A processing enzyme for prorenin in mouse submandibular gland. Purification and characterization

Renin is produced from an inactive precursor, prorenin, through proteolytic cleavage at paired basic amino acid residues. In this study, an enzyme which specifically cleaves mouse Ren 2 prorenin at the paired basic residues has been purified from mouse submandibular gland by CM-Toyopearl chromatography, antipain-Sepharose chromatography, and isoelectric focusing. This enzyme, named prorenin converting enzyme, consists of two polypeptide chains of 17 and 10 kDa. The enzyme has an isoelectric point of 9.5-9.8, and its pH optimum is between 7.5 and 8.5. It specifically cleaves the peptide bond on the carboxyl side of the Arg at the Lys-Arg pair of mouse Ren 2 prorenin to yield mature renin but does not cleave mouse Ren 1 and human prorenins. Studies on the effects of inhibitors indicate that this enzyme is a serine protease that differs from the enzymes processing other prohormones at paired basic amino acid residues.     

Substrate specificity of prorenin converting enzyme of mouse submandibular gland. Analysis using site-directed mutagenesis

Renin is produced from a larger, inactive precursor, prorenin, through endoproteolytic cleavage at paired basic amino acids. Recently, we have purified and characterized an enzyme, which catalyzes the endoproteolytic process, from mouse submandibular gland. The enzyme, named prorenin converting enzyme, specifically cleaves the peptide bond on the COOH-side of the Arg residue at the Lys-Arg pair of mouse Ren 2 prorenin, but does not cleave mouse Ren 1 and human prorenins. In this study, by synthesizing a series of mutant mouse prorenins using site-directed mutagenesis and the Xenopus oocyte expression system, we have investigated the role of the basic pair as the recognition signal for the enzyme as well as the determinant of the substrate specificity. The results indicate that the basic amino acid at the COOH-side but not at the NH2-side of the basic pair of Ren 2 prorenin is essential for processing directed by prorenin converting enzyme, and that the Arg residue at the COOH-side is more preferable for processing than the Lys. The results also demonstrated that the presence of a Pro residue next to the Lys-Arg pair prevents the processing of Ren 1 prorenin.     

Isolation and characterization of native human renin derived from Chinese hamster ovary cells

Transfection of Chinese hamster ovary (CHO) cells with a plasmid containing the cDNA for human preprorenin has provided cell lines that secrete 15 mg of native prorenin per liter of culture medium. Tryptic activation of the prorenin occurs by selective cleavage of the Arg66-Leu67 bond (numbering as in preprorenin). The renin product, purified in a single step and in high yield by affinity chromatography, is fully stable for as long as 8 months when stored in solution at 4 degrees C and pH 6.5. Purity of the renin was judged to be greater than 95% by gel electrophoresis, compositional and N-terminal sequence analyses, and specific enzyme activity. An important aspect of the present work is the development of a direct assay for renin which permits accurate and reproducible evaluation of enzyme units and kinetic parameters. Application of methods described herein, combined with appropriate scale-up fermentation capabilities, provides the means for generating gram quantities of human renin and its zymogen.     

A targeting sequence for dense secretory granules resides in the active renin protein moiety of human preprorenin

Human renin plays an important role in blood pressure homeostasis and is secreted in a regulated manner from the juxtaglomerular apparatus of the kidney in response to various physiological stimuli. Many aspects of the regulated release of renin (including accurate processing of prorenin to renin, subcellular targeting of renin to dense secretory granules, and regulated release of active renin) can be reproduced in mouse pituitary AtT-20 cells transfected with a human preprorenin expression vector. Using protein engineering, we have attempted to define the roles of various structures in prorenin that affect its production and trafficking to dense core secretory granules, resulting in its activation and regulated secretion. Replacement of the native signal peptide of human preprorenin with that of a constitutively secreted protein (immunoglobulin M) had no apparent effect on either the constitutive secretion of prorenin or the regulated secretion of active renin in transfected AtT-20 cells. Removal of the pro segment resulted in a marked reduction in total renin secretion, but did not prevent renin from entering the regulated secretory pathway. Single or combined mutations in the two glycosylation sites of human renin did not prevent its regulated secretion; however, the complete elimination of glycosylation resulted in a significant increase in the ratio of renin/prorenin secreted by the transfected cells. Thus, these results suggest that 1) at least one of the sequences that target human renin to dense secretory granules lies within the protein moiety of active renin; 2) the presence of the pro segment is important for efficient prorenin and renin production; and 3) glycosylation can quantitatively affect the proportion of active renin secreted.     

Sequence requirements for prohormone processing in mouse pituitary AtT-20 cells. Analysis using prorenins as model substrates

Although cleavage of peptides at sites marked by paired basic amino acids is a common feature of prohormone processing, little is known about the properties of endoprotease(s) responsible for cleavage of the precursor. To examine the cleavage specificity of a processing endoprotease, we have altered the Lys-Arg cleavage site of human prorenin to Arg-Arg, Lys-Lys and Arg-Lys by site-directed mutagenesis, and expressed the native and mutated precursors in mouse pituitary AtT-20 cells which are known to process foreign prohormones, including prorenin, at paired basic sites during the regulated secretory process. All native and mutated human prorenins were sorted into the regulated secretory pathway. The mutated precursor with Arg-Arg instead of the Lys-Arg native pair was processed at about half the efficiency of the native one, while the Lys-Lys and Arg-Lys mutants were not processed. Rat prorenin, which naturally has a Lys-Lys pair, was not processed in the cells. In addition, mouse Ren2 prorenin, which has a Ser residue next to the Lys-Arg pair, but not mouse Ren1 prorenin, which has a Pro residue next to the pair, was processed. These results suggest that the Arg residue at the COOH side of the basic pair is essential for cleavage of prorenins by a processing enzyme during the regulated secretory process in AtT-20 cells, although the NH2-side Lys residue also plays a role. The results also demonstrate that the processing enzyme cannot cleave the Arg-Pro peptide bond.     

The purification and characterization of recombinant human renin expressed in the human kidney cell line 293

The cDNA encoding human preprorenin has been introduced into the adenovirus-transformed human kidney cell line 293. The recombinant 293 cells expressed and secreted prorenin; trypsin was used to activate the secreted prorenin to renin in vitro. The recombinant protein was purified to homogeneity by a single affinity chromatographic step. Using synthetic tetradecapeptide, the Km was 57.1 +/- 9.3 microM and the kcat was (7.48 +/- 1.57) x 10(3)/hr. Activation with trypsin resulted in a secondary cleavage between Arg53 and Leu54 generating a two chain form held together via a disulfide between Cys51 and Cys58. This secondary cleavage did not affect enzyme activity as determined by the ability of renin to degrade a synthetic tetradecapeptide substrate. Our paper demonstrates the potential for producing large quantities of renin from human kidney cells and also suggests that the use of trypsin, which has been widely used to convert prorenin to renin in vitro, causes a secondary cleavage in the renin peptide chain.     

Evolution of aspartyl proteases by gene duplication: the mouse renin gene is organized in two homologous clusters of four exons.

Overlapping recombinant clones that appear to encompass the entire renin gene, named Ren 1, have been isolated from a library of BALB/c mouse genomic DNA fragments. Based on restriction endonuclease mapping and DNA sequence analysis, Ren 1 spans 9.6 kb and contains nine exons interrupted by eight intervening sequences of highly variable size. The first exon, encoding the signal peptide of preprorenin, is separated from the eight following exons by a 3-kb intron. These eight exons are organized into two clusters of four separated by a 2-kb intron. DNA stretches encoding the aspartyl residues, which are part of the active site of renin, are located at homologous positions in both clusters. Our results show that aspartyl protease genes have arisen by duplication and fusion of an ancestral gene containing five exons. The estimated date of the duplication event of the mouse renin genes Ren 1 and Ren 2 is discussed.     

Pure human inactive renin. Evidence that native inactive renin is prorenin

To clarify contradicting observations on the identity of inactive renin and prorenin, inactive renin was completely purified from native human chorion laeve and the culture medium of human chorion cells. A 720,000-fold purification with 14% recovery was achieved from chorion laeve in 6 steps, including immunoaffinity chromatography on a monoclonal antibody to human renin coupled to Protein A-Sepharose CL-4B. A 3,100-fold purification with 40% recovery was achieved from chorion culture medium in 4 steps, including immunoaffinity chromatography. Inactive renin purified from the two different sources migrated as a single protein band with the same molecular weight of 47,000 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis and consisted of multiple components that could be resolved by isoelectric focusing. Both had the same pI values which shifted downward upon activation by trypsin; however, relative peak heights were different between the two preparations. The purified inactive renin from chorion laeve was completely inactive and did not bind to pepstatin-aminohexyl-Sepharose; however, that from chorion culture medium was partially active and completely bound to the pepstatin gel, indicating that each molecule is partially activated. Trypsin-activated inactive renins from both sources were identical with human renal renin in terms of pH optimum and Km. Specific activities of trypsin-activated inactive renin from chorion laeve and chorion culture medium were 529 Goldblatt units/mg of protein and 449 Goldblatt units/mg of protein, respectively. Amino acid sequence analysis of both of the purified inactive renin preparations demonstrated a leucine residue at the amino terminus. The sequence of 11 additional amino acids was identical in both and agreed with that predicted from the base sequence of the renin gene. These findings indicate that preprorenin is converted to prorenin following removal of a 23-amino acid signal peptide and that the native inactive renin, whose amino acid sequence commences with Leu-Pro-Thr..., is prorenin.     

Recombinant human prorenin from CHO cells: Expression and purification

The gene for human preprorenin was obtained from total RNA prepared from primary human chorion cells. An expression vector was constructed containing an SV40 early promoter, a human preprorenin cDNA, bovine growth hormone poly-A addition signal, and a dihydrofolate reductase (dhfr) expression cassette. This vector was inserted into the DXB-11 Chinese hamster ovary (CHO) cell line. The recombinant protein was exported by CHO cells into the tissue culture media. At harvest the prorenin levels ranged from 1–5 mg/L. For prorenin isolation the cell culture supernatants were processed by filtration, concentration, dialysis, and batch extraction. Preparative-scale isolation of prorenin was accomplished using blue-dye chromatography and size-exclusion chromatography. The isolated prorenin yielded a single SDS-gel band with Mr 40,000. The proprotein was characterized with respect to N-terminal sequence and N-linked sugar composition. Trypsin-activated renin prepared from the proprotein was characterized with respect to N-terminal sequence andpH-activity profile. Enzyme activity was measured with a newly developed fluorogenic peptide substrate containing the P₆-P₃ sequence of human angiotensinogen.     

The molecular biology of human renin and its gene

The molecular biology of renin, prorenin, and the renin gene have been studied. A tissue-specific pattern of expression was found in rat and human tissues. In the human placenta, the transfected and endogenous renin promoters are active, and renin mRNA levels and transfected promoter activity are increased by a calcium ionophore plus cAMP. Cultured pituitary AtT-20 cells transfected with a preprorenin expression vector mimick renal renin release by converting prorenin to renin and releasing renin in response to 8Br-cAMP. Studies with mutant renin genes suggest that the body of renin directs renin to the regulated secretory pathway, and renin glycosylation affects its trafficking. Chinese hamster ovary cells were used to produce recombinant prorenin. Infused prorenin was not converted to renin in monkeys. Renin crystals were used to determine its three-dimensional structure. Renin resembles other aspartyl proteases in the active site and core, but it differs in other regions that probably explain renin's unique substrate specificity. Based on structural and mutational analysis, a model for human prorenin was built that suggests lysine -2 of the prosegment interacts with active site aspartate residues, and that the prosegment inactivation of renin is stabilized by binding of an amino terminal beta strand into a groove on renin.     

In vitro biosynthesis of human renin and identification of plasma inactive renin as an activation intermediate

The biosynthesis and post-translational modifications, including proteolytic processing and core glycosylation, of the human renin precursor have been studied in vitro in a cell-free system. For this purpose, highly enriched renin mRNA was isolated from a renin-producing juxtaglomerular cell tumor and translated in rabbit reticulocyte lysate containing [35S]methionine in the presence or absence of dog pancreas microsomal membranes. Fluorographic analysis of the radioactive translation products, immunoprecipitated and then resolved on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, revealed that the primary translation product, preprorenin (Mr = 45,000), is initially processed to glycosylated prorenin (Mr = 47,000) during or shortly after its sequestration into the lumen of the microsomal membranes. The vectorial translocation across the membrane was confirmed by the observation that the proform was resistant to digestion with trypsin while preprorenin was sensitive. Radiosequencing and the use of prorenin-specific antibodies established the cleavage points of the pre- and profragment and showed that the in vitro precursor of human renin contains a 23-residue signal peptide and a 43-residue prosegment. The post-translational modification which, despite the removal of signal peptide, resulted in an increase in apparent Mr, reflects the glycosylation as examined using Xenopus oocytes microinjected with renin mRNA in the presence of tunicamycin, an inhibitor of protein glycosylation. Four anti-peptide antibodies which specifically recognize the NH2 terminus (Pro 1), two middle parts (Pro 2A and Pro 2B), and COOH terminus (Pro 3) of the prosegment, respectively, have been raised and used to characterize plasma prorenin. Renin precursors (pre- and prorenin) synthesized in vitro or in the kidney reacted with these antibodies (anti-Pro 1, anti-Pro 2A, anti-Pro 2B, and anti-Pro 3). However, quite unexpectedly, human plasma prorenin was recognized only by anti-Pro 3, indicating that plasma prorenin is a truncated version of intact prorenin, which lacks a large portion of the NH2 terminus of the prosegment and may represent an activation intermediate. This somewhat surprising result may lead to a better understanding of the exact roles and activation mechanisms of plasma prorenin existing in a relatively large amount.     

Amino-terminal amino acid sequence and heterogeneity in glycosylation of rat renal renin

We isolated 7.4 mg of pure renin from 2 kg of rat kidneys using affinity chromatography on pepstatin-aminohexyl-Sepharose and an octapeptide renin inhibitor, H-77-Sepharose. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that renin consists of two polypeptide chains linked by a disulfide bond, one of Mr = 36,000 (heavy chain) and the other of Mr = 3,000 (light chain). The amino-terminal 10-amino acid sequences of the heavy and the light chains were identical to the sequences beginning at Ser72 and Asp355, respectively, of the amino acid sequence of preprorenin deduced from the renin cDNA sequence. Amino acid sequencing of the carboxyl-terminal peptide of the heavy chain, generated by digestion with lysyl endopeptidase, showed that the carboxyl-terminal residue of the heavy chain is Phe. Thus, the propeptide of prorenin is cleaved after Thr71, followed by removal of two amino acids, Arg353 and Asn354, the result being formation of the heavy and light chains. Thus, the site of cleavage of rat prorenin is after a nonbasic amino acid, in contrast to the cleavage of the propeptide after a pair of basic amino acids in mouse submaxillary renin, human renal renin, and many secretory proteins. Treatment of renin with neuraminidase or glycopeptidase F had no apparent effect on the charge heterogeneity of renin. Glycosylation probably does not contribute to charge heterogeneity.     

Human placental chorionic renin: production, purification and characterization

Native human renin, produced from the culture of human chorionic trophoblasts, has been purified to homogeneity on a milligram scale using a five-step purification scheme. The chorion cells secrete 50-200 milliGoldblatt Units of trypsin-activatable prorenin per ml into the medium. The pro-enzyme is partially purified by ammonium sulfate fractionation and chromatographies on QAE-Sephadex and cibracon blue-agarose. Following conversion of prorenin to the active enzyme by porcine trypsin, the renin is purified to homogeneity by affinity chromatography and gel filtration. Chorionic prorenin has a molecular weight of 43,000; the active enzyme 40,000. Both proteins exist as a single polypeptide chain as determined by SDS-polyacrylamide gel electrophoresis under reducing conditions. The average specific activity of six different preparations was found to be 1072 Goldblatt Units/mg. The amino acid composition and N-terminal sequence of the active enzyme has been determined and is identical to the human kidney enzyme. Microheterogeneity of chorionic renin was demonstrated by isoelectrofocusing analysis. The physical characterization of chorionic renin is compared with that reported for the human kidney enzyme.     

Efficient production of recombinant human (pro)renin utilizing a decahistidine tag attached at the C-terminus

Human prorenin attached by a decahistidine tag at the C-terminus was produced in Chinese hamster ovary cells. The tagged protein secreted into the culture medium was in the inactive prorenin form, and was activated to mature renin by proteolytic removal of its prosegment by trypsin in the same manner as native prorenin. The tagged (pro)renin was efficiently purified by metal-chelate affinity chromatography. The enzymatic properties of mature renin carrying the tag were similar to native renin. These results indicate that the introduction of a decahistidine tag at the C-terminus does not interfere with either the correct folding of prorenin or the catalytic activity of mature renin.     

Efficient Production of Recombinant Human (Pro)renin Utilizing a Decahistidine Tag Attached at the C-Terminus

Human prorenin attached by a decahistidine tag at the C-terminus was produced in Chinese hamster ovary cells. The tagged protein secreted into the culture medium was in the inactive prorenin form, and was activated to mature renin by proteolytic removal of its prosegment by trypsin in the same manner as native prorenin. The tagged (pro)renin was efficiently purified by metal-chelate affinity chromatography. The enzymatic properties of mature renin carrying the tag were similar to native renin. These results indicate that the introduction of a decahistidine tag at the C-terminus does not interfere with either the correct folding of prorenin or the catalytic activity of mature renin.     

Expression of Protein Complex Comprising the Human Prorenin and (Pro)Renin Receptor in Silkworm Larvae Using <Emphasis Type="Italic">Bombyx mori</Emphasis> Nucleopolyhedrovirus (BmNPV) Bacmids for Improving Biological Function

Three forms of recombinant protein complexes comprising the human prorenin (hPro) and (pro)renin receptor (hPRR) (hPRR/prorenin) were successfully expressed in the silkworm larvae using Bombyx mori nucleopolyhedrovirus (BmNPV) bacmids. They were localized in the fat body cells and formed a prorenin-bound hPRR complex. The expressed levels of hPro and hPRR were similar judging from Western blotting. The hPRR/prorenin complex containing 40 μg of hPRR (yield, 43%) and 30 μg of hPro (yield, 34%) was purified from 15 silkworm larvae by a series of purification using anti-FLAG and Strep-Tactin affinity chromatography. The renin activity of the purified hPRR/prorenin complex was 3.8-fold that of the mixture of hPRR and hPro expressed individually in vitro judging from the renin assay. These results show that the unstable transmembrane protein, hPRR, was coexpressed stably with ligand, hPro, and formed a stable protein, hPRR/prorenin complex that showed a high catalytic active form.     

Mouse submandibular gland prorenin-converting enzyme is a member of glandular kallikrein family

Mouse submandibular gland prorenin-converting enzyme (PRECE) consists of the two polypeptide chains of 17 and 10 kDa and cleaves mouse Ren-2 prorenin at a dibasic site to yield mature renin. Western blot analysis using an antiserum against this enzyme gave rise to multiple bands in mouse submandibular glands, suggesting that PRECE is a member of a protease family. Partial amino acid sequence analysis of purified PRECE and cloning and sequence analyses of its cDNA indicated that it is identical to the mGK-13 gene product, epidermal growth factor-binding protein type B, which is a member of the glandular kallikrein family and is involved in maturation of epidermal growth factor. Conditioned medium from Chinese hamster ovary cells transfected with an expression plasmid for PRECE had prorenin converting activity. These results indicate that PRECE is involved in the maturation of two bioactive polypeptides expressed in mouse submandibular glands, Ren-2 renin and epidermal growth factor.     

Prorenins activation by an enzyme from rat plasma (PreR-Co)

The aim of the present research was to explore the capacity of PreR-Co to process prorenin purified from kidney and corpora lutea (CL) and to study its action on extrarenal tissues. The PreR-Co was obtained from plasma as a single electrophoretic band by (NH4)2SO4 precipitation, gel filtration, anti-rat albumin immunoaffinity, and ion-exchange chromatography. Prorenin free of renin was obtained after (NH4)2SO4 precipitation, gel filtration, and ion-exchange chromatography by a passage through an affinity gel of H-77 Sepharose. SDS-PAGE of supernatant and of acidic elution from gel, exhibited a single band of 43 kDa and 35 kDa, respectively; both recognized by the specific anti rat renin antibody. The isolated renin was not attacked by PreR-Co; on the contrary prorenin was completely activated. The product of PreR-Co-activated prorenin showed an analogous MW to that of renin and was recognized by the specific antibody. In addition to processing kidney prorenin, PreR-Co was able to cleave inactive renin from ovary, CL, uterus and adrenal gland homogenates. However, the amount of active renin generated from these tissues was lower than those produced by trypsin activation. PreR-Co is a good candidate for the role of the enzyme involved in tissues prorenin activation.     

Pregnancy-induced changes in renin gene expression in mice.

A puzzling feature of the renin-angiotensin system during pregnancy is the appearance in the maternal circulation of a large increase in the concentration of prorenin and renin. The physiologic role of these changes is not understood. We determined that high levels of renin protein occur in the circulation of pregnant mice, thereby establishing the mouse as a valuable model for understanding gestation-induced changes in the renin-angiotensin system. We used the murine model to show that high levels of renin gene expression occur at the mother-fetus interface, first in maternal decidua and subsequently in placentas. These results were obtained using ICR mice that have 2 related renin genes, Ren1 and Ren2. We also examined renin gene expression in C57Bl/6 mice that have only the Ren1 gene. In these mice, very little renin gene expression was observed in placentas but instead was upregulated in kidneys during pregnancy. In both ICR and C57Bl/6 mice, there is an increase in renin protein in the maternal circulation during pregnancy. However, these mice differ with regard to gestation-induced sites of increased renin gene expression. These studies suggest that mice are a convenient and valuable model for studying renin gene expression during pregnancy.