The lysosomal–mitochondrial axis theory of postmitotic aging and cell death

Alexei Terman, Bertil Gustafsson, Ulf T. Brunk

Aging (senescence) is characterized by a progressive accumulation of macromolecular damage, supposedly due to a continuous minor oxidative stress associated with mitochondrial respiration. Aging mainly affects long-lived postmitotic cells, such as neurons and cardiac myocytes, which neither divide and dilute damaged structures, nor are replaced by newly differentiated cells. Because of inherent imperfect lysosomal degradation (autophagy) and other self-repair mechanisms, damaged structures (biological "garbage") progressively accumulate within such cells, both extra- and intralysosomally. Defective mitochondria and aggregated proteins are the most typical forms of extralysosomal "garbage", while lipofuscin that forms due to iron-catalyzed oxidation of autophagocytosed or heterophagocytosed material, represents intralysosomal "garbage". Based on findings that autophagy is diminished in lipofuscin-loaded cells and that cellular lipofuscin content positively correlates with oxidative stress and mitochondrial damage, we have proposed the mitochondrial-lysosomal axis theory of aging, according to which mitochondrial turnover progressively declines with age, resulting in decreased ATP production and increased oxidative damage. Due to autophagy of ferruginous material, lysosomes contain a pool of redox-active iron, which makes these organelles particularly susceptible to oxidative damage. Oxidant-mediated destabilization of lysosomal membranes releases hydrolytic enzymes to the cytosol, eventuating in cell death (either apoptotic or necrotic depending on the magnitude of the insult), while chelation of the intralysosomal pool of redox-active iron prevents these effects. In relation to the onset of oxidant-induced apoptosis, but after the initiating lysosomal rupture, cytochrome c is released from mitochondria and caspases are activated. Mitochondrial damage follows the release of lysosomal hydrolases, which may act either directly or indirectly, through activation of phospholipases or pro-apoptotic proteins such as Bid. Additional lysosomal rupture seems to be a consequence of a transient oxidative stress of mitochondrial origin that follows the attack by lysosomal hydrolases and/or phospholipases, creating an amplifying loop system.