Hypothesis

Aging is propagated not only by soluble SASP but by immune‑cell exosomes that deliver damaged mitochondrial DNA (mtDNA) and oxidative lipids to distant tissues, where they trigger the cGAS‑STING interferon pathway and lock cells into a senescent state.

Mechanistic Rationale

• Senescent immune cells accumulate mitochondrial dysfunction and release exosomes enriched in oxidized mtDNA, cardiolipin, and specific miRNAs (e.g., miR‑34a) that are known to modulate inflammation [1].
• Extracellular mtDNA is a potent agonist of the cytosolic DNA sensor cGAS; its uptake by stromal or parenchymal cells leads to STING‑dependent type I interferon production, which has been shown to reinforce senescence and tissue fibrosis [2].
• Blocking exosome biogenesis with neutral sphingomyelinase 2 (nSMase2) inhibitors reduces the cargo of oxidized mtDNA without affecting overall immune cell numbers, providing a way to test whether the paracrine signal, rather than the immune cell itself, drives aging [3].
• If the hypothesis is correct, lowering circulating exosomal mtDNA should decrease cGAS‑STING activation in liver, kidney, and brain, lower SASP factor levels, and improve functional readouts (grip strength, treadmill endurance, cognitive performance) in aged mice.

Experimental Plan

1. Animal model – Use 20‑month‑old C57BL/6 mice; treat half with GW4869 (nSMase2 inhibitor) administered intraperitoneally twice weekly for 8 weeks; controls receive vehicle.
2. Readouts
   • Plasma exosome isolation; quantify mtDNA copy number by qPCR and oxidized lipids by LC‑MS [4].
   • Measure cGAS‑STING pathway activation (phospho‑TBK1, IRF3, IFN‑β) in isolated hepatocytes, renal tubular cells, and cortical neurons via Western blot and immunofluorescence.
   • Assess tissue senescence (p16^Ink4a^, SA‑β‑gal) and classic SASP proteins (IL‑6, CCL2, SERPINE1) by ELISA and immunohistochemistry.
   • Functional assays: grip strength, rotarod, treadmill exhaustion, and novel object recognition test.
3. Predicted outcome – GW4869‑treated mice will show ≥30 % reduction in exosomal mtDNA, ↓cGAS‑STING signaling, ↓p16^Ink4a^ and SASP levels, and significant improvement in at least two functional assays compared with vehicle.
4. Falsification – If exosome inhibition lowers plasma vesicle count but does not alter cGAS‑STING activation, senescence markers, or functional performance, the hypothesis that immune‑derived exosomal mtDNA drives systemic aging is not supported.

Broader Implications

Positive results would reposition exosome cargo—not just the secreting cell—as a therapeutic target. Combining nSMase2 inhibition with senolytics could attack both the source and the spread of aging signals, potentially accelerating rejuvenation strategies beyond immune cell replacement alone.