cAMP-GEFII is a direct target of cAMP in regulated exocytosis

Abstract

Although cAMP is well known to regulate exocytosis in many secretory cells, its direct target in the exocytotic machinery is not known. Here we show that cAMP-GEFII, a cAMP sensor, binds to Rim (Rab3-interacting molecule, Rab3 being a small G protein) and to a new isoform, Rim2, both of which are putative regulators of fusion of vesicles to the plasma membrane. We also show that cAMP-GEFII, through its interaction with Rim2, mediates cAMP-induced, Ca2+-dependent secretion that is not blocked by an inhibitor of cAMP-dependent protein kinase (PKA). Accordingly, cAMP-GEFII is a direct target of cAMP in regulated exocytosis and is responsible for cAMP-dependent, PKA-independent exocytosis.

Cellular expression of Noc2, a Rab effector protein, in endocrine and exocrine tissues in the mouse

“…Using the specific antibodies against mouse Noc2, the present immunohistochemical study is the first to identify the cell types expressing All endocrine cell types in the adenohypophysis, pancreatic islets, and adrenal gland exhibited Noc2 immunoreactivity. Thus, the cellular distribution of Noc2 is broader than in the other two endocrine-type Rab effector proteins: granuphilin, which is detected in pancreatic β-cells and the pituitary gland (Wang et al 1999;Yi et al 2002), and Rim2, which is expressed in insulin cell lines, the pituitary gland, and PC12 cells, but not in the adrenal gland (Ozaki et al 2000). The association of Noc2 with secretory granules has been demonstrated in insulin-secreting β-cell lines (Cheviet et al 2004a) and PC12 cells (Fukuda et al 2004) (Garrett 1998).…”

“…This non-critical phenotye in β-cells may be explained by the multi-regulation of insulin secretion by different sets of Rab/effector. Endocrine cells including pancreatic β cells possess some potential Rab effectors other than Noc2, such as granuphilin (Wang et al 1999;Yi et al 2002) and Rim2 (Ozaki et al 2000), which were identified first in β cells and are suggested to be present in other amine/peptide-secreting endocrine cells. The normal insulin release in response to glucose and the normal morphology of β-cells in Noc2-knockout mice suggests a compensation for the Noc2 deficiency by granuphilin and Rim2 although the expression levels of granuphilin and Rim2, as assessed by RT-PCR, were similar in the knockout mice (Matsumoto et al 2004).…”

Oscillatory control of insulin secretion

“…Epac has also been reported to increase the number of fusion sites (Kwan et al, 2007;Shibasaki et al, 2007) and along with PKA the number of granule-granule fusion events (Kwan et al, 2007). The effects of Epac are mediated not only by its guanine nucleotide exchange activity on Rap1 (Shibasaki et al, 2007), but also by interactions with several other proteins, including the K ATP channel regulatory subunit SUR1 (Ozaki et al, 2000;Eliasson et al, 2003), the voltage-dependent Ca 2+ channel (Shibasaki et al, 2004) and the insulin granule proteins Rim2 (Ozaki et al, 2000) and Piccolo (Fujimoto et al, 2002). (Fig.4).…”

Mathematical modeling and statistical analysis of calcium-regulated insulin granule exocytosis in β-cells from mice and humans

“…Adenylate cyclases can also be activated by the pharmacological compound forskolin to generate cAMP. Potentiation of insulin exocytosis by cAMP involves both activation of protein kinase A (PKA) and Epac 2 (Exchange protein associated with cAMP 2) (Ozaki et al, 2000). Using capacitance measurements, we have suggested that PKA stimulates mobilization whereas Epac 2 is involved in rapid exocytosis of IRP/RRP granules (Eliasson et al, 2003).…”