Living organisms perform and control complex behaviours by using webs of chemical reactions organized in precise networks. This powerful system concept, which is at the very core of biology, has recently become a new foundation for bioengineering. Remarkably, however, it is still extremely difficult to rationally create such network architectures in artificial, non-living and well-controlled settings. We introduce here a method for such a purpose, on the basis of standard DNA biochemistry. This approach is demonstrated by assembling de novo an efficient chemical oscillator: we encode the wiring of the corresponding network in the sequence of small DNA templates and obtain the predicted dynamics. Our results show that the rational cascading of standard elements opens the possibility to implement complex behaviours in vitro. Because of the simple and well-controlled environment, the corresponding chemical network is easily amenable to quantitative mathematical analysis. These synthetic systems may thus accelerate our understanding of the underlying principles of biological dynamic modules.

Abstract-Apelin has been identified as the endogenous ligand of the human orphan G protein-coupled receptor APJ. This peptide exerts a variety of cardiovascular effects and particularly acts as an activator of angiogenesis. Importantly, hypoxia has been reported to regulate apelin expression but the molecular mechanism underlying hypoxia-induced apelin expression and the relationship with the physiological response of the apelin/APJ system are still not established. Here, we demonstrate that apelin expression is induced by hypoxia in cultured endothelial and vascular smooth muscle cells as well as in lung from mice exposed to acute hypoxia. Transient transfection experiments show that hypoxia-inducible transcriptional activation of apelin requires an intact hypoxia-responsive element (ϩ813/ϩ826) located within the first intron of the human apelin gene. Chromatin immunoprecipitation assay reveals that hypoxia-inducible factor-1␣ binds to the endogenous hypoxia-responsive element site of the apelin gene. Moreover, overexpression of hypoxia-inducible factor-1␣ increases the transcriptional activity of a reporter construct containing this hypoxia-responsive element, whereas small interfering RNA-mediated hypoxia-inducible factor-1␣ knockdown abolishes hypoxia-induced apelin expression. Finally, microinterfering RNA-mediated apelin or APJ receptor knockdown inhibits both hypoxia-induced endothelial cell proliferation in vitro and hypoxia-induced vessel regeneration in the caudal fin regeneration of Fli-1 transgenic zebrafish. The hypoxia-induced apelin expression may, thus, provide a new mechanism involved in adaptive physiological and pathophysiological response of vascular cells to low oxygen level. Key Words: hypoxia Ⅲ transcriptional regulation Ⅲ endothelial cell growth-APJ Ⅲ zebrafish model T he apelin peptide, originally isolated from bovine stomach tissue extracts, has been identified as the endogenous ligand of the human orphan G protein-coupled receptor APJ, a receptor related to the angiotensin II receptor type 1. Apelin and APJ are distributed in various tissues where it exerts a broad range of physiological actions, including heart contractility, appetite, and drinking behavior or the hypothalamicpituitary-adrenal axis. 1 The role of apelin in the cardiovascular system is currently the most documented. Apelin and APJ were reported to be expressed in cardiomyocytes, endothelial, and vascular smooth muscle cells. 2,3 Apelin-deficient mice are viable, but they present impaired cardiac contractility with aging and pressure overload, indicating that apelin/ APJ pathway is important for maintaining cardiac function. 4 Apelin was reported to positively regulate angiogenesis both in vitro and in vivo. Apelin stimulates endothelial cell proliferation, migration, and tube formation in vitro. 5,6 In the frog embryo, apelin is crucial for the normal development of the embryonic vasculature. 7,8 Apelin expression is upregulated during tumor angiogenesis, and its overexpression was reported to increase the in vivo ...