Integrated biophysical and spatial remodeling during insulin secretory granule maturation at the mitochondrial network.

Effective insulin secretion and blood glucose homeostasis depend on the multistep maturation of insulin secretory granules (ISGs), a process that includes lumen acidification, enzymatic insulin processing, and biophysical remodeling of the granule. An under studied aspect of ISG maturation is the role of inter-organelle contacts in organelle remodeling. While a correlation between ISG-mitochondria contacts and ISG maturation has been observed, many questions remain on how this interaction may impact maturation (1-5). We sought to address this gap in knowledge by using multi-scale imaging approaches (fluorescent microscopy, soft X-ray tomography, and cryo-electron tomography) to examine how the biophysical properties and spatial organization of ISGs change around the mitochondrial network. Our data suggests that ISGs in proximity to mitochondria exhibit lower pH, higher biomolecular density, and smaller vesicle diameter. Time-resolved imaging using a SNAP tag labelling system also shows that as ISGs age, their proximity to the mitochondria network is increased between 3-6 hours after biosynthesis, suggesting that ISG-mitochondria association is dynamically spatiotemporally regulated in pancreatic β-cells. These data suggest that mitochondrial proximity contributes to the maturation and remodeling of ISGs in pancreatic beta cells.