Glucocorticoid-induced polycystic kidney disease--a threshold trait

Novel zebrafish polycystic kidney disease models reveal functions of the Hippo pathway in renal cystogenesis

The Hippo signaling pathway is a kinase cascade that plays an important role in organ size control. As the main effectors of the Hippo pathway, transcription coactivators Yap1/Wwtr1 are regulated by the upstream kinase Stk3. Recent studies in mammals have implicated the Hippo pathway in kidney development and kidney diseases. To further illustrate its roles in vertebrate kidney, we generated a series of zebrafish mutants targeting stk3, yap1 and wwtr1 genes. The stk3^−/− mutant exhibited edema, formation of glomerular cysts and pronephric tubule dilation during the larval stage. Interestingly, disruption of wwtr1, but not yap1, significantly alleviated the renal phenotypes of the stk3^−/− mutant, and overexpression of Wwtr1 with the CMV promoter also induced pronephric phenotypes, similar to those of the stk3^−/− mutant, during larval stage. Notably, adult fish with Wwtr1 overexpression developed phenotypes similar to those of human polycystic kidney disease (PKD). Overall, our analyses revealed roles of Stk3 and Wwtr1 in renal cyst formation. Using a pharmacological approach, we further demonstrated that Stk3-deficient zebrafish could serve as a PKD model for drug development.

Summary: A zebrafish stk3 mutant line and Wwtr1 overexpression line provide evidence for functions of the Hippo signaling pathway in renal cyst formation and represent potential models for polycystic kidney disease.

11β-hydroxysteroid dehydrogenases: intracellular gate-keepers of tissue glucocorticoid action

Glucocorticoid action on target tissues is determined by the density of "nuclear" receptors and intracellular metabolism by the two isozymes of 11β-hydroxysteroid dehydrogenase (11β-HSD) which catalyze interconversion of active cortisol and corticosterone with inert cortisone and 11-dehydrocorticosterone. 11β-HSD type 1, a predominant reductase in most intact cells, catalyzes the regeneration of active glucocorticoids, thus amplifying cellular action. 11β-HSD1 is widely expressed in liver, adipose tissue, muscle, pancreatic islets, adult brain, inflammatory cells, and gonads. 11β-HSD1 is selectively elevated in adipose tissue in obesity where it contributes to metabolic complications. Similarly, 11β-HSD1 is elevated in the ageing brain where it exacerbates glucocorticoid-associated cognitive decline. Deficiency or selective inhibition of 11β-HSD1 improves multiple metabolic syndrome parameters in rodent models and human clinical trials and similarly improves cognitive function with ageing. The efficacy of inhibitors in human therapy remains unclear. 11β-HSD2 is a high-affinity dehydrogenase that inactivates glucocorticoids. In the distal nephron, 11β-HSD2 ensures that only aldosterone is an agonist at mineralocorticoid receptors (MR). 11β-HSD2 inhibition or genetic deficiency causes apparent mineralocorticoid excess and hypertension due to inappropriate glucocorticoid activation of renal MR. The placenta and fetus also highly express 11β-HSD2 which, by inactivating glucocorticoids, prevents premature maturation of fetal tissues and consequent developmental "programming." The role of 11β-HSD2 as a marker of programming is being explored. The 11β-HSDs thus illuminate the emerging biology of intracrine control, afford important insights into human pathogenesis, and offer new tissue-restricted therapeutic avenues.

Dominant role of monocytes in control of tissue function and aging

We propose that monocyte-derived cells regulate expression of epitopes of specific tissue cells, and in that way control recognition of tissue cells by autoreactive T lymphocytes and autoantibodies. Such T cells and antibodies are suggested to participate in stimulation of tissue cell differentiation. This may ultimately result in the aging and degeneration of tissue cells. By the end of their adaptation in early ontogeny, the monocyte-derived cells are supposed to encounter the most differentiated tissue cells in a tissue specific manner, and then prevent tissue cells to differentiate beyond the encoded state. Retardation or acceleration of certain tissue differentiation during adaptation results in a rigid and permanent alteration of this tissue function. The ability of monocytes to preserve tissue cells in the functional state declines with age, and this is accompanied by functional decline of various tissues within the body, and an increased incidence of degenerative diseases.

Development of autosomal recessive polycystic kidney disease in BALB/c-cpk/cpk mice

Autosomal recessive polycystic kidney disease (ARPKD) is a rare but devastating inherited disease in humans. Various strains of mice that are homozygous for the cpk gene display renal pathology similar to that seen in human ARPKD. The PKD progresses to renal insufficiency, azotemia, and ultimately a uremic death by approximately 3 wk of age. This study characterizes PKD in mice that are homozygous for the cpk gene on a BALB/c inbred mouse background. The BALB/c-cpk/cpk murine model displays renal as well as extrarenal pathology similar to that found in human ARPKD. The renal pathology includes the well-characterized early proximal tubule and, later, massive collecting duct cysts. The extrarenal defects in this murine model include common bile duct dilation, intrahepatic biliary duct cysts with periductal hyperplasia, and pancreatic dysplasia with cysts. Renal mRNA expression of c-myc, a proto-oncogene, and clusterin (SGP-2), a marker associated with immature collecting ducts, decreases during normal development but is upregulated in murine ARPKD. Expression of epidermal growth factor (EGF) mRNA is significantly diminished, whereas EGF receptor mRNA is upregulated in the BALB/c-cpk/cpk kidney compared with phenotypically normal littermates. To determine whether the altered EGF expression contributes to the development of PKD, neonatal mice were treated with exogenous EGF (1 microg/g body wt injected subcutaneously on postnatal days 3 through 9). EGF treatment reduced the relative kidney weight and common bile duct dilation and downregulated renal expression of clusterin and EGF receptor. However, exogenous EGF did not affect the degree of renal failure, the pancreatic pathology, or the misregulated renal expression of c-myc. In summary, the present study characterizes the renal and extrarenal pathology in the BALB/c-cpk/cpk murine model of ARPKD. Renal mRNA expression of EGF is diminished in this mouse model. EGF treatment did not prevent renal failure but ameliorated pathologic changes in the kidney and the biliary ducts of the BALB/c-cpk/cpk mouse.

Epithelin mRNA expression in polycystic kidney disease

Epithelins are polypeptides that are preferentially expressed in epithelial cells and modulate growth. Epithelin expression is predominant in tissues of epithelial origin such as the kidney, spleen, lung, placenta, and colon. Because polycystic kidney disease involves abnormal proliferation of the proximal and/or distal tubule epithelial cells, we investigated epithelin mRNA expression in polycystic kidneys of mice homozygous for the mutation. Epithelin mRNA was highly expressed in the polycystic kidneys of homozygous mice when compared with the heterozygotes or wild type controls. A study on the time course of epithelin expression indicated that epithelin mRNA expression paralleled cyst formation and progression of the disease. A 2-fold increase in expression was observed at Day 15, a stage when cystic changes were first visible. This increase in expression was also observed at Day 21, a stage of maximum disease pathology, which ultimately results in the death of the animal. In situ hybridization localized epithelin mRNA predominantly to the epithelial cell layer surrounding the cysts. The high levels of epithelin in epithelial cells suggest a role in renal epithelial cell proliferation and cyst formation in polycystic kidney disease.

A new in vitro bioassay for cyst formation by renal cells from an autosomal dominant rat model of polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is one of the most frequent human inherited diseases. The main feature of the disease is the development of renal cysts, first occurring in the proximal tubules, and with time, dominating all segments of the nephron, leading to end-stage renal disease in 50% of the patients in their fifth decade of life. A therapy for polycystic kidney disease (PKD) has not yet been developed. Patients coming to end-stage ADPKD require long-term dialysis and/or transplantation. A suitable animal model to study ADPKD is the spontaneously mutated Han:SPRD (cy/+) rat, but a method to cultivate Han:SPRD (cy/+) derived renal cells which preserves their ability to form cyst-like structures in vitro has previously not been reported. Based on this well-characterized animal model, we developed a cell culture model of renal cyst formation in vitro. When renal cells of the Han:SPRD (cy/+) rat were isolated and cultured under conditions that prevent cell-substratum adhesion, large amounts of cyst-like structures were formed de novo from Han:SPRD (cy/+) derived renal cells, but only a few from control rat renal cells. In contrast, when cultivated on plastic as monolayer cultures, Han:SPRD (cy/+)-derived and control rat-derived renal cells were indistinguishable and did not form cyst-like structures. Immunohistochemical characterization of the cyst-like structures suggests tubular epithelial origin of the cyst-forming cells. The amount of cysts formed from Han:SPRD (cy/+)-derived renal cells grown in a stationary suspension culture is susceptible to modulation by different conditions. Human cyst fluid and epidermal growth factor both stimulated the formation of cysts from Han:SPRD (cy/+)-derived renal cells whereas taxol inhibited cystogenesis. In contrast, neither human cyst fluid nor epidermal growth factor affected the amount of cysts formed by control rat renal cells. As the culture model reported here allows not only the distinction of PKD-derived tubular epithelium from its normal counterpart, but also the modulation of cyst formation especially by Han:SPRD (cy/+)-derived renal cells, it might be a useful prescreening protocol for potential treatments for PKD and thus reduce the need for animal experiments.

Characterization of Ke 6, a new 17beta-hydroxysteroid dehydrogenase, and its expression in gonadal tissues

The abnormal regulation of the Ke 6 gene has been linked to the development of recessive polycystic kidney disease in the mouse. In this report, we have shown that Ke 6 is a 17beta-hydroxysteroid dehydrogenase and can regulate the concentration of biologically active estrogens and androgens. The Ke 6 enzyme is preferentially an oxidative enzyme and inactivates estradiol, testosterone, and dihydrotestosterone. However, the enzyme has some reductive activity and can synthesize estradiol from estrone. We find that the Ke 6 gene is expressed within the ovaries and testes. The presence of Ke 6 protein within the cumulus cells surrounding the oocyte places it in a strategic location to control the level of steroids to which the egg is exposed. Previously, it had been shown that glucocorticoids can induce renal cysts in the neonatal rodent, only when given at a narrow time window of postnatal kidney development. We propose that the reduction in the level of Ke 6 enzyme, which occurs in the cpk, jck, and pcy mice, may lead to abnormal elevations in local level of sex steroids, which either directly or indirectly via abnormal glucocorticoid metabolism result in recessive renal cystic disease, a developmental disorder of the kidney.

Abnormal regulation of the Ke 6 gene, a new 17beta-hydroxysteroid dehydrogenase in the cpk mouse kidney

The function encoded by the Ke 6 gene has been recently determined to be 17beta-hydroxysteroid dehydrogenase. Previously, the abnormal expression of the Ke 6 gene has been intimately associated with development of recessive polycystic kidney disease. The Ke 6 gene is normally expressed at very high levels in the kidney and liver and is severely down regulated in all recessive murine models of polycystic kidney disease that have been examined to date. Here, we report a detailed examination of the promoter region of the Ke 6 gene in normal mouse kidney cells (CTA) and in cells derived from mouse kidneys homozygous for the cpk (congenital polycystic kidney) mutation, using transfection analysis and DNA-protein gel shift assays. The minimal promoter region, P1 (+1 to -96), and a putative enhancer site, P3 (-165 to -256), within the Ke 6 gene 5' flanking sequence have been identified. We have also identified another region, P2 (-97 to -165), that may be responsible for the lower promoter activity of the Ke 6 gene in cpk cells. Furthermore, absence of binding of a 38 kDa nuclear protein to a 16 bp sequence element (P1A) within the minimal promoter of the Ke 6 gene suggests that the P1A element could be responsible for the overall reduction in promoter function in cpk cells.

Murine autosomal recessive polycystic kidney disease with multiorgan involvement induced by the cpk gene

BACKGROUND

Autosomal recessive (AR) polycystic kidney disease (PKD) is characterized in humans and mice as a rapidly progressive, collecting duct cystic disease usually leading to uremia in the neonatal or infantile period. In humans, ARPKD renal pathology can be variable in severity and is associated with the development of prominent bile duct and liver pathology. The C57BL/6J-cpk/cpk mouse model of ARPKD is the most extensively studied murine model of inherited infantile PKD; however, these mice lack extrarenal pathology.

METHODS

In the present study, the cpk gene was backbreed onto CD1 mice to examine the development of cpk-induced ARPKD in this outbred mouse background. Resulting cystic offspring were examined morphologically and their serum urea nitrogen levels were assessed.

RESULTS

The rapid development of PKD in CD1 mice homozygous for the cpk gene appears to be slightly more rapid but otherwise comparable to that seen in inbred C57BL/6J mice. In CD1-cpk/cpk mice, the principal renal pathological finding is collecting duct cysts, which are lined by a relatively uniform epithelium. This epithelium appears to be relatively undifferentiated based on almost total absence of intercalated cells. Proximal tubule cysts are prominent in the first postnatal week while collecting duct cysts predominate in the later stages of the disease. Extrarenal manifestations of the cpk gene are evident in the CD1 strain and include cysts of pancreatic, common bile, and major hepatic ducts. Intrahepatic bile ducts also have focal dilations. Primary (thymus) and secondary (spleen) lymphoid tissues become hypoplastic as azotemia progresses. The strain-related variability in renal and liver changes in cpk-induced ARPKD may reflect the influence of other genes (possibly modifier genes) expressed in this mouse strain. In older CD1-cpk/+ mice, renal (proximal tubular) cysts and prominent liver cysts (lined by a biliary epithelium) develop, indicating that the heterozygous state (cpk/+ genotype) causes renal and hepatic pathology.

CONCLUSIONS

The cpk gene, when placed on an appropriate mouse strain background, causes multiorgan disease that more closely mimics human ARPKD than when the cpk gene is expressed on the C57BL/6J strain. A gene dose effect is present as cystic pathology is present in kidney and liver of both suckling homozygous (cpk/cpk) and old heterozygous (cpk/+) mice.

Transforming growth factor alpha and epidermal growth factor expression in experimental murine polycystic kidney disease

Cystic change in polycystic kidney disease (PKD) is associated with epithelial hyperplasia, altered fluid and electrolyte transport, and de-differentiation of renal tubular epithelium. The role of polypeptide growth factors as potential modulators of cystic change remains an area of controversy. In this study, the expression of epidermal growth factor (EGF) and transforming growth factor-alpha (TGF alpha) were assessed by immunohistochemistry and image analysis in glucocorticoid-induced PKD in the newborn mouse. Newborn C3H mice received either 200 mg/kg methylprednisolone acetate (MPA) or 0.9% saline as a control. EGF expression was not detected in significant quantities in either MPA-treated or control animals. TGF alpha, however, was expressed in immature control kidney in a largely basolateral distribution. Expression increased significantly in association with cystic change in MPA-treated animals and was localized to the apical cell surface, implying altered polarity of secretion. There is no evidence that EGF is a mitogen in this early developmental model of PKD. TGF alpha, however, may be an important mediator of cystic change in immature or de-differentiated renal tubular epithelium.

Ke 6 gene. Sequence and organization and aberrant regulation in murine polycystic kidney disease

Ke 6 gene is a newly identified gene located in the major histocompatibility complex and is a candidate steroid dehydrogenase gene because of structural homology and regulatory similarities with mammalian steroid dehydrogenases. We report here the complete nucleotide sequence and intron-exon organization of the Ke 6 gene and cloning of the alternatively spliced Ke 6b transcript. We find that Ke 6 gene expression is down-regulated in pcy mice which is a murine model of polycystic kidney disease (PKD). Thus far, Ke 6 gene expression is down-regulated in all murine models of PKD we have examined. Abnormal steroid metabolism as a possible cause of PKD is discussed.

Animal models of polycystic kidney disease

Polycystic kidney disease (PKD) is one of the most prevalent causes of heritable renal failure. The disease is characterized by the occurrence of numerous fluid-filled cysts within the parenchyma of kidney. The cysts are epithelial in origin and expand in size, leading to crowding of normal kidney tissue. Ultimately, there is gross enlargement of the kidneys with loss of normal functions, and death usually occurs because of complications related to renal failure. Animal models of polycystic kidney disease are proving to be extremely useful for studying the molecular basis of renal cyst formation and for the isolation of genes carrying the mutations. This article describes the various animal models of polycystic kidney disease, spontaneously and experimentally derived, that have recently been identified.

Renal, cardiovascular and hormonal characteristics of young adults with autosomal dominant polycystic kidney disease

We studied young adults with autosomal dominant polycystic kidney disease (ADPKD) to determine the characteristics that precede renal impairment. Nineteen affected (A) and 20 unaffected (U) offspring from families with ADPKD showed no significant differences in basal glomerular filtration rate (A: mean 97, SD 19; U: 100, SD 23 ml/min/1.73 m2) or renal functional reserve, but effective renal plasma flow was significantly lower in affected offspring (A: 532, SD 86; U: 605, SD 118 ml/min/1.73 m2, P less than 0.01). Plasma renin activity [A: median 26 (95% CI: 15 to 37); U: 14 (11 to 27) microU/ml, P less than 0.05, one-tailed test] and aldosterone [A: 2.5 (2.0 to 3.0), U: 1.0 (1.5 to 2.0) micrograms/100 ml, P less than 0.04, one-tailed test] were increased in affected offspring despite the higher systolic blood pressure (A: mean 123, SD 5; U: 115, SD 3 mm Hg, P less than 0.02) and significant expansion of total exchangeable sodium (A: 40.8, SD 2.3; U: 38.0, SD 3.5 mmol/kg, P less than 0.01). The ouabain-sensitive component of red cell sodium efflux was less in affected offspring (A: 0.258; SD 0.040; U: 0.288, SD 0.042 hr-1, P less than 0.04) and in both groups was correlated inversely with total exchangeable sodium. Echocardiography revealed no difference in left ventricular mass index nor prevalence of mitral valve prolapse. Potential cyst growth factors such as the glucocorticoids and somatomedin C were similar in both affected and unaffected groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Ontogeny of dexamethasone binding and sodium potassium ATPase activity in experimental murine polycystic kidney disease

The induction of polycystic kidney disease (PKD) by glucocorticoids in newborn mice behaves as a "threshold" trait, with prevalence of PKD varying in different inbred strains after exposure to an inducing steroid. C3H mice (low threshold for PKD) demonstrated greater specific dexamethasone binding than DBA mice (high threshold) on the second day of life. Treatment with methylprednisolone acetate (MPA), a cyst-inducing steroid, down regulated dexamethasone binding earlier than in DBA mice. C3H mice demonstrated greater whole kidney homogenate Na-K ATPase activity than DBA mice within 24h of MPA injection. Specific renal glucocorticoid binding may be a regulator of threshold for murine glucocorticoid induced PKD. Our findings support in vitro evidence that glucocorticoid induced Na-K ATPase activity during critical periods of nephron development is an important regulatory point of this model.