Isolation of subcellular agonist-sensitive calcium stores from the pancreatic acinar cell

Calcium, mitochondria and the initiation of acute pancreatitis

Acute pancreatitis is characterized by necrosis of its parenchymal cells and influx and activation of inflammatory cells that further promote injury and necrosis. This review is intended to discuss the central role of disorders of calcium metabolism and mitochondrial dysfunction in the mechanism of pancreatitis development. The disorders are placed in context of calcium and mitochondria in physiologic function of the pancreas. Moreover, we discuss potential therapeutics for preventing pathologic calcium signals that injure mitochondria and interventions that promote the removal of injured mitochondria and regenerate new and heathy populations of mitochondria.

Regulation of inositol 1,4,5-trisphosphate receptors during endoplasmic reticulum stress

The endoplasmic reticulum (ER) performs multiple functions in the cell: it is the major site of protein and lipid synthesis as well as the most important intracellular Ca(2+) reservoir. Adverse conditions, including a decrease in the ER Ca(2+) level or an increase in oxidative stress, impair the formation of new proteins, resulting in ER stress. The subsequent unfolded protein response (UPR) is a cellular attempt to lower the burden on the ER and to restore ER homeostasis by imposing a general arrest in protein synthesis, upregulating chaperone proteins and degrading misfolded proteins. This response can also lead to autophagy and, if the stress can not be alleviated, to apoptosis. The inositol 1,4,5-trisphosphate (IP3) receptor (IP3R) and IP3-induced Ca(2+) signaling are important players in these processes. Not only is the IP3R activity modulated in a dual way during ER stress, but also other key proteins involved in Ca(2+) signaling are modulated. Changes also occur at the structural level with a strengthening of the contacts between the ER and the mitochondria, which are important determinants of mitochondrial Ca(2+) uptake. The resulting cytoplasmic and mitochondrial Ca(2+) signals will control cellular decisions that either promote cell survival or cause their elimination via apoptosis. This article is part of a Special Issue entitled: 12th European Symposium on Calcium.

Expression and regulation of calpain in rat pancreatic acinar cells

INTRODUCTION

Calpains, cytosolic Ca(2+)-dependent cysteine proteases, are expressed in a variety of mammalian cells and have been found to participate in stimulus-secretion coupling in platelets and alveolar cells.

AIMS

In pancreatic acinar cells, expression of calpains and their role in the secretory process have not yet been elucidated. Both subjects, therefore, were examined in the current study.

METHODOLOGY

mu-calpain and m-calpain were detected immunochemically. Calpain activation was measured by fluorescence spectrophotometry and single-cell fluorometry using Suc-Leu-Leu-Val-Tyr-AMC as substrate. Amylase secretion and cell damage, characterized by lactate dehydrogenase release, were measured by colorimetric assays.

RESULTS

Immunochemistry revealed cytoplasmic localization of both calpain isoforms. Immediately after increasing the cytosolic Ca(2+) concentration with ionomycin, a marked dose-dependent protease activation and cellular damage were observed. Inhibition of ionomycin-mediated enzyme activation through preincubation of cells with Ca(2+)-free medium, BAPTA-AM, or Z-Leu-Leu-Tyr-CHN(2) significantly reduced cell injury. Cholecystokinin (100 pM) also induced proteolytic activity, preceding cholecystokinin-stimulated amylase secretion. Protease activity and amylase release were significantly inhibited by Z-Leu-Leu-Tyr-CHN(2 ) retreatment.

CONCLUSION

Calpains are expressed in pancreatic acinar cells and may participate in stimulus-secretion coupling. In addition, our study indicates that pathologic calpain activation may contribute to Ca(2+)-mediated acinar cell damage.

Mammalian sperm contain a Ca(2+)-sensitive phospholipase C activity that can generate InsP(3) from PIP(2) associated with intracellular organelles

We have previously described a phospholipase C (PLC) activity in mammalian sperm cytosolic extracts. Here we have examined the Ca(2+) dependency of the enzyme, whether there is enough in a single sperm to account for Ca(2+) release at fertilization, and finally where in the egg is the phosphatidyl 4,5-bisphosphate, the substrate for the enzyme. As for all PLCs examined so far in vitro, we found that the boar sperm PLC activity was Ca(2+) dependent. Specific activity increased when free Ca(2+) levels were micromolar. However, even at nanomolar free Ca(2+) concentration the boar sperm PLC activity was considerable, being two orders of magnitude greater than PLC activities in other tissues. We calculated that PLC activity of a single boar sperm in a mammalian egg is enough to generate 400 nM inositol 1,4,5-trisphosphate (InsP(3)) in 1 min, which may be sufficient to account for the observed Ca(2+) changes in an egg at fertilization. We fractionated sea urchin egg homogenate and examined the ability of boar sperm extract to generate InsP(3) from these fractions. The sperm PLC activity triggered InsP(3) production from a PIP(2)-enriched nonmicrosomal egg compartment that contained yolk platelets. We propose that this sperm PLC activity, which is active at nanomolar Ca(2+) levels and hydrolyzes PIP(2) from intracellular membranes, could be involved in the Ca(2+) changes observed at fertilization.

Multiple isoforms of the ryanodine receptor are expressed in rat pancreatic acinar cells

Regulation of cytosolic Ca(2+) is important for a variety of cell functions. The ryanodine receptor (RyR) is a Ca(2+) channel that conducts Ca(2+) from internal pools to the cytoplasm. To demonstrate the presence of the RyR in the pancreatic acinar cell, we performed reverse transcriptase (RT)-PCR, Western blot, immunocytochemistry and microscopic Ca(2+)-release measurements on these cells. RT-PCR showed the presence of mRNA for RyR isoforms 1, 2 and 3 in both rat pancreas and dispersed pancreatic acini. Furthermore, mRNA expression for RyR isoforms 1 and 2 was demonstrated by RT-PCR in individual pancreatic acinar cells selected under the microscope. Western-blot analysis of acinar cell immunoprecipitates, using antibodies against RyR1 and RyR2, showed a high-molecular-mass (>250 kDa) protein band that was much less intense when immunoprecipitated in the presence of RyR peptide. Functionally, permeablized acinar cells stimulated with the RyR activator, palmitoyl-CoA, released Ca(2+) from both basolateral and apical regions. These data show that pancreatic acinar cells express multiple isoforms of the RyR and that there are functional receptors throughout the cell.

Endoplasmic reticulum Ca(2+)-ATPase inhibitors stimulate membrane guanylate cyclase in pancreatic acinar cells

In this study, we show that particulate guanylate cyclase (GC) is present in rat pancreatic acinar cells and is located both on plasma membrane and membranes of endoplasmic reticulum (ER). Western blot analysis indicates that the enzyme isoform GC-A is present in the acinar cell membranes. The specific inhibitors of ER Ca(2+)-ATPase thapsigargin, 2,5-di-(t-butyl)-1,4-hydroquinone (BHQ), and cyclopiazonic acid all activated particulate GC in pancreatic acini, both in membrane fractions and intact cells. These inhibitors also induced dephosphorylation of GC. Dose dependencies of Ca(2+)-ATPase inhibition and GC activation by BHQ are very similar, and those for thapsigargin partially overlap. ER Ca(2+)-ATPase and GC are coimmunoprecipitated both by antisera against membrane GC and by antisera against ER Ca(2+)-ATPase, suggesting a physical association between the two enzymes. The results suggest that thapsigargin and the other inhibitors act through ER Ca(2+)-ATPase to activate membrane GC in pancreatic acinar cells, although their direct effect on GC cannot be excluded.

Distribution of inositol 1,4,5-trisphosphate receptor isoforms, SERCA isoforms and Ca2+ binding proteins in RBL-2H3 rat basophilic leukemia cells

RBL-2H3 rat basophilic leukemia cells were homogenized and fractionated. A fraction F3 obtained by differential centrifugation was 6-fold enriched in [3H]-inositol 1,4,5-trisphosphate (InsP3) binding activity, while the NADH-cytochrome c oxidoreductase and sulphatase-C activities were only 3.8- and 2.9-fold enriched, respectively. Furthermore, the three InsP3 receptor (InsP3R) isoforms, two sarco/endoplasmic reticulum Ca2+ ATPase (SERCA) isoforms (2b and 3) as well as four Ca2+ binding proteins (calreticulin, calnexin, protein disulfide isomerase (PDI) and BiP), were present in this fraction. Fraction F3 was, therefore, further purified on a discontinuous sucrose density gradient, and the 3 resulting fractions were analyzed. The InsP3 binding sites were distributed over the gradient and did not co-migrate with the RNA. We examined the relative content of the three InsP3R isoforms, of both SERCA2b and 3, as well as that of the four Ca2+ binding proteins in fraction F3 and the sucrose density gradient fractions. InsP3R-1 and InsP3R-2 showed a similar distribution, with the highest level in the light and intermediate density fractions. InsP3R-3 distributed differently, with the highest level in the intermediate density fraction. Both SERCA isoforms distributed similarly to InsP3R-1 and InsP3R-2. SERCA3 was present at a very low level in the high density fraction. Calreticulin and BiP showed a pattern similar to that of InsP3R-1 and InsP3R-2 and the SERCAs. PDI was clearly enriched in the light density fraction while calnexin was broadly distributed. These results indicate a heterogeneous distribution of the three InsP3R isoforms, the two SERCA isoforms and the four Ca2+ binding proteins investigated. This heterogeneity may underlie specialization of the Ca2+ stores and the subsequent initiation of intracellular Ca2+ signals.

Spatial domains of Ca2+ signaling in secretory epithelial cells

Secretory epithelial cells are found in exocrine organs such as the pancreas and are also found in the lining of the lungs and gut. One important regulator of cell function in epithelial cells is the concentration of cytosolic Ca2+. The study of Ca2+ signaling in these cells has a long history and recent work has now identified, at the molecular level, key components in the Ca2+ signaling cascade. Furthermore, advances in fluorescent imaging techniques has enabled a detailed insight into the subcellular distribution of the agonist-evoked [Ca2+]i signal. A number of spatially different [Ca2+]i responses have been identified. Firstly, global [Ca2+]i signals are observed in response to high agonist concentrations. Secondly, at lower agonist concentrations trains of local [Ca2+]i spikes, restricted to the secretory pole region of pancreatic acinar cells, have been identified. Finally, these local [Ca2+]i spikes have now been further devolved into microdomains of [Ca2+]i elevation. The [Ca2+]i signal within a single microdomain has been shown to be the crucial trigger in the regulation of the ion channels important in fluid secretion.