Epac2 signaling at the β-cell plasma membrane
Sammanfattning: Secretion of appropriate amounts of insulin from pancreatic β-cells is crucial for glucose homeostasis. The β-cells release insulin in response to glucose and other nutrients, hormones and neurotransmitters, which trigger intracellular signaling cascades, that result in exocytotic fusion of insulin-containing vesicles with the plasma membrane. Increases of the intracellular concentration of calcium ions ([Ca2+]i) trigger exocytosis, whereas the messenger cyclic adenosine monophosphate (cAMP) amplifies various steps of the secretion process. The protein Epac2 mediates some effects of cAMP, but little is known about its regulation in β-cells. In this study, the spatio-temporal dynamics of Epac2 was investigated in insulin-secreting MIN6-cells and primary β-cells using various cell signaling biosensors and live-cell fluorescence microscopy approaches. Increases in the cAMP concentration triggered translocation of Epac2 from the cytoplasm to the plasma membrane. Oscillations of cAMP induced by glucose and the insulin-releasing hormone GLP-1 were associated with cyclic translocation of Epac2. Analyses of Epac2 mutants showed that the high-affinity cyclic nucleotide-binding domain and Ras-association domains were crucial for the translocation, whereas neither the DEP domain, nor the low-affinity cAMP-binding domain were required for membrane binding. However, the latter domain targeted Epac2 to insulin granules at the plasma membrane, which promoted their priming for exocytosis. Depolarization-induced elevations of [Ca2+]i also stimulated Epac2 translocation, but the effects were complex and in the presence of high cAMP concentrations, [Ca2+]i increases often reduced membrane binding. The stimulatory effect of Ca2+ was mediated by increased Ras activity, while the inhibitory effect reflected reduced concentrations of the membrane phospholipid PtdIns(4,5)P2. Anti-diabetic drugs of the sulfonylurea class, suggested to directly activate Epac2, induced translocation indirectly by depolarizing β-cells to increase [Ca2+]i. Epac2 is an activator of Rap GTPases, and its translocation increased Rap activity at the plasma membrane. It is concluded that the subcellular localization of Epac2 is controlled by a complex interplay between cAMP, Ca2+ and PtdIns(4,5)P2 and that the protein controls insulin release by binding to the exocytosis machinery. These results provide new insights into the regulation of β-cell function and may facilitate the development of new anti-diabetic drugs that amplify insulin secretion.
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