Secretogranin II: regulation of synthesis and post-translational proteolysis in bovine adrenal chromaffin cells

Granin-derived peptides

The granin family comprises altogether 7 different proteins originating from the diffuse neuroendocrine system and elements of the central and peripheral nervous systems. The family is dominated by three uniquely acidic members, namely chromogranin A (CgA), chromogranin B (CgB) and secretogranin II (SgII). Since the late 1980s it has become evident that these proteins are proteolytically processed, intragranularly and/or extracellularly into a range of biologically active peptides; a number of them with regulatory properties of physiological and/or pathophysiological significance. The aim of this comprehensive overview is to provide an up-to-date insight into the distribution and properties of the well established granin-derived peptides and their putative roles in homeostatic regulations. Hence, focus is directed to peptides derived from the three main granins, e.g. to the chromogranin A derived vasostatins, betagranins, pancreastatin and catestatins, the chromogranin B-derived secretolytin and the secretogranin II-derived secretoneurin (SN). In addition, the distribution and properties of the chromogranin A-derived peptides prochromacin, chromofungin, WE14, parastatin, GE-25 and serpinins, the CgB-peptide PE-11 and the SgII-peptides EM66 and manserin will also be commented on. Finally, the opposing effects of the CgA-derived vasostatin-I and catestatin and the SgII-derived peptide SN on the integrity of the vasculature, myocardial contractility, angiogenesis in wound healing, inflammatory conditions and tumors will be discussed.

Evidence that the chromogranin B fragment 368-417 extracted from a pheochromocytoma is phosphorylated

A rabbit antiserum was raised against a synthetic peptide corresponding to residues 403 to 417 of human chromogranin B. This peptide was chosen to match the potential C-terminal end of a putative proteolytic fragment of the protein located between dibasic doublets in positions 366-367 and in positions 418-419 of the precursor. A radioimmunoassay based on this antiserum was developed and used to detect the protein or a fragment thereof in a pheochromocytoma tumor extract. One fragment was purified to homogeneity by successive reverse-phase HPLC chromatographies. The N-terminal sequence established by automated Edman degradation, was N-Y-P-S-L-E-L-D-K-M-A-H-G-Y-G-E-E-S-E-E-E-R corresponding to the 368-389 sequence of human chromogranin B. Taking into account the specificity of the antiserum used for peptide identification and alignment with the precursor sequence, we deduced that the purified peptide was chromogranin B (368-417) and represented a new peptide generated by limited proteolysis of chromogranin B. Combining electrospray mass-spectrometry and enzymatic dephosphorylation, we demonstrated that this peptide was phosphorylated.

Regulation of the biosynthesis and processing of chromogranins in organotypic slices: influence of depolarization, forskolin and differentiating factors

Slices from rat hippocampus in organotypic culture were used to study the biosynthesis regulation of chromogranins A and B and secretogranin II. Additionally, we investigated the proteolytic conversion of secretogranin II and the levels of prohormone convertases putatively involved. Forskolin treatment and depolarization with potassium plus BayK 8644 led to significant increases in secretogranin II mRNA in the principal cells of the hippocampus. Enhanced expression of secretogranin II was also reflected by a rise in peptide levels. Despite this induction of biosynthesis the extensive processing to secretoneurin normally observed in brain was maintained. Both forskolin and depolarization upregulated the prohormone convertase (PC)1, but not PC2, indicating that PC1 levels are critical for secretoneurin production under stimulating conditions. Results obtained for chromogranins A and B were less consistent. For chromogranin A mRNA, changes were restricted to granule cells; for chromogranin B, a response in granule cells was observed to depolarization but not to forskolin, and effects in pyramidal neurons were weak. Accordingly, we were unable to detect alterations in chromogranin A and B protein levels. Furthermore, we tested several neurotrophic growth factors and found that only basic fibroblast growth factor raised secretogranin II expression without affecting chromogranins A and B. The hippocampal slice preparation allowed well controlled treatment with identification of neuronal subpopulations and yielded data largely matching experiments in vivo and in cell culture. The pronounced regulation of secretogranin II and its effective processing underlines the importance of the resulting peptide secretoneurin as an active neuropeptide in the nervous system.

Antibacterial and antifungal activities of vasostatin-1, the N-terminal fragment of chromogranin A

Vasostatin-1, the natural N-terminal 1-76 chromogranin A (CGA)-derived fragment in bovine sequence, has been purified from chromaffin secretory granules and identified by sequencing and matrix-assisted laser desorption time-of-flight mass spectrometry. This peptide, which displays antibacterial activity against Gram-positive bacteria at micromolar concentrations, is also able to kill a large variety of filamentous fungi and yeast cells in the 1-10 microM range. We have found that the C-terminal moiety of vasostatin-1 is essential for the antifungal activity, and shorter active peptides have been synthesized. In addition, from the comparison with the activity displayed by related peptides (human recombinant and rat synthetic fragments), we could determine that antibacterial and antifungal activities have different structural requirements. To assess for such activities in vivo, CGA and CGA-derived fragments were identified in secretory material released from human polymorphonuclear neutrophils upon stimulation. Vasostatin-1, which is stored in a large variety of cells (endocrine, neuroendocrine, and neurons) and which is liberated from stimulated chromaffin and immune cells upon stress, may represent a new component active in innate immunity.

Phosphorylation and O-glycosylation sites of human chromogranin A (CGA79-439) from urine of patients with carcinoid tumors

Because of their water-soluble properties, chromogranins (CGs) and chromogranin-derived fragments are released together with catecholamines from adrenal chromaffin cells during stress situations and can be detected in the blood by radiochemical and enzyme assays. It is well known that chromogranins can serve as immunocytochemical markers for neuroendocrine tissues and as a diagnostic tool for neuroendocrine tumors. In 1993, large CGA-derived fragments have been shown to be excreted into the urine in patients with carcinoid tumors and the present study deals with the characterization of the post-translational modifications (phosphorylation and O-glycosylation) located along the largest natural CGA-derived fragment CGA79-439. Using mild proteolysis of peptidic material, high performance liquid chromatography, sequencing, and mass spectrometry analysis, six post-translational modifications were detected along the C-terminal CGA-derived fragment CGA79-439. Three O-linked glycosylation sites were located in the core of the protein on Thr163, Thr165, and Thr233, consisting in di-, tri-, and tetrasaccharides. Three phosphorylation sites were located in the middle and C-terminal domain, on serine residues Ser200, Ser252, and Ser315. These modified sites were compared with sequences of others species and discussed in relation with the post-translational modifications that we have reported previously for bovine CGA.

Exocytosis in chromaffin cells of the adrenal medulla

The chromaffin cell has been used as a model to characterize releasable components present in secretory granules and to understand the cellular mechanisms involved in catecholamine release. Recent physiological and biochemical developments have revealed that molecular mechanisms implicated in granule trafficking are conserved in all eukaryotic species: a rise in intracellular calcium triggers regulated exocytosis, and highly conserved proteins are essential elements which interact with each other to form a molecular scaffolding, ensuring the docking of granules at the plasma membrane, and perhaps membrane fusion. However, the mechanisms regulating secretion are multiple and cell specific. They operate at different steps along the life of a granule, from the time of granule biosynthesis up to the last step of exocytosis. With regard to cell specificity, noradrenaline and adrenaline chromaffin cells display different receptor and signaling characteristics that may be important to exocytosis. Characterization of regulated exocytosis in chromaffin cells provides not only fundamental knowledge of neurosecretion but is of additional importance as these cells are used for therapeutic purposes.

Effects of secretogranin II-derived peptides on the release of neurotransmitters monitored in the basal ganglia of the rat with in vivo microdialysis

In vivo microdialysis was used to study the effect of secretogranin II-derived peptides on dynorphin B (Dyn B), dopamine, gamma-aminobutyric acid (GABA), glutamate and aspartate release in the substantia nigra and neostriatum of halothane-anaesthesized rats. In the substantia nigra, local infusion of secretoneurin (secretogranin II 154-186) (1-50 microM) increased, in a concentration-dependent manner, extracellular aspartate, glutamate, Dyn B, dopamine and GABA levels. The effect was particularly prominent on aspartate and glutamate levels which, following 50 microM of secretoneurin, were increased by > 20 and > 10 fold, respectively. However, the effect of secretoneurin on Dyn B release appeared to be more specific, since a significant increase (> 20 fold) was already observed following 1 microM of secretoneurin. In the neostriatum, Dyn B, glutamate, aspartate and GABA levels were also increased by local secretoneurin infusion, but the effect was less prominent than in the substantia nigra. In the substantia nigra, only Dyn B levels were significantly increased following infusion of 10 microM of the secretoneurin-C terminal (secretoneurin-15C), whereas Dyn B and GABA levels were increased by the same concentration of the secretogranin II C terminus (YM). Only glutamate and aspartate levels were increased by local infusion of 10 microM of secretogranin II 133-151 (LF), a peptide adjacent to secretoneurin in the primary amino acid sequence. In the neostriatum, Dyn B and GABA levels were increased by 10 microM of secretoneurin-15C. Dyn B levels were also increased by 10 microM of YM, and glutamate and aspartate levels were increased by 10 microM of both YM and LF. Thus secretogranin II-derived peptides affect extracellular levels of several putative neurotransmitter systems monitored in the basal ganglia of the rat with in vivo microdialysis. The effect of Dyn B appears to be specific and related to a physiological role of secretoneurin, since (i) it occurs in an area where secretoneurin-immunocytochemistry has been observed, (ii) is exerted at comparatively low concentrations, and (iii) is mimicked by secretoneurin-15C. The increases in excitatory amino acid levels produced by high concentrations of secretoneurin and other secretogranin II-derived peptides reflect, perhaps, a potential neurotoxicity produced by abnormal accumulation of these peptides.

Molecular cloning of frog secretogranin II reveals the occurrence of several highly conserved potential regulatory peptides

Secretogranin II (SgII) is an acidic secretory protein present in large dense core vesicles of neuronal and endocrine cells. Based on the sequence of a peptide derived from the processing of SgII in the brain of the frog Rana ridibunda, degenerate oligonucleotides were used to clone the cDNA encoding frog SgII from a pituitary cDNA library. This cDNA encodes a 574 amino acid protein which exhibits 46-48% sequence identity with mammalian SgII and contains 11 pairs of basic amino acids. Four potential processing products delimited by pairs of basic residues exhibited a much higher degree of identity (68-82%) with the corresponding mammalian SgII sequences. The frog SgII mRNA is approximately 4 kb in length and is differentially expressed in the brain and endocrine tissues. The present data reveal that several SgII-derived peptides have been highly conserved during evolution, suggesting that these peptides may play important neuroendocrine regulatory functions.

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.

Biosynthesis of large dense-core vesicles in PC12 cells: effects of depolarization and second messengers on the mRNA levels of their constituents

mRNA levels of various constituents of large dense-core vesicles were determined in PC12 cells during depolarization and/or in the presence of BayK 8644, forskolin or phorbolester. For the soluble (secretory) proteins of the vesicles the mRNAs of chromogranin A and B, secretogranin II, neuropeptide Y and VGF were analyzed. Depolarization in the presence of BayK induced a strong up-regulation of the messages for chromogranin B, neuropeptide Y and VGF. Addition of forskolin enhanced this response for neuropeptide Y and VGF, phorbolester did the same only for VGF. Partly membrane-bound and membrane-spanning components analyzed were carboxypeptidase H, dopamine beta-hydroxylase and glycoprotein III (clusterin), peptidylglycine alpha-amidating mono-oxygenase and cytochrome b-561, respectively. Changes of mRNAs for these components were in general smaller and delayed. Six days of depolarization caused an up-regulation of glycoprotein III, peptidylglycine alpha-amidating mono-oxygenase and carboxypeptidase H mRNA levels which were not further increased by cyclic AMP and phorbolester. The dopamine beta-hydroxylase message increased after 6 days of depolarization, however, addition of phorbolester reduced this effect. For cytochrome b-561 there was no change after any of the conditions employed. These in vitro results are compared with those obtained for the biosynthesis regulation of large dense-core vesicles under in vivo conditions. It is suggested that in vivo acetylcholine and vasoactive intestinal polypeptide released from splanchnic nerve induce a differential change in the biosynthesis of large dense-core vesicles by acting via calcium and protein kinase A and C.

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.

Processing of secretogranin II by prohormone convertases: importance of PC1 in generation of secretoneurin

Secretoneurin is a recently characterized neuropeptide present in the primary amino acid sequence of secretogranin II. We investigated the proteolytic processing of secretogranin II by prohormone convertases in vivo in a cellular system using the vaccinia virus system. Both PC1 and PC2 can cleave the secretogranin II precursor at sites of pairs of basic amino acids to yield intermediate-sized fragments. Other convertases like PACE4, PC5 and furin were not active. For the formation of the free neuropeptide secretoneurin a different pattern was found. Only PC1 but none of the other convertases tested including PC2 were capable of generating secretoneurin. Our results demonstrate that the prohormone convertases PC1 and PC2 are involved in proteolytic processing of secretogranin II. The neuropeptide secretoneurin can only be generated by PC1 suggesting tissue-specific processing of secretogranin II in neurons expressing different subsets of the prohormone convertases.

Antibodies to secretogranin II reveal potential processing sites

Several events occur during secretory granule maturation in endocrine and neuronal cells, one of the most important being the processing of prohormones. In addition, secretory granules undergo several changes during storage and maturation within the cell. We have been investigating the maturation of secretory granules in the neuroendocrine cell line PC12. Our working hypothesis postulates that fusion of newly budded secretory granules occurs during maturation and results in a larger secretory granule. We have been investigating the kinetics and specificity of the prohormone processing enzymes towards secretogranin II. Processing of secretogranin II must occur predominantly in the maturing secretory granule and could be used as a method to monitor secretory granule maturation.

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.