Expression and cell type--specific processing of human preproenkephalin with a vaccinia recombinant

The Path to Therapeutic Furin Inhibitors: From Yeast Pheromones to SARS-CoV-2

The spurious acquisition and optimization of a furin cleavage site in the SARS-CoV-2 spike protein is associated with increased viral transmission and disease, and has generated intense interest in the development and application of therapeutic furin inhibitors to thwart the COVID-19 pandemic. This review summarizes the seminal studies that informed current efforts to inhibit furin. These include the convergent efforts of endocrinologists, virologists, and yeast geneticists that, together, culminated in the discovery of furin. We describe the pioneering biochemical studies which led to the first furin inhibitors that were able to block the disease pathways which are broadly critical for pathogen virulence, tumor invasiveness, and atherosclerosis. We then summarize how these studies subsequently informed current strategies leading to the development of small-molecule furin inhibitors as potential therapies to combat SARS-CoV-2 and other diseases that rely on furin for their pathogenicity and progression.

Direct cellular peptidomics of hypothalamic neurons

The chemical complexity of cell-to-cell communication has emerged as a fundamental challenge to understanding brain systems. This is certainly true for the hypothalamus, where neuropeptide signals are heterogeneous, localized and dynamic. Thus far, most hypothalamic peptidomic studies have centered on the entire structure; however, recent advances in collection strategies and analytical technologies have enabled direct, high-resolution peptidomic profiles focused on two regions of interest, the suprachiasmatic and supraoptic nuclei, including their sub-regions and individual cells. Suites of peptides now can be identified and probed for function. High spatial and analytical sensitivities reveal that discrete hypothalamic nuclei have distinct peptidomic signatures. Peptidomic discovery not only reveals unanticipated complexity, but also peptides previously unknown that act as key circuit components. Analysis of tissue releasates identifies peptides secreted into the extracellular environment and available for transmitting intercellular signals. Direct sampling techniques define peptide-releasate profiles in spatial, temporal and event-dependent patterns. These approaches are providing remarkable new insights into the complexity of neuropeptidergic cell-to-cell signaling central to neuroendocrine physiology.

Neurexophilins form a conserved family of neuropeptide-like glycoproteins

Neurexophilin was discovered as a neuronal glycoprotein that is copurified with neurexin Ialpha during affinity chromatography on immobilized alpha-latrotoxin (Petrenko et al., 1996). We have now investigated how neurexophilin interacts with neurexins, whether it is post-translationally processed by site-specific cleavage similar to neuropeptides, and whether related neuropeptide-like proteins are expressed in brain. Our data show that mammalian brains contain four genes for neurexophilins the products of which share a common structure composed of five domains: an N-terminal signal peptide, a variable N-terminal domain, a highly conserved central domain that is N-glycosylated, a short linker region, and a conserved C-terminal domain that is cysteine-rich. When expressed in pheochromocytoma (PC12) cells with a replication-deficient adenovirus, neurexophilin 1 was rapidly N-glycosylated and then slowly processed to a smaller mature form, probably by endoproteolytic cleavage. Similar expression experiments in other neuron-like cells and in fibroblastic cells revealed that N-glycosylation of neurexophilin 1 occurred in all cell types tested, whereas proteolytic processing was observed only in neuron-like cells. Finally, only recombinant neurexin Ialpha and IIIalpha but not neurexin Ibeta interacted with neurexophilin 1 and were preferentially bound to the processed mature form of neurexophilin. Together our data demonstrate that neurexophilins form a family of related glycoproteins that are proteolytically processed after synthesis and bind to alpha-neurexins. The structure and characteristics of neurexophilins indicate that they function as neuropeptides that may signal via alpha-neurexins.

Essential role of the disulfide-bonded loop of chromogranin B for sorting to secretory granules is revealed by expression of a deletion mutant in the absence of endogenous granin synthesis

Sorting of regulated secretory proteins in the TGN to immature secretory granules (ISG) is thought to involve at least two steps: their selective aggregation and their interaction with membrane components destined to ISG. Here, we have investigated the sorting of chromogranin B (CgB), a member of the granin family present in the secretory granules of many endocrine cells and neurons. Specifically, we have studied the role of a candidate structural motif implicated in the sorting of CgB, the highly conserved NH2-terminal disulfide- bonded loop. Sorting to ISG of full-length human CgB and a deletion mutant of human CgB (Deltacys-hCgB) lacking the 22-amino acid residues comprising the disulfide-bonded loop was compared in the rat neuroendocrine cell line PC12. Upon transfection, i.e., with ongoing synthesis of endogenous granins, the sorting of the deletion mutant was only slightly impaired compared to full-length CgB. To investigate whether this sorting was due to coaggregation of the deletion mutant with endogenous granins, we expressed human CgB using recombinant vaccinia viruses, under conditions in which the synthesis of endogenous granins in the infected PC12 cells was shut off. In these conditions, Deltacys-hCgB, in contrast to full-length hCgB, was no longer sorted to ISG, but exited from the TGN in constitutive secretory vesicles. Coexpression of full-length hCgB together with Deltacys-hCgB by double infection, using the respective recombinant vaccinia viruses, rescued the sorting of the deletion mutant to ISG. In conclusion, our data show that (a) the disulfide-bonded loop is essential for sorting of CgB to ISG and (b) the lack of this structural motif can be compensated by coexpression of loop-bearing CgB. Furthermore, comparison of the two expression systems, transfection and vaccinia virus-mediated expression, reveals that analyses under conditions in which host cell secretory protein synthesis is blocked greatly facilitate the identification of sequence motifs required for sorting of regulated secretory proteins to secretory granules.

cDNA expression and human two-dimensional gel protein databases: towards integrating DNA and protein information

The rapid progress in characterizing genes and mRNAs (expressed sequence tags, ESTs) as a result of the Human Genome Project makes it imperative to develop strategies to interface DNA mapping and sequencing data with protein information, as the latter orchestrate most cellular functions. Presently, the only technique able to resolve and record the thousands of proteins present in cells and tissues is two-dimensional (2-D) gel electrophoresis in combination with computer-aided technology to scan the gels, make synthetic images, assign numbers to individual spots as well as to enter qualitative and quantitative information. To date, comprehensive 2-D gel databases containing information about various properties of proteins (cellular localization, identification, regulatory properties, partial amino acid sequences, etc.) have been established (available on the internet: http:@biobase.dk/cgi-bin/celis). What remains is to provide a link between these data and the forthcoming information from the Human Genome Project. We are pursuing two approaches to achieve this goal: (i) microsequencing and mass spectrometry analysis of proteins resolved from 2-D gels and (ii) expression of cDNAs in the vaccinia virus expression system. Using the latter approach we have expressed about 60 cDNAs in human cells under conditions that faithfully reproduce post-translational trimmings and modifications of the proteins. The method, in combination with 2-D gel electrophoresis, allows precise matching of almost any cDNA to its protein product, irrespective of the protein abundance.

An efficient cellular system for mutational analysis of prohormone processing

A novel system for heterologous expression of prohormones based on transient transfection of the HIT beta-cell line was established using human progastrin as a model. Progastrin was expressed at high levels compared to other gene transfer systems in endocrine cells, and the processing pattern was similar to that of normal antral gastrin cells. Thus, gastrin was partially tyrosine O-sulfated and carboxyamidated. Cell extracts contained mainly gastrin-17 and gastrin-34 and the corresponding glycine-extended forms. In contrast, the media contained more incompletely processed gastrin forms. This suggests that gastrin was directed to the regulated secretory pathway but that some progastrin products were constitutively secreted. Glucose increased both the level of gastrin gene expression and maturation to carboxyamidated peptides, indicating that glucose influences the activity of the amidation enzyme complex, peptidylglycine alpha-amidating mono-oxygenase (PAM), in insulin cells. Mutational analysis of tyrosine sulfation of gastrin demonstrated that substitution of the uncharged residue carboxy-terminal to the tyrosine with an acidic residue does not increase sulfation in contrast to previous results, where the amino-terminal residue was replaced with an acidic residue. The mutant peptides displayed sulfation-dependent processing, supporting our recent suggestion that tyrosine sulfation increases the proteolytic processing of prohormones.

Effect of serum type and concentration on the expression ofβ-galactosidase in recombinant vaccinia virus infected HeLa S3 cells

The effect of serum type and concentration on recombinant protein expression in vaccinia virus infected HeLa S3 cells was studied in both static and suspension culture. A model heterologous protein,β-galactosidase (β-gal), was used. Calf and horse sera in the range of 0.5-10%(v/v) were investigated. In static culture, the calf serum concentration did not show any significant influence on the β-gal production which was almost completed within 24h postinfection (pi). Higher horse serum concentration, on the other hand, resulted in higher β-gal concentration which continued to increase until 48 h pi. Total β-gal concentrations in 0.5% calf serum at 24 h pi and 10% horse serum at 48 h pi were 2.2±0.7 and 2.2±0.1 IU/ml, respectively. In suspension culture, both sera showed their respective effects on the β-gal production similar to those observed in static culture, indicating that the cultivation method had little influence on β-gal production. Accordingly, the use of 0.5% calf serum after virus infection in recommended for economical β-gal production.

Production of bioactive enkephalin from the nonendocrine cell lines COS-7, NIH3T3, Ltk-, and C2C12

Enkephalin is synthesized from proenkephalin in neuroendocrine cells. For the attempt to induce nonneuroendocrine origin cells to produce enkephalin, we used a mammalian expression vector for fusion peptides, pMEproCT beta, in which a fused peptide is designed to be cleaved by a yeast Kex2-like endoprotease furin. Met-Enkephalin was expressed in four nonneuroendocrine cell lines: COS-7, C2C12, Ltk-, and NIH3T3. The four cell lines produced a marked amount of Met-enkephalin, which appeared as a single peak on reverse-phase HPLC. Because transplantation of adrenal medullary cells to the subarachnoid space has been used to alleviate terminal cancer pain, and enkephalin appears to play a central role in relieving pain, this enkephalin expression vector may be useful for direct enkephalin expression in pericancerous tissues.

pp60c-src enhances the acetylcholine receptor-dependent catecholamine release in vaccinia virus-infected bovine adrenal chromaffin cells

Secretion of catecholamines by adrenal chromaffin cells is a highly regulated process that involves serine/threonine and tyrosine phosphorylations. The nonreceptor tyrosine kinase pp60c-src is expressed at high levels and localized to plasma membranes and secretory vesicle membranes in these cells, suggesting an interaction of this enzyme with components of the secretory process. To test the hypothesis that pp60c-src is involved in exocytosis, we transiently expressed exogenous c-src cDNA using a vaccinia virus vector in primary cultures of bovine adrenomedullary chromaffin cells. Chromaffin cells infected with a c-src recombinant virus restored the diminished secretory activity accompanying infection by wild type virus alone or a control recombinant virus. The level of enhanced catecholamine release correlated directly with the time and level of exogenous c-src expression. These results could not be attributed to differences in cytopathic effects of wild type versus recombinant viruses as assessed by cell viability assays, nor to differences in norepinephrine uptake or basal release, suggesting that pp60c-src is involved in stimulus-secretion coupling in infected cells. Surprisingly, exogenous expression of an enzymatically inactive mutant c-src also restored catecholamine release, indicating that regions of the introduced c-src protein other than the kinase domain may affect catecholamine release. Secretory activity was elevated by both forms of c-src in response to either nicotine or carbachol (which activate the nicotinic and the nicotinic/muscarinic receptors, respectively). In contrast, release of catecholamines upon membrane depolarization (as elicited by 55 mM K+) or by treatment with the calcium ionophore A23187 was unaffected by either vaccinia infection or increased levels of pp60c-src. These results suggest that pp60c-src affects secretory processes in vaccinia-infected cells that are activated through ligand-gated, but not voltage-gated, ion channels.

Deletions of the synenkephalin domain which do not alter cell-specific proteolytic processing or secretory targeting of human proenkephalin

To identify signals that direct the proteolytic processing and regulated secretion of human proenkephalin (hPE), we have transfected the hPE gene or minigene constructs into pituitary tumor cells, either rat GH4Cl cells or mouse AtT-20 cells. Cells transfected with either the hPE gene or minigene contained similar levels of methionine-enkephalin (ME)-containing peptides and hPE mRNA. In the GH4Cl clones, ME was present predominantly in high-molecular-mass forms (5-25 kDa). In contrast, the AtT-20 clones contained almost exclusively free ME and low-molecular-mass forms (< 5 kDa), with very little high-molecular-mass species present. Thus, among pituitary cells, corticotroph-derived cells appear better equipped to process hPE than lactotroph-derived cells. Despite limited proteolytic processing, GH4Cl clones secreted large amounts of unprocessed (> 20 kDa) hPE into the medium, making up to 10% of endogenous rat prolactin secretion. Both precursor and processed forms of ME were cosecreted acutely (< 1 h) with rat prolactin, and release of both polypeptides was stimulated up to 12-fold by secretagogues. Thus, complete proteolytic processing was not required for accurate targeting of hPE to the regulated secretory pathway. When transfected with constructs bearing deletions of amino-terminal amino acids 2-43 or 2-67, i.e., part or nearly all of the synenkephalin moiety, GH4Cl cells handled the modified protein much like cells expressing the complete protein. They did not process the modified hPE extensively, but the protein was correctly targeted to the regulated secretory pathway. AtT-20 cells transfected with truncated hPE cDNA constructs expressed and processed the protein as efficiently as cells expressing unmodified hPE and expressed predominantly low-molecular-mass forms of ME. Therefore, the structural features required for correct targeting and processing are not present in the cysteine-rich amino-terminal third of the prohormone. It is interesting that the deletions did not include the SHLL peptide motif in synenkephalin, a motif that has been proposed as a sorting signal.

Efficient amidation of C-peptide deleted NPY precursors by non-endocrine cells is affected by the presence of Lys-Arg at the C-terminus

Post-translational processing of peptide precursors producing amidated, biologically active peptides generally occurs in specially differentiated endocrine or neural cells. However, we have previously shown that a C-peptide-deleted precursor of neuropeptide Y (NPY1-39) in which the precursor terminates in the sequence Gly-Lys-Arg was partially amidated by the non-endocrine cell line, CHO. In the present study we show that two other non-endocrine cell lines, NIH 3T3 and BHK, also possess amidating activities and that the NPY1-39 precursor was completely converted to NPY1-36 amide by the NIH 3T3 cell line. The role of the two basic residues (Lys-Arg) in the C-terminus was studied by transfection of a construct encoding a NPY precursor terminating with glycine alone. Both the CHO and NIH 3T3 cell lines, transfected with this construct, secreted a significantly smaller fraction of NPY reactive material as amidated NPY compared to the fraction of amidated NPY secreted by the cells transfected with the NPY1-39 precursor. It is concluded that the capacity to perform C-terminal amidation appears to be a universal feature of eukaryotic cells and that the carboxypeptidase E-like enzyme influences the amidation process, beyond its known ability to remove the C-terminal basic residues.

Proinsulin processing by the subtilisin-related proprotein convertases furin, PC2, and PC3

Experiments using recombinant vaccinia viruses expressing rat proinsulin I coinfected into COS-7 cells with recombinant vaccinia virus expressing human furin, human PC2, mouse PC3 (subtilisin-related proprotein convertases 1-3, respectively), or yeast Kex2 indicate that in this system both Kex2 and furin produce mature insulin, whereas PC2 selectively cleaves proinsulin at the C-peptide-A-chain junction. This is a property consistent with its probable identity with the rat insulinoma granule type II proinsulin processing activity as described by Davidson et al. [Davidson, H. W., Rhodes, C. J. & Hutton, J. C. (1988) Nature (London) 333, 93-96]. PC3 generates mature insulin but cleaves preferentially at the proinsulin B-chain-C-peptide junction. This pattern of cleavage by PC3 is similar, but not identical, to that of the highly B-chain-C-peptide junction-selective type I activity as described by Davidson et al., perhaps due to the presence of a P4 arginine residue near the C-peptide-A-chain junction unique to the rat proinsulins. These results along with data presented on the expression of both PC2 and PC3 in islet beta cells strongly support the conclusion that these proteases are involved in the conversion of proinsulin to insulin in vivo.

Expression of human pituitary adenylate cyclase activating polypeptide (PACAP) cDNA in CHO cells and characterization of the products

cDNA encoding human PACAP precursor was expressed in non-neuroendocrine Chinese hamster ovary cells, CHO-K1, The cells were transfected with expression vector (pTS705) containing the human PACAP cDNA by electroporation. A cell line which produced more than 80 ng/ml of immunoreactive PACAP (ir-PACAP) into the conditioned medium was established. RP-HPLC analysis of culture medium of this established cell line exhibited the presence of two types of PACAP, i.e. PACAP38 and PACAP27. At the same time, it was also revealed that immunoreactive PACAP-related peptide (ir-PRP) was secreted into the cultured medium. The ir-PACAPs were confirmed to ahve biological activities such as induction of cAMP and neurite outgrowth in rat pheochromocytoma PC12h cells.

Investigation of a possible role of the amino-terminal pro-region of proopiomelanocortin in its processing and targeting to secretory granules

Proopiomelanocortin (POMC) is a polyprotein which is targeted to the regulated secretory pathway of neuroendocrine cells where it undergoes tissue-specific proteolysis to yield peptides such as adrenocorticotropic hormone, beta-lipotropin and beta-endorphin. The pro-region of POMC is 49 amino acid long with two disulfide bonds between cysteine residues 2 and 24 and 8 and 20. These cysteine residues are conserved across the species. The pro-region contains no known hormonal sequence. Sorting to the regulated secretory pathway is thought to involve targeting signals encoded in the structure of secretory proteins. In the present study, we have examined the possibility that the disulfide bridges located in the NH2-terminal portion of the pro-region of POMC are essential for maintaining a determinant involved in the sorting of POMC to the regulated secretory pathway. Using site-directed and deletion mutagenesis of the porcine POMC cDNA, we created mutants in which one or both disulfide bridges were disrupted or in which the first 26 amino acid residues of the pro-region were deleted. Recombinant retroviruses carrying the mutated POMC cDNAs were used to infect Neuro2A cells. Immunofluorescence and immunoelectron microscopy studies performed on infected cells revealed that the unmutated and mutated POMC-immunoreactive peptides were localized in dense-core vesicles at the tips of cellular extensions. Analysis of the POMC-immunoreactive peptides extracted from the infected Neuro2A cells indicated that the mutated precursors in which one disulfide bridge was disrupted (POMC-S2 or POMC-S8) were stored and processed as efficiently as the unmutated POMC. By contrast, the mutated precursor in which both disulfide bridges were disrupted (POMC-S2,8) did not accumulate in intracellular compartments to the same extent as unmutated POMC. Moreover, this mutant was very inefficiently processed and no release could be observed upon stimulation of the cells with K+/Ca2+. These results suggest that POMC-S2,8 entered the regulated secretory pathway less efficiently than the unmutated precursor. However, when both disulfide bridges were removed from the precursor from the precursor by deletion of the first 26 amino acid residues of POMC, the truncated precursor (POMC delta 1-26) behaved as the unmutated POMC. Taken together our results indicate that the NH2-terminal portion of the pro-region including both disulfide bridges can be deleted without affecting the targeting of the molecule to secretory granules. However, when the entire POMC sequence is expressed in Neuro2A cells, the proper folding of the NH2-terminal region might be important for efficient processing and targeting.

Targeting of frog prodermorphin to the regulated secretory pathway by fusion to proenkephalin

We have investigated the sorting and processing of the amphibian precursor prepro-dermorphin in mammalian cells. Dermorphin, a D-alanine-containing peptide with potent opioid activity, has been isolated from the skin of the frog Phyllomedusa sauvagei. The maturation of this peptide from the precursor involves several posttranslational steps. Recombinant vaccinia viruses were used to infect AtT-20, PC12, and HeLa cells to study the sorting and processing of prepro-dermorphin. While this precursor was not processed in any of the examined cell lines, AtT-20 cells were able to process approximately 40% of a chimeric precursor consisting of the first 241 amino acids of prepro-enkephalin fused to a carboxy-terminal part of pro-dermorphin. By immunogold-EM, we could show that the chimeric protein, but not pro-dermorphin, was sorted to dense-core secretion granules. The processing products could be released upon stimulation by 8-Br-cAMP. We conclude that the pro-enkephalin part of the fusion protein contains the information for targeting to the regulated pathway of secretion, while this sorting information is missing in pro-dermorphin. This indicates that sorting mechanisms may differ between amphibian and mammalian cells.

A recombinant vaccinia virus expressing human carcinoembryonic antigen (CEA)

Carcinoembryonic antigen (CEA) is a 180-kDa glycoprotein expressed on most gastrointestinal carcinomas. A 2.4-kb cDNA clone, containing the complete coding sequence, was isolated from a human colon tumor cell library and inserted into a vaccinia virus genome. This newly developed construct was characterized by Southern blotting, DNA hybridization studies, and polymerase chain reaction analysis. The CEA gene was stably integrated into the vaccinia virus thymidine kinase gene. The recombinant was efficiently replicated upon serial passages in cell cultures and in animals. The recombinant virus expresses on the surface of infected cells a protein product recognized by a monoclonal antibody (COL-I) directed against CEA. Immunization of mice with the vaccinia construct elicited a humoral immune response against CEA. Pilot studies also showed that administration of the recombinant CEA vaccinia construct was able to greatly reduce the growth in mice of a syngeneic murine colon adenocarcinoma which had been transduced with the human CEA gene. The use of this new recombinant CEA vaccinia construct may thus provide an approach in the specific active immunotherapy of human GI cancer and other CEA expressing carcinoma types.

Expression and sorting of rat plasma kallikrein in POMC-producing AtT-20 cells

A vaccinia virus (VV) vector was used to express rat plasma kallikrein (rPK) in the constitutively secreting cells, BSC-40, and in the endocrine regulated cells, AtT-20. Using a specific rPK antibody and a fluorogenic substrate, Phe-Phe-Arg-AMC, we demonstrated that in both cell lines VV infections resulted in the synthesis of an immunoreactive enzyme predominantly present as a zymogen which can be activated with trypsin. Stimulation of VV:rPK-infected AtT-20 cells with either 5mM 8-bromo-cAMP or 56 mM KCl resulted in a different pattern of rPK and ACTH secretion, strongly suggesting that rPK follows the constitutive secretory pathway. Finally, the 10% rPK activity found within AtT-20 cell extracts had no effect on pro-opiomelanocortin (POMC) processing either intracellularly or extracellularly. The above data show that the biosynthetic machinery of both cell lines analyzed does not allow the efficient activation of plasma prekallikrein. Finally, despite the PK's demonstrated ability to cleave various hormone precursors in vitro at pairs of basic residues, in vivo, we did not obtain evidence that this hepatic enzyme can also act as an intracellular pro-protein processing enzyme.

Peptide processing and targeting in the neuronal secretory pathway

The abdominal ganglion of the marine mollusk Aplysia contains a pair of identified neuronal clusters, the bag cells, which control egg laying by means of a number of unique regulatory mechanisms. Each neuron in the bag cell clusters synthesizes several peptides derived from a single prohormone and packages them into separate vesicles. These vesicles are then differentially localized in specific neuronal processes, thus segregating peptides destined for autocrine and hormonal release sites. Therefore in this system, protein trafficking through the secretory pathway organizes multiple peptide neurochemical messengers to efficiently regulate simple behaviors.

Human fur gene encodes a yeast KEX2-like endoprotease that cleaves pro-beta-NGF in vivo

Extracts from BSC-40 cells infected with vaccinia recombinants expressing either the yeast KEX2 prohormone endoprotease or a human structural homologue (fur gene product) contained an elevated level of a membrane-associated endoproteolytic activity that could cleave at pairs of basic amino acids (-LysArg- and -ArgArg-). The fur-directed activity (furin) shared many properties with Kex2p including activity at pH 7.3 and a requirement for calcium. By using antifurin antibodies, immunoblot analysis detected two furin translation products (90 and 96 kD), while immunofluorescence indicated localization to the Golgi apparatus. Coexpression of either Kex2p or furin with the mouse beta-nerve growth factor precursor (pro-beta-NGF) resulted in greatly enhanced conversion of the precursor to mature nerve growth factor. Thus, the sequence homology shared by furin and the yeast KEX2 prohormone processing enzyme is reflected by significant functional homology both in vitro and in vivo.

Proenkephalin products are stored in the sperm acrosome and may function in fertilization

Previous studies have shown that spermatogenic cells are a major source of testicular RNA encoding the opioid peptide precursor proenkephalin, suggesting that proenkephalin-derived peptides may function as intratesticular paracrine factors produced by male germ cells. However, direct evidence for the production of proenkephalin by spermatogenic cells has been lacking. In this report, we have used polysome profile analysis, peptide quantitation, and immunocytochemistry to show that proenkephalin products are synthesized during spermatogenesis and are retained within spermatozoa of humans, hamsters, rats, and sheep. We further show that these peptides are stored in the sperm acrosome and are depleted from sperm following the acrosome reaction, an exocytotic event required for fertilization. Proenkephalin products thus may serve a dual function as sperm acrosomal factors released during the fertilization process as well as intratesticular regulators secreted by spermatogenic cells.

Vaccinia virus vectors: new strategies for producing recombinant vaccines

The development and continued refinement of techniques for the efficient insertion and expression of heterologous DNA sequences from within the genomic context of infectious vaccinia virus recombinants are among the most promising current approaches towards effective immunoprophylaxis against a variety of protozoan, viral, and bacterial human pathogens. Because of its medical relevance, this area is the subject of intense research interest and has evolved rapidly during the past several years. This review (i) provides an updated overview of the technology that exists for assembling recombinant vaccinia virus strains, (ii) discusses the advantages and disadvantages of these approaches, (iii) outlines the areas of outgoing research directed towards overcoming the limitations of current techniques, and (iv) provides some insight (i.e., speculation) about probable future refinements in the use of vaccinia virus as a vector.

Partial processing of the neuropeptide Y precursor in transfected CHO cells

The activation of regulatory peptides by post-translational modification of their biosynthetic precursors is generally thought to occur only in neuroendocrine cells. We have selected clones of Chinese hamster ovary cells, a non-neuroendocrine cell line, which were transfected with a eukaryotic expression vector coding for the precursor for neuropeptide Y. Although the majority of the immunoreactive NPY was found in the form of pro-NPY, some degree of intracellular proteolytic processing of the precursor occurred in all clones. Part of the intracellular NPY immunoreactivity was even correctly amidated. Extracellular degradation of pro-NPY in the tissue culture medium generated immunoreactivity which corresponded in size to NPY. It is concluded that precursor processing can occur in non-neuroendocrine cells both as a biological process within the cells and as apparent processing, degradation in the tissue culture medium.

Expression of Drosophila Shaker potassium channels in mammalian cells infected with recombinant vaccinia virus

A recombinant vaccinia virus containing a Drosophila potassium channel (Shaker H4) cDNA was constructed by homologous recombination between wild-type vaccinia virus DNA and a transfer plasmid. The new virus was used to infect four types of mammalian cells in culture. Electrophysiological recording 24-72 hr after infection revealed the expression of voltage-gated transient potassium channels in all four cell types. The properties of the induced currents were identical to those previously observed following injection of the Shaker H4 transcript into oocytes. Vaccinia promises to be an effective vehicle for the heterologous expression of transmembrane ion channels in a variety of cell types.

Expression and Post-Translational Processing of Preprodynorphin in the Rat Anterior Pituitary Cell Line, GH4C1

Abstract A recombinant plasmid containing the rat preprodynorphin cDNA was introduced into the rat anterior pituitary cell line, GH4C1. These cells normally express growth hormone and prolactin but not prodynorphin. Stable transformants were isolated and analyzed for the expression and processing of prodynorphin. Chromatographic analyses demonstrated that the prodynorphin was incompletely processed in GH4C1 cells. Analyses of the peptides by specific radioimmunoassays to chemically synthesized peptides showed that the cells have the ability to process both at dibasic and monobasic cleavage sites. The release of prodynorphin-derived peptides paralleled that of prolactin upon stimulation with thyrotropin-releasing hormone, forskolin or carbachol suggesting that the prodynorphin-derived peptides and prolactin are sequestered in similar physiologically responsive compartments. These data suggest that the GH4C1 cells incompletely process prodynorphin. The processing in GH4C1 cells occurs both at monobasic and dibasic cleavage sites.

Production of pro-opiomelanocortin (POMC) by a vaccinia virus transient expression system and in vitro processing of the expressed prohormone by POMC-converting enzyme

Pro-opiomelanocortin (POMC) was expressed in CV-1 (green monkey kidney) cells using a vaccinia virus transient expression system [(1986) Proc. Natl. Acad. Sci. USA 83, 8122]. The system involved infection of cells with a recombinant vaccinia virus carrying the T7 RNA polymerase gene and transfection with a plasmid containing the mouse POMC sequence flanked by the T7 RNA polymerase promoter at its 5'-end and the T7 RNA polymerase terminator at its 3'-end. Assay of the medium from transfected cells showed that 1-2 micrograms of immunoreactive ACTH was produced/10(6) cells. Analysis of the same medium by SDS-PAGE/Western blots revealed a band of 30-36 kDa, which was immunostained with both ACTH and beta-endorphin antisera. Labeling the transfected cells with [3H]Arg, followed by immunoprecipitation and SDS-PAGE showed the synthesis of a major peak of POMC, 33 kDa. Purified [3H]POMC expressed by CV-1 cells was cleaved in vitro by bovine intermediate lobe secretory vesicle pro-opiomelanocortin-converting enzyme to ACTH intermediates (19-25 kDa), beta-lipotropin and beta-endorphin. Thus, this work has demonstrated a technique for expressing microgram quantities of prohormones in mammalian cells, suitable for use as substrates for prohormone-converting enzymes in vitro.

The propeptide of preprosomatostatin mediates intracellular transport and secretion of alpha-globin from mammalian cells

We have investigated the role of the somatostatin propeptide in mediating intracellular transport and sorting to the regulated secretory pathway. Using a retroviral expression vector, two fusion proteins were expressed in rat pituitary (GH3) cells: a control protein consisting of the beta-lactamase signal peptide fused to chimpanzee alpha-globin (142 amino acids); and a chimera of the somatostatin signal peptide and proregion (82 amino acids) fused to alpha-globin. Control globin was translocated into the endoplasmic reticulum as determined by accurate cleavage of its signal peptide; however, alpha-globin was not secreted but was rapidly and quantitatively degraded intracellularly with a t 1/2 of 4-5 min. Globin degradation was insensitive to chloroquine, a drug which inhibits lysosomal proteases, but was inhibited at 16 degrees C suggesting proteolysis occurred during transport to the cis-Golgi apparatus. In contrast to the control globin, approximately 30% of the somatostatin propeptide-globin fusion protein was transported to the distal elements of the Golgi apparatus where it was endoproteolytically processed. Processing of the chimera occurred in an acidic intracellular compartment since cleavage was inhibited by 25 microM chloroquine. 60% of the transported chimera was cleaved at the Arg-Lys processing site in native prosomatostatin yielding "mature" alpha-globin. Most significantly, approximately 50% of processed alpha-globin was sorted to the regulated pathway and secreted in response to 8-Br-cAMP. We conclude that the somatostatin propeptide mediated transport of alpha-globin from the endoplasmic reticulum to the trans-Golgi network by protecting molecules from degradation and in addition, facilitated packaging of alpha-globin into vesicles whose secretion was stimulated by cAMP.

Targeting and processing of pro-opiomelanocortin in neuronal cell lines

Pro-opiomelanocortin (POMC) is the precursor to several pituitary hormones including adrenocorticotropic hormone and beta-endorphin (beta-END). POMC is also expressed in the brain, predominantly in discrete neuronal cell populations of the hypothalamus. In the pituitary and brain, POMC undergoes tissue-specific proteolysis to release different bioactive peptides. POMC processing in neuronal cell lines was studied after infection of PC12 and Neuro2A cells with a recombinant retrovirus carrying the porcine POMC cDNA. Our results indicate that both cell lines synthesize and target POMC to the regulated secretory pathway. Only the Neuro2A cells, however, can achieve proteolytic processing of POMC. Chromatographic and immunological characterization of the POMC-related material showed that beta-lipotropin (beta-LPH) and nonacetylated beta-END(1-31) are major maturation products of POMC in these cells. Release of both beta-LPH and beta-END(1-31) from infected Neuro2A cells can be stimulated by secretagogues in a calcium-dependent manner. Taken together, our results suggest that the cellular machinery of Neuro2A cells can recognize a foreign prohormone, target it to neurosecretory vesicles, process it into biologically active peptides, and secrete it in a manner characteristic to peptidergic neurons.

Posttranslational processing of endogenous and of baculovirus-expressed human gastrin-releasing peptide precursor

The 27-amino-acid gastrin-releasing peptide (GRP1-27) is a neuropeptide and growth factor that is synthesized by various neural and neuroendocrine cells. The major pro-GRP hormone (isoform I) contains both GRP1-27 and a novel C-terminal extension peptide termed pro-GRP31-125. In order to define potentially active neuropeptides that could be generated from this novel protein domain, we analyzed the posttranslational processing of endogenous human pro-GRP1-125 in a small-cell lung cancer cell line. Because such studies are much easier in an overexpression system, we investigated at the same time the posttranslational processing of baculovirus-expressed human pro-GRP1-125 in an insect ovary cell line. In the small-cell lung cancer cell line, GRP1-27 was cleaved as expected from the endogenous prohormone at a pair of basic amino acids (29 and 30) and alpha-amidated at its C-terminal methionine; however, a number of novel peptides were generated by additional cleavages in the pro-GRP31-125 domain. In the insect ovary cell line, GRP1-27 was cleaved from the expressed prohormone by a different mechanism, as were a number of other peptides that appeared to be similar in size to those produced by the human neuroendocrine tumor cell line. These data show for the first time that an insect ovary cell line that is widely used to overexpress proteins can process a human neuropeptide precursor. They also reveal the existence of novel pro-GRP-derived peptides that are candidates for biologically active ligands.

The polyprotein nature of substance P precursors

Substance P and related tachykinin peptides probably act as neurotransmitters or modulators of neurotransmission, and regulate biological processes as diverse as salivary secretion and transmission of pain signals. Substance P peptide sequences are expressed in three distinct mRNAs that are generated from one gene by differential RNA splicing. In addition to substance P, as many as three other tachykinin peptides can be generated from the polyprotein precursors by differential posttranslational processing. Three tachykinin receptor subtypes have been extensively characterized which differentially interact with the naturally occurring tachykinin peptides. Therefore, the generation of diversity of tachykinin peptides results from differential precursor RNA splicing and differential posttranslational processing. The specificity of peptide responses is the result of selective receptor subtype expression.

Retrovirus-mediated expression of preprosomatostatin: posttranslational processing, intracellular storage, and secretion in GH3 pituitary cells

Somatostatin (SRIF) is a 14-amino acid peptide hormone that is synthesized as part of a larger precursor, preproSRIF, consisting of a signal peptide and a proregion of 80-90 amino acids. The mature hormone, which is located at the carboxyl terminus of the precursor, is preceded by a single pair of basic amino acids. We are studying preproSRIF to investigate intracellular sorting, proteolytic processing, and storage of peptide hormone precursors in the secretory pathway. We used a retroviral expression vector to achieve the high levels of precursor synthesis which are necessary for detailed characterization of processing intermediates and mature somatostatin. Recombinant retroviruses containing RNA transcripts encoding anglerfish preproSRIF I were used to infect rat pituitary GH3 cells which secrete growth hormone and prolactin, neither of which are substrates for endoproteolytic cleavage. In these cells preproSRIF was accurately processed to the mature hormone with an efficiency of approximately 75%. Of the newly synthesized mature SRIF, 55% was sorted into the regulated secretory pathway and released in response to the secretagogue 8-Br-cAMP. The remaining 45% of mature SRIF and residual unprocessed precursor was rapidly secreted. In contrast to SRIF, only 5% of newly synthesized endogenous growth hormone was stored intracellularly, whereas 95% was sorted to the constitutive pathway and secreted rapidly with kinetics identical to proSRIF. Our results show that proSRIF processing is not necessarily dependent on a specific protease found only in SRIF-producing cells and suggest that proteolytic cleavage is not restricted to cells that process endogenous hormones. Moreover, these results demonstrate that GH3 cells have the capacity to discriminate between endogenous and foreign hormones and target the foreign molecule significantly more efficiently to the regulated secretory pathway.

Processing and secretion of nerve growth factor: expression in mammalian cells with a vaccinia virus vector

To study posttranslational mechanisms for the control of nerve growth factor (NGF), we used a recombinant vaccinia virus vector to independently express the two major NGF transcripts in a variety of mammalian cell lines. The two major transcripts contain NGF (12.5 kilodaltons [kDa]) at the C-terminus and differ by alternative splicing of an N-terminal exon, so that the large precursor (34 kDa) had 67 amino acids upstream of an internal signal peptide and the smaller precursor (27 kDa) had this signal peptide at its N-terminus. In L929 cells, expression of either NGF transcript with the vaccinia virus vector gave rise to an apparently identical intracellular 35-kDa glycosylated precursor formed by cleavage of the primary gene product after the signal peptide. These cells also secreted biologically active NGF. To determine whether NGF processing is restricted by cell type, we infected a variety of mammalian cell lines with both recombinant viruses; all accumulated the same 35-kDa precursor and secreted NGF. Thus, many types of cells have the machinery to process and secrete NGF. However, NGF accumulated intracellularly (presumably in secretory granules) in cells with a regulated pathway of secretion (e.g., AtT-20 and HIT cells). In these cells, a membrane-permeable cyclic AMP analog, 8-bromo-cyclic AMP, stimulated NGF secretion. This suggests a mechanism for the regulation of NGF levels in which specific secretagogues, e.g., neurotransmitters, control NGF secretion.