Oligomerization of G protein-coupled receptors: biochemical and biophysical methods.

Dimerization and oligomerization of G protein-coupled receptors (GPCRs), proposed almost 30 years ago, have crucial relevance for drug design. Indeed, formation of GPCR oligomers may affect the diversity and performance by which extracellular signals are transferred to G proteins in the process of receptor transduction. Thus, the control of oligomer assembly/disassembly and signaling will be a powerful pharmacological tool. This, however, requires (i) the determination that oligomerization takes place between particular receptors, (ii) the confirmation that the oligomer has pharmacological importance and (iii) the availability of the oligomer 3D structure. This review aims at presenting experimental methods which unveil the complexity of GPCR dimerization/oligomerization focusing on biochemical and biophysical approaches. In total, we review 22 methods, including biochemical methods (radiation inactivation technique, receptor co-expression and trans-complementation studies, cross-linking experiments, co-immunoprecipitation and immunoblotting studies and analysis of receptor mutants and chimeras) and biophysical methods (Fluorescence Resonance Energy Transfer, (FRET), including photobleaching FRET (pb-FRET) and Time-Resolved FRET (TR-FRET), Luminescence Resonance Energy Transfer (LRET), Bioluminescence Resonance Energy Transfer (BRET), Bimolecular Fluorescence Complementation (BiFC), Luminescence Fluorescence Complementation (BiLC), Fluorescence Recovery after Photobleaching (FRAP), Confocal Microscopy, Immunofluorescence Microscopy, Single Fluorescent-Molecule Imaging, Transmission Electron Microscopy, Immunoelectron Microscopy, Atomic Force Microscopy, Total Internal Reflectance Fluorescence Microscopy (TIRFM) and X-ray Crystallography). For each method the scientific basis of the approach is shortly described followed by the extensive description of its application for studying GPCR oligomers presented according to their classes and families. Based on the wealth of experimental evidence, there is no doubt about the existence of GPCR dimers, oligomers and receptor mosaics which constitute a new and highly promising group of novel drug targets for more selective and safer drugs.

Oligomerization of G protein-coupled receptors: computational methods.

Recent research has unveiled the complexity of mechanisms involved in G protein-coupled receptor (GPCR) functioning in which receptor dimerization/oligomerization may play an important role. Although the first high-resolution X-ray structure for a likely functional chemokine receptor dimer has been deposited in the Protein Data Bank, the interactions and mechanisms of dimer formation are not yet fully understood. In this respect, computational methods play a key role for predicting accurate GPCR complexes. This review outlines computational approaches focusing on sequence- and structure-based methodologies as well as discusses their advantages and limitations. Sequence-based approaches that search for possible protein-protein interfaces in GPCR complexes have been applied with success in several studies, but did not yield always consistent results. Structure-based methodologies are a potent complement to sequence-based approaches. For instance, protein-protein docking is a valuable method especially when guided by experimental constraints. Some disadvantages like limited receptor flexibility and non-consideration of the membrane environment have to be taken into account. Molecular dynamics simulation can overcome these drawbacks giving a detailed description of conformational changes in a native-like membrane. Successful prediction of GPCR complexes using computational approaches combined with experimental efforts may help to understand the role of dimeric/oligomeric GPCR complexes for fine-tuning receptor signaling. Moreover, since such GPCR complexes have attracted interest as potential drug target for diverse diseases, unveiling molecular determinants of dimerization/oligomerization can provide important implications for drug discovery.

Molecular structure of ionotropic glutamate receptors.

L-glutamate is the major excitatory neurotransmitter in the central nervous system (CNS). Although just a few glutamate receptor ligands have turned out to be clinically useful, primarily because of unfavorable psychotropic side effects, the glutamate system remains an attractive molecular target in the treatment of epilepsy, neurodegenerative diseases (Alzheimer's disease, Parkinson's disease, Huntington's chorea), schizophrenia, ischemia, pain, alcoholism and mood disorders. Knowledge about the structure of ionotropic glutamate receptors (iGluRs) at atomic resolution is vital for the determination of their physiological and pathological importance and, thus, for drug design. Recently, tremendous progress has been made in structure elucidation and understanding of the functioning of iGluRs. The data about general topology and modular composition of iGluRs as well as numerous crystal structures of ligand binding domains of many iGluR subtypes has been supplemented with the first molecular models of the whole receptor protein, followed by the first crystal structures of N-terminal domains and finally by the first crystal structure of the whole tetrameric iGluR. This review summarizes experimental and computational efforts to determine iGluR molecular architecture and focus on the above listed achievements of the last years. In particular, the aspects of iGluR structure which are important for drug design, like the molecular characterstics of the ligand binding sites, are depicted in detail.