Differential Expression of Hypothalamic and Gill-crh System With Osmotic Stress in the Euryhaline Black Porgy, Acanthopagrus schlegelii

Osmoregulatory contributions of the corticotropin-releasing factor system in the intestine of Atlantic salmon

The ability of euryhaline fishes to tolerate different environmental salinities depends upon the flexibility of their osmoregulatory organs, including the intestine. Several endocrine pathways contribute to the coordination of osmoregulatory processes in the teleost intestine; however, while the corticotropin-releasing factor (CRF) system has established osmoregulatory actions in the mammalian intestine, it is unclear whether the intestinal CRF system serves similar functions in teleosts. Therefore, we sought to determine whether the CRF system contributes to osmoregulatory processes in the intestine of Atlantic salmon (Salmo salar). We first showed using in vitro sac preparations that activation of CRF receptor type 2 (CRFR2) in the middle and posterior regions of the intestine reduces water, Na+ and Cl- absorption. However, co-activation of CRFR1 and CRFR2 inhibited water and Na+ absorption without affecting net Cl- absorption. We then assessed how the CRF system in the middle and posterior regions of the intestine was transcriptionally regulated during the seasonal acquisition of seawater tolerance (i.e. smoltification) and following changes in environmental salinity. Compared with parr, smolts had higher transcript levels of CRF ligands and this difference persisted following seawater transfer. Additionally, seawater transfer caused transient increases in transcription of urocortin 2 (ucn2) and crfr2 (posterior intestine only). Similar increases in ucn2 and crfr2 mRNA were observed following seawater to freshwater transfer of post-smolts. Our results indicate that the intestinal CRF system of Atlantic salmon contributes to osmoregulation during the initial days following changes in environmental salinity and that osmoregulatory actions of the intestinal CRF system are conserved across vertebrates.

Regulation and function of the gill corticotropin-releasing factor system during osmoregulatory disturbances in Atlantic salmon

While corticosteroids, including cortisol, have conserved osmoregulatory functions, the relative involvement of other stress-related hormones in osmoregulatory processes remains unclear. To address this gap, we initially characterized the gill corticotropin-releasing factor (CRF) system of Atlantic salmon (Salmo salar) and then determined: (1) how it is influenced by osmotic disturbances; (2) whether it is affected by cortisol; and (3) which physiological processes it regulates in the gills. Most CRF system components were expressed in the gills, with CRF receptor 2 (crfr2a), CRF binding protein (crfbp1 and crfbp2) and urocortin 2 (ucn2a) being the most abundant. The development of seawater tolerance in migratory juveniles (i.e. smolts) was associated with a general transcriptional upregulation of CRF ligands, but transcript levels of crfr2a, crfbp2, crfb2 and ucn2a decreased by ∼50% following seawater transfer. Accordingly, transfer of seawater-acclimated fish into freshwater increased crfr2a and ucn2a levels. Cortisol treatment of cultured gill filaments had marked effects on the CRF system; however, these effects failed to fully replicate changes observed during in vivo experiments, suggesting direct contributions of the gill CRF system during osmotic disturbances. Indeed, activation of the CRF system in cultured filaments from freshwater-acclimated (but not seawater-acclimated) salmon had transcriptional effects on several physiological systems (e.g. endothelial permeability, angiogenesis and immune regulation) which involved contributions by both CRF receptor subtypes. Overall, our results indicate that the gill CRF system is more active in hypoosmotic environments and directly contributes to the coordination of physiological responses following osmotic disturbances.

Isolation and characterisation of two epithelial-like cell lines from the gills of Chrysophrys auratus (Australasian snapper) and Oncorhynchus tshawytscha (Chinook salmon) and their use in aquatic toxicology

In vitro gill models are becoming increasingly important in aquatic toxicology, yet the fish gill invitrome is underrepresented, encompassing approximately 0.1% of extant species. Here, we describe the establishment and characterisation of two gill-derived, epithelial-like cell lines isolated from fish species of significant importance to New Zealand: Chrysophrys auratus (Australasian snapper) and Oncorhynchus tshawytscha (Chinook salmon). Designated CAgill1PFR (Chrysophrys auratus, gill 1, Plant & Food Research) and OTgill1PFR (Oncorhynchus tshawytscha, gill 1, Plant & Food Research), these cell lines have each been passaged greater than each 70 times over several years and are considered spontaneously immortalised. Both cell lines required serum for growth and exhibited differential responses to basal media formulations. CAgill1PFR was sensitive to low temperatures (4 °C) but replicated at high temperatures (30 °C), whereas OTgill1PFR was sensitive to high temperatures but remained viable at low temperatures, mirroring the natural environment of their host species. Immunostaining revealed expression of epithelial cell markers cytokeratin and E-cadherin, alongside positivity for the mesenchymal cell marker, vimentin. CAgill1PFR was more sensitive to the environmental toxin 3,4 dichloroaniline than OTgill1PFR through measurements of metabolic activity, membrane integrity, and lysosomal function. Furthermore, CAgill1PFR produced less CYP1A activity, indicative of ongoing biotransformation processes, in response to beta-naphthoflavone than OTgill1PFR. These cell lines expand the toolbox of resources and emphasise the need for species-specific aquatic toxicology research.

Response of CRH system in brain and gill of marine medaka to seawater acidification

Corticotropin-releasing hormone (CRH) is mainly secreted by the hypothalamus to regulate stress when environmental factors change. Gills contact with water directly and may also secrete CRH to maintain local homeostasis. Ocean acidification changes water chemical parameters and is becoming an important environmental stressor for marine fish. The response of brain and gill CRH systems to ocean acidification remains unclear. In this study, marine medaka were exposed to CO2-acidified seawater (440 ppm, 1000 ppm, and 1800 ppm CO2) for 2 h, 4 h, 24 h, and 7 d, respectively. At 2 h and 4 h, the expression of crh mRNA in gills increased with increasing CO2 concentration. Crh protein is expressed mainly in the lamellae cells. crhbp and crhr1 expression also increased significantly. However, at 2 h and 4 h, acidification caused little changes in these genes and Crh protein expression in the brain. At 7 d, Crh-positive cells were detected in the hypothalamus; moreover, Crh protein expression in the whole brain increased. It is suggested that CRH autocrine secretion in gills is responsible for local acid-base regulation rather than systemic mobilization after short-term acidification stress, which may help the rapid regulation of body damage caused by environmental stress.

Effects of Osmotic Stress on the mRNA Expression of prl, prlr, gr, gh, and ghr in the Pituitary and Osmoregulatory Organs of Black Porgy, Acanthopagrus schlegelii

In euryhaline teleost black porgy, Acanthopagrus schlegelii, the glucocorticoid receptor (gr), growth hormone receptor (ghr), prolactin (prl)-receptor (prlr), and sodium–potassium ATPase alpha subunit (α-nka) play essential physiological roles in the osmoregulatory organs, including the gill, kidney, and intestine, during osmotic stress. The present study aimed to investigate the impact of pituitary hormones and hormone receptors in the osmoregulatory organs during the transfer from freshwater (FW) to 4 ppt and seawater (SW) and vice versa in black porgy. Quantitative real-time PCR (Q-PCR) was carried out to analyze the transcript levels during salinity and osmoregulatory stress. Increased salinity resulted in decreased transcripts of prl in the pituitary, α-nka and prlr in the gill, and α-nka and prlr in the kidney. Increased salinity caused the increased transcripts of gr in the gill and α-nka in the intestine. Decreased salinity resulted in increased pituitary prl, and increases in α-nka and prlr in the gill, and α-nka, prlr, and ghr in the kidney. Taken together, the present results highlight the involvement of prl, prlr, gh, and ghr in the osmoregulation and osmotic stress in the osmoregulatory organs (gill, intestine, and kidney). Pituitary prl, and gill and intestine prlr are consistently downregulated during the increased salinity stress and vice versa. It is suggested that prl plays a more significant role in osmoregulation than gh in the euryhaline black porgy. Furthermore, the present results highlighted that the gill gr transcript’s role was solely to balance the homeostasis in the black porgy during salinity stress.

"Fishcide" effect of the fungicide difenoconazole in freshwater fish (Labeo rohita): A multi-endpoint approach

Difenoconazole is widely used to protect crops, fruits, and vegetables. However, this fungicide can enter aquatic environments and cause harmful effects to non-target organisms and induce little-known biological disorders. Thus, aiming to expand our knowledge about the ecotoxicity of difenoconazole on freshwater ichthyofauna, we aimed to determine the median lethal concentration (LC₅₀) of difenoconazole and evaluate its possible impacts from different toxicity biomarkers, using freshwater fish Labeo rohita as a model system. Using the probit analysis method, the 96 h LC₅₀ value of difenoconazole in the fish was calculated as 4.5 mg L^-1. Posteriorly, fish were exposed to two sublethal concentrations (0.45 mg L^-1 1/10th and 0.9 mg L^-1 1/5th LC₅₀ value) for 21 days. A significant reduction of superoxide dismutase (SOD) and catalase (CAT) activity was noted in the gill, liver, and kidneys of fish compared to the control groups. The level of glutathione-S-transferase (GST) and lipid peroxidation (LPO) activity was higher in all vital tissues of difenoconazole-treated fish. Histological alterations in the gill include epithelial lifting, lamellar fusion, hypertrophy, and epithelial necrosis. At the same time, the liver showed pyknotic nucleus, vacuolation, cellular edema and tubular necrosis, shrinkage of glomeruli, vacuolation, and pyknotic nuclei in the kidney. DNA damage was increased significantly with tail formation based on the concentration and time-dependent manner. Therefore, our study confirms that the exposure of L. rohita to difenoconazole induces negative biological consequences and sheds light on the danger of this fungicide for freshwater fish species. We believe that studies like ours can support actions and strategies for the remediation/mitigation of aquatic pollution by difenoconazole and for the conservation of freshwater ichthyofauna.

Localization of the Neuropeptide Arginine Vasotocin and Its Receptor in the Osmoregulatory Organs of Black Porgy, Acanthopagrus schlegelii: Gills, Kidneys, and Intestines

The neurohypophysial hormone arginine vasotocin (avt) and its receptor (avtr) regulates ions in the osmoregulatory organs of euryhaline black porgy (Acanthopagrus schlegelii). The localization of avt and avtr transcripts in the osmoregulatory organs has yet to be demonstrated. Thus, in the present study, we performed an in situ hybridization analysis to determine the localization of avt and avtr in the gills, kidneys, and intestines of the black porgy. The avt and avtr transcripts were identified in the filament and lamellae region of the gills in the black porgy. However, the basal membrane of the filament contained more avt and avtr transcripts. Fluorescence double tagging analysis revealed that avt and avtr mRNAs were partially co-localized with α-Nka-ir cells in the gill filament. The proximal tubules, distal tubules, and collecting duct of the kidney all had positive hybridization signals for the avt and avtr transcripts. Unlike the α-Nka immunoreactive cells, the avt and avtr transcripts were found on the basolateral surface of the distal convoluted tubule and in the entire cells of the proximal convoluted tubules of the black porgy kidney. In the intestine, the avt and avtr transcripts were found in the basolateral membrane of the enterocytes. Collectively, this study provides a summary of evidence suggesting that the neuropeptides avt and avtr with α-Nka-ir cells may have functions in the gills, kidneys, and intestines via ionocytes.

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.

Expression and Transcript Localization of star, sf-1, and dax-1 in the Early Brain of the Orange-Spotted Grouper Epinephelus coioides

We investigated the developmental expression and localization of sf-1 and dax-1 transcripts in the brain of the juvenile orange-spotted grouper in response to steroidogenic enzyme gene at various developmental ages in relation to gonadal sex differentiation. The sf-1 transcripts were significantly higher from 110-dah (day after hatching) and gradually increased up to 150-dah. The dax-1 mRNA, on the other hand, showed a decreased expression during this period, in contrast to sf-1 expression. At the same time, the early brain had increased levels of steroidogenic gene (star). sf-1 and star hybridization signals were found to be increased in the ventromedial hypothalamus at 110-dah; however, dax-1 mRNA signals decreased in the early brain toward 150-dah. Furthermore, the exogenous estradiol upregulated star and sf-1 transcripts in the early brain of the grouper. These findings suggest that sf-1 and dax-1 may have an antagonistic expression pattern in the early brain during gonadal sex differentiation. Increased expression of steroidogenic gene together with sf-1 during gonadal differentiation strongly suggests that sf-1 may play an important role in the juvenile grouper brain steroidogenesis and brain development.

Neurohypophysial Hormones Associated with Osmotic Challenges in the Brain and Pituitary of the Euryhaline Black Porgy, Acanthopagrus schlegelii

Our study showed differential expression of the arginine vasotocin (avt)/isotocin (it) in the brain and pituitary gland of the euryhaline black porgy (Acanthopagrus schlegelii) during osmotic stress. A decrease in serum osmolality and increased cortisol levels were observed after acute transfer from seawater (SW) to freshwater (FW). The increased expressions of avt, avt receptor (avtr: v1a), and isotocin receptor (itr: itr1) transcripts on day 1 and it and itr transcripts on days 7 and 30 were found in the brains and pituitary glands of FW fish. Increased levels of avt mRNA in the diencephalon and avtr mRNA in the pituitary together with serum cortisol on day 1 of FW exposure indicated activation of the hypothalamic-pituitary-interrenal (HPI) axis. The expression levels of avtr and itr after FW transfer were increased in the pituitary on days 7 and 30. Furthermore, in situ hybridization demonstrated spatially differential expression of avt and itr transcripts in nucleus preopticus parvocellularis of pars gigantocellularis (PMgc), magnocellularis (PMmc), and parvocellularis (PMpc) of the preoptic area (POA). Positive signals for avt and it were highly abundant in PMpc after FW exposure. The data suggest involvement of neurohypophysial hormones in the brain (telencephalon and diencephalon) and pituitary for osmotic stress.