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.

Activation of angiotensin-generating systems in the developing rat kidney.

The present study was designed to determine the developmental changes in intrarenal angiotensin (Ang) peptides in the rat. Kidney Ang I and II levels were threefold and sixfold higher in newborn than adult kidneys, respectively (Ang I, 678 +/- 180 versus 243 +/- 38 fmol/g, P < .01; Ang II, 667 +/- 75 versus 103 +/- 6 fmol/g, P < .001). Intrarenal Ang II levels correlated positively with the temporal changes in renin gene expression (r = .93, P < .001). However, no correlation was found between renal Ang II content and angiotensin-converting enzyme (ACE) expression during development, which prompted us to evaluate whether renal enzymes, other than renin and ACE, contribute to Ang II formation in the developing kidney. Angiotensin peptide levels were measured in newborn and adult kidney homogenates incubated with human angiotensinogen (a poor rat renin substrate) for 30 minutes at 37 degrees C. Inhibitors of aspartyl proteases and metalloproteases were ineffective in preventing the formation of Ang II in either newborn or adult kidneys. However, addition of the serine protease inhibitors soybean trypsin inhibitor and phenylmethylsulfonyl fluoride inhibited Ang II generation in the newborn kidneys only. In contrast, Ang I generation was not affected by inhibition of serine proteases in either newborn or adult kidneys. We conclude that Ang I and II synthesis is activated in the developing rat kidney. In addition to renin and ACE, the newborn rat kidney expresses serine protease activity that is capable of generating Ang II directly from angiotensinogen. This putative enzyme is induced in the newborn kidney and may cooperate with renin in the activation of Ang II synthesis during early development.

Ontogeny of somatic angiotensin-converting enzyme.

Angiotensin-converting enzyme or kininase II (ACE-KII) plays a central role in the control of circulating and tissue levels of angiotensin II and kinins. Both peptides have been implicated in the regulation of renal function and growth during normal development. We tested the hypothesis that the developing rat kidney expresses ACE-KII mRNA transcripts and the active enzyme and evaluated whether the developmental expression of the ACE-KII gene is related to changes in circulating angiotensin II and tissue kallikrein. ACE-KII mRNA and enzymatic activity were low in the newborn kidney; peak expression occurred on days 15 and 20 of postnatal life (16-fold versus day 1). In extrarenal tissues, ACE-KII activity and mRNA levels were also low during the newborn period in the following order of abundance: lung > kidney > aorta > heart. The lung showed a higher age-related increase in active ACE-KII and mRNA abundance (15-fold) than heart and aorta (activity, 3- to 4-fold; mRNA, 6- to 10-fold). The developmental profile of ACE-KII correlated temporally with changes in circulating angiotensin II and tissue kallikrein. Plasma angiotensin II levels were 2.5-fold higher in newborn than adult rats, whereas renal and extrarenal kallikrein-like activity increased twofold to fivefold from birth to adulthood. These results demonstrate that the ACE-KII gene is developmentally regulated in a tissue-specific manner. Tissue kinin generation and degradation, reflected by kallikrein and ACE-KII activities, are coordinately regulated during development, whereas circulating angiotensin II and tissue ACE-KII change in a reciprocal manner.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of bradykinin B2 receptors in neonatal kidney growth.

The kallikrein-kinin system is developmentally expressed in newborn kidneys. In addition, bradykinin (BK) is mitogenic in cultured glomerular mesangial cells. However, the role of endogenous BK in postnatal renal development has not been defined. In this study, the role of the BK-B2 receptor in neonatal kidney growth in the rat was examined. RNA blot analysis and semiquantitative reverse transcription-polymerase chain reaction showed that BK-B2 mRNA levels were approximately 30- to 40-fold higher in newborn than adult kidneys. Treatment of newborn rats with the selective BK-B2 antagonist, Hoe 140 (600 micrograms/kg per day, sc), from days 1 through 14 of life significantly reduced body weight, kidney-to-body weight ratios, and kidney DNA content, compared with saline-treated controls. Hoe 140 treatment had no effect on kidney protein or RNA content or the expression of transforming growth factor-beta mRNA. The growth retardation induced by BK-B2 blockade was observed only in the kidney and, to a lesser extent, in the heart. BK-B2 blockade had no effect on renal growth in adult rats, suggesting that these effects are developmentally regulated. In contrast to Hoe 140 treatment, neonatal protein undernutrition resulted in a generalized reduction in kidney DNA, RNA, and protein contents; increased renal transforming growth factor-beta gene expression; and decreased renal kallikrein expression and enzymatic activity. The results suggest that activation of BK-B2 receptor expression in the neonatal kidney plays an important role in the regulation of DNA synthesis during the latter stages of nephrogenesis.

Developmental biology of angiotensin-converting enzyme.

Molecular, biochemical, and evolutionary studies indicate that somatic angiotensin-converting enzyme (ACE) is developmentally regulated in a tissue-specific manner. However, many important questions remain unanswered. For example, the regulatory mechanisms that control the cell- and stage-specific expression of ACE remain largely unknown. The nature, location, and role of the enzyme involved in the release of plasma membrane-anchored ACE have not been elucidated. Although the expression and localization of ACE are developmentally regulated, the physiological implications of these changes in segmental nephron differentiation remain to be elucidated. Investigations of the cellular and molecular mechanisms mediating the developmental co-regulation of the renin-angiotensin and Kallikrein-kinin system by ACE are a formidable challenge for future research.

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.

Developmental regulation of ACE gene expression by endogenous kinins and angiotensin II.

Angiotensin converting enzyme (ACE, i.e., kininase II), a key regulator of kinins and angiotensin II (ANG II) generation, is developmentally regulated and its expression is induced at a specific time point (day 15) of postnatal kidney development. The present study tested the hypothesis that endogenous kinins and ANG II regulate the developmental expression of the renal ACE gene. In the first protocol, newborn rats received the kallikrein inhibitor, aprotinin (100,000 KIU.kg-1.day-1 sc), or the kinin B2 receptor antagonist, HOE-140 (600 micrograms.kg-1.day-1 sc), or 0.9% saline, from birth until postnatal days 5, 15, or 20. Aprotinin prevented the postnatal rise in renal kallikrein activity without affecting blood pressure in either developing or adult rats. Chronic kallikrein blockade significantly attenuated the postnatal induction of both serum ACE activity (-11% vs. controls) and kidney ACE activity and mRNA (-50% vs. controls). In addition, aprotinin attenuated the postnatal rise of ACE activity in the developing lungs. Kidney renin mRNA and ANG II contents were not altered by aprotinin. HOE-140 also attenuated the postnatal rise in kidney ACE mRNA (-25%) and activity (-40%) without affecting blood pressure. Infusion of aprotinin or HOE-140 via osmotic minipumps for 7 days in adult rats was not associated with any changes in renal or pulmonary ACE.(ABSTRACT TRUNCATED AT 250 WORDS)

Salt intake modulates the developmental expression of renal kallikrein and bradykinin B2 receptors.

The mechanisms involved in the postnatal induction of renal kallikrein gene transcription and enzymatic activity are unknown. The present study was designed to test the hypothesis that salt (NaCl) intake regulates the ontogeny of renal kallikrein gene expression and enzymatic activity and urinary kallikrein excretion. Newborn rats were artificially fed via a gastric tube with a milk formula containing either normal (25 meq/l, same as in maternal milk) or high (145 meq/l) NaCl content from day 7 to 14 of postnatal life. High-salt feeding decreased renal kallikrein mRNA levels (P < 0.05) and kallikrein-like activity (P < 0.05) compared with rat pups on normal salt intake. However, urinary kallikrein excretion (Ukal) was not different on chronic high vs. normal salt intake. Furthermore, acute volume expansion (0.9% saline, 1% body wt iv) did not alter Ukal in either group of developing rats. In adult rats, 1% NaCl in the drinking water for 10 days decreased renal active kallikrein contents (P < 0.05) but did not alter kallikrein mRNA levels compared with pair-fed rats on normal salt diet. Acute volume expansion in adult rats decreased active Ukal in the high-salt group only (P < 0.05). High-salt feeding upregulated bradykinin B2 receptor mRNA in the developing rats (P < 0.05). We conclude that chronic salt loading suppresses the postnatal rise in renal kallikrein gene expression and enzymatic activity, indicating that sodium intake is an important factor in the maturation of renal kallikrein synthesis. The data also suggest that bradykinin B2 receptor gene expression in the developing kidney may be subject to reciprocal feedback regulation by endogenous kallikrein-kinin activity.

Role of bradykinin B2 receptors in the developmental changes of renal hemodynamics in the neonatal rat.

The present study was performed to evaluate the role of bradykinin, acting via B2 receptors, in the developmental rise in renal blood flow (RBF) and glomerular filtration rate (GFR) in the rat. Newborn rats were chronically treated from birth with the kinin B2 receptor antagonist HOE-140 (600 micrograms/kg sc, every 12 h, n = 9) or 0.9% saline (n = 7). Weanling rats (mean age 23 days) were anesthetized with pentobarbital sodium (50 mg/kg ip) for measurements of mean arterial pressure (MAP), GFR, and renal plasma flow estimated from p-aminohippurate (PAH) clearance (ERPF). Outer cortical RBF (OCBF) was measured by laser-Doppler flowmetry. Baseline MAP was similar in HOE-140- and saline-treated rats (96 +/- 4 vs. 97 +/- 4 mmHg). Also, baseline GFR (0.65 +/- 0.05 vs. 0.52 +/- 0.08 ml.min-1.g-1) and ERPF (1.6 +/- 0.2 vs. 1.3 +/- 0.1 ml.min-1.g-1) were not different in HOE-140- and saline-treated rats, respectively. Intravenous infusion of 200 ng bradykinin did not change MAP or OCBF in HOE-140 rats but decreased MAP (-29 +/- 3%, P < 0.05) and OCBF (-20 +/- 2%, P < 0.05) in controls. Intravenous infusion of 25 ng angiotensin II increased MAP equally in both groups (-32 +/- 4%) and caused a similar reduction in OCBF (-37 +/- 14 vs. -46 +/- 5%). The angiotensin type 1 (AT1) receptor antagonist losartan (10 mg/kg iv) decreased MAP equally in both groups (-22 +/- 2%). However, AT1 blockade increased ERPF to 3.1 +/- 0.8 ml.min-1.g-1 (P < 0.05 vs. baseline) in saline but not in HOE-140 rats (1.9 +/- 0.4 ml.min-1.g-1). Kidney renin mRNA and angiotensin II contents were not different in HOE-140 vs. saline groups. The present findings indicate that bradykinin is not a primary mediator of the maturational rise in RBF or GFR in the rat. However, the data suggest that under control conditions, angiotensin II, acting via AT1 receptors, counteracts the renal vasodilatory effects of endogenous bradykinin in the developing kidney.

Bradykinin stimulates c-fos expression, AP-1-DNA binding activity and proliferation of rat glomerular mesangial cells.

An important role for bradykinin (BK) in nephrogenesis has been suggested based on impairment of renal growth in developing rats treated with a kinin antagonist. However, direct effects of BK on renal cell mitogenesis have not been reported. In the present study, we examined the mitogenic effects of BK on cultured rat mesangial cells. Transcripts encoding BK-B2 receptors were detected in quiescent and proliferating mesangial cells by reverse transcription-coupled polymerase chain reaction. In quiescent mesangial cell cultures (0.5% FCS for 48 hr), BK (10(-9) to 10 (-7)M) caused a significant increase in DNA synthesis (3H-thymidine incorporation into DNA) and cell number. BK-induced DNA synthesis was preceded by activation of c-fos gene expression and both of these effects were inhibited by Hoe-140, a specific BK-B2 antagonist. Electrophoretic gel mobility shift assays revealed enhanced binding of AP-1 complexes to a consensus AP-1 DNA sequence in BK-stimulated cells. Gel supershift assays confirmed that the AP-1 complexes contained the fos protein. These data document a direct mitogenic effect of BK, acting on B2 receptors, on mesangial cells.

Activation of kininogen expression during distal nephron differentiation.

Previous studies have shown that the epithelial precursors of the connecting tubule and collecting duct express tissue kallikrein and bradykinin B2 receptors, respectively, suggesting the presence of a local kinin-producing/responsive system in the maturing distal nephron. However, evidence for the existence of kininogen in the developing nephron is still lacking. This study examined the spatiotemporal relationships between segmental nephron differentiation and the ontogeny of kininogen and kinins in the rat. Kininogen immunoreactivity is detectable in the metanephros as early as embryonic day 15. In the nephrogenic zone, the terminal ureteric bud branches are the main kinin-expressing segments. Kininogen is also observed in the stromal mesenchyme. In contrast, proximal ureteric bud branches, metanephrogenic mesenchyme, and pretubular aggregates express little or no kininogen. After completion of nephrogenesis, kininogen distribution assumes its classic "adult" pattern in the collecting ducts. Peak kininogen mRNA and protein expression occur perinatally, corresponding to the period of active nephrogenesis in the rat, and declines gradually thereafter. Estimations made by RT-PCR, Western blotting, and radioimmunoassays indicate that renal kininogen mRNA and protein levels are at least 20-fold higher in newborn than adult rats. Likewise, immunoreactive tissue kinin levels are 2.3-fold higher in newborn than adult kidneys (P < 0.05). In summary, the present study demonstrates the activation of kininogen gene expression and kinin production in the developing kidney. The terminal ureteric bud branches and their epithelial derivatives are the principal kinin-producing segments in the maturing nephron. The results suggest an autocrine/paracrine role for the kallikrein-kinin system in distal nephron maturation.