Chromogranin A: osmotically active fragments and their susceptibility to proteolysis during lysis of the bovine chromaffin granules

Chromaffin cells: the peripheral brain

Chromaffin cells probably are the most intensively studied of the neural crest derivates. They are closely related to the nervous system, share with neurons some fundamental mechanisms and thus were the ideal model to study the basic mechanisms of neurobiology for many years. The lessons we have learned from chromaffin cell biology as a peripheral model for the brain and brain diseases pertain more than ever to the cutting edge research in neurobiology. Here, we highlight how studying this cell model can help unravel the basic mechanisms of cell renewal and regeneration both in the central nervous system (CNS) and neuroendocrine tissue and also can help in designing new strategies for regenerative therapies of the CNS.

Defective secretion of islet hormones in chromogranin-B deficient mice

Granins are major constituents of dense-core secretory granules in neuroendocrine cells, but their function is still a matter of debate. Work in cell lines has suggested that the most abundant and ubiquitously expressed granins, chromogranin A and B (CgA and CgB), are involved in granulogenesis and protein sorting. Here we report the generation and characterization of mice lacking chromogranin B (CgB-ko), which were viable and fertile. Unlike neuroendocrine tissues, pancreatic islets of these animals lacked compensatory changes in other granins and were therefore analyzed in detail. Stimulated secretion of insulin, glucagon and somatostatin was reduced in CgB-ko islets, in parallel with somewhat impaired glucose clearance and reduced insulin release, but normal insulin sensitivity in vivo. CgB-ko islets lacked specifically the rapid initial phase of stimulated secretion, had elevated basal insulin release, and stored and released twice as much proinsulin as wildtype (wt) islets. Stimulated release of glucagon and somatostatin was reduced as well. Surprisingly, biogenesis, morphology and function of insulin granules were normal, and no differences were found with regard to beta-cell stimulus-secretion coupling. We conclude that CgB is not required for normal insulin granule biogenesis or maintenance in vivo, but is essential for adequate secretion of islet hormones. Consequentially CgB-ko animals display some, but not all, hallmarks of human type-2 diabetes. However, the molecular mechanisms underlying this defect remain to be determined.

Measurement of plasma chromogranin A concentrations for assessment of stress responses in dogs with insulin-induced hypoglycemia

OBJECTIVE

To determine whether cross-reactivity exists between canine chromogranin A (CgA) and anti-human CgA antibody and investigate the usefulness of plasma CgA concentration measurements as an index of acute stress responses in dogs.

ANIMALS

12 healthy Beagles.

PROCEDURE

Canine CgA was extracted and purified from canine adrenal glands of cadaver dogs for studying cross-reactivity with anti-human CgA antibody. Western blotting with anti-human CgA antibody was performed. Blood samples were collected from dogs at 0, 10, 20, 30, 40, 60, 120, and 180 minutes after IV administration of saline (0.9% NaCl) solution or insulin. Canine plasma CgA concentrations were determined by use of a CgA ELISA kit with rabbit antiserum against the carboxy-terminal fragment of human CgA. Plasma cortisol and catecholamine (ie, norepinephrine and epinephrine) concentrations were measured by use of an ELISA and a high-performance liquid chromatography method, respectively.

RESULTS

Purified canine CgA was specifically detected by use of western blot analysis and an ELISA with anti-human CgA antibody. An increase in plasma CgA concentrations was observed in insulin-induced hypoglycemic dogs. Changes in plasma CgA concentration were correlated with changes in plasma cortisol or catecholamine concentrations of hypoglycemic dogs.

CONCLUSIONS AND CLINICAL RELEVANCE

Use of the CgA ELISA kit for determination of human plasma CgA concentrations is applicable to the measurement of canine plasma CgA concentrations. Canine plasma CgA concentrations, along with measurements of plasma cortisol and catecholamine concentrations, correctly reflect insulin-induced hypoglycemic stressed conditions in dogs. Measurement of canine plasma CgA concentrations may provide a useful index for evaluation of an acute stress response.

A panel of 11 region-specific radioimmunoassays for measurements of human chromogranin A

INTRODUCTION

The primary structure of human chromogranin A (CgA) not only contains 10 pairs of basic amino acids, which are potential cleavage sites for specific endogenous proteases, but also other sites in the molecule can be subjected to cleavage. Several CgA-related peptides have been identified in tissue, and many of the biological effects attributed to CgA seem to be mediated by these peptides.

MATERIALS AND METHODS

Peptides homologous to defined parts of the human CgA molecule were selected and synthesised. Antibodies were raised, and 11 specific radioimmunoassays were developed. Plasma samples from 20 patients with neuroendocrine tumours were collected and measured in all assays.

RESULTS

All assays measured circulating levels of CgA-derived peptides. Only four of the assays measured concentrations that correlated with that of total CgA. However, concentrations of the individual CgA-related peptides were generally lower than the concentration of total CgA. Different neuroendocrine tumours seem to process CgA differently. The ratio between a given region-specific assay and total CgA is inversely correlated to tumour activity.

CONCLUSION

The assays presented allow measurements of defined regions of CgA and will thus become important tools for further studies of processing of CgA.

Modulatory mechanism of the endogenous peptide catestatin on neuronal nicotinic acetylcholine receptors and exocytosis

The catestatin fragment of chromogranin A is the first known endogenous compound able to inhibit catecholamine release elicited by the activation of neuronal nicotinic acetylcholine receptors (nAChRs) of different animal species and catecholaminergic cell types. However, how catestatin regulates the receptor activity, which subunit combination of the heteropentameric forms of receptor is better blocked by the peptide, or how it affects the different stages of the exocytotic process have not yet been evaluated. To address these questions, we have assayed the effects of catestatin: (first) on the inward currents elicited by ACh (I(ACh)) in voltage-clamped oocytes expressing different combinations of nAChR subunits; and (second) on the cytosolic Ca2+ concentration, [Ca2+]c, and quantal release of catecholamines simultaneously monitored in single adrenal chromaffin cells stimulated with ACh. Catestatin potently blocks all the subtypes of nAChRs studied. Furthermore, it inhibits the alpha3beta4 current in a reversible, noncompetitive, voltage-, and use-dependent manner, a behavior compatible with open-channel blockade. In fura-2-loaded single chromaffin cells, the peptide reduced the [Ca2+]c signal and the total release of catecholamines elicited by ACh; however, catestatin did not modify the kinetics or the last step of the exocytotic process. Our results suggest that catestatin might play an autocrine regulatory role in neuroendocrine secretion through its interaction with different native nAChR subtypes; the extent of receptor blockade by the peptide could be acutely regulated by the intensity and duration of the presynaptic stimulus.

Analysis of the post-translational processing of chromogranin A in rat neuroendocrine tissue employing an N-terminal site-specific antiserum

Chromogranin A (CgA) is a complex prohormone expressed as a constituent of the regulated secretory pathway of numerous neuroendocrine cells. Recent investigations have demonstrated that CgA is selectively cleaved to generate distinct peptides in different neuroendocrine tissues. This investigation employed a site-specific antiserum that detects residues 98-106 rat CgA to examine the amino-terminal processing of CgA to generate beta-granin and related peptides in rat neuroendocrine tissues. Immunohistochemistry revealed moderate to intense beta-granin-like immunostaining in cells scattered throughout the anterior pituitary, thyroid, in the islets of Langerhans and in the mucosa of the gastrointestinal tract. Variable intensities of immunostaining were observed in distinct clusters of chromaffin cells. Quantitatively, the highest concentration of beta-granin-like immunoreactivity was detected in pituitary extracts. Significantly lower concentrations were detected in adrenal and thyroid glands, brain, ventral and dorsal pancreatic lobes and gastrointestinal tissue extracts. Chromatography resolved three distinct beta-granin-like immunoreactants; a large CgA-like form, an intermediate molecular form presumably corresponding to beta-granin (rat CgA1-128) and small immunoreactants that coeluted with the synthetic peptide. Two beta-granin-like immunoreactants, 21 and 22 kDa, were detected following immunoblot analysis of pituitary extracts. This study has demonstrated that chromogranin A is subject to distinct amino-terminal patterns of tissue-and cell-specific processing to generate a beta-granin-like immunoreactant which is additionally cleaved in pancreatic, fundic and colonic tissue to generate previously unidentified peptides.

Nitric oxide modulates a late step of exocytosis

The effects of nitric oxide (NO) on the late phase of exocytosis have been studied, by amperometry, on Ba(2+)-stimulated chromaffin cells. Acute incubation with NO or NO donors (sodium nitroprusside, spermine-NO, S-nitrosoglutathione) produced a drastic slowdown of the granule emptying. Conversely, cell treatment with N(omega)-nitro-l-arginine methyl ester (a NO synthase inhibitor) or with NO scavengers (methylene blue, 2-(4-carboxyphenyl)-4,4,5, 5-tetramethyl-imidazoline-1-oxyl-3-oxide potassium) accelerated the extrusion of catecholamines from chromaffin granules, suggesting the presence of a NO modulatory tone. The incubation with phosphodiesterase inhibitors (3-isobutyl-1-methylxanthine or zaprinast) or with the cell-permeant cGMP analog 8-bromo-cGMP, mimicked the effects of NO, suggesting the involvement of the guanylate cyclase cascade. NO effects were not related to changes in intracellular Ba(2+). NO did not modify the duration of feet. Effects were evident even on pre-fusioned granules, observed under hypertonic conditions, suggesting that the fusion pore is not the target for NO, which probably acts by modifying the affinity of catecholamines for the intragranular matrix. NO could modify the synaptic transmitter efficacy through a novel mechanism, which involves the regulation of the emptying of secretory vesicles.

Amine weak bases disrupt vesicular storage and promote exocytosis in chromaffin cells

The vesicular contents in bovine chromaffin cells are maintained at high levels owing to the strong association of its contents, which is promoted by the low vesicular pH. The association is among the catecholamines, Ca2+, ATP, and vesicular proteins. It was found that transient application of a weak base, methylamine (30 mM), amphetamine (10 microM), or tyramine (10 microM), induced exocytotic release. Exposure to these agents was also found to increase both cytosolic catecholamine and intracellular Ca2+ concentration, as measured by amperometry and fura-2 fluorescence. Amphetamine, the most potent amine with respect to evoking exocytosis, was found to be effective even in buffer without external Ca2+; however, the occurrence of spikes was suppressed when BAPTA-acetoxymethyl ester was used to complex intracellular Ca2+. Amphetamine-induced spikes in Ca2+-free medium were not suppressed by thapsigargin or ruthenium red, inhibitors of the sarco(endo)plasmic reticulum Ca2+-ATPase and mitochondrial Ca2+ stores. Atomic absorption measurements of amphetamine- and methylamine-treated vesicles reveal that intravesicular Ca2+ stores are decreased after a 15-min incubation. Taken together, these data indicate that amphetamine and methylamine can disrupt vesicular stores to a sufficient degree that Ca2+ can escape and trigger exocytosis.

Immunohistochemical distribution of chromogranins A and B and secretogranin II in neuroendocrine tumours of the gastrointestinal tract

The aim of the present study was to investigate immunohistochemically the distribution of chromogranin A, chromogranin B, and secretogranin II in a series of 152 neuroendocrine tumours of the gastrointestinal tract. Tumour tissues from 25 argyrophil gastric carcinoids, 18 gastrin and 5 somatostatin-producing tumours, 4 'gangliocytic paragangliomas', 49 classical argentaffin and 2 L cell appendiceal carcinoids, 27 classical ileal carcinoids, 17 rectal carcinoids, and 5 poorly differentiated neuroendocrine tumours of the stomach and rectum were immunostained with antibodies against chromogranin A, chromogranin B, and secretogranin II. Chromogranin A was the major granin expressed in gastric carcinoids and in serotonin-producing carcinoids of the appendix and the ileum. In contrast, strong chromogranin B and secretogranin II immunoreactivity was found in rectal carcinoids, in which chromogranin A was rarely expressed. Since chromogranin A is a widely used marker for neuroendocrine differentiation, it is of diagnostic importance that some gastrin-producing tumours, 'gangliocytic paragangliomas', poorly differentiated neuroendocrine carcinomas, and appendiceal L cell carcinoids completely lacked chromogranin A positivity. It is concluded that the various neuroendocrine tumours of the gastrointestinal tract show distinctly different patterns of granin expression, probably reflecting their histogenetical origin.

Synaptic vesicles and release of transmitters: new insights at the molecular level

Neurotransmitter release from transmitter storage vesicles is a regulated signalling event that has properties in common with other secretory systems. Biochemical characterization of mammalian synaptic vesicle proteins has recently converged with studies of protein traffic in non-neuronal cells and the genetic dissection of the yeast secretory pathway to give us a considerable amount of new data. Many new synaptic vesicle proteins have been characterized together with plasma membrane proteins with which they interact, and it appears that many of the participating components may be part of a general machinery for secretion. The new results significantly improve our understanding of the molecular mechanisms governing transmitter release. This review discusses the recent progress in terms of synaptic vesicle components and the proposed mechanisms for exocytosis.

Recombinant human chromogranin A: expression, purification and characterization of the N-terminal derived peptides

Chromogranin A (CGA) is an ubiquitous 48 kDa secretory protein stored and released from most endocrine cells and is present in nanomolar concentration in the human vascular system. Recent data suggest that CGA may be the precursor of several peptides with a defined biological activity. The present report describes the expression of human CGA in Escherichia coli using the pET3a vector system, the purification and characterization of the recombinant protein and the production of antibody against the expressed protein. The expressed CGA was purified by a multi-step protocol including heat treatment, gel filtration and high performance-anion exchange chromatography and two-dimensional gel electrophoresis. Two major forms of recombinant human CGA (rhCGA) were purified from the bacterial cytosol: a 70 kDa form which corresponded to the native full-length CGA and a major proteolytic 63 kDa product recognized by antibodies raised against the 70 kDa rhCGA or to synthetic peptides localized in the N-terminal part of the bovine CGA sequence. This E. coli expression system provides a method for producing a suitable protein which will permit the identification of CGA-derived peptides with defined biological function in human. Fragments containing the N-terminal domain were generated by acidic cleavage of the two forms of rhCGA. A two-step purification using high-performance reverse-phase chromatography yielded 6 peptide bands ranging in apparent molecular mass from 7 to 18 kDa. Four components (molecular mass range 12-18 kDa) were immunostained with antibodies directed against synthetic sequences of bovine vasostatin II (bCGA1-113) while the two others (molecular mass range 7-8 kDa) were immunostained only with antibodies directed against vasostatin I (bCGA1-76). From protein staining the ratio vasostatins II/I was 10:1. The vasoinhibitory activity of this preparation was examined on isolated human saphenous vein segments. An inhibitory effect was obtained in paired vessel segments from 7 patients undergoing surgery for coronary artery bypass, however with low potency for supression of the endothelin-1 evoked sustained tension in these vessels.

Vasoinhibitory activity of synthetic peptides from the amino terminus of chromogranin A

Naturally occurring amino terminal fragments of chromogranin A (CGA), the calcium-binding protein found in all endocrine secretory vesicles, have vasoinhibitory activity when tested in isolated segments of the endothelium-denuded human saphenous vein. Synthetic peptides corresponding to sequences within the first 76 residues of chromogranin A have been made and tested for biological activity. Full length vasostatin I (CGA1-76) (40 nM), but not the truncated vasostatin I, CGA1-40 (100 nM) mimics natural chromogranin A fragments in its inhibition of contractions induced by endothelin-1 (ET-1) in calcium containing medium. CGA1-40 (100 nM) mimics the inhibitory effect of the vasostatins on the contractions induced in the absence of extracellular calcium by high potassium and noradrenaline, but not by ET-1. The iodinated peptides both exhibit saturable binding in an aortic smooth muscle cell line, indicative of a single class of high affinity binding protein ('receptor' with an apparent KD of approximately 45 nM. This binding is not affected by endothelin-1. Iodinated peptides can be crosslinked to a single polypeptide in binding experiments performed on intact calf aortic smooth muscle cells.

Chromogranin A: secretion of processed products from the stimulated retrogradely perfused bovine adrenal gland

Chromogranin A (CGA) is a member of a family of highly acidic proteins co-stored and co-secreted with adrenaline and noradrenaline in the adrenal medulla. A number of biologically active fragments of CGA (CGAFs) have been characterized including a group of small N-terminal fragments collectively named vasostatins due to their vascular inhibitory activity. In the present study, the release of CGAFs, including CGA N-terminal fragments, from the isolated, retrogradely perfused bovine adrenal gland, has been studied under basal conditions and during nerve stimulation and perfusion with acetylcholine. The CGAFs were characterized by SDS-PAGE followed by immunoblotting with antisera to specific sequences within the CGA molecule. Many different CGAFs were released during stimulation of the glands. Antisera to CGA1-40 and CGA44-76 detected a 7 kD protein whose release was increased during stimulation. This component co-migrated with synthetic CGA1-76, was not immunoreactive to antisera to CGA79-113 or CGA124-143, and was seen whether or not the serine protease inhibitor aprotinin was present in the perfusion medium. The release of an approximately 18 kD component, which stained with antisera to CGA1-40, CGA44-76 and CGA79-113, but not to chromostatin (CGA124-143), was also increased during stimulation. Components of 22 kD and larger were detected with antisera to chromostatin, but not with antisera to CGA1-40, CGA44-76 and CGA79-113. Two of these components of 22 to 24 kD were enhanced during nerve stimulation in the presence of aprotinin. The results indicate that processed chromogranin A fragments are secreted from the bovine adrenal medulla during stimulation of chromaffin cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Vasostatins, comprising the N-terminal domain of chromogranin A, suppress tension in isolated human blood vessel segments

Chromogranin A (CGA) belongs to a family of highly acidic proteins which are co-stored and co-released with the catecholamines from the mammalian adrenal gland and occur in nmolar concentrations in the human circulation. A vascular function for the adrenomedullary released and circulating CGA has yet to be established. The present study reports on the novel vasoinhibitory effect of the N-terminal domain of the adrenomedullary CGA in isolated segments of the human internal thoracic artery (ITA) and saphenous vein (SV). The collective term vasostatin(s) refers to N-terminal fragments (CGA1-76 and CGA1-113) of apparent molecular weights 7 to 22 kD, to indicate their vascular inhibitory effects. The sustained contractions evoked by the potent vasoconstrictor peptide, endothelin-1 (ET-1) were suppressed when ITA and SV segments were preincubated for 15 min with vasostatins (72 nM). The vasoinhibitory effects were not dependent on an intact endothelium and suppression of the response to 35 nM ET-1 was approximately 77% and approximately 40% in endothelium-denuded ITA and SV segments, respectively. In endothelium-denuded SV segments the vasostatins suppressed the maximal sustained tension response but not the potency for ET-1, indicating that the vasostatin effect did not involve interference with ET-1 binding to its vascular receptor. Preincubation of endothelium-denuded SV segments with nifedipine (1 microM) inhibited the sustained response to ET-1 > or = 10 nM by 50%.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Chromogranin A processing in sympathetic neurons and release of chromogranin A fragments from sheep spleen

Chromogranin A (CGA) has been localized to the large dense cored vesicles (LDV) of sympathetic neurons. SDS-PAGE and immunoblotting of soluble LDV proteins from ox and dog adrenergic neuronal cell bodies, axons and nerve terminals, revealed an increasing number of CGA-immunoreactive forms, consistent with proteolytic processing during axonal transport. Splenic nerve electrical stimulation (10 Hz, 2 min) revealed that, apart from CGA, these CGA-processing products are released from the sheep spleen. The secretion of CGA-derived fragments from sympathetic neurons might suggest a role in the regulation of synaptic transmission.

The vasoinhibitory activity of bovine chromogranin A fragment (vasostatin) and its independence of extracellular calcium in isolated segments of human blood vessels

Endothelium-independent vasoconstrictor responses in isolated segments of human internal thoracic artery (ITA) and saphenous vein (SV) were used as a bioassay system for the vasoinhibitory activity of bovine chromogranin A (CGA). Preincubation with vasostatin (0.8 micrograms/ml), containing the N-terminal domain of CGA, (CGA1-76, CGA1-113 and CGA1-143ff), inhibited the contractile responses evoked by 80 mM K+, 2.6 microM noradrenaline (NA), or 65 nM endothelin-1 (ET-1) in Ca(2+)-free solution in SV but not in ITA. The results demonstrate a vasoinhibitory activity in vasostatin and show that there is a marked difference between the arterial and venous segments in the Ca2+ independent component of the inhibitory response. A vascular role for the N-terminal domain of CGA is indicated, presumably by inhibiting Ca2+ release from intracellular stores in the human vein but not the artery.

Pancreastatin and bovine parathyroid cell secretion

Chromogranin A (CgA) is an acidic glycoprotein found in secretory granules of multiple peptidergic tissues and cosecreted with the resident peptide hormones. Pancreastatin is an amidated, biologically active peptide whose sequence is contained within CgA. We investigated the effect of the C-terminal fragment of bovine pancreastatin (bP32-47) on bovine parathyroid cell secretion. bP32-47 amide inhibited low-calcium-stimulated PTH secretion by 44% and chromogranin A (CgA) secretion by 33%. We were able to identify a pancreastatin-like peptide as a very minor component of the endogenous breakdown peptides from CgA. However, using several approaches, we were unable to detect pancreastatin in secretory granule extracts or in incubation media. We conclude that although exogenous bovine pancreastatin has inhibitory effects on secretion, detectable pancreastatin is not secreted under normal incubation conditions. Based on our current data, we would question the physiologic importance of pancreastatin in bovine parathyroid glands.

Isolation and primary structure of a novel chromogranin A-derived peptide, WE-14, from a human midgut carcinoid tumour

The primary structure of a novel human chromogranin A-derived tetradecapeptide, WE-14, possessing N-terminal tryptophanyl (W) and C-terminal glutamyl (E) residues was isolated from a hepatic metastasis of an human ileal carcinoid tumour. Human and bovine WE-14 are structurally identical, while rat, mouse and porcine analogues exhibit 93% homology. WE-14 is flanked by paired basic residues (KR) in all known chromogranin A sequences.

Pancreastatin--a novel regulatory peptide?

Pancreastatin is a 49 amino acid peptide originally isolated from porcine pancreas on the basis of its C-terminal glycinamide as isolation criterion. It is derived by proteolytic processing from chromogranin A, an acidic protein component of secretory granules in endocrine and neuronal cells. The primary structures of human, porcine, bovine and rat pancreastatin have been determined on the protein or cDNA level and show 70% sequence homology. By immunocytochemistry, pancreastatin has been detected in the pituitary, adrenal gland, pancreas, CNS and throughout the gastrointestinal tract. In pancreatic islets, pancreastatin is co-localized with insulin, glucagon and somatostatin. The principle biological activities of this peptide are: inhibition of insulin release and of exocrine pancreatic secretion. These effects which can be assigned to the amidated C-terminal part of the molecule have been demonstrated in several species. Whether or not pancreastatin can be classified as a novel peptide hormone that under physiological conditions plays a role in the regulation of the endocrine and exocrine pancreas, is still a matter of controversy.