We used to think that differences in blood flow determined whether a blood vessel developed as an artery or a vein, but recently it became clear that this decision is genetically predetermined through vascular endothelial growth factor (Vegf) signalling. Using transgenic zebrafish, Nathan Lawson and colleagues have now identified plcg1 as a key gene, downstream of Vegf, which directs endothelial cells along the arterial developmental pathway.

In a classic forward-genetic approach, Lawson et al. screened a zebrafish line that had fluorescently labelled blood vessels for developing embryos that lacked blood vessels between the somites — blocks of cells that give rise to the backbone and body muscle. In this way they identified a mutant (y10) with defects in the development of arteries but not veins, and reduced expression of artery-specific genes in the embryonic dorsal aorta.

The y10 mutant mimicked mouse and zebrafish Vegf-knockout phenotypes but mapped to a location distinct from that of vegf or its receptor. Segregation analysis of mutant and wild-type embryos allowed Lawson et al. to place it in a region in which several zebrafish ESTs similar to rat Plcg1 also mapped.

Linkage and expression analyses of plcg1 provided strong indirect evidence that it was a mutant in this gene that led to the y10 phenotype. Subsequent sequence analysis of plcg1 in mutant and and wild-type embryos identified a G-to-A transition that eliminated a splice acceptor site as the putative causal mutation.

The authors then used an antisense morpholino to the intron–exon junction to mimic the effects of the putative mutation. The reduced number of segmental vessel sprouts in the embryos injected with this morpholino corresponded to the y10 phenotype. The clincher came when they showed that injection of wild-type plcg1 mRNA could rescue y10 mutants, which nailed down the zebrafish homologue of PLCG1 as the gene involved.

However, the most significant aspect of this work is that by showing that the y10 mutant phenotype could not be rescued by injecting wild-type vegf it provides the first definitive in vivo evidence that plcg1 acts downstream of Vegf. Moreover, because mice that lack Plcg1 also have severe defects in blood-vessel formation it seems that this developmental pathway might be conserved throughout vertebrates.

Given this apparent conservation, the zebrafish, with its unique developmental characteristics, a genome sequence on the way and a growing arsenal of genetic tools, might just be the future model of choice for dissecting blood-vessel development and the diseases associated with it.