α-Synuclein binds the KATP channel at insulin-secretory granules and inhibits insulin secretion

Geng X, Lou H, Wang J, Li L, Swanson AL, Sun M, Beers-Stolz D, Watkins S, Perez RG, Drain P. alpha-Synuclein binds the K-ATP channel at insulin-secretory granules and inhibits insulin secretion. Am J Physiol Endocrinol Metab 300: E276-E286, 2011. First published September 21, 2010; doi: 10.1152/ajpendo.00262.2010.-alpha-Synuclein has been studied in numerous cell types often associated with secretory processes. In pancreatic beta-cells, alpha-synuclein might therefore play a similar role by interacting with organelles involved in insulin secretion. We tested for alpha-synuclein localizing to insulin-secretory granules and characterized its role in glucose-stimulated insulin secretion. Immunohistochemistry and fluorescent sulfonyl-ureas were used to test for alpha-synuclein localization to insulin granules in beta-cells, immunoprecipitation with Western blot analysis for interaction between alpha-synuclein and K-ATP channels, and ELISA assays for the effect of altering alpha-synuclein expression up or down on insulin secretion in INS1 cells or mouse islets, respectively. Differences in cellular phenotype between alpha-synuclein knockout and wild-type beta-cells were found by using confocal microscopy to image the fluorescent insulin biosensor Ins-C-emGFP and by using transmission electron microscopy. The results show that anti-alpha-synuclein antibodies labeled secretory organelles within beta-cells. Anti-alpha-synuclein antibodies colocalized with K-ATP channel, anti-insulin, and anti-C-peptide antibodies. alpha-Synuclein coimmunoprecipitated in complexes with K-ATP channels. Expression of alpha-synuclein downregulated insulin secretion at 2.8 mM glucose with little effect following 16.7 mM glucose stimulation. alpha-Synuclein knockout islets upregulated insulin secretion at 2.8 and 8.4 mM but not 16.7 mM glucose, consistent with the depleted insulin granule density at the beta-cell surface membranes observed in these islets. These findings demonstrate that alpha-synuclein interacts with K-ATP channels and insulin-secretory granules and functionally acts as a brake on secretion that glucose stimulation can override. alpha-Synuclein might play similar roles in diabetes as it does in other degenerative diseases, including Alzheimer's and Parkinson's diseases.