Specific Aims Transforming growth factor (TGF-?) superfamily signaling in endothelial cells regulates essential components of angiogenesis and vascular morphogenesis, including proliferation and capillary tube formation. TGF-? superfamily ligands exert their regulatory effects through the endothelial cell specific TGF-? receptor complex, ALK1 (type I receptor) and endoglin (co-receptor), along with the ubiquitous type I TGF-? receptor, ALK5, to activate the canonical Smad 1/5/8 and Smad 2/3 pathways, respectively. TGF-? ligands also signal through non-Smad pathways such as MAPKs and PI3K/Akt, although the underlying mechanisms remain obscure. A critical role for endoglin and ALK1 in TGF-? signaling in endothelial cells is supported by their mutation resulting in the human vascular disease, hereditary hemorrhagic telangiectasia (HHT1 and 2), embryonic lethal phenotype due to defects in angiogenesis when either endoglin or ALK1 is targeted for deletion in mice, and by the elevated expression of endoglin during inflammation and tumor-induced angiogenesis. While important biological roles for endoglin have been established, the molecular basis for endoglin function in vascular biology remains poorly characterized. Here in vitro angiogenesis assays were employed to investigate precisely when and how endoglin regulates endothelial capillary sprouting and tube formation. Comparison of endoglin-null and wild type endothelial cells revealed that endoglin differentially regulates the stability of capillary sprouts and tubes in response to its physiologically relevant high-affinity ligands, TGF-? and BMP-9. Specifically, TGF-? resulted in regression of the capillary sprouts and tube structures, primarily through suppression of endoglin-dependent Akt signaling. Conversely, endoglin enhanced Akt signaling in response to BMP-9 to promote capillary stability. These outcomes are attributed to the association between endoglin and the scaffolding/trafficking protein, GIPC, since disrupting their interaction abrogated such endoglin-dependent effects. Given that I recently reported the enhancement of Smad 1/5/8 signaling through GIPC and endoglin, there exists a potential crosstalk between Akt and endoglin-dependent Smad 1/5/8 signaling, which I propose to investigate. Lastly, I discovered a novel interaction between endogenous endoglin and Akt, a finding that will likely yield new facets of endoglin biology. Based upon these preliminary data, I propose the following hypothesis: Endoglin associates with GIPC to promote angiogenesis by stabilizing endothelial capillaries via BMP-9-dependent Akt activation while destabilizing capillaries via TGF-?-dependent suppression of Akt signaling and cell survival mechanisms. This hypothesis will be addressed by the objectives outlined in two specific aims.
A distinctly innovative aspect of my application is in defining a new signaling role for endoglin in altering endothelial cell behavior. The current paradigm describes endoglin as an auxiliary TGF-? co-receptor that regulates the balance of two opposing signaling pathways, Smad1/5/8 and Smad2/3, which invoke pro- or antiangiogenic responses, respectively [12-22]. However, several lines of evidence challenge this overly simplistic role for endoglin. First, an embryonic lethal phenotype is observed in mice bearing homozygous deletions [1,2]. Second, the significance of endoglin signaling to the Smad pathways during angiogenesis is not fully understood, since these canonical signal transducers can be activated without endoglin expression . My preliminary findings indicate that endoglin engages other important signaling and cellular functions such as regulating Akt activation and downstream cell survival mechanisms. I have so far characterized the ability for endoglin to differentially regulate Akt activation in response to its direct ligands, TGF-? and BMP-9. A very intriguing aspect my research proposal tries to address, therefore, is how endoglin recognizes two structurally related ligands to elicit such divergent effects on Akt activation. Another innovative aspect of my research proposal involves the plan to use in vitro model systems that incorporate many, if not all, of the separate components of the angiogenic process, such as endothelial cell proliferation, degradation of basement membrane, migration, alignment, and capillary tube formation. Several groups have employed cell proliferation and migration assays to dissect the mechanisms by which endoglin regulates angiogenesis. However, appropriate in vitro systems that comprise most of the sequential stages of angiogenesis would be favorable to monitor the effects of endoglin during angiogenic progression. Manipulating various conditions such as ligand treatment or effects of altering protein levels, would all be possible. I have begun employing such methods to study endoglin function. My preliminary findings based on capillary tube formation assay suggest that endoglin plays an important role in regulating the stability of sprouted capillary tubes, and that this dynamic process is largely governed by Akt signaling and cell survival mechanisms. How endoglin might contribute to the stability of these maturing vessels during angiogenesis is an entirely unexplored area of research, and the basis of my research plan. By employing several new in vitro angiogenesis assays including micro- carrier and co-culturing methods, the proposed work will yield significant insight into when and how endoglin exerts its effects during angiogenesis.
|Kumar, S; Pan, C C; Bloodworth, J C et al. (2014) Antibody-directed coupling of endoglin and MMP-14 is a key mechanism for endoglin shedding and deregulation of TGF-? signaling. Oncogene 33:3970-9|