Copper Overload Hijacks Microglia, Fueling Alzheimer's Inflammation

Alzheimer's disease (AD) is characterized by amyloid-beta (Aβ) plaque accumulation and chronic neuroinflammation driven by microglia, the brain's resident immune cells. While copper (Cu) dyshomeostasis has long been linked to AD, the precise molecular mechanisms by which excess Cu corrupts microglial function have remained unclear. This study, published in Redox Biology, provides a detailed mechanistic account of how sub-lethal copper overload transforms microglia from protective to neurotoxic cells.

Using the SIM-A9 spontaneously immortalized mouse microglial cell line, researchers exposed cells to sublethal doses of copper sulfate for 24 hours. They found that copper preferentially accumulated in mitochondria, where it depleted mitochondrial glutathione (mtGSH) and dramatically elevated reactive oxygen species (ROS). This mitochondrial oxidative stress triggered the cytosolic release of oxidized mitochondrial DNA (ox-mtDNA), a potent damage-associated molecular pattern (DAMP) that activates the NLRP3 inflammasome. The result was robust caspase-1 activation and secretion of mature IL-1β and IL-18—hallmarks of inflammasome-driven neuroinflammation. Critically, depleting mtDNA with ddC or inhibiting the inflammasome with MCC950 blunted this response, confirming ox-mtDNA as the key trigger.

In parallel, copper overload upregulated the sterol regulatory element-binding transcription factor 2 (SREBF2) pathway, increasing cholesterol biosynthesis and its mitochondrial transport via STAR, STARD3, and TSPO. Elevated mitochondrial cholesterol further compromised mtGSH levels, creating a vicious cycle of oxidative stress. Importantly, this cholesterol accumulation downregulated ABCA7, an ATP-binding cassette transporter critical for microglial Aβ phagocytosis. Copper-overloaded microglia showed significantly impaired ability to engulf Aβ oligomers, an effect rescued by cholesterol depletion with HP-β-CD or by restoring mtGSH with GSH ethyl ester (GSHee).

To assess downstream neuronal consequences, conditioned media from copper-primed, Aβ-stimulated microglia was applied to primary cortical-hippocampal neurons. Neuronal viability was markedly reduced compared to media from Aβ-stimulated microglia alone. This neurotoxicity was prevented by pre-treating microglia with MCC950 (NLRP3 inhibitor) or GSHee, directly linking mitochondrial oxidative stress and inflammasome activation to neuronal death. The study also validated key findings in APP-PSEN1 transgenic AD mice and SREBF2 overexpressing mice, strengthening translational relevance.

Collectively, this work maps a coherent pathway: environmental copper → mitochondrial copper accumulation → mtGSH depletion → ox-mtDNA release → NLRP3 inflammasome activation + cholesterol-mediated ABCA7 downregulation → impaired Aβ clearance + neuroinflammation → neurodegeneration. The identification of mtGSH restoration and NLRP3 inhibition as intervention points offers concrete therapeutic directions for AD, particularly in populations with chronic copper exposure.