Structure-function studies with G protein-coupled receptors as a paradigm for improving drug discovery and development of therapeutics

Abstract

There are a great variety of human membrane proteins, and these currently form the largest group of targets for marketed drugs. Despite the advances in drug design, however, promiscuity between drug molecules and targets often leads to undesired signaling effects, which result in unintended side effects. In this review, one family of membrane proteins - the G protein-coupled receptors (GPCRs) - is used as a model to review experimental techniques that may be used to examine the activity of membrane proteins. As these receptors are highly relevant to healthy human physiology and represent the largest family of drug targets, they represent an excellent model for membrane proteins in general. We also review experimental evidence that suggests there may be multiple ways to target a GPCR - and by extension, membrane proteins - to more effectively target unhealthy phenotypes while reducing the occurrence and severity of side effects.

Figure 1

Schematic of the membrane environment and GPCR signaling. (I) Single receptor molecules are generally capable of transducing a signal to the intracellular medium. Several regions of the receptor may modulate this signal. The most common means is modulation by orthosteric ligand binding, which is caused by the binding of agonists or antagonists (site a). Allosteric ligand binding sites may allow either soluble species (site b) or membrane-soluble species, including lipids, to access the receptor (site c). (II) Protein-protein interactions, here between two receptor molecules, a receptor and a heterotrimeric G protein, and a receptor activity modulating protein (RAMP). These inter-protein interactions may also modulate signaling behavior upon ligand binding to the receptor. As with all proteins in the cellular milieu, these are subject to regulatory proteins, which further modulate the overall signaling potential of receptors at the cell surface. There is also therapeutic potential in designing molecules, peptide or otherwise, that modulate signaling analogously to RAMP-like species. (III) The membrane itself has intrinsic mechanical properties the receptor must accommodate, in addition to specific interactions. These properties may shift the conformation of the receptor toward or away from active conformation.

Figure 2

Development of human therapeutics is by necessity built upon many experimental foundations. GPCR interactions with other proteins, ligands, or the cell membrane are best identified in isolation and then combined to form a holistic snapshot of receptor behavior. Animal models (manuscript section §3.1) show how tissues, or whole organisms, may react to treatment, but require substantial resources and studies are typically directed by results from cell cultures (§3.2). Therapeutic effects may be even more intensively examined through structural analysis (§3.3), as well as biophysical studies of protein in isolation in liposomes, micelles, or other biomimetic systems (§3.4-3.5). Finally, molecular dynamics and docking studies (§3.6) are used to study the movements of individual receptors. The collection of data from all of these experiments is required for the development of drugs that more specifically treat a condition, while minimizing the risk of adverse effects.