Resistance to inflammation underlies enhanced fitness in clonal hematopoiesis

[Figure: see text].

Chemical screening in zebrafish for novel biological and therapeutic discovery

Zebrafish chemical screening allows for an in vivo assessment of small molecule modulation of biological processes. Compound toxicities, chemical alterations by metabolism, pharmacokinetic and pharmacodynamic properties, and modulation of cell niches can be studied with this method. Furthermore, zebrafish screening is straightforward and cost effective. Zebrafish provide an invaluable platform for novel therapeutic discovery through chemical screening.

Transplantation in zebrafish

Tissue or cell transplantation is an invaluable technique with a multitude of applications including studying the developmental potential of certain cell populations, dissecting cell-environment interactions, and identifying stem cells. One key technical requirement for performing transplantation assays is the capability of distinguishing the transplanted donor cells from the endogenous host cells and tracing the donor cells over time. The zebrafish has emerged as an excellent model organism for performing transplantation assays, thanks in part to the transparency of embryos and even adults when pigment mutants are employed. Using transgenic techniques and fast-evolving imaging technology, fluorescence-labeled donor cells can be readily identified and studied during development in vivo. In this chapter, we will discuss the rationale of different types of zebrafish transplantation in both embryos and adults and then focus on four detailed methods of transplantation: blastula/gastrula transplantation for mosaic analysis, hematopoietic stem cell transplantation, chemical screening using a transplantation model, and tumor transplantation.

Chromatin immunoprecipitation and an open chromatin assay in zebrafish erythrocytes

Zebrafish is an excellent genetic and developmental model for the study of vertebrate development and disease. Its ability to produce an abundance of transparent, externally developed embryos has facilitated large-scale genetic and chemical screens for the identification of critical genes and chemical factors that modulate developmental pathways. These studies can have profound implications for the diagnosis and treatment of a variety of human diseases. Recent advancements in molecular and genomic studies have provided valuable tools and resources for comprehensive and high-resolution analysis of epigenomes during cell specification and lineage differentiation throughout development. In this chapter, we describe two simple methods to evaluate protein-DNA interaction and chromatin architecture in erythrocytes from adult zebrafish. These are chromatin immunoprecipitation coupled with next-generation sequencing (ChIP-seq) and an assay for transposase-accessible chromatin with high-throughput sequencing (ATAC-seq). These techniques, together with gene expression profiling, are useful for analyzing epigenomic regulation of cell specification, differentiation, and function during zebrafish development in both normal and disease models.

Tissue-specific gene targeting using CRISPR/Cas9

The zebrafish has been a powerful model in forward genetic screens to identify genes essential for organogenesis and embryonic development. Conversely, using reverse genetics to investigate specific gene function requires phenotypic analysis of complete gene inactivation. Despite the availability and efficacy of morpholinos, the lack of tractable and efficient knockout technologies has impeded reverse genetic studies in the zebrafish, particularly in adult animals. The recent development of genome-editing technologies such as CRISPR/Cas9 greatly widened the scope of loss-of-function studies in the zebrafish, allowing for the rapid phenotypic assessment of gene silencing in embryos, the generation of knockout lines, and large-scale reverse genetic screens. Tissue-specific gene inactivation would be ideal for these studies given the caveats of whole-embryo gene silencing, yet spatial control of gene targeting remains a challenge. In this chapter, we focus on tissue-specific gene inactivation using the CRISPR/Cas9 technology. We first explain the rationale for this technique, including some of its potential applications to tackle important biological issues and the inability of current technologies to address these issues. We then present a method to target genes in a tissue-specific manner in the zebrafish. Finally, we discuss technical difficulties and limitations of this method as well as possible future developments.

Embryonic cell culture in zebrafish

Zebrafish embryonic cell cultures have many useful properties that make them complementary to intact embryos for a wide range of studies. Embryonic cell cultures allow for maintenance of transient cell populations, control of chemical and mechanical cues received by cells, and facile chemical screening. Zebrafish cells can be cultured in either heterogeneous or homogeneous cultures from a wide range of developmental time points. Here we describe two methods with particular applicability to chemical screening: a method for the culture of blastomeres for directed differentiation toward the myogenic lineage and a method for the culture of neural crest cells in heterogeneous cultures from early somitogenesis embryos.

Generating and evaluating a ranked candidate gene list for potential vertebrate heart field regulators

The vertebrate heart develops from two distinct lineages of cardiomyocytes that arise from the first and second heart fields (FHF and SHF, respectively). The FHF forms the primitive heart tube, while adding cells from the SHF allows elongation at both poles of the tube. Initially seen as an exclusive characteristic of higher vertebrates, recent work has demonstrated the presence of a distinct FHF and SHF in lower vertebrates, including zebrafish. We found that key transcription factors that regulate septation and chamber formation in higher vertebrates, including Tbx5 and Pitx2, influence relative FHF and SHF contributions to the zebrafish heart tube. To identify molecular modulators of heart field migration, we used microarray-based expression profiling following inhibition of tbx5a and pitx2ab in embryonic zebrafish (Mosimann & Panakova, et al, 2015; GSE70750). Here, we describe in more detail the procedure used to process, prioritize, and analyze the expression data for functional enrichment.

A point mutation of zebrafish c-cbl gene in the ring finger domain produces a phenotype mimicking human myeloproliferative disease

Controlled self-renewal and differentiation of hematopoietic stem/progenitor cells (HSPCs) are critical for vertebrate development and survival. These processes are tightly regulated by the transcription factors, signaling molecules and epigenetic factors. Impaired regulations of their function could result in hematological malignancies. Using a large-scale zebrafish N-ethyl-N-nitrosourea mutagenesis screening, we identified a line named LDD731, which presented significantly increased HSPCs in hematopoietic organs. Further analysis revealed that the cells of erythroid/myeloid lineages in definitive hematopoiesis were increased while the primitive hematopoiesis was not affected. The homozygous mutation was lethal with a median survival time around 14-15 days post fertilization. The causal mutation was located by positional cloning in the c-cbl gene, the human ortholog of which, c-CBL, is found frequently mutated in myeloproliferative neoplasms (MPN) or acute leukemia. Sequence analysis showed the mutation in LDD731 caused a histidine-to-tyrosine substitution of the amino acid codon 382 within the RING finger domain of c-Cbl. Moreover, the myeloproliferative phenotype in zebrafish seemed dependent on the Flt3 (fms-like tyrosine kinase 3) signaling, consistent with that observed in both mice and humans. Our study may shed new light on the pathogenesis of MPN and provide a useful in vivo vertebrate model of this syndrome for screening drugs.

Mlck1a is expressed in zebrafish thrombocytes and is an essential component of thrombus formation

BACKGROUND

We have used the advantages of the zebrafish model system to demonstrate which of the vertebrate myosin light chain kinase (MLCK) genes is expressed in thrombocytes and important for thrombus formation.

METHODS AND RESULTS

Here we report that Mlck1a is an essential component of thrombus formation. Phylogenetic data revealed four zebrafish orthologous for three human MLCK genes. To investigate expression of the zebrafish mlck genes in thrombocytes we compared GFP-tagged platelets with other cells by microarray analysis, and showed that mlck1a expression was 4.5-fold enriched in platelets. Furthermore, mlck1a mRNA and mRNA for the platelet-specific cd41 co-localized in thrombi. Expression of other mlck subtypes was lower in GFP-tagged platelets (mlck1b; 0.77-fold enriched) and absent in thrombi (mlck1b, -2, -3). To investigate the role of Mlck1a in thrombus formation, we knocked down mlck1a using two morpholinos. This resulted in impaired morphology changes of platelets adhering on fibrinogen. In a thrombosis model, in which thrombocytes adhere to the vessel wall damaged by laser irradiation, thrombus formation was slowed down in mlck1a-deficient embryos.

CONCLUSION

We conclude that Mlck1a is the subtype of MLCK that contributes to platelet shape change and thrombus formation.

Regulation of stem cells in the zebra fish hematopoietic system

Hematopoietic stem cells (HSCs) have been used extensively as a model for stem cell biology. Stem cells share the ability to self-renew and differentiate into multiple cell types, making them ideal candidates for tissue regeneration or replacement therapies. Current applications of stem cell technology are limited by our knowledge of the molecular mechanisms that control their proliferation and differentiation, and various model organisms have been used to fill these gaps. This chapter focuses on the contributions of the zebra fish model to our understanding of stem cell regulation within the hematopoietic system. Studies in zebra fish have been valuable for identifying new genetic and signaling factors that affect HSC formation and development with important implications for humans, and new advances in the zebra fish toolbox will allow other aspects of HSC behavior to be investigated as well, including migration, homing, and engraftment.

Co-injection strategies to modify radiation sensitivity and tumor initiation in transgenic Zebrafish

The zebrafish has emerged as a powerful genetic model of cancer, but has been limited by the use of stable transgenic approaches to induce disease. Here, a co-injection strategy is described that capitalizes on both the numbers of embryos that can be microinjected and the ability of transgenes to segregate together and exert synergistic effects in forming tumors. Using this mosaic transgenic approach, gene pathways involved in tumor initiation and radiation sensitivity have been identified.

Small molecule screening in the zebrafish

The zebrafish is an ideal organism for small molecule studies. The ability to use the whole organism allows complex in vivo phenotypes to be assayed and combines animal testing with screening. Embryos are easily treatable by waterborne exposure. The small size and abundance of embryos make zebrafish suitable for screening in a high-throughput manner in 96- or 48-well plates. Zebrafish embryos have successfully been used in chemical genetic screens to elucidate biological pathways and find chemical suppressors. Small molecules discovered by screening zebrafish disease models may also be useful as lead compounds for drug development as there appears to be a high level of conservation of drug activity between mammals and zebrafish. Here we provide the technical aspects of treating embryos with small molecules and performing chemical screens with zebrafish.

The dystrophin associated protein complex in zebrafish

Many cases of muscular dystrophy in humans are caused by mutations in members of the dystrophin associated protein complex (DAPC). Zebrafish are small vertebrates whose bodies are composed predominantly of skeletal muscle, making them attractive models for studying mammalian muscle disorders. Potential orthologs to most of the human DAPC proteins have been found in zebrafish by database screening. Expression of the sarcoglycans, dystroglycan and dystrophin has been confirmed by western blotting. Immunohistochemical and biochemical techniques localize these proteins to the muscle cell membrane in adult zebrafish. Morpholino (MO) experiments designed to inhibit the translation of dystrophin mRNA produce juvenile zebrafish that are less active than zebrafish injected with control morpholinos. Western blot analysis of the dystrophin morpholino-injected zebrafish shows concurrent reduction of dystrophin and the sarcoglycans, suggesting that these proteins, like those in mammals, are part of a complex whose integrity is dependent on dystrophin expression. These results indicate that the zebrafish is an excellent animal model in which to approach the study of dystrophin and its associated proteins.

Molecular cloning and developmental expression of Tlx (Hox11) genes in zebrafish (Danio rerio)

Tlx (Hox11) genes are orphan homeobox genes that play critical roles in the regulation of early developmental processes in vertebrates. Here, we report the identification and expression patterns of three members of the zebrafish Tlx family. These genes share similar, but not identical, expression patterns with other vertebrate Tlx-1 and Tlx-3 genes. Tlx-1 is expressed early in the developing hindbrain and pharyngeal arches, and later in the putative splenic primordium. However, unlike its orthologues, zebrafish Tlx-1 is not expressed in the cranial sensory ganglia or spinal cord. Two homologues of Tlx-3 were identified: Tlx-3a and Tlx-3b, which are both expressed in discrete regions of the developing nervous system, including the cranial sensory ganglia and Rohon-Beard neurons. However, only Tlx-3a is expressed in the statoacoustic cranial ganglia, enteric neurons and non-neural tissues such as the fin bud and pharyngeal arches and Tlx-3b is only expressed in the dorsal root ganglia.

Molecular cloning, genetic mapping, and expression analysis of four zebrafish c/ebp genes

The CCAAT/enhancer binding protein family (C/EBP) are transcription factors that play integral roles in the development and function of many organ systems, including hematopoietic cells, adipose tissues, and liver. We have identified and characterized putative zebrafish orthologs of mammalian C/EBP alpha, beta, gamma, and delta using low-stringency hybridization screening and computer searches of the GenBank EST database. c/ebpa and g were mapped within 1 cM of each other on linkage group (LG) 7, syntenic with human CEBPA and G genes on chromosome 19. c/ebpb was mapped to LG8, and c/ebpd was mapped to LG24, on the same LG as a recently identified unique c/ebp in zebrafish, c/ebp1. The mapping of these genes established new syntenic relationships between LG8 and human chromosome 20, extended existing synteny between LG7 and human chromosome 19, and confirmed the synteny between LG24 and human chromosome 8. In addition, these syntenies between zebrafish and human chromosomes are also conserved in the mouse genome. To characterize the expression of these genes, RNA in situ hybridization in embryos of wild type and a hematopoietic mutant, cloche, was performed. The results showed that zebrafish c/ebpa, b, g, and d were expressed in many embryonic tissues. c/ebpa and b were expressed in a subset of hematopoietic cells in a region consistent with myeloid expression. In addition, there was expression of c/ebpa and b in the liver and c/ebpa, b, and d in regions of the gastrointestinal tract. The expression of the c/ebps may serve as important markers for analysis of myelopoiesis, hepatic development, and other developmental processes in the future.

The LN54 radiation hybrid map of zebrafish expressed sequences

To increase the density of a gene map of the zebrafish, Danio rerio, we have placed 3119 expressed sequence tags (ESTs) and cDNA sequences on the LN54 radiation hybrid (RH) panel. The ESTs and genes mapped here join 748 SSLp markers and 459 previously mapped genes and ESTs, bringing the total number of markers on the LN54 RH panel to 4226. Addition of these new markers brings the total LN54 map size to 14,372 cR, with 118 kb/cR. The distribution of ESTs according to linkage groups shows relatively little variation (minimum, 73; maximum, 201). This observation, combined with a relatively uniform size for zebrafish chromosomes, as previously indicated by karyotyping, indicates that there are no especially gene-rich or gene-poor chromosomes in this species. We developed an algorithm to provide a semiautomatic method for the selection of additional framework markers for the LN54 map. This algorithm increased the total number of framework markers to 1150 and permitted the mapping of a high percentage of sequences that could not be placed on a previous version of the LN54 map. The increased concentration of expressed sequences on the LN54 map of the zebrafish genome will facilitate the molecular characterization of mutations in this species.

The art and design of genetic screens: zebrafish

Inventive genetic screens in zebrafish are revealing new genetic pathways that control vertebrate development, disease and behaviour. By exploiting the versatility of zebrafish, biological processes that had been previously obscured can be visualized and many of the responsible genes can be isolated. Coupled with gene knockdown and overexpression technologies, and small-molecule-induced phenotypes, genetic screens in zebrafish provide a powerful system by which to dissect vertebrate gene function and gene networks.

Development. Endothelium--chicken soup for the endoderm

Endothelial cells in blood vessels are known to be important during the later stages of organ development in the embryo. However, their involvement at the induction stage of organ formation has not been previously documented. As Bahary and Zon explain in their Perspective, new work demonstrates that endothelial cells secrete factors early in development that induce embryonic endoderm to become liver or pancreas (Matsumoto et al., Lammert et al.).

neptune, a Krüppel-like transcription factor that participates in primitive erythropoiesis in Xenopus

The specification of the erythroid lineage from hematopoietic stem cells requires the expression and activity of lineage-specific transcription factors. One transcription factor family that has several members involved in hematopoiesis is the Krüppel-like factor (KLF) family [1]. For example, erythroid KLF (EKLF) regulates beta-globin expression during erythroid differentiation [2-6]. KLFs share a highly conserved zinc finger-based DNA binding domain (DBD) that mediates binding to CACCC-box and GC-rich sites, both of which are frequently found in the promoters of hematopoietic genes. Here, we identified a novel Xenopus KLF gene, neptune, which is highly expressed in the ventral blood island (VBI), cranial ganglia, and hatching and cement glands. neptune expression is induced in response to components of the BMP-4 signaling pathway in injected animal cap explants. Similar to its family member, EKLF, Neptune can bind CACCC-box and GC-rich DNA elements. We show that Neptune cooperates with the hematopoietic transcription factor XGATA-1 to enhance globin induction in animal cap explants. A fusion protein comprised of Neptune's DBD and the Drosophila engrailed repressor domain suppresses the induction of globin in ventral marginal zones and in animal caps. These studies demonstrate that Neptune is a positive regulator of primitive erythropoiesis in Xenopus.

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.

Molecular cloning of a human Vent-like homeobox gene

We have isolated a previously unknown human homeobox-containing cDNA, VENT-like homeobox-2 (VENTX2), using PCR with a bone marrow cDNA library and primers designed from the VENTX1 (alias HPX42) homeobox sequence. Here we describe the molecular cloning, chromosomal localization to 10q26.3, and functional analysis of this gene. The 2.4-kb human VENTX2 cDNA encoded a protein with a predicted molecular weight of 28 kDa containing a homeodomain with 65% identity to the Xenopus laevis ventralizing gene Xvent2B. VENTX2 antisera detected a 28-kDa protein in cells transfected with a VENTX2 expression construct, in a human erythroleukemic cell line and in bone marrow samples obtained from patients in recovery phase after chemotherapy. The similarity of the homeodomains from VENTX2 and the X. laevis Vent gene family places them in the same homeodomain class. Consistent with this structural classification, overexpression of VENTX2 in zebrafish embryos led to anterior truncations and failure to form a notochord, which are characteristics of ventralization.

Myelopoiesis in the zebrafish, Danio rerio

Genome-wide chemical mutagenesis screens in the zebrafish (Danio rerio) have led to the identification of novel genes affecting vertebrate erythropoiesis. In determining if this approach could also be used to clarify the molecular genetics of myelopoiesis, it was found that the developmental hierarchy of myeloid precursors in the zebrafish kidney is similar to that in human bone marrow. Zebrafish neutrophils resembled human neutrophils, possessing segmented nuclei and myeloperoxidase-positive cytoplasmic granules. The zebrafish homologue of the human myeloperoxidase (MPO) gene, which is specific to cells of the neutrophil lineage, was cloned and used to synthesize antisense RNA probes for in situ hybridization analyses of zebrafish embryos. Granulocytic cells expressing zebrafish mpo were first evident at 18 hours after fertilization (hpf) in the posterior intermediate cell mass (ICM) and on the anterior yolk sac by 20 hpf. By 24 hpf, mpo-expressing cells were observed along the ICM and within the developing vascular system. Thus, the mpo gene should provide a useful molecular probe for identifying zebrafish mutants with defects in granulopoiesis. The expression of zebrafish homologues was also examined in 2 other mammalian hematopoietic genes, Pu.1, which appears to initiate a commitment step in normal mammalian myeloid development, and L-Plastin, a gene expressed by human monocytes and macrophages. The results demonstrate a high level of conservation of the spatio-temporal expression patterns of these genes between zebrafish and mammals. The morphologic and molecular genetic evidence presented here supports the zebrafish as an informative model system for the study of normal and aberrant human myelopoiesis. (Blood. 2001;98:643-651)

Primitive erythropoiesis in theXenopusembryo: the synergistic role of LMO-2, SCL and GATA-binding proteins

Hematopoietic stem cells are derived from ventral mesoderm during vertebrate development. Gene targeting experiments in the mouse have demonstrated key roles for the basic helix-loop-helix transcription factor SCL and the GATA-binding protein GATA-1 in hematopoiesis. When overexpressed in Xenopus animal cap explants, SCL and GATA-1 are each capable of specifying mesoderm to become blood. Forced expression of either factor in whole embryos, however, does not lead to ectopic blood formation. This apparent paradox between animal cap assays and whole embryo phenotype has led to the hypothesis that additional factors are involved in specifying hematopoietic mesoderm. SCL and GATA-1 interact in a transcriptional complex with the LIM domain protein LMO-2. We have cloned the Xenopus homolog of LMO-2 and show that it is expressed in a similar pattern to SCL during development. LMO-2 can specify hematopoietic mesoderm in animal cap assays. SCL and LMO-2 act synergistically to expand the blood island when overexpressed in whole embryos. Furthermore, co-expression of GATA-1 with SCL and LMO-2 leads to embryos that are ventralized and have blood throughout the dorsal-ventral axis. The synergistic effect of SCL, LMO-2 and GATA-1, taken together with the findings that these factors can form a complex in vitro, suggests that this complex specifies mesoderm to become blood during embryogenesis.

Hepatic maturation in differentiating embryonic stem cells in vitro

We investigated the potential of mouse embryonic stem (ES) cells to differentiate into hepatocytes in vitro. Differentiating ES cells expressed endodermal-specific genes, such as alpha-fetoprotein, transthyretin, alpha 1-anti-trypsin and albumin, when cultured without additional growth factors and late differential markers of hepatic development, such as tyrosine aminotransferase (TAT) and glucose-6-phosphatase (G6P), when cultured in the presence of growth factors critical for late embryonic liver development. Further, induction of TAT and G6P expression was induced regardless of expression of the functional SEK1 gene, which is thought to provide a survival signal for hepatocytes during an early stage of liver morphogenesis. The data indicate that the in vitro ES differentiation system has a potential to generate mature hepatocytes. The system has also been found useful in analyzing the role of growth factors and intracellular signaling molecules in hepatic development.

Fishing for lymphoid genes

Thymic organogenesis and T-cell lymphopoiesis are crucial interdependent processes that establish a functional vertebrate immune system. The current understanding of vertebrate thymic development during embryogenesis remains incomplete and would benefit from novel approaches. The zebrafish Danio rerio is a powerful developmental and genetic system for the dissection of early events in the ontogeny of the immune system. Forward genetic screens have uncovered genes involved in hematopoiesis, and specific screens are being designed to examine the genes that regulate T-cell development and the origin of the thymus. Studies of the zebrafish should improve our understanding of lymphoid development in vertebrates.

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.

Role of MEKK2-MEK5 in the regulation of TNF-alpha gene expression and MEKK2-MKK7 in the activation of c-Jun N-terminal kinase in mast cells

Cross-linking of the high-affinity IgE receptor (FcepsilonRI) on mast cells with IgE and multivalent antigen triggers mitogen-activated protein (MAP) kinase activation and cytokine gene expression. We report here that MAP kinase kinase 4 (MKK4) gene disruption does not affect either MAP kinase activation or cytokine gene expression in response to cross-linking of FcepsilonRI in embryonic stem cell-derived mast cells. MKK7 is activated in response to cross-linking of FcepsilonRI, and this activation is inhibited by MAP/ERK kinase (MEK) kinase 2 (MEKK2) gene disruption. In addition, expression of kinase-inactive MKK7 in the murine mast cell line MC/9 inhibits c-Jun NH(2)-terminal kinase (JNK) activation in response to cross-linking of FcepsilonRI, whereas expression of kinase-inactive MKK4 does not affect JNK activation by this stimulus. However, FcepsilonRI-induced activation of the tumor necrosis factor-alpha (TNF-alpha) gene promoter is not affected by expression of kinase-inactive MKK7. We describe an alternative pathway by which MEKK2 activates MEK5 and big MAP kinase1/extracellular signal-regulated kinase 5 in addition to MKK7 and JNK, and interruption of this pathway inhibits TNF-alpha promoter activation. These findings suggest that JNK activation by antigen cross-linking is dependent on the MEKK2-MKK7 pathway, and cytokine production in mast cells is regulated in part by the signaling complex MEKK2-MEK5-ERK5.

Turning mesoderm into blood: the formation of hematopoietic stem cells during embryogenesis

The formation of hematopoietic stem cells during development occurs by a multistep process that begins with the induction of ventral mesoderm. This mesoderm is patterned during gastrulation by a bone morphogenetic protein (BMP) signaling pathway that is mediated, at least in part, by members of the Mix and Vent families of homeobox transcription factors. Following gastrulation, a subset of ventral mesoderm is specified to become hematopoietic stem cells. Key determinants of hematopoietic fate include the product of the zebrafish cloche gene and the basic helix-loop-helix transcription factor SCL. Future studies in Xenopus and zebrafish should reveal other critical factors in this developmental pathway.

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.

Hereditary spherocytosis in zebrafish riesling illustrates evolution of erythroid beta-spectrin structure, and function in red cell morphogenesis and membrane stability

Spectrins are key cytoskeleton proteins with roles in membrane integrity, cell morphology, organelle transport and cell polarity of varied cell types during development. Defects in erythroid spectrins in humans result in congenital hemolytic anemias with altered red cell morphology. Although well characterized in mammals and invertebrates, analysis of the structure and function of non-mammalian vertebrate spectrins has been lacking. The zebrafish riesling (ris) suffers from profound anemia, where the developing red cells fail to assume terminally differentiated erythroid morphology. Using comparative genomics, erythroid beta-spectrin (sptb) was identified as the gene mutated in ris. Zebrafish Sptb shares 62.3% overall identity with the human ortholog and phylogenetic comparisons suggest intragenic duplication and divergence during evolution. Unlike the human and murine orthologs, the pleckstrin homology domain of zebrafish Sptb is not removed in red cells by alternative splicing. In addition, apoptosis and abnormal microtubule marginal band aggregation contribute to hemolysis of mutant erythrocytes, which are features not present in mammalian red cells with sptb defects. This study presents the first genetic characterization of a non-mammalian vertebrate sptb and demonstrates novel features of red cell hemolysis in non-mammalian red cells. Further, we propose that the distinct mammalian erythroid morphology may have evolved from specific modifications of Sptb structure and function.

Effects of infectious hematopoietic necrosis virus and infectious pancreatic necrosis virus infection on hematopoietic precursors of the zebrafish

The zebrafish Danio rerio is a new model system for studying the genetics of hematopoiesis. To define naturally occurring viruses which could infect and replicate within hematopoietic precursors of the zebrafish, infectious hematopoietic necrosis virus (IHNV) and infectious pancreatic necrosis virus (IPNV) were studied. Infection of whole fish with viral supernatants demonstrated infectious replicants for both viruses, indicating that the virus host range includes the zebrafish. In other species, infection with these viruses leads to prominent hematopoietic necrosis of the head kidney, the major site of adult hematopoiesis. We detected a transient toxicity of the virus to hematopoietic precursors and terminally differentiated red cells after viral infections. The kinetics of hematopoietic defects between IHNV and IPNV infection differed; fish infected with either virus, however, recovered by 6 days postinfection. In contrast to other fish infected with the virus, hematocrit did not change appreciably during this time. These studies are the first to demonstrate IHNV and IPNV infection of the zebrafish and reveal the potential for use of such viruses for gene transfer experiments to infect zebrafish hematopoietic cells.

The gene encoding TBC1D1 with homology to the tre-2/USP6 oncogene, BUB2, and cdc16 maps to mouse chromosome 5 and human chromosome 4

TBC1D1 is the founding member of a family of related proteins with homology to tre-2/UPS6, BUB2, and cdc16 and containing the tbc box motif of 180-220 amino acids. This protein family is thought to have a role in differentiation and in regulating cell growth. We set out to map the TBC1D1 gene in mouse and human. Segregation analysis of a TBC1D1 RFLP in two independent mouse RI (recombinant inbred) lines reveals that mouse Tbc1d1 is closely linked to Pgm1 on chromosome 5. The human TBC1D1 gene was assigned to human chromosome 4p15.1-->4q21 using Southern blot analyses of genomic DNAs from rodent-human somatic cell lines. A human-specific genomic fragment was observed in the somatic cell lines containing human chromosome 4 or the 4p15.1-->4q21 region of the chromosome. TBC1D1 maps to the region containing the ortholog of mouse Pgm1 adding another locus to this long region of conserved synteny between mouse and man.

In vivo immunological function of mast cells derived from embryonic stem cells: an approach for the rapid analysis of even embryonic lethal mutations in adult mice in vivo

An important goal of tissue engineering is to achieve reconstitution of specific functionally active cell types by transplantation of differentiated cell populations derived from normal or genetically altered embryonic stem cells in vitro. We find that mast cells derived in vitro from wild-type or genetically manipulated embryonic stem cells can survive and orchestrate immunologically specific IgE-dependent reactions after transplantation into mast cell-deficient Kit(W)/Kit(W-v) mice. These findings define a unique approach for analyzing the effects of mutations of any genes that are expressed in mast cells, including embryonic lethal mutations, in vitro or in vivo.

An orthologue of the kit-related gene fms is required for development of neural crest-derived xanthophores and a subpopulation of adult melanocytes in the zebrafish, Danio rerio

Developmental mechanisms underlying traits expressed in larval and adult vertebrates remain largely unknown. Pigment patterns of fishes provide an opportunity to identify genes and cell behaviors required for postembryonic morphogenesis and differentiation. In the zebrafish, Danio rerio, pigment patterns reflect the spatial arrangements of three classes of neural crest-derived pigment cells: black melanocytes, yellow xanthophores and silver iridophores. We show that the D. rerio pigment pattern mutant panther ablates xanthophores in embryos and adults and has defects in the development of the adult pattern of melanocyte stripes. We find that panther corresponds to an orthologue of the c-fms gene, which encodes a type III receptor tyrosine kinase and is the closest known homologue of the previously identified pigment pattern gene, kit. In mouse, fms is essential for the development of macrophage and osteoclast lineages and has not been implicated in neural crest or pigment cell development. In contrast, our analyses demonstrate that fms is expressed and required by D. rerio xanthophore precursors and that fms promotes the normal patterning of melanocyte death and migration during adult stripe formation. Finally, we show that fms is required for the appearance of a late developing, kit-independent subpopulation of adult melanocytes. These findings reveal an unexpected role for fms in pigment pattern development and demonstrate that parallel neural crest-derived pigment cell populations depend on the activities of two essentially paralogous genes, kit and fms.

Cloning and genetic mapping of zebrafish BMP-2

The BMP family of polypeptide growth factors has been shown to play diverse roles in establishing embryonic patterning and tissue fates. We report the cloning of the zebrafish homologue of BMP-2, examine its expression during embryogenesis, and find that it is localized to the distal end of the long arm of zebrafish chromosome 20. A missense mutation of the bmp2 gene has recently been shown to be responsible for the early dorsalized phenotype of the zebrafish swirl mutant [Kishimoto et al., 1997]. Given the dynamic expression of bmp2 in the developing embryo and the complex interactions of BMP signaling response in vertebrates, it is possible that other mutant phenotypes, due to altered bmp2 gene expression, will eventually map to or interact with this genetic locus.

Zebrafish: a model system for the study of human disease

The zebrafish (Danio rerio) is a powerful model organism for the study of vertebrate biology, being well suited to both developmental and genetic analysis. Large-scale genetic screens have identified hundreds of mutant phenotypes, many of which resemble human clinical disorders. The creation of critical genetic reagents, coupled with the rapid progress of the zebrafish genome initiative directed by the National Institutes of Health, are bringing this model system to its full potential for the study of vertebrate biology, physiology and human disease.

FOG acts as a repressor of red blood cell development in Xenopus

Members of the GATA family of zinc-finger transcription factors have critical roles in a variety of cell types. GATA-1, GATA-2 and GATA-3 are required for proliferation and differentiation of several hematopoietic lineages, whereas GATA-4, GATA-5 and GATA-6 activate cardiac and endoderm gene expression programs. Two GATA cofactors have recently been identified. Friend of GATA-1 (FOG-1) interacts with GATA-1 and is expressed principally in hematopoietic lineages, whereas FOG-2 is expressed predominantly in heart and brain. Although gene targeting experiments are consistent with an essential role for FOG-1 as an activator of GATA-1 function, reporter assays in transfected cells indicate that FOG-1 and FOG-2 can act as repressors. We have cloned a Xenopus laevis homologue of FOG that is structurally most similar to FOG-1, but is expressed predominantly in heart and brain, as well as the ventral blood island and adult spleen. Ectopic expression and explant assays demonstrate that FOG proteins can act as repressors in vivo, in part through interaction with the transcriptional co-repressor, C-terminal Binding Protein (CtBP). FOG may regulate the differentiation of red blood cells by modulating expression and activity of GATA-1 and GATA-2. We propose that the FOG proteins participate in the switch from progenitor proliferation to red blood cell maturation and differentiation.

Realizing the potential of zebrafish as a model for human disease

The value of the zebrafish (Danio rerio) as a model for human disease has been substantiated by a number of recently published papers. Several zebrafish mutants with "human" diseases have been found, spanning a variety of human pathologies. These successful studies utilizing the zebrafish have been made possible by the development of key reagents such as YAC, PAC, and BAC libraries, as well as radiation hybrid panels. With the further establishment of new tools and access to the newly generated resources, the zebrafish is poised to serve as a novel model for human disease.

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.

Positional cloning of zebrafish ferroportin1 identifies a conserved vertebrate iron exporter

Defects in iron absorption and utilization lead to iron deficiency and overload disorders. Adult mammals absorb iron through the duodenum, whereas embryos obtain iron through placental transport. Iron uptake from the intestinal lumen through the apical surface of polarized duodenal enterocytes is mediated by the divalent metal transporter, DMTi. A second transporter has been postulated to export iron across the basolateral surface to the circulation. Here we have used positional cloning to identify the gene responsible for the hypochromic anaemia of the zebrafish mutant weissherbst. The gene, ferroportin1, encodes a multiple-transmembrane domain protein, expressed in the yolk sac, that is a candidate for the elusive iron exporter. Zebrafish ferroportin1 is required for the transport of iron from maternally derived yolk stores to the circulation and functions as an iron exporter when expressed in Xenopus oocytes. Human Ferroportin1 is found at the basal surface of placental syncytiotrophoblasts, suggesting that it also transports iron from mother to embryo. Mammalian Ferroportin1 is expressed at the basolateral surface of duodenal enterocytes and could export cellular iron into the circulation. We propose that Ferroportin1 function may be perturbed in mammalian disorders of iron deficiency or overload.

Developmental derivation of embryonic and adult macrophages

The macrophage cell lineage continually arises from hematopoietic stem cells during embryonic, fetal, and adult life. Previous theories proposed that macrophages are the recent progeny of bone marrow-derived monocytes and that they function primarily in phagocytosis. More recently, however, observations have shown that the ontogeny of macrophages in early mouse and human embryos is different from that occurring during adult development, and that the embryonic macrophages do not follow the monocyte pathway. Fetal macrophages are thought to differentiate from yolk sac-derived primitive macrophages before the development of adult monocytes. Further support for a separate lineage of fetal macrophages has come from studies of several species, including chicken, zebrafish, Xenopus, Drosophila, and C. elegans. The presence of fetal macrophages in PU.1-null mice indicates their independence from monocyte precursors and their existence as an alternative macrophage lineage.

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.

Concentration-dependent positive and negative regulation of a MAP kinase by a MAP kinase kinase

There are at least three distinct MAP kinase signaling modules in mammalian cells, distinguished by the family of kinases (Erk, SAPK/JNK, or p38) that is ultimately activated. Many input signals activate multiple MAP kinase cascades, and the mechanisms that control the specificity of signal output are not well understood. We show that SEK1/MKK4, a MAP kinase kinase proposed to activate SAPK/JNK, is a very potent inhibitor of p54 SAPK beta/JNK3 both in vitro and in vivo if present at equimolar or higher ratios. In contrast SEK can activate SAPK when present in substoichiometric amounts, but this activation is slow, consistent with the rate-limiting step in activation being the dissociation of an inactive SEK:SAPK complex. The N-terminal unique region of SEK is both necessary and partially sufficient for inhibition of SAPK, and is also necessary for activation of SAPK by SEK in vitro. We have also used the p38 MAP kinase and its activator MKK6 to examine the regulatory relationships among different kinases involved in stress responses. We show using purified kinases that inhibitory activity is specific for the combination of SEK and SAPK: SEK can activate but not inhibit p38, and MKK6 can activate but not inhibit SAPK beta and p38. These results reveal a potential mechanism for regulating stress-activated kinases, adding to a growing body of evidence suggesting that MAP kinases are controlled by relatively stable interactions with their activators.

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 JAK2 gene may play a role in leukemia. We have isolated and characterized jak 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 cloche and 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 mammalian Jak2, with a role in early erythropoiesis.

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.

Radiation hybrid mapping of the zebrafish genome

The zebrafish is an excellent genetic system for the study of vertebrate development and disease. In an effort to provide a rapid and robust tool for zebrafish gene mapping, a panel of radiation hybrids (RH) was produced by fusion of irradiated zebrafish AB9 cells with mouse B78 cells. The overall retention of zebrafish sequences in the 93 RH cell lines that constitute the LN54 panel is 22%. Characterization of the LN54 panel with 849 simple sequence length polymorphism markers, 84 cloned genes and 122 expressed sequence tags allowed the production of an RH map whose total size was 11,501 centiRays. From this value, we estimated the average breakpoint frequency of the LN54 RH panel to correspond to 1 centiRay = 148 kilobase. Placement of a group of 235 unbiased markers on the RH map suggests that the map generated for the LN54 panel, at present, covers 88% of the zebrafish genome. Comparison of marker positions in RH and meiotic maps indicated a 96% concordance. Mapping expressed sequence tags and cloned genes by using the LN54 panel should prove to be a valuable method for the identification of candidate genes for specific mutations in zebrafish.