Prox1 is a novel coregulator of Ff1b and is involved in the embryonic development of the zebra fish interrenal primordium

Modeling Podocyte Ontogeny and Podocytopathies with the Zebrafish

Podocytes are exquisitely fashioned kidney cells that serve an essential role in the process of blood filtration. Congenital malformation or damage to podocytes has dire consequences and initiates a cascade of pathological changes leading to renal disease states known as podocytopathies. In addition, animal models have been integral to discovering the molecular pathways that direct the development of podocytes. In this review, we explore how researchers have used the zebrafish to illuminate new insights about the processes of podocyte ontogeny, model podocytopathies, and create opportunities to discover future therapies.

Interrenal development and function in zebrafish

In this article we aim to provide an overview of the zebrafish interrenal development and function, as well as a review of its contribution to basic and translational research. A search of the PubMed database identified 41 relevant papers published over the last 20 years. Based on the common themes identified, we discuss the organogenesis of the interrenal gland and its functional development and we review what is known about the genes involved in zebrafish steroidogenesis. We also outline the consequences of specific defects in steroid biosynthesis, as revealed by evidence from genetically engineered zebrafish models, including cyp11a2, cyp21a2, hsd3b1, cyp11c1 and fdx1b deficiency. Finally, we summarise the impact of different chemicals and environmental factors on steroidogenesis. Our review highlights the utility of zebrafish as a research model for exploring important areas of basic science and human disease, especially in the current context of rapid technological progress in the field of Molecular Biology.

A fish with no sex: gonadal and adrenal functions partition between zebrafish NR5A1 co-orthologs

People with NR5A1 mutations experience testicular dysgenesis, ovotestes, or adrenal insufficiency, but we do not completely understand the origin of this phenotypic diversity. NR5A1 is expressed in gonadal soma precursor cells before expression of the sex-determining gene SRY. Many fish have two co-orthologs of NR5A1 that likely partitioned ancestral gene subfunctions between them. To explore ancestral roles of NR5A1, we knocked out nr5a1a and nr5a1b in zebrafish. Single-cell RNA-seq identified nr5a1a-expressing cells that co-expressed genes for steroid biosynthesis and the chemokine receptor Cxcl12a in 1-day postfertilization (dpf) embryos, as does the mammalian adrenal-gonadal (interrenal-gonadal) primordium. In 2dpf embryos, nr5a1a was expressed stronger in the interrenal-gonadal primordium than in the early hypothalamus but nr5a1b showed the reverse. Adult Leydig cells expressed both ohnologs and granulosa cells expressed nr5a1a stronger than nr5a1b. Mutants for nr5a1a lacked the interrenal, formed incompletely differentiated testes, had no Leydig cells, and grew far larger than normal fish. Mutants for nr5a1b formed a disorganized interrenal and their gonads completely disappeared. All homozygous mutant genotypes lacked secondary sex characteristics, including male breeding tubercles and female sex papillae, and had exceedingly low levels of estradiol, 11-ketotestosterone, and cortisol. RNA-seq showed that at 21dpf, some animals were developing as females and others were not, independent of nr5a1 genotype. By 35dpf, all mutant genotypes greatly under-expressed ovary-biased genes. Because adult nr5a1a mutants form gonads but lack an interrenal and conversely, adult nr5a1b mutants lack a gonad but have an interrenal, the adrenal, and gonadal functions of the ancestral nr5a1 gene partitioned between ohnologs after the teleost genome duplication, likely owing to reciprocal loss of ancestral tissue-specific regulatory elements. Identifying such elements could provide hints to otherwise unexplained cases of Differences in Sex Development.

Disodium Cromolyn and Anti-podoplanin Antibodies Strongly Inhibit Growth of BHK 21/C13-derived Fibrosarcoma in a Chick Embryo Chorioallantoic Membrane Model

AIM

To characterize baby hamster kidney fibroblast (BHK 21/C13) cells and test the effects of antibodies against podoplanin and disodium cromolyn on BHK 21/C13 cell line-derived tumors grown on chick embryo chorioallantoic membrane (CAM).

MATERIAL AND METHODS

BHK 21/C13 cell-derived fibrosarcomas developed in hamsters were implanted on CAM and treated with anti-podoplanin antibodies and disodium cromolyn. BHK 21/C13 cell immunophenotype was assessed.

RESULTS

Fibrosarcoma cells were positive for vimentin, CD117, smooth muscle actin, vascular endothelial growth factor epidermal growth factor receptor, homebox prospero gene 1 and negative for platelet-derived growth factor B, neuron-specific enolase, S100, CD34, Ewing sarcoma and podoplanin. CAM-grown fibrosarcomas were highly sensitive to disodium cromolyn and anti-podoplanin antibodies.

CONCLUSION

Immunophenotyping BHK 21/C13 cells and their response to drugs represent the first step in revealing cell line utility and a reliable tool for experimental cancer research.

Disodium Cromolyn and Anti-podoplanin Antibodies Strongly Inhibit Growth of BHK 21/C13-derived Fibrosarcoma in a Chick Embryo Chorioallantoic Membrane Model

AIM

To characterize baby hamster kidney fibroblast (BHK 21/C13) cells and test the effects of antibodies against podoplanin and disodium cromolyn on BHK 21/C13 cell line-derived tumors grown on chick embryo chorioallantoic membrane (CAM).

MATERIAL AND METHODS

BHK 21/C13 cell-derived fibrosarcomas developed in hamsters were implanted on CAM and treated with anti-podoplanin antibodies and disodium cromolyn. BHK 21/C13 cell immunophenotype was assessed.

RESULTS

Fibrosarcoma cells were positive for vimentin, CD117, smooth muscle actin, vascular endothelial growth factor epidermal growth factor receptor, homebox prospero gene 1 and negative for platelet-derived growth factor B, neuron-specific enolase, S100, CD34, Ewing sarcoma and podoplanin. CAM-grown fibrosarcomas were highly sensitive to disodium cromolyn and anti-podoplanin antibodies.

CONCLUSION

Immunophenotyping BHK 21/C13 cells and their response to drugs represent the first step in revealing cell line utility and a reliable tool for experimental cancer research.

Visualizing the Interrenal Steroidogenic Tissue and Its Vascular Microenvironment in Zebrafish

This protocol introduces how to detect differentiated interrenal steroidogenic cells through a simple whole-mount enzymatic activity assay. Identifying differentiated steroidogenic tissues through chromogenic histochemical staining of 3-β-Hydroxysteroid dehydrogenase /Δ^5-4 isomerase (3β-Hsd) activity-positive cells is critical for monitoring the morphology and differentiation of adrenocortical and interrenal tissues in mammals and teleosts, respectively. In the zebrafish model, the optical transparency and tissue permeability of the developing embryos and larvae allow for whole-mount staining of 3β-Hsd activity. This staining protocol, as performed on transgenic fluorescent reporter lines marking the developing pronephric and endothelial cells, enables the detection of the steroidogenic interrenal tissue in addition to the kidney and neighboring vasculature. In combination with vibratome sectioning, immunohistochemistry, and confocal microscopy, we can visualize and assay the vascular microenvironment of interrenal steroidogenic tissues. The 3β-Hsd activity assay is essential for studying the cell biology of the zebrafish interrenal gland because to date, no suitable antibody is available for labeling zebrafish steroidogenic cells. Furthermore, this assay is rapid and simple, thus providing a powerful tool for mutant screens targeting adrenal (interrenal) genetic disorders as well as for determining disruption effects of chemicals on steroidogenesis in pharmaceutical or toxicological studies.

The endoderm indirectly influences morphogenetic movements of the zebrafish head kidney through the posterior cardinal vein and VegfC

Integration of blood vessels and organ primordia determines organ shape and function. The head kidney in the zebrafish interacts with the dorsal aorta (DA) and the posterior cardinal vein (PCV) to achieve glomerular filtration and definitive hematopoiesis, respectively. How the head kidney co-develops with both the axial artery and vein remains unclear. We found that in endodermless sox32-deficient embryos, the head kidney associated with the PCV but not the DA. Disrupted convergent migration of the PCV and the head kidney in sox32-deficient embryos was rescued in a highly coordinated fashion through the restoration of endodermal cells. Moreover, grafted endodermal cells abutted the host PCV endothelium in the transplantation assay. Interestingly, the severely-disrupted head kidney convergence in the sox32-deficient embryo was suppressed by both the cloche mutation and the knockdown of endothelial genes, indicating that an interaction between the endoderm and the PCV restricts the migration of the head kidney. Furthermore, knockdown of either vegfC or its receptor vegfr3 suppressed the head kidney convergence defect in endodermless embryos and perturbed the head kidney-PCV association in wild-type embryos. Our findings thus underscore a role for PCV and VegfC in patterning the head kidney prior to organ assembly and function.

Studying the peripheral sympathetic nervous system and neuroblastoma in zebrafish

The zebrafish serves as an excellent model to study vertebrate development and disease. Optically clear embryos, combined with tissue-specific fluorescent reporters, permit direct visualization and measurement of peripheral nervous system formation in real time. Additionally, the model is amenable to rapid cellular, molecular, and genetic approaches to determine how developmental mechanisms contribute to disease states, such as cancer. In this chapter, we describe the development of the peripheral sympathetic nervous system (PSNS) in general, and our current understanding of genetic pathways important in zebrafish PSNS development specifically. We also illustrate how zebrafish genetics is used to identify new mechanisms controlling PSNS development and methods for interrogating the potential role of PSNS developmental pathways in neuroblastoma pathogenesis in vivo using the zebrafish MYCN-driven neuroblastoma model.

Dedifferentiation of neurons precedes tumor formation in Lola mutants

The ability to reprogram differentiated cells into a pluripotent state has revealed that the differentiated state is plastic and reversible. It is evident, therefore, that mechanisms must be in place to maintain cells in a differentiated state. Transcription factors that specify neuronal characteristics have been well studied, but less is known about the mechanisms that prevent neurons from dedifferentiating to a multipotent, stem cell-like state. Here, we identify Lola as a transcription factor that is required to maintain neurons in a differentiated state. We show that Lola represses neural stem cell genes and cell-cycle genes in postmitotic neurons. In lola mutants, neurons dedifferentiate, turn on neural stem cell genes, and begin to divide, forming tumors. Thus, neurons rather than stem cells or intermediate progenitors are the tumor-initiating cells in lola mutants.

Zebrafish yap1 plays a role in differentiation of hair cells in posterior lateral line

The evolutionarily conserved Hippo signaling pathway controls organ size by regulating cell proliferation and apoptosis and this process involves Yap1. The zebrafish Yap1 acts during neural differentiation, but its function is not fully understood. The detailed analysis of yap1 expression in proliferative regions, revealed it in the otic placode that gives rise to the lateral line system affected by the morpholino-mediated knockdown of Yap1. The comparative microarray analysis of transcriptome of Yap1-deficient embryos demonstrated changes in expression of many genes, including the Wnt signaling pathway and, in particular, prox1a known for its role in development of mechanoreceptors in the lateral line. The knockdown of Yap1 causes a deficiency of differentiation of mechanoreceptors, and this defect can be rescued by prox1a mRNA. Our studies revealed a role of Yap1 in regulation of Wnt signaling pathway and its target Prox1a during differentiation of mechanosensory cells.

Osmotic regulation of seamless tube growth

Most organs are composed of tubes of differing cellular architectures, including intracellular 'seamless' tubes. Two studies examining the morphogenesis of the seamless tubes formed by the excretory canal cell in Caenorhabditis elegans reveal a previously unappreciated role for osmoregulation of tubulogenesis: hyperosmotic shock recruits canalicular vesicles to the lumenal membrane to promote seamless tube growth.

Role of glucocorticoid in developmental programming: evidence from zebrafish

The vertebrate corticosteroid stress response is highly conserved and a key function is to restore homeostasis by mobilizing and reallocating energy stores. This process is primarily initiated by activation of the hypothalamus-pituitary-adrenal axis, leading to the release of corticosteroids into the circulation. In teleosts, cortisol is the primary corticosteroid that is released into the circulation in response to stress. This steroid activates corticosteroid receptors that are ligand-bound transcription factors, modulating downstream gene expression in target tissues. Recent research in zebrafish (Danio rerio) has identified novel roles for cortisol in early developmental processes, including organogenesis and mesoderm formation. As cortisol biosynthesis commences only around the time of hatch in teleosts, the early developmental events are orchestrated by cortisol that is maternally deposited prior to fertilization. This review will highlight the molecular events leading to the development of the corticosteroid stress axis, and the possible role of cortisol in the developmental programming of stress axis function. Use of zebrafish as a model may lead to significant insights into the conserved role of glucocorticoids during early development with potential implications in biomedical research, including fetal stress syndromes in humans.

Lymphatic reprogramming by Kaposi sarcoma herpes virus promotes the oncogenic activity of the virus-encoded G-protein-coupled receptor

Kaposi sarcoma, the most common cancer in HIV-positive individuals, is caused by endothelial transformation mediated by the Kaposi sarcoma herpes virus (KSHV)-encoded G-protein-coupled receptor (vGPCR). Infection of blood vascular endothelial cells (BEC) by KSHV reactivates an otherwise silenced embryonic program of lymphatic differentiation. Thus, Kaposi sarcoma tumors express numerous lymphatic endothelial cell (LEC) signature genes. A key unanswered question is how lymphatic reprogramming by the virus promotes tumorigenesis leading to Kaposi sarcoma formation. In this study, we present evidence that this process creates an environment needed to license the oncogenic activity of vGPCR. We found that the G-protein regulator RGS4 is an inhibitor of vGPCR that is expressed in BECs, but not in LECs. RGS4 was downregulated by the master regulator of LEC differentiation PROX1, which is upregulated by KSHV and directs KSHV-induced lymphatic reprogramming. Moreover, we found that KSHV upregulates the nuclear receptor LRH1, which physically interacts with PROX1 and synergizes with it to mediate repression of RGS4 expression. Mechanistic investigations revealed that RGS4 reduced vGPCR-enhanced cell proliferation, migration, VEGF expression, and Akt activation and suppressed tumor formation induced by vGPCR. Our findings resolve long-standing questions about the pathologic impact of KSHV-induced reprogramming of host cell identity, and they offer biologic and mechanistic insights supporting the hypothesis that a lymphatic microenvironment is more favorable for Kaposi sarcoma tumorigenesis.

Interrenal organogenesis in the zebrafish model

In recent years, many genes that participate in the specification, differentiation and steroidogenesis of the interrenal organ, the teleostean homologue of the adrenal cortex, have been identified and characterized in zebrafish. In-depth studies of these genes have helped to delineate the morphogenetic steps of interrenal organ formation, as well as some of the molecular and cellular mechanisms that govern these processes. The co-development of interrenal tissue with the embryonic kidney (pronephros), surrounding endothelium and invading chromaffin cells has been analyzed, by virtue of the amenability of zebrafish embryos to a variety of genetic, developmental and histological approaches. Moreover, zebrafish embryos can be subject to molecular as well as biochemical assays for the unraveling of the transcriptional regulation program underlying interrenal development. To this end, the key mechanisms that control organogenesis and steroidogenesis of the zebrafish interrenal gland have been shown to resemble those in mammals, justifying the future utilization of zebrafish model for discovering novel genes associated with adrenal development and disease.

Prox1 regulates the notch1-mediated inhibition of neurogenesis

During development of the spinal cord, Prox1 controls the balance between proliferation and differentiation of neural progenitor cells via suppression of Notch1 gene expression.

Activation of Notch1 signaling in neural progenitor cells (NPCs) induces self-renewal and inhibits neurogenesis. Upon neuronal differentiation, NPCs overcome this inhibition, express proneural genes to induce Notch ligands, and activate Notch1 in neighboring NPCs. The molecular mechanism that coordinates Notch1 inactivation with initiation of neurogenesis remains elusive. Here, we provide evidence that Prox1, a transcription repressor and downstream target of proneural genes, counteracts Notch1 signaling via direct suppression of Notch1 gene expression. By expression studies in the developing spinal cord of chick and mouse embryo, we showed that Prox1 is limited to neuronal precursors residing between the Notch1+ NPCs and post-mitotic neurons. Physiological levels of Prox1 in this tissue are sufficient to allow binding at Notch1 promoter and they are critical for proper Notch1 transcriptional regulation in vivo. Gain-of-function studies in the chick neural tube and mouse NPCs suggest that Prox1-mediated suppression of Notch1 relieves its inhibition on neurogenesis and allows NPCs to exit the cell cycle and differentiate. Moreover, loss-of-function in the chick neural tube shows that Prox1 is necessary for suppression of Notch1 outside the ventricular zone, inhibition of active Notch signaling, down-regulation of NPC markers, and completion of neuronal differentiation program. Together these data suggest that Prox1 inhibits Notch1 gene expression to control the balance between NPC self-renewal and neuronal differentiation.

Early during development, neural progenitor cells (NPCs) can either proliferate or differentiate into neurons. Thus, generation of the correct number of neurons is governed by a tightly regulated balance between proliferation and differentiation, and disruption of this balance can result in severe developmental deficits, malformations, or cancers. Notch1 is a member of the Notch family of receptors, which make up a highly conserved cell signaling system. Notch1 signaling has been shown to inhibit NPC differentiation and to promote self-renewal, thereby allowing NPCs to divide and progressively generate the enormous number of neurons present in the central nervous system. The molecular mechanism by which NPCs overcome Notch1-mediated inhibition in order to differentiate into neurons, however, is not completely understood. In this study, we show that Prox1, a homeobox transcriptional repressor, plays a fundamental role in the switch to differentiation by suppressing the expression of Notch1 receptor, thereby preventing newly produced neuronal precursors from receiving inhibitory signals from Notch ligands present in neighboring cells. This transcriptional repression may regulate cell cycle exit and differentiation of NPCs as they migrate towards different regions and adopt their final cell fates. We suggest that Prox1 may exert its known influence on embryonic development, organ morphogenesis, and cancer through its ability to counteract Notch1 signaling.

prox1b Activity is essential in zebrafish lymphangiogenesis

Background

The lymphatic vascular system, draining interstitial fluids from most tissues and organs, exerts crucial functions in several physiological and pathological processes. Lymphatic system development depends on Prox1, the first marker to be expressed in the endothelial cells of the cardinal vein from where lymph vessels originate. Prox1 ortholog in the optically clear, easily manipulated zebrafish model has been previously isolated and its contribution to lymphangiogenesis has been clarified. Because of a round of genome duplication occurred at the base of teleosts radiation, several zebrafish genes have been retained in duplicate through evolution. We investigated for the presence of additional prox1 genes and determined their role in zebrafish lymphangiogenesis.

Methodology/Principal Findings

We isolated a second ortholog, named prox1b, and analyzed its expression during development by whole mount in situ hybridization (WISH). We detected strong prox1b expression in the endothelium of the posterior cardinal vein (PCV) from where lymphatic precursors originate. To analyze prox1b involvement in lymphangiogenesis we utilized the fli1:GFP transgenics and followed the formation of the toracic duct (TD), the primary lymph vessel in fish, after prox1b knockdown. Our findings clearly demonstrated that the absence of prox1b activity severely hampers the formation of the TD.

Conclusions/Significance

This work provides substantial progress toward the understanding of zebrafish lymphangiogenesis. In light of the features shared by the lymphatic systems of zebrafish and higher vertebrates, the establishment of such lymphatic model will provide a powerful tool to study, for instance, disorders of body fluid homeostasis, inflammation and cancer metastasis, and may ultimately contribute to novel therapies.

Heterogeneity and plasticity of lymphatic endothelial cells

Endothelial cells are found in most organs and tissues in our body. Despite their apparent morphological and functional similarities, endothelial cells exhibit remarkable heterogeneity and plasticity. In a strict sense, no two endothelial cells are identical in terms of their biological, immunological, functional, metabolic, morphological, and anatomical aspects. Their heterogeneity and plasticity are now known to be dependent upon and conferred by their microenvironments, arteriovenous-lymphatic cell identity, organ-specific vascular beds, fluid dynamics, vessel sizes, anatomical locations, physiological and pathological states, and more. Although abundant evidence is available to demonstrate endothelial heterogeneity in the blood vascular system, studies of heterogeneity and plasticity of lymphatic endothelial cells are limited because of the short history of lymphatic research. Nonetheless, a growing body of exciting work has begun to discover that lymphatic endothelial cells are as heterogeneous as blood vascular endothelial cells. In this article, we discuss the heterogeneity and plasticity of lymphatic endothelial cells.

The nuclear hormone receptor Coup-TFII is required for the initiation and early maintenance of Prox1 expression in lymphatic endothelial cells

The homeobox gene Prox1 is crucial for mammalian lymphatic vascular development. In the absence of Prox1, lymphatic endothelial cells (LECs) are not specified. The maintenance of LEC identity also requires the constant expression of Prox1. However, the mechanisms controlling the expression of this gene in LECs remain poorly understood. The SRY-related gene Sox18 is required to induce Prox1 expression in venous LEC progenitors. Although Sox18 is also expressed in embryonic arteries, these vessels do not express Prox1, nor do they give rise to LECs. This finding suggests that some venous endothelial cell-specific factor is required for the activation of Prox1. Here we demonstrate that the nuclear hormone receptor Coup-TFII is necessary for the activation of Prox1 in embryonic veins by directly binding a conserved DNA domain in the regulatory region of Prox1. In addition, we show that the direct interaction between nuclear hormone receptors and Prox1 is also necessary for the maintenance of Prox1 expression during early stages of LEC specification and differentiation.

Studying peripheral sympathetic nervous system development and neuroblastoma in zebrafish

The combined experimental attributes of the zebrafish model system, which accommodates cellular, molecular, and genetic approaches, make it particularly well-suited for determining the mechanisms underlying normal vertebrate development as well as disease states, such as cancer. In this chapter, we describe the advantages of the zebrafish system for identifying genes and their functions that participate in the regulation of the development of the peripheral sympathetic nervous system (PSNS). The zebrafish model is a powerful system for identifying new genes and pathways that regulate PSNS development, which can then be used to genetically dissect PSNS developmental processes, such as tissue size and cell numbers, which in the past haves proved difficult to study by mutational analysis in vivo. We provide a brief review of our current understanding of genetic pathways important in PSNS development, the rationale for developing a zebrafish model, and the current knowledge of zebrafish PSNS development. Finally, we postulate that knowledge of the genes responsible for normal PSNS development in the zebrafish will help in the identification of molecular pathways that are dysfunctional in neuroblastoma, a highly malignant cancer of the PSNS.

The zebrafish prospero homolog prox1 is required for mechanosensory hair cell differentiation and functionality in the lateral line

Background

The lateral line system in zebrafish is composed of a series of organs called neuromasts, which are distributed over the body surface. Neuromasts contain clusters of hair cells, surrounded by accessory cells.

Results

In this report we describe zebrafish prox1 mRNA expression in the migrating primordium and in the neuromasts of the posterior lateral line. Furthermore, using an antibody against Prox1 we characterize expression of the protein in different cell types within neuromasts, and we show distribution among the supporting cells and hair cells.

Conclusion

Functional analysis using antisense morpholinos indicates that prox1 activity is crucial for the hair cells to differentiate properly and acquire functionality, while having no role in development of other cell types in neuromasts.

Zebrafish cyp11a1 and hsd3b genes: structure, expression and steroidogenic development during embryogenesis

Zebrafish has been used increasingly as a good animal model for a number of studies. To facilitate the use of this zebrafish model, the current report put emphasis on the study of two steroidogenic genes: cyp11a1 and hsd3b. These two genes encode enzymes that catalyze the first two steps of the steroidogenic pathway, and both enzymes are important for the synthesis of all steroids. Zebrafish cyp11a1 and hsd3b genes are expressed in the same cells in the gonads and interrenal gland. The interrenal gland is the counterpart of mammalian adrenal; it is located inside the head kidney and is developed parallel to the development of the pronephros. In addition, cyp11a1 and hsd3b are also expressed in the blastomeres of the early embryos before gastrulation, and in the extra-embryonic yolk syncytial layer during gastrulation. This early expression implies a novel role of steroids at gastrulation.

Long and winding roads: testis differentiation in zebrafish

Zebrafish sex determination, gonad differentiation and reproduction are far from being fully understood. Although the mode of sex determination is still being disputed, most experimental data point towards the lack of sex chromosomes and a multigenic sex determination system. Secondary effects from the environment and/or (xeno)hormones may influence the process, resulting in biased sex ratios. The exact time point of sex determination is unknown. Gonad differentiation involves a compulsory 'juvenile ovary' stage with subsequent transformation of the gonad into a testis in males. As the latter is a late event, there is a delay between sex determination and testis differentiation in zebrafish, in contrast to mammals. Information on the expression of several candidate genes thought to be involved in these processes has been supplemented with data from large-scale gonadal transcriptomic studies. New approaches and methodologies provide hope that answers to a number of important questions will be deciphered in the future.

Repression of interferon-gamma expression in T cells by Prospero-related homeobox protein

Interferon-gamma (IFN-gamma) is a major proinflammatory effector and regulatory cytokine produced by activated T cells and NK cells. IFN-gamma has been shown to play pivotal roles in fundamental immunological processes such as inflammatory reactions, cell-mediated immunity and autoimmunity. A variety of human disorders have now been linked to irregular IFN-gamma expression. In order to achieve proper IFN-gamma-mediated immunological effects, IFN-gamma expression in T cells is subject to both positive and negative regulation. In this study, we report for the first time the negative regulation of IFN-gamma expression by Prospero-related Homeobox (Prox1). In Jurkat T cells and primary human CD4+ T cells, Prox1 expression decreases quickly upon T cell activation, concurrent with a dramatic increase in IFN-gamma expression. Reporter analysis and chromatin immunoprecipitation (ChIP) revealed that Prox1 associates with and inhibits the transcription activity of IFN-,gammapromoter in activated Jurkat T cells. Co-immunoprecipitation and GST pull-down assay demonstrated a direct binding between Prox1 and the nuclear receptor peroxisome proliferator-activated receptor gamma (PPPARgamma, which is also an IFN-gamma repressor in T cells. By introducing deletions and mutations into Prox1, we show that the repression of IFN-gamma promoter by Prox1 is largely dependent upon the physical interaction between Prox1 and PPPARgamma Furthermore, PPPARgammaantagonist treatment removes Prox1 from IFN-gamma promoter and attenuates repression of IFN-gamma expression by Prox1. These findings establish Prox1 as a new negative regulator of IFN-gamma expression in T cells and will aid in the understanding of IFN-gamma transcription regulation mechanisms.

PCNA interacts with Prox1 and represses its transcriptional activity

PURPOSE

Prox1 is a transcription factor which can function either as a transcriptional activator, transcriptional repressor or a transcriptional corepressor. This paper seeks to better understand the role of protein-protein interactions in this multitude of functions.

METHODS

We performed a yeast two-hybrid screen of an 11.5 day post coitum (dpc) mouse embryo cDNA library using the homeo-Prospero domain of Prox1 as bait. Computer modeling, cotransfection analysis and confocal immunolocalization were used to investigate the significance of one of the identified interactions.

RESULTS

Proliferating cell nuclear antigen (PCNA) was identified as a Prox1 interacting protein. Prox1 interactions with PCNA require the PCNA interacting protein motif (PIP box), located in the Prospero domain of Prox1. Computer modeling of this interaction identified the apparent geometry of this interface which maintains the accessibility of Prox1 to DNA. Prox1 activated the chicken betaB1-crystallin promoter in cotransfection tests as previously reported, while PCNA squelched this transcriptional activation.

CONCLUSIONS

Since PCNA is expressed in the lens epithelium where Prox1 levels are low, while chicken betaB1-crystallin expression activates in lens fibers where Prox1 expression is high and PCNA levels are low, these data suggest that Prox1-PCNA interactions may in part prevent the activation of betaB1-crystallin expression in the lens epithelium.

Prox1 physically and functionally interacts with COUP-TFII to specify lymphatic endothelial cell fate

Specification of endothelial cell (EC) fate during vascular development is controlled by distinct key regulators. While Notch plays an essential role in induction of arterial phenotypes, COUP-TFII is required to maintain the venous EC identity. Homeodomain transcription factor Prox1 functions to reprogram venous ECs to lymphatic endothelial cells (LECs). Here, we report that the venous EC fate regulator COUP-TFII is expressed in LECs throughout development and physically interacts with Prox1 to form a stable complex in various cell types including LECs. We found that COUP-TFII functions as a coregulator of Prox1 to control several lineage-specific genes including VEGFR-3, FGFR-3, and neuropilin-1 and is required along with Prox1 to maintain LEC phenotype. Together, we propose that the physical and functional interactions of the 2 proteins constitute an essential part in the program specifying LEC fate and may provide the molecular basis for the hypothesis of venous EC identity being the prerequisite for LEC specification.

Dual roles for Prox1 in the regulation of the chicken betaB1-crystallin promoter

PURPOSE

Lens fiber cell differentiation is marked by the onset of betaB1-crystallin expression and is controlled by the cooperative action of a set of transcription factors including Prox1, an atypical homeodomain protein. Previously, the authors reported that Prox1 directly interacts with the OL2 element found in the chicken betaB1-crystallin basal promoter to activate the expression of this gene. Here they mapped the location of activating and repressing sequences of the full-length chicken betaB1-crystallin promoter (-432/+30) in lens epithelial cells, annular pad cells, and intact lens and characterized Prox1-binding sites found in this region.

METHODS

Transfection analysis and transgenic mice were used to characterize upstream regions of the chicken betaB1-crystallin gene. DNaseI footprinting and chromatin immunoprecipitation was performed to identify Prox1-binding sites, and transfection analyses were used to characterize these sites functionally.

RESULTS

Sequences between -152 and -432 of the chicken betaB1-crystallin promoter mediated either promoter activation or repression, depending on the stage of lens differentiation tested. Two new Prox1-binding sites were found in this region that bound Prox1 more avidly than the OL2 element. However, neither binding site conferred Prox1-mediated activation on a heterologous promoter; instead, each allowed Prox1 to repress promoter function.

CONCLUSIONS

The function of the upstream region of the chicken betaB1-crystallin promoter changes depending on cellular context. These data suggest that Prox1 function as a transcriptional activator could be regulated at the DNA level based on the characteristics of the responsive elements.

Crucial role of zebrafish prox1 in hypothalamic catecholaminergic neurons development

Background

Prox1, the vertebrate homolog of prospero in Drosophila melanogaster, is a divergent homeogene that regulates cell proliferation, fate determination and differentiation during vertebrate embryonic development.

Results

Here we report that, in zebrafish, prox1 is widely expressed in several districts of the Central Nervous System (CNS). Specifically, we evidenced prox1 expression in a group of neurons, already positive for otp1, located in the hypothalamus at the level of the posterior tuberculum (PT). Prox1 knock-down determines the severe loss of hypothalamic catecholaminergic (CA) neurons, identified by tyrosine hydroxylase (TH) expression, and the synergistic prox1/otp1 overexpression induces the appearance of hypothalamic supernumerary TH-positive neurons and ectopic TH-positive cells on the yolk epitelium.

Conclusion

Our findings indicate that prox1 activity is crucial for the proper development of the otp1-positive hypothalamic neuronal precursors to their terminal CA phenotype.

Lymphatic vasculature: a molecular perspective

The lymphatic vasculature comprises an intricate network of vessels critical for fluid homeostasis, immune surveillance and fat absorption. Recent studies have provided insights into the developmental processes and molecular mechanisms controlling the formation and remodelling of the lymphatic vessels. These studies have further demonstrated the essential and active role of the lymphatic vessels in various pathological conditions and advanced our understanding of the progression of human diseases, such as inflammation and tumorigenesis. In the context of the latest exciting findings, we review here the current understanding of the mechanisms of lymphatic development and contribution of lymphatic vessels to pathological conditions.

Histone deacetylase 3 (hdac3) is specifically required for liver development in zebrafish

Histone deacetylases (HDACs) are key transcription regulators that function by deacetylating histones/transcription factors and modifying chromatin structure. In this work, we showed that chemical inhibition of HDACs by valproic acid (VPA) led to impaired liver development in zebrafish mainly by inhibiting specification, budding, and differentiation. Formation of exocrine pancreas but not endocrine pancreas was also inhibited. The liver defects induced by VPA correlate with suppressed total HDAC enzymatic activity, but are independent of angiogenesis inhibition. Gene knockdown by morpholino demonstrated that hdac3 is specifically required for liver formation while hdac1 is more globally required for multiple development processes in zebrafish including liver/exocrine pancreas formation. Furthermore, overexpression of hdac3 but not hdac1 partially rescued VPA induced small liver. One mechanism by which hdac3 regulates zebrafish liver growth is through inhibiting growth differentiation factor 11 (gdf11), a unique target of hdac3 and a member of the transforming growth factor beta family. Simultaneous overexpression or morpholino knockdown showed that hdac3 and gdf11 function antagonistically in zebrafish liver development. These results revealed a novel and specific role of hdac3 in liver development and the distinct functions between hdac1 and hdac3 in zebrafish embryonic development.

Localized rbp4 expression in the yolk syncytial layer plays a role in yolk cell extension and early liver development

BACKGROUND

The number of genes characterized in liver development is steadily increasing, but the origin of liver precursor cells and the molecular control of liver formation remain poorly understood. Existing theories about formation of zebrafish visceral organs emphasize either their budding from the endodermal rod or formation of independent anlage followed by their later fusion, but none of these is completely satisfactory in explaining liver organogenesis in zebrafish.

RESULTS

Expression of a gene encoding the retinol binding protein 4 (Rbp4) was analyzed in zebrafish. rbp4, which is expressed mainly in the liver in adults, was shown to be expressed in the yolk syncytial layer (YSL) during early embryogenesis. At 12-16 hpf rbp4 expression was restricted to the ventro-lateral YSL and later expanded to cover the posterior YSL. We demonstrated that rbp4 expression was negatively regulated by Nodal and Hedgehog (Hh) signalling and positively controlled by retinoic acid (RA). Knockdown of Rbp4 in the YSL resulted in shortened yolk extension as well as the formation of two liver buds, which could be due to impaired migration of liver progenitor cells. rbp4 appears also to regulate the extracellular matrix protein Fibronectin1 (Fn1) specifically in the ventro-lateral yolk, indicating a role of Fn1 in liver progenitor migration. Since exocrine pancreas, endocrine pancreas, intestine and heart developed normally in Rbp4 morphants, we suggest that rbp4 expression in the YSL is required only for liver development.

CONCLUSION

The characteristic expression pattern of rbp4 suggests that the YSL is patterned despite its syncytial nature. YSL-expressed Rbp4 plays a role in formation of both yolk extension and liver bud, the latter may also require migration of liver progenitor cells.

Neurturin-GFRalpha2 signaling controls liver bud migration along the ductus venosus in the chick embryo

During chick liver development, the liver bud arises from the foregut, invaginates into the septum transversum, and elongates along and envelops the ductus venosus. However, the mechanism of liver bud migration is only poorly understood. Here, we demonstrate that a GDNF family ligand involved in neuronal outgrowth and migration, neurturin (NRTN), and its receptor, GFRalpha2, are essential for liver bud migration. In the chick embryo, we found that GFRalpha2 was expressed in the liver bud and that NRTN was expressed in the endothelial cells of the ductus venosus. Inhibition of GFRalpha2 signaling suppressed liver bud elongation along the ductus venous without affecting cell proliferation and apoptosis. Moreover, ectopic expression of NRTN perturbed the directional migration along the ductus venosus, leading to splitting or ectopic branching of the liver. We showed that liver buds selectively migrated toward an NRTN-soaked bead in vitro. These data represent a new model for liver bud migration: NRTN secreted from endothelial cells functions as a chemoattractant to direct the migration of the GFRalpha2-expressing liver bud in early liver development.

Pituitary-interrenal interaction in zebrafish interrenal organ development

To further elucidate pituitary adrenal interactions during development, we studied the organogenesis of the interrenal organ, the teleost homolog of the mammalian adrenal gland, in zebrafish. To this end we compared wild-type zebrafish interrenal development with that of mutants lacking pituitary cell types including corticotrophs. In addition, we studied the effects of ACTH receptor (Mc2r) knockdown and dexamethasone (dex) on interrenal development and pituitary feedback. Until 2 d post fertilization (2 dpf) interrenal development assessed by transcripts of key steroidogenic genes (cyp11a1, mc2r, star) is independent of proopiomelanocortin (Pomc) as demonstrated in aal/eya1and lia/fgf3 mutants. However, at 5 dpf lack of pituitary cells leads to reduced expression of steroidogenic genes at both the transcriptional and the protein level. Pituitary control of interrenal development resides in corticotrophs, because pit1 mutants lacking pituitary cells except corticotrophs have a phenotype similar to that of wild-type controls. Furthermore, development in mc2r knockdown morphants does not differ from aal/eya1 and lia/fgf3 mutants. Inhibition of steroidogenesis by mc2r knockdown induces up-regulation of pomc expression in the anterior domain of pituitary corticotrophs. Accordingly, dex suppresses pomc in the anterior domain only, leading to impaired expression of steroidogenic genes commencing at 3 dpf and interrenal hypoplasia via reduced interrenal proliferation. In contrast, negative feedback on pituitary corticotrophs by dex is evident at 2 dpf and precedes effects of Pomc on the interrenal primordium. These data demonstrate a gradual transition from early pituitary-independent interrenal organogenesis to developmental control by the anterior domain of pituitary corticotrophs acting via Mc2 receptors.

Identification and characterization of a novel fushi tarazu factor 1 (FTZ-F1) nuclear receptor in Schistosoma mansoni

Fushi-tarazu factor-1 (FTZ-F1) is an orphan nuclear receptor involved in gene regulation of various developmental processes and physiological activities. We identified a new member of ftz-f1 gene in Schistosoma mansoni, termed Smftz-f1alpha. The Smftz-f1alpha gene has a complex structure with 15 exons interrupted by 14 introns. It encodes an unusually long SmFTZ-F1alpha protein of 1892 amino acids containing all the modular domains found in nuclear receptors. The DNA-binding domain (DBD) of SmFTZ-F1alpha is conserved and most similar to those of human and mouse FTZ-F1 orthologues, exhibiting a 76% identity. The ligand-binding domain (LBD) is less conserved than the DBD; it shares more diverse identity scores in different regions ranging from 23% to 42% in region II and 28% to 72% in region III. A conserved activation function-2 (AF-2) sequence is present in the SmFTZ-F1alpha LBD. This protein also contains a long hinge region (1027 aa) and an F region (220 aa) at the carboxyl end. Phylogenetic analysis suggests that SmFTZ-F1alpha is the orthologue of Drosophila FTZ-F1alpha and vertebrate NR5 members. Western blot analysis of a schistosome extract identified two proteins, one with a size (206 kDa) predicted by the SmFTZ-F1alpha cDNA sequence and a smaller component of 120 kDa. Smftz-f1alpha is expressed throughout the schistosome life cycle with the highest expression in the egg stage. SmFTZ-F1alpha mRNA is widely distributed in adult worms but does not appear in vitelline cells of female worms. SmFTZ-F1alpha localizes to a variety of tissues but is most abundant in the testis of the male and the ovary of female worms. Our results suggest that SmFTZ-F1alpha plays a role in regulating schistosome development and sexual differentiation similar to other FTZ-F1 family members.

Endothelium is required for the promotion of interrenal morphogenetic movement during early zebrafish development

The adrenal cortex has a complex vasculature that is essential for growth, tissue maintenance, and access of secreted steroids to the bloodstream. However, the interaction between vasculature and adrenal cortex during early organogenesis remains largely unclear. In this study, we focused on the zebrafish counterpart of adrenal cortex, interrenal tissue, to explore the possible role of endothelium in the development of steroidogenic tissues. The ontogeny of interrenal tissue was found to be tightly associated with the endothelial cells (ECs) that constitute the axial vessels. The early interrenal primordia emerge as two clusters of cells that migrate centrally and converge at the midline, whereas the central convergence was abrogated in the avascular cloche (clo) mutant. Neither loss of blood circulation nor perturbations of vessel assembly could account for the interrenal convergence defect, implying a role of endothelial signaling prior to the formation of axial blood vessels. Moreover, as the absence of trunk endothelium in clo mutant was rescued by the forced expression of SCL, the interrenal fusion defect could be alleviated. We thus conclude that endothelial signaling is involved in the morphogenetic movement of early interrenal tissue.

Live imaging of lymphatic development in the zebrafish

The lymphatic system has become the subject of great interest in recent years because of its important role in normal and pathological processes. Progress in understanding the origins and early development of this system, however, has been hampered by difficulties in observing lymphatic cells in vivo and in performing defined genetic and experimental manipulation of the lymphatic system in currently available model organisms. Here, we show that the optically clear developing zebrafish provides a useful model for imaging and studying lymphatic development, with a lymphatic system that shares many of the morphological, molecular and functional characteristics of the lymphatic vessels found in other vertebrates. Using two-photon time-lapse imaging of transgenic zebrafish, we trace the migration and lineage of individual cells incorporating into the lymphatic endothelium. Our results show lymphatic endothelial cells of the thoracic duct arise from primitive veins through a novel and unexpected pathway.

HNF factors form a network to regulate liver-enriched genes in zebrafish

Defects in some of liver-enriched genes in mammals will cause liver- and/or blood-related diseases. However, due to the fact that embryogenesis happens intrauterinally in the mammals, the function of these liver-enriched genes during liver organogenesis is poorly studied. We report here the identification of 129 genuine liver-enriched genes in adult zebrafish and show that, through in situ hybridization, 69 of these genes are also enriched in the embryonic liver. External embryogenesis coupled with the well-established morpholino-mediated gene knock-down technique in zebrafish offers us a unique opportunity to study if this group of genes plays any role during liver organogenesis in the future. As an example, preliminary study using morpholino-mediated gene knock-down method revealed that a novel liver-enriched gene leg1 is crucial for the liver expansion growth. We also report the analysis of promoter regions of 51 liver-enriched genes by searching putative binding sites for Hnf1, Hnf3, Hnf4 and Hnf6, four key transcription factors enriched in the liver. We found that promoter regions of majority of liver-enriched genes contain putative binding sites for more than one HNF factors, suggesting that most of liver-enriched genes are likely co-regulated by different combination of HNF factors. This observation supports the hypothesis that these four liver-enriched transcription factors form a network in controlling the expression of liver-specific or -enriched genes in the liver.

A Prospero-related homeodomain protein is a novel co-regulator of hepatocyte nuclear factor 4alpha that regulates the cholesterol 7alpha-hydroxylase gene

Prox1, an early specific marker for developing liver and pancreas in foregut endoderm has recently been shown to interact with alpha-fetoprotein transcription factor and repress cholesterol 7alpha-hydroxylase (CYP7A1) gene transcription. Using a yeast two-hybrid assay, we found that Prox1 strongly and specifically interacted with hepatocyte nuclear factor (HNF)4alpha, an important transactivator of the human CYP7A1 gene in bile acid synthesis and phosphoenolpyruvate carboxykinase (PEPCK) gene in gluconeogenesis. A real time PCR assay detected Prox1 mRNA expression in human primary hepatocytes and HepG2 cells. Reporter assay, GST pull-down, co-immunoprecipitation, and yeast two-hybrid assays identified a specific interaction between the N-terminal LXXLL motif of Prox1 and the activation function 2 domain of HNF4alpha. Prox1 strongly inhibited HNF4alpha and peroxisome proliferators-activated receptor gamma coactivator-1alpha co-activation of the CYP7A1 and PEPCK genes. Knock down of the endogenous Prox1 by small interfering RNA resulted in significant increase of CYP7A1 and PEPCK mRNA expression and the rate of bile acid synthesis in HepG2 cells. These results suggest that Prox1 is a novel co-regulator of HNF4alpha that may play a key role in the regulation of bile acid synthesis and gluconeogenesis in the liver.

Characterization of a novel prospero-related homeobox gene, Prox2

Prospero-related homeobox genes have been identified from various multi-cellular organisms and play important roles in development as a cell fate determinant. Mouse Prox1 is essential for embryogenesis and is required to differentiate horizontal cells in the retina. Here we describe a novel prospero family member, Prox2. Transcriptional reporter assays demonstrated that mouse Prox2 is a transcriptional activator and the N-terminal region has been identified as an activation domain. The expression of mouse Prox2 was detected in postnatal eyes and adult testes as well as embryos. To investigate the in vivo role of Prox2, we generated the Prox2 mutant allele, Prox2-, by homologous recombination in mouse ES cells. Prox2- lacks the first coding exon that encodes a translational start site and a part of homeodomain. In spite of the Prox2 expression during embryogenesis, Prox2- homozygous mutant mice were born at the expected Mendelian ratio without overt abnormalities. Histological analyses revealed that Prox2- homozygous eyes retained the organized layer structure including three nuclear layers and differentiated horizontal cells. Prox2- homozygous mutant males produced elongated spermatids and were fertile. These results demonstrate that mouse Prox2 is dispensable for embryonic development, horizontal cell generation and fertility in contrast to mouse Prox1.

Loss of function of def selectively up-regulates Delta113p53 expression to arrest expansion growth of digestive organs in zebrafish

Transcription factor p53 forms a network with associated factors to regulate the cell cycle and apoptosis in response to environmental stresses. However, there is currently no direct genetic evidence to show if or how the p53 pathway functions during organogenesis. Here we present evidence to show that the zebrafish def (digestive-organ expansion factor) gene encodes a novel pan-endoderm-specific factor. A loss-of-function mutation in def confers hypoplastic digestive organs and selectively up-regulates the expression of Delta113p53, counterpart to a newly identified isoform of p53 produced by an alternative internal promoter in intron 4 of the p53 gene in human. The increased Delta113p53 expression is limited to within the mutant digestive organs, and this increase selectively induces the expression of p53-responsive genes to trigger the arrest of the cell cycle but not apoptosis, resulting in compromised organ growth in the mutant. Our data demonstrate that, while induction of expression of p53 and/or its isoforms is crucial to suppress abnormal cell growth, Delta113p53 is tightly regulated by an organ/tissue-specific factor Def, especially during organogenesis, to prevent adverse inhibition of organ/tissue growth.

Coactivation of Liver Receptor Homologue-1 by Peroxisome Proliferator-Activated Receptor γ Coactivator-1α on Aromatase Promoter II and Its Inhibition by Activated Retinoid X Receptor Suggest a Novel Target for Breast-Specific Antiestrogen Therapy

Aromatase inhibitors target the production of estrogen in breast adipose tissue, but in doing so, also decrease estrogen formation in bone and other sites, giving rise to deleterious side effects, such as bone loss and arthralgia. Thus, it would be clinically useful to selectively inhibit aromatase production in breast. In this regard, we have determined that the orphan nuclear receptor liver receptor homologue-1 (LRH-1) is a specific transcriptional activator of aromatase gene expression in human breast preadipocytes but not in other tissues of postmenopausal women. In this study, we show that the coactivator peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1alpha) is a physiologically relevant modulator of LRH-1, and that its transcriptional activity can be inhibited effectively using receptor-interacting peptide antagonists that prevent PGC-1alpha recruitment. Interestingly, we note that all of these peptides also interact in an agonist-dependent manner with retinoid X receptor alpha (RXRalpha), suggesting that these two receptors may compete for limiting cofactors within target cells. In support of this hypothesis, we show that 9-cis-retinoic acid, acting through RXR, inhibits both the basal and PGC-1alpha-induced transcriptional activity of LRH-1. The importance of this finding was confirmed by showing that LRH-1-dependent, PGC-1alpha-stimulated regulation of aromatase gene expression in primary human breast preadipocytes was effectively suppressed by RXR agonists. We infer from these data that LRH-1 is a bona fide target whose inhibition would selectively block aromatase expression in breast, while sparing other sites of expression.

Gene duplication, gene loss and evolution of expression domains in the vertebrate nuclear receptor NR5A (Ftz-F1) family

Fushi tarazu factor 1 (Ftz-F1, NR5A) is a zinc-finger transcription factor that belongs to the nuclear receptor superfamily and regulates genes that are involved in sterol and steroid metabolism in gonads, adrenals, liver and other tissues. To understand the evolutionary origins and developmental genetic relationships of the Ftz-F1 genes, we have cloned four homologous Ftz-f1 genes in zebrafish, called ff1a, ff1b, ff1c and ff1d. These four genes have different temporal and spatial expression patterns during development, indicating that they have distinct mechanisms of genetic regulation. Among them, the ff1a expression pattern is similar to mammalian Nr5a2, while the ff1b pattern is similar to that of mammalian Nr5a1. Genetic mapping experiments show that these four ff1 genes are located on chromosome segments conserved between the zebrafish and human genomes, indicating a common ancestral origin. Phylogenetic and conserved synteny analysis show that ff1a is the orthologue of NR5A2, and that ff1b and ff1d genes are co-orthologues of NR5A1 that arose by a gene-duplication event, probably a whole-genome duplication, in the ray-fin lineage, and each gene is located next to an NR6A1 co-orthologue as in humans, showing that the tandem duplication occurred before the divergence of human and zebrafish lineages. ff1c does not have a mammalian counterpart. Thus we have characterized the phylogenetic relationships, expression patterns and chromosomal locations of these Ftz-F1 genes, and have demonstrated their identities as NR5A genes in relation to the orthologous genes in other species.

Prox1 promotes lineage-specific expression of fibroblast growth factor (FGF) receptor-3 in lymphatic endothelium: a role for FGF signaling in lymphangiogenesis

Fibroblast growth factors play important roles in angiogenesis, but their functions in lymphangiogenesis remain poorly understood. The homeodomain transcription factor Prox1 is essential for development of the lymphatic system by specifying lymphatic endothelial cell (LEC) fate. Here, we identify fibroblast growth factor (FGF) receptor (FGFR)-3 as a novel Prox1 target gene. Ectopic overexpression of Prox1 in blood vascular endothelial cells up-regulates FGFR-3. Prox1 induces the expression of the IIIc isoform, which we also found to be the major isoform of FGFR-3 expressed in LECs. This transcriptional activation is mediated by a direct binding of Prox1 to newly identified Prox1-response elements in the FGFR-3 promoter. Consistently, FGFR-3 is up-regulated in Prox1-positive newly formed lymphatic vessels during embryogenesis and its lymphatic-specific expression is maintained throughout development. We also found that FGF-1 and FGF-2 promote proliferation, migration, and survival of cultured LECs without involvement of vascular endothelial cell growth factor receptor-3. We show that FGF-2 binds to low- and high-affinity receptors on LECs and is efficiently internalized and processed. Moreover, functional inhibition of FGFR-3 using small interfering RNA represses LEC proliferation. Together, these results indicate that FGFR-3 is an initial target of Prox1 during the lymphatic cell fate specification and that FGF signaling may play an important role in lymphatic vessel development.

Zebrafish sex determination and differentiation: involvement of FTZ-F1 genes

Sex determination is the process deciding the sex of a developing embryo. This is usually determined genetically; however it is a delicate process, which in many cases can be influenced by environmental factors. The mechanisms controlling zebrafish sex determination and differentiation are not known. To date no sex linked genes have been identified in zebrafish and no sex chromosomes have been identified. However, a number of genes, as presented here, have been linked to the process of sex determination or differentiation in zebrafish. The zebrafish FTZ-F1 genes are of central interest as they are involved in regulating interrenal development and thereby steroid biosynthesis, as well as that they show expression patterns congruent with reproductive tissue differentiation and function. Zebrafish can be sex reversed by exposure to estrogens, suggesting that the estrogen levels are crucial during sex differentiation. The Cyp19 gene product aromatase converts testosterone into 17 beta-estradiol, and when inhibited leads to male to female sex reversal. FTZ-F1 genes are strongly linked to steroid biosynthesis and the regulatory region of Cyp19 contains binding sites for FTZ-F1 genes, further linking FTZ-F1 to this process. The role of FTZ-F1 and other candidates for zebrafish sex determination and differentiation is in focus of this review.

Zebrafish ftz-f1a (nuclear receptor 5a2) functions in skeletal muscle organization

Fushi-tarazu factor 1a (Ftz-F1a, Ff1a, Nr5a2) is a nuclear receptor with diverse functions in many tissues. Here, we report the function of ff1a in zebrafish muscle differentiation. In situ hybridization revealed that ff1a mRNA was present in the adaxial and migrating slow muscle precursors and was down-regulated when slow muscle cells matured. This expression was under the control of hedgehog genes, expanded when hedgehog was increased and missing in mutants defective in genes in the Hedgehog pathway like you-too (yot), sonic you (syu), and u-boot (ubo). Blocking ff1a activity by injecting a deleted form of ff1a or an antisense ff1a morpholino oligo into fish embryos caused thinner and disorganized fibers of both slow and fast properties. Transient expression of ff1a in syu, ubo, and yot embryos led to more fibril bundles, even when slow myoblasts were transfated into fast properties. We showed that ff1a and prox1 complemented each other in slow myofibril assembly, but they did not affect the expression of each other. These results demonstrate that ff1a functions in both slow and fast muscle morphogenesis in response to Hedgehog signaling, and this function parallels the activity of another slow muscle gene, prox1.

Genomic, cDNA and embryonic expression analysis of zebrafish IRF6, the gene mutated in the human oral clefting disorders Van der Woude and popliteal pterygium syndromes

Van der Woude syndrome (VWS) and popliteal pterygium syndrome (PPS) are autosomal dominant clefting disorders recently discovered to be caused by mutations in the IRF6 (Interferon Regulatory Factor 6) gene. The IRF gene family consists of nine members encoding transcription factors that share a highly conserved helix-turn-helix DNA-binding domain and a less conserved protein-binding domain. Most IRFs regulate the expression of interferon-alpha and -beta after viral infection, but the function of IRF6 remains unknown. We have isolated a full-length zebrafish irf6 cDNA, which encodes a 492 amino acid protein that contains a Smad-IRF interaction motif and a DNA-binding domain. The zebrafish irf6 gene consists of eight exons and maps to linkage group 22 closest to marker unp1375. By in situ hybridization analysis of embryo whole-mounts and cryosections, we demonstrate that irf6 is first expressed as a maternal transcript. During gastrulation, irf6 expression was concentrated in the forerunner cells. From the bud stage to the 3-somite stage, irf6 expression was observed in the Kupffer's vesicle. No expression could be detected at the 6-somite and 10-somite stages. At the 14-somite stage, expression was detected in the otic placode. At the 17-somite stage, strong expression was also observed in the cloaca. During the pharyngula, hatch and larva periods up to 5 days post-fertilization, irf6 was expressed in the pharyngeal arches, olfactory and otic placodes, and in the epithelial cells of endoderm derived tissues. The latter tissues include the mouth, pharynx, esophagus, endodermal lining of swim bladder, liver, exocrine pancreas, and associated ducts. Overall, the zebrafish expression data are consistent with the observations of lip pits in VWS patients, as well as more recent reports of alae nasi, otitis media and sensorineural hearing loss documented in some patients.

Maternal diet programs embryonic kidney gene expression

Human epidemiological data associating birth weight with adult disease suggest that organogenesis is "programmed" by maternal diet. In rats, protein restriction in pregnancy produces offspring with fewer renal glomeruli and higher systemic blood pressures than controls. We tested the hypothesis that maternal diet alters gene expression in the metanephros, the precursor of the definitive mammalian kidney. We demonstrated that maternal low-protein diet initiated when pregnancy starts and maintained to embryonic day 13, when the metanephros consists of mesenchyme surrounding a once-branched ureteric bud, is sufficient to significantly reduce glomerular numbers in offspring by about 20%. As assessed by representational difference analyses and real-time quantitative polymerase chain reactions, low-protein diet modulated gene expression in embryonic day 13 metanephroi. In particular, levels of prox-1, the ortholog of Drosophila transcription factor prospero, and cofilin-1, a regulator of the actin cytoskeleton, were reduced. During normal metanephrogenesis, prox-1 protein was first detected in mesenchymal cells around the ureteric tree and thereafter in nascent nephron epithelia, whereas cofilin-1 immunolocalized to bud derivatives and condensing mesenchyme. Previously, we reported that low-protein diets increased mesenchymal apoptosis cells when metanephrogenesis began and thereafter reduced numbers of precursor cells. Collectively, these studies prove that the maternal diet programs the embryonic kidney, altering cell turnover and gene expression at a time when nephrons and glomeruli have yet to form. The human implication is that the maternal diet ingested between conception and 5- 6-wk gestation contributes to the variation in glomerular numbers that are known to occur between healthy and hypertensive populations.

Novel steroidogenic factor-1 homolog (ff1d) is coexpressed with anti-Mullerian hormone (AMH) in zebrafish

ff1d is a novel zebrafish FTZ-F1 gene with sequence characteristics indicating similar basic regulatory mechanisms as the previously characterized ff1 based on the presence of an FTZ-F1 box in the DNA binding domain and an interactive domain (I-Box) and an AF-2 in the ligand binding domain. The highest sequence similarity was found between ff1d and ff1b (NR5A4), a gene previously shown to be a functional homolog to the steroidogenic factor 1 (SF-1). The expression pattern of ff1d was comparable to ff1b both in brain and gonads in adults and in the pituitary and interrenal cells in embryos. SF-1 is crucial in mammalian steroidogenesis and in sex determination by regulating the anti-Mullerian hormone (AMH). In fish, AMH has not been described previously. In this study, we cloned a partial zebrafish AMH. AMH was detected in growing oocytes, the ovarian follicular layer and testicular Sertoli cells, similar to the mammalian pattern, suggesting a conserved role between zebrafish and mammalian AMH. Teleosts lack a gene homolog to SRY, which constitute the universal testis-determining factor in mammalian sex determination. Comparison of sequences and expression patterns indicate that ff1d is a new candidate for sex determination and differentiation in a way similar to SF-1, possibly involving AMH.