Mitochondrial RNA Leaking Into Cells Drives Senescence Inflammation
Senescent cells leak mitochondrial RNA into the cytosol, triggering an antiviral response that fuels the inflammatory SASP and age-related disease.
Summary
Researchers at Mayo Clinic discovered that senescent cells accumulate mitochondrial RNA (mtRNA) in the cytosol, activating RNA sensors RIG-I and MDA5, which aggregate the MAVS protein and drive the senescence-associated secretory phenotype (SASP). This cytosolic mtRNA leakage depends on pro-apoptotic proteins BAX and BAK. Blocking these sensors or deleting BAX/BAK significantly reduced SASP factors both in cell culture and in a mouse model of metabolic liver disease (MASH). These findings reveal a previously underappreciated mitochondria-to-cytosol RNA signaling axis that promotes chronic inflammation in aging tissues.
Detailed Summary
Cellular senescence — the state in which stressed cells permanently stop dividing — is increasingly recognized as a major driver of aging and age-related disease. Senescent cells secrete a cocktail of inflammatory proteins called the SASP (senescence-associated secretory phenotype), which damages surrounding tissue. While mitochondrial DNA leakage into the cytosol was already known to activate the cGAS/STING inflammatory pathway, this study reveals an entirely parallel mechanism: mitochondrial RNA (mtRNA) also leaks out and triggers a distinct antiviral signaling cascade.
Using super-resolution Airyscan confocal microscopy and subcellular fractionation, the Mayo Clinic team showed that senescent human fibroblasts (induced by radiation, replicative exhaustion, or chemotherapy drugs) accumulate significantly more cytosolic double-stranded mtRNA compared to proliferating cells. Specific mitochondrial transcripts — MT-ND5, MT-ND6, MT-CYB, and MT-COI — were elevated in the cytosol. This leakage coincided with upregulated expression of RNA pattern-recognition receptors RIG-I, MDA5, and TLR3, both at mRNA and protein levels. Consistent findings were replicated in IMR90 fibroblasts and in aged mouse tissues (kidney, heart, liver, spleen), where RNA sensor expression correlated positively with senescence markers p16 and p21 and with SASP factor expression.
To confirm causality, the researchers transfected proliferating fibroblasts with isolated mtRNA and observed a significant induction of SASP factors and RNA sensors — demonstrating that cytosolic mtRNA is itself sufficient to trigger inflammation. Conversely, using cells stripped of mitochondria via Parkin-mediated mitophagy, they confirmed that SASP was abolished. Knockdown or inhibition of RIG-I, MDA5, or their downstream adapter MAVS (which forms prion-like aggregates upon activation) substantially reduced SASP secretion without eliminating the cell cycle arrest, suggesting these are selective, therapeutically tractable targets.
The mechanism of mtRNA release was traced to BAX and BAK, pro-apoptotic proteins that form pores in the mitochondrial outer membrane during sublethal (non-lethal) apoptosis. Deletion of BAX and BAK reduced cytosolic mtRNA, RNA sensor activation, and SASP expression. In a mouse model of Metabolic Dysfunction-Associated Steatohepatitis (MASH) — a disease characterized by liver senescence and inflammation — genetic deletion of BAX/BAK or MAVS reduced SASP factors and improved liver function parameters.
These findings establish a new mtRNA-RIG-I/MDA5-MAVS signaling axis as a key driver of senescence-associated inflammation and identify multiple intervention points for suppressing the SASP in aging and metabolic disease. The distinction from cGAS/STING means that both DNA and RNA mitochondrial alarmins contribute independently, expanding the toolkit for anti-senescence therapy.
Key Findings
- Senescent cells accumulate cytosolic mitochondrial dsRNA, detected by super-resolution microscopy and subcellular fractionation.
- Cytosolic mtRNA activates RNA sensors RIG-I, MDA5, and TLR3, driving MAVS aggregation and SASP induction.
- Knocking down RIG-I, MDA5, or MAVS substantially reduces SASP without eliminating cell cycle arrest.
- Pro-apoptotic proteins BAX and BAK mediate mtRNA leakage; their deletion diminishes SASP in vitro and in a MASH mouse model.
- RNA sensor expression increases with age in multiple mouse tissues and correlates with senescence markers and SASP factors.
Methodology
Human fibroblasts (MRC5, IMR90) were rendered senescent by ionizing radiation, replicative exhaustion, or chemotherapy. Super-resolution Airyscan confocal imaging, subcellular fractionation, qPCR, Western blotting, siRNA knockdown, and genetic knockout models were used. In vivo validation was performed in a mouse model of MASH with BAX/BAK and MAVS conditional deletion.
Study Limitations
The in vivo MASH model used genetic deletion rather than pharmacological inhibition, limiting immediate translatability. The study does not fully resolve whether mtRNA and mtDNA/cGAS-STING pathways act additively or synergistically. All human cell work used fibroblast lines, so findings may not fully generalize to all senescent cell types in vivo.
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