Strain-dependent influences on the hypothalamo-pituitary-adrenal axis profoundly affect the 7B2 and PC2 null phenotypes

Two null mouse models have previously been created to study the role of the prohormone convertase (PC2) and its helper protein 7B2; unexpectedly, the phenotypes of these two nulls differ profoundly, with the 7B2 but not the PC2 null dying at 5 wk. The genetic backgrounds of these two models differ, with the 7B2 null in a 129/SvEv (129) background and the PC2 null in a mixed C57BL/N6:129/SvEv (B6:129) background. Because background can contribute greatly to phenotype, we have here examined strain influence on the hypothalamo-pituitary-adrenal (HPA) axis and glucose levels in wild-type, 7B2 null, and PC2 null mice. Wild-type B6 and 129 mice differed in basal corticosterone and glucose levels. When 7B2 nulls were transferred onto the B6 background, they survived and showed greatly decreased circulating corticosterone and increased blood glucose levels, most likely due to the comparatively higher adrenal resistance of the B6 strain to ACTH stimulation. Circulating ACTH levels were increased over wild-type in the B6 7B2 null but did not reach levels as high as the 129 7B2 null. Conversely, when the mixed-strain PC2 nulls were bred into the 129 background at the N6 generation, they began to exhibit the Cushing's-like phenotype characteristic of 129 7B2 null mice and died before 6 wk of age. Taken together, these results indicate that background effects are critical because they increase the phenotypic differences between the 7B2 and PC2 nulls and play a life-or-death role in the ACTH hypersecretion syndrome present in both 129 nulls.

A fully functional proopiomelanocortin/melanocortin-1 receptor system regulates the differentiation of human scalp hair follicle melanocytes

The proopiomelanocortin (POMC)-derived peptides, ACTH and alpha-MSH, are the principal mediators of human skin pigmentation via their action at the melanocortin-1 receptor (MC-1R). Recent data have demonstrated the existence of a functionally active beta-endorphin/mu-opiate receptor system in both epidermal and hair follicle melanocytes, whereby beta-endorphin can regulate melanogenesis, dendricity, and proliferation in these cells. However, a role for ACTH and alpha-MSH in the regulation of the human follicular pigmentary unit has not been determined. This study was designed to examine the involvement of ACTH and the alpha-MSH/MC-1R system in human follicular melanocyte biology. To address this question we employed RT-PCR and immunohisto/cytochemistry, and a functional role for these POMC peptides was assessed in follicular melanocyte cultures. Human scalp hair follicle melanocytes synthesized and processed POMC. ACTH and alpha-MSH in association with their processing enzymes and MC-1R are expressed in human follicular melanocytes at the message level in vitro and at the protein level both in situ and in vitro. The expression of the POMC/MC-1R receptor system was confined only to subpopulations of poorly and moderately differentiated melanocytes. In addition, functional studies revealed that ACTH and alpha-MSH are able to promote follicular melanocyte differentiation by up-regulating melanogenesis, dendricity, and proliferation in less differentiated melanocyte subpopulations. Thus, these findings suggest a role for these POMC peptides in regulating human hair follicle melanocyte differentiation.

Biosynthesis of proopiomelanocortin-derived peptides in prohormone convertase 2 and 7B2 null mice

Prohormone convertases (PCs) are thought to represent the major proteinases involved in the biosynthetic processing of peptide hormone precursors to bioactive peptide products. The maturation of PC2 requires the aid of a helper protein, 7B2, in order for the zymogen to become an active enzyme species. The 7B2 and PC2 nulls should thus be functionally equivalent with regard to deficits in precursor processing. In this article, we have examined this proposition through the study of proopiomelanocortin (POMC) biosynthesis and granule content in both null models. RIA data indicate that both PC2 and 7B2 nulls lack pituitary alpha-MSH; interestingly, 7B2 nulls are still able to generate beta-endorphin from beta-lipotropin, whereas PC2 nulls contain little if any beta-endorphin. Labeling experiments demonstrate a build-up of POMC, high molecular weight intermediates, and intact ACTH, as well as the disappearance of alpha-MSH, in both null models. Electron microscopy of neurointermediate lobe melanotrophs reveals the presence of a significantly greater number of secretory granules in both 7B2 and PC2 nulls compared with wild-type controls. However, PC2 null melanotrophs contain twice as many granules as 7B2 null melanotrophs. Another difference between the two null models is a relatively enhanced accumulation of precursors in the PC2 null compared with the 7B2 null; these include not only PC2 substrates, but also presumed PC1 substrates. These data indicate that the two nulls are not phenotypically equivalent.

Regulation of cell growth and expression of 7B2, PC2, and PC1/3 by TGFbeta 1 and sodium butyrate in a human pituitary cell line (HP75)

Recent studies have shown that 7B2 and the neuroendocrine- specific proconvertase PC2 have important roles in pituitary cell proliferation and hormone secretion. Studies from our laboratory have also shown that TGFb1 regulates anterior pituitary cell proliferation and hormone secretion. To study the regulation of 7B2 in human pituitary tumors, we used a cell line derived from a human pituitary adenoma (HP75) that has been shown to express 7B2, PC1, PC2, and TGFbeta receptors to analyze the effects of TGFbeta1 and the histone deacetylase inhibitor (HDACI) sodium butyrate (NaB) treatment on 7B2 mRNA expression along with the neuroendocrine-specific proconvertases 1/3 (PC1) and PC2 mRNA and protein expression. RNA was quantified by real-time PCR and proteins were detected by immunohistochemistry and Western blotting. Treatment of cells with 1 mM NaB or 1 nM TGF 1 for 4 d decreased cell proliferation with a concomitant increase in the cell cycle protein p21. Real-time PCR analysis showed a significant increase in 7B2 mRNA after NaB and TGFbeta1 treatment. PC2 mRNA was down regulated by NaB while PC1 mRNA was unchanged. TGFbeta1 stimulated PC1, but not PC2 mRNA levels. Changes in PC1 and PC2 protein were similar to changes in the mRNAs, but the differences were not significant. These results indicated that NaB and TGFbeta1 inhibit pituitary cell proliferation and regulate the expression of 7B2, PC1, and PC2 in a cell culture model of pituitary tumors. Our results also indicate that inhibition of pituitary cell proliferation is associated with increased expression of 7B2 mRNA.

Frequent appearance of autoantibodies against prohormone convertase 1/3 and neuroendocrine protein 7B2 in patients with nonfunctioning pituitary macroadenoma

Among pituitary disorders having mass effect of the pituitary gland, nonfunctioning pituitary macroadenoma and lymphocytic hypophysitis are difficult to differentiate without histological examination. In order to efficiently distinguish lymphocytic hypophysitis and pituitary tumors, we studied the presence of autoantibodies against prohormone-processing enzymes, prohormone convertase (PC) 1/3, PC2, carboxypeptidase E (CPE), and PC2 regulatory protein, 7B2, by radioligand assay using recombinant human 35S-labeled protein in patients with clinically nonfunctioning pituitary macroadenoma, lymphocytic hypophysitis, and other pituitary diseases. The indexes for anti-PC1/3 antibodies (Ab) were significantly higher in patients with nonfunctioning pituitary macroadenoma than in patients with lymphocytic hypophysitis. Patients positive for either anti-PC1/3 or anti-7B2 Ab were significantly frequent among patients with nonfunctioning pituitary macroadenoma than in other pituitary diseases and healthy controls. None of the patients was positive for anti-PC2 Ab or anti-CPE Ab. These results suggest that autoantibodies against PC1/3 and 7B2 are novel tumor-associated autoantibodies and can be helpful in the diagnosis of clinically nonfunctioning pituitary macroadenoma.

Neuropeptides in anterior pituitary development

Recent studies using biotechnological methods have achieved significant advances in our knowledge of molecular mechanisms underlying pituitary gland development and the differentiation of pituitary cytotypes. A large number of neuropeptides have been reported in the adult pituitary gland as well as in the central and peripheral nervous system. The early presence of neuropeptides during pituitary development is reviewed here. Neuromedin U (NmU), galanin and the polypeptide 7B2 have been localised to different endocrine cells of the gland. Their expression seems to be manifold even though it is temporally and spatially regulated. There is now firm immunocytochemical evidence that neuropeptides are present during morphogenesis of the pituitary and can be present simultaneously with all pituitary hormones.

Increased synthesis but decreased processing of neuronal proCCK in prohormone convertase 2 and 7B2 knockout animals

In addition to its role as a gut hormone, cholecystokinin (CCK) is a widespread and potent neurotransmitter. Its biosynthesis requires endoproteolytic cleavage of proCCK at several mono- and dibasic sites by subtilisin-like prohormone convertases (PCs). Of these, PC1 and PC2 are specific for neuroendocrine cells. We have now examined the role of PC2 and its binding protein, 7B2, in the neuronal processing of proCCK by measurement of precursor, processing-intermediates and bioactive end-products in brain extracts from PC2- and 7B2-null mice and from corresponding controls. PC2-null mice displayed a nine-fold increase of cerebral proCCK concentrations, and a two-fold increase in the concentrations of the processing-intermediate, glycine-extended CCK, whereas the concentrations of transmitter-active (i.e. alpha-amidated and O-sulfated) CCK peptides were reduced (61%). Chromatography showed that O-sulfated CCK-8 still is the predominant transmitter-active CCK in PC2-null brains, but that the fraction of intermediate-sized CCK-peptides (CCK-58, -33 and -22) was eight-fold increased. 7B2-null brains displayed a similar pattern but with less pronounced precursor accumulation. In contrast with the cerebral changes, PC2 deficiency was without effect on proCCK synthesis and processing in intestinal endocrine cells, whereas 7B2 deficiency halved the concentration of bioactive CCK in the intestine. The results show that PC2 plays a major neuron-specific role in the processing of proCCK.

Proinsulin processing in the diabetic Goto-Kakizaki rat

The biosynthesis and processing of proinsulin was investigated in the diabetic Goto-Kakizaki (GK) rat. Immunofluorescence microscopy comparing GK and Wistar control rat pancreata revealed marked changes in the distribution of alpha-cells and pronounced beta-cell heterogeneity in the expression patterns of insulin, prohormone convertases PC1, PC2, carboxypeptidase E (CPE) and the PC-binding proteins 7B2 and ProSAAS. Western blot analyses of isolated islets revealed little difference in PC1 and CPE expression but PC2 immunoreactivity was markedly lower in the GK islets. The processing of the PC2-dependent substrate chromogranin A was reduced as evidenced by the appearance of intermediates. No differences were seen in the biosynthesis and post-translational modification of PC1, PC2 or CPE following incubation of islets in 16.7 mM glucose, but incubation in 3.3 mM glucose resulted in decreased PC2 biosynthesis in the GK islets. The rates of biosynthesis, processing and secretion of newly synthesized (pro)insulin were comparable. Circulating insulin immunoreactivity in both Wistar and GK rats was predominantly insulin 1 and 2 in the expected ratios with no (pro)insulin evident. Thus, the marked changes in islet morphology and PC2 expression did not impact the rate or extent of proinsulin processing either in vitro or in vivo in this experimental model.

Proteolytic processing of chromogranins is modified in brains of transgenic mice

In normal brain, secretogranin II and chromogranins A and S are extensively converted by endopeptidases to the peptides secretoneurin, GE-19, and PE-11, respectively. After genetic knockout of PC2 or its helper protein 7B2, but not after mutation of carboxypeptidase E, endoproteolytic processing decreased, as indicated by appearance of intermediate-sized processing products.

Intrapituitary adenoviral administration of 7B2 can extend life span and reverse endocrinological deficiencies in 7B2 null mice

The prohormone convertase PC2 requires the aid of a helper protein, known as 7B2, for production of active enzyme. Deletion of 7B2 results in a lethal phenotype resembling Cushing's disease. In this study, we have investigated the effect of a single low dose of recombinant adenovirus vector encoding 7B2 and delivered directly to the pituitary of 7B2 nulls on pituitary ACTH, plasma ACTH, corticosterone, alpha MSH and glucose, and survival time. We show that after injection of recombinant adenovirus encoding 27-kDa 7B2 into 7B2 nulls, transgene expression, as measured by RIA for 7B2, exhibits a transient elevation in the pituitary and blood, with a slight but significant elevation of PC2 activity in pituitaries of 7B2 nulls and a drop in the level of circulating ACTH concomitant with a small increase in circulating alpha MSH. The level of circulating blood glucose was increased, and that of corticosterone was decreased. Lastly, slight but significantly prolonged survival times were observed. These data showing partial rescue of 7B2 nulls support the idea that adenoviral administration of 7B2 will represent an effective means to study the role of this interesting neuroendocrine protein on endocrine function in vivo.

The lethal form of Cushing's in 7B2 null mice is caused by multiple metabolic and hormonal abnormalities

The neuroendocrine-specific protein 7B2, which serves as a molecular escort for proPC2 in the secretory pathway, promotes the production of enzymatically active PC2 and may have non-PC2 related endocrine roles. Mice null for 7B2 exhibit a lethal phenotype with a complex Cushing's-like pathology, which develops from intermediate lobe ACTH hypersecretion as a consequences of interruption of PC2-mediated peptide processing as well as undefined consequences of the loss of 7B2. In this study we investigated the endocrine and metabolic alterations of 7B2 null mice from pathological and biochemical points of view. Our results show that 7B2 nulls exhibit a multisystem disorder that includes severe pathoanatomical and histopathologic alterations of vital organs, including the heart and spleen but most notably the liver, in which massive steatosis and necrosis are observed. Metabolic derangements in glucose metabolism result in glycogen and fat deposition in liver under conditions of chronic hypoglycemia. Liver failure is also likely to contribute to abnormalities in blood coagulation and blood chemistry, such as lactic acidosis. A hypoglycemic crisis coupled with respiratory distress and intensive internal thrombosis most likely results in rapid deterioration and death of the 7B2 null.

Mortality in 7B2 null mice can be rescued by adrenalectomy: involvement of dopamine in ACTH hypersecretion

The serine protease prohormone convertase 2 (PC2), principally involved in the processing of polypeptide hormone precursors in neuroendocrine tissues, requires interaction with the neuroendocrine protein 7B2 to generate an enzymatically active form. 7B2 null mice express no PC2 activity and release large quantities of uncleaved ACTH, resulting in a lethal endocrine condition that resembles pituitary Cushing's (Westphal, C. H., Muller, L., Zhou, A., Bonner-Weir, S., Schambelan, M., Steiner, D. F., Lindberg, I. & Leder, P. (1999) Cell 96, 689). Here, we have compared the 7B2 and PC2 null mouse models to determine why the 7B2 null, but not the PC2 null, exhibits a lethal disease state. Both 7B2 and PC2 nulls contained highly elevated pituitary adrenocorticotropic hormone (ACTH); the neurointermediate lobe content of ACTH in 7B2 nulls was 13-fold higher than in WT mice; that of the PC2 null was 65-fold higher. However, circulating ACTH levels were much higher in the 7B2 null than in the PC2 null. Because hypothalamic inhibitory dopaminergic control represents the major influence on intermediate lobe proopiomelanocortin-derived peptide secretion, dopamine levels were measured, and they revealed that 7B2 null pituitaries contained only one-fourth of WT pituitary dopamine. Adrenalectomized 7B2 null animals survived past the usual time of death at 5 weeks; a month after adrenalectomy, they exhibited normal levels of pituitary dopamine, circulating ACTH, and corticosterone. Elevated corticosterone, therefore, seems to play a central role in the lethal phenotype of the 7B2 null, whereas a 7B2-mediated dopaminergic deficiency state may be involved in the actual ACTH hypersecretion phenomenon. Interestingly, adrenalectomized 7B2 nulls also developed unexpectedly severe obesity.

Functional characterization of ProSAAS: similarities and differences with 7B2

Prohormone convertases (PC) 1 and 2, enzymes found primarily in neuroendocrine tissues, are thought to mediate the proteolytic cleavage of many peptide precursors. To date, endogenous binding proteins for both PC2 (7B2) and PC1 (proSAAS) have been identified. Although 7B2 represents a potent inhibitor of PC2, the most important function of 7B2 as regards this enzyme appears to be the absolute requirement of PC2 for 7B2 in the generation of active enzyme, recently corroborated through production of a null animal that lacks PC2 activity. The purpose of the present study was to determine whether proSAAS exerts effects on PC1 other than inhibition, and to establish functional similarities and differences between 7B2 and proSAAS. We first asked whether the N-terminal domain of proSAAS (proSAAS-(1-180)) could stabilize PC1 activity, similar to the effect of the N-terminal domain of 7B2 on PC2. Recombinant His-tagged proSAAS-(1-180) had no effect on PC1 activity in vitro and was unable to protect PC1 from thermal denaturation. Transient cotransfection of proSAAS-(1-225) cDNA with PC1 cDNA into HEK 293 cells reduced the amount of PC1 activity detected in the medium. Surprisingly, cotransfection of proSAAS-(1-180) cDNA, encoding a protein that lacks the inhibitory C-terminal domain peptide, also reduced the activity of PC1 detected in the medium, but the mass of PC1 secreted into the medium was increased, suggesting a proSAAS-mediated inactivation reaction. Similar results were observed in CHO/PC1 cells stably transfected with pro-SAAS-(1-180). Stable transfection of SAAS cDNAs into AtT-20 cells was used to examine the role of proSAAS in a neuroendocrine setting. Unlike 7B2, proSAAS-(1-225) was able to slow convertase-mediated processing of proopiomelanocortin and proenkephalin; however, similarly to 7B2, proSAAS expression did not result in any accumulated differences in the content of cellular processed peptide. In summary, although both proSAAS and 7B2 potently inhibit PC enzymes via a C-terminal peptide, their intracellular interactions with PCs appear to differ significantly, with each binding protein exhibiting unique properties.

ACTH secretion by mouse corticotroph AtT20 cells is negatively modulated by the intracellular level of 7B2

7B2 is a pan-neuroendocrine protein known to facilitate the trafficking and activation of the prohormone proprotein convertase-2 (PC2). 7B2-null mice not only lack PC2 activity, but they also develop an adrenocorticotropic hormone (ACTH) hypersecretion syndrome, suggesting that 7B2 may regulate hormone secretion. To verify this possibility, we introduced into mouse corticotroph AtT20 cells a retroviral vector carrying either a sense or an antisense 7B2 transgene to induce higher and lower 7B2 expression, respectively. Relative to control AtT20 cells, 7B2-overexpressing cells released less ACTH following KCl-induced membrane depolarization, whereas cells expressing lower levels of 7B2 released relatively more, suggesting that 7B2-related peptides modulate regulated secretion in neuroendocrine cells.

Glucocorticoid treatment is associated with decreased expression of processed AVP but not of proAVP, neurophysin or oxytocin in the human hypothalamus: are PC1 and PC2 involved?

OBJECTIVES

We reported earlier that vasopressin (AVP) peptide expression is significantly decreased in the postmortem hypothalamus of glucocorticoid (GC) treated patients, while such a decrease was not observed in AVP prohormone (proAVP) expression. This indicated a GC-induced suppression of AVP synthesis at the posttranslational level. Here, we investigated in detail whether this decreased levels of AVP expression in GC treated patients might be due to the down regulation of the prohormone convertases PC-1 and PC-2, and the molecular chaperone 7B2, as was reported previously in some AVP-related disorders.

MATERIALS & METHODS

An immunocytochemical study was performed on post-mortem hypothalami of GC exposed patients and controls, in which quantification of proAVP, AVP, neurophysin (NP) and oxytocin (OXT) expression were done along with the quantification of PC1, PC2 and 7B2 expression in the paraventricular nucleus, by using a computerized image analysis system.

RESULTS

Expression of processed AVP in GC exposed patients was significantly decreased (p=0.021), while the amount of proAVP expression was unchanged. Despite the strong correlation between AVP and NP (the other cleavage product of proAVP) expression in the GC group (r=0.917, p=0.004), the mean NP immunoreactivity did not show a significant decrease in this group. Also the OXT expression was similar in both groups. Although in most of the GC treated patients, the expression intensities of PC1 and PC2 were decreased parallel to the decrease in AVP, the mean expression levels of neither of PC1 and PC2, nor of 7B2 were statistically different between the groups (p=0.20-0.80).

CONCLUSION

We conclude that the suppression of AVP expression by GCs is not mediated solely by the down regulation of PC1, PC2 or 7B2. Other mechanisms, which may contribute to the GC-induced posttranslational suppression of AVP, are discussed.

Progastrin processing differs in 7B2 and PC2 knockout animals: a role for 7B2 independent of action on PC2

Cellular synthesis of neuroendocrine peptides requires prohormone convertases (PCs). In order to determine the role of PC2 for gastrin synthesis, we examined antral extracts from mice lacking PC2 or its chaperone, 7B2. The overall concentrations of precursors and alpha-amidated gastrins were similar in all mice. Chromatography, however, revealed that while the K(53)-K(54) site was almost fully cleaved in controls and half cleaved in PC2 null mice, only 23% was cleaved in 7B2 null mice. The results show that PC2 and 7B2 both are required for synthesis of the main form of gastrin (gastrin-17), and that 7B2 exhibits effects beyond PC2-mediated cleavages.

Inactivation of the 7B2 inhibitory CT peptide depends on a functional furin cleavage site

The eukaryotic subtilisin prohormone convertase 2 (PC2) is known to require in vivo exposure to the neuroendocrine protein 7B2 in order to produce an enzymatically active species capable of proteolytic action on prohormone substrates. In the present study, we examined the role of the pentabasic site within 27-kDa 7B2 in this process. We prepared two His-tagged recombinant 7B2s by overexpression in bacteria: 7B2-Ser-Ser (SS), with an inactivating mutation in the CT peptide from Lys171-Lys172 (KK) to SS, rendering the CT peptide non-inhibitory; blockade-SS, a double mutant of both the CT peptide as well as of the pentabasic furin cleavage site. These purified proteins were used in a cell-free proPC2 activation assay. Both 7B2-SS as well as blockade-SS were able to facilitate the activation of proPC2 (as judged by efficient production of enzyme activity), suggesting that cleavage at the furin site is not required for 7B2s lacking inhibitory CT peptides. Plasmids encoding proPC2 and various 7B2s were transiently transfected into human embryonic kidney (HEK293) cells and PC2 enzymatic activity and CT forms in each overnight conditioned medium were measured. Cells transfected with proPC2 and wild-type 7B2 secreted CT peptide cleavage products, but cells transfected with proPC2 and the blockade mutant overwhelmingly secreted intact, 27-kDa, blockaded 7B2. Medium obtained from HEK293 cells transfected with proPC2 and either wild-type 7B2, 7B2-SS, or blockade-SS exhibited PC2 activity, but medium from cells expressing the 7B2 blockade mutant did not. We conclude that cleavage at the 7B2 furin consensus site is required to produce PC2 capable of efficient proteolytic inactivation of the CT peptide.

Coexpression of proprotein convertase SPC3 and the neuroendocrine precursor proSAAS

The subtilisin-like proprotein convertases are a family of serine proteinases involved in the processing of secreted proteins via cleavage at paired basic residues. Until recently, only one natural inhibitor had been demonstrated, the neuropeptide 7B2, which contains a C-terminal domain with inhibitory activity against SPC2. A novel granin-like peptide precursor, named proSAAS, has recently been identified that contains potent and specific inhibitory activity on SPC3 in vitro. To exert such an inhibitory action of SPC3 activity, it would be important to demonstrate that proSAAS and SPC3 are colocalized. We have studied the expression of proSAAS and SPC3 mRNAs in the rat central nervous system and various peripheral tissues by in situ hybridization histochemistry. Our results show that, like 7B2, proSAAS is expressed with a panneuronal distribution. In the periphery, proSAAS is an excellent marker of endocrine cells. Double labeling studies show that SPC3 expression is nearly always accompanied by proSAAS expression. However, proSAAS was also found to be expressed in endocrine cells and neurons that did not express SPC3, suggesting that proSAAS could have additional functions other than the modulation of SPC3 activity. These data support the hypothesis that one of the roles of proSAAS may be to modulate the activity of SPC3.

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.

Peptide repertoire of human cerebrospinal fluid: novel proteolytic fragments of neuroendocrine proteins

Polypeptides in human cerebrospinal fluid (CSF), isolated by phase separation in chloroform-methanol-water and reversed-phase HPLC, were characterised by sequence analysis and mass spectrometry. This identified the presence of peptide fragments of testican, neuroendocrine specific protein VGF, neuroendocrine protein 7B2, chromogranin B/secretogranin I, chromogranin A, osteopontin, IGF-II E-peptide and proenkephalin. The majority of these fragments were generated by proteolysis at dibasic sites, suggesting that they are derived by activities related to prohormone convertase(s). Several of the fragments have previously not been detected, and their functions in CSF or elsewhere are unknown. A characteristic feature of all these fragments is a very high content of acidic residues, in particular glutamic acid. In addition to the fragments of neuroendocrine proteins, endothelin-binding receptor-like protein 2, ribonuclease 1, IGF-binding protein 6, albumin, alpha1-acid glycoprotein 1, prostaglandin-H2 D-isomerase, apolipoprotein A1, transthyretin, beta2-microglobulin, ubiquitin, fibrinopeptide A, and C4A anaphylatoxin were found.

Expression of proopiomelanocortin peptides in human dermal microvascular endothelial cells: evidence for a regulation by ultraviolet light and interleukin-1

Proopiomelanocortin peptides such as alpha-melanocyte-stimulating hormone and adrenocorticotropin are expressed in the epidermal and dermal compartment of the skin after noxious stimuli and are recognized as modulators of immune and inflammatory reactions. Human dermal microvascular endothelial cells mediate leukocyte-endothelial interactions during cutaneous inflammation by the expression of cellular adhesion molecules and cytokines such as interleukin-1. This study addresses the hypothesis that human dermal microvascular endothelial cells express both proopiomelanocortin and prohormone convertases, which are required to generate proopiomelanocortin peptides. Semiquantitative reverse transcriptase polymerase chain reaction and northern blot studies revealed a constitutive expression of proopiomelanocortin mRNA by human dermal microvascular endothelial cells in vitro that was time- and concentration-dependently upregulated by interleukin-1 beta. Furthermore, irradiation of human dermal microvascular endothelial cells with ultraviolet A1 (30J per cm(2)) or ultraviolet B (12.5 mJ per cm(2)) enhanced proopiomelanocortin expression as well as the production and release of the proopiomelanocortin peptides adrenocorticotropin and alpha-melanocyte-stimulating hormone. In addition to proopiomelanocortin, prohormone convertase 1 mRNA expression was detected by reverse transcriptase polymerase chain reaction in unstimulated human dermal microvascular endothelial cells and was augmented after exposure to alpha-melanocyte- stimulating hormone, interleukin-1 beta, or irradiation with ultraviolet. These findings demonstrate that human dermal microvascular endothelial cells express proopiomelanocortin and prohormone convertase 1 required for the generation of adrenocorticotropin. Additionally, human dermal microvascular endothelial cells express mRNA for the prohormone convertase 2 binding protein 7B2. Taken together these findings indicate that human dermal microvascular endothelial cells upon stimulation express both proopiomelanocortin and prohormone convertases required for the generation of alpha-melanocyte-stimulating hormone. As proopiomelanocortin peptides were found to regulate the production of human dermal microvascular endothelial cell cytokines and adhesion molecules and to have a variety of anti-inflammatory properties these peptides may significantly contribute to the modulation of skin inflammation. J Invest Dermatol 115:1021-1028 2000

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.

Development of hormonal peptides and processing enzymes in the embryonic avian pancreas with special reference to co-localisation

Studies on the developing mammalian pancreas have suggested that insulin and glucagon co-exist in a transient cell population and that peptide YY (PYY) marks the earliest developing endocrine cells. We have investigated this in the embryonic avian pancreas, which is characterised by anatomical separation of insulin and glucagon islets. Moreover, we have compared the development of the endocrine cells to that of processing enzymes involved in pancreatic hormone biosynthesis. PYY-like immunoreactivity occurred in islet cells from the youngest stages examined: it increased in amount from approximately 5 days of incubation and was co-localised with glucagon and to a lesser extent with insulin. Insulin and glucagon cells were numerous: co-existence of the two peptides in the same cells was but rarely observed. From the youngest stages examined, prohormone convertase (PC) 1/3-like immunoreactivity was detected in insulin cells and PC2-, 7B2- and carboxypeptidase E-like immunoreactivity in both glucagon and insulin cells. We conclude that: (1) PYY-like immunoreactivity occurs in avian islet cells but generally in lesser amounts than in mammals at the earlier stages, (2) the paucity of cells co-expressing insulin and glucagon indicate that all avian insulin cells do not pass through a stage where they co-express glucagon and (3) the early expression of the enzymes responsible for the processing of prohormones suggests that this process is initiated soon after islet cells first differentiate.

PC2 and 7B2 null mice demonstrate that PC2 is essential for normal pro-CCK processing

Analysis of CCK content in extracts of whole forebrain from PC2 and 7B2 null mouse brain showed a significant decrease relative to wild-type brains. More detailed analysis revealed that CCK 8 amide levels in cerebral cortex and forebrain regions were more decreased than in hypothalamus. CCK 8 content in PC2 null mouse intestines was identical to control. Null mutant brains contained less CCK 8 than wild type and no other forms were seen when analyzed by gel filtration chromatography. No brain area examined was completely devoid of CCK, suggesting that other enzymes can partially compensate for the loss of PC2. This is the first demonstration that any endoprotease is important for CCK processing but also suggest the presence of a redundant system to ensure production of active CCK in the brain.

Interaction of Drosophila melanogaster prohormone convertase 2 and 7B2. Insect cell-specific processing and secretion

The prohormone convertases (PCs) are an evolutionarily ancient group of proteases required for the maturation of neuropeptide and peptide hormone precursors. In Drosophila melanogaster, the homolog of prohormone convertase 2, dPC2 (amontillado), is required for normal hatching behavior, and immunoblotting data indicate that flies express 80- and 75-kDa forms of this protein. Because mouse PC2 (mPC2) requires 7B2, a helper protein for productive maturation, we searched the fly data base for the 7B2 signature motif PPNPCP and identified an expressed sequence tag clone encoding the entire open reading frame for this protein. dPC2 and d7B2 cDNAs were subcloned into expression vectors for transfection into HEK-293 cells; mPC2 and rat 7B2 were used as controls. Although active mPC2 was detected in medium in the presence of either d7B2 or r7B2, dPC2 showed no proteolytic activity upon coexpression of either d7B2 or r7B2. Labeling experiments showed that dPC2 was synthesized but not secreted from HEK-293 cells. However, when dPC2 and either d7B2 or r7B2 were coexpressed in Drosophila S2 cells, abundant immunoreactive dPC2 was secreted into the medium, coincident with the appearance of PC2 activity. Expression and secretion of dPC2 enzyme activity thus appears to require insect cell-specific posttranslational processing events. The significant differences in the cell biology of the insect and mammalian enzymes, with 7B2 absolutely required for secretion of dPC2 and zymogen conversion occurring intracellularly in the case of dPC2 but not mPC2, support the idea that the Drosophila enzyme has specific requirements for maturation and secretion that can be met only in insect cells.

The SAAS granin exhibits structural and functional homology to 7B2 and contains a highly potent hexapeptide inhibitor of PC1

Prohormone convertases (PCs) 1 and 2 are thought to mediate the proteolytic cleavage of many peptide precursors. Endogenous inhibitors of both PC1 and PC2 have now been identified; the 7B2 protein is a nanomolar inhibitor of PC2, while the novel protein proSAAS was recently reported to be a micromolar inhibitor of PC1 [Fricker et al. (2000) J. Neurosci. 20, 639-648]. We here report evidence that 7B2 and proSAAS exhibit several elements of structural and functional homology. Firstly, 26 kDa human, mouse and rat proSAAS, like all vertebrate 7B2s, contain a proline-rich sequence within the first half of the molecule and also contain a C-terminal 40 residue peptide (SAAS CT peptide) separated from the remainder of the protein by a furin consensus sequence. The SAAS CT peptide contains the precise sequence of a hexapeptide previously identified by combinatorial peptide library screening as a potent inhibitor of PC1, and the vast majority of the inhibitory potency of proSAAS can be attributed to this hexapeptide. Further, like the 7B2 CT peptide, SAAS CT-derived peptides represent tight-binding competitive convertase inhibitors with nanomolar potencies. Lastly, recombinant PC1 is able to cleave the proSAAS CT peptide to a product with a mass consistent with cleavage following the inhibitory hexapeptide. Taken together, our results indicate that proSAAS and 7B2 may comprise two members of a functionally homologous family of convertase inhibitor proteins.

Mutations in the catalytic domain of prohormone convertase 2 result in decreased binding to 7B2 and loss of inhibition with 7B2 C-terminal peptide

Prohormone convertases 1 (PC1) and 2 (PC2) are members of a family of subtilisin-like proprotein convertases responsible for proteolytic maturation of a number of different prohormones and proneuropeptides. Although sharing more than 50% homology in their catalytic domains, PC1 and PC2 exhibit differences in substrate specificity and susceptibility to inhibitors. In addition to these differences, PC2, unlike PC1 and other members of the family, specifically binds the neuroendocrine protein 7B2. In order to identify determinants responsible for the specific properties of the PC2 catalytic domain, we compared its primary sequence with that of other PCs. This allowed us to distinguish a PC2-specific sequence at positions 242-248. We constructed two PC2 mutants in which residues 242 and 243 and residues 242-248 were replaced with the corresponding residues of PC1. Studies of in vivo cleavage of proenkephalin, in vivo production of alpha-MSH from proopiomelanocortin, and in vitro cleavage of a PC2-specific artificial substrate by mutant PC2s did not reveal profound alterations. On the other hand, both mutant pro-PC2s exhibited a considerably reduced ability to bind to 21-kDa 7B2. In addition, inhibition of mutant PC2-(242-248) by the potent natural inhibitor 7B2 CT peptide was almost completely abolished. Taken together, our results show that residues 242-248 do not play a significant role in defining the substrate specificity of PC2 but do contribute greatly to binding 7B2 and are critical for inhibition with the 7B2 CT peptide.

Coordinate regulation of neuroendocrine convertase PC2 and peptide 7B2 in P19 neurons

Convertases are proteases responsible for the bioactivation of many proteins and peptides having a potential role in ontogenesis. As a model to study regulation of convertases in embryo, we use the P19 embryonal carcinoma cell line, which can differentiate into various cell types. The expression of convertase PC2 and its specific binding peptide 7B2 are co-induced during neuronal differentiation of P19 cells. We investigated the possibility that expression of both proteins may be coregulated by T3 and dexamethasone, activators of nuclear receptors, isobutylmethylxanthine, and dibutyryl cAMP, activators of protein kinase A, and phorbol 12-myristate 13-acetate, an activator of protein kinase C. Western blotting results show that expression of PC2 and 7B2 can be upregulated by modulators of the protein kinases, and upregulation needs not be strictly stoichiometric.

Pro-opiomelanocortin-related peptides, prohormone convertases 1 and 2 and the regulatory peptide 7B2 are present in melanosomes of human melanocytes

Recently, it has been shown that alpha-melanocyte stimulating hormone can directly activate tyrosinase by removing the allosteric regulator 6(R)-L-erythro 5,6,7,8 tetrahydrobiopterin resulting in a stable alpha-melanocyte stimulating hormone/6(R)-L-erythro 5,6,7,8 tetrahydrobiopterin complex. As melanin production occurs in the melanosome, a specific organelle of the melanocyte, it seemed important to investigate whether these organelles themselves actually produce pro-opiomelanocortin-related peptides in their acidic environment. The presence of alpha-melanocyte stimulating hormone and adrenocorticotropin in the epidermis and melanocytes has been shown by several investigators. In order to follow possible pro-opiomelanocortin processing in the melanosome, human melanocytes were established in MCDB 153 medium and utilized for immunohistochemistry, immunogold electron microscopy, and western blotting. For this purpose antibodies against alpha-melanocyte stimulating hormone, adrenocorticotropin, prohormone convertases 1 and 2 (PC1 and PC2) and the PC2 regulatory protein 7B2 were used. Our results demonstrated the presence of the entire system for pro-opiomelanocortin processing in the melanosome. Considering the pH optima of these convertases, the results are in agreement with an autocrine intramelanosomal production of pro- opiomelanocortin-related peptides and an autocrine production and recycling of the cofactor 6(R)-L- erythro 5,6,7,8 tetrahydrobiopterin in melanocytes. Based on these novel observations, we would like to propose that the pigmentation process may not necessarily involve a melanocortin-1 receptor-mediated mechanism.

Structure-function analysis of the 7B2 CT peptide

Prohormone convertases play important roles in the proteolytic conversion of many protein precursors. The neuroendocrine protein 7B2 and its 31-residue carboxyl-terminal (CT) peptide potently and specifically inhibit prohormone convertase 2 (PC2). We have analyzed the residues contributing to inhibition using N-terminal truncation and alanine scanning. Removal of more than 3 residues from the amino-terminal end of CT1-18 resulted in a more than 190-fold drop in inhibitory activity, showing that most of the residues between 3 and 18 are required for inhibition. In agreement, an Ala scan indicated that only 4 residues could be replaced with Ala without losing mid-nanomolar inhibitory potency; in particular, Gln7, Gln9, and Asp12 could be Ala-substituted to yield peptides with a similar inhibitory potency to the starting peptide. The all-d-retro-inverso, all-l-inverso, and all-d analogues of CT peptide were completely inactive, indicating that amino acid side chains and the CT peptide main chain interact with PC2. CT peptide inhibition could not be competitively blocked by preincubation with truncated CT peptide forms, supporting an absolute requirement for the Lys-Lys pair in initial binding of the CT peptide to the active site.

Differential onset of expression of mRNAs encoding proopiomelanocortin, prohormone convertases 1 and 2, and granin family members during Xenopus laevis development

The production of peptide hormones through proteolytic cleavage of prohormones, e.g., proopiomelanocortin (POMC), involves a number of regulated secretory proteins, such as prohormone convertase PC1, PC2 and granin family members, that are co-expressed with the prohormone. Although the expression of these proteins has been well-studied in adult animals, data on their expression during development are limited. We used whole-mount in situ hybridization to visualize POMC mRNA expression in the intermediate and anterior pituitary of Xenopus tadpoles. A more sensitive analysis, namely semi-quantitative reverse-transcription polymerase chain reaction (RT-PCR) on total RNA isolated from Xenopus developmental stages, revealed that the expression of POMC, PC1 and PC2 mRNA commenced at stages 13 (neural plate stage), 15 (neural fold stage) and 19 (neural tube stage), respectively, with a gradual increase in their expression levels during further development. Surprisingly, and in contrast to what holds for POMC and the convertases, mRNAs for secretogranin II and III (SgII, SgIII) and 7B2 were not only expressed during neural development, but could already be detected in unfertilized mature oocytes, the first cleavage stages and in blastula-stage embryos. These granins are thus maternally present in Xenopus embryos suggesting that they may have a role during oogenesis and/or early embryonic development.

Peptide hormone precursor processing: getting sorted?

Processing of proproteins to biologically active peptides and, in the case of peptide hormones and neuropeptides, their sorting to granules of the regulated secretory pathway, requires the concerted action of a cascade of enzymes and chaperones. The purpose of this review is to summarize the recent emerging knowledge of how these molecules affect specific endocrine systems. This has come about through the study of gene knockout mice as well as endocrinopathies resulting from mutated genes in humans.

The cell biology of the prohormone convertases PC1 and PC2

Mature peptide hormones and neuropeptides are typically synthesized from much larger precursors and require several posttranslational processing steps--including proteolytic cleavage--for the formation of the bioactive species. The subtilisin-related proteolytic enzymes that accomplish neuroendocrine-specific cleavages are known as prohormone convertases 1 and 2 (PC1 and PC2). The cell biology of these proteases within the regulated secretory pathway of neuroendocrine cells is complex, and they are themselves initially synthesized as inactive precursor molecules. ProPC1 propeptide cleavage occurs rapidly in the endoplasmic reticulum, yet its major site of action on prohormones takes place later in the secretory pathway. PC1 undergoes an interesting carboxyl terminal processing event whose function appears to be to activate the enzyme. ProPC2, on the other hand, exhibits comparatively long initial folding times and exits the endoplasmic reticulum without propeptide cleavage, in association with the neuroendocrine-specific protein 7B2. Once the proPC2/7B2 complex arrives at the trans-Golgi network, 7B2 is internally cleaved into two domains, the 21-kDa fragment and a carboxy-terminal 31 residue peptide. PC2 propeptide removal occurs in the maturing secretory granule, most likely through autocatalysis, and 7B2 association does not appear to be directly required for this cleavage event. However, if proPC2 has not encountered 7B2 intracellularly, it cannot generate a catalytically active mature species. The molecular mechanism behind the intriguing intracellular association of 7B2 and proPC2 is still unknown, but may involve conformational rearrangement or stabilization of a proPC2 conformer mediated by a 36-residue internal segment of 21-kDa 7B2.

The role of the 7B2 CT peptide in the inhibition of prohormone convertase 2 in endocrine cell lines

Prohormone convertase (PC) 2 plays an important role in the processing of neuropeptide precursors via the regulated secretory pathway in neuronal and endocrine tissues. PC2 interacts with 7B2, a neuroendocrine protein that is cleaved to a 21-kDa domain involved in proPC2 maturation and a carboxyl-terminal peptide (CT peptide) that represents a potent inhibitor of PC2 in vitro. A role for the CT peptide as an inhibitor in vivo has not yet been established. To study the involvement of the CT peptide in PC2-mediated cleavages in neuroendocrine cells, we constructed a mutant proenkephalin (PE) expression vector containing PE with its carboxyl-terminal peptide (peptide B) replaced with the 7B2 inhibitory CT peptide. This PECT chimera was stably transfected into two PC2-expressing cell lines, AtT-20/PC2 and Rin cells. Although recombinant PECT proved to be a potent (nM) inhibitor of PC2 in vitro, cellular PC2-mediated cleavages of PE were not inhibited by the PECT chimera, nor was proopiomelanocortin cleavage (as assessed by adrenocorticotropin cleavage to alpha-melanocyte-stimulating hormone) inhibited further than in control cells expressing only the competitive substrate PE. Tests of stimulated secretion showed that both the CT peptide and the PE portion of the chimera were stored in regulated secretory granules of transfected clones. In both AtT-20/PC2 and Rin cells expressing the chimera, the CT peptide was substantially internally hydrolyzed, potentially accounting for the observed lack of inhibition. Taken together, our data suggest that overexpressed CT peptide derived from PECT is unable to inhibit PC2 in mature secretory granules, most likely due to its inactivation by PC2 or by other enzyme(s).

Protein 7B2 is essential for the targeting and activation of PC2 into the regulated secretory pathway of rMTC 6-23 cells

Among the prohormone convertases, PC2 is unique in that it specifically binds to the neuroendocrine-specific protein 7B2 in the endoplasmic reticulum (ER) and is activated late in the regulated secretory pathway of neuroendocrine cells. Several roles, sometimes contradictory, have been suggested for 7B2 with regard to PC2 cellular fate. Thus, 7B2 was proposed to act as a PC2 chaperone in the ER, or to facilitate 7B2 transport from the ER to the trans-Golgi network and to be necessary for proPC2 activation, or to inhibit PC2 enzymatic activity until the latter reaches the secretory granules. To gain insight into the function of 7B2, we sought to block its expression in PC2-expressing endocrine cells using antisense strategies. We have previously shown that the endocrine rMTC 6-23 cell line expresses PC2 and that the enzyme is responsible for the processing of pro-neurotensin/neuromedin N (proNT/NN). Here, we show that rMTC 6-23 cells express 7B2 and that the protein was coordinately induced with PC2 and proNT/NN by dexamethasone. Stable transfection of rMTC 6-23 cells with 7B2 antisense cDNA led to a marked reduction (>90%) in 7B2 levels. ProPC2 was expressed to normal levels and cleaved to yield a PC2 form that was constitutively released, was not stored within secretory granules and was unable to process proNT/NN. We conclude that 7B2 is essential for the sorting and activation of PC2 into the regulated secretory pathway of endocrine cells.

The neuroendocrine protein 7B2 is required for peptide hormone processing in vivo and provides a novel mechanism for pituitary Cushing's disease

The neuroendocrine protein 7B2 has been implicated in activation of prohormone convertase 2 (PC2), an important neuroendocrine precursor processing endoprotease. To test this hypothesis, we created a null mutation in 7B2 employing a novel transposon-facilitated technique and compared the phenotypes of 7B2 and PC2 nulls. 7B2 null mice have no demonstrable PC2 activity, are deficient in processing islet hormones, and display hypoglycemia, hyperproinsulinemia, and hypoglucagonemia. In contrast to the PC2 null phenotype, these mice show markedly elevated circulating ACTH and corticosterone levels, with adrenocortical expansion. They die before 9 weeks of severe Cushing's syndrome arising from pituitary intermediate lobe ACTH hypersecretion. We conclude that 7B2 is indeed required for activation of PC2 in vivo but has additional important functions in regulating pituitary hormone secretion.

The proteolytic maturation of prohormone convertase 2 (PC2) is a pH-driven process

Recombinant proPC2 purified from the medium of CHO cells overexpressing both the prohormone convertase (PC) precursor proPC2 and the 21-kDa amino terminal portion of the neuroendocrine protein 7B2 can spontaneously convert to an active species. In the present report, we have characterized the proPC2 zymogen conversion process. Sequencing of the mature 66 kDa enzyme revealed a single site of cleavage at the paired basic site amino terminal to the GYRDI sequence. In contrast to mature PC2 activity, proPC2 conversion was inhibited neither by the eukaryotic subtilisin inhibitor pCMS nor by the specific PC2 inhibitor, 7B2 CT peptide, suggesting significant differences between the proPC2 conversion reaction and the hydrolysis of synthetic substrates by mature PC2. In support of this idea, proPC2 conversion was not calcium dependent and was unaffected by 5 mM EDTA. The rate of conversion of proPC2 remained similar with a 10-fold difference in zymogen concentration, implicating an intramolecular rather than intermolecular mechanism of activation. Interestingly, the rate of proPC2 conversion was extremely pH dependent, occurring most extensively between pHs 4.0 and 4.9. Taken together, our results suggest that cellular proPC2 maturation occurs via an autocatalytic, intramolecular process controlled not by 7B2 inhibition nor by calcium levels, but by the decreasing pH gradient along the secretory pathway.

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.

Functioning human insulinomas. An immunohistochemical analysis of intracellular insulin processing

Sixty-seven insulinomas were investigated by immunohistochemistry using site-directed antibodies against insulin, proinsulin, chromogranin A, HISL-19, and four proteins directly or indirectly involved in the proteolytic processing of proinsulin: the prohormone convertases PC2 and PC3, carboxypeptidase H (CPH) and 7B2. Results were expressed in a six-grade score according to the frequency of immunoreactive tumour cells. Insulin was expressed by all tumours, appearing in either a diffuse or a polarized pattern and being detected in more than 30% of tumour cells in all cases but three. Proinsulin was also expressed in all tumours, with more than 50% of tumour cells immunoreactive in all cases but 5. It was consistently localized in the Golgi apparatus. In about half the cases, moreover, it also showed diffuse cytoplasmic staining, usually with a very sparse distribution. Trabecular and solid insulinomas did not present specific, homogeneous patterns of insulin immunostaining. However, insulin immunoreactivity was much more abundant in trabecular than in solid neoplasms, being present in virtually all tumour cells (score 6) in 50% and 8% of cases, respectively. Virtually all insulinomas expressed PC2, PC3, CPH and 7B2, usually in 30-100% of tumour cells, with a frequency significantly related to that of insulin. However, detection of PC2 and 7B2 was slightly less frequent than that of PC3 and CPH. In consecutive sections these proteins were found to be mostly co-localized with insulin and chromogranin A but not with proinsulin. They were heavily expressed in all 10 tumours with more than 10% of cells showing cytoplasmic proinsulin immunoreactivity, indicating that the leakage of proinsulin from the Golgi compartment is not associated with faulty expression of converting enzymes and possibly reflects a saturated processing capacity. HISL-19 immunoreactivity was found in both Golgi apparatus and insulin stores, indicating that the relevant antigen is different from all other proteins investigated. These results do not support a defect in expression or localization of proinsulin-processing enzymes in most insulinomas.

The vasopressin precursor is not processed in the hypothalamus of Wolfram syndrome patients with diabetes insipidus: evidence for the involvement of PC2 and 7B2

Wolfram syndrome (WS) is characterized by optic atrophy, insulin-dependent diabetes mellitus, vasopressin (VP)-sensitive diabetes insipidus, and neurosensory hearing loss. Here we report a disturbance in VP precursor processing in the supraoptic and paraventricular nuclei of WS patients. In these patients with diabetes insipidus we could hardly detect any cellular immunoreactivity for processed VP in the supraoptic and paraventricular nuclei. On the other hand, in the paraventricular nucleus a considerable number of cells immunoreactive for the VP precursor were present. In addition, the proprotein convertase PC2 and the molecular chaperone 7B2 were absent. As expression of PC2 and 7B2 was detected in the nearby nucleus basalis of Meynert of one WS patient and in the anterior lobe of the other WS patient, the absence of the two proteins in the paraventricular nucleus was not due to mutations in their genes. These results indicate that in WS patients with diabetes insipidus, not only does VP neuron loss occur in the supraoptic nucleus, but there is also a defect in VP precursor processing.

Cloning and functional analysis of C. elegans 7B2

The neuroendocrine protein 7B2 is a binding protein for the prohormone convertase 2 (PC2) and is required for the intracellular conversion of proPC2 to active PC2. Both full-length 7B2 and its carboxy-terminal 31-residue peptide (CT peptide) are capable of potent inhibition of PC2; the 7B2 protein thus regulates both the biosynthesis and the activity of PC2. Vertebrate 7B2s are highly conserved (92%-97% homology), and thus, species comparison has not been informative in assessing the crucial protein domains responsible for bioactivity. We here report the cloning of the Caenorhabditis elegans 7B2 protein. Although weakly conserved with the vertebrate sequences (23% similarity with mouse 7B2), C. elegans 7B2 contains the signature PPNPCP motif as well as a highly conserved heptapeptide within the CT peptide. In in vitro assays, C. elegans 7B2 possessed significant inhibitory activity against recombinant vertebrate PC2 (IC50 130 nM), and in two functional tests, the amino-terminal domain of C. elegans 7B2 facilitated the activation of proPC2. We conclude that despite low amino acid conservation overall, both functional domains within 7B2 have been conserved between the C. elegans and the vertebrate proteins.

Manipulation of disulfide bonds differentially affects the intracellular transport, sorting, and processing of neuroendocrine secretory proteins

To investigate if the prevention of disulfide bond formation affects the intracellular transport, sorting, and processing of a distinct set of neuroendocrine proteins in the regulated secretory pathway, we have treated Xenopus intermediate pituitaries with the thiol-reducing agent dithiothreitol. Pulse-chase incubations in combination with immunoprecipitation analysis were used to monitor the fates of the prohormone proopiomelanocortin (POMC), prohormone convertase PC2 and its helper protein 7B2, as well as secretogranin III. Manipulation of the disulfide bonds in POMC and proPC2 blocked their transport to the trans-Golgi network and strongly inhibited their processing. Reduction of the single disulfide bond in 7B2 did not disturb its transport and cleavage, but caused its missorting to the constitutive secretory pathway. Moreover, the liaison between proPC2 and 7B2 was prevented. Dithiothreitol did not affect transport, sorting, and cleavage of secretogranin III, which lacks disulfide bonds. When the reducing agent was washed away, POMC processing, proPC2 maturation, and the association between proPC2 and 7B2 were reestablished. Collectively, our findings indicate that manipulation of disulfide bonds differentially affects the fates of neuroendocrine proteins during their transit through the secretory pathway.

Residues unique to the pro-hormone convertase PC2 modulate its autoactivation, binding to 7B2 and enzymatic activity

The prohormone convertase PC2 is one of the major subtilisin/kexin-like enzymes responsible for the formation of small bioactive peptides in neural and endocrine cells. This convertase is unique among the members of the subtilisin/kexin-like mammalian serine proteinase family in that it undergoes zymogen processing of its inactive precursor proPC2 late along the secretory pathway and requires the help of a PC2-specific binding protein known as 7B2. We hypothesized that some of these unique properties of PC2 are dictated by the presence of PC2-specific amino acids, which in the six other known mammalian convertases are otherwise conserved but distinct. Accordingly, six sites were identified within the catalytic segment of PC2. Herein we report on the site-directed mutagenesis of Tyr194 and of the oxyanion hole Asp309 and the consequences of such mutations on the cellular expression and enzyme activity of PC2. The data show that the Y194D mutation markedly increases the ex vivo ability of PC2 to process proopiomelanocortin (POMC) into beta-endorphin in cells devoid of 7B2, e.g. BSC40 cells. In these cells, expression of native PC2 does not result in the secretion of measurable in vitro activity against a pentapeptide fluorogenic substrate. In contrast, secreted Y194D-PC2 exhibited significant enzymatic activity, even in the absence of 7B2. Based on co-immunoprecipitations and Western blots, binding assays indicate that Tyr194 participates in the interaction of PC2 with 7B2, and that the oxyanion hole Asp309 is critical for the binding of proPC2 with pro7B2.

Mapping of the domain in the neuroendocrine protein 7B2 important for its helper function towards prohormone convertase PC2

In the secretory pathway, the neuroendocrine polypeptide 7B2 transiently associates with its physiological target, the prohormone convertase PC2. The 7B2 protein carries two functional domains involved in the control of proPC2 maturation and activation, an inhibitory C-terminal region and an N-terminal domain displaying facilitative activity. Here, we show that a short segment in 7B2 (His120-Pro131) is important for its facilitative activity. Site-directed mutagenesis indicates that within this segment. Tyr130 is particularly important for the ability of 7B2 to act as a helper protein for PC2.

Attenuation of the polypeptide 7B2, prohormone convertase PC2, and vasopressin in the hypothalamus of some Prader-Willi patients: indications for a processing defect

7B2 is a neuroendocrine chaperone interacting with the prohormone convertase PC2 in the regulated secretory pathway. Its gene is located near the Prader-Willi syndrome (PWS) region on chromosome 15. In a previous study we were able to show 7B2 immunoreactivity in the supraoptic nucleus (SON) or the paraventricular nucleus (PVN) in only three of five PWS patients. Here we report that in contrast with five other PWS patients, the neurons in the hypothalamic SON and PVN of the two 7B2-immunonegative PWS patients also failed to show any reaction using two antibodies directed against processed vasopressin (VP). On the other hand, even these two cases reacted normally with five antibodies that recognize different parts of the VP precursor. This finding pointed to a processing defect. Indeed, the same patients had no PC2 immunoreactivity in the SON or PVN, whereas PC1 immunoreactivity was only slightly diminished. In conclusion, in the VP neurons of two PWS patients, greatly reduced amounts of 7B2 and PC2 are present, resulting in diminished VP precursor processing.

Structural elements of PC2 required for interaction with its helper protein 7B2

The structures of the eukaryotic subtilisin protease family members can be divided into four distinct domains as follows: the proregion, the catalytic domain, the P domain, and the carboxyl-terminal region. Although these enzymes are evolutionarily related, only prohormone convertase 2 (PC2) requires 7B2 for activation. To examine the potential contribution of each domain of PC2 to PC2-7B2 interactions, we performed sequential deletions, site-directed mutagenesis, and domain swapping to replace individual domains or particular amino acids of pro-PC2 with the corresponding segments/amino acids of pro-PC1. These chimeras and mutant enzyme molecules were then expressed in AtT-20 cells and analyzed for 7B2 binding, maturation ability, and enzymatic activity. The results revealed that 1) the PC2 proregion is required but is not sufficient to confer 7B2 binding; 2) the P domain is required for the stabilization of PC2 structure and is not exchangeable with the P domain of PC1; and 3) the carboxyl-terminal domain is not involved in 7B2 binding. Site-directed mutagenesis of pro-PC2 further showed that a single residue replacement in the catalytic domain, Tyr-194 --> Asp, prevented pro-PC2 from binding 7B2 and blocked activation. This residue is present within a loop rich in aromatic amino acids which appears to be on the surface of the molecule as extrapolated from the crystal structure of subtilisin. This loop may represent the primary recognition site for 7B2 within the catalytic domain.

Lack of translation of normal 7B2 mRNA levels in hypothalamic mutant vasopressin cells of the homozygous Brattleboro rat

The homozygous Brattleboro rat (di/di) synthesizes a vasopressin (VP) precursor with an aberrant C-terminus, which causes a hypothalamic form of diabetes insipidus. The neuroendocrine polypeptide 7B2 is present in VP and oxytocin (OT) neurons of the supraoptic and paraventricular nucleus of the hypothalamus in wild type rats. However, in the di/di rat 7B2 immunoreactivity is absent in the VP cell population, whereas 7B2 levels within the OT cells are unaffected. Remarkably, there is no obvious difference in 7B2 transcript levels between VP and OT neurons in the di/di rat hypothalamus. This study shows that the presence of mRNA does not automatically result in the subsequent synthesis of its protein. Cellular mechanisms underlying this discrepancy are discussed.

Mechanism of the facilitation of PC2 maturation by 7B2: involvement in ProPC2 transport and activation but not folding

Among the members of the prohormone convertase (PC) family, PC2 has a unique maturation pattern: it is retained in the ER for a comparatively long time and its propeptide is cleaved in the TGN/ secretory granules rather than in the ER. It is also unique by its association with the neuroendocrine protein 7B2. This interaction results in the facilitation of proPC2 maturation and in the production of activatable proPC2 from CHO cells. In the present study, we have investigated the mechanism of this interaction. ProPC2 binds 7B2 in the ER, but exits this compartment much more slowly than 7B2. We found that proPC2 was also slow to acquire the capacity to bind 7B2, whereas 7B2 could bind proPC2 rapidly after synthesis. This indicated that proPC2 folding was the limiting step in the formation of the complex. Indeed, sensitivity of native proPC2 to N-glycanase F digestion and inhibition of proPC2 folding supported the notion that 7B2 is not involved in the early steps of proPC2 folding, and that proPC2 must fold before binding 7B2. Under experimental conditions that prevent propeptide cleavage, 7B2 expression increased proPC2 transport to the Golgi. This increase exhibited the same kinetics as the facilitation of the removal of the propeptide. Finally, proPC2 activation could be reconstituted in Golgi- enriched subcellular fractions. In vitro, 7B2 was required for proPC2 activation at an acidic pH. Taken together, our results demonstrate that rather than promoting proPC2 folding, 7B2 acts as a helper protein involved in proPC2 transport and is required in the proPC2 activation process. We propose, therefore, that 7B2 stabilizes proPC2 in a conformation already competent for these two events.

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.