Synaptotagmin-9 in Insulin Secretion Figure 4. p-S9X mice exhibit similar islet architecture and b-cell ultrastructure as their control mice. (A) Histological sections stained with hematoxylin and eosin (H&E). Representative sections are shown for p-S9X and control islets. N = 4 mice for each genotype. Scale bar = 50 mm. (B) Transmission electron micrographs showing b-cells of p-S9X and control pancreas. Scale bars = 2 mm. doi:10.1371/journal.pone.0015414.g004 for fast insulin release, and a high affinity sensor in charge of insulin release at low calcium concentrations. We earlier identified synaptotagmin-7 as a high affinity calcium sensor for insulin secretion, whose deletion resulted in ,40% reduction in insulin release [15]. In this study, we focused on synaptotagmin-9, the second most highly expressed calcium- binding synaptotagmin in pancreatic islets, and tested our hypothesis that synaptotagmin-9 functions as a low affinity calcium sensor in insulin granule exocytosis. Pancreas-specific synaptotagmin-9 KO mice showed no differences in body weight, glucose tolerance, or glucose-induced insulin secretion in vivo and from isolated islets when compared to control mice. Deletion of synaptotagmin-9 also had no effects on pancreas development, islet insulin content, islet architecture or b-cell ultrastructure. Furthermore, calcium-triggered insulin granule exocytosis as detected by membrane capacitance measurements was normal in the absence of synaptotagmin-9. These results demonstrated that synaptotagmin-9 was not involved in the regulation of glucose homeostasis in general, or insulin secretion in particular. Although synaptotagmin-9 exhib- ited proper calcium binding properties required for insulin secretion, our study does not support our initial hypothesis that Figure 5. Normal membrane capacitance measurements in p-S9X mouse b-cells. (A) Membrane capacitance was recorded in functionally identified b-cells from p-S9X and control mice. Capacitance increase was elicited by 500 ms depolarizations from -70 to 0 mV. (B) Capacitance increases shown as means 6 SEM. N = 7 for control mouse b-cells (white bar) and 9 for p-S9X (grey bar). doi:10.1371/journal.pone.0015414.g005 PLoS ONE | www.plosone.org 6 November 2010 | Volume 5 | Issue 11 | e15414