Molecular pathogenesis of autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is one of the commonest inherited human disorders yet remains relatively unknown to the wider medical, scientific and public audience. ADPKD is characterised by the development of bilateral enlarged kidneys containing multiple fluid-filled cysts and is a leading cause of end-stage renal failure (ESRF). ADPKD is caused by mutations in two genes: PKD1 and PKD2. The protein products of the PKD genes, polycystin-1 and polycystin-2, form a calcium-regulated, calcium-permeable ion channel. The polycystin complex is implicated in regulation of the cell cycle via multiple signal transduction pathways as well as the mechanosensory function of the renal primary cilium, an enigmatic cellular organelle whose role in normal physiology is still poorly understood. Defects in cilial function are now documented in several other human diseases including autosomal recessive polycystic kidney disease, nephronophthisis, Bardet-Biedl syndrome and many animal models of polycystic kidney disease. Therapeutic trials in these animal models of polycystic kidney disease have identified several promising drugs that ameliorate disease severity. However, elucidation of the function of the polycystins and the primary cilium will have a major impact on our understanding of renal cystic diseases and will create exciting new opportunities for the design of disease-specific therapies.     

Defective epidermal growth factor gene expression in mice with polycystic kidney disease

The C57BL/6J-cpk mouse has an inheritable form of polycystic kidney disease similar to the autosomal recessive disorder seen in humans. Between approximately 1 and 3 weeks of age, affected cpk mice develop numerous large cysts in the collecting tubule segment of kidney nephrons. The present study examined the ontogeny of renal and submandibular gland prepro-epidermal growth factor (preproEGF) gene expression in the cpk mouse using Northern blot hybridization and immunohistochemistry. There was a virtual absence of renal preproEGF gene expression in cystic kidneys over the 3-week postnatal period, during which time renal preproEGF mRNA and proEGF/EGF protein normally reach significant levels. PreproEGF mRNA was expressed in salivary glands of cystic mice; however, this mRNA could not be further elevated with testosterone suggesting that there are abnormalities in the regulation of the preproEGF gene in the submandibular gland, as well as in the kidney. Since renal preproEGF expression during the early postnatal period occurs when collecting duct cysts form, it is possible that a deficiency in renal proEGF or EGF contributes to the rapid development of collecting duct cysts and the concomitant renal failure in the C57BL/6J-cpk cystic mouse.     

Multiple cyst formation in the liver and kidneys of a lion (<Emphasis Type="Italic">Panthera leo</Emphasis>): a case of polycystic kidney disease?

The present report describes the case of a 19-year-old male lion (Panthera leo) with polycystic lesions in liver and kidney. The animal suffered from inappetence and progressing hind limb weakness and was euthanized due to poor prognosis. Necropsy revealed multiple large, cystic cavities in the liver and numerous cysts in the renal cortex. Hepatic and renal cysts were lined by a flattened or cuboidal single-layered epithelium and derived from hepatic bile ducts and renal tubular structures, respectively, as confirmed by immunohistochemistry for the detection of different cytokeratins and carcinoembryonic antigen. Furthermore, a marked bile duct proliferation and fibrosis was found in the portal areas. Occasionally, renal cysts contained variable amounts of partly mineralized proteinaceous material, detached epithelial cells, and cellular debris. Additional findings included a moderate suppurative bronchopneumonia and other minor age-related degenerative changes. A mutation screen of exon 29 of the PKD1 gene did not reveal causative mutations like in domestic cats. This is the first report of polycystic kidney disease (PKD)-like lesions in a lion. The features of these lesions are similar to those of the adult-onset form of PKD in Persian cats.     

Identification of apolipoproteinA1 reduction in the polycystic kidney by proteomics analysis of the Mxi1‐deficient mouse

Autosomal dominant polycystic kidney disease is one of the most common human monogenic diseases in which extensive epithelial‐lined cysts develop in kidney and other organs. Affected kidneys are not only characterized by the formation of cysts, but also by changes associated with the extracellular matrix and interstitial inflammation, which can progress to fibrosis and loss of renal function. Mxi1 protein, which is a c‐myc antagonist, may be essential in controlling cellular growth and differentiation. Previously, multiple tubular cysts were observed in kidney of Mxi1‐deficient mice aged 6 months and more. Presently, 2‐DE and MALDI‐TOF MS was employed to identify the differentially expressed proteins in the kidney. Several proteins were identified, among them, apolipoproteinA1 which is a major component of the high‐density lipoprotein complex and has anti‐inflammation effects, was significantly decreased in the Mxi1‐deficient mouse. We confirm the development of inflammation and renal fibrosis and the expression of extracellular matrix molecules including transforming growth factor were also increased in cystic kidney. These results indicate that expression of proteins related with inflammation and renal fibrosis changes by Mxi1 inactivation in polycystic kidney.     

Distinct oxylipin alterations in diverse models of cystic kidney diseases

Cystic kidney diseases are characterized by multiple renal cysts and are the leading cause of inherited renal disease. Oxylipins are bioactive lipids derived from fatty acids formed via cyclooxygenase, lipoxygenase and cytochrome P450 activity, and are important regulators of renal health and disease. Oxylipins are altered in nephronophthisis, a type of cystic kidney disease. To further investigate and to determine whether other cystic renal diseases share these abnormalities, a targeted lipidomic analysis of renal oxylipins was performed in orthologous models of autosomal dominant polycystic kidney disease 1 (Mx1Cre⁺ Pkd1^flox/flox mouse) and 2 (Pkd2^ws25/− mouse), autosomal recessive polycystic kidney disease (PCK rat) and nephronophthisis (jck/jck mouse). Kidney cyclooxygenase oxylipins were consistently higher in all diseased kidneys, even in very early stage disease. On the other hand, cytochrome P450 epoxygenase derived oxylipins were lower only in the autosomal recessive polycystic kidney disease and nephronophthisis models, while lipoxygenase and cytochrome P450 hydroxylase derived oxylipins were lower only in nephronophthisis. Sex effects on renal oxylipin alterations were observed but they did not always coincide with sex effects on disease. For oxylipins with sex effects, arachidonic acid derived oxylipins formed via cyclooxygenases and lipoxygenases were higher in females, while oxylipins from other fatty acids and via cytochrome P450 enzymes were higher in males. The consistent and unique patterns of oxylipin alterations in the different models indicates the importance of these bioactive lipids in cystic renal diseases, suggesting that pharmacological agents (e.g. cyclooxygenase inhibitors) may be useful in treating these disorders, for which effective treatment remains elusive.     

PPAR-gamma agonist ameliorates kidney and liver disease in an orthologous rat model of human autosomal recessive polycystic kidney disease

In autosomal recessive polycystic kidney disease (ARPKD), progressive enlargement of fluid-filled cysts is due to aberrant proliferation of tubule epithelial cells and transepithelial fluid secretion leading to extensive nephron loss and interstitial fibrosis. Congenital hepatic fibrosis associated with biliary cysts/dilatations is the most common extrarenal manifestation in ARPKD and can lead to massive liver enlargement. Peroxisome proliferator-activated receptor γ (PPAR-γ), a member of the ligand-dependent nuclear receptor superfamily, is expressed in a variety of tissues, including the kidneys and liver, and plays important roles in cell proliferation, fibrosis, and inflammation. In the current study, we determined that pioglitazone (PIO), a PPAR-γ agonist, decreases polycystic kidney and liver disease progression in the polycystic kidney rat, an orthologous model of human ARPKD. Daily treatment with 10 mg/kg PIO for 16 wk decreased kidney weight (% of body weight), renal cystic area, serum urea nitrogen, and the number of Ki67-, pERK1/2-, and pS6-positive cells in the kidney. There was also a decrease in liver weight (% of body weight), liver cystic area, fibrotic index, and the number of Ki67-, pERK1/2-, pERK5-, and TGF-β-positive cells in the liver. Taken together, these data suggest that PIO inhibits the progression of polycystic kidney and liver disease in a model of human ARPKD by inhibiting cell proliferation and fibrosis. These findings suggest that PPAR-γ agonists may have therapeutic value in the treatment of the renal and hepatic manifestations of ARPKD.     

Dietary potassium citrate does not harm the pcy mouse

Formation of multiple cysts in the kidneys occurs in several inherited diseases and often leads to terminal kidney failure. Because there is no definitive therapy to halt or slow the progression of renal cystic disease in people, numerous studies have examined possible therapies in animal models. Autosomal-dominant polycystic kidney disease (ADPKD) in the Han:SPRD rat is ameliorated when alkalinizing citrate salts are provided in drinking solutions. By contrast, pcy mice with cystic disease fare worse with the same treatment. We tested the hypothesis that pcy mice ingesting citrate salts in the feed would not be adversely affected by this treatment. Male homozygous pcy mice were given regular feed or 6% potassium citrate-supplemented feed and ad libitum access to water starting at 3 weeks of age. The survival curves of the treated and untreated mice were not significantly different. We conclude that treatment with potassium citrate in the feed does not affect the progression of renal cystic disease in the pcy mouse. This model closely resembles human adolescent nephronophthisis (NPHP3). Based on these findings, citrate treatment cannot be recommended for NPHP3. The fact that it did no harm, however, removes a significant barrier to its consideration as a therapy for ADPKD.     

Structural and functional analyses of liver cysts from the BALB/c-cpk mouse model of polycystic kidney disease

Liver cysts arising from hepatic bile ducts are a common extra-renal pathology associated with both autosomal dominant and recessive polycystic kidney disease in humans. To elucidate the functional and structural changes inherent in cyst formation and growth, hepatic bile duct epithelia were isolated from the BALB/ c-cpk mouse model of polycystic kidney disease. Light and transmission electron microscopy revealed substantial fibrosis in the basal lamina surrounding hepatic bile duct cysts isolated from heterozygous (BALB/c-cpk/+) and homozygous (BALB/c-cpk/cpk) animals. Scanning electron microscopy and length analysis of normal, precystic and cystic bile ducts provided the unique observation that primary cilia in cholangiocytes isolated from bile ducts and cysts of animals expressing the mutated cpk gene had lengths outside the minimal and maximal ranges of those in cells lining bile ducts of wild-type animals. Based on the hypothesis that PKD is one of several diseases characterized as ciliopathies, this abnormal variability in the length of the primary cilia may have functional implications. Electrophysiological analyses of freshly isolated cysts indicate that the amiloride-sensitive epithelial Na(+) channel (ENaC) is inactive/absent and cAMP-mediated anion secretion is the electrogenic transport process contributing to cyst fluid accumulation. Anion secretion can be stimulated by the luminal stimulation of adenylyl cyclase.     

Putative roles of cilia in polycystic kidney disease

The last 10 years has witnessed an explosion in research into roles of cilia in cystic renal disease. Cilia are membrane-enclosed finger-like projections from the cell, usually on the apical surface or facing into a lumen, duct or airway. Ten years ago, the major recognised functions related to classical “9 + 2” cilia in the respiratory and reproductive tracts, where co-ordinated beating clears secretions and assists fertilisation respectively. Primary cilia, which have a “9 + 0” arrangement lacking the central microtubules, were anatomical curiosities but several lines of evidence have implicated them in both true polycystic kidney disease and other cystic renal conditions: ranging from the homology between Caenorhabditis elegans proteins expressed on sensory cilia to mammalian polycystic kidney disease (PKD) 1 and 2 proteins, through the discovery that orpk cystic mice have structurally abnormal cilia to numerous recent studies wherein expression of nearly all cyst-associated proteins has been reported in the cilia or its basal body. Functional studies implicate primary cilia in mechanosensation, photoreception and chemosensation but it is the first of these which appears most important in polycystic kidney disease: in the simplest model, fluid flow across the apical surface of renal cells bends the cilia and induces calcium influx, and this is perturbed in polycystic kidney disease. Downstream effects include changes in cell differentiation and polarity. Pathways such as hedgehog and Wnt signalling may also be regulated by cilia. These data support important roles for cilia in the pathogenesis of cystic kidney diseases but one must not forget that the classic polycystic kidney disease proteins are expressed in several other locations where they may have equally important roles, such as in cell-cell and cell-matrix interactions, whilst it is not just aberrant cilia signalling that can lead to de-differentiation, loss of polarity and other characteristic features of polycystic kidney disease. Understanding how cilia fit into the other aspects of polycystic kidney disease biology is the challenge for the next decade. This article is part of a Special Issue entitled: Polycystic Kidney Disease.     

Contribution of renal innervation to hypertension in rat autosomal dominant polycystic kidney disease

The kidney has both afferent (sensory) and efferent (sympathetic) nerves that can influence renal function. Renal innervation has been shown to play a role in the pathogenesis of many forms of hypertension. Hypertension and flank pain are common clinical manifestations of autosomal dominant (AD) polycystic kidney disease (PKD). We hypothesize that renal innervation contributes to the hypertension and progression of cystic change in rodent PKD. In the present study, the contribution of renal innervation to hypertension and progression of renal histopathology and dysfunction was assessed in male Han:SPRD-Cy/+ rats with ADPKD. At 4 weeks of age, male offspring from crosses of heterozygotes (Cy/+) were randomized into either 1) bilateral surgical renal denervation, 2) surgical sham denervation control, or 3) nonoperated control groups. A midline laparotomy was performed to allow the renal denervation (i.e., physical stripping of the nerves and painting the artery with phenol/alcohol). Blood pressure (tail cuff method), renal function (BUN) and histology were assessed at 8 weeks of age. Bilateral renal denervation reduced the cystic kidney size, cyst volume density, systolic blood pressure, and improved renal function (BUN) as compared with nonoperated controls. Operated control cystic rats had kidney weights, cyst volume densities, systolic blood pressures, and plasma BUN levels that were intermediate between those in the denervated animals and the nonoperated controls. The denervated group had a reduced systolic blood pressure compared with the operated control animals, indicating that the renal innervations was a major contributor to the hypertension in this model of ADPKD. Renal denervation was efficacious in reducing some pathology, including hypertension, renal enlargement, and cystic pathology. However, sham operation also affected the cystic disease but to a lesser extent. We hypothesize that the amelioration of hypertension in Cy/+ rats was due to the effects of renal denervation on the renin angiotensin system.     

Advances in polycystic kidney disease

Until recently, the nature of the molecules involved in inherited cystic disease of the kidney remained unknown. These diseases are characterized by the development of multiple abnormal fluid-filled sacs or dilations in the kidney parenchyma, often leading to significant renal failure. The recent characterization of the PKD1 gene product and of other genes involved in murine polycystic models underscores the complexity of the pathways that lead to renal cystic disease.     

Apoptosis in polycystic kidney disease: involvement of caspases

Polycystic kidney disease (PKD) is characterized by the development of large renal cysts and progressive loss of renal function. Although the cause of the development of renal cysts is unknown, recent evidence suggests that excessive apoptosis occurs in PKD. With the use of terminal deoxynucleotidyl transferase dUTP nick-end labeling staining, we have confirmed the presence of apoptotic bodies in cystic kidneys of congenital polycystic kidney (cpk) disease mice carrying a homozygous mutation at 3 wk of age. Apoptosis was localized primarily to the interstitium with little evidence of cell death in cyst epithelium or noncystic tubules. In addition, we observed that the expression of various caspases, bax and bcl-2, was upregulated in cystic kidneys. With the use of various substrates in enzyme activity assays, we have demonstrated a greater than sevenfold increase in caspase 4 activity and a sixfold increase in caspase 3 activity. These data suggest that there is a caspase-dependent apoptosis pathway associated with PKD and support the hypothesis that apoptotic cell death contributes to cyst formation in PKD.     

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.     

Characterization of purinergic receptor expression in ARPKD cystic epithelia

Polycystic kidney diseases (PKDs) are a group of inherited nephropathies marked by formation of fluid-filled cysts along the nephron. Growing evidence suggests that in the kidney formation of cysts and alteration of cystic electrolyte transport are associated with purinergic signaling. PCK/CrljCrl-Pkhd1pck/CRL (PCK) rat, an established model of autosomal recessive polycystic kidney disease (ARPKD), was used here to test this hypothesis. Cystic fluid of PCK rats and their cortical tissues exhibited significantly higher levels of ATP compared to Sprague Dawley rat kidney cortical interstitium as assessed by highly sensitive ATP enzymatic biosensors. Confocal calcium imaging of the freshly isolated cystic monolayers revealed a stronger response to ATP in a higher range of concentrations (above 100 μM). The removal of extracellular calcium results in the profound reduction of the ATP evoked transient, which suggests calcium entry into the cyst-lining cells is occurring via the extracellular (ionotropic) P2X channels. Further use of pharmacological agents (α,β-methylene-ATP, 5-BDBD, NF449, isoPPADS, AZ10606120) and immunofluorescent labeling of isolated cystic epithelia allowed us to narrow down potential candidate receptors. In conclusion, our ex vivo study provides direct evidence that the profile of P2 receptors is shifted in ARPKD cystic epithelia in an age-related manner towards prevalence of P2X₄ and/or P2X₇ receptors, which opens new avenues for the treatment of this disease.     

Zystennieren – eine Übersicht

Cystic kidney diseases are a clinically and genetically heterogeneous group of disorders, representing one of the most frequent genetic conditions with a prevalence of about 1 in 1000. The most important forms include autosomal dominant polycystic kidney disease (ADPKD) caused by mutations in the PKD1 and PKD2 genes and the autosomal recessive polycystic kidney disease (ARPKD) caused by mutations in the PKHD1 gene. The proteins encoded by the involved genes are summarized as cystoproteins. On the cellular level, the majority of these cystoproteins co-localize in primary cilia, the basal body or the centrosome of renal epithelial cells. Inherited polycystic kidney diseases belong to the increasing number of reported ciliopathies which include many syndromic forms, e.g. Bardet-Biedl syndrome, Meckel syndrome and Joubert syndrome. Identifying the genetic defect can help establish the correct diagnosis, define the clinical prognosis and forms the basis for genetic counselling. In addition to establishing a clinical, ultrasonographic and morphological picture of the underlying kidney disease, the algorithm of genetic diagnosis should take the presence of further organ dysfunction or malformation as well as family history into consideration.     

Implementing Patch Clamp and Live Fluorescence Microscopy to Monitor Functional Properties of Freshly Isolated PKD Epithelium

Cyst initiation and expansion during polycystic kidney disease is a complex process characterized by abnormalities in tubular cell proliferation, luminal fluid accumulation and extracellular matrix formation. Activity of ion channels and intracellular calcium signaling are key physiologic parameters which determine functions of tubular epithelium. We developed a method suitable for real-time observation of ion channels activity with patch-clamp technique and registration of intracellular Ca²⁺ level in epithelial monolayers freshly isolated from renal cysts. PCK rats, a genetic model of autosomal recessive polycystic kidney disease (ARPKD), were used here for ex vivo analysis of ion channels and calcium flux. Described here is a detailed step-by-step procedure designed to isolate cystic monolayers and non-dilated tubules from PCK or normal Sprague Dawley (SD) rats, and monitor single channel activity and intracellular Ca²⁺ dynamics. This method does not require enzymatic processing and allows analysis in a native setting of freshly isolated epithelial monolayer. Moreover, this technique is very sensitive to intracellular calcium changes and generates high resolution images for precise measurements. Finally, isolated cystic epithelium can be further used for staining with antibodies or dyes, preparation of primary cultures and purification for various biochemical assays.     

Renal cystic disease in tuberous sclerosis: role of the polycystic kidney disease 1 gene.

Tuberous sclerosis is an autosomal dominant trait characterized by the development of hamartomatous growths in many organs. Renal cysts are also a frequent manifestation. Major genes for tuberous sclerosis and autosomal dominant polycystic kidney disease, TSC2 and PKD1, respectively, lie adjacent to each other at chromosome 16p13.3, suggesting a role for PKD1 in the etiology of renal cystic disease in tuberous sclerosis. We studied 27 unrelated patients with tuberous sclerosis and renal cystic disease. Clinical histories and radiographic features were reviewed, and renal function was assessed. We sought mutations at the TSC2 and PKD1 loci, using pulsed field- and conventional-gel electrophoresis and FISH. Twenty-two patients had contiguous deletions of TSC2 and PKD1. In 17 patients with constitutional deletions, cystic disease was severe, with early renal insufficiency. One patient with deletion of TSC2 and of only the 3' UTR of PKD1 had few cysts. Four patients were somatic mosaics; the severity of their cystic disease varied considerably. Mosaicism and mild cystic disease also were demonstrated in parents of 3 of the constitutionally deleted patients. Five patients without contiguous deletions had relatively mild cystic disease, 3 of whom had gross rearrangements of TSC2 and 2 in whom no mutation was identified. Significant renal cystic disease in tuberous sclerosis usually reflects mutational involvement of the PKD1 gene, and mosaicism for large deletions of TSC2 and PKD1 is a frequent phenomenon.     

A loss-of-function model for cystogenesis in human autosomal dominant polycystic kidney disease type 2.

Autosomal dominant polycystic kidney disease (ADPKD) is genetically heterogeneous, with at least three chromosomal loci (PKD1, PKD2, and PKD3) that account for the disease. Mutations in the PKD2 gene, on the long arm of chromosome 4, are expected to be responsible for approximately 15% of cases of ADPKD. Although ADPKD is a systemic disease, it shows a focal expression, because <1% of nephrons become cystic. A feasible explanation for the focal nature of events in PKD1, proposed on the basis of the two-hit theory, suggests that cystogenesis results from the inactivation of the normal copy of the PKD1 gene by a second somatic mutation. The aim of this study is to demonstrate that somatic mutations are present in renal cysts from a PKD2 kidney. We have studied 30 renal cysts from a patient with PKD2 in which the germline mutation was shown to be a deletion that encompassed most of the disease gene. Loss-of-heterozygosity (LOH) studies showed loss of the wild-type allele in 10% of cysts. Screening of six exons of the gene by SSCP detected eight different somatic mutations, all of them expected to produce truncated proteins. Overall, >/=37% of the cysts studied presented somatic mutations. No LOH for the PKD1 gene or locus D3S1478 were observed in those cysts, which demonstrates that somatic alterations are specific. We have identified second-hit mutations in human PKD2 cysts, which suggests that this mechanism could be a crucial event in the development of cystogenesis in human ADPKD-type 2.     

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.