Harnessing zebrafish for the study of white blood cell development and its perturbation

Observation and characterisation of macrophages in zebrafish liver

Kupffer cells are liver-resident macrophages that play an important role in mediating immune-related functions in mammals and humans. They are well-known for their capacity to phagocytose large amounts of waste complexes, cell debris, microbial particles and even malignant cells. Location, appearance and functional aspects are important features used to identify these characteristic cells of the liver sinusoid. To-date, there is limited information on the occurrence of macrophages in zebrafish liver. Therefore, we aimed to characterise the ultrastructural and functional aspects of liver-associated macrophages in the zebrafish model by taking advantage of the latest advances in zebrafish genetics and multimodal correlative imaging. Herein, we report on the occurrence of macrophages within the zebrafish liver exhibiting conventional ultrastructural features (e.g. presence of pseudopodia, extensive lysosomal apparatus, a phagolysosome and making up ∼3% of the liver volume). Intriguingly, these cells were not located within the sinusoidal vascular bed of hepatic tissue but instead resided between hepatocytes and lacked phagocytic function. While our results demonstrated the presence and structural similarities with liver macrophages from other experimental models, their functional characteristics were distinctly different from Kupffer cells that have been described in rodents and humans. These findings illustrate that the innate immune system of the zebrafish liver has some distinctly different characteristics compared to other animal experimental models. This conclusion underpins our call for future studies in order to have a better understanding of the physiological role of macrophages residing between the parenchymal cells of the zebrafish liver.

Jak2a regulates erythroid and myeloid hematopoiesis during zebrafish embryogenesis

Zebrafish (Danio rerio) are an attractive vertebrate model for the molecular dissection of disease mechanisms. Janus kinase (JAK)/signal transducer and activator of transcription (stat) has been defined through studies of cytokine signaling pathways in mammals. Here, we examined the expression level of Jak2a, which is a homolog of mammalian jak2 in zebrafish, by quantitative reverse transcriptase (RT)-PCR, and the peak of mRNA expression occurred at 3.75 hours post fertilization (hpf). The overexpression of Jak2a was proven by real-time Q-PCR and Western blot in 1-4-cell stage embryos injected with 400 ng/µl full-length jak2a mRNA as well as gfi1.1, gata1, mpo and β-embryonic hemoglobin as detected by real-time Q-PCR. Moreover, jak2a mRNA significantly increased the GFP⁺ population in the transgenic zebrafish lines Tg (gata1: gfp) (uninjected embryos: 17.22±1.70%; embryos injected with jak2a mRNA: 21.31±2.11%, p<0.01) and Tg (mpo: gfp) (uninjected embryos: 3.86±1.94; embryos injected with jak2a mRNA: 6.64±1.30%, p<0.01) compared with the control group. Thus, our data indicate that Jak2a plays an important role in erythropoiesis and myeloid hematopoiesis.

The resolution of inflammation: Principles and challenges

The concept that chemokines, cytokines and pro-inflammatory mediators act in a co-ordinated fashion to drive the initiation of the inflammatory reaction is well understood. The significance of such networks acting during the resolution of inflammation however is poorly appreciated. In recent years, specific pro-resolving mediators were discovered which activate resolution pathways to return tissues to homeostasis. These mediators are diverse in nature, and include specialized lipid mediators (lipoxins, resolvins, protectins and maresins) proteins (annexin A1, galectins) and peptides, gaseous mediators including hydrogen sulphide, a purine (adenosine), as well as neuromodulator release under the control of the vagus nerve. Functionally, they can act to limit further leukocyte recruitment, induce neutrophil apoptosis and enhance efferocytosis by macrophages. They can also switch macrophages from classical to alternatively activated cells, promote the return of non-apoptotic cells to the lymphatics and help initiate tissue repair mechanisms and healing. Within this review we highlight the essential cellular aspects required for successful tissue resolution, briefly discuss the pro-resolution mediators that drive these processes and consider potential challenges faced by researchers in the quest to discover how inflammation resolves and why chronic inflammation persists.

Pegasus, the 'atypical' Ikaros family member, influences left-right asymmetry and regulates pitx2 expression

Members of the Ikaros family of zinc-finger transcription factors have been shown to be critical for immune and blood cell development. However, the role of the most divergent family member, Pegasus, has remained elusive, although it shows conservation to invertebrate Hunchback proteins that influence embryonic patterning through regulation of homeodomain genes. Zebrafish was employed as a relevant model to investigate the function of Pegasus since it possesses a single pegasus orthologue with high homology to its mammalian counterparts. During zebrafish embryogenesis pegasus transcripts were initially maternally-derived and later replaced by zygotic expression in the diencephalon, tectum, hindbrain, thymus, eye, and ultimately the exocrine pancreas and intestine. Morpholino-mediated knockdown of the zebrafish pegasus gene resulted in disrupted left-right asymmetry of the gut and pancreas. Molecular analysis indicated that zebrafish Pegasus localised to the nucleus in discrete non-nucleolar structures and bound the 'atypical' DNA sequence GN3GN2G, confirming its presumed role as a transcriptional regulator. In vivo transcriptome analysis identified candidate target genes, several of which encoded homeodomain transcription factors. One of these, pitx2, implicated in left-right asymmetry, possessed appropriate 'atypical' Pegasus binding sites in its promoter. Knockdown of Pegasus affected both the level and asymmetry of pitx2 expression, as well as disrupting the asymmetry of the lefty2 and spaw genes, explaining the perturbed left-right patterning in pegasus morphants. Collectively these results provide the first definitive insights into the in vivo role of Pegasus, supporting the notion that it acts as a broader regulator of development, with potential parallels to the related invertebrate Hunchback proteins.

Suppressor of cytokine signaling 1 regulates embryonic myelopoiesis independently of its effects on T cell development

Suppressor of cytokine signaling 1 (SOCS1) has been shown to play important roles in the immune system. It acts as a key negative regulator of signaling via receptors for IFNs and other cytokines controlling T cell development, as well as Toll receptor signaling in macrophages and other immune cells. To gain further insight into SOCS1, we have identified and characterized the zebrafish socs1 gene, which exhibited sequence and functional conservation with its mammalian counterparts. Initially maternally derived, the socs1 gene showed early zygotic expression in mesodermal structures, including the posterior intermediate cell mass, a site of primitive hematopoiesis. At later time points, expression was seen in a broad anterior domain, liver, notochord, and intersegmental vesicles. Morpholino-mediated knockdown of socs1 resulted in perturbation of specific hematopoietic populations prior to the commencement of lymphopoiesis, ruling out T cell involvement. However, socs1 knockdown also lead to a reduction in the size of the developing thymus later in embryogenesis. Zebrafish SOCS1 was shown to be able to interact with both zebrafish Jak2a and Stat5.1 in vitro and in vivo. These studies demonstrate a conserved role for SOCS1 in T cell development and suggest a novel T cell-independent function in embryonic myelopoiesis mediated, at least in part, via its effects on receptors using the Jak2-Stat5 pathway.

Spatiotemporal photolabeling of neutrophil trafficking during inflammation in live zebrafish

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

Haemopoiesis in the head kidney of tilapia, Oreochromis niloticus (Teleostei: Cichlidae): a morphological (optical and ultrastructural) study

The present work focused on the histological and ultrastructural studies on haemopoiesis in the kidney of tilapia, Oreochromis niloticus. Haemopoietic tissue was found mainly in the head kidney and a small amount occurred in the mesonephros. The haemopoiesis of tilapia had the following series: erythropoiesis, granulopoiesis, thrombopoiesis, monopoiesis and lymphoplasmopoiesis. Erythropoiesis includes proerythroblasts, basophilic erythroblasts, polychromatic erythroblasts, acidophilic erythroblasts and young and mature erythrocytes. The proerythroblasts were the largest cells in the erythropoietic series. During the maturation process both the nuclear and cellular size decreased gradually due to the chromatin condensation and the progressive substitution of cytoplasmic matrix with a large amount of haemoglobin. Granulopoietic series consisted of cells with variable shape and size at different stages of maturity from myeloblasts to mature granulocytes. The promyelocytes were the largest cells in the series and were characterised by the appearance of primary (azoruphilic) granules. The maturation process involved the appearance of specific granules in the heterophilic, eosinophilic and basophilic series. It is important to mention that eosinophilic granulocytes were the dominant granulopoietic series in the haemopoietic tissue (Ht) of tilapia. Lymphopoietic series consisted of lymphoblasts, large lymphocytes, small lymphocytes and active and inactive plasma cells. Thrombopoietic series consisted of thromboblasts, prothromboblasts and thrombocytes. Thrombocytes of tilapia were nucleated and possessed a spindle shape. Melanomacrophage centres were dominant among the Ht of the head kidney. Also, monocytes were detected and shown to be large cells with an indented nucleus and cytoplasm containing numerous vesicles of different sizes and a few lysosomes.

Fate mapping embryonic blood in zebrafish: multi- and unipotential lineages are segregated at gastrulation

Vertebrate hematopoiesis first produces primitive (embryonic) lineages and ultimately generates the definitive (adult) blood. Whereas definitive hematopoiesis may produce many diverse blood types via a common multipotent progenitor, primitive hematopoiesis has been thought to produce only erythrocytes or macrophages via progenitors that are unipotent for single blood lineages. Using a variety of in vivo cell-tracing techniques, we show that primitive blood in zebrafish derives from two different progenitor types. On the dorsal gastrula, blood progenitors are unipotential cells that divide infrequently, populate the rostral blood islands, and differentiate into macrophages. In contrast, on the ventral gastrula, blood progenitors are multipotential cells with rapid cell cycles; populate the intermediate cell mass; and differentiate into erythrocytes, neutrophils, and thrombocytes. Our results demonstrate the existence of primitive hematopoietic progenitors that are segregated very early in development and that are specified to produce either a unipotent or a multipotent blood cell lineage.

Muscle degeneration and leukocyte infiltration caused by mutation of zebrafish Fad24

Factor for adipocyte differentiation 24 (fad24) is a novel gene that has been implicated in adipocyte differentiation and DNA replication. In a screen for zebrafish mutants that have an abnormal tissue distribution of neutrophils, we identified an insertional allele of fad24, fad24hi1019. Homozygous fad24hi1019 larvae exhibit muscle degeneration accompanied by leukocyte infiltration. Muscle degeneration was extensive and included tissue apoptosis and disorganized, poorly striated muscle fibers. Blocking apoptosis using pan-caspase inhibitors resulted in decreased neutrophil recruitment into the body of the larva, suggesting a causative link between apoptosis and leukocyte infiltration. These findings suggest that zebrafish is a powerful genetic model system to address the interplay between muscle degeneration and leukocyte infiltration, and indicate that tissue apoptosis may contribute to neutrophil recruitment in some inflammatory states.

Live imaging of chronic inflammation caused by mutation of zebrafish Hai1

The hallmark of chronic inflammation is the infiltration and persistence of leukocytes within inflamed tissue. Here, we describe the first zebrafish chronic inflammation mutant identified in an insertional mutagenesis screen for mutants that exhibit abnormal tissue distribution of neutrophils. We identified a mutant line with an insertion in the Hepatocyte growth factor activator inhibitor 1 gene (hai1; also known as Spint1) that showed accumulation of neutrophils in the fin. The mutant embryos exhibited inflammation in areas of epidermal hyperproliferation that was rescued by knock-down of the type II transmembrane serine protease Matriptase 1 (also known as St14), suggesting a novel role for Hai1-Matriptase 1 pathway in regulating inflammation. Using time-lapse microscopy of mutant embryos that express GFP from a neutrophil-specific promoter, we found that individual neutrophils in inflamed tissue displayed random motility characterized by periods of pausing alternating with periods of motility. During periods of persistent movement the cells were highly polarized, while the pausing modes were characterized by a loss of cell polarity. In contrast to responses to acute injury, neutrophils did not exhibit clear retrograde chemotaxis or resolution of inflammation in the mutant. These findings illustrate the utility of zebrafish as a new model system to study chronic inflammation and to visualize immune responses with high resolution in vivo.

The role of jak2a in zebrafish hematopoiesis

Janus kinase 2 (Jak2) transduces signals from hematopoietic cytokines, and a gain-of-function mutation (Jak2(617V>F)) is associated with myeloproliferative diseases, particularly polycythemia vera. In this study, we examined the role of jak2a in zebrafish embryos in knock-down and overexpression studies using morpholinos (MOs) targeting the 5' untranslated region (UTR) (jak2a(UTR)-MO) and splice-site junction (jak2a(SS)-MO) of jak2a, a Jak inhibitor AG490 and a constitutive-active form of jak2a (jak2a(ca)). At 18 and 24 hours after fertilization (hpf), jak2a is expressed predominantly in the intermediate cell mass (ICM; site of primitive hematopoiesis) of wild-type and chordin morphant embryos (characterized by expansion of ICM). Both jak2a MOs and AG490 reduced gata1(+) (erythroid) cells in Tg(gata1:GFP) embryos, signal transducer and activation of transcription 5 (stat5) phosphorylation, and gene expression associated with early progenitors (scl and lmo2) and erythroid (gata1, alphahe1 and betahe1) and myeloid (spi1 [early] and mpo [late]) lineages. The chordin morphant is associated with increased stat5 phosphorylation, and both jak2a MOs and treatment with AG490 significantly ameliorated ICM expansion and hematopoietic gene up-regulation in these embryos. Injection of plasmid encoding jak2a(ca) significantly increased erythropoiesis and expression of gata1, alphahe1 and betahe1, spi1, mpo, and l-plastin. In conclusion, zebrafish jak2a is involved in primitive hematopoiesis under normal and deregulated conditions.

Zebrafish kidney marrow contains ABCG2-dependent side population cells exhibiting hematopoietic stem cell properties

Zebrafish (Danio rerio) has emerged as a powerful genetic model for the study of vertebrate hematopoiesis. However, methods for detection and isolation of hematopoietic stem cells (HSCs) have not yet been reported. In mammals, the combination of Hoechst 33342 staining with flow cytometry can be used for separation of a bone marrow side population (SP), which is highly enriched for HSCs. We applied a similar procedure to hematopoietic kidney marrow cells from adult zebrafish, and identified a segregated cohort of SP cells, which demonstrate a set of features typical of stem cells. SP cells show extremely low scatter characteristics, and are small in size with a minimum of cytoplasm. Treatment of zebrafish kidney marrow cells with reserpine or fumitremorgin C, which inhibit the ABCG2 transporter responsible for Hoechst 33342 efflux, caused a clear reduction in the number of SP cells. Consistent with the quiescent state of HSCs, the SP cells are strongly resistant to the myelosuppressive agent 5-fluorouracil. In addition, SP cells specifically demonstrate higher expression of genes known to be markers of HSCs of mammals. Hence, our results show that the SP phenotype is conserved between mammals and teleosts, and the properties of the zebrafish SP cells indicate a significant enrichment for HSCs. These rapid flow cytometric methods for purification of HSCs from zebrafish may greatly facilitate genetic analysis of stem cells using the advantages of this vertebrate model.

Resolution of inflammation by retrograde chemotaxis of neutrophils in transgenic zebrafish

Neutrophil chemotaxis to sites of inflammation is a critical process during normal immune responses to tissue injury and infection and pathological immune responses leading to chronic inflammation. Although progress has been made in understanding the mechanisms that promote neutrophil recruitment to inflamed tissue, the mechanisms that regulate the resolution phase of the inflammatory response have remained relatively elusive. To define the mechanisms that regulate neutrophil-mediated inflammation in vivo, we have developed a novel transgenic zebrafish in which the neutrophils express GFP under control of the myeloperoxidase promoter (zMPO:GFP). Tissue injury induces a robust, inflammatory response, which is characterized by the rapid chemotaxis of neutrophils to the wound site. In vivo time-lapse imaging shows that neutrophils subsequently display directed retrograde chemotaxis back toward the vasculature. These findings implicate retrograde chemotaxis as a novel mechanism that regulates the resolution phase of the inflammatory response. The zMPO:GFP zebrafish provides unique insight into the mechanisms of neutrophil-mediated inflammation and thereby offers opportunities to identify new regulators of the inflammatory response in vivo.

Constitutive activation of zebrafish Stat5 expands hematopoietic cell populations in vivo

OBJECTIVE

Constitutive activation of Stat5 has been observed in a variety of malignancies, particularly myeloid leukemias. To directly investigate the in vivo consequences of Stat5 perturbation, we expressed constitutively active forms in zebrafish.

METHODS

We generated mutants of the zebrafish stat5.1 protein (N646H, H298R/N714F, and N714F) based on previously identified constitutively active mutants of murine Stat5a. The in vitro properties of these mutants were determined using phosphorylation-specific antibodies and luciferase reporter assays, and their in vivo effects were analyzed through microinjection of zebrafish embryos.

RESULTS

Two of these stat5.1 mutants (N646H and H298R/N714F) showed increased tyrosine phosphorylation and transactivation activity compared to the wild-type protein. Expression of either mutant led to a range of hematological perturbations, which were more pronounced for the H298R/N714F mutant. Interestingly, expression of wild-type also produced generally similar phenotypes. Further analysis showed that expression of the H298R/N714F mutant led to increased numbers of early and late myeloid cells, erythrocytes, and B cells. Some nonhematopoietic developmental perturbations were also observed, but these were equally prominent with wild-type or mutant forms.

CONCLUSION

These data implicate Stat5 activity as a direct critical regulator of hematological cell proliferation, suggesting a causal role for constitutively-active Stat5 in the etiology of hematological malignancies.