Notch signaling functions as a cell-fate switch between the endothelial and hematopoietic lineages

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

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

Inflammation in Development and Aging: Insights from the Zebrafish Model

Zebrafish are an emergent animal model to study human diseases due to their significant genetic similarity to humans, swift development, and genetic manipulability. Their utility extends to the exploration of the involvement of inflammation in host defense, immune responses, and tissue regeneration. Additionally, the zebrafish model system facilitates prompt screening of chemical compounds that affect inflammation. This study explored the diverse roles of inflammatory pathways in zebrafish development and aging. Serving as a crucial model, zebrafish provides insights into the intricate interplay of inflammation in both developmental and aging contexts. The evidence presented suggests that the same inflammatory signaling pathways often play instructive or beneficial roles during embryogenesis and are associated with malignancies in adults.

A splicing factor switch controls hematopoietic lineage specification of pluripotent stem cells

Alternative splicing (AS) leads to transcriptome diversity in eukaryotic cells and is one of the key regulators driving cellular differentiation. Although AS is of crucial importance for normal hematopoiesis and hematopoietic malignancies, its role in early hematopoietic development is still largely unknown. Here, by using high-throughput transcriptomic analyses, we show that pervasive and dynamic AS takes place during hematopoietic development of human pluripotent stem cells (hPSCs). We identify a splicing factor switch that occurs during the differentiation of mesodermal cells to endothelial progenitor cells (EPCs). Perturbation of this switch selectively impairs the emergence of EPCs and hemogenic endothelial progenitor cells (HEPs). Mechanistically, an EPC-induced alternative spliced isoform of NUMB dictates EPC specification by controlling NOTCH signaling. Furthermore, we demonstrate that the splicing factor SRSF2 regulates splicing of the EPC-induced NUMB isoform, and the SRSF2-NUMB-NOTCH splicing axis regulates EPC generation. The identification of this splicing factor switch provides a new molecular mechanism to control cell fate and lineage specification.

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.

Anatomical map of the cranial vasculature and sensory ganglia

There is growing evidence of a direct influence of vasculature on the development of neurons in the brain. The development of the cranial vasculature has been well described in zebrafish but its anatomical relationship with the adjacent developing sensory ganglia has not been addressed. Here, by 3D imaging of fluorescently labelled blood vessels and sensory ganglia, we describe for the first time the spatial organization of the cranial vasculature in relation to the cranial ganglia during zebrafish development. We show that from 24 h post-fertilization (hpf) onwards, the statoacoustic ganglion (SAG) develops in direct contact with two main blood vessels, the primordial hindbrain channel and the lateral dorsal aortae (LDA). At 48 hpf, the LDA is displaced medially, losing direct contact with the SAG. The relationship of the other cranial ganglia with the vasculature is evident for the medial lateral line ganglion and for the vagal ganglia that grow along the primary head sinus (PHS). We also observed that the innervation of the anterior macula runs over the PHS vessel. Our spatiotemporal anatomical map of the cranial ganglia and the head vasculature indicates physical interactions between both systems and suggests a possible functional interaction during development.

Dual Roles of Fer Kinase Are Required for Proper Hematopoiesis and Vascular Endothelium Organization during Zebrafish Development

Fer kinase, a protein involved in the regulation of cell-cell adhesion and proliferation, has been shown to be required during invertebrate development and has been implicated in leukemia, gastric cancer, and liver cancer. However, in vivo roles for Fer during vertebrate development have remained elusive. In this study, we bridge the gap between the invertebrate and vertebrate realms by showing that Fer kinase is required during zebrafish embryogenesis for normal hematopoiesis and vascular organization with distinct kinase dependent and independent functions. In situ hybridization, quantitative PCR and fluorescence activated cell sorting (FACS) analyses revealed an increase in both erythrocyte numbers and gene expression patterns as well as a decrease in the organization of vasculature endothelial cells. Furthermore, rescue experiments have shown that the regulation of hematopoietic proliferation is dependent on Fer kinase activity, while vascular organizing events only require Fer in a kinase-independent manner. Our data suggest a model in which separate kinase dependent and independent functions of Fer act in conjunction with Notch activity in a divergent manner for hematopoietic determination and vascular tissue organization.

The molecular regulation of arteriovenous specification and maintenance

The formation of a hierarchical vascular network, composed of arteries, veins, and capillaries, is essential for embryogenesis and is required for the production of new functional vasculature in the adult. Elucidating the molecular mechanisms that orchestrate the differentiation of vascular endothelial cells into arterial and venous cell fates is requisite for regenerative medicine, as the directed formation of perfused vessels is desirable in a myriad of pathological settings, such as in diabetes and following myocardial infarction. Additionally, this knowledge will enhance our understanding and treatment of vascular anomalies, such as arteriovenous malformations (AVMs). From studies in vertebrate model organisms, such as mouse, zebrafish, and chick, a number of key signaling pathways have been elucidated that are required for the establishment and maintenance of arterial and venous fates. These include the Hedgehog, Vascular Endothelial Growth Factor (VEGF), Transforming Growth Factor-β (TGF-β), Wnt, and Notch signaling pathways. In addition, a variety of transcription factor families acting downstream of, or in concert with, these signaling networks play vital roles in arteriovenous (AV) specification. These include Notch and Notch-regulated transcription factors (e.g., HEY and HES), SOX factors, Forkhead factors, β-Catenin, ETS factors, and COUP-TFII. It is becoming apparent that AV specification is a highly coordinated process that involves the intersection and carefully orchestrated activity of multiple signaling cascades and transcriptional networks. This review will summarize the molecular mechanisms that are involved in the acquisition and maintenance of AV fate, and will highlight some of the limitations in our current knowledge of the molecular machinery that directs AV morphogenesis.

Complex regulation of HSC emergence by the Notch signaling pathway

Hematopoietic stem cells are formed during embryonic development, and serve as the foundation of the definitive blood program for life. Notch signaling has been well established as an essential direct contributor to HSC specification. However, several recent studies have indicated that the contribution of Notch signaling is complex. HSC specification requires multiple Notch signaling inputs, some received directly by hematopoietic precursors, and others that occur indirectly within neighboring somites. Of note, proinflammatory signals provided by primitive myeloid cells are needed for HSC specification via upregulation of the Notch pathway in hemogenic endothelium. In addition to multiple requirements for Notch activation, recent studies indicate that Notch signaling must subsequently be repressed to permit HSC emergence. Finally, Notch must then be reactivated to maintain HSC fate. In this review, we discuss the growing understanding of the dynamic contributions of Notch signaling to the establishment of hematopoiesis during development.

An unexpected link between notch signaling and ROS in restricting the differentiation of hematopoietic progenitors in Drosophila

A fundamental question in hematopoietic development is how multipotent progenitors achieve precise identities, while the progenitors themselves maintain quiescence. In Drosophila melanogaster larvae, multipotent hematopoietic progenitors support the production of three lineages, exhibit quiescence in response to cues from a niche, and from their differentiated progeny. Infection by parasitic wasps alters the course of hematopoiesis. Here we address the role of Notch (N) signaling in lamellocyte differentiation in response to wasp infection. We show that Notch activity is moderately high and ubiquitous in all cells of the lymph gland lobes, with crystal cells exhibiting the highest levels. Wasp infection reduces Notch activity, which results in fewer crystal cells and more lamellocytes. Robust lamellocyte differentiation is induced even in N mutants. Using RNA interference knockdown of N, Serrate, and neuralized (neur), and twin clone analysis of a N null allele, we show that all three genes inhibit lamellocyte differentiation. However, unlike its cell-autonomous function in crystal cell development, Notch's inhibitory influence on lamellocyte differentiation is not cell autonomous. High levels of reactive oxygen species in the lymph gland lobes, but not in the niche, accompany N(RNAi)-induced lamellocyte differentiation and lobe dispersal. Our results define a novel dual role for Notch signaling in maintaining competence for basal hematopoiesis: while crystal cell development is encouraged, lamellocytic fate remains repressed. Repression of Notch signaling in fly hematopoiesis is important for host defense against natural parasitic wasp infections. These findings can serve as a model to understand how reactive oxygen species and Notch signals are integrated and interpreted in vivo.

BMP-mediated specification of the erythroid lineage suppresses endothelial development in blood island precursors

The developmental relationship between the blood and endothelial cell (EC) lineages remains unclear. In the extra-embryonic blood islands of birds and mammals, ECs and blood cells are closely intermixed, and blood island precursor cells in the primitive streak express many of the same molecular markers, leading to the suggestion that both lineages arise from a common precursor, called the hemangioblast. Cells within the blood island of Xenopus also coexpress predifferentiation markers of the blood and EC lineages. However, using multiple assays, we find that precursor cells in the Xenopus blood island do not normally differentiate into ECs, suggesting that classic hemangioblasts are rare or nonexistent in Xenopus. What prevents these precursor cells from developing into mature ECs? We have found that bone morphogenetic protein (BMP) signaling is essential for erythroid differentiation, and in the absence of BMP signaling, precursor cells adopt an EC fate. Furthermore, inhibition of the erythroid transcription pathway leads to endothelial differentiation. Our results indicate that bipotential endothelial/erythroid precursor cells do indeed exist in the Xenopus blood island, but BMP signaling normally acts to constrain EC fate. More generally, these results provide evidence that commitment to the erythroid lineage limits development of bipotential precursors toward an endothelial fate.

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.

Stem cells living with a Notch

Notch signaling has been shown over the past few decades to play fundamental roles in a plethora of developmental processes in an evolutionarily conserved fashion. Notch-mediated cell-to-cell signaling is involved in many aspects of embryonic development and control of tissue homeostasis in a variety of adult tissues, and regulates stem cell maintenance, cell differentiation and cellular homeostasis. The focus of this Review is the role of Notch signaling in stem cells, comparing insights from flies, fish and mice to highlight similarities, as well as differences, between species, tissues and stem cell compartments.

Vascular endothelial growth factor signaling regulates the segregation of artery and vein via ERK activity during vascular development

Segregation of two axial vessels, the dorsal aorta and caudal vein, is one of the earliest patterning events occur during development of vasculature. Despite the importance of this process and recent advances in our understanding on vascular patterning during development, molecular mechanisms that coordinate the segregation of axial vessels remain largely elusive. In this report, we find that vascular endothelial growth factor-A (Vegf-A) signaling regulates the segregation of dorsal aorta and axial vein during development. Inhibition of Vegf-A pathway components including ligand Vegf-A and its cognate receptor Kdrl, caused failure in segregation of axial vessels in zebrafish embryos. Similarly, chemical inhibition of Mitogen-activated protein kinase kinase (Map2k1)/Extracellular-signal-regulated kinases (Erk) and phosphatidylinositol 3-kinases (PI3K), which are downstream effectors of Vegf-A signaling pathway, led to the fusion of two axial vessels. Moreover, we find that restoring Erk activity by over-expression of constitutively active MEK in embryos with a reduced level of Vegf-A signaling can rescue the defects in axial vessel segregation. Taken together, our data show that segregation of axial vessels requires the function of Vegf-A signaling, and Erk may function as the major downstream effector in this process.

β(2)-Integrin and Notch-1 differentially regulate CD34(+)CD31(+) cell plasticity in vascular niches

AIMS

The implication of circulating haematopoietic CD34(+) progenitors in the vasculature is unclear due to the lack of understanding of their characteristics and plasticity mediated by their cellular microenvironment. We investigated how vascular smooth muscle cells (SMCs) and their interactions with endothelial cells (ECs) affect the behaviour and plasticity of CD34(+)CD31(+) progenitors and the underlying mechanisms.

METHODS AND RESULTS

Human peripheral blood-derived CD34(+)CD31(+) cells were directly transplanted into injured arteries in vivo and co-cultured with ECs and SMCs in vitro. CD34(+)CD31(+) progenitors injected into wire-injured mouse arteries differentiate into ECs and macrophages in the neoendothelial layer and neointima, respectively. SMC-co-culture increases CD34(+)CD31(+) cell mobility and adhesion to and transmigration across ECs. Sorted CD34(+)CD31(+) progenitors that adhered to ECs co-cultured with SMCs have the capacity to form capillary-like structures in Matrigel and chimeric blood vessels in vivo. Sorted transmigrated progenitors give rise to macrophages with increased pro-angiogenic activity. These differentiations of CD34(+)CD31(+) progenitors into ECs and macrophages are mediated by β(2)-integrin and Notch-1, respectively. β(2)-Integrin and Notch-1 are activated by their counterligands, intercellular adhesion molecule-1 (ICAM-1) and jagged-1, which are highly expressed in the neoendothelium and neointima in injured arteries. Intra-arterial injection of β(2)-integrin-activated CD34(+)CD31(+) progenitors into wire-injured mouse arteries inhibits neointima formation.

CONCLUSION

Our findings indicate that the peripheral vascular niches composed of ECs and SMCs may predispose haematopoietic CD34(+)CD31(+) progenitors to differentiate into ECs and macrophages through the activations of the ICAM-1/β(2)-integrin and jagged-1/Notch-1 cascades, respectively.

Hematopoietic stem cell development and regulatory signaling in zebrafish

BACKGROUND

Hematopoietic stem cells (HSCs) are a population of multipotent cells that can self-renew and differentiate into all blood lineages. HSC development must be tightly controlled from cell fate determination to self-maintenance during adulthood. This involves a panel of important developmental signaling pathways and other factors which act synergistically within the HSC population and/or in the HSC niche. Genetically conserved processes of HSC development plus many other developmental advantages make the zebrafish an ideal model organism to elucidate the regulatory mechanisms underlying HSC programming.

SCOPE OF REVIEW

This review summarizes recent progress on zebrafish HSCs with particular focus on how developmental signaling controls hemogenic endothelium-derived HSC development. We also describe the interaction of different signaling pathways during these processes.

MAJOR CONCLUSIONS

The hematopoietic stem cell system is a paradigm for stem cell studies. Use of the zebrafish model to study signaling regulation of HSCs in vivo has resulted in a great deal of information concerning HSC biology in vertebrates.

GENERAL SIGNIFICANCE

These new findings facilitate a better understanding of molecular mechanisms of HSC programming, and will provide possible new strategies for the treatment of HSC-related hematological diseases, such as leukemia. This article is part of a Special Issue entitled Biochemistry of Stem Cells.

On the origin of hematopoietic stem cells: progress and controversy

Hematopoietic Stem Cells (HSCs) are responsible for the production and replenishment of all blood cell types during the entire life of an organism. Generated during embryonic development, HSCs transit through different anatomical niches where they will expand before colonizing in the bone marrow, where they will reside during adult life. Although the existence of HSCs has been known for more than fifty years and despite extensive research performed in different animal models, there is still uncertainty with respect to the precise origins of HSCs. We review the current knowledge on embryonic hematopoiesis and highlight the remaining questions regarding the anatomical and cellular identities of HSC precursors.

Developmental and pathological angiogenesis

The formation of the vascular network is an intricate and complex process that is an obligate requirement during vertebrate development. The cardiovascular system is the first organ to develop and reach a functional state, which underscores the crucial role of the vasculature in the developing embryo. The development of the vasculature into highly branched conduits needs to occur in numerous sites and in precise patterns to supply oxygen and nutrients to the rapidly expanding tissue of the embryo. This process is mediated by the coordinated response of vascular endothelial and mural cells to the heterogeneous angiogenic cues provided by tissues and organs, whereas aberrant regulation and coordination of angiogenic signals during development result in lethality, impaired organ development, or disease states. This article reviews the essential signaling pathways required for establishment of the vertebrate vasculature with a major focus on a key regulatory factor, vascular endothelial growth factor (VEGF). We also discuss current knowledge of physiological angiogenic processes as well as their disruptions in pathological processes, particularly tumorigenesis.

Functional characterization and expression profiling of human induced pluripotent stem cell- and embryonic stem cell-derived endothelial cells

With regard to human induced pluripotent stem cells (hiPSCs), in which adult cells are reprogrammed into embryonic-like cells using defined factors, their functional and transcriptional expression pattern during endothelial differentiation has yet to be characterized. In this study, hiPSCs and human embryonic stem cells (hESCs) were differentiated using the embryoid body method, and CD31(+) cells were sorted. Fluorescence activated cell sorting analysis of hiPSC-derived endothelial cells (hiPSC-ECs) and hESC-derived endothelial cells (hESC-ECs) demonstrated similar endothelial gene expression patterns. We showed functional vascular formation by hiPSC-ECs in a mouse Matrigel plug model. We compared the gene profiles of hiPSCs, hESCs, hiPSC-ECs, hESC-ECs, and human umbilical vein endothelial cells (HUVECs) using whole genome microarray. Our analysis demonstrates that gene expression variation of hiPSC-ECs and hESC-ECs contributes significantly to biological differences between hiPSC-ECs and hESC-ECs as well as to the "distances" among hiPSCs, hESCs, hiPSC-ECs, hESC-ECs, and HUVECs. We further conclude that hiPSCs can differentiate into functional endothelial cells, but with limited expansion potential compared with hESC-ECs; thus, extensive studies should be performed to explore the cause and extent of such differences before clinical application of hiPSC-ECs can begin.

Fli+ etsrp+ hemato-vascular progenitor cells proliferate at the lateral plate mesoderm during vasculogenesis in zebrafish

BACKGROUND

Vasculogenesis, the de novo formation of blood vessels from precursor cells is critical for a developing embryo. However, the signals and events that dictate the formation of primary axial vessels remain poorly understood.

METHODOLOGY/PRINCIPAL FINDINGS

In this study, we use ets-related protein-1 (etsrp), which is essential for vascular development, to analyze the early stages of vasculogenesis in zebrafish. We found etsrp(+) cells of the head, trunk and tail follow distinct developmental sequences. Using a combination of genetic, molecular and chemical approaches, we demonstrate that fli(+)etsrp(+) hemato-vascular progenitors (FEVPs) are proliferating at the lateral plate mesoderm (LPM). The Shh-VEGF-Notch-Hey2 signaling pathway controls the proliferation process, and experimental modulation of single components of this pathway alters etsrp(+) cell numbers at the LPM.

CONCLUSIONS/SIGNIFICANCE

This study for the first time defines factors controlling proliferation, and cell numbers of pre-migratory FEVPs in zebrafish.