Defective vascular development in connexin 45-deficient mice

In order to reveal the biological function(s) of the gap-junction protein c onnexin 45 (Cx45), we generated Cx45-deficient mice with targeted replaceme nt of the Cx45-coding region with the lacZ reporter gene, Heterozygous Cx45 (+/-) mice showed strong expression of the reporter gene in vascular and vi sceral smooth muscle cells. Cx45-deficient embryos exhibited striking abnor malities in vascular development and died between embryonic day (E) 9.5 and 10.5. Differentiation and positioning of endothelial cells appeared to be normal, but subsequent development of blood vessels revealed impaired forma tion of vascular trees in the yolk sac, impaired allantoic mesenchymal ingr owth and capillary formation in the labyrinthine part of the placenta, and arrest of arterial growth, including a failure to develop a smooth muscle l ayer surrounding the major arteries of the embryo proper. As a consequence, the hearts of most Cx45-deficient embryos were dilated. The abnormal devel opment of the vasculature in the yolk sac of Cx45(-/-) embryos could be cau sed by defective TGF beta signalling, as the amount of TGF beta1 protein in the epithelial layer of the yolk sac was largely decreased in the E9.5 Cx4 5(-/-) embryo, compared with the wild-type embryo. The defective vascular d evelopment was accompanied by massive apoptosis, which began in some embryo s at E8.5 and was abundant in virtually all tissues of the embryos at E9.5. We conclude that in Cx45(-/-) embryos, vasculogenesis was normal, but subs equent transformation into mature vessels was interrupted. Development of d ifferent types of vessels was impaired to a varying extent, which possibly reflects the complementation by other connexin(s).