Exit of nonglycosylated secretory proteins from the rough endoplasmic reticulum is asynchronous in the exocrine pancreas

Protein Production and Secretion in Exocrine Cells

Acinar cells of exocrine glands are highly specialized for producing, storing, and discharging secretory proteins for use on surfaces that represent interfaces between the organism and the surrounding environment. These functions are achieved through the secretory pathway that includes a series of functionally distinct intracellular compartments — the endoplasmic reticulum, subcompartments of the Golgi complex, and the secretion granule in which exportable macromolecules are stored at high concentrations. Most secretion occurs by granule exocytosis in response to external hormonal or neural stimuli. Although these processes have been traced in a variety of morphological and biochemical studies, very Utile is known about the mechanisms involved in facilitating and maintaining secretory storage, orchestrating discharge at the apical cell surface, and in ensuring conservation and re-internalization of the granule membrane. Recent studies initiated on cell fractions obtained from the rat parotid gland have provided significant insight into the protein storage conditions that prevail in the granule interior and the components of the granule membrane that are likely to be involved in general secretory function such as exocytosis.

Pancreatic Islets: Methods for Isolation and Purification of Juvenile and Adult Pig Islets

The current situation of organ transplantation is mainly determined by the disbalance between the number of available organs and the number of patients on the waiting list. This obvious dilemma might be solved by the transplantation of porcine organs into human patients. The metabolic similarities which exist between both species made pancreatic islets of Langerhans to that donor tissue which will be most likely transplanted in human recipients. Nevertheless, the successful isolation of significant yields of viable porcine islets is extremely difficult and requires extensive experiences in the field. This review is focussing on the technical challenges, pitfalls and particularities that are associated with the isolation of islets from juvenile and adult pigs considering donor variables that can affect porcine islet isolation outcome.

Day-to-day variations of serum pancreatic enzymes in benign pancreatic hyperenzymemia

BACKGROUND & AIMS

Benign pancreatic hyperenzymemia is a newly identified syndrome that is characterized by a chronic increase of serum pancreatic enzymes in the absence of pancreatic disease. When checked at intervals of months or years, the enzyme levels show considerable variation from one test to another, and enzyme normalization is sometimes seen. The purpose of this study was to determine whether these variations can occur on a day-to-day basis.

METHODS

Forty-two subjects with benign pancreatic hyperenzymemia, 28 men and 14 women, mean age 50 years, range 16-68 years, were studied. In each subject serum amylase, pancreatic isoamylase, and lipase levels were determined once daily on each of 5 consecutive days; in 15 of the 42 subjects studied, serum trypsin level was also measured.

RESULTS

In 8 (19%) of the 42 subjects, the serum enzyme levels were abnormally elevated for all 5 days of the study but showed wide fluctuations. In 33 (78.6%) subjects, the hyperenzymemia showed considerable day-to-day variations that included normalizations. In the remaining subject (2.4%), all enzymes were normal. In most of the subjects with hyperenzymemia, 37 of 41, all enzyme levels were elevated, with lipase, and trypsin when performed, showing the highest values. Of the remaining 4 subjects, in 3 only amylase and pancreatic isoamylase were increased and in 1 only lipase.

CONCLUSIONS

The results show that there are marked day-to-day variations, with frequent normalizations, of serum enzyme levels in subjects with benign pancreatic hyperenzymemia. The reason for these wide day-to-day fluctuations is not known.

Effect of secretin on serum pancreatic enzymes and on the Wirsung duct in chronic nonpathological pancreatic hyperenzymemia

AIM

Chronic nonpathological pancreatic hyperenzymemia (CNPH) is a new syndrome characterized by an increase in serum pancreatic enzymes in the absence of pancreatic disease. The aim of this study was to increase our understanding of this condition by determining the serum pancreatic enzyme response as well as changes in the caliber of the Wirsung duct after secretin stimulation.

METHODS

Twenty subjects with CNPH and 9 healthy subjects without CNPH were studied. Blood samples were drawn 5 and 0 min before and 5, 10, 15, 30, 45, and 60 min after intravenous injection of secretin (1 U/kg). Amylase, pancreatic isoamylase, and lipase concentrations were determined. The caliber of the Wirsung duct was measured by ultrasonography.

RESULTS

The injection of secretin caused a marked and statistically significant (p < 0.05) increase in serum pancreatic enzymes in the subjects with CNPH that persisted for the duration of the study. The increase over the basal value was in the range of 1.2- to 1.6-fold for amylase, 1.4- to 2.1-fold for pancreatic isoamylase, and 2.6- to 4.2-fold for lipase. In the control subjects the increase was mild, but statistically significant (p < 0.05), ranging from 1.1- to 1.2-fold for amylase, 1.2- to 1.4-fold for pancreatic isoamylase, and 1.5- to 2.2-fold for lipase. The injection of secretin caused a slight increase in the diameter of the Wirsung duct in both groups of subjects, but this was statistically significant only during the first 30 min of the study.

CONCLUSIONS

The serum pancreatic enzyme response to secretin was more marked in CNPH than in controls. The Wirsung duct showed no alterations after secretin injection that would help to explain the hyperenzymemia.

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).

Vesicular tubular clusters between the ER and Golgi mediate concentration of soluble secretory proteins by exclusion from COPI-coated vesicles

We have determined the concentrations of the secretory proteins amylase and chymotrypsinogen and the membrane proteins KDELr and rBet1 in COPII- and COPI-coated pre-Golgi compartments of pancreatic cells by quantitative immunoelectron microscopy. COPII was confined to ER membrane buds and adjacent vesicles. COPI occurred on vesicular tubular clusters (VTCs), Golgi cisternae, the trans-Golgi network, and immature secretory granules. Both secretory proteins exhibited a first, significant concentration step in noncoated segments of VTC tubules and were excluded from COPI-coated tips. By contrast, KDELr and rBet1 showed a first, significant concentration in COPII-coated ER buds and vesicles and were prominently present in COPI-coated tips of VTC tubules. These data suggest an important role of VTCs in soluble cargo concentration by exclusion from COPI-coated domains.

Kinetic analysis of secretory protein traffic and characterization of golgi to plasma membrane transport intermediates in living cells

Quantitative time-lapse imaging data of single cells expressing the transmembrane protein, vesicular stomatitis virus ts045 G protein fused to green fluorescent protein (VSVG-GFP), were used for kinetic modeling of protein traffic through the various compartments of the secretory pathway. A series of first order rate laws was sufficient to accurately describe VSVG-GFP transport, and provided compartment residence times and rate constants for transport into and out of the Golgi complex and delivery to the plasma membrane. For ER to Golgi transport the mean rate constant (i.e., the fraction of VSVG-GFP moved per unit of time) was 2.8% per min, for Golgi to plasma membrane transport it was 3.0% per min, and for transport from the plasma membrane to a degradative site it was 0.25% per min. Because these rate constants did not change as the concentration of VSVG-GFP in different compartments went from high (early in the experiment) to low (late in the experiment), secretory transport machinery was never saturated during the experiments. The processes of budding, translocation, and fusion of post-Golgi transport intermediates carrying VSVG- GFP to the plasma membrane were also analyzed using quantitative imaging techniques. Large pleiomorphic tubular structures, rather than small vesicles, were found to be the primary vehicles for Golgi to plasma membrane transport of VSVG-GFP. These structures budded as entire domains from the Golgi complex and underwent dynamic shape changes as they moved along microtubule tracks to the cell periphery. They carried up to 10,000 VSVG-GFP molecules and had a mean life time in COS cells of 3.8 min. In addition, they fused with the plasma membrane without intersecting other membrane transport pathways in the cell. These properties suggest that the post-Golgi intermediates represent a unique transport organelle for conveying large quantities of protein cargo from the Golgi complex directly to the plasma membrane.

Intracellular protein transport to the thyrocyte plasma membrane: potential implications for thyroid physiology

We present a snapshot of developments in epithelial biology that may prove helpful in understanding cellular aspects of the machinery designed for the synthesis of thyroid hormones on the thyroglobulin precursor. The functional unit of the thyroid gland is the follicle, delimited by a monolayer of thyrocytes. Like the cells of most simple epithelia, thyrocytes exhibit specialization of the cell surface that confronts two different extracellular environments-apical and basolateral, which are separated by tight junctions. Specifically, the basolateral domain faces the interstitium/bloodstream, while the apical domain is in contact with the lumen that is the primary target for newly synthesized thyroglobulin secretion and also serves as a storage depot for previously secreted protein. Thyrocytes use their polarity in several important ways, such as for maintaining basolaterally located iodide uptake and T4 deiodination, as well apically located iodide efflux and iodination machinery. The mechanisms by which this organization is established, fall in large part under the more general cell biological problem of intracellular sorting and trafficking of different proteins en route to the cell surface. Nearly all exportable proteins begin their biological life after synthesis in an intracellular compartment known as the endoplasmic reticulum (ER), upon which different degrees of difficulty may be encountered during nascent polypeptide folding and initial export to the Golgi complex. In these initial stages, ER molecular chaperones can assist in monitoring protein folding and export while themselves remaining as resident proteins of the thyroid ER. After export from the ER, most subsequent sorting for protein delivery to apical or basolateral surfaces of thyrocytes occurs within another specialized intracellular compartment known as the trans-Golgi network. Targeting information encoded in secretory proteins and plasma membrane proteins can be exposed or buried at different stages along the export pathway, which is likely to account for sorting and specific delivery of different newly-synthesized proteins. Defects in either burying or exposing these structural signals, and consequent abnormalities in protein transport, may contribute to different thyroid pathologies.

Association of bile-salt-dependent lipase with membranes of human pancreatic microsomes

Immunolocalization studies indicated that, in contrast to other enzyme markers of human pancreatic secretion, bile-salt-dependent lipase (BSDL) was partly but specifically associated with endoplasmic reticulum membranes. In microsomes, temperature-induced phase separation using Triton X-114 elucidated the partition of BSDL between the aqueous phase and the detergent-rich phase containing hydrophilic and membrane proteins, respectively. The size of the membrane-associated BSDL (approx. 100 kDa) is compatible with that of the fully processed enzyme. Fucosylated O- and N-linked oligosaccharide structures were detected by means of specific lectins. The membrane-associated BSDL might therefore be released from membranes between the trans-Golgi compartment (where terminal fucose residues were added) and the zymogen granules where BSDL was mainly found in the soluble fraction. Even though BSDL associated with membranes was enzymically active, it appeared less efficient than the soluble form. The association of BSDL with membranes was pH-dependent and optimal association occurred between pH 5-6. The membrane-associated BSDL was released by KBr which suggests that the association of BSDL with microsomal membranes involves ionic interactions. Lipid-protein interactions are probably not involved in this association as BSDL did not associate with liver microsome membranes. We attempted to characterize the putative ligand and showed that BSDL and a 94-kDa protein, immunologically related to a glucose-regulated protein of 94 kDa (Grp94), were co-immunoprecipitated by specific antibodies directed against each individual species. It is suggested that the biogenesis of the human pancreatic BSDL involves an association with intracellular membranes and that its folding may be assisted by molecular chaperones.

Chaperone function of a Grp 94-related protein for folding and transport of the pancreatic bile salt-dependent lipase

In its fundamental attributes, the secretion pathway of the pancreatic bile salt-dependent lipase (BSDL) followed that described for all enzymes involved in regulated secretion. This route was inhibited by drugs that affect protein synthesis and intracellular transport. In the presence of monensin, BSDL was solely detected in microsome membrane fractions. The association of BSDL with intracellular membranes involved a protein complex, formed by at least two proteins of 94 and 56 kDa. In cells experiencing the metabolic stress due to azetidine-2-carboxylic acid, BSDL was additionally associated with a protein of 46 kDa. Affinity blotting showed that BSDL bound directly to the 94-kDa protein (p94). It was suggested that p94 could be a molecular chaperone, further identified as related to the 94-kDa glucose regulated protein (Grp 94). The membrane-associated BSDL (i.e. BSDL bound to the Grp 94-related p94) was O- and N-glycosylated and consequently appeared released from membranes in the trans-Golgi compartment. Therefore and for the first time, it is suggested that a multiprotein complex including the chaperone Grp 94-related p94 protein may play an essential role in the folding and transport of BSDL. One hypothesis is that the association of BSDL with membrane via the Grp 94-related p94 along its secretion pathway is required for its complete O-glycosylation, which occurs on the extended mucin-like structures present on the C-terminal part of the protein.

Rapid cleavage of the endogenous PC3 prosegment and slow conversion to 74 kDa and 66 kDa proteins in AtT-20 cells

AtT-20 cells synthesize 87 kDa and 66 kDa forms of the prohormone convertase PC3 (also known as PC1). In the present work, using biosynthetic labeling experiments (performed both at 20 degrees C and at 37 degrees C), followed by immunoprecipitation with aminoterminally and carboxyterminally-directed antisera, we have found that the first PC3 translational product was a 94 kDa protein that was then converted to an 84 kDa form. This processing was extremely rapid, occurring with a half-life of less than 2 min at 20 degrees C. The 84 kDa form was endoglycosidase H-sensitive, indicating a lack of acquisition of sugar transferred in the medial golgi. Dithiothreitol, a reducing agent that prevents the disulfide bond formation of newly synthesized proteins in the endoplasmic reticulum (ER), inhibited the processing of the 94 kDa to the 84 kDa form. However, brefeldin A (BFA), an inhibitor of ER/golgi transport, and monensin, an inhibitor of the medial/trans-golgi transport, did not affect the cleavage of the 94 kDa to the 84 kDa protein. The 84 kDa protein was converted to an endoglycosidase H-resistant form of 87 kDa that was sequentially processed to 74 kDa and 66 kDa proteins. The 87 kDa protein was immunoprecipitated by the PC3 aminoterminally and carboxyterminally-directed antisera, while the 74 kDa and 66 kDa protein were only detected with the aminoterminally-directed antibody. Radiosequencing of the 87 kDa and 66 kDa proteins indicated that the biosynthesis of the 87 kDa proteins involves the removal of the 83 amino acid prosegment, and that the processing of the 87 kDa to 66 kDa form occurred by cleavage at the carboxyterminal portion. BFA and monensin effectively interrupted the processing of the 84-87 kDa protein to the 74 and 66 kDa species. In addition, while the 84-87 kDa protein produced in monensin-treated cells was still sensitive to endoglycosidase H, the 66 kDa protein was resistant to this enzyme. These results indicate that the post-translational processing of PC3 occurs in three steps: (1) rapid conversion, probably in the ER, of the 94 kDa precursor to the 84 kDa protein by removal of the aminoterminus prosegment; (2) cleavage of the 87 kDa protein to an intermediate product of 74 kDa; and (3) production of the 66 kDa protein. The second and third steps occur in late cellular compartments such as the trans-golgi network or secretory granules and involve sequential cleavages at the carboxyterminus.(ABSTRACT TRUNCATED AT 400 WORDS)

Long-acting somatostatin analogue therapy and protein metabolism in patients with jejunostomies

BACKGROUND/AIMS

Previous studies have shown that secretory losses in patients with end jejunostomy syndrome (EJS) on home parenteral nutrition (HPN) can be suppressed by the somatostatin analogue, octreotide, thus facilitating fluid balance. However, the hormone also has antianabolic actions that may interfere with the use of infused amino acids.

METHODS

Amino acid metabolism, pancreatic enzyme synthesis and secretion, and mucosal protein turnover were measured by primed/continuous intravenous infusion of [1-14C] leucine tracer, duodenal aspiration, and endoscopic mucosal biopsy techniques during hormonal stimulation with pentagastrin and cholecystokinin 8.

RESULTS

In comparison with normal healthy controls, baseline measurements of amino acid metabolism were normal in patients with EJS/HPN, but pancreatic enzyme synthesis and secretion were elevated. Octreotide therapy improved fluid balance but suppressed gut hormone (insulin, gastrin, glucagon, peptide YY) levels in blood and the uptake of amino acids into pancreatic enzyme and mucosal proteins, increasing oxidative losses.

CONCLUSIONS

Octreotide improves fluid balance in patients who have undergone jejunostomy but reduces the use of amino acids for splanchnic protein synthesis. This may interfere with the physiological process of adaptation to intestinal resection.

Heterogeneous distribution of the precursor of type I and type III collagen and fibronectin in the rough endoplasmic reticulum of palatal mesenchymal cells of the mouse embryo cultured in ascorbate-depleted medium

In order to examine the intracellular distribution of precursors of type I and type III collagen and fibronectin in the palatal mesenchymal (MEPM) cells of the mouse embryo cultured under ascorbate-deficient conditions, immuno-electron-microscopic studies were carried out by use of affinity purified antibodies for these proteins. MEPM cells were obtained from the palatal shelves of 14-day-old mouse fetuses and cultured for 3-7 days in medium, either with or without 50 ng/dish/day ascorbic acid. Results obtained were as follows: (1) Although the rough endoplasmic reticulum (rER) of MEPM cells cultured for 5 days in ascorbate-supplemented medium was flattened, that in cells cultured in ascorbate-deficient medium had a distended or vesicular appearance. (2) Vesicular or distended rER showed heterogeneous staining for both type I and type III collagen, namely, some parts of rER showed positive staining for both types of collagen, while others showed negative staining. (3) Both type I and type III collagen showed codistribution in the same vesicular rER. (4) Vesicular rER showed negative or very faint labelling for fibronectin. These results may suggest regional differences in the function of rER.

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 rate of bulk flow from the Golgi to the plasma membrane

A truncated analog of the backbone of sphingomyelin and glycolipids was synthesized. This truncated C8C8 ceramide was soluble in water (but was still able to cross cell membranes) and was utilized by the Golgi apparatus of living cells to produce water-soluble truncated phospholipids and glycolipids that were then secreted into the medium. Sphingomyelin is synthesized in a proximal (likely the cis) Golgi compartment. At 37 degrees C in CHO cells, the sphingomyelin analog is secreted with a half time of about 10 min. With this rate of bulk flow, no special signal is needed to pass through the Golgi to the plasma membrane. At 30 degrees C the half time of secretion of a lumenal ER marker is about 18 min, and that of the truncated sphingomyelin is about 14 min. Comparison of these rates sets an upper limit of about 4 min for half of the ER to be drained into the proximal Golgi at 30 degrees C.

Synthesis of chromogranin A, dopamine beta-hydroxylase, and chromaffin vesicles

Primary cultures of bovine adrenal medullary cells synthesize chromogranin A (CgA) and dopamine beta-hydroxylase (DBH) and incorporate them into chromaffin vesicles. The incorporation of L-[35S]methionine into CgA, DBH, and total protein was approximately linear for 8 h at methionine concentrations of 12.5, 25, and 50 microM. Newly synthesized CgA and DBH were initially incorporated into vesicles of low buoyant density that matured over 24 h into vesicles having the greater buoyant density of chromaffin vesicles. Approximately 10% of the newly synthesized CgA is released constitutively within 4 h of formation, approximately 30-40% appears to be degraded, and the remainder is incorporated into chromaffin vesicles, which can secrete CgA in response to nicotinic stimulation. Newly synthesized DBH follows a similar course. Once incorporated into chromaffin vesicles, the newly synthesized CgA and DBH appear to be stable for 2-3 days and then decline with a half-time of 3-4 days. Primary cultures of bovine adrenal medullary cells are a good model system for studying factors regulating CgA and DBH synthesis and the formation of chromaffin vesicles.

Microtubule-dependent retrograde transport of proteins into the ER in the presence of brefeldin A suggests an ER recycling pathway

Characteristics of brefeldin A (BFA)-induced redistribution of Golgi proteins into the endoplasmic reticulum (ER) and its relationship to an ER retrieval pathway were investigated. Retrograde movement of Golgi proteins into the ER occurred via long, tubulovesicular processes extending out of the Golgi along microtubules. Microtubule-disrupting agents (i.e., nocodazole), energy poisons, and reduced temperatures inhibited this pathway. In BFA-treated cells Golgi proteins appeared to cycle between the ER and an intermediate compartment marked by a 53 kd protein. Addition of nocodazole disrupted this dynamic cycle by preferentially inhibiting retrograde movement, causing Golgi proteins to accumulate in the intermediate compartment. In the absence of BFA, such an ER cycling pathway appeared to be followed normally by the 53 kd protein but not by Golgi proteins, as revealed by temperature shift experiments. We propose that BFA induces the interaction of the Golgi with an intermediate "recycling" compartment that utilizes a microtubule-dependent pathway into the ER.

Perifusion of isolated rat pancreatic acini: carbamylcholine-induced biphasic amylase release

To solve many problems in other in-vitro methods such as perfusion and static incubation, perifusion of isolated rat pancreatic acini was established. Continuous stimulation by carbamylcholine induced a biphasic secretory response, a sharp initial phase and a slow-varying second phase. The junction of these two phases occurred at almost the same time (in about 7 min of stimulation) at any concentration of carbamylcholine. The slow-rising former part of a second phase indicated that a biphasic pattern is produced by overlapping of two kinds of real phases and the beginning of a real second phase occurs at the time of the junction. The concentration dependence of amylase release showed a bell-shaped pattern and the maximal amylase release at an optimal concentration (10(-6)M) was 5.43 +/- 0.24% total/30 min. The sustained latter part of a second phase showed a slow decline at optimal or less concentrations but a plateau at supraoptimal concentrations.

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.

Monoclonal antibodies localize events in the folding, assembly, and intracellular transport of the influenza virus hemagglutinin glycoprotein

We used monoclonal antibodies that recognize monomeric and/or trimeric forms of the influenza virus hemagglutinin (HA) to study biosynthesis of this integral membrane protein in influenza virus-infected cells. We find the following: First, the globular head of the HA folds into its mature conformation in the endoplasmic reticulum prior to the assembly of HA monomers into trimers. Second, trimerization begins within 1 to 2 min following synthesis, with a half-time of approximately 5 min. Third, trimerization occurs only after the HA has been transported from the endoplasmic reticulum. Fourth, newly formed trimers are sensitive to acid-induced conformational alterations associated with viral fusion activity.

A study on the intracellular transport of prothrombin, albumin and transferrin in rat

The intracellular transport of prothrombin in rat has been studied and compared with the transport of albumin and transferrin. The proteins were immunoisolated from plasma samples after pulse labelling with [3H]leucine and the secretion kinetics were determined. The half-times for secretion (t1/2) were approx. 30, 53 and 75 min for albumin, prothrombin and transferrin, respectively, whereas the minimal transit time for prothrombin was approx. 30 min, and those for albumin and transferrin 15-20 min. After injection of vitamin K-1 into warfarin-treated rats, the accumulated prothrombin precursor was gamma-carboxylated and secreted with a t1/2 of 37 min. This indicates that the gamma-carboxylation of prothrombin in rough endoplasmic reticulum cannot account for the delay in the transport of prothrombin as compared to albumin. Comparison of the incorporation of [3H]leucine and [3H]glucosamine into plasma prothrombin and transferrin suggested that transferrin is secreted randomly from an intracellular pool, whereas prothrombin is transported in a more orderly sequence. Moreover, treatment of rough microsomes with 0.05% sodium deoxycholate indicated that prothrombin is more tightly associated with the membranes of rough endoplasmic reticulum than albumin and transferrin.

Information transfer in biological systems: targeting of proteins to specific organelles or to the extracellular environment (secretion)

Orderliness is the salient characteristic of living systems. Cells are intolerant of disorder. They express this by rapidly eliminating or degrading out-of-place molecules. When cells are broken apart and their constituent organelles separated and analysed, the same types of macromolecules are always associated with the same subcellular structures. One finds, for example, the same proteins in mitochondria time after time, and these differ from the sets of proteins found in nuclei, secretory granules, or plasma membranes. The information necessary to target each protein to its appropriate intracellular destination is determined primarily by the gene for that protein. Encoded within the DNA structure of genes are signals that specify where each protein molecule belongs. Thus, it is the transfer of information from one macromolecule to another that maintains the integrity and orderliness of living cells.

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.

Protein production and secretion in exocrine cells

Acinar cells of exocrine glands are highly specialized for producing, storing, and discharging secretory proteins for use on surfaces that represent interfaces between the organism and the surrounding environment. These functions are achieved through the secretory pathway that includes a series of functionally distinct intracellular compartments--the endoplasmic reticulum, subcompartments of the Golgi complex, and the secretion granule in which exportable macromolecules are stored at high concentrations. Most secretion occurs by granule exocytosis in response to external hormonal or neural stimuli. Although these processes have been traced in a variety of morphological and biochemical studies, very little is known about the mechanisms involved in facilitating and maintaining secretory storage, orchestrating discharge at the apical cell surface, and in ensuring conservation and re-internalization of the granule membrane. Recent studies initiated on cell fractions obtained from the rat parotid gland have provided significant insight into the protein storage conditions that prevail in the granule interior and the components of the granule membrane that are likely to be involved in general secretory function such as exocytosis.

Exocytosis occurs at the lateral plasma membrane of the pancreatic acinar cell during supramaximal secretagogue stimulation

In vitro and in vivo studies indicate that the secretory response to both caerulein and carbamylcholine stimulation is biphasic. Over the range of submaximal to maximal concentrations of secretagogues, discharge of exocrine proteins in vitro into the incubation medium and in vivo into the pancreatic duct increased and morphologic analysis indicated that exocytosis of zymogen granules occurred exclusively at the luminal membrane. Under in vivo conditions, supramaximal stimulation with caerulein or carbamylcholine resulted in a dose-dependent decrease in amylase release into the pancreatic duct and increase in the appearance of amylase in the blood circulation. Under in vitro or in vivo conditions, supramaximal secretagogue stimulation resulted in marked inhibition of exocytotic activity at the luminal plasma membrane, the appearance of intergranule contacts and fusions within the cytoplasm, and the appearance of exocytotic activity at the lateral plasma membrane. Lateral exocytotic images were observed with individual and fused zymogen granules and autophagic vacuoles. This redirection in the final step of the secretory pathway provides in part the biological basis for the increased appearance of pancreatic (pro)enzymes in the interstitial fluid and serum during supramaximal secretagogue stimulation.

Pancreatic secretion in the rat influenced by the low molecular weight serine proteinase inhibitor Gabexate mesilate

The effect of intravenous or intragastric administration of the synthetic proteinase inhibitor Gabexate mesilate (GM) on the pancreas of rats was investigated. Infused intravenously at 4 mg kg-1 h-1, GM inhibited both basal or cerulein (0.2 microgram kg-1 h-1)-stimulated pancreatic protein secretion. Intracellular transport and secretion of newly synthesized pancreatic enzymes was not influenced by intravenous infusion of GM. Intragastric administration of GM (400 mg kg-1) on four consecutive days increased pancreatic wet weight, protein and enzyme content of the gland. A preferential increase of proteinases above glucosidases was observed. Pancreatic lobules from inhibitor-treated rats released 30% less amylase in response to cerulein or carbachol when the rate of discharge was expressed in percent of initial content. Expressed in ku amylase/microgram DNA secretion rate was two-fold higher than in controls. In pancreatic duct cannulated rats GM (400 mg kg-1 h-1), introduced intragastrically on five consecutive days, stimulated volume-bicarbonate and protein secretion rate, the secretory response on the fifth day being significantly higher than on the first day. Enzyme pattern in pancreatic juice changed characteristically: mainly the amount of acidic proteinases increased, whereas the amount of the basic isoforms was altered only slightly.