Reversible cryoactivation of recombinant human prorenin

The Clinical Impact of Sample Storage at -20 °C on Renin Reference Intervals and Aldosterone-Renin Ratio Calculations

Cryoactivation is known to occur in whole blood and plasma samples when kept between +4 and -5 °C, leading to falsely high renin concentrations. In 2022 it has been clearly shown that cryoactivation can also occur in samples stored at -20 °C. Based on these new findings, here we discuss how this can influence the clinical diagnosis of patients. First, we show that storage of renin plasma samples can affect the renin measurements and thereby the aldosterone to renin ratio (ARR) calculation, which might explain the high intraindividual variability in ARR also recently demonstrated. Second, we discuss the existing studies on the establishment of renin reference intervals and note the lack of attention given to this recently revealed preanalytical condition. Our literature review of the reference intervals for renin suggest that cryoactivation might have influenced the published data.

Quantitation of Renin Activity in Plasma Using Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS)

Accurate determination of plasma renin activity (PRA) is essential for the development and maintenance of an effective screening program for primary aldosteronism (PA). PRA measurement can also be useful in the investigation of renal artery stenosis, syndrome of mineralocorticoid excess, Addison's disease, congenital adrenal hyperplasia, Bartters and Gitelman syndromes, and for inherited defects in the renin angiotensin aldosterone system (RAAS). We describe a semiautomated and simple method for the accurate and precise measurement of PRA from 500 μL of plasma (250 μL if blank subtraction is omitted, as discussed) using a liquid chromatography and tandem mass spectrometry (LC-MS/MS) method for angiotensin I (AngI) in 96-well format. After a 3 h AngI generation step at 37 °C in buffering conditions at pH 6, the reaction is quenched with 10% formic acid containing AngI internal standard. Sample preparation then proceeds with offline solid phase extraction, two wash steps, and methanol elution followed by injection into the LC-MS/MS system. Quantitation is performed against a 7-point calibration linear curve prepared in buffer. The assay calibration range is 0.34-30.0 ng/mL, which corresponds to PRA values of 0.11-10.0 ng/mL/h: much wider than was possible using traditional competitive antibody-based methods. Total precision in clinical production has been observed to be 5.8-5.0% for BioRad Hypertension Control materials having nominal PRA values ranging from 1.73 to 12.43 ng/mL/h. At AngI concentrations of 0.06 ng/L (corresponding to a PRA of 0.02 ng/mL/h), signal-to-noise ratio is 50:1, indicating that the limit of quantitation is well below the level required for clinical use.

Stability of direct renin concentration and plasma renin activity in EDTA whole blood and plasma at ambient and refrigerated temperatures from 0 to 72 hours

OBJECTIVES

The aim of this study was to determine the appropriate transport and storage conditions for blood taken for direct renin concentration and plasma renin activity measurement, and whether cryoactivation of prorenin is seen at time points relevant to clinical practice.

METHODS

Blood was extracted from n=10 volunteers into K₂-EDTA tubes. Stability of renin was assessed in whole blood stored at room temperature (15-25 °C) and in the refrigerator (2-8 °C) at 0 h, 8 h, and 24 h. The stability of renin in plasma was determined under the same conditions at 0 h, 24 h and 72 h.

RESULTS

Stability of plasma renin activity and direct renin concentration in whole blood stored at room temperature was found to be acceptable for up to 24 h. At refrigerated temperature, whole blood stability was acceptable for measurement of direct renin concentration up to 8 h and plasma renin activity up to 24 h. In contrast, plasma renin activity was not stable in plasma stored at either room or refrigerated temperatures up to 24 h; however, direct renin concentration had acceptable stability in plasma stored at room temperature for up to 24 h, but stability was unacceptable at refrigerated temperatures.

CONCLUSIONS

Samples collected for plasma renin activity and direct renin concentration should be transported as whole blood to optimise stability. After sample processing, plasma can be kept at room temperature for up to 24 h for direct renin concentration, however, for determination of plasma renin activity separated plasma should be analysed or frozen as soon as possible.

Interactions amongst inflammation, renin-angiotensin-aldosterone and kallikrein-kinin systems: suggestive approaches for COVID-19 therapy

Coronavirus disease 2019 (COVID-19) is a rapid-spread infectious disease caused by the SARS-CoV-2 virus, which can culminate in the renin-angiotensin-aldosterone (RAAS) and kallikrein-kinin (KKS) systems imbalance, and in serious consequences for infected patients. This scoping review of published research exploring the RAAS and KKS was undertaken in order to trace the history of the discovery of both systems and their multiple interactions, discuss some aspects of the viral-cell interaction, including inflammation and the system imbalance triggered by SARS-CoV-2 infection, and their consequent disorders. Furthermore, we correlate the effects of continued use of the RAAS blockers in chronic diseases therapies with the virulence and physiopathology of COVID-19. We also approach the RAAS and KKS-related proposed potential therapies for treatment of COVID-19. In this way, we reinforce the importance of exploring both systems and the application of their components or their blockers in the treatment of coronavirus disease.

Human prorenin determination by hybrid immunocapture liquid chromatography/mass spectrometry: A mixed-solvent-triggered digestion utilizing D-optimal design

RATIONALE

Human prorenin, representing the precursor of mature renin, has been discussed as a potential biomarker, e.g. in diagnosing primary hyperaldosteronism or diabetes-induced nephropathy. Currently, only immunoassays are available for prorenin quantification. As the similarity of prorenin to active renin impedes its accurate determination by immunoassay, mass spectrometry appears as an accurate alternative for differentiation of that protein.

METHODS

Immunoaffinity purification plus a mixed-solvent-triggered digestion was combined with liquid chromatography/mass spectrometry (LC/MS) to enable a fast, sensitive, and less laboratory-intensive approach to the quantification of prorenin. Statistical experimental planning, which is known as Design of Experiments (DOE), was used to identify the optimal conditions for the generation of the signature peptides within a manageable number of experiments. The efficiency of the mixed-solvent-triggered digestion by trypsin was investigated using four different organic solvents: acetonitrile, acetone, tetrahydrofuran and methanol.

RESULTS

By utilizing a D-optimal design, we found that the optimal mixed-solvent type for the generation of both signature peptides was acetonitrile at a concentration of 84% and an incubation temperature of 16°C. Using the mixed-solvent-triggered digestion, the procedure time allowed a fast analysis of active renin and prorenin with a short digestion time of 98 min. This optimized mixed-solvent-triggered digestion procedure was applied to detect renin and prorenin successfully in human plasma by the newly developed hybrid approach.

CONCLUSIONS

The identification of unique surrogates for human prorenin enabled the mass spectrometric differentiation between the two similar proteins. The novel hybrid approach successfully proved its ability to purify, detect and distinguish between prorenin and active renin in human plasma.

Activation mechanism of plasmepsins, pepsin-like aspartic proteases from Plasmodium, follows a unique trans-activation pathway

Plasmodium parasites that cause malaria produce plasmepsins (PMs), pepsin-like aspartic proteases that are important antimalarial drug targets due to their role in host hemoglobin degradation. The enzymes are synthesized as inactive zymogens (pro-PMs), and the mechanism of their conversion to the active, mature forms has not been clearly elucidated. Our structural investigations of vacuolar pro-PMs with truncated prosegment (pro-tPMs) reveal that the formation of the S-shaped dimer is their innate property. Further structural studies, biochemical analysis, and molecular dynamics simulations indicate that disruption of the Tyr-Asp loop (121p-4), coordinated with the movement of the loop L1 (237-247) and helix H2 (101p-113p), is responsible for the extension of the pro-mature region (harboring the cleavage site). Consequently, under acidic pH conditions, these structural changes result in the dissociation of the dimers to monomers and the protonation of the residues in the prosegment prompts its unfolding. Subsequently, we demonstrated that the active site of the monomeric pro-tPMs with the unfolded prosegment is accessible for peptide substrate binding; in contrast, the active site is blocked in folded prosegment form of pro-tPMs. Thus, we propose a novel mechanism of auto-activation of vacuolar pro-tPMs that under acidic conditions can form a catalytically competent active site. One monomer cleaves the prosegment of the other one through a trans-activation process, resulting in formation of mature enzyme. As a result, once a mature enzyme is generated, it leads to the complete conversion of all the inactive pro-tPMs to their mature form. DATABASE: Atomic coordinates and structure factors have been submitted in the Protein Data Bank (PDB) under the PDB IDs 6KUB, 6KUC, and 6KUD.

Renin Activity in Heart Failure with Reduced Systolic Function-New Insights

Regardless of the cause, symptomatic heart failure (HF) with reduced ejection fraction (rEF) is characterized by pathological activation of the renin–angiotensin–aldosterone system (RAAS) with sodium retention and extracellular fluid expansion (edema). Here, we review the role of active renin, a crucial, upstream enzymatic regulator of the RAAS, as a prognostic and diagnostic plasma biomarker of heart failure with reduced ejection fraction (HFrEF) progression; we also discuss its potential as a pharmacological bio-target in HF therapy. Clinical and experimental studies indicate that plasma renin activity is elevated with symptomatic HFrEF with edema in patients, as well as in companion animals and experimental models of HF. Plasma renin activity levels are also reported to be elevated in patients and animals with rEF before the development of symptomatic HF. Modulation of renin activity in experimental HF significantly reduces edema formation and the progression of systolic dysfunction and improves survival. Thus, specific assessment and targeting of elevated renin activity may enhance diagnostic and therapeutic precision to improve outcomes in appropriate patients with HFrEF.

Intrarenal renin-angiotensin system activity is augmented after initiation of dialysis

Circulating renin-angiotensin system (RAS) activation is maintained after renal function has deteriorated. The activation of the intrarenal RAS plays a critical role in the pathophysiology of chronic kidney disease (CKD), independently of the circulating RAS. However, the activation of intrarenal RAS and the chymase-dependent pathway after initiation of dialysis has not been clarified. We recruited 19 CKD patients (10 without dialysis and 9 with dialysis) who underwent a heminephrectomy. Circulating RAS was investigated before nephrectomy. The levels of intrarenal RAS components and chymase-positive cells were investigated using radioimmunoassay or immunoblot analysis on samples collected from the removed kidney. Renal damage was evaluated by the extent of tubulointerstitial fibrosis. No significant differences in circulating RAS between nondialysis and dialysis patients were found. However, intrarenal angiotensin II (AngII) and the extent of tubulointerstitial fibrosis in dialysis patients were significantly increased when compared with nondialysis patients. Prorenin and angiotensin-converting enzyme (ACE) levels were dramatically decreased in accordance with renal dysfunction. On the other hand, chymase-positive cells and AngII type 1 receptor (AT1R) expression was significantly increased in dialysis patients when compared with nondialysis patients. In multiple linear regression analyses, there were significant positive and negative relationships between the extent of interstitial fibrosis and angiotensinogen (β=0.45, P=0.042) and prorenin levels (β=-0.85, P<0.01), respectively. In summary, a decrease in prorenin and ACE expression and an increase in chymase, angiotensinogen and AT1R expression in the kidney may augment the intrarenal RAS activation and be associated with renal damage, even after initiation of dialysis.

Decidualisation of human endometrial stromal cells is associated with increased expression and secretion of prorenin

BACKGROUND

In pregnancy, the decidualised endometrium expresses high levels of prorenin and other genes of the renin-angiotensin system (RAS) pathway. In this study we aimed to determined if the RAS was present in endometrial stromal cells and if decidualisation upregulated the expression of prorenin, the prorenin receptor ((P)RR) and associated RAS pathways. Immortalised human endometrial stromal cells (HESCs) can be stimulated to decidualise by combined treatment with medroxyprogesterone acetate (MPA), 17β-estradiol (E2) and cAMP (MPA-mix) or with 5-aza-2'-deoxycytidine (AZA), a global demethylating agent.

METHODS

HESCs were incubated for 10 days with one of the following treatments: vehicle, MPA-mix, a combination of medroxyprogesterone acetate (MPA) and estradiol-17β alone, or AZA. Messenger RNA abundance and protein levels of prorenin (REN), the (P)RR (ATP6AP2), angiotensinogen (AGT), angiotensin converting enzyme (ACE), angiotensin II type 1 receptor (AGTR1), vascular endothelial growth factor (VEGF), and plasminogen activator inhibitor-1 (PAI-1) were measured by real-time PCR and ELISA's, respectively. Promyelocytic zinc finger (PLZF) and phospho-inositol-3 kinase (PIK3R1) mRNA abundances were also measured.

RESULTS

HESCs expressed the prorenin receptor (ATP6AP2), REN, AGT, ACE and low levels of AGTR1. MPA-mix and AZA stimulated expression of REN. Prorenin protein secretion was increased in MPA-mix treated HESCs. E2 + MPA had no effect on any RAS genes. MPA-mix treatment was associated with increased VEGF (VEGFA) and PAI-1 (SERPINE1) mRNA and VEGF protein.

CONCLUSIONS

An endometrial prorenin receptor/renin angiotensin system is activated by decidualisation. Since (P)RR is abundant, the increase in prorenin secretion could have stimulated VEGF A and SERPINE1 expression via Ang II, as both ACE and AGTR1 are present, or by Ang II independent pathways. Activation of the RAS in human endometrium with decidualisation, through stimulation of VEGF expression and secretion, could be critical in establishing an adequate blood supply to the developing maternal placental vascular bed.

Acid-activated prorenin binds to (pro)renin receptor in vitro

Binding properties of acid-activated prorenin to (pro)renin receptor [(P)RR] was investigated in vitro to discuss possible roles of such reversibly acid-activated prorenin in the renin angiotensin (RA) system. Prorenin was acidified at pH 3.3, 4.5, 5.5, 6.5, and its activation level was measured at 1, 2, 4, 8, 12, and 25 h. Prorenin, activated non-proteolytically in time- and pH-dependent manners, was verified by Western blot analyses. Acidification of prorenin for 25 h at pH 3.3, 4.5, 5.5, and 6.5 showed 78%, 54%, 34%, and 20% activities, respectively when compared with the renin activity of trypsinized prorenin as 100%. Additionally, the binding properties of acidified prorenin to (P)RR were elucidated both at the equilibrium state and in the kinetic state using BIAcore. BIAcore assay showed that acidified prorenin at pH 3.3, 4.5, 5.5, and 6.5 had apparent K(D) of 1.57 × 10(4), 14.1, 8.29, and 8.04 nM, respectively while native prorenin at pH 7.4 had a K(D) of 7.8 nM. At equilibrium state, K(D) of native prorenin was 1.42 nM whereas apparent K(D) varied from 1.25 to 5.0 nM for the prorenin acidified at pH 4.5, 5.5, and 6.5. The K(m) values of free forms of acidified prorenin at different pH (0.33-0.5 μM) was almost similar to those of (P)RR-bound forms of acidified prorenin (0.5-0.77 μM). These in vitro data indicate that prorenin acidified in vivo possibly modulate RA system in receptor-dependent and/or -independent manners which could ultimately lead to the pathogenesis of diseases.

[Clinical evaluation of the renin-aldosterone system: comparison of two methods in different clinical conditions]

INTRODUCTION

Measurement of plasma aldosterone/renin ratio is the key step in the diagnosis of primary aldosteronism.

AIM

The aim of the authors was to analyze and compare the diagnostic utility of plasma aldosterone/renin activity and plasma aldosterone/renin concentration ratios.

METHODS

Plasma aldosterone and plasma renin activity were determined by radioimmunoassays and plasma renin concentration was measured by immunoradiometric assay in 134 subjects (80 women and 54 men, aged 46±15.5 years) including 49 healthy blood donors (control group), 59 patients with hypertension (25 treated and 34 untreated) and 26 patients with incidentally discovered adrenal adenomas.

RESULTS

There was a weak correlation (r = 0.59) between plasma renin activity and plasma renin concentration in the lower range (plasma renin activity, 0.63±0.41 ng/ml/h; plasma renin concentration, 8.1±4.9 ng/l). Considering the cut-off value of plasma aldosterone/renin ratios determined in controls (plasma aldosterone/renin activity ratio, 30 ng/dl/ng/ml/h; plasma aldosterone/renin concentration ratio, 3.0 ng/dl/ng/l), high proportion of falsely positive results were found among patients on beta-receptor blocker therapy (plasma aldosterone/renin activity ratio, 22.2%; plasma aldosterone/renin concentration ratio, 44.4%)

CONCLUSION

The widely used plasma aldosterone/renin activity ratio can only be replaced with plasma aldosterone/renin concentration ratio with precaution on different clinical conditions.

Molecular cloning and characterization of procirsin, an active aspartic protease precursor from Cirsium vulgare (Asteraceae)

Typical aspartic proteinases from plants of the Astereaceae family like cardosins and cyprosins are well-known milk-clotting enzymes. Their effectiveness in cheesemaking has encouraged several studies on other Astereaceae plant species for identification of new vegetable rennets. Here we report on the cloning, expression and characterization of a novel aspartic proteinase precursor from the flowers of Cirsium vulgare (Savi) Ten. The isolated cDNA encoded a protein product with 509 amino acids, termed cirsin, with the characteristic primary structure organization of plant typical aspartic proteinases. The pro form of cirsin was expressed in Escherichia coli and shown to be active without autocatalytically cleaving its pro domain. This contrasts with the acid-triggered autoactivation by pro-segment removal described for several recombinant plant typical aspartic proteinases. Recombinant procirsin displayed all typical proteolytic features of aspartic proteinases as optimum acidic pH, inhibition by pepstatin, cleavage between hydrophobic amino acids and strict dependence on two catalytic Asp residues for activity. Procirsin also displayed a high specificity towards κ-casein and milk-clotting activity, suggesting it might be an effective vegetable rennet. The findings herein described provide additional evidences for the existence of different structural arrangements among plant typical aspartic proteinases.

The plasma renin test reveals the contribution of body sodium-volume content (V) and renin-angiotensin (R) vasoconstriction to long-term blood pressure

Body sodium works together with the plasma renin-angiotensin system to ensure adequate blood flow to the tissues. Body sodium content determines the extracellular fluid (ECF) volume ensuring that, with each heart beat, a sufficient volume of fluid is delivered into the arterial space. At the same time the kidneys monitor ECF volume and blood pressure (BP), so that the juxtaglomerular cells can adjust their net secretion rate of renin to maintain an appropriate plasma renin activity (PRA) level. Plasma renin produces angiotensin II (Ang II) to constrict the arterioles and thereby ensure sufficient BP to deliver an appropriate rate of flow for cardiovascular homeostasis. The low renin, sodium-volume dependent (V) form of essential hypertension occurs whenever body sodium content increases beyond the point where plasma renin-angiotensin vasoconstrictor activity is turned off. In contrast, medium to high renin (R) hypertension occurs when too much renin is secreted relative to the body sodium content. Thus, BP = V × R. This volume-vasoconstriction dual support of long-term hypertension is validated by the fact that all effective long-term antihypertensive drug types are either (i) natriuretic to reduce body salt and volume content (anti-V), or (ii) antirenin to reduce or block the activity of the circulating renin-angiotensin system (anti-R). The PRA test defines the relative participation of the concurrent volume and vasoconstrictor factors. In the hypertensive patient PRA testing can guide initiation, addition or subtraction of anti-V or anti-R antihypertensive drug types to thereby improve BP control and prognosis while reducing drug type usage and cost.

Biochemical properties of renin and prorenin binding to the (pro)renin receptor

The discovery of (pro)renin receptor, (P)RR, has made the renin-angiotensin system (RAS) more multifaceted. Interaction of renin and prorenin with this receptor has set a new perspective about the physiological functions, activation mechanism and pathophysiological roles of renin/prorenin. Uses of peptides mimicking the structure of the ligands have been very effective for determining structure-function relationship between the ligands and receptor. The probable pivotal role of decoy peptide region (R(10P)IFLKRMPSI(19P)) of prorenin prosegment was suggested for higher binding affinity of prorenin to (P)RR than that of mature renin. Recently, 'hinge' region peptide (S(149)QGVLKEDVF(158)) in renin/prorenin molecule has been reported. Bothrenin and prorenin can interact with (P)RR through the 'hinge' region. Furthermore, it has been proposed that prorenin has multiple binding sites whereas renin has a single binding site for (P)RR. To comprehend the activation mechanism of renin and prorenin after receptor binding, it is very important to understand their interaction with the receptor. Several kinds of peptides designed from the regions of the tertiary structure of renin and predicted model of prorenin facilitated the study of the in vitro binding mechanisms for renin and prorenin to (P)RR. Here, a series of recent in vitro studies was reviewed to discuss a possible binding mechanism of renin/prorenin to the (P)RR.

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.

Characterization of recombinant CDR1, an Arabidopsis aspartic proteinase involved in disease resistance

The Arabidopsis thaliana constitutive disease resistance 1 (CDR1) gene product is an aspartic proteinase that has been implicated in disease resistance signaling (Xia, Y., Suzuki, H., Borevitz, J., Blount, J., Guo, Z., Patel, K., Dixon, R. A., and Lamb, C. (2004) EMBO J. 23, 980-988). This apoplastic enzyme is a member of the group of "atypical" plant aspartic proteinases. As for other enzymes of this subtype, CDR1 has remained elusive until recently as a result of its unusual properties and localization. Here we report on the heterologous expression and characterization of recombinant CDR1, which displays unique enzymatic properties among plant aspartic proteinases. The highly restricted specificity requirements, insensitivity toward the typical aspartic proteinase inhibitor pepstatin A, an unusually high optimal pH of 6.0-6.5, proteinase activity without irreversible prosegment removal, and dependence of catalytic activity on formation of a homo-dimer are some of the unusual properties observed for recombinant CDR1. These findings unveil a pattern of unprecedented functional complexity for Arabidopsis CDR1 and are consistent with a highly specific and regulated biological function.

Conformational changes in prorenin during renin inhibition in vitro and in vivo

BACKGROUND

Some renin inhibitors induce changes in the conformation of prorenin in vitro and influence the quantification of active renin by immunoradiometric assays. Whether such changes in renin recognition by monoclonal antibodies exist after oral administration of aliskiren, the first orally available renin inhibitor, is not known.

METHODS

Two commercially available immunoradiometric assays (Cisbio and Nichols) were compared to determine immunoreactive active renin concentrations in plasma samples collected in a single oral dose crossover study comparing the renin inhibitor, aliskiren (300 mg), with the angiotensin II antagonist, valsartan (160 mg), in healthy male subjects.

RESULTS

The addition of aliskiren to plasma samples in vitro, at concentrations of 1-100 micromol/l, increased active renin immunoreactivity in both the Cisbio and Nichols assays. In the crossover study, the two assays gave similar values for the plasma immunoreactive active renin concentration before treatment and following valsartan administration (intraclass coefficient for agreement between the two assays = 0.92). However, a Bland-Altman plot showed a systematic bias towards higher values (1.75-fold higher; 95% confidence interval = 1.02-3.01) in the Nichols than in the Cisbio assay following aliskiren administration. The difference between the results obtained with the two assays depended on incubation time.

CONCLUSIONS

Depending on incubation conditions, circulating renin inhibitors interfere with the recognition of active renin molecules by the monoclonal antibodies used in commercially available assays. Careful consideration must therefore be given to the methodology used for quantifying immunoreactive plasma active renin when patients are treated with renin inhibitors, to avoid an overestimation of the magnitude of active renin release attributable to conformational changes in plasma prorenin.

Plasma renin and aldosterone measurements in low renin hypertensive states

Plasma renin concentrations are extremely low, requiring high sensitivity methods to detect low renin hypertensive states. Moreover, plasma prorenin must not cryoactivate to renin to avoid falsely high values. The enzyme kinetic plasma renin activity (PRA) test has the required sensitivity, whereas direct renin assays and PRA tests with short incubation times are usually not accurate enough. Test specificity is essential for plasma aldosterone. The Nichols Advantage aldosterone assay is fast and automated but requires great attention to quality control. Here, the impact of renin on the aldosterone:renin ratio as a screening test for primary aldosteronism is reviewed. A sensitive plasma renin test is essential for the diagnosis of low renin hypertensive states and, currently, can be consistently achieved only with the PRA radioimmunoassay.

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.

Rat renal prorenin concentration in experimental hypertension

The amount of renal prorenin in models of hypertension in rats was studied by using a novel enzyme (PreR-Co). Ten microgrames of PreR-Co promoted a complete conversion of inactive renin, and during the first 15-min incubation the reaction was under initial velocity conditions. The enzyme-substrate reaction obeyed Michaelis-Menten kinetics, with a Vmax of 0.97 x 10(-5) pmol Ang I/min and a Km of 5.03 x 10(-5) pmol prorenin. The difference between the total renin concentration (TRC) and active renin concentration (ARC) in the normal rat kidney (356.4 +/- 20.6 and 105.3 +/- 7.6 ng Ang I/mg tissue/h respectively), indicated that inactive renin comprised 70% of TRC. In the aortic coarctation model, inactive renin comprised 68 % of TRC in the right kidney and no or very little prorenin was found in the left kidney. In the Goldblatt 2-kidney, 1-clip rats, the right kidney prorenin comprised 61% of the TRC and 54% in the clamped left kidney. After DOCA-Salt treatment prorenin was almost absent in the rat kidneys. In conclusion, we have developed an easy and sensitive method to measure inactive renin in the kidney that may be useful to study the biochemical events of renin maturation in physiological and pathological states.

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.

Improved Immunoradiometric Assay for Plasma Renin

Background: Our renin IRMA overestimated renin in plasmas with high prorenin-to-renin ratios. We suspected that the overestimation of renin was caused less by cross-reactivity of the renin-specific antibody with prorenin than by a conformational change of prorenin into an enzymatically active form during the assay.

Methods: Because the inactive form of prorenin converts slowly into an active form at low temperature, we raised the assay temperature from 22 °C to 37 °C, simultaneously shortening the incubation time from 24 to 6 h. The former IRMA was performed in &lt;1 working day with these modifications.

Results: The comeasurement of prorenin as renin was eliminated. Reagents were stable at 37 °C, and the new and old IRMAs were comparable in terms of precision and accuracy. The functional lower limit of the assay (4 mU/L) was below the lower reference limit (9 mU/L). The modified IRMA agreed closely with the activities measured with an enzyme-kinetic assay. Results were not influenced by the plasma concentration of angiotensinogen. At normal angiotensinogen concentrations, the IRMA closely correlated with the classical enzyme-kinetic assay of plasma renin activity.

Conclusion: The modified IRMA, performed at 37 °C, avoids interference by prorenin while retaining the desirable analytical characteristics of the older IRMA and requiring less time.

In vivo enzymatic assay reveals catalytic activity of the human renin precursor in tissues

The aspartyl protease renin is secreted into the circulation of mammals in 2 forms: the proteolytically processed active form of the enzyme and the precursor form, prorenin. Prorenin has no detectable enzymatic activity in the circulation, but it is the exclusive form of the enzyme produced by several tissues that also produce the other components of the renin enzymatic cascade (renin-angiotensin system). To test whether prorenin might be enzymatically active in these tissues, transgenic mice expressing the human renin substrate (angiotensinogen) exclusively in the pituitary gland were mated to mice expressing either active human renin or prorenin in the same tissue. Measurement of in vivo product formation in pituitary glands of double-transgenic mice revealed that human prorenin was enzymatically active, and Western blot analysis demonstrated that this prorenin was in the precursor form with its prosegment attached. This in vivo enzymatic assay demonstrates for the first time that human prorenin can be activated within tissues by nonproteolytic means, where it could contribute to the activity of a localized renin-angiotensin system.

Uptake and proteolytic activation of prorenin by cultured human endothelial cells

OBJECTIVE

To investigate the mechanisms of vascular uptake of prorenin and renin and to explore the possibility of vascular activation of prorenin.

DESIGN AND METHODS

Human umbilical vein endothelial cells (HUVECs) cultured in a chemically defined medium were incubated with recombinant human prorenin or renin in the presence or absence of putative inhibitors of renin internalization. Cell surface-bound and internalized prorenin or renin were separated by the acid-wash method and were quantified by enzyme-kinetic assays. The activation of prorenin was also monitored by a direct immunoradiometric assay (IRMA) with use of a monoclonal antibody directed against the -p24-Arg to -1p-Arg C-terminal propeptide sequence of prorenin.

RESULTS

Prorenin and renin were internalized at 37 degrees C in a dose-dependent manner; with 1000 microU prorenin/ml medium, the quantity of cell-associated prorenin after 3 h of incubation was 9.3 +/- 1.0 microU/4 x 10(5) cells, and with 75,000 microU/ml medium it was 670 +/- 75 microU/4 x 10(5) cells (mean +/- SD; n = 5). Results for renin were similar. Prorenin that had been treated with endoglycosidase H to remove N-linked oligosaccharides was not internalized. Addition of mannose 6-phosphate (M-6-P) to the medium caused a dose-dependent inhibition of renin and prorenin internalization. Fifty per cent inhibition was observed at 70 micromol/M-6-P, whereas mannose 1-phosphate, glucose 6-phosphate and alpha-methylmannoside at this concentration had no effect Ammonium chloride (50 mmol/l) and monensin (10 micromol/l) also inhibited internalization. Prorenin was activated by HUVECs, and cell-activated prorenin was only found in the internalized fraction, whereas the surface-bound prorenin remained inactive. Thus, it appears that the activation of prorenin took place at the time of its internalization or thereafter. The results of the prorenin IRMA indicated that activation was associated with proteolytic cleavage of the propeptide.

CONCLUSIONS

Our findings provide evidence for M-6-P receptor-dependent endocytosis of (pro)renin and proteolytic prorenin activation by vascular endothelial cells.

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.

Ovarian origin of plasma and peritoneal fluid prorenin in early pregnancy and in patients with ovarian hyperstimulation syndrome

Prorenin is the major product of renin gene expression in the ovary. Plasma levels of prorenin are elevated in ovarian-stimulated patients and during early pregnancy. To further elucidate the source of the elevated plasma levels of prorenin, we measured prorenin, renin activity, angiotensinogen, and steroid hormone levels in the plasma, luteal fluids (luteal cysts), ascitic fluid, and in ovarian venous samples collected from a patient with severe ovarian hyperstimulation syndrome (OHSS) and ectopic pregnancy. Prorenin/renin was also measured in plasma and in peritoneal fluid obtained during, therapeutic paracentesis from four patients with OHSS. Several corpora luteal fluids were obtained that were rich in estradiol (E2) and progesterone (P). Ovarian venous E2 and P were 20-fold higher than in arterial blood and as high or higher than the levels detected in the luteal fluids. The ratios of the hormonal levels in ascitic fluid and plasma were 1.9 for P and 1.4 for E2. A wide range of prorenin concentrations [1279 +/- 918 SD ng/mL/hr, n = 6] were found in corpora luteal fluids, but in each the prorenin concentration was higher than in plasma (494 ng/mL/hr). Prorenin but not renin was higher (+23%) in ovarian venous than arterial blood. Prorenin in the 7 liters of ascitic fluid aspirated (2686 ng/mL/hr) was 5-fold higher than in plasma and similar to the levels measured in the corpora lutea with the highest prorenin concentrations. Renin in luteal cysts and ascitic fluid constituted 3% and 6% of the total renin (renin+prorenin), respectively. Total renin was also higher in peritoneal fluid (1538 +/- 925 ng/mL/hr) than in plasma (375 +/- 237 ng/mL/hr) of the 4 additional patients with severe OHSS. These findings indicate that the ovary secretes prorenin during early pregnancy and that its secretion is directed preferentially from the luteal cysts into the peritoneal cavity. In light of recent evidence of an effect of prorenin on the vascular system, the presence of a huge reservoir of prorenin in the peritoneal cavity of patients with OHSS suggests a potential role for prorenin in the pathogenesis of this syndrome.

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.

Regulation of utero-placental prorenin

Prorenin (Pro) is synthesized in a number of human utero-placental tissues, including chorion, decidua, villous placenta and probably mesenchymal cells. The release of Pro from these extra-renal tissues follows new protein synthesis and appears to utilize the constitutive secretory pathway. Unlike processing in the kidney, very little of the Pro is subsequently cleaved to the smaller product (active renin). Primary signals which regulate Pro include protein hormones and peptides (relaxin, endothelin, hCG), amines (epinephrine, norepinephrine, and related beta adrenergic agents), and eicosanoids. These agents increase the mRNA for prorenin at a time before peak secretory effects are noted. Other extracellular signals have negative regulatory effects. These include angiotensin, endotoxin and cytokines (TNF-alpha and interleukin-1 B). There is also evidence that glucocorticoid receptor activation has an inhibitory effects on Pro release in placenta. Second messengers involved in the regulation of Pro include cyclic AMP and protein kinase A (PKA), protein kinase C (PKC), and calcium. The possible biological effect(s) of the extracellular Pro are unknown but may be due to direct generation of angiotensin I. Since angiotensin-peptides have a number of trophic effect on both vascular and non-vascular tissues, regulation of utero-placental Pro by autocrine, paracrine or endocrine signalling may be critical in normal fetal and/or placental development.

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.

Prorenin in diabetes mellitus

A rise in plasma prorenin often precedes the onset of vascular injury in patients with diabetes mellitus. Plasma prorenin measurements may be useful for predicting which patients will develop vascular injury and for monitoring the progression of the disease. A hypothesis is presented that accounts for these relationships and for the cosecretion of prorenin and renin into the circulation.