The aging brain exhibits progressive synaptic loss that correlates with cognitive decline, yet the trigger for this loss remains debated. While developmental synaptic pruning is tightly coupled to neural activity, aging‑associated pruning appears to lose this gating, leading to indiscriminate elimination of synapses. We hypothesize that accumulated mitochondrial DNA (mtDNA) damage in neurons initiates a chronic innate immune response in neighboring microglia via the cytosolic DNA sensor cGAS and its adaptor STING, resulting in sustained production of type I interferons and downstream up‑regulation of complement component C1q. This microglial state tags synapses for removal irrespective of their activity levels, effectively converting a damage signal into a synaptotoxic program.

Mechanistic reasoning: Neurons with compromised mtDNA release oxidized mtDNA fragments into the extracellular space through stress‑induced mitochondrial efflux or vesicle shedding. Microglia express cGAS, which binds cytosolic DNA and activates STING‑TBK1‑IRF3 signaling, driving IFN‑β transcription. Chronic IFN‑β exposure primes microglia to adopt a phenotype characterized by elevated C1q expression and phagocytic propensity, as demonstrated in models of neurodegeneration where IFN signaling exacerbates synaptic loss. Importantly, this pathway is independent of neuronal firing patterns, providing a mechanistic basis for the observed activity‑independent pruning in aged tissue.

Testable predictions:

1. Genetic inhibition of cGAS or STING specifically in microglia (e.g., using Cx3cr1‑CreERT2; cGAS^fl/fl mice) will prevent age‑related increases in hippocampal C1q immunoreactivity and preserve synaptic density markers (synaptophysin, PSD‑95) without altering neuronal mtDNA damage levels.
2. Pharmacological blockade of the STING antagonist H‑151 administered intracerebroventricularly in aged wild‑type mice will rescue activity‑dependent synaptic plasticity measured by LTP in acute slices, while leaving baseline neuronal activity unchanged.
3. Exposure of primary microglia to purified oxidized mtDNA will induce C1q expression via a cGAS‑STING‑dependent cascade, an effect abrogated by IFN‑β neutralizing antibodies or microglial IFNAR knockdown.
4. Longitudinal in vivo two‑photon imaging of Thy1‑YFP mice crossed with microglial reporters will show that synapses adjacent to neurons bearing mtDNA damage (detected via mito‑ROS sensors) are eliminated at a higher rate than distal synapses, and this selectivity disappears in microglia‑specific STING knockouts.

Falsifiable outcomes: If microglial cGAS/STING deletion fails to attenuate synaptic loss or if exogenous mtDNA does not elevate microglial C1q, the hypothesis would be refuted, suggesting alternative drivers such as nuclear DNA‑damage‑induced SASP or microglial senescence. Conversely, confirmation would position the cGAS‑STING‑IFN‑C1q axis as a causal link between mitochondrial genome instability and the maladaptive synaptic pruning that underlies age‑related cognitive decline, offering a target for interventions aimed at preserving synaptic integrity without attempting to "reverse" a supposedly efficient pruning process.