Runx1 is required for zebrafish blood and vessel development and expression of a human RUNX1-CBF2T1 transgene advances a model for studies of leukemogenesis

Roles of RUNX in Hypoxia-Induced Responses and Angiogenesis

During the past two decades, Runt domain transcription factors (RUNX1, 2, and 3) have been investigated in regard to their function, structural elements, genetic variants, and roles in normal development and pathological conditions. The Runt family proteins are evolutionarily conserved from Drosophila to mammals, emphasizing their physiological importance. A hypoxic microenvironment caused by insufficient blood supply is frequently observed in developing organs, growing tumors, and tissues that become ischemic due to impairment or blockage of blood vessels. During embryonic development and tumor growth, hypoxia triggers a stress response that overcomes low-oxygen conditions by increasing erythropoiesis and angiogenesis and triggering metabolic changes. This review briefly introduces hypoxic conditions and cellular responses, as well as angiogenesis and its related signaling pathways, and then describes our current knowledge on the functions and molecular mechanisms of Runx family proteins in hypoxic responses, especially in angiogenesis.

Developmental HSC Microenvironments: Lessons from Zebrafish

Hematopoietic stem cells (HSCs) posses the ability to maintain the blood system of an organism from birth to adulthood. The behavior of HSCs is modulated by its microenvironment. During development, HSCs acquire the instructions to self-renew and differentiate into all blood cell fates by passing through several developmental microenvironments. In this chapter, we discuss the signals and cell types that inform HSC decisions throughout ontogeny with a focus on HSC specification, mobilization, migration, and engraftment.

Somite-Derived Retinoic Acid Regulates Zebrafish Hematopoietic Stem Cell Formation

Hematopoietic stem cells (HSCs) are multipotent progenitors that generate all vertebrate adult blood lineages. Recent analyses have highlighted the importance of somite-derived signaling factors in regulating HSC specification and emergence from dorsal aorta hemogenic endothelium. However, these factors remain largely uncharacterized. We provide evidence that the vitamin A derivative retinoic acid (RA) functions as an essential regulator of zebrafish HSC formation. Temporal analyses indicate that RA is required for HSC gene expression prior to dorsal aorta formation, at a time when the predominant RA synthesis enzyme, aldh1a2, is strongly expressed within the paraxial mesoderm and somites. Previous research implicated the Cxcl12 chemokine and Notch signaling pathways in HSC formation. Consequently, to understand how RA regulates HSC gene expression, we surveyed the expression of components of these pathways in RA-depleted zebrafish embryos. During somitogenesis, RA-depleted embryos exhibit altered expression of jam1a and jam2a, which potentiate Notch signaling within nascent endothelial cells. RA-depleted embryos also exhibit a severe reduction in the expression of cxcr4a, the predominant Cxcl12b receptor. Furthermore, pharmacological inhibitors of RA synthesis and Cxcr4 signaling act in concert to reduce HSC formation. Our analyses demonstrate that somite-derived RA functions to regulate components of the Notch and Cxcl12 chemokine signaling pathways during HSC formation.

Zebrafish Models of Human Leukemia: Technological Advances and Mechanistic Insights

Insights concerning leukemic pathophysiology have been acquired in various animal models and further efforts to understand the mechanisms underlying leukemic treatment resistance and disease relapse promise to improve therapeutic strategies. The zebrafish (Danio rerio) is a vertebrate organism with a conserved hematopoietic program and unique experimental strengths suiting it for the investigation of human leukemia. Recent technological advances in zebrafish research including efficient transgenesis, precise genome editing, and straightforward transplantation techniques have led to the generation of a number of leukemia models. The transparency of the zebrafish when coupled with improved lineage-tracing and imaging techniques has revealed exquisite details of leukemic initiation, progression, and regression. With these advantages, the zebrafish represents a unique experimental system for leukemic research and additionally, advances in zebrafish-based high-throughput drug screening promise to hasten the discovery of novel leukemia therapeutics. To date, investigators have accumulated knowledge of the genetic underpinnings critical to leukemic transformation and treatment resistance and without doubt, zebrafish are rapidly expanding our understanding of disease mechanisms and helping to shape therapeutic strategies for improved outcomes in leukemic patients.

A gene trap transposon eliminates haematopoietic expression of zebrafish Gfi1aa, but does not interfere with haematopoiesis

A transposon-mediated gene trap screen identified the zebrafish line qmc551 that expresses a GFP reporter in primitive erythrocytes and also in haemogenic endothelial cells, which give rise to haematopoietic stem and progenitor cells (HSPCs) that seed sites of larval and adult haematopoiesis. The transposon that mediates this GFP expression is located in intron 1 of the gfi1aa gene, one of three zebrafish paralogs that encode transcriptional repressors homologous to mammalian Gfi1 and Gfi1b proteins. In qmc551 transgenics, GFP expression is under the control of the endogenous gfi1aa promoter, recapitulates early gfi1aa expression and allows live observation of gfi1aa promoter activity. While the transposon integration interferes with the expression of gfi1aa mRNA in haematopoietic cells, homozygous qmc551 fish are viable and fertile, and display normal primitive and definitive haematopoiesis. Retained expression of Gfi1b in primitive erythrocytes and up-regulation of Gfi1ab at the onset of definitive haematopoiesis in homozygous qmc551 carriers, are sufficient to allow normal haematopoiesis. This finding contradicts previously published morpholino data that suggested an essential role for zebrafish Gfi1aa in primitive erythropoiesis.

Pharmacological activation of lysophosphatidic acid receptors regulates erythropoiesis

Lysophosphatidic acid (LPA), a growth factor-like phospholipid, regulates numerous physiological functions, including cell proliferation and differentiation. In a previous study, we have demonstrated that LPA activates erythropoiesis by activating the LPA 3 receptor subtype (LPA₃) under erythropoietin (EPO) induction. In the present study, we applied a pharmacological approach to further elucidate the functions of LPA receptors during red blood cell (RBC) differentiation. In K562 human erythroleukemia cells, knockdown of LPA₂ enhanced erythropoiesis, whereas knockdown of LPA₃ inhibited RBC differentiation. In CD34⁺ human hematopoietic stem cells (hHSC) and K526 cells, the LPA₃ agonist 1-oleoyl-2-methyl-sn-glycero-3-phosphothionate (2S-OMPT) promoted erythropoiesis, whereas the LPA₂ agonist dodecyl monophosphate (DMP) and the nonlipid specific agonist GRI977143 (GRI) suppressed this process. In zebrafish embryos, hemoglobin expression was significantly increased by 2S-OMPT treatment but was inhibited by GRI. Furthermore, GRI treatment decreased, whereas 2S-OMPT treatment increased RBC counts and amount of hemoglobin level in adult BALB/c mice. These results indicate that LPA₂ and LPA₃ play opposing roles during RBC differentiation. The pharmacological activation of LPA receptor subtypes represent a novel strategies for augmenting or inhibiting erythropoiesis.

Cellular dissection of zebrafish hematopoiesis

Zebrafish as a model system have been instrumental in understanding early vertebrate development, especially of the hematopoietic system. The external development of zebrafish and their genetic amenability have allowed in-depth studies of multiple blood cell types and their respective genetic regulation. This chapter highlights some new data in zebrafish hematopoiesis regarding primitive and definitive hematopoiesis in the embryonic and adult fish, allowing the isolation of prospective progenitor subsets. It also highlights assays developed to examine the function of these progenitors in vivo and in vitro, allowing an evolutionary understanding of the hematopoietic system and how zebrafish can be better utilized as a model system for a multitude of hematopoietic disorders.

Conserved hemopoietic transcription factor Cg-SCL delineates hematopoiesis of Pacific oyster Crassostrea gigas

Hemocytes are the effective immunocytes in bivalves, which have been reported to be derived from stem-like cells in gill epithelium of oyster. In the present work, a conserved haematopoietic transcription factor Tal-1/Scl (Stem Cell Leukemia) was identified in Pacific oyster (Cg-SCL), and it was evolutionarily close to the orthologs in deuterostomes. Cg-SCL was highly distributed in the hemocytes as well as gill and mantle. The hemocyte specific genes Integrin, EcSOD and haematopoietic transcription factors GATA3, C-Myb, c-kit, were down-regulated when Cg-SCL was interfered by dsRNA. During the larval developmental stages, the mRNA transcripts of Cg-SCL gradually increased after fertilization and peaked at early trochophore larvae stage (10 hpf, hours post fertilization), then sharply decreased in late trochophore larvae stage (15 hpf) before resuming in umbo larvae (120 hpf). Whole-mount immunofluorescence assay further revealed that the immunoreactivity of Cg-SCL appeared in blastula larvae with two approximate symmetric spots, and this expression pattern lasted in gastrula larvae. By trochophore, the immunoreactivity formed a ring around the dorsal region and then separated into two remarkable spots at the dorsal side in D-veliger larvae. After bacterial challenge, the mRNA expression levels of Cg-SCL were significantly up-regulated in the D-veliger and umbo larvae, indicating the available hematopoietic regulation in oyster larvae. These results demonstrated that Cg-SCL could be used as haematopoietic specific marker to trace potential developmental events of hematopoiesis during ontogenesis of oyster, which occurred early in blastula stage and maintained until D-veliger larvae.

Sprouting Buds of Zebrafish Research in Malaysia: First Malaysia Zebrafish Disease Model Workshop

Zebrafish is gaining prominence as an important vertebrate model for investigating various human diseases. Zebrafish provides unique advantages such as optical clarity of embryos, high fecundity rate, and low cost of maintenance, making it a perfect complement to the murine model equivalent in biomedical research. Due to these advantages, researchers in Malaysia are starting to take notice and incorporate the zebrafish model into their research activities. However, zebrafish research in Malaysia is still in its infancy stage and many researchers still remain unaware of the full potential of the zebrafish model or have limited access to related tools and techniques that are widely utilized in many zebrafish laboratories worldwide. To overcome this, we organized the First Malaysia Zebrafish Disease Model Workshop in Malaysia that took place on 11th and 12th of November 2015. In this workshop, we showcased how the zebrafish model is being utilized in the biomedical field in international settings as well as in Malaysia. For this, notable international speakers and those from local universities known to be carrying out impactful research using zebrafish were invited to share some of the cutting edge techniques that are used in their laboratories that may one day be incorporated in the Malaysian scientific community.

Recent findings on vertebrate developmental immunity using the zebrafish model

To grant survival against sterile or microbe induced inflammation, all animals rely on correct immune system functioning. The development of immunity occurs in vertebrates during embryogenesis in a process called hematopoiesis, which is characterized by the formation of blood cellular components such as embryonic erythrocytes and primitive macrophages. These cells are formed in a sterile environment from a rare subset of pluripotent hematopoietic stem cells (HSC) during a brief period of the primitive hematopoietic wave. Diverse signals, like Notch, are indispensable in HSC emergence and differentiation. However, to successfully replicate the process in vitro using pluripotent precursors, the full set of required signals is still a matter of debate. Among the latest findings, proinflammatory signals produced by transient primitive myelocites in zebrafish have been seen to act as essential mediators in establishing the HSC program of the adult vertebrate hematopoietic system. In this regard, the zebrafish immune model has emerged as a feasible live vertebrate model for examining developmental immunity and related host-microbe interactions, both at the molecular and cellular level. Thus, using the zebrafish embryo, this review summarizes recent findings, on the signals required for immune development and further maturation of the system, in a context where no adaptive immune response has yet been developed.

K. Rothman A, Elworthy S, van Eeden FJM, Chico TJA. klf2ash317 Mutant Zebrafish Do Not Recapitulate Morpholino-Induced Vascular and Haematopoietic Phenotypes

Introduction and Objectives

The zinc-finger transcription factor Krϋppel-like factor 2 (KLF2) transduces blood flow into molecular signals responsible for a wide range of responses within the vasculature. KLF2 maintains a healthy, quiescent endothelial phenotype. Previous studies report a range of phenotypes following morpholino antisense oligonucleotide-induced klf2a knockdown in zebrafish. Targeted genome editing is an increasingly applied method for functional assessment of candidate genes. We therefore generated a stable klf2a mutant zebrafish and characterised its cardiovascular and haematopoietic development.

Methods and Results

Using Transcription Activator-Like Effector Nucleases (TALEN) we generated a klf2a mutant (klf2a^sh317) with a 14bp deletion leading to a premature stop codon in exon 2. Western blotting confirmed loss of wild type Klf2a protein and the presence of a truncated protein in klf2a^sh317 mutants. Homozygous klf2a^sh317 mutants exhibit no defects in vascular patterning, survive to adulthood and are fertile, without displaying previously described morphant phenotypes such as high-output cardiac failure, reduced haematopoetic stem cell (HSC) development or impaired formation of the 5^th accessory aortic arch. Homozygous klf2a^sh317 mutation did not reduce angiogenesis in zebrafish with homozygous mutations in von Hippel Lindau (vhl), a form of angiogenesis that is dependent on blood flow. We examined expression of three klf family members in wildtype and klf2a^sh317 zebrafish. We detected vascular expression of klf2b (but not klf4a or biklf/klf4b/klf17) in wildtypes but found no differences in expression that might account for the lack of phenotype in klf2a^sh317 mutants. klf2b morpholino knockdown did not affect heart rate or impair formation of the 5^th accessory aortic arch in either wildtypes or klf2a^sh317 mutants.

Conclusions

The klf2a^sh317 mutation produces a truncated Klf2a protein but, unlike morpholino induced klf2a knockdown, does not affect cardiovascular development.

Fishing the targets of myeloid malignancies in the era of next generation sequencing

Recent advent in next generation sequencing (NGS) and bioinformatics has generated an unprecedented amount of genetic information in myeloidmalignancies. This information may shed lights to the pathogenesis, diagnosis and prognostication of these diseases and provide potential targets for therapeutic intervention. However, the rapid emergence of genetic information will quickly outpace their functional validation by conventional laboratory platforms. Foundational knowledge about zebrafish hematopoiesis accumulated over the past two decades and novel genomeediting technologies and research strategies in thismodel organismhavemade it a unique and timely research tool for the study of human blood diseases. Recent studies modeling human myeloid malignancies in zebrafish have also highlighted the technical feasibility and clinical relevance of thesemodels. Careful validation of experimental protocols and standardization among laboratorieswill further enhance the application of zebrafish in the scientific communities and provide important insights to the personalized treatment ofmyeloid malignancies.

The promise of zebrafish as a chemical screening tool in cancer therapy

Cancer progression in zebrafish recapitulates many aspects of human cancer and as a result, zebrafish have been gaining popularity for their potential use in basic and translational cancer research. Human cancer can be modeled in zebrafish by induction using chemical mutagens, xenotransplantation or by genetic manipulation. Chemical screens based on zebrafish cancer models offer a rapid, powerful and inexpensive means of evaluating the potential of suppression or prevention on cancer. The identification of small molecules through such screens will serve as ideal entry points for novel chemical therapies for cancer treatment. This article outlines advances that have been made within the growing field of zebrafish cancer models and presents their advantages for chemical drug screening.

Cannabinoid Receptor-2 Regulates Embryonic Hematopoietic Stem Cell Development via Prostaglandin E2 and P-Selectin Activity

Cannabinoids (CB) modulate adult hematopoietic stem and progenitor cell (HSPCs) function, however, impact on the production, expansion, or migration of embryonic HSCs is currently uncharacterized. Here, using chemical and genetic approaches targeting CB-signaling in zebrafish, we show that CB receptor (CNR) 2, but not CNR1, regulates embryonic HSC development. During HSC specification in the aorta-gonad-mesonephros (AGM) region, CNR2 stimulation by AM1241 increased runx1;cmyb(+) HSPCs, through heightened proliferation, whereas CNR2 antagonism decreased HSPC number; FACS analysis and absolute HSC counts confirmed and quantified these effects. Epistatic investigations showed AM1241 significantly upregulated PGE2 synthesis in a Ptgs2-dependent manner to increase AGM HSCs. During the phases of HSC production and colonization of secondary niches, AM1241 accelerated migration to the caudal hematopoietic tissue (CHT), the site of embryonic HSC expansion, and the thymus; however these effects occurred independently of PGE2. Using a candidate approach for HSC migration and retention factors, P-selectin was identified as the functional target of CNR2 regulation. Epistatic analyses confirmed migration of HSCs into the CHT and thymus was dependent on CNR2-regulated P-selectin activity. Together, these data suggest CNR2-signaling optimizes the production, expansion, and migration of embryonic HSCs by modulating multiple downstream signaling pathways.

Methods for studying the zebrafish brain: past, present and future

The zebrafish (Danio rerio) is one of the most promising new model organisms. The increasing popularity of this amazing small vertebrate is evident from the exponentially growing numbers of research articles, funded projects and new discoveries associated with the use of zebrafish for studying development, brain function, human diseases and screening for new drugs. Thanks to the development of novel technologies, the range of zebrafish research is constantly expanding with new tools synergistically enhancing traditional techniques. In this review we will highlight the past and present techniques which have made, and continue to make, zebrafish an attractive model organism for various fields of biology, with a specific focus on neuroscience.

Zebrafish as a Model for the Study of Human Myeloid Malignancies

Myeloid malignancies are heterogeneous disorders characterized by uncontrolled proliferation or/and blockage of differentiation of myeloid progenitor cells. Although a substantial number of gene alterations have been identified, the mechanism by which these abnormalities interact has yet to be elucidated. Over the past decades, zebrafish have become an important model organism, especially in biomedical research. Several zebrafish models have been developed to recapitulate the characteristics of specific myeloid malignancies that provide novel insight into the pathogenesis of these diseases and allow the evaluation of novel small molecule drugs. This report will focus on illustrative examples of applications of zebrafish models, including transgenesis, zebrafish xenograft models, and cell transplantation approaches, to the study of human myeloid malignancies.

Zebrafish as a model for leukemia and other hematopoietic disorders

Zebrafish is an established model for the study of vertebrate development, and is especially amenable for investigating hematopoiesis, where there is strong conservation of key lineages, genes, and developmental processes with humans. Over recent years, zebrafish has been increasingly utilized as a model for a range of human hematopoietic diseases, including malignancies. This review provides an overview of zebrafish hematopoiesis and describes its application as a model of leukemia and other hematopoietic disorders.

Molecular identification of transcription factor Runx1 variants in grass carp (Ctenopharyngodon idella) and their responses to immune stimuli

The Runt-related transcription factor (Runx) family consists of three members, Runx1, Runx2 and Runx3 in mammals, which are involved in various biological processes. Recent studies have demonstrated that Runx1 plays critical roles in the immunity of higher vertebrates. In fish, zebrafish and fugu Runx family members have been identified, and their chromosome location, promoter usage and expression patterns have been elucidated. However, their expression profiles in immune responses are still unknown. In this study, we identified grass carp five Runx1 (gcRunx1) variants (v1-5) possibly generated through alternative promoter usage and alternative splicing. The gcRunx1 v1-3 encodes the proteins possessing intact structural characteristics of Runx family, but the putative proteins of gcRunx1 v4-5 lack a transactivation domain, an inhibitory domain and a C-terminal pentapeptide motif (VWRPY). Tissue distribution assays revealed that gcRunx1 was preferentially expressed in some immune-related tissues including thymus and spleen, indicating its potential roles in teleost immunity. The changes of gcRunx1 expression to various immune stimuli was examined in periphery blood lymphocytes, showing that gcRunx1 v1-3 mRNA levels were increased after LPS, poly I:C and PHA treatment, whereas gcRunx1 v4-5 mRNA expression were stimulated only by LPS and PHA. Furthermore, in vivo studies confirmed that bacterial challenge enhanced gcRunx1 mRNA levels. In particular, in vitro and in vivo studies revealed that gcRunx1 v4-5 mRNA expression was induced with a delayed kinetics compared with that of gcRunx1 v1-3. These findings not only provide the evidence for the involvement of gcRunx1 in immune response, but also reveal the inducible expression diversity of fish Runx1 splicing variants, thereby facilitating further elucidating the role of Runx1 in piscine immunity.

Oceans of opportunity: exploring vertebrate hematopoiesis in zebrafish

Exploitation of the zebrafish model in hematology research has surged in recent years, becoming one of the most useful and tractable systems for understanding regulation of hematopoietic development, homeostasis, and malignancy. Despite the evolutionary distance between zebrafish and humans, remarkable genetic and phenotypic conservation in the hematopoietic system has enabled significant advancements in our understanding of blood stem and progenitor cell biology. The strengths of zebrafish in hematology research lie in the ability to perform real-time in vivo observations of hematopoietic stem, progenitor, and effector cell emergence, expansion, and function, as well as the ease with which novel genetic and chemical modifiers of specific hematopoietic processes or cell types can be identified and characterized. Further, myriad transgenic lines have been developed including fluorescent reporter systems to aid in the visualization and quantification of specified cell types of interest and cell-lineage relationships, as well as effector lines that can be used to implement a wide range of experimental manipulations. As our understanding of the complex nature of blood stem and progenitor cell biology during development, in response to infection or injury, or in the setting of hematologic malignancy continues to deepen, zebrafish will remain essential for exploring the spatiotemporal organization and integration of these fundamental processes, as well as the identification of efficacious small molecule modifiers of hematopoietic activity. In this review, we discuss the biology of the zebrafish hematopoietic system, including similarities and differences from mammals, and highlight important tools currently utilized in zebrafish embryos and adults to enhance our understanding of vertebrate hematology, with emphasis on findings that have impacted our understanding of the onset or treatment of human hematologic disorders and disease.

Smoc2 modulates embryonic myelopoiesis during zebrafish development

BACKGROUND

SMOC2 is a member of the BM-40 (SPARC) family of matricellular proteins, reported to influence signaling in the extracellular compartment. In mice, Smoc2 is expressed in many different tissues and was shown to enhance the response to angiogenic growth factors, mediate cell adhesion, keratinocyte migration, and metastasis. Additionally, SMOC2 is associated with vitiligo and craniofacial and dental defects. The function of Smoc2 during early zebrafish development has not been determined to date.

RESULTS

In pregastrula zebrafish embryos, smoc2 is expressed ubiquitously. As development progresses, the expression pattern becomes more anteriorly restricted. At the onset of blood cell circulation, smoc2 morphants presented a mild ventralization of posterior structures. Molecular analysis of the smoc2 morphants indicated myelopoietic defects in the rostral blood islands during segmentation stages. Hemangioblast development and further specification of the myeloid progenitor cells were shown to be impaired. Additional experiments indicated that Bmp target genes were down-regulated in smoc2 morphants.

CONCLUSIONS

Our findings reveal that Smoc2 is an essential player in the development of myeloid cells of the anterior lateral plate mesoderm during embryonic zebrafish development. Furthermore, our data show that Smoc2 affects the transcription of Bmp target genes without affecting initial dorsoventral patterning or mesoderm development.

Drl.3 governs primitive hematopoiesis in zebrafish

The molecular program controlling hematopoietic differentiation is not fully understood. Here, we describe a family of zebrafish genes that includes a novel hematopoietic regulator, draculin-like 3 (drl.3). We found that drl.3 is expressed in mesoderm-derived hematopoietic cells and is retained during erythroid maturation. Moreover, drl.3 expression correlated with erythroid development in gata1a- and spi1b-depleted embryos. Loss-of-function analysis indicated that drl.3 plays an essential role in primitive erythropoiesis and, to a lesser extent, myelopoiesis that is independent of effects on vasculature, emergence of primitive and definitive progenitor cells and cell viability. While drl.3 depletion reduced gata1a expression and inhibited erythroid development, enforced expression of gata1a was not sufficient to rescue erythropoiesis, indicating that the regulation of hematopoiesis by drl.3 extends beyond control of gata1a expression. Knockdown of drl.3 increased the proportion of less differentiated, primitive hematopoietic cells without affecting proliferation, establishing drl.3 as an important regulator of primitive hematopoietic cell differentiation.

Regulation of embryonic hematopoiesis by a cytokine-inducible SH2 domain homolog in zebrafish

Cytokine-inducible SH2 domain-containing protein (CISH), a member of the suppressor of cytokine signaling family of negative feedback regulators, is induced by cytokines that activate STAT5 and can inhibit STAT5 signaling in vitro. However, demonstration of a definitive in vivo role for CISH during development has remained elusive. This study employed expression analysis and morpholino-mediated knockdown in zebrafish in concert with bioinformatics and biochemical approaches to investigate CISH function. Two zebrafish CISH paralogs were identified, cish.a and cish.b, with high overall conservation (43-46% identity) with their mammalian counterparts. The cish.a gene was maternally derived, with transcripts present throughout embryogenesis, and increasing at 4-5 d after fertilization, whereas cish.b expression commenced at 8 h after fertilization. Expression of cish.a was regulated by the JAK2/STAT5 pathway via conserved tetrameric STAT5 binding sites (TTCN3GAA) in its promoter. Injection of morpholinos targeting cish.a, but not cish.b or control morpholinos, resulted in enhanced embryonic erythropoiesis, myelopoiesis, and lymphopoiesis, including a 2- 3-fold increase in erythrocytic markers. This occurred concomitantly with increased activation of STAT5. This study indicates that CISH functions as a conserved in vivo target and regulator of STAT5 in the control of embryonic hematopoiesis.

Protein tyrosine phosphatase PTPN9 regulates erythroid cell development through STAT3 dephosphorylation in zebrafish

Protein tyrosine phosphatases (PTPs) are involved in hematopoiesis, but the function of many PTPs is not well characterized in vivo. Here, we have identified Ptpn9a, an ortholog of human PTPN9, as a crucial regulator of erythroid cell development in zebrafish embryos. ptpn9a, but not ptpn9b, was expressed in the posterior lateral plate mesoderm and intermediate cell mass - two primitive hematopoietic sites during zebrafish embryogenesis. Morpholino-mediated knockdown of ptpn9a caused erythrocytes to be depleted by inhibiting erythroid cell maturation without affecting erythroid proliferation and apoptosis. Consistently, both dominant-negative PTPN9 (with mutation C515S) and siRNA against PTPN9 inhibited erythroid differentiation in human K562 cells. Mechanistically, depletion of ptpn9 in zebrafish embryos in vivo or in K562 cells in vitro increased phosphorylated STAT3, and the hyper-phosphorylated STAT3 entrapped and prevented the transcription factors GATA1 and ZBP-89 (also known as ZNF148) from regulating erythroid gene expression. These findings imply that PTPN9 plays an important role in erythropoiesis by disrupting an inhibitory complex of phosphorylated STAT3, GATA1 and ZBP-89, providing new cellular and molecular insights into the role of ptpn9a in developmental hematopoiesis.

Danio rerio: Small Fish Making a Big Splash in Leukemia

Zebrafish (Danio rerio) are widely used for developmental biology studies. In the past decade, D. rerio have become an important oncology model as well. Leukemia is one type of cancer where zebrafish are particularly valuable. As vertebrates, fish have great anatomic and biologic similarity to humans, including their hematopoietic and immune systems. As an experimental platform, D. rerio offer many advantages that mammalian models lack. These include their ease of genetic manipulation, capacity for imaging, and suitability for large-scale phenotypic and drug screens. In this review, we present examples of these strategies and others to illustrate how zebrafish have been and can be used to study leukemia. Besides appraising the techniques researchers apply and introducing the leukemia models they have created, we also highlight recent and exciting discoveries made using D. rerio with an eye to where the field is likely headed.

Aggf1 acts at the top of the genetic regulatory hierarchy in specification of hemangioblasts in zebrafish

The hemangioblast is a multipotential progenitor, which is derived from the mesoderm and can further differentiate into hematopoietic and endothelial lineages. The molecular mechanism governing the specification of hemangioblasts is fundamental to regenerative medicine based on embryonic stem cells for the treatment of various hematologic and vascular diseases. Here we show that aggf1 acts at the top of the genetic regulatory hierarchy in the specification of hemangioblasts in zebrafish. Knockdown of aggf1 expression decreases expression of endothelial cell-specific markers (cdh5, admr) and disrupts primitive hematopoiesis as shown by a decreased number of erythroid cells and reduced expression of gata1 (marker for erythroid progenitors) and pu.1 (myeloid progenitors). Aggf1 knockdown also decreases expression of runx1 and c-myb, indicating that it is required for specification of hematopoietic stem cells (definitive hematopoiesis). Aggf1 knockdown led to dramatically reduced expression of hemangioblast markers fli1, etsrp, lmo2, and scl, and hematopoietic/endothelial defects in aggf1 morphants were rescued by messenger RNA for scl, fli-vp16, or etsrp. Taken together, these data indicate that aggf1 is involved in differentiation of both hematopoietic and endothelial lineages and that aggf1 acts upstream of scl, fli1, and etsrp in specification of hemangioblasts.

Cohesin and CTCF differentially regulate spatiotemporal runx1 expression during zebrafish development

Runx1 is a transcription factor essential for definitive hematopoiesis. In all vertebrates, the Runx1 gene is transcribed from two promoters: a proximal promoter (P2), and a distal promoter (P1). We previously found that runx1 expression in a specific hematopoietic cell population in zebrafish embryos depends on cohesin. Here we show that zebrafish runx1 is directly bound by cohesin and CCCTC binding factor (CTCF) at the P1 and P2 promoters, and within the intron between P1 and P2. Cohesin initiates expression of runx1 in the posterior lateral mesoderm and influences promoter use, while CTCF represses its expression in the newly emerging cells of the tail bud. The intronic binding sites for cohesin and CTCF coincide with histone modifications that confer enhancer-like properties, and two of the cohesin/CTCF sites behaved as insulators in an in vivo assay. The identified cohesin and CTCF binding sites are likely to be cis-regulatory elements (CREs) for runx1 since they also recruit RNA polymerase II (RNAPII). CTCF depletion excluded RNAPII from two intronic CREs but not the promoters of runx1. We propose that cohesin and CTCF have distinct functions in the regulation of runx1 during zebrafish embryogenesis, and that these regulatory functions are likely to involve runx1 intronic CREs. Cohesin (but not CTCF) depletion enhanced RUNX1 expression in a human leukemia cell line, suggesting conservation of RUNX1 regulation through evolution.

Etv5a regulates the proliferation of ventral mesoderm cells and the formation of hemato-vascular derivatives

Hematopoietic and vascular endothelial cells constitute the circulatory system and are both generated from the ventral mesoderm. However, the molecules and signaling pathways involved in ventral mesoderm formation and specification remain unclear. We found that zebrafish etv5a was expressed in the ventral mesoderm during gastrulation. Knockdown of Etv5a using morpholinos increased the proliferation of ventral mesoderm cells and caused defects in hematopoietic derivatives and in vascular formation. By contrast, the formation of other mesodermal derivatives, such as pronephros, somites and the gut wall, was not affected. Knockdown specificity was further confirmed by overexpression of an etv5a construct lacking its acidic domain. In conclusion, our data reveal that etv5a is essential for the inhibition of ventral mesoderm cell proliferation and for the formation of the hemato-vascular lineage.

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.

Hematopoiesis

Hematopoiesis - the process by which blood cells are formed - has been studied intensely for over a century using a variety of model systems. There is conservation of the overall hematopoietic process between vertebrates, although some differences do exist. Over the last decade, the zebrafish has come to the forefront as a new model in hematopoiesis research, as it allows the use of large-scale genetics, chemical screens and transgenics. This comparative approach to understanding hematopoiesis has led to fundamental knowledge about the process and to the development of new therapies for disease. Here, we provide a broad overview of vertebrate hematopoiesis. We also highlight the benefits of using zebrafish as a model.

Signalling pathways that control vertebrate haematopoietic stem cell specification

Haematopoietic stem cells (HSCs) are tissue-specific stem cells that replenish all mature blood lineages during the lifetime of an individual. Clinically, HSCs form the foundation of transplantation-based therapies for leukaemias and congenital blood disorders. Researchers have long been interested in understanding the normal signalling mechanisms that specify HSCs in the embryo, in part because recapitulating these requirements in vitro might provide a means to generate immune-compatible HSCs for transplantation. Recent embryological work has demonstrated the existence of previously unknown signalling requirements. Moreover, it is now clear that gene expression in the nearby somite is integrally involved in regulating the transition of the embryonic endothelium to a haemogenic fate. Here, we review current knowledge of the intraembryonic signals required for the specification of HSCs in vertebrates.

Emergence of zebrafish models in oncology for validating novel anticancer drug targets and nanomaterials

The in vivo zebrafish models have recently attracted great attention in molecular oncology to investigate multiple genetic alterations associated with the development of human cancers and validate novel anticancer drug targets. Particularly, the transparent zebrafish models can be used as a xenotransplantation system to rapidly assess the tumorigenicity and metastatic behavior of cancer stem and/or progenitor cells and their progenies. Moreover, the zebrafish models have emerged as powerful tools for an in vivo testing of novel anticancer agents and nanomaterials for counteracting tumor formation and metastases and improving the efficacy of current radiation and chemotherapeutic treatments against aggressive, metastatic and lethal cancers.

The zebrafish as a model for paediatric diseases

The rapid increase in information about genes and their associations with human diseases has highlighted the need for model organisms suitable for genetic manipulation and drug testing. The zebrafish is a valuable vertebrate animal model that offers many advantages, including the relative ease of husbandry and genetic manipulation and the capacity for high-throughput screens. In this review, we describe the zebrafish as a model for paediatric diseases, with particular emphasis on haematopoietic and infectious diseases.

CONCLUSION

The zebrafish has become an established vertebrate model in which to elucidate the molecular mechanisms of various human diseases.

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.

Signaling axis involving Hedgehog, Notch, and Scl promotes the embryonic endothelial-to-hematopoietic transition

During development, the hematopoietic lineage transits through hemogenic endothelium, but the signaling pathways effecting this transition are incompletely characterized. Although the Hedgehog (Hh) pathway is hypothesized to play a role in patterning blood formation, early embryonic lethality of mice lacking Hh signaling precludes such analysis. To determine a role for Hh signaling in patterning of hemogenic endothelium, we assessed the effect of altered Hh signaling in differentiating mouse ES cells, cultured mouse embryos, and developing zebrafish embryos. In differentiating mouse ES cells and mouse yolk sac cultures, addition of Indian Hh ligand increased hematopoietic progenitors, whereas chemical inhibition of Hh signaling reduced hematopoietic progenitors without affecting primitive streak mesoderm formation. In the setting of Hh inhibition, induction of either Notch signaling or overexpression of Stem cell leukemia (Scl)/T-cell acute lymphocytic leukemia protein 1 rescued hemogenic vascular-endothelial cadherin(+) cells and hematopoietic progenitor formation. Together, our results reveal that Scl overexpression is sufficient to rescue the developmental defects caused by blocking the Hh and Notch pathways, and inform our understanding of the embryonic endothelial-to-hematopoietic transition.

PDCD2 controls hematopoietic stem cell differentiation during development

Programmed cell death 2 (Pdcd2) is a highly conserved protein of undefined function, and is widely expressed in embryonic and adult tissues. The observations that knockout of Pdcd2 in the mouse is embryonic lethal at preimplantation stages, and that in Drosophila, Zfrp8, the ortholog of Pdcd2, is required for normal lymph gland development suggest that Pdcd2 is important for regulating hematopoietic development. Through genetic and functional studies, we investigated pdcd2 function during the zebrafish ontogeny. Knockdown of pdcd2 expression in zebrafish embryos resulted in defects in embryonic hematopoietic development. Loss of pdcd2 function caused increased expression of progenitor markers, and accumulation of erythroid progenitors during primitive hematopoiesis. Additionally, hematopoietic stem cells (HSCs) failed to appear in the aorta-gonad mesonephros, and were not able to terminally differentiate or reconstitute hematopoiesis. Pdcd2 effects on HSC emergence were cell autonomous and P53-independent, and loss of pdcd2 function was associated with mitotic defects and apoptosis. Restoration of runx1 function(s) and modulation of apoptosis through the inhibition of Jak/Stat signaling rescued the hematopoietic and erythroid defects resulting from pdcd2 knockdown. Our studies suggest that pdcd2 plays a critical role in regulating the transcriptional hierarchy controlling hematopoietic lineage determination. Furthermore, the effects of pdcd2 in regulating mitotic cell death may contribute to its role(s) in directing hematopoietic differentiation during development.

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.

A novel complex, RUNX1-MYEF2, represses hematopoietic genes in erythroid cells

RUNX1 is known to be an essential transcription factor for generating hematopoietic stem cells (HSC), but much less is known about its role in the downstream process of hematopoietic differentiation. RUNX1 has been shown to be part of a large transcription factor complex, together with LDB1, GATA1, TAL1, and ETO2 (N. Meier et al., Development 133:4913-4923, 2006) in erythroid cells. We used a tagging strategy to show that RUNX1 interacts with two novel protein partners, LSD1 and MYEF2, in erythroid cells. MYEF2 is bound in undifferentiated cells and is lost upon differentiation, whereas LSD1 is bound in differentiated cells. Chromatin immunoprecipitation followed by sequencing (ChIP-seq) and microarray expression analysis were used to show that RUNX1 binds approximately 9,000 target sites in erythroid cells and is primarily active in the undifferentiated state. Functional analysis shows that a subset of the target genes is suppressed by RUNX1 via the newly identified partner MYEF2. Knockdown of Myef2 expression in developing zebrafish results in a reduced number of HSC.

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.

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.

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.

In vivo chemical screening for modulators of hematopoiesis and hematological diseases

In vivo chemical screening is a broadly applicable approach not only for dissecting genetic pathways governing hematopoiesis and hematological diseases, but also for finding critical components in those pathways that may be pharmacologically modulated. Both high-throughput chemical screening and facile detection of blood-cell-related phenotypes are feasible in embryonic/larval zebrafish. Two recent studies utilizing phenotypic chemical screens in zebrafish have identified several compounds that promote hematopoietic stem cell formation and reverse the hematopoietic phenotypes of a leukemia oncogene, respectively. These studies illustrate efficient drug discovery processes in zebrafish and reveal novel biological roles of prostaglandin E2 in hematopoietic and leukemia stem cells. Furthermore, the compounds discovered in zebrafish screens have become promising therapeutic candidates against leukemia and included in a clinical trial for enhancing hematopoietic stem cells during hematopoietic cell transplantation.

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.