CNS distribution of a novel pituitary protein '7B2': localization in secretory and synaptic vesicles

The neuroendocrine protein 7B2 is intrinsically disordered

The small neuroendocrine protein 7B2 has been shown to be required for the productive maturation of proprotein convertase 2 (proPC2) to an active enzyme form; this action is accomplished via its ability to block aggregation of proPC2 into nonactivatable forms. Recent data show that 7B2 can also act as a postfolding chaperone to block the aggregation of a number of other proteins, for example, α-synuclein. To gain insight into the mechanism of action of 7B2 in blocking protein aggregation, we performed structural studies of this protein using gel filtration chromatography, intrinsic tryptophan fluorescence, 1-anilino-8-naphthalenesulfonate (ANS) binding, circular dichroism (CD), and nuclear magnetic resonance (NMR) spectroscopy. Gel filtration studies indicated that 7B2 exists as an extended monomer, eluting at a molecular mass higher than that expected for a globular protein of similar size. However, chemical cross-linking showed that 7B2 exhibits concentration-dependent oligomerization. CD experiments showed that both full-length 27 kDa 7B2 and the C-terminally truncated 21 kDa form lack appreciable secondary structure, although the longer protein exhibited more structural content than the latter, as demonstrated by intrinsic and ANS fluorescence studies. NMR spectra confirmed the lack of structure in native 7B2, but a disorder-to-order transition was observed upon incubation with one of its client proteins, α-synuclein. We conclude that 7B2 is a natively disordered protein whose function as an antiaggregant chaperone is likely facilitated by its lack of appreciable secondary structure and tendency to form oligomers.

The proprotein convertases, 20 years later

The proprotein convertases (PCs) are secretory mammalian serine proteinases related to bacterial subtilisin-like enzymes. The family of PCs comprises nine members, PC1/3, PC2, furin, PC4, PC5/6, PACE4, PC7, SKI-1/S1P, and PCSK9 (Fig. 3.1). While the first seven PCs cleave after single or paired basic residues, the last two cleave at non-basic residues and the last one PCSK9 only cleaves one substrate, itself, for its activation. The targets and substrates of these convertases are very varied covering many aspects of cellular biology and communication. While it took more than 22 years to begin to identify the first member in 1989-1990, in less than 14 years they were all characterized. So where are we 20 years later in 2011? We have now reached a level of maturity needed to begin to unravel the mechanisms behind the complex physiological functions of these PCs both in health and disease states. We are still far away from comprehensively understanding the various ramifications of their roles and to identify their physiological substrates unequivocally. How do these enzymes function in vivo? Are there other partners to be identified that would modulate their activity and/or cellular localization? Would non-toxic inhibitors/silencers of some PCs provide alternative therapies to control some pathologies and improve human health? Are there human SNPs or mutations in these PCs that correlate with disease, and can these help define the finesses of their functions and/or cellular sorting? The more we know about a given field, the more questions will arise, until we are convinced that we have cornered the important angles. And yet the future may well reserve for us many surprises that may allow new leaps in our understanding of the fascinating biology of these phylogenetically ancient eukaryotic proteases (Fig. 3.2) implicated in health and disease, which traffic through the cells via multiple sorting pathways (Fig. 3.3).

The proprotein convertase PC2 is involved in the maturation of prosomatostatin to somatostatin-14 but not in the somatostatin deficit in Alzheimer's disease

A somatostatin deficit occurs in the cerebral cortex of Alzheimer's disease patients without a major loss in somatostatin-containing neurons. This deficit could be related to a reduction in the rate of proteolytic processing of peptide precursors. Since the two proprotein convertases (PC)1 and PC2 are responsible for the processing of neuropeptide precursors directed to the regulated secretory pathway, we examined whether they are involved first in the proteolytic processing of prosomatostatin in mouse and human brain and secondly in somatostatin defect associated with Alzheimer's disease. By size exclusion chromatography, the cleavage of prosomatostatin to somatostatin-14 is almost totally abolished in the cortex of PC2 null mice, while the proportions of prosomatostatin and somatostatin-28 are increased. By immunohistochemistry, PC1 and PC2 were localized in many neuronal elements in human frontal and temporal cortex. The convertases levels were quantified by Western blot, as well as the protein 7B2 which is required for the production of active PC2. No significant change in PC1 levels was observed in Alzheimer's disease. In contrast, a marked decrease in the ratio of the PC2 precursor to the total enzymatic pool was observed in the frontal cortex of Alzheimer patients. This decrease coincides with an increase in the binding protein 7B2. However, the content and enzymatic activity of the PC2 mature form were similar in Alzheimer patients and controls. Therefore, the cortical somatostatin defect is not due to convertase alteration occuring during Alzheimer's disease. Further studies will be needed to assess the mechanisms involved in somatostatin deficiency in Alzheimer's disease.

Neuroendocrine secretory protein 7B2: structure, expression and functions

7B2 is an acidic protein residing in the secretory granules of neuroendocrine cells. Its sequence has been elucidated in many phyla and species. It shows high similarity among mammals. A Pro-Pro-Asn-Pro-Cys-Pro polyproline motif is its most conserved feature, being carried by both vertebrate and invertebrate sequences. It is biosynthesized as a precursor protein that is cleaved into an N-terminal fragment and a C-terminal peptide. In neuroendocrine cells, 7B2 functions as a specific chaperone for the proprotein convertase (PC) 2. Through the sequence around its Pro-Pro-Asn-Pro-Cys-Pro motif, it binds to an inactive proPC2 and facilitates its transport from the endoplasmic reticulum to later compartments of the secretory pathway where the zymogen is proteolytically matured and activated. Its C-terminal peptide can inhibit PC2 in vitro and may contribute to keep the enzyme transiently inactive in vivo. The PC2-7B2 model defines a new neuroendocrine paradigm whereby proteolytic activation of prohormones and proneuropeptides in the secretory pathway is spatially and temporally regulated by the dynamics of interactions between converting enzymes and their binding proteins. Interestingly, unlike PC2-null mice, which are viable, 7B2-null mutants die early in life from Cushing's disease due to corticotropin ('ACTH') hypersecretion by the neurointermediate lobe, suggesting a possible involvement of 7B2 in secretory granule formation and in secretion regulation. The mechanism of this regulation is yet to be elucidated. 7B2 has been shown to be a good marker of several neuroendocrine cell dysfunctions in humans. The possibility that anomalies in its structure and expression could be aetiological causes of some of these dysfunctions warrants investigation.

Cathepsin-B fusion proteins misroute secretory protein partners such as the proprotein convertase PC2-7B2 complex toward the lysosomal degradation pathways

A general strategy is presented for the dominant negative reduction in the levels of heterodimeric soluble proteins within the secretory pathway through fusion of one of its partners C-terminal to the lysosomal enzyme cathepsin B (CB). Stable transfectants of CB-7B2 chimeras in AT20 cells result in a drastic reduction of the endogenous levels of its partner, the proprotein convertase PC2. This dominant negative suppressive effect requires active CB. It was partially reversed by NH(4)Cl, the cell-permeable CB inhibitor CA-074Me, but not by the proteasome inhibitor Lactacystin, suggesting the potential participation of the lysosomal/endosomal degradative pathway in this process.

Carboxypeptidase D is a potential candidate to carry out redundant processing functions of carboxypeptidase E based on comparative distribution studies in the rat central nervous system

Post-translational processing is essential for the biological activation of many proteins and peptides. After precursor cleavage at specific single residues or pairs of basic residues by the proprotein convertases, the C-terminal basic residues are removed. Carboxypeptidase E was thought to be the only enzyme responsible. Recent studies with carboxypeptidase E-deficient mice, Cpe(fat)/Cpe(fat), indicated the existence of carboxypeptidase E-like carboxypeptidases, such as carboxypeptidase D. In order to define potential redundant functions in vivo, we compared the distributions of both carboxypeptidases in the rat central nervous system and selected endocrine tissues. Carboxypeptidase D messenger RNA was abundantly expressed in glial cells in the gray and white matter, while neurons in several brain regions, such as the piriform cortex, basolateral amygdala and hippocampus, also expressed high levels of carboxypeptidase D messenger RNA. Co-localization of carboxypeptidases E and D messenger RNAs was observed in many brain regions, the spinal cord and endocrine tissues. Immunohistochemistry showed the intracellular distribution of carboxypeptidase D with a perinuclear pattern. The extensive distribution of carboxypeptidase D in both glial and neuronal cells indicates the important role of carboxypeptidase D in peptide processing, possibly working together with furin, a ubiquitously expressed proprotein convertase. The co-localization of carboxypeptidases D and E suggests that carboxypeptidase D may, at least partially, compensate for carboxypeptidase E processing functions in Cpe(fat)/Cpe(fat) mice.

Neuroendocrine protein 7B2 is essential for proteolytic conversion and activation of proprotein convertase 2 in vivo

The 7B2 protein is widely distributed in neural and endocrine tissues. Its biological function was found to be related to the processing enzyme proprotein convertase 2 (PC2), a mammalian subtilisin/kexin-like endoproteinase that cleaves at specific single or multiple basic amino-acid residues. In order to examine the proposed function of 7B2 on PC2 in in vivo models, we first compared the distribution of 7B2 and PC2 mRNAs in the rat brain. Expression of 7B2 mRNA was found to be pan-neuronal, but additionally, we observed 7B2 mRNA in ependymal cells and in the subcommissural organ. Although the expression of PC2 mRNA was exclusively neuronal, it was more restricted, sparing some regions expressing high levels of 7B2. This finding suggests that 7B2 has an additional function in non-PC2-expressing cells. No evidence of PC2-positive/7B2-negative cells could be obtained in the adult rat brain. However, in the developing rat brain (E17), such regions were easily observed, showing higher levels of pro-PC2 (75 kD). Similarly, in the animal model of insulin-induced hypoglycemic shock, where adrenomedullary 7B2 expression is decreased, the ratio of pro-PC2 to mature PC2 (75 kD:68 kD) was observed to be increased. Finally, the human neuroepithelioma SK-N-MCIXC expresses PC2 but not 7B2. Accordingly, only inactive pro-PC2 forms were observed: 75-kD intracellular and 71-kD extracellular. After stable transfection of SK-N-MCIXC cells with 27-kD pro-7B2, mature and active (68-kD) PC2 was secreted into the medium. Our data demonstrate a critical role of 7B2 in the proteolytic conversion and activation of PC2 in vivo.

Cellular localization of the prohormone convertases in the hypothalamic paraventricular and supraoptic nuclei: selective regulation of PC1 in corticotrophin-releasing hormone parvocellular neurons mediated by glucocorticoids

The prohormone convertases (PCs) are processing enzymes that activate proproteins via cleavage at specific single or pairs of basic residues. The hypothalamic paraventricular nucleus (PVN) and supraoptic nucleus (SON) are primary sites of biosynthesis of several neuroendocrine hormone precursors, including provasopressin (pro-AVP), pro-oxytocin (pro-OT), and procorticotrophin-releasing hormone (pro-CRH), which require post-translational processing to yield active products. Using in situ hybridization, we observed PC1 and PC5 mRNAs in PVN and SON magnocellular neurons, while PC2 mRNA was observed in both magnocellular and parvocellular PVN neurons as well as magnocellular SON neurons. Similar to furin, PC7 mRNA was expressed throughout the PVN and SON, whereas PACE4 mRNA levels were undetectable. Both immunohistochemical and Western blot studies were performed to demonstrate the presence of PC proteins and forms in the PVN and SON. Using double-labeling in situ hybridization, we examined the cellular colocalization of each PC mRNA with pro-AVP, pro-OT, and pro-CRH mRNAs in PVN and SON. PC1 mRNA was colocalized with both AVP and OT mRNA in PVN and SON magnocellular neurons. All AVP, OT, and CRH neurons expressed PC2. In contrast, PC5 mRNA was colocalized only with OT mRNA. We examined the effects of adrenalectomy (ADX) on PVN PC mRNA levels. PC1 mRNA levels were increased selectively within CRH/AVP parvocellular neurons but were unchanged in PVN magnocellular AVP or OT neurons. These results established the anatomical organization of each convertase and proneuropeptide substrates in the PVN and SON and suggested potential roles for each enzyme under resting and stimulated conditions.

Convertase PC2 and the neuroendocrine polypeptide 7B2 are co-induced and processed during neuronal differentiation of P19 embryonal carcinoma cells

Convertases of the subtilisin/kexin family are responsible for the biological activation of a variety of pro-proteins, pro-hormones, and pro-trophic factors, and thus can modulate various aspects of embryonic development. We investigated the expression of each convertase by Northern hybridization during cell differentiation in vitro, using the mouse embryonal carcinoma cell line P19 as a model. The neuroendocrine convertase PC2 and 7B2, its specific binding protein, are co-induced during neuronal differentiation of P19 cells with retinoic acid, whereas the other convertases are not or follow different patterns of temporal expression. The mature forms of PC2 and 7B2 proteins are detected together by immunoblotting following induction of mRNA expression, indicating that these proteins are processed early during brain development. These results demonstrate that PC2 and 7B2 gene expression and protein processing are in a close temporal association during neuronal differentiation and point to the value of the P19 cell model to study the significance and the regulation of this relationship in mammalian brain development.

The integrity of the RRGDL sequence of the proprotein convertase PC1 is critical for its zymogen and C-terminal processing and for its cellular trafficking

In order to define the functional importance of the conserved RRGDL motif in the P-domain of the mammalian proprotein convertases(PCs) we generated and cellularly expressed three mutant PC1 vaccinia-virus (VV) recombinants: ARGDL-PC1, RAGDL-PC1 and RRGEL-PC1. Functionally, these mutants caused a decreased level of processing of pro-opiomelanocortin (POMC) into beta-lipotropic pituitary hormone (beta-LPH), especially in the constitutively secreting BSC40 cells. Pulse-chase analyses demonstrated that, in part, this effect was due to both an increased degradation of the mutant PC1s within the endoplasmic reticulum and to a diminished level of zymogen processing in the same compartment. In addition, within cells containing secretory granules such as PC12 and GH4C1 cells, such mutations prevented the C-terminal auto-processing of PC1 into the fully mature 66 kDa form stored in the secretory granules of regulated cells. Since the 66 kDa PC1 is the most active form of the enzyme, it is proposed that the RRGDL sequence is critical for the generation of maximal intracellular PC1 activity. In regulated cells, co-expression of POMC with PC1 or its mutants together with the general PC inhibitor alpha1-antitrypsin Portland (alpha1-PDX), which acts primarily within the constitutive secretory pathway, demonstrated that the latter completely inhibited the formation of beta-LPH by PC1 mutants, whereas it only partially inhibited the ability of wild-type PC1 to process POMC. This suggests that RRGDL mutations prevent PC1 from entering secretory granules and hence the formation of the 66 kDa PC1, and result in the mis-sorting of PC1 mutants towards the constitutive secretory pathway. This conclusion was further supported by immunocytochemical data demonstrating that RRGDL mutants exhibit an intracellular localization pattern different from that of the granule-associated wild-type PC1,but similar to that of the Golgi-localized convertase PC5-B.

The neuroendocrine chaperone 7B2 can enhance in vitro POMC cleavage by prohormone convertase PC2

We previously showed that the neuroendocrine polypeptide 7B2 transiently interacts with prohormone convertase PC2 in the secretory pathway of neuroendocrine cells. Here we demonstrate that the processed, but not the intact, form of 7B2 can enhance the in vitro cleavage of newly synthesized prohormone proopiomelanocortin (POMC) in lysates of Xenopus intermediate pituitary cells. PC2 is presumably the cleavage enzyme involved since intact 7B2 abolishes the enhancing effect of processed 7B2 and is known to act as a specific inhibitor of PC2. Furthermore, processed 7B2 stimulates in vitro POMC cleavage by immunopurified Xenopus PC2. Our results indicate that 7B2 can display chaperone activity towards PC2.

Spatial and temporal distribution of 7B2 in the pulmonary diffuse neuroendocrine system of the cat

The postnatal developmental profile of the presence of 7B2 was studied immunocytochemically in the pulmonary diffuse neuroendocrine system of the cat. Labelling was found in neuroepithelial bodies (NEB) but not in solitary neuroendocrine cells. The identity of NEB was confirmed on adjacent serial sections by the immunostaining for protein gene product 9.5, a pan-neuroendocrine marker. The first 3 weeks after birth 7B2 stained NEB were most abundant with a subsequent decline thereafter. The positive correlation between the variation of 7B2 immunoreactivity and number of NEB suggests an important role for 7B2 in NEB.

7B2 expression in intrapulmonary neuroepithelial bodies: immunocytochemical detection and colocalization with serotonin and calcitonin gene-related peptide

In this study, the occurrence of 7B2, a highly conserved pituitary protein present in many neuroendocrine tissues and tumors, was investigated for the first time in the neuroendocrine cells (NEC) and neuroepithelial bodies (NEB) of hamster, rat and cat lung, as well as its colocalization with serotonin (5-HT) and calcitonin gene-related peptide (CGRP). Bouin fixed and wax embedded lung tissue was serially cut and immunoreactive sites for 7B2, 5-HT and CGRP were demonstrated on adjacent sections with polyclonal rabbit antibodies, using the streptavidin-biotin peroxidase method. 7B2 immunostained NEB were numerous in the intrapulmonary airways and the alveolar parenchyma, always representing a subpopulation of CGRP labelled NEB, but outnumbering those containing 5-HT. NEC, on the other hand, were only immunoreactive for CGRP and 5-HT. Our results suggest that mammalian NEB are storehouses of heterogenous bioactive substances, which may be important components in the development of bronchopulmonary tumors. Moreover, the ubiquitous presence of 7B2 in NEB supports the idea of a pre- and/or post-exocytotic role in the cellular secretory processes while the lack of 7B2 immunoreactivity in NEC might indicate a distinct function for each of both neuroendocrine cell types.

Differential expression of the neuroendocrine polypeptide 7B2 in hypothalami of Prader-(Labhart)-Willi syndrome patients

Prader-(Labhart-)Willi syndrome (PWS) is characterized by infantile hypotonia, early childhood obesity, mental deficiency, short stature, small hands and feet and hypogonadism. In 70% of the cases this syndrome is associated with a defect of chromosome 15 at 15q11-q13, close to the location of the 7B2 gene (15q13-q14). The majority of the remaining PWS patients display maternal uniparental disomy on chromosome 15. Since the 7B2 gene products are expressed in neuroendocrine cells that are probably affected in PWS, e.g. by a pleiotrophic influence of the neighboring deletion, the presence of 7B2 was studied in the supraoptic and paraventricular nucleus of the hypothalamus of five subjects clinically diagnosed as PWS patients using five antibodies against various parts of the 7B2 precursor polypeptide. Three of the five PWS patients studied showed no reaction to the 7B2 antibody MON-102, whereas all 30 control patients did. However, one of the three MON-102 non-reacting PWS patients reacted to other 7B2 antibodies. In conclusion, the vanishing of 7B2 gene products is not obligatory for PWS, possibly due to the variable genetic background of PWS patients. However, in most patients there is a clear modification of 7B2 expression, pointing to altered neuroendocrine functions.

Pan-neuronal mRNA expression of the secretory polypeptide 7B2

The polypeptide 7B2 exhibits a widespread distribution in the CNS and in the endocrine tissues. By in situ hybridization in the mouse tissues, we detected 7B2-mRNA transcripts in most, if not all, neurons of the brain and spinal cord, and in the cranial and spinal ganglia. 7B2-mRNA was undetectable in supportive glial cells, ependymal cells and endothelial cells. In embryonic tissues, 7B2-mRNA expression was observed at midgestation, starting on day 11. Both differentiated neurons and neuronal precursors have been shown to express 7B2 transcript. We conclude that 7B2-mRNA is a good molecular marker of developing and definitive neurons.

Depolarizing action of secretory granule protein 7B2 on rat supraoptic neurosecretory neurons

A novel precursor neuropeptide termed 7B2 is present within specific brain areas, including the hypothalamic magnocellular neurosecretory neurons, and appears to be processed to smaller fragments. In order to determine whether specific C-terminal fragments of 7B2 might exert local effects on neurosecretory cells, we used intracellular current-clamp recordings in supraoptic neurons maintained in superfused hypothalamic explants to evaluate membrane potential and resistance changes in 25 supraoptic nucleus neurons during bolus applications of 7B2 174-186 and two other C-terminal peptide fragments 7B2 156-173 and 7B2 141-150. In 15 supraoptic neurons, only the 7B2 174-186 fragment induced a gradual 2-8 mV membrane depolarization that lasted for 4 to 30 min and was accompanied by 15+/-8% reduction in input resistance. Immunocytochemical identification of the recorded cells revealed that both vasopressin (VP)- and oxytocin (OT)-containing neurons were depolarized by 7B2 174-186. These data suggest that 7B2 174-186 is a biologically active fragment of 7B2 and may regulate the excitability of magnocellular supraoptic nucleus neurons.

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.

Dynamics of 7B2 and galanin expression in solitary magnocellular hypothalamic vasopressin neurons of the homozygous Brattleboro rat

The homozygous Brattleboro rat (di/di), displaying a hypothalamic form of diabetes insipidus, synthesizes a vasopressin (VP) precursor with an abnormal C-terminus. The phenotypic expression of coexisting peptides in mutant magnocellular VP cells shows a differential pattern. 7B2 is one of the peptides which is not detectable, whereas there is a clear galanin expression. During postnatal life a small but increasing number of solitary post-mitotic VP neurons of the di/di rat undergoes a switch to a heterozygous phenotype. Here we report the presence of 7B2 and galanin in these heterozygous cells, which suggests that for the expression of 7B2, but not for that of galanin, the relative amount of mutant VP precursor must be diminished. Possible underlying mechanisms for this differential phenotypic expression of coexisting peptides are compartmentalization of precursor synthesis within the RER or a competition for sites involved in the translocation of the functionally reduced RER.

Expression of the neuroendocrine cell marker 7B2 in human ACTH secreting tumours

OBJECTIVE AND DESIGN

Pro-opiomelanocortin gene expression is a ubiquitous phenomenon which takes place not only in the pituitary but also in many normal and tumoral non-pituitary tissues. However, the clinical features of the ectopic ACTH syndrome are rarely encountered. To further investigate this problem we examined series of normal human pituitaries and endocrine tumours evaluating the tissue content of pro-opiomelanocortin peptides, and the state of neuroendocrine differentiation as indicated by the biochemical marker 7B2.

PATIENTS AND MEASUREMENTS

Tissue concentration of 7B2, pro-opiomelanocortin products (joining peptide and beta-endorphin) were measured in 13 pituitary corticotrophic adenomas and 13 non-pituitary tumours associated with the ectopic ACTH syndrome (five out of 20 bronchial carcinoid tumours, two out of 19 phaeochromocytomas, one out of 11 medullary thyroid carcinomas, three pancreatic and two thymic carcinoid tumours). Molecular weight forms of immunoreactive 7B2 and 7B2 RNA messenger were determined using Western and Northern blot analysis respectively.

RESULTS

In all tissues examined, concentrations of immunoreactive beta-endorphin (fmol/mg tissue wet weight) showed widely distributed values from less than 0.7 to 1,340,000, which were correlated (r = 0.975, P less than 0.01) with that of immunoreactive joining peptide, another pro-opiomelanocortin fragment. In the 13 non-pituitary tumours associated with the ectopic ACTH syndrome, immunoreactive beta-endorphin concentrations ranged from 8.6 to 548,000, whereas in normal and tumoral pituitaries they varied from 16,600 to 364,800, and 5000 to 1,340,000, respectively. Immunoreactive 7B2 was detected in 67 of 68 neuroendocrine tumours. Tissue concentrations (fmol/mg tissue wet weight) of immunoreactive 7B2 varied from 135 to 1787 in pituitary tumours; from less than 0.5 to 555 in bronchial carcinoids; from 21.7 to 793 in phaeochromocytomas; from less than 1.6 to 948 in medullary thyroid carcinomas. Western blot analysis showed a predominant molecular weight form of immunoreactive 7B2 at 22 kDa. Northern blot analysis of RNA extracted from ACTH secreting pituitary and non-pituitary tumours showed a predominant signal hybridizing at 1.5 kb with a 7B2 probe.

CONCLUSION

These results show that all ACTH secreting tumours have biochemical markers for neuroendocrine differentiation. Tissue concentrations of pro-opiomelanocortin peptides are variable, being extremely high in the most benign tumours and low in those with an aggressive growing pattern, and are not correlated with the biochemical neuroendocrine markers.

The Secretory Granule Protein 7B2 is Secreted in Parallel with Proopiomelanocortin and its End-Products by the Mouse Corticotroph Tumour Cells AtT-20

First isolated in porcine pituitary glands the protein 7B2 subsequently proved to be a specific biochemical marker of the secretory granules. Likewise 7B2 was detected in almost all normal and tumoral endocrine tissues. Unexpectedly, several authors failed to demonstrate its presence in rat and human adrenocorticotrophin (ACTH)-secreting cells. In order to definitely establish whether this cell type also produces 7B2 we chose the mouse pituitary corticotroph tumour cell line AtT-20 as a model. Serial dilutions of the mouse culture medium generated displacement curves parallel to that of the standard in a specific 7B2 RIA directed against the human 7B2(23-39) fragment. Under basal secretory conditions immunoreactive 7B2 accumulated in the culture medium in parallel with proopiomelanocortin (POMC) and its fragments N-terminal-joining peptide (NT-JP), joining peptide (JP), beta-lipotropin (beta-LPH), and beta-endorphin (beta-end), although at a much lower (approximately 100-fold) molar concentration. As expected mouse corticotroph cells responded to the stimulatory action of cyclic AMP (3.5 mM) with a preferential increase in the release of POMC end-products, JP and beta-end, which was accompanied by a parallel increase in immunoreactive 7B2 release.

The production by alternate splicing of two mRNAs differing by one codon could be an intrinsic property of neuroendocrine protein 7B2 gene expression in man

Two types of mRNAs for neuroendocrine protein 7B2 were deduced from the sequence of cDNAs clones isolated from a human pituitary cDNA library. One type lacks an Ala100 codon present in the other. The difference is located at an intron site within the human 7B2 gene and can be explained by the transcriptional utilization of two alternate acceptor splice sites, three nucleotides apart. Heteroduplex analysis of DNA fragments amplified by the polymerase chain reaction indicated that this 7B2 mRNA dimorphism occurs in several human endocrine tissues as well as in other species, suggesting that the alternate processing of 7B2 gene transcripts may be an intrinsic mechanism of its expression and could underlie some yet unknown biological functions.

The granin (chromogranin/secretogranin) family

The chromogranins/secretogranins, referred to in abbreviated form as granins, are a family of acidic secretory proteins that are found in the secretory granules of a wide variety of endocrine cells and neurons, being stored together with many different peptide hormones and neuropeptides. The recent elucidation of their primary structure has provided insights into possible functions of these proteins. Moreover, the granins have been successfully used as markers for normal and neoplastic endocrine and neuronal cells, as well as model proteins to understand the sorting mechanism involved in the formation of secretory granules.

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.

CNS connections with the median raphe nucleus: retrograde tracing with WGA-apoHRP-Gold complex in the rat

In this work we examined the neuronal input to one of the serotoninergic centers in the brain, median raphe nucleus (MR). Special consideration is given to projections of the hypothalamus. To describe the afferents to MR, a retrograde transport technique was used after microinjection of WGA-apoHRP-Gold complex under pressure and subsequent gold-silver intensification on formaldehyde-fixed rat brain sections. Optimal conditions were obtained when the coordinates of the injection site were A +/- 1.5, L +/- 0.15, and H +/- 2.7 according to Paxinos and Watson (The Rat Brain in Stereotaxic Coordinates. New York: Academic Press, '82). Results obtained under these conditions show a heterogeneous distribution of labeled neurons throughout the brain, including a large proportion (+/- 65%) of hypothalamic neurons. Extra-hypothalamic neurons projecting to MR were from the prefrontal cortex, lateral and medial habenular nuclei, the pontine area of the central grey, interpeduncular nucleus, dorsal raphe nucleus, oculomotor and trochlear nuclei, dorsal and laterodorsal tegmental nuclei, parabrachial nuclei, and lateral and interpositus cerebellar nuclei. Hypothalamic neurons connected to MR were found to be from medial and lateral preoptic areas, lateral hypothalamus, dorsomedian nucleus, the perifornical area, and the complex of mammillary bodies. Many other discrete regions contained different densities of labeled perikarya: the medial preoptic nucleus, paraventricular nucleus, retrochiasmatic area, arcuate nucleus, lateral magnocellular nucleus, and the posterior area. The MR appears as an integrative center receiving many neuroanatomically and functionally heterogeneous inputs from the whole brain.

Development of a monoclonal antibody against recombinant neuroendocrine 7B2 protein

Mouse monoclonal antibody MON-100 was raised against the neuroendocrine protein 7B2 using bacterially produced hybrid proteins. In Western blot analysis, MON-100 reacted with 3 different 7B2 hybrid proteins and not with the respective carrier proteins. Furthermore, MON-100 was reactive with recombinant 7B2 cleaved from a hybrid protein. In an immunohistochemical study, MON-100 exhibited strong reactivity with the intermediate lobe of the Xenopus pituitary gland, a tissue previously shown to contain 7B2 mRNA. Using MON-100, immunoprecipitation analysis of newly synthesized proteins produced by in vitro incubated Xenopus neurointermediate lobes revealed the biosynthesis of a single protein of Mr 24 kDa, the expected size of the 7B2 protein. It appears, therefore, that the anti-7B2 monoclonal antibody MON-100 can be successfully used for Western blot analysis and immunohistochemical analysis as well as for immunoprecipitation experiments.

The novel pituitary polypeptide 7B2 is a highly-conserved protein coexpressed with proopiomelanocortin

In the amphibian intermediate pituitary gland the biosynthetic activity for production of the precursor protein proopiomelanocortin (POMC) can be physiologically manipulated; POMC synthesis is high in animals adapted to a black background and low in white-adapted animals. In order to study genes associated with POMC gene expression we applied a differential hybridization technique involving screening of a pituitary cDNA library with probes derived from RNA of inactive and physiologically activated intermediate pituitary cells of the amphibian Xenopus laevis. A differentially hybridizing Xenopus pituitary cDNA clone encoded the novel polypeptide 7B2. This Mr-21,000 secretory granule-associated protein of unknown function is shown to be highly conserved between Xenopus and human (83% amino acid sequence similarity). Conserved segments within the 7B2 structure included the N-terminal portion, three pairs of basic amino acids which are potential recognition sites for proteolytic enzymes, and three regions sharing similarity with putative GTP-binding domains. Levels of 7B2 mRNA were about 3% of POMC mRNA levels in Xenopus pituitary glands. In the intermediate pituitary the amount of both POMC and 7B2 mRNA was much higher in black-adapted toads than in white-adapted animals. These physiologically-induced changes in POMC and 7B2 mRNA levels were not found in the anterior pituitary. We conclude that the POMC and 7B2 genes are coexpressed and that modulation of the activity of these genes is tissue-specific.

cDNA sequence of neuroendocrine protein 7B2 expressed in beta cell tumors of transgenic mice

The cDNA for a widely distributed neuroendocrine protein called 7B2 has been cloned from beta cell tumors of transgenic mice and sequenced. As deduced from the cDNA sequence, 7B2 is a secretory protein of 186 amino acids, nearly identical to its human and porcine homologs. The presence of several pairs of basic residues in the carboxyl terminal portion of the protein suggests that 7B2 can undergo proteolytic maturation in secretory granules and thus generate potential bioactive peptides. 7B2 mRNA is about 1.5 kilobase long and is apparently transcribed from a single gene per haploid genome. The use of tissue-specific promoters to express oncogenes in rare cell types of transgenic mice is a powerful tool for immortalization and expansion of these cells, and it facilitates the isolation and the study of rare proteins such as 7B2.

Two-dimensional immunoblotting analysis and immunocytochemical localization of the secretory polypeptide 7B2 in adrenal medulla

The soluble proteins of bovine chromaffin granules were subjected to 2D-electrophoresis followed by immunoblotting with an antiserum against the pituitary peptide 7B2. One immunoreactive spot was visualized at a position corresponding to a molecular weight of 24,000 and to a pI of 5.2. Using peroxidase-antiperoxidase (PAP) pre-embedding immunocytochemical technique for electron microscopy, 7B2 has been localized within secretory granules with diameters of approximately 115 and 190 nm in noradrenergic and adrenergic cells respectively. These data establish that in chromaffin granules 7B2 represents a minor component of the acidic proteins which include the chromogranins A and B, secretogranin II and the enkephalin-containing peptides.

Cloning, characterization, and sequence of a porcine cDNA encoding a secreted neuronal and endocrine protein

This report describes the cloning, sequence, and characterization of a cDNA which encodes a protein synthesized in the brain and endocrine tissue, including pituitary, adrenal medulla, and ovary. The deduced 207-amino-acid sequence of the 23-kD protein contains a hydrophobic signal peptide suggesting that it is secreted. Northern blot analysis utilizing the cDNA clone identifies a single RNA of approximately 1400 nucleotides in porcine brain, adrenal medulla, pituitary, and ovary, as well as in human endocrine tumors. Very high levels of RNA were observed in one human pancreatic tumor. Southern blot analysis suggests that sequences homologous to the porcine cDNA are present in human, cow, rat, and salmon DNA, indicating that the gene(s) have been highly conserved during evolution.

Cloning and sequence analysis of human pituitary cDNA encoding the novel polypeptide 7B2

Application of a differential hybridization technique led to the isolation of a human pituitary cDNA clone encoding the complete structure of the polypeptide 7B2. This protein of unknown function, which is sorted to secretory granules, appears to be present selectively in neurons and endocrine cells. The polypeptide chain of human 7B2, preceded by a cleaved signal peptide, comprises 185 amino acids (a calculated Mr of 20,793). Interesting features of the highly-conserved 7B2 structure include (i) a serine phosphorylation consensus sequence, (ii) the occurrence of three pairs of dibasic amino acids representing potential proteolytic cleavage sites and, in particular, (iii) the presence of three regions homologous to GTP-binding domains giving 7B2 structural characteristics of a GTP-binding protein.

Plasma levels of 7B2 (a novel pituitary polypeptide) and its molecular forms in plasma and urine in patients with chronic renal failure: possible degradation by the kidney

Plasma immunoreactive (IR)-7B2 was measured in patients with chronic renal failure (CRF), using a specific radioimmunoassay. The mean (+/- S.E.M.) concentration of plasma IR-7B2 in CRF patients under hemodialysis (502 +/- 36 pg/ml, n = 27) was significantly higher than that in normal subjects (men, 52.9 +/- 1.7 pg/ml (n = 179); women, 55.8 +/- 1.3 pg/ml (n = 198]. Significant correlations between plasma levels of IR-7B2 and those of blood urea nitrogen, creatinine and beta 2-microglobulin were evident in non-dialyzed CRF patients. In the analyses of pooled plasma and urine obtained from normal subjects on gel permeation chromatography, a major peak of IR-7B2 was observed at an apparent molecular weight of 20,000 in the plasma, and at a position of a smaller molecular weight in the urine. These results suggest that 7B2 is degraded mainly in the kidney and that measurement of plasma 7B2 may serve as an appropriate tool for assessing renal function.

Structural and immunological homology of human and porcine pituitary and plasma IRCM-serine protease 1 to plasma kallikrein: marked selectivity for pairs of basic residues suggests a widespread role in pro-hormone and pro-enzyme processing

IRCM-serine protease 1 (SP1), originally isolated from porcine pituitaries and exhibiting preference for cleavage at pairs of basic residues has now been isolated in sufficient quantities to be structurally characterized from both porcine and human pituitaries and plasmas. Whereas the porcine protease shows a high degree of amino acid sequence homology to human plasma pre-kallikrein, the human homologue exhibits an identity of sequence in the first 25 residues of each chain (regulatory and catalytic chains). In addition, human plasma and pituitary IRCM-SP1 and human plasma pre-kallikrein show virtually identical immunological and molecular properties. These data strongly suggest that IRCM-SP1 and plasma pre-kallikrein originate from the same gene product. Purified extracts from perfused rat pituitaries show that 32% of the IRCM-SP1 activity found in normal rat pituitaries, still remain. These data together with the demonstrated association of IRCM-SP1 with particulate fractions of the pituitary suggest that IRCM-SP1 represents a tissue form of plasma pre-kallikrein. The characterization of the digestion products obtained upon reaction of IRCM-SP1 with pro-insulin, ACTH1-39, pro-dynorphin and pro-enkephalin-derived peptides, somatostatin-28, and a pro-renin-like peptide confirmed the high degree of cleavage selectivity of this enzyme for pairs of basic residues.

Presence of the novel pituitary protein "7B2" in bovine chromaffin granules: possible co-release of 7B2 and catecholamine as induced by nicotine

We observed the presence of the novel pituitary protein "7B2" and its release in the bovine adrenal medulla. The 7B2 concentration (mean +/- SEM) in extracts of the bovine adrenal medulla was 952 +/- 155 pg/mg tissue (n = 6). 7B2 was distributed in the chromaffin granule fraction prepared from the bovine adrenal medulla and was released by high K+ and/or nicotine from cultured cells of the bovine adrenal medulla. Co-release of 7B2 with catecholamine induced by nicotine from the cultured bovine chromaffin cells was also observed. In an analysis of the bovine adrenal medulla chromaffin granule fraction on gel permeation chromatography, there was a major peak with an apparent molecular weight of 45,000, whereas a major peak with an apparent molecular weight of 20,000 was found in that on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. On reverse-phase HPLC, a major peak with a retention time of 35 min was observed in the bovine chromaffin granule fraction and in the bovine anterior pituitary extract. These findings indicate that 7B2 is a secretory protein in the bovine adrenal medulla. The possibility that 7B2 might be released with catecholamine, possibly in response to stress, warrants investigation.

The pituitary polypeptide "7B2" is associated with LH/FSH and TSH cells and is localized within secretory vesicles

A highly conserved polypeptide termed "7B2", isolated from human and porcine pituitaries, has been reported by immunoreactivity to be distributed in various organs. However, the highest concentration has been found in the pituitary as demonstrated by a specific radioimmunoassay. In order to determine the type of cells within the pituitary that contain 7B2 and to analyse its intracellular localization, specific immunocytochemistry techniques (unlabeled antibody, peroxidase-antiperoxidase) were used both for light and electron microscopy. Immunocytochemistry of both explants and monolayer-cell cultures of the adenohypophysis was studied. Immunoreactivity to 7B2 has been found in 21.9% of the total number of cells. After simultaneous staining of serial sections with appropriate antibodies, 7B2 was found to be colocalized with beta-LH/beta-FSH in gonadotrophs and with beta-TSH in thyrotrophs. In situ immunocytochemistry at the electron-microscopic level showed that immunoreactive 7B2 is compartmentalized within secretory granules. The small (130 to 250 nm) but not the large granules (400 to 700 nm) were labeled in gonadotroph-like cells and small granules (90 to 150 nm) were also labeled in thyroph-like cells. Study of the gonadotrophs in cell culture after Zamboni's fixation revealed weak to moderate immunoreaction in rough endoplasmic reticulum. The current findings as well as previous data indicate that 7B2 is synthesized, stored and possibly released from the adenohypophysis similarly to many other secretory products.

Immunocytochemical localization of a novel pituitary polypeptide "7B2" in the gastro-intestinal tract of the rat

Immunoreactivity to the polypeptide designated "7B2" recently isolated from human and porcine pituitary glands, appears to be consistently confined to neuroendocrine and endocrine cells in various tissues. In rat gut, immunoreactive 7B2 was found in endocrine-paracrine cells. Highly labeled cells were found in the antrum of the stomach and, cells with lower concentrations, in the fundus, duodenum, jejunum and ileum. Except for a few cells which were simultaneously positive for 5-hydroxytryptamine, and a few others showing Grimelius's reaction, "7B2" cells do not exhibit argentaffin and/or argyrophil character. The 7B2 polypeptide seems to be distributed amongst several different types of endocrine cells in the gut.

Some aspects of neuroendocrine pathology

The advent of modern microscopical investigative methods for the determination of neuroendocrine differentiation has increasingly given credence to the original concept of a "diffuse endocrine system". These methods include a variety of silver impregnation techniques, technologically advanced light and electron microscopical immunocytochemistry, and, lately, the ability to localise specific binding sites by in vitro autoradiography and mRNA species by in situ hybridisation. Further insight has been gained into the possible role of regulatory peptides contained in the so called "diffuse endocrine system" and into the nature of disease processes by investigating the role of the system in benign and malignant disease.