Salt-tolerant fish like tilapia, Orechromis mossambicus, revert from actively absorbing salt in freshwater to secreting salt across their gill epithelium in seawater. This requires a fast remodelling of the cellular gill epithelium upon transfer from freshwater to seawater and is accompanied by changes in mRNA levels of several ATPases, ion transporters and water channels. Given what we know about mammalian osmotic regulation, it seems reasonable that transcriptional regulation plays a role in inducing these changes. So far, only one transcription factor (TonEBP) is known to be involved in vertebrate osmoregulation. Diego Fiol and colleagues from the University of California Davis now have evidence for two novel transcription factors that are specifically induced in response to hyperosmotic stress in fish.
Fiol's team reasoned that any transcriptional changes observed after they moved tilapia from freshwater to seawater would reflect a response to help the fish cope with increased saltiness. They got their first glimpse of the transcription factors that potentially mediate this response when they screened a cDNA library obtained by subtracting cDNA of a freshwater fish from cDNA of a fish that the authors had exposed to seawater for four hours. Among the induced transcripts they found two novel transcription factors. One of these they called osmotic stress transcription factor 1 (OSTF1), and to find some clues regarding its function they compared its gene sequence to other vertebrate gene sequences. They found that OSTF1 was similar to a splice variant of a glucocorticoid-induced leucine zipper (GILZ) transcription factor that responds to glucocorticoid treatment. This suggests that cortisol, a glucocorticoid hormone that plays a major role in the acclimation of fish to high salinities, may have a similar effect on OSTF1. The second novel factor was a homologue of TFIIB, a general transcription factor found in vertebrates. Tilapia TFIIB showed high sequence identity (89.6%) with other vertebrate TFIIBs and, thus, is likely to be very similar in function to these TFIIB homologues. Importantly, the team found that OSTF1 and TFIIB possess similar phosphorylation sites for stress-signalling kinases, indicating that these two novel factors mediate stress responses and appear to be affected by similar stress signals.
But were these two transcription factors really induced by hyperosmotic stress caused by a move from freshwater to seawater? The authors used quantitative PCR to follow the time course of mRNA expression before and after they transferred fish from freshwater to seawater, and found that OSTF1 and TFIIB levels significantly increased between one and four hours after the transfer. So these two factors are indeed co-induced when saltiness increases. Since there are many stress-induced genes, the authors took great care to show that these transcription factors were specifically linked to osmotic stress. Exposure to oxidative stress (H2O2) and heat-shock led to strong induction of heat-shock protein70, a general stress protein, but did not affect OSTF1 and TFIIB levels. So OSTF1 and TFIIB are not part of the general stress response, but are induced specifically by hyperosmotic stress.
Although the exact roles of these two novel transcription factors and the genes they affect need to be elucidated, homologies and sites of post-translational regulation provide insights into their possible functions. Their rapid induction suggests a role in the transduction of the osmosensory signal to the acclimation response to hyperosmotic conditions. Since TFIIB is known to interact with other transcription factors, its co-induction with OSTF1 in tilapia gill cells may indicate that these two transcription factors interact during osmotic stress.