CD41+ cmyb+ precursors colonize the zebrafish pronephros by a novel migration route to initiate adult hematopoiesis

The zebrafish granulocyte colony-stimulating factors (Gcsfs): 2 paralogous cytokines and their roles in hematopoietic development and maintenance

Granulocyte colony-stimulating factor (Gcsf) drives the proliferation and differentiation of granulocytes, monocytes, and macrophages (mφs) from hematopoietic stem and progenitor cells (HSPCs). Analysis of the zebrafish genome indicates the presence of 2 Gcsf ligands, likely resulting from a duplication event in teleost evolution. Although Gcsfa and Gcsfb share low sequence conservation, they share significant similarity in their predicted ligand/receptor interaction sites and structure. Each ligand displays differential temporal expression patterns during embryogenesis and spatial expression patterns in adult animals. To determine the functions of each ligand, we performed loss- and gain-of-function experiments. Both ligands signal through the Gcsf receptor to expand primitive neutrophils and mφs, as well as definitive granulocytes. To further address their functions, we generated recombinant versions and tested them in clonal progenitor assays. These sensitive in vitro techniques indicated similar functional attributes in supporting HSPC growth and differentiation. Finally, in addition to supporting myeloid differentiation, zebrafish Gcsf is required for the specification and proliferation of hematopoietic stem cells, suggesting that Gcsf represents an ancestral cytokine responsible for the broad support of HSPCs. These findings may inform how hematopoietic cytokines evolved following the diversification of teleosts and mammals from a common ancestor.

Erythro-myeloid progenitors: "definitive" hematopoiesis in the conceptus prior to the emergence of hematopoietic stem cells

Erythro-myeloid progenitors (EMP) serve as a major source of hematopoiesis in the developing conceptus prior to the formation of a permanent blood system. In this review, we summarize the current knowledge regarding the emergence, fate, and potential of this hematopoietic stem cell (HSC)-independent wave of hematopoietic progenitors, focusing on the murine embryo as a model system. A better understanding of the temporal and spatial control of hematopoietic emergence in the embryo will ultimately improve our ability to derive hematopoietic stem and progenitor cells from embryonic stem cells and induced pluripotent stem cells to serve therapeutic purposes.

Expressions of transcription factors in goldfish (Carassius auratus L.) macrophages and their progenitors

The development of macrophages is a highly regulated process requiring coordination amongst transcription factors. The presence/absence, relative levels, antagonism, or synergy of all transcription factors involved is critical to directing lineage cell fate and differentiation. While relative levels of many key myeloid transcription factors have been determined in mammalian macrophage differentiation, a similar set of studies have yet to be conducted in a teleost system. In this study, we report on the mRNA levels of transcription factors (cebpa, cjun, cmyb, egr1, gata1, gata2, gata3, lmo2, mafb, pax5, pu.1 and runx1) in sorted goldfish progenitor cells, monocytes, and macrophages from primary kidney macrophage cultures. The mRNA levels of runx1 and pu.1 were significantly higher, gata3 and pax5 mRNA levels were lower, in monocytes compared to progenitors, and the mRNA levels of cjun, egr1, gata2, gata3, mafb and pax5 were significantly decreased in macrophages compared to progenitor cells. The relative mRNA levels of the interferon regulatory factor family of transcription factors, irf1, irf2, irf5, irf7, irf8 and irf9 in sorted progenitors, monocytes and macrophages were also measured. In contrast to other irf family transcription factors examined, irf8 mRNA levels were increased in monocytes compared to progenitors by greater than three-fold, suggesting that irf8 is important for monopoiesis. Lastly, we show the differential regulation of myeloid transcription factor mRNA levels in sorted progenitor cells from 1, 2, or 3-day old cultures in response to the recombinant goldfish growth factors, rgCSF-1 and rgKITLA.

Effect of radiation dose-rate on hematopoietic cell engraftment in adult zebrafish

Although exceptionally high radiation dose-rates are currently attaining clinical feasibility, there have been relatively few studies reporting the biological consequences of these dose-rates in hematopoietic cell transplant (HCT). In zebrafish models of HCT, preconditioning before transplant is typically achieved through radiation alone. We report the comparison of outcomes in adult zebrafish irradiated with 20 Gy at either 25 or 800 cGy/min in the context of experimental HCT. In non-transplanted irradiated fish we observed no substantial differences between dose-rate groups as assessed by fish mortality, cell death in the kidney, endogenous hematopoietic reconstitution, or gene expression levels of p53 and ddb2 (damage-specific DNA binding protein 2) in the kidney. However, following HCT, recipients conditioned with the higher dose rate showed significantly improved donor-derived engraftment at 9 days post transplant (p ≤ 0.0001), and improved engraftment persisted at 31 days post transplant. Analysis for sdf-1a expression, as well as transplant of hematopoietic cells from cxcr4b -/- zebrafish, (odysseus), cumulatively suggest that the sdf-1a/cxcr4b axis is not required of donor-derived cells for the observed dose-rate effect on engraftment. Overall, the adult zebrafish model of HCT indicates that exceptionally high radiation dose-rates can impact HCT outcome, and offers a new system for radiobiological and mechanistic interrogation of this phenomenon. Key words: Radiation dose rate, Total Marrow Irradiation (TMI), Total body irradiation (TBI), SDF-1, Zebrafish, hematopoietic cell transplant.

Effects of nanosilver exposure on cholinesterase activities, CD41, and CDF/LIF-like expression in zebrafish (Danio rerio) larvae

Metal nanosolicoparticles are suspected to cause diseases in a number of organisms, including man. In this paper, we report the effects of nanosilver (Ag, 1-20 nm particles) on the early development of the zebrafish, a well-established vertebrate model. Embryos at the midgastrula stage were exposed to concentrations ranging from 100 to 0.001 mg/L to verify the effects on different endpoints: lethality, morphology, expression of cholinergic molecules, and development of the immune system. (1) Relative risk of mortality was exponential in the range between 0.001 and 10 mg/L. Exposure to 100 mg/L caused 100% death of embryos before reaching the tail-bud stage. (2) Developmental anomalies were present in the 72 h larvae obtained from embryos exposed to nanosilver: whole body length, decreased eye dimension, and slow response to solicitation by gentle touch with a needle tip, with a significant threshold at 0.1 mg/L. (3) Dose-dependent inhibition of acetylcholinesterase activity was significant among the exposures, except between 1 mg/L and 10 mg/L. (4) The distribution of CD41+ cells and of CDF/LIF-like immunoreactivity was altered according to the Ag concentration. The possible effect of nanosilver in impairing immune system differentiation through the inhibition of molecules related to the cholinergic system is discussed.

Assaying hematopoiesis using zebrafish

The zebrafish has become a commonly used model for studying hematopoiesis as a result of its unique attributes. Zebrafish are highly suitable for large-scale genetic and chemical screens compared to other vertebrate systems. It is now possible to analyze hematopoietic lineages in zebrafish and validate cell function via transplantation assays. Here, we review advancements over the past decade in forward genetic screens, chemical screens, fluorescence-activated cell sorting analysis, and transplantation assays. Integrating these approaches enables new chemical and genetic screens that assay cell function within the hematopoietic system. Studies in zebrafish will continue to contribute and expand our knowledge about hematopoiesis, and develop novel treatments for clinical applications.

An evolutionarily conserved program of B-cell development and activation in zebrafish

Teleost fish are among the most ancient vertebrates possessing an adaptive immune system with B and T lymphocytes that produce memory responses to pathogens. Most bony fish, however, have only 2 types of B lymphocytes, in contrast to the 4 types available to mammals. To better understand the evolution of adaptive immunity, we generated transgenic zebrafish in which the major immunoglobulin M (IgM(+)) B-cell subset expresses green fluorescence protein (GFP) (IgM1:eGFP). We discovered that the earliest IgM(+) B cells appear between the dorsal aorta and posterior cardinal vein and also in the kidney around 20 days postfertilization. We also examined B-cell ontogeny in adult IgM1:eGFP;rag2:DsRed animals, where we defined pro-B, pre-B, and immature/mature B cells in the adult kidney. Sites of B-cell development that shift between the embryo and adult have previously been described in birds and mammals. Our results suggest that this developmental shift occurs in all jawed vertebrates. Finally, we used IgM1:eGFP and cd45DsRed;blimp1:eGFP zebrafish to characterize plasma B cells and investigate B-cell function. The IgM1:eGFP reporter fish are the first nonmammalian B-cell reporter animals to be described. They will be important for further investigation of immune cell evolution and development and host-pathogen interactions in zebrafish.

miR-34b regulates multiciliogenesis during organ formation in zebrafish

Multiciliated cells (MCCs) possess multiple motile cilia and are distributed throughout the vertebrate body, performing important physiological functions by regulating fluid movement in the intercellular space. Neither their function during organ development nor the molecular mechanisms underlying multiciliogenesis are well understood. Although dysregulation of members of the miR-34 family plays a key role in the progression of various cancers, the physiological function of miR-34b, especially in regulating organ formation, is largely unknown. Here, we demonstrate that miR-34b expression is enriched in kidney MCCs and the olfactory placode in zebrafish. Inhibiting miR-34b function using morpholino antisense oligonucleotides disrupted kidney proximal tubule convolution and the proper distribution of distal transporting cells and MCCs. Microarray analysis of gene expression, cilia immunostaining and a fluid flow assay revealed that miR-34b is functionally required for the multiciliogenesis of MCCs in the kidney and olfactory placode. We hypothesize that miR-34b regulates kidney morphogenesis by controlling the movement and distribution of kidney MCCs and fluid flow. We found that cmyb was genetically downstream of miR-34b and acted as a key regulator of multiciliogenesis. Elevated expression of cmyb blocked membrane docking of centrioles, whereas loss of cmyb impaired centriole multiplication, both of which resulted in defects in the formation of ciliary bundles. Thus, miR-34b serves as a guardian to maintain the proper level of cmyb expression. In summary, our studies have uncovered an essential role for miR-34b-Cmyb signaling during multiciliogenesis and kidney morphogenesis.

Control of hematopoietic stem cell emergence by antagonistic functions of ribosomal protein paralogs

It remains controversial whether the highly homologous ribosomal protein (RP) paralogs found in lower eukaryotes have distinct functions and this has not been explored in vertebrates. Here we demonstrate that despite ubiquitous expression, the RP paralogs, Rpl22 and Rpl22-like1 (Rpl22l1) play essential, distinct, and antagonistic roles in hematopoietic development. Knockdown of Rpl22 in zebrafish embryos selectively blocks the development of T lineage progenitors after they have seeded the thymus. In contrast, knockdown of the Rpl22 paralog, Rpl22l1, impairs the emergence of hematopoietic stem cells (HSC) in the aorta-gonad-mesonephros by abrogating Smad1 expression and the consequent induction of essential transcriptional regulator, Runx1. Indeed, despite the ability of both paralogs to bind smad1 RNA, Rpl22 and Rpl22l1 have opposing effects on Smad1 expression. Accordingly, circumstances that tip the balance of these paralogs in favor of Rpl22 (e.g., Rpl22l1 knockdown or Rpl22 overexpression) result in repression of Smad1 and blockade of HSC emergence.

SUMO1-activating enzyme subunit 1 is essential for the survival of hematopoietic stem/progenitor cells in zebrafish

In vertebrates, establishment of the hematopoietic stem/progenitor cell (HSPC) pool involves mobilization of these cells in successive developmental hematopoietic niches. In zebrafish, HSPCs originate from the ventral wall of the dorsal aorta (VDA), the equivalent of the mammalian aorta-gonad-mesonephros (AGM). The HSPCs subsequently migrate to the caudal hematopoietic tissue (CHT) for transitory expansion and differentiation during the larval stage, and they finally colonize the kidney, where hematopoiesis takes place in adult fish. Here, we report the isolation and characterization of a zebrafish mutant, tango(hkz5), which shows defects of definitive hematopoiesis. In tango(hkz5) mutants, HSPCs initiate normally in the AGM and subsequently colonize the CHT. However, definitive hematopoiesis is not sustained in the CHT owing to accelerated apoptosis and diminished proliferation of HSPCs. Positional cloning reveals that tango(hkz5) encodes SUMO1-activating enzyme subunit 1 (Sae1). A chimera generation experiment and biochemistry analysis reveal that sae1 is cell-autonomously required for definitive hematopoiesis and that the tango(hkz5) mutation produces a truncated Sae1 protein (ΔSae1), resulting in systemic reduction of sumoylation. Our findings demonstrate that sae1 is essential for the maintenance of HSPCs during fetal hematopoiesis in zebrafish.

Regulatory mechanisms of thymus and T cell development

The thymus is a central hematopoietic organ which produces mature T lymphocytes with diverse antigen specificity. During development, the thymus primordium is derived from the third pharyngeal endodermal pouch, and then differentiates into cortical and medullary thymic epithelial cells (TECs). TECs represent the primary functional cell type that forms the unique thymic epithelial microenvironment which is essential for intrathymic T-cell development, including positive selection, negative selection and emigration out of the thymus. Our understanding of thymopoiesis has been greatly advanced by using several important animal models. This review will describe progress on the molecular mechanisms involved in thymus and T cell development with particular focus on the signaling and transcription factors involved in this process in mouse and zebrafish.

Zebrafish erythropoiesis and the utility of fish as models of anemia

Erythrocytes contain oxygen-carrying hemoglobin to all body cells. Impairments in the generation of erythrocytes, a process known as erythropoiesis, or in hemoglobin synthesis alter cell function because of decreased oxygen supply and lead to anemic diseases. Thus, understanding how erythropoiesis is regulated during embryogenesis and adulthood is important to develop novel therapies for anemia. The zebrafish, Danio rerio, provides a powerful model for such study. Their small size and the ability to generate a large number of embryos enable large-scale analysis, and their transparency facilitates the visualization of erythroid cell migration. Importantly, the high conservation of hematopoietic genes among vertebrates and the ability to successfully transplant hematopoietic cells into fish have enabled the establishment of models of human anemic diseases in fish. In this review, we summarize the current progress in our understanding of erythropoiesis on the basis of zebrafish studies and highlight fish models of human anemias. These analyses could enable the discovery of novel drugs as future therapies.

Human ESC-derived hemogenic endothelial cells undergo distinct waves of endothelial to hematopoietic transition

Several studies have demonstrated that hematopoietic cells originate from endotheliumin early development; however, the phenotypic progression of progenitor cells during human embryonic hemogenesis is not well described. Here, we define the developmental hierarchy among intermediate populations of hematopoietic progenitor cells (HPCs) derived from human embryonic stem cells (hESCs). We genetically modified hESCs to specifically demarcate acquisition of vascular (VE-cadherin) and hematopoietic (CD41a) cell fate and used this dual-reporting transgenic hESC line to observe endothelial to hematopoietic transition by real-time confocal microscopy. Live imaging and clonal analyses revealed a temporal bias in commitment of HPCs that recapitulates discrete waves of lineage differentiation noted during mammalian hemogenesis. Specifically, HPCs isolated at later time points showed reduced capacity to form erythroid/ megakaryocytic cells and exhibited a tendency toward myeloid fate that was enabled by expression of the Notch ligand Dll4 on hESC-derived vascular feeder cells. These data provide a framework for defining HPC lineage potential, elucidate a molecular contribution from the vascular niche in promoting hematopoietic lineage progression, and distinguish unique subpopulations of hemogenic endothelium during hESC differentiation.

KEY POINTS

Live imaging of endothelial to hematopoietic conversion identifies distinct subpopulations of hESC-derived hemogenic endothelium. Expression of the Notch ligand DII4 on vascular ECs drives induction of myeloid fate from hESC-derived hematopoietic progenitors.

Kidney organogenesis in the zebrafish: insights into vertebrate nephrogenesis and regeneration

Vertebrates form a progressive series of up to three kidney organs during development-the pronephros, mesonephros, and metanephros. Each kidney derives from the intermediate mesoderm and is comprised of conserved excretory units called nephrons. The zebrafish is a powerful model for vertebrate developmental genetics, and recent studies have illustrated that zebrafish and mammals share numerous similarities in nephron composition and physiology. The zebrafish embryo forms an architecturally simple pronephros that has two nephrons, and these eventually become a scaffold onto which a mesonephros of several hundred nephrons is constructed during larval stages. In adult zebrafish, the mesonephros exhibits ongoing nephrogenesis, generating new nephrons from a local pool of renal progenitors during periods of growth or following kidney injury. The characteristics of the zebrafish pronephros and mesonephros make them genetically tractable kidney systems in which to study the functions of renal genes and address outstanding questions about the mechanisms of nephrogenesis. Here, we provide an overview of the formation and composition of these zebrafish kidney organs, and discuss how various zebrafish mutants, gene knockdowns, and transgenic models have created frameworks in which to further delineate nephrogenesis pathways.

Regulation of a vascular plexus by gata4 is mediated in zebrafish through the chemokine sdf1a

Using the zebrafish model we describe a previously unrecognized requirement for the transcription factor gata4 controlling embryonic angiogenesis. The development of a vascular plexus in the embryonic tail, the caudal hematopoietic tissue (CHT), fails in embryos depleted of gata4. Rather than forming a normal vascular plexus, the CHT of gata4 morphants remains fused, and cells in the CHT express high levels of osteogenic markers ssp1 and runx1. Definitive progenitors emerge from the hemogenic aortic endothelium, but fail to colonize the poorly vascularized CHT. We also found abnormal patterns and levels for the chemokine sdf1a in gata4 morphants, which was found to be functionally relevant, since the embryos also show defects in development of the lateral line, a mechano-sensory organ system highly dependent on a gradient of sdf1a levels. Reduction of sdf1a levels was sufficient to rescue lateral line development, circulation, and CHT morphology. The result was surprising since neither gata4 nor sdf1a is obviously expressed in the CHT. Therefore, we generated transgenic fish that conditionally express a dominant-negative gata4 isoform, and determined that gata4 function is required during gastrulation, when it is co-expressed with sdf1a in lateral mesoderm. Our study shows that the gata4 gene regulates sdf1a levels during early embryogenesis, which impacts embryonic patterning and subsequently the development of the caudal vascular plexus.

Through the looking glass: visualizing leukemia growth, migration, and engraftment using fluorescent transgenic zebrafish

Zebrafish have emerged as a powerful model of development and cancer. Human, mouse, and zebrafish malignancies exhibit striking histopathologic and molecular similarities, underscoring the remarkable conservation of genetic pathways required to induce cancer. Zebrafish are uniquely suited for large-scale studies in which hundreds of animals can be used to investigate cancer processes. Moreover, zebrafish are small in size, optically clear during development, and amenable to genetic manipulation. Facile transgenic approaches and new technologies in gene inactivation have provided much needed genomic resources to interrogate the function of specific oncogenic and tumor suppressor pathways in cancer. This manuscript focuses on the unique attribute of labeling leukemia cells with fluorescent proteins and directly visualizing cancer processes in vivo including tumor growth, dissemination, and intravasation into the vasculature. We will also discuss the use of fluorescent transgenic approaches and cell transplantation to assess leukemia-propagating cell frequency and response to chemotherapy.

L-Leucine improves the anemia and developmental defects associated with Diamond-Blackfan anemia and del(5q) MDS by activating the mTOR pathway

Haploinsufficiency of ribosomal proteins (RPs) has been proposed to be the common basis for the anemia observed in Diamond-Blackfan anemia (DBA) and myelodysplastic syndrome with loss of chromosome 5q [del(5q) MDS]. We have modeled DBA and del(5q) MDS in zebrafish using antisense morpholinos to rps19 and rps14, respectively, and have demonstrated that, as in humans, haploinsufficient levels of these proteins lead to a profound anemia. To address the hypothesis that RP loss results in impaired mRNA translation, we treated Rps19 and Rps14-deficient embryos with the amino acid L-leucine, a known activator of mRNA translation. This resulted in a striking improvement of the anemia associated with RP loss. We confirmed our findings in primary human CD34⁺ cells, after shRNA knockdown of RPS19 and RPS14. Furthermore, we showed that loss of Rps19 or Rps14 activates the mTOR pathway, and this is accentuated by L-leucine in both Rps19 and Rps14 morphants. This effect could be abrogated by rapamycin suggesting that mTOR signaling may be responsible for the improvement in anemia associated with L-leucine. Our studies support the rationale for ongoing clinical trials of L-leucine as a therapeutic agent for DBA, and potentially for patients with del(5q) MDS.

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.

Histocompatibility and hematopoietic transplantation in the zebrafish

The zebrafish has proven to be an excellent model for human disease, particularly hematopoietic diseases, since these fish make similar types of blood cells as humans and other mammals. The genetic program that regulates the development and differentiation of hematopoietic cells is highly conserved. Hematopoietic stem cells (HSCs) are the source of all the blood cells needed by an organism during its lifetime. Identifying an HSC requires a functional assay, namely, a transplantation assay consisting of multilineage engraftment of a recipient and subsequent serial transplant recipients. In the past decade, several types of hematopoietic transplant assays have been developed in the zebrafish. An understanding of the major histocompatibility complex (MHC) genes in the zebrafish has lagged behind transplantation experiments, limiting the ability to perform unbiased competitive transplantation assays. This paper summarizes the different hematopoietic transplantation experiments performed in the zebrafish, both with and without immunologic matching, and discusses future directions for this powerful experimental model of human blood diseases.

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.

Intravital imaging of thymopoiesis reveals dynamic lympho-epithelial interactions

T cell development occurs in the thymus. The thymic microenvironment attracts hematopoietic progenitors, specifies them toward the T cell lineage, and orchestrates their differentiation and egress into the periphery. The anatomical location of the thymus and the intrauterine development of mouse embryos have so far precluded a direct visualization of the initial steps of thymopoiesis. Here, we describe transgenic zebrafish lines enabling the in vivo observation of thymopoiesis. The cell-autonomous proliferation of thymic epithelial cells, their morphological transformation into a reticular meshwork upon contact with hematopoietic cells, and the multiple migration routes of thymus-settling cells could be directly visualized. The unexpectedly dynamic thymus homing process is chemokine driven and independent of blood circulation. Thymocyte development appears to be completed in less than 4 days. Our work establishes a versatile model for the in vivo observation and manipulation of thymopoiesis.

The zebrafish reveals dependence of the mast cell lineage on Notch signaling in vivo

We used the opportunities afforded by the zebrafish to determine upstream pathways regulating mast cell development in vivo and identify their cellular origin. Colocalization studies demonstrated zebrafish notch receptor expression in cells expressing carboxypeptidase A5 (cpa5), a zebrafish mast cell-specific marker. Inhibition of the Notch pathway resulted in decreased cpa5 expression in mindbomb mutants and wild-type embryos treated with the γ-secretase inhibitor, Compound E. A series of morpholino knockdown studies specifically identified notch1b and gata2 as the critical factors regulating mast cell fate. Moreover, hsp70::GAL4;UAS::nicd1a transgenic embryos overexpressing an activated form of notch1, nicd1a, displayed increased cpa5, gata2, and pu.1 expression. This increase in cpa5 expression could be reversed and reduced below baseline levels in a dose-dependent manner using Compound E. Finally, evidence that cpa5 expression colocalizes with lmo2 in the absence of hematopoietic stem cells revealed that definitive mast cells initially delineate from erythromyeloid progenitors. These studies identify a master role for Notch signaling in vertebrate mast cell development and establish developmental origins of this lineage. Moreover, these findings postulate targeting the Notch pathway as a therapeutic strategy in mast cell diseases.

Transplantation in zebrafish

Tissue or cell transplantation has been an extremely valuable technique for studying developmental potential of certain cell population, dissecting cell-environment interaction relationship, identifying stem cells, and many other applications. One key technical requirement for performing transplantation assay is the capability of distinguishing the transplanted donor cells from the endogenous host cells, and tracing the donor cells over time. Zebrafish has emerged as an excellent model organism for performing transplantation assay, thanks to the transparency of embryos during development and even certain adults. Using transgenic techniques and fast-evolving imaging technology, fluorescence-labeled donor cells can be easily identified and studied in vivo. In this chapter, we will first discuss the rationale of different types of zebrafish transplantation in both embryos and adults, and then focus on detailed methods of three types of transplantation: blastula/gastrula transplantation for mosaic analysis, stem cell transplantation, and tumor transplantation.

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.

Dominant-negative C/ebpα and polycomb group protein Bmi1 extend short-lived hematopoietic stem/progenitor cell life span and induce lethal dyserythropoiesis

The primitive hematopoietic stem/progenitor cells (HSPCs) during embryonic hematopoiesis are thought to be short-lived (SL) with limited self-renewal potential. The fate and consequence of these short-lived HSPCs, once reprogrammed into “long-lived” in a living animal body, remain unknown. Here we show that targeted expression of a dominant-negative C/ebpα (C/ebpαDN) in the primitive SL-HSPCs during zebrafish embryogenesis extends their life span, allowing them to survive to later developmental stage to colonize the definitive hematopoietic sites, where they undergo a proliferative expansion followed by erythropoietic dysplasia and embryonic lethality because of circulation congestion. Mechanistically, C/ebpαDN binds to a conserved C/EBP-binding motif in the promoter region of bmi1 gene, associated with a specific induction of bmi1 transcription in the transgenic embryos expressing C/ebpαDN. Targeted expression of Bmi1 in the SL-HSPCs recapitulates nearly all aberrant phenotypes induced by C/ebpαDN, whereas knockdown of bmi1 largely rescues these abnormalities. The results indicate that Bmi1 acts immediately downstream of C/ebpαDN to regulate the survival and self-renewal of HSPCs and contribute to the erythropoietic dysplasia.

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.

A somitic Wnt16/Notch pathway specifies haematopoietic stem cells

Haematopoietic stem cells (HSCs) are a self-renewing population that continuously replenish all blood and immune cells during the lifetime of an individual^{1, 2}. HSCs are used clinically to treat a wide array of diseases, including acute leukaemias and congenital blood disorders, but obtaining suitable numbers of cells and finding immune compatible donors remain serious problems. These concerns have led to an interest in the conversion of embryonic stem cells or induced pluripotent stem cells into HSCs, which is not possible using current methodologies. To accomplish this goal, it is critical to understand the native mechanisms involved in specification of HSCs during embryonic development. Here we demonstrate that Wnt16 controls a novel genetic regulatory network required for HSC specification. Non-canonical signaling by Wnt16 is required for somitic expression of the Notch ligands deltaC (dlc) and deltaD (dld), and these ligands are in turn required for establishment of definitive haematopoiesis. Notch signalling downstream of Dlc/Dld is earlier than, and distinct from known cell-autonomous requirements for Notch, strongly suggesting that novel Notch-dependent relay signal(s) induce the first HSCs in parallel to other established pathways. Our results demonstrate that somite-specific gene expression is required for the production of haemogenic endothelium.

The identification and characterization of zebrafish hematopoietic stem cells

HSCs are defined by their ability to self-renew and maintain hematopoiesis throughout the lifespan of an organism. The optical clarity of their embryos and the ease of genetic manipulation make the zebrafish (Danio rerio) an excellent model for studying hematopoiesis. Using flow cytometry, we identified 2 populations of CD41-GFP(+) cells (GFP(hi) and GFP(lo)) in the whole kidney marrow of Tg(CD41:GFP) zebrafish. Past studies in humans and mice have shown that CD41 is transiently expressed in the earliest hematopoietic progenitors and is then silenced, reappearing in the platelet/thrombocyte lineage. We have transplanted flow-sorted GFP(hi) and GFP(lo) cells into irradiated adult zebrafish and assessed long-term hematopoietic engraftment. Transplantation of GFP(hi) cells did not reconstitute hematopoiesis. In contrast, we observed multilineage hematopoiesis up to 68 weeks after primary and secondary transplantation of GFP(lo) cells. We detected the CD41-GFP transgene in all major hematopoietic lineages and CD41-GFP(+) cells in histologic sections of kidneys from transplant recipients. These studies show that CD41-GFP(lo) cells fulfill generally accepted criteria for HSCs. The identification of fluorescent zebrafish HSCs, coupled with our ability to transplant them into irradiated adult recipients, provide a valuable new tool to track HSC homing, proliferation, and differentiation into hematopoietic cells.

Embryonic origin of the adult hematopoietic system: advances and questions

Definitive hematopoietic stem cells (HSCs) lie at the foundation of the adult hematopoietic system and provide an organism throughout its life with all blood cell types. Several tissues demonstrate hematopoietic activity at early stages of embryonic development, but which tissue is the primary source of these important cells and what are the early embryonic ancestors of definitive HSCs? Here, we review recent advances in the field of HSC research that have shed light on such questions, while setting them into a historical context, and discuss key issues currently circulating in this field.

Microarray analysis of XOPS-mCFP zebrafish retina identifies genes associated with rod photoreceptor degeneration and regeneration

PURPOSE

XOPS-mCFP transgenic zebrafish experience a continual cycle of rod photoreceptor development and degeneration throughout life, making them a useful model for investigating the molecular determinants of rod photoreceptor regeneration. The purpose of this study was to compare the gene expression profiles of wild-type and XOPS-mCFP retinas and identify genes that may contribute to the regeneration of the rods.

METHODS

Adult wild-type and XOPS-mCFP retinal mRNA was subjected to microarray analysis. Pathway analysis was used to identify biologically relevant processes that were significantly represented in the dataset. Expression changes were verified by RT-PCR. Selected genes were further examined during retinal development and in adult retinas by in situ hybridization and immunohistochemistry and in a transgenic fluorescent reporter line.

RESULTS

More than 600 genes displayed significant expression changes in XOPS-mCFP retinas compared with expression in wild-type controls. Many of the downregulated genes were associated with phototransduction, whereas upregulated genes were associated with several biological functions, including cell cycle, DNA replication and repair, and cell development and death. RT-PCR analysis of a subset of these genes confirmed the microarray

RESULTS

Three transcription factors (sox11b, insm1a, and c-myb), displaying increased expression in XOPS-mCFP retinas, were also expressed throughout retinal development and in the persistently neurogenic ciliary marginal zone.

CONCLUSIONS

This study identified numerous gene expression changes in response to rod degeneration in zebrafish and further suggests a role for the transcriptional regulators sox11b, insm1a, and c-myb in both retinal development and rod photoreceptor regeneration.

Characterization of the mononuclear phagocyte system in zebrafish

The evolutionarily conserved immune system of the zebrafish (Danio rerio), in combination with its genetic tractability, position it as an excellent model system in which to elucidate the origin and function of vertebrate immune cells. We recently reported the existence of antigen-presenting mononuclear phagocytes in zebrafish, namely macrophages and dendritic cells (DCs), but have been impaired in further characterizing the biology of these cells by the lack of a specific transgenic reporter line. Using regulatory elements of a class II major histocompatibility gene, we generated a zebrafish reporter line expressing green fluorescent protein (GFP) in all APCs, macrophages, DCs, and B lymphocytes. Examination of mhc2dab:GFP; cd45:DsRed double-transgenic animals demonstrated that kidney mhc2dab:GFP(hi); cd45:DsRed(hi) cells were exclusively mature monocytes/macrophages and DCs, as revealed by morphologic and molecular analyses. Mononuclear phagocytes were found in all hematolymphoid organs, but were most abundant in the intestine and spleen, where they up-regulate the expression of inflammatory cytokines upon bacterial challenge. Finally, mhc2dab:GFP and cd45:DsRed transgenes mark mutually exclusive cell subsets in the lymphoid fraction, enabling the delineation of the major hematopoietic lineages in the adult zebrafish. These findings suggest that mhc2dab:GFP and cd45:DsRed transgenic lines will be instrumental in elucidating the immune response in the zebrafish.

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. In this study, 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 microRNAs (miRNAs) miR-126 and miR-150. Both miRNAs 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 miRNAs not only function 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 gata1/pu.1 lineage fate paradigm varies between blood populations and is modulated by tif1γ

Lineage fate decisions underpin much of development as well as tissue homeostasis in the adult. A mechanistic paradigm for such decisions is the erythroid versus myeloid fate decision controlled by cross-antagonism between gata1 and pu.1 transcription factors. In this study, we have systematically tested this paradigm in blood-producing populations in zebrafish embryos, including the haematopoietic stem cells (HSCs), and found that it takes a different form in each population. In particular, gata1 activity varies from autostimulation to autorepression. In addition, we have added a third member to this regulatory kernel, tif1γ (transcription intermediate factor-1γ). We show that tif1γ modulates the erythroid versus myeloid fate outcomes from HSCs by differentially controlling the levels of gata1 and pu.1. By contrast, tif1γ positively regulates both gata1 and pu.1 in primitive erythroid and prodefinitive erythromyeloid progenitors. We therefore conclude that the gata1/pu.1 paradigm for lineage decisions takes different forms in different cellular contexts and is modulated by tif1γ.

cpsf1 is required for definitive HSC survival in zebrafish

A comprehensive understanding of the genes and pathways regulating hematopoiesis is needed to identify genes causally related to bone marrow failure syndromes, myelodysplastic syndromes, and hematopoietic neoplasms. To identify novel genes involved in hematopoiesis, we performed an ethyl-nitrosourea mutagenesis screen in zebrafish (Danio rerio) to search for mutants with defective definitive hematopoiesis. We report the recovery and analysis of the grechetto mutant, which harbors an inactivating mutation in cleavage and polyadenylation specificity factor 1 (cpsf1), a gene ubiquitously expressed and required for 3' untranslated region processing of a subset of pre-mRNAs. grechetto mutants undergo normal primitive hematopoiesis and specify appropriate numbers of definitive HSCs at 36 hours postfertilization. However, when HSCs migrate to the caudal hematopoietic tissue at 3 days postfertilization, their numbers start decreasing as a result of apoptotic cell death. Consistent with Cpsf1 function, c-myb:EGFP(+) cells in grechetto mutants also show defective polyadenylation of snrnp70, a gene required for HSC development. By 5 days postfertilization, definitive hematopoiesis is compromised and severely decreased blood cell numbers are observed across the myeloid, erythroid, and lymphoid cell lineages. These studies show that cpsf1 is essential for HSC survival and differentiation in caudal hematopoietic tissue.

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.

Stem cell migration: a zebrafish model

Compared with other vertebrate animal models, zebrafish (Danio rerio) has its superior advantages for studying stem cell migration. Zebrafish have similar tissues and organs as mammals, where tissue-specific stem cells reside in. Zebrafish eggs are externally fertilized and remain transparent until most of the organs are fully developed. This allows imaging stem cells in vivo very easily. Recently, a zebrafish double pigmentation mutant, casper, became a new popular imaging model in the zebrafish field due to its completely transparent bodies in adulthood. It has been used as an excellent model to study adult hematopoietic stem cell (HSC) in the transplantation setting. The unparalleled imaging power of zebrafish provides great opportunities of tracing stem cells in vivo in the developmental and regenerative context. In this chapter, we use HSC as an example and combine the powerful imaging techniques in zebrafish, to provide protocols for in vivo imaging fluorescence-labeled stem cell migration, stem cell fate tracing in zebrafish embryos, HSC transplantation, and in vivo imaging in both zebrafish embryos and adults. These techniques can also be applied to other types of stem cells in zebrafish embryos and adults.

Cellular dissection of zebrafish hematopoiesis

The zebrafish is an excellent model system to study vertebrate blood cell development due to a highly conserved hematopoietic system, optical transparency, and amenability to both forward and reverse genetic approaches. The development of functional assays to analyze the biology of hematopoietic mutants and diseased animals remains a work in progress. Here we discuss recent advances in zebrafish hematology, prospective isolation techniques, cellular transplantation, and culture-based assays that now provide more rigorous tests of hematopoietic stem and progenitor cell function. Together with the proven strengths of the zebrafish, the development and refinement of these assays further enable efforts to better understand the development and evolution of the vertebrate hematopoietic system.

Hematopoietic stem cell development: using the zebrafish to identify the signaling networks and physical forces regulating hematopoiesis

Hematopoietic stem cells (HSC) form the basis of the hematopoietic hierarchy, giving rise to each of the blood lineages found throughout the lifetime of the organism. The genetic programs regulating HSC development are highly conserved between vertebrate species. The zebrafish has proven to be an excellent model for discovering and characterizing the signaling networks and physical forces regulating vertebrate hematopoietic development.

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.

The chicken miR-150 targets the avian orthologue of the functional zebrafish MYB 3'UTR target site

BACKGROUND

The c-myb proto-oncogene is the founding member of a family of transcription factors involved principally in haematopoiesis, in diverse organisms, from zebrafish to mammals. Its deregulation has been implicated in human leukaemogenesis and other cancers. The expression of c-myb is tightly regulated by post-transcriptional mechanisms involving microRNAs. MicroRNAs are small, highly conserved non-coding RNAs that inhibit translation and decrease mRNA stability by binding to regulatory motifs mostly located in the 3'UTR of target mRNAs conserved throughout evolution. MYB is an evolutionarily conserved miR-150 target experimentally validated in mice, humans and zebrafish. However, the functional miR-150 sites of humans and mice are orthologous, whereas that of zebrafish is different.

RESULTS

We identified the avian mature miRNA-150-5P, Gallus gallus gga-miR-150 from chicken leukocyte small-RNA libraries and showed that, as expected, the gga-miR-150 sequence was highly conserved, including the seed region sequence present in the other miR-150 sequences listed in miRBase. Reporter assays showed that gga-miR-150 acted on the avian MYB 3'UTR and identified the avian MYB target site involved in gga-miR-150 binding. A comparative in silico analysis of the miR-150 target sites of MYB 3'UTRs from different species led to the identification of a single set of putative target sites in amphibians and zebrafish, whereas two sets of putative target sites were identified in chicken and mammals. However, only the target site present in the chicken MYB 3'UTR that was identical to that in zebrafish was functional, despite the additional presence of mammalian target sites in chicken. This specific miR-150 site usage was not cell-type specific and persisted when the chicken c-myb 3'UTR was used in the cell system to identify mammalian target sites, showing that this miR-150 target site usage was intrinsic to the chicken c-myb 3'UTR.

CONCLUSION

Our study of the avian MYB/gga-miR-150 interaction shows a conservation of miR-150 target site functionality between chicken and zebrafish that does not extend to mammals.

Identification of dendritic antigen-presenting cells in the zebrafish

In mammals, dendritic cells (DCs) form the key link between the innate and adaptive immune systems. DCs act as immune sentries in various tissues and, upon encountering pathogen, engulf and traffic foreign antigen to secondary lymphoid tissues, stimulating antigen-specific T lymphocytes. Although DCs are of fundamental importance in orchestrating the mammalian immune response, their presence and function in nonmammalian vertebrates is largely unknown. Because teleosts possess one of the earliest recognizable adaptive immune systems, we sought to identify antigen-presenting cells (APCs) in the zebrafish to better understand the potential origins of DCs and their evolutionary relationship to lymphocytes. Here we present the identification and characterization of a zebrafish APC subset strongly resembling mammalian DCs. Rare DCs are present in various adult tissues, and can be enriched by their affinity for the lectin peanut agglutinin (PNA). We show that PNA(hi) myeloid cells possess the classical morphological features of mammalian DCs as revealed by histochemical and ultrastructural analyses, phagocytose-labeled bacterial preparations in vivo, and exhibit expression of genes associated with DC function and antigen presentation, including il12, MHC class II invariant chain iclp1, and csf1r. Importantly, we show that PNA(hi) cells can activate T lymphocytes in an antigen-dependent manner. Together, these studies suggest that the cellular constituents responsible for antigen presentation are remarkably conserved from teleosts to mammals, and indicate that the zebrafish may serve as a unique model to study the origin of APC subsets and their evolutionary role as the link between the innate and adaptive immune systems.

Eosinophils in the zebrafish: prospective isolation, characterization, and eosinophilia induction by helminth determinants

Eosinophils are granulocytic leukocytes implicated in numerous aspects of immunity and disease. The precise functions of eosinophils, however, remain enigmatic. Alternative models to study eosinophil biology may thus yield novel insights into their function. Eosinophilic cells have been observed in zebrafish but have not been thoroughly characterized. We used a gata2:eGFP transgenic animal to enable prospective isolation and characterization of zebrafish eosinophils, and demonstrate that all gata2(hi) cells in adult hematopoietic tissues are eosinophils. Although eosinophils are rare in most organs, they are readily isolated from whole kidney marrow and abundant within the peritoneal cavity. Molecular analyses demonstrate that zebrafish eosinophils express genes important for the activities of mammalian eosinophils. In addition, gata2(hi) cells degranulate in response to helminth extract. Chronic exposure to helminth- related allergens resulted in profound eosinophilia, demonstrating that eosinophil responses to allergens have been conserved over evolution. Importantly, infection of adult zebrafish with Pseudocapillaria tomentosa, a natural nematode pathogen of teleosts, caused marked increases in eosinophil number within the intestine. Together, these observations support a conserved role for eosinophils in the response to helminth antigens or infection and provide a new model to better understand how parasitic worms activate, co-opt, or evade the vertebrate 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.