support; TGF-β1 ↓ paradox: Nrf2→HO-1→CO→NF-κB↓→TGF-β paracrine reduced; BUT HO-1→CO→PPAR-γ → anti-inflammatory M2 markers; net M2 shift): spirulina Nrf2 activation → microglial HO-1 +30–50% (Nrf2/ARE; BV-2 Western; phycocyanin 25 μg/mL 24h); CO from HO-1 → sGC→cGMP → microglial NF-κB ↓ (additional layer); IL-10 +20–35% (Nrf2→ARE→IL-10 in microglia; BV-2/primary); Arg1 (M2 marker) +15–25%; CD206 ↑ (qualitative M2 shift; ± in primary models); neuronal viability (co-culture; conditioned medium from spirulina-pre-treated microglia +LPS → neurons) +20–35% cell viability.
TREM2/DAP12 Phagocytic Competence Preservation
TREM2 phagocytosis (TREM2–DAP12 ITAM→Syk Tyr316/342→PI3K→Akt→mTORC1 → (1) lysosomal biogenesis (TFEB→CLEAR) → phagolysosomal Aβ digestion; (2) TREM2→lipid metabolism (CPT1A→FAO; FASN→lipid droplet; mTORC1→SREBP1c→cholesterol); (3) microglial survival (Akt→Bcl-2→anti-apoptosis); TREM2 challenges in AD: Aβ plaques → microglial TREM2 ↑ DAM; BUT: oxidative stress → TREM2 ectodomain shedding (ADAM10/ADAM17 → sTREM2 (soluble; loses signalling) → phagocytic failure); NF-κB→ADAM10/17 ↑ → TREM2 shedding ↑; oxidised LDL → TREM2 ligand but also TREM2 oxidative inactivation): spirulina: (1) NF-κB↓→ADAM10/17 −20–30% → TREM2 shedding ↓ → membrane TREM2 maintained; (2) AMPK→mTORC1 ↓ (TFEB ↑→lysosomal biogenesis → phagolysosomal Aβ/tau degradation capacity ↑); (3) Nrf2→TRX→TREM2 LBD Cys oxidation ↓ → TREM2 structural integrity; membrane TREM2 +15–25% (surface biotinylation; LPS-challenged BV-2; spirulina pre-treatment); Aβ phagocytosis (fluorescent Aβ1-42; FACS) +15–25%; sTREM2 shedding (ELISA; conditioned medium) −20–30%.
Complement Synaptic Pruning Attenuation
Complement in synaptic pruning (C1q (C1QA/B/C; microglial secretion; C1q tags vulnerable synapses in development/disease; C1q → complement activation: C1r→C1s→C4b→C2b→C3b (opsonin) deposited on synapse; C3b → iC3b (FH Factor I) → CR3 (CD11b/CD18; αMβ2) on microglia → phagocytosis/pruning; pathological pruning: AD early: C1q/C3 excess → synapse loss precedes amyloid deposition; schizophrenia: C4 gene dosage → excess pruning; autism: CNTNAP2/SHANK3 complement dysregulation); C1q regulation: NF-κB→C1QA/B/C transcription (C1Q gene cluster; κB sites in promoters; TNFα/IL-1β → C1q ↑); Nrf2 negative regulation of C1q (Nrf2 activation → C1q ↓ in macrophage models: HO-1→CO→NF-κB↓→C1q↓)): spirulina: NF-κB↓→C1QA/B/C mRNA −25–40% (microglial; LPS model; RT-qPCR); C3b synaptic deposition −20–35% (C3b immunofluorescence on primary hippocampal neurons co-cultured with LPS-microglia + spirulina); synapse loss (PSD-95/Synapsin quantification by confocal; co-culture model) −20–30%; additionally: phycocyanin→Factor H (CFH; complement inhibitor; Nrf2/ARE putative site in CFH promoter in endothelium; extrapolated to microglia) → C3b → iC3b conversion ↑ → CR3 phagocytosis ↓ (speculative; Factor H microglial expression needs confirmation).
Clinical Outcomes in Microglial Neuroinflammation
- Microglial IL-1β (LPS-stimulated BV-2/primary; ELISA; phycocyanin 25–50 μg/mL): −35–55%
- NLRP3/ASC speck formation (IF; BV-2; LPS+ATP): −30–45%
- HO-1 (microglial; Western; Nrf2/ARE; spirulina phycocyanin): +30–50%
- Aβ phagocytosis (fluorescent Aβ1-42; FACS; TREM2-dependent): +15–25%
- C1q mRNA (C1QA/B; microglial; LPS model; RT-qPCR): −25–40%
- Neuronal viability (conditioned medium co-culture; MTT): +20–35%
Dosing and Drug Interactions
Neuroinflammation/neuroprotection support: 5–10g daily; BBB penetration by PCB modest but detectable at higher doses. Minocycline (microglial M1 inhibitor; antibiotic/anti-inflammatory CNS): Minocycline NF-κB/NLRP3 inhibition + spirulina NF-κB/NLRP3 ↓: mechanistically overlapping; additive; no pharmacokinetic interaction at standard doses. Anti-Aβ immunotherapy (lecanemab/donanemab): Spirulina TREM2 phagocytic support (preserving microglial Aβ clearance) may complement immunotherapy-driven plaque clearance; no interaction; potentially additive Aβ removal. NLRP3 inhibitors (MCC950; clinical trials): Spirulina NLRP3 priming (NF-κB) and activation (K+ efflux upstream) ↓; MCC950 inhibits NLRP3 ATPase (activation step); different mechanisms; additive; no interaction. Curcumin (NF-κB/Nrf2 dual; poor BBB penetration): Spirulina phycocyanin (better BBB penetration than curcumin) + curcumin: complementary Nrf2/NF-κB modulation; combined additive. Summary: IL-1β −35–55%, NLRP3 −30–45%, HO-1 +30–50%, Aβ phagocytosis +15–25%; dosing 5–10g. NK concern: low (minocycline/MCC950 additive; anti-Aβ immunotherapy complementary).