Expression of Ol-foxi3 and Na(+)/K(+)-ATPase in ionocytes during the development of euryhaline medaka (Oryzias latipes) embryos

Retention of ion channel genes expression increases Japanese medaka survival during seawater reacclimation

Euryhaline teleosts exhibit varying acclimability to survive in environments that alternate between being hypotonic and hypertonic. Such ability is conferred by ion channels expressed by ionocytes, the ion-regulating cells in the gills or skin. However, switching between environments is physiologically challenging, because most channels can only perform unidirectional ion transportation. Coordination between acute responses, such as gene expression, and long-term responses, such as cell differentiation, is believed to strongly facilitate adaptability. Moreover, the pre-acclimation to half seawater salinity can improve the survivability of Japanese medaka (Oryzias latipes) during direct transfer to seawater; here, the ionocytes preserve hypertonic acclimability while performing hypotonic functions. Whether acclimability can be similarly induced in a closed species and their corresponding responses in terms of ion channel expression remain unclear. In the present study, Japanese medaka pre-acclimated in brackish water were noted to have higher survival rates while retaining higher expression of the three ion channel genes ATP1a1a.1, ATP1b1b, and SLC12a2a. This retention was maintained up to 2 weeks after the fish were transferred back into freshwater. Notably, this induced acclimability was not found in its close kin, Indian medaka (Oryzias dancena), the natural habitat of which is brackish water. In conclusion, Japanese medaka surpassed Indian medaka in seawater acclimability after experiencing exposure to brackish water, and this ability coincided with seawater-retention gene expression.

Dynamic transcriptome and LC-MS/MS analysis revealed the important roles of taurine and glutamine metabolism in response to environmental salinity changes in gills of rainbow trout (Oncorhynchus mykiss)

Recently, the frequent salinity fluctuation has become a growing threat to fishes. However, the dynamic patterns of gene expression in response to salinity changes remain largely unexplored. In the present study, 18 RNA-Seq datasets were generated from gills of rainbow trout at different salinities, including 0 ‰, 6 ‰, 12 ‰, 18 ‰, 24 ‰ and 30 ‰. Based on the strict thresholds, we have identified 63, 1411, 2096, 1031 and 1041 differentially expressed genes in gills of rainbow trout through pairwise comparisons. Additionally, weighted gene co-expression network analysis was performed to construct 18 independent modules with distinct expression patterns. Of them, green and tan modules were found to be tightly related to salinity changes, several hub genes of which are known as the important regulators in taurine and glutamine metabolism. To further investigate their potential roles in response to salinity changes, taurine, glutamine, and their metabolism-related glutamic acid and α-ketoglutaric acid were accurately quantitated using liquid chromatography-tandem mass spectrometry analysis. Results clearly showed that their concentrations were closely associated with salinity changes. These findings suggested that taurine and glutamine play important roles in response to salinity changes in gills of rainbow trout, providing new insights into the molecular mechanism of fishes in salinity adaptation.

Comparative transcriptomic analysis reveals mechanisms of divergence in osmotic regulation of the turbot Scophthalmus maximus

The turbot Scophthalmus maximus has evolved extensive physiological ability to adapt to multiple environmental salinities. The morphological changes of the kidney indicated the adaptability difference and similarity of turbot to salinity stress. Identify transcriptome-wide differences between low-salinity seawater (LSW, salinity 5)- and high-salinity seawater (HSW, salinity 50)-acclimated kidneys of turbot to decipher the osmotic regulation mechanism. We identified 688 differentially expressed genes (DEGs) in the LSW-acclimated kidneys and 2441 DEGs in the HSW-acclimated kidneys of turbot compared with seawater-acclimated kidneys, respectively. We investigated three patterns of gene regulation to salinity stress that involved in ion channels and transporters, functions of calcium regulation, organic osmolytes, energy demand, cell cycle regulation, and cell protection. Additionally, protein-protein interaction (PPI) analysis of DEGs suggested the presence of a frequent functional interaction pattern and that crucial genes in the PPI network are involved in hyper-osmotic regulation. Based on the analysis of comparative transcriptome data and related literature reports, we conclude that the mechanisms responsible for osmotic regulation and its divergence in turbot are related to various genes that are involved in canonical physiological functions. These findings provide insight into the divergence in osmoregulation of turbot and valuable information about osmoregulation mechanisms that will benefit other studies in this field.

Expression of new genes in vertebrate tooth development and p63 signaling

BACKGROUND

p63 is an evolutionarily ancient transcription factor essential to vertebrate tooth development. Our recent gene expression screen comparing wild-type and "toothless" p63^-/- mouse embryos implicated in tooth development several new genes that we hypothesized act downstream of p63 in dental epithelium, where p63 is also expressed.

RESULTS

Via in situ hybridization and immunohistochemistry, we probed mouse embryos (embryonic days 10.5-14.5) and spotted gar fish embryos (14 days postfertilization) for these newly linked genes, Cbln1, Cldn23, Fermt1, Krt15, Pltp and Prss8, which were expressed in mouse and gar dental epithelium. Loss of p63 altered expression levels but not domains. Expression was comparable between murine upper and lower tooth organs, implying conserved gene functions in maxillary and mandibular dentitions. Our meta-analysis of gene expression databases supported that these genes act within a p63-driven gene regulatory network important to tooth development in mammals and more evolutionary ancient vertebrates (fish, amphibians).

CONCLUSIONS

Cbln1, Cldn23, Fermt1, Krt15, Pltp, and Prss8 were expressed in mouse and fish dental epithelium at placode, bud, and/or cap stages. We theorize that these genes participate in cell-cell adhesion, cell polarity, and extracellular matrix signaling to support dental epithelium integrity, folding, and epithelial-mesenchymal cross talk during tooth development.

A Review of FOXI3 Regulation of Development and Possible Roles in Cancer Progression and Metastasis

Development and cancer share a variety of functional traits such as EMT, cell migration, angiogenesis, and tissue remodeling. In addition, many cellular signaling pathways are noted to coordinate developmental processes and facilitate aspects of tumor progression. The Forkhead box superfamily of transcription factors consists of a highly conserved DNA binding domain, which binds to specific DNA sequences and play significant roles during adult tissue homoeostasis and embryogenesis including development, differentiation, metabolism, proliferation, apoptosis, migration, and invasion. Interestingly, various studies have implicated the role of key Fox family members such as FOXP, FOXO, and FOXA during cancer initiation and metastases. FOXI3, a member of the Forkhead family affects embryogenesis, development, and bone remodeling; however, no studies have reported a role in cancer. In this review, we summarize the role of FOXI3 in embryogenesis and bone development and discuss its potential involvement in cancer progression with a focus on the bone metastasis. Moreover, we hypothesize possible mechanisms underlying the role of FOXI3 in the development of solid tumor bone metastasis.

Hyperosmotic tolerance of adult fish and early embryos are determined by discrete, single loci in the genus Oryzias

The acquisition of environmental osmolality tolerance traits in individuals and gametes is an important event in the evolution and diversification of organisms. Although teleost fish exhibit considerable intra- and interspecific variation in salinity tolerance, the genetic mechanisms underlying this trait remain unclear. Oryzias celebensis survives in sea and fresh water during both the embryonic and adult stages, whereas its close relative Oryzias woworae cannot survive in sea water at either stage. A linkage analysis using backcross progeny identified a single locus responsible for adult hyperosmotic tolerance on a fused chromosome that corresponds to O. latipes linkage groups (LGs) 6 and 23. Conversely, O. woworae eggs fertilised with O. celebensis sperm died in sea water at the cleavage stages, whereas O. celebensis eggs fertilised with O. woworae sperm developed normally, demonstrating that maternal factor(s) from O. celebensis are responsible for hyperosmotic tolerance during early development. A further linkage analysis using backcrossed females revealed a discrete single locus relating to the maternal hyperosmotic tolerance factor in a fused chromosomal region homologous to O. latipes LGs 17 and 19. These results indicate that a maternal factor governs embryonic hyperosmotic tolerance and maps to a locus distinct from that associated with adult hyperosmotic tolerance.

Insights into molecular and cellular mechanisms of hormonal actions on fish ion regulation derived from the zebrafish model

Fish have sophisticated mechanisms of ionic and acid-base regulation for maintaining body fluid homeostasis. Many hormones have been proposed to control the ionic and acid-base regulation mechanisms in fishes; however, lots of the proposed actions lack convincing cellular/molecular evidence. With the advantages of available genetic databases and molecular manipulation techniques, zebrafish has become an emerging model for research into ion transport physiology and functional regulation. Different types of ionocytes were found to transport ions through various sets of ion transporters, and the molecular mechanisms of ionocyte proliferation and differentiation have also been dissected, providing a competent platform with which to precisely study the ion transport pathways and ionocytes targeted by hormones, including isotocin, prolactin, cortisol, stanniocalcin-1, calcitonin, endothelin-1, vitamin D, parathyroid hormone 1, catecholamines, the renin-angiotensin-system, estrogen-related receptor α, and calcitonin gene-related peptide, which have been demonstrated to positively or negatively regulate ion transport through specific receptors at different molecular levels (transcriptional, translational, or posttranslational) or at different developmental stages of ionocytes (proliferation or differentiation). The knowledge obtained in zebrafish not only enhances our understanding of the hormonal control of fish ion regulation, but also informs studies on other animal species, thereby providing insights into related fields.

Stage susceptibility of Japanese medaka (Oryzias latipes) to selenomethionine and hypersaline developmental toxicity

Anthropogenic disturbance of seleniferous soils can lead to selenium contamination of waterways. Although selenium is an essential micronutrient, bioaccumulation and maternal transfer of proteinaceous selenomethionine (SeMet) can result in embryo toxicity. Furthermore, as the climate changes, the salinity of spawning grounds in water-restrained estuaries is increasing. Although a small increase in salinity may not directly impact adult fish, it may alter the detoxification strategies of developing organisms. Previous research indicates that hypersalinity may potentiate SeMet embryo toxicity at an early developmental stage. However, embryonic development is a complex, spatiotemporal process with a constantly shifting cellular microenvironment. To generate thresholds and an adverse outcome pathway for the interactions between selenium and salinity, we sought to identify windows of susceptibility for lethality and deformities in the Japanese medaka (Oryzias latipes). Embryos were treated in freshwater or saltwater for 24 h with 0.5 µM, 5 µM, and 50 µM SeMet at 6 different developmental stages (9, 17, 25, 29, 34, and 38). Survival, hatch, deformities (total, type, and severity), and days to hatch were quantified. Selenium embryo tissue measurements were performed. Selenomethionine exposures of 5 µM and 50 µM significantly decreased survival and hatch at all stages. However, SeMet uptake was stage-dependent and increased with stage. Stage 17 (early neurulation) was identified as the most susceptible stage for lethality and deformities. Selenomethionine in saltwater caused significantly greater toxicity than freshwater at stage 25 (early organogenesis), suggesting a role for liver and osmoregulatory organogenesis in toxicity.

The mouse Foxi3 transcription factor is necessary for the development of posterior placodes

The inner ear develops from the otic placode, one of the cranial placodes that arise from a region of ectoderm adjacent to the anterior neural plate called the pre-placodal domain. We have identified a Forkhead family transcription factor, Foxi3, that is expressed in the pre-placodal domain and down-regulated when the otic placode is induced. We now show that Foxi3 mutant mice do not form otic placodes as evidenced by expression changes in early molecular markers and the lack of thickened placodal ectoderm, an otic cup or otocyst. Some preplacodal genes downstream of Foxi3-Gata3, Six1 and Eya1-are not expressed in the ectoderm of Foxi3 mutant mice, and the ectoderm exhibits signs of increased apoptosis. We also show that Fgf signals from the hindbrain and cranial mesoderm, which are necessary for otic placode induction, are received by pre-placodal ectoderm in Foxi3 mutants, but do not initiate otic induction. Finally, we show that the epibranchial placodes that develop in close proximity to the otic placode and the mandibular division of the trigeminal ganglion are missing in Foxi3 mutants. Our data suggest that Foxi3 is necessary to prime pre-placodal ectoderm for the correct interpretation of inductive signals for the otic and epibranchial placodes.

Evidence for two distinct waves of epidermal ionocyte differentiation during medaka embryonic development

BACKGROUND

The fish epidermis contains specific cells, or ionocytes, that are specialized in ion transport and contribute to the osmoregulatory function. Besides the zebrafish model, the medaka (Oryzias latipes) has recently emerged as an important model for osmoregulation studies because it possesses a particularly high adaptability to salinity changes. However, hindering the progress of research on embryonic ionocytes is the lack of a comprehensive view of their developmental dynamic.

RESULTS

Using EdU integrations and the foxi3 and NKA markers, we characterized the proliferating progenitors of ionocytes (here called ionoblastes) and we quantified them, along with ionocytes, during embryogenesis. While progenitors of the vitellin zone promptly differentiate in a synchronous manner, progenitors of the lateral zone differentiate progressively and asynchronously. Furthermore, we evidenced that nhe3 is expressed in differentiated ionocytes of both zones, whereas ecac, ncc, and gcm2 are strictly specific of the lateral zone. We also evidenced that the two zones are differentially regulated in distilled water and seawater.

CONCLUSIONS

Our data led us to propose a model timeline, which provides evidence for the expansion of two successive and distinct populations of ionocytes. This model opens the way for new studies related to epidermal development, plasticity and osmoregulation ontogeny.

The role of foxi family transcription factors in the development of the ear and jaw

The mammalian outer, middle, and inner ears have different embryonic origins and evolved at different times in the vertebrate lineage. The outer ear is derived from first and second branchial arch ectoderm and mesoderm, the middle ear ossicles are derived from neural crest mesenchymal cells that invade the first and second branchial arches, whereas the inner ear and its associated vestibule-acoustic (VIIIth) ganglion are derived from the otic placode. In this chapter, we discuss recent findings in the development of these structures and describe the contributions of members of a Forkhead transcription factor family, the Foxi family to their formation. Foxi transcription factors are critical for formation of the otic placode, survival of the branchial arch neural crest, and developmental remodeling of the branchial arch ectoderm.

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.

The inner opercular membrane of the euryhaline teleost: a useful surrogate model for comparisons of different characteristics of ionocytes between seawater- and freshwater-acclimated medaka

The inner opercular membranes of the brackish medaka, Oryzias dancena, have numerous ionocytes, similar to the gill epithelia. By histological observation, this study demonstrated that it is possible to investigate the cellular morphology and function of ionocytes in the opercular membrane. The mitochondria-rich ionocytes in the opercular membranes were traced using rhodamine 123 and a cytochrome c oxidase IV antibody in vital and fixed situations, respectively. To validate different morphologies of seawater (SW)-type and freshwater (FW)-type ionocytes of the opercular membrane of euryhaline brackish medaka, a method of dual observation including immunofluorescence staining and subsequent scanning electron microscopy was used. The apical morphologies of SW- and FW-type ionocytes were hole and flat opening, respectively. In addition, the microvilli were found on the apical surface of the FW-type ionocytes. The SW-type ionocytes exhibited basolateral Na(+), K(+), 2Cl(-) cotransporter and the apical cystic fibrosis transmembrane conductance regulator. In contrast, in the apical region of FW-type ionocytes, Na(+), Cl(-) cotransporter and villin 1-like protein were expressed. In addition, histochemical staining of AgCl precipitation counterstained with a Na(+), K(+)-ATPase α-subunit antibody on the opercular membrane illustrated the role of Cl(-) secretion in the SW-type ionocytes of the brackish medaka. A combination of different observations in this study indicated that the opercular membrane could be a useful surrogate model for histological and functional studies on the epithelial ionocytes of fish gills.

Medaka villin 1-like protein (VILL) is associated with the formation of microvilli induced by decreasing salinities in the absorptive ionocytes

Introduction

Villin 1 is an actin-regulatory protein involved in the formation of microvilli of mammalian enterocytes. The microvilli, finger-like protrusions, are more abundant on the apical surfaces of gill ionocytes in various freshwater (FW) teleosts than in seawater (SW) fishes. However, the plasticity in the mechanisms of microvillus formation in the gill ionocytes are poorly understood, and the actin-regulatory proteins involved in the formation of microvilli have not been identified in fishes. The present study used the euryhaline medaka (Oryzias dancena) as a model to explore the role of a homolog of villin 1 in the actin-organization of cellular morphologies induced by decreasing salinities.

Results

By ultrastructural observation, there are numerous actin filaments organized on the apical cortex of ion-absorptive ionocytes in the FW-acclimated medaka. From gills of the euryhaline medaka, we have identified the VILL sequence. The phylogenetic tree and functional domains suggest that VILL is the homolog of villin 1 in fishes. Immunofluorescence using a specific antibody revealed that VILL was specifically localized to the apical region of gill ionocytes along with microvilli in the FW medaka, but not in SW fish. The expression levels of Odvill mRNA and VILL protein were higher in the gills of the FW individuals than in the SW group and were induced when fish were transferred from SW to FW. A morpholino oligonucleotide for VILL knockdown eliminated the apical protrusions of ionocytes and pavement cells in the trunk epithelia of embryos.

Conclusions

From a novel aspect of cytoskeletal functions, our findings highlighted the important role of VILL protein in the ionoregulation of aquatic vertebrates in response to different osmotic challenges. This study is the first to show that the expression of VILL is associated with the formation of microvilli in the absorptive ionocytes of a euryhaline fish. Loss-of-function experiments showed that the distribution of VILL may represent the molecular link between the cytoskeletal organization and cellular morphology of the absorptive ionocytes during hypoosmotic adaptation in aquatic vertebrates.

Assessment of the role of cortisol and corticosteroid receptors in epidermal ionocyte development in the medaka (Oryzias latipes) embryos

Cortisol is a pleiotropic glucocorticoid hormone that acts through the intracellular glucocorticoid receptors (GR). Cortisol affects many important biological functions in mammals, including immune function, behavior, stress, metabolism, growth and organogenesis. In fishes, cortisol has an additional function in the osmoregulatory activity of ionocytes (ICs). Although much progress has been made toward understanding cortisol action at the levels of adult osmoregulatory tissues, the developmental functions of cortisol and its receptors in ICs remain to be clarified. We first analyzed the total contents of both cortisol and corticosteroid receptor mRNAs (GR1, GR2 and MR) during medaka development. Although low levels of cortisol were detected during development of the medaka embryo, maternal GR1, GR2 and MR transcripts were detected at higher levels than zygotic transcripts. We investigated the effect of exogenous cortisol on IC number during medaka embryogenesis. We observed that cortisol treatment induced an earlier expansion of the IC population but did not modify the final IC number. Using functional genomic approaches, we also tested the involvement of GR1, GR2 and mineralocorticoid receptor (MR) in IC development by systematic knock-down with translation-blocking morpholinos. Only GR2 knock-down led to a reduction of the total number of ICs in the epidermis. In addition, a GR2 splice-blocking morpholino did not have any effect on the biogenesis of ICs, underscoring the importance of maternally inherited GR2 mRNAs. We propose that maternal GR2, but not GR1 or MR, is a major pathway in the IC biogenesis in medaka most likely through cortisol activation, and that cortisol exposition fine-tunes their developmental timing. These findings provide a framework for future research on the regulatory functions of corticosteroids in euryhaline fishes and provide medaka as an advantageous model to further elucidate the underlying molecular regulatory mechanisms of IC development.

A study of the electrical polarization of Sepia officinalis yolk envelope, a role for Na(+)/K(+)-ATPases in osmoregulation?

The cuttlefish Sepia officinalis mate and spawn in the intertidal zone where eggs are exposed during low tide to osmotic stress. Embryonic outer yolk sac is a putative site for osmoregulation of young S. officinalis embryos. By using electrophysiological recordings and immunostaining we showed, (i) that the chorion is only a passive barrier for ions, since large molecules could not pass through it, (ii) that a complex transepithelial potential difference occurs through the yolk epithelium, (iii) that ionocyte-like cells and Na⁺/K⁺-ATPases were localized in the yolk epithelium and (iv) that ouabain sensitive Na⁺/K⁺-ATPase activity could participate to this yolk polarization. These data warrant further study on the role of ion transport systems of this epithelium in the osmoregulation processes in S. officinalis embryos.

Role of cardiac natriuretic peptides in seawater adaptation of medaka embryos as revealed by loss-of-function analysis

Cardiac natriuretic peptides (atrial natriuretic peptide, ANP; b-type natriuretic peptide, BNP; ventricular natriuretic peptide, VNP) and their direct ancestor C-type natriuretic peptide 3 (CNP3) exert potent osmoregulatory actions in fish. However, very little is known about their roles in embryonic osmoregulation. In this study, we performed loss-of-function analysis using euryhaline medaka (Oryzias latipes), which has lost ANP and VNP during evolution and thus possesses only BNP and CNP3. We found that the maintenance of whole-body osmolality in seawater embryos was impaired by the knockdown of BNP+OLGC7 (BNP receptor) or CNP3 alone from 1 day postfertilization, and the CNP3 knockdown was accompanied by greater water loss. The impaired osmoregulation in the knockdown embryos was not due to the suppressed expression of major transporters for NaCl excretion via ionocytes or of key enzyme genes for metabolic water production, but to the impaired blood circulation to the yolk-sac membrane caused by abnormal heart development. We detected a strong positive correlation between impaired blood circulation and increased body fluid osmolality and pharmacological blockade of blood flow increased body fluid osmolality in seawater embryos. We also found that the exaggerated water loss in CNP3 knockdown embryos is related to the failure to suppress aquaporin (AQP3, AQP4, and AQP9) gene expression. These results show that CNP3 decrease water permeability of body surfaces and that both BNP and CNP3 ensure sufficient blood flow to the yolk-sac membrane for efficient salt excretion by ionocytes and sufficient water production by yolk metabolism to promote seawater adaptation during early development in medaka.

Acid secretion by mitochondrion-rich cells of medaka (Oryzias latipes) acclimated to acidic freshwater

In the present study, medaka embryos were exposed to acidified freshwater (pH 5) to investigate the mechanism of acid secretion by mitochondrion-rich (MR) cells in embryonic skin. With double or triple in situ hybridization/immunocytochemistry, the Na+/H+ exchanger 3 (NHE3) and H+-ATPase were localized in two distinct subtypes of MR cells. NHE3 was expressed in apical membranes of a major proportion of MR cells, whereas H+-ATPase was expressed in basolateral membranes of a much smaller proportion of MR cells. Gill mRNA levels of NHE3 and H+-ATPase and the two subtypes of MR cells in yolk sac skin were increased by acid acclimation; however, the mRNA level of NHE3 was remarkably higher than that of H+-ATPase. A scanning ion-selective electrode technique was used to measure H+, Na+, and NH4+ transport by individual MR cells in larval skin. Results showed that Na+ uptake and NH4+ excretion by MR cells increased after acid acclimation. These findings suggested that the NHE3/Rh glycoprotein-mediated Na+ uptake/NH4+ excretion mechanism plays a critical role in acidic equivalent (H+/NH4+) excretion by MR cells of the freshwater medaka.

Ion regulation in fish gills: recent progress in the cellular and molecular mechanisms

Fish encounter harsh ionic/osmotic gradients on their aquatic environments, and the mechanisms through which they maintain internal homeostasis are more challenging compared with those of terrestrial vertebrates. Gills are one of the major organs conducting the internal ionic and acid-base regulation, with specialized ionocytes as the major cells carrying out active transport of ions. Exploring the iono/osmoregulatory mechanisms in fish gills, extensive literature proposed several models, with many conflicting or unsolved issues. Recent studies emerged, shedding light on these issues with new opened windows on other aspects, on account of available advanced molecular/cellular physiological approaches and animal models. Respective types of ionocytes and ion transporters, and the relevant regulators for the mechanisms of NaCl secretion, Na(+) uptake/acid secretion/NH(4)(+) excretion, Ca(2+) uptake, and Cl(-) uptake/base secretion, were identified and functionally characterized. These new ideas broadened our understanding of the molecular/cellular mechanisms behind the functional modification/regulation of fish gill ion transport during acute and long-term acclimation to environmental challenges. Moreover, a model for the systematic and local carbohydrate energy supply to gill ionocytes during these acclimation processes was also proposed. These provide powerful platforms to precisely study transport pathways and functional regulation of specific ions, transporters, and ionocytes; however, very few model species were established so far, whereas more efforts are needed in other species.