New insights into the molecular pathophysiology of polycystic kidney disease

Ciliary and Non-Ciliary Roles of IFT88 in Development and Diseases

Cilia are highly specialized cellular projections emanating from the cell surface, whose defects contribute to a spectrum of diseases collectively known as ciliopathies. Intraflagellar transport protein 88 (IFT88) is a crucial component of the intraflagellar transport-B (IFT-B) subcomplex, a protein complex integral to ciliary transport. The absence of IFT88 disrupts the formation of ciliary structures; thus, animal models with IFT88 mutations, including the oak ridge polycystic kidney (ORPK) mouse model and IFT88 conditional allelic mouse model, are frequently employed in molecular and clinical studies of ciliary functions and ciliopathies. IFT88 plays a pivotal role in a variety of cilium-related processes, including organ fibrosis and cyst formation, metabolic regulation, chondrocyte development, and neurological functions. Moreover, IFT88 also exhibits cilium-independent functions, such as spindle orientation, planar cell polarity establishment, and actin organization. A deeper understanding of the biological events and molecular mechanisms mediated by IFT88 is anticipated to advance the development of diagnostic and therapeutic strategies for related diseases.

Kidney organoid models reveal cilium-autophagy metabolic axis as a therapeutic target for PKD both in vitro and in vivo

Human pluripotent stem cell-derived kidney organoids offer unprecedented opportunities for studying polycystic kidney disease (PKD), which still has no effective cure. Here, we developed both in vitro and in vivo organoid models of PKD that manifested tubular injury and aberrant upregulation of renin-angiotensin aldosterone system. Single-cell analysis revealed that a myriad of metabolic changes occurred during cystogenesis, including defective autophagy. Experimental activation of autophagy via ATG5 overexpression or primary cilia ablation significantly inhibited cystogenesis in PKD kidney organoids. Employing the organoid xenograft model of PKD, which spontaneously developed tubular cysts, we demonstrate that minoxidil, a potent autophagy activator and an FDA-approved drug, effectively attenuated cyst formation in vivo. This in vivo organoid model of PKD will enhance our capability to discover novel disease mechanisms and validate candidate drugs for clinical translation.

Co-Inheritance of Pathogenic Variants in PKD1 and PKD2 Genes Determined by Parental Segregation and De Novo Origin: A Case Report

Autosomal dominant polycystic kidney disease (ADPKD) is the most common hereditary renal disease, and it is typically caused by PKD1 and PKD2 heterozygous variants. Nonetheless, the extensive phenotypic variability observed among affected individuals, even within the same family, suggests a more complex pattern of inheritance. We describe an ADPKD family in which the proband presented with an earlier and more severe renal phenotype (clinical diagnosis at the age of 14 and end-stage renal disease aged 24), compared to the other affected family members. Next-generation sequencing (NGS)-based analysis of polycystic kidney disease (PKD)-associated genes in the proband revealed the presence of a pathogenic PKD2 variant and a likely pathogenic variant in PKD1, according to the American College of Medical Genetics and Genomics (ACMG) criteria. The PKD2 nonsense p.Arg872Ter variant was segregated from the proband's father, with a mild phenotype. A similar mild disease presentation was found in the proband's aunts and uncle (the father's siblings). The frameshift p.Asp3832ProfsTer128 novel variant within PKD1 carried by the proband in addition to the pathogenic PKD2 variant was not found in either parent. This report highlights that the co-inheritance of two or more PKD genes or alleles may explain the extensive phenotypic variability among affected family members, thus emphasizing the importance of NGS-based techniques in the definition of the prognostic course.

Cystic Diseases of the Kidneys: From Bench to Bedside

Exploration into the causes of hereditary renal cystic diseases demonstrates a deep-rooted connection with the proteomic components of the cellular organelle cilia. Cilia are essential to the signaling cascades, and their dysfunction has been tied to a range of renal cystic diseases initiating with studies on the oak ridge polycystic kidney (ORPK) mouse model. Here, we delve into renal cystic pathologies that have been tied with ciliary proteosome and highlight the genetics associated with each. The pathologies are grouped based on the mode of inheritance, where inherited causes that result in cystic kidney disease phenotypes include autosomal dominant and autosomal recessive polycystic kidney disease, nephronophthisis (Bardet-Biedl syndrome and Joubert Syndrome), and autosomal dominant tubulointerstitial kidney disease. Alternatively, phakomatoses-, also known as neurocutaneous syndromes, associated cystic kidney diseases include tuberous sclerosis (TS) and Von Hippel-Lindau (VHL) disease. Additionally, we group the pathologies by the mode of inheritance to discuss variations in recommendations for genetic testing for biological relatives of a diagnosed individual.

Octodon degus: a natural model of multimorbidity for ageing research

Integrating the multifactorial processes co-occurring in both physiological and pathological human conditions still remains one of the main challenges in translational investigation. Moreover, the impact of age-associated disorders has increased, which underlines the urgent need to find a feasible model that could help in the development of successful therapies. In this sense, the Octodon degus has been indicated as a 'natural' model in many biomedical areas, especially in ageing. This rodent shows complex social interactions and high sensitiveness to early-stressful events, which have been used to investigate neurodevelopmental processes. Interestingly, a high genetic similarity with some key proteins implicated in human diseases, such as apolipoprotein-E, β-amyloid or insulin, has been demonstrated. On the other hand, the fact that this animal is diurnal has provided important contribution in the field of circadian biology. Concerning age-related diseases, this rodent could be a good model of multimorbidity since it naturally develops cognitive decline, neurodegenerative histopathological hallmarks, visual degeneration, type II diabetes, endocrinological and metabolic dysfunctions, neoplasias and kidneys alterations. In this review we have collected and summarized the studies performed on the Octodon degus through the years that support its use as a model for biomedical research, with a special focus on ageing.

Apoptosis and autophagy in polycystic kidney disease (PKD)

Apoptosis in the cystic epithelium is observed in most rodent models of polycystic kidney disease (PKD) and in human autosomal dominant PKD (ADPKD). Apoptosis inhibition decreases cyst growth, whereas induction of apoptosis in the kidney of Bcl-2 deficient mice increases proliferation of the tubular epithelium and subsequent cyst formation. However, alternative evidence indicates that both induction of apoptosis as well as increased overall rates of apoptosis are associated with decreased cyst growth. Autophagic flux is suppressed in cell, zebra fish and mouse models of PKD and suppressed autophagy is known to be associated with increased apoptosis. There may be a link between apoptosis and autophagy in PKD. The mammalian target of rapamycin (mTOR), B-cell lymphoma 2 (Bcl-2) and caspase pathways that are known to be dysregulated in PKD, are also known to regulate both autophagy and apoptosis. Induction of autophagy in cell and zebrafish models of PKD results in suppression of apoptosis and reduced cyst growth supporting the hypothesis autophagy induction may have a therapeutic role in decreasing cyst growth, perhaps by decreasing apoptosis and proliferation in PKD. Future research is needed to evaluate the effects of direct autophagy inducers on apoptosis in rodent PKD models, as well as the cause and effect relationship between autophagy, apoptosis and cyst growth in PKD.

Investigation of epigenetics in kidney cell biology

Epigenetics is the study of heritable changes in DNA or its associated proteins except mutations in gene sequence. Epigenetic regulation plays fundamental roles in the processes of kidney cell biology through the action of DNA methylation, chromatin modifications via epigenetic regulators and interaction via transcription factors, and noncoding RNA species. Kidney diseases, including acute kidney injury, chronic kidney disease, nephritic and nephrotic syndromes, pyelonephritis and polycystic kidney diseases are driven by aberrant activity in numerous signaling pathways in even individual kidney cell. Epigenetic alterations, including DNA methylation, histone acetylation and methylation, noncoding RNAs, and protein posttranslational modifications, could disrupt essential pathways that protect the renal cells from uncontrolled growth, apoptosis and establishment of other renal associated syndromes, which have been recognized as one of the critical mechanisms for regulating functional changes that drive and maintain the kidney disease phenotype. In this chapter, we briefly summarize the epigenetic mechanisms in kidney cell biology and epigenetic basis of kidney development, and introduce epigenetic techniques that can be used in investigating the molecular mechanism of kidney cell biology and kidneys diseases, primarily focusing on the integration of DNA methylation and chromatin immunoprecipitation technologies into kidney disease associated studies. Future studies using these emerging technologies will elucidate how alterations in the renal cell epigenome cooperate with genetic aberrations for kidney disease initiation and progression. Incorporating epigenomic testing into the clinical research is essential to future studies with epigenetics biomarkers and precision medicine using emerging epigenetic therapies.

Gαi-mediated TRPC4 activation by polycystin-1 contributes to endothelial function via STAT1 activation

Hypertension and aneurysm are frequently associated with autosomal dominant polycystic kidney disease (ADPKD) caused by polycystin-1 (PC1) mutations, which is closely related to endothelial dysfunction. PC1 is an atypical G-protein-coupled receptor that activates G-proteins by self-cleavage; currently, however, the molecular and cellular mechanisms of the associated intracellular signaling and ion channel activation remain poorly elucidated. Here, we report an activation mechanism of a calcium-permeable canonical transient receptor potential 4 (TRPC4) channel by PC1 and its endothelial function. We found that the inhibitory Gα_i3 protein selectively bound to the G-protein-binding domain on the C-terminus of PC1. The dissociation of Gα_i3 upon cleavage of PC1 increased TRPC4 activity. Calcium influx through TRPC4 activated the transcription factor STAT1 to regulate cell proliferation and death. The down-regulation of PC1/TRPC4/STAT1 disrupted migration of endothelial cell monolayers, leading to an increase in endothelial permeability. These findings contribute to greater understanding of the high risk of aneurysm in patients with ADPKD.

Sclt1 deficiency causes cystic kidney by activating ERK and STAT3 signaling

Ciliopathies form a group of inherited disorders sharing several clinical manifestations because of abnormal cilia formation or function, and few treatments have been successful against these disorders. Here, we report a mouse model with mutated Sclt1 gene, which encodes a centriole distal appendage protein important for ciliogenesis. Sodium channel and clathrin linker 1 (SCLT1) mutations were associated with the oral-facial-digital syndrome (OFD), an autosomal recessive ciliopathy. The Sclt1-/- mice exhibit typical ciliopathy phenotypes, including cystic kidney, cleft palate and polydactyly. Sclt1-loss decreases the number of cilia in kidney; increases proliferation and apoptosis of renal tubule epithelial cells; elevates protein kinase A, extracellular signal-regulated kinases, SMAD and signal transducer and activator of transcription 3 (STAT3) pathways; and enhances pro-inflammation and pro-fibrosis pathways with disease progression. Embryonic kidney cyst formation of Sclt1-/- mice was effectively reduced by an anti-STAT3 treatment using pyrimethamine. Overall, we reported a new mouse model for the OFD; and our data suggest that STAT3 inhibition may be a promising treatment for SCLT1-associated cystic kidney.

Aberrant Smad3 phosphoisoforms in cyst-lining epithelial cells in the cpk mouse, a model of autosomal recessive polycystic kidney disease

Cystic epithelia acquire mesenchymal-like features in polycystic kidney disease (PKD). In this phenotypic alteration, it is well known that transforming growth factor (TGF)-β/Smad3 signaling is involved; however, there is emerging new data on Smad3 phosphoisoforms: Smad3 phosphorylated at linker regions (pSmad3L), COOH-terminal regions (pSmad3C), and both (pSmad3L/C). pSmad3L/C has a pathological role in colorectal cancer. Mesenchymal phenotype-specific cell responses in the TGF-β/Smad3 pathway are implicated in carcinomas. In this study, we confirmed mesenchymal features and examined Smad3 phosphoisoforms in the cpk mouse, a model of autosomal recessive PKD. Kidney sections were stained with antibodies against mesenchymal markers and domain-specific phospho-Smad3. TGF-β, pSmad3L, pSmad3C, JNK, cyclin-dependent kinase (CDK) 4, and c-Myc were evaluated by Western blotting. Cophosphorylation of pSmad3L/C was assessed by immunoprecipitation. α-Smooth muscle actin, which indicates mesenchymal features, was expressed higher in cpk mice. pSmad3L expression was increased in cpk mice and was predominantly localized in the nuclei of tubular epithelial cells in cysts; however, pSmad3C was equally expressed in both cpk and control mice. Levels of pSmad3L, JNK, CDK4, and c-Myc protein in nuclei were significantly higher in cpk mice than in controls. Immunoprecipitation showed that Smad3 was cophosphorylated (pSmad3L/C) in cpk mice. Smad3 knockout/ cpk double-mutant mice revealed amelioration of cpk abnormalities. These findings suggest that upregulating c-Myc through the JNK/CDK4-dependent pSmad3L pathway may be key to the pathophysiology in cpk mice. In conclusion, a qualitative rather than a quantitative abnormality of the TGF-β/Smad3 pathway is involved in PKD and may be a target for disease-specific intervention.

Autosomal Dominant Polycystic Disease is Associated with Depressed Levels of Soluble Tumor Necrosis Factor-Related Apoptosis-Inducing Ligand

BACKGROUND

Autosomal dominant polycystic kidney disease (ADPKD) is characterized by multiple, large renal cysts and impaired kidney function. Although the reason for the development of kidney cysts is unknown, ADPKD is associated with cell cycle arrest and abundant apoptosis of renal tubular epithelial cells.

AIMS

We asked whether serum-soluble TNF-related apoptosis-inducing ligand (sTRAIL) might underlie ADPKD.

STUDY DESIGN

Case-control study.

METHODS

Serum sTRAIL levels were measured in 44 patients with ADPKD and 18 healthy volunteers. The human soluble TRAIL/Apo2L ELISA kit was used for the in vitro quantitative determination of sTRAIL in serum samples.

RESULTS

Mean serum sTRAIL levels were lower in patients with ADPKD as compared to the control group (446.9±103.1 and 875.9±349.6 pg/mL, p<0.001). Serum sTRAIL levels did not differ among stages of renal failure in patients with ADPKD. There was no correlation between serum sTRAIL levels and estimated glomerular filtration rate in patients with ADPKD (p>0.05).

CONCLUSION

Our results show that ADPKD patients have depressed sTRAIL levels, indicating apoptosis unrelated to the stage of chronic renal failure.

Ouabain Enhances ADPKD Cell Apoptosis via the Intrinsic Pathway

Progression of autosomal dominant polycystic kidney disease (ADPKD) is highly influenced by factors circulating in blood. We have shown that the hormone ouabain enhances several characteristics of the ADPKD cystic phenotype, including the rate of cell proliferation, fluid secretion and the capacity of the cells to form cysts. In this work, we found that physiological levels of ouabain (3 nM) also promote programmed cell death of renal epithelial cells obtained from kidney cysts of patients with ADPKD (ADPKD cells). This was determined by Alexa Fluor 488 labeled-Annexin-V staining and TUNEL assay, both biochemical markers of apoptosis. Ouabain-induced apoptosis also takes place when ADPKD cell growth is blocked; suggesting that the effect is not secondary to the stimulatory actions of ouabain on cell proliferation. Ouabain alters the expression of BCL family of proteins, reducing BCL-2 and increasing BAX expression levels, anti- and pro-apoptotic mediators respectively. In addition, ouabain caused the release of cytochrome c from mitochondria. Moreover, ouabain activates caspase-3, a key “executioner” caspase in the cell apoptotic pathway, but did not affect caspase-8. This suggests that ouabain triggers ADPKD cell apoptosis by stimulating the intrinsic, but not the extrinsic pathway of programmed cell death. The apoptotic effects of ouabain are specific for ADPKD cells and do not occur in normal human kidney cells (NHK cells). Taken together with our previous observations, these results show that ouabain causes an imbalance in cell growth/death, to favor growth of the cystic cells. This event, characteristic of ADPKD, further suggests the importance of ouabain as a circulating factor that promotes ADPKD progression.

Implementing Patch Clamp and Live Fluorescence Microscopy to Monitor Functional Properties of Freshly Isolated PKD Epithelium

Cyst initiation and expansion during polycystic kidney disease is a complex process characterized by abnormalities in tubular cell proliferation, luminal fluid accumulation and extracellular matrix formation. Activity of ion channels and intracellular calcium signaling are key physiologic parameters which determine functions of tubular epithelium. We developed a method suitable for real-time observation of ion channels activity with patch-clamp technique and registration of intracellular Ca2+ level in epithelial monolayers freshly isolated from renal cysts. PCK rats, a genetic model of autosomal recessive polycystic kidney disease (ARPKD), were used here for ex vivo analysis of ion channels and calcium flux. Described here is a detailed step-by-step procedure designed to isolate cystic monolayers and non-dilated tubules from PCK or normal Sprague Dawley (SD) rats, and monitor single channel activity and intracellular Ca2+ dynamics. This method does not require enzymatic processing and allows analysis in a native setting of freshly isolated epithelial monolayer. Moreover, this technique is very sensitive to intracellular calcium changes and generates high resolution images for precise measurements. Finally, isolated cystic epithelium can be further used for staining with antibodies or dyes, preparation of primary cultures and purification for various biochemical assays.

Selective dicer suppression in the kidney alters GSK3β/β-catenin pathways promoting a glomerulocystic disease

Dicer is a crucial enzyme for the maturation of miRNAs. Mutations in the Dicer gene are highly associated with Pleuro Pulmonary Blastoma-Family Dysplasia Syndrome (PPB-FDS, OMIM 601200), recently proposed to be renamed Dicer syndrome. Aside from the pulmonary phenotype (blastoma), renal nephroma and thyroid goiter are frequently part of Dicer syndrome. To investigate the renal phenotype, conditional knockout (cKO) mice for Dicer in Pax8 expressing cells were generated. Dicer cKO mice progressively develop a glomerulocystic phenotype coupled with urinary concentration impairment, proteinuria and severe renal failure. Higher cellular turnover of the parietal cells of Bowman's capsule precedes the development of the cysts and the primary cilium progressively disappears with cyst-enlargement. Upregulation of GSK3β precedes the development of the glomerulocystic phenotype. Downregulation of β-catenin in the renal cortex and its cytosolic removal in the cells lining the cysts may be associated with observed accumulation of GSK3β. Alterations of β-catenin regulating pathways could promote cystic degeneration as in other models. Thus, miRNAs are fundamental in preserving renal morphology and function. Alteration of the GSK3β/β-catenin pathway could be a crucial mechanism linking miRNA dysregulation and the development of a glomerulocystic disease.

Berberine slows cell growth in autosomal dominant polycystic kidney disease cells

Autosomal dominant polycystic kidney disease (ADPKD) is the most common hereditary monogenic disorder characterized by development and enlargement of kidney cysts that lead to loss of renal function. It is caused by mutations in two genes (PKD1 and PKD2) encoding for polycystin-1 and polycystin-2 proteins which regulate different signals including cAMP, mTOR and EGFR pathways. Abnormal activation of these signals following PC1 or PC2 loss of function causes an increased cell proliferation which is a typical hallmark of this disease. Despite the promising findings obtained in animal models with targeted inhibitors able to reduce cystic cell growth, currently, no specific approved therapy for ADPKD is available. Therefore, the research of new more effective molecules could be crucial for the treatment of this severe pathology. In this regard, we have studied the effect of berberine, an isoquinoline quaternary alkaloid, on cell proliferation and apoptosis in human and mouse ADPKD cystic cell lines. Berberine treatment slows cell proliferation of ADPKD cystic cells in a dose-dependent manner and at high doses (100 μg/mL) it induces cell death in cystic cells as well as in normal kidney tubule cells. However, at 10 μg/mL, berberine reduces cell growth in ADPKD cystic cells only enhancing G0/G1 phase of cell cycle and inhibiting ERK and p70-S6 kinases. Our results indicate that berberine shows a selected antiproliferative activity in cellular models for ADPKD, suggesting that this molecule and similar natural compounds could open new opportunities for the therapy of ADPKD patients.

Generation of c-Myc transgenic pigs for autosomal dominant polycystic kidney disease

After several decades of research, autosomal dominant polycystic kidney disease (ADPKD) is still incurable and imposes enormous physical, psychological, and economic burdens on patients and their families. Murine models of ADPKD represent invaluable tools for studying this disease. These murine forms of ADPKD can arise spontaneously, or they can be induced via chemical or genetic manipulations. Although these models have improved our understanding of the etiology and pathogenesis of ADPKD, they have not led to effective treatment strategies. The mini-pig represents an effective biomedical model for studying human diseases, as the pig's human-like physiological processes help to understand disease mechanisms and to develop novel therapies. Here, we tried to generate a transgenic model of ADPKD in pigs by overexpressing c-Myc in kidney tissue. Western-blot analysis showed that c-Myc was overexpressed in the kidney, brain, heart, and liver of transgenic pigs. Immunohistochemical staining of kidney tissue showed that exogenous c-Myc predominantly localized to renal tubules. Slightly elevated blood urea nitrogen levels were observed in transgenic pigs 1 month after birth, but no obvious abnormalities were detected after that time. In the future, we plan to subject this model to renal injury in an effort to promote ADPKD progression.

Tg737 signaling is required for hypoxia-enhanced invasion and migration of hepatoma cells

BACKGROUND

Although hypoxia is known to promote hepatoma cell invasion and migration, little is known regarding the molecular mechanisms of this process. Our previous research showed that loss of Tg737 is associated with hepatoma cell invasion and migration; therefore, we hypothesized that the Tg737 signal might be required for hypoxia-enhanced invasion and migration.

METHODS

We established in vitro normoxic or hypoxic models to investigate the role of Tg737 in the hypoxia-enhanced invasion and migration of hepatoma cells. The hepatoma cell lines HepG2 and MHCC97-H were subjected to normoxic or hypoxic conditions, and the cell adhesion, invasion, and migration capabilities were tested. The expression of Tg737 under normoxia or hypoxia was detected using western blot assays; cell viability was determined using flow cytometry. Furthermore, we created HepG2 and MHCC97-H cells that over expressed Tg737 prior to incubation under hypoxia and investigated their metastatic characteristics. Finally, we analyzed the involvement of critical molecular events known to regulate invasion and migration.

RESULTS

In this study, Tg737 expression was significantly inhibited in HepG2 and MHCC97-H cells following exposure to hypoxia. The down regulation of Tg737 expression corresponded to significantly decreased adhesion and increased invasion and migration. Hypoxia also decreased the expression/secretion of polycystin-1, increased the secretion of interleukin-8 (IL-8), and increased the levels of active and total transforming growth factor β 1 (TGF-β1), critical regulators of cell invasion and migration. Moreover, the decrease in adhesiveness and the increase in the invasive and migratory capacities of hypoxia-treated hepatoma cells were attenuated by pcDNA3.1-Tg737 transfection prior to hypoxia. Finally, following the up regulation of Tg737, the expression/secretion of polycystin-1 increased, and the secretion of IL-8 and the levels of active and total TGF-β1 decreased correspondingly.

CONCLUSIONS

These data provide evidence that Tg737 contributes to hypoxia-induced invasion and migration, partially through the polycystin-1, IL-8, and TGF-β1 pathway. Taken together, this work suggests that Tg737 is involved in the invasion and migration of hepatoma cells under hypoxia, with the involvement of the polycystin-1, IL-8, and TGF-β1 signaling pathway. Tg737 is a potential therapeutic target for inhibiting the high invasion and migration potential of hepatoma cells in hypoxic regions.

Indoxyl sulfate-induced epithelial-to-mesenchymal transition and apoptosis of renal tubular cells as novel mechanisms of progression of renal disease

Indoxyl sulfate (IS), one of the uremic toxins, is regarded to have a substantial role in the progression of chronic kidney disease (CKD). Epithelial-to-mesenchymal transition (EMT) and apoptosis of renal tubular cells are known to be the critical mechanisms of the development and aggravation of CKD. We investigated the effect of IS on EMT and apoptosis in renal proximal tubular cells, NRK-52E cells. IS significantly inhibited cell proliferation and induced cell migration with a morphological transition from cuboidal epithelial cells to spindle-shaped scattered fibroblast-like cells. IS downregulated the expressions of zonula occluden-1 and E-cadherin, whereas upregulated α-SMA expression at 48 h, which was blocked by a pretreatment of the organic anion transporter, probenecid. IS also induced apoptosis of NRK cells from a concentration of 25 μg/ml with an activation of ERK1/2 and p38 MAP kinase (MAPK). Pretreatment of ERK1/2 or p38 MAPK inhibitors, PD98059 or SB203580, resulted in no significant effect on IS-induced EMT, whereas it ameliorated IS-induced apoptosis of NRK cells. These findings suggested phenotypic transition and apoptosis as potential mechanisms of IS-induced renal damage and the differential role of MAPK activation in IS-induced EMT and apoptosis of renal tubular cells.

Soluble klotho and autosomal dominant polycystic kidney disease

BACKGROUND AND OBJECTIVES

Fibroblast growth factor 23 (FGF23) levels are elevated in patients with autosomal dominant polycystic kidney disease (ADPKD) and X-linked hypophosphatemia (XLH), but only the latter is characterized by a renal phosphate wasting phenotype. This study explored potential mechanisms underlying resistance to FGF23 in ADPKD.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

FGF23 and klotho levels were measured, and renal phosphate transport was evaluated by calculating the ratio of the maximum rate of tubular phosphate reabsorption to GFR (TmP/GFR) in 99 ADPKD patients, 32 CKD patients, 12 XLH patients, and 20 healthy volunteers. ADPKD and CKD patients were classified by estimated GFR (CKD stage 1, ≥90 ml/min per 1.73 m(2); CKD stage 2, 60-89 ml/min per 1.73 m(2)).

RESULTS

ADPKD patients had 50% higher FGF23 levels than did XLH patients; TmP/GFR was near normal in most ADPKD patients and very low in XLH patients. Serum klotho levels were lowest in the ADPKD group, whereas the CKD and XLH groups and volunteers had similar levels. ADPKD patients with an apparent renal phosphate leak had two-fold higher klotho levels than those without. Serum klotho values correlated inversely with cyst volume and kidney growth.

CONCLUSIONS

Loss of klotho might be a consequence of cyst growth and constrain the phosphaturic effect of FGF23 in most patients with ADPKD. Normal serum klotho levels were associated with normal FGF23 biologic activity in all XLH patients and a minority of ADPKD patients. Loss of klotho and FGF23 increase appear to exceed and precede the changes that can be explained by loss of GFR in patients with ADPKD.

Primary cilia regulates the directional migration and barrier integrity of endothelial cells through the modulation of hsp27 dependent actin cytoskeletal organization

Cilia are mechanosensing organelles that communicate extracellular signals into intracellular responses. Altered functions of primary cilia play a key role in the development of various diseases including polycystic kidney disease. Here, we show that endothelial cells from the oak ridge polycystic kidney (Tg737(orpk/orpk) ) mouse, with impaired cilia assembly, exhibit a reduction in the actin stress fibers and focal adhesions compared to wild-type (WT). In contrast, endothelial cells from polycystin-1 deficient mice (pkd1(null/null) ), with impaired cilia function, display robust stress fibers, and focal adhesion assembly. We found that the Tg737(orpk/orpk) cells exhibit impaired directional migration and endothelial cell monolayer permeability compared to the WT and pkd1(null/null) cells. Finally, we found that the expression of heat shock protein 27 (hsp27) and the phosphorylation of focal adhesion kinase (FAK) are downregulated in the Tg737(orpk/orpk) cells and overexpression of hsp27 restored both FAK phosphorylation and cell migration. Taken together, these results demonstrate that disruption of the primary cilia structure or function compromises the endothelium through the suppression of hsp27 dependent actin organization and focal adhesion formation, which may contribute to the vascular dysfunction in ciliopathies.

Polycystic kidney disease: the complexity of planar cell polarity and signaling during tissue regeneration and cyst formation

Autosomal Dominant Polycystic Kidney Disease (ADPKD) is an inherited systemic disease with intrarenal cystogenesis as its primary characteristic. A variety of mouse models provided information on the requirement of loss of balanced polycystin levels for initiation of cyst formation, the role of proliferation in cystogenesis and the signaling pathways involved in cyst growth and expansion. Here we will review the involvement of different signaling pathways during renal development, renal epithelial regeneration and cyst formation in ADPKD, focusing on planar cell polarity (PCP) and oriented cell division (OCD). This will be discussed in context of the hypothesis that aberrant PCP signaling causes cyst formation. In addition, the role of the Hippo pathway, which was recently found to be involved in cyst growth and tissue regeneration, and well-known for regulating organ size control, will be reviewed. The fact that Hippo signaling is linked to PCP signaling makes the Hippo pathway a novel cascade in cystogenesis. The newly gained understanding of the complex signaling network involved in cystogenesis and disease progression, not only necessitates refining of the current hypothesis regarding initiation of cystogenesis, but also has implications for therapeutic intervention strategies. This article is part of a Special Issue entitled: Polycystic Kidney Disease.

Apoptosis in polycystic kidney disease

Apoptosis is the process of programmed cell death. It is a ubiquitous, controlled process consuming cellular energy and designed to avoid cytokine release despite activation of local immune cells, which clear the cell fragments. The process occurs during organ development and in maintenance of homeostasis. Abnormalities in any step of the apoptotic process are associated with autoimmune diseases and malignancies. Polycystic kidney disease (PKD) is the most common inherited kidney disease leading to end-stage renal disease (ESRD). Cyst formation requires multiple mechanisms and apoptosis is considered one of them. Abnormalities in apoptotic processes have been described in various murine and rodent models of PKD as well as in human PKD kidneys. The purpose of this review is to outline the role of apoptosis in progression of PKD as well as to describe the mechanisms involved. This article is part of a Special Issue entitled: Polycystic Kidney Disease.

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

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

mTORC1/2 and rapamycin in female Han:SPRD rats with polycystic kidney disease

Rapamycin slows disease progression in the male Han:SPRD (Cy/+) rat with polycystic kidney disease (PKD). The aim of this study was to determine the effect of rapamycin on PKD and the relative contributions of the proproliferative mammalian target of rapamycin complexes 1 and 2 (mTORC1 and mTORC2) in female Cy/+ rats. Female Cy/+ rats were treated with rapamycin from 4 to 12 wk of age. In vehicle-treated Cy/+ rats, kidney volume increased by 40% and cyst volume density (CVD) was 19%. Phosphorylated S6 (p-S6) ribosomal protein, a marker of mTORC1 activity, was increased in Cy/+ rats compared with normal littermate controls (+/+) and decreased by rapamycin. Despite activation of mTORC1 in female Cy/+ rats, rapamycin had no effect on kidney size, CVD, number of PCNA-positive cystic tubular cells, caspase-3 activity, or the number of terminal deoxynucleotidyl transferase dUTP-mediated nick-end label-positive apoptotic cells. To determine a reason for the lack of effect of rapamycin, we studied the mTORC2 signaling pathway. On immunoblot of kidney, phosphorylated (Ser473) Akt (p-Akt), a marker of mTORC2 activity, was increased in female Cy/+ rats treated with rapamycin. Phosphorylated (Ser657) PKCα, a substrate of mTORC2, was unaffected by rapamycin in females. In contrast, in male rats, where rapamycin significantly decreases PKD, p-Akt (Ser473) was decreased by rapamcyin. PKCα (Ser657) was increased in male Cy/+ rats but was unaffected by rapamycin. In summary, in female Cy/+ rats, rapamycin had no effect on PKD and proproliferative p-Akt (Ser473) activity was increased by rapamycin. There were differential effects of rapamycin on mTORC2 signaling in female vs. male Cy/+ rats.

Altered renal proximal tubular endocytosis and histology in mice lacking myosin-VI

Myosin VI (Myo6) is an actin-based molecular motor involved in clathrin-mediated endocytosis that is highly expressed in the renal proximal tubule brush border. We investigated the renal physiological consequences of loss of Myo6 function by performing renal clearance and physiological measurements on Myo6 functional null Snell's waltzer (sv/sv) and control heterozygous (+/sv) mice. Sv/sv mice showed reduced body weight and elevated blood pressure compared with controls; no differences were observed for glomerular flow rate, urine volume, blood acid-base parameters, and plasma concentrations and urinary excretions of Na(+) and K(+). To assess the integrity of endocytosis-mediated protein absorption by the kidney, urinary albumin excretion was measured, and the proximal tubular uptake of intravenously injected endocytic marker horseradish peroxidase (HRP) was examined. Albumin excretion was increased nearly 4-fold in sv/sv mice relative to controls. Conversely, HRP uptake was reduced and delayed in proximal tubule cells of the sv/sv kidney observed by electron microscopy at 5 and 30 min after injection. Consistent with impaired endocytosis, we also observed defects indicating alterations along the endocytic pathway in sv/sv proximal tubule cells: (1) decreased membrane association of the clathrin adaptor subunit, adaptin beta, and Disabled-2 (Dab2) after sedimentation of renal homogenates and (2) reduced apical vacuole number. In addition, proximal tubular dilation and fibrosis, likely secondary effects of the loss of Myo6, were observed in sv/sv kidneys. These results indicate that Myo6 plays a key role in endocytosis-mediated protein absorption in the mouse kidney proximal tubule.

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

Background

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

Methods

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

Results

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

Conclusions

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

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

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

Microarray-based approach identifies microRNAs and their target functional patterns in polycystic kidney disease

Background

MicroRNAs (miRNAs) play key roles in mammalian gene expression and several cellular processes, including differentiation, development, apoptosis and cancer pathomechanisms. Recently the biological importance of primary cilia has been recognized in a number of human genetic diseases. Numerous disorders are related to cilia dysfunction, including polycystic kidney disease (PKD). Although involvement of certain genes and transcriptional networks in PKD development has been shown, not much is known how they are regulated molecularly.

Results

Given the emerging role of miRNAs in gene expression, we explored the possibilities of miRNA-based regulations in PKD. Here, we analyzed the simultaneous expression changes of miRNAs and mRNAs by microarrays. 935 genes, classified into 24 functional categories, were differentially regulated between PKD and control animals. In parallel, 30 miRNAs were differentially regulated in PKD rats: our results suggest that several miRNAs might be involved in regulating genetic switches in PKD. Furthermore, we describe some newly detected miRNAs, miR-31 and miR-217, in the kidney which have not been reported previously. We determine functionally related gene sets, or pathways to reveal the functional correlation between differentially expressed mRNAs and miRNAs.

Conclusion

We find that the functional patterns of predicted miRNA targets and differentially expressed mRNAs are similar. Our results suggest an important role of miRNAs in specific pathways underlying PKD.

NCAM as a cystogenesis marker gene of PKD2 overexpression

ADPKD (Autosomal Dominant Polycystic Kidney Disease) is characterized by the progressive expansion of multiple cystic lesions in the kidneys. ADPKD is caused by mutations in Ed-pl. consider PKD1 and PKD2. Recently a relation between c-myc and the pathogenesis of ADPKD was reported. In addition, c-Myc is a downstream effector of PKD1. To identify the gene regulated by PKD2 and c-Myc, we performed gene expression profiling in PKD2 and c-Myc overexpressing cells using a human 8K cDNA microarray. NCAM (neuronal cell adhesion molecule) levels were significantly reduced in PKD2 overexpressing systems in vitro and in vivo. These results suggest that NCAM is an important molecule in the cystogenesis induced by PKD2 overexpression.

Polycystin-2 down-regulates cell proliferation via promoting PERK-dependent phosphorylation of eIF2alpha

Autosomal dominant polycystic kidney disease (ADPKD) is characterized by the formation of renal, hepatic and pancreatic cysts and by non-cystic manifestations such as abnormal vasculature and embryo left-right asymmetry development. Polycystin-2 (PC2), in which mutations account for 10-15% of ADPKD, was previously shown to down-regulate cell proliferation, but the underlying mechanism was not elucidated. Here, we demonstrate that PC2, but not pathogenic mutants E837X and R872X, represses cell proliferation through promoting the phosphorylation of eukaryotic translation initiation factor eIF2alpha by pancreatic ER-resident eIF2alpha kinase (PERK). ER stress is known to enhance eIF2alpha phosphorylation through up-regulating PERK kinase activity (assessed by phosphorylated PERK). During ER stress, PC2 knockdown also repressed eIF2alpha phosphorylation but did not alter PERK phosphorylation, indicating that PC2 facilitates the eIF2alpha phosphorylation by PERK. PC2 was found to be in the same complex as PERK and eIF2alpha. Together, we demonstrate that PC2 negatively controls cell growth by promoting PERK-mediated eIF2alpha phosphorylation, presumably through physical interaction, which may underlie a pathogenesis mechanism of ADPKD and indicates that PC2 is an important regulator of the translation machinery.

Small-molecule CFTR inhibitors slow cyst growth in polycystic kidney disease

Cyst expansion in polycystic kidney disease (PKD) involves progressive fluid accumulation, which is believed to require chloride transport by the cystic fibrosis transmembrane conductance regulator (CFTR) protein. Herein is reported that small-molecule CFTR inhibitors of the thiazolidinone and glycine hydrazide classes slow cyst expansion in in vitro and in vivo models of PKD. More than 30 CFTR inhibitor analogs were screened in an MDCK cell model, and near-complete suppression of cyst growth was found by tetrazolo-CFTR(inh)-172, a tetrazolo-derived thiazolidinone, and Ph-GlyH-101, a phenyl-derived glycine hydrazide, without an effect on cell proliferation. These compounds also inhibited cyst number and growth by >80% in an embryonic kidney cyst model involving 4-d organ culture of embryonic day 13.5 mouse kidneys in 8-Br-cAMP-containing medium. Subcutaneous delivery of tetrazolo-CFTR(inh)-172 and Ph-GlyH-101 to neonatal, kidney-specific PKD1 knockout mice produced stable, therapeutic inhibitor concentrations of >3 microM in urine and kidney tissue. Treatment of mice for up to 7 d remarkably slowed kidney enlargement and cyst expansion and preserved renal function. These results implicate CFTR in renal cyst growth and suggest that CFTR inhibitors may hold therapeutic potential to reduce cyst growth in PKD.

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

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

Expression of the Pkd1 gene is momentously regulated by Sp1

BACKGROUND

Autosomal dominant polycystic kidney disease (ADPKD) is a common human genetic disease that is caused by a mutation of a single gene inherited from either parent. Mutations in the Pkd1 gene result in the formation of multiple fluid-filled cysts in kidneys. In previous studies, the functional regulatory sequences of Pkd1 promoter region were detected by the use of comparative genome analysis.

METHODS

To investigate the transcriptional regulation of the Pkd1 gene, the Pkd1 promoter was isolated. This promoter contains three Sp1-binding sites. Two of the sites which are found in a 300 bp fragment (-127 to +157) were mutated. An electrophoretic mobility shift assay (EMSA) was performed to determine which transcription factors are bound to Pkd1.

RESULTS

Based on studies using a luciferase assay, the Sp1-A site (the nearest Sp1 to the ATG start codon) is more important for activation of Pkd1. The result of EMSA showed that Sp1 transcription factor binds with Pkd1 promoter regions.

CONCLUSIONS

Two of the Sp1 sites were found in a proximal promoter region of Pkd1 (-127 to +157). Sp1 sites affect an important role in the activation of the gene. Especially, the Sp1-A site is more important for expression of Pkd1.

Ultrasound-Assisted Drainage and Alcoholization of Hepatic and Renal Cysts: 22 Cases

Twenty-two dogs and cats with symptomatic renal or hepatic cysts that had undergone ultrasound-assisted drainage and alcoholization were retrospectively evaluated. Common presenting complaints were anorexia, reluctance to move, and vomiting. Abdominal pain was observed in all cases. Systemic hypertension was identified in four dogs and four cats with renal cysts. Cyst drainage and alcoholization were achieved without complications in 19 animals, and all clinical signs resolved after the procedure. In three cases, transient bleeding was observed during alcoholization, and the procedure was interrupted. Blood pressure normalized in the four dogs with renal cysts, but it remained elevated in the four cats.

TRPP2 and autosomal dominant polycystic kidney disease

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

Ouabain binds with high affinity to the Na,K-ATPase in human polycystic kidney cells and induces extracellular signal-regulated kinase activation and cell proliferation

In autosomal dominant polycystic kidney disease (ADPKD), cyst formation and enlargement require proliferation of mural renal epithelial cells and the transepithelial secretion of fluid into the cyst cavity. Na,K-ATPase is essential for solute and water transport in ADPKD cells, and ouabain blocks fluid secretion in these cells. By binding to the Na,K-ATPase, ouabain also induces proliferation in some cell types. Surprisingly, it was found that nanomolar concentrations of ouabain, similar to those circulating in blood, induced ADPKD cell proliferation but had no statistically significant effect on normal human kidney (NHK) cells. Ouabain, acting from the basolateral side of the cells, also caused an increase in the level of phosphorylated extracellular signal-regulated kinases (ERK). Mitogen-activated protein kinase kinase (MEK) inhibitor U0126 blocked ouabain-induced ERK activation and cell proliferation, suggesting that ouabain effect is mediated through the MEK-ERK pathway. In contrast to NHK cells, the dose-response curve for ouabain inhibition of Na,K-ATPase activity indicated that approximately 20% of the enzyme in ADPKD cells exhibits a higher affinity for ouabain. The increased ouabain affinity of ADPKD cells was not due to differences in Na,K-ATPase isoform expression because these cells, like NHK cells, possess only the alpha1 and beta1 subunits. The gamma variants of the Na,K-ATPase also are expressed in the cells but are elevated in ADPKD cells. Currently, the basis for the differences in ouabain sensitivity of NHK and ADPKD cells is unknown. It is concluded that ouabain stimulates proliferation in ADPKD cells by binding to the Na,K-ATPase with high affinity and via activation of the MEK-ERK pathway.

Molecular and cellular pathophysiology of autosomal recessive polycystic kidney disease (ARPKD)

Autosomal recessive polycystic kidney disease (ARPKD) belongs to a group of congenital hepatorenal fibrocystic syndromes characterized by dual renal and hepatic involvement of variable severity. Despite the wide clinical spectrum of ARPKD (MIM 263200), genetic linkage studies indicate that mutations at a single locus, PKHD1 (polycystic kidney and hepatic disease 1), located on human chromosome region 6p21.1-p12, are responsible for all phenotypes of ARPKD. Identification of cystic disease genes and their encoded proteins has provided investigators with critical tools to begin to unravel the molecular and cellular mechanisms of PKD. PKD cystic epithelia share common phenotypic abnormalities despite the different genetic mutations that underlie the disease. Recent studies have shown that many cyst-causing proteins are expressed in multimeric complexes at distinct subcellular locations within epithelia. This co-expression of cystoproteins suggests that cyst formation, regardless of the underlying disease gene, results from perturbations in convergent and/or integrated signal transduction pathways. To date, no specific therapies are in clinical use for ameliorating cyst growth in ARPKD. However, studies noted in this review suggest that therapeutic targeting of the cAMP and epidermal growth factor receptor (EGFR)-axis abnormalities in cystic epithelia may translate into effective therapies for ARPKD and, by analogy, autosomal dominant polycystic kidney disease (ADPKD). A particularly promising approach appears to be the targeting of downstream intermediates of both the cAMP and EGFR axis. This review focuses on ARPKD and presents a concise summary of the current understanding of the molecular genetics and cellular pathophysiology of this disease. It also highlights phenotypic and mechanistic similarities between ARPKD and ADPKD.

Extracellular signal-regulated kinase inhibition slows disease progression in mice with polycystic kidney disease

The expression of mitogen-activated protein kinases (MAPK) in DBA/2-pcy/pcy (pcy) mice, a murine model of polycystic kidney disease was investigated. Proliferating cell nuclear antigen-positive cells were recognized in cyst epithelium from embryonic day 14.5 to 25 wk of age. Extracellular signal-regulated kinase (ERK) was expressed in the renal tubules of control and pcy mice, but stronger immunostaining was observed in cyst epithelium. Phosphorylated ERK was detected only in pcy mice and was localized predominantly in the cysts. p38 MAPK (p38) was no longer expressed after birth in controls but was detected in the cyst epithelium and in occasional tubular cells of pcy mice at all stages examined. c-Jun N-terminal kinase (JNK) was expressed in all tubular segments of controls after neonatal day 7, whereas in pcy kidneys, tubules became positive for JNK after 8 wk, and the cysts expressed little JNK. Administration of an oral MAP/ERK kinase inhibitor, PD184352, 400 mg/kg per d, to 10-wk-old pcy mice daily for the first week and then every third day for 6 additional weeks significantly decreased BP, kidney weight, serum creatinine level, and water intake and significantly increased urine osmolality. The cystic index and expression of phosphorylated ERK and ERK were significantly lower in PD184352-treated pcy mice. These results demonstrate that the expression of MAPK is dysregulated in cyst epithelium and that inhibition of ERK slowed the progression of renal disease in pcy mice.

Lixazinone stimulates mitogenesis of Madin-Darby canine kidney cells

Polycystic kidney diseases (PKD) are characterized by excessive proliferation of renal tubular epithelial cells, development of fluid-filled cysts, and progressive renal insufficiency. cAMP inhibits proliferation of normal renal tubular epithelial cells but stimulates proliferation of renal tubular epithelial cells derived from patients with PKD. Madin-Darby canine kidney (MDCK) epithelial cells, which are widely used as an in vitro model of cystogenesis, also proliferate in response to cAMP. Intracellular cAMP levels are tightly regulated by phosphodiesterases (PDE). Isoform-specific PDE inhibitors have been developed as therapeutic agents to regulate signaling pathways directed by cAMP. In other renal cell types, we have previously demonstrated that cAMP is hydrolyzed by PDE3 and PDE4, but only PDE3 inhibitors suppress proliferation by inhibiting Raf-1 activity (Cheng J, Thompson MA, Walker HJ, Gray CE, Diaz Encarnacion MM, Warner GM, Grande JP. Am J Physiol Renal Physiol 287:F940-F953, 2004.) A potential role for PDE isoform(s) in cAMP-mediated proliferation of MDCK cells has not previously been established. Similar to what we have previously found in several other renal cell types, cAMP hydrolysis in MDCK cells is directed primarily by PDE4 (85% of total activity) and PDE3 (15% of total activity). PDE4 inhibitors are more effective than PDE3 inhibitors in increasing intracellular cAMP levels in MDCK cells. However, only PDE3 inhibitors, and not PDE4 inhibitors, stimulate mitogenesis of MDCK cells. PDE3 but not PDE4 inhibitors activate B-Raf but not Raf-1, as assessed by an in vitro kinase assay. PDE3 but not PDE4 inhibitors activate the ERK pathway and activate cyclins D and E, as assessed by histone H1 kinase assay. We conclude that mitogenesis of MDCK cells is regulated by a functionally compartmentalized intracellular cAMP pool directed by PDE3. Pharmacologic agents that stimulate PDE3 activity may provide the basis for new therapies directed toward reducing cystogenesis in patients with PKD.

Dietary modulation of p-nonylphenol-induced polycystic kidneys in male Sprague-Dawley rats

We had previously found that p-nonylphenol (NP) at 1000-2000 ppm in a soy- and alfalfa-free diet induced severe polycystic kidney disease (PKD) in both male and female pups exposed from gestation day 7 through postnatal day (PND) 50 and hypothesized that differences in dietary components contributed to the severity of lesions relative to those reported in other studies using similar doses of NP. The present study investigated the dietary modulation of NP-induced PKD using the same exposure regimen with 2000 ppm NP in four different diets: the natural ingredient soy- and alfalfa-free diet that had been used in the earlier study, Purina 5K96; two defined diets AIN-93G, designated AIN-CAS, and a modified AIN-93G with soy protein isolate replacing casein as the protein source (AIN-SPI); and the commonly used natural ingredient diet Purina 5001 (P5001). Serum isoflavone levels were negligible in animals fed the soy-free AIN-CAS and 5K96 diets and were 2- to 18-fold higher in animals fed P5001 than in those fed AIN-SPI. Consumption of P5001 was significantly greater than consumption of the other diets, and those animals fed P5001 were generally significantly heavier than animals receiving the other diets. NP significantly reduced body weight gain in male pups regardless of the diet fed. There was no evidence of NP-induced kidney toxicity in male pups at PND 2, 14, or 21 or in the dams. In PND 50 male pups, serum blood urea nitrogen was significantly elevated by NP in all diet groups. Urine volume and urinary N-acetyl beta-glucuronidase were significantly increased by NP in the soy-free 5K96 and AIN-CAS diet groups. Relative kidney weights were increased by NP in all diet groups except P5001, with the greatest increase in AIN-CAS and 5K96 diet groups. Microscopic evaluation of kidneys from the PND 50 males showed that NP induced PKD in all diet groups but with marked variation in the severity depending on the diet. PKD was severe in 100% of the NP-treated animals in the AIN-CAS and 5K96 groups, moderate in 88% of the AIN-SPI diet group, and mild in only 40% of the P5001 diet group. Thus, diet can significantly modulate the development of PKD induced by dietary NP in rats. Soy components, as well as other complex dietary factors, may account for the level of protection afforded by the P5001 diet.

The relationship between cell proliferation, Cl- secretion, and renal cyst growth: a study using CFTR inhibitors

BACKGROUND

In autosomal-dominant polycystic kidney disease (ADPKD), cAMP-stimulated cell proliferation and Cl- secretion via the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel drive the enlargement of fluid-filled epithelial cysts. To investigate how CFTR blockers inhibit cyst growth, we studied cAMP-dependent Cl- secretion, cell proliferation, and cyst growth using type I Madin Darby canine kidney (MDCK) cells as a model of renal cyst development and growth.

METHODS

We grew MDCK cysts in collagen gels in the presence of the cAMP agonist forskolin, measured Cl- secretion with the Ussing chamber technique, and assayed cell proliferation using nonpolarized and polarized MDCK cells. To inhibit CFTR, we used glibenclamide, 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB), genistein, and the specific CFTR inhibitor CFTRinh-172. As controls, we tested the effects of blockers of other types of apical membrane Cl- channels and inhibitors of basolateral membrane ion channels and transporters.

RESULTS

In the absence of inhibitors of transepithelial ion transport, forskolin stimulated dramatic cyst growth. CFTR blockers and inhibitors of basolateral membrane ion channels and transporters retarded cyst growth. In contrast, blockers of other types of apical membrane Cl- channels, which were without effect on CFTR, failed to inhibit cyst growth. Inhibition of cyst growth by CFTR blockers was correlated with inhibition of cAMP-stimulated Cl- current (correlation coefficient = 0.81; P < 0.05), but not cell proliferation (correlation coefficient = 0.50; P > 0.05).

CONCLUSION

Our data suggest that CFTR blockers might retard cyst growth predominantly by inhibiting fluid accumulation within the cyst lumen.

Alpha-actinin associates with polycystin-2 and regulates its channel activity

Polycystin-2 (PC2) is the product of the PKD2 gene, which is mutated in 10-15% patients of autosomal dominant polycystic kidney disease (ADPKD). PC2 is an integral transmembrane protein and acts as a calcium-permeable cation channel. The functional modulation of this channel by other protein partners remains largely unknown. In the present study, using a yeast two-hybrid approach, we discovered that both intracellular N- and C-termini of PC2 associate with alpha-actinins, actin-binding and actin-bundling proteins important in cytoskeleton organization, cell adhesion, proliferation and migration. The PC2-alpha-actinin association was confirmed by in vitro glutathione S-transferase pull-down and dot blot overlay assays. In addition, the in vivo interaction between endogenous PC2 and alpha-actinins was demonstrated by co-immunoprecipitation in human embryonic kidney 293 and Madin-Darby canine kidney (MDCK) cells, rat kidney and heart tissues and human syncytiotrophoblast (hST) apical membrane vesicles. Immunofluorescence experiments showed that PC2 and alpha-actinin were partially co-localized in epithelial MDCK and inner medullary collecting duct cells, NIH 3T3 fibroblasts and hST vesicles. We studied the functional modulation of PC2 by alpha-actinin in a lipid bilayer electrophysiology system using in vitro translated PC2 and found that alpha-actinin substantially stimulated the channel activity of reconstituted PC2. A similar stimulatory effect of alpha-actinin on PC2 was also observed when hST vesicles were reconstituted in lipid bilayer. Thus, physical and functional interactions between PC2 and alpha-actinin may play an important role in abnormal cell adhesion, proliferation and migration observed in ADPKD.

Activation of the MEK5/ERK5 cascade is responsible for biliary dysgenesis in a rat model of Caroli's disease

Polycystic kidney (PCK) rats exhibit a multiorgan cyst pathology similar to human autosomal recessive polycystic kidney disease, and are proposed as an animal model of Caroli's disease with congenital hepatic fibrosis (CHF). This study investigated the expression and function of selected components of the mitogen activated protein kinase (MAPK) pathway in cultured intrahepatic biliary epithelial cells (BECs) of PCK rats. Compared to the proliferative activity of cultured BECs of control rats, those of the PCK rats were hyperresponsive to epidermal growth factor (EGF). The increase in BEC proliferation was accompanied by overexpression of MAPK/extracellular signal-regulated protein kinase (ERK) kinase 5 (MEK5), and subsequent phosphorylation of ERK5 in vitro. The increased proliferative activity was significantly inhibited by the transfection of short interfering RNA against MEK5 mRNA. An EGF receptor tyrosine kinase inhibitor, gefitinib ("Iressa", ZD1839), also significantly inhibited the abnormal growth of cultured BECs of PCK rats. By contrast, treatment with PD98059 and U0126, inhibitors for MEK1/2, was less effective. These results suggest that the activation of the MEK5-ERK5 cascade plays a pivotal role in the biliary dysgenesis of PCK rats, and also provide insights into the pathogenesis of Caroli's disease with CHF. As the MEK5-ERK5 interaction is highly specific, it may represent a potential target of therapy.

Calcium absorption across epithelia

Ca(2+) is an essential ion in all organisms, where it plays a crucial role in processes ranging from the formation and maintenance of the skeleton to the temporal and spatial regulation of neuronal function. The Ca(2+) balance is maintained by the concerted action of three organ systems, including the gastrointestinal tract, bone, and kidney. An adult ingests on average 1 g Ca(2+) daily from which 0.35 g is absorbed in the small intestine by a mechanism that is controlled primarily by the calciotropic hormones. To maintain the Ca(2+) balance, the kidney must excrete the same amount of Ca(2+) that the small intestine absorbs. This is accomplished by a combination of filtration of Ca(2+) across the glomeruli and subsequent reabsorption of the filtered Ca(2+) along the renal tubules. Bone turnover is a continuous process involving both resorption of existing bone and deposition of new bone. The above-mentioned Ca(2+) fluxes are stimulated by the synergistic actions of active vitamin D (1,25-dihydroxyvitamin D(3)) and parathyroid hormone. Until recently, the mechanism by which Ca(2+) enter the absorptive epithelia was unknown. A major breakthrough in completing the molecular details of these pathways was the identification of the epithelial Ca(2+) channel family consisting of two members: TRPV5 and TRPV6. Functional analysis indicated that these Ca(2+) channels constitute the rate-limiting step in Ca(2+)-transporting epithelia. They form the prime target for hormonal control of the active Ca(2+) flux from the intestinal lumen or urine space to the blood compartment. This review describes the characteristics of epithelial Ca(2+) transport in general and highlights in particular the distinctive features and the physiological relevance of the new epithelial Ca(2+) channels accumulating in a comprehensive model for epithelial Ca(2+) absorption.

orpkmouse model of polycystic kidney disease reveals essential role of primary cilia in pancreatic tissue organization

Polycystic kidney disease (PKD) includes a group of disorders that are characterized by the presence of cysts in the kidney and other organs,including the pancreas. Here we show that in orpk mice, a model system for PKD that harbors a mutation in the gene that encodes the polaris protein, pancreatic defects start to occur at the end of gestation, with an initial expansion of the developing pancreatic ducts. Ductal dilation continues rapidly after birth and results in the formation of large,interconnected cysts. Expansion of pancreatic ducts is accompanied by apoptosis of neighboring acinar cells, whereas endocrine cell differentiation and islet formation appears to be unaffected. Polaris has been shown to co-localize with primary cilia, and these structures have been implicated in the formation of renal cysts. In the orpk pancreas, cilia numbers are reduced and cilia length is decreased. Expression of polycystin-2, a protein involved in PKD, is mislocalized in orpk mice. Furthermore, the cellular localization of β-catenin, a protein involved in cell adhesion and Wnt signaling, is altered. Thus, polaris and primary cilia function are required for the maturation and maintenance of proper tissue organization in the pancreas.

Regulation of calcium signaling by polycystin-2

Autosomal dominant PKD (ADPKD) is a common lethal genetic disorder characterized by progressive development of fluid-filled cysts in the kidney and other target organs. ADPKD is caused by mutations in the PKD1 and PKD2 genes, encoding the transmembrane proteins polycystin-1 (PC1) and polycystin-2 (PC2), respectively. Although the function and putative interacting ligands of PC1 are largely unknown, recent evidence indicates that PC2 behaves as a TRP-type Ca2+-permeable nonselective cation channel. The PC2 channel is implicated in the transient increase in cytosolic Ca2+in renal epithelial cells and may be linked to the activation of subsequent signaling pathways. Recent studies also indicate that PC1 functionally interacts with PC2 such that the PC1-PC2 channel complex is an obligatory novel signaling pathway implicated in the transduction of environmental signals into cellular events. The present review purposely avoids issues of regulation of PC2 expression and trafficking and focuses instead on the evidence for the TRP-type cation channel function of PC2. How its role as a cation channel may unmask mechanisms that trigger Ca2+transport and regulation is the focus of attention. PC2 channel function may be essential in renal cell function and kidney development. Nonrenal-targeted expression of PC2 and related proteins, including the cardiovascular system, also suggests previously unforeseeable roles in signal transduction.

Polycystic kidney disease: the cilium as a common pathway in cystogenesis

PURPOSE OF REVIEW

Polycystic kidney disease (PKD) is characterized by the formation and progressive expansion of cysts in the kidney, frequently leading to renal failure. The purpose of this review is to summarize recent studies that have provided insight into the mechanisms of cytogenesis.

RECENT FINDINGS

Mutations in cilia-associated proteins have been identified in a number of diseases associated with cyst formation, including autosomal dominant and recessive PKD, and nephronophthisis. The primary cilia are finger-like projections on the surface of all kidney cells, except acid-base transporting intercalated cells in the collecting duct. Cilia have been proposed to serve as mechano- or chemosensors, responding to and interacting with the microenvironment. Abnormal cilia structure or function or both may lead to abnormalities in cell proliferation and tubular differentiation, ultimately leading to cyst formation. In addition to ciliary dysfunction, other potential mechanisms of cystogenesis need to be explored.

SUMMARY

Our understanding of the importance of the primary cilium in renal cyst formation may guide potential therapy for cystic kidney diseases by targeting the structural and functional integrity of the cilia.

Zebrafish pronephros: A model for understanding cystic kidney disease

The embryonic kidney of the zebrafish is the pronephros. The ease of genetic analysis and experimentation in zebrafish, coupled with the simplicity of the pronephros, make the zebrafish an ideal model system for studying kidney development and function. Several mutations have been isolated in zebrafish genetic screens that result in cyst formation in the pronephros. Cloning and characterization of these mutations will provide insight into kidney development but may also provide understanding of the molecular basis of cystic kidney diseases. In this review, we focus on the zebrafish as a model for understanding cystic kidney disease and the links between cystic kidney disease and left-right patterning.