Milieu-induced, selective aggregation of regulated secretory proteins in the trans-Golgi network

Molecular probing of the secretory pathway in peptide hormone-producing cells

The biosynthetic machinery in the melanotrope cells of the Xenopus intermediate pituitary is primarily dedicated to the generation of proopiomelanocortin (POMC)-derived, melanophore-stimulating peptides. Transfer of the animal to a black background stimulates the production of these peptides and causes a dramatic increase in POMC mRNA levels. To identify genes involved in the biosynthesis and regulated release of peptide hormones, we differentially screened an intermediate pituitary cDNA library of toads adapted to a black background with cDNA probes derived from intermediate pituitary mRNA of black- and white-adapted animals. Here we report the identification of twelve distinct genes whose expression levels in the melanotropes are regulated in coordination with that of POMC. Four of these genes are novel while the others code for translocon-associated proteins, a lumenal cysteine protease of the endoplasmic reticulum, prohormone-processing enzymes, members of the granin family and a transmembrane protein presumably involved in the assembly and/or specific functioning of vacuolar H(+)-ATPase from secretory granules. Our results indicate that a wide variety of both soluble and membrane-associated components of the secretory pathway is recruited in physiologically activated, peptide hormone-producing cells.

Characterization of the endopeptidase PC2 activity towards secretogranin II in stably transfected PC12 cells

To study the processing of secretogranin II (SgII) by the prohormone convertase PC2 we have generated a stable PC12 cell line which expresses mouse PC2. We here present the characteristics of the PC12/PC2 cell line and demonstrate that the exogenous PC2 is sorted and stored in secretory granules in the PC12/PC2 cell line as efficiently as the endogenous granins. By indirect immunofluorescence with antibodies specific for chromogranin B (CgB) and PC2 we were able to establish that the PC2 is stored in secretory granules in the PC12/PC2 cell line. After subcellular fractionation, followed by immunoblotting, the mature 68 kDa form of PC2 was found co-sedimented with SgII in fractions containing secretory granules. Two-dimensional gel electrophoresis was used to characterize a secretory granule fraction obtained from the PC12/PC2 cells, and a comparison was done of the electrophoretic pattern obtained from the PC12/PC2 cells with the parent cell line PC12. The products derived from the processing of SgII by PC2 were identified by immunoblotting with a panel of antibodies directed against SgII. Using [35S]sulphate to label the newly synthesized SgII, we performed a time course to monitor the appearance of the lower-molecular-mass fragments of SgII: beginning 15 min after a 5 min pulse of [35S]sulphate we were able to detect the first proteolytic fragment of SgII. Our results demonstrate that SgII is proteolytically processed by PC2 in the immature secretory granule into several lower-molecular-mass proteins, the major ones being an 18 kDa sulphated fragment and a 28 kDa fragment.

Production and secretion of N-terminal secretogranin II derived peptides in GH3B6 prolactin cells

Chromo/secreto-granins are proteins specific of neuroendocrine cells. Chromogranin B (CgB) and secretogranin II (SgII) are both present in normal and in tumoral (GH3B6) prolactin cells, in which they are colocalized in the same secretory granules. These proteins contain multiple dibasic cleavage sites and are considered as potential precursors of active peptides, though their exact function remains unknown. SgII is sulfated on tyrosine-126. We took advantage of this feature to study its post-translational processing in anterior pituitary cells in primary culture and in GH3B6 cells. Pulse-chase experiments with [35S]sulfate demonstrated the precursor-product relationship between SgII and four N-terminal-derived peptides. Kinetic experiments showed the sequential cleavage of SgII from the C-terminus to the N-terminus. Mature SgII and the derived peptides were secreted by both cell models, and their release was stimulated by TRH (30 nM), a secretagogue of prolactin. These data show that SgII is proteolytically processed into different secreted peptides in prolactin cells and demonstrate that GH3B6 cells provide a good cell model for studying the maturation of SgII in anterior pituitary.

Calcitonin secretion, C cell differentiation and proliferation during the spontaneous development of murine medullary thyroid carcinoma

Medullary thyroid carcinoma (MTC), a C cell neoplasm, synthesizes large amounts of calcitonin (CT), its biological marker. However, in some cases with a poor prognosis, MTC is associated with low basal CT levels owing to a decrease in the thyroid CT content. Using a murine model of human MTC, we studied the relationships between CT biosynthesis, C cell proliferation, and the circulating CT level during MTC progression. Cell proliferation was revealed by autoradiography of radioactive thymidine incorporation in dividing nuclei, after CT or CT mRNA detection by immunocytochemistry (ICC) or in situ hybridization (ISH). All rat thyroids showed a severe hyperplasia of C cells containing significant amounts of CT and CT mRNA, and a very low mitotic index. Tumours were found in 68% of the thyroids. In the strongly immunoreactive small nodules (ICC+), many labelled nuclei were observed. Subsequently some nodular cells, still containing detectable CT mRNA (ISH+), were not detected by immunocytochemistry (ICC-) owing to a dramatic decrease in secretory granules. Their mitotic index increased, and a rise of the basal CT plasma level was noted. These ISH+, ICC- tumour MTC cells represent a modified aggressive tumour C cell population exhibiting an increased ability to proliferate and were detected by the rise in the basal circulating CT level.

pH- and Ca(2+)-induced conformational change and aggregation of chromogranin B. Comparison with chromogranin A and implication in secretory vesicle biogenesis

Chromogranins A and B have been known to undergo pH- and Ca(2+)-dependent aggregation, and this property is considered essential for the proper sorting of the vesicular matrix proteins. In the present study, purified native chromogranin B (CGB) from bovine adrenal medulla was used to study the pH- and Ca(2+)-dependent conformational changes and aggregation property. Similar to chromogranin A (CGA), which had been shown to undergo pH- and Ca(2+)-dependent conformational changes and to be composed of 60-65% random coil with 25-40% alpha-helicity, chromogranin B was also shown to consist of 65-70% random coil, 15-25% alpha-helix, and 10-15% beta-sheet structures. The high percentage of random coil suggests that CGB behaves hydrodynamically as an asymmetric molecule, thus explaining its anomalous migration on SDS-polyacrylamide gels. Further, CGB eluted from a gel filtration column in the volume indicative of a globular protein with molecular weight of approximately 200,000 at both the intravesicular pH of 5.5 and a near physiological pH of 7.5. Considering that dimeric CGA eluted from a gel filtration column in the position suggestive of a 300-kDa protein, this result indicated that CGB exists in a monomeric state at both pH levels. Like CGA, which exhibited greater aggregation at pH 5.5 than at pH 7.5 upon Ca2+ binding, CGB also aggregated much more readily at pH 5.5 than at pH 7.5. However, there was a marked difference in the aggregation properties of CGA and CGB with regard to their sensitivity to Ca2+: CGB was at least 2 orders of magnitude more sensitive to Ca2+ than CGA. This suggested that, in spite of the low concentration of CGB (approximately one-tenth that of CGA) in bovine adrenal chromaffin cells, CGB would start to aggregate well ahead of CGA in the trans-Golgi network. In view of the proposed importance of the pH- and Ca(2+)-induced chromogranin aggregation in vesicle biogenesis, the extreme sensitivity of CGB aggregation to Ca2+ appears to underline the potential importance of CGB aggregation in the early stages of vesicle biogenesis.

Secretogranin II: molecular properties, regulation of biosynthesis and processing to the neuropeptide secretoneurin

Secretogranin II is an acidic secretory protein in large dense core vesicles of endocrine, neuroendocrine and neuronal tissues. It comprises, together with chromogranins A and B, the class of proteins collectively called chromogranins. In this review the physico-chemical properties, genomic organization, tissue distribution, synthesis regulation, ontogeny and physiological function of this protein are discussed. Secretogranin II gained interest recently for mainly three reasons: (1) secretogranin II is an excellent marker for the regulated secretory pathway due to its simple and specific metabolic labeling by inorganic sulfate; (2) secretogranin II occurs in a variety of neoplasms arising from endocrine and neuroendocrine cells and was shown to be a useful histological tumor marker for these cells; (3) secretogranin II is the precursor of the recently discovered neuropeptide secretoneurin which induces dopamine release in the striatum of the rat brain.

Calcium- and pH-dependent aggregation of carboxypeptidase E

Carboxypeptidase E (CPE) is involved with the biosynthesis of numerous peptide hormones and neurotransmitters. Several forms of CPE have been previously detected in neuroendocrine cells, including a form which is soluble at pH 5.5 (S-CPE), a form which can be extracted from membranes with 1 M NaCl at pH 5.5 (M1-CPE), and a form which requires both 1% Triton X-100 and 1 M NaCl for extraction from membranes at pH 5.5 (M2-CPE). Like other peptide processing enzymes, CPE is known to be sorted into peptide-containing secretory vesicles of the regulated pathway. One mechanism that has been proposed to be important for sorting of regulated pathway proteins is Ca2+ and pH-induced aggregation. CPE purified from bovine pituitary membranes aggregates at pH 5.5 when the concentration of CPE is 0.3 micrograms/microliters or higher, but not when the concentration is 0.01 microgram/microliters. Aggregation of CPE is pH-dependent, with very little aggregation occurring at pH 6 or above. At pH 5.0-5.5, the M2 form of CPE shows a greater tendency to aggregate than the other two forms. At pH 6, Ca2+ concentrations from 1-30 mM increase the aggregation of M1- and M2-CPE, but not S-CPE. The aggregation of M2-CPE does not explain the apparent membrane binding of this protein since the aggregate is solubilized by 1% Triton X-100 at pH 5.5 or by pH 6.0, whereas M2-CPE is not extracted from membranes under these conditions. Taken together, these results are consistent with a model in which the decreasing pH and increasing Ca2+ levels in the trans Golgi network induce the aggregation of CPE, which contributes to the sorting of this protein into regulated pathway secretory vesicles.

Direct measurement of trans-Golgi pH in living cells and regulation by second messengers

In the endocytic compartment, an acidic pH plays a key role in receptor and ligand sorting, vesicular transport, and protein degradation. In the secretory compartment, indirect estimates of trans-Golgi pH based on partitioning of weak bases and following viral infection suggest a mildly acidic pH of > 6.0. We developed a liposome microinjection method to introduce fluorescent indicators into the aqueous compartment of trans-Golgi in living cells. In the presence of ATP and at 37 degrees C, 70-nm diameter liposomes delivered their fluid-phase contents selectively into the trans-Golgi compartment as assessed by colocalization with the trans-Golgi stain N-[6-[(7-nitrobenz-2-oxa-1, 3-diazol-4-yl)amino]caproyl]-sphingosine (C6-NBD-ceramide). Liposome fusion was ATP- and temperature-dependent and blocked by N-ethylmaleimide but not by guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S). trans-Golgi pH in skin fibroblasts was 6.17 +/- 0.02 (S.E., n = 174) as measured by ratio imaging confocal microscopy using fluorescein and rhodamine-based indicators and an in vivo calibration procedure. trans-Golgi pH increased to 6.8 +/- 0.1 by cAMP agonists and to 6.5 +/- 0.1 by protein kinase C activation. These results provide the first direct measurement of trans-Golgi pH in living cells and demonstrate pH regulation by second messengers.

Secretory pathways of neuropeptides in rat lumbar dorsal root ganglion neurons and effects of peripheral axotomy

Using immunocytochemistry combined with confocal and electron microscopy, the secretory pathways related to substance P (SP), calcitonin gene-related peptide (CGRP), galanin (GAL), and neuropeptide Y (NPY) were investigated in neurons in rat lumbar (L) 4 and L5 dorsal root ganglia (DRGs) before and after peripheral axotomy. All four peptides were processed through the regulated secretory pathway in many small neurons in normal DRGs, and CGRP through this pathway also in some large neurons. In many small neurons, two neuropeptides could be sorted into the same or separate large dense-core vesicles (LDCVs). The LDCVs had a significantly larger diameter in small as compared to large DRG neurons. Fourteen days after sciatic nerve cut, the levels of SP- and CGRP-like immunoreactivities (-LIs) and the number of LDCVs containing these peptides were markedly reduced, but SP- and CGRP-LIs were still seen in the regulated pathway. GAL-LI was markedly increased in many small neurons and some large neurons and NPY-LI mainly in large neurons. Both peptides were particularly abundant in the Golgi region. In small neurons, the number of LDCVs containing GAL- or NPY-LI was increased, but did not appear to reach the numbers containing SP- or CGRP-LI in normal DRG neurons. After axotomy, CGRP-LI and GAL-LI were often in separate LDCVs. One type of NPY-positive large neurons showed budding off of LDCVs after axotomy, but also some "scattered" labeling in the cytoplasm. In the second type, NPY-LI was mainly found in multivesicular bodies. In several myelinated nerve fibers a "diffuse" distribution of NPY was seen together with some LDCVs containing NPY-LI. In contrast, in unmyelinated nerve fibers, NPY-, GAL-, SP-, and CGRP-LIs were always observed in LDCVs. Thus, both in normal and axotomized DRG neurons, peptides are processed through the regulated pathway. However, in some large neurons, NPY is, in addition, secreted through the constitutive pathway, perhaps as a consequence of limited sorting mechanisms for NPY, i.e., the plasticity of the secretory mechanisms does not match the rate of peptide synthesis after axotomy.

Invariant chain induces a delayed transport from early to late endosomes

Invariant chain associated with class II molecules is proteolytically processed in several distinct intermediates during its transport through the endocytic pathway. Using subcellular fractionation, early and late endosomal compartments were separated in human fibroblasts transfected with HLA-DR (4N5 cells) and supertransfected with invariant chain (4N5Ii cells) or invariant chain lacking most of the cytoplasmic tail (4N5 delta 20Ii cells). Early and late endosome membrane fractions were characterized by morphology and by analyzing the presence of the Rab5 and Rab7 GTPases as markers of early and late endosomes, respectively. The transfer of endocytosed horseradish peroxidase from early to late endosomes proceeded relatively rapid both in 4N5 and 4N5 delta 20Ii cells (t1/2 = 25 min), whereas this transfer was significantly delayed (t1/2 = 2 h) in 4N5Ii cells. Pulse-chase experiments showed that invariant chain and its degradation products were first observed in early endosomes and thereafter in late endosomes. Our results strongly suggest that invariant chain induces a retention mechanism in the endocytic pathway.

Transport via the regulated secretory pathway in semi-intact PC12 cells: role of intra-cisternal calcium and pH in the transport and sorting of secretogranin II

To gain insight into the mechanisms governing protein sorting, we have developed a system that reconstitutes both the formation of immature secretory granules and their fusion with the plasma membrane. Semi-intact PC12 cells were incubated with ATP and cytosol for 15 min to allow immature granules to form, and then in a buffer containing 30 microM [Ca2+]free to induce exocytosis. Transport via the regulated pathway, as assayed by the release of secretogranin II (SgII) labeled in the TGN, was inhibited by depletion of ATP, or by the inclusion of 100 microM GTP gamma S, 50 microM AlF3-5 or 5 micrograms/ml BFA. When added after immature granules had formed, GTP gamma S stimulated rather than inhibited exocytosis. Thus, exocytosis of immature granules in this system resembles the characteristics of fully matured granules. Transport of SgII via the regulated pathway occurred at a fourfold higher efficiency than glycosaminoglycan chains, indicating that SgII is sorted to some extent upon exit from the TGN. Addition of A23187 to release Ca2+ from the TGN had no significant effect on sorting of SgII into immature granules. In contrast, depletion of lumenal calcium inhibited the endoproteolytic cleavage of POMC and proinsulin. These results establish the importance of intra-cisternal Ca2+ in prohormone processing, but raise the question whether lumenal calcium is required for proper sorting of SgII into immature granules. Disruption of organelle pH gradients with an ionophore or a weak base resulted in the inhibition of transport via both the constitutive and the regulated pathways.

Characterization of the major sulfated protein of mouse pancreatic acinar cells: a high molecular weight peripheral membrane glycoprotein of zymogen granules

The major sulfated protein of the mouse pancreatic acinar cell, gp300, has been identified and characterized with monoclonal and polyclonal antibodies. gp300 is a glycoprotein of M(r) = 300,000 which contains approximately 40% of metabolically incorporated [35S]sulfate in the acinar cell. Sulfate on gp300 is resistant to hot 1N HCl, but sensitive to alkaline hydrolysis, demonstrating that the sulfate is carbohydrate-linked rather than tyrosine-linked. gp300 metabolically labeled with [3H]glucosamine and [35S]sulfate was chemically and enzymatically treated followed by Bio-Gel P-10 gel filtration. Both labels were resistant to treatments which degrade glycosaminoglycans. Treatment of dual-labeled gp300 with PNGase F to cleave N-linked oligosaccharides released approximately 17% of [3H] and little [35S]. Mild alkaline borohydride treatment after removal of N-linked sugar released the remainder of both labels, indicating the presence of sulfated O-linked oligosaccharides. Biosynthesis studies and PNGase F digestion indicate that the core protein is approximately 210 kDa, with apparent contributions of approximately 35 kDa N-linked sugar, and approximately 55 kDa O-linked sugar. Lectin blotting and glycosidase digestion demonstrated the presence of Gal beta(1-3)GalNAc and sialic acid alpha(2-3)Gal in O-linked oligosaccharide, and Gal beta(1-4)GlcNAc in N-linked oligosaccharide. Immunolocalization and subcellular fractionation showed that gp300 is a peripheral membrane protein localized to the lumenal face of the zymogen granule membrane. gp300 was not secreted in response to hormone stimulation of acini, so it is not a secretory product. Immunoblot analysis showed that gp300 is present in other gastrointestinal tissues and parotid glands. Localization of this nonsecreted sulfated glycoprotein to exocrine secretory granule membranes suggests that gp300 may have a role in granule biogenesis.

The disulfide bond in chromogranin B, which is essential for its sorting to secretory granules, is not required for its aggregation in the trans-Golgi network

Chromogranin B (secretogranin I), a protein sorted to secretory granules in many endocrine cells and neurons, undergoes selective aggregation during the sorting process in the trans-Golgi network. Reduction of the single, highly conserved intramolecular disulfide bond of chromogranin B by exposure of intact PC12 cells to the thiol reducing agent dithiothreitol has previously been shown to cause its missorting to the constitutive pathway of secretion. Using saponin perforation of membrane vesicles in aggregative buffer mimicking the milieu in the lumen of the trans-Golgi network (pH 6.4, 10 mM calcium), we show here that treatment with dithiothreitol does not prevent the aggregation of chromogranin B in this compartment. This implies that the loop in the chromogranin B polypeptide that is formed by the disulfide bond has a critical role in the membrane recognition of aggregated chromogranin B during secretory granule formation.

High-molecular-weight forms of tyrosinase and the tyrosinase-related proteins: evidence for a melanogenic complex

Tyrosinase, tyrosinase-related protein-1 (TRP-1), and tyrosinase-related protein-2, (TRP-2, dopachrome tautomerase) were shown by immunoblotting and enzyme assays to copurify from extracts of Cloudman S91 melanoma cells. Antibodies to TRP-1 and TRP-2 immunoprecipitated tyrosinase activity, suggesting a stable interaction (complex) among these proteins. The tyrosine hydroxylase activity of tyrosinase was reduced in the complexed form; treatment with Triton X-100 dissociated the complex and activated the tyrosinase present within it. To further study this complex, we employed sucrose gradient density centrifugation of extracts from cultured murine melanocytes. Tyrosinase, TRP-1, and TRP-2 all existed in high molecular weight "multimers" of approximately 200 to > 700 kilodaltons. Extraction of cells with buffers containing the detergent CHAPS preserved the high molecular weight multimers; Triton X-100 caused their dissociation into monomers. Low pH, low ionic strength, and millimolar concentrations of calcium ions favored the maintenance of multimers. The results of this study demonstrate that the participation of tyrosinase, TRP-1, and TRP-2 in a multimeric complex could have important physiologic consequences, and raise the possibility that some of the well-known interactions between coat color genes may be explained by intermolecular interactions between the gene products.

7B2 is a neuroendocrine chaperone that transiently interacts with prohormone convertase PC2 in the secretory pathway

The neuroendocrine polypeptide 7B2 is a highly conserved secretory protein selectively present in prohormone-producing cells equipped with a regulated secretory pathway. We find that the amino-terminal half of 7B2 is distantly related to chaperonins, a subclass of molecular chaperones. When incubated in vitro with newly synthesized pituitary proteins, recombinant 7B2 specifically associates with prohormone convertase PC2. Metabolic cell labeling combined with coimmunoprecipitation studies showed that, in vivo, the precursor form of 7B2 interacts with the proform of PC2. Pulse-chase analysis revealed that this association is transient in that it commences early in the secretory pathway, while dissociation in the later stages appears to coincide with the cleavages of 7B2, proPC2, and prohormone. Our results suggest that 7B2 is a novel type of molecular chaperone preventing premature activation of proPC2 in the regulated secretory pathway.

Sorting and processing of secretory proteins

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Intracellular Ca2+ stores in neurons. Identification and functional aspects

Various aspects of the rapidly exchanging intracellular Ca2+ stores of neurons and nerve cells are reviewed: their multiplicity, with separate sensitivity to either the second messenger, inositol 1,4,5-trisphosphate, or ryanodine-caffeine (the latter stores are probably activated via Ca(2+)-induced Ca2+ release); their control of the plasma membrane Ca2+ permeability, via the activation of a peculiar type of cation channels; their ability to sustain localized heterogeneities of the [Ca2+]i that could be of physiological key-importance. Finally, the molecular composition of these stores is discussed. They are shown (by high resolution immunocytochemistry and subcellular fractionation) to express: i) a Ca2+ ATPase responsible for the accumulation of the cation; ii) Ca2+ binding protein(s) of low affinity and high capacity to keep Ca2+ stored; and iii) a Ca2+ channel, activated by either one of the mechanisms mentioned above, to release Ca2+ to the cytosol. Results obtained in Purkinje neurons document the heterogeneity of the stores and the strategical distribution of the corresponding organelles (calciosomes; specialized portions of the ER) within the cell body, dendrites and dendritic spines.

Evidence for defects in membrane traffic in Paramecium secretory mutants unable to produce functional storage granules

The ciliated protozoan Paramecium has a regulated secretory system amenable to genetic analysis. The secretory storage granules, known as trichocysts, enclose a crystalline matrix with a genetically determined shape whose biogenesis involves proteolytic maturation of a family of precursor molecules into a heterogeneous set of small acidic polypeptides that crystallize within the maturing vesicles. We have developed an original pulse-chase protocol for monoxenic Paramecium cultures using radiolabeled bacteria to study the processing of trichocyst matrix proteins in wild-type and mutant cells. In wild-type cells, proteolytic processing is blocked in the presence of monensin and otherwise rapidly completed after approximately 20 min of chase, suggesting that the conversion occurs in the trans-Golgi and/or in small vesicles soon after sorting to the regulated pathway, probably before crystallization begins. In trichless mutant cells, which contain no visible trichocysts, secretory proteins are synthesized but not processed and we report constitutive secretion of the uncleaved precursor molecules. The mutation thus appears to affect sorting to the regulated pathway and should prove useful for analysis of the sorting machinery and of the relationship between sorting and proteolytic processing of secretory proteins. In mutants bearing misshapen trichocysts with poorly crystallized contents (tam33, tam38, stubbyA), the proteolytic processing of the trichocyst matrix proteins appears to be normal, while both pulse-chase and morphological data indicate that intracellular transport is perturbed, probably between ER and Golgi. Precursor molecules are present in the mutant trichocysts but not in wild-type trichocysts and may account for the defective crystallization. Our analysis of these mutants suggests that the temporal coordination of intracellular traffic plays a regulatory role in granule maturation.

Differential effects of temperature blockade on the proteolytic processing of three secretory granule-associated proteins

Vesicular transport within the secretory pathway can be arrested by incubating cells at 15 degrees C or 20 degrees C to block exit from the endoplasmic reticulum or trans-Golgi network, respectively. Using this powerful tool we have compared the intracellular sites of endoproteolytic processing of proopiomelanocortin and two prohormone processing enzymes in AtT-20 mouse pituitary corticotrope tumor cells. For comparison, proopiomelanocortin processing was also evaluated in primary neurointermediate pituitary cultures. AtT-20 cells synthesize and store endogenous proopiomelanocortin and prohormone convertase 1; AtT-20 cells expressing high levels of integral membrane or soluble peptidylglycine alpha-amidating monooxygenase were generated by stable transfection. Cells were incubated with [35S]methionine and chased at 4 degrees C, 15 degrees C, 20 degrees C or 37 degrees C. The endoproteolytic processing of peptidylglycine alpha-amidating mono-oxygenase, prohormone convertase 1, and proopiomelanocortin was compared following immunoprecipitation. Endoproteolytic processing of integral membrane and soluble peptidylglycine alpha-amidating monooxygenase proteins was completely blocked by incubation of cells at 20 degrees C. In contrast, prohormone convertase 1 processing from the 87 kDa precursor to the 81 kDa intermediate proceeded to completion at both 15 degrees C and 20 degrees C, while cleavage to generate the 63 kDa prohormone convertase 1 protein was completely blocked at 20 degrees C. In AtT-20 cells and neurointermediate pituitary cultures, generation of beta-lipotropin from proopiomelanocortin continued at a slow but significant rate at 20 degrees C, while processing of beta-lipotropin to beta-endorphin was blocked. Thus prohormone convertase 1 processing begins in the endoplasmic reticulum and is not completed until after the trans-Golgi network, while peptidylglycine alpha-amidating monooxygenase processing begins after the trans-Golgi network. Selected proopiomelanocortin cleavages begin before entry into immature granules.

Regulation of secretory granule formation in chronically hypersecretory melanotrophs in the rat pituitary

The formation of secretory granules in chronically hypersecretory melanotrophs in the rat pituitary was studied. Hypersecretion was induced by treatment with the dopamine antagonist haloperidol (1.5 mg/kg daily for 7 days), which releases the normal neural dopaminergic inhibition of secretion from the melanotroph. Morphometric analysis showed a 100% increase in the volume fraction of granular endoplasmic reticulum after haloperidol treatment, while the volume fractions of electron-dense granules, electron-lucent granules and the Golgi apparatus were unaltered. The mean diameter of the mature secretory granules was increased by 10%, indicating a 30% increase in mean granule volume. A similar increase in diameter was observed in condensing granules within the Golgi area. With earlier results on the effect of chronic inhibition the study shows that a main adaptive response of the melanotroph to altered secretory conditions is a change in the volume of the secretory granules, regulated by a mechanism that operates at an early stage of granule formation.

Proteolytic processing of the Aplysia A peptide precursor in AtT-20 cells

When the Aplysia ELH precursor is expressed in AtT-20 cells, the carboxyterminal derived peptides are packaged and stored in secretory vesicles, while the aminoterminal region of the precursor is constitutively secreted. In contrast, when the highly homologous A peptide precursor is transfected into AtT-20 cells, both aminoterminal and carboxyterminal derived peptides are packaged in storage granules. We propose that this is due to the fact that the initial cleavage of the A peptide precursor occurs more slowly, and perhaps later in the secretory pathway, than the ELH precursor. We further suggest that in the A peptide precursor, the first cleavage occurs after the sorting site resulting in co-packaging of the multiple products derived from a single precursor protein. To determine the structural features of the prohormones responsible for this differential sorting, we made chimeric precursors and determined the rates of the initial cleavage as well as the efficiency of storing the peptide products. From these studies, we conclude that the differential sorting is regulated both by the amino acid sequence of the first processing site, and by more global aspects of the precursor structure.

Identification of a sorting signal for the regulated secretory pathway at the N-terminus of pro-opiomelanocortin

The N-terminal 26 amino acids of the prohormone pro-opiomelanocortin (POMC) were investigated to determine whether this region has the capacity to act as a sorting signal for the regulated secretory pathway. Constructs were made using the N-terminal 101, 50, 26 or 10 amino acids of POMC fused to the chloramphenicol acetyltransferase (CAT) reporter protein and expressed in AtT20 cells to show that at least the first 26 amino acids were required to sort CAT to the regulated secretory pathway. Full length POMC was mutated by deleting amino acids 2-26 from the N-terminal region. Analysis of Neuro-2a cells expressing this mutation compared to wild type POMC indicated that these 26 amino acids contain information essential for sorting POMC to the regulated secretory pathway. The results presented here suggest the presence of a conformation-dependent signal in the N-terminal 26 amino acids of POMC responsible for sorting POMC to the regulated secretory pathway.

Differential trafficking of soluble and integral membrane secretory granule-associated proteins

The posttranslational processing enzyme peptidylglycine alpha-amidating monooxygenase (PAM) occurs naturally in integral membrane and soluble forms. With the goal of understanding the targeting of these proteins to secretory granules, we have compared the maturation, processing, secretion, and storage of PAM proteins in stably transfected AtT-20 cells. Integral membrane and soluble PAM proteins exit the ER and reach the Golgi apparatus with similar kinetics. Biosynthetic labeling experiments demonstrated that soluble PAM proteins were endoproteolytically processed to a greater extent than integral membrane PAM; this processing occurred in the regulated secretory pathway and was blocked by incubation of cells at 20 degrees C. 16 h after a biosynthetic pulse, a larger proportion of soluble PAM proteins remained cell-associated compared with integral membrane PAM, suggesting that soluble PAM proteins were more efficiently targeted to storage granules. The nonstimulated secretion of soluble PAM proteins peaked 1-2 h after a biosynthetic pulse, suggesting that release was from vesicles which bud from immature granules during the maturation process. In contrast, soluble PAM proteins derived through endoproteolytic cleavage of integral membrane PAM were secreted in highest amount during later times of chase. Furthermore, immunoprecipitation of cell surface-associated integral membrane PAM demonstrated that very little integral membrane PAM reached the cell surface during early times of chase. However, when a truncated PAM protein lacking the cytoplasmic tail was expressed in AtT-20 cells, > 50% of the truncated PAM-1 protein reached the cell surface within 3 h. We conclude that the trafficking of integral membrane and soluble secretory granule-associated enzymes differs, and that integral membrane PAM proteins are less efficiently retained in maturing secretory granules.

Peptides derived from the granins (chromogranins/secretogranins)

This review summarizes the role of granins (chromogranins/secretogranins) as precursors to biologically active peptides. We discuss the structural basis of the proteolytic processing of the granins, the consequences of their processing and the biological effects of granin-derived peptides.

Intracellular degradation of prohormone-chloramphenicol-acetyl-transferase chimeras in a pre-lysosomal compartment

Small peptide hormones (less than 50 amino acids) are synthesized as larger inactive precursors. Work from several laboratories, including our own, has implicated the propeptide of various precursors in mediating intracellular transport and targeting to secretory granules. We previously demonstrated that the proregion of prosomatostatin, one of the simplest peptide hormone precursors, when fused to alpha-globin, enabled the globin polypeptide to be transported to the regulated secretory pathway. To identify sorting motifs in this propeptide, we have now constructed a chimera comprising the somatostatin signal peptide and proregion fused to chloramphenicol acetyl transferase (CAT) and a control protein consisting of the signal peptide fused to CAT, both of which were expressed in rat anterior-pituitary GH3 cells. Both molecules were translocated into the endoplasmic reticulum (ER) efficiently and core-glycosylated on the single cryptic N-linked glycosylation site present in CAT. Surprisingly, the glycosylated propeptide-CAT and signal without CAT were degraded intracellularly with half-lives of 30 min and 90 min, respectively. Based on the kinetics of degradation, temperature sensitivity, and resistance to lysosomotrophic agents, we suggest that degradation occurred in the ER. Our data imply that the pro-region is not an a priori universal sorter, but only directs heterologous peptides to the secretory pathway when the passenger peptide assumes a secretion-competent conformation.

Differential effect of brefeldin A on phosphorylation of the caseins in lactating mouse mammary epithelial cells

The major milk proteins, the caseins, contain multiple phosphorylation sites. Phosphorylation of the caseins is necessary to allow Ca2+ binding and aggregation of the caseins to form micelles. We have followed the phosphorylation of the caseins in isolated acini from lactating mouse mammary gland. Incubation of mammary cells with [32P]orthophosphate revealed that phosphorylation of newly synthesised caseins was complete within 20 minutes of synthesis. Extensive secretion of alpha-, beta- and gamma-caseins occurred over a 2 hour period. Activation of the regulated secretory pathway using ionomycin over the last hour resulted in a preferential increase in secretion of alpha- and gamma-caseins. Brefeldin A (BFA) inhibited protein secretion and synthesis in mammary cells in prolonged incubations. An examination of short-term treatments with BFA on 32P incorporation into the caseins revealed a differential effect of BFA in which the drug inhibited phosphorylation of beta- and gamma- but not alpha-caseins. These results suggest that phosphorylation of alpha-casein normally occurs in Golgi cisternae whereas that of beta- and gamma-caseins occurs in the trans-Golgi network. Phosphorylation of specific secretory proteins may, therefore, occur in different Golgi compartments.

pH-dependent binding of chromogranin B and secretory vesicle matrix proteins to the vesicle membrane

Contrary to the notion that the soluble intravesicular matrix proteins of the secretory vesicles of adrenal medullary chromaffin cells freely float in the vesicle, several vesicle matrix proteins of the secretory vesicles, including chromogranins A and B, bound to the vesicle membrane at intravesicular pH (5.5) and were freed from it when the pH was raised to a near physiological pH (7.5). Estimation of the fraction of vesicle matrix proteins that might remain bound to the vesicle membrane in the vesicle suggested that the majority (> 50-80%) of chromogranins A and B, as well as several other proteins, will stay bound to the membrane in the vesicle. Comparison of the amino-acid sequences of chromogranins A and B revealed two highly conserved regions, i.e., one near the N-terminus and the other being the C-terminal region. Since it has been demonstrated with chromogranin A that the conserved near N-terminal region of chromogranin A exhibited the pH-dependent membrane-binding activity (Yoo, S. H. (1993) Biophys. J., 64, A195), the same region in chromogranin B (residues 17-36) was tested using a synthetic chromogranin B peptide, and found to exhibit the pH-dependent membrane-binding activity. The pH-dependent binding of the matrix proteins at pH 5.5 and the automatic untethering at a physiological pH accord well with the rapid release and circulation of the vesicular contents in the bloodstream.

Targeting of pro-opiomelanocortin to the regulated secretory pathway may involve cooperation between different protein domains

The structure of pro-opiomelanocortin (POMC) can be divided into three main domains: an NH2-terminal domain formed by the NH2-terminal glycopeptide and the joining peptide, a central domain corresponding to the adrenocorticotropin sequences and a COOH-terminal domain containing the beta-lipotropin sequences. Expression of POMC in neuroendocrine cell lines such as the mouse neuroblastoma Neuro2A cells results in its targeting to the regulated secretory pathway of these cells. Intracellular targeting of proteins along non default pathways are widely believed to involve the recognition of specific structural features by a sorting machinery. To understand the nature of the signal involved in targeting prohormone to the regulated secretory pathway, we have constructed mutants of POMC in which sequences from the NH2-terminal, the central and the COOH-terminal domains were deleted and examined the sorting of these mutant POMC molecules in Neuro2A cells by immunofluorescence and immunoelectron microscopy. Our results indicate that POMC NH2-terminal glycopeptide or beta-LPH domain do not contain sufficient information for targeting to the regulated pathway since these peptides are not sorted to secretory vesicles when expressed in Neuro2A cells: Similarly, the ACTH domain does not contain essential targeting information since POMC mutants lacking these sequences were sorted to secretory vesicles. Mutant POMCs containing the sequences of more than one of the main protein domains were, however, correctly targeted to the regulated secretory pathway. Our results indicate that POMC is not targeted to the regulated secretory pathway through recognition of a unique continuous 'molecular address'.(ABSTRACT TRUNCATED AT 250 WORDS)

Biogenesis of constitutive secretory vesicles, secretory granules and synaptic vesicles

The formation of constitutive secretory vesicles and secretory granules from the trans-Golgi network is inhibited by brefeldin A, suggesting a role for ADP ribosylation factor and other coat proteins in the budding process, and is regulated by multiple heterotrimeric G proteins. Membrane proteins are sorted to secretory granules through signals in their cytoplasmic domains and, like secretory proteins, by aggregation of their lumenal domains. Synaptic vesicle membrane proteins undergo exocytotic-endocytotic recycling before synapse formation, consistent with the proposed biogenesis of synaptic vesicles from early endosomes.

Secretory protein traffic. Chromogranin A contains a dominant targeting signal for the regulated pathway

Secretory proteins are targeted into either constitutive (secreted upon synthesis) or regulated (stored in vesicles and released in response to a secretagogue) pathways. To investigate mechanisms of protein targeting into catecholamine storage vesicles (CSV), we stably expressed human chromogranin A (CgA), the major soluble protein in human CSV, in the rat pheochromocytoma PC-12 cell line. Chromaffin cell secretagogues (0.1 mM nicotinic cholinergic agonist, 55 mM K+, or 2 mM Ba++) caused cosecretion of human CgA and catecholamines from human CgA-expressing cells. Sucrose gradients colocalized human CgA and catecholamines to subcellular particles of the same buoyant density. Chimeric proteins, in which human CgA (either full-length [457 amino acids] or truncated [amino-terminal 226 amino acids]) was fused in-frame to the ordinarily nonsecreted protein chloramphenicol acetyltransferase (CAT), were expressed transiently in PC-12 cells. Both constructs directed CAT activity into regulated secretory vesicles, as judged by secretagogue-stimulated release. These data demonstrate that human CgA expressed in PC-12 cells is targeted to regulated secretory vesicles. In addition, human CgA can divert an ordinarily non-secreted protein into the regulated secretory pathway, consistent with the operation of a dominant targeting signal for the regulated pathway within the peptide sequence of CgA.

Protein targeting to dense-core secretory granules

Regulated secretory proteins are stored within specialized vesicles known as secretory granules. It is not known how proteins are sorted into these organelles. Regulated proteins may possess targeting signals which interact with specific sorting receptors in the lumen of the trans-Golgi network (TGN) prior to their aggregation to form the characteristic dense-core of the granule. Alternatively, sorting may occur as the result of specific aggregation of regulated proteins in the TGN. Aggregates may be directed to secretory granules by interaction of a targeting signal on the surface with a sorting receptor. Novel targeting signals which confer on regulated proteins a tendency to aggregate under certain conditions, and in so doing cause them to be incorporated into secretory granules, have been implicated. Specific targeting signals may also play a role in directing membrane proteins to secretory granules.

Reconstitution in vitro of the pH-dependent aggregation of pancreatic zymogens en route to the secretory granule: implication of GP-2

Regulated secretory proteins are thought to be sorted in the trans-Golgi network (TGN) via selective aggregation. To elucidate the biogenesis of the secretory granule in the exocrine pancreas, we reconstituted in vitro the conditions of pH and ions believed to exist in the TGN using the end product of this sorting process, the zymogen granule contents. Protein aggregation was dependent on pH (acidic) and on the presence of cations (10 mM Ca2+, 150 mM K+) to reproduce the pattern of proteins found in the granule. The constitutive secretory protein IgG was excluded from these aggregates. Zymogen aggregation correlated with the relative proportion of the major granule membrane protein GP-2 in the assay. These results show that the glycosylphosphatidylinositol-anchored protein GP-2 co-aggregates with zymogens in the acidic environment believed to exist in the pancreatic TGN, and thus suggest that GP-2 would function as a membrane anchor for zymogen aggregates, facilitating their entrapment in budding vesicles directed towards the regulated secretory pathway.

Posttranslational processing of proenkephalins and chromogranins/secretogranins

Posttranslational processing of peptide-precursors is nowadays believed to play an important role in the functioning of neurons and endocrine cells. Both proenkephalins and chromogranins/secretogranins are considered as precursor molecules in these tissues, resulting in posttranslationally formed degradation products with potential biological activities. Among the proteins and peptides of neuronal and endocrine secretory granules, the enkephalins and enkephalin-containing peptides have been most extensively studied. The characterization of the post-translationally formed degradation products of the proenkephalins have enabled the understanding of their processing pathway. Chromogranins/secretogranins represent a group of acidic glycoproteins, contained within hormone storage granules. The biochemistry, biogenesis and molecular properties of these proteins have already been studied for 25 years. The chromogranins/secretogranins have a widespread distribution throughout the neuroendocrine system, the adrenal medullary chromaffin granules being the major source of these storage components. Recent data provide evidence for a precursor role for all members of the chromogranins/secretogranins family although also several other functions have been proposed. In this review, some of the methods applied to study proteolytic processing are described. In addition, the posttranslational processing of chromogranins/secretogranins and proenkephalins, especially the biochemical aspects, will be discussed and compared. Recent exciting developments on the generation and identification of potential physiologically active fragments will be covered.

Granins markers of the regulated secretory pathway

The granins are a family of acidic secretory proteins made up of chromogranin A, chromogranin B, and secretogranin II, which exhibit widespread distribution in endocrine and neuronal cells. The numerous potential sites for proteolytic processing have suggested a role for these peptides as prohormones: several potential degradation products of chromogranin A, pancreastatin, and chromostatin have autocrine activity. On the other hand, an intracellular role for the granins is supported by their propensity to aggregate in a low-pH, high-calcium environment such as found in the trans-Golgi network followed by their efficient sorting to the regulated pathway. As a result, the granins are considered markers for the regulated pathway and may play a role in secretory granule formation.

Immunological characterization of trichocyst proteins in the ciliate Pseudomicrothorax dubius

Ejectable trichocysts were isolated from the ciliate Pseudomicrothorax dubius. Polyclonal antibodies were raised against three groups of trichocyst proteins: G1 (30-31 kDa), G2 (26-27 kDa) and G3 (15-20 kDa). By indirect immunofluorescence, the three antisera strongly label the shafts of ejected trichocysts and the proximal ends of condensed trichocysts within the cells. By immunogold labeling for electron microscopy, the three sera specifically recognize the shafts of both extended and condensed trichocysts and shaft precursors in pretrichocysts as well. On one-dimensional immunoblots of isolated trichocysts, anti-G1 serum recognizes the G1 proteins, anti-G2 serum detects G2 proteins and some G1 proteins, and anti-G3 serum reacts with 15 bands, mainly the G3 and (30-41)-kDa proteins. In cells with and without trichocysts, the sera recognize non-ejectable trichocyst proteins at 41-42 kDa and 47 kDa. On two-dimensional immunoblots of isolated trichocysts, anti-G1 serum labels proteins with a pI of 4.75-5.7, anti-G2 serum labels proteins with a pI of 4.75-6.25 and anti-G3 serum labels proteins with a pI of 4.7-6.6. Analyses of cells with and without trichocysts allow identification of possible precursors between 41 and 47 kDa. Some are in the same pI range as their putative products, but others, labeled by anti-G3 serum, are less acidic than most of their mature products.