Reversing the Signal to Stimulate Insulin Secretion

Abstract

For β cells in the pancreas to respond in a coordinated manner to changes in glucose status, cell communication is essential. In fact, the cells need to be in contact with one another for maximal secretion, because when β cells are separated (in culture, for example) insulin secretion is lower. Konstantinova et al . show that an Eph and ephrin system of cell-attached ligands and receptors mediates β cell communication regulating insulin secretion. The Eph-ephrin system exhibits bidirectional signaling, with a receptor tyrosine kinase pathway downstream of Eph (each Eph is activated by specific ephrin) and another signaling pathway downstream of the glycosylphosphatidylinositol-anchored ephrin (activated by the cognate Eph). Ephrin-A5 and EphA5 were present in higher abundance in the islets than in the surrounding exocrine tissue in mouse and human pancreas. In the MIN6 β cell line, although ephrin-A5 and EphA5 were colocalized in regions of cell contact, ephrin-A5 was more abundant at the plasma membrane and EphA5 in insulin secretory granules. Ephrin-A5 signaling was necessary for proper regulation of insulin secretion because islets from ephrin-A5 −/− mice or MIN6 cells in which ephrin-A5 was knocked down with RNAi showed increased basal insulin secretion and reduced glucose-stimulated insulin secretion. Forward signaling through the receptor tyrosine kinase EphA5 and reverse signaling through ephrin-A5 were isolated using fusion-protein ligands (ephrin-A5-Fc to stimulate EphA5 forward signaling and EphA5-FC to stimulate ephrin-A5 reverse signaling). In mouse and human pancreatic islets or in the MIN6 cells, activation of EphA5 inhibited glucose-stimulated insulin secretion, and activation of ephrin-A5 enhanced glucose-stimulated insulin secretion. Secretion was monitored by measuring released insulin and by monitoring vesicle fusion by total internal reflection-fluorescence microscopy, thereby showing that Eph-ephrin signaling regulates vesicle release, not insulin production. Consistent with a role in regulating vesicle fusion, EphA5 forward signaling increased F-actin polymerization (known to inhibit vesicle fusion), whereas ephrin-A5 reverse signaling promoted F-actin rearrangements and decreased F-actin intensity. The cytoskeletal regulator Rac1 was involved in Eph-ephrin regulation of insulin secretion, with ephrin-A5 stimulating Rac1 activity and EphA5 inhibiting Rac1 activity. How, if these two proteins are present on adjacent cells and delivering opposing signals, does one outsignal the other? Dephosphorylation of EphA5 in response to glucose appears to tip the balance in favor of the stimulatory reverse signaling through ephrin-A5. Kulkarni and Kahn discuss the implications of this work. I. Konstantinova, G. Nikolova, M. Ohara-Imaizumi, P. Meda, T. Kučera, K. Zarbalis, W. Wurst, S. Nagamatsu, E. Lammert, EphA-ephrin-A-mediated β cell communication regulates insulin secretion from pancreatic islets. Cell 129 , 359-370 (2007). [Online Journal] R. N. Kulkarni, C. R. Kahn, Ephs and ephrins keep pancreatic β cells connected. Cell 129 , 241-243 (2007). [Online Journal]