Expression of aquaporin-3 and -8 mRNAs in the parr and smolt stages of sockeye salmon, Oncorhynchus nerka: effects of cortisol treatment and seawater acclimation

Endocrine control of gill ionocyte function in euryhaline fishes

The endocrine system is an essential regulator of the osmoregulatory organs that enable euryhaline fishes to maintain hydromineral balance in a broad range of environmental salinities. Because branchial ionocytes are the primary site for the active exchange of Na+, Cl-, and Ca2+ with the external environment, their functional regulation is inextricably linked with adaptive responses to changes in salinity. Here, we review the molecular-level processes that connect osmoregulatory hormones with branchial ion transport. We focus on how factors such as prolactin, growth hormone, cortisol, and insulin-like growth-factors operate through their cognate receptors to direct the expression of specific ion transporters/channels, Na+/K+-ATPases, tight-junction proteins, and aquaporins in ion-absorptive (freshwater-type) and ion-secretory (seawater-type) ionocytes. While these connections have historically been deduced in teleost models, more recently, increased attention has been given to understanding the nature of these connections in basal lineages. We conclude our review by proposing areas for future investigation that aim to fill gaps in the collective understanding of how hormonal signaling underlies ionocyte-based processes.

Salinity effects on expression and localization of aquaporin 3 in gills of the euryhaline milkfish (Chanos chanos)

Milkfish (Chanos chanos) are important euryhaline fish in Southeast Asian countries that can tolerate a wide range of salinity changes. Previous studies have revealed that milkfish have strong ion regulation and survival abilities under osmotic stress. In addition to ion regulation, water homeostasis in euryhaline teleosts is important during environmental salinity shifts. Aquaporins (AQP) are vital water channels in fish, and different AQPs can transport water influx or outflux from the body. AQP3 is one of the AQP channels, and the function of AQP3 in the gills of euryhaline milkfish is still unknown. The aim of this study was to investigate the expression and localization of AQP3 in the gills of euryhaline milkfish to contribute to our understanding of the physiological role and localization of AQP3 in fish. The AQP3 sequence was found in the milkfish next-generation sequencing (NGS) database and is mainly distributed in the gills of freshwater (FW)-acclimated milkfish. Under hypoosmotic and hyperosmotic stress, the osmolality of milkfish immediately shifted, similar to the aqp3 gene expression. Moreover, the abundance of AQP3 protein significantly decreased 3 h after transferring milkfish from FW to seawater (SW). However, there was no change within 7 days when the milkfish experienced hypoosmotic stress. Moreover, double immunofluorescence staining of milkfish gills showed that AQP3 colocalized with Na+ /K+ ATPase at the basolateral membrane of ionocytes. These results combined indicate that milkfish have a strong osmoregulation ability under acute osmotic stress because of the quick shift in the gene and protein expression of AQP3 in their gills.

Molecular Characterization of Aquaporins Genes from the Razor Clam Sinonovacula constricta and Their Potential Role in Salinity Tolerance

Aquaporins (AQPs) play crucial roles in osmoregulation, but the knowledge about the functions of AQPs in Sinonovacula constricta is unclear. In this study, Sc-AQP1, Sc-AQP8, and Sc-AQP11 were identified from S. constricta, and the three Sc-AQPs are highly conserved compared to the known AQPs. The qRT-PCR analysis revealed that the highest mRNA expressions of Sc-AQP1, Sc-AQP8, and Sc-AQP11 were detected in the gill, digestive gland, and adductor muscle, respectively. In addition, the highest mRNA expression of Sc-AQP1 and Sc-AQP11 was detected in the D-shaped larvae stage, whereas that of SC-AQP8 was observed in the umbo larvae stage. The mRNA expression of Sc-AQP1, Sc-AQP8, and Sc-AQP11 significantly increased to 12.45-, 12.36-, and 27.44-folds post-exposure of low salinity (3.5 psu), while only Sc-AQP1 and Sc-AQP11 significantly increased post-exposure of high salinity (35 psu) (p < 0.01). The fluorescence in situ hybridization also showed that the salinity shift led to the boost of Sc-AQP1, Sc-AQP8, and Sc-AQP11 mRNA expression in gill filament, digestive gland, and adductor muscle, respectively. Knockdown of the Sc-AQP1 and Sc-AQP8 led to the decreased osmotic pressure in the hemolymph. Overall, these findings would contribute to the comprehension of the osmoregulation pattern of AQPs in S. constricta.

Intestinal osmoregulatory mechanisms differ in Mediterranean and Atlantic European sea bass: A focus on hypersalinity

European sea bass (Dicentrarchus labrax) migrate towards habitats where salinity can reach levels over 60‰, notably in Mediterranean lagoons. D. labrax are genetically subdivided in Atlantic and Mediterranean lineages and have evolved in slightly different salinities. We compared Atlantic and West-Mediterranean populations regarding their capacity to tolerate hypersalinity with a focus on the involvement of the intestine in solute-driven water reabsorption. Fish were analyzed following a two-week transfer from seawater (SW, 36‰) to either SW or hypersaline water (HW, 55‰). Differences among lineages were observed in posterior intestines of fish maintained in SW regarding NKA activities and mRNA expressions of nkaα1a, aqp8b, aqp1a and aqp1b with systematic higher levels in Mediterranean sea bass. High salinity transfer triggered similar responses in both lineages but at different magnitudes which may indicate slight different physiological strategies between lineages. High salinity transfer did not significantly affect the phenotypic traits measured in the anterior intestine. In the posterior intestine however, the size of enterocytes and NKA activity were higher in HW compared to SW. In this tissue, nka-α1a, nkcc2, aqp8ab and aqp8aa mRNA levels were higher in HW compared to SW as well as relative protein expression of AQP8ab. For aqp1a, 1b, 8aa and 8b, an opposite trend was observed. The sub-apical localization of AQP8ab in enterocytes suggests its role in transepithelial water reabsorption. Strong apical NKCC2/NCC staining indicates an increased Na⁺ and Cl^- reuptake by enterocytes which could contribute to solute-coupled water reuptake in cells where AQP8ab is expressed.

Genome-Wide Characterization of Aquaporins (aqps) in Lateolabrax maculatus: Evolution and Expression Patterns During Freshwater Acclimation

Aquaporin (aqp) proteins are a group of small integral membrane proteins that play crucial roles as pore channels for the transport of water and other small solutes across the cell membrane. In our study, we identified 17 aqp genes from the spotted sea bass (Lateolabrax maculatus) genomic database. Gene organization, motif distribution, and selection pressure analyses were performed to investigate their evolutionary characteristics. The aqp mRNA displayed tissue-specific expression pattern in ten selected tissues of healthy spotted sea bass. To investigate the potential involvement of spotted sea bass aqps in osmoregulation, the expression profiles of aqp genes in gills were examined during freshwater (FW) acclimation using qRT-PCR. The mRNA level of aqp3a was dramatically induced during 1-3 day of the FW transition period (77-fold and 15-fold upregulated on 1 day and 3 day than in the control group), indicating that aqp3a may play an important hypo-osmoregulatory role in spotted sea bass. In addition, the expression levels of aqp1aa, aqp1ab, aqp3b, aqp7, and aqp9b increased to various degrees at 1 day after transferring to FW, suggesting their potential involvement in the FW acclimation process. Our study provides a valuable foundation for future studies aimed at uncovering the specific roles of aqp genes during salinity acclimation in spotted sea bass and other teleost species.

Freshening effect on the osmotic response of the Antarctic spiny plunderfish Harpagifer antarcticus

Global warming is having a significant impact around the world, modifying environmental conditions in many areas, including in zones that have been thermally stable for thousands of years, such as Antarctica. Stenothermal sedentary intertidal fish species may suffer due to warming, notably if this causes water freshening from increased freshwater inputs. Acute decreases in salinity, from 33 down to 5, were used to assess osmotic responses to environmental salinity fluctuations in Antarctic spiny plunderfish Harpagifer antarcticus, in particular to evaluate if H. antarcticus is able to cope with freshening and to describe osmoregulatory responses at different levels (haematological variables, muscle water content, gene expression, NKA activity). H. antarcticus were acclimated to a range of salinities (33 as control, 20, 15, 10 and 5) for 1 week. At 5, plasma osmolality and calcium concentration were both at their lowest, while plasma cortisol and percentage muscle water content were at their highest. At the same salinity, gill and intestine Na⁺ -K⁺ -ATPase (NKA) activities were at their lowest and highest, respectively. In kidney, NKA activity was highest at intermediate salinities (15 and 10). The salinity-dependent NKA mRNA expression patterns differed depending on the tissue. Marked changes were also observed in the expression of genes coding membrane proteins associated with ion and water transport, such as NKCC2, CFTR and AQP8, and in the expression of mRNA for the regulatory hormone prolactin (PRL) and its receptor (PRLr). Our results demonstrate that freshening causes osmotic imbalances in H. antarcticus, apparently due to reduced capacity of both transport and regulatory mechanisms of key organs to maintain homeostasis. This has implications for fish species that have evolved in stable environmental conditions in the Antarctic, now threatened by climate change.

Genomic and expression characterization of aquaporin genes from Siniperca chuatsi

Aquaporins (AQPs) are major intrinsic proteins that form pores in the membranes of biological cells. We first cloned the full-length sequences of aqp0, 1, 3, 4, 7, 8, 9, 10, 11, and 12 genes in Siniperca chuatsi. The 10 S. chuatsi aqp (Sc-aqp) genes included complete open reading frames and exhibited different exon-intron organizations. Sc-aqp1, 3, 8, 9, 10, and 11 were mostly expressed in the gallbladder, gills, gastric cecum, liver, ovaries, and spleen, respectively; Sc-aqp0 and 4 were mostly expressed in larvae at 1 day after hatching and in gastrula; Sc-aqp7 and 12 were mostly expressed in 2K-cell embryos. The expression levels of Sc-aqp1, 3, 7, 8, 9, and 10 after 10 part per thousand (ppt) salt treatment had significantly changed compared with those after 0 ppt salt treatment. Real-time quantitative PCR analysis further showed that in the intestines, the mRNA levels of Sc-aqp1 and 10 significantly decreased by approximately 2.07- and 2.85-fold, respectively, whereas those of Sc-aqp8 and 9 significantly increased by approximately 7.08- and 4.14-fold, respectively. Sc-aqp1, 8, 9, and 10 showed no significant differences in the gills. Sc-aqp3 significantly decreased by approximately 1.51- and 1.67-fold in the gills and intestines, respectively. Sc-aqp7 significantly increased by approximately 4.18- and 7.04-fold in the gills and intestines, respectively. This study was the first to investigate the tissue expression profiles and response to salt stress of aqp genes in S. chuatsi. Moreover, altering diet and suffering from immune stress could cause changes in the expression level of aqps. This study provided valuable reference information for AQPs' roles in osmoregulation in freshwater fish.

Cloning and Expression of Four Aquaporin Homologs from the Chinese Black Sleeper (Bostrychus sinensis): The Effects of Salinity Acclimation

Several fish species are known to possess mechanisms that allow them to adapt to environments with different salinities. The aim of this study was to investigate the effects of salinity on the expression of aquaporins (aqp1a, aqp3a, aqp8a, and aqp9a) in the gills and intestines of Chinese black sleeper. After 30 days of acclimation, the expression of aqp1a, aqp3a, and aqp9a in the gills was significantly higher in fish transferred to 5 ppt than in those transferred to 40 ppt seawater, whereas aqp8 expression was lower. In contrast, aqp1a, aqp3a, and aqp8a expression in the intestines was higher in fish acclimated in 40 ppt than in those acclimated in 5 ppt. During abrupt salinity acclimation, the levels of aqp1a and aqp9a in the gills varied over time in fish acclimated in 5 ppt, but not in 40 ppt. The aqp3a levels in gills were higher in the 5 ppt group after 24 h than in the 40 ppt. The expression level of aqp8a in gills was higher in 40 ppt than in 5 ppt, except for that at 12 h. In the intestines, expression level of aqp1a and aqp8a were significantly upregulated from 12 to 48 h following acclimation in 40 ppt and aqp3a was higher in 40 ppt group than in 5 ppt, while aqp9a expression exhibited an opposite trend. These findings suggest that aqp1a, aqp3a, aqp8a and aqp9a may play a major osmoregulatory role in water transport in the gills and intestines during acclimation to different salinity environment.

Comparative transcriptome profiling of selected osmotic regulatory proteins in the gill during seawater acclimation of chum salmon (Oncorhynchus keta) fry

Salmonid fishes, chum salmon (Oncorhynchus keta) have the developed adaptive strategy to withstand wide salinity changes from the early life stage. This study investigated gene expression patterns of cell membrane proteins in the gill of chum salmon fry on the transcriptome level by tracking the salinity acclimation of the fish in changing environments ranging from freshwater (0 ppt) to brackish water (17.5 ppt) to seawater (35 ppt). Using GO analysis of DEGs, the known osmoregulatory genes and their functional groups such as ion transport, transmembrane transporter activity and metal ion binding were identified. The expression patterns of membrane protein genes, including pump-mediated protein (NKA, CFTR), carrier-mediated protein (NKCC, NHE3) and channel-mediated protein (AQP) were similar to those of other salmonid fishes in the smolt or adult stages. Based on the protein-protein interaction analysis between transmembrane proteins and other related genes, we identified osmotic-related genes expressed with salinity changes and analyzed their expression patterns. The findings of this study may facilitate the disentangling of the genetic basis of chum salmon and better able an understanding of the osmophysiology of the species.

Differential Expression and Localization of Branchial AQP1 and AQP3 in Japanese Medaka (Oryzias latipes)

Aquaporins (AQPs) facilitate transmembrane water and solute transport, and in addition to contributing to transepithelial water transport, they safeguard cell volume homeostasis. This study examined the expression and localization of AQP1 and AQP3 in the gills of Japanese medaka (Oryzias latipes) in response to osmotic challenges and osmoregulatory hormones, cortisol, and prolactin (PRL). AQP3 mRNA was inversely regulated in response to salinity with high levels in ion-poor water (IPW), intermediate levels in freshwater (FW), and low levels in seawater (SW). AQP3 protein levels decreased upon SW acclimation. By comparison, AQP1 expression was unaffected by salinity. In ex vivo gill incubation experiments, AQP3 mRNA was stimulated by PRL in a time- and dose-dependent manner but was unaffected by cortisol. In contrast, AQP1 was unaffected by both PRL and cortisol. Confocal microscopy revealed that AQP3 was abundant in the periphery of gill filament epithelial cells and co-localized at low intensity with Na⁺,K⁺-ATPase in ionocytes. AQP1 was present at a very low intensity in most filament epithelial cells and red blood cells. No epithelial cells in the gill lamellae showed immunoreactivity to AQP3 or AQP1. We suggest that both AQPs contribute to cellular volume regulation in the gill epithelium and that AQP3 is particularly important under hypo-osmotic conditions, while expression of AQP1 is constitutive.

Aquaporin 1 Is Involved in Acid Secretion by Ionocytes of Zebrafish Embryos through Facilitating CO2 Transport

Mammalian aquaporin 1 (AQP1) is well known to function as a membrane channel for H₂O and CO₂ transport. Zebrafish AQP1a.1 (the homologue of mammalian AQP1) was recently identified in ionocytes of embryos; however its role in ionocytes is still unclear. In this study, we hypothesized that zebrafish AQP1a.1 is involved in the acid secretion by ionocytes through facilitating H₂O and CO₂ diffusion. A real-time PCR showed that mRNA levels of AQP1a.1 in embryos were induced by exposure to 1% CO₂ hypercapnia for 3 days. In situ hybridization and immunohistochemistry showed that the AQP1a.1 transcript was highly expressed by acid-secreting ionocytes, i.e., H⁺-ATPase-rich (HR) cells. A scanning ion-selective electrode technique (SIET) was applied to analyze CO₂-induced H⁺ secretion by individual ionocytes in embryos. H⁺ secretion by HR cells remarkably increased after a transient loading of CO₂ (1% for 10 min). AQP1a.1 knockdown with morpholino oligonucleotides decreased the H⁺ secretion of HR cells by about half and limited the CO₂ stimulated increase. In addition, exposure to an AQP inhibitor (PCMB) for 10 min also suppressed CO₂-induced H⁺ secretion. Results from this study support our hypothesis and provide in vivo evidence of the physiological role of AQP1 in CO₂ transport.

Water transport and functional dynamics of aquaporins in osmoregulatory organs of fishes

Aquaporins play distinct roles for water transport in fishes as they do in mammals-both at the cellular, organ, and organismal levels. However, with over 32,000 known species of fishes inhabiting almost every aquatic environment, from tidal pools, small mountain streams, to the oceans and extreme salty desert lakes, the challenge to obtain consensus as well as specific knowledge about aquaporin physiology in these vertebrate clades is overwhelming. Because the integumental surfaces of these animals are in intimate contact with the surrounding milieu, passive water loss and uptake represent two of the major osmoregulatory challenges that need compensation. However, neither obligatory nor regulatory water transport nor their mechanisms have been elucidated to the same degree as, for example, ion transport in fishes. Currently fewer than 60 papers address fish aquaporins. Most of these papers identify "what is present" and describe tissue expression patterns in various teleosts. The agnathans, chondrichthyans, and functionality of fish aquaporins generally have received little attention. This review emphasizes the functional physiology of aquaporins in fishes, focusing on transepithelial water transport in osmoregulatory organs in euryhaline species - primarily teleosts, but covering other taxonomic groups as well. Most current knowledge comes from teleosts, and there is a strong need for related information on older fish clades. Our survey aims to stimulate new, original research in this area and to bring together new collaborations across disciplines.

Profiles of hypothalamus-pituitary-interrenal axis gene expression in the parr and smolt stages of rainbow trout, Oncorhynchus mykiss: effects of recombinant aquaporin 3 and seawater acclimation

The objective of this investigation was to quantify how the hypothalamus-pituitary-interrenal (HPI) axis in the rainbow trout, Oncorhynchus mykiss (parr/smolt), responds to salinity changes during transfer from freshwater (FW) to seawater (SW) and recombinant aquaporin 3 (rAQP3) injection. mRNA expression levels of HPI axis genes [corticotropic-releasing hormone (CRH) and adrenocorticotropic hormone (ACTHα and ACTHβ)] significantly increased when the fish were transferred from FW to SW (parr: 16.4-, 13.2-, 21.4-, and 11.9-fold higher than FW; smolt: 2.3-, 2.7-, 13.6-, and 6.2-fold higher than FW, respectively). Furthermore, and the plasma ACTH, Na(+), Cl(-), and K(+) levels were the highest at 50% SW. Moreover, these parameters were significantly lower in the rAQP3-treated group than those in the control (parr: 2.0-, 2.4-, 2.1-, and 2.0-fold lower than SW; smolt: 4.2-, 1.9-, 2.4-, and 2.3-fold lower than SW, respectively). Hence, HPI axis genes may play a role in SW adaptation during migration from FW to SW environments. We showed that there was a negative correlation between rAQP3, HPI axis genes, and ion levels when the fish were transferred to SW, with levels being significantly lower in the rAQP3-injected group. Hence, cortisol appears to be a stress hormone and plasma Na(+) and Cl(-) levels significantly increased when the fish were transferred to SW, with levels being significantly lower in the rAQP3-treated group. These results indicate that rAQP3 modulates the HPI axis and ion transportation in rainbow trout.

Aquaporin expression in the Japanese medaka (Oryzias latipes) in freshwater and seawater: challenging the paradigm of intestinal water transport?

We investigated the salinity-dependent expression dynamics of seven aquaporin paralogs (aqp1a, aqp3a, aqp7, aqp8ab, aqp10a, aqp10b and aqp11a) in several tissues of euryhaline Japanese medaka (Oryzias latipes). All paralogs except aqp7 and aqp10a had a broad tissue distribution, and several were affected by salinity in both osmoregulatory and non-osmoregulatory tissues. In the intestine, aqp1a, aqp7, aqp8ab and aqp10a decreased upon seawater (SW) acclimation in both long-term acclimated fish and during 1-3 days of the transition period. In the gill, aqp3a was lower and aqp10a higher in SW than in freshwater (FW). In the kidney no aqps were affected by salinity. In the skin, aqp1a and aqp3a were lower in SW than in FW. In the liver, aqp8ab and aqp10a were lower in SW than in FW. Furthermore, six Na(+),K(+)-ATPase α-subunit isoform transcripts were analysed in the intestine but none showed a consistent response to salinity, suggesting that water transport is not regulated at this level. In contrast, mRNA of the Na(+),K(+),2Cl(-)-cotransporter type-2 strongly increased in the intestine in SW compared with FW fish. Using custom-made antibodies, Aqp1a, Aqp8ab and Aqp10a were localized in the apical region of enterocytes of FW fish. Apical staining intensity strongly decreased, vanished or moved to subapical regions, when fish were acclimated to SW, supporting the lower mRNA expression in SW. Western blots confirmed the decrease in Aqp1a and Aqp10a in SW. The strong decrease in aquaporin expression in the intestine of SW fish is surprising, and challenges the paradigm for transepithelial intestinal water absorption in SW fishes.

Isolation and mRNA expression analysis of aquaporin isoforms in marine medaka Oryzias dancena, a euryhaline teleost

We have identified six putative aquaporin (AQP) genes from marine medaka Oryzias dancena (named odAQPs 1, 3, 8, 10, 11 and 12). The marine medaka AQP cDNAs encode polypeptides of 259-298 amino acids, respectively. Topology predictions showed six transmembrane domains, five connecting loops, and cytoplasmic N- and C-terminal domains, all of which is conserved among AQP molecules. Although asparagine-proline-alanine (NPA) motifs are highly conserved in most odAQP isoforms, several AQPs revealed variant types of motifs such as asparagine-proline-proline (NPP), asparagine-proline-valine (NPV) or/and asparagine-proline-serine (NPS) motifs. The phylogenic analysis showed that marine medaka AQPs had closet relationship with Japanese ricefish (medaka; Oryzias latipes) counterparts. Reverse transcription (RT)-PCR analyses showed that marine medaka AQP transcripts would be expressed in not only osmoregulatory tissues but also nonosmoregulatory tissues, and also that the expression levels of certain AQP isoforms in nonosmoregulatory tissues were readily comparable or even higher than those in typically known osmoregulatory organs. Although the overall tissue distribution patterns of AQPs were not significantly different between 0- and 30-ppt acclimated fish, the expression levels under different salinities were largely variable among isoforms and tissues. This is the first report to investigate tissue expression profiles of teleostean AQPs 11 and 12 during the long-term acclimation to freshwater and salted water.