Hemato-vascular specification requires arnt1 and arnt2 genes in zebrafish embryos
During embryonic development, a subset of cells in the mesoderm germ layer are specified as hemato- vascular progenitor cells, which then differentiate into endothelial cells and hematopoietic stem and progenitor cells. In zebrafish, the transcription factor npas4l, also known as cloche, is required for the specification of hemato-vascular progenitor cells. However, it is unclear if npas4l is the sole factor at the top of the hemato-vascular specification cascade. Here we show that arnt1 and arnt2 genes are required for hemato-vascular specification. We found that arnt1;arnt2 double homozygous mutant zebrafish embryos (herein called arnt1/2 mutants), but not arnt1 or arnt2 single mutants, lack blood cells and most vascular endothelial cells. arnt1/2 mutants have reduced or absent expression of etsrp and tal1, the earliest known endothelial and hematopoietic transcription factor genes. npas4l and the arnt genes are PAS domain-containing bHLH transcription factors that function as dimers. We found that Npas4l binds both Arnt1 and Arnt2 proteins in vitro, consistent with the idea that PAS domain- containing bHLH transcription factors act in a multimeric complex to regulate gene expression. Our results demonstrate that npas4l, arnt1 and arnt2 act together to regulate endothelial and hematopoietic cell fate, where each gene is necessary, but by itself not sufficient, to drive hemato-vascular specification. Our results also demonstrate that arnt1 and arnt2 act redundantly in a transcriptional complex containing npas4l, but do not act redundantly when interacting with another PAS domain- containing bHLH transcription factor, the aryl hydrocarbon receptor. Altogether, our data enhance our understanding of hemato-vascular specification and the function of PAS domain-containing bHLH transcription factors.