Hypothesis

Damaged mitochondria release mtDNA not only into the cytosol of the originating cell but also packaged into extracellular vesicles (EVs) that act as paracrine danger signals. These mtDNA‑laden EVs are taken up by distal stromal and immune cells, where they activate cGAS‑STING‑IFN‑I signaling in a cell‑non‑autonomous manner, amplifying systemic inflammaging independent of local mtDNA mutation load.

Rationale

Recent work shows that cytosolic mtDNA triggers cGAS‑STING‑IFN‑I, leading to NRF2 repression, SASP, and hematopoietic reprogramming [2, 3]. However, the source of circulating mtDNA in aging remains unclear. Mitochondria are known to shed vesicles (mitovesicles) under stress, and EVs can transport nucleic acids, proteins, and lipids [4]. If mtDNA within EVs retains its immunostimulatory capacity, it could explain why IFN‑I signatures are elevated in tissues with low mitochondrial mutation burden (e.g., brain parenchyma) and why systemic IFN‑I blockade rescues phenotypes in POLG mutators despite persistent mtDNA damage [2].

Novel Mechanistic Insight

We propose that EV‑associated mtDNA is protected from cytosolic nucleases by vesicular membranes, allowing it to reach distant cells intact. Upon endosomal escape or vesicle fusion, mtDNA is released into the recipient cell cytosol, where it binds cGAS. This triggers a feed‑forward loop: IFN‑I signaling upregulates Rab GTPases and SNAREs that increase EV biogenesis, thereby amplifying the signal. Importantly, this pathway bypasses the need for mitophagy failure in the recipient cell, linking mitochondrial stress in one tissue to inflammaging elsewhere.

Testable Predictions

1. EV mtDNA enrichment: Plasma EVs from aged mice (or humans) will contain higher mtDNA copy number relative to nuclear DNA compared with young controls.
2. Cell‑non‑autonomous activation: Transfer of EVs from aged donor mice to young recipient mice will induce cGAS‑STING‑IFN‑I signaling in recipient tissues (e.g., spleen, liver) without increasing recipient mtDNA mutations.
3. Functional blockade: Pharmacological inhibition of EV release (e.g., GW4869) or neutralization of EV‑associated mtDNA (using antisense oligos packaged in EVs) will reduce circulating IFN‑I signatures and ameliorate age‑related phenotypes in POLG mutator mice.
4. Rescue by cGAS knockout: Recipient‑specific cGAS deletion will abolish EV‑induced IFN‑I signaling, confirming that the EV cargo acts via cGAS‑STING.

Experimental Approach

• Isolation and characterization: Ultracentrifuge plasma from young (3 mo) and aged (24 mo) POLG mutor and wild‑type mice. Quantify mtDNA/nDNA ratio in EV fractions by qPCR.
• EV labeling and transfer: Label EVs with fluorescent lipophilic dye (PKH26) and inject intravenously into young recipients. Track EV uptake in tissues via confocal microscopy and assess cGAS activation (phospho‑TBK1, IRF3) and IFN‑β mRNA at 6 h, 24 h, and 72 h.
• Functional readouts: Measure serum IFN‑α/β, SASP cytokines (IL‑6, CCL2), and markers of hematopoietic skewing (Flow cytometry for Ly6C^hi monocytes, CD8^+ T‑cell exhaustion).
• Intervention groups: Treat aged donors with GW4869 or administer EV‑targeted antisense oligos against mtDNA before EV isolation; assess whether transferred EVs lose immunostimulatory capacity.
• Genetic validation: Generate bone‑marrow chimeras where recipient hematopoietic cells lack cGAS (cGAS^−/−) and repeat EV transfer experiments.

Potential Outcomes and Interpretation

• If predictions hold: Demonstrates a transmissible, vesicle‑mediated mtDNA danger signal that drives systemic IFN‑I signaling, positioning EVs as a therapeutic target upstream of cGAS‑STING.
• If EV mtDNA does not elevate with age or fails to activate cGAS in recipients: Suggests alternative sources (e.g., passive release from necrotic cells) dominate circulating mtDNA, redirecting focus to clearance mechanisms.
• If EV blockade reduces IFN‑I but does not improve longevity: Indicates that while EV‑mediated signaling contributes to inflammaging, additional parallel pathways sustain aging phenotypes.

This hypothesis directly extends the mtDNA‑cGAS‑STING‑IFN‑I axis by adding an intercellular dimension, offers clear falsifiable experiments, and could reshape strategies aimed at neutralizing mitochondrial‑derived innate immune activation in aging.