Enhanced inter-compartmental Ca²⁺ flux modulates mitochondrial metabolism and apoptotic threshold during aging

Senescence is characterized by a gradual decline in cellular functions, including changes in energy homeostasis and decreased proliferation activity. As cellular power plants, contributors to signal transduction, sources of reactive oxygen species (ROS) and executors of programmed cell death, mitochondria are in a unique position to affect aging-associated processes of cellular decline. Notably, metabolic activation of mitochondria is tightly linked to Ca²⁺ due to the Ca²⁺ -dependency of several enzymes in the Krebs cycle, however, overload of mitochondria with Ca²⁺ triggers cell death pathways. Consequently, a machinery of proteins tightly controls mitochondrial Ca²⁺ homeostasis as well as the exchange of Ca²⁺ between the different cellular compartments, including Ca²⁺ flux between mitochondria and the endoplasmic reticulum (ER).

In this study, we investigated age-related changes in mitochondrial Ca²⁺ homeostasis, mitochondrial-ER linkage and the activity of the main ROS production site, the mitochondrial respiration chain, in an in vitro aging model based on porcine aortic endothelial cells (PAECs), using high-resolution live cell imaging, proteomics and various molecular biological methods.

We describe that in aged endothelial cells, increased ER-mitochondrial Ca²⁺ crosstalk occurs due to enhanced ER-mitochondrial tethering. The close functional inter-organelle linkage increases mitochondrial Ca²⁺ uptake and thereby the activity of the mitochondrial respiration, but also makes senescent cells more vulnerable to mitochondrial Ca²⁺-overload-induced cell death. Moreover, we identified the senolytic properties of the polyphenol resveratrol, triggering cell death via mitochondrial Ca²⁺ overload exclusively in senescent cells.

By unveiling aging-related changes in the inter-organelle tethering and Ca²⁺ communications we have advanced the understanding of endothelial aging and highlighted a potential basis to develop drugs specifically targeting senescent cells.