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Insulin secretion is the process whereby insulin-containing granules fuse with the plasma membrane of β-cells. Exocytosis is preceded by cargo loading, maturation and transport of the secretory granules; processes that require modification of both the protein and lipid composition of the granules. We recently identified phosphatidylinositol-4 phosphate (PI[4]P) dephosphorylation by INPP5F/Sac2 on the surface of insulin granules as a key step that precedes stable granule docking at the plasma membrane and that is required for normal insulin secretion. Here, we show that PI(4)P is used to target the lipid exchange protein oxysterol-binding protein (OSBP) to the granule surface where it is involved in PI(4)P/cholesterol exchange. Loss of Sac2 resulted in excess accumulation of cholesterol on insulin granules that was normalized when OSBP expression was reduced. Acute inhibition of OSBP resulted in dramatic cellular redistribution of OSBP to insulin granules where it colocalized with the ER-resident protein VAP-A at ER-granule contact sites. Stimulation of insulin secretion also resulted in translocation of OSBP to the insulin granule surface in a process that depended on Ca2+-induced acidification of the cytosol. Similar to Sac2 knockdown, inhibition of OSBP suppressed insulin secretion without affecting insulin production. In conclusion, we show that lipid exchange at ER-granule contacts sites is involved in the exocytic process, and propose that these contacts act as reaction centers with multimodal functions during insulin granule maturation.

A wide range of fluorescent sensors with different properties have been developed for imaging of cAMP signals in living cells and tissues. Most cAMP reporters have been designed to undergo changes in fluorescence resonance energy transfer but there are alternative techniques with advantages for certain applications. Here, we describe protocols for cAMP recordings in the sub-plasma membrane space based on detection of translocation of engineered, fluorescent protein-tagged protein kinase A subunits between the plasma membrane and the cytoplasm. Changes in reporter localization can be detected with either confocal or total internal reflection fluorescence microscopy but signal changes are more robust and image analyses less complicated with the latter technique. We show how translocation reporters can be used to study sub-plasma membrane cAMP signals, including oscillations, in insulin-secreting β-cells stimulated with glucose and G-protein-coupled receptor agonists. We also demonstrate how translocation reporters can be combined with other sensors for simultaneous recordings of the cytosolic Ca