Targeting the hemangioblast with a novel cell type-specific enhancer

We report the identification of a hemangioblast-specific enhancer (Hb) located in the cis-regulatory region of chick Cerberus gene (cCer) that is able to direct the expression of enhanced green fluorescent protein (eGFP) to the precursors of yolk sac blood and endothelial cells in electroporated chick embryos. Moreover, we present the Hb-eGFP reporter as a powerful live imaging tool for visualizing hemangioblast cell fate and blood island morphogenesis.We hereby introduce the Hb enhancer as a valuable resource for genetically targeting the hemangioblast population as well as for studying the dynamics of vascular and blood cell development.In the early vertebrate embryo, both hematopoietic and endothelial lineages derive from aggregates of mesodermal cells that form the blood islands in the extraembryonic yolk sac [1]. This observation led to the hypothesis that both lineages derive from a common precursor named the hemangioblast [2]. Although still debatable, the existence of hemangioblasts is mainly supported by in vitro differentiation studies [3,4] as well as by evidence that blood and endothelial progenitors express a number of genes in common, such as VEGFR2, GATA2, Lmo2 and Scl/Tal1 [5], some of which regulate the differentiation of both cell lineages [6-8]. Other than these in vitro and genetic studies, further insight into hemangioblast cell fate will require time-lapse imaging studies using a lineage-specific marker.Hemangioblast reporters have been described in transgenic mouse and zebrafish models [9,10]. However, a particularly suitable system for visualizing live hematovascular development is the yolk sac of the avian embryo [11,12]. During the study of chick Cerberus (cCer) transcriptional regulation [13], we isolated a cis-regulatory region that drives reporter gene expression specifically in blood-island progenitors or hemangioblasts. Here, we report the identification and characterization of this novel hemangioblast-specific enhancer and reveal its