Suppressive Effects of Neuromedin U Receptor 2-Selective Peptide Agonists on Appetite and Prolactin Secretion in Mice

Neuromedin U receptor 2 (NMUR2), which is expressed in the central nervous system (CNS) including the hypothalamus, has been noted as a therapeutic target against obesity. We previously reported that intranasal administration of CPN-219, a NMUR2-selective hexapeptide agonist, suppresses body weight gain in mice; however, there is no detailed information regarding its CNS effects. Recently, in addition to appetite suppression, stress responses and regulation of prolactin (PRL) secretion have also attracted attention. NMUR2 expressed in the hypothalamic tuberoinfundibular dopaminergic neurons has emerged as an alternative target for treating hyperprolactinemia. Here, CPN-219 decreased food intake up to 24 h after administration at a dose of 200 nmol, resulting in body weight gain suppression, although grooming and anxiety-like behaviors were transiently induced. Interestingly, the restraint stress-induced increase in plasma PRL levels was significantly suppressed at a lower dose of 20 nmol, indicating the potential for drug development as an anti-PRL agent of NMUR2-selective agonists.

Structural Insights into the Unique Modes of Relaxin-Binding and Tethered-Agonist Mediated Activation of RXFP1 and RXFP2

Our poor understanding of the mechanism by which the peptide-hormone H2 relaxin activates its G protein coupled receptor, RXFP1 and the related receptor RXFP2, has hindered progress in its therapeutic development. Both receptors possess large ectodomains, which bind H2 relaxin, and contain an N-terminal LDLa module that is essential for receptor signaling and postulated to be a tethered agonist. Here, we show that a conserved motif (GDxxGWxxxF), C-terminal to the LDLa module, is critical for receptor activity. Importantly, this motif adopts different structures in RXFP1 and RXFP2, suggesting distinct activation mechanisms. For RXFP1, the motif is flexible, weakly associates with the LDLa module, and requires H2 relaxin binding to stabilize an active conformation. Conversely, the GDxxGWxxxF motif in RXFP2 is more closely associated with the LDLa module, forming an essential binding interface for H2 relaxin. These differences in the activation mechanism will aid drug development targeting these receptors.

Osteocalcin: A Protein Hormone Connecting Metabolism, Bone and Testis Function

During the last decade, the disclosure of systemic effects of osteocalcin (OCN) in its undercarboxylated form contributed to switch the concept of bone from a merely structural apparatus to a fully endocrine organ involved in the regulation of systemic functions. Since that time, the role of OCN as osteokine has been more and more widened appreciated and detailed by the major use of animal models, starting from the original function in the bone extracellular matrix as Gla-protein and spanning from the protective effects towards weight gain, insulin sensitivity and glucose homeostasis, to the anabolic and metabolic roles in skeletal muscle, to the stimulating effects on the testis endocrine function and male fertility, to the most recent preservation from anxious and depressive states through a direct activity on the central nervous system. In this review, experimental data supporting the inter-organ communication roles of this protein are discussed, together with the available data supporting the consistency between experimental data obtained in animals and those reported in humans. In addition, a specific session has been devoted to the possible significance the OCN as a template agonist on its receptor GPRC6A, for the development of novel therapeutic and pharmacological approaches for the treatment of dismetabolic states and male infertility.

Current Mechanistic and Pharmacodynamic Understanding of Melanocortin-4 Receptor Activation

In this work we summarize our understanding of melanocortin 4 receptor (MC4R) pathway activation, aiming to define a safe and effective therapeutic targeting strategy for the MC4R. Delineation of cellular MC4R pathways has provided evidence for distinct MC4R signaling events characterized by unique receptor activation kinetics. While these studies remain narrow in scope, and have largely been explored with peptidic agonists, the results provide a possible correlation between distinct ligand groups and differential MC4R activation kinetics. In addition, when a set of small-molecule and peptide MC4R agonists are compared, evidence of biased signaling has been reported. The results of such mechanistic studies are discussed.

Determination of Etelcalcetide Biotransformation and Hemodialysis Kinetics to Guide the Timing of Its Dosing

Introduction

Etelcalcetide, a novel calcimimetic agonist of the calcium-sensing receptor for treatment of secondary hyperparathyroidism in chronic kidney disease patients on hemodialysis, is a d-amino acid linear heptapeptide with a d-cysteine that is linked to an l-cysteine by a disulfide bond. In addition to binding to the calcium-sensing receptor, etelcalcetide is biotransformed by disulfide exchange in whole blood to predominantly form a covalent serum albumin peptide conjugate (SAPC). Key factors anticipated to affect the pharmacokinetics and disposition of etelcalcetide in chronic kidney disease patients on hemodialysis are the drug’s intrinsic dialytic properties and biotransformation kinetics.

Methods

These factors were investigated using in vitro methods, and the findings were modeled to derive corresponding kinetic rate constants.

Results

Biotransformation was reversible after incubation of etelcalcetide or SAPC in human whole blood. The rate of SAPC formation from etelcalcetide was 18-fold faster than the reverse process. Clearance of etelcalcetide by hemodialysis was rapid in the absence of blood and when hemodialysis was initiated immediately after addition of etelcalcetide to blood. Preincubation of etelcalcetide in blood for 3 hours before hemodialysis resulted in formation of SAPC and decreased its clearance due to the slow rate of etelcalcetide formation from SAPC. Etelcalcetide hemodialysis clearance was >16-fold faster than its biotransformation.

Discussion

These results indicate that etelcalcetide should be administered after hemodialysis to avoid elimination of a significant fraction of the dose.

The constitutive activity of the adhesion GPCR GPR114/ADGRG5 is mediated by its tethered agonist

Adhesion GPCRs (aGPCRs) form the second largest, yet most enigmatic class of the GPCR superfamily. Although the physiologic importance of aGPCRs was demonstrated in several studies, the majority of these receptors is still orphan with respect to their agonists and signal transduction. Recent studies reported that aGPCRs are activated through a tethered peptide agonist, coined the Stachel sequence. The Stachel sequence is the most C-terminal part of the highly conserved GPCR autoproteolysis-inducing domain. Here, we used cell culture-based assays to investigate 2 natural splice variants within the Stachel sequence of the orphan Gs coupling aGPCR GPR114/ADGRG5. There is 1 variant constitutively active in cAMP assays (∼25-fold over empty vector) and sensitive to mechano-activation. The other variant has low basal activity in cAMP assays (6-fold over empty vector) and is insensitive to mechano-activation. In-depth mutagenesis studies of these functional differences revealed that the N-terminal half of the Stachel sequence confers the agonistic activity, whereas the C-terminal part orientates the agonistic core sequence to the transmembrane domain. Sequence comparison and functional testing suggest that the proposed mechanism of Stachel-mediated activation is relevant not only to GPR114 but to aGPCRs in general.

Peptide ligand recognition by G protein-coupled receptors

The past few years have seen spectacular progress in the structure determination of G protein-coupled receptors (GPCRs). We now have structural representatives from classes A, B, C, and F. Within the rhodopsin-like class A, most structures belong to the α group, whereas fewer GPCR structures are available from the β, γ, and δ groups, which include peptide GPCRs such as the receptors for neurotensin (β group), opioids, chemokines (γ group), and protease-activated receptors (δ group). Structural information on peptide GPCRs is restricted to complexes with non-peptidic drug-like antagonists with the exception of the chemokine receptor CXCR4 that has been crystallized in the presence of a cyclic peptide antagonist. Notably, the neurotensin receptor 1 is to date the only peptide GPCR whose structure has been solved in the presence of a peptide agonist. Although limited in number, the current peptide GPCR structures reveal great diversity in shape and electrostatic properties of the ligand binding pockets, features that play key roles in the discrimination of ligands. Here, we review these aspects of peptide GPCRs in view of possible models for peptide agonist binding.

Stapled Vasoactive Intestinal Peptide (VIP) Derivatives Improve VPAC2 Agonism and Glucose-Dependent Insulin Secretion

Agonists of vasoactive intestinal peptide receptor 2 (VPAC2) stimulate glucose-dependent insulin secretion, making them attractive candidates for the treatment of hyperglycaemia and type-II diabetes. Vasoactive intestinal peptide (VIP) is an endogenous peptide hormone that potently agonizes VPAC2. However, VIP has a short serum half-life and poor pharmacokinetics in vivo and is susceptible to proteolytic degradation, making its development as a therapeutic agent challenging. Here, we investigated two peptide cyclization strategies, lactamisation and olefin-metathesis stapling, and their effects on VPAC2 agonism, peptide secondary structure, protease stability, and cell membrane permeability. VIP analogues showing significantly enhanced VPAC2 agonist potency, glucose-dependent insulin secretion activity, and increased helical content were discovered; however, neither cyclization strategy appeared to effect proteolytic stability or cell permeability of the resulting peptides.