Adhesion of Arg-Gly-Asp (RGD) peptide vesicles onto an integrin surface: Visualization of the segregation of RGD ligands into the adhesion plaques by fluorescence

Integrins are adhesion receptors that mediate cell adhesion and play an important function in many biological processes such as morphogenesis and tissue remodeling. These membrane proteins specifically interact with a short tripeptide sequence, RGD (Arg-Gly-Asp), present in numerous extracellular macromolecules. Model systems have been developed in order to understand how membrane adhesion is induced by this specific RGD peptide ligand/integrin recognition system. We have previously shown that RGD giant vesicles selectively adhere to endothelial cells by formation of pinning centers. Nevertheless, the nature of the lipids located in the adhesion contact zone is unknown. One hypothesis is that the lipidic ligands migrate to the contact zone where they are confined after binding to the receptor. To study the possible formation of ligand domains within the vesicle bilayer, we synthesized a fluorescently labeled RGD lipid that can be easily incorporated in giant vesicles. Adhesion of giant RGD vesicles onto an integrin-functionalized surface was followed simultaneously by reflection interference contrast microscopy and fluorescence microscopy. For the first time, it was possible to observe the microsegregation of RGD lipids in the contact zone during adhesion. Additionally, we observed interesting photosensitive properties of the chalcone chromophore that could lead to a new method of analyzing the lipid organization within the membrane during adhesion and to the design of new ligand lipids and vesicle vectors for cell targeting.