Mitochondria are required for pro-ageing features of the senescent phenotype.

Cell senescence is an important tumour suppressor mechanism and driver of ageing. Both functions are dependent on the development of the senescent phenotype, which involves an overproduction of pro-inflammatory and pro-oxidant signals. However, the exact mechanisms regulating these phenotypes remain poorly understood. Here, we show the critical role of mitochondria in cellular senescence. In multiple models of senescence, absence of mitochondria reduced a spectrum of senescence effectors and phenotypes while preserving ATP production via enhanced glycolysis. Global transcriptomic analysis by RNA sequencing revealed that a vast number of senescent-associated changes are dependent on mitochondria, particularly the pro-inflammatory phenotype. Mechanistically, we show that the ATM, Akt and mTORC1 phosphorylation cascade integrates signals from the DNA damage response (DDR) towards PGC-1ß-dependent mitochondrial biogenesis, contributing to aROS-mediated activation of the DDR and cell cycle arrest. Finally, we demonstrate that the reduction in mitochondrial content in vivo, by either mTORC1 inhibition or PGC-1ß deletion, prevents senescence in the ageing mouse liver. Our results suggest that mitochondria are a candidate target for interventions to reduce the deleterious impact of senescence in ageing tissues.

Quantitative assessment of markers for cell senescence.

Cellular senescence, the irreversible loss of replicative capacity, might be a tumour suppressor and a contributor to age-related loss of tissue function. The absence of quantitative tests for reliability of candidate markers for senescent cells is a major drawback in cell population studies. Fibroblasts in culture constitute mixed populations of proliferation-competent and senescent cells, with transition between these with increasing population doublings (PD). We estimated senescent fraction in human and mouse fibroblasts with high precision from easily observed growth curves using a dynamic simulation model. We also determined senescent fractions, at various PD (over a wide range of senescent cell frequencies) using candidate senescence markers: Ki67, p21 (CDKN1A), γH2AX, SAHF and Sen-ß-Gal either alone or in combination, and compared with those derived from growth curves. This comparison allowed ranking of candidate markers. High rankings were obtained for Sen-ß-Gal, SAHFs and the combination of Ki67 negativity with high (>5 per nucleus) γH2A.X foci density in MRC5 fibroblasts. We demonstrate that this latter marker combination, which can easily be performed in paraffin-embedded tissue, gives quantitative senescent cell frequency estimates in mouse embryonic fibroblast cultures and in mouse intestinal sections. The technique presented is a framework for quantitative assessment of markers for senescence.