Seven-transmembrane receptors (7TMRs), also called G protein-coupled receptors (GPCRs), are the most common class of receptors, with more than 800 members identified in the human genome1
. They are also the most commonly targeted receptor class for medicinal therapeutics2
. Drugs that activate 7TMRs are thought to modulate the proportion of receptors that are in an active signalling conformation relative to those in an inactive, non-signalling conformation. Based on the classical model for 7TMR activity, agonist binding to the 7TMR causes the receptor to adopt a conformation that results in the activation of associated heterotrimeric G proteins. This activation involves the exchange of bound GDP for GTP by the Gα subunit of the G protein, leading to dissociation of the heterotrimeric protein complex into Gα and Gβγ subunits. This dissociation then promotes the production of and consequent signalling by second messenger systems, such as those involving cyclic AMP, diacylglycerol and calcium3
. Signalling by the activated conformation of the 7TMR is terminated by phosphorylation of the cytoplasmic loops and tail of the 7TMR, which is catalysed predominantly by GPCR kinases (GRKs). This results in the binding of arrestins (most commonly β-arrestin 1 and β-arrestin 2) and consequent desensitization followed by internalization into clathrin-coated pits4
. Thus, in the classical model, heterotrimeric G proteins mediate signal transduction via the receptor, and β-arrestins mediate receptor desensitization and internalization ().
G protein and β-arrestin-mediated signalling
This classical model, however, is both over-simplified and incomplete. Over the past decade there has been a new appreciation regarding the capacity of β-arrestins to act not only as regulators of 7TMR desensitization, but also as multifunctional adaptor proteins that have the ability to signal through multiple mediators such as mitogen-activated protein kinases (MAPKs), SRC, nuclear factor-κB (NF-κB) and phosphoinositide 3-kinase (PI3K). This new perspective on 7TMR signalling represented a paradigm shift5
(). In this model, both heterotrimeric G proteins and β-arrestins are capable of interacting with and recruiting intracellular signalling molecules, with desensitization of the ligand-bound receptor mediated by β-arrestins. It is still unclear whether the same receptor conformations that result in β-arrestin-mediated signal transduction also lead to receptor desensitization. Biochemical data suggest that the signalling mediated by β-arrestins has distinct functional and physiological consequences from that mediated by G proteins5
. However, much of the work comparing β-arrestin- and G protein-mediated signalling has been performed in transiently transfected cell lines and not in primary cells or animal models, which limits our current understanding of the physiology of β-arrestin-mediated signalling.
It was originally thought that most ligands that bind to 7TMRs have balanced or unbiased activity for signalling through β-arrestins and G protein pathways; that is, they signal equally through both6
. However, some receptor–ligand systems display bias towards one pathway over the other; that is, they preferentially signal through either the G protein- or β-arrestin-mediated pathway7
. This behaviour is an example of biased agonism (), which is also referred to as collateral efficacy, functional selectivity or stimulus trafficking8
. Biased agonism has important implications for the design of therapeutics that target 7TMRs, as signalling through these parallel pathways is thought to have distinct functional consequences. For example, two drugs could both act as agonists of G protein-mediated functions, but may have differing effects on β-arrestin-mediated signalling. This could result in markedly different signalling profiles in vivo
, examples of which will be discussed in this Review. Frequently, drug screening assays are designed to detect signalling downstream of G proteins, such as an increase in intracellular calcium levels typical of Gq
-coupled receptors or increased cAMP levels typical of Gs
-coupled receptors. Such assays, however, may not be sensitive to β-arrestin-mediated signalling, the target pathways of which have yet to be fully characterized. In this Review, we discuss recent advances in the characterization of β-arrestin-mediated signalling and biased agonism at 7TMRs, and address the implications of these for drug discovery and design involving this ubiquitous superfamily of receptors.