Macromolecular crowding as a cell volume sensor.

The non-ideal properties of solutions containing high concentrations of macromolecules can result in enormous increases in the activity of the individual macromolecules. There is considerable evidence that macromolecular crowding and confinement not only occur in cells, but that these are major determinants of the activity of the proteins and other intracellular macromolecules. This concept has important implications for cell volume regulation because, under crowded conditions, relatively small changes in concentration, consequent to alterations of water content, lead to large changes in macromolecular activity which could provide a mechanism by which cells sense changes in their volume. This brief review considers 1) direct demonstrations that introducing a high concentration of appropriate macromolecules into cells in vitro produced volume regulatory changes, 2) the physical chemical principles involved in the effects of crowding of macromolecules on their activity, 3) estimates of the actual intracellular activity of macromolecules, 4) a proposed model of how changes in macromolecular crowding could signal volume regulation in cells, and 5) brief consideration of the complexities introduced by interactions between macromolecules, water and cosolutes. The hypothesis that macromolecular crowding provides a mechanism by which cells sense changes in their volume is plausible and is supported by striking observations in red blood cell ghosts and perfused barnacle muscle cells. However, the signaling molecules involved have not been identified, the proposed model is not fully consistent with the experiments, experimental verification in intact cells is lacking, and numerous alternative or additional mechanisms are not excluded.