Genetics and pathogenesis of polycystic kidney disease

Polycystic kidney disease in Han:SPRD Cy rats is associated with elevated expression and mislocalization of SamCystin

Polycystic kidney disease (PKD) in Han:SPRD Cy rats is caused by a missense mutation in Anks6 (also called Pkdr1), leading to an R823W substitution in SamCystin, a protein that contains ankyrin repeats and a sterile alpha motif (SAM). The cellular function of SamCystin and the role of the Cy (R823W) mutation in cyst formation are unknown. In normal SPRD rats, SamCystin was found to be expressed in proximal tubules and glomeruli; protein expression was highest at 7 days of age and declined by ∼50-60% at 45-84 days of age. In Cy/+ and Cy/Cy kidneys, expression of SamCystin was lower than in +/+ kidneys at 3 and 7 days but became elevated at 21 days. Immunohistochemical analysis revealed that SamCystin was distributed on the brush border of proximal tubules in normal rat kidneys. In Cy/+ kidneys, there were robust SamCystin staining in cyst-lining epithelial cells and loss of apical localization, and increased number of PCNA-positive cells in cyst-lining epithelia. Verapamil, an L-type Ca(2+) channel blocker, accelerated PKD progression in this model and caused a further increase in the expression and abnormal distribution of SamCystin. We conclude that aberrant expression and mislocalization of R823W SamCystin lead to increased cell proliferation and renal cyst formation.

Identification of the structural motif responsible for trimeric assembly of the C-terminal regulatory domains of polycystin channels PKD2L1 and PKD2

Polycystin 2-type cation channels PKD2 and PKD2L1 interact with polycystin 1-type proteins PKD1 and PKD1L3 respectively, to form receptor-cation-channel complexes. The PKD2L1-PKD1L3 complex perceives sour taste, whereas disruption of the PKD2-PKD1 complex, responsible for mechanosensation, leads to development of ADPKD (autosomal-dominant polycystic kidney disease). Besides modulating channel activity and related signalling events, the CRDs (C-terminal regulatory domains) of PKD2 and PKD2L1 play a central role in channel oligomerization. The present study investigates the aggregation state of purified full-length PKD2L1-CRD as well as truncations of CRDs from PKD2 channels. Far- and near-UV CD spectroscopy show that the full-length PKD2L1 CRD (PKD2L1-198) and the truncated PKD2 CRD (PKD2-244) are alpha-helical with no beta-sheet, the alpha-helix content agrees with sequence-based predictions, and some of its aromatic residues are in an asymmetric environment created at least by partially structured regions. Additionally, the CRD truncations exhibit an expected biochemical function by binding Ca2+ in a physiologically relevant range with Kd values of 2.8 muM for PKD2-244 and 0.51 muM for PKD2L1-198. Complimentary biophysical and biochemical techniques establish that truncations of the PKD2 and PKD2L1 CRDs are elongated molecules that assemble as trimers, and the trimeric aggregation state is independent of Ca2+ binding. Finally, we show that a common coiled-coil motif is sufficient and necessary to drive oligomerization of the PKD2 and PKD2L1 CRD truncations under study. Despite the moderate sequence identity (39%) between CRDs of PKD2 and PKD2L1, they both form trimers, implying that trimeric organization of CRDs may be true of all polycystin channels.

Inhibition of glucosylceramide accumulation results in effective blockade of polycystic kidney disease in mouse models

Polycystic kidney disease (PKD) represents a family of genetic disorders characterized by renal cystic growth and progression to kidney failure. No treatment is currently available for people with PKD, although possible therapeutic interventions are emerging. Despite genetic and clinical heterogeneity, PKDs have in common defects of cystic epithelia, including increased proliferation, apoptosis and activation of growth regulatory pathways. Sphingolipids and glycosphingolipids are emerging as major regulators of these cellular processes. We sought to evaluate the therapeutic potential for glycosphingolipid modulation as a new approach to treat PKD. Here we demonstrate that kidney glucosylceramide (GlcCer) and ganglioside GM3 levels are higher in human and mouse PKD tissue as compared to normal tissue, regardless of the causative mutation. Blockade of GlcCer accumulation with the GlcCer synthase inhibitor Genz-123346 effectively inhibits cystogenesis in mouse models orthologous to human autosomal dominant PKD (Pkd1 conditional knockout mice) and nephronophthisis (jck and pcy mice). Molecular analysis in vitro and in vivo indicates that Genz-123346 acts through inhibition of the two key pathways dysregulated in PKD: Akt protein kinase-mammalian target of rapamycin signaling and cell cycle machinery. Taken together, our data suggest that inhibition of GlcCer synthesis represents a new and effective treatment option for PKD.

Autosomal recessive polycystic kidney disease epithelial cell model reveals multiple basolateral epidermal growth factor receptor sorting pathways

We have discovered that multiple basolateral pathways mediate EGF receptor sorting in renal epithelial cells. The polycystic kidney disease allele in the BPK mouse model, Bicc1, interferes with one specific EGF receptor pathway, causing nonpolar delivery of the receptor without affecting overall cell polarity.

Sorting and maintenance of the EGF receptor on the basolateral surface of renal epithelial cells is perturbed in polycystic kidney disease and apical expression of receptors contributes to severity of disease. The goal of these studies was to understand the molecular basis for EGF receptor missorting using a well-established mouse model for the autosomal recessive form of the disease. We have discovered that multiple basolateral pathways mediate EGF receptor sorting in renal epithelial cells. The polycystic kidney disease allele in this model, Bicc1, interferes with one specific EGF receptor pathway without affecting overall cell polarity. Furthermore one of the pathways is regulated by a latent basolateral sorting signal that restores EGF receptor polarity in cystic renal epithelial cells via passage through a Rab11-positive subapical compartment. These studies give new insights to possible therapies to reconstitute EGF receptor polarity and function in order to curb disease progression. They also indicate for the first time that the Bicc1 gene that is defective in the mouse model used in these studies regulates cargo-specific protein sorting mediated by the epithelial cell specific clathrin adaptor AP-1B.

Renal ultrasonographic evaluation in children at risk of autosomal dominant polycystic kidney disease

BACKGROUND

To date, there are no criteria for diagnosing autosomal dominant polycystic kidney disease (ADPKD) in at-risk children 15 years or younger.

STUDY DESIGN

Longitudinal (retrospective cohort study).

SETTING & PARTICIPANTS

420 children (mean age, 8.3 +/- 4.2 years) with a family history of ADPKD were studied.

MEASUREMENTS

Renal ultrasonography was performed for cyst detection. Urine protein was measured using two 24-hour urine collections. Glomerular filtration rate was calculated using the Schwartz formula. Blood pressure measurements were performed in the arm with the highest blood pressure, using an appropriate cuff size. Standard 2-dimensional and Doppler echocardiography was performed for measuring left ventricular mass index.

PREDICTORS

None.

OUTCOME

Presence of renal cysts.

RESULTS

Renal cysts were detected in 193 children and no cysts were detected in 227 children. In children with renal cysts, 150 had bilateral and 43 had unilateral cysts. Children with bilateral cysts had larger kidneys and more hypertension than children with unilateral or no cysts. Follow-up in 77 children 15 years or younger showed bilateral cysts in 14 and unilateral cysts in 4 of the children who had no detectable renal cysts using ultrasonography at baseline. Similar follow-up of 26 children 15 years or younger with unilateral cysts detected at baseline showed bilateral cysts in 17 children using ultrasonography. By 15 years of age, 181 patients in the total group of 420 showed bilateral cysts. Overall, 193 of 304 children (63.4%) who had follow-up at any age developed bilateral cysts detected using ultrasonography.

LIMITATIONS

Follow-up unavailable for all participants.

CONCLUSION

The present results in 420 at-risk children with ADPKD 15 years or younger detected bilateral renal cysts using ultrasonography in 181 of the children who had a family history of this genetic disease.

Glomerulocystic kidney: one hundred-year perspective

CONTEXT

Glomerular cysts, defined as Bowman space dilatation greater than 2 to 3 times normal size, are found in disorders of diverse etiology and with a spectrum of clinical manifestations. The term glomerulocystic kidney (GCK) refers to a kidney with greater than 5% cystic glomeruli. Although usually a disease of the young, GCK also occurs in adults.

OBJECTIVE

To assess the recent molecular genetics of GCK, review our files, revisit the literature, and perform in silico experiments.

DATA SOURCES

We retrieved 20 cases from our files and identified more than 230 cases published in the literature under several designations.

CONCLUSIONS

Although GCK is at least in part a variant of autosomal dominant or recessive polycystic kidney disease (PKD), linkage analysis has excluded PKD-associated gene mutations in many cases of GCK. A subtype of familial GCK, presenting with cystic kidneys, hyperuricemia, and isosthenuria is due to uromodullin mutations. In addition, the familial hypoplastic variant of GCK that is associated with diabetes is caused by mutations in TCF2, the gene encoding hepatocyte nuclear factor-1beta. The term GCK disease (GCKD) should be reserved for the latter molecularly recognized/inherited subtypes of GCK (not to include PKD). Review of our cases, the literature, and our in silico analysis of the overlapping genetic entities integrates established molecular-genetic functions into a proposed model of glomerulocystogenesis; a classification scheme emerged that (1) emphasizes the clinical significance of glomerular cysts, (2) provides a pertinent differential diagnosis, and (3) suggests screening for probable mutations.

Cystic diseases of the kidney: molecular biology and genetics

CONTEXT

Cystic diseases of the kidney are a very heterogeneous group of renal inherited conditions, with more than 33 genes involved and encompassing X-linked, autosomal dominant, and autosomal recessive inheritance. Although mostly monogenic with mendelian inheritance, there are clearly examples of oligogenic inheritance, such as 3 mutations in 2 genes, while the existence of genetic modifiers is perhaps the norm, based on the extent of variable expressivity and the broad spectrum of symptoms.

OBJECTIVES

To present in the form of a mini review the major known cystic diseases of the kidney for which genes have been mapped or cloned and characterized, with some information on their cellular and molecular biology and genetics, and to pay special attention to commenting on the issues of molecular diagnostics, in view of the genetic and allelic heterogeneity. Data Sources.-We used major reviews that make excellent detailed presentation of the various diseases, as well as original publications.

CONCLUSIONS

There is already extensive genetic heterogeneity in the group of cystic diseases of the kidney; however, there are still many more genes awaiting to be discovered that are implicated or mutated in these diseases. In addition, the synergism and interaction among this repertoire of gene products is largely unknown, while a common unifying aspect is the expression of nearly all of them at the primary cilium or the basal body. A major interplay of functions is anticipated, while mutations in all converge in the unifying phenotype of cyst formation.

Genetic kidney diseases

Knowledge of the primary cause of a disease is essential for elucidation of its mechanisms, and for adequate classification, prognosis, and treatment. Recently, the causes of many kidney diseases have been shown to be single-gene defects-eg, steroid-resistant nephrotic syndrome, which is caused by podocin mutations in about 25% of children and nearly 15% of adults with the disease. Knowledge of a disease-causing mutation in a single-gene disorder represents one of the most robust diagnostic examples of personalised medicine because the mutation conveys an almost 100% risk of developing the disease by a defined age. Whereas single-gene diseases are rare disorders, polygenic risk alleles arise in common adult-onset diseases. In this Review, I will discuss prominent renal single-gene kidney disorders, and polygenic risk alleles of common disorders. I delineate how emerging techniques of total exome capture and large-scale sequencing will assist molecular genetic diagnosis, prognosis, and specific treatment, and lead to an improved elucidation of disease mechanisms, thus enabling development of new targeted drugs.

Diagnosis and screening of autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is the most common inherited cause of kidney failure and accounts for approximately 5% of ESRD population in the United States. The disorder is characterized by the focal and sporadic development of renal cysts, which increase in size and number with age. Mutations of PKD1 and PKD2 account for most of the cases. Although the clinical manifestations of both gene types overlap completely, PKD1 is associated with more severe disease than PKD2, with larger kidneys and earlier onset of ESRD. In general, renal ultrasonography is commonly used for the diagnosis of ADPKD, and age-dependent criteria have been defined for subjects at risk of PKD1. However, the utility of the PKD1 ultrasound criteria in the clinic setting is unclear because their performance characteristics have not been defined for the milder PKD2 and the gene type for most test subjects is unknown. Recently, highly predictive ultrasound diagnostic criteria have been derived for at-risk subjects of unknown gene type. Additionally, both DNA linkage or gene-based direct sequencing are now available for the diagnosis of ADPKD, especially in subjects with equivocal imaging results, subjects with a negative or indeterminate family history, or in younger at-risk individuals being evaluated as potential living-related kidney donors. Here, we review the clinical utilities and limitations of both imaging- and molecular-based diagnostic tests and outline our approach for the evaluation of individuals suspected to have ADPKD.

Unravelling the genetic basis of renal diseases; from single gene to multifactorial disorders

Chronic kidney disease is common with up to 5% of the adult population reported to have an estimated glomerular filtration rate of < 60 ml/min/1.73 m(2). A large number of pathogenic mutations have been identified that are responsible for 'single gene' renal disorders, such as autosomal dominant polycystic kidney disease and X-linked Alport syndrome. These single gene disorders account for < 15% of the burden of end-stage renal disease that requires dialysis or kidney transplantation. It has proved more difficult to identify the genetic susceptibility underlying common, complex, multifactorial kidney conditions, such as diabetic nephropathy and hypertensive nephrosclerosis. This review describes success to date and explores strategies currently employed in defining the genetic basis for a number of renal disorders. The complementary use of linkage studies, candidate gene and genome-wide association analyses are described and a collation of renal genetic resources highlighted.

Carboxy terminal tail of polycystin-1 regulates localization of TSC2 to repress mTOR

Autosomal dominant polycystic kidney disease (ADPKD) is a commonly inherited renal disorder caused by defects in the PKD1 or PKD2 genes. ADPKD is associated with significant morbidity, and is a major underlying cause of end-stage renal failure (ESRF). Commonly, treatment options are limited to the management of hypertension, cardiovascular risk factors, dialysis, and transplantation when ESRF develops, although several new pharmacotherapies, including rapamycin, have shown early promise in animal and human studies. Evidence implicates polycystin-1 (PC-1), the gene product of the PKD1 gene, in regulation of the mTOR pathway. Here we demonstrate a mechanism by which the intracellular, carboxy-terminal tail of polycystin-1 (CP1) regulates mTOR signaling by altering the subcellular localization of the tuberous sclerosis complex 2 (TSC2) tumor suppressor, a gatekeeper for mTOR activity. Phosphorylation of TSC2 at S939 by AKT causes partitioning of TSC2 away from the membrane, its GAP target Rheb, and its activating partner TSC1 to the cytosol via 14-3-3 protein binding. We found that TSC2 and a C-terminal polycystin-1 peptide (CP1) directly interact and that a membrane-tethered CP1 protects TSC2 from AKT phosphorylation at S939, retaining TSC2 at the membrane to inhibit the mTOR pathway. CP1 decreased binding of 14-3-3 proteins to TSC2 and increased the interaction between TSC2 and its activating partner TSC1. Interestingly, while membrane tethering of CP1 was required to activate TSC2 and repress mTOR, the ability of CP1 to inhibit mTOR signaling did not require primary cilia and was independent of AMPK activation. These data identify a unique mechanism for modulation of TSC2 repression of mTOR signaling via membrane retention of this tumor suppressor, and identify PC-1 as a regulator of this downstream component of the PI3K signaling cascade.

Challenges of genomics and proteomics in nephrology

An increasing number of patients suffering from renal diseases and limitations in standard diagnostic and therapeutic approaches has created an intense interest in applying genomics and proteomics in the field of nephrology. Genomics has provided a vast amount of information, linking the gene activity with disease. However, proteomic technologies allow us to understand proteins and their modifications, elucidating properties of cellular behavior that may not be reflected in analysis of gene expression. The application of these innovative approaches has recently yielded the promising new urinary biomarkers for acute kidney injury and chronic kidney disease, thus providing a better insight in renal pathophysiology and establishing the basis for new therapeutic strategies. Despite significant improvements in therapeutics, the mortality and morbidity associated with acute renal failure (ARF) remain high. The lack of early markers for ARF causes an unacceptable delay in initiating therapy. These biomarker panels will probably be useful for assessing the duration and severity of ARF, and for predicting progression and adverse clinical outcomes. Kidney failure leads to the uremic syndrome characterized by accumulation of uremic toxins, which are normally cleared by the kidneys. Proteomics has gained considerable interest in this field, as a new and promising analytical approach to identify new uremic toxins. The urinary proteome as a tool for biomarker discovery is still in its early phase. A major challenge will be the integration of proteomics with genomics data and their functional interpretation in conjunction with clinical results and epidemiology.

A mitotic transcriptional switch in polycystic kidney disease

Hepatocyte nuclear factor-1beta (HNF-1beta) is a transcription factor required for the expression of several renal cystic genes and whose prenatal deletion leads to polycystic kidney disease (PKD). We show here that inactivation of Hnf1b from postnatal day 10 onward does not elicit cystic dilations in tubules after their proliferative morphogenetic elongation is over. Cystogenic resistance is intrinsically linked to the quiescent state of cells. In fact, when Hnf1b deficient quiescent cells are forced to proliferate by an ischemia-reperfusion injury, they give rise to cysts, owing to loss of oriented cell division. Remarkably, in quiescent cells, the transcription of crucial cystogenic target genes is maintained even in the absence of HNF-1beta. However, their expression is lost as soon as cells proliferate and the chromatin of target genes acquires heterochromatin marks. These results unveil a previously undescribed aspect of gene regulation. It is well established that transcription is shut off during the mitotic condensation of chromatin. We propose that transcription factors such as HNF-1beta might be involved in reprogramming gene expression after transcriptional silencing is induced by mitotic chromatin condensation. Notably, HNF-1beta remains associated with the mitotically condensed chromosomal barrels. This association suggests that HNF-1beta is a bookmarking factor that is necessary for reopening the chromatin of target genes after mitotic silencing.

Conditional mutation of Pkd2 causes cystogenesis and upregulates beta-catenin

Loss of polycystin-2 (PC2) in mice (Pkd2(-/-)) results in total body edema, focal hemorrhage, structural cardiac defects, abnormal left-right axis, hepatorenal and pancreatic cysts, and embryonic lethality. The molecular mechanisms by which loss of PC2 leads to these phenotypes remain unknown. We generated a model to allow targeted Pkd2 inactivation using the Cre-loxP system. Global inactivation of Pkd2 produced a phenotype identical to Pkd2(-/-) mice with undetectable PC2 protein and perinatal lethality. Using various Cre mouse lines, we found that kidney, pancreas, or time-specific deletion of Pkd2 led to cyst formation. In addition, we developed an immortalized renal collecting duct cell line with inactive Pkd2; these cells had aberrant cell-cell contact, ciliogenesis, and tubulomorphogenesis. They also significantly upregulated beta-catenin, axin2, and cMyc. Our results suggest that loss of PC2 disrupts normal behavior of renal epithelial cells through dysregulation of beta-catenin-dependent signaling, revealing a potential role for this signaling pathway in PC2-associated ADPKD.

The role of the mammalian target of rapamycin (mTOR) in renal disease

The mammalian target of rapamycin (mTOR) is a serine/threonine kinase that plays a pivotal role in mediating cell size and mass, proliferation, and survival. mTOR has also emerged as an important modulator of several forms of renal disease. mTOR is activated after acute kidney injury and contributes to renal regeneration and repair. Inhibition of mTOR with rapamycin delays recovery of renal function after acute kidney injury. Activation of mTOR within the kidney also occurs in animal models of diabetic nephropathy and other causes of progressive kidney disease. Rapamycin ameliorates several key mechanisms believed to mediate changes associated with the progressive loss of GFR in chronic kidney disease. These include glomerular hypertrophy, intrarenal inflammation, and interstitial fibrosis. mTOR also plays an important role in mediating cyst formation and enlargement in autosomal dominant polycystic kidney disease. Inhibition of mTOR by rapamycin or one of its analogues represents a potentially novel treatment for autosomal dominant polycystic kidney disease. Finally, inhibitors of mTOR improve survival in patients with metastatic renal cell carcinoma.

The CIL-1 PI 5-phosphatase localizes TRP Polycystins to cilia and activates sperm in C. elegans

BACKGROUND

C. elegans male sexual behaviors include chemotaxis and response to hermaphrodites, backing, turning, vulva location, spicule insertion, and sperm transfer, culminating in cross-fertilization of hermaphrodite oocytes with male sperm. The LOV-1 and PKD-2 transient receptor potential polycystin (TRPP) complex localizes to ciliated endings of C. elegans male-specific sensory neurons and mediates several aspects of male mating behavior. TRPP complex ciliary localization and sensory function are evolutionarily conserved. A genetic screen for C. elegans mutants with PKD-2 ciliary localization (Cil) defects led to the isolation of a mutation in the cil-1 gene.

RESULTS

Here, we report that a phosphoinositide (PI) 5-phosphatase, CIL-1, regulates TRPP complex ciliary receptor localization and sperm activation. cil-1 does not regulate the localization of other ciliary proteins, including intraflagellar transport (IFT) components, sensory receptors, or other TRP channels in different cell types. Rather, cil-1 specifically controls TRPP complex trafficking in male-specific sensory neurons and does so in a cell-autonomous fashion. In these cells, cil-1 is required for normal PI(3)P distribution, indicating that a balance between PI(3,5)P2 and PI(3)P is important for TRPP localization. cil-1 mutants are infertile because of sperm activation and motility defects. In sperm, the CIL-1 5-phosphatase and a wortmannin-sensitive PI 3-kinase act antagonistically to regulate the conversion of sessile spermatids into motile spermatozoa, implicating PI(3,4,5)P3 signaling in nematode sperm activation.

CONCLUSION

Our studies identify the CIL-1 5-phosphatase as a key regulator of PI metabolism in cell types that are important in several aspects of male reproductive biology.

Aberrant planar cell polarity induced by urinary tract obstruction

Flow sensing by primary cilia of the epithelial cells is involved in cystogenesis in polycystic kidney disease. We investigate whether a similar mechanism applies to the pathogenesis of cyst-like tubular dilatation induced by ureteral obstruction in mice. Robust proliferation occurs in the obstructed tubules when urine flow is interrupted as well as in the repairing tubules when urine flow is reestablished after relief of the obstruction, suggesting a urine flow-independent mechanism of proliferation. In the urothelium, proliferation is only detected above the obstruction, although urine flow ceased both above and below the obstruction. Our results support mechanical strain- rather than flow-mediated proliferation in obstructive uropathy. To understand the mechanism of cell proliferation leading to increased tubular diameter in cyst-like tubular dilatation, we examine planar cell polarity (PCP), which is necessary for oriented cell division and maintenance of tubular diameter. In dilated tubules, the orientation of cell division is randomized, atypical PKC (aPKC) is mislocalized, and the pattern of the expression of a core PCP protein, Frizzled3 (Fz3), is altered. In addition, the level of Fz3 expression is increased. These results indicate that aberrant PCP may contribute to cyst-like tubular dilatation in obstructive uropathy. Interestingly, the orientation of cell division, localization of aPKC, and Fz3 expression return to normal when obstruction is relieved, which suggest a role of normal PCP signaling in tubular repair.

Loss of Tsc1, but not Pten, in renal tubular cells causes polycystic kidney disease by activating mTORC1

Tuberous sclerosis complex (TSC) is a genetic disorder linked to mutations of either the TSC1 or TSC2 gene, which encode proteins that form a complex to negatively regulate mammalian target of rapamycin complex 1 (mTORC1). Clinically, a small percentage of TSC patients develop severe infantile polycystic kidney disease (PKD), which is believed to be caused by deletion of the contiguous TSC2 and PKD1 genes on human chromosome 16. Recent studies have implicated the TSC/mTORC1 signaling pathway in PKD, but how dysfunction of the TSC/mTORC1 pathway induces PKD is not clear. We report a PKD mouse model created by knocking out Tsc1 in a subset of renal tubular cells. Extensive renal cyst formation in these mice is accompanied by broadly elevated mTORC1 activity in both cell autonomous and non-cell autonomous compartments. Furthermore, cyst development requires mTORC1 activation, as low dosage of rapamycin administration effectively blocks cyst formation. Interestingly, disruption of Pten, an upstream regulator of TSC1/TSC2, in the same cells, does not lead to PKD seemingly due to limited activation of mTORC1, suggesting that PTEN may not be a major upstream regulator of TSC/mTORC1 during early postnatal kidney development.

Advances in the pathogenesis and treatment of polycystic kidney disease

PURPOSE OF REVIEW

Polycystic kidney disease (PKD) is the most common genetic cause of chronic renal failure. Mouse models of PKD, especially those with mutations in genes that are orthologous to human disease genes, have provided insights into the pathogenesis of cyst formation and advanced the preclinical testing of new drugs.

RECENT FINDINGS

PKD is a ciliopathy that arises from abnormalities in the primary cilium, a sensory organelle present on the surface of most cells. The primary cilium is required for the maintenance of planar cell polarity, which regulates tubular diameter. Acute kidney injury stimulates cell proliferation and promotes cyst formation in a mouse model of PKD. Studies of signaling pathways that are perturbed in PKD have identified new potential therapeutic targets. Drugs that have shown beneficial effects in orthologous animal models of PKD include tolvaptan, octreotide, src inhibitors, CFTR inhibitors, pioglitazone, etanercept, and triptolide.

SUMMARY

Abnormalities in the primary cilium perturb signaling pathways that regulate renal epithelial cell growth and differentiation and lead to the formation of kidney cysts. Acute kidney injury promotes cyst formation and may underlie the variability in disease progression that is observed in affected individuals. Several promising new therapeutic agents that have been validated in orthologous animal models have entered clinical trials in humans.

Phosphoinositide regulation of non-canonical transient receptor potential channels

Transient receptor potential (TRP) channels are involved in a wide range of physiological processes, and characterized by diverse activation mechanisms. Phosphoinositides, especially phosphatidylinositol 4,5-bisphosphate [PIP(2), or PtdIns(4,5)P(2)] recently emerged as regulators of many TRP channels. Several TRP channels require PIP(2) for activity, and depletion of the lipid inhibits them. For some TRP channels, however, phosphoinositide regulation seems more complex, both activating and inhibitory effects have been reported. This review will discuss phosphoinositide regulation of members of the TRPM (Melastatin), TRPV (Vanilloid), TRPA (Ankyrin) and TRPP (Polycystin) families. Lipid regulation of TRPC (Canonical) channels is discussed elsewhere in this volume.

Regulation of cell proliferation and differentiation in the kidney

The mammalian cut proteins are a broadly expressed family of nuclear transcription factors related to the Drosophila protein cut. One member of the cut family, Cux1, has been shown to function as a cell cycle dependent transcription factor, regulating the expression of a number of cell cycle regulatory proteins. Cux1 expression is developmentally regulated in multiple tissues suggesting an important regulatory function. Cux1 exists as multiple isoforms that arise from proteolytic processing of a 200 kD protein or use of an alternate promoter. Several mouse models of Cux1 have been generated that suggest important roles for this gene in cell cycle regulation during hair growth, lung development and maturation, and genitourinary tract development. Moreover, the aberrant expression of Cux1 may contribute to diseases such as polycystic kidney disease and cancer. In this review, we will focus on the phenotypes observed in the five existing transgenic mouse models of Cux1, and discuss the role of Cux1 in kidney development and disease.

Experience with autosomal dominant polycystic kidney disease in patients before and after renal transplantation: a 7-year observation

OBJECTIVE

Autosomal dominant polycystic kidney disease (ADPKD) is characterized by the presence of multiple cysts in both kidneys. Symptoms of the disease may arise either from the presence of cysts or from increasing loss of kidney function. First symptoms usually appear in the third decade of life: lumbar pain, urinary tract infections, arterial hypertension, or renal colic due to cyst rupture or coexistent nephrolithiasis. An early diagnosis, male gender, large kidneys by sonography, arterial hypertension, hematuria, and urinary tract infections are predictive factors of a faster progression of the disease. Our aim was to establish the indications for nephrectomy among symptomatic ADPKD patients before kidney transplantation and to assess the risks of posttransplantation complications among ADPKD patients without nephrectomy.

PATIENTS AND METHODS

The observed group consisted of 183 patients with ADPKD among whom 50 (27.3%) underwent kidney transplantation during a 7-year observation period (2000-2007). Among those subjects were 3 groups: (I) nephrectomy preceding transplantation; (II) nephrectomy during kidney transplantation; and (III) without nephrectomy.

RESULTS

Among group I before transplantation we observed: arterial hemorrhage, wound infections, and splenectomy 4 weeks after ADPKD nephrectomy; afterward we observed: urinary tract infections and contralateral cyst infection. Among group II we only observed 1 case of wound infection. Among group III we observed: ascending urinary tract infections, cyst infections, and cyst hemorrhage. Cyst hemorrhage and cyst infections led mainly to ADPKD kidney nephrectomy. During the observation time, 80.95% of grafts were functioning.

CONCLUSIONS

Unilateral nephrectomy is a well-founded preliminary surgical treatment before kidney transplantation. Bilateral nephrectomy before or during transplantation eliminates ADPKD complications and does not significantly increase general complications. The greatest numbers of complications and of graft losses were observed among the group without pretransplantation nephrectomy.

Polycystic kidney disease in adult Brazilian agoutis (Dasyprocta leporina)

During the last 21 years, 7 adult captive Brazilian agoutis (Dasyprocta leporina) from 4 different zoologic gardens were necropsied and histologically examined at the Leibniz Institute for Zoo and Wildlife Research, Berlin, Germany. All animals had polycystic kidney disease as the major pathologic change. Except in 1 case, no clinical signs were recognized prior to death. The animals had macroscopic bilateral alterations of the kidneys ranging from granulated surfaces to severe polycystic changes. Microscopic examination revealed multifocal to generalized, moderate to severe cystic dilatations of Bowman's capsules and renal tubules, moderate mesangial and capsular proliferation of the renal corpuscles, mild interstitial fibrosis, and mild to moderate interstitial lympho-plasmacytic infiltrations. Little information is known about the genetic relationships of these animals, but breeding practice indicates a high possibility of inbred agouti zoo populations in Germany. This is the first report on polycystic kidney disease in Brazilian agoutis with possible genetic background.

Evidence for pathogenicity of atypical splice mutations in autosomal dominant polycystic kidney disease

BACKGROUND AND OBJECTIVES

Mutation-based molecular diagnostics of autosomal dominant polycystic kidney disease (ADPKD) is complicated by locus and allelic heterogeneity, large multi-exon gene structure and duplication in PKD1, and a high level of unclassified variants. Comprehensive screening of PKD1 and PKD2 by two recent studies have shown that atypical splice mutations account for 3.5% to 5% of ADPKD. We evaluated the role of bioinformatic prediction of atypical splice mutations and determined the pathogenicity of an atypical PKD2 splice variant from a multiplex ADPKD (TOR101) family.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Using PubMed, we identified 17 atypical PKD1 and PKD2 splice mutations. We found that bioinformatics analysis was often useful for evaluating the pathogenicity of these mutations, although RT-PCR is needed to provide the definitive proof.

RESULTS

Sequencing of both PKD1 and PKD2 in an affected subject of TOR101 failed to identify a definite mutation, but revealed several UCVs, including an atypical PKD2 splice variant. Linkage analysis with microsatellite markers indicated that TOR101 was PKD2-linked and IVS8 + 5G-->A was shown to cosegregate only with affected subjects. RT-PCR of leukocyte mRNA from an affected subject using primers from exons 7 and 9 revealed six splice variants that resulted from activation of different combinations of donor and acceptor cryptic splice sites, all terminating with premature stop codons.

CONCLUSIONS

The data provide strong evidence that IVS8 + 5G-->A is a pathogenic mutation for PKD2. This case highlights the importance of functional analysis of UCVs.

Nephronophthisis: disease mechanisms of a ciliopathy

Nephronophthisis (NPHP), a recessive cystic kidney disease, is the most frequent genetic cause of end-stage kidney disease in children and young adults. Positional cloning of nine genes (NPHP1 through 9) and functional characterization of their encoded proteins (nephrocystins) have contributed to a unifying theory that defines cystic kidney diseases as "ciliopathies." The theory is based on the finding that all proteins mutated in cystic kidney diseases of humans or animal models are expressed in primary cilia or centrosomes of renal epithelial cells. Primary cilia are sensory organelles that connect mechanosensory, visual, and other stimuli to mechanisms of epithelial cell polarity and cell-cycle control. Mutations in NPHP genes cause defects in signaling mechanisms that involve the noncanonical Wnt signaling pathway and the sonic hedgehog signaling pathway, resulting in defects of planar cell polarity and tissue maintenance. The ciliary theory explains the multiple organ involvement in NPHP, which includes retinal degeneration, cerebellar hypoplasia, liver fibrosis, situs inversus, and mental retardation. Positional cloning of dozens of unknown genes that cause NPHP will elucidate further signaling mechanisms involved. Nephrocystins are highly conserved in evolution, thereby allowing the use of animal models to develop future therapeutic approaches.

Microarray-based approach identifies microRNAs and their target functional patterns in polycystic kidney disease

Background

MicroRNAs (miRNAs) play key roles in mammalian gene expression and several cellular processes, including differentiation, development, apoptosis and cancer pathomechanisms. Recently the biological importance of primary cilia has been recognized in a number of human genetic diseases. Numerous disorders are related to cilia dysfunction, including polycystic kidney disease (PKD). Although involvement of certain genes and transcriptional networks in PKD development has been shown, not much is known how they are regulated molecularly.

Results

Given the emerging role of miRNAs in gene expression, we explored the possibilities of miRNA-based regulations in PKD. Here, we analyzed the simultaneous expression changes of miRNAs and mRNAs by microarrays. 935 genes, classified into 24 functional categories, were differentially regulated between PKD and control animals. In parallel, 30 miRNAs were differentially regulated in PKD rats: our results suggest that several miRNAs might be involved in regulating genetic switches in PKD. Furthermore, we describe some newly detected miRNAs, miR-31 and miR-217, in the kidney which have not been reported previously. We determine functionally related gene sets, or pathways to reveal the functional correlation between differentially expressed mRNAs and miRNAs.

Conclusion

We find that the functional patterns of predicted miRNA targets and differentially expressed mRNAs are similar. Our results suggest an important role of miRNAs in specific pathways underlying PKD.

Cyst formation in kidney via B-Raf signaling in the PKD2 transgenic mice

The pathogenic mechanisms of human autosomal dominant polycystic kidney disease (ADPKD) have been well known to include the mutational inactivation of PKD2. Although haploinsufficiency and loss of heterozygosity at the Pkd2 locus can cause cyst formation in mice, polycystin-2 is frequently expressed in the renal cyst of human ADPKD, raising the possibility that deregulated activation of PKD2 may be associated with the cystogenesis of human ADPKD. To determine whether increased PKD2 expression is physiologically pathogenic, we generated PKD2-overexpressing transgenic mice. These mice developed typical renal cysts and an increase of proliferation and apoptosis, which are reflective of the human ADPKD phenotype. These manifestations were first observed at six months, and progressed with age. In addition, we found that ERK activation was induced by PKD2 overexpression via B-Raf signaling, providing a possible molecular mechanism of cystogenesis. In PKD2 transgenic mice, B-Raf/MEK/ERK sequential signaling was up-regulated. Additionally, the transgenic human polycystin-2 partially rescues the lethality of Pkd2 knock-out mice and therefore demonstrates that the transgene generated a functional product. Functional strengthening or deregulated activation of PKD2 may be a direct cause of ADPKD. The present study provides evidence for an in vivo role of overexpressed PKD2 in cyst formation. This transgenic mouse model should provide new insights into the pathogenic mechanism of human ADPKD.

Deletion of IFT20 in the mouse kidney causes misorientation of the mitotic spindle and cystic kidney disease

Primary cilia project from the surface of most vertebrate cells and are thought to be sensory organelles. Defects in primary cilia lead to cystic kidney disease, although the ciliary mechanisms that promote and maintain normal renal function remain incompletely understood. In this work, we generated a floxed allele of the ciliary assembly gene Ift20. Deleting this gene specifically in kidney collecting duct cells prevents cilia formation and promotes rapid postnatal cystic expansion of the kidney. Dividing collecting duct cells in early stages of cyst formation fail to properly orient their mitotic spindles along the tubule, whereas nondividing cells improperly position their centrosomes. At later stages, cells lacking cilia have increased canonical Wnt signaling and increased rates of proliferation. Thus, IFT20 functions to couple extracellular events to cell proliferation and differentiation.

Identification of genes involved in the ciliary trafficking of C. elegans PKD-2

Ciliary membrane proteins are important extracellular sensors, and defects in their localization may have profound developmental and physiological consequences. To determine how sensory receptors localize to cilia, we performed a forward genetic screen and identified 11 mutants with defects in the ciliary localization (cil) of C. elegans PKD-2, a transient receptor potential polycystin (TRPP) channel. Class A cil mutants exhibit defects in PKD-2::GFP somatodendritic localization while Class B cil mutants abnormally accumulate PKD-2::GFP in cilia. Further characterization reveals that some genes mutated in cil mutants act in a tissue-specific manner while others are likely to play more general roles in such processes as intraflagellar transport (IFT). To this end, we identified a Class B mutation that disrupts the function of the cytoplasmic dynein light intermediate chain gene xbx-1. Identification of the remaining mutations will reveal novel molecular pathways required for ciliary receptor localization and provide further insight into mechanisms of ciliary signaling.

Unified criteria for ultrasonographic diagnosis of ADPKD

Individuals who are at risk for autosomal dominant polycystic kidney disease are often screened by ultrasound using diagnostic criteria derived from individuals with mutations in PKD1. Families with mutations in PKD2 typically have less severe disease, suggesting a potential need for different diagnostic criteria. In this study, 577 and 371 at-risk individuals from 58 PKD1 and 39 PKD2 families, respectively, were assessed by renal ultrasound and molecular genotyping. Using sensitivity data derived from genetically affected individuals and specificity data derived from genetically unaffected individuals, various diagnostic criteria were compared. In addition, data sets were created to simulate the PKD1 and PKD2 case mix expected in practice to evaluate the performance of diagnostic criteria for families of unknown genotype. The diagnostic criteria currently in use performed suboptimally for individuals with mutations in PKD2 as a result of reduced test sensitivity. In families of unknown genotype, the presence of three or more (unilateral or bilateral) renal cysts is sufficient for establishing the diagnosis in individuals aged 15 to 39 y, two or more cysts in each kidney is sufficient for individuals aged 40 to 59 y, and four or more cysts in each kidney is required for individuals > or = 60 yr. Conversely, fewer than two renal cysts in at-risk individuals aged > or = 40 yr is sufficient to exclude the disease. These unified diagnostic criteria will be useful for testing individuals who are at risk for autosomal dominant polycystic kidney disease in the usual clinical setting in which molecular genotyping is seldom performed.

Retinoic acid-dependent activation of the polycystic kidney disease-1 (PKD1) promoter

The retinoic acids all-trans retinoic acid (AT-RA) and 9-cis retinoic acid (9C-RA) and the retinoic acid receptors RAR and RXR significantly induce transcriptional activity from a 200-bp PKD1 proximal promoter in transfected mammalian cells. This PKD1 promoter region contains Ets, p53, and GC box motifs, but lacks a canonical RAR/RXR motif. Mutagenesis of the Ets sites did not affect RA induction. In contrast, GC box mutations completely blocked stimulation by AT-RA and by RXRbeta or RARbeta. Mithramycin A, which prevents Sp1 binding, significantly reduced basal promoter activity and suppressed upregulation by AT-RA and RXR. The 200-bp proximal promoter could not be induced by AT-RA in Drosophila SL2 cells, which lack Sp1, but could be activated in these cells transfected with exogenous Sp1. Small interfering RNA knockdown of Sp1 in mammalian cells completely blocked RXRbeta upregulation of the promoter. These data indicate that induction of the PKD1 promoter by retinoic acid is mediated through Sp1 elements. RT-PCR showed that AT-RA treatment of HEK293T cells increased the levels of endogenous PKD1 RNA, and chromatin immunoprecipitation showed the presence of both RXR and Sp1 at the PKD1 proximal promoter. These results suggest that retinoids and their receptors may play a role in PKD1 gene regulation.

Acceleration of polycystic kidney disease progression in cpk mice carrying a deletion in the homeodomain protein Cux1

Polycystic kidney diseases (PKD) are inherited as autosomal dominant (ADPKD) or autosomal recessive (ARPKD) traits and are characterized by progressive enlargement of renal cysts. Aberrant cell proliferation is a key feature in the progression of PKD. Cux1 is a homeobox gene that is related to Drosophila cut and is the murine homolog of human CDP (CCAAT Displacement Protein). Cux1 represses the cyclin kinase inhibitors p21 and p27, and transgenic mice ectopically expressing Cux1 develop renal hyperplasia. However, Cux1 transgenic mice do not develop PKD. Here, we show that a 246 amino acid deletion in Cux1 accelerates PKD progression in cpk mice. Cystic kidneys isolated from 10-day-old cpk/Cux1 double mutant mice were significantly larger than kidneys from 10-day-old cpk mice. Moreover, renal function was significantly reduced in the Cux1 mutant cpk mice, compared with cpk mice. The mutant Cux1 protein was ectopically expressed in cyst-lining cells, where expression corresponded to increased cell proliferation and apoptosis, and a decrease in expression of the cyclin kinase inhibitors p27 and p21. While the mutant Cux1 protein altered PKD progression, kidneys from mice carrying the mutant Cux1 protein alone were phenotypically normal, suggesting the Cux1 mutation modifies PKD progression in cpk mice. During cell cycle progression, Cux1 is proteolytically processed by a nuclear isoform of the cysteine protease cathepsin-L. Analysis of the deleted sequences reveals that a cathepsin-L processing site in Cux1 is deleted. Moreover, nuclear cathepsin-L is significantly reduced in both human ADPKD cells and in Pkd1 null kidneys, corresponding to increased levels of Cux1 protein in the cystic cells and kidneys. These results suggest a mechanism in which reduced Cux1 processing by cathepsin-L results in the accumulation of Cux1, downregulation of p21/p27, and increased cell proliferation in PKD.

Polycystin-2 expression is regulated by a PC2-binding domain in the intracellular portion of fibrocystin

Autosomal dominant (ADPKD) and autosomal recessive (ARPKD) polycystic kidney disease are caused by mutations in Pkd1/Pkd2 and Pkhd1, which encode polycystins (PCs) and fibrocystin/polyductin (FPC). Our recent study reported that a deficiency in FPC increases the severity of cystic disease in Pkd2 mutants and down-regulates PC2 in vivo, but the precise molecular mechanism of these effects is unknown (Kim, I., Fu, Y., Hui, K., Moeckel, G., Mai, W., Li, C., Liang, D., Zhao, P., Ma, J., Chen, X.-Z., George, A. L., Jr., Coffey, R. J., Feng, Z. P., and Wu, G. (2008) J. Am. Soc. Nephrol. 19, 455-468). In this study, through the use of deletion and mutagenesis strategies, we identified a PC2-binding domain in the intracellular C terminus of FPC and an FPC-binding domain in the intracellular N terminus of PC2. These binding domains provide a molecular basis for the physical interaction between PC2 and FPC. In addition, we also found that physical interaction between the binding domains of PC2 and FPC is able to prevent down-regulation of PC2 induced by loss of FPC. In vivo, we generated a mouse model of ADPKD with hypomorphic Pkd2 alleles (Pkd2nf3/nf3) and show that PC2 down-regulation is accompanied by a phenotype similar to that of Pkhd1(-/-) mice. These findings demonstrate a common mechanism underlying cystogenesis in ADPKD and ARPKD and provide insight into the molecular relationship between PC2 and FPC.

Neutrophil gelatinase-associated lipocalin suppresses cyst growth by Pkd1 null cells in vitro and in vivo

Cyst growth in patients with autosomal dominant polycystic kidney disease is thought to be due to increased tubular cell proliferation. One model to explain this altered proliferation suggests that the polycystin proteins PC1 and PC2 localize to apical cilia and serve as an integral part of the flow-sensing pathway thus modulating the proliferative response. We measured proliferation and apoptosis in proximal tubule derived cell lines lacking PC1. These cells showed increased rates of proliferation, a decreased rate of apoptosis, compared to control heterozygous cell lines, and spontaneously formed cysts rather than tubules in an in vitro tubulogenesis assay. Addition of neutrophil gelatinase associated lipocalin (NGAL), a small secreted protein that binds diverse ligands, to the cells lacking PC1 inhibited proliferation and increased apoptosis leading to slower cyst growth in vitro. Sustained over-expression at low level of NGAL by an adenoviral delivery system suppressed cyst enlargement without improving renal function in the Pkd1 mutant mice. Our studies show that renal epithelial cells lacking PC1 have an inherent tendency to hyper-proliferate forming cysts in vitro independent of a flow stimulus. The potential benefit of attenuating cyst growth with NGAL remains to be determined.

Polycystin-2 down-regulates cell proliferation via promoting PERK-dependent phosphorylation of eIF2alpha

Autosomal dominant polycystic kidney disease (ADPKD) is characterized by the formation of renal, hepatic and pancreatic cysts and by non-cystic manifestations such as abnormal vasculature and embryo left-right asymmetry development. Polycystin-2 (PC2), in which mutations account for 10-15% of ADPKD, was previously shown to down-regulate cell proliferation, but the underlying mechanism was not elucidated. Here, we demonstrate that PC2, but not pathogenic mutants E837X and R872X, represses cell proliferation through promoting the phosphorylation of eukaryotic translation initiation factor eIF2alpha by pancreatic ER-resident eIF2alpha kinase (PERK). ER stress is known to enhance eIF2alpha phosphorylation through up-regulating PERK kinase activity (assessed by phosphorylated PERK). During ER stress, PC2 knockdown also repressed eIF2alpha phosphorylation but did not alter PERK phosphorylation, indicating that PC2 facilitates the eIF2alpha phosphorylation by PERK. PC2 was found to be in the same complex as PERK and eIF2alpha. Together, we demonstrate that PC2 negatively controls cell growth by promoting PERK-mediated eIF2alpha phosphorylation, presumably through physical interaction, which may underlie a pathogenesis mechanism of ADPKD and indicates that PC2 is an important regulator of the translation machinery.

Polycystin-1 C-terminal tail associates with beta-catenin and inhibits canonical Wnt signaling

Polycystin-1 (PC1), the product of the PKD1 gene mutated in the majority of autosomal dominant polycystic kidney disease (ADPKD) cases, undergoes a cleavage resulting in the intracellular release of its C-terminal tail (CTT). Here, we demonstrate that the PC1 CTT co-localizes with and binds to beta-catenin in the nucleus. This interaction requires a nuclear localization motif present in the PC1 CTT as well as the N-terminal portion of beta-catenin. The PC1 CTT inhibits the ability of both beta-catenin and Wnt ligands to activate T-cell factor (TCF)-dependent gene transcription, a major effector of the canonical Wnt signaling pathway. The PC1 CTT may produce this effect by reducing the apparent affinity of the interaction between beta-catenin and the TCF protein. DNA microarray analysis reveals that the canonical Wnt signaling pathway is activated in ADPKD patient cysts. Our results suggest a novel mechanism through which PC1 cleavage may impact upon Wnt-dependent signaling and thereby modulate both developmental processes and cystogenesis.

Diagnosis of autosomal dominant polycystic kidney disease

BACKGROUND

Autosomal dominant polycystic kidney disease (ADPKD) is the most common hereditary kidney disease and accounts for 5 - 10% of end stage renal disease. Mutations of two genes, PKD1 and PKD2, account for ∼ 85 and ∼ 15% of cases, respectively.

OBJECTIVE

This paper reviews the clinical features of ADPKD, highlights the current roles for image- and molecular-based diagnostics, and the potential for new innovations to improve the clinical diagnostics for ADPKD.

METHODS

This paper reviews the literature on the clinical features, differential diagnosis, and image- and molecular-based diagnostics for ADPKD.

RESULTS/CONCLUSION

At present, presymptomatic diagnosis of ADPKD in subjects born with 50% risk is typically performed by renal ultrasonography. Renal MRI, with improved sensitivity for detecting smaller cysts, is a promising modality. There is also a clear role for molecular diagnostics, especially in patients with equivocal imaging results, in those with a negative family history and in younger at-risk subjects with a negative ultrasound study being evaluated as a living-related kidney donor. Also, several classes of promising disease-modifying drugs are being tested in clinical trials and, if proved effective, some of them will be used in early disease. Therefore, it is likely that there will be an increased demand for accurate and early diagnosis of ADPKD in the not so distant future.

Loss of polycystin-1 causes centrosome amplification and genomic instability

Autosomal dominant polycystic kidney disease (ADPKD) is the most common monogenetic disease predominantly caused by alteration or dysregulation of the PKD1 gene, which encodes polycystin-1 (PC1). The disease is characterized by the progressive expansion of bilateral fluid-filled renal cysts that ultimately lead to renal failure. Individual cysts, even within patients with germline mutations, are genetically heterogeneous, displaying diverse chromosomal abnormalities. To date, the molecular mechanisms responsible for this genetic heterogeneity remain unknown. Using a lentiviral-mediated siRNA expression model of Pkd1 hypomorphism, we show that loss of PC1 function is sufficient to produce centrosome amplification and multipolar spindle formation. These events lead to genomic instability characterized by gross polyploidism and mitotic catastrophe. Following these dramatic early changes, the cell population rapidly converges toward a stable ploidy in which centrosome amplification is significantly decreased, though cytological abnormalities such as micronucleation, chromatin bridges and aneuploidy remain common. In agreement with our in vitro findings, we provide the first in vivo evidence that significant centrosome amplification occurs in kidneys from conditional Pkd1 knockout mice at early and late time during the disease progression as well as in human ADPKD patients. These findings establish a novel function of PC1 in ADPKD pathogenesis and a genetic mechanism that may underlie the intrafamilial variability of ADPKD progression.

A tumor necrosis factor-alpha-mediated pathway promoting autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is caused by heterozygous mutations in either PKD1 or PKD2, genes that encode polycystin-1 and polycystin-2, respectively. We show here that tumor necrosis factor-alpha (TNF-alpha), an inflammatory cytokine present in the cystic fluid of humans with ADPKD, disrupts the localization of polycystin-2 to the plasma membrane and primary cilia through a scaffold protein, FIP2, which is induced by TNF-alpha. Treatment of mouse embryonic kidney organ cultures with TNF-alpha resulted in formation of cysts, and this effect was exacerbated in the Pkd2(+/-) kidneys. TNF-alpha also stimulated cyst formation in vivo in Pkd2(+/-) mice. In contrast, treatment of Pkd2(+/-) mice with the TNF-alpha inhibitor etanercept prevented cyst formation. These data reveal a pathway connecting TNF-alpha signaling, polycystins and cystogenesis, the activation of which may reduce functional polycystin-2 below a critical threshold, precipitating the ADPKD cellular phenotype.

KCa3.1 potassium channels are critical for cAMP-dependent chloride secretion and cyst growth in autosomal-dominant polycystic kidney disease

Autosomal-dominant polycystic kidney disease (ADPKD) is characterized by numerous fluid-filled kidney cysts. Net fluid secretion into renal cysts is caused by transepithelial transport mediated by the apical cystic fibrosis transmembrane conductance regulator chloride channel, which leads to cyst enlargement. Here we found that forskolin, a potent adenylyl cyclase agonist, stimulated anion secretion by monolayers of kidney cells derived from patients with ADPKD. TRAM-34, a specific KCa3.1 potassium channel blocker, inhibited this current, and in vitro cyst formation and enlargement by the cells cultured within a collagen gel. Net chloride secretion was enhanced by the KCa3.1 activator DCEBIO and both chloride secretion and in vitro cyst growth were inhibited by overexpression of myotubularin-related protein-6, a phosphatase that specifically inhibits KCa3.1 channel activity. Our study suggests that KCa3.1 channels play a critical role in transcellular chloride secretion and net fluid transport into the kidney cysts of patients with ADPKD by maintaining the electrochemical driving force for chloride efflux through apical chloride channels. Pharmacological inhibitors of KCa3.1 channels may provide a novel and effective therapy to delay progression to kidney failure in patients with ADPKD.

Ciliary dysfunction in polycystic kidney disease: an emerging model with polarizing potential

The majority of different cell types in the human body have a cilium, a thin rod-like structure of uniquely arranged microtubules that are encapsulated by the surface plasma membrane. The cilium originates from a basal body, a mature centriole that has migrated and docked to the cell surface. The non-motile cilia are microtubule-based organelles that are generally considered sensory structures. The purpose of this review is to discuss the practicality of the ciliary hypothesis as a unifying concept for polycystic kidney disease and to review current literature in the field of cilium biology, as it relates to mechanosensation and planar cell polarity. The polycystins and fibrocystin localization at the cilium and other subcellular localizations are discussed, followed by a hypothetical model for the cilium's role in mechanosensing, planar cell polarity, and cystogenesis.

Kidney cysts, pancreatic cysts, and biliary disease in a mouse model of autosomal recessive polycystic kidney disease

Mutations in PKHD1 cause autosomal recessive polycystic kidney disease (ARPKD). We produced a mouse model of ARPKD by replacing exons 1-3 of Pkhd1 with a lacZ reporter gene utilizing homologous recombination. This approach yielded heterozygous Pkhd1 (lacZ/+) mice, that expressed beta-galactosidase in tissues where Pkhd1 is normally expressed, and homozygous Pkhd1 (lacZ/lacZ) knockout mice. Heterozygous Pkhd1 (lacZ/+) mice expressed beta-galactosidase in the kidney, liver, and pancreas. Homozygous Pkhd1 (lacZ/lacZ) mice lacked Pkhd1 expression and developed progressive renal cystic disease involving the proximal tubules, collecting ducts, and glomeruli. In the liver, inactivation of Pkhd1 resulted in dilatation of the bile ducts and periportal fibrosis. Dilatation of pancreatic exocrine ducts was uniformly seen in Pkhd1 (lacZ/lacZ ) mice, with pancreatic cysts arising less frequently. The expression of beta-galactosidase, Pkd1, and Pkd2 was reduced in the kidneys of Pkhd1 (lacZ/lacZ ) mice compared with wild-type littermates, but no changes in blood urea nitrogen (BUN) or liver function tests were observed. Collectively, these results indicate that deletion of exons 1-3 leads to loss of Pkhd1 expression and results in kidney cysts, pancreatic cysts, and biliary ductal plate malformations. The Pkhd1 (lacZ/lacZ ) mouse represents a new orthologous animal model for studying the pathogenesis of kidney cysts and biliary dysgenesis that characterize human ARPKD.

Small-molecule CFTR inhibitors slow cyst growth in polycystic kidney disease

Cyst expansion in polycystic kidney disease (PKD) involves progressive fluid accumulation, which is believed to require chloride transport by the cystic fibrosis transmembrane conductance regulator (CFTR) protein. Herein is reported that small-molecule CFTR inhibitors of the thiazolidinone and glycine hydrazide classes slow cyst expansion in in vitro and in vivo models of PKD. More than 30 CFTR inhibitor analogs were screened in an MDCK cell model, and near-complete suppression of cyst growth was found by tetrazolo-CFTR(inh)-172, a tetrazolo-derived thiazolidinone, and Ph-GlyH-101, a phenyl-derived glycine hydrazide, without an effect on cell proliferation. These compounds also inhibited cyst number and growth by >80% in an embryonic kidney cyst model involving 4-d organ culture of embryonic day 13.5 mouse kidneys in 8-Br-cAMP-containing medium. Subcutaneous delivery of tetrazolo-CFTR(inh)-172 and Ph-GlyH-101 to neonatal, kidney-specific PKD1 knockout mice produced stable, therapeutic inhibitor concentrations of >3 microM in urine and kidney tissue. Treatment of mice for up to 7 d remarkably slowed kidney enlargement and cyst expansion and preserved renal function. These results implicate CFTR in renal cyst growth and suggest that CFTR inhibitors may hold therapeutic potential to reduce cyst growth in PKD.

Genomic organization and mutation screening of the human ortholog of Pkdr1 associated with polycystic kidney disease in the rat

Autosomal dominant polycystic kidney disease (ADPKD) is one of the most common inherited disorders in humans. Although disease-causing mutations have been found in two genes, PKD1 and PKD2, a small number of ADPKD families exist that are unlinked to either of these genes, suggesting involvement of a third, as yet unidentified PKD3 gene. Susceptibility to renal cyst formation in the (cy/+) rat is caused by a missense mutation in Pkdr1 encoding the novel protein SamCystin. To initiate studies of the human orthologous gene, we determined the location and the organization of human PKDR1. We genotyped microsatellite markers flanking the human ortholog in PKD families that either are unlinked to known PKD genes, or in which mutations have not yet been identified and carried out mutation analysis in PKD patients. We identified eight novel single nucleotide polymorphisms, including three leading to amino acid changes. These variants are unlikely to account for PKD in these patients, yet the screening of other affected populations may provide information about the involvement of PKDR1 as a modifier gene in cystic kidney disease.

The Caenorhabditis elegans nephrocystins act as global modifiers of cilium structure

Nephronophthisis (NPHP) is the most common genetic cause of end-stage renal disease in children and young adults. In Chlamydomonas reinhardtii, Caenorhabditis elegans, and mammals, the NPHP1 and NPHP4 gene products nephrocystin-1 and nephrocystin-4 localize to basal bodies or ciliary transition zones (TZs), but their function in this location remains unknown. We show here that loss of C. elegans NPHP-1 and NPHP-4 from TZs is tolerated in developing cilia but causes changes in localization of specific ciliary components and a broad range of subtle axonemal ultrastructural defects. In amphid channel cilia, nphp-4 mutations cause B tubule defects that further disrupt intraflagellar transport (IFT). We propose that NPHP-1 and NPHP-4 act globally at the TZ to regulate ciliary access of the IFT machinery, axonemal structural components, and signaling molecules, and that perturbing this balance results in cell type-specific phenotypes.