Stability of Intracellular Protein Concentration under Extreme Osmotic Challenge

Abstract

Cell volume (CV) regulation is typically studied in short-term experiments to avoid complications resulting from cell growth and division. By combining quantitative phase imaging (by transport-of-intensity equation) with CV measurements (by the exclusion of an external absorbing dye), we were able to monitor the intracellular protein concentration (PC) in HeLa and 3T3 cells for up to 48 h. Long-term PC remained stable in solutions with osmolarities ranging from one-third to almost twice the normal. When cells were subjected to extreme hypoosmolarity (one-quarter of normal), their PC did not decrease as one might expect, but increased; a similar dehydration response was observed at high concentrations of ionophore gramicidin. Highly dilute media, or even moderately dilute in the presence of cytochalasin, caused segregation of water into large protein-free vacuoles, while the surrounding cytoplasm remained at normal density. These results suggest that: (1) dehydration is a standard cellular response to severe stress; (2) the cytoplasm resists prolonged dilution. In an attempt to investigate the mechanism behind the homeostasis of PC, we tested the inhibitors of the protein kinase complex mTOR and the volume-regulated anion channels (VRAC). The initial results did not fully elucidate whether these elements are directly involved in PC maintenance.

Intracellular Potassium in Cell Growth and Proliferation of Human Mesenchymal Stem Cells and Blood Lymphocytes

“…Recent studies using quantitative Raman microscopy demonstrate that under the same physiological conditions, the concentrations of cellular proteins, lipids, and water are maintained within a narrow range and change in different physiological states, such as stages of the cell cycle, attachment to substrates, or entering senescence [63]. It is assumed that the accumulation of macromolecules may play a role in the signaling associated with cell volume changes [64][65][66]. The relationship between cell hydration and proliferation may reflect the effect of macromolecule accumulation on cell metabolism and gene expression [67][68][69][70][71][72].…”

Nucleic Acids Regulate Intracellular Ions and Membrane Potential

“…Furthermore, on a longer time scale, cell volume can only be conserved as the average over cell population or in terminally differentiated cells. For a growing culture, the very notion of cell volume regulation may have to be revised: it appears that it is rather the dry mass concentration (or cell water content) that is subject to control [25,136]. Similarly, a strict separation of intracellular osmolytes into permeant and impermeant does not apply to proliferating cells, because both the amount of organic osmolytes and their average charge are liable to change.…”

“…Long-term osmotic adaptation is different from the response to acute osmotic shock [23]. In particular, the constantly changing cell volume can hardly serve as a set point in growing and dividing cells, and there is evidence that cells are guided by the density of the dry mass instead (or the equivalent measure of cell water content) [24,25]. No osmolytes can be considered unalterable during slow processes, and their role in cell regulation must be examined.…”

Trimethylamine N-Oxide (TMAO): a Unique Counteracting Osmolyte?

“…The 275-400 mosmol/kg range may have originated to balance the ancient oceans [4,7] or to optimize cell water content, protein concentration, density and/or macromolecular crowding for universal cellular processes [3,11]). It must be noted, however, that there are significant exceptions to the 275-400 range in some freshwater organisms; e.g., algal cells as low as 62 mosmol/kg [12]), amoebas at 101 mosmol/kg [13] and bivalve hemolymph as low as 32 mosmol/kg (reviewed by [14]).…”