SERCA3 Ablation Does Not Impair Insulin Secretion but Suggests Distinct Roles of Different Sarcoendoplasmic Reticulum Ca2+ Pumps for Ca2+ Homeostasis in Pancreatic β-cells

Abstract

Two sarcoendoplasmic reticulum Ca(2+)-ATPases, SERCA3 and SERCA2b, are expressed in pancreatic islets. Immunocytochemistry showed that SERCA3 is restricted to beta-cells in the mouse pancreas. Control and SERCA3-deficient mice were used to evaluate the role of SERCA3 in beta-cell cytosolic-free Ca(2+) concentration ([Ca(2+)](c)) regulation, insulin secretion, and glucose homeostasis. Basal [Ca(2+)](c) was not increased by SERCA3 ablation. Stimulation with glucose induced a transient drop in basal [Ca(2+)](c) that was suppressed by inhibition of all SERCAs with thapsigargin (TG) but unaffected by selective SERCA3 ablation. Ca(2+) mobilization by acetylcholine was normal in SERCA3-deficient beta-cells. In contrast, [Ca(2+)](c) oscillations resulting from intermittent glucose-stimulated Ca(2+) influx and [Ca(2+)](c) transients induced by pulses of high K(+) were similarly affected by SERCA3 ablation or TG pretreatment of control islets; their amplitude was increased and their slow descending phase suppressed. This suggests that, during the decay of each oscillation, the endoplasmic reticulum releases Ca(2+) that was pumped by SERCA3 during the upstroke phase. SERCA3 ablation increased the insulin response of islets to 15 mmol/l glucose. However, basal and postprandial plasma glucose and insulin concentrations in SERCA3-deficient mice were normal. In conclusion, SERCA2b, but not SERCA3, is involved in basal [Ca(2+)](c) regulation in beta-cells. SERCA3 becomes operative when [Ca(2+)](c) rises and is required for normal [Ca(2+)](c) oscillations in response to glucose. However, a lack of SERCA3 is insufficient in itself to alter glucose homeostasis or impair insulin secretion in mice.

cAMP Induces Stromal Interaction Molecule 1 (STIM1) Puncta but neither Orai1 Protein Clustering nor Store-operated Ca2+ Entry (SOCE) in Islet Cells

“…The glucose sensitivity of STIM1 retranslocation to the ER was strikingly higher in ␣-than in ␤-cells with maximal effect at 3 mM reinforcing indirect observations that this concentration is sufficient for maximal Ca 2ϩ filling of the ␣-cell ER (22). The different glucose sensitivities cannot be attributed to the fact that ␣-cells only express SERCA2b, whereas the ␤-cell also express the lower affinity Ca 2ϩ transporter SERCA3 (51). The glucose concentration dependence of ER Ca 2ϩ filling in ␤-cells is thus unaffected after SERCA3 knock-out (52).…”

Islet Secretory Defect in Insulin Receptor Substrate 1 Null Mice Is Linked With Reduced Calcium Signaling and Expression of Sarco(endo)plasmic Reticulum Ca2+-ATPase (SERCA)-2b and -3

“…One potential explanation for the selective reduction in SERCA3 but not other isoforms in the islets is the presence of independent regulatory mechanisms for each isoform that has been reported to occur in other cell types (19,21). Recently, ablation of SERCA3 in mice has been reported not to impair insulin secretion (59), whereas studies by Borge and Wolf (33) indicate that IRS-1 and SERCA-3b are colocalized in the endoplasmic reticulum and play a role in the insulin secretory process. Furthermore, IRS-1 overexpressing ␤-cells show an increase in insulin secretion that is associated with altered levels of SERCA-3b expression but without affecting the levels of SERCA-2b mRNA levels (57).…”

Ca2+-induced Ca2+ release by activation of inositol 1,4,5-trisphosphate receptors in primary pancreatic β-cells

“…Ca 2+ uptake into the -cell ER is due to a high affinity SERCA2 and a low affinity SERCA3 mechanism [58]. Whereas the high affinity mechanism explains active accumulation of Ca 2+ in the IP 3 -releasable pool at basal [Ca 2+ ] i concentrations [37], the low affinity uptake together with the leakage allows the ER to function as a "passive" Ca 2+ buffer at elevated levels of [Ca 2+ ] i [58,59]. A high Ca 2+ permeability is probably a factor enabling the ER to exert this dual function.…”