Isolation of stable pancreatic zymogen granules

Analysis of endoscopic pancreatic function test (ePFT)-collected pancreatic fluid proteins precipitated via ultracentrifugation

CONTEXT

We have shown previously that trichloroacetic acid precipitation is an effective method of protein extraction from pancreatic fluid for downstream biomarker discovery, compared to other common extraction methods tested.

OBJECTIVE

We aim to assess the utility of ultracentrifugation as an alternative method of protein extraction from pancreatic fluid.

DESIGN

Proteins extracted from trichloroacetic acid- and ultracentrifugation-precipitated pancreatic fluid were identified using mass spectrometry techniques (in-gel tryptic digestion followed by liquid chromatography-tandem mass spectrometry; GeLC-MS/MS). Data were analyzed using Proteome Discoverer and Scaffold 3.

SETTING

This is a proteomic analysis experiment of endoscopically collected fluid in an academic center.

PATIENTS

The study population included adult patients referred to the Center for Pancreatic Disease at Brigham and Women's Hospital, Boston, MA, USA for the evaluation of abdominal pain and gastrointestinal symptoms.

INTERVENTIONS

Secretin-stimulated pancreatic fluid was collected as standard of care for the evaluation of abdominal pain and gastrointestinal symptoms.

MAIN OUTCOME MEASURES

We compared proteins identified via standard trichloroacetic acid precipitation and this alternative ultracentrifugation strategy.

RESULTS

A subset of pancreatic fluid proteins was identified via the ultracentrifugation method. Of these proteins, similar numbers were obtained from fully tryptic or semi-tryptic database searching. Proteins identified in the ultracentrifugation-precipitated samples included previously identified biomarker candidates of chronic pancreatitis.

CONCLUSIONS

This alternative ultracentrifugation strategy requires less time and fewer handling procedures than standard trichloroacetic acid precipitation, at the expense of higher sample volume. As such, this method is well suited for targeted assays (i.e., dot blotting or targeted mass spectrometry) if the protein of interest is among those readily identified by ultracentrifugation-promoted precipitation.

Salmeterol restores secretory functions in cystic fibrosis airway submucosal gland serous cells

The activity of the cystic fibrosis (CF) transmembrane conductance regulator (CFTR) can be mediated by surface G protein-coupled receptors such as the beta(2)-adrenergic receptor. In this study, we explored the effect of a long-acting beta(2)-adrenergic agonist, salmeterol, on the CFTR-dependent secretory capacity of a human CF tracheal gland serous cell line (CF-KM4), homozygous for the delF508 mutation. We showed that, compared with the untreated CF serous cells, a 24-hour pre-incubation period with 200 nM salmeterol induced an 83% increase in delF508-CFTR-mediated chloride efflux. The restoration of the bioelectric properties is associated with increased apical surface pool of delF508-CFTR. Salmeterol induced a decrease in ion concentration and an increase in the level of hydration of the mucus packaged inside the CF secretory granules. The effects of salmeterol are not associated with a persistent production of cAMP. Western blotting on isolated secretory granules demonstrated immunoreactivity for CFTR and lysozyme. In parallel, we measured by atomic force microscopy an increased size of secretory granules isolated from CF serous cells compared with non-CF serous cells (MM39 cell line) and showed that salmeterol was able to restore a CF cell granule size similar to that of non-CF cells. To demonstrate that the salmeterol effect was a CFTR-dependent mechanism, we showed that the incubation of salmeterol-treated CF serous cells with CFTR-inh172 suppressed the restoration of normal secretory functions. The capacity of salmeterol to restore the secretory capacity of glandular serous cells suggests that it could also improve the airway mucociliary clearance in patients with CF.

Effect of parenteral and early intrajejunal nutrition on pancreatic digestive enzyme synthesis, storage and discharge in dog models of acute pancreatitis

AIM: To study the effect of early intrajejunal nutrition on enzyme-protein synthesis and secretion during acute pancreatitis.

METHODS: Fifteen dogs were randomly divided into parenteral nutrition (n = 7) and early intrajejunal nutrition groups (n = 8). An acute pancreatitis model was induced by injecting 5% sodium taurocholate and trypsin into the pancreas via the pancreatic duct. Intrajejunal nutrition was delivered with a catheter via a jejunostomy tube after the model was established for 24 h. On d 1 and 7 and at the beginning of nutritional support, radioactive tracing and electron microscopes were used to evaluate the enzyme-protein synthesis in acinar cells, the subcellular fractionation and the change in zymogen granules after 1.85 × 10⁶ Bq L-³H phenylalanine was infused at 30, 60, 120, and 180 min.

RESULTS: The ³H radioactivity in pancreatic acinar cells reached its peak level at 60 min, and the contents in the early intrajejunal nutrition group were higher than those in the parenteral nutrition group, which were then decreased. The mean number and area of zymogen granules did not show any significant statistical difference in both groups on d 1 or on d 7 (P > 0.05).

CONCLUSION: Early intrajejunal nutrition might be effective in dogs with acute pancreatitis.

Expression of pro-Muclin in pancreatic AR42J cells induces functional regulated secretory granules

It is not clear how protein cargo is sorted to and retained in forming regulated secretory granules (RSG). Here, the sulfated mucin-type glycoprotein pro-Muclin was tested for its ability to induce RSG in the poorly differentiated rat pancreatic cell line AR42J. AR42J cells express RSG content proteins, but they fail to make granules. Adenovirus-pro-Muclin-infected AR42J cells store amylase, accumulate RSG, and respond to hormonal stimulation by secreting the stored protein. Expression of pro-Muclin combined with the inducing effect of dexamethasone resulted in a significant enhancement of the efficiency of regulated secretion. The effect of pro-Muclin was a strong decrease in constitutive secretion compared with dexamethasone-induction alone. A pro-Muclin construct missing the cytosolic tail domain was less effective at improving the efficiency of regulated secretion compared with the full-length construct. Increased expression of cargo (using adenovirus amylase) also modestly enhanced regulated secretion, indicating that part of pro-Muclin's effect may be due to increased expression of cargo protein. Overall, the data show that pro-Muclin acts as a sorting receptor that can induce RSG, and that its cytosolic tail is important in this process.

Protection from pancreatitis by the zymogen granule membrane protein integral membrane-associated protein-1

Pancreatitis is a common disease with substantial morbidity and mortality. To better understand the mechanisms conferring sensitivity or resistance to pancreatitis, we have initiated the analysis of novel acinar cell proteins. Integral membrane-associated protein-1 (Itmap1) is a CUB (complement subcomponents C1r/C1s, sea urchin Uegf protein, bone morphogenetic protein-1) and zona pellucida (ZP) domain-containing protein we find prominently expressed in pancreatic acinar cells. Within the acinar cell, Itmap1 localizes to zymogen granule membranes. Although roles in epithelial polarity, granule assembly, and mucosal protection have been postulated for CUB/ZP proteins, in vivo functions for these molecules have not been proven. To determine the function of Itmap1, we generated Itmap1-deficient mice. Itmap1(-/-) mice demonstrate increased severity of secretagogue- and diet-induced pancreatitis in comparison to Itmap1(+/+) mice. In contrast to previous animal models exhibiting altered severity of pancreatitis, Itmap1 deficiency results in impaired activation of trypsin, an enzyme believed critical for initiating a cascade of digestive zymogen activation during pancreatitis. Itmap1 deficiency does not alter zymogen granule size, appearance, or the composition of zymogen granule contents. Our results demonstrate that Itmap1 plays an essential role in trypsinogen activation and that both impaired and augmented trypsinogen activation can be associated with increased severity of pancreatitis.

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.

Changes in the exocrine pancreas secondary to altered small intestinal function in the CF mouse

The exocrine pancreas of the cystic fibrosis (CF) mouse (cftr(m1UNC)) is only mildly affected compared with the human disease, providing a useful model to study alterations in exocrine function. The CF mouse pancreas has approximately 50% of normal amylase levels and approximately 200% normal Muclin levels, the major sulfated glycoprotein of the pancreas. Protein biosynthetic rates and mRNA levels for amylase were not altered in CF compared with normal mice, and increases in Muclin biosynthesis and mRNA paralleled the increased protein content. Stimulated pancreatic amylase secretion in vitro and in vivo tended to be increased in CF mice but was not statistically significant compared with normal mice. We show for the first time that the CF mouse duodenum is abnormally acidic (normal intestinal pH = 6.47 +/- 0.05; CF intestinal pH = 6.15 +/- 0.07) and hypothesize that this may result in increased signaling to the exocrine pancreas. There were significant increases in CF intestinal mRNA levels for secretin (310% of normal, P < 0.001) and vasoactive intestinal peptide (148% of normal, P < 0.05). Furthermore, CF pancreatic cAMP levels were 147% of normal (P < 0.01). These data suggest that the CF pancreas may be chronically stimulated by cAMP-mediated signals, which in turn may exacerbate protein plugging in the acinar/ductal lumen, believed to be the primary cause of destruction of the pancreas in CF.

The recovery of acute pancreatitis depends on the enzyme amount stored in zymogen granules at early stages

Little is known about the changes in pancreatic enzyme storage in acute pancreatitis. We have performed flow cytometric studies of zymogen granules from rats with acute pancreatitis induced by hyperstimulation with caerulein. A comparison was made with rats treated with hydrocortisone (10 mg/kg/day) over 7 days before inducing pancreatitis in order to find out whether the amount of enzymes stored in the pancreas plays a key role in the development of pancreatitis. The potentially therapeutic effect of L-364,718 (0.1 mg/kg/day, for 7 days), a CCK receptor antagonist, was assayed in the rats with caerulein-induced pancreatitis which had previously received the hydrocortisone treatment. A significant increase in the intragranular enzyme content was observed 5 h after hyperstimulation with caerulein. The highest values were reached in the rats previously treated with hydrocortisone. The greatest pancreatic enzyme load was parallel to the highest values in plasma amylase, edema and haematocrit observed. Acute pancreatitis was reversed seven days later. At this stage smaller granules appeared in the pancreas whose enzyme content was similar to that of controls when no treatment was applied after pancreatitis. In contrast, L-364,718 administration prevented the favourable evolution of pancreatitis since the antagonism exerted on CCK receptors induced a blockade of secretion of the large amounts of enzymes stored in the pancreas. Moreover, the enzyme content in zymogen granules was below normal values since the stimulatory CCK action on enzyme synthesis can be inhibited by L-364,718. Our results suggest that the efficiency of CCK antagonists, as potential therapy, would also depend on the load of enzymes in the pancreas when acute pancreatitis is produced.

Effect of cholecystokinin blockade on the recovery of alterations induced by acute pancreatitis in glycoconjugates of rat zymogen granules

Lectin-binding studies have been performed on rat zymogen granules to investigate alterations in the carbohydrate membrane composition that occur in acute pancreatitis induced by caerulein. The influence of treatment with hydrocortisone for seven days before inducing pancreatitis was also studied. Lectin labeling on zymogen granules was also analyzed seven days after inducing pancreatitis in rats that had previously received a hydrocortisone treatment. During this period L 364,718 (0.1 mg/kg)--specific cholecystokinin (CCK) receptor antagonist--was administered daily to some of the rats, and no treatment was applied to others. Using fluorescein-labelled T. purpureus (TP)lectin, a significant decrease in the amount of L-fucose in the granule membrane was observed in rats with caerulein-induced pancreatitis. This effect was directly caused by the pancreatitis and was not influenced by previous hydrocortisone treatment. Seven days later, the density of TP receptors in the granule membrane was similar to the controls both in L-364,718-treated and untreated rats. Therefore, we suggest that endogenous CCK is not an essential factor in the recovery of L-fucose containing glycoconjugates the granule membrane after pancreatitis. Acute pancreatitis did not alter the expression of wheat germ agglutinin (WGA) receptors in the zymogen granule membrane. WGA specifically binds N-acetyl glucosamine and sialic acids. L 364,718 administered for seven days after inducing pancreatitis significantly reduced WGA binding, untreated rats showed a normal zymogen granule membrane. Therefore, the blockade of CCK-induced alterations in membrane glycoconjugates enriched in N-acetyl glucosamine and sialic acid of newly formed granules after pancreatitis, a finding that could explain the delay in the regression of the disease.

Chromogranin B (secretogranin I), a neuroendocrine-regulated secretory protein, is sorted to exocrine secretory granules in transgenic mice

Chromogranin B (CgB, secretogranin I) is a secretory granule matrix protein expressed in a wide variety of endocrine cells and neurons. Here we generated transgenic mice expressing CgB under the control of the human cytomegalovirus promoter. Northern and immunoblot analyses, in situ hybridization and immunocytochemistry revealed that the exocrine pancreas was the tissue with the highest level of ectopic CgB expression. Upon subcellular fractionation of the exocrine pancreas, the distribution of CgB in the various fractions was indistinguishable from that of amylase, an endogenous constituent of zymogen granules. Immunogold electron microscopy of pancreatic acinar cells showed co-localization of CgB with zymogens in Golgi cisternae, condensing vacuoles/immature granules and mature zymogen granules; the ratio of immunoreactivity of CgB to zymogens being highest in condensing vacuoles/immature granules. CgB isolated from zymogen granules of the pancreas of the transgenic mice aggregated in a mildly acidic (pH 5.5) milieu in vitro, suggesting that low pH-induced aggregation contributed to the observed concentration of CgB in condensing vacuoles. Our results show that a neuroendocrine-regulated secretory protein can be sorted to exocrine secretory granules in vivo, and imply that a key feature of CgB sorting in the trans-Golgi network of neuroendocrine cells, i.e. its aggregation-mediated concentration in the course of immature secretory granule formation, also occurs in exocrine cells although secretory protein sorting in these cells is thought to occur largely in the course of secretory granule maturation.

Cellular alterations of parotid gland of rats with acute pancreatitis induced by cerulein

To explore the cellular and subcellular alterations of the parotid gland during acute pancreatitis, we examined the redistribution of lysosomal enzyme, cathepsin B, along with the discharge of the LDH and cathepsin B from parotid acini of rats with acute pancreatitis induced by a supramaximal dose of cerulein (5 micrograms/kg/h for 3.5 h). Both the serum amylase level and parotid-gland amylase content were increased significantly in rats with acute pancreatitis. The dry-/wet-wt ratio ratio was significantly lower than in the control. In vitro studies showed that discharge of LDH from the parotid acini and leakage of cathepsin B from lysosomes in the acini were significantly higher than in the control. In addition, there was redistribution of the cathepsin B (shifting from the lysosomal pellet to the zymogen pellet) in the parotid gland. These results indicate that in acute pancreatitis there is edema and accumulation of amylase in the parotid glands, along with increased cellular and lysosomal fragility. Thus, there seems to be a close relationship between the exocrine pancreas and the parotid glands. Gut hormones, such as cerulein, also appear to play an important role in the pathophysiology of the parotid glands.

Dual modulation of chloride conductance by nucleotides in pancreatic and parotid zymogen granules

The regulation of Cl- conductance by cytoplasmic nucleotides was investigated in pancreatic and parotid zymogen granules. Cl- conductance was assayed by measuring the rate of cation-ionophore-induced osmotic lysis of granules suspended in iso-osmotic salt solutions. Both inhibition and stimulation were observed, depending on the type and concentration of nucleotide. Under optimal conditions, the average inhibition measured in different preparations was 1.6-fold, whereas the average stimulation was 4.4-fold. ATP was inhibitory at 1-10 microM but stimulated Cl- conductance above 50 microM. Stimulation by ATP was more pronounced in granules with low endogenous Cl- conductance. The potency of nucleotides in terms of inhibition was ATP greater than adenosine 5'-[gamma-thio]triphosphate (ATP[S]) greater than UTP much greater than or equal to CTP much greater than or equal to GTP much greater than or equal to guanosine 5'-[gamma-thio]triphosphate (GTP[S]) much greater than or equal to ITP. The potency with respect to stimulation had the following order: adenosine 5'-[beta gamma-methylene]triphosphate (App[CH2]p) greater than ATP greater than guanosine 5'-[beta-thio]diphosphate (GDP[S]). Adenosine 5'-[beta gamma-imido]triphosphate (App[NH]p) was also stimulatory, and was more potent than ATP in the parotid granules, but less potent in the pancreatic granules. Aluminium fluoride stimulated Cl- conductance maximally at 15-30 microM-Al3+ and 10-15 mM-F. F was less effective at higher concentrations. Protein phosphorylation by kinases was apparently not involved, since the nucleotide effects (1) could be mimicked by non-hydrolysable analogues of ATP and GTP, (2) showed reversibility, and (3) were not abolished by the protein kinase inhibitors 1-(5-isoquinolinesulphonyl)-2-methylpiperazine (H-7) or staurosporine. The data suggest the presence of at least two binding sites for nucleotides, whereby occupancy of one induces inhibition and occupancy of the other induces stimulation.

Effects of a cytosolic protein on the interaction of rat pancreatic zymogen granules in vitro

Photon correlation spectroscopy has been used to study the kinetics of aggregation of isolated rat pancreatic zymogen granules in vitro by monitoring time-dependent changes in mean particle size derived from the photon count autocorrelation function, g2(tau). Isolated granules were stable in isotonic sucrose (pH 5.4-7.0). At pH 6.0 they maintained a mean diameter of 1225 +/- 18 nm with a polydispersity index of 0.199 +/- 0.007. The mean granule diameter showed a limited decrease (approx. 20%) with increasing pH within the range 5.4-7.0, but the polydispersity index was unaltered. At pH greater than 7.0 granule instability was indicated by a rapid reduction in total photon counts. In solutions of monovalent cations ([M+] greater than 10 mM) and divalent cations ([M2+] greater than 0.5 mM) zymogen granules aggregated at a rate dependent upon both ion and granule concentration. These effects were consistent with the bimolecular nature of the interaction mechanism and were clearly distinguishable from the limited size changes associated with osmolarity. At concentrations of Na+ or K+ salts greater than 50 mM granule aggregation was accompanied by anion-dependent solubilisation. A soluble protein fraction separated from the pancreatic acinar cell cytosol by gel filtration reduced the mean diameter and polydispersity index of zymogen granules suspended in isotonic sucrose, inhibited cation-induced aggregation and stabilised granules to solubilisation induced by raising pH greater than 7.0 or exposure to high ionic strength media. The inhibitory effects of this protein were apparent at concentrations less than or equal to 10 micrograms X ml-1 (i.e. at inhibitor: granule protein ratios less than 1:20) and could not be mimicked by bovine serum albumin, the Ca2+-binding proteins calmodulin and troponin C (less than or equal to 100 micrograms X ml-1), nor the highly negatively charged polymer polyglutamate (less than or equal to 10 micrograms X ml-1). Inhibitory activity was also absent from fractions of rat liver cytosol prepared identically to pancreatic acinar cytosol. These observations are consistent with the presence in pancreatic acinar cells of a specific cytosolic granule stabilisation factor (or factors) that normally restricts zymogen granule interaction and may therefore play an important role in the regulation of granule mobility and exocytosis.

Protein kinase activity associated with pancreatic zymogen granules

Purified zymogen granules were prepared from rat pancreas by using an iso-osmotic Percoll gradient. In the presence of [gamma-32P]ATP, phosphorylation of several granule proteins was induced by Ca2+, most notably a Mr-13 000 protein, whereas addition of cyclic AMP was without effect. When phosphatidylserine was also added, Ca2+ increased the phosphorylation of additional proteins, with the largest effect on a protein of Mr 62 000. Purified granules were also able to phosphorylate exogenous substrates. Ca2+-induced phosphorylation of lysine-rich histone was enhanced over 3-fold in the presence of phosphatidylserine, and cyclic AMP-activated protein kinase activity was revealed with mixed histone as substrate. The concentrations of free Ca2+ and cyclic AMP required for half-maximal phosphorylation of both endogenous and exogenous proteins were 1-3 microM and 57 nM respectively. Treatment of granules with 0.25 M-KCl resulted in the release of phosphatidylserine-dependent kinase activity into a high-speed granule supernatant. In contrast, granule-protein substrates of Ca2+-activated kinase activity were resistant to KCl extraction, and in fact were present in purified granule membranes. Kinase activity activated by cyclic AMP was not extracted by KCl treatment. It is concluded that phosphorylation of integral membrane proteins in the zymogen granule can be induced by one or more Ca2+-activated protein kinases. Such a reaction is a potential mechanism by which exocytosis may be regulated in the exocrine pancreas by Ca2+-mediated secretagogues.