G protein-coupled receptor signaling complexity in neuronal tissue: implications for novel therapeutics

The manipulation of transmembrane signaling by G protein-coupled receptors (GPCRs) constitutes perhaps the single most important therapeutic target in medicine. Therapeutics acting on GPCRs have traditionally been classified as agonists, partial agonists, or antagonists based on a two state model of receptor function embodied in the ternary complex model. Over the past decade, however, many lines of investigation have shown that GPCR signaling exhibits greater diversity and 'texture' than previously appreciated. Signal diversity arises from numerous factors, among them the ability of receptors to adopt multiple 'active' states with different effector coupling profiles, the formation of receptor dimers that exhibit unique pharmacology, signaling, and trafficking, the dissociation of receptor 'activation' from desensitization and internalization, and the discovery that non-G protein effectors mediate some aspects of GPCR signaling. At the same time, clustering of GPCRs with their downstream effectors in membrane microdomains, and interactions between receptors and a plethora of multidomain scaffolding proteins and accessory/chaperone molecules confers signal preorganization, efficiency, and specificity. More importantly it is likely that alteration in the interactions of these proteins with GPCRs may occur in aging or neurodegenerative disorders, thus defining a distinct 'pharmacology' from that seen in young organisms or normal physiology. In this context, the concept of agonist selective trafficking of receptor signaling, which recognizes that a bound ligand may select between a menu of 'active' receptor conformations and induce only a subset of the possible response profile, presents the opportunity to develop drugs that change the quality as well as the quantity of efficacy and enhance these qualities for specific disorders or other paradigms. As a more comprehensive understanding of the complexity of GPCR signaling is developed, the rational design of ligands possessing increased specific efficacy and attenuated side effects may become the standard mode of drug development.