The main scientific interest of my laboratory is to understand molecular and cellular mechanisms that modulate specification and maintenance of the endothelial lineage. Specifically, we are interested in the following areas of research.
In vertebrates, there are two main types of blood vessels, arteries and veins. During development, arteries and veins undergo morphogenesis to create elaborate vascular networks. While morphogenesis of embryonic arteries has been relatively well-understood, how veins undergo similar process remains largely unknown.
We have previously reported that BMP2 signaling functions as a venous specific pro-angiogenic cue during zebrafish development (Wiley et al, Nat. Cell. Biol., 2011). Over-expression of BMP2 ligand in post-natal zebrafish induced the formation of exuberant angiogenic sprouts from venous endothelial cells within the cardinal vein without affecting adjacent arterial endothelial cells (Fig. 1 top middle column). In contrast, ectopic expression of VEGF-A signaling preferentially promotes angiogenesis from arterial endothelial cells (Fig. 1 top right column), indicating that these two ligands may selectively activate venous or arterial endothelial cells respectively (Fig. 1 bottom).
To extend our findings in zebrafish to mammalian system, we examined whether BMP2 signaling similarly affects vascular development in mammalian systems. Our preliminary data suggest that venous endothelial cells may respond to BMP2 signaling more sensitively than arterial endothelial cells in mammalian systems as in zebrafish. To further examine this idea, we have generated inducible endothelial specific knock-outs of two BMP Type I receptors, Alk2/Acvr1 and Alk3/Bmpr1, as well as BMP Type II receptor, BMPR2, and are currently analyzing the vascular phenotypes.
Based on our previous analyses, BMP2 signaling can be an attractive target to manipulate angiogenesis from venous endothelial cells. However, considering diverse roles of BMP2 signaling during development and pathogenesis, global inhibition or activation of BMP2 signaling will undoubtedly lead to adverse side-effects. To modulate the activity of BMP2 signaling in endothelial cells in a more specific manner, identification of endothelial specific targets of BMP2 signaling is essential. We plan to perform phosphor-proteomics screen to identify immediate target of BMP2 signaling in endothelial cells.
During development, lymphatic endothelial cells emerge from the venous endothelial cells within the cardinal vein. Dysfunctions in lymphatic endothelial cells led to lymphedema, a debilitating condition that adversely affects the quality of life. Approximately 140 million people worldwide suffer from various types of lymphedema.
However, there are no effective treatment options available. We have recently demonstrated that inhibition of BMP2 signaling can substantially increase the number of lymphatic endothelial cells with a concomitant decrease in blood endothelial cells in zebrafish (Fig.2 top row) (Dunworth et al, Circ. Res., 2014). Based on our data, we postulate that BMP2 signaling may suppress the lymphatic fate while promoting venous fate in developing vein.
To explore the therapeutic application of our finding, we examined whether manipulation of BMP2 signaling can promote the differentiation of lymphatic endothelial cells from human induced pluripotent stem (iPS) cells. Our preliminary data suggest that inhibition of BMP signaling by endogenous antagonist, Noggin, can substantially increase the number of lymphatic endothelial cells (Fig. 2 bottom row). As an extension of our finding, we plan to investigate the relevance of BMP2 signaling in a clinical setting including primary and secondary lymphedema. In addition, to fully harness the potential of BMP2 signaling as the modulator of lymphatic endothelial cells, we plan to perform high throughput high content small chemical screen using transgenic zebrafish and cell culture as model systems.
The importance of BMP signaling during development and disease has been well established and key factors such as SMADs and MAPK that mediate BMP signaling have been identified. However, it is seemingly paradoxical that large numbers of BMP ligands and receptors converge into a small number of downstream effectors. Therefore, a fine tuning process of BMP signaling must exist in order to distinguish distinctive signal inputs.
Our preliminary data suggest that the internalization process of BMP signaling complex may determine the outcomes of BMP signaling within cells (Kim et al, Dev. Cell, 2012). In order to test this idea, we plan to trace the internalization process of activated BMP signaling complex in live cells by super-resolution confocal microscopy.
Recent advances in the field present that complex interaction among signaling pathways regulate angiogenesis and lymphangiogenesis. To effectively target angiogenesis for therapeutic purpose, it is essential to identify additional molecules and novel signaling pathways that interact with previously identified key signaling pathways. Using zebrafish and cell culture models, we plan to screen for novel modifiers and interactors of previously identified signaling pathways. For instance, we have recently shown that a well characterized G protein coupled receptor, APJ, is essential for lymphatic development (Kim et al, ATVB, 2013).
Pulmonary arterial hypertension is a relatively rare disease with an extremely high mortality. Nearly a half of diagnosed patients succumb to the disease within the first three years. Unfortunately, there are no effective treatment options currently available. The most common cause of pulmonary arterial hypertension is dysfunction of BMPR2. However, more than half of pulmonary arterial hypertension patients does not have any mutations in BMPR2 locus.
We have recently found that a well characterized receptor tyrosine kinase, VEGFR3, may contribute to the onset of pulmonary arterial hypertension by modulating the level of BMP signaling in endothelial cells. We are currently collaborating to investigate the role of VEGFR3 in BMP signaling in developmental and pathological angiogenesis.