(A) Whole islets from SNAP23^fl/fl mice (24) show that SNAP23 is abundant in β cells (confocal imaging), shown more clearly in the enlarged box 1. Scale bars: 100 μm. Quantitation can be found in Supplemental Figure 1A, bottom left. (B) Single islet β cells from human (first row) and mouse (second row) show that SNAP23 is abundant in the insulin SGs. SNAP23 is partly colocalized with Stx-1A (third row) and SNAP25 (fourth row) on the PM. Scale bars: 5 μm. Quantitation can be found in Supplemental Figure 1A, bottom middle. SNAP23 antibody controls (without primary antibody) are shown in Supplemental Figure 1A, top left, and A, top right. (C) SNAP23 is also present in glucagon-containing α cells located in the periphery of the mouse islet. Scale bars: 10 μm. Quantitation in Supplemental Figure 1A, bottom right. (D) AAV8-RIP1-Cre drives Cre expression only in β cells (top) and not in α cells (bottom). Scale bars: 100 μm. Note the cytosolic insulin surrounding the nuclear Cre shown clearly in the enlarged box 2. (E) Efficient knockdown of islet β cell SNAP23 expression is shown in (top) where Cre-positive cells are SNAP23-negative, indicating SNAP23 deletion in those cells, and (bottom) the majority of insulin-positive cells are SNAP23-negative. The few SNAP23-positive cells are likely α cells. Scale bars: 100 μm. (F) Western blots of islets from SNAP23^fl/fl mice injected with the AAV8 (βSNAP23-KO) or not (Control) showed reduction of SNAP23 but not other exocytotic proteins. SNAP23 and VAMP8 are not abundant in mouse brain. Blots are representative from 3 independent experiments; analysis of n = 3 in Supplemental Figure 1B. (G) βSNAP23-KO versus SNAP23^fl/fl (Control) mice show SNAP23 to be reduced only in islets but not in fat, muscle, or liver, wherein SNAP23 and cognate Munc18c and Stx-4 are putative exocytotic proteins. Shown are representative of 3 independent experiments; analysis of n = 3 in Supplemental Figure 1C.