Molecular pathways governing development of vascular endothelial cells from ES/iPS cells

Assembly of complex vascular networks occurs in numerous biological systems through morphogenetic processes such as vasculogenesis, angiogenesis and vascular remodeling. Pluripotent stem cells such as embryonic stem (ES) and induced pluripotent stem (iPS) cells can differentiate into any cell type, including endothelial cells (ECs), and have been extensively used as in vitro models to analyze molecular mechanisms underlying EC generation and differentiation. The emergence of these promising new approaches suggests that ECs could be used in clinical therapy. Much evidence suggests that ES/iPS cell differentiation into ECs in vitro mimics the in vivo vascular morphogenic process. Through sequential steps of maturation, ECs derived from ES/iPS cells can be further differentiated into arterial, venous, capillary and lymphatic ECs, as well as smooth muscle cells. Here, we review EC development from ES/iPS cells with special attention to molecular pathways functioning in EC specification

Bone morphogenetic protein 4 differently promotes distinct VE-cadherin+ precursor stages during the definitive hematopoietic development from embryonic stem cell-derived mesodermal cells

Definitive hematopoietic cells develop from the fetal liver kinase 1 (Flk1)+ mesodermal cells during the in vitro differentiation of mouse embryonic stem cells (ESCs). VE-cadherin+CD41-CD45- (V+41-45-) hemogenic endothelial cells (HECs) and VE-cadherin+CD41+CD45- (V+41+45-) cells mediate the definitive hematopoietic development from Flk1+ cells. Bone morphogenetic protein 4 (BMP4) is known to be essential for the formation of mesoderm. However, the role of BMP4 in the differentiation of the VE-cadherin+ definitive hematopoietic precursors from the mesoderm has been elusive. We addressed this issue using a co-aggregation culture of ESC-derived Flk1+ cells with OP9 stromal cells. This culture method induced V+41-45- cells, V+41+45- cells, and CD45+ cells from Flk1+ cells. V+41+45- cells possessed the potentials of erythromyeloid and T-lymphoid differentiation. When Flk1+ cells were cultured in the presence of a high concentration of BMP4, the generation of V+41-45- cells was enhanced. The increase of V+41-45- cells led to the subsequent increase of V+41+45- cells and CD45+ cells. The addition of BMP4 also increased hematopoietic colony-forming cells of various lineages. Furthermore, BMP4 promoted the expansion of V+41+45- cells independently of the preceding V+41-45- cell stage. These results suggest that BMP4 has promotive effects on the differentiation of V+41-45- HECs from Flk1+ mesodermal cells and the subsequent proliferation of V+41+45- hematopoietic precursors. These findings may provide insights for establishing a culture system to induce definitive hematopoietic stem cells from ESCs

Morphology regulation in vascular endothelial cells

Abstract Morphological change in endothelial cells is an initial and crucial step in the process of establishing a functional vascular network. Following or associated with differentiation and proliferation, endothelial cells elongate and assemble into linear cord-like vessels, subsequently forming a perfusable vascular tube. In vivo and in vitro studies have begun to outline the underlying genetic and signaling mechanisms behind endothelial cell morphology regulation. This review focuses on the transcription factors and signaling pathways regulating endothelial cell behavior, involved in morphology, during vascular development

The canonical smooth muscle cell marker TAGLN is present in endothelial cells and is involved in angiogenesis

Elongation of vascular endothelial cells (ECs) is an important process in angiogenesis; however, the molecular mechanisms remain unknown. The actin-crosslinking protein TAGLN (transgelin, also known as SM22 or SM22α) is abundantly expressed in smooth muscle cells (SMCs) and is widely used as a canonical marker for this cell type. In the course of studies using mouse embryonic stem cells (ESCs) carrying an Tagln promoter-driven fluorescence marker, we noticed activation of the Tagln promoter during EC elongation. Tagln promoter activation co-occurred with EC elongation in response to vascular endothelial growth factor A (VEGF-A). Inhibition of phosphoinositide 3-kinase (PI3K)–Akt signaling and mTORC1 also induced EC elongation and Tagln promoter activation. Human umbilical vein endothelial cells (HUVECs) elongated, activated the TAGLN promoter and increased TAGLN transcripts in an angiogenesis model. Genetic disruption of TAGLN augmented angiogenic behaviors of HUVECs, as did the disruption of TAGLN2 and TAGLN3 genes. Tagln expression was found in ECs in mouse embryos. Our results identify TAGLN as a putative regulator of angiogenesis whose expression is activated in elongating ECs. This finding provides insight into the cytoskeletal regulation of EC elongation and an improved understanding of the molecular mechanisms underlying the regulation of angiogenesis

Identification of CD19-B220+c-Kit+Flt3/Flk-2+cells as early B lymphoid precursors before pre-B-I cells in juvenile mouse bone marrow

The combined analysis of the expression of receptor tyrosine kinases c-Kit and Flt3/Flk-2 and of the human CD25 gene expressed as a transgene under the regulation of the mouse λ5 promoter in the bone marrow of 1-week-old mice allows us to identify three stages of B lymphocyte development before the CD19+c-Kit+ pre-B-I cells. Single-cell PCR analysis of the rearrangement status of the Ig heavy chain alleles allows us to order these early stages of B cell development as follows: (i) B220+CD19-c-KitloFlt3/Flk-2hiλ5-, (ii) B220+CD19-c-KitloFlt3/Flk-2hiλ5+ and (iii) B220+CD19+c-KitloFlt3/Flk-2loλ5+ before B220+CD19+c-KitloFlt3/Flk-2-λ5+ pre-B-I cells. All these progenitors are clonable on stromal cells in the presence of IL-7 and can differentiate to CD19+c-Kit- B-lineage cells. A combination of stem cell factor, Flt3 ligand and IL-7 was also able to support the proliferation and differentiation of the progenitors in a suspension culture. Furthermore, the analyses indicate that the onset of DHJH rearrangements precedes the expression of the λ5 gene. These progenitor populations were characteristic of juvenile mice and could not be detected in the bone marrow of adult mice. Hence the expression pattern, and probably the function, of the receptor tyrosine kinases in early B cell differentiation appears to be different in juvenile and adult mic

SCL/tal-1-dependent process determines a competence to select the definitive hematopoietic lineage prior to endothelial differentiation

Hematopoiesis in most vertebrate species occurs in two distinct phases, primitive and definitive, which diverge from FLK1(+)VE-cadherin(–) mesoderm and FLK1(+)VE-cadherin(+) endothelial cells (EC), respectively. This study aimed at determining the stage at which hematopoietic lineage fate is determined by manipulating the SCL/tal-1 expression that is known to be essential for the early development of the primitive and definitive hematopoietic systems. We established SCL-null ES cell lines in which SCL expression is rescued by tamoxifen-inducible Cre recombinase-loxP site-mediated recombination. While no hematopoietic cells (HPC) were detected in SCL-null ES cell differentiation cultures, SCL gene reactivation from day 2 to day 4 after initiation of differentiation could rescue both primitive and definitive hematopoiesis. SCL reactivation at later phases was ineffective. Moreover, generation of VE-cadherin(+) EC that can give rise to definitive HPC required SCL reactivation prior to VE-cadherin expression. These results indicated that the competence to become HPC is acquired at the mesodermal stage by a SCL-dependent process that takes place independently of determination of endothelial fate