Reduced Pax2 gene dosage increases apoptosis and slows the progression of renal cystic disease

Shared features in ear and kidney development - implications for oto-renal syndromes

The association between ear and kidney anomalies has long been recognized. However, little is known about the underlying mechanisms. In the last two decades, embryonic development of the inner ear and kidney has been studied extensively. Here, we describe the developmental pathways shared between both organs with particular emphasis on the genes that regulate signalling cross talk and the specification of progenitor cells and specialised cell types. We relate this to the clinical features of oto-renal syndromes and explore links to developmental mechanisms.

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.

Molecular Mechanisms of Epigenetic Regulation, Inflammation, and Cell Death in ADPKD

Autosomal dominant polycystic kidney disease (ADPKD) is a genetic disorder, which is caused by mutations in the PKD1 and PKD2 genes, characterizing by progressive growth of multiple cysts in the kidneys, eventually leading to end-stage kidney disease (ESKD) and requiring renal replacement therapy. In addition, studies indicate that disease progression is as a result of a combination of factors. Understanding the molecular mechanisms, therefore, should facilitate the development of precise therapeutic strategies for ADPKD treatment. The roles of epigenetic modulation, interstitial inflammation, and regulated cell death have recently become the focuses in ADPKD. Different epigenetic regulators, and the presence of inflammatory markers detectable even before cyst growth, have been linked to cyst progression. Moreover, the infiltration of inflammatory cells, such as macrophages and T cells, have been associated with cyst growth and deteriorating renal function in humans and PKD animal models. There is evidence supporting a direct role of the PKD gene mutations to the regulation of epigenetic mechanisms and inflammatory response in ADPKD. In addition, the role of regulated cell death, including apoptosis, autophagy and ferroptosis, have been investigated in ADPKD. However, there is no consensus whether cell death promotes or delays cyst growth in ADPKD. It is therefore necessary to develop an interactive picture between PKD gene mutations, the epigenome, inflammation, and cell death to understand why inherited PKD gene mutations in patients may result in the dysregulation of these processes that increase the progression of renal cyst formation.

Factors influencing the development of Multicystic Dysplastic Kidney (MCDK) following urinary tract obstruction in the fetal lamb

BACKGROUND

Creating obstructive uropathy (OU) during glomerulogenesis in the fetal lamb results in multicystic dysplastic kidney (MCDK) at term. We explored this using immunohistochemical techniques.

METHOD

OU was created in fetal lambs at 60-day gestation, ligating the urethra and urachus. The kidneys of MCDK lambs, 60-day gestation fetal lambs, full-term lamb (145 days), term sham-operated lambs, and adult ewes were evaluated by HE staining, and immunohistochemistry with paired box genes 2 (PAX2) and CD10.

RESULTS

Multiple cysts were found in the MCDK model. CD10 was expressed in proximal tubular epithelial cells, glomerular epithelial cells, and medullary stromal cells in the kidneys of 60-day gestation fetal lambs and full-term lambs and adult ewes. PAX2 expression was found in ureteric buds, C- and S-shaped bodies, epithelial cells of collecting ducts, and Bowman's capsule of fetal kidneys at 60-day gestation, but only in the collecting ducts of full-term fetal lambs and adult ewes. Both CD10 and PAX2 were expressed in the cystic epithelial cells of the MCDK model.

DISCUSSION

PAX2 expression in cystic epithelial cells suggests that cyst formation is associated with disturbed down-regulation of PAX2 in the nephrogenic zone epithelial cells during the renal development in the OU model.

Identification of Pax protein inhibitors that suppress target gene expression and cancer cell proliferation

The Pax family of developmental control genes are frequently deregulated in human disease. In the kidney, Pax2 is expressed in developing nephrons but not in adult proximal and distal tubules, whereas polycystic kidney epithelia or renal cell carcinoma continues to express high levels. Pax2 reduction in mice or cell culture can slow proliferation of cystic epithelial cells or renal cancer cells. Thus, inhibition of Pax activity may be a viable, cell-type-specific therapy. We designed an unbiased, cell-based, high-throughput screen that identified triazolo pyrimidine derivatives that attenuate Pax transactivation ability. We show that BG-1 inhibits Pax2-positive cancer cell growth and target gene expression but has little effect on Pax2-negative cells. Chromatin immunoprecipitation suggests that these inhibitors prevent Pax protein interactions with the histone H3K4 methylation complex at Pax target genes in renal cells. Thus, these compounds may provide structural scaffolds for kidney-specific inhibitors with therapeutic potential.

Epithelial proliferation and cell cycle dysregulation in kidney injury and disease

Various cellular insults and injury to renal epithelial cells stimulate repair mechanisms to adapt and restore the organ homeostasis. Renal tubular epithelial cells are endowed with regenerative capacity, which allows for a restoration of nephron function after acute kidney injury. However, recent evidence indicates that the repair is often incomplete, leading to maladaptive responses that promote the progression to chronic kidney disease. The dysregulated cell cycle and proliferation is also a key feature of renal tubular epithelial cells in polycystic kidney disease and HIV-associated nephropathy. Therefore, in this review, we provide an overview of cell cycle regulation and the consequences of dysregulated cell proliferation in acute kidney injury, polycystic kidney disease, and HIV-associated nephropathy. An increased understanding of these processes may help define better targets for kidney repair and combat chronic kidney disease progression.

Epigenetic regulation of arginine vasopressin receptor 2 expression by PAX2 and Pax transcription interacting protein

Renal arginine vasopressin receptor 2 (AVPR2) plays a crucial role in osmoregulation. Engagement of ligand with AVPR2 results in aquaporin 2 movement to the apical membrane and water reabsorption from the urinary filtrate. Despite this essential role, little is known about transcriptional regulation of Avpr2. Here, we identify novel roles for PAX2, a transcription factor crucial for kidney development, and its adaptor protein, Pax transcription interacting protein (PTIP), for epigenetic regulation of Avpr2 and thus body water balance. Chromatin immunoprecipitation (ChIP) from murine inner medulla cells (IMCD-3) identified the minimal DNA-binding region of PAX2 on the Avpr2 promoter. Regulation of Avpr2 by PAX2 was confirmed using a heterologous DNA expression system. PAX2 recruits the adaptor protein PTIP and its associated histone methyltransferase (HMT) complex to Avpr2 promoter, imposing epigenetic marks on this region and throughout the coding sequence that modulate Avpr2 gene transcription. Reduction of PAX2 or PTIP protein levels by siRNA prevented histone lysine methylation and expression of Avpr2. ChIP using mouse or human kidneys determined that PAX2 is highly enriched in the AVPR2 promoter alongside PTIP and HMT proteins, leading to high levels of histone H3 lysine trimethylation within the promoter and throughout the gene. In conclusion, PAX2 provides locus specificity for PTIP, allowing the HMT complex to impart epigenetic changes at the Avpr2 locus and regulate Avpr2 transcription. These finding have major implications for understanding regulation of body water balance.NEW & NOTEWORTHY The transcription factor PAX2 plays an indispensable role in kidney development. In the adult kidney, we identified the first described protein this protein regulates. PAX2 and its interacting partner Pax transcription interacting protein recruit a histone methyltransferase complex to the promoter and epigentically regulate the expression of arginine vasopressin receptor 2, a protein that plays a crucial role in osmoregulation in the distal tubule.

Autosomal dominant polycystic kidney disease: Disrupted pathways and potential therapeutic interventions

Autosomal dominant polycystic kidney disease (ADPKD) is a monogenic inherited renal cystic disease that occurs in different races worldwide. It is characterized by the development of a multitude of renal cysts, which leads to massive enlargement of the kidney and often to renal failure in adulthood. ADPKD is caused by a mutation in PKD1 or PKD2 genes encoding the proteins polycystin-1 and polycystin-2, respectively. Recent studies showed that cyst formation and growth result from deregulation of multiple cellular pathways like proliferation, apoptosis, metabolic processes, cell polarity, and immune defense. In ADPKD, intracellular cyclic adenosine monophosphate (cAMP) promotes cyst enlargement by stimulating cell proliferation and transepithelial fluid secretion. Several interventions affecting many of these defective signaling pathways have been effective in animal models and some are currently being tested in clinical trials. Moreover, the stem cell therapy can improve nephropathies and according to studies were done in this field, can be considered as a hopeful therapeutic approach in future for PKD. This study provides an in-depth review of the relevant molecular pathways associated with the pathogenesis of ADPKD and their implications in development of potential therapeutic strategies.

New PAX2 heterozygous mutation in a child with chronic kidney disease: a case report and review of the literature

BACKGROUND

We herein report a 3-year-old boy presented with chronic kidney disease (CKD) due to PAX2 missense mutation (C to G transversion at position 418 in exon 4).

CASE PRESENTATION

He attended our clinic with a 3-month history of foamy urine. Upon examination, he had reduced estimated glomerular filtration rate (GFR) and renal atrophy. Genetic investigations revealed that he has inherited a mutated PAX2 gene from his father, who had renal failure at the age of 20. We searched the literature and confirmed that this mutation site has not been reported by any other group before.

CONCLUSIONS

Although renal coloboma syndrome (RCS) with simultaneous kidney and eye involvement is the most common phenotype of PAX2 mutations, current literature supports that such mutations may have profuse clinical manifestations and renal hypoplasia is one distinct entity in the spectrum.

Possibilities and limitations of three-dimensional reconstruction and simulation techniques to identify patterns, rhythms and functions of apoptosis in the early developing neural tube

The now classical idea that programmed cell death (apoptosis) contributes to a plethora of developmental processes still has lost nothing of its impact. It is, therefore, important to establish effective three-dimensional (3D) reconstruction as well as simulation techniques to decipher the exact patterns and functions of such apoptotic events. The present study focuses on the question whether and how apoptosis promotes neurulation-associated processes in the spinal cord of Tupaia belangeri (Tupaiidae, Scandentia, Mammalia). Our 3D reconstructions demonstrate that at least two craniocaudal waves of apoptosis consecutively pass through the dorsal spinal cord. The first wave appears to be involved in neural fold fusion and/or in selection processes among premigratory neural crest cells. The second one seems to assist in establishing the dorsal signaling center known as the roof plate. In the hindbrain, in contrast, apoptosis among premigratory neural crest cells progresses craniocaudally but discontinuously, in a segment-specific manner. Unlike apoptosis in the spinal cord, these segment-specific apoptotic events, however, precede later ones that seemingly support neural fold fusion and/or postfusion remodeling. Arguing with Whitehead that biological patterns and rhythms differ in that biological rhythms depend "upon the differences involved in each exhibition of the pattern" (Whitehead in An enquiry concerning the principles of natural knowledge. Cambridge University Press, London, 1919, p. 198) we show that 3D reconstruction and simulation techniques can contribute to distinguish between (static) patterns and (dynamic) rhythms of apoptosis. By deciphering novel patterns and rhythms of developmental apoptosis, our reconstructions help to reconcile seemingly inconsistent earlier findings in chick and mouse embryos, and to create rules for computer simulations.

Are Pax proteins potential therapeutic targets in kidney disease and cancer?

Pax genes encode developmental regulators that are expressed in a variety of tissues and control critical events in morphogenesis. In the kidney, Pax2 and Pax8 are expressed in embryonic development and in specific renal diseases associated with aberrant epithelial cell proliferation. Prior genetic and cell biological studies suggest that reducing the activity of Pax proteins in renal cancer or in polycystic kidney disease can slow the progression of these conditions. The Pax proteins may be critical for providing tissue and locus specificity to recruit epigenetic modifiers that control gene expression and chromatin structure. Although they are nuclear, targeting Pax proteins to inhibit function may be feasible with small molecules. Such inhibition of Pax protein function may provide novel therapies for subsets of renal disorders that are tissue- and cell type-specific and avoid systemic effects on non-Pax-expressing cells and tissues. Given the paucity of effective treatments for renal cancer and cystic disease, the Pax family of proteins represents new pharmaceutical targets that merit exploration and further development.

Inhibition of Pax2 Transcription Activation with a Small Molecule that Targets the DNA Binding Domain

The Pax gene family encodes DNA binding transcription factors that control critical steps in embryonic development and differentiation of specific cell lineages. Often, Pax proteins are re-expressed or ectopically expressed in cancer and other diseases of abnormal proliferation, making them attractive targets for tissue specific inhibition by small molecules. In this report, we used a homology model of the Pax2 paired domain and a virtual screen to identify small molecules that can inhibit binding of the paired domain to DNA and Pax2 mediated transcription activation. Candidates from the virtual screen were then confirmed in a cell based Pax2 transactivation assay. Subsequently, we tested analogs of these hits to identify a single compound that effectively blocked Pax2 activity and DNA binding with a K_d of 1.35-1.5 μM. The compound, termed EG1, was used to inhibit embryonic kidney development, a process directly dependent on Pax2 activity. Furthermore, we show that EG1 can inhibit proliferation of Pax2 positive renal and ovarian cancer cell lines but has little effect on Pax2 negative cancer cells. These data confirm that small molecules targeting the DNA binding paired domain can be identified and may be good lead compounds for developing tissue and cell-type specific anticancer therapies.

Pax2/Pax8-defined subdomains and the occurrence of apoptosis in the posterior placodal area of mice

The present work aims to improve our understanding of the causes and functions of apoptosis during the morphogenesis of epibranchial placodes in mice. Schematic maps helped to compare the spatiotemporal sequence of apoptotic events with the protein expression patterns of general (Six1) and specific placodal markers (Pax2, Pax8). Our findings challenge the view that, in mammals, all three epibranchial placodes spring from the original posterior placodal area (PPA) of presomite or early somite embryos. Instead, close-meshed analysis of the Pax2/Pax8 expression patterns demonstrates the stepwise emergence of two subdomains which both belong to the gradually expanding PPA, and which largely give rise to the otic placode and epibranchial placode 1 (anterior subdomain), or to the caudal epibranchial placodes (posterior subdomain). Our observations reinforce previous doubts raised on the PPA progeny of early somite Xenopus embryos (Schlosser and Ahrens, Dev Biol 271:439-466, 2004). They also demonstrate that partly different Pax2/Pax8 codes accompany epibranchial placode development in Xenopus laevis and mice. In mice, interplacodal apoptosis assists in the establishment of the two PPA subdomains and, subsequently, of individualized placodes by predominantly eliminating Six1⁺ placodal precursor cells. Onset of interplacodal and intraplacodal large-scale apoptosis is almost always preceded and/or paralleled by Pax2/Pax8 expression minima in the very same region. Future work will demand the use of knock-out mice and whole embryo culture to experimentally test, whether the combined action of differentially expressed Pax2 and Pax8 genes exerts antiapoptotic effects in the mammalian PPA.

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.

Evidence for intermediate mesoderm and kidney progenitor cell specification by Pax2 and PTIP dependent mechanisms

Activation of the Pax2 gene marks the intermediate mesoderm shortly after gastrulation, as the mesoderm becomes compartmentalized into paraxial, intermediate, and lateral plate. Using an EGFP knock-in allele of Pax2 to identify and sort cells of the intermediate mesodermal lineage, we compared gene expression patterns in EGFP positive cells that were heterozygous or homozygous null for Pax2. Thus, we identified critical regulators of intermediate mesoderm and kidney development whose expression depended on Pax2 function. In cell culture models, Pax2 is thought to recruit epigenetic modifying complex to imprint activating histone methylation marks through interactions with the adaptor protein PTIP. In kidney organ culture, conditional PTIP deletion showed that many Pax2 target genes, which were activated early in renal progenitor cells, remained on once activated, whereas Pax2 target genes expressed later in kidney development were unable to be fully activated without PTIP. In Pax2 mutants, we also identified a set of genes whose expression was up-regulated in EGFP positive cells and whose expression was consistent with a cell fate transformation to paraxial mesoderm and its derivatives. These data provide evidence that Pax2 specifies the intermediate mesoderm and renal epithelial cells through epigenetic mechanisms and in part by repressing paraxial mesodermal fate.

Kidney: polycystic kidney disease

Polycystic kidney disease (PKD) is a life-threatening genetic disorder characterized by the presence of fluid-filled cysts primarily in the kidneys. PKD can be inherited as autosomal recessive (ARPKD) or autosomal dominant (ADPKD) traits. Mutations in either the PKD1 or PKD2 genes, which encode polycystin 1 and polycystin 2, are the underlying cause of ADPKD. Progressive cyst formation and renal enlargement lead to renal insufficiency in these patients, which need to be managed by lifelong dialysis or renal transplantation. While characteristic features of PKD are abnormalities in epithelial cell proliferation, fluid secretion, extracellular matrix and differentiation, the molecular mechanisms underlying these events are not understood. Here we review the progress that has been made in defining the function of the polycystins, and how disruption of these functions may be involved in cystogenesis.

Genome-wide analysis of gestational gene-environment interactions in the developing kidney

The G protein-coupled bradykinin B2 receptor (Bdkrb2) plays an important role in regulation of blood pressure under conditions of excess salt intake. Our previous work has shown that Bdkrb2 also plays a developmental role since Bdkrb2(-/-) embryos, but not their wild-type or heterozygous littermates, are prone to renal dysgenesis in response to gestational high salt intake. Although impaired terminal differentiation and apoptosis are consistent findings in the Bdkrb2(-/-) mutant kidneys, the developmental pathways downstream of gene-environment interactions leading to the renal phenotype remain unknown. Here, we performed genome-wide transcriptional profiling on embryonic kidneys from salt-stressed Bdkrb2(+/+) and Bdkrb2(-/-) embryos. The results reveal significant alterations in key pathways regulating Wnt signaling, apoptosis, embryonic development, and cell-matrix interactions. In silico analysis reveal that nearly 12% of differentially regulated genes harbor one or more Pax2 DNA-binding sites in their promoter region. Further analysis shows that metanephric kidneys of salt-stressed Bdkrb2(-/-) have a significant downregulation of Pax2 gene expression. This was corroborated in Bdkrb2(-/-);Pax2(GFP+/tg) mice, demonstrating that Pax2 transcriptional activity is significantly repressed by gestational salt-Bdkrb2 interactions. We conclude that gestational gene (Bdkrb2) and environment (salt) interactions cooperate to impact gene expression programs in the developing kidney. Suppression of Pax2 likely contributes to the defects in epithelial survival, growth, and differentiation in salt-stressed BdkrB2(-/-) mice.

Lhx1 is required in Müllerian duct epithelium for uterine development

The female reproductive tract organs of mammals, including the oviducts, uterus, cervix and upper vagina, are derived from the Müllerian ducts, a pair of epithelial tubes that form within the mesonephroi. The Müllerian ducts form in a rostral to caudal manner, guided by and dependent on the Wolffian ducts that have already formed. Experimental embryological studies indicate that caudal elongation of the Müllerian duct towards the urogenital sinus occurs in part by proliferation at the ductal tip. The molecular mechanisms that regulate the elongation of the Müllerian duct are currently unclear. Lhx1 encodes a LIM-homeodomain transcription factor that is essential for male and female reproductive tract development. Lhx1 is expressed in both the Wolffian and Müllerian ducts. Wolffian duct-specific knockout of Lhx1 results in degeneration of the Wolffian duct and consequently the non-cell-autonomous loss of the Müllerian duct. To determine the role of Lhx1 specifically in the Müllerian duct epithelium, we performed a Müllerian duct-specific knockout study using Wnt7a-Cre mice. Loss of Lhx1 in the Müllerian duct epithelium led to a block in Müllerian duct elongation and uterine hypoplasia characterized by loss of the entire endometrium (luminal and glandular epithelium and stroma) and inner circular but not the outer longitudinal muscle layer. Time-lapse imaging and molecular analyses indicate that Lhx1 acts cell autonomously to maintain ductal progenitor cells for Müllerian duct elongation. These studies identify LHX1 as the first transcription factor that is essential in the Müllerian duct epithelial progenitor cells for female reproductive tract development. Furthermore, these genetic studies demonstrate the requirement of epithelial-mesenchymal interactions for uterine tissue compartment differentiation.

An unusual presentation of Denys-Drash syndrome due to bigenic disease

We report a case of Denys-Drash syndrome (DDS) in a 3-month-old girl presenting with bilateral renal cortical cysts mimicking polycystic kidney disease. Genetic analysis revealed a de novo heterozygous missense mutation c.1186G>A (p.Asp396Asn) in the WT1 gene, confirming the diagnosis of DDS. Because multiple renal cysts have never been reported in DDS, we explored several genes responsible for these renal manifestations, such as HNF-1β, PAX2, PKD1, and PKD2. Remarkably, we identified a heterozygous missense variant c.12439A>G (p.Lys4147Glu) in the PKD1 gene. The same variant was found in the patient's mother, who had no renal cysts, and in the grandfather, who had several renal cysts. Mutation prediction programs classified the c.12439A>G variant as being "likely pathogenic." We hypothesize that the severe cystic phenotype in the index patient could be due to the WT1 mutation, enhancing pathogenicity of the "hypomorph" PKD1 allele. A possible role for Wilms tumor suppressor 1 (WT1) in renal cyst development should be considered. From a conceptual point of view, this case shows that an unusual presentation of a known genetic syndrome might point to bigenic inheritance, with unexpected interference of mutated genes causing an uncommon clinical phenotype.

Strategies targeting cAMP signaling in the treatment of polycystic kidney disease

Polycystic kidney disease (PKD) is a leading cause of ESRD worldwide. In PKD, excessive cell proliferation and fluid secretion, pathogenic interactions of mutated epithelial cells with an abnormal extracellular matrix and alternatively activated interstitial macrophages, and the disruption of mechanisms controlling tubular diameter contribute to cyst formation. Studies with animal models suggest that several diverse pathophysiologic mechanisms, including dysregulation of intracellular calcium levels and cAMP signaling, mediate these cystogenic mechanisms. This article reviews the evidence implicating calcium and cAMP as central players in a network of signaling pathways underlying the pathogenesis of PKD and considers the therapeutic relevance of treatment strategies targeting cAMP signaling.

Ouabain protects against Shiga toxin-triggered apoptosis by reversing the imbalance between Bax and Bcl-xL

Hemolytic uremic syndrome, a life-threatening disease often accompanied by acute renal failure, usually occurs after gastrointestinal infection with Shiga toxin 2 (Stx2)-producing Escherichia coli. Stx2 binds to the glycosphingolipid globotriaosylceramide receptor, expressed by renal epithelial cells, and triggers apoptosis by activating the apoptotic factor Bax. Signaling via the ouabain/Na,K-ATPase/IP3R/NF-κB pathway increases expression of Bcl-xL, an inhibitor of Bax, suggesting that ouabain might protect renal cells from Stx2-triggered apoptosis. Here, exposing rat proximal tubular cells to Stx2 in vitro resulted in massive apoptosis, upregulation of the apoptotic factor Bax, increased cleaved caspase-3, and downregulation of the survival factor Bcl-xL; co-incubation with ouabain prevented all of these effects. Ouabain activated the NF-κB antiapoptotic subunit p65, and the inhibition of p65 DNA binding abolished the antiapoptotic effect of ouabain in Stx2-exposed tubular cells. Furthermore, in vivo, administration of ouabain reversed the imbalance between Bax and Bcl-xL in Stx2-treated mice. Taken together, these results suggest that ouabain can protect the kidney from the apoptotic effects of Stx2.

PAX2 in human kidney malformations and disease

Human PAX2 mutations have been associated with abnormalities in the developing and adult kidney ranging from congenital abnormalities of the kidney and urinary tract (CAKUT) to oncogenic processes. Defining the relationship of PAX2 to human renal disease requires an appreciation of its fundamental role in renal development. Given the highly conserved nature of the PAX2 gene in vertebrates, it is not surprising that much of our understanding of PAX2 involvement in renal disease has been derived from animal models. The following review will outline the current evidence supporting involvement of PAX2 in the pathologic processes involving the kidney.

c-Met and NF-κB-dependent overexpression of Wnt7a and -7b and Pax2 promotes cystogenesis in polycystic kidney disease

The mechanisms of cystogenesis in autosomal dominant polycystic kidney disease (ADPKD) are not fully understood. Hyperactivation of the tyrosine kinase c-Met contributes to cyst formation, but we do not know the downstream mediators. Here, we found that hyperactivated c-Met led to increased NF-κB signaling, which in turn, drove de novo expression of Wnt7a and overexpression of Wnt7b in Pkd1(-/-) mouse kidneys. Hyperactivated Wnt signaling increased expression of the transcription factor Pax2 in the cells lining cysts. Furthermore, blocking Wnt signaling with DKK1 decreased cyst formation in an organ culture model of ADPKD. In summary, these results suggest that the c-Met/NF-κB/Wnt/Pax2 signaling transduction axis may provide pharmacological targets for the treatment of ADPKD.

Expression of Fraser syndrome genes in normal and polycystic murine kidneys

BACKGROUND

Fraser syndrome (FS) features renal agenesis and cystic kidneys. Mutations of FRAS1 (Fraser syndrome 1)and FREM2 (FRAS1-related extracellular matrix protein 2)cause FS. They code for basement membrane proteins expressed in metanephric epithelia where they mediate epithelial/mesenchymal signalling. Little is known about whether and where these molecules are expressed in more mature kidneys.

METHODS

In healthy and congenital polycystic kidney (cpk)mouse kidneys we sought Frem2 expression using a LacZ reporter gene and quantified Fras family transcripts. Fras1 immunohistochemistry was undertaken in cystic kidneys from cpk mice and PCK (Pkhd1 mutant) rats (models of autosomal recessive polycystic kidney disease) and in wildtype metanephroi rendered cystic by dexamethasone.

RESULTS

Nascent nephrons transiently expressed Frem2 in both tubule and podocyte epithelia. Maturing and adult collecting ducts also expressed Frem2. Frem2 was expressed in cpk cystic epithelia although Frem2 haploinsufficiency did not significantly modify cystogenesis in vivo. Fras1 transcripts were significantly upregulated, and Frem3 downregulated, in polycystic kidneys versus the non-cystic kidneys of littermates. Fras1 was immunodetected in cpk, PCK and dexamethasone-induced cystepithelia.

CONCLUSIONS

These descriptive results are consistent with the hypothesis that Fras family molecules play diverse roles in kidney epithelia. In future, this should be tested by conditional deletion of FS genes in nephron segments and collecting ducts.

LGR4 is required for the cell survival of the peripheral mesenchyme at the embryonic stages of nephrogenesis

In mice, homozygous Lgr4 inactivation results in hypoplastic kidneys. To understand better the role of LGR4 in kidney development, we performed an analysis of kidneys in Lgr4-/- embryos. We stained Lgr4-/- kidneys with anti-WT1 and anti-Cleaved Caspase3 antibodies at E16.5, and observed that the structures of the cap mesenchyme were disrupted and that apoptosis increased. In addition, the expression of PAX2, an anti-apoptotic factor in kidney development, was also significantly decreased at E16.5. We found that the LGR4 defect caused an increase in apoptosis in the peripheral mesenchyme during kidney development.

Disruption of IFT complex A causes cystic kidneys without mitotic spindle misorientation

Intraflagellar transport (IFT) complexes A and B build and maintain primary cilia. In the mouse, kidney-specific or hypomorphic mutant alleles of IFT complex B genes cause polycystic kidneys, but the influence of IFT complex A proteins on renal development is not well understood. In the present study, we found that HoxB7-Cre-driven deletion of the complex A gene Ift140 from collecting ducts disrupted, but did not completely prevent, cilia assembly. Mutant kidneys developed collecting duct cysts by postnatal day 5, with rapid cystic expansion and renal dysfunction by day 15 and little remaining parenchymal tissue by day 20. In contrast to many models of polycystic kidney disease, precystic Ift140-deleted collecting ducts showed normal centrosomal positioning and no misorientation of the mitotic spindle axis, suggesting that disruption of oriented cell division is not a prerequisite to cyst formation in these kidneys. Precystic collecting ducts had an increased mitotic index, suggesting that cell proliferation may drive cyst expansion even with normal orientation of the mitotic spindle. In addition, we observed significant increases in expression of canonical Wnt pathway genes and mediators of Hedgehog and tissue fibrosis in highly cystic, but not precystic, kidneys. Taken together, these studies indicate that loss of Ift140 causes pronounced renal cystic disease and suggest that abnormalities in several different pathways may influence cyst progression.

Patterning and early cell lineage decisions in the developing kidney: the role of Pax genes

Specification of the intermediate mesoderm and the epithelial derivatives that will make the mammalian kidney depends on the concerted action of many transcription factors and signaling proteins. Among the earliest genes expressed in the nephric duct and surrounding mesenchyme is Pax2, whose function is essential for making and maintaining the epithelium. The Pax2 protein is subject to phosphorylation in response to signals that activate the c-Jun N-terminal kinase pathway, including Wnts and BMPs. In cell culture systems, Pax2 is know to recruit components of a histone H3 lysine 4 methyltransferase complex to specific DNA sites to alter the pattern of histone modifications and determine gene expression. This epigenetic function may underlie the ability of Pax2 and similar proteins to maintain cell lineages during development.

The transcription factor PAX2 regulates ADAM10 expression in renal cell carcinoma

ADAM10 is a metalloprotease that plays an important role in the progression and metastasis of various cancers. In the present study, we present compelling evidence that PAX2 can bind to the promotor of ADAM10 and regulate ADAM10 protein expression in renal cancer cells. We further show that ADAM10 is the major sheddase for the constitutive cleavage of L1-CAM and c-Met, two important proteins involved in the progression of renal cancer. The downregulation of ADAM10 led to a more scattered cell phenotype, which was accompanied by the induction of Slug and the loss of E-cadherin, which is observed during epithelial-to-mesenchymal transition (EMT). In addition, the downregulation of ADAM10 reduced the proliferation but induced the migration of renal cancer cells. Notably, the downregulation of PAX2 led to an increased L1-CAM expression, which was accompanied by a massive metalloprotease-mediated release of soluble L1-CAM. Importantly, soluble L1-CAM induced the proliferation of endothelial cells and the migration of renal cancer cells. Finally, we can demonstrate that the silencing of PAX2 led to an L1-CAM-dependent activation of the PI3K/Akt pathway, one important pathway mediating cancer cell survival. In summary, we identified PAX2 as a regulator of L1-CAM and ADAM10, which play crucial roles in the progression of various cancers including renal cell carcinoma and the downregulation of ADAM10 maybe an earlier step in renal cancer development as it seems to be involved in processes of EMT.

VHL mutations and dysregulation of pVHL- and PTEN-controlled pathways in multilocular cystic renal cell carcinoma

Multilocular cystic renal cell carcinoma is a rare renal cell carcinoma with an excellent prognosis. To clarify the relationship with typical clear cell renal cell carcinoma, we evaluated 15 cases of multilocular cystic renal cell carcinomas diagnosed according to the 2004 WHO classification. Von Hippel Lindau (VHL) gene mutations were determined by whole genome amplification and direct sequencing. Carbonic anhydrase 9 (CAIX), a hypoxia-inducible factor (HIF) target, paired box gene 2 (PAX2), cyclin-dependent kinase inhibitor p27 and glycogen synthase kinase 3-β (GSK3β) were immunohistochemically evaluated as members of the VHL protein (pVHL)- and phosphatase and tensin homolog (PTEN)-controlled pathways. VHL mutations were identified in 3 of 12 (25%) tumors. Inactivated GSK3β, decreased PTEN expression and PAX2 positivity were observed in the vast majority of the multilocular cystic renal cell carcinomas. Strong nuclear staining of p27 was seen in 14 of 15 cases. Compared with multilocular cystic renal cell carcinomas, expression frequencies of PAX2, p-GSK3β, PTEN and CAIX were similar in a set of low-grade, early-stage clear cell renal cell carcinomas, whereas only 30% had strong p27 positivity. These results are consistent with the hypothesis that multilocular cystic renal cell carcinomas are related at the molecular level with clear cell renal cell carcinomas. Maintenance of a strong subcellular p27 expression in all multilocular cystic renal cell carcinomas analyzed may in part explain the excellent prognosis of these tumor patients.

PAX2 gene mutations in pediatric and young adult transplant recipients: kidney and urinary tract malformations without ocular anomalies

Heterozygous humans for PAX2 mutations show autosomal dominant papillorenal syndrome (PRS), consisting of ocular colobomas, renal hypo/dysplasia and progressive renal failure in childhood. PAX2 mutations have also been identified in patients with isolated renal hypo/dysplasia. Twenty unrelated children and young adults with kidney and urinary tract malformations and no ocular abnormalities were retrospectively recruited for PAX2 mutational analysis. All patients had undergone renal transplantation after end-stage renal disease. We identified two new sequence variations: (i) a deletion causing a frameshift (c.69delC) and (ii) a nucleotide substitution determining a splice site mutation (c.410+5 G/A) by predictive analysis. Therefore, we suggest PAX2 molecular analysis to be extended to all patients with congenital malformations of kidney and urinary tract (CAKUT).

Cyclic stretch induces proliferation and TGF-β1-mediated apoptosis via p38 and ERK in ureteric bud cells

We previously reported that p38 mitogen-activated protein kinase (p38) and phosphorylated ERK are upregulated in cyst epithelium of human renal dysplasia and obstructive uropathy in fetal lambs (Omori S, Fukuzawa R, Hida M, Awazu M. Kidney Int 61: 899–906, 2002; Omori S, Kitagawa H, Koike J, Fujita H, Hida M, Pringle KC, Awazu M. Kidney Int 73: 1031–1037, 2008). Dysplastic epithelium is characterized by proliferation, apoptosis, and upregulation of Pax2 and transforming growth factor (TGF)-β1. In the present study, we investigated whether cyclic mechanical stretching of ureteric bud cells, a mimic of the hydrodynamic derangement after fetal urinary tract obstruction, reproduces events seen in vivo. Cyclic stretch activated p38 and ERK and upregulated Pax2 expression in a time-dependent manner in ureteric bud cells. Stretch-stimulated Pax2 expression was suppressed by a p38 inhibitor, SB203580, or a MEK inhibitor, PD98059. 5-Deoxyuridine incorporation was increased by stretch at 24 h, which was also abolished by SB203580 or PD98059. On the other hand, apoptosis was not induced at 24 h by stretch but was significantly increased at 48 h. TGF-β1 secretion was increased by stretch at 24 h, which was inhibited by SB203580 or PD98059. Inhibition of p38 or ERK as well as anti-TGF-β antibody abolished the stretch-induced apoptosis. Finally, exogenous TGF-β1 induced apoptosis of ureteric bud cells, which was inhibited by SB203580 and PD98059. In conclusion, cyclic stretch induces Pax2 upregulation, proliferation, and TGF-β1-mediated apoptosis, features characteristic of dysplastic epithelium, via p38 and ERK in ureteric bud cells.

PAX2 mutations in fetal renal hypodysplasia

Papillorenal syndrome also known as renal-coloboma syndrome (OMIM 120330) is an autosomal dominant condition comprising optic nerve anomaly and renal oligomeganephronic hypoplasia. This reduced number of nephron generations with compensatory glomerular hypertrophy leads towards chronic insufficiency with renal failure. We report on two fetuses with PAX2 mutations presenting at 24 and 18 weeks' gestation, respectively, born into two different sibships. In our first patient, termination of pregnancy was elected for anhydramnios and suspicion of renal agenesis in the healthy couple with an unremarkable previous clinical history. This fetus had bilateral asymmetric kidney anomalies including a small multicystic left kidney, and an extremely hypoplastic right kidney. Histology showed dysplastic lesions in the left kidney, contrasting with rather normal organization in the hypoplastic right kidney. Ocular examination disclosed bilateral optic nerve coloboma. The association of these anomalies, highly suggestive of the papillorenal syndrome, led us to perform the molecular study of the PAX2 gene. Direct sequencing of the PAX2 coding sequence identified a de novo single G deletion of nucleotide 935 in exon 3 of the PAX2 resulting in a frameshift mutation (c.392delG, p.Ser131Thrfs*28). In the second family, the presence of a maternally inherited PAX2 mutation led to a decision for termination of pregnancy. The 18-week gestation fetus presented the papillorenal syndrome including hypoplastic kidneys and optic nerve coloboma. In order to address the PAX2 involvement in isolated renal "disease," 18 fetuses fulfilling criteria were screened: 10/18 had uni- or bilateral agenesis, 6/18 had bilateral multicystic dysplasia with enlarged kidneys, and 2/18 presented bilateral severe hypodysplasia confirmed on fetopathological examination. To the best of our knowledge, our first patient represents an unreported fetal diagnosis of papillorenal syndrome, and another example of the impact of oriented fetopathological examination in genetic counseling of the parents.

Calcineurin a-binding protein, a novel modulator of the calcineurin-nuclear factor of activated T-cell signaling pathway, is overexpressed in wilms' tumors and promotes cell migration

Current therapeutic strategies against Wilms' tumor (WT) reach 80% to 85% success rate. In spite of this, a remaining 15% to 20% of tumors relapse and are associated with increased metastasis and poor prognosis. To identify new regulators of WT progression, we screened for developmental target genes of Pax2, a key regulator of kidney development and a WT signature gene. We show that one of these target genes, calcineurin A-binding protein (CnABP), is coexpressed with Pax2 during kidney development and is overexpressed in >70% of WT samples analyzed. The CnABP gene encodes a novel protein product conserved in higher vertebrates. We show that CnABP promotes cell proliferation and migration in cell culture experiments. Biochemical analyses additionally identified an interaction between CnABP and calcineurin Abeta, the catalytic subunit of the calcium-responsive serine/threonine phosphatase calcineurin. We show that this interaction leads to the inhibition of calcineurin phosphatase activity and prevents nuclear factor of activated T-cell (NFAT) nuclear translocation. Inhibition of NFAT nuclear localization results in decreased NFAT transcriptional response. Together, these data identify a new modulator of calcineurin signaling up-regulated in WTs.

Mammalian target of rapamycin and caspase inhibitors in polycystic kidney disease

One of the most important abnormalities of the tubular epithelial cells lining the cysts as well as noncystic tubular epithelium is a disturbance in the balance between tubular cell proliferation and apoptosis. Activation of the mammalian target of rapamycin signaling pathway results in increased cell proliferation. Recent studies suggested abnormalities of the mammalian target of rapamycin signaling pathway in polycystic kidney disease. Mammalian target of rapamycin inhibition with sirolimus or everolimus results in attenuation of cyst formation in rat and mouse models of polycystic kidney disease. Apoptosis is a pathologic feature of most models of polycystic kidney disease, including human polycystic kidneys. Caspases, the major mediators of apoptosis, are increased in polycystic kidney disease kidneys. Both in vitro and in vivo studies suggest that caspase or apoptosis inhibition attenuates cyst formation. This review focuses on mammalian target of rapamycin and apoptosis signaling pathways in polycystic kidney disease and the role of mammalian target of rapamycin inhibitors and apoptosis inhibitors as potential therapies to reduce cyst formation.

Neutrophil gelatinase-associated lipocalin suppresses cyst growth by Pkd1 null cells in vitro and in vivo

Cyst growth in patients with autosomal dominant polycystic kidney disease is thought to be due to increased tubular cell proliferation. One model to explain this altered proliferation suggests that the polycystin proteins PC1 and PC2 localize to apical cilia and serve as an integral part of the flow-sensing pathway thus modulating the proliferative response. We measured proliferation and apoptosis in proximal tubule derived cell lines lacking PC1. These cells showed increased rates of proliferation, a decreased rate of apoptosis, compared to control heterozygous cell lines, and spontaneously formed cysts rather than tubules in an in vitro tubulogenesis assay. Addition of neutrophil gelatinase associated lipocalin (NGAL), a small secreted protein that binds diverse ligands, to the cells lacking PC1 inhibited proliferation and increased apoptosis leading to slower cyst growth in vitro. Sustained over-expression at low level of NGAL by an adenoviral delivery system suppressed cyst enlargement without improving renal function in the Pkd1 mutant mice. Our studies show that renal epithelial cells lacking PC1 have an inherent tendency to hyper-proliferate forming cysts in vitro independent of a flow stimulus. The potential benefit of attenuating cyst growth with NGAL remains to be determined.

Activated extracellular signal-regulated kinase correlates with cyst formation and transforming growth factor-beta expression in fetal obstructive uropathy

Human renal dysplasia is frequently associated with urinary tract obstruction and the abnormal expression of mitogen-activated protein kinase (MAPK). Here, we determined the renal responses and MAPK expression in developing kidneys that were obstructed in fetal lambs. Kidneys were harvested at various times after obstruction (gestation day 60) through normal term (day 145). Dilation of Bowman's capsule and proximal tubules was seen 2 days after obstruction and involved the whole cortex 18 days later, with numerous cysts present throughout the kidney at term. The proliferation marker Ki-67 and transforming growth factor-beta (TGF-beta) were detected 2 days after obstruction and progressively increased in tubules, cysts, and the interstitium. In control kidneys, p38 was expressed in tubules only during the fetal stage, whereas phosphorylated extracellular signal-regulated kinase (P-ERK) was limited to ureteric buds and collecting ducts at all stages examined. However, Jun-N-terminal kinase (JNK) was absent in the fetal kidney but present in tubules at term. In obstructed kidneys, cyst epithelia were positive for p38 and P-ERK but negative for JNK throughout all stages. These studies show that P-ERK correlated spatially and temporally with Ki-67 and TGF-beta expression, which suggests that ERK may contribute to cyst formation and fibrosis in the obstructed fetal kidney.

Pax2andPax8Regulate Branching Morphogenesis and Nephron Differentiation in the Developing Kidney

Pax genes are important regulators of kidney development. In the mouse, homozygous Pax2 inactivation results in renal agenesis, a phenotype that has largely precluded the analysis of Pax gene function during metanephric kidney development. To address this later function, kidney development was analyzed in embryos that were compound heterozygous for Pax2 and for Pax8, a closely related member of the Pax gene family. Both genes are coexpressed in differentiating nephrons and collecting ducts. At the morphological level, Pax2(+/-)Pax8(+/-) metanephric kidneys are severely hypodysplastic and characterized by a reduction in ureter tips and nephron number in comparison with wild-type or Pax2(+/-) kidneys. In developing nephrons, the molecular analysis of Pax2(+/-)Pax8(+/-) kidneys reveals a strong reduction in the expression levels of Lim1, a key regulator of nephron differentiation, accompanied by an increase in apoptosis. At a more mature stage, the reduction of Pax2/8 gene dosage severely affects distal tubule formation, revealing a role for Pax genes in the differentiation of specific nephron segments. At the ureter tips, the expression of Wnt11, a target of glial cell-derived neurotrophic factor-Ret signaling, is significantly reduced, whereas the expression levels of Ret and GDNF remain normal. Together, these results demonstrate a crucial role for Pax2 and Pax8 in nephron differentiation and branching morphogenesis of the metanephros.

The transcription factor PAX4 acts as a survival gene in INS-1E insulinoma cells

The paired/homeodomain transcription factor Pax4 is essential for islet beta-cell generation during pancreas development and their survival in adulthood. High Pax4 expression was reported in human insulinomas indicating that deregulation of the gene may be associated with tumorigenesis. We report that rat insulinoma INS-1E cells express 25-fold higher Pax4 mRNA levels than rat islets. In contrast to primary beta-cells, activin A but not betacellulin or glucose induced Pax4 mRNA levels indicating dissociation of Pax4 expression from insulinoma cell proliferation. Short hairpin RNA adenoviral constructs targeted to the paired domain or homeodomain (viPax4PD and viPax4HD) were generated. Pax4 mRNA levels were lowered by 73 and 50% in cells expressing either viPax4PD or viPax4HD. Transcript levels of the Pax4 target gene bcl-xl were reduced by 53 and 47%, whereas Pax6 and Pdx1 mRNA levels were unchanged. viPax4PD-infected cells displayed a twofold increase in spontaneous apoptosis and were more susceptible to cytokine-induced cell death. In contrast, proliferation was unaltered. RNA interference-mediated repression of insulin had no adverse effects on either Pax4 or Pdx1 expression as well as on cell replication or apoptosis. These results indicate that Pax4 is redundant for proliferation of insulinoma cells, whereas it is essential for survival through upregulation of the antiapoptotic gene bcl-xl.

Galectin-3 associates with the primary cilium and modulates cyst growth in congenital polycystic kidney disease

Several lines of evidence implicate the beta-galactoside-binding lectin galectin-3 in development and pathological processes in renal collecting ducts: galectin-3 is expressed in the ureteric bud/collecting duct lineage during nephrogenesis, modulates collecting duct growth/differentiation in vitro, and is expressed in human autosomal recessive polycystic kidney disease in cyst epithelia, almost all of which arise from collecting ducts. Moreover, exogenous galectin-3 restricts growth of cysts generated by Madin-Darby canine kidney collecting duct-derived cells in three-dimensional culture in collagen. Using the cpk mouse model of recessively inherited polycystic kidney disease, we observed widespread galectin-3 mRNA and protein in cyst epithelia. Exogenous galectin-3 reduced cyst formation in suspension culture, and mice-null mutant for galectin-3 had more extensive renal cysts in vivo. Galectin-3 was also detected for the first time in the centrosome/primary cilium, which has been implicated in diverse polycystic kidney disease. Cilia structure/number appeared normal in galectin-3-null mutants. Finally, paclitaxel, a therapy that retards polycystic kidney disease in cpk mice, increased extracellular galectin-3, in which the lectin could potentially interact with cilia. These data raise the possibility that galectin-3 may act as a natural brake on cystogenesis in cpk mice, perhaps via ciliary roles.

Sustained cell proliferation of renal epithelial cells in mice with inv mutation

A tubule system is an important component of the nephron, which is the structural and functional unit of the kidney. Expansion of renal tubules results in renal cysts. Hereditary forms of renal cystic diseases suggest that tubular size is determined genetically. The inv was discovered as a mutant with renal cysts and situs inversus. Inv/inv, inv deltaC::GFP (inv deltaC) mouse was created by the introduction of the inv gene lacking the C-terminus (inv deltaC) into inv/inv mice. The mouse develops multiple renal cysts without situs abnormality, giving us an opportunity to study inv function in renal tubular structure maintenance. In the present study, we showed that inv suppresses cyst progression in a dose-dependent manner and that the inv deltaC cystic kidneys showed increased cell proliferation and apoptosis. Cell cycle regulators for G1-S progression were activated in the cystic kidney. Furthermore, cDNA microarray and semiquantitative RT-PCR analysis showed that growth-related genes maintained a high level of expression in the cystic kidney at 4 weeks of age whereas they were decreased in control kidneys, suggesting that cells in inv deltaC kidney are still active in the cell cycle. One of the inv protein functions may provide a stop signal for renal epithelial cell proliferation.

Pax2 gene dosage influences cystogenesis in autosomal dominant polycystic kidney disease

Mutations in PKD1 cause dominant polycystic kidney disease (PKD), characterized by large fluid-filled kidney cysts in adult life, but the molecular mechanism of cystogenesis remains obscure. Ostrom et al. [Dev. Biol., 219, 250-258 (2000)] showed that reduced dosage of Pax2 caused increased apoptosis, and ameliorated cystogenesis in Cpk mutant mice with recessive PKD. Pax2 is expressed in condensing metanephrogenic mesenchyme and arborizing ureteric bud, and plays an important role in kidney development. Transient Pax2 expression during fetal kidney mesenchyme-to-epithelial transition, as well as in nascent tubules, is followed by marked down-regulation of Pax2 expression. Here, we show that in humans with PKD, as well as in Pkd1(del34/del34) mutant mice, Pax2 was expressed in cyst epithelial cells, and facilitated cyst growth in Pkd1(del34/del34) mutant mice. In Pkd1(del34/del34) mutant kidneys, the expression of Pax2 persisted in nascent collecting ducts. In contrast, homozygous Pkd1(del34/del34) fetal mice carrying mutant Pax2 exhibited ameliorated cyst growth, although reduced cystogenesis was not associated with increased apoptosis. Pax2 expression was attenuated in nascent collecting ducts and absent from remnant cysts of Pkd1(del34/del34)/Pax2(1Neu/+) mutant mice. To investigate whether the Pkd1 gene product, Polycystin-1, regulates Pax2, MDCK cells were engineered constitutively expressing wild-type Pkd1; Pax2 protein levels and promoter activity were both repressed in MDCK cells over-expressing Pkd1, but not in cells without transgenic Pkd1. These data suggest that polycystin-1-deficient tubular epithelia persistently express Pax2 in ADPKD, and that Pax2 or its pathway may be an appropriate target for the development of novel therapies for ADPKD.

Inhibition of PAX2 expression results in alternate cell death pathways in prostate cancer cells differing in p53 status

Inhibition of apoptosis is a critical pathophysiological factor that contributes to the development of prostate cancer. Recently, PAX2, a transcriptional regulator implicated in oncogenesis, has been demonstrated to be expressed by prostate cancer. However, its downstream molecular pathways for suppression of apoptosis, other than the tumor suppressor gene p53, have yet to be elucidated. Here, we examine the effects of inhibiting PAX2 expression by prostate cancer cells that differ in p53 gene status. These data collectively demonstrate that PAX2 inhibition results in cell death independent of p53, and that additional tumor suppressors or cell death pathways may be inhibited by PAX2 in prostate cancer cells.

Neuronal apoptosis inhibitory protein is expressed in developing kidney and is regulated by PAX2

During fetal kidney development, the extent of ureteric bud (UB) branching will determine final nephron endowment for life. Nephron number varies widely among normal humans and those who are born at the low end of the nephron number spectrum may be at risk for essential hypertension in adulthood. Little is known about how nephron number is set. However, we previously showed that the transcription factor, Pax2, suppresses apoptosis in UB cells during kidney development and optimizes branching morphogenesis. Here, we report that PAX2 directly binds to a specific recognition motif in the human neuronal apoptosis inhibitory protein (NAIP) gene promoter. NAIP is an endogenous inhibitor of apoptosis, inactivating caspase-3 and caspase-7 in neuronal tissues. PAX2 activates NAIP gene transcription (7-fold) in vitro and NAIP transcript level is increased fourfold in HEK293 cells stably transfected with PAX2. We show that Naip is expressed in embryonic day 15 (E15) fetal kidney tissue (RT-PCR) and NAIP protein is demonstrated by immunohistochemistry in E15 mouse kidney collecting ducts and P1 proximal tubules. Naip mRNA is significantly reduced (50%) in heterozygous Pax2 mutant mice. Finally, we show that an antisense Naip1 cDNA transfected into murine collecting duct cells doubles caspase-3/7 activity induced by Baxalpha. These observations suggest that the powerful effects of PAX2 on renal branching morphogenesis and final nephron number may be mediated by activation of Naip which then suppresses apoptosis in UB cells.

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.