Studies on the pancreas of the guinea pig. Parallel processing and discharge of exocrine proteins

Glycoprotein 2 as a gut gate keeper for mucosal equilibrium between inflammation and immunity

Glycoprotein 2 (GP2) is a widely distributed protein in the digestive tract, contributing to mucosal barrier maintenance, immune homeostasis, and antigen-specific immune response, while also being linked to inflammatory bowel disease (IBD) pathogenesis. This review sheds light on the extensive distribution of GP2 within the gastrointestinal tract and its intricate interplay with the immune system. Furthermore, the significance of GP2 autoantibodies in diagnosing and categorizing IBD is underscored, alongside the promising therapeutic avenues for modulating GP2 to regulate immunity and maintain mucosal balance.

Induction of Pancreatic Inflammation Accelerates Pancreatic Tumorigenesis in Mice

Pancreatitis is a major risk factor for the development of pancreatic cancer. In genetically engineered mouse models, induction of pancreatic inflammation dramatically accelerates oncogenic KRas-induced fibrosis, precancerous PanIN formation, and tumorigenesis. Here we describe simple methods of secretagogue-induced experimental acute and chronic pancreatitis, the most commonly used pancreatitis models, and their applications in pancreatic cancer research. Additionally, the preparation of primary pancreatic acinar cells is introduced. Primary acinar cells can be used to study the early events of pancreatic inflammation and pancreatic acinar-to-ductal (ADM) metaplasia.

Proteoglycans support proper granule formation in pancreatic acinar cells

Zymogen granules (ZG) are specialized organelles in the exocrine pancreas which allow digestive enzyme storage and regulated secretion. The molecular mechanisms of their biogenesis and the sorting of zymogens are still incompletely understood. Here, we investigated the role of proteoglycans in granule formation and secretion of zymogens in pancreatic AR42J cells, an acinar model system. Cupromeronic Blue cytochemistry and biochemical studies revealed an association of proteoglycans primarily with the granule membrane. Removal of proteoglycans by carbonate treatment led to a loss of membrane curvature indicating a supportive role in the maintenance of membrane shape and stability. Chemical inhibition of proteoglycan synthesis impaired the formation of normal electron-dense granules in AR42J cells and resulted in the formation of unusually small granule structures. These structures still contained the zymogen carboxypeptidase, a cargo molecule of secretory granules, but migrated to lighter fractions after density gradient centrifugation. Furthermore, the basal secretion of amylase was increased in AR42J cells after inhibitor treatment. In addition, irregular-shaped granules appeared in pancreatic lobules. We conclude that the assembly of a proteoglycan scaffold at the ZG membrane is supporting efficient packaging of zymogens and the proper formation of stimulus-competent storage granules in acinar cells of the pancreas.

The guinea pig pancreas secretes a single trypsinogen isoform, which is defective in autoactivation

OBJECTIVES

The aim of the present study was to purify and clone the trypsinogen isoforms from the guinea pig pancreas and characterize their activation properties.

METHODS

Trypsinogens from pancreatic homogenates were isolated by ecotin-affinity chromatography, followed by cation-exchange chromatography. Activation of trypsinogens was tested with enteropeptidase, cathepsin B, and trypsin. Complementary DNAs for pretrypsinogens were cloned from total RNA after reverse transcription and polymerase chain reaction amplification.

RESULTS

Purification of trypsinogens yielded a single peak with an N-terminal amino-acid sequence of LPIDD. Cloning of pretrypsinogen cDNAs revealed 2 distinct but nearly identical isoforms. At the amino acid level, the only difference between the 2 isoforms is an Ala/Ser change at position 15 within the signal peptide. Thus, both cDNA variants give rise to the same mature trypsinogen upon secretion. Guinea pig trypsinogen is readily activated by enteropeptidase and cathepsin B but exhibits essentially no autoactivation, under conditions where human cationic and anionic trypsinogens rapidly autoactivate.

CONCLUSIONS

The observations suggest that multiple trypsinogen isoforms and their ability to autoactivate are not required universally for normal digestive physiology in mammals. Furthermore, the inability of guinea pig trypsinogen to undergo autoactivation suggests that this species might be more resistant to pancreatitis than humans, where increased autoactivation of cationic trypsinogen mutants has been linked to hereditary pancreatitis.

Effects of exogenous cholecystokinin and gastrin on the secretion of trypsin and chymotrypsin from yellowtail (Seriola quinqueradiata) isolated pyloric caeca

The humoral control of secretion of the proteolytic enzymes trypsin and chymotrypsin was studied in yellowtail (Seriola quinqueradiata). In vitro trials were performed to investigate the effects of cholecystokinin (CCK) and two commercially available gastrin peptides. Isolated preparations of pyloric caeca/pancreas release trypsin and chymotrypsin when incubated with cholecystokinin (CCK) at 10 microM and gastrin I (G1) at 50 microM after 15 min of incubation. On the other hand, G1 at 10 microM and gastrin-related peptide (G2) did not enhance trypsin and chymotrypsin secretion. The studies concerning the CCK effects at different incubation temperatures have shown that trypsin and chymotrypsin secretion at 25 degrees C was stimulated by CCK after 15 min, while at 10, 15 and 20 degrees C the stimulatory effects of CCK were observed only after 30 min of incubation. The CCK effects were increased at higher incubation temperatures and longer incubation periods.

Changes in cholecystokinin and peptide Y gene expression with feeding in yellowtail (Seriola quinqueradiata): relation to pancreatic exocrine regulation

In fish, the regulation of digestive enzyme secretion by hormonal control such as cholecystokinin (CCK) and neuropeptide Y (NPY)-related peptide is not well understood. To investigate the roles of fish CCK and peptide Y (PY) in digestive enzyme secretion, mRNA levels of CCK and PY, pyloric caeca enzyme activities and mRNA levels of pancreatic digestive enzymes (lipase, trypsin and amylase) were measured at pre- and post-prandial stages in yellowtail. Pyloric caeca were sampled at 0, 0.5, 1.5, 3, 6, 12, 24 and 48 h after feeding. The mRNA levels of trypsin and amylase increased after feeding, suggesting that transcription was induced by feed ingestion. Digestive enzyme activities decreased in exocrine pancreas after feeding, suggesting the stored enzyme was secreted from pancreas post-prandially. mRNA levels for CCK displayed a time-dependent increase, peaking between 1.5 and 3 h after-feeding followed by a rapid decrease 3 to 6 h after feeding. The mRNA expression pattern of PY was inverse to the pattern of CCK, decreasing until 1.5 h after feeding and then rising to initial levels by 12 h after feeding. These results suggest that CCK and PY work antagonistically in the exocrine pancreas of yellowtail.

The ER to Golgi interface is the major concentration site of secretory proteins in the exocrine pancreatic cell

By using quantitative immuno-electron microscopy of two-sided labeled resin sections of rat exocrine pancreatic cells, we have established the relative concentrations of the secretory proteins amylase and chymotrypsinogen in the compartments of the secretory pathway. Their total concentration over the entire pathway was approximately 11 and approximately 460 times, respectively. Both proteins exhibited their largest increase in concentration between the endoplasmic reticulum and cis-Golgi, where they were concentrated 3-4 and 50-70 times, respectively. Over the further pathway, increases in concentration were moderate, albeit two times higher for chymotrypsinogen than for amylase. From trans-Golgi to secretory granules, where the main secretory protein concentration is often thought to occur, relatively small concentration increases were observed. Additional observations on a third secretory protein, procarboxypeptidase A, showed a concentration profile very similar to chymotrypsinogen. The relatively high concentration of amylase in the early compartments of the secretory route is consistent with its exceptionally slow intracellular transport. Our data demonstrate that secretory proteins undergo their main concentration between the endoplasmic reticulum and cis-Golgi, where we have previously found concentration activity associated with vesicular tubular clusters (Martínez-Menárguez JA, Geuze HJ, Slot JW, Klumperman J. Cell 1999; 98: 81-90).

Cadmium inhibits stimulated amylase secretion from isolated pancreatic lobules of the guinea-pig

The effect of cadmium chloride on pancreatic exocrine secretion 'in vitro' was examined using guinea-pig isolated lobules. Cadmium (10(-3)M) stimulated amylase release when added alone to the incubation medium and the increase of amylase was unaffected by atropine. Cadmium (10(-4)M) did not significantly modify the basal amylase release. Depolarization of pancreatic nerves with potassium stimulated amylase secretion; the stimulant effect of KCl was completely inhibited by atropine. Cadmium (10(-4)M) inhibited, but did not abolish, the stimulant effect of KCl, indicating a direct effect of the metal on the acinar cell. Cadmium (10(-4)M) also inhibited the amylase release evoked by the secretagogues carbachol and caerulein, which are known to act directly on the acinar cell. Taken together with previous data reporting a large increase of pancreatic cadmium concentration following cadmium ingestion, the strong inhibition of pancreatic secretion observed in our experiments suggests that the exocrine pancreas may be regarded as a possible target organ of cadmium toxicity.

Real-time studies of zymogen granule exocytosis in intact rat pancreatic acinar cells

An adequate understanding of secretion requires the measurement of exocytosis on the same time scale as that used for second messenger dynamics. To investigate the kinetics of ACh-evoked secretion in pancreatic acinar cells, exocytosis of zymogen granules was quantified by continuous, time-differential analysis of digital images. The validity of this method was confirmed by simultaneous fluorescence imaging of quinacrine-loaded zymogen granules. Basal rates of exocytosis were low (0.2 events min(-1)). ACh stimulated a biphasic increase in secretory activity, maximal rates exceeding 20 events min(-1) after 10 s of ACh application (10 microM). Over the next 15 s the rate of exocytosis fell to less than 4 events min(-1); then began a second phase of secretion that peaked 15 s later at approximately 11 events min(-1), but subsequently declined in the continued presence of agonist. Measurements of fura-2 fluorescence demonstrated a biphasic increase in intracellular [Ca2+] ([Ca2+]i). Comparison of the [Ca2+]i records and time-differential analysis revealed that the fall in exocytotic rate following the initial burst occurred despite the fact that [Ca2+]i remained high. The second phase of secretion depended on both [Ca2+]i and [ACh]. At 10 microM ACh there was a decrease in the steepness of the relationship between [Ca2+]i and exocytosis that led to an enhancement of the slow secretory phase. We propose that acinar cells contain two pools of secretory vesicles: a small pool of granules that is exocytosed rapidly, but is quickly depleted; and a reserve pool of granules that can be recruited by ACh in a process that is modulated by second messengers other than calcium.

A submembranous matrix of proteoglycans on zymogen granule membranes is involved in granule formation in rat pancreatic acinar cells

The secretory lectin ZG16p mediated the binding of aggregated zymogens to the granule membrane in pancreatic acinar cells. Using a recently established in vitro condensation-sorting assay, we now show that pretreatment of zymogen granule membranes (ZGM) with either sodium bicarbonate at pH 10 or with phosphatidyl inositol-specific phospholipase C (PI-PLC) reduced the binding efficiency of zymogens to the same extent, as distinct components were liberated from ZGM. Analysis of the composition of the bicarbonate extract revealed the presence of the secretory lectin ZG16p, the serpin ZG46p and the GPI-linked glycoprotein GP-2, together with several unknown proteins, and small amounts of lipase and carboxylester lipase. The unknown proteins detected in 2-D gels represented a group of acidic and basic protein spots, which were positive in a glycan staining reaction and were soluble in methanol. One protein spot of the acidic group and several of the basic group reacted with a monoclonal antibody directed against chondroitin sulfate, indicating that the proteins represented proteoglycans. A staining pattern similar to the glycan reaction was observed in immunoblots using a polyclonal antibody directed against the whole bicarbonate extract. Immunogold electron microscopy revealed that this antibody reacted with components in the periphery of zymogen granules and strongly stained ZGM in the pellet fraction of a standard in vitro condensation-sorting assay. The amino acid composition of isolated components of both the acidic and basic group showed similarities to aggrecan, a cartilage-specific proteoglycan, and to glycine-rich glycoproteins, respectively. We therefore conclude that a submembranous matrix on the ZGM composed of proteoglycans and glycoproteins is involved in granule formation in pancreatic acinar cells.

Effect of exogenous cholecystokinin on the discharge of the gallbladder and the secretion of trypsin and chymotrypsin from the pancreas of the Atlantic salmon, Salmo salar L

The humoral control of release of the proteases trypsin and chymotrypsin was investigated in the Atlantic salmon (Salmo salar L.). Intraperitoneal injection of a purified preparation of the peptide cholecystokinin (CCK) from pig into starved fish produces a dose-dependent release of both enzymes from the pyloric caeca/pancreas tissues which accumulate in the intestinal contents (digesta). It also induces release of the contents of the gallbladder. Isolated preparations of pyloric caeca/pancreas when incubated with CCK release trypsin and chymotrypsin. It is concluded that while a possible role for a neuronal component to the control and regulation of these enzymes cannot be ruled out, humoral control by a CCK-like peptide has been established. The fact that a mammalian-derived extract of CCK induces this response in fish indicates an early evolution and subsequent conservation of this control mechanism in the vertebrates.

Secretory pathways in animal cells: with emphasis on pancreatic acinar cells

Studies over the past three decades have clearly established the existence of at least two distinct pathways for the intracellular transport and release of secretory proteins by animal cells. These have been identified as the regulated and constitutive pathways. Many observations have indicated that in certain cells, such as those of the exocrine pancreas and parotid glands at least, these pathways coexist in the same cells. Although the general scheme of protein transport within these pathways is well established, many fundamental aspects of intracellular transport remain to be unraveled. How are proteins transported through the endoplasmic reticulum? How are the transitional vesicles formed and what are the underlying mechanisms involved in their fusion with the cis-Golgi cisterna? Even the general mode of transfer through the Golgi stack is debated: Is there a diffusion through the stack by flow through intercisternal tubules and openings or is there a vesicle transfer system where membrane quanta hop from one cisterna to the other? What is the fate of secretory proteins in the trans-Golgi area and by what mechanisms is a fraction of newly synthesized molecules of a given secretory protein released spontaneously while the majority of such nascent molecules are diverted into a secretory granule compartment? In this review, we have examined these and other aspects of intracellular transport of secretory proteins using pancreatic acinar cells as our reference model and we present some evidence to support the existence of a paragranular pathway of secretion associated with secretory granule maturation.

The asynchronous transport of secretory proteins in the exocrine pancreas. Compatibility with the hypothesis of a paragranular pathway?

An asynchronous transport of individual secretory proteins has been recently described in the pancreas. This asynchrony was observed in both unstimulated and stimulated conditions. It has also been proposed that unstimulated and stimulated secretions correspond to distinct secretory processes. Indeed according to that hypothesis, under resting conditions, a small fraction of the newly synthesized secretory proteins are channeled into a paragranular (vesicle) pathway while the residual proteins are packaged in the zymogen granules. These zymogen granules eventually move to the cell surface where their content is extruded by exocytosis. Under stimulated conditions the latter process is accelerated. Since the same type of asynchrony is observed under resting and stimulated conditions in the pancreatic juice, one can wonder if the hypothesis of a paragranular pathway is compatible with the observed asynchrony. In this review, an explanation is presented to account for the facts that following pulse and chase labelling, two waves of labelled proteins are released under resting secretions and secondly that asynchrony is maintained in both resting and stimulated conditions.

Evidence in vivo of asynchronous intracellular transport of rat pancreatic secretory proteins

Labeled proteins which appeared in pancreatic juice after the intravenous (i.v.) injection of [35S]methionine into conscious rats with chronic pancreatic duct fistulae were separated by gel electrophoresis and measured by determination of the radioactivity of each of the separated bands. Radioactivity appeared in the secreted proteins 20 min after injection of the label. In the subsequent 10 min, 6.44% of the radioactivity was found in trypsinogen, whereas 100 min later only 3.4% of the radioactivity was associated with this enzyme. The values at 10 and 100 min for amylase were 10.85% and 21%, respectively, showing an earlier appearance of labeled trypsinogen than of amylase. Chymotrypsinogen behaved similarly to trypsinogen. Early secretion of labeled proteases was also demonstrated by separation of pancreatic proteins by two-dimensional gel electrophoresis followed by fluorography. In pancreatic duct cannulated rats, zymogen granules were prepared 30 and 60 min after injection of the labeled methionine. Determination of the radioactivity of the individual proteins demonstrated a similar time course of the labeling pattern in the zymogen granule fraction to that in pancreatic juice. The results of the experiments suggest an asynchronous secretion of newly synthesized rat pancreatic proteins.

Identification and localization of synaptophysin, an integral membrane glycoprotein of Mr 38,000 characteristic of presynaptic vesicles

A polypeptide of Mr 38,000 has been identified as a specific component of the membrane of presynaptic vesicles, using the monoclonal antibody SY38. This protein, which is acidic (isoelectric at approximately pH 4.8) and glycosylated, appears to be an integral membrane protein, as suggested by its solubilization with the nonionic detergent Triton X-100 and the finding that the epitope recognized by antibody SY38 is located on the cytoplasmic surface of those vesicles. It is found in presynaptic vesicles of neurons of the brain, spinal cord, and retina as well as at neuromuscular junctions. It is also found in the adrenal medulla. Its occurrence in diverse vertebrate species indicates its stability during evolution. This protein, for which we propose the name synaptophysin*, provides a molecular marker for the presynaptic vesicle membrane and may be involved in synaptic vesicle formation and exocytosis.

Pancreatic secretion by nonparallel exocytosis: potential resolution of a long controversy

The idea that pancreatic digestive enzyme secretion can occur in a nonparallel manner has been controversial because of its presumed incompatibility with the exocytosis secretory mechanism. Correlation and regression analysis of enzyme output by the rabbit pancreas after it is stimulated with cholecystokinin and chymodenin revealed that digestive enzymes are secreted in a highly linked fashion, compatible with exocytosis and with nonparallel secretion. Thus, exocytosis and nonparallel secretion are not contradictory processes, but rather nonparallel secretion is due to exocytosis from heterogeneous sources within the pancreas.

Isoelectric focusing studies on human pancreatic secretion

Pure bile, pancreatic and duodenal human juices have been analyzed by isoelectric focusing, either at rest or upon stimulation with caerulein. In rats, stimulation has also been performed by secretin. Twenty bands have been resolved and quantified in the pancreatic secretion. By developing zymograms, a number of isozymes have been identified: 6 iso-amylases [pl's 7.2, 7.1 and 6.6 (major) and pl's 7.4, 6.7 and 5.8 (minor)], 3 lipases [pl's 7.0 and 6.8 (major) and 6.4 (minor)], two major alkaline proteases (pl's 9.8 and 8.4) and one major acidic protease (pl 4.3) and one band of RNAase activity (pl 8.6). The stimulation kinetics follow a mechanism according to Palade, indicating uniform response to secretogogues, parallel intracellular transport and parallel discharge of pancreatic exocrine proteins.

Kinetics of biosynthesis of acetylcholine receptor and subsequent incorporation into plasma membrane of cultured chick skeletal muscle

20% of the acetylcholine receptors in cultured chick skeletal muscle remain unbound following long-term growth of muscle in medium containing a potent, essentially irreversible receptor-blocking agent, alpha-bungarotoxin. About half the receptors which are unavailable for interaction with extracellular alpha-bungarotoxin are newly synthesized molecules which presumably are being processed and transported to the plasma membrane. When the muscle cultures are switched to a medium containing 2H, 13C, 15N-amino acids, these receptors are rapidly labeled, the fraction of labeled molecules beginning to plateau at 3 hr. Few labeled receptors appear in the plasma membrane during the first 3 hr of labeling with 2H, 13C, 15N-amino acids. After 3.5 hr of labeling, virtually all the receptors being incorporated into the plasma membrane are labeled receptors. The kinetics of labeling of the "pool" and "surface" receptors with 2H, 13C, 15N-amino acids confirm the "precursor-product" type relationship of pool and surface acetylcholine receptors. In this study, receptors synthesized in medium containing 2H, 13C, 15N-amino acids were resolved from 1H, 12C, 14N-receptors by velocity sedimentation in sucrose-deuterium oxide and sucrose-H2O gradients, and their densities were estimated from sedimentation rates in shallow gradients of various average density. Estimated densities were 1.32 g/cm3 for 1H, 12C, 14N-receptors and 1.41 g/cm3 for 2H, 13C, 15N-receptors. This density difference corresponds to 80% substitution of normal aminoacyl residues by 2H, 13C, 15N-residues in the denser receptor.