Purification of time-resolved insulin granules reveals proteomic and lipidomic changes during granule aging

Endocrine cells employ regulated exocytosis of secretory granules to secrete hormones and neurotransmitters. Secretory granule exocytosis depends on spatiotemporal variables such as proximity to the plasma membrane and age, with newly generated granules being preferentially released. Despite recent advances, we lack a comprehensive view of the molecular composition of insulin granules and associated changes over their lifetime. Here, we report a strategy for the purification of insulin secretory granules of distinct age from insulinoma INS-1 cells. Tagging the granule-resident protein phogrin with a cleavable CLIP tag, we obtain intact fractions of age-distinct granules for proteomic and lipidomic analyses. We find that the lipid composition changes over time, along with the physical properties of the membrane, and that kinesin-1 heavy chain (KIF5b) as well as Ras-related protein 3a (RAB3a) associate preferentially with younger granules. Further, we identify the Rho GTPase-activating protein (ARHGAP1) as a cytosolic factor associated with insulin granules.

A fluorescent timer reporter enables sorting of insulin secretory granules by age

Within the pancreatic β-cells, insulin secretory granules (SGs) exist in functionally distinct pools, displaying variations in motility as well as docking and fusion capability. Current therapies that increase insulin secretion do not consider the existence of these distinct SG pools. Accordingly, these approaches are effective only for a short period, with a worsening of glycemia associated with continued decline in β-cell function. Insulin granule age is underappreciated as a determinant for why an insulin granule is selected for secretion and may explain why newly synthesized insulin is preferentially secreted from β-cells. Here, using a novel fluorescent timer protein, we aimed to investigate the preferential secretion model of insulin secretion and identify how granule aging is affected by variation in the β-cell environment, such as hyperglycemia. We demonstrate the use of a fluorescent timer construct, syncollin-dsRedE5TIMER, which changes its fluorescence from green to red over 18 h, in both microscopy and fluorescence-assisted organelle-sorting techniques. We confirm that the SG-targeting construct localizes to insulin granules in β-cells and does not interfere with normal insulin SG behavior. We visualize insulin SG aging behavior in MIN6 and INS1 β-cell lines and in primary C57BL/6J mouse and nondiabetic human islet cells. Finally, we separated young and old insulin SGs, revealing that preferential secretion of younger granules occurs in glucose-stimulated insulin secretion. We also show that SG population age is modulated by the β-cell environment in vivo in the db/db mouse islets and ex vivo in C57BL/6J islets exposed to different glucose environments.