cdx4, meis1.1 and Hox gene expression in hoxd4a morphants, and a genetic pathway for specification of hemangioblasts and unipotential stem cells.
(A) qRT-PCR showing decreased expression of meis1.1 and many, but not all, hox genes in hoxd4a morphants at 26–28 hpf. By contrast, cdx4 levels are unchanged. Samples were normalized to β-actin. Error bars indicate standard error. All pairs marked with an asterisk meet statistical significance (p≤0.02). (B) Based on the results presented here and those in the literature, we propose a pathway in which hoxd4a and meis1.1 occupy sequential steps downstream of cloche and cdx1/4 in a genetic programme leading to the specification of hemangioblasts and unipotential angiogenic and hematopoietic stem cells. The effects of hoxd4a knockdown may be magnified through positive cross-regulatory interactions with meis1.1. The observed effects on the expression of multiple Hox genes could be due to the direct action of hoxd4a and meis1.1. Non-exclusively, cdx1 and cdx4 may act in conjunction with hoxd4a and meis1.1 in a feed-forward type of mechanism to regulate one or more of these same Hox genes with widespread consequences for vasculogenesis, angiogenesis and hematopoiesis at all levels.