PKA activation bypasses the requirement for UNC-31 in the docking of dense core vesicles from C. elegans neurons.

The nematode C. elegans provides a powerful model system for exploring the molecular basis of synaptogenesis and neurotransmission. However, the lack of direct functional assays of release processes has largely prevented an in depth understanding of the mechanism of vesicular exocytosis and endocytosis in C. elegans. We address this technical limitation by developing direct electrophysiological assays, including membrane capacitance and amperometry measurements, in primary cultured C. elegans neurons. In addition, we have succeeded in monitoring the docking and fusion of single dense core vesicles (DCVs) employing total internal reflection fluorescence microscopy. With these approaches and mutant perturbation analysis, we provide direct evidence that UNC-31 is required for the docking of DCVs at the plasma membrane. Interestingly, the defect in DCV docking caused by UNC-31 mutation can be fully rescued by PKA activation. We also demonstrate that UNC-31 is required for UNC-13-mediated augmentation of DCV exocytosis.

Synaptotagmin IV regulates dense core vesicle (DCV) release in LbetaT2 cells.

Synaptotagmins (Syts) are calcium-binding proteins which are conserved from nematodes to humans. Fifteen Syts have been identified in mammalian species. Syt I is recognized as a Ca(2+) sensor for the synchronized release of synaptic vesicles in some types of neurons, but its role in the secretion of dense core vesicles (DCVs) remains unclear. The function of Syt IV is of particular interest because it is rapidly up-regulated by chronic depolarization and seizures. Using RNAi-mediated gene silencing, we have explored the role of Syt I and IV on secretion in a pituitary gonadotrope cell line. Downregulation of Syt IV clearly reduced Ca(2+)-triggered exocytosis of dense core vesicles (DCVs) in LbetaT2 cells. Syt I silencing, however, had no effect on vesicular release.

Characteristics of Ca2+-exocytosis coupling in isolated mouse pancreatic beta cells.

AIM: To characterize Ca2+-stimulated exocytosis in isolated mouse pancreatic beta cells. METHODS: An improved method was described for isolation of mouse pancreatic beta cells by collagenase P. The Ca2+ channel current and the membrane capacitance were examined by using the whole-cell patch clamp recording technique. RESULTS: Using depolarization and flash photolysis of caged Ca2+ to induce Ca2+-dependent exocytosis in beta cell from KM mouse, we have explored the characteristics of the Ca2+ channel current and the relationship between Ca2+ signals and exocytosis. The averaged peak Ca2+ current measured at +20 mV was -60+/-6 pA (n=13). CONCLUSION: We characterized three kinetically different pools of vesicles in mouse pancreatic beta cells, namely an immediately releasable pool, a readily releasable pool, and a reserve pool.

Kir6.2DeltaC26 channel traffics to plasma membrane by constitutive exocytosis.

Adenosine triphosphate (ATP)-sensitive K+ (KATP) channels regulate many cellular functions by coupling the metabolic state of the cell to the changes in membrane potential. Truncation of C-terminal 26 amino acid residues of Kir6.2 protein (Kir6.2DeltaC26) deletes its endoplasmic reticulum retention signal, allowing functional expression of Kir6.2 in the absence of sulfonylurea receptor subunit. pEGFP-Kir6.2DeltaC26 and pKir6.2DeltaC26-IRES2-EGFP expression plasmids were constructed and transfected into HEK293 cells. We identified that Kir6.2DeltaC26 was localized on the plasma membrane and trafficked to the plasmalemma by means of constitutive exocytosis of Kir6.2DeltaC26 transport vesicles, using epi-fluorescence and total internal reflection fluorescence microscopy. Our electrophysiological data showed that Kir6.2DeltaC26 alone expressed KATP currents, whereas EGFP-Kir6.2DeltaC26 fusion protein displayed no KATP channel activity.