Role of the angiotensin receptor in the development of the mammalian kidney and urinary tract

The Impact of Functional and Structural Maturation of the Kidney on Susceptibility to Drug and Chemical Toxicity in Neonatal Rodents

Drug responses are often unpredictable in juvenile animal toxicity studies; hence, optimizing dosages is challenging. Renal functional differences based on age of development will often result in vastly different toxicologic responses. Developmental changes in renal function can alter plasma clearance of compounds with extensive renal elimination. Absorption, distribution, metabolism, and excretion of drugs vary depending on animal age and kidney maturation. Toxicity can result in malformations or renal degeneration. Although renal morphologic development in humans generally occurs in utero, maximal levels of tubular secretion, acid-base equilibrium, concentrating ability, or glomerular filtration rate (GFR) are reached postnatally in humans and animals and subject to drug effects. Maturation of renal metabolism and transporters occurs postnatally and plays a critical role in detoxification and excretion. Maturation times must be considered when designing juvenile toxicity studies and may require cohorts of animals of specific ages to achieve optimal dosing schemes and toxicokinetics. In recent years, critical end points and windows of susceptibility have been established comparatively between species to better model pharmacokinetics and understand pediatric nephrotoxicity. There are examples of agents where toxicity is enhanced in neonates, others where it is diminished, and others where rat nephrotoxicity is expressed as juvenile toxicity, but in humans as gestational toxicity.

Severe toxicity and cyclooxygenase (COX)-2 mRNA increase by lithium in the neonatal mouse kidney

Functions of the kidney of mammals are immature during the neonatal period, and the neonatal kidney could be susceptible to chemicals, including drugs and environmental toxicants. Among these chemicals, cyclooxygenase (COX)-inducing chemicals should be given attentions as the potential kidney toxicants during the period, and we hypothesized that lithium chloride (LiCl) has such toxicity. Neonatal mice of C57BL/J strain were intraperitoneally injected with LiCl (2 mmol/kg body weight) daily until 21 days of age, and examined on 7 days and 21 days of age. Neonatal treatment of LiCl caused a significant increase in COX-2 mRNA and a decrease in mRNAs of aquaporins on day 7 of age. Osmolarity of urine from LiCl-treated neonates was significantly lower than that of control neonate. Most of the LiCl-treated neonates died during the second week of age. Histological examination revealed renal cysts on day 7 and hydronephrosis on day 21. in the surviving neonates. The present results showed that the kidney of mouse neonates is vulnerable to lithium, and suggested the possibility that COX-2 upregulation is responsible for the severe renal toxicity including hydronephrosis.

Magnetic resonance imaging of urea transporter knockout mice shows renal pelvic abnormalities

Many transgenic and knockout mice with increased urine flow have structural abnormalities of the renal pelvis and inner medulla. Here, we used high resolution contrast enhanced T1-weighted magnetic resonance imaging of mice whose urea transporters UT-A1 and UT-A3 were deleted (UT-A1/3(-/-) mice) as a model for the in vivo study of such abnormalities. Three distinct variations in the appearance of the renal pelvis were found. These included normal kidneys with no accumulation of contrast agent in the renal pelvis; infrequent frank right-sided unilateral hydronephrosis with marked atrophy of the renal medulla; and a renal pelvic reflux pattern characterized by the presence of contrast agent in the renal pelvis surrounding the renal inner medulla but no substantial atrophy of the medulla. This last pattern was found in most of the advanced age UT-A1/3(-/-) mice and in aquaporin-1 knockout mice. The UT-A1/3(-/-) mice also had increased mean arterial blood pressures. Feeding the mice a low protein diet did not prevent development of their renal pelvic abnormalities. Our studies show that real time imaging of renal pelvic structure in genetically manipulated mice provides a tool for the non-destructive, temporal evaluation of kidney structure.

Localization of genetic loci controlling hydronephrosis in the Brown Norway rat and its association with hematuria

The aim of this study was to investigate the genetic basis of congenital hydronephrosis (HN), a poorly defined pathological entity, with a rat model. The Brown Norway (BN) strain spontaneously presents a high incidence of apparently asymptomatic HN, whereas the LOU strain does not. A backcross was established between these two strains [BN x (BN x LOU)] and a genomewide scan was performed with 193 microsatellite markers on 121 males and 118 females of this population, which had been phenotyped and scored for HN severity (defined as degree of renal pelvic dilation), followed by linkage analysis with Mapmaker/QTL software. Bilateral HN score was significantly linked to a locus on chromosome 6 (Z scores 4.4 and 4.8 for all rats and for females, respectively). Suggestive loci were identified on chromosomes 2 (for only right-sided HN) and 4. This is the first study in rats to identify genetic loci for HN. Three candidate genes present in these loci were sequenced and insertions detected in Id2 and Agtr1b genes in BN, which did not, however, lead to modified expression as measured by quantitative PCR. Production of a congenic line for part of the chromosome 6 locus confirmed its involvement in HN, but the phenotype was mild. Evidence of hematuria was observed in 9.6% of the backcross rats, mostly males and only in kidneys with HN, but not necessarily in the most severely affected. Hematuria also occurs in the BN colony used here, where it is due to papilloma-like lesions involving pelvic epithelial proliferation, but not in the LOU rat.

Cardiovascular phenotype of mice lacking all three subtypes of angiotensin II receptors

Angiotensin II activates two distinct receptors, the angiotensin II receptors type 1 (AT(1)) and type 2 (AT(2)). In rodents, two AT(1) subtypes were identified (AT(1a) and AT(1b)). To determine receptor-specific functions and possible angiotensin II effects independent of its three known receptors we generated mice deficient in either one of the angiotensin II receptors, in two, or in all three (triple knockouts). Triple knockouts were vital and fertile, but survival was impaired. Hypotension and renal histological abnormalities in triple knockouts were comparable to those in mice lacking both AT(1) subtypes. All combinations lacking AT(1a) were distinguished by reduced heart rate. AT(1a) deletion impaired the in vivo pressor response to angiotensin II bolus injection, whereas deficiency for AT(1b) and/or AT(2) had no effect. However, the additional lack of AT(1b) in AT(1a)-deficient mice further impaired the vasoconstrictive capacity of angiotensin II. Although general vasoconstrictor properties were not changed, angiotensin II failed to alter blood pressure in triple knockouts, indicating that there are no other receptors involved in direct angiotensin II pressor effects. Our data identify mice deficient in all three angiotensin II receptors as an ideal tool to better understand the structure and function of the renin-angiotensin system and to search for angiotensin II effects independent of AT(1) and AT(2).

A genome search for primary vesicoureteral reflux shows further evidence for genetic heterogeneity

Vesicoureteral reflux (VUR) is the most common disease of the urinary tract in children. In order to identify gene(s) involved in this complex disorder, we performed a genome-wide search in a selected sample of 31 patients with primary VUR from eight families originating from southern Italy. Sixteen additional families with 41 patients were included in a second stage. Nonparametric, affected-only linkage analysis identified four genomic areas on chromosomes 1, 3, and 4 (p < 0.05); the best result corresponded to the D3S3681-D3S1569 interval on chromosome 3 (nonparametric linkage score, NPL = 2.75, p = 0.008). This region was then saturated with 26 additional markers, tested in the complete group of 72 patients from 24 families (NPL = 2.01, p = 0.01). We identified a genomic area on 3q22.2-23, where 26 patients from six multiplex families shared overlapping haplotypes. However, we did not find evidence for a common ancestral haplotype. The region on chromosome 1 was delimited to 1p36.2-34.3 (D1S228-D1S255, max. NPL = 1.70, p = 0.03), after additional fine typing. Furthermore, on chromosome 22q11.22-12.3, patients from a single family showed excess allele sharing (NPL = 3.35, p = 0.015). Only the chromosome 3q region has been previously reported in the single genome-wide screening available for primary VUR. Our results suggest the presence of several novel loci for primary VUR, giving further evidence for the genetic heterogeneity of this disorder.

Localization of cytochrome P450 1A1 in a specific region of hydronephrotic kidney of rat neonates lactationally exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin

Hydronephrosis is typically observed in terata caused by in utero and lactational exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), via the arylhydrocarbon receptor, but the molecular mechanism underlying its pathogenesis is largely unknown. In the present study, pregnant Holtzman rats were treated once by gavage with TCDD (1.0 microg/kg bw) or corn oil on gestation day 15. All dams were allowed to litter, and standardized litters in terms of litter size were then reciprocally cross-fostered on postnatal day (PND) 1. On PND1, pups were divided into four experimental groups: pups exposed only in utero, pups exposed only lactationally, pups not exposed via either route (vehicle control), and pups exposed via both routes. Pups were euthanized on PND21 for further analyses. The TCDD dose used was not overtly toxic to the dams or neonates. The incidence and severity of hydronephrosis were markedly high in pups exposed to TCDD lactationally, but not those exposed in utero. On PND21, cytochrome P450 (CYP) 1A1 was detected predominantly in the outer zone of the medulla of the kidney from all the pups lactationally exposed to TCDD, regardless of the occurrence of hydronephrosis. Interestingly, TCDD concentrations in the cortex, the outer zone of the medulla and the inner zone of the medulla were similar. When adult Holtzman rats were administered TCDD, the induction of CYP1A1 was immunohistochemically detected in the liver but not in the kidney 7 days postadministration. The present findings suggest that TCDD-inducible genes via an AhR-dependent mechanism may be associated with the etiology of hydronephrosis in a particular region of the kidney.

Angiotensin-converting enzyme and angiotensin type 2 receptor gene genotype distributions in Italian children with congenital uropathies

Angiotensin I-converting enzyme (ACE) and angiotensin type 2 receptor (AT2R) gene polymorphisms have been associated with an increased incidence of congenital anomalies of the kidney and urinary tract (CAKUT). We investigated the genotype distribution of these polymorphisms in Italian children with CAKUT. We also evaluated the association between the ACE insertion/deletion and the AT2R gene polymorphisms with the progression of renal damage in subgroups of CAKUT patients. We recruited 102 Italian children with CAKUT; 27 with vesicoureteral reflux; 12 with hypoplastic kidneys; 20 with multicystic dysplastic kidneys; 13 with ureteropelvic junctions stenosis/atresia; 18 with nonobstructed, nonrefluxing primary megaureters; and 12 with posterior urethral valves and compared them with 92 healthy control subjects. ACE and AT2R gene polymorphisms were analyzed by PCR. The identification of AT2R gene polymorphisms in intron 1 and in exon 3 was revealed by enzymatic digestion. ACE genotype distribution in children with CAKUT was no different from that of the control subjects, but the subgroup of patients with radiographic renal parenchymal abnormalities showed an increased occurrence of the D/D genotype. The frequency of the G allele of AT2R gene in children with CAKUT was increased in respect to that of the control subjects. By contrast, no significant difference in the frequency of the C and A alleles of the AT2R gene was found. Our findings indicate that the ACE gene can be a risk factor in the progression of renal parenchymal damage in CAKUT patients. Moreover, a major role of the AT2R gene in the development of CAKUT has been found, at least in Italian children.

Neonatal RAS inhibition changes the phenotype of the developing thick ascending limb of Henle

Pharmacological interruption of angiotensin II type 1 (AT(1)) receptor signaling during nephrogenesis in rats perturbs renal tubular development. Perturbed tubulogenesis may contribute to long-term impairment of urinary concentrating ability, which is the main functional irreversible defect. The aim of this study was to further characterize tubular developmental deficits in neonatal rats, focusing on the thick ascending limb of Henle (TALH), known to undergo profound developmental changes and to be involved in urine-concentrating mechanisms. We have carried out immunohistochemistry and Western immunoblotting using antibodies directed against the major histocompatibility complex class II (MHC II) molecule and different TALH-specific markers, namely, cyclooxygenase-2 (COX-2), Tamm-Horsfall glycoprotein (THP), and the bumetanide-sensitive Na(+)-K(+)-2Cl(-) cotransporter (BSC-1/NKCC2). Immunohistochemistry demonstrated expression of MHC II, COX-2, THP, and BSC-1/NKCC2 proteins in normally developing TALH cells. The AT(1)-receptor antagonist losartan abolished MHC II expression exclusively in the developing TALH cells. Increased expression of COX-2 and THP was observed in the TALH cells of losartan-treated rats. Western immunoblotting confirmed increases in cortical and medullary COX-2 and THP abundance and revealed a decrease in cortical BSC-1/NKCC2 abundance in response to losartan treatment. We conclude that neonatal losartan treatment causes significant changes in the phenotype of the developing TALH in the rat.

Genetic disorders of human congenital anomalies of the kidney and urinary tract (CAKUT)

Congenital abnormalities of the kidney and urinary tract, CAKUT are common in humans, occurring at a frequency of approximately 1 in 500 fetal ultrasound examinations. CAKUT are major causes of chronic renal failure in infants and young children, but little is known about the molecular pathogenesis of these disorders. To date, several gene mutations have been identified as a cause of human CAKUT: these include PAX2, KAL, EYA1, AGTR2 and HNF-1beta. At present, there is only limited information regarding how mutations alter gene expression during development to cause some CAKUT. The most convincing information comes from the multiorgan malformation syndromes with specific gene mutations. However, these syndromes are relatively rare, and most CAKUT appear to occur in isolation. The goal of this review is to provide an overview of these genetic disorders for CAKUT. An understanding of the genetic aspects of human CAKUT will help to unravel the pathogenesis of these disorders and may facilitate the design of genetic screening tests for early diagnosis and appropriate genetic counseling. Moreover, a deeper insight into the relationship between abnormal genes and the pathogenesis of abnormalities of CAKUT will provide an etiological classification of CAKUT. In addition, the importance of developing a registry of patients with various forms of CAKUT is discussed. This information will allow us to combine molecular biology and classical epidemiologic methods, and to continue expanding our knowledge regarding CAKUT.

Maturation-dependent changes of angiotensin receptor expression in fowl

An angiotensin (ANG) receptor homologous to the type 1 receptor (AT1) has been cloned in chickens (cAT1). We investigated whether cAT1 expression in various tissues shows maturation/age-dependent changes. cAT1 mRNA levels detected in renal glomeruli [in situ hybridization (ISH)] and kidney extract (RT-PCR) are significantly (P < 0.01) higher in 19-day embryos (EB) than in chicks (CH, 2-3 wk) and pullets/cockerels (PL/CK, 14-16 wk). The levels in adrenal glands (concentrated in subcapsular regions) are high in EB and further increased in CH and PL/CK. cAT1 mRNA is also detectable in smooth muscle (SM)/adventitia of EB and CH aorta and in the adventitia, but not SM, from PL/CK aortas. The endothelia from small arteries and arterioles, but not from aorta, express cAT1 mRNA (ISH). In all age groups, ANG II induces profound endothelium-dependent relaxation of abdominal aorta, partly (37-47%) inhibitable (P < 0.01) by Nomega-nitro-l-arginine methyl ester (l-NAME, 10(-4) M), suggesting the presence of ANG receptor in endothelium. l-NAME-resistant ANG II relaxation, examined in a limited number of EB or CH aortas, was reduced by 125 mM K+ or apamin plus charybdotoxin. The results suggest that 1) cAT1 is present in kidney, adrenal gland, and vascular endothelium (heterogeneity exists among arteries) of EB, CH, and PL/CK, and in aortic SM/adventitia of EB/CH but only in adventitia of PL/CK; 2) levels of cAT1 gene expression change during maturation in a tissue-specific manner; and 3) ANG II-induced relaxation may be partly attributable to nitric oxide and potassium channel activation.

Angiotensin II type 2 receptor gene is not responsible for familial vesicoureteral reflux

PURPOSE

The renin-angiotensin system has an important role in the development of the kidney and ureter. It has been reported that disruption of the angiotensin II type 2 receptor (AT2) gene leads to congenital anomalies of the kidney and ureter in mice, including vesicoureteral reflux. In humans a single base A to G transition at position -1332 in intron 1 (A-1332G) of the AT2 gene has been reported to occur significantly more often in patients with ureteropelvic junction obstruction and primary obstructive megaureter than in controls. We investigate the incidence of A-1332G transition in patients with primary familial vesicoureteral reflux to determine if AT2 gene is involved in pathogenesis of this disease.

MATERIALS AND METHODS

We evaluated the incidence of A-1332G transition in 82 male and 110 female patients, 111 male and 124 female nonaffected family members from 88 families in which 2 or more members had primary vesicoureteral reflux, and 106 male and 107 female controls with no unselected for reflux status. Genomic DNA was extracted from whole blood samples. Polymerase chain reaction method modified for fluorescent detection was used to type all samples for the A-1332G variant. Furthermore, to identify mutations in the coding sequence of the AT2 gene, we selected 61 patients from different families as well as 15 controls with no vesicoureteral reflux status.

RESULTS

The incidence of A-1332G transition in male patients with primary familial vesicoureteral reflux and controls was 33% (27 of 82 patients) and 38% (41 of 106 controls), respectively, and, the incidence of A-1332G substituted allele in female patients and controls was 47% (104 of 220 total alleles) and 50% (107 of 214 total alleles), respectively. Moreover, the transmission/disequilibrium test revealed no significant skewing of genotype transmission from mother to children. None of the 61 patients or 15 controls carried by mutations or polymorphisms in the coding sequence of the AT2 gene.

CONCLUSIONS

Although the AT2 gene has been reported to have a role in developmental anomalies of the kidney and ureter, our data indicate that it is not involved in the pathogenesis of primary familial vesicoureteral reflux.