Understanding the interplay between osmotic stress response and ion homeostasis in Saccharomyces cerevisiae: An approach integrating in-vivo imaging, microfluidics and mathematical modeling.
Final Report Abstract
The hyperosmotic stress response in Saccharomyces cerevisiae is one of the best studied stress adaptation mechanisms in a microorganism. It permits cells to grow in conditions where the external osmolarity increases, for example in case the liquid surrounding of the cell is evaporating. Such a drop in osmolarity poses a problem to the cell, because it leads to a drop in turgor pressure and to cell shrinkage. Adaptation is achieved via the high osmolarity glycerol (HOG) signaling pathway, which promotes the production of the small osmolyte glycerol, thereby increasing intracellular osmolarity. A hyposmotic shock on the other hand, leads to an increase in turgor pressure and a swelling of the cell, which can even lead to bursting of the cell. Tightly coupled to osmotic stress response, is the regulation of ion concentrations within the cell. Cells have to ensure that intracellular ion concentrations are held within specific bounds, since certain ions are required for different physiological functions, while others are toxic at high concentrations. Ion homeostasis is achieved via various ion specific membrane transporters and pumps, which are regulated according to ionic conditions. The two regulation mechanisms are linked, since ion concentration changes will also have an impact on osmotic pressure. In addition there are several known interactions between the two adaptation mechanisms, for example changes in ion transporter expression in response to HOG pathway activation. The project “Understanding the interplay between osmotic stress response and ion homeostasis in Saccharomyces cerevisiae” had the aim to explore the interplay between these two cellular regulation systems, employing a combination of mathematical modeling and live cell experimentation. The idea was to measure ion concentrations in living cells while varying the external osmotic conditions using a microfluidic device. The ion concentrations should then be quantified using genetically encoded fluorescent ion reporters, that have recently been developed, mostly with the aim to be employed in neurological studies. The first result of this project was a negative one, since expression of these genetically encoded sensors in S. cerevisiae was more difficult than expected. All sensors tested in this study either showed no expression or no ion sensitivity in yeast. Due to the problems with quantifying ion concentrations, the focus of the project was slightly shifted. Related to the interaction between the osmotic and ion regulatory systems is the question of how osmotic effects influence cellular shape and growth. Using a combination of mathematical modeling, atomic force microscopy and microfluidic experiments, we were able to show that local alterations in cell wall elasticity can describe the cell shape changes that are observed during yeast mating response. We observed that during mating, the cell wall softens in the protrusion region, while the material at the tip of the emerging shmoo is stiffer. With the help of a mathematical model this can explain the cellular shape changes observed during mating.
Publications
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(2016). Dynamics of cell wall elasticity pattern shapes the cell during yeast mating morphogenesis. Open Biology, 6(9), 160136
Goldenbogen, B., Giese, W., Hemmen, M., Uhlendorf, J., Herrmann, A., & Klipp, E.
