Mitochondrial Dysfunction Accounts for the Stochastic Heterogeneity in Telomere-Dependent Senescence

TLDR

Aging is inherently stochastic, producing heterogeneity across organisms, cell types, and clonal populations, yet the telomere‑dependent replicative lifespan of primary human cells remains poorly understood. The authors aim to determine whether mitochondrial reactive oxygen species (ROS) are a major determinant of telomere‑dependent senescence at the single‑cell level. They propose that mitochondrial ROS drives telomere‑dependent senescence, accounting for cell‑to‑cell variation in replicative lifespan. Mitochondrial superoxide increases with replicative age, and mild uncoupling reduces ROS, slows telomere shortening, and delays senescence; early senescent cells show higher ROS, dysfunctional mitochondria, shorter telomeres, and telomeric γ‑H2A.X foci, indicating ROS as a key cause of replicative senescence.

Abstract

Aging is an inherently stochastic process, and its hallmark is heterogeneity between organisms, cell types, and clonal populations, even in identical environments. The replicative lifespan of primary human cells is telomere dependent; however, its heterogeneity is not understood. We show that mitochondrial superoxide production increases with replicative age in human fibroblasts despite an adaptive UCP-2–dependent mitochondrial uncoupling. This mitochondrial dysfunction is accompanied by compromised [Ca2+]i homeostasis and other indicators of a retrograde response in senescent cells. Replicative senescence of human fibroblasts is delayed by mild mitochondrial uncoupling. Uncoupling reduces mitochondrial superoxide generation, slows down telomere shortening, and delays formation of telomeric γ-H2A.X foci. This indicates mitochondrial production of reactive oxygen species (ROS) as one of the causes of replicative senescence. By sorting early senescent (SES) cells from young proliferating fibroblast cultures, we show that SES cells have higher ROS levels, dysfunctional mitochondria, shorter telomeres, and telomeric γ-H2A.X foci. We propose that mitochondrial ROS is a major determinant of telomere-dependent senescence at the single-cell level that is responsible for cell-to-cell variation in replicative lifespan.