Insulin secretion from pancreatic β cells is regulated by the blood glucose concentration and occurs through Ca(2+)-triggered exocytosis. The activities of multiple ion channels in the β cell plasma membrane are required to fine-tune insulin secretion in order to maintain normoglycemia. Phosphoinositide lipids in the plasma membrane often gate ion channels, and variations in the concentration of these lipids affect ion-channel open probability and conductance. Using light-regulated synthesis or depletion of plasma membrane phosphatidylinositol 4,5-bisphosphate (PI[4,5]P2), we found that this lipid positively regulated both depolarization- and glucose-triggered Ca(2+) influx in a dose-dependent manner. Small reductions of PI(4,5)P2 caused by brief illumination resulted in partial suppression of Ca(2+) influx that followed the kinetics of the lipid, whereas depletion resulted in marked inhibition of both Ca(2+) influx and insulin secretion.

Endoplasmic reticulum (ER)—plasma membrane (PM) contacts are dynamic structures with important roles in the regulation of calcium (Ca2+) and lipid homeostasis. The extended synaptotagmins (E‐Syts) are ER‐localized lipid transport proteins that interact with PM phosphatidylinositol 4,5‐bisphosphate in a Ca2+‐dependent manner. E‐Syts bidirectionally transfer glycerolipids, including diacylglycerol (DAG), between the 2 juxtaposed membranes, but the biologic significance of this transport is still unclear. Using insulin‐secreting cells and live‐cell imaging, we now show that Ca2+‐triggered exocytosis of insulin granules is followed, in sequence, by PM DAG formation and E‐Syt1 recruitment. E‐Syt1 counteracted the depolarization‐induced DAG formation through a mechanism that required both voltage‐dependent Ca2+ influx and Ca2+ release from the ER. E‐Syt1 knockdown resulted in prolonged accumulation of DAG in the PM, resulting in increased glucose‐stimulated insulin secretion. We conclude that Ca2+‐triggered exocytosis is temporally coupled to Ca2+‐triggered E‐Syt1 PM recruitment and removal of DAG to negatively regulate the same process.—Xie, B., Nguyen, P. M., Idevall‐Hagren, O. Feedback regulation of insulin secretion by extended synaptotagmin‐1. FASEB J. 33, 4716–4728 (2019). http://www.fasebj.org

Nguyen et al. find that the PI(4)P phosphatase Sac2 localizes to insulin granules, where it controls granule docking to the plasma membrane. Loss of Sac2 results in impaired insulin secretion, and Sac2 mRNA levels are reduced in type 2 diabetes.

The primary cilium is an organelle present in most adult mammalian cells that is considered as an antenna for sensing the local microenvironment. Here, we use intact mouse pancreatic islets of Langerhans to investigate signaling properties of the primary cilium in insulin-secreting β-cells. We find that GABAB1 receptors are strongly enriched at the base of the cilium, but are mobilized to more distal locations upon agonist binding. Using cilia-targeted Ca2+ indicators, we find that activation of GABAB1 receptors induces selective Ca2+ influx into primary cilia through a mechanism that requires voltage-dependent Ca2+ channel activation. Islet β-cells utilize cytosolic Ca2+ increases as the main trigger for insulin secretion, yet we find that increases in cytosolic Ca2+ fail to propagate into the cilium, and that this isolation is largely due to enhanced Ca2+ extrusion in the cilium. Our work reveals local GABA action on primary cilia that involves Ca2+ influx and depends on restricted Ca2+ diffusion between the cilium and cytosol.

The primary cilium is an organelle present in most adult mammalian cells and is thought of as an antenna for detection of a variety of signals. Here we use intact mouse pancreatic islets of Langerhans to investigate signalling properties of the primary cilium in β-cells. Using cilia-targeted Ca2+ indicators we find that the resting Ca2+ concentration in the cilium is lower than that of the cytosol, and we uncover a Ca2+ extrusion mechanism in the cilium that effectively insulates the cilium from changes in cytosolic Ca2+. Stimuli that give rise to pronounced cytosolic Ca2+ concentration increases, such as glucose- and depolarization-induced Ca2+ influx, and mobilization of Ca2+ from the ER, was accompanied by minor increases in cilia Ca2+ concentrations that were spatially restricted to a small compartment at the base. Conversely, we observe pronounced Ca2+ concentration changes in the primary cilia of islet β-cells that do not propagate into the cytosol and show that paracrine GABA signalling via cilia-localized GABA- B1-receptors is responsible for this Ca2+ signalling. Finally, we demonstrate that the cilia response to GABA involves ligand-dependent transport of GABA-B1 receptors into the cilium.