Proprotein and prohormone convertases: a family of subtilases generating diverse bioactive polypeptides

Novel insights into cadherin processing by subtilisin-like convertases

Proprotein convertases (PCs) are known to activate many important molecules and their overexpression plays a significant role in tumor progression. Only little is known about the involvement of PCs in the processing of cadherin adhesion molecules, which are potent tumor suppressors. Here we show in a baculovirus overexpression system that the desmosomal cadherins Dsg1 and Dsg3 are substrates for the PC furin. Accordingly, inhibition of PCs in differentiating mouse keratinocytes by alpha 1-anti-trypsin Portland (alpha 1-PDX) negatively interfered with pro-epithelial (proE)-cadherin processing, but unexpectedly also resulted in a dramatic reduction of E-cadherin, Dsg1 and Dsg3 protein and Dsg1 mRNA. Because loss of intercellular adhesion is a rate-limiting step in the transition from benign to malignant tumors, these results have significant implications for the use of PC inhibitors as possible therapeutic tools.

The secretory proprotein convertase neural apoptosis-regulated convertase 1 (NARC-1): liver regeneration and neuronal differentiation

Seven secretory mammalian kexin-like subtilases have been identified that cleave a variety of precursor proteins at monobasic and dibasic residues. The recently characterized pyrolysin-like subtilase SKI-1 cleaves proproteins at nonbasic residues. In this work we describe the properties of a proteinase K-like subtilase, neural apoptosis-regulated convertase 1 (NARC-1), representing the ninth member of the secretory subtilase family. Biosynthetic and microsequencing analyses of WT and mutant enzyme revealed that human and mouse pro-NARC-1 are autocatalytically and intramolecularly processed into NARC-1 at the (Y,I)VV(V,L)(L,M) downward arrow motif, a site that is representative of its enzymic specificity. In vitro peptide processing studies andor Ala substitutions of the P1-P5 sites suggested that hydrophobicaliphatic residues are more critical at P1, P3, and P5 than at P2 or P4. NARC-1 expression is highest in neuroepithelioma SK-N-MCIXC, hepatic BRL-3A, and in colon carcinoma LoVo-C5 cell lines. In situ hybridization and Northern blot analyses of NARC-1 expression during development in the adult and after partial hepatectomy revealed that it is expressed in cells that have the capacity to proliferate and differentiate. These include hepatocytes, kidney mesenchymal cells, intestinal ileum, and colon epithelia as well as embryonic brain telencephalon neurons. Accordingly, transfection of NARC-1 in primary cultures of embryonic day 13.5 telencephalon cells led to enhanced recruitment of undifferentiated neural progenitor cells into the neuronal lineage, suggesting that NARC-1 is implicated in the differentiation of cortical neurons.

Expression, purification, and PC1-mediated processing of (H10D, P28K, and K29P)-human proinsulin

Our previous methods for the generation of recombinant human proinsulin were inadequate in terms of reproducibility and yield. In addition, it was difficult to perform structure/function studies on proinsulin because of its tendency to form hexamers. We have developed an improved procedure, which overcomes many of the technical purification problems, and results in a potentially monomeric version of modified proinsulin. Inclusion bodies were prepared using a commercial bacterial lysis solution. The inclusion bodies were solubilized and the fusion protein's affinity tag was removed by chemical cleavage. The polypeptide was then reduced and transferred into a refolding buffer. Following an overnight incubation, only a single form of proinsulin was detected using analytical reversed-phase high-performance liquid chromatography. The refolded (H10D, P28K, and K29P)-human proinsulin (DKP-hPI) was subjected to a final purification step using reversed-phase chromatography. The method is reproducible and produces milligram quantities of purified DKP-hPI from a single liter of bacterial culture. The final product is greater than 95% pure and is suitable for use as a substrate for the propeptide convertase PC1.

Structure-function analysis of the prosegment of the proprotein convertase PC5A

To investigate if some residues within the prosegment of PC5A are important for its optimal proteolytic function, various PC5A mutants were cellularly expressed, and their processing activities were compared using pro-vascular endothelial growth factor C (pro-VEGF-C) as a substrate. Although wild type PC5A almost completely processes pro-VEGF-C, a prosegment deletion as well as both P1 mutants of the primary (R116A) and secondary (R84A) autocatalytic cleavage sites are inactive. The in vitro inhibitory potency of various decapeptides mimicking the C-terminal sequence of PC5 prosegment (pPC5) revealed that the native (107)QQVVKKRTKR(116) peptide is a nanomolar inhibitor, whereas its P6 mutant K111H is more selective toward PC5A than Furin. In vitro activity assays using the bacterially expressed pPC5 and its mutants revealed them to be very potent nanomolar inhibitors (IC(50)) and only approximately 6-fold more selective inhibitors of PC5A versus Furin. Expression of the preprosegment of PC5 (ppPC5) and its mutants in Chinese hamster ovary FD11 cells overexpressing pro-VEGF-C with either PC5A or Furin showed them to be as good inhibitors of PC5A as the serpin alpha1-antitrypsin Portland (alpha1-PDX), ppFurin, or ppPACE4 but less potent toward overexpressed Furin. In conclusion, cleavages of the prosegment of PC5A at both Arg(116) and Arg(84) are required for PC5A cellular activity, and ppPC5 is a very potent but modestly selective cellular inhibitor of PC5A.

Inactivation of Kex2p diminishes the virulence of Candida albicans

Deletion of the kexin gene (KEX2) in Candida albicans has a pleiotropic effect on phenotype and virulence due partly to a defect in the expression of two major virulence factors: the secretion of active aspartyl proteinases and the formation of hyphae. kex2/kex2 mutants are highly attenuated in a mouse systemic infection model and persist within cultured macrophages for at least 24 h without causing damage. Pathology is modest, with little disruption of kidney matrix. The infecting mutant cells are largely confined to glomeruli, and are aberrant in morphology. The complex phenotype of the deletion mutants reflects a role for kexin in a wide range of cellular processes. Taking advantage of the specificity of Kex2p cleavage, an algorithm we developed to scan the 9168 open reading frames in Assembly 6 of the C. albicans genome identified 147 potential substrates of Kex2p. These include all previously identified substrates, including eight secreted aspartyl proteinases, the exoglucanase Xog1p, the immunodominant antigen Mp65, and the adhesin Hwp1p. Other putative Kex2p substrates identified include several adhesins, cell wall proteins, and hydrolases previously not implicated in pathogenesis. Kexins also process fungal mating pheromones; a modification of the algorithm identified a putative mating pheromone with structural similarities to Saccharomyces cerevisiae alpha-factor.

Expression and distribution of pituitary adenylate cyclase-activating peptide in human prostate and prostate cancer tissues

The presence, expression and distribution of pituitary adenylate cyclase-activating peptide (PACAP) in human prostate cancer and healthy tissue were investigated by means of biochemical and morphological procedures. Reverse transcription-polymerase chain reaction (RT-PCR) analysis demonstrated the presence of its precursor encoding mRNA in both normal and pathological conditions (amplification products with 577 or 226 bp were identified). Immunochemistry using an appropriate antibody served to detect in both classes of tissues a 19.9-kDa product corresponding to the PACAP preproprotein and another protein of 14.6 kDa that may represent a product partially processed by convertases. However, a 5-kDa band characteristic of PACAP-38 peptide was not observed. Immunohistochemistry on tissue sections indicated the location of PACAP in the epithelial layer of prostate glands (and in some scarce leucocytes) but not in the stroma, either in normal or carcinomatous tissues. No clear differences could be established when comparing samples from patients with different tumor Gleason grades. These results are the first demonstration of the localization of PACAP or its precursors and its mRNA in the human prostate gland and their presence during the progression of prostate carcinoma.

Proteolytic processing of chromogranins is modified in brains of transgenic mice

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

Furin at the cutting edge: from protein traffic to embryogenesis and disease

Furin catalyses a simple biochemical reaction--the proteolytic maturation of proprotein substrates in the secretory pathway. But the simplicity of this reaction belies furin's broad and important roles in homeostasis, as well as in diseases ranging from Alzheimer's disease and cancer to anthrax and Ebola fever. This review summarizes various features of furin--its structural and enzymatic properties, intracellular localization, trafficking, substrates, and roles in vivo.

Deficiencies in pro-thyrotropin-releasing hormone processing and abnormalities in thermoregulation in Cpefat/fat mice

Cpe(fat/fat) mice are obese, diabetic, and infertile. They have a mutation in carboxypeptidase E (CPE), an enzyme that converts prohormone intermediates to bioactive peptides. The Cpe(fat) mutation leads to rapid degradation of the enzyme. To test whether pro-thyrotropin-releasing hormone (TRH) conversion to TRH involves CPE, processing was examined in the Cpe(fat/fat) mouse. Hypothalamic TRH is depressed by at least 75% compared with wild-type controls. Concentrations of pro-TRH forms are increased in homozygotes. TRH-[Gly(4)-Lys(5)-Arg(6)] and TRH-[Gly(4)-Lys(5)] represent approximately 45% of the total TRH-like immunoreactivity in Cpe(fat/fat) mice; they constitute approximately 1% in controls. Levels of TRH-[Gly(4)] were depressed in homozygotes. Because the hypothalamus contains some TRH, another carboxypeptidase must be responsible for processing. Immunocytochemical studies indicate that TRH neurons contain CPE- and carboxypeptidase D-like immunoreactivity. Recombinant CPE or carboxypeptidase D can convert synthetic TRH-[Gly(4)-Lys(5)] and TRH-[Gly(4)-Lys(5)-Arg(6)] to TRH-[Gly(4)]. When Cpe(fat/fat) mice are exposed to cold, they cannot maintain their body temperatures, and this loss is associated with hypothalamic TRH depletion and reduction in thyroid hormone. These findings demonstrate that the Cpe(fat) mutation can affect not only carboxypeptidase activity but also endoproteolysis. Because Cpe(fat/fat) mice cannot sustain a cold challenge, and because alterations in the hypothalamic-pituitary-thyroid axis can affect metabolism, deficits in pro-TRH processing may contribute to the obese and diabetic phenotype in these mice.

Identification and characterization of the feeding circuit-activating peptides, a novel neuropeptide family of aplysia

We use a multidisciplinary approach to identify, map, and characterize the bioactivity of modulatory neuropeptides in the circuitry that generates feeding behavior in Aplysia. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry of the cerebral-buccal connective (CBC), a nerve containing axons of many interneurons that control feeding behavior of Aplysia, was used to identify neuropeptides that may participate in generation and shaping of feeding motor programs. Using this functionally oriented search, we identified a novel family of peptides that we call the feeding circuit-activating peptides (FCAPs). Two peptides with masses identical to those observed in the CBCs (molecular weight 1387 and 1433) were purified from buccal ganglia and partially sequenced using mass spectrometry. The amino acid sequence was then used to clone the FCAP precursor, which encodes multiple copies of eight different FCAPs. The two FCAPs present in highest copy number correspond to those observed in the CBC. The distribution of FCAP expression was mapped using Northern analysis, whole-mount in situ hybridization, and immunocytochemistry. Consistent with our initial findings, FCAP-immunopositive axons were observed in the CBC. Furthermore, we found that FCAP was present in some cerebral-buccal and buccal-cerebral interneurons. As their name suggests, FCAPs are capable of initiating rhythmic feeding motor programs and are the first neuropeptides with such activity in this circuit. The actions of FCAPs suggest that these peptides may contribute to the induction and maintenance of food-induced arousal. FCAPs were also localized to several other neuronal systems, suggesting that FCAPs may play a role in the regulation of multiple behaviors.

Human kallikrein 10 expression in normal tissues by immunohistochemistry

The normal epithelial cell-specific 1 (NES1) gene (official name kallikrein gene 10, KLK10) was recently cloned and encodes for a putative secreted serine protease (human kallikrein 10, hK10). Several studies have confirmed that hK10 shares many similarities with the other kallikrein members at the DNA, mRNA, and protein levels. The enzyme was found in biological fluids, tissue extracts, and serum. Here we report the first detailed immunohistochemical (IHC) localization of hK10 in normal human tissues. We used the streptavidin-biotin method with two hK10-specific antibodies, a polyclonal rabbit and a monoclonal mouse antibody, developed in house. We analyzed 184 paraffin blocks from archival, current, and autopsy material, prepared from almost every normal human tissue. The staining pattern, the distribution of the immunostaining, and its intensity were studied in detail. Previously, we reported the expression of another novel human kallikrein, hK6, by using similar techniques. The IHC expression of hK10 was generally cytoplasmic and not organ-specific. A variety of normal human tissues expressed the protein. Glandular epithelia constituted the main immunoexpression sites, with representative organs being the breast, prostate, kidney, epididymis, endometrium, fallopian tubes, gastrointestinal tract, bronchus, salivary glands, bile ducts, and gallbladder. The choroid plexus epithelium, the peripheral nerves, and some neuroendocrine organs (including the islets of Langerhans, cells of the adenohypophysis, the adrenal medulla, and Leydig cells) expressed the protein strongly and diffusely. The spermatic epithelium of the testis expressed the protein moderately. A characteristic immunostaining was observed in Hassall's corpuscles of the thymus, oxyphilic cells of the thyroid and parathyroid glands, and chondrocytes. Comparing these results with those of hK6, we observed that both kallikreins had a similar IHC expression pattern.

Severe block in processing of proinsulin to insulin accompanied by elevation of des-64,65 proinsulin intermediates in islets of mice lacking prohormone convertase 1/3

The neuroendocrine processing endoproteases PC2 and PC1/3 are expressed in the beta cells of the islets of Langerhans and participate in the processing of proinsulin to insulin and C-peptide. We have previously shown that disruption of PC2 (SPC2) expression significantly impairs proinsulin processing. Here we report that disruption of the expression of PC1/3 (SPC3) produces a much more severe block in proinsulin conversion. In nulls, pancreatic and circulating proinsulin-like components comprise 87% and 91%, respectively, of total insulin-related immunoreactivity. Heterozygotes also show a more than 2-fold elevation in proinsulin levels to approximately 12%. Immunocytochemical and ultrastructural studies of the beta cells reveal the nearly complete absence of mature insulin immunoreactivity and its replacement by that of proinsulin in abundant immature-appearing secretory granules. In contrast, alpha cell morphology and glucagon processing are normal, and there is also no defect in somatostatin-14 generation. Pulse-chase labeling studies confirm the existence of a major block in proinsulin processing in PC1/3 nulls with prolongation of half-times of conversion by 7- and 10-fold for proinsulins I and II, respectively. Lack of PC1/3 also results in increased levels of des-64,65 proinsulin intermediates generated by PC2, in contrast to PC2 nulls, in which des- 31,32 proinsulin intermediates predominate. These results confirm that PC1/3 plays a major role in processing proinsulin, but that its coordinated action with PC2 is necessary for the most efficient and complete processing of this prohormone.

Altered processing of the neurotensin/neuromedin N precursor in PC2 knock down mice: a biochemical and immunohistochemical study

Neurotensin (NT) and neuromedin N (NN) are generated by endoproteolytic cleavage of a common precursor molecule, pro-NT/NN. To gain insight into the role of prohormone convertases PC1, PC2, and PC7 in this process, we investigated the maturation of pro-NT/NN in the brain of PC7 (PC7-/-), PC2 (PC2-/-), and/or PC1 (PC1+/- and PC2-/-; PC1+/-) knock down mice. Inactivation of the PC7 gene was without effect, suggesting that this convertase is not involved in the processing of pro-NT/NN. By contrast, there was a 15% decrease in NT and a 50% decrease in NN levels, as measured by radioimmunoassay, in whole brain extracts from PC2 null as compared with wild type mice. Using immunohistochemistry, we found that this decrease in pro-NT/NN maturation products was uneven and that it was most pronounced in the medial preoptic area, lateral hypothalamus, and paraventricular hypothalamic nuclei. These results suggest that PC2 plays a critical role in the processing of pro-NT/NN in mouse brain and that its deficiency may be compensated to a regionally variable extent by other convertases. Previous data have suggested that PC1 might be subserving this role. However, there was no change in the maturation of pro-NT/NN in the brain of mice in which the PC1 gene had been partially inactivated, implying that complete PC1 knock down may be required for loss of function.

Processing of proendothelin-1 by members of the subtilisin-like pro-protein convertase family

Endothelial cells (ECs) secrete numerous bioactive peptides that are initially synthesized as inactive precursor proteins. One of these, proendothelin-1 (proET-1), undergoes proteolysis at specific pairs of basic amino acids. Here, we wished to examine the role of mammalian convertases in this event. Northern blot analysis shows that only furin and PC7 are expressed in ECs. In vitro cleavage of proET-1 by furin or PC7 demonstrated that both enzymes efficiently and specifically process proET-1. These data reveal that furin and PC7 have similar specificities towards proET-1 and suggest that both enzymes may participate in the maturation of proET-1 in ECs.

Analyzing the evolution of the opioid/orphanin gene family

Advances in molecular biology have made it possible to rapidly obtain the amino acid sequence of neuropeptide precursors-either by cloning and sequencing the cDNA that encodes the precursor, or by reconstructing the arrangement of exons and introns in a neuropeptide-coding gene through genomic approaches. The databases generated from these molecular approaches have been used to design probes to identify the cells that express the gene, or to ascertain the rate of expression of the gene, and even to predict the post-translational modifications that can generate functional neuropeptides from a biologically inert precursor. Although the power of these approaches is substantial, it is appreciated that a gene sequence or an mRNA sequence reflects the potential products that may be assembled in a secretory cell. To understand the functional capabilities of the secretory cell, the molecular genetics approaches must be combined with procedures that actually characterize the end-products generated by the secretory cell. Recent advances in two-dimensional gel electrophoresis and mass spectrometry now make it possible to analyze neuropeptides from a relatively small amount of tissue. These procedures can reveal novel end-products, tissue-specific endoproteolytic cleavage events, and developmental shifts in post-translational processing schemes. A gene family that illustrates all of these processes and the advantages of combining genomics with proteomics is the opioid/orphanin gene family.

Imaging secretory vesicles by fluorescent protein insertion in propeptide rather than mature secreted peptide

We combined confocal and live-cell imaging with a novel molecular strategy aimed at revealing mechanisms underlying glucose-regulated insulin vesicle secretion. The 'Ins-C-GFP' reporter monitors secretory peptide targeting, trafficking, and exocytosis without directly tagging the mature secreted peptide. We trapped a green fluorescent protein (GFP) reporter in equimolar quantity within the secretory vesicle by fusing it within the C peptide of proinsulin which only after nascent vesicle sealing and acidification is cleaved from the mature secreted A and B chains of insulin. Ins-C-GFP expression in mouse islets without fail exhibited punctate distribution of green fluorescence by confocal microscopy. Ins-C-GFP colocalized GFP with insulin at vesicle dense cores by immuno-electron microscopy. Glucose stimulation decreased vesicle fluorescence coordinately with enhanced secretion from islets of C-GFP detected by anti-GFP Western blots, and of insulin detected by anti-insulin radioimmunoassay. An insulin secretagogue with a red fluorescent label, glibenclamide BODIPY TR, was applied to islets expressing Ins-C-GFP. The stimulus response was imaged as a rise in red secretagogue leading to marked loss in green granules. Since neuropeptides as well as peptide hormones are processed from propeptides after sealing of secretory granules, vesicle trapping likely is widely applicable for studies on targeting, trafficking, and regulated release of secretory peptides.

Human proprotein convertase 2 homologue from a plant nematode: cloning, characterization, and comparison with other species

Proprotein convertases (PCs) are evolutionarily conserved enzymes responsible for processing the precursors of many bioactive peptides in mammals. The invertebrate homologues of PC2 play important roles during development that makes the enzyme a good target for practical applications in pest management. Screening of a plant nematode Heterodera glycines cDNA library resulted in isolation of a full-length clone encoding a PC2-like precursor. The deduced protein (74.2 kD) exhibits strong amino acid homology to all known PC2s, including human, and shares the main structural characteristics: signal peptide; prosegment; catalytic domain, with D/H/S catalytic triad, PC2-specific residues, and 7B2 binding sites; P domain (with RRGDT pentapeptide); and carboxyl terminus. Comparative analysis of PC2s from 15 species discloses the presence of an insert in the catalytic domain unique to nematodes. Expression of PC2-like mRNA found in eggs and juveniles was undetectable in adult stages of H. glycines. Nucleotide analysis reveals distinctive differences in base composition and codon usage between H. glycines and Caenorhabditis elegans PC2s. The H. glycines cDNA clone encoding PC2 is the first one isolated from plant-parasitic nematodes.

Characterization of a repressor element in the promoter region of proprotein convertase 2 (PC2) gene

The proprotein convertase PC2 is primarily expressed in neuroendocrine cells where it mediates the proteolytic maturation of prohormones and proneuropeptides. We have identified in the upstream sequence of its gene a conserved domain partially homologous to the repressor element RE1/NRSE found in several genes for neuronal proteins. RE1/NRSE binds the silencing transcription factor REST/NRSF, a nuclear protein primarily found in nonneuronal cells. To determine the functionality of the PC2 gene RE1-like sequence (RE1-lk), we examined by electrophoretic mobility shift assays its ability to attach nuclear factors from PC2-expressing and nonexpressing cells. Specific binding factors were mostly detectable in PC2-non-expressing cells. These factors differ from REST/NRSF, as molar excess of competing RE1/NRSE could not prevent their binding to RE1-lk. Reciprocally, molar excess of RE1-lk could not prevent the binding of RE1/NRSE to the DNA-binding domain of a recombinant REST/NRSF. The presence of RE1-lk in cis reduced the ability of the PC2 promoter and the heterologous phosphoglycerate kinase promoter to drive expression of a green fluorescent protein reporter gene in transiently transfected PC2-nonexpressing cells, but not in PC2-expressing cells. These observations suggest that binding of transcription-silencing factors to the RE1-lk element may contribute to repression of the PC2 gene in nonneuroendocrine cells.

Cloning and characterization of feline brain natriuretic peptide

Brain (B-type) natriuretic peptide (BNP) is a cardiac hormone involved in regulation of fluid balance and blood pressure homeostasis of mammalian species. BNP sequence is species-specific and considered to be a significant prognostic and diagnostic marker for cardiac dysfunction. Using conventional polymerase chain reaction and amplification of cDNA 3'- and 5'-ends, a total of 1500 nucleotides encompassing the entire feline BNP gene were characterized. The feline BNP gene is organized in three exons separated by two introns. The complete transcript of 736 nucleotides was characterized, including 396 nucleotides encoding feline preproBNP. The preproBNP consisted of a signal peptide of 26 amino acids and a proBNP of 106 residues. The predicted mature BNP comprised 35 amino acids with likely 26- and 29-aa isomers, including a histidine residue at the C-terminus. Based on the similarity of BNP prepropeptide sequences, a phylogenetic relationship is presented for mammalian species including human, cat, cattle, dog, mouse, rat, sheep and swine.

Proprotein convertases in tumor progression and malignancy: novel targets in cancer therapy

The mammalian subtilisin/kexin-like proprotein convertase (PC) family has been implicated in the activation of a wide spectrum of proteins. These proteins are usually synthesized as inactive precursors before their conversion to fully mature bioactive forms. A large majority of these active proteins such as matrix metalloproteases, growth factors, and adhesion molecules are crucial in the processes of cellular transformation, acquisition of the tumorigenic phenotype, and metastases formation. Inhibition of PCs significantly affects the malignant phenotype of various tumor cells. In addition to direct tumor cell proliferation and migration blockade, PC inhibitors can also be used to target tumor angiogenesis. In this Review article we discuss a number of recent findings on the clinical relevance of PCs in cancer patients, their implication in the regulation of multiple cellular functions that impact on the invasive/metastatic potential of cancer cells. Thus, PC inhibitors may constitute new promising agents for the treatment of multiple tumors and/or in adjuvant therapy to prevent recurrence.

Processing of pro-atrial natriuretic peptide by corin in cardiac myocytes

Corin is a type II transmembrane serine protease abundantly expressed in the heart. In a previous study using transfected 293 cells, we showed that corin converted pro-atrial natriuretic peptide (pro-ANP) to atrial natriuretic peptide (ANP), suggesting that corin is likely the pro-ANP convertase. Because other serine proteases such as thrombin and kallikrein had previously also been shown to cleave pro-ANP in vitro, it remained to demonstrate that corin is indeed the endogenous pro-ANP convertase in cardiomyocytes. In this study, we examined pro-ANP processing in a murine cardiac muscle cell line, HL-5. Northern analysis showed that corin mRNA was present in HL-5 cells. In HL-5 cells transfected with a plasmid expressing pro-ANP, recombinant pro-ANP was converted to mature ANP as determined by Western analysis, indicating the presence of the endogenous pro-ANP convertase in these cells. The processed recombinant ANP was shown to be active in an enzyme-linked immunosorbent assay-based cGMP assay in baby hamster kidney cells. The processing of recombinant pro-ANP in HL-5 cells was highly sequence-specific, because mutation R98A, but not mutations R101A and R102A, in pro-ANP prevented the conversion of pro-ANP to ANP. Expression of recombinant wild-type corin enhanced the processing of pro-ANP in HL-5 cells. In contrast, overexpression of active site mutant corin S985A or transfection of oligonucleotide small interfering RNA duplexes directed against the mouse corin gene completely inhibited the processing of recombinant pro-ANP in HL-5 cells. These results indicate that corin is the physiological pro-ANP convertase in cardiac myocytes.

Isolation and characterization of VGF peptides in rat brain. Role of PC1/3 and PC2 in the maturation of VGF precursor

The neurotrophin responsive gene vgf is widely expressed in central and peripheral neurones, and in certain neuroendocrine cell populations. Its encoded VGF precursor protein (proVGF1: 617 amino acids in rat, 615 in man, > 85% homology) gives rise to several low molecular weight species. We studied a range of neuroendocrine and neuronal cells, in which VGF-processing products were prominent with an apparent molecular weight of 20 and 10 kDa (VGF20 and VGF10, respectively). Such peptides were recognized by antibodies specific for the C-terminal rat VGF nonapeptide, thus indicating that they included the C-terminus of proVGF. Ectopic expression of the neuroendocrine-specific prohormone convertases PC1/3 or PC2 in GH3 cells showed that both could generate VGF20, while VGF10 was preferentially produced by PC1/3. Site-directed mutagenesis was used to identify the KRKRKK(488) motif as the target within VGF sequence which leads to the production of VGF20. Molecular characterization of rat VGF10, on the other hand, revealed that this peptide is produced by cleavage at the RPR(555) site. By the combined use of high-resolution separation techniques, matrix-assisted laser desorption/ionization time of flight (MALDI-ToF) mass spectrometry and manual Edman degradation we identified in rat brain a VGF fragment analogous to bovine peptide V and two novel peptides also derived from the C-terminal region of proVGF.

New insights in Adipokinetic Hormone (AKH) precursor processing in Locusta migratoria obtained by capillary liquid chromatography-tandem mass spectrometry

After translation, the AKH I and AKH II precursors form three dimeric constructs prior to further processing into the respective AKHs and three dimeric Adipokinetic Hormone Precursor Related Peptides or APRPs (two homodimers and one heterodimer). By capillary liquid chromatography-tandem mass spectrometry we demonstrate that the APRPs in Locusta migratoria are further processed to form two smaller neuropeptides: DAADFADPYSFL (residue 36 to 47 of the AKH I precursor) and YADPNADPMAFL (residue 34 to 45 of the AKH II precursor). The peptides are designated as Adipokinetic Hormone Joining Peptide 1 (AKH-JP I) and 2 (AKH-JP II) respectively. Within the AKH I and AKH II precursor molecules, the classic KK and RR processing sites separate the AKH-JPs from the AKH I and II respectively. At the carboxyterminus, both AKH-JP I and II are flanked by Tyr-Arg, a cleaving site not described before. Such an unusual cleavage site suggests the presence, in the corpora cardiaca, of specific convertases. The AKH-JP-II does not stimulate lipid release from the fat body nor does it stimulate glycogen phosphorylase activity, both key functions of AKH.

Enhanced endotoxin sensitivity in fps/fes-null mice with minimal defects in hematopoietic homeostasis

The fps/fes proto-oncogene encodes a cytoplasmic protein tyrosine kinase implicated in growth factor and cytokine receptor signaling and thought to be essential for the survival and terminal differentiation of myeloid progenitors. Fps/Fes-null mice were healthy and fertile, displayed slightly reduced numbers of bone marrow myeloid progenitors and circulating mature myeloid cells, and were more sensitive to lipopolysaccharide (LPS). These phenotypes were rescued using a fps/fes transgene. This confirmed that Fps/Fes is involved in, but not required for, myelopoiesis and that it plays a role in regulating the innate immune response. Bone marrow-derived Fps/Fes-null macrophages showed no defects in granulocyte-macrophage colony-stimulating factor-, interleukin 6 (IL-6)-, or IL-3-induced activation of signal transducer and activator of transcription 3 (Stat3) and Stat5A or LPS-induced degradation of I kappa B or activation of p38, Jnk, Erk, or Akt.

Biosynthesis and cellular trafficking of the convertase SKI-1/S1P: ectodomain shedding requires SKI-1 activity

Subtilisin kexin isozyme-1 (SKI-1)/site 1 protease is a mammalian subtilase composed of distinct functional domains. Among the major substrates of SKI-1 are the sterol regulatory element-binding proteins, regulating cholesterol and fatty acid homeostasis. Other substrates include the stress response factor activating transcription factor-6, the brain-derived neurotrophic factor, and the surface glycoproteins of highly infectious viruses belonging to the family of Arenaviridae. Domain deletion and/or point mutants were used to gauge the role of the various domains of SKI-1. Biosynthesis, cellular trafficking, and sterol regulatory element-binding protein-2 cleavage activity were used as diagnostic tools. Results revealed that Arg(130) and Arg(134) are critical for the autocatalytic primary processing of the prosegment and for the subsequent efficient exit of SKI-1 from the endoplasmic reticulum. Functional mapping of the growth factor cytokine receptor motif suggested a folding role within the endoplasmic reticulum. Microsequencing of the remaining membrane-bound stub following ectodomain shedding of SKI-1 localized the shedding site to KHQKLL(953) downward arrow. Site-directed mutagenesis, in vitro cleavage of a synthetic peptide containing the shedding site, and inhibitor studies favor an autocatalytic event occurring at a non-canonical SKI-1 recognition sequence, with P2 and P1 Leu being very critical. In conclusion, multiple domains ensuring optimal functional characteristics control SKI-1 activity and cellular trafficking.

Proprotein convertases are important mediators of the adipocyte differentiation of mouse 3T3-L1 cells

Mouse 3T3-L1 cells are widely used to study adipocyte differentiation in vitro. When treated with insulin, dexamethasone and isobutylmethylxanthine these fibroblastic cells differentiate into round triglyceride-rich adipocytes. Because several proteins implicated in adipocyte differentiation(e.g. type 1 IGF receptors) are proteolytically activated by endoproteinases of the proprotein convertase family, we sought to determine whether these endoproteinases are crucial for adipose conversion. In this study, we show that expression of the proprotein convertases PACE4, PC7 and furin increases when 3T3-L1 cells are induced to differentiate into adipocytes. The differentiation was blocked in transfected cells expressingα1-antitrypsin Portland or in normal cells pre-treated with the synthetic inhibitor decanoyl-RVKR-chloromethylketone. Both inhibitors are known to specifically inactivate proprotein convertases. The block was associated with impaired proteolytic activation of proIGF-1 receptor, absence of induction of the adipogenic transcriptional factor PPARγ and marked reduction of the nuclear translocation of the C/EBPβ factor. Taken together, these data constitute evidence that proprotein convertases are crucial mediators of adipogenesis.

A rapid fluorometric assay for the proteolytic activity of SKI-1/S1P based on the surface glycoprotein of the hemorrhagic fever Lassa virus

The subtilase subtilisin kexin isozyme-1 (SKI-1)/site 1 protease (S1P), has been implicated in the processing of Lassa virus glycoprotein C (GP-C) precursor into GP1 and GP2 that are responsible for viral fusion with the host cell membrane. Here, we studied in vitro the kinetics of this cleavage by hSKI-1 using an intramolecularly quenched fluorogenic (IQF) peptide, Q-GPC(251-263) [Abz-(251)Asp-Ile-Tyr-Ile-Ser-Arg-Arg-Leu-Leu/Gly-Thr-Phe-Thr(263)-3-NitroTyr-Ala-CONH(2)], containing the identified site. The measured V(max (app))/K(m (app)) was compared to those for other IQF SKI-substrates. Q-GPC(251-263) is cleaved 10-fold more efficiently than the previously known best SKI-substrate, Q-hproSKI(134-142). This study confirmed the role of SKI-1 in GP-C processing and provides a novel, rapid and efficient enzymatic assay of SKI-1.

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

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

Elevated stratum corneum hydrolytic activity in Netherton syndrome suggests an inhibitory regulation of desquamation by SPINK5-derived peptides

Netherton syndrome is a congenital ichthyosis associated with erythroderma, hair shaft defects, and atopic features. The mutations of the secretory serine protease inhibitor Kazal-type 5 gene have been identified in Netherton syndrome patients; however, the actual physiologic substrates of the serine protease inhibitor Kazal-type 5 proprotein are unknown, and how the genetic defects cause characteristic skin phenotype remains uncertain. Here, we describe the serine protease inhibitor Kazal-type 5 gene mutations, including two novel non-sense mutations, and genotype-phenotype correlation in three Netherton syndrome patients in two unrelated Japanese families. Furthermore, based on the reappraisal of the structure of the serine protease inhibitor Kazal-type 5 proprotein, demonstration of the presence of carboxypeptidase in normal keratinocytes, and the observation of mRNA localization of the serine protease inhibitor Kazal-type 5 transcripts in the uppermost epidermis as well as pilosebaceous units, we propose a hypothetical model of proteolytic processing of the serine protease inhibitor Kazal-type 5 proprotein in the epidermis and inhibitory regulation of corneocyte desquamation by a set of serine protease inhibitor Kazal-type 5-derived peptides. This hypothesis is supported by the marked increase of trypsin-like hydrolytic activity demonstrated in stratum corneum samples from our Netherton syndrome patients. The findings in this study suggest that the defective inhibitory regulation of desquamation due to the serine protease inhibitor Kazal-type 5 gene mutations may cause over-desquamation of corneocytes in Netherton syndrome, leading to severe skin permeability barrier dysfunction.

Proteolytic processing of familial British dementia-associated BRI variants: evidence for enhanced intracellular accumulation of amyloidogenic peptides

Different mutations in the BRI(2) gene cause rare neurodegenerative conditions, termed familial British dementia (FBD) and familial Danish dementia (FDD). The mutant genes encode BRI-L and BRI-D, the precursors of fibrillogenic ABri and ADan peptides, respectively. We previously reported that furin processes both BRI-L and its wild type counterpart, BRI, resulting in the secretion of C-terminal peptides; elevated levels of peptides were generated from BRI-L. In the present study, we show that inducible expression of alpha1-antitrypsin Portland, a furin inhibitor, inhibits the endoproteolysis of BRI and BRI-L in a dose-dependent manner. Moreover, comparison of the activities of several proprotein convertases reveals that furin is most efficient in endoproteolysis of BRI and BRI-L; PACE4, PC6A, PC6B, and LPC show much lower activities. Interestingly, LPC also exhibits enhanced cleavage of BRI-L compared with BRI. Finally, we demonstrate that BRI-D is also processed by furin and, like BRI-L, the cleavage of BRI-D is more efficient than that of BRI. Interestingly, while the ABri peptide is detected both intracellularly and in the medium, the ADan peptide accumulates predominantly in intracellular compartments. We propose that intracellular accumulation of amyloidogenic ADan or ABri peptides results in the neuronal damage leading to FDD and FBD, respectively.

Biosynthesis and post-translational processing of site-directed endoproteolytic cleavage mutants of Pro-CCK in AtT-20 cells

Site-directed mutagenesis in which individual cleavage site P1 amino acids were changed to Ala was performed to delineate their importance in the processing of pro-CCK in mouse pituitary tumor AtT-20 cells. Individual substitution of cleavage sites on pro-CCK, viz., CCK 58 cleavage site R/A to A/A, CCK 33 cleavage site R/K to A/K, CCK 22 cleavage site K/N to A/N, and CCK 8 cleavage site R/D to A/D, did not inhibit pro-CCK expression or the production of some form of amidated CCK. Wild-type CCK cDNA expression in these cells results in production and secretion of CCK 8 and CCK 22. Substitution of the 58R/A cleavage site with A/A produces only CCK 33; 33A/K and 22A/N produce only CCK 8, whereas 8A/D produces CCK 12 and some CCK 22. Where the GRR residues on the C-terminus of CCK 8 were mutated to GAA, no amidated CCK was produced. Significant amounts of the pro-CCK, C-terminal peptide S9S was found in the medium of cells transfected with GAA mutant cDNA, indicating that this pro-CCK was cleaved at the GAA site probably by a nonprohormone convertase enzyme. Further analysis of the cells expressing the GAA mutant demonstrated that it is not extensively cleaved at other sites to produce CCK 8 GAA or larger peptides. In the mutant where the entire pro-CCK, C-terminal S9S was deleted, CCK 8 is processed and secreted normally. Thus, the cleavage at the C-terminal GRR site is essential for subsequent cleavages, and modification of other cleavage sites (58, 33, 22, and 8) has a major impact on pro-CCK processing. These results suggest that there is a temporal order of cleavages, and the structure of pro-CCK has a strong influence on where and whether pro-CCK is processed.

New approaches for high-yield purification of Müllerian inhibiting substance improve its bioactivity

We have established a new method to purify Müllerian inhibiting substance (MIS) with higher purity and recovery over existing procedures. Recombinant human MIS was expressed in Chinese hamster ovary cells and secreted into chemically defined serum-free media. The secreted products were concentrated by either precipitation with ammonium sulfate or lectin-affinity chromatography, each of which was followed by anion-exchange chromatography. Further separation of the active carboxy-terminal domain of MIS was achieved after cleavage by plasmin followed by lectin-affinity chromatography. This method may be applicable to other members of the transforming growth factor beta family with which MIS shares sequence homology.

Thekex2 gene from the dimorphic and human pathogenic fungusParacoccidioides brasiliensis

Kexin-like protein is a component of the subtilase family of proteinases involved in the processing of proproteins to their active forms. Kexin-like proteins are also synthesized as a propeptide and this is involved in (auto)inhibition, correct folding and subcellular sorting of proteins. The kexin-like protein was described as the product of the kex2 gene for Aspergillus niger, Candida albicans, Saccharomyces cerevisiae, Yarrowia lipolytica and other fungi. Disruption of the kex2 gene in C. albicans and Y. lipolytica affects hyphae production and induces morphological cell defects, strongly suggesting a possible role of kexin-like proteins in dimorphism of human pathogenic fungi. In this work, we report the nucleotide sequence of the kex2 gene cloned from the dimorphic and human pathogenic fungus Paracoccidioides brasiliensis (Pbkex2). An open reading frame (ORF) of 2622 bp was identified in the complete sequence, interrupted by only one intron of 93 bp. The 5' non-coding region contains consensus sequences such as canonical TATA, CAAT boxes and putative motifs for transcriptional factors binding sites, such as HSE-like regulating genes involved in thermo-dependent processes; Xbp1, reported as a transcriptional factor that may control genes involved in cell morphology; and StuAp, which may regulate spore differentiation and pseudohyphal growth in fungi. In the 3' non-coding region were observed the canonical motifs necessary for correct mRNA processing and polyadenylation. The deduced protein sequence consists of 842 amino acid residues, showing identity to kexin-like proteinases from A. niger (55%), Emericella nidulans (53%) and C. albicans (48%). Comparative sequence analysis of P. brasiliensis kexin-like protein reveals the presence of homologous regions related to a signal peptide, a propeptide, a subtilisin-like catalytic domain, a P domain, a S/T rich region and a transmembrane domain. A putative Golgi retrieval signal (YEFEMI) has also been found in the cytoplasmic tail. The complete nucleotide sequence of Pbkex2 and its flanking regions have been submitted to GenBank database under Accession No. AF486805.

Up-regulation of splenic prohormone convertases PC1 and PC2 in diabetic rats

Organisms respond to infection in a complex manner involving bidirectional interactions between the neuroendocrine and immune systems. Many of the bioactive endocrine/immune factors are synthesized in a precursor form and are expected to be activated by prohormone convertases (PCs). Since patients with both type 1 and type 2 diabetes have an increased incidence and severity of infections, we hypothesized that in a condition of hyperglycemia, these processing enzymes would be activated in an immune tissue, the spleen. To test this hypothesis, we treated rats with intraperitoneal streptozotocin (STZ) (50 mg/kg/day) daily for 5 days and measured splenic PC1 and PC2 mRNA by ribonuclease protection assay. We found that PC1 mRNA was increased 6.0+/-0.02-fold (P<0.05) and PC2 mRNA was increased 1.80+/-0.01-fold (P<0.005) in the spleen of rats that received STZ compared to rats that received vehicle. Western blot indicated that the 75-kDa form of PC1 was the only form of PC1 present in the spleen and that this form increased with STZ treatment. Immunohistochemistry revealed that PC1 was found in both the white pulp (T-lymphocytes) and red pulp (monocytes and macrophages) and that its increase in immunoreactivity occurred primarily in the white pulp. PC2 and pro-opiomelanocortin (POMC, a possible splenic substrate for PC1/PC2) immunoreactivity was found predominantly in the red pulp. STZ induced an increase in splenic PC1 and POMC, but not PC2 protein levels. We conclude that in the STZ model of diabetes, splenic PCs are induced, which could lead to an increased activation of many immune-derived hormones. We speculate that this up-regulation of prohormone converting enzymes may be related to the increased infections seen in patients with both type 1 and type 2 diabetes.

Peptide signals and their receptors in higher plants

At least four peptides play a vital role in plant cell-cell communication by means of their specific receptors. Two of these receptors have been identified as receptor kinases, which form a large family of receptor molecules in plants. These findings highlight the significance of receptor-mediated peptide signaling in various physiological events in plants, and predict the existence of further peptide-signal-interacting receptor kinases. Some candidates have been found in plant genomes. Here, we outline recent progress and future challenges in the signaling peptide analysis, which began with systemin, phytosulfokine, CLAVATA3 and S-locus cysteine-rich protein (also called S-locus protein 11).

Cloning, expression and processing of the CP2 neuropeptide precursor of Aplysia

The cDNA sequence encoding the CP2 neuropeptide precursor is identified and encodes a single copy of the neuropeptide that is flanked by appropriate processing sites. The distribution of the CP2 precursor mRNA is described and matches the CP2-like immunoreactivity described previously. Single cell RT-PCR independently confirms the presence of CP2 precursor mRNA in selected neurons. MALDI-TOF MS is used to identify additional peptides derived from the CP2 precursor in neuronal somata and nerves, suggesting that the CP2 precursor may give rise to additional bioactive neuropeptides.

Altered expression of the neuropeptide-processing enzyme carboxypeptidase E in the rat brain after global ischemia

Carboxypeptidase E, an exoprotease involved in the processing of bioactive peptides released by a regulated secretory pathway, was identified in a subtractive complementary DNA library derived from an ischemic rat brain by differential screening. In situ hybridization and immunocytochemical analysis showed the presence of carboxypeptidase E messenger RNA and protein in the cerebral cortex, thalamus, striatum, and hippocampus of a healthy rat brain. After 15 minutes of transient global ischemia followed by 8 hours of reperfusion, increased levels of carboxypeptidase E messenger RNA and protein were observed in the hippocampal CA1 and CA3 regions and in the cortex, as detected by Northern and Western blot analyses and in situ hybridization. After extended reperfusion (24 to 72 hours), both carboxypeptidase E messenger RNA and protein levels were decreased. The ischemia-induced changes in carboxypeptidase E expression suggest that this enzyme may play a role in modulating the brain's response to ischemia.

Autosomal-dominant hypophosphatemic rickets (ADHR) mutations stabilize FGF-23

BACKGROUND

The gene for the renal phosphate wasting disorder autosomal-dominant hypophosphatemic rickets (ADHR) is FGF23, which encodes a secreted protein related to the fibroblast growth factors (FGFs). We previously detected missense mutations R176Q, R179W, and R179Q in FGF23 from ADHR kindreds. The mutations replace R residues within a subtilisin-like proprotein convertase (SPC) cleavage site 176RHTR-179 (RXXR motif). The goal of these studies was to determine if the ADHR mutations lead to protease resistance of FGF-23.

METHODS

The ADHR mutations were introduced into human FGF-23 cDNA clones with or without an N-terminal FLAG tag by site-directed mutagenesis and were transiently transfected into HEK293 cells. Protein expression was determined by Western analyses.

RESULTS

Antibodies directed toward the C-terminal portion of FGF-23 revealed that the native FGF-23 protein resolved as 32 kD and 12 kD species in HEK293 conditioned media; however, the three mutated proteins were detected only as the 32 kD band. An N-terminal FLAG-tagged native FGF-23 resolved as two bands of 36 kD and 26 kD when detected with a FLAG antibody, whereas the R176Q mutant resolved primarily as the 36 kD protein species. Cleavage of FGF-23 was not enhanced by extracellular incubation of FGF-23 with HEK293 cells. Native and mutant FGF-23s bound heparin.

CONCLUSIONS

FGF-23 proteins containing the ADHR mutations are secreted, and produce polypeptides less sensitive to protease cleavage than wild-type FGF-23. Therefore, the ADHR mutations may protect FGF-23 from proteolysis, thereby potentially elevating circulating concentrations of FGF-23 and leading to phosphate wasting in ADHR patients.

The spectrum of human kallikrein 6 (zyme/protease M/neurosin) expression in human tissues as assessed by immunohistochemistry

The KLK6 gene is a new member of the human kallikrein gene family and encodes for a secreted protease, human kallikrein 6 (hK6; also known as zyme/protease M/neurosin). No study has as yet reported detailed immunohistochemical localization of hK6 in human tissues. Our purpose was to examine the expression of hK6 in human tissues by immunohistochemistry. We have analyzed 199 paraffin blocks from archival, current, and autopsy material prepared from almost every normal human tissue. We employed an hK6-specific polyclonal rabbit antibody and avidin-biotin to localize hK6 by IHC. The staining pattern, the distribution of the immunostaining, and its intensity were studied in detail. The IHC expression of zyme was generally cytoplasmic. Various normal human tissues expressed the protein abundantly. Glandular epithelia constituted the main immunoexpression sites, with representative organs being the breast, prostate, kidney, endometrium, colon, appendix, salivary glands, bile ducts, and gallbladder. The small intestine, stomach, endocervix, Fallopian tube, epididymis, bronchus, and upper respiratory tract showed a focal expression as well. Choroid plexus epithelium, peripheral nerves, and some neuroendocrine cells (including the islets of Langerhans, cells in the anterior pituitary gland, and adrenal medulla) expressed the protein strongly and diffusely. A characteristic immunostaining was observed in the Hassall's corpuscles of the thymus, the oxyphilic cells of the thyroid and parathyroid glands, the primordial follicles of the ovary, dendritic cells mainly in the spleen, and in various cells of the placenta.

The enterins: a novel family of neuropeptides isolated from the enteric nervous system and CNS of Aplysia

To identify neuropeptides that have a broad spectrum of actions on the feeding system of Aplysia, we searched for bioactive peptides that are present in both the gut and the CNS. We identified a family of structurally related nonapeptides and decapeptides (enterins) that are present in the gut and CNS of Aplysia, and most of which share the HSFVamide sequence at the C terminus. The structure of the enterin precursor deduced from cDNA cloning predicts 35 copies of 20 different enterins. Northern analysis, in situ hybridization, and immunocytochemistry show that the enterins are abundantly present in the CNS and the gut of Aplysia. Using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry we characterized the enterin-precursor processing, demonstrated that all of the precursor-predicted enterins are present, and determined post-translational modifications of various enterins. Enterin-positive neuronal somata and processes were found in the gut, and enterins inhibited contractions of the gut. In the CNS, the cerebral and buccal ganglia, which control feeding, contained the enterins. Enterin was also present in the nerve that connects these two ganglia. Enterins reduced the firing of interneurons B4/5 during feeding motor programs. Such enterin-induced reduction of firing also occurred when excitability of B4/5 was tested directly. Because reduction of B4/5 activity corresponds to a switch from egestive to ingestive behaviors, enterin may contribute to such program switching. Furthermore, because enterins are present throughout the nervous system, they may also play a regulatory role in nonfeeding behaviors of Aplysia.

Biogenesis of processing-competent secretory organelles in vitro

Propeptide processing occurs in specific compartments of the secretory pathway, but how these processing-competent organelles are generated from their processing-incompetent precursor compartments is unknown. To dissect the process biochemically, we have developed a novel cell-free system reconstituting the production of processing-competent secretory granules in AtT-20 cells. Using donor membranes containing [(35)S]sulfate labeled pro-opiomelanocortin (POMC)(5) in the trans-Golgi, we can reconstitute cytosol- and ATP-dependent prohormone processing as well as incorporation of processed ACTH into immature secretory granules (ISGs). Under limiting cytosol conditions, both reactions are greatly stimulated by ADP-ribosylation factor 1 (ARF1) but not by the GDP-bound ARF1 T31N mutant. pH studies show that lumenal acidification, most likely due to ARF-mediated sorting of proton pumps and leaks during budding, confers processing competency to the resulting organelle. Surprisingly, comparison of onset of processing and ISG release reveals that they are distinct biochemical processes with different kinetics and separate cytosolic requirements. Moreover, ARF regulates the onset of prohormone processing but not ISG release. Our data suggest a two-step mechanism (onset of processing followed by ISG release) for the production of processing-competent organelles from the trans-Golgi and provide the first system with which these two steps may be individually dissected.

Oxytocin and autistic disorder: alterations in peptide forms

BACKGROUND

Oxytocin (OT) is synthesized as a prohormone that is sequentially processed to peptides. These peptides are the bioactive amidated form (OT) and the C-terminal extended peptides, OT-Gly, OT-Gly-Lys and OT-Gly-Lys-Arg, which are designated together as OT-X. As an extension of our previous study finding decreased plasma OT in autism, studies were conducted to determine whether there were changes in OT peptide forms in autistic children.

METHODS

Twenty eight male subjects (97 +/- 20 months; range, 70-139 months), diagnosed with DSM-IV autistic disorder through observation and semi-structured interview, were compared with 31 age-matched nonpsychiatric control subjects (106 +/- 22 months; range, 74-140 months). Using OT antisera with different specificity for the peptide forms, we measured plasma OT and OT-X in each group.

RESULTS

T tests showed that there was a decrease in plasma OT (t = 4.4, p <.0001), an increase in OT-X (t = 2.3, p <.03) and an increase in the ratio of OT-X/OT (t = 4.5, p <.0001) in the autistic sample, compared with control subjects.

CONCLUSIONS

The results suggest that children with autistic disorder show alterations in the endocrine OT system. Deficits in OT peptide processing in children with autism may be important in the development of this syndrome.

Shedding of membrane type matrix metalloproteinase 5 by a furin-type convertase: a potential mechanism for down-regulation

The shedding of membrane-associated proteins has been recognized as a regulatory mechanism to either up-regulate or down-regulate cellular functions by releasing membrane-bound growth factors or removing ectodomains of adhesion molecules and receptors. We have reported previously that the ectoenzyme of membrane type matrix metalloproteinase 5 (MT5-MMP) is shed into extracellular milieu (Pei, D. (1999) J. Biol. Chem. 274, 8925-8932). Here we present evidence that MT5-MMP is shed by a furin-type convertase activity in the trans-Golgi network. Among proteinase inhibitors screened, only decanoyl-Arg-Val-Lys-Arg-chloromethylketone, a known inhibitor for furin-type convertases, blocked the shedding of MT5-MMP in a dose-dependent manner. As expected, decanoyl-Arg-Val-Lys-Arg-chloromethylketone also prevented the activation of MT5-MMP, raising the possibility that the observed shedding could be autolytic. However, an active site mutant devoid of any catalytic activity, is also shed efficiently, thus ruling out the autolytic pathway. The shedding cleavage was subsequently mapped to the stem region immediately upstream of the transmembrane domain, where a cryptic furin recognition site, (545)RRKERR, was recognized. Indeed, MT5-MMP and furin are co-localized in the trans-Golgi network and the shed species could be detected inside the cells. Furthermore, deletion mutations removing this cryptic site prevented MT5-MMP from shedding. The resulting mutants express a gain-of-function phenotype by mediating more robust activation of proMMP-2 than the wild type molecule. Thus, shedding provides a potential mechanism to regulate proteolytic activity of membrane-bound MMPs.

Cross-talk between the p42/p44 MAP kinase and Smad pathways in transforming growth factor beta 1-induced furin gene transactivation

Furin, a predominant convertase of the cellular constitutive secretory pathway, is known to be involved in the maturation of a number of growth/differentiation factors, but the mechanisms governing its expression remain elusive. We have previously demonstrated that transforming growth factor (TGF) beta 1, through the activation of Smad transducers, regulates its own converting enzyme, furin, creating a unique activation/regulation loop of potential importance in a variety of cell fate and functions. Here we studied the involvement of the p42/p44 MAPK pathway in such regulation. Using HepG2 cells transfected with fur P1 LUC (luciferase) promoter construct, we observed that forced expression of a dominant negative mutant form of the small G protein p21(ras) (RasN17) inhibited TGF beta 1-induced fur gene transcription, suggesting the involvement of the p42/p44 MAPK cascade. In addition, TGF beta induced sustained activation/phosphorylation of endogenous p42/p44 MAPK. Further-more, the role of MAPK cascade in fur gene transcription was highlighted by the use of the MEK1/2 inhibitors, PD98059 or U0126, or co-expression of a p44 antisense construct that repressed the induction of fur promoter transactivation. Conversely, overexpression of a constitutively active form of MEK1 increased unstimulated, TGF beta 1-stimulated, and Smad2-stimulated promoter P1 transactivation, and the universal Smad inhibitor, Smad7, inhibited this effect. Activation of Smad2 by MEK1 or TGF beta 1 resulted in an enhanced nuclear localization of Smad2, which was inhibited upon blocking MEK1 activity. Our findings clearly show that the activation of the p42/p44 MAPK pathway is involved in fur gene expression and led us to propose a co-operative model whereby TGF beta 1-induced receptor activation stimulates not only a Smad pathway but also a parallel p42/p44 MAPK pathway that targets Smad2 for an increased nuclear translocation and enhanced fur gene transactivation. Such an uncovered mechanism may be a key determinant for the regulation of furin in embryogenesis and growth-related physiopathological conditions.

Inhibitory specificity and potency of proSAAS-derived peptides toward proprotein convertase 1

Prohormone convertase 1 (PC1), mediating the proteolytic processing of neural and endocrine precursors, is thought to be regulated by the neuroendocrine protein proSAAS. The PC1 inhibitory sequence is mostly confined within a 10-12-amino acid segment near the C terminus of the conserved human proSAAS and contains the critical KR(244) dibasic motif. Our results show that the decapeptide proSAAS-(235-244)( 235)VLGALLRVKR(244) is the most potent reversible competitive PC1-inhibitor (K(i) approximately 9 nm). The C-terminally extended proSAAS-(235-246) exhibits a 5-6-fold higher K(i) ( approximately 51 nm). The additional LE sequence at P1'-P2', resulted in a competitive substrate cleaved by PC1 at KR(244) downward arrowLE(246). Systematic alanine scanning and in some cases lysine scanning tested the contribution of each residue within proSAAS-(235-246) toward the PC1-inhibition's specificity and potency. The amino acids P1 Arg, P2 Lys, and P4 Arg are all critical for inhibition. Moreover, the aliphatic P3 Val and P5, P6, and P1' Leu significantly affect the degree of enzyme inactivation and PC1 specificity. Interestingly, a much longer N- and C-terminally extended endogenous rat proSAAS-(221-254) called little PenLen, was found to be a 3-fold less potent PC1 inhibitor with reduced selectivity but a much better substrate than proSAAS-(235-246). Molecular modeling studies and circular dichroism analysis indicate an extended and poly-l-proline II type structural conformation for proSAAS-(235-244), the most potent PC1 inhibitor, a feature not present in poor PC1 inhibitors.

Inhibition of proprotein convertases is associated with loss of growth and tumorigenicity of HT-29 human colon carcinoma cells: importance of insulin-like growth factor-1 (IGF-1) receptor processing in IGF-1-mediated functions

Proprotein convertases (PCs) of the subtilisin/kexin family are responsible for the activation of prohormones, protrophic factors, and their receptors. We sought to determine whether loss of PC-mediated activities might affect the malignant phenotypes of cancer cells. Stable transfectants of alpha(1)-antitrypsin Portland (alpha(1)-PDX) cDNA, coding for a potent PC inhibitor, were analyzed in model HT-29 cells (HT-29/PDX) and in other cell lines. Expression of alpha(1)-PDX resulted in a proinsulin-like growth factor-1 receptor (pro-IGF-1R) processing blockade, hence inhibiting the ability of exogenous IGF-1 to induce tyrosine phosphorylation of its beta-subunit and insulin-related substrate-1. Coexpression of IGF-1R with four different PCs or the novel convertase SKI-1 in the furin-defective LoVo-C5 cells demonstrated that pro-IGF-1R ( approximately 200 kDa) cleavage into IGF-1R (beta-subunit, approximately 105 kDa) can be achieved by furin and PC5A, but not by PACE4, PC7, or SKI-1. Expression of alpha(1)-PDX resulted in reduction of DNA synthesis and in anchorage-independent growth. Following serum deprivation, the alpha(1)-PDX transfectants exhibited an enhanced apoptotic phenotype and were insensitive to IGF-1-mediated [(3)H]thymidine incorporation and protection against apoptosis. These cells showed reduced invasiveness that paralleled decreased mRNA levels of urokinase-type plasminogen activator and its receptor, tissue-type plasminogen activator, and plasminogen activator inhibitor-1. Comparative subcutaneous inoculation of cells in nude mice revealed that animals injected with HT-29/PDX cells exhibited delayed and lower incidence of tumor development as well as reduced tumor size. Immunohistochemical analysis of CD31 antigen expression, a marker of endothelial cells, revealed reduced HT-29/PDX tumor vascularization. These findings indicate that PCs actively contribute to the growth and malignant phenotypes of HT-29 tumors, suggesting that PC inhibition strategies may be a useful adduct to the arsenal of colorectal anticancer gene therapies.

Response of an integral granule membrane protein to changes in pH

A key feature of the regulated secretory pathway in neuroendocrine cells is lumenal pH, which decreases between trans-Golgi network and mature secretory granules. Because peptidylglycine alpha-amidating monooxygenase (PAM) is one of the few membrane-spanning proteins concentrated in secretory granules and is a known effector of regulated secretion, we examined its sensitivity to pH. Based on antibody binding experiments, the noncatalytic linker regions between the two enzymatic domains of PAM show pH-dependent conformational changes; these changes occur in the presence or absence of a transmembrane domain. Integral membrane PAM-1 solubilized from rat anterior pituitary or from transfected AtT-20 cells aggregates reversibly at pH 5.5 while retaining enzyme activity. Over 35% of the PAM-1 in anterior pituitary extracts aggregates at pH 5.5, whereas only about 5% aggregates at pH 7.5. PAM-1 recovered from secretory granules and endosomes is highly responsive to low pH-induced aggregation, whereas PAM-1 recovered from a light, intracellular recycling compartment is not. Mutagenesis studies indicate that a transmembrane domain is necessary but not sufficient for low pH-induced aggregation and reveal a short lumenal, juxtamembrane segment that also contributes to pH-dependent aggregation. Taken together, these results demonstrate that several properties of membrane PAM serve as indicators of granule pH in neuroendocrine cells.