Joanna Tober

Inflammatory signaling regulates embryonic hematopoietic stem and progenitor cell production

Identifying signaling pathways that regulate hematopoietic stem and progenitor cell (HSPC) formation in the embryo will guide efforts to produce and expand HSPCs ex vivo. Here we show that sterile tonic inflammatory signaling regulates embryonic HSPC formation. Expression profiling of progenitors with lymphoid potential and hematopoietic stem cells (HSCs) from aorta/gonad/mesonephros (AGM) regions of midgestation mouse embryos revealed a robust innate immune/inflammatory signature. Mouse embryos lacking interferon γ (IFN-γ) or IFN-α signaling and zebrafish morphants lacking IFN-γ and IFN-ϕ activity had significantly fewer AGM HSPCs. Conversely, knockdown of IFN regulatory factor 2 (IRF2), a negative regulator of IFN signaling, increased expression of IFN target genes and HSPC production in zebrafish. Chromatin immunoprecipitation (ChIP) combined with sequencing (ChIP-seq) and expression analyses demonstrated that IRF2-occupied genes identified in human fetal liver CD34(+) HSPCs are actively transcribed in human and mouse HSPCs. Furthermore, we demonstrate that the primitive myeloid population contributes to the local inflammatory response to impact the scale of HSPC production in the AGM region. Thus, sterile inflammatory signaling is an evolutionarily conserved pathway regulating the production of HSPCs during embryonic development.

Human definitive haemogenic endothelium and arterial vascular endothelium represent distinct lineages

The generation of haematopoietic stem cells (HSCs) from human pluripotent stem cells (hPSCs) will depend on the accurate recapitulation of embryonic haematopoiesis. In the early embryo, HSCs develop from the haemogenic endothelium (HE) and are specified in a Notch-dependent manner through a process named endothelial-to-haematopoietic transition (EHT). As HE is associated with arteries, it is assumed that it represents a subpopulation of arterial vascular endothelium (VE). Here we demonstrate at a clonal level that hPSC-derived HE and VE represent separate lineages. HE is restricted to the CD34+CD73−CD184− fraction of day 8 embryoid bodies and it undergoes a NOTCH-dependent EHT to generate RUNX1C+ cells with multilineage potential. Arterial and venous VE progenitors, in contrast, segregate to the CD34+CD73medCD184+ and CD34+CD73hiCD184− fractions, respectively. Together, these findings identify HE as distinct from VE and provide a platform for defining the signalling pathways that regulate their specification to functional HSCs.

15-deoxy-Δ12,14-PGJ2 enhances platelet production from megakaryocytes

Thrombocytopenia is a critical problem that occurs in many hematologic diseases, as well as after cancer therapy and radiation exposure. Platelet transfusion is the most commonly used therapy but has limitations of alloimmunization, availability, and expense. Thus, the development of safe, small, molecules to enhance platelet production would be advantageous for the treatment of thrombocytopenia. Herein, we report that an important lipid mediator and a peroxisome proliferator-activated receptor gamma (PPARgamma) ligand called 15-deoxy-Delta(12,14) prostaglandin J(2) (15d-PGJ(2)), increases Meg-01 maturation and platelet production. 15d-PGJ(2) also promotes platelet formation from culture-derived mouse and human megakaryocytes and accelerates platelet recovery after in vivo radiation-induced bone marrow injury. Interestingly, the platelet-enhancing effects of 15d-PGJ(2) in Meg-01 cells are independent of PPARgamma, but dependent on reactive oxygen species (ROS) accumulation; treatment with antioxidants such as glutathione ethyl ester (GSH-EE); or N-acetylcysteine (NAC) attenuate 15d-PGJ(2)-induced platelet production. Collectively, these data support the concept that megakaryocyte redox status plays an important role in platelet generation and that small electrophilic molecules may have clinical efficacy for improving platelet numbers in thrombocytopenic patients.

Trisomy 21-associated defects in human primitive hematopoiesis revealed through induced pluripotent stem cells

Patients with Down syndrome (trisomy 21, T21) have hematologic abnormalities throughout life. Newborns frequently exhibit abnormal blood counts and a clonal preleukemia. Human T21 fetal livers contain expanded erythro-megakaryocytic precursors with enhanced proliferative capacity. The impact of T21 on the earliest stages of embryonic hematopoiesis is unknown and nearly impossible to examine in human subjects. We modeled T21 yolk sac hematopoiesis using human induced pluripotent stem cells (iPSCs). Blood progenitor populations generated from T21 iPSCs were present at normal frequency and proliferated normally. However, their developmental potential was altered with enhanced erythropoiesis and reduced myelopoiesis, but normal megakaryocyte production. These abnormalities overlap with those of T21 fetal livers, but also reflect important differences. Our studies show that T21 confers distinct developmental stage- and species-specific hematopoietic defects. More generally, we illustrate how iPSCs can provide insight into early stages of normal and pathological human development.

Distinct temporal requirements for Runx1 in hematopoietic progenitors and stem cells

The transcription factor Runx1 is essential for the formation of yolk sac-derived erythroid/myeloid progenitors (EMPs) and hematopoietic stem cells (HSCs) from hemogenic endothelium during embryogenesis. However, long-term repopulating HSCs (LT-HSCs) persist when Runx1 is conditionally deleted in fetal liver cells, demonstrating that the requirement for Runx1 changes over time. To define more precisely when Runx1 transitions from an essential factor to a homeostatic regulator of EMPs and HSCs, and whether that transition requires fetal liver colonization, we performed conditional, timed deletions of Runx1 between E7.5 and E13.5. We determined that Runx1 loss reduces the formation or function of EMPs up through E10.5. The Runx1 requirement in HSCs ends later, as deletion up to E11.5 eliminates HSCs. At E11.5, there is an abrupt transition to Runx1 independence in at least a subset of HSCs that does not require fetal liver colonization. The transition to Runx1 independence in EMPs is not mediated by other core binding factors (Runx2 and/or Runx3); however, deleting the common non-DNA-binding β subunit (CBFβ) severely compromises LT-HSC function. Hence, the requirements for Runx1 in EMP and HSC formation are temporally distinct, and LT-HSC function is highly reliant on continued core binding factor activity.

Primitive erythropoiesis and megakaryopoiesis in the yolk sac are independent of c-myb

Hematopoiesis initiates within the yolk sac of mammalian embryos in overlapping primitive and definitive waves, each containing erythroid and megakaryocyte progenitors. c-myb-null mouse fetuses lack definitive erythrocytes but contain primitive erythroblasts and hepatic megakaryocytes. However, it is unclear if c-myb-null embryos harbor definitive erythroid or any megakaryocyte progenitors. We determined that c-myb was not expressed in primitive erythroid precursors and that c-myb-null embryos had normal primitive erythroid and megakaryocyte progenitor numbers and kinetics between embryonic day (E) 7.0 and E9.0. While primitive hematopoiesis is c-myb-independent, no definitive erythroid potential was detected in c-myb-null embryos, confirming that definitive erythropoiesis, beginning at E8.25 in the yolk sac, is completely c-myb-dependent. In contrast, reduced numbers of megakaryocyte progenitors with restricted proliferative capacity persist in E10.5 yolk sac and E11.5 liver. Despite this impaired megakaryocyte potential, c-myb-null fetuses had normal platelet numbers at E12.5 but became thrombocytopenic by E15.5, suggesting that c-myb is required for sustained thrombopoiesis.

Taking the Leap: Runx1 in the Formation of Blood from Endothelium

Blood cell formation in the embryo occurs from multiple anatomic sites and results in the production of hematopoietic stem and progenitor cells that appear in overlapping waves. The transcription factor Runx1 is involved in a dramatic step of this process, for the transition from an endothelial cell that is integrated in a monolayer to a nonadherent circulating blood cell, a process conceptually similar to the epithelial to mesenchymal cell transition. Here we will review the role of Runx1 in the so-called hemogenic endothelium. We will describe the blood cell progenitors for which Runx1 is required, the proximal upstream transcription factors and signaling events that regulate its expression, and some of its important downstream targets in the hemogenic endothelium.

Maturation of hematopoietic stem cells from prehematopoietic stem cells is accompanied by up-regulation of PD-L1

Hematopoietic stem cells (HSCs) mature from pre-HSCs that originate in the major arteries of the embryo. To identify HSCs from in vitro sources, it will be necessary to refine markers of HSCs matured ex vivo. We purified and compared the transcriptomes of pre-HSCs, HSCs matured ex vivo, and fetal liver HSCs. We found that HSC maturation in vivo or ex vivo is accompanied by the down-regulation of genes involved in embryonic development and vasculogenesis, and up-regulation of genes involved in hematopoietic organ development, lymphoid development, and immune responses. Ex vivo matured HSCs more closely resemble fetal liver HSCs than pre-HSCs, but are not their molecular equivalents. We show that ex vivo–matured and fetal liver HSCs express programmed death ligand 1 (PD-L1). PD-L1 does not mark all pre-HSCs, but cell surface PD-L1 was present on HSCs matured ex vivo. PD-L1 signaling is not required for engraftment of embryonic HSCs. Hence, up-regulation of PD-L1 is a correlate of, but not a requirement for, HSC maturation.