7B2 is a neuroendocrine chaperone that transiently interacts with prohormone convertase PC2 in the secretory pathway

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.

Biosynthesis of secretogranin II in Xenopus intermediate pituitary

Secretogranin II (SgII) is a sulphated secretory protein found in a broad variety of neuroendocrine cells. We have raised an antiserum against SgII to monitor its fate in Xenopus intermediate pituitary. Pulse-chase incubations in combination with immunoprecipitation analysis showed that SgII was synthesised as an 84-kDa precursor protein which was processed to fragments of 69, 54, 34, 21 and 15 kDa. Secretion of these cleavage products was sensitive to the dopamine D2 receptor agonist apomorphine, and thus occurred via the regulated secretory pathway. When cells were treated with the fungal metabolite brefeldin A or with the specific vacuolar H+-ATPase inhibitor bafilomycin A1, the processing of SgII and the release of its cleavage products were strongly inhibited, indicating that its processing commenced in the later compartments of the secretory pathway. Pulse-chase and immunoblot analysis showed that the 21-kDa fragment was the major SgII-derived cleavage and release product, and carried secretoneurin, a highly conserved peptide flanked by potential dibasic processing sites. Hence, SgII is cleaved to a variety of products that are released via the regulated secretory pathway, while secretoneurin does not seem to represent a major end-product of SgII processing in Xenopus intermediate pituitary.

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.

Differences in the autocatalytic cleavage of pro-PC2 and pro-PC3 can be attributed to sequences within the propeptide and Asp310 of pro-PC2

PC2 and PC3 are subtilisin-like proteases involved in the maturation of prohormones and proneuropeptides within neuroendocrine cells. They are synthesized as zymogens that undergo autocatalytic maturation within the secretory pathway. Maturation of pro-PC2 is slow (t12 >8 h), exhibits a pH optimum of 5.5 and is dependent on calcium (K0.5 2 mM), while pro-PC3 maturation is relatively rapid (t12 15 min), exhibits a neutral pH optimum and is not calcium dependent. These differences in the rates and optimal conditions for activation of the proteases may contribute to the diversity of products generated by these proteases in different cell types. Although highly similar, there are two major differences between pro-PC2 and pro-PC3: the presence of an aspartate at position 310 in pro-PC2 compared with asparagine at the equivalent position in pro-PC3 (and all other members of the subtilisin family), and the N-terminal propeptides, which exhibit low sequence identity (30%). With a view to establishing the structural features that might be responsible for these differences in the maturation of pro-PC2 and pro-PC3, Asp310 in pro-PC2 was mutated to Asn, and Asn309 in pro-PC3 was mutated to Asp. Chimaeric proteins were also made consisting of the pro-region of PC2 fused to the mature portion of PC3 and the pro-region of PC3 fused to the mature region of PC2. The wild-type and mutant DNA constructs were then transcribed and translated in an in vitro system capable of supporting maturation of pro-PC2 and pro-PC3. The results demonstrated that Asp310 of pro-PC2 is responsible for the acidic pH optimum for maturation. Thus changing Asp310 to Asn shifted the pH optimum for maturation to pH 7.0. However, changing Asn309 of pro-PC3 to Asp had no effect on the optimum pH for maturation of pro-PC3. A chimaeric construct containing the propeptide of pro-PC2 attached to PC3 shifted the pH optimum for maturation from pH 7.0 to 6.0 and slowed down the rate of maturation (t12 >8 h). When attached to PC2, the pro-region of pro-PC3 had no effect on the optimum pH for maturation (pH 5.5-6.0), but it did accelerate the rate of maturation (t12 2 h). These results demonstrate that Asp310 and the pro-region of pro-PC2 contribute to the acidic pH optimum and low rate of maturation of this zymogen relative to its closely related homologue PC3.

Biosynthetic processing and quaternary interactions of proprotein convertase SPC4 (PACE4)

SPC4 (PACE4), a member of the eukaryotic family of subtilisin-like proprotein convertases, is synthesized as a proenzyme (proSPC4) which undergoes proteolytic removal of N-terminal propeptide during transit through the secretory pathway. As this propeptide processing seems to be a key event in the functional expression of SPC4, we have investigated its mechanism and the intracellular site where it occurs. In transfected fibroblast cells, the 110-kDa proSPC4 undergoes slow cleavage to generate a 103-kDa mature enzyme in the endoplasmic reticulum (ER). Site-directed mutagenesis studies demonstrate that the proteolytic activation of SPC4 occurs mainly through a unimolecular autocatalytic process and propeptide cleavage is a prerequisite for its export from the ER. Sedimentation velocity and chemical cross-linking analysis demonstrate that the precursor protein in the cells exists as both a monomer and a dimer-sized complex whereas mature SPC4 exists only as a monomer. These results suggest that the cleavage of the N-terminal propeptide of SPC4 plays a regulatory role in its activation and secretion through the change in its oligomeric state.

S-phase DNA damage checkpoint in budding yeast

Eukaryotic cells must be able to coordinate DNA repair, replication and cell cycle progression in response to DNA damage. A failure to activate the checkpoints which delay the cell cycle in response to internal and external cues and to repair the DNA lesions results in an increase in genetic instability and cancer predisposition. The use of the yeast Saccharomyces cerevisiae has been invaluable in isolating many of the genes required for the DNA damage response, although the molecular mechanisms which couple this regulatory pathway to different DNA transactions are still largely unknown. In analogy with prokaryotes, we propose that DNA strand breaks, caused by genotoxic agents or by replication-related lesions, trigger a replication coupled repair mechanism, dependent upon recombination, which is induced by the checkpoint acting during S-phase.

Identification of a novel secretogranin II-derived peptide (SgII(187-252)) in adult and fetal human adrenal glands using antibodies raised against the human recombinant peptide

Molecular cloning of secretogranin II (SgII) in phylogenetically distant species has recently revealed the existence of a highly conserved 66-amino acid peptide flanked by preserved pairs of basic residues. This observation suggested that this peptide, named EM66, which had not been described to date, could be an important processing product of SgII. The aim of the present study was to investigate the possible occurrence of EM66 in the human adrenal gland. The EM66 peptide was generated in Escherichia coli, which was programmed to make a fusion protein containing the human EM66 sequence. The affinity-purified fusion protein was used to raise polyclonal antibodies in rabbits. The free EM66 peptide was obtained by cleavage of the fusion protein followed by high performance liquid chromatography purification. Immunohistochemical analysis using the EM66 antibodies revealed intense labeling of adrenochromaffin cells in the adult adrenal medulla and the fetal adrenal gland. A sensitive and specific RIA was developed and applied to the detection of EM66-like immunoreactivity in extracts of adult adrenal medulla and whole fetal adrenal gland after high performance liquid chromatographic analysis. A major immunoreactive species exhibiting the same retention time as recombinant EM66 was detected in both adult and fetal adrenal extracts. Taken together, these data demonstrate that posttranslational processing of SgII actually generates EM66 in the adrenal gland. The strong conservation of the amino acid sequence of EM66 in the vertebrate phylum and the occurrence of the mature peptide in both fetal and adult chromaffin cells suggest that EM66 could play an important physiological role in the human adrenal gland.

Identification of a novel secretogranin II-derived peptide in the adult and fetal human adrenal gland

Molecular cloning of secretogranin II (SgII) in different species has revealed the existence of a highly conserved 66-amino acid peptide (EM66) flanked by preserved pairs of basic residues. In the present study we have localized and characterized EM66 in the human adrenal gland. A fusion protein containing the human EM66 peptide was produced in E. coli and used to raise polyclonal antibodies in rabbits. Immunohistochemical staining of human adrenal slices revealed intense labeling of adrenochromaffin cells in the adult and fetal gland. HPLC analysis of adrenal extracts showed the presence of an immunoreactive peak exhibiting the same retention time as recombinant EM66 in both adult and fetus. These data demonstrate that post-translational processing of SgII actually generates a novel peptide in the human adrenal gland. The conservation of the sequence of EM66 in vertebrates and the occurrence of the mature peptide during early ontogenesis of the human adrenal gland strongly suggest that EM66 could exert physiological activities.

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.

RAP, a novel type of ER chaperone

Members of the low-density lipoprotein (LDL) receptor gene family play an important role in cellular uptake of various extracellular ligands. Recent studies have shown that a 39-kDa protein known as RAP (receptor-associated protein) serves as a molecular chaperone to assist the folding of certain LDL-receptor family proteins and their passage through the secretory pathway. In this review, the authors discuss our current understanding of the roles of RAP as a molecular chaperone/escort protein and present a model of how RAP might carry out these functions.

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.

Molecular diversity in neurosecretion: reflections on the hypothalamo-neurohypophysial system

1. The diversity of molecules involved in various aspects of neurosecretion, such as proprotein processing, axonal transport of large dense core vesicles (LDCVs), and regulated secretion, is discussed in the context of the hypothalamo-neurohypophysial system (HNS). 2. Recent studies have uncovered a family of at least seven processing enzymes known as proprotein convertases (PCs) which are involved in proteolytically cleaving protein precursors at paired basic amino acid motifs to yield biologically active peptides. Three of these, PC1(3), 2, and 5, are found in neurons and are involved in producing regulated secretory peptide products. 3. The axonal transport of LDCVs occurs on microtubule tracks by still unknown mechanisms. There are over 11 distinct kinesin-related molecules that have now been identified as possible microtubule motor candidates. 4. Calcium channels in the nervous system are known to be derived from at least five alpha-subunit and four beta-subunit genes with multiple alternatively spliced isoforms in each case. These could account, in part, for the varied calcium currents found in the HNS. 5. The large number of proteins and isoforms now demonstrated to be involved in regulated secretion are discussed, with a focus on LDCV compositions and the synaptotagmin gene family.

Multifactorial modulation of TRH metabolism

1. Thyrotropin releasing hormone (TRH), synthesized in the paraventricular nucleus of the hypothalamus (PVN), is released in response to physiological stimuli through median eminence nerve terminals to control thyrotropin or prolactin secretion from the pituitary. 2. Several events participate in the metabolism of this neuropeptide: regulation of TRH biosynthesis and release as well as modulation of its inactivation by the target cell. 3. Upon a physiological stimulus such as cold stress or suckling, TRH is released and levels of TRH mRNA increase in a fast and transient manner in the PVN; a concomitant increase in cfos is observed only with cold exposure. 4. Hypothalamic cell cultures incubated with cAMP or phorbol esters show a rise in TRH mRNA levels; dexamethasone produces a further increase at short incubation times. TRH mRNA are thus controlled by transsynaptic and hormonal influences. 5. Once TRH is released, it is inactivated by a narrow specificity ectoenzyme, pyroglutamyl peptidase II (PPII). 6. In adenohypophysis, PPII is subject to stringent control: positive by thyroid hormones and negative by TRH; other hypothalamic factors such as dopamine and somatostatin also influence its activity. 7. These combined approaches suggest that TRH action is modulated in a coordinate fashion.

Regulation of the biosynthesis of large dense-core vesicles in chromaffin cells and neurons

1. The proteins of large dense-core vesicles (LDV) in neuroendocrine tissues are well characterized. Secretory components comprise chromogranins and neuropeptides. Intrinsic membrane proteins include cytochrome b-561, transporters, SV2, synaptotagmin, and synaptobrevin. 2. The effects of stimulation and of second messengers on the biosynthesis of LDV have been studied in detail. 3. Regulation of biosynthesis is complex. The cell can adapt to prolonged stimulation either by producing vesicles of normal size filled with a higher quantum of secretory peptides or by forming larger vesicles. In addition, some components, e.g., enzymes, can be upregulated specifically.

The proprotein convertases

The major endoproteolytic processing enzymes of the secretory pathway are the subtilisin-like proprotein convertases (SPCs). Furin (SPC1) has emerged as one of the major processing enzymes of the constitutive secretory pathway and its localization in the trans-Golgi network and mechanism of autoactivation have been studied in considerable detail. Recent gene disruption experiments and the study of naturally-occurring mutations underscore the importance of PC2 (prohormones convertase 2, or SPC2) and PC1/PC3 (prohormone convertase 1/3, or SPC3) in the processing of a wide variety of hormone and neuropeptide precursors. The role of Carboxypeptidase E (CPE) in the removal of carboxy-terminal basic residues exposed by the endoproteases was shown to be necessary for efficient endoproteolytic processing of proinsulin and several other protein precursors. Many biologically active peptides are also amidated after their proteolytic processing by peptidylglycine alpha-amidating monooxygenase (PAM) and recent X-ray studies of the peptidyl alpha-hydroxylating monooxygenase component of PAM have shed new light on the role of copper in the mechanism of this reaction.

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.

Immunocytochemical localization of the prohormone convertases PC1 and PC2 in rat prolactin cells

The prohormone convertases PC1 and PC2 are subtilisin-related endopeptidases that process prohormone and neuropeptide precursors. Using different ultrastructural immunocytochemical approaches, we have investigated their intracellular distribution in a neuroendocrine cell type that has not been examined thus far, the rat anterior pituitary lactotrope. These cells secrete mainly prolactin and also express the neuroendocrine-specific protein secretogranin II, which is considered a peptide precursor. Our study provides evidence for the expression of PC1 and PC2 in rat lactotropes and provides new information on their subcellular localization. Apart from their presence in the secretory granules, PC1 and PC2 displayed different major localization along the secretory pathway. PC1 immunoreactivity was concentrated in the Golgi apparatus, whereas PC2 immunoreactivity was prominent in the rough endoplasmic reticulum (RER). These observations provide morphological support for previous biochemical analysis of proPC1 and proPC2 post-translational processing, which has demonstrated that PC1 exits very rapidly from the RER, whereas PC2 is retained much longer in this compartment. (J Histochem Cytochem 46:101-108, 1998)

Intracellular transport, sorting, and proteolytic processing of regulated secretory proteins does not require protein sulfation

The biological significance of protein sulfation is poorly understood. To study a possible role of protein sulfation in the regulated secretory pathway, neurointermediate lobes (NIL) of the pituitary of South-African clawed toads (Xenopus laevis) were treated with chlorate, a potent in vivo inhibitor of protein sulfation. We monitored the fates of newly synthesized proopiomelanocortin (POMC), prohormone convertase (PC2), and secretogranin III (SgIII), which are sulfated and regulated secretory proteins. Inhibition of protein sulfation had no effect on the proteolytic processing of these precursor proteins and the kinetics of release of their cleavage products. The release was sensitive to apomorphine, a drug that blocks the release of proteins via the regulated secretory pathway, indicating that no missorting to the constitutive pathway had occurred. Our results suggest that protein sulfation is not required for the intracellular transport, sorting, and proteolytic processing of regulated secretory proteins.

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.

Background adaptation by Xenopus laevis: a model for studying neuronal information processing in the pituitary pars intermedia

This review is concerned with recent literature on the neural control of the pituitary pars intermedia of the amphibian Xenopus laevis. This aquatic toad adapts skin colour to the light intensity of its environment, by releasing the proopiomelanocortin (POMC)-derived peptide alpha-MSH (alpha-melanophore-stimulating hormone) from melanotrope cells. The activity of these cells is controlled by brain centers of which the hypothalamic suprachiasmatic and magnocellular nuclei, respectively, inhibit and stimulate both biosynthesis and release of alpha-MSH. The suprachiasmatic nucleus secretes dopamine, GABA, and NPY from synaptic terminals on the melanotropes. The structure of the synapses depends on the adaptation state of the animal. The inhibitory transmitters act via cAMP. Under inhibition conditions, melanotropes actively export cAMP, which might have a first messenger action. The magnocellular nucleus produces CRH and TRH. CRH, acting via cAMP, and TRH stimulate POMC-biosynthesis and POMC-peptide release. ACh is produced by the melanotrope cell and acts in an autoexcitatory feedback on melanotrope M1 muscarinic receptors to activate secretory activity. POMC-peptide secretion is driven by oscillations of the [Ca2+]i, which are initiated by receptor-mediated stimulation of Ca2+ influx via N-type calcium channels. The hypothalamic neurotransmitters and ACh control Ca2+ oscillatory activity. The structural and functional aspects of the various neural and endocrine steps in the regulation of skin colour adaptation by Xenopus reveal a high degree of plasticity, enabling the animal to respond optimally to the external demands for physiological adaptation.

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.

Somatostatin-14, somatostatin-28, and prosomatostatin[1-10] are independently and efficiently processed from prosomatostatin in the constitutive secretory pathway in islet somatostatin tumor cells (1027B2)

We have characterized the biosynthetic origin of somatostatin-14 (SS-14), SS-28, and pro-SS[1-10] from pro-SS (PSS) in 1027B2 rat islet tumor cells. Because these cells lack regulated secretion and show unresponsiveness of the SS gene to cAMP, we have additionally carried out morphological and functional studies to elucidate the molecular defect in cAMP signalling and to localize the sites of PSS maturation along the secretory pathway. Cell extracts and secretion media were analysed by high performance liquid chromatography and specific C- and N-terminal radioimmunoassays. Electron microscopic sampling of 1027B2 cell cultures showed that most cells had very few dense core secretory granules for heterogeneous sizes. The cells expressed the endoproteases furin, PC1, and PC2 and contained large quantities of fully processed SS-14 and SS-28 with very little unprocessed PSS (ratio SS-14:SS-28:PSS = 39:51:10%). They secreted high concentrations of SS-14, SS-28, and PSS[1-10] constitutively along with PC1 and PC2. Pulse-chase studies demonstrated that PSS is rapidly (within 15 min), and efficiently processed to SS-14, SS-28, and PSS[1-10] via separate biosynthetic pathways: PSS --> SS-14 + 8 kDa; PSS --> SS-28 + 7 kDa; PSS --> PSS[1-10]. Monensin reduced intracellular SS-like immunoreactivity without altering processing efficiency. Transfection with the catalytic subunit of protein kinase A (PKA-C) activated SS promoter-CAT activating indicating that the defect in cAMP-dependent signaling in 1027B2 cells lies at the level of PKA-C. PKA-C overexpression failed to alter the ratio of processed SS-14 and SS-28. These results demonstrate that SS-14, SS-28, and PSS[1-10] are independently synthesized from PSS and that efficient precursor processing can occur within the constitutive secretory pathway in the relative absence of dense core secretory vesicles.

Regulation by gastric acid of the processing of progastrin-derived peptides in rat antral mucosa

1. Inhibition of gastric acid secretion by proton pump inhibitors like omeprazole increases the synthesis and secretion of the pyloric antral hormone gastrin. We report here how omeprazole influences the conversion of the gastrin precursor to its final products, and the abundance of mRNAs encoding proteins associated with progastrin processing in rat antral mucosa. 2. Progastrin processing was studied using a pulse-chase protocol in antral mucosa, incubated in vitro, from rats treated with omeprazole for up to 5 days. Labelled peptides were detected by on-line scintillation counting after immunoprecipitation and HPLC. The mRNAs encoding prohormone-processing enzymes were identified by Northern blot, polymerase chain reaction or ribonuclease protection assay, and their cellular origins identified by immunocytochemistry. 3. Cleavage of [3H]- and [35S]-labelled progastrins at Arg-94-95 or Arg-57-58, and amidation at Phe-92 were not influenced by pretreatment with omeprazole. In contrast, cleavage of G34 (the thirty-four amino acid amidated gastrin) at Lys-74-75 to give G17 (the seventeen amino acid amidated gastrin), and of G34-Gly to G17-Gly (G34 and G17 with COOH-terminal glycine), was increased 3-fold after treatment with omeprazole for either 1 or 5 days. 4. Approximately 20% of newly synthesized amidated and Gly-extended gastrins were secreted within 240 min of the labelling period in omeprazole-treated samples, but secretion of labelled gastrins from control tissue was undetectable over a comparable period. 5. The amidating enzyme, peptidyglycine alpha-amidating mono-oxygenase (PAM), the prohormone convertases PC1/3, PC2, PC5 and the PC2 chaperone 7B2 were localized to rat antral gastrin cells by immunocytochemistry. The relative abundance of mRNA species encoding 7B2, PC5 and PAM were unchanged after treatment with omeprazole for 5 days, whereas gastrin, PC1/3 and PC2 mRNAs are known to increase at this time. 6. The main consequence of increased cleavage at Lys-74-75 is the production of G17 and G17-Gly at the expense of G34 and G34-Gly, respectively. The latter have longer plasma half-lives, and so their increased cleavage may serve to limit the rise in plasma gastrin concentration after inhibition of acid secretion. Changes in the abundance of mRNAs encoding prohormone-processing enzymes cannot account for the rapidity of the changes in cleavage of progastrin at Lys residues after omeprazole.

Transposon-generated 'knock-out' and 'knock-in' gene-targeting constructs for use in mice

The conventional technique for targeted mutation of mouse genes entails placing a genomic DNA fragment containing the gene of interest into a vector for fine mapping, followed by cloning of two genomic arms around a selectable neomycin-resistance cassette in a vector containing thymidine kinase [1]; this generally requires 1-2 months of work for each construct. The single 'knock-out' construct is then transfected into mouse embryonic stem (ES) cells, which are subsequently subjected to positive selection (using G418 to select for neomycin-resistance) and negative selection (using FIAU to exclude cells lacking thymidine kinase), allowing the selection of cells which have undergone homologous recombination with the knockout vector. This approach leads to inactivation of the gene of interest [2]. Recently, an in vitro reaction was developed, on the basis of the yeast Ty transposon, as a useful technique in shotgun sequencing [3]. An artificial transposable element, integrase enzyme and the target plasmid are incubated together to engender transposition. The DNA is then purified, and subsequently electroporated into bacteria. The transposon and the target plasmid bear distinct antibiotic resistance markers (trimethoprim and ampicillin, respectively), allowing double selection for transposition events. In the present study, we have modified this system to allow the rapid, simultaneous generation of a palette of potential gene targeting constructs. Our approach led from genomic clone to completed construct ready for transfection in a matter of days. The results presented here indicate that this technique should also be applicable to the generation of gene fusion constructs [4-8], simplifying this technically demanding method.

Expression of the proprotein convertases PC1 and PC2 mRNAs in thyrotropin releasing hormone neurons of the rat paraventricular nucleus of hypothalamus

PC1 and PC2 are subtilisin-like processing enzymes capable of cleaving thyrotropin releasing hormone (TRH) precursor (pro-TRH) at paired basic residues in vitro. In the paraventricular nucleus of the hypothalamus (PVN), pro-TRH is synthesized to control adenohypophysial thyrotropin and prolactin release. Biochemical and immunological approaches have shown that in the hypothalamus, pro-TRH is extensively cleaved at pairs of basic amino acids. We quantified, by two different approaches, in situ hybridization (ISH) on consecutive cryostat sections or double label ISH, the proportion of PVN TRH neurons containing either PC1 or PC2 mRNAs. Both techniques gave similar results: PC2 mRNA was present in 60-70% of TRH neurons, and PC1 mRNA in 37-46%. Values were similar in the anterior and medial parts of the parvocellular PVN. TRH neurons containing either PC1 or PC2 mRNA were found throughout the areas containing TRH cells without any evidence of anatomical segregation. These results suggest a biochemical heterogeneity in PVN TRH biosynthetic machinery.

Defective prohormone processing and altered pancreatic islet morphology in mice lacking active SPC2

The prohormone convertase SPC2 (PC2) participates in the processing of proinsulin, proglucagon, and a variety of other neuroendocrine precursors, acting either alone or in conjunction with the structurally related dense-core granule convertase SPC3 (PC3/PC1). We have generated a strain of mice lacking active SPC2 by introducing the neomycin resistance gene (Neor) into the third exon of the mSPC2 gene. This gene insertion results in the synthesis of an exon 3-deleted form of SPC2 that does not undergo autoactivation and is not secreted. The homozygous mutant mice appear to be normal at birth. However, they exhibit a small decrease in rate of growth. They also have chronic fasting hypoglycemia and a reduced rise in blood glucose levels during an intraperitoneal glucose tolerance test, which is consistent with a deficiency of circulating glucagon. The processing of proglucagon, prosomatostatin, and proinsulin in the alpha, delta, and beta cells, respectively, of the pancreatic islets is severely impaired. The islets in mutant mice at 3 months of age show marked hyperplasia of alpha and delta cells and a relative diminution of beta cells. SPC2-defective mice offer many possibilities for further delineating neuroendocrine precursor processing mechanisms and for exploring more fully the physiological roles of many neuropeptides and peptide hormones.

Two activation states of the prohormone convertase PC1 in the secretory pathway

PC1, a neuroendocrine member of the prohormone convertase family of serine proteinases, is implicated in the processing of proproteins in the secretory pathway. PC1 is synthesized as a zymogen and cleaves not only its own profragment in the endoplasmic reticulum, but a subset of protein substrates in the Golgi apparatus and in the Golgi-distal compartments of the regulated secretory pathway. Likewise, mouse PC1 (mPC1) has previously been shown to cleave human prorenin in GH4 cells (that contain secretory granules) while being unable to cleave prorenin in cells, such as Chinese hamster ovary (CHO) or BSC-40, which are devoid of secretory granules. In the current study, we show that removal of a C-terminal tail of mPC1 allows the efficient cleavage of prorenin in the constitutive secretory pathway of CHO cells. The C-terminal tail thus appears to act as an inhibitor of PC1 activity against certain substrates in the endoplasmic reticulum and Golgi apparatus, and its removal, which occurs naturally in secretory granules, may explain the observed granule-specific processing of certain proproteins. These results also demonstrate that PC1 is present in a partially active state prior to the secretory granules where it is processed to a maximally active state.

Ontogeny of prohormone convertases in rat prenatal development

It has been well established that peptide precursors usually undergo limited proteolysis at pairs or single basic amino acids during their biosynthetic process. This posttranslational modification paradigm is common for numerous membrane-spanning and secreted proteins, neuropeptides, and peptide hormones of physiological significance, in which endoproteolytic cleavage is invariably essential for the accurate biosynthesis and full activity of the mature products. Establishment of an effective peptide profile is dependent on not only the presence of peptide precursor, but also the presence and the enzymatic specificities of cleavage enzymes. We have, therefore, characterized the spatial and temporal patterns of six subtilisin-like serine endoproteases known to be involved in proprotein processing, including furin, PC1, PC2, PC4, PC5, and PACE4, in rat prenatal development and related the results to the expression patterns of several peptide precursors. We have observed largely distinct and sometimes complementary expression patterns of individual PCs in various embryonic structures, suggesting PCs may be functionally distinct in processing different sets of proprotein substrates in development. From these studies, numerous tentative enzyme-substrate relationships in various embryonic structures have been proposed and should encourage more studies to test the in vitro cleavage potentialities of individual PCs toward these precursors. In the future, knowledge gained from these studies, when combined with insights gained from in vivo perturbation and genetic ablation studies, should lead to final comprehensive understanding of specific precursors cleaved by specific enzymes at specific cleavage sites in known spatial and temporal expression patterns during development.

Endoplasmic reticulum Ca2+ is important for the proteolytic processing and intracellular transport of proinsulin in the pancreatic beta-cell

The role of intracellular Ca2+ in the proteolytic processing and intracellular transport of secretory granule proproteins was investigated by pulse-chase radiolabelling of isolated rat islets of Langerhans. The conversion of proinsulin was inhibited by depletion of medium Ca2+ with EGTA and by blocking the transport of Ca2+ into cells with the Ca2+-channel antagonists verapamil, nifedipine and NiCl2. Proinsulin conversion was also reduced by the endoplasmic reticulum Ca2+-ATPase inhibitor thapsigargin, indicating that the process requires transport of Ca2+ into the endoplasmic reticulum. This was supported by the finding that proinsulin processing was inhibited when Ca2+ was depleted before or during pulse-labelling, but not after transport of the protein to post-endoplasmic-reticulum compartments. Similarly, the inhibition of proinsulin processing was reversed by re-introduction of medium Ca2+ around the time of radiolabelling, but not after 15 min of chase incubation. Ca2+ depletion also decreased proteolytic maturation of the prohormone convertases PC1, PC2 and carboxypeptidase H. Secretion experiments suggested that the rate and extent of proinsulin transport into secretory granules were inhibited marginally by Ca2+ depletion, whereas those of the convertases were markedly impeded. Inhibition of proinsulin conversion by Ca2+ depletion was thus not simply related to the Ca2+-dependencies of mature PC1 and PC2, but also to a requirement for endoplasmic reticulum Ca2+ in proteolytic maturation of the convertases and in their transfer to secretory granules. The results also suggest that the Ca2+ required for prohormone processing in the granules enters the secretory pathway via the endoplasmic reticulum.

Activation of the furin endoprotease is a multiple-step process: requirements for acidification and internal propeptide cleavage

Activation of furin requires autoproteolytic cleavage of its 83-amino acid propeptide at the consensus furin site, Arg-Thr-Lys-Arg107/. This RER-localized cleavage is necessary, but not sufficient, for enzyme activation. Rather, full activation of furin requires exposure to, and correct routing within, the TGN/endosomal system. Here, we identify the steps in addition to the initial propeptide cleavage necessary for activation of furin. Exposure of membrane preparations containing an inactive RER-localized soluble furin construct to either: (i) an acidic and calcium-containing environment characteristic of the TGN; or (ii) mild trypsinization at neutral pH, resulted in the activation of the endoprotease. Taken together, these results suggest that the pH drop facilitates the removal of a furin inhibitor. Consistent with these findings, following cleavage in the RER, the furin propeptide remains associated with the enzyme and functions as a potent inhibitor of the endoprotease. Co-immunoprecipitation studies coupled with analysis by mass spectrometry show that release of the propeptide at acidic pH, and hence activation of furin, requires a second cleavage within the autoinhibitory domain at a site containing a P6 arginine (-Arg70-Gly-Val-Thr-Lys-Arg75/-). The significance of this cleavage in regulating the compartment-specific activation of furin, and the relationship of the furin activation pathway to those of other serine endoproteases are discussed.

Identification of a molluscan homologue of the neuroendocrine polypeptide 7B2

In vertebrates, interaction of prohormone convertase 2 (PC2) with the highly conserved polypeptide 7B2 is essential for transport and maturation of proPC2 in the regulated secretory pathway. In vitro, 7B2 displays a strong inhibitory activity toward PC2. Here, we characterize a cDNA encoding the first invertebrate 7B2-related protein (L7B2) from the brain of the mollusc Lymnaea stagnalis. The overall amino acid sequence identity between L7B2 and its vertebrate counterparts is surprisingly low (29%) and is restricted to a few small stretches of amino acid residues. Of particular interest are a conserved proline-rich region in the middle portion of the L7B2 sequence and a repeated conserved region in the carboxyl-terminal domain. Synthetic peptides corresponding to the carboxyl-terminal regions inhibit Lymnaea PC2 enzyme activity in extracts of insulin-producing neurons, in which both L7B2 and Lymnaea PC2 are abundantly expressed. Moreover, the peptides inhibit mouse PC2 enzyme activity. Our cloning of invertebrate 7B2 helps to delineate residues that are important for 7B2-PC2 interaction.

PACE4: a subtilisin-like endoprotease with unique properties

PACE4 is one of the neuroendocrine-specific mammalian subtilisin-related endoproteases believed to function in the secretory pathway. The biosynthesis and secretion of PACE4 have been studied using transfected neuroendocrine and fibroblast cell lines. as well as primary pituitary cultures. ProPACE4 (approx. 106 kDa) is cleaved intracellularly before secretion of PACE4 (approx. 97 kDa); the N-terminal propeptide cleavage is accelerated in a truncated form of PACE4 lacking the Cys-rich C-terminal region (PACE4s). Neither PACE4 nor PACE4s is stored in regulated neuroendocrine secretory granules, whereas pro-opiomelanocortin-derived peptides and prohormone convertase I enter the regulated secretory pathway efficiently. The relatively slow cleavage of the proregion of proPACE4 in primary anterior pituitary cells, followed by rapid secretion of PACE4, is similar to the results for proPACE4 in transfected cell lines. The enzyme activity of PACE4 is distinct from furin and prohormone convertases, both in the marked sensitivity of PACE4 to inhibition by leupeptin and the relative insensitivity of PACE4 to inhibition by Ca2+ chelators and dithiothreitol; PACE4 is not inhibited by the alpha1-antitrypsin Portland variant that is very potent at inhibiting furin. The unique biosynthetic and enzymic patterns seen for PACE4 suggest a role for this neuroendocrine-specific subtilisin-like endoprotease outside the pathway for peptide biosynthesis.

pH-dependent processing of secretogranin II by the endopeptidase PC2 in isolated immature secretory granules

We have previously characterized the processing of secretogranin II (SgII) in PC12 cells that were stably transfected with the endopeptidase PC2. Here we show that processing of SgII can be observed in isolated immature secretory granules (ISGs) derived from this cell line in a temperature- and ATP-dependent manner. The stimulatory effect of ATP on processing can be attributed to the activation of the vacuolar H(+)-ATPase and a concomitant decrease in intragranular pH. The immature secretory granule therefore provides an adequate environment for correct processing of SgII by PC2. The rate of SgII processing was strongly dependent on the intragranular pH, suggesting that processing of SgII can be used as a pH indicator for the granule interior. A standard curve was prepared using SgII processing in ISGs equilibrated at a range of pH values. The extent of processing in ISGs incubated in the presence of ATP at physiological pH was compared with the standard curve, and the intragranular pH was determined. From these observations, we propose an intragranular pH of 6.3 +/- 0.1 for ISGs in a physiological buffer in the presence of ATP. Hence, the pH of ISGs seems to be similar to the pH of the trans-Golgi network (TGN) and is clearly higher than the pH of mature secretory granules (pH 5.0-5.5). Interestingly, no processing of SgII could be observed in a membrane fraction that is highly enriched in TGN under conditions for which processing was readily obtained in isolated ISGs.

Functional analysis of human PACE4-A and PACE4-C isoforms: identification of a new PACE4-CS isoform

There are seven known subtilisin/kexin-like proprotein convertases responsible for the processing of numerous precursors at either pairs or specific single basic residues. Three members, PACE4, PC4 and PC5, exhibit alternative splicing of their RNAs resulting in the generation of multiple isoforms differing in their C- or N-terminal segments. In this study we examined the biosynthesis, functional activity and cellular localization of two of these isoforms, namely the full length PACE4-A and the C-terminally truncated PACE4-C which lacks 11 amino acids at the end of its chaperone-like P-domain. We report the existence of a new isoform, termed PACE4-CS, which is a C-terminally shortened version of PACE4-C. Cellular expression results demonstrated that PACE4-A codes for a functional secretable enzyme capable of cleaving pro7B2 into 7B2. In contrast, PACE4-CS is not secreted since it remains in the endoplasmic reticulum as an inactive zymogen form, thereby emphasizing the importance of the integrity of the P-domain. Microsequencing of the intracellular PACE4-CS protein in two cell lines revealed that it is proPACE4-CS with an N-terminal trimming reminiscent of the action of a dipeptidylpeptidase recognizing the motifs X-Ala and X-Pro.

Involvement of a polyproline helix-like structure in the interaction of 7B2 with prohormone convertase 2

The neuroendocrine protein 7B2 is known to be involved in the biosynthesis and activity of prohormone convertase 2 (PC2). Previous studies have demonstrated that while the carboxyl-terminal portion of 7B2 (residues 155-186) regulates the enzymatic activity of PC2, the amino terminus of the molecule (residues 1-151) is required for maturation of proPC2. In this study we employed four different experimental approaches (co-immunoprecipitation with proPC2, facilitation of pro-PC2 maturation, acquisition of enzymatic activity, and thermal protection assays) to identify structural elements of 7B2 important for bioactivity. Inspection of the sequence of 7B2 indicated potential involvement of a polyproline helix-like (PPII) structure, with similarities to those present within SH3 domain ligands, in the interaction of 7B2 with proPC2. Site-directed point mutagenesis of this proline-rich region confirmed the involvement of this area. Replacement of prolines in positions critical to helix formation (Pro90, Pro91, Pro93, and Pro95) either severely impaired or totally abolished 7B2 bioactivity, as gauged by the four assays described. In addition, constructs longer than residues 1-121 were still functional, whereas those shorter than residues 1-109 were not. Computer-assisted analysis predicts the presence of an alpha-helix structure between residues 107 and 123. We conclude that both the proline-rich region and the alpha-helix contribute to 7B2 activity. Polyproline-containing peptides have been shown to be involved in cytoplasmic protein-protein interactions; our results suggest that the polyproline helix motif may also be used to mediate protein-protein interactions within the secretory pathway.

Secretogranin III is a sulfated protein undergoing proteolytic processing in the regulated secretory pathway

Secretogranin III (SgIII) is an acidic protein of unknown function that is present in the storage vesicles of many neuroendocrine cells. It is coexpressed with the prohormone proopiomelanocortin in the intermediate pituitary of Xenopus laevis. We developed an antiserum to investigate the biosynthesis of SgIII in pulse-chase incubated Xenopus neurointermediate lobes. SgIII was synthesized as a 61- or 63-kDa (N-glycosylated) protein and processed to a 48-kDa form which, in turn, was partially cleaved to fragments of 28 and 20 kDa. The 48-, 28-, and 20-kDa cleavage products, but not their precursors, were secreted. This secretion is regulated and can be blocked in parallel with that of proopiomelanocortin-derived peptides by the hypothalamic factors dopamine, gamma-aminobutyric acid, and neuropeptide Y. Coexpression of Xenopus SgIII with prohormone convertase (PC)1 or PC2 in transfected fibroblasts was sufficient to reconstitute the processing events observed in the neurointermediate lobes. Site-directed mutagenesis revealed that Xenopus SgIII is cleaved at two dibasic sites, namely Lys68-Arg69 and Arg237-Arg238. Pulse-chase incubations of lobes with Na2[35S]SO4 showed that SgIII is sulfated in the trans-Golgi network before it is processed. Finally, SgIII processing was found in several neuroendocrine cell types from various species. We conclude that SgIII is a precursor protein and that the intact molecule can only have an intracellular function, whereas an extracellular role can only be attributed to its cleavage products.

Isolation of the human PC6 gene encoding the putative host protease for HIV-1 gp160 processing in CD4+ T lymphocytes

Production of infectious HIV-1 virions is dependent on the processing of envelope glycoprotein gp160 by a host cell protease. The protease in human CD4+ T lymphocytes has not been unequivocally identified, yet members of the family of mammalian subtilisin-like protein convertases (SPCs), which are soluble or membrane-bound proteases of the secretory pathway, best fulfill the criteria. These proteases are required for proprotein maturation and cleave at paired basic amino acid motifs in numerous cellular and viral glycoprotein precursors, both in vivo and in vitro. To identify the gp160 processing protease, we have used reverse transcription-PCR and Northern blot analyses to ascertain the spectrum of SPC proteases in human CD4+ T cells. We have cloned novel members of the SPC family, known as the human PC6 genes. Two isoforms of the hPC6 protease are expressed in human T cells, hPC6A and the larger hPC6B. The patterns of SPC gene expression in human T cells has been compared with the furin-defective LoVo cell line, both of which are competent in the production of infectious HIV virions. This comparison led to the conclusion that the hPC6 gene products are the most likely candidates for the host cell protease responsible for HIV-1 gp160 processing in human CD4+ T cells.

Dissociation of the complex between the neuroendocrine chaperone 7B2 and prohormone convertase PC2 is not associated with proPC2 maturation

7B2 is a highly conserved neuroendocrine protein that is associated with the proform of the prohormone convertase PC2 in the early stages of the secretory pathway in intermediate pituitary cells of Xenopus laevis. Subsequent processing of 7B2 and dissociation of the 7B2/proPC2 complex is thought to be associated with the conversion of proPC2 to the mature enzyme. Here, we report that, in both Xenopus and mouse intermediate cells, proPC2 maturation does not take place when the proenzyme is associated with the 7B2 precursor and that, in contrast to the previous notion, dissociation of the complex between proPC2 and the N-terminal 7B2 fragment precedes, and is thus not directly linked to, proPC2 maturation. In vitro, conversion of newly synthesized proPC2 was efficiently blocked by recombinant 7B2 and studies with truncation mutants indicated that a short segment in the C-terminal region of 7B2 is necessary and sufficient for this inhibitory effect. Our results indicate that, after 7B2 precursor processing and dissociation of the N-terminal fragment, the C-terminal fragment of 7B2 may remain associated with proPC2, thereby preventing autocatalytic conversion of the proenzyme until the appropriate site for activation in the secretory pathway is reached.

Internal cleavage of the inhibitory 7B2 carboxyl-terminal peptide by PC2: a potential mechanism for its inactivation

The neuroendocrine protein 7B2 contains two domains, a 21-kDa protein required for prohormone convertase 2 (PC2) maturation and a carboxyl-terminal (CT) peptide that inhibits PC2 at nanomolar concentrations. To determine how the inhibition of PC2 is terminated, we studied the metabolic fate of the 7B2 CT peptide in RinPE-7B2, AtT-20/PC2-7B2, and alphaTC1-6 cells. Extracts obtained from cells labeled for 6 h with [3H]valine were subjected to immunoprecipitation using an antibody raised against the extreme carboxyl terminus of r7B2, and immunoprecipitated peptides were separated by gel filtration. All three cell lines yielded two distinct peaks at about 3.5 kDa and 1.5 kDa, corresponding to the CT peptide and a smaller fragment consistent with cleavage at an interior Lys-Lys site. These results were corroborated using a newly developed RIA against the carboxyl terminus of the CT peptide which showed that the intact CT peptide represented only about half of the stored CT peptide immunoreactivity, with the remainder present as the 1.5-kDa peptide. Both peptides could be released upon phorbol 12-myristate 13-acetate stimulation. We investigated the possibility that PC2 itself could be responsible for this cleavage by performing in vitro experiments. When 125I-labeled CT peptide was incubated with purified recombinant PC2, a smaller peptide was generated. Analysis of CT peptide derivatives for their inhibitory potency revealed that CT peptide 1-18 (containing Lys-Lys at the carboxyl terminus) represented a potent inhibitor, but that peptide 1-16 was inactive. Inclusion of carboxypeptidase E (CPE) in the reaction greatly diminished the inhibitory potency of the CT peptide against PC2, in line with the notion that the CT peptide cleavage product is not inhibitory after the removal of terminal lysines by CPE. In summary, our data support the idea that PC2 cleaves the 7B2 CT peptide at its internal Lys-Lys site within secretory granules; deactivation of the cleavage product is then accomplished by CPE, thus providing an efficient mechanism for intracellular inactivation of the CT peptide.

Evidence that PC2 is the endogenous pro-neurotensin convertase in rMTC 6-23 cells and that PC1- and PC2-transfected PC12 cells differentially process pro-neurotensin

The neuropeptide precursor proneurotensin/neuromedin N (pro-NT/NN) is mainly expressed and differentially processed in the brain and in the small intestine. We showed previously that rMTC 6-23 cells process pro-NT/NN with a pattern similar to brain tissue and increase pro-NT/NN expression in response to dexamethasone, and that PC12 cells also produce pro-NT/NN but are virtually unable to process it. In addition, PC12 cells were reported to be devoid of the prohormone convertases PC1 and PC2. The present study was designed to identify the proprotein convertase(s) (PC) involved in pro-NT/NN processing in rMTC 6-23 cells and to compare PC1- and PC2-transfected PC12 cells for their ability to process pro-NT/NN. rMTC 6-23 cells were devoid of PC1, PC4, and PC5 but expressed furin and PC2. Stable expression of antisense PC2 RNA in rMTC 6-23 cells led to a 90% decrease in PC2 protein levels that correlated with a > 80% reduction of pro-NT/NN processing. PC2 expression was stimulated by dexamethasone in a time- and concentration-dependent manner. Stable PC12/PC2 transfectants processed pro-NT/NN with a pattern similar to that observed in the brain and in rMTC 6-23 cells. In contrast, stable PC12/PC1 transfectants reproduced the pro-NT/NN processing pattern seen in the gut. We conclude that (i) PC2 is the major pro-NT/NN convertase in rMTC 6-23 cells; (ii) its expression is coregulated with that of pro-NT/NN in this cell line; and (iii) PC2 and PC1 differentially process pro-NT/NN with brain and intestinal phenotype, respectively.

Development of the sympathoadrenal system in the chick embryo: an immunocytochemical study with antibodies to pan-neuroendocrine markers, catecholamine-synthesizing enzymes, proprotein-processing enzymes, and neuropeptides

BACKGROUND

The adrenal chromaffin cells synthesize, store and secrete a complex mixture containing amines, structural proteins, enzymes, and neurohormonal polypeptides. Most of the studies dealing with the development of the avian sympathoadrenal system have been based on antibodies recognizing signal molecules like HNK-1, NC-1, and N-CAM.

METHODS

The development of the chick sympathoadrenal system was studied from 3 1/2 to 21 days of incubation, both morphologically and immunocytochemically, using antibodies to 17 separate antigens, including antibodies to pan-neuroendocrine markers, catecholamine synthesizing enzymes, proprotein-processing enzymes, and neuropeptides.

RESULTS

Some of the antigens studied were heavily expressed from the first days of development, e.g., chromogranin-A, chromogranin-B, Go protein-alpha subunit, tyrosine hydroxylase, and galanin, while for others a strong heterogeneity both in number of immunoreactive cells and intensity of immunostaining was recorded at the different stages, e.g., dopamine-beta-hydroxylase,, 7B2 protein, proprotein convertase 2, somatostatin, met-enkephalin, secretogranin II, proprotein convertase 3, neuropeptide Y, phenyl-N-methyl transferase, and neuron-specific enolase. The first immunoreactivities to appear at day 3 1/2 were those for HNK-1, tyrosine hydroxylase, chromogranin-A, and chromogranin-B. Except for HNK-1, immunoreactivity for all the remaining antigens showed a steady increase up to the hatching.

CONCLUSIONS

Three expression patterns were found, in the developmental adrenal-gland: defining early permanent markers (chromogranin-A, chromogranin-B, Go protein-alpha subunit, tyrosine hydroxylase, and galanin), others that show a progressively increased expression until the day 10 of development (dopamine-beta-hydroxylase, 7B2 protein, proprotein convertase 2, somatostatin, met-enkephalin), and late-appearing antigens (secretogranin II, proprotein convertase 3, neuropeptide Y, phenyl-N-methyl transferase, and neuron-specific enolase).

Chromogranin B (secretogranin I) promotes sorting to the regulated secretory pathway of processing intermediates derived from a peptide hormone precursor

Chromogranin B (CgB, secretogranin I) is a widespread constituent of neuroendocrine secretory granules whose function is unknown. To determine whether CgB affects the sorting of peptide hormone and neuropeptide precursors to secretory granules, we overexpressed CgB in AtT-20 cells, which exhibit an only moderate capacity to sort proopiomelanocortin and proteolytic fragments derived therefrom. In mock-transfected AtT-20 cells, a substantial proportion of newly synthesized proopiomelanocortin and its two primary proteolytic products generated in the trans-Golgi network, the N-terminal 23-kDa fragment containing adrenocorticotropin and the C-terminal beta-lipotropin fragment, was secreted via the constitutive pathway. Two- to three-fold overexpression of CgB markedly reduced the constitutive secretion of the 23-kDa fragment, but not beta-lipotropin and tripled the amount of adrenocorticotropin generated and stored in secretory granules. Our results indicate the existence of neuroendocrine-specific helper proteins which promote the sorting from the trans-Golgi network to secretory granules of certain processing intermediates derived from peptide hormone and neuropeptide precursors and demonstrate that CgB functions as such.

Structural organization of the gene encoding the neuroendocrine chaperone 7B2

The neuroendocrine-specific polypeptide 7B2 is a constituent of the regulated secretary pathway. Recently, 7B2 was found to function as a molecular chaperone for prohormone convertase PC2. This report describes the genomic organization of the 7B2 gene which consists of six exons. Exon I corresponds to the 5'-untranslated mRNA region, while exons 2 and 3 encode the signal peptide and the amino-terminal half of the 7B2 protein that is distantly related to a subclass of molecular chaperones. The carboxy-terminal half of 7B2, responsible for its inhibitory action on PC2, is encoded by exons 4-6. Primer-extension analysis showed that the human 7B2 gene is transcribed from multiple transcription-initiation sites. The human 7B2 gene promoter contains a cAMP-responsive element, an AP-1 site, and several Pit-1/GHF-1-binding domains and heat-shock-element-like sequences but no obvious TATA or CAAT boxes. Of further interest is the finding of two DNA elements which are common to the promoter regions of the 7B2 gene and other genes selectively expressed in neuroendocrine tissues.

Calcium-induced aggregation of neuroendocrine protein 7B2 in vitro and its modulation by ATP

To study the behavior of the neuroendocrine polypeptide 7B2 in the presence of calcium, various fragments of this molecule were produced in Escherichia coli as fusion proteins to glutathione S-transferase (GST). Addition of millimolar concentrations of Ca2+ to purified preparations of hybrid molecules carrying the N-terminal segment of 7B2 induced precipitation in a manner dependent on protein and cation concentrations. This precipitation occurred at pH 7.5 but not at pH 5.2. It was augmented by 4 and 8 mM ATP, and reduced by 12 and 24 mM ATP. ADP had a similar but weaker effect. Calcium failed to cause precipitation of GST alone or of GST fused to the C-terminal peptide 7B2(156-186). However, when the latter protein was mixed with a GST protein carrying a short fragment of the N-terminal region of 7B2, both proteins were precipitated by calcium. Except for the pH dependence, the behavior of 7B2 fusion proteins in the presence of calcium and adenosine nucleotides are reminiscent of those exhibited by chromogranins and secretogranins, which, like 7B2, are acidic proteins found in the secretory granules of a variety of neuroendocrine cells. As suggested for other granins, this property may underlie the segregation of 7B2 fragments into secretory granules.

Molecular probing of the secretory pathway in peptide hormone-producing cells

The biosynthetic machinery in the melanotrope cells of the Xenopus intermediate pituitary is primarily dedicated to the generation of proopiomelanocortin (POMC)-derived, melanophore-stimulating peptides. Transfer of the animal to a black background stimulates the production of these peptides and causes a dramatic increase in POMC mRNA levels. To identify genes involved in the biosynthesis and regulated release of peptide hormones, we differentially screened an intermediate pituitary cDNA library of toads adapted to a black background with cDNA probes derived from intermediate pituitary mRNA of black- and white-adapted animals. Here we report the identification of twelve distinct genes whose expression levels in the melanotropes are regulated in coordination with that of POMC. Four of these genes are novel while the others code for translocon-associated proteins, a lumenal cysteine protease of the endoplasmic reticulum, prohormone-processing enzymes, members of the granin family and a transmembrane protein presumably involved in the assembly and/or specific functioning of vacuolar H(+)-ATPase from secretory granules. Our results indicate that a wide variety of both soluble and membrane-associated components of the secretory pathway is recruited in physiologically activated, peptide hormone-producing cells.

Ionic milieu controls the compartment-specific activation of pro-opiomelanocortin processing in AtT-20 cells

Newly synthesized prohormones and their processing enzymes transit through the same compartments before being packaged into regulated secretory granules. Despite this coordinated intracellular transport, prohormone processing does not occur until late in the secretory pathway. In the mouse pituitary AtT-20 cell line, conversion of pro-opiomelanocortin (POMC) to mature adrenocorticotropic hormone involves the prohormone convertase PC1. The mechanism by which this proteolytic processing is restricted to late secretory compartments is unknown; PC1 activity could be regulated by compartment-specific activators/inhibitors, or through changes in the ionic milieu that influence its activity. By arresting transport in a semi-intact cell system, we have addressed whether metabolically labeled POMC trapped in early secretory compartments can be induced to undergo conversion if the ionic milieu in these compartments is experimentally manipulated. Prolonged incubation of labeled POMC trapped in the endoplasmic reticulum or Golgi/trans-Golgi network did not result in processing, thereby supporting the theory that processing is normally a post-Golgi/trans-Golgi network event. However, acidification of these compartments allowed effective processing of POMC to the intermediate and mature forms. The observed processing increased sharply at a pH below 6.0 and required millimolar calcium, regardless of the compartment in which labeled POMC resided. These conditions also resulted in the coordinate conversion of PC1 from the 84/87 kDa into the 74-kDa and 66-kDa forms. We propose that POMC processing is predominantly restricted to acidifying secretory granules, and that a change in pH within these granules is both necessary and sufficient to activate POMC processing.

Heterologous processing of rat prosomatostatin to somatostatin-14 by PC2: requirement for secretory cell but not the secretion granule

The role of PC2 in prosomatostatin (PSS) processing was investigated in GH3/GH4C1 pituitary cells. These cells are sparsely granulated, express different amounts of PC2 and no PC1. We described heterologous processing of rat PSS (rPSS) co-expressed with PC2 in stably transfected cells, correlate PC2 protein levels under different conditions of transfection with efficiency of PSS processing to somatostatin-14 (SS-14), determine the effect of modulating cell granularity on enzyme expression and PSS processing, and compare the relative potency of PC2 with that of PC1, PSS and cleavage products were monitored by HPLC and radioimmunoassay of SS-like immunoreactivity (SSLI). Radioimmunoassay analysis of N-terminal PC2-like immunoreactivity (PC2 LI) in GH4C1:rPSS, GH4C1:rPSS + PC2 and GH3:rPSS transfectants showed a gradient of PC2 protein of 1:2.6:3.4 in cell extracts and 1:4.7:9 in secretion media from these cells respectively. The concentration of PC2 protein correlated with SS-14 conversion efficiency was 36 +/- 3% in GH4C1:rPSS cells, 56 +/- 7% in GH4C1:rPSS-PC2 cells and 100% in GH3:rPSS cells. Treatment of GH4C1:rPSS + PC2 cells with epidermal growth factor, insulin, and beta-estradiol to induce granules, significantly increased basal and forskolin-stimulated co-release of SS LI and PC2 LI, but had no influence on SS-14 processing efficiency. Hormone treatment led to a small increase in the ratio of mature PC2 (68 kDa) to proPC2 (75 kDa) forms. PC1 stably transfected in GH4C1 cells produced significantly greater SS-14 conversion (62% in cells, 66% in media) compared with PC2 transfectants (53% in cells, 47% in media) These results provide the first proof that PC2 can effect dibasic processing of mammalian PSS, and, along with PC1, qualifies as an authentic SS-14 convertase. The activity of PC2 requires the milieu of the secretory cell but not the secretory granule.

Characterization of the endopeptidase PC2 activity towards secretogranin II in stably transfected PC12 cells

To study the processing of secretogranin II (SgII) by the prohormone convertase PC2 we have generated a stable PC12 cell line which expresses mouse PC2. We here present the characteristics of the PC12/PC2 cell line and demonstrate that the exogenous PC2 is sorted and stored in secretory granules in the PC12/PC2 cell line as efficiently as the endogenous granins. By indirect immunofluorescence with antibodies specific for chromogranin B (CgB) and PC2 we were able to establish that the PC2 is stored in secretory granules in the PC12/PC2 cell line. After subcellular fractionation, followed by immunoblotting, the mature 68 kDa form of PC2 was found co-sedimented with SgII in fractions containing secretory granules. Two-dimensional gel electrophoresis was used to characterize a secretory granule fraction obtained from the PC12/PC2 cells, and a comparison was done of the electrophoretic pattern obtained from the PC12/PC2 cells with the parent cell line PC12. The products derived from the processing of SgII by PC2 were identified by immunoblotting with a panel of antibodies directed against SgII. Using [35S]sulphate to label the newly synthesized SgII, we performed a time course to monitor the appearance of the lower-molecular-mass fragments of SgII: beginning 15 min after a 5 min pulse of [35S]sulphate we were able to detect the first proteolytic fragment of SgII. Our results demonstrate that SgII is proteolytically processed by PC2 in the immature secretory granule into several lower-molecular-mass proteins, the major ones being an 18 kDa sulphated fragment and a 28 kDa fragment.