Endothelial nitric oxide synthase gene expression during murine embryogenesis: commencement of expression in the embryo occurs with the establishment of a unidirectional circulatory system.

To elucidate the role of endothelial NO synthase (eNOS)-derived NO during mammalian embryogenesis, we assessed the expression of the eNOS gene during development. Using transgenic eNOS promoter/reporter mice (with beta-galactosidase and green fluorescent protein reporters), in situ cRNA hybridization, and immunohistochemistry to assess transcription, steady-state mRNA levels, and protein expression, respectively, we noted that eNOS expression in the developing cardiovascular system was highly restricted to endothelial cells of medium- and large-sized arteries and the endocardium. The onset of transcription of the native eNOS gene and reporters coincided with the establishment of robust, unidirectional blood flow at embryonic day 9.5, as assessed by Doppler ultrasound biomicroscopy. Interestingly, reporter transgene expression and native eNOS mRNA were also observed in discrete regions of the developing skeletal musculature and the apical ectodermal ridge of developing limbs, suggesting a role for eNOS-derived NO in limb development. In vitro studies of promoter/reporter constructs indicated that similar eNOS promoter regions operate in both embryonic skeletal muscle and vascular endothelial cells. In summary, transcriptional activity of the eNOS gene in the murine circulatory system occurred following the establishment of embryonic blood flow. Thus, the eNOS gene is a late-onset gene in endothelial ontogeny.