INTEGRIN AND CADHERIN LIGANDS: INTERACTION STUDIES BY COMPUTATIONAL METHODS AND BIOAFFINITY NMR ON INTACT CELLS

On a molecular level protein – ligand interactions are central to a number of biological processes, but their investigation is inherently difficult due to several problems, especially for membrane proteins. The study of this type of interactions poses a whole set of challenges, including the characterization of the dynamic behaviour and of the conformational properties of the ligands in complex with the target macromolecules. A variety of biophysical methods have been developed to study protein – ligand interactions and several NMR spectroscopic techniques have emerged as powerful methods to identify and characterize the binding of ligands with receptor proteins. Ligand-based methods do not require labeled protein, since only the ligand NMR signals are detected and only a small amount of protein is required. These techniques are particularly useful in the medium–low affinity range and, therefore they have been adopted to detect ligand interactions in various systems. Among the ligand-based NMR techniques, Saturation Transfer Difference (STD) and transferred-NOE focus on the NMR signals of the ligand and utilize NOE effects between protein and ligand. They are used for: i) the definition of the bioactive conformation of the ligand in the bound state (tr-NOESY), ii) the identification and characterization of the binding mode of the ligand to the receptor with epitope mapping of the ligand itself (STD). The use of the technique is limited to molecules that exhibit a dissociation constant Kd between 10-3 M and 10-7 M. During my PhD, I had the highly qualifying opportunity to grasp these new potent NMR methods, and to apply them for assessing the interactions of cell surface proteins with peptidomimetics. Since membrane proteins, such as integrins, change their conformation if extracted from their environment, it is clear the importance of working in the biophysical neighbourhood of the membrane itself and not in an isotropic extracellular medium. For this reason, when appropriate to the project, I have carried out NMR experiments using intact cells overexpressing the proteins of interest. Specifically, two main topics have been addressed: 1. The first and second year of my PhD have been mainly focused on the conformational study of peptidomimetic integrin ligands and on the investigation of their interaction with platelets and cancer cell overexpressing integrins on their membrane. This study has been developed within the framework of a PRIN project (MIUR-PRIN project 2010NRREPL “Synthesis and Biomedical Applications of Tumor-Targeting Peptidomimetics”) in collaboration with the research groups of Proff. Gennari and Piarulli (University of Insubria) as regards the synthetic activities and with the group of Dr. Belvisi as regards the computational and design studies. 2. The second part of my PhD was mainly focused on cadherins, a class of cell adhesion proteins that promote homophilic interactions. This work is at an early stage and has been developed within the framework of a FIRB project coordinated by Dr. Civera (MIUR-FIRB “Futuro in Ricerca” RBFR088ITV “Computer-aided design, synthesis and biological evaluation of peptidomimetics targeting N-cadherin as anticancer agents”). The NMR study has been aimed at obtaining a thorough understanding of the interaction of peptidomimetic molecules with isolated cadherin constructs containing relevant extracellular domains.