Protein misfolding in hypertonic stress: new insights into an old idea. Focus on "genome-wide RNAi screen and in vivo protein aggregation reporters identify degradation of damaged protein as an essential hypertonic stress response".

WATER is the most abundant molecule of the body, around 60% by weight. Volume and composition of cellular water, also known as internal milieu, are critical factors that affect cellular function because cellular water is the medium in which the majority of biochemical reactions take place. How cells adapt to hypertonic environment is an important subject that has been studied for a long time. The immediate consequence of exposure to a hypertonic environment is loss of cellular water due to osmosis. The loss of water results in two profound changes in the intracellular milieu: reduced volume and increased ionic strength. Macromolecular crowding (7) due to reduced cell volume and elevated ionic strength (9) have been considered to be the central elements of “hypertonic stress” because they are believed to perturb protein function based on observations made in test tubes and theoretical considerations. In a stunning revelation, a recent report (1) provides direct support for this old idea plus unexpected new insights. Upon exposure to extreme hypertonic environment, animals or cells die in a dose-dependent manner. The threshold is a little over 400 mM NaCl for the nematode Caenorhabditis elegans (1) and 700 mosmol/kg for cultured mammalian cells (6). Both apoptosis and necrosis have been shown in the hypertonicity-induced cell death (2, 6). Choe and Strange (1) present an elegant and comprehensive body of work which demonstrates that indeed protein misfolding and protein aggregates are central features of hypertonic stress in C. elegans. The investigators performed genome-wide screen in C. elegans to find those genes of which their expression was required for survival under hypertonic conditions. The expression of 19,000 genes was individually knocked down using RNA interference followed by exposure to a hypertonic condition in which control animals display better than 90% survival. A total of 49 genes were found to significantly reduce survival when knocked down with the RNA interference. The investigators infer that these 49 genes are required for survival in hypertonicity. They are named Hos (hypertonic sensitive). Nearly half of the Hos genes (22 of 49) are predicted to function in removal of damaged proteins as they are either critical components of lysosomes and proteasomes, two organelles that degrade damaged proteins, or proteins that sort and deliver substrates to the organelles. Using model polyglutamine substrates in vivo, the investigators directly showed that hypertonicity induced protein aggregation in a manner dependent on reduced volume and elevated ionic strength. There was an excellent correlation between protein aggregation and reduced cell survival when expression of Hos genes were individually knocked down providing undisputable support that survival under hypertonicity required the ability of remove damaged proteins. In addition, hypertonicity caused a global increase in ubiquitination of cellular proteins. These results demonstrate that protein aggregates are produced under hypertonic conditions, and their removal by lysosomes and proteasomes is critical for survival. Much effort has been directed to understanding how animals and cells adapt to hypertonicity. Most animals and cells are Address for reprint requests and other correspondence: H. Moo Kwon, 655 West Baltimore St., Bressler 8029, Baltimore, MD 21201 (e-mail: mkwon @medicine.umaryland.edu).