Osmotic stress is a frequent experience, both for organisms and the cells that make them up. An organism's ability to osmoregulate successfully is key to its survival, and mechanisms of osmoregulation are intensively studied by some authors in this journal. A recent paper from Kevin Strange's group uses the tiny nematode worm to implicate a very unexpected gene in the ability to survive hypertonic stress, linking osmoregulation to the ageing process.
Caenorhabditis elegans is a tiny soil-dwelling nematode that has become a favoured model organism for genetics. It might seem unlikely that useful physiology could be performed on something only a few tens of microns in diameter, but Strange nonetheless showed that C. elegans could readily adapt to hyper- or hypotonic environments. Previous work had shown that a class of C. elegans mutants that altered the organism's ability to form a long-lived, stress-tolerant larval stage, called the dauer larva, could greatly alter the animal's resistance to thermal, oxidative or hypoxic stress. Strange reasoned that this might also extend to osmotic stress and, sure enough, showed that one such mutant, daf-2, conferred greatly increased survival on agar plates containing up to 400 mmol l–1 NaCl. daf-2 encodes the worm's insulin receptor,so the implication is clear; disrupting insulin signalling actually enhances survival under stress.
There are other players in the insulin signalling cascade, so to explore the genes responsible for stress tolerance in C. elegans, the authors tried these other genes singly and in combination. Mutations in age-1, a gene implicated in longevity that encodes PI3-kinase, showed a similar salt resistance phenotype to daf-2. However, mutations in daf-16, also identified in screens for genes that affect lifespan,could suppress these effects in double mutants carrying daf-2 and daf-16 or carrying age-1 and daf-16. That is,normal daf-16 is required for daf-2 or age-1mutants to be effective at conferring salt resistance. Over-expression of daf-16, which leads to increased longevity, also increases salt tolerance. So longevity, ageing and resistance to osmotic and other stressors are all interlinked.
daf-16 encodes a FOXO transcription factor, which switches on other genes, and other workers had recently published a microarray list of genes upregulated by daf-16 over-expression. Some might be candidates for increasing survival after salt stress. Conspicuous in the list were the heat shock protein (hsp) genes, which protect organisms from stress-induced damage; by knocking these down with RNAi (particularly easy in C. elegans), several of the hsps were shown to be necessary for successful adaptation to high salt. Inspired by this result, the authors screened a further 222 target genes of daf-16 by RNAi and found that 10 of them were also important in salt response. Two of these were trehalose-6-phosphate synthases, which catalyse the formation of trehalose, an important osmolyte in many organisms.
In summary, the authors propose that in normal worms, insulin signalling through the daf-2 insulin receptor activates the age-1 PI3 kinase, which in turn normally phosphorylates the daf-16/FOXO transcription factor to keep it out of the nucleus, stopping it from activating downstream genes. In mutants of insulin signalling, daf-16is able to enter the nucleus, allowing it to switch on a variety of genes that allow the animal to adapt to stress more easily. It is interesting to note that these genes are presently the darlings of the ageing research community,suggesting that ageing and osmoregulation may be linked. And as these genes are conserved right across to humans, there is the prospect that work on the tiny worm may have uncovered a general mechanism for response to hypertonic stress.
