Renal development: perspectives on a Wnt-dependent process

Wnt Pathway: An Integral Hub for Developmental and Oncogenic Signaling Networks

The Wnt pathway is an integral cell-to-cell signaling hub which regulates crucial development processes and maintenance of tissue homeostasis by coordinating cell proliferation, differentiation, cell polarity, cell movement, and stem cell renewal. When dysregulated, it is associated with various developmental diseases, fibrosis, and tumorigenesis. We now better appreciate the complexity and crosstalk of the Wnt pathway with other signaling cascades. Emerging roles of the Wnt signaling in the cancer stem cell niche and drug resistance have led to development of therapeutics specifically targeting various Wnt components, with some agents currently in clinical trials. This review highlights historical and recent findings on key mediators of Wnt signaling and how they impact antitumor immunity and maintenance of cancer stem cells. This review also examines current therapeutics being developed that modulate Wnt signaling in cancer and discusses potential shortcomings associated with available therapeutics.

Ciliary Genes in Renal Cystic Diseases

Cilia are microtubule-based organelles, protruding from the apical cell surface and anchoring to the cytoskeleton. Primary (nonmotile) cilia of the kidney act as mechanosensors of nephron cells, responding to fluid movements by triggering signal transduction. The impaired functioning of primary cilia leads to formation of cysts which in turn contribute to development of diverse renal diseases, including kidney ciliopathies and renal cancer. Here, we review current knowledge on the role of ciliary genes in kidney ciliopathies and renal cell carcinoma (RCC). Special focus is given on the impact of mutations and altered expression of ciliary genes (e.g., encoding polycystins, nephrocystins, Bardet-Biedl syndrome (BBS) proteins, ALS1, Oral-facial-digital syndrome 1 (OFD1) and others) in polycystic kidney disease and nephronophthisis, as well as rare genetic disorders, including syndromes of Joubert, Meckel-Gruber, Bardet-Biedl, Senior-Loken, Alström, Orofaciodigital syndrome type I and cranioectodermal dysplasia. We also show that RCC and classic kidney ciliopathies share commonly disturbed genes affecting cilia function, including VHL (von Hippel-Lindau tumor suppressor), PKD1 (polycystin 1, transient receptor potential channel interacting) and PKD2 (polycystin 2, transient receptor potential cation channel). Finally, we discuss the significance of ciliary genes as diagnostic and prognostic markers, as well as therapeutic targets in ciliopathies and cancer.

Intracellular Communication among Morphogen Signaling Pathways during Vertebrate Body Plan Formation

During embryonic development in vertebrates, morphogens play an important role in cell fate determination and morphogenesis. Bone morphogenetic proteins (BMPs) belonging to the transforming growth factor-β (TGF-β) family control the dorsal-ventral (DV) patterning of embryos, whereas other morphogens such as fibroblast growth factor (FGF), Wnt family members, and retinoic acid (RA) regulate the formation of the anterior-posterior (AP) axis. Activation of morphogen signaling results in changes in the expression of target genes including transcription factors that direct cell fate along the body axes. To ensure the correct establishment of the body plan, the processes of DV and AP axis formation must be linked and coordinately regulated by a fine-tuning of morphogen signaling. In this review, we focus on the interplay of various intracellular regulatory mechanisms and discuss how communication among morphogen signaling pathways modulates body axis formation in vertebrate embryos.

A High-Fat/High-Sucrose Diet Induces WNT4 Expression in Mouse Pancreatic β-cells

Aims/Introduction: Several lines of evidence suggest that dysregulation of the WNT signaling pathway is involved in the pathogenesis of type 2 diabetes. This study was performed to elucidate the effects of a high-fat/high-sucrose (HF/HS) diet on pancreatic islet functions in relation to modulation of WNT ligand expression in β-cells.

MATERIALS AND METHODS

Mice were fed either standard mouse chow or a HF/HS diet from 8 weeks of age. At 20 weeks of age, intraperitoneal glucose tolerance tests were performed in both groups of mice, followed by euthanasia and isolation of pancreatic islets. WNT-related gene expression in islets and MIN6 cells was measured by quantitative real-time RT-PCR. To explore the direct effects of WNT signals on pancreatic β-cells, MIN6 cells were exposed to recombinant mouse WNT4 protein (rmWNT4) for 48 h, and glucose-induced insulin secretion was measured. Furthermore, Wnt4 siRNAs were transfected into MIN6 cells, and cell viability and insulin secretion were measured in control and Wnt4 siRNA-transfected MIN6 cells.

RESULTS

Mice fed the HF/HS diet were heavier and their plasma glucose and insulin levels were higher compared with mice fed standard chow. Wnt4, Wnt5b, Ror1, and Ror2 expression was upregulated, while Fzd4, Fzd5, Fzd6, Lrp5, and Lrp6 expression was downregulated in the islets of mice fed the HF/HS diet. Wnt4 was the most abundantly expressed WNT ligand in β-cells, and its expression was increased by the HF/HS diet. Although exposure to recombinant mouse WNT4 protein for 48 h did not alter glucose-induced insulin secretion, it was significantly reduced by knockdown of Wnt4 in MIN6 cells.

CONCLUSIONS

We demonstrated that the HF/HS diet-induced increase of WNT4 signaling in β-cells is involved in augmentation of glucose-induced insulin secretion and impaired β-cell proliferation. These results strongly indicate an essential role of WNT4 in the regulation of β-cell functions in mouse pancreatic islets.

Directed Differentiation of Human Pluripotent Stem Cells to Podocytes under Defined Conditions

A major cause of chronic kidney disease (CKD) is glomerular disease, which can be attributed to a spectrum of podocyte disorders. Podocytes are non-proliferative, terminally differentiated cells. Thus, the limited supply of primary podocytes impedes CKD research. Differentiation of human pluripotent stem cells (hPSCs) into podocytes has the potential to produce podocytes for disease modeling, drug screening, and cell therapies. In the podocyte differentiation process described here, hPSCs are first induced to primitive streak-like cells by activating canonical Wnt signaling. Next, these cells progress to mesoderm precursors, proliferative nephron progenitors, and eventually become mature podocytes by culturing in a serum-free medium. Podocytes generated via this protocol adopt podocyte morphology, express canonical podocyte markers, and exhibit podocyte phenotypes, including albumin uptake and TGF-β1 triggered cell death. This study provides a simple, defined strategy to generate podocytes for in vitro modeling of podocyte development and disease or for cell therapies.

Atmin modulates Pkhd1 expression and may mediate Autosomal Recessive Polycystic Kidney Disease (ARPKD) through altered non-canonical Wnt/Planar Cell Polarity (PCP) signalling

Autosomal Recessive Polycystic Kidney Disease (ARPKD) is a genetic disorder with an incidence of ~1:20,000 that manifests in a wide range of renal and liver disease severity in human patients and can lead to perinatal mortality. ARPKD is caused by mutations in PKHD1, which encodes the large membrane protein, Fibrocystin, required for normal branching morphogenesis of the ureteric bud during embryonic renal development. The variation in ARPKD phenotype suggests that in addition to PKHD1 mutations, other genes may play a role, acting as modifiers of disease severity. One such pathway involves non-canonical Wnt/Planar Cell Polarity (PCP) signalling that has been associated with other cystic kidney diseases, but has not been investigated in ARPKD. Analysis of the AtminGpg6 mouse showed kidney, liver and lung abnormalities, suggesting it as a novel mouse tool for the study of ARPKD. Further, modulation of Atmin affected Pkhd1 mRNA levels, altered non-canonical Wnt/PCP signalling and impacted cellular proliferation and adhesion, although Atmin does not bind directly to the C-terminus of Fibrocystin. Differences in ATMIN and VANGL2 expression were observed between normal human paediatric kidneys and age-matched ARPKD kidneys. Significant increases in ATMIN, WNT5A, VANGL2 and SCRIBBLE were seen in human ARPKD versus normal kidneys; no substantial differences were seen in DAAM2 or NPHP2. A striking increase in E-cadherin was also detected in ARPKD kidneys. This work indicates a novel role for non-canonical Wnt/PCP signalling in ARPKD and suggests ATMIN as a modulator of PKHD1.

Application of Muse Cell Therapy for Kidney Diseases

The kidney plays an essential role in the maintenance of homeostasis in healthy individuals, e.g., by regulating the amount of water and concentration of electrolyte in the body. Owing to the structural complexity, renal dysfunction is caused by a myriad of diseases and conditions, and in severe cases, it progresses to end-stage renal disease in which patients require renal replacement therapy, i.e., maintenance dialysis or kidney transplantation. The currently available therapeutic modalities, with the exception of renal transplantation, cannot recover severely deteriorated renal function. Thus, regenerative medicine holds considerable promise as a potential means for developing next-generation renal therapeutics. Mesenchymal stem cell (MSC) transplantation has been investigated in acute kidney injury and chronic kidney disease models, and clinical studies have already been started for some kinds of kidney diseases. However, most of these studies concluded that the main underlying mechanism of therapeutic effect of MSC transplantation was paracrine. Recently, we reported that Muse cell therapy in a murine model of chronic kidney disease resulted in differentiation of intravenously injected Muse cells into glomerular cells after preferential homing to damaged glomerulus and improvement in renal function. The result suggested the potentiality of Muse cell therapy for glomerular regeneration. Muse cells are a promising cell source for regenerative therapy for kidney diseases.

Antennas of organ morphogenesis: the roles of cilia in vertebrate kidney development

Cilia arose early during eukaryotic evolution, and their structural components are highly conserved from the simplest protists to complex metazoan species. In recent years, the role of cilia in the ontogeny of vertebrate organs has received increasing attention due to a staggering correlation between human disease and dysfunctional cilia. In particular, the presence of cilia in both the developing and mature kidney has become a deep area of research due to ciliopathies common to the kidney, such as polycystic kidney disease (PKD). Interestingly, mutations in genes encoding proteins that localize to the cilia cause similar cystic phenotypes in kidneys of various vertebrates, suggesting an essential role for cilia in kidney organogenesis and homeostasis as well. Importantly, the genes so far identified in kidney disease have conserved functions across species, whose kidneys include both primary and motile cilia. Here, we aim to provide a comprehensive description of cilia and their role in kidney development, as well as highlight the usefulness of the zebrafish embryonic kidney as a model to further understand the function of cilia in kidney health.

Tissue specificity of decellularized rhesus monkey kidney and lung scaffolds

Initial steps in establishing an optimal strategy for functional bioengineered tissues is generation of three-dimensional constructs containing cells with the appropriate organization and phenotype. To effectively utilize rhesus monkey decellularized kidney scaffolds, these studies evaluated two key parameters: (1) residual scaffold components after decellularization including proteomics analysis, and (2) the use of undifferentiated human embryonic stem cells (hESCs) for recellularization in order to explore cellular differentiation in a tissue-specific manner. Sections of kidney and lung were selected for a comparative evaluation because of their similar pattern of organogenesis. Proteomics analysis revealed the presence of growth factors and antimicrobial proteins as well as stress proteins and complement components. Immunohistochemistry of recellularized kidney scaffolds showed the generation of Cytokeratin+ epithelial tubule phenotypes throughout the scaffold that demonstrated a statistically significant increase in expression of kidney-associated genes compared to baseline hESC gene expression. Recellularization of lung scaffolds showed that cells lined the alveolar spaces and demonstrated statistically significant upregulation of key lung-associated genes. However, overall expression of kidney and lung-associated markers was not statistically different when the kidney and lung recellularized scaffolds were compared. These results suggest that decellularized scaffolds have an intrinsic spatial ability to influence hESC differentiation by physically shaping cells into tissue-appropriate structures and phenotypes, and that additional approaches may be needed to ensure consistent recellularization throughout the matrix.

The Meckel syndrome protein meckelin (TMEM67) is a key regulator of cilia function but is not required for tissue planar polarity

Meckel syndrome (MKS) is a lethal disorder associated with renal cystic disease, encephalocele, ductal plate malformation and polydactyly. MKS is genetically heterogeneous and part of a growing list of syndromes called ciliopathies, disorders resulting from defective cilia. TMEM67 mutation (MKS3) is a major cause of MKS and the related ciliopathy Joubert syndrome, although the complete etiology of the disease is not well understood. To further investigate MKS3, we analyzed phenotypes in the Tmem67 null mouse (bpck) and in zebrafish tmem67 morphants. Phenotypes similar to those in human MKS and other ciliopathy models were observed, with additional eye, skeletal and inner ear abnormalities characterized in the bpck mouse. The observed disorganized stereociliary bundles in the bpck inner ear and the convergent extension defects in zebrafish morphants are similar to those found in planar cell polarity (PCP) mutants, a pathway suggested to be defective in ciliopathies. However, analysis of classical vertebrate PCP readouts in the bpck mouse and ciliary organization analysis in tmem67 morphants did not support a global loss of planar polarity. Canonical Wnt signaling was upregulated in cyst linings and isolated fibroblasts from the bpck mouse, but was unchanged in the retina and cochlea tissue, suggesting that increased Wnt signaling may only be linked to MKS3 phenotypes associated with elevated proliferation. Together, these data suggest that defective cilia loading, but not a global loss of ciliogenesis, basal body docking or PCP signaling leads to dysfunctional cilia in MKS3 tissues.

Wnt signaling in vertebrate axis specification

The Wnt pathway is a major embryonic signaling pathway that controls cell proliferation, cell fate, and body-axis determination in vertebrate embryos. Soon after egg fertilization, Wnt pathway components play a role in microtubule-dependent dorsoventral axis specification. Later in embryogenesis, another conserved function of the pathway is to specify the anteroposterior axis. The dual role of Wnt signaling in Xenopus and zebrafish embryos is regulated at different developmental stages by distinct sets of Wnt target genes. This review highlights recent progress in the discrimination of different signaling branches and the identification of specific pathway targets during vertebrate axial development.

β-Catenin signaling regulates Foxa2 expression during endometrial hyperplasia formation

The Wnt/β-catenin signaling is essential for various organogenesis and is often implicated during tumorigenesis. Dysregulated β-catenin signaling is associated with the formation of endometrial adenocarcinomas (EACs), which is considered as the common form of endometrial cancer in women. In the current study, we investigate the downstream target of Wnt/β-catenin signaling in the uterine epithelia and the mechanism leading to the formation of endometrial hyperplasia. We report that conditional ablation and activation of β-catenin in the uterine epithelia lead to aberrant epithelial structures and endometrial hyperplasia formation, respectively. We demonstrate that β-catenin regulates Foxa2 with its candidate upstream region for the uterine epithelia. Furthermore, knockdown of Foxa2 leads to defects in cell cycle regulation, suggesting a possible function of Foxa2 in the control of cell proliferation. We also observe that β-catenin and Foxa2 expression levels are augmented in the human specimens of complex atypical endometrial hyperplasia, which is considered to have a greater risk of progression to EACs. Thus, our study indicates that β-catenin regulates Foxa2 expression, and this interaction is possibly essential to control cell cycle progression during endometrial hyperplasia formation. Altogether, the augmented expression levels of β-catenin and Foxa2 are essential features during the formation of endometrial hyperplasia.

Pescadillo homologue 1 and Peter Pan function during Xenopus laevis pronephros development

BACKGROUND INFORMATION

pes1 (pescadillo homologue 1) and ppan (Peter Pan) are multifunctional proteins involved in ribosome biogenesis, cell proliferation, apoptosis, cell migration and regulation of gene expression. Both proteins are required for early neural development in Xenopus laevis, as previously demonstrated.

RESULTS

We show that the expression of both genes in the developing pronephros depends on wnt4 and fzd3 (frizzled homologue 3) function. Loss of pes1 or ppan by MO (morpholino oligonucleotide)-based knockdown approaches resulted in strong malformations during pronephric tubule formation. Defects were already notable during specification of pronephric progenitor cells, as shown by lhx1 expression. Moreover, we demonstrated that Xenopus pes1 and ppan interact physically and functionally and that pes1 and ppan can cross-rescue the loss of function phenotype of one another. Interference with rRNA synthesis, however, did not result in a similar early pronephros phenotype.

CONCLUSION

These results demonstrate that pes1 and ppan are required for Xenopus pronephros development and indicate that their function in the pronephros is independent of their role in ribosome biosynthesis.

Gli-similar (Glis) Krüppel-like zinc finger proteins: insights into their physiological functions and critical roles in neonatal diabetes and cystic renal disease

GLI-similar (Glis) 1-3 proteins constitute a subfamily of the Krüppel-like zinc finger transcription factors that are closely related to the Gli family. Glis1-3 play critical roles in the regulation of a number of physiological processes and have been implicated in several pathologies. Mutations in GLIS2 have been linked to nephronophthisis, an autosomal recessive cystic kidney disease. Loss of Glis2 function leads to renal atrophy and fibrosis that involves epithelial-mesenchymal transition (EMT) of renal tubule epithelial cells. Mutations in human GLIS3 have been implicated in a syndrome characterized by neonatal diabetes and congenital hypothyroidism (NDH) and in some patients accompanied by polycystic kidney disease, glaucoma, and liver fibrosis. In addition, the GLIS3 gene has been identified as a susceptibility locus for the risk of type 1 and 2 diabetes. Glis3 plays a key role in pancreatic development, particularly in the generation of ß-cells and in the regulation of insulin gene expression. Glis2 and Glis3 proteins have been demonstrated to localize to the primary cilium, a signaling organelle that has been implicated in several pathologies, including cystic renal diseases. This association suggests that Glis2/3 are part of primary cilium-associated signaling pathways that control the activity of Glis proteins. Upon activation in the primary cilium, Glis proteins may translocate to the nucleus where they subsequently regulate gene transcription by interacting with Glis-binding sites in the promoter regulatory region of target genes. In this review, we discuss the current knowledge of the Glis signaling pathways, their physiological functions, and their involvement in several human pathologies.

Crosstalk between Wnt and bone morphogenic protein signaling: a turbulent relationship

The Wnt and the bone morphogenic protein (BMP) pathways are evolutionarily conserved and essentially independent signaling mechanisms, which, however, often regulate similar biological processes. Wnt and BMP signaling are functionally integrated in many biological processes, such as embryonic patterning in Drosophila and vertebrates, formation of kidney, limb, teeth and bones, maintenance of stem cells, and cancer progression. Detailed inspection of regulation in these and other tissues reveals that Wnt and BMP signaling are functionally integrated in four fundamentally different ways. The molecular mechanism evolved to mediate this integration can also be summarized in four different ways. However, a fundamental aspect of functional and mechanistic interaction between these pathways relies on tissue-specific mechanisms, which are often not conserved and cannot be extrapolated to other tissues. Integration of the two pathways contributes toward the sophisticated means necessary for creating the complexity of our bodies and the reliable and healthy function of its tissues and organs.

Targeting the Met signaling pathway in renal cancer

Renal cell carcinoma (RCC), the most common form of kidney cancer, accounts for 3% of all adult malignancies and its incidence has significantly increased over the last 20 years. RCC claims 13,000 lives annually in the USA and more than 100,000 worldwide. A better understanding of the molecular basis of RCC has facilitated the development of novel and more selective therapeutic approaches. An important role in RCC oncogenesis is played by the receptor for HGF, Met, which has attracted considerable attention, more recently as a molecular target for cancer therapy, and several drugs selectively targeting this pathway are now in clinical trials. This review will focus on efforts to understand the role of the Met signaling pathway in renal cancer and how this has contributed to the development of potent and selective drug candidates.

The effect of vitamin A deficiency in maternal rats on tumor formation in filial rats

PURPOSE

We established a vitamin A-deficient (VAD) model of pregnant rats to evaluate the effect of vitamin A deficiency in maternal rats on tumor formation in filial rats.

METHODS

Ten pregnant Wistar rats were divided into 2 groups: (1) VAD group, 6 rats were given nonvitamin A diet from 2 weeks before mating till delivery and (2) normal diet (ND) group, 4 rats were given normal diet. Twenty random neonatal rats from each group were killed on the next day of delivery. The rest neonates were given normal diet for 1 year until killed. Serum levels of vitamin A, morphology of the kidney, incidence of tumor formation, and retinoid X receptor (RXR) alpha messenger RNA (mRNA) expression in renal tissue were assessed for the filial rats.

RESULTS

Fifty-six and 49 neonatal rats were born for VAD group and ND group, respectively. The detection rate of nephrogenic rests (NRs) from neonates in VAD group (50%) was significantly higher than that in ND group (20%; P < .05). The incidence of nephroblastoma was 13.9% in filial rats of VAD group and 0% for ND group. The detection rate of NRs for filial rats of VAD group (30.6%) was significantly higher than that of ND group (6.9%; P < .01). The expression of RXRalpha mRNA in tumor tissue of the filial rats of VAD group (3.17 +/- 0.15) was significantly lower than that in kidney tissue of ND group (3.58 +/- 0.20; P < .01).

CONCLUSION

Deficiency in vitamin A for pregnant rats resulted in renal dysplasia, increased NRs, and higher incidence of nephroblastoma.

VHL loss of function and its impact on oncogenic signaling networks in clear cell renal cell carcinoma

Loss of von Hippel-Lindau tumor suppressor gene function occurs in familial and most sporadic clear cell renal cell carcinoma, resulting in the aberrant expression of genes that control cell proliferation, metabolism, invasion and angiogenesis. The molecular mechanisms by which loss of function leads to tumorigenesis are not yet fully defined. The von Hippel-Lindau gene product is part of an ubiquitin ligase complex that targets hypoxia inducible factors for polyubiquitination and proteasomal degradation, linking hypoxia response genes to renal cell carcinoma oncogenesis. Loss von Hippel-Lindau gene function also promotes cell invasiveness in response to hepatocyte growth factor, an important regulator of kidney development and renal homeostasis. Increased cell invasiveness is mediated by another ubiquitin ligase target with relevance to the molecular pathogenesis of renal cell carcinoma: beta-catenin. This discovery and other recent insights into kidney cancer oncogenesis implicate convergent developmental and homeostatic signaling pathways in tumorigenesis, tumor invasiveness and metastasis.

Accumulation of malignant renal stem cells is associated with epigenetic changes in normal renal progenitor genes

Recent studies indicate a dual epigenetic role of the Polycomb group (PcG) proteins in self-renewal of stem cells and oncogenesis. Their elevation in our previous human kidney microarray screen led us examine whether they participate in processes involving normal and malignant renal progenitors. We therefore analyzed the expression of the PcG genes (EZH2, BMI-1, EED, SUZ12) in relation to that of the nephric-progenitor genes (WT1, PAX2, SALL1, SIX2, CITED1) using real-time polymerase chain reaction and methylation assays during renal development, regeneration, and tumorigenesis. Although all of the nephric-progenitor genes were shown to be developmentally regulated, analysis of polycomb gene expression during murine nephrogenesis and in an in vitro induction model of the nephrogenic mesenchyme indicated dynamic regulation only for EZH2 in the normal renal progenitor population. In contrast, induction of adult kidney regeneration by ischemia/reperfusion injury resulted primarily in rapid elevation of BMI-1, whereas EZH2 was silenced. Analysis of renal tumorigenesis in stem cell-like tumor xenografts established by serial passage of Wilms' tumor (WT) in immunodeficient mice showed cooperative upregulation of all PcG genes. This was accompanied by upregulation of WT1, PAX2, and SALL1 but downregulation of SIX2. Accordingly, methylation-specific quantitative polymerase chain reaction demonstrated promoter hypomethylation of WT1, PAX2, and SIX2 in primary WT and fetal kidneys, whereas progressive WT xenografts showed hypermethylation of SIX2, possibly leading to loss of renal differentiation. PcG genes vary in expression during renal development, regeneration, and tumorigenesis. We suggest a link between polycomb activation and epigenetic alterations of the renal progenitor population in initiation and progression of renal cancer.

Beta-catenin is necessary to keep cells of ureteric bud/Wolffian duct epithelium in a precursor state

Differentiation is the process by which tissues/organs take on their final, physiologically functional form. This process is mediated in part by the silencing of embryonic genes and the activation of terminal, differentiation gene products. Mammalian kidney development is initiated when the Wolffian duct branches and invades the overlying metanephric mesenchyme. The newly formed epithelial bud, known as the ureteric bud, will continue to branch ultimately differentiating into the collecting duct system and ureter. Here, we show that Hoxb7-Cre mediated removal of beta-catenin from the mouse Wolffian duct epithelium leads to the premature expression of gene products normally associated with the differentiated kidney collecting duct system including the water channel protein, Aquaporin-3 and the tight junction protein isoform, ZO-1 alpha+. Mutant cells fail to maintain expression of some genes associated with embryonic development, including several mediators of branching morphogenesis, which subsequently leads to kidney aplasia or hypoplasia. Reciprocally, expression of a stabilized form of beta-catenin appears to block differentiation of the collecting ducts. All of these defects occur in the absence of any effects on the adherens junctions. These data indicate a role for beta-catenin in maintaining cells of the Wolffian ducts and the duct derived ureteric bud/collecting duct system in an undifferentiated or precursor state.

Role of primary cilia in the pathogenesis of polycystic kidney disease

Cysts in the kidney are among the most common inherited human pathologies, and recent research has uncovered that a defect in cilia-mediated signaling activity is a key factor that leads to cyst formation. The cilium is a microtubule-based organelle that is found on most cells in the mammalian body. Multiple proteins whose functions are disrupted in cystic diseases have now been localized to the cilium or at the basal body at the base of the cilium. Current data indicate that the cilium can function as a mechanosensor to detect fluid flow through the lumen of renal tubules. Flow-mediated deflection of the cilia axoneme induces an increase in intracellular calcium and alters gene expression. Alternatively, a recent finding has revealed that the intraflagellar transport 88/polaris protein, which is required for cilia assembly, has an additional role in regulating cell-cycle progression independent of its function in ciliogenesis. Further research directed at understanding the relationship between the cilium, cell-cycle, and cilia-mediated mechanosensation and signaling activity will hopefully provide important insights into the mechanisms of renal cyst pathogenesis and lead to better approaches for therapeutic intervention.

Wnt signaling in polycystic kidney disease

Wnt signaling cascades activate morphogenetic programs that range from cell migration and proliferation to cell fate determination and stem cell renewal. These pathways enable cells to translate environmental cues into the complex cellular programs that are needed to organize tissues and build organs. Wnt signaling is essential for renal development; however, the specific molecular underpinnings involved are poorly understood. Recent research has revealed an unexpected intersection between Wnt signaling and polycystic kidney disease. Some polycystic kidney disease proteins, such as Inversin and Bardet-Biedl syndrome family members, were found to use components of the Wnt signaling cascade to orient cells along a secondary polarity axis within the plane of the epithelium. These spatial cues may be needed to position nascent tubules with a defined geometry.

Wnt signaling and human diseases: what are the therapeutic implications?

Wnt signaling plays an important role in regulating cell proliferation and differentiation. De-regulation of these signaling pathways has been implicated in many human diseases, ranging from cancers to skeletal disorders. Wnt proteins are a large family of secreted factors that bind to the Frizzled receptors and LRP5/6 co-receptors and initiate complex signaling cascades. Over the past two decades, our understanding of Wnt signaling has been significantly improved due to the identification of many key regulators and mediators of these pathways. Given that Wnt signaling is tightly regulated at multiple cellular levels, these pathways themselves offer ample nodal points for targeted therapeutics. Here, we focus on our current understanding of these pathways, the associations of Wnt signaling with human disorders, and the opportunities to target key components of Wnt signaling for rational drug discovery.

WNTS and WNT receptors as therapeutic tools and targets in human disease processes

The body of scientific literature linking Wnts and Wnt-associated proteins to human disease processes continues to grow in parallel with new discoveries from basic science laboratories that further characterize the elaborate cellular events following the binding of Wnts to their receptors. While Wnt-mediated signaling has long been known to play a major role in human carcinogenesis, accumulating evidence indicates that Wnts are also important mediators of inflammation and recovery from injury. The binding of secreted Wnt ligands to their receptors offers an attractive and accessible target for therapeutic regulation of these signaling pathways. Several promising preliminary studies have already addressed potential avenues for the manipulation of Wnt signaling in disease processes. This review will focus on disease processes involving the regulation of Wnt signaling at the level of Wnt binding to its target receptors. Wnt proteins, Wnt receptors, and secreted Wnt inhibitors are attractive as potential therapeutic agents and targets due to their extracellular location. In addition, since Wnt signaling results in a diverse array of downstream intracellular events, many of which are not fully understood, the targeting of this pathway at the most upstream site of pathway activation also provides a strategic advantage for therapy. As the list of Wnt-related diseases continues to grow, advances in our understanding of the biochemical and molecular mechanisms underlying Wnt signaling may ultimately translate into innovative ways to treat Wnt-related disease processes in patients.

The roles of cilia in developmental disorders and disease

Cilia are highly conserved organelles that have diverse motility and sensory functions. Recent discoveries have revealed that cilia also have crucial roles in cell signaling pathways and in maintaining cellular homeostasis. As such, defects in cilia formation or function have profound effects on the development of body pattern and the physiology of multiple organ systems. By categorizing syndromes that are due to cilia dysfunction in humans and from studies in vertebrate model organisms, molecular pathways that intersect with cilia formation and function have come to light. Here, we summarize an emerging view that in order to understand some complex developmental pathways and disease etiologies, one must consider the molecular functions performed by cilia.

The von Hippel-Lindau tumor suppressor gene product represses oncogenic beta-catenin signaling in renal carcinoma cells

Loss of von Hippel-Lindau (VHL) tumor suppressor gene function occurs in familial and most sporadic clear cell renal cell carcinoma (RCC), resulting in the aberrant expression of genes that control cell proliferation, invasion, and angiogenesis. The molecular mechanisms by which VHL loss leads to tumorigenesis are not yet fully defined. VHL loss has been shown to allow robust RCC cell motility, invasiveness, and morphogenesis in response to hepatocyte growth factor (HGF) stimulation, processes that are known to contribute to tumor invasiveness and metastatic potential. Among the most likely intracellular mediators of these HGF-driven activities is beta-catenin, a structural link between cadherens and the actin cytoskeleton, as well as a gene transactivator. We show that reconstitution of VHL expression in RCC cells repressed HGF-stimulated beta-catenin tyrosyl phosphorylation, adherens junction disruption, cytoplasmic beta-catenin accumulation, and reporter gene transactivation in RCC cells. Ectopic expression of a ubiquitination-resistant beta-catenin mutant specifically restored HGF-stimulated invasion and morphogenesis in VHL-transfected RCC cells. VHL gene silencing in non-RCC renal epithelial cells phenotypically mimicked VHL loss in RCC, and HGF-driven invasiveness was blocked by the expression of a dominant-negative mutant of Tcf. We conclude that, unlike many other cancers, where HGF pathway activation contributes to malignancy through the acquisition of autocrine signaling, receptor overexpression, or mutation, in RCC cells VHL loss enables HGF-driven oncogenic beta-catenin signaling. These findings identify beta-catenin as a potential target in biomarker and drug development for RCC.

[Changes of the glomerular size during the human fetal kidney development]

INTRODUCTION

Newborns adaptation on postnatal conditions includes significant morphological and functional renal changes. Every kidney contains a constant number of nephrons, at the end of the nephrogenesis period, which extends from week 8 to 34 of gestation. Mature juxtamedullary nephrons possess higher filtration capacity than primitive superficial nephrons, which have insufficient vascularization.

OBJECTIVE

The objective of the study was to calculate an average glomerular diameter in cortical zones of the kidney during development, to define periods of their most intensive growth, and to record differences of glomerular size between different cortical zones.

METHOD

A total of 30 human fetal kidneys aged from IV to X lunar months were analyzed. Stereological methods were used for calculating the average glomerular diameter in superficial, intermediate and juxtamedullary zone of the kidney cortex.

RESULTS

Glomeruli in the superficial cortical zone had the lowest average diameter. The average glomerular diameter continually increased from IV lunar month (0.057 +/- 0.004 mm) to X lunar month (0.082 +/- 0.004 mm), with highly significant correlation with gestational age (r=0.755; p<0.01). The average glomerular diameter in the intermediate zone increased from 0.081 +/- 0.004 mm (IV lunar month) to 0.096 +/- 0.004 mm (X lunar month) with low linear correlation with gestational age (r=0.161). Juxtamedullary glomeruli were the biggest ones. Their average diameter, during the IV LM ranged from 0.093 +/- 0.006 mm to 0.101 +/- 0.004 mm. In the newborns (X lunar month), juxtamedullary glomeruli had spherical structures with an average diameter of 0.103 +/- 0.004 mm, and low negative correlation (r=-0.032) with gestational age. In the IV and V lunar months of gestation, there was significant difference (p<0.01; p<0.05) between the average glomerular diameter in the different zones of the kidney cortex.

CONCLUSION

Superficial glomeruli had the smallest diameter, while juxtamedullary glomeruli were the largest. The average glomerular diameter increased during intrauterine development in all zones, most intensive in the X lunar month. There was a significant difference of the glomeruli between different cortical zones in the young fetuses. Such significant difference receded as gestational age increased.

Wnt-4 signaling is involved in the control of smooth muscle cell fate via Bmp-4 in the medullary stroma of the developing kidney

Wnt-4, a member of the Wnt family of secreted signaling molecules, is essential for nephrogenesis, but its expression in the presumptive medulla suggests additional developmental roles in kidney organogenesis. We demonstrate here that Wnt-4 signaling plays also a role in the determination of the fate of smooth muscle cells in the medullary stroma of the developing kidney, as a differentiation marker, smooth muscle alpha-actin (alpha-SMA), is markedly reduced in the absence of its signaling. Wnt-4 probably performs this function by activating the Bmp-4 gene encoding a known differentiation factor for smooth muscle cells, since Bmp-4 gene expression was lost in the absence of Wnt-4 while Wnt-4 signaling led to a rescue of Bmp-4 expression and induction of alpha-SMA-positive cells in vitro. Recombinant Bmp-4 similarly rescued the differentiation of alpha-SMA-expressing cells in cultured Wnt-4-deficient embryonic kidney. The lack of smooth muscle cell differentiation leads to an associated deficiency in the pericytes around the developing vessels of the Wnt-4-deficient kidney and apparently leads to a secondary defect in the maturation of the kidney vessels. Thus, besides being critical for regulating mesenchymal to epithelial transformation in the cortical region in nephrogenesis, Wnt-4 signaling regulates the fate of smooth muscle cells in the developing medullary region.

Essential Roles of Sall Family Genes in Kidney Development

We isolated a mouse Sall1, a mammalian homologue of the Drosophila region-specific homeotic gene spalt (sal), and found that mice deficient in Sall1 die in the perinatal period from kidney agenesis. Sall1 is expressed in the metanephric mesenchyme surrounding the ureteric bud, and the homozygous deletion of Sall1 results in an incomplete ureteric bud outgrowth. Therefore Sall1 is essential for ureteric bud invasion, the initial key step for metanephros development. We also set up an in vitro culture system, using NIH3T3 cells stably expressing Wnt4 as a feeder layer, to identify kidney progenitors in the metanephric mesenchyme. In this culture condition, a single renal progenitor in the mesenchyme forms colonies consisting of several types of epithelial cells that exist in glomeruli and renal tubules. We found that only cells strongly expressing Sall1 (Sall1-GFP(high) cells) form colonies and that they reconstitute a three-dimensional kidney structure in an organ culture setting. Thus our colony-forming assay, which identifies multipotent progenitors in the embryonic mouse kidney, can be used for examining mechanisms of renal progenitor differentiation.

Canonical Wnt signaling negatively regulates branching morphogenesis of the lung and lacrimal gland

Key gene families such as FGFs and BMPs are important mediators of branching morphogenesis. To understand whether Wnt genes, and in particular, the canonical Wnt signaling pathway also function in the branching process, we have used a combination of experimental and genetic gain and loss of function approaches to perturb the levels of canonical Wnt signaling in two arborized structures, the lung and the lacrimal gland. Here, we show that the addition of Wnt3a conditioned medium or LiCl strongly represses growth and proliferation of the lung and lacrimal gland, a result that was confirmed in vivo using a dominant stable mutation of beta-catenin conditionally expressed in the lacrimal gland epithelium. In agreement with these data, knockdown of Wnt signaling with beta-catenin morpholinos results in a greater number of branches and increased cell proliferation. In addition, we show that canonical Wnt signaling is able to modulate the levels of Fgf10 and suppress BMP-induced proliferation in the lacrimal gland. Thus, canonical Wnt signaling negatively regulates branching morphogenesis providing a balance to FGFs and BMPs which positively regulate this process. This multilayered control of growth and proliferation ensures that branched structures attain the morphology required to function efficiently.

Inactivation of FGF8 in early mesoderm reveals an essential role in kidney development

To bypass the essential gastrulation function of Fgf8 and study its role in lineages of the primitive streak, we have used a new mouse line,T-Cre, to generate mouse embryos with pan-mesodermal loss of Fgf8expression. Surprisingly, despite previous models in which Fgf8 has been assigned a pivotal role in segmentation/somite differentiation, Fgf8 is not required for these processes. However, mutant neonates display severe renal hypoplasia with deficient nephron formation. In mutant kidneys, aberrant cell death occurs within the metanephric mesenchyme (MM),particularly in the cortical nephrogenic zone, which provides the progenitors for recurring rounds of nephron formation. Prior to mutant morphological changes, Wnt4 and Lim1 expression, which is essential for nephrogenesis, is absent in MM. Furthermore, comparative analysis of Wnt4-null homozygotes reveals concomitant downregulation of Lim1 and diminished tubule formation. Our data support a model whereby FGF8 and WNT4 function in concert to induce the expression of Lim1 for MM survival and tubulogenesis.

Quantitative analysis of the nephron during human fetal kidney development

BACKGROUND

The development of human kidney is a complex process. The number, shape, size, and distribution of nephrons as functional units in a kidney, provide some important information about the organization of the kidney. The aim of this study was to extend the knowledge of the developing human kidney by studying nephrons in the kidney's cortex during gestation.

METHODS

Kidney tissue specimens of 32 human fetuses, the gestational age from IV lunar month (LM IV) to LM X, were analysed. Specimens were divided in ten groups based on gestational age. Stereological methods were used at the light microscopic level to estimate the volume densities of the corpuscular and tubular components of the nephron in the cortex of the developing human kidney.

RESULTS

Nephron polymorphism was the main characteristic of the human fetal kidney during development. In younger fetuses, just below the renal capsule, there was a wide nephrogenic zone. It contained the condensed mesenchyme and terminal ends of the ureteric bud. Nephrons, in the different stages of development, were located around the ureteric bud which branched in the cortical nephrogenic zone and induced nephrogenesis. More mature nephrons were located in the deeper part of the cortex, close to the juxta-medullary junction. During gestation, nephrogenesis continually advanced, and the number of nephrons increased. Glomeruli changed their size and shape, while the tubules changed their length and convolution. Renal cortex became wider and contained the more mature glomeruli and the more convoluted tubules. The volume density of the tubular component of the nephron increased continually from 10.53% (LM IVa) to 27.7% (LM X). Renal corpuscles changed their volume density irregularly during gestation, increasing from 13% (LM IVa) to 15.5% (LM IVb). During the increase of gestational age, the volume density of corpuscular component of the nephron decreased to 11.7% (LM VIII), then went on increasing until the end of the intrauterine development (LM X) when corpuscles occupied 16.73% of the cortical volume. The volume density of the developing nephrons (corpuscular and tubular portion) showed the significant positive correlation (r = 0.85; p<0.01) with gestational age.

CONCLUSION

The present study was one of few quantitative studies of the human developing nephron. Knowledge about the normal development of the human kidney should be important for the future medical practice.

Inversin, the gene product mutated in nephronophthisis type II, functions as a molecular switch between Wnt signaling pathways

Cystic renal diseases are caused by mutations of proteins that share a unique subcellular localization: the primary cilium of tubular epithelial cells. Mutations of the ciliary protein inversin cause nephronophthisis type II, an autosomal recessive cystic kidney disease characterized by extensive renal cysts, situs inversus and renal failure. Here we report that inversin acts as a molecular switch between different Wnt signaling cascades. Inversin inhibits the canonical Wnt pathway by targeting cytoplasmic dishevelled (Dsh or Dvl1) for degradation; concomitantly, it is required for convergent extension movements in gastrulating Xenopus laevis embryos and elongation of animal cap explants, both regulated by noncanonical Wnt signaling. In zebrafish, the structurally related switch molecule diversin ameliorates renal cysts caused by the depletion of inversin, implying that an inhibition of canonical Wnt signaling is required for normal renal development. Fluid flow increases inversin levels in ciliated tubular epithelial cells and seems to regulate this crucial switch between Wnt signaling pathways during renal development.

Cited1 Is a Bifunctional Transcriptional Cofactor That Regulates Early Nephronic Patterning

In a screen to identify factors that regulate the conversion of mesenchyme to epithelium during the early stages of nephrogenesis, it was found that the Smad4-interacting transcriptional cofactor, Cited1, is expressed in the condensed cap mesenchyme surrounding the tip of the ureteric bud (UB), is downregulated after differentiation into epithelia, and has the capacity to block UB branching and epithelial morphogenesis in cultured metanephroi. Cited1 represses Wnt/beta-catenin but activates Smad4-dependent transcription involved in TGF-beta and Bmp signaling. By modifying these pathways, Cited1 may coordinate cellular differentiation and survival signals that regulate nephronic patterning in the metanephros.

Signals which build a tubule

The phenomenon of branching morphogenesis is a fundamental process critical for development of several tubular organs including lung, mammary gland, and kidney. In the case of kidney, the ureteric bud (UB) that extends out from a pre-existing epithelial tube, the Wolffian duct, gives rise to the branched collecting duct system while the surrounding metanephric mesenchyme undergoes mesenchymal-epithelial transition to form the proximal parts of the nephron. These events are mediated by several soluble factors that act in a cooperative fashion either as pro or anti tubulogenic factors. Among the growing list of such molecules are the members of the FGF, TGF-beta, and Wnt families as well as GDNF, HGF, and EGF. Cells respond to these soluble factors by initiating signaling pathways that regulate cell proliferation, cell migration and cell morphogenesis. These signaling pathways are also regulated in parallel by cell-cell and cell-matrix interactions, leading to the complex events necessary for tubule formation. Recent in-vitro and in-vivo studies have begun to shed light on the overall regulation of this phenomenon while the specific subcellular mechanisms are only beginning to be understood. This review focuses on our understanding of the morphogenic responses that regulate in-vitro tubulogenesis and how they may help us to ultimately understand this process in vivo in the kidney.

WNT and beta-catenin signalling: diseases and therapies

WNT signalling has been studied primarily in developing embryos, in which cells respond to WNTs in a context-dependent manner through changes in survival and proliferation, cell fate and movement. But WNTs also have important functions in adults, and aberrant signalling by WNT pathways is linked to a range of diseases, most notably cancer. What is the full range of diseases that involve WNT pathways? Can inhibition of WNT signalling form the basis of an effective therapy for some cancers? Could activation of WNT signalling provide new therapies for other clinical conditions? Finally, on the basis of recent experiments, might WNTs normally participate in self-renewal, proliferation or differentiation of stem cells? If so, altering WNT signalling might be beneficial to the use of stem cells for therapeutic means.

The role of Xenopus frizzled-8 in pronephric development

Vertebrates use two or three forms of kidney successively during development and the nephric duct is essential for this succession of kidney induction. While transcripts of many Wnt ligands and Wnt receptor Frizzled genes have been localized in developing kidney, the relationship between Wnt signaling and nephric duct development remains unknown. This study investigated the role of Xenopus frizzled-8 (Xfz8) in pronephros development. Translational inhibition of Xfz8 caused a significant reduction in the staining of a duct-specific antibody, but did not affect the expression of early pronephric maker genes in the duct region. Defects in pronephric tubule branching were also observed following inhibition of Xfz8. Histological analysis revealed that the Xfz8-inhibited cells failed to form a normal epithelium structure. These results suggest that Xfz8 is involved in the process of normal epithelium formation in the developing pronephric duct and tubules after specification.