Optogenetic Analysis of Depolarization-Dependent Glucagonlike Peptide-1 Release

04 Oct 2017

Incretin hormones play an important role in the regulation of food intake and glucose homeostasis. Glucagon-like peptide-1 (GLP-1) secreting cells have been demonstrated to be electrically excitable and to fire action potentials (APs) with increased frequency in response to nutrient exposure. However, nutrients can also be metabolised or activate G-protein-coupled receptors, thus potentially stimulate GLP-1 secretion independent of their effects on the plasma membrane potential. Here we used channelrhodopsins to manipulate the membrane potential of GLUTag cells, a well established model of GLP-1 secreting enteroendocrine L-cells. Using channelrhodopsins with fast or slow on/off kinetics (CheTA and SSFO, respectively), we found that trains of light pulses could trigger APs and calcium elevation in GLUTag cells stably expressing either CheTA or SSFO. Tetrodotoxin reduced light-triggered AP frequency but did not impair calcium responses, whereas further addition of the calcium channel blockers nifedipine and ω-conotoxin GVIA abolished both APs and calcium transients. Light pulse trains did not trigger GLP-1 secretion from CheTA-expressing cells under basal conditions, but were an effective stimulus when cAMP concentrations were elevated by forskolin plus IBMX. In SSFO-expressing cells, light-stimulated GLP-1 release was observed at resting and elevated cAMP concentrations and was blocked by nifedipine plus ω-conotoxin GVIA but not tetrodotoxin. We conclude that cAMP elevation or cumulative membrane depolarisation triggered by SSFO enhance the efficiency of light-triggered action potential firing, voltage gated calcium entry and GLP-1 secretion.