ABSTRACT
The extracellular matrix (ECM) critically affects ß cell functions via integrin activation. But whether these ECM actions drive the spatial organization of ß cells, as they do in epithelial cells, is unknown. Here, we show that within islets of Langerhans, focal adhesion activation in ß cells occurs exclusively where they contact the capillary ECM (vascular face). In cultured ß cells, 3D mapping shows enriched insulin granule fusion where the cells contact ECM-coated coverslips, which depends on ß1 integrin receptor activation. Culture on micro-contact printed stripes of E-cadherin and fibronectin shows that ß cell contact at the fibronectin stripe selectively activates focal adhesions and enriches exocytic machinery and insulin granule fusion. Culture of cells in high glucose, as a model of glucotoxicity, abolishes granule targeting. We conclude that local integrin activation targets insulin secretion to the islet capillaries. This mechanism might be important for islet function and may change in disease.
Subject(s)
Insulin-Secreting Cells/metabolism , Islets of Langerhans/metabolism , Animals , Cadherins/metabolism , Cell Adhesion/physiology , Cell Line, Tumor , Exocytosis/physiology , Extracellular Matrix/metabolism , Focal Adhesions/metabolism , Insulin/metabolism , Insulin Secretion/physiology , Integrins/metabolism , Male , Mice , Mice, Inbred C57BLABSTRACT
AIMS/HYPOTHESIS: We set out to test the hypothesis that insulin secretion from beta cells is targeted towards the vasculature. METHODS: The spatial location of granule fusion was identified by live-cell two-photon imaging of mouse pancreatic beta cells within intact islets, using sulforhodamine B labelling. Three-dimensional (3D) immunofluorescence of pancreatic slices was used to identify the location of proteins associated with neuronal synapses. RESULTS: We demonstrated an asymmetric, non-random, distribution of sites of insulin granule fusion in response to glucose and focal targeting of insulin granule secretion to the beta cell membrane facing the vasculature. 3D immunofluorescence of islets showed that structural proteins, such as liprin, piccolo and Rab2-interacting molecule, normally associated with neuronal presynaptic targeting, were present in beta cells and enriched at the vascular face. In contrast, we found that syntaxin 1A and synaptosomal-associated protein 25 kDa (SNAP25) were relatively evenly distributed across the beta cells. CONCLUSIONS/INTERPRETATION: Our results show that beta cells in situ, within intact islets, are polarised and target insulin secretion. This evidence for an 'endocrine synapse' has wide implications for our understanding of stimulus-secretion coupling in healthy islets and in disease.
