Rab26 restricts insulin secretion via sequestering Synaptotagmin-1

Abstract Rab26 is known to regulate multiple membrane trafficking events, but its role in insulin secretion of pancreatic β-cells remains unclear despite it was first identified in the pancreas. In this study, we generated Rab26−/− mice through CRISPR/Cas9 technique. Surprisingly, insulin levels in the blood of the Rab26−/− mice do not decrease upon glucose stimulation, but conversely increases. Deficiency of Rab26 promotes insulin secretion, which was independently verified by Rab26 knockdown in pancreatic β-cells. Conversely, over-expression of Rab26 suppresses insulin secretion in both β-cell lines and isolated mouse islets. Islets over-expressing Rab26, upon transplantation, also failed to restore glucose homeostasis in type 1 diabetic mice. Immuno-fluorescence microscopy revealed that over-expression of Rab26 results in clustering of insulin granules. GST-pulldown experiments reveal Rab26 interacts with synaptotagmin-1(Syt1) through directly binding to its C2A domain. This interaction interferes with the interaction between Syt1 and SNAP25. TIRF microscopy revealed Rab26 inhibits the exocytosis of newcomer insulin granules. Our results suggest that Rab26 serves as a negative regulator of insulin secretion, via suppressing insulin granule fusion with plasma membrane through sequestering Syt1.