Form Of Renin Research Articles

Case-inspired exploration of renin mutations in autosomal dominant tubulointerstitial kidney disease: not all paths lead to the endoplasmic reticulum.

Autosomal dominant tubulointerstitial kidney disease (ADTKD) results from mutations in various genes, including REN, UMOD, MUC1, and HNF1B. ADTKD due to REN mutations (ADTKD-REN) is often characterized as a proteinopathy that triggers the endoplasmic reticulum stress (ERS) cascade, potentially sharing similarities with ADTKD-UMOD and ADTKD-MUC1 at the cellular level. This study, inspired by a patient harboring a W17R mutation, investigates ERS activation by this mutation alongside two other renin variants, W10R and L381P. We established stable cell lines expressing both wild-type and mutated renin forms (W17R, W10R, and L381P). Using luciferase reporter assays, RT-qPCR, and confocal microscopy, we evaluated ERS activation, determined the cellular localization of the renin variants, and characterized the mitochondrial network in the W17R line. The L381P line exhibited ERS activation, including transcriptional upregulation of MANF and CRELD2. No ERS activation was observed in the W17R line, while the W10R line exhibited intermediate characteristics. Notably, the W17R variant was misrouted to the mitochondria resulting in changes of the mitochondrial network organisation. ERS activation is not a universal response to different renin mutations in ADTKD-REN. The pathogenesis of the W17R mutation may involve mitochondrial dysfunction rather than the ER pathway, albeit further research is needed to substantiate this hypothesis fully. Testing CRELD2 and MANF as targeted therapy markers for a specific subgroup of ADTKD-REN patients is recommended. Additionally, fludrocortisone treatment has shown efficacy in stabilizing the renal function of our patient over a four-year period without significant side effects.

Low renin forms of monogenic hypertension: review of the evidence

<b>Background:</b> Monogenic hypertension syndromes result from a single genetic mutation and present with severe, refractory hypertension, distinct laboratory abnormalities, and a positive family history. These syndromes are often unrecognized or misdiagnosed as essential hypertension, thus preventing proper treatment. The rise of molecular genetics has brought these conditions to the limelight, and physicians must be kept abreast of the latest in this field. This paper aims to educate doctors to recognize and institute appropriate management early to prevent end-organ damage.<br /> <b>Discussion: </b>These syndromes all affect sodium transport in the distal nephron of the kidneys. However, they are divided based on the location of the primary disorder, i.e., the adrenal glands or the distal nephron and discussed in that manner. Tables provide an overview of the different syndromes and provide essential information in a snapshot.<br /> <b>Conclusion:</b> The widespread availability of genetic testing facilities will aid in the earlier diagnosis of these conditions to prevent morbidity.

Modulation of 11β-hydroxysteroid dehydrogenase functions by the cloud of endogenous metabolites in a local microenvironment: The glycyrrhetinic acid-like factor (GALF) hypothesis

11β-Hydroxysteroid dehydrogenase (11β-HSD)-dependent conversion of cortisol to cortisone and corticosterone to 11-dehydrocorticosterone are essential in regulating transcriptional activities of mineralocorticoid receptors (MR) and glucocorticoid receptors (GR). Inhibition of 11β-HSD by glycyrrhetinic acid metabolites, bioactive components of licorice, causes sodium retention and potassium loss, with hypertension characterized by low renin and aldosterone. Essential hypertension is a major disease, mostly with unknown underlying mechanisms. Here, we discuss a putative mechanism for essential hypertension, the concept that endogenous steroidal compounds acting as glycyrrhetinic acid-like factors (GALFs) inhibit 11β-HSD dehydrogenase, and allow for glucocorticoid-induced MR and GR activation with resulting hypertension. Initially, several metabolites of adrenally produced glucocorticoids and mineralocorticoids were shown to be potent 11β-HSD inhibitors. Such GALFs include modifications in the A-ring and/or at positions 3, 7 and 21 of the steroid backbone. These metabolites may be formed in peripheral tissues or by gut microbiota. More recently, metabolites of 11β-hydroxy-Δ4androstene-3,17-dione and 7-oxygenated oxysterols have been identified as potent 11β-HSD inhibitors. In a living system, 11β-HSD isoforms are not exposed to a single substrate but to several substrates, cofactors, and various inhibitors simultaneously, all at different concentrations depending on physical state, tissue and cell type. We propose that this “cloud” of steroids and steroid-like substances in the microenvironment determines the 11β-HSD-dependent control of MR and GR activity. A dysregulated composition of this cloud of metabolites in the respective microenvironment needs to be taken into account when investigating disease mechanisms, for forms of low renin, low aldosterone hypertension.

Abstract P278: On the Origin of Brain Renin

A renin-angiotensin system (RAS) in the brain is believed to contribute to blood pressure regulation. Given the presence of the blood-brain barrier, brain RAS activity most likely depends on synthesis of (pro)renin in the brain. In support of this concept, an intracellular, non-secreted form of renin has been described in the brain, depending on an alternative transcript of the renin gene. To what degree this intracellular truncated prorenin truly displays angiotensin (Ang) I-generating activity (AGA) is unknown. In the present study, we set out to quantify brain (pro)renin, both before and after buffer perfusion of the brain, in normal mice, renin knockout (KO) mice, DOCA-salt-treated mice (which have been reported to display brain RAS activation), and angiotensin II-infused mice. Brain nuclei were homogenized and incubated with excess angiotensinogen to detect AGA, both before and after acid activation of prorenin, with or without the renin inhibitor aliskiren to correct for non-renin-mediated AGA. Renin-dependent (i.e., aliskiren-inhibitable) AGA was readily detectable in brain nuclei, the highest AGA being present in brainstem (>thalamus ≈cerebellum ≈striatum ≈midbrain >hippocampus ≈cortex). Brain AGA increased non-significantly after prorenin activation, suggesting that brain prorenin levels are low or absent. Buffer perfusion reduced AGA in all brain areas by >60%. Plasma renin (expressed per mL plasma) was 40-800 x higher than brain renin (expressed per g tissue). Plasma prorenin levels resembled plasma renin levels. AGA was undetectable in plasma and brain of renin KO mice, both before and after prorenin activation. DOCA-salt and Ang II suppressed plasma renin, and parallel decreases were observed for brain renin. In conclusion, brain renin levels (per g tissue) correspond with the amount of renin present in 1-20 μL blood plasma. Brain renin disappears after buffer perfusion, and varies in association with plasma renin. This indicates that renin detected in brain nuclei represents plasma renin and/or locally activated plasma prorenin. DOCA-salt exposure does not selectively increase brain renin expression.

Elucidation of the binding mechanism of renin using a wide array of computational techniques and biological assays

We investigate the binding mechanism in renin complexes, involving three drugs (remikiren, zankiren and enalkiren) and one lead compound, which was selected after screening the ZINC database. For this purpose, we used ab initio methods (the effective fragment potential, the variational perturbation theory, the energy decomposition analysis, the atoms-in-molecules), docking, molecular dynamics, and the MM-PBSA method. A biological assay for the lead compound has been performed to validate the theoretical findings. Importantly, binding free energy calculations for the three drug complexes are within 3kcal/mol of the experimental values, thus further justifying our computational protocol, which has been validated through previous studies on 11 drug-protein systems. The main elements of the discovered mechanism are: (i) minor changes are induced to renin upon drug binding, (ii) the three drugs form an extensive network of hydrogen bonds with renin, whilst the lead compound presented diminished interactions, (iii) ligand binding in all complexes is driven by favorable van der Waals interactions and the nonpolar contribution to solvation, while the lead compound is associated with diminished van der Waals interactions compared to the drug-bound forms of renin, and (iv) the environment (H2O/Na+) has a small effect on the renin–remikiren interaction.

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.

RENIN ANGIOTENSIN SYSTEM-DEPENDENT AND -INDEPENDENT FUNCTIONS OF (PRO)RENIN RECEPTOR AND THE ROLES OF ITS BLOCKERS

(Pro)renin receptor [(P)RR] is one of the major components of the renin angiotensin (RA) system, which is involved in theregulation of blood pressure. (P)RR binds with and increases the activity of mature renin, the key regulatory enzyme of RA system,which catalyzes the conversion of angiotensinogen to angiotensin-I. (P)RR also binds to the inactive form of renin called prorenin.Receptor-bound prorenin exerts biochemical and molecular effects in either an angiotensin II-dependent or -independent manner.RA system-independent function and/or involvement of (P)RR has been well reported. Inhibition of binding and activation of renin/prorenin to (P)RR has been found to improve proteinuria in diabetic model rats. This article reviews the action of (P)RR blockers aswell as its associated mechanism.

RENIN ANGIOTENSIN SYSTEM IN THE CONTEXT OF RENIN, PRORENIN, AND THE (PRO)RENIN RECEPTOR

Since the discovery of renin by Tigerstedt in 1898, the complex roles of the components of the renin angiotensin system (RAS)have been extensively studied. RAS is involved in maintaining the homeostasis of body fluids and thus regulates the blood pressure.Renin is the key regulatory enzyme of RAS that catalyzes its only substrate angiotensinogen into the decapeptide angiotensin I,which is then converted into the octapeptide angiotensin II by angiotensin converting enzyme. Angiotensin II exerts its physiologicaleffects through angiotensin type 1 and type 2 receptors. Elucidation of the three-dimensional structure of renin has led to the designand synthesis of several synthetic peptides that inhibit renin activity, controlling blood pressure. Nephrectomized animals haveno detectable level of renin in their blood plasma, indicating that the kidney is the only source of renin. Renin is synthesized aspreprorenin, which is processed into prorenin. Prorenin is the preactive form of renin, with an extra 43 amino acid residues that blockthe active site of renin. Prorenin can be converted into renin proteolytically by the action of trypsin, cathepsin, and other proteases.However, non-proteolytically activated prorenin (using acidic pH, low temperature, or protein-protein interactions) is also possible,resulting in enzymatically active renin. The (pro)renin receptor [(P)RR], a RAS member, activates prorenin through protein–proteininteractions and exhibits angiotensin II-dependent as well as angiotensin II-independent functions. (P)RR in conjunction with (pro)reninexerts pathophysiological activities in human and animal models, especially in end-stage organ damage. Design of peptides that mimicparts of renin/prorenin can help to explain the binding interaction between (P)RR and its ligands. This article includes a discussion ofthe complex current challenges of studying renin, prorenin, and (P)RR.

Abstract 238: Nuclear Renin-Angiotensin System in Renal Proximal Tubule Cells

Compelling evidence for an intracellular renin-angiotensin system (RAS) is evident in multiple tissues including the heart, vasculature, brain and kidney. Indeed, the full complement of angiotensin receptors is localized on renal cortical nuclei; however, elucidation of the intracellular pathways that contribute to the expression of Ang II or Ang-(1-7) is incomplete. The current study sought to identify a proximal tubule epithelial cell model to facilitate our understanding of the regulation and function of this intracellular system. Utilizing the rat-derived NRK52E epithelial cells, immunofluorescent staining and western blots revealed nuclear expression of angiotensinogen (Aogen), renin and the prorenin receptor (PRR). Further characterization of Aogen expression utilizing an antibody against the protein region distal to Ang I ([des-Ang I]-Aogen) revealed a single 60 kDa band evident in the nuclear and the cytosolic fractions of the cells. A second antibody to the Ang I sequence of Aogen revealed expression in the cytosol and collected cell media, but not the nuclear fraction. The large molecular form of renin in the nuclear fraction suggests the predominance of prorenin in the nucleus. Basal renin activity in the nuclear fraction as measured by Ang I generation was significantly inhibited by the specific renin inhibitor aliskiren [770 ± 46 vs. 129 ± 14 pg Ang I/mg protein, n=3; p<0.01]. Conversion of the proform to active renin (total renin) by trypsin treatment of the nuclear fraction increased renin activity 2-fold [1933 ± 230 pg Ang I/mg; n=3; p<0.01]; aliskiren significantly reduced total renin activity by 92% [167 ± 24 pg Ang I/mg, n=3; p<0.01]. Utilizing specific radioimmunoassays, we detected both Ang II and Ang-(1-7) at concentrations of 50 ± 24 and 51 ± 39 pg/mg protein, respectively (n=3) in nuclear extracts of NRK cells. Collectively, these data suggest that processing of Aogen from either prorenin or renin may occur on the nucleus forming [des-Ang I]-Aogen and the active peptides Ang II and Ang-(1-7). We conclude that a nuclear prorenin/renin-Aogen pathway may constitute one source of peptide ligands for intracellular angiotensin receptors.

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].

Aliskiren binds to renin and prorenin bound to (pro)renin receptor in vitro

Human (pro)renin receptor ((P)RR) has been implicated in the augmentation of many biological and cellular processes through bindings to its ligands, renin and prorenin. In this study, we investigated the effects of aliskiren, a direct oral renin inhibitor, on the activities of free and (P)RR-bound forms of human mature renin. We also elucidated the effect of aliskiren on the 'renin activity' of the receptor-bound form of prorenin. Aliskiren had an IC(50) of 0.72 nmol l(-1) against renin. The compound competitively inhibited renin activity with an inhibitory constant (K(i)) of 0.18 nmol l(-1). Furthermore, the dissociation constants (K(D)) for aliskiren from renin and prorenin bound to (P)RR were determined using surface plasmon resonance in a BIAcore assay system (Uppsala, Sweden). These values were estimated to be 0.46 ± 0.03 and 0.25 ± 0.01 nmol l(-1), respectively. The compound competitively inhibited the renin activities of (P)RR-bound forms of both renin and prorenin with a K(i) of 0.14 and 0.15 nmol l(-1), respectively. These results indicate that aliskiren could be a potent inhibitor of the free forms of mature renin and of the receptor-bound forms of renin and prorenin.

118. THE PRORENIN RECEPTOR/PLZF PATHWAY IN HUMAN AMNION

Prorenin, despite being inactive, is the major form of renin found in amniotic fluid and reproductive tissues. Prorenin becomes active if it binds to the novel prorenin receptor (ATP6AP2). The prorenin-ATP6AP2 complex has been found to stimulate translocation of Promyelocytic Zinc Finger (PLZF) protein to the nucleus where it increases expression of the p85α subunit of PI3 kinase (PI3K-p85α) and represses the expression of ATP6AP21. Progesterone and glucocorticoids have also been shown to stimulate PLZF2, 3. We aimed to find out if PLZF and the prorenin-ATP6AP2 pathway interact in human reproductive tissues. Human amnion was cultured for 24 h in media containing vehicle, dexamethasone, amniotic fluid or recombinant human (rh) prorenin. Total RNA was extracted using TRIZol® and converted to cDNA for quantitative real-time PCR using SuperScript III and random hexamers. mRNA abundances for PLZF, PI3K-p85α and ATP6AP2 were calculated relative to Alien RNA using the ΔΔCT method. Our preliminary data show that exposure of amnion explants to dexamethasone upregulates PLZF and PI3K-p85α mRNA but has no effect on ATP6AP2. Culture of amnion explants with amniotic fluid also increases PLZF but does not change PI3K or ATP6AP2. In contrast, culture of amnion explants with (rh) prorenin increases PI3K mRNA but not PLZF or ATP6AP2. As expected, dexamethasone affects PLZF expression, however in amnion there is no interaction with the ATP6AP2 pathway. In addition, we believe we have identified a novel prorenin/ATP6AP2 signalling pathway which acts on PI3K-p85α independent of PLZF. In contrast to these data, amniotic fluid increases PLZF but not PI3K-p85α mRNA levels suggesting that amniotic fluid contains other factors that oppose prorenin and glucocorticoid effects on PI3K-p85α. (1) Schefe, J.H., et al. Circ Res, 2006. 99(12): 1355–1366.(2) Fahnenstich, J., et al. Mol. Hum. Reprod., 2003. 9(10): 611–623.(3) Conquest, A. et al. Fetal and Neonatal Physiology Workshop, 2010. Wellington, New Zealand.

Truncated Prorenin Comes Up … Short

In the present issue of Hypertension , Xu et al1 report on a pivotal set of experiments that test the contribution of a key component to the action of the putative “intracellular renin-angiotensin system” (RAS; iRAS) in its normal physiological setting. In contrast to the classic circulating RAS, the iRAS has been proposed to be wholly contained within cells, where it produces angiotensin peptides that trigger noncanonical signaling (reviewed in Reference 2). The results reported by Xu et al1 suggest that an intracellular form of active prorenin cannot compensate for the loss of the classic secreted form of renin in some of the primordial roles of the RAS: directing kidney development, blood pressure regulation, and hematopiesis. The average mammalian cell makes many thousands of different proteins, each of which has to be delivered to the appropriate destination to exercise its function. The first major triage comes at the level of the secretory apparatus: all of the proteins that are destined to be released from the cell, expressed on the cell surface, or retained in one of the several vacuolar structures within the cell (secretory pathway, lysosomes, etc) first enter the membrane “bags” of the secretory pathway. This is accomplished by the recognition of a signal peptide on the newly made protein by a complex that docks the synthesis machinery on the membranes of the endoplasmic reticulum and ensures the extrusion of the protein into the lumen of the membrane sac. Proteins without a signal peptide are made within the cell cytoplasm, where they will either remain to exercise their function or where they are retargeted to other locations within the cell, such as the nucleus. Thus, whereas proteins …

CYP450 4A inhibition attenuates O 2 induced arteriolar constriction in chronic but not acute Goldblatt hypertension

We explored the role of 20-hydroxy-5Z, 8Z, 11Z, 14Z-eicosatetraenoic acid (20-HETE) in oxygen-induced vasoconstriction in a normal renin form of hypertension [the 1 kidney-1 clip Goldblatt hypertensive rat (1K1C)] and a high renin form of hypertension [the 2 kidney-1 clip Goldblatt hypertensive rat (2K1C)]. A silver clip was placed around the left renal artery of adult Sprague–Dawley males. The right kidney was removed in the 1K1C group and left intact in the 2K1C group. Arteriolar responses to elevation of O 2 concentration in the superfusion solution from 0% O 2 to 21% O 2 were determined in the in situ cremaster muscle before and after inhibition of cytochrome P450 4A ω-hydroxylase (CYP450 4A) with N-methyl-sulfonyl-12, 12-dibromododec-11-enamide (DDMS). Arteriolar constriction to elevated PO 2 was enhanced in the chronic 1K1C but not the acute 1K1C or 2K1C. DDMS eliminated O 2-induced arteriolar constriction in the 9-week 1K1C, but had no effect in the 2-week 1K1C, and only partially inhibited O 2-induced constriction of arterioles in the 4-week 2K1C rat. These findings indicate that although the CYP4A/20-HETE system contributes to arteriolar constriction in response to elevated PO 2 in the established stage of 1K1C renovascular hypertension, physiological alterations in other mechanisms are the primary determinants of O 2-induced constriction of arterioles in the early and developing stages of 1K1C and 2K1C hypertension.

Exaggerated Blood Pressure Variability Superimposed on Hypertension Aggravates Cardiac Remodeling in Rats via Angiotensin II System-Mediated Chronic Inflammation

Hypertensive patients with large blood pressure variability (BPV) have aggravated end-organ damage. However, the pathogenesis remains unknown. We investigated whether exaggerated BPV aggravates hypertensive cardiac remodeling and function by activating inflammation and angiotensin II-mediated mechanisms. A model of exaggerated BPV superimposed on chronic hypertension was created by performing bilateral sinoaortic denervation (SAD) in spontaneously hypertensive rats (SHRs). SAD increased BPV to a similar extent in Wistar Kyoto rats and SHRs without significant changes in mean blood pressure. SAD aggravated left ventricular and myocyte hypertrophy and myocardial fibrosis to a greater extent and impaired left ventricular systolic function in SHRs. SAD induced monocyte chemoattractant protein-1, transforming growth factor-beta, and angiotensinogen mRNA upregulations and macrophage infiltration of the heart in SHRs. The effects of SAD on cardiac remodeling and inflammation were much smaller in Wistar Kyoto rats compared with SHRs. Circulating levels of norepinephrine, the active form of renin, and inflammatory cytokines were not affected by SAD in Wistar Kyoto rats and SHRs. A subdepressor dose of candesartan abolished the SAD-induced left ventricular/myocyte hypertrophy, myocardial fibrosis, macrophage infiltration, and inductions of monocyte chemoattractant protein-1, transforming growth factor-beta, and angiotensinogen and subsequently prevented systolic dysfunction in SHRs with SAD. These findings suggest that exaggerated BPV induces chronic myocardial inflammation and thereby aggravates cardiac remodeling and systolic function in hypertensive hearts. The cardiac angiotensin II system may play a role in the pathogenesis of cardiac remodeling and dysfunction induced by a combination of hypertension and exaggerated BPV.

Newly developed renin and prorenin assays and the clinical evaluation of renin inhibitors

The last decade has seen the introduction of renin inhibitors and new plasma renin and prorenin assays, which has led to a better understanding of the tissue renin-angiotensin system. To clarify the consequences of these developments for the methodology and interpretation of measurements of renin and prorenin. The principles and application of the newly developed immunosorbent assays (ISAs) are surveyed and the results are compared with those of enzyme-kinetic assays (EKAs). Angiotensin (Ang) II in cardiac, renal and adrenal tissue is known to originate mainly from locally produced Ang I. Experimental evidence and theoretical considerations show that a simple relation between Ang II receptor occupancy, in tissue micromilieu, and the circulating levels of Ang II or renin may not exist. This supports the clinicians' view that the plasma level of renin tells more about the mechanisms regulating its release into the circulation than about the Ang II-dependency of hypertension. ISAs are a welcome addition to clinical studies of renin inhibitors. By comparing the results of ISAs with those of EKAs, the inhibitor-bound forms of renin and prorenin can be distinguished from the unbound forms. ISAs also provide important information on the molecular basis of prorenin activation. We propose a single kinetic model to incorporate the conformational changes of prorenin induced by cryo-activation and acid-activation, and by binding to renin inhibitors. It explains why renin ISAs can overestimate the rise of renin in response to these drugs, and shows how to deal with this artefact.

The (pro)renin receptors

Two (pro)renin receptors have been characterized so far, the mannose-6-phosphate receptor (M6P-R) and a specific receptor called (P)RR for (pro)renin receptor. Each receptor controls a different aspect of renin and prorenin metabolism. The M6P-R is a clearance receptor, whereas (P)RR mediates their cellular effects by activating intracellular signaling and up-regulating gene expression. Moreover, binding to (P)RR increases renin enzymatic activity and fully activates prorenin, the inactive proenzyme form of renin. Experimental models suggest that increased (P)RR synthesis and/or activation may be relevant to diseases, especially to high blood pressure, to cardiac fibrosis associated with hypertension, and to diabetic nephropathy.

Renin/prorenin receptors

The existence of a tissue renin-angiotensin (RAS) system independent of the circulating RAS has prompted the search for cellular binding sites for angiotensinogen and for renin in order to explain their tissue uptake. Two receptors that bind with similar affinity mature renin and prorenin were identified, the mannose-6-phosphate receptor (M6P-R) and a specific receptor. The M6P-R is a clearance receptor that binds exclusively the glycosylated forms of renin and prorenin. Binding of renin and prorenin to the M6P-R is followed by internalization and degradation, and the intracellular proteolysis of prorenin in mature renin did not provoke any generation of intracellular angiotensins. In contrast to the M6P-R, (pro)renin bound to the specific receptor was not degraded. Instead, receptor-bound renin showed increased catalytic activity, and receptor-bound prorenin exhibited full catalytic activity. This 'gain of activity' was explained by a conformational change of the (pro)renin molecule upon binding. Furthermore, (pro)renin binding provoked a rapid activation of the mitogen-activated protein kinases p44/p42, indicating that the receptor has mediated specific, angiotensin II-independent effects of (pro)renin. This receptor represents an elegant concept to explain the existence of active prorenin in vivo, and it provides a pathological role for prorenin in situations with paradoxical low renin and high prorenin concentrations such as in diabetes. Experimental models of rats overexpressing the receptor either in vascular smooth muscle cells and developing high blood pressure or with ubiquitous expression associated with glomerulosclerosis and proteinuria confirm a role for the receptor in cardiovascular and renal diseases.

Prorenin

Prorenin, the precursor of renin, exists in circulating blood at concentrations that are ≈5 to 10× higher than those of renin. For many years, prorenin was considered to be an inactive form of renin with no physiological role. Then, in the mid-80s of the last century, Luetscher et al reported that the levels of circulating prorenin (but not renin) are increased in diabetic subjects.1 Subsequent studies revealed that these high levels correlated with the presence of microvascular complications, and it was proposed that prorenin might be used to predict the occurrence of microalbuminuria.2 Simultaneously, high prorenin levels were observed in pregnant women, and evidence was obtained for the local synthesis of prorenin at extrarenal tissue sites such as the ovary and eye. Until today, however, the exact function of prorenin remains unknown. An attractive concept is that in tissues not synthesizing renin locally, circulating prorenin, after its local conversion to renin, contributes to angiotensin (Ang) generation. This would not only provide a role for prorenin in vivo, but also explain why tissues, in contrast with plasma, contain predominantly renin.3 Studies in transgenic animals support this idea. Veniant et al4 described that transgenic rats expressing prorenin exclusively in the liver (resulting in a 400-fold rise in plasma prorenin) display cardiac hypertrophy and vascular damage in the absence of hypertension. Prescott et al5 detected increased cardiac Ang I levels in mice overexpressing human prorenin in the liver and human angiotensinogen in the heart. The question then arises how tissues sequester and activate circulating prorenin. Sequestration may involve diffusion into the interstitial fluid and/or binding to a receptor.3 …

Evidence Supporting a Functional Role for Intracellular Renin in the Brain

The brain renin-angiotensin system is implicated in the regulation of blood pressure (BP) and fluid homeostasis. Recent studies reveal that 2 forms of renin are expressed in the brain of rodents and humans: secreted prorenin and a nonsecreted intracellular form of active renin (icREN). Although the intracellular action of renin has long been postulated, no data supporting its role in BP regulation has been reported. Therefore, we directly evaluated whether this form of renin has physiological implications for BP regulation by characterizing transgenic mice expressing human icREN driven by the glial fibrillary acidic protein (GFAP) promoter and comparing it with similar mice expressing the secreted form of renin. GFAP-icREN mice express hREN primarily in the brain and at the same level of expression as GFAP-secreted prorenin. Unlike the secreted form, which can be detected in cerebrospinal fluid, no human renin could be detected in the cerebrospinal fluid of GFAP-icREN mice. GFAP-icREN mice were then bred with transgenic mice expressing human angiotensinogen, also driven by the GFAP promoter. Double-transgenic mice expressing either the intracellular renin (2.0+/-0.12 mL/10 g/day) or secreted renin (2.8+/-0.3 mL/10 g/day) exhibited an increase in drinking volume compared with nontransgenic littermates (1.5+/-0.1 mL/10 g/day). Both models exhibited an increase in mean arterial pressure (137+/-5 and 133+/-8 mm Hg, respectively) compared with control littermates (115+/-3 mm Hg), which could be rapidly reduced after ICV injection of losartan. These data support the concept of an intracellular form of renin in the brain, which may provoke functional changes in fluid homeostasis and BP regulation.