Cell-Demanded Liberation of VEGF 121 From Fibrin Implants Induces Local and Controlled Blood Vessel Growth

Abstract

Although vascular endothelial growth factor (VEGF) has been described as a potent angiogenic stimulus, its application in therapy remains difficult: blood vessels formed by exposure to VEGF tend to be malformed and leaky. In nature, the principal form of VEGF possesses a binding site for ECM components that maintain it in the immobilized state until released by local cellular enzymatic activity. In this study, we present an engineered variant form of VEGF, α 2 PI 1–8 -VEGF 121 , that mimics this concept of matrix-binding and cell-mediated release by local cell-associated enzymatic activity, working in the surgically-relevant biological matrix fibrin. We show that matrix-conjugated α 2 PI 1–8 -VEGF 121 is protected from clearance, contrary to native VEGF 121 mixed into fibrin, which was completely released as a passive diffusive burst. Grafting studies on the embryonic chicken chorioallantoic membrane (CAM) and in adult mice were performed to assess and compare the quantity and quality of neovasculature induced in response to fibrin implants formulated with matrix-bound α 2 PI 1–8 -VEGF 121 or native diffusible VEGF 121 . Our CAM measurements demonstrated that cell-demanded release of α 2 PI 1–8 -VEGF 121 increases the formation of new arterial and venous branches, whereas exposure to passively released wild-type VEGF 121 primarily induced chaotic changes within the capillary plexus. Specifically, our analyses at several levels, from endothelial cell morphology and endothelial interactions with periendothelial cells, to vessel branching and network organization, revealed that α 2 PI 1–8 -VEGF 121 induces vessel formation more potently than native VEGF 121 and that those vessels possess more normal morphologies at the light microscopic and ultrastructural level. Permeability studies in mice validated that vessels induced by α 2 PI 1–8 -VEGF 121 do not leak. In conclusion, cell-demanded release of engineered VEGF 121 from fibrin implants may present a therapeutically safe and practical modality to induce local angiogenesis.