An integrated view on a eukaryotic osmoregulation system

Osmoregulation encompasses active homeostatic processes that ensure proper cell volume, shape and turgor as well as an intercellular milieu optimal for the diverse biochemical processes. Recent studies demonstrate that yeast cells operate within a tight window of cellular water concentrations that still allows rapid diffusion of biomolecules while already moderate cell compression following hyper-osmotic stress leads to macromolecular crowding and a slow-down of cellular processes. Yeast cells accumulate glycerol as compatible osmolyte under hyper-osmotic stress to regain cell volume and turgor and release glycerol following a hypo-osmotic shock. The high osmolarity glycerol (HOG) response pathway controls glycerol accumulation at various levels, where each mechanism contributes to the temporal and quantitative pattern of volume recovery: inhibition of glycerol efflux, direct activation of the first enzyme in glycerol biosynthesis, stimulation of glycolytic flux as well as upregulation of expression of genes encoding enzymes in glycerol biosynthesis and an active glycerol uptake system. The HOG mitogen-activated protein kinase (MAPK) pathway communicates with the other yeast MAPK pathways to control cell morphogenesis. Cross-talk between the MAPK pathways has recently been used to re-wire osmostress-controlled expression of glycerol biosynthesis genes from Hog1 to Kss1-Fus3. The results of this study further illustrate the key importance of glycerol accumulation under osmostress and allow studying Hog1-dependent and independent processes as well as redundancy and robustness of the MAPK system.