Polycystin-1, the PKD1 gene product, is in a complex containing E-cadherin and the catenins

Cilium, centrosome and cell cycle regulation in polycystic kidney disease

Polycystic kidney disease is the defining condition of a group of common life-threatening genetic disorders characterized by the bilateral formation and progressive expansion of renal cysts that lead to end stage kidney disease. Although a large body of information has been acquired in the past years about the cellular functions that characterize the cystic cells, the mechanisms triggering the cystogenic conversion are just starting to emerge. Recent findings link defects in ciliary functions, planar cell polarity pathway, and centrosome integrity in early cystic development. Many of the signals dysregulated during cystogenesis may converge on the centrosome for its central function as a structural support for cilia formation and a coordinator of protein trafficking, polarity, and cell division. Here, we will discuss the contribution of proliferation, cilium and planar cell polarity to the cystic signal and will analyze in particular the possible role that the basal bodies/centrosome may play in the cystogenetic mechanisms. This article is part of a Special Issue entitled: Polycystic Kidney Disease.

Effect of PKD1 gene missense mutations on polycystin-1 membrane topogenesis

Polycystin-1 (PC1), the product of the polycystic kidney disease-1 (PKD1) gene, has a number of reported missense mutations whose pathogenicity is indeterminate. Previously, we utilized N-linked glycosylation reporter tags along with membrane insertion and topology assays to define the 11 membrane-spanning domains (I-XI) of PC1. In this report, we utilize glycosylation assays to determine whether two reported human polymorphisms/missense mutations within transmembrane (TM) domains VI and X affect the membrane topology of PC1. M3677T within TM VI had no effect on the topology of this TM domain as shown by the ability of two native N-linked glycosylation sites within the extracellular loop following TM VI to be glycosylated. In contrast, G4031D, within TM X, decreased the glycosylation of TM X reporter constructs, demonstrating that the substitution affected the C-terminal translocating activity of TM X. Furthermore, G4031D reduced the membrane association of TM X and XI together. These results suggest that G4031D affects the membrane insertion and topology of the C-terminal portion of polycystin-1 and represents a bona fide pathogenic mutation.

The cell biology of polycystic kidney disease

Polycystic kidney disease is a common genetic disorder in which fluid-filled cysts displace normal renal tubules. Here we focus on autosomal dominant polycystic kidney disease, which is attributable to mutations in the PKD1 and PKD2 genes and which is characterized by perturbations of renal epithelial cell growth control, fluid transport, and morphogenesis. The mechanisms that connect the underlying genetic defects to disease pathogenesis are poorly understood, but their exploration is shedding new light on interesting cell biological processes and suggesting novel therapeutic targets.

TRPP channels and polycystins

The founding member of the TRPP family, TRPP2, was identified as one of the disease genes causing autosomal dominant polycystic kidney disease (ADPKD). ADPKD is the most prevalent, potentially lethal, monogenic disorder in humans, with an average incidence of one in 400 to one in 1,000 individuals worldwide. Here we give an overview of TRPP ion channels and Polycystin-1 receptor proteins focusing on more recent studies. We include the Polycystin-1 family since these proteins are functionally linked to TRPP channels.

Receptor protein tyrosine phosphatases are novel components of a polycystin complex

Autosomal dominant polycystic kidney disease (ADPKD) is caused by mutation of PKD1 and PKD2 that encode polycystin-1 and polycystin-2. Polycystin-1 is tyrosine phosphorylated and modulates multiple signaling pathways including AP-1, and the identity of the phosphatases regulating polycystin-1 are previously uncharacterized. Here we identify members of the LAR protein tyrosine phosphatase (RPTP) superfamily as members of the polycystin-1complex mediated through extra- and intracellular interactions. The first extracellular PKD1 domain of polycystin-1 interacts with the first Ig domain of RPTPσ, while the polycystin-1 C-terminus of polycystin-1 interacts with the regulatory D2 phosphatase domain of RPTPγ. Additional homo- and heterotypic interactions between RPTPs recruit RPTPδ. The multimeric polycystin protein complex is found localised in cilia. RPTPσ and RPTPδ are also part of a polycystin-1/E-cadherin complex known to be important for early events in adherens junction stabilisation. The interaction between polycystin-1 and RPTPγ is disrupted in ADPKD cells, while RPTPσ and RPTPδ remain closely associated with E-cadherin, largely in an intracellular location. The polycystin-1 C-terminus is an in vitro substrate of RPTPγ, which dephosphorylates the c-Src phosphorylated Y4237 residue and activates AP1-mediated transcription. The data identify RPTPs as novel interacting partners of the polycystins both in cilia and at adhesion complexes and demonstrate RPTPγ phosphatase activity is central to the molecular mechanisms governing polycystin-dependent signaling. This article is part of a Special Issue entitled: Polycystic Kidney Disease.

Molecular genetics and pathogenic mechanisms for the severe ciliopathies: insights into neurodevelopment and pathogenesis of neural tube defects

Meckel-Gruber syndrome (MKS) is a severe autosomal recessively inherited disorder characterized by developmental defects of the central nervous system that comprise neural tube defects that most commonly present as occipital encephalocele. MKS is considered to be the most common syndromic form of neural tube defect. MKS is genetically heterogeneous with six known disease genes: MKS1, MKS2/TMEM216, MKS3/TMEM67, RPGRIP1L, CEP290, and CC2D2A with the encoded proteins all implicated in the correct function of primary cilia. Primary cilia are microtubule-based organelles that project from the apical surface of most epithelial cell types. Recent progress has implicated the involvement of cilia in the Wnt and Shh signaling pathways and has led to an understanding of their role in normal mammalian neurodevelopment. The aim of this review is to provide an overview of the molecular genetics of the human disorder, and to assess recent insights into the etiology and molecular cell biology of severe ciliopathies from mammalian animal models of MKS.

Epithelial-to-mesenchymal transition in cyst lining epithelial cells in an orthologous PCK rat model of autosomal-recessive polycystic kidney disease

In polycystic kidney disease (PKD), cyst lining cells show polarity abnormalities. Recent studies have demonstrated loss of cell contact in cyst cells, suggesting induction of epithelial-to-mesenchymal transition (EMT). Recently, EMT has been implicated in the pathogenesis of PKD. To explore further evidence of EMT in PKD, we examined age- and segment-specific expression of adhesion molecules and mesenchymal markers in PCK rats, an orthologous model of human autosomal-recessive PKD. Kidneys from 5 male PCK and 5 control rats each at 0 days, 1, 3, 10, and 14 wk, and 4 mo of age were serially sectioned and stained with segment-specific markers and antibodies against E-cadherin, Snail1, β-catenin, and N-cadherin. mRNAs for E-cadherin and Snail1 were quantified by real-time PCR. Vimentin, fibronectin, and α-smooth muscle actin (α-SMA) expressions were assessed as mesenchymal markers. E-cadherin expression pattern was correlated with the disease pathology in that tubule segments showing the highest expression in control had much severer cyst formation in PCK rats. In PCK rats, E-cadherin and β-catenin in cystic tubules was attenuated and localized to lateral areas of cell-cell contact, whereas nuclear expression of Snail1 increased in parallel with cyst enlargement. Some epithelial cells in large cysts derived from these segments, especially in adjacent fibrotic areas, showed positive immunoreactivity for vimentin and fibronectin. In conclusion, these findings suggest that epithelial cells in cysts acquire mesenchymal features in response to cyst enlargement and participate in progressive renal fibrosis. Our study clarified the nephron segment-specific cyst profile related to EMT in PCK rats. EMT may play a key role in polycystic kidney disease.

Novel targets for the treatment of autosomal dominant polycystic kidney disease

IMPORTANCE OF THE FIELD

Autosomal dominant (AD) polycystic kidney disease (PKD) is the most common life-threatening hereditary disorder. There is currently no therapy that slows or prevents cyst formation and kidney enlargement in humans. An increasing number of animal studies have advanced our understanding of molecular and cellular targets of PKD.

AREAS COVERED IN THE REVIEW

The purpose of this review is to summarize the molecular and cellular targets involved in cystogenesis and to update on the promising therapies that are being developed and tested based on knowledge of these molecular and cellular targets.

WHAT THE READER WILL GAIN

Insight into the pathogenesis of PKD and how a better understanding of the pathogenesis of PKD has led to the development of potential therapies to inhibit cyst formation and/or growth and improve kidney function.

TAKE HOME MESSAGE

The results of animal studies in PKD have led to the development of clinical trials testing potential new therapies to reduce cyst formation and/or growth. A vasopressin V2 receptor antagonist, mTOR inhibitors, blockade of the renin-angiotensin system and statins that reduce cyst formation and improve renal function in animal models of PKD are being tested in interventional studies in humans.

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.

Polycystin-1 is a microtubule-driven desmosome-associated component in polarized epithelial cells

In this study, we have analyzed the expression and localization of polycystin-1 in intestinal epithelial cells, a system lacking primary cilia. Polycystin-1 was found to be expressed in the epithelium of the small intestine during development and levels remained elevated in the adult. Dual-labelling indirect immunofluorescence revealed polycystin-1 at sites of cell-cell contact co-localizing with the desmosomes both in situ as well as in polarized Caco-2/15 cells. In unpolarized cultures of Caco-2/15 cells, polycystin-1 was recruited to the cell surface early during initiation of cell junction assembly. In isolated Caco-2/15 cells and HIEC-6 cell cultures, where junctional complexes are absent, polycystin-1 was found predominantly associated with the cytoskeletal elements of the intermediate filaments and microtubule networks. More precisely, polycystin-1 was seen as brightly labelled puncta decorating the keratin-18 positive filaments as well as the beta-tubulin positive microtubules, which was particularly obvious in the lamellipodia. Treatment with the microtubule-disrupting agent, nocodazole, eliminated the microtubule association of polycystin-1 but did not seem to affect its association with keratin or the desmosomes. Taken together these data suggest that polycystin-1 is involved with the establishment of cell-cell junctions in absorptive intestinal epithelial cells and exploits the microtubule-based machinery in order to be transported to the plasma membrane.

Polycystin-1 interacts with inositol 1,4,5-trisphosphate receptor to modulate intracellular Ca2+ signaling with implications for polycystic kidney disease

The PKD1 or PKD2 genes encode polycystins (PC) 1 and 2, which are associated with polycystic kidney disease. Previously we demonstrated that PC2 interacts with the inositol 1,4,5-trisphosphate receptor (IP(3)R) to modulate Ca(2+) signaling. Here, we investigate whether PC1 also regulates IP(3)R. We generated a fragment encoding the last six transmembrane (TM) domains of PC1 and the C-terminal tail (QIF38), a section with the highest homology to PC2. Using a Xenopus oocyte Ca(2+) imaging system, we observed that expression of QIF38 significantly reduced the initial amplitude of IP(3)-induced Ca(2+) transients, whereas a mutation lacking the C-terminal tail did not. Thus, the C terminus is essential to QIF38 function. Co-immunoprecipitation assays demonstrated that through its C terminus, QIF38 associates with the IP(3)-binding domain of IP(3)R. A shorter PC1 fragment spanning only the last TM and the C-terminal tail also reduced IP(3)-induced Ca(2+) release, whereas another C-terminal fragment lacking any TM domain did not. Thus, only endoplasmic reticulum-localized PC1 can modulate IP(3)R. Finally, we show that in the polarized Madin-Darby canine kidney cells, heterologous expression of full-length PC1 resulted in a smaller IP(3)-induced Ca(2+) response. Overexpression of the IP(3)-binding domain of IP(3)R reversed the inhibitory effect of PC1, suggesting interaction of full-length PC1 (or its cleavage forms) with endogenous IP(3)R in Madin-Darby canine kidney cells. These results indicate that the behavior of full-length PC1 in mammalian cells is congruent with that of PC1 C-terminal fragments in the oocyte system. These data demonstrate that PC1 inhibits Ca(2+) release, perhaps opposing the effect of PC2, which facilitates Ca(2+) release through the IP(3)R.

Renal involvement in tuberous sclerosis complex and von Hippel-Lindau disease: shared disease mechanisms?

Tuberous sclerosis complex and von Hippel-Lindau disease are distinct autosomal dominant tumor suppressor syndromes that can exhibit similar renal phenotypes and seem to share some signaling pathway components. Similarities exist in the current clinical management of, and the newly identified potential therapeutic approaches for, these conditions. This Review summarizes the pathophysiologic and therapeutic overlap between tuberous sclerosis complex and von Hippel-Lindau disease and highlights the results of recent drug trials in these settings.

Homophilic and heterophilic polycystin 1 interactions regulate E-cadherin recruitment and junction assembly in MDCK cells

Autosomal dominant polycystic kidney disease (ADPKD) is the most common inherited human renal disease and is caused by mutations in two genes, PKD1 (85%) and PKD2 (15%). Cyst epithelial cells are characterised by a complex cellular phenotype including changes in proliferation, apoptosis, basement membrane composition and apicobasal polarity. Since polycystin 1 (PC1), the PKD1 protein, has been located in the basolateral membrane of kidney epithelial cells, we hypothesised that it might have a key role in mediating or stabilising cell-cell interactions. In non-ciliated L929 cells, stable or transient surface expression of the PC1 extracellular domain was sufficient to confer an adhesive phenotype and stimulate junction formation. In MDCK cells, we found that PC1 was recruited to the lateral membranes coincident with E-cadherin within 30 minutes after a ;calcium switch'. Recruitment of both proteins was significantly delayed when cells were treated with a PC1 blocking antibody raised to the PKD domains. Finally, PC1 and E-cadherin could be coimmunoprecipitated together from MDCK cells. We conclude that PC1 has a key role in initiating junction formation via initial homophilic interactions and facilitates junction assembly and the establishment of apicobasal polarity by E-cadherin recruitment.

Beta-catenin gets jaded and von Hippel-Lindau is to blame

Numerous studies have pointed to interactions between the tumor suppressor von Hippel-Lindau (VHL) and the oncogenic Wnt-beta-catenin signaling cascade; however, the mechanism of this crosstalk has remained elusive. Among other roles, VHL can promote the stabilization of Jade-1. Now, recent findings provide compelling evidence that Jade-1 ubiquitylates beta-catenin, leading to its degradation. Thus, the loss of VHL, as seen in clear cell renal cell carcinoma, could lead to tumor formation through beta-catenin de-repression.

A short carboxy-terminal domain of polycystin-1 reorganizes the microtubular network and the endoplasmic reticulum

Mutations of PKD1 cause autosomal dominant polycystic kidney disease (ADPKD), a syndrome characterized by kidney cysts and progressive renal failure. Polycystin-1, the protein encoded by PKD1, is a large integral membrane protein with a short carboxy-terminal cytoplasmic domain that appears to initiate multiple cellular programs. We report now that this polycystin-1 domain contains a novel motif responsible for rearrangements of intermediate filaments, microtubules and the endoplasmic reticulum (ER). This motif reveals homology to CLIMP-63, a microtubule-binding protein that rearranges the ER. Our findings suggest that polycystin-1 influences the shape and localization of both the microtubular network and the ER.

Vasopressin antagonists in polycystic kidney disease

Increased cell proliferation and fluid secretion, probably driven by alterations in intracellular calcium homeostasis and cyclic adenosine 3,5-phosphate, play an important role in the development and progression of polycystic kidney disease. Hormone receptors that affect cyclic adenosine monophosphate and are preferentially expressed in affected tissues are logical treatment targets. There is a sound rationale for considering the arginine vasopressin V2 receptor as a target. The arginine vasopressin V2 receptor antagonists OPC-31260 and tolvaptan inhibit the development of polycystic kidney disease in cpk mice and in three animal orthologs to human autosomal recessive polycystic kidney disease (PCK rat), autosomal dominant polycystic kidney disease (Pkd2/WS25 mice), and nephronophthisis (pcy mouse). PCK rats that are homozygous for an arginine vasopressin mutation and lack circulating vasopressin are markedly protected. Administration of V2 receptor agonist 1-deamino-8-D-arginine vasopressin to these animals completely recovers the cystic phenotype. Administration of 1-deamino-8-D-arginine vasopressin to PCK rats with normal arginine vasopressin aggravates the disease. Suppression of arginine vasopressin release by high water intake is protective. V2 receptor antagonists may have additional beneficial effects on hypertension and chronic kidney disease progression. A number of clinical studies in polycystic kidney disease have been performed or are currently active. The results of phase 2 and phase 2-3 clinical trials suggest that tolvaptan is safe and well tolerated in autosomal dominant polycystic kidney disease. A phase 3, placebo-controlled, double-blind study in 18- to 50-yr-old patients with autosomal dominant polycystic kidney disease and preserved renal function but relatively rapid progression, as indicated by a total kidney volume >750 ml, has been initiated and will determine whether tolvaptan is effective in slowing down the progression of this disease.

Disrupted cell adhesion but not proliferation mediates cyst formation in polycystic liver disease

The pathogenesis of polycystic liver disease is not well understood. The putative function of the associated proteins, hepatocystin and Sec63p, do not give insight in their role in cystogenesis and their tissue-wide expression does not fit with the liver-specific phenotype of the disease. We designed this study with the specific aim to dissect whether pathways involved in polycystic kidney diseases are also implicated in polycystic liver disease. Therefore, we immunohistochemically stained cyst tissue specimen with antibodies directed against markers for apoptosis, proliferation, growth receptors, signaling and adhesion. We analyzed genotyped polycystic liver disease cyst tissue (n=21) compared with normal liver tissue (n=13). None of the cysts showed proliferation of epithelial cells. In addition, anti-apoptosis marker Bcl-2 revealed slight increase in expression, with variable increase of apoptosis marker active caspase 3. Growth factor receptors, EGFR and c-erbB-2, were overexpressed and mislocalized. We found EGFR staining in the nuclei of cyst epithelial cells regardless of mutational state of the patient. Further, in hepatocystin-mutant polycystic liver disease patients, apical membranous staining of c-erbB-2 and adhesion markers, MUC1 and CEA, was lost and the proteins appeared to be retained in cytoplasm of cyst epithelia. Finally, we found loss of adhesion molecules E-cadherin and Ep-CAM in cyst epithelium of all patients. Nevertheless, we observed normal beta-catenin expression. Our results show that polycystic liver disease cystogenesis is different from renal cystogenesis. Polycystic liver disease involves overexpression of growth factor receptors and loss of adhesion. In contrast, proliferation or deregulated apoptosis do not seem to be implicated. Moreover differential findings for PRKCSH- and SEC63-associated polycystic liver disease suggest a divergent mechanism for cystogenesis in these two groups.

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.

Tight junction composition is altered in the epithelium of polycystic kidneys

Kidney cysts in autosomal dominant polycystic kidney disease (ADPKD) undergo progressive enlargement together with luminal fluid secretion. This involves active, uphill transcellular Cl(-) transport which drives passive Na(+) and water secretion. Implicit in this mechanism is the assumption that the paracellular permeability of the cyst epithelium to Cl(-) must be very low. Claudins are tight junction (TJ) transmembrane proteins that determine the ion selectivity of paracellular barriers. The aim of this study was to determine the expression and localization of claudins within renal cysts in a mouse hypomorphic model of ADPKD and in human patients. We found that the majority of cysts were of collecting duct origin. Claudins normally expressed in collecting duct (3, 4, 7, 8, and 10) were found in small cysts. However, only claudin-7 persisted at substantive levels in the dedifferentiated epithelium of large, presumably late-stage cysts, where it was localized both at the TJ and basolaterally. The constitutively expressed TJ proteins, ZO-1 and occludin, were also abundantly expressed and correctly localized, suggesting that the basic infrastructure of the TJ is preserved. A previous study suggested that claudin-7 may function as a paracellular Cl(-) barrier. We postulate that the role of claudin-7 in ADPKD is to seal the paracellular route in Cl(-)-secreting cyst epithelium, preventing backleak of Cl(-), and that it thereby plays a permissive role in fluid secretion and cyst growth.

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.

Polycystic kidney diseases: from molecular discoveries to targeted therapeutic strategies

Polycystic kidney diseases (PKDs) represent a large group of progressive renal disorders characterized by the development of renal cysts leading to end-stage renal disease. Enormous strides have been made in understanding the pathogenesis of PKDs and the development of new therapies. Studies of autosomal dominant and recessive polycystic kidney diseases converge on molecular mechanisms of cystogenesis, including ciliary abnormalities and intracellular calcium dysregulation, ultimately leading to increased proliferation, apoptosis and dedifferentiation. Here we review the pathobiology of PKD, highlighting recent progress in elucidating common molecular pathways of cystogenesis. We discuss available models and challenges for therapeutic discovery as well as summarize the results from preclinical experimental treatments targeting key disease-specific pathways.

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.

Physiology and pathophysiology of the vasopressin-regulated renal water reabsorption

To prevent dehydration, terrestrial animals and humans have developed a sensitive and versatile system to maintain their water homeostasis. In states of hypernatremia or hypovolemia, the antidiuretic hormone vasopressin (AVP) is released from the pituitary and binds its type-2 receptor in renal principal cells. This triggers an intracellular cAMP signaling cascade, which phosphorylates aquaporin-2 (AQP2) and targets the channel to the apical plasma membrane. Driven by an osmotic gradient, pro-urinary water then passes the membrane through AQP2 and leaves the cell on the basolateral side via AQP3 and AQP4 water channels. When water homeostasis is restored, AVP levels decline, and AQP2 is internalized from the plasma membrane, leaving the plasma membrane watertight again. The action of AVP is counterbalanced by several hormones like prostaglandin E2, bradykinin, dopamine, endothelin-1, acetylcholine, epidermal growth factor, and purines. Moreover, AQP2 is strongly involved in the pathophysiology of disorders characterized by renal concentrating defects, as well as conditions associated with severe water retention. This review focuses on our recent increase in understanding of the molecular mechanisms underlying AVP-regulated renal water transport in both health and disease.

TRP channels in disease

The transient receptor potential (TRP) channels are a large family of proteins with six main subfamilies termed the TRPC (canonical), TRPV (vanilloid), TRPM (melastatin), TRPP (polycystin), TRPML (mucolipin), and TRPA (ankyrin) groups. The sheer number of different TRPs with distinct functions supports the statement that these channels are involved in a wide range of processes ranging from sensing of thermal and chemical signals to reloading intracellular stores after responding to an extracellular stimulus. Mutations in TRPs are linked to pathophysiology and specific diseases. An understanding of the role of TRPs in normal physiology is just beginning; the progression from mutations in TRPs to pathophysiology and disease will follow. In this review, we focus on two distinct aspects of TRP channel physiology, the role of TRP channels in intracellular Ca2+ homeostasis, and their role in the transduction of painful stimuli in sensory neurons.

PKD1 inhibits cancer cells migration and invasion via Wnt signaling pathway in vitro

The approximately 14 kb mRNA of the polycystic kidney disease gene PKD1 encodes a large ( approximately 460 kDa) protein, termed polycystin-1 (PC-1), that is responsible for autosomal dominant polycystic kidney disease (ADPKD). The unique organization of its multiple adhesive domains (16 Ig-like domains/PKD domains) suggests that it may play an important role in cell-cell/cell-matrix interactions. Here we demonstrated that PKD1 promoted cell-cell and cell-matrix interactions in cancer cells, indicating that PC-1 is involved in the cell adhesion process. Furthermore in this study, we showed that PKD1 inhibited cancer cells migration and invasion. And we also showed that PC-1 regulated these processes in a process that may be at least partially through the Wnt pathway. Collectively, our data suggest that PKD1 may act as a novel member of the tumor suppressor family of genes.

Pkd1 and Nek8 mutations affect cell-cell adhesion and cilia in cysts formed in kidney organ cultures

Development of novel therapies for polycystic kidney disease (PKD) requires assays that adequately reflect disease biology and are adaptable to high-throughput screening. Here we describe an embryonic cystic kidney organ culture model and demonstrate that a new mutant allele of the Pkd1 gene ( Pkd1 tm1Bdgz) modulates cystogenesis in this model. Cyst formation induced by cAMP is influenced by the dosage of the mutant allele: Pkd1 tm1Bdgz −/− cultures develop a larger cystic area compared with +/+ counterparts, while Pkd1 tm1Bdgz +/− cultures show an intermediate phenotype. A similar relationship between the degree of cystogenesis and mutant gene dosage is seen in cystic kidney organ cultures derived from mice with a mutated Nek8 gene ( Nek8 jck). Both Pkd1− and Nek8− cultures display altered cell-cell junctions, with reduced E-cadherin expression and altered desmosomal protein expression, similar to ADPKD epithelia. Additionally, characteristic ciliary abnormalities are identified in cystic kidney cultures, with elevated ciliary polycystin 1 expression in Nek8 homozygous cultures and elevated ciliary Nek8 protein expression in Pkd1 homozygotes. These data suggest that the Nek8 and Pkd1 genes function in a common pathway to regulate cystogenesis. Moreover, compound Pkd1 and Nek8 heterozygous adult mice develop a more aggressive cystic disease than animals with a mutation in either gene alone. Finally, we validate the kidney organ culture cystogenesis assay as a therapeutic testing platform using the CDK inhibitor roscovitine. Therefore, embryonic kidney organ culture represents a relevant model for studying molecular cystogenesis and a rapid tool for the screening for therapies that block cystic growth.

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.

Polycystin-1 induces cell migration by regulating phosphatidylinositol 3-kinase-dependent cytoskeletal rearrangements and GSK3beta-dependent cell cell mechanical adhesion

Polycystin-1 (PC-1) is a large plasma-membrane receptor encoded by the PKD1 gene mutated in autosomal dominant polycystic kidney disease (ADPKD). Although the disease is thought to be recessive on a molecular level, the precise mechanism of cystogenesis is unclear, although cytoarchitecture defects seem to be the most likely initiating events. Here we show that PC-1 regulates the actin cytoskeleton in renal epithelial cells (MDCK) and induces cell scattering and cell migration. All of these effects require phosphatidylinositol 3-kinase (PI3-K) activity. Consistent with these observations Pkd1-/- mouse embryonic fibroblasts (MEFs) have reduced capabilities to migrate compared with controls. PC-1 overexpressing MDCK cells are able to polarize normally with proper adherens and tight junctions formation, but show quick reabsorption of ZO-1, E-cadherin, and beta-catenin upon wounding of a monolayer and a transient epithelial-to-mesenchymal transition (EMT) that favors a rapid closure of the wound and repolarization. Finally, we show that PC-1 is able to control the turnover of cytoskeletal-associated beta-catenin through activation of GSK3beta. Expression of a nondegradable form of beta-catenin in PC-1 MDCK cells restores strong cell-cell mechanical adhesion. We propose that PC-1 might be a central regulator of epithelial plasticity and its loss results in impaired normal epithelial homeostasis.

Autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease is the most prevalent, potentially lethal, monogenic disorder. It is associated with large interfamilial and intrafamilial variability, which can be explained to a large extent by its genetic heterogeneity and modifier genes. An increased understanding of the disorder's underlying genetic, molecular, and cellular mechanisms and a better appreciation of its progression and systemic manifestations have laid out the foundation for the development of clinical trials and potentially effective treatments.

Annexin A5 interacts with polycystin-1 and interferes with the polycystin-1 stimulated recruitment of E-cadherin into adherens junctions

Polycystin-1 is the gene product of PKD1, the first gene identified to be causative for the condition of autosomal dominant polycystic kidney disease (ADPKD). Mutations in PKD1 are responsible for the majority of ADPKD cases worldwide. Polycystin-1 is a protein of the transient receptor potential channels superfamily, with 11 transmembrane spans and an extracellular N-terminal region of approximately 3109 amino acid residues, harboring multiple putative ligand binding domains. We demonstrate here that annexin A5 (ANXA5), a Ca(2+) and phospholipid binding protein, interacts with the N-terminal leucine-rich repeats of polycystin-1, in vitro and in a cell culture model. This interaction is direct and specific and involves a conserved sequence of the ANXA5 N-terminal domain. Using Madin-Darby canine kidney cells expressing polycystin-1 in an inducible manner we also show that polycystin-1 colocalizes with E-cadherin at cell-cell contacts and accelerates the recruitment of intracellular E-cadherin to reforming junctions. This polycystin-1 stimulated recruitment is significantly delayed by extracellular annexin A5.

Temperature-dependent expression of Listeria monocytogenes internalin and internalin-like genes suggests functional diversity of these proteins among the listeriae

The Listeria monocytogenes genome contains genes encoding several internalins and internalin-like proteins. As L. monocytogenes is present in many environments and can infect numerous, diverse host species, the environmental temperature was hypothesized to be a signal that might affect internalin gene transcription. A subgenomic microarray was used to investigate temperature-dependent transcription of 24 members of the internalin gene family in L. monocytogenes 10403S. The levels of internalin gene transcripts for cells grown at 37 degrees C were compared to the levels of transcripts for cells grown at 16, 30, and 42 degrees C using competitive microarray hybridization, and the results were confirmed by performing quantitative reverse transcriptase PCR for 14 internalin genes. Based on these studies, the internalin genes can be grouped into the following five temperature-dependent categories: (i) four sigma(B)-dependent internalin genes (inlC2, inlD, lmo0331, and lmo0610) with the highest levels of transcripts at 16 degrees C and generally the lowest levels of transcripts at 37 degrees C; (ii) three partially PrfA-dependent internalin genes (inlA, inlB, and inlC) with the lowest levels of transcripts at 16 degrees C and the highest levels of transcripts at 37 and 42 degrees C; (iii) four genes (inlG, inlJ, lmo0514, and lmo1290) with the lowest levels of transcripts at 16 degrees C and the highest levels of transcripts at 30 and/or 37 degrees C; (iv) one gene (lmo0327) with the highest levels of transcripts at 16 degrees C and low levels of transcripts at higher temperatures; and (v) 12 internalin genes with no differences in the levels of transcripts at the temperatures used in this study. The temperature-dependent transcription patterns suggest that the relative importance of different internalins varies by environment, which may provide insight into the specific functions of these proteins.

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

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

Epithelial paracellular proteins in health and disease

PURPOSE OF REVIEW

Tight junctions are intercellular seams sealing and preventing the entrance of microorganisms or unwanted substances from the luminal compartment. They also define the border between the basolateral and apical membranes of polarized cells, thus enabling the normal transcellular vectorial transport typical of epithelial function. Their major components are claudin and occludin proteins. Tight junctions are now recognized as having more specific properties in cell function. This review will concentrate on novel findings related to tight junctions in polarized cells.

RECENT FINDINGS

Tight junctions are regulated, interacting with the cell cytoskeleton and being responsible for the reabsorption of molecules. The latter has been exemplified by the discovery of claudin 16 (paracellin-1) as the gene product responsible for the hypomagnesaemia with hypercalciuria syndrome. Pathogenic bacteria and viruses target and use the tight-junction machinery to invade multicellular organisms.

SUMMARY

Tight-junction-targeted research may help not only in the future design of novel therapies against invading microorganisms, but also may promote passage of medications through the normally sealed epithelial barrier. In addition, the regulatory properties of tight junctions may help in the understanding of still unanswered aspects of epithelial ion transport.

Stable knockdown of polycystin-1 confers integrin-alpha2beta1-mediated anoikis resistance

The mechanisms of action of polycystin-1 (PC1) have been difficult to dissect because of its interaction with multiple factors, the heterogeneity of the genetic mutations, and the complexity of the experimental animal models. Here, stable knockdown of PC1 in MDCK epithelial cells was achieved by lentiviral-mediated delivery of a specific small interfering RNA for PKD1. The reduction of PC1 expression prevented tubulogenesis in three-dimensional collagen type I culture in response to hepatocyte growth factor and induced formation of cysts. PC1 knockdown created a condition of haploinsufficiency that led to hyperproliferation, increased adhesion to collagen type I, and increased apoptosis. It was shown that the suppression of PC1 was associated with the increased expression of integrin-alpha2beta1 and reduced apoptosis in cells grown on collagen type I. The engagement of integrin-alpha2beta1 seemed to be essential for the survival because PC1 knockdown cells were significantly less susceptible to anoikis by a mechanism that was reversible by anti-integrin-alpha2beta1 blocking antibodies. Overall, these data link integrin-alpha2beta1 to some of the biologic functions that are ascribed to PC1 and establish the potential of this approach for the direct study of PC1 functions in a genetically defined background. Furthermore, these findings indicate that reduction of PC1 expression levels, rather than the loss of heterozygosity, may be sufficient to induce cystogenesis.

Polycystic kidney disease: Cell division without a c(l)ue?

Polycystic kidneys are caused by an amazingly broad array of genetic mutations and manipulations. The ciliary hypothesis has evolved as the unifying concept of cystogenesis: cilia, bend by fluid flow, initiate a calcium influx that prevents cyst formation. The integrity of ciliary functions has been linked to the polycystic kidney disease gene products localizing to the cilium or the basal body/centrosome. Until recently, the signals and cellular programs located downstream of the ciliary-mediated calcium flux have remained elusive. Now, several reports point towards a role of the cilium or the basal body/centrosome complex in planar cell polarity, a pathway that orients cell in the plane of a tissue layer. First, Inversin, a protein mutated in nephronophthisis type II was found to act as a switch between the canonical and the noncanonical Wnt cascade, suggesting that beta-catenin/TCF-dependent gene transcription has to be curtailed to allow normal tubular differentiation. Second, heterozygote deletions of Bardet-Biedl syndrome proteins affect neural tube closure and disrupt the cochlear sterociliary bundles, two typical planar cell polarity defects. Third, tubular epithelial cells undergo oriented cell division during tubular elongation, along the axis of the anterior-posterior axis of the nephron. Thus, the cilium or the basal body/centrosome complex may provide the spatial cues to position the centrosome and the mitotic spindle before the next cell division. Failure to communicate this spatial information may condemn the tubular epithelial cells to proliferate and to form cysts.

[Polycystic kidney diseases: molecular genetics and counselling]

Autosomal dominant polycystic kidney disease (ADPKD) affects 1 newborn in 400 to 1000 making it the most common inherited form of genetic kidney disease and an important cause of medical morbidity and account for about 10% of end-stage renal disease. Autosomal recessive polycystic kidney disease (ARPKD) is a rare (1/20,000 to 1/40,000) inherited disease in children characterized by the association of dilation of collecting ducts and biliary dysgenesis. The clinical spectrum is variable but it represents an important cause of renal and liver-related morbidity and mortality in neonates and infancy. Symptoms of autosomal recessive PKD can begin before birth. ARPKD is genetically different from ADPKD. Parents who do not have the disease can have a child with the disease if both parents carry the abnormal gene and both pass the gene to their baby. Recently important advances in understanding the molecular basis of ADPKD (i.e. ADPKD1 and ADPKD2) and autosomal recessive PKD (i.e. PKHD1) have been done and are reported here. Genetic counselling is particularly advised in early onset disease families. It permits to determine the type of transmission, to describe the course and the major complications of the disease and to explain currents therapeutics possibilities.

New developments in the epidemiology and genetics of gout

The prevalence of gout appears to be rapidly increasing worldwide and is no longer a disorder suffered primarily by over-fed alcohol consumers. Emerging risk factors include longevity, metabolic syndrome, and new classes of pharmacologic agents. In some ethnic populations, no obvious risk factors can explain the high incidence of hyperuricemia and gout, suggesting a genetic liability. Studies to identify genes associated with gout have included families with defects in purine metabolism, as well as families in whom the occurrence of gout is secondary to renal disorders such as juvenile hyperuricemic nephropathy and medullary cystic kidney disease. Case-control studies of isolated aboriginal cohorts suffering from primary gout have revealed several chromosomal loci that may harbor genes that are important to the development and/or progression of gout.

Medullary cystic kidney disease type 1: mutational analysis in 37 genes based on haplotype sharing

Medullary cystic kidney disease type 1 (MCKD1) is an autosomal dominant, tubulo-interstitial nephropathy that causes renal salt wasting and end-stage renal failure in the fourth to seventh decade of life. MCKD1 was localized to chromosome 1q21. We demonstrated haplotype sharing and confirmed the telomeric border by a recombination of D1S2624 in a Belgian kindred. Since the causative gene has been elusive, high resolution haplotype analysis was performed in 16 kindreds. Clinical data and blood samples of 257 individuals (including 75 affected individuals) from 26 different kindreds were collected. Within the defined critical region mutational analysis of 37 genes (374 exons) in 23 MCKD1 patients was performed. In addition, for nine kindreds RT-PCR analysis for the sequenced genes was done to screen for mutations activating cryptic splice sites. We found consistency with the haplotype sharing hypothesis in an additional nine kindreds, detecting three different haplotype subsets shared within a region of 1.19 Mb. Mutational analysis of all 37 positional candidate genes revealed sequence variations in 3 different genes, AK000210, CCT3, and SCAMP3, that were segregating in each affected kindred and were not found in 96 healthy individuals, indicating, that a single responsible gene causing MCKD1 remains elusive. This may point to involvement of different genes within the MCKD1 critical region.

More than colocalizing with polycystin-1, polycystin-L is in the centrosome

Polycystin-1 and polycystin-2 are involved in autosomal dominant polycystic kidney disease by unknown mechanisms. These two proteins are located in primary cilia where they mediate mechanosensation, suggesting a link between cilia function and renal disease. In this study, we sought to characterize the subcellular localization of polycystin-L, a closely related member of polycystin-2, in epithelial renal cell lines. We have shown that endogenous polycystin-l subcellular distribution is different in proliferative and nonproliferative cultures. Polycystin-L is found mostly in the endoplasmic reticulum in subconfluent cell cultures, while in confluent cells it is redistributed to sites of cell-cell contact and to the primary cilium as is polycystin-1. Subcellular fractionation confirmed a common distribution of polycystin-L and polycystin-1 in the fractions corresponding to those containing the plasma membrane of postconfluent cells. Reciprocal coimmunoprecipitation experiments showed that polycystin-L was associated with polycystin-1 in a common complex in both subconfluent and confluent cell cultures. Interestingly, we also identified a novel site for a polycystin member (polycystin-L) in unciliated cells, the centrosome, which allowed us to reveal an involvement of polycystin-l in cell proliferation.

Defining a link with autosomal-dominant polycystic kidney disease in mice with congenitally low expression of Pkd1

Mouse models for autosomal-dominant polycystic kidney disease (ADPKD), derived from homozygous targeted disruption of Pkd1 gene, generally die in utero or perinatally because of systemic defects. We introduced a loxP site and a loxP-flanked mc1-neo cassette into introns 30 and 34, respectively, of the Pkd1 locus to generate a conditional, targeted mutation. Significantly, before excision of the floxed exons and mc1-neo from the targeted locus by Cre recombinase, mice homozygous for the targeted allele appeared normal at birth but developed polycystic kidney disease with a slower progression than that of Pkd-null mice. Further, the homozygotes continued to produce low levels of full-length Pkd1-encoded protein, suggesting that slight Pkd1 expression is sufficient for renal cyst formation in ADPKD. In this viable model, up-regulation of heparin-binding epidermal growth factor-like growth factor accompanied increased epidermal growth factor receptor signaling, which may be involved in abnormal proliferation of the cyst-lining epithelia. Increased apoptosis in cyst epithelia was only observed in the later period that correlated with the cyst regression. Abnormalities in Na(+)/K(+)-ATPase, aquaporin-2, and vasopressin V2 receptor expression were also identified. This mouse model may be suitable for further studies of progression and therapeutic interventions of ADPKD.

Ets factors regulate the polycystic kidney disease-1 promoter

The Ets family of transcription factors consists of a group of highly conserved sequence-specific DNA binding proteins that functionally cooperate with other transcription factors to regulate a number of diverse cellular processes including proliferation, differentiation, and apoptosis. We have analyzed a 3.3kb 5'-upstream region of the human PKD1 promoter, using transient transfection in HEK293T cells and Drosophila SL2 cells, to demonstrate that the PKD1 promoter is a target of Ets family transcription factors. Our studies showed that PKD1 promoter-luciferase reporter gene expression is downregulated by cotransfected Fli-1 and is upregulated by cotransfected Ets-1. Using deletion constructs, we demonstrated that the sequences responding to Fli-1 and Ets-1 lie within the -200 to +33bp proximal promoter. This region was found to contain two putative Ets response elements (EREs): an upstream (Ets-A) sequence 5'-CGGAA-3' (-181 to -185) and a downstream (Ets-B) sequence 5'-CGGAT-3' (-129 to -133). Site-directed mutagenesis indicated that both EREs are functional. A Fli-1 DNA binding domain mutant construct (W321R), which is incapable of binding DNA, was unable to inhibit basal promoter activity. In contrast, a Fli-1 DNA binding domain truncation mutant construct, which only contains the DNA binding domain and lacks the transactivation domain, was able to inhibit. These results suggest that the effect of Fli-1 is through direct binding to these EREs. Direct binding of Fli-1 and Ets-1 to the Ets-A and Ets-B sites was supported by electrophoretic mobility shift assays. Lastly, competition between Fli-1 and Ets-1 for the two EREs was demonstrated by showing that increasing amounts of Ets-1 could overcome Fli-1 repression of promoter activity. Taken together, these experiments define the proximal PKD1 promoter region as a potential target of Ets family transcription factors.

ADPKD: molecular characterization and quest for treatment

Autosomal-dominant polycystic kidney disease (ADPKD) is a common hereditary disease that features multiple cystogenesis in various organs and vascular defects. The genes responsible for ADPKD, PKD1, and PKD2 have been identified, and the pathological processes of the disease are becoming clearer. This review focuses on recent findings about the molecular and cellular biology of ADPKD, and especially on PKD1. PKD1 and its product, polycystin-1, play pivotal roles in cellular differentiation because they regulate the cell cycle and because polycystin-1 is a component of adherens junctions. A possible link between polycystin-1 and PPARgamma is discussed. The extraordinarily fast research progress in this area in the last decade has now reached a stage where the development of a remedy for ADPKD might become possible in the near future.

Tight junction biology and kidney dysfunction

The epithelial tight junction (TJ) has three major functions. As a "gate," it serves as a regulatory barrier separating and maintaining biological fluid compartments of different composition. As a "fence," it generates and maintains the apicobasal polarity of cells that form the confluent epithelium. Finally, the TJ proteins form a trafficking and signaling platform that regulates cell growth, proliferation, differentiation, and dedifferentiation. Six examples are selected that illustrate the emerging link between TJ dysfunction and kidney disease. First, the glomerular slit diaphragm (GSD) is evolved, in part, from the TJ and, on maturation, exhibits all three functions of the TJ. GSD dysfunction leads to proteinuria and, in some instances, podocyte dedifferentiation and proliferation. Second, accumulating evidence supports epithelial-mesenchymal transformation (EMT) as a major player in renal fibrosis, the final common pathway that leads to end-stage renal failure. EMT is characterized by a loss of cell-cell contact and apicobasal polarity, which are hallmarks of TJ dysfunction. Third, in autosomal dominant polycystic kidney disease, mutations of the polycystins may disrupt their known interactions with the apical junction complex, of which the TJ is a major component. This can lead to disturbances in epithelial polarity regulation with consequent abnormal tubulogenesis and cyst formation. Fourth, evidence for epithelial barrier and polarity dysregulation in the pathogenesis of ischemic acute renal failure will be summarized. Fifth, the association between mutations of paracellin-1, the first TJ channel identified, and clinical disorders of magnesium and calcium wasting and bovine renal fibrosis will be used to highlight an integral TJ protein that can serve multiple TJ functions. Finally, the role of WNK4 protein kinase in shunting chloride across the TJ of the distal nephron will be addressed.

PKD1 and PKD2 mutations in Slovenian families with autosomal dominant polycystic kidney disease

Background

Autosomal dominant polycystic kidney disease (ADPKD) is a genetically heterogeneous disorder caused by mutations in at least two different loci. Prior to performing mutation screening, if DNA samples of sufficient number of family members are available, it is worthwhile to assign the gene involved in disease progression by the genetic linkage analysis.

Methods

We collected samples from 36 Slovene ADPKD families and performed linkage analysis in 16 of them. Linkage was assessed by the use of microsatellite polymorphic markers, four in the case of PKD1 (KG8, AC2.5, CW3 and CW2) and five for PKD2 (D4S1534, D4S2929, D4S1542, D4S1563 and D4S423). Partial PKD1 mutation screening was undertaken by analysing exons 23 and 31–46 and PKD2 .

Results

Lod scores indicated linkage to PKD1 in six families and to PKD2 in two families. One family was linked to none and in seven families linkage to both genes was possible. Partial PKD1 mutation screening was performed in 33 patients (including 20 patients from the families where linkage analysis could not be performed). We analysed PKD2 in 2 patients where lod scores indicated linkage to PKD2 and in 7 families where linkage to both genes was possible. We detected six mutations and eight polymorphisms in PKD1 and one mutation and three polymorphisms in PKD2.

Conclusion

In our study group of ADPKD patients we detected seven mutations: three frameshift, one missense, two nonsense and one putative splicing mutation. Three have been described previously and 4 are novel. Three newly described framesfift mutations in PKD1 seem to be associated with more severe clinical course of ADPKD. Previously described nonsense mutation in PKD2 seems to be associated with cysts in liver and milder clinical course.

Protein Kinase X Activates Ureteric Bud Branching Morphogenesis in Developing Mouse Metanephric Kidney

The human protein kinase X (PRKX) gene was identified previously as a cAMP-dependent serine/threonine kinase that is aberrantly expressed in autosomal dominant polycystic disease kidneys and normally expressed in fetal kidneys. The PRKX kinase belongs to a serine/threonine kinase family that is phylogenetically and functionally distinct from classical protein kinase A kinases. Expression of PRKX activates cAMP-dependent renal epithelial cell migration and tubular morphogenesis in cell culture, suggesting that it might regulate branching growth of the collecting duct system in the fetal kidney. With the use of a mouse embryonic kidney organ culture system that recapitulates early kidney development in vitro, it is demonstrated that lentiviral vector-driven expression of a constitutively active, cAMP-independent PRKX in the ureteric bud epithelium stimulates branching morphogenesis and results in a 2.5-fold increase in glomerular number. These results suggest that PRKX stimulates epithelial branching morphogenesis by activating cell migration and support a role for this kinase in the regulation of nephrogenesis and of collecting system development in the fetal kidney.

Disruption of polycystin-1 function interferes with branching morphogenesis of the ureteric bud in developing mouse kidneys

The polycystic kidney disease (PKD1) gene-encoded protein, polycystin-1, is developmentally regulated, with highest expression levels seen in normal developing kidneys, where it is distributed in a punctate pattern at the basal surface of ureteric bud epithelia. Overexpression in ureteric epithelial cell membranes of an inhibitory pMyr-GFP-PKD1 fusion protein via a retroviral (VVC) delivery system and microinjection into the ureteric bud lumen of embryonic day 11 mouse metanephric kidneys resulted in disrupted branching morphogenesis. Using confocal quantitative analysis, significant reductions were measured in the numbers of ureteric bud branch points and tips, as well as in the total ureteric bud length, volume and area, while significant increases were seen as dilations of the terminal branches, where significant increases in outer diameter and volumes were measured. Microinjection of an activating 5TM-GFP-PKD1 fusion protein had an opposite effect and showed significant increases in ureteric bud length and area. These are the first studies to experimentally manipulate polycystin-1 expression by transduction in the embryonic mouse kidney and suggest that polycystin-1 plays a critical role in the regulation of epithelial morphogenesis during renal development.