Abstract
Insulin secretion is the process whereby insulin-containing granules fuse with the plasma membrane of pancreatic β-cells. Exocytosis is preceded by cargo loading and granule biogenesis at the Golgi, followed by maturation and transport of the secretory granules; processes that require modification of both the protein and lipid composition of the granules. Here, we show that insulin-containing secretory granules form physical contacts with the endoplasmic reticulum. The lipid exchange protein OSBP dynamically redistributes to ER-SG contacts in a process regulated by Ca2+and cytosolic pH, and contributes to cholesterol loading of the granules. This process depends on granular PI(4)P and ER-localized VAPs, and is positively regulated by granular PI4-kinases and negatively regulated by granule-localized Sac2. Loss of Sac2 results in excess accumulation of cholesterol on insulin granules that is normalized when OSBP expression is reduced, and both acute inhibition and siRNA-mediated knockdown of OSBP suppresses glucose-stimulated insulin secretion without affecting insulin production or intracellular Ca2+ signaling. In conclusion, we show that lipid exchange at ER-granule contact sites is involved in the exocytic process, and propose that these contacts act as reaction centers with multimodal functions during insulin granule maturation.
Publisher
Cold Spring Harbor Laboratory
Reference67 articles.
1. LABKIT: Labeling and Segmentation Toolkit for Big Image Data;Front. Comput. Sci,2022
2. A Plasma Membrane Pool of Phosphatidylinositol 4-Phosphate Is Generated by Phosphatidylinositol 4-Kinase Type-III Alpha: Studies with the PH Domains of the Oxysterol Binding Protein and FAPP1;Mol. Biol. Cell,2008
3. Barg, S. , Eliasson, L. , Renström, E. and Rorsman, P. (2002). A subset of 50 secretory granules in close contact with L-type Ca2+ channels accounts for first-phase insulin secretion in mouse β-cells. Diabetes 51,.
4. Syntaxin clusters assemble reversibly at sites of secretory granules in live cells