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Mitochondrial membrane potential directly powers many critical functions of mitochondria, including ATP production, mitochondrial protein import, and metabolite transport. Its loss is a cardinal feature of aging and mitochondrial diseases, and cells closely monitor membrane potential as an indicator of mitochondrial health. Given its central importance, it is logical that cells would modulate mitochondrial membrane potential in response to demand and environmental cues, but there has been little exploration of this question. We report that loss of the Sit4 protein phosphatase in yeast increases mitochondrial membrane potential, both through inducing the electron transport chain and the phosphate starvation response. Indeed, a similarly elevated mitochondrial membrane potential is also elicited simply by phosphate starvation or by abrogation of the Pho85-dependent phosphate sensing pathway. This enhanced membrane potential is primarily driven by an unexpected activity of the ADP/ATP carrier. We also demonstrate that this connection between phosphate limitation and enhancement of the mitochondrial membrane potential is also observed in primary and immortalized mammalian cells as well as inDrosophila. These data suggest that mitochondrial membrane potential is subject to environmental stimuli and intracellular signaling regulation and raise the possibility for therapeutic enhancement of mitochondrial functions even with defective mitochondria.

Luengo et al. (2020) demonstrate that pyruvate dehydrogenase (PDH) overactivation blunts NAD+ regeneration by overcharging the mitochondrial membrane potential and driving ATP synthesis beyond demand. Under these conditions, some cells prioritize aerobic glycolysis to meet the need for oxidized cofactors in biosynthetic metabolism.

Summary Reactive oxygen species (ROS), despite having damaging roles, serve as signaling molecules regulating diverse biological and physiological processes. Employing in vivo genetic studies in Drosophila, we show that besides causing G1-S arrest by activation of Dacapo, ROS can simultaneously inhibit cell growth by regulating the expression of 4EBP and S6K. This is achieved by triggering a signaling cascade that includes Ask1, JNK, and FOXO independent of the Tsc-TOR growth regulatory pathway. Qualitative and quantitative differences in the types of ROS molecules generated dictate whether cells undergo G1-S arrest only or experience blocks in both cell proliferation and growth. Importantly, during normal development, this signaling cascade is triggered by ecdysone in late larval fat body cells to restrict their growth prior to pupation by antagonizing insulin signaling. The present work reveals an unexpected role of ROS in systemic control of growth in response to steroid hormone signaling to establish organismal size.