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dc.contributor.authorLobna Adien_US
dc.description.abstractincretin receptors, glucagon-like peptide-1 receptor (GLP-1R) and glucose-dependent insulinotropic polypeptide receptor (GIPR) are members of the glucagon receptor family of class B (secretin) G-protein coupled receptors (GPCRs). At comparable levels, GIPR displays significantly higher ligand-independent (constitutive) activity than GLP-1R. A naturally existing E354Q polymorphism in human GIPR is characterized with significantly lowered constitutive activity. In addition, substituting the third extracellular loop (ECL3) of the glucagon receptor (GCGR) with that of GLP-1R produces a chimeric receptor with significantly higher constitutive activity. GCGR could adopt both a closed (inactive) conformation and an open (active) conformation stabilized by peptide ligand binding. The aim of this study was to identify the amino acid residues that contribute to the constitutive activity of GIPR. A proline (P) at position 359 was substituted with a phenylalanine (F) and the ECL3 of GIPR was mutated to that of the GLP-1R. A glutamate (E) at position 354 was substituted with glutamine (Q) to generate the E354Q mutant. These mutants were transfected into HEK 293 cells and characterized using Cre Luciferase and Receptor-Surface Expression assays. Exchanging the ECL3 of GIPR with that of GLP-1R created a model that was consistent with the notion that the extracellular domain (ECD) – ECL3 interaction in GIPR plays a major role in maintaining the receptor’s closed (inactive) conformation. In addition, it was found that the P359F mutation significantly compromised the effect of the ECL3 mutation on the basal activity of GIPR. Interestingly, GIP was more potent at the E354Q mutant than it was on the WT GIPR which was also in agreement with a recent study that demonstrated that the equivalent mutation in GLP-1R (E364Q) resulted in a receptor with increased affinity for GLP-1. This suggests that the increased potency of GIP at GIPR E354Q was related to an increase in affinity for GIP.en_US
dc.publisher Kuwait university - college of graduate studiesen_US
dc.subjectglucagon receptoren_US
dc.subjectconstitutive activity of GIPRen_US
dc.titleInvestigating Amino Acid Residues that Contribute to the Constitutive Activity of the Glucose-Dependent Insulinotropic Polypeptideen_US
dc.contributor.supervisorProf. Yunus Luqmanien_US
dc.description.conclusionsThe data presented in this study show that replacing the ECL3 of GIPR with that of GLP-1R results in a chimera with higher basal activity at comparable levels of expression and accordingly, a GIPR model that is compatible with the GCGR model proposed by Yang et al (2016) has been presented. The P359F mutation in GIPR P359F had no significant effect on the basal activity of GIPR; however in GIPR P359F ECL3, it significantly reduced the effect of the ECL3 mutation on the basal activity of GIPR. The reciprocal mutation of GIPR P359F in GLP-1R (GLP-1R F369P) had no significant effect on the basal activity of GLP-1R. In addition, the lowered potency displayed by the ECL3 chimera compared to WT GIPR may indicate a role for the ECL3 of GIPR in influencing the binding affinity for GIP. It was also confirmed that at comparable levels of expression, GIPR E354Q displays significantly lower basal activity than WT GIPR. The potency of GIP at the E354Q mutant was significantly higher than that at WT GIPR, which is in agreement with a recent study (Yang et al, 2016) that shows that the equivalent mutation in GLP-1R (E364Q) results in a receptor with higher binding affinity for GLP-1. Another recent study (Cordomi et al, 2015) reported that GIP exhibits a significantly higher potency at the constitutively active E354A mutant than WT GIPR. Taken together, these differences in potency may be due to conformational changes within GIPR that influence the binding affinity for GIP. Therefore, the data presented in this study should be analyzed in the context of how each mutation affects the overall 98 structure of GIPR and how these induced structural (conformational) changes affect GIPR signaling. Since GIPR E354Q polymorphism is associated with insulin resistance and an increased risk of bone fracture and cardiovascular disease, this study should also provide insights into the development of small molecule drugs that modify the basal activity of GIPR.en_US
dc.contributor.cosupervisorDr. Suleiman Al-Sabahen_US
Appears in Programs:1100 Pharmaceutical Sciences
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