Prof Marella de Bruijn

Research Area: Developmental and Stem Cell Biology
Technology Exchange: Cell sorting, ES cell / homologous recombination, Flow cytometry, Immunohistochemistry, In situ hybridisation, Microscopy (Confocal), Mouse models, Transcript profiling and Transgenesis
Scientific Themes: Developmental Biology & Stem Cells and Haematology
Keywords: Hematopoietic stem cells, Hemogenic endothelium, Developmental biology, Lineage specification and Transcriptional regulation
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A central question in the stem cell field is what are the cellular and molecular mechanisms that underlie the generation and maintenance of the different types of stem cells. The focus of our research is the origin of the haemopoietic stem cells (HSCs) during mouse embryonic development. The first HSCs appear in the aorta-gonad-mesonephros (AGM) region and in the vitelline and umbilical arteries of the midgestation mouse embryo. Runx1, the gene encoding the DNA- binding subunit of the heterodimeric transcription factor Runx1:CBFb, is expressed at these sites prior to the generation of functional HSCs, and is absolutely required for HSCs generation. The specific temporal and spatial pattern of Runx1 expression suggests a critical role for Runx1 in the specification of precursor cells towards the haemopoietic lineage and in the establishment and maintenance of haemopoietic differentiation programs. It also suggests that factors regulating the expression of Runx1 are important to the development of the haemopoietic system. Because of its pivotal position at the onset of haemopoiesis, our lab uses Runx1 as an entry point for studies aimed at building a roadmap of HSC development in the embryo and identifying the signals and transcription factors required for HSC specification and maintenance. These studies provide a basis for building gene regulatory networks underlying HSC specification. Such insights are expected to contribute to the development of new therapeutic strategies for blood-related disorders. RUNX1 mutations are involved in human leukaemia, and a deeper understanding of the role of Runx1 in the regulatory network underlying HSC emergence could also shed light the molecular mechanisms of leukaemia onset and progression.

Name Department Institution Country
Prof Berthold Gottgens Cambridge Stem Cell Institute University of Cambridge United Kingdom
Dr John Pimanda University of New South Wales, Sydney Australia
Prof Roger Patient Nuffield Division of Clinical Laboratory Sciences Oxford University, Weatherall Institute of Molecular Medicine United Kingdom
Prof Catherine Porcher Nuffield Division of Clinical Laboratory Sciences Oxford University, Weatherall Institute of Molecular Medicine United Kingdom
Prof Sten Eirik Jacobsen Nuffield Division of Clinical Laboratory Sciences Oxford University, Weatherall Institute of Molecular Medicine United Kingdom
Prof Thomas Milne Nuffield Division of Clinical Laboratory Sciences Oxford University, Weatherall Institute of Molecular Medicine United Kingdom
Migueles RP, Shaw L, Rodrigues NP, May G, Henseleit K, Anderson KG, Goker H, Jones CM, de Bruijn MF, Brickman JM, Enver T. 2017. Transcriptional regulation of Hhex in hematopoiesis and hematopoietic stem cell ontogeny. Dev Biol, 424 (2), pp. 236-245. | Show Abstract | Read more

Hematopoietic stem cells (HSCs) emerge during development via an endothelial-to-hematopoietic transition from hemogenic endothelium of the dorsal aorta (DA). Using in situ hybridization and analysis of a knock-in RedStar reporter, we show that the transcriptional regulator Hhex is expressed in endothelium of the dorsal aorta (DA) and in clusters of putative HSCs as they are specified during murine development. We exploited this observation, using the Hhex locus to define cis regulatory elements, enhancers and interacting transcription factors that are both necessary and sufficient to support gene expression in the emerging HSC. We identify an evolutionarily conserved non-coding region (ECR) in the Hhex locus with the capacity to bind the hematopoietic-affiliated transcriptional regulators Gata2, SCL, Fli1, Pu.1 and Ets1/2. This region is sufficient to drive the expression of a transgenic GFP reporter in the DA endothelium and intra-aortic hematopoietic clusters. GFP-positive AGM cells co-expressed HSC-associated markers c-Kit, CD34, VE-Cadherin, and CD45, and were capable of multipotential differentiation and long term engraftment when transplanted into myelo-ablated recipients. The Hhex ECR was also sufficient to drive expression at additional blood sites including the yolk sac blood islands, fetal liver, vitelline and umbilical arteries and the adult bone marrow, suggesting a common mechanism for Hhex regulation throughout ontogenesis of the blood system. To explore the physiological requirement for the Hhex ECR region during hematoendothelial development, we deleted the ECR element from the endogenous locus in the context of a targeted Hhex-RedStar reporter allele. Results indicate a specific requirement for the ECR in blood-associated Hhex expression during development and further demonstrate a requirement for this region in the adult HSC compartment. Taken together, our results identified the ECR region as an enhancer both necessary and sufficient for gene expression in HSC development and homeostasis. The Hhex ECR thus appears to be a core node for the convergence of the transcription factor network that governs the emergence of HSCs.

de Bruijn M, Dzierzak E. 2017. Runx transcription factors in the development and function of the definitive hematopoietic system. Blood, 129 (15), pp. 2061-2069. | Show Abstract | Read more

The Runx family of transcription factors (Runx1, Runx2, and Runx3) are highly conserved and encode proteins involved in a variety of cell lineages, including blood and blood-related cell lineages, during developmental and adult stages of life. They perform activation and repressive functions in the regulation of gene expression. The requirement for Runx1 in the normal hematopoietic development and its dysregulation through chromosomal translocations and loss-of-function mutations as found in acute myeloid leukemias highlight the importance of this transcription factor in the healthy blood system. Whereas another review will focus on the role of Runx factors in leukemias, this review will provide an overview of the normal regulation and function of Runx factors in hematopoiesis and focus particularly on the biological effects of Runx1 in the generation of hematopoietic stem cells. We will present the current knowledge of the structure and regulatory features directing lineage-specific expression of Runx genes, the models of embryonic and adult hematopoietic development that provide information on their function, and some of the mechanisms by which they affect hematopoietic function.

Luis TC, Luc S, Mizukami T, Boukarabila H, Thongjuea S, Woll PS, Azzoni E, Giustacchini A, Lutteropp M, Bouriez-Jones T et al. 2016. Initial seeding of the embryonic thymus by immune-restricted lympho-myeloid progenitors. Nat Immunol, 17 (12), pp. 1424-1435. | Show Abstract | Read more

The final stages of restriction to the T cell lineage occur in the thymus after the entry of thymus-seeding progenitors (TSPs). The identity and lineage potential of TSPs remains unclear. Because the first embryonic TSPs enter a non-vascularized thymic rudiment, we were able to directly image and establish the functional and molecular properties of embryonic thymopoiesis-initiating progenitors (T-IPs) before their entry into the thymus and activation of Notch signaling. T-IPs did not include multipotent stem cells or molecular evidence of T cell-restricted progenitors. Instead, single-cell molecular and functional analysis demonstrated that most fetal T-IPs expressed genes of and had the potential to develop into lymphoid as well as myeloid components of the immune system. Moreover, studies of embryos deficient in the transcriptional regulator RBPJ demonstrated that canonical Notch signaling was not involved in pre-thymic restriction to the T cell lineage or the migration of T-IPs.

de Bruijn M. 2016. Turning it down a Notch Blood, 128 (12), pp. 1541-1542. | Show Abstract | Read more

© 2016 by The American Society of Hematology. In this issue of Blood, Souilhol et al pinpoint the requirement for Notch signaling to precisely defined stages in hematopoietic stem cell (HSC) emergence and implicate both Notch1 and Notch2 in this process. 1

Pereira CF, Chang B, Gomes A, Bernitz J, Papatsenko D, Niu X, Swiers G, Azzoni E, de Bruijn MF, Schaniel C et al. 2016. Hematopoietic Reprogramming In Vitro Informs In Vivo Identification of Hemogenic Precursors to Definitive Hematopoietic Stem Cells. Dev Cell, 36 (5), pp. 525-539. | Show Abstract | Read more

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.

Schütte J, Wang H, Antoniou S, Jarratt A, Wilson NK, Riepsaame J, Calero-Nieto FJ, Moignard V, Basilico S, Kinston SJ et al. 2016. An experimentally validated network of nine haematopoietic transcription factors reveals mechanisms of cell state stability. Elife, 5 (FEBRUARY2016), pp. e11469. | Show Abstract | Read more

Transcription factor (TF) networks determine cell-type identity by establishing and maintaining lineage-specific expression profiles, yet reconstruction of mammalian regulatory network models has been hampered by a lack of comprehensive functional validation of regulatory interactions. Here, we report comprehensive ChIP-Seq, transgenic and reporter gene experimental data that have allowed us to construct an experimentally validated regulatory network model for haematopoietic stem/progenitor cells (HSPCs). Model simulation coupled with subsequent experimental validation using single cell expression profiling revealed potential mechanisms for cell state stabilisation, and also how a leukaemogenic TF fusion protein perturbs key HSPC regulators. The approach presented here should help to improve our understanding of both normal physiological and disease processes.

Blobel GA, Bodine D, Brand M, Crispino J, de Bruijn MF, Nathan D, Papayannopoulou T, Porcher C, Strouboulis J, Zon L et al. 2015. An international effort to cure a global health problem: A report on the 19th Hemoglobin Switching Conference. Exp Hematol, 43 (10), pp. 821-837. | Show Abstract | Read more

Every 2 years since 1978, an international group of scientists, physicians, and other researchers meet to discuss the latest developments in the underlying etiology, mechanisms of action, and developmental acquisition of cellular and systemic defects exhibited and elicited by the most common inherited human disorders, the hemoglobinopathies. The 19th Hemoglobin Switching Conference, held in September 2014 at St. John's College in Oxford, once again exceeded all expectations by describing cutting edge research in cellular, molecular, developmental, and genomic advances focused on these diseases. The conference comprised about 60 short talks over 3 days by leading investigators in the field. This meeting report describes the highlights of the conference.

Marks-Bluth J, Khanna A, Chandrakanthan V, Thoms J, Bee T, Eich C, Kang YC, Knezevic K, Qiao Q, Fitch S et al. 2015. SMAD1 and SMAD5 Expression Is Coordinately Regulated by FLI1 and GATA2 during Endothelial Development. Mol Cell Biol, 35 (12), pp. 2165-2172. | Show Abstract | Read more

The bone morphogenetic protein (BMP)/SMAD signaling pathway is a critical regulator of angiogenic sprouting and is involved in vascular development in the embryo. SMAD1 and SMAD5, the core mediators of BMP signaling, are vital for this activity, yet little is known about their transcriptional regulation in endothelial cells. Here, we have integrated multispecies sequence conservation, tissue-specific chromatin, in vitro reporter assay, and in vivo transgenic data to identify and validate Smad1+63 and the Smad5 promoter as tissue-specific cis-regulatory elements that are active in the developing endothelium. The activity of these elements in the endothelium was dependent on highly conserved ETS, GATA, and E-box motifs, and chromatin immunoprecipitation showed high levels of enrichment of FLI1, GATA2, and SCL at these sites in endothelial cell lines and E11 dorsal aortas in vivo. Knockdown of FLI1 and GATA2 but not SCL reduced the expression of SMAD1 and SMAD5 in endothelial cells in vitro. In contrast, CD31(+) cKit(-) endothelial cells harvested from embryonic day 9 (E9) aorta-gonad-mesonephros (AGM) regions of GATA2 null embryos showed reduced Smad1 but not Smad5 transcript levels. This is suggestive of a degree of in vivo selection where, in the case of reduced SMAD1 levels, endothelial cells with more robust SMAD5 expression have a selective advantage.

Gomez Perdiguero E, Klapproth K, Schulz C, Busch K, Azzoni E, Crozet L, Garner H, Trouillet C, de Bruijn MF, Geissmann F, Rodewald HR. 2015. Tissue-resident macrophages originate from yolk-sac-derived erythro-myeloid progenitors. Nature, 518 (7540), pp. 547-551. | Show Abstract | Read more

Most haematopoietic cells renew from adult haematopoietic stem cells (HSCs), however, macrophages in adult tissues can self-maintain independently of HSCs. Progenitors with macrophage potential in vitro have been described in the yolk sac before emergence of HSCs, and fetal macrophages can develop independently of Myb, a transcription factor required for HSC, and can persist in adult tissues. Nevertheless, the origin of adult macrophages and the qualitative and quantitative contributions of HSC and putative non-HSC-derived progenitors are still unclear. Here we show in mice that the vast majority of adult tissue-resident macrophages in liver (Kupffer cells), brain (microglia), epidermis (Langerhans cells) and lung (alveolar macrophages) originate from a Tie2(+) (also known as Tek) cellular pathway generating Csf1r(+) erythro-myeloid progenitors (EMPs) distinct from HSCs. EMPs develop in the yolk sac at embryonic day (E) 8.5, migrate and colonize the nascent fetal liver before E10.5, and give rise to fetal erythrocytes, macrophages, granulocytes and monocytes until at least E16.5. Subsequently, HSC-derived cells replace erythrocytes, granulocytes and monocytes. Kupffer cells, microglia and Langerhans cells are only marginally replaced in one-year-old mice, whereas alveolar macrophages may be progressively replaced in ageing mice. Our fate-mapping experiments identify, in the fetal liver, a sequence of yolk sac EMP-derived and HSC-derived haematopoiesis, and identify yolk sac EMPs as a common origin for tissue macrophages.

Vierstra J, Rynes E, Sandstrom R, Zhang M, Canfield T, Hansen RS, Stehling-Sun S, Sabo PJ, Byron R, Humbert R et al. 2014. Mouse regulatory DNA landscapes reveal global principles of cis-regulatory evolution. Science, 346 (6212), pp. 1007-1012. | Show Abstract | Read more

To study the evolutionary dynamics of regulatory DNA, we mapped >1.3 million deoxyribonuclease I-hypersensitive sites (DHSs) in 45 mouse cell and tissue types, and systematically compared these with human DHS maps from orthologous compartments. We found that the mouse and human genomes have undergone extensive cis-regulatory rewiring that combines branch-specific evolutionary innovation and loss with widespread repurposing of conserved DHSs to alternative cell fates, and that this process is mediated by turnover of transcription factor (TF) recognition elements. Despite pervasive evolutionary remodeling of the location and content of individual cis-regulatory regions, within orthologous mouse and human cell types the global fraction of regulatory DNA bases encoding recognition sites for each TF has been strictly conserved. Our findings provide new insights into the evolutionary forces shaping mammalian regulatory DNA landscapes.

Yue F, Cheng Y, Breschi A, Vierstra J, Wu W, Ryba T, Sandstrom R, Ma Z, Davis C, Pope BD et al. 2014. A comparative encyclopedia of DNA elements in the mouse genome. Nature, 515 (7527), pp. 355-364. | Show Abstract | Read more

The laboratory mouse shares the majority of its protein-coding genes with humans, making it the premier model organism in biomedical research, yet the two mammals differ in significant ways. To gain greater insights into both shared and species-specific transcriptional and cellular regulatory programs in the mouse, the Mouse ENCODE Consortium has mapped transcription, DNase I hypersensitivity, transcription factor binding, chromatin modifications and replication domains throughout the mouse genome in diverse cell and tissue types. By comparing with the human genome, we not only confirm substantial conservation in the newly annotated potential functional sequences, but also find a large degree of divergence of sequences involved in transcriptional regulation, chromatin state and higher order chromatin organization. Our results illuminate the wide range of evolutionary forces acting on genes and their regulatory regions, and provide a general resource for research into mammalian biology and mechanisms of human diseases.

de Bruijn M. 2014. The hemangioblast revisited. Blood, 124 (16), pp. 2472-2473. | Show Abstract | Read more

In this issue of Blood, Padrón-Barthe et al explore the role of the hemangioblast as the cell of origin for yolk sac blood and endothelium.

Wilkinson AC, Kawata VK, Schütte J, Gao X, Antoniou S, Baumann C, Woodhouse S, Hannah R, Tanaka Y, Swiers G et al. 2014. Single-cell analyses of regulatory network perturbations using enhancer-targeting TALEs suggest novel roles for PU.1 during haematopoietic specification. Development, 141 (20), pp. 4018-4030. | Show Abstract | Read more

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.

Liakhovitskaia A, Rybtsov S, Smith T, Batsivari A, Rybtsova N, Rode C, de Bruijn M, Buchholz F, Gordon-Keylock S, Zhao S, Medvinsky A. 2014. Runx1 is required for progression of CD41+ embryonic precursors into HSCs but not prior to this. Development, 141 (17), pp. 3319-3323. | Show Abstract | Read more

Haematopoiesis in adult animals is maintained by haematopoietic stem cells (HSCs), which self-renew and can give rise to all blood cell lineages. The AGM region is an important intra-embryonic site of HSC development and a wealth of evidence indicates that HSCs emerge from the endothelium of the embryonic dorsal aorta and extra-embryonic large arteries. This, however, is a stepwise process that occurs through sequential upregulation of CD41 and CD45 followed by emergence of fully functional definitive HSCs. Although largely dispensable at later stages, the Runx1 transcription factor is crucially important during developmental maturation of HSCs; however, exact points of crucial involvement of Runx1 in this multi-step developmental maturation process remain unclear. Here, we have investigated requirements for Runx1 using a conditional reversible knockout strategy. We report that Runx1 deficiency does not preclude formation of VE-cad+CD45-CD41+ cells, which are phenotypically equivalent to precursors of definitive HSCs (pre-HSC Type I) but blocks transition to the subsequent CD45+ stage (pre-HSC Type II). These data emphasise that developmental progression of HSCs during a very short period of time is regulated by precise stage-specific molecular mechanisms.

Swiers G, Baumann C, O'Rourke J, Giannoulatou E, Taylor S, Joshi A, Moignard V, Pina C, Bee T, Kokkaliaris KD et al. 2013. Early dynamic fate changes in haemogenic endothelium characterized at the single-cell level. Nat Commun, 4 pp. 2924. | Show Abstract | Read more

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.

Moignard V, Macaulay IC, Swiers G, Buettner F, Schütte J, Calero-Nieto FJ, Kinston S, Joshi A, Hannah R, Theis FJ et al. 2013. Characterization of transcriptional networks in blood stem and progenitor cells using high-throughput single-cell gene expression analysis. Nat Cell Biol, 15 (4), pp. 363-372. | Show Abstract | Read more

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.

Mirshekar-Syahkal B, Haak E, Kimber GM, van Leusden K, Harvey K, O'Rourke J, Laborda J, Bauer SR, de Bruijn MF, Ferguson-Smith AC et al. 2013. Dlk1 is a negative regulator of emerging hematopoietic stem and progenitor cells. Haematologica, 98 (2), pp. 163-171. | Show Abstract | Read more

The first mouse adult-repopulating hematopoietic stem cells emerge in the aorta-gonad-mesonephros region at embryonic day (E) 10.5. Their numbers in this region increase thereafter and begin to decline at E12.5, thus pointing to the possible existence of both positive and negative regulators of emerging hematopoietic stem cells. Our recent expression analysis of the aorta-gonad-mesonephros region showed that the Delta-like homologue 1 (Dlk1) gene is up-regulated in the region of the aorta-gonad-mesonephros where hematopoietic stem cells are preferentially located. To analyze its function, we studied Dlk1 expression in wild-type and hematopoietic stem cell-deficient embryos and determined hematopoietic stem and progenitor cell activity in Dlk1 knockout and overexpressing mice. Its role in hematopoietic support was studied in co-culture experiments using stromal cell lines that express varying levels of Dlk1. We show here that Dlk1 is expressed in the smooth muscle layer of the dorsal aorta and the ventral sub-aortic mesenchyme, where its expression is dependent on the hematopoietic transcription factor Runx1. We further demonstrate that Dlk1 has a negative impact on hematopoietic stem and progenitor cell activity in the aorta-gonad-mesonephros region in vivo, which is recapitulated in co-cultures of hematopoietic stem cells on stromal cells that express varying levels of Dlk1. This negative effect of Dlk1 on hematopoietic stem and progenitor cell activity requires the membrane-bound form of the protein and cannot be recapitulated by soluble Dlk1. Together, these data suggest that Dlk1 expression by cells of the aorta-gonad-mesonephros hematopoietic microenvironment limits hematopoietic stem cell expansion and is, to our knowledge, the first description of such a negative regulator in this tissue.

Böiers C, Carrelha J, Lutteropp M, Luc S, Green JC, Azzoni E, Woll PS, Mead AJ, Hultquist A, Swiers G et al. 2013. Lymphomyeloid contribution of an immune-restricted progenitor emerging prior to definitive hematopoietic stem cells. Cell Stem Cell, 13 (5), pp. 535-548. | Show Abstract | Read more

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.

Swiers G, Rode C, Azzoni E, de Bruijn MF. 2013. A short history of hemogenic endothelium. Blood Cells Mol Dis, 51 (4), pp. 206-212. | Show Abstract | Read more

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.

Wilkinson AC, Ballabio E, Geng H, North P, Tapia M, Kerry J, Biswas D, Roeder RG, Allis CD, Melnick A et al. 2013. RUNX1 is a key target in t(4;11) leukemias that contributes to gene activation through an AF4-MLL complex interaction. Cell Rep, 3 (1), pp. 116-127. | Show Abstract | Read more

The Mixed Lineage Leukemia (MLL) protein is an important epigenetic regulator required for the maintenance of gene activation during development. MLL chromosomal translocations produce novel fusion proteins that cause aggressive leukemias in humans. Individual MLL fusion proteins have distinct leukemic phenotypes even when expressed in the same cell type, but how this distinction is delineated on a molecular level is poorly understood. Here, we highlight a unique molecular mechanism whereby the RUNX1 gene is directly activated by MLL-AF4 and the RUNX1 protein interacts with the product of the reciprocal AF4-MLL translocation. These results support a mechanism of transformation whereby two oncogenic fusion proteins cooperate by activating a target gene and then modulating the function of its downstream product.

Luc S, Luis TC, Boukarabila H, Macaulay IC, Buza-Vidas N, Bouriez-Jones T, Lutteropp M, Woll PS, Loughran SJ, Mead AJ et al. 2012. The earliest thymic T cell progenitors sustain B cell and myeloid lineage potential. Nat Immunol, 13 (4), pp. 412-419. | Show Abstract | Read more

The stepwise commitment from hematopoietic stem cells in the bone marrow to T lymphocyte-restricted progenitors in the thymus represents a paradigm for understanding the requirement for distinct extrinsic cues during different stages of lineage restriction from multipotent to lineage-restricted progenitors. However, the commitment stage at which progenitors migrate from the bone marrow to the thymus remains unclear. Here we provide functional and molecular evidence at the single-cell level that the earliest progenitors in the neonatal thymus had combined granulocyte-monocyte, T lymphocyte and B lymphocyte lineage potential but not megakaryocyte-erythroid lineage potential. These potentials were identical to those of candidate thymus-seeding progenitors in the bone marrow, which were closely related at the molecular level. Our findings establish the distinct lineage-restriction stage at which the T cell lineage-commitment process transits from the bone marrow to the remote thymus.

Knezevic K, Bee T, Wilson NK, Janes ME, Kinston S, Polderdijk S, Kolb-Kokocinski A, Ottersbach K, Pencovich N, Groner Y et al. 2011. A Runx1-Smad6 rheostat controls Runx1 activity during embryonic hematopoiesis. Mol Cell Biol, 31 (14), pp. 2817-2826. | Show Abstract | Read more

The oncogenic transcription factor Runx1 is required for the specification of definitive hematopoietic stem cells (HSC) in the developing embryo. The activity of this master regulator is tightly controlled during development. The transcription factors that upregulate the expression of Runx1 also upregulate the expression of Smad6, the inhibitory Smad, which controls Runx1 activity by targeting it to the proteasome. Here we show that Runx1, in conjunction with Fli1, Gata2, and Scl, directly regulates the expression of Smad6 in the aorta-gonad-mesonephros (AGM) region in the developing embryo, where HSCs originate. Runx1 regulates Smad6 activity via a novel upstream enhancer, and Runx1 null embryos show reduced Smad6 transcripts in the yolk-sac and c-Kit-positive fetal liver cells. By directly regulating the expression of Smad6, Runx1 sets up a functional rheostat to control its own activity. The perturbation of this rheostat, using a proteasomal inhibitor, results in an increase in Runx1 and Smad6 levels that can be directly attributed to increased Runx1 binding to tissue-specific regulatory elements of these genes. Taken together, we describe a scenario in which a key hematopoietic transcription factor controls its own expression levels by transcriptionally controlling its controller.

Wilson NK, Foster SD, Wang X, Knezevic K, Schütte J, Kaimakis P, Chilarska PM, Kinston S, Ouwehand WH, Dzierzak E et al. 2010. Combinatorial transcriptional control in blood stem/progenitor cells: genome-wide analysis of ten major transcriptional regulators. Cell Stem Cell, 7 (4), pp. 532-544. | Show Abstract | Read more

Combinatorial transcription factor (TF) interactions control cellular phenotypes and, therefore, underpin stem cell formation, maintenance, and differentiation. Here, we report the genome-wide binding patterns and combinatorial interactions for ten key regulators of blood stem/progenitor cells (SCL/TAL1, LYL1, LMO2, GATA2, RUNX1, MEIS1, PU.1, ERG, FLI-1, and GFI1B), thus providing the most comprehensive TF data set for any adult stem/progenitor cell type to date. Genome-wide computational analysis of complex binding patterns, followed by functional validation, revealed the following: first, a previously unrecognized combinatorial interaction between a heptad of TFs (SCL, LYL1, LMO2, GATA2, RUNX1, ERG, and FLI-1). Second, we implicate direct protein-protein interactions between four key regulators (RUNX1, GATA2, SCL, and ERG) in stabilizing complex binding to DNA. Third, Runx1(+/-)::Gata2(+/-) compound heterozygous mice are not viable with severe hematopoietic defects at midgestation. Taken together, this study demonstrates the power of genome-wide analysis in generating novel functional insights into the transcriptional control of stem and progenitor cells.

Bee T, Swiers G, Muroi S, Pozner A, Nottingham W, Santos AC, Li PS, Taniuchi I, de Bruijn MF. 2010. Nonredundant roles for Runx1 alternative promoters reflect their activity at discrete stages of developmental hematopoiesis. Blood, 115 (15), pp. 3042-3050. | Show Abstract | Read more

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.

Swiers G, Speck NA, de Bruijn MF. 2010. Visualizing blood cell emergence from aortic endothelium. Cell Stem Cell, 6 (4), pp. 289-290. | Show Abstract | Read more

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.

Swiers G, de Bruijn M, Speck NA. 2010. Hematopoietic stem cell emergence in the conceptus and the role of Runx1. Int J Dev Biol, 54 (6-7), pp. 1151-1163. | Show Abstract | Read more

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.

Peeters M, Ottersbach K, Bollerot K, Orelio C, de Bruijn M, Wijgerde M, Dzierzak E. 2009. Ventral embryonic tissues and Hedgehog proteins induce early AGM hematopoietic stem cell development. Development, 136 (15), pp. 2613-2621. | Show Abstract | Read more

Hematopoiesis is initiated in several distinct tissues in the mouse conceptus. The aorta-gonad-mesonephros (AGM) region is of particular interest, as it autonomously generates the first adult type hematopoietic stem cells (HSCs). The ventral position of hematopoietic clusters closely associated with the aorta of most vertebrate embryos suggests a polarity in the specification of AGM HSCs. Since positional information plays an important role in the embryonic development of several tissue systems, we tested whether AGM HSC induction is influenced by the surrounding dorsal and ventral tissues. Our explant culture results at early and late embryonic day 10 show that ventral tissues induce and increase AGM HSC activity, whereas dorsal tissues decrease it. Chimeric explant cultures with genetically distinguishable AGM and ventral tissues show that the increase in HSC activity is not from ventral tissue-derived HSCs, precursors or primordial germ cells (as was previously suggested). Rather, it is due to instructive signaling from ventral tissues. Furthermore, we identify Hedgehog protein(s) as an HSC inducing signal.

Bee T, Liddiard K, Swiers G, Bickley SR, Vink CS, Jarratt A, Hughes JR, Medvinsky A, de Bruijn MF. 2009. Alternative Runx1 promoter usage in mouse developmental hematopoiesis. Blood Cells Mol Dis, 43 (1), pp. 35-42. | Show Abstract | Read more

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.

Bee T, Ashley EL, Bickley SR, Jarratt A, Li PS, Sloane-Stanley J, Göttgens B, de Bruijn MF. 2009. The mouse Runx1 +23 hematopoietic stem cell enhancer confers hematopoietic specificity to both Runx1 promoters. Blood, 113 (21), pp. 5121-5124. | Show Abstract | Read more

The transcription factor Runx1 plays a pivotal role in hematopoietic stem cell (HSC) emergence, and studies into its transcriptional regulation should give insight into the critical steps of HSC specification. Recently, we identified the Runx1 +23 enhancer that targets reporter gene expression to the first emerging HSCs of the mouse embryo when linked to the heterologous hsp68 promoter. Endogenous Runx1 is transcribed from 2 alternative promoters, P1 and P2. Here, we examined the in vivo cis-regulatory potential of these alternative promoters and asked whether they act with and contribute to the spatiotemporal specific expression of the Runx1 +23 enhancer. Our results firmly establish that, in contrast to zebrafish runx1, mouse Runx1 promoter sequences do not confer any hematopoietic specificity in transgenic embryos. Yet, both mouse promoters act with the +23 enhancer to drive reporter gene expression to sites of HSC emergence and colonization, in a +23-specific pattern.

Landry JR, Kinston S, Knezevic K, de Bruijn MF, Wilson N, Nottingham WT, Peitz M, Edenhofer F, Pimanda JE, Ottersbach K, Göttgens B. 2008. Runx genes are direct targets of Scl/Tal1 in the yolk sac and fetal liver. Blood, 111 (6), pp. 3005-3014. | Show Abstract | Read more

Transcription factors such as Scl/Tal1, Lmo2, and Runx1 are essential for the development of hematopoietic stem cells (HSCs). However, the precise mechanisms by which these factors interact to form transcriptional networks, as well as the identity of the genes downstream of these regulatory cascades, remain largely unknown. To this end, we generated an Scl(-/-) yolk sac cell line to identify candidate Scl target genes by global expression profiling after reintroduction of a TAT-Scl fusion protein. Bioinformatics analysis resulted in the identification of 9 candidate Scl target transcription factor genes, including Runx1 and Runx3. Chromatin immunoprecipitation confirmed that both Runx genes are direct targets of Scl in the fetal liver and that Runx1 is also occupied by Scl in the yolk sac. Furthermore, binding of an Scl-Lmo2-Gata2 complex was demonstrated to occur on the regions flanking the conserved E-boxes of the Runx1 loci and was shown to transactivate the Runx1 element. Together, our data provide a key component of the transcriptional network of early hematopoiesis by identifying downstream targets of Scl that can explain key aspects of the early Scl(-/-) phenotype.

Nottingham WT, Jarratt A, Burgess M, Speck CL, Cheng JF, Prabhakar S, Rubin EM, Li PS, Sloane-Stanley J, Kong-A-San J, de Bruijn MF. 2007. Runx1-mediated hematopoietic stem-cell emergence is controlled by a Gata/Ets/SCL-regulated enhancer. Blood, 110 (13), pp. 4188-4197. | Show Abstract | Read more

The transcription factor Runx1/AML1 is an important regulator of hematopoiesis and is critically required for the generation of the first definitive hematopoietic stem cells (HSCs) in the major vasculature of the mouse embryo. As a pivotal factor in HSC ontogeny, its transcriptional regulation is of high interest but is largely undefined. In this study, we used a combination of comparative genomics and chromatin analysis to identify a highly conserved 531-bp enhancer located at position + 23.5 in the first intron of the 224-kb mouse Runx1 gene. We show that this enhancer contributes to the early hematopoietic expression of Runx1. Transcription factor binding in vivo and analysis of the mutated enhancer in transient transgenic mouse embryos implicate Gata2 and Ets proteins as critical factors for its function. We also show that the SCL/Lmo2/Ldb-1 complex is recruited to the enhancer in vivo. Importantly, transplantation experiments demonstrate that the intronic Runx1 enhancer targets all definitive HSCs in the mouse embryo, suggesting that it functions as a crucial cis-regulatory element that integrates the Gata, Ets, and SCL transcriptional networks to initiate HSC generation.

Pimanda JE, Donaldson IJ, de Bruijn MF, Kinston S, Knezevic K, Huckle L, Piltz S, Landry JR, Green AR, Tannahill D, Göttgens B. 2007. The SCL transcriptional network and BMP signaling pathway interact to regulate RUNX1 activity. Proc Natl Acad Sci U S A, 104 (3), pp. 840-845. | Show Abstract | Read more

Hematopoietic stem cell (HSC) development is regulated by several signaling pathways and a number of key transcription factors, which include Scl/Tal1, Runx1, and members of the Smad family. However, it remains unclear how these various determinants interact. Using a genome-wide computational screen based on the well characterized Scl +19 HSC enhancer, we have identified a related Smad6 enhancer that also targets expression to blood and endothelial cells in transgenic mice. Smad6, Bmp4, and Runx1 transcripts are concentrated along the ventral aspect of the E10.5 dorsal aorta in the aorta-gonad-mesonephros region from which HSCs originate. Moreover, Smad6, an inhibitor of Bmp4 signaling, binds and inhibits Runx1 activity, whereas Smad1, a positive mediator of Bmp4 signaling, transactivates the Runx1 promoter. Taken together, our results integrate three key determinants of HSC development; the Scl transcriptional network, Runx1 activity, and the Bmp4/Smad signaling pathway.

Jaffredo T, Nottingham W, Liddiard K, Bollerot K, Pouget C, de Bruijn M. 2005. From hemangioblast to hematopoietic stem cell: an endothelial connection? Exp Hematol, 33 (9), pp. 1029-1040. | Show Abstract | Read more

The developmental origin of hematopoietic stem cells has been the subject of much research. Now that the developmental link between the hematopoietic system and the vasculature has been well established, questions remain regarding the precise cellular origin of definitive hematopoietic cells and at what point they branch off from the endothelial lineage. Do they emerge directly from a hemangioblast-type cell, similar to what is proposed for primitive yolk sac hematopoiesis, or are they generated via an endothelial intermediate, the hemogenic endothelium? In this review, we will give an overview of the data obtained from the mouse and avian models on the cellular origins of the hematopoietic system.

de Bruijn MF, Speck NA. 2004. Core-binding factors in hematopoiesis and immune function. Oncogene, 23 (24), pp. 4238-4248. | Show Abstract | Read more

Core binding factors are heterodimeric transcription factors containing a DNA binding Runx1, Runx2, or Runx3 subunit, along with a non DNA binding CBF beta subunit. All four subunits are required at one or more stages of hematopoiesis. This review describes the role of Runx1 and CBF beta in the initiation of hematopoiesis in the embryo, and in the emergence of hematopoietic stem cells. We also discuss the later stages of hematopoiesis for which members of the core binding factor family are required, as well as the recently described roles for these proteins in autoimmunity.

Schmidt U, van den Akker E, Parren-van Amelsvoort M, Litos G, de Bruijn M, Gutiérrez L, Hendriks RW, Ellmeier W, Löwenberg B, Beug H, von Lindern M. 2004. Btk is required for an efficient response to erythropoietin and for SCF-controlled protection against TRAIL in erythroid progenitors. J Exp Med, 199 (6), pp. 785-795. | Show Abstract | Read more

Regulation of survival, expansion, and differentiation of erythroid progenitors requires the well-controlled activity of signaling pathways induced by erythropoietin (Epo) and stem cell factor (SCF). In addition to qualitative regulation of signaling pathways, quantitative control may be essential to control appropriate cell numbers in peripheral blood. We demonstrate that Bruton's tyrosine kinase (Btk) is able to associate with the Epo receptor (EpoR) and Jak2, and is a substrate of Jak2. Deficiency of Btk results in reduced and delayed phosphorylation of the EpoR, Jak2, and downstream signaling molecules such as Stat5 and PLCgamma1 as well as in decreased responsiveness to Epo. As a result, expansion of erythroid progenitors lacking Btk is impaired at limiting concentrations of Epo and SCF. In addition, we show that SCF induces Btk to interact with TNF-related apoptosis-inducing ligand (TRAIL)-receptor 1 and that lack of Btk results in increased sensitivity to TRAIL-induced apoptosis. Together, our results indicate that Btk is a novel, quantitative regulator of Epo/SCF-dependent expansion and survival in erythropoiesis.

North TE, Stacy T, Matheny CJ, Speck NA, de Bruijn MF. 2004. Runx1 is expressed in adult mouse hematopoietic stem cells and differentiating myeloid and lymphoid cells, but not in maturing erythroid cells. Stem Cells, 22 (2), pp. 158-168. | Show Abstract | Read more

The transcription factor Runx1 marks all functional hematopoietic stem cells (HSCs) in the embryo and is required for their generation. Mutations in Runx1 are found in approximately 25% of acute leukemias and in familial platelet disorder, suggesting a role for Runx1 in adult hematopoiesis as well. A comprehensive analysis of Runx1 expression in adult hematopoiesis is lacking. Here we show that Runx1 is expressed in functional HSCs in the adult mouse, as well as in cells with spleen colony-forming unit (CFU) and culture CFU capacities. Additionally, we document Runx1 expression in all hematopoietic lineages at the single cell level. Runx1 is expressed in the majority of myeloid cells and in a smaller proportion of lymphoid cells. Runx1 expression substantially decreases during erythroid differentiation. We also document effects of reduced Runx1 levels on adult hematopoiesis.

Baumeister T, Rössner S, Pech G, de Bruijn MF, Leenen PJ, Schuler G, Lutz MB. 2003. Interleukin-3Ralpha+ myeloid dendritic cells and mast cells develop simultaneously from different bone marrow precursors in cultures with interleukin-3. J Invest Dermatol, 121 (2), pp. 280-288. | Show Abstract | Read more

The distinct developmental routes of dendritic cells and mast cells from murine bone marrow cultures with interleukin-3 are unclear. We found that short-term bone marrow cultures with interleukin-3 after 8-10 d consist of about 10%-30% dendritic cells and 70%-90% mast cell precursors, and only after 4-6 wk do homogeneous populations of mast cells emerge. Phenotypical and functional analysis of interleukin-3/dendritic cells revealed a high similarity with myeloid dendritic cells generated with granulocyte-macrophage colony stimulating factor in the expression of myeloid dendritic cell markers (CD11c+ B220- CD8alpha- CD11b+), major histocompatibility complex II and costimulatory molecules, endocytosis, maturation potential, interleukin-12 production, and T cell priming. Interleukin-3/dendritic cells expressed higher levels of interleukin-3 receptor, however. To dissect the interleukin-3/dendritic cell and mast cell development, we sorted fresh bone marrow cells into six subsets by the antibodies ER-MP12 (CD31) and ER-MP20 (Ly-6C). Both interelukin-3/dendritic cells and granulocyte-macrophage colony stimulating factor/dendritic cells develop from the same bone marrow populations, including the ER-MP12neg, ER-MP20high bone marrow monocytes. In contrast, mast cells only developed from ER-MP12(int+high), ER-MP20neg bone marrow cell subsets, indicating that different precursors exist for interleukin-3/dendritic cells and mast cells. Established mast cell cultures could not be converted to dendritic cells or stimulated to express major histocompatibility complex II molecules in vitro or home to lymph node T cell areas in vivo. In summary, we show that dendritic cells generated from bone marrow precursors with interleukin-3 are clearly myeloid and develop via a different pathway compared to bone marrow mast cells.

Nikolic T, de Bruijn MF, Lutz MB, Leenen PJ. 2003. Developmental stages of myeloid dendritic cells in mouse bone marrow. Int Immunol, 15 (4), pp. 515-524. | Show Abstract | Read more

The lineage relationship of dendritic cells (DC) with other hematopoietic cell types has been studied extensively, resulting in the identification of different bone marrow (BM) progenitors that give rise to distinct DC types. However, the identity of the different maturation stages of DC precursors in the BM remains unclear. In this study we define the in vivo developmental steps of the myeloid DC lineage in mouse BM. To this end, BM cells were separated according to their expression of CD31 (ER-MP12), Ly-6C (ER-MP20) and ER-MP58 antigens, and stimulated to develop into myeloid DC, using granulocyte macrophage colony stimulating factor as a specific growth factor. DC developed from three BM subpopulations: ER-MP12(hi)/20(-) (early blast cells), ER-MP12(+)/20(+) (myeloid blasts) and ER-MP12(-)/20(hi) (monocytes). The kinetic and phenotypic features of DC developing in vitro indicate that the three populations represent successive maturation stages of myeloid DC precursors. Within the earliest ER-MP12(hi)/20(-) population, DC precursors exclusively occurred in the myeloid-restricted ER-MP58(hi) subset. By using switch cultures, we show that these BM precursor subpopulations, when stimulated to develop into macrophages using macrophage colony stimulating factor, retain the ability to develop into myeloid DC until advanced stages of maturation. Together, these findings support a common ER-MP12/20-defined differentiation pathway for both macrophages and myeloid DC throughout their BM development.

Robin C, Ottersbach K, de Bruijn M, Ma X, van der Horn K, Dzierzak E. 2003. Developmental origins of hematopoietic stem cells. Oncol Res, 13 (6-10), pp. 315-321. | Show Abstract | Read more

Hematopoietic stem cells (HSCs) are at the foundation of the hematopoietic hierarchy and give rise to all blood lineages in the adult organism. A thorough understanding of the molecular, cellular, and developmental biology of HSCs is of fundamental importance, but is also clinically relevant for the advancement of cell replacement therapies and transplantation protocols in blood-related genetic disease and leukemias. While the major anatomical sites of hematopoiesis change during ontogeny, it was long believed that the developmental origin of the adult mammalian hematopoietic system was the yolk sac. However, current studies have shown that the first adult-type HSCs are autonomously generated in the intrabody portion of the mouse embryo, the aorta-gonads-mesonephros (AGM) region, and sublocalize to the dorsal aorta. HSCs are also found in the other large embryonic vessels, the vitelline and umbilical arteries. The intraluminal hematopoietic clusters along these vessels, together with the role of the Runx1 transcription factor in cluster and HSC formation and the HSC/endothelial/mesenchymal Runxl expression pattern, strongly suggest a vascular endothelial/mesenchymal origin for the first HSCs. Moreover, a transgenic mouse line expressing the GFP marker under the control of the Sca-1 transcriptional regulatory elements (GFP expression marks all HSCs) shows a clear localization of GFP-expressing cells to the endothelial cell layer of the dorsal aorta. Thus, highly enriched GFP-positive AGM HSCs will serve as a basis for the future examination of the cellular and molecular factors involved in the induction and expansion of adult HSCs.

North TE, de Bruijn MF, Stacy T, Talebian L, Lind E, Robin C, Binder M, Dzierzak E, Speck NA. 2002. Runx1 expression marks long-term repopulating hematopoietic stem cells in the midgestation mouse embryo. Immunity, 16 (5), pp. 661-672. | Show Abstract | Read more

Hematopoietic stem cells (HSCs) are first found in the aorta-gonad-mesonephros region and vitelline and umbilical arteries of the midgestation mouse embryo. Runx1 (AML1), the DNA binding subunit of a core binding factor, is required for the emergence and/or subsequent function of HSCs. We show that all HSCs in the embryo express Runx1. Furthermore, HSCs in Runx1(+/-) embryos are heterogeneous and include CD45(+) cells, endothelial cells, and mesenchymal cells. Comparison with wild-type embryos showed that the distribution of HSCs among these various cell populations is sensitive to Runx1 dosage. These data provide the first morphological description of embryonic HSCs and contribute new insight into their cellular origin.

de Bruijn MFTR, Ma X, Robin C, Ottersbach K, Sanchez M-J, Dzierzak E. 2002. Hematopoietic Stem Cells Localize to the Endothelial Cell Layer in the Midgestation Mouse Aorta Immunity, 16 (5), pp. 673-683. | Read more

Oostendorp RA, Harvey KN, Kusadasi N, de Bruijn MF, Saris C, Ploemacher RE, Medvinsky AL, Dzierzak EA. 2002. Stromal cell lines from mouse aorta-gonads-mesonephros subregions are potent supporters of hematopoietic stem cell activity. Blood, 99 (4), pp. 1183-1189. | Show Abstract | Read more

The aorta-gonads-mesonephros (AGM) region autonomously generates the first adult repopulating hematopoietic stem cells (HSCs) in the mouse embryo. HSC activity is initially localized to the dorsal aorta and mesenchyme (AM) and vitelline and umbilical arteries. Thereafter, HSC activity is found in the urogenital ridges (UGs), yolk sac, and liver. As increasing numbers of HSCs are generated, it is thought that these sites provide supportive microenvironments in which HSCs are harbored until the bone marrow microenvironment is established. However, little is known about the supportive cells within these midgestational sites, and particularly which microenvironment is most supportive for HSC growth and maintenance. Thus, to better understand the cells and molecules involved in hematopoietic support in the midgestation embryo, more than 100 stromal cell lines and clones were established from these sites. Numerous stromal clones were found to maintain hematopoietic progenitors and HSCs to a similar degree as, or better than, previously described murine stromal lines. Both the AM and UG subregions of the AGM produced many supportive clones, with the most highly HSC-supportive clone being derived from the UGs. Interestingly, the liver at this stage yielded only few supportive stromal clones. These results strongly suggest that during midgestation, not only the AM but also the UG subregion provides a potent microenvironment for growth and maintenance of the first HSCs.

Ma X, de Bruijn M, Robin C, Peeters M, Kong-A-San J, de Wit T, Snoijs C, Dzierzak E. 2002. Expression of the Ly-6A (Sca-1) lacZ transgene in mouse haematopoietic stem cells and embryos. Br J Haematol, 116 (2), pp. 401-408. | Show Abstract | Read more

The Sca-1 surface glycoprotein is used routinely as a marker for haematopoietic stem cell enrichment. Two allelic genes, Ly-6A and Ly-6E, encode this marker and appear to be differentially regulated in haematopoietic cells and haematopoietic stem cells. The Sca-1 protein has been shown to be expressed at a greater frequency in these cells from Ly-6A strains of mice. To study the specific expression pattern and haematopoietic regulation of the Ly-6A gene, we constructed a 14 kb cassette from a genomic Ly-6A fragment, inserted a lacZ reporter gene and created transgenic mice. We found that the Ly-6A lacZ transgene was expressed in the haematopoietic tissues and predominantly in the T-lymphoid lineage. Some expression was also found in the B-lymphoid and myeloid lineages. We demonstrated functional haematopoietic stem cell enrichment by sorting for beta-galactosidase-expressing cells from the bone marrow. In addition, we found an interesting embryonic expression pattern in the AGM region, the site of the first haematopoietic stem cell generation. Surprisingly, when compared with data from Ly-6E lacZ transgenic mice, our results suggest that the Ly-6A cassette does not improve lacZ marker gene expression in haematopoietic cells.

Cai Z, de Bruijn M, Ma X, Dortland B, Luteijn T, Downing RJ, Dzierzak E. 2000. Haploinsufficiency of AML1 affects the temporal and spatial generation of hematopoietic stem cells in the mouse embryo. Immunity, 13 (4), pp. 423-431. | Show Abstract | Read more

The AML1:CBFbeta transcription factor complex is essential for definitive hematopoiesis. Null mutations in mouse AML1 result in midgestational lethality with a complete lack of fetal liver hematopoiesis. While the cell autonomous nature and expression pattern of AML1 suggest an intrinsic role for this transcription factor in the developing hematopoietic system, no direct link to a functional cell type has been made. Here, we examine the consequences of AML1 loss in hematopoietic stem cells (HSC) of the mouse embryo. We demonstrate an absolute requirement for AML1 in functional HSCs. Moreover, haploinsufficiency results in a dramatic change in the temporal and spatial distribution of HSCs, leading to their early appearance in the normal position in the aorta-gonad-mesonephros region and also in the yolk sac.

Whyatt D, Lindeboom F, Karis A, Ferreira R, Milot E, Hendriks R, de Bruijn M, Langeveld A, Gribnau J, Grosveld F, Philipsen S. 2000. An intrinsic but cell-nonautonomous defect in GATA-1-overexpressing mouse erythroid cells. Nature, 406 (6795), pp. 519-524. | Show Abstract | Read more

GATA-1 is a tissue-specific transcription factor that is essential for the production of red blood cells. Here we show that overexpression of GATA-1 in erythroid cells inhibits their differentiation, leading to a lethal anaemia. Using chromosome-X-inactivation of a GATA-1 transgene and chimaeric animals, we show that this defect is intrinsic to erythroid cells, but nevertheless cell nonautonomous. Usually, cell nonautonomy is thought to reflect aberrant gene function in cells other than those that exhibit the phenotype. On the basis of our data, we propose an alternative mechanism in which a signal originating from wild-type erythroid cells restores normal differentiation to cells overexpressing GATA-1 in vivo. The existence of such a signalling mechanism indicates that previous interpretations of cell-nonautonomous defects may be erroneous in some cases and may in fact assign gene function to incorrect cell types.

de Bruijn MFTR, van Vianen W, Ploemacher RE, Bakker-Woudenberg IAJM, Campbell PA, van Ewijk W, Leenen PJM. 1998. Bone marrow cellular composition in Listeria monocytogenes infected mice detected using ER-MP12 and ER-MP20 antibodies: a flow cytometric alternative to differential counting Journal of Immunological Methods, 217 (1-2), pp. 27-39. | Read more

Dzierzak E, Medvinsky A, de Bruijn M. 1998. Qualitative and quantitative aspects of haematopoietic cell development in the mammalian embryo Immunology Today, 19 (5), pp. 228-236. | Read more

de Bruijn MFTR, Slieker WAT, van der Loo JCM, Voerman JSA, van Ewijk W, Leenen PJM. 1994. Distinct mouse bone marrow macrophage precursors identified by differential expression of ER-MP12 and ER-MP20 antigens European Journal of Immunology, 24 (10), pp. 2279-2284. | Read more

Leenen PJM, de Bruijn MFTR, Voerman JSA, Campbell PA, van Ewijk W. 1994. Markers of mouse macrophage development detected by monoclonal antibodies Journal of Immunological Methods, 174 (1-2), pp. 5-19. | Read more

Slieker WAT, de Rijk-de Bruijn MFTR, Leenen PJM, van Ewijk W. 1993. ER-MP12 antigen, a new cell surface marker on mouse bone marrow cells with thymus-repopulating ability: I. Intrathymic repopulating ability of ER-MP12-positive bone marrow cells International Immunology, 5 (9), pp. 1093-1098. | Read more

Slieker WAT, van der Loo JCM, de Rlik-de Bruijn MFTR, Godfrey DI, Leenen PJM, van Ewijk W. 1993. ER-MP12 antigen, a new cell surface marker on mouse bone marrow cells with thymus-repopulating ability: II. Thymus-homing ability and phenotypic characterization of ER-MP12-positive bone marrow cells International Immunology, 5 (9), pp. 1099-1107. | Read more

Arraf AA, De Bruijn MFTR, Schultheiss TM. 2017. Disruption of the aortic wall by coelomic lining-derived mesenchymal cells accompanies the onset of aortic hematopoiesis International Journal of Developmental Biology, 61 (3-5), pp. 329-335. | Show Abstract | Read more

© 2017 UPV/EHU Press. In vertebrates, definitive hematopoietic stem cells (HSCs) first emerge in the ventral wall of the aorta in the Aorta-Gonad-Mesonephros (AGM) region of the embryo, where they differentiate from a specialized type of endothelium termed Hemogenic Endothelium (HE). While the transition from HE to hematopoietic tissue has received much experimental attention, much less is known regarding generation of HE itself. The current study investigates the emergence of the HE in the chick embryo aorta. Using the HE marker Runx1 as well as a new chicken-reactive antibody to the endothelial marker VE-Cadherin, we document the relationship between the emerging HE and surrounding tissues, particularly the coelomic epithelium (CE) and CE-derived sub-aortic mesenchyme. In addition, the fate of the CE cells was traced by electroporation of a GFP-expressing plasmid into the CE, followed by analysis using immunofluorescence and in situ hybridization. We make the novel observation that CE-derived mesenchyme transiently invades through the ventral wall of the aorta during the period of establishment of HE and just prior to the emergence of hematopoietic cell clusters in the ventral aortic wall. These observations emphasize a hitherto unappreciated dynamism in the aortic wall during the period of HE generation, and open the door to future studies regarding the role of invasive CE-derived cells during aortic hematopoiesis.

de Bruijn MF, Speck NA, Peeters MC, Dzierzak E. 2000. Definitive hematopoietic stem cells first develop within the major arterial regions of the mouse embryo. EMBO J, 19 (11), pp. 2465-2474. | Show Abstract | Read more

The aorta-gonad-mesonephros (AGM) region is a potent hematopoietic site within the mammalian embryo body, and the first place from which hematopoietic stem cells (HSCs) emerge. Within the complex embryonic vascular, excretory and reproductive tissues of the AGM region, the precise location of HSC development is unknown. To determine where HSCs develop, we subdissected the AGM into aorta and urogenital ridge segments and transplanted the cells into irradiated adult recipients. We demonstrate that HSCs first appear in the dorsal aorta area. Furthermore, we show that vitelline and umbilical arteries contain high frequencies of HSCs coincident with HSC appearance in the AGM. While later in development and after organ explant culture we find HSCs in the urogenital ridges, our results strongly suggest that the major arteries of the embryo are the most important sites from which definitive HSCs first emerge.

Hendriks RW, de Bruijn MF, Maas A, Dingjan GM, Karis A, Grosveld F. 1996. Inactivation of Btk by insertion of lacZ reveals defects in B cell development only past the pre-B cell stage. EMBO J, 15 (18), pp. 4862-4872. | Show Abstract

Bruton's tyrosine kinase (Btk) is a cytoplasmic protein kinase that is defective in X-linked agammaglobulinaemia in man and in X-linked immunodeficiency in the mouse. There is controversy regarding the stages of B cell development that are dependent on Btk function. To determine the point in B cell differentiation at which defects in Btk become apparent, we generated a mouse model by inactivating the Btk gene through an in-frame insertion of a lacZ reporter by homologous recombination in embryonic stem cells. The phenomenon of X-chromosome inactivation in Btk+/- heterozygous female mice enabled us to evaluate the competition between B cell progenitors expressing wild-type Btk and those expressing the Btk-/lacZ allele in each successive step of development. Although Btk was already expressed in pro-B cells, the first selective disadvantage only became apparent at the transition from small pre-B cells to immature B cells in the bone marrow. A second differentiation arrest was found during the maturation from IgD(low)IgM(high) to IgD(high)IgM(low) stages in the periphery. Our results show that Btk expression is essential at two distinct differentiation steps, both past the pre-B cell stage.