Early onset salt-sensitive hypertension in bradykinin B(2) receptor null mice

Role of Kinins in Hypertension and Heart Failure

The kallikrein-kinin system (KKS) is proposed to act as a counter regulatory system against the vasopressor hormonal systems such as the renin-angiotensin system (RAS), aldosterone, and catecholamines. Evidence exists that supports the idea that the KKS is not only critical to blood pressure but may also oppose target organ damage. Kinins are generated from kininogens by tissue and plasma kallikreins. The putative role of kinins in the pathogenesis of hypertension is discussed based on human mutation cases on the KKS or rats with spontaneous mutation in the kininogen gene sequence and mouse models in which the gene expressing only one of the components of the KKS has been deleted or over-expressed. Some of the effects of kinins are mediated via activation of the B2 and/or B1 receptor and downstream signaling such as eicosanoids, nitric oxide (NO), endothelium-derived hyperpolarizing factor (EDHF) and/or tissue plasminogen activator (T-PA). The role of kinins in blood pressure regulation at normal or under hypertension conditions remains debatable due to contradictory reports from various laboratories. Nevertheless, published reports are consistent on the protective and mediating roles of kinins against ischemia and cardiac preconditioning; reports also demonstrate the roles of kinins in the cardiovascular protective effects of the angiotensin-converting enzyme (ACE) and angiotensin type 1 receptor blockers (ARBs).

Unraveling the Pivotal Role of Bradykinin in ACE Inhibitor Activity

Historically, the first described effect of an angiotensin converting enzyme (ACE) inhibitor was an increased activity of bradykinin, one of the substrates of ACE. However, in the subsequent years, molecular models describing the mechanism of action of ACE inhibitors in decreasing blood pressure and cardiovascular risk have focused mostly on the renin-angiotensin system. Nonetheless, over the last 20 years, the importance of bradykinin in regulating vasodilation, natriuresis, oxidative stress, fibrinolysis, inflammation, and apoptosis has become clearer. The affinity of ACE appears to be higher for bradykinin than for angiotensin I, thereby suggesting that ACE inhibitors may be more effective inhibitors of bradykinin degradation than of angiotensin II production. Data describing the effect of ACE inhibition on bradykinin signaling support the hypothesis that the most cardioprotective benefits attributed to ACE inhibition may be due to increased bradykinin signaling rather than to decreased angiotensin II signaling, especially when high dosages of ACE inhibitors are considered. In particular, modulation of bradykinin in the endothelium appears to be a major target of ACE inhibition. These new mechanistic concepts may lead to further development of strategies enhancing the bradykinin signaling.

Conditional knockout of collecting duct bradykinin B2 receptors exacerbates angiotensin II-induced hypertension during high salt intake

We elucidated the role of collecting duct kinin B2 receptor (B2R) in the development of salt-sensitivity and angiotensin II (ANG II)-induced hypertension. To this end, we used a Cre-Lox recombination strategy to generate mice lacking Bdkrb2 gene for B2R in the collecting duct (Hoxb7-Cre(tg/+):Bdkrb2(flox/flox)). In 3 groups of control (Bdkrb2(flox/flox)) and 3 groups of UB(Bdkrb2-/-) mice, systolic blood pressure (SBP) responses to high salt intake (4 or 8% NaCl; HS) were monitored by radiotelemetry in comparison with standard salt diet (0.4% NaCl) prior to and during subcutaneous ANG II infusion (1000 ng/min/kg) via osmotic minipumps. High salt intakes alone for 2 weeks did not alter SBP in either strain. ANG II significantly increased SBP equally in control (121 ± 2 to 156 ± 3 mmHg) and UB(Bdkrb2-/-) mice (120 ± 2 to 153 ± 2 mmHg). The development of ANG II-induced hypertension was exacerbated by 4%HS in both control (125 ± 3 to 164 ± 5 mmHg) and UB(Bdkrb2-/-) mice (124 ± 2 to 162 ± 3 mmHg) during 2 weeks. Interestingly, 8%HS caused a more profound and earlier ANG II-induced hypertension in UB(Bdkrb2-/-) (129 ± 2 to 166 ± 3 mmHg) as compared to control (128 ± 2 to 158 ± 2 mmHg) and it was accompanied by body weight loss and increased mortality. In conclusion, targeted inactivation of B2R in the renal collecting duct does not cause salt-sensitivity; however, collecting duct B2R attenuates the hypertensive actions of ANG II under conditions of very high salt intake.

High salt-induced hypertension in B2 knockout mice is corrected by the ETA antagonist, A127722

BACKGROUND AND PURPOSE

The contribution of endothelin-1 (ET-1) in a B2KO mouse model of a high salt-induced arterial hypertension was investigated.

EXPERIMENTAL APPROACH

Wild-type (WT) or B2KO mice receiving a normal diet (ND) or a high-salt diet (HSD) were monitored by radiotelemetry up to a maximum of 18 weeks. At the 12th week of diet, subgroups under ND or HSD received by gavage the ETA antagonist A127722 during 5 days. In addition, blood samples were collected and, following euthanasia, the lungs, heart and kidneys were extracted, homogenized and assayed for ET-1 by RIA. In a separate series of experiments, the ETA antagonist, BQ123 was tested against the pressor responses to a NOS inhibitor L-N(G)-nitroarginine methyl ester (L-NAME) in anaesthetized WT and B2KO mice.

KEY RESULTS

In B2KO, but not WT mice, 12 weeks of HSD triggered a maximal increase of the mean arterial pressure (MAP) of 19.1 ± 2.8 mmHg, which was corrected by A127722 to MAP levels found in B2KO mice under ND. Significant increases in immunoreactive ET-1 were detected only in the lungs of B2KO mice under HSD. On the other hand, metabolic studies showed that sodium urinary excretion was markedly reduced in B2KO compared with WT mice under ND. Finally, BQ123 (2 mg·kg(-1)) reduced by 50% the pressor response to L-NAME (2 mg·kg(-1)) in B2KO, but not WT mice under anaesthesia.

CONCLUSIONS AND IMPLICATIONS

Our results support the concept that functional B2 receptors oppose high salt-induced increments in MAP, which are corrected by an ETA receptor antagonist in this mouse model of experimental hypertension.

Genome-wide analysis of gestational gene-environment interactions in the developing kidney

The G protein-coupled bradykinin B2 receptor (Bdkrb2) plays an important role in regulation of blood pressure under conditions of excess salt intake. Our previous work has shown that Bdkrb2 also plays a developmental role since Bdkrb2(-/-) embryos, but not their wild-type or heterozygous littermates, are prone to renal dysgenesis in response to gestational high salt intake. Although impaired terminal differentiation and apoptosis are consistent findings in the Bdkrb2(-/-) mutant kidneys, the developmental pathways downstream of gene-environment interactions leading to the renal phenotype remain unknown. Here, we performed genome-wide transcriptional profiling on embryonic kidneys from salt-stressed Bdkrb2(+/+) and Bdkrb2(-/-) embryos. The results reveal significant alterations in key pathways regulating Wnt signaling, apoptosis, embryonic development, and cell-matrix interactions. In silico analysis reveal that nearly 12% of differentially regulated genes harbor one or more Pax2 DNA-binding sites in their promoter region. Further analysis shows that metanephric kidneys of salt-stressed Bdkrb2(-/-) have a significant downregulation of Pax2 gene expression. This was corroborated in Bdkrb2(-/-);Pax2(GFP+/tg) mice, demonstrating that Pax2 transcriptional activity is significantly repressed by gestational salt-Bdkrb2 interactions. We conclude that gestational gene (Bdkrb2) and environment (salt) interactions cooperate to impact gene expression programs in the developing kidney. Suppression of Pax2 likely contributes to the defects in epithelial survival, growth, and differentiation in salt-stressed BdkrB2(-/-) mice.

Renal (tissue) kallikrein-kinin system in the kidney and novel potential drugs for salt-sensitive hypertension

A large variety of antihypertensive drugs, such as angiotensin converting enzyme inhibitors, diuretics, and others, are prescribed to hypertensive patients, with good control of the condition. In addition, all individuals are generally believed to be salt sensitive and, thus, severe restriction of salt intake is recommended to all. Nevertheless, the physiological defense mechanisms in the kidney against excess salt intake have not been well clarified. The present review article demonstrated that the renal (tissue) kallikrein-kinin system (KKS) is ideally situated within the nephrons of the kidney, where it functions to inhibit the reabsorption of NaCl through the activation of bradykinin (BK)-B2 receptors localized along the epithelial cells of the collecting ducts (CD). Kinins generated in the CD are immediately inactivated by two kidney-specific kinin-inactivating enzymes (kininases), carboxypeptidase Y-like exopeptidase (CPY), and neutral endopeptidase (NEP). Our work demonstrated that ebelactone B and poststatin are selective inhibitors of these kininases. The reduced secretion of the urinary kallikrein is linked to the development of salt-sensitive hypertension, whereas potassium ions and ATP-sensitive potassium channel blockers ameliorate salt-sensitive hypertension by accelerating the release of renal kallikrein. On the other hand, ebelactone B and poststatin prolong the life of kinins in the CD after excess salt intake, thereby leading to the augmentation of natriuresis and diuresis, and the ensuing suppression of salt-sensitive hypertension. In conclusion, accelerators of the renal kallikrein release and selective renal kininase inhibitors are both novel types of antihypertensive agents that may be useful for treatment of salt-sensitive hypertension.

The kallikrein-kinin system as a regulator of cardiovascular and renal function

Autocrine, paracrine, endocrine, and neuroendocrine hormonal systems help regulate cardio-vascular and renal function. Any change in the balance among these systems may result in hypertension and target organ damage, whether the cause is genetic, environmental or a combination of the two. Endocrine and neuroendocrine vasopressor hormones such as the renin-angiotensin system (RAS), aldosterone, and catecholamines are important for regulation of blood pressure and pathogenesis of hypertension and target organ damage. While the role of vasodepressor autacoids such as kinins is not as well defined, there is increasing evidence that they are not only critical to blood pressure and renal function but may also oppose remodeling of the cardiovascular system. Here we will primarily be concerned with kinins, which are oligopeptides containing the aminoacid sequence of bradykinin. They are generated from precursors known as kininogens by enzymes such as tissue (glandular) and plasma kallikrein. Some of the effects of kinins are mediated via autacoids such as eicosanoids, nitric oxide (NO), endothelium-derived hyperpolarizing factor (EDHF), and/or tissue plasminogen activator (tPA). Kinins help protect against cardiac ischemia and play an important part in preconditioning as well as the cardiovascular and renal protective effects of angiotensin-converting enzyme (ACE) and angiotensin type 1 receptor blockers (ARB). But the role of kinins in the pathogenesis of hypertension remains controversial. A study of Utah families revealed that a dominant kallikrein gene expressed as high urinary kallikrein excretion was associated with a decreased risk of essential hypertension. Moreover, researchers have identified a restriction fragment length polymorphism (RFLP) that distinguishes the kallikrein gene family found in one strain of spontaneously hypertensive rats (SHR) from a homologous gene in normotensive Brown Norway rats, and in recombinant inbred substrains derived from these SHR and Brown Norway rats this RFLP cosegregated with an increase in blood pressure. However, humans, rats and mice with a deficiency in one or more components of the kallikrein-kinin-system (KKS) or chronic KKS blockade do not have hypertension. In the kidney, kinins are essential for proper regulation of papillary blood flow and water and sodium excretion. B2-KO mice appear to be more sensitive to the hypertensinogenic effect of salt. Kinins are involved in the acute antihypertensive effects of ACE inhibitors but not their chronic effects (save for mineralocorticoid-salt-induced hypertension). Kinins appear to play a role in the pathogenesis of inflammatory diseases such as arthritis and skin inflammation; they act on innate immunity as mediators of inflammation by promoting maturation of dendritic cells, which activate the body's adaptive immune system and thereby stimulate mechanisms that promote inflammation. On the other hand, kinins acting via NO contribute to the vascular protective effect of ACE inhibitors during neointima formation. In myocardial infarction produced by ischemia/reperfusion, kinins help reduce infarct size following preconditioning or treatment with ACE inhibitors. In heart failure secondary to infarction, the therapeutic effects of ACE inhibitors are partially mediated by kinins via release of NO, while drugs that activate the angiotensin type 2 receptor act in part via kinins and NO. Thus kinins play an important role in regulation of cardiovascular and renal function as well as many of the beneficial effects of ACE inhibitors and ARBs on target organ damage in hypertension.

Angiotensin 1-7 and Mas decrease thrombosis in Bdkrb2-/- mice by increasing NO and prostacyclin to reduce platelet spreading and glycoprotein VI activation

Bradykinin B2 receptor-deleted mice (Bdkrb2(-/-)) have delayed carotid artery thrombosis times and prolonged tail bleeding time resulting from elevated angiotensin II (AngII) and angiotensin receptor 2 (AT2R) producing increased plasma nitric oxide (NO) and prostacyclin. Bdkrb2(-/-) also have elevated plasma angiotensin-(1-7) and messenger RNA and protein for its receptor Mas. Blockade of Mas with its antagonist A-779 in Bdkrb2(-/-) shortens thrombosis times (58 ± 4 minutes to 38 ± 4 minutes) and bleeding times (170 ± 13 seconds to 88 ± 8 seconds) and lowers plasma nitrate (22 ± 4 μM to 15 ± 5 μM), and 6-keto-PGF1α (259 ± 103 pg/mL to 132 ± 58 pg/mL). Bdkrb2(-/-) platelets express increased NO, guanosine 3',5'-cyclic monophosphate, and cyclic adenosine monophosphate with reduced spreading on collagen, collagen peptide GFOGER, or fibrinogen. In vivo A-779 or combined L-NAME and nimesulide treatment corrects it. Bdkrb2(-/-) platelets have reduced collagen-related peptide-induced integrin α2bβ3 activation and P-selectin expression that are partially corrected by in vivo A-779, nimesulide, or L-NAME. Bone marrow transplantations show that the platelet phenotype and thrombosis time depends on the host rather than donor bone marrow progenitors. Transplantation of wild-type bone marrow into Bdkrb2(-/-) hosts produces platelets with a spreading defect and delayed thrombosis times. In Bdkrb2(-/-), combined AT2R and Mas overexpression produce elevated plasma prostacyclin and NO leading to acquired platelet function defects and thrombosis delay.

Vascular mechanisms involved in angiotensin II-induced venoconstriction in hypertensive rats

To investigate the venoconstrictor effect of angiotensin II (Ang II) in spontaneously hypertensive rats (SHR), we used preparations of mesenteric venular beds and the circular muscle of the portal veins. Vessels were tested with Ang II in the presence or absence of losartan, PD 123319, HOE 140, L-NAME, indomethacin, or celecoxib. In the mesenteric venular bed of SHR, the effect of Ang II (0.1 nmol) was nearly abolished by losartan and enhanced by HOE 140, indomethacin, and celecoxib, while PD123319 and L-NAME had no effect. In portal vein preparations, cumulative-concentration response curves (CCRC) to Ang II (0.1-100 nmol/L) exhibited a lower maximal response (E(max)) in SHR compared to Wistar rats. AT(1) receptor expression was similar in the two strains, while AT(2) receptor levels were lower in SHR portal veins when compared to Wistar. In SHR portal veins, losartan shifted the CCRC to Ang II to the right, while indomethacin and HOE 140 increased the E(max) to Ang II. PD 123319, celecoxib, and L-NAME had no effect. Taken together, our results suggest that Ang II-induced venoconstriction in SHR is mediated by activation of AT(1) receptors and this effect may be counterbalanced by kinin B(2) receptor and COX metabolites. Furthermore, our data indicate that there are different cellular and molecular mechanisms involved in the regulation of venous tonus of normotensive and hypertensive rats. These differences probably reflect distinct factors that influence arterial and venous bed in hypertension.

Role of vascular Kinin B1 and B2 receptors in endothelial nitric oxide metabolism

Kinin B(1) and B(2) receptors play an essential role in inflammatory process and cardiovascular homeostasis. The present study investigated the vascular reactivity and nitric oxide (NO) generation in the isolated mesenteric arteriolar bed from B(1) (B(1)(-/-)) and B(2) receptor (B(2)(-/-)) knockout mice. Endothelial-dependent relaxation was significantly decreased in arterioles from both B(1)(-/-) and B(2)(-/-) in comparison to wild type (WT) mice, with no differences for endothelial-independent relaxating or vasoconstrictor agents. Plasmatic and vascular NO production were markedly reduced in both B(1)(-/-) and B(2)(-/-). In contrast, in the presence of l-arginine, Ca(2+) and co-factors for the enzyme, NO synthase activity was higher in homogenates of mesenteric vessels of B(1)(-/-) and B(2)(-/-). The present study demonstrated that targeted deletion of B(1) or B(2) receptor gene in mice induces important alterations in the vascular reactivity of resistance vessels and NO metabolism. The severe impairment in the endothelial-mediated vasodilation accompanied by decreased NO bioavailability, despite the augmented NOS activity, strongly indicates an exacerbation of NO inactivation in B(1)(-/-) and B(2)(-/-) vessels. The present data provide valuable information in order to clarify the relevance of kinin receptors in regulating vascular physiology and may point to new approaches regarding its correlation with endothelial dysfunction, oxidative stress and NO availability.

Genetics of salt-sensitive hypertension

The assessment of salt sensitivity of blood pressure is difficult because of the lack of universal consensus on definition. Regardless of the variability in the definition of salt sensitivity, increased salt intake, independent of the actual level of blood pressure, is also a risk factor for cardiovascular morbidity and mortality and kidney disease. A modest reduction in salt intake results in an immediate decrease in blood pressure, with long-term beneficial consequences. However, some have suggested that dietary sodium restriction may not be beneficial to everyone. Thus, there is a need to distinguish salt-sensitive from salt-resistant individuals, but it has been difficult to do so with phenotypic studies. Therefore, there is a need to determine the genes that are involved in salt sensitivity. This review focuses on genes associated with salt sensitivity, with emphasis on the variants associated with salt sensitivity in humans that are not due to monogenic causes. Special emphasis is given to gene variants associated with salt sensitivity whose protein products interfere with cell function and increase blood pressure in transgenic mice.

The B2 receptor of bradykinin is not essential for the post-exercise increase in glucose uptake by insulin-stimulated mouse skeletal muscle

Bradykinin can enhance skeletal muscle glucose uptake (GU), and exercise increases both bradykinin production and muscle insulin sensitivity, but bradykinin's relationship with post-exercise insulin action is uncertain. Our primary aim was to determine if the B2 receptor of bradykinin (B2R) is essential for the post-exercise increase in GU by insulin-stimulated mouse soleus muscles. Wildtype (WT) and B2R knockout (B2RKO) mice were sedentary or performed 60 minutes of treadmill exercise. Isolated soleus muscles were incubated with [³H]-2-deoxyglucose +/-insulin (60 or 100 microU/ml). GU tended to be greater for WT vs. B2RKO soleus with 60 microU/ml insulin (P=0.166) and was significantly greater for muscles with 100 microU/ml insulin (P<0.05). Both genotypes had significant exercise-induced reductions (P<0.05) in glycemia and insulinemia, and the decrements for glucose (approximately 14 %) and insulin (approximately 55 %) were similar between genotypes. GU tended to be greater for exercised vs. sedentary soleus with 60 microU/ml insulin (P=0.063) and was significantly greater for muscles with 100 microU/ml insulin (P<0.05). There were no significant interactions between genotype and exercise for blood glucose, plasma insulin or GU. These results indicate that the B2R is not essential for the exercise-induced decrements in blood glucose or plasma insulin or for the post-exercise increase in GU by insulin-stimulated mouse soleus muscle.

Physiology of Kidney Renin

The protease renin is the key enzyme of the renin-angiotensin-aldosterone cascade, which is relevant under both physiological and pathophysiological settings. The kidney is the only organ capable of releasing enzymatically active renin. Although the characteristic juxtaglomerular position is the best known site of renin generation, renin-producing cells in the kidney can vary in number and localization. (Pro)renin gene transcription in these cells is controlled by a number of transcription factors, among which CREB is the best characterized. Pro-renin is stored in vesicles, activated to renin, and then released upon demand. The release of renin is under the control of the cAMP (stimulatory) and Ca2+(inhibitory) signaling pathways. Meanwhile, a great number of intrarenally generated or systemically acting factors have been identified that control the renin secretion directly at the level of renin-producing cells, by activating either of the signaling pathways mentioned above. The broad spectrum of biological actions of (pro)renin is mediated by receptors for (pro)renin, angiotensin II and angiotensin-( 1 – 7 ).

A Novel Category of Anti-Hypertensive Drugs for Treating Salt-Sensitive Hypertension on the Basis of a New Development Concept

Terrestrial animals must conserve water and NaCl to survive dry environments. The kidney reabsorbs 95% of the sodium filtered from the glomeruli before sodium reaches the distal connecting tubules. Excess sodium intake requires the renal kallikrein-kinin system for additional excretion. Renal kallikrein is secreted from the distal connecting tubule cells of the kidney, and its substrates, low molecular kininogen, from the principal cells of the cortical collecting ducts (CD). Formed kinins inhibit reabsorption of NaCl through bradykinin (BK)-B₂ receptors, localized along the CD. Degradation pathway of BK by kinin-destroying enzymes in urine differs completely from that in plasma, so that ACE inhibitors are ineffective. Urinary BK is destroyed mainly by a carboxypeptidase-Y-like exopeptidase (CPY) and partly by a neutral endopeptidase (NEP). Inhibitors of CPY and NEP, ebelactone B and poststatin, respectively, were found. Renal kallikrein secretion is accelerated by potassium and ATP-sensitive potassium (KATP) channel blockers, such as PNU-37883A. Ebelactone B prevents DOCA-salt hypertension in rats. Only high salt intake causes hypertension in animals deficient in BK-B2 receptors, tissue kallikrein, or kininogen. Hypertensive patients, and spontaneously hypertensive rats, excrete less kallikrein than normal subjects, irrespective of races, and become salt-sensitive. Ebelactone B, poststatin, and KATP channel blockers could become novel antihypertensive drugs by increase in urinary kinin levels. Roles of kinin in cardiovascular diseases were discussed.

Ontogeny of bradykinin B1 receptors in the mouse kidney

Kinins are vasoactive peptides that stimulate two G-protein coupled bradykinin receptors (B1R and B2R). B2R-knockout mice are salt sensitive and develop renal dysgenesis and hypertension if salt stressed during embryogenesis. B1R-knockout mice, on the other hand, are protected from inflammation and fibrosis. This study examined the spatiotemporal expression of B1R during renal organogenesis. The segmental nephron identity of B1R immunoreactivity was determined by costaining with markers of the collecting duct (Dolichos biflorus), proximal tubule (Dolichos tetraglonus), and nephron progenitors (Pax2). At E14.5, the B1R was confined to few cells in the metanephric mesenchyme. Abundance of B1R increased progressively during development. On E17.5, B1R was enriched in differentiating proximal tubular cells and by postnatal day 1, B1R was clearly expressed on the luminal aspect of the proximal tubule. Quantitative real-time PCR revealed that the levels of B1R mRNA more than double during renal maturation. We conclude that 1) B1R expression correlates closely with nephron maturation; 2) lack of B1R in nephron progenitors suggests that B1R is unlikely to play a role in early nephrogenesis; and 3) enrichment of B1R in maturing proximal tubule suggests a potential role for this receptor in terminal differentiation of the proximal nephron.

Bradykinin B2 receptor null mice harboring a Ser23-to-Ala substitution in the p53 gene are protected from renal dysgenesis

A physiological cross talk operates between the tumor suppressor protein p53 and the bradykinin B2 receptor (BdkrB2) during renal organogenesis. Thus, although BdkrB2 is a target for p53-mediated transcriptional activation, BdkrB2 is required to restrict p53 proapoptotic activity. We previously demonstrated that BdkrB2(-/-) embryos exposed to gestational salt stress develop renal dysgenesis as a result of p53-mediated apoptosis of nephron progenitors and repression of the terminal differentiation program. Compared with wild-type kidneys, BdkrB2(-/-) express abnormally high levels of the Checkpoint kinase (Chk1), which activates p53 via Ser23 phosphorylation. To define the functional relevance of p53S23 phosphorylation, we generated a compound strain of BdkrB2(-/-) mice harboring a homozygous Ser23-to-Ala (S23A) mutation in the p53 gene by crossing BdkrB2(-/-) with p53S23A knockin mice. Unlike salt-stressed BdkrB2(-/-) pups, which exhibit renal dysgenesis, homozygous S23A;BdkrB2(-/-) littermates are protected and have normal renal development. Heterozygous S23A;BdkrB2(-/-) mice have an intermediate phenotype. The p53-S23A substitution was associated with amelioration of apoptosis and restored markers of nephrogenesis and tubulogenesis. Real-time quantitative RT-PCR of terminal differentiation genes demonstrated that the S23A substitution restored normal expression patterns of aquaporin-2, Na-Cl cotransporter, Na-K-2Cl cotransporter, Na-bicarbonate cotransporter, and Sglt1. We conclude that p53 phosphorylation on Ser23 is an essential step in the signaling pathway mediating the susceptibility of BdkrB2(-/-) mutants to renal dysgenesis.

Genetically altered animals in the study of the metabolic functions of peptide hormone systems

PURPOSE OF REVIEW

Here we review the use of genetically altered animals to address the roles of peptide hormone systems in the modulation of energy homeostasis. Despite the disseminated use of transgenic techniques to establish the functional relevance of several peptide hormone systems, we focus on two multifunctional systems, the renin-angiotensin and the kallikrein-kinin systems. Initially, we explored the background information supporting the functional aspects of these systems, followed by novel knowledge obtained with the phenotypic characterization of genetically altered animals.

RECENT FINDINGS

A role for the renin-angiotensin system in the regulation of adiposity and glucose metabolism has been suggested. Studies using genetically altered animals not only confirmed the physiological relevance of angiotensin II in the control of energy homeostasis, but also revealed that the adipose tissue renin-angiotensin system participates in the endocrine modulation of cardiovascular and renal function. On the other hand, the involvement of the kallikrein-kinin system with metabolic processes was not so obvious. Recent reports using genetically altered animals, however, provided strong evidence to support an important role for kinins in the control of glucose homeostasis and energy balance.

SUMMARY

Here we present examples of how genetically altered animals contribute to a final postulation of the physiological roles of certain hormone systems, bringing new insights into the field.

The plasma kallikrein-kinin system: its evolution from contact activation

The plasma kallikrein-kinin system consists of the proteins factor XII (FXII), prekallikrein (PK), and high molecular weight kininogen. It was first recognized as a surface-activated coagulation system that is activated when blood or plasma interacts with artificial surfaces. Although surface-activated contact activation occurs in vivo in the case of tissue destruction or a developing thrombus, the physiologic basis for the activation and function of this system has not been delineated. New investigations indicate that there is a proteolytic pathway on cells for PK activation independent of FXII. This pathway for PK with subsequent FXII activation indicates physiologic activities. These activities include blood pressure regulation and modulation of thrombosis risk independently of hemostasis. Furthermore, they include regulation of endothelial cell proliferation, angiogenesis and apoptosis through a cellular-based, outside-in signaling system. The present characterizations of this system, which incorrectly had been thought to initiate coagulation, represent an evolution of understanding in this field.

Mechanisms of Disease: the tissue kallikrein–kinin system in hypertension and vascular remodeling

The pathogenesis of arterial hypertension often involves a rise in systemic vascular resistance (vasoconstriction and vascular remodeling) and impairment of salt excretion in the kidney (inappropriate salt retention despite elevated blood pressure). Experimental and clinical evidence implicate an imbalance between endogenous vasoconstrictor and vasodilator systems in the development and maintenance of hypertension. Kinins (bradykinin and lys-bradykinin) are endogenous vasodilators and natriuretic peptides known best for their ability to antagonize angiotensin-induced vasoconstriction and sodium retention. In humans, angiotensin-converting enzyme inhibitors, a potent class of antihypertensive agents, lower blood pressure at least partially by favoring enhanced kinin accumulation in plasma and target tissues. The beneficial actions of kinins in renal and cardiovascular disease are largely mediated by nitric oxide and prostaglandins, and extend beyond their recognized role in lowering blood pressure to include cardioprotection and nephroprotection. This article is a review of exciting, recently generated genetic, biochemical and clinical data from studies that have examined the importance of the tissue kallikrein-kinin system in protection from hypertension, vascular remodeling and renal fibrosis. Development of novel therapeutic approaches to bolster kinin activity in the vascular wall and in specific compartments in the kidney might be a highly effective strategy for the treatment of hypertension and its complications, including cardiac hypertrophy and renal failure.

The Bradykinin B2 receptor gene is a target of angiotensin II type 1 receptor signaling

Cross-talk between G protein-coupled receptors (GPCR) is known to occur at multiple levels, including receptor heterodimerization and intracellular signaling. This study tested the hypothesis that GPCR cross-talk occurs at the transcriptional level. It was demonstrated that the bradykinin B2 receptor gene (BdkrB2) is a direct transcriptional target of the angiotensin II (AngII) type 1 receptor (AT(1)R) in collecting duct cells. AngII induced BdkrB2 mRNA expression in mouse inner medullary collecting duct cells, and this effect was abrogated by AT(1)R blockade; in contrast, AT(2)R blockade was ineffective. Actinomycin D, an inhibitor of gene transcription, abrogated AngII-stimulated BdkrB2 expression. In addition, AngII produced dosage- and time-dependent increases in B2 receptor protein levels (2.9 +/- 0.4 fold; P < 0.05). AngII stimulated phosphorylation of cAMP response element binding protein (CREB) on Ser-133 and assembly of p-CREB on the BdkrB2 promoter in vivo. Moreover, AngII induced hyperacetylation of BdkrB2 promoter-associated H4 histones, a chromatin modification that is associated with gene activation. Mutations of the CRE abrogated AngII-induced activation of the BdkrB2 promoter. AngII-treated inner medullary collecting duct cells exhibited augmented intracellular calcium signaling in response to bradykinin, confirming the functional relevance of AT(1)-B2 receptor signaling. Finally, studies that were conducted in angiotensin type 1 receptor (Agtr1)-null mice revealed that BdkrB2 mRNA levels were significantly lower in the renal medulla of Agtr1(A)(-/-) and Agtr1(A/B)(-/-) than in Agtr1(+/+) and Agtr1(B)(-/-) mice. It is concluded that BdkrB2 is a downstream target of the AT(1)R-CREB signaling pathway. Transcriptional regulation represents a novel form of cross-talk between GPCR that link the renin-angiotensin and kallikrein-kinin systems.

Susceptibility to metanephric apoptosis in bradykinin B2 receptor null mice via the p53-Bax pathway

In response to gestational high salt intake, BdkrB2−/− embryos acquire an aberrant renal phenotype mimicking renal dysplasia in humans. Genetic analysis identified p53 as a mediator of the renal dysplasia in salt-stressed BdkrB2−/− mice, acting partly via repression of terminal epithelial differentiation genes. The present study tested the hypothesis that inactivation of BdkrB2 predisposes the salt-stressed embryo to p53-mediated metanephric apoptosis. Newborn BdkrB2−/− pups exhibited hyperphosphorylation of metanephric p53 on serine 20 (mouse serine 23), a modification known to increase p53 stability and apoptotic activity. As a result, there was widespread, ectopic expression of p53 in the BdkrB2−/− kidney. However, no differences were found in the apoptosis index or gene expression in BdkrB2−/− and +/+ kidneys, indicating that p53 stabilization as a result of BdkrB2 inactivation is not sufficient to induce metanephric apoptosis. On gestational salt stress, fulminant metanephric apoptosis and enhanced Bax gene expression occurred in BdkrB2−/− but not their +/− or +/+ littermates. Germline deletion of p53 from BdkrB2−/− mice prevented Bax activation and normalized the apoptosis index. Rescue of metanephric apoptosis in BdkrB2−/− mice was similarly achieved by Bax gene deletion. Aberrant apoptosis in salt-stressed BdkrB2−/− mice was triggered on embryonic day E15.5 and involved both ureteric bud (UB) and metanephric mesenchyme-derived nephron elements. Cultured E12.5 salt-stressed BdkrB2−/− metanephroi manifested stunted UB branching compared with +/− and +/+ littermates; the abnormal UB branching was corrected by p53 deletion. Our results suggest a model whereby a seemingly silent genetic mutation of BdkrB2 predisposes mice to renal dysplasia by creating a “preapoptotic” state through p53 activation.

Genetically altered animal models in the kallikrein-kinin system

Transgenic and gene-targeting technologies allowing the generation of genetically altered animal models have greatly advanced our understanding of the function of specific genes. This is also true for the kallikrein-kinin system (KKS), in which some, but not yet all, components have been functionally characterized using such techniques. The first genetically altered animal model for a KKS component was supplied by nature, the brown Norway rat carrying an inactivating mutation in the kininogen gene. Mice deficient in tissue kallikrein, B1 and B2 receptors, some kinin-degrading enzymes, and factor XII followed, together with transgenic rat and mouse strains overexpressing tissue kallikrein, B1 and B2 receptors, and degrading enzymes. There are still no animal models with genetic alterations in plasma kallikrein, kininases I and some other degrading enzymes. The models have confirmed an important role of the KKS in cardiovascular pathology, inflammation, and pain, and have partially elucidated the distinct function of the two receptors. This created the basis for rational decisions concerning the putative use of kinin receptor agonists and antagonists in therapeutic applications. However, a more thorough analysis of the existing models and the generation of new, more sophisticated transgenic models will be necessary to clarify the still elusive issue as to where and by which mechanisms the kinins exert their actions.

Putative roles of kinin receptors in the therapeutic effects of angiotensin 1-converting enzyme inhibitors in diabetes mellitus

The role of endogenous kinins and their receptors in diabetes mellitus is being confirmed with the recent developments of molecular and genetic animal models. Compelling evidence suggests that the kinin B(2) receptor is organ-protective and partakes to the therapeutic effects of angiotensin 1-converting enzyme inhibitors (ACEI) and angiotensin AT(1) receptor antagonists. Benefits derive primarily from vasodilatory, antihypertensive, antiproliferative, antihypertrophic, antifibrotic, antithrombotic and antioxidant properties of kinin B(2) receptor activation. Mechanisms include the formation of nitric oxide and prostacyclin and the inhibition of NAD(P)H oxidase activity involving classical and novel signalling pathways. Kinin B(2) receptor also ameliorates insulin resistance by increasing glucose uptake and supply, and by inducing glucose transporter-4 translocation either directly or through phosphorylation of insulin receptor. The kinin B(1) receptor, which is induced by the cytokine network, growth factors and hyperglycaemia, mediates hyperalgesia, vascular hyperpermeability and leukocytes infiltration in diabetic animals. However, emerging data highlight reno- and cardio-protective effects mediated by kinin B(1) receptor under chronic ACEI therapy in diabetes mellitus. Thus, the Janus-faced of kinin receptors needs to be taken into account in future drug development. For instance, locally acting kinin B(1)/B(2) receptor agonists if used in a safe therapeutic window may represent a more rationale strategy in the prevention and management of diabetic complications. Because kinin B(2) receptor antagonists may further increase insulin resistance, the persisting dogma that restricts the development of kinin receptor analogues to antagonists (that is still relevant to abrogate pain and inflammation) needs to be revisited.

International union of pharmacology. XLV. Classification of the kinin receptor family: from molecular mechanisms to pathophysiological consequences

Kinins are proinflammatory peptides that mediate numerous vascular and pain responses to tissue injury. Two pharmacologically distinct kinin receptor subtypes have been identified and characterized for these peptides, which are named B1 and B2 and belong to the rhodopsin family of G protein-coupled receptors. The B2 receptor mediates the action of bradykinin (BK) and lysyl-bradykinin (Lys-BK), the first set of bioactive kinins formed in response to injury from kininogen precursors through the actions of plasma and tissue kallikreins, whereas the B(1) receptor mediates the action of des-Arg9-BK and Lys-des-Arg9-BK, the second set of bioactive kinins formed through the actions of carboxypeptidases on BK and Lys-BK, respectively. The B2 receptor is ubiquitous and constitutively expressed, whereas the B1 receptor is expressed at a very low level in healthy tissues but induced following injury by various proinflammatory cytokines such as interleukin-1beta. Both receptors act through G alpha(q) to stimulate phospholipase C beta followed by phosphoinositide hydrolysis and intracellular free Ca2+ mobilization and through G alpha(i) to inhibit adenylate cyclase and stimulate the mitogen-activated protein kinase pathways. The use of mice lacking each receptor gene and various specific peptidic and nonpeptidic antagonists have implicated both B1 and B2 receptors as potential therapeutic targets in several pathophysiological events related to inflammation such as pain, sepsis, allergic asthma, rhinitis, and edema, as well as diabetes and cancer. This review is a comprehensive presentation of our current understanding of these receptors in terms of molecular and cell biology, physiology, pharmacology, and involvement in human disease and drug development.

Combinatorial control of the bradykinin B2 receptor promoter by p53, CREB, KLF-4, and CBP: implications for terminal nephron differentiation

Despite a wealth of knowledge regarding the early steps of epithelial differentiation, little is known about the mechanisms responsible for terminal nephron differentiation. The bradykinin B2 receptor (B2R) regulates renal function and integrity, and its expression is induced during terminal nephron differentiation. This study investigates the transcriptional regulation of the B2R during kidney development. The rat B2R 5′-flanking region has a highly conserved cis-acting enhancer in the proximal promoter consisting of contiguous binding sites for the transcription factors cAMP response element binding protein (CREB), p53, and Krüppel-like factor (KLF-4). The B2R enhancer drives reporter gene expression in inner medullary collecting duct-3 cells but is considerably weaker in other cell types. Site-directed mutagenesis and expression of dominant negative mutants demonstrated the requirement of CREB DNA binding and Ser-133 phosphorylation for optimal enhancer function. Moreover, helical phasing experiments showed that disruption of the spatial organization of the enhancer inhibits B2R promoter activity. Several lines of evidence indicate that cooperative interactions among the three transcription factors occur in vivo during terminal nephron differentiation: 1) CREB, p53, and KLF-4 are coexpressed in B2R-positive differentiating cells; 2) the maturational expression of B2R correlates with CREB/p53/KLF-4 DNA-binding activity; 3) assembly of CREB, p53, and KLF-4 on chromatin at the endogenous B2R promoter is developmentally regulated and is accompanied by CBP recruitment and histone hyperacetylation; and 4) CREB and p53 occupancy of the B2R enhancer is cooperative. These results demonstrate that combinatorial interactions among the transcription factors, CREB, p53, and KLF-4, and the coactivator CBP, may be critical for the regulation of B2R gene expression during terminal nephron differentiation.

A novel pathological role of p53 in kidney development revealed by gene-environment interactions

Gene-environment interactions are implicated in congenital human disorders. Accordingly, there is a pressing need to develop animal models of human disease, which are the product of defined gene-environment interactions. Previously, our laboratory demonstrated that gestational salt stress of bradykinin B(2) receptor (B(2)R)-null mice induces renal dysgenesis and early death of the offspring. In contrast, salt-stressed B(2)R +/+ or +/- littermates have normal development. The present study investigates the mechanisms underlying the susceptibility of B(2)R-null mice to renal dysgenesis. Proteomic and conventional Western blot screens identified E-cadherin among the differentially repressed proteins in B(2)R-/- kidneys, whereas the checkpoint kinase Chk1 and its substrate P-Ser(20) p53 were induced. We tested the hypothesis that p53 mediates repression of E-cadherin gene expression and is causally linked to the renal dysgenesis. Genetic crosses between B(2)R -/- and p53+/- mice revealed that germline reduction of p53 gene dosage rescues B(2)R-/- mice from renal dysgenesis and restores kidney E-cadherin gene expression. Furthermore, gamma-irradiation induces repression of E-cadherin gene expression in p53+/+ but not -/- cells. In transient transfection assays, p53 repressed human E-cadherin promoter-driven reporter activity, whereas a mutant p53, which cannot bind DNA, did not. Functional promoter analysis indicated the presence of a p53-responsive element in exon 1, which partially mediates p53-induced repression. Chromatin immunoprecipitation assays revealed that p53 inhibits histone acetylation of the E-cadherin promoter. Treatment with a histone deacetylase inhibitor reversed both p53-mediated promoter repression and deacetylation. In conclusion, this study demonstrates that gene-environment interactions cooperate to induce congenital defects through p53 activation.

Loss of Sodium Modulation of Plasma Kinins in Human Hypertension

We studied the effect of salt intake and hypertension on the systemic kallikrein-kinin system (KKS), as measured by bradykinin (BK) 1-5, a stable circulating bradykinin metabolite, and the tissue KKS, as measured by urinary kallikrein excretion. Venous BK 1-5, urinary kallikrein, and components of the renin-angiotensin-aldosterone system were measured in 35 normotensive and 19 hypertensive subjects who were maintained on a high (200 mmol/day) or low (10 mmol/day) salt diet. Salt restriction decreased mean arterial pressure (MAP) (P < 0.001 overall) and the plasma angiotensin-converting enzyme (P = 0.017) and increased plasma renin activity (P < 0.001) and serum aldosterone (P < 0.001). There was an interactive effect of salt intake and hypertension on plasma BK 1-5 (P = 0.043), with BK 1-5 significantly lower during low compared with high salt intake in normotensive (24.7 +/- 2.6 versus 34.9 +/- 5.6 fmol/ml, P = 0.002) but not hypertensive subjects (30.6 +/- 4.6 versus 27.5 +/- 2.8 fmol/ml, P = 0.335). In normotensives, the change in plasma BK 1-5 from high to low salt intake correlated with the change in MAP (r = 0.533, P = 0.004). Urinary kallikrein was higher during low compared with high salt intake (P < 0.001) in both groups. There was no effect of salt intake on urinary BK 1-5. In summary, the systemic and renal KKSs act in tandem to modulate the response to salt intake. The systemic system is activated during high salt intake and counterbalances increased vascular response to pressors. With sodium restriction, the renal system is activated and counterbalances the increased sodium-retaining state induced by activation of the renin-angiotensin-aldosterone system. With hypertension, these modulating effects are diminished or lost, supporting a role for both systems in the development/maintenance of hypertension.

Renal and blood pressure phenotype in 18-mo-old bradykinin B2R(-/-)CRD mice

Aberrant gene-environment interactions are implicated in the pathogenesis of congenital renal dysgenesis (CRD), a leading cause of renal failure in infants and children. We have recently developed an animal model of CRD that is caused by gestational salt stress (5% NaCl diet; HS) of bradykinin B2R null mice [B2R(-/-)CRD; El-Dahr SS, Harrison-Bernard LM, Dipp S, Yosipiv IV, and Meleg-Smith S. Physiol Genomics 3: 121-131, 2000.]. Developing B2R(-/-)CRD mice exhibit tubular and glomerular cysts, stromal expansion, and loss of corticomedullary differentiation. In addition, B2R(-/-)CRD mice exhibit transient hypertension from 2 to 4 mo of age. The present study was designed to determine the long-term consequences of CRD on renal morphology and salt sensitivity of blood pressure in B2R(-/-)CRD mice. One-year- and 18-mo-old B2R(-/-)CRD mice exhibited stunted renal growth, glomerular cystic abnormalities, and collecting duct ectasia. Moreover, tumors of mesenchymal cell origin emerged in the dysplastic kidneys of 90% of 1-yr-old and 100% of 18-mo-old B2R(-/-)CRD mice but not in age-matched B2R(-/-) or wild-type mice. When challenged with an HS diet, 18-mo-old B2R(-/-)CRD exhibited a significant rise in systolic and diastolic blood pressures and more pronounced natriuresis and diuresis compared with salt-loaded 18-mo-old wild-type mice. Kidney aquaporin-2 expression was decreased by 50%, whereas renin, ANG type 1 receptor, and Na+-K+-ATPase levels were not different in B2R(-/-)CRD mice compared with controls. In conclusion, this study demonstrates that B2R(-/-)CRD mice exhibit permanent phenotypic and functional abnormalities in renal growth and differentiation. This novel model of human disease links gene-environment interactions with renal development and blood pressure homeostasis.

Renal kallikrein-kinin system damage and salt sensitivity: insights from experimental models

The importance of tubulointerstitial injury in the pathophysiology of human essential hypertension, and particularly salt sensitivity, is increasingly recognized. Since the renal kallikrein-kinin system (KKS) is located in the tubulointerstitial region of the kidney it is reasonable to expect that injury to this area, whatever the cause, may impair KKS production and compromise its role in blood pressure regulation. In this review we discuss evidence of injury in the renal kallikrein-producing structures in three different experimental models characterized by prominent tubulointerstitial lesions: subtotal nephrectomy; inhibition of nitric oxide synthase; and overload proteinuria. These three experimental models have in common the development of important tubulointerstitial damage and salt-sensitive hypertension expressed after the initial injury has ceased. In these three models, reduced KKS activity may contribute to the establishment of a pathophysiologic state characterized by unopposed hyperactivity of the renin-angiotensin system, resulting in salt retention.

The Bradykinin B2 receptor is required for full expression of renal COX-2 and renin

Angiotensin converting enzyme (ACE) inhibition leads to increased levels of bradykinin, cyclooxygenase-2 (COX-2), and renin. Since bradykinin stimulates prostaglandin release, renin synthesis may be regulated through a kinin-COX-2 pathway. To test this hypothesis, we examined the impact of bradykinin B2 receptor (B2R) gene disruption in mice on kidney COX-2 and renin gene expression. Kidney COX-2 mRNA and protein levels were significantly lower in B2R-/- mice by 40-50%. On the other hand, renal COX-1 levels were similar in B2R-/- and +/+ mice. Renal renin protein was 61% lower in B2R-/- compared to B2R+/+ mice. This was accompanied by a significant reduction in renin mRNA levels in B2R-/- mice. Likewise, intrarenal angiotensin I levels were significantly lower in B2R-/- mice compared to B2R+/+ mice. In contrast, kidney angiotensin II levels were not different and averaged 261+/-16 and 266+/-15fmol/g in B2R+/+ and B2R-/- mice, respectively. Kidney angiotensinogen, AT1 receptor and ACE activity were not different between B2R+/+ and B2R-/- mice. The results of these studies demonstrate suppression of renal renin synthesis in mice lacking the bradykinin B2R and support the notion that B2R regulation of COX-2 participates in the steady-state control of renin gene expression.

Decreased renal NO excretion and reduced glomerular tuft area in mice lacking the bradykinin B2 receptor

Bradykinin B(2) receptor knockout mice (B(2)-/-) have been useful to study the role of bradykinin under pathological conditions. With the use of these mice, it was shown that bradykinin plays an important role in angiogenesis, heart failure, salt-induced hypertension, and kidney fibrosis. Data on the role of the bradykinin B(2) receptor under physiological conditions using these mice are controversial and scarce, because these mice have no typical phenotype. For this reason, we have studied, under physiological conditions, renal hemodynamics as well as a number of morphometric glomerular parameters of B(2)-/- mice on a homogenized genetic background and on mice bred in a pathogen-free environment. Backcrossed B(2)-/- mice had normal blood pressure and normal apparent renal hemodynamics and morphology. However, reduced renal nitrite excretion and glomerular cGMP content were found, which was associated with a reduced glomerular capillary surface area. These differences had, however, no detectable effects on renal hemodynamics. These differences between B(2)-/- and wild-type mice might become important under pathological conditions as shown by a number of studies using these bradykinin B(2) receptor knockout mice.

Role of bradykinin in angiotensin-converting enzyme knockout mice

Angiotensin-converting enzyme (ACE) plays a central role in the renin-angiotensin system. Whereas ACE is responsible for the production of angiotensin II, it is also important in the elimination of bradykinin. Constitutively, the biological function of bradykinin is mediated through the bradykinin B(2) receptor. ACE knockout mice have a complicated phenotype including very low blood pressure. To investigate the role of bradykinin in the expression of the ACE knockout phenotype, we bred B(2) receptor knockout mice with ACE knockout mice, thus generating a line of mice deficient in both the B(2) receptor and ACE. Surprisingly, these mice did not differ from ACE knockout mice in blood pressure, urine concentrating ability, renal pathology, and hematocrit. Thus abnormalities of bradykinin accumulation do not play an important role in the ACE knockout phenotype. Rather, this phenotype appears due to the defective production of angiotensin II.

The renal kallikrein-kinin system: its role as a safety valve for excess sodium intake, and its attenuation as a possible etiologic factor in salt-sensitive hypertension

The distal tubules of the kidney express the full set of the components of the kallikrein-kinin system, which works independently from the plasma kallikrein-kinin system. Studies on the role of the renal kallikrein-kinin system, using congenitally kininogen-deficient Brown-Norway Katholiek rats and also bradykinin B2 receptor knockout mice, revealed that this system starts to function and to induce natriuresis and diuresis when sodium accumulates in the body as a result of excess sodium intake or aldosterone release, for example, by angiotensin II. Thus, it can be hypothesized that the system works as a safety valve for sodium accumulation. The large numbers of studies on hypertensive animal models and on essential hypertensive patients, particularly those with salt sensitivity, indicate a tendency toward the reduced excretion of urinary kallikrein, although this reduction is modified by potassium intake and impaired renal function. We hypothesize that the reduced excretion of the renal kallikrein may be attributable to a genetic defect of factor(s) in renal kallikrein secretion process and may cause salt-sensitive hypertension after salt intake.

A practical guide to evaluating cardiovascular, renal, and pulmonary function in mice

The development and widespread use of genetically altered mice to study the role of various proteins in biological control systems have led to a renewed interest in methodologies and approaches for evaluating physiological phenotypes. As a result, cross-disciplinary approaches have become essential for fully realizing the potential of these new and powerful animal models. The combination of classical physiological approaches and modern innovative technology has given rise to an impressive arsenal for evaluating the functional results of genetic manipulation in the mouse. This review attempts to summarize some of the techniques currently being used for measuring cardiovascular, renal, and pulmonary variables in the intact mouse, with specific attention to practical considerations useful for their successful implementation.

Targeted disruption of the bradykinin B(2) receptor gene in mice alters the ontogeny of the renin-angiotensin system

Angiotensin II type 1 (AT(1)) receptor knockout (KO) mice exhibit an activated kallikrein-kinin system (KKS) that serves to attenuate the severity of the renal vascular phenotype in these mice (Tsuchida S, Miyazaki Y, Matsusaka T, Hunley TE, Inagami T, Fogo A, and Ichikawa I, Kidney Int 56: 509-516, 1999). Conversely, gestational high salt suppresses the fetal renin-angiotensin system (RAS) and provokes aberrant renal development in bradykinin B(2)-KO mice (El-Dahr SS, Harrison-Bernard LM, Dipp S, Yosipiv IV, and Meleg-Smith S, Physiol Genomics 3: 121-131, 2000). Thus the cross talk between the RAS and KKS may be critical for normal renal maturation. To further define the developmental interactions between the KKS and RAS, we examined the consequences of B(2) receptor gene ablation on the expression of RAS components. Renal renin mRNA levels are 50% lower in newborn B(2)-KO than wild-type (WT) mice. Also, the age-related decline in renin mRNA is greater in B(2)-KO than WT mice (3.5- vs. 2-fold, P < 0.05). Although renal angiotensinogen (Ao) protein levels are higher in newborn B(2)-KO than WT mice, Ao mRNA levels are not, suggesting accumulation of Ao as a result of decreased renin-mediated cleavage. Similar age-related increases (8-fold) in angiotensin I-converting enzyme (ACE) activity are observed in B(2)-KO and WT mice. Renal AT(1) protein levels are not different in B(2)-KO and WT mice. Furthermore, the developmental increases in renal kallikrein mRNA and enzymatic activity are more pronounced in B(2)-KO compared with WT mice (mRNA: 8- vs. 3-fold; activity: 13- vs. 6-fold, P < 0.05). We conclude that 1) bradykinin stimulates renin gene expression, 2) renal kallikrein is regulated via a negative feedback loop involving the B(2) receptor, and 3) Ao, ACE, and AT(1) are not bradykinin-target genes.

Targeted disruption of the bradykinin B2 receptor gene in mice alters the ontogeny of the renin-angiotensin system

First published July 12, 2001; 10.1152/ajprenal.0020.2001.—Angiotensin II type 1 (AT1) receptor knockout (KO) mice exhibit an activated kallikrein-kinin system (KKS) that serves to attenuate the severity of the renal vascular phenotype in these mice (Tsuchida S, Miyazaki Y, Matsusaka T, Hunley TE, Inagami T, Fogo A, and Ichikawa I, Kidney Int 56: 509–516, 1999). Conversely, gestational high salt suppresses the fetal renin-angiotensin system (RAS) and provokes aberrant renal development in bradykinin B2-KO mice (El-Dahr SS, Harrison-Bernard LM, Dipp S, Yosipiv IV, and Meleg-Smith S, Physiol Genomics 3: 121–131, 2000). Thus the cross talk between the RAS and KKS may be critical for normal renal maturation. To further define the developmental interactions between the KKS and RAS, we examined the consequences of B2 receptor gene ablation on the expression of RAS components. Renal renin mRNA levels are 50% lower in newborn B2-KO than wild-type (WT) mice. Also, the age-related decline in renin mRNA is greater in B2-KO than WT mice (3.5- vs. 2-fold, P < 0.05). Although renal angiotensinogen (Ao) protein levels are higher in newborn B2-KO than WT mice, Ao mRNA levels are not, suggesting accumulation of Ao as a result of decreased renin-mediated cleavage. Similar age-related increases (8-fold) in angiotensin I-converting enzyme (ACE) activity are observed in B2-KO and WT mice. Renal AT1 protein levels are not different in B2-KO and WT mice. Furthermore, the developmental increases in renal kallikrein mRNA and enzymatic activity are more pronounced in B2-KO compared with WT mice (mRNA: 8- vs. 3-fold; activity: 13- vs. 6-fold, P < 0.05). We conclude that 1) bradykinin stimulates renin gene expression, 2) renal kallikrein is regulated via a negative feedback loop involving the B2 receptor, and 3) Ao, ACE, and AT1 are not bradykinin-target genes.

Importance of quantitative genetic variations in the etiology of hypertension

Recent progress has been remarkable in identifying mutations which cause diseases (mostly uncommon) that are inherited simply. Unfortunately, the common diseases of humankind with a strong genetic component, such as those affecting cardiovascular function, have proved less tractable. Their etiology is complex with substantial environmental components and strong indications that multiple genes are implicated. In this article, we consider the genetic etiology of essential hypertension. After presenting the distribution of blood pressures in the population, we propose the hypothesis that essential hypertension is the consequence of different combinations of genetic variations that are individually of little consequence. The candidate gene approach to finding relevant genes is exemplified by studies that identified potentially causative variations associated with quantitative differences in the expression of the angiotensinogen gene (AGT). Experiments to test causation directly are possible in mice, and we describe their use to establish that blood pressures are indeed altered by genetic changes in AGT expression. Tests of differences in expression of the genes coding for the angiotensin-converting enzyme (ACE) and for the natriuretic peptide receptor A are also considered, and we provide a tabulation of all comparable experiments in mice. Computer simulations are presented that resolve the paradoxical finding that while ACE inhibitors are effective, genetic variations in the expression of the ACE gene do not affect blood pressure. We emphasize the usefulness of studying animals heterozygous for an inactivating mutation and a wild-type allele, and briefly discuss a way of establishing causative links between complex phenotypes and single nucleotide polymorphisms.

Bradykinin B2 null mice are prone to renal dysplasia: gene-environment interactions in kidney development

Congenital abnormalities of the kidney and urinary tract are a common cause of end-stage renal disease in children. Host and environment factors are implicated in the pathogenesis of aberrant renal development. However, direct evidence linking gene-environment interactions with congenital renal disease is lacking. We report an animal model of renal dysgenesis that is dependent on a defined genetic defect and specific embryonic stressor. Specifically, mice that are deficient in the bradykinin type 2 receptor gene (B(2)) and salt loaded during embryogenesis acquire an aberrant kidney phenotype and die shortly after birth. In contrast, B(2) mutant mice maintained on normal sodium intake or salt-loaded wild-type mice do not develop kidney abnormalities. The kidney abnormality is evident histologically on embryonic day 16, shortly after the onset of metanephric B(2) gene expression, and consists of distorted renal architecture, foci of tubular dysgenesis, and cyst formation. The dysplastic tubules are of distal nephron origin [Dolichos biflorus agglutinin (DBA)- and aquaporin-2 (AQP2) positive, and angiotensinogen negative]. Neonatal antihypertensive therapy fails to ameliorate the renal abnormalities, arguing against the possibility that the nephropathy is a consequence of early hypertension. Moreover, the nephropathy is intrinsic to the embryo, because B(2) homozygous offspring from heterozygous parents exhibit the same renal phenotype as offspring from homozygous null parents. Further characterization of the renal phenotype revealed an important genetic background effect since the penetrance of the congenital nephropathy is increased substantially upon backcrossing of 129/BL6 B(2) mutants to a uniform C57BL/6J. We conclude that the type 2 bradykinin receptor is required for the maintenance of metanephric structure and epithelial integrity in the presence of fetal stress. This study provides a "proof-of-principle" that defined gene-environment interactions are a cause of congenital renal disease.

The bradykinin type 2 receptor is a target for p53-mediated transcriptional activation

The bradykinin type 2 receptor (BK2) is a developmentally regulated G protein-coupled receptor that mediates diverse actions such as vascular reactivity, salt and water excretion, inflammatory responses, and cell growth. However, little is known regarding regulation of the BK2 gene. We report here that the rat BK2 receptor is transcriptionally regulated by the tumor suppressor protein p53. The 5'-flanking region of the rat BK2 gene contains two p53-like binding sites: a sequence at -70 base pairs (P1 site) that is conserved in the murine and human BK2 genes; and a sequence at -707 (P2) that is not. The P1 and P2 motifs bind specifically to p53, as assessed by gel mobility shift assays. Transient transfection into HeLa cells of a CAT reporter construct driven by 1.2-kilobases of the BK2 gene 5'-flanking region demonstrated that the BK2 promoter is dose dependently activated by co-expression of wild-type p53. Co-expression of a dominant negative mutant p53 suppresses the activation of BK2 by wild-type p53. Promoter truncation localized the p53-responsive element to the region between -38 and -94 base pairs encompassing the p53-binding P1 sequence. Furthermore, p53-mediated activation of the BK2 promoter is augmented by the transcriptional co-activators, CBP/p300. Interestingly, removal of the P2 site by truncation or site-directed deletion amplifies p53-mediated activation of the BK2 promoter. These results demonstrate that the rat BK2 promoter is a target for p53-mediated activation and suggest a new physiological role for p53 in the regulation of G protein-coupled receptor gene expression.