Organization and dynamics of the cortical complexes controlling insulin secretion in β-cells

Insulin secretion in pancreatic beta-cells is regulated by cortical complexes that are enriched at the sites of adhesion to extracellular matrix facing the vasculature. Many components of these complexes, including bassoon, RIM, ELKS and liprins, are shared with neuronal synapses. Here, we show that insulin secretion sites also contain the non-neuronal proteins LL5 beta (also known as PHLDB2) and KANK1, which, in migrating cells, organize exocytotic machinery in the vicinity of integrin-based adhesions. Depletion of LL5 beta or focal adhesion disassembly triggered by myosin II inhibition perturbed the clustering of secretory complexes and attenuated the first wave of insulin release. Although previous analyses in vitro and in neurons have suggested that secretory machinery might assemble through liquid-liquid phase separation, analysis of endogenously labeled ELKS in pancreatic islets indicated that its dynamics is inconsistent with such a scenario. Instead, fluorescence recovery after photobleaching and single-molecule imaging showed that ELKS turnover is driven by binding and unbinding to low-mobility scaffolds. Both the scaffold movements and ELKS exchangewere stimulated by glucose treatment. Our findings help to explain how integrin-based adhesions control spatial organization of glucose-stimulated insulin release.

Automated summary

Insulin secretion in pancreatic beta-cells is regulated by cortical complexes that are enriched at the sites of adhesion to the extracellular matrix. Non-neuronal proteins LL5 beta and KANK1 are also present at insulin secretion sites and are involved in organizing exocytotic machinery. The dynamics of ELKS, an essential component of secretory complexes, is driven by binding and unbinding to low-mobility scaffolds, which are stimulated by glucose treatment. This study provides insights into the spatial organization of glucose-stimulated insulin release.