The zebrafish gene cloche acts upstream of a flk-1 homologue to regulate endothelial cell differentiation

Dissection of angiogenic signaling in zebrafish using a chemical genetic approach

Striking homology between signaling molecules in zebrafish and humans suggests that compounds known to inhibit human kinases may enable a chemical genetic approach to dissect signaling pathways in the zebrafish embryo. We tested this hypothesis using a vascular endothelial growth factor receptor inhibitor, PTK787/ZK222584. Zebrafish embryos treated with this compound lacked all major blood vessels. Overexpression of AKT/PKB, a putative effector of vascular endothelial growth factor signaling, allowed blood vessels to form in the presence of drug. Endothelial cell apoptosis induced by the drug is prevented by increasing AKT/PKB activity, thus establishing the physiological relevance of AKT/PKB in the angiogenic process. This approach allowed us to examine the effects of blood flow and the role of endothelial signals in organogenesis.

Organogenesis--heart and blood formation from the zebrafish point of view

Organs are specialized tissues used for enhanced physiology and environmental adaptation. The cells of the embryo are genetically programmed to establish organ form and function through conserved developmental modules. The zebrafish is a powerful model system that is poised to contribute to our basic understanding of vertebrate organogenesis. This review develops the theme of modules and illustrates how zebrafish have been particularly useful for understanding heart and blood formation.

Transcriptional regulation of hemopoiesis

The regulation of blood cell formation, or hemopoiesis, is central to the replenishment of mature effector cells of innate and acquired immune responses. These cells fulfil specific roles in the host defense against invading pathogens, and in the maintenance of homeostasis. The development of hemopoietic cells is under stringent control from extracellular and intracellular stimuli that result in the activation of specific downstream signaling cascades. Ultimately, all signal transduction pathways converge at the level of gene expression where positive and negative modulators of transcription interact to delineate the pattern of gene expression and the overall cellular hemopoietic response. Transcription factors, therefore, represent a nodal point of hemopoietic control through the integration of the various signaling pathways and subsequent modulation of the transcriptional machinery. Transcription factors can act both positively and negatively to regulate the expression of a wide range of hemopoiesis-relevant genes including growth factors and their receptors, other transcription factors, as well as various molecules important for the function of developing cells. The expression of these genes is dependent on the complex interactions between transcription factors, co-regulatory molecules, and specific binding sequences on the DNA. Recent advances in various vertebrate and invertebrate systems emphasize the importance of transcription factors for hemopoiesis control and the evolutionary conservation of several of such mechanisms. In this review we outline some of the key issues frequently identified in studies of the transcriptional regulation of hemopoietic gene expression. In teleosts, we expect that the characterization of several of these transcription factors and their regulatory mechanisms will complement recent advances in a number of fish systems where identification of cytokine and other hemopoiesis-relevant factors are currently under investigation.

The role of vascular endothelial growth factor (VEGF) in vasculogenesis, angiogenesis, and hematopoiesis in zebrafish development

Vascular endothelial growth factor (VEGF, VEGF-A), a selective mitogen for endothelial cells is a critical factor for vascular development. Two isoforms that differ in the presence of exons 6 and 7, Vegf(165) and Vegf(121), are the dominant forms expressed in zebrafish embryo. Simultaneous overexpression of both isoforms in the embryo results in increased production of flk1, tie1, scl, and gata1 transcripts, indicating a stimulation of both endothelial and hematopoietic lineages. We also demonstrate that vegf can stimulate hematopoiesis in zebrafish by promoting the formation of terminally differentiated red blood cells. Simultaneous overexpression of both isoforms also causes ectopic vasculature and blood cells in many of the injected embryos as well as pericardial edema in later stage embryos. Overexpression of vegf also resulted in earlier onset of flk1, tie1, scl, and gata1 expression in the embryo, indicating a possible role of vegf in stimulating the differentiation of both vascular and hematopoietic lineages. Co-injection of RNAs for both isoforms results in increased expression of three of these markers over and above that observed when either RNA is singly injected and analysis of vegf expression in the notochord mutants no tail and floating head suggests that the notochord patterns the formation of the dorsal aorta by stimulating adjacent somite cells to express vegf, which in turn functions as a signal in dorsal aorta patterning. Finally, studies of vegf expression in cloche mutant indicate that vegf expression is generally independent of cloche function. These results show that in the zebrafish embryo, vegf can not only stimulate endothelial cell differentiation but also hematopoiesis. Moreover, these effects are most dramatic when both vegf isoforms are co-expressed, indicating a synergistic effect of the expression of the two forms of the VEGF protein.

The zebrafish klf gene family

The Krüppel-like factor (KLF) family of genes encodes transcriptional regulatory proteins that play roles in differentiation of a diverse set of cells in mammals. For instance, the founding member KLF1 (also known as EKLF) is required for normal globin production in mammals. Five new KLF genes have been isolated from the zebrafish, Danio rerio, and the structure of their products, their genetic map positions, and their expression during development of the zebrafish have been characterized. Three genes closely related to mammalian KLF2 and KLF4 were found, as was an ortholog of mammalian KLF12. A fifth gene, apparently missing from the genome of mammals and closely related to KLF1 and KLF2, was also identified. Analysis demonstrated the existence of novel conserved domains in the N-termini of these proteins. Developmental expression patterns suggest potential roles for these zebrafish genes in diverse processes, including hematopoiesis, blood vessel function, and fin and epidermal development. The studies imply a high degree of functional conservation of the zebrafish genes with their mammalian homologs. These findings further the understanding of the KLF genes in vertebrate development and indicate an ancient role in hematopoiesis for the Krüppel-like factor gene family.

Zebrafish--an emerging genetic model for the study of cytokines and hematopoiesis in the era of functional genomics

Now that whole genomes are sequenced, the identification of gene function rather than gene discovery is a major challenge. Saturation mutagenesis and screening for mutant phenotypes are methods that allow sampling of the genome for lesions in genes critical for particular physiological processes. This approach promises to provide new insights into gene function, even for molecularly well-characterized processes such as hematopoiesis and cytokine signaling. Animal models for such genetic approaches have traditionally included Drosophila and the mouse. Recently, the zebrafish (Danio rerio) has emerged as a flexible and informative vertebrate for genetic studies. Zebrafish hematopoiesis has a morphological and molecular complexity closer to that of mammals than does Drosophila, providing scope for recognizing mutant zebrafish phenotypes representing finely tuned lesions in these processes. Compared to mice, zebrafish represent an economical, flexible, and genetically tractable animal model for mutagenesis studies. The structure of the teleost genome creates several phylogenetic issues in assessing zebrafish and piscine orthologues and paralogues of known mammalian genes, here exemplified by a cytokine ligand (interleukin-1beta), kinase receptors (c-kit and c-fins), and a family of intracellular signaling molecules (JAK kinases). Several anemic zebrafish mutants are now genetically characterized, and others present hematopoietic phenotypes that promise novel insights into the regulation of hematopoiesis.

Isolation and expression analysis of three zebrafish angiopoietin genes

The Tie1 and Tie2 receptor tyrosine kinases and the Tie2 ligands, the angiopoietins, play critical roles in vertebrate vascular embryogenesis, helping to mediate the interaction between endothelial cells and the pericytes or vascular smooth muscle cells that envelop and support them. We have obtained full-length cDNA sequences for zebrafish orthologs of angiopoietin-1 (ang1), angiopoietin-2 (ang2), and angiopoietin-like-3 (angptl3). Ang1 is expressed in head ventral mesenchyme, in the ventromedial region of somites, in mesenchyme surrounding trunk axial vessels, and in the hypochord, a transient embryonic structure of endodermal origin that has been implicated in dorsal aorta assembly in both zebrafish and Xenopus. Ang2 is expressed in head and anterior trunk ventral mesenchyme and the developing pronephric glomeruli. Angptl3 is expressed in the yolk syncytial layer.

A novel myeloid-restricted zebrafish CCAAT/enhancer-binding protein with a potent transcriptional activation domain

The CCAAT/enhancer-binding protein (C/EBP) family consists of transcription factors essential for hematopoiesis. The defining feature of the C/EBPs is a highly conserved carboxy-terminal bZIP domain that is necessary and sufficient for dimerization and DNA binding, whereas their amino-terminal domains are unique. This study reports a novel c/ebp gene (c/ebp1) from zebrafish that encodes a protein homologous to mammalian C/EBPs within the bZIP domain, but with an amino terminus lacking homology to any C/EBP or to any known sequence. In zebrafish embryos, c/ebp1 expression was initially observed in cells within the yolk sac circulation valley at approximately the 16-to 18-somite stage, and at 24 hours postfertilization (hpf), also in circulating cells. Most c/ebp1(+) cells also expressed a known early macrophage marker, leukocyte-specific plastin (l-plastin). Expression of both markers was lost in cloche, a mutant affecting hematopoiesis at the level of the hemangioblast. Expression of both markers was retained in m683 and spadetail, mutants affecting erythropoiesis, but not myelopoiesis. Further, c/ebp1 expression was lost in a mutant with defective myelopoiesis, but intact erythropoiesis. These data suggest that c/ebp1 is expressed exclusively in myeloid cells. In electrophoretic mobility shift assays, c/ebp1 was able to bind a C/EBP consensus DNA site. Further, a chimeric protein containing the amino-terminal domain of c/ebp1 fused to the DNA-binding domain of GAL4 induced a GAL4 reporter 4000-fold in NIH3T3 cells. These results suggest that c/ebp1 is a novel member of the C/EBP family that may function as a potent transcriptional activator in myeloid cells.

Use of the zebrafish (Danio rerio) to define hematopoiesis

Hematopoiesis in the vertebrate is characterized by the induction of ventral mesoderm to form hematopoietic stem cells and the eventual differentiation of these progenitors to form the peripheral blood lineages. Several genes have been implicated in the differentiation and development of hematopoietic and vascular progenitor cells, yet our understanding of the discrete steps involved in the induction of these cells from the ventral mesoderm is still incomplete. One method of delineating these processes is based on the use of lower vertebrates. The zebrafish (Danio rerio) is an especially robust vertebrate system for both isolating and characterizing genes involved in these processes. Hematopoietic mutants have been generated with defects in many of the steps of both the primitive and definitive hematopoietic programs. Cloning of the genes that underlie these mutations should yield valuable details of hematopoiesis and may have therapeutic implications for bone marrow transplantation and stem cell gene therapy.

Zebrafish genetics and vertebrate heart formation

Forward-genetic analyses in Drosophila and Caenorhabditis elegans have given us unprecedented insights into many developmental mechanisms. To study the formation of organs that contain cell types and structures not present in invertebrates, a vertebrate model system amenable to forward genetics would be very useful. Recent work shows that a newly initiated genetic approach in zebrafish is already making significant contributions to understanding the development of the vertebrate heart, an organ that contains several vertebrate-specific features. These and other studies point to the utility of the zebrafish system for studying a wide range of vertebrate-specific processes.

Hhex and scl function in parallel to regulate early endothelial and blood differentiation in zebrafish

During embryogenesis, endothelial and blood precursors are hypothesized to arise from a common progenitor, the hemangioblast. Several genes that affect the differentiation of, or are expressed early in, both the endothelial and blood lineages may in fact function at the level of the hemangioblast. For example, the zebrafish cloche mutation disrupts the differentiation of both endothelial and blood cells. The transcription factor gene scl is expressed in both endothelial and blood lineages from an early stage and can regulate their differentiation. Here we report that in zebrafish the homeobox gene hhex (previously called hex) is also expressed in endothelial and blood lineages from an early stage. We find that hhex expression in these lineages is significantly reduced in cloche mutant embryos, indicating that hhex functions downstream of cloche to regulate endothelial and blood differentiation. Ectopic expression of hhex through injection of a DNA construct leads to the premature and ectopic expression of early endothelial and blood differentiation genes such as fli1, flk1 and gata1, indicating that Hhex can positively regulate endothelial and blood differentiation. However, analysis of a hhex deficiency allele shows that hhex is not essential for early endothelial and blood differentiation, suggesting that another gene, perhaps scl, compensates for the absence of Hhex function. Furthermore, we find that hhex and scl can induce each other's expression, suggesting that these two genes cross-regulate each other during early endothelial and blood differentiation. Together, these data provide the initial framework of a pathway that can be used to further integrate the molecular events regulating hemangioblast differentiation.

Vascular developmental biology: getting nervous

First described in the developing nervous system, Semaphorin III/Neuropilin, Ephrin/Eph, and Delta/Notch signaling relays have now been implicated in the elaboration of the blood vessel network during embryogenesis.

Building the vertebrate vasculature: research is going swimmingly

The vertebrate vasculature develops in remarkably similar fashion in all vertebrates. A cohort of unspecified mesodermal cells differentiates into primitive endothelial cells, which migrate to and occupy positions within the stereotypical blueprint of the primitive vasculature. Once in position, these cells coalesce and form cords, which lumenize and become ensheathed by supporting pericytes and smooth muscle cells. This primitive vascular network is extensively remodeled in some places, and expanded by sprouting in others. Various studies using the mouse, quail/chick, and frog have uncovered a number of signals that guide these complex processes but many gaps still exist in our understanding of the mechanisms by which the embryonic vasculature is built. Because many questions will require in vivo studies to be properly addressed, the zebrafish, with its unique accessibility to analysis by combined embryological, molecular, and genetic methods, should prove invaluable in identifying new molecules involved in blood vessel development and integrating pathways that influence embryonic blood vessel formation.

The endoderm plays an important role in patterning the segmented pharyngeal region in zebrafish (Danio rerio)

The development of the vertebrate head is a highly complex process involving tissues derived from all three germ layers. The endoderm forms pharyngeal pouches, the paraxial mesoderm gives rise to endothelia and muscles, and the neural crest cells, which originate from the embryonic midbrain and hindbrain, migrate ventrally to form cartilage, connective tissue, sensory neurons, and pigment cells. All three tissues form segmental structures: the hindbrain compartmentalizes into rhombomeres, the mesoderm into somitomeres, and the endoderm into serial gill slits. It is not known whether the different segmented tissues in the head develop by the same molecular mechanism or whether different pathways are employed. It is also possible that one tissue imposes segmentation on the others. Most recent studies have emphasized the importance of neural crest cells in patterning the head. Neural crest cells colonize the segmentally arranged arches according to their original position in the brain and convey positional information from the hindbrain into the periphery. During the screen for mutations that affect embryonic development of zebrafish, one mutant, called van gogh (vgo), in which segmentation of the pharyngeal region is absent, was isolated. In vgo, even though hindbrain segmentation is unaffected, the pharyngeal endoderm does not form reiterated pouches and surrounding mesoderm is not patterned correctly. Accordingly, migrating neural crest cells initially form distinct streams but fuse when they reach the arches. This failure to populate distinct pharyngeal arches is likely due to the lack of pharyngeal pouches. The results of our analysis suggest that the segmentation of the endoderm occurs without signaling from neural crest cells but that tissue interactions between the mesendoderm and the neural crest cells are required for the segmental appearance of the neural crest-derived cartilages in the pharyngeal arches. The lack of distinct patches of neural crest cells in the pharyngeal region is also seen in mutants of one-eyed pinhead and casanova, which are characterized by a lack of endoderm, as well as defects in mesodermal structures, providing evidence for the important role of the endoderm and mesoderm in governing head segmentation.

Molecular mechanism of angiogenesis transcription factors and their therapeutic relevance

Controlling angiogenesis, either positively or negatively, is considered to be useful for the treatment of various diseases. Angiogenesis primarily depends on the phenotypic changes of vascular endothelial cells (ECs). Various genes are expressed in ECs during angiogenesis, and thus, understanding of the transcriptional control of these genes in ECs is very important. This review focuses on transcription factors that may play important roles in angiogenesis.

tbx20, a new vertebrate T-box gene expressed in the cranial motor neurons and developing cardiovascular structures in zebrafish

The T-box genes constitute a family of transcriptional regulator genes that have been implicated in a variety of developmental processes ranging from the formation of germ layers to the regionalization of the central nervous system. In this report we describe the cloning and expression pattern of a new T-box gene from zebrafish, which we named tbx20. tbx20 is an ortholog of two other T-box genes isolated from animals of different phyla - H15 of Drosophila melanogaster and tbx-12 of Caenorhabditis elegans, suggesting that the evolutionary origin of this gene predates the divergence between the protostomes and deuterostomes. During development, tbx20 is expressed in embryonic structures of both mesodermal and ectodermal origins, including the heart, cranial motor neurons, and the roof of the dorsal aorta.

Screening mosaic F1 females for mutations affecting zebrafish heart induction and patterning

The genetic pathways underlying the induction and anterior-posterior patterning of the heart are poorly understood. The recent emergence of the zebrafish model system now allows a classical genetic approach to such challenging problems in vertebrate development. Two large-scale screens for mutations affecting zebrafish embryonic development have recently been completed; among the hundreds of mutations identified were several that affect specific aspects of cardiac morphogenesis, differentiation, and function. However, very few mutations affecting induction and/or anterior-posterior patterning of the heart were identified. We hypothesize that a directed approach utilizing molecular markers to examine these particular steps of heart development will uncover additional such mutations. To test this hypothesis, we are conducting two parallel screens for mutations that affect either the induction or the anterior-posterior patterning of the zebrafish heart. As an indicator of cardiac induction, we examine expression of nkx2.5, the earliest known marker of precardiac mesoderm; to assess anterior-posterior patterning, we distinguish ventricle from atrium with antibodies that recognize different myosin heavy chain isoforms. In order to expedite the examination of a large number of mutations, we are screening the haploid progeny of mosaic F1 females. In these ongoing screens, we have identified four mutations that affect nkx2.5 expression as well as 21 that disrupt either ventricular or atrial development and thus far have recovered several of these mutations, demonstrating the value of our approach. Future analysis of these and other cardiac mutations will provide further insight into the processes of induction and anterior-posterior patterning of the heart.

Podocyte differentiation in the absence of endothelial cells as revealed in the zebrafish avascular mutant, cloche

The physiological functions of the zebrafish pronephros are blood plasma filtration and osmoregulation. The pronephric glomerulus is vascularized through a capillary network sprouting from the dorsal aorta. Vascularization of the glomerulus, visualized by flk-1 expression and alkaline phosphatase reactivity, involves the intimate association between podocytes and endothelial cells and the formation of an intervening glomerular basement membrane (GBM). Cell-cell interactions between podocytes and endothelial cells are thought to play an important role in glomerular angiogenesis. In order to determine whether endothelial cell-derived signals were required for podocyte differentiation, we employed in situ hybridization and electron microscopy to investigate glomerulogenesis in the zebrafish mutant cloche (clo), where endothelial cell development is blocked at an early stage. In clo mutants, glomerular epithelial cells expressing the podocyte specific marker wt1 display well-formed foot processes and are able to form a GBM, suggesting podocytes are able to morphologically differentiate in the absence of endothelia or endothelial-derived signals. The presence of irregular aggregates in the clo GBM as well as the apparent effacement of podocyte foot processes implies a role for endothelial cells in the maintenance of the mature glomerular filtration barrier.

Roles for Fgf signaling during zebrafish fin regeneration

Following amputation of a urodele limb or teleost fin, the formation of a blastema is a crucial step in facilitating subsequent regeneration. Using the zebrafish caudal fin regeneration model, we have examined the hypothesis that fibroblast growth factors (Fgfs) initiate blastema formation from fin mesenchyme. We find that fibroblast growth factor receptor 1 (fgfr1) is expressed in mesenchymal cells underlying the wound epidermis during blastema formation and in distal blastemal tissue during regenerative outgrowth. fgfr1 transcripts colocalize with those of msxb and msxc, putative markers for undifferentiated, proliferating cells. A zebrafish Fgf member, designated wfgf, is expressed in the regeneration epidermis during outgrowth. Furthermore, we show that a specific inhibitor of Fgfr1 applied immediately following fin amputation blocks blastema formation, without obvious effects on wound healing. This inhibitor blocks the proliferation of blastemal cells and the onset of msx gene transcription. Inhibition of Fgf signaling during ongoing fin regeneration prevents further outgrowth while downregulating the established expression of blastemal msx genes and epidermal sonic hedgehog. Our findings indicate that zebrafish fin blastema formation and regenerative outgrowth require Fgf signaling.

Advances in renal development

Branching morphogenesis, mesenchymal cell condensation and mesenchymal-to-epithelial conversion are key steps in kidney development and morphogenesis of several other organs. Review articles describing our current knowledge of the genetic and molecular regulation of these processes have been published. Therefore, this review focuses on the important question of cell lineage specification during kidney development. We describe how new insights are being made into the specification of kidney cells in the intermediate mesoderm, as well as specification of cells that will form the renal mesenchyme, ureteric duct, nephron tubule epithelium and renal vasculature.

Endoderm and heart development

Since the first half of the 20th century, experimental embryologists have noted a relationship between endoderm cells and the development of cardiac tissue from mesoderm. During the past decade, the accumulation of evidence for an obligatory interaction between endoderm and mesoderm during the specification and terminal differentiation of myocardial, and more recently endocardial, cells has markedly accelerated. Moreover, the endoderm-derived molecules that may regulate these processes are being identified. It now appears that endoderm-derived growth factors regulate the formation of both myocardial and endocardial cells during specification, terminal differentiation, and perhaps morphogenesis of cells in the developing embryonic heart.

gridlock, an HLH gene required for assembly of the aorta in zebrafish

The first artery and vein of the vertebrate embryo assemble in the trunk by migration and coalescence of angioblasts to form endothelial tubes. The gridlock (grl) mutation in zebrafish selectively perturbs assembly of the artery (the aorta). Here it is shown that grl encodes a basic helix-loop-helix (bHLH) protein belonging to the Hairy/Enhancer of the split family of bHLH proteins. The grl gene is expressed in lateral plate mesoderm before vessel formation, and thereafter in the aorta and not in the vein. These results suggest that the arterial endothelial identity is established even before the onset of blood flow and implicate the grl gene in assignment of vessel-specific cell fate.

Zebrafish: a genetic approach in studying hematopoiesis

The zebrafish (Danio rerio) has emerged in recent years as an exciting animal model system for studying vertebrate organ development and, in particular, the development of the hematopoietic system. The combined advantages of developmental biology and genetic screens for mutations in zebrafish have provided insights into early events in hematopoiesis and identified several genes required for normal blood development in vertebrates. As a result of the large-scale mutagenesis screens for developmental mutants, several zebrafish mutants with defects in blood development have been recovered. This review discusses how these blood mutations in zebrafish have given new perspectives on hematopoietic development.

Insights into early vasculogenesis revealed by expression of the ETS-domain transcription factor Fli-1 in wild-type and mutant zebrafish embryos

Fli-1 is an ETS-domain transcription factor whose locus is disrupted in Ewing's Sarcoma and F-MuLV induced erythroleukaemia. To gain a better understanding of its normal function, we have isolated the zebrafish homologue. Similarities with other vertebrates, in the amino acid sequence and DNA binding properties of Fli-1 from zebrafish, suggest that its function has been conserved during vertebrate evolution. The initial expression of zebrafish fli-1 in the posterior lateral mesoderm overlaps with that of gata2 in a potential haemangioblast population which likely contains precursors of blood and endothelium. Subsequently, fli-1 and gata2 expression patterns diverge, with separate fli-1 and gata2 expression domains arising in the developing vasculature and in sites of blood formation respectively. Elsewhere in the embryo, fli-1 is expressed in sites of vasculogenesis. The expression of fli-1 was investigated in a number of zebrafish mutants, which affect the circulatory system. In cloche, endothelium is absent and blood is drastically reduced. In contrast to the blood and endothelial markers that have been studied previously, fli-1 expression was initiated normally in cloche embryos, indicating that induction of fli-1 is one of the earliest indicators of haemangioblast formation. Furthermore, although fli-1 expression in the trunk was not maintained, the normal expression pattern in the anterior half of the embryo was retained. These anterior cells did not, however, condense to form blood vessels. These data indicate that cloche has previously unsuspected roles at multiple stages in the formation of the vasculature. Analysis of fli-1 expression in midline patterning mutants floating head and squint, confirms a requirement for the notochord in the formation of the dorsal-aorta. The formation of endothelium in one-eyed pinhead, cyclops and squint embryos indicates a novel role for the endoderm in the formation of the axial vein. The phenotype of sonic-you mutants implies a likely role for Sonic Hedgehog in mediating these processes.

Induction and differentiation of the zebrafish heart requires fibroblast growth factor 8 (fgf8/acerebellar)

Vertebrate heart development is initiated from bilateral lateral plate mesoderm that expresses the Nkx2.5 and GATA4 transcription factors, but the extracellular signals specifying heart precursor gene expression are not known. We describe here that the secreted signaling factor Fgf8 is expressed in and required for development of the zebrafish heart precursors, particularly during initiation of cardiac gene expression. fgf8 is mutated in acerebellar (ace) mutants, and homozygous mutant embryos do not establish normal circulation, although vessel formation is only mildly affected. In contrast, heart development, in particular of the ventricle, is severely abnormal in acerebellar mutants. Several findings argue that Fgf8 has a direct function in development of cardiac precursor cells: fgf8 is expressed in cardiac precursors and later in the heart ventricle. Fgf8 is required for the earliest stages of nkx2.5 and gata4, but not gata6, expression in cardiac precursors. Cardiac gene expression is restored in acerebellar mutant embryos by injecting fgf8 RNA, or by implanting a Fgf8-coated bead into the heart primordium. Pharmacological inhibition of Fgf signalling during formation of the heart primordium phenocopies the acerebellar heart phenotype, confirming that Fgf signaling is required independently of earlier functions during gastrulation. These findings show that fgf8/acerebellar is required for induction and patterning of myocardial precursors.

Dissecting hematopoiesis and disease using the zebrafish

The study of blood has often defined paradigms that are relevant to the biology of other vertebrate organ systems. As examples, stem cell physiology and the structure of the membrane cytoskeleton were first described in hematopoietic cells. Much of the reason for these successes resides in the ease with which blood cells can be isolated and manipulated in vitro. The cell biology of hematopoiesis can also be illuminated by the study of human disease states such as anemia, immunodeficiency, and leukemia. The sequential development of the blood system in vertebrates is characterized by ventral mesoderm induction, hematopoietic stem cell specification, and subsequent cell lineage differentiation. Some of the key regulatory steps in this process have been uncovered by studies in mouse, chicken, and Xenopus. More recently, the genetics of the zebrafish (Danio rerio) have been employed to define novel points of regulation of the hematopoietic program. In this review, we describe the advantages of the zebrafish system for the study of blood cell development and the initial success of the system in this pursuit. The striking similarity of zebrafish mutant phenotypes and human diseases emphasizes the utility of this model system for elucidating pathophysiologic mechanisms. New screens for lineage-specific mutations are beginning, and the availability of transgenics promises a better understanding of lineage-specific gene expression. The infrastructure of the zebrafish system is growing with an NIH-directed genome initiative, providing a detailed map of the zebrafish genome and an increasing number of candidate genes for the mutations. The zebrafish is poised to contribute greatly to our understanding of normal and disease-related hematopoiesis.

casanova plays an early and essential role in endoderm formation in zebrafish

The cellular and molecular mechanisms that regulate endoderm development in vertebrates have only recently begun to be explored. Here we show that the zebrafish locus casanova plays an early and essential role in this process. casanova mutants lack a gut tube and do not express any molecular markers of endoderm differentiation. The early endodermal expression of genes such as axial, gata5, and fkd2 does not initiate in casanova mutants, indicating that the endoderm is defective from the onset of gastrulation. Mosaic analysis demonstrates that casanova functions cell autonomously within the endodermal progenitors. We also report the isolation of a zebrafish homologue of Mixer, a gene important for early endoderm formation in Xenopus. casanova does not encode zebrafish Mixer, and mixer expression is normal in casanova mutants, indicating that casanova acts downstream of, or parallel to, mixer to promote endoderm formation. We further find that the forerunner cells, a specialized group of noninvoluting dorsal mesendodermal cells, do not form in casanova mutants. Studies of casanova mutants do not support an important role for the forerunner cells in either dorsal axis or tail development, as has been previously proposed. In addition, although different populations of mesodermal precursors are generated normally in casanova mutants, morphogenetic defects in the heart, vasculature, blood, and kidney are apparent, suggesting a possible role for the endoderm in morphogenesis of these organs.

Specification of hematopoietic and vascular development by the bHLH transcription factor SCL without direct DNA binding

Transcription factors, such as those of the basic-helix-loop-helix (bHLH) and homeodomain classes, are primary regulators of cell fate decisions and differentiation. It is considered axiomatic that they control their respective developmental programs via direct binding to cognate DNA sequences in critical targets genes. Here we test this widely held paradigm by in vivo functional assay of the leukemia oncoprotein SCL, a bHLH factor that resembles myogenic and neurogenic proteins and is essential for both hematopoietic and vascular development in vertebrates. Contrary to all expectation, we find that SCL variants unable to bind DNA rescue hematopoiesis from gene-targeted SCL(−)(/)(−) embryonic stem cells and complement hematopoietic and vascular deficits in the zebrafish mutant cloche. Our findings establish DNA-binding-independent functions of SCL critical for transcriptional specification, and should encourage reassessment of presumed requirements for direct DNA binding by other transcription factors during initiation of developmental programs.

Gene Duplication of Zebrafish JAK2 Homologs Is Accompanied by Divergent Embryonic Expression Patterns: Only jak2a Is Expressed During Erythropoiesis

Members of the JAK family of protein tyrosine kinase (PTK) proteins are required for the transmission of signals from a variety of cell surface receptors, particularly those of the cytokine receptor family. JAK function has been implicated in hematopoiesis and regulation of the immune system, and recent data suggest that the vertebrate JAK2gene may play a role in leukemia. We have isolated and characterizedjak cDNAs from the zebrafish Danio rerio. The zebrafish genome possesses 2 jak2 genes that occupy paralogous chromosome segments in the zebrafish genome, and these segments conserve syntenic relationships with orthologous genes in mammalian genomes, suggesting an ancient duplication in the zebrafish lineage. The jak2a gene is expressed at high levels in erythroid precursors of primitive and definitive waves and at a lower level in early central nervous system and developing fin buds. jak2b is expressed in the developing lens and nephritic ducts, but not in hematopoietic tissue. The expression of jak2a was examined in hematopoietic mutants and found to be disrupted in clocheand spadetail, suggesting an early role in hematopoiesis. Taken together with recent gene knockout data in the mouse, we suggest that jak2a may be functionally equivalent to mammalianJak2, with a role in early erythropoiesis.

An SCL 3′ enhancer targets developing endothelium together with embryonic and adult haematopoietic progenitors

The SCL gene encodes a basic helix-loop-helix transcription factor which is expressed in early haematopoietic progenitors throughout ontogeny and is essential for the normal development of blood and blood vessels. Transgenic studies have characterised spatially distinct 5′ enhancers which direct lacZ expression to subdomains of the normal SCL expression pattern, but the same elements failed to produce appropriate haematopoietic expression. We now describe an SCL 3′ enhancer with unique properties. It directed lacZ expression in transgenic mice to extra-embryonic mesoderm and subsequently to both endothelial cells and to a subset of blood cells at multiple sites of embryonic haematopoiesis including the yolk sac, para-aortic splanchnopleura and AGM region. The 3′ enhancer also targeted expression to haematopoietic progenitors in both foetal liver and adult bone marrow. Purified lacZ(+)cells were highly enriched for clonogenic myeloid and erythroid progenitors as well as day-12 spleen colony forming units (CFU-S). Within the total gated population from bone marrow, 95% of the myeloid and 90% of the erythroid colony-forming cells were contained in the lacZ(+) fraction, as were 98% of the CFU-S. Activation of the enhancer did not require SCL protein. On the contrary, transgene expression in yolk sacs was markedly increased in an SCL−/− background, suggesting that SCL is subject to negative autoregulation. Alternatively the SCL−/− environment may alter differentiation of extra-embryonic mesoderm and result in an increased number of cells capable of expressing high levels of the transgene. Our data represents the first description of an enhancer that integrates information necessary for expression in developing endothelium and early haematopoietic progenitors at distinct times and sites throughout ontogeny. This enhancer provides a potent tool for the manipulation of haematopoiesis and vasculogenesis in vivo.

Regulation of midline development by antagonism of lefty and nodal signaling

The embryonic midline is crucial for the development of embryonic pattern including bilateral symmetry and left-right asymmetry. In zebrafish, lefty1 (lft1) and lefty2 (lft2) have distinct midline expression domains along the anteroposterior axis that overlap with the expression patterns of the nodal-related genes cyclops and squint. Altered expression patterns of lft1 and lft2 in zebrafish mutants that affect midline development suggests different upstream pathways regulate each expression domain. Ectopic expression analysis demonstrates that a balance of lefty and cyclops signaling is required for normal mesendoderm patterning and goosecoid, no tail and pitx2 expression. In late somite-stage embryos, lft1 and lft2 are expressed asymmetrically in the left diencephalon and left lateral plate respectively, suggesting an additional role in laterality development. A model is proposed by which the vertebrate midline, and thus bilateral symmetry, is established and maintained by antagonistic interactions among co-expressed members of the lefty and nodal subfamilies of TGF-beta signaling molecules.

The role of tolloid/mini fin in dorsoventral pattern formation of the zebrafish embryo

A highly conserved TGF-β signaling pathway is involved in the establishment of the dorsoventral axis of the vertebrate embryo. Specifically, Bone Morphogenetic Proteins (Bmps) pattern ventral tissues of the embryo while inhibitors of Bmps, such as Chordin, Noggin and Follistatin, are implicated in dorsal mesodermal and neural development. We investigated the role of Tolloid, a metalloprotease that can cleave Chordin and increase Bmp activity, in patterning the dorsoventral axis of the zebrafish embryo. Injection of tolloid mRNA into six dorsalized mutants rescued only one of these mutants, mini fin. Through chromosomal mapping, linkage and cDNA sequence analysis of several mini fin alleles, we demonstrate that mini fin encodes the tolloid gene. Characterization of the mini fin mutant phenotype reveals that Mini fin/Tolloid activity is required for patterning ventral tissues of the tail: the ventral fin, and the ventroposterior somites and vasculature. Gene expression studies show that mfn mutants exhibit reduced expression of ventrally restricted markers at the end of gastrulation, suggesting that the loss of ventral tail tissues is caused by a dorsalization occurring at the end of gastrulation. Based on the mini fin mutant phenotype and the expression of tolloid, we propose that Mini fin/Tolloid modifes the Bmp activity gradient at the end of gastrulation, when the ventralmost marginal cells of the embryo are in close proximity to the dorsal Chordin-expressing cells. At this time, unimpeded Chordin may diffuse to the most ventral marginal regions and inhibit high Bmp activity levels. In the presence of Mini fin/Tolloid, however, Chordin activity would be negatively modulated through proteolytic cleavage, thereby increasing Bmp signaling activity. This extracellular mechanism is amplified by an autoregulatory loop for bmp gene expression.

Conditional requirement for the Flk-1 receptor in the in vitro generation of early hematopoietic cells

Genetic studies in mice have previously demonstrated an intrinsic requirement for the vascular endothelial growth factor (VEGF) receptor Flk-1 in the early development of both the hematopoietic and endothelial cell lineages. In this study, embryonic stem (ES) cells homozygous for a targeted null mutation in flk-1 (flk-1 (-/-)) were examined for their hematopoietic potential in vitro during embryoid body (EB) formation or when cultured on the stromal cell line OP9. Surprisingly, in EB cultures flk-1 (-/-) ES cells were able to differentiate into all myeloid-erythroid lineages, albeit at half the frequency of heterozygous lines. In contrast, although flk-1 (-/-) ES cells formed mesodermal-like colonies on OP9 monolayers, they failed to generate hematopoietic clusters even in the presence of exogenous cytokines. However, flk-1 (-/-) OP9 cultures did contain myeloid precursors, albeit at greatly reduced percentages. This defect was rescued by first allowing flk-1 (-/-) ES cells to differentiate into EBs and then passaging these cells onto OP9 stroma. Thus, the requirement for Flk-1 in early hematopoietic development can be abrogated by alterations in the microenvironment. This finding is consistent with a role for Flk-1 in regulating the migration of early mesodermally derived precursors into a microenvironment that is permissive for hematopoiesis.

Cell-autonomous and non-autonomous requirements for the zebrafish gene cloche in hematopoiesis

Vertebrate embryonic hematopoiesis is a complex process that involves a number of cellular interactions, notably those occurring between endothelial and blood cells. The zebrafish cloche mutation affects both the hematopoietic and endothelial lineages from an early stage (Stainier, D. Y. R., Weinstein, B. M., Detrich, H. W. R., Zon, L. I. and Fishman, M. C. (1995) Development 121, 3141–3150). cloche mutants lack endocardium, as well as head and trunk endothelium, and nearly all blood cells. Cell transplantation studies have revealed that the endocardial defect in cloche is cell-autonomous: wild-type cells can form endocardium in mutant hosts, but mutant cells never contribute to the endocardium in wild-type or mutant hosts. In this paper, we analyze the cell-autonomy of the blood defect in cloche. The blood cell deficiency in cloche mutants could be an indirect effect of the endothelial defects. Alternatively, cloche could be required cell-autonomously in the blood cells themselves. To distinguish between these possibilities, we cotransplanted wild-type and mutant cells into a single wild-type host in order to compare their respective hematopoietic capacity. We found that transplanted wild-type cells were much more likely than mutant cells to contribute to circulating blood in a wild-type host. Furthermore, in the few cases where both wild-type and mutant donors contributed to blood in a wild-type host, the number of blood cells derived from the wild-type donor was always much greater than the number of blood cells derived from the mutant donor. These data indicate that cloche is required cell-autonomously in blood cells for their differentiation and/or proliferation. When we assessed early expression of the erythropoietic gene gata-1 in transplant recipients, we found that mutant blastomeres were as likely as wild-type blastomeres to give rise to gata-1-expressing cells in a wild-type host. Together, these two sets of data argue that cloche is not required cell-autonomously for the differentiation of red blood cells, as assayed by gata-1 expression, but rather for their proliferation and/or survival, as assayed by their contribution to circulating blood. In addition, we found that transplanted wild-type cells were less likely to express gata-1 in a mutant environment than in a wild-type one, suggesting that cloche also acts non-autonomously in red blood cell differentiation. This non-autonomous function of cloche in red blood cell differentiation may reflect its cell-autonomous requirement in the endothelial lineage. Thus, cloche appears to be required in erythropoiesis cell non-autonomously at a step prior to gata-1 expression, and cell-autonomously subsequently.

alyron, an insertional mutation affecting early neural crest development in zebrafish

alyronz12 (aln) is a recessive lethal mutation that affects early stages of neural crest development in the zebrafish. alyron appears to be an insertional mutation as the mutation was generated following microinjection of plasmid DNA into one-cell embryos and the stably integrated transgenic sequences are closely linked to the mutation. The insertion site harbors multiple copies of the plasmid sequence that have experienced complex rearrangements. Host-insert junction fragments have been molecularly cloned and host sequences adjacent to the transgene have been used to map the mutation to the distal arm of linkage group 15. alyron function is required cell-autonomously in the neural crest lineage. alyron mutants have a severe but not complete deficit of premigratory neural crest as judged by reduced expression of several markers associated with early stages of neural crest development. Lack of premigratory neural crest is likely to account for the two most conspicuous characteristics of alyron mutants: the absence of body pigmentation and the inability to affect blood circulation. The neural crest phenotype of alyron mutants resembles that observed in mouse mutants that lack Pax-3 or both Wnt-1 and Wnt-3a function, and expression of the zebrafish homologues of these genes is greatly reduced in the dorsal neural keels of alyron mutants. In contrast, ventral neural keel identity appears unaffected. Given our findings that the mutation is unlinked to pax or wnt genes that have been described in the zebrafish, we propose that alyron is a novel gene function required for the specification and/or proliferative expansion of neural crest progenitors.

Elucidating the origins of the vascular system: a fate map of the vascular endothelial and red blood cell lineages in Xenopus laevis

Required to supply nutrients and oxygen to the growing embryo, the vascular system is the first functional organ system to develop during vertebrate embryogenesis. Although there has been substantial progress in identifying the genetic cascade regulating vascular development, the initial stages of vasculogenesis, namely, the origin of vascular endothelial cells within the early embryo, remain unclear. To address this issue we constructed a fate map for specific vascular structures, including the aortic arches, endocardium, dorsal aorta, cardinal veins, and lateral abdominal veins, as well as for the red blood cells at the 16-cell stage and the 32-cell stage of Xenopus laevis. Using genetic markers to identify these cell types, our results suggest that vascular endothelial cells can arise from virtually every blastomere of the 16-cell-stage and the 32-cell-stage embryo, with different blastomeres preferentially, though not exclusively, giving rise to specific vascular structures. Similarly, but more surprisingly, every blastomere in the 16-cell-stage embryo and all but those in the most animal tier of the 32-cell-stage embryo serve as progenitors for red blood cells. Taken together, our results suggest that during normal development, both dorsal and ventral blastomeres contribute significantly to the vascular endothelial and red blood cell lineages.

What guides early embryonic blood vessel formation?

Survival of vertebrate embryos depends on their ability to assemble a correctly patterned, integrated network of blood vessels to supply oxygen and nutrients to developing tissues. The arrangement of larger caliber intraembryonic vessels, specification of arterial-venous identity, and proper placement of major branch points and arterial-venous connections are all precisely determined. A number of recent studies in both mammalian and nonmammalian vertebrate species, reviewed here, have now begun to reveal the major role played by genetically predetermined extrinsic cues in guiding the formation of early embryonic blood vessels and determining the global pattern of the vasculature.

Early hematopoiesis and developing lymphoid organs in the zebrafish

In zebrafish, the transparent and rapidly developing embryo and the potential for genetic screening offer a unique opportunity to investigate the early development of the vertebrate immune system. Here we describe the initial appearance of various blood lineages and the nature of accumulating hematopoietic tissue in the thymus and kidney, the main lymphoid organs of adult teleosts. The ultrastructure of the first site of hematopoiesis, the intermediate cell mass (ICM), is described from the 5-somite stage, about 11.5 hours post-fertilization (hpf) until 24 hpf. The ICM gives rise to the primitive erythroid lineage, which accounts for all circulating erythrocytes for the first 4 days pf. From 24 to 72 hpf, a few developing granulocytes are seen close to the yolk sac walls and in the caudal axial vein. The heart, previously proposed as an early blood-forming organ in zebrafish, did not contain hematopoietic cells. The thymic primordium, consisting of two layers of epithelial cells, appears at 60 hpf. By 65 hpf, it is colonized by immature lymphoblasts. The thymus gradually accumulates lymphocytes, and the lymphocytes and epithelial cells progressively differentiate for 3 weeks pf. At 96 hr, the pronephros contains hematopoietic cells, mainly developing erythrocytes and granulocytes. The amount of renal hematopoietic tissue increases rapidly; however, lymphocytes were not detected until 3 weeks pf.

Lymphocyte development in fish and amphibians

Recently, molecular markers such as recombination activating genes (RAG), terminal deoxynucleotidyl transferase (TdT), stem cell leukemia hematopoietic transcription factor (SCL), Ikaros and gata-binding protein (Gata)-family members have been isolated and characterized from key lower vertebrates, adding to our growing knowledge of lymphopoiesis in ectotherms. In all gnathostomes there appear to be two main embryonic locations derived from the early mesoderm, both intra- and extraembryonic, which contribute to primitive and definitive hematopoiesis based upon their differential expression of SCL, Gata-1, Gata-2 and myeloblastosis oncogene (c-myb). In teleosts, a unique intraembryonic location for hematopoietic stem cells termed the intermediate cell mass (ICM) of Oellacher appears to be responsible for primitive or definitive hematopoiesis depending upon the species being investigated. In Xenopus, elegant grafting studies in combination with specific molecular markers has led to a better definition of the roles that ventral blood islands and dorsal lateral plate play in amphibian hematopoiesis, that of primitive and definitive lymphopoiesis. After the early embryonic contribution to hematopoiesis, specialized tissues must assume the role of providing the proper microenvironment for T and B-lymphocyte development from progenitor stem cells. In all gnathostomes, the thymus is the major site for T-cell maturation as evidenced by strong expression of developmental markers such as Ikaros, Rag and TdT plus expression of T-cell specific markers such as T-cell receptor beta and lck. In this respect, several zebrafish mutants have provided new insights on the development of the thymopoietic environment. On the other hand, the sites for B-cell lymphopoiesis are less clear among the lower vertebrates. In elasmobranchs, the spleen, Leydig's organ and the spiral valve may all contribute to B-cell development, although pre-B cells have yet to be fully addressed in fish. In teleosts, the kidney is undeniably the major source of B-cell development based upon functional, cellular and molecular indices. Amphibians appear to use several different sites (spleen, bone marrow and/or kidney) depending upon the species in question.

Molecular Identity of Hematopoietic Precursor Cells Emerging in the Human Embryo

It is now accepted from studies in animal models that hematopoietic stem cells emerge in the para-aortic mesoderm-derived aorta-gonad-mesonephros region of the vertebrate embryo. We have previously identified the equivalent primitive hematogenous territory in the 4- to 6-week human embryo, under the form of CD34+CD45+Lin− high proliferative potential hematopoietic cells clustered on the ventral endothelium of the aorta. To characterize molecules involved in initial stem cell emergence, we first investigated the expression in that territory of known early hematopoietic regulators. We herein show that aorta-associated CD34+ cells coexpress the tal-1/SCL, c-myb, GATA-2, GATA-3, c-kit, and flk-1/KDR genes, as do embryonic and fetal hematopoietic progenitors later present in the liver and bone marrow. Next, CD34+CD45+ aorta-associated cells were sorted by flow cytometry from a 5-week embryo and a cDNA library was constructed therefrom. Differential screening of that library with total cDNA probes obtained from CD34+embryonic liver cells allowed the isolation of a kinase-related sequence previously identified in KG-1 cells. In addition to emerging blood stem cells, KG-1 kinase is also strikingly expressed in all developing endothelial cells in the yolk sac and embryo, which suggests its involvement in the genesis of both hematopoietic and vascular cell lineages in humans.

Morphogenesis of the first blood vessels

The initial phase of vessel formation is the establishment of nascent endothelial tubes from mesodermal precursor cells. Development of the vascular epithelium is examined using the transcription factor TAL1 as a marker of endothelial precursor cells (angioblasts), and a functional assay based on intact, whole-mounted quail embryos. Experimental studies examining the role(s) of integrins and vascular endothelial growth factor (VEGF) establish that integrin-mediated cell adhesion is necessary for normal endothelial tube formation and that stimulation of embryonic endothelial cells with exogenous VEGF results in a massive "fusion" of vessels and the obliteration of normally avascular zones. The second phase of vessel morphogenesis is assembly of the vessel wall. To understand the process by which mesenchyme gives rise to vascular smooth muscle, a novel monoclonal antibody, 1E12, that recognizes smooth muscle precursor cells was used. Additionally, development of the vessel wall was examined using the expression fo extracellular matrix proteins as markers. Comparison of labeling patterns of 1E12 and the extracellular matrix molecules fibulin-1 and fibrillin-2 indicate vessel wall heterogeneity at the earliest stages of development; thus smooth muscle cell diversity is manifested during the differentiation and assembly of the vessel wall. From these studies it is postulated that the extracellular matrix composition of the vessel wall may prove to be the best marker of smooth muscle diversity. The data are discussed in the context of recent work by others, especially provocative new studies suggesting an endothelial origin for vascular smooth muscle cells. Also discussed is recent work that provides clues to the mechanism of vascular smooth muscle induction and recruitment. Based on these findings, vascular smooth muscle cells can be thought of as existing along a continuum of phenotypes. This spectrum varies from mainly matrix-producing cells to primarily contractile cells; thus no one cell type typifies vascular smooth muscle. This view of the smooth muscle cell is considered in terms of a contrasting opinion that views smooth muscle cell as existing in either a synthetic or proliferative state.

The cloche and spadetail genes differentially affect hematopoiesis and vasculogenesis

In vertebrates, hematopoietic and vascular progenitors develop from ventral mesoderm. The first primitive wave of hematopoiesis yields embryonic red blood cells, whereas progenitor cells of subsequent definitive waves form all hematopoietic cell lineages. In this report we examine the development of hematopoietic and vasculogenic cells in normal zebrafish and characterize defects in cloche and spadetail mutant embryos. The zebrafish homologs of lmo2, c-myb, fli1, flk1, and flt4 have been cloned and characterized in this study. Expression of these genes identifies embryonic regions that contain hematopoietic and vascular progenitor cells. The expression of c-myb also identifies definitive hematopoietic cells in the ventral wall of the dorsal aorta. Analysis of b316 mutant embryos that carry a deletion of the c-myb gene demonstrates that c-myb is not required for primitive erythropoiesis in zebrafish even though it is expressed in these cells. Both cloche and spadetail mutant embryos have defects in primitive hematopoiesis and definitive hematopoiesis. The cloche mutants also have significant decreases in vascular gene expression, whereas spadetail mutants expressed normal levels of these genes. These studies demonstrate that the molecular mechanisms that regulate hematopoiesis and vasculogenesis have been conserved throughout vertebrate evolution and the clo and spt genes are key regulators of these programs.

Isolation of the zebrafish homologues for the tie-1 and tie-2 endothelium-specific receptor tyrosine kinases

Several characteristics of the zebrafish embryo make it an attractive model in which to study the development of the cardiovascular system. The utility of the zebrafish as a model of mammalian vascular development will depend on the conservation of molecular and morphogenetic mechanisms of vessel growth. Here, we report the cloning of the zebrafish homologues of the endothelium-specific receptor tyrosine kinases tie-1 and tie-2. The Z tie-2 clone represents the first report of a full-length zebrafish endothelium-specific gene. The zebrafish tie family members have significant structural homology with their murine and human counterparts. In addition, like the murine tie-1 and tie-2 genes, expression was found predominantly in endothelial cells. At 24-hr postfertilization (HPF), Z tie-1 was expressed in all observed populations of endothelial cells. Interestingly, Z tie-2 exhibited a similar, although slightly more restricted, expression pattern. Taken together, these data strongly suggest that mechanisms of vascular development are highly conserved across species and that zebrafish will continue to be a useful model for the investigation of vertebrate embryonic vascular development.

Use of the zebrafish (Danio rerio) to define hematopoiesis

Hematopoiesis in the vertebrate is characterized by the induction of ventral mesoderm to form hematopoietic stem cells and the eventual differentiation of these progenitors to form the peripheral blood lineages. Several genes have been implicated in the differentiation and development of hematopoietic and vascular progenitor cells, yet our understanding of the discrete steps involved in the induction of these cells from the ventral mesoderm is still incomplete. One method of delineating these processes is based on the use of lower vertebrates. The zebrafish (Danio rerio) is an especially robust vertebrate system for both isolating and characterizing genes involved in these processes. Hematopoietic mutants have been generated with defects in many of the steps of both the primitive and definitive hematopoietic programs. Cloning of the genes that underlie these mutations should yield valuable details of hematopoiesis and may have therapeutic implications for bone marrow transplantation and stem cell gene therapy.

Molecular insights into cardiac development

Recent discoveries have led to a greater appreciation of the diverse mechanisms that underlie cardiac morphogenesis. Genetic strategies (primarily gene targeting approaches in mice) have significantly broadened research in cardiovascular developmental biology by illuminating new pathways involved in heart development and by allowing the genetic evaluation of pathways that have previously been implicated in these events. Advances have also been made using biochemical and cell- and tissue-based approaches. This review summarizes the author's interpretation of current trends in the effort to understand the molecular basis of cardiac-development, with an emphasis on insights obtained from genetic models.