Effects of salinity and prolactin on gene transcript levels of ion transporters, ion pumps and prolactin receptors in Mozambique tilapia intestine

Changes in cortisol and corticosteroid receptors during dynamic salinity challenges in Mozambique tilapia

In estuarine environments, euryhaline fish maintain a narrow range of internal osmolality despite daily changes in environmental salinity that can range from fresh water (FW) to seawater (SW). The capacity of euryhaline fish to maintain homeostasis in a range of environmental salinities is primarily facilitated by the neuroendocrine system. One such system, the hypothalamic-pituitary-interrenal (HPI) axis, culminates in the release of corticosteroids such as cortisol into circulation. Cortisol functions as both a mineralocorticoid and glucocorticoid in fish because of its roles in osmoregulation and metabolism, respectively. The gill, a key site for osmoregulation, and the liver, the primary storage site for glucose, are known targets of cortisol's actions during salinity stress. While cortisol facilitates acclimation to SW environments, less is known on its role during FW adaptation. In this study, we characterized the responses of plasma cortisol, mRNA expression of pituitary pro-opiomelanocortin (pomc), and mRNA expression of liver and gill corticosteroid receptors (gr1, gr2, and mr) in the euryhaline Mozambique tilapia (Oreochromis mossambicus) under salinity challenges. Specifically, tilapia were subjected to salinity transfer regimes from steady-state FW to SW, SW to FW (experiment 1) or steady state FW or SW to tidal regimen (TR, experiment 2). In experiment 1, fish were sampled at 0 h, 6 h, 1, 2, and 7 d post transfer; while in experiment 2, fish were sampled at day 0 and day 15. We found a rise in pituitary pomc expression and plasma cortisol following transfer to SW while branchial corticosteroid receptors were immediately downregulated after transfer to FW. Moreover, branchial expression of corticosteroid receptors changed with each salinity phase of the TR, suggesting rapid environmental modulation of corticosteorid action. Together, these results support the role of the HPI-axis in promoting salinity acclimation, including in dynamically-changing environments.

Osmosensitive transcription factors in the prolactin cell of a euryhaline teleost

In euryhaline fish, prolactin (Prl) plays a key role in freshwater acclimation. Prl release in the rostral pars distalis (RPD) of the pituitary is directly stimulated by a fall in extracellular osmolality. Recently, we identified several putative transcription factor modules (TFM) predicted to bind to the promoter regions of the two prl isoforms in Mozambique tilapia, Oreochromis mossambicus. We characterized the effects of extracellular osmolality on the activation of these TFMs from RPDs, in vivo and in vitro. OCT1_PIT1 01, CEBP_CEBP 01 and BRNF_RXRF 01 were significantly activated in freshwater (FW)- acclimated tilapia RPDs while SORY_PAX3 02 and SP1F_SP1F 06, SP1F_SP1F 09 were significantly activated in seawater (SW)- counterparts. Short-term incubation of SW- acclimated tilapia RPDs in hyposmotic media (280 mOsm/kg) resulted in activation of CAAT_AP1F 01, OCT1_CEBP 01, AP1F_SMAD 01, GATA_SP1F 01, SORY_PAX6 01 and CREB_EBOX 02, EBOX_AP2F 01, EBOX_MITF 01 while hyperosmotic media (420 mOsm/kg) activated SORY_PAX3 02 and AP1F_SMAD 01 in FW- tilapia. Short-term incubation of dispersed Prl cells from FW- acclimated fish exposed to hyperosmotic conditions decreased pou1f1, pou2f1b, stat3, stat1a and ap1b1 expression, while pou1f1, pou2f1b, and stat3 were inversely related to osmolality in their SW- counterparts. Further, in Prl cells of SW- tilapia, creb3l1 was suppressed in hyposmotic media. Collectively, our results indicate that multiple TFMs are involved in regulating prl transcription at different acclimation salinities and, together, they modulate responses of Prl cells to changes in extracellular osmolality. These responses reflect the complexity of osmosensitive molecular regulation of the osmoreceptive Prl cell of a euryhaline teleost.

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.

Effects of acute low-salinity stress on osmoregulation, antioxidant capacity, and growth of the black sea bream (Acanthopagrus schlegelii)

The black sea bream (Acanthopagrus schlegelii) is an important marine economic fish found on the southeast coast of China. Because of the frequent climate change, the salinity of the waters inhabited by A. schlegelii often decreases, which interferes with the fish's physiological homeostasis. The isotonic salinity of teleosts are usually lower than that of seawater, so maximum economic benefits cannot be obtained from conventional mariculture. This study was performed to preliminarily clarify the osmotic regulation and antioxidant mechanism of juvenile A. schlegelii and find an appropriate culture salinity value. We selected 5 psu, 10 psu, 15 psu, and 25 psu (control) to conduct physiological experiments for 96 h and growth experiments for 60 days. We found that the juvenile A. schlegelii could adjust their osmotic pressure within 12 h. The growth hormone and cortisol were found to be seawater-acclimating hormones, whereas prolactin was freshwater-acclimating hormone. The activity and mRNA expression of Na⁺/K⁺-ATPase showed a U-shaped trend with the decrease of in salinity at 12-96 h. Serum ion concentration and osmotic pressure remained at a relatively stable level after being actively adjusted from 6 to 12 h. At 96 h, the osmotic pressure of the serum isotonic point of juvenile A. schlegelii was approximately equal to that of water with 14.94 salinity. The number and volume of Cl^--secreting cells in the gills decreased. The glomeruli were more developed and structurally sound, with the renal tubules increasing in diameter and the medial brush border being more developed; this may indicate a decrease in salt secretion and an enhanced reabsorption function in the low salinity groups. The activities of superoxide dismutase and catalase and concentration of malondialdehyde were the lowest in the 15 psu group. In addition, the culture conditions of the 15 psu group improved the feed conversion rate without significant differences in weight gain when compared with the control group. Our results show that 15 psu salinity may be the best parameter for obtaining the maximum economic benefits.

Age-Dependent Decline in Salinity Tolerance in a Euryhaline Fish

Euryhaline teleost fish are characterized by their ability to tolerate a wide range of environmental salinities by modifying the function of osmoregulatory cells and tissues. In this study, we experimentally addressed the age-related decline in the sensitivity of osmoregulatory transcripts associated with a transfer from fresh water (FW) to seawater (SW) in the euryhaline teleost, Mozambique tilapia, Oreochromis mossambicus. The survival rates of tilapia transferred from FW to SW were inversely related with age, indicating that older fish require a longer acclimation period during a salinity challenge. The relative expression of Na⁺/K⁺/2Cl⁻ cotransporter 1a (nkcc1a), which plays an important role in hyposmoregulation, was significantly upregulated in younger fish after SW transfer, indicating a clear effect of age in the sensitivity of branchial ionocytes. Prolactin (Prl), a hyperosmoregulatory hormone in O. mossambicus, is released in direct response to a fall in extracellular osmolality. Prl cells of 4-month-old tilapia were sensitive to hyposmotic stimuli, while those of >24-month-old fish did not respond. Moreover, the responsiveness of branchial ionocytes to Prl was more robust in younger fish. Taken together, multiple aspects of osmotic homeostasis, from osmoreception to hormonal and environmental control of osmoregulation, declined in older fish. This decline appears to undermine the ability of older fish to survive transfer to hyperosmotic environments.

The digestive tract as an essential organ for water acquisition in marine teleosts: lessons from euryhaline eels

Adaptation to a hypertonic marine environment is one of the major topics in animal physiology research. Marine teleosts lose water osmotically from the gills and compensate for this loss by drinking surrounding seawater and absorbing water from the intestine. This situation is in contrast to that in mammals, which experience a net osmotic loss of water after drinking seawater. Water absorption in fishes is made possible by (1) removal of monovalent ions (desalinization) by the esophagus, (2) removal of divalent ions as carbonate (Mg/CaCO₃) precipitates promoted by HCO₃^- secretion, and (3) facilitation of NaCl and water absorption from diluted seawater by the intestine using a suite of unique transporters. As a result, 70-85% of ingested seawater is absorbed during its passage through the digestive tract. Thus, the digestive tract is an essential organ for marine teleost survival in the hypertonic seawater environment. The eel is a species that has been frequently used for osmoregulation research in laboratories worldwide. The eel possesses many advantages as an experimental animal for osmoregulation studies, one of which is its outstanding euryhalinity, which enables researchers to examine changes in the structure and function of the digestive tract after direct transfer from freshwater to seawater. In recent years, the molecular mechanisms of ion and water transport across epithelial cells (the transcellular route) and through tight junctions (the paracellular route) have been elucidated for the esophagus and intestine. Thanks to the rapid progress in analytical methods for genome databases on teleosts, including the eel, the molecular identities of transporters, channels, pumps and junctional proteins have been clarified at the isoform level. As 10 y have passed since the previous reviews on this subject, it seems relevant and timely to summarize recent progress in research on the molecular mechanisms of water and ion transport in the digestive tract in eels and to compare the mechanisms with those of other teleosts and mammals from comparative and evolutionary viewpoints. We also propose future directions for this research field to achieve integrative understanding of the role of the digestive tract in adaptation to seawater with regard to pathways/mechanisms including the paracellular route, divalent ion absorption, metabolon formation and cellular trafficking of transporters. Notably, some of these have already attracted practical attention in laboratories.

Fluoride exposure changed the expression of microRNAs in gills of male zebrafish (Danio rerio)

Fluoride has been found to cause detrimental effects on fish gills. Despite essential roles in various metabolism activities, whether and how miRNAs participate in the course of toxicity caused by fluoride in gills is still unclear. In this study, male zebrafish were exposed to 0, 20, 40 mg/L fluoride for 60 days to study the underlying osmotic regulatory mechanism by determining the influences of fluoride on the miRNAs and regulated genes in gills. mRNAs were isolated from the gills and the expression profiles were analyzed by using Illumina Hiseq 2500 platforms. Expressions of 7 differentially miRNAs and some related-genes in gills were validated by qRT-PCR. The results showed that miRNAs expressions were notably altered by fluoride. A total of 584 and 327 miRNAs were remarkably changed after 20 and 40 mg/L fluoride exposure, of which 322 were increased and 262 were decreased in 20 mg/L fluoride group, whereas 219 were elevated and 108 were reduced in 40 mg/L fluoride group. The differentially expressive miRNAs confirmed by qRT-PCR were consistent with micro-assay data. Cluster of Orthologous Groups of proteins (COG) function classification showed that the target genes of differentially expressive miRNAs are mainly related to signal transduction mechanisms, replication, transcription, inorganic ion transport and metabolism, repair and recombination, and energy formation and transformation. In addition, fluoride disturbed the expressions of target genes involved in the osmoregulation of the gill in the fluoride-exposed zebrafish, such as the increased expressions of OSTF1 and the decreased expressions of Na⁺-K⁺-ATPase, CFTR, and AQP-3, which provides a possibility that miRNAs regulation induced by fluoride has an effects on osmotic regulation, providing new hints to the osmotic regulatory mechanism of the toxicity caused by fluoride in zebrafish, and distinguishes new biomarkers of miRNAs for fluoride toxicity.

Transcriptional regulation of prolactin in a euryhaline teleost: Characterisation of gene promoters through in silico and transcriptome analyses

The sensitivity of prolactin (Prl) cells of the Mozambique tilapia (Oreochromis mossambicus) pituitary to variations in extracellular osmolality enables investigations into how osmoreception underlies patterns of hormone secretion. Through the actions of their main secretory products, Prl cells play a key role in supporting hydromineral balance of fishes by controlling the major osmoregulatory organs (ie, gill, intestine and kidney). The release of Prl from isolated cells of the rostral pars distalis (RPD) occurs in direct response to physiologically relevant reductions in extracellular osmolality. Although the particular signal transduction pathways that link osmotic conditions to Prl secretion have been identified, the processes that underlie hyposmotic induction of prl gene expression remain unknown. In this short review, we describe two distinct tilapia gene loci that encode Prl177 and Prl188 . From our in silico analyses of prl177 and prl188 promoter regions (approximately 1000 bp) and a transcriptome analysis of RPDs from fresh water (FW)- and seawater (SW)-acclimated tilapia, we propose a working model for how multiple transcription factors link osmoreceptive processes with adaptive patterns of prl177 and prl188 gene expression. We confirmed via RNA-sequencing and a quantitative polymerase chain reaction that multiple transcription factors emerging as predicted regulators of prl gene expression are expressed in the RPD of tilapia. In particular, gene transcripts encoding pou1f1, stat3, creb3l1, pbxip1a and stat1a were highly expressed; creb3l1, pbxip1a and stat1a were elevated in fish acclimated to SW vs FW. Combined, our in silico and transcriptome analyses set a path for resolving how adaptive patterns of Prl secretion are achieved via the integration of osmoreceptive processes with the control of prl gene transcription.

Osmoregulatory actions of prolactin in the gastrointestinal tract of fishes

In fishes, prolactin (Prl) signaling underlies the homeostatic regulation of hydromineral balance by controlling essential solute and water transporting functions performed by the gill, gastrointestinal tract, kidney, urinary bladder, and integument. Comparative studies spanning over 60 years have firmly established that Prl promotes physiological activities that enable euryhaline and stenohaline teleosts to reside in freshwater environments; nonetheless, the specific molecular and cellular targets of Prl in ion- and water-transporting tissues are still being resolved. In this short review, we discuss how particular targets of Prl (e.g., ion cotransporters, tight-junction proteins, and ion pumps) confer adaptive functions to the esophagus and intestine. Additionally, in some instances, Prl promotes histological and functional transformations within esophageal and intestinal epithelia by regulating cell proliferation. Collectively, the demonstrated actions of Prl in the gastrointestinal tract of teleosts indicate that Prl operates to promote phenotypes supportive of freshwater acclimation and to inhibit phenotypes associated with seawater acclimation. We conclude our review by underscoring that future investigations are warranted to determine how growth hormone/Prl-family signaling evolved in basal fishes to support the gastrointestinal processes underlying hydromineral balance.

Growth hormone regulates intestinal gene expression of nutrient transporters in tilapia (Oreochromis mossambicus)

Among the various ways that growth hormone (GH) underlies the growth physiology of teleost fishes, GH stimulates transport pathways that facilitate the absorption of nutrients across intestinal epithelia. The current study investigated the effects of GH on the gene expression of nutrient transporters in an omnivorous teleost, the Mozambique tilapia (Oreochromis mossambicus). We employed pituitary gland removal (hypophysectomy) and hormone replacement to assess whether GH directs the gene expression of the GH receptor (ghr2), the peptide transporters, pept1a, pept1b and pept2, the amino acid transporter, slc7a9, the Na+/glucose cotransporter, sglt1, the glucose transporter, glut2, and the myo-inositol transporter, smit2, in anterior, middle, and posterior intestine. ghr2 was predominantly expressed in posterior intestine, while pept1a, pept1b, slc7a9, sglt1, glut2, and smit2 exhibited the highest mRNA levels in anterior and/or middle intestine. While hypophysectomized tilapia exhibited diminished expression of ghr2, pept1a, pept1b, slc7a9, and glut2 compared with intact and sham-operated controls, only ghr2, pept1a, pept1b and glut2 levels were restored by GH replacement. Our findings indicate that GH supports growth, at least in part, by stimulating the gene expression of its cognate receptor and key nutrient transporters in the intestine.

Cloning and molecular characterization of PRL and PRLR from turbot (Scophthalmus maximus) and their expressions in response to short-term and long-term low salt stress

The pituitary hormone prolactin (PRL) regulates salt and water homeostasis by altering ion retention and water uptake through peripheral osmoregulatory organs. To understand the role of PRL and its receptor (PRLR) in hypoosmoregulation of turbot (Scophthalmus maximus), we characterized the PRL and PRLR gene and analyzed the tissue distribution of the two genes and their gene transcriptional patterns in the main expressed tissues under long-term and short-term low salt stress. The PRL cDNA is 1486 bp in length, incorporating an ORF of 636 bp with a putative primary structure of 211 residues. And the PRLR cDNA is 2849 bp in length, incorporating an ORF of 1944 bp with a putative primary structure of 647 residues. The deduced amino acid sequences of these two genes shared highly conserved structures with those from other teleosts. Quantitative real-time PCR results showed that PRL transcripts were strongly expressed in the pituitary and very weakly in brain, but were hardly expressed in other tissues. PRLR transcripts were most abundant in the kidney, to a lesser extent in the gill, intestine, brain, and spleen, and at low levels in the pituitary and other tissues examined. The expression of PRL in the pituitary increased after short-term or long-term low salt stress, and the highest expression level appeared 12 h after stress (P < 0.05). And there is no significant difference between both low salt group (5 ppt and 10 ppt) at each sampling point. The variation of PRLR expression in gill under short-term low salt stress is similar to that of PRL gene in pituitary, with highest value in 12 h (P < 0.05). However, the expression under long-term low salt stress was significantly higher than control group even than 12 h group under 5 ppt (P < 0.05). The expression of PRLR in the kidney increased first and then decreased after low salt stress, and the highest value also appeared in 12 h after stress and there was no significant difference between the salinity groups. After long-term low salt stress, the expression level also increased significantly (P < 0.05), but it was flat with 24 h, which was lower than 12 h. The variation of PRLR expression in the intestine was basically consistent with that in the kidney. The difference was that the expression level of 24 h after stress in the 5 ppt group was significantly higher than that of the 10 ppt group (P < 0.05). After a comprehensive analysis of the expression levels of the two genes, it can be found that the expression level increased and peaked at 12 h after short-term low salt stress, indicating that this time point is the key point for the regulation of turbot in response to low salt stress. This also provides very important information for studying the osmotic regulation of turbot. In addition, our results also showed that the expression of PRLR was stable in the kidney and intestine after long-term low salt stress, while the expression in the gill was much higher than short-term stress. It suggested that PRL and its receptors mainly exert osmotic regulation function in the gill under long-term low salt stress. At the same time, such a result also brings a hint for the low salt selection of turbot, focusing on the regulation of ion transport in the gill.

Embryonic exposure to pentabromobenzene inhibited the inflation of posterior swim bladder in zebrafish larvae

The emerging flame retardants pentabromobenzene (PBB) has been frequently detected in recent years and may pose exposure risks to wild animals and human beings. In this study, the inflation of posterior swim bladder of zebrafish larvae was used as an endpoint to study the developmental toxicity and putative mechanisms associated with PBB toxicity. Our results showed that embryonic exposure to PBB could significantly inhibit the inflation of posterior swim bladders. Reduced T3 levels and transcriptional changes of crh and pomc were observed in PBB treated zebrafish larvae at 120 hpf. However, key regulators of thyroid and adrenocortical system involved in the synthesis (tsh), biological conversion (ugt1ab, dio2) and functional regulation (trα, trβ, gr) showed no significant changes. Further data revealed that prlra was the only gene that was altered among the detected genes at 96 h post fertilization (hpf). At 120 hpf, the morphology of swim bladder indicated deflation in treatments at 0.25 μM and higher. In addition, the mRNA levels of anxa5, prlra, prlrb, atp1b2 and slc12a10 were all significantly changed at 120 hpf. Taken together, we suppose that embryonic exposure to PBB inhibited the inflation of swim bladder in zebrafish probably via prlra mediated pathways. The observed changes of thyroid and adrenocortical parameters might be indirect effects evoked by PBB exposure. Overall, our results provide important data and indications for future toxicological study and risk assessment of the emerging flame retardants PBB.

Divergent genes encoding the putative receptors for growth hormone and prolactin in sea lamprey display distinct patterns of expression

Growth hormone receptor (GHR) and prolactin receptor (PRLR) in jawed vertebrates were thought to arise after the divergence of gnathostomes from a basal vertebrate. In this study we have identified two genes encoding putative GHR and PRLR in sea lamprey (Petromyzon marinus) and Arctic lamprey (Lethenteron camtschaticum), extant members of one of the oldest vertebrate groups, agnathans. Phylogenetic analysis revealed that lamprey GHR and PRLR cluster at the base of gnathostome GHR and PRLR clades, respectively. This indicates that distinct GHR and PRLR arose prior to the emergence of the lamprey branch of agnathans. In the sea lamprey, GHR and PRLR displayed a differential but overlapping pattern of expression; GHR had high expression in liver and heart tissues, whereas PRLR was expressed highly in the brain and moderately in osmoregulatory tissues. Branchial PRLR mRNA levels were significantly elevated by stage 5 of metamorphosis and remained elevated through stage 7, whereas levels of GHR mRNA were only elevated in the final stage (7). Branchial expression of GHR increased following seawater (SW) exposure of juveniles, but expression of PRLR was not significantly altered. The results indicate that GHR and PRLR may both participate in metamorphosis and that GHR may mediate SW acclimation.

Systemic versus tissue-level prolactin signaling in a teleost during a tidal cycle

Euryhaline Mozambique tilapia (Oreochromis mossambicus) are native to estuaries where they encounter tidal fluctuations in environmental salinity. These fluctuations can be dramatic, subjecting individuals to salinities characteristic of fresh water (FW < 0.5‰) and seawater (SW 35‰) within a single tidal cycle. In the current study, we reared tilapia under a tidal regimen that simulated the dynamic conditions of their native habitat. Tilapia were sampled every 3 h over a 24 h period to temporally resolve how prolactin (PRL) signaling is modulated in parallel with genes encoding branchial effectors of osmoregulation. The following parameters were measured: plasma osmolality, plasma PRL₁₇₇ and PRL₁₈₈ concentrations, pituitary prl₁₇₇ and prl₁₈₈ gene expression, and branchial prl receptor (prlr1 and prlr2), Na⁺/Cl^--cotransporter (ncc2), Na⁺/K⁺/2Cl^--cotransporter (nkcc1a), Na⁺/K⁺-ATPase (nkaα1a and nkaα1b), cystic fibrosis transmembrane regulator (cftr), and aquaporin 3 (aqp3) gene expression. Throughout the 24 h sampling period, plasma osmolality reflected whether tilapia were sampled during the FW or SW phases of the tidal cycle, whereas pituitary prl gene expression and plasma PRL levels remained stable. Branchial patterns of ncc2, nkcc1a, nkaα1a, nkaα1b, cftr, and aqp3 gene expression indicated that fish exposed to tidally changing salinities regulate the expression of these gene transcripts in a similar fashion as fish held under static SW conditions. By contrast, branchial prlr1 and prlr2 levels were highly labile throughout the tidal cycle. We conclude that local (branchial) regulation of endocrine signaling underlies the capacity of euryhaline fishes, such as Mozambique tilapia, to thrive under dynamic salinity conditions.

Effects of the acclimation to high salinity on intestinal ion and peptide transporters in two tilapia species that differ in their salinity tolerance

Tilapiine species, widely distributed across habitats with diverse water salinities, are important to aquaculture as well as a laboratory model. The effects of water salinity on two tilapia species, that differ in their salinity tolerance, was evaluated. Oreochromis niloticus reared in brackish-water, showed a significant decrease in growth and feed efficiency, whereas O. mossambicus reared in seawater did not show any significant changes. The expression and activity of Na⁺/K⁺-ATPase (NKA), V-type H⁺-ATPase (VHA) and carbonic anhydrase (CA), as well as expression levels of genes encoding two HCO₃^- and three peptide transporters (nbc1, slc26a6, slc15a1a, slc15a1b and slc15a2) were measured in three intestinal sections of these two species, grown in freshwater and brackish/sea-water. Overall, the spatial distribution along the intestine of the genes examined in this study was similar between the two species, with the exception of tcaIV. The salinity response, on the other hand, varied greatly between these species. In O. mossambicus, there was a salinity-dependent increased expression of most of the examined genes (except slc26a6 and slc15a2), while in O. niloticus the expression of most genes did not change, or even decreased (tcaIV, nbc1 and slc15a1b). This study highlighted differences in the intestinal response to salinity acclimation between closely- related species that differ in their salinity tolerance. O. mossambicus, which has a high salinity tolerance, showed expression patterns and responses similar to marine species, and differed from the low-salinity-tolerance O. niloticus, which showed a response that differed from the accepted models, that are based on marine and diadromous fishes.

Role of Na+/K+-ATPase in Natriuretic Effect of Prolactin in a Model of Cholestasis of Pregnancy

Participation of Na+/K+-ATPase in the natriuretic effect of prolactin in a cholestasis of pregnancy model was investigated. The Na+/K+-ATPase activity in rat kidney medulla, where active sodium reabsorption occurs, decreased in the model of cholestasis of pregnancy and other hyperprolactinemia types compared with intact animals. This effect was not connected with the protein level of α1- and β-subunits of Na+/K+-ATPase measured by Western blotting in the kidney medulla. Decrease in Na+/K+-ATPase activity in the kidney cortex was not significant, as well as decrease in the quantity of mRNA and proteins of the α1- and β-subunits of Na+/K+-ATPase. There were no correlations between the Na+/K+-ATPase activity and sodium clearance, although sodium clearance increased significantly in the model of cholestasis of pregnancy and other hyperprolactinemia groups under conditions of stable glomerular filtration rate measured by creatinine clearance. We conclude that the Na+/K+-ATPase is not the only mediator of the natriuretic effect of prolactin in the model of cholestasis of pregnancy.

High rates of intestinal bicarbonate secretion in seawater tilapia (Oreochromis mossambicus)

Osmoregulation in fish is a complex process that requires the orchestrated cooperation of many tissues. In fish facing hyperosmotic environments, the intestinal absorption of some monovalent ions and the secretion of bicarbonate are key processes to favor water absorption. In the present study, we showed that bicarbonate levels in the intestinal fluid are several fold higher in seawater than in freshwater acclimated tilapia (Oreochromis mossambicus). In addition, we analyzed gene expression of the main molecular mechanisms involved in HCO₃^- movements i.e. slc26a6, slc26a3, slc4a4 and v-type H-ATPase sub C in the intestine of tilapia acclimated to both seawater and freshwater. Our results show an anterior/posterior functional regionalization of the intestine in tilapia in terms of expression patterns, which is affected by environmental salinity mostly in the anterior and mid intestine. Analysis of bicarbonate secretion using pH-Stat in tissues mounted in Ussing chambers reveals high rates of bicarbonate secretion in tilapia acclimated to seawater from anterior intestine to rectum ranging between ~900 and ~1700nmolHCO₃^-cm^-2h^-1. However, a relationship between the expression of slc26a6, slc26a3, slc4a4 and the rate of bicarbonate secretion seems to be compromised in the rectum. In this region, the low expression of the bicarbonate transporters could not explain the high bicarbonate secretion rates here described. However, we postulate that the elevated v-type H-ATPase mRNA expression in the rectum could be involved in this process.

Salinity-Dependent Shift in the Localization of Three Peptide Transporters along the Intestine of the Mozambique Tilapia (Oreochromis mossambicus)

The peptide transporter (PepT) systems are well-known for their importance to protein absorption in all vertebrate species. These symporters use H⁺ gradient at the apical membrane of the intestinal epithelial cells to mediate the absorption of small peptides. In fish, the intestine is a multifunctional organ, involved in osmoregulation, acid-base regulation, and nutrient absorption. Therefore, we expected environmental stimuli to affect peptide absorption. We examined the effect of three environmental factors; salinity, pH and feeding, on the expression, activity and localization of three PepT transporters (PepT1a, PepT1b, PepT2) along the intestine of the Mozambique tilapia (Oreochromis mossambicus). Quantitative real time PCR (qPCR) analysis demonstrated that the two PepT1 variants are typical to the proximal intestinal section while PepT2 is typical to the distal intestinal sections. Immunofluorescence analysis with custom-made antibodies supported the qPCR results, localized both transporters on the apical membrane of enterocytes and provided the first evidence for the participation of PepT2 in nutrient absorption. This first description of segment-specific expression and localization points to a complementary role of the different peptide transporters, corresponding to the changes in nutrient availability along the intestine. Both gene expression and absorption activity assays showed that an increase in water salinity shifted the localization of the PepT genes transcription and activity down along the intestinal tract. Additionally, an unexpected pH effect was found on the absorption of small peptides, with increased activity at higher pH levels. This work emphasizes the relationships between different functions of the fish intestine and how they are affected by environmental conditions.

Effect of acute salinity stress on ion homeostasis, Na+/K+-ATPase and histological structure in sea cucumber Apostichopus japonicus

Background

Sea cucumbers (Apostichopus japonicus) are an imperiled fauna exposed to a variety of environmental condition such as salinity and studies are urgently needed to assess their effects to guide aquaculture efforts. The effects of acute salinity stress on coelomic fluid osmotic pressure, ion concentrations, the activity of Na⁺/K⁺-ATPase in respiratory trees and the histological variations were measured to evaluate the salinity tolerance of sea cucumbers.

Results

Significant correlations in osmotic pressure were observed between coelomic fluid and ambient environmental salinity. In coelomic fluid, Na⁺ concentration was observed fluctuated during salinity 18 psu and the inflection point presented at the 6 h. The Na⁺/K⁺-ATPase activity in respiratory trees indicated the “U-shaped” fluctuant change and the change trend was opposite with the Na⁺ concentration. The ions (K⁺, Cl⁻) concentration decreased and showed the same tendency at salinity 40 psu with salinity 18 psu. The total coelomocytes counts and phagocytosis of coelomic fluid Na⁺/K⁺-ATPase activity indicated fluctuating changes under different salinity stress. Histological variation revealed a negative relation between decreasing salt concentration and tissue integrity. Tissue damages were significantly observed in intestines, muscles and tube feet under low salinity environment (18, 23 and 27 psu). The connective tissue in intestines of A. japonicus exposed to 18 and 23 psu damaged and partly separated from the mucosal epithelium. The significant variations occurred in tube feet, which presented the swelling in connective tissue and a fracture in longitudinal muscles under low salinity (18 psu). The morphological change of tube feet showed the shrinkage of connective tissue under high salinity (40 psu). The amount of infusoria in the respiratory trees decreased or even disappeared in salinity treatment groups (18 and 23 psu).

Conclusion

The results inferred that osmoconformity and ionoregulation were seen in sea cucumbers, which contributed to understand the salinity regulatory mechanisms of A. japonicus under acute salinity stress.

The basal function of teleost prolactin as a key regulator on ion uptake identified with zebrafish knockout models

Prolactin (PRL) is an anterior pituitary hormone with a broad range of functions. Its ability to stimulate lactogenesis, maternal behavior, growth and development, osmoregulation, and epithelial ion transport has been reported in many vertebrates. In our present study, we have targeted the zebrafish prl locus via transcription activator-like effector nucleases (TALENs). Two independent targeted mutant lines with premature termination of the putative sequence of PRL peptides were generated. All prl-deficient zebrafish progeny died at 6–16 days post-fertilization stage (dpf) in egg water. However, the prl-deficient larvae thrived and survived through adulthood in brackish water (5175 mg/L ocean salts), without obvious defects in somatic growth or reproduction. When raised in egg water, the expression levels of certain key Na⁺/Cl⁻ cotransporters in the gills and Na⁺/K⁺-ATPase subunits, Na⁺/H⁺ exchangers and Na⁺/Cl⁻ transporters in the pronephros of prl-deficient larvae were down-regulated at 5 dpf, which caused Na⁺/K⁺/Cl⁻ uptake defects in the mutant fish at 6 dpf. Our present results demonstrate that the primary function of zebrafish prl is osmoregulation via governing the uptake and homeostasis of Na⁺, K⁺ and Cl⁻. Our study provides valuable evidence to understand the mechanisms of PRL function better through both phylogenetic and physiological perspectives.