Understanding the interplay between osmotic stress response and ion homeostasis in Saccharomyces cerevisiae: An approach integrating in-vivo imaging, microfluidics and mathematical modeling.

• Applicant: Dr. Jannis Uhlendorf
• Subject Area: Biophysics
• Term: from 2014 to 2017
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 261044771

The hyperosmotic stress response in Saccharomyces cerevisiae is one of the best studied stress adaptation mechanisms in a microorganism. It allows cells to grow in conditions where the external osmolarity increases, for example in case the liquid surrounding of the cell is evaporating. Adaptation is achieved via the high osmolarity glycerol (HOG) signaling cascade, which promotes the production of the small osmolyte glycerol, thereby increasing intracellular osmolarity. In addition to keeping osmotic balance, cells also 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. Cells achieve ion homeostasis 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. While both systems have been studied in great detail in isolation, a holistic picture of how the two work together in order to achieve homeostasis is remains elusive.In this study the interplay of the osmoregulation and the ion homeostasis system will be explored by developing an integrative mathematical model of the complete system. The model will be based on a wealth set of in vivo ion concentration measurements in response to various perturbations of external osmolarity and ion concentrations. Ion concentration- and membrane potential changes will be monitored in vivo using fluorescent sensor proteins or dyes, which even allow to quantify ion concentration changes in single cells. Also signaling activity and gene expression will be observed using fluorescent proteins. External ion concentrations and osmotic pressure will be controlled using a microfluidic device, which provides the possibility to generate precise and time-varying input signals, which will facilitate deciphering the dynamics of the system. These previously unavailable in vivo measurements of ion concentrations in combination with the advanced stimulation methods of a microfluidic device will make it possible to develop an integrative model of the osmo- and ion regulation systems, that will help to understand how the two systems act together in order to achieve homeostasis.

• DFG Programme: Research Grants