Zymogen granule acidity is not required for stimulated pancreatic protein secretion

STAT5-glucocorticoid receptor interaction and MTF-1 regulate the expression of ZnT2 (Slc30a2) in pancreatic acinar cells

The exocrine pancreas plays an important role in endogenous zinc loss by regulating excretion into the intestinal tract and hence influences the dietary zinc requirement. The present experiments show that the zinc transporter ZnT2 (Slc30a2) is localized to the zymogen granules and that dietary zinc restriction in mice decreased the zinc concentration of zymogen granules and ZnT2 expression. Excess zinc given orally increased ZnT2 expression and was associated with increased pancreatic zinc accumulation. Rat AR42J acinar cells when induced into a secretory phenotype, using the glucocorticoid analog dexamethasone (DEX), exhibited increased ZnT2 expression and labile zinc as measured with a fluorophore. DEX administrated to mice also induced ZnT2 expression that accompanied a reduction of the pancreatic zinc content. ZnT2 promoter analyses identified elements required for responsiveness to zinc and DEX. Zinc regulation was traced to a MRE located downstream from the ZnT2 transcription start site. Responsiveness to DEX is produced by two upstream STAT5 binding sites that require the glucocorticoid receptor for activation. ZnT2 knockdown in the AR42J cells using siRNA resulted in increased cytoplasmic zinc and decreased zymogen granule zinc that further demonstrated that ZnT2 may mediate the sequestration of zinc into zymogen granules. We conclude, based upon experiments with intact mice and pancreatic acinar cells in culture, that ZnT2 participates in zinc transport into pancreatic zymogen granules through a glucocorticoid pathway requiring glucocorticoid receptor and STAT5, and zinc-regulated signaling pathways requiring MTF-1. The ZnT2 transporter appears to function in a physiologically responsive manner involving entero-pancreatic zinc trafficking.

Chloride channels of intracellular membranes

Proteins implicated as intracellular chloride channels include the intracellular ClC proteins, the bestrophins, the cystic fibrosis transmembrane conductance regulator, the CLICs, and the recently described Golgi pH regulator. This paper examines current hypotheses regarding roles of intracellular chloride channels and reviews the evidence supporting a role in intracellular chloride transport for each of these proteins.

Systematic colocalization errors between acridine orange and EGFP in astrocyte vesicular organelles

Dual-color imaging of acridine orange (AO) and EGFP fused to a vesicular glutamate transporter or the vesicle-associated membrane proteins 2 or 3 has been used to visualize a supposedly well-defined subpopulation of glutamatergic astrocytic secretory vesicles undergoing regulated exocytosis. However, AO metachromasy results in the concomitant emission of green and red fluorescence from AO-stained tissue. Therefore, the question arises whether AO and EGFP fluorescence can be distinguished reliably. We used evanescent-field imaging with spectral fluorescence detection as well as fluorescence lifetime imaging microscopy to demonstrate that green fluorescent AO monomers inevitably coexist with red fluorescing AO dimers, at the level of single astroglial vesicles. The green monomer emission spectrally overlaps with that of EGFP and produces a false apparent colocalization on dual-color images. On fluorophore abundance maps calculated from spectrally resolved and unmixed single-vesicle spectral image stacks, EGFP is obscured by the strong green monomer fluorescence, precluding the detection of EGFP. Hence, extreme caution is required when deriving quantitative colocalization information from images of dim fluorescing EGFP-tagged organelles colabeled with bright and broadly emitting dyes like AO. We finally introduce FM4-64/EGFP dual-color imaging as a remedy for imaging a distinct population of astroglial fusion-competent secretory vesicles.

Two phases of zymogen granule lifetime in mouse pancreas: ghost granules linger after exocytosis of contents

Different cell types show widely divergent mechanisms and kinetics of exocytosis. We investigated these processes in pancreatic acinar cells by using video-rate 2-photon microscopy to image entry of extracellular dye into individual zymogen granules undergoing exocytosis. Fluorescence signals display two distinct phases; an initial peak that then decays over several seconds to a prolonged plateau. Several observations suggest that the first component reflects the binding of dye to the granule contents and their subsequent release into the acinar duct. These observations include: the peak/plateau fluorescence ratio differs between different dyes; the initial fluorescence decay mirrors the loss of granule contents as monitored by differential interference contrast microscopy; and the fall in vesicular fluorescence is accompanied by a rise in fluorescence in the adjacent duct lumen. We thus propose the use of extracellular fluorescent probes as a convenient means to monitor the kinetics of loss of proteinaceous content from secretory granules. In pancreatic acinar cells the fusion pore remains open much longer than required to ensure secretion of the granule contents, and instead the persistent empty 'ghost-granule' may act as a conduit to which secondary granules can fuse and release their contents by compound exocytosis.

Ion channels in secretory granules of the pancreas and their role in exocytosis and release of secretory proteins

Regulated secretion in exocrine and neuroendocrine cells occurs through exocytosis of secretory granules and the subsequent release of stored small molecules and proteins. The introduction of biophysical techniques with high temporal and spatial resolution, and the identification of Ca(2+)-dependent and -independent "docking" and "fusion" proteins, has greatly enhanced our understanding of exocytosis. The cloning of families of ion channel proteins, including intracellular ion channels, has also revived interest in the role of secretory granule ion channels in exocytotic secretion. Thus secretory granules of pancreatic acinar cell express a ClC-2 Cl(-) channel, a HCO-permeable member of the CLCA Ca(2+)-dependent anion channel family, and a KCNQ1 K(+) channel. Evidence suggests that these channels may facilitate the release of digestive enzymes and/or prevent exocytosed granules from collapsing during "kiss and run" recycling. In pancreatic beta-cells, a granular ClC-3 Cl(-) channel provides a shunt pathway for a vacuolar-type H(+)-ATPase. Acidification "primes" the granules for Ca(2+)-dependent exocytosis and release of insulin. In summary, secretory granules are equipped with specific sets of ion channels, which modulate regulated exocytosis and the release of macromolecules. These channels could represent excellent targets for therapeutic interventions to control exocytotic secretion in relevant diseases, such as pancreatitis, cystic fibrosis, or diabetes mellitus.

Modulation of calcium signals by intracellular pH in isolated rat pancreatic acinar cells

1. We have investigated the interactions between intracellular pH (pH1) and the intracellular free calcium concentration ([Ca2+]i) in isolated rat pancreatic acinar cells. The fluorescent dyes fura-2 and BCECF were used to measure [Ca2+]i and pHi, respectively. 2. Sodium acetate and ammonium chloride (NH4Cl) were used to acidify and alkalinize pHi, respectively. Cytosolic acidification had no effect on [Ca2+]i in resting pancreatic acinar cells, whereas cytosolic alkalinization released Ca2+ from intracellular stores. 3. Cytosolic acidification using either acetate or a CO2-HCO3(-)-buffered medium enhanced Ca2+ signals evoked by acetylcholine (ACh) and cholecystokinin (CCK). In contrast, both NH4Cl and trimethylamine (TMA) inhibited Ca2+ signals during stimulation with either ACh or CCK. This inhibitory effect was also observed in the absence of extracellular Ca2+, and was therefore not due to changes in Ca2+ entry. 4. Calcium oscillations evoked by physiological concentrations of CCK were enhanced by cytosolic acidification and inhibited by cytosolic alkalinization. 5. In order to determine the effects of pHi upon Ca2+ handling by intracellular Ca2+ stores, intraorganellar [Ca2+] was monitored using the low affinity Ca2+ indicator mag-fura-2 in permeabilized cells. Addition of NH4Cl, which is expected to alkalinize intraorganellar pH, did not alter intraorganellar [Ca2+] in permeabilized cells, suggesting that changing intraorganellar pH does not release Ca2+ from intracellular stores. Addition of NH4Cl or acetate also did not affect the rate of Ca2+ release induced by inositol 1,4,5-trisphosphate (InsP3). 6. Modification of extraorganellar ('cytosolic') pH did not affect the rate of ATP-dependent Ca2+ uptake into stores, but did modify the rate of Ca2+ release evoked by submaximal concentrations of InsP3. The rate of Ca2+ release was increased at more alkaline extraorganellar pHs. These results would suggest that manipulation of intraorganellar pH does not affect Ca2+ handling by the intracellular stores. In contrast, extraorganellar ('cytosolic') pH does affect InsP3-induced Ca2+ release from the stores. 7. In conclusion, changes in intracellular pH in pancreatic acinar cells can profoundly alter cytosolic [Ca2+]. This may shed light on earlier observations whereby cell-permeant weak acids and bases can modulate fluid secretion in epithelia.

Effect of monensin on secretory granules and basal beta-endorphin secretion in the melanotroph of the rat pituitary

The effect of monensin on the Golgi complex, formation of secretory granules and basal beta-endorphin secretion in cultured melanotrophs from the rat pituitary was studied. Earlier studies on the effect of monensin on regulated secretion have generally showed only minor effects on secretory granules. The initial (within 5 min) effect of monensin on the melanotroph was the appearance of large vacuoles at the trans-side of the thiamine pyrophosphatase-positive trans-most Golgi cisternae. This was associated with a dose-dependent inhibition of the condensation of electron-dense secretory products. After 1 h of treatment with 1 microM monensin the Golgi stack was completely vacuolized. At the same time mature secretory granules were enlarged to severalfold their original size, and after 4 h of treatment secretory granules were no longer observed. Despite the marked effects on granule formation and mature secretory granules monensin did not affect the basal release of beta-endorphin-immunoreactive material during continued incubation for up to 4 h, indicating that basal peptide secretion can bypass the monensin block.

Reconstitution in vitro of the pH-dependent aggregation of pancreatic zymogens en route to the secretory granule: implication of GP-2

Regulated secretory proteins are thought to be sorted in the trans-Golgi network (TGN) via selective aggregation. To elucidate the biogenesis of the secretory granule in the exocrine pancreas, we reconstituted in vitro the conditions of pH and ions believed to exist in the TGN using the end product of this sorting process, the zymogen granule contents. Protein aggregation was dependent on pH (acidic) and on the presence of cations (10 mM Ca2+, 150 mM K+) to reproduce the pattern of proteins found in the granule. The constitutive secretory protein IgG was excluded from these aggregates. Zymogen aggregation correlated with the relative proportion of the major granule membrane protein GP-2 in the assay. These results show that the glycosylphosphatidylinositol-anchored protein GP-2 co-aggregates with zymogens in the acidic environment believed to exist in the pancreatic TGN, and thus suggest that GP-2 would function as a membrane anchor for zymogen aggregates, facilitating their entrapment in budding vesicles directed towards the regulated secretory pathway.