Salinity changes in the anadromous river pufferfish, Takifugu obscurus, mediate gene regulation

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.

Differential expression of aquaporin genes and the influence of environmental hypertonicity on their expression in juveniles of air-breathing stinging catfish (Heteropneustes fossilis)

Aquaporins (AQPs) are a superfamily of transmembrane channel proteins that are responsible for the transport of water and some other molecules to and from the cell, mainly for osmoregulation under anisotonicity. We investigated here the expression patterns of different AQP isoforms and also during exposure to hypertonicity (300 mOsmol/L) for 48 h in juvenile stages of air-breathing stinging catfish (Heteropneustes fossilis). A total of 8 mRNA transcripts for different isoforms of AQPs and their translated proteins could be detected in the anterior and posterior regions of S1, S2, and S3 stages of juveniles of stinging catfish at variable levels. In general, more expression of mRNAs for different aqp genes was seen in the S2 and S3 juveniles than in the S1 juveniles. Most interestingly, exposure to hypertonicity of S2 juveniles for a period of 48 h led to increased expression of most of the aqp genes both at transcriptional and translational levels, except for aqp3 in the anterior and posterior regions and aqp1 in the anterior region, showing maximum expression at later stages of hypertonic exposure. Thus, it is evident that AQPs play crucial roles in maintaining the water and ionic balances under anisotonic conditions even at the early developmental stages of stinging catfish as a biochemical adaptational strategy to survive and grow in anisotonic environment.

Immunosuppression and apoptosis activation mediated by p53-Bcl2/Bax signaling pathway -The potential mechanism of goldfish (Carassius auratus Linnaeus) gill disease caused by Myxobolus ampullicapsulatus

Myxobolus, a major harmful type of myxospora, is one of the main parasitic pathogens of freshwater fish. Once myxoboliosis occurs, treatment can be extremely difficult. Therefore, clear understandings of the etiology of myxoboliosis and its pathological mechanism are keys for prevention and control. Here, histology, transmission electron microscopy, transcriptome study, tunel assay, and immunohistochemistry were carried out, revealing the morphology, pathological effects as well as host response mechanism of goldfish gill to Myxobolus ampullicapsulatus. Histological studies showed that the mature spores of Myxobolus ampullicapsulatus were composed of three parts, the spore shell, sporoplasm and bottle shaped polar capsule containing double S-shaped polar filaments. Transcriptome analysis revealed that Myxobolus ampullicapsulatus -infected (Myx) goldfish gills were characterized by apoptosis activation mediated by "p53 signaling pathway" with significantly up-regulated apoptosis-related differential genes dominated by p53-Bcl2/Bax signaling pathway. In addition, tunel assay revealed severe gill apoptosis in the Myx group. Transcriptome analysis also revealed that Myx group showed changes in immune response and significantly down-regulated immune-related differential genes. Beyond that, immunohistochemistry showed that there was no significant increase in the number of gill lymphocyte after parasite infection. These results suggest that the pathological mechanism of Myxobolus ampullicapsulatus infection on gills of goldfish may be related to apoptosis and immunosuppression. Subsequent qRT-PCR showed that apoptosis-related genes (Caspase3,Bad, Bax) and anti-inflammatory gene IL-10 were significantly increased, while immune-related pro-inflammatory genes (IL-1β, IL-8) were markedly down-regulated, further verifying the transcriptome results. Based on the above results, we concluded that p53-Bcl2/Bax related networks that dominant the expression of apoptosis genes were activated while immunity was suppressed in the gills of Myxobolus ampullicapsulatus infected goldfish. Our study is not only of benefit to enrich the taxonomy of Myxobolus but also clarifies its pathogenic mechanism, thus providing targets for prevention and control of myxoboliosis.

CDK5/NFAT5-Regulated Transporters Involved in Osmoregulation in Fejervarya cancrivora

Crab-eating frogs (Fejervarya cancrivora) can live in brackish water with a salinity of up to 18‱, although most amphibians are not able to tolerate such high saline environments. To investigate its potential osmoregulation, we conducted experiments in F. cancrivora and F. multistriata. The results showed that F. cancrivora made use of ions (such as Na+ and Cl−) to increase intracellular concentrations via the Na+/K+-ATPase (NKA) enzyme. The mRNA expression of aldose reductase (AR) was significantly higher in F. cancrivora (p < 0.05), indicating that more organic osmolytes were produced and transported to maintain cellular homeosis. The mRNA expressions of Aquaporin 1 (AQP1) and AQP3 in kidney were significantly higher in F. cancrivora, while AQP expression in skin was higher in F. multistriata (p < 0.05). The mRNA level in activating the transcription of the nuclear factor of activated T cells-5 (NFAT5) which is one of the target genes of regulating the cellular response to hypertonicity, was higher in F. cancrivora. The protein expression of CDK5, the upstream protein of the NFAT5 pathway, was 2 times higher in F. cancrivora. Therefore, we can conclude that CDK5/NFAT5-regulated transporters might be involved in osmoregulation in F. cancrivora.

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.

Immunohistochemical characterization and change in location of branchial ionocytes after transfer from freshwater to seawater in the euryhaline obscure puffer, Takifugu obscurus

The obscure puffer Takifugu obscurus is a euryhaline fish species suitable for studying the molecular mechanism of osmoregulation. The distributional changes of branchial ionocytes were detected following the transfer from freshwater (FW) to seawater (SW) based on two main ion transporters, Na⁺/K⁺-ATPase (NKA) and Na⁺/K⁺/ 2Cl^- cotransporter 1 (NKCC1). The mRNA and protein expression levels of NKA and NKCC1 in the gills all increased rapidly in the first four days after transfer to SW. Double immunofluorescence staining showed that NKCC1 and NKA were colocalized in the branchial ionocytes and the immunoreaction of NKCC1 was stronger after transfer. Moreover, following transfer to SW, the number of lamellar ionocytes in the gills is reduced and the number of filament ionocytes is increased significantly. Taken together, these findings indicated that SW transfer of obscure puffer promotes the changes of distribution, function and size of branchial ionocytes.

Salinity stress-induced differentially expressed miRNAs and target genes in sea cucumbers Apostichopus japonicus

Environmental salinity is an important abiotic factor influencing normal physiological functions and productive performance in the sea cucumber Apostichopus japonicus. It is therefore important to understand how changes in salinity affect sea cucumbers in the face of global climate change. In this study, we investigated the responses to salinity stress in sea cucumbers using mRNA and miRNA sequencing. The regulatory network of mRNAs and miRNAs involved in salinity stress was examined, and the metabolic pathways enriched for differentially expressed miRNAs and target mRNAs were identified. The top 20 pathways were involved in carbohydrate metabolism, fatty acid metabolism, degradation, and elongation, amino acid metabolism, genetic information processing, metabolism of cofactors and vitamins, transport and catabolism, and environmental information processing. A total of 22 miRNAs showed differential expression during salinity acclimation. The predicted 134 target genes were enriched in functions consistent with the results of gene enrichment based on transcriptome analysis. These results suggested that sea cucumbers deal with salinity stress via changes in amino acid metabolism, ion channels, transporters, and aquaporins, under stimulation by environmental signals, and that this process requires energy from carbohydrate and fatty acid metabolism. Salinity challenge also induced miRNA expression. These results provide a valuable genomic resource that extends our understanding of the unique biological characteristics of this economically important species under conditions of salinity stress.

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.

Bioaccumulation of trace metals in octocorals depends on age and tissue compartmentalization

Trace metal dynamics have not been studied with respect to growth increments in octocorals. It is particularly unknown whether ontogenetic compartmentalization of trace metal accumulation is species-specific. We studied here for the first time the intracolonial distribution and concentrations of 18 trace metals in the octocorals Subergorgia suberosa, Echinogorgia complexa and E. reticulata that were retrieved from the northern coast of Taiwan. Levels of trace metals were considerably elevated in corals collected at these particular coral habitats as a result of diverse anthropogenic inputs. There was a significant difference in the concentration of metals among octocorals except for Sn. Both species of Echinogorgia contained significantly higher concentrations of Cu, Zn and Al than Subergorgia suberosa. We used for the first time exponential growth curves that describe an age-specific relationship of octocoral trace metal concentrations of Cu, Zn, Cd, Cr and Pb where the distance from the grip point was reflecting younger age as linear regressions. The larger colony (C7) had a lower accumulation rate constant than the smaller one (C6) for Cu, Zn, Cd, Cr and Pb, while other trace metals showed an opposite trend. The Cu concentration declined exponentially from the grip point, whereas the concentrations of Zn, Cd, Cr and Pb increased exponentially. In S. suberosa and E. reticulata, Zn occurred primarily in coenosarc tissues and Zn concentrations increased with distance from the grip point in both skeletal and coenosarc tissues. Metals which appeared at high concentrations (e.g. Ca, Zn and Fe) generally tended to accumulate in the outer coenosarc tissues, while metals with low concentrations (e.g. V) tended to accumulate in the soft tissues of the inner skeleton.

Mitigation of nitrite toxicity by increased salinity is associated with multiple physiological responses: A case study using an economically important model species, the juvenile obscure puffer (Takifugu obscurus)

Nitrite is a common pollutant in water and is highly toxic to aquatic animals. To reveal the mechanism of salinity in attenuating nitrite toxicity to fish, we measured the physiological responses of juvenile Takifugu obscurus exposed to nitrite concentrations (0, 10, 20, 50, and 100 mg/L) under different salinity levels (0, 10, and 20 ppt) for 96 h. Salinity increased the survival rates of juvenile T. obscurus exposed to nitrite. Changes in key hematological parameters, antioxidant system, malondialdehyde, Na⁺/K⁺-ATPase, and HSP70 indicated that nitrite induced considerable damage to juveniles; salinity mitigated the harmful effects. This finding reflects similar changing trends in both antioxidants and their gene expressions among different tissues. We applied an overall index, an integrated biomarker response (IBR), that increased under high-nitrite condition but recovered to the normal levels under salinity treatment. Analysis of the selected detection indices and IBR values showed that the overall mitigating effect of salinity on nitrite toxicity seems to be at sub-cellular level and associated with complicated physiological responses.

Apoptosis in fish: environmental factors and programmed cell death

Apoptosis, a form of programmed cell death, is a critical component in maintaining homeostasis and growth in all tissues and plays a significant role in immunity and cytotoxicity. In contrast to necrosis or traumatic cell death, apoptosis is a well-controlled and vital process characterized mainly by cytoplasmic shrinkage, chromatin condensation, DNA fragmentation, membrane blebbing and apoptotic bodies. Our understanding of apoptosis is partly based on observations in invertebrates but mainly in mammals. Despite the great advantages of fish models in studying vertebrate development and diseases and the tremendous interest observed in recent years, reports on apoptosis in fish are still limited. Although apoptotic machinery is well conserved between aquatic and terrestrial organisms throughout the history of evolution, some differences exist in key components of apoptotic pathways. Core parts of apoptotic machinery in fish are virtually expressed as equivalent to the mammalian models. Some differences are, however, evident, such as the extrinsic and intrinsic pathways of apoptosis including lack of a C-terminal region in the Fas-associated protein with a death domain in fish. Aquatic species inhabit a complex and highly fluctuating environment, making these species good examples to reveal features of apoptosis that may not be easily investigated in mammals. Therefore, in order to gain a wider view on programmed cell death in fish, interactions between the main environmental factors, chemicals and apoptosis are discussed in this review. It is indicated that apoptosis can be induced in fish by exposure to environmental stressors during different stages of the fish life cycle.

Molecular Characterization of Aquaporin 1 and Aquaporin 3 from the Gills of the African Lungfish, Protopterus annectens, and Changes in Their Branchial mRNA Expression Levels and Protein Abundance during Three Phases of Aestivation

African lungfishes can undergo long periods of aestivation on land during drought. During aestivation, lungfishes are confronted with desiccation and dehydration, and their gills become non-functional and covered with a thick layer of dried mucus. Aquaporins (Aqps) are a superfamily of integral membrane proteins which generally facilitate the permeation of water through plasma membranes. This study aimed to obtain the complete cDNA coding sequences of aqp1 and aqp3 from the gills of Protopterus annectens, and to determine their branchial mRNA and protein expression levels during the induction, maintenance and arousal phases of aestivation. Dendrogramic analyses of the deduced Aqp1 and Aqp3 amino acid sequences of P. annectens revealed their close relationships with those of Latimeria chalumnae and tetrapods. During the induction phase, there were significant decreases in the transcript levels of aqp1 and aqp3 in the gills of P. annectens, but the branchial Aqp1 and Aqp3 protein abundance remained unchanged. As changes in transcription might precede changes in translation, this could be regarded as an adaptive response to decrease the protein abundance of Aqp1 and Aqp3 in the subsequent maintenance phase of aestivation. As expected, the branchial transcript levels and protein abundance of aqp1/Aqp1 and aqp3/Aqp3 were significantly down-regulated during the maintenance phase, probably attributable to the shutdown of branchial functions and the cessation of volume regulation of branchial epithelial cells. Additionally, these changes could reduce the loss of water through branchial epithelial surfaces, supplementing the anti-desiccating property of the dried mucus. Upon arousal, it was essential for the lungfish to restore branchial functions. Indeed, the protein abundance of Aqp1 recovered partially, with complete recovery of mRNA expression level and protein abundance of Aqp3, in the gills of P. annectens after 3 days of arousal. These results provide insights into how P. annectens regulates branchial Aqp expression to cope with desiccation and rehydration during different phases of aestivation.

Evaluation of potential candidate genes involved in salinity tolerance in striped catfish (Pangasianodon hypophthalmus) using an RNA-Seq approach

Increasing salinity levels in freshwater and coastal environments caused by sea level rise linked to climate change is now recognized to be a major factor that can impact fish growth negatively, especially for freshwater teleost species. Striped catfish (Pangasianodon hypophthalmus) is an important freshwater teleost that is now widely farmed across the Mekong River Delta in Vietnam. Understanding the basis for tolerance and adaptation to raised environmental salinity conditions can assist the regional culture industry to mitigate predicted impacts of climate change across this region. Attempt of next generation sequencing using the ion proton platform results in more than 174 million raw reads from three tissue libraries (gill, kidney and intestine). Reads were filtered and de novo assembled using a variety of assemblers and then clustered together to generate a combined reference transcriptome. Downstream analysis resulted in a final reference transcriptome that contained 60,585 transcripts with an N50 of 683 bp. This resource was further annotated using a variety of bioinformatics databases, followed by differential gene expression analysis that resulted in 3062 transcripts that were differentially expressed in catfish samples raised under two experimental conditions (0 and 15 ppt). A number of transcripts with a potential role in salinity tolerance were then classified into six different functional gene categories based on their gene ontology assignments. These included; energy metabolism, ion transportation, detoxification, signal transduction, structural organization and detoxification. Finally, we combined the data on functional salinity tolerance genes into a hypothetical schematic model that attempted to describe potential relationships and interactions among target genes to explain the molecular pathways that control adaptive salinity responses in P. hypophthalmus. Our results indicate that P. hypophthalmus exhibit predictable plastic regulatory responses to elevated salinity by means of characteristic gene expression patterns, providing numerous candidate genes for future investigations.

Harpin Hpa1 Interacts with Aquaporin PIP1;4 to Promote the Substrate Transport and Photosynthesis in Arabidopsis

Harpin proteins produced by plant-pathogenic Gram-negative bacteria are the venerable player in regulating bacterial virulence and inducing plant growth and defenses. A major gap in these effects is plant sensing linked to cellular responses, and plant sensor for harpin Hpa1 from rice bacterial blight pathogen points to plasma membrane intrinsic protein (PIP). Here we show that Arabidopsis AtPIP1;4 is a plasma membrane sensor of Hpa1 and plays a dual role in plasma membrane permeability of CO2 and H2O. In particular, AtPIP1;4 mediates CO2 transport with a substantial contribute to photosynthesis and further increases this function upon interacting with Hpa1 at the plasma membrane. As a result, leaf photosynthesis rates are increased and the plant growth is enhanced in contrast to the normal process without Hpa1-AtPIP1;4 interaction. Our findings demonstrate the first case that plant sensing of a bacterial harpin protein is connected with photosynthetic physiology to regulate plant growth.

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.

Biological mosquito control is affected by alternative prey

BACKGROUND

Mosquitofish were introduced to several countries of the tropics and subtropics as biological agents for the control of mosquito larvae. Meanwhile, they became a threat to native communities and fish worldwide, similar to other invasive species through resource competition, overexploitation, or habitat alteration. We investigated prey selectivity patterns of Gambusia affinis (mosquitofish) preying on larvae of the two Indian major carps (Catla catla and Labeo rohita) in the presence of varied proportions of alternative prey (rotifers, cladocerans, chironomid and mosquito larvae) under laboratory conditions.

RESULTS

The patterns of prey selectivity in mosquitofish were influenced by the presence of alternative prey and theirrelative abundance in the environment. Carp larvae, when present in equal proportions, were randomly selected by mosquitofish, however, positively selected when present in higher proportions.Inthe presence of Hexarthramira, Daphnia similoides or the mosquito larval instar-IV as an alternative prey, the mosquitofish preferred fish larvae regardless of prey proportions. In the medium where either mosquito larval instar-I or chironomid larvae were givenas alternative prey, the mosquitofish either rejected or randomly selected the carp larvae. Given a multispecies prey combination, mosquitofish primarily selected the larvae of L.rohita and mosquito larval instar-I. We also found a prey switching ability of mosquitofish in relation to varying abundances of prey species in the environment.

CONCLUSIONS

Thepresent results suggest that mosquito immatures are not the preferred food of mosquitofish when fish larvae are present in their natural habitats. Since mosquitofish and carp larvae have overlapping natural habitats and prey preferences are the invasive mosquitofish may have a substantial impact on native communities of invertebrates and fish. This way, they are equally important for extensive fisheries and conservation management.

Prolactin and teleost ionocytes: new insights into cellular and molecular targets of prolactin in vertebrate epithelia

The peptide hormone prolactin is a functionally versatile hormone produced by the vertebrate pituitary. Comparative studies over the last six decades have revealed that a conserved function for prolactin across vertebrates is the regulation of ion and water transport in a variety of tissues including those responsible for whole-organism ion homeostasis. In teleost fishes, prolactin was identified as the "freshwater-adapting hormone", promoting ion-conserving and water-secreting processes by acting on the gill, kidney, gut and urinary bladder. In mammals, prolactin is known to regulate renal, intestinal, mammary and amniotic epithelia, with dysfunction linked to hypogonadism, infertility, and metabolic disorders. Until recently, our understanding of the cellular mechanisms of prolactin action in fishes has been hampered by a paucity of molecular tools to define and study ionocytes, specialized cells that control active ion transport across branchial and epidermal epithelia. Here we review work in teleost models indicating that prolactin regulates ion balance through action on ion transporters, tight-junction proteins, and water channels in ionocytes, and discuss recent advances in our understanding of ionocyte function in the genetically and embryonically accessible zebrafish (Danio rerio). Given the high degree of evolutionary conservation in endocrine and osmoregulatory systems, these studies in teleost models are contributing novel mechanistic insight into how prolactin participates in the development, function, and dysfunction of osmoregulatory systems across the vertebrate lineage.