👤 Noushafarin Khajavi

Unimolecular peptides targeting the receptors for glucagon-like peptide-1 (GLP-1), glucose-dependent insulinotropic polypeptide (GIP) and glucagon (GCG) have been shown to improve glycaemic management in both mice and humans. Yet the identity of the downstream signalling events mediated by these peptides remain to be elucidated. Here, we aimed to assess the mechanisms by which a validated peptide triagonist for GLP-1/GIP/GCG receptors (IUB447) stimulates insulin secretion in murine pancreatic islets. Islets were isolated from wild-type (WT), Gipr-knockout (Gipr The triagonist promoted glucose-stimulated insulin secretion (GSIS) to a greater degree than co-administration of conventional mono-agonists in WT mouse islets. The triagonist-induced increase in GSIS was unchanged in the absence of either Gipr or Gcgr. However, the triagonist failed to enhance insulin secretion in islets lacking both Glp-1r and Gipr and upon treatment with the GLP-1 receptor-specific antagonist exendin-3 (9-39). Similarly, the specific blocking of Gαq signalling with YM254890 or transient receptor potential melastatin 5 (TRPM5) with triphenylphosphine oxide (TPPO) suppressed the triagonist-induced enhancement of GSIS. In vivo assessment of high-fat-fed Trpm5 Triagonist-induced augmentation of GSIS is primarily mediated through its interaction with the GLP-1 receptor and subsequent activation of the Gαq-TRPM5 signalling pathway. Given that Gαq is a key player in the amplification of GSIS, particularly under diabetic conditions, these findings highlight a GLP-1 receptor-centric pharmacological profile that underlies the potent effects of this multi-receptor agonist. Show less

Glucose-dependent insulinotropic polypeptide (GIP) is an incretin hormone that augments insulin secretion in pancreatic β-cells via its glucose-dependent insulinotropic polypeptide receptor (GIPR). Recent genome-wide association studies identified a single nucleotide variant (SNV) in the GIPR encoding gene (GIPR), rs1800437, that is associated with obesity and insulin resistance. In the present study, we tested whether GIPR variants contribute to obesity and disturb glucose homeostasis or diabetes in specific patient populations. Exon sequencing of GIPR was performed in 164 children with obesity and insulin resistance and in 80 children with paediatric-onset diabetes of unknown origin. The Study of Health in Pomerania (SHIP) cohort, comprising 8320 adults, was screened for the GIPR variant Arg217Leu. GIPR variants were expressed in COS-7 cells and cAMP production was measured upon stimulation with GIP. Cell surface expression was determined by ELISA. Protein homology modelling of the GIPR variants was performed to extract three-dimensional information of the receptor. A heterozygous missense GIPR variant Arg217Leu (rs200485112) was identified in a patient of Asian ancestry. Functional characterization of Arg217Leu revealed reduced surface expression and signalling after GIP challenge. The homology model of the GIPR structure supports the observed functional relevance of Arg217Leu. In vitro functional studies and protein homology modelling indicate a potential relevance of the GIPR variant Arg217Leu in receptor function. The heterozygous variant displayed partial co-segregation with diabetes. Based on these findings, we suggest that GIPR variants may play a role in disturbed glucose homeostasis and may be of clinical relevance in homozygous patients. Show less