Genetically encoded voltage indicator proteins revealed differential effects of hyperosmotic stress on yeast plasma membrane potential imposed by different stress conditions

Hyperosmotic stress from different stress agents had different effects on plasma membrane potential and the morphology of living yeast cells as determined by optical physiology and confocal microscopy. Cells can be affected by several causes of osmotic stress, by which they are forced to adapt. An essential aspect of adaptation is ion regulation, and many insights into such complex processes can be obtained through measurement of the plasma membrane potential (PMP) of cells during stress. We recently established genetically encoded voltage indicator proteins that could be utilized to report the yeast PMP change in real time. In this work, we employed this method to monitor the early change in the PMP of yeast Saccharomyces cerevisiae with intact cell wall, immediately following hyperosmotic up-shock due to various stress agents. The results pointed to differential effects of NaCl, sorbitol and polyethylene glycol (PEG) 6000. Yeast PMP was more responsive toward PEG 6000 than NaCl and sorbitol at comparable osmotic pressure, and PEG 6000 stimulated the largest response magnitude, followed by sorbitol and NaCl, respectively. After prolonged treatment, PEG 6000 also instigated distinct cell morphology from NaCl and sorbitol. Accordingly, this study presents new evidence supporting multiple pathways underlying yeast adaptation to varying hyperosmotic conditions, enabled through the optical physiology approach. Our findings promote better understanding of the yeast cellular response to hyperosmotic stress, with tenable relevance to the physiologically related plant cells.