Towards understanding bias at GPCRs

Abstract

Pharmacology as a scientific discipline was born in the mid-nineteenth century, before chemical structures were elucidated, before receptors were even conceptualized. Scientists knew that cells were exposed to a variety of chemical signals, but how the cells interpreted and reacted to these signals was a mystery. The idea that cell-surface receptors might act as detectors and transmitters of external chemical signals originated with German bacteriologist and immunologist Paul Ehrlich and British physiologist John Newport Langley at the turn of the twentieth century, but in the absence of direct proof, debate over the existence of receptors lingered and competing theories remained popular. By the early 1930s, considerable support for the concept of drug receptors emerged from the quantitative analysis of drug action on cells by Alfred Clark. Clark's work formed the beginnings of the receptor occupancy theory, in which the number of receptors occupied by a specific compound is proportional to the response of the cell to that compound. In 1948, Raymond P. Ahlquist proposed distinct α- and β-adrenoceptors to explain differing responses to adrenergic stimulation in different tissues. However, it was not until James Black's discovery in 1965 of propranolol as the first clinically useful β-receptor blocker that receptor theory took hold as a powerful driver of pharmacological innovation.However, receptors were still very much a ‘black box’, and the enormous complexity of their signaling was not yet appreciated. Receptors as molecular entities began to come into focus with the advent of receptor-specific radioligands that allowed them to be directly studied and purified in a highly selective manner. Understanding and appreciation of G protein-coupled receptors (GPCRs) in particular then took a large step forward in 1986, when Brian Kobilka and Robert Lefkowitz and their colleagues cloned the hamster β2-adrenoceptor. The homology between the β2-adrenoceptor and the recently cloned rhodopsin receptor sparked the insight that these two receptors were part of a larger family; and indeed, there are 800 human genes encoding GPCRs, comprising roughly 4% of the entire human genome.GPCRs act as both the gatekeepers and molecular messengers of the cell, transmitting signals from outside to inside. The signal can consist of a vast array of stimuli, from photons to neurotransmitters to hormones. Nearly all GCPRs function as ligand-activated guanine nucleotide exchange factors (GEFs) for heterotrimeric G proteins. In ‘classical’ GPCR signaling, agonist binding stabilizes the receptor in an ‘active’ conformation, in which it catalyzes GTP-for-GDP exchange on the Gα-subunits, promoting dissociation of the GTP-bound Gα-subunit from the Gβγ-subunit heterodimer. However, recent work has shown that signaling through GPCRs is more complex than originally appreciated.GPCRs can generate ‘G-protein-independent’ signals through adapter or scaffold proteins that link the receptor to novel, non-G protein-regulated effectors. Chief of these are the arrestins, a small family of GPCR-binding proteins that were originally discovered as a result of their role in receptor desensitization. Whereas heterotrimeric G proteins act through the activation of second messengers, such as cAMP, diacylglycerol, or calcium, β-arrestins serve as scaffolds for various signaling proteins, including mitogen-activated protein kinases and E3 ubiquitin ligases. Furthermore, G protein and β-arrestin pathways are distinct and can be pharmacologically modulated independently with ‘biased ligands’.The discovery of ligand bias has spurred speculation that biased ligands could offer novel pharmacology distinct from classical agonists or antagonists. For example, biased ligands could increase efficacy or reduce side effects by targeting only the desired signaling pathway. Interest in understanding and identifying ligand bias has thus triggered a barrage of new questions in the GPCR field. What precisely is meant by ‘bias’? How can it best be detected through assays or modeled through in silico design? What do the recent crystal structures of GPCRs reveal about how ligand bias functions at these receptors? Finally, where do we stand in the process of clinical translation of these discoveries?We at Trends in Pharmacological Sciences are addressing these questions and more in a special series devoted to ligand bias at GPCRs. We are delighted to partner in this effort with Guest Editor Robert Lefkowitz, whose seminal contributions to GPCR research continue to shape the field today. The series kicks off in this issue with ‘Modeling G protein-coupled receptors in complex with biased agonists’ by Stefano Costanzi. Later contributions will feature work from Jonathan Violin, Tommaso Costa, Michel Bouvier, and Brian Kobilka. We would like to take this opportunity to thank all the authors and reviewers for their efforts in putting the series together, and we hope you enjoy the resulting articles.