The zebrafish gene cloche acts upstream of a flk-1 homologue to regulate endothelial cell differentiation

Tuning apicobasal polarity and junctional recycling in the hemogenic endothelium orchestrates the morphodynamic complexity of emerging pre-hematopoietic stem cells

Hematopoietic stem cells emerge in the embryo from an aortic-derived tissue called the hemogenic endothelium (HE). The HE appears to give birth to cells of different nature and fate but the molecular principles underlying this complexity are largely unknown. Here we show, in the zebrafish embryo, that two cell types emerge from the aortic floor with radically different morphodynamics. With the support of live imaging, we bring evidence suggesting that the mechanics underlying the two emergence types rely, or not, on apicobasal polarity establishment. While the first type is characterized by reinforcement of apicobasal polarity and maintenance of the apical/luminal membrane until release, the second type emerges via a dynamic process reminiscent of trans-endothelial migration. Interfering with Runx1 function suggests that the balance between the two emergence types depends on tuning apicobasal polarity at the level of the HE. In support of this and unexpectedly, we show that Pard3ba - one of the four Pard3 proteins expressed in the zebrafish - is sensitive to interference with Runx1 activity, in aortic endothelial cells. This supports the idea of a signaling cross talk controlling cell polarity and its associated features, between aortic and hemogenic cells. In addition, using new transgenic fish lines that express Junctional Adhesion Molecules and functional interference, we bring evidence for the essential role of ArhGEF11/PDZ-RhoGEF in controlling the HE-endothelial cell dynamic interface, including cell-cell intercalation, which is ultimately required for emergence completion. Overall, we highlight critical cellular and dynamic events of the endothelial-to-hematopoietic transition that support emergence complexity, with a potential impact on cell fate.

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.

A tail of their own: regulation of cardiolipin and phosphatidylinositol fatty acyl profile by the acyltransferase LCLAT1

Cardiolipin and phosphatidylinositol along with the latter's phosphorylated derivative phosphoinositides, control a wide range of cellular functions from signal transduction, membrane traffic, mitochondrial function, cytoskeletal dynamics, and cell metabolism. An emerging dimension to these lipids is the specificity of their fatty acyl chains that is remarkably distinct from that of other glycerophospholipids. Cardiolipin and phosphatidylinositol undergo acyl remodeling involving the sequential actions of phospholipase A to hydrolyze acyl chains and key acyltransferases that re-acylate with specific acyl groups. LCLAT1 (also known as LYCAT, AGPAT8, LPLAT6, or ALCAT1) is an acyltransferase that contributes to specific acyl profiles for phosphatidylinositol, phosphoinositides, and cardiolipin. As such, perturbations of LCLAT1 lead to alterations in cardiolipin-dependent phenomena such as mitochondrial respiration and dynamics and phosphoinositide-dependent processes such as endocytic membrane traffic and receptor signaling. Here we examine the biochemical and cellular actions of LCLAT1, as well as the contribution of this acyltransferase to the development and specific diseases.

Zebrafish as a Useful Model System for Human Liver Disease

Liver diseases represent a significant global health challenge, thereby necessitating extensive research to understand their intricate complexities and to develop effective treatments. In this context, zebrafish (Danio rerio) have emerged as a valuable model organism for studying various aspects of liver disease. The zebrafish liver has striking similarities to the human liver in terms of structure, function, and regenerative capacity. Researchers have successfully induced liver damage in zebrafish using chemical toxins, genetic manipulation, and other methods, thereby allowing the study of disease mechanisms and the progression of liver disease. Zebrafish embryos or larvae, with their transparency and rapid development, provide a unique opportunity for high-throughput drug screening and the identification of potential therapeutics. This review highlights how research on zebrafish has provided valuable insights into the pathological mechanisms of human liver disease.

Endothelial-derived angiocrine factors as instructors of embryonic development

Blood vessels are well-known to play roles in organ development and repair, primarily owing to their fundamental function in delivering oxygen and nutrients to tissues to promote their growth and homeostasis. Endothelial cells however are not merely passive conduits for carrying blood. There is now evidence that endothelial cells of the vasculature actively regulate tissue-specific development, morphogenesis and organ function, as well as playing roles in disease and cancer. Angiocrine factors are growth factors, cytokines, signaling molecules or other regulators produced directly from endothelial cells to instruct a diverse range of signaling outcomes in the cellular microenvironment, and are critical mediators of the vascular control of organ function. The roles of angiocrine signaling are only beginning to be uncovered in diverse fields such as homeostasis, regeneration, organogenesis, stem-cell maintenance, cell differentiation and tumour growth. While in some cases the specific angiocrine factor involved in these processes has been identified, in many cases the molecular identity of the angiocrine factor(s) remain to be discovered, even though the importance of angiocrine signaling has been implicated. In this review, we will specifically focus on roles for endothelial-derived angiocrine signaling in instructing tissue morphogenesis and organogenesis during embryonic and perinatal development.

Hemato-vascular specification requires arnt1 and arnt2 genes in zebrafish embryos

During embryonic development, a subset of cells in the mesoderm germ layer are specified as hemato-vascular progenitor cells, which then differentiate into endothelial cells and hematopoietic stem and progenitor cells. In zebrafish, the transcription factor npas4l (cloche) is required for the specification of hemato-vascular progenitor cells. However, it is unclear whether npas4l is the sole factor at the top of the hemato-vascular specification cascade. Here, we show that arnt1 and arnt2 genes are required for hemato-vascular specification. We found that arnt1;arnt2 double mutant zebrafish embryos, but not arnt1 or arnt2 single mutants, lack blood cells and most endothelial cells. arnt1/2 mutants have reduced or absent expression of etsrp and tal1, the earliest known endothelial and hematopoietic transcription factor genes. We found that Npas4l binds both Arnt1 and Arnt2 proteins in vitro, consistent with the idea that PAS domain-containing bHLH transcription factors act in a multimeric complex to regulate gene expression. Our results demonstrate that npas4l, arnt1 and arnt2 act together to regulate endothelial and hematopoietic cell fate, where each gene is necessary, but not sufficient, to drive hemato-vascular specification.

Three-dimensional cerebral vasculature topological parameter extraction of transgenic zebrafish embryos with a filling-enhancement deep learning network

Quantitative analysis of zebrafish cerebral vasculature is essential for the study of vascular development and disease. We developed a method to accurately extract the cerebral vasculature topological parameters of transgenic zebrafish embryos. The intermittent and hollow vascular structures of transgenic zebrafish embryos, obtained from 3D light-sheet imaging, were transformed into continuous solid structures with a filling-enhancement deep learning network. The enhancement enables the extraction of 8 vascular topological parameters accurately. Quantitation of the zebrafish cerebral vasculature vessels with the topological parameters show a developmental pattern transition from 2.5 to 5.5 dpf.

Endothelial versus pronephron fate decision is modulated by the transcription factors Cloche/Npas4l, Tal1, and Lmo2

Endothelial specification is a key event during embryogenesis; however, when, and how, endothelial cells separate from other lineages is poorly understood. In zebrafish, Npas4l is indispensable for endothelial specification by inducing the expression of the transcription factor genes etsrp, tal1, and lmo2. We generated a knock-in reporter in zebrafish npas4l to visualize endothelial progenitors and their derivatives in wild-type and mutant embryos. Unexpectedly, we find that in npas4l mutants, npas4l reporter-expressing cells contribute to the pronephron tubules. Single-cell transcriptomics and live imaging of the early lateral plate mesoderm in wild-type embryos indeed reveals coexpression of endothelial and pronephron markers, a finding confirmed by creERT2-based lineage tracing. Increased contribution of npas4l reporter-expressing cells to pronephron tubules is also observed in tal1 and lmo2 mutants and is reversed in npas4l mutants injected with tal1 mRNA. Together, these data reveal that Npas4l/Tal1/Lmo2 regulate the fate decision between the endothelial and pronephron lineages.

Endothelial Cell Differentiation and Hemogenic Specification

Formation of the vasculature is a critical step within the developing embryo and its disruption causes early embryonic lethality. This complex process is driven by a cascade of signaling events that controls differentiation of mesodermal progenitors into primordial endothelial cells and their further specification into distinct subtypes (arterial, venous, hemogenic) that are needed to generate a blood circulatory network. Hemogenic endothelial cells give rise to hematopoietic stem and progenitor cells that generate all blood cells in the body during embryogenesis and postnatally. We focus our discussion on the regulation of endothelial cell differentiation, and subsequent hemogenic specification, and highlight many of the signaling pathways involved in these processes, which are conserved across vertebrates. Gaining a better understanding of the regulation of these processes will yield insights needed to optimize the treatment of vascular and hematopoietic disease and generate human stem cell-derived vascular and hematopoietic cells for tissue engineering and regenerative medicine.

Single-cell transcriptomic analysis of vascular endothelial cells in zebrafish embryos

Vascular endothelial cells exhibit substantial phenotypic and transcriptional heterogeneity which is established during early embryogenesis. However, the molecular mechanisms involved in establishing endothelial cell diversity are still not well understood. Zebrafish has emerged as an advantageous model to study vascular development. Despite its importance, the single-cell transcriptomic profile of vascular endothelial cells during zebrafish development is still missing. To address this, we applied single-cell RNA-sequencing (scRNA-seq) of vascular endothelial cells isolated from zebrafish embryos at the 24 hpf stage. Six distinct clusters or subclusters related to vascular endothelial cells were identified which include arterial, two venous, cranial, endocardial and endothelial progenitor cell subtypes. Furthermore, we validated our findings by characterizing novel markers for arterial, venous, and endocardial cells. We experimentally confirmed the presence of two transcriptionally different venous cell subtypes, demonstrating heterogeneity among venous endothelial cells at this early developmental stage. This dataset will be a valuable resource for future functional characterization of vascular endothelial cells and interrogation of molecular mechanisms involved in the establishment of their heterogeneity and cell-fate decisions.

Vascular Regulation of Developmental Neurogenesis

Evolutionary studies indicate that the nervous system evolved prior to the vascular system, but the increasing complexity of organisms prompted the vascular system to emerge in order to meet the growing demand for oxygen and nutrient supply. In recent years, it has become apparent that the symbiotic communication between the nervous and the vascular systems goes beyond the exclusive covering of the demands on nutrients and oxygen carried by blood vessels. Indeed, this active interplay between both systems is crucial during the development of the central nervous system (CNS). Several neural-derived signals that initiate and regulate the vascularization of the CNS have been described, however less is known about the vascular signals that orchestrate the development of the CNS cytoarchitecture. Here, we focus on reviewing the effects of blood vessels in the process of neurogenesis during CNS development in vertebrates. In mammals, we describe the spatiotemporal features of vascular-driven neurogenesis in two brain regions that exhibit different neurogenic complexity in their germinal zone, the hindbrain and the forebrain.

Molecular and Cellular Mechanisms of Vascular Development in Zebrafish

The establishment of a functional cardiovascular system is crucial for the development of all vertebrates. Defects in the development of the cardiovascular system lead to cardiovascular diseases, which are among the top 10 causes of death worldwide. However, we are just beginning to understand which signaling pathways guide blood vessel growth in different tissues and organs. The advantages of the model organism zebrafish (Danio rerio) helped to identify novel cellular and molecular mechanisms of vascular growth. In this review we will discuss the current knowledge of vasculogenesis and angiogenesis in the zebrafish embryo. In particular, we describe the molecular mechanisms that contribute to the formation of blood vessels in different vascular beds within the embryo.

Autonomous Calcium Signaling in Human and Zebrafish Podocytes Controls Kidney Filtration Barrier Morphogenesis

BACKGROUND

Podocytes are critical to maintaining the glomerular filtration barrier, and mutations in nephrotic syndrome genes are known to affect podocyte calcium signaling. However, the role of calcium signaling during podocyte development remains unknown.

METHODS

We undertook live imaging of calcium signaling in developing podocytes, using zebrafish larvae and human kidney organoids. To evaluate calcium signaling during development and in response to channel blockers and genetic defects, the calcium biosensor GCaMP6s was expressed in zebrafish podocytes. We used electron microscopy to evaluate filtration barrier formation in zebrafish, and Fluo-4 to detect calcium signals in differentiating podocytes in human kidney organoids.

RESULTS

Immature zebrafish podocytes (2.5 days postfertilization) generated calcium transients that correlated with interactions with forming glomerular capillaries. Calcium transients persisted until 4 days postfertilization, and were absent after glomerular barrier formation was complete. We detected similar calcium transients in maturing human organoid glomeruli, suggesting a conserved mechanism. In both models, inhibitors of SERCA or IP3 receptor calcium-release channels blocked calcium transients in podocytes, whereas lanthanum was ineffective, indicating the calcium source is from intracellular podocyte endoplasmic-reticulum stores. Calcium transients were not affected by blocking heartbeat or by blocking development of endothelium or endoderm, and they persisted in isolated glomeruli, suggesting podocyte-autonomous calcium release. Inhibition of expression of phospholipase C-γ1, but not nephrin or phospholipase C-ε1, led to significantly decreased calcium activity. Finally, blocking calcium release affected glomerular shape and podocyte foot process formation, supporting the critical role of calcium signaling in glomerular morphogenesis.

CONCLUSIONS

These findings establish podocyte cell-autonomous calcium signaling as a prominent and evolutionarily conserved feature of podocyte differentiation and demonstrate its requirement for podocyte foot process formation.

Sensory Neuroblast Quiescence Depends on Vascular Cytoneme Contacts and Sensory Neuronal Differentiation Requires Initiation of Blood Flow

In many organs, stem cell function depends on communication with their niche partners. Cranial sensory neurons develop in close proximity to blood vessels; however, whether vasculature is an integral component of their niches is yet unknown. Here, two separate roles for vasculature in cranial sensory neurogenesis in zebrafish are uncovered. The first involves precise spatiotemporal endothelial-neuroblast cytoneme contacts and Dll4-Notch signaling to restrain neuroblast proliferation. The second, instead, requires blood flow to trigger a transcriptional response that modifies neuroblast metabolic status and induces sensory neuron differentiation. In contrast, no role of sensory neurogenesis in vascular development is found, suggesting unidirectional signaling from vasculature to sensory neuroblasts. Altogether, we demonstrate that the cranial vasculature constitutes a niche component of the sensory ganglia that regulates the pace of their growth and differentiation dynamics.

The effect of combined exposure of zinc and nickel on the development of zebrafish

Excessive accumulation of Zn²⁺ or Ni²⁺ can cause various problems to aquatic animals. In this study, the developmental toxicity induced by individual or combined exposure of Zn²⁺ and Ni²⁺ to zebrafish embryos and larvae were evaluated to better understand the interaction between Zn²⁺ and Ni²⁺ . Both of individual and combined exposure of Zn²⁺ and Ni²⁺ could cause obvious developmental toxicity, which mainly occurred after hatching, at a concentration-dependent manner. The calculated 168-h LC₅₀ were 2.79 mg/L for Zn²⁺ and 7.44 mg/L for Ni²⁺ . The interaction of Zn²⁺ and Ni²⁺ based on mortality was found to be an antagonism. Various malformations, including tail curving, spinal curvature, pericardial edema, and yolk sac edema, were observed with significant effects on body length and heartbeat rates after exposure of Zn²⁺ and Ni²⁺ . Meanwhile, some genes related to cardiovascular development and bone formation were mainly down-regulated by the individual and combined exposure of Zn²⁺ and Ni²⁺ . The individual exposure was more toxic than combined exposure because the interaction of Zn²⁺ and Ni²⁺ was determined to be an antagonism. The down-regulation of genes related to cardiovascular development and bone formation may contribute to the observed malformation and decreases of body length and heartbeat rates.

Genome-wide strategies reveal target genes of Npas4l associated with vascular development in zebrafish

The development of a vascular network is essential to nourish tissues and sustain organ function throughout life. Endothelial cells (ECs) are the building blocks of blood vessels, yet our understanding of EC specification remains incomplete. Zebrafish cloche/npas4l mutants have been used broadly as an avascular model, but little is known about the molecular mechanisms of action of the Npas4l transcription factor. Here, to identify its direct and indirect target genes, we have combined complementary genome-wide approaches, including transcriptome analyses and chromatin immunoprecipitation. The cross-analysis of these datasets indicates that Npas4l functions as a master regulator by directly inducing a group of transcription factor genes that are crucial for hematoendothelial specification, such as etv2, tal1 and lmo2 We also identified new targets of Npas4l and investigated the function of a subset of them using the CRISPR/Cas9 technology. Phenotypic characterization of tspan18b mutants reveals a novel player in developmental angiogenesis, confirming the reliability of the datasets generated. Collectively, these data represent a useful resource for future studies aimed to better understand EC fate determination and vascular development.

Zebrafish facial lymphatics develop through sequential addition of venous and non-venous progenitors

Lymphatic vessels are known to be derived from veins; however, recent lineage-tracing experiments propose that specific lymphatic networks may originate from both venous and non-venous sources. Despite this, direct evidence of a non-venous lymphatic progenitor is missing. Here, we show that the zebrafish facial lymphatic network is derived from three distinct progenitor populations that add sequentially to the developing facial lymphatic through a relay-like mechanism. We show that while two facial lymphatic progenitor populations are venous in origin, the third population, termed the ventral aorta lymphangioblast (VA-L), does not sprout from a vessel; instead, it arises from a migratory angioblast cell near the ventral aorta that initially lacks both venous and lymphatic markers, and contributes to the facial lymphatics and the hypobranchial artery. We propose that sequential addition of venous and non-venous progenitors allows the facial lymphatics to form in an area that is relatively devoid of veins. Overall, this study provides conclusive, live imaging-based evidence of a non-venous lymphatic progenitor and demonstrates that the origin and development of lymphatic vessels is context-dependent.

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.

A new mode of pancreatic islet innervation revealed by live imaging in zebrafish

Pancreatic islets are innervated by autonomic and sensory nerves that influence their function. Analyzing the innervation process should provide insight into the nerve-endocrine interactions and their roles in development and disease. Here, using in vivo time-lapse imaging and genetic analyses in zebrafish, we determined the events leading to islet innervation. Comparable neural density in the absence of vasculature indicates that it is dispensable for early pancreatic innervation. Neural crest cells are in close contact with endocrine cells early in development. We find these cells give rise to neurons that extend axons toward the islet as they surprisingly migrate away. Specific ablation of these neurons partly prevents other neurons from migrating away from the islet resulting in diminished innervation. Thus, our studies establish the zebrafish as a model to interrogate mechanisms of organ innervation, and reveal a novel mode of innervation whereby neurons establish connections with their targets before migrating away.

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.

Dietary Safety Assessment of Flk1-Transgenic Fish

Genetic engineering, also called genetic modification, is facing with growing demands of aquaculture and aquatic products. Although various genetically modified (GM) aquatics have been generated, it is important to evaluate biosafety of GM organisms on the human health before entering into our food chain. For this purpose, we establish a zebrafish wild adult feeding Flk1-transgenic larvae model to examine the predatory fish's histology in multiple tissues, and the global gene expression profile in the liver. 180 days of feeding trial show that there are no significantly morphological changes in intestine, liver, kidney, and sex gonads between fish fed with Flk1 transgenic fish diet (TFD) and fish fed with regular food meal (RFM). However, a characteristic skin spot and autofluorescence increase in the theca of follicle are observed in F1 generation of TFD fish. Liver RNA-sequencing analyses demonstrate that 53 out of 56712 genes or isoforms are differentially transcribed, and mostly involved in proteolysis in extracellular region. According to GO enrichment terms these deregulated genes function in catalytic activity, steroid storing, lipid metabolic process and N-Glycan biosynthesis. These results suggest that a long term of Flk1-transgenic fish diet could alter certain metabolic pathways and possibly cause related tissue deformation. Compared to the previous reports, our feasible transgenic dietary assess system could evaluate subchronic and potential health impact of transgenic fish diet by combining multi-tissue histology and liver transcriptome analyses.

Wnt9a Is Required for the Aortic Amplification of Nascent Hematopoietic Stem Cells

All mature blood cell types in the adult animal arise from hematopoietic stem and progenitor cells (HSPCs). However, the developmental cues regulating HSPC ontogeny are incompletely understood. In particular, the details surrounding a requirement for Wnt/β-catenin signaling in the development of mature HSPCs are controversial and difficult to consolidate. Using zebrafish, we demonstrate that Wnt signaling is required to direct an amplification of HSPCs in the aorta. Wnt9a is specifically required for this process and cannot be replaced by Wnt9b or Wnt3a. This proliferative event occurs independently of initial HSPC fate specification, and the Wnt9a input is required prior to aorta formation. HSPC arterial amplification occurs prior to seeding of secondary hematopoietic tissues and proceeds, in part, through the cell cycle regulator myca (c-myc). Our results support a general paradigm, in which early signaling events, including Wnt, direct later HSPC developmental processes.

Hematopoietic stem and progenitor cells (HSPCs) give rise to all of the blood cells of the adult organism; however, how these cells are derived in vivo is still incompletely understood. Using zebrafish, Grainger et al. find that Wnt9a mediates amplification of HSPCs prior to their migration to secondary hematopoietic sites.

Re-evaluating the functional landscape of the cardiovascular system during development

The cardiovascular system facilitates body-wide distribution of oxygen, a vital process for the development and survival of virtually all vertebrates. However, the zebrafish, a vertebrate model organism, appears to form organs and survive mid-larval periods without a functional cardiovascular system. Despite such dispensability, it is the first organ to develop. Such enigma prompted us to hypothesize other cardiovascular functions that are important for developmental and/or physiological processes. Hence, systematic cellular ablations and functional perturbations were performed on the zebrafish cardiovascular system to gain comprehensive and body-wide understanding of such functions and to elucidate the underlying mechanisms. This approach identifies a set of organ-specific genes, each implicated for important functions. The study also unveils distinct cardiovascular mechanisms, each differentially regulating their expressions in organ-specific and oxygen-independent manners. Such mechanisms are mediated by organ-vessel interactions, circulation-dependent signals, and circulation-independent beating-heart-derived signals. A comprehensive and body-wide functional landscape of the cardiovascular system reported herein may provide clues as to why it is the first organ to develop. Furthermore, these data could serve as a resource for the study of organ development and function.

Summary: The body-wide landscape of the cardiovascular functions during development is reported. Such landscape may provide clues as to why the cardiovascular system is the first organ to develop.

Endothelial cells are not required for specification of respiratory progenitors

Crosstalk between mesenchymal and epithelial cells influences organogenesis in multiple tissues, such as lung, pancreas, liver, and the nervous system. Lung mesenchyme comprises multiple cell types, however, and precise identification of the mesenchymal cell type(s) that drives early events in lung development remains unknown. Endothelial cells have been shown to be required for some aspects of lung epithelial patterning, lung stem cell differentiation, and regeneration after injury. Furthermore, endothelial cells are involved in early liver and pancreas development. From these observations we hypothesized that endothelial cells might also be required for early specification of the respiratory field and subsequent lung bud initiation. We first blocked VEGF signaling in E8.5 cultured foreguts with small molecule VEGFR inhibitors and found that lung specification and bud formation were unaltered. However, when we examined E9.5 mouse embryos carrying a mutation in the VEGFR Flk-1, which do not develop endothelial cells, we found that respiratory progenitor specification was impeded. Because the E9.5 embryos were substantially smaller than control littermates, suggesting the possibility of developmental delay, we isolated and cultured foreguts from mutant and control embryos on E8.5, when no size differences were apparent. We found that both specification of the respiratory field and lung bud formation occurred in mutant and control explants. These observations were unaffected by the presence or absence of serum. We also observed that hepatic specification and initiation occurred in the absence of endothelial cells, and that expansion of the liver epithelium in culture did not differ between mutant and control explants. Consistent with previously published results, we also found that pancreatic buds were not maintained in cultured foreguts when endothelial cells were absent. Our observations support the conclusion that endothelial cells are not required for early specification of lung progenitors and bud initiation, and that the diminished lung specification seen in E9.5 Flk-/- embryos is likely due to developmental delay resulting from the insufficient delivery of oxygen, nutrients, and other factors in the absence of a vasculature.

Bipotent progenitors as embryonic origin of retinal stem cells

In lower vertebrates, retinal stem cells (RSCs) capable of producing all retinal cell types are a resource for retinal tissue growth throughout life. However, the embryonic origin of RSCs remains largely elusive. Using a Zebrabow-based clonal analysis, we characterized the RSC niche in the ciliary marginal zone of zebrafish retina and illustrate that blood vessels associated with RSCs are required for the maintenance of actively proliferating RSCs. Full lineage analysis of RSC progenitors reveals lineage patterns of RSC production. Moreover, in vivo lineage analysis demonstrates that these RSC progenitors are the direct descendants of a set of bipotent progenitors in the medial epithelial layer of developing optic vesicles, suggesting the involvement of the mixed-lineage states in the RSC lineage specification.

Disruption of pdgfra alters endocardial and myocardial fusion during zebrafish cardiac assembly

Cardiac development in vertebrates is a finely tuned process regulated by a set of conserved signaling pathways. Perturbations of these processes are often associated with congenital cardiac malformations. Platelet-derived growth factor receptor α (PDGFRα) is a highly conserved tyrosine kinase receptor, which is essential for development and organogenesis. Disruption of Pdgfrα function in murine models is embryonic lethal due to severe cardiovascular defects, suggesting a role in cardiac development, thus necessitating the use of alternative models to explore its precise function. In this study, we generated a zebrafish pdgfra mutant line by gene trapping, in which the Pdgfra protein is truncated and fused with mRFP (Pdgfra-mRFP). Our results demonstrate that pdgfra mutants have defects in cardiac morphology as a result of abnormal fusion of myocardial precursors. Expression analysis of the developing heart at later stages suggested that Pdgfra-mRFP is expressed in the endocardium. Further examination of the endocardium in pdgfra mutants revealed defective endocardial migration to the midline, where cardiac fusion eventually occurs. Together, our data suggests that pdgfra is required for proper medial migration of both endocardial and myocardial precursors, an essential step required for cardiac assembly and development.

Summary: The molecular mechanisms regulating cardiac fusion are not well understood. Here, we show that platelet-derived growth factor receptor alpha is essential for normal endocardial and myocardial fusion during zebrafish development.

Development: For cloche the Bell Tolls

A recent publication identifies npas4l as the gene defective in the well-known cloche mutant that lacks most endothelial as well as hematopoietic cells. This work poses intriguing questions as to the genetic and molecular nature of the origin of hemato-vascular lineages during early embryogenesis.

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.

The hyaloid vasculature facilitates basement membrane breakdown during choroid fissure closure in the zebrafish eye

A critical aspect of vertebrate eye development is closure of the choroid fissure (CF). Defects in CF closure result in colobomas, which are a significant cause of childhood blindness worldwide. Despite the growing number of mutated loci associated with colobomas, we have a limited understanding of the cell biological underpinnings of CF closure. Here, we utilize the zebrafish embryo to identify key phases of CF closure and regulators of the process. Utilizing Laminin-111 as a marker for the basement membrane (BM) lining the CF, we determine the spatial and temporal patterns of BM breakdown in the CF, a prerequisite for CF closure. Similarly, utilizing a combination of in vivo time-lapse imaging, β-catenin immunohistochemistry and F-actin staining, we determine that tissue fusion, which serves to close the fissure, follows BM breakdown closely. Periocular mesenchyme (POM)-derived endothelial cells, which migrate through the CF to give rise to the hyaloid vasculature, possess distinct actin foci that correlate with regions of BM breakdown. Disruption of talin1, which encodes a regulator of the actin cytoskeleton, results in colobomas and these correlate with structural defects in the hyaloid vasculature and defects in BM breakdown. cloche mutants, which entirely lack a hyaloid vasculature, also possess defects in BM breakdown in the CF. Taken together, these data support a model in which the hyaloid vasculature and/or the POM-derived endothelial cells that give rise to the hyaloid vasculature contribute to BM breakdown during CF closure.

Cloche is a bHLH-PAS transcription factor that drives haemato-vascular specification

Vascular and haematopoietic cells organize into specialized tissues during early embryogenesis to supply essential nutrients to all organs and thus play critical roles in development and disease. At the top of the haemato-vascular specification cascade lies cloche, a gene that when mutated in zebrafish leads to the striking phenotype of loss of most endothelial and haematopoietic cells and a significant increase in cardiomyocyte numbers. Although this mutant has been analysed extensively to investigate mesoderm diversification and differentiation and continues to be broadly used as a unique avascular model, the isolation of the cloche gene has been challenging due to its telomeric location. Here we used a deletion allele of cloche to identify several new cloche candidate genes within this genomic region, and systematically genome-edited each candidate. Through this comprehensive interrogation, we succeeded in isolating the cloche gene and discovered that it encodes a PAS-domain-containing bHLH transcription factor, and that it is expressed in a highly specific spatiotemporal pattern starting during late gastrulation. Gain-of-function experiments show that it can potently induce endothelial gene expression. Epistasis experiments reveal that it functions upstream of etv2 and tal1, the earliest expressed endothelial and haematopoietic transcription factor genes identified to date. A mammalian cloche orthologue can also rescue blood vessel formation in zebrafish cloche mutants, indicating a highly conserved role in vertebrate vasculogenesis and haematopoiesis. The identification of this master regulator of endothelial and haematopoietic fate enhances our understanding of early mesoderm diversification and may lead to improved protocols for the generation of endothelial and haematopoietic cells in vivo and in vitro.

4-Dimensional light-sheet microscopy to elucidate shear stress modulation of cardiac trabeculation

Hemodynamic shear forces are intimately linked with cardiac development, during which trabeculae form a network of branching outgrowths from the myocardium. Mutations that alter Notch signaling also result in trabeculation defects. Here, we assessed whether shear stress modulates trabeculation to influence contractile function. Specifically, we acquired 4D (3D + time) images with light sheets by selective plane illumination microscopy (SPIM) for rapid scanning and deep axial penetration during zebrafish morphogenesis. Reduction of blood viscosity via gata1a morpholino oligonucleotides (MO) reduced shear stress, resulting in downregulation of Notch signaling and attenuation of trabeculation. Arrest of cardiomyocyte contraction either by troponin T type 2a (tnnt2a) MO or in weak atriumm58 (wea) mutants resulted in reduced shear stress and downregulation of Notch signaling and trabeculation. Integrating 4D SPIM imaging with synchronization algorithm demonstrated that coinjection of neuregulin1 mRNA with gata1 MO rescued trabeculation to restore contractile function in association with upregulation of Notch-related genes. Crossbreeding of Tg(flk:mCherry) fish, which allows visualization of the vascular system with the Tg(tp1:gfp) Notch reporter line, revealed that shear stress-mediated Notch activation localizes to the endocardium. Deleting endocardium via the clochesk4 mutants downregulated Notch signaling, resulting in nontrabeculated ventricle. Subjecting endothelial cells to pulsatile flow in the presence of the ADAM10 inhibitor corroborated shear stress-activated Notch signaling to modulate trabeculation.

SCL/TAL1 in Hematopoiesis and Cellular Reprogramming

SCL, a transcription factor of the basic helix-loop-helix family, is a master regulator of hematopoiesis. Scl specifies lateral plate mesoderm to a hematopoietic fate and establishes boundaries by inhibiting the cardiac lineage. A combinatorial interaction between Scl and Vegfa/Flk1 sets in motion the first wave of primitive hematopoiesis. Subsequently, definitive hematopoietic stem cells (HSCs) emerge from the embryo proper via an endothelial-to-hematopoietic transition controlled by Runx1, acting with Scl and Gata2. Past this stage, Scl in steady state HSCs is redundant with Lyl1, a highly homologous factor. However, Scl is haploinsufficient in stress response, when a rare subpopulation of HSCs with very long term repopulating capacity is called into action. SCL activates transcription by recruiting a core complex on DNA that necessarily includes E2A/HEB, GATA1-3, LIM-only proteins LMO1/2, LDB1, and an extended complex comprising ETO2, RUNX1, ERG, or FLI1. These interactions confer multifunctionality to a complex that can control cell proliferation in erythroid progenitors or commitment to terminal differentiation through variations in single component. Ectopic SCL and LMO1/2 expression in immature thymocytes activates of a stem cell gene network and reprogram cells with a finite lifespan into self-renewing preleukemic stem cells (pre-LSCs), an initiating event in T-cell acute lymphoblastic leukemias. Interestingly, fate conversion of fibroblasts to hematoendothelial cells requires not only Scl and Lmo2 but also Gata2, Runx1, and Erg, indicating a necessary collaboration between these transcription factors for hematopoietic reprogramming. Nonetheless, full reprogramming into self-renewing multipotent HSCs may require additional factors and most likely, a permissive microenvironment.

Macrophage Recruitment Contributes to Regeneration of Mechanosensory Hair Cells in the Zebrafish Lateral Line

In vertebrates, damage to mechanosensory hair cells elicits an inflammatory response, including rapid recruitment of macrophages and neutrophils. While hair cells in amniotes usually become permanently lost, they readily regenerate in lower vertebrates such as fish. Damage to hair cells of the fish lateral line is followed by inflammation and rapid regeneration; however the role of immune cells in this process remains unknown. Here, we show that recruited macrophages are required for normal regeneration of lateral line hair cells after copper damage. We found that genetic ablation or local ablation using clodronate liposomes of macrophages recruited to the site of injury, significantly delays hair cell regeneration. Neutrophils, on the other hand, are not needed for this process. We anticipate our results to be a starting point for a more detailed description of extrinsic signals important for regeneration of mechanosensory cells in vertebrates. J. Cell. Biochem. 117: 1880-1889, 2016. © 2016 Wiley Periodicals, Inc.

Recent findings on vertebrate developmental immunity using the zebrafish model

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

Abnormal retinal development in Cloche mutant zebrafish

BACKGROUND

Functions for the early embryonic vasculature in regulating development of central nervous system tissues, such as the retina, have been suggested by in vitro studies and by in vivo manipulations that caused additional ocular vessels to develop. Here, we use an avascular zebrafish embryo, cloche-/- (clo-/-), to begin to identify necessary developmental functions of the ocular vasculature in regulating development and patterning of the neural retina, in vivo. These studies are possible in zebrafish embryos, which do not yet rely upon the vasculature for tissue oxygenation.

RESULTS

clo-/- embryos lacked early ocular vasculature and were microphthalmic, with reduced retinal cell proliferation and cell survival. Retinas of clo mutants were disorganized, with irregular synaptic layers, mispatterned expression domains of retinal transcription factors, morphologically abnormal Müller glia, reduced differentiation of specific retinal cell types, and sporadically distributed cone photoreceptors. Blockade of p53-mediated cell death did not completely rescue this phenotype and revealed ectopic cones in the inner nuclear layer. clo-/- embryos did not upregulate a molecular marker for hypoxia.

CONCLUSIONS

The disorganized retinal phenotype of clo-/- embryos is consistent with a neural and glial developmental patterning role for the early ocular vasculature that is independent of its eventual function in gas exchange.

Renin expression in developing zebrafish is associated with angiogenesis and requires the Notch pathway and endothelium

Although renin is a critical regulatory enzyme of the cardiovascular system, its roles in organogenesis and the establishment of cardiovascular homeostasis remain unclear. Mammalian renin-expressing cells are widespread in embryonic kidneys but are highly restricted, specialized endocrine cells in adults. With a functional pronephros, embryonic zebrafish are ideal for delineating the developmental functions of renin-expressing cells and the mechanisms governing renin transcription. Larval zebrafish renin expression originates in the mural cells of the juxtaglomerular anterior mesenteric artery and subsequently at extrarenal sites. The role of renin was determined by assessing responses to renin-angiotensin system blockade, salinity variation, and renal perfusion ablation. Renin expression did not respond to renal flow ablation but was modulated by inhibition of angiotensin-converting enzyme and altered salinity. Our data in larval fish are consistent with conservation of renin's physiological functions. Using transgenic renin reporter fish, with mindbomb and cloche mutants, we show that Notch signaling and the endothelium are essential for developmental renin expression. After inhibition of angiogenesis, renin-expressing cells precede angiogenic sprouts. Arising from separate lineages, but relying on mutual interplay with endothelial cells, renin-expressing cells are among the earliest mural cells observed in larval fish, performing both endocrine and paracrine functions.

Whole-mount single molecule FISH method for zebrafish embryo

Noise in gene expression renders cells more adaptable to changing environment by imposing phenotypic and functional heterogeneity on genetically identical individual cells. Hence, quantitative measurement of noise in gene expression is essential for the study of biological processes in cells. Currently, there are two complementary methods for quantitatively measuring noise in gene expression at the single cell level: single molecule FISH (smFISH) and single cell qRT-PCR (or single cell RNA-seq). While smFISH has been developed for culture cells, tissue sections and whole-mount invertebrate organisms, the method has not been reported for whole-mount vertebrate organisms. Here, we report an smFISH method that is suitable for whole-mount zebrafish embryo, a popular vertebrate model organism for the studies of development, physiology and disease. We show the detection of individual transcripts for several cell-type specific and ubiquitously expressed genes at the single cell level in whole-mount zebrafish embryo. We also demonstrate that the method can be adapted to detect two different genes in individual cells simultaneously. The whole-mount smFISH method described in this report is expected to facilitate the study of noise in gene expression and its role in zebrafish, a vertebrate animal model relevant to human biology.

Zebrafish models of dyslipidemia: relevance to atherosclerosis and angiogenesis

Lipid and lipoprotein metabolism in zebrafish and in humans are remarkably similar. Zebrafish express all major nuclear receptors, lipid transporters, apolipoproteins and enzymes involved in lipoprotein metabolism. Unlike mice, zebrafish express cetp and the Cetp activity is detected in zebrafish plasma. Feeding zebrafish a high cholesterol diet, without any genetic intervention, results in significant hypercholesterolemia and robust lipoprotein oxidation, making zebrafish an attractive animal model to study mechanisms relevant to early development of human atherosclerosis. These studies are facilitated by the optical transparency of zebrafish larvae and the availability of transgenic zebrafish expressing fluorescent proteins in endothelial cells and macrophages. Thus, vascular processes can be monitored in live animals. In this review article, we discuss recent advances in using dyslipidemic zebrafish in atherosclerosis-related studies. We also summarize recent work connecting lipid metabolism with regulation of angiogenesis, the work that considerably benefited from using the zebrafish model. These studies uncovered the role of aibp, abca1, abcg1, mtp, apoB, and apoC2 in regulation of angiogenesis in zebrafish and paved the way for future studies in mammals, which may suggest new therapeutic approaches to modulation of excessive or diminished angiogenesis contributing to the pathogenesis of human disease.

Transcriptional regulation of endothelial cell and vascular development

The establishment and maintenance of the vascular system is critical for embryonic development and postnatal life. Defects in endothelial cell development and vessel formation and function lead to embryonic lethality and are important in the pathogenesis of vascular diseases. Here, we review the underlying molecular mechanisms of endothelial cell differentiation, plasticity, and the development of the vasculature. This review focuses on the interplay among transcription factors and signaling molecules that specify the differentiation of vascular endothelial cells. We also discuss recent progress on reprogramming of somatic cells toward distinct endothelial cell lineages and its promise in regenerative vascular medicine.

Signalling pathways that control vertebrate haematopoietic stem cell specification

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

Transgenic zebrafish illuminate the dynamics of thyroid morphogenesis and its relationship to cardiovascular development

Among the various organs derived from foregut endoderm, the thyroid gland is unique in that major morphogenic events such as budding from foregut endoderm, descent into subpharyngeal mesenchyme and growth expansion occur in close proximity to cardiovascular tissues. To date, research on thyroid organogenesis was missing one vital tool-a transgenic model that allows to track the dynamic changes in thyroid size, shape and location relative to adjacent cardiovascular tissues in live embryos. In this study, we generated a novel transgenic zebrafish line, tg(tg:mCherry), in which robust and thyroid-specific expression of a membrane version of mCherry enables live imaging of thyroid development in embryos from budding stage throughout formation of functional thyroid follicles. By using various double transgenic models in which EGFP expression additionally labels cardiovascular structures, a high coordination was revealed between thyroid organogenesis and cardiovascular development. Early thyroid development was found to proceed in intimate contact with the distal ventricular myocardium and live imaging confirmed that thyroid budding from the pharyngeal floor is tightly coordinated with the descent of the heart. Four-dimensional imaging of live embryos by selective plane illumination microscopy and 3D-reconstruction of confocal images of stained embryos yielded novel insights into the role of specific pharyngeal vessels, such as the hypobranchial artery (HA), in guiding late thyroid expansion along the pharyngeal midline. An important role of the HA was corroborated by the detailed examination of thyroid development in various zebrafish models showing defective cardiovascular development. In combination, our results from live imaging as well es from 3D-reconstruction of thyroid development in tg(tg:mCherry) embryos provided a first dynamic view of late thyroid organogenesis in zebrafish-a critical resource for the design of future studies addressing the molecular mechanisms of these thyroid-vasculature interactions.

Importance of myocyte-nonmyocyte interactions in cardiac development and disease

Emerging data in the field of cardiac development as well as repair and regeneration indicate a complex and important interplay between endocardial, epicardial, and myofibroblast populations that is critical for cardiomyocyte differentiation and postnatal function. For example, epicardial cells have been shown to generate cardiac myofibroblasts and may be one of the primary sources for this cell lineage during development. Moreover, paracrine signaling from the epicardium and endocardium is critical for proper development of the heart and pathways such as Wnt, fibroblast growth factor, and retinoic acid signaling have been shown to be key players in this process. Despite this progress, interactions between nonmyocyte cells and cardiomyocytes in the heart are still poorly understood. We review the various nonmyocyte-myocyte interactions that occur in the heart and how these interactions, primarily through signaling networks, help direct cardiomyocyte differentiation and regulate postnatal cardiac function.

Transgenic line with gal4 insertion useful to study morphogenesis of craniofacial perichondrium, vascular endothelium-associated cells, floor plate, and dorsal midline radial glia during zebrafish development

Zebrafish is a good model for studying vertebrate development because of the availability of powerful genetic tools. We are interested in the study of the craniofacial skeletal structure of the zebrafish. For this purpose, we performed a gene trap screen and identified a Gal4 gene trap line, SAGFF(LF)134A. We then analyzed the expression pattern of SAGFF(LF)134A;Tg(UAS:GFP) and found that green fluorescent protein (GFP) was expressed not only in craniofacial skeletal elements but also in the vascular system, as well as in the nervous system. In craniofacial skeletal elements, strong GFP expression was detected not only in chondrocytes but also in the perichondrium. In the vascular system, GFP was expressed in endothelium-associated cells. In the spinal cord, strong GFP expression was found in the floor plate, and later in the dorsal radial glia located on the midline. Taking advantage of this transgenic line, which drives Gal4 expression in specific tissues, we crossed SAGFF(LF)134A with several UAS reporter lines. In particular, time-lapse imaging of photoconverted floor-plate cells of SAGFF(LF)134A;Tg(UAS:KikGR) revealed that the floor-plate cells changed their shape within 36 h from cuboidal/trapezoidal to wine glass shaped. Moreover, we identified a novel mode of association between axons and glia. The putative paths for the commissural axons, including pax8-positive CoBL interneurons, were identified as small openings in the basal endfoot of each floor plate. Our results indicate that the transgenic line would be useful for studying the morphogenesis of less-well-characterized tissues of interest, including the perichondrium, dorsal midline radial glia, late-stage floor plate, and vascular endothelium-associated cells.

Retinoic acid signaling plays a restrictive role in zebrafish primitive myelopoiesis

Retinoic acid (RA) is known to regulate definitive myelopoiesis but its role in vertebrate primitive myelopoiesis remains unclear. Here we report that zebrafish primitive myelopoiesis is restricted by RA in a dose dependent manner mainly before 11 hpf (hours post fertilization) when anterior hemangioblasts are initiated to form. RA treatment significantly reduces expressions of anterior hemangioblast markers scl, lmo2, gata2 and etsrp in the rostral end of ALPM (anterior lateral plate mesoderm) of the embryos. The result indicates that RA restricts primitive myelopoiesis by suppressing formation of anterior hemangioblasts. Analyses of ALPM formation suggest that the defective primitive myelopoiesis resulting from RA treatment before late gastrulation may be secondary to global loss of cells for ALPM fate whereas the developmental defect resulting from RA treatment during 10–11 hpf should be due to ALPM patterning shift. Overexpressions of scl and lmo2 partially rescue the block of primitive myelopoiesis in the embryos treated with 250 nM RA during 10–11 hpf, suggesting RA acts upstream of scl to control primitive myelopoiesis. However, the RA treatment blocks the increased primitive myelopoiesis caused by overexpressing gata4/6 whereas the abolished primitive myelopoiesis in gata4/5/6 depleted embryos is well rescued by 4-diethylamino-benzaldehyde, a retinal dehydrogenase inhibitor, or partially rescued by knocking down aldh1a2, the major retinal dehydrogenase gene that is responsible for RA synthesis during early development. Consistently, overexpressing gata4/6 inhibits aldh1a2 expression whereas depleting gata4/5/6 increases aldh1a2 expression. The results reveal that RA signaling acts downstream of gata4/5/6 to control primitive myelopoiesis. But, 4-diethylamino-benzaldehyde fails to rescue the defective primitive myelopoiesis in either cloche embryos or lycat morphants. Taken together, our results demonstrate that RA signaling restricts zebrafish primitive myelopoiesis through acting downstream of gata4/5/6, upstream of, or parallel to, cloche, and upstream of scl.