Endosymbiotic theory of aging revisited: Age-related leakage of mitochondrial dsDNA/RNA stimulates cytosolic nucleic acid sensors which remodel the immune network and promote the aging process.

Abstract

About 1.5-2 billion years ago, an endosymbiosis between aerobic α-proteobacteria and anaerobic archaeal cells generated mitochondria, i.e., organelles capable of producing oxidative energy. The bacterial genome was fundamentally reduced and a circular mitochondrial genome evolved containing mainly the genes coding for the subunits of the electron transport chain. Before the symbiotic event, there existed a virus-host co-evolution which involved the development of sensors for detecting dangerous viral DNA/RNA molecules. Endosymbiosis supplied eukaryotic cells not only with an oxidative powerhouse to allow the evolution of more complex multicellular organisms but it also meant that cells now housed an organelle which was able to generate reactive oxygen species (ROS) and to leak mitochondrial DNA (mtDNA) and double-stranded RNA (dsRNA) into the cytoplasm. There is now abundant evidence that during aging and age-related diseases mitochondria are prone to release both mtDNA and dsRNA. In the cytoplasm, mtDNA/dsRNA molecules activate a number of cytosolic nucleic acid sensors leading to the secretion of type-1 interferons (IFN) and many other cytokines which promote an age-related proinflammatory state. Currently, it is known that mtDNA can activate the cGAS-STING pathway, AIM2 inflammasomes, IFI16 receptors, and ZBP1 sensors and in addition mitochondrial dsRNA stimulates RIG-1/MDA5 signaling. Interestingly, there is abundant evidence that all these receptors are drivers of cellular senescence and inflammaging. For decades, there has been mounting evidence that mitochondria have a crucial role in the aging process. We will examine this question from the perspective of evolution and propose that mitochondrial evolution created an endogenic source for the leakage of dangerous mtDNA/dsRNA which subsequently stimulated cytosolic DNA/RNA sensors, an evolutionarily conserved viral defence mechanism. It seems that these two evolutionary events provided not only the basis for the inevitable process of aging but also ensuring the death of parental organisms.