ORF21-04 - Calcium-Driven Gs-to-Gq Switching of GLP-1 Receptor Signaling in Pancreatic Β-Cells under High-Glucose Conditions

Glucagon-like peptide-1 (GLP-1) is an incretin hormone that enhances glucose-stimulated insulin secretion from pancreatic β-cells. The GLP-1 receptor (GLP-1R) is a class B G protein–coupled receptor (GPCR) that couples to both Gs and Gq proteins. The insulinotropic effect of GLP-1 is attenuated under chronic hyperglycemic conditions, potentially through multiple mechanisms, including reduced GLP-1R expression. Thus, ambient glucose levels critically influence GLP-1R signaling efficacy, and glucose-dependent functional modulation of GLP-1R warrants further investigation. Recently, a novel concept termed "Gs-to-Gq switching" has been proposed, in which GLP-1R predominantly couples to Gs proteins under normoglycemic conditions, whereas elevated glucose levels shift GLP-1R signaling toward Gq coupling (Oduori et al., J Clin Invest, 2020). However, the molecular mechanism underlying this phenomenon remains unclear. In this study, we sought to elucidate the mechanism governing glucose-dependent modulation of GLP-1R signaling.
Here, we demonstrate that the apparent Gs-to-Gq switch reflects effector-biased, rather than G protein–biased, modulation, with intracellular calcium playing a central role. In the glucose-responsive β-cell line MIN6, GLP-1–induced cAMP accumulation (a Gs-dependent response) was attenuated, while IP1 accumulation (a Gq-dependent response) was enhanced under high-glucose conditions, consistent with the reported switching phenomenon. Notably, similar glucose-dependent modulation–suppression of cAMP and augmentation of IP1–was observed following stimulation with multiple ligands, including GIP and glucagon (Gs-dependent) as well as carbachol and LPI (Gq-dependent). These findings indicate that the observed signaling switch is not specific to the GLP-1R–G protein axis but is shared among diverse GPCR signaling pathways. Importantly, this glucose-induced signaling modulation was reversed by treatment with the calcium-channel blocker nifedipine or the potassium-channel opener diazoxide, both of which reduce calcium influx. This suggests a pivotal role for intracellular calcium in GPCR signaling modulation. Indeed, adenylyl cyclase (AC) and phospholipase C (PLC), the principal downstream effectors of Gs and Gq signaling, respectively, are known to be regulated by calcium ions: calcium suppresses specific AC isoforms while enhancing the activity of certain PLC isoforms. Using CRISPR-Cas9–mediated gene editing, we further confirmed the involvement of AC6 in this regulatory mechanism.
While GPCRs are generally capable of coupling to multiple G proteins, the relative strength of downstream signaling is finely tuned by cellular context. To our knowledge, this study provides the first evidence of calcium-driven, effector-level biased modulation of GPCR signaling, mediated through differential regulation of AC and PLC.