The classic opinion regarding PI(3)P’s role in intracellular trafficking is that it is synthesized and functions primarily on early endosomes. Several recent publications have highlighted a potential second role for this lipid in exocytosis. Extending previous work by this group and others on the positive role of PI3K-C2a in neurosecretory pathways (1,2) and GLUT4 exoctyosis (3,4) this paper from Tania Maffucci's group interrogates the role of this lipid kinase in insulin secretion in INS1 cells. Combined these results suggest a general role for PI3K-C2a and its product, PI(3)P in exocytotic events.
This group has been investigating the role of the class II PI3K isoforms in exocytosis in a variety of systems. In this paper they investigate the role of PI3K-C2a in insulin secretion using INS1 rat insulinoma cells as a model. Stable knockdown cells did not show any defects in proliferation, calcium signaling, intracellular insulin levels or the expression levels and sub-cellular localization of exocytotic proteins. The PI3K-C2a knockdown cells did however show significant defects in insulin secretion, stimulated by either a secretagogue cocktail or potassium chloride.
Mechanistically, the authors show that there is no defect in insulin granules proximal to the plasma membrane at the resting state. They do detect a decrease in the amount of SNAP25 hydrolysis induced by the secretagogue cocktail. SNAP25 hydrolysis has been proposed to be an important step in the fusion of exocytic vesicles (5). This proteolytic event has not been established as a major mechanism in exocytosis, and it may only correlate with defects in PI3K-C2a signaling. However if reduced SNAP25 proteolysis is the mechanistic defect resulting from PI3K-C2a knockdown, then this suggests a role in protease regulation by PI(3)P or another PI(3)P-derived molecule as a key part of the general exocytotic machinery. It is also possible that there is another, as of yet unstudied role of PI3K-C2a and PI(3)P in exocytosis.
Dominguez, V., Raimondi, C., Somanath, S., Bugliani, M., Loder, M., Edling, C., Divecha, N., da Silva-Xavier, G., Marselli, L., Persaud, S., Turner, M., Rutter, G., Marchetti, P., Falasca, M., & Maffucci, T. (2010). Class II Phosphoinositide 3-Kinase Regulates Exocytosis of Insulin Granules in Pancreatic Cells Journal of Biological Chemistry, 286 (6), 4216-4225 DOI: 10.1074/jbc.M110.200295
References
(1) Meunier, Frederic, Shona Osborne, Gerald Hammond, Frank Cooke, Peter Parker, Jan Domin, and Giampietro Schiavo. “Phosphatidylinositol 3-Kinase C2{alpha} Is Essential for ATP-dependent Priming of Neurosecretory Granule Exocytosis.” Molecular Biology of the Cell 16, no. 10 (2005): 4841-4851. PubMed, DOI.
(2) Wen, Peter J, Shona L Osborne, Isabel C Morrow, Robert G Parton, Jan Domin, and Frederic A Meunier. “Ca2+-regulated pool of phosphatidylinositol-3-phosphate produced by phosphatidylinositol 3-kinase C2alpha on neurosecretory vesicles.” Molecular biology of the cell 19, no. 12 (December 2008): 5593-603. PMC, PubMed, DOI.
(3) Maffucci, Tania, Anna Brancaccio, Enza Piccolo, Robert C Stein, and Marco Falasca. “Insulin induces phosphatidylinositol-3-phosphate formation through TC10 activation.” The EMBO journal 22, no. 16 (August 15, 2003): 4178-89. PubMed, DOI.
(4) Falasca, Marco, William E Hughes, Veronica Dominguez, Gianluca Sala, Florentia Fostira, Michelle Q Fang, Rosanna Cazzolli, Peter R Shepherd, David E James, and Tania Maffucci. “The role of phosphoinositide 3-kinase C2alpha in insulin signaling.” The Journal of biological chemistry 282, no. 38 (September 21, 2007): 28226-36. PubMed, DOI.
