A 36-residue peptide contains all of the information required for 7B2-mediated activation of prohormone convertase 2

Secreted Chaperones in Neurodegeneration

Protein homeostasis, or proteostasis, is a combination of cellular processes that govern protein quality control, namely, protein translation, folding, processing, and degradation. Disruptions in these processes can lead to protein misfolding and aggregation. Proteostatic disruption can lead to cellular changes such as endoplasmic reticulum or oxidative stress; organelle dysfunction; and, if continued, to cell death. A majority of neurodegenerative diseases involve the pathologic aggregation of proteins that subverts normal neuronal function. While prior reviews of neuronal proteostasis in neurodegenerative processes have focused on cytoplasmic chaperones, there is increasing evidence that chaperones secreted both by neurons and other brain cells in the extracellular – including transsynaptic – space play important roles in neuronal proteostasis. In this review, we will introduce various secreted chaperones involved in neurodegeneration. We begin with clusterin and discuss its identification in various protein aggregates, and the use of increased cerebrospinal fluid (CSF) clusterin as a potential biomarker and as a potential therapeutic. Our next secreted chaperone is progranulin; polymorphisms in this gene represent a known genetic risk factor for frontotemporal lobar degeneration, and progranulin overexpression has been found to be effective in reducing Alzheimer’s- and Parkinson’s-like neurodegenerative phenotypes in mouse models. We move on to BRICHOS domain-containing proteins, a family of proteins containing highly potent anti-amyloidogenic activity; we summarize studies describing the biochemical mechanisms by which recombinant BRICHOS protein might serve as a therapeutic agent. The next section of the review is devoted to the secreted chaperones 7B2 and proSAAS, small neuronal proteins which are packaged together with neuropeptides and released during synaptic activity. Since proteins can be secreted by both classical secretory and non-classical mechanisms, we also review the small heat shock proteins (sHsps) that can be secreted from the cytoplasm to the extracellular environment and provide evidence for their involvement in extracellular proteostasis and neuroprotection. Our goal in this review focusing on extracellular chaperones in neurodegenerative disease is to summarize the most recent literature relating to neurodegeneration for each secreted chaperone; to identify any common mechanisms; and to point out areas of similarity as well as differences between the secreted chaperones identified to date.

Phosphorylation and Alternative Splicing of 7B2 Reduce Prohormone Convertase 2 Activation

FAM20C is a secretory kinase responsible for the phosphorylation of multiple secreted proteins in mammalian cells; it has been shown to phosphorylate serine residues within a variety of different bone proteins. In this work we demonstrate that FAM20C also phosphorylates threonines, specifically those within the N-terminal domain of the neuroendocrine chaperone 7B2. Analysis of the primary sequence of 7B2 revealed that three threonine residues in its N-terminal domain are located within FAM20C consensus motifs: Thr73, Thr99, and Thr111. The individual substitution of Thr73 and Thr111 residues by neutral alanines caused a marked decrease in the total phosphorylation of 7B2. Furthermore, the phosphomimetic substitution of Thr111 by Glu clearly diminished the ability of 7B2 to activate pro-prohormone convertase 2 (PC2) in 7B2-lacking SK-N-MC neuroblastoma cells, suggesting that the phosphorylation of this residue critically impacts the 7B2-proPC2 interaction. However, the phosphomimetic mutation did not alter 7B2's ability to function as an antiaggregant for human islet amyloid polypeptide. FAM20C-mediated phosphorylation of a common alternatively spliced variant of human 7B2 that lacks Ala100 (thus eliminating the Thr99 phosphorylation consensus site) was similar to the Ala-containing protein, but this variant did not activate proPC2 as efficiently as the Ala-containing protein. Although threonines within 7B2 were phosphorylated efficiently, FAM20C was incapable of performing the well-known regulatory threonine phosphorylation of the molecular chaperone binding immunoglobulin protein. Taken together, these results indicate that FAM20C plays a role in 7B2-mediated proPC2 activation by phosphorylating residue Thr111; and that 7B2 function is regulated by alternative splicing.

Defective transport of the obesity mutant PC1/3 N222D contributes to loss of function

Mutations in the PCSK1 gene encoding prohormone convertase 1/3 (PC1/3) are strongly associated with obesity in humans. The PC1/3(N222D) mutant mouse thus far represents the only mouse model that mimics the PC1/3 obesity phenotype in humans. The present investigation addresses the cell biology of the N222D mutation. Metabolic labeling experiments reveal a clear defect in the kinetics of insulin biosynthesis in islets from PC1/3(N222D) mutant mice, resulting in an increase in both proinsulin and its processing intermediates, predominantly lacking cleavage at the Arg-Arg site. Although the mutant PC1/3 zymogen is correctly processed to the 87-kDa form, pulse-chase immunoprecipitation experiments, labeling, and immunohistochemical experiments using uncleavable variants all demonstrate that the PC1/3-N222D protein is largely mislocalized compared with similar wild-type (WT) constructs, being predominantly retained in the endoplasmic reticulum. The PC1/3-N222D mutant also undergoes more efficient degradation via the ubiquitin-proteasome system than the WT enzyme. Lastly, the mutant PC1/3-N222D protein coimmunoprecipitates with WT PC1/3 and exerts a modest effect on intracellular retention of the WT enzyme. These profound alterations in the cell biology of PC1/3-N222D are likely to contribute to the defective insulin biosynthetic events observed in the mutant mice and may be relevant to the dramatic contributions of polymorphisms in this gene to human obesity.

SUPPRESSOR OF LLP1 1-mediated C-terminal processing is critical for CLE19 peptide activity

Cell-to-cell communication is essential for the coordinated development of multicellular organisms. Members of the CLAVATA3/EMBRYO-SURROUNDING REGION-RELATED (CLE) family, a group of small secretory peptides, are involved in these processes in plants. Although post-translational modifications are considered to be indispensable for their activity, the detailed mechanisms governing these modifications are not well understood. Here, we report that SUPPRESSOR OF LLP1 1 (SOL1), a putative Zn²⁺ carboxypeptidase previously isolated as a suppressor of the CLE19 over-expression phenotype, functions in C-terminal processing of the CLE19 proprotein to produce the functional CLE19 peptide. Newly isolated sol1 mutants are resistant to CLE19 over-expression, consistent with the previous report (Casamitjana-Martinez, E., Hofhuis, H.F., Xu, J., Liu, C.M., Heidstra, R. and Scheres, B. (2003) Curr. Biol. 13, 1435-1441). As expected, our experiment using synthetic CLE19 peptide revealed that the sol1 mutation does not compromise CLE signal transduction pathways per se. SOL1 possesses enzymatic activity to remove the C-terminal arginine residue of CLE19 proprotein in vitro, and SOL1-dependent cleavage of the C-terminal arginine residue is necessary for CLE19 activity in vivo. Additionally, the endosomal localization of SOL1 suggests that this processing occurs in endosomes in the secretory pathway. Thus, our data indicate the importance of C-terminal processing of CLE proproteins to ensure CLE activities.

Blockade of islet amyloid polypeptide fibrillation and cytotoxicity by the secretory chaperones 7B2 and proSAAS

The deposition of fibrillated human islet β-cell peptide islet amyloid polypeptide (hIAPP) into amyloid plaques is characteristic of the pathogenesis of islet cell death during type 2 diabetes. We investigated the effects of the neuroendocrine secretory proteins 7B2 and proSAAS on hIAPP fibrillation in vitro and on cytotoxicity. In vitro, 21-kDa 7B2 and proSAAS blocked hIAPP fibrillation. Structure-function studies showed that a central region within 21-kDa 7B2 is important in this effect and revealed the importance of the N-terminal region of proSAAS. Both chaperones blocked the cytotoxic effects of exogenous hIAPP on Rin5f cells; 7B2 generated by overexpression was also effective. ProSAAS and 7B2 may perform a chaperone role as secretory anti-aggregants in normal islet cell function and in type 2 diabetes.

The neuroendocrine protein 7B2 suppresses the aggregation of neurodegenerative disease-related proteins

Neurodegenerative diseases such as Alzheimer (AD) and Parkinson (PD) are characterized by abnormal aggregation of misfolded β-sheet-rich proteins, including amyloid-β (Aβ)-derived peptides and tau in AD and α-synuclein in PD. Correct folding and assembly of these proteins are controlled by ubiquitously expressed molecular chaperones; however, our understanding of neuron-specific chaperones and their involvement in the pathogenesis of neurodegenerative diseases is limited. We here describe novel chaperone-like functions for the secretory protein 7B2, which is widely expressed in neuronal and endocrine tissues. In in vitro experiments, 7B2 efficiently prevented fibrillation and formation of Aβ_1–42, Aβ_1–40, and α-synuclein aggregates at a molar ratio of 1:10. In cell culture experiments, inclusion of recombinant 7B2, either in the medium of Neuro-2A cells or intracellularly via adenoviral 7B2 overexpression, blocked the neurocytotoxic effect of Aβ_1–42 and significantly increased cell viability. Conversely, knockdown of 7B2 by RNAi increased Aβ_1–42-induced cytotoxicity. In the brains of APP/PSEN1 mice, a model of AD amyloidosis, immunoreactive 7B2 co-localized with aggregation-prone proteins and their respective aggregates. Furthermore, in the hippocampus and substantia nigra of human AD- and PD-affected brains, 7B2 was highly co-localized with Aβ plaques and α-synuclein deposits, strongly suggesting physiological association. Our data provide insight into novel functions of 7B2 and establish this neural protein as an anti-aggregation chaperone associated with neurodegenerative disease.

The neuroendocrine protein 7B2 is intrinsically disordered

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

The extended granin family: structure, function, and biomedical implications

The chromogranins (chromogranin A and chromogranin B), secretogranins (secretogranin II and secretogranin III), and additional related proteins (7B2, NESP55, proSAAS, and VGF) that together comprise the granin family subserve essential roles in the regulated secretory pathway that is responsible for controlled delivery of peptides, hormones, neurotransmitters, and growth factors. Here we review the structure and function of granins and granin-derived peptides and expansive new genetic evidence, including recent single-nucleotide polymorphism mapping, genomic sequence comparisons, and analysis of transgenic and knockout mice, which together support an important and evolutionarily conserved role for these proteins in large dense-core vesicle biogenesis and regulated secretion. Recent data further indicate that their processed peptides function prominently in metabolic and glucose homeostasis, emotional behavior, pain pathways, and blood pressure modulation, suggesting future utility of granins and granin-derived peptides as novel disease biomarkers.

Modulation of prohormone convertase 1/3 properties using site-directed mutagenesis

Prohormone convertase (PC)1/3 and PC2 cleave active peptide hormones and neuropeptides from precursor proteins. Compared with PC2, recombinant PC1/3 exhibits a very low specific activity against both small fluorogenic peptides and recombinant precursors, even though the catalytic domains in mouse PC1/3 and PC2 share 56% amino acid sequence identity. In this report, we have designed PC2-specific mutations into the catalytic domain of PC1/3 in order to investigate the molecular contributions of these sequences to PC1/3-specific properties. The exchange of residues RQG(314) with the SY sequence present in the same location within PC2 paradoxically shifted the pH optimum of PC1/3 upward into the neutral range; other mutations in the catalytic domain had no effect. Although none of the full-length PC1/3 mutants examined exhibited increased specific activity, the 66-kDa form of the RQG(314)SY mutant was two to four times more active than the 66-kDa form of wild-type PC1/3. However, stable transfection of RQG(314)SY into PC12 cells did not result in greater activity against the endogenous substrate proneurotensin, implying unknown cellular controls of PC1/3 activity. Mutation of GIVTDA(243-248) to QPFMTDI, a molecular determinant of 7B2 binding, resulted in increased zymogen expression but no propeptide cleavage or secretion, suggesting that this mutant is trapped in the endoplasmic reticulum due to an inability to cleave its own propeptide. We conclude that many convertase-specific properties are attributable less to convertase-specific catalytic cleft residues than to convertase-specific domain interactions.

Cell-surface processing of the metalloprotease pro-ADAMTS9 is influenced by the chaperone GRP94/gp96

A disintegrin-like and metalloprotease domain with thrombospondin type 1 motifs 9 (ADAMTS9) is a highly conserved metalloprotease that has been identified as a tumor suppressor gene and is required for normal mouse development. The secreted ADAMTS9 zymogen undergoes proteolytic excision of its N-terminal propeptide by the proprotein convertase furin. However, in contrast to other metalloproteases, propeptide excision occurs at the cell surface and leads to decreased activity of the zymogen. Here, we investigated the potential cellular mechanisms regulating ADAMTS9 biosynthesis and cell-surface processing by analysis of molecular complexes formed by a construct containing the propeptide and catalytic domain of pro-ADAMTS9 (Pro-Cat) in HEK293F cells. Cross-linking of cellular proteins bound to Pro-Cat followed by mass spectrometric analysis identified UDP-glucose:glycoprotein glucosyltransferase I, heat shock protein gp96 (GRP94), BiP (GRP78), and ERdj3 (Hsp40 homolog) as associated proteins. gp96 and BiP were present at the cell surface in an immunoprecipitable complex with pro-ADAMTS9 and furin. Treatment with geldanamycin, an inhibitor of the HSP90alpha family (including gp96), led to decreased furin processing of pro-ADAMTS9 and accumulation of the unprocessed pro-ADAMTS9 at the cell surface. gp96 siRNA down-regulated the levels of cell-surface pro-ADAMTS9 and furin, whereas the levels of cell-surface pro-ADAMTS9, but not of cell-surface furin, were decreased upon treatment with BiP siRNA. These data identify for the first time the cellular chaperones associated with secretion of an ADAMTS protease and suggest a role for gp96 in modulating pro-ADAMTS9 processing.

Identification of proSAAS homologs in lower vertebrates: conservation of hydrophobic helices and convertase-inhibiting sequences

The prohormone convertases (PCs) 1/3 and 2 accomplish the major proteolytic cleavage events in neuroendocrine tissues; each of these convertases has a small associated binding protein that inhibits convertase action in the secretory pathway. The proSAAS protein binds to PC1/3, whereas the 7B2 protein binds to PC2. However, both convertase-binding proteins are more widely expressed than their cognate enzymes, suggesting that they may perform other functions as well. All known mammalian proSAASs are over 85% conserved; thus, identifying functionally important segments has been impossible. Here, we report the first identification of nonmammalian proSAAS molecules, from Xenopus and zebrafish (Danio rerio). Although these two proteins show an overall amino acid sequence identity of only 29 and 30% with mouse proSAAS, two 14-16 residue hydrophobic segments (predicted to form alpha-helices) and two, nine through 11 residue sequences containing basic convertase cleavage sites are highly conserved; therefore, these sequences may be of functional importance. Confidence that these nonmammalian molecules represent authentic proSAAS is supported by the finding that both inhibit mouse PC1/3 with nanomolar inhibition constants; human furin was not inhibited. In vitro, the two proteins were cleaved by PC2 and furin to three or more peptide products. Both zebrafish and Xenopus proSAAS exhibited neural and endocrine distributions, as assessed by in situ and PCR experiments, respectively. In summary, the identification of proSAAS molecules in lower vertebrates provides clues as to functional regions within this widely expressed neuroendocrine protein.

Pax6 regulates the proglucagon processing enzyme PC2 and its chaperone 7B2

Pax6 is important in the development of the pancreas and was previously shown to regulate pancreatic endocrine differentiation, as well as the insulin, glucagon, and somatostatin genes. Prohormone convertase 2 (PC2) is the main processing enzyme in pancreatic alpha cells, where it processes proglucagon to produce glucagon under the spatial and temporal control of 7B2, which functions as a molecular chaperone. To investigate the role of Pax6 in glucagon biosynthesis, we studied potential target genes in InR1G9 alpha cells transfected with Pax6 small interfering RNA and in InR1G9 clones expressing a dominant-negative form of Pax6. We now report that Pax6 controls the expression of the PC2 and 7B2 genes. By binding and transactivation studies, we found that Pax6 indirectly regulates PC2 gene transcription through cMaf and Beta2/NeuroD1 while it activates the 7B2 gene both directly and indirectly through the same transcription factors, cMaf and Beta2/NeuroD1. We conclude that Pax6 is critical for glucagon biosynthesis and processing by directly and indirectly activating the glucagon gene through cMaf and Beta2/NeuroD1, as well as the PC2 and 7B2 genes.

7B2 prevents unfolding and aggregation of prohormone convertase 2

Prohormone convertase 2 (PC2) requires interaction with the neuroendocrine protein 7B2 for the production of an activatable zymogen; the mechanism for this effect is unknown. 7B2 could act proactively to generate an activation-competent form of pro-PC2 during synthesis, or block spontaneous generation of activation-incompetent forms. We here demonstrate that addition of exogenous recombinant 7B2 to CHO cells expressing pro-PC2 prevented the unfolding and aggregation of secreted PC2 forms in a dose-dependent manner, as assessed by aggregation assays, activity assays, cross-linking experiments, and sucrose density gradients. Intracellular pro-PC2 was also found to exist in part as higher-order oligomers that were reduced in the presence of coexpressed 7B2. 7B2 addition did not result in the acquisition of enzymatic competence unless added before or very rapidly after pro-PC2 secretion, indicating that an activation-competent structure cannot be maintained in the absence of 7B2. Velocity sedimentation experiments showed that addition of extracellular 7B2 solubilized three different PC2 species from a precipitable aggregate: two activatable pro-PC2 species, the intact zymogen and a zymogen with a partially cleaved propeptide, and an inactive 66-kDa form. Our results suggest that 7B2 possesses chaperone activity that blocks partially unfolded pro-PC2 forms from losing catalytic competence and then aggregating. The loss of the catalytically competent conformer appears to represent the earliest indicator of pro-PC2 unfolding and is followed on a slower time scale by the appearance of aggregates. Because 7B2 expression is not confined to areas expressing pro-PC2, 7B2 may represent a general intracellular and extracellular secretory chaperone.

Proteases for processing proneuropeptides into peptide neurotransmitters and hormones

Peptide neurotransmitters and peptide hormones, collectively known as neuropeptides, are required for cell-cell communication in neurotransmission and for regulation of endocrine functions. Neuropeptides are synthesized from protein precursors (termed proneuropeptides or prohormones) that require proteolytic processing primarily within secretory vesicles that store and secrete the mature neuropeptides to control target cellular and organ systems. This review describes interdisciplinary strategies that have elucidated two primary protease pathways for prohormone processing consisting of the cysteine protease pathway mediated by secretory vesicle cathepsin L and the well-known subtilisin-like proprotein convertase pathway that together support neuropeptide biosynthesis. Importantly, this review discusses important areas of current and future biomedical neuropeptide research with respect to biological regulation, inhibitors, structural features of proneuropeptide and protease interactions, and peptidomics combined with proteomics for systems biological approaches. Future studies that gain in-depth understanding of protease mechanisms for generating active neuropeptides will be instrumental for translational research to develop pharmacological strategies for regulation of neuropeptide functions. Pharmacological applications for neuropeptide research may provide valuable therapeutics in health and disease.

Neuropeptidergic signaling in the nematode Caenorhabditis elegans

The nematode Caenorhabditis elegans joins the menagerie of behavioral model systems next to the fruit fly Drosophila melanogaster, the marine snail Aplysia californica and the mouse. In contrast to Aplysia, which contains 20,000 neurons having cell bodies of hundreds of microns in diameter, C. elegans harbors only 302 tiny neurons from which the cell lineage is completely described, as is the case for all the other somatic cells. As such, this nervous system appears at first sight incommensurable with those of higher organisms, although genome-wide comparison of predicted C. elegans genes with their counterparts in vertebrates revealed many parallels. Together with its short lifespan and ease of cultivation, suitability for high-throughput genetic screenings and genome-wide RNA interference approaches, access to an advanced genetic toolkit and cell-ablation techniques, it seems that this tiny transparent organism of only 1mm in length has nothing to hide. Recently, highly exciting developments have occurred within the field of neuropeptidergic signaling in C. elegans, not only because of the availability of a sequenced genome since 1998, but especially because of state of the art post genomic technologies, that allow for molecular characterization of the signaling molecules. Here, we will focus on endogenous, bioactive (neuro)peptides and mainly discuss biosynthesis, peptide sequence information, localization and G-protein coupled receptors of the three major peptide families in C. elegans.

Strain-specific steroidal control of pituitary function

We have previously shown that 7B2 null mice on the 129/SvEvTac (129) genetic background die at 5 weeks of age with hypercorticosteronemia due to a Cushing's-like disease unless they are rescued by adrenalectomy; however, 7B2 nulls on the C57BL/6NTac (B6) background remain healthy, with normal steroid levels. Since background exerts such a profound influence on the phenotype of this mutation, we have evaluated whether these two different mouse strains respond differently to high circulating steroids by chronically treating wild-type 129 and B6 mice with the synthetic steroid dexamethasone (Dex). Dex treatment decreased the dopamine content of the neurointermediate lobes (NIL) of 129 mice, leading to NIL enlargement and increased total D(2)R mRNA in the 129, but not the B6, NIL. Despite the decrease in this inhibitory transmitter, Dex-treated 129 mice exhibited reduced circulating alpha-melanocyte-stimulating hormone (alpha-MSH) along with reduced POMC-derived peptides compared with controls, possibly due to reduced POMC content in the NIL. In contrast, Dex-treated B6 mice showed lowered cellular ACTH, unchanged alpha-MSH and beta-endorphin, and increased circulating alpha-MSH, most likely due to increased cleavage of NIL ACTH by increased PC2. Dex-treated 129 mice exhibited hyperinsulinemia and lowered blood glucose, whereas Dex-treated B6 mice showed slightly increased glucose levels despite their considerably increased insulin levels. Taken together, our results suggest that the endocrinological response of 129 mice to chronic Dex treatment is very different from that of B6 mice. These strain-dependent differences in steroid sensitivity must be taken into account when comparing different lines of transgenic or knockout mice.

Neuroendocrine protein 7B2 can be inactivated by phosphorylation within the secretory pathway

The prohormone convertases play important roles in the maturation of neuropeptides and peptide hormone precursors. Prohormone convertase-2 (PC2) is the only convertase that requires the expression of another neuroendocrine protein, 7B2, for expression of enzyme activity. In this study, we determined that 7B2 can be phosphorylated in Rin cells (a rat insulinoma cell line) and cultured chromaffin cells, but not in AtT-20 cells (derived from mouse anterior pituitary). Phosphoamino acid analysis of Rin cell 7B2 indicated the presence of phosphorylated serine and threonine. Phosphorylation of Ser115 (located within the minimally active 36-residue peptide) was confirmed by mutagenesis, although Ser115 did not represent the sole residue phosphorylated. Two independent assays were used to investigate the effect of phosphorylated 7B2 on PC2 activation: the ability of 7B2 to bind to pro-PC2 was assessed by co-immunoprecipitation, and activation of pro-PC2 was assessed in a cell-free assay. Phosphorylated 7B2 was unable to bind pro-PC2, and the phosphorylated 7B2 peptide (residues 86-121, known to be the minimally active peptide for pro-PC2 activation) was impaired in its ability to facilitate the generation of PC2 activity in membrane fractions containing pro-PC2. In vitro phosphorylation experiments using Golgi membrane fractions showed that 7B2 could be phosphorylated by endogenous Golgi kinases. Golgi kinase activity was strongly inhibited by the broad-range kinase inhibitor staurosporine and partially inhibited by the protein kinase C inhibitor bisindolylmaleimide I, but not by the other protein kinase A, Ca2+/calmodulin-dependent kinase II, myosin light chain kinase, and protein kinase G inhibitors tested. We conclude that phosphorylation of 7B2 functionally inactivates this protein and suggest that this may be analogous to the phosphorylating inactivation of BiP, which impairs its ability to bind substrate.

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.

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.

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.

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.

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.

Cell type-specific storage of dopamine beta-monooxygenase

Expression of dopamine beta-monooxygenase (DBM), the enzyme that converts dopamine into norepinephrine, is limited to adrenal chromaffin cells and a small population of neurons. We studied DBM trafficking to regulated granules by stably expressing rat DBM in AtT-20 corticotrope tumor cells, which contain regulated granules, and in Chinese hamster ovary (CHO) cells, which lack regulated granules. The behavior of exogenous DBM in both cell lines was compared with endogenous DBM in adrenal chromaffin cells. CHO cells secreted active DBM, indicating that production of active enzyme does not require features unique to neuroendocrine cells. Pulse-chase experiments indicated that early steps in DBM maturation followed a similar time course in AtT-20, CHO, and adrenal chromaffin cells. Use of a conformation-sensitive DBM antiserum indicated that acquisition of a folded structure occurred with a similar time course in all three cell types. Cell type-specific differences in DBM trafficking became apparent only when storage in granules was examined. As expected, DBM was stored in secretory granules in chromaffin cells; CHO cells failed to store DBM. Despite the fact that AtT-20 cells have regulated granules, exogenous DBM was not stored in these granules. Thus storage of DBM in secretory granules requires cell type specific factors.