Conformational Sensors and Domain Swapping Reveal Structural and Functional Differences between ß-Arrestin Isoforms.

Desensitization, signaling, and trafficking of G-protein-coupled receptors (GPCRs) are critically regulated by multifunctional adaptor proteins, ß-arrestins (ßarrs). The two isoforms of ßarrs (ßarr1 and 2) share a high degree of sequence and structural similarity; still, however, they often mediate distinct functional outcomes in the context of GPCR signaling and regulation. A mechanistic basis for such a functional divergence of ßarr isoforms is still lacking. By using a set of complementary approaches, including antibody-fragment-based conformational sensors, we discover structural differences between ßarr1 and 2 upon their interaction with activated and phosphorylated receptors. Interestingly, domain-swapped chimeras of ßarrs display robust complementation in functional assays, thereby linking the structural differences between receptor-bound ßarr1 and 2 with their divergent functional outcomes. Our findings reveal important insights into the ability of ßarr isoforms to drive distinct functional outcomes and underscore the importance of integrating this aspect in the current framework of biased agonism.

Partial ligand-receptor engagement yields functional bias at the human complement receptor, C5aR1.

The human complement component, C5a, binds two different seven-transmembrane receptors termed C5aR1 and C5aR2. C5aR1 is a prototypical G-protein-coupled receptor that couples to the Gαi subfamily of heterotrimeric G-proteins and ß-arrestins (ßarrs) following C5a stimulation. Peptide fragments derived from the C terminus of C5a can still interact with the receptor, albeit with lower affinity, and can act as agonists or antagonists. However, whether such fragments might display ligand bias at C5aR1 remains unexplored. Here, we compare C5a and a modified C-terminal fragment of C5a, C5apep, in terms of G-protein coupling, ßarr recruitment, endocytosis, and extracellular signal-regulated kinase 1/2 mitogen-activated protein kinase activation at the human C5aR1. We discover that C5apep acts as a full agonist for Gαi coupling as measured by cAMP response and extracellular signal-regulated kinase 1/2 phosphorylation, but it displays partial agonism for ßarr recruitment and receptor endocytosis. Interestingly, C5apep exhibits full-agonist efficacy with respect to inhibiting lipopolysaccharide-induced interleukin-6 secretion in human macrophages, but its ability to induce human neutrophil migration is substantially lower compared with C5a, although both these responses are sensitive to pertussis toxin treatment. Taken together, our data reveal that compared with C5a, C5apep exerts partial efficacy for ßarr recruitment, receptor trafficking, and neutrophil migration. Our findings therefore uncover functional bias at C5aR1 and also provide a framework that can potentially be extended to chemokine receptors, which also typically interact with chemokines through a biphasic mechanism.

A synthetic intrabody-based selective and generic inhibitor of GPCR endocytosis.

Beta-arrestins (ßarrs) critically mediate desensitization, endocytosis and signalling of G protein-coupled receptors (GPCRs), and they scaffold a large number of interaction partners. However, allosteric modulation of their scaffolding abilities and direct targeting of their interaction interfaces to modulate GPCR functions selectively have not been fully explored yet. Here we identified a series of synthetic antibody fragments (Fabs) against different conformations of ßarrs from phage display libraries. Several of these Fabs allosterically and selectively modulated the interaction of ßarrs with clathrin and ERK MAP kinase. Interestingly, one of these Fabs selectively disrupted ßarr-clathrin interaction, and when expressed as an intrabody, it robustly inhibited agonist-induced endocytosis of a broad set of GPCRs without affecting ERK MAP kinase activation. Our data therefore demonstrate the feasibility of selectively targeting ßarr interactions using intrabodies and provide a novel framework for fine-tuning GPCR functions with potential therapeutic implications.

Core engagement with ß-arrestin is dispensable for agonist-induced vasopressin receptor endocytosis and ERK activation.

G protein-coupled receptors (GPCRs) exhibit highly conserved activation and signaling mechanisms by which agonist stimulation leads to coupling of heterotrimeric G proteins and generation of second messenger response. This is followed by receptor phosphorylation, primarily in the carboxyl terminus but also in the cytoplasmic loops, and subsequent binding of arrestins. GPCRs typically recruit arrestins through two different sets of interactions, one involving phosphorylated receptor tail and the other mediated by the receptor core. The engagement of both set of interactions (tail and core) is generally believed to be necessary for arrestin-dependent functional outcomes such as receptor desensitization, endocytosis, and G protein-independent signaling. Here we demonstrate that a vasopressin receptor (V2R) mutant with truncated third intracellular loop (V2RΔICL3) can interact with ß-arrestin 1 (ßarr1) only through the phosphorylated tail without engaging the core interaction. Of interest, such a partially engaged V2RΔICL3-ßarr1 complex can efficiently interact with clathrin terminal domain and ERK2 MAPK in vitro. Furthermore, this core interaction-deficient V2R mutant exhibits efficient endocytosis and ERK activation upon agonist stimulation. Our data suggest that core interaction with ßarr is dispensable for V2R endocytosis and ERK activation and therefore provide novel insights into refining the current understanding of functional requirements in biphasic GPCR-ßarr interaction.

Functional competence of a partially engaged GPCR-ß-arrestin complex.

G Protein-coupled receptors (GPCRs) constitute the largest family of cell surface receptors and drug targets. GPCR signalling and desensitization is critically regulated by ß-arrestins (ßarr). GPCR-ßarr interaction is biphasic where the phosphorylated carboxyl terminus of GPCRs docks to the N-domain of ßarr first and then seven transmembrane core of the receptor engages with ßarr. It is currently unknown whether fully engaged GPCR-ßarr complex is essential for functional outcomes or partially engaged complex can also be functionally competent. Here we assemble partially and fully engaged complexes of a chimeric ß2V2R with ßarr1, and discover that the core interaction is dispensable for receptor endocytosis, ERK MAP kinase binding and activation. Furthermore, we observe that carvedilol, a ßarr biased ligand, does not promote detectable engagement between ßarr1 and the receptor core. These findings uncover a previously unknown aspect of GPCR-ßarr interaction and provide novel insights into GPCR signalling and regulatory paradigms.

Emerging Approaches to GPCR Ligand Screening for Drug Discovery.

The superfamily of G-protein-coupled receptors (GPCRs) represents the largest class of cell surface receptors and, thus, a prominent family of drug targets. Recently, there has been significant progress in determination of GPCR crystal structures. The structure-based ligand discovery of GPCRs is emerging as a powerful path to drug development. Sensor surface-immobilized GPCRs can identify direct receptor-ligand interactions of a range of chemical libraries. This type of screening shows great promise as an alternative strategy for ligand discovery. Here, we summarize the most recent developments of structure- and sensor-based GPCR ligand discovery. We also highlight certain areas where GPCRs harbor great potential for the development of novel therapeutics, emphasizing the strategic approaches that may yield significant breakthroughs.

From Recombinant Expression to Crystals: A Step-by-Step Guide to GPCR Crystallography.

G protein-coupled receptors (GPCRs) are the primary targets of drugs prescribed for many human pathophysiological conditions such as hypertension, allergies, schizophrenia, asthma, and various types of cancer. High-resolution structure determination of GPCRs has been a key focus area in GPCR biology to understand the basic mechanism of their activation and signaling and to materialize the long-standing dream of structure-based drug design on these versatile receptors. There has been tremendous effort at this front in the past two decades and it has culminated into crystal structures of 27 different receptors so far. The recent progress in crystallization and structure determination of GPCRs has been driven by innovation and cutting-edge developments at every step involved in the process of crystallization. Here, we present a step-by-step description of various steps involved in GPCR crystallization starting from recombinant expression to obtaining diffracting crystals. We also discuss the next frontiers in GPCR biology that are likely to be a primary focus for crystallography efforts in the next decade or so.

Methodological advances: the unsung heroes of the GPCR structural revolution.

G protein-coupled receptors (GPCRs) are intricately involved in a diverse array of physiological processes and pathophysiological conditions. They constitute the largest class of drug target in the human genome, which highlights the importance of understanding the molecular basis of their activation, downstream signalling and regulation. In the past few years, considerable progress has been made in our ability to visualize GPCRs and their signalling complexes at the structural level. This is due to a series of methodological developments, improvements in technology and the use of highly innovative approaches, such as protein engineering, new detergents, lipidic cubic phase-based crystallization and microfocus synchrotron beamlines. These advances suggest that an unprecedented amount of structural information will become available in the field of GPCR biology in the coming years.

SnapShot: GPCR-Ligand Interactions.

G-protein-coupled receptors enable cells to recognize numerous external stimuli and to transmit corresponding signals across the plasma membrane to trigger appropriate cellular responses. Crystal structures of a number of these receptors have now been determined in inactive and active conformations bound to chemically and functionally distinct ligands. These crystal structures illustrate overall receptor organization and atomic details of ligand-receptor interactions.

Emerging structural insights into biased GPCR signaling.

The discovery of biased signaling at G protein-coupled receptors (GPCRs), the largest class of cell surface receptors and primary drug targets for numerous human diseases, has redefined the classical concepts of receptor pharmacology. It not only highlights the depth of signaling diversity within the GPCR system, but also offers possibilities for novel and more-effective therapeutics. Here, we highlight the recent biophysical and structural advances in our understanding of ligand-receptor interactions and conformational changes in the receptors, which provide novel mechanistic insights into biased GPCR signaling. We also underline key aspects of GPCR-biased signaling that remain to be investigated in greater detail to develop a complete molecular understanding of this process and overall GPCR signaling.