Ca2+ is the key intracellular regulator of insulin secretion, acting in the β-cell as the ultimate trigger for exocytosis. In response to high glucose, ATP-sensitive K+ channel closure and plasma membrane depolarization engage a sophisticated machinery to drive pulsatile cytosolic Ca2+ changes. Voltage-gated Ca2+ channels, Ca2+ -activated K+ channels and Na+ /Ca2+ exchange all play important roles. The use of targeted Ca2+ probes has revealed that during each cytosolic Ca2+ pulse, uptake of Ca2+ by mitochondria, endoplasmic reticulum (ER), secretory granules and lysosomes fine-tune cytosolic Ca2+ dynamics and control organellar function. For example, changes in the expression of the Ca2+ -binding protein Sorcin appear to provide a link between ER Ca2+ levels and ER stress, affecting β-cell function and survival. Across the islet, intercellular communication between highly interconnected "hubs," which act as pacemaker β-cells, and subservient "followers," ensures efficient insulin secretion. Loss of connectivity is seen after the deletion of genes associated with type 2 diabetes (T2D) and follows metabolic and inflammatory insults that characterize this disease. Hubs, which typically comprise ~1%-10% of total β-cells, are repurposed for their specialized role by expression of high glucokinase (Gck) but lower Pdx1 and Nkx6.1 levels. Single cell-omics are poised to provide a deeper understanding of the nature of these cells and of the networks through which they communicate. New insights into the control of both the intra- and intercellular Ca2+ dynamics may thus shed light on T2D pathology and provide novel opportunities for therapy.
Keywords: Ca2+; connectivity; imaging; insulin; organelle.
© 2017 John Wiley & Sons Ltd.