Regulation of human renin mRNA expression and protein release in transgenic mice

Long-Range Control of Renin Gene Expression in Tsukuba Hypertensive Mice

Renin, a rate-limiting enzyme in the renin–angiotensin system, is regulated to maintain blood pressure homeostasis: renin gene expression in the kidney is suppressed in a hypertensive environment. We found that expression of a 15-kb human RENIN (hREN) transgene was aberrantly upregulated (>4.2-fold), while the endogenous mouse renin (mRen) gene was suppressed (>1.7-fold) in Tsukuba hypertensive mice (THM), a model for genetically induced hypertension. We then generated transgenic mice using a 13-kb mRen gene fragment that was homologous to the 15-kb hREN transgene and found that its expression was also upregulated (>3.1-fold) in THM, suggesting that putative silencing elements of the renin genes were distally located in the loci. We next examined the possible role of a previously identified mouse distal enhancer (mdE) located outside of the 13-kb mRen gene fragment. Deletion of the mdE in the context of a 156-kb mRen transgene did not affect its transcriptional repression in THM, implying that although the silencing element of the mRen gene is located within the 156-kb fragment tested, it is distinct from the mdE. Consistent with these results, deletion of the 63-kb region upstream of the mdE from the endogenous mRen gene locus abrogated its transcriptional repression in THM. We finally tested whether dysregulation of the short renin transgenes also occurred in the fetal or neonatal kidneys of THM and found that their expression was not aberrantly upregulated, demonstrating that aberrant regulation of short renin transgenes commences sometime between neonate and adult periods.

Cardiovascular consequences of genetic variation at -6/235 in human angiotensinogen using "humanized" gene-targeted mice

Genetic and functional data support a role for angiotensinogen in blood pressure control, and many population studies have suggested that polymorphisms in the angiotensinogen gene contribute to hypertension. Two common haplotypes of the human angiotensinogen gene are -6A/235T and -6G/235M. To study their contributions to blood pressure regulation in a controlled model system, we developed triple-transgenic mice expressing either -6A/235T or -6G/235M human angiotensinogen, expressing either an overexpressed and poorly regulated (REN9) or a tightly regulated (PAC160) human renin, and all carrying a null mutation in the endogenous murine angiotensinogen gene. These humanized mice were then examined for blood pressure differences at baseline and after a high-salt diet, changes in cardiovascular organ weight, and differences in angiotensinogen and renin gene expression. Mice expressing the -6G/235M haplotype on the PAC160 background exhibited increased blood pressure and cardiac hypertrophy at baseline. In contrast, all of the mice with the REN9 background had equivalent baseline blood pressures. On the REN9 background, there was a greater increase in blood pressure in -6A/235T in response to a high-salt diet, providing evidence it may be a susceptibility allele. There were no differences in angiotensinogen expression between haplotypes on either background strain. The data suggest that the impact of angiotensinogen haplotypes on cardiovascular end points may be dependent on renin status and environmental influences, such as dietary sodium. These insights may help explain the discrepancies among observational studies that have examined roles for the -6A/235T and -6G/235M angiotensinogen haplotypes in varied human populations.

Cellular expression of renal, cardiac and pulmonary inducible nitric oxide synthase in double-transgenic mice expressing human renin and angiotensinogen genes

1. Hypertensive mice expressing the human renin (REN) and angiotensinogen (AGT) genes are used as a model for human hypertension. 2. The aim of the present study was to investigate the cellular expression and distribution of inducible nitric oxide synthase (iNOS) using immunohistochemistry in lung, heart and kidney tissues from a model of human hypertension using male and female double-transgenic (h-Ang 204/1h-Ren6) mice and wild-type C57/BI6J mice as controls. 3. In the kidney, the pattern of iNOS expression in various renal microanatomical regions during hypertension was similar to that of age-matched controls, except in the medullary ascending limb (MAL). In hypertension, iNOS expression was downregulated in the MAL. No significant differences in iNOS expression were seen between control or hypertensive mice in various cardiac microanatomical locations. In the lungs of hypertensive mice, iNOS expression was upregulated in bronchial airway epithelium and bronchial and vascular smooth muscle cells, but downregulated in alveolar macrophages, alveolar septa and pulmonary vascular endothelial cells. Expression of iNOS was similar between male and female mice in the kidney, heart and lungs. 4. In conclusion, iNOS regulation in hypertension is complex and depends on the cell type in which it is expressed and the localization of the cell type in the cardiorenal and pulmonary systems.

Pulmonary and cardiorenal cyclooxygenase-1 (COX-1), -2 (COX-2), and microsomal prostaglandin E synthase-1 (mPGES-1) and -2 (mPGES-2) expression in a hypertension model

Hypertensive mice that express the human renin and angiotensinogen genes are used as a model for human hypertension because they develop hypertension secondary to increased renin-angiotensin system activity. Our study investigated the cellular localization and distribution of COX-1, COX-2, mPGES-1, and mPGES-2 in organ tissues from a mouse model of human hypertension. Male (n = 15) and female (n = 15) double transgenic mice (h-Ang 204/1 h-Ren 9) were used in the study. Lung, kidney, and heart tissues were obtained from mice at necropsy and fixed in 10% neutral buffered formalin followed by embedding in paraffin wax. Cut sections were stained immunohistochemically with antibodies to COX-1, COX-2, mPGES-1, and mPGES-2 and analyzed by light microscopy. Renal expression of COX-1 was the highest in the distal convoluted tubules, cortical collecting ducts, and medullary collecting ducts; while proximal convoluted tubules lacked COX-1 expression. Bronchial and bronchiolar epithelial cells, alveolar macrophages, and cardiac vascular endothelial cells also had strong COX-1 expression, with other renal, pulmonary, or cardiac microanatomic locations having mild-to-moderate expression. mPGES-2 expression was strong in the bronchial and bronchiolar epithelial cells, mild to moderate in various renal microanatomic locations, and absent in cardiac tissues. COX-2 expression was strong in the proximal and distal convoluted tubules, alveolar macrophages, and bronchial and bronchiolar epithelial cells. Marked mPGES-1 was present only in bronchial and bronchiolar epithelial cells; while mild-to-moderate expression was present in other pulmonary, renal, or cardiac microanatomic locations. Expression of these molecules was similar between males and females. Our work suggests that in hypertensive mice, there are (a) significant microanatomic variations in the pulmonary, renal, and cardiac distribution and cellular localization of COX-1, COX-2, mPGES-1, and mPGES-2, and (b) no differences in expression between genders.

Development of hypertension and kidney hypertrophy in transgenic mice overexpressing ARAP1 gene in the kidney

Angiotensin II regulates blood pressure via activation of the type 1 receptor. We previously identified a novel angiotensin II type 1 receptor-associated protein and demonstrated that it promotes receptor recycling to the plasma membrane. To delineate the pathophysiological function of the ARAP1 in the kidneys, we generated transgenic mice that overexpress rat ARAP1 cDNA specifically in proximal tubules and tested the hypothesis that proximal tubule-specific overexpression of ARAP1 causes hypertension. Two lines of male transgenic mice, 650 and 670, displayed kidney-specific transgene expression. Systolic blood pressure was significantly elevated by &20 to 25 mm Hg in these lines of mice at 20 weeks of age compared with their nontransgenic litter mates. Urine volume, but not water intake, was significantly decreased in both lines compared with nontransgenic controls. The kidney/body weight ratio was significantly increased in both lines compared with their nontransgenic litter mates at 12 and 20 weeks of age. In contrast, no difference was observed in the ratio of brain, spleen, heart, and testis to body weight between male transgenic and nontransgenic animals. Inhibitions of the renin-angiotensin system completely normalized the systolic blood pressure of transgenic mice. Moreover, low salt intake prevented the development of hypertension, whereas high salt intake exacerbated the increase in blood pressure in transgenic mice. Therefore, our data show that proximal tubule-specific overexpression of ARAP1 leads to hypertension, suggesting that renal ARAP1 plays an important role in the regulation of blood pressure and renal function via activation of the intrarenal renin-angiotensin system.

Transgenic mice for studies of the renin-angiotensin system in hypertension

Hypertension is a polygenic and multi-factorial disorder that is extremely prevalent in western societies, and thus has received a great deal of attention by the research community. The renin-angiotensin system has a strong impact on the control of blood pressure both in the short- and long-term, making it one of the most extensively studied physiological systems. Nevertheless, despite decades of research, the specific mechanisms implicated in its action on blood pressure and electrolyte balance, as well as its integration with other cardiovascular pathways remains incomplete. The production of transgenic models either over-expressing or knocking-out specific components of the renin-angiotensin system has given us a better understanding of its role in the pathogenesis of hypertension. Moreover, our attention has recently been refocused on local tissue renin-angiotensin systems and their physiological effect on blood pressure and end-organ damage. Herein, we will review studies using genetic manipulation of animals to determine the role of the endocrine and tissue renin-angiotensin system in hypertension. We will also discuss some untraditional approaches to target the renin-angiotensin system in the kidney.

Increased blood pressure in transgenic mice expressing both human renin and angiotensinogen in the renal proximal tubule

The purpose of this study was to evaluate the physiological significance of a tissue renin-angiotensin system in the proximal tubule of the kidney. To accomplish this, we produced mice that express human renin (hREN) under the control of the kidney androgen-regulated promoter (KAP), which is androgen responsive. One of the lines expressed the hREN transgene primarily in the kidney. Renal expression of the transgene was undetectable in females but could be induced by testosterone treatment. Because the renin-angiotensin system is species specific, we bred KAP2-hREN mice with the mice expressing human angiotensinogen under the same promoter (KAP-hAGT) to produce offspring that expressed both transgenes. We measured mean arterial blood pressure (MAP) in the carotid artery of double-transgenic and control mice using radiotelemetry. Double-transgenic female mice had a normal baseline MAP (116 +/- 4 mmHg, n = 8), which increased by 15 mmHg after 2 wk of testosterone treatment, and returned to baseline after elimination of the testosterone pellet. The change in arterial pressure paralleled the change in plasma testosterone. There was no MAP change in testosterone-treated control littermates. We conclude that dual production of renin and angiotensinogen in the renal proximal tubule can result in a systemic increase in arterial pressure. These data support a role for a tissue-specific renin-angiotensin system in the renal proximal tubule that contributes to the regulation of systemic blood pressure.

Glia- and neuron-specific expression of the renin-angiotensin system in brain alters blood pressure, water intake, and salt preference

The purpose of this study is to examine the regulation of blood pressure and fluid and electrolyte homeostasis in mice overexpressing angiotensin II (Ang-II) in the brain and to determine whether there are significant physiologic differences in Ang-II production in neurons or glia. Therefore, we generated and characterized transgenic mice overexpressing human renin (hREN) under the control of the glial fibrillary acidic protein (GFAP) promoter (GFAP-hREN) and synapsin-I promoter (SYN-hREN) and bred them with mice expressing human angiotensinogen (hAGT) under the control of the same promoters (GFAP-hAGT and SYN-hAGT). Both GFAP-hREN and SYN-hREN mice exhibited the highest hREN mRNA expression in the brain and had undetectable levels of hREN protein in the systemic circulation. In the brain of GFAP-hREN and SYN-hREN mice, hREN protein was observed almost exclusively in astrocytes and neurons, respectively. Transgenic mice overexpressing both hREN and hAGT transgenes in either glia or neurons were moderately hypertensive. In the glia-targeted mice, blood pressure could be corrected by intracerebroventricular injection of the Ang-II type 1 receptor antagonist losartan, and intravenous injection of a ganglion blocking agent, but not an arginine vasopressin V1 receptor antagonist, lowered blood pressure. These data suggest that stimulation of Ang-II type 1 receptors in the brain by Ang-II derived from local synthesis of renin and angiotensinogen can cause an elevation in blood pressure via a mechanism involving enhanced sympathetic outflow. Glia- and neuron-targeted mice also exhibited an increase in drinking volume and salt preference, suggesting that chronic overexpression of renin and angiotensinogen locally in the brain can result in hypertension and alterations in fluid homeostasis.

Neuron-specific expression of human angiotensinogen in brain causes increased salt appetite

The brain renin-angiotensin system (RAS) has an important role in the regulation of cardiovascular function. In the brain, angiotensinogen (AGT) is expressed mainly in astrocytes (glia) and in some neurons in regions controlling cardiovascular activities. Because of the inability to dissect the functional role of astrocyte- vs. neuron-derived AGT in vivo by pharmacological approaches, the exact role of neuron-derived AGT in the regulation of blood pressure (BP) and fluid and electrolyte balance remains unclear. Therefore, we generated a transgenic mouse model overexpressing human AGT under the control of a neuron-specific (synapsin I) promoter (SYN-hAGT). These mice exhibited high-level expression of human AGT mRNA in the brain, with lower expression in the kidney and heart. Human AGT was not detected in plasma, but in the brain it was expressed exclusively in neurons. Intracerebroventricular (30 ng) but not intravenous (500 ng) injection of purified human renin (hREN) caused a pressor response, which was prevented by intracerebroventricular preinjection of the angiotensin II type 1 receptor antagonist losartan, indicating an AT(1) receptor-dependent functional role of neuron-derived AGT in the regulation of BP in response to exogenous REN. Double transgenic mice expressing both the hREN gene and SYN-hAGT transgene exhibited normal BP and water intake but had an increased preference for salt. These data suggest that neuronal AGT may play an important role in regulating salt intake and salt appetite.

Angiotensin mutant mice: a focus on the brain renin-angiotensin system

With advances in genetic manipulation and molecular biological and physiological techniques, the mouse has become the animal model of choice for studying the genetic basis of human diseases. The two most commonly used methods for analyzing the function of a gene in vivo, overexpression (transgenic mouse) and deletion (knockout mouse), have been extremely useful in establishing the importance of genes in genetic disorders. The renin-angiotensin system (RAS) is one of the most widely studied systems controlling blood pressure. Although the primary site of Ang-II production is the plasma, all the components of the RAS cascade are expressed in many tissues, including the brain. This review briefly summarizes systemic and tissue-specific transgenic and knockout mouse models of the RAS for determining the role of this system in the regulation of blood pressure and in the pathogenesis of hypertension, with a focus on the RAS in the brain.

Nonimmune hydrops fetalis and activation of the renin-angiotensin system after asphyxia in preterm fetal sheep

This study examined the hypothesis that the development of hydrops fetalis after asphyxia in the 0.6 gestation sheep fetus would be associated with activation of the fetal renin-angiotensin system (RAS). Fetuses were randomly assigned to either sham occlusion (n = 7) or to 30 min of asphyxia induced by complete umbilical cord occlusion for 30 min (n = 8). Asphyxia led to severe bradycardia and hypotension that resolved after release of occlusion. After occlusion, plasma renin concentration was significantly increased in the asphyxia group compared with controls (P < 0.005) after 3 min (16.3 +/- 5.3 vs. 4.1 +/- 1.3 ng. ml(-1). h(-1)), and 72 h (30.6 +/- 6.3 vs. 3.7 +/- 1.2 ng. ml(-1). h(-1)). Renal renin concentrations and mRNA levels were significantly greater in the asphyxia group after 72 h of recovery. All fetuses in the asphyxia group showed generalized tissue edema, ascites, and pleural effusions after 72 h of recovery. In conclusion, asphyxia in the preterm fetus caused sustained activation of the RAS, which was associated with hydrops fetalis.

Identification of three human renin mRNA isoforms from alternative tissue-specific transcriptional initiation

We have reported that mice transgenic for 140- and 160-kb P1 phage artificial chromosomes (PACs) containing the human renin gene express the gene in a highly tissue-restricted and regulated manner. Herein, we demonstrate that the transgene is also expressed appropriately throughout development. In the course of this investigation, we identified the existence of three transcriptional isoforms of human renin mRNA derived from the utilization of alternative transcription start sites. The first isoform is the kidney-specific isoform, which utilizes the classic renin promoter. The second is a brain-specific isoform, which when previously identified in rats and mice was due to a transcription initiation site within intron A. However, the start site in the human gene resides approximately 1,325 bp upstream of the classic promoter and encodes a new exon 1 (termed exon 1b) that splices directly to exon 2. The third isoform is lung specific and is due to transcriptional initiation 79 bp directly upstream of exon 2, fusing additional DNA within intron A (termed exon 1c) directly to exon 2 without splicing. Importantly, the alternative first exons observed in the PAC transgenic mice were identical to those used to transcribe renin in human fetal kidney, brain, and lung, suggesting these sites are bona fide isoforms of human renin mRNA and not artifacts of transgenesis. Moreover, the subtle differences in tissue-specific transcriptional initiation observed in the renin gene of rats and humans can be faithfully and accurately emulated in a transgenic model.

Transgenic models as tools for studying the regulation of human renin expression

Transgenic mice and rats have become popular tools to study the regulation of gene expression and the consequences of protein over-production. Over the past decade, numerous transgenic models have been developed to study the mechanisms of human renin gene expression and the participation of the renin-angiotensin system in the development of hypertension. Herein we will provide an overview of what has been learned from the use of transgenic models for studying the human renin gene.

Highly regulated cell type-restricted expression of human renin in mice containing 140- or 160-kilobase pair P1 phage artificial chromosome transgenes

We generated transgenic mice with two P1 artificial chromosomes, each containing the human renin (HREN) gene and extending to -35 and -75 kilobase pairs, respectively. HREN protein production was restricted to juxtaglomerular cells of the kidney, and its expression was tightly regulated by angiotensin II and sodium. The magnitude of the up- and down-regulation in HREN mRNA caused by the stimuli tested was identical to the endogenous renin gene, suggesting tight physiological regulation. P1 artificial chromosome mice were mated with transgenic mice overexpressing human angiotensinogen to determine if there was a chronic compensatory down-regulation of the transgene. Despite a 3-fold down-regulation of HREN mRNA, plasma angiotensin II and blood pressure was modestly elevated in the double transgenic mice. Nevertheless, this elevation was significantly less than a different double transgenic model containing a poorly regulated HREN transgene. The increase in blood pressure, despite the decrease in HREN mRNA, suggests that the HREN gene can partially, but not completely, compensate for excess circulating angiotensinogen. These data suggest the possibility that increases in circulating or tissue angiotensinogen may cause an increase in blood pressure in humans, even in the presence of a functionally active servo-mechanism to down-regulate HREN expression.

JG cell expression and partial regulation of a human renin genomic transgene driven by a minimal renin promoter

In the kidney, renin gene expression is exquisitely localized to the juxtaglomerular (JG) cells lining the afferent arteriole, having the capacity to regulate renin synthesis in response to a variety of physiological cues. We investigated human renin gene expression in transgenic mice containing a genomic construct driven by 149 bp of its proximal promoter to elucidate whether this was sufficient to confer JG-specific expression. Whereas human renin mRNA was permissively expressed in most tissues, the transgene was expressed mainly in JG cells in the kidney. Active human renin and human prorenin were found in the systemic circulation at levels consistent with previous transgenic models. Remarkably, two lines displayed an appropriate upregulation of transgene mRNA in response to angiotensin-converting enzyme inhibition, and two lines exhibited a downregulation of transgene mRNA in response to subpressor and pressor doses of ANG II. Our results suggest that 149 bp of the human renin proximal promoter, in a context of a genomic construct, are sufficient to confer human renin expression in renal JG cells and at least some aspects of appropriate regulation.

Transgenic animal models for the functional analysis of vasoactive peptides

The interplay of vasoactive peptide systems is an essential determinant of blood pressure regulation in mammals. While the endothelin and the renin-angiotensin systems raise blood pressure by inducing vasoconstriction and sodium retention, the kallikrein-kinin and the natriuretic-peptide systems reduce arterial pressure by eliciting vasodilatation and natriuresis. Transgenic technology has proven to be very useful for the functional analysis of vasoactive peptide systems. As an outstanding example, transgenic rats overexpressing the mouse Ren-2 renin gene in several tissues become extremely hypertensive. Several other transgenic rat and mouse strains with genetic modifications of components of the renin-angiotensin system have been developed in the past decade. Moreover, in recent years gene-targeting technology was employed to produce mouse strains lacking these proteins. The established animal models as well as the main insights gained by their analysis are summarized in this review.

Complementation of reduced survival, hypotension, and renal abnormalities in angiotensinogen-deficient mice by the human renin and human angiotensinogen genes

The aim of this study was to determine whether elements of the human renin-angiotensin system (RAS) could functionally replace elements of the mouse RAS by complementing the reduced survival and renal abnormalities observed in mice carrying a gene-targeted deletion of the mouse angiotensinogen gene (mAgt). Double transgenic mice containing the human renin (HREN) and human angiotensinogen (HAGT) genes were bred to mice heterozygous for the mAgt deletion and the compound heterozygotes were identified and intercrossed. The resulting progeny (n = 139) were genotyped at each locus and the population was stratified into two groups: the first containing both human transgenes (RA+) and the second containing zero or one, but not both human transgenes (RA-). Despite appropriate Mendelian ratios of RA- mice that were wildtype (+/+), heterozygous (+/-), and homozygous (-/-) for the deletion of mAgt at birth, there was reduced survival of RA- mAgt-/- mice to adulthood (P < 0.001 by chi2). In contrast, we observed appropriate Mendelian ratios of RA+ mAgt+/+, RA+ mAgt+/-, and RA+ mAgt-/- mice at birth and in adults (P > 0.05 by chi2). These results demonstrate that the presence of both human transgenes rescues the postnatal lethality in mAgt-/- mice. The renal histopathology exhibited by RA- mAgt-/- mice, including thickened arterial walls, severe fibrosis, lymphocytic infiltration, and atrophied parenchyma, was also rescued in the RA+ mAgt-/- mice. Direct arterial blood pressure recordings in conscious freely moving mice revealed that BP (in mmHg) varied proportionally to mAgt gene copy number in RA+ mice (approximately 20 mmHg per mAgt gene copy, P < 0.001). BP in RA+ mAgt-/- mice (132+/-3, n = 14) was intermediate between wild-type (RA- mAgt+/+, 105+/-2, n = 9) and RA+ mAgt+/+ (174+/-3, n = 10) mice. These studies establish that the human renin and angiotensinogen genes can functionally replace the mouse angiotensinogen gene, and provides proof in principle that we can examine the regulation of elements of the human RAS and test the significance of human RAS gene variants by a combined transgenic and gene targeting approach.