Foxi3 transcription factors and Notch signaling control the formation of skin ionocytes from epidermal precursors of the zebrafish embryo

Sox10 is required for systemic initiation of bone mineralization

Heterozygous variants in SOX10 cause congenital syndromes affecting pigmentation, digestion, hearing, and neural development, primarily attributable to failed differentiation or loss of non-skeletal neural crest derivatives. We report here an additional, previously undescribed requirement for Sox10 in bone mineralization. Neither crest- nor mesoderm-derived bones initiate mineralization on time in zebrafish sox10 mutants, despite normal osteoblast differentiation and matrix production. Mutants are deficient in the Trpv6+ ionocytes that take up calcium from the environment, resulting in severe calcium deficiency. As these ionocytes derive from ectoderm, not crest, we hypothesized that the primary defect resides in a separate organ that systemically regulates ionocyte numbers. RNA sequencing revealed significantly elevated stanniocalcin (Stc1a), an anti-hypercalcemic hormone, in sox10 mutants. Stc1a inhibits calcium uptake in fish by repressing trpv6 expression and Trpv6+ ionocyte proliferation. Epistasis assays confirm excess Stc1a as the proximate cause of the calcium deficit. The pronephros-derived glands that synthesize Stc1a interact with sox10+ cells, but these cells are missing in mutants. We conclude that sox10+ crest-derived cells non-autonomously limit Stc1a production to allow the inaugural wave of calcium uptake necessary to initiate bone mineralization.

Paired and solitary ionocytes in the zebrafish olfactory epithelium

The sense of smell is generated by electrical currents that are influenced by the concentration of ions in olfactory sensory neurons and mucus. In contrast to the extensive morphological and molecular characterization of sensory neurons, there has been little description of the cells that control ion concentrations in the zebrafish olfactory system. Here, we report the molecular and ultrastructural characterization of zebrafish olfactory ionocytes. Transcriptome analysis suggests that the zebrafish olfactory epithelium contains at least three different ionocyte types, which resemble Na + /K + -ATPase-rich (NaR), Na + /Cl - cotransporter (NCC), and H + -ATPase-rich (HR) cells, responsible for calcium, chloride, and pH regulation, respectively, in the zebrafish skin. NaR-like and HR-like ionocytes are usually adjacent to one another, whereas NCC-like cells are usually solitary. The distinct subtypes are differentially distributed: NaR-like/HR-like cell pairs are found broadly within the olfactory epithelium, whereas NCC-like cells reside within the peripheral non-sensory multiciliated cell zone. Comparison of gene expression and serial-section electron microscopy analysis indicates that the NaR-like cells wrap around the HR-like cells and are connected to them by shallow tight junctions. The development of olfactory ionocyte subtypes is also differentially regulated, as pharmacological Notch inhibition leads to a loss of NaR-like and HR-like cells, but does not affect NCC-like ionocyte number. These results provide a molecular and anatomical characterization of olfactory ionocytes in a stenohaline freshwater teleost. The paired ionocytes suggest that both transcellular and paracellular transport regulate ion concentrations in the olfactory epithelium, while the solitary ionocytes may enable independent regulation of multiciliated cells.

Foxi1 regulates multiple steps of mucociliary development and ionocyte specification through transcriptional and epigenetic mechanisms

Foxi1 is a master regulator of ionocytes (ISCs / INCs) across species and organs. Two subtypes of ISCs exist, and both α- and β-ISCs regulate pH- and ion-homeostasis in epithelia. Gain and loss of FOXI1 function are associated with human diseases, including Pendred syndrome, male infertility, renal acidosis and cancers. Foxi1 functions were predominantly studied in the context of ISC specification, however, reports indicate additional functions in early and ectodermal development. Here, we re-investigated the functions of Foxi1 in Xenopus laevis embryonic mucociliary epidermis development and found a novel function for Foxi1 in the generation of Notch-ligand expressing mucociliary multipotent progenitors (MPPs). We demonstrate that Foxi1 has multiple concentration-dependent functions: At low levels, Foxi1 confers ectodermal competence through transcriptional and epigenetic mechanisms, while at high levels, Foxi1 induces a multi-step process of ISC specification and differentiation. We further describe how foxi1 expression is affected through auto- and Notch-regulation, how Ubp1 and Dmrt2 regulate ISC subtype differentiation, and how this developmental program affects Notch signaling as well as mucociliary patterning. Together, we reveal novel functions for Foxi1 in Xenopus mucociliary epidermis formation, relevant to our understanding of vertebrate development and human disease.

Environmental and molecular control of tissue-specific ionocyte differentiation in zebrafish

Organisms cope with environmental fluctuations and maintain fitness in part via reversible phenotypic changes (acclimation). Aquatic animals are subject to dramatic seasonal fluctuations in water salinity, which affect osmolarity of their cells and consequently cellular function. Mechanosensory lateral line hair cells detect water motion for swimming behavior and are especially susceptible to salinity changes due to their direct contact with the environment. To maintain hair cell function when salinity decreases, neuromast (Nm)-associated ionocytes differentiate and invade lateral line neuromasts. The signals that trigger the adaptive differentiation of Nm ionocytes are unknown. We demonstrate that new Nm ionocytes are rapidly specified and selectively triggered to proliferate by low Ca2+ and Na+/Cl- levels. We further show that Nm ionocyte recruitment and induction is affected by hair cell activity. Once specified, Nm ionocyte differentiation and survival are associated with sequential activation of different Notch pathway components, a process different from other tissue-specific ionocytes. In summary, we show how environmental changes activate a signaling cascade that leads to physiological adaptation. This may prove essential for survival not only in seasonal changing environments but also in changing climates.

Cell proliferation and regeneration in the gill

Seminal studies from the early 20th century defined the structural changes associated with development and regeneration of the gills in goldfish at the gross morphological and cellular levels using standard techniques of light and electron microscopy. More recently, investigations using cell lineage tracing, molecular biology, immunohistochemistry and single-cell RNA-sequencing have pushed the field forward and have begun to reveal the cellular and molecular processes that orchestrate cell proliferation and regeneration in the gills. The gill is a multifunctional organ that mediates an array of important physiological functions, including respiration, ion regulation and excretion of waste products. It is comprised of unique cell types, such as pavement cells, ionocytes, chemoreceptors and undifferentiated stem or progenitor cells that regulate growth and replenish cell populations. The gills develop from the embryonic endoderm and are rich in cell types derived from the neural crest. The gills have the capacity to remodel themselves in response to environmental change, such as in the case of ionocytes, chemoreceptors and the interlamellar cell mass, and can completely regenerate gill filaments and lamellae. Both processes of remodeling and regeneration invariably involve cell proliferation. Although gill regeneration has been reported in only a limited number of fish species, the process appears to have many similarities to regeneration of other organs in fish and amphibians. The present article reviews the studies that have described gill development and growth, and that demonstrate a suite of genes, transcription factors and other proteins involved in cell proliferation and regeneration in the gills.

Environmental and molecular control of tissue-specific ionocyte differentiation in zebrafish

Organisms adjust their physiology to cope with environmental fluctuations and maintain fitness. These adaptations occur via genetic changes over multiple generations or through acclimation, a set of reversible phenotypic changes that confer resilience to the individual. Aquatic organisms are subject to dramatic seasonal fluctuations in water salinity, which can affect the function of lateral line mechanosensory hair cells. To maintain hair cell function when salinity decreases, ion-regulating cells, Neuromast-associated ionocytes (Nm ionocytes), increase in number and invade lateral line neuromasts. How environmental changes trigger this adaptive differentiation of Nm ionocytes and how these cells are specified is still unknown. Here, we identify Nm ionocyte progenitors as foxi3a/foxi3b-expressing skin cells and show that their differentiation is associated with sequential activation of different Notch pathway components, which control ionocyte survival. We demonstrate that new Nm ionocytes are rapidly specified by absolute salinity levels, independently of stress response pathways. We further show that Nm ionocyte differentiation is selectively triggered by depletion of specific ions, such as Ca2+ and Na+/Cl-, but not by low K+ levels, and is independent of media osmolarity. Finally, we demonstrate that hair cell activity plays a role in Nm ionocyte recruitment and that systemic factors are not necessary for Nm ionocyte induction. In summary, we have identified how environmental changes activate a signaling cascade that triggers basal skin cell progenitors to differentiate into Nm ionocytes and invade lateral line organs. This adaptive behavior is an example of physiological plasticity that may prove essential for survival in changing climates.

CFTR high expresser cells in cystic fibrosis and intestinal diseases

Cystic Fibrosis Transmembrane Conductance Regulator (CFTR), the Cl^-/HCO₃ ^- channel implicated in Cystic Fibrosis, is critical to the pathophysiology of many gastrointestinal diseases. Defects in CFTR lead to intestinal dysfunction, malabsorption, obstruction, infection, inflammation, and cancer that increases morbidity and reduces quality of life. This review will focus on CFTR in the intestine and the implications of the subpopulation of CFTR High Expresser Cells (CHEs) in Cystic Fibrosis (CF), intestinal physiology and pathophysiology of intestinal diseases.

A tessellated lymphoid network provides whole-body T cell surveillance in zebrafish

Homeostatic trafficking to lymph nodes allows T cells to efficiently survey the host for cognate antigen. Non-mammalian jawed vertebrates lack lymph nodes but maintain similarly diverse T cell pools. Here, we exploit in vivo imaging of transparent zebrafish to investigate how T cells organize and survey for antigen in an animal devoid of lymph nodes. We find that naïve-like T cells in zebrafish organize into a previously undescribed whole-body lymphoid network that supports streaming migration and coordinated trafficking through the host. This network has the cellular hallmarks of a mammalian lymph node, including naïve T cells and CCR7-ligand expressing non-hematopoietic cells, and facilitates rapid collective migration. During infection, T cells transition to a random walk that supports antigen presenting cell interactions and subsequent activation. Our results reveal that T cells can toggle between collective migration and individual random walks to prioritize either large-scale trafficking or antigen search in situ . This novel lymphoid network thus facilitates whole-body T cell trafficking and antigen surveillance in the absence of a lymph node system.

Significance Statement

In mammals, lymph nodes play a critical role in the initiation of adaptive immune responses by providing a dedicated place for T cells to scan antigen-presenting cells. Birds, reptiles, amphibians, and fish all maintain diverse repertoires of T cells but lack lymph nodes, raising questions about how adaptive immunity functions in lower jawed vertebrates. Here, we describe a novel network of lymphocytes in zebrafish that supports whole-body T cell trafficking and provides a site for antigen search, mirroring the function of mammalian lymph nodes. Within this network, T cells can prioritize large-scale trafficking or antigen scanning by toggling between two distinct modes of migration. This network provides valuable insights into the evolution of adaptive immunity.

Single-cell transcriptomic analysis of neuroepithelial cells and other cell types of the gills of zebrafish (Danio rerio) exposed to hypoxia

The fish gill is a multifunctional organ involved in numerous physiological processes, such as gas exchange and sensing of hypoxia by respiratory chemoreceptors, called neuroepithelial cells (NECs). Many studies have focused on zebrafish (Danio rerio) to investigate the structure, function and development of the gills, yet the transcriptomic profile of most gill cells remains obscure. We present the results of a comprehensive transcriptomic analysis of the gills of zebrafish using single-cell RNA sequencing (scRNA‐seq). Gill cells from ETvmat2:EGFP zebrafish were individually labelled before scRNA‐seq library construction using 10× Genomics Chromium technology. 12,819 cells were sequenced with an average depth of over 27,000 reads per cell. We identified a median of 485 genes per cell and 16 cell clusters, including NECs, neurons, pavement cells, endothelial cells and mitochondrion-rich cells. The identity of NECs was confirmed by expression of slc18a2, encoding the vesicular monoamine transporter, Vmat2. Highly differentially-expressed genes in NECs included tph1a, encoding tryptophan hydroxylase, sv2 (synaptic vesicle protein), and proteins implicated in O₂ sensing (ndufa4l2a, cox8al and epas1a). In addition, NECs and neurons expressed genes encoding transmembrane receptors for serotonergic, cholinergic or dopaminergic neurotransmission. Differential expression analysis showed a clear shift in the transcriptome of NECs following 14 days of acclimation to hypoxia. NECs in the hypoxia group showed high expression of genes involved in cell cycle control and proliferation. The present article provides a complete cell atlas for the zebrafish gill and serves as a platform for future studies investigating the molecular biology and physiology of this organ.

DNA-damage induced cell death in yap1;wwtr1 mutant epidermal basal cells

In a previous study, it was reported that Yap1 and Wwtr1 in zebrafish regulates the morphogenesis of the posterior body and epidermal fin fold (Kimelman et al., 2017). We report here that DNA damage induces apoptosis of epidermal basal cells (EBCs) in zebrafish yap1^-/-;wwtr1^-/- embryos. Specifically, these mutant EBCs exhibit active Caspase-3, Caspase-8, and γH2AX, consistent with DNA damage serving as a stimulus of the extrinsic apoptotic pathway in epidermal cells. Live imaging of zebrafish epidermal cells reveals a steady growth of basal cell size in the developing embryo, but this growth is inhibited in mutant basal cells followed by apoptosis, leading to the hypothesis that factors underscoring cell size play a role in this DNA damage-induced apoptosis phenotype. We tested two of these factors using cell stretching and substrate stiffness assays, and found that HaCaT cells cultured on stiff substrates exhibit more numerous γH2AX foci compared to ones cultured on soft substrates. Thus, our experiments suggest that substrate rigidity may modulate genomic stress in epidermal cells, and that Yap1 and Wwtr1 promotes their survival.

Connexinplexity: the spatial and temporal expression of connexin genes during vertebrate organogenesis

Animal development requires coordinated communication between cells. The Connexin family of proteins is a major contributor to intercellular communication in vertebrates by forming gap junction channels that facilitate the movement of ions, small molecules, and metabolites between cells. Additionally, individual hemichannels can provide a conduit to the extracellular space for paracrine and autocrine signaling. Connexin-mediated communication is widely used in epithelial, neural, and vascular development and homeostasis, and most tissues likely use this form of communication. In fact, Connexin disruptions are of major clinical significance contributing to disorders developing from all major germ layers. Despite the fact that Connexins serve as an essential mode of cellular communication, the temporal and cell-type-specific expression patterns of connexin genes remain unknown in vertebrates. A major challenge is the large and complex connexin gene family. To overcome this barrier, we determined the expression of all connexins in zebrafish using single-cell RNA-sequencing of entire animals across several stages of organogenesis. Our analysis of expression patterns has revealed that few connexins are broadly expressed, but rather, most are expressed in tissue- or cell-type-specific patterns. Additionally, most tissues possess a unique combinatorial signature of connexin expression with dynamic temporal changes across the organism, tissue, and cell. Our analysis has identified new patterns for well-known connexins and assigned spatial and temporal expression to genes with no-existing information. We provide a field guide relating zebrafish and human connexin genes as a critical step toward understanding how Connexins contribute to cellular communication and development throughout vertebrate organogenesis.

Ontogenetic Changes in Blood Osmolality During the Postembryonic Development of Zebrafish (Danio rerio)

The zebrafish Danio rerio is a teleost model species widely used in developmental genetics, biomedical studies, toxicology, and drug screening. Despite the interest of this species in research, little is known through indirect observations about its blood osmolality, which is a key parameter for diverse experiments. In this study, we directly measured blood osmolality using nano-osmometry at different stages of zebrafish postembryonic development. We found that blood osmolality is close to 240 mOsm·kg⁻¹ in early larvae. It progressively increased to ∼270 mOsm·kg⁻¹ during the larval development before reaching ∼300 mOsm·kg⁻¹ after metamorphosis in juveniles and later in adults. These ontogenetic changes in blood osmolality illustrate the physiological changes in osmoregulation associated with postembryonic development, including metamorphosis. These values are of practical interest for adjusting the osmolality of fixatives and cell and tissue culture media for research using zebrafish as a model.

Cystic Fibrosis and the Cells of the Airway Epithelium: What Are Ionocytes and What Do They Do?

Cystic fibrosis (CF) is caused by defects in an anion channel, the cystic fibrosis transmembrane conductance regulator (CFTR). Recently, a new airway epithelial cell type has been discovered and dubbed the pulmonary ionocyte. Unexpectedly, these ionocytes express higher levels of CFTR than any other airway epithelial cell type. However, ionocytes are not the sole CFTR-expressing airway epithelial cells, and CF-associated disease genes are in fact expressed in multiple airway epithelial cell types. The experimental depletion of ionocytes perturbs epithelial physiology in the mouse trachea, but the role of these rare cells in the pathogenesis of human CF remains mysterious. Ionocytes have been described in diverse tissues(kidney and inner ear) and species (frog and fish). We draw on these prior studies to suggest potential roles of airway ionocytes in health and disease. A complete understanding of ionocytes in the mammalian airway will ultimately depend on cell type-specific genetic manipulation.

Triphenyl phosphate-induced pericardial edema is associated with elevated epidermal ionocytes within zebrafish embryos

Triphenyl phosphate (TPHP) is an organophosphate ester-based plasticizer and flame retardant. The objective of this study was to identify the potential role of epidermal ionocytes in mediating TPHP-induced pericardial edema within zebrafish embryos. Exposure to TPHP from 24 to 72 h post fertilization (hpf) resulted in a significant increase in pericardial edema and the number of ionocytes at 72 hpf relative to time-matched embryos treated with vehicle. In addition, co-exposure of embryos to mannitol (an osmotic diuretic) blocked TPHP-induced pericardial edema and effects on ionocyte abundance. However, knockdown of ATPase1a1.4 - an abundant Na⁺/K⁺-ATPase localized to epidermal ionocytes - mitigated TPHP-induced effects on ionocyte abundance but not pericardial edema, whereas co-exposure of embryos to ouabain - a Na⁺/K⁺-ATPase inhibitor - enhanced TPHP-induced pericardial edema but not ionocyte abundance. Overall, our findings suggest that TPHP may have multiple mechanisms of toxicity leading to an increase in ionocyte abundance and pericardial edema within developing zebrafish embryos.

Delta/Jagged-mediated Notch signaling induces the differentiation of agr2-positive epidermal mucous cells in zebrafish embryos

Teleosts live in aquatic habitats, where they encounter ionic and acid-base fluctuations as well as infectious pathogens. To protect from these external challenges, the teleost epidermis is composed of living cells, including keratinocytes and ionocytes that maintain body fluid ionic homeostasis, and mucous cells that secret mucus. While ionocyte progenitors are known to be specified by Delta-Notch-mediated lateral inhibition during late gastrulation and early segmentation, it remains unclear how epidermal mucous cells (EMCs) are differentiated and maintained. Here, we show that Delta/Jagged-mediated activation of Notch signaling induces the differentiation of agr2-positive (agr2⁺) EMCs in zebrafish embryos during segmentation. We demonstrated that agr2⁺ EMCs contain cytoplasmic secretory granules and express muc5.1 and muc5.2. Reductions in agr2⁺ EMC number were observed in mib mutants and notch3 MOs-injected notch1a mutants, while increases in agr2⁺ cell number were detected in notch1a- and X-Su(H)/ANK-overexpressing embryos. Treatment with γ-secretase inhibitors further revealed that Notch signaling is required during bud to 15 hpf for the differentiation of agr2⁺ EMCs. Increased agr2⁺ EMC numbers were also observed in jag1a-, jag1b-, jag2a- and dlc-overexpressing, but not jag2b-overexpressing embryos. Meanwhile, reductions in agr2⁺ EMC numbers were detected in jag1a morphants, jag1b mutants, jag2a mutants and dlc morphants, but not jag2b mutants. Reduced numbers of pvalb8-positive epidermal cells were also observed in mib or jag2a mutants and jag1a or jag1b morphants, while increased pvalb8-positive epidermal cell numbers were detected in notch1a-overexpressing, but not dlc-overexpressing embryos. BrdU labeling further revealed that the agr2⁺ EMC population is maintained by proliferation. Cell lineage experiments showed that agr2⁺ EMCs are derived from the same ectodermal precursors as keratinocytes or ionocytes. Together, our results indicate that specification of agr2⁺ EMCs in zebrafish embryos is induced by DeltaC/Jagged-dependent activation of Notch1a/3 signaling, and the cell population is maintained by proliferation.

As aquatic organisms, fish must tolerate environmental challenges that include acid-base fluctuations and water-borne pathogens. The skin provides a first line of defense against these challenges, and specific cell types in the tissue are responsible for different protective functions. For example, keratinocytes provide body coverage, ionocytes are responsible for maintaining body fluid ionic homeostasis, and epidermal mucous cells generate a protective layer of mucus that covers the entire fish surface. In this study, we uncovered the developmental process in zebrafish that underlies the generation of epidermal mucous cells. First, we characterized epidermal mucous cells according to their expression of a particular gene, agr2. Then, we found that these cells differentiate soon after ionocytes and keratinocytes, and the molecular pathways that guide differentiation of all three cell types involve similar signals. While ionocytes and keratinocytes are known to be specified by Delta-Notch-mediated lateral inhibition, we found that epidermal mucous cells are specified by activation of Notch by Delta and Jagged ligands. Thus, our results suggest that the specification of these major cell types in the epidermis occurs via a streamlined Notch-dependent process. This utilization of temporally distinct signaling events can therefore generate diverse cell types in the fish epidermis.

Knockout of mafba Causes Inner-Ear Developmental Defects in Zebrafish via the Impairment of Proliferation and Differentiation of Ionocyte Progenitor Cells

Zebrafish is an excellent model for exploring the development of the inner ear. Its inner ear has similar functions to that of humans, specifically in the maintenance of hearing and balance. Mafba is a component of the Maf transcription factor family. It participates in multiple biological processes, but its role in inner-ear development remains poorly understood. In this study, we constructed a mafba knockout (mafba^−/−) zebrafish model using CRISPR/Cas9 technology. The mafba^−/− mutant inner ear displayed severe impairments, such as enlarged otocysts, smaller or absent otoliths, and insensitivity to sound stimulation. The proliferation of p63⁺ epidermal stem cells and dlc⁺ ionocyte progenitors was inhibited in mafba^−/− mutants. Moreover, the results showed that mafba deletion induces the apoptosis of differentiated K⁺-ATPase-rich (NR) cells and H⁺-ATPase-rich (HR) cells. The activation of p53 apoptosis and G0/G1 cell cycle arrest resulted from DNA damage in the inner-ear region, providing a mechanism to account for the inner ear deficiencies. The loss of homeostasis resulting from disorders of ionocyte progenitors resulted in structural defects in the inner ear and, consequently, loss of hearing. In conclusion, the present study elucidated the function of ionic channel homeostasis and inner-ear development using a zebrafish Mafba model and clarified the possible physiological roles.

Characterization of a member of the CEACAM protein family as a novel marker of proton pump-rich ionocytes on the zebrafish epidermis

In humans, several members of the CEACAM receptor family have been shown to interact with intestinal pathogens in an inflammatory context. While CEACAMs have long been thought to be only present in mammals, recent studies have identified ceacam genes in other vertebrates, including teleosts. The function of these related genes remains however largely unknown. To gain insight into the function of CEACAM proteins in fish, we undertook the study of a putative member of the family, CEACAMz1, identified in Danio rerio. Sequence analysis of the ceacamz1 gene product predicted a GPI-anchored extracellular protein containing eleven immunoglobulin domains but revealed no evident orthology with human CEACAMs. Using a combination of RT-PCR analyses and in situ hybridization experiments, as well as a fluorescent reporter line, we showed that CEACAMz1 is first expressed in discrete cells on the ventral skin of zebrafish larvae and later on in the developing gills. This distribution remains constant until juvenile stage is reached, at which point CEACAMz1 is almost exclusively expressed in gills. We further observed that at late larval stages, CEACAMz1-expressing cells mostly localize on the afferent side of the branchial filaments and possibly in the inter-lamellar space. Using immunolabelling and 3D-reconstructions, we showed that CEACAMz1 is expressed in cells from the uppermost layer of skin epidermis. These cells are embedded within the keratinocytes pavement and we unambiguously identified them as proton-pump rich ionocytes (HR cells). As the expression of ceacamz1 is turned on concomitantly to that of other known markers of HR cells, we propose that ceacamz1 may serve as a novel marker of mature HR cells from the zebrafish epidermis.

Adaptive cell invasion maintains lateral line organ homeostasis in response to environmental changes

Mammalian inner ear and fish lateral line sensory hair cells (HCs) detect fluid motion to transduce environmental signals. Actively maintained ionic homeostasis of the mammalian inner ear endolymph is essential for HC function. In contrast, fish lateral line HCs are exposed to the fluctuating ionic composition of the aqueous environment. Using lineage labeling, in vivo time-lapse imaging and scRNA-seq, we discovered highly motile skin-derived cells that invade mature mechanosensory organs of the zebrafish lateral line and differentiate into Neuromast-associated (Nm) ionocytes. This invasion is adaptive as it is triggered by environmental fluctuations. Our discovery of Nm ionocytes challenges the notion of an entirely placodally derived lateral line and identifies Nm ionocytes as likely regulators of HC function possibly by modulating the ionic microenvironment. Nm ionocytes provide an experimentally accessible in vivo system to study cell invasion and migration, as well as the physiological adaptation of vertebrate organs to changing environmental conditions.

Py3-FITC: a new fluorescent probe for live cell imaging of collagen-rich tissues and ionocytes

Polypyrrole-based polyamides are used as sequence-specific DNA probes. However, their cellular uptake and distribution are affected by several factors and have not been extensively studied in vivo. Here, we generated a series of fluorescence-conjugated polypyrrole compounds and examined their cellular distribution using live zebrafish and cultured human cells. Among the evaluated compounds, Py3-FITC was able to visualize collagen-rich tissues, such as the jaw cartilage, opercle and bulbus arteriosus, in early-stage living zebrafish embryos. Then, we stained cultured human cells with Py3-FITC and found that the staining became more intense as the amount of collagen was increased. In addition, Py3-FITC-stained HR cells, which represent a type of ionocyte on the body surface of living zebrafish embryos. Py3-FITC has low toxicity, and collagen-rich tissues and ionocytes can be visualized when soaked in Py3-FITC solution. Therefore, Py3-FITC may be a useful live imaging tool for detecting changes in collagen-rich tissue and ionocytes, including their mammalian analogues, during both normal development and disease progression.

Foxi1 inactivation rescues loss of principal cell fate selection in Hes1-deficient kidneys but does not ensure maintenance of principal cell gene expression

The distal nephron and collecting duct segments of the mammalian kidney consist of intercalated cell types intermingled among principal cell types. Notch signaling ensures that a sufficient number of cells select a principal instead of an intercalated cell fate. However, the precise mechanisms by which Notch signaling patterns the distal nephron and collecting duct cell fates is unknown. Here we observed that Hes1, a direct target of Notch signaling pathway, is required within the mouse developing collecting ducts for repression of Foxi1 expression, an essential intercalated cell specific transcription factor. Interestingly, inactivation of Foxi1 in Hes1-deficient collecting ducts rescues the deficiency in principal cell fate selection, overall urine concentrating deficiency, and reduces the occurrence of hydronephrosis. However, Foxi1 inactivation does not rescue the reduction in expression of all principal cell genes in the Hes1-deficient kidney collecting duct cells that select the principal cell fate. Additionally, suppression of Notch/Hes1 signaling in mature principal cells reduces principal cell gene expression without activating Foxi1. We conclude that Hes1 is a Notch signaling target that is essential for normal patterning of the collecting ducts with intermingled cell types by repressing Foxi1, and for maintenance of principal cell gene expression independent of repressing Foxi1.

Expression of Protein-Coding Gene Orthologs in Zebrafish and Mouse Inner Ear Non-sensory Supporting Cells

Non-mammalian vertebrates, including zebrafish, retain the ability to regenerate hair cells (HCs) due to unknown molecular mechanisms that regulate proliferation and conversion of non-sensory supporting cells (nsSCs) to HCs. This regenerative capacity is not conserved in mammals. Identification of uniquely expressed orthologous genes in zebrafish nsSCs may reveal gene candidates involved in the proliferation and transdifferentiation of zebrafish nsSCs to HCs in the inner ear. A list of orthologous protein-coding genes was generated based on an Ensembl Biomart comparison of the zebrafish and mouse genomes. Our previously published RNA-seq-based transcriptome datasets of isolated inner ear zebrafish nsSCs and HCs, and mouse non-sensory supporting pillar and Deiters’ cells, and HCs, were merged to analyze gene expression patterns between the two species. Out of 17,498 total orthologs, 11,752 were expressed in zebrafish nsSCs and over 10,000 orthologs were expressed in mouse pillar and Deiters’ cells. Differentially expressed genes common among the zebrafish nsSCs and mouse pillar and Deiters’ cells, compared to species-specific HCs, included 306 downregulated and 314 upregulated genes; however, over 1,500 genes were uniquely upregulated in zebrafish nsSCs. Functional analysis of genes uniquely expressed in nsSCs identified several transcription factors associated with cell fate determination, cell differentiation and nervous system development, indicating inherent molecular properties of nsSCs that promote self-renewal and transdifferentiation into new HCs. Our study provides a means of characterizing these orthologous genes, involved in proliferation and transdifferentiation of nsSCs to HCs in zebrafish, which may lead to identification of potential targets for HC regeneration in mammals.

Cell-autonomous regulation of epithelial cell quiescence by calcium channel Trpv6

Epithelial homeostasis and regeneration require a pool of quiescent cells. How the quiescent cells are established and maintained is poorly understood. Here, we report that Trpv6, a cation channel responsible for epithelial Ca²⁺ absorption, functions as a key regulator of cellular quiescence. Genetic deletion and pharmacological blockade of Trpv6 promoted zebrafish epithelial cells to exit from quiescence and re-enter the cell cycle. Reintroducing Trpv6, but not its channel dead mutant, restored the quiescent state. Ca²⁺ imaging showed that Trpv6 is constitutively open in vivo. Mechanistically, Trpv6-mediated Ca²⁺ influx maintained the quiescent state by suppressing insulin-like growth factor (IGF)-mediated Akt-Tor and Erk signaling. In zebrafish epithelia and human colon carcinoma cells, Trpv6/TRPV6 elevated intracellular Ca²⁺ levels and activated PP2A, which down-regulated IGF signaling and promoted the quiescent state. Our findings suggest that Trpv6 mediates constitutive Ca²⁺ influx into epithelial cells to continuously suppress growth factor signaling and maintain the quiescent state.

Xenobiotic transporter activity in zebrafish embryo ionocytes

Ionocytes are specialized cells in the epidermis of embryonic zebrafish (Danio rerio) that play important roles in ion homeostasis and have functional similarities to mammalian renal cells. Here, we examined whether these cells might also share another functional similarity with renal cells, which is the presence of efflux transporter activities useful for elimination of toxic small molecules. Xenobiotic transporters (XTs), including the ATP-Binding Cassette (ABC) family, are a major defense mechanism against diffusible toxic molecules in aquatic embryos, including zebrafish, but their activity in the ionocytes has not previously been studied. Using fluorescent small molecule substrates of XT, we observed that specific populations of ionocytes uptake and efflux fluorescent small molecules in a manner consistent with active transport. We specifically identified a P-gp/ABCB1 inhibitor-sensitive efflux activity in the H⁺-ATPase-rich (HR) ionocytes, and show that these cells exhibit enriched expression of the ABCB gene, abcb5. The results extend our understanding of the functional significance of zebrafish ionocytes and indicate that these cells could play an important role in protection of the fish embryo from harmful small molecules.

Zebrafish Klf4 maintains the ionocyte progenitor population by regulating epidermal stem cell proliferation and lateral inhibition

In the skin and gill epidermis of fish, ionocytes develop alongside keratinocytes and maintain body fluid ionic homeostasis that is essential for adaptation to environmental fluctuations. It is known that ionocyte progenitors in zebrafish embryos are specified from p63+ epidermal stem cells through a patterning process involving DeltaC (Dlc)-Notch-mediated lateral inhibition, which selects scattered dlc+ cells into the ionocyte progenitor fate. However, mechanisms by which the ionocyte progenitor population is modulated remain unclear. Krüppel-like factor 4 (Klf4) transcription factor was previously implicated in the terminal differentiation of mammalian skin epidermis and is known for its bifunctional regulation of cell proliferation in a tissue context-dependent manner. Here, we report novel roles for zebrafish Klf4 in the ventral ectoderm during embryonic skin development. We found that Klf4 was expressed in p63+ epidermal stem cells of the ventral ectoderm from 90% epiboly onward. Knockdown or knockout of klf4 expression reduced the proliferation rate of p63+ stem cells, resulting in decreased numbers of p63+ stem cells, dlc-p63+ keratinocyte progenitors and dlc+ p63+ ionocyte progenitor cells. These reductions subsequently led to diminished keratinocyte and ionocyte densities and resulted from upregulation of the well-known cell cycle regulators, p53 and cdkn1a/p21. Moreover, mutation analyses of the KLF motif in the dlc promoter, combined with VP16-klf4 or engrailed-klf4 mRNA overexpression analyses, showed that Klf4 can bind the dlc promoter and modulate lateral inhibition by directly repressing dlc expression. This idea was further supported by observing the lateral inhibition outcomes in klf4-overexpressing or knockdown embryos. Overall, our experiments delineate novel roles for zebrafish Klf4 in regulating the ionocyte progenitor population throughout early stem cell stage to initiation of terminal differentiation, which is dependent on Dlc-Notch-mediated lateral inhibition.

Stem cell proliferation is induced by apoptotic bodies from dying cells during epithelial tissue maintenance

Epithelial tissues require the removal and replacement of damaged cells to sustain a functional barrier. Dying cells provide instructive cues that can influence surrounding cells to proliferate, but how these signals are transmitted to their healthy neighbors to control cellular behaviors during tissue homeostasis remains poorly understood. Here we show that dying stem cells facilitate communication with adjacent stem cells by caspase-dependent production of Wnt8a-containing apoptotic bodies to drive cellular turnover in living epithelia. Basal stem cells engulf apoptotic bodies, activate Wnt signaling, and are stimulated to divide to maintain tissue-wide cell numbers. Inhibition of either cell death or Wnt signaling eliminated the apoptosis-induced cell division, while overexpression of Wnt8a signaling combined with induced cell death led to an expansion of the stem cell population. We conclude that ingestion of apoptotic bodies represents a regulatory mechanism linking death and division to maintain overall stem cell numbers and epithelial tissue homeostasis.

Damaged epithelial tissues are known to compensate for cell death through compensatory cell divisions to maintain epithelial integrity. Here, the authors show in living epithelia that dying cells stimulate adjacent stem cells to divide through caspase-dependent production of Wnt8a-containing apoptotic bodies.

Tuberous sclerosis complex exhibits a new renal cystogenic mechanism

Tuberous sclerosis complex (TSC) is a tumor predisposition syndrome with significant renal cystic and solid tumor disease. While the most common renal tumor in TSC, the angiomyolipoma, exhibits a loss of heterozygosity associated with disease, we have discovered that the renal cystic epithelium is composed of type A intercalated cells that have an intact Tsc gene that have been induced to exhibit Tsc-mutant disease phenotype. This mechanism appears to be different than that for ADPKD. The murine models described here closely resemble the human disease and both appear to be mTORC1 inhibitor responsive. The induction signaling driving cystogenesis may be mediated by extracellular vesicle trafficking.

Endogenous Notch Signaling in Adult Kidneys Maintains Segment-Specific Epithelial Cell Types of the Distal Tubules and Collecting Ducts to Ensure Water Homeostasis

BACKGROUND

Notch signaling is required during kidney development for nephron formation and principal cell fate selection within the collecting ducts. Whether Notch signaling is required in the adult kidney to maintain epithelial diversity, or whether its loss can trigger principal cell transdifferentiation (which could explain acquired diabetes insipidus in patients receiving lithium) is unclear.

METHODS

To investigate whether loss of Notch signaling can trigger principal cells to lose their identity, we genetically inactivated Notch1 and Notch2, inactivated the Notch signaling target Hes1, or induced expression of a Notch signaling inhibitor in all of the nephron segments and collecting ducts in mice after kidney development. We examined renal function and cell type composition of control littermates and mice with conditional Notch signaling inactivation in adult renal epithelia. In addition, we traced the fate of genetically labeled adult kidney collecting duct principal cells after Hes1 inactivation or lithium treatment.

RESULTS

Notch signaling was required for maintenance of Aqp2-expressing cells in distal nephron and collecting duct segments in adult kidneys. Fate tracing revealed mature principal cells in the inner stripe of the outer medulla converted to intercalated cells after genetic inactivation of Hes1 and, to a lesser extent, lithium treatment. Hes1 ensured repression of Foxi1 to prevent the intercalated cell program from turning on in mature Aqp2⁺ cell types.

CONCLUSIONS

Notch signaling via Hes1 regulates maintenance of mature renal epithelial cell states. Loss of Notch signaling or use of lithium can trigger transdifferentiation of mature principal cells to intercalated cells in adult kidneys.

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.

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.

Development and Diseases of the Collecting Duct System

The collecting duct of the mammalian kidney is important for the regulation of extracellular volume, osmolarity, and pH. There are two major structurally and functionally distinct cell types: principal cells and intercalated cells. The former regulates Na⁺ and water homeostasis, while the latter participates in acid-base homeostasis. In vivo lineage tracing using Cre recombinase or its derivatives such as CreGFP and CreER^T2 is a powerful new technique to identify stem/progenitor cells in their native environment and to decipher the origins of the tissue that they give rise to. Recent studies using this technique in mice have revealed multiple renal progenitor cell populations that differentiate into various nephron segments and collecting duct. In particular, emerging evidence suggests that like principal cells, most of intercalated cells originate from the progenitor cells expressing water channel Aquaporin 2. Mutations or malfunctions of the channels, pumps, and transporters expressed in the collecting duct system cause various human diseases. For example, gain-of-function mutations in ENaC cause Liddle's syndrome, while loss-of-function mutations in ENaC lead to Pseudohypoaldosteronism type 1. Mutations in either AE1 or V-ATPase B1 result in distal renal tubular acidosis. Patients with disrupted AQP2 or AVPR2 develop nephrogenic diabetes insipidus. A better understanding of the function and development of the collecting duct system may facilitate the discovery of new therapeutic strategies for treating kidney disease.

Elf5 is a principal cell lineage specific transcription factor in the kidney that contributes to Aqp2 and Avpr2 gene expression

The mammalian kidney collecting ducts are critical for water, electrolyte and acid-base homeostasis and develop as a branched network of tubular structures composed of principal cells intermingled with intercalated cells. The intermingled nature of the different collecting duct cell types has made it challenging to identify unique and critical factors that mark and/or regulate the development of the different collecting duct cell lineages. Here we report that the canonical Notch signaling pathway components, RBPJ and Presinilin1 and 2, are involved in patterning the mouse collecting duct cell fates by maintaining a balance between principal cell and intercalated cell fates. The relatively reduced number of principal cells in Notch-signaling-deficient kidneys offered a unique genetic leverage to identify critical principal cell-enriched factors by transcriptional profiling. Elf5, which codes for an ETS transcription factor, is one such gene that is down-regulated in kidneys with Notch-signaling-deficient collecting ducts. Additionally, Elf5 is among the earliest genes up regulated by ectopic expression of activated Notch1 in the developing collecting ducts. In the kidney, Elf5 is first expressed early within developing collecting ducts and remains on in mature principal cells. Lineage tracing of Elf5-expressing cells revealed that they are committed to the principal cell lineage by as early as E16.5. Over-expression of ETS Class IIa transcription factors, including Elf5, Elf3 and Ehf, increase the transcriptional activity of the proximal promoters of Aqp2 and Avpr2 in cultured ureteric duct cell lines. Conditional inactivation of Elf5 in the developing collecting ducts results in a small but significant reduction in the expression levels of Aqp2 and Avpr2 genes. We have identified Elf5 as an early maker of the principal cell lineage that contributes to the expression of principal cell specific genes.

The transcriptome of metamorphosing flatfish

BACKGROUND

Flatfish metamorphosis denotes the extraordinary transformation of a symmetric pelagic larva into an asymmetric benthic juvenile. Metamorphosis in vertebrates is driven by thyroid hormones (THs), but how they orchestrate the cellular, morphological and functional modifications associated with maturation to juvenile/adult states in flatfish is an enigma. Since THs act via thyroid receptors that are ligand activated transcription factors, we hypothesized that the maturation of tissues during metamorphosis should be preceded by significant modifications in the transcriptome. Targeting the unique metamorphosis of flatfish and taking advantage of the large size of Atlantic halibut (Hippoglossus hippoglossus) larvae, we determined the molecular basis of TH action using RNA sequencing.

RESULTS

De novo assembly of sequences for larval head, skin and gastrointestinal tract (GI-tract) yielded 90,676, 65,530 and 38,426 contigs, respectively. More than 57 % of the assembled sequences were successfully annotated using a multi-step Blast approach. A unique set of biological processes and candidate genes were identified specifically associated with changes in morphology and function of the head, skin and GI-tract. Transcriptome dynamics during metamorphosis were mapped with SOLiD sequencing of whole larvae and revealed greater than 8,000 differentially expressed (DE) genes significantly (p < 0.05) up- or down-regulated in comparison with the juvenile stage. Candidate transcripts quantified by SOLiD and qPCR analysis were significantly (r = 0.843; p < 0.05) correlated. The majority (98 %) of DE genes during metamorphosis were not TH-responsive. TH-responsive transcripts clustered into 6 groups based on their expression pattern during metamorphosis and the majority of the 145 DE TH-responsive genes were down-regulated.

CONCLUSIONS

A transcriptome resource has been generated for metamorphosing Atlantic halibut and over 8,000 DE transcripts per stage were identified. Unique sets of biological processes and candidate genes were associated with changes in the head, skin and GI-tract during metamorphosis. A small proportion of DE transcripts were TH-responsive, suggesting that they trigger gene networks, signalling cascades and transcription factors, leading to the overt changes in tissue occurring during metamorphosis.

A toolbox to study epidermal cell types in zebrafish

Epithelia provide a crucial protective barrier for our organs and are also the sites where the majority of carcinomas form. Most studies on epithelia and carcinomas use cell culture or organisms where high-resolution live imaging is inaccessible without invasive techniques. Here, we introduce the developing zebrafish epidermis as an excellent in vivo model system for studying a living epithelium. We developed tools to fluorescently tag specific epithelial cell types and express genes in a mosaic fashion using five Gal4 lines identified from an enhancer trap screen. When crossed to a variety of UAS effector lines, we can now track, ablate or monitor single cells at sub-cellular resolution. Using photo-cleavable morpholino oligonucleotides that target gal4, we can also express genes in a mosaic fashion at specific times during development. Together, this system provides an excellent in vivo alternative to tissue culture cells, without the intrinsic concerns of culture conditions or transformation, and enables the investigation of distinct cell types within living epithelial tissues.

Phenotype anchoring in zebrafish reveals a potential role for matrix metalloproteinases (MMPs) in tamoxifen's effects on skin epithelium

The zebrafish is a powerful alternative model used to link phenotypes with molecular effects to discover drug mode of action. Using a zebrafish embryo-larval toxicity bioassay, we evaluated the effects of tamoxifen--a widely used anti-estrogen chemotherapeutic. Zebrafish exposed to ≥ 10 μM tamoxifen exhibited a unique necrotic caudal fin phenotype that was rapidly induced regardless of developmental life-stage when treatment was applied. To define tamoxifen's bioactivity resulting in this phenotype, targeted gene expression was used to evaluate 100 transcripts involved in tissue remodeling, calcium signaling, cell cycle and cell death, growth factors, angiogenesis and hypoxia. The most robustly misregulated transcripts in the tail were matrix metalloproteinases mmp9 and mmp13a, induced 127 and 1145 fold, respectively. Expression of c-fos, c-jun, and ap1s1 were also moderately elevated (3-7 fold), consistent with AP-1 activity--a transcription factor that regulates MMP expression. Immunohistochemistry confirmed high levels of induction for MMP13a in affected caudal fin skin epithelial tissue. The necrotic caudal fin phenotype was significantly attenuated or prevented by three functionally unique MMP inhibitors: EDTA (metal chelator), GM 6001 (broad MMP inhibitor), and SR 11302 (AP-1 transcription factor inhibitor), suggesting MMP-dependence. SR 11302 also inhibited induction of mmp9, mmp13a, and a putative MMP target, igfbp1a. Overall, our studies suggest that tamoxifen's effect is the result of perturbation of the MMP system in the skin leading to ectopic expression, cytotoxicity, and the necrotic caudal fin phenotype. These studies help advance our understanding of tamoxifen's non-classical mode of action and implicate a possible role for MMPs in tissues such as skin.

Recent advances in understanding trans-epithelial acid-base regulation and excretion mechanisms in cephalopods

Cephalopods have evolved complex sensory systems and an active lifestyle to compete with fish for similar resources in the marine environment. Their highly active lifestyle and their extensive protein metabolism has led to substantial acid-base regulatory abilities enabling these organisms to cope with CO2 induced acid-base disturbances. In convergence to teleost, cephalopods possess an ontogeny-dependent shift in ion-regulatory epithelia with epidermal ionocytes being the major site of embryonic acid-base regulation and ammonia excretion, while gill epithelia take these functions in adults. Although the basic morphology and excretory function of gill epithelia in cephalopods were outlined almost half a century ago, modern immunohistological and molecular techniques are bringing new insights to the mechanistic basis of acid-base regulation and excretion of nitrogenous waste products (e.g. NH3/NH4 (+)) across ion regulatory epithelia of cephalopods. Using cephalopods as an invertebrate model, recent findings reveal partly conserved mechanisms but also novel aspects of acid-base regulation and nitrogen excretion in these exclusively marine animals. Comparative studies using a range of marine invertebrates will create a novel and exciting research direction addressing the evolution of pH regulatory and excretory systems.

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.

Osmoregulation in zebrafish: ion transport mechanisms and functional regulation

Fish, like mammals, have to maintain their body fluid ionic and osmotic homeostasis through sophisticated iono-/osmoregulation mechanisms, which are conducted mainly by ionocytes of the gill (the skin in embryonic stages), instead of the renal tubular cells in mammals. Given the advantages in terms of genetic database availability and manipulation, zebrafish is an emerging model for research into regulatory and integrative physiology. At least five types of ionocytes, HR, NaR, NCC, SLC26, and KS cells, have been identified to carry out Na(+) uptake/H(+) secretion/NH4 (+) excretion, Ca(2+) uptake, Na(+)/Cl(-) uptake, K(+) secretion, and Cl(-) uptake/HCO3 (-) secretion, respectively, through distinct sets of transporters. Several hormones, namely isotocin, prolactin, cortisol, stanniocalcin-1, calcitonin, endothelin-1, vitamin D, parathyorid hormone 1, catecholamines, and the renin-angiotensin-system, have been demonstrated to positively or negatively regulate ion transport through specific receptors at different ionocytes stages, at either the transcriptional/translational or posttranslational level. The knowledge obtained using zebrafish answered many long-term contentious or unknown issues in the field of fish iono-/osmoregulation. The homology of ion transport pathways and hormone systems also means that the zebrafish model informs studies on mammals or other animal species, thereby providing insights into related fields.

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.

Stanniocalcin-1 controls ion regulation functions of ion-transporting epithelium other than calcium balance

Stanniocalcin-1 (STC-1) was first identified to involve in Ca(2+) homeostasis in teleosts, and was thought to act as a hypocalcemic hormone in vertebrate. Recent studies suggested that STC-1 exhibits broad effects on ion balance, not confines to Ca(2+), but the mechanism of this regulation process remains largely unknown. Here, we used zebrafish embryos as an alternative in vivo model to investigate how STC-1 regulates transepithelial ion transport function in ion-transporting epithelium. Expression of stc-1 mRNA in zebrafish embryos was increased in high-Ca(2+) environments but decreased by acidic and ion-deficient treatments while overexpression of stc-1 impaired the hypotonic acclimation by decreasing whole body Ca(2+), Na(+), and Cl(-) contents and H(+) secretion ability. Injection of STC-1 mRNA also down-regulated mRNA expressions of epithelial Ca(2+) channel, H(+)-ATPase, and Na(+)-Cl(-) cotransporter, suggesting the roles of STC-1 in regulation of ions other than Ca(2+). Knockdown of STC-1 caused an increase in ionocyte progenitors (foxi3a as the marker) and mature ionocytes (ion transporters as the markers), but did not affect epithelium stem cells (p63 as the marker) in the embryonic skin. Overexpression of STC-1 had the corresponding opposite effect on ionocyte progenitors, mature ionocytes in the embryonic skin. Taken together, STC-1 negatively regulates the number of ionocytes to reduce ionocyte functions. This process is important for body fluid ionic homeostasis, which is achieved by the regulation of ion transport functions in ionocytes. The present findings provide new insights into the broader functions of STC-1, a hypocalcemic hormone.

Renal acid-base regulation: new insights from animal models

Because majority of biological processes are dependent on pH, maintaining systemic acid-base balance is critical. The kidney contributes to systemic acid-base regulation, by reabsorbing HCO3 (-) (both filtered by glomeruli and generated within a nephron) and acidifying urine. Abnormalities in those processes will eventually lead to a disruption in systemic acid-base balance and provoke metabolic acid-base disorders. Research over the past 30 years advanced our understanding on cellular and molecular mechanisms responsible for those processes. In particular, a variety of transgenic animal models, where target genes are deleted either globally or conditionally, provided significant insights into how specific transporters are contributing to the renal acid-base regulation. Here, we broadly overview the mechanisms of renal ion transport participating to acid-base regulation, with emphasis on data obtained from transgenic mice models.

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.

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

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

A role for transcription factor glial cell missing 2 in Ca2+ homeostasis in zebrafish, Danio rerio

The present study investigated the role of the transcription factor, glial cell missing 2 (gcm2), in Ca(2+) regulation in zebrafish larvae. Translational gene knockdown of gcm2 decreased Ca(2+) uptake and the density of ionocytes expressing the epithelial Ca(2+) channel (ecac), and disrupted the overall Ca(2+) balance. Ca(2+) uptake and the expression of gcm2 messenger RNA (mRNA) were significantly elevated in larvae acclimated to low Ca(2+) water (25 μM); the stimulation of Ca(2+) uptake was not observed in fish experiencing gcm2 knockdown. Acclimation to acidic water (pH 4) significantly reduced whole-body Ca(2+) content owing to reduced Ca(2+) uptake and increased Ca(2+) efflux. However, ecac mRNA levels and the density of ecac-expressing ionocytes were increased in fish acclimated to acidic water, and maximal Ca(2+) uptake capacity (J MAX) was significantly increased when measured in control water (pH ~7.4). Acclimation of larvae to acidic water significantly increased gcm2 mRNA expression, and in gcm2 morphants, no such stimulation in Ca(2+) uptake was observed after their return to control water. Overexpression of gcm2 mRNA resulted in a significant increase in the numbers of ecac-expressing ionocytes and Ca(2+) uptake. These observations reveal a critical role for gcm2 in Ca(2+) homeostasis in zebrafish larvae.

The physiology of fish at low pH: the zebrafish as a model system

Ionic regulation and acid–base balance are fundamental to the physiology of vertebrates including fish. Acidification of freshwater ecosystems is recognized as a global environmental problem, and the physiological responses to acid exposure in a few fish species are well characterized. However, the underlying mechanisms promoting ionic and acid–base balance for most fish species that have been investigated remain unclear. Zebrafish (Danio rerio) has emerged as a powerful model system to elucidate the molecular basis of ionic and acid–base regulation. The utility of zebrafish is related to the ease with which it can be genetically manipulated, its suitability for state-of-the-art molecular and cellular approaches, and its tolerance to diverse environmental conditions. Recent studies have identified several key regulatory mechanisms enabling acclimation of zebrafish to acidic environments, including activation of the sodium/hydrogen exchanger (NHE) and H+-ATPase for acid secretion and Na+ uptake, cortisol-mediated regulation of transcellular and paracellular Na+ movements, and ionocyte proliferation controlled by specific cell-fate transcription factors. These integrated physiological responses ultimately contribute to ionic and acid–base homeostasis in zebrafish exposed to acidic water. In the present review, we provide an overview of the general effects of acid exposure on freshwater fish, the adaptive mechanisms promoting extreme acid tolerance in fishes native to acidic environments, and the mechanisms regulating ionic and acid–base balance during acid exposure in zebrafish.

Ephrin-B2 governs morphogenesis of endolymphatic sac and duct epithelia in the mouse inner ear

Control over ionic composition and volume of the inner ear luminal fluid endolymph is essential for normal hearing and balance. Mice deficient in either the EphB2 receptor tyrosine kinase or the cognate transmembrane ligand ephrin-B2 (Efnb2) exhibit background strain-specific vestibular-behavioral dysfunction and signs of abnormal endolymph homeostasis. Using various loss-of-function mouse models, we found that Efnb2 is required for growth and morphogenesis of the embryonic endolymphatic epithelium, a precursor of the endolymphatic sac (ES) and duct (ED), which mediate endolymph homeostasis. Conditional inactivation of Efnb2 in early-stage embryonic ear tissues disrupted cell proliferation, cell survival, and epithelial folding at the origin of the endolymphatic epithelium. This correlated with apparent absence of an ED, mis-localization of ES ion transport cells relative to inner ear sensory organs, dysplasia of the endolymph fluid space, and abnormally formed otoconia (extracellular calcite-protein composites) at later stages of embryonic development. A comparison of Efnb2 and Notch signaling-deficient mutant phenotypes indicated that these two signaling systems have distinct and non-overlapping roles in ES/ED development. Homozygous deletion of the Efnb2 C-terminus caused abnormalities similar to those found in the conditional Efnb2 null homozygote. Analyses of fetal Efnb2 C-terminus deletion heterozygotes found mis-localized ES ion transport cells only in the genetic background exhibiting vestibular dysfunction. We propose that developmental dysplasias described here are a gene dose-sensitive cause of the vestibular dysfunction observed in EphB-Efnb2 signaling-deficient mice.

Basal keratinocytes contribute to all strata of the adult zebrafish epidermis

The epidermis of terrestrial vertebrates is a stratified epithelium and forms an essential protective barrier. It is continually renewed, with dead corneocytes shed from the surface and replaced from a basal keratinocyte stem cell population. Whilst mouse is the prime model system used for epidermal studies, there is increasing employment of the zebrafish to analyse epidermis development and homeostasis, however the architecture and ontogeny of the epidermis in this system are incompletely described. In particular, it is unclear if adult zebrafish epidermis is derived entirely from the basal epidermal stem cell layer, as in the mouse, or if the most superficial keratinocyte layer is a remnant of the embryonic periderm. Furthermore, a relative paucity of cellular markers and genetic reagents to label and manipulate the basal epidermal stem cell compartment has hampered research. Here we show that the type I keratin, krtt1c19e, is a suitable marker of the basal epidermal layer and identify a krtt1c19e promoter fragment able to drive strong and specific expression in this cell type. Use of this promoter to express an inducible Cre recombinase allowed permanent labelling of basal cells during embryogenesis, and demonstrated that these cells do indeed generate keratinocytes of all strata in the adult epidermis. Further deployment of the Cre-Lox system highlighted the transient nature of the embryonic periderm. We thus show that the epidermis of adult zebrafish, as in the mouse, derives from basal stem cells, further expanding the similarities of epidermal ontogeny across vertebrates. Future use of this promoter will assist genetic analysis of basal keratinocyte biology in zebrafish.

Origin and differentiation of ionocytes in gill epithelium of teleost fish

This paper focuses on the environmental cues that transform the gills of euryhaline teleost fish from an oxygen exchange structure into a bifunctional organ that can control both gaseous movement and water/ion transport. The cellular development that allows this structure to accomplish these tasks begins shortly after fertilization of the egg. It involves alterations of structure and function of embryonic cells [ionoblasts (IB)] that are shed from the pharyngeal anlage area of the embryo. These IB contain unique protein-receptor domains in the plasma membrane. These receptors respond specifically to the environmental cues effecting a calcium-binding protein receptor [calcium-sensing receptor (CaSR)]. The CaSR containing IB act as stem cells and are acted upon by isotocin, a heteroprotein regulator which induces them to form progenitor ionocytes (pIC). The pIC form two types of cells. The first type becomes an aquaphilic ionocyte which regulates uptake of ions and through aquaporin molecules transports water out of the cell and controls body fluids of the fish. This mechanism is essential for freshwater living. The second type becomes a halophilic ionocyte and transports ions out of the cell and controls cell shrinkage by uptake of water via aquaporin molecules. This mechanism is essential for seawater living. These differentiating events in the pIC are controlled by the cross talking of genomic mechanisms found in the precursor IB. To unravel the cross talking events it is necessary to uncover how these genetic pathways are regulated by transcriptional and translational events coming from complementary DNA. Various gene families are involved such as those found in apoptosis mechanisms, regulatory volume regulators and ionic transport systems (cystic fibrosis transmembrane conductance regulator).