Transmembrane Signaling Analysis in Model Membrane Systems

Studying the complex mechanisms of transducing an extracellular signal across a plasma membrane to initiate an intracellular responce is of fundamental importance for a cell’s function. Transmembrane signaling is the key allowing cells to sense and communicate with its environment. Signal transduction is primarily mediated by numerous membrane receptors. In this thesis we investigated the function of G-protein-coupled receptors (GPCR). GPCRs represent the largest and most diverse group of membrane proteins, encoded by more than 800 genes. They allow cells to recognize as diverse extracellular stimuli as photons, organic odorants, nucleotides, nucleosides, peptides, lipids and proteins, conferring to GPCRs a fundamental role in signal transduction. They are broadly distributed across the entire body and thus involved in a wide range of physiological and pathological processes, which makes GPCRs one of the main targets of modern medicines. Activation of GPCRs initiate numerous intracellular signaling pathways, involving a large number of proteins and molecules. They act as a central molecular activators and integrators of a complex network of signaling pathways. While most of the studies have been performed on cells, the complexity of cellular systems has raised the need to develop simpler model systems to assess the role of individual signaling com- ponents. In this thesis we present different model systems to study the functionality of GPCR signal transduction from the ligand binding to the activation of downstream cellular reactions. The first part of the thesis concerns the isolation of single receptors from cultured cells by detergent solubilization followed by purification and the subsequent receptor reconstitution into giant unilamellar vesicles (GUV). Detergent micelle receptors were analyzed by fluorescence correlation spectroscopy (FCS) and surface plasmon resonance (SPR) measurements. We could determine the conditions of functional solubilization of GPCRs by characterizing the amount of obtained receptor-detergent micelles, their homogeneity and the capacity of the receptor to selectively bind ligands. The receptors were then reconstituted into GUVs and ligand binding was monitored by confocal microscopy. In the second part of the thesis, we investigated different aspects properties of native plasma membrane vesicles. These vesicles are derived from live mammalian cells using v cytochalasin B or optical tweezers; they comprise parts of a cell’s plasma membrane and cytosol. After the characterization of their overall composition, we investigated their ability to mediated transmembrane activation of intracellular signaling reactions upon agonist binding to a GPCR. We could successfully monitor the ligand binding to receptor by FCS, the subsequent G-protein activation by intermolecular FRET and the receptor desensitization by monitoring the arrestin recruitment to the plasma membrane. These vesicles thus represent the smallest autonomous container performing cellular signaling reactions thus functioning like a minimized cell. Finally, preliminary experiments demonstrated the potential of native vesicles to serve as novel drug delivery system since they meet most of the requirements for an efficient drug delivery platform.