AIMS/HYPOTHESIS: G-protein-coupled receptors (GPCRs) play an important role in maintaining systemic glucose homeostasis by regulating insulin secretion, with protein kinase A (PKA) signalling serving as a key downstream effector. Our previous work identified specific expression of type IIB PKA in pancreatic beta cells. Based on these findings, we propose that type IIB PKA is involved in mediating the GPCR signalling in pancreatic beta cells.
METHODS: The glucagon-like peptide-1 (GLP-1) analogue liraglutide was administered to mice 30 min before glucose injection during an IPGTT, whereas the glucose levels and insulin levels were measured in wild-type and RIIβ-knockout mice. The isolated islets were subjected to both perifusion assay and static batch incubations following stimulation with liraglutide, glucagon and follicle-stimulating hormone (FSH). RNA-seq analysis was performed to identify molecular changes in islets with RIIβ ablation. Both western blotting and quantitative PCR were employed to quantify the gene expression. Whole-cell patch-clamp recordings were conducted to measure Kand Cacurrents. Insulin granule morphology and abundance were evaluated by electron microscopy and flow cytometry using EGFP-labelled Syncollin, respectively. ATP2+
RESULTS: RIIβ-knockout mice exhibited impaired glucose tolerance and attenuated insulin secretion in response to liraglutide. Islets isolated from RIIβ-knockout mice showed reduced insulin secretion following liraglutide stimulation. Similarly, RIIβ-ablated islets displayed decreased insulin secretion in response to both glucagon and FSH. Further mechanistic studies revealed that RIIβ deficiency impaired liraglutide-mediated PKA signalling activation. Specifically, RIIβ-ablated beta cells exhibited reduced basal Kchannel activity and lack of liraglutide-mediated channel inhibition. Multiple voltage-gated Cachannel genes were downregulated in RIIβ-ablated islets, leading to a mild reduction in basal Cacurrent and a significant decrease following liraglutide treatment. RIIβ-knockout beta cells also exhibited reduced insulin granule size, decreased total granule number and fewer granules docked at the plasma membrane. ATP2+ 2+
CONCLUSIONS/INTERPRETATION: Our results highlight type IIB PKA as a primary mediator of Gs-coupled receptor-potentiated insulin secretion, providing a new molecular framework for metabolic regulation research.
