Polycystin-1 interacts with E-cadherin and the catenins--clues to the pathogenesis of cyst formation in ADPKD?

Role of ciliopathy protein TMEM107 in eye development: insights from a mouse model and retinal organoid

Primary cilia are cellular surface projections enriched in receptors and signaling molecules, acting as signaling hubs that respond to stimuli. Malfunctions in primary cilia have been linked to human diseases, including retinopathies and ocular defects. Here, we focus on TMEM107, a protein localized to the transition zone of primary cilia. TMEM107 mutations were found in patients with Joubert and Meckel-Gruber syndromes. A mouse model lacking Tmem107 exhibited eye defects such as anophthalmia and microphthalmia, affecting retina differentiation. Tmem107 expression during prenatal mouse development correlated with phenotype occurrence, with enhanced expression in differentiating retina and optic stalk. TMEM107 deficiency in retinal organoids resulted in the loss of primary cilia, down-regulation of retina-specific genes, and cyst formation. Knocking out TMEM107 in human ARPE-19 cells prevented primary cilia formation and impaired response to Smoothened agonist treatment because of ectopic activation of the SHH pathway. Our data suggest TMEM107 plays a crucial role in early vertebrate eye development and ciliogenesis in the differentiating retina.

Development of Glycerol-Rose Bengal-Polidocanol (GRP) foam for enhanced sclerosis of a cyst for cystic diseases

Polycystic kidney disease (PKD) is a common genetic disorder that results in a proliferating and enlarging cyst and ultimately leads to loss of kidney function. Because an enlarged cyst is a primary factor for limited kidney function, the large cyst is surgically removed by laparoscopic deroofing or sclerosant. This a relatively nascent treatment method entails complications and sometimes fail due to the cyst fluid refilling and infection. This study proposes using a more stable and effective polidocanol foam with glycerol and Rose Bengal (GRP form) to prevent cyst regeneration and irritation, which is caused by the required body movement during the treatment. Specifically, the foam retention time and viscosity were increased by adding glycerol up to 10% (w/v). The GRP form inhibited cellular proliferation and disrupted cellular junctions, e-cadherin, and cyst formation, demonstrated by the LDH, Live and Dead, and re-plating culture assays. The GRP foam was shown to be a safe and effective treatment as a commercial grade polidocanol foam form by an in vivo study in which subcutaneously injected mice injected with commercial 3% polidocanol, and the GRP foam showed no difference in inflammation. Thus, this study provides an advanced polidocanol form by adding glycerol and Rose-Bengal to help existing sclerotherapy.

Virtual-tissue computer simulations define the roles of cell adhesion and proliferation in the onset of kidney cystic disease

Virtual-tissue modeling is used to model emergent cyst growth in polycystic kidney disease. Model predictions, confirmed experimentally, show that decreased cell adhesion is necessary to produce stalk cysts, and loss of contact inhibition causes saccular cysts. Virtual-tissue modeling can be used to fully explore cell- and tissue-based behaviors.

In autosomal dominant polycystic kidney disease (ADPKD), cysts accumulate and progressively impair renal function. Mutations in PKD1 and PKD2 genes are causally linked to ADPKD, but how these mutations drive cell behaviors that underlie ADPKD pathogenesis is unknown. Human ADPKD cysts frequently express cadherin-8 (cad8), and expression of cad8 ectopically in vitro suffices to initiate cystogenesis. To explore cell behavioral mechanisms of cad8-driven cyst initiation, we developed a virtual-tissue computer model. Our simulations predicted that either reduced cell–cell adhesion or reduced contact inhibition of proliferation triggers cyst induction. To reproduce the full range of cyst morphologies observed in vivo, changes in both cell adhesion and proliferation are required. However, only loss-of-adhesion simulations produced morphologies matching in vitro cad8-induced cysts. Conversely, the saccular cysts described by others arise predominantly by decreased contact inhibition, that is, increased proliferation. In vitro experiments confirmed that cell–cell adhesion was reduced and proliferation was increased by ectopic cad8 expression. We conclude that adhesion loss due to cadherin type switching in ADPKD suffices to drive cystogenesis. Thus, control of cadherin type switching provides a new target for therapeutic intervention.

Is standardized cardiac assessment of asymptomatic high-risk renal transplant candidates beneficial?

BACKGROUND

Perioperative cardiovascular events in renal transplantation are common and non-invasive cardiac stress tests are recommended in high-risk renal transplant candidates. In 2004, we introduced a standardized preoperative cardiac risk assessment programme with the aim of reducing perioperative cardiac events.

METHODS

Since 2004, all asymptomatic high-risk renal transplant candidates had to undergo non-invasive cardiac stress testing. Patients with a positive stress test went for a coronary angiography and if indicated for revascularization. The incidence of perioperative cardiac events (≤30 days of transplantation) was analysed in all high-risk patients who received a transplantation (screening group) and compared with high-risk renal transplant recipients evaluated in the 4 years before the introduction of the cardiac assessment programme (historical control group).

RESULTS

Since 2004, 227 of 349 asymptomatic high-risk renal transplant candidates underwent non-invasive cardiac stress testing. In 15 patients (6.6%), significant ischaemia was found. Ten of these 15 patients underwent coronary angiography (eight patients had significant coronary artery disease and in five patients, percutaneous coronary intervention was performed). One hundred and sixty of 349 renal transplant candidates have received renal transplantation so far (screening group). In the screening group, 6 perioperative cardiac events (3.8%) occurred compared to 13 perioperative events (7.6%) in the historical control group (n = 172) (P = 0.136).

CONCLUSIONS

The incidence of significant cardiac ischaemia in high-risk renal transplant patients was low and was followed by revascularization in a small percentage of patients. No significant decrease in perioperative cardiac events was observed after the introduction of the standardized cardiac assessment programme.

Rosiglitazone attenuates development of polycystic kidney disease and prolongs survival in Han:SPRD rats

Although pioglitazone, a PPAR-gamma (peroxisome-proliferator-activated receptor-gamma) agonist, has been shown to prolong survival in two rapidly progressive pkd1 (polycystic kidney disease 1)-knockout mice models through disparate mechanisms, these studies lacked data on therapeutic potential and long-term safety because of a short observation period. In the present study, we have used another potent PPAR-gamma agonist, rosiglitazone, to treat Han:SPRD rats, a slowly progressive ADPKD (autosomal dominant PKD) animal model, and confirmed that short-term treatment was able to delay the progression of kidney cysts and protect renal function, which may relate to down-regulating the abnormally activated beta-catenin signalling pathway and its anti-inflammatory and anti-fibrosis effects. Long-term administration significantly prolonged the survival of Han:SPRD rats. Moreover, early therapy in rats with normal renal function had a better outcome than delayed therapy, while initiating therapy in rats with mild impaired renal function still protected renal function. The efficacy of rosiglitazone depended on continuous drug administration; withdrawal of the drug caused accelerated deterioration of renal function in effectively treated rats and shortened their survival to an untreated state. Long-term administration led to cardiac enlargement, probably due to rosiglitazone-mediated sodium re-absorption. In conclusion, these results indicate that rosiglitazone was able to effectively delay the progression of kidney disease and protect renal function in Han:SPRD rats, but its adverse effect of inducing cardiac enlargement should also be monitored closely.

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.

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.

Regulation of ryanodine receptor-dependent calcium signaling by polycystin-2

Mutations in polycystin-2 (PC2) cause autosomal dominant polycystic kidney disease. A function for PC2 in the heart has not been described. Here, we show that PC2 coimmunoprecipitates with the cardiac ryanodine receptor (RyR2) from mouse heart. Biochemical assays showed that the N terminus of PC2 binds the RyR2, whereas the C terminus only binds to RyR2 in its open state. Lipid bilayer electrophysiological experiments indicated that the C terminus of PC2 functionally inhibited RyR2 channel activity in the presence of calcium (Ca(2+)). Pkd2(-/-) cardiomyocytes had a higher frequency of spontaneous Ca(2+) oscillations, reduced Ca(2+) release from the sarcoplasmic reticulum stores, and reduced Ca(2+) content compared with Pkd2(+/+) cardiomyocytes. In the presence of caffeine, Pkd2(-/-) cardiomyocytes exhibited decreased peak fluorescence, a slower rate of rise, and a longer duration of Ca(2+) transients compared with Pkd2(+/+). These data suggest that PC2 is important for regulation of RyR2 function and that loss of this regulation of RyR2, as occurs when PC2 is mutated, results in altered Ca(2+) signaling in the heart.

Molecular regulation of kidney development: is the answer blowing in the Wnt?

Development of the metanephric kidney is a complicated process regulated by reciprocal signals from the ureteric bud and the metanephric mesenchyme that regulate tubule formation and epithelial branching morphogenesis. Over the past several years, several studies have suggested that Wnt signaling is involved in multiple aspects of normal kidney development as well as injury response and cancer progression. We will review these data here.

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.

Aberrant polycystin-1 expression results in modification of activator protein-1 activity, whereas Wnt signaling remains unaffected

Polycystin-1, the polycystic kidney disease 1 gene product, has been implicated in several signaling complexes that are known to regulate essential cellular functions. We investigated the role of polycystin-1 in Wnt signaling and activator protein-1 (AP-1) activation. To this aim, a membrane-targeted construct encoding the conserved C-terminal region of mouse polycystin-1 reported to mediate signal transduction activity was expressed in human embryonic and renal epithelial cells. To ensure specificity and minimal cotransfection effects, we focused our study on the endogenous proteins that actually transduce the signals, beta-catenin and T-cell factor/lymphoid-enhancing factor for Wnt signaling and (phosphorylated) c-Jun, ATF2, and c-Fos for AP-1. Our data indicate that the C-terminal region of polycystin-1 activates AP-1 by inducing phosphorylation and expression of at least c-Jun and ATF2, whereas c-Fos was not affected. Under our experimental conditions, polycystin-1 did not modulate Wnt signaling. AP-1 activity was aberrant in human autosomal dominant polycystic kidney disease (ADPKD) renal cystic epithelial cells and in renal epithelial cells expressing transgenic full-length polycystin-1, resulting in decreased Jun-ATF and increased Jun-Fos activity, whereas Wnt signaling remained unaffected. Since our data indicate that aberrant polycystin-1 expression results in altered AP-1 activity, polycystin-1 may be required for adequate AP-1 activity.

A polycystin-1 multiprotein complex is disrupted in polycystic kidney disease cells

Autosomal dominant polycystic kidney disease (ADPKD) is typified by the accumulation of fluid-filled cysts and abnormalities in renal epithelial cell function. The disease is principally caused by mutations in the gene encoding polycystin-1, a large basolateral plasma membrane protein expressed in kidney epithelial cells. Our studies reveal that, in normal kidney cells, polycystin-1 forms a complex with the adherens junction protein E-cadherin and its associated catenins, suggesting a role in cell adhesion or polarity. In primary cells from ADPKD patients, the polycystin-1/polycystin-2/E-cadherin/beta-catenin complex was disrupted and both polycystin-1 and E-cadherin were depleted from the plasma membrane as a result of the increased phosphorylation of polycystin-1. The loss of E-cadherin was compensated by the transcriptional upregulation of the normally mesenchymal N-cadherin. Increased cell surface N-cadherin in the disease cells in turn stabilized the continued plasma membrane localization of beta-catenin in the absence of E-cadherin. The results suggest that enhanced phosphorylation of polycystin-1 in ADPKD cells precipitates changes in its localization and its ability to form protein complexes that are critical for the stabilization of adherens junctions and the maintenance of a fully differentiated polarized renal epithelium.

The polycystic kidney disease-1 promoter is a target of the beta-catenin/T-cell factor pathway

Polycystic kidney disease (PKD) results from loss-of-function mutations in the PKD1 gene. There are also reports showing abnormally high levels of PKD1 expression in cystic epithelial cells. At present, nothing is known about the molecular mechanisms regulating the normal expression of the PKD1 gene or whether transcriptional disregulation of the PKD1 gene has a role in cyst formation. We have analyzed a 3.3-kb 5'-proximal portion of the human PKD1 gene. Sequence analysis revealed the presence of consensus sequences for numerous transactivating factors, including four T-cell factor (TCF) binding elements (TBEs). Transcriptional activity of the 3.3-kb fragment and a series of deletion constructs was assayed in HEK293T cells. A 2.0-kb proximal promoter region containing one of the four TBEs (TBE1) was inducible up to 6-fold by cotransfection with beta-catenin. beta-catenin-mediated induction was inhibited by dominant-negative TCF and by deletion of the TBE1 sequence. 15- or 109-bp sequences containing the TBE1 site, when cloned upstream of a minimal promoter, were shown to respond to beta-catenin induction. Gel shift assays confirmed that the TBE1 site is capable of forming complexes with TCF and beta-catenin. To determine whether expression of the endogenous PKD1 gene responds to beta-catenin, HT1080 cells were treated with LiCl, and HeLa cells were stably transfected with beta-catenin. In both cases, endogenous PKD1 mRNA levels were elevated in response to these treatments. Taken together, these studies define an active PKD1 promoter region and suggest that the PKD1 gene is a target of the beta-catenin/TCF pathway.