Tracing the origin of the HSC hierarchy reveals an SCF-dependent, IL-3-independent CD43(-) embryonic precursor

Abstract

Definitive hematopoietic stem cells (HSCs) develop in the aorta gonad mesonephros (AGM) region in a stepwise manner. Type I pre-HSCs express CD41 but lack CD45 expression, which is subsequently upregulated in type II pre-HSCs prior to their maturation into definitive HSCs. Here, using ex vivo modeling of HSC development, we identify precursors of definitive HSCs in the trunk of the embryonic day 9.5 (E9.5) mouse embryo. These precursors, termed here pro-HSCs, are less mature than type I and II pre-HSCs. Although pro-HSCs are CD41(+), they lack the CD43 marker, which is gradually upregulated in the developing HSC lineage. We show that stem cell factor (SCF), but not interleukin-3 (IL-3), is a major effector of HSC maturation during E9-E10. This study extends further the previously established hierarchical organization of the developing HSC lineage and presents it as a differentially regulated four-step process and identifies additional targets that could facilitate the generation of transplantable HSCs from pluripotent cells for clinical needs.

Figure 1

Pro-HSC Maturation into dHSCs (A) While dHSCs from E10.5 precursors appeared after 4 days in culture (green diamonds), E9.5 caudal part precursors matured into dHSCs only by day 7 in culture (red diamonds) (two independent experiments). (B) Pro-HSCs are localized to the caudal part of the embryo body, but not in other intra- or extraembryonic tissues (three independent experiments). (C) The yolk sac, but not the caudal part, contains large numbers of CFU-C (comparisons between yolk sac and caudal parts of individually staged embryos are shown; two independent experiments). (D) Dissection strategy for E9.5 embryo (24 sp): a, umbilical cord; b, tip of the tail; c, posteriormost one-third of the caudal part excluding the tail tip; d, intermediate one-third of the caudal part; and e, rostralmost one-third of the caudal part immediately below the heart level; anterior part (AP) including the heart, neck, and head. (E) Subdissected regions from individual embryos were separately cocultured with OP9 cells. Presence of pro-HSCs in different regions (x axis) and level of donor-derived engraftment (y axis) is indicated. Age of individual embryos (24–28 sp) is indicated (z axis). Each color depicts one embryo (four independent experiments). (F) Pro-HSCs stably emerge in the embryonic caudal part from 24 sp stage. On rare occasions, dHSCs developed in embryos between 15 and 23 sp (three independent experiments). Diamond symbols represent individual recipients. Number of e.e. transplanted per recipient (ee/rec) is shown. Dissection scheme shown on CD41Cre::sGFP embryo for better orientation (Rybtsov et al., 2011). For multilineage analyses, see Figure S1. Each plot represents two to four independent experiments.

Figure 2

Stepwise Phenotypic Progression of the HSC Lineage during E9.5 to E11.5 (A) Sorting strategies for the developing HSC lineage (plots show 7AAD⁻Lin⁻ cells). (B) Functional identification of members of the developing HSC hierarchy. Sorting strategy (see also Figure S2) and functional validation for: (a and a′) E9 VC⁺CD45⁻: repopulation achieved with R2 cells (note that the VC-negative population was also tested and gave no repopulation), four independent experiments; (b and b′) E10 VC⁺CD45⁻: repopulation achieved with R2 and R3 cells, two independent experiments; (c and c′) E11 VC⁺CD45⁻: repopulation achieved only with R1 cells, three independent experiments; (d and d′) E11 VC⁺CD45⁺: repopulation achieved only with R2 cells, three independent experiments; (C and D) Tracking CD43 and CD45 upregulation in cultured pro-HSCs. Sorting strategy of pro-HSCs (Lin⁻VC⁺CD41^loCD43⁻CD45⁻) from fresh E9 caudal parts (C, left). Sorting purity of this population (C, right). Pure, sorted, Lin⁻VC⁺ CD41^loCD43⁻CD45⁻ pro-HSCs show gradual upregulation of CD43 in cultured during 7 days ex vivo (note that CD45 upregulation occurs with a slight delay) (D) (two independent experiments). (E and F) By the end of culture, E9 caudal parts produce definitive HSCs with the CD43⁺CD45⁺ phenotype (E) (located in R1), as demonstrated by transplantation of sorted live cells (F) (two independent experiments). (G) Demonstration of coexpression of CD41 and RUNX1 in E9 live VC⁺CD45⁻ population shown by intracellular RUNX1 antibody staining. (H) CFU-C potential of sorted E11 populations. (a) CFU-C are generated predominantly by R1 cells sorted as in (Ac) (three independent experiments). Error bars represent SD. (b) CFU-C are generated predominantly by R2 cells sorted as in (Ad) (three independent experiments). Error bars represent SD.

Figure 3

Phenotypic Mapping of Pro-HSC Localization (A) VC⁺CD43⁺RUNX1⁺ cells localize to the ventral floor of the E9 dorsal aorta. Zoomed boxed area is shown in (A′). Yellow arrowheads indicate cells with pro-HSC phenotype. Red arrows show RUNX1⁺VC⁺ pro-HSCs localized to the ventral endothelium. (B) Hematopoietic development in the E9 omphalomesenteric artery (OA). The OA, but not the dorsal aorta (DA), contains strings of large VC⁺CD43⁺RUNX1⁺ clusters. Note the presence of VC⁺CD43⁻RUNX1⁺cells in small vessels connecting OA and DA (shown by yellow arrowheads). Zoomed image of the boxed area is shown in (B′). Yellow arrow indicates a large cluster inside the OA composed of VC⁺CD43⁺RUNX1⁺ cells. (C) Small ventrally located intra-aortic clusters appearing at E10 contain cells expressing CD43, but not CD45. (D) E10 rare VC⁺CD43⁺ cells in small intra-aortic clusters show expression of CD45 (yellow arrow). Subendothelial (aortic) mesenchyme (SM) contains large clusters of VE-cad⁺CD45⁻CD43⁺ (blue arrow) not associated with vessels. (E) E10 intra-aortic clusters are VE-cad⁺CD43⁺RUNX1⁺. (F) Fine spatial localization of pro-HSCs in the caudal part of the body. Dissection strategy shown on a thick transverse slice in the area of UA as indicated schematically by double dotted line in (G) (green dotted line indicates subdissections). DA, OA, and UA are indicated by red color. Violet color depicts vitelline veins (VV). (H) Dorsal aorta is the main site of pro-HSC residence. Dissected subregions of E9.5 caudal part (as shown in (F) and (G) were cocultured with OP9 cells and transplanted into irradiated recipients. (I) CFU-C contents in UA + OA and DA. UA + OA contains significantly more CFU-C than dorsal aorta (comparisons between UA + OA and DA of individually staged embryos are shown). Abbreviations: OA, omphalomesenteric artery; DA, dorsal aorta; UA, umbilical artery; VV, vitelline vein; SCM, subcardiac mesenchyme; Som, somites; Hrt, heart. Crossed double-headed arrows indicate dorsoventral (D-V) and posterior-arterial (P-A) orientations. Double-headed arrow indicates the lumen of dorsal aorta (DA). Each plot represents two independent experiments.

Figure 4

CFU-C in the E9.5 Embryo Are Marked by High cKIT and CD43 Expression (A) Gating strategy for identification of CFU-C and endothelial progenitors in E9.5 caudal part; live Lin⁻VC⁺CD45⁻ cells were sorted on the basis of CD41 and CD43 expression. (B) Four sorted populations (as indicated in A) were tested for hematopoietic CFU-C and endothelial progenitors CFU-En (frequencies per e.e. are shown). While CFU-C resided in R2 (note the large numbers of multilineage colonies, CFU-GEMM generated), CFU-En resided in R4. Error bars represent SD. (C) CFU-C population (R2) shows high level of cKIT expression, whereas pro-HSCs (R3) express cKIT at intermediate levels (cKIT expression and control FMO are depicted by blue and red colors, respectively). Each plot represents two independent experiments.

Figure 5

Growth Factor Requirements for Pro-/Pre-HSC Development (A) E9 pro-HSCs mature into dHSCs in presence of SCF (S), but not IL-3 (I) (1 e.e./recipient; four independent experiments). (B) E11 type I pre-HSCs respond by maturing into dHSCs in the presence of SCF, but not IL-3. Meanwhile, type II pre-HSCs respond equally well to SCF and IL-3 (left and right, respectively). Transplantations: type I pre-HSCs: 0.2 ee/recipient (left) and type II pre-HSCs: 0.3 ee/recipient (right) (three independent experiments). (C) Scf expression in the dorsal aorta, as measured by quantitative PCR, increases 2.5-fold between E9.5 and E10.5. Note that cKit expression remains at the same level during this period of time. Expression levels are normalized to Tbp housekeeping gene expression (three independent RNA preparations). Error bars represent SD. (D) Long-term donor-derived lymphoid and myeloid contribution in recipient blood derived from E9.5 caudal parts. While SCF on its own generates dHSCs with balanced lymphomyeloid differentiation (∼15%–20% of lymphoid cells), IL-3 on its own or in combination with SCF tends to shift balance more toward myeloid differentiation. Each bar represents an individual recipient (two independent experiments).

Figure 6

Schematic Development of the HSC Lineage in the Context of Growth Factor—SCF and IL-3—Dependency Proposed stepwise development of the HSC lineage in the dorsal aorta beginning from E9.5 stage based on cumulative data obtained from current research and previous publications (Rybtsov et al., 2011; Taoudi et al., 2008). Color coding shows a transient increase in CD41 expression at E9.5 followed by stable gradual upregulation of CD43 and subsequent elevation of CD45 expression. Modulation of VE-cadherin, cKIT, and RUNX1 expression was not studied in detail, and these are presented schematically at steady levels throughout E9–E11 stages. Stage-specific dependency on SCF and IL-3 is indicated within arrows showing transition of one stage to another. Abbreviations: HE, hematogenic endothelium; Pro, pro-HSC; pre- I, type I pre-HSCs; pre- II, type II pre-HSCs; HSC, definitive HSCs. Images of cells were downloaded from Servier Medical Art (http://www.servier.com/) with the owner’s permission.