Chromogranin A-processing proteinases in purified chromaffin granules: contaminants or endogenous enzymes?

Processing of chromogranin A in the parathyroid: generation of parastatin-related peptides

Chromogranin A (CgA) is a glycoprotein present in secretory granules of endocrine cells. In the parathyroid, it is costored and cosecreted with parathormone (PTH) in response to hypocalcemia. CgA is the precursor of several bioactive peptides including pancreastatin and betagranin. Parastatin (PARA, pCgA(347-419)) is a novel peptide that we generated in vitro by enzymatic digestion of pCgA. In vitro, it inhibits low Ca(2+)-stimulated parathyroid secretion. Full activity resides in its first 19 residues. In order to determine if PARA or PARA-derived peptides are natural products of the parathyroid, we generated an antiserum directed against pCgA(347-359) corresponding to the bioactive N-terminal sequence of pPARA (pPARA(1-13) antiserum), and developed a specific radioimmunoassay that we used in conjunction with various chromatographic separations. We identified small peptides carrying the pPARA(1-13) immunoactivity in extracts and secretion medium of porcine parathyroid glands. Continuous and pulse-chase radiolabeling studies, along with immunoprecipitation using PARA(1-13) antiserum demonstrate that a newly-synthesized PARA-related peptide fraction with a Mr of 11 kDa is secreted by the parathyroid cells and accumulates in the secretion medium. Edman degradation of the 11 kDa PARA-related peptide band by Edman degradation yielded three major N-terminal sequences: S-K-M-D-R-L-A-K-E-L-(residues 313-322), D-R-L-A-K-E-L-T-A-E-(residues 316-325), and A-K-E-L-T-A-E-K-R-L-(residues 319-329), in a molar ratio of approximately 1:2:1. The peptide bonds required to be cleaved to yield these peptides, Trp-Ser, Met-Asp and Leu-Ala, suggest that a chymotrypsin-like endopeptidase participated in their formation. The molecular size and the results of amino acid compositional analysis, indicate that the C-termini of these peptides extended variably to residues 384-401 of pCgA. These results demonstrate that processing of CgA by the parathyroid gland generates bioactive PARA-related peptides that could affect the gland's secretory activity.

Study of the enzymatic degradation of vasostatin I and II and their precursor chromogranin A by dipeptidyl peptidase IV using high-performance liquid chromatography/electrospray mass spectrometry

The interaction of dipeptidyl peptidase IV with structurally related proteins differing in chain length, namely vasostatin I and II and their precursor protein chromogranin A, was examined using high-performance liquid chromatography in combination with electrospray mass spectrometry. Suitable analytical procedures were developed involving the use of reversed-phase high-performance liquid chromatography for purification of the enzymatic degradation products and a peptide mapping procedure for evaluating the enzymatic degradation of the large precursor protein chromogranin A. While vasostatin I was found to be a substrate for dipeptidyl peptidase IV, no N-terminal cleavage of Leu-Pro could be noted for chromogranin A. With respect to vasostatin II, N-terminal degradation was only observed after degradation in the C-terminal domain to proteins containing < or = 78 amino acids. The specificity of the N-terminal release of Leu-Pro was proved by addition of a DPP IV specific inhibitor.

Chromogranin A: its clinical value as marker of neuroendocrine tumours

Chromogranin A (CgA) belongs to a family of secretory proteins that are present in densecore vesicles of neuroendocrine cells. Owing to its widespread distribution in neuroendocrine tissues, it can be used as an excellent immunohistochemical marker of neoplasms of neuroendocrine origin. It can also serve as serum marker of neuroendocrine activity because it is co-released with the peptide hormone content of the secretory granules. The serum concentration of CgA is elevated in patients with various neuroendocrine tumours. Elevated levels are strongly correlated with tumour volume. Although its sensitivity and specificity cannot compete with that of the specific hormonal secretion products of most of these tumours, it can nevertheless have useful clinical applications. Neuroendocrine tumours for which no peptide marker is available usually retain the capacity to secrete CgA. CgA can thus be used as serum marker for these so-called 'non-functioning' endocrine tumours. Moreover, in patients with carcinoids and phaeochromocytomas, CgA is a more stable and thus more easily manageable marker than plasma levels of respectively serotonin and catecholamines and their urinary metabolites. Its role as an important general neuroendocrine marker may be extended in the future by the development of immunoscintigraphy of membrane-bound CgA, allowing in vivo visualization of neuroendocrine neoplasms.

Regulation of proteolytic activity in tissues

Degradation of tissue proteins is controlled by multiple means. These include regulation of the synthesis of proteinases, activation of the zymogen forms, the activity of the mature proteinase, and the degradation of these enzymes and the substrates. Mature proteinases can be controlled by pH, calcium ions, ATP, lipids and the formation of complexes with other proteinases, proteoglycans, and inhibitors.

Intracellular and extracellular processing of chromogranin A. Determination of cleavage sites

Chromogranins are a family of acidic soluble proteins which exhibit widespread distribution in endocrine cells and neurons. Chromogranin A (CGA), the major soluble component of the secretory granules in chromaffin cells of the adrenal medulla, is a single polypeptide chain of 431 residues with an apparent molecular mass of 70-75 kDa and a pI of 4.5-5. In mature bovine chromaffin granules about 50% of the CGA has been processed. In the present paper, the structural features of the proteolytic degradation mechanism have been characterized with regard to the possible function of CGA as a prohormone, as suggested by recent studies. CGA-derived components present in chromaffin granules were subjected to either two-dimensional gel electrophoresis or HPLC and the N-terminal of each fragment was sequenced. Immunoblotting with antisera to specific sequences within the CGA molecule were used to characterize these fragments further at their C-terminal. In addition, a similar approach was performed to characterize CGA-derived fragments released into the extracellular space from directly depolarized bovine cultured chromaffin cells. Our results identified several proteolytic cleavage sites involved in CGA degradation. Intragranular processing occurs at 12 cleavage sites along the peptide chain located in both N- and C-terminal moieties of the protein; a preferential proteolytic attack in the C-terminal part was noted. We found that CGA processing also occurs in the extracellular space after release, generating new shorter fragments. The proteolytic cleavage sites identified in this study were compared with the cleavage points which are thought to be involved in generating CGA fragments with specific biological activity: pancreastatin, chromostatin and N-terminal vasostatin fragments. In addition, a new 12-amino-acid CGA-derived peptide corresponding to the sequence 65-76 was identified in the soluble core of purified chromaffin granules. This short peptide was released, together with catecholamines, after stimulation of cultured chromaffin cells suggesting its presence within the storage complex of chromaffin granules. The specific biological activity of this CGA-derived fragment remains to be determined.

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)

Processing of chromaffin granule proteins: a profusion of proteases?

Evidence suggests that proenkephalin and members of the chromogranin/secretogranin family of proteins are prohormone precursors, giving rise to a variety of peptides with biologic activity. However, the specific proteases responsible for cleaving these proteins in vivo have not been fully established. Several candidate proteases have been described, some of which have been shown to cleave these proteins in vitro. Proteolytic processing of the chromogranins may be particularly complex, occurring in specific tissue-dependent patterns. To account for this level of complexity several protease systems may be operative, either alone or in concert, both within the neurosecretory granule and in the extracellular space. Specific proteases which are available within neurosecretory cells or in the local extracellular environment, and which may cleave these prohormones include PC1 and PC2 (recently described members of the Kex2/furin family of endoproteases), as well as kallikrein, acetylcholinesterase, and, more recently, the plasminogen/plasmin protease system. The potential role of these specific proteases in the processing of proenkephalin and the chromogranins is discussed, in particular, in the context of possible processing clues available from recent analysis of cDNA and genomic intron/exon structure.

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.

Kex2-like proteolytic activity in adrenal medullary chromaffin granules

This study demonstrates the presence of boc-Gln-Arg-Arg-MCA cleaving activity in bovine chromaffin granule membranes that resembles yeast Kex2 proteolytic activity. The chromaffin granule boc-Gln-Arg-Arg-MCA cleaving activity, like Kex2 proteolytic activity, shows calcium dependence, optimum activity at pH 7.5-8.2, inhibition by serine protease inhibitors, and preference for cleavage at the COOH-terminal side of Arg-Arg and Lys-Arg, over Lys-Lys, paired basic residues. Potent inhibition by the active-site directed inhibitor [D-Tyr]-Glu-Phe-Lys-Arg-CK (20 microM) provided further evidence for dibasic residue cleavage site specificity. These results are the first report of endogenous mammalian Kex2-like proteolytic activity that may be related to PC1/PC3 and PC2 enzymes, the newly discovered mammalian homologues of Kex2 protease. It will be important to determine the role of this Kex2-like proteolytic activity in processing the precursors of adrenal medullary neuropeptides.

Characterization of proenkephalin-cleaving proteinases in bovine adrenal chromaffin granules using [35S]proenkephalin copolymerized into sodium dodecyl sulfate-polyacrylamide gel electrophoresis

Proteinases capable of cleaving proenkephalin into smaller peptides have been identified in bovine adrenal chromaffin granules using [35S]methionine-labeled recombinant rat proenkephalin as a selective substrate in sodium dodecyl sulfate-polyacrylamide gel electrophoresis proteinase radiozymography. This technique was used for the screening of subcellular fractions, general characterization of pH optima, and the mechanistic characterization of proteinases with both reversible and irreversible inhibitors. Two enzymes with approximate molecular masses of 76 and 30 kDa were shown to be localized to the highest-density fractions of chromaffin granules by sucrose density gradient fractionation. Both were enriched in a 1 M NaCl wash of purified chromaffin granule membranes, were active at high pH, and were characterized as serine proteinases based on inhibition by soybean trypsin inhibitor. The 30-kDa enzyme was also inhibited by diisopropyl fluorophosphate, D-Phe-Pro-Arg-CH2Cl, and D-Val-Phe-Lys-CH2Cl and appeared to be the previously described adrenal trypsin-like enzyme. A third enzyme, of 66 kDa, was also associated with the 1 M NaCl wash of purified chromaffin granule membranes but was not localized exclusively to chromaffin granules in sucrose gradients. This proteinase was found to be Ca2+ activated and inhibited by EDTA but not diisopropyl fluorophosphate, soybean trypsin inhibitor, p-chloromercuriphenylsulfonic acid, 1,10-phenanthroline, or pepstatin.

Sex-Related Differences in Chromogranin A, Chromogranin B and Secretogranin II Gene Expression in Rat Pituitary

Chromogranin A, an acidic secretory protein, is widely distributed throughout diverse endocrine cells and the central and peripheral nervous systems. Chromogranin A is co-stored and co-secreted from secretory vesicles together with the endogenous hormones or neurotransmitters. Recently, two peptides derived from the Chromogranin A precursor have been shown to inhibit secretion from endocrine cells. In the present study, we investigated the regulation of the biosynthesis of Chromogranin A by estrogen in various tissues. In the pituitary, steady-state levels of Chromogranin A mRNA were markedly reduced by 64% in estrogen-treated male rats. At the protein level, a comparable decrease was found. Chromogranin B and secretogranin II, two other secretory proteins co-stored with Chromogranin A, were slightly increased by estrogen. In pituitaries of female rats Chromogranin A mRNA and protein levels were significantly lower than in males. For Chromogranin B on the other hand, a 2-fold increase of mRNA levels was found. Our observations demonstrate that physiologic concentrations of estrogen strongly affect Chromogranin A levels in the pituitary resulting in a sex-related difference in Chromogranin A gene expression. Based on these and previous results demonstrating increased biosynthesis of Chromogranin A by glucocorticoids and calciferol, we suggest that a typical and characteristic feature of the Chromogranin A gene is its regulation by at least three different classes of steroid hormones.

Chromostatin, a 20-amino acid peptide derived from chromogranin A, inhibits chromaffin cell secretion

Chromogranin A (CGA) is a ubiquitous 48-kDa secretory protein present in adrenal medulla, anterior pituitary, central and peripheral nervous system, endocrine gut, thyroid, parathyroid, and endocrine pancreas. Recently, we have demonstrated that the protein could be a precursor of bioactive peptides capable of modulating catecholamine secretion from cultured adrenal medullary chromaffin cells. Here we cleaved CGA purified from bovine chromaffin granules with endoproteinase Lys-C, and we isolated and partially sequenced the peptide inhibiting catecholamine secretion from cultured chromaffin cells. A corresponding synthetic peptide composed of the first 20 N-terminal amino acids produced a dose-dependent inhibition in the 10(-9) to 10(-6) M range (with an ID50 of 5 nM) of the catecholamine secretion evoked by carbamoylcholine or by potassium at a depolarizing concentration. This peptide affected secretagogue-induced calcium fluxes but did not alter sodium fluxes. It was found to increase desensitization of cell responses and to modify the kinetics of catecholamine release. Our results indicate that the peptide is extracellularly generated from CGA by a calcium-dependent proteolytic mechanism. We suggest that this peptide, named chromostatin, may be an endocrine modulator of catecholamine-associated responses.

Chromaffin granule and PC12 cell chondroitin sulfate proteoglycans and their relation to chromogranin A

Two major proteoglycans, which appear to be structurally closely related, were isolated from bovine chromaffin granule matrix proteins by ion-exchange chromatography. On sodium dodecyl sulfate-polyacrylamide gel electrophoresis they have apparent average molecular sizes of 35-40 kDa (range of 23-75 kDa) and generate a 14-kDa core glycoprotein after chondroitinase treatment. Previous studies demonstrated that these two major chromaffin granule proteoglycans are very similar in terms of their peptide mapping patterns and carbohydrate composition (having a high proportion of tri- and tetraantennary N-glycosidic oligosaccharides, and O-glycosidic oligosaccharides consisting predominantly of disialyl derivatives of galactosyl(beta 1-3)N-acetylgalactosamine), and that they differed in these respects from the chromogranins. By using antisera to five synthetic peptide fragments of chromogranin A to stain immunoblots of purified chromaffin granule proteoglycans before and after chondroitinase treatment, we have now shown that these major proteoglycans are not immunochemically related to chromogranin A. However, it has recently been reported that some chromogranin A-immunoreactive material disappears after chondroitinase treatment, and our studies demonstrate that approximately 1-2% of the chromogranin A occurs in the form of a 110-kDa proteoglycan, which is converted to a 95-kDa core glycoprotein after chondroitinase treatment. Similar chromogranin A proteoglycans could be detected in rat PC12 pheochromocytoma cells, where they have a molecular size of 115-145 kDa and yield a 105-kDa core protein after chondroitinase treatment. Studies using antibodies to synthetic peptide fragments of chromogranin B (secretogranin I) did not provide any evidence that this related protein occurs in a proteoglycan form.