Piccolo, a Ca2+ sensor in pancreatic β-cells -: Involvement of cAMP-GEFII•Rim2•Piccolo complex in cAMP-dependent exocytosis

We have previously shown that cAMP-binding protein cAMP-guanidine nucleotide exchange factor II (GEFII) (or Epac2) interacting with Rim2 is involved in cAMP-dependent, protein kinase A-independent exocytosis in pancreatic beta-cells. The action of the cAMP-GEFII.Rim2 complex requires both intracellular cAMP and Ca2+. Although Rim2 has C-2 domains, its role as a Ca2+ sensor has remained unclear. In the present investigation, we have discovered that Piccolo, a CAZ ((c) under bar ytoskeletal matrix associated with the (a) under bar ctive (z) under bar one) protein in neurons that is structurally related to Rim2, also binds to cAMP-GEFII and that it forms both homodimer and heterodimer with Rim2 in a Ca2+-dependent manner, whereas Rim2 alone does not form the homodimer. The association of Piccolo.Rim2 heterodimerization is stronger than Piccolo.Piccolo homodimerization. Treatment of pancreatic islets with antisense oligodeoxynucleotides against Piccolo inhibits insulin secretion induced by cAMP analog 8-bromo-cyclic AMP plus high glucose stimulation. These results suggest that Piccolo serves as a Ca2+ sensor in exocytosis in pancreatic beta-cells and that the formation of a cAMP-GEFII.Rim2.Piccolo complex is important in cAMP-induced insulin secretion. In addition, this study suggests that CAZ proteins similar to those in neurons are also function in pancreatic beta-cells.