Faculty of 1000 evaluation for Biased agonism: An emerging paradigm in GPCR drug discovery (original) (raw)
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Faculty Opinions recommendation of Biased agonism: An emerging paradigm in GPCR drug discovery
Faculty Opinions – Post-Publication Peer Review of the Biomedical Literature, 2017
G protein coupled receptors have historically been one of the most druggable classes of cellular proteins. The members of this large receptor gene family couple to primary effectors, G proteins, that have built in mechanisms for regeneration and amplification of signaling with each engagement of receptor and ligand, a kinetic event in itself. In recent years GPCRs, have been found to interact with arrestin proteins to initiate signal propagation in the absence of G protein interactions. This pinnacle observation has changed a previously held notion of the linear spectrum of GPCR efficacy and uncovered a new paradigm in GPCR research and drug discovery that relies on multidimensionality of GPCR signaling. Ligands were found that selectively confer activity in one pathway over another, and this phenomenon has been referred to as 'biased agonism' or 'functional selectivity'. While great strides in the understanding of this phenomenon have been made in recent years, two critical questions still dominate the field: How can we rationally design biased GPCR ligands, and ultimately, which physiological responses are due to G protein versus arrestin interactions? This review will discuss the current understanding of some of the key aspects of biased signaling that are related to these questions, including mechanistic insights in the nature of biased signaling and methods for measuring ligand bias, as well as relevant examples of drug discovery applications and medicinal chemistry strategies that highlight the challenges and opportunities in this rapidly evolving field.
Fulfilling the Promise of "Biased" G Protein–Coupled Receptor Agonism
Molecular Pharmacology, 2015
The fact that over 30% of current pharmaceuticals target heptahelical G protein-coupled receptors (GPCRs) attests to their tractability as drug targets. Although GPCR drug development has traditionally focused on conventional agonists and antagonists, the growing appreciation that GPCRs mediate physiologically relevant effects via both G protein and non-G protein effectors has prompted the search for ligands that can "bias" downstream signaling in favor of one or the other process. Biased ligands are novel entities with distinct signaling profiles dictated by ligand structure, and the potential prospect of biased ligands as better drugs has been pleonastically proclaimed. Indeed, preclinical proof-of-concept studies have demonstrated that both G protein and arrestin pathwayselective ligands can promote beneficial effects in vivo while simultaneously antagonizing deleterious ones. But along with opportunity comes added complexity and new challenges for drug discovery. If ligands can be biased, then ligand classification becomes assay dependent, and more nuanced screening approaches are needed to capture ligand efficacy across several dimensions of signaling. Moreover, because the signaling repertoire of biased ligands differs from that of the native agonist, unpredicted responses may arise in vivo as these unbalanced signals propagate. For any given GPCR target, establishing a framework relating in vitro efficacy to in vivo biologic response is crucial to biased drug discovery. This review discusses approaches to describing ligand efficacy in vitro, translating ligand bias into biologic response, and developing a systemslevel understanding of biased agonism in vivo, with the overall goal of overcoming current barriers to developing biased GPCR therapeutics.
Fulfilling the Promise of 'Biased' GPCR Agonism
Molecular pharmacology, 2015
The fact that over 30% of current pharmaceuticals target heptahelical G protein-coupled receptors (GPCRs) attests to their tractability as drug targets. While GPCR drug development has traditionally focused on conventional agonists and antagonists, the growing appreciation that GPCRs mediate physiologically relevant effects via both G protein and non-G protein effectors has prompted the search for ligands that can 'bias' downstream signaling in favor of one or the other process. Biased ligands are novel entities with distinct signaling profiles dictated by ligand structure, and the potential prospect of biased ligands as better drugs has been pleonastically proclaimed. Indeed, preclinical proof-of-concept studies have demonstrated that both G protein and arrestin pathway-selective ligands can promote beneficial effects in vivo while simultaneously antagonizing deleterious ones. But along with opportunity comes added complexity and new challenges for drug discovery. If ligand...
Biased G Protein-Coupled Receptor Signaling: New Player in Modulating Physiology and Pathology
Biomolecules & therapeutics, 2017
G protein-coupled receptors (GPCRs) are a family of cell-surface proteins that play critical roles in regulating a variety of pathophysiological processes and thus are targeted by almost a third of currently available therapeutics. It was originally thought that GPCRs convert extracellular stimuli into intracellular signals through activating G proteins, whereas β-arrestins have important roles in internalization and desensitization of the receptor. Over the past decade, several novel functional aspects of β-arrestins in regulating GPCR signaling have been discovered. These previously unanticipated roles of β-arrestins to act as signal transducers and mediators of G protein-independent signaling have led to the concept of biased agonism. Biased GPCR ligands are able to engage with their target receptors in a manner that preferentially activates only G protein- or β-arrestin-mediated downstream signaling. This offers the potential for next generation drugs with high selectivity to th...
ACS Pharmacology & Translational Science
G protein-coupled receptors (GPCRs) form the largest family of membrane proteins involved in signal transduction. Because of their ability to regulate a wide range of cellular responses and their dysregulation being associated with many diseases, GPCRs remain a key therapeutic target for several clinical indications. In recent years, it has been demonstrated that ligands for a given receptor can engage distinct pathways with different relative efficacies, a concept known as biased signaling or functional selectivity. However, the structural determinants of this phenomenon remain poorly understood. Using the β2-adrenergic receptor as a model, we identified a linker residue (L124 3.43) between the known PIF and NPxxY structural motifs, that plays a central role in the differential efficacy of biased ligands toward the Gs and β-arrestin pathways. Given the high level of conservation of this linker residue, the study provides structural explanations for biased signaling that can be extrapolated to other GPCRs.
Loss of biased signaling at a G protein-coupled receptor in overexpressed systems
PLOS ONE, 2023
G protein-coupled receptors (GPCRs) regulate cellular signaling pathways by coupling to two classes of transducers: heterotrimeric G proteins and β-arrestins. [Sarcosine 1 Ile 4 Ile 8 ]-angiotensin II (SII), an analog of the endogenous ligand angiotensin II (AngII) for the angiotensin II type 1 receptor (AT 1 R), fails to activate G protein in physiologically relevant models. Despite this, SII and several derivatives induce cellular signaling outcomes through β-arrestin-2dependent mechanisms. However, studies reliant on exogenous AT 1 R overexpression indicate that SII is a partial agonist for G protein signaling and lacks β-arrestin-exclusive functional specificity. We investigated this apparent discrepancy by profiling changes in functional specificity at increasing expression levels of AT 1 R using a stably integrated tetracycline-titratable expression system stimulated with AngII, SII, and four other AngII analogs displaying different signaling biases. Unbiased and G protein-biased ligands activated dose-dependent calcium responses at all tested receptor concentrations. In contrast, β-arrestin-biased ligands induced dose-dependent calcium signaling only at higher AT 1 R overexpression levels. Using inhibitors of G proteins, we demonstrated that both G i and G q/11 mediated overexpressiondependent calcium signaling by β-arrestin-biased ligands. Regarding β-arrestin-mediated cellular events, the β-arrestin-biased ligand TRV026 induced receptor internalization at low physiological receptor levels insufficient for it to initiate calcium signaling. In contrast, unbiased AngII exhibited no relative preference between these outcomes under such low receptor conditions. However, with high receptor overexpression, TRV026 lost its functional selectivity. These results suggest receptor overexpression misleadingly distorts the bias of AT 1 R ligands and highlight the risks of using overexpressed systems to infer the signaling bias of GPCR ligands in physiologically relevant contexts.
Journal of Biomolecular Screening, 2013
A variety of G-protein-coupled receptor (GPCR) screening technologies have successfully partnered a number of GPCRs with their cognate ligands. GPCR-mediated β-arrestin recruitment is now recognized as a distinct intracellular signaling pathway, and ligand-receptor interactions may show a bias toward β-arrestin over classical GPCR signaling pathways. We hypothesized that the failure to identify native ligands for the remaining orphan GPCRs may be a consequence of biased β-arrestin signaling. To investigate this, we assembled 10 500 candidate ligands and screened 82 GPCRs using PathHunter β-arrestin recruitment technology. High-quality screening assays were validated by the inclusion of liganded receptors and the detection and confirmation of these established ligand-receptor pairings. We describe a candidate endogenous orphan GPCR ligand and a number of novel surrogate ligands. However, for the majority of orphan receptors studied, measurement of β-arrestin recruitment did not lead to the identification of cognate ligands from our screening sets. β-Arrestin recruitment represents a robust GPCR screening technology, and ligand-biased signaling is emerging as a therapeutically exploitable feature of GPCR biology. The identification of cognate ligands for the orphan GPCRs and the extent to which receptors may exist to preferentially signal through β-arrestin in response to their native ligand remain to be determined.
A Novel Method for Analyzing Extremely Biased Agonism at G Protein-Coupled Receptors
Molecular pharmacology, 2015
Seven transmembrane receptors were originally named and characterized based on their ability to couple to heterotrimeric G proteins. The assortment of coupling partners for G protein-coupled receptors (GPCR) has subsequently expanded to include other effectors (most notably the βarrestins). This diversity of partners available to the receptor has prompted the pursuit of ligands that selectively activate only a subset of the available partners. A biased or functionally selective ligand may be able to distinguish between different active states of the receptor and this would result in the preferential activation of one signaling cascade more than another. While application of the "standard" operational model for analyzing ligand bias is useful and suitable, in most cases, there are limitations that arise when the biased agonist fails to induce a significant response in one of the assays being compared. In this manuscript we describe a quantitative method for measuring ligand...
Many faces of the GPCR-arrestin interaction
Archives of Pharmacal Research, 2020
G protein-coupled receptors (GPCRs) comprise the largest family of cell surface receptors that mediate various physiological functions such as vision, smell, taste, cardiovascular regulation, neurological regulation, and immune responses (Sriram and Insel 2018). Owing to their roles in normal physiology and pathology, approximately one-third of the drugs currently prescribed target GPCRs (Sriram and Insel 2018). Extracellular signals, such as light, hormones, and neurotransmitters, bind to the extracellular part of the receptor, which induces conformational changes in the receptor, therefore enabling its interaction with downstream intracellular signaling molecules (Fig. 1). The most well-characterized downstream signaling molecule for GPCRs is the heterotrimeric G protein (hereafter referred to as the G protein) (Milligan and Kostenis 2006). Agonist-activated GPCRs interact with G proteins, and this interaction induces the release of guanidine diphosphate (GDP) (Fig. 1). The empty nucleotide-binding pocket of the G protein is then rapidly occupied by guanidine triphosphate (GTP), which results in the dissociation of the Gα subunit from the receptor and Gβγ subunits (Fig. 1). The dissociated Gα and βγ subunits interact and regulate further downstream molecules, such as adenylyl cyclase or phospholipase C (Milligan and Kostenis 2006). Agonist-activated GPCRs are then desensitized and/or internalized through several molecular mechanisms such as phosphorylation by G protein-coupled receptor kinase (GRKs) followed by interaction with arrestins (Fig. 1). Arrestins were first identified as "arresting" GPCR-mediated G protein signaling (Benovic et al. 1987), and to date, four arrestin subtypes have been identified in the human genome and designated as arrestin1-4. The expression of arrestin1 and 4 (also called visual and cone arrestin, respectively) is Abstract G protein-coupled receptors (GPCRs) belong to a major receptor family and regulate important physiological and pathological functions. Upon agonist activation, GPCRs couple to G proteins and induce the activation of G protein-dependent signaling pathways. The agonist-activated GPCRs are also phosphorylated by G protein-coupled receptor kinases (GRKs), which promote their interaction with arrestins. Arrestin binding induces desensitization (i.e., inability to couple to G proteins) and/or internalization of GPCRs. Arrestins not only desensitize and/or internalize GPCRs but also mediate other downstream signals such as mitogen-activated protein kinases. G protein-mediated signaling and arrestin-mediated signaling often result in different functional outcomes, and therefore, it has been suggested that signaling-selective regulation of GPCRs could lead to the development of more effective treatments with fewer side effects. Thus, studies have attempted to develop functionally biased (i.e., signaling-selective) GPCR-targeting drugs. To this end, it is important to elucidate the structural mechanism underlying functionally biased GPCR signaling, which includes understanding the structural mechanism underlying the GPCR-arrestin interaction. This review aims discuss the structural aspects of the GPCR-arrestin interaction, focusing on the differences between reported GPCR-arrestin complex structures.