CRC 629 B15 - Control of angiogenic signaling by endothelial endocytosis

Basic data for this project

Description

Blood vessels form an extensive, hierarchical network in the vertebrate body to transport gases, liquids, nutrients, signaling molecules and circulating cells between tissues and organs. With a few exceptions in early embryonic development, growth of the vasculature is predominantly achieved through angiogenesis, a process that involves the sprouting of endothelial cells from existing vessels, the generation of new tubular connections and branch points in response to increased local oxygen supply or tissue damage. Since angiogenesis involves very dynamic processes, it is critical to understand how the underlying molecular pathways are regulated in a temporal and spatial fashion. To achieve new growth without disrupting essential blood transport through excisting vessels, extensive communication between endothelial cells (ECs) as well as with the tissue environment is required. Vascular Endothelial Growth Factor (VEGF-A), a growth factor released by hypoxic tissues, leads to the activation of pro-angiogenic receptor tyrosine kinases in the endothelium. Conversely, communication between neighboring ECs through Notch receptors and the ligands Delta-like 4 (Dll4) and Jagged1 controls VEGF receptor (VEGFR) expression and can suppress or enhance new angiogenic growth. Recent work from our group has shown that the transmembrane protein Jagged1, has the opposite effect as Dll4 and promotes angiogenesis in the embryo and the postnatal retina by blocking Dll4-Notch signaling. In addition to the well-established pro-angiogenic function of VEGF-A/VEGFR-2 signaling, VEGFR-3 (Flt4), another tyrosine kinase within the VEGF receptor family, contributes to the growth of blood vessels. VEGFR-3 is activated by the ligands VEGF-C and VEGF-D and appears to be positively regulated in blood vessels by VEGFR-2 signaling. Tip cells which express high levels of VEGFR-3 are found at the angiogenic front, but VEGFR-3 is also expressed in a fraction of neighboring stalk cells. During blood vessel growth both VEGFR-2 and VEGFR-3 are functionally important and cooperate. Given the prominent role of the Notch and VEGF receptors in angiogenesis, it is important to know how the surface presentation and signaling activity of these proteins is controlled on a molecular level. Internalization and degradation are possible mechanisms that allow the termination of intercellular signaling and facilitate dynamic cellular responses. A second alternative pathway is the recycling of internalized (activated) receptors back to the plasma membrane which enables dynamic changes but preserves responsiveness to external signals. To better understand the role of endocytosis of cell surface receptors in endothelial cells during angiogenic sprouting and vessel growth, we will examine the dynamics and interacting partners of clathrin-associate proteins, in particular, Numb, Numb-like and Dab2. Differences in the trafficking and intracellular localization of these signaling receptors might affect the downstream signal transduction cascades and thereby lead to distinct biological readouts. We are specifically interested in the question how these molecules control the activity of known angiogenic regulators such as the VEGF, Notch or Eph-ephrin signaling pathways, but will also screen for novel cargo proteins that might regulate the behavior of endothelial cells in normal and pathological conditions.