The 'definitive' (and 'primitive') guide to zebrafish hematopoiesis

Maternal pak4 expression is required for primitive myelopoiesis in zebrafish

Transcripts of pak4, the zebrafish ortholog of p21-activated kinase 4 (PAK4), are most abundant in the egg and fall to low levels by the end of gastrulation, after which expression is essentially ubiquitous. Translation of maternal mRNA into pak4 protein is first detectable at high stage (3.3hpf). Splice-blocking morpholino oligonucleotides (MOs) were used to prevent zygotic pak4 expression. This had no discernable effect on development through larval stages. In contrast, a translation-blocking MO, alone or in combination with the splice MOs, resulted in a complex lethal phenotype. In addition to disrupted somite development and other morphogenetic abnormalities, the knockdown of maternal pak4 expression led to alterations in regulatory gene expression in the primitive hematopoietic domains, leading to deficiencies in granulocyte and leukocyte lineages. At least some of the effects of pak4 knockdown on gene expression could be mimicked by treatment with actin depolymerization agents, suggesting a mechanistic link between regulation of microfilament dynamics by pak4 and regulation of gene expression in primitive myeloid cell differentiation.

Teleost growth factor independence (gfi) genes differentially regulate successive waves of hematopoiesis

Growth Factor Independence (Gfi) transcription factors play essential roles in hematopoiesis, differentially activating and repressing transcriptional programs required for hematopoietic stem/progenitor cell (HSPC) development and lineage specification. In mammals, Gfi1a regulates hematopoietic stem cells (HSC), myeloid and lymphoid populations, while its paralog, Gfi1b, regulates HSC, megakaryocyte and erythroid development. In zebrafish, gfi1aa is essential for primitive hematopoiesis; however, little is known about the role of gfi1aa in definitive hematopoiesis or about additional gfi factors in zebrafish. Here, we report the isolation and characterization of an additional hematopoietic gfi factor, gfi1b. We show that gfi1aa and gfi1b are expressed in the primitive and definitive sites of hematopoiesis in zebrafish. Our functional analyses demonstrate that gfi1aa and gfi1b have distinct roles in regulating primitive and definitive hematopoietic progenitors, respectively. Loss of gfi1aa silences markers of early primitive progenitors, scl and gata1. Conversely, loss of gfi1b silences runx-1, c-myb, ikaros and cd41, indicating that gfi1b is required for definitive hematopoiesis. We determine the epistatic relationships between the gfi factors and key hematopoietic transcription factors, demonstrating that gfi1aa and gfi1b join lmo2, scl, runx-1 and c-myb as critical regulators of teleost HSPC. Our studies establish a comparative paradigm for the regulation of hematopoietic lineages by gfi transcription factors.

Large-scale forward genetic screening analysis of development of hematopoiesis in zebrafish

Zebrafish is a powerful model for the investigation of hematopoiesis. In order to isolate novel mutants with hematopoietic defects, large-scale mutagenesis screening of zebrafish was performed. By scoring specific hematopoietic markers, 52 mutants were identified and then classified into four types based on specific phenotypic traits. Each mutant represented a putative mutation of a gene regulating the relevant aspect of hematopoiesis, including early macrophage development, early granulopoiesis, embryonic myelopoiesis, and definitive erythropoiesis/lymphopoiesis. Our method should be applicable for other types of genetic screening in zebrafish. In addition, further study of the mutants we identified may help to unveil the molecular basis of hematopoiesis.

Novel insights into the genetic controls of primitive and definitive hematopoiesis from zebrafish models

Hematopoiesis is a dynamic process where initiation and maintenance of hematopoietic stem cells, as well as their differentiation into erythroid, myeloid and lymphoid lineages, are tightly regulated by a network of transcription factors. Understanding the genetic controls of hematopoiesis is crucial as perturbations in hematopoiesis lead to diseases such as anemia, thrombocytopenia, or cancers, including leukemias and lymphomas. Animal models, particularly conventional and conditional knockout mice, have played major roles in our understanding of the genetic controls of hematopoiesis. However, knockout mice for most of the hematopoietic transcription factors are embryonic lethal, thus precluding the analysis of their roles during the transition from embryonic to adult hematopoiesis. Zebrafish are an ideal model organism to determine the function of a gene during embryonic-to-adult transition of hematopoiesis since bloodless zebrafish embryos can develop normally into early larval stage by obtaining oxygen through diffusion. In this review, we discuss the current status of the ontogeny and regulation of hematopoiesis in zebrafish. By providing specific examples of zebrafish morphants and mutants, we have highlighted the contributions of the zebrafish model to our overall understanding of the roles of transcription factors in regulation of primitive and definitive hematopoiesis.

Ginger stimulates hematopoiesis via Bmp pathway in zebrafish

BACKGROUND

Anemia is a hematologic disorder with decreased number of erythrocytes. Erythropoiesis, the process by which red blood cells differentiate, are conserved in humans, mice and zebrafish. The only known agents available to treat pathological anemia are erythropoietin and its biologic derivatives. However, erythropoietin therapy elicits unwanted side-effects, high cost and intravenous or subcutaneous injection, warranting the development of a more cost effective and non-peptide alternative. Ginger (Zingiber officinale) has been widely used in traditional medicine; however, to date there is no scientific research documenting the potential of ginger to stimulate hematopoiesis.

METHODOLOGY/PRINCIPAL FINDINGS

Here, we utilized gata1:dsRed transgenic zebrafish embryos to investigate the effect of ginger extract on hematopoiesis in vivo and we identified its bioactive component, 10-gingerol. We confirmed that ginger and 10-gingerol promote the expression of gata1 in erythroid cells and increase the expression of hematopoietic progenitor markers cmyb and scl. We also demonstrated that ginger and 10-gingerol can promote the hematopoietic recovery from acute hemolytic anemia in zebrafish, by quantifying the number of circulating erythroid cells in the dorsal aorta using video microscopy. We found that ginger and 10-gingerol treatment during gastrulation results in an increase of bmp2b and bmp7a expression, and their downstream effectors, gata2 and eve1. At later stages ginger and 10-gingerol can induce bmp2b/7a, cmyb, scl and lmo2 expression in the caudal hematopoietic tissue area. We further confirmed that Bmp/Smad pathway mediates this hematopoiesis promoting effect of ginger by using the Bmp-activated Bmp type I receptor kinase inhibitors dorsomorphin, LND193189 and DMH1.

CONCLUSIONS/SIGNIFICANCE

Our study provides a strong foundation to further evaluate the molecular mechanism of ginger and its bioactive components during hematopoiesis and to investigate their effects in adults. Our results will provide the basis for future research into the effect of ginger during mammalian hematopoiesis to develop novel erythropoiesis promoting agents.

Zebrafish thrombocytes: functions and origins

Platelets play an important role in mammalian hemostasis. Thrombocytes of early vertebrates are functionally equivalent to mammalian platelets. A substantial amount of research has been done to study platelet function in humans as well as in animal models. However, to date only limited functional genomic studies of platelets have been performed but are low throughput and are not cost-effective. Keeping this in mind we introduced zebrafish, a vertebrate genetic model to study platelet function. We characterized zebrafish thrombocytes and established functional assays study not only their hemostatic function but to also their production. We identified a few genes which play a role in their function and production. Since we introduced the zebrafish model for the study of hemostasis and thrombosis, other groups have adapted this model to study genes that are associated with thrombocyte function and a few novel genes have also been identified. Furthermore, transgenic zebrafish with GFP-tagged thrombocytes have been developed which helped to study the production of thrombocytes and their precursors as well as their functional roles not only in hemostasis but also hematopoiesis. This paper integrates the information available on zebrafish thrombocyte function and its formation.

Hematopoietic stem cell development and regulatory signaling in zebrafish

BACKGROUND

Hematopoietic stem cells (HSCs) are a population of multipotent cells that can self-renew and differentiate into all blood lineages. HSC development must be tightly controlled from cell fate determination to self-maintenance during adulthood. This involves a panel of important developmental signaling pathways and other factors which act synergistically within the HSC population and/or in the HSC niche. Genetically conserved processes of HSC development plus many other developmental advantages make the zebrafish an ideal model organism to elucidate the regulatory mechanisms underlying HSC programming.

SCOPE OF REVIEW

This review summarizes recent progress on zebrafish HSCs with particular focus on how developmental signaling controls hemogenic endothelium-derived HSC development. We also describe the interaction of different signaling pathways during these processes.

MAJOR CONCLUSIONS

The hematopoietic stem cell system is a paradigm for stem cell studies. Use of the zebrafish model to study signaling regulation of HSCs in vivo has resulted in a great deal of information concerning HSC biology in vertebrates.

GENERAL SIGNIFICANCE

These new findings facilitate a better understanding of molecular mechanisms of HSC programming, and will provide possible new strategies for the treatment of HSC-related hematological diseases, such as leukemia. This article is part of a Special Issue entitled Biochemistry of Stem Cells.

Retinoic acid signaling plays a restrictive role in zebrafish primitive myelopoiesis

Retinoic acid (RA) is known to regulate definitive myelopoiesis but its role in vertebrate primitive myelopoiesis remains unclear. Here we report that zebrafish primitive myelopoiesis is restricted by RA in a dose dependent manner mainly before 11 hpf (hours post fertilization) when anterior hemangioblasts are initiated to form. RA treatment significantly reduces expressions of anterior hemangioblast markers scl, lmo2, gata2 and etsrp in the rostral end of ALPM (anterior lateral plate mesoderm) of the embryos. The result indicates that RA restricts primitive myelopoiesis by suppressing formation of anterior hemangioblasts. Analyses of ALPM formation suggest that the defective primitive myelopoiesis resulting from RA treatment before late gastrulation may be secondary to global loss of cells for ALPM fate whereas the developmental defect resulting from RA treatment during 10–11 hpf should be due to ALPM patterning shift. Overexpressions of scl and lmo2 partially rescue the block of primitive myelopoiesis in the embryos treated with 250 nM RA during 10–11 hpf, suggesting RA acts upstream of scl to control primitive myelopoiesis. However, the RA treatment blocks the increased primitive myelopoiesis caused by overexpressing gata4/6 whereas the abolished primitive myelopoiesis in gata4/5/6 depleted embryos is well rescued by 4-diethylamino-benzaldehyde, a retinal dehydrogenase inhibitor, or partially rescued by knocking down aldh1a2, the major retinal dehydrogenase gene that is responsible for RA synthesis during early development. Consistently, overexpressing gata4/6 inhibits aldh1a2 expression whereas depleting gata4/5/6 increases aldh1a2 expression. The results reveal that RA signaling acts downstream of gata4/5/6 to control primitive myelopoiesis. But, 4-diethylamino-benzaldehyde fails to rescue the defective primitive myelopoiesis in either cloche embryos or lycat morphants. Taken together, our results demonstrate that RA signaling restricts zebrafish primitive myelopoiesis through acting downstream of gata4/5/6, upstream of, or parallel to, cloche, and upstream of scl.

Zebrafish: model for the study of inflammation and the innate immune response to infectious diseases

The zebrafish (Danio rerio) has been extensively used in biomedical research as a model to study vertebrate development and hematopoiesis and recently, it has been adopted into varied fields including immunology. After fertilization, larvae survive with only the innate immune responses because adaptive immune system is morphologically and functionally mature only after 4-6 weeks postfertilization. This temporal separation provides a suitable system to study the vertebrate innate immune response in vivo, independently from the adaptive immune response. The transparency of early life stages allows a useful real-time visualization. Adult zebrafish which have complete (innate and adaptative) immune systems offer also advantages over other vertebrate infection models: small size, relatively rapid life cycle, ease of breeding, and a growing list of molecular tools for the study of infectious diseases. In this review, we have tried to give some examples of the potential of zebrafish as a valuable model in innate immunity and inflammation studies.

Using zebrafish (Danio rerio) to assess gene function in thrombus formation

Cardiovascular and cerebrovascular disease is the major cause of death in the developed world, with a high burden of disease and substantial pharmaceutical investment to manage it (WHO, Global Burden of Disease, 2004 Update, W.H. Organisation, Editor. 2008). Platelets, as the principal mediators of thrombus formation, are a primary pharmaceutical target, with attenuation of platelet function and thrombus formation significantly reducing the incidence of myocardial infarction and stroke. Haemostasis, however, may also be affected by antithrombotics, leading to spontaneous and/or prolonged bleeding as a potentially severe side effect. Developing a comprehensive understanding of the mechanisms involved in platelet function and thrombus formation is anticipated to identify drug targets that may effectively manage vascular disease without an impact on haemostasis. Despite the progress in characterising individual genes in platelet function and thrombosis, using gene knockout and transgenic mice over the past decade or so, there is still much to be uncovered. Investigating gene function using mouse models is a substantial investment and a considerable amount of work, with a relevant phenotype not guaranteed. As such, a new model is needed for the effective screening of novel genes that have been identified as having potential roles in platelet function or cardiovascular disease by genomic association and comparative expression studies (Nature, 447(7145): 661-678, 2007; Nat Genet, 41(11): 1182-1190, 2009; N Engl J Med, 357(5): 443-453, 2007; Blood, 109(8): 3260-3269, 2007). Here, we highlight and discuss the relevance of the zebrafish (Danio rerio) as a model for studying thrombosis, the current techniques that are employed to assess gene function in a zebrafish model of thrombosis, and how an effective genetic screen may be constructed.

Characterization of a weak allele of zebrafish cloche mutant

Hematopoiesis is a complicated and dynamic process about which the molecular mechanisms remain poorly understood. Danio rerio (zebrafish) is an excellent vertebrate system for studying hematopoiesis and developmental mechanisms. In the previous study, we isolated and identified a cloche¹⁷² (clo¹⁷²) mutant, a novel allele compared to the original cloche (clo) mutant, through using complementation test and initial mapping. Here, according to whole mount in-situ hybridization, we report that the endothelial cells in clo¹⁷² mutant embryos, although initially developed, failed to form the functional vascular system eventually. In addition, further characterization indicates that the clo¹⁷² mutant exhibited weaker defects instead of completely lost in primitive erythroid cells and definitive hematopoietic cells compared with the clo^s5 mutant. In contrast, primitive myeloid cells were totally lost in clo¹⁷² mutant. Furthermore, these reappeared definitive myeloid cells were demonstrated to initiate from the remaining hematopoietic stem cells (HSCs) in clo¹⁷² mutant, confirmed by the dramatic decrease of lyc in clo¹⁷²runx1^w84x double mutant. Collectively, the clo¹⁷² mutant is a weak allele compared to the clo^s5 mutant, therefore providing a model for studying the early development of hematopoietic and vascular system, as well as an opportunity to further understand the function of the cloche gene.

Zebrafish as a model for normal and malignant hematopoiesis

Zebrafish studies in the past two decades have made major contributions to our understanding of hematopoiesis and its associated disorders. The zebrafish has proven to be a powerful organism for studies in this area owing to its amenability to large-scale genetic and chemical screening. In addition, the externally fertilized and transparent embryos allow convenient genetic manipulation and in vivo imaging of normal and aberrant hematopoiesis. This review discusses available methods for studying hematopoiesis in zebrafish, summarizes key recent advances in this area, and highlights the current and potential contributions of zebrafish to the discovery and development of drugs to treat human blood disorders.

Deep sequencing of the innate immune transcriptomic response of zebrafish embryos to Salmonella infection

Salmonella enterica serovar Typhimurium (S. typhimurium) bacteria cause an inflammatory and lethal infection in zebrafish embryos. To characterize the embryonic innate host response at the transcriptome level, we have extended and validated previous microarray data by Illumina next-generation sequencing analysis. We obtained 10 million sequence reads from control and Salmonella-infected zebrafish embryos using a tag-based sequencing method (DGE or Tag-Seq) and 15 million reads using whole transcript sequencing (RNA-Seq), which respectively mapped to circa 65% and 85% of 28,716 known Ensembl transcripts. Both sequencing methods showed a strong correlation of sequence read counts per transcript and an overlap of 241 transcripts differentially expressed in response to infection. A lower overlap of 165 transcripts was observed with previous microarray data. Based on the combined sequencing-based and microarray-based transcriptome data we compiled an annotated reference set of infection-responsive genes in zebrafish embryos, encoding transcription factors, signal transduction proteins, cytokines and chemokines, complement factors, proteins involved in apoptosis and proteolysis, proteins with anti-microbial activities, as well as many known or novel proteins not previously linked to the immune response. Furthermore, by comparison of the deep sequencing data of S. typhimurium infection in zebrafish embryos with previous deep sequencing data of Mycobacterium marinum infection in adult zebrafish we derived a common set of infection-responsive genes. This gene set consists of known and putative innate host defense genes that are expressed both in the absence and presence of a fully developed adaptive immune system and that provide a valuable reference for future studies of host-pathogen interactions using zebrafish infection models.

Comparison of static immersion and intravenous injection systems for exposure of zebrafish embryos to the natural pathogen Edwardsiella tarda

Background

The zebrafish embryo is an important in vivo model to study the host innate immune response towards microbial infection. In most zebrafish infectious disease models, infection is achieved by micro-injection of bacteria into the embryo. Alternatively, Edwardsiella tarda, a natural fish pathogen, has been used to treat embryos by static immersion. In this study we used transcriptome profiling and quantitative RT-PCR to analyze the immune response induced by E. tarda immersion and injection.

Results

Mortality rates after static immersion of embryos in E. tarda suspension varied between 25-75%, while intravenous injection of bacteria resulted in 100% mortality. Quantitative RT-PCR analysis on the level of single embryos showed that expression of the proinflammatory marker genes il1b and mmp9 was induced only in some embryos that were exposed to E. tarda in the immersion system, whereas intravenous injection of E. tarda led to il1b and mmp9 induction in all embryos. In addition, microarray expression profiles of embryos subjected to immersion or injection showed little overlap. E. tarda-injected embryos displayed strong induction of inflammatory and defense genes and of regulatory genes of the immune response. E. tarda-immersed embryos showed transient induction of the cytochrome P450 gene cyp1a. This gene was also induced after immersion in Escherichia coli and Pseudomonas aeruginosa suspensions, but, in contrast, was not induced upon intravenous E. tarda injection. One of the rare common responses in the immersion and injection systems was induction of irg1l, a homolog of a murine immunoresponsive gene of unknown function.

Conclusions

Based on the differences in mortality rates between experiments and gene expression profiles of individual embryos we conclude that zebrafish embryos cannot be reproducibly infected by exposure to E. tarda in the immersion system. Induction of il1b and mmp9 was consistently observed in embryos that had been systemically infected by intravenous injection, while the early transcriptional induction of cyp1a and irg1l in the immersion system may reflect an epithelial or other tissue response towards cell membrane or other molecules that are shed or released by bacteria. Our microarray expression data provide a useful reference for future analysis of signal transduction pathways underlying the systemic innate immune response versus those underlying responses to external bacteria and secreted virulence factors and toxins.

Zebrafish models to study hypoxia-induced pathological angiogenesis in malignant and nonmalignant diseases

Most in vivo preclinical disease models are based on mouse and other mammalian systems. However, these rodent-based model systems have considerable limitations to recapitulate clinical situations in human patients. Zebrafish have been widely used to study embryonic development, behavior, tissue regeneration, and genetic defects. Additionally, zebrafish also provides an opportunity to screen chemical compounds that target a specific cell population for drug development. Owing to the availability of various genetically manipulated strains of zebrafish, immune privilege during early embryonic development, transparency of the embryos, and easy and precise setup of hypoxia equipment, we have developed several disease models in both embryonic and adult zebrafish, focusing on studying the role of angiogenesis in pathological settings. These zebrafish disease models are complementary to the existing mouse models, allowing us to study clinically relevant processes in cancer and nonmalignant diseases, which otherwise would be difficult to study in mice. For example, dissemination and invasion of single human or mouse tumor cells from the primary site in association with tumor angiogenesis can be studied under normoxia or hypoxia in zebrafish embryos. Hypoxia-induced retinopathy in the adult zebrafish recapitulates the clinical situation of retinopathy development in diabetic patients or age-related macular degeneration. These zebrafish disease models offer exciting opportunities to understand the mechanisms of disease development, progression, and development of more effective drugs for therapeutic intervention.

Methionine aminopeptidase 2 is required for HSC initiation and proliferation

In a chemical screening, we tested the antiangiogenic effects of fumagillin derivatives and identified fumagillin as an inhibitor of definitive hematopoiesis in zebrafish embryos. Fumagillin is known to target methionine aminopeptidase II (MetAP2), an enzyme whose function in hematopoiesis is unknown. We investigated the role of MetAP2 in hematopoiesis by using zebrafish embryo and human umbilical cord blood models. Zebrafish metap2 was expressed ubiquitously during early embryogenesis and later in the somitic region, the caudal hematopoietic tissue, and pronephric duct. metap2 was inhibited by morpholino and fumagillin treatment, resulting in increased mpo expression at 18 hours postfertilization and reduced c-myb expression along the ventral wall of dorsal aorta at 36 hours postfertilization. It also disrupted intersegmental vessels in Tg(fli1:gfp) embryos without affecting development of major axial vasculatures. Inhibition of MetAP2 in CB CD34(+) cells by fumagillin had no effect on overall clonogenic activity but significantly reduced their engraftment into immunodeficient nonobese diabetes/severe combined immunodeficiency mice. metap2 knock-down in zebrafish and inhibition by fumagillin in zebrafish and human CB CD34(+) cells inhibited Calmodulin Kinase II activity and induced ERK phosphorylation. This study demonstrated a hitherto-undescribed role of MetAP2 in definitive hematopoiesis and a possible link to noncanonical Wnt and ERK signaling.

Zebrafish stromal cells have endothelial properties and support hematopoietic cells

The goal of this study was to determine if we could establish a mesenchymal stromal line from zebrafish that would support hematopoietic cells. Such a coculture system would be a great benefit to study of the hematopoietic cell-stromal cell interaction in both in vitro and in vivo environments. Zebrafish stromal cells (ZStrC) were isolated from the "mesenchymal" tissue of the caudal tail and expanded in a specialized growth media. ZStrC were evaluated for phenotype, gene expression, and ability to maintain zebrafish marrow cells in coculture experiments. ZStrC showed mesenchymal and endothelial gene expression. Although ZStrC lacked the ability to differentiate into classic mesenchymal stromal cell lineages (i.e., osteocytes, adipocytes, chondrocytes), they did have the capacity for endotube formation on Matrigel and low-density lipoprotein uptake. ZStrC supported marrow cells for >2 weeks in vitro. Importantly, marrow cells were shown to retain homing ability in adoptive transfer experiments. ZStrC were also shown to improve hematopoietic recovery after sublethal irradiation after adoptive transfer. As the zebrafish model grows in popularity and importance in the study of hematopoiesis, new tools to aid in our understanding of the hematopoietic cell-stromal cell interaction are required. ZStrC represent an additional tool in the study of hematopoiesis and will be useful in understanding the factors that mediate the stromal cell-hematopoietic cell interactions that are important in hematopoietic cell maintenance.

Host-pathogen interactions made transparent with the zebrafish model

The zebrafish holds much promise as a high-throughput drug screening model for immune-related diseases, including inflammatory and infectious diseases and cancer. This is due to the excellent possibilities for in vivo imaging in combination with advanced tools for genomic and large scale mutant analysis. The context of the embryo’s developing immune system makes it possible to study the contribution of different immune cell types to disease progression. Furthermore, due to the temporal separation of innate immunity from adaptive responses, zebrafish embryos and larvae are particularly useful for dissecting the innate host factors involved in pathology. Recent studies have underscored the remarkable similarity of the zebrafish and human immune systems, which is important for biomedical applications. This review is focused on the use of zebrafish as a model for infectious diseases, with emphasis on bacterial pathogens. Following a brief overview of the zebrafish immune system and the tools and methods used to study host-pathogen interactions in zebrafish, we discuss the current knowledge on receptors and downstream signaling components that are involved in the zebrafish embryo’s innate immune response. We summarize recent insights gained from the use of bacterial infection models, particularly the Mycobacterium marinum model, that illustrate the potential of the zebrafish model for high-throughput antimicrobial drug screening.

Dissection of the adult zebrafish kidney

Researchers working in the burgeoning field of adult stem cell biology seek to understand the signals that regulate the behavior and function of stem cells during normal homeostasis and disease states. The understanding of adult stem cells has broad reaching implications for the future of regenerative medicine. For example, better knowledge about adult stem cell biology can facilitate the design of therapeutic strategies in which organs are triggered to heal themselves or even the creation of methods for growing organs in vitro that can be transplanted into humans. The zebrafish has become a powerful animal model for the study of vertebrate cell biology. There has been extensive documentation and analysis of embryonic development in the zebrafish. Only recently have scientists sought to document adult anatomy and surgical dissection techniques, as there has been a progressive movement within the zebrafish community to broaden the applications of this research organism to adult studies. For example, there are expanding interests in using zebrafish to investigate the biology of adult stem cell populations and make sophisticated adult models of diseases such as cancer. Historically, isolation of the zebrafish adult kidney has been instrumental for studying hematopoiesis, as the kidney is the anatomical location of blood cell production in fish. The kidney is composed of nephron functional units found in arborized arrangements, surrounded by hematopoietic tissue that is dispersed throughout the intervening spaces. The hematopoietic component consists of hematopoietic stem cells (HSCs) and their progeny that inhabit the kidney until they terminally differentiate. In addition, it is now appreciated that a group of renal stem/progenitor cells (RPCs) also inhabit the zebrafish kidney organ and enable both kidney regeneration and growth, as observed in other fish species. In light of this new discovery, the zebrafish kidney is one organ that houses the location of two exciting opportunities for adult stem cell biology studies. It is clear that many outstanding questions could be well served with this experimental system. To encourage expansion of this field, it is beneficial to document detailed methods of visualizing and then isolating the adult zebrafish kidney organ. This protocol details our procedure for dissection of the adult kidney from both unfixed and fixed animals. Dissection of the kidney organ can be used to isolate and characterize hematopoietic and renal stem cells and their offspring using established techniques such as histology, fluorescence activated cell sorting (FACS), expression profiling, and transplantation. We hope that dissemination of this protocol will provide researchers with the knowledge to implement broader use of zebrafish studies that ultimately can be translated for human application.

Is there a direct role for erythrocytes in the immune response?

Erythrocytes are highly abundant circulating cells in the vertebrates, which, with the notable exception of mammals, remain nucleated throughout the entire life cycle. The major function associated with these cells is respiratory gas exchange however other functions including interaction with the immune system have been attributed to these cells. Many viral, prokaryotic and eukaryotic pathogens directly target this cell type and across the vertebrate group a significant number of related pathologies have been reported. Across the primary literature mechanisms of interaction, invasion and replication between viruses and erythrocytes have been well described however the functional response of the erythrocyte has been poorly studied. A fragmented series of reports spanning the vertebrates suggests that these cells are capable of functional responses to viral infection. In contrast, in-depth proteomic studies using human erythrocytes have strongly progressed throughout the past decade providing a rich source of information related to protein expression and potential function. Furthermore information at the gene expression level is becoming available. Here we provide a review of erythrocyte-pathogen interactions, erythrocyte functions in immunity and propose in light of recent -omics research that the nucleated erythrocytes may have a direct role in the immune response.

Sumoylation of CCAAT/enhancer–binding protein α promotes the biased primitive hematopoiesis of zebrafish

Hematopoiesis is evolutionarily conserved from zebrafish to mammals, and this includes both primitive and definitive waves during embryogenesis. Primitive hematopoiesis is dominated by erythropoiesis with limited myelopoiesis. Protein sumoylation, a ubiquitination-like posttranslational protein modification, is implicated in a variety of biochemical processes, most notably in transcriptional repression. We show here that the loss of 6 small ubiquitin-related modifier (SUMO) paralogs triggers a sharp up-regulation of the myeloid-specific marker mpo and down-regulation of the erythroid-specific marker gata1 in myelo-erythroid progenitor cells (MPCs) in the intermediate cell mass (ICM) during primitive hematopoiesis. Accordingly, in transgenic zebrafish lines, hyposumoylation expands myelopoiesis at the expense of erythropoiesis. A SUMO–CCAAT/enhancer–binding protein α (SUMO-C/ebpα) fusion restores the normal myelopoiesis/erythropoiesis balance, suggesting that sumoylation status of C/ebpα contributes to myelo-erythroid lineage determination. Our results therefore implicate sumoylation in early lineage determination and reveal the possible molecular mechanism underlying the puzzling biased primitive hematopoiesis in vertebrates.

Lineage relationship of direct-developing melanocytes and melanocyte stem cells in the zebrafish

Previous research in zebrafish has demonstrated that embryonic and larval regeneration melanocytes are derived from separate lineages. The embryonic melanocytes that establish the larval pigment pattern do not require regulative melanocyte stem cell (MSC) precursors, and are termed direct-developing melanocytes. In contrast, the larval regeneration melanocytes that restore the pigment pattern after ablation develop from MSC precursors. Here, we explore whether embryonic melanocytes and MSCs share bipotent progenitors. Furthermore, we explore when fate segregation of embryonic melanocytes and MSCs occurs in zebrafish development. In order to achieve this, we develop and apply a novel lineage tracing method. We first demonstrate that Tol2-mediated genomic integration of reporter constructs from plasmids injected at the 1-2 cell stage occurs most frequently after the midblastula transition but prior to shield stage, between 3 and 6 hours post-fertilization. This previously uncharacterized timing of Tol2-mediated genomic integration establishes Tol2-mediated transposition as a means for conducting lineage tracing in zebrafish. Combining the Tol2-mediated lineage tracing strategy with a melanocyte regeneration assay previously developed in our lab, we find that embryonic melanocytes and larval regeneration melanocytes are derived from progenitors that contribute to both lineages. We estimate 50-60 such bipotent melanogenic progenitors to be present in the shield-stage embryo. Furthermore, our examination of direct-developing and MSC-restricted lineages suggests that these are segregated from bipotent precursors after the shield stage, but prior to the end of convergence and extension. Following this early fate segregation, we estimate approximately 100 embryonic melanocyte and 90 MSC-restricted lineages are generated to establish or regenerate the zebrafish larval pigment pattern, respectively. Thus, the dual strategies of direct-development and MSC-derived development are established in the early gastrula, via fate segregation of the two lineages.

Modulation of Tcf3 repressor complex composition regulates cdx4 expression in zebrafish

The caudal homeobox (cdx) gene family is critical for specification of caudal body formation and erythropoiesis. In zebrafish, cdx4 expression is controlled by the Wnt pathway, but the molecular mechanism of this regulation is not fully understood. Here, we provide evidence that Tcf3 suppresses cdx4 expression through direct binding to multiple sites in the cdx4 gene regulatory region. Tcf3 requires corepressor molecules such as Groucho (Gro)/TLE and HDAC1 for activity. Using zebrafish embryos and cultured mammalian cells, we show that the transcription factor E4f1 derepresses cdx4 by dissociating corepressor proteins from Tcf3 without inhibiting its binding to cis-regulatory sites in the DNA. Further, the E3 ubiquitin ligase Lnx2b, acting as a scaffold protein irrespective of its enzymatic activity, counteracts the effects of E4f1. We propose that the modulation of Tcf3 repressor function by E4f1 assures precise and robust regulation of cdx4 expression in the caudal domain of the embryo.

Characterization and gene expression of transcription factors, PU.1 and C/EBPα driving transcription from the tumor necrosis factor α promoter in Japanese flounder, Paralichthys olivaceus

Both PU.1 and C/EBPα transcription factors play important roles in myeloid development and inflammatory response. These transcripts were cloned from the Japanese flounder (Paralichthys olivaceus) and were highly conserved with those of other vertebrates. PU.1 mRNA was mainly expressed in lymphoid tissues while C/EBPα mRNA was widely expressed in all tissues examined. Higher levels of PU.1 mRNA were expressed in the IgM(+) cells of both PBL and KL, while C/EBPα expression was higher only in the IgM(-) cells of KL. The expression of C/EBPα mRNA was induced only in KL stimulated with LPS. Interestingly, PU.1 mRNA expression was induced by Edwardsiella tarda, whereas the expression of C/EBPα mRNA was induced by Streptococcus iniae infection. Both PU.1 and C/EBPα drove transcription from the LPS-responsive region of the Japanese flounder TNFα gene, suggesting that both PU.1 and C/EBPα induced by bacterial infection are involved in inflammation mediated through TNFα expression.

Characterization of immune-matched hematopoietic transplantation in zebrafish

Evaluating hematopoietic stem cell (HSC) function in vivo requires a long-term transplantation assay. Although zebrafish are a powerful model for discovering the genetics of hematopoiesis, hematopoietic transplantation approaches have been underdeveloped. Here we established a long-term reconstitution assay in adult zebrafish. Primary and secondary recipients showed multilineage engraftment at 3 months after transplantation. Limiting dilution data suggest that at least 1 in 65 000 zebrafish marrow cells contain repopulating activity, consistent with mammalian HSC frequencies. We defined zebrafish haplotypes at the proposed major histocompatibility complex locus on chromosome 19 and tested functional significance through hematopoietic transplantation. Matching donors and recipients dramatically increased engraftment and percentage donor chimerism compared with unmatched fish. These data constitute the first functional test of zebrafish histocompatibility genes, enabling the development of matched hematopoietic transplantations. This lays the foundation for competitive transplantation experiments with mutant zebrafish HSCs and chemicals to test for effects on engraftment, thereby providing a model for human hematopoietic diseases and treatments not previously available.

Rumba and Haus3 are essential factors for the maintenance of hematopoietic stem/progenitor cells during zebrafish hematopoiesis

The hallmark of vertebrate definitive hematopoiesis is the establishment of the hematopoietic stem/progenitor cell (HSPC) pool during embryogenesis. This process involves a defined ontogenic switching of HSPCs in successive hematopoietic compartments and is evolutionarily conserved from teleost fish to human. In zebrafish, HSPCs originate from the ventral wall of the dorsal aorta (VDA), from which they subsequently mobilize to an intermediate hematopoietic site known as the caudal hematopoietic tissue (CHT) and finally colonize the kidney for adult hematopoiesis. Despite substantial understanding of the ontogeny of HSPCs, the molecular basis governing migration, colonization and maintenance of HSPCs remains to be explored fully. Here, we report the isolation and characterization of two zebrafish mutants, rumbahkz1 and sambahkz2, that are defective in generating definitive hematopoiesis. We find that HSPC initiation in the VDA and subsequent homing to the CHT are not affected in these two mutants. However, the further development of HSPCs in the CHT is compromised in both mutants. Positional cloning reveals that Rumba is a novel nuclear C2H2 zinc-finger factor with unknown function and samba encodes an evolutionarily conserved protein that is homologous to human augmin complex subunit 3 (HAUS3). Furthermore, we show that these two factors independently regulate cell cycle progression of HSPCs and are cell autonomously required for HPSC development in the CHT. Our study identifies Rumba and Haus3 as two essential regulators of HSPC maintenance during zebrafish fetal hematopoiesis.

Spatiotemporal photolabeling of neutrophil trafficking during inflammation in live zebrafish

How neutrophils traffic during inflammation in vivo remains elusive. To visualize the origin and fate of neutrophils during induction and resolution of inflammation, we established a genetically encoded photolabeling system by generating transgenic zebrafish that express a photoconvertible fluorescent reporter Dendra2 in neutrophils. Spatiotemporal photolabeling of neutrophils in vivo demonstrates that they emerge from the hematopoietic tissue in close proximity to injured tissue and repeat forward and reverse migration between the wound and the vasculature. Subsequently, neutrophils disperse throughout the body as wound-healing proceeds, contributing to local resolution at injured tissue and systemic dissemination of wound-sensitized neutrophils. Tissue damage also alters the fate of neutrophils in the caudal hematopoietic tissue and promotes caudorostral mobilization of neutrophils via the circulation to the cephalic mesenchyme. This work provides new insight into neutrophil behaviors during inflammation and resolution within a multicellular organism.

Infectious disease modeling and innate immune function in zebrafish embryos

The major cell types of the innate immune system, macrophages and neutrophils, develop during the first two days of zebrafish embryogenesis. The interaction of these immune cells with pathogenic microbes can excellently be traced in the optically transparent zebrafish embryos. Various tools and methods have recently been developed for visualizing and isolating the zebrafish embryonic innate immune cells, for establishing infections by different micro-injection techniques, and for analyzing the host innate immune response following microbial recognition. Here we provide practical guidelines for the application of these methodologies and review the current state of the art in zebrafish infectious disease research.

Ribosomal protein L11 mutation in zebrafish leads to haematopoietic and metabolic defects

Mutations in ribosomal proteins are associated with a congenital syndrome, Diamond-Blackfan anaemia (DBA), manifested by red blood cell aplasia, developmental abnormalities and increased risk of malignancy. Recent studies suggest the involvement of p53 activation in DBA. However, which pathways are involved and how they contribute to the DBA phenotype remains unknown. Here we show that a zebrafish mutant for the rpl11 gene had defects both in the development of haematopoietic stem cells (HSCs) and maintenance of erythroid cells. The molecular signature of the mutant included upregulation of p53 target genes and global changes in metabolism. The changes in several pathways may affect haematopoiesis including upregulation of pro-apoptotic and cell cycle arrest genes, suppression of glycolysis, downregulation of biosynthesis and dysregulation of cytoskeleton. Each of these pathways has been individually implicated in haematological diseases. Inhibition of p53 partially rescued haematopoiesis in the mutant. Altogether, we propose that the unique phenotype of DBA is a sum of several abnormally regulated molecular pathways, mediated by the p53 protein family and p53-independent, which have synergistic impact on haematological and other cellular pathways affected in DBA. Our results provide new insights into the pathogenesis of DBA and point to the potential avenues for therapeutic intervention.

Zebrafish microRNA-126 determines hematopoietic cell fate through c-Myb

Precise regulatory mechanisms are required to appropriately modulate the cellular levels of transcription factors controlling cell fate decisions during blood cell development. Here, we show that miR-126 is a novel physiological regulator of the proto-oncogene c-myb during definitive hematopoiesis. We show that knockdown of miR-126 results in increased c-Myb levels and promotes erythropoiesis at the expense of thrombopoiesis in vivo. We further provide evidence that specification of thrombocyte versus erythrocyte cell lineages is altered by the concerted activities of the miRNAs miR-126 and miR-150. Both microRNAs are required but not sufficient individually to precisely regulate the cell fate decision between erythroid and megakaryocytic lineages during definitive hematopoiesis in vivo. These results support the notion that microRNAs not only act to provide precision to developmental programs but also are essential determinants in the control of variable potential functions of a single gene during hematopoiesis.

The role of meis1 in primitive and definitive hematopoiesis during zebrafish development

BACKGROUND

The Meis1 protein represents an important cofactor for Hox and Pbx1 and is implicated in human and murine leukemias. Though much is known about the role of meis1 in leukemogenesis, its function in normal hematopoiesis remains largely unclear. Here we characterized the role of the proto-oncogene, meis1, during zebrafish primitive and definitive hematopoiesis.

DESIGN AND METHODS

Zebrafish embryos were stained with o-dianisidine to detect hemoglobin-containing cells and Sudan black to quantify neutrophils. The numbers of other cells (scl-, gata1- and alas2-positive cells) were also quantified by measuring the corresponding stained areas of the embryos. We used anti-Meis1 antibody and whole mount immunohistochemistry to determine the pattern of expression of Meis1 during zebrafish development and then analyzed the functional role of Meis1 by knocking-down the meis1 gene.

RESULTS

Using antisense morpholino oligomers to interrupt meis1 expression we found that, although primitive macrophage development could occur unhampered, posterior erythroid differentiation required meis1, and its absence resulted in a severe decrease in the number of mature erythrocytes. Furthermore a picture emerged that meis1 exerts important effects on later stages of erythrocyte maturation and that these effects are independent of gata1, but under the control of scl. In addition, meis1 morpholino knock-down led to dramatic single arteriovenous tube formation. We also found that knock-down of pbx1 resulted in a phenotype that was strikingly similar to that of meis1 knock-down zebrafish.

CONCLUSIONS

These results imply that meis1, jointly with pbx1, regulates primitive hematopoiesis as well as vascular development.

Transcriptome analysis of Traf6 function in the innate immune response of zebrafish embryos

TRAF6 is a key player at the cross-roads of development and immunity. The analysis of its in vivo molecular function is a great challenge since severe developmental defects and early lethality caused by Traf6 deficiency in knock-out mice interfere with analyses of the immune response. In this study we have used a new strategy to analyze the function of Traf6 in a zebrafish-Salmonella infectious disease model. In our approach the effect of a Traf6 translation-blocking morpholino was titrated down to avoid developmental defects and the response to infection under these conditions was studied using the combination of microarray analysis and whole transcriptome deep sequencing. Transcriptome profiling of the traf6 knock-down allowed the identification of a gene set whose responsiveness during infection is highly dependent on Traf6. Expression trend analysis based on the resulting datasets identified nine clusters of genes with characteristic transcription response profiles, demonstrating Traf6 has a dynamic role as a positive and negative regulator. Among the Traf6-dependent genes was a large set of well known anti-microbial and inflammatory genes. Additionally, we identified several genes which were not previously linked to a response to microbial infection, such as the fertility hormone gene gnrh2 and the DNA-damage regulated autophagy modulator 1 gene dram1. With the use of the zebrafish embryo model we have now analyzed the in vivo function of Traf6 in the innate immune response without interference of adaptive immunity.

Essential role of c-myb in definitive hematopoiesis is evolutionarily conserved

The transcription factor c-myb has emerged as one of the key regulators of vertebrate hematopoiesis. In mice, it is dispensable for primitive stages of blood cell development but essentially required for definitive hematopoiesis. Using a conditional knock-out strategy, recent studies have indicated that c-myb is required for self-renewal of mouse hematopoietic stem cells. Here, we describe and characterize the c-myb mutant in a lower vertebrate, the zebrafish Danio rerio. The recessive loss-of-function allele of c-myb (c-myb(t25127)) was identified in a collection of N-ethyl-N-nitrosourea (ENU)-induced mutants exhibiting a failure of thymopoiesis. The sequence of the mutant allele predicts a missense mutation (I181N) in the middle of the DNA recognition helix of repeat 3 of the highly conserved DNA binding domain. In keeping with the findings in the mouse, primitive hematopoiesis is not affected in the c-myb mutant fish. By contrast, definitive hematopoiesis fails, resulting in the loss of all blood cells by day 20 of development. Thus, the mutant fish lack lymphocytes and other white and red blood cells; nonetheless, they survive for 2-3 mo but show stunted growth. Because the mutant fish survive into early adulthood, it was possible to directly show that their definitive hematopoiesis is permanently extinguished. Our results, therefore, suggest that the key role of c-myb in definitive hematopoiesis is similar to that in mammals and must have become established early in vertebrate evolution.

Cloning and characterization of a zebrafish homologue of human AQP1: a bifunctional water and gas channel

The mammalian aquaporins AQP1, AQP4, and AQP5 have been shown to function not only as water channels but also as gas channels. Zebrafish have two genes encoding an AQP1 homologue, aqp1a and aqp1b. In the present study, we cloned the cDNA that encodes the zebrafish protein Aqp1a from the 72-h postfertilization (hpf) embryo of Danio rerio, as well as from the swim bladder of the adult. The deduced amino-acid sequence of aqp1a consists of 260 amino acids and is 59% identical to human AQP1. By analyzing the genomic DNA sequence, we identified four exons in the aqp1a gene. By in situ hybridization, aqp1a is expressed transiently in the developing vasculature and in erythrocytes from 16 to 48 h of development. Later, at 72 hpf, aqp1a is expressed in dermal ionocytes and in the swim bladder. Western blot analysis of adult tissues reveals that Aqp1a is most highly expressed in the eye and swim bladder. Xenopus oocytes expressing aqp1a have a channel-dependent (*) osmotic water permeability (P(f)(*)) that is indistinguishable from that of human AQP1. On the basis of the magnitude of the transient change in surface pH (ΔpH(S)) that were recorded as the oocytes were exposed to either CO(2) or NH(3), we conclude that zebrafish Aqp1a is permeable to both CO(2) and NH(3). The ratio (ΔpH(S)(*))((CO)2)/P(f)(*) is about half that of human AQP1, and the ratio (ΔpH(S)(*))(NH3)/P(f)(*) is about one-quarter that of human AQP1. Thus, compared with human AQP1, zebrafish Aqp1a has about twice the selectivity for CO(2) over NH(3).

Specific resistance to Pseudomonas aeruginosa infection in zebrafish is mediated by the cystic fibrosis transmembrane conductance regulator

Cystic fibrosis (CF) is a genetic disease caused by recessive mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene and is associated with prevalent and chronic Pseudomonas aeruginosa lung infections. Despite numerous studies that have sought to elucidate the role of CFTR in the innate immune response, the links between CFTR, innate immunity, and P. aeruginosa infection remain unclear. The present work highlights the zebrafish as a powerful model organism for human infectious disease, particularly infection by P. aeruginosa. Zebrafish embryos with reduced expression of the cftr gene (Cftr morphants) exhibited reduced respiratory burst response and directed neutrophil migration, supporting a connection between cftr and the innate immune response. Cftr morphants were infected with P. aeruginosa or other bacterial species that are commonly associated with infections in CF patients, including Burkholderia cenocepacia, Haemophilus influenzae, and Staphylococcus aureus. Intriguingly, the bacterial burden of P. aeruginosa was found to be significantly higher in zebrafish Cftr morphants than in controls, but this phenomenon was not observed with the other bacterial species. Bacterial burden in Cftr morphants infected with a P. aeruginosa ΔLasR mutant, a quorum sensing-deficient strain, was comparable to that in control fish, indicating that the regulation of virulence factors through LasR is required for enhancement of infection in the absence of Cftr. The zebrafish system provides a multitude of advantages for studying the pathogenesis of P. aeruginosa and for understanding the role that innate immune cells, such as neutrophils, play in the host response to acute bacterial infections commonly associated with cystic fibrosis.

Macrophage-specific gene functions in Spi1-directed innate immunity

The Spi1/Pu.1 transcription factor plays a crucial role in myeloid cell development in vertebrates. Despite extensive studies of Spi1, the controlled gene group remains largely unknown. To identify genes dependent on Spi1, we used a microarray strategy using a knockdown approach in zebrafish embryos combined with fluorescence-activated cell sorting of myeloid cells from transgenic embryos. This approach of using knockdowns with specific green fluorescent protein-marked cell types was highly successful in identifying macrophage-specific genes in Spi1-directed innate immunity. We found a gene group down-regulated on spi1 knockdown, which is also enriched in fluorescence-activated cell-sorted embryonic myeloid cells of a spi1:GFP transgenic line. This gene group, representing putative myeloid-specific Spi1 target genes, contained all 5 previously identified Spi1-dependent zebrafish genes as well as a large set of novel immune-related genes. Colocalization studies with neutrophil and macrophage markers revealed that genes cxcr3.2, mpeg1, ptpn6, and mfap4 were expressed specifically in early embryonic macrophages. In a functional approach, we demonstrated that gene cxcr3.2, coding for chemokine receptor 3.2, is involved in macrophage migration to the site of bacterial infection. Therefore, based on our combined transcriptome analyses, we discovered novel early macrophage-specific marker genes, including a signal transducer pivotal for macrophage migration in the innate immune response.

Bef medaka mutant reveals the essential role of c-myb in both primitive and definitive hematopoiesis

Vertebrate hematopoiesis is characterized by two evolutionally conserved phases of development, i.e., primitive hematopoiesis, which is a transient phenomenon in the early embryo, and definitive hematopoiesis, which takes place in the later stages. Beni fuji (bef) was originally isolated as a medaka mutant that has an apparently reduced number of erythrocytes in its peripheral blood. Positional cloning revealed that the bef mutant has a nonsense mutation in the c-myb gene. Previous studies have shown that c-myb is essential for definitive hematopoiesis, and c-myb is now widely used as a marker gene for the onset of definitive hematopoiesis. To analyze the phenotypes of the bef mutant, we performed whole-mount in situ hybridization with gene markers of hematopoietic cells. The bef embryos showed decreased expression of alpha-globin and l-plastin, and a complete loss of mpo1 and rag1 expression, suggesting that the bef embryos had defects not only in erythrocytes but also in other myeloid cells, which indicates that their definitive hematopoiesis was aberrant. Interestingly, we observed a diminution in the number of primitive erythrocytes and a delay in the emergence of primitive macrophages in the bef embryos. These results suggest that c-myb also functions in the primitive hematopoiesis, potentially demonstrating a link between primitive and definitive hematopoiesis.

Zebrafish small molecule screen in reprogramming/cell fate modulation

Embryonic zebrafish have long been used for lineage-tracing studies. In zebrafish embryos, the cell fate identities can be determined by whole-mount in situ hybridization, or by visualization of live embryos if using fluorescent reporter lines. We use embryonic zebrafish to study the effects of a leukemic oncogene AML1-ETO on modulating hematopoietic cell fate. Induced expression of AML1-ETO is able to efficiently reprogram hematopoietic progenitor cells from erythroid to myeloid cell fate. Using the zebrafish model of AML1-ETO, we performed a chemical screen to identify small molecules that suppress the cell fate switch in the presence of AML1-ETO. The methods discussed herein may be broadly applicable for identifying small molecules that modulate other cell fate decisions.

Demonstration of T cell and macrophage progenitors in carp (Cyprinus carpio) kidney hematopoietic tissues. Development of clonal assay system for carp hematopoietic cells

Single hematopoietic cells from carp (Cyprinus carpio) kidney were seeded to each well of 96-well plates and cultured in the presence of a supporting cell layer and conditioned media (CM). The CM were obtained from bulk-cultured carp hematopoietic cells, in which T and macrophage-lineage cells rapidly proliferated as previously reported. After 2-3 weeks, colony formation was found in 0-4 wells of each plate. Three different morphological types of colonies were observed: "type I colonies", "type II colonies" and "mixed-type colonies". Type I colony cells were interpreted as composed by macrophage-lineage cells, since they expressed a specific macrophage marker, M-CSFR/csf1r gene, and most of them phagocytosed latex particles. Type II colony cells were interpreted as composed by T lineage cells, since they expressed several T cell marker genes including gata3, lck and TCRbeta, but did not engulf latex particles. Mixed-type colonies were interpreted as composed by both macrophages and T lineage cells. They expressed not only the M-CSFR gene but also a T cell marker gene, gata3, but not other T cell markers, such as lck and TCRbeta. These results indicated that the mixed-type colonies were developed from immature common progenitors of macrophage and T cell. In contrast, type I and type II colonies were developed from more mature and mono-potent progenitors of macrophage and T cell, respectively.

Interaction of retinoic acid and scl controls primitive blood development

Hematopoietic development during embryogenesis involves the interaction of extrinsic signaling pathways coupled to an intrinsic cell fate that is regulated by cell-specific transcription factors. Retinoic acid (RA) has been linked to stem cell self-renewal in adults and also participates in yolk sac blood island formation. Here, we demonstrate that RA decreases gata1 expression and blocks primitive hematopoiesis in zebrafish (Danio rerio) embryos, while increasing expression of the vascular marker, fli1. Treatment with an inhibitor of RA biosynthesis or a retinoic acid receptor antagonist increases gata1(+) erythroid progenitors in the posterior mesoderm of wild-type embryos and anemic cdx4(-/-) mutants, indicating a link between the cdx-hox signaling pathway and RA. Overexpression of scl, a DNA binding protein necessary for hematopoietic development, rescues the block of hematopoiesis induced by RA. We show that these effects of RA and RA pathway inhibitors are conserved during primitive hematopoiesis in murine yolk sac explant cultures and embryonic stem cell assays. Taken together, these data indicate that RA inhibits the commitment of mesodermal cells to hematopoietic fates, functioning downstream of cdx4 and upstream of scl. Our studies establish a new connection between RA and scl during development that may participate in stem cell self-renewal and hematopoietic differentiation.

Host-microbe interactions in the developing zebrafish

The amenability of the zebrafish to in vivo imaging and genetic analysis has fueled expanded use of this vertebrate model to investigate the molecular and cellular foundations of host-microbe relationships. Study of microbial encounters in zebrafish hosts has concentrated on developing embryonic and larval stages, when the advantages of the zebrafish model are maximized. A comprehensive understanding of these host-microbe interactions requires appreciation of the developmental context into which a microbe is introduced, as well as the effects of that microbial challenge on host ontogeny. In this review, we discuss how in vivo imaging and genetic analysis in zebrafish has advanced our knowledge of host-microbe interactions in the context of a developing vertebrate host. We focus on recent insights into immune cell ontogeny and function, commensal microbial relationships in the intestine, and microbial pathogenesis in zebrafish hosts.

Generation of clonal zebrafish lines and transplantable hepatic tumors

Transplantable tumors are an accepted gold standard in cancer studies in rodents. The progress of this model in zebrafish has long been constrained by the lack of true inbred lines in zebrafish. We have generated several lines of homozygous diploid clonal zebrafish lines, which allow serial transplantations of tumor cells from one fish to another without sublethal gamma-irradiation. The spectrum of transplantable tumors that were initially induced and maintained in inbred clonal zebrafish lines was limited to different types of spontaneous and diethylnitrosamine-induced hepatic tumors. However, this model can readily be extended to a broad range of extrahepatic tumors, transgenic tumors with defined mechanisms of induction and fluorescence-tagged tumor lines. These models will further facilitate in-depth analysis of invasive tumor growth, angiogenesis, metastasis and tumor-initiating cells by in vivo imaging and provide a cost-effective system for high-throughput (HTP) screening of anticancer therapeutics, including biological response modifiers. In addition, homozygous zebrafish lines are an indispensable tool for immunogenetics, mapping of quantitative trait loci and other genetic applications. The whole procedure, from generation of a gynogenetic female homozygous fish (a founder) to obtaining 3-4 consecutive passages of a syngeneic tumor, takes approximately 12-18 months. This time-frame largely depends on methods of tumor induction, tumor type and tumor growth rate.

Efficient transient rescue of hematopoietic mutant phenotypes in zebrafish using Tol2-mediated transgenesis

Phenotypic rescue experiments have been commonly used in zebrafish since it is convenient to study the causality of mutant phenotypes just by injecting mRNA into embryos. However, this strategy is only effective for phenotypes at early embryonic stages due to mRNA instability. For later developmental stages, DNA constructs are used to express exogenous genes, while it is usually ineffective owing to the problem of mosaicism. This study attempted to solve the problem by using Tol2-mediated transgenesis. As a model case, we used vlad tepes (vlt), a zebrafish gata1 mutant, whose phenotypes have never been able to be rescued at later stages by transient rescue experiments. Blood cell-specific transgenic expression of gata1 was driven by its own promoter/enhancer elements. The co-injection of a Tol2-donor plasmid containing gata1 cDNA and transposase mRNA efficiently rescued the bloodless phenotypes of vlt even in day 12 larvae when definitive erythropoiesis took place with primitive erythropoiesis. This Tol2-mediated rescue is therefore considered to be a quick and easy method for analyzing the mutant phenotypes in zebrafish.

Comparative gene expression analysis of zebrafish and mammals identifies common regulators in hematopoietic stem cells

Hematopoiesis in teleost fish is maintained in the kidney. We previously reported that Hoechst dye efflux activity of hematopoietic stem cells (HSCs) is highly conserved in vertebrates, and that Hoechst can be used to purify HSCs from teleost kidneys. Regulatory molecules that are strongly associated with HSC activity may also be conserved in vertebrates. In this study, we identified evolutionarily conserved molecular components in HSCs by comparing the gene expression profiles of zebrafish, murine, and human HSCs. Microarray data of zebrafish kidney side population cells (zSPs) showed that genes involved in cell junction and signal transduction tended to be up-regulated in zSPs, whereas genes involved in DNA replication tended to be down-regulated. These properties of zSPs were similar to those of mammalian HSCs. Overlapping gene expression analysis showed that 40 genes were commonly up-regulated in these 3 HSCs. Some of these genes, such as egr1, gata2, and id1, have been previously implicated in the regulation of HSCs. In situ hybridization in zebrafish kidney revealed that expression domains of egr1, gata2, and id1 overlapped with that of abcg2a, a marker for zSPs. These results suggest that the overlapping genes identified in this study are regulated in HSCs and play important roles in their functions.