The Copper-Gut-Brain Axis: A Triple Inflammatory Pathway Driving Neuroinflammation in Alzheimer's Disease.

Serum copper increases progressively with normal aging, yet its downstream consequences for the gut microbiome and neuroinflammation remain unexplored. Gut microbiota dysbiosis and elevated lipopolysaccharide levels are established features of Alzheimer's disease, and growing evidence indicates that this dysbiosis drives neuroinflammatory disease progression. Yet the upstream trigger initiating this dysbiosis remains unknown. We propose that age-related copper dyshomeostasis serves as this missing trigger. The redox-active copper content of ceruloplasmin increases across the adult lifespan, and copper is selectively toxic to anaerobic bacteria, preferentially affecting butyrate-producing genera including Faecalibacterium, Roseburia, and Coprococcus while sparing copper-resistant species. This selective toxicity is supported by animal studies demonstrating copper-induced elimination of butyrate producers with reversible gut barrier damage and by Wilson's disease cohorts showing consistent depletion of butyrate-producing genera due to elevated copper levels. The resulting dysbiosis creates a triple inflammatory pathway: butyrate loss compromises gut barrier integrity and removes histone deacetylase-mediated suppression of neuroinflammation; the increase of Gram-negative bacteria elevates lipopolysaccharide translocation through the compromised barrier; and impaired blood-brain barrier integrity reduces amyloid-β clearance. These three insults trigger microglial activation through NF-κB signaling, creating a 'triple hit' on a single transcription factor that may explain the magnitude of neuroinflammatory effects observed in Alzheimer's disease. This mechanism explains the increased acetate/butyrate ratio recently identified as a biomarker distinguishing Alzheimer's-related from non-Alzheimer's cognitive impairment (AUC 0.951), since copper disrupts microbial metabolic cross-feeding networks that convert acetate to butyrate. We present specific, falsifiable predictions that can be tested in human cohorts and propose copper as a novel upstream therapeutic target for Alzheimer's disease prevention.