Miller, Laurence J.

G protein-coupled receptors (GPCRs) are among the most common drug targets, with numerous agonists and antagonists approved for clinical use. More recently, it has been appreciated that drugs can also modulate the action of natural agonists of these receptors, thus providing unique clinical advantages. Here, we describe the identification and characterization of a small molecule positive allosteric modulator (PAM) of secretin action at the class B G protein-coupled secretin receptor. This phenyl(trifluoromethyl)-pyrimidine can occupy the secretin receptor without stimulating its internalization, yet priming it to enhance both the potency and efficacy of the action of natural secretin. This is also shown to exhibit its effects on cells expressing low numbers of these receptors, without enhancing the effects of other structurally related hormones acting at other class B GPCRs. The mechanism responsible for this PAM effect is the slowing of the off-rate of receptor-bound secretin. This compound can serve as a lead to the development of other drugs that enhance the action of natural endogenous secretin and can be utilized to explore the potential therapeutic utility of such compounds.

Abstract The functional significance of the interactions between proteins in living cells to form short-lived quaternary structures cannot be overemphasized. Yet, quaternary structure information is not captured by current methods, neither can those methods determine structure within living cells. The dynamic versatility, abundance, and functional diversity of G protein-coupled receptors (GPCRs) pose myriad challenges to existing technologies but also present these proteins as the ideal testbed for new technologies to investigate the complex inter-regulation of receptor-ligand, receptor-receptor, and receptor-downstream effector interfaces in living cells. Here, we present development and use of a novel method capable of overcoming existing challenges by combining distributions (or spectrograms) of FRET efficiencies from populations of fluorescently tagged proteins associating into oligomeric complexes in live cells with diffusion-like trajectories of FRET donors and acceptors obtained from molecular dynamics (MD) simulations. Our approach provides an atom-level picture of the binding interfaces within oligomers of the human secretin receptor (hSecR) in live cells and allows for extraction of mechanistic insights into the function of GPCRs oligomerization. This FRET-MD spectrometry approach is a robust platform for investigating protein-protein binding mechanisms and opens a new avenue for investigating stable as well as fleeting quaternary structures of any membrane proteins in living cells.

As part of an ongoing effort to develop a drug targeting the type 1 cholecystokinin receptor (CCK1R) to help prevent and/or treat obesity, we recently performed a high throughput screening effort of small molecules seeking candidates that enhanced the action of the natural agonist, CCK, thus acting as positive allosteric modulators without exhibiting intrinsic agonist action. Such probes would be expected to act in a temporally finite way to enhance CCK action to induce satiety during and after a meal and potentially even modulate activity at the CCK1R in a high cholesterol environment present in some obese patients. The current work focuses on the best scaffold, representing tetracyclic molecules identified through high throughput screening we previously reported. Extensive characterization of the two top “hits” from the previous effort demonstrated them to fulfill the desired pharmacologic profile. We undertook analog-by-catalog expansion of this scaffold using 65 commercially available analogs. In this effort, we were able to eliminate an off-target effect observed for this scaffold while retaining its activity as a positive allosteric modulator of CCK1R in both normal and high cholesterol membrane environments. These insights should be useful in the rational medicinal chemical enhancement of this scaffold and in the future development of candidates to advance to pre-clinical proof-of-concept and to clinical trials.

Pituitary Adenylate Cyclase-Activating Peptide (PACAP) and Vasoactive Intestinal Peptide (VIP) are neuropeptides involved in a diverse array of physiological and pathological processes through activating the PACAP subfamily of class B1 G protein-coupled receptors (GPCRs): VIP receptor 1 (VPAC1R), VIP receptor 2 (VPAC2R), and PACAP type I receptor (PAC1R). VIP and PACAP share nearly 70% amino acid sequence identity, while their receptors PAC1R, VPAC1R, and VPAC2R share 60% homology in the transmembrane regions of the receptor. PACAP binds with high affinity to all three receptors, while VIP binds with high affinity to VPAC1R and VPAC2R, and has a thousand-fold lower affinity for PAC1R compared to PACAP. Due to the wide distribution of VIP and PACAP receptors in the body, potential therapeutic applications of drugs targeting these receptors, as well as expected undesired side effects, are numerous. Designing selective therapeutics targeting these receptors remains challenging due to their structural similarities. This review discusses recent discoveries on the molecular mechanisms involved in the selectivity and signaling of the PACAP subfamily of receptors, and future considerations for therapeutic targeting.

How to catch a dynamic state AMP-activated protein kinase (AMPK) is a key sensor of energy status in eukaryotes. Its dynamic structure is regulated by allosteric factors including phosphorylation and binding of nucleotides and metabolites. Yan et al. developed conformation-specific antibodies that trap AMPK in a fully inactive state that has experienced a large, domain-level rotation. Biophysical experiments in cells and in vitro are consistent with the structural work and support a model in which the activation loop is fully exposed in the completely inactive, dephosphorylated state. These structures inform our understanding of the complex allosteric behavior in this crucial metabolic regulator. Science , abe7565, this issue p. 413

Class B G protein‐coupled receptors (GPCRs) are a highly important group of therapeutic targets with strong physiologic implications in cardiovascular, metabolic, or gastrointestinal systems. Endogenously activated by moderate‐length peptide hormones, class B GPCRs entertain a complex binding mechanism to induce stimulatory signaling processes. The secretin receptor (SCTR) is a prototypical member of this receptor family, having clinical relevance in multiple diseases including heart failure, digestive disorders, diabetes and obesity. [1‐4] However, its endogenous ligand secretin and peptide‐based analogs are the only ligands known to interact with SCTRs, likely due to challenges in class B GPCR drug discovery [5]. To find orally available therapeutic leads targeting this “undruggable” receptor, we focused on identifying positive allosteric modulators (PAMs), which act via structurally distinct binding pockets and potentiate hormonal signaling of native agonists. Thus, we developed a testing funnel tailored to discover SCTR small molecule modulators. After screening a 12,000‐compound library with a unique primary assay that used a mixture of stimulators, we identified three structurally related scaffolds (A‐C) enhancing peptidic SCTR activation [5]. Applying a comprehensive toolbox with binding and functional assays, we studied structure‐activity relationships of a well‐considered set of commercially available analogs incorporating these three scaffolds. Our investigations afforded an optimized analog B2, which showed substantial positive cooperativity to low‐potent peptide agonists and significantly decelerated secretin dissociation. Due to the innate electrophilicity of scaffold set A, we recognized a structural similarity to PAMs acting on glucagon‐like peptide‐1 receptor (GLP‐1R), a prominent member of class B GPCR family. We further demonstrated that a subset of SCTR PAMs was able to augment not only SCTR signaling but also GLP‐1R functions. A recent study described that dual activation of SCTRs and GLP‐1Rs was benefiting glucose homeostasis and appetite regulation while avoiding proliferative effects on pancreas mass [6]. Determining cooperativity between PAMs and selected orthosteric peptides, we established distinct selectivity and probe dependency profiles. To decipher reversible and covalent mechanisms‐of‐action as well as potential binding sites of novel PAMs, we conducted glutathione reactivity, cAMP‐washout and receptor mutational studies. [7] To conclude, we discovered a unique class of SCTR modulators providing useful pharmacologic tools to study SCTR‐ or GLP‐1R‐specific activation pathways and explore novel allosteric binding sites with potential to instigate the development of first‐in‐class therapeutics for treating comorbid conditions obesity and diabetes. [7] References [1] Annu Rev Pharmacol Toxicol, 2020 , 60 , 89‐107; [2] Trends in Biochemical Sciences, 2017 , 12 , 946‐960; [3] Experimental Physiology, 2013 , 5 , 973‐987; [4] Current Neuropharmacology, 2007 , 3 , 168‐179; [5] SLAS Discovery, 2021 , 26 , 1‐16; [6] Journal of Peptide Science, 2017 , 12 , 845‐854; [7] submitted to Biochemical Pharmacology 2021 .

The Secretin receptor (SCTR) has been cloned as the first member of class B G protein‐coupled receptors (GPCRs) , giving rise to this highly important group of therapeutic targets, which are crucial for hormonal homeostasis . Despite the wide recognition of their functional importance, the development of potent small molecule ligands for Class B GPCRs remained challenging . To date, no ligands other than secretin peptide and its close analogs are known to interact with SCTRs, despite its therapeutic relevance in multiple diseases, such as heart failure, obesity, diabetes and cancer . In order to enhance our chances for discovering orally bioavailable drug candidates targeting an ‚undruggable’ class B GPCR, we focused our efforts on the identification of positive allosteric modulators (PAMs), which are likely to interact via structurally distinct binding sites and potentiate effects of native hormones and neurotransmitters . We developed a comprehensive tool box with robust high‐throughput screening and lead characterization/optimization assays capable of identifying and characterizing biologically relevant SCTR small molecule modulators. The dominant physiological coupling of SCTRs occurs via Gs proteins. Hence, we utilized this signaling mechanism to establish diverse cAMP accumulation assays suitable for simultaneous detection of PAMs modulating secretin and its potential metabolites. We also established a TR‐FRET based binding assay to further characterize potential PAMs with respect to their kinetic properties. Since functional selectivity has been proven to be therapeutically relevant for this family of membrane proteins, we additionally designed assays to investigate signaling mechanisms beyond Gs protein coupling, such as β‐arrestin recruitment, receptor internalization and calcium flux. High‐throughput screening (HTS) of a 360,000 compound library provided promising sets of hits, which have been further investigated regarding their allosteric activity and mechanism of action. We identified three related scaffolds with significant PAM activity at SCTRs. Selectivity studies revealed that this specific set of compounds demonstrated positive modulation not only on SCTRs but also on GLP‐1Rs (glucagon‐like peptide 1 receptors). Interestingly, dual activation of SCTRs and GLP‐1Rs has been shown to be beneficial for appetite regulation and glucose homeostasis without having proliferative effects on exocrine or total pancreas mass . To conclude, HTS and the development of a diverse panel of binding and functional assays has led to the discovery of small molecules with a unique dual‐acting pharmacological profile and potential to serve as starting points for developing first‐in‐class drug candidates for the treatment of comorbid conditions obesity and diabetes. Support or Funding Information Funding: NHLBI award 5R01HL133501

Abnormal gastric accommodation (GA) and gastric emptying contribute to pathophysiology in functional dyspepsia (FD). Secretin is a key regulator of GA in animal studies. Our aim was to study the effects of secretin on gastric motility, satiation, postprandial symptoms, and key hormones. We performed two double-blind, randomized, saline-controlled crossover trials in 10 healthy volunteers and 10 patients with FD by Rome IV criteria. We used measured GA (by validated SPECT method) after a 111In radiolabeled Ensure 300-mL meal and quantified gastric emptying for 30 min by scintigraphy. Satiation was measured by volume to fullness (VTF) and maximum tolerated volume (MTV) on an Ensure nutrient drink test and postprandial symptoms 30 min post-MTV. Fasting and postprandial GLP-1, GIP, and HPP were measured. The ages and sex distribution of healthy controls and patients with FD were similar. Compared with placebo, secretin delayed gastric emptying at 30 min in both health [−11% (−16, −4), P = 0.004]; and FD [−8% (−9, 0), P = 0.03]. Satiation (VTF and MTV), GA, and plasma levels of GLP-1, GIP, and HPP did not differ between treatment arms in health or FD. On ANCOVA analysis (adjusting for age and sex), secretin did not consistently increase postprandial symptoms in health or FD. Secretin delayed gastric emptying in both health and FD without significantly altering GA, VTF, or MTV or selected hormones. Thus, secretin receptor activation may provide a novel therapeutic mechanism for patients with FD and rapid gastric emptying. NEW & NOTEWORTHY The naturally occurring hormone secretin retards gastric emptying of solids without deleteriously affecting gastric accommodation, satiation, other upper gastrointestinal hormones, or postprandial symptoms. Given these findings, a subset of patients with rapid gastric emptying (e.g., the estimated 20% of patients with functional dyspepsia) could be candidates for treatments that stimulate a secretin receptor such as sacubitril, which inhibits neprilysin, an enzyme that degrades secretin.

INTRODUCTION: Reduced gastric accommodation (GA) or accelerated or delayed gastric emptying (GE) contributes to pathophysiology in functional dyspepsia (FD). Secretin is an endogenous hormone secreted postprandially from the duodenal mucosa. Our objective was to determine the effects of secretin in human gastric motor functions and satiation in health and FD. METHODS: We conducted a double-blind, randomized, placebo-controlled, cross-over trial in 10 healthy participants and 10 patients with FD by Rome IV criteria. Subjects were randomized to IV secretin or normal saline with a 1-4 week washout period. We measured GA using validated 99m Tc-single photon emission computed tomography with a standardized 300 mL meal of 111 In radiolabeled Ensure®. We measured GE using scintigraphy of the same meal and symptoms 30 minutes post meal based on 100 mm visualized analog scale (VAS). To measure volume to fullness (VTF) and maximum tolerated volume (MTV), we performed Ensure® nutrient drink test (NDT) and recorded symptoms 30 min post-MTV. Wilcoxon Signed-Rank Test was used for comparison within healthy and FD groups and ANCOVA modeling was used for intention to treat comparison between groups, adjusting for age, sex, and treatment arm (secretin vs. placebo). Study drug was provided by ChiroClin, Inc. RESULTS: The median age for healthy volunteers (HV) was 45.0 years [IQR: 38.0, 49.0 (80% female)] and for FD was 50.5 years [42.0, 55.0 (70% female)]. There was no statistical difference in either group for secretin vs. placebo for GA. Secretin delayed GE in both HV and FD (Table 1). There was decrease in GE for secretin vs. placebo in HV at 20 minutes [-10% (-13, -7), P = 0.008] and 30 minutes [-11% (-16, -4), P = 0.004]. This finding was also seen in the FD group for GE at 25 minutes [-5% (-9, 0), P = 0.047] and 30 minutes [-8% (-9, 0), P = 0.03]. There were no statistically significant differences between treatment arms for VTF or MTV in either HV or FD. On ANCOVA analysis, there were statistically significant increases in nausea at 30 minutes ( P = 0.0016) and fullness at 30 minutes ( P = 0.0002) for secretin vs. placebo in FD compared to HV. CONCLUSION: Secretin delayed GE in both healthy volunteers and FD without significantly altering GA, VTF, or MTV. Secretin receptor stimulation deserves further study in FD patients with accelerated GE.