Understanding the molecular control of blood vessel growth, angiogenesis, is one of the frontiers in current biomedical research because of its broad implication in cancer, cardiovascular disease and many other human diseases. While the best studied angiogenesis model focuses on capillary growth, the development and re-growth of arteries remains poorly understood. Similarly, mechanisms responsible for arterio-venous (AV) differentiation and maintenance remain largely a mystery. Recent discovery that ephrinB2, a cell surface molecule, is expressed specifically in arteries prior to the onset of circulation establishes a genetic contribution to AV specification. Furthermore, ephrinB2 is linked genetically to the upstream transmembrane receptor Notch . My laboratory focuses on the cellular and molecular mechanisms underlying arterial growth and the AV interface, with an emphasis on the Notch and ephrinB2 signaling pathways.

To investigate the function of Notch in both embryos and adults, we have developed mouse models in which constitutively active forms of Notch are expressed exclusively in endothelial cells, in a temporally regulatable manner, thereby permitting examination in both developing and adult vasculatures. A major unresolved question is whether Notch determines the fate of angioblasts, endothelial cell stem cells, or acts later to simply maintain the differentiated features. In zebrafish, disruption of Notch signaling affects arterial marker expression but not the morphogenesis of the aorta, suggesting that Notch functions after angioblast differentiation. In contrast, we have shown that a gain-of-function Notch mutant exhibited enlargement of the dorsal aortae at the expense of cardinal veins, suggesting that Notch directs arterial differentiation of angioblasts. In addition, we have provided the first functional demonstration that Notch expression causes adult venous endothelial cells to exhibit arterial characteristics and dynamically regulates the AV interface of adult vasculature. Combining also loss-of-function approaches and time-lapse imaging, we aim to delineate the cellular mechanisms by which Notch specifies arterial identities and to identify the molecular events triggered by Notch in endothelial cells. These efforts will reveal the function of Notch in mammalian AV differentiation and maintenance and illuminate potential molecular mechanisms underlying these processes.

To identify the molecular regulation of AV specification during tumor vascularization, we utilize our TRE-hMET transgenic mouse model, in which hepatocellular carcinoma develops with 100% penetrance. In contrast to the current view that tumor vasculature consists only of nascent microvessels, elaborate vascular hierarchy develops in this tumor model similar to what is seen in human disease. We hypothesize that genes regulating normal AV specification, such as Notch and ephrinB2 , are crucial in generating the tumor AV system, therefore, inactivation of these genes may prevent AV communication, thereby effectively limiting tumor growth. We have shown that ephrinB2 is expressed specifically in tumor endothelium. We are investigating the effects of tumor vascular formation and tumorigenesis in mice with endothelial cell-specific deletions of ephrinB2 and Notch .

Because Notch and ephrinB2 are preferentially expressed in arteries and required for arterial morphogenesis in embryos, we hypothesize that they are also crucial for arteriogenesis in adult ischemia revascularization, a process that is complementary to tumor angiogenesis, because stimulating blood vessel growth is beneficial.

In summary, my laboratory studies AV specification and arteriogenesis in normal, tumorous, and ischemic circumstances through the utilization of mouse genetics (both gain- and loss-of-function models) as well as molecular, cellular, and imaging approaches. Our research may ultimately lead to the identification of novel drug targets and therapeutic interventions for ischemia and hepatocellular carcinoma -two of the world's most common and devastating diseases.