Hyposthenuria Research Articles

Prostanoids in health and disease

The prostanoids are a family of lipid mediators generated by the action of cyclooxygenase on a 20-carbon unsaturated fatty acid, arachidonic acid. Prostanoids are generated widely in response to diverse stimuli and, acting in a paracrine or autocrine manner, play important roles in normal physiology and disease. This review summarizes the current knowledge on prostanoid generation and the roles of individual mediators, their biosynthetic pathways, and their receptors in health and disease.

Altered Renal Expression of Aquaporin Water Channels and Sodium Transporters in Rats with Two-Kidney, One-Clip Hypertension

Aims: To determine whether the renal regulation of aquaporin (AQP) water channels and sodium transporters are altered in 2-kidney, 1-clip (2K1C) hypertension. Methods: Male Sprague-Dawley rats were used. They were made 2K1C hypertensive for 1 week. The renal expression of AQPs and sodium transporters was determined by semiquantitative immunoblotting and immunohistochemistry. The activity of adenylyl cyclase was measured by stimulated generation of cAMP. Results: Systolic blood pressure was increased in 2K1C rats. Experimental rats revealed impaired urinary concentration in association with increased urine volume. Urinary sodium excretion also increased. The expression of AQP1–3 was decreased in the clipped kidney compared with the control kidney, whereas it was unchanged in the non-clipped kidney. The adenylyl cyclase activity provoked by arginine vasopressin, sodium fluoride or forskolin was blunted in the clipped kidney, but remained unaltered in the contralateral kidney. The expressions of the Na,K-ATPase α1-subunit, type 3 Na⁺/H⁺ exchanger, Na-K-2Cl cotransporter and epithelial sodium channels were decreased in the clipped kidney, while remaining unchanged in the non-clipped kidney. Conclusion: The downregulation of AQPs and major sodium transporters/channels in the clipped kidney may play a role in the urinary concentration defect and impaired sodium reabsorption in 2K1C hypertension.

Jouberin localizes to collecting ducts and interacts with nephrocystin-1

Joubert syndrome and related disorders are autosomal recessive multisystem diseases characterized by cerebellar vermis aplasia/hypoplasia, retinal degeneration and cystic kidney disease. There are five known genes; mutations of which give rise to a spectrum of renal cystic diseases the most common of which is nephronophthisis, a disorder characterized by early loss of urinary concentrating ability, renal fibrosis, corticomedullary cyst formation and renal failure. Many of the proteins encoded by these genes interact with one another and are located at adherens junctions or the primary cilia and or basal bodies. Here we characterize Jouberin, a multi-domain protein encoded by the AHI1 gene. Immunohistochemistry with a novel antibody showed that endogenous Jouberin is expressed in brain, kidney and HEK293 cells. In the kidney, Jouberin co-localized with aquaporin-2 in the collecting ducts. We show that Jouberin interacts with nephrocystin-1 as determined by yeast-2-hybrid system and this was confirmed by exogenous and endogenous co-immunoprecipitation in HEK293 cells. Jouberin is expressed at cell-cell junctions, primary cilia and basal body of mIMCD3 cells while a Jouberin-GFP construct localized to centrosomes in subconfluent and dividing MDCK cells. Our results suggest that Jouberin is a protein whose expression pattern supports both the adherens junction and the ciliary hypotheses for abnormalities leading to nephronophthisis.

NF-κB Modulates Aquaporin-2 Transcription in Renal Collecting Duct Principal Cells

Renal tubulo-interstitial inflammation is frequently associated with polyuria and urine concentration defects. This led us to investigate the effects of the major pro-inflammatory nuclear factor kappaB (NF-kappaB) pathway on aquaporin 2 (AQP2) expression by the collecting duct. Using immortalized collecting duct principal cells (mpkCCDcl4), we found that, acting independently of vasopressin, activation of NF-kappaB by lipopolysaccharide (LPS) decreased AQP2 mRNA and protein levels in a time- and dose-dependent manner but did not decrease AQP2 mRNA stability. Consistently, constitutively active IkappaB kinase beta decreased AQP2 expression. The LPS-induced decrease in AQP2 mRNA levels was confirmed in rat kidney slices and was reproduced both under conditions of elevated cAMP concentration and V(2) receptor antagonism. Computer analysis of the AQP2 promoter revealed two putative kappaB elements. Mutation of either kappaB element abolished the LPS-induced decrease of luciferase activity in cells expressing AQP2 promoter-luciferase plasmid constructs. Chromatin immunoprecipitation revealed that LPS challenge decreased p65, increased p50 and p52, and had no effect on RelB and c-Rel binding to kappaB elements of the AQP2 promoter. RNA-mediated interference silencing of p65, p50, and p52 confirmed controlled AQP2 transcription by these NF-kappaB subunits. We additionally found that hypertonicity activated NF-kappaB in mpkCCDcl4 cells, an effect that may counteract the Tonicity-responsive enhancer binding protein (TonEBP)-dependent increase in AQP2 gene transcription. Taken together, these findings indicate that NF-kappaB is an important physiological regulator of AQP2 transcription.

Downregulation of UT-A1/UT-A3 Is Associated with Urinary Concentrating Defect in Glucocorticoid-Excess State

Excessive glucocorticoid hormone, as occurs with Cushing syndrome, is known to be associated with altered body water homeostasis, but the molecular mechanisms are unknown. In this study, rats treated with daily dexamethasone (Dex) for 14 d provided a model of Cushing syndrome. Compared with control rats, Dex-treated rats demonstrated increased mean arterial pressure, urine flow rate, and urinary excretion of both sodium and urea. Dex-treated rats had increased abundance of aquaporin 1 (AQP1), AQP3, and Na-K-2Cl co-transporter proteins and a marked reduction of the urea transporters UT-A1 and UT-A3. In response to an acute water load, Dex-treated rats increased water excretion more than control rats, but both groups exhibited similar AQP2 expression. In response to fluid deprivation, Dex-treated rats demonstrated an impaired urinary concentrating capacity: Urine flow rate was higher and urine osmolality was lower than control rats despite an increase in AQP1, AQP3, and Na-K-2Cl co-transporter expression. AQP2 expression was similar between the two groups, but UT-A1 and UT-A3 were decreased and urinary urea excretion was increased in Dex-treated rats. Because Dex-treated rats ingested less food and water compared with controls, paired food and water studies were performed; these substantiated the previous results. In summary, the alterations in body water observed with glucocorticoid excess may be a result, in part, of impaired urinary concentrating capacity; downregulation of UT-A1 and UT-A3 and increased urea excretion may contribute to this impairment.

Nephronophthisis

Nephronophthisis (NPH) is an autosomal recessive disease characterized by a chronic tubulointerstitial nephritis that progress to terminal renal failure during the second decade (juvenile form) or before the age of 5 years (infantile form). In the juvenile form, a urine concentration defect starts during the first decade, and a progressive deterioration of renal function is observed in the following years. Kidney size may be normal, but loss of corticomedullary differentiation is often observed, and cysts occur usually after patients have progressed to end-stage renal failure. Histologic lesions are characterized by tubular basement membrane anomalies, tubular atrophy, and interstitial fibrosis. The infantile form is characterized by cortical microcysts and progression to end-stage renal failure before 5 years of age. Some children present with extrarenal symptoms: retinitis pigmentosa (Senior-Løken syndrome), mental retardation, cerebellar ataxia, bone anomalies, or liver fibrosis. Positional cloning and candidate gene approaches led to the identification of eight causative genes (NPHP1, 3, 4, 5, 6, 7, 8, and 9) responsible for the juvenile NPH and one gene NPHP2 for the infantile form. NPH and associated disorders are considered as ciliopathies, as all NPHP gene products are expressed in the primary cilia, similarly to the polycystic kidney disease (PKD) proteins.

Vasopressin and Aquaporin 2 in Clinical Disorders of Water Homeostasis

Impaired urinary dilution leading to water retention and hyponatremia may occur in patients with cardiac failure, cirrhosis, pregnancy, oxytocin administration, hypothyroidism, glucocorticoid, and mineralocorticoid deficiency. The mechanisms for these defects predominantly involve the nonosmotic stimulation of arginine vasopressin release with up-regulation of aquaporin 2 water channel expression and trafficking to the apical membrane of the principal cells of the collecting duct. These perturbations are reversed by V2 vasopressin receptor antagonists. In contrast, urinary concentration defects leading to polyuria are vasopressin resistant. They may involve several factors, such as impaired countercurrent concentration secondary to down-regulation of Na-K-2Cl cotransporter. Vasopressin-resistant down-regulation of aquaporin 2 expression has also been described as a factor in impaired urinary concentration.

Multiple renal cysts, urinary concentration defects, and pulmonary emphysematous changes in mice lacking TAZ

TAZ (transcriptional coactivator with PDZ-binding motif), also called WWTR1 (WW domain containing transcription regulator 1), is a 14-3-3-binding molecule homologous to Yes-associated protein. TAZ acts as a coactivator for several transcription factors as well as a modulator of membrane-associated PDZ domain-containing proteins, but its (patho)physiological roles remain unknown. Here we show that gene inactivation of TAZ in mice resulted in pathological changes in the kidney and lung that resemble the common human diseases polycystic kidney disease and pulmonary emphysema. Taz-null/lacZ knockin mutant homozygotes demonstrated renal cyst formation as early as embryonic day 15.5 with dilatation of Bowman's capsules and proximal tubules, followed by pelvic dilatation and hydronephrosis. After birth, only one-fifth of TAZ-deficient homozygotes grew to adulthood and demonstrated multicystic kidneys with severe urinary concentrating defects and polyuria. Furthermore, adult TAZ-deficient homozygotes exhibited diffuse emphysematous changes in the lung. Thus TAZ is essential for developmental mechanisms involved in kidney and lung organogenesis, whose disturbance may lead to the pathogenesis of common human diseases.

Renal function in late survivors of Iranian children with cancer: Single centre experience

One of the side-effects of chemotherapy is nephrotoxicity, whose incidence rate has reportedly been higher in pediatrics. Early diagnosis of renal dysfunction may decrease the morbidity in those with partial or complete remission by avoiding nephrotoxic agents and promoting regular follow-up. We studied the frequency of renal dysfunction in pediatric patients whose therapy was completed regardless of the type of chemotherapy. Prospective cross sectional study in Hematoncology department of children's hospital. 0 One hundred and eight pediatric cancer patients (44 females, 64 males), who were at least one year off therapy, enrolled in the study from 2003 to 2005. Demographic data, mean dosages of anticancer drugs, history of other nephrotoxic agents, nephrectomy, radiotherapy and acute renal failure were recorded. Fasting blood and urine samples were collected to calculate fractional excretion of magnesium, calcium, phosphate, urine protein to urine creatinine ratio, creatinine clearance (using Schwartz formula), urine analysis, urine osmolality and blood gas analysis. T-test, Chi square and binary logistic regression were used to compare means, frequency and correlation respectively. P values Renal dysfunction was seen in 25.2% of cases. These abnormalities included hypercalciuria (7.2%), phosphaturia (13.5%), magnesuria (3.6%), glomerular filtration rate less than 90 ml/min (7.5%), metabolic acidosis (3%), metabolic alkalosis (10%), urinary concentration defect (19%), proteinuria (7.2%), glycosuria (2%), microscopic hematuria (4%), sterile pyuria (6%), and hypertension (8%). We found only Procarbazin as an independent variable of nephrotoxicity (P< 0.05). Mild to moderate tubular dysfunction has been observed in the survivors of cancer disease.

Evaluation of renal function in leprosy: a study of 59 consecutive patients

Renal abnormalities in leprosy have been largely described in medical literature, but there are few studies evaluating renal function in these patients. This is a cross-sectional study in 59 consecutive paucibacillary (PB) and multibacillary (MB) leprosy patients. Glomerular filtration rate (GFR) was estimated by simplified-MDRD formula. Microalbuminuria was determined by 24 h urine collection. Urinary acidification capacity was measured after water deprivation and acid-loading with CaCl(2). Urinary concentration capacity was evaluated after desmopressin acetate administration, using the urinary to plasma osmolality (U/P(osm)) ratio. All parameters except microalbuminuria were measured in a control group of 18 healthy volunteers. Age and gender were similar between leprosy (MB or PB) and control groups. GFR <or= 80 ml/min/1.73 m(2) was observed in 50% of the leprosy patients. GFR and U/P(osm) in leprosy patients were significantly lower than in controls (P<0.001). Urinary acidification defect was found in 32% of PB and in 29% of MB patients and urinary concentrating ability was abnormal in 83% of PB and 85% of MB patients. Microalbuminuria was found in 4 patients (8.5%), leukocyturia was found in 13 (22%) and haematuria was present in 16 patients (27%). Plasma creatinine (P(cr)) >1.2 mg/dl was observed in 17.9% of MB patients and in none of the controls (P=0.020). A negative correlation was observed between GFR and time of treatment (r= -0.339; P=0.002). Age and time of treatment were independent risk factors for GFR <or= 80 ml/min/1.73 m(2) in multivariate analysis. Asymptomatic GFR changes and renal tubular dysfunction, including urine concentration defect and impaired acidifying mechanisms, can be caused by leprosy on specific treatment and without any reaction episodes.

Reduced urea flux across the blood-testis barrier and early maturation in the male reproductive system in UT-B-null mice

A urea-selective urine-concentrating defect was found in transgenic mice deficient in urea transporter (UT)-B. To determine the role of facilitated urea transport in extrarenal organs expressing UT-B, we studied the kinetics of [(14)C]urea distribution in UT-B-null mice versus wild-type mice. After renal blood flow was disrupted, [(14)C]urea distribution was selectively reduced in testis in UT-B-null mice. Under basal conditions, total testis urea content was 335.4 +/- 43.8 microg in UT-B-null mice versus 196.3 +/- 18.2 microg in wild-type mice (P < 0.01). Testis weight in UT-B-null mice (6.6 +/- 0.8 mg/g body wt) was significantly greater than in wild-type mice (4.2 +/- 0.8 mg/g body wt). Elongated spermatids were observed earlier in UT-B-null mice compared with wild type mice on day 24 versus day 32, respectively. First breeding ages in UT-B knockout males (48 +/- 3 days) were also significantly earlier than that in wild-type males (56 +/- 2 days). In competing mating tests with wild-type males and UT-B-null males, all pups carried UT-B-targeted genes, which indicates that all pups were produced from breeding of UT-B-null males. Experiments of the expression of follicle-stimulating hormone receptor (FSHR) and androgen binding protein (ABP) indicated that the development of Sertoli cells was also earlier in UT-B-null mice than that in wild-type mice. These results suggest that UT-B plays an important role in eliminating urea produced by Sertoli cells and that UT-B deletion causes both urea accumulation in the testis and early maturation of the male reproductive system. The UT-B knockout mouse may be a useful experimental model to define the molecular mechanisms of early puberty.

Distal Renal Tubular Acidosis in Mice Lacking the AE1 (Band3) Cl−/HCO3 − Exchanger (slc4a1)

Mutations in the human gene that encodes the AE1 Cl(-)/HCO(3)(-) exchanger (SLC4A1) cause autosomal recessive and dominant forms of distal renal tubular acidosis (dRTA). A mouse model that lacks AE1/slc4a1 (slc4a1-/-) exhibited dRTA characterized by spontaneous hyperchloremic metabolic acidosis with low net acid excretion and, inappropriately, alkaline urine without bicarbonaturia. Basolateral Cl(-)/HCO(3)(-) exchange activity in acid-secretory intercalated cells of isolated superfused slc4a1-/- medullary collecting duct was reduced, but alternate bicarbonate transport pathways were upregulated. Homozygous mice had nephrocalcinosis associated with hypercalciuria, hyperphosphaturia, and hypocitraturia. A severe urinary concentration defect in slc4a1-/- mice was accompanied by dysregulated expression and localization of the aquaporin-2 water channel. Mice that were heterozygous for the AE1-deficient allele had no apparent defect. Thus, the slc4a1-/- mouse is the first genetic model of complete dRTA and demonstrates that the AE1/slc4a1 Cl(-)/HCO(3)(-) exchanger is required for maintenance of normal acid-base homeostasis by distal renal regeneration of bicarbonate in the mouse as well as in humans.

The Uromodulin C744G mutation causes MCKD2 and FJHN in children and adults and may be due to a possible founder effect

Autosomal dominant medullary cystic kidney disease type 2 (MCKD2) is a tubulo-in terstitial nephropathy that causes renal salt wasting, hyperuricemia, gout, and end-stage renal failure in the fifth decade of life. This disorder was described to have an age of onset between the age of 20-30 years or even later. Mutations in the Uromodulin (UMOD) gene were published in patients with familial juvenile hyperuricemic nephropathy (FJHN) and MCKD2. Clinical data and blood samples of 16 affected individuals from 11 different kindreds were collected. Mutational analysis of the UMOD gene was performed by exon polymerase chain reaction (PCR) and direct sequencing. We found the heterozygous C744G (Cys248Trp) mutation, which was originally published by our group, in an additional four kindreds from Europe and Turkey. Age of onset ranged from 3 years to 39 years. The phenotype showed a variety of symptoms such as urinary concentration defect, vesicoureteral reflux, urinary tract infections, hyperuricemia, hypertension, proteinuria, and renal hypoplasia. Haplotype analysis showed cosegragation with the phenotype in all eight affected individuals indicating that the C744G mutation may be due to a founder effect. Moreover, we describe a novel T229G (Cys77Gly) mutation in two affecteds of one kindred. Three of the affected individuals were younger than 10 years at the onset of MCKD2/FJHN. Symptoms include recurrent urinary tract infections compatible with the published phenotype of the Umod knockout mouse model. This emphasizes that MCKD2 is not just a disease of the young adult but is also relevant for children.

Regulation and dysregulation of aquaporins in water balance disorders*

The discovery of aquaporin-1 (AQP1) explained the long-standing biophysical question of how water specifically crosses biological membranes. These studies led to the identification of a whole new family of membrane proteins, the aquaporin water channels. At present, at least eight aquaporins are expressed at distinct sites in the kidney and four members of this family (AQP1-4) have been demonstrated to play pivotal roles in the physiology and pathophysiology for renal regulation of body water balance. In the present review, a number of inherited and acquired conditions characterized by urinary concentration defects as well as common diseases associated with severe water retention are discussed with relation to the role of aquaporins in regulation and dysregulation of renal water transport.

Polycystic kidney disease: genes, proteins, animal models, disease mechanisms and therapeutic opportunities

An increased understanding of the genetic, molecular and cellular mechanisms responsible for the development of polycystic kidney disease has laid out the foundation for the development of rational therapies. Many animal models where these therapies can be tested are currently available. This review summarizes the rationale for these treatments, the results of preclinical trials and the prospects for clinical trials, some already in early phases of implementation.

Néphronophtise

Nephronophthisis is a chronic tubulo-interstitial nephritis which progress to terminal renal failure. It is an heterogeneous entity at the clinical as well as at the genetic level. There are three main clinical forms of nephronophtisis which have been associated with five gene defects. Juvenile nephronophtisis, the most frequent, progress to end stage renal failure before age 15. It is an autosomal recessive disease which is responsible for a urine concentration defect starting after age 2, failure to thrive and a progressive deterioration of renal function without signs of glomerular disease. Kidney size is normal. Histologic lesions concern tubular basement membranes which are thickened and multilayered or thinned. There is an associated intertitial fibrosis. Some children present with extrarenal symptoms: tapetoretinal degeneration (Senior-Loken syndrome), mental retardation, cerebellar ataxia, bone anomalies or liver involvement. Infantile nephronophtisis is a recessive autosomic tubulo-interstitial nephritis with cortical microcysts which progress to end stage renal failure before age 5. Adolescent nephronophtisis is a less frequent form of nephronophtisis. Medullary cystic disease is transmitted as an autosomic dominant trait. Clinical and histological signs are similar to nephronophthisis, but the disease progress later to terminal renal failure and is not accompanied by extra-renal symptoms. Several genes which are involved in nephronophtisis, encode proteins that localize in different cell compartments, in particular to the primary apical cilia, as it is the case for many other cystic kidney diseases.

A 67-Year-Old Woman With Chronic Proteinuria

67-year-old woman presented with low-grade proteinuria (1040 mg/24 h) and hypertension (170/90 mm Hg). Urinalysis showed dark yellow, nonturbid urine that was negative for leukocyte esterase. The urine had a specific gravity of 1.030, pH 6.0, no glucose, and trace ketones. Laboratory findings also revealed negativity for bilirubin, blood, and nitrite in the urine. White blood cell count was 4 to 6/mm 3 . Other aspects of the physical examination were unremarkable. The patient’s medical history was significant for low-grade proteinuria (approximately 1000 mg/24 h) of 12 years and hypertension of 4 years. She had no previous history of microscopic or macroscopic hematuria, renal stones, epistaxis or hemoptysis, flank pain, or history suggestive of a connective tissue disease. She had no significant family history. A renal biopsy was performed to identify the etiology of her long-standing proteinuria. The specimen submitted for light microscopic study consisted of a single core of renal cortical tissue with up to 8 glomeruli per section, 3 of which were globally collapsed and sclerotic. The other glomeruli had patent glomerular capillary lumina; no glomerular hypercellularity was noted. A single glomerulus revealed a segmentally sclerotic segment. The visceral epithelial cells were distended by fine intracytoplasmic vacuoles (Figure 1, arrows). The interstitium exhibited slight focal widening

A case of Mikulicz’s disease complicated with interstitial nephritis successfully treated by high-dose corticosteroid

A 40-year-old woman who had bilateral swelling in the eyelids and submandibular region was admitted. Clinical findings suggested she had primary Sjögren's syndrome. Laboratory data showed glucosuria, positive CRP (0.50 mg/dl), liver dysfunction (AST 53 U/l, ALT 101 U/l, gamma-GTP 241 U/l, ALP 914 U/l, LAP 496 U/l), hyperglycemia, hypergammaglobulinemia (IgG 3450 mg/dl, IgA 91 mg/dl, IgM 80 mg/dl), hypocomplementemia (C3 73 mg/dl, C4 2 mg/dl, CH50 < 19.0 U/ml), renal tubular dysfunction (urine N-acetyl-beta-D: -glucosaminidase 8.6 U/l, urine (beta2)-microglobulin 83 microg/l), and urinary concentration defect. Ammonium chloride loading test was normal. Gallium-67 scintigram indicated abnormal uptake in bilateral lacrimal glands, submandibular glands, and kidneys. A diagnosis of Mikulicz's disease and interstitial nephritis was made, since biopsy specimens of her lacrimal gland and minor salivary gland showed diffuse infiltration of lymphocytes. Renal biopsy specimens also showed severe interstitial infiltration of lymphocytes. Symptoms and laboratory data normalized in response to methylprednisolone pulse therapy and prednisolone 60 mg/day. This case of Mikulicz's disease complicated with interstitial nephritis was successfully treated by high-dose corticosteroid.

Fanconi’s syndrome and distal (Type 1) renal tubular acidosis in a patient with primary Sjögren’s syndrome with monoclonal gammopathy of undetermined significance

Tubulointerstitial nephritis is a well-recognized complication in primary Sjögrens syndrome. Fanconi's syndrome is a far less frequent complication compared with distal tubular dysfunction. We here describe a 49-year-old woman with primary Sjögren's syndrome. In 1997, she was diagnosed with primary Sjögren's syndrome with tubulointerstitial nephritis, and was then treated with oral prednisolone for the tubulointerstitial nephritis. In 2002, she was referred to our hospital because of progressive fatigue. At that time, biclonal spike on serum protein (IgG-kappa and IgA-kappa) and Bence-Jones protein in urine were found. Bone marrow aspiration showed 1.0% plasma cell infiltration. Thus, a diagnosis of monoclonal gammopathy of undetermined significance (MGUS) was made. In 2004, she was again admitted to our hospital because of mild renal dysfunction and hypokalemia. Laboratory evaluation showed inappropriate, alkaline urine in hyperchloremic metabolic acidosis and a positive urine anion gap, indicating the presence of distal (Type 1) renal tubular acidosis (RTA). The urine concentration defect was also found. Further studies revealed proximal tubular dysfunction, including renal glycosuria, generalized aminoaciduria, phosphaturia, uricosuria and proximal RTA. The kidney biopsy represented diffuse and severe tubulointerstitial nephritis with dense infiltrates of lymphocytes and IgA and K light chain-positive plasma cells. No findings of multiple myeloma or malignant lymphoma were observed. In conclusion, our patient had Sjögren's syndrome with MGUS and exhibited dysfunction of both proximal tubule (Fanconi's syndrome) and distal tubule, which may be attributed to diffuse tubulointerstitial nephritis.

Congenital progressive hydronephrosis ( cph ) is caused by an S256L mutation in aquaporin-2 that affects its phosphorylation and apical membrane accumulation

Congenital progressive hydronephrosis (cph) is a spontaneous recessive mutation that causes severe hydronephrosis and obstructive nephropathy in affected mice. The mutation has been mapped to the distal end of mouse chromosome 15, but the mutated gene has not been found. Here, we describe the identification of a single base pair change in aquaporin-2 (Aqp2) in cph mutants through genetic linkage mapping. The C-T change led to the substitution of a Ser (S256) by a Leu in the cytoplasmic tail of the Aqp2 protein, preventing its phosphorylation at S256 and the subsequent accumulation of Aqp2 on the apical membrane of the collecting duct principal cells. The interference with normal trafficking of Aqp2 by this mutation resulted in a severe urine concentration defect. cph homozygotes demonstrated polydipsia and produced a copious amount of hypotonic urine. The urine concentration defect could not be corrected by [deamino-Cys1,D-Arg8]-vasopressin (DDAVP, a vasopressin analog), characteristic of nephrogenic diabetes insipidus. The nephrogenic diabetes insipidus symptoms and the absence of developmental defects in the pyeloureteral peristaltic machinery in the mutants before the onset of hydronephrosis suggest that the congenital obstructive nephropathy is most likely a result of the polyuria. This study has revealed the genetic basis for the classical cph mutation and has provided direct genetic evidence that S256 in Aqp2 is indispensable for the apical accumulation, but not the general glycosylation or membrane association, of Aqp2.