Cystic Kidney Diseases: All Roads Lead to the Cilium

Ciliary exclusion of Polycystin-2 promotes kidney cystogenesis in an autosomal dominant polycystic kidney disease model

The human PKD2 locus encodes Polycystin-2 (PC2), a TRPP channel that localises to several distinct cellular compartments, including the cilium. PKD2 mutations cause Autosomal Dominant Polycystic Kidney Disease (ADPKD) and affect many cellular pathways. Data underlining the importance of ciliary PC2 localisation in preventing PKD are limited because PC2 function is ablated throughout the cell in existing model systems. Here, we dissect the ciliary role of PC2 by analysing mice carrying a non-ciliary localising, yet channel-functional, PC2 mutation. Mutants develop embryonic renal cysts that appear indistinguishable from mice completely lacking PC2. Despite not entering the cilium in mutant cells, mutant PC2 accumulates at the ciliary base, forming a ring pattern consistent with distal appendage localisation. This suggests a two-step model of ciliary entry; PC2 first traffics to the cilium base before TOP domain dependent entry. Our results suggest that PC2 localisation to the cilium is necessary to prevent PKD.

The molecular role of ciliary Polycystin-2 (PC2) in cyst formation and polycystic kidney disease (ADKPD) is unclear. Here, the authors identify a PC2 mutant lacking ciliary localisation but with active Ca2+ channel function in mice, that is sufficient to generate an ADPKD phenotype.

Ciliogenesis is reciprocally regulated by PPARA and NR1H4/FXR through controlling autophagy in vitro and in vivo

The primary cilia are evolutionarily conserved microtubule-based cellular organelles that perceive metabolic status and thus link the sensory system to cellular signaling pathways. Therefore, ciliogenesis is thought to be tightly linked to autophagy, which is also regulated by nutrient-sensing transcription factors, such as PPARA (peroxisome proliferator activated receptor alpha) and NR1H4/FXR (nuclear receptor subfamily 1, group H, member 4). However, the relationship between these factors and ciliogenesis has not been clearly demonstrated. Here, we present direct evidence for the involvement of macroautophagic/autophagic regulators in controlling ciliogenesis. We showed that activation of PPARA facilitated ciliogenesis independently of cellular nutritional states. Importantly, PPARA-induced ciliogenesis was mediated by controlling autophagy, since either pharmacological or genetic inactivation of autophagy significantly repressed ciliogenesis. Moreover, we showed that pharmacological activator of autophagy, rapamycin, recovered repressed ciliogenesis in ppara^-/- cells. Conversely, activation of NR1H4 repressed cilia formation, while knockdown of NR1H4 enhanced ciliogenesis by inducing autophagy. The reciprocal activities of PPARA and NR1H4 in regulating ciliogenesis were highlighted in a condition where de-repressed ciliogenesis by NR1H4 knockdown was further enhanced by PPARA activation. The in vivo roles of PPARA and NR1H4 in regulating ciliogenesis were examined in greater detail in ppara^-/- mice. In response to starvation, ciliogenesis was facilitated in wild-type mice via enhanced autophagy in kidney, while ppara^-/- mice displayed impaired autophagy and kidney damage resembling ciliopathy. Furthermore, an NR1H4 agonist exacerbated kidney damage associated with starvation in ppara^-/- mice. These findings indicate a previously unknown role for PPARA and NR1H4 in regulating the autophagy-ciliogenesis axis in vivo.

The von Hippel–Lindau Tumor Suppressor Protein

The von Hippel–Lindau (VHL) gene is a two-hit tumor suppressor gene and is linked to the development of the most common form of kidney cancer, clear cell renal carcinoma; blood vessel tumors of the retina, cerebellum, and spinal cord called hemangioblastomas; and tumors of the sympathoadrenal nervous system called paragangliomas. The VHL gene product, pVHL, is the substrate recognition subunit of a cullin-dependent ubiquitin ligase that targets the α subunits of hypoxia-inducible factor (HIF) for destruction when oxygen is plentiful. Mounting evidence implicates HIF2 in the pathogenesis of pVHL-defective tumors and has provided a conceptual foundation for the development of drugs to treat them that inhibit HIF2-responsive gene products such as VEGF and, more recently, HIF2 itself. pVHL has additional, noncanonical functions that are cancer relevant, including roles related to the primary cilium, chromosome stability, extracellular matrix formation, and survival signaling.

Routes and machinery of primary cilium biogenesis

Primary cilia are solitary, microtubule-based protrusions of the cell surface that play fundamental roles as photosensors, mechanosensors and biochemical sensors. Primary cilia dysfunction results in a long list of developmental and degenerative disorders that combine to give rise to a large spectrum of human diseases affecting almost any major body organ. Depending on the cell type, primary ciliogenesis is initiated intracellularly, as in fibroblasts, or at the cell surface, as in renal polarized epithelial cells. In this review, we have focused on the routes of primary ciliogenesis placing particular emphasis on the recently described pathway in renal polarized epithelial cells by which the midbody remnant resulting from a previous cell division event enables the centrosome for initiation of primary cilium assembly. The protein machinery implicated in primary cilium formation in epithelial cells, including the machinery best known for its involvement in establishing cell polarity and polarized membrane trafficking, is also discussed.

Genetic spectrum of Saudi Arabian patients with antenatal cystic kidney disease and ciliopathy phenotypes using a targeted renal gene panel

Background

Inherited cystic kidney disorders are a common cause of end-stage renal disease. Over 50 ciliopathy genes, which encode proteins that influence the structure and function of the primary cilia, are implicated in cystic kidney disease.

Methods

To define the phenotype and genotype of cystic kidney disease in fetuses and neonates, we correlated antenatal ultrasound examination and postnatal renal ultrasound examination with targeted exon sequencing, using a renal gene panel. A cohort of 44 families in whom antenatal renal ultrasound scanning findings in affected cases included bilateral cystic kidney disease, echogenic kidneys or enlarged kidneys was investigated.

Results

In this cohort, disease phenotypes were severe with 36 cases of stillbirth or perinatal death. Extra renal malformations, including encephalocele, polydactyly and heart malformations, consistent with ciliopathy phenotypes, were frequently detected. Renal gene panel testing identified causative mutations in 21 out of 34 families (62%), where patient and parental DNA was available. In the remaining 10 families, where only parental DNA was available, 7 inferred causative mutations were found. Together, mutations were found in 12 different genes with a total of 13 novel pathogenic variants, including an inferred novel variant in NEK8. Mutations in CC2D2A were the most common cause of an antenatal cystic kidney disease and a suspected ciliopathy in our cohort.

Conclusions

In families with ciliopathy phenotypes, mutational analysis using a targeted renal gene panel allows a rapid molecular diagnosis and provides important information for patients, parents and their physicians.

Unconventional Functions of Mitotic Kinases in Kidney Tumorigenesis

Human tumors exhibit a variety of genetic alterations, including point mutations, translocations, gene amplifications and deletions, as well as aneuploid chromosome numbers. For carcinomas, aneuploidy is associated with poor patient outcome for a large variety of tumor types, including breast, colon, and renal cell carcinoma. The Renal cell carcinoma (RCC) is a heterogeneous carcinoma consisting of different histologic types. The clear renal cell carcinoma (ccRCC) is the most common subtype and represents 85% of the RCC. Central to the biology of the ccRCC is the loss of function of the Von Hippel–Lindau gene, but is also associated with genetic instability that could be caused by abrogation of the cell cycle mitotic spindle checkpoint and may involve the Aurora kinases, which regulate centrosome maturation. Aneuploidy can also result from the loss of cell–cell adhesion and apical–basal cell polarity that also may be regulated by the mitotic kinases (polo-like kinase 1, casein kinase 2, doublecortin-like kinase 1, and Aurora kinases). In this review, we describe the “non-mitotic” unconventional functions of these kinases in renal tumorigenesis.

Regulation of polycystin-1 ciliary trafficking by motifs at its C-terminus and polycystin-2 but not by cleavage at the GPS site

Failure to localize membrane proteins to the primary cilium causes a group of diseases collectively named ciliopathies. Polycystin-1 (PC1, also known as PKD1) is a large ciliary membrane protein defective in autosomal dominant polycystic kidney disease (ADPKD). Here, we developed a large set of PC1 expression constructs and identified multiple sequences, including a coiled-coil motif in the C-terminal tail of PC1, regulating full-length PC1 trafficking to the primary cilium. Ciliary trafficking of wild-type and mutant PC1 depends on the dose of polycystin-2 (PC2, also known as PKD2), and the formation of a PC1-PC2 complex. Modulation of the ciliary trafficking module mediated by the VxP ciliary-targeting sequence and Arf4 and Asap1 does not affect the ciliary localization of full-length PC1. PC1 also promotes PC2 ciliary trafficking. PC2 mutations truncating its C-terminal tail but not those changing the VxP sequence to AxA or impairing the pore of the channel, leading to a dead channel, affect PC1 ciliary trafficking. Cleavage at the GPCR proteolytic site (GPS) of PC1 is not required for PC1 trafficking to cilia. We propose a mutually dependent model for the ciliary trafficking of PC1 and PC2, and that PC1 ciliary trafficking is regulated by multiple cis-acting elements. As all pathogenic PC1 mutations tested here are defective in ciliary trafficking, ciliary trafficking might serve as a functional read-out for ADPKD.

Human kidney proximal tubule-on-a-chip for drug transport and nephrotoxicity assessment

Kidney toxicity is one of the most frequent adverse events reported during drug development. The lack of accurate predictive cell culture models and the unreliability of animal studies have created a need for better approaches to recapitulate kidney function in vitro. Here, we describe a microfluidic device lined by living human kidney epithelial cells exposed to fluidic flow that mimics key functions of the human kidney proximal tubule. Primary kidney epithelial cells isolated from human proximal tubule are cultured on the upper surface of an extracellular matrix-coated, porous, polyester membrane that splits the main channel of the device into two adjacent channels, thereby creating an apical 'luminal' channel and a basal 'interstitial' space. Exposure of the epithelial monolayer to an apical fluid shear stress (0.2 dyne cm(-2)) that mimics that found in living kidney tubules results in enhanced epithelial cell polarization and primary cilia formation compared to traditional Transwell culture systems. The cells also exhibited significantly greater albumin transport, glucose reabsorption, and brush border alkaline phosphatase activity. Importantly, cisplatin toxicity and Pgp efflux transporter activity measured on-chip more closely mimic the in vivo responses than results obtained with cells maintained under conventional culture conditions. While past studies have analyzed kidney tubular cells cultured under flow conditions in vitro, this is the first report of a toxicity study using primary human kidney proximal tubular epithelial cells in a microfluidic 'organ-on-a-chip' microdevice. The in vivo-like pathophysiology observed in this system suggests that it might serve as a useful tool for evaluating human-relevant renal toxicity in preclinical safety studies.

Uriniferous tubule: structural and functional organization

The uriniferous tubule is divided into the proximal tubule, the intermediate (thin) tubule, the distal tubule and the collecting duct. The present chapter is based on the chapters by Maunsbach and Christensen on the proximal tubule, and by Kaissling and Kriz on the distal tubule and collecting duct in the 1992 edition of the Handbook of Physiology, Renal Physiology. It describes the fine structure (light and electron microscopy) of the entire mammalian uriniferous tubule, mainly in rats, mice, and rabbits. The structural data are complemented by recent data on the location of the major transport- and transport-regulating proteins, revealed by morphological means(immunohistochemistry, immunofluorescence, and/or mRNA in situ hybridization). The structural differences along the uriniferous tubule strictly coincide with the distribution of the major luminal and basolateral transport proteins and receptors and both together provide the basis for the subdivision of the uriniferous tubule into functional subunits. Data on structural adaptation to defined functional changes in vivo and to genetical alterations of specified proteins involved in transepithelial transport importantly deepen our comprehension of the correlation of structure and function in the kidney, of the role of each segment or cell type in the overall renal function,and our understanding of renal pathophysiology.

Reduction of oxidative stress during recovery accelerates normalization of primary cilia length that is altered after ischemic injury in murine kidneys

The primary cilium is a microtubule-based nonmotile organelle that extends from the surface of cells, including renal tubular cells. Here, we investigated the alteration of primary cilium length during epithelial cell injury and repair, following ischemia/reperfusion (I/R) insult, and the role of reactive oxygen species in this alteration. Thirty minutes of bilateral renal ischemia induced severe renal tubular cell damage and an increase of plasma creatinine (PCr) concentration. Between 8 and 16 days following the ischemia, the increased PCr returned to normal range, although without complete histological restoration. Compared with the primary cilium length in normal kidney tubule cells, the length was shortened 4 h and 1 day following ischemia, increased over normal 8 days after ischemia, and then returned to near normal 16 days following ischemia. In the urine of I/R-subjected mice, acetylated tubulin was detected. The cilium length of proliferating cells was shorter than that in nonproliferating cells. Mature cells had shorter cilia than differentiating cells. Treatment with Mn(III) tetrakis(1-methyl-4-pyridyl) porphyrin (MnTMPyP), an antioxidant, during the recovery of damaged kidneys accelerated normalization of cilia length concomitant with a decrease of oxidative stress and morphological recovery in the kidney. In the Madin-Darby canine kidney (MDCK) cells, H2O2 treatment caused released ciliary fragment into medium, and MnTMPyP inhibited the deciliation. The ERK inhibitor U0126 inhibited elongation of cilia in normal and MDCK cells recovering from H2O2 stress. Taken together, our results suggest that primary cilia length reflects cell proliferation and the length of primary cilium is regulated, at least, in part, by reactive oxygen species through ERK.

The role of the cilium in normal and abnormal cell cycles: emphasis on renal cystic pathologies

The primary cilium protrudes from the cell surface and acts as a sensor for chemical and mechanical growth cues, with receptors for a number of growth factors (PDGFα, Hedgehog, Wnt, Notch) concentrated within the ciliary membrane. In normal tissues, the cilium assembles after cells exit mitosis and is resorbed as part of cell cycle re-entry. Although regulation of the cilium by cell cycle transitions has been appreciated for over 100 years, only recently have data emerged to indicate the cilium also exerts influence on the cell cycle. The resorption/protrusion cycle, regulated by proteins including Aurora-A, VHL, and GSK-3β, influences cell responsiveness to growth cues involving cilia-linked receptors; further, resorption liberates the ciliary basal body to differentiate into the centrosome, which performs discrete functions in S-, G2-, and M-phase. Besides these roles, the cilium provides a positional cue that regulates polarity of cell division, and thus directs cells towards fates of differentiation versus proliferation. In this review, we summarize the specific mechanisms mediating the cilia-cell cycle dialog. We then emphasize the examples of polycystic kidney disease (PKD), nephronopthisis (NPHP), and VHL-linked renal cysts as cases in which defects of ciliary function influence disease pathology, and may also condition response to treatment.

miR-129-3p controls cilia assembly by regulating CP110 and actin dynamics

Ciliogenesis requires the removal of CP110 from the mother centriole; actin dynamics also influence ciliation, at least partly by affecting the centrosomal accumulation of ciliogenic membrane vesicles. How these distinct processes are properly regulated remains unknown. Here we show that miR-129-3p, a microRNA conserved in vertebrates, controlled cilia biogenesis in cultured cells by concomitantly downregulating CP110 and repressing branched F-actin formation. Blocking miR-129-3p inhibited serum-starvation-induced ciliogenesis, whereas its overexpression potently induced ciliation in proliferating cells and also promoted cilia elongation. Gene expression analysis further identified ARP2, TOCA1, ABLIM1 and ABLIM3 as its targets in ciliation-related actin dynamics. Moreover, miR-129-3p inhibition in zebrafish embryos suppressed ciliation in Kupffer's vesicle and the pronephros, and induced developmental abnormalities including a curved body, pericardial oedema and defective left-right asymmetry. Therefore, our results reveal a mechanism that orchestrates both the centriole-to-basal body transition and subsequent cilia assembly through microRNA-mediated post-transcriptional regulation.

In vitro investigation of renal epithelial injury suggests that primary cilium length is regulated by hypoxia-inducible mechanisms

Primary cilia are non-motile sensory organelles that project from cells in many tissues. The role of renal primary cilium-based signalling in regulating epithelial cell proliferation and differentiation is highlighted by studies showing that defects of the cilium lead to epithelial de-differentiation, over proliferation and polycystic kidney disease. Recent studies show that renal primary cilia may also play a role in controlling epithelial differentiation during renal repair. After injury, renal cilium length increases dramatically and then undergoes a normalization that coincides with structural and functional repair in both human patients and mouse models of renal injury. These changes in cilium length are likely to modulate cilium-based signalling, but the injury-related factors that influence renal primary cilium length have yet to be determined. Here, we investigated the effect of three factors commonly associated with renal injury on renal cilium length in an in vitro setting. MDCK (Madin Darby canine kidney) cell cultures bearing primary cilia were treated with BSA to simulate albuminuria, cobalt chloride to simulate hypoxia and the inflammation-related cytokine tumour necrosis factor α. Primary cilium length was only increased in cultures treated with cobalt chloride. Our results suggest a role for hypoxia and the induction of HIF-1α (hypoxia-inducible factor 1α) in increasing renal primary cilium length following renal injury.

New insights into the biology of renal cell carcinoma

Ninety percent or more of kidney cancers are believed to be of epithelial cell origin, and are referred to as renal cell carcinoma (RCC), which are further subdivided based on histology into clear-cell RCC (75%), papillary RCC (15%), chromophobe tumor (5%), and oncocytoma (5%). Some genes confer an increased risk of these various histologic RCC subtypes. In practice, there is some overlap among the histologic subtypes, and there are some shared molecular features among these tumor types. This review focuses primarily on the most common form of RCC, clear-cell renal carcinoma, noting some recent advances in the other histologic subtypes.

Drosophila provides rapid modeling of renal development, function, and disease

The evolution of specialized excretory cells is a cornerstone of the metazoan radiation, and the basic tasks performed by Drosophila and human renal systems are similar. The development of the Drosophila renal (Malpighian) tubule is a classic example of branched tubular morphogenesis, allowing study of mesenchymal-to-epithelial transitions, stem cell-mediated regeneration, and the evolution of a glomerular kidney. Tubule function employs conserved transport proteins, such as the Na(+), K(+)-ATPase and V-ATPase, aquaporins, inward rectifier K(+) channels, and organic solute transporters, regulated by cAMP, cGMP, nitric oxide, and calcium. In addition to generation and selective reabsorption of primary urine, the tubule plays roles in metabolism and excretion of xenobiotics, and in innate immunity. The gene expression resource FlyAtlas.org shows that the tubule is an ideal tissue for the modeling of renal diseases, such as nephrolithiasis and Bartter syndrome, or for inborn errors of metabolism. Studies are assisted by uniquely powerful genetic and transgenic resources, the widespread availability of mutant stocks, and low-cost, rapid deployment of new transgenics to allow manipulation of renal function in an organotypic context.

Cyst formation in the PKD2 (1-703) transgenic rat precedes deregulation of proliferation-related pathways

Background

Polycystic Kidney Disease is characterized by the formation of large fluid-filled cysts that eventually destroy the renal parenchyma leading to end-stage renal failure. Although remarkable progress has been made in understanding the pathologic mechanism of the disease, the precise orchestration of the early events leading to cyst formation is still unclear. Abnormal cellular proliferation was traditionally considered to be one of the primary irregularities leading to cyst initiation and growth. Consequently, many therapeutic interventions have focused on targeting this abnormal proliferation, and some have even progressed to clinical trials. However, the role of proliferation in cyst development was primarily examined at stages where cysts are already visible in the kidneys and therefore at later stages of disease development.

Methods

In this study we focused on the cystic phenotype since birth in an attempt to clarify the temporal contribution of cellular proliferation in cyst development. Using a PKD2 transgenic rat model (PKD2 (1-703)) of different ages (0-60 days after birth) we performed gene expression profiling and phenotype analysis by measuring various kidney parameters.

Results

Phenotype analysis demonstrated that renal cysts appear immediately after birth in the PKD2 transgenic rat model (PKD2 (1-703)). On the other hand, abnormal proliferation occurs at later stages of the disease as identified by gene expression profiling. Interestingly, other pathways appear to be deregulated at early stages of the disease in this PKD model. Specifically, gene expression analysis demonstrated that at day 0 the RAS system is involved. This is altered at day 6, when Wnt signaling and focal adhesion pathways are affected. However, at and after 24 days, proliferation, apoptosis, altered ECM signaling and many other factors become involved.

Conclusions

Our data suggest that cystogenesis precedes deregulation of proliferation-related pathways, suggesting that proliferation abnormalities may contribute in cyst growth rather than cyst formation.

Mechanisms of p53-mediated repression of the human polycystic kidney disease-1 promoter

We previously reported that the tumor suppressor protein p53 participates in a negative feedback loop to fine-tune PKD1 gene expression. This physiological pathway is believed to prevent polycystin-1 overexpression and thus renal cysts. The present study examined the mechanisms of p53-mediated repression of PKD1. The 5'-upstream region of the human PKD1 gene is TATA-less, GC-rich, and contains four consensus p53 binding sites at positions -2.7 kb (BS4), -1.2 kb (BS3), -0.8 kb (BS2), and -0.2 kb (BS1), respectively. PKD1BS1-4 are bound to endogenous p53 in vivo and in vitro. Transient transfection assays in inner medullary collecting duct cells revealed that disruption of PKD1BS1 enhances baseline PKD1 promoter activity; in contrast, disruption of PKD1BS4 suppressed PKD1 transcription. PKD1BS1 confers p53-mediated repression when substituted for the p53 enhancer element in the bradykinin B2 receptor gene, indicating that PKD1BS1 is a bona fide p53 repressor element. Moreover, PKD1BS1 requires intact BS2-4 and cellular histone deacetylase activity for full functional activity. Indeed, the PKD1BS1/4 regions are occupied by a complex containing HDAC1/2 and mSin3. These findings suggest a model whereby p53 exerts a biphasic control on PKD1 gene transcription, depending on cellular context and the cognate cis-acting element.

Colchicine treatment in autosomal dominant polycystic kidney disease: many points in common

Autosomal dominant polycystic kidney disease (ADPKD) is the most common of the inherited renal cystic diseases and constitutes 10% of the end stage kidney disease population. ADPKD is caused by PKD1 and PKD2 gene mutations in 85% and 15% of the cases respectively. Its high prevalence and negative impact on health outcomes fostered efforts to explain pathophysiologic pathways of cyst formation in kidneys. Among these are increased apoptosis, unopposed proliferation of tubule cells, impaired polarization and planar cell polarity, impaired cAMP pathway, cilier dysfunction, activated mTOR pathway, increased tumor necrosis factor-alpha (TNF-alpha) production. Many drugs have been tried in an attempt to halt cystogenesis in some point. Despite success to some extent in experimental studies, none reached clinical armamentarium yet. Colchicine, originally extracted from Colchicum autunale, is an anti-inflammatory drug that has been in continuous use for more than 3000 years. It has been used successfully to prevent attacks of familial mediterranien fever and amyloidosis, to treat gout and pseudogout attacks for a few decades. Colchicine principally is a microtubule inhibitor, thus prevents cell migration, division, and polarization. It also has anti-apoptotic, anti-proliferative and anti-inflammatory effects and down-regulates (TNF-alpha) receptors. As can easily be seen, many of the effects of colchicine have pathophysiologic counterparts in ADPKD. Thus, we hypothesized that colchicine would be beneficial to prevent or at least delay cyst formation in ADPKD patients. Indirect evidence also support our hypothesis, in which taxol and paclitaxel, other two microtubule inhibitors, were shown to delay cyst formation in experimental models of ADPKD. To our opinion, despite its narrow therapeutic index, widespread experience makes colchicine a suitable candidate for prolonged clinical use, should experimental studies show any benefit in ADPKD.

A mouse model for Meckel syndrome type 3

Meckel-Gruber syndrome type 3 (MKS3; OMIM 607361) is a severe autosomal recessive disorder characterized by bilateral polycystic kidney disease. Other malformations associated with MKS3 include cystic changes in the liver, polydactyly, and brain abnormalities (occipital encephalocele, hydrocephalus, and Dandy Walker-type cerebellar anomalies). The disorder is hypothesized to be caused by defects in primary cilia. In humans, the underlying mutated gene, TMEM67, encodes transmembrane protein 67, also called meckelin (OMIM 609884), which is an integral protein of the renal epithelial cell and membrane of the primary cilium. Here, we describe a spontaneous deletion of the mouse ortholog, Tmem67, which results in polycystic kidney disease and death by 3 wk after birth. Hydrocephalus also occurs in some mutants. We verified the mutated gene by transgenic rescue and characterized the phenotype with microcomputed tomography, histology, scanning electron microscopy, and immunohistochemistry. This mutant provides a mouse model for MKS3 and adds to the growing set of mammalian models essential for studying the role of the primary cilium in kidney function.

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.

Ciliary syndromes and treatment

Abnormal visceral patterning has been known for centuries. However, it has not been associated with ciliary dysfunction until recently. Overlapping clinical entities including situs inversus, certain infertility disorders, as well as chronic respiratory infections have their roots in abnormal ciliary function. Current research focuses on causative factors and genes involved in signal transduction pathways that define ciliary function and structure, as well as treatment. In this review, attempts are made to outline selected, yet key topics related to ciliary function in health and disease.

Purinergic signaling in the lumen of a normal nephron and in remodeled PKD encapsulated cysts

The nephron is the functional unit of the kidney. Blood and plasma are continually filtered within the glomeruli that begin each nephron. Adenosine 5' triphosphate (ATP) and its metabolites are freely filtered by each glomerulus and enter the lumen of each nephron beginning at the proximal convoluted tubule (PCT). Flow rate, osmolality, and other mechanical or chemical stimuli for ATP secretion are present in each nephron segment. These ATP-release stimuli are also different in each nephron segment due to water or salt permeability or impermeability along different luminal membranes of the cells that line each nephron segment. Each of the above stimuli can trigger additional ATP release into the lumen of a nephron segment. Each nephron-lining epithelial cell is a potential source of secreted ATP. Together with filtered ATP and its metabolites derived from the glomerulus, secreted ATP and adenosine derived from cells along the nephron are likely the principal two of several nucleotide and nucleoside candidates for renal autocrine and paracrine ligands within the tubular fluid of the nephron. This minireview discusses the first principles of purinergic signaling as they relate to the nephron and the urinary bladder. The review discusses how the lumen of a renal tubule presents an ideal purinergic signaling microenvironment. The review also illustrates how remodeled and encapsulated cysts in autosomal dominant polycystic kidney disease (ADPKD) and remodeled pseudocysts in autosomal recessive PKD (ARPKD) of the renal collecting duct likely create an even more ideal microenvironment for purinergic signaling. Once trapped in these closed microenvironments, purinergic signaling becomes chronic and likely plays a significant epigenetic and detrimental role in the secondary progression of PKD, once the remodeling of the renal tissue has begun. In PKD cystic microenvironments, we argue that normal purinergic signaling within the lumen of the nephron provides detrimental acceleration of ADPKD once remodeling is complete.

Loss of apical monocilia on collecting duct principal cells impairs ATP secretion across the apical cell surface and ATP-dependent and flow-induced calcium signals

Renal epithelial cells release ATP constitutively under basal conditions and release higher quantities of purine nucleotide in response to stimuli. ATP filtered at the glomerulus, secreted by epithelial cells along the nephron, and released serosally by macula densa cells for feedback signaling to afferent arterioles within the glomerulus has important physiological signaling roles within kidneys. In autosomal recessive polycystic kidney disease (ARPKD) mice and humans, collecting duct epithelial cells lack an apical central cilium or express dysfunctional proteins within that monocilium. Collecting duct principal cells derived from an Oak Ridge polycystic kidney (orpk ( Tg737 ) ) mouse model of ARPKD lack a well-formed apical central cilium, thought to be a sensory organelle. We compared these cells grown as polarized cell monolayers on permeable supports to the same cells where the apical monocilium was genetically rescued with the wild-type Tg737 gene that encodes Polaris, a protein essential to cilia formation. Constitutive ATP release under basal conditions was low and not different in mutant versus rescued monolayers. However, genetically rescued principal cell monolayers released ATP three- to fivefold more robustly in response to ionomycin. Principal cell monolayers with fully formed apical monocilia responded three- to fivefold greater to hypotonicity than mutant monolayers lacking monocilia. In support of the idea that monocilia are sensory organelles, intentionally harsh pipetting of medium directly onto the center of the monolayer induced ATP release in genetically rescued monolayers that possessed apical monocilia. Mechanical stimulation was much less effective, however, on mutant orpk collecting duct principal cell monolayers that lacked apical central monocilia. Our data also show that an increase in cytosolic free Ca(2+) primes the ATP pool that is released in response to mechanical stimuli. It also appears that hypotonic cell swelling and mechanical pipetting stimuli trigger release of a common ATP pool. Cilium-competent monolayers responded to flow with an increase in cell Ca(2+) derived from both extracellular and intracellular stores. This flow-induced Ca(2+) signal was less robust in cilium-deficient monolayers. Flow-induced Ca(2+) signals in both preparations were attenuated by extracellular gadolinium and by extracellular apyrase, an ATPase/ADPase. Taken together, these data suggest that apical monocilia are sensory organelles and that their presence in the apical membrane facilitates the formation of a mature ATP secretion apparatus responsive to chemical, osmotic, and mechanical stimuli. The cilium and autocrine ATP signaling appear to work in concert to control cell Ca(2+). Loss of a cilium-dedicated autocrine purinergic signaling system may be a critical underlying etiology for ARPKD and may lead to disinhibition and/or upregulation of multiple sodium (Na(+)) absorptive mechanisms and a resultant severe hypertensive phenotype in ARPKD and, possibly, other diseases.

Failure of epithelial tube maintenance causes hydrocephalus and renal cysts in Dlg5-/- mice

Epithelial tubes represent fundamental building blocks of metazoan organisms; however, the mechanisms responsible for their formation and maintenance are not well understood. Here, we show that the evolutionarily conserved coiled-coil MAGUK protein Dlg5 is required for epithelial tube maintenance in mammalian brain and kidneys. We demonstrate that Dlg5(-/-) mice develop fully penetrant hydrocephalus and kidney cysts caused by a deficiency in membrane delivery of cadherin-catenin adhesion complexes and loss of cell polarity. Dlg5 travels with cadherin-containing vesicles and binds to syntaxin 4, a t-SNARE protein that regulates fusion of transport vesicles with the lateral membrane domain. We propose that Dlg5 functions in plasma membrane delivery of cadherins by linking cadherin-containing transport vesicles with the t-SNARE targeting complex. These findings show that Dlg5 is causally involved in hydrocephalus and renal cysts and reveal that targeted membrane delivery of cadherin-catenin adhesion complexes is critical for cell polarity and epithelial tube maintenance.

Polycystic kidney disease and renal injury repair: common pathways, fluid flow, and the function of polycystin-1

The root cause for most cases of autosomal-dominant polycystic kidney disease (ADPKD) is mutations in the polycystin-1 (PC1) gene. While PC1 has been implicated in a perplexing variety of protein interactions and signaling pathways, what its normal function is and why its disruption leads to the proliferation of renal epithelial cells are unknown. Recent results suggest that PC1 is involved in mechanotransduction by primary cilia measuring the degree of luminal fluid flow. PC1 has also recently been shown to regulate the mTOR and signal transducers and activators of transcription (STAT) 6 pathways. These two pathways are normally dormant in the healthy kidney but are activated in response to injury and appear to drive a proliferative repair response. This review develops the idea that a critical function of PC1 and primary cilia in the adult kidney may be to sense renal injury by detecting changes in luminal fluid flow and to trigger proliferation. Constitutive activation of these pathways in ADPKD would lead to the futile attempt to repair a nonexisting injury, resulting in cyst growth. The existence of many known cellular and molecular similarities between renal repair and ADPKD supports this model.

Polycystic disease caused by deficiency in xylosyltransferase 2, an initiating enzyme of glycosaminoglycan biosynthesis

The basic biochemical mechanisms underlying many heritable human polycystic diseases are unknown despite evidence that most cases are caused by mutations in members of several protein families, the most prominent being the polycystin gene family, whose products are found on the primary cilia, or due to mutations in posttranslational processing and transport. Inherited polycystic kidney disease, the most prevalent polycystic disease, currently affects approximately 500,000 people in the United States. Decreases in proteoglycans (PGs) have been found in tissues and cultured cells from patients who suffer from autosomal dominant polycystic kidney disease, and this PG decrease has been hypothesized to be responsible for cystogenesis. This is possible because alterations in PG concentrations would be predicted to disrupt many homeostatic mechanisms of growth, development, and metabolism. To test this hypothesis, we have generated mice lacking xylosyltransferase 2 (XylT2), an enzyme involved in PG biosynthesis. Here we show that inactivation of XylT2 results in a substantial reduction in PGs and a phenotype characteristic of many aspects of polycystic liver and kidney disease, including biliary epithelial cysts, renal tubule dilation, organ fibrosis, and basement membrane abnormalities. Our findings demonstrate that alterations in PG concentrations can occur due to loss of XylT2, and that reduced PGs can induce cyst development.

The role for HNF-1beta-targeted collectrin in maintenance of primary cilia and cell polarity in collecting duct cells

Collectrin, a homologue of angiotensin converting enzyme 2 (ACE2), is a type I transmembrane protein, and we originally reported its localization to the cytoplasm and apical membrane of collecting duct cells. Recently, two independent studies of targeted disruption of collectrin in mice resulted in severe and general defects in renal amino acid uptake. Collectrin has been reported to be under the transcriptional regulation by HNF-1α, which is exclusively expressed in proximal tubules and localized at the luminal side of brush border membranes. The deficiency of collectrin was associated with reduction of multiple amino acid transporters on luminal membranes. In the current study, we describe that collectrin is a target of HNF-1β and heavily expressed in the primary cilium of renal collecting duct cells. Collectrin is also localized in the vesicles near the peri-basal body region and binds to γ-actin-myosin II-A, SNARE, and polycystin-2-polaris complexes, and all of these are involved in intracellular and ciliary movement of vesicles and membrane proteins. Treatment of mIMCD3 cells with collectrin siRNA resulted in defective cilium formation, increased cell proliferation and apoptosis, and disappearance of polycystin-2 in the primary cilium. Suppression of collectrin mRNA in metanephric culture resulted in the formation of multiple longitudinal cysts in ureteric bud branches. Taken together, the cystic change and formation of defective cilium with the interference in the collectrin functions would suggest that it is necessary for recycling of the primary cilia-specific membrane proteins, the maintenance of the primary cilia and cell polarity of collecting duct cells. The transcriptional hierarchy between HNF-1β and PKD (polycystic kidney disease) genes expressed in the primary cilia of collecting duct cells has been suggested, and collectrin is one of such HNF-1β regulated genes.

Cell biology of polycystin-2

Naturally occurring mutations in two separate, but interacting loci, pkd1 and pkd2 are responsible for almost all cases of autosomal dominant polycystic kidney disease (ADPKD). ADPKD is one of the most common genetic diseases resulting primarily in the formation of large kidney, liver, and pancreatic cysts. Homozygous deletion of either pkd1 or pkd2 results in embryonic lethality in mice due to kidney and heart defects illustrating their indispensable roles in mammalian development. However, the mechanism by which mutations in these genes cause ADPKD and other developmental defects are unknown. Research in the past several years has revealed that PKD2 has multiple functions depending on its subcellular localization. It forms a receptor-operated, non-selective cation channel in the plasma membrane, a novel intracellular Ca2+ release channel in the endoplasmic reticulum (ER), and a mechanosensitive channel in the primary cilium. This review focuses on the functional compartmentalization of PKD2, its modes of activation, and PKD2-mediated signal transduction.

TRPP2 and autosomal dominant polycystic kidney disease

Mutations in TRPP2 (polycystin-2) cause autosomal dominant polycystic kidney disease (ADPKD), a common genetic disorder characterized by progressive development of fluid-filled cysts in the kidney and other organs. TRPP2 is a Ca(2+)-permeable nonselective cation channel that displays an amazing functional versatility at the cellular level. It has been implicated in the regulation of diverse physiological functions including mechanosensation, cell proliferation, polarity, and apoptosis. TRPP2 localizes to different subcellular compartments, such as the endoplasmic reticulum (ER), the plasma membrane and the primary cilium. The channel appears to have distinct functions in different subcellular compartments. This functional compartmentalization is thought to contribute to the observed versatility and specificity of TRPP2-mediated Ca(2+) signaling. In the primary cilium, TRPP2 has been suggested to function as a mechanosensitive channel that detects fluid flow in the renal tubule lumen, supporting the proposed role of the primary cilium as the unifying pathogenic concept for cystic kidney disease. This review summarizes the known and emerging functions of TRPP2, focusing on the question of how channel function translates into complex morphogenetic programs regulating tubular structure.

Diagnosis, pathogenesis, and treatment prospects in cystic kidney disease

Cystic kidney diseases (CKDs) are a clinically and genetically heterogeneous group of disorders characterized by progressive fibrocystic renal and hepatobiliary changes. Recent findings have proven the cystogenic process to be compatible with cellular dedifferentiation, i. e. increased apoptosis and proliferation rates, altered protein sorting and secretory characteristics, as well as disorganization of the extracellular matrix. Compelling evidence suggests that cilia play a central pathogenic role and most cystic kidney disorders converge into a common pathogenic pathway. Recently, several promising trials have further extended our understanding of the pathophysiology of CKD and may have the potential for rational personalized therapies in future years. This review aims to summarize the current state of knowledge of the structure and function of proteins underlying polycystic kidney disease, to explore the clinical consequences of changes in respective genes, and to discuss potential therapeutic approaches.

ADP-ribosylation factor-like 3 is involved in kidney and photoreceptor development

ADP-ribosylation factor-like 3 (Arl3) is a member of a small subfamily of G-proteins involved in membrane-associated vesicular and intracellular trafficking processes. Genetic studies in Leishmania have shown that the Arl3 homolog is essential for flagellum biogenesis. Mutations in a related human family member, Arl6, result in Bardet-Biedl syndrome in humans, which is characterized by genital, renal, and retinal abnormalities, obesity, and learning deficits. As part of our large-scale phenotypic screen, mice deficient for the Arl3 gene were generated and analyzed. Arl3 (-/-) mice were born at a sub-Mendelian ratio, were small and sickly, and had markedly swollen abdomens. These mutants failed to thrive, and all died by 3 weeks of age. The (-/-) mice exhibited abnormal development of renal, hepatic, and pancreatic epithelial tubule structures, which is characteristic of the renal-hepatic-pancreatic dysplasia found in autosomal recessive polycystic kidney disease. Absence of Arl3 was associated with abnormal epithelial cell proliferation and cyst formation. Moreover, mice lacking Arl3 exhibited photoreceptor degeneration as early as postnatal day 14. These results are the first to implicate Arl3 in a ciliary disease affecting the kidney, biliary tract, pancreas, and retina.

Algorithm for efficient PKHD1 mutation screening in autosomal recessive polycystic kidney disease (ARPKD)

Autosomal recessive polycystic kidney disease (ARPKD) is an important cause of childhood renal- and liver-related morbidity and mortality with variable disease expression. While most cases manifest peri-/neonatally with a high mortality rate in the first month of life, others survive to adulthood. ARPKD is caused by mutations in the Polycystic Kidney and Hepatic Disease 1 (PKHD1) gene on chromosome 6p12. PKHD1 is an exceptionally large gene (470 kb) with a longest open reading frame transcript of 67 exons predicted to encode a 4,074-amino acid (aa) (447 kDa) multidomain integral membrane protein (fibrocystin/polyductin) of unknown function. Recent DHPLC-based mutational studies have reported detection rates of about 80% and a minimum of one PKHD1 mutation in more than 95% of families. Thus far, a total of 263 different PKHD1 mutations (639 mutated alleles) are included in the locus-specific database (www.humgen.rwth-aachen.de). Except for a few population-specific founder alleles and the common c.107C>T (p.Thr36Met) missense change, PKHD1 is characterized by significant allelic diversity, making mutation screening time-consuming and labor-intensive. Mutations are distributed throughout the gene's coding sequence; however, they are not equally scattered. Thus, we aimed to set up an algorithm for efficient molecular genetic diagnostics in ARPKD. A total of 80% of known PKHD1 mutations can be identified if a subset of 27 out of 77 DHPLC fragments is screened. The current study provides an essential platform for PKHD1 mutation screening in a routine setting that will largely alleviate molecular genetic diagnostics in patients suspected to have ARPKD.

Polycystin-2 traffics to cilia independently of polycystin-1 by using an N-terminal RVxP motif

Primary cilia play a key role in the pathogenesis of autosomal dominant polycystic kidney disease (ADPKD). The affected proteins, polycystin-1 (PC1) and polycystin-2 (PC2), interact with each other and are expressed in cilia. We found that COOH-terminal truncated PC2 (PC2-L703X), lacking the PC1 interaction region, still traffics to cilia. We examined PC2 expression in several tissues and cells lacking PC1 and found that PC2 is expressed in cilia independently of PC1. We used N-terminal deletion constructs to narrow the domain necessary for cilia trafficking to the first 15 amino acids of PC2 and identified a conserved motif, R6VxP, that is required for cilial localization. The N-terminal 15 amino acids are also sufficient to localize heterologous proteins in cilia. PC2 has endogenous cilia trafficking information and is present in cilia of cells lining cysts that result from mutations in PKD1.

Polycystin-1, STAT6, and P100 function in a pathway that transduces ciliary mechanosensation and is activated in polycystic kidney disease

Primary cilia are implicated in the pathogenesis of autosomal-dominant polycystic kidney disease (ADPKD), which results from defects in polycystin-1 (PC1), but the function of PC1 remains poorly understood. Here, we show that PC1 undergoes proteolytic cleavage that results in nuclear translocation of its cytoplasmic tail. The PC1 tail interacts with the transcription factor STAT6 and the coactivator P100, and it stimulates STAT6-dependent gene expression. Under normal conditions, STAT6 localizes to primary cilia of renal epithelial cells. Cessation of apical fluid flow results in nuclear translocation of STAT6. Cyst-lining cells in ADPKD exhibit elevated levels of nuclear STAT6, P100, and the PC1 tail. Exogenous expression of the human PC1 tail results in renal cyst formation in zebrafish embryos. These results identify a novel mechanism of cilia function in the transduction of a mechanical signal to changes of gene expression involving PC1 and show that this pathway is inappropriately activated in ADPKD.

The transcription factor hepatocyte nuclear factor-6 controls the development of pancreatic ducts in the mouse

BACKGROUND & AIMS

A number of hereditary polycystic diseases are associated with formation of cysts within the pancreatic ducts. The cysts result from abnormal tubulogenesis, but how normal pancreatic duct development is controlled remains poorly understood. Here, we investigate the transcriptional mechanisms that control pancreatic duct development by addressing the role of the transcription factor hepatocyte nuclear factor (HNF)-6.

METHODS

Using immunostaining, we have determined the expression pattern of HNF-6 in pancreatic ducts during mouse development. Hnf6 null mice at various stages of development were studied by immunolocalization methods to assess the morphology, differentiation, and proliferation status of ductal cells. The expression of genes involved in hereditary polycystic diseases was determined by real-time, reverse-transcription polymerase chain reaction (RT-PCR).

RESULTS

We show that HNF-6 is expressed in the pancreatic duct epithelium throughout development and that, in the absence of HNF-6, duct morphogenesis is perturbed. Although development of the intercalated ducts is normal, cysts appear within the interlobular and intralobular ducts. This is associated with abnormal development of primary cilia at the apical pole of the duct cells and with reduced expression of a set of genes involved in polycystic diseases, namely those coding for HNF-1beta and for the cilium-associated proteins polyductin/fibrocystin and cystin.

CONCLUSIONS

We identify HNF-6 as the first transcriptional regulator of pancreatic duct development and reveal the existence of different regulatory mechanisms in distinct duct compartments. HNF-6 controls a network of genes involved in cilium formation and in hereditary polycystic diseases. Finally, HNF-6 deficiency represents a genetically defined model of pancreatic cystic disease.

The C. elegans homologs of nephrocystin-1 and nephrocystin-4 are cilia transition zone proteins involved in chemosensory perception

Nephronophthisis (NPH) is a cystic kidney disorder that causes end-stage renal failure in children. Five nephrocystin (nephrocystin-1 to nephrocystin-5) genes, whose function is disrupted in NPH patients, have been identified and data indicate they form a complex at cell junctions and focal adhesions. More recently, the nephrocystin proteins have also been identified in cilia, as have multiple other cystic kidney disease related proteins. Significant insights into this cilia and cystic kidney disease connection have come from analyses in simpler eukaryotic organisms such as Caenorhabditis elegans. In this regard, we became interested in the C. elegans homologs of nephrocystin-1 (nph-1) and nephrocystin-4 (nph-4) from a database screen to identify genes coordinately regulated by the ciliogenic transcription factor DAF-19. Here we show that expression of nph-1 and nph-4 is DAF-19 dependent, that their expression is restricted to ciliated sensory neurons, and that both NPH-1 and NPH-4 concentrate at the transition zones at the base of the cilia, but are not found in the cilium axoneme. In addition, NPH-4 is required for the localization of NPH-1 to this domain. Interestingly, nph-1 or nph-4 mutants have no obvious cilia assembly defects; however, they do have abnormalities in cilia-mediated sensory functions as evidenced by abnormal chemotaxis and lifespan regulation. Our data suggest that rather than having a ciliogenic role, the NPH proteins play an important function as part of the sensory or signaling machinery of this organelle. These findings suggest that the defects in human NPH patients may not be the result of aberrant ciliogenesis but abnormal cilia-sensory functions.

Fibrocystin interacts with CAML, a protein involved in Ca2+ signaling

The predicted structure of the autosomal recessive polycystic kidney disease protein, fibrocystin, suggests that it may function as a receptor, but its function remains unknown. To understand its function, we searched for proteins that interact with the intracellular C-terminus of fibrocystin using the yeast two-hybrid system. From the screening, we found calcium modulating cyclophilin ligand (CAML), a protein involved in Ca(2+) signaling. Immunofluorescent analysis showed that both proteins are co-localized in the apical membrane, primary cilia, and the basal body of cells derived from the distal nephron Epitope-tagged expression constructs of both proteins were co-immunoprecipitated from COS7 cells. The intracellular C-terminus of fibrocystin interacts with CAML, a protein with an intracellular distribution that is similar to that of PKD2. Fibrocystin may participate in regulation of intracellular Ca(2+) in the distal nephron in a manner similar to PKD1 and PKD2 that are involved in autosomal dominant polycystic kidney disease.

Mutant kinesin-2 motor subunits increase chromosome loss

The Chlamydomonas anterograde intraflagellar transport motor, kinesin-2, is isolated as a heterotrimeric complex containing two motor subunits and a nonmotor subunit known as kinesin-associated polypeptide or KAP. One of the two motor subunits is encoded by the FLA10 gene. The sequence of the second motor subunit was obtained by mass spectrometry and sequencing. It shows 46.9% identity with the Fla10 motor subunit and the gene maps to linkage group XII/XIII near RPL9. The temperature-sensitive flagellar assembly mutants fla1 and fla8 are linked to this kinesin-2 motor subunit. In each strain, a unique single point mutation gives rise to a unique single amino acid substitution within the motor domain. The fla8 strain is named fla8-1 and the fla1 strain is named fla8-2. The fla8 and fla10 alleles show a chromosome loss phenotype. To analyze this chromosome loss phenotype, intragenic revertants of fla8-1, fla8-2, and fla10-14 were generated. The analysis of the mutants and the revertants demonstrates the importance of a pocket in the amino terminus of these motor subunits for both motor activity and for a novel, dominant effect on the fidelity of chromosome segregation.