The regulation of salt and water balance in aquatic organisms

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Life in water can be challenging and a fundamental problem for most animals that reside there is the maintenance of salt and water balance. This is because aquatic animals maintain stable salt levels in the body (e.g. ions such as sodium, chloride, and calcium) despite the fact that many inhabit environments where salt levels are much lower or much higher than body fluids. For example, the salt content of freshwater is considerably lower than that of blood, causing passive ion loss (diffusion) from the body to the surroundings and water loading (excessive hydration). In contrast, for many seawater animals, their surroundings are saltier than their blood, resulting in ion loading and water loss (dehydration). For aquatic animals to maintain salt and water balance, epithelia of ionoregulatory organs function as tissue barriers that regulate the movement of ions and water between fluid compartments (i.e. water and blood). Ion transport across epithelia occurs via two routes. The first is through cells (i.e. the transcellular pathway) and the second is between cells (i.e. the paracellular pathway). Transcellular ion transport is governed by a complex suite of transcellular transport proteins and ion channels that are comparatively well documented in aquatic animals. The paracellular route is governed by proteins of the tight junction (TJ) complex in vertebrates or septate junction (SJ) complex in invertebrates, and in contrast to transcellular transport proteins, very little is known about TJ and SJ proteins in aquatic animals. This is alarming because the permeability of the paracellular pathway in aquatic animals is broadly acknowledged to play an important role in the regulation of salt and water balance. As such we are presented with a serious gap in our fundamental knowledge of how aquatic animals function in a natural setting and how these organisms are equipped to cope with environmental change. My research programs is focused on elucidating the functional properties of TJ and SJ proteins in aquatic animals by identifying new proteins in these animals, characterizing them, studying how they respond to environmental change and examining the functional consequences of knocking down/out TJ or SJ protein gene expression. In the coming cycle I also propose to examine sex-specific differences in the molecular physiology of the aquatic vertebrate TJ complex. This research is important to anyone who has an interest in how aquatic organisms maintain "homeostasis" (i.e. internal stability) or who are interested in how biological material moves across the epithelial barriers of aquatic animals. This is a broad populace that ranges from basic researchers to those who investigate the effects of pollutants and the impact of human activity on aquatic organisms. In addition, those who seek to optimize the delivery of biological material (e.g. drugs) to aquatic organisms via the paracellular pathway are also informed by this research.