ABSTRACT: Hematopoiesis occurs in distinct waves. ‘Definitive’ hematopoietic stem cells (HSC) with the potential for all blood lineages emerge in the aorta-gonado-mesonephros (AGM), while ‘primitive’ progenitors, whose potential is thought to be limited to erythrocytes, megakaryocytes and macrophages (MΦ), arise earlier in the yolk sac (YS). Here, we questioned whether other YS lineages exist that have not been identified, partially owing to limitations of current lineage tracing models. We established the use of Cdh5CreERT2 for hematopoietic fate mapping, which revealed the YS origin of mast cells (MC). YS derived MC are replaced by definitive MC, which maintain themselves independently from the BM in the adult. Replacement occurs with tissue specific kinetics. MC in the skin, but not other organs, remain largely YS derived prenatally and are phenotypically and transcriptomically distinct from definite adult MC. We conclude that dual hematopoietic origin is not MΦ specific, but shared between these two myeloid lineages.
Project description:The embryonic site of definitive hematopoietic stem cell (dHSC) origination has been debated for decades. Although an intra-embryonic origin is supported, recent data suggest that a large fraction of adult blood derives from the yolk sac (YS). Investigating the origins of hematopoiesis before heartbeat onset (i.e. 5-7 somite pairs (sp)) is precluded by a lack of assays that can distinguish dHSC precursors in early embryos. Here, we report robust, multi-lineage and serially transplantable dHSC activity from cultured 2-7sp murine embryonic explants (Em-Ex). dHSC were undetectable in 2-7sp YS explants (YS-Ex). Our work supports a model in which the embryo, not the YS, is the major source of lifelong hematopoiesis.
Project description:Normal arteries contain a large population of tissue resident macrophages (MΦ). Their origins, as well as the mechanisms that sustain them during homeostasis and disease, however, are poorly understood. Gene expression profiling, we show, identifies arterial MΦ as a distinct population among tissue MΦ. Ontologically, arterial MΦ arise before birth, though CX3CR1-, Csf1r-, and Flt3-driven fate mapping approaches demonstrate MΦ colonization occurs through successive contributions of yolk sac (YS) and conventional hematopoiesis. In adulthood, arterial MΦ renewal is driven by local proliferation rather than monocyte recruitment from the blood. Proliferation sustains MΦ not only during steady state conditions, but mediates their rebound after severe depletion following sepsis. Importantly, the return of arterial MΦ to functional homeostasis after infection is rapid; repopulated MΦ exhibit a transcriptional program similar to resting MΦ and efficiently phagocytose bacteria. Collectively, our data provide a detailed framework for future studies of arterial MΦ function in health and disease.
Project description:Extra-embryonic mesoderm (ExM), the earliest cells that traverse through the primitive streak, give rise to the endothelium as well as hematopoietic progenitors in the developing yolk-sac (YS). How a specific subset of ExM becomes committed to a hematopoietic fate remains unclear. Here we report that Eomesodermin (Eomes), a T-box transcription factor, is transiently expressed in ExM progenitors that generate virtually all YS hematopoietic and endothelial cells. Using an embryonic stem cell (ESC) differentiation system, we find that Eomes activity is essential for the production of primitive erythrocytes and for the normal development of Runx1+ HE that generates the definitive hematopoietic progenitors. RNA-Seq and ATAC-Seq experiments reveal that Eomes governs the accessibility of numerous hematopoietic enhancers that SCL normally utilizes to specify primitive erythrocytes and HE in Flk-1hi/PdgfRa- hematovascular mesoderm. ChIP-seq experiments suggest that Eomes coordinates the development of hemogenic competent mesoderm in the context of Activin/Nodal and Tead-Yap signaling. Finally, single-cell- RNA-seq (scRNAseq) shows that in the absence of Eomes function diversion towards an endothelial rather than hematopoietic fate occurs after the initial specification of Flk-1+/SCL+ hematovascular mesoderm. Collectively, these experiments demonstrate that Eomes sits at the top of the transcriptional hierarchy, functioning upstream of Runx1 expression and SCL functional activity, and promotes hemogenic competence of the entire YS mesodermal lineage.
Project description:Extra-embryonic mesoderm (ExM), the earliest cells that traverse through the primitive streak, give rise to the endothelium as well as hematopoietic progenitors in the developing yolk-sac (YS). How a specific subset of ExM becomes committed to a hematopoietic fate remains unclear. Here we report that Eomesodermin (Eomes), a T-box transcription factor, is transiently expressed in ExM progenitors that generate virtually all YS hematopoietic and endothelial cells. Using an embryonic stem cell (ESC) differentiation system, we find that Eomes activity is essential for the production of primitive erythrocytes and for the normal development of Runx1+ HE that generates the definitive hematopoietic progenitors. RNA-Seq and ATAC-Seq experiments reveal that Eomes governs the accessibility of numerous hematopoietic enhancers that SCL normally utilizes to specify primitive erythrocytes and HE in Flk-1hi/PdgfRa- hematovascular mesoderm. ChIP-seq experiments suggest that Eomes coordinates the development of hemogenic competent mesoderm in the context of Activin/Nodal and Tead-Yap signaling. Finally, single-cell- RNA-seq (scRNAseq) shows that in the absence of Eomes function diversion towards an endothelial rather than hematopoietic fate occurs after the initial specification of Flk-1+/SCL+ hematovascular mesoderm. Collectively, these experiments demonstrate that Eomes sits at the top of the transcriptional hierarchy, functioning upstream of Runx1 expression and SCL functional activity, and promotes hemogenic competence of the entire YS mesodermal lineage.
Project description:Extra-embryonic mesoderm (ExM), the earliest cells that traverse through the primitive streak, give rise to the endothelium as well as hematopoietic progenitors in the developing yolk-sac (YS). How a specific subset of ExM becomes committed to a hematopoietic fate remains unclear. Here we report that Eomesodermin (Eomes), a T-box transcription factor, is transiently expressed in ExM progenitors that generate virtually all YS hematopoietic and endothelial cells. Using an embryonic stem cell (ESC) differentiation system, we find that Eomes activity is essential for the production of primitive erythrocytes and for the normal development of Runx1+ HE that generates the definitive hematopoietic progenitors. RNA-Seq and ATAC-Seq experiments reveal that Eomes governs the accessibility of numerous hematopoietic enhancers that SCL normally utilizes to specify primitive erythrocytes and HE in Flk-1hi/PdgfRa- hematovascular mesoderm. ChIP-seq experiments suggest that Eomes coordinates the development of hemogenic competent mesoderm in the context of Activin/Nodal and Tead-Yap signaling. Finally, single-cell- RNA-seq (scRNAseq) shows that in the absence of Eomes function diversion towards an endothelial rather than hematopoietic fate occurs after the initial specification of Flk-1+/SCL+ hematovascular mesoderm. Collectively, these experiments demonstrate that Eomes sits at the top of the transcriptional hierarchy, functioning upstream of Runx1 expression and SCL functional activity, and promotes hemogenic competence of the entire YS mesodermal lineage.
Project description:Extra-embryonic mesoderm (ExM), the earliest cells that traverse through the primitive streak, give rise to the endothelium as well as hematopoietic progenitors in the developing yolk-sac (YS). How a specific subset of ExM becomes committed to a hematopoietic fate remains unclear. Here we report that Eomesodermin (Eomes), a T-box transcription factor, is transiently expressed in ExM progenitors that generate virtually all YS hematopoietic and endothelial cells. Using an embryonic stem cell (ESC) differentiation system, we find that Eomes activity is essential for the production of primitive erythrocytes and for the normal development of Runx1+ HE that generates the definitive hematopoietic progenitors. RNA-Seq and ATAC-Seq experiments reveal that Eomes governs the accessibility of numerous hematopoietic enhancers that SCL normally utilizes to specify primitive erythrocytes and HE in Flk-1hi/PdgfRa- hematovascular mesoderm. ChIP-seq experiments suggest that Eomes coordinates the development of hemogenic competent mesoderm in the context of Activin/Nodal and Tead-Yap signaling. Finally, single-cell- RNA-seq (scRNAseq) shows that in the absence of Eomes function diversion towards an endothelial rather than hematopoietic fate occurs after the initial specification of Flk-1+/SCL+ hematovascular mesoderm. Collectively, these experiments demonstrate that Eomes sits at the top of the transcriptional hierarchy, functioning upstream of Runx1 expression and SCL functional activity, and promotes hemogenic competence of the entire YS mesodermal lineage.
Project description:In mice, Kupffer cells (KCs) first derive from yolk sac (YS) hematopoietic progenitors prior to the emergence of the hematopoietic stem cell. To investigate human KC ontogeny, we recapitulated YS hematopoiesis from human pluripotent stem cells (hPSCs) and transplanted derivative macrophage progenitors into NSG mice previously humanized with hPSC-liver sinusoidal endothelial cells (LSECs). We demonstrate that hPSC-LSECs facilitate stable hPSC-YS-macrophage engraftment for at least 7 weeks. Single cell RNA sequencing of engrafted YS-macrophages revealed a homogenous MARCO-expressing KC gene signature and low expression of inflammatory macrophage genes. In contrast, human cord blood (CB)-derived macrophages generated grafts that contain multiple hematopoietic lineages in addition to KCs. Functional analyses showed that the engrafted KCs actively perform phagocytosis and erythrophagocytosis in vivo. Taken together, these findings demonstrate that it is possible to generate human KCs from hPSCs and show that the equivalent of the human YS hematopoietic programs represent enriched sources of progenitors for this lineage.
Project description:Multiple mammalian lineages independently evolved a definitive mammalian middle ear (DMME) through breakdown of Meckel’s cartilage (MC). However, the cellular and molecular drivers of this evolutionary transition remain unknown for most mammal groups. Here, we identify such drivers in the living marsupial opossum Monodelphis domestica, whose MC transformation during development anatomically mirrors the evolutionary transformation observed in fossils. Specifically, we link increases in cellular apoptosis and TGF-BR2 signalling to MC breakdown in opossums. We demonstrate that a simple change in TGF-b signalling is sufficient to inhibit MC breakdown during opossum development, indicating that changes in TGF-b signalling might be key during mammalian evolution. Furthermore, the apoptosis that we observe during opossum MC breakdown does not seemingly occur in mouse, consistent with homoplastic DMME evolution in the marsupial and placental lineages.
Project description:Although classified as hematopoietic cells, tissue-resident macrophages are selfrenewing and maintained independently of adult hematopoiesis. While most macrophages originate from embryonic precursors that seed tissues prior to birth, their exact origin is unknown. Using an in utero macrophage depletion strategy and fatemapping of yolk sac (YS) and fetal liver (FL) hematopoiesis, we found that YS macrophages are the main precursors of microglia, while most other macrophages derive from fetal monocytes. Both YS macrophages and fetal monocytes arise from erythro-myeloid progenitors (EMP) generated in the YS. In the YS, EMP gave rise to macrophages without monocytic intermediates, while EMP seeding the FL upon the establishment of blood circulation acquired c-Myb expression and gave rise to fetal monocytes that then seed embryonic tissues to differentiate into macrophages. Thus, adult tissue-resident macrophages established from HSC-independent embryonic precursors arise from two different developmental programs.
Project description:Definitive hematopoiesis generates hematopoietic stem/progenitor cells (HSPCs) that give rise to all mature blood and immune cells, but remains poorly defined in human. Here, we resolve human hematopoietic populations at the earliest hematopoiesis stage by single-cell RNA-seq. We characterize the distinct molecular profiling between early primitive and definitive hematopoiesis in both human embryonic stem cell (hESC) differentiation and early embryonic development. We generate definitive HSPCs from hESCs that hold the multipotency to differentiate various blood and immune cells, as validated by single-cell clonal assay. Strikingly, the generated HSPCs from hESCs give rise to various blood and lymphoid lineages in vivo. Lastly, we characterize gene-expression dynamics in definitive and primitive hematopoiesis and reveal an unreported role of ROCK-inhibition in enhancing human definitive hematopoiesis. Our study provides a prospect for understanding human early hematopoiesis and a firm basis to generate blood and immune cells for clinical purposes.