Directed insulin secretion from beta cells occurs at cortical sites devoid of microtubules at the edges of ELKS/LL5β patches

To maintain normal blood glucose levels, pancreatic beta cells secrete insulin into the bloodstream at specialized regions at the cell periphery, often called secretion hot spots. While many secretory machinery components are located all over the cell membrane, directed secretion relies on distinct cortical patches of the scaffolding protein ELKS and the microtubule (MT)-anchoring protein LL5{beta}. However, using TIRF microscopy of intact mouse islets to precisely localize secretion events within ELKS/LL5{beta} patches, we now show that secretion is restricted to only 5% of ELKS/LL5{beta} patch area. Moreover, the majority of secretion occurs at the margins of ELKS patches. This suggests that additional factor(s) must be responsible for hot spot definition. Because the MT cytoskeleton plays a regulatory role in the insulin secretion process via both delivery and removal of secretory granules from the secretion sites, we test whether local MT organization defines secretory activity at hot spots. We find that the majority of secretion events occur at regions devoid of MTs. Based on our findings, we present a model in which local MT disassembly and optimal ELKS content are strong predictors of directed insulin secretion. Significance StatementO_LIInsulin has to be secreted directly into the bloodstream for efficient regulation of glucose metabolism. Molecular requirements for precise secretion location and microtubule-mediated regulation in directed secretion are uncharacterized. C_LIO_LIUsing intact mouse islets and a live insulin secretion assay, we demonstrate that cortical patches containing ELKS and LL5{beta} display secretory heterogeneity and secretion hot spots are precisely localized to patch edges that are devoid of microtubules. C_LIO_LIThese findings suggest that microtubule absence is critical for secretion and secretion occurs away from presumed sites of secretory granule delivery. This expands the current knowledge of regulation and spatial characteristics of insulin secretion. C_LI