Neuroinflammation

Category: disease_cascade

Overview

Microglia are the resident innate immune cells of the CNS, originating from yolk-sac erythromyeloid progenitors during embryogenesis. Step 1: homeostatic state — surveillance of synaptic + parenchymal space; phagocytosis of debris + apoptotic cells; complement-tagged synaptic pruning (developmental and adult). Step 2: activation trigger — pathogen exposure, sterile injury, protein aggregates (Aβ, α-synuclein, mutant huntingtin), DAMPs (HMGB1, ATP, mitochondrial DNA). Step 3: priming — chronic low-grade stimulation lowers the activation threshold without producing full activation; primed microglia hyper-respond to subsequent triggers ("microglial reactivity hypersensitivity"). Step 4: activation phenotypes — historically M1 (pro-inflammatory: TNF-α, IL-1β, IL-6, NO, ROS) vs M2 (anti-inflammatory: IL-10, TGF-β, arginase, debris clearance); modern view recognizes continuum + disease-associated microglia (DAM) signature (Trem2-high). Step 5: chronic activation pathology — sustained pro-inflammatory output → bystander neuronal damage; complement-mediated synaptic pruning excess → cognitive decline (Alzheimer, schizophrenia, lupus-CNS); NLRP3 inflammasome → IL-1β + pyroptosis. Step 6: cross-talk — astrocyte reactivity (A1 neurotoxic vs A2 reparative); blood-brain barrier disruption recruits peripheral myeloid cells; chronic neuroinflammation contributes to AD, PD, MS, traumatic brain injury, depression. Therapeutic targets: NLRP3 (canakinumab, MCC950), microglial CSF1R (pexidartinib — preclinical CNS), TREM2 agonists (early clinical), classical anti-inflammatories (corticosteroids, minocycline — modest CNS penetration), psychobiotics + omega-3 + curcumin (chronic anti-inflammatory). Cross-links: tlr innate signaling (TLR4 + microglia), cgas sting type1 ifn (cytosolic DNA → IFN), nlrp3 inflammasome (canonical microglial inflammasome), alzheimer amyloid tau cascade, parkinson alpha synuclein aggregation.

Organ Systems

Pathway Steps

  1. homeostatic microglia (P2RY12+ TMEM119+) → surveillance + synaptic pruning + debris clearance — via process motility scanning the parenchyma; complement-tagged spine pruning. In the healthy CNS, microglia (marked by P2RY12, TMEM119) are not resting but actively surveilling — extending processes to monitor synapses, prune them, and clear debris. This homeostatic role makes them essential housekeepers; their dysfunction, not just their activation, contributes to disease.
  2. DAMPs / PAMPs / protein aggregates → TLR + RAGE + NLRP3 + cGAS engagement — via multiple PRRs converge on NF-κB + inflammasome priming. Microglia sense danger through pattern-recognition receptors: TLRs and RAGE detect DAMPs/PAMPs and protein aggregates (Aβ, α-synuclein), while NLRP3 and cGAS-STING detect crystalline and nucleic-acid danger. This repertoire lets them respond to both infection and the misfolded proteins of neurodegeneration.
  3. chronic low-grade stimulation → primed microglia (lowered activation threshold) — via epigenetic + metabolic reprogramming; hyper-respond to subsequent stimulus. Repeated or chronic low-grade stimulation “primes” microglia — leaving them sensitized with a lowered threshold, so a later trigger provokes an exaggerated response. Priming (by aging, prior infection, or ongoing pathology) links systemic inflammation to amplified CNS responses and to delayed neurodegeneration.
  4. full activation trigger → pro-inflammatory secretion (TNF-α, IL-1β, IL-6, NO, ROS) — via classical "M1" phenotype; bystander neuronal damage. On full activation, microglia secrete pro-inflammatory mediators — TNF-α, IL-1β, IL-6, nitric oxide, and ROS. Acutely these aid defense, but sustained release is neurotoxic, damaging neurons and reinforcing inflammation. The balance of protective versus harmful output is context- and duration-dependent.
  5. NLRP3 inflammasome assembly → caspase-1 → IL-1β maturation + GSDMD pyroptosis — via cross-link to pyroptosis gasdermin — cholesterol/Aβ/α-syn crystals are triggers. A central amplifier is the NLRP3 inflammasome: assembly activates caspase-1, which matures IL-1β and cleaves gasdermin D for pyroptotic release. This couples microglial sensing to potent IL-1β-driven inflammation and is implicated in Alzheimer’s and Parkinson’s — making NLRP3 a neuro-inflammatory target.
  6. chronic neuroinflammation → synaptic loss + neuronal injury + BBB compromise — via feedback amplification through astrocyte reactivity + peripheral myeloid recruitment. Chronic, unresolved neuroinflammation becomes a driver of pathology: it promotes synaptic loss (via complement-tagged pruning), direct neuronal injury, and blood-brain-barrier breakdown that admits peripheral immune cells. This self-sustaining cycle is now seen as an active contributor to neurodegeneration, not merely a reaction to it.

Known Modulators

References