Piccolo, a Ca2+ Sensor in Pancreatic β-Cells

Abstract

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 β-cells. The action of the cAMP-GEFII·Rim2 complex requires both intracellular cAMP and Ca2+. Although Rim2 has C2 domains, its role as a Ca2+ sensor has remained unclear. In the present investigation, we have discovered that Piccolo, a CAZ (cytoskeletal matrix associated with the active zone) protein in neurons that is structurally related to Rim2, also binds to cAMP-GE-FII 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 β-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 β-cells.