Combinatorial function of ETS transcription factors in the developing vasculature

Chapter 4. Using the zebrafish to study vessel formation

Danio rerio, commonly referred to as the zebrafish, is a powerful animal model for studying the formation of the vasculature. Zebrafish offer unique opportunities for in vivo analysis of blood and lymphatic vessels formation because of their accessibility to large-scale genetic and experimental analysis as well as the small size, optical clarity, and external development of zebrafish embryos and larvae. A wide variety of established techniques are available to study vessel formation in the zebrafish, from early endothelial cell differentiation to adult vessel patterning. In this chapter, we review methods used to functionally manipulate and visualize the vasculature in the zebrafish and illustrate how these methods have helped further understanding of the genetic components regulating formation and patterning of developing vessels.

Vessel and blood specification override cardiac potential in anterior mesoderm

Organ progenitors arise within organ fields, embryonic territories that are larger than the regions required for organ formation. Little is known about the regulatory pathways that define organ field boundaries and thereby limit organ size. Here we identify a mechanism for restricting heart size through confinement of the developmental potential of the heart field. Via fate mapping in zebrafish, we locate cardiac progenitors within hand2-expressing mesoderm and demonstrate that hand2 potentiates cardiac differentiation within this region. Beyond the rostral boundary of hand2 expression, we find progenitors of vessel and blood lineages. In embryos deficient in vessel and blood specification, rostral mesoderm undergoes a fate transformation and generates ectopic cardiomyocytes. Therefore, induction of vessel and blood specification represses cardiac specification and delimits the capacity of the heart field. This regulatory relationship between cardiovascular pathways suggests strategies for directing progenitor cell differentiation to facilitate cardiac regeneration.

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.

A transgene-assisted genetic screen identifies essential regulators of vascular development in vertebrate embryos

Formation of a functional vasculature during mammalian development is essential for embryonic survival. In addition, imbalance in blood vessel growth contributes to the pathogenesis of numerous disorders. Most of our understanding of vascular development and blood vessel growth comes from investigating the Vegf signaling pathway as well as the recent observation that molecules involved in axon guidance also regulate vascular patterning. In order to take an unbiased, yet focused, approach to identify novel genes regulating vascular development, we performed a three-step ENU mutagenesis screen in zebrafish. We first screened live embryos visually, evaluating blood flow in the main trunk vessels, which form by vasculogenesis, and the intersomitic vessels, which form by angiogenesis. Embryos that displayed reduced or absent circulation were fixed and stained for endogenous alkaline phosphatase activity to reveal blood vessel morphology. All putative mutants were then crossed into the Tg(flk1:EGFP)(s843) transgenic background to facilitate detailed examination of endothelial cells in live and fixed embryos. We screened 4015 genomes and identified 30 mutations affecting various aspects of vascular development. Specifically, we identified 3 genes (or loci) that regulate the specification and/or differentiation of endothelial cells, 8 genes that regulate vascular tube and lumen formation, 8 genes that regulate vascular patterning, and 11 genes that regulate vascular remodeling, integrity and maintenance. Only 4 of these genes had previously been associated with vascular development in zebrafish illustrating the value of this focused screen. The analysis of the newly defined loci should lead to a greater understanding of vascular development and possibly provide new drug targets to treat the numerous pathologies associated with dysregulated blood vessel growth.

FoxH1 negatively modulates flk1 gene expression and vascular formation in zebrafish

Flk1 is the major receptor for VEGF on endothelial cells. During embryogenesis, flk1 is required for both vasculogenesis and angiogenesis and abnormally elevated flk1 expression is often associated with pathological conditions in adults. While the biological function of flk1 has been studied extensively, very little is known about how the flk1 gene is regulated at the transcriptional level. Our transgenic study led to the identification of a flk1 endothelial enhancer positioned approximately 5 kb upstream of the flk1 translation initiation site. Binding sites for FoxH1, scl, ets and gata factors are found in the zebrafish flk1 endothelial enhancer, as well as in upstream sequences of mouse flk1 and human kdr genes, suggesting that the regulatory machinery for flk1/kdr is conserved from fish to mammals. The roles of scl, ets and gata factors in hemangioblasts have been well defined, but the significance of FoxH1 in vessel formation has not been explored previously. Here we show that FoxH1 binds to the flk1 endothelial enhancer in vitro and functions as a repressor for flk1 transcription in cultured cells. Consistent with these findings, the expression level of flk1 is elevated in embryos lacking both maternal and zygotic FoxH1. We further show that overexpression of FoxH1 has a negative effect on vascular formation that can be counteracted by the down-regulation of smad2 activity in zebrafish embryos. Taken together, our data provide the first evidence that flk1 is a direct target of FoxH1 and that FoxH1 is involved in vessel formation in zebrafish.