Snapshot of the cannabinoid receptor 1-arrestin complex unravels the biased signaling mechanism

Flavors of GPCR signaling bias

GPCRs are inherently flexible molecules existing in an equilibrium of multiple conformations. Binding of GPCR agonists shifts this equilibrium. Certain agonists can increase the fraction of active-like conformations that predispose the receptor to coupling to a particular signal transducer or a select group of transducers. Such agonists are called biased, in contrast to balanced agonists that facilitate signaling via all transducers the receptor couples to. These biased agonists preferentially channel the signaling of a GPCR to particular G proteins, GRKs, or arrestins. Preferential activation of particular G protein or arrestin subtypes can be beneficial, as it would reduce unwanted on-target side effects, widening the therapeutic window. However, biasing GPCRs has two important limitations: a) complete bias is impossible due to inherent flexibility of GPCRs; b) receptor-independent functions of signal transducer proteins cannot be directly affected by GPCR ligands or differential receptor barcoding by GRK phosphorylation. This article is part of the Special Issue on "Ligand Bias".

Advances in the molecular understanding of GPCR-arrestin complexes

Arrestins are essential proteins for the regulation of G protein-coupled receptors (GPCRs). They mediate GPCR desensitization after the activated receptor has been phosphorylated by G protein receptor kinases (GRKs). In addition, GPCR-arrestin interactions may trigger signaling pathways that are distinct and independent from G proteins. The non-visual GPCRs encompass hundreds of receptors with varying phosphorylation patterns and amino acid sequences, which are regulated by only two human non-visual arrestin isoforms. This review describes recent findings on GPCR-arrestin complexes, obtained by structural techniques, biophysical, biochemical, and cellular assays. The solved structures of complete GPCR-arrestin complexes are of limited resolution ranging from 3.2 to 4.7 Å and reveal a high variability in the relative receptor-arrestin orientation. In contrast, biophysical and functional data indicate that arrestin recruitment, activation and GPCR-arrestin complex stability depend on the receptor phosphosite sequence patterns and density. At present, there is still a manifest lack of high-resolution structural and dynamical information on the interactions of native GPCRs with both GRKs and arrestins, which could provide a detailed molecular understanding of the genesis of receptor phosphorylation patterns and the specificity GPCR-arrestin interactions. Such insights seem crucial for progress in the rational design of advanced, arrestin-specific therapeutics.

Structural basis of psychedelic LSD recognition at dopamine D1 receptor

Understanding the kinetics of LSD in receptors and subsequent induced signaling is crucial for comprehending both the psychoactive and therapeutic effects of LSD. Despite extensive research on LSD's interactions with serotonin 2A and 2B receptors, its behavior on other targets, including dopamine receptors, has remained elusive. Here, we present cryo-EM structures of LSD/PF6142-bound dopamine D1 receptor (DRD1)-legobody complexes, accompanied by a β-arrestin-mimicking nanobody, NBA3, shedding light on the determinants of G protein coupling versus β-arrestin coupling. Structural analysis unveils a distinctive binding mode of LSD in DRD1, particularly with the ergoline moiety oriented toward TM4. Kinetic investigations uncover an exceptionally rapid dissociation rate of LSD in DRD1, attributed to the flexibility of extracellular loop 2 (ECL2). Moreover, G protein can stabilize ECL2 conformation, leading to a significant slowdown in ligand's dissociation rate. These findings establish a solid foundation for further exploration of G protein-coupled receptor (GPCR) dynamics and their relevance to signal transduction.

Homodimerization of CB2 cannabinoid receptor triggered by a bivalent ligand enhances cellular signaling

G protein-coupled receptors (GPCRs) exist within a landscape of interconvertible conformational states and in dynamic equilibrium between monomers and higher-order oligomers, both influenced by ligand binding. Here, we show that a homobivalent ligand formed by equal chromenopyrazole moieties as pharmacophores, connected by 14 methylene units, can modulate the dynamics of the cannabinoid CB2 receptor (CB2R) homodimerization by simultaneously binding both protomers of the CB2R-CB2R homodimer. Computational and pharmacological experiments showed that one of the ligand pharmacophores binds to the orthosteric site of one protomer, and the other pharmacophore to a membrane-oriented pocket between transmembranes 1 and 7 of the partner protomer. This results in unique pharmacological properties, including increased potency in Gi-mediated signaling and enhanced recruitment of β-arrestin. Thus, by modulating dimerization dynamics, it may be possible to fine-tune CB2R activity, potentially leading to improved therapeutic outcomes.

Entropy drives the ligand recognition in G-protein-coupled receptor subtypes

Achieving ligand subtype selectivity within highly homologous subtypes of G-protein-coupled receptor (GPCR) is critical yet challenging for GPCR drug discovery, primarily due to the unclear mechanism underlying ligand subtype selectivity, which hampers the rational design of subtype-selective ligands. Herein, we disclose an unusual molecular mechanism of entropy-driven ligand recognition in cannabinoid (CB) receptor subtypes, revealed through atomic-level molecular dynamics simulations, cryoelectron microscopy structure, and mutagenesis experiments. This mechanism is attributed to the distinct conformational dynamics of the receptor's orthosteric pocket, leading to variations in ligand binding entropy and consequently, differential binding affinities, which culminate in specific ligand recognition. We experimentally validated this mechanism and leveraged it to design ligands with enhanced or ablated subtype selectivity. One such ligand demonstrated favorable pharmacokinetic properties and significant efficacy in rodent inflammatory analgesic models. More importantly, it is precisely due to the high subtype selectivity obtained based on this mechanism that this ligand does not show addictive properties in animal models. Our findings elucidate the unconventional role of entropy in CB receptor subtype selectivity and suggest a strategy for rational design of ligands to achieve entropy-driven subtype selectivity for many pharmaceutically important GPCRs.

GPCR-dependent and -independent arrestin signaling

Biological activity of free arrestins is often overlooked. Based on available data, we compare arrestin-mediated signaling that requires and does not require binding to G-protein-coupled receptors (GPCRs). Receptor-bound arrestins activate ERK1/2, Src, and focal adhesion kinase (FAK). Yet, arrestin-3 regulation of Src family member Fgr does not appear to involve receptors. Free arrestin-3 facilitates the activation of JNK family kinases, preferentially binds E3 ubiquitin ligases Mdm2 and parkin, and facilitates parkin-dependent mitophagy. The binding of arrestins to microtubules and calmodulin and their function in focal adhesion disassembly and apoptosis also do not involve receptors. Biased GPCR ligands and the phosphorylation barcode can only affect receptor-dependent arrestin signaling. Thus, elucidation of receptor dependence or independence of arrestin functions has important scientific and therapeutic implications.

Arrestins: A Small Family of Multi-Functional Proteins

The first member of the arrestin family, visual arrestin-1, was discovered in the late 1970s. Later, the other three mammalian subtypes were identified and cloned. The first described function was regulation of G protein-coupled receptor (GPCR) signaling: arrestins bind active phosphorylated GPCRs, blocking their coupling to G proteins. It was later discovered that receptor-bound and free arrestins interact with numerous proteins, regulating GPCR trafficking and various signaling pathways, including those that determine cell fate. Arrestins have no enzymatic activity; they function by organizing multi-protein complexes and localizing their interaction partners to particular cellular compartments. Today we understand the molecular mechanism of arrestin interactions with GPCRs better than the mechanisms underlying other functions. However, even limited knowledge enabled the construction of signaling-biased arrestin mutants and extraction of biologically active monofunctional peptides from these multifunctional proteins. Manipulation of cellular signaling with arrestin-based tools has research and likely therapeutic potential: re-engineered proteins and their parts can produce effects that conventional small-molecule drugs cannot.

Biased signalling in analgesic research and development

Ligand bias offers a novel means to improve the therapeutic profile of drugs. With regard to G protein-coupled receptors involved in analgesia, it could be advantageous to develop such drugs if the analgesic effect is mediated by a different cellular signalling pathway than the adverse effects associated with the drug. Whilst this has been explored over a number of years for the μ receptor, it remains unclear whether this approach offers significant benefit for the treatment of pain. Nevertheless, the development of biased ligands at other G protein-coupled receptors in the CNS does offer some promise for the development of novel analgesic drugs in the future. Here we summarise and discuss the recent evidence to support this.

Natural products in osteoarthritis treatment: bridging basic research to clinical applications

Osteoarthritis (OA) is the most prevalent degenerative musculoskeletal disease, severely impacting the function of patients and potentially leading to disability, especially among the elderly population. Natural products (NPs), obtained from components or metabolites of plants, animals, microorganisms etc., have gained significant attention as important conservative treatments for various diseases. Recently, NPs have been well studied in preclinical and clinical researches, showing promising potential in the treatment of OA. In this review, we summed up the main signaling pathways affected by NPs in OA treatment, including NF-κB, MAPKs, PI3K/AKT, SIRT1, and other pathways, which are related to inflammation, anabolism and catabolism, and cell death. In addition, we described the therapeutic effects of NPs in different OA animal models and the current clinical studies in OA patients. At last, we discussed the potential research directions including in-depth analysis of the mechanisms and new application strategies of NPs for the OA treatment, so as to promote the basic research and clinical transformation in the future. We hope that this review may allow us to get a better understanding about the potential bioeffects and mechanisms of NPs in OA therapy, and ultimately improve the effectiveness of NPs-based clinical conservative treatment for OA patients.