Probes for GLP1R

Glucagon-like peptide-1 receptor (GLP-1R) agonists, used to treat type 2 diabetes and obesity, reduce rates of myocardial infarction and cardiovascular death. The GLP-1R has been localized to the human sinoatrial node; however, its expression in ventricular tissue remains uncertain. Here we studied GLP-1R expression in the human heart using GLP-1R-directed antisera, quantitative PCR, reverse transcription PCR to detect full length mRNA transcripts, and in situ hybridization. GLP1R mRNA transcripts, encompassing the entire open reading frame, were detected in all four cardiac chambers from 15 hearts at levels approximating those detected in human pancreas. In contrast, cardiac GLP2R expression was relatively lower, whereas cardiac GCGR expression was sporadic and not detected in the left ventricle. GLP1R mRNA transcripts were not detected in RNA from human cardiac fibroblasts, coronary artery endothelial, or vascular smooth muscle cells. Human Brunner's glands and pancreatic islets exhibited GLP-1R-immunopositivity and abundant expression of GLP1R mRNA transcripts by in situ hybridization. GLP1R transcripts were also detected by in situ hybridization in human cardiac sinoatrial node tissue. However definitive cellular localization of GLP1R mRNA transcripts or immunoreactive GLP-1R protein within human cardiomyocytes (CMs) or cardiac blood vessels remained elusive. Moreover, validated GLP-1R antisera lacked sufficient sensitivity to detect expression of the endogenous islet or cardiac GLP-1R by Western blotting. Hence, although human cardiac ventricles express the GLP1R, the identity of one or more ventricular cell type(s) that express a translated GLP1R protein requires further clarification with highly sensitive methods of detection.

Pharmacological activation of the hypothalamic glucagon-like peptide-1 (GLP-1) receptor (GLP-1R) promotes weight loss and improves glucose tolerance. This demonstrates that the hypothalamic GLP-1R is sufficient but does not show whether it is necessary for the effects of exogenous GLP-1R agonists (GLP-1RA) or endogenous GLP-1 on these parameters. To address this, we crossed mice harboring floxed Glp1r alleles to mice expressing Nkx2.1-Cre to knock down Glp1r expression throughout the hypothalamus (GLP-1RKDΔNkx2.1cre). We also generated mice lacking Glp1r expression specifically in two GLP-1RA-responsive hypothalamic feeding nuclei/cell types, the paraventricular nucleus (GLP-1RKDΔSim1cre) and proopiomelanocortin neurons (GLP-1RKDΔPOMCcre). Chow -fed GLP-1RKDΔNkx2.1cre mice exhibited increased food intake and energy expenditure with no net effect on body weight. When fed a high fat diet (HFD), these mice exhibited normal food intake but elevated energy expenditure, yielding reduced weight gain. None of these phenotypes were observed in GLP-1RKDΔSim1creand GLP-1RKDΔPOMCcre mice. The acute anorectic and glucose tolerance effects of peripherally-dosed GLP-1RA exendin-4 and liraglutide were preserved in all mouse lines. Chronic liraglutide treatment reduced body weight in chow-fed GLP-1RKDΔNkx2.1cre mice, but this effect was attenuated upon HFD feeding. In sum, classical homeostatic control regions are sufficient but not individually necessary for the effects of GLP-1RA on nutrient homeostasis.

Abstract

Objectives

G protein-coupled receptors (GPCRs) act as transmembrane molecular sensors of neurotransmitters, hormones, nutrients, and metabolites. Because unmyelinated vagalafferents richly innervate the gastrointestinal mucosa, gut-derived molecules may directly modulate the activity of vagal afferents through GPCRs. However, the types of GPCRs expressed in vagal afferents are largely unknown. Here, we determined the expression profile of all GPCRs expressed in vagal afferents of the mouse, with a special emphasis on those innervating the gastrointestinal tract.

Methods

Using a combination of high-throughput quantitative PCR, RNA sequencing, and in situhybridization, we systematically quantified GPCRs expressed in vagal unmyelinated Nav1.8-expressing afferents.

Results

GPCRs for gut hormones that were the most enriched in Nav1.8-expressing vagal unmyelinated afferents included NTSR1, NPY2R, CCK1R, and to a lesser extent, GLP1R, but not GHSR and GIPR. Interestingly, both GLP1R and NPY2R were coexpressed with CCK1R. In contrast, NTSR1 was coexpressed with GPR65, a marker preferentially enriched in intestinal mucosal afferents. Only few microbiome-derived metabolite sensors such as GPR35 and, to a lesser extent, GPR119 and CaSR were identified in the Nav1.8-expressing vagal afferents. GPCRs involved in lipid sensing and inflammation (e.g. CB1R, CYSLTR2, PTGER4), and neurotransmitters signaling (CHRM4, DRD2, CRHR2) were also highly enriched in Nav1.8-expressing neurons. Finally, we identified 21 orphan GPCRs with unknown functions in vagal afferents.

Conclusion

Overall, this study provides a comprehensive description of GPCR-dependent sensing mechanisms in vagal afferents, including novel coexpression patterns, and conceivably coaction of key receptors for gut-derived molecules involved in gut-brain communication.

Glucagon-like peptide-1 (GLP-1) is an incretin hormone increasing postprandial insulin release. GLP-1 also induces diuresis and natriuresis in humans and rodents. The GLP-1 receptor is extensively expressed in the renal vascular tree in normotensive rats where acute GLP-1 treatment leads to increased mean arterial pressure (MAP) and increased renal blood flow (RBF). In hypertensive animal models, GLP-1 has been reported both to increase and decrease MAP. The aim of this study was to examine expression of renal GLP-1 receptors in spontaneously hypertensive rats (SHR) and to assess the effect of acute intrarenal infusion of GLP-1. We hypothesized that GLP-1 would increase diuresis and natriuresis and reduce MAP in SHR. Immunohistochemical staining and in situ hybridization for the GLP-1 receptor were used to localize GLP-1 receptors in the kidney. Sevoflurane-anesthetized normotensive Sprague-Dawley rats and SHR received a 20 min intrarenal infusion of GLP-1 and changes in MAP, RBF, heart rate, dieresis, and natriuresis were measured. The vasodilatory effect of GLP-1 was assessed in isolated interlobar arteries from normo- and hypertensive rats. We found no expression of GLP-1 receptors in the kidney from SHR. However, acute intrarenal infusion of GLP-1 increased MAP, RBF, dieresis, and natriuresis without affecting heart rate in both rat strains. These results suggest that the acute renal effects of GLP-1 in SHR are caused either by extrarenal GLP-1 receptors activating other mechanisms (e.g., insulin) to induce the renal changes observed or possibly by an alternative renal GLP-1 receptor.