SVEP1 enables efficient binding of Angiopoietin-2 to the TIE1 receptor, allowing receptor phosphorylation and downstream signaling

The molecular mechanisms that drive (lymph-)angiogenesis are crucial to understand diseases, such as lymphedema, that are caused due to malformations of the lymphatic vasculature. Recently an interaction between the secreted protein Svep1, a key regulator in lymphangiogenesis, and the transmembrane receptor Tie1 was shown in zebrafish, human and mice. Here, guided by in silico AlphaFold-multimer structure predictions of highly confident SVEP1 complexes, we assert with protein binding studies that the human CCP20 domain is the primary binding site for TIE1, and also show that in addition the CCP5-EGFL7 fragment of SVEP1 engages TIE1 as a secondary site. We further demonstrate that SVEP1 mediates strong binding of ANG2 and TIE1, and that combined stimulation of hdLECs with SVEP1 and ANG2 leads to phosphorylation of TIE1. TIE1 activation by SVEP1 and ANG2 enables downstream signaling and in turn potentiates nuclear exclusion of FOXO1 and phosphorylation of AKT compared to SVEP1 or ANG2 alone. We present a model where ANG1/2 dimers bind to both SVEP1 and TIE1, resulting in multiple TIE1 receptor molecules being recruited to a multimeric complex at the cell membrane, potentially amplifying its signaling capacity.