Molecular mechanism of agonism and inverse agonism in ghrelin receptor

In vitro pharmacological characterization of growth hormone secretagogue receptor ligands using the dynamic mass redistribution and calcium mobilization assays

Ghrelin modulates several biological functions via selective activation of the growth hormone secretagogue receptor (GHSR). GHSR agonists may be useful for the treatment of anorexia and cachexia, while antagonists and inverse agonists may represent new drugs for the treatment of metabolic and substance use disorders. Thus, the identification and pharmacodynamic characterization of new GHSR ligands is of high interest. In the present work the label-free dynamic mass redistribution (DMR) assay has been used to evaluate the pharmacological activity of a panel of GHSR ligands. This includes the endogenous peptides ghrelin, desacyl-ghrelin and LEAP2(1-14). Among synthetic compounds, the agonists anamorelin and HM01, the antagonists HM04 and YIL-781, and the inverse agonist PF-05190457 have been tested, together with HM03, R011, and H1498 from patent literature. The DMR results have been compared to those obtained in parallel experiments with the calcium mobilization assay. Ghrelin, anamorelin, HM01, and HM03 behaved as potent full GHSR agonists. YIL-781 behaved as a partial GHSR agonist and R011 as antagonist in both the assays. LEAP2(1-14) resulted a GHSR inverse agonist in DMR but not in calcium mobilization assay. PF-05190457, HM04, and H1498 behaved as GHSR inverse agonists in DMR experiments, while they acted as antagonists in calcium mobilization studies. In conclusion, this study provided a systematic pharmacodynamic characterization of several GHSR ligands in two different pharmacological assays. It demonstrated that the DMR assay can be successfully used particularly to discriminate between antagonists and inverse agonists. This study may be useful for the selection of the most appropriate compounds to be used in future studies.

Effects of an external static EF on the conformational transition of 5-HT1A receptor: A molecular dynamics simulation study

The serotonin receptor subtype 1A (5-HT1AR), one of the G-protein-coupled receptor (GPCR) family, has been implicated in several neurological conditions. Understanding the activation and inactivation mechanism of 5-HT1AR at the molecular level is critical for discovering novel therapeutics in many diseases. Recently there has been a growing appreciation for the role of external electric fields (EFs) in influencing the structure and activity of biomolecules. In this study, we used molecular dynamics (MD) simulations to examine conformational features of active states of 5-HT1AR and investigate the effect of an external static EF with 0.02 V/nm applied on the active state of 5-HT1AR. Our results showed that the active state of 5-HT1AR maintained the native structure, while the EF led to structural modifications in 5-HT1AR, particularly inducing the inward movement of transmembrane helix 6 (TM6). Furthermore, it disturbed the conformational switches associated with activation in the CWxP, DRY, PIF, and NPxxY motifs, consequently predisposing an inclination towards the inactive-like conformation. We also found that the EF led to an overall increase in the dipole moment of 5-HT1AR, encompassing TM6 and pivotal amino acids. The analyses of conformational properties of TM6 showed that the changed secondary structure and decreased solvent exposure occurred upon the EF condition. The interaction of 5-HT1AR with the membrane lipid bilayer was also altered under the EF. Our findings reveal the molecular mechanism underlying the transition of 5-HT1AR conformation induced by external EFs, which offer potential novel insights into the prospect of employing structure-based EF applications for GPCRs.

Revealing the graded activation mechanism of neurotensin receptor 1

Graded activation contributes to the precise regulation of GPCR activity, presenting new opportunities for drug design. In this work, a total of 10 μs enhanced-sampling simulations are performed to provide molecular insights into the binding dynamics differences of the neurotensin receptor 1 (NTSR1) to the full agonist SRI-9829, partial agonist RTI-3a and inverse agonist SR48692. The possible graded activation mechanism of NTSR1 is revealed by an integrated analysis utilizing the reweighted potential of mean force (PMF), deep learning (DL) and transfer entropy (TE). Specifically, the orthosteric pocket is observed to undergo expansion and contraction, with the G-protein-binding site experiencing interconversions among the inactive, intermediate and active-like states. Detailed structural comparisons capture subtle conformational differences arising from ligand binding in allosteric signaling, which can well explain the graded activation. Critical microswitches that contribute to graded activation are efficiently identified with the DL model. TE calculations enable the visualization of allosteric communication networks within the receptor, elucidating the driver-responder relationships associated with signal transduction. Fortunately, the dissociation of the full agonist from the orthosteric pocket is observed. The current findings systematically reveal the mechanism of NTSR1 graded activation, and also provide implications for structure-based drug design.

Structural basis of tethered agonism and G protein coupling of protease-activated receptors

Protease-activated receptors (PARs) are a unique group within the G protein-coupled receptor superfamily, orchestrating cellular responses to extracellular proteases via enzymatic cleavage, which triggers intracellular signaling pathways. Protease-activated receptor 1 (PAR1) is a key member of this family and is recognized as a critical pharmacological target for managing thrombotic disorders. In this study, we present cryo-electron microscopy structures of PAR1 in its activated state, induced by its natural tethered agonist (TA), in complex with two distinct downstream proteins, the Gq and Gi heterotrimers, respectively. The TA peptide is positioned within a surface pocket, prompting PAR1 activation through notable conformational shifts. Contrary to the typical receptor activation that involves the outward movement of transmembrane helix 6 (TM6), PAR1 activation is characterized by the simultaneous downward shift of TM6 and TM7, coupled with the rotation of a group of aromatic residues. This results in the displacement of an intracellular anion, creating space for downstream G protein binding. Our findings delineate the TA recognition pattern and highlight a distinct role of the second extracellular loop in forming β-sheets with TA within the PAR family, a feature not observed in other TA-activated receptors. Moreover, the nuanced differences in the interactions between intracellular loops 2/3 and the Gα subunit of different G proteins are crucial for determining the specificity of G protein coupling. These insights contribute to our understanding of the ligand binding and activation mechanisms of PARs, illuminating the basis for PAR1's versatility in G protein coupling.

Enhancement of transcription efficiency by TAR-Tat system increases the functional expression of human olfactory receptors

Humans have approximately 400 different olfactory receptors (hORs) and recognize odorants through the repertoire of hOR responses. Although the cell surface expression of hORs is critical to evaluate their response, hORs are poorly expressed on the surface of heterologous cells. To address this problem, previous studies have focused on hOR transportation to the membrane. Nevertheless, the response pattern of hORs to odorants has yet to be successfully linked, and the response sensitivity still remains to be improved. In this study, we demonstrate that increasing the transcriptional level can result in a significant increase in cell surface and functional expression of hORs. We used the TAR-Tat system, which increases the transcription efficiency through positive feedback, and found that OR1A1, OR6N2, and OR51M1 exhibited robust expression. Moreover, this system induces enhanced hOR responses to odorants, thus defining four hORs as novel n-hexanal receptors and n-hexanal is an inverse agonist to one of them. Our results suggested that using the TAR-Tat system and increasing the transcriptional level of hORs can help understanding the relationship between hORs and odorants that were previously undetectable. This finding could facilitate the understanding of the sense of smell by decoding the repertoire of hOR responses.

Backbone Modification Provides a Long-Acting Inverse Agonist of Pathogenic, Constitutively Active PTH1R Variants

Parathyroid hormone 1 receptor (PTH1R) plays a key role in mediating calcium homeostasis and bone development, and aberrant PTH1R activity underlies several human diseases. Peptidic PTH1R antagonists and inverse agonists have therapeutic potential in treating these diseases, but their poor pharmacokinetics and pharmacodynamics undermine their in vivo efficacy. Herein, we report the use of a backbone-modification strategy to design a peptidic PTH1R inhibitor that displays prolonged activity as an antagonist of wild-type PTH1R and an inverse agonist of the constitutively active PTH1R-H223R mutant both in vitro and in vivo. This peptide may be of interest for the future development of therapeutic agents that ameliorate PTH1R malfunction.

Allosteric crosstalk in modular proteins: Function fine-tuning and drug design

Modular proteins are regulatory proteins that carry out more than one function. These proteins upregulate or downregulate a biochemical cascade to establish homeostasis in cells. To switch the function or alter the efficiency (based on cellular needs), these proteins require different facilitators that bind to a site different from the catalytic (active/orthosteric) site, aka 'allosteric site', and fine-tune their function. These facilitators (or effectors) are allosteric modulators. In this Review, we have discussed the allostery, characterized them based on their mechanisms, and discussed how allostery plays an important role in the activity modulation and function fine-tuning of proteins. Recently there is an emergence in the discovery of allosteric drugs. We have also emphasized the role, significance, and future of allostery in therapeutic applications.

Dynamic Insights into the Self-Activation Pathway and Allosteric Regulation of the Orphan G-Protein-Coupled Receptor GPR52

Within over 800 members of G-protein-coupled receptors, there are numerous orphan receptors whose endogenous ligands are largely unknown, providing many opportunities for novel drug discovery. However, the lack of an in-depth understanding of the intrinsic working mechanism for orphan receptors severely limits the related rational drug design. The G-protein-coupled receptor 52 (GPR52) is a unique orphan receptor that constitutively increases cellular 5'-cyclic adenosine monophosphate (cAMP) levels without binding any exogenous agonists and has been identified as a promising therapeutic target for central nervous system disorders. Although recent structural biology studies have provided snapshots of both active and inactive states of GPR52, the mechanism of the conformational transition between these states remains unclear. Here, an acceptable self-activation pathway for GPR52 was proposed through 6 μs Gaussian accelerated molecular dynamics (GaMD) simulations, in which the receptor spontaneously transitions from the active state to that matching the inactive crystal structure. According to the three intermediate states of the receptor obtained by constructing a reweighted potential of mean force, how the allosteric regulation occurs between the extracellular orthosteric binding pocket and the intracellular G-protein-binding site is revealed. Combined with the independent gradient model, several important microswitch residues and the allosteric communication pathway that directly links the two regions are both identified. Transfer entropy calculations not only reveal the complex allosteric signaling within GPR52 but also confirm the unique role of ECL2 in allosteric regulation, which is mutually validated with the results of GaMD simulations. Overall, this work elucidates the allosteric mechanism of GPR52 at the atomic level, providing the most detailed information to date on the self-activation of the orphan receptor.

Analysis of the Dynamics of the Human Growth Hormone Secretagogue Receptor Reveals Insights into the Energy Landscape of the Molecule

G protein-coupled receptors initiate signal transduction in response to ligand binding. Growth hormone secretagogue receptor (GHSR), the focus of this study, binds the 28 residue peptide ghrelin. While structures of GHSR in different states of activation are available, dynamics within each state have not been investigated in depth. We analyze long molecular dynamics simulation trajectories using "detectors" to compare dynamics of the apo and ghrelin-bound states yielding timescale-specific amplitudes of motion. We identify differences in dynamics between apo and ghrelin-bound GHSR in the extracellular loop 2 and transmembrane helices 5-7. NMR of the GHSR histidine residues reveals chemical shift differences in these regions. We evaluate timescale specific correlation of motions between residues of ghrelin and GHSR, where binding yields a high degree of correlation for the first 8 ghrelin residues, but less correlation for the helical end. Finally, we investigate the traverse of GHSR over a rugged energy landscape via principal component analysis.

Desacyl-ghrelin, not just an inactive form of ghrelin?-A review of current knowledge on the biological actions of desacyl-ghrelin

Desacyl-ghrelin is a form of ghrelin which lacks acyl-modification of the third serine residue of ghrelin. Originally, desacyl-ghrelin was considered to be just an inactive form of ghrelin. More recently, however, it has been suggested to have various biological activities, including control of food intake, growth hormone, glucose metabolism, and gastric movement, and is involved in cell survival. In this review, we summarize the current knowledge of the biological actions of desacyl-ghrelin and the proposed mechanisms by which it exerts the effects.

New insights into GPCR coupling and dimerisation from cryo-EM structures

Over the past three years (2020-2022) more structures of GPCRs have been determined than in the previous twenty years (2000-2019), primarily of GPCR complexes that are large enough for structure determination by single-particle cryo-EM. This review will present some structural highlights that have advanced our molecular understanding of promiscuous G protein coupling, how a G protein receptor kinase and β-arrestins couple to GPCRs, and GPCR dimerisation. We will also discuss advances in the use of gene fusions, nanobodies, and Fab fragments to facilitate the structure determination of GPCRs in the inactive state that, on their own, are too small for structure determination by single-particle cryo-EM.

Ghrelin receptor signaling in health and disease: a biased view

As allosteric complexes, G-protein-coupled receptors (GPCRs) respond to extracellular stimuli and pleiotropically couple to intracellular transducers to elicit signaling pathway-dependent effects in a process known as biased signaling or functional selectivity. One such GPCR, the ghrelin receptor (GHSR1a), has a crucial role in restoring and maintaining metabolic homeostasis during disrupted energy balance. Thus, pharmacological modulation of GHSR1a bias could offer a promising strategy to treat several metabolism-based disorders. Here, we summarize current evidence supporting GHSR1a functional selectivity in vivo and highlight recent structural data. We propose that precise determinations of GHSR1a molecular pharmacology and pathway-specific physiological effects will enable discovery of GHSR1a drugs with tailored signaling profiles, thereby providing safer and more effective treatments for metabolic diseases.

Structural Understanding of Peptide-Bound G Protein-Coupled Receptors: Peptide-Target Interactions

The activation of G protein-coupled receptors (GPCRs) is triggered by ligand binding to their orthosteric sites, which induces ligand-specific conformational changes. Agonists and antagonists bound to GPCR orthosteric sites provide detailed information on ligand-binding modes. Among these, peptide ligands play an instrumental role in GPCR pharmacology and have attracted increased attention as therapeutic drugs. The recent breakthrough in GPCR structural biology has resulted in the remarkable availability of peptide-bound GPCR complexes. Despite the several structural similarities shared by these receptors, they exhibit distinct features in terms of peptide recognition and receptor activation. From this perspective, we have summarized the current status of peptide-bound GPCR structural complexes, largely focusing on the interactions between the receptor and its peptide ligand at the orthosteric site. In-depth structural investigations have yielded valuable insights into the molecular mechanisms underlying peptide recognition. This study would contribute to the discovery of GPCR peptide drugs with improved therapeutic effects.

Thyrotropin receptor antagonists and inverse agonists, and their potential application to thyroid diseases

The thyrotropin receptor (TSHR) plays critical roles in thyroid growth and function and in the pathogenesis of several thyroid diseases including Graves' hyperthyroidism and ophthalmopathy, non-autoimmune hyperthyroidism and thyroid cancer. Several low-molecular weight compounds (LMWCs) and anti-TSHR monoclonal antibodies (mAbs) with receptor antagonistic and inverse agonistic activities have been reported. The former binds to the pocket formed by the receptor transmembrane bundle, and the latter to the extracellular TSH binding site. Both are effective inhibitors of TSH/thyroid stimulating antibody-stimulated cAMP and/or hyaluronic acid production in TSHR-expressing cells. Anti-insulin-like growth factor 1 inhibitors are also found to inhibit TSHR signaling. Each agent has advantages and disadvantages; for example, mAbs have a higher affinity and longer half-life but are more costly than LMWCs. At present, mAbs appear most promising, yet the development of more efficacious LMWCs is desirable. These agents are anticipated to be efficacious not only for the above-mentioned diseases but also for resistance to thyroid hormone and have utility for thyroid cancer radionuclide scintigraphy/therapy as a new theranostic.

Structural basis for recognition of antihistamine drug by human histamine receptor

The histamine receptors belong to the G protein-coupled receptor (GPCR) superfamily, and play important roles in the regulation of histamine and other neurotransmitters in the central nervous system, as potential targets for the treatment of neurologic and psychiatric disorders. Here we report the crystal structure of human histamine receptor H₃R bound to an antagonist PF-03654746 at 2.6 Å resolution. Combined with the computational and functional assays, our structure reveals binding modes of the antagonist and allosteric cholesterol. Molecular dynamic simulations and molecular docking of different antihistamines further elucidate the conserved ligand-binding modes. These findings are therefore expected to facilitate the structure-based design of novel antihistamines.

Binding domain characterization of growth hormone secretagogue receptor

Background and Objectives

Activation of ghrelin receptor growth hormone secretagogue receptor (GHS-R) by endogenous or synthetic ligands amplifies pulsatile release of growth hormone (GH) and enhances food intake, very relevant to development and growth. GHS-R is a G-protein coupled receptor that has great druggable potential. Understanding the precise ligand and receptor interactions is crucial to advance the application of GHS-R.

Materials and Methods

We used radiolabeled ligand-binding assay and growth hormone release assay to assess the binding and functional characteristics of GHS-R to synthetic agonists MK-0677 and GHS-25, as well as to endogenous peptide ligand ghrelin. We analyzed the ligand-dependent activity of GHS-R by measuring aequorin-based [Ca⁺⁺]_i responses. To define a ligand-binding pocket of GHS-R, we generated a series of human/puffer fish GHS-R chimeras by domain swapping, as well as a series of mutants by site-directed mutagenesis.

Results

We found that the synthetic ligands have high binding affinity to GHS-R in the in vitro competitive binding assay. Remarkably, the in vivo GH secretagogue activity is higher with the synthetic agonists MK-0677 and GHS-25 than that of ghrelin. Importantly, the activity was completely abolished in GHS-R knockout mice. In GHS-R chimera analysis, we identified the C-terminal region, particularly the transmembrane domain 6 (TM6), to be critical for the ligand-dependent activity. Our site-directed mutagenesis study further revealed that amino acid residues D99 and W276 in GHS-R are essential for ligand binding. Interestingly, critical residues distinctively interact with different ligands, MK-0677 activation depends on E124, while ghrelin and GHS-25 preferentially interact with F279.

Conclusion

The ligand-binding pocket of human GHS-R is mainly defined by interactive residues in TM6 and the adjacent region of the receptor. This novel finding in GHS-R binding domains advances the structural/ functional understanding of GHS-R, which will help to select/design better GHS-R agonists/ antagonists for future therapeutic applications.

Systemic infusion of exogenous ghrelin in male broiler chickens (Gallus gallus domesticus). The effect of pulse frequency, doses, and ghrelin forms on feed intake, average daily gain, corticosterone, and growth hormone concentrations

There is limited information on the effect of exogenous ghrelin infusion on feed intake (FI) in chickens. Therefore, male broilers were used in 3 factorial experiments to determine the relationships between doses (0, 1, or 4 nM; Dose), frequency (once every two h; 2 h), once every 4th h (4 h) or continuous infusion, and ghrelin forms including acylated-ghrelin (AG) and desacylated-ghrelin (DAG) on FI, ADG, and concentrations of corticosterone and Growth Hormone (GH). Treatments were delivered via a jugular cannula, using programmable pumps for 11 consecutive days. FI and ADG were recorded, and plasma was collected. Data were analyzed using a factorial design. In Experiment 1 the effect of AG pulse frequency and doses were evaluated. There was a linear decrease in FI (P = 0.002) and a linear increase in corticosterone (P = 0.033) and GH (P = 0.011) concentrations when AG was infused. However, ADG decreased with doses (P = 0.011) only when AG was given at 2 h. In Experiment 2 the effect of ghrelin forms and doses given at 2 h was evaluated. There was a linear decrease in FI when AG was infused and a linear increase in FI when DAG was infused (P < 0.05). Birds infused with DAG gained more weight than those infused with AG. There was a linear increase in corticosterone and GH concentrations only when AG was infused (P < 0.01). In Experiment 3 the effect of continuous infusion of 2 doses (0 and 1 nM) of AG and DAG were evaluated. There was a linear decrease in FI and ADG when AG (P < 0.001) was infused and a linear increase in FI and ADG when DAG was infused (P < 0.05). There was an increase in corticosterone concentrations only when AG was infused (P = 0.022). However, GH concentrations were not affected by treatments. We concluded that AG and DAG pulse frequency and doses had a differential effect on FI, ADG, corticosterone, and GH concentrations in broiler chickens.

Advances in the Development of Nonpeptide Small Molecules Targeting Ghrelin Receptor

Ghrelin is an octanoylated peptide acting by the activation of the growth hormone secretagogue receptor, namely, GHS-R1a. The involvement of ghrelin in several physiological processes, including stimulation of food intake, gastric emptying, body energy balance, glucose homeostasis, reduction of insulin secretion, and lipogenesis validates the considerable interest in GHS-R1a as a promising target for the treatment of numerous disorders. Over the years, several GHS-R1a ligands have been identified and some of them have been extensively studied in clinical trials. The recently resolved structures of GHS-R1a bound to ghrelin or potent ligands have provided useful information for the design of new GHS-R1a drugs. This perspective is focused on the development of recent nonpeptide small molecules acting as GHS-R1a agonists, antagonists, and inverse agonists, bearing classical or new molecular scaffolds, as well as on radiolabeled GHS-R1a ligands developed for imaging. Moreover, the pharmacological effects of the most studied ligands have been discussed.