AIMS/HYPOTHESIS: Postprandial glucagon-like peptide-1 (GLP-1) secretion by enteroendocrine L cells of the gut plays an important role in glucose homeostasis, thus representing a therapeutic option of ever-growing significance for type 2 diabetes. However, the precise mechanisms linking nutrient sensing and GLP-1 secretion are incompletely understood. In this study, we focused on a potential new role for endoplasmic reticulum (ER)-mitochondria contact sites, called mitochondria-associated membranes (MAMs), in nutrient-induced GLP-1 secretion by L cells, as they are dynamically regulated by nutrients, they influence cellular calcium homeostasis crucial for hormone secretion, and their miscommunication has been implicated in alterations of glucose homeostasis in several tissues.
METHODS: We combined biochemical and imaging approaches to investigate nutrient-induced GLP-1 secretion, and ER-mitochondria interaction and calcium exchange in the STC-1 cell line, ex vivo ileal mouse organoids, and/or in vivo in gut enteroendocrine cells from Glu-Venus mice, both in acute conditions and after diet-induced obesity and type 2 diabetes.
RESULTS: We show here that ER-mitochondria interactions are dynamically induced by two GLP-1 secretagogues, glucose and deoxycholic acid (DCA), in STC-1 cells (1.8- and 2.1-fold, respectively), ileal mouse organoids (1.7- and 1.3-fold, respectively), and in vivo in colonic L cells of Glu-Venus mice (1.3- and 1.2-fold, respectively). In addition, glucose increased ER-mitochondria calcium exchange in STC-1 cells (1.2-fold). A paracrine action of secreted GLP-1 was also involved in the regulation of MAMs by glucose and DCA in STC-1 cells. Dynamic reinforcement of MAMs by glucose and DCA played a causal role in GLP-1 release, as both pharmacological and genetic disruption of organelle communication blocked L cell secretory response to the two stimuli in STC-1 cells. In agreement, depleting ER calcium levels or inhibiting mitochondrial calcium entry decreased glucose-induced GLP-1 secretion (-37.5% and -30.9%, respectively), whereas inducing ER or mitochondrial stress prevented it (-47.9% and -51.8%, respectively). Mechanistically, glucose induces ER-mitochondria communication through a sodium-glucose cotransporter 1-mediated electrogenic effect, whereas DCA acts through a Takeda G protein-coupled receptor 5 (TGR5)-cAMP-protein kinase A (PKA) pathway. Finally, we demonstrated in C57Bl/6J mice and in Glu-Venus mice that diet-induced obesity reinforced basal ER-mitochondria interactions in colonic L cells and blocked their ability to respond to oral glucose in terms of both GLP-1 secretion and MAM upregulation.
CONCLUSIONS/INTERPRETATION: These results point to a new role for ER-mitochondria calcium coupling in glucose-induced GLP-1 secretion in L cells of the gut, which is impaired in obesity and type 2 diabetes, providing a novel target for the modulation of GLP-1 secretion. Therefore, these data reinforce the potential targeting of MAMs to improve glycaemic outcomes in metabolic diseases.
