Atp6ap2 deletion causes extensive vacuolation that consumes the insulin content of pancreatic β cells

Significance Cells maintain several mechanisms to ensure their survival, including the removal of old or damaged proteins and organelles. This process must be balanced: too little turnover results in the accumulation of cellular “junk,” while excessive removal can deplete the cell and organism of key components. Here, we show that Atp6ap2 in the pancreatic β cell is essential for insulin granule turnover, as its absence resulted in the accumulation of oversized cytosolic vacuoles which conceivably account for excessive granule degradation, and thereby leads to impaired insulin secretion and diabetes. Pancreatic β cells store insulin within secretory granules which undergo exocytosis upon elevation of blood glucose levels. Crinophagy and autophagy are instead responsible to deliver damaged or old granules to acidic lysosomes for intracellular degradation. However, excessive consumption of insulin granules can impair β cell function and cause diabetes. Atp6ap2 is an essential accessory component of the vacuolar ATPase required for lysosomal degradative functions and autophagy. Here, we show that Cre recombinase-mediated conditional deletion of Atp6ap2 in mouse β cells causes a dramatic accumulation of large, multigranular vacuoles in the cytoplasm, with reduction of insulin content and compromised glucose homeostasis. Loss of insulin stores and gigantic vacuoles were also observed in cultured insulinoma INS-1 cells upon CRISPR/Cas9-mediated removal of Atp6ap2. Remarkably, these phenotypic alterations could not be attributed to a deficiency in autophagy or acidification of lysosomes. Together, these data indicate that Atp6ap2 is critical for regulating the stored insulin pool and that a balanced regulation of granule turnover is key to maintaining β cell function and diabetes prevention.