Biased GPCR signaling: Possible mechanisms and inherent limitations
Introduction
The classical paradigm of G protein-coupled receptor (GPCR) signaling posited that GPCRs communicate with the cell via heterotrimeric G proteins, and their signaling is terminated by receptor phosphorylation followed by arrestin binding (reviewed in (Carman & Benovic, 1998)). However, in the last two decades, numerous studies have suggested that receptor-bound arrestins initiate a second round of signaling, which is quite different from the G protein-mediated one (reviewed in (Gurevich & Gurevich, 2006a; Peterson & Luttrell, 2017)). This phenomenon suggested the idea that GPCR signaling can be biased towards either G proteins or arrestins by using ligands that promote receptor interaction with one type of signal transducer but not with the other. Although the first reported examples of bias of a ligand or a GPCR described differential activation of different G protein subtypes (Eason, Jacinto, & Liggett, 1994; Spengler et al., 1993), in recent years discussions of GPCR signaling bias have mostly focused on the G protein-arrestin dichotomy (DeWire & Violin, 2011; Violin & Lefkowitz, 2007; Whalen, Rajagopal, & Lefkowitz, 2011; Wisler, Xiao, Thomsen, & Lefkowitz, 2014). In many cases, it has been proposed that one signaling branch is responsible for the therapeutic effects of drugs, while the other mediates unwanted side effects. Therefore, the idea that biased drugs will be therapeutically superior to the traditional unbiased ones has become popular (extensively reviewed in (DeWire & Violin, 2011; Khoury, Clément, & Laporte, 2014; Luttrell, Maudsley, & Bohn, 2015; Rankovic, Brust, & Bohn, 2016; Whalen et al., 2011; Wisler et al., 2014)). For a number of pathological conditions, drugs that might specifically direct GPCR signaling to G protein- or arrestin-mediated pathways have been identified, and their beneficial effects have been demonstrated (reviewed in (Whalen et al., 2011)). Here we consider the issue of GPCR signaling bias from a structural and mechanistic perspective, discussing the possibilities opened by this approach and its inherent limitations. We do not attempt to provide a comprehensive review of all reported cases where signaling bias has been documented, as this has already been done in the excellent reviews cited above.
Section snippets
Conformational heterogeneity of GPCRs
The classical view of GPCRs considered these receptors as on-off switches that oscillate between two conformations, active and inactive. Agonists were believed to shift the equilibrium towards the active conformation, inverse agonists – towards the inactive one, thereby suppressing constitutive activity, whereas neutral antagonists occupy the ligand binding site without affecting the conformational equilibrium. The classical extended ternary complex model of GPCR signaling (Samama, Cotecchia,
Structures of GPCR complexes with signal transducers
The first solved structure of a GPCR bound to a cognate G protein (β2AR complex with Gs (Rasmussen et al., 2011)) was followed by several GPCR-Gs structures in which the receptor-G protein interfaces looked remarkably similar (Carpenter, Nehmé, Warne, Leslie, & Tate, 2016; Liang et al., 2017; Zhang et al., 2017). These structures confirmed the activation-dependent outward movement of the cytoplasmic ends of the transmembrane helices V and VI earlier described in rhodopsin (Farrens, Altenbach,
What does the mechanism of GPCR desensitization tell us about biased signaling?
Let's recall the classical paradigm of two-step GPCR desensitization (Carman & Benovic, 1998). It posits that active GPCRs are phosphorylated by GRKs, and then active phosphorylated receptors bind arrestins. The original concept assumed that the same active conformation of the receptor activates G proteins, is recognized and phosphorylated by GRKs, and then binds arrestins (Fig. 1, classical paradigm). Since GRK/arrestin-mediated desensitization has been demonstrated by numerous labs working
Is there G protein-independent arrestin-mediated signaling?
The ERK pathway is the second, after c-Src, signaling mechanism identified as activated via arrestin-dependent scaffolding (Luttrell et al., 2001). It is arguably the best studied arrestin-dependent signaling pathway and often serves as a classic example of arrestin-mediated signaling. ERK is activated by a variety of mechanisms, some of which are GPCR-dependent (Luttrell, 2003), while others involve growth factor receptors (Garrington & Johnson, 1999; McKay & Morrison, 2007), death receptors (
6. Diversity of arrestin and GRK subtypes
Another issue routinely ignored in the discussion of bias is the existence of arrestin isoforms: all vertebrates, including mammals, have two non-visual arrestins (bony fish that underwent an extra whole genome duplication have three) (Gurevich & Gurevich, 2006a; Indrischek, Prohaska, Gurevich, Gurevich, & Stadler, 2017). While they are highly homologous (Attramadal et al., 1992; Indrischek et al., 2017; Sterne-Marr et al., 1993), the fact that vertebrate evolution kept them for millions of
How typical is the ERK pathway for arrestin-dependent signaling?
Arrestins are known to regulate a large and extremely diverse set of signaling pathways (Peterson & Luttrell, 2017). It is hardly conceivable that all these pathways depend in a similar fashion on the arrestin interactions with GPCRs. However, the dependence of a particular signaling function of arrestins on their interactions with GPCRs is rarely studied or discussed. Even though both ERK1/2 and JNK1/2/3 are MAP kinases activated by three-tiered MAP kinase cascades, the modes of activation of
Can changes in the kinetics of G protein activation be confused with arrestin-mediated signaling?
In many cases when ligand bias is discussed, arrestin-mediated signaling is proposed as an alternative to the G protein-dependent signaling. However, it is critical to analyze what was experimentally demonstrated in each study, rather than what was implied or assumed. Let us consider some of the most recent examples. Accessory protein MRAP2 was shown to bias ghrelin receptor GHSR1a towards G proteins (Rouault et al., 2020). In this study, G protein-mediated signaling was measured directly,
The promise and limitations of biased GPCR ligands
Most GPCRs couple to more than one type of G protein and also bind one or more arrestin variant upon phosphorylation by several distinct GRKs. In quite a few cases, some branches of GPCR signaling were shown to be therapeutically beneficial, whereas others caused unwanted on-target side effects. Thus, the development of biased ligands that can initiate certain branches of signaling, but not others, has an obvious value for therapy (reviewed in (Gesty-Palmer & Luttrell, 2011; Luttrell et al.,
Conclusions
Biased GPCR ligands that preferentially direct signaling to particular G proteins, GRKs, and/or arrestins hold promise for better therapeutic outcomes. However, the bias is unlikely to be absolute: other (unwanted) signaling pathways would likely be activated along with the preferred one. Moreover, in the quest to reduce or eliminate the unwanted signaling, the preferred pathway is also likely to be impaired. A huge technical hurdle in the development of biased GPCR-targeting drugs is the issue
Declaration of Competing Interest
The authors declare no conflict of interest.
Acknowledgments
This project was supported by NIH grants RO1 EY011500, R35 GM122491, and Cornelius Vanderbilt Endowed Chair (Vanderbilt University) (VVG), as well as NIH grants RO1 NS065868 and RO1 DA030103 (EVG).
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2023, Drug Discovery TodayCitation Excerpt :There is a growing consensus that ligands and their cognate receptors can selectively target either the G protein or β-arrestin pathways; such ligands are referred to as ‘biased ligands’ and the GPCRs as ‘biased receptors’. Thus, biased ligands and biased receptors modulate different physiological and pathological signal transduction pathways compared with their nonbiased counterparts.16–18 As proof, the AT1R ligand TRV027 does not activate the Gαq signaling pathway but activates β-arrestin 2, which can block the harmful effects of AT1R activation while preserving or enhancing the beneficial effects.19
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2022, Archives of Medical ResearchCitation Excerpt :These proteins decrease G protein-mediated events and promote those independent of these heterotrimeric GTPases. In addition, they interact with a large variety of proteins participating in many functions, including, among many others, regulation of gene expression, proliferation, and differentiation (41,42). Analysis of the phosphorylation codes that might promote β-arrestin binding indicates putative multiple binding sites for these proteins in the GPCR third intracellular loops and the carboxyl termini, which complicates the exploration of the possible functional consequences (43).
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2022, Trends in Biochemical SciencesCitation Excerpt :The data accumulated over the past ~20 years indicate that GPCR-bound arrestins also regulate several branches of signaling [7,8], with potential therapeutic implications [9,10]. For most GPCRs, the binding of arrestins is contingent upon receptor phosphorylation by GPCR kinases (GRKs) [11]. Although receptor-bound GRKs have not been reported to participate in cell signaling directly, receptor phosphorylation by distinct GRKs apparently plays a critical role in controlling the signaling [12].
Agonist-induced phosphorylation of orthologues of the orphan receptor GPR35 functions as an activation sensor
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