199-OR: Optogenetic Control of Pancreatic Delta Cells Reveals Their Central Role in Hormone Release from Pancreatic Islets Free

Diabetes mellitus is a metabolic disorder resulting from insufficient insulin release and associated with dysregulation of glucagon secretion. Somatostatin secreted from pancreatic δ-cells is a strong paracrine factor for both insulin and glucagon secretion in islets. Here, we developed an optogenetic approach, through specific expressing light-sensitive channels/pumps in δ-cells, to investigate the paracrine control of hormone release from islets. We generated mouse models that express either a light-sensitive cation channel, channelrhodopsin-2 (ChR2), or a chloride pump, halorhodopsin (HR), specifically in the pancreatic δ-cells (SST-ChR2 and SST-HR mouse). Their specific expression was confirmed by immunostaining. Optoactivation of ChR2 at 1mM glucose rapidly depolarized δ-cells by 37mV (n=5) and stimulated electrical activity and somatostatin release. At high glucose or tolbutamide, light activation of HR repolarized δ-cells by 15 mV (n=3), suppressing action potentials. Optoactivation of SST-ChR2 δ-cells suppressed, after a variable delay depending on the distance from δ-cells, action potential firing in neighbouring α-cells, inhibited exocytosis by 30% (n=5) and glucagon secretion by 55% at 1mM glucose (n=7). In a subset (35%) α-cells with no spontaneous action potentials, opto-inhibition of SST-HR δ-cells caused a 9mV depolarization (n=7) and induced action potential firing, which may account for the stimulation of glucagon secretion. Neither optoactivation, nor opto-inhibition of δ-cells affected insulin secretion at hypo- and hyperglycaemia. However, at normoglycaemia (6 mM glucose), optoactivation of δ-cells depolarized neighbouring β-cells (via gap-junctions) and evoked action potential firing, leading to a paradoxical 160% stimulation of insulin secretion (n=9). We conclude that δ-cells play a central role in the regulation of both insulin and glucagon release but that the effects are more complex than appreciated to date.