Profiling the Physiological Roles in Fish Primary Cell Culture

Fish primary cell culture has emerged as a valuable tool for investigating the physiological roles and responses of various cell types found in fish species. This review aims to provide an overview of the advancements and applications of fish primary cell culture techniques, focusing on the profiling of physiological roles exhibited by fish cells in vitro. Fish primary cell culture involves the isolation and cultivation of cells directly derived from fish tissues, maintaining their functional characteristics and enabling researchers to study their behavior and responses under controlled conditions. Over the years, significant progress has been made in optimizing the culture conditions, establishing standardized protocols, and improving the characterization techniques for fish primary cell cultures. The review highlights the diverse cell types that have been successfully cultured from different fish species, including gonad cells, pituitary cells, muscle cells, hepatocytes, kidney and immune cells, adipocyte cells and myeloid cells, brain cells, primary fin cells, gill cells, and other cells. Each cell type exhibits distinct physiological functions, contributing to vital processes such as metabolism, tissue regeneration, immune response, and toxin metabolism. Furthermore, this paper explores the pivotal role of fish primary cell culture in elucidating the mechanisms underlying various physiological processes. Researchers have utilized fish primary cell cultures to study the effects of environmental factors, toxins, pathogens, and pharmaceutical compounds on cellular functions, providing valuable insights into fish health, disease pathogenesis, and drug development. The paper also discusses the application of fish primary cell cultures in aquaculture research, particularly in investigating fish growth, nutrition, reproduction, and stress responses. By mimicking the in vivo conditions in vitro, primary cell culture has proven instrumental in identifying key factors influencing fish health and performance, thereby contributing to the development of sustainable aquaculture practices.

Osmotic Gradient Is a Factor That Influences the Gill Microbiota Communities in Oryzias melastigma

Simple Summary

This study was applied to the laboratory medaka to understand how the osmotic gradient could influence the composition of the gill microbiota communities. The data suggested that the shift of the gill microbiota community has relied on the first sense of osmolality differences, and such changes were accomplished by the enriched osmosensing and metabolic pathways.

Abstract

The fish gill is the first tissue that is exposed to the external media and undergoes continuous osmotic challenges. Recently, our group published an article entitled “Integrated Omics Approaches Revealed the Osmotic Stress-Responsive Genes and Microbiota in Gill of Marine Medaka” in the journal mSystems (e0004722, 2022), and suggested the possible host-bacterium interaction in the fish gill during osmotic stress. The previous study was performed by the progressive fresh water transfer (i.e., seawater to fresh water transfer via 50% seawater (FW)). Our group hypothesized that osmotic gradient could be a factor that determines the microbiota communities in the gill. The current 16S rRNA metagenomic sequencing study found that the direct transfer (i.e., seawater to fresh water (FWd)) could result in different gill microbiota communities in the same fresh water endpoints. Pseduomonas was the dominant bacteria (more than 55%) in the FWd gill. The Kyoto Encyclopedia of Genes and Genomes and MetaCyc analysis further suggested that the FWd group had enhanced osmosensing pathways, such as the ATP-binding cassette transporters, taurine degradation, and energy-related tricarboxylic acid metabolism compared to the FW group.

Modulation of physiological oxidative stress and antioxidant status by abiotic factors especially salinity in aquatic organisms

Exposure to a variety of environmental factors such as temperature, pH, oxygen and salinity may influence the oxidative status in aquatic organisms. The present review article focuses on the modulation of oxidative stress with reference to the generation of reactive oxygen species (ROS) in aquatic animals from different phyla. The focus of the review article is to explore the plausible mechanisms of physiological changes occurring in aquatic animals due to altered salinity in terms of oxidative stress. Apart from the seasonal variations in salinity, global warming and anthropogenic activities have also been found to influence oxidative health status of aquatic organisms. These effects are discussed with an objective to develop precautionary measures to protect the diversity of aquatic species with sustainable conservation. Comparative analyses among different aquatic species suggest that salinity alone or in combination with other abiotic factors are intricately associated with modulation in oxidative stress in a species-specific manner in aquatic animals. Osmoregulation under salinity stress in relation to energy demand and supply are also discussed. The literature survey of >50 years (1960-2020) indicates that oxidative stress status and comparative analysis of redox modulation have evolved from the analysis of various biotic and/or abiotic factors to the study of cellular signalling pathways in these aquatic organisms.

Genome-wide analysis of MicroRNA-messenger RNA interactome in ex-vivo gill filaments, Anguilla japonica

Background

Gills of euryhaline fishes possess great physiological and structural plasticity to adapt to large changes in external osmolality and to participate in ion uptake/excretion, which is essential for the re-establishment of fluid and electrolyte homeostasis. The osmoregulatory plasticity of gills provides an excellent model to study the role of microRNAs (miRs) in adaptive osmotic responses. The present study is to characterize an ex-vivo gill filament culture and using omics approach, to decipher the interaction between tonicity-responsive miRs and gene targets, in orchestrating the osmotic stress-induced responses.

Results

Ex-vivo gill filament culture was exposed to Leibovitz’s L-15 medium (300 mOsmol l^− 1) or the medium with an adjusted osmolality of 600 mOsmol l^− 1 for 4, 8 and 24 h. Hypertonic responsive genes, including osmotic stress transcriptional factor, Na⁺/Cl⁻-taurine transporter, Na⁺/H⁺ exchange regulatory cofactor, cystic fibrosis transmembrane regulator, inward rectifying K⁺ channel, Na⁺/K⁺-ATPase, and calcium-transporting ATPase were significantly upregulated, while the hypo-osmotic gene, V-type proton ATPase was downregulated. The data illustrated that the ex-vivo gill filament culture exhibited distinctive responses to hyperosmotic challenge. In the hyperosmotic treatment, four key factors (i.e. drosha RNase III endonuclease, exportin-5, dicer ribonuclease III and argonaute-2) involved in miR biogenesis were dysregulated (P < 0.05). Transcriptome and miR-sequencing of gill filament samples at 4 and 8 h were conducted and two downregulated miRs, miR-29b-3p and miR-200b-3p were identified. An inhibition of miR-29b-3p and miR-200b-3p in primary gill cell culture led to an upregulation of 100 and 93 gene transcripts, respectively. Commonly upregulated gene transcripts from the hyperosmotic experiments and miR-inhibition studies, were overlaid, in which two miR-29b-3p target-genes [Krueppel-like factor 4 (klf4), Homeobox protein Meis2] and one miR-200b-3p target-gene (slc17a5) were identified. Integrated miR-mRNA-omics analysis revealed the specific binding of miR-29b-3p on Klf4 and miR-200b-3p on slc17a5. The target-genes are known to regulate differentiation of gill ionocytes and cellular osmolality.

Conclusions

In this study, we have characterized the hypo-osmoregulatory responses and unraveled the modulation of miR-biogenesis factors/the dysregulation of miRs, using ex-vivo gill filament culture. MicroRNA-messenger RNA interactome analysis of miR-29b-3p and miR-200b-3p revealed the gene targets are essential for osmotic stress responses.

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.

Liver transcriptome analysis reveals extensive transcriptional plasticity during acclimation to low salinity in Cynoglossus semilaevis

BACKGROUND

Salinity is an important abiotic stress that influences the physiological and metabolic activity, reproduction, growth and development of marine fish. It has been suggested that half-smooth tongue sole (Cynoglossus semilaevis), a euryhaline fish species, uses a large amount of energy to maintain osmotic pressure balance when exposed to fluctuations in salinity. To delineate the molecular response of C. semilaevis to different levels of salinity, we performed RNA-seq analysis of the liver to identify the genes and molecular and biological processes involved in responding to salinity changes.

RESULTS

The present study yielded 330.4 million clean reads, of which 83.9% were successfully mapped to the reference genome of C. semilaevis. One hundred twenty-eight differentially expressed genes (DEGs), including 43 up-regulated genes and 85 down-regulated genes, were identified. These DEGs were highly represented in metabolic pathways, steroid biosynthesis, terpenoid backbone biosynthesis, butanoate metabolism, glycerolipid metabolism and the 2-oxocarboxylic acid metabolism pathway. In addition, genes involved in metabolism, osmoregulation and ion transport, signal transduction, immune response and stress response, and cytoskeleton remodeling were affected during acclimation to low salinity. Genes acat2, fdps, hmgcr, hmgcs1, mvk, pmvk, ebp, lss, dhcr7, and dhcr24 were up-regulated and abat, ddc, acy1 were down-regulated in metabolic pathways. Genes aqp10 and slc6a6 were down-regulated in osmoregulation and ion transport. Genes abat, fdps, hmgcs1, mvk, pmvk and dhcr7 were first reported to be associated with salinity adaptation in teleosts.

CONCLUSIONS

Our results revealed that metabolic pathways, especially lipid metabolism were important for salinity adaptation. The candidate genes identified from this study provide a basis for further studies to investigate the molecular mechanism of salinity adaptation and transcriptional plasticity in marine fish.

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.

MicroRNA-Sequence Profiling Reveals Novel Osmoregulatory MicroRNA Expression Patterns in Catadromous Eel Anguilla marmorata

MicroRNAs (miRNAs) are a class of endogenous small non-coding RNAs that regulate gene expression by post-transcriptional repression of mRNAs. Recently, several miRNAs have been confirmed to execute directly or indirectly osmoregulatory functions in fish via translational control. In order to clarify whether miRNAs play relevant roles in the osmoregulation of Anguilla marmorata, three sRNA libraries of A. marmorata during adjusting to three various salinities were sequenced by Illumina sRNA deep sequencing methods. Totally 11,339,168, 11,958,406 and 12,568,964 clear reads were obtained from 3 different libraries, respectively. Meanwhile, 34 conserved miRNAs and 613 novel miRNAs were identified using the sequence data. MiR-10b-5p, miR-181a, miR-26a-5p, miR-30d and miR-99a-5p were dominantly expressed in eels at three salinities. Totally 29 mature miRNAs were significantly up-regulated, while 72 mature miRNAs were significantly down-regulated in brackish water (10‰ salinity) compared with fresh water (0‰ salinity); 24 mature miRNAs were significantly up-regulated, while 54 mature miRNAs were significantly down-regulated in sea water (25‰ salinity) compared with fresh water. Similarly, 24 mature miRNAs were significantly up-regulated, while 45 mature miRNAs were significantly down-regulated in sea water compared with brackish water. The expression patterns of 12 dominantly expressed miRNAs were analyzed at different time points when the eels transferred from fresh water to brackish water or to sea water. These miRNAs showed differential expression patterns in eels at distinct salinities. Interestingly, miR-122, miR-140-3p and miR-10b-5p demonstrated osmoregulatory effects in certain salinities. In addition, the identification and characterization of differentially expressed miRNAs at different salinities can clarify the osmoregulatory roles of miRNAs, which will shed lights for future studies on osmoregulation in fish.

Physiological mechanisms used by fish to cope with salinity stress

Salinity represents a critical environmental factor for all aquatic organisms, including fishes. Environments of stable salinity are inhabited by stenohaline fishes having narrow salinity tolerance ranges. Environments of variable salinity are inhabited by euryhaline fishes having wide salinity tolerance ranges. Euryhaline fishes harbor mechanisms that control dynamic changes in osmoregulatory strategy from active salt absorption to salt secretion and from water excretion to water retention. These mechanisms of dynamic control of osmoregulatory strategy include the ability to perceive changes in environmental salinity that perturb body water and salt homeostasis (osmosensing), signaling networks that encode information about the direction and magnitude of salinity change, and epithelial transport and permeability effectors. These mechanisms of euryhalinity likely arose by mosaic evolution involving ancestral and derived protein functions. Most proteins necessary for euryhalinity are also critical for other biological functions and are preserved even in stenohaline fish. Only a few proteins have evolved functions specific to euryhaline fish and they may vary in different fish taxa because of multiple independent phylogenetic origins of euryhalinity in fish. Moreover, proteins involved in combinatorial osmosensing are likely interchangeable. Most euryhaline fishes have an upper salinity tolerance limit of approximately 2× seawater (60 g kg−1). However, some species tolerate up to 130 g kg−1 salinity and they may be able to do so by switching their adaptive strategy when the salinity exceeds 60 g kg−1. The superior salinity stress tolerance of euryhaline fishes represents an evolutionary advantage favoring their expansion and adaptive radiation in a climate of rapidly changing and pulsatory fluctuating salinity. Because such a climate scenario has been predicted, it is intriguing to mechanistically understand euryhalinity and how this complex physiological phenotype evolves under high selection pressure.

The role of osmotic stress transcription factor 1 in fishes

Osmotic stress transcription factor 1 (Ostf1) was first discovered by subtractive hybridization in the gills of Mozambique tilapia (Oreochromis mossambicus) transferred from fresh water (FW) to seawater (SW). It is a putative transcriptional regulator and the “early hyperosmotic regulated protein”. In the 2 hours after FW to SW transfer, ostf1 mRNA levels increase six fold. It is believed that, as a fast-response gene, Ostf1 plays a critical role in fish osmoregulation. Since its discovery, numerous studies have been performed to understand the nature and osmoregulatory mechanism of Ostf1. A decade has passed since the discovery of Ostf1, and it is a good time to summarize our current understanding of this gene. Different fish models have been used to study Ostf1, which is not limited to the traditional euryhaline fishes, such as eels and tilapia. Ostf1 can be found in modern fish models such as medaka and zebrafish. This review covers and summarizes the findings from different fishes, and provides a perspective for future Ostf1 studies.

iTRAQ-based quantitative proteomic analysis reveals acute hypo-osmotic responsive proteins in the gills of the Japanese eel (Anguilla japonica)

Osmoregulation in fish has been a classical research topic for several decades. Salmon and eels are the widely used model animals because of their wide distribution in different geographical locations and spawning migration between fresh- and salt-water habitats. Numerous fish osmoregulatory hormones and ion transporters were identified for their essential roles in acclimation and adaptation to waters of different salinities. Because of the lack of a genomic database, the scope of most studies, however, is very limited. Recently, our group reported the first high-throughput transcriptomic and proteomic studies to identify hyperosmotic-responsive genes/proteins in gills of Japanese eels. In this study, we aimed to decipher changes in hypo-osmotic-responsive proteins in fish acclimating from seawater (SW) to freshwater (FW) conditions. We collected gill samples from SW-adapted and SW-to-FW-acclimating fish. The respective gill proteins were extracted and labeled using isobaric tags for relative and absolute quantitation (iTRAQ) and analyzed using a high-resolution mass spectrometer. In the short-term transfer from SW to FW, 51 hypo-responsive proteins were detected, and 24 unique hypo-osmotic-responsive proteins were identified (15 up-regulated and nine down-regulated proteins). Our data support the use of an omics approach to facilitate the application of functional genomics in non-model organisms.

BIOLOGICAL SIGNIFICANCE

By combining transcriptomic and proteomic approaches, the study has provided the most comprehensive, targeted investigation of eel gill hypo-osmotic responsive proteins that provides molecular insights of osmoregulation mechanisms in a non-model organism, eel. This article is part of a Special Issue entitled: Proteomics of non-model organisms.

Seawater acclimation and inositol monophosphatase isoform expression in the European eel (Anguilla anguilla) and Nile tilapia (Orechromis niloticus)

Inositol monophosphatase (IMPA) is responsible for the synthesis of inositol, a polyol that can function as an intracellular osmolyte helping re-establish cell volume when exposed to hypertonic environments. Some epithelial tissues in euryhaline teleosts such as the eel and tilapia encounter considerable hyperosmotic challenge when fish move from freshwater (FW) to seawater (SW) environments; however, the roles played by organic osmolytes, such as inositol, have yet to be determined. Syntenic analysis has indicated that, as a result of whole genome- and tandem-duplication events, up to six IMPA isoforms can exist within teleost genomes. Four isoforms are homologs of the mammalian IMPA1 gene, and two isoforms are homologs of the mammalian IMPA2 gene. Although the tissue-dependent isoform expression profiles of the teleost isoforms appear to be species-specific, it was primarily mRNA for the IMPA1.1 isoform that was upregulated in epithelial tissues after fish were transferred to SW (up to 16-fold in eel and 90-fold in tilapia). Although up-regulation of IMPA1.1 expression was evident in many tissues in the eel, more substantial increases in IMPA1.1 expression were found in tilapia tissues, where SW acclimation resulted in up to 2,000-fold increases in protein expression, 16-fold increases in enzyme activity and 15-fold increases in tissue inositol contents. Immunohistochemical studies indicated that the tissue and cellular distribution of IMPA1.1 protein differed slightly between eels and tilapia; however, in both species the basal epithelial cell layers within the skin and fin, and the branchial epithelium and interstitial cells within the kidney, exhibited high levels of IMPA1.1 protein expression.

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.

Dexamethasone (DEX) induces Osmotic stress transcription factor 1 (Ostf1) through the Akt-GSK3β pathway in freshwater Japanese eel gill cell cultures

Osmosensing and osmoregulatory processes undertaken in gills of euryhaline fish are coordinated by integrative actions of various signaling molecules/transcriptional factors. Considerable numbers of studies report the hyper- and hypo-osmoregulatory functions of fish gills, by illustrating the process of gill cell remodeling and the modulation of the expression of ion channels/transporters. Comparatively mechanistic information relayed from signal integration to transcriptional regulation in mediating gill cell functions has not yet been elucidated. In this study we demonstrate the functional links from cortisol stimulation, to Akt activation, to the expression of the transcriptional factor, Ostf1. Using the synthetic glucocorticoid receptor agonist, dexamethasone (DEX), Ostf1 expression is found to be activated via glucocorticoid receptor (GR) and mediated by the Akt-GSK3β signaling pathway. Pharmacological experiments using kinase inhibitors reveal that the expression of Ostf1 is negatively regulated by Akt activation. The inhibition of PI3K or Akt activities, by the specific kinase inhibitors (wortmannin, LY294002 or SH6), stimulates Ostf1 expression, while a reduction of GSK3β activity by LiCl reduces Ostf1 expression. Collectively, our report for the first time indicates that DEX can induce Ostf1 via GR, with the involvement of the Akt-GSK3β signaling pathway in primary eel gill cell cultures. The data also suggest that Ostf1 may play different roles in gill cell survival during seawater acclimation.

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.

Taurine plays an important role in the protection of spermatogonia from oxidative stress

It has been demonstrated that taurine has various physiological functions in the body. We demonstrated that taurine is abundant in the serum, liver, muscle and testis of the Japanese eel (Anguilla japonica). In the eel testis, taurine is found mainly in spermatogonia and is weakly expressed also in the Sertoli cells. We have further found in the eel testis that taurine is actively accumulated via the sodium/chloride-dependent taurine transporter (TauT; SLC6A6), which is expressed in germ cells. In our current study, the effects of taurine on the anti-oxidant response were examined. Taurine was found to promote the total superoxide dismutase (SOD) activity in the testis. Moreover, our results indicate that taurine does not affect the mRNA levels of copper-zinc (Cu/Zn) SOD or manganese SOD, but promotes the translation of Cu/Zn SOD. Overall, our present data suggest that taurine may modulate Cu/Zn SOD at the translational level and thereby may play an important role in the protection of germ cells from oxidative stress.

Regulatory function of hyperosmotic stress-induced signaling cascades in the expression of transcription factors and osmolyte transporters in freshwater Japanese eel primary gill cell culture

In the present study, we investigated the early activation of osmotic stress-related protein kinases, with the aim of characterizing their functional links with downstream effectors (i.e. transcription factors and osmolyte transporters). Freshwater eel primary gill cells were cultured in hypertonic medium (500 mosmol l(-1)) for 6 h. Protein lysates and total RNA were collected for western blotting and quantitative real-time PCR assays. In this study, the osmotic challenge stimulated histone H3 phosphorylation, various signaling pathways (i.e. ERK1/2, p38 MAPK, JNK, CREB, MARCKS and MLCK) and expression of some downstream effectors (i.e. Na(+)/K(+)-ATPase, TauT and Ostf). Increased phosphorylation of acetylated histone is known to promote chromatin relaxation for global gene transcription, probably leading to the activation of downstream effectors for osmotic responses. In addition, the importance of the p38 MAPK and MLCK pathways in the regulation of the expression of Na(+)/K(+)-ATPase and TauT was demonstrated. Inhibition of the p38 MAPK pathway by SB202190 reduced histone H3 phosphorylation and TauT mRNA expression. Moreover, inhibition of the MLCK pathway by ML-7 decreased the expression level of Na(+)/K(+)-ATPase but increased the transcript level of TauT. Collectively, the present study reveals possible functional links of osmosensing signaling cascades to the regulation of downstream effectors.

Cloning, tissue and ontogenetic expression of the taurine transporter in the flatfish Senegalese sole (Solea senegalensis)

Flatfish species seem to require dietary taurine for normal growth and development. Although dietary taurine supplementation has been recommended for flatfish, little is known about the mechanisms of taurine absorption in the digestive tract of flatfish throughout ontogeny. This study described the cloning and ontogenetic expression of the taurine transporter (TauT) in the flatfish Senegalese sole (Solea senegalensis). Results showed a high similarity between TauT in Senegalese sole and other vertebrates, but a change in TauT amino acid sequences indicates that taurine transport may differ between mammals and fish, reptiles or birds. Moreover, results showed that Senegalese sole metamorphosis is an important developmental trigger to promote taurine transport in larvae, especially in muscle tissues, which may be important for larval growth. Results also indicated that the capacity to uptake dietary taurine in the digestive tract is already established in larvae at the onset of metamorphosis. In Senegalese sole juveniles, TauT expression was highest in brain, heart and eye. These are organs where taurine is usually found in high concentrations and is believed to play important biological roles. In the digestive tract of juveniles, TauT was more expressed in stomach and hindgut, indicating that dietary taurine is quickly absorbed when digestion begins and taurine endogenously used for bile salt conjugation may be recycled at the posterior end of the digestive tract. Therefore, these results suggest an enterohepatic recycling pathway for taurine in Senegalese sole, a process that may be important for maintenance of the taurine body levels in flatfish species.