Simulation of regulated exocytosis of amylase from salivary parotid acinar cells by a consecutive reaction model comprising two sequential first-order reactions

Complexin 2 modulates vesicle-associated membrane protein (VAMP) 2-regulated zymogen granule exocytosis in pancreatic acini

Complexins are soluble proteins that regulate the activity of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complexes necessary for vesicle fusion. Neuronal specific complexin 1 has inhibitory and stimulatory effects on exocytosis by clamping trans-SNARE complexes in a prefusion state and promoting conformational changes to facilitate membrane fusion following cell stimulation. Complexins are unable to bind to monomeric SNARE proteins but bind with high affinity to ternary SNARE complexes and with lower affinity to target SNARE complexes. Far less is understood about complexin function outside the nervous system. Pancreatic acini express the complexin 2 isoform by RT-PCR and immunoblotting. Immunofluorescence microscopy revealed complexin 2 localized along the apical plasma membrane consistent with a role in secretion. Accordingly, complexin 2 was found to interact with vesicle-associated membrane protein (VAMP) 2, syntaxins 3 and 4, but not with VAMP 8 or syntaxin 2. Introduction of recombinant complexin 2 into permeabilized acini inhibited Ca(2+)-stimulated secretion in a concentration-dependent manner with a maximal inhibition of nearly 50%. Mutations of the central α-helical domain reduced complexin 2 SNARE binding and concurrently abolished its inhibitory activity. Surprisingly, mutation of arginine 59 to histidine within the central α-helical domain did not alter SNARE binding and moreover, augmented Ca(2+)-stimulated secretion by 130% of control. Consistent with biochemical studies, complexin 2 colocalized with VAMP 2 along the apical plasma membrane following cholecystokinin-8 stimulation. These data demonstrate a functional role for complexin 2 outside the nervous system and indicate that it participates in the Ca(2+)-sensitive regulatory pathway for zymogen granule exocytosis.

Cyclic AMP potentiates Ca2+-dependent exocytosis in pancreatic duct epithelial cells

Exocytosis is evoked by intracellular signals, including Ca²⁺ and protein kinases. We determined how such signals interact to promote exocytosis in exocrine pancreatic duct epithelial cells (PDECs). Exocytosis, detected using carbon-fiber microamperometry, was stimulated by [Ca²⁺]_i increases induced either through Ca²⁺ influx using ionomycin or by activation of P2Y2 or protease-activated receptor 2 receptors. In each case, the exocytosis was strongly potentiated when cyclic AMP (cAMP) was elevated either by activating adenylyl cyclase with forskolin or by activating the endogenous vasoactive intestinal peptide receptor. This potentiation was completely inhibited by H-89 and partially blocked by Rp-8-Br-cAMPS, inhibitors of protein kinase A. Optical monitoring of fluorescently labeled secretory granules showed slow migration toward the plasma membrane during Ca²⁺ elevations. Neither this Ca²⁺-dependent granule movement nor the number of granules found near the plasma membrane were detectably changed by raising cAMP, suggesting that cAMP potentiates Ca²⁺-dependent exocytosis at a later stage. A kinetic model was made of the exocytosis stimulated by UTP, trypsin, and Ca²⁺ ionophores with and without cAMP increase. In the model, without a cAMP rise, receptor activation stimulates exocytosis both by Ca²⁺ elevation and by the action of another messenger(s). With cAMP elevation the docking/priming step for secretory granules was accelerated, augmenting the releasable granule pool size, and the Ca²⁺ sensitivity of the final fusion step was increased, augmenting the rate of exocytosis. Presumably both cAMP actions require cAMP-dependent phosphorylation of target proteins. cAMP-dependent potentiation of Ca²⁺-induced exocytosis has physiological implications for mucin secretion and, possibly, for membrane protein insertion in the pancreatic duct. In addition, mechanisms underlying this potentiation of slow exocytosis may also exist in other cell systems.

The expression and localization of osteopontin in the mouse major salivary glands

The present study investigated the expression and distribution of osteopontin in the mouse major salivary glands. The level of osteopontin expression in the mouse submandibular gland was higher (12.7-fold) than that in parotid and sublingual glands at the mRNA level. By Western blot analysis, intense positive bands were seen at the predicted molecular mass (about 55 kDa) in all the major salivary glands, while an approximately 30 kDa band of osteopontin was detected only in the submandibular gland. Indirect immunofluorescent and immuno-electron microscopy analyses demonstrated the localization of osteopontin in the luminal (apical) membranes of acinar cells in all the salivary glands. Osteopontin was also localized at the lumen of acini in the submandibular gland. These results suggest that the expression of osteopontin in the submandibular gland is different from that in the parotid and sublingual glands and that osteopontin may be degraded in the mouse submandibular gland.

A primary culture of parotid acinar cells retaining capacity for agonists-induced amylase secretion and generation of new secretory granules

Exocrine acinar cells, like parotid cells, have difficulty in maintaining their functions in cell lines or in primary cultures. For this reason, molecular studies on exocrine cell functions are unsatisfactory. To examine the mechanisms whereby the functions of parotid acinar cells are maintained, we attempted to establish a system for primary culture and transfection of exogenous genes. Acinar cells were dispersed from rat parotid glands by digestion with enzymes and were cultured in a medium containing rat serum. Most of the cultured cells had secretory granules that contained amylase, suggesting that they were derived from acinar cells, although they spread on the dish surface and formed filopodia. The cultured cells retained both granules and the ability to release amylase in response to beta-adrenergic and cholinergic agonists, even 48 h after dispersion. However, the total amount of amylase in the cells decreased rapidly from 24 to 48 h after dispersion. These results suggested that amylase synthesis was more damaged than the machinery for exocytosis during culture in vitro. VAMP2 gene fused with enhanced green fluorescence protein was transfected into the dispersed acinar cells, and VAMP2 protein was expressed and localized to amylase-containing granules, as normally seen for endogenous VAMP2 protein. This indicated that new granules were generated, and that protein sorting was functional. The cells cultured by this method maintained their functions for at least 48 h. They can be used for examining the effects of exogenous genes on parotid acinar cell functions, such as regulated exocytosis and the maturation of secretory granules.

Involvement of phospholipase D in the cAMP-regulated exocytosis of rat parotid acinar cells

The activation of beta-adrenergic receptors in rat parotid acinar cells causes intracellular cAMP elevation and appreciably stimulates the exocytotic release of amylase into saliva. The activation of Ca(2+)-mobilizing receptors also induces some exocytosis. We investigated the role of phospholipase D (PLD) in regulated exocytosis in rat parotid acinar cells. A transphosphatidylation assay detected GTPgammaS (a nonhydrolyzable analogue of GTP)-dependent PLD activity in lysates of rat parotid acinar cells, suggesting that PLD is activated by small molecular mass GTP-binding proteins. The PLD inhibitor, neomycin, suppressed cAMP-dependent exocytosis in saponin-permeabilized cells. Signaling downstream of PLD was disrupted by 1-butanol due to conversion of the PLD reaction product (phosphatidic acid) to phosphatidylbutanol. The stimulation of exocytosis by isoproterenol as well as by a Ca(2+)-mobilizing agonist (methacholine) was inhibited by 1-butanol. These results suggest that PLD is important for regulated exocytosis in rat parotid acinar cells.

cAMP modulation of Ca(2+)-regulated exocytosis in ACh-stimulated antral mucous cells of guinea pig

Effects of cAMP accumulation on ATP-dependent priming and Ca(2+)-dependent fusion in Ca(2+)-regulated exocytosis were examined in antral mucous cells of guinea pigs by using video-enhanced contrast microscopy. The Ca(2+)-regulated exocytosis activated by 1 microM ACh consisted of two phases, an initial transient phase followed by a sustained phase, which were potentiated by cAMP accumulation. Depletion of ATP by 100 microM dinitrophenol (uncoupler of oxidative phosphorylation) or anoxia induced the sustained phase without the initial transient phase in Ca(2+)-regulated exocytosis. However, accumulation of cAMP before depletion of ATP induced and potentiated an initial transient phase followed by a sustained phase in Ca(2+)-regulated exocytosis. This suggests that the initial transient phase of Ca(2+)-regulated exocytosis is induced by fusion of all primed granules maintained by ATP and that accumulation of cAMP accelerates ATP-dependent priming of the exocytotic cycle. Moreover, ACh and Ca(2+) dose-response studies showed that accumulation of cAMP shifted the dose-response curves to the low concentration side, suggesting that it increases Ca(2+) sensitivity in the fusion of the exocytotic cycle. In conclusion, cAMP accumulation increases the number of primed granules and Ca(2+) sensitivity of the fusion, which potentiates Ca(2+)-regulated exocytosis in antral mucous cells.