Identification and pathway analysis of immediate hyperosmotic stress responsive molecular mechanisms in tilapia (Oreochromis mossambicus) gill

Hormonal regulation and expression dynamics of Bmp4 in the reproductive cycle of Pampus argenteus

As a member of the bone morphogenetic protein (BMP) family, Bmp4 plays a crucial role in bone formation, early developmental stage and the regulation of gonadal development. This study aims to investigate the expression patterns and potential functions of the Bmp4 in Pampus argenteus during its reproductive cycle. A systematic evolutionary analysis of the Bmp4 was performed, and its expression was quantified and localized using real-time polymerase chain reaction (PCR), Western blotting and immunohistochemistry. Additionally, the effects of various hormones on hormone-related genes were analysed in vitro using cultured testes, ovaries and oocytes. The results revealed that Bmp4 is evolutionarily conserved and broadly expressed across tissues. Notably, its expression significantly increased at 4 and 13 days post-hatching (dph), with no significant differences observed at other developmental stages. Western blotting identified a specific Bmp4 target band in Stage III ovaries, and immunohistochemistry localized Bmp4 expression in oogonia, oocytes and spermatogonia. In vitro hormonal treatments demonstrated that oestradiol (E2) significantly upregulated bmp4 expression in both testes and ovaries, with expression levels being notably higher in ovaries. In immature vitellogenic oocytes cultured in vitro, sex hormone treatments did not significantly affect the expression of bmp4, lhr, fshr and lhβ. However, under LH + hCG treatment (50 ng/mL and 5 IU/mL), the expression of fshβ was upregulated. In summary, our findings reveal that Bmp4 serves as a pivotal factor in the early development of the gonads and germ cells in P. argenteus. The expression of Bmp4 within the ovary is likely regulated by E2. During the early-stage development of the ovary, E2 and Bmp4 may jointly promote the early-stage development and differentiation of germ cells. This study offers a valuable theoretical foundation for addressing challenges in the breeding process.

Comparative proteomic profiling represents an inhibition of protein synthesis to regulate osmotic stress in Nile tilapia (Oreochromis niloticus) embryos

Seawater (SW)-acclimated Nile tilapia, Oreochromis niloticus, can tolerate up to 30 g.L-1 SW but rarely produce offspring. The embryos of SW-acclimated O. niloticus survived equally well from 0- to 10-g.L-1 environment but not under 20-g. L-1. However, when the embryos were incubated under 10 g.L-1 during days 0-3, and then the salinity was suddenly shifted to and maintained at 20 g.L-1 during days 4-6, their survival rate was comparable to those incubated under 0 and 10 g.L-1. To elucidate a molecular adaptation of the embryos that survived different salinity environments, the proteomic profiles of the newly hatched embryos, or early larvae, hatched under 0 g.L-1, 10 g.L-1, and those being incubated at 10 g.L-1 during days 0-3 followed at 20 g.L-1 during days 4-6 were compared. Total proteins extracted from the samples were identified with a gel-free shot-gun proteomics approach using the Nile tilapia protein database. The early larvae from the three groups expressed 2295 proteins, and 279 proteins showed statistically different expressions among groups. Downregulation of the 182 proteins in the larvae hatched under 10 and 20 g.L-1 was found to include 22 proteins that are responsible for cellular responses to osmotic stress. This adaptation may be a crucial factor in reducing cellular metabolism and ion transport between the intra- and extra-cellular environment to stabilize cellular osmolality. In addition, some of these proteins suppress cellular damage from oxygen free radicals generated from the osmotic stress. Eighty-seven proteins significantly changed in the larvae hatched under 20 g.L-1 were clustered. Nineteen of the cellular stress response proteins, which were considered to be mortality induction, were described.

Application of Transcriptome Analysis to Understand the Adverse Effects of Hypotonic Stress on Different Development Stages in the Giant Freshwater Prawn Macrobrachium rosenbergii Post-Larvae

Salinity is one of the important environmental factors affecting survival and growth of aquatic animals. However, the impact of low-salinity stress on M. rosenbergii post-larvae at different development stages remains elusive. Therefore, the aim of this study was to explore the underlying mechanisms of hypotonic stress at different development stages of M. rosenbergii post-larvae through transcriptome analysis and antioxidant parameters detection. The salinity of the control group was 15 psu (S15) and the hypotonic stress group was 6 psu (S6). Samples were collected at 7 days-post-hatch (dph), 14 dph and 21 dph larvae. The results showed that hypotonic stress caused oxidative damage in post-larvae evidenced by decreased glutathione peroxidase (GSH-Px); superoxide dismutase (SOD); anti-superoxide anion free radical (ASAFR); and increased malondialdehyde (MDA); nitric oxide (NO); and inducible nitric oxide synthase (iNOS) levels. Transcriptome analysis showed that there were 1428, 1187, 132 DEGs including 301, 366, 4 up-regulated genes and 1127, 821, 128 down-regulated genes at 7 dph, 14 dph and 21 dph larvae under hypotonic stress, respectively. Furthermore, GO and KEGG enrichment indicated that hypotonic stress led to dysregulation of immune signals including lysosome and autophagy in the 7 dph larvae. The autophagy-related genes including beclin 1-associated autophagy-related key regulator (Barkor); ubiquitin-like modifier-activating enzyme ATG7 (ATG7); Beclin; autophagy-related protein 13 (ATG13); nuclear receptor-binding factor 2 (Nrbf2); ubiquitin-like-conjugating enzyme ATG3 (ATG3); vacuole membrane protein 1 (VMP1); and autophagy-related protein 2 (ATG2) decreased at 7 dph, and 14 dph larvae, and then increased at 21 dph larvae under hypotonic stress. In the 14 dph and 21 dph larvae, the renin-angiotensin system was activated. In conclusion, our data indicated that hypotonic stress reduced the antioxidant capacity and impaired the immune system in post-larvae, but as development progresses, the adaptability of post-larvae to hypotonic stress gradually increased, and might reach a new homeostasis through the RAS signaling pathway.

RNA Sequencing Analysis Reveals Divergent Adaptive Response to Hypo- and Hyper-Salinity in Greater Amberjack (Seriola dumerili) Juveniles

Salinity significantly affects physiological and metabolic activities, breeding, development, survival, and growth of marine fish. The greater amberjack (Seriola dumerili) is a fast-growing species that has immensely contributed to global aquaculture diversification. However, the tolerance, adaptation, and molecular responses of greater amberjack to salinity are unclear. This study reared greater amberjack juveniles under different salinity stresses (40, 30, 20, and 10 ppt) for 30 days to assess their tolerance, adaptation, and molecular responses to salinity. RNA sequencing analysis of gill tissue was used to identify genes and biological processes involved in greater amberjack response to salinity stress at 40, 30, and 20 ppt. Eighteen differentially expressed genes (DEGs) (nine upregulated and nine downregulated) were identified in the 40 vs. 30 ppt group. Moreover, 417 DEGs (205 up-regulated and 212 down-regulated) were identified in the 20 vs. 30 ppt group. qPCR and transcriptomic analysis indicated that salinity stress affected the expression of genes involved in steroid biosynthesis (ebp, sqle, lss, dhcr7, dhcr24, and cyp51a1), lipid metabolism (msmo1, nsdhl, ogdh, and edar), ion transporters (slc25a48, slc37a4, slc44a4, and apq4), and immune response (wnt4 and tlr5). Furthermore, KEGG pathway enrichment analysis showed that the DEGs were enriched in steroid biosynthesis, lipids metabolism, cytokine-cytokine receptor interaction, tryptophan metabolism, and insulin signaling pathway. Therefore, this study provides insights into the molecular mechanisms of marine fish adaptation to salinity.

Transcriptome changes for Nile tilapia (Oreochromis niloticus) in response to alkalinity stress

Nile tilapia is an important economic fish in the world because of its fast growth, high meat yield and strong adaptability. It is more adaptable to high alkalinity than common freshwater fish and provides valuable material for developing alkaline-tolerant strains and understanding the adaptation mechanism of fish to extreme environmental stress. In this study, we employed high throughput RNA sequencing to reveal the tissues (gill, kidney and liver) transcriptome differences of O. niloticus at different carbonate alkalinities (FW, AW40 and AW60). A total of 1,369,381,790 raw reads were obtained, including 496,441,232 reads in FW group, 437,907,696 reads in AW40 and 435,032,862 reads in AW60. In addition, 484,555,626 reads in gill, 451,618,224 reads in kidney and 433,207,940 reads in liver. A large number of stress-regulated changes were detected comprehensively. We focused on 3 significantly change pathways (steroid biosynthesis, drug metabolism and protein digestion/absorption) and 17 DEGs (HMG-CoA reductase, UDP-glucuronosyltransferase, and carbonic anhydrase etc.) which were shared among compared groups (AW40 vs FW, AW60 vs FW, AW40 vs 60 AW60) in gill, kidney and liver, respectively. These pathways/genes are sensitive to alkalinity stress and crucial to the alkalinity adaptation of tilapia. Overall, we found a large number of candidate genes, which encode important regulators of stress tolerance and ultimately contribute to future alkaline-tolerant fish breeding. Among these genes, lipid metabolism (involving signal transduction), detoxification and immune related genes are more prominent to the response and adaptability of fish to alkalinity stress.

Expression of Na+/K+-ATPase Was Affected by Salinity Change in Pacific abalone Haliotis discus hannai

Na⁺/K⁺-ATPase (NKA) belongs to the P-type ATPase family, whose members are located in the cell membrane and are distributed in diverse tissues and cells. The main function of the NKA is to regulate osmotic pressure. To better understand the role of NKA in osmoregulation, we first cloned and characterized the full-length cDNAs of NKA α subunit and β subunit from Pacific abalone Haliotis discus hannai in the current study. The predicted protein sequence of the NKA α subunit, as the catalytic subunit, was well conserved. In contrast, the protein sequence of the β subunit had low similarity with those of other species. Phylogenetic analysis revealed that both the α and β subunits of the NKA protein of Pacific abalone were clustered with those of the Gastropoda. Then, the relationship between salinity changes and the NKA was investigated. Sudden salinity changes (with low-salinity seawater (LSW) or high-salinity seawater (HSW)) led to clear changes in ion concentration (Na⁺ and K⁺) in hemolymph; however, the relative stability of ion concentrations in tissue revealed that Pacific abalone has a strong osmotic pressure regulation ability when faced with these salinity changes. Meanwhile, the expression and activity of the NKA was significantly decreased (in LSW group) or increased (in HSW group) during the ion concentration re-establishing stages, which was consistent with the coordinated regulation of ion concentration in hemolymph. Moreover, a positive correlation between cyclic adenosine monophosphate (cAMP) concentrations and NKA mRNA expression (NKA activity) was observed in mantle and gill. Therefore, the sudden salinity changes may affect NKA transcription activation, translation and enzyme activity via a cAMP-mediated pathway.

Identification of immune-related genes in gill cells of Japanese eels (Anguilla japonica) in adaptation to water salinity changes

The changes in ambient salinity influence ion and water homeostasis, hormones secretion, and immune response in fish gills. The physiological functions of hormones and ion transporters in the regulation of gill-osmoregulation have been widely studied, however the modulation of immune response under salinity changes is not determined. Using transcriptome sequencing, we obtained a comprehensive profile of osmo-responsive genes in gill cells of Japanese eel (Anguilla japonica). Herein, we applied bioinformatics analysis to identify the immune-related genes that were significantly higher expressed in gill pavement cells (PVCs) and mitochondrial-rich cells (MRCs) in freshwater (FW) than seawater (SW) adapted fish. We validated the data using the real-time qPCR, which showed a high correlation between the RNA-seq and real-time qPCR data. In addition, the immunohistochemistry results confirmed the changes of the expression of selected immune-related genes, including C-reactive protein (CRP) in PVCs, toll-like receptor 2 (TLR2) in MRCs and interleukin-1 receptor type 2 (IL-1R2) in both PVCs and MRCs. Collectively our results demonstrated that those immune-related genes respond to salinity changes, and might trigger related special signaling pathways and network. This study provides new insights into the impacts of ambient salinity changes on adaptive immune response in fish gill cells.

Osmolality/salinity-responsive enhancers (OSREs) control induction of osmoprotective genes in euryhaline fish

Fish respond to salinity stress by transcriptional induction of many genes, but the mechanism of their osmotic regulation is unknown. We developed a reporter assay using cells derived from the brain of the tilapia Oreochromis mossambicus (OmB cells) to identify osmolality/salinity-responsive enhancers (OSREs) in the genes of Omossambicus Genomic DNA comprising the regulatory regions of two strongly salinity-induced genes, inositol monophosphatase 1 (IMPA1.1) and myo-inositol phosphate synthase (MIPS), was isolated and analyzed with dual luciferase enhancer trap reporter assays. We identified five sequences (two in IMPA1.1 and three in MIPS) that share a common consensus element (DDKGGAAWWDWWYDNRB), which we named "OSRE1." Additional OSREs that were less effective in conferring salinity-induced trans-activation and do not match the OSRE1 consensus also were identified in both MIPS and IMPA1.1 Although OSRE1 shares homology with the mammalian osmotic-response element/tonicity-responsive enhancer (ORE/TonE) enhancer, the latter is insufficient to confer osmotic induction in fish. Like other enhancers, OSRE1 trans-activates genes independent of orientation. We conclude that OSRE1 is a cis-regulatory element (CRE) that enhances the hyperosmotic induction of osmoregulated genes in fish. Our study also shows that tailored reporter assays developed for OmB cells facilitate the identification of CREs in fish genomes. Knowledge of the OSRE1 motif allows affinity-purification of the corresponding transcription factor and computational approaches for enhancer screening of fish genomes. Moreover, our study enables targeted inactivation of OSRE1 enhancers, a method superior to gene knockout for functional characterization because it confines impairment of gene function to a specific context (salinity stress) and eliminates pitfalls of constitutive gene knockouts (embryonic lethality, developmental compensation).

Transcriptomic analysis reveals specific osmoregulatory adaptive responses in gill mitochondria-rich cells and pavement cells of the Japanese eel

BACKGROUND

Homeostasis of ions and water is important for the maintenance of cellular functions. The regulation of the homeostasis is particularly important in euryhaline fish that migrate between freshwater (FW) and seawater (SW) environments. The fish gill, the major tissue that forms an interface separating the extracellular fluids and external water environment, has an effective transport system to maintain and regulate a constant body osmolality. In fish gills, the two major epithelial cells, pavement cells (PVCs) and mitochondria-rich cells (MRCs), are known to play key and complementary roles in ion transport at the interface. Discovering the robust mechanisms underlying the two cell types' response to osmotic stress would benefit our understanding of the fundamental mechanism allowing PVCs and MRCs to handle osmotic stress. Owing to the limited genomic data available on estuarine species, existing knowledge in this area is slim. In this study, transcriptome analyses were conducted using PVCs and MRCs isolated from Japanese eels adapted to FW or SW environments to provide a genome-wide molecular study to unravel the fundamental processes at work.

RESULTS

The study identified more than 12,000 transcripts in the gill cells. Interestingly, remarkable differential expressed genes (DEGs) were identified in PVCs (970 transcripts) instead of MRCs (400 transcripts) in gills of fish adapted to FW or SW. Since PVCs cover more than 90 % of the gill epithelial surface, the greater change in gene expression patterns in PVCs in response to external osmolality is anticipated. In the integrity pathway analysis, 19 common biological functions were identified in PVCs and MRCs. In the enriched signaling pathways analysis, most pathways differed between PVCs and MRCs; 14 enriched pathways were identified in PVCs and 12 in MRCs. The results suggest that the osmoregulatory responses in PVCs and MRCs are cell-type specific, which supports the complementary functions of the cells in osmoregulation.

CONCLUSIONS

This is the first study to provide transcriptomic analysis of PVCs and MRCs in gills of eels adapted to FW or SW environments. It describes the cell-type specific transcriptomic network in different tonicity. The findings consolidate the known osmoregulatory pathways and provide molecular insight in osmoregulation. The presented data will be useful for researchers to select their targets for further studies.

De novo assembly and transcriptome analysis of osmoregulation in Litopenaeus vannamei under three cultivated conditions with different salinities

Litopenaeus vannamei, one of the most important euryhaline crustaceans, is cultured in seawater, brackish water, and freshwater worldwide. We performed Illumina RNA sequencing of L. vannamei gills, generating 124,914,870; 119,250,450; and 105,487,350 raw reads from the shrimps cultured in seawater, brackish water, and freshwater, respectively. From these reads, 466,293 transcripts were de novo assembled and annotated. Comparative genomic analysis showed that 1752 genes were significantly differentially expressed in the freshwater group compared with the seawater group, including 1242 upregulated and 510 downregulated genes. In addition, 1246 genes were differentially expressed in the brackish group vs. the seawater water group, including 659 upregulated and 587 downregulated genes. These differentially expressed genes were mainly involved in energy metabolism, substance metabolism, ion transport and signal transduction, and genetic process. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway enrichment analysis were used to analyze the functional significance of the differentially expressed genes, included those responding to salinity through diverse biological functions and processes and numerous potential genes associated with the osmotic response. L. vannamei responses to the three cultivated salinities were analyzed using next-generation sequencing. The transcriptional database established from the current research adds to the information available on L. vannamei and the findings expand our knowledge of the molecular basis of osmoregulation mechanisms in this species.

Differential Gene Expression in Liver, Gill, and Olfactory Rosettes of Coho Salmon (Oncorhynchus kisutch) After Acclimation to Salinity

Most Pacific salmonids undergo smoltification and transition from freshwater to saltwater, making various adjustments in metabolism, catabolism, osmotic, and ion regulation. The molecular mechanisms underlying this transition are largely unknown. In the present study, we acclimated coho salmon (Oncorhynchus kisutch) to four different salinities and assessed gene expression through microarray analysis of gills, liver, and olfactory rosettes. Gills are involved in osmotic regulation, liver plays a role in energetics, and olfactory rosettes are involved in behavior. Between all salinity treatments, liver had the highest number of differentially expressed genes at 1616, gills had 1074, and olfactory rosettes had 924, using a 1.5-fold cutoff and a false discovery rate of 0.5. Higher responsiveness of liver to metabolic changes after salinity acclimation to provide energy for other osmoregulatory tissues such as the gills may explain the differences in number of differentially expressed genes. Differentially expressed genes were tissue- and salinity-dependent. There were no known genes differentially expressed that were common to all salinity treatments and all tissues. Gene ontology term analysis revealed biological processes, molecular functions, and cellular components that were significantly affected by salinity, a majority of which were tissue-dependent. For liver, oxygen binding and transport terms were highlighted. For gills, muscle, and cytoskeleton-related terms predominated and for olfactory rosettes, immune response-related genes were accentuated. Interaction networks were examined in combination with GO terms and determined similarities between tissues for potential osmosensors, signal transduction cascades, and transcription factors.

Prolactin 177, prolactin 188, and extracellular osmolality independently regulate the gene expression of ion transport effectors in gill of Mozambique tilapia

This study characterized the local effects of extracellular osmolality and prolactin (PRL) on branchial ionoregulatory function of a euryhaline teleost, Mozambique tilapia (Oreochromis mossambicus). First, gill filaments were dissected from freshwater (FW)-acclimated tilapia and incubated in four different osmolalities, 280, 330, 380, and 450 mosmol/kg H2O. The mRNA expression of Na(+)/K(+)-ATPase α1a (NKA α1a) and Na(+)/Cl(-) cotransporter (NCC) showed higher expression with decreasing media osmolalities, while Na(+)/K(+)/2Cl(-) cotransporter 1a (NKCC1a) and PRL receptor 2 (PRLR2) mRNA levels were upregulated by increases in media osmolality. We then incubated gill filaments in media containing ovine PRL (oPRL) and native tilapia PRLs (tPRL177 and tPRL188). oPRL and the two native tPRLs showed concentration-dependent effects on NCC, NKAα1a, and PRLR1 expression; Na(+)/H(+) exchanger 3 (NHE3) expression was increased by 24 h of incubation with tPRLs. Immunohistochemical observation showed that oPRL and both tPRLs maintained a high density of NCC- and NKA-immunoreactive ionocytes in cultured filaments. Furthermore, we found that tPRL177 and tPRL188 differentially induce expression of these ion transporters, according to incubation time. Together, these results provide evidence that ionocytes of Mozambique tilapia may function as osmoreceptors, as well as directly respond to PRL to modulate branchial ionoregulatory functions.

Direct Ionic Regulation of the Activity of Myo-Inositol Biosynthesis Enzymes in Mozambique Tilapia

Myo-inositol (Ins) is a major compatible osmolyte in many cells, including those of Mozambique tilapia (Oreochromis mossambicus). Ins biosynthesis is highly up-regulated in tilapia and other euryhaline fish exposed to hyperosmotic stress. In this study, enzymatic regulation of two enzymes of Ins biosynthesis, Ins phosphate synthase (MIPS) and inositol monophosphatase (IMPase), by direct ionic effects is analyzed. Specific MIPS and IMPase isoforms from Mozambique tilapia (MIPS-160 and IMPase 1) were selected based on experimental, phylogenetic, and structural evidence supporting their role for Ins biosynthesis during hyperosmotic stress. Recombinant tilapia IMPase 1 and MIPS-160 activity was assayed in vitro at ionic conditions that mimic changes in the intracellular milieu during hyperosmotic stress. The in vitro activities of MIPS-160 and IMPase 1 are highest at alkaline pH of 8.8. IMPase 1 catalytic efficiency is strongly increased during hyperosmolality (particularly for the substrate D-Ins-3-phosphate, Ins-3P), mainly as a result of [Na+] elevation. Furthermore, the substrate-specificity of IMPase 1 towards D-Ins-1-phosphate (Ins-1P) is lower than towards Ins-3P. Because MIPS catalysis results in Ins-3P this results represents additional evidence for IMPase 1 being the isoform that mediates Ins biosynthesis in tilapia. Our data collectively demonstrate that the Ins biosynthesis enzymes are activated under ionic conditions that cells are exposed to during hypertonicity, resulting in Ins accumulation, which, in turn, results in restoration of intracellular ion homeostasis. We propose that the unique and direct ionic regulation of the activities of Ins biosynthesis enzymes represents an efficient biochemical feedback loop for regulation of intracellular physiological ion homeostasis during hyperosmotic stress.

Differential transcriptomic analyses revealed genes and signaling pathways involved in iono-osmoregulation and cellular remodeling in the gills of euryhaline Mozambique tilapia, Oreochromis mossambicus

BACKGROUND

The Mozambique tilapia Oreochromis mossambicus has the ability to adapt to a broad range of environmental salinities and has long been used for investigating iono-osmoregulation. However, to date most studies have focused mainly on several key molecules or parameters hence yielding a limited perspective of the versatile iono-osmoregulation in the euryhaline fish. This study aimed to capture transcriptome-wide differences between the freshwater- and seawater-acclimated gills of the Mozambique tilapia.

RESULTS

We have identified over 5000 annotated gene transcripts with high homology (E-value <1.0E-50) to human genes that were differentially expressed in freshwater- and seawater-acclimated gills of the Mozambique tilapia. These putative human homologs were found to be significantly associated with over 50 canonical signaling pathways that are operating in at least 23 biological processes in relation to branchial iono-osmoregulation and cellular remodeling. The analysis revealed multiple signaling pathways in freshwater-acclimated gills acting in concert to maintain cellular homeostasis under hypo-osmotic environment while seawater-acclimated gills abounded with molecular signals to cope with the higher cellular turn-over rate, energetics and iono-regulatory demands under hyper-osmostic stress. Additionally, over 100 transcripts encoding putative inorganic ion transporters/channels were identified, of which several are well established in gill iono-regulation while the remainder are lesser known. We have also validated the expression profiles of 47 representative genes in freshwater- and seawater-acclimated gills, as well as in hypersaline-acclimated (two-fold salinity of seawater) gills. The findings confirmed that many of these responsive genes retained their expression profiles in hypersaline-acclimated gills as in seawater-acclimated gills, although several genes had changed significantly in their expression level/direction in hypersaline-acclimated gills.

CONCLUSIONS

This is the first study that has provided an unprecedented transcriptomic-wide perspective of gill iono-osmoregulation since such studies were initiated more than 80 years ago. It has expanded our molecular perspective from a relatively few well-studied molecules to a plethora of gene transcripts and a myriad of canonical signaling pathways driving various biological processes that are operating in gills under hypo-osmotic and hyper-osmotic stresses. These findings would provide insights and resources to fuel future studies on gill iono-osmoregulation and cellular remodeling in response to salinity challenge and acclimation.

Derivation and osmotolerance characterization of three immortalized tilapia (Oreochromis mossambicus) cell lines

Fish cell cultures are becoming more widely used models for investigating molecular mechanisms of physiological response to environmental challenge. In this study, we derived two immortalized Mozambique tilapia (Oreochromis mossambicus) cell lines from brain (OmB) and lip epithelium (OmL), and compared them to a previously immortalized bulbus arteriosus (TmB) cell line. The OmB and OmL cell lines were generated without or with Rho-associated kinase (ROCK) inhibitor/3T3 feeder layer supplementation. Although both approaches were successful, ROCK inhibitor/feeder layer supplementation was found to offer the advantages of selecting for epithelial-like cell type and decreasing time to immortalization. After immortalization (≥ passage 5), we characterized the proteomes of the newly derived cell lines (OmB and OmL) using LCMS and identified several unique cell markers for each line. Subsequently, osmotolerance for each of the three cell lines following acute exposure to elevated sodium chloride was evaluated. The acute maximum osmotolerance of these tilapia cell lines (>700 mOsm/kg) was markedly higher than that of any other known vertebrate cell line, but was significantly higher in the epithelial-like OmL cell line. To validate the physiological relevance of these tilapia cell lines, we quantified the effects of acute hyperosmotic challenge (450 mOsm/kg and 700 mOsm/kg) on the transcriptional regulation of two enzymes involved in biosynthesis of the compatible organic osmolyte, myo-inositol. Both enzymes were found to be robustly upregulated in all three tilapia cell lines. Therefore, the newly established tilapia cells lines represent valuable tools for studying molecular mechanisms involved in the osmotic stress response of euryhaline fish.

Transcriptome sequencing revealed the genes and pathways involved in salinity stress of Chinese mitten crab, Eriocheir sinensis

A total of 276.9 million reads were obtained and assembled into 206, 371 contigs with an average length of 614 bp and N50 of 1,470 bp. Comparison of digital gene expression between treatment and control group reveals 1,151 and 941 genes were significantly differentially expressed in crab gill and muscle, respectively. In gill and muscle, protein ubiquitination, ubiquinone biosynthesis, oxidative phosphorylation, and mitochondria dysfunction pathways were the top pathways differentially expressed following the challenge. EIF 2 signaling pathway and IGF-1 signaling pathway were the top ones among the signal-related pathways. Most of the amino acid metabolism pathways were found to be involved in this process. The expression patterns of 15 differentially expressed genes were validated by quantitative real-time RT-PCR (average correlation coefficient 0.80). This is the first report of expression analysis of genes and pathways involved in osmoregulation of Eriocheir sinensis through transcriptome sequencing. The findings of this study will further promote the understanding of the underlying molecular mechanism of salinity stress adaptation for crustacean species.

Functional categorization of transcriptome in the species Symphysodon aequifasciatus Pellegrin 1904 (Perciformes: Cichlidae) exposed to benzo[a]pyrene and phenanthrene

This study aims to evaluate the transcriptome alterations, through cDNA libraries, associated with the combined effects of two PAHs, benzo[a]pyrene (0.5 µg/L) and phenanthrene (50 µg/L), present in crude oil, on specimens of Symphysodon aequifasciatus (discus fish) after 48 h of exposure. The cDNA libraries were constructed according to the SOLiD™ SAGE™ protocol for sequencing in the SOLiD v.3 Plus sequencer. The results were analyzed by bioinformatics and differentially expressed genes were categorized using the gene ontology program. The functional categories (terms) found in the gene ontology and the gene network generated using STRING software were used to predict the adverse effects of benzo[a]pyrene and phenanthrene in the liver. In the present study, 27,127 genes (compared to Danio rerio database) were identified. Considering only those genes with a p-value less than or equal to 0.05 and greater than or equal to two-fold change in expression across libraries, we found 804 genes, 438 down-regulated (54%) and 366 up-regulated (46%), in the experimental group compared to the control. Out of this total, 327 genes were successfully categorized, 174 down-regulated and 153 up-regulated, using gene ontology. Using String, the gene network was composed by 199 nodes, 124 of them resulting in 274 interactions. The results showed that even an acute exposure of 48 h caused metabolic change in response to environmental contaminants, resulting in changes of cell integrity, in oxidation-reduction processes, in the immune response and disturbances of intracellular signaling of discus fish. Also the gene network has showed no central interplay cluster, exhibiting instead interconnected clusters interactions and connected sub-networks. These findings highlight that even an acute sublethal exposure of PAHs can cause metabolism changes that may affect survival of discus. Our findings using SOLiD coupled with SAGE-method resulted in a powerful and reliable means for gene expression analysis in discus, a non-model Amazonian fish.

Tilapia (Oreochromis mossambicus) brain cells respond to hyperosmotic challenge by inducing myo-inositol biosynthesis

This study aimed to determine the regulation of the de novo myo-inositol biosynthetic (MIB) pathway in Mozambique tilapia (Oreochromis mossambicus) brain following acute (25 ppt) and chronic (30, 60 and 90 ppt) salinity acclimations. The MIB pathway plays an important role in accumulating the compatible osmolyte, myo-inositol, in cells in response to hyperosmotic challenge and consists of two enzymes, myo-inositol phosphate synthase and inositol monophosphatase. In tilapia brain, MIB enzyme transcriptional regulation was found to robustly increase in a time (acute acclimation) or dose (chronic acclimation) dependent manner. Blood plasma osmolality and Na(+) and Cl(-) concentrations were also measured and significantly increased in response to both acute and chronic salinity challenges. Interestingly, highly significant positive correlations were found between MIB enzyme mRNA and blood plasma osmolality in both acute and chronic salinity acclimations. Additionally, a mass spectrometry assay was established and used to quantify total myo-inositol concentration in tilapia brain, which closely mirrored the hyperosmotic MIB pathway induction. Thus, myo-inositol is a major compatible osmolyte that is accumulated in brain cells when exposed to acute and chronic hyperosmotic challenge. These data show that the MIB pathway is highly induced in response to environmental salinity challenge in tilapia brain and that this induction is likely prompted by increases in blood plasma osmolality. Because the MIB pathway uses glucose-6-phosphate as a substrate and large amounts of myo-inositol are being synthesized, our data also illustrate that the MIB pathway likely contributes to the high energetic demand posed by salinity challenge.

Salinity-induced regulation of the myo-inositol biosynthesis pathway in tilapia gill epithelium

The myo-inositol biosynthesis (MIB) pathway converts glucose-6-phosphate to the compatible osmolyte myo-inositol that protects cells from osmotic stress. Using proteomics, the enzymes that constitute the MIB pathway, myo-inositol phosphate synthase (MIPS) and inositol monophosphatase 1 (IMPA1), are identified in tilapia (Oreochromis mossambicus) gill epithelium. Targeted, quantitative, label-free proteomics reveals that they are both upregulated during salinity stress. Upregulation is stronger when fish are exposed to severe (34 ppt acute and 90 ppt gradual) relative to moderate (70 ppt gradual) salinity stress. IMPA1 always responds more strongly than MIPS, suggesting that MIPS is more stable during salinity stress. MIPS is N-terminally acetylated and the corresponding peptide increases proportionally to MIPS protein, while non-acetylated N-terminal peptide is not detectable, indicating that MIPS acetylation is constitutive and may serve to stabilize the protein. Hyperosmotic induction of MIPS and IMPA1 is confirmed using western blot and real-time qPCR and is much higher at the mRNA than at the protein level. Two distinct MIPS mRNA variants are expressed in the gill, but one is more strongly regulated by salinity than the other. A single MIPS gene is encoded in the tilapia genome whereas the zebrafish genome lacks MIPS entirely. The genome of euryhaline tilapia contains four IMPA genes, two of which are expressed, but only one is salinity regulated in gill epithelium. The genome of stenohaline zebrafish contains a single IMPA gene. We conclude that the MIB pathway represents a major salinity stress coping mechanism that is regulated at multiple levels in euryhaline fish but absent in stenohaline zebrafish.

Transcriptomic and iTRAQ proteomic approaches reveal novel short-term hyperosmotic stress responsive proteins in the gill of the Japanese eel (Anguilla japonica)

Osmoregulation is critical for the survival of fishes that migrate between freshwater (FW) and seawater (SW). The eel, as a catadromous fish, has been studied for decades to reveal the mechanisms of osmoregulation. These studies, however, have been limited by the lack of a genomic database to decipher the mechanism of osmoregulation at a molecular level. In this study, using high-throughput transcriptomic and proteomic technologies, we have provided the first genome-wide study to identify hyperosmotic responsive proteins in the gills of the Japanese eel. Deep sequencing using the 454 platform produced over 660,000 reads with a mean length of 385 bp. For the proteomic study, we collected gill samples from three different treatment groups of fish that had fully adapted to FW/SW or were transferred from FW to SW for 6h. The respective group of gill proteins were extracted and labeled using an isobaric tag for relative and absolute quantitation (iTRAQ) using LTQ-Orbitrap, a high resolution mass spectrometer. Among the 1519 proteins identified from the gill samples, 96 proteins were differentially expressed between FW and SW adapted fish. Nineteen hyperosmotic responsive proteins were detected (10 up-regulated and 9 down-regulated proteins) after 6h post FW to SW transfer.

BIOLOGICAL SIGNIFICANCE

The study has provided the most comprehensive, targeted investigation of eel gill proteins to date, and shown the powerfulness of combining transcriptomic and proteomic approaches to provide molecular insights of osmoregulation mechanisms in a non-model organism, eel.

The Combinatorial Nature of Osmosensing in Fishes

Organisms exposed to altered salinity must be able to perceive osmolality change because metabolism has evolved to function optimally at specific intracellular ionic strength and composition. Such osmosensing comprises a complex physiological process involving many elements at organismal and cellular levels of organization. Input from numerous osmosensors is integrated to encode magnitude, direction, and ionic basis of osmolality change. This combinatorial nature of osmosensing is discussed with emphasis on fishes.

Mechanosensitive signalling in fish gill and other ion transporting epithelia

Epithelia involved in vectorial salt transport respond to apical and basolateral changes in osmotic activity by moderating the transmural solute transport rate simultaneously with underlying volume regulatory mechanisms involved in regulatory volume increase (RVI) and decrease (RVD). This review examines rapid osmotic responses in salt secreting epithelia of marine and euryhaline teleost fish, with inclusion of recent results from other ion transporting epithelia that also respond rapidly to osmotic shock. Mitochondrion-rich chloride secreting cells of marine teleost fish gills and skin, when exposed to hypertonic shock, activate NaCl secretion via phosphorylation of Na(+), K(+), 2Cl(-) cotransporter (NKCC1) in the basolateral membrane and activation of anion channels in the apical membrane. Conversely, NaCl secretion is inhibited when chloride secreting cells are swollen osmotically. Mammalian airway epithelial cells also possess NKCC1 basally and apical anion channels [Cystic Fibrosis Transmembrane conductance Regulator (CFTR)]; with hypotonic shock, this epithelium releases ATP and NaCl secretion is stimulated via purinergic receptors, while hypertonic shock inhibits Na(+) uptake. In the eye, the ciliary epithelium activates Cl(-) channels in response to hypotonic shock as RVD, an effect that modulates transepithelial fluid transport rates. In the renal A6 cell line, K(+) and Cl(-) effluxes activate during RVD and RVI Na(+) transepithelial absorption. A common theme in these systems is ATP release in hypotonic shock with subsequent RVD-effective mechanisms such as NKCC1 inhibition and K(+) and Cl(-) efflux, but there are different effects of osmotic changes on transepithelial transport, apparently depending on the role of the epithelial system.

Genomic mechanisms of evolved physiological plasticity in killifish distributed along an environmental salinity gradient

Adaptive variation tends to emerge clinally along environmental gradients or discretely among habitats with limited connectivity. However, in Atlantic killifish (Fundulus heteroclitus), a population genetic discontinuity appears in the absence of obvious barriers to gene flow along parallel salinity clines and coincides with a physiologically stressful salinity. We show that populations resident on either side of this discontinuity differ in their abilities to compensate for osmotic shock and illustrate the physiological and functional genomic basis of population variation in hypoosmotic tolerance. A population native to a freshwater habitat, upstream of the genetic discontinuity, exhibits tolerance to extreme hypoosmotic challenge, whereas populations native to brackish or marine habitats downstream of the discontinuity lose osmotic homeostasis more severely and take longer to recover. Comparative transcriptomics reveals a core transcriptional response associated with acute and acclimatory responses to hypoosmotic shock and posits unique mechanisms that enable extreme osmotic tolerance. Of the genes that vary in expression among populations, those that are putatively involved in physiological acclimation are more likely to exhibit nonneutral patterns of divergence between freshwater and brackish populations. It is not the well-known effectors of osmotic acclimation, but rather the lesser-known immediate-early responses, that appear important in contributing to population differences.

Functional genomics of physiological plasticity and local adaptation in killifish

Evolutionary solutions to the physiological challenges of life in highly variable habitats can span the continuum from evolution of a cosmopolitan plastic phenotype to the evolution of locally adapted phenotypes. Killifish (Fundulus sp.) have evolved both highly plastic and locally adapted phenotypes within different selective contexts, providing a comparative system in which to explore the genomic underpinnings of physiological plasticity and adaptive variation. Importantly, extensive variation exists among populations and species for tolerance to a variety of stressors, and we exploit this variation in comparative studies to yield insights into the genomic basis of evolved phenotypic variation. Notably, species of Fundulus occupy the continuum of osmotic habitats from freshwater to marine and populations within Fundulus heteroclitus span far greater variation in pollution tolerance than across all species of fish. Here, we explore how transcriptome regulation underpins extreme physiological plasticity on osmotic shock and how genomic and transcriptomic variation is associated with locally evolved pollution tolerance. We show that F. heteroclitus quickly acclimate to extreme osmotic shock by mounting a dramatic rapid transcriptomic response including an early crisis control phase followed by a tissue remodeling phase involving many regulatory pathways. We also show that convergent evolution of locally adapted pollution tolerance involves complex patterns of gene expression and genome sequence variation, which is confounded with body-weight dependence for some genes. Similarly, exploiting the natural phenotypic variation associated with other established and emerging model organisms is likely to greatly accelerate the pace of discovery of the genomic basis of phenotypic variation.

A novel tilapia prolactin receptor is functionally distinct from its paralog

A novel tilapia prolactin (PRL) receptor (OmPRLR2) was identified based on its induction during hyperosmotic stress. OmPRLR2 protein shows 28% identity to tilapia OmPRLR1 and 26% identity to human PRLR. Comparison of OmPRLR1 and OmPRLR2 revealed conserved features of cytokine class I receptors (CKR1): a WS domain and transmembrane domain, two pairs of cysteines and N-glycosylation motifs in the extracellular region, CKR1 boxes I and II, and three tyrosines in the intracellular region. However, OmPRLR2 lacked the ubiquitin ligase and 14-3-3 binding motifs. OmPRLR2 mRNA was present in all tissues analyzed, with highest expression in gills, intestine, kidney and muscle, similar to OmPRLR1. Transfer of fish from fresh water to sea water transiently increased gill OmPRLR2 mRNA levels within 4 h but decreased its protein abundance in the long term. OmPRLR2 is expressed in part as a truncated splice variant of 35 kDa in addition to the 55 kDa full-length protein. Cloning of the mRNA encoding the 35 kDa variant revealed that it lacks the extracellular region. It is expressed at significantly higher levels in males than in females. In stably transfected HEK293 cells over-expressing tetracycline-inducible OmPRLR1 and OmPRLR2, activation of these receptors by tilapia PRL177 and PRL188 triggered different downstream signaling pathways. Moreover, OmPRLR2 significantly increased HEK293 salinity tolerance. Our data reveal that tilapia has two PRLR genes whose protein products respond uniquely to PRL and activate different downstream pathways. Expression of a short PRLR2 variant may serve to inhibit PRL binding during osmotic stress and in male tissues.

A microarray-based transcriptomic time-course of hyper- and hypo-osmotic stress signaling events in the euryhaline fish Gillichthys mirabilis: osmosensors to effectors

Cells respond to changes in osmolality with compensatory adaptations that re-establish ion homeostasis and repair disturbed aspects of cell structure and function. These physiological processes are highly complex, and require the coordinated activities of osmosensing, signal transducing and effector molecules. Although the critical role of effector proteins such as Na+, K+-ATPases and Na+/K+/Cl(-) co-transporters during osmotic stress are well established, comparatively little information is available regarding the identity or expression of the osmosensing and signal transduction genes that may govern their activities. To better resolve this issue, a cDNA microarray consisting of 9207 cDNA clones was used to monitor gene expression changes in the gill of the euryhaline fish Gillichthys mirabilis exposed to hyper- and hypo-osmotic stress. We successfully annotated 168 transcripts differentially expressed during the first 12 h of osmotic stress exposure. Functional classifications of genes encoding these transcripts reveal that a variety of biological processes are affected. However, genes participating in cell signaling events were the dominant class of genes differentially expressed during both hyper- and hypo-osmotic stress. Many of these genes have had no previously reported role in osmotic stress adaptation. Subsequent analyses used the novel expression patterns generated in this study to place genes within the context of osmotic stress sensing, signaling and effector events. Our data indicate multiple major signaling pathways work in concert to modify diverse effectors, and that these molecules operate within a framework of regulatory proteins.

Prolonged apoptosis in mitochondria-rich cells of tilapia (Oreochromis mossambicus) exposed to elevated salinity

The time-course of programmed cell death (apoptosis) during reorganization of gill epithelium in salinity-stressed tilapia was analyzed using a recently developed method based on laser scanning cytometry (LSC) of dissociated gill cells. Apoptosis in mitochondria-rich cells (MRC) was distinguished from that in other cell types using Na(+)/K(+) ATPase (NKA) as a cell-specific marker. Caspase 3/7 activity in MRC, assessed using LSC and microplate assays, increased significantly starting at 6 h of salinity stress and remained elevated for at least 5 days. This time-course of apoptosis in MRC during acute salinity stress was reflected in elevated apoptotic DNA fragmentation. In parallel to induction of apoptosis, MRC showed a pronounced shift to G2 phase of the cell cycle, which is indicative of G2/M cell cycle arrest, and an increase in NKA abundance per MRC. Unlike in MRC, apoptosis was not significantly increased in other gill cell types, although there was a small transient increase in DNA fragmentation at 6 h. G2 arrest was also observed. Overall, we interpret our data as evidence for a significant role of apoptosis in the extensive reorganization of MRC populations that takes place during salinity acclimation, perhaps similar to its well-established role during organismal development.

Osmotic stress sensing and signaling in fishes

In their aqueous habitats, fish are exposed to a wide range of osmotic conditions and differ in their abilities to respond adaptively to these variations in salinity. Fish species that inhabit environments characterized by significant salinity fluctuation (intertidal zone, estuaries, salt lakes, etc.) are euryhaline and able to adapt to osmotic stress. Adaptive and acclimatory responses of fish to salinity stress are based on efficient mechanisms of osmosensing and osmotic stress signaling. Multiple osmosensors, including calcium sensing receptor likely act in concert to convey information about osmolality changes to downstream signaling and effector mechanisms. The osmosensory signal transduction network in fishes is complex and includes calcium, mitogen-activated protein kinase, 14-3-3 and macromolecular damage activated signaling pathways. This network controls, among other targets, osmosensitive transcription factors such as tonicity response element binding protein and osmotic stress transcription factor 1, which, in turn, regulate the expression of genes involved in osmotic stress acclimation. In addition to intracellular signaling mechanisms, the systemic response to osmotic stress in euryhaline fish is coordinated via hormone- and paracrine factor-mediated extracellular signaling. Overall, current insight into osmosensing and osmotic stress-induced signal transduction in fishes is limited. However, euryhaline fish species represent excellent models for answering critical emerging questions in this field and for elucidating the underlying molecular mechanisms of osmosensory signal transduction.

Functional genomics and proteomics of the cellular osmotic stress response in `non-model' organisms

All organisms are adapted to well-defined extracellular salinity ranges. Osmoregulatory mechanisms spanning all levels of biological organization, from molecules to behavior, are central to salinity adaptation. Functional genomics and proteomics approaches represent powerful tools for gaining insight into the molecular basis of salinity adaptation and euryhalinity in animals. In this review, we discuss our experience in applying such tools to so-called`non-model' species, including euryhaline animals that are well-suited for studies of salinity adaptation. Suppression subtractive hybridization,RACE-PCR and mass spectrometry-driven proteomics can be used to identify genes and proteins involved in salinity adaptation or other environmental stress responses in tilapia, sharks and sponges. For protein identification in non-model species, algorithms based on sequence homology searches such as MSBLASTP2 are most powerful. Subsequent gene ontology and pathway analysis can then utilize sets of identified genes and proteins for modeling molecular mechanisms of environmental adaptation. Current limitations for proteomics in non-model species can be overcome by improving sequence coverage, N- and C-terminal sequencing and analysis of intact proteins. Dependence on information about biochemical pathways and gene ontology databases for model species represents a more severe barrier for work with non-model species. To minimize such dependence, focusing on a single biological process (rather than attempting to describe the system as a whole) is key when applying `omics'approaches to non-model organisms.

Regulation of osmotic stress transcription factor 1 (Ostf1) in tilapia(Oreochromis mossambicus) gill epithelium during salinity stress

Mechanisms of induction of osmotic stress transcription factor 1 (Ostf1)were analyzed in gill epithelium of tilapia exposed to salinity stress. Experiments with primary cultures of gill epithelial cells revealed that hyperosmotic Ostf1 induction was independent of systemic factors. In addition,the synthetic glucocorticoid receptor agonist dexamethasone did not affect Ostf1 levels, arguing against cortisol being the signal for Ostf1 induction during hyperosmotic stress. Exposure of primary gill cell cultures to a hyperosmotic agent that is cell permeable and non-hypertonic (glycerol) did not trigger Ostf1 induction. However, when gill cells were exposed to hypertonicity (either in the form of NaCl or other forms) Ostf1 was rapidly and significantly induced. Analysis of hnRNA and mRNA levels revealed that Ostf1 upregulation in gill cells of intact fish and primary cultures of gill epithelial cells was mediated by transient mRNA stabilization. In addition to the initial transient mRNA stabilization a subsequent transcriptional induction of Ostf1 was observed. In cultured gill cells increase in Ostf1 mRNA synthesis was stable and very potent, whereas in gill cells of intact fish this increase was transient. This observation suggests positive feedback by Ostf1 or one of its targets and negative feedback by systemic factors on Ostf1 transcription.

We conclude that Ostf1 induction in gill epithelial cells of tilapia exposed to salinity stress (1) is independent of cortisol or other systemic factors; (2) depends on hypertonicity as the signal; and (3) is based on transient mRNA stabilization. Moreover, our data on primary cell cultures show that systemic signals are necessary to prevent sustained transcriptional induction of Ostf1 during hyperosmotic stress, indicating feedback regulation and a high degree of complexity of osmosensing and signaling networks in euryhaline fishes.