Molecular insights into peptide agonist engagement with the PTH receptor

Prolonging parathyroid hormone analog action in vitro and in vivo through peptide lipidation

Parathyroid hormone (PTH) analogs with improved actions in vivo could lead to optimized treatments for bone and mineral ion diseases. Rapid clearance from the circulation and short dwell times on the PTH receptor limit the efficacies of conventional PTH peptides currently in medical use. Here, we seek to enhance PTH peptide efficacy using two distinct peptide lipidation strategies. First, we append a lipid chain to the peptide's C-terminus in a fashion to promote binding to serum albumin and hence prolong the peptide's circulation half-life in vivo. Second, we append a lipid chain to a lysine side chain in a fashion designed to anchor the peptide to the cell membrane as the ligand is bound to the receptor and hence increase its dwell time on the receptor. We find that both strategies of lipidation can profoundly enhance the efficacy of PTH peptides in vitro and in mice. Our results could lead to the development of modified PTH analogs with optimized therapeutic utility.

Allosteric communication mechanism in the glucagon receptor

Recent drug development suggests agonists may be more promising against glucagon receptor dysregulation in metabolic disorders. Allosteric modulation may pave an alternative way to initiate responses that are required to target these metabolic disorders. Here, we investigated the allosteric communication mechanisms within the glucagon receptor using molecular dynamics simulations on five receptor states. Results highlighted that the extracellular domain is dynamic in the absence of an orthosteric agonist. In the presence of a partial agonist, we observed increased flexibility in the N terminus of the receptor compared with the full agonist-bound receptor. Class B1 G protein-coupled receptor (GPCR) microswitches showed repacking going from the inactive state to the active state, allowing for G protein coupling. In the full agonist- and G protein-bound state, Gαs showed both translational and rotational movement in the N terminus, core, and α5-helix, thereby forming key interactions between the core of the G protein and the receptor. Finally, the allosteric communication from the extracellular region to the G protein coupling region of the receptor was the strongest in the intracellular negative allosteric modulator-bound state, the full agonist and G protein-bound state, and the full agonist-bound G protein-free state. The residue positions predicted to play a significant role in the allosteric communication mechanism showed overlap with disease-associated mutations. Overall, our study provides insights into the allosteric communication mechanism in a class B1 GPCR, which sets the foundation for future design of allosteric modulators targeting the glucagon receptor.

Prolonged signaling of backbone-modified glucagon-like peptide- analogues with diverse receptor trafficking

Signal duration and subcellular location are emerging as important facets of G protein-coupled receptor (GPCR) function. The glucagon-like peptide-1 receptor (GLP-1R), a clinically relevant class B1 GPCR, stimulates production of the second messenger cyclic adenosine monophosphate (cAMP) upon activation by the native hormone, GLP-1. cAMP production continues after the hormone-receptor complex has been internalized via endocytosis. Here, we report GLP-1 analogues that induce prolonged signaling relative to GLP-1. A single β-amino acid substitution at position 18, with the residue derived from (S,S)-trans-2-aminocyclopentanecarboxylic acid (ACPC), enhances signaling duration with retention of receptor endocytosis. Pairing ACPC at position 18 with a second substitution, α-aminoisobutyric acid (Aib) at position 16, abrogates endocytosis, but prolonged signaling is maintained. Prolonged signaling is sensitive to the structure of the β residue at position 18. Cryoelectron microscopy structures of two GLP-1 analogues bound to the GLP-1R:Gs complex suggest substantial alterations to bound peptide structure and dynamics compared to the GLP-1:GLP-1R:Gs complex. These structural findings strengthen an emerging view that agonist dynamics in the receptor-bound state influence signaling profiles. Our results advance understanding of the structural underpinnings of receptor activation and introduce tools for exploring the impact of spatiotemporal signaling profiles following GLP-1R activation.

Allosteric mechanism in the distinctive coupling of Gq and Gs to the parathyroid hormone type 1 receptor

The mechanism determining the preferential stimulation of one heterotrimeric G protein signaling pathway over another by a ligand remains undetermined. By reporting the cryogenic electron microscopy (cryo-EM) structure of the parathyroid hormone (PTH) type 1 receptor (PTH1R) complexed with Gq and comparing its allosteric dynamics with that of PTH1R in complex with Gs, we uncover a mechanism underlying such preferences. We show that an allosteric coupling between the ligand PTH and the C-terminal helix α5 of the Gα subunit controls the stability of the PTH1R complex with the specific G protein, Gs or Gq. Single-cell-level experiments further validate the G protein-selective effects of the PTH binding pose by demonstrating the differential, G protein-dependent residence times and affinity of this ligand at the PTH1R binding site. The findings deepen our understanding of the selective coupling of PTH1R to Gs or Gq and how it relates to the stability and kinetics of ligand binding. They explain the observed variability in the ligand-binding affinity of a GPCR when coupled to different G proteins.

Cholesterol-dependent dynamic changes in the conformation of the type 1 cholecystokinin receptor affect ligand binding and G protein coupling

Development of optimal therapeutics for disease states that can be associated with increased membrane cholesterol requires better molecular understanding of lipid modulation of the drug target. Type 1 cholecystokinin receptor (CCK1R) agonist actions are affected by increased membrane cholesterol, enhancing ligand binding and reducing calcium signaling, while agonist actions of the closely related CCK2R are not. In this work, we identified a set of chimeric human CCK1R/CCK2R mutations that exchange the cholesterol sensitivity of these 2 receptors, providing powerful tools when expressed in CHO and HEK-293 model cell lines to explore mechanisms. Static, low energy, high-resolution structures of the mutant CCK1R constructs, stabilized in complex with G protein, were not substantially different, suggesting that alterations to receptor dynamics were key to altered function. We reveal that cholesterol-dependent dynamic changes in the conformation of the helical bundle of CCK receptors affects both ligand binding at the extracellular surface and G protein coupling at the cytosolic surface, as well as their interrelationships involved in stimulus-response coupling. This provides an ideal setting for potential allosteric modulators to correct the negative impact of membrane cholesterol on CCK1R.

Deciphering the role of glycosaminoglycans in GPCR signaling

G protein-coupled receptors (GPCR) and glycosaminoglycans (GAGs) are two essential components of the cell surface that regulate physiological processes in the body. GPCRs are the most extensive family of transmembrane receptors that control cellular responses to extracellular stimuli, while GAGs are polysaccharides that contribute to the function of the extracellular matrix (ECM). Due to their proximity to the plasma membrane, GAGs participate in signal transduction by interacting with various extracellular molecules and cell surface receptors. GAGs can directly interact with certain GPCRs or their ligands (chemokines, peptide hormones and neuropeptides, structural proteins, and enzymes) from the glutamate receptor family, the rhodopsin receptor family, the adhesion receptor family, and the secretin receptor family. These interactions have recently become an emerging topic, providing a new avenue for understanding how GPCR signaling is regulated. This review discusses our current state of knowledge about the role of GAGs in GPCR signaling and function.

Impact of secretin receptor homo-dimerization on natural ligand binding

Class B G protein-coupled receptors can form dimeric complexes important for high potency biological effects. Here, we apply pharmacological, biochemical, and biophysical techniques to cells and membranes expressing the prototypic secretin receptor (SecR) to gain insights into secretin binding to homo-dimeric and monomeric SecR. Spatial proximity between peptide and receptor residues, probed by disulfide bond formation, demonstrates that the secretin N-terminus moves from adjacent to extracellular loop 3 (ECL3) at wild type SecR toward ECL2 in non-dimerizing mutants. Analysis of fluorescent secretin analogs demonstrates stable engagement of the secretin C-terminal region within the receptor extracellular domain (ECD) for both dimeric and monomeric receptors, while the mid-region exhibits lower mobility while docked at the monomer. Moreover, decoupling of G protein interaction reduces mobility of the peptide mid-region at wild type receptor to levels similar to the mutant, whereas it has no further impact on the monomer. These data support a model of peptide engagement whereby the ability of SecR to dimerize promotes higher conformational dynamics of the peptide-bound receptor ECD and ECLs that likely facilitates more efficient G protein recruitment and activation, consistent with the higher observed functional potency of secretin at wild type SecR relative to the monomeric mutant receptor.

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.

Harnessing Aromatic-Histidine Interactions through Synergistic Backbone Extension and Side Chain Modification

Peptide engineering efforts have delivered drugs for diverse human diseases. Side chain alteration is among the most common approaches to designing new peptides for specific applications. The peptide backbone can be modified as well, but this strategy has received relatively little attention. Here we show that new and favorable contacts between a His side chain on a target protein and an aromatic side chain on a synthetic peptide ligand can be engineered by rational and coordinated side chain modification and backbone extension. Side chain modification alone was unsuccessful. Binding measurements, high-resolution structural studies and pharmacological outcomes all support the synergy between backbone and side chain modification in engineered ligands of the parathyroid hormone receptor-1, which is targeted by osteoporosis drugs. These results should motivate other structure-based designs featuring coordinated side chain modification and backbone extension to enhance the engagement of peptide ligands with target proteins.

Hormone Analogues with Unique Signaling Profiles from Replacement of α-Residue Triads with β/γ Diads

We have applied an underexplored backbone modification strategy to generate new analogues of peptides that activate two clinically important class B1 G protein-coupled receptors (GPCRs). Most peptide modification strategies involve changing side chains or, less commonly, changing the configuration at side chain-bearing carbons (i.e., l residues replaced by d residues). In contrast, backbone modifications alter the number of backbone atoms and the identities of backbone atoms relative to a poly-α-amino acid backbone. Starting from the peptide agonists PTH(1-34) (the first 34 residues of the parathyroid hormone, used clinically as the drug teriparatide) and glucagon-like peptide-1 (7-36) (GLP-1(7-36)), we replaced native α-residue triads with a diad composed of a β-amino acid residue and a γ-amino acid residue. The β/γ diad retains the number of backbone atoms in the ααα triad. Because the β and γ residue each bear a single side chain, we implemented ααα→βγ replacements at sites that contained a Gly residue (i.e., at α-residue triads that presented only two side chains). All seven of the α/β/γ-peptides derived from PTH(1-34) or GLP-1(7-36) bind to the cognate receptor (the PTHR1 or the GLP-1R), but they vary considerably in their activity profiles. Outcomes include functional mimicry of the all-α agonist, receptor-selective agonist activity, biased agonism, or strong binding with weak activation, which could lead to antagonist development. Collectively, these findings demonstrate that ααα→βγ replacements, which are easily implemented via solid-phase synthesis, can generate peptide hormone analogues that display unique and potentially useful signaling behavior.

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.