A multistep computational approach reveals a neuro-mesenchymal cell population in the embryonic hematopoietic stem cell niche.

The first hematopoietic stem and progenitor cells (HSPCs) emerge in the Aorta-Gonad-Mesonephros (AGM) region of the mid-gestation mouse embryo. However, the precise nature of their supportive mesenchymal microenvironment remains largely unexplored. Here, we profiled transcriptomes of laser micro-dissected aortic tissues at three developmental stages and individual AGM cells. Computational analyses allowed the identification of several cell subpopulations within the E11.5 AGM mesenchyme, with the presence of a yet unidentified subpopulation characterized by the dual expression of genes implicated in adhesive or neuronal functions. We confirmed the identity of this cell subset as a neuro-mesenchymal population, through morphological and lineage tracing assays. Loss of function in the zebrafish confirmed that Decorin, a characteristic extracellular matrix component of the neuro-mesenchyme, is essential for HSPC development. We further demonstrated that this cell population is not merely derived from the neural crest, and hence, is a bona fide novel subpopulation of the AGM mesenchyme.

Dermomyotome-derived endothelial cells migrate to the dorsal aorta to support hematopoietic stem cell emergence.

Development of the dorsal aorta is a key step in the establishment of the adult blood-forming system, since hematopoietic stem and progenitor cells (HSPCs) arise from ventral aortic endothelium in all vertebrate animals studied. Work in zebrafish has demonstrated that arterial and venous endothelial precursors arise from distinct subsets of lateral plate mesoderm. Here, we profile the transcriptome of the earliest detectable endothelial cells (ECs) during zebrafish embryogenesis to demonstrate that tissue-specific EC programs initiate much earlier than previously appreciated, by the end of gastrulation. Classic studies in the chick embryo showed that paraxial mesoderm generates a subset of somite-derived endothelial cells (SDECs) that incorporate into the dorsal aorta to replace HSPCs as they exit the aorta and enter circulation. We describe a conserved program in the zebrafish, where a rare population of endothelial precursors delaminates from the dermomyotome to incorporate exclusively into the developing dorsal aorta. Although SDECs lack hematopoietic potential, they act as a local niche to support the emergence of HSPCs from neighboring hemogenic endothelium. Thus, at least three subsets of ECs contribute to the developing dorsal aorta: vascular ECs, hemogenic ECs, and SDECs. Taken together, our findings indicate that the distinct spatial origins of endothelial precursors dictate different cellular potentials within the developing dorsal aorta.

Zebrafish snai2 mutants fail to phenocopy morphant phenotypes.

Snail2 is a zinc-finger transcription factor best known to repress expression of genes encoding cell adherence proteins to facilitate induction of the epithelial-to-mesenchymal transition. While this role has been best documented in the developmental migration of the neural crest and mesoderm, here we expand on previously reported preliminary findings that morpholino knock-down of snai2 impairs the generation of hematopoietic stem cells (HSCs) during zebrafish development. We demonstrate that snai2 morphants fail to initiate HSC specification and show defects in the somitic niche of migrating HSC precursors. These defects include a reduction in sclerotome markers as well as in the Notch ligands dlc and dld, which are known to be essential components of HSC specification. Accordingly, enforced expression of the Notch1-intracellular domain was capable of rescuing HSC specification in snai2 morphants. To parallel our approach, we obtained two mutant alleles of snai2. In contrast to the morphants, homozygous mutant embryos displayed no defects in HSC specification or in sclerotome development, and mutant fish survive into adulthood. However, when these homozygous mutants were injected with snai2 morpholino, HSCs were improperly specified. In summary, our morpholino data support a role for Snai2 in HSC development, whereas our mutant data suggest that Snai2 is dispensable for this process. Together, these findings further support the need for careful consideration of both morpholino and mutant phenotypes in studies of gene function.

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/ß-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.

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.

Gata2b is a restricted early regulator of hemogenic endothelium in the zebrafish embryo.

The adult blood system is established by hematopoietic stem cells (HSCs), which arise during development from an endothelial-to-hematopoietic transition of cells comprising the floor of the dorsal aorta. Expression of aortic runx1 has served as an early marker of HSC commitment in the zebrafish embryo, but recent studies have suggested that HSC specification begins during the convergence of posterior lateral plate mesoderm (PLM), well before aorta formation and runx1 transcription. Further understanding of the earliest stages of HSC specification necessitates an earlier marker of hemogenic endothelium. Studies in mice have suggested that GATA2 might function at early stages within hemogenic endothelium. Two orthologs of Gata2 exist in zebrafish: gata2a and gata2b. Here, we report that gata2b expression initiates during the convergence of PLM, becoming restricted to emerging HSCs. We observe Notch-dependent gata2b expression within the hemogenic subcompartment of the dorsal aorta that is in turn required to initiate runx1 expression. Our results indicate that Gata2b functions within hemogenic endothelium from an early stage, whereas Gata2a functions more broadly throughout the vascular system.

FGF signalling specifies haematopoietic stem cells through its regulation of somitic Notch signalling.

Haematopoietic stem cells (HSCs) derive from haemogenic endothelial cells of the primitive dorsal aorta (DA) during vertebrate embryogenesis. The molecular mechanisms governing this unique endothelial to haematopoietic transition remain unclear. Here, we demonstrate a novel requirement for fibroblast growth factor (FGF) signalling in HSC emergence. This requirement is non-cell-autonomous, and acts within the somite to bridge the Wnt and Notch signalling pathways. We previously demonstrated that Wnt16 regulates the somitic expression of two Notch ligands, deltaC (dlc) and deltaD (dld), whose combined function is required for HSC fate. How Wnt16 connects to Notch function has remained an open question. Our current studies demonstrate that FGF signalling, via FGF receptor 4 (Fgfr4), mediates a signal-transduction pathway between Wnt16 and Dlc, but not Dld, to regulate HSC specification. Our findings demonstrate that FGF signalling acts as a key molecular relay within the developmental HSC niche to instruct HSC fate.

FGF signalling restricts haematopoietic stem cell specification via modulation of the BMP pathway.

Haematopoietic stem cells (HSCs) are produced during embryogenesis from the floor of the dorsal aorta. The localization of HSCs is dependent on the presence of instructive signals on the ventral side of the vessel. The nature of the extrinsic molecular signals that control the aortic haematopoietic niche is currently poorly understood. Here we demonstrate a novel requirement for FGF signalling in the specification of aortic haemogenic endothelium. Our results demonstrate that FGF signalling normally acts to repress BMP activity in the subaortic mesenchyme through transcriptional inhibition of bmp4, as well as through activation of two BMP antagonists, noggin2 and gremlin1a. Taken together, these findings demonstrate a key role for FGF signalling in establishment of the developmental HSC niche via its regulation of BMP activity in the subaortic mesenchyme. These results should help inform strategies to recapitulate the development of HSCs in vitro from pluripotent precursors.

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 and requires intimate contact between signal-emitting and signal-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 specification. It has remained unclear, however, how these somitic requirements are connected to the later emergence of HSCs from the dorsal aorta. Here we show in zebrafish 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 (also known as f11r) and migrate axially across the ventral somite, where Jam2a and the 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 dorsal aorta.

A systems biology approach for defining the molecular framework of the hematopoietic stem cell niche.

Despite progress in identifying the cellular composition of hematopoietic stem/progenitor cell (HSPC) niches, little is known about the molecular requirements of HSPC support. To address this issue, we used a panel of six recognized HSPC-supportive stromal lines and less-supportive counterparts originating from embryonic and adult hematopoietic sites. Through comprehensive transcriptomic meta-analyses, we identified 481 mRNAs and 17 microRNAs organized in a modular network implicated in paracrine signaling. Further inclusion of 18 additional cell strains demonstrated that this mRNA subset was predictive of HSPC support. Our gene set contains most known HSPC regulators as well as a number of unexpected ones, such as Pax9 and Ccdc80, as validated by functional studies in zebrafish embryos. In sum, our approach has identified the core molecular network required for HSPC support. These cues, along with a searchable web resource, will inform ongoing efforts to instruct HSPC ex vivo amplification and formation from pluripotent precursors.

The gata1/pu.1 lineage fate paradigm varies between blood populations and is modulated by tif1γ.

Lineage fate decisions underpin much of development as well as tissue homeostasis in the adult. A mechanistic paradigm for such decisions is the erythroid versus myeloid fate decision controlled by cross-antagonism between gata1 and pu.1 transcription factors. In this study, we have systematically tested this paradigm in blood-producing populations in zebrafish embryos, including the haematopoietic stem cells (HSCs), and found that it takes a different form in each population. In particular, gata1 activity varies from autostimulation to autorepression. In addition, we have added a third member to this regulatory kernel, tif1γ (transcription intermediate factor-1γ). We show that tif1γ modulates the erythroid versus myeloid fate outcomes from HSCs by differentially controlling the levels of gata1 and pu.1. By contrast, tif1γ positively regulates both gata1 and pu.1 in primitive erythroid and prodefinitive erythromyeloid progenitors. We therefore conclude that the gata1/pu.1 paradigm for lineage decisions takes different forms in different cellular contexts and is modulated by tif1γ.

Aortic remodelling during hemogenesis: is the chicken paradigm unique?

Since the era of the ancient Egyptians and Greeks, the avian embryo has been a subject of intense interest to visualize the first steps of development. It has served as a pioneer model to scrutinize the question of hematopoietic development from the beginning of the 20th century. It's large size and easy accessibility have permitted the development of techniques dedicated to following the origins and fates of different cell populations. Here, we shall review how the avian model has brought major contributions to our understanding of the development of the hematopoietic system in the past four decades and how these discoveries have influenced our knowledge of mammalian hematopoietic development. The discovery of an intra-embryonic source of hematopoietic cells and the developmental link between endothelial cells and hematopoietic cells will be presented. We shall then point to the pivotal role of the somite in the construction of the aorta and hematopoietic production and demonstrate how two somitic compartments cooperate to construct the definitive aorta. We shall finish by showing how fate-mapping experiments have allowed the identification of the tissue which gives rise to the sub-aortic mesenchyme. Taken together, this review aims to give an overview of how and to what extent the avian embryo has contributed to our knowledge of developmental hematopoiesis.

Hedgehog and Bmp polarize hematopoietic stem cell emergence in the zebrafish dorsal aorta.

Hematopoietic stem cells (HSCs) are first detected in the floor of the embryonic dorsal aorta (DA), and we investigate the signals that induce the HSC program there. We show that while continued Hedgehog (Hh) signaling from the overlying midline structures maintains the arterial program characteristic of the DA roof, a ventral Bmp4 signal induces the blood stem cell program in the DA floor. This patterning of the DA by Hh and Bmp is the mirror image of that in the neural tube, with Hh favoring dorsal rather than ventral cell types, and Bmp favoring ventral rather than dorsal. With the majority of current data supporting a model whereby HSCs derive from arterial endothelium, our data identify the signal driving this conversion. These findings are important for the study of the production of HSCs from embryonic stem cells and establish a paradigm for the development of adult stem cells.

Are intra-aortic hemopoietic cells derived from endothelial cells during ontogeny?

The aorta is recognized as an intraembryonic site that produces adult-type hemopoietic stem cells. A corpus of data indicates that hemopoietic cells arranged as clusters attached to the aortic floor derive from an endothelial intermediate. This review reports on experimental approaches carried out in the avian embryo to establish the developmental history of the aortic endothelium and trace the origin of associated hemopoietic cells.

Somite-derived cells replace ventral aortic hemangioblasts and provide aortic smooth muscle cells of the trunk.

We have previously shown that endothelial cells of the aortic floor give rise to hematopoietic cells, revealing the existence of an aortic hemangioblast. It has been proposed that the restriction of hematopoiesis to the aortic floor is based on the existence of two different and complementary endothelial lineages that form the vessel: one originating from the somite would contribute to the roof and sides, another from the splanchnopleura would contribute to the floor. Using quail/chick orthotopic transplantations of paraxial mesoderm, we have traced the distribution of somite-derived endothelial cells during aortic hematopoiesis. We show that the aortic endothelium undergoes two successive waves of remodeling by somitic cells: one when the aortae are still paired, during which the initial roof and sides of the vessels are renewed; and a second, associated to aortic hematopoiesis, in which the hemogenic floor is replaced by somite endothelial cells. This floor thus appears as a temporary structure, spent out and replaced. In addition, the somite contributes to smooth muscle cells of the aorta. In vivo lineage tracing experiments with non-replicative retroviral vectors showed that endothelial cells do not give rise to smooth muscle cells. However, in vitro, purified endothelial cells acquire smooth muscle cells characteristics. Taken together, these data point to the crucial role of the somite in shaping the aorta and also give an explanation for the short life of aortic hematopoiesis.

From hemangioblast to hematopoietic stem cell: an endothelial connection?

The developmental origin of hematopoietic stem cells has been the subject of much research. Now that the developmental link between the hematopoietic system and the vasculature has been well established, questions remain regarding the precise cellular origin of definitive hematopoietic cells and at what point they branch off from the endothelial lineage. Do they emerge directly from a hemangioblast-type cell, similar to what is proposed for primitive yolk sac hematopoiesis, or are they generated via an endothelial intermediate, the hemogenic endothelium? In this review, we will give an overview of the data obtained from the mouse and avian models on the cellular origins of the hematopoietic system.

The embryonic origins of hematopoietic stem cells: a tale of hemangioblast and hemogenic endothelium.

The developmental origin of hematopoietic stem cells has been for decades the subject of great interest. Once thought to emerge from the yolk sac, hematopoietic stem cells have now been shown to originate from the embryonic aorta. Increasing evidence suggests that hematopoietic stem cells are produced from an endothelial intermediate designated by the authors as hemangioblast or hemogenic endothelium. Recently, the allantois in the avian embryo and the placenta in the mouse embryo were shown to be a site of hematopoietic cell production/expansion and thus appear to play a critical role in the formation of the hematopoietic system. In this review we shall give an overview of the data obtained from human, mouse and avian models on the cellular origins of the hematopoietic system and discuss some aspects of the molecular mechanisms controlling hematopoietic cell production.

[From the primitive to the definitive aorta: angioblasts and hemangioblasts during aorta-associated haematopoiesis]. / De l'aorte primitive à l'aorte definitive: angioblastes et hémangioblastes au cours de l'hématopoïèse.

Intra-aortic haematopoiesis is a transient phenomenon, present in all the vertebrate species examined. Aorta-associated haematopoiesis produces Haematopoietic Stem Cells (HSC) that emerge from the ventral aortic endothelium through endothelial cells (EC) that switch to HSC. HSC emergence is followed by the colonization of definitive haematopoietic organs. Since intra-aortic haematopoiesis is born from EC of the aortic floor, we wondered how vascular integrity was maintained during haematopoietic production. Transplantation experiments have brought about evidence according to which two distinct endothelial lineages contribute to the embryonic vasculature. One comes from the splanchnic mesoderm and gives rise to EC and haematopoietic cells (HC). The other originates from the somite and is restricted to EC differentiation. We have used interspecific quail/chick grafts to study aortic organogenesis during the course of haematopoiesis. We demonstrate that: 1) before haematopoiesis, the aorta, originally entirely of splanchnic origin, is colonized by EC from the somite. This colonization contributes to create a new roof and sides, which are hence formed by somite-derived EC whereas the floor is contributed by splanchnopleural-derived EC; 2) as haematopoiesis proceeds, somite-derived EC begin to colonize the aortic floor and are found beneath HSC clusters; 3) after haematopoiesis, aortic hemangioblasts disappear from the endothelium and are replaced by somite-derived EC. At this stage, the whole aortic endothelium is derived from somitic cells; 4) we have identified a new cell population from the somite that contributes to the vascular smooth muscle cells (VSMC). This population appears distinct from the somite-derived EC. Using lineage tracing with non-replicative retroviral vectors, we show that EC do not give rise to VSMC as previously thought. Taken together, our results bring about new lights on aorta morphogenesis and the time-restricted production of haematopoiesis.