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

Effect of Angiotensin II on Chondrocyte Degeneration and Protection via Differential Usage of Angiotensin II Receptors

The renin–angiotensin system (RAS) controls not only systemic functions, such as blood pressure, but also local tissue-specific events. Previous studies have shown that angiotensin II receptor type 1 (AT₁R) and type 2 (AT₂R), two RAS components, are expressed in chondrocytes. However, the angiotensin II (ANG II) effects exerted through these receptors on chondrocyte metabolism are not fully understood. In this study, we investigated the effects of ANG II and AT₁R blockade on chondrocyte proliferation and differentiation. Firstly, we observed that ANG II significantly suppressed cell proliferation and glycosaminoglycan content in rat chondrocytic RCS cells. Additionally, ANG II decreased CCN2, which is an anabolic factor for chondrocytes, via increased MMP9. In Agtr1a-deficient RCS cells generated by the CRISPR-Cas9 system, Ccn2 and Aggrecan (Acan) expression increased. Losartan, an AT₁R antagonist, blocked the ANG II-induced decrease in CCN2 production and Acan expression in RCS cells. These findings suggest that AT₁R blockade reduces ANG II-induced chondrocyte degeneration. Interestingly, AT₁R-positive cells, which were localized on the surface of the articular cartilage of 7-month-old mice expanded throughout the articular cartilage with aging. These findings suggest that ANG II regulates age-related cartilage degeneration through the ANG II–AT₁R axis.

Longitudinal Changes in Prorenin and Renin in the Chronic Renal Insufficiency Cohort

INTRODUCTION

Prorenin, a precursor of renin, and renin play an important role in regulation of the renin-angiotensin system. More recently, receptor-bound prorenin has been shown to activate intracellular signaling pathways that mediate fibrosis, independent of angiotensin II. Prorenin and renin may thus be of physiologic significance in CKD, but their plasma concentrations have not been well characterized in CKD.

METHODS

We evaluated distribution and longitudinal changes of prorenin and renin concentrations in the plasma samples collected at follow-up years 1, 2, 3, and 5 of the Chronic Renal Insufficiency Cohort (CRIC) study, an ongoing longitudinal observational study of 3,939 adults with CKD. Descriptive statistics and multivariable regression of log-transformed values were used to describe cross-sectional and longitudinal variation and associations with participant characteristics.

RESULTS

A total of 3,361 CRIC participants had plasma available for analysis at year 1. The mean age (±standard deviation, SD) was 59 ± 11 years, and the mean estimated glomerular filtration rate (eGFR, ± SD) was 43 ± 17 mL/min per 1.73 m2. Median (interquartile range) values of plasma prorenin and renin at study entry were 4.4 (2.1, 8.8) ng/mL and 2.0 (0.8, 5.9) ng/dL, respectively. Prorenin and renin were positively correlated (Spearman correlation 0.51, p < 0.001) with each other. Women and non-Hispanic blacks had lower prorenin and renin values at year 1. Diabetes, lower eGFR, and use of angiotensin-converting enzyme inhibitors, angiotensin receptor blockers, statins, and diuretics were associated with higher levels. Prorenin and renin decreased by a mean of 2 and 5% per year, respectively. Non-Hispanic black race and eGFR <30 mL/min/1.73 m2 at year 1 predicted a steeper decrease in prorenin and renin over time. In addition, each increase in urinary sodium excretion by 2 SDs at year 1 increased prorenin and renin levels by 4 and 5% per year, respectively.

DISCUSSION/CONCLUSIONS

The cross-sectional clinical factors associated with prorenin and renin values were similar. Overall, both plasma prorenin and renin concentrations decreased over the years, particularly in those with severe CKD at study entry.

An international cohort study of autosomal dominant tubulointerstitial kidney disease due to REN mutations identifies distinct clinical subtypes

There have been few clinical or scientific reports of autosomal dominant tubulointerstitial kidney disease due to REN mutations (ADTKD-REN), limiting characterization. To further study this, we formed an international cohort characterizing 111 individuals from 30 families with both clinical and laboratory findings. Sixty-nine individuals had a REN mutation in the signal peptide region (signal group), 27 in the prosegment (prosegment group), and 15 in the mature renin peptide (mature group). Signal group patients were most severely affected, presenting at a mean age of 19.7 years, with the prosegment group presenting at 22.4 years, and the mature group at 37 years. Anemia was present in childhood in 91% in the signal group, 69% prosegment, and none of the mature group. REN signal peptide mutations reduced hydrophobicity of the signal peptide, which is necessary for recognition and translocation across the endoplasmic reticulum, leading to aberrant delivery of preprorenin into the cytoplasm. REN mutations in the prosegment led to deposition of prorenin and renin in the endoplasmic reticulum-Golgi intermediate compartment and decreased prorenin secretion. Mutations in mature renin led to deposition of the mutant prorenin in the endoplasmic reticulum, similar to patients with ADTKD-UMOD, with a rate of progression to end stage kidney disease (63.6 years) that was significantly slower vs. the signal (53.1 years) and prosegment groups (50.8 years) (significant hazard ratio 0.367). Thus, clinical and laboratory studies revealed subtypes of ADTKD-REN that are pathophysiologically, diagnostically, and clinically distinct.

Angiotensin generation in the brain: a re-evaluation

The existence of a so-called brain renin-angiotensin system (RAS) is controversial. Given the presence of the blood-brain barrier, angiotensin generation in the brain, if occurring, should depend on local synthesis of renin and angiotensinogen. Yet, although initially brain-selective expression of intracellular renin was reported, data in intracellular renin knockout animals argue against a role for this renin in angiotensin generation. Moreover, renin levels in brain tissue at most represented renin in trapped blood. Additionally, in neurogenic hypertension brain prorenin up-regulation has been claimed, which would generate angiotensin following its binding to the (pro)renin receptor. However, recent studies reported no evidence for prorenin expression in the brain, nor for its selective up-regulation in neurogenic hypertension, and the (pro)renin receptor rather displays RAS-unrelated functions. Finally, although angiotensinogen mRNA is detectable in the brain, brain angiotensinogen protein levels are low, and even these low levels might be an overestimation due to assay artefacts. Taken together, independent angiotensin generation in the brain is unlikely. Indeed, brain angiotensin levels are extremely low, with angiotensin (Ang) I levels corresponding to the small amounts of Ang I in trapped blood plasma, and Ang II levels at most representing Ang II bound to (vascular) brain Ang II type 1 receptors. This review concludes with a unifying concept proposing the blood origin of angiotensin in the brain, possibly resulting in increased levels following blood-brain barrier disruption (e.g. due to hypertension), and suggesting that interfering with either intracellular renin or the (pro)renin receptor has consequences in an RAS-independent manner.

Brain Renin-Angiotensin System: Does It Exist?

Because of the presence of the blood-brain barrier, brain renin-angiotensin system activity should depend on local (pro)renin synthesis. Indeed, an intracellular form of renin has been described in the brain, but whether it displays angiotensin (Ang) I-generating activity (AGA) is unknown. Here, we quantified brain (pro)renin, before and after buffer perfusion of the brain, in wild-type mice, renin knockout mice, deoxycorticosterone acetate salt-treated mice, and Ang II-infused mice. Brain regions were homogenized and incubated with excess angiotensinogen to detect AGA, before and after prorenin activation, using a renin inhibitor to correct for nonrenin-mediated AGA. Renin-dependent AGA was readily detectable in brain regions, the highest AGA being present in brain stem (>thalamus=cerebellum=striatum=midbrain>hippocampus=cortex). Brain AGA increased marginally after prorenin activation, suggesting that brain prorenin is low. Buffer perfusion reduced AGA in all brain areas by >60%. Plasma renin (per mL) was 40× to 800× higher than brain renin (per gram). Renin was undetectable in plasma and brain of renin knockout mice. Deoxycorticosterone acetate salt and Ang II suppressed plasma renin and brain renin in parallel, without upregulating brain prorenin. Finally, Ang I was undetectable in brains of spontaneously hypertensive rats, while their brain/plasma Ang II concentration ratio decreased by 80% after Ang II type 1 receptor blockade. In conclusion, brain renin levels (per gram) correspond with the amount of renin present in 1 to 20 μL of plasma. Brain renin disappears after buffer perfusion and varies in association with plasma renin. This indicates that brain renin represents trapped plasma renin. Brain Ang II represents Ang II taken up from blood rather than locally synthesized Ang II.

A Genetic Variant in the Distal Enhancer Region of the Human Renin Gene Affects Renin Expression

Background

The high heritability of plasma renin activity was confirmed in recent investigations. A variation located near the strong enhancer of the human renin gene (REN), C-5312T, has been shown to have different transcription activity levels depending on its allele: the 5312T allele shows transcription levels that are 45% greater than those of the 5312C allele. The purpose of this study was to confirm the hypothesis that variations in the enhancer region of the REN gene are involved in regulating renal expression of renin.

Methods

Sixty-four subjects with biopsy-proven renal diseases were included in this study (male/female: 35/29, age 41.9 ± 20.9 years, SBP/DBP 123.1 ± 23.7/73.4 ± 14.8 mmHg, s-Cr 0.93 ± 0.63 mg/dl). A genetic variant of REN, C-5312T, was assayed by PCR-RFLP and the TaqMan method. Total RNAs from a small part of the renal cortex were reverse-transcribed and amplified for REN and GAPDH with a real-time PCR system.

Results

Logarithmically transformed expression values of the relative ratio of REN to GAPDH (10⁻³) were as follows (mean ± SE): CC (26 cases), 0.016 ± 0.005; CT (33 cases), 0.047 ± 0.021 (p = 0.41 vs. CC); TT (5 cases), 0.198 ± 0.194 (p = 0.011 vs. CC, p < 0.031 vs. CT). Thus, significant differences in REN expression were observed among the genetic variants.

Conclusion

The results suggest that variants in the enhancer region of the human renin gene have an effect on the expression levels of renin in renal tissue; this observation is in good accordance with the results of the transcriptional assay.

Determinants of plasma renin activity: role of a human renin gene variant as a genetic factor

The plasma renin activity (PRA) is affected by a number of environmental factors. However, significant heritability has been shown for the activity. A hypothesis that a candidate regulatory single-nucleotide polymorphism, C-5312T, of human renin gene should have a significant effect on PRA was elucidated and updating of independent determinants of PRA was attempted. Cross sectional study. Outpatient study. We enrolled consecutive 810 subjects who had consulted our hospitals for lifestyle-related diseases. Genotypes were assayed with genomic DNA for C-5312T. Among the genetic variants, the difference of PRA was evaluated. Monovariate linear regression analysis was performed to test the correlation between PRA and clinical variables. Finally, stepwise multiple regression analysis was performed to evaluate the independent determinants. On comparing 2 genotype groups, CC/CT and T allele homozygote, the geometric means of PRA were 0.778 and 0.941 ng/ml/h, respectively (F = 5.992, P = 0.015). Monovariate linear regression analysis revealed that a number of variables have a significant correlation with the activity, including urinary salt excretion. A stepwise multivariate regression analysis revealed that renin C-5312T variant (TT) is one of the independent determinants of PRA. Thus, for the first time, a human renin gene variant was associated with a significant increase in PRA as a genetic factor and the independent determinants for the activity were updated including genetic factor.

General lysosomal hydrolysis can process prorenin accurately

Renin, an aspartyl protease that catalyzes the rate-limiting step of the renin-angiotensin system, is first synthesized as an inactive precursor, prorenin. Prorenin is activated by the proteolytic removal of an amino terminal prosegment in the dense granules of the juxtaglomerular (JG) cells of the kidney by one or more proteases whose identity is uncertain but commonly referred to as the prorenin-processing enzyme (PPE). Because several extrarenal tissues secrete only prorenin, we tested the hypothesis that the unique ability of JG cells to produce active renin might be explained by the existence of a PPE whose expression is restricted to JG cells. We found that inducing renin production by the mouse kidney by up to 20-fold was not associated with the concomitant induction of candidate PPEs. Because the renin-containing granules of JG cells also contain several lysosomal hydrolases, we engineered mouse Ren1 prorenin to be targeted to the classical vesicular lysosomes of cultured HEK-293 cells, where it was accurately processed and stored. Furthermore, we found that HEK cell lysosomes hydrolyzed any artificial extensions placed on the protein and that active renin was extraordinarily resistant to proteolytic degradation. Altogether, our results demonstrate that accurate processing of prorenin is not restricted to JG cells but can occur in classical vesicular lysosomes of heterologous cells. The implication is that renin production may not require a specific PPE but rather can be achieved by general hydrolysis in the lysosome-like granules of JG cells.

Identification of bona fide alternative renin transcripts expressed along cortical tubules and potential roles in promoting insulin resistance in vivo without significant plasma renin activity elevation

Renin belongs to a family of aspartyl proteases and is the rate-limiting enzyme in the synthesis of the potent vasoactive peptide angiotensin II. Processing of renal renin has been extensively investigated in juxtaglomerular granular cells, in which prorenin and active renin are present in secretory condensed granules. Previous studies demonstrated alternative renin transcription in rat adrenal glands. Different studies reported novel intracellular forms of renin deduced from novel 5' variants derived from renin mRNA in both mice and humans. Comprehensive detailed studies in genetically engineered mice showed that both a secreted and an intracellular form of renin plays divergent mechanism regulating fluid intake and metabolism by the brain renin-angiotensin system; however, the presence, regulation, and functions of these renin isoforms in kidney and adrenal gland are not fully understood in mice. To investigate the characteristics of renin isoforms in mice, we performed a systematic inventory of renin transcripts of mice with and without a duplication of the renin gene alternatively from previous studies. We discovered a novel isoform of renin of the Ren2 gene, which conserved functionally important residues of the prosegment and incomplete isoforms of the Ren1C/D gene lacking a pre-pro segment. In situ hybridization assays revealed alternative renin isoforms expressed along cortical tubules. Newly generated transgenic mice with systemic overexpression of alternative renin transcript showed enhanced local angiotensin II generation without elevation of plasma renin activity and systemic insulin resistance in vivo, providing new pathophysiological insights into insulin resistance exaggerated by bona fide renin isoform.

Antenatal betamethasone exposure is associated with lower ANG-(1-7) and increased ACE in the CSF of adult sheep

Antenatal betamethasone (BM) therapy accelerates lung development in preterm infants but may induce early programming events with long-term cardiovascular consequences. To elucidate these events, we developed a model of programming whereby pregnant ewes are administered BM (2 doses of 0.17 mg/kg) or vehicle at the 80th day of gestation and offspring are delivered at term. BM-exposed (BMX) offspring develop elevated blood pressure; decreased baroreflex sensitivity; and alterations in the circulating, renal, and brain renin-angiotensin systems (RAS) by 6 mo of age. We compared components of the choroid plexus fourth ventricle (ChP4) and cerebral spinal fluid (CSF) RAS between control and BMX male offspring at 6 mo of age. In the choroid plexus, high-molecular-weight renin protein and ANG I-intact angiotensinogen were unchanged between BMX and control animals. Angiotensin-converting enzyme 2 (ACE2) activity was threefold higher than either neprilysin (NEP) or angiotensin 1-converting enzyme (ACE) in control and BMX animals. Moreover, all three enzymes were equally enriched by approximately 2.5-fold in ChP4 brush-border membrane preparations. CSF ANG-(1-7) levels were significantly lower in BMX animals (351.8 ± 76.8 vs. 77.5 ± 29.7 fmol/mg; P < 0.05) and ACE activity was significantly higher (6.6 ± 0.5 vs. 8.9 ± 0.5 fmol·min(-1)·ml(-1); P < 0.05), whereas ACE2 and NEP activities were below measurable limits. A thiol-sensitive peptidase contributed to the majority of ANG-(1-7) metabolism in the CSF, with higher activity in BMX animals. We conclude that in utero BM exposure alters CSF but not ChP RAS components, resulting in lower ANG-(1-7) levels in exposed animals.

The Prorenin and (Pro)renin Receptor: New Players in the Brain Renin-Angiotensin System?

It is well known that the brain renin-angiotensin (RAS) system plays an essential role in the development of hypertension, mainly through the modulation of autonomic activities and vasopressin release. However, how the brain synthesizes angiotensin (Ang) II has been a debate for decades, largely due to the low renin activity. This paper first describes the expression of the vasoconstrictive arm of RAS components in the brain as well as their physiological and pathophysiological significance. It then focus on the (pro)renin receptor (PRR), a newly discovered component of the RAS which has a high level in the brain. We review the role of prorenin and PRR in peripheral organs and emphasize the involvement of brain PRR in the pathogenesis of hypertension. Some future perspectives in PRR research are heighted with respect to novel therapeutic target for the treatment of hypertension and other cardiovascular diseases.

Human prorenin structure sheds light on a novel mechanism of its autoinhibition and on its non-proteolytic activation by the (pro)renin receptor

Antibodies and prorenin mutants have long been used to structurally characterize prorenin, the inactive proenzyme form of renin. They were designed on the basis of homology models built using other aspartyl protease proenzyme structures since no structure was available for prorenin. Here, we present the first X-ray structure of a prorenin. The current structure of prorenin reveals that, in this zymogene, the active site of renin is blocked by the N-terminal residues of the mature version of the renin molecule, which are, in turn, covered by an Ω-shaped prosegment. This prevents access of substrates to the active site. The departure of the prosegment on activation induces an important global conformational change in the mature renin molecule with respect to prorenin: similar to other related enzymes such as pepsin or gastricsin, the segment that constitutes the N-terminal β-strand in renin is displaced from the renin active site by about 180° straight into the position that corresponds to the N-terminal β-strand of the prorenin prosegment. This way, the renin active site will become completely exposed and capable of carrying out its catalytic functions. A unique inactivation mechanism is also revealed, which does not make use of a lysine against the catalytic aspartates, probably in order to facilitate pH-independent activation [e.g., by the (pro)renin receptor].

Evaluation of a direct prorenin assay making use of a monoclonal antibody directed against residues 32-39 of the prosegment

BACKGROUND

Prorenin is an early marker of microvascular complications in diabetes. However, it can only be measured indirectly (following its conversion to renin), with a renin immunoradiometric assay (IRMA). Unfortunately, treatment with a renin inhibitor interferes with this assay, because renin inhibitors induce a conformational change in prorenin, thereby allowing its detection as renin.

METHODS

We evaluated Molecular Innovation's new direct prorenin ELISA, which makes use of an antibody that recognizes an epitope near prorenin's putative cleavage site (R 43 L 44), thus no longer requiring prorenin activation. Plasma samples of 41 diabetic individuals treated with aliskiren (renin inhibitor) or irbesartan were tested. Semi-purified recombinant prorenin was used as standard, because the ELISA standard yielded approximately 10-fold lower values in the renin IRMA following its conversion to renin.

RESULTS

The ELISA detected prorenin levels that were identical to those determined by the IRMA in untreated and irbesartan-treated individuals. Yet, it yielded higher prorenin levels in aliskiren-treated individuals. Aliskiren, at levels reached in plasma during treatment, did not interfere with the ELISA, but allowed the detection of up to 20-30% of prorenin as renin in the IRMA, thereby resulting in a significant overestimation of renin and an underestimation of prorenin. The ELISA rendered results within 2 h and did not require a pretreatment period of several days to convert prorenin to renin.

CONCLUSION

The new direct assay allows rapid prorenin detection, is not hampered by aliskiren when used at clinically relevant doses, and might be used to identify diabetic patients developing retinopathy and/or nephropathy.

A specific binding site for the prorenin propart peptide Arg10-Arg20 does not occur on human endothelial cells

INTRODUCTION

We looked for novel binding sites for the human prorenin 'decoy peptide' sometimes called 'handle region peptide' on human endothelial cells.

METHOD

The biotinylated peptide biotin-Acp-RIFLKRMPSIR (B-PR), an unlabelled peptide PR1 (RIFLKRMPSIR) and a scrambled peptide scPR1 (SRRMIFPIKLR) were synthesized. B-PR was added to human umbilical cord endothelial cells (HUVECs) maintained in serum-free medium, with or without excess unlabelled peptide or 'scrambled' peptide as blocker. Biotin-labelled HUVEC proteins were extracted, the amount of bound tracer was measured, and the identity of the binding proteins was analysed by sodium-dodecyl-sulphate polyacrylamide gel electrophoresis (SDS-PAGE) and by liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS).

RESULTS

Biotinylated peptide bound to the HUVEC proteins with a major labelled band at 68,600 ± 1503 kDa (mean ± SEM, n = 5 runs). Unlabelled peptide and scrambled peptide equally displaced the labelled peptide, indicating that the binding was non-specific for amino acid sequence. LC-MS/MS showed that binding was mainly to cytoskeletal proteins.

CONCLUSION

The binding of the human prorenin peptide R¹⁰IFLKRMPSIR²⁰ to HUVEC proteins is not specific for amino acid sequence and probably involves a general peptide/protein uptake mechanism. We could not detect a specific prorenin propart binding site in these cells.

Cathepsin B is not the processing enzyme for mouse prorenin

Renin, an aspartyl protease that catalyzes the rate-limiting step in the renin-angiotensin system (RAS), is proteolytically activated by a second protease [referred to as the prorenin processing enzyme (PPE)] before its secretion from the juxtaglomerular cells of the kidney. Although several enzymes are capable of activating renin in vitro, the leading candidate for the PPE in the kidney is cathepsin B (CTSB) due to is colocalization with the renin precursor (prorenin) in juxtaglomerular cell granules and because of its site-selective activation of human prorenin both in vitro and in transfected tissue culture cell models. To verify the role of CTSB in prorenin processing in vivo, we tested the ability of CTSB-deficient (CTSB−/−) mice to generate active renin. CTSB−/− mice do not exhibit any overt symptoms (renal malformation, preweaning mortality) typical of an RAS deficiency and have normal levels of circulating active renin, which, like those in control animals, rise more than 15-fold in response to pharmacologic inhibition of the RAS. The mature renin enzyme detected in kidney lysates of CTSB−/− mice migrates at the same apparent molecular weight as that in control mice, and the processing to active renin is not affected by chloroquine treatment of the animals. Finally, the distribution and morphology of renin-producing cells in the kidney is normal in CTSB−/− mice. In conclusion, CTSB-deficient mice exhibit no differences compared with controls in their ability to generate active renin, and our results do not support CTSB as the PPE in mice.

Telomere length and outcome in heart failure

BACKGROUND

Telomeres are causally involved in senescence. Senescence is a potential factor in the pathogenesis and progression of heart failure. In heart failure telomeres are shorter, but the prognostic value associated with telomere length has not been defined.

METHODS

Telomere length was prospectively determined by quantitative polymerase chain reaction in 890 patients with New York Heart Association (NYHA) functional class II to IV heart failure. After 18 months, we examined the association between telomere length and the predefined primary end-point: time to death or hospitalization for heart failure.

RESULTS

Mean age of the patients was 71 years, 39% were women, 51% were in NYHA class II, and 49% were in class III/IV. A total of 344 patients reached the primary end-point (130 deaths and 214 hospitalizations). Patients with shorter telomeres were at an increased risk of reaching the primary end-point (hazard ratio 1.79; 95% confidence interval (CI) 1.21-2.63). In multivariate analysis shorter telomere length remained associated with a higher risk for death or hospitalization (hazard ratio, 1.74; 95% CI 1.07-2.95) after adjustment for age of heart failure onset, gender, hemoglobin, renal function, and N-terminal pro-B-type natriuretic peptide level, a history of stroke, atrial fibrillation, and diabetes.

CONCLUSIONS

Shorter length of telomeres predicts the occurrence of death or hospitalization in patients with chronic heart failure.

A current view of brain renin-angiotensin system: Is the (pro)renin receptor the missing link?

The renin-angiotensin system (RAS) plays a central role in the brain to regulate blood pressure (BP). This role includes the modulation of sympathetic nerve activity (SNA) that regulates vascular tone; the regulation of secretion of neurohormones that have a critical role in electrolyte as well as fluid homeostasis; and by influencing behavioral processes to increase salt and water intake. Based on decades of research it is clear that angiotensin II (Ang II), the major bioactive product of the RAS, mediates these actions largely via its Ang II type 1 receptor (AT1R), located within hypothalamic and brainstem control centers. However, the mechanisms of brain RAS function have been questioned, due in large part to low expression levels of the rate limiting enzyme renin within the central nervous system. Tissue localized RAS has been observed in heart, kidney tubules and vascular cells. Studies have also given rise to the hypothesis for localized RAS function within the brain, so that Ang II can act in a paracrine manner to influence neuronal activity. The recently discovered (pro)renin receptor (PRR) may be key in this mechanism as it serves to sequester renin and prorenin for localized RAS activity. Thus, the PRR can potentially mitigate the low levels of renin expression in the brain to propagate Ang II action. In this review we examine the regulation, expression and functional properties of the various RAS components in the brain with particular focus on the different roles that PRR may have in BP regulation and hypertension.

Preservation of intracellular renin expression is insufficient to compensate for genetic loss of secreted renin

The primary product of the renin gene is preprorenin. A signal peptide sorts renin to the secretory pathway in juxtaglomerular cells where it is released into the circulation to initiate the renin-angiotensin system cascade. In the brain, transcription of renin occurs from an alternative promoter encoding an mRNA starting with a new first exon (exon 1b). Exon 1b initiating transcripts skip over the classical first exon (exon 1a) containing the initiation codon for preprorenin. Exon 1b transcripts are predicted to use a highly conserved initiation codon within exon 2, producing renin, which should remain intracellular, because it lacks the signal peptide. To evaluate the roles of secreted and intracellular renin, we took advantage of the organization of the renin locus to generate a secreted renin (sRen)-specific knockout, which preserves intracellular renin expression. Expression of sRen mRNA was ablated in the brain and kidney, whereas intracellular renin mRNA expression was preserved in fetal and adult brains. We noted a developmental shift from the expression of sRen mRNA in the fetal brain to intracellular renin mRNA in the adult brain. Homozygous sRen knockout mice exhibited very poor survival at weaning. The survivors exhibited renal lesions, low hematocrit, an inability to generate a concentrated urine, decreased arterial pressure, and impaired aortic contraction. These results suggest that preservation of intracellular renin expression in the brain is not sufficient to compensate for a loss of sRen, and sRen plays a pivotal role in renal development and function, survival, and the regulation of arterial pressure.

Recombinant prosegment peptide acts as a folding catalyst and inhibitor of native pepsin

Porcine pepsin A, a gastric aspartic peptidase, is initially produced as the zymogen pepsinogen that contains an N-terminal, 44 residue prosegment (PS) domain. In the absence of the PS, native pepsin (Np) is irreversibly denatured and when placed under refolding conditions, folds to a thermodynamically stable denatured state. This denatured, refolded pepsin (Rp) state can be converted to Np by the exogenous addition of the PS, which catalyzes the folding of Rp to Np. In order to thoroughly study the mechanism by which the PS catalyzes pepsin folding, a soluble protein expression system was developed to produce recombinant PS peptide in a highly pure form. Using this system, the wild-type and three-mutant PS forms, in which single residue substitutions were made (V4A, R8A and K36A), were expressed and purified. These PS peptides were characterized for their ability to inhibit Np enzymatic activity and to catalyze the folding of Rp to Np. The V4A, R8A and K36A mutant PS peptides were found to have nanomolar inhibition constants, Ki, of 82.4, 58.3 and 95.6 nM, respectively, approximately a two-fold increase from that of the wild-type PS (36.2 nM). All three-mutant PS peptides were found to catalyze Np folding with a rate constant of 0.06 min(-1), five-fold lower than that of the wild-type. The observation that the mutant PS peptides retained their inhibition and folding-catalyst functionality suggests a high level of resilience to mutations of the pepsin PS.

Multifunctional aspartic peptidase prosegments

The structure-function relationships of aspartic peptidases (APs) (EC 3.4.23.X) have been extensively investigated, yet much remains to be elucidated regarding the various molecular mechanisms of these enzymes. Over the past years, APs have received considerable interest for food applications (e.g. cheese, fermented foods) and as potential targets for pharmaceutical intervention in human diseases including hypertension, cancer, Alzheimer's disease, AIDS (acquired immune deficiency syndrome), and malaria. A deeper understanding of the structure and function of APs, therefore, will have a direct impact on the design of peptidase inhibitors developed to treat such diseases. Most APs are synthesized as zymogens which contain an N-terminal prosegment (PS) domain that is removed at acidic pH by proteolytic cleavage resulting in the active enzyme. While the nature of the AP PS function is not entirely understood, the PS can be important in processes such as the initiation of correct folding, protein stability, blockage of the active site, pH-dependence of activation, and intracellular sorting of the zymogen. This review summarizes the current knowledge of AP PS function (especially within the A1 family), with particular emphasis on protein folding, cellular sorting, and inhibition.

An intracellular renin-angiotensin system in neurons: fact, hypothesis, or fantasy

The renin-angiotensin system in the brain acts to regulate a number of physiological processes. Evidence suggests that angiotensin peptides may act as neurotransmitters, although their biosynthetic pathways are poorly understood. We review evidence for neuronal production of angiotensin peptides and hypothesize that angiotensin may be synthesized intracellularly in neurons.

Twenty years of the (pro)renin receptor

The (pro)renin receptor [(P)RR] specifically binds renin and prorenin and mediates their intracellular effects. It acts as co-factor for renin and prorenin by increasing their enzymatic activity on the cell-surface and it activates the mitogen activated protein kinases ERK1/2 (extracellular signal regulated kinase) cascade leading to cell proliferation and to upregulation of profibrotic genes expression. Studies in genetically modified animals over-expressing ubiquitously (P)RR or specifically in smooth-muscle cells suggest a direct role for (P)RR cardiovascular and renal pathologies. A putative (P)RR blocker consisting in part of the prosegment of prorenin gave spectacular results in the prevention of diabetic nephropathy and cardiac fibrosis but its mechanism of action and its specificity for (P)RR remain controversial. Unexpectedly, the total ablation of (P)RR gene is impossible in contrast to the other components of the renin angiotensin system (RAS) and studies in zebra fish and in embryonic stem cells indicate that (P)RR is necessary to cell survival and proliferation. Furthermore, a mutation of (P)RR is associated with mental retardation and epilepsy, pointing to an essential role of (P)RR in brain development. If the role of (P)RR in cardiovascular and renal diseases can be confirmed in (P)RR knockout animals, the benefit of a (P)RR blocker in order to optimize the tissue RAS blockade should really be addressed but not without a good understanding of all its functions and not only those related to the RAS.

The (pro)renin receptor: a new kid in town

Renin inhibitors are now available in therapeutic doses and it is accepted that they decrease blood pressure as efficiently as the classic inhibitors of the renin-angiotensin system (RAS): angiotensin converting enzyme inhibitors and angiotensin II-receptor blockers (ARBs). One major issue will be to know how, beyond the normalization of blood pressure, renin inhibitors (RIs) will compare with angiotensin converting enzyme inhibitors and ARBs for their ability to protect the organs against the tissue damage associated with overactivation of the RAS. The mechanism(s) of tissue protection may involve the inhibition of a direct cellular effect of renin and prorenin mediated by the (pro)renin receptor ([P]RR). This review updates the recent findings on (P)RR; its role in hypertension, cardiac fibrosis, diabetic nephropathy, and retinopathy; and the effects of a putative (P)RR antagonist.

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.

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.

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.

Cloning and characterization of marmoset renin: comparison with human renin

The poor interspecies conservation of the renin-angiotensin system prevents the use of nonprimate in vivo models to test renin inhibitors. Thus the small New-World monkey marmoset is used in many instances as a model. However, large differences between the potencies of renin inhibitors as measured in human and marmoset plasma were observed. To understand this phenomenon, we cloned marmoset renin and angiotensinogen. They were highly homologous to their human counterparts, except for a six-residue deletion in the marmoset renin propeptide. Human and marmoset recombinant renins were found in vitro to display comparable activities, suggesting that the observed differences in plasma apparent affinity of inhibitors could be due to different plasma protein binding of the inhibitors.

Human renin gene BglI dimorphism associated with hypertension in two independent populations

The renin (REN) gene is a good candidate that could underlie an individual's genetic susceptibility to human essential hypertension (EHT). We describe here a polymerase chain reaction-based assay for detection of a BglI dimorphic site located in the first intron of the REN gene. In this retrospective, case-control, association study, we investigated BglI allele and genotype distributions in 554 subjects (280 hypertensives and 274 normotensives) from the United Arab Emirates (UAE) - a genetically homogeneous ethnic population with no history of smoking or alcohol consumption - and in 485 hypercholesterolemic, US Caucasian subjects (250 hypertensives and 235 normotensives). A statistically significant association was found between alleles on which the BglI site is present [BglI(+)] and clinical diagnosis of EHT in the UAE sample group (odds ratio = 2.69, p = 0.0006), and a similar trend was observed in the US group (odds ratio = 1.97, p = 0.01). BglI(+) homozygous status was also investigated in the US group and found to be associated with elevated systolic and diastolic blood pressure values (respectively, 144.8+/-26.1 vs. 134.1+/-23.0 mm Hg, p = 0.04; and 91.0+/-12.5 vs. 82.2+/-12.7 mm Hg, p = 0.009). In conclusion, variations of the REN (or of a nearby) gene that may be in linkage disequilibrium with the REN BglI(+) marker could play a role in contributing to an increased individual's genetic susceptibility to EHT in the UAE population and amongst US hypercholesterolemic Caucasians. Such a genetic influence, which seems to show a recessive mode of inheritance, could also be implicated in raising both systolic and diastolic blood pressures.

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.

Non-proteolytic activation of human prorenin by anti-prorenin prosegment (pf#1: 1P-15P) antiserum

Recombinant human prorenin was activated by incubation with anti-prorenin prosegment (L1PPTDTTTFKRIFLKR15P) antiserum at 4 degrees C. This activation was dependent on the concentration of the antiserum and incubation time. After the activation no molecular weight alteration of prorenin was observed by immunoblotting analysis. A peptide of L1PPTDTTTF8P as well as L1PPTDTTTFKRIFLKR15P potently interfered with the activation. Most of the activated prorenin bound to Protein A Sepharose CL 4B. The Km and Vmax values of the activated prorenin were 0.2 microM and 23.7 micrograms Ang I/ml/h, respectively, which were similar in level to those of mature renin obtained by trypsinization.

Prorenin processing by cathepsin B in vitro and in transfected cells

Renin, which catalyzes the initial proteolytic cleavage reaction in the production of angiotensins, is first synthesized as a zymogen, prorenin, and requires the proteolytic removal of an amino-terminal prosegment for activation in vivo. The lysosomal hydrolase cathepsin B has been proposed as a prorenin processing enzyme based on reports of its co-localization with renin in the secretory granules of certain tissues and its ability to activate prorenin in vitro. In the current study, scanning mutagenesis was used to identify the amino acids which determine the site selectivity of prorenin cleavage by human cathepsin B in vitro. Co-expression assays in AtT-20 cells were also used to test for the ability of cathepsin B to cleave human prorenin within cells. Our results suggest that a basic lysine residue at the -2 position from the cleavage site is required for cathepsin B cleavage of prorenin in vitro and that the structure of prorenin itself may account for the selection of the proper cleavage site. In addition, although cathepsin B appears to be correctly sorted to lysosomes, the enzyme exhibits prorenin processing activity in transfected AtT-20 cells, raising the question of the cellular localization in which the processing event occurs.

The application of molecular genetic techniques to the study of hypertensive diseases

The techniques of modern molecular genetics are shedding new light on hypertension and its sequelae. This article discusses techniques which have identified genes associated with hypertension and have pointed the way toward identifying the full cohort of genes operative in all forms of human hypertension. These techniques have expanded understanding of the pathophysiology of hypertension as well as its prevention.

Prorenin activation and prohormone convertases in the mouse As4.1 cell line

The precise identification of prorenin-processing enzymes has been hampered by the very low abundance of juxtaglomerular cells in the kidney. Recently, an immortalized renin-producing renal tumor cell line (As4.1) has been proposed as a model to carry out such studies. Despite the fact that they contain secretory granules, we found no evidence (on the basis of enzymatic assays of renin activity in the supernatant of the cells and of immunoprecipitations experiments) that the As4.1 cells can secrete active renin through the regulated pathway. As4.1 cells produce only renin-1, as they derive from a strain of mice expressing only one renin gene. However, stable transfection of these cells with a renin-2 expression plasmid increased the capacity of this cell line to secrete active renin in the regulated pathway. Northern blot and reverse transcriptase-polymerase chain reaction amplification (RT-PCR) assays revealed that furin, PACE4 and PC5 were the only members of the proprotein convertase (PC) family to be present in these cells. As PC5 is the only such enzyme with the demonstrated ability to process mouse prorenin 2, it may constitute a candidate enzyme for the processing of prorenin-2 in mouse juxtaglomerular cells. However, it is not likely to be involved in the processing of mouse prorenin 1.

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.