Chromogranin A, B and C: immunohistochemical localization in ovine pituitary and the relationship with hormone-containing cells

Secretoneurin is a secretogranin-2 derived hormonal peptide in vertebrate neuroendocrine systems

Secretogranin-2 (SCG2) is a large precursor protein that is processed into several potentially bioactive peptides, with the 30-43 amino acid central domain called secretoneurin (SN) being clearly evolutionary conserved in vertebrates. Secretoneurin exerts a diverse array of biological functions including regulating nervous, endocrine, and immune systems in part due to its wide tissue distribution. Expressed in some neuroendocrine neurons and pituitary cells, SN is a stimulator of the synthesis and release of luteinizing hormone from both goldfish pituitary cells and the mouse LβT2 cell line. Neuroendocrine, paracrine and autocrine signaling pathways for the stimulation of luteinizing hormone release indicate hormone-like activities to regulate reproduction. Mutation of the scg2a and scg2b genes using TALENs in zebrafish reduces sexual behavior, ovulation, oviposition, and fertility. A single injection of the SNa peptide enhanced reproductive outcomes in scg2a/scg2b double mutant zebrafish. Evidence in goldfish suggests a new role for SN to stimulate food intake by actions on other feeding-related neuropeptides. Expression and regulation of the Scg2a precursor mRNA in goldfish gut also supports a role in feeding. In rodent models, SN has trophic-like properties promoting both neuroprotection and neuronal plasticity and has chemoattractant properties that regulate neuroinflammation. Data obtained from several cellular models suggest that SN binds to and activates a G-protein coupled receptor (GPCR), but a bona fide SN receptor protein needs to be identified. Other signaling pathways for SN have been reported which provides alternatives to the GPCR hypothesis. These include AMP-activated protein kinase (AMPK), extracellular signal-regulated kinases (ERK), mitogen-activated protein kinase (MAPK)and calcium/calmodulin-dependent protein kinase II in cardiomyocytes, phosphatidylinositol 3-kinase (PI3K) and Akt/Protein Kinase B (AKT, and MAPK in endothelial cells and Janus kinase 2/signal transducer and activator of transcription protein (JAK2-STAT) signaling in neurons. Some studies in cardiac cells provide evidence for cellular internalization of SN by an unknown mechanism. Many of the biological functions of SN remain to be fully characterized, which could lead to new and exciting applications.

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.

Research Resource: A Chromogranin A Reporter for Serotonin and Histamine Secreting Enteroendocrine Cells

Chromogranin A (ChgA) is an acidic protein found in large dense-core secretory vesicles and generally considered to be expressed in all enteroendocrine cells of the gastrointestinal (GI) tract. Here, we characterize a novel reporter mouse for ChgA, ChgA-humanized Renilla reniformis (hr)GFP. The hrGFP reporter was found in the monoamine-storing chromaffin cells of the adrenal medulla, where ChgA was originally discovered. hrGFP also was expressed in enteroendocrine cells throughout the GI tract, faithfully after the expression of ChgA, as characterized by immunohistochemistry and quantitative PCR analysis of fluorescence-activated cell sorting-purified cells, although the expression in the small intestine was weak compared with that of the stomach and colon. In the stomach, hrGFP was highly expressed in almost all histamine-storing enterochromaffin (EC)-like cells, at a lower level in the majority of serotonin-storing EC cells and ghrelin cells, in a small fraction of somatostatin cells, but was absent from gastrin cells. In the small intestine, the hrGFP reporter was selectively, but weakly expressed in EC cells, although not in any peptide-storing enteroendocrine cells. In the colon, hrGFP was exclusively expressed in EC cells but absent from the peptide-storing enteroendocrine cells. In contrast, in the pancreas, hrGFP was expressed in β-cells, α-cells, and a fraction of pancreatic polypeptide cells. It is concluded that ChgA-hrGFP in the GI tract functions as an effective reporter, particularly for the large populations of still poorly characterized monoamine-storing enteroendocrine cells. Furthermore, our findings substantiate the potential function of ChgA as a monoamine-binding protein that facilitates the regulated endocrine secretion of large amounts of monoamines from enteroendocrine cells.

Regulation and distribution of squirrel monkey chorionic gonadotropin and secretogranin II in the pituitary

Secretogranin II (SgII) is a member of the granin family of proteins found in neuroendocrine and endocrine cells. The expression and storage of SgII in the pituitary gland of Old World primates and rodents have been linked with those of luteinizing hormone (LH). However, New World primates including squirrel monkeys do not express LH in the pituitary gland, but rather CG is expressed. If CG takes on the luteotropic role of LH in New World primates, SgII may be associated with the expression and storage of CG in the pituitary gland. The goal of this study was to evaluate the regulation and distribution of CG and SgII in the squirrel monkey. A DNA fragment containing approximately 750 bp of squirrel monkey SgII promoter was isolated from genomic DNA and found to contain a cyclic-AMP response element that is also present in the human SgII promoter and important for GnRH responsiveness. The squirrel monkey and human SgII promoters were similarly activated by GnRH in luciferase reporter gene assays in LβT2 cells. Double immunofluorescence microscopy demonstrated close association of SgII and CG in gonadotrophs of squirrel monkey pituitary gland. These results suggest that CG and SgII have a similar intercellular distribution and are coregulated in squirrel monkey pituitary gland.

Secretoneurin as a hormone regulator in the pituitary

Secretoneurin (SN) is a 33-34 amino acid peptide derived from the most conserved sequence of the secretogranin (SgII) precursor. SgII is a granin protein found in the secretory granules of neuroendocrine tissues. There are two paralogs of teleost SgII that we name here SgIIa and SgIIb. Processing of these proteins would yield SNa and SNb in fish. Secretoneurin immunoreactivity is found within all the major pituitary cell types in mammals. In goldfish, it appears to be mainly expressed in the prolactin cells of the rostral pars distalis. We have investigated the paracrine role of goldfish SN (SNa) to stimulate luteinizing hormone from gonadotrophs in the neighboring proximal pars distalis. Another source of SN is the hypophysiotropic neurons that may deliver SN to target cells by direct pituitary innervation. Little else is known about the neuroendocrine role of SN. We also discuss the evolution, distribution and production of SN in the pituitary.

Distribution of mRNAs for Chromogranins A and B and Secretogranin II in Rat Brain

The mRNA distribution of chromogranins A and B and secretogranin II was determined in rat brain. In Northern blots the oligonucleotide probes used hybridized with single mRNA species of the expected sizes. With tissue hybridization the mRNA signals for these three proteins were found throughout the brain. However, each of the three messages had a distinct distribution, which was exemplified by the fact that in the various regions either all three proteins, a combination of two or only one of them were apparently synthesized. Significant levels of all three mRNAs were found in several regions of the hippocampus and of the amygdala, in some thalamic nuclei and in the pyriform cortex. On the other hand the subiculum contained only the message for chromogranin A, the granule cell layer of the cerebellum only that for chromogranin B, and in posterior intralaminar thalamic and medial geniculate nuclei and in the nucleus of the solitary tract only secretogranin II mRNA was found. The distinct distributions of mRNAs for the chromogranins in various brain regions support the concept that these proteins are propeptides giving rise to functionally active components.

Granin proteins (chromogranin A and secretogranin II C23-3 and C26-3) in the endocrine pancreas of amphibians

The occurrence and cellular distribution of chromogranin A (CgA) and of two synthetic secretogranin II (SgII)-fragments (termed C23-3 and C26-3) has been investigated immunohistochemically in the endocrine pancreas of five amphibian species. Immunoreactivity for CgA was detected only in specimens of the genus Rana, whereas for SgII it was found in all the urodeles and anurans studied. Either CgA or the SgII-fragment displayed its own cellular distribution patterns in the endocrine pancreas of a given species. Moreover, immunoreactivity for both regions (C23-3 and C26-3) of the SgII-molecule exhibited by the same endocrine cell population have been encountered in newt and frog organs. Besides the interspecific heterogeneous distribution of CgA and of the two SgII-fragments in relation to the insular cell types, a striking heterogeneity of their immunostaining density among the endocrine cells of the same type was also revealed. The above findings entirely support the concept of a good conservation of granins during phylogeny; they do not support, however, the previously ascribed usefulness of these anionic glycoproteins as markers for all neuro-endocrine cells.

Granin proteins (chromogranin A and secretogranin II C23-3 and C26-3) in the intestine of reptiles

The occurrence, distribution and the possible cellular co-localizations of chromogranin A (CgA) and of two synthetic secretogranin II-peptides (SgIIC23-3 and SgIIC26-3) with several enteric neuropeptides and serotonin have been investigated immunohistochemically in turtles, lizards and snakes. The distribution of CgA-immunoreactivity was restricted only to the enteroendocrine cells in all the reptiles studied. SgII-immunoreactivity--absent in turtle--revealed nerve cells and fibers, besides enteroendocrine cells in lizard and snake guts. Moreover, the two antisera (C23-3 and C26-3) raised against the different regions of the SgII-molecule yielded distinct distribution patterns of immunoreactivity both in the lizard and snake organs. Small amounts of enteric serotonin cells co-stored CgA or SgIIC23-3 in lizards and snakes and only SgIIC26-3-peptide in snakes. CgA was found co-stored with somatostatin in a few enterocytes of the turtle duodenum. In the same gut segment of lizards and throughout the snake organ, neurotensin and the SgIIC23-3-peptide co-existed in a small number of endocrine cells. The pancreatic polypeptide-containing cells were devoid of immunoreactivity both for CgA and SgII. Bombesin immunopositive cells were absent throughout the intestines of the reptiles investigated. The above findings entirely support the heterogenous distribution of granins in neuroendocrine organs and tissues and also within the same neuroendocrine cell population. They further support the concept of a good conservation of granins during phylogeny.

Secretogranin II (SgII) distribution and processing studies in human normal and adenomatous anterior pituitaries using new polyclonal antibodies

Studies concerning the identification of Secretogranin II (SgII) and its processed forms in human pituitary remain scarce since no anti-human SgII antisera has been available. In the present report, a specific hSgII antiserum was used in immunohistochemistry experiments to determine the distribution of SgII in normal anterior pituitaries and pituitary adenomas (5 gonadotroph, 3 non-functioning and 5 mammotroph tumors). In normal pituitaries SgII was detected in gonadotrophs, thyrotrophs and corticotrophs but was absent from somatotrophs and mammotrophs. In tumor tissues, the SgII protein was found in gonadotroph and non-functioning adenomas but not in the mammotroph tumors. Northern blot analyses demonstrated the same 2.5 kb SgII mRNA species in all types of tumors as in normal anterior pituitaries. In Western blotting experiments, apart from the 97 K polypeptide. SgII antiserum detected two lower Mr proteins, 46 K and 31 K. These were observed in gonadotroph and in non-functioning adenomas and were absent from the mammotroph adenomas. Four new antisera were raised against sequential regions of SgII (N-terminal, two internal and C-terminal sequences). Western blotting experiments revealed that both the 46 K and 31 K polypeptides arose from the second half (C-terminal) of the molecule, thus suggesting that SgII may be processed by cleavage of short N-terminal polypeptides not detected in our conditions. Our results indicate that SgII may represent not only a valuable histological marker for non-functioning pituitary adenomas, but also a pertinent tool to study the proteolytic processing mechanisms in various neuroendocrine tumors.

Large variations in the proteolytic formation of a chromogranin A-derived peptide (GE-25) in neuroendocrine tissues

We have established a radioimmunoassay for GE-25, a peptide present in the C-terminal end of the primary amino acid sequence of chromogranin A where it is flanked by typical proteolytic cleavage sites. Gel-filtration HPLC was used to characterize the molecular sizes of the immunoreactive molecules. The antiserum recognized not only the free peptide but also larger precursors including the proprotein chromogranin A. The tissues with the highest levels of GE-25 immunoreactivity were in decreasing order: the adrenal medulla, the three lobes of the pituitary gland, intestinal mucosa, pancreas and various brain regions. In adrenal medulla and parathyroid gland most of the immunoreactivity was found to be present as intact chromogranin A and some intermediate-sized peptides, without significant amounts of the free peptide. In anterior pituitary, and even more so in intestine, a shift to smaller peptides was seen. In the posterior and intermediate pituitary and in pancreas the predominant immunoreactive material was apparently represented by the free peptide GE-25. In reverse-phase chromatography this peptide eluted exactly like the synthetic standard, which allows a tentative identification as GE-25. In brain tissue the processing of chromogranin A was intermediate, with significant amounts of immunoreactivity corresponding to GE-25 as well as precursor proteins being present. We suggest that in those organs (endocrine pancreas, intermediate and posterior pituitary) where the major hormones are proteolytically processed there is also a concomitant proteolysis of further susceptible peptides. Since GE-25 is apparently formed in vivo and is well conserved between species it seems a good candidate for having specific physiological functions.

Occurrence of secretogranin II in the prolactin-immunoreactive neurons of the rat lateral hypothalamus: an in situ hybridization and immunocytochemical study

The occurrence of secretogranin II in a neuron population of the rat lateral hypothalamus specifically detected by an anti-serum to ovine prolactin was examined. As this population was previously reported to synthesize dynorphin, the distribution of neurons recognized by ovine prolactin-, dynorphin B- and secretogranin II anti-sera was investigated on adjacent sections of hypothalami. The prolactin immunoreactive neurons were the only cells in the lateral hypothalamus to be stained by secretogranin II anti-serum. Moreover, coupling immunocytochemical detection and in situ hybridization with an oligonucleotide probe complementary to secretogranin II mRNA showed that these neurons expressed the secretogranin II gene. These new findings should help to study the physiological role of the prolactin immunoreactive neurons of the lateral hypothalamus.

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.

Measurement of secretogranin II release from individual adenohypophysial gonadotropes

Secretogranin II (SG-II) is an acidic 86-kDa protein found in high abundance in the anterior pituitary gland. In the present studies, we investigated the secretion and the localization of SG-II using pituitary cells from female rats at all stages of the estrous cycle. Double immunofluorescence staining revealed that SG-II immunoreactivity was localized in low abundance in about half of all pituitary cells and in high abundance in all of the luteinizing hormone (LH)-immunoreactive cells (which represent approximately 5% of all pituitary cells). Using a reverse hemolytic plaque assay for measurement of SG-II release from individual pituitary cells in culture, we found that SG-II secretion was strongly stimulated by gonadotropin-releasing hormone in a dose-related fashion, and the amount of SG-II secretion was also related to the stage of the estrous cycle: it was highest at proestrus and lowest at estrus. SG-II plaque assay followed by LH immunofluorescence staining further revealed that all the SG-II-secreting cells contained LH immunoreactivity. At proestrus all the LH-immunoreactive cells secreted SG-II, whereas another days of the estrous cycle only a fraction of them did so. Thus our findings demonstrate a striking resemblance between SG-II and LH with regard to cell localization and secretory regulation.

Secretoneurin--a neuropeptide generated in brain, adrenal medulla and other endocrine tissues by proteolytic processing of secretogranin II (chromogranin C)

Secretogranin II (chromogranin C), originally described as tyrosine sulfated protein of the anterior pituitary, is present in large dense core vesicles of several endocrine cells and neurons. We raised antisera in rabbits to conjugates of two synthetic peptides (bovine secretogranin 133-151 and rat secretogranin 154-186) flanked in the primary structure of secretogranin II by pairs of basic residues and used them to investigate the proteolytic processing of this protein by immunoblotting and a newly developed radioimmunoassay. The sensitivity of this assay was 30 fmol for secretogranin 154-186 and 60 fmol for secretogranin 133-151. The highest degree of processing of secretogranin II (> 90%) occurs in brain. One of the peptides (secretogranin 133-151) is not generated to any significant extent. The other peptide, secretogranin 154-186, however, is formed in vivo, and in brain the free peptide apparently represents the predominant form. The highest concentrations of secretogranin 154-186 are found in the hypothalamus, two- to six-fold lower levels are present in the hippocampus, caudate nucleus, thalamus and brainstem. These concentrations are comparable to those of established neuropeptides. In order to indicate the special relevance of secretogranin II and of this peptide for brain we have named this peptide secretoneurin. The newly developed radioimmunoassay for this peptide will be a useful tool to establish its physiologic role in brain.

In situ hybridization: mRNA levels of secretogranin II, neuropeptides and carboxypeptidase H in brains of salt-loaded and Brattleboro rats

In situ hybridization was used to study the mRNA levels for vasopressin, galanin, secretogranin II and carboxypeptidase H in salt-loaded and Brattleboro rats. These animals represent an in vivo model for the chronic stimulation of the hypothalamo-neurohypophyseal neurons. As shown by immunelectron microscopy secretogranin II is co-stored with vasopressin in these neurons. In salt-loaded rats the levels of mRNA for vasopressin, galanin and secretogranin II are increased in the paraventricular and supraoptic nuclei. Analogous changes were observed for Brattleboro rats with the exception of the vasopressin message which was decreased in these animals. The secretogranin II message was also increased in neurons which do not contain the vasopressin mRNA, i.e. in magnocellular neurons of the lateral hypothalamus and in the subfornical organ. Carboxypeptidase H message was also found in the paraventricular and supraoptic nuclei and in the subfornical organ; however, in both models the changes in mRNA in these nuclei were much lower than those observed for the secretory peptides or non-existent. We conclude that chronic stimulation of vasopressin neurons leads to a concomitant up-regulation of the biosynthesis of neuropeptides and secretogranin II. We suggest that the secretogranin II message might be a useful general marker for identifying chronically stimulated neurons.

Immunohistochemical demonstration of chromogranins A and B in neuroendocrine tumors of the lung

Fifty neuroendocrine tumors of the lung (16 carcinoids, two atypical carcinoids/well-differentiated neuroendocrine carcinomas [WDNCs], 13 neuroendocrine carcinomas of intermediate cell type [SCNCs], and 19 neuroendocrine carcinomas of small cell type [SCNs]) were immunohistochemically investigated with antibodies against chromogranins A and B. All carcinoids and WDNCs were positive for both chromogranins A and B, whereas in cases of ICNC and SCNC both markers were only expressed in six and five cases, respectively. One ICNC was only positive for chromogranin A. In cases of SCNC five tumors were exclusively positive for chromogranin A and six were positive only for chromogranin B. Chromogranins are therefore excellent markers for the immunohistochemical demonstration of carcinoids and WDNCs. It may be speculated that expression of chromogranins in cases of ICNC and SCNC represents a higher degree of differentiation in these tumors.

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.

Topology of chromogranin A and secretogranin II in the rat anterior pituitary: potential marker proteins for distinct secretory pathways in gonadotrophs

Chromogranins (Cg)/secretogranins (Sg) are representative acidic glycoproteins in secretory granules of many endocrine cells where they are co-stored and co-released with resident amines or peptides. The exact distribution of these proteins in the rat anterior pituitary is unknown. Therefore, pituitaries from untreated male rats were investigated by light- and electron-microscopical immunocytochemistry for the cellular and subcellular localization of CgA, CgB, and SgII. Endocrine cells, identified light-microscopically as gonadotrophs in adjacent semithin sections immunostained for follicle-stimulating hormone (FSH) and luteinizing hormone (LH), concomitantly were immunoreactive for CgA, CgB, and SgII. Ultrastructurally, gonadotrophs exhibited two types of secretory granules which varied in their immunoreactivities for gonadotropins and Cg/Sg. Large-sized (500 nm), moderately electron-dense granules showed antigenicities for FSH, LH, and CgA. Smaller-sized (200 nm), electron-dense granules were immunoreactive exclusively for LH and SgII. The distinct localization of CgA and SgII to morphologically and hormonally different secretory granules indicates the existence of two regulated secretory pathways in rat pituitary gonadotrophs. Hence, these proteins are considered as valuable tools to analyze the intracellular trafficking during granule biogenesis and the possible different regulation of FSH and LH secretion.

Topology of chromogranins in secretory granules of endocrine cells

Chromogranins A and B are glycoproteins originally detected in the adrenal medulla. These proteins are also present in a variety of neuroendocrine cells. The subcellular distribution of the chromogranins, and particularly their intra-granular topology are of special interest with respect to their putative functions. Endocrine cells of the guinea pig adrenal medulla, pancreas and gastric mucosa were investigated immunoelectron microscopically for the subcellular distribution of both chromogranins. Out of 13 established endocrine cell types in all locations, only two endocrine cell types showed immunoreactivity for both chromogranin A and B, and eight endocrine cell types showed immunoreactivities only for chromogranin A. These immunoreactivities varied inter-cellularly. Three endocrine cell types were unreactive for the chromogranins. Moreover, some hormonally non-identified endocrine cells in the pancreas and the gastric mucosa also contained chromogranin A immunoreactivities. Subcellularly, chromogranin A or B were confined to secretory granules. In most endocrine cells, the secretory granules showed chromogranin immunoreactivities of varying densities. Furthermore, the intra-granular topology of chromogranin A or B in the secretory granules varied considerably: in some endocrine cell types, i.e. chromaffin-, gastrin- and enterochromaffin-like-cells, chromogranin A immunoreactivity was localized in the perigranular and/or dense core region of the secretory granules; in others, i.e. insulin-, pancreatic polypeptide- and bovine adrenal medulla dodecapeptide-cells, it was present preferentially in the electron-opaque centre of the secretory granules; chromogranin B immunoreactivity was localized preferentially in the perigranular region of the secretory granules of chromaffin cells and gastrin-cells. The inter-cellular and inter-granular variations of chromogranin A and B immunoreactivities point to differences in biosynthesis or processing of the chromogranins among endocrine cells and their secretory granules.

Immunoreactivities for chromogranin A and B, and secretogranin II in the guinea pig entero-endocrine system: cellular distributions and intercellular heterogeneities

The family of the chromogranin/secretogranin proteins consists of three major subtypes: chromogranin A (CgA), chromogranin B (CgB) and secretogranin II (SgII). These proteins are present in various endocrine cells and organs. Using immunohistochemistry on serial semithin sections, we have investigated ten endocrine cell types of the guinea pig gastro-intestinal tract for their content of chromogranin/secretogranin proteins. The gastrin cell was the only cell type containing immunoreactivities for all three chromogranin subtypes. The majority of entero-endocrine cells showed immunoreactivities for CgA and SgII. Somatostatin cells lacked immunoreactivities for any of the chromogranins. Moreover, the densities of the corresponding immunoreactivities varied among the different endocrine cell types or even among endocrine cells of a given population. Aminergic endocrine cells (e.g., enterochromaffin and enterochromaffin-like cells) regularly exhibited strong immunoreactivities for CgA but failed to react for SgII. In peptidergic endocrine cells, the immunoreactivities for both CgA and SgII ranged from dense to faint. This was also true for CgB in gastrin cells. Hence, only CgA and SgII can be considered as regular constituents of entero-endocrine cells. The intercellular differences in immunoreactivities for all three chromogranin subtypes indicate that every endocrine cell has its own composition of chromogranin/secretogranin proteins. This may be due to differences in the regulation of biosynthesis or processing of the chromogranins in individual endocrine cells; this in turn might be related to the functional states of endocrine cells.

Hybridization studies of cultured human pituitary prl and gh producing adenoma cells: Effects of thyrotropin-releasing hormone, somatostatin, and phorbol ester

The effects of the hypothalamic hormones, thyrotropin-releasing hormone (TRH), and somatostatin (SRIH), and of phorbol 12-myristate 13-acetate (PMA) on PRL and GH secretion and messenger RNA (mRNA) levels were analyzed in 10 GH and/or PRL producing adenomas after culturing the tumor cells in the presence of these secretagogues for 7 days. The expression of chromogranin A and B mRNAs was also examined. All four of the clinically diagnosed GH adenomas expressed or secreted both GH and PRL while four of six clinically diagnosed prolactinomas produced or secreted both PRL and GH. Prolactinomas had less than 10% of tumor cells expressing chromogranin A mRNA while more than 40% of the adenoma cells expressed chromogranin B mRNA. TRH stimulated PRL secretion and increased PRL mRNA levels while SRIH decreased GH secretion and mRNA expression in some cases. Unexpectedly, PMA stimulated PRL mRNA levels four- to sevenfold above control levels in two adenomas and generally stimulated chromogranin A and B mRNA expression but not GH mRNA, as determined by Northern hybridization and in situ hybridization analyses.These results indicate that cultured prolactinoma cells express significantly more chromogranin B mRNA than chromogranin A mRNA, and that PMA increases PRL mRNA expression in some prolactinomas, although the effect of PMA on various adenomas reflects the heterogeneity of these tumors with respect to protein kinase C stimulation.

Light and electron microscopical immunocytochemical localization of pancreastatin-like immunoreactivity in porcine tissues

Pancreastatin is a 49 amino acid comprising peptide isolated from porcine pancreas that is derived by proteolytic processing from chromogranin A. Using an antibody against the synthetic C-terminal fragment pancreastatin (33-49), we examined the light and electron microscopical immunocytochemical localization of this peptide in porcine tissues. Pancreastatin-like immunoreactivity (PLI) was found in pancreatic somatostatin-, insulin- and glucagon cells in varying intensities; pancreatic polypeptide cells were always negative. At the electron microscopical (EM) level the immunoreactivity was confined to the electron dense core of the secretory granules in the case of somatostatin and insulin cells or to the less electron dense "halo" of the glucagon granules. In the antrum PLI positive cells represented gastrin (G), somatostatin (D) and enterochromaffin (EC) cells, in the duodenum in addition to EC- and G-cells a small number of PLI positive cells showed a positive immunoreaction for glucagon-like peptide (GLP) I and secretin in serial sections. Both norepinephrine and epinephrine containing cells of the adrenal medulla exhibited a strong reaction for PLI. In the pituitary several cell populations stained with varying intensities, including gonadotrophs and thyrotrophys. PLI is present in a distinct and characteristic subpopulation of neuroendocrine cells in various organs. The subcellular localization may indicate a function in the granular concentration, packaging and storage of peptides and amines in the brain-gut endocrine system.

Synaptophysin and chromogranins/secretogranins--widespread constituents of distinct types of neuroendocrine vesicles and new tools in tumor diagnosis

Normal and neoplastic neuroendocrine (NE) cells have been identified for many years by morphological criteria only. With the advent of immunocytochemistry, antibodies against NE-specific polypeptides have been used to identify NE cells that had been missed by conventional techniques, thus improving the diagnosis of NE cells. In this review article we discuss (i) the biochemical, cell biological and molecular biological data obtained so far for two major types of NE markers, synaptophysin, which is characteristic of the small "transparent-looking" neurosecretory vesicles, and the chromogranins/secretogranins, which are widespread constituents of the larger "dense-cored" secretory granules; (ii) the immunohistochemical data obtained for these marker proteins in normal and neoplastic human NE cells and tissues; and (iii) future possible developments involving these as well as other proteins that are associated with these two distinct secretory organelles of NE cells and may serve as potential markers in NE cell diagnosis.

Biochemistry of the chromogranin A protein family

Images

Nucleotide sequence and cellular distribution of rat chromogranin B (secretogranin I) mRNA in the neuroendocrine system

The mRNA of rat secretory-vesicle protein chromogranin B is abundant in brain, adrenal medulla, and anterior pituitary. The primary translation product predicted from the cDNA sequence of this 2,337-nucleotide transcript corresponds to a hydrophilic 655-residue protein preceded by a signal peptide. Both termini of the mature 75-kD protein show extensive similarity to other chromogranins; the more variable internal region is characterized by glutamic acid clusters and numerous pairs of basic residues. In rodent brain, mRNA accumulation starts around embryonic days 13-14 and peaks by postnatal day 20. In situ hybridization in brain sections shows that the mRNA is enriched in the hippocampal formation, the endocrine hypothalamus, the olfactory system, and in anatomically distinct structures in the pons-medulla.

An immunological study on chromogranin A and B in human endocrine and nervous tissues

Antisera were raised against synthetic peptides derived from the primary amino acid sequence of human chromogranin B. These antisera recognized in one- and two-dimensional immunoblotting a component previously designated as chromogranin B. In human chromaffin granules, the major endogenous processing product of chromogranin B is formed by proteolytic cleavage of the protein near the C-terminus. Immunohistochemical localizations were obtained with antisera against human chromogranins A and B and against a synthetic peptide corresponding to the B sequence. In human tissues, chromogranin B is co-stored with chromogranin A in the adrenal medulla, the anterior pituitary, parafollicular cells of the thyroid, in some cells of the endocrine pancreas and in some enterochromaffin cells, whereas only chromogranin A is found in the parathyroid gland and enterochromaffin cells of the gastric corpus mucosa. In the nervous system, no immunostaining was observed for chromogranin A and only a weak one for chromogranin B in some cells of the spinal cord. However, the Purkinje cells of the cerebellum were strongly positive for chromogranin B.

Preparation and characterization of anti-human chromogranin A and chromogranin B (secretogranin I) monoclonal antibodies

Chromogranin A, chromogranin B/secretogranin I and chromogranin C/secretogranin II are acidic sulphated and phosphorylated secretory proteins present in a large number of endocrine and neuronal tissues. It has been suggested that these proteins may be useful immunohistochemical markers for human tumours of endocrine origin and their measurement in plasma has been proposed as a diagnostic tool in patients with these tumours. In order to obtain anti-human chromogranins/secretogranins antibodies for clinical applications, we immunized mice with whole chromaffin granules isolated from human pheochromocytoma. The immune sera analysed by two-dimensional immunoblotting were found to recognize chromogranins/secretogranins and other unidentified proteins and to react in immunocytochemistry with pheochromocytoma as well as with a number of endocrine cells of different types. Hybridoma supernatants obtained from the splenocytes of a hyperimmune mouse, screened with an enzyme-linked immunosorbent assay, were analysed by both immunocytochemistry and two-dimensional immunoblotting. By using this experimental approach we were able to identify several monoclonal antibodies against human chromaffin granule components. In particular, we have characterized one anti-human chromogranin A and one anti-human chromogranin B/secretogranin I monoclonal antibody which showed a very specific pattern both in immunocytochemistry and in two-dimensional immunoblotting.

Regional distribution of chromogranin B 420-493-like immunoreactivity in the pituitary gland and central nervous system of man, guinea-pig and rat

The distribution of chromogranin B 420-493-like immunoreactivity was investigated in the pituitary gland and central nervous system of man, guinea-pig and rat, by using two different antibodies. In man, highest chromogranin B 420-493-like immunoreactivity concentrations were found in the pituitary gland, chromogranin B 420-493 (1-17)-like immunoreactivity 74.3 +/- 8.5 (mean +/- S.E.M. pmol/g wet wt tissue) and chromogranin B 420-493 (20-38)-like immunoreactivity 2017.0 +/- 142.3, followed by the hypothalamus and substantia nigra. In the spinal cord, the highest concentration was found in the sacral dorsal area. Chromogranin B 420-493-like immunoreactivity was detected in human cerebrospinal fluid, chromogranin B 420-493 (1-17)-like immunoreactivity 376.7 +/- 77.9 and chromogranin B 420-493 (20-38)-like immunoreactivity 1174.7 +/- 259.3 pmol/l cerebrospinal fluid. Chromogranin B 420-493-like immunoreactivity was also found in high concentrations in the guinea-pig and rat pituitary glands, but very low chromogranin B 420-493-like immunoreactivity levels were present in different parts of the brain of both species. Gel permeation chromatography and fast protein liquid chromatography analysis of eight different regions of human CNS showed multiple chromogranin B 420-493-like immunoreactivity peaks. The profile of pituitary extracts was different from those of other parts of the CNS and the cerebrospinal fluid. The profiles of guinea-pig and rat pituitary on gel chromatography and fast protein liquid chromatography were different again from those of human CNS. Examination of subcellular fractionation of whole rat brain showed highest concentrations of chromogranin B 420-493-like immunoreactivity in the synaptosome fractions. Chromogranin B 420-493-like immunoreactivity release was stimulated from rat pituitary cells by high potassium ion concentrations. These findings show that (1) chromogranin B 420-493-like immunoreactivity is widely distributed throughout the pituitary gland and CNS of three mammalian species, with the highest concentration in the pituitary gland, and it is also present in human cerebrospinal fluid, and (2) the processing of chromogranin B in the pituitary gland may be different from that in the other area of the CNS, so that it is possible chromogranin B or its fragments may play a neuroeffector role in the mammalian CNS.

The distribution of GAWK-like immunoreactivity in neuroendocrine cells of the human gut, pancreas, adrenal and pituitary glands and its co-localisation with chromogranin B

GAWK is a recently discovered peptide isolated from extracts of human pituitary gland and subsequently shown to be identical to sequence 420-493 of human chromogranin B. The distribution of this peptide was studied in human gut, pancreas, adrenal and pituitary glands using antisera to two portions of the 74 amino acid peptide (sequences 1-17 and 20-38). In addition, the co-existence of GAWK immunoreactivity with other peptides and chromogranin B was investigated using comparative immunocytochemistry. In the gut, GAWK was localised mainly to serotonin-containing cells of the mucosal epithelium, where electron microscopy showed it to be stored in typical electron-dense (250 nm diameter) granules, and to a moderate population of nerve fibres in the gut wall. Considerable quantities of GAWK-like immunoreactivity were measured in the gut, up to 36.3 +/- 18 pmol GAWK 1-17/g wet weight of tissue (mean +/- SEM) and 12.4 +/- 2.9 pmol GAWK 20-38/g. Chromatography of gut extracts revealed several GAWK-like immunoreactive peaks. GAWK-like immunoreactivity was also detected in endocrine cells of pancreas, pituitary gland and adrenal medulla, where the highest concentrations of GAWK-like immunoreactivity were measured (GAWK 1-17 2071.8 +/- 873.2 and GAWK 20-38 1292.7 +/- 542.7 pmol/g). Endocrine cells containing GAWK-like immunoreactivity were found also to be immunoreactive for chromogranin B. Our results define a discrete distribution of GAWK immunoreactivity in human endocrine cells and nerves and provide morphological support for the postulated precursor-product relationship between chromogranin B and GAWK. Details of the functions of this peptide are awaited.

Phylogenetic distribution of peptides related to chromogranins A and B

The presence of chromogranin-related peptides in a wide range of species was investigated by one and two-dimensional electrophoresis followed by immunoblotting. Antisera against bovine chromogranins A and B and the peptide WE-14 (chromogranin A316-329) were used. Chromogranins were identified by their heat stability, by their electrophoretic behavior, and by immunological cross-reaction with antisera. In all species investigated ranging from mammals to birds, amphibians, fish, and arthropods, chromogranin A- and B-like proteins could be demonstrated. For all species, there was an immunological cross-reaction with antisera against bovine chromogranins. The molecular sizes and isoelectric points of the chromogranins were similar in all species. The antiserum against WE-14 cross-reacted with pig, rat, and chicken chromogranins. It is concluded that the chromogranins A and B have a widespread phylogenetic distribution with a significant conservation of molecular size, isoelectric points, and immunological epitopes. This is consistent with the concept that these peptides have a specific function.

Chromogranin B (secretogranin I), a putative precursor of two novel pituitary peptides through processing at paired basic residues

During the course of reversed-phase high-pressure liquid chromatography (RP-HPLC) purification of the 7B2 peptide originally isolated in our laboratory from human pituitary gland extracts, two novel peptides were identified and purified to homogeneity. The complete amino acid sequence of the first one was established in 1985 and recently found to be entirely homologous to positions 420-493 of the just published chromogranin B sequence. This peptide, denoted GAWK, could originate from chromogranin B following specific cleavage at the basic amino acids flanking both termini of GAWK. Moreover, another peptide isolated in our laboratory from the same source and denoted CCB has been discovered and its sequence is also part of the same chromogranin B molecule. Here again, this peptide, occupying positions 597-653 and located at the COOH-terminal region of chromogranin B, could derive from specific processing at basic amino acids, Arg-Lys-Lys, present at positions 594-596. In a manner reminiscent of the relationship between pancreastatin and chromogranin A, it is proposed that both GAWK and CCB are produced from chromogranin B after specific processing at basic amino acids. These data are thus in favor of a putative role of chromogranins as precursors to potentially bioactive peptides.

Chromogranin A and B and secretogranin II in bronchial and intestinal carcinoids

Carcinoid tumours (bronchial and intestinal) were analyzed by immunoblotting for the presence of chromogranin A, B and secretogranin II. In all tumours an antigen corresponding in electrophoretic behaviour to adrenal chromogranin A was present. Lung carcinoids (3 out of 5) contained a relatively high concentration of a proteoglycan form of this antigen in addition. Chromogranin B was found in all tumours. In one and two dimensional immunoblotting it appeared identical to the corresponding adrenal antigen. Secretogranin II was also present, however concentrations (especially in intestinal carcinoids) were low and variable. Furthermore, in intestinal tumours it differed from the adrenal antigen by having a slightly higher molecular size and a more alkaline pI. Immunohistochemistry revealed that the tumour tissues stained positively for all three antigens. For secretogranin II the staining in intestinal tumours was relatively weak and quite variable. These results should provide a defined basis for immunohistochemical screening of carcinoids for the chromogranin/secretogranin antigens.