Formation of cysts by principal-like MDCK cells depends on the synergy of cAMP- and ATP-mediated fluid secretion

Advances and Challenges in Modeling Autosomal Dominant Polycystic Kidney Disease: A Focus on Kidney Organoids

Autosomal dominant polycystic kidney disease (ADPKD) is a prevalent hereditary disorder characterized by distinct phenotypic variability that has posed challenges for advancing in-depth research. Recent advancements in kidney organoid construction technologies have enabled researchers to simulate kidney development and create simplified in vitro experimental environments, allowing for more direct observation of how genetic mutations drive pathological phenotypes and disrupt physiological functions. Emerging technologies, such as microfluidic bioreactor culture systems and single-cell transcriptomics, have further supported the development of complex ADPKD organoids, offering robust models for exploring disease mechanisms and facilitating drug discovery. Nevertheless, significant challenges remain in constructing more accurate ADPKD disease models. This review will summarize recent advances in ADPKD organoid construction, focusing on the limitations of the current techniques and the critical issues that need to be addressed for future breakthroughs. New and Noteworthy: This review presents recent advancements in ADPKD organoid construction, particularly iPSC-derived models, offering new insights into disease mechanisms and drug discovery. It focuses on challenges such as limited vascularization and maturity, proposing potential solutions through emerging technologies. The ongoing optimization of ADPKD organoid models is expected to enhance understanding of the disease and drive breakthroughs in disease mechanisms and targeted therapy development.

P2Y2R and Cyst Growth in Polycystic Kidney Disease

Key Points

Polycystic kidney disease (PKD) is characterized by continuous cyst growth, which results in a decline in kidney function. Deletion of P2Y2R and pharmacological antagonism of purinergic signaling significantly reduced cyst growth in an orthologous PKD mouse model. P2Y2R was expressed in cysts of human PKD nephrectomies, which makes P2Y2R a reasonable target for treatment of PKD.

Background

Autosomal dominant polycystic kidney disease (ADPKD) is characterized by multiple bilateral kidney cysts that gradually enlarge, resulting in a decline in kidney function. Cyst growth is significantly driven by ATP-dependent chloride secretion mediated by the ion channel TMEM16A. This pathway is further augmented in advanced stages of the disease by hypoxia and activation of hypoxia-inducible factor (HIF)-1α . The mechanisms by which ATP leads to activation of TMEM16A and how HIF-1α contributes to cyst growth in vivo have remained elusive.

Methods

Mice with an inducible tubule-specific deletion of Pkd1 were compared with mice with an additional codeletion of the purinergic receptor P2y2r . Furthermore, animals were challenged by pharmacological activation of HIF-1α and Pkd1 -deficient mice were treated with suramin, an antagonist of purinergic signaling. In addition, expression of P2Y2R, TMEM16A, and HIF-1α was analyzed in nephrectomy samples from 27 patients with ADPKD.

Results

Genetic deletion of P2y2r significantly inhibited cyst growth in vivo . In addition, aggravation of the polycystic phenotype mediated by pharmacological activation of HIF-1α was reduced by deletion of P2y2r . Application of suramin to pharmacologically inhibit purinergic signaling also suppressed cyst enlargement in vivo . Analysis of kidney samples from 27 patients with ADPKD revealed significant expression of P2Y2R at the luminal site of the cyst-lining epithelium.

Conclusions

P2Y2R was significantly expressed in human and mouse polycystic kidneys. Deletion and antagonism of P2Y2R reduced cyst enlargement in an ADPKD mouse model.

Benzothiazole derivatives as histone deacetylase inhibitors for the treatment of autosomal dominant polycystic kidney disease

Recent evidence suggests that histone deacetylases (HDACs) are important regulators of autosomal dominant polycystic kidney disease (ADPKD). In the present study, a series of benzothiazole-bearing compounds were designed and synthesized as potential HDAC inhibitors. Given the multiple participation of HDACs in ADPKD cyst progression, we embarked on a targeted screen using HeLa nuclear extracts to identify potent pan-HDAC inhibitors. Compound 26 emerged as the most efficacious candidate. Subsequent pharmacological characterization showed that compound 26 effectively inhibits several HDACs, notably HDAC1, HDAC2, and HDAC6 (IC50 < 150 nM), displaying a particularly high sensitivity towards HDAC6 (IC50 = 11 nM). The selected compound significantly prevented cyst formation and expansion in an in vitro cyst model and was efficacious in reducing cyst growth in both an embryonic kidney cyst model and an in vivo ADPKD mouse model. Our results provided compelling evidence that compound 26 represents a new HDAC inhibitor for the treatment of ADPKD.

The Han:SPRD Rat: A Preclinical Model of Polycystic Kidney Disease

Autosomal Dominant Polycystic Kidney Disease (ADPKD) stands as the most prevalent hereditary renal disorder in humans, ultimately culminating in end-stage kidney disease. Animal models carrying mutations associated with polycystic kidney disease have played an important role in the advancement of ADPKD research. The Han:SPRD rat model, carrying an R823W mutation in the Anks6 gene, is characterized by cyst formation and kidney enlargement. The mutated protein, named Samcystin, is localized in cilia of tubular epithelial cells and seems to be involved in cystogenesis. The homozygous Anks6 mutation leads to end-stage renal disease and death, making it a critical factor in kidney development and function. This review explores the utility of the Han:SPRD rat model, highlighting its phenotypic similarity to human ADPKD. Specifically, we discuss its role in preclinical trials and its importance for investigating the pathogenesis of the disease and developing new therapeutic approaches.

Pathogenic Relationships in Cystic Fibrosis and Renal Diseases: CFTR, SLC26A9 and Anoctamins

The Cl--transporting proteins CFTR, SLC26A9, and anoctamin (ANO1; ANO6) appear to have more in common than initially suspected, as they all participate in the pathogenic process and clinical outcomes of airway and renal diseases. In the present review, we will therefore concentrate on recent findings concerning electrolyte transport in the airways and kidneys, and the role of CFTR, SLC26A9, and the anoctamins ANO1 and ANO6. Special emphasis will be placed on cystic fibrosis and asthma, as well as renal alkalosis and polycystic kidney disease. In essence, we will summarize recent evidence indicating that CFTR is the only relevant secretory Cl- channel in airways under basal (nonstimulated) conditions and after stimulation by secretagogues. Information is provided on the expressions of ANO1 and ANO6, which are important for the correct expression and function of CFTR. In addition, there is evidence that the Cl- transporter SLC26A9 expressed in the airways may have a reabsorptive rather than a Cl--secretory function. In the renal collecting ducts, bicarbonate secretion occurs through a synergistic action of CFTR and the Cl-/HCO3- transporter SLC26A4 (pendrin), which is probably supported by ANO1. Finally, in autosomal dominant polycystic kidney disease (ADPKD), the secretory function of CFTR in renal cyst formation may have been overestimated, whereas ANO1 and ANO6 have now been shown to be crucial in ADPKD and therefore represent new pharmacological targets for the treatment of polycystic kidney disease.

Spotlight on Genetic Kidney Diseases: A Call for Drug Delivery and Nanomedicine Solutions

Nanoparticles as drug delivery carriers have benefited diseases, including cancer, since the 1990s, and more recently, their promise to quickly and efficiently be mobilized to fight against global diseases such as in the COVID-19 pandemic have been proven. Despite these success stories, there are limited nanomedicine efforts for chronic kidney diseases (CKDs), which affect 844 million people worldwide and can be linked to a variety of genetic kidney diseases. In this Perspective, we provide a brief overview of the clinical status of genetic kidney diseases, background on kidney physiology and a summary of nanoparticle design that enable kidney access and targeting, and emerging technological strategies that can be applied for genetic kidney diseases, including rare and congenital kidney diseases. Finally, we conclude by discussing gaps in knowledge remaining in both genetic kidney diseases and kidney nanomedicine and collective efforts that are needed to bring together stakeholders from diverse expertise and industries to enable the development of the most relevant drug delivery strategies that can make an impact in the clinic.

In vitro cyst puncture and injury-induced tubule formation using renal epithelial cells

Collecting-duct-derived renal epithelial cells switch from tubule to cyst formation; however, the cysts still form tubules after injury of the cyst-lining epithelium. Here, we provide a protocol that describes in vitro cyst growth with focus on glass-capillary-induced cyst wall injury to induce tubule formation. We detail steps for the establishment of the in vitro cyst assay, followed by puncture of the cysts in the collagen matrix. We further describe live imaging and steps to analyze the tubule growth. For complete details on the use and execution of this protocol, please refer to Scholz et al. (2022).1.

A novel direct adenosine monophosphate kinase activator ameliorates disease progression in preclinical models of Autosomal Dominant Polycystic Kidney Disease

Autosomal dominant polycystic kidney disease (ADPKD) mainly results from mutations in the PKD1 gene, which encodes polycystin 1. It is the most common inherited kidney disease and is characterized by a progressive bilateral increase in cyst number and size, often leading to kidney failure. The cellular energy sensor and regulator adenosine monophosphate stimulated protein kinase (AMPK) has been implicated as a promising new therapeutic target. To address this hypothesis, we determined the effects of a potent and selective clinical stage direct allosteric AMPK activator, PXL770, in canine and patient-derived 3D cyst models and an orthologous mouse model of ADPKD. PXL770 induced AMPK activation and dose-dependently reduced cyst growth in principal-like Madin-Darby Canine Kidney cells stimulated with forskolin and kidney epithelial cells derived from patients with ADPKD stimulated with desmopressin. In an inducible, kidney epithelium-specific Pkd1 knockout mouse model, PXL770 produced kidney AMPK pathway engagement, prevented the onset of kidney failure (reducing blood urea by 47%), decreased cystic index by 26% and lowered the kidney weight to body weight ratio by 35% compared to untreated control Pkd1 knockout mice. These effects were accompanied by a reduction of markers of cell proliferation (-48%), macrophage infiltration (-53%) and tissue fibrosis (-37%). Thus, our results show the potential of direct allosteric AMPK activation in the treatment of ADPKD and support the further development of PXL770 for this indication.

Emerging therapies for autosomal dominant polycystic kidney disease with a focus on cAMP signaling

Autosomal dominant polycystic kidney disease (ADPKD), with an estimated genetic prevalence between 1:400 and 1:1,000 individuals, is the third most common cause of end stage kidney disease after diabetes mellitus and hypertension. Over the last 3 decades there has been great progress in understanding its pathogenesis. This allows the stratification of therapeutic targets into four levels, gene mutation and polycystin disruption, proximal mechanisms directly caused by disruption of polycystin function, downstream regulatory and signaling pathways, and non-specific pathophysiologic processes shared by many other diseases. Dysfunction of the polycystins, encoded by the PKD genes, is closely associated with disruption of calcium and upregulation of cyclic AMP and protein kinase A (PKA) signaling, affecting most downstream regulatory, signaling, and pathophysiologic pathways altered in this disease. Interventions acting on G protein coupled receptors to inhibit of 3',5'-cyclic adenosine monophosphate (cAMP) production have been effective in preclinical trials and have led to the first approved treatment for ADPKD. However, completely blocking cAMP mediated PKA activation is not feasible and PKA activation independently from cAMP can also occur in ADPKD. Therefore, targeting the cAMP/PKA/CREB pathway beyond cAMP production makes sense. Redundancy of mechanisms, numerous positive and negative feedback loops, and possibly counteracting effects may limit the effectiveness of targeting downstream pathways. Nevertheless, interventions targeting important regulatory, signaling and pathophysiologic pathways downstream from cAMP/PKA activation may provide additive or synergistic value and build on a strategy that has already had success. The purpose of this manuscript is to review the role of cAMP and PKA signaling and their multiple downstream pathways as potential targets for emergent therapies for ADPKD.

Effects of Suramin on Polycystic Kidney Disease in a Mouse Model of Polycystin-1 Deficiency

The aberrant activation of the purinergic signaling pathway has been shown to promote cyst growth and fluid secretion in autosomal dominant polycystic kidney disease (ADPKD). Suramin is an anti-parasitic drug that has strong anti-purinergic properties. Whether suramin could have a therapeutic effect on ADPKD has not been fully investigated. We examined the effect of suramin on cyst progression in a Pkd1 microRNAs transgenic mouse model that presented stable Pkd1 knockdown and moderate disease progression. The Pkd1-deficient mice were treated with suramin (60 mg/kg) by intraperitoneal injection twice a week from postnatal days 35 to 90. Kidney-to-body weight ratios, cyst indices, and blood urea nitrogen (BUN) levels were measured. Cell proliferation and macrophage infiltration were determined by immunohistochemistry. The suramin-treated group had significantly lower renal cyst densities, cell proliferation, and macrophage infiltration compared with saline-treated controls. Suramin significantly inhibited ERK phosphorylation and the expression of Il1b, Il6, Nlrp3, Tgfb, Fn1, P2rx7, and P2ry2 mRNAs in the kidneys. However, BUN levels remained high despite the reduction in cyst growth. Furthermore, plasma cystatin C and neutrophil gelatinase-associated lipocalin (NGAL) levels were significantly higher in the suramin-treated group compared with the control group. Periodic acid-Schiff staining revealed degenerative changes and epithelial cell vacuolation in the non-cystic renal tubules, which indicated phospholipidosis following suramin treatment. These results suggest that suramin may reduce renal cyst growth and inflammation, but the associated tubular cell injuries could limit its therapeutic potential. Other purinergic receptor antagonists with less nephrotoxicity may deserve further investigation for the treatment of ADPKD.

Loss of Polycystin-1 causes cAMP-dependent switch from tubule to cyst formation

Autosomal dominant polycystic kidney disease is the most common monogenic disease that causes end-stage renal failure. It primarily results from mutations in the PKD1 gene that encodes for Polycystin-1. How loss of Polycystin-1 translates into bilateral renal cyst development is mostly unknown. cAMP is significantly involved in cyst enlargement but its role in cyst initiation has remained elusive. Deletion of Polycystin-1 in collecting duct cells resulted in a switch from tubule to cyst formation and was accompanied by an increase in cAMP. Pharmacological elevation of cAMP in Polycystin-1-competent cells caused cyst formation, impaired plasticity, nondirectional migration, and mis-orientation, and thus strongly resembled the phenotype of Polycystin-1-deficient cells. Mis-orientation of developing tubule cells in metanephric kidneys upon loss of Polycystin-1 was phenocopied by pharmacological increase of cAMP in wildtype kidneys. In vitro, cAMP impaired tubule formation after capillary-induced injury which was further impaired by loss Polycystin-1.

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Loss of Polycystin-1 switches renal cells from tubule to cyst formation

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Deletion of Polycystin-1 leads to increase in cAMP

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Elevation of cAMP in wildtype cells phenocopies Polycystin-1-deficient features

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Features are: impaired plasticity, nondirectional migration, and mis-orientation

Recent advances in understanding ion transport mechanisms in polycystic kidney disease

This review focuses on the most recent advances in the understanding of the electrolyte transport-related mechanisms important for the development of severe inherited renal disorders, autosomal dominant (AD) and recessive (AR) forms of polycystic kidney disease (PKD). We provide here a basic overview of the origins and clinical aspects of ARPKD and ADPKD and discuss the implications of electrolyte transport in cystogenesis. Special attention is devoted to intracellular calcium handling by the cystic cells, with a focus on polycystins and fibrocystin, as well as other calcium level regulators, such as transient receptor potential vanilloid type 4 (TRPV4) channels, ciliary machinery, and purinergic receptor remodeling. Sodium transport is reviewed with a focus on the epithelial sodium channel (ENaC), and the role of chloride-dependent fluid secretion in cystic fluid accumulation is discussed. In addition, we highlight the emerging promising concepts in the field, such as potassium transport, and suggest some new avenues for research related to electrolyte handling.

The chloride channel CFTR is not required for cyst growth in an ADPKD mouse model

Autosomal dominant polycystic kidney disease (ADPKD) is characterized by the development of bilateral renal cysts which enlarge continuously, leading to compression of adjacent intact nephrons. The growing cysts lead to a progressive decline in renal function. Cyst growth is driven by enhanced cell proliferation and chloride secretion into the cyst lumen. Chloride secretion is believed to occur mainly by the cAMP-activated cystic fibrosis transmembrane conductance regulator (CFTR), with some contribution by the calcium-activated chloride channel TMEM16A. However, our previous work suggested TMEM16A as a major factor for renal cyst formation. The contribution of CFTR to cyst formation has never been demonstrated in an adult ADPKD mouse model. We used mice with an inducible tubule-specific Pkd1 knockout, which consistently develop polycystic kidneys upon deletion of Pkd1. Cellular properties, ion currents, and cyst development in these mice were compared with that of mice carrying a co-deletion of Pkd1 and Cftr. Knockout of Cftr did not reveal any significant impact on cyst formation in the ADPKD mouse model. Furthermore, knockout of Cftr did not attenuate the largely augmented cell proliferation observed in Pkd1 knockout kidneys. Patch clamp analysis on primary renal epithelial cells lacking expression of Pkd1 indicated an only marginal contribution of CFTR to whole cell Cl^- currents, which were clearly dominated by calcium-activated TMEM16A currents. In conclusion, CFTR does not essentially contribute to renal cyst formation in mice caused by deletion of Pkd1. Enhanced cell proliferation and chloride secretion is caused primarily by upregulation of the calcium-activated chloride channel TMEM16A.

Gender-Dependent Phenotype in Polycystic Kidney Disease Is Determined by Differential Intracellular Ca2+ Signals

Autosomal dominant polycystic kidney disease (ADPKD) is caused by loss of function of PKD1 (polycystin 1) or PKD2 (polycystin 2). The Ca²⁺-activated Cl⁻ channel TMEM16A has a central role in ADPKD. Expression and function of TMEM16A is upregulated in ADPKD which causes enhanced intracellular Ca²⁺ signaling, cell proliferation, and ion secretion. We analyzed kidneys from Pkd1 knockout mice and found a more pronounced phenotype in males compared to females, despite similar levels of expression for renal tubular TMEM16A. Cell proliferation, which is known to be enhanced with loss of Pkd1^−/−, was larger in male when compared to female Pkd1^−/− cells. This was paralleled by higher basal intracellular Ca²⁺ concentrations in primary renal epithelial cells isolated from Pkd1^−/− males. The results suggest enhanced intracellular Ca²⁺ levels contributing to augmented cell proliferation and cyst development in male kidneys. Enhanced resting Ca²⁺ also caused larger basal chloride currents in male primary cells, as detected in patch clamp recordings. Incubation of mouse primary cells, mCCDcl1 collecting duct cells or M1 collecting duct cells with dihydrotestosterone (DHT) enhanced basal Ca²⁺ levels and increased basal and ATP-stimulated TMEM16A chloride currents. Taken together, the more severe cystic phenotype in males is likely to be caused by enhanced cell proliferation, possibly due to enhanced basal and ATP-induced intracellular Ca²⁺ levels, leading to enhanced TMEM16A currents. Augmented Ca²⁺ signaling is possibly due to enhanced expression of Ca²⁺ transporting/regulating proteins.

Macrophage migration inhibitory factor is regulated by HIF-1α and cAMP and promotes renal cyst cell proliferation in a macrophage-independent manner

Progressive cyst growth leads to decline of renal function in polycystic kidney disease. Macrophage migration inhibitory factor (MIF) was found to be upregulated in cyst-lining cells in a mouse model of polycystic kidney disease and to promote cyst growth. In addition, MIF can be secreted by tubular cells and may contribute to cyst growth in an autocrine manner. However, the underlying mechanisms leading to induction of MIF in cyst-lining cells remained elusive. Here, we demonstrate that hypoxia-inducible transcription factor (HIF) 1α upregulates MIF in cyst-lining cells in a tubule-specific PKD1 knockout mouse. Pharmacological stabilization of HIF-1α resulted in significant increase of MIF in cyst epithelial cells whereas tubule-specific knockout of HIF-1α prevented MIF upregulation. Identical regulation could be found for ABCA1, which has been shown to act as a transport protein for MIF. Furthermore, we show that MIF and ABCA1 are direct target genes of HIF-1α in human primary tubular cells. Next to HIF-1α and hypoxia, we found MIF being additionally regulated by cAMP which is a strong promotor of cyst growth. In line with these findings, HIF-1α- and cAMP-dependent in vitro cyst growth could be decreased by the MIF-inhibitor ISO-1 which resulted in reduced cyst cell proliferation. In conclusion, HIF-1α and cAMP regulate MIF in primary tubular cells and cyst-lining epithelial cells, and MIF promotes cyst growth in the absence of macrophages. In line with these findings, the MIF inhibitor ISO-1 attenuates HIF-1α- and cAMP-dependent in vitro cyst enlargement. KEY MESSAGES: • MIF is upregulated in cyst-lining cells in a polycystic kidney disease mouse model. • MIF upregulation is mediated by hypoxia-inducible transcription factor (HIF) 1α. • ABCA1, transport protein for MIF, is also regulated by HIF-1α in vitro and in vivo. • MIF is additionally regulated by cAMP, a strong promotor of cyst growth. • MIF-inhibitor ISO-1 reduces HIF-1α- and cAMP-dependent cyst growth.

Cyst growth in ADPKD is prevented by pharmacological and genetic inhibition of TMEM16A in vivo

In autosomal dominant polycystic kidney disease (ADPKD) multiple bilateral renal cysts gradually enlarge, leading to a decline in renal function. Transepithelial chloride secretion through cystic fibrosis transmembrane conductance regulator (CFTR) and TMEM16A (anoctamin 1) are known to drive cyst enlargement. Here we demonstrate that loss of Pkd1 increased expression of TMEM16A and CFTR and Cl^- secretion in murine kidneys, with TMEM16A essentially contributing to cyst growth. Upregulated TMEM16A enhanced intracellular Ca²⁺ signaling and proliferation of Pkd1-deficient renal epithelial cells. In contrast, increase in Ca²⁺ signaling, cell proliferation and CFTR expression was not observed in Pkd1/Tmem16a double knockout mice. Knockout of Tmem16a or inhibition of TMEM16A in vivo by the FDA-approved drugs niclosamide and benzbromarone, as well as the TMEM16A-specific inhibitor Ani9 largely reduced cyst enlargement and abnormal cyst cell proliferation. The present data establish a therapeutic concept for the treatment of ADPKD.

Modulation of polycystic kidney disease by G-protein coupled receptors and cyclic AMP signaling

Autosomal Dominant Polycystic Kidney Disease (ADPKD) is a systemic disorder associated with polycystic liver disease (PLD) and other extrarenal manifestations, the most common monogenic cause of end-stage kidney disease, and a major burden for public health. Many studies have shown that alterations in G-protein and cAMP signaling play a central role in its pathogenesis. As for many other diseases (35% of all approved drugs target G-protein coupled receptors (GPCRs) or proteins functioning upstream or downstream from GPCRs), treatments targeting GPCR have shown effectiveness in slowing the rate of progression of ADPKD. Tolvaptan, a vasopressin V2 receptor antagonist is the first drug approved by regulatory agencies to treat rapidly progressive ADPKD. Long-acting somatostatin analogs have also been effective in slowing the rates of growth of polycystic kidneys and liver. Although no treatment has so far been able to prevent the development or stop the progression of the disease, these encouraging advances point to G-protein and cAMP signaling as a promising avenue of investigation that may lead to more effective and safe treatments. This will require a better understanding of the relevant GPCRs, G-proteins, cAMP effectors, and of the enzymes and A-kinase anchoring proteins controlling the compartmentalization of cAMP signaling. The purpose of this review is to provide an overview of general GPCR signaling; the function of polycystin-1 (PC1) as a putative atypical adhesion GPCR (aGPCR); the roles of PC1, polycystin-2 (PC2) and the PC1-PC2 complex in the regulation of calcium and cAMP signaling; the cross-talk of calcium and cAMP signaling in PKD; and GPCRs, adenylyl cyclases, cyclic nucleotide phosphodiesterases, and protein kinase A as therapeutic targets in ADPKD.

TMEM16A drives renal cyst growth by augmenting Ca2+ signaling in M1 cells

Abstract

Polycystic kidney disease (PKD) leads to continuous decline of renal function by growth of renal cysts. Enhanced proliferation and transepithelial chloride secretion through cystic fibrosis transmembrane conductance regulator (CFTR) and Ca²⁺-activated TMEM16A Cl⁻ channels is thought to cause an increase in cyst volume. Recent work shows the pro-proliferative role of the Ca²⁺ activated Cl⁻ channel TMEM16A (anoctamin 1), and demonstrates the essential contribution of TMEM16A to CFTR-dependent Cl⁻ secretion. The present data demonstrate an increase in intracellular Ca²⁺ ([Ca²⁺]i) signals and Cl⁻ secretion by TMEM16A, in renal collecting duct principle cells from dog (MDCK) and mouse (M1) as well as primary tubular epithelial cells from PKD1−/− knockout mice. M1 organoids proliferated, increased expression of TMEM16A, and secreted Cl⁻ upon knockdown of endogenous polycystin 1 or 2 (PKD1,2), by retroviral transfection with shPKD1 and shPKD2, respectively. Knockdown of PKD1 or PKD2 increased basal intracellular Ca²⁺ levels and enhanced purinergic Ca²⁺ release from endoplasmic reticulum. In contrast, ryanodine receptors were found not to be expressed in mouse renal epithelial cells and caffeine had no effects on [Ca²⁺]i. Ca²⁺ signals, proliferation, and Cl⁻ secretion were largely reduced by knockdown or blockade of TMEM16A. TMEM16A may be therefore important for enhanced Ca²⁺ release from IP₃-sensitive Ca²⁺ stores in polycystic kidney disease.

Key messages

• ADPKD leads to continuous decline of renal function by growth of renal cysts.

• Knockdown of PKD1 or PKD2 increases TMEM16A expression.

• TMEM16A enhanced intracellular Ca²⁺ signals, Cl⁻ secretion, and proliferation.

• TMEM16A contributes to cyst growth in ADPKD.

Electronic supplementary material

The online version of this article (10.1007/s00109-020-01894-y) contains supplementary material, which is available to authorized users.

Role of oxygen and the HIF-pathway in polycystic kidney disease

Kidney cyst growth in ADPKD is associated with regional hypoxia, presumably due to a mismatch between enlarged cysts and the peritubular capillary blood supply and compression of peritubular capillaries in cyst walls. Regional hypoxia leads to activation of hypoxia-inducible transcription factors, with the two main HIF isoforms, HIF-1 and HIF-2 expressed in cyst epithelia and pericystic interstitial cells, respectively. While HIF-2 activation is linked to EPO production, mitigating the anemia that normally accompanies chronic kidney disease, HIF-1 promotes cyst growth. HIF-dependent cyst growth is primarily due to an increase in chloride-dependent fluid secretion into the cyst lumen. However, given the broad spectrum of HIF-target genes, additional HIF-mediated pathways may also contribute to cyst progression. Furthermore, hypoxia can influence cyst growth through the generation of reactive oxygen species. Since cyst expansion aggravates regional hypoxia, a feedforward loop is established that accelerates cyst expansion and disease progression. Inhibiting the HIF pathway and/or HIF target genes that are of particular relevance for HIF-dependent cyst fluid secretion may therefore represent novel therapeutic approaches to retard the progression of APDKD.

Prioritization of novel ADPKD drug candidates from disease-stage specific gene expression profiles

Background

Autosomal Dominant Polycystic Kidney Disease (ADPKD) is one of the most common causes of end-stage renal failure, caused by mutations in PKD1 or PKD2 genes. Tolvaptan, the only drug approved for ADPKD treatment, results in serious side-effects, warranting the need for novel drugs.

Methods

In this study, we applied RNA-sequencing of Pkd1cko mice at different disease stages, and with/without drug treatment to identify genes involved in ADPKD progression that were further used to identify novel drug candidates for ADPKD. We followed an integrative computational approach using a combination of gene expression profiling, bioinformatics and cheminformatics data.

Findings

We identified 1162 genes that had a normalized expression after treating the mice with drugs proven effective in preclinical models. Intersecting these genes with target affinity profiles for clinically-approved drugs in ChEMBL, resulted in the identification of 116 drugs targeting 29 proteins, of which several are previously linked to Polycystic Kidney Disease such as Rosiglitazone. Further testing the efficacy of six candidate drugs for inhibition of cyst swelling using a human 3D-cyst assay, revealed that three of the six had cyst-growth reducing effects with limited toxicity.

Interpretation

Our data further establishes drug repurposing as a robust drug discovery method, with three promising drug candidates identified for ADPKD treatment (Meclofenamic Acid, Gamolenic Acid and Birinapant). Our strategy that combines multiple-omics data, can be extended for ADPKD and other diseases in the future.

Funding

European Union's Seventh Framework Program, Dutch Technology Foundation Stichting Technische Wetenschappen and the Dutch Kidney Foundation.

Small-molecule allosteric activators of PDE4 long form cyclic AMP phosphodiesterases

Cyclic AMP (cAMP) phosphodiesterase-4 (PDE4) enzymes degrade cAMP and underpin the compartmentalization of cAMP signaling through their targeting to particular protein complexes and intracellular locales. We describe the discovery and characterization of a small-molecule compound that allosterically activates PDE4 long isoforms. This PDE4-specific activator displays reversible, noncompetitive kinetics of activation (increased V _max with unchanged K _m), phenocopies the ability of protein kinase A (PKA) to activate PDE4 long isoforms endogenously, and requires a dimeric enzyme assembly, as adopted by long, but not by short (monomeric), PDE4 isoforms. Abnormally elevated levels of cAMP provide a critical driver of the underpinning molecular pathology of autosomal dominant polycystic kidney disease (ADPKD) by promoting cyst formation that, ultimately, culminates in renal failure. Using both animal and human cell models of ADPKD, including ADPKD patient-derived primary cell cultures, we demonstrate that treatment with the prototypical PDE4 activator compound lowers intracellular cAMP levels, restrains cAMP-mediated signaling events, and profoundly inhibits cyst formation. PDE4 activator compounds thus have potential as therapeutics for treating disease driven by elevated cAMP signaling as well as providing a tool for evaluating the action of long PDE4 isoforms in regulating cAMP-mediated cellular processes.

Lipid Peroxidation Drives Renal Cyst Growth In Vitro through Activation of TMEM16A

BACKGROUND

Transepithelial chloride^- secretion, through the chloride channels cystic fibrosis transmembrane conductance regulator (CFTR) and TMEM16A (anoctamin 1), drives cyst enlargement in polycystic kidney disease (PKD). Polycystic kidneys are hypoxic, and oxidative stress activates TMEM16A. However, mechanisms for channel activation in PKD remain obscure.

METHODS

Using tissue samples from patients with autosomal dominant PKD, embryonic kidney cultures, and an MDCK in vitro cyst model, we assessed peroxidation of plasma membrane phospholipids in human and mouse polycystic kidneys. We also used electrophysiologic Ussing chamber and patch clamp experiments to analyze activation of TMEM16A and growth of renal cysts.

RESULTS

Peroxidation of phospholipids in human and mouse kidneys as well as MDCK cysts in vitro is probably due to enhanced levels of reactive oxygen species. Lipid peroxidation correlated with increased cyst volume as shown in renal cultures and MDCK cysts in three-dimensional cultures. Reactive oxygen species and lipid peroxidation strongly activated TMEM16A, leading to depletion of calcium ion stores and store-operated calcium influx. Activation of TMEM16A- and CFTR-dependent chloride secretion strongly augmented cyst growth. Exposure to scavengers of reactive oxygen species, such as glutathione, coenzyme Q10, or idebenone (a synthetic coenzyme Q10 homolog), as well as inhibition of oxidative lipid damage by ferrostatin-1 largely reduced activation of TMEM16A. Inhibition of TMEM16A reduced proliferation and fluid secretion in vitro.

CONCLUSIONS

These findings indicate that activation of TMEM16A by lipid peroxidation drives growth of renal cysts. We propose direct inhibition of TMEM16A or inhibition of lipid peroxidation as potentially powerful therapeutic approaches to delay cyst development in PKD.

Characterization of purinergic receptor expression in ARPKD cystic epithelia

Polycystic kidney diseases (PKDs) are a group of inherited nephropathies marked by formation of fluid-filled cysts along the nephron. Growing evidence suggests that in the kidney formation of cysts and alteration of cystic electrolyte transport are associated with purinergic signaling. PCK/CrljCrl-Pkhd1pck/CRL (PCK) rat, an established model of autosomal recessive polycystic kidney disease (ARPKD), was used here to test this hypothesis. Cystic fluid of PCK rats and their cortical tissues exhibited significantly higher levels of ATP compared to Sprague Dawley rat kidney cortical interstitium as assessed by highly sensitive ATP enzymatic biosensors. Confocal calcium imaging of the freshly isolated cystic monolayers revealed a stronger response to ATP in a higher range of concentrations (above 100 μM). The removal of extracellular calcium results in the profound reduction of the ATP evoked transient, which suggests calcium entry into the cyst-lining cells is occurring via the extracellular (ionotropic) P2X channels. Further use of pharmacological agents (α,β-methylene-ATP, 5-BDBD, NF449, isoPPADS, AZ10606120) and immunofluorescent labeling of isolated cystic epithelia allowed us to narrow down potential candidate receptors. In conclusion, our ex vivo study provides direct evidence that the profile of P2 receptors is shifted in ARPKD cystic epithelia in an age-related manner towards prevalence of P2X₄ and/or P2X₇ receptors, which opens new avenues for the treatment of this disease.

HIF-1α promotes cyst progression in a mouse model of autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is mainly caused by mutations of the PKD1 gene and characterized by growth of bilateral renal cysts. Cyst growth is accompanied by regional hypoxia and induction of hypoxia-inducible factor (HIF)-1α in cyst-lining epithelial cells. To determine the relevance of HIF-1α for cyst growth in vivo we used an inducible kidney epithelium-specific knockout mouse to delete Pkd1 at postnatal day 20 or 35 to induce polycystic kidney disease of different severity and analyzed the effects of Hif-1α co-deletion and HIF-1α stabilization using a prolyl-hydroxylase inhibitor. HIF-1α expression was enhanced in kidneys with progressive cyst growth induced by early Pkd1 deletion, but unchanged in the milder phenotype induced by later Pkd1 deletion. Hif-1α co-deletion significantly attenuated cyst growth in the severe, but not in the mild, phenotype. Application of a prolyl-hydroxylase inhibitor resulted in severe aggravation of the mild phenotype with rapid loss of renal function. HIF-1α expression was associated with induction of genes that mediate calcium-activated chloride secretion. Thus, HIF-1α does not seem to play a role in early cyst formation, but accelerates cyst growth during progressive polycystic kidney disease. This novel mechanism of cyst growth may qualify as a therapeutic target.

Three-dimensional in vitro models answer the right questions in ADPKD cystogenesis

Novel technologies, new understanding of the basement membrane composition, and better comprehension of the embryonic development of the mammalian kidney have led to explosive growth in the use of three-dimensional in vitro models to study a range of human disease pathologies (Clevers H. Cell 165: 1586-1597, 2016; Shamir ER, Ewald AJ. Nat Rev Mol Cell Biol 15: 647-664, 2014). The development of these effective model systems represents a new tool to study the progressive cystogenesis of autosomal dominant polycystic kidney disease (ADPKD). ADPKD is a prevalent and complex monogenetic disease, characterized by the pathological formation of fluid fill cysts in renal tissue (Grantham JJ, Mulamalla S, Swenson-Fields KI. Nat Rev Nephrol 7: 556-566, 2011; Takiar V, Caplan MJ. Biochim Biophys Acta 1812: 1337-1343, 2011). ADPKD cystogenesis is attributed to loss of function mutations in either PKD1 or PKD2, which encode for two transmembrane proteins, polycystin-1 and polycystin-2, and progresses with loss of both copies of either gene through a proposed two-hit mechanism with secondary somatic mutations (Delmas P, Padilla F, Osorio N, Coste B, Raoux M, Crest M. Biochem Biophys Res Commun 322: 1374-1383, 2004; Pei Y, Watnick T, He N, Wang K, Liang Y, Parfrey P, Germino G, St George-Hyslop P. Am Soc Nephrol 10: 1524-1529, 1999; Wu G, D'Agati V, Cai Y, Markowitz G, Park JH, Reynolds DM, Maeda Y, Le TC, Hou H Jr, Kucherlapati R, Edelmann W, Somlo S. Cell 93: 177-188, 1998). The exaggerated consequences of large fluid filled cysts result in fibrosis and nephron injury, leading initially to functional compensation but ultimately to dysfunction (Grantham JJ. Am J Kidney Dis 28: 788-803, 1996; Norman J. Biochim Biophys Acta 1812: 1327-1336, 2011; Song CJ, Zimmerman KA, Henke SJ, Yoder BK. Results Probl Cell Differ 60: 323-344, 2017). The complicated disease progression has scattered focus and resources across the spectrum of ADPKD research.

Functional and therapeutic importance of purinergic signaling in polycystic kidney disease

Polycystic kidney diseases (PKD) are a group of inherited nephropathies marked with the formation of fluid-filled cysts along the nephron. This renal disorder affects millions of people worldwide, but current treatment strategies are unfortunately limited to supportive therapy, dietary restrictions, and, eventually, renal transplantation. Recent advances in PKD management are aimed at targeting exaggerated cell proliferation and dedifferentiation to interfere with cyst growth. However, not nearly enough is known about the ion transport properties of the cystic cells, or specific signaling pathways modulating channels and transporters in this condition. There is growing evidence that abnormally elevated concentrations of adenosine triphosphate (ATP) in PKD may contribute to cyst enlargement; change in the profile of purinergic receptors may also result in promotion of cystogenesis. The current mini-review is focused on the role of ATP and associated signaling affecting ion transport properties of the renal cystic epithelia.

P2Y2R is a direct target of HIF-1α and mediates secretion-dependent cyst growth of renal cyst-forming epithelial cells

Polycystic kidney diseases are characterized by numerous renal cysts that continuously enlarge resulting in compression of intact nephrons and tissue hypoxia. Recently, we have shown that hypoxia-inducible factor (HIF)-1α promotes secretion-dependent cyst expansion, presumably by transcriptional regulation of proteins that are involved in calcium-activated chloride secretion. Here, we report that HIF-1α directly activates expression of the purinergic receptor P2Y2R in human primary renal tubular cells. In addition, we found that P2Y2R is highly expressed in cyst-lining cells of human ADPKD kidneys as well as PKD1 orthologous mouse kidneys. Knockdown of P2Y2R in renal collecting duct cells inhibited calcium-dependent chloride secretion in Ussing chamber analyses. In line with these findings, knockdown of P2Y2R retarded cyst expansion in vitro and prevented ATP- and HIF-1α-dependent cyst growth. In conclusion, P2Y2R mediates ATP-dependent cyst growth and is transcriptionally regulated by HIF-1α. These findings provide further mechanistic evidence on how hypoxia promotes cyst growth.

Chloride secretion by renal collecting ducts

PURPOSE OF REVIEW

Renal collecting ducts maintain NaCl homeostasis by fine-tuning urinary excretion to balance dietary salt intake. This review focuses on recent studies on transcellular Cl secretion by collecting ducts, its regulation and its role in cyst growth in autosomal dominant polycystic kidney disease (ADPKD).

RECENT FINDINGS

Lumens of nonperfused rat medullary collecting ducts collapse in control media but expand with fluid following treatment with cAMP, demonstrating the capacity for both salt absorption and secretion. Recently, inhibition of apical epithelial Na channels (ENaC) unmasked Cl secretion in perfused mouse cortical collecting ducts (CCDs), involving Cl uptake by basolateral NKCC1 and efflux through apical Cl channels. AVP, the key hormone for osmoregulation, promotes cystic fibrosis transmembrane conductance regulator (CFTR)-mediated Cl secretion. In addition, prostaglandin E2 stimulates Cl secretion through both CFTR and Ca-activated Cl channels.

SUMMARY

Renal Cl secretion has been commonly overlooked because of the overwhelming capacity for the nephron to reabsorb NaCl from the glomerular filtrate. In ADPKD, Cl secretion plays a central role in the accumulation of cyst fluid and the remarkable size of the cystic kidneys. Investigation of renal Cl secretion may provide a better understanding of NaCl homeostasis and identify new approaches to reduce cyst growth in PKD.

New Insights into the Molecular Mechanisms Targeting Tubular Channels/Transporters in PKD Development

BACKGROUND

Autosomal dominant polycystic kidney disease (PKD) or autosomal recessive PKD is caused by a mutation in the PKD1, PKD2 or PKHD1 gene, which encodes polycystin-1, polycystin-2 or fibrocystin, respectively. Embryonic and postnatal mutation studies show that transport or channel function is dysregulated before the initiation of cystogenesis, suggesting that the abnormality of transport or channel function plays a critical role in the pathology of PKD.

SUMMARY

Polycystin-2 by itself is a calcium-permeable cation channel, and its channel function can be regulated by polycystin-1 or fibrocystin. In this paper, we reviewed the current knowledge about calcium transports and cyclic adenosine monophosphate (cAMP)-driven chloride transports in PKD. In addition, the function and the underlining mechanism of glucose transporters, phosphate transporters and water channels in PKD are also discussed.

KEY MESSAGES

Abnormalities in calcium handling and exuberant cAMP-dependent cystic fibrosis transmembrane conductance regulator-mediated fluid secretion in the collecting duct are the most important issues in the pathogenesis of PKD.

Anoctamin 6 is localized in the primary cilium of renal tubular cells and is involved in apoptosis-dependent cyst lumen formation

Primary cilia are antenna-like structures projected from the apical surface of various mammalian cells including renal tubular cells. Functional or structural defects of the cilium lead to systemic disorders comprising polycystic kidneys as a key feature. Here we show that anoctamin 6 (ANO6), a member of the anoctamin chloride channel family, is localized in the primary cilium of renal epithelial cells in vitro and in vivo. ANO6 was not essential for cilia formation and had no effect on in vitro cyst expansion. However, knockdown of ANO6 impaired cyst lumen formation of MDCK cells in three-dimensional culture. In the absence of ANO6, apoptosis was reduced and epithelial cells were incompletely removed from the center of cell aggregates, which form in the early phase of cystogenesis. In line with these data, we show that ANO6 is highly expressed in apoptotic cyst epithelial cells of human polycystic kidneys. These data identify ANO6 as a cilium-associated protein and suggest its functional relevance in cyst formation.

Glucose promotes secretion-dependent renal cyst growth

Polycystic kidney diseases are characterized by the development of numerous bilateral renal cysts that continuously enlarge resulting in a decline of kidney function due to compression of intact nephrons. Cyst growth is driven by transepithelial chloride secretion which depends on both intracellular cAMP and calcium. Mechanisms that are involved in the regulation of the underlying secretory pathways remain incompletely understood. Here we show that glucose concentration has a strong impact on cyst growth of renal tubular cells within a collagen matrix as well as in embryonic kidneys deficient or competent for Pkd1. Glucose-dependent cyst growth correlates with the transcriptional induction of the calcium-activated chloride channel anoctamin 1 (ANO1) and its increased expression in the apical membrane of cyst-forming cells. Inhibition of ANO1 with the specific inhibitor CaCCinh-AO1 significantly decreases glucose-dependent cyst growth in both models. Ussing chamber analyses revealed increased apical chloride secretion of renal tubular cells upon exposure to high glucose medium which can also be inhibited by the use of CaCCinh-AO1. These data suggest that glycemic control may help to reduce renal cyst growth in patients with polycystic kidney disease.

KEY MESSAGE

Renal cyst growth depends on glucose concentration in two in vitro cyst models. High glucose leads to upregulation of the calcium-activated chloride channel ANO1. High glucose promotes calcium-activated chloride secretion via ANO1. Glucose-dependent secretion can be inhibited by a specific inhibitor of ANO1.

Vasopressin and disruption of calcium signalling in polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is the most common monogenic kidney disease and is responsible for 5-10% of cases of end-stage renal disease worldwide. ADPKD is characterized by the relentless development and growth of cysts, which cause progressive kidney enlargement associated with hypertension, pain, reduced quality of life and eventual kidney failure. Mutations in the PKD1 or PKD2 genes, which encode polycystin-1 (PC1) and polycystin-2 (PC2), respectively, cause ADPKD. However, neither the functions of these proteins nor the molecular mechanisms of ADPKD pathogenesis are well understood. Here, we review the literature that examines how reduced levels of functional PC1 or PC2 at the primary cilia and/or the endoplasmic reticulum directly disrupts intracellular calcium signalling and indirectly disrupts calcium-regulated cAMP and purinergic signalling. We propose a hypothetical model in which dysregulated metabolism of cAMP and purinergic signalling increases the sensitivity of principal cells in collecting ducts and of tubular epithelial cells in the distal nephron to the constant tonic action of vasopressin. The resulting magnified response to vasopressin further enhances the disruption of calcium signalling that is initiated by mutations in PC1 or PC2, and activates downstream signalling pathways that cause impaired tubulogenesis, increased cell proliferation, increased fluid secretion and interstitial inflammation.

Hypoxia inhibits nephrogenesis through paracrine Vegfa despite the ability to enhance tubulogenesis

Reduced nephron number predisposes to hypertension and kidney disease. Interaction of the branching ureteric bud and surrounding mesenchymal cells determines nephron number. Since oxygen supply may be critical for intrauterine development, we tested whether hypoxia and hypoxia-inducible factor-1α (HIF-1α) influence nephrogenesis. We found that HIF-1α is required for branching of MDCK cells. In addition, culture of metanephric mouse kidneys with ureteric bud cell-specific stabilization or knockout of HIF-1α revealed a positive impact of HIF-1α on nephrogenesis. In contrast, widespread stabilization of HIF-1α in metanephric kidneys through hypoxia or HIF stabilizers impaired nephrogenesis, and pharmacological HIF inhibition enhanced nephrogenesis. Several lines of evidence suggest an inhibitory effect through the hypoxia response of mesenchymal cells. HIF-1α was expressed in mesenchymal cells during nephrogenesis. Expression of the anti-branching factors Bmp4 and Vegfa, secreted by mesenchymal cells, was increased upon HIF stabilization. The conditioned medium from hypoxic metanephric kidneys inhibited MDCK branching, which was partially rescued by Vegfa antibodies. Thus, the effect of HIF-1α on nephrogenesis appears context dependent. While HIF-1α in the ureteric bud is of importance for proper branching morphogenesis, the net effect of hypoxia-induced HIF activation in the embryonic kidney appears to be mesenchymal cell-dependent inhibition of ureter branching.

ANO1: an additional key player in cyst growth

Chloride secretion driven by calcium-dependent chloride channels has a major impact on cyst enlargement in polycystic kidney disease. Buchholz et al. studied the functional role of anoctamin 1 (ANO1), a recently identified calcium-activated chloride channel, in kidney cyst growth, using principal-like cells forming cysts within a collagen matrix and an embryonic kidney cyst model. Their results support a potential role for this chloride channel in cyst generation.

Vasopressin-2 receptor signaling and autosomal dominant polycystic kidney disease: from bench to bedside and back again

Blockade of the vasopressin-2 receptor (V2R) in the kidney has recently emerged as a promising therapeutic strategy in autosomal dominant polycystic kidney disease. The pathophysiologic basis of V2R-dependent cyst proliferation and disease progression, however, is not fully understood. Recent evidence suggests that polycystic kidney disease is characterized by defects in urinary concentrating mechanisms and subsequent deregulation of vasopressin excretion by the neurohypophysis. On the cellular level, several recent studies revealed unexpected crosstalk of signaling pathways downstream of V2R activation in the kidney epithelium. This review summarizes some of the unexpected roles of V2R signaling and suggests that vasopressin signaling itself may contribute crucially to loss of polarity and enhanced proliferation in cystic kidney epithelium.

Purinergic signalling in the kidney in health and disease

The involvement of purinergic signalling in kidney physiology and pathophysiology is rapidly gaining recognition and this is a comprehensive review of early and recent publications in the field. Purinergic signalling involvement is described in several important intrarenal regulatory mechanisms, including tuboglomerular feedback, the autoregulatory response of the glomerular and extraglomerular microcirculation and the control of renin release. Furthermore, purinergic signalling influences water and electrolyte transport in all segments of the renal tubule. Reports about purine- and pyrimidine-mediated actions in diseases of the kidney, including polycystic kidney disease, nephritis, diabetes, hypertension and nephrotoxicant injury are covered and possible purinergic therapeutic strategies discussed.

Hypoxia-inducible factor-1α causes renal cyst expansion through calcium-activated chloride secretion

Polycystic kidney diseases are characterized by numerous bilateral renal cysts that continuously enlarge and, through compression of intact nephrons, lead to a decline in kidney function over time. We previously showed that cyst enlargement is accompanied by regional hypoxia, which results in the stabilization of hypoxia-inducible transcription factor-1α (HIF-1α) in the cyst epithelium. Here we demonstrate a correlation between cyst size and the expression of the HIF-1α-target gene, glucose transporter 1, and report that HIF-1α promotes renal cyst growth in two in vitro cyst models-principal-like MDCK cells (plMDCKs) within a collagen matrix and cultured embryonic mouse kidneys stimulated with forskolin. In both models, augmenting HIF-1α levels with the prolyl hydroxylase inhibitor 2-(1-chloro-4-hydroxyisoquinoline-3-carboxamido) acetate enhanced cyst growth. In addition, inhibition of HIF-1α degradation through tubule-specific knockdown of the von Hippel-Lindau tumor suppressor increased cyst size in the embryonic kidney cyst model. In contrast, inhibition of HIF-1α by chetomin and knockdown of HIF-1α both decreased cyst growth in these models. Consistent with previous reports, plMDCK cyst enlargement was driven largely by transepithelial chloride secretion, which consists, in part, of a calcium-activated chloride conductance. plMDCKs deficient for HIF-1α almost completely lacked calcium-activated chloride secretion. We conclude that regional hypoxia in renal cysts contributes to cyst growth, primarily due to HIF-1α-dependent calcium-activated chloride secretion. These findings identify the HIF system as a novel target for inhibition of cyst growth.

Osmoregulation, vasopressin, and cAMP signaling in autosomal dominant polycystic kidney disease

PURPOSE OF REVIEW

Autosomal dominant polycystic kidney disease (ADPKD) is the most frequent inherited nephropathy. This review will focus on the vasopressin and 3'-5'-cyclic adenosine monophosphate (cAMP) signaling pathways in ADPKD and will discuss how these insights offer new possibilities for the follow-up and treatment of the disease.

RECENT FINDINGS

Defective osmoregulation is an early manifestation of ADPKD and originates from both peripheral (renal effect of vasopressin) and central (release of vasopressin) components. Copeptin, which is released from the vasopressin precursor, may identify ADPKD patients at risk for rapid disease progression. Increased levels of cAMP in tubular cells, reflecting modifications in intracellular calcium homeostasis and abnormal stimulation of the vasopressin V2 receptor (V2R), play a central role in cystogenesis. Blocking the V2R lowers cAMP in cystic tissues, slows renal cystic progression and improves renal function in preclinical models. A phase III clinical trial investigating the effect of the V2R antagonist tolvaptan in ADPKD patients has shown that this treatment blunts kidney growth, reduces associated symptoms and slows kidney function decline when given over 3 years.

SUMMARY

These advances open perspectives for the understanding of cystogenesis in ADPKD, the mechanisms of osmoregulation, the role of polycystins in the brain, and the pleiotropic action of vasopressin.

Anoctamin 1 induces calcium-activated chloride secretion and proliferation of renal cyst-forming epithelial cells

Polycystic kidney diseases are characterized by multiple bilateral renal cysts that gradually enlarge and lead to a decline in renal function. Cyst enlargement is driven by transepithelial chloride secretion, stimulated by enhanced levels of cyclic adenosine monophosphate, which activates apical cystic fibrosis transmembrane conductance regulator chloride channels. However, chloride secretion by calcium-dependent chloride channels, activated through stimulation of purinergic receptors, also has a major impact. To identify the molecular basis of calcium-dependent chloride secretion in cyst expansion, we determined the role of anoctamin 1 and 6, two recently discovered calcium-activated chloride channels both of which are expressed in epithelial cells. We found that anoctamin 1, which plays a role in epithelial fluid secretion and proliferation, is strongly expressed in principal-like MDCK cells (PLCs) forming cysts within a collagen matrix, in an embryonic kidney cyst model, and in human autosomal dominant polycystic kidney disease tissue. Knockdown of anoctamin 1 but not anoctamin 6 strongly diminished the calcium-dependent chloride secretion of PLCs. Moreover, two inhibitors of anoctamin ion channels, tannic acid and a more selective inhibitor of anoctamin 1, significantly inhibited PLC cyst growth and cyst enlargement in an embryonic kidney cyst model. Knockdown of ANO1 by morpholino analogs also attenuated embryonic cyst growth. Thus, calcium-activated chloride secretion by anoctamin 1 appears to be a crucial component of renal cyst growth.

Heterogeneity of the MDCK cell line and its applicability for influenza virus research

Single-cell clones have been established from the MDCK cell line, characterized for their morphology and evaluated for their suitability for influenza virus research. Three discrete cell morphotypes were identified using light microscopy. Besides morphological features, the cell types can be distinguished by the level of expression of surface glycans recognized by peanut agglutinin (PNA). All clones were susceptible to infection by influenza viruses of different subtypes of influenza A virus (H1N1, H1N1pdm09, H3N2, H5N1) and influenza B virus, and all possessed on their surface terminally sialylated glycans with both types of glycosidic linkage (α2-3 and α2-6). The Type-1 cell lines were able to support a multicycle replication of influenza A and B viruses without help of an exogenous trypsin. In contrast, cell lines exhibiting Type-2 morphology were unable to support multicycle replication of influenza A viruses without trypsin supplementation. Western blot analysis of the hemagglutinin of H1N1 strains demonstrated that Type-2 cells were deficient in production of proteolytically activated hemagglutinin (no cleavage between HA1/HA2 was observed). HA1/HA2 cleavage of influenza B viruses in the Type-2 cells was also significantly impaired, but not completely abrogated, producing sufficient amount of activated HA to support efficient virus replication without trypsin. In contrast, all clones of Type-1 cells were able to produce proteolytically activated hemagglutinin of influenza A and B viruses. However, the growth kinetics and plaque size of influenza A viruses varied significantly in different clones. Influenza B virus also showed different plaque size, with the biggest plaque formation in the Type-2 cells, although the growth kinetics and peak infectivity titers were similar in all clones. Taken together, the study demonstrates that the population of original MDCK cells is represented by various types of cells that differ in their capacities to support replication of influenza A and B viruses.

Role of extracellular ATP and P2 receptor signaling in regulating renal cyst growth and interstitial inflammation in polycystic kidney disease

Polycystic kidney diseases (PKD) are a group of inherited ciliopathies in which the formation and growth of multiple cysts derived from the distal nephron and collecting duct leads to the disruption of normal kidney architecture, chronic interstitial inflammation/fibrosis and hypertension. Kidney failure is the most life-threatening complication of PKD, and is the consequence of cyst expansion, renal interstitial disease and loss of normal kidney tissue. Over the last decade, accumulating evidence suggests that the autocrine and paracrine effects of ATP (through its receptor family P2X and P2Y), could be detrimental for the progression of PKD. (2009). In vitro, ATP-P2 signaling promotes cystic epithelial cell proliferation, chloride-driven fluid secretion and apoptosis. Furthermore, dysfunction of the polycystin signal transduction pathways promotes the secretagogue activity of extracellular ATP by activating a calcium-activated chloride channel via purinergic receptors. Finally, ATP is a danger signal and could potentially contribute to interstitial inflammation associated with PKD. These data suggest that ATP-P2 signaling worsens the progression of cyst enlargement and interstitial inflammation in PKD.

Kif3a guides microtubular dynamics, migration and lumen formation of MDCK cells

The microtubular motor Kinesin-2 and its subunit Kif3a are essential for the formation of primary cilia, an organelle implicated in a wide spectrum of developmental abnormalities. Outside cilia, Kinesin-2 mediated transport has been implicated in vesicle and N-cadherin transport, but it is unknown if and how extraciliary Kif3a affects basic cellular functions such as migration or the formation of multicellular structures. Here we show that tetracycline inducible depletion of Kif3a in MDCK cells slows epithelial cell migration. Microtubules at the leading edge of Kif3a depleted cells failed to grow perpendicularly into the leading edge and microtubular dynamics were dampened in Kif3a depleted cells. Loss of Kif3a retarded lateral membrane specification and completely prevented the formation of three-dimensional spheres in collagen. These data uncover that Kif3a regulates the microtubular cytoskeleton in the cell periphery and imply that extra-ciliary Kif3a has an unexpected function in morphogenesis.

The Raf kinase inhibitor PLX5568 slows cyst proliferation in rat polycystic kidney disease but promotes renal and hepatic fibrosis

BACKGROUND

Autosomal dominant polycystic kidney disease (ADPKD) is a common cause of renal failure. Aberrant epithelial cell proliferation is a major cause of progressive cyst enlargement in ADPKD. Since activation of the Ras/Raf signaling system has been detected in cyst-lining epithelia, inhibition of Raf kinase has been proposed as an approach to retard the progression of ADPKD. Methods and results. PLX5568, a novel selective small molecule inhibitor of Raf kinases, attenuated proliferation of human ADPKD cyst epithelial cells. It reduced in vitro cyst growth of Madin-Darby Canine Kidney cells and of human ADPKD cells within a collagen gel. In male cy/+ rats with polycystic kidneys, PLX5568 inhibited renal cyst growth along with a significant reduction in the number of proliferating cell nuclear antigen- and phosphorylated extracellular signal-regulated kinase-positive cyst-lining epithelial cells. Furthermore, treated animals showed increased capacity to concentrate urine. However, PLX5568 did not lead to a consistent improvement of renal function. Moreover, although relative cyst volume was decreased, total kidney-to-body weight ratio was not significantly reduced by PLX5568. Further analyses revealed a 2-fold increase of renal and hepatic fibrosis in animals treated with PLX5568.

CONCLUSIONS

PLX5568 attenuated cyst enlargement in vitro and in a rat model of ADPKD without improving kidney function, presumably due to increased renal fibrosis. These data suggest that effective therapies for the treatment of ADPKD will need to target fibrosis as well as the growth of cysts.