Mechanisms of ligand recognition and activation of melanin-concentrating hormone receptors.

Melanin-concentrating hormone (MCH) is a cyclic neuropeptide that regulates food intake, energy balance, and other physiological functions by stimulating MCHR1 and MCHR2 receptors, both of which are class A G protein-coupled receptors. MCHR1 predominately couples to inhibitory G protein, Gi/o, and MCHR2 can only couple to Gq/11. Here we present cryo-electron microscopy structures of MCH-activated MCHR1 with Gi and MCH-activated MCHR2 with Gq at the global resolutions of 3.01 Å and 2.40 Å, respectively. These structures reveal that MCH adopts a consistent cysteine-mediated hairpin loop configuration when bound to both receptors. A central arginine from the LGRVY core motif between the two cysteines of MCH penetrates deeply into the transmembrane pocket, triggering receptor activation. Integrated with mutational and functional insights, our findings elucidate the molecular underpinnings of ligand recognition and MCH receptor activation and offer a structural foundation for targeted drug design.

Dimerization and antidepressant recognition at noradrenaline transporter.

The noradrenaline transporter has a pivotal role in regulating neurotransmitter balance and is crucial for normal physiology and neurobiology1. Dysfunction of noradrenaline transporter has been implicated in numerous neuropsychiatric diseases, including depression and attention deficit hyperactivity disorder2. Here we report cryo-electron microscopy structures of noradrenaline transporter in apo and substrate-bound forms, and as complexes with six antidepressants. The structures reveal a noradrenaline transporter dimer interface that is mediated predominantly by cholesterol and lipid molecules. The substrate noradrenaline binds deep in the central binding pocket, and its amine group interacts with a conserved aspartate residue. Our structures also provide insight into antidepressant recognition and monoamine transporter selectivity. Together, these findings advance our understanding of noradrenaline transporter regulation and inhibition, and provide templates for designing improved antidepressants to treat neuropsychiatric disorders.

Cryo-EM structures of adenosine receptor A3AR bound to selective agonists.

The adenosine A3 receptor (A3AR), a key member of the G protein-coupled receptor family, is a promising therapeutic target for inflammatory and cancerous conditions. The selective A3AR agonists, CF101 and CF102, are clinically significant, yet their recognition mechanisms remained elusive. Here we report the cryogenic electron microscopy structures of the full-length human A3AR bound to CF101 and CF102 with heterotrimeric Gi protein in complex at 3.3-3.2 Å resolution. These agonists reside in the orthosteric pocket, forming conserved interactions via their adenine moieties, while their 3-iodobenzyl groups exhibit distinct orientations. Functional assays reveal the critical role of extracellular loop 3 in A3AR's ligand selectivity and receptor activation. Key mutations, including His3.37, Ser5.42, and Ser6.52, in a unique sub-pocket of A3AR, significantly impact receptor activation. Comparative analysis with the inactive A2AAR structure highlights a conserved receptor activation mechanism. Our findings provide comprehensive insights into the molecular recognition and signaling of A3AR, paving the way for designing subtype-selective adenosine receptor ligands.

Flexible scaffold-based cheminformatics approach for polypharmacological drug design.

Effective treatments for complex central nervous system (CNS) disorders require drugs with polypharmacology and multifunctionality, yet designing such drugs remains a challenge. Here, we present a flexible scaffold-based cheminformatics approach (FSCA) for the rational design of polypharmacological drugs. FSCA involves fitting a flexible scaffold to different receptors using different binding poses, as exemplified by IHCH-7179, which adopted a "bending-down" binding pose at 5-HT2AR to act as an antagonist and a "stretching-up" binding pose at 5-HT1AR to function as an agonist. IHCH-7179 demonstrated promising results in alleviating cognitive deficits and psychoactive symptoms in mice by blocking 5-HT2AR for psychoactive symptoms and activating 5-HT1AR to alleviate cognitive deficits. By analyzing aminergic receptor structures, we identified two featured motifs, the "agonist filter" and "conformation shaper," which determine ligand binding pose and predict activity at aminergic receptors. With these motifs, FSCA can be applied to the design of polypharmacological ligands at other receptors.

Cryo-electron microscopy for GPCR research and drug discovery in endocrinology and metabolism.

G protein-coupled receptors (GPCRs) are the largest family of cell surface receptors, with many GPCRs having crucial roles in endocrinology and metabolism. Cryogenic electron microscopy (cryo-EM) has revolutionized the field of structural biology, particularly regarding GPCRs, over the past decade. Since the first pair of GPCR structures resolved by cryo-EM were published in 2017, the number of GPCR structures resolved by cryo-EM has surpassed the number resolved by X-ray crystallography by 30%, reaching >650, and the number has doubled every ~0.63 years for the past 6 years. At this pace, it is predicted that the structure of 90% of all human GPCRs will be completed within the next 5-7 years. This Review highlights the general structural features and principles that guide GPCR ligand recognition, receptor activation, G protein coupling, arrestin recruitment and regulation by GPCR kinases. The Review also highlights the diversity of GPCR allosteric binding sites and how allosteric ligands could dictate biased signalling that is selective for a G protein pathway or an arrestin pathway. Finally, the authors use the examples of glycoprotein hormone receptors and glucagon-like peptide 1 receptor to illustrate the effect of cryo-EM on understanding GPCR biology in endocrinology and metabolism, as well as on GPCR-related endocrine diseases and drug discovery.

AlphaFold: Research accelerator and hypothesis generator.

To celebrate the 50th anniversary of Cell Press and the Cell focus issue on structural biology, we discussed with scientists working across diverse fields how AlphaFold has changed their research and brought structural biology to the masses.

Molecular recognition and activation of the prostacyclin receptor by anti-pulmonary arterial hypertension drugs.

The prostacyclin (PGI2) receptor (IP) is a Gs-coupled receptor associated with blood pressure regulation, allergy, and inflammatory response. It is a main therapeutic target for pulmonary arterial hypertension (PAH) and several other diseases. Here we report cryo-electron microscopy (cryo-EM) structures of the human IP-Gs complex bound with two anti-PAH drugs, treprostinil and MRE-269 (active form of selexipag), at global resolutions of 2.56 and 2.41 angstrom, respectively. These structures revealed distinct features governing IP ligand binding, receptor activation, and G protein coupling. Moreover, comparison of the activated IP structures uncovered the mechanism and key residues that determine the superior selectivity of MRE-269 over treprostinil. Combined with molecular docking and functional studies, our structures provide insight into agonist selectivity, ligand recognition, receptor activation, and G protein coupling. Our results provide a structural template for further improving IP-targeting drugs to reduce off-target activation of prostanoid receptors and adverse effects.

Autoimmune diseases: targets, biology, and drug discovery.

Autoimmune diseases (AIDs) arise from a breakdown in immunological self-tolerance, wherein the adaptive immune system mistakenly attacks healthy cells, tissues and organs. AIDs impose excessive treatment costs and currently rely on non-specific and universal immunosuppression, which only offer symptomatic relief without addressing the underlying causes. AIDs are driven by autoantigens, targeting the autoantigens holds great promise in transforming the treatment of these diseases. To achieve this goal, a comprehensive understanding of the pathogenic mechanisms underlying different AIDs and the identification of specific autoantigens are critical. In this review, we categorize AIDs based on their underlying causes and compile information on autoantigens implicated in each disease, providing a roadmap for the development of novel immunotherapy regimens. We will focus on type 1 diabetes (T1D), which is an autoimmune disease characterized by irreversible destruction of insulin-producing ß cells in the Langerhans islets of the pancreas. We will discuss insulin as possible autoantigen of T1D and its role in T1D pathogenesis. Finally, we will review current treatments of TID and propose a potentially effective immunotherapy targeting autoantigens.

Repurposing of Rutan showed effective treatment for COVID-19 disease.

Previously, from the tannic sumac plant (Rhus coriaria), we developed the Rutan 25 mg oral drug tablets with antiviral activity against influenza A and B viruses, adenoviruses, paramyxoviruses, herpes virus, and cytomegalovirus. Here, our re-purposing study demonstrated that Rutan at 25, 50, and 100 mg/kg provided a very effective and safe treatment for COVID-19 infection, simultaneously inhibiting two vital enzyme systems of the SARS-CoV-2 virus: 3C-like proteinase (3CLpro) and RNA-dependent RNA polymerase (RdRp). There was no drug accumulation in experimental animals' organs and tissues. A clinical study demonstrated a statistically significant decrease in the C-reactive protein and a reduction of the viremia period. In patients receiving Rutan 25 mg (children) and 100 mg (adults), the frequency of post-COVID-19 manifestations was significantly less than in the control groups not treated with Rutan tablets. Rutan, having antiviral activity, can provide safe treatment and prevention of COVID-19 in adults and children. Clinical Trial Registration: ClinicalTrials.gov, ID NCT05862883.

Molecular recognition of niacin and lipid-lowering drugs by the human hydroxycarboxylic acid receptor 2.

Niacin, an age-old lipid-lowering drug, acts through the hydroxycarboxylic acid receptor 2 (HCAR2), a G-protein-coupled receptor (GPCR). Yet, its use is hindered by side effects like skin flushing. To address this, specific HCAR2 agonists, like MK-6892 and GSK256073, with fewer adverse effects have been created. However, the activation mechanism of HCAR2 by niacin and these new agonists is not well understood. Here, we present three cryoelectron microscopy structures of Gi-coupled HCAR2 bound to niacin, MK-6892, and GSK256073. Our findings show that different ligands induce varying binding pockets in HCAR2, influenced by aromatic amino acid clusters (W91ECL1, H1614.59, W1885.38, H1895.39, and F1935.43) from receptors ECL1, TM4, and TM5. Additionally, conserved residues R1113.36 and Y2847.43, unique to the HCA receptor family, likely initiate activation signal propagation in HCAR2. This study provides insights into ligand recognition, receptor activation, and G protein coupling mediated by HCAR2, laying the groundwork for developing HCAR2-targeted drugs.

Recognition of methamphetamine and other amines by trace amine receptor TAAR1.

Trace amine-associated receptor 1 (TAAR1), the founding member of a nine-member family of trace amine receptors, is responsible for recognizing a range of biogenic amines in the brain, including the endogenous ß-phenylethylamine (ß-PEA)1 as well as methamphetamine2, an abused substance that has posed a severe threat to human health and society3. Given its unique physiological role in the brain, TAAR1 is also an emerging target for a range of neurological disorders including schizophrenia, depression and drug addiction2,4,5. Here we report structures of human TAAR1-G-protein complexes bound to methamphetamine and ß-PEA as well as complexes bound to RO5256390, a TAAR1-selective agonist, and SEP-363856, a clinical-stage dual agonist for TAAR1 and serotonin receptor 5-HT1AR (refs. 6,7). Together with systematic mutagenesis and functional studies, the structures reveal the molecular basis of methamphetamine recognition and underlying mechanisms of ligand selectivity and polypharmacology between TAAR1 and other monoamine receptors. We identify a lid-like extracellular loop 2 helix/loop structure and a hydrogen-bonding network in the ligand-binding pockets, which may contribute to the ligand recognition in TAAR1. These findings shed light on the ligand recognition mode and activation mechanism for TAAR1 and should guide the development of next-generation therapeutics for drug addiction and various neurological disorders.

GPCR activation and GRK2 assembly by a biased intracellular agonist.

Phosphorylation of G-protein-coupled receptors (GPCRs) by GPCR kinases (GRKs) desensitizes G-protein signalling and promotes arrestin signalling, which is also modulated by biased ligands1-6. The molecular assembly of GRKs on GPCRs and the basis of GRK-mediated biased signalling remain largely unknown owing to the weak GPCR-GRK interactions. Here we report the complex structure of neurotensin receptor 1 (NTSR1) bound to GRK2, Gαq and the arrestin-biased ligand SBI-5537. The density map reveals the arrangement of the intact GRK2 with the receptor, with the N-terminal helix of GRK2 docking into the open cytoplasmic pocket formed by the outward movement of the receptor transmembrane helix 6, analogous to the binding of the G protein to the receptor. SBI-553 binds at the interface between GRK2 and NTSR1 to enhance GRK2 binding. The binding mode of SBI-553 is compatible with arrestin binding but clashes with the binding of Gαq protein, thus providing a mechanism for its arrestin-biased signalling capability. In sum, our structure provides a rational model for understanding the details of GPCR-GRK interactions and GRK2-mediated biased signalling.

Conserved class B GPCR activation by a biased intracellular agonist.

Class B G-protein-coupled receptors (GPCRs), including glucagon-like peptide 1 receptor (GLP1R) and parathyroid hormone 1 receptor (PTH1R), are important drug targets1-5. Injectable peptide drugs targeting these receptors have been developed, but orally available small-molecule drugs remain under development6,7. Here we report the high-resolution structure of human PTH1R in complex with the stimulatory G protein (Gs) and a small-molecule agonist, PCO371, which reveals an unexpected binding mode of PCO371 at the cytoplasmic interface of PTH1R with Gs. The PCO371-binding site is totally different from all binding sites previously reported for small molecules or peptide ligands in GPCRs. The residues that make up the PCO371-binding pocket are conserved in class B GPCRs, and a single alteration in PTH2R and two residue alterations in GLP1R convert these receptors to respond to PCO371. Functional assays reveal that PCO371 is a G-protein-biased agonist that is defective in promoting PTH1R-mediated arrestin signalling. Together, these results uncover a distinct binding site for designing small-molecule agonists for PTH1R and possibly other members of the class B GPCRs and define a receptor conformation that is specific only for G-protein activation but not arrestin signalling. These insights should facilitate the design of distinct types of class B GPCR small-molecule agonist for various therapeutic indications.

ARID3a from the ARID family: structure, role in autoimmune diseases and drug discovery.

The AT-rich interaction domain (ARID) family of DNA-binding proteins is a group of transcription factors and chromatin regulators with a highly conserved ARID domain that recognizes specific AT-rich DNA sequences. Dysfunction of ARID family members has been implicated in various human diseases including cancers and intellectual disability. Among them, ARID3a has gained increasing attention due to its potential involvement in autoimmunity. In this article we provide an overview of the ARID family, focusing on the structure and biological functions of ARID3a. It explores the role of ARID3a in autoreactive B cells and its contribution to autoimmune diseases such as systemic lupus erythematosus and primary biliary cholangitis. Furthermore, we also discuss the potential for drug discovery targeting ARID3a and present a plan for future research in this field.

Structural insights into ligand recognition and activation of the medium-chain fatty acid-sensing receptor GPR84.

GPR84 is an orphan class A G protein-coupled receptor (GPCR) that is predominantly expressed in immune cells and plays important roles in inflammation, fibrosis, and metabolism. Here, we present cryo-electron microscopy (cryo-EM) structures of Gαi protein-coupled human GPR84 bound to a synthetic lipid-mimetic ligand, LY237, or a putative endogenous ligand, a medium-chain fatty acid (MCFA) 3-hydroxy lauric acid (3-OH-C12). Analysis of these two ligand-bound structures reveals a unique hydrophobic nonane tail -contacting patch, which forms a blocking wall to select MCFA-like agonists with the correct length. We also identify the structural features in GPR84 that coordinate the polar ends of LY237 and 3-OH-C12, including the interactions with the positively charged side chain of R172 and the downward movement of the extracellular loop 2 (ECL2). Together with molecular dynamics simulations and functional data, our structures reveal that ECL2 not only contributes to direct ligand binding, but also plays a pivotal role in ligand entry from the extracellular milieu. These insights into the structure and function of GPR84 could improve our understanding of ligand recognition, receptor activation, and Gαi-coupling of GPR84. Our structures could also facilitate rational drug discovery against inflammation and metabolic disorders targeting GPR84.

Structural insights into neurokinin 3 receptor activation by endogenous and analogue peptide agonists.

Neurokinin 3 receptor (NK3R) is a tachykinin receptor essential for the hypothalamic-pituitary-gonadal axis. The endogenous peptide agonist neurokinin B (NKB) preferentially activates NK3R, while substance P (SP) binds preferentially to NK1R. In addition, the SP analogue senktide more potently activates NK3R than NKB and SP. However, the mechanisms of preferential binding of peptide and NK3R activation remain elusive. Herein, we determined the cryogenic electron microscopy (cryo-EM) structures of the NK3R-Gq complex bound to NKB, SP and senktide. The three NK3R-Gq/peptide complexes utilize a class of noncanonical receptor activation mechanisms. Combining the structural analysis and functional assay illustrated that the consensus C-termini of the three peptide agonists share a conserved binding mode to NK3R, while the divergent N-termini of the peptides confer the preferential binding of the agonist to NK3R. In addition, the specific interactions between the N-terminus of senktide and the N-terminus and extracellular loops (ECL2 and ECL3) of NK3R lead to the improved activation displayed by senktide compared to SP and NKB. These findings pave the way to understand tachykinin receptor subtype selectivity and provide ideas to rationally develop drugs targeting NK3R.

Conformational Dynamics of the D53-D3-D14 Complex in Strigolactone Signaling.

Strigolactones (SLs) play fundamental roles in regulating plant architecture, which is a major factor determining crop yield. The perception and signal transduction of SLs require the formation of a complex containing the receptor DWARF14 (D14), an F-box protein D3 and a transcriptional regulator D53 in an SL-dependent manner. Structural and biochemical analyses of D14 and its orthologs DAD2 and AtD14, D3 and the complexes of ASK1-D3-AtD14 and D3CTH-D14 have made great contributions to understanding the mechanisms of SL perception. However, structural analyses of D53 and the D53-D3-D14 holo-complex are challenging, and the biochemical mechanism underlying the complex assembly remains poorly understood. Here, we found that apo-D53 was rather flexible and reconstituted the holo-complex containing D53, S-phase kinase-associated protein 1 (SKP1), D3 and D14 with rac-GR24. The cryo-electron microscopy (cryo-EM) structure of SKP1-D3-D14 in the presence of D53 was analyzed and superimposed on the crystal structure of ASK1-D3-AtD14 without D53. No large conformational rearrangement was observed, but a 9Å rotation appeared between D14 and AtD14. Using hydrogen-deuterium exchange monitored by mass spectrometry, we analyzed dynamic motifs of D14, D3 and D53 in the D53-SKP1-D3-D14 complex assembly process and further identified two potential interfaces in D53 that are located in the N and D2 domains, respectively. Together, our results uncovered the dynamic conformational changes and built a model of the holo-complex D53-SKP1-D3-D14, offering valuable information for the biochemical and genetic mechanisms of SL perception and signal transduction.

Revealing the signaling of complement receptors C3aR and C5aR1 by anaphylatoxins.

The complement receptors C3aR and C5aR1, whose signaling is selectively activated by anaphylatoxins C3a and C5a, are important regulators of both innate and adaptive immune responses. Dysregulations of C3aR and C5aR1 signaling lead to multiple inflammatory disorders, including sepsis, asthma and acute respiratory distress syndrome. The mechanism underlying endogenous anaphylatoxin recognition and activation of C3aR and C5aR1 remains elusive. Here we reported the structures of C3a-bound C3aR and C5a-bound C5aR1 as well as an apo-C3aR structure. These structures, combined with mutagenesis analysis, reveal a conserved recognition pattern of anaphylatoxins to the complement receptors that is different from chemokine receptors, unique pocket topologies of C3aR and C5aR1 that mediate ligand selectivity, and a common mechanism of receptor activation. These results provide crucial insights into the molecular understanding of C3aR and C5aR1 signaling and structural templates for rational drug design for treating inflammation disorders.