Rapid communicationA human glucagon-like peptide-1 receptor polymorphism results in reduced agonist responsiveness
Introduction
Glucagon-like peptide-1 (GLP-1, 7–36 amide) is an enteroendocrine hormone that plays an important physiological role in maintaining blood glucose homeostasis [1], [2]. This hormone is produced by L-cells primarily localized in the ileal/colonic mucosa. Following food ingestion, GLP-1 is secreted into the circulation and acts on multiple target tissues to attenuate the postprandial increase in blood glucose levels. In the endocrine pancreas, GLP-1 enhances glucose-induced insulin secretion as well as stimulates the growth of insulin-producing beta cells [3], [4]. This peptide also has additional peripheral and central functions, including delay of gastric emptying and induction of satiety [5].
GLP-1 acts via a cognate G-protein-coupled receptor (GLP-1R) that falls within the class B subfamily. Stimulation of the receptor with endogenous agonist (GLP-1) triggers cAMP production as the primary signal transduction pathway [6]. Given the physiological importance of GLP-1, derivatives of this peptide which extend its extremely short biological half life provide a promising new class of drugs for the treatment of diabetes [7], [8]. One of these stable peptides, exendin-4, was identified as a naturally occurring receptor agonist that was isolated from the salivary gland of the lizard reptile, Heloderma suspectum [9]. As a complementary strategy to enhance GLP-1R activity, drugs are also under development which inhibit the rapid enzymatic degradation of endogenously produced GLP-1 [10].
In addition to drug discovery efforts, studies of the GLP-1R have been directed at identifying polymorphisms in the GLP-1R gene that may be a contributing factor in the pathogenesis of diabetes. Although conclusive evidence for this link remains to be established, it is of note that a recent study of 791 Japanese individuals with type 2 diabetes identified a patient with a heterozygous GLP-1R missense polymorphism that resulted in substitution of threonine 149 by methionine (T149M). In contrast to several other GLP-1R polymorphisms that were found in this study [11], the T149M substitution was not observed in a control population of 318 non-diabetic individuals, and did not correspond to any sequence variation that occurs in known mammalian homologues of this receptor.
In the current study, we demonstrate that the T149M substitution, when introduced into the human GLP-1R by recombinant DNA techniques, causes a significant loss of function vs. the wild type protein. This finding warrants further investigation, to address the question whether this receptor polymorphism plays a role in the development of diabetes.
Section snippets
Generation of mutant GLP-1R cDNA
Experiments were performed using the human GLP-1 receptor cDNA in the expression vector pcDNA1 [12], [13]. The T149M substitution was introduced using a quick-change mutagenesis kit (Stratagene, La Jolla, CA). Dideoxynucleotide sequencing confirmed that the deduced amino acid sequence of the protein coding region was otherwise identical to the published receptor cDNA (Swiss-Prot Database, P43220).
Functional characterization of the recombinant GLP-1R
COS-7 cells, which lack endogenous GLP-1Rs, were transiently transfected with receptor cDNA using
Results
Properties of the T149M variant and the wild type receptor were compared using several well-established GLP-1R peptide ligands (Fig. 1). Among these, GLP-1 and exendin-4 are known full GLP-1R agonists, whereas the amino-terminally truncated derivative exendin (9-39) has been used as a pharmacological antagonist of this receptor [12], [16]. GLP-1 (9-36)amide is the primary endogenous metabolite of GLP-1, resulting from rapid enzymatic cleavage of its two amino-terminal residues [10]. GLP-1
Discussion
The T149M substitution in the human GLP-1R induces a marked decrease in the affinity and potency of both GLP-1 and exendin-4. In contrast, this polymorphism does not alter the affinity of corresponding amino-terminally truncated peptides, GLP-1 (9-36)amide and exendin (9-39) (see Fig. 1 for sequence comparisons). It thus appears that ligand sensitivity to the GLP-1R polymorphism depends on an intact amino terminus (of GLP-1 or exendin-4). As a possible explanation for this requirement, one may
Acknowledgement
The excellent technical assistance by Ci Chen is greatly appreciated. This work was supported by grants from the National Institutes of Health, National Institute of Diabetes, Digestive and Kidney Diseases (DK56674, DK46767 and P30 DK34928).
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2021, Molecular MetabolismCentral GLP-1 receptors: Novel molecular targets for cocaine use disorder
2019, Physiology and BehaviorCitation Excerpt :Moreover, there are several known single nucleotide polymorphisms (SNPs) in the human GLP-1 receptor [173]. One SNP in particular, the T149 M variant, displayed reduced GLP-1 binding and cAMP signaling as well as differences in the extent of reduced functional responses to various GLP-1 receptor ligands [173,174]. It is possible that expression of the T149M variant and/or decreased baseline circulating GLP-1 levels in human cocaine addicts may serve as predictors of treatment response and/or relapse, as these phenotypes are associated with decreased endogenous GLP-1 signaling in the body.