8 The Roles of Gill Permeability and Transport Mechanisms in Euryhalinity

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.

Olfactory receptor OR52N2 for PGE2 in mediation of guppy courtship behaviors

Prostaglandins (PGs) are a type of physiologically active unsaturated fatty acids. As an important sex pheromone, PGs play a vital role in regulating the reproductive behaviors of species by mediating nerve and endocrine responses. In this study, guppy (Poecilia reticulate) was used as the model specie to detect the function of PGE₂ in inducing the onset of courtship behaviors. Our results showed that adding PGE₂ into the water environment could activate the courtship behavior of male guppy, indicating that the peripheral olfactory system mediated the PGE₂ function. Thereafter, the open reading frame (ORF) of olfactory receptor or52n2 was cloned, which was 936 bp in length, coding 311 amino acids. As a typical G protein-coupled receptor, OR52N2 had a conservative seven α-helix transmembrane domains. To confirm the regulatory relationship between OR52N2 and PGE₂, dual-luciferase reporter assay was employed to verify the activation of downstream CREB signaling pathways. Results showed that PGE₂ significantly enhanced CRE promoter activity in or52n2 ORF transient transfected HEK-293 T cells. Finally, localization of or52n2 mRNA were observed in ciliated receptor cells of the olfactory epithelium using in situ hybridization. Our results provide a novel insight into sex pheromone signaling transduction in reproductive behavior.

Transcriptome Analysis Revealed Osmoregulation Related Regulatory Networks and Hub Genes in the Gills of Hilsa shad, Tenualosa ilisha, during the Migratory Osmotic Stress

Tenualosa ilisha (Hilsa shad), an anadromous fish, usually inhabits coastal and estuarine waters, and migrates to freshwater for spawning. In this study, large-scale gill transcriptome analyses from three salinity regions, i.e., fresh, brackish and marine water, revealed 3277 differentially expressed genes (DEGs), out of which 232 were found to be common between marine vs freshwater and brackish vs freshwater. These genes were mapped into 54 KEGG Pathways, and the most significant of these were focal adhesion, adherens junction, tight junction, and PI3K-Akt signaling pathways. A total of 24 osmoregulatory genes were found to be differentially expressed in different habitats. The gene members of slc16 and slc2 families showed a dissimilar pattern of expressions, while two claudin genes (cldn11 & cldn10), transmembrane tm56b, and voltage-gated potassium channel gene kcna10 were downregulated in freshwater samples, as compared to that of brackish and marine environment. Protein-protein interaction (PPI) network analysis of 232 DEGs showed 101 genes to be involved in PPI, while fn1 gene was found to be interacting with the highest number of genes (36). Twenty-five hub genes belonged to 12 functional groups, with muscle structure development with seven genes, forming the major group. These results provided valuable information about the genes, potentially involved in the molecular mechanisms regulating water homeostasis in gills, during migration for spawning and low-salinity adaptation in Hilsa shad. These genes may form the basis for the bio-marker development for adaptation to the stress levied by major environmental changes, due to hatchery/culture conditions.

Ion Transporters and Osmoregulation in the Kidney of Teleost Fishes as a Function of Salinity

Euryhaline teleosts exhibit major changes in renal function as they move between freshwater (FW) and seawater (SW) environments, thus tolerating large fluctuations in salinity. In FW, the kidney excretes large volumes of water through high glomerular filtration rates (GFR) and low tubular reabsorption rates, while actively reabsorbing most ions at high rates. The excreted product has a high urine flow rate (UFR) with a dilute composition. In SW, GFR is greatly reduced, and the tubules reabsorb as much water as possible, while actively secreting divalent ions. The excreted product has a low UFR, and is almost isosmotic to the blood plasma, with Mg²⁺, SO₄^2–, and Cl^– as the major ionic components. Early studies at the organismal level have described these basic patterns, while in the last two decades, studies of regulation at the cell and molecular level have been implemented, though only in a few euryhaline groups (salmonids, eels, tilapias, and fugus). There have been few studies combining the two approaches. The aim of the review is to integrate known aspects of renal physiology (reabsorption and secretion) with more recent advances in molecular water and solute physiology (gene and protein function of transporters). The renal transporters addressed include the subunits of the Na⁺, K⁺- ATPase (NKA) enzyme, monovalent ion transporters for Na⁺, Cl^–, and K⁺ (NKCC1, NKCC2, CLC-K, NCC, ROMK2), water transport pathways [aquaporins (AQP), claudins (CLDN)], and divalent ion transporters for SO₄^2–, Mg²⁺, and Ca²⁺ (SLC26A6, SLC26A1, SLC13A1, SLC41A1, CNNM2, CNNM3, NCX1, NCX2, PMCA). For each transport category, we address the current understanding at the molecular level, try to synthesize it with classical knowledge of overall renal function, and highlight knowledge gaps. Future research on the kidney of euryhaline fishes should focus on integrating changes in kidney reabsorption and secretion of ions with changes in transporter function at the cellular and molecular level (gene and protein verification) in different regions of the nephrons. An increased focus on the kidney individually and its functional integration with the other osmoregulatory organs (gills, skin and intestine) in maintaining overall homeostasis will have applied relevance for aquaculture.

Energy and nitrogenous waste from glutamate/glutamine catabolism facilitates acute osmotic adjustment in non-neuroectodermal branchial cells

Maintenance of homeostasis is one of the most important physiological responses for animals upon osmotic perturbations. Ionocytes of branchial epithelia are the major cell types responsible for active ion transport, which is mediated by energy-consuming ion pumps (e.g., Na⁺-K⁺-ATPase, NKA) and secondary active transporters. Consequently, in addition to osmolyte adjustments, sufficient and immediate energy replenishment is essenttableial for acclimation to osmotic changes. In this study, we propose that glutamate/glutamine catabolism and trans-epithelial transport of nitrogenous waste may aid euryhaline teleosts Japanese medaka (Oryzias latipes) during acclimation to osmotic changes. Glutamate family amino acid contents in gills were increased by hyperosmotic challenge along an acclimation period of 72 hours. This change in amino acids was accompanied by a stimulation of putative glutamate/glutamine transporters (Eaats, Sat) and synthesis enzymes (Gls, Glul) that participate in regulating glutamate/glutamine cycling in branchial epithelia during acclimation to hyperosmotic conditions. In situ hybridization of glutaminase and glutamine synthetase in combination with immunocytochemistry demonstrate a partial colocalization of olgls1a and olgls2 but not olglul with Na⁺/K⁺-ATPase-rich ionocytes. Also for the glutamate and glutamine transporters colocalization with ionocytes was found for oleaat1, oleaat3, and olslc38a4, but not oleaat2. Morpholino knock-down of Sat decreased Na⁺ flux from the larval epithelium, demonstrating the importance of glutamate/glutamine transport in osmotic regulation. In addition to its role as an energy substrate, glutamate deamination produces NH₄⁺, which may contribute to osmolyte production; genes encoding components of the urea production cycle, including carbamoyl phosphate synthetase (CPS) and ornithine transcarbamylase (OTC), were upregulated under hyperosmotic challenges. Based on these findings the present work demonstrates that the glutamate/glutamine cycle and subsequent transepithelial transport of nitrogenous waste in branchial epithelia represents an essential component for the maintenance of ionic homeostasis under a hyperosmotic challenge.

Behavioural salinity preference of juvenile yellow perch Perca flavescens

The present study determined the behavioural salinity preference of a freshwater stock of juvenile yellow perch Perca flavescens acclimated to salinities of 0 and 10. The preferred salinities ranged between 7·3 and 13·0 (mean ± s.d. = 10·4 ± 1·7; n = 13) with no significant effect of acclimation salinity. The results showed that juvenile P. flavescens prefers near isoosmotic salinities, which could be due to a lowered energetic cost of osmoregulation.

Exploration of the mechanisms of protein quality control and osmoregulation in gills of Chromis viridis in response to reduced salinity

Fish gills are the vital multifunctional organ in direct contact with external environment. Therefore, activation of the cytoprotective mechanisms to maintain branchial cell viability is important for fish upon stresses. Salinity is one of the major factors strongly affecting cellular and organismal functions. Reduction of ambient salinity may occur in coral reef and leads to osmotic stress for reef-associated stenohaline fish. However, the physiological responses to salinity stress in reef-associated fish were not examined substantially. With this regard, the physiological parameters and the responses of protein quality control (PQC) and osmoregulatory mechanisms in gills of seawater (SW; 33-35 ‰)- and brackish water (BW; 20 ‰)-acclimated blue-green damselfish (Chromis viridis) were explored. The results showed that the examined physiological parameters were maintained within certain physiological ranges in C. viridis acclimated to different salinities. In PQC mechanism, expression of heat-shock protein (HSP) 90, 70, and 60 elevated in response to BW acclimation while the levels of ubiquitin-conjugated proteins were similar between the two groups. Thus, it was presumed that upregulation of HSPs was sufficient to prevent the accumulation of aggregated proteins for maintaining the protein quality and viability of gill cells when C. viridis were acclimated to BW. Moreover, gill Na(+)/K(+)-ATPase expression and protein amounts of basolaterally located Na(+)/K(+)/2Cl(-) cotransporter were higher in SW fish than in BW fish. Taken together, this study showed that the cytoprotective and osmoregulatory mechanisms of blue-green damselfish were functionally activated and modulated to withstand the challenge of reduction in salinity for maintaining physiological homeostasis.

Divergent immunity and energetic programs in the gills of migratory and resident Oncorhynchus mykiss

Divergent life history strategies occur in steelhead or rainbow trout Oncorhynchus mykiss, and many populations produce both migrant (anadromous fish that move to the ocean after rearing) and resident (do not migrate and remain in fresh water) individuals. Mechanisms leading to each type are only partially understood; while the general tendency of a population is heritable, individual tendency may be plastic, influenced by local environment. Steelhead hatchery programmes aim to mitigate losses in wild stocks by producing trout that will migrate to the ocean and not compete with wild trout for limited freshwater resources. To increase our understanding of gill function in these migratory or resident phenotypes, here we compare gill transcriptome profiles of hatchery-released fish either at the release site (residents) or five river kilometres downstream while still in full fresh water (migrants). To test whether any of these genes can be used as predictive markers for smoltification, we compared these genes between migrant-like and undifferentiated trout while still in the hatchery in a common environment (prerelease). Results confirmed the gradual process of smoltification, and the importance of energetics, gill remodelling and ion transport capacity for migrants. Additionally, residents overexpressed transcripts involved in antiviral defences, potentially for immune surveillance via dendritic cells in the gills. The best smoltification marker candidate was protein s100a4, expression of which was highly correlated with Na(+) , K(+) ATPase (NKA) activity and smolt-like morphology in pre- and postrelease trout gills.

Effect of salinity on oxygen consumption in fishes: a review

The effect of salinity on resting oxygen uptake was measured in the perch Perca fluviatilis and available information on oxygen uptake in teleost species at a variety of salinities was reviewed. Trans-epithelial ion transport against a concentration gradient requires energy and exposure to salinities osmotically different from the body fluids therefore imposes an energetic demand that is expected to be lowest in brackish water compared to fresh and sea water. Across species, there is no clear trend between oxygen uptake and salinity, and estimates of cost of osmotic and ionic regulation vary from a few per cent to >30% of standard metabolism.

Time course of the acute response of the North Pacific spiny dogfish shark (Squalus suckleyi) to low salinity

Dogfish are considered stenohaline sharks but are known to briefly enter estuaries. The acute response of North Pacific spiny dogfish (Squalus suckleyi) to lowered salinity was tested by exposing sharks to 21‰ salinity for 48 h. Temporal trends in blood pH, plasma osmolality, CO2, HCO3(-), Na(+), Cl(-), K(+), and urea concentrations, and in the rates of urea efflux and O2 consumption, were quantified. The rate of O2 consumption exhibited cyclic variation and was significantly depressed by lowered salinity. After 9 h, plasma [Cl(-)] stabilized at 9% below initial levels, while plasma [Na(+)] decreased by more than 20% within the first 12 h. Plasma [urea] dropped by 15% between 4 and 6 h, and continued to decrease. The rate of urea efflux increased over time, peaking after 36 h at 72% above the initial rate. Free-swimming sharks subjected to the same salinity challenge survived over 96 h and differed from cannulated sharks with respect to patterns of Na(+) and urea homeostasis. This high-resolution study reveals that dogfish exposed to 21‰ salinity can maintain homeostasis of Cl(-) and pH, but Na(+) and urea continue to be lost, likely accounting for the inability of the dogfish to fully acclimate to reduced salinity.

Osmoregulation and expression of ion transport proteins and putative claudins in the gill of southern flounder (Paralichthys lethostigma)

The southern flounder is a euryhaline teleost that inhabits ocean, estuarine, and riverine environments. We investigated the osmoregulatory strategy of juvenile flounder by examining the time-course of homeostatic responses, hormone levels, and gill Na(+),K(+)-ATPase and Na(+),K(+),2Cl(-) cotransporter protein expression after salinity challenge. Transfer of freshwater (FW)-acclimated flounder to sea water (SW) induced an increase in plasma osmolality and cortisol and a decrease in muscle water content, plasma insulin-like growth factor I (IGF-I) and hepatic IGF-I mRNA, all returning to control levels after 4 days. Gill Na(+),K(+)-ATPase and Na(+),K(+),2Cl(-) cotransporter protein levels were elevated in response to SW after 4 days. Transfer of SW-acclimated flounder to FW reduced gill Na(+),K(+)-ATPase and Na(+),K(+),2Cl(-) cotransporter protein, increased plasma IGF-I, but did not alter hepatic IGF-I mRNA or plasma cortisol levels. Gill claudin-3 and claudin-4 immunoreactive proteins were elevated in FW versus SW acclimated flounder. The study demonstrates that successful acclimation of southern flounder to SW or FW occurs after an initial crisis period and that the salinity adaptation process is associated with changes in branchial expression of ion transport and putative tight junction claudin proteins known to regulate epithelial permeability in mammalian vertebrates.

The effect of environmental salinity on the protein expression of Na+/K+-ATPase, Na+/K+/2Cl- cotransporter, cystic fibrosis transmembrane conductance regulator, anion exchanger 1, and chloride channel 3 in gills of a euryhaline teleost, Tetraodon nigroviridis

Chloride transport mechanisms in the gills of the estuarine spotted green pufferfish (Tetraodon nigroviridis) were investigated. Protein abundance of Na(+)/K(+)-ATPase (NKA) and the other four chloride transporters, i.e., Na(+)/K(+)/2Cl(-) cotransporter (NKCC), cystic fibrosis transmembrane conductance regulator (CFTR), Cl(-)/HCO(3)(-) anion exchanger 1 (AE1), and chloride channel 3 (CLC-3) in gills of the seawater- (SW; 35 per thousand) or freshwater (FW)-acclimatized fish were examined by immunoblot analysis. Appropriate negative controls were used to confirm the specificity of the antibodies to the target proteins. The relative protein abundance of NKA was higher (i.e., 2-fold) in gills of the SW group compared to the FW group. NKCC and CFTR were expressed in gills of the SW group but not in the FW group. In contrast, the levels of relative protein abundance of branchial AE1 and CLC-3 in the FW group were 23-fold and 2.7-fold higher, respectively, compared to those of the SW group. This study is first of its kind to provide direct in vivo evidence of the protein expression of CLC-3 in teleostean gills, as well as to examine the simultaneous protein expression of the Cl(-) transporters, especially AE1 and CLC-3 of FW- and SW-acclimatized teleosts. The differential protein expression of NKA, chloride transporters in gills of the FW- and SW-acclimatized T. nigroviridis observed in the present study shows their close relationship to the physiological homeostasis (stable blood osmolality), as well as explains the impressive ionoregulatory ability of this euryhaline species in response to salinity challenges.

Physiological responses to hyper-saline waters in sailfin mollies (Poecilia latipinna)

We examined the ionoregulatory physiology and biochemistry of the teleost sailfin molly (Poecilia latipinna), an inhabitant of salt marshes along the gulf coast, during exposure to hyper-saline waters (salinity range 35-95 ppt). Mollies were able to tightly control plasma Na(+) and Cl(-) concentrations and tissue water levels up to 65 ppt, but at higher salinities plasma ion levels began to rise and muscle water content dropped. Still, even at the highest salinity (90 ppt) plasma Na(+) and Cl(-) levels were only 32% and 39%, respectively, above levels at 35 ppt. Drinking rates at 60 ppt climbed 35%, while gut Na(+)/K(+)-ATPase (NAK) activity rose 70% and branchial NAK activity jumped 200%. The relatively small rise in drinking rate, in the face of a more than doubling of the osmotic gradient, suggests that a reduction in branchial water permeability significantly limited water loss and associated salt load. At 80 ppt, a salinity where plasma ion levels just begin to rise, drinking rate rose more rapidly, but gut and gill NAK activity did not, suggesting that mollies employed other pathways (perhaps renal) of salt excretion. At higher salinities, plasma ion levels continued to rise and muscle water content fell slightly indicating the beginnings of internal osmotic disturbances. To evaluate the energetic costs of hyper-salinity on mollies we measured the rate of O(2) consumption and found it rose with salinity, in sharp contrast to virtually all species previously examined. Interestingly, despite higher metabolism, growth was unaffected by hyper-salinity.

The natriuretic peptide system in eels: a key endocrine system for euryhalinity?

The natriuretic peptide system of a euryhaline teleost, the Japanese eel (Anguilla japonica), consists of three types of hormones [atrial natriuretic peptide (ANP), ventricular natriuretic peptide (VNP), and C-type natriuretic peptide (CNP)] and four types of receptors [natriuretic peptide receptors (NPR)-A, -B, -C, and -D]. Although ANP is recognized as a volume-regulating hormone that extrudes both Na(+) and water in mammals, ANP more specifically extrudes Na(+) in eels. Accumulating evidence shows that ANP is secreted in response to hypernatremia and acts to inhibit the uptake and to stimulate the excretion of Na(+) but not water, thereby promoting seawater (SW) adaptation. In fact, ANP is secreted immediately after transfer of eels to SW and ameliorates sudden increases in plasma Na(+) concentration through inhibition of drinking and intestinal absorption of NaCl. ANP also stimulates the secretion of cortisol, a long-acting hormone for SW adaptation, whereas ANP itself disappears quickly from the circulation. Thus ANP is a primary hormone responsible for the initial phase of SW adaptation. By contrast, CNP appears to be a hormone involved in freshwater (FW) adaptation. Recent data show that the gene expression of CNP and its specific receptor, NPR-B, is much enhanced in FW eels. In fact, CNP infusion increases (22)Na uptake from the environment in FW eels. These results show that ANP and CNP, despite high sequence identity, have opposite effects on salinity adaptation in eels. This difference apparently originates from the difference in their specific receptors, ANP for NPR-A and CNP for NPR-B. VNP may compensate the effects of ANP and CNP for adaptation to respective media, because it has high affinity to both receptors. On the basis of these data, the authors suggest that the natriuretic peptide system is a key endocrine system that allows this euryhaline fish to adapt to diverse osmotic environments, particularly in the initial phase of adaptation.

The cultured branchial epithelium of the rainbow trout as a model for diffusive fluxes of ammonia across the fish gill

A novel branchial epithelial preparation grown in L-15 medium in culture was used as a model system for understanding the diffusion of ammonia across the gills of the rainbow trout Oncorhynchus mykiss. The epithelium is known to contain both respiratory and mitochondria-rich cells in the approximate proportion in which they occur in vivo and to exhibit diffusive fluxes of Na+ and Cl– similar to in vivo values, but does not exhibit active apical-to-basolateral transport of Na+. Transepithelial resistance and paracellular permeability are also known to increase when the apical medium is changed from L-15 medium (symmetrical conditions) to fresh water (asymmetrical conditions). In the present study, net basolateral-to-apical ammonia fluxes increased as basolateral total ammonia concentration, basolateral-to-apical pH gradients and basolateral-to-apical PNH3 gradients were experimentally increased and were greater under asymmetrical than under symmetrical conditions. The slope of the relationship between ammonia flux and PNH3 gradient (i.e. NH3 permeability) was the same under both conditions and similar to values for other epithelia. The higher fluxes under asymmetrical conditions were explained by an apparent diffusive flux of NH4+ that was linearly correlated with transepithelial conductance and was probably explained by the higher electrochemical gradient and higher paracellular permeability when fresh water was present on the apical surface. In this situation, NH4+ diffusion was greater than NH3 diffusion under conditions representative of in vivo values, but overall fluxes amounted to only approximately 20 % of those in vivo. These results suggest that branchial ammonia excretion in the intact animal is unlikely to be explained by diffusion alone and, therefore, that carrier-mediated transport may play an important role.

Hormonal control of osmoregulation in the channel catfish Ictalurus punctatus

Prolactin (PRL) is an important hormone for freshwater adaptation in many teleost species. In some euryhaline fishes, growth hormone (GH) and cortisol are involved in seawater adaptation by stimulating ion extrusion. When channel catfish (Ictalurus punctatus) were transferred from fresh water to dilute seawater (300-400 mOsm), their plasma osmolality was always higher than the environmental salinity. In correlation with the increase in plasma osmolality, significant increases in plasma cortisol were observed. However, no effect of ovine GH or cortisol was seen in plasma osmolality or gill Na, K-ATPase activity when the hormones were given during the course of acclimation to dilute seawater. When catfish in fresh water were hypophysectomized, plasma osmolality was significantly decreased by 24 h, reaching a minimum level after 2 days. When they were transferred to dilute seawater, the plasma osmolality of the sham-operated fish was consistently higher than that of environmental water, whereas the osmolality of the hypophysectomized fish was equivalent to the environmental salinity. Ovine PRL restored the plasma osmolality of the hypophysectomized fish in fresh water to the level of sham-operated fish. Cortisol was also effective, but the effect was less pronounced than the effect of PRL. Injection of PRL in combination with cortisol resulted in a marked additive increase in plasma osmolality to a level even above that of the sham-operated fish. Ovine GH was without effect. These treatments in hypophysectomized fish transferred to dilute seawater produced essentially the same results as those in fish in fresh water. Plasma osmolality was also increased after PRL treatment of the intact fish in fresh water. There was a synergistic effect between PRL and cortisol in hypophysectomized fish in dilute seawater as well as in intact fish in fresh water. PRL did not stimulate cortisol secretion either in hypophysectomized fish or in intact fish. In the stenohaline catfish, both PRL and cortisol seem to be involved importantly in ion uptake from the environment not only in fresh water but also in brackish water.

A novel natriuretic peptide isolated from eel cardiac ventricles

A new natriuretic peptide, which exhibits the entire spectrum of actions known to be characteristic of atrial and brain natriuretic peptides (ANP and BNP), was isolated from eel cardiac ventricles and has been named ventricular natriuretic peptide (VNP). The primary structure of eel VNP is characterized by its uniquely long C-terminal 'tail' that extends from the second half-cystine. Thus, eel VNP appears to be a novel natriuretic peptide of a type not found in mammals. With respect to natriuretic (rat) and vasodepressor (rat and eel) activities, eel VNP is much more potent than human ANP in eels and almost equipotent in rats. Strong tachyphylaxis is observed for the vasodepressor effect in both rats and eels, whereas it is not observed for the natriuretic effect in rats.