Study of G-Protein Coupled Receptor Signaling in Membrane Environment by Plasmon Waveguide Resonance

Here we describe an experimental technique, termed plasmon waveguide resonance (PWR) spectroscopy that enables the characterization of molecular interactions occurring at the level of anisotropic thin films as lipid membranes and therein inserted or interacting molecules. PWR allows one to characterize such molecular interactions at different levels: (1) acquire binding curves and calculate dissociation constants; (2) obtain kinetic information; (3) obtain information about associated anisotropy changes and changes in membrane thickness; (4) obtain insight about lateral homogeneity (formation of domains). Points 1, 2, and 4 can be directly obtained from the data. Point 3 requires spectral fitting procedures so that the different optical parameters characterizing thin films as proteolipid membranes, namely refractive index and extinction coefficient for both p- (TM component of light that is parallel to the incident light) and s- (TE component of light that is perpendicular to the incident light) polarizations and thickness, can be determined. When applied to membrane proteins as the G-protein coupled receptor (GPCR) family, both ligand-induced conformational changes of the receptor can be followed as well as interactions with effectors (e.g., G-proteins). Additionally, by either altering the lipid composition in cellular membranes or specifically controlling its composition in the case of lipid model membranes with reconstituted proteins, the role of the lipid environment in receptor activation and signaling can be determined. Additionally, the eventual partition of receptors in different lipid microdomains (e.g., lipid rafts) can be followed. Such information can be obtained  ex cellulo with mammalian cell membrane fragments expressing the protein of interest and/or in vitro with lipid model systems where the protein under investigation has been reconstituted. Moreover, PWR can also be applied to directly follow the reconstitution of membrane proteins in lipid model membranes. The measurements are performed directly (no labeling of molecular partners), in real time and with very high sensitivity. Here we will discuss different aspects of GPCR activation and signaling where PWR brought important information in parallel with other approaches. The utility of PWR is not limited to GPCRs but can be applied to any membrane protein. PWR is also an excellent tool to characterize the interaction of membrane active molecules (as cell penetrating, antimicrobial, viral and amyloid peptides) with lipids. A brief section is dedicated to such applications, with particular emphasis on amyloid peptides. To finalize, as PWR is a homemade technology, ongoing instrument developments aiming at breaking current experimental limitations are briefly discussed, namely, the coupling of PWR with electrochemical measurements and the expansion of measurements from the visible to the infrared region.