Senescence is a complex cellular response to various stressors characterised by stable cell cycle arrest and secretion of a complex mixture of inflammatory, fibrogenic, and mitogenic soluble factors known as senescence-associated secretory phenotype (SASP). While the development of this phenotype is physiologically important during embryonic development and tissue regeneration, the unrestricted accumulation of senescent cells is a hallmark of ageing-related diseases and cancer. Indeed, in pathological conditions, the reduced clearance of these cells by the immune system can lead to chronic inflammation and impaired tissue regeneration abilities.
The selective elimination of senescent cells has been shown to delay or prevent numerous age-associated pathologies, thus, the generation of novel therapeutics, termed senolytics, able to clear senescent cells while being non-toxic for other cell populations, is a highly attractive strategy in the treatment of these disorders. To develop successful senolytic approaches, a deep understanding of senescent cells’ biology is necessary, to identify which unique aspects can be targeted.
Numerous studies have reported that senescent cells are characterised by highly altered mitochondrial and metabolic adaptations, including changes in morphology and dynamics, altered respiratory capacity, and reactive oxygen species (ROS) and calcium accumulation. These differences between senescent and non-senescent cells’ mitochondrial biology could offer a promising and still largely unexplored research field for senolytic therapies.
Through a CRISPR-based screening, we identified PPIF, the gene encoding for the mitochondrial matrix isomerase cyclophilin D (CypD), as a novel senolytic candidate. Reduced steady-state levels of CypD, indeed, significantly decrease the viability of senescent human fibroblasts and cancer cell lines but not of their proliferative counterparts. CypD is known to be the main modulator of the mitochondrial permeability transition pore (mPTP), triggering its opening by sensitising it to calcium, inorganic phosphate, and reactive oxygen species (ROS). The nature of the pore is still largely debated, since its structural components have not been fully identified, and the knowledge around its biological function is still evolving. The sustained and irreversible opening of the mPTP, which takes place in conditions of elevated and irreparable damage, leads to mitochondrial swelling, uncontrolled diffusion of molecules of <1,500 Da across the inner mitochondrial membrane, and loss of mitochondrial membrane potential, resulting in cell death. On the other side, the transient and intermitting opening of the pore is believed to maintain mitochondrial homeostasis and favour the exchange of molecules between the mitochondrial matrix and the cytoplasm. In our cellular models, we expect the downregulation of CypD to lead to decreased transient opening of the mPTP in senescent cells and a consequential increase in the mitochondrial matrix calcium levels.
Our final aim is to identify and characterise in vitro and in vivo a new senolytic target that can be use to develop therapeutics for the treatment of ageing-associated diseases and cancer.
