Insights into early vasculogenesis revealed by expression of the ETS-domain transcription factor Fli-1 in wild-type and mutant zebrafish embryos

Abstract

Fli-1 is an ETS-domain transcription factor whose locus is disrupted in Ewing's Sarcoma and F-MuLV induced erythroleukaemia. To gain a better understanding of its normal function, we have isolated the zebrafish homologue. Similarities with other vertebrates, in the amino acid sequence and DNA binding properties of Fli-1 from zebrafish, suggest that its function has been conserved during vertebrate evolution. The initial expression of zebrafish fli-1 in the posterior lateral mesoderm overlaps with that of gata2 in a potential haemangioblast population which likely contains precursors of blood and endothelium. Subsequently, fli-1 and gata2 expression patterns diverge, with separate fli-1 and gata2 expression domains arising in the developing vasculature and in sites of blood formation respectively. Elsewhere in the embryo, fli-1 is expressed in sites of vasculogenesis. The expression of fli-1 was investigated in a number of zebrafish mutants, which affect the circulatory system. In cloche, endothelium is absent and blood is drastically reduced. In contrast to the blood and endothelial markers that have been studied previously, fli-1 expression was initiated normally in cloche embryos, indicating that induction of fli-1 is one of the earliest indicators of haemangioblast formation. Furthermore, although fli-1 expression in the trunk was not maintained, the normal expression pattern in the anterior half of the embryo was retained. These anterior cells did not, however, condense to form blood vessels. These data indicate that cloche has previously unsuspected roles at multiple stages in the formation of the vasculature. Analysis of fli-1 expression in midline patterning mutants floating head and squint, confirms a requirement for the notochord in the formation of the dorsal-aorta. The formation of endothelium in one-eyed pinhead, cyclops and squint embryos indicates a novel role for the endoderm in the formation of the axial vein. The phenotype of sonic-you mutants implies a likely role for Sonic Hedgehog in mediating these processes.

Migratory path of definitive hematopoietic stem/progenitor cells during zebrafish development

“…Fluorescence images were acquired with Olympus Fluoview confocal microscope. Based on the temporal and spatial expression pattern of transcription factor fli1, 30 we reasoned that the Tg(fli1:eGFP) transgenic line, in which the EGFP expression is under the control of the fli1 promoter, 26 might provide a unique tool to investigate hematopoietic stem cell development in early zebrafish embryo. First we investigated whether green fluorescence protein could mark putative definitive HSPCs presumably located in the ventral wall of DA during zebrafish embryogenesis.…”

Molecular and developmental biology of the hemangioblast

“…To address roles of the endoderm in angioblast and hematopoietic development, Vokes and Krieg have assessed the presence of hematopoietic and endothelial gene expression in endoderm-depleted gastrulation embryos, either by surgical removal of endoderm or treatment of VegT antisense oligonucleotides in the frog gastrula, or in animal caps treated with bFGF that have mesoderm but no endoderm (Vokes and Krieg, 2002). Similarly several investigators have reported that endothelial and hematopoietic lineages are present in zebrafish genetic mutants such as one-eyed pinhead, casanova, cyclops, and squint, which show complete absence or strong reduction of the endoderm (Brown et al, 2000;Lawson et al, 2001;Jin et al, 2005). In addition, hematopoiesis and vasculogenesis were observed in Gata4 Ϫ/Ϫ ESC-derived EBs that lack the visceral yolk sac endoderm (Bielinska et al, 1996).…”

“…In zebrafish and frog embryos, the endoderm is not required for generation of hematopoietic and endothelial lineages (Brown et al, 2000;Lawson et al, 2001;Vokes and Krieg, 2002;Jin et al, 2005). To address roles of the endoderm in angioblast and hematopoietic development, Vokes and Krieg have assessed the presence of hematopoietic and endothelial gene expression in endoderm-depleted gastrulation embryos, either by surgical removal of endoderm or treatment of VegT antisense oligonucleotides in the frog gastrula, or in animal caps treated with bFGF that have mesoderm but no endoderm (Vokes and Krieg, 2002).…”

“…This represents the first single gene mutation that eliminates both endothelial and hematopoietic lineages, probably in the level of hemangioblasts. Epistasis analyses have shown that cloche acts upstream of zbp-89, scl, lmo2, gata1, gata2, runx1, c-myb, fli1, flk1, tie2, hhex, and etsrp in hematopoietic and endothelial cell development in zebrafish (Stainier et al, 1995;Fouquet et al, 1997;Liao et al, 1997Liao et al, , 1998Liao et al, , 2000Brown et al, 2000;Burns et al, 2002;Patterson et al, 2005Patterson et al, , 2007Li et al, 2006;Pham et al, 2006;Sumanas and Lin, 2006). In addition, microarray analyses by comparing cloche mutant and wild-type embryos have revealed that cloche specifically regulates a panel of hematopoietic and endothelial genes (Qian et al, 2005;Sumanas et al, 2005;Weber et al, 2005).…”

Egfl7 knockdown causes defects in the extension and junctional arrangements of endothelial cells during zebrafish vasculogenesis

“…In this mutant, the angioblasts show a delayed migration to the midline, after which they form a PCV with dilations (Jin et al, 2005). Another mutant with deficient endoderm and derivatives, the one-eyed pinhead (oep) mutant, forms an intact DA, but no PCV, suggesting that signals from the endoderm are necessary for axial vein formation (Brown et al, 2000).…”