A structural model to explain the partial catalytic activity of human prorenin

Structural libraries of protein models for multiple species to understand evolution of the renin-angiotensin system

The details of protein pathways at a structural level provides a bridge between genetics/molecular biology and physiology. The renin-angiotensin system is involved in many physiological pathways with informative structural details in multiple components. Few studies have been performed assessing structural knowledge across the system. This assessment allows use of bioinformatics tools to fill in missing structural voids. In this paper we detail known structures of the renin-angiotensin system and use computational approaches to estimate and model components that do not have their protein structures defined. With the subsequent large library of protein structures, we then created a species specific protein library for human, mouse, rat, bovine, zebrafish, and chicken for the system. The rat structural system allowed for rapid screening of genetic variants from 51 commonly used rat strains, identifying amino acid variants in angiotensinogen, ACE2, and AT1b that are in contact positions with other macromolecules. We believe the structural map will be of value for other researchers to understand their experimental data in the context of an environment for multiple proteins, providing pdb files of proteins for the renin-angiotensin system in six species. With detailed structural descriptions of each protein, it is easier to assess a species for use in translating human diseases with animal models. Additionally, as whole genome sequencing continues to decrease in cost, tools such as molecular modeling will gain use as an initial step in designing efficient hypothesis driven research, addressing potential functional outcomes of genetic variants with precompiled protein libraries aiding in rapid characterizations.

Structural analysis of the intracellular domain of (pro)renin receptor fused to maltose-binding protein

The (pro)renin receptor (PRR) is an important component of the renin-angiotensin system (RAS), which regulates blood pressure and cardiovascular function. The integral membrane protein PRR contains a large extracellular domain (∼310 amino acids), a single transmembrane domain (∼20 amino acids) and an intracellular domain (∼19 amino acids). Although short, the intracellular (IC) domain of the PRR has functionally important roles in a number of signal transduction pathways activated by (pro)renin binding. Meanwhile, together with the transmembrane domain and a small portion of the extracellular domain (∼30 amino acids), the IC domain is also involved in assembly of V(0) portion of the vacuolar proton-translocating ATPase (V-ATPase). To better understand structural and multifunctional roles of the PRR-IC, we report the crystal structure of the PRR-IC domain as maltose-binding protein (MBP) fusion proteins at 2.0Å (maltose-free) and 2.15Å (maltose-bound). In the two separate crystal forms having significantly different unit-cell dimensions and molecular packing, MBP-PRR-IC fusion protein was found to be a dimer, which is different with the natural monomer of native MBP. The PRR-IC domain appears as a relatively flexible loop and is responsible for the dimerization of MBP fusion protein. Residues in the PRR-IC domain, particularly two tyrosines, dominate the intermonomer interactions, suggesting a role for the PRR-IC domain in protein oligomerization.

Activity Assays and Immunoassays for Plasma Renin and Prorenin: Information Provided and Precautions Necessary for Accurate Measurement

Background: Measurement of plasma renin is important for the clinical assessment of hypertensive patients. The most common methods for measuring plasma renin are the plasma renin activity (PRA) assay and the renin immunoassay. The clinical application of renin inhibitor therapy has thrown into focus the differences in information provided by activity assays and immunoassays for renin and prorenin measurement and has drawn attention to the need for precautions to ensure their accurate measurement.

Content: Renin activity assays and immunoassays provide related but different information. Whereas activity assays measure only active renin, immunoassays measure both active and inhibited renin. Particular care must be taken in the collection and processing of blood samples and in the performance of these assays to avoid errors in renin measurement. Both activity assays and immunoassays are susceptible to renin overestimation due to prorenin activation. In addition, activity assays performed with peptidase inhibitors may overestimate the degree of inhibition of PRA by renin inhibitor therapy. Moreover, immunoassays may overestimate the reactive increase in plasma renin concentration in response to renin inhibitor therapy, owing to the inhibitor promoting conversion of prorenin to an open conformation that is recognized by renin immunoassays.

Conclusions: The successful application of renin assays to patient care requires that the clinician and the clinical chemist understand the information provided by these assays and of the precautions necessary to ensure their accuracy.

A hydrophobic patch in a charged alpha-helix is sufficient to target proteins to dense core secretory granules

Many endocrine and neuroendocrine cells contain specialized secretory organelles called dense core secretory granules. These organelles are the repository of proteins and peptides that are secreted in a regulated manner when the cell receives a physiological stimulus. The targeting of proteins to these secretory granules is crucial for the generation of certain peptide hormones, including insulin and ACTH. Although previous work has demonstrated that proteins destined to a variety of cellular locations, including secretory granules, contain targeting sequences, no single consensus sequence for secretory granule-sorting signals has emerged. We have shown previously that alpha-helical domains in the C-terminal tail of the prohormone convertase PC1/3 play an important role in the ability of this region of the protein to direct secretory granule targeting (Jutras, I. Seidah, N. G., and Reudelhuber, T. L. (2000) J. Biol. Chem. 275, 40337-40343). In this study, we show that a variety of alpha-helical domains are capable of directing a heterologous secretory protein to granules. By testing a series of synthetic alpha-helices, we also demonstrate that the presence of charged (either positive or negative) amino acids spatially segregated from a hydrophobic patch in the alpha-helices of secretory proteins likely plays a critical role in the ability of these structures to direct secretory granule sorting.

Physiology of Local Renin-Angiotensin Systems

Since the first identification of renin by Tigerstedt and Bergmann in 1898, the renin-angiotensin system (RAS) has been extensively studied. The current view of the system is characterized by an increased complexity, as evidenced by the discovery of new functional components and pathways of the RAS. In recent years, the pathophysiological implications of the system have been the main focus of attention, and inhibitors of the RAS such as angiotensin-converting enzyme (ACE) inhibitors and angiotensin (ANG) II receptor blockers have become important clinical tools in the treatment of cardiovascular and renal diseases such as hypertension, heart failure, and diabetic nephropathy. Nevertheless, the tissue RAS also plays an important role in mediating diverse physiological functions. These focus not only on the classical actions of ANG on the cardiovascular system, namely, the maintenance of cardiovascular homeostasis, but also on other functions. Recently, the research efforts studying these noncardiovascular effects of the RAS have intensified, and a large body of data are now available to support the existence of numerous organ-based RAS exerting diverse physiological effects. ANG II has direct effects at the cellular level and can influence, for example, cell growth and differentiation, but also may play a role as a mediator of apoptosis. These universal paracrine and autocrine actions may be important in many organ systems and can mediate important physiological stimuli. Transgenic overexpression and knock-out strategies of RAS genes in animals have also shown a central functional role of the RAS in prenatal development. Taken together, these findings may become increasingly important in the study of organ physiology but also for a fresh look at the implications of these findings for organ pathophysiology.

Human prorenin has "gate and handle" regions for its non-proteolytic activation

We investigated the mechanism for non-proteolytic activation of human prorenin using five kinds of antibodies. Each of the antigens, L1PPTDTTTFKRI11P, T7PFKRIFLKRMP17P, I11PFLKRMPSIRESLKER26P, M16PPSIRESLKER26P, and G27PVDMARLGPEWSQPM41P, was designed from the tertiary structure of predicted prorenin. These antibodies were labeled anti-01/06, anti-07/10, anti-11/26, anti-16/26, and anti-27/41, respectively, for their binding specificities. Inactive recombinant human prorenin (0.1 nM) bound to various concentrations of anti-01/06, anti-11/26, and anti-27/41 antibodies at 4 degrees C with equilibrium dissociation constants of 138, 41, and 22 nM, respectively. However, intact prorenin (0.1 nM) did not show significant binding to 200 nM anti-07/10 and anti-16/26 antibodies for 20 h. Ninety percent of prorenin (0.1 nM) was found to be non-proteolytically activated by incubation with anti-11/26 antibodies (200 nM) at 4 degrees C for 20 h. Prorenin was not active even under complex with either anti-01/06 or anti-27/41 antibodies. Prorenin was also reversibly activated at pH 3.3 and 4 degrees C for 25 h. The acid-activated prorenin bound to anti-07/10 and anti-16/26 antibodies as well as to anti-01/06, anti-11/15, and anti-27/41 antibodies at neutral pH and 4 degrees C in 2 h. Their dissociation constants were 13, 40, 8.6, 3.6, and 14 nM, respectively. The acid-activated prorenin was re-inactivated by incubation at pH 7.4 and 4 degrees C in 50 h. Anti-07/10 and anti-11/26 antibodies inhibited such re-inactivation at 25 degrees C by more than 90% and 50%, respectively, whereas other kinds of antibodies did not prevent the re-inactivation at 25 degrees C. These results indicate that prorenin has "gate" (T7PFKR10P) and "handle" (I11PFLKR15P) regions critical for its non-proteolytic activation.

Functional significance of prorenin internalization in the rat heart

Intracardiac renin is considered to be involved in the pathogenesis of cardiac hypertrophy, fibrosis, and myocardial infarction. Cardiac renin is predominantly derived from the circulation, because preprorenin is not expressed locally and uptake of renin has been demonstrated. One mechanism of internalization recently described involves the mannose-6-phosphate receptor and requires glycosylation of renin. Based on previous observations, we considered the existence of another pathway of uptake, not requiring glycosylation and predominantly involving prorenin. This hypothesis and its functional consequences were investigated in vitro and in vivo. We demonstrate that isolated adult cardiomyocytes internalize unglycosylated prorenin, which is followed by the generation of angiotensins. We further show that transgenic rats, expressing the ren-2(d) renin gene in an inducible manner, exhibit markedly enhanced levels of unglycosylated renin within intracellular compartments in the heart as a consequence of the induction of hepatic transgene expression and the rise of circulating unglycosylated prorenin levels. Because in this model severe cardiac damage occurs as a consequence of the rise of circulating prorenin levels, internalization of prorenin into cardiac cells is likely to play a key role in this process.

Prorenin accumulation and activation in human endothelial cells: importance of mannose 6-phosphate receptors

ACE inhibitors improve endothelial dysfunction, possibly by blocking endothelial angiotensin production. Prorenin, through its binding and activation by endothelial mannose 6-phosphate (M6P) receptors, may contribute to this production. Here, we investigated this possibility as well as prorenin activation kinetics, the nature of the prorenin-activating enzyme, and M6P receptor-independent prorenin binding. Human umbilical vein endothelial cells (HUVECs) were incubated with wild-type prorenin, K/A-2 prorenin (in which Lys42 is mutated to Ala, thereby preventing cleavage by known proteases), M6P-free prorenin, and nonglycosylated prorenin, with or without M6P, protease inhibitors, or angiotensinogen. HUVECs bound only M6P-containing prorenin (K(d) 0.9+/-0.1 nmol/L, maximum number of binding sites [B(max)] 1010+/-50 receptors/cell). At 37 degrees C, because of M6P receptor recycling, the amount of prorenin internalized via M6P receptors was >25 times B(max). Inside the cells, wild-type and K/A-2 prorenin were proteolytically activated to renin. Renin was subsequently degraded. Protease inhibitors interfered with the latter but not with prorenin activation, thereby indicating that the activating enzyme is different from any of the known prorenin-activating enzymes. Incubation with angiotensinogen did not lead to endothelial angiotensin generation, inasmuch as HUVECs were unable to internalize angiotensinogen. Most likely, therefore, in the absence of angiotensinogen synthesis or endocytosis, M6P receptor-mediated prorenin internalization by endothelial cells represents prorenin clearance.

Proteolytic activation of recombinant pro-memapsin 2 (pro-beta-secretase) studied with new fluorogenic substrates

Memapsin 2 (beta-secretase), a membrane-anchored aspartic protease, is involved in the cleavage of beta-amyloid precursor protein to form beta-amyloid peptide. The primary structure of memapsin 2 suggests that it is synthesized in vivo as pro-memapsin 2 and converted to memapsin 2 by an activating protease [Lin et al. (2000) Proc. Natl. Acad. Sci. U.S.A. 97, 1456-1460]. To simulate this activation mechanism and to produce stable mature memapsin 2 for kinetic/specificity studies, we have investigated the activation of recombinant pro-memapsin 2 by several proteases with trypsin-like specificity. Clostripain, kallikrein, and trypsin increased the activity of pro-memapsin 2. Clostripain activation was accompanied by the cleavage of the pro region to form mainly two activation products, Leu(30p)- and Gly(45p)-memapsin 2. Another activation product, Leu(28p)-memapsin 2, was also purified. Kinetics of the activated memapsin 2 were compared with pro-memapsin 2 using two new fluorogenic substrates, Arg-Glu(5-[(2-aminoethyl)amino]naphthalene-1-sulfonic acid (EDANS))-Glu-Val-Asn-Leu-Asp-Ala-Glu-Phe-Lys(4-(4-dimethylaminophe nyl azo)benzoic acid (DABCYL))-Arg and (7-methoxycoumarin-4-yl)acetyl (MCA))-Ser-Glu-Val-Asn-Leu-Asp-Ala-Glu-Phe-Lys(2,4-dinitrophenyl (DNP)). These results establish that the activity of pro-memapsin 2 stems from a part-time and reversible uncovering of its active site by its pro region. Proteolytic removal of part of the pro-peptide at Leu(28p) or Gly(45p), which diminishes the affinity of the shortened pro-peptide to the active site, results in activated memapsin 2. These results also suggest that Glu(33p)-memapsin 2 observed in the cells expressing this enzyme [Vassar et al. (1999) Science 286, 735-741; Yan et al. (1999) Nature 402, 533-537] is an active intermediate of in vivo activation, or that the peptide Glu(33p)-Arg(44p) may serve a regulatory role.

The dominant role of the prosegment of prorenin in determining the rate of activation by acid or trypsin: studies with molecular chimeras

Human prorenin activation by acid or trypsin is faster than rat prorenin by two orders of magnitude. No plausible mechanism exists to explain the difference. Two chimeric mutant prorenins were produced in CHO cells. A chimera, hPro/rRen, composed of human prorenin prosegment and rat active renin segment, was activated as fast as wild-type human prorenin at pH 3.3 and 25 degrees C or by trypsin (1 microg/ml). The other chimera, rPro/hRen, composed of rat prorenin prosegment and human active renin segment, was activated as slowly as wild-type rat prorenin at pH 3.3 and 25 degrees C or by trypsin (50 microg/ml). These results indicate that the rate of activation of prorenin is predominantly determined by the N-terminal pro-sequence. Plausible mechanisms are discussed.

Measurement of the renin-angiotensin system in heart failure

Angiotensin II (ANG II), the effector hormone of the renin-angiotensin system (RAS), has been implicated in the pathophysiology and progression of heart failure. Therefore, the measurement of ANG II has become important to characterize the role of this neurohormone in heart failure. However, because ANG II has been difficult to measure, other components of the RAS have been measured to characterize ANG II production. The RAS components (e.g., renin, angiotensin I-converting enzyme [ACE], angiotensin II) have been measured with a variety of techniques. In this review, RAS physiology and the techniques used to measure the RAS components are discussed. In addition, the advantages and disadvantages of the RAS measurement methods are described.

Plasma renin and prorenin and renin gene variation in patients with insulin-dependent diabetes mellitus and nephropathy

BACKGROUND

The most striking abnormality in the renin angiotensin system in diabetic nephropathy (DN) is increased plasma prorenin. Renin is thought to be low or normal in DN. In spite of altered (pro)renin regulation the renin gene has not been studied for contribution to the development of DN.

METHODS

We studied plasma renin, prorenin, and four polymorphic markers of the renin gene in 199 patients with IDDM and DN, and in 192 normoalbuminuric IDDM controls matched for age, sex, and duration of diabetes. Plasma renin and total renin were measured by immunoradiometric assays. Genotyping was PCR-based.

RESULTS

Plasma renin was increased in patients with nephropathy (median (range), 26.3 (5.2-243.3) vs 18.3 (4.2-373.5) microU/ml in the normoalbuminuric group, P<0.0001). Prorenin levels were elevated out of proportion to renin levels in nephropathic patients (789 (88-5481) vs 302 (36-2226) microU/ml, P<0.0001). Proliferative retinopathy had an additive effect on plasma prorenin, but not on renin. DN was associated with a BglI RFLP in the first intron of the renin gene (bb-genotype: n=106 vs 82 in DN and normoalbuminuric patients respectively, P=0.037), but not with three other polymorphisms in the renin gene. A trend for association of higher prorenin levels with the DN-associated allele of this renin polymorphism was observed in a subgroup of patients with DN (bb vs Bb+BB, P=0.07).

CONCLUSIONS

The results indicate that in DN there is an increase in both renin and prorenin levels. A renin gene polymorphism may contribute weakly to DN. Although speculative, one of the renin gene alleles could lead to increased renin gene expression, leading to higher renin and prorenin levels. These may play a role in the pathogenesis of DN.

Probing epitopes on human prorenin during its proteolytic and non-proteolytic activation

The conformational changes of prorenin (PR) that are associated with its reversible non-proteolytic activation and irreversible proteolytic activation were monitored with immunoradiometric assays, using antibodies against epitopes belonging to the propeptide or the renin part of PR. Binding of PR to the renin inhibitor remikiren or protonation of PR resulted in the slowly progressive and simultaneous expression (t1/2 congruent with3.5-5.0 h at 4 degreesC) of epitopes of the N-terminal and C-terminal halves of the propeptide and an epitope that is manifest on renin but not on native non-activated PR. During reversible PR activation-inactivation, expression and disappearance of these epitopes coincided with the appearance and disappearance of enzyme activity. Cleavage of the propeptide from the renin part of PR by plasmin, as demonstrated by the failure of remikiren to unmask the N-terminal and C-terminal propeptide epitopes, was, with some time lag, followed by the simultaneous expression (t1/2 congruent with60 min at 4 degreesC) of the renin-specific epitope and enzymatic activity. Based on these findings we propose a model for the non-proteolytic activation of PR that involves the formation of an intermediary form of activated PR with the following properties: (1) the covalently bound propeptide has moved out of the active-site cleft, so that binding sites are exposed to active site ligands, (2) the propeptide is still not in the 'relaxed' conformation that is characteristic for fully, non-proteolytically, activated PR, and (3) the N-terminal part of the renin polypeptide chain has not yet attained the proper location that is required for enzymatic activity.

Vascular damage without hypertension in transgenic rats expressing prorenin exclusively in the liver

We have developed a transgenic animal model to investigate the effects of overexpression of rat prorenin on the cardiovascular system. Two transgenic rat lines were generated in which rat prorenin expression was directed to the liver by a human alpha1-antitrypsin promoter. Liver-specific expression was confirmed by RNase protection assay. Plasma prorenin concentrations in transgenic rats were increased 400-fold in the males of both lines but were increased only two- to threefold in the females. Thus, transgene expression exhibited sexual dimorphism. Blood pressures were not significantly higher in transgenic rats than in nontransgenic controls. The ratio of heart weight to body weight was greater in male transgenic rats than in the nontransgenic controls. Histological analysis revealed severe renal lesions and hypertrophic cardiomyocytes in transgenic males only. This transgenic model demonstrates a likely role of prorenin in the development of cardiac and renal pathology independent of hypertension. These animals will facilitate studies of the effects of blockade of the renin-angiotensin system and other pharmacological interventions on the development and treatment of cardiac, vascular, and renal lesions induced by changes in this system in the absence of chronic hypertension.

A protease processing site is essential for prorenin sorting to the regulated secretory pathway

Transfected mouse pituitary AtT-20 cells were used to examine the sorting of human prorenin to dense core secretory granules and the regulated secretory pathway. These cells secrete prorenin constitutively and sort a portion of the prorenin to secretory granules, where it is converted to active renin by proteolytic processing. Pulse-chase labeling of transfected AtT-20 cells demonstrated that regulated secretion of prorenin was prevented by: 1) the mutagenic deletion of the prosegment, 2) the premature proteolytic removal of the prosegment by a Golgi-resident processing protease, or 3) the mutation of the native cleavage site so as to prevent removal of the prosegment. In addition, expression of fusion proteins containing portions of the prorenin prosegment demonstrated that exposure of potential proteolytic cleavage sites was sufficient to confer cleavage-dependent regulated secretion of the corresponding protein. These data implicate the protease cleavage event in the regulated secretion of prorenin and are consistent with the involvement of a subclass of processing proteases in the sorting of certain proteins to secretory granules in AtT-20 cells.

Molecular analysis of human prorenin prosegment variants in vitro and in vivo

The aspartyl protease renin, an important modulator of blood pressure in humans, is present in the circulation not only in its active form, but also as an inactive precursor, prorenin, in which a 43-amino acid prosegment blocks access of the substrate to the active site of the enzyme. Site-directed mutagenesis of the prosegment has led to the following conclusions. 1) Maintenance of the enzymatically inactive state of prorenin requires a short peptide sequence between positions 10P and 20P (where P denotes prosegment and numbering is relative to amino terminus) of the prosegment; and 2) there is an inverse relationship between the ability of prosegment mutations to activate and their effect on the secretion of the various prorenins, suggesting that this same region of the prosegment plays a critical role in the biosynthesis of human prorenin. Since these results demonstrated that single amino acid mutations could activate human prorenin to varying degrees, mutations in this region of the renin gene could be clinically important in humans. To test this hypothesis, genomic screening was carried out on the corresponding region of the human renin gene (exon 2) in a cohort of patients selected for a likely familial component to their hypertension. While this study identified a novel polymorphism in exon 2 of the human renin gene, evidence was not obtained for either the presence of prosegment mutations or the association of the novel polymorphism with hypertension in the patient population studied. In conclusion, both structure-function studies and genetic screening suggest that mutation of the prorenin prosegment is an unlikely factor in activation of the renin-angiotensin system in humans.

Renin processing and secretion in adrenal and retina of transgenic (mREN-2)27 rats

Two extrarenal tissue sources of renin were studied using quantitative assays and immunocytochemical methods during 12 hours following binephrectomy (BNx) in anesthetized hypertensive homozygous Ren-2 transgenic (TG) rats maintained off hypotensive drugs for three weeks. Compared to normal rats, circulating active renin was depressed 50% in conscious TG rats and prorenin was 5- to 10-fold higher. Post-BNx, arterial active and prorenin increased progressively to 10-fold, at which time adrenal venous outputs were 0.1 and 20 mGU/min, respectively. The ratio of active to prorenin (3.1 +/- 0.6%) remained unchanged with increasing plasma levels. Thus, either intrinsic enzyme activity of the transgenic prorenin contributed a constant proportion to the measured active renin, or processing to mature renin was coupled to prorenin synthesis and secretion in extrarenal tissues. In the TG rat eye, renin protein labeling was localized throughout retinal Müller cells with prosequence more obvious posteriorly, consistent with directional processing. Immunogold studies are in progress. In adrenal following BNx, labeling for renin and prosequence increased uniformaly in all zones of the cortex and in scattered medullary chromaffin cells. In cortex, both renin and prosequence were strongly located in intramitochondrial dense bodies. In chromaffin cells, renin labeling was present in both cytoplasmic vesicles and electron-dense granules, while prosequence was predominantly in cytoplasmic vesicles, consistent with processing of prorenin prior to storage in chromaffin granules.

Prorenin secretion from villous placenta: regulation by cyclic AMP and angiotensin

Renin synthesis and secretion from human chorion and decidua have previously been shown to be stimulated by agents which increase cellular cyclic adenosine monophosphate (cAMP). We have now used organ culture of villous placenta, incubated for periods up to 72 h, to investigate the cellular regulation of renin in this tissue. The placental tissues release renin (92-96% in the form of prorenin) and human chorionic gonadotrophin (hCG), but not prolactin. We found that cholera toxin and forskolin markedly stimulate the synthesis and release of renin in a time-dependent manner. This stimulation was potentiated by phosphodiesterase inhibitors and inhibited by an angiotensin II agonist, sar-1-angiotensin II. The inhibitory action of the angiotensin agonist on renin release was blocked by sar-1-leu-8-angiotensin II, a selective angiotensin receptor antagonist. The potential for stimulation of renin expression by cyclic AMP-regulated elements is supported by the dramatic (two-orders of magnitude) increase in renin release observed with cholera and forskolin at 72 h. There are several possible candidates for primary signals for adenylyl cyclase-coupled renin secretion from the placenta, including relaxin and epinephrine. The extremely low concentration of renin in term villous placenta may be related to activation of negative regulatory elements on the renin gene. We propose that angiotensin II is one negative regulator of this system.

Analysis of latent forms of renin using antibodies raised against the propart segment of human prorenin: validation with representative samples of ovarian cyst and follicular fluids

Antisera were raised against synthetic peptides from the prosegment of human prorenin. The use of each of these for detection of the appropriate prosegment region of prorenin was validated by development of an ELISA protocol standardised with recombinant prorenin present in culture medium conditioned by myeloma cells transfected with a prorenin expression plasmid. Detection of the respective epitopes in the prosegment required prior exposure of the prorenin in the medium to acid pH in order to partially unfold the prorenin molecule by dislodging the prosegment from the main body of the protein. By these ELISA protocols, the form of latent renin present in representative samples from ovarian cyst and follicular fluids was analysed; one follicular cyst fluid was found to contain full-length prorenin whereas the fluid from a benign cyst and ovarian follicular fluid samples contained the precursor in truncated form.

Molecular determinants of human prorenin processing

In humans, active renin is generated by the removal of a 43-amino acid prosegment from the zymogen prorenin. This cleavage event is highly specific, occurring at only one of the seven pairs of basic amino acids in the body of preprorenin. This cleavage site selectivity is also displayed by a number of other proteases in vitro and in mouse pituitary AtT-20 cells transfected with a human preprorenin expression vector, suggesting that specificity of cleavage is directed in part by the primary sequence, the higher order structure, or both of prorenin itself. To test this hypothesis, single amino acid mutations were introduced in the region of human preprorenin surrounding the natural cleavage site, and the resultant recombinant proteins were expressed in cultured Chinese hamster ovary and AtT-20 cells. The results suggest that amino acids in addition to the pair of basic amino acids surrounding the cleavage site affect the ability of both trypsin and the endogenous AtT-20 processing enzyme to cleave prorenin. Notably, although a proline at position -4 is essential for processing of prorenin in AtT-20 cells and is correlated with predicted formation of a beta-turn at this position, site-directed mutations suggest that this structural feature in addition to a pair of basic amino acids is not sufficient to lead to proteolytic activation of prorenin. Displacement of sequences surrounding the cleavage site to a position 10 amino acids toward the amino terminus led to partial processing of a mutated prorenin.(ABSTRACT TRUNCATED AT 250 WORDS)

Lewis K. Dahl Memorial Lecture. The renin system and four lines fo hypertension research. Nephron heterogeneity, the calcium connection, the prorenin vasodilator limb, and plasma renin and heart attack

As the major regulator of arterial blood pressure and sodium balance, the renin axis supports normotension or hypertension via angiotensin-mediated vasoconstriction and angiotensin plus aldosterone-induced renal sodium retention. In this endocrine servo control, renal renin is released by hypotension or salt depletion; conversely, with hypertension or volume excess, plasma renin activity falls to zero. Accordingly, any renal renin secretion is abnormal in the face of arterial hypertension. Human hypertensive disorders comprise a spectrum of abnormal vasoconstriction-volume products (renin-sodium profiles). Excess plasma renin activity for the sodium balance is created by nephron heterogeneity in which a subpopulation of ischemic nephrons hypersecretes renin and retains sodium. This excess renin impairs adaptive natriuresis of neighboring normal nephrons. Research defining the pivotal role of vascular cytosolic calcium for transducing sodium or renin-mediated vasoconstriction explains the selective value of calcium antagonists for correcting the sodium-volume-mediated, and beta-blockers or angiotensin converting enzyme inhibitors for correcting renin-mediated, arteriolar vasoconstriction. The renin precursor prorenin appears to be physiologically active, causing selective vasodilation that offsets renin-mediated vasoconstriction. Overactivity of prorenin may be involved in the hyperperfusion vascular injuries of diabetes mellitus and toxemias. Prorenin underactivity may facilitate renin-mediated ischemic vascular injury. In essential hypertension, undue plasma renin activity is powerfully and independently associated with heart attack risk. Conversely, patients with low renin activity are protected from heart attack despite higher blood pressures and greater age. Also, renin or angiotensin administration consistently causes vascular injury in the heart, brain, and kidneys of animals. These data suggest new potentials for the prevention of cardiovascular sequelae (heart attack and stroke) by using explicit strategies to curtail plasma renin activity.

Purification and characterization of human truncated prorenin

Posttranslational processing of enzymatically inactive prorenin to an active form participates in the control of the activity of a key system involved in blood pressure regulation, growth, and other important functions. The issue is complicated because renin can be produced by a number of tissues throughout the body, in addition to the kidney, but the mechanism by which they process prorenin to renin is unknown and difficult to determine because of the small amounts of renin present. In the juxtaglomerular cell of the kidney, a 43 amino acid prosegment is cleaved from the amino terminus of prorenin to generate renin of molecular weight 44,000 [Do, Y. S., Shinagawa, T., Tam, H., Inagami, T., & Hsueh, W. A. (1987) J. Biol. Chem. 262, 1037-1043]. Using human uterine lining or a recombinant human prorenin system, we employed the same approach as that used in kidney, ammonium sulfate precipitation at pH 3.1 followed by pepstatin and H-77 affinity chromatography or gel filtration, to purify to homogeneity a 45,500-MW totally active renin. The specific activity of the active truncated prorenin was 850 Goldblatt units (GU)/mg of protein for chorion-decidua renin and 946 GU/mg of protein for recombinant renin, both similar to that reported for pure human renal renin. Both forms of renin cross-reacted with an antibody generated against 44,00-MW pure human renal renin and with an antibody generated against a peptide identical to the carboxy-terminal one-third of the prosegment.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of aspartic proteinases by synthetic peptides derived from the propart region of human prorenin

1. Five synthetic peptides which together spanned the propart segment of human prorenin were tested for their ability to interact with human renin, pepsin, gastricsin, cathepsin D, cathepsin E, calf chymosin and the aspartic proteinase from Endothia parasitica. 2. While two peptides showed no significant effect with any of the enzymes, a further two were cleaved by several enzymes. 3. Only one (corresponding to the 32P-43P residues in the propart sequence) acted as a weak competitive inhibitor of most of the enzymes.

Similarity between physicochemical properties of recombinant rat prorenin and native inactive renin

Rat prorenin was synthesized by Chinese-hamster ovary cells transfected with an expression vector containing rat preprorenin cDNA sequences, then purified by concanavalin A-Sepharose chromatography and h.p.l.c. on G3000SW. The molecular mass of purified prorenin was 46,000 Da, as determined by h.p.l.c. on G3000SW. Immunoblot analysis indicated that recombinant prorenin cross-reacted with anti-(mature renin) antibody and two kinds of antibodies recognizing the N-terminus and C-terminus of the prosegment of rat prorenin. Recombinant prorenin was bound to a Cibacron Blue-Sepharose column and eluted with 1.4 M-NaCl, but was not retained by an octapeptide renin inhibitor (H-77)-Sepharose column. Trypsin activation of prorenin increased the renin activity 110-fold, caused binding to an H-77-Sepharose column and nullified the reactivity to the above two kinds of anti-prosegment antibodies, findings indicating that the activation of prorenin with trypsin is due to the cleavage of the prosegment. Rat plasma inactive renin, partially purified by h.p.l.c. on G3000SW, had much the same physicochemical characteristics as the recombinant prorenin. These results provide evidence that rat plasma inactive renin is prorenin. Recombinant prorenin is a useful material for examining the physiological role of circulating prorenin.

Human prorenin

Human prorenin is the enzymatically inactive biosynthetic precursor of renin. Recent interest has focused on the posttranslational sorting and processing of prorenin to renin since markedly increased levels of circulating prorenin have been associated with both physiological and pathological changes. These observations raise the question of whether prorenin processing may be a regulatory event in renin production in the kidney. In the juxtaglomerular cells of the kidney, prorenin can be sorted to either of two pathways: 1) the regulated pathway, which is mediated by secretory granules, where a thiol protease resembling cathepsin B processes prorenin to renin by cleavage of the amino terminal 43-amino acid prosegment, which allows exposure of the active site of renin, or 2) the constitutive pathway, which is not regulated and does not involve conversion of prorenin to renin. Studies in which segments of prorenin are modified by site-directed mutagenesis suggest that the prosegment and glycosylation are not required for sorting, although they may influence or participate in sorting, or both. Certain areas in the prosegment are important determinants of enzyme activity and ability to cleave the prosegment. Further structural analysis of prorenin will be useful to assess details of its sorting and processing. In addition, a number of extrarenal tissues such as uterine lining, ovarian theca, corpus luteum, pituitary, and adrenal, express the renin gene. These tissues have different capabilities to sort and process prorenin compared with kidney, and some tissues secrete only prorenin. Whether prorenin-to-renin conversion is necessary to activate these local renin-angiotensin systems is a key issue.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular biology of human renin and its gene

This article describes investigations of several aspects of the molecular biology of the human renin gene and the three-dimensional structure of renin and its precursor, prorenin. Because of the importance of the RAS in hypertension, heart failure, renal failure, and possibly other disorders such as atherosclerosis, it is critical to understand the detailed control of this system. This control involves regulation at the transcriptional level, folding of prorenin, sorting of prorenin to a regulated pathway where it is proteolytically cleaved to renin and released in response to secretogogues, constitutive release of uncleaved prorenin, and nonproteolytic activation of prorenin. Currently there is great interest not only in the control of renin in the kidney, the sole source of circulating renin, but also at extrarenal sites where RAS activity may regulate cardiovascular functions. The renin gene was found to be expressed significantly in the renal juxtaglomerular cells and several other cell types. Most tissue culture cells did not express the gene; exceptions were cultured SK-LMS-1 cells and cAMP-stimulated human lung fibroblasts. Cultured human uterine-placental cells expressed the human renin gene at levels higher than in other cell types assessed. Renin mRNA had the same start site in the placental cells as the kidney and was regulated by calcium ionophores and cAMP. Thus, these cells provide primary nontransformed human cells to study the homologous human promoter. Transfected renin promoters showed cell type-specific expression and cAMP responsiveness in these cells in constructs containing as few as 102 bp of 5'-flanking DNA. DNA upstream from this appears to contain an inhibitory element(s) that may have some tissue specificity in its distribution. The cAMP response is not due to cAMP induction of a transcription factor that secondarily affects the renin promoter. A novel element may be involved, since the promoter does not contain a CRE element that mediates many cAMP responses, and the cells do not appear to respond to another known cAMP-responsive transcription factor, AP-2. Studies with transfected vectors expressing a mutant cAMP-responsive protein kinase A regulatory subunit suggest that cAMP is not responsible for basal renin promoter activity in the placental cells. By contrast, cAMP induces in essence gene activation in WI26VA4 transformed human lung fibroblasts in which renin mRNA levels increase by up to 150-fold in response to forskolin. Thus, cAMP may activate renin gene expression under certain circumstances and tissue-specific renin gene expression may be directed by more than one mechanism.(ABSTRACT TRUNCATED AT 400 WORDS)

Increase in plasma prorenin during the menstrual cycle of a bilaterally nephrectomized woman

To investigate the origin of the prorenin peak that occurs during the normal menstrual cycle, plasma levels of prorenin, renin, oestradiol, progesterone, LH and FSH were measured serially in a nephrectomized woman having regular haemodialysis. The prorenin peak coincided with the LH surge and preceded the rise of progesterone, whereas renin was below the detection limit during the whole cycle. These findings indicate that the rise in prorenin during the menstrual cycle is not of renal origin but is probably due to increased production by the ovary and supports the increasing evidence for the existence of a local renin-angiotensin system in the ovary.

Protein modeling of human prorenin using the molecular dynamics method

To study the activation-inactivation mechanism of the renin zymogen, prorenin, a tertiary structural model of human prorenin was constructed using computer graphics and molecular dynamics calculations, based on the pepsinogen structure. This prorenin model shows that the folded prosegment polypeptide can fit into the substrate binding cleft of the renin moiety. The three positively charged residues, Arg10, Arg15, and Arg20, in the prosegment make salt bridges with Asp225, Glu331, and Asp60, respectively, in renin. Arg43, which is in the processing site, forms salt bridges with the catalytic residues of Asp81 and Asp269. These ionic interactions between the prosegment and the renin may contribute to keeping the prorenin structure as an inactive form.

Rat angiotensinogen is secreted only constitutively when transfected into AtT-20 cells

To test whether angiotensinogen might be targeted to dense core secretory granules in cells containing a regulated secretory pathway, we expressed rat angiotensinogen in AtT-20 cells, a mouse pituitary cell line that has the demonstrated ability to correctly sort proteins to the constitutive or regulated pathway. We compared the pattern of secretion of angiotensinogen with that of endogenous adrenocorticotropin hormone, which is secreted by AtT-20 cells through the regulated pathway. When cells were incubated for 5 hours with dibutyryladenosine cyclic monophosphate or KCl, adrenocorticotropin hormone secretion was significantly higher than control, whereas monensin had no effect. In contrast, angiotensinogen secretion was markedly reduced by monensin, but no stimulation was observed with dibutyryladenosine cyclic monophosphate or KCl. These results make it unlikely that angiotensinogen could be cotargeted with active renin in the dense core granules of the regulated pathway. Alternative mechanisms must explain how angiotensin II is synthesized locally by tissue renin-angiotensin systems.

Identification of an enzyme in human kidney that correctly processes prorenin

Using pure recombinant human prorenin as a substrate, we have identified an enzyme in human kidney that accurately processes prorenin to active renin (EC 3.4.23.15). In the crude homogenate, the predominant activity of this potential renin-processing enzyme (RPE) converted the Mr 47,000 inactive prorenin to Mr 44,000 active renin and had a pH optimum of approximately 6. The activity was blocked by cysteine protease inhibitors, but not by pepstatin, EDTA, or serine protease inhibitors. This RPE activity was not detected in a similarly prepared homogenate of human chorion decidua tissue, which produces primarily prorenin, or in human plasma. The activity was purified 100-fold by ammonium sulfate precipitation, p-chloromercuribenzoate affinity chromatography, and chromatofocusing. The partially purified enzyme has a Mr of approximately 27,000 and an isoelectric point in the pH 4.8-5.6 range. The activity in the purified RPE preparation had the same pH optimum as that in crude homogenate, cleaved the prosegment at the same site used by the kidney in vivo based on amino-terminal sequencing of the processed renin, and did not degrade prorenin or renin. These data suggest that the cysteine protease we have isolated is a candidate for authentic renal RPE.