Scientists have found that leakage of mitochondrial DNA in senescent cells is a major cause of their pro-inflammatory activity, and it can be targeted without clearing those cells out [1].
A new aspect of senescence
Clearing senescent cells with drugs called senolytics is one of the most promising avenues in geroscience. However, cellular senescence is highly heterogeneous across various cell types and senescence inductors, and many of its mechanisms are still unknown or under-researched.
As we reported earlier, at the ARDD 2023 conference in Copenhagen, João Passos of Mayo Clinic presented results from a new study elucidating a previously unknown aspect of cellular senescence: the role of leaked mitochondrial DNA (mtDNA), which appears to contribute to SASP secretion. Now, this paper has been published in Nature, and his presentation’s full context has been brought to light.
Regulating SASP but not senescence
Mitochondrial dysfunction is a hallmark of cellular senescence [2]. Previous work by the same group of scientists revealed that when mitochondria are depleted in senescent cells, SASP production plummets while the cells remain in cell cycle arrest. However, mitochondria are also well-known triggers of apoptosis, a widespread type of programmed cellular death [3].
With apoptosis, massive leakage of mtDNA into the cytosol triggers a cascade of events that causes the cell to die quietly without disturbing its neighbors, while SASP production is dampened by cleaved caspase, a central mediator of apoptosis. In the case of cellular senescence, however, mtDNA leakage only occurs in a small subset of peripheral mitochondria [4]. The researchers suspected that this mechanism, which resembles a weaker version of apoptosis, is essential for continuous SASP production, which is probably the most deleterious aspect of senescence.
mtDNA leakage happens through pores in the mitochondrial membrane created by the closely related proteins BAK and BAX in mitochondrial outer membrane permeabilization (MOMP). The researchers confirmed that BAX becomes activated in senescent cells, regardless of type or reason for senescence, which coincides with mtDNA leakage from some of the cell’s mitochondria.
Combined deletion of BAX and BAK suppressed mtDNA release in senescent cells induced by DNA damage and led to a decrease in the expression of several common SASP genes. However, it did not alter the levels of senescence markers, suggesting that BAX and BAK regulate the SASP but not the senescence-associated cell cycle arrest.
To investigate this mechanism further in vivo, the researchers created genetically engineered BAK- and BAX-deficient mice and irradiated them with a small dose of radiation known to induce cellular senescence in the liver. In the treated mice, levels of several pro-inflammatory factors were much lower than in controls, but senescence markers were not. This suggests that while about the same number of cells became senescent, they produced less SASP.
Stronger grip, healthier bones
In the next experiment, the researchers deleted BAK and BAX in old mice. Aging is known to cause increased expression of pro-inflammatory factors in the liver, which was significantly attenuated by the treatment. Consistent with this, less infiltration of immune cells in the livers of the treated animals was observed.
The researchers then wanted to see whether inhibiting BAX (which had been shown to be a more essential component than BAK) can improve health indicators in mice. Treating aged animals with two BAX inhibitors working via different pathways led to significant improvements in several parameters of healthspan, such as rotarod latency, grip strength, and bone density. The treatment also reduced overall frailty and decreased inflammation, but it did not cause lifespan extension.
As João Passos reported in his talk, these findings mean that BAX inhibition or mitochondrial clearance can potentially modify senescent cells (making them senomorphics) and that this approach might have several advantages over the more widely explored senolytic one, in which senescent cells are removed. mtDNA leakage might also be used as a much-needed marker of senescence universal across cell and trigger types, although this requires further investigation.
In summary, MOMP is often essential for apoptotic cell death, a terminal cell fate that is considered to be independent from cellular senescence and immunologically silent. Our findings indicate that miMOMP occurs during cellular senescence and can drive the SASP through the release of mtDNA into the cytosol. Importantly, we show that inhibition of miMOMP may be a therapeutic target to counteract age-associated sterile inflammation and improve healthspan.
Literature
[1] Victorelli, S., Salmonowicz, H., Chapman, J. et al. Apoptotic stress causes mtDNA release during senescence and drives the SASP. Nature (2023).
[2] Correia-Melo, C., Marques, F. D., Anderson, R., Hewitt, G., Hewitt, R., Cole, J., … & Passos, J. F. (2016). Mitochondria are required for pro-ageing features of the senescent phenotype. The EMBO journal, 35(7), 724-742.
[3] Bock, F. J., & Tait, S. W. (2020). Mitochondria as multifaceted regulators of cell death. Nature reviews Molecular cell biology, 21(2), 85-100.
[4] Dou, Z., Ghosh, K., Vizioli, M. G., Zhu, J., Sen, P., Wangensteen, K. J., … & Berger, S. L. (2017). Cytoplasmic chromatin triggers inflammation in senescence and cancer. Nature, 550(7676), 402-406.
