Na+ K+ ATPase isoform switching in zebrafish during transition to dilute freshwater habitats

Cellular mechanisms of ion and acid-base regulation in teleost gill ionocytes

The mechanism(s) of sodium, chloride and pH regulation in teleost fishes has been the subject of intense interest for researchers over the past 100 years. The primary organ responsible for ionoregulatory homeostasis is the gill, and more specifically, gill ionocytes. Building on the theoretical and experimental research of the past, recent advances in molecular and cellular techniques in the past two decades have allowed for substantial advances in our understanding of mechanisms involved. With an increased diversity of teleost species and environmental conditions being investigated, it has become apparent that there are multiple strategies and mechanisms employed to achieve ion and acid-base homeostasis. This review will cover the historical developments in our understanding of the teleost fish gill, highlight some of the recent advances and conflicting information in our understanding of ionocyte function, and serve to identify areas that require further investigation to improve our understanding of complex cellular and molecular machineries involved in iono- and acid-base regulation.

Exposure to Total Suspended Solids (TSS) Impacts Gill Structure and Function in Adult Zebrafish

Total suspended solids (TSS) are a major contributor of anthropogenic impacts to aquatic systems. TSS exposure have been shown to affect the function of gills, but the mode of action is unclear. Zebrafish (Danio rerio) is emerging as an excellent model for mechanistic toxicology, and as there are no baseline studies on TSS effects in zebrafish gills, we tested the hypothesis that environmental concentrations of TSS damages gill structure and function in this species. Adult zebrafish were exposed to either 0, 10, 100, 500, 1000, or 2000 mg/L TSS for 4 days to assess the gill morphology. The minimal concentration that affected the gill structure was further tested for the distribution of key ion transporters, including Na+/K+- ATPase (NKA) and vacuolar-type H+-ATPase (VHA), using confocal microscopy. Our results reveal that TSS concentration as low as 100 mg/L alters the morphology of gills, including greater filament thickness, lamellae thickness, and epithelial lifting. This was also associated with a reduction in NKA immunoreactive (IR) cell count and intensity in the 100 mg/L TSS group, while there was neither a change in the VHA-IR cell count or expression nor the transcript abundance of atp6v1a and atp1a1a4 in the gills. Markers of stress response in these animals, including levels of cortisol, glucose, lactate, and glycogen were not altered after 4 days of TSS exposure. Overall, environmentally relevant concentrations of TSS can damage the gill structure and function in zebrafish and has the potential to enhance the toxicity of contaminants acting via the gills.

Impacts of sequential salinity and heat stress are recovery time-specific in freshwater crustacean, Daphnia pulicaria

Stressors can interact to affect animal fitness, but we have limited knowledge about how temporal variation in stressors may impact their combined effect. This limits our ability to predict the outcomes of pollutants and future dynamic environmental changes. Elevated salinity in freshwater ecosystems has been observed worldwide. Meanwhile, heatwaves have become more frequent and intensified as an outcome of climate change. These two stressors can jointly affect organisms; however, their interaction has rarely been explored in the context of freshwater ecosystems. We conducted lab experiments using Daphnia pulicaria, a key species in lakes, to investigate how elevated salinity and heatwave conditions collectively affect freshwater organisms. We also monitored the impacts of various recovery times between the two stressors. Daphnia physiological conditions (metabolic rate, Na+-K+-ATPase (NKA) activity, and lipid peroxidation level) and life history traits (survival, fecundity, and growth) in response to salt stress as well as mortality in heat treatment were examined. We found that Daphnia responded to elevated salinity by upregulating NKA activity and increasing metabolic rate, causing a high lipid peroxidation level. Survival, fecundity, and growth were all negatively affected by this stressor. These impacts on physiological conditions and life history traits persisted for a few days after the end of the exposure. Heat treatments caused mortality in Daphnia, which increased with rising temperature. Results also showed that individuals that experienced salt exposure were more susceptible to subsequent heat stress, but this effect decreased with increasing recovery time between stressors. Findings from this work suggest that the legacy effects from a previous stressor can reduce individual resistance to a subsequent stressor, adding great difficulties to the prediction of outcomes of multiple stressors. Our work also demonstrates that cross-tolerance/susceptibility and the associated mechanisms remain unclear, necessitating further investigation.

The role of salinity in recovery of white sturgeon (Acipenser transmontanus) from stimulated angling stress

White sturgeon (Acipenser transmontanus) in the Lower Fraser River are the focus of a catch-and-release angling fishery in British Columbia, Canada. However, the lower region of the catch area includes areas where tidal waters invade, and the consequence of salinity levels on recovery from an angling challenge are not characterized in sturgeon, despite theoretical implications of its import. We acclimated white sturgeon to various salinities (0, 10 and 20‰ (parts per thousand)) to investigate the effects of acclimation on recovery from stimulated angling stress that was induced through manual chasing. This challenge elicited the traditional physiological responses such as ion homeostasis disturbance, increases in secondary stress indicators and metabolic acidosis; however, environmental salinity altered the timing of recovery in some of the parameters measured. In addition, the severity of the intracellular pH disturbance in both heart and red blood cell seemed to be mediated in fresh water, yet the recovery pattern of plasma chloride and bicarbonate ions seemed to be facilitated by higher salinity. In general, responses were similar but not identical, leading us to conclude that the role of salinity on recovery from exercise is complex but not insignificant. Salinity may be important to behaviours exhibited by white sturgeon (such as migrations) in their respective saline environments, but less so around the impact of an angling stressor. Further exploration of this response may provide insight on whether the current tidal boundaries for angling white sturgeon are appropriate.

Triphenyl phosphate-induced pericardial edema in zebrafish embryos is dependent on the ionic strength of exposure media

Pericardial edema is commonly observed in zebrafish embryo-based chemical toxicity screens, and a mechanism underlying edema may be disruption of embryonic osmoregulation. Therefore, the objective of this study was to identify whether triphenyl phosphate (TPHP) - a widely used aryl phosphate ester-based flame retardant - induces pericardial edema via impacts on osmoregulation within embryonic zebrafish. In addition to an increase in TPHP-induced microridges in the embryonic yolk sac epithelium, an increase in ionic strength of exposure media exacerbated TPHP-induced pericardial edema when embryos were exposed from 24 to 72 h post-fertilization (hpf). However, there was no difference in embryonic sodium concentrations in situ within TPHP-exposed embryos relative to embryos exposed to vehicle (0.1% DMSO) from 24 to 72 hpf. Interestingly, increasing the osmolarity of exposure media with mannitol (an osmotic diuretic which mitigates TPHP-induced pericardial edema) and increasing the ionic strength of the exposure media (which exacerbates TPHP-induced pericardial edema) did not affect embryonic doses of TPHP, suggesting that TPHP uptake was not altered under these varying experimental conditions. Overall, our findings suggest that TPHP-induced pericardial edema within zebrafish embryos is dependent on the ionic strength of exposure media, underscoring the importance of further standardization of exposure media and embryo rearing protocols in zebrafish-based chemical toxicity screening assays.

Repeated Genetic Targets of Natural Selection Underlying Adaptation of Fishes to Changing Salinity

Ecological transitions across salinity boundaries have led to some of the most important diversification events in the animal kingdom, especially among fishes. Adaptations accompanying such transitions include changes in morphology, diet, whole-organism performance, and osmoregulatory function, which may be particularly prominent since divergent salinity regimes make opposing demands on systems that maintain ion and water balance. Research in the last decade has focused on the genetic targets underlying such adaptations, most notably by comparing populations of species that are distributed across salinity boundaries. Here, we synthesize research on the targets of natural selection using whole-genome approaches, with a particular emphasis on the osmoregulatory system. Given the complex, integrated and polygenic nature of this system, we expected that signatures of natural selection would span numerous genes across functional levels of osmoregulation, especially salinity sensing, hormonal control, and cellular ion exchange mechanisms. We find support for this prediction: genes coding for V-type, Ca2+, and Na+/K+-ATPases, which are key cellular ion exchange enzymes, are especially common targets of selection in species from six orders of fishes. This indicates that while polygenic selection contributes to adaptation across salinity boundaries, changes in ATPase enzymes may be of particular importance in supporting such transitions.

Triphenyl phosphate-induced pericardial edema is associated with elevated epidermal ionocytes within zebrafish embryos

Triphenyl phosphate (TPHP) is an organophosphate ester-based plasticizer and flame retardant. The objective of this study was to identify the potential role of epidermal ionocytes in mediating TPHP-induced pericardial edema within zebrafish embryos. Exposure to TPHP from 24 to 72 h post fertilization (hpf) resulted in a significant increase in pericardial edema and the number of ionocytes at 72 hpf relative to time-matched embryos treated with vehicle. In addition, co-exposure of embryos to mannitol (an osmotic diuretic) blocked TPHP-induced pericardial edema and effects on ionocyte abundance. However, knockdown of ATPase1a1.4 - an abundant Na⁺/K⁺-ATPase localized to epidermal ionocytes - mitigated TPHP-induced effects on ionocyte abundance but not pericardial edema, whereas co-exposure of embryos to ouabain - a Na⁺/K⁺-ATPase inhibitor - enhanced TPHP-induced pericardial edema but not ionocyte abundance. Overall, our findings suggest that TPHP may have multiple mechanisms of toxicity leading to an increase in ionocyte abundance and pericardial edema within developing zebrafish embryos.

Wild Zebrafish Sentinels: Biological Monitoring of Site Differences Using Behavior and Morphology

Environmental change poses a devastating risk to human and environmental health. Rapid assessment of water conditions is necessary for monitoring, evaluating, and addressing this global health danger. Sentinels or biological monitors can be deployed in the field using minimal resources to detect water quality changes in real time, quickly and cheaply. Zebrafish (Danio rerio) are ideal sentinels for detecting environmental changes due to their biomedical tool kit, widespread geographic distribution, and well-characterized phenotypic responses to environmental disturbances. Here, we demonstrate the utility of zebrafish sentinels by characterizing phenotypic differences in wild zebrafish between two field sites in India. Site 1 was a rural environment with flowing water, low-hypoxic conditions, minimal human-made debris, and high iron and lead concentrations. Site 2 was an urban environment with still water, hypoxic conditions, plastic pollution, and high arsenic, iron, and chromium concentrations. We found that zebrafish from Site 2 were smaller, more cohesive, and less active than Site 1 fish. We also found sexually dimorphic body shapes within the Site 2, but not the Site 1, population. Advancing zebrafish sentinel research and development will enable rapid detection, evaluation, and response to emerging global health threats.

Reductionist approaches to the study of ionoregulation in fishes

The mechanisms underlying ionoregulation in fishes have been studied for nearly a century, and reductionist methods have been applied at all levels of biological organization in this field of research. The complex nature of ionoregulatory systems in fishes makes them ideally suited to reductionist methods and our collective understanding has been dramatically shaped by their use. This review provides an overview of the broad suite of techniques used to elucidate ionoregulatory mechanisms in fishes, from the whole-animal level down to the gene, discussing some of the advantages and disadvantages of these methods. We provide a roadmap for understanding and appreciating the work that has formed the current models of organismal, endocrine, cellular, molecular, and genetic regulation of ion balance in fishes and highlight the contribution that reductionist techniques have made to some of the fundamental leaps forward in the field throughout its history.

Expression of ion transport genes in ionocytes isolated from larval zebrafish (Danio rerio) exposed to acidic or Na+-deficient water

In zebrafish (Danio rerio), a specific ionocyte subtype, the H⁺-ATPase-rich (HR) cell, is presumed to be a significant site of transepithelial Na⁺ uptake/acid secretion. During acclimation to environments differing in ionic composition or pH, ionic and acid-base regulations are achieved by adjustments to the activity level of HR cell ion transport proteins. In previous studies, the quantitative assessment of mRNA levels for genes involved in ionic and acid-base regulations relied on measurements using homogenates derived from the whole body (larvae) or the gill (adult). Such studies cannot distinguish whether any differences in gene expression arise from adjustments of ionocyte subtype numbers or transcriptional regulation specifically within individual ionocytes. The goal of the present study was to use fluorescence-activated cell sorting to separate the HR cells from other cellular subpopulations to facilitate the measurement of gene expression of HR cell-specific transporters and enzymes from larvae exposed to low pH (pH 4.0) or low Na⁺ (5 μM) conditions. The data demonstrate that treatment of larvae with acidic water for 4 days postfertilization caused cell-specific increases in H⁺-ATPase (atp6v1aa), ca17a, ca15a, nhe3b, and rhcgb mRNA in addition to increases in mRNA linked to cell proliferation. In fish exposed to low Na⁺, expression of nhe3b and rhcgb was increased owing to HR cell-specific regulation and elevated numbers of HR cells. Thus, the results of this study demonstrate that acclimation to low pH or low Na⁺ environmental conditions is facilitated by HR cell-specific transcriptional control and by HR cell proliferation.

Differential regulation of select osmoregulatory genes and Na+/K+-ATPase paralogs may contribute to population differences in salinity tolerance in a semi-anadromous fish

The Sacramento splittail (Pogonichthys macrolepidotus) is a species of special concern that is native to the San Francisco Estuary, USA. Two genetically distinct populations exist and differ in maximal salinity tolerances. We examined the expression of 12 genes representative of osmoregulatory functions in the gill over a 14  day time course at two different salinities [11 or 14 PSU (Practical Salinity Units)] and revealed that each population showed distinct patterns of gene expression consistent with population differences in response to osmotic regimes. The relatively more salinity-tolerant San Pablo population significantly upregulated nine out of the 12 transcripts investigated on day 1 of 11 PSU salinity exposure in comparison to the day zero freshwater control. Three transcripts (nka1a, nka1b, and mmp13) were differentially expressed between the populations at 7 and 14 days of salinity exposure, suggesting a reduced ability of the relatively salinity-intolerant Central Valley population to recover. Additionally, a phylogenetic analysis of several Sacramento splittail Na⁺/K⁺-ATPase α1 sequences resulted in grouping by proposed paralog rather than species, suggesting that different paralogs of this gene may exist. These findings, together with prior research conducted on the Sacramento splittail, suggest that the San Pablo population may be able to preferentially regulate select osmoregulatory genes, including different Na⁺/K⁺-ATPase α1 paralogs, to better cope with salinity challenges.

Reassessing the contribution of the Na+/H+ exchanger Nhe3b to Na+ uptake in zebrafish (Danio rerio) using CRISPR/Cas9 gene editing

Freshwater fishes absorb Na+ from their dilute environment using ion-transporting cells. In larval zebrafish (Danio rerio), Na+ uptake is coordinated by (1) Na+/H+-exchanger 3b (Nhe3b) and (2) H+-ATPase-powered electrogenic uptake in H+-ATPase-rich (HR) cells and by (3) Na+-Cl−­­-cotransporter (Ncc) expressed in NCC cells. The present study aimed to better understand the roles of these 3 proteins in Na+ uptake by larval zebrafish under ‘normal’ (800 µmol/L) and ‘low’ (10 µmol/L) Na+ conditions. We hypothesized that Na+ uptake would be reduced by CRISPR/Cas9 knockout (KO) of slc9a3.2 (encoding Nhe3b), particularly in low Na+ where Nhe3b is believed to play a dominant role. Contrary to this hypothesis, Na+ uptake was sustained in nhe3b KO larvae under both Na+ conditions, which led to the exploration of whether compensatory regulation of H+-ATPase or Ncc was responsible for maintaining Na+ uptake in nhe3b KO larvae. mRNA expression of the genes encoding H+-ATPase and Ncc were not altered in nhe3b KO. Moreover, morpholino knockdown of H+-ATPase, which significantly reduced H+ flux by HR cells, did not reduce Na+ uptake in nhe3b KO larvae, nor did rearing larvae in chloride-free conditions, thereby eliminating any driving force for Na+-Cl−-cotransport via Ncc. Finally, simultaneously treating nhe3b KO larvae with H+-ATPase morpholino and chloride-free conditions did not reduce Na+ uptake under normal or low Na+. These findings highlight the flexibility of the Na+ uptake system and demonstrate that Nhe3b is expendable to Na+ uptake in zebrafish and that our understanding of Na+ uptake mechanisms in this species is incomplete.