HoxA3 is an apical regulator of haemogenic endothelium.
During development, haemogenesis occurs invariably at sites of vasculogenesis. Between embryonic day (E) 9.5 and E10.5 in mice, endothelial cells in the caudal part of the dorsal aorta generate haematopoietic stem cells and are referred to as haemogenic endothelium. The mechanisms by which haematopoiesis is restricted to this domain, and how the morphological transformation from endothelial to haematopoietic is controlled are unknown. We show here that HoxA3, a gene uniquely expressed in the embryonic but not yolk sac vasculature, restrains haematopoietic differentiation of the earliest endothelial progenitors, and induces reversion of the earliest haematopoietic progenitors into CD41-negative endothelial cells. This reversible modulation of endothelial-haematopoietic state is accomplished by targeting key haematopoietic transcription factors for downregulation, including Runx1, Gata1, Gfi1B, Ikaros, and PU.1. Through loss-of-function, and gain-of-function epistasis experiments, and the identification of antipodally regulated targets, we show that among these factors, Runx1 is uniquely able to erase the endothelial program set up by HoxA3. These results suggest both why a frank endothelium does not precede haematopoiesis in the yolk sac, and why haematopoietic stem cell generation requires Runx1 expression only in endothelial cells.
PubMed ID: 21170035
PMC ID: PMC3079247
Grant support: 1R01HL081186-01 NHLBI NIH HHS , R01 HL081186-04 NHLBI NIH HHS
Genes referenced: cad cald1 cdh5 gapdh gata1 gfi1b hoxa3 ikzf1 itga2b.2 kdr kit ptprc runx1 spi1 tspan32
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|Figure 2. HoxA3 expression in early mesoderm and committed hemogenic endothelium restrains hematopoeisis(A) Representative flow cytometric profiles of EBs at day 6 without doxycycline (No Dox) or with 1 μg/mL doxycycline (+Dox) to induce HoxA3 expression from day 4 to day 6. VE-cadherin (VE-cad)/Flk-1 antibody staining or c-Kit/CD41 and c-Kit/CD45 staining were performed to identify vascular and hematopoietic progenitor populations. (B) Frequencies of cells expressing endothelial surface markers (Flk-1+/VE-cadherin+, F/V), hematopoietic markers CD41+ and CD45+ cells during EB differentiation in 7 independent experiments (for CD41 p=0.0004 and for CD45, p=0.0031). (C) 50,000 cells from day 6 EBs (induced with 1 μg/mL dox to express HoxA3 continually from EB day 4-6 or not) were plated in methylcellulose with hematopoietic cytokines. n=3. Black bar: no dox treatment, gray bar: dox treatment. Colonies: GEMM (granulocyte/erythrocyte/macrophage/megakaryocyte) GM (granulocyte/macrophage) M (macrophage only) Ery-D (definitive erythroid) p=0.032, Ery-P (primitive erythroid) p=0.0002 Ery-Meg (erythrocyte-megakaryocyte) p=0.0009. (D) Brightfield and fluorescence images showing both endothelial (+Dox) and hematopoietic colonies (No Dox or Dox removal) derived from Flk1+/VE-cadherin+ (F/V) endothelial progenitors from day 6 EBs. Immunofluorescence for VE-cadherin is shown in adherent cells growing in the presence of doxycycline. Bar 100 μm. (E) Equivalent analysis of cultures derived from day 6 EB c-Kit+/CD41+ (K/41) hematopoietic progenitors. (F) Representative flow cytometric profile of 100,000 Flk-1/VE-cadherin double positive cells or (G) c-Kit/CD41 double positive cells from day 6 uninduced EBs (left), cultured on OP9 for 5 days, in the presence or absence of 1 μg/mL doxycycline. Dox-induced cells were cultured for an additional 4 days in the absence of dox to test the effect of HoxA3 down-regulation. Hematopoietic surface markers, c-Kit, CD41 and CD45 and endothelial markers Flk-1 and VE-cadherin are plotted. (H) AGM tissue dissected from E10.5 embryos, dissociated and transduced with control ires-GFP or HoxA3-ires-GFP retrovirus, cultured on OP9 for 5 days. Bright field images are shown at left, GFP at right. Both hematopoietic and endothelial colonies that acquired GFP were observed with the control, but GFP segregated with endothelial colonies in the HoxA3-ires-GFP transduced sample, indicating skewing of differentiation towards endothelial by HoxA3. Bar 100 μm. (I) Representative flow cytometric profile of AGM cells co-cultured on OP9, and statistical analysis of 5 independent experiments (histogram CD41 p=0.053 CD45 p=0.02).|
|Figure 3. Global expression changes upon HoxA3 induction(A) Venn diagram of regulated genes in endothelial (F/V) and hematopoietic (K/41) progenitor cells. Arrow up upregulated genes, arrow down downregulated genes. (B) Clustering of genes upregulated in the c-Kit CD41 double-positive cells upon HoxA3 induction, during EB differentiation in the hematopoietic c-Kit CD41 double positive cells, based on their expression levels in the Flk-1 VE-cadherin double positive cells (F/V) and in the c-Kit CD41 double positive cells (K/41) of uninduced EBs. (C) Clustering of genes downregulated genes in the Flk-1 VE-cadherin double-positive population upon 6-hour HoxA3 induction, in day 6 EB cells, based on their baseline (control) levels in the Flk-1 VE-cadherin double-positive population (F/V). (D) Real time RTPCR measurements of gene expression changes following HoxA3 induction in sorted endothelial (F/V black bars) or hematopoietic (K/41 grey bars) progenitors. n=5 independent experiments.|
|Figure 5. Regulation of Runx1 by HoxA3(A) In situ hybridization showing Runx1 expression in HoxA3 +/+, +/− and −/− E8.5 embryos. Runx1 expression is absent in the dorsal aortae of HoxA3 +/+ (i) and HoxA3+/− (ii) embryos, but robustly expressed in yolk sac. Runx1 is ectopically expressed in the dorsal aortae of HoxA3−/− (iii) embryos. Stippled red lines outline dorsal aortae. Penetrance of this phenotype is indicated at lower left. (i′-iii′) Sections of the embryos shown above. Both endothelial and hematopoietic cells are negative for Runx1 in wild-type or heterozygous embryos, while Runx1-expressing cells are found in HoxA3−/− embryos. Arrows indicate Runx1-expressing endothelial cells. a, aorta; ys, yolk sac. Scale bar = 50 μm for whole mounts, 10 μm for sections. (B) Model for regulation of endothelial hemogenesis by HoxA3 and Runx1. HoxA3 represses a cascade of transcription factors that promote hemogenesis and induces a set of genes that maintain endothelial character. Runx1 is a positive regulator of most of these transcription factors, and a negative regulator of genes essential for endothelial character, thus transient expression of Runx1 erases the endothelial program and initiates the hematopoietic.|