Structural perspective of class B1 GPCR signaling

Abstract

Class B1 G protein-coupled receptors (GPCRs) play important roles in human physiology and disease pathology. Using cryo-electron microscopy (cryo-EM) and X-ray crystallography, the 3D structures of all 15 members of this receptor subfamily have been determined in recent years at the near-atomic level. Although they share many structural commonalities, they show distinct features in terms of ligand recognition and receptor activation. In-depth structural analyses have yielded valuable insights into the N termini of both peptide hormones and cognate receptors, the outward movement of transmembrane helix 6 (TM6), the allosteric modulation sites located in the transmembrane domain (TMD), and the constitutive signaling bias mediated by receptor splice variants. These provide new directions for the design of better therapeutic agents, thereby making these targets more druggable. Rapid advancesG protein-coupled receptors (GPCRs, see Glossary) are the most abundant membrane proteins involved in numerous physiological functions [1]. One GPCR class, the B1 or secretin family, comprises 15 receptors for peptide hormones that regulate a variety of biological processes ranging from growth and development to metabolism and neuroactivities, thereby making them important therapeutic targets for many diseases [2]. Several drugs against this class have been successfully launched to the market, and glucagon-like peptide 1 (GLP-1) and parathyroid hormone (PTH) receptor agonists have reached 'blockbuster' status [3,4]. In earlier years X-ray crystallography and nuclear magnetic resonance (NMR) structural studies on class B1 GPCRs mainly focused on the apo state and ligand-receptor complex structures (Table S1 in the supplemental material online), providing substantial information about the conformations of these receptor extracellular domains (ECDs) and transmembrane domains (TMDs), as well as about the structural basis of ligand recognition. To deepen our knowledge on the structure-activity relationship of these ligand-receptor pairs, the first two full-length structures were determined using single-particle cryo-electron microscopy (cryo-EM) in 2017 [5,6]. Since then the cryo-EM structures for all 15 members of class B1 GPCRs have been solved (Figure 1), not only giving valuable information about hormone recognition and receptor activation from a class-wide perspective but also providing a useful template for the design and development of better peptidic and/or small-molecule drugs [7][8][9][10][11][12][13][14][15][16].

Molecular insights into the distinct signaling duration for the peptide-induced PTH1R activation

“…By contrast, the short-acting peptide easily escapes from the receptor during the activation cycles because of its incompatible binding with "intermediate state" and its unstable R G state, therefore leading to transient signaling 49 . Given the relatively similar activation mechanisms of class B1 receptors 50 , the mechanism might be applicable to other receptors, pointing the way toward the development of the long-or short-acting ligands for class B1 GPCRs. Declarations and subsequently lysed.…”

Kinetic and Thermodynamic Insights on Agonist Interactions with the Parathyroid Hormone Receptor-1 from a New NanoBRET assay

“…However, significant variations were observed in the relationship between the C-terminal portion of LA-PTH and the ECD, with one of the three states showing no interaction in this region of the complex. The difference in koff values at the full-length receptor for LA-PTH vs. M-PTH (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14) suggests that interactions between the C-terminal portion of LA-PTH and the ECD contribute significantly to the overall affinity of the receptor for LA-PTH.…”

“…Family B GPCRs feature a large extracellular domain (ECD) that engages the C-terminal portion of the long polypeptide agonist. [8][9][10][11][12] The N-terminal region of the agonist binds into the core of the family B GPCR transmembrane domain (TMD), which contains the seven-helix bundle tertiary structure that is characteristic of all GPCRs. 13,14 Strategies to monitor GPCR activation are valuable for characterizing relationships between agonist structure and function.…”

“…However, significant variations were observed in the relationship between the C-terminal portion of LA-PTH and the ECD, with one of the three states showing no interaction in this region of the complex. The difference in koff values at the full-length receptor for LA-PTH vs. M-PTH(1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14) suggests that interactions between the C-terminal portion of LA-PTH and the ECD contribute significantly to the overall affinity of the receptor for LA-PTH.Sequence variants of known PTHR1 agonists. We selected several peptides derived from PTH(1-34) and/or PTHrP(1-36) for evaluation with the new assay based on previous reports documenting effects of sequence modifications on agonist activity and/or PTHR1 binding.…”

A unique peptide recognition mechanism by the human relaxin family peptide receptor 4 (RXFP4)

“…Endogenous peptides mainly bind to class A and B1 GPCRs 42,43 . Unlike its class B1 counterparts that have large extracellular domains, class A GPCRs usually adopt extended loop conformations during their insertion into the orthosteric pocket by the peptide N terminus [e.g., DAMGO 44 , C-C chemokine ligand 15 (CCL15) 28 , C-X-C motif chemokine ligand 8 (CXCL8) 45 , Aβ 42 46 , N-formyl humanin 46 and ghrelin 36 ], the peptide C terminus [e.g., angiotensin II 47,48 , bradykinin 27 , cholecystokinin-8 (CCK-8) 49 , Des-Arg 10 -kallidin 27 , gastrin-17 25 , IMV449 50 , neuromedin U 51 and neuromedin S 51 ] or the peptide middle region [e.g., α-melanocyte-stimulating hormone (α-MSH) 52 , arginine-vasopressin (AVP) 53 and somatostain-14 54 ], thereby achieving a significantly larger peptide-receptor interface area (>1500 Å 2 ) compared to that displayed by interaction with small molecules (<1000 Å 2 ) (Fig.…”