Arun K. Shukla

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.

Emerging Functional Divergence of β-Arrestin Isoforms in GPCR Function

G protein-coupled receptors (GPCRs) are tightly regulated by multifunctional protein β-arrestins. Two isoforms of β-arrestin sharing more than 70% sequence identity and overall very similar 3D structures, β-arrestins 1 and 2, were originally expected to be functionally redundant. However, in recent years multiple lines of emerging evidence suggest they have distinct roles in various aspects of GPCR regulation and signaling. We summarize selected examples of GPCRs where β-arrestin isoforms are discovered to display non-overlapping and sometimes even antagonistic functions. We also discuss potential mechanistic basis for their functional divergence and highlight new frontiers that are likely to form the focal points of research in this area in coming years.

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.

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

For a prototypical GPCR, the human vasopressin receptor type 2 (V2R), βarr1 can form a stable complex associated via only the phosphorylated carboxyl terminus of the receptor. Such a partially engaged complex is functionally competent with respect to supporting receptor internalization and ERK MAPK activation.

Novel Structural Insights into GPCR–β-Arrestin Interaction and Signaling

G protein-coupled receptors (GPCRs) are major signal recognition and transmission units in the plasma membrane. The interaction of activated and phosphorylated GPCRs with the multifunctional adaptor proteins β-arrestins (βarrs) is crucial for regulation of their signaling and functional outcomes. Over the past few years, a range of structural, biochemical, and cellular studies have revealed novel insights into GPCR-βarr interaction and signaling. Some of these findings have come as a surprise and therefore have the potential to significantly refine the conceptual framework of the GPCR-βarr system. Here we discuss these recent advances with particular emphasis on biphasic GPCR-βarr interaction, the formation of GPCR-G-protein-βarr supercomplexes, and receptor-specific conformational signatures in βarrs. We also underline the emerging research areas that are likely to be at the center stage of investigations 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.

Antibody fragments for stabilization and crystallization of G protein-coupled receptors and their signaling complexes.

G protein-coupled receptors (GPCRs) are one of the key players in extracellular signal recognition and their subsequent communications with cellular signaling machinery. Crystallization and high-resolution structure determination of GPCRs has been one of the major advances in the area of GPCR biology over the last 7-8 years. There have primarily been three approaches to GPCR crystallization till date. These are fusion protein strategy, thermostabilization, and antibody fragment-mediated crystallization. Of these, antibody fragment-mediated crystallization has not only provided the first breakthrough in structure determination of a non-rhodopsin GPCR but it has also assisted in obtaining structures of fully active conformations of GPCRs. Antibody fragment approach has also been crucial in obtaining structural information on GPCR signaling complexes. Here, we highlight the specific examples of GPCR crystal structures that have utilized antibody fragments for promoting crystallogenesis and structure solution. We also discuss emerging powerful technologies such as the nanobody technology and the synthetic phage display libraries in the context of GPCR crystallization and underline how these tools are likely to propel key GPCR structural studies in future.

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.

Molecular mechanism of modulating arrestin conformation by GPCR phosphorylation.

Arrestins regulate the signaling of ligand-activated, phosphorylated G-protein-coupled receptors (GPCRs). Different patterns of receptor phosphorylation (phosphorylation barcode) can modulate arrestin conformations, resulting in distinct functional outcomes (for example, desensitization, internalization, and downstream signaling). However, the mechanism of arrestin activation and how distinct receptor phosphorylation patterns could induce different conformational changes on arrestin are not fully understood. We analyzed how each arrestin amino acid contributes to its different conformational states. We identified a conserved structural motif that restricts the mobility of the arrestin finger loop in the inactive state and appears to be regulated by receptor phosphorylation. Distal and proximal receptor phosphorylation sites appear to selectively engage with distinct arrestin structural motifs (that is, micro-locks) to induce different arrestin conformations. These observations suggest a model in which different phosphorylation patterns of the GPCR C terminus can combinatorially modulate the conformation of the finger loop and other phosphorylation-sensitive structural elements to drive distinct arrestin conformation and functional outcomes.Analysis of 18 available structures and other data reveals a new, conserved structural motif in arrestins and suggests that different phosphorylation patterns of the GPCR C terminus can drive distinct arrestin conformations and functional outcomes.

GPCR Signaling: The Interplay of Gαi and β-arrestin

Biased agonism at G-protein-coupled receptors is generally conceptualized as the ability of certain stimuli to trigger downstream signaling exclusively through one of two effectors. Recent studies reveal that signaling downstream of the β1 adrenergic receptor and the angiotensin II type 1 receptor induced by biased stimuli actually involves both effectors.