ER calcium stores contribute to glucose-induced Ca2+ waves and intercellular connectivity in mouse pancreatic islets
Defective insulin secretion is a hallmark of diabetes mellitus. Glucose-induced Ca2+ oscillations are critical for the stimulation of insulin secretion, though the mechanisms through which these propagate across the islet are poorly understood. Here, we use beta cell-targeted GCaMP6f to explore the role of endoplasmic reticulum (ER) Ca2+ mobilization in response to submaximal (11mM) and hyperglycemic (25mM) glucose concentrations. Inhibition of inositol 1,4,5 trisphosphate (IP3) receptors, and other ion channels, with 2-aminoethoxydiphenyl borate (2-APB) had minimal effects on the initial peak or intercellular connectivity provoked by 11mM glucose. However, 2-APB lowered subsequent glucose-induced cytosolic Ca2+ increases and connectivity at both 11 and 25mM glucose. Unexpectedly, the activation of IP3 receptors with the muscarinic acetylcholine receptor agonist carbachol had minimal impact on the initial peak elicited by 11 mM glucose, but Ca2+ waves at 11 and 25 mM glucose were more poorly coordinated. To determine whether ER calcium mobilization was sufficient to initiate Ca2+ waves we next blocked sarco(endo)plasmic Ca2+ ATPase (SERCA) pumps with thapsigargin, whilst preventing plasma membrane depolarization with the KATP-channel opener, diazoxide. Under these conditions, an initial cytosolic Ca2+ increase was followed by secondary Ca2+ waves that slowly subsided. The application of carbachol alongside diazoxide still enhanced Ca2+ dynamics, though this activity was uncoordinated and beta cells were poorly connected. Our results show that ER Ca2+ mobilization plays a relatively minor role in the initiation and propagation of Ca2+ waves in response to glucose. On the other hand, ER stores are required to transition to sustained Ca2+ waves. HighlightsO_LIIP3R inhibition or activation perturbs glucose-induced Ca2+ waves in islets C_LIO_LIER store mobilization is insufficient to generate Ca2+ waves C_LIO_LIER Ca2+ stores are required for sustained Ca2+ waves and beta cell connectivity C_LI GRAPHICAL ABSTRACT O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=171 SRC="FIGDIR/small/643121v1_ufig1.gif" ALT="Figure 1"> View larger version (35K): org.highwire.dtl.DTLVardef@e7ed00org.highwire.dtl.DTLVardef@78db10org.highwire.dtl.DTLVardef@7627e4org.highwire.dtl.DTLVardef@1351300_HPS_FORMAT_FIGEXP M_FIG C_FIG