Abstract
BackgroundDictyostelium cells exhibit an unusual response to hyperosmolarity that is distinct from the response in other organisms investigated: instead of accumulating compatible osmolytes as it has been described for a wide range of organisms, Dictyostelium cells rearrange their cytoskeleton and thereby build up a rigid network which is believed to constitute the major osmoprotective mechanism in this organism. To gain more insight into the osmoregulation of this amoeba, we investigated physiological processes affected under hyperosmotic conditions in Dictyostelium.ResultsWe 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.ConclusionTogether, 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.
Highlights
Dictyostelium cells exhibit an unusual response to hyperosmolarity that is distinct from the response in other organisms investigated: instead of accumulating compatible osmolytes as it has been described for a wide range of organisms, Dictyostelium cells rearrange their cytoskeleton and thereby build up a rigid network which is believed to constitute the major osmoprotective mechanism in this organism
Hyperosmotic stress leads to intracellular acidification It was shown recently, that the external medium was acidified, when Dictyostelium cells were exposed to high tonicity [18]
SPB buffer/2 M sorbitol was added to a final concentration of 400 mM sorbitol and flow cytometry measurements were performed at intervals of 30 sec
Summary
Dictyostelium cells exhibit an unusual response to hyperosmolarity that is distinct from the response in other organisms investigated: instead of accumulating compatible osmolytes as it has been described for a wide range of organisms, Dictyostelium cells rearrange their cytoskeleton and thereby build up a rigid network which is believed to constitute the major osmoprotective mechanism in this organism. Cells steadily face fluctuations of the external osmolarity due to dehydration. Compatible osmolytes, e.g. polyols or amines are http://www.biomedcentral.com/1471-2121/2/9 accumulated inside the cells [3]. These osmolytes exhibit a stabilizing effect on proteins and thereby avoid the deleterious effect of protein aggregation. The expression of stress proteins, as chaperones and DNA repair proteins was observed in various organisms in response to hypertonicity [4,5,6]
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