Diel Changes in the Molecular Physiology of Rainbow Trout Gill

Abstract

The regulation of salt and water balance is a basic physiological challenge in vertebrates. In aquatic vertebrates, such as fishes, ionoregulation is strongly influenced by water conditions which, in turn, can be altered by diel changes and abiotic factors such as light. In this study, the effects of light and dark cycles on gene expression in the whole gill of rainbow trout (Oncorhynchus mykiss) was quantified using real-time quantitative PCR and western blot analysis. It was hypothesized that diel changes would be observed in core-clock genes, which drive the circadian expression of many physiological processes in vertebrates, as well as in tight junction (TJ) proteins, which regulate the movement of salt and water between epithelial cells in the gill. As suggested, it was observed that mRNA abundance of several core-clock genes had diel rhythms that peaked at the dark-to-light transition or in the middle of the light period. Similarly, changes in select TJ proteins also followed a rhythmic pattern, with some transcripts showcasing a different oscillatory period than the expected 24 hours. Although mRNA abundance was rhythmic, interestingly, the protein abundance for some TJ proteins remained constant throughout the 24-hour experiment. Together, these results suggest that, at the molecular level, factors that generally contribute to regulating osmoregulation exhibit diel changes. These results will help bridge the current gap in knowledge regarding ionoregulation in the freshwater gill within the context of light and dark cycles, which may provide clues to evolutionary adaptations of fish and whether fish can anticipate changes in their environment in order to improve their survival.