The aging brain actively removes metabolically inefficient neurons through an interferon‑complement axis, but the consequences of this eviction extend beyond simple loss. We hypothesize that neurons earmarked for elimination release mitochondrial DNA (mtDNA) into the extracellular space during phagocytosis by interferon‑responsive microglia (IRM). This mtDNA activates the cytosolic cGAS‑STING pathway in neighboring microglia and astrocytes, amplifying type I interferon (IFN‑I) production and thereby increasing the threshold for further neuronal eviction. In young brains, efficient lysosomal degradation and astrocytic mtDNA scavenging keep this loop in check. With age, microglial accumulation of myelin debris impairs lysosomal function, reducing mtDNA clearance and lowering the activation threshold of cGAS‑STING. Consequently, a feed‑forward cycle emerges: IFN‑I signaling drives neuronal eviction, evicted neurons release mtDNA, mtDNA fuels more IFN‑I via cGAS‑STING, and microglial metabolic exhaustion sustains the loop, accelerating cognitive decline.

Key predictions that make the hypothesis testable and falsifiable:

1. In aged mice, CSF and hippocampal extracellular fluid will show elevated mtDNA fragments that correlate with IFN‑Istimulated gene (ISG) expression in microglia; neutralizing extracellular mtDNA with DNase I or blocking cGAS with small‑molecule inhibitors (e.g., RU.521) will reduce ISG signatures and preserve neuronal numbers despite aged microglia.
2. Microglia‑specific knockout of STING (Tmem173) in aged mice will break the feed‑forward loop: IFN‑I levels will rise initially after neuronal eviction but will not be sustained, leading to less complementary tagging (C1q/C3) and attenuated neuronal loss compared with wild‑type controls.
3. Enhancing astrocytic mtDNA uptake via overexpression of the mtDNA transporter SLC25A26 will mitigate the loop, resulting in lower hippocampal mtDNA burden, reduced IFN‑I signaling, and rescued performance in spatial memory tests (Morris water maze) in aged mice.
4. In human post‑mortem tissue, individuals with high hippocampal IFN‑I/ISG signatures will also show increased extracellular mtDNA (measured by qPCR for mitochondrial genes in tissue extracts) and heightened cGAS‑STING activation (phospho‑STING immunostaining) specifically in microglia adjacent to evicted neurons.

Experimental approach: Use CX3CR1‑CreER;Ifnar1fl/fl mice to confirm that microglial IFN‑I signaling is necessary for mtDNA release, then cross with Stingfl/fl or treat with cGAS inhibitor. Measure mtDNA in CSF by digital droplet PCR, quantify ISG expression via RNAscope, assess synaptic tagging via immunofluorescence for C1q/C3, and track neuronal survival with NeuN staining. Cognitive outcomes will be evaluated at 3, 6, and 12 months post‑treatment.

If extracellular mtDNA proves to be a critical amplifier of IFN‑I driven neuronal eviction, therapeutically targeting the mtDNA‑cGAS‑STING axis could uncouple the beneficial quality‑control function of IFN‑I from its deleterious, self‑propagating form in the aging brain.