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The first artery and vein of the vertebrate embryo assemble in the trunk by migration and coalescence of angioblasts to form endothelial tubes. The gridlock (grl) mutation in zebrafish selectively perturbs assembly of the artery (the aorta). Here it is shown that gri encodes a basic helix-loop-helix (bHLH) protein belonging to the Hairy/Enhancer of the split family of bHLH proteins. The gri gene is expressed in lateral plate mesoderm before vessel formation, and thereafter in the aorta and not in the vein. These results suggest that the arterial endothelial identity is established even before the onset of blood flow and implicate the grl gene in assignment of vessel-specific cell fate.
Arteries and veins are morphologically and functionally very distinct. For example, arteries deliver oxygenated blood at high pressure from the heart, whereas veins serve as capacitance vessels for blood return. Some of the morphological differences may be imposed after, and depend upon, onset of function. However, a complete vascular loop, composed of the trunk dorsal aorta and posterior cardinal vein, is needed to accommodate the output of the first heartbeat. These simple tubes of endothelium form by local aggregation of angioblasts, a process termed vasculogenesis (1). Neither mutations nor molecular markers have revealed whether there are arterial-venous distinctions between angioblast progenitors. In the mouse, ephrinB2 is selectively expressed on the arteries and EphB3 and EphB4 on the veins, but this occurs after vasculogenesis (2). Furthermore, mutation of ephrinB2 does not affect vasculogenesis, although it does disrupt later vessel formation and remodeling, a process termed angiogenesis (2).
The gridlock mutation (grl^sup m145^) was originally isolated in a large-scale chemical mutagenesis screen (3) for developmental mutations of the zebrafish, Danio rerio. Homozygous mutant embryos have no circulation to the posterior trunk and tail because of a localized block to caudal blood flow at the base of the dorsal aorta, the region where the two anterior lateral dorsal aortae merge to form the single midline dorsal aorta. Cranial vessets and other trunk vessels appear to form normally in the mutants.
To clone the grl mutation, we first established its position on the genetic map, using single-strand length polymorphism (SSLP) markers and random amplified polymorphic DNAs (4, 5), and then generated a physical map of the...