Lipid bilayers as osmotic response elements.

The importance of osmolarity in the regulation of a wide range of cellular functions and processes is well established. However, much less is known about the molecular mechanisms imparting sensitivity to osmotic forces to cells. The findings summarized in this brief review demonstrate that the principal structural element of all biomembranes, the lipid bilayer, provides a highly sensitive machinery for conveying information in the osmotic conditions of a cell to the relevant regulatory machineries. More specifically, osmotic shrinkage, swelling, as well as applied osmotic stress all have pronounced effects on the physical state and molecular interactions in the bilayer, influencing lipid packing and dynamics, and also altering the 2-dimensional (lateral) ordering in the membrane into compositionally distinct microdomains. Because of the cooperative behavior of lipid bilayers integrative regulation of the functions embedded in the different organelle membranes by the physical properties of lipids is possible. Organelle membranes should thus be understood as adaptive platforms harboring specialized metabolic pathways and functions, whose activities (physiological state) can be controlled by the physical state of the membrane lipids. Finally, it is important to keep in mind that virtually all biological macromolecules maintain a hydration shell. Living cells thus constitute highly complex supramolecular assemblies, their numerous components responding to osmotic forces in unison. Copyright 2000 S. Karger AG, Basel