Zebrafish as a Model System for Investigating the Compensatory Regulation of Ionic Balance during Metabolic Acidosis

Ammonia excretion by the fish gill: discoveries and ideas that shaped our current understanding

The fish gill serves many physiological functions, among which is the excretion of ammonia, the primary nitrogenous waste in most fishes. Although it is the end-product of nitrogen metabolism, ammonia serves many physiological functions including acting as an acid equivalent and as a counter-ion in mechanisms of ion regulation. Our current understanding of the mechanisms of ammonia excretion have been influenced by classic experimental work, clever mechanistic approaches, and modern molecular and genetic techniques. In this review, I will overview the history of the study of ammonia excretion by the gills of fishes, highlighting the important advancements that have shaped this field with a nearly 100-year history. The developmental and evolutionary implications of an ammonia and gill-dominated nitrogen regulation strategy in most fishes will also be discussed. Throughout the review, I point to areas in which more work is needed to push forward this field of research that continues to produce novel insights and discoveries that will undoubtedly shape our overall understanding of fish physiology.

Effects of sodium dodecylbenzene sulfonate (SDBS) on zebrafish (Danio rerio) gills and blood

Sodium dodecylbenzene sulfonate (SDBS) is an important surfactant used as a cleaning agent and industrial additive to remove unwanted chemicals which have been detected in the aquatic environment. The aim of this study was to examine the toxicological potential of SDBS on the gills of adult male zebrafish (Danio rerio) exposed to this chemical. For the 96 hr acute exposure, fish were divided into three groups: control, 0.25 mg/L, and 0.5 mg/L of SDBS. After the experiment, morphophysiological analyses (gill histopathology and histochemistry), oxidative stress (determination of gill activities of superoxide dismutase (SOD) and catalase (CAT)), and hematological analyses (leukocyte differentiation) were conducted. Data demonstrated that SDBS at both tested concentrations altered the histopathological index and initiated circulatory disturbances, as well as adverse, progressive, and immunological changes in the gills. In the 0.5 mg/L group, SOD activity decreased significantly, but CAT activity was not altered. Prominent blood changes observed in this group were neutrophilia and lymphocytosis. The number of mucous and chloride cells increased significantly in both groups. Taken together, our findings demonstrated that exposure of D. rerio to SDBS, even for 96 hr, produced adverse morphological and hematological effects associated with a reduction in SOD activity. Our findings indicate that exposure of aquatic species to the anionic surfactant SDBS may lead to adverse consequences associated with oxidative stress. Therefore, this study highlights the risks that this substance may pose to aquatic ecosystems and emphasizes the need for further investigations and strict regulations on its disposal.

Molecular Physiological Evidence for the Role of Na+-Cl− Co-Transporter in Branchial Na+ Uptake in Freshwater Teleosts

The gills are the major organ for Na+ uptake in teleosts. It was proposed that freshwater (FW) teleosts adopt Na+/H+ exchanger 3 (Nhe3) as the primary transporter for Na+ uptake and Na+-Cl− co-transporter (Ncc) as the backup transporter. However, convincing molecular physiological evidence to support the role of Ncc in branchial Na+ uptake is still lacking due to the limitations of functional assays in the gills. Thus, this study aimed to reveal the role of branchial Ncc in Na+ uptake with an in vivo detection platform (scanning ion-selective electrode technique, SIET) that has been recently established in fish gills. First, we identified that Ncc2-expressing cells in zebrafish gills are a specific subtype of ionocyte (NCC ionocytes) by using single-cell transcriptome analysis and immunofluorescence. After a long-term low-Na+ FW exposure, zebrafish increased branchial Ncc2 expression and the number of NCC ionocytes and enhanced gill Na+ uptake capacity. Pharmacological treatments further suggested that Na+ is indeed taken up by Ncc, in addition to Nhe, in the gills. These findings reveal the uptake roles of both branchial Ncc and Nhe under FW and shed light on osmoregulatory physiology in adult fish.

Comparative toxicokinetics and toxicity of PFOA and its replacement GenX in the early stages of zebrafish

PER: and poly-fluoroalkyl substances (PFAS) are receiving attention due to their persistence, and potential adverse effects on environmental and human health. Efforts to reduce long-chained PFAS (≥C8) compounds were implemented in 2006 as a part of "PFOA Stewardship Program Initiative" (PFOA-perfluorooctanoic acid). Short-chained PFAS (<C8) were introduced as replacements, which were believed to have lower potential for environmental persistence and bioaccumulation. Little is known about the uptake and elimination, and potential toxic effects of these replacement compounds. Hence, it is important to compare toxicokinetics and toxicity of long-chain PFAS to their replacement compounds. To this end, zebrafish (ZF), Danio rerio, embryos were exposed to PFOA and its short-chain replacement perfluoro (2-methyl-3-oxahexanoic) acid (GenX) with the aim to assess uptake and elimination kinetics, hatching success, morphology, startle response, and survival. At 24 hpf, LC50 was 82 μM for PFOA and 170 μM for GenX. At 54 hpf, GenX but not PFOA showed an increase in hatching success. At 120 hpf, no statistically significant differences were seen in white light startle response below the LC50. PFAS internal concentrations were measured at 72 and 120 hpf during exposure phase, and at 168 hpf during depuration phase. GenX and PFOA internal concentrations in 120 hpf larvae exposed to highest concentration (20 μM) were 35.02 and 44.51 μM, respectively. Concentrations were eliminated almost completely at 168 hpf for GenX up to 95%, while for PFOA up to 50%. As steady-state was not reached, we estimated kinetic bioconcentration factors (BCFkin). BCFkin for GenX was lower than PFOA at equimolar concentrations. However, bioconcentration factors were higher at the lower exposure concentrations for both chemicals, suggesting a concentration-dependent uptake of PFASs. The predicted internal effect concentrations, accounting for the differences in bioconcentration factors, were 229 μM for GenX and 226 μM for PFOA, suggesting similar toxic potencies.

Differential Branchial Response of Low Salinity Challenge Induced Prolactin in Active and Passive Coping Style Olive Flounder

Stress coping styles are very common in fish, and investigations into this area can greatly improve fish welfare and promote the sustainable development of aquaculture. Although most studies have focused on the behavioral and physiological differences of these fishes, the endocrine response of different coping styles fish when undergoing salinity challenge is still unclear. We examined the physiological response in olive flounder with active coping (AC) style and passive coping (PC) style after transferred from seawater (SW) to freshwater for 0, 2, 5, 8, and 14 days. The results showed that: 1) the plasma prolactin level of FW-acclimated AC flounder was substantially higher than that of FW-acclimated PC flounder at 5, 8, and 14 days, and the branchial gene expression of prolactin receptor (PRLR) in AC flounder was slightly higher than PC flounder after transfer. While there was no remarkable difference observed in cortisol (COR) levels between AC and PC flounder. After transfer, glucocorticoid receptor (GR) expression in AC flounder was significantly higher compared with PC flounder at 8 days. 2) Branchial NKA-IR ionocytes numbers were reduced in PC flounder after transfer, while ionocytes number remain stable in AC flounder. 3) The branchial stem cell transcription factor foxi1 gene expression of AC flounder was significantly higher than PC flounder at 2, 5, and 14 days after transfer, while branchial stem cell transcription factor p63 gene expression of FW-acclimated AC flounder was only substantially higher than that of PC flounder at 5 days. 4) As an apoptosis upstream initiator, the branchial gene expression of caspase-9 in PC flounder was considerably higher than in AC flounder after transfer at 8 days. This study revealed that olive flounder with active and passive coping styles have different endocrine coping strategies after facing the low-salinity challenge. AC flounder adopt an active endocrine strategy by increasing ionocyte differentiation and prolactin secretion significantly. In contrast, PC flounder employ a passive strategy of reducing ionocytes differentiation and retaining prolactin content at a low level to reduce branchial ionocytes number.

In Vivo Functional Assay in Fish Gills: Exploring Branchial Acid-Excreting Mechanisms in Zebrafish

Molecular and physiological analyses in ionoregulatory organs (e.g., adult gills and embryonic skin) are essential for studying fish ion regulation. Recent progress in the molecular physiology of fish ion regulation was mostly obtained in embryonic skin; however, studies of ion regulation in adult gills are still elusive and limited because there are no direct methods for in vivo functional assays in the gills. The present study applied the scanning ion-selective electrode technique (SIET) in adult gills to investigate branchial H⁺-excreting functions in vivo. We removed the opercula from zebrafish and then performed long-term acid acclimation experiments. The results of Western blot and immunofluorescence showed that the protein expression of H⁺-ATPase (HA) and the number of H⁺-ATPase-rich ionocytes were increased under acidic situations. The SIET results proved that the H⁺ excretion capacity is indeed enhanced in the gills acclimated to acidic water. In addition, both HA and Na⁺/H⁺ exchanger (Nhe) inhibitors suppressed the branchial H⁺ excretion capacity, suggesting that H⁺ is excreted in association with HA and Nhe in zebrafish gills. These results demonstrate that SIET is effective for in vivo detection in fish gills, representing a breakthrough approach for studying the molecular physiology of fish ion regulation.

A novel K+ -dependent Na+ uptake mechanism during low pH exposure in adult zebrafish (Danio rerio): New tricks for old dogma

AIM

To determine whether Na⁺ uptake in adult zebrafish (Danio rerio) exposed to acidic water adheres to traditional models reliant on Na⁺ /H⁺ Exchangers (NHEs), Na⁺ channels and Na⁺ /Cl^- Cotransporters (NCCs) or if it occurs through a novel mechanism.

METHODS

Zebrafish were exposed to control (pH 8.0) or acidic (pH 4.0) water for 0-12 hours during which ²² Na⁺ uptake ( J Na in ), ammonia excretion, net acidic equivalent flux and net K⁺ flux ( J H net ) were measured. The involvement of NHEs, Na⁺ channels, NCCs, K⁺ -channels and K⁺ -dependent Na⁺ /Ca²⁺ exchangers (NCKXs) was evaluated by exposure to Cl^- -free or elevated [K⁺ ] water, or to pharmacological inhibitors. The presence of NCKXs in gill was examined using RT-PCR.

RESULTS

J Na in was strongly attenuated by acid exposure, but gradually recovered to control rates. The systematic elimination of each of the traditional models led us to consider K⁺ as a counter substrate for Na⁺ uptake during acid exposure. Indeed, elevated environmental [K⁺ ] inhibited J Na in during acid exposure in a concentration-dependent manner, with near-complete inhibition at 10 mM. Moreover, J H net loss increased approximately fourfold at 8-10 hours of acid exposure which correlated with recovered J Na in in 1:1 fashion, and both J Na in and J H net were sensitive to tetraethylammonium (TEA) during acid exposure. Zebrafish gills expressed mRNA coding for six NCKX isoforms.

CONCLUSIONS

During acid exposure, zebrafish engage a novel Na⁺ uptake mechanism that utilizes the outwardly directed K⁺ gradient as a counter-substrate for Na⁺ and is sensitive to TEA. NKCXs are promising candidates to mediate this K⁺ -dependent Na⁺ uptake, opening new research avenues about Na⁺ uptake in zebrafish and other acid-tolerant aquatic species.

Bioconcentration and toxicological impacts of fipronil and 2,4-D commercial formulations (single and in mixture) in the tropical fish, Danio rerio

The insecticide fipronil and the herbicide 2,4-D are the most applied pesticides in sugarcane crops leading to aquatic contamination. The whole-body bioconcentration of fipronil and 2,4-D, single and in mixture, was evaluated in Danio rerio after 96-h exposure. The activities of catalase (CAT) and glutathione S-transferase(GST) in whole body and in the gills and the acetylcholinesterase (AChE) in muscle were determined. The gill histopathology and the morphology of the pavement (PVC) and the mitochondria-rich(MRC) cells at gill surface were analyzed. Bioconcentration occurred after exposure to fipronil (2.69 L kg^-1) and 2,4-D (1.73 L kg^-1) single and in mixture of fipronil (3.10 L kg^-1) and 2,4-D (1.27 L kg^-1). Whole-body CAT activity was unchanged, and its activity decreased in the gills after exposure to fipronil and increased after exposure to 2,4-D and mixture. GST and AChE increased after single exposure to each pesticide and mixture of both. Fish exposed to mixture increased the MRC fractional area (MRCFA) which suggested possible ionic regulation disturbance and reduced the microridge of the PVC surface. Synergistic interactions occurred in the CAT activity and MRCFA after exposure to mixture of pesticides. The results indicate that the recommended application dose of fipronil and 2,4-D, single or in mixture, for sugarcane crops affects this fish species altering its homeostasis.

Effects of Bicarbonate Stress on Serum Ions and Gill Transporters in Alkali and Freshwater Forms of Amur Ide (Leuciscus waleckii)

The Amur ide (Leuciscus waleckii) is a fish in the Cyprinidae family. Compared with other Amur ide living in freshwater ecosystems, the Amur ide population in Lake Dali Nor of China is famous for its high tolerance to the alkaline conditions of 54 mM (pH 9.6). Yet, surprisingly, the ionoregulatory mechanism responsible for this remarkable alkaline adaptation remains unclear. Therefore, this study sought to investigate how bicarbonate affects the acid-base balancing and ionoregulatory responses of this animal. Here, using a comparative approach, the alkali form of Amur ide and its ancestral freshwater form living in other freshwater basins were each exposed to 50 mM (pH 9.59 ± 0.09), a level close to the alkalinity of Lake Dali Nor, and their physiological (AE1) adjustment of ions and acid-base regulation were investigated. This study highlighted differences in blood pH and serum ions (e.g., Na⁺, K⁺, Cl⁻, and Ca²⁺), Na⁺/K⁺ ATPase (NKA) activity and its mRNA level, and mRNA expression of gill transporters (Na⁺/H⁺ exchanger member 2 and/or 3, Na⁺/HCO3- cotransporter (NBC1), Cl⁻/HCO3- exchanger, Na⁺/Cl⁻ cotransporter (NCC), Na⁺/K⁺/2Cl⁻ (NKCC1), SLC26A5, and SLC26A6) for alkalinity adaptation between the two forms of Amur ide differing in alkalinity tolerance. Specifically, close relationships among the serum Na⁺ and mRNA levels of NCC, NKCC1, and NHE, and also NKA and NBC1, in addition to serum Cl⁻ and bicarbonate transporters (e.g., SLC26A5 and SLC26A6), characterized the alkali form of Amur ide. We propose that this ecotype can ensure its transepithelial Cl⁻ and Na⁺ uptake/base secretions are highly functional, by its basolateral NKA with NBC1 and apical ionic transporters, and especially NCC incorporated with other transporters (e.g., SLC26). This suggests an evolved strong ability to maintain an ion osmotic and acid-base balance for more effectively facilitating its adaptability to the high alkaline environment. This study provides new insights into the physiological responses of the alkaline form of the Amur ide fish for adapting to extreme alkaline conditions. This information could be used as a reference to cultivating alkaline-tolerant fish species in abandoned alkaline waters.

A Transcriptome Insight During Early Fish Larval Development Followed by Starvation in Seriola rivoliana

We investigated a time-course larval transcriptional analysis (RNA-seq) in the longfin yellowtail Seriola rivoliana, from hatching to day four at 22 °C, without providing zooplankton as food. Larval starvation is a critical physiological stage that must be prevented to ensure survival. However, the transcriptional mechanisms to endure starvation have not been investigated in marine fish. Differential gene expression showed newly day-specific transcriptome events during larval development. On day 1 (yolk sac absorption), the predominant upregulated developmental processes were larval growth, muscle and vision development, cytoskeletal structure, protein synthesis, protein and fat digestion-absorption, and hormone biosynthesis, whereas the cell cycle was suppressed. On day 2 (yolk sac exhaustion), a new stage of energy regeneration (ATP) was supplied by the oil drop reserve, whereas protein digestion-absorption and growth were suppressed. On day 3 (mouth opening and starvation), stress signals and nutrition deprivation upregulated the p53 signal and triggered autophagy and the AMP-activated protein kinase (AMPK) pathways as an alternative catabolic pathway to enduring starvation, and the circadian rhythm was established. On day 4 (starving and weakened larvae condition), autophagy supported subsequent protein synthesis, activated the immune system, and promoted estrogen signaling and skeleton renovation. However, larvae suppressed muscle development, vision and carbohydrate, and fat digestion-absorption and became lethargic, evidencing limited physiological support by autophagy to maintain survival without exogenous nutrition in this species.

Toxicities of copper oxide nanomaterial and copper sulphate in early life stage zebrafish: Effects of pH and intermittent pulse exposure

Effort has been made to standardise regulatory ecotoxicity tests for engineered nanomaterials (ENMs), but the environmental realism of altered water quality and/or pulse exposure to these pollutants should be considered. This study aimed to investigate the relative toxicity to early life-stage zebrafish of CuO ENMs at acid pH and then under pulse exposure conditions, all compared to CuSO₄. At all pH values, CuSO₄ was more toxic to zebrafish than CuO ENMs. Additions of H⁺ were protective of CuSO₄ toxicity, with median lethal concentrations LC₅₀ (with 95% confidence intervals) of: 0.36 (0.33-0.40), 0.22 (0.20-0.24) and 0.27 (0.25-0.29) mg L^-1 at pH 5, pH 6 and pH 7, respectively. In contrast, the toxicity of CuO ENMs increased with acidity; LC₅₀ values were: 6.6 (4.5-8.5), 19.4 (11.6-27.2) and >100 mg L^-1 at pH 5, pH 6 and pH 7, respectively. The increased toxicity of the CuO ENMs in acid water corresponded with greater dissolution of dissolved Cu from the particles at low pH, suggesting free Cu²⁺ ion delivery to the zebrafish was responsible for the pH-effect. In continuous 96 h exposures to the substances at the LC₁₀ values and at pH 6, both CuSO₄ and CuO ENMs caused Cu accumulation, inhibition of Na⁺/K⁺-ATPase and depletion of total glutathione in zebrafish. However, two 24 h pulses of CuSO₄ or CuO ENMs at the same peak concentration caused similar effects to the continuous 96 h exposure, despite the shorter exposure durations of the former; suggesting that the pulses were more hazardous than the continuous exposure. In conclusion, the current water quality correction for pH with respect to Cu toxicity to freshwater fish should not be applied to the nano form. Crucially, CuO ENMs are more toxic in pulse than continuous exposure and new corrections for both water pH and the Cu exposure profile are needed for environmental risk assessment.

Zebrafish models of diabetes-related CNS pathogenesis

Diabetes mellitus (DM) is a common metabolic disorder that affects multiple organ systems. DM also affects brain processes, contributing to various CNS disorders, including depression, anxiety and Alzheimer's disease. Despite active research in humans, rodent models and in-vitro systems, the pathogenetic link between DM and brain disorders remains poorly understood. Novel translational models and new model organisms are therefore essential to more fully study the impact of DM on CNS. The zebrafish (Danio rerio) is a powerful novel model species to study metabolic and CNS disorders. Here, we discuss how DM alters brain functions and behavior in zebrafish, and summarize their translational relevance to studying DM-related CNS pathogenesis in humans. We recognize the growing utility of zebrafish models in translational DM research, as they continue to improve our understanding of different brain pathologies associated with DM, and may foster the discovery of drugs that prevent or treat these diseases.

Novel discoveries in acid-base regulation and osmoregulation: A review of selected hormonal actions in zebrafish and medaka

Maintenance of internal ionic and acid-base homeostasis is critical for survival in all biological systems. Similar to mammals, aquatic fishes have developed sophisticated homeostatic mechanisms to mitigate metabolic or environmental disruptions in ionic and acid-base status of systemic body fluids via hormone-controlled transport of ions or acid equivalents. The present review summarizes newly discovered actions of several hormones in zebrafish (Danio rerio) and medaka (Oryzias latipes) that have greatly contributed to our overall understanding of ionic/acid-base regulation. For example, isotocin and cortisol were reported to enhance transport of various ions by stimulating the proliferation and/or differentiation of ionocyte progenitors. Meanwhile, stanniocalcin-1, a well-documented hypocalcemic hormone, was found to suppress ionocyte differentiation and thus downregulate secretion of H⁺ and uptake of Na⁺ and Cl^-. Estrogen-related receptor and calcitonin gene-related peptide also regulate the differentiation of certain types of ionocytes to either stimulate or suppress H⁺ secretion and Cl^- uptake. On the other hand, endothelin and insulin-like growth factor 1 activate the respective secretion of H⁺ and Na⁺/Cl through fast actions. These new findings enhance our understanding of how hormones regulate fish ionic and acid-base regulation while further providing new insights into vertebrate evolution, mammalian endocrinology and human disease-related therapeutics.

Cisplatin exposure impairs ionocytes and hair cells in the skin of zebrafish embryos

This study aimed to assess the sublethal effects of a platinum-based compound, cisplatin, using a zebrafish model. Zebrafish embryos were incubated in different concentrations of cisplatin at 0-96 h post-fertilization. Using a non-invasive, scanning ion-selective electrode technique (SIET), we measured the functions of hair cells (Ca²⁺ influx) and ionocytes ([H⁺] gradients). The survival rate, hatching rate, phenotype, body length, whole-body ion (Na⁺, Cl^-, and Ca²⁺) and Pt contents were also determined. The effects of cisplatin on zebrafish embryos were demonstrated as first impairing hair cell function (at 1 μM of cisplatin), the hair cell number, and body ion content of Cl^- (at 10 μM of cisplatin), then decreasing ionocyte acid secretion and overall body ion contents of Na⁺ and Ca²⁺ (at 50 μM of cisplatin). The body length and ionocyte density decreased at 100 μM of cisplatin, and survival decreased at 500 μM of cisplatin. As the cisplatin concentration increased, the accumulation of Pt in fish embryos also increased. These results revealed that hair cells are significantly more susceptible to cisplatin toxicity than ionocytes. By determining the lowest observed effective concentration of cisplatin that caused in vivo functional alterations of zebrafish hair cells and skin ionocytes, this model demonstrated 500-fold greater sensitivity than by detecting changes in survival, for early assessment of the effects of platinum-based chemotherapeutic drugs on fish.