Definitive hematopoietic stem/progenitor cells manifest distinct differentiation output in the zebrafish VDA and PBI

Establishment of a Diamond-Blackfan anemia like model in zebrafish

BACKGROUND

Anemia is defined as a lack of erythrocytes, low hemoglobin levels, or abnormal erythrocyte morphology. Diamond-Blackfan anemia (DBA) is a rare and severe congenital hypoplastic anemia that occurs due to the dominant inheritance of a ribosomal protein gene mutation. Even rarer is a case described as Diamond-Blackfan anemia like (DBAL), which occurs due to a loss-of-function EPO mutation recessive inheritance. The effective cures for DBAL are bone marrow transfusion and treatment with erythropoiesis-stimulating agents (ESAs). To effectively manage the condition, construction of DBAL models to identify new medical methods or screen drugs are necessary.

RESULTS

Here, an epoa-deficient mutant zebrafish called epoaszy8 was generated to model DBAL. The epoa-deficiency in zebrafish caused developmental defects in erythroid cells, leading to severe congenital anemia. Using the DBAL model, we validated a loss-of-function EPO mutation using an in vivo functional analysis and explored the ability of ESAs to alleviate congenital anemia.

CONCLUSIONS

Together, our study demonstrated that epoa deficiency in zebrafish leads to a phenotype resembling DBAL. The DBAL zebrafish model was found to be beneficial for the in vivo assessment of patient-derived EPO variants with unclear implications and for devising potential therapeutic approaches for DBAL.

The embryonic zebrafish brain is exclusively colonized by pu.1-dependent and lymphatic-independent population of microglia

Microglia, the crucial immune cells inhabiting the central nervous system (CNS), perform a range of vital functions, encompassing immune defense and neuronal regulation. Microglia subsets with diverse functions and distinct developmental regulations have been identified recently. It is generally accepted that all microglia originate from hematopoiesis and depend on the myeloid transcription factor PU.1. However, a recent study reported the existence of mrc1+ microglia in zebrafish embryos, which are seemingly independent of Pu.1 and reliant on lymphatic vessels, sparking great interest in the possibility of lymphatic-originated microglia. To address this, we took advantage of a pu.1 knock-in zebrafish allele for a detailed investigation. Our results conclusively showed that almost all zebrafish embryonic microglia (~95% on average) express pu.1. Further, lineage tracing and mutant analysis revealed that these microglia neither emerged from nor depended on lymphatic vessels. In essence, our study refutes the presence of pu.1-independent but lymphatic-dependent microglia.

Targeting TNF/IL-17/MAPK pathway in hE2A-PBX1 leukemia: effects of OUL35, KJ-Pyr-9, and CID44216842

t(1;19)(q23;p13) is one of the most common translocation genes in childhood acute lymphoblastic leukemia (ALL) and is also present in acute myeloid leukemia (AML) and mixed-phenotype acute leukemia (MPAL). This translocation results in the formation of the oncogenic E2A-PBX1 fusion protein, which contains a trans-activating domain from E2A and a DNA-binding homologous domain from PBX1. Despite its clear oncogenic potential, the pathogenesis of E2A-PBX1 fusion protein is not fully understood (especially in leukemias other than ALL), and effective targeted clinical therapies have not been developed. To address this, we established a stable and heritable zebrafish line expressing human E2A-PBX1 (hE2A-PBX1) for high-throughput drug screening. Blood phenotype analysis showed that hE2A-PBX1 expression induced myeloid hyperplasia by increasing myeloid differentiation propensity of hematopoietic stem cells (HSPC) and myeloid proliferation in larvae, and progressed to AML in adults. Mechanistic studies revealed that hE2A-PBX1 activated the TNF/IL-17/MAPK signaling pathway in blood cells and induced myeloid hyperplasia by upregulating the expression of runx1. Interestingly, through high-throughput drug screening, three small molecules targeting the TNF/IL-17/MAPK signaling pathway were identified, including OUL35, KJ-Pyr-9, and CID44216842, which not only alleviated the hE2A-PBX1-induced myeloid hyperplasia in zebrafish but also inhibited the growth and oncogenicity of human pre-B ALL cells with E2A-PBX1. Overall, this study provides a novel hE2APBX1 transgenic zebrafish leukemia model and identifies potential targeted therapeutic drugs, which may offer new insights into the treatment of E2A-PBX1 leukemia.

Styxl2 regulates de novo sarcomere assembly by binding to non-muscle myosin IIs and promoting their degradation

Styxl2, a poorly characterized pseudophosphatase, was identified as a transcriptional target of the Jak1-Stat1 pathway during myoblast differentiation in culture. Styxl2 is specifically expressed in vertebrate striated muscles. By gene knockdown in zebrafish or genetic knockout in mice, we found that Styxl2 plays an essential role in maintaining sarcomere integrity in developing muscles. To further reveal the functions of Styxl2 in adult muscles, we generated two inducible knockout mouse models: one with Styxl2 being deleted in mature myofibers to assess its role in sarcomere maintenance, and the other in adult muscle satellite cells (MuSCs) to assess its role in de novo sarcomere assembly. We find that Styxl2 is not required for sarcomere maintenance but functions in de novo sarcomere assembly during injury-induced muscle regeneration. Mechanistically, Styxl2 interacts with non-muscle myosin IIs, enhances their ubiquitination, and targets them for autophagy-dependent degradation. Without Styxl2, the degradation of non-muscle myosin IIs is delayed, which leads to defective sarcomere assembly and force generation. Thus, Styxl2 promotes de novo sarcomere assembly by interacting with non-muscle myosin IIs and facilitating their autophagic degradation.

What can we learn about fish neutrophil and macrophage response to immune challenge from studies in zebrafish

Fish rely, to a high degree, on the innate immune system to protect them against the constant exposure to potential pathogenic invasion from the surrounding water during homeostasis and injury. Zebrafish larvae have emerged as an outstanding model organism for immunity. The cellular component of zebrafish innate immunity is similar to the mammalian innate immune system and has a high degree of sophistication due to the needs of living in an aquatic environment from early embryonic stages of life. Innate immune cells (leukocytes), including neutrophils and macrophages, have major roles in protecting zebrafish against pathogens, as well as being essential for proper wound healing and regeneration. Zebrafish larvae are visually transparent, with unprecedented in vivo microscopy opportunities that, in combination with transgenic immune reporter lines, have permitted visualisation of the functions of these cells when zebrafish are exposed to bacterial, viral and parasitic infections, as well as during injury and healing. Recent findings indicate that leukocytes are even more complex than previously anticipated and are essential for inflammation, infection control, and subsequent wound healing and regeneration.

Interplay of Zeb2a, Id2a and Batf3 regulates microglia and dendritic cell development in the zebrafish brain

In vertebrates, the central nervous system (CNS) harbours various immune cells, including parenchymal microglia, perivascular macrophages and dendritic cells, which act in coordination to establish an immune network to regulate neurogenesis and neural function, and to maintain the homeostasis of the CNS. Recent single cell transcriptomic profiling has revealed that the adult zebrafish CNS contains microglia, plasmacytoid dendritic cells (pDCs) and two conventional dendritic cells (cDCs), ccl35+ cDCs and cnn3a+cDCs. However, how these distinct myeloid cells are established in the adult zebrafish CNS remains incompletely defined. Here, we show that the Inhibitor of DNA binding 2a (Id2a) is essential for the development of pDCs and cDCs but is dispensable for the formation of microglia, whereas the Basic leucine zipper transcription factor ATF-like 3 (Batf3) acts downstream of id2a and is required exclusively for the formation of the cnn3a+ cDC subset. In contrast, the Zinc finger E-box-binding homeobox 2a (Zeb2a) promotes the expansion of microglia and inhibits the DC specification, possibly through repressing id2a expression. Our study unravels the genetic networks that govern the development of microglia and brain-associated DCs in the zebrafish CNS.

Cell competition for neuron-derived trophic factor controls the turnover and lifespan of microglia

Microglia are brain-resident macrophages capable of long-term maintenance through self-renewal. Yet the mechanism governing the turnover and lifespan of microglia remains unknown. In zebrafish, microglia arise from two sources, rostral blood island (RBI) and aorta-gonad-mesonephros (AGM). The RBI-derived microglia are born early but have a short lifespan and diminish in adulthood, while the AGM-derived microglia emerge later and are capable of long-term maintenance in adulthood. Here, we show that the attenuation of RBI microglia is due to their less competitiveness for neuron-derived interleukin-34 (Il34) caused by age-dependent decline of colony-stimulating factor-1 receptor a (csf1ra). Alterations of Il34/Csf1ra levels and removal of AGM microglia revamp the proportion and lifespan of RBI microglia. The csf1ra/CSF1R expression in zebrafish AGM-derived microglia and murine adult microglia also undergo age-dependent decline, leading to the elimination of aged microglia. Our study reveals cell competition as a general mechanism controlling the turnover and lifespan of microglia.

Activation of lineage competence in hemogenic endothelium precedes the formation of hematopoietic stem cell heterogeneity

Hematopoietic stem and progenitor cells (HSPCs) are considered as a heterogeneous population, but precisely when, where and how HSPC heterogeneity arises remain largely unclear. Here, using a combination of single-cell multi-omics, lineage tracing and functional assays, we show that embryonic HSPCs originate from heterogeneous hemogenic endothelial cells (HECs) during zebrafish embryogenesis. Integrated single-cell transcriptome and chromatin accessibility analysis demonstrates transcriptional heterogeneity and regulatory programs that prime lymphoid/myeloid fates at the HEC level. Importantly, spi2+ HECs give rise to lymphoid/myeloid-primed HSPCs (L/M-HSPCs) and display a stress-responsive function under acute inflammation. Moreover, we uncover that Spi2 is required for the formation of L/M-HSPCs through tightly controlling the endothelial-to-hematopoietic transition program. Finally, single-cell transcriptional comparison of zebrafish and human HECs and human induced pluripotent stem cell-based hematopoietic differentiation results support the evolutionary conservation of L/M-HECs and a conserved role of SPI1 (spi2 homolog in mammals) in humans. These results unveil the lineage origin, biological function and molecular determinant of HSPC heterogeneity and lay the foundation for new strategies for induction of transplantable lineage-primed HSPCs in vitro.

Learning from Zebrafish Hematopoiesis

Hematopoiesis is a complex process that tightly regulates the generation, proliferation, differentiation, and maintenance of hematopoietic cells. Disruptions in hematopoiesis can lead to various diseases affecting both hematopoietic and non-hematopoietic systems, such as leukemia, anemia, thrombocytopenia, rheumatoid arthritis, and chronic granuloma. The zebrafish serves as a powerful vertebrate model for studying hematopoiesis, offering valuable insights into both hematopoietic regulation and hematopoietic diseases. In this chapter, we present a comprehensive overview of zebrafish hematopoiesis, highlighting its distinctive characteristics in hematopoietic processes. We discuss the ontogeny and modulation of both primitive and definitive hematopoiesis, as well as the microenvironment that supports hematopoietic stem/progenitor cells. Additionally, we explore the utility of zebrafish as a disease model and its potential in drug discovery, which not only advances our understanding of the regulatory mechanisms underlying hematopoiesis but also facilitates the exploration of novel therapeutic strategies for hematopoietic diseases.

Hematopoietic Stem and Progenitor Cell Identification and Transplantation in Zebrafish

The zebrafish as a model organism is well known for its versatile genetics, rapid development, and straightforward live imaging. It is an excellent model to study hematopoiesis because of its highly conserved ontogeny and gene regulatory networks. Recently developed highly specific transgenic reporter lines have allowed direct imaging and tracking of hematopoietic stem and progenitor cells (HSPCs) in live zebrafish. These reporter lines can also be used for fluorescence-activated cell sorting (FACS) of HSPCs. Similar to mammalian models, HSPCs can be transplanted to reconstitute the entire hematopoietic system of zebrafish recipients. However, the zebrafish provides unique advantages to study HSPC biology, such as transplants into embryos and high-throughput chemical screening. This chapter will outline the methods needed to identify, isolate, and transplant HSPCs in zebrafish.

The ETS transcription factor Spi2 regulates hematopoietic cell development in zebrafish

The E26 transformation-specific or E-twenty-six (ETS) genes encode a superfamily of transcription factors involved in diverse biological processes. Here, we report the identification and characterization of a previously unidentified member of the ETS transcription factors, Spi2, that is found exclusively in the ray-finned fish kingdom. We show that the expression of spi2 is restricted to hemogenic endothelial cells (HECs) and to hematopoietic stem and progenitor cells (HSPCs) in zebrafish. Using bacteria artificial chromosome transgenesis, we generate a spi2 reporter line, TgBAC(spi2:P2a-GFP), which manifests the GFP pattern recapitulating the endogenous spi2 expression. Genetic ablation of spi2 has little effect on HEC formation and the endothelial-to-hematopoietic transition, but results in compromised proliferation of HSPCs in the caudal hematopoietic tissue (CHT) during early development and in severe myeloid lineage defect in adulthood. Epistatic analysis shows that spi2 acts downstream of runx1 in regulating HSPC development in the CHT. Our study identifies Spi2 as an essential regulator for definitive hematopoietic cell development and creates a TgBAC(spi2:P2a-GFP) reporter line for tracking HECs, HSPCs, myeloid cells and thrombocytes from early development to adulthood.

Mafba and Mafbb regulate microglial colonization of zebrafish brain via controlling chemotaxis receptor expression

Microglia are a subpopulation of macrophages residing in the central nervous system (CNS). Because microglial precursors/peripheral macrophages are born in peripheral hematopoietic tissues, the establishment of a microglia pool in the CNS involves two processes: colonization, the homing of macrophages from peripheral tissues to the CNS, and maturation, the differentiation of brain-colonizing macrophages into microglia. This study aims to investigate the molecular mechanisms underlying microglial colonization during early development. Utilizing a zebrafish model system, we show that Mafba and Mafbb, two zebrafish orthologs of mammalian MAFB essential for macrophage differentiation and phagocytosis, regulate microglial colonization of the brain via modulating the lysoPS-Gpr34a signaling pathway during early embryogenesis. Our findings reveal a previously unappreciated genetic mechanism involved in microglial colonization of the brain.

Microglia are the central nervous system (CNS)–resident macrophages involved in neural inflammation, neurogenesis, and neural activity regulation. Previous studies have shown that naturally occurring neuronal apoptosis plays a critical role in regulating microglial colonization of the brain in zebrafish. However, the molecular signaling cascades underlying neuronal apoptosis-mediated microglial colonization and the regulation of these cascades remain undefined. Here, we show that basic leucine zipper (b-Zip) transcription factors, Mafba and Mafbb, two zebrafish orthologs of mammalian MAFB, are key regulators in neuronal apoptosis-mediated microglial colonization of the brain in zebrafish. We document that the loss of Mafba and Mafbb function perturbs microglial colonization of the brain. We further demonstrate that Mafba and Mafbb act cell-autonomously and cooperatively to orchestrate microglial colonization, at least in part, by regulating the expression of G protein–coupled receptor 34a (Gpr34a), which directs peripheral macrophage recruitment into the brain through sensing the lysophosphatidylserine (lysoPS) released by the apoptotic neurons. Our study reveals that Mafba and Mafbb regulate neuronal apoptosis-mediated microglial colonization of the brain in zebrafish via the lysoPS-Gpr34a pathway.

Generation of a thrombopoietin-deficient thrombocytopenia model in zebrafish

BACKGROUND

The production of platelets is tightly regulated by thrombopoietin (THPO). Mutations in the THPO gene cause thrombocytopenia. Although mice lacking Thpo present with thrombocytopenia, predicting phenotypes and pathogenicity of novel THPO mutations in mice is limited. Zebrafish can be a powerful tool for fast validation and study of candidate genes of human hematological diseases and have already been used as a model of human thrombocytopenia.

OBJECTIVES

We aim to investigate the role of Thpo in zebrafish thrombopoiesis and to establish a Thpo-deficient zebrafish model. The model could be applied for illustrating the clinically discovered human THPO variants of which the clinical significance is not known and to evaluate the effect of THPO receptor agonists (THPO-Ras), as well as a screening platform for new drugs.

METHODS

We generated a thpo loss-of-function zebrafish model using CRISPR/Cas9. After disruption of zebrafish thpo, thposzy6 zebrafish presented with a significant reduction of thpo expression and developed thrombocytopenia. Furthermore, we performed in vivo studies with zebrafish with the thposzy6 mutation and found two human clinical point mutations (c.091C > T and c.112C > T) that were responsible for the thrombocytopenia phenotype. In addition, effects of THPO-RAs used as therapeutics against thrombocytopenia were evaluated in the Tg(mpl:eGFP);thposzy6 line.

RESULTS AND CONCLUSIONS

Zebrafish with the mutation thposzy6 presented with a significant reduction of thpo expression and developed thrombocytopenia. Thpo loss-of-function zebrafish model can serve as a valuable preclinical model for thrombocytopenia caused by thpo-deficiency, as well as a tool to study human clinical THPO variants and evaluate the effect of THPO-RAs.

Rapid reverse genetics systems for Nothobranchius furzeri, a suitable model organism to study vertebrate aging

The African turquoise killifish Nothobranchius furzeri (N. furzeri) is a useful model organism for studying aging, age-related diseases, and embryonic diapause. CRISPR/Cas9-mediated gene knockout and Tol2 transposon-mediated transgenesis in N. furzeri have been reported previously. However, these methods take time to generate knockout and transgenic fish. In addition, knock-in technology that inserts large DNA fragments as fluorescent reporter constructs into the target gene in N. furzeri has not yet been established. Here, we show that triple-target CRISPR-mediated single gene disruption efficiently produces whole-body biallelic knockout and enables the examination of gene function in the F0 generation. In addition, we developed a method for creating the knock-in reporter N. furzeri without crossing by optimizing the CRISPR/Cas9 system. These methods drastically reduce the duration of experiments, and we think that these advances will accelerate aging and developmental studies using N. furzeri.

Opportunities presented by zebrafish larval models to study neutrophil function in tissues

Appropriate neutrophil function is essential for innate immune defence and to avoid inflammatory pathology. Neutrophils can adapt their responses according to their environment and recently, the existence of multiple distinct neutrophil populations has been confirmed in both health and disease. However, the study of neutrophil functions in their tissue environment has remained challenging, and for instance, the relationship between neutrophil maturity and function is not fully understood. Many neutrophil morphological and functional features are highly conserved between mammals and non-mammalian vertebrates. This enables the use of the transparent and genetically tractable zebrafish larvae to study neutrophil biology. We review data on the development and function of zebrafish larval neutrophils and advances zebrafish have brought to studies of neutrophil biology. In addition, we discuss opportunities and aspects to be considered when using the larval zebrafish model to further enhance our understanding of neutrophil function in health and disease.

Lympho-Hematopoietic Microenvironments and Fish Immune System

In the last 50 years information on the fish immune system has increased importantly, particularly that on species of marked commercial interest (i.e., salmonids, cods, catfish, sea breams), that occupy a key position in the vertebrate phylogenetical tree (i.e., Agnatha, Chondrichtyes, lungfish) or represent consolidated experimental models, such as zebrafish or medaka. However, most obtained information was based on genetic sequence analysis with little or no information on the cellular basis of the immune responses. Although jawed fish contain a thymus and lympho-hematopoietic organs equivalents to mammalian bone marrow, few studies have accounted for the presumptive relationships between the organization of these cell microenvironments and the known immune capabilities of the fish immune system. In the current review, we analyze this topic providing information on: (1) The origins of T and B lymphopoiesis in Agnatha and jawed fish; (2) the remarkable organization of the thymus of teleost fish; (3) the occurrence of numerous, apparently unrelated organs housing lympho-hematopoietic progenitors and, presumably, B lymphopoiesis; (4) the existence of fish immunological memory in the absence of germinal centers.

Vascular endothelial growth factor c regulates hematopoietic stem cell fate in the dorsal aorta

Hematopoietic stem and progenitor cells (HSPCs) are multipotent cells that self-renew or differentiate to establish the entire blood hierarchy. HSPCs arise from the hemogenic endothelium of the dorsal aorta (DA) during development in a process called endothelial-to-hematopoietic transition. The factors and signals that control HSPC fate decisions from the hemogenic endothelium are not fully understood. We found that Vegfc has a role in HSPC emergence from the zebrafish DA. Using time-lapse live imaging, we show that some HSPCs in the DA of vegfc loss-of-function embryos display altered cellular behavior. Instead of typical budding from the DA, emergent HSPCs exhibit crawling behavior similar to myeloid cells. This was confirmed by increased myeloid cell marker expression in the ventral wall of the DA and the caudal hematopoietic tissue. This increase in myeloid cells corresponded with a decrease in HSPCs that persisted into larval stages. Together, our data suggest that Vegfc regulates HSPC emergence in the hemogenic endothelium, in part by suppressing a myeloid cell fate. Our study provides a potential signal for modulation of HSPC fate in stem cell differentiation protocols.

Redundant mechanisms driven independently by RUNX1 and GATA2 for hematopoietic development

RUNX1 is essential for the generation of hematopoietic stem cells (HSCs). Runx1-null mouse embryos lack definitive hematopoiesis and die in mid-gestation. However, although zebrafish embryos with a runx1 W84X mutation have defects in early definitive hematopoiesis, some runx1W84X/W84X embryos can develop to fertile adults with blood cells of multilineages, raising the possibility that HSCs can emerge without RUNX1. Here, using 3 new zebrafish runx1-/- lines, we uncovered the compensatory mechanism for runx1-independent hematopoiesis. We show that, in the absence of a functional runx1, a cd41-green fluorescent protein (GFP)+ population of hematopoietic precursors still emerge from the hemogenic endothelium and can colonize the hematopoietic tissues of the mutant embryos. Single-cell RNA sequencing of the cd41-GFP+ cells identified a set of runx1-/--specific signature genes during hematopoiesis. Significantly, gata2b, which normally acts upstream of runx1 for the generation of HSCs, was increased in the cd41-GFP+ cells in runx1-/- embryos. Interestingly, genetic inactivation of both gata2b and its paralog gata2a did not affect hematopoiesis. However, knocking out runx1 and any 3 of the 4 alleles of gata2a and gata2b abolished definitive hematopoiesis. Gata2 expression was also upregulated in hematopoietic cells in Runx1-/- mice, suggesting the compensatory mechanism is conserved. Our findings indicate that RUNX1 and GATA2 serve redundant roles for HSC production, acting as each other's safeguard.

A novel conditioning-free hematopoietic stem cell transplantation model in zebrafish

Transplantation is the most common assay for measuring the in vivo functionality of hematopoietic stem cells (HSCs). Although various HSC transplantation strategies have been developed in zebrafish, they are underutilized because of challenges related to immune matching and preconditioning toxicity. To circumvent these limitations, we developed a simple and robust transplantation model using HSC-deficient hosts. Homozygous runx1W84X mutants are devoid of definitive hematopoietic cells, including HSCs and adaptive immune cells; thus, they require no preconditioning regimen for transplantation. Marrow cell transplantation into runx1-mutant zebrafish 2 days after fertilization significantly improved their survival to adulthood and resulted in robust, multilineage, long-lasting, serially repopulating engraftment. Furthermore, we demonstrated that engraftment into runx1 homozygous mutants was significantly higher than into runx1 heterozygotes, demonstrating that the improved transplantation success is attributable to the empty HSC niche in mutants and not just the embryonic environment. Competitive transplantation of marrow cells into runx1 mutants revealed a stem cell frequency similar to that of murine marrow cells, which demonstrates the utility of this model for quantifying HSC function. The streamlined approach and robustness of this assay will help broaden its feasibility for future high-throughput transplantation experiments in zebrafish and will enable further novel discoveries in the biology of HSCs.

A single-cell resolution developmental atlas of hematopoietic stem and progenitor cell expansion in zebrafish

The caudal hematopoietic tissue (CHT) is characterized as a hematopoietic organ for fetal hematopoietic stem and progenitor cell (HSPC) expansion in zebrafish. In this study, we used scRNA-seq combined with functional assays to decode the developing CHT. First, we resolved fetal HSPC heterogeneity, manifested as lineage priming and metabolic gene signatures. We further analyzed the cellular interactions among nonhematopoietic niche components and HSPCs and identified an endothelial cell-specific factor, Gpr182, followed by experimental validation of its role in promoting HSPC expansion. Finally, we uncovered the conservation and divergence of developmental hematopoiesis between human fetal liver and zebrafish CHT. Our study provides a valuable resource for fetal HSPC development and clues to establish a supportive niche for HSPC expansion in vitro.

During vertebrate embryogenesis, fetal hematopoietic stem and progenitor cells (HSPCs) exhibit expansion and differentiation properties in a supportive hematopoietic niche. To profile the developmental landscape of fetal HSPCs and their local niche, here, using single-cell RNA-sequencing, we deciphered a dynamic atlas covering 28,777 cells and 9 major cell types (23 clusters) of zebrafish caudal hematopoietic tissue (CHT). We characterized four heterogeneous HSPCs with distinct lineage priming and metabolic gene signatures. Furthermore, we investigated the regulatory mechanism of CHT niche components for HSPC development, with a focus on the transcription factors and ligand–receptor networks involved in HSPC expansion. Importantly, we identified an endothelial cell-specific G protein–coupled receptor 182, followed by in vivo and in vitro functional validation of its evolutionally conserved role in supporting HSPC expansion in zebrafish and mice. Finally, comparison between zebrafish CHT and human fetal liver highlighted the conservation and divergence across evolution. These findings enhance our understanding of the regulatory mechanism underlying hematopoietic niche for HSPC expansion in vivo and provide insights into improving protocols for HSPC expansion in vitro.

Innate immune evasion revealed in a colorectal zebrafish xenograft model

Cancer immunoediting is a dynamic process of crosstalk between tumor cells and the immune system. Herein, we explore the fast zebrafish xenograft model to investigate the innate immune contribution to this process. Using multiple breast and colorectal cancer cell lines and zAvatars, we find that some are cleared (regressors) while others engraft (progressors) in zebrafish xenografts. We focus on two human colorectal cancer cells derived from the same patient that show contrasting engraftment/clearance profiles. Using polyclonal xenografts to mimic intra-tumor heterogeneity, we demonstrate that SW620_progressors can block clearance of SW480_regressors. SW480_regressors recruit macrophages and neutrophils more efficiently than SW620_progressors; SW620_progressors however, modulate macrophages towards a pro-tumoral phenotype. Genetic and chemical suppression of myeloid cells indicates that macrophages and neutrophils play a crucial role in clearance. Single-cell-transcriptome analysis shows a fast subclonal selection, with clearance of regressor subclones associated with IFN/Notch signaling and escaper-expanded subclones with enrichment of IL10 pathway. Overall, our work opens the possibility of using zebrafish xenografts as living biomarkers of the tumor microenvironment.

Runx1 regulates zebrafish neutrophil maturation via synergistic interaction with c-Myb

Neutrophils play an essential role in the innate immune defense system in vertebrates. During hematopoiesis, the full function of neutrophils involves maturation of granules and related enzymes. Yet, transcription regulators that promote neutrophil maturation remain largely undefined. Here, two hematopoiesis-defective zebrafish mutants, runx1^w84x and c-myb^hkz3, were used to investigate the in vivo roles of Runx1 in cooperation with c-Myb in regulating neutrophil maturation. Loss of runx1 impairs primitive neutrophil development. Additional regulation of c-myb^+/− and c-myb^−/− induces a more severe phenotypes suggesting a synergistic genetic interaction between c-myb and runx1 in neutrophil maturation. Further studies revealed that the two transcription factors act cooperatively to control neutrophil maturation processes via transactivating a series of neutrophil maturation-related genes. These data reveal the in vivo roles of Runx1 in regulating primitive neutrophil maturation while also indicating a novel genetic and molecular orchestration of Runx1 and c-Myb in myeloid cell development. The study will provide new evidence on the regulation of neutrophil maturation during hematopoiesis.

Pax9 is essential for granulopoiesis but dispensable for erythropoiesis in zebrafish

Paired Box (Pax) gene family, a group of transcription regulators have been implicated in diverse physiological processes. However, their role during hematopoiesis which generate a plethora of blood cells remains largely unknown. Using a previously reported single cell transcriptomics data, we analyzed the expression of individual Pax family members in hematopoietic cells in zebrafish. We have identified that Pax9, which is an essential regulator for odontogenesis and palatogenesis, is selectively localized within a single cluster of the hematopoietic lineage. To further analyze the function of Pax9 in hematopoiesis, we generated two independent pax9 knock-out mutants using the CRISPR-Cas9 technique. We found that Pax9 appears to be an essential regulator for granulopoiesis but dispensable for erythropoiesis during development, as lack of pax9 selectively decreased the number of neutrophils with a concomitant decrease in the expression level of neutrophil markers. In addition, embryos, where pax9 was functionally disrupted by injecting morpholinos, failed to increase the number of neutrophils in response to pathogenic bacteria, suggesting that Pax9 is not only essential for developmental granulopoiesis but also emergency granulopoiesis. Due to the inability to initiate emergency granulopoiesis, innate immune responses were severely compromised in pax9 morpholino-mediated embryos, increasing their susceptibility and mortality. Taken together, our data indicate that Pax9 is essential for granulopoiesis and promotes innate immunity in zebrafish larvae.

Two phenotypically and functionally distinct microglial populations in adult zebrafish

Microglia are the tissue-resident macrophages in the central nervous system and are critically involved in immune defense, neural development and function, and neuroinflammation. The versatility of microglia has long been attributed to heterogeneity. Recent studies have revealed possible heterogeneity in human but not in murine microglia, yet a firm demonstration linking microglial heterogeneity to functional phenotypes remains scarce. Here, we identified two distinct microglial populations in adult zebrafish that differ in morphology, distribution, development, and function. The predominant population, phagocytotic microglia, which expresses ccl34b.1, is broadly distributed, amoeboid in shape, highly mobile, and phagocytotic. The other white matter-enriched ccl34b.1^- population, regulatory microglia, has ramified protrusions but has limited mobility and phagocytosis capability. These functional differences are further supported by distinct transcriptomes and responses to bacterial infection, where ccl34b.1⁺ microglia function in tissue clearance and ccl34b.1^- microglia release immune regulators. Our study sheds light on the heterogeneity and functional diversification of microglia.

Runx1 promotes scar deposition and inhibits myocardial proliferation and survival during zebrafish heart regeneration

Runx1 is a transcription factor that plays a key role in determining the proliferative and differential state of multiple cell types, during both development and adulthood. Here, we report how Runx1 is specifically upregulated at the injury site during zebrafish heart regeneration, and that absence of runx1 results in increased myocardial survival and proliferation, and overall heart regeneration, accompanied by decreased fibrosis. Using single cell sequencing, we found that the wild-type injury site consists of Runx1-positive endocardial cells and thrombocytes that induce expression of smooth muscle and collagen genes. Both these populations cannot be identified in runx1 mutant wounds that contain less collagen and fibrin. The reduction in fibrin in the mutant is further explained by reduced myofibroblast formation and upregulation of components of the fibrin degradation pathway, including plasminogen receptor annexin 2A as well as downregulation of plasminogen activator inhibitor serpine1 in myocardium and endocardium, resulting in increased levels of plasminogen. Our findings suggest that Runx1 controls the regenerative response of multiple cardiac cell types and that targeting Runx1 is a novel therapeutic strategy for inducing endogenous heart repair.

Identification and spatiotemporal expression of gpr161 genes in zebrafish

G protein coupled Receptor 161 (GPR161) is a ciliary orphan GPCR. It is reported to play critical roles in regulating vertebrate Hedgehog (Hh) signaling pathway, that is conserved in metazoan and functions in earlier embryogenesis and homeostasis of adult metabolism. However, to date, all GPR161 functional studies were performed only in mouse. Knock out gpr161 in NIH3T3 cell lines, the common material for Hh mechanism research, failed to give any obvious Hh pathway defects, raising the question that whether GPR161 functions in Hh pathway is conserved in vertebrate system. Here, we described the characterization and spatiotemporal expression of two zebrafish gpr161 homologs, gpr161a and gpr161b. gpr161a was renamed of the gpr161 previously identified, while gpr161b was novel identified. The whole-mount in situ hybridization and quantitative PCR results showed that gpr161a is initially expressed in maternal manner while gpr161b is not. Although these two gpr161 showed ubiquitously expressed at early embryonic stages, each of them had tissue specific accumulation. gpr161a is abundant in the central nervous system (CNS) and adaxial cells, where are rich of Hh responding cells. Together gpr161a was highly expressed in muscle and intestine in adult fishes. These results strongly suggest the regulating roles of Gpr161 a in zebrafish Hh signal transduction. gpr161b was also accumulated in the CNS but mainly at the midline in the neural tube, similar pattern as wnt5b expression in such area, suggesting its potential function correlated with WNT signaling pathway. Interestingly, we also found the specific accumulation of gpr161 in posterior blood island (PBI) at 24 hours post fertilization (hpf), indicating the gpr161 may play roles in early hematopoiesis in zebrafish. Our work provides a starting point to unveil the divergent functions of gpr161 in vertebrate and will shed light on the studies of mechanism of Hh and WNT pathways, as well as early hematopoiesis.

Human BCR/ABL1 induces chronic myeloid leukemia-like disease in zebrafish

Chronic myeloid leukemia (CML) is induced by the BCR/ABL1 oncogene, which encodes a protein tyrosine kinase. We examined the effect of direct overexpression of the human p210 BCR/ABL1 oncoprotein in zebrafish. Humanized p210 BCR/ABL1 protein was detectable in Tg(hsp70: p210BCR/ABL1 ) transgenic zebrafish embryos and adult kidney marrow. Transgenic zebrafish developed CML, which could be induced via cells transplanted into recipients. The expression of human BCR/ABL1 promoted myeloid lineages in Tg(hsp70:p210BCR/ABL1) transgenic embryos. A total of 77 of 101 (76.24%) Tg(hsp70:p210BCR/ABL1) adult transgenic zebrafish (age 6 months-1 year) developed CML. CML in zebrafish showed a triphasic phenotype, similar to that in humans, involving a chronic phase predominantly characterized by neutrophils in various degrees of maturation, an accelerated phase with an increase in blasts and immature myeloid elements, and a blast phase with >90% blasts in both the peripheral blood and kidney marrow. Tyrosine kinase inhibitors, as the standard drug treatment for human CML, effectively reduced the expanded myeloid population in Tg(hsp70:p210BCR/ABL1) transgenic embryos. Moreover, we screened a library of 171 compounds and identified ten new drugs against BCR/ABL1 kinase-dependent or -independent pathways that could also reduce lcp1+ myeloid cell numbers in Tg(hsp70:p210BCR/ABL1) transgenic embryos. In summary, we generated the first humanized zebrafish CML model that recapitulates many characteristics of human CML. This novel in vivo model will help to elucidate the mechanisms of CML disease progression and allow high-throughput drug screening of possible treatments for this disease.

RNA 5-Methylcytosine Facilitates the Maternal-to-Zygotic Transition by Preventing Maternal mRNA Decay

The maternal-to-zygotic transition (MZT) is a conserved and fundamental process during which the maternal environment is converted to an environment of embryonic-driven development through dramatic reprogramming. However, how maternally supplied transcripts are dynamically regulated during MZT remains largely unknown. Herein, through genome-wide profiling of RNA 5-methylcytosine (m⁵C) modification in zebrafish early embryos, we found that m⁵C-modified maternal mRNAs display higher stability than non-m⁵C-modified mRNAs during MZT. We discovered that Y-box binding protein 1 (Ybx1) preferentially recognizes m⁵C-modified mRNAs through π-π interactions with a key residue, Trp45, in Ybx1's cold shock domain (CSD), which plays essential roles in maternal mRNA stability and early embryogenesis of zebrafish. Together with the mRNA stabilizer Pabpc1a, Ybx1 promotes the stability of its target mRNAs in an m⁵C-dependent manner. Our study demonstrates an unexpected mechanism of RNA m⁵C-regulated maternal mRNA stabilization during zebrafish MZT, highlighting the critical role of m⁵C mRNA modification in early development.

Nlrc3-like is required for microglia maintenance in zebrafish

Microglia are tissue-resident macrophages residing in the central nervous system (CNS) and play critical roles in removing cellular debris and infectious agents as well as regulating neurogenesis and neuronal activities. Yet, the molecular basis underlying the establishment of microglia pool and the maintenance of their homeostasis in the CNS remain largely undefined. Here we report the identification and characterization of a mutant zebrafish, which harbors a point mutation in the nucleotide-binding oligomerization domain (NOD) like receptor gene nlrc3-like, resulting in the loss of microglia in a temperature sensitive manner. Temperature shift assay reveals that the late onset of nlrc3-like deficiency leads to excessive microglia cell death. Further analysis shows that the excessive microglia death in nlrc3-like deficient mutants is attributed, at least in part, to aberrant activation of canonical inflammasome pathway. Our study indicates that proper regulation of inflammasome cascade is critical for the maintenance of microglia homeostasis.

Loss of runx1 function results in B cell immunodeficiency but not T cell in adult zebrafish

Transcription factor RUNX1 holds an integral role in multiple-lineage haematopoiesis and is implicated as a cofactor in V(D)J rearrangements during lymphocyte development. Runx1 deficiencies resulted in immaturity and reduction of lymphocytes in mice. In this study, we found that runx1^W84X/W84X mutation led to the reduction and disordering of B cells, as well as the failure of V(D)J rearrangements in B cells but not T cells, resulting in antibody-inadequate-mediated immunodeficiency in adult zebrafish. By contrast, T cell development was not affected. The decreased number of B cells mainly results from excessive apoptosis in immature B cells. Disrupted B cell development results in runx1^W84X/W84X mutants displaying a similar phenotype to common variable immunodeficiency—a primary immunodeficiency disease primarily characterized by frequent susceptibility to infection and deficient immune response, with marked reduction of antibody production of IgG, IgA and/or IgM. Our studies demonstrated an evolutionarily conserved function of runx1 in maturation and differentiation of B cells in adult zebrafish, which will serve as a valuable model for the study of immune deficiency diseases and their treatments.

VCAM-1+ macrophages guide the homing of HSPCs to a vascular niche

Haematopoietic stem and progenitor cells (HSPCs) give rise to all blood lineages that support the entire lifespan of vertebrates¹. After HSPCs emerge from endothelial cells within the developing dorsal aorta, homing allows the nascent cells to anchor in their niches for further expansion and differentiation^2-5. Unique niche microenvironments, composed of various blood vessels as units of microcirculation and other niche components such as stromal cells, regulate this process^6-9. However, the detailed architecture of the microenvironment and the mechanism for the regulation of HSPC homing remain unclear. Here, using advanced live imaging and a cell-labelling system, we perform high-resolution analyses of the HSPC homing in caudal haematopoietic tissue of zebrafish (equivalent to the fetal liver in mammals), and reveal the role of the vascular architecture in the regulation of HSPC retention. We identify a VCAM-1⁺ macrophage-like niche cell population that patrols the inner surface of the venous plexus, interacts with HSPCs in an ITGA4-dependent manner, and directs HSPC retention. These cells, named 'usher cells', together with caudal venous capillaries and plexus, define retention hotspots within the homing microenvironment. Thus, the study provides insights into the mechanism of HSPC homing and reveals the essential role of a VCAM-1⁺ macrophage population with patrolling behaviour in HSPC retention.

Gfi1aa and Gfi1b set the pace for primitive erythroblast differentiation from hemangioblasts in the zebrafish embryo

The transcriptional repressors Gfi1(a) and Gfi1b are epigenetic regulators with unique and overlapping roles in hematopoiesis. In different contexts, Gfi1 and Gfi1b restrict or promote cell proliferation, prevent apoptosis, influence cell fate decisions, and are essential for terminal differentiation. Here, we show in primitive red blood cells (prRBCs) that they can also set the pace for cellular differentiation. In zebrafish, prRBCs express 2 of 3 zebrafish Gfi1/1b paralogs, Gfi1aa and Gfi1b. The recently identified zebrafish gfi1aa gene trap allele qmc551 drives erythroid green fluorescent protein (GFP) instead of Gfi1aa expression, yet homozygous carriers have normal prRBCs. prRBCs display a maturation defect only after splice morpholino-mediated knockdown of Gfi1b in gfi1aa qmc551 homozygous embryos. To study the transcriptome of the Gfi1aa/1b double-depleted cells, we performed an RNA-Seq experiment on GFP-positive prRBCs sorted from 20-hour-old embryos that were heterozygous or homozygous for gfi1aa qmc551 , as well as wt or morphant for gfi1b We subsequently confirmed and extended these data in whole-mount in situ hybridization experiments on newly generated single- and double-mutant embryos. Combined, the data showed that in the absence of Gfi1aa, the synchronously developing prRBCs were delayed in activating late erythroid differentiation, as they struggled to suppress early erythroid and endothelial transcription programs. The latter highlighted the bipotent nature of the progenitors from which prRBCs arise. In the absence of Gfi1aa, Gfi1b promoted erythroid differentiation as stepwise loss of wt gfi1b copies progressively delayed Gfi1aa-depleted prRBCs even further, showing that Gfi1aa and Gfi1b together set the pace for prRBC differentiation from hemangioblasts.

Rare Genetic Blood Disease Modeling in Zebrafish

Hematopoiesis results in the correct formation of all the different blood cell types. In mammals, it starts from specific hematopoietic stem and precursor cells residing in the bone marrow. Mature blood cells are responsible for supplying oxygen to every cell of the organism and for the protection against pathogens. Therefore, inherited or de novo genetic mutations affecting blood cell formation or the regulation of their activity are responsible for numerous diseases including anemia, immunodeficiency, autoimmunity, hyper- or hypo-inflammation, and cancer. By definition, an animal disease model is an analogous version of a specific clinical condition developed by researchers to gain information about its pathophysiology. Among all the model species used in comparative medicine, mice continue to be the most common and accepted model for biomedical research. However, because of the complexity of human diseases and the intrinsic differences between humans and other species, the use of several models (possibly in distinct species) can often be more helpful and informative than the use of a single model. In recent decades, the zebrafish (Danio rerio) has become increasingly popular among researchers, because it represents an inexpensive alternative compared to mammalian models, such as mice. Numerous advantages make it an excellent animal model to be used in genetic studies and in particular in modeling human blood diseases. Comparing zebrafish hematopoiesis to mammals, it is highly conserved with few, significant differences. In addition, the zebrafish model has a high-quality, complete genomic sequence available that shows a high level of evolutionary conservation with the human genome, empowering genetic and genomic approaches. Moreover, the external fertilization, the high fecundity and the transparency of their embryos facilitate rapid, in vivo analysis of phenotypes. In addition, the ability to manipulate its genome using the last genome editing technologies, provides powerful tools for developing new disease models and understanding the pathophysiology of human disorders. This review provides an overview of the different approaches and techniques that can be used to model genetic diseases in zebrafish, discussing how this animal model has contributed to the understanding of genetic diseases, with a specific focus on the blood disorders.

Divergent Hemogen genes of teleosts and mammals share conserved roles in erythropoiesis: analysis using transgenic and mutant zebrafish

Hemogen is a vertebrate transcription factor that performs important functions in erythropoiesis and testicular development and may contribute to neoplasia. Here we identify zebrafish Hemogen and show that it is considerably smaller (∼22 kDa) than its human ortholog (∼55 kDa), a striking difference that is explained by an underlying modular structure. We demonstrate that Hemogens are largely composed of 21-25 amino acid repeats, some of which may function as transactivation domains (TADs). Hemogen expression in embryonic and adult zebrafish is detected in hematopoietic, renal, neural and gonadal tissues. Using Tol2- and CRISPR/Cas9-generated transgenic zebrafish, we show that Hemogen expression is controlled by two Gata1-dependent regulatory sequences that act alone and together to control spatial and temporal expression during development. Partial depletion of Hemogen in embryos by morpholino knockdown reduces the number of erythrocytes in circulation. CRISPR/Cas9-generated zebrafish lines containing either a frameshift mutation or an in-frame deletion in a putative, C-terminal TAD display anemia and embryonic tail defects. This work expands our understanding of Hemogen and provides mutant zebrafish lines for future study of the mechanism of this important transcription factor.

Summary: Transgenic and mutant zebrafish lines were created to characterize the expression and functions of Hemogen, a transcription factor involved in the formation of red blood cells and other processes.

Adult zebrafish Langerhans cells arise from hematopoietic stem/progenitor cells

The origin of Langerhans cells (LCs), which are skin epidermis-resident macrophages, remains unclear. Current lineage tracing of LCs largely relies on the promoter-Cre-LoxP system, which often gives rise to contradictory conclusions with different promoters. Thus, reinvestigation with an improved tracing method is necessary. Here, using a laser-mediated temporal-spatial resolved cell labeling method, we demonstrated that most adult LCs originated from the ventral wall of the dorsal aorta (VDA), an equivalent to the mouse aorta, gonads, and mesonephros (AGM), where both hematopoietic stem cells (HSCs) and non-HSC progenitors are generated. Further fine-fate mapping analysis revealed that the appearance of LCs in adult zebrafish was correlated with the development of HSCs, but not T cell progenitors. Finally, we showed that the appearance of tissue-resident macrophages in the brain, liver, heart, and gut of adult zebrafish was also correlated with HSCs. Thus, the results of our study challenged the EMP-origin theory for LCs.

The caudal dorsal artery generates hematopoietic stem and progenitor cells via the endothelial-to-hematopoietic transition in zebrafish

Zebrafish hematopoietic stem and progenitor cells (HSPCs) originate from the hemogenic endothelium of the ventral wall of the dorsal aorta (DA) through the endothelial-to-hematopoietic transition (EHT) from approximately 30 to 60 hours post fertilization (hpf). However, whether other artery sites can generate HSPCs de novo remains unclear. In this study, using live imaging and lineage tracing, we found that the caudal dorsal artery (CDA) in the caudal hematopoietic tissue directly gave rise to HSPCs through EHT. This process initiated from approximately 60 hpf and terminated at approximately 156 hpf. Compared with that in the DA, fewer EHT events were observed in the CDA. The EHT events in the DA and CDA were similarly regulated by Runx1 but differentially influenced by blood flow (i.e., the EHT frequency in CDA was affected to a lesser extent when circulation was compromised in the tnnt2a-/- mutant). Therefore, the whole artery, including both DA and CDA, was endowed with the ability to produce HSPCs during a much longer time period. Coincidently, the lineage tracing results indicated that adult hematopoietic cells originated from the embryonic endothelium, and those produced later preferentially colonized the adult thymus. Collectively, our study revealed that the CDA serves as an additional source of hematopoiesis, and it shows similar but not identical properties with the DA.

An optimised pipeline for parallel image-based quantification of gene expression and genotyping after in situ hybridisation

Advances in genome engineering have resulted in the generation of numerous zebrafish mutant lines. A commonly used method to assess gene expression in the mutants is in situ hybridisation (ISH). Because the embryos can be distinguished by genotype after ISH, comparing gene expression between wild-type and mutant siblings can be done blinded and in parallel. Such experimental design reduces the technical variation between samples and minimises the risk of bias. This approach, however, requires an efficient method of genomic DNA extraction from post-ISH fixed zebrafish samples to ascribe phenotype to genotype. Here we describe a method to obtain PCR-quality DNA from 95-100% of zebrafish embryos, suitable for genotyping after ISH. In addition, we provide an image analysis protocol for quantifying gene expression of ISH-probed embryos, adaptable for the analysis of different expression patterns. Finally, we show that intensity-based image analysis enables accurate representation of the variability of gene expression detected by ISH and that it can complement quantitative methods like qRT-PCR. By combining genotyping after ISH and computer-based image analysis, we have established a high-confidence, unbiased methodology to assign gene expression levels to specific genotypes, and applied it to the analysis of molecular phenotypes of newly generated lmo4a mutants.

Summary: Our optimised protocol to genotype zebrafish mutant embryos after in situ hybridisation and digitally quantify the in situ signal will help to standardise existing experimental designs and methods of analysis.

The first wave of T lymphopoiesis in zebrafish arises from aorta endothelium independent of hematopoietic stem cells

Tian et al. demonstrate that, in addition to giving rise to hematopoietic stem cells, the ventral endothelium of aorta in zebrafish also directly converts to non–hematopoietic stem cell hematopoietic precursors capable of generating a transient wave of CD4 T_αβ lymphocytes.

T lymphocytes are key cellular components of the adaptive immune system and play a central role in cell-mediated immunity in vertebrates. Despite their heterogeneities, it is believed that all different types of T lymphocytes are generated exclusively via the differentiation of hematopoietic stem cells (HSCs). Using temporal–spatial resolved fate-mapping analysis and time-lapse imaging, here we show that the ventral endothelium in the zebrafish aorta–gonad–mesonephros and posterior blood island, the hematopoietic tissues previously known to generate HSCs and erythromyeloid progenitors, respectively, gives rise to a transient wave of T lymphopoiesis independent of HSCs. This HSC-independent T lymphopoiesis occurs early and generates predominantly CD4 T_αβ cells in the larval but not juvenile and adult stages, whereas HSC-dependent T lymphopoiesis emerges late and produces various subtypes of T lymphocytes continuously from the larval stage to adulthood. Our study unveils the existence, origin, and ontogeny of HSC-independent T lymphopoiesis in vivo and reveals the complexity of the endothelial-hematopoietic transition of the aorta.

Irf8 regulates the progression of myeloproliferative neoplasm-like syndrome via Mertk signaling in zebrafish

Interferon regulatory factor (IRF)-8 is a critical transcription factor involved in the pathogenesis of myeloid neoplasia. However, the underlying mechanisms in vivo are not well known. Investigation of irf8-mutant zebrafish in this study indicated that Irf8 is evolutionarily conserved as an essential neoplastic suppressor through tight control of the proliferation and longevity of myeloid cells. Surviving irf8 mutants quickly developed a myeloproliferative neoplasm (MPN)-like disease with enhanced output of the myeloid precursors, which recurred after transplantation. Multiple molecules presented notable alteration and Mertk signaling was aberrantly activated in the hematopoietic cells in irf8 mutants. Transgenic mertk overexpression in Tg(coro1a:mertk) zebrafish recapitulated the myeloid neoplasia-like syndrome in irf8 mutants. Moreover, functional interference with Mertk, via morpholino knockdown or genetic disruption, attenuated the myeloid expansion phenotype caused by Irf8 deficiency. Therefore, Mertk signaling is a critical downstream player in the Irf8-mediated regulation of the progression of myeloid neoplasia. Our study extends the understanding of the mechanisms underlying leukemogenesis.

Rhein attenuates inflammation through inhibition of NF-κB and NALP3 inflammasome in vivo and in vitro

Rhein is an important component in traditional Chinese herbal medicine formulations for gastrointestinal disorders, including inflammatory bowel diseases such as ulcerative colitis. In this study, we investigated the beneficial effects of rhein in inflammation models in the transgenic zebrafish line TG (corolla eGFP), in which both macrophages and neutrophils express eGFP and RAW264.7 macrophages. We found that the tail-cutting-induced migration of immune cells was significantly reduced in transgenic zebrafish treated with rhein. In addition, the production of proinflammatory cytokines, including IL-6, IL-1β, and tumor necrosis factor-α, were significantly reduced in lipopolysaccharide (LPS)-induced RAW264.7 macrophages treated with rhein. Parallel to the inhibition of proinflammatory cytokines, rhein significantly reduced phosphorylation levels of NF-κB p65 and inducible nitric oxide synthase, as well as COX-2 protein expression levels. Furthermore, rhein significantly reduced NALP3 and cleaved IL-1β expression in LPS + ATP-induced RAW264.7 macrophages. Thus, the present study demonstrates that rhein may exhibit its anti-inflammatory action via inhibition of NF-κB and NALP3 inflammasome pathways.

Multiple origins of embryonic and tadpole myeloid cells in Xenopus laevis

Rabbit anti-serum against a myeloid-cell-specific peroxidase (Mpo) of Xenopus laevis was generated to identify myeloid cells in adult and larval animals. Smears of blood samples from adult hematopoietic organs were co-stained with Mpo and with XL-2, a mouse monoclonal antibody against a leukocyte common antigen. Lymphocytes found in the thymus and spleen were XL-2⁺Mpo^- and granulocytes found in peripheral blood cells and the spleen were XL-2⁺Mpo⁺, indicating that double-staining with these two antibodies allowed classification of the leukocyte lineages. Immunohistochemical analysis of larval organs showed that XL-2⁺Mpo^- cells were scattered throughout the liver, whereas XL-2⁺Mpo⁺ cells were present mainly in the cortex region. Interestingly, a cluster of XL-2⁺Mpo⁺ cells was found in the region of the larval mesonephric rudiment. The ratio of XL-2⁺Mpo⁺ cells to XL-2⁺ cells in the mesonephric region was approximately 80%, which was much higher than that found in other hematopoietic organs. In order to elucidate the embryonic origin of the myeloid cells in the tadpole mesonephros, grafting experiments between X. laevis and X. borealis embryos were performed to trace the X. borealis cells as donor cells. Among the embryonic tissues examined, the tailbud tissue at the early neurula stage contributed greatly to the myeloid cluster in the mesonephric region at stage 48. Therefore, at least four independent origins of the myeloid cell population can be traced in the Xenopus embryo.

Cellular responses to recurrent pentylenetetrazole-induced seizures in the adult zebrafish brain

A seizure is a sustained increase in brain electrical activity that can result in loss of consciousness and injury. Understanding how the brain responds to seizures is important for development of new treatment strategies for epilepsy, a neurological condition characterized by recurrent and unprovoked seizures. Pharmacological induction of seizures in rodent models results in a myriad of cellular alterations, including inflammation, angiogenesis, and adult neurogenesis. The purpose of this study is to investigate the cellular responses to recurrent pentylenetetrazole seizures in the adult zebrafish brain. We subjected zebrafish to five once-daily pentylenetetrazole-induced seizures and characterized the cellular consequences of these seizures. In response to recurrent seizures, we found histologic evidence of vasodilatation, perivascular leukocyte egress and leukocyte proliferation suggesting seizure-induced acute CNS inflammation. We also found evidence of increased proliferation, neurogenesis, and reactive gliosis following pentylenetetrazole-induced seizures. Collectively, our results suggest that the cellular responses to seizures in the adult zebrafish brain are similar to those observed in mammalian brains.

Zebrafish models of leukemia

The zebrafish, Danio rerio, is a well-established, invaluable model system for the study of human cancers. The genetic pathways that drive oncogenesis are highly conserved between zebrafish and humans, and multiple unique attributes of the zebrafish make it a tractable tool for analyzing the underlying cellular processes that give rise to human disease. In particular, the high conservation between human and zebrafish hematopoiesis (Jing & Zon, 2011) has stimulated the development of zebrafish models for human hematopoietic malignancies to elucidate molecular pathogenesis and to expedite the preclinical investigation of novel therapies. While T-cell acute lymphoblastic leukemia was the first transgenic cancer model in zebrafish (Langenau et al., 2003), a wide spectrum of zebrafish models of human hematopoietic malignancies has been established since 2003, largely through transgenesis and genome-editing approaches. This chapter presents key examples that validate the zebrafish as an indispensable model system for the study of hematopoietic malignancies and highlights new models that demonstrate recent advances in the field.

Spatiotemporal expression and transcriptional regulation of heme oxygenase and biliverdin reductase genes in zebrafish (Danio rerio) suggest novel roles during early developmental periods of heightened oxidative stress

Heme oxygenase 1 (HMOX1) degrades heme into biliverdin, which is subsequently converted to bilirubin by biliverdin reductase (BVRa or BVRb) in a manner analogous to the classic anti-oxidant glutathione-recycling pathway. To gain a better understanding of the potential antioxidant roles the BVR enzymes may play during development, the spatiotemporal expression and transcriptional regulation of zebrafish hmox1a, bvra and bvrb were characterized under basal conditions and in response to pro-oxidant exposure. All three genes displayed spatiotemporal expression patterns consistent with classic hematopoietic progenitors during development. Transient knockdown of Nrf2a did not attenuate the ability to detect bvra or bvrb by ISH, or alter spatial expression patterns in response to cadmium exposure. While hmox1a:mCherry fluorescence was documented within the intermediate cell mass, a transient location of primitive erythrocyte differentiation, expression was not fully attenuated in Nrf2a morphants, but real-time RT-PCR demonstrated a significant reduction in hmox1a expression. Furthermore, Gata-1 knockdown did not attenuate hmox1a:mCherry fluorescence. However, while there was a complete loss of detection of bvrb expression by ISH at 24hpf, bvra expression was greatly attenuated but still detectable in Gata-1 morphants. In contrast, 96 hpf Gata-1 morphants displayed increased bvra and bvrb expression within hematopoietic tissues. Finally, temporal expression patterns of enzymes involved in the generation and maintenance of NADPH were consistent with known changes in the cellular redox state during early zebrafish development. Together, these data suggest that Gata-1 and Nrf2a play differential roles in regulating the heme degradation enzymes during an early developmental period of heightened cellular stress.

Distinct regulatory networks control the development of macrophages of different origins in zebrafish

Macrophages are key components of the innate immune system and play pivotal roles in immune response, organ development, and tissue homeostasis. Studies in mice and zebrafish have shown that tissue-resident macrophages derived from different hematopoietic origins manifest distinct developmental kinetics and colonization potential, yet the genetic programs controlling the development of macrophages of different origins remain incompletely defined. In this study, we use zebrafish, where tissue-resident macrophages arise from the rostral blood island (RBI) and ventral wall of dorsal aorta (VDA), the zebrafish hematopoietic tissue equivalents to the mouse yolk sac and aorta-gonad-mesonephros for myelopoiesis, to address this issue. We show that RBI- and VDA-born macrophages are orchestrated by distinctive regulatory networks formed by the E-twenty-six (Ets) transcription factors Pu.1 and Spi-b, the zebrafish ortholog of mouse spleen focus forming virus proviral integration oncogene B (SPI-B), and the helix-turn-helix DNA-binding domain containing protein Irf8. Epistatic studies document that during RBI macrophage development, Pu.1 acts upstream of Spi-b, which, upon induction by Pu.1, partially compensates the function of Pu.1. In contrast, Pu.1 and Spi-b act in parallel and cooperatively to regulate the development of VDA-derived macrophages. Interestingly, these two distinct regulatory networks orchestrate the RBI- and VDA-born macrophage development largely by regulating a common downstream gene, Irf8. Our study indicates that macrophages derived from different origins are governed by distinct genetic networks formed by the same repertoire of myeloid-specific transcription factors.

Systemic inoculation of Escherichia coli causes emergency myelopoiesis in zebrafish larval caudal hematopoietic tissue

Emergency granulopoiesis occurs in response to severe microbial infection. However, whether and how other blood components, particularly monocytes/macrophages and their progenitors, including hematopoietic stem/progenitor cells (HSPCs), participate in the process and the underlying molecular mechanisms remain unknown. In this study, we challenged zebrafish larvae via direct injection of Escherichia coli into the bloodstream, which resulted in systemic inoculation with this microbe. The reaction of hematopoietic cells, including HSPCs, in the caudal hematopoietic tissue was carefully analysed. Both macrophages and neutrophils clearly expanded following the challenge. Thus, emergency myelopoiesis, including monopoiesis and granulopoiesis, occurred following systemic bacterial infection. The HSPC reaction was dependent on the bacterial burden, manifesting as a slight increase under low burden, but an obvious reduction following the administration of an excessive volume of bacteria. Pu.1 was important for the effective elimination of the microbes to prevent excessive HSPC apoptosis in response to stress. Moreover, Pu.1 played different roles in steady and emergency monopoiesis. Although Pu.1 was essential for normal macrophage development, it played suppressive roles in emergency monopoiesis. Overall, our study established a systemic bacterial infection model that led to emergency myelopoiesis, thereby improving our understanding of the function of Pu.1 in this scenario.

Cebpα is essential for the embryonic myeloid progenitor and neutrophil maintenance in zebrafish

In vertebrates, myeloid cells arise from multiple waves of development: the first or embryonic wave of myelopoiesis initiates early from non-hematopoietic stem cell (HSC) precursors and gives rise to myeloid cells transiently during early development; whereas the second or adult wave of myelopoiesis emerges later from HSCs and produces myeloid cells continually during fetal and adult life. In the past decades, a great deal has been learnt about the development of myeloid cells from adult myelopoiesis, yet the genetic network governing embryonic myelopoiesis remains poorly defined. In this report, we present an in vivo study to delineate the role of Cebpα during zebrafish embryonic myelopoiesis. We show that embryonic myelopoiesis in cebpα-deficient zebrafish mutants initiates properly but fails to produce macrophages and neutrophils. The lack of macrophages and neutrophils in the mutants is largely attributed to the cell cycle arrest of embryonic myeloid progenitors, resulting in the impairment of their maintenance and subsequent differentiation. We further show that Cebpα, perhaps acting cooperatively with Runx1, plays a critical role in embryonic neutrophil maintenance. Our findings reveal a new role of Cebpα in embryonic myelopoiesis.

tfec controls the hematopoietic stem cell vascular niche during zebrafish embryogenesis

In mammals, embryonic hematopoiesis occurs in successive waves, culminating with the emergence of hematopoietic stem cells (HSCs) in the aorta. HSCs first migrate to the fetal liver (FL), where they expand, before they seed the bone marrow niche, where they will sustain hematopoiesis throughout adulthood. In zebrafish, HSCs emerge from the dorsal aorta and colonize the caudal hematopoietic tissue (CHT). Recent studies showed that they interact with endothelial cells (ECs), where they expand, before they reach their ultimate niche, the kidney marrow. We identified tfec, a transcription factor from the mitf family, which is highly enriched in caudal endothelial cells (cECs) at the time of HSC colonization in the CHT. Gain-of-function assays indicate that tfec is capable of expanding HSC-derived hematopoiesis in a non-cell-autonomous fashion. Furthermore, tfec mutants (generated by CRISPR/Cas9) showed reduced hematopoiesis in the CHT, leading to anemia. Tfec mediates these changes by increasing the expression of several cytokines in cECs from the CHT niche. Among these, we found kitlgb, which could rescue the loss of HSCs observed in tfec mutants. We conclude that tfec plays an important role in the niche to expand hematopoietic progenitors through the modulation of several cytokines. The full comprehension of the mechanisms induced by tfec will represent an important milestone toward the expansion of HSCs for regenerative purposes.

Smyd5 plays pivotal roles in both primitive and definitive hematopoiesis during zebrafish embryogenesis

Methylation of histone tails plays a pivotal role in the regulation of a wide range of biological processes. SET and MYND domain-containing protein (SMYD) is a methyltransferase, five family members of which have been identified in humans. SMYD1, SMYD2, SMYD3, and SMYD4 have been found to play critical roles in carcinogenesis and/or the development of heart and skeletal muscle. However, the physiological functions of SMYD5 remain unknown. To investigate the function of Smyd5 in vivo, zebrafish were utilised as a model system. We first examined smyd5 expression patterns in developing zebrafish embryos. Smyd5 transcripts were abundantly expressed at early developmental stages and then gradually decreased. Smyd5 was expressed in all adult tissues examined. Loss-of-function analysis of Smyd5 was then performed in zebrafish embryos using smyd5 morpholino oligonucleotide (MO). Embryos injected with smyd5-MO showed normal gross morphological development, including of heart and skeletal muscle. However, increased expression of both primitive and definitive hematopoietic markers, including pu.1, mpx, l-plastin, and cmyb, were observed. These phenotypes of smyd5-MO zebrafish embryos were also observed when we introduced mutations in smyd5 gene with the CRISPR/Cas9 system. As the expression of myeloid markers was elevated in smyd5 loss-of-function zebrafish, we propose that Smyd5 plays critical roles in hematopoiesis.