Newly synthesized secretory proteins from pig pancreas are not released from a homogeneous granule compartment

Sorting and storage during secretory granule biogenesis: looking backward and looking forward

Secretory granules are specialized intracellular organelles that serve as a storage pool for selected secretory products. The exocytosis of secretory granules is markedly amplified under physiologically stimulated conditions. While granules have been recognized as post-Golgi carriers for almost 40 years, the molecular mechanisms involved in their formation from the trans-Golgi network are only beginning to be defined. This review summarizes and evaluates current information about how secretory proteins are thought to be sorted for the regulated secretory pathway and how these activities are positioned with respect to other post-Golgi sorting events that must occur in parallel. In the first half of the review, the emerging role of immature secretory granules in protein sorting is highlighted. The second half of the review summarizes what is known about the composition of granule membranes. The numerous similarities and relatively limited differences identified between granule membranes and other vesicular carriers that convey products to and from the plasmalemma, serve as a basis for examining how granule membrane composition might be established and how its unique functions interface with general post-Golgi membrane traffic. Studies of granule formation in vitro offer additional new insights, but also important challenges for future efforts to understand how regulated secretory pathways are constructed and maintained.

Protein targeting via the "constitutive-like" secretory pathway in isolated pancreatic islets: passive sorting in the immature granule compartment

We have suggested the existence of a novel "constitutive-like" secretory pathway in pancreatic islets, which preferentially conveys a fraction of newly synthesized C-peptide, insulin, and proinsulin, and is related to the presence of immature secretory granules (IGs). Regulated exocytosis of IGs results in an equimolar secretion of C-peptide and insulin; however an assay of the constitutive-like secretory pathway recently demonstrated that this route conveys newly synthesized C-peptide in molar excess of insulin (Arvan, P., R. Kuliawat, D. Prabakaran, A.-M. Zavacki, D. Elahi, S. Wang, and D. Pilkey. J. Biol. Chem. 266:14171-14174). We now use this assay to examine the kinetics of constitutive-like secretion. Though its duration is much shorter than the life of mature granules under physiologic conditions, constitutive-like secretion appears comparatively slow (t1/2 approximately equal to 1.5 h) compared with the rate of proinsulin traffic through the ER and Golgi stacks. We have examined whether this slow rate is coupled to the rate of IG exit from the trans-Golgi network (TGN). Escape from the 20 degrees C temperature block reveals a t1/2 less than or equal to 12 min from TGN exit to stimulated release of IGs; the time required for IG formation is too rapid to be rate limiting for constitutive-like secretion. Further, conditions are described in which constitutive-like secretion is blocked yet regulated discharge of IGs remains completely intact. Thus, constitutive-like secretion appears to represent an independent secretory pathway that is kinetically restricted to a specific granule maturation period. The data support a model in which passive sorting due to insulin crystallization results in enrichment of C-peptide in membrane vesicles that bud from IGs to initiate the constitutive-like secretory pathway.

[Detailed analysis of human pancreatic secretions collected by retrograde catheterization. Parallel or non-parallel excretion of digestive enzymes?]

Pancreatic juices were collected by selective reverse catherism of the chief pancreatic duct in two patients, one free from pancreatic disease and the other having a pancreas cancer. They were analysed in detail especially in order to get information on the mechanism of enzyme excretion. The variations of the digestive enzyme activities (amylase, lipase, trypsin, chymotrypsin, carboxypeptidase A and B) were not superimposable among them or with the fluctuations of the protein concentration in the pancreatic juice samples. These results agree with a non-parallel enzyme-excretion mechanism by the pancreas. However deep electrophoresis analyses of pancreatic juice samples showed that the ratio of each digestive enzyme concentration remained almost constant in the same patient. This observation disagrees with the above conclusion and suggests that the data obtained by using classical methods for estimating digestive enzyme activities have to be considered prudently. By another way, two main significant differences were reported by analysing the ionic composition of the pancreatic juice samples following their origin. The pancreatic juice samples of the patient having a pancreas cancer had a lower and more variable Na+ concentration than those coming from the patient who was free from pancreas disease. They had a HCO3- concentration which was almost constant, contrary to what was observed for the pancreatic juice secreted by the other patient.

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.

Sorting and secretory pathways in exocrine cells

Exocrine secretory cells contain multiple post-Golgi pathways from protein secretion. The major pathway in pancreatic and parotid acinar cells involves protein sorting into storage granules that undergo exocytosis with or without stimulation by secretagogues. This route of release is paralleled by a minor nongranular (but vesicular) pathway that originates by budding from maturing secretory granules. The nongranular pathway carries the same polypeptides that undergo storage in the granules but in different relative amounts. These features indicate that sorting into the stimulus-regulated pathway reflects not only the deposition of secretory proteins into immature granules but may also involve selective aggregation of proteins along with exclusion and vesicle-mediated secretion of other polypeptides that are inefficiently retained. Storage granules represent a distinct compartment of the secretory pathway, as indicated by the specific composition of their limiting membranes. Little is known about processes that maintain the low content and limited diversity of integral proteins of the granule membrane as compared to the membranes with which it fuses during exocytosis and formation. Future studies will examine the role of the nongranular secretory pathway in acinar cells, the branchpoint of pathways that are directed to the apical or basolateral cell surfaces, the structural determinants of secretory sorting, and the distribution and function of specific granule membrane polypeptides.

Secretion of gamma-glutamyltranspeptidase by the pancreas: evidence for a membrane shedding process during exocytosis

Gamma Glutamyltranspeptidase (GGT) is a membrane-bound enzyme involved in glutathione metabolism. It is present in rat exocrine pancreas at a level which is only exceeded by the kidney. It has been previously shown that most of the enzyme activity is located in the apical area of the acinar cell, more precisely at the level of zymogen granules and plasma membrane. The aim of the present study was to examine the secretory behavior of that enzyme. Under resting conditions, in vivo, high levels of GGT were found in pancreatic juice and its level was not related to protein concentration. Under secretin infusion, a relatively constant level of GGT was released, and again, there was no correlation between enzyme activity and protein concentration. Following a bolus injection of caerulein, an analog of cholecystokinin, marked and concomitant rises in protein and GGT levels were observed. Ultracentrifugation, as well as gel filtration on Sepharose 4B, demonstrated that the enzyme was not released in a soluble form. This observation is in agreement with in vitro determinations on isolated zymogen granules showing that GGT is totally associated with the ZG membrane and undetect-able in the content of these organelles. The present data show that 1 degree GGT is released from the rat pancreas acinar cells in a particulate form; 2 degree GGT release is elicited by hormonal stimulation coinciding with the exocytotic release of secretory proteins. Our observations lead us to propose that in rat pancreas, ZG membrane fragments are released along with secretory proteins during exocytosis.

Differences in the protein composition of rat parotid salivas evoked by sympathetic and parasympathetic nerve stimulation

1. Parotid salivas were collected from rats following electrical stimulation of either the sympathetic or parasympathetic nervous supplies. 2. The protein compositions of these salivas were compared using high performance ion-exchange chromatography (FPLC), SDS electrophoresis and biochemical assay. 3. Chromatography and electrophoresis indicated differences between the protein compositions of sympathetic and parasympathetic salivas. 4. Protein secretion derived solely from the exocytosis of parotid acinar cells cannot account for these differences.

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.

In adrenal medulla synaptophysin (protein p38) is present in chromaffin granules and in a special vesicle population

We have analyzed the properties and subcellular localization of synaptophysin (protein p38) in bovine adrenal medulla. In one-dimensional immunoblotting the adrenal antigen appears identical to synaptophysin of rat synaptic vesicles. In two-dimensional immunoblotting it migrates as a heterogeneous band varying in pI from 4.5 to 5.8. Subcellular fractionation by various sucrose gradients revealed that synaptophysin was present in two different cell particles. More than half of the antigens present in adrenal medulla were confined to special membranes that sedimented both with the "large granules" and with microsomal elements. These membranes could be removed from the large granule sediment by washing. In gradients it equilibrated in regions of low sucrose density. These membranes did not contain any markers for chromaffin granules. Less than half of the amount of synaptophysin present in adrenal medulla copurified with chromaffin granules. Despite several variations in the fractionation scheme synaptophysin could not be removed from chromaffin granules. After washing of granule membranes with alkaline solution synaptophysin still cosedimented in gradients with typical granule markers. The concentration of synaptophysin in membranes of chromaffin granules is low (less than 10%) when compared with synaptic vesicles. It is concluded that in adrenal medulla synaptophysin is present in special membranes, probably in high concentration, and in membranes of chromaffin granules, either in a low concentration in all or in a higher concentration in some of them.

Lipid analysis of a novel type of cell secretion in the exocrine pancreas: the pancreasomes

A novel type of cell secretion termed 'microvesicular secretion' has been described recently in the exocrine pancreas. According to this process, microvesicles are released by acinocytes in the pancreas acinar lumen. These microvesicles, identified as 'pancreasomes', were characterized by the presence of a major glycoprotein component originating in the exocrine acinar cell. In the present work, phospholipids of pancreasomes have been identified. Five classes of phospholipid were found: phosphatidylcholines, phosphatidylethanolamines, lysophosphatidylcholines, sphingomyelins and another minor class of ninhydrin-positive phospholipid (phosphatidylserines or lysophosphatidylethanolamines). The ratios of neutral lipids to phospholipids were particularly high (3:1), as estimated by GLC of their fatty acid content. Analysis of fatty acid composition of pancreasomes lipids revealed a very high proportion of two saturated fatty acids, palmitic (40%) and stearic (24%), whereas two main unsaturated fatty acids, oleic (17%) and linoleic (8%), were found in smaller proportions. Differential scanning calorimeter studies on washed pancreasomes indicated that there was no lipid phase transition in their membrane, despite the absence of cholesterol. Our observations show that pancreasomes have an unusual lipid composition and confirm our previous conclusion based on protein analysis that the release of pancreasomes occurs according to an hitherto undescribed type of secretion, in which a glycoprotein is released associated with specific domains of the luminal plasma membrane.

Synthesis and secretion of amylase in the rat parotid gland following autonomic nerve stimulation in vivo

Parasympathetic and sympathetic nerve stimulation in vivo, individually and in combination, was used to study secretion and synthesis of amylase in the rat parotid gland. After 30 min with sympathetic nerve stimulation (3 Hz) a decrease in glandular amylase was seen, which corresponded approximately to the salivary output. On the other hand, after parasympathetic stimulation (10 Hz), chosen to obtain comparable amylase output, there was no decrease in glandular amylase, which points to synthesis during such activation. Experiments with incorporation of [3H]leucine, reflecting amylase synthesis, showed that both types of nerve stimulations increased such uptake in parotid protein. These results indicate that beside sympathetic activity, which is the main stimulus for granular amylase secretion, parasympathetic nerve impulses can evoke considerable amylase secretion because amylase synthesis is stimulated and amylase is rapidly available from a special, possibly non-granular pool. As expected from previous experiments an augmentation of amylase secretion was found, and the present experiments also indicated an augmentation at the level of synthesis when the two nerves were stimulated at the same time.

Pathways of secretion in the exocrine pancreas: the status of resting secretion

In the last decade the concept of two distinct pathways of secretion in the exocrine pancreas has slowly emerged. According to this concept, one pathway is involved in stimulated (regulated) conditions and another under resting (constitutive) conditions. This hypothesis was elaborated at first from the comparison of the specific radioactivities of secretory proteins released by the gland under resting and stimulated conditions. Analysis of the protein composition of the juice released under these two physiological conditions further supported that hypothesis. More recent studies compared the kinetic of accumulation of newly synthesized proteins in zymogen granule and their release in the gland lumen. The latter results are in agreement with a model in which secretory proteins are channelled in two separate pathways, one regulated, and one constitutive. Essentially, the constitutive pathway would correspond to a paragranular route in which the proteins would be immediately secreted instead of being stored in zymogen granules. In addition, some of the proteins released in the juice under "resting" conditions are associated to microvesicles. The term "microvesicular secretion" is used to designate that type of secretion.

Phasic release of newly synthesized secretory proteins in the unstimulated rat exocrine pancreas

Pancreatic lobules from fasted rats secrete pulse-labeled proteins in two phases comprising 15 and 85% of basal output, respectively. The first (0-6.5 h) is initially (less than or equal to 0.5 h) unstimulated by secretagogues, probably represents vesicular traffic of Golgi and post-Golgi origin (including condensing vaculoles/immature granules), and notably contains two groups of polypeptides with distinct release rates: zymogens (t1/2 approximately 2.4 h) and minor nonzymogens plus one unique zymogen (t1/2 approximately 1 h). The second phase (peak at 9-10 h) is stimulable, probably represents basal granule exocytosis (t1/2 approximately 5 h), and contains zymogens exclusively. Newly synthesized proteins released in both phases appear asynchronously, reiterating their asynchronous transport through intracellular compartments. Zymogens in both phases are secreted apically. The sorting of first from second phase zymogen release does not appear to be carrier-mediated, although the sorting of zymogens from other secretory proteins may use this process. Finally, data are presented that suggest that both secretory phases are subject to physiologic regulation.

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.

Nonparallel discharge of digestive enzymes from isolated pancreatic acini

The secretion of amylase, trypsinogen, chymotrypsinogen and proelastase from isolated rat dispersed pancreatic acini was investigated in the absence (basal) and presence of two concentrations of CCK8 (50 and 500 pM), carbachol (2.5 and 7.5 microM) and secretin (10 nM and 1 microM). The unstimulated (basal) rate of release of each of the digestive enzymes was essentially the same. However, whereas both doses of CCK8 and carbachol caused a preferential release of chymotrypsinogen over that of amylase and trypsinogen, the magnitude of stimulated release of amylase, trypsinogen and chymotrypsinogen by 1 microM secretin was found to be similar for each of the enzymes. Furthermore, none of the secretagogues caused a significant enhancement in proelastase release. The present data demonstrate that whereas CCK8 and carbachol induce a greater release of chymotrypsinogen over that of amylase or trypsinogen, release of all three enzymes was equally stimulated by secretin from isolated pancreatic acini.

Electrophoretic and cytological evidence for heterogeneity of pancreatic acinar cell responsiveness to carbachol, caerulein and secretin

Incubation of rat pancreatic lobules for 90 min with optimal concentrations of caerulein, carbachol or secretin caused the release of about 30% of the amylase content. Combination of secretin with carbachol or caerulein increased the amylase output to about 40%. With secretin, as with carbachol or caerulein, heterogeneity of cellular responsiveness was observed, some acini being partially or completely depleted of their zymogen granules, whereas others appeared to be resting. When secretin was combined with carbachol or caerulein, granule depletion, originally confined to small groups of neighbouring acini, spread to form large areas of degranulated cells, sometimes comprising a whole section of a lobule. In dispersed acini, under the same conditions, carbachol caused the release of about 60% of the amylase content, and secretin 40%. When both secretagogues were combined, a significant increase to 78% was observed. Under these conditions, there was some important cellular damage, as indicated by the release of 20% of the amylase content and between 6 and 12% of lactate dehydrogenase into the media, in the absence of stimulus. These results were corroborated by cytological observations. On the basis of their secretory response two groups of acini can be distinguished, those that respond to carbachol, caerulein or secretin and those that respond to the combination of secretin with carbachol or caerulein. Electrophoretic patterns of secretory proteins released by lobules stimulated by these different types of secretagogues were essentially similar. The pattern was quite different, however, in the absence of a stimulus.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of circulating catecholamines on protein secretion into rat parotid saliva during parasympathetic stimulation

Secretion of proteins by rat parotid glands in response to parasympathetic nerve stimulation was studied in vivo during pentobarbitone anaesthesia. Parasympathetic stimulation (3-10 Hz) via the auriculotemporal nerve resulted in a copious flow of saliva low in protein. In contrast, sympathetic stimulation (5 Hz) via the cervical sympathetic trunk evoked saliva low in volume but high in protein. Nevertheless, the specific concentrations of amylase and peroxidase (mg/mg protein) and the ratio of amylase to peroxidase remained constant. Sodium dodecyl sulphate (SDS) polyacrylamide gel electrophoresis revealed a single, rapidly migrating protein band of unknown identity in proportionately greater amounts in parasympathetic saliva than in sympathetic saliva. Bilateral adrenalectomy led to reduced amylase and peroxidase secretion in response to parasympathetic stimulation both on a mg/ml and a mg/mg protein basis. SDS gel electrophoresis also demonstrated the decrease in specific amylase concentration following adrenalectomy. The ratio of amylase to peroxidase, however, was not significantly affected. Administration of 6-hydroxydopamine 17-72 h prior to adrenalectomy caused no further reduction in the secretion of amylase and peroxidase. Chronic sympathectomy of 2.5-4 months duration resulted in an increased protein secretion (mg/ml) by the parotid gland in response to parasympathetic stimulation. This increase was only slightly reduced by bilateral adrenalectomy. However, as observed in non-sympathectomized rats, adrenalectomy caused a significant reduction in the specific concentrations of both amylase and peroxidase, but did not affect the amylase to peroxidase ratios. We conclude that parasympathetic nerve stimulation of rat parotid glands after overnight starvation causes secretion of proteins in proportions similar to, but in significantly lower concentrations than those found in sympathetic saliva. Circulating catecholamines, however, influence the amount of amylase and peroxidase secreted by the rat parotid gland in response to parasympathetic nerve stimulation and account for most of the increased secretion of these enzymes following chronic sympathectomy.

Amino acid composition of salivary protein secreted by the parotid glands of rats in response to parasympathomimetic and sympathomimetic drugs

Although the total protein concentration in the saliva varied markedly, depending on the nature of the stimulus, the proportions of the amino-acid residues were independent on the nature of the stimuli, where they were cholinergic, adrenergic or dopaminergic. However, the difference in protein composition of saliva previously shown in chronically isoprenaline-treated rats electrophoretically could be determined by amino-acid analysis.