Citrate trafficking supports rewiring of mitochondrial metabolism via RTG signaling in yeast osmoadaptation

Inter-organellar cross-talk is an important component of cellular stress response enabling adaptation and survival. We have demonstrated the activation of RTG retrograde signaling to sustain the peroxisomesmitochondria- nucleus axis in a model of osmostressed Saccharomyces cerevisiae yeast cells. In this work, we aimed to gain insight into the molecular mechanisms regulating the communication between these organelles upon NaCl treatment. A metabolomic analysis revealed that the homeostasis of citrate is a pivotal factor in the osmoadaptive response. Gene expression analysis and citrate synthase activity showed that the synthesis of citrate mainly derives from peroxisomes, as indicated by the up-regulation of CIT2, and not CIT1 and CIT3, under the control of the RTG pathway. Furthermore, the involvement of the mitochondrial citrate transporter, encoded by YHM2, in the osmoadaptive response, as judged by gene and protein expression analysis together with growth assay, is demonstrated. In the absence of YHM2, alternative pathways relying on ODC2 and ACO1 are activated, indicating possible compensatory mechanisms for osmoadaptation. We propose a model in which peroxisome-derived citrate is converted to cytosolic 2-oxoglutarate to replenish TCA cycle and promote its rewiring. This work reveals a new layer of metabolic co-ordination among organelles and identifies citrate shuttling as a crucial adaptive mechanism to osmotic stress.