The thromboxane A2 receptor (TP), expressed in platelets and smooth muscle, plays an important role in blood clotting and muscle contraction. The endogenous ligand of this G protein-coupled receptor (GPCR), thromboxane A2 (TXA2), is a short-lived arachidonic acid metabolite with a half-life of ∼30 seconds, which makes investigating the TP structure and activation mechanism highly challenging. Here we determine the structures of the TP in complex with the synthetic agonists, U46619 and I-BOP, stable analogues of the natural ligand, in the presence of the signalling protein partner, Gq. The structures reveal a unique activation switch for the receptor that differs from typical class A GPCR family members. Complemented by functional studies, mutational analysis, docking, and molecular dynamics (MD) simulations, our investigation highlights the differences between agonist and antagonist binding and explores the ligand entry mechanism to the binding pocket from within the membrane via a molecular gate composed of two transmembrane helices. In addition, our study provides crucial information to aid in the rational design of compounds targeting the TP, and offers mechanistic insights into inherited disorders associated with mutations in the TP.

Abstract Understanding how allosteric modulators influence protein dynamics is essential for guiding drug design. This work analyses a total of 45 μs of classical molecular dynamics simulations for four class A G-protein-coupled receptors (GPCRs), namely the Complement C5a receptor (C5AR1), the Purinergic Receptor P2Y (P2RY1), and the Cannabinoid Receptors 1 and 2 (CNR1/CNR2). Protein dynamics is essential to detect the shallow extrahelical binding sites, such as the one found in P2RY1. Current methods for computing Allosteric Communication Networks (ACNs) produce complex outputs requiring expert interpretation. To address this, we focus on the shortest paths of information transfer between the orthosteric and G-protein binding sites in Class A GPCRs. Our retrospective analysis reveals state- and bias ligand-dependent residue interactions along these communication pathways. Furthermore, focusing on the predicted binding site of allosteric modulator EC21a at cannabinoid receptors, the ACN framework was used to prioritize two residues for mutational analysis that may contribute to allosteric communication.

G protein-coupled receptors orchestrate numerous physiological processes and represent the largest class of drug targets, yet their intracellular regulators, the β-arrestins, remain largely underexplored. Despite their crucial roles in receptor desensitization, trafficking, and signaling, few modulators have been identified, with limited isoform selectivity. Therapeutic efforts have mainly focused on receptor-level biased ligands to indirectly influence arrestin pathways. However, advances in small-molecule discovery and peptide design are now expanding the feasibility of directly modulating β-arrestins using structurally tailored ligands, primarily as research tools and potential therapeutic leads. Along with the recent identification of disease-associated mutations and first-generation modulators, these developments create new opportunities for selective and mutation-specific targeting. In this review, we summarize β-arrestin biology and signaling, highlight recent discoveries of disease-associated mutations and β-arrestin modulators, and discuss emerging strategies for precision drug development of arrestin-targeting compounds, with a focus on peptides.

Emerging evidence has demonstrated that cannabinoid receptor 2 (CB2) is involved in a number of diseases, such as neurodegenerative disorders and various types of cancer, making it an attractive pharmacological target. Classically, a protein active site or an orthosteric binding site, where the endogenous ligand binds to, is used as a target for the design of most small-molecule drugs. This can present challenges when it comes to phylogenetically related proteins that have similar orthosteric binding sites, such as the cannabinoid receptors. An alternative approach is to target sites that are unique to these receptors yet still impact receptor function, known as allosteric binding sites. Using an inactive-state human cannabinoid receptor 2 crystal structure (PDB ID:5ZTY), we identified a putative CB2 allosteric site using computational approaches. In vitro signaling assays using known allosteric modulators and CB2 agonists have been used to verify the in silico results. This identification opens promising avenues for the development of selective and specific CB2 ligands for therapeutic purposes.

Patients diagnosed with pancreatic ductal adenocarcinoma (PDAC) have a dismal 5-year survival (∼13%). Thus, new, effective, and ideally, less toxic therapies are desperately needed. Epidemiologic studies have found that patients with PDAC prescribed H1-antihistamines have improved survival. Expression of the histamine H1 receptor (HRH1), a G protein-coupled receptor which is blocked by approved H1-antihistamines, is increased by ∼20-fold in PDAC tumors compared with normal pancreas. Here, we used bioinformatic and molecular biological techniques to identify the cellular localization of HRH1 in the PDAC tumor microenvironment, assess functional responses to HRH1 activation, and define its potential biological roles in PDAC. We found that HRH1 is primarily expressed in cancer cells of PDAC tumors in humans and KPC mice (mice engineered to develop PDAC) and signals via G protein q/11 to increase intracellular Ca2+. HRH1 activation increases migration and invasion by PDAC cancer cells. Orally administered fexofenadine, an H1-antihistamine, was bioavailable in the tumors of KPC mice and yielded smaller pancreatic tumor tissue weights and lower expression of immunomodulatory (interleukin 6 and PD-1) and fibrotic (Col1A1) genes than in vehicle-control KPC mice. Thus, PDAC cancer cells express HRH1, which is functional in vitro and in vivo, suggesting that the repurposing of approved H1-antihistamines may be an efficacious and safe therapeutic approach for patients with PDAC. SIGNIFICANCE STATEMENT: Pancreatic ductal adenocarcinoma (PDAC) has a ∼13% 5-year survival rate, highlighting the need for new therapies. The HRH1 (histamine) receptor, associated with poorer survival, is upregulated in PDAC tumors. This study found that HRH1 is functional in PDAC cells, increasing intracellular Ca2+ via Gq/11 and promoting tumorigenic responses. KPC mice treated with an H1-antihistamine have reduced pancreas weight and lower proinflammatory and fibrotic markers in PDAC tumors. Thus, HRH1 may be a potential target for repurposing approved H1-antihistamines to treat PDAC.

Patients diagnosed with pancreatic ductal adenocarcinoma (PDAC) have a dismal 5-year survival (∼13%). Thus, new, effective, and ideally, less toxic therapies are desperately needed. Epidemiologic studies have found that patients with PDAC prescribed H1-antihistamines have improved survival. Expression of the histamine H1 receptor (HRH1), a G protein-coupled receptor which is blocked by approved H1-antihistamines, is increased by ∼20-fold in PDAC tumors compared with normal pancreas. Here, we used bioinformatic and molecular biological techniques to identify the cellular localization of HRH1 in the PDAC tumor microenvironment, assess functional responses to HRH1 activation, and define its potential biological roles in PDAC. We found that HRH1 is primarily expressed in cancer cells of PDAC tumors in humans and KPC mice (mice engineered to develop PDAC) and signals via G protein q/11 to increase intracellular Ca2+. HRH1 activation increases migration and invasion by PDAC cancer cells. Orally administered fexofenadine, an H1-antihistamine, was bioavailable in the tumors of KPC mice and yielded smaller pancreatic tumor tissue weights and lower expression of immunomodulatory (interleukin 6 and PD-1) and fibrotic (Col1A1) genes than in vehicle-control KPC mice. Thus, PDAC cancer cells express HRH1, which is functional in vitro and in vivo, suggesting that the repurposing of approved H1-antihistamines may be an efficacious and safe therapeutic approach for patients with PDAC. SIGNIFICANCE STATEMENT: Pancreatic ductal adenocarcinoma (PDAC) has a ∼13% 5-year survival rate, highlighting the need for new therapies. The HRH1 (histamine) receptor, associated with poorer survival, is upregulated in PDAC tumors. This study found that HRH1 is functional in PDAC cells, increasing intracellular Ca2+ via Gq/11 and promoting tumorigenic responses. KPC mice treated with an H1-antihistamine have reduced pancreas weight and lower proinflammatory and fibrotic markers in PDAC tumors. Thus, HRH1 may be a potential target for repurposing approved H1-antihistamines to treat PDAC.

The complement system is a complex network of proteins that plays a crucial role in the innate immune response. One important component of this system is the C5a-C5aR1 complex, which is critical in the recruitment and activation of immune cells. In-depth investigation of the activation mechanism as well as biased signaling of the C5a-C5aR1 system will facilitate the elucidation of C5a-mediated pathophysiology. In this study, we determined the structure of C5a-C5aR1-Gi complex at a high resolution of 3 & Aring; using cryo-electron microscopy (Cryo-EM). Our results revealed the binding site of C5a, which consists of a polar recognition region on the extracellular side and an amphipathic pocket within the transmembrane domain. Furthermore, we found that C5a binding induces conformational changes of C5aR1, which subsequently leads to the activation of G protein signaling pathways. Notably, a key residue (M265) located on transmembrane helix 6 (TM6) was identified to play a crucial role in regulating the recruitment of beta-arrestin driven by C5a. This study provides more information about the structure and function of the human C5a-C5aR1 complex, which is essential for the proper functioning of the complement system. The findings of this study can also provide a foundation for the design of new pharmaceuticals targeting this receptor with bias or specificity.

Astrocytes play a key role in modulating synaptic transmission by controlling extracellular gamma-aminobutyric acid (GABA) levels via GAT-1 and GAT-3 GABA transporters (GATs). Using primary cultures of rat astrocytes, we show here that a further level of regulation of GABA uptake occurs via modulation of the GATs by the adenosine A1 (A1R) and A2A (A2AR) receptors. This regulation occurs through A1R-A2AR heteromers that signal via two different G proteins, Gs and Gi/0, and either enhances (A2AR) or inhibits (A1R) GABA uptake. These results provide novel mechanistic insight into how GPCR heteromers signal. Furthermore, we uncover a previously unknown mechanism where adenosine, in a concentration-dependent manner, acts via a heterocomplex of adenosine receptors in astrocytes to significantly contribute to neurotransmission at the tripartite (neuron-glia-neuron) synapse.

The melanocortin-4 receptor (MC4R) is a G protein-coupled receptor (GPCR) and a key regulator of appetite and metabolism. It can interact with the melanocortin-receptor accessory protein 2 (MRAP2), a single transmembrane helix protein known to interplay with several different GPCRs. However, the consequences of this interaction are not completely understood. Here we report that co-expression of MRAP2 has multiple effects on the MC4R: it enhances G protein-mediated signaling and simultaneously impairs β-arrestin2 recruitment and, consequently, internalization. In addition, co-expression of MRAP2 leads to an increased number of monomers of MC4R by disrupting receptor oligomers. A structural model of the active state MC4R–MRAP2 complex supports the experimentally observed monomerization. Taken together, our data indicate that MRAP2 is an accessory protein that interacts with and influences MC4R structure, biasing its signaling towards G protein-mediated effects. ### Competing Interest Statement The authors have declared no competing interest.

GPR68 is a proton-sensing G-protein Coupled Receptor (GPCR) involved in a variety of physiological processes and disorders including neoplastic pathologies. While GPR68 and few other GPCRs have been shown to be activated by a decrease in the extracellular pH, the molecular mechanism of their activation remains largely unknown. In this work, we used a combined computational and in vitro approach to provide new insight into the activation mechanism of the receptor. Molecular Dynamics simulations of GPR68 were used to model the changes in residue interactions and motions triggered by pH. Global and local rearrangements consistent with partial activation were observed upon protonation of the inactive state. Selected extracellular histidine and transmembrane acidic residues were found to have significantly upshifted pKa values during the simulations, consistently with their previously hypothesised role in activation through changes in protonation state. Moreover, a novel pairing between histidine and acidic residues in the extracellular region was highlighted by both sequence analyses and simulation data and tested through site-directed mutagenesis. At last, we identified a previously unknown hydrophobic lock in the extracellular region that might stabilise the inactive conformation and regulate the transition to the active state.

HCN channels are important for regulating heart rhythm and nerve activity and have been studied as potential drug targets for treating depression, arrhythmia, nerve pain and epilepsy.Despite possessing unique pharmacological properties, HCN channels share common characteristics in that they are activated by hyperpolarization and modulated by cAMP and other membrane lipids. However, the mechanisms of how these ligands bind and modulate HCN channels are unclear. In this study, we solved structures of full-length human HCN3 using cryo-EM and captured two different states, including a state without any ligand bound and a state with cAMP bound. Our structures reveal the novel binding sites for cholesteryl hemisuccinate in apo-state and show how cholesteryl hemisuccinate and cAMP binding cause conformational changes in different states. These findings explain how these small modulators are sensed in mammals at the molecular level.The results of our study could help design more potent and specific compounds to influence HCN channel activity and offer new therapeutic possibilities for diseases that lack effective treatment.

We report the structure-based design of cannabinoid receptor type 2 (CB2R)-selective inverse agonists (S)-1 and (R)-1, which were derived from privileged agonist HU-308 by introduction of a phenyl group at the gem-dimethylheptyl sidechain. Epimer (R)-1 exhibits high affinity for CB2R with Kd = 39 nM and serves as a platform for the synthesis of a wide variety of probes. Notably, the fluorescent probes, for the first time, retain their inverse agonist functionality, high affinity, and selectivity for CB2R independent of linker and fluorophore substitution. Ligands (S)-1, (R)-1, and their derivatives act as inverse agonists in CB2R-mediated cAMP as well as G protein recruitment assays, and do not trigger β-arrestin–receptor association. Furthermore, no receptor activation was detected in live cell ERK1/2 phosphorylation and Ca2+-release assays. Confocal fluorescence imaging experiments with (R)-7 (Alexa488) and (R)-9 (Alexa647) probes employing BV-2 microglial cells visualized CB2R expressed at endogenous levels. Finally, molecular dynamics simulations corroborate the initial docking data in which inverse agonists restrict movement of toggle switch, Trp2586.48, and thereby stabilize CB2R in its inactive state. The present study serves as a blueprint for the rational design of GPCR ligands beyond CB2R with a tailored functional response.

Bitter taste receptors (TAS2Rs), a subfamily of G-protein coupled receptors (GPCRs) expressed orally and extraorally, elicit signaling in response to a large set of ligands. Among the 25 functional TAS2Rs encoded in the human genome, TAS2R14 is the most promiscuous, and responds to hundreds of chemically diverse agonists. Here, we present the cryo–electron microscopy (cryo-EM) structure of the human TAS2R14 (hTAS2R14) in complex with its cognate signaling partner gustducin, and bound to flufenamic acid (FFA), a clinically approved nonsteroidal anti-inflammatory drug. The structure reveals an unusual binding mode for FFA, where two copies are bound at distinct binding pockets: one at the canonical GPCR site within the trans-membrane bundle, and the other in the intracellular facet, bridging the receptor with gustducin. Combined with site-directed mutagenesis and the design of a fluorescent FFA derivative for pocket-specific ligand binding BRET assays, our studies support a dual binding mode for FFA in TAS2R14. These results fill a gap in the understanding of bitter taste signaling and provide tools for guided design of TAS2R-targeted compounds.### Competing Interest StatementThe authors have declared no competing interest.

SARS-CoV-2 and its variants continue to threaten public health. Nanobodies that block the attachment of the RBD to host cell angiotensin-converting enzyme 2 (ACE2) represent promising drug candidates. In this study, we reported the identification and structural biological characterization of a nanobody from a RBD-immunized alpaca. The nanobody, termed as 2S-1-19, shows outstanding neutralizing activity against both pseudotyped and authentic SARS-CoV-2 viruses. The crystal structure of 2S-1-19 bound to SARS-CoV-2 RBD reveals an epitope that overlaps with the binding site for ACE2. We also showed that 2S-1-19 reserves promising, though compromised, neutralizing activity against the Delta variant and that the trivalent form of 2S-1-19 remarkably increases its neutralizing capacity. Despite this, neither the monomeric or trimeric 2S-1-19 could neutralize the Omicron BA.1.1 variant, possibility due to the E484A and Q493K mutations found within this virus variant. These data provide insights into immune evasion caused by SARS-CoV-2 variants.

Bitter taste receptors (TAS2Rs), a subfamily of G-protein coupled receptors (GPCRs) expressed orally and extraorally, elicit signaling in response to a large set of tastants. Among 25 functional TAS2Rs encoded in the human genome, TAS2R14 is the most promiscuous, and responds to hundreds of chemically diverse ligands. Here we present the cryo-electron microscopy (cryo-EM) structure of the human TAS2R14 in complex with its signaling partner gustducin, and bound to flufenamic acid (FFA), a clinically approved nonsteroidal anti-inflammatory drug. The structure reveals an unusual binding mode, where two copies of FFA are bound at distinct pockets: one at the canonical receptor site within the trans-membrane bundle, and the other in the intracellular facet, bridging the receptor with gustducin. Together with a pocket-specific BRET-based ligand binding assay, these results illuminate bitter taste signaling and provide tools for a site-targeted compound design. Bitter taste receptors (TAS2Rs) are a subfamily of G-protein coupled receptors (GPCRs). Here, the authors report a cryo-EM structure of the human TAS2R14 in complex with its signaling partner gustducin, and bound to an anti-inflammatory drug flufenamic acid (FFA).

G protein-coupled receptors are among the most widely studied classes of drug targets. A major challenge in this field is to develop ligands that will selectively modulate a single receptor subtype to overcome the disadvantages of undesired "off target" effects caused by lack of target and thus signaling specificity. In the current study, we explored ligand design for the melanocortin 4 receptor (MC4R) since it is an attractive target for developing antiobesity drugs. Endogenously, the receptor is activated by peptide ligands, i.e., three melanocyte-stimulating hormones (α-MSH, β-MSH, and γ-MSH) and by adrenocorticotropic hormone. Therefore, we utilized a peptide drug design approach, utilizing "molecular grafting" of pharmacophore peptide sequence motifs onto a stable nature-derived peptide scaffold. Specifically, protegrin-4-like-peptide-1 (Pr4LP1) and arenicin-1-like-peptide-1 (Ar3LP1) fully activated MC4R in a functional cAMP assay with potencies of 3.7 and 1.0 nM, respectively. In a nanoluciferase complementation assay with less signal amplification, the designed peptides fully recruited mini-Gs with subnanomolar and nanomolar potencies. Interestingly, these novel peptide MC4R ligands recruited β-arrestin-2 with ∼2-fold greater efficacies and ∼20-fold increased potencies as compared to the endogenous α-MSH. The peptides were inactive at related MC1R and MC3R in a cAMP accumulation assay. These findings highlight the applicability of animal-derived disulfide-rich scaffolds to design pathway and subtype selective MC4R pharmacological probes. In the future, this approach could be exploited to develop functionally selective ligands that could offer safer and more effective obesity drugs.