Distinct functions for different scl isoforms in zebrafish primitive and definitive hematopoiesis

Tango6 regulates HSPC proliferation and definitive haematopoiesis via Ikzf1 and Cmyb in caudal haematopoietic tissue

Haematopoietic stem and progenitor cells (HSPCs) arise from the aorta-gonad-mesonephros and migrate to the caudal haematopoietic tissue (CHT) in zebrafish, where nascent HSPCs undergo tightly controlled proliferation and differentiation to promote definitive haematopoiesis. Effective expansion of HSPCs requires the coordination of well-established vesicle trafficking systems and appropriate transcription factors. However, the underlying molecules are yet to be identified. Using large-scale genetic screening of zebrafish larvae, Tango6 of the coat protein complex I (COPI) vesicle trafficking system was found to be indispensable for HSPC proliferation and definitive haematopoiesis. Homozygous tango6cq72 mutants display defective expansion of HSPCs in the CHT and compromised haematopoiesis. However, haematopoietic overexpression of Tango6 promoted haematopoietic expansion. tango6 deficiency caused a decline in RNA polymerase II subunit B and accumulation of DNA damage, which suppressed cell expansion in a P53-dependent manner. ikzf1 and cmyb (myb), two indispensable haematopoietic transcription factors, are targets of P53 and are used by tango6 in haematopoiesis. The haematopoietic phenotype was partially recovered by compensating for loss of ikzf1 and cmyb in tango6cq72 mutants. This study reveals a vesicle trafficking-mediated Tango6-P53-Ikzf1/Cmyb axis in zebrafish definitive haematopoiesis.

Developmental regulation of primitive erythropoiesis

PURPOSE OF REVIEW

In this review, we present an overview of recent studies of primitive erythropoiesis, focusing on advances in deciphering its embryonic origin, defining species-specific differences in its developmental regulation, and better understanding the molecular and metabolic pathways involved in terminal differentiation.

RECENT FINDINGS

Single-cell transcriptomics combined with state-of-the-art lineage tracing approaches in unperturbed murine embryos have yielded new insights concerning the origin of the first (primitive) erythroid cells that arise from mesoderm-derived progenitors. Moreover, studies examining primitive erythropoiesis in rare early human embryo samples reveal an overall conservation of primitive erythroid ontogeny in mammals, albeit with some interesting differences such as localization of erythropoietin (EPO) production in the early embryo. Mechanistically, the repertoire of transcription factors that critically regulate primitive erythropoiesis has been expanded to include regulators of transcription elongation, as well as epigenetic modifiers such as the histone methyltransferase DOT1L. For the latter, noncanonical roles aside from enzymatic activity are being uncovered. Lastly, detailed surveys of the metabolic and proteomic landscape of primitive erythroid precursors reveal the activation of key metabolic pathways such as pentose phosphate pathway that are paralleled by a striking loss of mRNA translation machinery.

SUMMARY

The ability to interrogate single cells in vivo continues to yield new insights into the birth of the first essential organ system of the developing embryo. A comparison of the regulation of primitive and definitive erythropoiesis, as well as the interplay of the different layers of regulation - transcriptional, epigenetic, and metabolic - will be critical in achieving the goal of faithfully generating erythroid cells in vitro for therapeutic purposes.

Endocardium gives rise to blood cells in zebrafish embryos

Previous studies have suggested that the endocardium contributes to hematopoiesis in murine embryos, although definitive evidence to demonstrate the hematopoietic potential of the endocardium is still missing. Here, we use a zebrafish embryonic model to test the emergence of hematopoietic progenitors from the endocardium. By using a combination of expression analysis, time-lapse imaging, and lineage-tracing approaches, we demonstrate that myeloid cells emerge from the endocardium in zebrafish embryos. Inhibition of Etv2/Etsrp or Scl/Tal1, two known master regulators of hematopoiesis and vasculogenesis, does not affect the emergence of endocardial-derived myeloid cells, while inhibition of Hedgehog signaling results in their reduction. Single-cell RNA sequencing analysis followed by experimental validation suggests that the endocardium is the major source of neutrophilic granulocytes. These findings will promote our understanding of alternative mechanisms involved in hematopoiesis, which are likely to be conserved between zebrafish and mammalian embryos.

Exploring hematopoiesis in zebrafish using forward genetic screening

Zebrafish have emerged as a powerful animal model for investigating the genetic basis of hematopoiesis. Owing to its close genetic and developmental similarities to humans, combined with its rapid reproduction and extensive genomic resources, zebrafish have become a versatile and efficient platform for genetic studies. In particular, the forward genetic screening approach has enabled the unbiased identification of novel genes and pathways related to blood development, from hematopoietic stem cell formation to terminal differentiation. Recent advances in mutant gene mapping have further expanded the scope of forward genetic screening, facilitating the identification of previously unknown genes and pathways relevant to hematopoiesis. In this review, we provide an overview of the zebrafish forward screening approach for hematopoietic gene discovery and highlight the key genes and pathways identified using this method. This review emphasizes the importance of zebrafish as a model system for understanding the genetic basis of hematopoiesis and its associated disorders.

Transcription Factor TAL1 in Erythropoiesis

Lineage-specific transcription factors (TFs) regulate differentiation of hematopoietic stem cells (HSCs). They are decisive for the establishment and maintenance of lineage-specific gene expression programs during hematopoiesis. For this they create a regulatory network between TFs, epigenetic cofactors, and microRNAs. They activate cell-type specific genes and repress competing gene expression programs. Disturbance of this process leads to impaired lineage fidelity and diseases of the blood system. The TF T-cell acute leukemia 1 (TAL1) is central for erythroid differentiation and contributes to the formation of distinct gene regulatory complexes in progenitor cells and erythroid cells. A TAL1/E47 heterodimer binds to DNA with the TFs GATA-binding factor 1 and 2 (GATA1/2), the cofactors LIM domain only 1 and 2 (LMO1/2), and LIM domain-binding protein 1 (LDB1) to form a core TAL1 complex. Furthermore, cell-type-dependent interactions of TAL1 with other TFs such as with runt-related transcription factor 1 (RUNX1) and Kruppel-like factor 1 (KLF1) are established. Moreover, TAL1 activity is regulated by the formation of TAL1 isoforms, posttranslational modifications (PTMs), and microRNAs. Here, we describe the function of TAL1 in normal hematopoiesis with a focus on erythropoiesis.

Cooperative contributions of the klf1 and klf17 genes in zebrafish primitive erythropoiesis

Krüppel-like transcription factors (Klfs), which are characterized by the three conserved C-terminal zinc fingers, are involved in various biological processes, such as haematopoiesis and angiogenesis. However, how the Klf family of transcription factors cooperate in organogenesis remains elusive. During zebrafish embryogenesis, both klf1 and klf17 are expressed in the intermediate cell mass (ICM), where primitive erythroid cells are produced. Using CRISPR-Cas9 genome editing technology, we established klf1-klf17 double mutant zebrafish to investigate the functionally interactive roles of the klf1 and klf17 genes. The klf1-klf17 mutant exhibited a diminished number of circulating primitive erythroid cells at 2 days postfertilization (dpf), while klf1 or klf17 single mutants and wild-type embryos produced comparable numbers of primitive erythroid cells. Circulating erythroid cells from the klf1-klf17 mutant possessed larger nuclei at 2 dpf than wild-type cells, suggesting the impairment of primitive erythroid cell maturation. The expression of the erythroid cell maturation markers band3 and mitoferrin, but not the haematopoietic progenitor markers c-myb and scl, was decreased in the klf1-klf17 mutant at 1 dpf. Thus, these results illustrate the cooperative function of klf1 and klf17 in the maturation processes of zebrafish primitive erythroid cells.

Haematopoietic stem and progenitor cell heterogeneity is inherited from the embryonic endothelium

Definitive haematopoietic stem and progenitor cells (HSPCs) generate erythroid, lymphoid and myeloid lineages. HSPCs are produced in the embryo via transdifferentiation of haemogenic endothelial cells in the aorta-gonad-mesonephros (AGM). HSPCs in the AGM are heterogeneous in differentiation and proliferative output, but how these intrinsic differences are acquired remains unanswered. Here we discovered that loss of microRNA (miR)-128 in zebrafish leads to an expansion of HSPCs in the AGM with different cell cycle states and a skew towards erythroid and lymphoid progenitors. Manipulating miR-128 in differentiating haemogenic endothelial cells, before their transition to HSPCs, recapitulated the lineage skewing in both zebrafish and human pluripotent stem cells. miR-128 promotes Wnt and Notch signalling in the AGM via post-transcriptional repression of the Wnt inhibitor csnk1a1 and the Notch ligand jag1b. De-repression of cskn1a1 resulted in replicative and erythroid-biased HSPCs, whereas de-repression of jag1b resulted in G2/M and lymphoid-biased HSPCs with long-term consequence on the respective blood lineages. We propose that HSPC heterogeneity arises in the AGM endothelium and is programmed in part by Wnt and Notch signalling.

Isoforms of the TAL1 transcription factor have different roles in hematopoiesis and cell growth

T-cell acute lymphoblastic leukemia (T-ALL) protein 1 (TAL1) is a central transcription factor in hematopoiesis. The timing and level of TAL1 expression orchestrate the differentiation to specialized blood cells and its overexpression is a common cause of T-ALL. Here, we studied the 2 protein isoforms of TAL1, short and long, which are generated by the use of alternative promoters as well as by alternative splicing. We analyzed the expression of each isoform by deleting an enhancer or insulator, or by opening chromatin at the enhancer location. Our results show that each enhancer promotes expression from a specific TAL1 promoter. Expression from a specific promoter gives rise to a unique 5' UTR with differential regulation of translation. Moreover, our study suggests that the enhancers regulate TAL1 exon 3 alternative splicing by inducing changes in the chromatin at the splice site, which we demonstrate is mediated by KMT2B. Furthermore, our results indicate that TAL1-short binds more strongly to TAL1 E-protein partners and functions as a stronger transcription factor than TAL1-long. Specifically TAL1-short has a unique transcription signature promoting apoptosis. Finally, when we expressed both isoforms in mice bone marrow, we found that while overexpression of both isoforms prevents lymphoid differentiation, expression of TAL-short alone leads to hematopoietic stem cell exhaustion. Furthermore, we found that TAL1-short promoted erythropoiesis and reduced cell survival in the CML cell line K562. While TAL1 and its partners are considered promising therapeutic targets in the treatment of T-ALL, our results show that TAL1-short could act as a tumor suppressor and suggest that altering TAL1 isoform's ratio could be a preferred therapeutic approach.

The Role of SCL Isoforms in Embryonic Hematopoiesis

Three waves of hematopoiesis occur in the mouse embryo. The primitive hematopoiesis appears as blood islands in the extra embryonic yolk sac at E7.5. The extra embryonic pro-definitive hematopoiesis launches in late E8 and the embryonic definitive one turns on at E10.5 indicated by the emergence of hemogenic endothelial cells on the inner wall of the extra embryonic arteries and the embryonic aorta. To study the roles of SCL protein isoforms in murine hematopoiesis, the SCL-large (SCL-L) isoform was selectively destroyed with the remaining SCL-small (SCL-S) isoform intact. It was demonstrated that SCL-S was specifically expressed in the hemogenic endothelial cells (HECs) and SCL-L was only detected in the dispersed cells after budding from HECs. The SCLΔ/Δ homozygous mutant embryos only survived to E10.5 with normal extra embryonic vessels and red blood cells. In wild-type mouse embryos, a layer of neatly aligned CD34+ and CD43+ cells appeared on the endothelial wall of the aorta of the E10.5 fetus. However, the cells at the same site expressed CD31 rather than CD34 and/or CD43 in the E10.5 SCLΔ/Δ embryo, indicating that only the endothelial lineage was developed. These results reveal that the SCL-S is sufficient to sustain the primitive hematopoiesis and SCL-L is necessary to launch the definitive hematopoiesis.

Hemogenic and aortic endothelium arise from a common hemogenic angioblast precursor and are specified by the Etv2 dosage

SignificanceHematopoietic stem cells (HSCs) are generated from specialized endothelial cells, called hemogenic endothelial cells (HECs). It has been debated whether HECs and non-HSC-forming conventional endothelial cells (cECs) arise from a common precursor or represent distinct lineages. Moreover, the molecular basis underlying their distinct fate determination is poorly understood. We use photoconvertible labeling, time-lapse imaging, and single-cell RNA-sequencing analysis to trace the lineage of HECs. We discovered that HECs and cECs arise from a common hemogenic angioblast precursor, and their distinct fate is determined by high or low dosage of Etv2, respectively. Our results illuminate the lineage origin and a mechanism on the fate determination of HECs, which may enhance the understanding on the ontogeny of HECs in vertebrates.

SLUG and Truncated TAL1 Reduce Glioblastoma Stem Cell Growth Downstream of Notch1 and Define Distinct Vascular Subpopulations in Glioblastoma Multiforme

Simple Summary

Glioblastoma multiforme is the most aggressive form of brain tumor and is still incurable. These neoplasms are particularly difficult to treat efficiently because of their highly heterogeneous and resistant characteristics. Advances in genomics have highlighted the complex molecular landscape of these tumors and the need to further develop effective and targeted therapies for each patient. A specific population of cells with enriched stem cell properties within tumors, i.e., glioblastoma stem cells (GSC), drives this cellular heterogeneity and therapeutical resistance, and thus constitutes an attractive target for the design of innovative treatments. However, the signals driving the maintenance and resistance of these cells are still unclear. We provide new findings regarding the expression of two transcription factors in these cells and directly in glioblastoma patient samples. We show that these proteins downregulate GSC growth and ultimately participate in the progression of gliomas. The forthcoming results will contribute to a better understanding of gliomagenesis.

Abstract

Glioblastomas (GBM) are high-grade brain tumors, containing cells with distinct phenotypes and tumorigenic potentials, notably aggressive and treatment-resistant multipotent glioblastoma stem cells (GSC). The molecular mechanisms controlling GSC plasticity and growth have only partly been elucidated. Contact with endothelial cells and the Notch1 pathway control GSC proliferation and fate. We used three GSC cultures and glioma resections to examine the expression, regulation, and role of two transcription factors, SLUG (SNAI2) and TAL1 (SCL), involved in epithelial to mesenchymal transition (EMT), hematopoiesis, vascular identity, and treatment resistance in various cancers. In vitro, SLUG and a truncated isoform of TAL1 (TAL1-PP22) were strongly upregulated upon Notch1 activation in GSC, together with LMO2, a known cofactor of TAL1, which formed a complex with truncated TAL1. SLUG was also upregulated by TGF-β1 treatment and by co-culture with endothelial cells. In patient samples, the full-length isoform TAL1-PP42 was expressed in all glioma grades. In contrast, SLUG and truncated TAL1 were preferentially overexpressed in GBMs. SLUG and TAL1 are expressed in the tumor microenvironment by perivascular and endothelial cells, respectively, and to a minor extent, by a fraction of epidermal growth factor receptor (EGFR) -amplified GBM cells. Mechanistically, both SLUG and truncated TAL1 reduced GSC growth after their respective overexpression. Collectively, this study provides new evidence for the role of SLUG and TAL1 in regulating GSC plasticity and growth.

Germline competent mesoderm: the substrate for vertebrate germline and somatic stem cells?

In vitro production of tissue-specific stem cells [e.g. haematopoietic stem cells (HSCs)] is a key goal of regenerative medicine. However, recent efforts to produce fully functional tissue-specific stem cells have fallen short. One possible cause of shortcomings may be that model organisms used to characterize basic vertebrate embryology (Xenopus, zebrafish, chick) may employ molecular mechanisms for stem cell specification that are not conserved in humans, a prominent example being the specification of primordial germ cells (PGCs). Germ plasm irreversibly specifies PGCs in many models; however, it is not conserved in humans, which produce PGCs from tissue termed germline-competent mesoderm (GLCM). GLCM is not conserved in organisms containing germ plasm, or even in mice, but understanding its developmental potential could unlock successful production of other stem cell types. GLCM was first discovered in embryos from the axolotl and its conservation has since been demonstrated in pigs, which develop from a flat-disc embryo like humans. Together these findings suggest that GLCM is a conserved basal trait of vertebrate embryos. Moreover, the immortal nature of germ cells suggests that immortality is retained during GLCM specification; here we suggest that the demonstrated pluripotency of GLCM accounts for retention of immortality in somatic stem cell types as well.

This article has an associated Future Leaders to Watch interview with the author of the paper.

Summary: Recent findings that germline and stem cell specification may differ between species may have important implications for regenerative medicine and the future of stem cell biology.

Tpr Deficiency Disrupts Erythroid Maturation With Impaired Chromatin Condensation in Zebrafish Embryogenesis

Vertebrate erythropoiesis involves nuclear and chromatin condensation at the early stages of terminal differentiation, which is a unique process to distinguish mature erythrocytes from erythroblasts. However, the underlying mechanisms of chromatin condensation during erythrocyte maturation remain elusive. Here, we reported a novel zebrafish mutant cas7 with erythroid maturation deficiency. Positional cloning showed that a single base mutation in tprb gene, which encodes nucleoporin translocated promoter region (Tpr), is responsible for the disrupted erythroid maturation and upregulation of erythroid genes, including ae1-globin and be1-globin. Further investigation revealed that deficient erythropoiesis in tprb cas7 mutant was independent on HIF signaling pathway. The proportion of euchromatin was significantly increased, whereas the percentage of heterochromatin was markedly decreased in tprb cas7 mutant. In addition, TPR knockdown in human K562 cells also disrupted erythroid differentiation and dramatically elevated the expression of globin genes, which suggests that the functions of TPR in erythropoiesis are highly conserved in vertebrates. Taken together, this study revealed that Tpr played vital roles in chromatin condensation and gene regulation during erythroid maturation in vertebrates.

Regulation of Hemogenic Endothelial Cell Development and Function

Embryonic definitive hematopoiesis generates hematopoietic stem and progenitor cells (HSPCs) essential for establishment and maintenance of the adult blood system. This process requires the specification of a subset of vascular endothelial cells to become blood-forming, or hemogenic, and the subsequent endothelial-to-hematopoietic transition to generate HSPCs therefrom. The mechanisms that regulate these processes are under intensive investigation, as their recapitulation in vitro from human pluripotent stem cells has the potential to generate autologous HSPCs for clinical applications. In this review, we provide an overview of hemogenic endothelial cell development and highlight the molecular events that govern hemogenic specification of vascular endothelial cells and the generation of multilineage HSPCs from hemogenic endothelium. We also discuss the impact of hemogenic endothelial cell development on adult hematopoiesis.

Transient activation of the Notch-her15.1 axis plays an important role in the maturation of V2b interneurons

In the vertebrate ventral spinal cord, p2 progenitors give rise to two interneuron subtypes: excitatory V2a interneurons and inhibitory V2b interneurons. In the differentiation of V2a and V2b cells, Notch signaling promotes V2b fate at the expense of V2a fate. Later, V2b cells extend axons along the ipsilateral side of the spinal cord and express the inhibitory transmitter GABA. Notch signaling has been reported to inhibit the axonal outgrowth of mature neurons of the central nervous system; however, it remains unknown how Notch signaling modulates V2b neurite outgrowth and maturation into GABAergic neurons. Here, we have investigated neuron-specific Notch functions regarding V2b axon growth and maturation into zebrafish GABAergic neurons. We found that continuous neuron-specific Notch activation enhanced V2b fate determination but inhibited V2b axonal outgrowth and maturation into GABAergic neurons. These results suggest that Notch signaling activation is required for V2b fate determination, whereas its downregulation at a later stage is essential for V2b maturation. Accordingly, we found that a Notch signaling downstream gene, her15.1, showed biased expression in V2 linage cells and downregulated expression during the maturation of V2b cells, and continuous expression of her15.1 repressed V2b axogenesis. Our data suggest that spatiotemporal control of Notch signaling activity is required for V2b fate determination, maturation and axogenesis.

The nuclear gene rpl18 regulates erythroid maturation via JAK2-STAT3 signaling in zebrafish model of Diamond-Blackfan anemia

Diamond-Blackfan anemia (DBA) is a rare, inherited bone marrow failure syndrome, characterized by red blood cell aplasia, developmental abnormalities, and enhanced risk of malignancy. However, the underlying pathogenesis of DBA is yet to be understood. Recently, mutations in the gene encoding ribosomal protein (RP) L18 were identified in DBA patients. RPL18 is a crucial component of the ribosomal large subunit but its role in hematopoiesis remains unknown. To genetically model the ribosomal defect identified in DBA, we generated a rpl18 mutant line in zebrafish, using CRISPR/Cas9 system. Molecular characterization of this mutant line demonstrated that Rpl18 deficiency mirrored the erythroid defects of DBA, namely a lack of mature red blood cells. Rpl18 deficiency caused an increase in p53 activation and JAK2-STAT3 activity. Furthermore, we found inhibitors of JAK2 or STAT3 phosphorylation could rescue anemia in rpl18 mutants. Our research provides a new in vivo model of Rpl18 deficiency and suggests involvement of signal pathway of JAK2-STAT3 in the DBA pathogenesis.

Loss of rps9 in Zebrafish Leads to p53-Dependent Anemia

Ribosome is a vital molecular machine for protein translation in the cell. Defects in several ribosomal proteins including RPS19, RPL11 and RPS14 have been observed in two types of anemia: Diamond Blackfan Anemia and 5q- syndrome. In zebrafish, deficiency of these ribosomal proteins shows similar anemic phenotype. It remains to be determined if any other ribosome proteins are similarly involved in regulating erythropoiesis. Here we generated mutations in zebrafish rps9, a rarely studied ribosomal protein gene, and investigated its function. Analysis of this mutant demonstrates that rps9 disruption leads to impairment of erythrocyte maturation, resulting in anemia. In addition, the overall phenotype including the anemic state is p53-dependent in rps9 mutants. Furthermore, this anemic state can be partially relieved by the treatment of L-leucine, and dexamethasone, which have been previously used in rescuing the phenotype of other ribosomal protein mutants. Finally, by comparing the phenotype, we show that there are considerable differences in morphology, cytomorphology, and hemoglobin levels for four ribosomal protein mutants in zebrafish. Based on the observed difference, we suggest that the level of anemic severity correlates with the delayed status of erythrocyte maturation in zebrafish models.

Blood stem cell-forming haemogenic endothelium in zebrafish derives from arterial endothelium

Haematopoietic stem cells are generated from the haemogenic endothelium (HE) located in the floor of the dorsal aorta (DA). Despite being integral to arteries, it is controversial whether HE and arterial endothelium share a common lineage. Here, we present a transgenic zebrafish runx1 reporter line to isolate HE and aortic roof endothelium (ARE)s, excluding non-aortic endothelium. Transcriptomic analysis of these populations identifies Runx1-regulated genes and shows that HE initially expresses arterial markers at similar levels to ARE. Furthermore, runx1 expression depends on prior arterial programming by the Notch ligand dll4. Runx1-/- mutants fail to downregulate arterial genes in the HE, which remains integrated within the DA, suggesting that Runx1 represses the pre-existing arterial programme in HE to allow progression towards the haematopoietic fate. These findings strongly suggest that, in zebrafish, aortic endothelium is a precursor to HE, with potential implications for pluripotent stem cell differentiation protocols for the generation of transplantable HSCs.

Cul4a promotes zebrafish primitive erythropoiesis via upregulating scl and gata1 expression

CUL4A and CUL4B are closely related members in Cullin family and can each assemble a Cullin-RING E3 ligase complex (Cullin-RING Ligase 4A or 4B, CRL4A, or CRL4B) and participate in a variety of biological processes. Previously we showed that zebrafish cul4a, but not cul4b, is essential for cardiac and pectoral fin development. Here, we have identified cul4a as a crucial regulator of primitive erythropoiesis in zebrafish embryonic development. Depletion of cul4a resulted in a striking reduction of erythroid cells due to the inhibition of erythroid differentiation. Transcript levels for early hematopoietic regulatory genes including scl, lmo2, and gata1 are significantly reduced in cul4a-deficient embryos. Mechanistically, we demonstrated that scl and gata1, the central regulators of primitive hematopoiesis for erythroid determination, are transcriptionally upregulated by cul4a. These findings demonstrate an important role for cul4a in primitive erythropoiesis and may bear implications in regeneration medicine of anemia and related diseases.

Dual role of Jam3b in early hematopoietic and vascular development

In order to efficiently derive hematopoietic stem cells (HSCs) from pluripotent precursors, it is crucial to understand how mesodermal cells acquire hematopoietic and endothelial identities, two divergent, but closely related cell fates. Although Npas4 has been recently identified as a conserved master regulator of hemato-vascular development, the molecular mechanisms underlying cell fate divergence between hematopoietic and vascular endothelial cells are still unclear. Here, we show in zebrafish that mesodermal cell differentiation into hematopoietic and vascular endothelial cells is regulated by Junctional adhesion molecule 3b (Jam3b) via two independent signaling pathways. Mutation of jam3b led to a reduction in npas4l expression in the posterior lateral plate mesoderm and defects in both hematopoietic and vascular development. Mechanistically, we uncover that Jam3b promotes endothelial specification by regulating npas4l expression through repression of the Rap1a-Erk signaling cascade. Jam3b subsequently promotes hematopoietic development, including HSCs, by regulating lrrc15 expression in endothelial precursors through the activation of an integrin-dependent signaling cascade. Our data provide insight into the divergent mechanisms for instructing hematopoietic or vascular fates from mesodermal cells.

The oyster immunity

Oysters, the common name for a number of different bivalve molluscs, are the worldwide aquaculture species and also play vital roles in the function of ecosystem. As invertebrate, oysters have evolved an integrated, highly complex innate immune system to recognize and eliminate various invaders via an array of orchestrated immune reactions, such as immune recognition, signal transduction, synthesis of antimicrobial peptides, as well as encapsulation and phagocytosis of the circulating haemocytes. The hematopoietic tissue, hematopoiesis, and the circulating haemocytes have been preliminary characterized, and the detailed annotation of the Pacific oyster Crassostrea gigas genome has revealed massive expansion and functional divergence of innate immune genes in this animal. Moreover, immune priming and maternal immune transfer are reported in oysters, suggesting the adaptability of invertebrate immunity. Apoptosis and autophagy are proved to be important immune mechanisms in oysters. This review will summarize the research progresses of immune system and the immunomodulation mechanisms of the primitive catecholaminergic, cholinergic, neuropeptides, GABAergic and nitric oxidase system, which possibly make oysters ideal model for studying the origin and evolution of immune system and the neuroendocrine-immune regulatory network in lower invertebrates.

The zebrafish: A fintastic model for hematopoietic development and disease

Hematopoiesis is a complex process with a variety of different signaling pathways influencing every step of blood cell formation from the earliest precursors to final differentiated blood cell types. Formation of blood cells is crucial for survival. Blood cells carry oxygen, promote organ development and protect organs in different pathological conditions. Hematopoietic stem and progenitor cells (HSPCs) are responsible for generating all adult differentiated blood cells. Defects in HSPCs or their downstream lineages can lead to anemia and other hematological disorders including leukemia. The zebrafish has recently emerged as a powerful vertebrate model system to study hematopoiesis. The developmental processes and molecular mechanisms involved in zebrafish hematopoiesis are conserved with higher vertebrates, and the genetic and experimental accessibility of the fish and the optical transparency of its embryos and larvae make it ideal for in vivo analysis of hematopoietic development. Defects in zebrafish hematopoiesis reliably phenocopy human blood disorders, making it a highly attractive model system to screen small molecules to design therapeutic strategies. In this review, we summarize the key developmental processes and molecular mechanisms of zebrafish hematopoiesis. We also discuss recent findings highlighting the strengths of zebrafish as a model system for drug discovery against hematopoietic disorders. This article is categorized under: Adult Stem Cells, Tissue Renewal, and Regeneration > Stem Cell Differentiation and Reversion Vertebrate Organogenesis > Musculoskeletal and Vascular Nervous System Development > Vertebrates: Regional Development Comparative Development and Evolution > Organ System Comparisons Between Species.

Splicing factor SF3B1K700E mutant dysregulates erythroid differentiation via aberrant alternative splicing of transcription factor TAL1

More than 60% of myeloid dysplasia syndrome (MDS) contains mutations in genes encoding for splicing factors such as SF3B1, U2AF, SRSF2 and ZRSR2. Mutations in SF3B1 are associated with 80% cases of refractory anemia with ring sideroblast (RARS), a subtype of MDS. SF3B1K700E is the most frequently mutated site among mutations on SF3B1. Yet the molecular mechanisms on how mutations of splicing factors lead to defective erythropoiesis are not clear. SF3B1K700E mutant binds to an RNA binding protein, RBM15, stronger than the wild type SF3B1 protein in co-immunoprecipitation assays. In addition, K700E mutant alters the RNA splicing of transcription factors TAL1 and GATA1. Via alternative RNA splicing, a novel short TAL1 transcript variant (TAL1s) is generated. Enhanced interaction between SF3B1 and RBM15 promotes the production of full-length TAL1 (TAL1fl) mRNA, while reduction of RBM15 protein level via PRMT1-mediated degradation pathway changes TAL1s/TAL1fl ratio in favor of TAL1s. TAL1s contains the helix-loop-helix DNA binding domain but not the N terminal region upstream of the DNA binding domain. The TAL1s protein loses its interaction with ETO2, which represses early erythropoiesis. In this vein, overexpression of TAL1s stimulates the transcription of β-hemoglobin in human leukemia K562 cells and promotes erythroid differentiation of human cord blood CD34+ cells cultured in erythropoietin-containing medium. Therefore, mutations of SF3B1 may block erythropoiesis via dysregulation of alternative RNA splicing of transcription factor TAL1, and targeting PRMT1 may alleviate the anemic symptoms in MDS patients.

SCL/TAL1: a multifaceted regulator from blood development to disease

SCL/TAL1 (stem cell leukemia/T-cell acute lymphoblastic leukemia [T-ALL] 1) is an essential transcription factor in normal and malignant hematopoiesis. It is required for specification of the blood program during development, adult hematopoietic stem cell survival and quiescence, and terminal maturation of select blood lineages. Following ectopic expression, SCL contributes to oncogenesis in T-ALL. Remarkably, SCL’s activities are all mediated through nucleation of a core quaternary protein complex (SCL:E-protein:LMO1/2 [LIM domain only 1 or 2]:LDB1 [LIM domain-binding protein 1]) and dynamic recruitment of conserved combinatorial associations of additional regulators in a lineage- and stage-specific context. The finely tuned control of SCL’s regulatory functions (lineage priming, activation, and repression of gene expression programs) provides insight into fundamental developmental and transcriptional mechanisms, and highlights mechanistic parallels between normal and oncogenic processes. Importantly, recent discoveries are paving the way to the development of innovative therapeutic opportunities in SCL+ T-ALL.

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.

Understanding the regulation of vertebrate hematopoiesis and blood disorders - big lessons from a small fish

Hematopoietic stem cells (HSCs) give rise to all differentiated blood cells. Understanding the mechanisms that regulate self-renewal and lineage specification of HSCs is key for developing treatments for many human diseases. Zebrafish have emerged as an excellent model for studying vertebrate hematopoiesis. This review will highlight the unique strengths of zebrafish and important findings that have emerged from studies of blood development and disorders using this system. We discuss recent advances in our understanding of hematopoiesis, including the origin of HSCs, molecular control of their development, and key signaling pathways involved in their regulation. We highlight significant findings from zebrafish models of blood disorders and discuss their application for investigating stem cell dysfunction in disease and for the development of new therapeutics.

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

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

Memory of cell shape biases stochastic fate decision-making despite mitotic rounding

Cell shape influences function, and the current model suggests that such shape effect is transient. However, cells dynamically change their shapes, thus, the critical question is whether shape information remains influential on future cell function even after the original shape is lost. We address this question by integrating experimental and computational approaches. Quantitative live imaging of asymmetric cell-fate decision-making and their live shape manipulation demonstrates that cellular eccentricity of progenitor cell indeed biases stochastic fate decisions of daughter cells despite mitotic rounding. Modelling and simulation indicates that polarized localization of Delta protein instructs by the progenitor eccentricity is an origin of the bias. Simulation with varying parameters predicts that diffusion rate and abundance of Delta molecules quantitatively influence the bias. These predictions are experimentally validated by physical and genetic methods, showing that cells exploit a mechanism reported herein to influence their future fates based on their past shape despite dynamic shape changes.

Cell shape influences function but during mitotic cell rounding the original shape is lost. Here the authors show that the cellular eccentricity of progenitor cell biases stochastic fate-decisions using a combination of quantitative live imaging, genetic manipulations and computational simulations.

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

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

Inactivation of 3-hydroxybutyrate dehydrogenase 2 delays zebrafish erythroid maturation by conferring premature mitophagy

Mitochondria are the site of iron utilization, wherein imported iron is incorporated into heme or iron-sulfur clusters. Previously, we showed that a cytosolic siderophore, which resembles a bacterial siderophore, facilitates mitochondrial iron import in eukaryotes, including zebrafish. An evolutionarily conserved 3-hydroxy butyrate dehydrogenase, 3-hydroxy butyrate dehydrogenase 2 (Bdh2), catalyzes a rate-limiting step in the biogenesis of the eukaryotic siderophore. We found that inactivation of bdh2 in developing zebrafish embryo results in heme deficiency and delays erythroid maturation. The basis for this erythroid maturation defect is not known. Here we show that bdh2 inactivation results in mitochondrial dysfunction and triggers their degradation by mitophagy. Thus, mitochondria are prematurely lost in bdh2-inactivated erythrocytes. Interestingly, bdh2-inactivated erythroid cells also exhibit genomic alterations as indicated by transcriptome analysis. Reestablishment of bdh2 restores mitochondrial function, prevents premature mitochondrial degradation, promotes erythroid development, and reverses altered gene expression. Thus, mitochondrial communication with the nucleus is critical for erythroid development.

Genome-wide analysis of the zebrafish Klf family identifies two genes important for erythroid maturation

Krüppel-like transcription factors (Klfs), each of which contains a CACCC-box binding domain, have been investigated in a variety of developmental processes, such as angiogenesis, neurogenesis and somatic-cell reprogramming. However, the function and molecular mechanism by which the Klf family acts during developmental hematopoiesis remain elusive. Here, we report identification of 24 Klf family genes in zebrafish using bioinformatics. Gene expression profiling shows that 6 of these genes are expressed in blood and/or vascular endothelial cells during embryogenesis. Loss of function of 2 factors (klf3 or klf6a) leads to a decreased number of mature erythrocytes. Molecular studies indicate that both Klf3 and Klf6a are essential for erythroid cell differentiation and maturation but that these two proteins function in distinct manners. We find that Klf3 inhibits the expression of ferric-chelate reductase 1b (frrs1b), thereby promoting the maturation of erythroid cells, whereas Klf6a controls the erythroid cell cycle by negatively regulating cdkn1a expression to determine the rate of red blood cell proliferation. Taken together, our study provides a global view of the Klf family members that contribute to hematopoiesis in zebrafish and sheds new light on the function and molecular mechanism by which Klf3 and Klf6a act during erythropoiesis in vertebrates.

Altered Proteome of Burkholderia pseudomallei Colony Variants Induced by Exposure to Human Lung Epithelial Cells

Burkholderia pseudomallei primary diagnostic cultures demonstrate colony morphology variation associated with expression of virulence and adaptation proteins. This study aims to examine the ability of B. pseudomallei colony variants (wild type [WT] and small colony variant [SCV]) to survive and replicate intracellularly in A549 cells and to identify the alterations in the protein expression of these variants, post-exposure to the A549 cells. Intracellular survival and cytotoxicity assays were performed followed by proteomics analysis using two-dimensional gel electrophoresis. B. pseudomallei SCV survive longer than the WT. During post-exposure, among 259 and 260 protein spots of SCV and WT, respectively, 19 were differentially expressed. Among SCV post-exposure up-regulated proteins, glyceraldehyde 3-phosphate dehydrogenase, fructose-bisphosphate aldolase (CbbA) and betaine aldehyde dehydrogenase were associated with adhesion and virulence. Among the down-regulated proteins, enolase (Eno) is implicated in adhesion and virulence. Additionally, post-exposure expression profiles of both variants were compared with pre-exposure. In WT pre- vs post-exposure, 36 proteins were differentially expressed. Of the up-regulated proteins, translocator protein, Eno, nucleoside diphosphate kinase (Ndk), ferritin Dps-family DNA binding protein and peptidyl-prolyl cis-trans isomerase B were implicated in invasion and virulence. In SCV pre- vs post-exposure, 27 proteins were differentially expressed. Among the up-regulated proteins, flagellin, Eno, CbbA, Ndk and phenylacetate-coenzyme A ligase have similarly been implicated in adhesion, invasion. Protein profiles differences post-exposure provide insights into association between morphotypic and phenotypic characteristics of colony variants, strengthening the role of B. pseudomallei morphotypes in pathogenesis of melioidosis.

Jam1a-Jam2a interactions regulate haematopoietic stem cell fate through Notch signalling

Notch signalling plays a key role in the generation of haematopoietic stem cells (HSCs) during vertebrate development^1-3 and requires intimate contact between signal emitting and receiving cells, although little is known regarding when, where, and how these intercellular events occur. We previously reported that the somitic Notch ligands, Dlc and Dld, are essential for HSC specification⁴. It has remained unclear, however, how these somitic requirements are connected to the later emergence of HSCs from the dorsal aorta (DA). Here we show that Notch signalling establishes HSC fate as their shared vascular precursors migrate across the ventral face of the somite and that Junctional adhesion molecules (JAMs) mediate this required Notch signal transduction. HSC precursors express jam1a and migrate axially across the ventral somite, where Jam2a and Notch ligands Dlc and Dld are expressed. Despite no alteration in the expression of Notch ligand or receptor genes, loss of function of jam1a led to loss of Notch signalling and loss of HSCs. Enforced activation of Notch in shared vascular precursors rescued HSCs in jam1a or jam2a deficient embryos. Together, these results indicate that Jam1a – Jam2a interactions facilitate the transduction of requisite Notch signals from the somite to the precursors of HSCs, and that these events occur well before formation of the DA.

Lysophosphatidic acid acts as a nutrient-derived developmental cue to regulate early hematopoiesis

Primitive hematopoiesis occurs in the yolk sac blood islands during vertebrate embryogenesis, where abundant phosphatidylcholines (PC) are available as important nutrients for the developing embryo. However, whether these phospholipids also generate developmental cues to promote hematopoiesis is largely unknown. Here, we show that lysophosphatidic acid (LPA), a signaling molecule derived from PC, regulated hemangioblast formation and primitive hematopoiesis. Pharmacological and genetic blockage of LPA receptor 1 (LPAR1) or autotoxin (ATX), a secretory lysophospholipase that catalyzes LPA production, inhibited hematopoietic differentiation of mouse embryonic stem cells and impaired the formation of hemangioblasts. Mechanistic experiments revealed that the regulatory effect of ATX-LPA signaling was mediated by PI3K/Akt-Smad pathway. Furthermore, during in vivo embryogenesis in zebrafish, LPA functioned as a developmental cue for hemangioblast formation and primitive hematopoiesis. Taken together, we identified LPA as an important nutrient-derived developmental cue for primitive hematopoiesis as well as a novel mechanism of hemangioblast regulation.

Different combinations of Notch ligands and receptors regulate V2 interneuron progenitor proliferation and V2a/V2b cell fate determination

The broad diversity of neurons is vital to neuronal functions. During vertebrate development, the spinal cord is a site of sensory and motor tasks coordinated by interneurons and the ongoing neurogenesis. In the spinal cord, V2-interneuron (V2-IN) progenitors (p2) develop into excitatory V2a-INs and inhibitory V2b-INs. The balance of these two types of interneurons requires precise control in the number and timing of their production. Here, using zebrafish embryos with altered Notch signaling, we show that different combinations of Notch ligands and receptors regulate two functions: the maintenance of p2 progenitor cells and the V2a/V2b cell fate decision in V2-IN development. Two ligands, DeltaA and DeltaD, and three receptors, Notch1a, Notch1b, and Notch3 redundantly contribute to p2 progenitor maintenance. On the other hand, DeltaA, DeltaC, and Notch1a mainly contribute to the V2a/V2b cell fate determination. A ubiquitin ligase Mib, which activates Notch ligands, acts in both functions through its activation of DeltaA, DeltaC, and DeltaD. Moreover, p2 progenitor maintenance and V2a/V2b fate determination are not distinct temporal processes, but occur within the same time frame during development. In conclusion, V2-IN cell progenitor proliferation and V2a/V2b cell fate determination involve signaling through different sets of Notch ligand-receptor combinations that occur concurrently during development in zebrafish.

Hyaluronic acid receptor Stabilin-2 regulates Erk phosphorylation and arterial--venous differentiation in zebrafish

The hyaluronic acid receptor for endocytosis Stabilin-2/HARE mediates systemic clearance of multiple glycosaminoglycans from the vascular and lymphatic circulations. In addition, recent in vitro studies indicate that Stab2 can participate in signal transduction by interacting with hyaluronic acid (HA), which results in Erk phosphorylation. However, it is not known whether Stab2 function or HA-Stab2 signaling play any role in embryonic development. Here we show that Stab2 functions in a signal transduction pathway regulating arterial-venous differentiation during zebrafish embryogenesis. Stab2 morpholino knockdown embryos (morphants) display an absence of intersegmental vessels and defects in the axial vessel formation. In addition, Stab2 morphants show defects in arterial-venous differentiation including the expansion of venous marker expression. Simultaneous knockdown of Stabilin-2 and Has2, an HA synthetase, results in a synergistic effect, arguing that HA and Stab2 interact during vasculature formation. Stab2 morphants display reduced Erk phosphorylation in the arterial progenitors, which is a known transducer of VEGF signaling, previously associated with arterial-venous differentiation. In addition, VEGF signaling acts as a negative feedback loop to repress stab2 expression. These results argue that Stab2 is involved in a novel signaling pathway that plays an important role in regulating Erk phosphorylation and establishing arterial-venous identity.

MiR-144 regulates hematopoiesis and vascular development by targeting meis1 during zebrafish development

Hematopoiesis is a dynamic process by which peripheral blood lineages are developed. It is a process tightly regulated by many intrinsic and extrinsic factors, including transcriptional factors and signaling molecules. However, the epigenetic regulation of hematopoiesis, for example, regulation via microRNAs (miRNAs), remains incompletely understood. Here we show that miR-144 regulates hematopoiesis and vascular development in zebrafish. Overexpression of miR-144 inhibited primitive hematopoiesis as demonstrated by a reduced number of circulating blood cells, reduced o-dianisidine staining of hemoglobin, and reduced expression of hbαe1, hbβe1, gata1 and pu.1. Overexpression of miR-144 also inhibited definitive hematopoiesis as shown by reduced expression of runx1 and c-myb. Mechanistically, miR-144 regulates hematopoiesis by repressing expression of meis1 involved in hematopoiesis. Both real-time RT-PCR and Western blot analyses showed that overexpression of miR-144 repressed expression of meis1. Bioinformatic analysis predicts a target binding sequence for miR-144 at the 3'-UTR of meis1. Deletion of the miR-144 target sequence eliminated the repression of meis1 expression mediated by miR-144. The miR-144-mediated abnormal phenotypes were partially rescued by co-injection of meis1 mRNA and could be almost completely rescued by injection of both meis1 and gata1 mRNA. Finally, because meis1 is involved in vascular development, we tested the effect of miR-144 on vascular development. Overexpression of miR-144 resulted in abnormal vascular development of intersegmental vessels in transgenic zebrafish with Flk1p-EGFP, and the defect was rescued by co-injection of meis1 mRNA. These findings establish miR-144 as a novel miRNA that regulates hematopoiesis and vascular development by repressing expression of meis1.

TET2 plays an essential role in erythropoiesis by regulating lineage-specific genes via DNA oxidative demethylation in a zebrafish model

Although epigenetic modulation is critical for a variety of cellular activities, its role in erythropoiesis remains poorly understood. Ten-eleven translocation (TET) molecules participate in methylcytosine (5mC) hydroxylation, which results in DNA demethylation in several biological processes. In this research, the role of TETs in erythropoiesis was investigated by using the zebrafish model, where three TET homologs were identified. These homologs share conserved structural domains with their mammalian counterparts. Zebrafish TETs mediate the conversion of 5mC to hydroxymethylcytosine (5hmC) in zebrafish embryos, and the deletion of TET2 inhibits erythropoiesis by suppressing the expression of the scl, gata-1, and cmyb genes. TET2-upregulated lineage-specific genes and erythropoiesis are closely associated with the occurrence of 5hmC and demethylation in the intermediate CpG promoters (ICPs) of scl, gata-1, cmyb, which frequently occur at specific regions or CpG sites of these ICPs. Moreover, TET2 regulates the formation and differentiation of erythroid progenitors, and deletion of TET2 leads to erythrocyte dysplasia and anemia. Here, we preliminarily proved that TET2 plays an essential role in erythrocyte development by regulating lineage-specific genes via DNA oxidative demethylation. This report is anticipated to broaden current information on hematopoiesis and pathogenesis of hematopoiesis-related diseases.

Pivotal role of Pten in the balance between proliferation and differentiation of hematopoietic stem cells in zebrafish

Self-renewing hematopoietic stem/progenitor cells (HSPCs) produce blood cells of all lineages throughout life. Phosphatase and tensin homolog (PTEN), a tumor suppressor that antagonizes phosphatidylinositol 3-kinase (PI3K) signaling, is frequently mutated in hematologic malignancies such as bone marrow failure and leukemia. We set out to investigate whether Pten is required for hematopoiesis. Analysis of zebrafish mutants lacking functional Pten revealed that HSPCs colonized the caudal hematopoietic tissue normally. There, HSPCs hyperproliferated and engaged in all blood lineages. However, they failed to differentiate into mature blood cells. Hence, Pten mutant zebrafish embryos displayed hallmarks of leukemia in humans. Inhibition of PI3K signaling in mutants lacking functional Pten suppressed hyperproliferation and released the differentiation arrest. We conclude that Pten has an essential role in the balance between proliferation and differentiation of blood cells.

Hemogenic endothelium specification and hematopoietic stem cell maintenance employ distinct Scl isoforms

Recent studies have shown that nascent hematopoietic stem cells (HSCs) derive directly from the ventral aortic endothelium (VAE) via endothelial to hematopoietic transition (EHT). However, whether EHT initiates from a random or predetermined subpopulation of VAE, as well as the molecular mechanism underlying this process, remain unclear. We previously reported that different zebrafish stem cell leukemia (scl) isoforms are differentially required for HSC formation in the ventral wall of the dorsal aorta. However, the exact stage at which these isoforms impact HSC development was not defined. Here, using in vivo time-lapse imaging of scl isoform-specific reporter transgenic zebrafish lines, we show that prior to EHT scl-β is selectively expressed in hemogenic endothelial cells, a unique subset of VAE cells possessing hemogenic potential, whereas scl-α is expressed later in nascent HSCs as they egress from VAE cells. In accordance with their expression, loss-of-function studies coupled with in vivo imaging analysis reveal that scl-β acts earlier to specify hemogenic endothelium, which is later transformed by runx1 into HSCs. Our results also reveal a previously unexpected role of scl-α in maintaining newly born HSCs in the aorta-gonads-mesonephros. Thus, our data suggest that a defined hemogenic endothelial population preset by scl-β supports the deterministic emergence of HSCs, and unravel the cellular mechanisms by which scl isoforms regulate HSC development.

Piezo1 plays a role in erythrocyte volume homeostasis

Mechanosensitivity is an inherent property of virtually all cell types, allowing them to sense and respond to physical environmental stimuli. Stretch-activated ion channels represent a class of mechanosensitive proteins which allow cells to respond rapidly to changes in membrane tension; however their identity has remained elusive. The piezo genes have recently been identified as a family of stretch-activated mechanosensitive ion channels. We set out to determine the role of piezo1 during zebrafish development. Here we report that morpholino-mediated knockdown of piezo1 impairs erythrocyte survival without affecting hematopoiesis or differentiation. Our results demonstrate that piezo1 is involved in erythrocyte volume homeostasis, disruption of which results in swelling/lysis of red blood cells and consequent anemia.

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

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

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

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

Live imaging reveals differing roles of macrophages and neutrophils during zebrafish tail fin regeneration

Macrophages and neutrophils are the pivotal immune phagocytes that enter the wound after tissue injury to remove the cell debris and invaded microorganisms, which presumably facilitate the regrowth of injured tissues. Taking advantage of the regeneration abilities of zebrafish and the newly generated leukocyte-specific zebrafish lines with labeling of both leukocyte lineages, we assessed the behaviors and functions of neutrophils and macrophages during tail fin regeneration. Live imaging showed that within 6 hours post amputation, the inflammatory stage, neutrophils were the primary cells scavenging apoptotic bodies and small cell debris, although they had limited phagocytic capacity and quickly underwent apoptosis. From 6 hours post amputation on, the resolution and regeneration stage, macrophages became the dominant scavengers, efficiently resolving inflammation and facilitating tissue remodeling and regrowth. Ablation of macrophages but not neutrophils severely impaired the inflammatory resolution and tissue regeneration, resulting in the formation of large vacuoles in the regenerated fins. In contrast, removal of neutrophils slightly accelerates the regrowth of injured fin. Our study documents the differing behaviors and functions of macrophages and neutrophils during tissue regeneration.

Identification of cis regulatory features in the embryonic zebrafish genome through large-scale profiling of H3K4me1 and H3K4me3 binding sites

An organism's genome sequence serves as a blueprint for the proteins and regulatory RNAs essential for cellular function. The genome also harbors cis-acting non-coding sequences that control gene expression and are essential to coordinate regulatory programs during embryonic development. However, the genome sequence is largely identical between cell types within a multi-cellular organism indicating that factors such as DNA accessibility and chromatin structure play a crucial role in governing cell-specific gene expression. Recent studies have identified particular chromatin modifications that define functionally distinct cis regulatory elements. Among these are forms of histone 3 that are mono- or tri-methylated at lysine 4 (H3K4me1 or H3K4me3, respectively), which bind preferentially to promoter and enhancer elements in the mammalian genome. In this work, we investigated whether these modified histones could similarly identify cis regulatory elements within the zebrafish genome. By applying chromatin immunoprecipitation followed by deep sequencing, we find that H3K4me1 and H3K4me3 are enriched at transcriptional start sites in the genome of the developing zebrafish embryo and that this association correlates with gene expression. We further find that these modifications associate with distal non-coding conserved elements, including known active enhancers. Finally, we demonstrate that it is possible to utilize H3K4me1 and H3K4me3 binding profiles in combination with available expression data to computationally identify relevant cis regulatory sequences flanking syn-expressed genes in the developing embryo. Taken together, our results indicate that H3K4me1 and H3K4me3 generally mark cis regulatory elements within the zebrafish genome and indicate that further characterization of the zebrafish using this approach will prove valuable in defining transcriptional networks in this model system.

The hemangioblast: from concept to authentication

The hemangioblast hypothesis has been hotly debated for over a century. Hemangioblasts are defined as multipotent cells that can give rise to both hematopoietic cells and endothelial cells. The existence of hemangioblasts has now been confirmed and many important molecules and several signaling pathways are involved in their generation and differentiation. Fibroblast growth factor, renin-angiotensin system and runt-related transcription factor 1 (Runx1) direct the formation of hemangioblasts through highly selective gene expression patterns. On the other hand, the hemogenic endothelium theory and a newly discovered pattern of hematopoietic/endothelial differentiation make the genesis of hemangioblasts more complicated. But how hemangioblasts are formed and how hematopoietic cells or endothelial cells are derived from remains largely unknown. Here we summarize the current knowledge of the signaling pathways and molecules involved in hemangioblast development and suggest some future clinical applications.

A novel miRNA processing pathway independent of Dicer requires Argonaute2 catalytic activity

Dicer is a central enzyme in microRNA (miRNA) processing. We identified a Dicer-independent miRNA biogenesis pathway that uses Argonaute2 (Ago2) slicer catalytic activity. In contrast to other miRNAs, miR-451 levels were refractory to dicer loss of function but were reduced in MZago2 (maternal-zygotic) mutants. We found that pre-miR-451 processing requires Ago2 catalytic activity in vivo. MZago2 mutants showed delayed erythropoiesis that could be rescued by wild-type Ago2 or miR-451-duplex but not by catalytically dead Ago2. Changing the secondary structure of Dicer-dependent miRNAs to mimic that of pre-miR-451 restored miRNA function and rescued developmental defects in MZdicer mutants, indicating that the pre-miRNA secondary structure determines the processing pathway in vivo. We propose that Ago2-mediated cleavage of pre-miRNAs, followed by uridylation and trimming, generates functional miRNAs independently of Dicer.

DeltaA mRNA and protein distribution in the zebrafish nervous system

Physical interaction between the transmembrane proteins Delta and Notch allows only a subset of neural precursors to become neurons, as well as regulating other aspects of neural development. To examine the localization of Delta protein during neural development, we generated an antibody specific to zebrafish Delta A (Dla). Here, we describe for the first time the subcellular localization of Dla protein in distinct puncta at cell cortex and/or membrane, supporting the function of Dla in direct cell-cell communication. In situ RNA hybridization and immunohistochemistry revealed dynamic, coordinated expression patterns of dla mRNA and Dla protein in the developing and adult zebrafish nervous system. Dla expression is mostly excluded from differentiated neurons and is maintained in putative precursor cells at least until larval stages. In the adult brain, dla mRNA and Dla protein are expressed in proliferative zones normally associated with stem cells.

Scl isoforms act downstream of etsrp to specify angioblasts and definitive hematopoietic stem cells

Recent lineage studies suggest that hematopoietic stem cells (HSCs) may be derived from endothelial cells. However, the genetic hierarchy governing the emergence of HSCs remains elusive. We report here that zebrafish ets1-related protein (etsrp), which is essential for vascular endothelial development, also plays a critical role in the initiation of definitive hematopoiesis by controlling the expression of 2 stem cell leukemia (scl) isoforms (scl-alpha and scl-beta) in angioblasts. In etsrp morphants, which are deficient in endothelial and HSC development, scl-alpha alone partially rescues angioblast specification, arterial-venous differentiation, and the expression of HSC markers, runx1 and c-myb, whereas scl-beta requires angioblast rescue by fli1a to restore runx1 expression. Interestingly, when vascular endothelial growth factor (Vegf) signaling is inhibited, HSC marker expression can still be restored by scl-alpha in etsrp morphants, whereas the rescue of arterial ephrinb2a expression is blocked. Furthermore, both scl isoforms partially rescue runx1 but not ephrinb2a expression in embryos deficient in Vegf signaling. Our data suggest that downstream of etsrp, scl-alpha and fli1a specify the angioblasts, whereas scl-beta further initiates HSC specification from this angioblast population, and that Vegf signaling acts upstream of scl-beta during definitive hematopoiesis.

Mta3-NuRD complex is a master regulator for initiation of primitive hematopoiesis in vertebrate embryos

Metastasis-associated antigens 1/2/3 (Mta1/2/3) are components of nucleosome remodeling and deacetylase (NuRD) complexes and have been found to play roles in embryonic development and homeostasis. However, their functions in primitive hematopoiesis are unknown. In this study, we demonstrate that knockdown of mta3 by antisense morpholinos abolishes primitive hematopoietic lineages and causes abnormal angiogenesis in zebrafish embryos. However, the expression of the pronephric duct and paraxial mesoderm markers is unaltered and the specification of angioblasts is unaffected in mta3 morphants. The results suggest that mta3 is specifically required for primitive hematopoiesis. Furthermore, inhibition of deacetylase activity with the inhibitors valproic acid (VPA) or trichostatin A (TSA) in zebrafish embryos completely blocks primitive hematopoiesis, resulting in hematopoietic defects almost identical to those seen in mta3 morphants. Importantly, overexpression of scl or scl and lmo2, 2 master genes for primitive hematopoiesis, is able to overturn effects of mta3 knockdown or VPA/TSA treatment; and overexpression of mta3, and human MBD3 or HDAC1, 2 other components of NuRD complex, enhances the expression of scl and lmo2 in the posterior lateral plate mesoderm during early primitive hematopoiesis. We conclude that Mta3-NuRD complex is essential for the initiation of primitive hematopoiesis. Thus, our findings provide new insight into the regulatory hierarchy of primitive hematopoiesis in vertebrates.