Tie2 (green) and Ang1 (red) co-localize at cell–cell contacts upon Ang1 stimulation (bottom row: merged images).

From Nat. Cell Biol. 10, 513-526 (2008).

Vascular development is regulated by endothelial-specific receptor tyrosine kinases and their ligands. One system that controls both angiogenesis and vascular quiescence involves the Tie2 receptor and Angiopoietin-1 (Ang1), but the molecular mechanisms underlying this dual function remain unknown. Two studies published in Nature Cell Biology now show that Ang1 and Tie2 form distinct complexes at cell–matrix and cell–cell contacts to activate the AKT and ERK signalling cascades, respectively.

Mature vessel endothelial cells maintain cell-cell contact and do not proliferate. However, during angiogenesis, the cells lose their cell-cell contacts to proliferate and migrate, resulting in the formation of new vessels through the sprouting and branching of pre-existing ones. Ang1-Tie2 signalling has been shown to inhibit apoptosis and enhance cell adhesion to maintain vascular integrity, and is also thought to promote endothelial cell migration and neovascularization. But how does Tie2 function in quiescent or angiogenic endothelia?

Fukuhara et al. and Saharinen et al. analysed the subcellular localization of Tie2 in confluent and sparse human endothelial cell cultures that mimic quiescent and angiogenic conditions, respectively. Upon Ang1 stimulation, Tie2 accumulated at the cell–cell contacts of confluent cells, where it co-localized with Ang1. Interestingly, the relocalization of Tie2, as well as its phosphorylation, only occurred when both contacting cells expressed the Tie2 receptor, showing that Ang1 induces in trans activation and association of Tie2 at cell–cell contacts. In the absence of cell-cell contacts, Ang1 induced Tie2 localization to cell_matrix contacts. Tie2 was anchored by extracellular matrix-bound Ang1 to form adhesive structures that were close but distinct from focal adhesions (FAs).

To address the biological significance of Tie2 activation at different subcellular locations, the authors then analysed signalling downstream of Tie2. The AKT pathway was preferentially activated in the presence of cell-cell contacts, whereas ERK activation was induced in matrix-attached cells. AKT activation led to phosphorylation of the forkhead transcription factor Foxo1 and eNOS (endothelial nitric oxide synthase). Both factors are known to play critical roles in endothelial cell survival and stability, thus suggesting Tie2 may control endothelium quiescence through AKT. Conversely, Ang1-bound substratum promoted the formation of lamellipodia and FAs, and enhanced cell migration in an ERK-dependent manner, suggesting that Ang1-Tie2 regulates endothelial cell migration through ERK signalling at cell–matrix contacts. Futhermore, Fukuhara et al. report that ERK activation by Tie2 is partially dependent on focal adhesion kinase (FAK), which is known to localize at FAs and mediate signalling by integrins.

Together these studies identify two different signalling pathways that are differentially activated downstream of the Ang1–Tie2 system depending on the subcellular localization. Tie2 receptors bridged by Ang1 at cell–cell contacts may favour vascular quiescence via AKT activation, whereas Ang1-activated receptors at cell-matrix contacts may promote angiogenesis through ERK signalling.

Author: Kim Baumann