Rh proteins and H+ transporters involved in ammonia excretion in Amur Ide (Leuciscus waleckii) under high alkali exposure

High alkaline environment can lead to respiratory alkalosis and ammonia toxification to freshwater fish. However, the Amur ide (Leuciscus waleckii), which inhabits an extremely alkaline lake in China with titratable alkalinity up to 53.57 mM (pH 9.6) has developed special physiological and molecular mechanisms to adapt to such an environment. Nevertheless, how the Amur ide can maintain acid-base balance and perform ammonia detoxification effectively remains unclear. Therefore, this study was designed to study the ammonia excretion rate (Tamm), total nitrogen accumulation in blood and tissues, including identification, expression, and localization of ammonia-related transporters in gills of both the alkali and freshwater forms of the Amur ide. The results showed that the freshwater form Amur ide does not have a perfect ammonia excretion mechanism exposed to high-alkaline condition. Nevertheless, the alkali form of Amur ide was able to excrete ammonia better than freshwater from Amur ide, which was facilitated by the ionocytes transporters (Rhbg, Rhcg1, Na+/H+ exchanger 2 (NHE2), and V-type H+ ATPase (VHA)) in the gills. Converting ammonia into urea served as an ammonia detoxication strategy to reduced endogenous ammonia accumulation under high-alkaline environment.

Transcriptomic and proteomic analyses provide insights into the adaptive responses to the combined impact of salinity and alkalinity in Gymnocypris przewalskii

Gymnocypris przewalskii is the only high-land endemic teleost living in Qinghai Lake, the largest saline-alkaline lake in China. Its osmoregulatory physiology remains elusive due to a lack of precise identification of the response proteins. In the present study, DIA/SWATH was used to identify differentially expressed proteins (DEPs) under alkaline (pH = 10.1, carbonate buffer), saline (12‰, sodium chloride), and saline-alkaline [carbonate buffer (pH = 10.1) plus 11‰ sodium chloride] stresses. A total of 66,056 unique peptides representing 7,150 proteins and 230 DEPs [the false discovery rate (FDR) ≤ 0.05, fold change (FC) ≥ 1.5] were identified under different stresses. Comparative analyses of the proteome and transcriptome indicated that over 86% of DEPs did not show consistent trends with mRNA. In addition to consistent enrichment results under different stresses, the specific DEPs involved in saline-alkaline adaptation were primarily enriched in functions of homeostasis, hormone synthesis and reactions of defense response, complement activation and reproductive development. Meanwhile, a protein-protein interaction (PPI) network analysis of these specific DEPs indicated that the hub genes were ITGAX, MMP9, C3, F2, CD74, BTK, ANXA1, NCKAP1L, and CASP8. This study accurately isolated the genes that respond to stress, and the results could be helpful for understanding the physiological regulation mechanisms regarding salinity, alkalinity, and salinity-alkalinity interactions.

The Rhesus glycoprotein Rhcgb is expendable for ammonia excretion and Na+ uptake in zebrafish (Danio rerio)

In zebrafish (Danio rerio), the ammonia-transporting Rhesus glycoprotein Rhcgb is implicated in mechanisms of ammonia excretion and Na⁺ uptake. In particular, Rhcgb is thought to play an important role in maintaining ammonia excretion in response to alkaline conditions and high external ammonia (HEA) exposure, in addition to facilitating Na⁺ uptake via a functional metabolon with the Na⁺/H⁺-exchanger Nhe3b, specifically under low Na⁺ conditions. In the present study, we hypothesized that CRISPR/Cas9 knockout of rhcgb would reduce ammonia excretion and Na⁺ uptake capacity, particularly under the conditions listed above that have elicited increases in Rhcgb-mediated ammonia excretion and/or Na⁺ uptake. Contrary to this hypothesis, however, larval and juvenile rhcgb knockout (KO) mutants showed no reductions in ammonia excretion or Na⁺ uptake under any of the conditions tested in our study. In fact, under control conditions, rhcgb KO mutants generally displayed an increase in ammonia excretion, potentially due to increased transcript abundance of another rh gene, rhbg. Under alkaline conditions, rhcgb KO mutants were also able to maintain ammonia excretion, similar to wild-type fish, and stimulation of ammonia excretion after HEA exposure also was not affected by rhcgb KO. Surprisingly, ammonia excretion and Na⁺ uptake were unaffected by rhcgb or nhe3b KO in juvenile zebrafish acclimated to normal (800 μmol/L) or low (10 μmol/L) Na⁺ conditions. These results demonstrate that Rhcgb is expendable for ammonia excretion and Na⁺ uptake in zebrafish, highlighting the plasticity and flexibility of these physiological systems in this species.

The toxic effect of triclosan and methyl-triclosan on biological pathways revealed by metabolomics and gene expression in zebrafish embryos

The omnipresence of antimicrobial triclosan (TCS) and by-products in aquatic environments is a threat to aquatic organisms. Traditionally, the adverse effects of TCS and its by-products have been evaluated by examining the phenotypic output relevant to predicting acute toxicity rather than studying the perturbation of biological pathways. Identifying alterations in the key pathways and molecular mechanisms caused by toxic chemicals helps researchers assess the ecological risks of TCS and its by-products to aquatic environments. In this study, we used metabolomics and reverse transcription qPCR to investigate the adverse effects of a wide range of concentrations of triclosan and its derivative methyl-triclosan (MTCS), ranging from relatively low environmentally relevant levels (ng/L) to high-dose concentrations (sublethal concentration), on zebrafish (Danio rerio) embryos. The metabolism and transcriptome analysis revealed changes in the metabolite and transcripts expression of zebrafish embryos after 96 h exposure at 30 μg/L and 300 μg/L of TCS, 400 μg/L of MTCS and the TCS/MTCS mixture (30 μg/L TCS + 3 μg/L MTCS and 300 μg/L TCS + 30 μg/L MTCS). Significant dysregulations in the expression of the urea transporter (UT), glucose-6-phosphate dehydrogenase (G6PD), alanine transaminase (ALT), glutamate dehydrogenase (GDH), phosphoglucomutase (PGM), and fatty acid synthase (FASN), together with changes in alanine, urea, glucose, 6-phosphogluconalactone, and palmitic acid were observed in the TCS, MTCS, and TCS/MTCS treatments. Particularly, the MTCS treatment group showed fold changes in the mRNA expression of nitrogen metabolism, energy metabolism, and fatty acid synthesis, indicating a disruption of the zebrafish embryos' biological pathways. The changes in the metabolites and gene expressions induced by the TCS, MTCS and the TCS/MTCS mixture treatment demonstrate the pathway changes in starch and sucrose metabolism, nitrogen metabolism, fatty acid synthesis, and phenylalanine, tyrosine and tryptophan biosynthesis. Therefore, our study provides better insights into the risks of the parental compound (TCS) and its by-product (MTCS), as well as the perturbation in biological pathways induced by these two compounds in aquatic environments.

Assessment of the effect of methyl-triclosan and its mixture with triclosan on developing zebrafish (Danio rerio) embryos using mass spectrometry-based metabolomics

Methyl-triclosan (MTCS), as a biodegradation product from antibacterial triclosan (TCS), has been detected in water catchments, and it has also been verified to accumulate in biota due to its hydrophobicity. There is a lack, however, of toxicity studies on MTCS and its effects on organisms in conjunction with TCS. In this study, exposure experiments were conducted to assess the toxicity to embryonic zebrafish of selected concentrations of MTCS (from 1 ng/L to 400 μg/L) and MTCS/TCS mixtures (from 1 μg/L TCS and 100 ng/L MTCS to 300 μg/L TCS and 30 μg/L MTCS). Specimens were extracted using acetonitrile: isopropanol: water (3:3:2; v/v/v) and then analyzed using Gas chromatography-mass spectrometry (GC-MS) to identify the metabolites based on the Fiehn library database. The results showed that MTCS exposure led to the alterations of the metabolomes of the zebrafish embryos, including level changes of l-valine, d-mannose, d-glucose, and other metabolites. Multivariate analysis (PCA, PLS-DA, sPLS-DA) and univariate analysis (one-way ANOVA) indicated differences between the control and exposure groups of the metabolites, indicating that biological pathways, such as amino acid synthesis, pentose phosphate pathway (PPP), starch and sucrose metabolism were influenced. Moreover, when the embryos were exposed to a mix of TCS and MTCS, TCS dominated the mixture's effect on biological pathways because the concentration ratio within the mixture, which mimics environmental ratio of 10 TCS : 1 MTCS, leads to high bioavailability of TCS.

Metabolomic profiling of zebrafish (Danio rerio) embryos exposed to the antibacterial agent triclosan

Triclosan, a widely used antibacterial and antifungal agent, is ubiquitously detected in the natural environment. There is increasing evidence that triclosan can produce cytotoxic, genotoxic, and endocrine disruptor effects in aquatic biota, including algae, crustaceans, and fish. Metabolomics can provide important information regarding molecular-level effects and toxicity of xenobiotic chemicals in aquatic organisms. The aim of the present study was to assess the toxicity of triclosan in developing zebrafish (Danio rerio) embryos using gas chromatography-mass spectrometry (GC-MS)-based metabolomics. The embryos were exposed to a wide range of triclosan concentrations (10 ng/L-500 µg/L). Endogenous metabolites were extracted using acetonitrile:isopropanol:water (3:3:2, v/v/v). Derivatization of metabolites was performed prior to identification and quantification via GC-MS analysis. A total of 29 metabolites were positively identified in embryos. Univariate (one-way analysis of variance) and multivariate (principal components analysis and projection to latent structure-discriminant analysis) analyses were employed to determine metabolic profile changes in triclosan-exposed embryos. Eight metabolites were significantly altered (p < 0.05) in embryos exposed to triclosan (urea, citric acid, D-(+)-galactose, D-glucose, stearic acid, L-proline, phenylalanine, and L-glutamic acid). The results suggest that triclosan exposure can result in impairment of several pathways in developing zebrafish embryos, with implications for energy metabolism and amino acid metabolism, as well as nitrogen metabolism and gill function. These findings will benefit future risk assessments of triclosan and other contaminants of emerging concern. Environ Toxicol Chem 2019;38:240-249. © 2018 SETAC.