Mitochondrial RNA Emerges as a Key Inflaming Signal in Senescent Cells

Cellular senescence, a permanent state of growth arrest, acts as a crucial barrier against cancer, halting cells that have sustained severe DNA damage or have exhausted their replicative potential. However, this protective mechanism comes at a steep long-term cost. Senescent cells are not inert. They become biochemical factories, secreting a potent cocktail of inflammatory cytokines, chemokines, and tissue-remodeling factors. This pro-inflammatory output is collectively known as the Senescence-Associated Secretory Phenotype, or SASP. While initially beneficial for tissue repair and immune clearance, the chronic, persistent SASP from accumulated senescent cells drives tissue dysfunction, chronic inflammation, and the progression of numerous age-related diseases, from fibrosis to metabolic disorders.

For years, scientists have sought to unravel the precise molecular triggers that launch this destructive secretory program. Mitochondria, the cell's power plants, have been implicated, with the leakage of mitochondrial DNA (mtDNA) into the cell's cytoplasm – and its subsequent detection by the cGAS-STING immune pathway – identified as one key driver. Now, a collaborative study by researchers in the United States and Spain has uncovered a powerful and previously overlooked accomplice: mitochondrial RNA (mtRNA). Their findings, published in a leading scientific journal, reveal how escaped mtRNA acts as a primary danger signal, igniting inflammatory pathways that fuel the SASP and offering a novel target for therapeutic intervention.

The research team employed multiple established models of senescence in human fibroblasts, including replicative exhaustion and stress-induced senescence from radiation or chemotherapy. Using advanced super-resolution microscopy and meticulous subcellular fractionation techniques, they made a critical observation: double-stranded mitochondrial RNA (mtRNA) abnormally accumulated in the cytosol of senescent cells, but not in their healthy, proliferating counterparts.

Parallel to this escape, the cells showed a significant increase in the expression of cytoplasmic RNA sensors, key components of our innate antiviral defense system, particularly the proteins RIG-I and MDA5. This correlation suggested a potential link. To test if mtRNA was merely a bystander or an active initiator, scientists took a direct approach. They artificially introduced purified mtRNA into normal, young fibroblasts. The result was clear and striking: the treated cells began producing classic SASP factors, mirroring the inflammatory state of genuine senescence. This experiment positioned mtRNA not as a passive byproduct but as a potent trigger of inflammatory signaling.

To definitively prove mtRNA's specific role, the researchers turned to a clever genetic tool: they used cells engineered to rapidly eliminate all their mitochondria (a process called mitophagy). When these "mitochondria-free" cells were driven into senescence, they still underwent cell cycle arrest but developed a profoundly blunted SASP. The inflammatory engine had stalled. Importantly, re-introducing mtRNA into these cells partially revived the inflammatory gene expression profile, including genes dependent on the central inflammatory regulators NF-κB and interferon signaling.

The next step was to trace the exact signaling cascade. Biochemical assays confirmed that in senescent cells, the leaked mtRNA physically bound to the sensors RIG-I and MDA5. This binding activated the pathway, causing the aggregation of the adapter protein MAVS – a critical event that amplifies the danger signal. When the team used genetic tools to knock out RIG-I, MDA5, or MAVS, the expression of SASP factors plummeted. Crucially, the cells still remained senescent (growth-arrested), proving that this RNA-sensing pathway is specifically responsible for the inflammatory secretory aspect of senescence, not the arrest itself.

A major question remained: how does mtRNA escape the tightly sealed double membrane of the mitochondria? The answer lies in a process known as mitochondrial outer membrane permeabilization (MOMP), typically associated with apoptosis. The researchers discovered that in senescence, a partial, sub-lethal form of MOMP occurs, orchestrated by the pro-apoptotic proteins BAX and BAK. These proteins form large pores in the mitochondrial outer membrane. Genetic deletion of BAX and BAK in senescent cells effectively plugged these leaks: it reduced mtRNA escape, prevented RIG-I/MDA5 activation, inhibited MAVS aggregation, and ultimately suppressed the SASP. This identified the BAX/BAK pore complex as the essential gateway for this inflammatory signal.

The ultimate test of any cellular mechanism is its relevance in a living organism. The team investigated this in a mouse model of metabolic dysfunction-associated steatohepatitis (MASH), a severe liver disease driven by inflammation and cell stress. In mice fed a disease-inducing diet, hepatocytes (liver cells) showed clear signs of senescence, activation of the RIG-I/MDA5 pathway, and severe tissue inflammation.

The researchers then genetically engineered mice to lack BAX/BAK specifically in their hepatocytes. The result was a dramatic alleviation of the SASP and inflammatory landscape in the liver. Similarly, suppressing the MAVS adapter protein yielded a comparable anti-inflammatory effect. These interventions reduced immune cell infiltration and markers of tissue damage without affecting liver size or increasing apoptosis, highlighting the specificity of targeting this senescence-associated pathway.

This work fundamentally expands our understanding of what fuels the chronic, low-grade inflammation of aging. It establishes mitochondrial RNA as a major and independent danger signal that works in concert with, but distinct from, mtDNA to drive the SASP. The identified pathway – from BAX/BAK-dependent mitochondrial leakage, through cytosolic mtRNA, to activation of RIG-I/MDA5 and MAVS – provides a detailed new map of senescence-associated inflammation.

Publication date: 15.12.2025

Source:
Victorelli S. et al. Mitochondrial RNA cytosolic leakage drives the SASP. Nat Commun. 2025 Dec 15;16(1):10992. doi: 10.1038/s41467-025-66159-z. PMID: 41398033; PMCID: PMC12705736.
https://www.nature.com/articles/s41467-025-66159-z