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.

Voltage-dependent calcium (Ca2+) channels are involved in many specialized cellular functions, and are controlled by intracellular signals such as heterotrimeric G-proteins, protein kinases and calmodulin (CaM). However, the direct role of small G-proteins in the regulation of Ca2+ channels is unclear. We report here that the GTP-bound form of kir/Gem, identified originally as a Ras-related small G-protein that binds CaM, inhibits high-voltage-activated Ca2+ channel activities by interacting directly with the beta-subunit. The reduced channel activities are due to a decrease in alpha1-subunit expression at the plasma membrane. The binding of Ca2+/CaM to kir/Gem is required for this inhibitory effect by promoting the cytoplasmic localization of kir/Gem. Inhibition of L-type Ca2+ channels by kir/Gem prevents Ca2+-triggered exocytosis in hormone-secreting cells. We propose that the small G-protein kir/Gem, interacting with beta-subunits, regulates Ca2+ channel expression at the cell surface.

Glucose-responsive (GR) neurons in the hypothalamus are thought to be critical in glucose homeostasis, but it is not known how they function in this context. Kir6.2 is the pore-forming subunit of K(ATP) channels in many cell types, including pancreatic beta-cells and heart. Here we show the complete absence of both functional ATP-sensitive K+ (K(ATP)) channels and glucose responsiveness in the neurons of the ventromedial hypothalamus (VMH) in Kir6.2-/- mice. Although pancreatic alpha-cells were functional in Kir6.2-/-, the mice exhibited a severe defect in glucagon secretion in response to systemic hypoglycemia. In addition, they showed a complete loss of glucagon secretion, together with reduced food intake in response to neuroglycopenia. Thus, our results demonstrate that KATP channels are important in glucose sensing in VMH GR neurons, and are essential for the maintenance of glucose homeostasis.

Incretins such as glucagon-like peptide-1 and gastric inhibitory polypeptide/glucose-dependent insulinotropic peptide are known to potentiate insulin secretion mainly through a cAMP/protein kinase A (PKA) signaling pathway in pancreatic ␤-cells, but the mechanism is not clear. We recently found that the cAMP-binding protein cAMP-GEFII (or Epac 2), interacting with Rim2, a target of the small G protein Rab3, mediates cAMP-dependent, PKA-independent exocytosis in a reconstituted system. In the present study, we investigated the role of the cAMP-GEFII⅐Rim2 pathway in incretin-potentiated insulin secretion in native pancreatic ␤-cells. Treatment of pancreatic islets with antisense oligodeoxynucleotides (ODNs) against cAMP-GEFII alone or with the PKA inhibitor H-89 alone inhibited incretinpotentiated insulin secretion ϳ50%, while a combination of antisense ODNs and H-89 inhibited the secretion ϳ80 -90%. The effect of cAMP-GEFII on insulin secretion is mediated by Rim2 and depends on intracellular calcium as well as on cAMP. Treatment of the islets with antisense ODNs attenuated both the first and second phases of insulin secretion potentiated by the cAMP analog 8-bromo-cAMP. These results indicate that the PKA-independent mechanism involving the cAMP-GEFII⅐Rim2 pathway is critical in the potentiation of insulin secretion by incretins.

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