Hematopoietic Reprogramming In Vitro Informs In Vivo Identification of Hemogenic Precursors to Definitive Hematopoietic Stem Cells.

Definitive hematopoiesis emerges via an endothelial-to-hematopoietic transition in the embryo and placenta; however, the precursor cells to hemogenic endothelium are not defined phenotypically. We previously demonstrated that the induction of hematopoietic progenitors from fibroblasts progresses through hemogenic precursors that are Prom1(+)Sca1(+)CD34(+)CD45(-) (PS34CD45(-)). Guided by these studies, we analyzed mouse placentas and identified a population with this phenotype. These cells express endothelial markers, are heterogeneous for early hematopoietic markers, and localize to the vascular labyrinth. Remarkably, global gene expression profiles of PS34CD45(-) cells correlate with reprogrammed precursors and establish a hemogenic precursor cell molecular signature. PS34CD45(-) cells are also present in intra-embryonic hemogenic sites. After stromal co-culture, PS34CD45(-) cells give rise to all blood lineages and engraft primary and secondary immunodeficient mice. In summary, we show that reprogramming reveals a phenotype for in vivo precursors to hemogenic endothelium, establishing that direct in vitro conversion informs developmental processes in vivo.

Single-cell analyses of regulatory network perturbations using enhancer-targeting TALEs suggest novel roles for PU.1 during haematopoietic specification.

Transcription factors (TFs) act within wider regulatory networks to control cell identity and fate. Numerous TFs, including Scl (Tal1) and PU.1 (Spi1), are known regulators of developmental and adult haematopoiesis, but how they act within wider TF networks is still poorly understood. Transcription activator-like effectors (TALEs) are a novel class of genetic tool based on the modular DNA-binding domains of Xanthomonas TAL proteins, which enable DNA sequence-specific targeting and the manipulation of endogenous gene expression. Here, we report TALEs engineered to target the PU.1-14kb and Scl+40kb transcriptional enhancers as efficient new tools to perturb the expression of these key haematopoietic TFs. We confirmed the efficiency of these TALEs at the single-cell level using high-throughput RT-qPCR, which also allowed us to assess the consequences of both PU.1 activation and repression on wider TF networks during developmental haematopoiesis. Combined with comprehensive cellular assays, these experiments uncovered novel roles for PU.1 during early haematopoietic specification. Finally, transgenic mouse studies confirmed that the PU.1-14kb element is active at sites of definitive haematopoiesis in vivo and PU.1 is detectable in haemogenic endothelium and early committing blood cells. We therefore establish TALEs as powerful new tools to study the functionality of transcriptional networks that control developmental processes such as early haematopoiesis.

Early dynamic fate changes in haemogenic endothelium characterized at the single-cell level.

Haematopoietic stem cells (HSCs) are the founding cells of the adult haematopoietic system, born during ontogeny from a specialized subset of endothelium, the haemogenic endothelium (HE) via an endothelial-to-haematopoietic transition (EHT). Although recently imaged in real time, the underlying mechanism of EHT is still poorly understood. We have generated a Runx1 +23 enhancer-reporter transgenic mouse (23GFP) for the prospective isolation of HE throughout embryonic development. Here we perform functional analysis of over 1,800 and transcriptional analysis of 268 single 23GFP(+) HE cells to explore the onset of EHT at the single-cell level. We show that initiation of the haematopoietic programme occurs in cells still embedded in the endothelial layer, and is accompanied by a previously unrecognized early loss of endothelial potential before HSCs emerge. Our data therefore provide important insights on the timeline of early haematopoietic commitment.

A short history of hemogenic endothelium.

Definitive hematopoietic cells are generated de novo during ontogeny from a specialized subset of endothelium, the so-called hemogenic endothelium. In this review we give a brief overview of the identification of hemogenic endothelium, explore its links with the HSC lineage, and summarize recent insights into the nature of hemogenic endothelium and the microenvironmental and intrinsic regulators contributing to its transition into blood. Ultimately, a better understanding of the processes controlling the transition of endothelium into blood will advance the generation and expansion of hematopoietic stem cells for therapeutic purposes.

Lymphomyeloid contribution of an immune-restricted progenitor emerging prior to definitive hematopoietic stem cells.

In jawed vertebrates, development of an adaptive immune-system is essential for protection of the born organism against otherwise life-threatening pathogens. Myeloid cells of the innate immune system are formed early in development, whereas lymphopoiesis has been suggested to initiate much later, following emergence of definitive hematopoietic stem cells (HSCs). Herein, we demonstrate that the embryonic lymphoid commitment process initiates earlier than previously appreciated, prior to emergence of definitive HSCs, through establishment of a previously unrecognized entirely immune-restricted and lymphoid-primed progenitor. Notably, this immune-restricted progenitor appears to first emerge in the yolk sac and contributes physiologically to the establishment of lymphoid and some myeloid components of the immune-system, establishing the lymphomyeloid lineage restriction process as an early and physiologically important lineage-commitment step in mammalian hematopoiesis.

Characterization of transcriptional networks in blood stem and progenitor cells using high-throughput single-cell gene expression analysis.

Cellular decision-making is mediated by a complex interplay of external stimuli with the intracellular environment, in particular transcription factor regulatory networks. Here we have determined the expression of a network of 18 key haematopoietic transcription factors in 597 single primary blood stem and progenitor cells isolated from mouse bone marrow. We demonstrate that different stem/progenitor populations are characterized by distinctive transcription factor expression states, and through comprehensive bioinformatic analysis reveal positively and negatively correlated transcription factor pairings, including previously unrecognized relationships between Gata2, Gfi1 and Gfi1b. Validation using transcriptional and transgenic assays confirmed direct regulatory interactions consistent with a regulatory triad in immature blood stem cells, where Gata2 may function to modulate cross-inhibition between Gfi1 and Gfi1b. Single-cell expression profiling therefore identifies network states and allows reconstruction of network hierarchies involved in controlling stem cell fate choices, and provides a blueprint for studying both normal development and human disease.

Hematopoietic stem cell emergence in the conceptus and the role of Runx1.

Hematopoietic stem cells (HSCs) are functionally defined as cells that upon transplantation into irradiated or otherwise immunocompromised adult organisms provide long-term reconstitution of the entire hematopoietic system. They emerge in the vertebrate conceptus around midgestation. Genetic studies have identified a number of transcription factors and signaling molecules that act at the onset of hematopoiesis, and have begun to delineate the molecular mechanisms underlying the formation of HSCs. One molecule that has been a particularly useful marker of this developmental event in multiple species is Runx1 (also known as AML1, Pebp2alpha). Runx1 is a sequence-specific DNA-binding protein, that along with its homologues Runx2 and Runx3 and their shared non-DNA binding subunit CBFbeta, constitute a small family of transcription factors called core-binding factors (CBFs). Runx1 is famous for its role in HSC emergence, and notorious for its involvement in leukemia, as chromosomal rearrangements and inactivating mutations in the human RUNX1 gene are some of the most common events in de novo and therapy-related acute myelogenous leukemia, myelodysplastic syndrome and acute lymphocytic leukemia. Here we will review the role of Runx1 in HSC emergence in the mouse conceptus and describe some of the genetic pathways that operate upstream and downstream of this gene. Where relevant, we will include data obtained from other species and embryonic stem (ES) cell differentiation cultures.

Visualizing blood cell emergence from aortic endothelium.

Three recent Nature papers use time-lapse confocal imaging to visualize the birth of blood cells from the aortic endothelium. Two studies (Bertrand et al., 2010; Kissa and Herbomel, 2010) utilize the zebrafish embryo, while the third (Boisset et al., 2010) develops a novel technique to image the mouse aorta.

A conserved mechanism for vertebrate mesoderm specification in urodele amphibians and mammals.

Understanding how mesoderm is specified during development is a fundamental issue in biology, and it has been studied intensively in embryos from Xenopus. The gene regulatory network (GRN) for Xenopus is surprisingly complex and is not conserved in vertebrates, including mammals, which have single copies of the key genes Nodal and Mix. Why the Xenopus GRN should express multiple copies of Nodal and Mix genes is not known. To understand how these expanded gene families evolved, we investigated mesoderm specification in embryos from axolotls, representing urodele amphibians, since urodele embryology is basal to amphibians and was conserved during the evolution of amniotes, including mammals. We show that single copies of Nodal and Mix are required for mesoderm specification in axolotl embryos, suggesting the ancestral vertebrate state. Furthermore, we uncovered a novel genetic interaction in which Mix induces Brachyury expression, standing in contrast to the relationship of these molecules in Xenopus. However, we demonstrate that this functional relationship is conserved in mammals by showing that it is involved in the production of mesoderm from mouse embryonic stem cells. From our results, we produced an ancestral mesoderm (m)GRN, which we suggest is conserved in vertebrates. The results are discussed within the context of a theory in which the evolution of mechanisms governing early somatic development is constrained by the ancestral germ line-soma relationship, in which germ cells are produced by epigenesis.

Nonredundant roles for Runx1 alternative promoters reflect their activity at discrete stages of developmental hematopoiesis.

The transcription factor Runx1 is a pivotal regulator of definitive hematopoiesis in mouse ontogeny. Vertebrate Runx1 is transcribed from 2 promoters, the distal P1 and proximal P2, which provide a paradigm of the complex transcriptional and translational control of Runx1 function. However, very little is known about the biologic relevance of alternative Runx1 promoter usage in definitive hematopoietic cell emergence. Here we report that both promoters are active at the very onset of definitive hematopoiesis, with a skewing toward the P2. Moreover, functional and morphologic analysis of a novel P1-null and an attenuated P2 mouse model revealed that although both promoters play important nonredundant roles in the emergence of definitive hematopoietic cells, the proximal P2 was most critically required for this. The nature of the observed phenotypes is indicative of a differential contribution of the P1 and P2 promoters to the control of overall Runx1 levels, where and when this is most critically required. In addition, the dynamic expression of P1-Runx1 and P2-Runx1 points at a requirement for Runx1 early in development, when the P2 is still the prevalent promoter in the emerging hemogenic endothelium and/or first committed hematopoietic cells.

Alternative Runx1 promoter usage in mouse developmental hematopoiesis.

The interest in stem cell based therapies has emphasized the importance of understanding the cellular and molecular mechanisms by which stem cells are generated in ontogeny and maintained throughout adult life. Hematopoietic stem cells (HSCs) are first found in clusters of hematopoietic cells budding from the luminal wall of the major arteries in the developing mammalian embryo. The transcription factor Runx1 is critical for their generation and is specifically expressed at sites of HSC generation, prior to their formation. To understand better the transcriptional hierarchies that converge on Runx1 during HSC emergence, we have initiated studies into its transcriptional regulation. Here we systematically analyzed Runx1 P1 and P2 alternative promoter usage in hematopoietic sites and in sorted cell populations during mouse hematopoietic development. Our results indicate that Runx1 expression in primitive erythrocytes is largely P2-derived, whilst in definitive hematopoietic stem and/or progenitor cells from the yolk sac or AGM and vitelline and umbilical arteries both the distal P1 and proximal P2 promoters are active. After cells have migrated to the fetal liver, the P1 gradually becomes the main hematopoietic promoter and remains this into adulthood. In addition, we identified a novel P2-derived Runx1 isoform.

Transcriptional networks regulating hematopoietic cell fate decisions.

PURPOSE OF REVIEW: We provide a summary of the temporal cascade of transcriptional networks giving rise to the hematopoietic stem cell (HSC) and controlling differentiation of the erythroid lineage from it. We focus on the mechanisms by which cell fate decisions are made and comment on recent developments and additions to the networks. RECENT FINDINGS: A role for an SCL/LMO2 complex in HSC emergence, as well as in subsequent erythroid differentiation, has received support. Connections between the transcriptional networks and signaling molecules are being made but more work is needed in this area. Evidence that transcriptional cross-antagonistic switches underlie the choice between lineage pathways is increasing, and we highlight how the dynamics of earlier lineage decisions can influence later ones. Mathematical models are being built and reveal a surprising degree of power in these simple motifs to explain lineage choices. SUMMARY: New links in the transcriptional networks underlying cell-fate decisions are constantly emerging, and their incorporation into the evolving networks will make mathematical modeling more precise in its predictions of cell behavior, which can be tested experimentally.

Genetic regulatory networks programming hematopoietic stem cells and erythroid lineage specification.

Erythroid cell production results from passage through cellular hierarchies dependent on differential gene expression under the control of transcription factors responsive to changing niches. We have constructed Genetic Regulatory Networks (GRNs) describing this process, based predominantly on mouse data. Regulatory network motifs identified in E. coli and yeast GRNs are found in combination in these GRNs. Feed-forward motifs with autoregulation generate forward momentum and also control its rate, which is at its lowest in hematopoietic stem cells (HSCs). The simultaneous requirement for multiple regulators in multi-input motifs (MIMs) provides tight control over expression of target genes. Combinations of MIMs, exemplified by the SCL/LMO2 complexes, which have variable content and binding sites, explain how individual regulators can have different targets in HSCs and erythroid cells and possibly also how HSCs maintain stem cell functions while expressing lineage-affiliated genes at low level, so-called multi-lineage priming. MIMs combined with cross-antagonism describe the relationship between PU.1 and GATA-1 and between two of their target genes, Fli-1 and EKLF, with victory for GATA-1 and EKLF leading to erythroid lineage specification. These GRNs are useful repositories for current regulatory information, are accessible in interactive form via the internet, enable the consequences of perturbation to be predicted, and can act as seed networks to organize the rapidly accumulating microarray data.