Stepwise commitment from embryonic stem to hematopoietic and endothelial cells

ETS Transcription Factor ETV2/ER71/Etsrp in Hematopoietic and Vascular Development

Effective establishment of the hematopoietic and vascular systems is prerequisite for successful embryogenesis. The ETS transcription factor Etv2 has proven to be essential for hematopoietic and vascular development. Etv2 expression marks the onset of the hematopoietic and vascular development and its deficiency leads to an absolute block in hematopoietic and vascular development. Etv2 is transiently expressed during development and is mainly expressed in testis in adults. Consistent with its expression pattern, Etv2 is transiently required for the generation of the optimal levels of the hemangiogenic cell population. Deletion of this gene after the hemangiogenic progenitor formation leads to normal hematopoietic and vascular development. Mechanistically, ETV2 induces the hemangiogenic program by activating blood and endothelial cell lineage specifying genes and enhancing VEGF signaling. Moreover, ETV2 establishes an ETS hierarchy by directly activating other Ets genes, which in the face of transient Etv2 expression, presumably maintain blood and endothelial cell program initiated by ETV2 through an ETS switching mechanism. Current studies suggest that the hemangiogenic progenitor population is exclusively sensitive to ETV2-dependent FLK1 signaling. Any perturbation in the ETV2, VEGF, and FLK1 balance causing insufficient hemangiogenic progenitor cell generation would lead to defects in hematopoietic and endothelial cell development.

Lysophosphatidic acid acts as a nutrient-derived developmental cue to regulate early hematopoiesis

Primitive hematopoiesis occurs in the yolk sac blood islands during vertebrate embryogenesis, where abundant phosphatidylcholines (PC) are available as important nutrients for the developing embryo. However, whether these phospholipids also generate developmental cues to promote hematopoiesis is largely unknown. Here, we show that lysophosphatidic acid (LPA), a signaling molecule derived from PC, regulated hemangioblast formation and primitive hematopoiesis. Pharmacological and genetic blockage of LPA receptor 1 (LPAR1) or autotoxin (ATX), a secretory lysophospholipase that catalyzes LPA production, inhibited hematopoietic differentiation of mouse embryonic stem cells and impaired the formation of hemangioblasts. Mechanistic experiments revealed that the regulatory effect of ATX-LPA signaling was mediated by PI3K/Akt-Smad pathway. Furthermore, during in vivo embryogenesis in zebrafish, LPA functioned as a developmental cue for hemangioblast formation and primitive hematopoiesis. Taken together, we identified LPA as an important nutrient-derived developmental cue for primitive hematopoiesis as well as a novel mechanism of hemangioblast regulation.

Synergism of matrix stiffness and vascular endothelial growth factor on mesenchymal stem cells for vascular endothelial regeneration

Mesenchymal stem cells (MSCs) hold tremendous potential for vascular tissue regeneration. Research has demonstrated that individual factors in the cell microenvironment such as matrix elasticity and growth factors regulate MSC differentiation to vascular lineage. However, it is not well understood how matrix elasticity and growth factors combine to direct the MSC fate. This study examines the combined effects of matrix elasticity and vascular endothelial growth factor (VEGF) on both MSC differentiation into endothelial lineage and MSC paracrine signaling. MSCs were seeded in soft nanofibrous matrices with or without VEGF, and in Petri dishes with or without VEGF. Only MSCs seeded in three-dimensional soft matrices with VEGF showed significant increases in the expression of endothelial markers (vWF, eNOS, Flt-1, and Flk-1), while eliminating the expression of smooth muscle marker (SM-α-actin). MSCs cultured in VEGF alone on two-dimensional dishes showed increased expression of both early-stage endothelial and smooth muscle markers, indicating immature vascular differentiation. Furthermore, MSCs cultured in soft matrices with VEGF showed faster upregulation of endothelial markers compared with MSCs cultured in VEGF alone. Paracrine signaling studies found that endothelial cells cultured in the conditioned media from MSCs differentiated in the soft matrix and VEGF condition exhibited increased migration and formation of capillary-like structures. These results demonstrate that VEGF and soft matrix elasticity act synergistically to guide MSC differentiation into mature endothelial phenotype while enhancing paracrine signaling. Therefore, it is critical to control both mechanical and biochemical factors to safely regenerate vascular tissues with MSCs.

Standardization of pluripotent stem cell cultures for toxicity testing

INTRODUCTION

Pluripotent stem cell (PSC) lines offer a unique opportunity to derive various human cell types that can be exploited for human safety assessments in vitro and as such contribute to modern mechanistically oriented toxicity testing.

AREAS COVERED

This article reviews the two major types of PSC cultures that are currently most promising for toxicological applications: human embryonic stem cell lines and human induced PSC lines. Through the review, the article explains how these cell types will improve the current safety evaluations of chemicals and will allow a more efficient selection of drug candidates. Additionally, the article discusses the important issues of maintaining PSCs as well as their differentiation efficiency.

EXPERT OPINION

The demonstration of the reliability and relevance of in vitro toxicity tests for a given purpose is mandatory for their use in regulatory toxicity testing. Given the peculiar nature of PSCs, a high level of standardization of undifferentiated cell cultures as well as of the differentiation process is required in order to ensure the establishment of robust test systems. It is, therefore, of pivotal importance to define and internationally agree on crucial parameters to judge the quality of the cellular models before enrolling them for toxicity testing.

The hemangioblast: from concept to authentication

The hemangioblast hypothesis has been hotly debated for over a century. Hemangioblasts are defined as multipotent cells that can give rise to both hematopoietic cells and endothelial cells. The existence of hemangioblasts has now been confirmed and many important molecules and several signaling pathways are involved in their generation and differentiation. Fibroblast growth factor, renin-angiotensin system and runt-related transcription factor 1 (Runx1) direct the formation of hemangioblasts through highly selective gene expression patterns. On the other hand, the hemogenic endothelium theory and a newly discovered pattern of hematopoietic/endothelial differentiation make the genesis of hemangioblasts more complicated. But how hemangioblasts are formed and how hematopoietic cells or endothelial cells are derived from remains largely unknown. Here we summarize the current knowledge of the signaling pathways and molecules involved in hemangioblast development and suggest some future clinical applications.

Differential Hox expression in murine embryonic stem cell models of normal and malignant hematopoiesis

The Hox family are master transcriptional regulators of developmental processes, including hematopoiesis. The Hox regulators, caudal homeobox factors (Cdx1-4), and Meis1, along with several individual Hox proteins, are implicated in stem cell expansion during embryonic development, with gene dosage playing a significant role in the overall function of the integrated Hox network. To investigate the role of this network in normal and aberrant, early hematopoiesis, we employed an in vitro embryonic stem cell differentiation system, which recapitulates mouse developmental hematopoiesis. Expression profiles of Hox, Pbx1, and Meis1 genes were quantified at distinct stages during the hematopoietic differentiation process and compared with the effects of expressing the leukemic oncogene Tel/PDGFRβ. During normal differentiation the Hoxa cluster, Pbx1 and Meis1 predominated, with a marked reduction in the majority of Hox genes (27/39) and Meis1 occurring during hematopoietic commitment. Only the posterior Hoxa cluster genes (a9, a10, a11, and a13) maintained or increased expression at the hematopoietic colony stage. Cdx4, Meis1, and a subset of Hox genes, including a7 and a9, were differentially expressed after short-term oncogenic (Tel/PDGFRβ) induction. Whereas Hoxa4-10, b1, b2, b4, and b9 were upregulated during oncogenic driven myelomonocytic differentiation. Heterodimers between Hoxa7/Hoxa9, Meis1, and Pbx have previously been implicated in regulating target genes involved in hematopoietic stem cell (HSC) expansion and leukemic progression. These results provide direct evidence that transcriptional flux through the Hox network occurs at very early stages during hematopoietic differentiation and validates embryonic stem cell models for gaining insights into the genetic regulation of normal and malignant hematopoiesis.

Differentiation of mouse embryonic stem cells into blood

Embryonic stem (ES) cells can be maintained as pluripotent stem cells or induced to differentiate into many different somatic cell types. As ES-derived somatic cells can potentially be used for cell transplantation or cell-based therapy, ES cells have gained much scientific and general public attention. Successful derivation of blood from ES cells for tissue engineering will require a comprehensive understanding of inductive signals and downstream effectors involved in blood lineage development. Ideally, directed differentiation of ES cells into blood and isolation of pure hematopoietic progenitors will enhance our ability to utilize ES-derived blood cells for future clinical applications. The protocols provided in this unit describe methods of maintaining and differentiating mouse ES cells as well as identifying and isolating hematopoietic progenitors by utilizing flow cytometry and progenitor assays.

Molecular and developmental biology of the hemangioblast

The hemangioblast hypothesis was proposed a century ago. The existence of hemangioblasts is now demonstrated in mouse and human embryonic stem cell (ESC)-derived embryoid bodies (EBs), in the mouse and zebrafish gastrula, and in adults. The hemangioblast is believed to derive from mesodermal cells, and is enriched in the Bry+Flk1+ and Flk1+Scl+ cell populations in EBs and in the posterior primitive streak of the mouse gastrula and in the ventral mesoderm of the zebrafish gastrula. However, recent studies suggest that the hemangioblast does not give rise to all endothelial and hematopoietic lineages in mouse and zebrafish embryos. Although several signaling pathways are known to involve the generation of hemangioblasts, it remains largely unknown how the hemangioblast is formed and what are the master genes controlling hemangioblast development. This review will summarize our current knowledge, challenges, and future directions on molecular and developmental aspects of the hemangioblast.

Mouse lysocardiolipin acyltransferase controls the development of hematopoietic and endothelial lineages during in vitro embryonic stem-cell differentiation

The blast colony-forming cell (BL-CFC) was identified as an equivalent to the hemangioblast during in vitro embryonic stem (ES) cell differentiation. However, the molecular mechanisms underlying the generation of the BL-CFC remain largely unknown. Here we report the isolation of mouse lysocardiolipin acyltransferase (Lycat) based on homology to zebrafish lycat, a candidate gene for the cloche locus. Mouse Lycat is expressed in hematopoietic organs and is enriched in the Lin(-)C-Kit(+)Sca-1(+) hematopoietic stem cells in bone marrow and in the Flk1(+)/hCD4(+)(Scl(+)) hemangioblast population in embryoid bodies. The forced Lycat transgene leads to increased messenger RNA expression of hematopoietic and endothelial genes as well as increased blast colonies and their progenies, endothelial and hematopoietic lineages. The Lycat small interfering RNA transgene leads to a decrease expression of hematopoietic and endothelial genes. An unbiased genomewide microarray analysis further substantiates that the forced Lycat transgene specifically up-regulates a set of genes related to hemangioblasts and hematopoietic and endothelial lineages. Therefore, mouse Lycat plays an important role in the early specification of hematopoietic and endothelial cells, probably acting at the level of the hemangioblast.

GATA2 functions at multiple steps in hemangioblast development and differentiation

Molecular mechanisms that regulate the generation of hematopoietic and endothelial cells from mesoderm are poorly understood. To define the underlying mechanisms, we compared gene expression profiles between embryonic stem (ES) cell-derived hemangioblasts (Blast-Colony-Forming Cells, BL-CFCs) and their differentiated progeny, Blast cells. Bioinformatic analysis indicated that BL-CFCs resembled other stem cell populations. A role for Gata2, one of the BL-CFC-enriched transcripts, was further characterized by utilizing the in vitro model of ES cell differentiation. Our studies revealed that Gata2 was a direct target of BMP4 and that enforced GATA2 expression upregulated Bmp4, Flk1 and Scl. Conditional GATA2 induction resulted in a temporal-sensitive increase in hemangioblast generation, precocious commitment to erythroid fate, and increased endothelial cell generation. GATA2 additionally conferred a proliferative signal to primitive erythroid progenitors. Collectively, we provide compelling evidence that GATA2 plays specific, contextual roles in the generation of Flk-1+ mesoderm, the Flk-1+Scl+ hemangioblast, primitive erythroid and endothelial cells.