Identification and localization of polycystin, the PKD1 gene product

Elevating microRNA levels by targeting biogenesis with steric-blocking antisense oligonucleotides

MicroRNAs (miRNAs) are regulators of gene expression, and their dysregulation is linked to cancer and other diseases, making them important therapeutic targets. Several strategies for targeting and modulating miRNA activity are being explored. For example, steric-blocking antisense oligonucleotides (ASOs) can reduce miRNA activity by either blocking binding sites on specific mRNAs or base-pairing to the miRNA itself to prevent its interaction with the target mRNAs. ASOs have been less explored as a tool to elevate miRNA levels, which could also be beneficial for treating disease. In this study, using the PKD1/miR-1225 gene locus as an example, where miR-1225 is located within a PKD1 intron, we demonstrate an ASO-based strategy that increases miRNA abundance by enhancing biogenesis from the primary miRNA transcript. Disruptions in PKD1 and miR-1225 are associated with autosomal dominant polycystic kidney disease (ADPKD) and various cancers, respectively, making them important therapeutic targets. We investigated PKD1 sequence variants reported in ADPKD that are located within the sequence shared by miR-1225 and PKD1, and identified one that causes a reduction in miR-1225 without affecting PKD1 We show that this reduction in miR-1225 can be recovered by treatment with a steric-blocking ASO. The ASO-induced increase in miR-1225 correlates with a decrease in the abundance of predicted miR-1225 cellular mRNA targets. This study demonstrates that miRNA abundance can be elevated using ASOs targeted to the primary transcript. This steric-blocking ASO-based approach has broad potential application as a therapeutic strategy for diseases that could be treated by modulating miRNA biogenesis.

Cleavage fragments of the C-terminal tail of polycystin-1 are regulated by oxidative stress and induce mitochondrial dysfunction

Mutations in the gene encoding polycystin-1 (PC1) are the most common cause of autosomal dominant polycystic kidney disease (ADPKD). Cysts in ADPKD exhibit a Warburg-like metabolism characterized by dysfunctional mitochondria and aerobic glycolysis. PC1 is an integral membrane protein with a large extracellular domain, a short C-terminal cytoplasmic tail and shares structural and functional similarities with G protein-coupled receptors. Its exact function remains unclear. The C-terminal cytoplasmic tail of PC1 undergoes proteolytic cleavage, generating soluble fragments that are overexpressed in ADPKD kidneys. The regulation, localization, and function of these fragments is poorly understood. Here, we show that a ∼30 kDa cleavage fragment (PC1-p30), comprising the entire C-terminal tail, undergoes rapid proteasomal degradation by a mechanism involving the von Hippel-Lindau tumor suppressor protein. PC1-p30 is stabilized by reactive oxygen species, and the subcellular localization is regulated by reactive oxygen species in a dose-dependent manner. We found that a second, ∼15 kDa fragment (PC1-p15), is generated by caspase cleavage at a conserved site (Asp-4195) on the PC1 C-terminal tail. PC1-p15 is not subject to degradation and constitutively localizes to the mitochondrial matrix. Both cleavage fragments induce mitochondrial fragmentation, and PC1-p15 expression causes impaired fatty acid oxidation and increased lactate production, indicative of a Warburg-like phenotype. Endogenous PC1 tail fragments accumulate in renal cyst-lining cells in a mouse model of PKD. Collectively, these results identify novel mechanisms regarding the regulation and function of PC1 and suggest that C-terminal PC1 fragments may be involved in the mitochondrial and metabolic abnormalities observed in ADPKD.

The cytoplasmic tail of the mechanosensitive channel Pkd2 regulates its internalization and clustering in eisosomes

Polycystins are a family of conserved ion channels, mutations of which lead to one of the most common human genetic disorders, namely, autosomal dominant polycystic kidney disease. Schizosacchromyces pombe possesses an essential polycystin homologue, Pkd2, which directs Ca2+ influx on the cell surface in response to membrane tension, but its structure remains unsolved. Here, we analyzed the structure-function relationship of Pkd2 based on its AlphaFold-predicted structure. Pkd2 consists of three domains, the extracellular lipid-binding domain (LBD), nine-helix transmembrane domain (TMD) and C-terminal cytoplasmic domain (CCD). Our genetic and microscopy data revealed that LBD and TMD are essential for targeting Pkd2 to the plasma membrane from the endoplasmic reticulum. In comparison, CCD ensures the polarized distribution of Pkd2 by promoting its internalization and preventing its clustering in the eisosome, a caveolae-like membrane compartment. The domains of Pkd2 and their functions are conserved in other fission yeast species. We conclude that both extracellular and cytoplasmic domains of Pkd2 are crucial for its intracellular trafficking and function. We propose that mechanosensitive channels can be desensitized through either internalization or clustering in low-tension membrane compartments.

Calcium signaling in polycystic kidney disease- cell death and survival

Polycystic kidney disease is typified by cysts in the kidney and extra-renal manifestations including hypertension and heart failure. The main genetic underpinning this disease are loss-of function mutations to the two polycystin proteins, polycystin 1 and polycystin 2. Molecularly, the disease is characterized by changes in multiple signaling pathways including down regulation of calcium signaling, which, in part, is contributed by the calcium permeant properties of polycystin 2. These signaling pathways enable the cystic cells to survive and avoid cell death. This review focuses on the studies that have emerged in the past 5 years describing how the structural insights gained from PC-1 and PC-2 inform the calcium dependent molecular pathways of autophagy and the unfolded protein response that are regulated by the polycystin proteins and how it leads to cell survival and/or cell death.

PKD1 and PKD2 mRNA cis-inhibition drives polycystic kidney disease progression

Autosomal dominant polycystic kidney disease (ADPKD), among the most common human genetic conditions and a frequent etiology of kidney failure, is primarily caused by heterozygous PKD1 mutations. Kidney cyst formation occurs when PKD1 dosage falls below a critical threshold. However, no framework exists to harness the remaining allele or reverse PKD1 decline. Here, we show that mRNAs produced by the noninactivated PKD1 allele are repressed via their 3'-UTR miR-17 binding element. Eliminating this motif (Pkd1∆17) improves mRNA stability, raises Polycystin-1 levels, and alleviates cyst growth in cellular, ex vivo, and mouse PKD models. Remarkably, Pkd2 is also inhibited via its 3'-UTR miR-17 motif, and Pkd2∆17-induced Polycystin-2 derepression retards cyst growth in Pkd1-mutant models. Moreover, acutely blocking Pkd1/2 cis-inhibition, including after cyst onset, attenuates murine PKD. Finally, modeling PKD1∆17 or PKD2∆17 alleles in patient-derived primary ADPKD cultures leads to smaller cysts, reduced proliferation, lower pCreb1 expression, and improved mitochondrial membrane potential. Thus, evading 3'-UTR cis-interference and enhancing PKD1/2 mRNA translation is a potentially mutation-agnostic ADPKD-arresting approach.

Generation of heterozygous PKD1 mutant pigs exhibiting early-onset renal cyst formation

Autosomal dominant polycystic kidney disease (ADPKD) is the most common inherited kidney disease, manifesting as the progressive development of fluid-filled renal cysts. In approximately half of all patients with ADPKD, end-stage renal disease results in decreased renal function. In this study, we used CRISPR-Cas9 and somatic cell cloning to produce pigs with the unique mutation c.152_153insG (PKD1^insG/+). Pathological analysis of founder cloned animals and progeny revealed that PKD1^insG/+ pigs developed many pathological conditions similar to those of patients with heterozygous mutations in PKD1. Pathological similarities included the formation of macroscopic renal cysts at the neonatal stage, number and cystogenic dynamics of the renal cysts formed, interstitial fibrosis of the renal tissue, and presence of a premature asymptomatic stage. Our findings demonstrate that PKD1^insG/+ pigs recapitulate the characteristic symptoms of ADPKD.

Recent Discoveries in Epigenetic Modifications of Polycystic Kidney Disease

Autosomal Dominant Polycystic Kidney Disease (ADPKD) is a heritable renal disease that results in end-stage kidney disease, due to the uncontrolled bilateral growth of cysts throughout the kidneys. While it is known that a mutation within a PKD-causing gene is required for the development of ADPKD, the underlying mechanism(s) causing cystogenesis and progression of the disease are not well understood. Limited therapeutic options are currently available to slow the rate of cystic growth. Epigenetic modifications, including DNA methylation, are known to be altered in neoplasia, and several FDA-approved therapeutics target these disease-specific changes. As there are many similarities between ADPKD and neoplasia, we (and others) have postulated that ADPKD kidneys contain alterations to their epigenetic landscape that could be exploited for future therapeutic discovery. Here we summarise the current understanding of epigenetic changes that are associated with ADPKD, with a particular focus on the burgeoning field of ADPKD-specific alterations in DNA methylation.

Retarding Progression of Chronic Kidney Disease in Autosomal Dominant Polycystic Kidney Disease with Metformin and Other Therapies: An Update of New Insights

Autosomal dominant polycystic kidney disease (ADPKD) is the most frequent single-gene disorder leading to renal failure. Current therapies are aimed to treat renal and extrarenal complications of ADPKD, but improved knowledge of the pathophysiological mechanisms leading to the generation and growth of cysts has permitted the identification of new drug candidates for clinical trials. Among these, in this review, we will examine above all the role of metformin, hypothesized to be able to activate the AMP-activated protein kinase (AMPK) pathway and potentially modulate some mechanisms implicated in the onset and the growth of the cysts.

Mitral Valve Prolapse and Its Motley Crew-Syndromic Prevalence, Pathophysiology, and Progression of a Common Heart Condition

Mitral valve prolapse (MVP) is a commonly occurring heart condition defined by enlargement and superior displacement of the mitral valve leaflet(s) during systole. Although commonly seen as a standalone disorder, MVP has also been described in case reports and small studies of patients with various genetic syndromes. In this review, we analyzed the prevalence of MVP within syndromes where an association to MVP has previously been reported. We further discussed the shared biological pathways that cause MVP in these syndromes, as well as how MVP in turn causes a diverse array of cardiac and noncardiac complications. We found 105 studies that identified patients with mitral valve anomalies within 18 different genetic, developmental, and connective tissue diseases. We show that some disorders previously believed to have an increased prevalence of MVP, including osteogenesis imperfecta, fragile X syndrome, Down syndrome, and Pseudoxanthoma elasticum, have few to no studies that use up-to-date diagnostic criteria for the disease and therefore may be overestimating the prevalence of MVP within the syndrome. Additionally, we highlight that in contrast to early studies describing MVP as a benign entity, the clinical course experienced by patients can be heterogeneous and may cause significant cardiovascular morbidity and mortality. Currently only surgical correction of MVP is curative, but it is reserved for severe cases in which irreversible complications of MVP may already be established; therefore, a review of clinical guidelines to allow for earlier surgical intervention may be warranted to lower cardiovascular risk in patients with MVP.

Regulation of polycystin expression, maturation and trafficking

The major autosomal dominant polycystic kidney disease (ADPKD) genes, PKD1 and PKD2, are wildly expressed at the organ and tissue level. PKD1 encodes polycystin 1 (PC1), a large membrane associated receptor-like protein that can complex with the PKD2 product, PC2. Various cellular locations have been described for both PC1, including the plasma membrane and extracellular vesicles, and PC2, especially the endoplasmic reticulum (ER), but compelling evidence indicates that the primary cilium, a sensory organelle, is the key site for the polycystin complex to prevent PKD. As with other membrane proteins, the ER biogenesis pathway is key to appropriately folding, performing quality control, and exporting fully folded PC1 to the Golgi apparatus. There is a requirement for binding with PC2 and cleavage of PC1 at the GPS for this folding and export to occur. Six different monogenic defects in this pathway lead to cystic disease development, with PC1 apparently particularly sensitive to defects in this general protein processing pathway. Trafficking of membrane proteins, and the polycystins in particular, through the Golgi to the primary cilium have been analyzed in detail, but at this time, there is no clear consensus on a ciliary targeting sequence required to export proteins to the cilium. After transitioning though the trans-Golgi network, polycystin-bearing vesicles are likely sorted to early or recycling endosomes and then transported to the ciliary base, possibly via docking to transition fibers (TF). The membrane-bound polycystin complex then undergoes facilitated trafficking through the transition zone, the diffusion barrier at the base of the cilium, before entering the cilium. Intraflagellar transport (IFT) may be involved in moving the polycystins along the cilia, but data also indicates other mechanisms. The ciliary polycystin complex can be ubiquitinated and removed from cilia by internalization at the ciliary base and may be sent back to the plasma membrane for recycling or to lysosomes for degradation. Monogenic defects in processes regulating the protein composition of cilia are associated with syndromic disorders involving many organ systems, reflecting the pleotropic role of cilia during development and for tissue maintenance. Many of these ciliopathies have renal involvement, likely because of faulty polycystin signaling from cilia. Understanding the expression, maturation and trafficking of the polycystins helps understand PKD pathogenesis and suggests opportunities for therapeutic intervention.

c-Myc is a regulator of the PKD1 gene and PC1-induced pathogenesis

Autosomal dominant polycystic kidney disease (ADPKD) is among the most common monogenic disorders mainly associated with PKD1/PC1 mutations. We show herein that renal regulation in Pc1 dosage-reduced and -increased mouse models converge toward stimulation of c-Myc expression along with β-catenin, delineating c-Myc as a key Pkd1 node in cystogenesis. Enhanced renal c-Myc-induced ADPKD in SBM transgenic mice lead conversely to striking upregulation of Pkd1/Pc1 expression and β-catenin activation, lending credence for reciprocal crosstalk between c-Myc and Pc1. In adult SBM kidneys, c-Myc is strongly enriched on Pkd1 promoter with RNA pol II, consistent with Pkd1 upregulation during cystogenesis. Similar c-Myc direct binding at birth uncovers an equivalent role on Pkd1 regulation during renal developmental program. Concurrent with enriched c-Myc binding, recruitment of active chromatin modifying co-factors by c-Myc at the Pkd1 regulatory region probably opens chromatin to stimulate transcription. A similar transcriptional activation by c-Myc is also likely operant on endogenous human PKD1 gene from our transactivation analysis in response to human c-MYC upregulation. Genetic ablation of c-Myc in Pc1-reduced and -increased mouse models significantly attenuates cyst growth, proliferation and PKD progression. Our study determined a dual role for c-Myc, as a major contributor in Pc1-induced cystogenesis and in a feed-forward regulatory Pkd1-c-Myc loop mechanism that may also prevail in human ADPKD.

The master regulators Myc and p53 cellular signaling and functions in polycystic kidney disease

The transcription factors Myc and p53 associated with oncogenesis play determinant roles in a human genetic disorder, autosomal dominant polycystic kidney disease (ADPKD), that was coined early in ADPKD etiology a «neoplasia in disguise ». These factors are interdependent master cell regulators of major biological processes including proliferation, apoptosis, cell growth, metabolism, inflammation, fibrosis and differentiation that are all modulated in ADPKD. Myc and p53 proteins evolved to respond and carry out overlapping functions via opposing mechanisms of action. Studies in human ADPKD kidneys, caused by mutations in the PKD1 or PKD2 genes, reveal reduced p53 expression and high expression of Myc in the cystic tubular epithelium. Myc and p53 via direct interaction act respectively, as transcriptional activator and repressor of PKD1 gene expression, consistent with increased renal PKD1 levels in ADPKD. Mouse models generated by Pkd1 and Pkd2 gene dosage dysregulation reproduce renal cystogenesis with activation of Myc expression and numerous signaling pathways, strikingly similar to those determined in human ADPKD. In fact, upregulation of renal Myc expression is also detected in virtually all non-orthologous animal models of PKD. A definitive causal connection of Myc with cystogenesis was established by renal overexpression of Myc in transgenic mice that phenocopies human ADPKD. The network of activated signaling pathways in human and mouse cystogenesis individually or in combination can target Myc as a central node of PKD pathogenesis. One or many of the multiple functions of Myc upon activation can play a role in every phases of ADPKD development and lend credence to the notion of "Myc addiction" for cystogenesis. We propose that the residual p53 levels are conducive to an ADPKD biological program without cancerogenesis while a "p53 dependent annihilation" mechanism would be permissive to oncogenesis. Of major importance, Myc ablation in orthologous mouse models or direct inhibition in non-orthologous mouse model significantly delays cystogenesis consistent with pharmacologic or genetic inhibition of Myc upstream regulator or downstream targets in the mouse. Together, these studies on PKD proteins upon dysregulation not only converged on Myc as a focal point but also attribute to Myc upregulation a causal and « driver » role in pathogenesis. This review will present and discuss our current knowledge on Myc and p53, focused on PKD mouse models and ADPKD.

Monkeys mutant for PKD1 recapitulate human autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) caused by PKD1 mutations is one of the most common hereditary disorders. However, the key pathological processes underlying cyst development and exacerbation in pre-symptomatic stages remain unknown, because rodent models do not recapitulate critical disease phenotypes, including disease onset in heterozygotes. Here, using CRISPR/Cas9, we generate ADPKD models with PKD1 mutations in cynomolgus monkeys. As in humans and mice, near-complete PKD1 depletion induces severe cyst formation mainly in collecting ducts. Importantly, unlike in mice, PKD1 heterozygote monkeys exhibit cyst formation perinatally in distal tubules, possibly reflecting the initial pathology in humans. Many monkeys in these models survive after cyst formation, and cysts progress with age. Furthermore, we succeed in generating selective heterozygous mutations using allele-specific targeting. We propose that our models elucidate the onset and progression of ADPKD, which will serve as a critical basis for establishing new therapeutic strategies, including drug treatments.

Disease modeling in genetic kidney diseases: zebrafish

Growing numbers of translational genomics studies are based on the highly efficient and versatile zebrafish (Danio rerio) vertebrate model. The increasing types of zebrafish models have improved our understanding of inherited kidney diseases, since they not only display pathophysiological changes but also give us the opportunity to develop and test novel treatment options in a high-throughput manner. New paradigms in inherited kidney diseases have been developed on the basis of the distinct genome conservation of approximately 70 % between zebrafish and humans in terms of existing gene orthologs. Several options are available to determine the functional role of a specific gene or gene sets. Permanent genome editing can be induced via complete gene knockout by using the CRISPR/Cas-system, among others, or via transient modification by using various morpholino techniques. Cross-species rescues succeeding knockdown techniques are employed to determine the functional significance of a target gene or a specific mutation. This article summarizes the current techniques and discusses their perspectives.

Loss of Zeb2 in mesenchyme-derived nephrons causes primary glomerulocystic disease

Primary glomerulocystic kidney disease is a special form of renal cystic disorder characterized by Bowman's space dilatation in the absence of tubular cysts. ZEB2 is a SMAD-interacting transcription factor involved in Mowat-Wilson syndrome, a congenital disorder with an increased risk for kidney anomalies. Here we show that deletion of Zeb2 in mesenchyme-derived nephrons with either Pax2-cre or Six2-cre causes primary glomerulocystic kidney disease without tubular cysts in mice. Glomerulotubular junction analysis revealed many atubular glomeruli in the kidneys of Zeb2 knockout mice, which explains the presence of glomerular cysts in the absence of tubular dilatation. Gene expression analysis showed decreased expression of early proximal tubular markers in the kidneys of Zeb2 knockout mice preceding glomerular cyst formation, suggesting that defects in proximal tubule development during early nephrogenesis contribute to the formation of congenital atubular glomeruli. At the molecular level, Zeb2 deletion caused aberrant expression of Pkd1, Hnf1β, and Glis3, three genes causing glomerular cysts. Thus, Zeb2 regulates the morphogenesis of mesenchyme-derived nephrons and is required for proximal tubule development and glomerulotubular junction formation. Our findings also suggest that ZEB2 might be a novel disease gene in patients with primary glomerular cystic disease.

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.

A Case of New Familiar Genetic Variant of Autosomal Dominant Polycystic Kidney Disease-2: A Case Study

BACKGROUND

Autosomal dominant polycystic kidney disease (ADPKD) is characterized by renal cyst formation due to mutations in genes coding for polycystin-1 [PKD1 (85-90% of cases), on ch 16p13.3] and polycystin-2 [PKD2 (10-15% of cases), on ch 4q13-23] and PKD3 gene (gene unmapped). It is also associated with TSC2/PKD1 contiguous gene syndrome. ADPKD is usually inherited, but new mutations without a family history occur in approximately 10% of the cases.

CASE PRESENTATION

A 17-year-old boy was followed up for bilateral cystic kidney disease, hypertension, and obesity since he was 13 years old. The diagnosis was an accidental finding during abdominal CT at age 13 to rule out appendicitis. A renal ultrasonogram also demonstrated a multiple bilateral cysts. Because of parental history of bilateral renal cysts, PKD1 and PKD2, genetic testing was ordered. Results showed, PKD2 variant 1:3 bp deletion of TGT; nucleotide position: 1602-1604; codon position: 512-513; mRNA reading frame maintained. The same mutation was later identified in his father.

CONCLUSION

A smaller number of patients have a defect in the PKD2 locus on chromosome 4 (resulting in PKD2 disease). There are no known published cases on this familiar genetic variant of ADPKD-2 cystic kidney disease. In this case, the disease is present unusually early in life.

Autopsy report with clinical and pathophysiologic discussion of autosomal dominant adult polycystic kidney disease

The average weight of a kidney is approximately 135 gm, measuring on average 10 × 6 × 4 cm. In hereditary conditions, autosomal dominant and autosomal recessive polycystic kidney disease, the shape, size, and the weight can be significantly abnormal, causing progressive renal failure, often necessitating dialysis or renal transplant for survival. We report a case of adult polycystic kidney disease in a 50-year-old female without a family history, who died of complications of the disease which included accelerated hypertension, and renal and cardiac failure.

Novel functional complexity of polycystin-1 by GPS cleavage in vivo: role in polycystic kidney disease

Polycystin-1 (Pc1) cleavage at the G protein-coupled receptor (GPCR) proteolytic site (GPS) is required for normal kidney morphology in humans and mice. We found a complex pattern of endogenous Pc1 forms by GPS cleavage. GPS cleavage generates not only the heterodimeric cleaved full-length Pc1 (Pc1(cFL)) in which the N-terminal fragment (NTF) remains noncovalently associated with the C-terminal fragment (CTF) but also a novel (Pc1) form (Pc1(deN)) in which NTF becomes detached from CTF. Uncleaved Pc1 (Pc1(U)) resides primarily in the endoplasmic reticulum (ER), whereas both Pc1(cFL) and Pc1(deN) traffic through the secretory pathway in vivo. GPS cleavage is not a prerequisite, however, for Pc1 trafficking in vivo. Importantly, Pc1(deN) is predominantly found at the plasma membrane of renal epithelial cells. By functional genetic complementation with five Pkd1 mouse models, we discovered that CTF plays a crucial role in Pc1(deN) trafficking. Our studies support GPS cleavage as a critical regulatory mechanism of Pc1 biogenesis and trafficking for proper kidney development and homeostasis.

Bardet-Biedl syndrome proteins 1 and 3 regulate the ciliary trafficking of polycystic kidney disease 1 protein

Bardet-Biedl syndrome (BBS) and autosomal dominant polycystic kidney disease (ADPKD) are two genetically distinct ciliopathies but share common phenotypes such as renal cysts. Seven BBS proteins form a complex called the BBSome which is localized at the basal body or ciliary axoneme and regulates the ciliary entry or flagellar exit of several signaling molecules. Here, we demonstrate that, unlike the seven-span somatostatin receptor 3 or the leptin receptor that interacts with all subunits of the BBSome, the ADPKD protein polycystin-1 (PC1) interacts with BBS1, BBS4, BBS5 and BBS8, four of the seven components of the BBSome. Only depletion or mutation of BBS1, but not depletion of BBS5 and BBS8, or knockout of BBS4, impairs ciliary trafficking of PC1 in kidney epithelial cells. Depletion of these BBS proteins affects neither the ciliary length nor the plasma membrane targeting of PC1. Expression of a pathogenic BBS3/Arl6 mutant (T31R) that locks Arl6 in the GDP form leads to stunted cilia and inhibition of PC1 on primary cilia. We propose that the 11-span membrane protein PC1 is a BBSome cargo and that the components of the BBSome may possess subunit-specific functions. Moreover, physical interactions between the BBS and ADPKD proteins may underline the overlapping renal phenotypes in these two diseases.

Role of cilia in normal pancreas function and in diseased states

Primary cilia play an essential role in modulating signaling cascades that shape cellular responses to environmental cues to maintain proper tissue development. Mutations in primary cilium proteins have been linked to several rare developmental disorders, collectively known as ciliopathies. Together with other disorders associated with dysfunctional cilia/centrosomes, affected individuals have increased risk of developing metabolic syndrome, neurologic disorders, and diabetes. In pancreatic tissues, cilia are found exclusively in islet and ductal cells where they play an essential role in pancreatic tissue organization. Their absence or disorganization leads to pancreatic duct abnormalities, acinar cell loss, polarity defects, and dysregulated insulin secretion. Cilia in pancreatic tissues are hubs for cellular signaling. Many signaling components, such as Hh, Notch, and Wnt, localize to pancreatic primary cilia and are necessary for proper development of pancreatic epithelium and β-cell morphogenesis. Receptors for neuroendocrine hormones, such as Somatostatin Receptor 3, also localize to the cilium and may play a more direct role in controlling insulin secretion due to somatostatin's inhibitory function. Finally, unique calcium signaling, which is at the heart of β-cell function, also occurs in primary cilia. Whereas voltage-gated calcium channels trigger insulin secretion and serve a variety of homeostatic functions in β-cells, transient receptor potential channels regulate calcium levels within the cilium that may serve as a feedback mechanism, regulating insulin secretion. This review article summarizes our current understanding of the role of primary cilia in normal pancreas function and in the diseased state.

The primary cilium calcium channels and their role in flow sensing

The primary cilium has been the focus of intense research since it was discovered that mutations in ciliary/basal body localized proteins give rise to a multitude of disorders. While these studies have revealed the contribution of this sensory organelle to multiple signalling pathways, little is known about how it actually mediates downstream events and why its loss causes disease states. Ciliopathies are linked to defects in either structure or function of cilia and are often associated with kidney cysts. The ciliopathy, autosomal dominant polycystic kidney disease (ADPKD), is caused by mutations to the PKD1 or PKD2 gene. The PKD gene products localize to the primary cilium, where they have been proposed to form a mechanosensory complex, sensitive to flow. Since mouse knockout models of Pkd1 or Pkd2 develop structurally normal cilia, it has been hypothesized that the loss of polycystins may lead to an impairment of flow sensing. Today, technically challenging patch clamp recordings of the primary cilium have become available, and the genetic relationship between polycystins (TRPPs) and the primary cilium has recently been dissected in detail.

Polycystic kidney disease - where gene dosage counts

Gene dosage effects have emerged as playing a central role in the pathogenesis of polycystic kidney disease. Yet, how gene dosage can ultimately have an impact on the formation of kidney cysts remains unknown. In this commentary we review the evidence for the role of gene dosage effects versus the “2-hit” mutation model in polycystic kidney disease (PKD), and also discuss how gene networks may potentially make intertwined contributions to PKD.

Polycystin-1 negatively regulates Polycystin-2 expression via the aggresome/autophagosome pathway

Mutations of the PKD1 and PKD2 genes, encoding polycystin-1 (PC1) and polycystin-2 (PC2), respectively, lead to autosomal dominant polycystic kidney disease. Interestingly, up-regulation or down-regulation of PKD1 or PKD2 leads to polycystic kidney disease in animal models, but their interrelations are not completely understood. We show here that full-length PC1 that interacts with PC2 via a C-terminal coiled-coil domain regulates PC2 expression in vivo and in vitro by down-regulating PC2 expression in a dose-dependent manner. Expression of the pathogenic mutant R4227X, which lacks the C-terminal coiled-coil domain, failed to down-regulate PC2 expression, suggesting that PC1-PC2 interaction is necessary for PC2 regulation. The proteasome and autophagy are two pathways that control protein degradation. Proteins that are not degraded by proteasomes precipitate in the cytoplasm and are transported via histone deacetylase 6 (HDAC6) toward the aggresomes. We found that HDAC6 binds to PC2 and that expression of full-length PC1 accelerates the transport of the HDAC6-PC2 complex toward aggresomes, whereas expression of the R4227X mutant fails to do so. Aggresomes are engulfed by autophagosomes, which then fuse with the lysosome for degradation; this process is also known as autophagy. We have now shown that PC1 overexpression leads to increased degradation of PC2 via autophagy. Interestingly, PC1 does not activate autophagy generally. Thus, we have now uncovered a new pathway suggesting that when PC1 is expressed, PC2 that is not bound to PC1 is directed to aggresomes and subsequently degraded via autophagy, a control mechanism that may play a role in autosomal dominant polycystic kidney disease pathogenesis.

Polycystin-1 regulates actin cytoskeleton organization and directional cell migration through a novel PC1-Pacsin 2-N-Wasp complex

How epithelial cells form a tubule with defined length and lumen diameter remains a fundamental question in cell and developmental biology. Loss of control of tubule lumen size in multiple organs including the kidney, liver and pancreas features polycystic kidney disease (PKD). To gain insights into autosomal dominant polycystic kidney disease, we performed yeast two-hybrid screens using the C-terminus of polycystin-1 (PC1) as bait. Here, we report that PC1 interacts with Pacsin 2, a cytoplasmic phosphoprotein that has been implicated in cytoskeletal organization, vesicle trafficking and more recently in cell intercalation during gastrulation. PC1 binds to a 107-residue fragment containing the α3 helix of the F-BAR domain of Pacsin 2 via a coiled-coil domain in its C-tail. PC1 and Pacsin 2 co-localize on the lamellipodia of migrating kidney epithelial cells. PC1 and Pacsin 2-deficient kidney epithelial cells migrate at a slower speed with reduced directional persistency. We further demonstrate that PC1, Pacsin 2 and N-Wasp are in the same protein complex, and both PC1 and Pacsin 2 are required for N-Wasp/Arp2/3-dependent actin remodeling. We propose that PC1 modulates actin cytoskeleton rearrangements and directional cell migration through the Pacsin 2/N-Wasp/Arp2/3 complex, which consequently contributes to the establishment and maintenance of the sophisticated tubular architecture. Disruption of this complex contributes to cyst formation in PKD.

Reduced ciliary polycystin-2 in induced pluripotent stem cells from polycystic kidney disease patients with PKD1 mutations

Heterozygous mutations in PKD1 or PKD2, which encode polycystin-1 (PC1) and polycystin-2 (PC2), respectively, cause autosomal dominant PKD (ADPKD), whereas mutations in PKHD1, which encodes fibrocystin/polyductin (FPC), cause autosomal recessive PKD (ARPKD). However, the relationship between these proteins and the pathogenesis of PKD remains unclear. To model PKD in human cells, we established induced pluripotent stem (iPS) cell lines from fibroblasts of three ADPKD and two ARPKD patients. Genetic sequencing revealed unique heterozygous mutations in PKD1 of the parental ADPKD fibroblasts but no pathogenic mutations in PKD2. Undifferentiated PKD iPS cells, control iPS cells, and embryonic stem cells elaborated primary cilia and expressed PC1, PC2, and FPC at similar levels, and PKD and control iPS cells exhibited comparable rates of proliferation, apoptosis, and ciliogenesis. However, ADPKD iPS cells as well as somatic epithelial cells and hepatoblasts/biliary precursors differentiated from these cells expressed lower levels of PC2 at the cilium. Additional sequencing confirmed the retention of PKD1 heterozygous mutations in iPS cell lines from two patients but identified possible loss of heterozygosity in iPS cell lines from one patient. Furthermore, ectopic expression of wild-type PC1 in ADPKD iPS-derived hepatoblasts rescued ciliary PC2 protein expression levels, and overexpression of PC1 but not a carboxy-terminal truncation mutant increased ciliary PC2 expression levels in mouse kidney cells. Taken together, these results suggest that PC1 regulates ciliary PC2 protein expression levels and support the use of PKD iPS cells for investigating disease pathophysiology.

Cell polarity and cystic kidney disease

Epithelial cell polarity is essential for organ development; aberrations in this process have been implicated in various diseases, including polycystic kidney disease. Establishment and maintenance of cell polarity is governed by a number of molecular processes and how these processes operate remains an interesting question. Conserved protein complexes guide both apical-basolateral polarity and planar cell polarity. In this review we discuss the recent findings that provide insights into polarity mechanisms and the intriguing crosstalk between apical-basolateral polarity and planar cell polarity, and their relationship to cystic kidney disease.

Primary cilia: the chemical antenna regulating human adipose-derived stem cell osteogenesis

Adipose-derived stem cells (ASC) are multipotent stem cells that show great potential as a cell source for osteogenic tissue replacements and it is critical to understand the underlying mechanisms of lineage specification. Here we explore the role of primary cilia in human ASC (hASC) differentiation. This study focuses on the chemosensitivity of the primary cilium and the action of its associated proteins: polycystin-1 (PC1), polycystin-2 (PC2) and intraflagellar transport protein-88 (IFT88), in hASC osteogenesis. To elucidate cilia-mediated mechanisms of hASC differentiation, siRNA knockdown of PC1, PC2 and IFT88 was performed to disrupt cilia-associated protein function. Immunostaining of the primary cilium structure indicated phenotypic-dependent changes in cilia morphology. hASC cultured in osteogenic differentiation media yielded cilia of a more elongated conformation than those cultured in expansion media, indicating cilia-sensitivity to the chemical environment and a relationship between the cilium structure and phenotypic determination. Abrogation of PC1, PC2 and IFT88 effected changes in both hASC proliferation and differentiation activity, as measured through proliferative activity, expression of osteogenic gene markers, calcium accretion and endogenous alkaline phosphatase activity. Results indicated that IFT88 may be an early mediator of the hASC differentiation process with its knockdown increasing hASC proliferation and decreasing Runx2, alkaline phosphatase and BMP-2 mRNA expression. PC1 and PC2 knockdown affected later osteogenic gene and end-product expression. PC1 knockdown resulted in downregulation of alkaline phosphatase and osteocalcin gene expression, diminished calcium accretion and reduced alkaline phosphatase enzymatic activity. Taken together our results indicate that the structure of the primary cilium is intimately associated with the process of hASC osteogenic differentiation and that its associated proteins are critical players in this process. Elucidating the dynamic role of the primary cilium and its associated proteins will help advance the application of hASC in generating autologous tissue engineered therapies in critical defect bone injuries.

Construction of a transgenic pig model overexpressing polycystic kidney disease 2 (PKD2) gene

Autosomal dominant polycystic kidney disease (ADPKD) is a common human genetic disease, affecting millions of people worldwide. The progressive growth of cysts in kidneys eventually leads to renal failure in 50 % of patients, and there is currently no effective treatment. Various murine models have been studied to elucidate the disease mechanisms, and much information has been acquired. However, the course of the disease cannot be fully recapitulated using these models. The pig is a suitable model for biomedical research, and pig PKD2 has high similarity to the human ortholog at the molecular level. Here, a mini-pig PKD2 transgenic model was generated, driven by a ubiquitous cytomegalovirus enhancer/promoter. Using somatic cell nuclear transfer, four transgenic pigs with approximately 10 insertion events each were generated. Quantitative real-time PCR and western blotting showed that PKD2 was more highly expressed in transgenic pigs than in wild-type counterparts. Because of the chronic nature of ADPKD, blood urea nitrogen and serum creatinine levels were continuously measured to assess the pig kidney function. The transgenic pigs continue to show no significant alteration in kidney function; it is estimated that 1-2 more years may be required for manifestation of renal cystogenesis in these pigs.

Calcium signaling in renal tubular cells

The kidney handles calcium by filtration and reabsorption. About 60% of the plasma calcium is filterable, and 99% is reabsorbed in the tubule. In the proximal tubule, the reabsorption is passive and paracellular, but in the distal tubule is active and transcellular. Thus, renal tubular cells are exposed to very high concentrations of calcium in both, the extracellular and the intracellular compartments. Extracellular calcium signaling is transmitted by the calcium sensing receptor, located both in the luminal and basolateral sides of tubular cells. This receptor is able to control levels of extracellular calcium and acts in consequence to maintain calcium homeostasis. Furthermore, renal tubular cells possess several calcium channels that regulate some of the cell functions. Among those, voltage gated calcium channels, transient receptor potential channels and N-methyl-D-aspartate receptor channels have been reported to control several functions. Those functions include survival, apoptosis, differentiation, epithelial-mesenchymal transition, and active vitamin D and renin synthesis.

Ciliopathies: an expanding disease spectrum

Ciliopathies comprise a group of disorders associated with genetic mutations encoding defective proteins, which result in either abnormal formation or function of cilia. As cilia are a component of almost all vertebrate cells, cilia dysfunction can manifest as a constellation of features that include characteristically, retinal degeneration, renal disease and cerebral anomalies. Additional manifestations include congenital fibrocystic diseases of the liver, diabetes, obesity and skeletal dysplasias. Ciliopathic features have been associated with mutations in over 40 genes to date. However, with over 1,000 polypeptides currently identified within the ciliary proteome, several other disorders associated with this constellation of clinical features will likely be ascribed to mutations in other ciliary genes. The mechanisms underlying many of the disease phenotypes associated with ciliary dysfunction have yet to be fully elucidated. Several elegant studies have crucially demonstrated the dynamic ciliary localisation of components of the Hedgehog and Wnt signalling pathways during signal transduction. Given the critical role of the cilium in transducing "outside-in" signals, it is not surprising therefore, that the disease phenotypes consequent to ciliary dysfunction are a manifestation of aberrant signal transduction. Further investigation is now needed to explore the developmental and physiological roles of aberrant signal transduction in the manifestation of ciliopathy phenotypes. Utilisation of conditional and inducible murine models to delete or overexpress individual ciliary genes in a spatiotemporal and organ/cell-specific manner should help clarify some of the functional roles of ciliary proteins in the manifestation of phenotypic features.

Drug discovery for polycystic kidney disease

In polycystic kidney disease (PKD), a most common human genetic diseases, fluid-filled cysts displace normal renal tubules and cause end-stage renal failure. PKD is a serious and costly disorder. There is no available therapy that prevents or slows down the cystogenesis and cyst expansion in PKD. Numerous efforts have been made to find drug targets and the candidate drugs to treat PKD. Recent studies have defined the mechanisms underlying PKD and new therapies directed toward them. In this review article, we summarize the pathogenesis of PKD, possible drug targets, available PKD models for screening and evaluating new drugs as well as candidate drugs that are being developed.

Expression and physiological roles of TRP channels in smooth muscle cells

Smooth muscles are widely distributed in mammal body through various systems such as circulatory, respiratory, gastro-intestinal and urogenital systems. The smooth muscle cell (SMC) is not only a contractile cell but is able to perform other important functions such as migration, proliferation, production of cytokines, chemokines, extracellular matrix proteins, growth factors and cell surface adhesion molecules. Thus, SMC appears today as a fascinating cell with remarkable plasticity that contributes to its roles in physiology and disease. Most of the SMC functions are dependent on a key event: the increase in intracellular calcium concentration ([Ca(2+)](i)). Calcium entry from the extracellular space is a major step in the elevation of [Ca(2+)](i) in SMC and involves a variety of plasmalemmal calcium channels, among them is the superfamily of transient receptor potential (TRP) proteins. TRPC (canonical), TRPM (melastatin), TRPV (vanilloid) and TRPP (polycystin), are widely expressed in both visceral (airways, gastrointestinal tract, uterus) and vascular (systemic and pulmonary circulation) smooth muscles. Mainly, TRPC, TRPV and TRPM are implicated in a variety of physiological and pathophysiological processes such as: SMC contraction, relaxation, growth, migration and proliferation; control of blood pressure, arterial myogenic tone, pulmonary hypertension, intestinal motility, gastric acidity, uterine activity during parturition and labor. Thus it is becoming evident that TRP are major element of SMC calcium homeostasis and, thus, appear as novel drug targets for a better management of diseases originating from SMC dysfunction.

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.

Identification of polycystin-1 and Gα12 binding regions necessary for regulation of apoptosis

Most patients with autosomal dominant polycystic kidney disease (ADPKD) harbor mutations in PKD1, the gene for polycystin-1 (PC1), a transmembrane protein with a cytoplasmic C-terminus that interacts with numerous signaling molecules, including Gα12. The functions of PC1 and the mechanisms of cyst development leading to renal failure are complex. Recently, we reported that PC1 expression levels modulate activity of Gα12-stimulated apoptosis (Yu et al., J. Biol. Chem. 2010 285(14):10243-51). Herein, a mutational analysis of Gα12 and PC1 was undertaken to identify regions required for their interaction and ability to modulate apoptosis. A set of Gα12 mutations with systematic replacement of six amino acids with NAAIRS was tested for binding to the PC1 C-terminus in GST pulldowns. Additionally, a series of deletions within the PC1 C-terminus was examined for binding to Gα12. We identified 3 NAAIRS substitutions in Gα12 that completely abrogated binding, and identified a previously described 74 amino acid Gαi/o binding domain in the PC1 C-terminus as necessary for Gα12 interaction. The functional consequences of uncoupling PC1/Gα12 binding were studied in apoptosis assays utilizing HEK293 cells with inducible PC1 overexpression. Gα12 mutants deficient in PC1 binding were refractory to PC1 inhibition of Gα12-stimulated apoptosis. Likewise, deletion of the Gα12-interacting sequence from the PC1 cytoplasmic domain abrogated its inhibition of Gα12-stimulated apoptosis. Based on the crystal structure of Gα12, the PC1 interaction sites are likely to reside on exposed regions within the G protein helical domain. These structural details should facilitate the design of reagents to uncouple PC1/Gα12 signaling in ADPKD.

The role of noise and positive feedback in the onset of autosomal dominant diseases

Background

Autosomal dominant (AD) diseases result when a single mutant or non-functioning gene is present on an autosomal chromosome. These diseases often do not emerge at birth. There are presently two prevailing theories explaining the expression of AD diseases. One explanation originates from the Knudson two-hit theory of hereditary cancers, where loss of heterozygosity or occurrence of somatic mutations impairs the function of the wild-type copy. While these somatic second hits may be sufficient for stable disease states, it is often difficult to determine if their occurrence necessarily marks the initiation of disease progression. A more direct consequence of a heterozygous genetic background is haploinsufficiency, referring to a lack of sufficient gene function due to reduced wild-type gene copy number; however, haploinsufficiency can involve a variety of additional mechanisms, such as noise in gene expression or protein levels, injury and second hit mutations in other genes. In this study, we explore the possible contribution to the onset of autosomal dominant diseases from intrinsic factors, such as those determined by the structure of the molecular networks governing normal cellular physiology.

Results

First, simple models of single gene insufficiency using the positive feedback loops that may be derived from a three-component network were studied by computer simulation using Bionet software. The network structure is shown to affect the dynamics considerably; some networks are relatively stable even when large stochastic variations in are present, while others exhibit switch-like dynamics. In the latter cases, once the network switches over to the disease state it remains in that state permanently. Model pathways for two autosomal dominant diseases, AD polycystic kidney disease and mature onset diabetes of youth (MODY) were simulated and the results are compared to known disease characteristics.

Conclusions

By identifying the intrinsic mechanisms involved in the onset of AD diseases, it may be possible to better assess risk factors as well as lead to potential new drug targets. To illustrate the applicability of this study of pathway dynamics, we simulated the primary pathways involved in two autosomal dominant diseases, Polycystic Kidney Disease (PKD) and mature onset diabetes of youth (MODY). Simulations demonstrate that some of the primary disease characteristics are consistent with the positive feedback - stochastic variation theory presented here. This has implications for new drug targets to control these diseases by blocking the positive feedback loop in the relevant pathways.

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.

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.

Autosomal dominant polycystic kidney disease: risk factor for nonmelanoma skin cancer following kidney transplantation

Nonmelanoma skin cancers (NMSC) are the most common malignant tumors following solid organ transplantation. Risk factors for NMSC mainly include immunosuppression, age, sun exposure and patient phototype. Recent findings have suggested that autosomal dominant polycystic kidney disease (ADPKD) may increase the risk of developing NMSC. We performed a monocenter retrospective study including all kidney recipients between 1985 and 2006 (n = 1019). We studied the incidence of NMSC, solid cancers and post-transplantation lymphoproliferative disease (PTLD), and analyzed the following parameters: age, gender, phototype, time on dialysis, graft rank, immunosuppressive regimen, history of cancer and kidney disease (ADPKD versus others). Median follow-up was 5.5 years (range: 0.02-20.6; 79 838 patient-years). The cumulated incidence of NMSC 10 years after transplantation was 12.7% (9.3% for solid cancers and 3.5% for PTLD). Autosomal dominant polycystic kidney disease and age were risk factors for NMSC (HR 2.63; P < 0.0001 and HR 2.21; P < 0.001, respectively) using univariate analysis. The association between ADPKD and NMSC remained significant after adjustments for age, gender and phototype using multivariate analysis (HR 1.71; P = 0.0145) and for immunosuppressive regimens (P < 0.0001). Autosomal dominant polycystic kidney disease was not a risk factor for the occurrence of solid cancers after transplantation (HR 0.96; P = 0.89). Our findings suggest that ADPKD is an independent risk factor for developing NMSC after kidney transplantation.

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

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

Pkd1 transgenic mice: adult model of polycystic kidney disease with extrarenal and renal phenotypes

While high levels of Pkd1 expression are detected in tissues of patients with autosomal dominant polycystic kidney disease (ADPKD), it is unclear whether enhanced expression could be a pathogenetic mechanism for this systemic disorder. Three transgenic mouse lines were generated from a Pkd1-BAC modified by introducing a silent tag via homologous recombination to target a sustained wild-type genomic Pkd1 expression within the native tissue and temporal regulation. These mice specifically overexpressed the Pkd1 transgene in extrarenal and renal tissues from approximately 2- to 15-fold over Pkd1 endogenous levels in a copy-dependent manner. All transgenic mice reproducibly developed tubular and glomerular cysts leading to renal insufficiency. Interestingly, Pkd1(TAG) mice also exhibited renal fibrosis and calcium deposits in papilla reminiscent of nephrolithiasis as frequently observed in ADPKD. Similar to human ADPKD, these mice consistently displayed hepatic fibrosis and approximately 15% intrahepatic cysts of the bile ducts affecting females preferentially. Moreover, a significant proportion of mice developed cardiac anomalies with severe left-ventricular hypertrophy, marked aortic arch distention and/or valvular stenosis and calcification that had profound functional impact. Of significance, Pkd1(TAG) mice displayed occasional cerebral lesions with evidence of ruptured and unruptured cerebral aneurysms. This Pkd1(TAG) mouse model demonstrates that overexpression of wild-type Pkd1 can trigger the typical adult renal and extrarenal phenotypes resembling human ADPKD.

The polycystic kidney disease 1 (Pkd1) gene is required for the responses of osteochondroprogenitor cells to midpalatal suture expansion in mice

Mechanical stress is known to modulate postnatal skeletal growth and development. However, the mechanisms underlying the mechanotransduction are not fully understood. Polycystin-1 (PC1) is a promising candidate among proteins that may play a role in the process as it has been shown to function as a flow sensor in renal epithelium and it is known to be important for skeletal development. To investigate whether PC1 is involved in mechanotransduction in skeletal tissues, mice with a conditional deficiency for PC1 in neural crest cells, osteoblasts or chondrocytes were subjected to midpalatal suture expansion. Dynamic bone labeling revealed that new bone formation in response to expansion was significantly reduced in Wnt1Cre;Pkd1 mice, as the suture area containing new bone was 14.0+/-3.4% in mutant mice versus 65.0+/-3.8% in control mice at 2 weeks (p<0.001). In contrast, stress-induced new bone formation was not affected in OsxCre;Pkd1 mice. The increase in cell proliferation and differentiation into osteoblasts, seen in wild-type mice 1 day after force delivery, was not observed until 14 days in Wnt1Cre;Pkd1 mice. TUNEL labeling showed a significant increase in apoptotic suture cells at days 1 and 3 (from 7.0+/-0.5% to 13.5+/-1.4% at day 1 and from 4.6+/-1.1% to 10.5+/-1.7% at day 3, p<0.05). Abnormal ossification of nasal cartilage of Wnt1Cre;Pkd1 mice was accelerated upon suture expansion. Such ossification was also observed, but to a lesser extent in Col2a1-ERCre;Pkd1 mice. Transcript levels of Runx2 and MMP13 were significantly increased in the nasal cartilage of Wnt1Cre;Pkd1 mice compared to controls (p<0.05 and p<0.001, respectively), and in mutant mice with expansion versus without expansion (p<0.05 and p<0.001, respectively). Lack of PC1 in chondroprogenitor cells also resulted in increased cell apoptosis and an altered arrangement of chondrocytes in nasal cartilage. These results indicate that PC1 plays a critical role in the response of osteochondroprogenitor cells to the mechanical tissue stress induced by midpalatal suture expansion. They also suggest that the combination of an in vivo mechanical model, such as midpalatal suture expansion, with conditional deficiency for proteins that play a role in mechanotransduction, represents a powerful experimental strategy to explore underlying mechanisms.

A novel dephosphorylation-activated conductance in a mouse renal collecting duct cell line

Autosomal dominant polycystic kidney disease (ADPKD) is one of the most common inherited renal diseases. It is associated with the progressive development of renal tubular cysts, which may subsequently lead to renal failure. Studies into the genetic basis of ADPKD have identified two genes, PKD1 and PKD2, that are mutated in ADPKD patients. The PKD1 and PKD2 genes encode for two different proteins, TRPP1 and TRPP2. Previous studies have demonstrated the presence of both TRPP1 and TRPP2 in the renal collecting duct cell line M8. The aim of the following study was to investigate the functional properties of cation currents in these cells and to examine the effect of overexpression of TRPP1 using a transgenic cell model (M7). In M8 cells, initial whole cell currents were low. However, over time there was activation of a flow-sensitive current, which was inhibited by gadolinium (I(Gd)). The I(Gd) was more selective for cations over anions, but did not discriminate between monovalent cations and was Ca2+ permeable. Activation of I(Gd) was dependent on the presence of Ca2+ and also required dephosphorylation. The protein phosphatase 2A inhibitor okadaic acid prevented activation of I(Gd), suggesting that protein phosphatase 2A plays an important role in channel activation. The properties and magnitude of I(Gd) were unaffected in M7 cells, suggesting that overexpression of TRPP1 was without effect. I(Gd) was selectively inhibited by an antibody raised against the C-terminus of TRPP2. However, its selectivity profile was different to TRPP2, suggesting that it is attributable to a TRPP2-like channel or a TRPP2-containing heteromeric channel. In conclusion, these data describe the functional identification of a novel dephosphorylation- and flow-activated TRPP2-related channel in mouse collecting duct cells.

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.

Pkd1 inactivation induced in adulthood produces focal cystic disease

Autosomal dominant polycystic kidney disease, the most common monogenetic disorder, is characterized by gradual replacement of normal renal parenchyma by fluid-filled cysts. Mutations in either PKD1 or PKD2 cause autosomal dominant polycystic kidney disease. Pkd1(-/-) or Pkd2(-/-) mice develop rapid renal cystic disease and exhibit embryonic lethality; this supports the "two-hit" hypothesis, which proposes that a germline mutation in PKD1 (or PKD2) followed by a second somatic mutation later in life is responsible for the phenotype. Here, for investigation of the loss of Pkd1 at specific times of development, an inducible Pkd1-knockout mouse model was generated. Inactivation of Pkd1 in 5-wk-old mice resulted in formation of only focal renal cysts 6 to 9 wk later but in a severe polycystic phenotype nearly 1 yr later. Cysts derived from either collecting tubules or distal tubules but not from proximal tubules, which correlated with sites of Cre-mediated recombination. Inactivation of Pkd1 in 1-wk-old mice, however, resulted in massive cyst disease 6 wk later, despite a similar pattern of Cre-mediated recombination between 1- and 5-wk-old kidneys. Moreover, a germline heterozygous Pkd1 mutation facilitated cyst formation when a somatic Pkd1 mutation was induced. A marked increase in proliferating cell nuclear antigen expression was observed in cyst-lining epithelia and in normal-looking tubules adjacent to but not in those distant from cysts. These data suggest that Pkd1 inactivation is not sufficient to initiate the cell proliferation necessary for cyst formation; a paracrine mechanism may account for focal cell proliferation and regional disease progression. We propose that an additional genetic or nongenetic "third hit" may be required for rapid development of cysts in polycystic kidney disease.

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.