Cytosolic acidification as a signal mediating hyperosmotic stress responses in Dictyostelium discoideum

We determined pH changes in response to hyperosmotic stress using FACS or 31P-NMR. Hyperosmolarity was found to acidify the cytosol from pH 7.5 to 6.8 within 5 minutes, whereas the pH of the endo-lysosomal compartment remained constant. Fluid-phase endocytosis was identified as a possible target of cytosolic acidification, as the inhibition of endocytosis observed under hypertonic conditions can be fully attributed to cytosolic acidification. In addition, a deceleration of vesicle mobility and a decrease in the NTP pool was observed.Together, these results indicate that hyperosmotic stress triggers pleiotropic effects, which are partially mediated by a pH signal and which all contribute to the downregulation of cellular activity. The comparison of our results with the effect of hyperosmolarity and intracellular acidification on receptor-mediated endocytosis in mammalian cells reveals striking similarities, suggesting the hypothesis of the same mechanism of inhibition by low internal pH.Cells steadily face fluctuations of the external osmolarity due to dehydration. Occasionally, dramatic changes in osmolarity can occur, resulting in a stress condition [1]. Hyperosmolarity of the external medium leads to the extrusion of water and the concomitant shrinkage of cells [2]. Within a few minutes, the cells activate mechanisms, termed "regulatory volume increase" (RVI), to regain their volume [3]. Under prolonged hyperosmotic conditions, compatible osmolytes, e.g. polyols or amines are accumulated inside the cells [3]. These osmolytes exhibit a stabilizing effect on proteins and thereby avoid the deleterious effect of protein aggregation. In addition, the expression of stress proteins, as chaperones and DNA repair proteins was observed in various organisms in response to hypertonicity [4,5,6].Recently it could be shown, that the amoeba Dictyostelium discoideum exhibits an unusual response to hypertonic stress which is distinct from the response observed in other organisms [