Glucose and NAADP trigger elementary intracellular β-cell Ca2+signals

Pancreatic β-cells release insulin upon a rise in blood glucose. The precise mechanisms of stimulus-secretion coupling, and its failure in Diabetes Mellitus Type 2, remain to be elucidated. The consensus model, as well as a class of currently prescribed anti-diabetic drugs, are based around the observation that glucose-evoked ATP production in β-cells leads to closure of cell membrane ATP-gated potassium (K ATP ) channels, plasma membrane depolarisation, Ca 2+ influx, and finally the exocytosis of insulin granules (Ashcroft et al., 1984; Cook and Hales, 1984). However, it has been demonstrated by the inactivation of this pathway using genetic and pharmacological means that closure of the K ATP channel alone may not be sufficient to explain all β-cell responses to glucose elevation (Henquin, 1998; Seghers et al., 2000). Here we show using total internal reflection fluorescence (TIRF) microscopy (Axelrod, 1981) that glucose as well as the Ca 2+ mobilising messenger nicotinic acid adenine dinucleotide phosphate (NAADP), known to operate in β-cells (Johnson and Misler, 2002; Masgrau et al., 2003), lead to highly localised elementary intracellular Ca 2+ signals. These were found to be obscured by measurements of global Ca 2+ signals and the action of powerful SERCA-based sequestration mechanisms at the endoplasmic reticulum (ER). This is the first demonstration of elemental Ca 2+ signals in response to NAADP, although they have been suspected (Davis et al., 2020). Optical quantal analysis of these events reveals a unitary event amplitude equivalent to that of known elementary Ca 2+ signalling events, inositol trisphosphate (IP 3 ) receptor mediated blips (Parker et al., 1996; Parker and Ivorra, 1990), and ryanodine receptor mediated sparks (Cheng et al., 1993). We propose that a mechanism based on these highly localised intracellular Ca 2+ signalling events mediated by NAADP may initially operate in β-cells when they respond to elevations in blood glucose.