Elevated SMAD1/beta-catenin molecular complexes and renal medullary cystic dysplasia in ALK3 transgenic mice

Deciphering the cellular and molecular landscapes of Wnt/β-catenin signaling in mouse embryonic kidney development

Background

The Wnt/β-catenin signaling pathway is critical in kidney development, yet its specific effects on gene expression in different embryonic kidney cell types are not fully understood.

Methods

Wnt/β-catenin signaling was activated in mouse E12.5 kidneys in vitro using CHIR99021, with RNA sequencing performed afterward, and the results were compared to DMSO controls (dataset GSE131240). Differential gene expression in ureteric buds and cap mesenchyme following pathway activation (datasets GSE20325 and GSE39583) was analyzed. Single-cell RNA-seq data from the Mouse Cell Atlas was used to link differentially expressed genes (DEGs) with kidney cell types. β-catenin ChIP-seq data (GSE39837) identified direct transcriptional targets.

Results

Activation of Wnt/β-catenin signaling led to 917 significant DEGs, including the upregulation of Notum and Apcdd1 and the downregulation of Crym and Six2. These DEGs were involved in kidney development and immune response. Single-cell analysis identified 787 DEGs across nineteen cell subtypes, with Macrophage_Apoe high cells showing the most pronounced enrichment of Wnt/β-catenin-activated genes. Gene expression profiles in ureteric buds and cap mesenchyme differed significantly upon β-catenin manipulation, with cap mesenchyme showing a unique set of DEGs. Analysis of β-catenin ChIP-seq data revealed 221 potential direct targets, including Dpp6 and Fgf12.

Conclusion

This study maps the complex gene expression driven by Wnt/β-catenin signaling in embryonic kidney cell types. The identified DEGs and β-catenin targets elucidate the molecular details of kidney development and the pathway's role in immune processes, providing a foundation for further research into Wnt/β-catenin signaling in kidney development and disease.

Enhanced reno-protective effects of CHIR99021 modified mesenchymal stem cells against rat acute kidney injury model

Introduction

Mesenchymal stem cells of human umbilical cord origin (hucMSCs) appear to be an attractive candidate for cell-based therapies. However, their efficacy requires improvement as poor survival and specific homing to the site of injury are the major barriers to their effective implementation in cell therapy. As Wnt signaling pathway is involved in the development and repair of organs, we adopted a preconditioning strategy of stem cells by using CHIR99021 compound (a Wnt pathway agonist) to potentiate hucMSCs beneficial effects and circumvent their therapeutic limitations.

Methods

We treated hucMSCs with 5 µM of CHIR99021 and evaluated the expression levels of genes involved in different biological processes through qRT-PCR. Subsequently, we examined the effectiveness of preconditioned cells (CHIR99021-hucMSCs) in a cisplatin-induced rat acute kidney injury model. Amelioration in tissue injury was evaluated by histopathology, immunohistochemistry and renal functional assessment.

Results

In treated groups, we observed preserved renal tissue architecture in terms of lesser epithelial cells necrosis (P ≤ 0.001) and cast formation ( ≤ 0.05). Accelerated proliferation of injured tubular cells (P ≤ 0.001) and low serum creatinine values (P ≤ 0.01) were observed in preconditioned hucMSCs group compared to untreated AKI rats. In addition, administration of preconditioned hucMSCs in kidney injury model offered better restoration of tubular cell membrane β-catenin molecules. Our findings showed that CHIR99021-modified hucMSCs may exhibit better capacity for cell migration and proliferation.

Conclusion

The results showed that preconditioning of stem cells with Wnt pathway activators could provide advanced benefits for organ repair, which may contribute to design a more effective therapeutic approach for renal regeneration.

Androgens downregulate BMP2 impairing the inductive role of dermal papilla cells on hair follicle stem cells differentiation

Hair follicle cyclical regeneration is regulated by epithelial-mesenchymal interactions. During androgenetic alopecia (AGA), hair follicle stem cells (HFSC) differentiation is impaired by deregulation of dermal papilla cells (DPC) secreted factors. We analyzed androgen influence on BMPs expression in DPC and their effect on HFSC differentiation to hair lineage. Androgens downregulated BMP2 and BMP4 in DPC spheroids. Addition of BMP2 restored alkaline phosphatase activity, marker of hair-inductivity in DPC, and DPC-induced HFSC differentiation, both inhibited by androgens. Concomitantly, in differentiating HFSC, an upregulation of BMPRIa and BMPRII receptors and nuclear β-catenin accumulation, indicative of Wnt/β-catenin pathway activation, were detected. Our results present BMP2 as an androgen-downregulated paracrine factor that contributes to DPC inductivity and favors DPC-induced HFSC differentiation to hair lineage, possibly through a crosstalk with Wnt/β-catenin pathway. A comprehensive understanding of androgen-deregulated DPC factors and their effects on differentiating HFSC would help to improve treatments for AGA.

Quercetin treatment reduces the severity of renal dysplasia in a beta-catenin dependent manner

Renal dysplasia, the major cause of childhood renal failure, is characterized by defective branching morphogenesis and nephrogenesis. Beta-catenin, a transcription factor and cell adhesion molecule, is markedly increased in the nucleus of kidney cells in human renal dysplasia and contributes to its pathogenesis by altering target genes that are essential for kidney development. Quercetin, a naturally occurring flavonoid, reduces nuclear beta-catenin levels and reduces beta-catenin transcriptional activity. In this study, we utilized wild type and dysplastic mouse kidney organ explants to determine if quercetin reduces beta-catenin activity during kidney development and whether it improves the severity of renal dysplasia. In wild type kidney explants, quercetin treatment resulted in abnormal branching morphogenesis and nephrogenesis in a dose dependent manner. In wild type embryonic kidneys, quercetin reduced nuclear beta-catenin expression and decreased expression of beta-catenin target genes Pax2, Six2, and Gdnf, which are essential for kidney development. Our RD^B mouse model of renal dysplasia recapitulates the overexpression of beta-catenin and histopathological changes observed in human renal dysplasia. RD^B kidneys treated with quercetin resulted in improvements in the overall histopathology, tissue organization, ureteric branching morphogenesis, and nephrogenesis. Quercetin treatment also resulted in reduced nuclear beta-catenin and reduced Pax2 expression. These improvements were associated with the proper organization of vimentin, NCAM, and E-cadherin, and a 45% increase in the number of developing and maturing nephrons. Further, our results show that in human renal dysplasia, beta-catenin, vimentin, and e-cadherin also have abnormal expression patterns. Taken together, these data demonstrate that quercetin treatment reduces nuclear beta-catenin and this is associated with improved epithelial organization of developing nephrons, resulting in increased developing nephrons and a partial rescue of renal dysplasia.

Polycystic Kidney Disease and Renal Fibrosis

Polycystic kidney disease (PKD) is a common genetic disorder characterized by formations of numerous cysts in kidneys and most caused by PKD1 or PKD2 mutations in autosomal dominant polycystic kidney disease (ADPKD). The interstitial inflammation and fibrosis is one of the major pathological changes in polycystic kidney tissues with an accumulation of inflammatory cells, chemokines, and cytokines. The immune response is observed across different stages and occurs prior to or coincident with cyst formation in ADPKD. Evidence for inflammation as an important contributor to cyst growth and fibrosis includes increased interstitial macrophages, upregulated expressions of pro-inflammatory cytokines, activated complement system, and activated pathways including NF-κB and JAK-STAT signaling in polycystic kidney tissues. Inflammatory cells are responsible for overproduction of several pro-fibrotic growth factors which promote renal fibrosis in ADPKD. These growth factors trigger epithelial mesenchymal transition and myofibroblast/fibrocyte activation, which stimulate the expansion of extracellular matrix (ECM) including collagen I, III, IV, V, and fibronectin, leading to renal fibrosis and reduced renal function. Besides, there are imbalanced ECM turnover regulators which lead to the increased ECM production and inadequate degradation in polycystic kidney tissues. Several fibrosis associated signaling pathways, such as TGFβ-SMAD, Wnt, and periostin-integrin-linked kinase are also activated in polycystic kidney tissues. Although the effective anti-fibrotic treatments are limited at the present time, slowing the cyst expansion and fibrosis development is very important for prolonging life span and improving the palliative care of ADPKD patients. The inhibition of pro-fibrotic cytokines involved in fibrosis might be a new therapeutic strategy for ADPKD in the future.

Three Optimized Methods for In Situ Quantification of Progenitor Cell Proliferation in Embryonic Kidneys Using BrdU, EdU, and PCNA

Background:

Nephron progenitor cells derived from the metanephric mesenchyme undergo a complex balance of self-renewal and differentiation throughout kidney development to give rise to the mature nephron. Cell proliferation is an important index of progenitor population dynamics. However, accurate and reproducible in situ quantification of cell proliferation within progenitor populations can be technically difficult to achieve due to the complexity and harsh tissue treatment required of certain protocols.

Objective:

To optimize and compare the performance of the 3 most accurate S phase–specific labeling methods used for in situ detection and quantification of nephron progenitor and ureteric bud cell proliferation in the developing kidney, namely, 5-bromo-2’-deoxyuridine (BrdU), 5-ethynyl-2’-deoxyuridine (EdU), and proliferating cell nuclear antigen (PCNA).

Methods:

Protocols for BrdU, EdU, and PCNA were optimized for fluorescence labeling on paraformaldehyde-fixed, paraffin-embedded mouse kidney tissue sections, with co-labeling of nephron progenitor cells and ureteric bud with Six2 and E-cadherin antibodies, respectively. Image processing and analysis, including quantification of proliferating cells, were carried out using free ImageJ software.

Results:

All 3 methods detect similar ratios of nephron progenitor and ureteric bud proliferating cells. The BrdU staining protocol is the lengthiest and most complex protocol to perform, requires tissue denaturation, and is most subject to interexperimental signal variability. In contrast, bound PCNA and EdU protocols are relatively more straightforward, consistently yield clear results, and far more easily lend themselves to co-staining; however, the bound PCNA protocol requires substantive additional postexperimental analysis to distinguish the punctate nuclear PCNA staining pattern characteristic of proliferating cells.

Conclusions:

All 3 markers exhibit distinct advantages and disadvantages in quantifying cell proliferation in kidney progenitor populations, with EdU and PCNA protocols being favored due to greater technical ease and reproducibility of results associated with these methods.

TGF-β Family Signaling in Ductal Differentiation and Branching Morphogenesis

Epithelial cells contribute to the development of various vital organs by generating tubular and/or glandular architectures. The fully developed forms of ductal organs depend on processes of branching morphogenesis, whereby frequency, total number, and complexity of the branching tissue define the final architecture in the organ. Some ductal tissues, like the mammary gland during pregnancy and lactation, disintegrate and regenerate through periodic cycles. Differentiation of branched epithelia is driven by antagonistic actions of parallel growth factor systems that mediate epithelial-mesenchymal communication. Transforming growth factor-β (TGF-β) family members and their extracellular antagonists are prominently involved in both normal and disease-associated (e.g., malignant or fibrotic) ductal tissue patterning. Here, we discuss collective knowledge that permeates the roles of TGF-β family members in the control of the ductal tissues in the vertebrate body.

Novel Insights into the Pathogenesis of Monogenic Congenital Anomalies of the Kidney and Urinary Tract

Congenital anomalies of the kidneys and urinary tract (CAKUT) comprise a large spectrum of congenital malformations ranging from severe manifestations, such as renal agenesis, to potentially milder conditions, such as vesicoureteral reflux. CAKUT causes approximately 40% of ESRD that manifests within the first three decades of life. Several lines of evidence indicate that CAKUT is often caused by recessive or dominant mutations in single (monogenic) genes. To date, approximately 40 monogenic genes are known to cause CAKUT if mutated, explaining 5%-20% of patients. However, hundreds of different monogenic CAKUT genes probably exist. The discovery of novel CAKUT-causing genes remains challenging because of this pronounced heterogeneity, variable expressivity, and incomplete penetrance. We here give an overview of known genetic causes for human CAKUT and shed light on distinct renal morphogenetic pathways that were identified as relevant for CAKUT in mice and humans.

Inflammation and Fibrosis in Polycystic Kidney Disease

Polycystic kidney disease (PKD) is a commonly inherited disorder characterized by cyst formation and fibrosis (Wilson, N Engl J Med 350:151-164, 2004) and is caused by mutations in cilia or cilia-related proteins, such as polycystin 1 or 2 (Oh and Katsanis, Development 139:443-448, 2012; Kotsis et al., Nephrol Dial Transplant 28:518-526, 2013). A major pathological feature of PKD is the development of interstitial inflammation and fibrosis with an associated accumulation of inflammatory cells (Grantham, N Engl J Med 359:1477-1485, 2008; Zeier et al., Kidney Int 42:1259-1265, 1992; Ibrahim, Sci World J 7:1757-1767, 2007). It is unclear whether inflammation is a driving force for cyst formation or a consequence of the pathology (Ta et al., Nephrology 18:317-330, 2013) as in some murine models cysts are present prior to the increase in inflammatory cells (Phillips et al., Kidney Blood Press Res 30:129-144, 2007; Takahashi et al., J Am Soc Nephrol JASN 1:980-989, 1991), while in other models the increase in inflammatory cells is present prior to or coincident with cyst initiation (Cowley et al., Kidney Int 43:522-534, 1993, Kidney Int 60:2087-2096, 2001). Additional support for inflammation as an important contributor to cystic kidney disease is the increased expression of many pro-inflammatory cytokines in murine models and human patients with cystic kidney disease (Karihaloo et al., J Am Soc Nephrol JASN 22:1809-1814, 2011; Swenson-Fields et al., Kidney Int, 2013; Li et al., Nat Med 14:863-868, 2008a). Based on these data, an emerging model in the field is that disruption of primary cilia on tubule epithelial cells leads to abnormal cytokine cross talk between the epithelium and the inflammatory cells contributing to cyst growth and fibrosis (Ta et al., Nephrology 18:317-330, 2013). These cytokines are produced by interstitial fibroblasts, inflammatory cells, and tubule epithelial cells and activate multiple pathways including the JAK-STAT and NF-κB signaling (Qin et al., J Am Soc Nephrol JASN 23:1309-1318, 2012; Park et al., Am J Nephrol 32:169-178, 2010; Bhunia et al., Cell 109:157-168, 2002). Indeed, inflammatory cells are responsible for producing several of the pro-fibrotic growth factors observed in PKD patients with fibrosis (Nakamura et al., Am J Nephrol 20:32-36, 2000; Wilson et al., J Cell Physiol 150:360-369, 1992; Song et al., Hum Mol Genet 18:2328-2343, 2009; Schieren et al., Nephrol Dial Transplant 21:1816-1824, 2006). These growth factors trigger epithelial cell proliferation and myofibroblast activation that stimulate the production of extracellular matrix (ECM) genes including collagen types 1 and 3 and fibronectin, leading to reduced glomerular function with approximately 50% of ADPKD patients progressing to end-stage renal disease (ESRD). Therefore, treatments designed to reduce inflammation and slow the rate of fibrosis are becoming important targets that hold promise to improve patient life span and quality of life. In fact, recent studies in several PKD mouse models indicate that depletion of macrophages reduces cyst severity. In this chapter, we review the potential mechanisms of interstitial inflammation in PKD with a focus on ADPKD and discuss the role of interstitial inflammation in progression to fibrosis and ESRD.

Multicystic degeneration of the Cowper's gland in a Large White boar

The present report describes a case of multicystic degeneration of the Cowper's gland in a 1.3-year-old purebred Large White intact boar with reduced fertility in Switzerland. Based on the case history, a general physical examination, an andrological investigation and a transrectal ultrasonography combined with a subsequent thorough pathological examination including histology a multicystic degeneration of the Cowper's gland were diagnosed. The case indicates that cystic degeneration of the bulbourethral gland should be contemplated in the differential diagnoses of andrological disorders even though it has not been described in pigs so far. While selecting breeding boars, a morphological check of the bulbourethral gland can be performed, since degeneration of the gland would potentially have an impact on future fertility.

BMP-7 Signaling and its Critical Roles in Kidney Development, the Responses to Renal Injury, and Chronic Kidney Disease

Chronic kidney disease (CKD) is a significant health problem that most commonly results from congenital abnormalities in children and chronic renal injury in adults. The therapeutic potential of BMP-7 was first recognized nearly two decades ago with studies demonstrating its requirement for kidney development and ability to inhibit the pathogenesis of renal injury in models of CKD. Since this time, our understanding of CKD has advanced considerably and treatment strategies have evolved with the identification of many additional signaling pathways, cell types, and pathologic processes that contribute to disease progression. The purpose of this review is to revisit the seminal studies that initially established the importance of BMP-7, highlight recent advances in BMP-7 research, and then integrate this knowledge with current research paradigms. We will provide an overview of the evolutionarily conserved roles of BMP proteins and the features that allow BMP signaling pathways to function as critical signaling nodes for controlling biological processes, including those related to CKD. We will discuss the multifaceted functions of BMP-7 during kidney development and the potential for alterations in BMP-7 signaling to result in congenital abnormalities and pediatric kidney disease. We will summarize the renal protective effects of recombinant BMP-7 in experimental models of CKD and then propose a model to describe the potential physiological role of endogenous BMP-7 in the innate repair mechanisms of the kidneys that respond to renal injury. Finally, we will highlight emerging clinical approaches for applying our knowledge of BMP-7 toward improving the treatment of patients with CKD.

Stromal β-catenin overexpression contributes to the pathogenesis of renal dysplasia

Renal dysplasia, the leading cause of renal failure in children, is characterized by disrupted branching of the collecting ducts and primitive tubules, with an expansion of the stroma, yet a role for the renal stroma in the genesis of renal dysplasia is not known. Here, we demonstrate that expression of β-catenin, a key transcriptional co-activator in renal development, is markedly increased in the expanded stroma in human dysplastic tissue. To understand its contribution to the genesis of renal dysplasia, we generated a mouse model that overexpresses β-catenin specifically in stromal progenitors, termed β-cat(GOF-S) . Histopathological analysis of β-cat(GOF) (-S) mice revealed a marked expansion of fibroblast cells surrounding primitive ducts and tubules, similar to defects observed in human dysplastic kidneys. Characterization of the renal stroma in β-cat(GOF) (-S) mice revealed altered stromal cell differentiation in the expanded renal stroma demonstrating that this is not renal stroma but instead a population of stroma-like cells. These cells overexpress ectopic Wnt4 and Bmp4, factors necessary for endothelial cell migration and blood vessel formation. Characterization of the renal vasculature demonstrated disrupted endothelial cell migration, organization, and vascular morphogenesis in β-cat(GOF) (-S) mice. Analysis of human dysplastic tissue demonstrated a remarkably similar phenotype to that observed in our mouse model, including altered stromal cell differentiation, ectopic Wnt4 expression in the stroma-like cells, and disrupted endothelial cell migration and vessel formation. Our findings demonstrate that the overexpression of β-catenin in stromal cells is sufficient to cause renal dysplasia. Further, the pathogenesis of renal dysplasia is one of disrupted stromal differentiation and vascular morphogenesis. Taken together, this study demonstrates for the first time the contribution of stromal β-catenin overexpression to the genesis of renal dysplasia. Copyright © 2016 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Mutant GDF5 enhances ameloblast differentiation via accelerated BMP2-induced Smad1/5/8 phosphorylation

Bone morphogenetic proteins (BMPs) regulate hard tissue formation, including bone and tooth. Growth differentiation factor 5 (GDF5), a known BMP, is expressed in cartilage and regulates chondrogenesis, and mutations have been shown to cause osteoarthritis. Notably, GDF5 is also expressed in periodontal ligament tissue; however, its role during tooth development is unclear. Here, we used cell culture and in vivo analyses to determine the role of GDF5 during tooth development. GDF5 and its associated BMP receptors are expressed at the protein and mRNA levels during postnatal tooth development, particularly at a stage associated with enamel formation. Furthermore, whereas BMP2 was observed to induce evidently the differentiation of enamel-forming ameloblasts, excess GDF5 induce mildly this differentiation. A mouse model harbouring a mutation in GDF5 (W408R) showed enhanced enamel formation in both the incisors and molars, but not in the tooth roots. Overexpression of the W408R GDF5 mutant protein was shown to induce BMP2-mediated mRNA expression of enamel matrix proteins and downstream phosphorylation of Smad1/5/8. These results suggest that mutant GDF5 enhances ameloblast differentiation via accelerated BMP2-signalling.

The Good and Bad of β-Catenin in Kidney Development and Renal Dysplasia

Congenital renal malformations are a major cause of childhood and adult onset chronic kidney disease. Identifying the etiology of these renal defects is often challenging since disruptions in the processes that drive kidney development can result from disruptions in environmental, genetic, or epigenetic cues. β-catenin is an intracellular molecule involved in cell adhesion, cell signaling, and regulation of gene transcription. It plays essential roles in kidney development and in the pathogenesis of renal dysplasia. Here, we review the function of β-catenin during kidney development and in the genesis of renal dysplasia.

The ureteric bud epithelium: morphogenesis and roles in metanephric kidney patterning

The mammalian metanephric kidney is composed of two epithelial components, the collecting duct system and the nephron epithelium, that differentiate from two different tissues -the ureteric bud epithelium and the nephron progenitors, respectively-of intermediate mesoderm origin. The collecting duct system is generated through reiterative ureteric bud branching morphogenesis, whereas the nephron epithelium is formed in a process termed nephrogenesis, which is initiated with the mesenchymal-epithelial transition of the nephron progenitors. Ureteric bud branching morphogenesis is regulated by nephron progenitors, and in return, the ureteric bud epithelium regulates nephrogenesis. The metanephric kidney is physiologically divided along the corticomedullary axis into subcompartments that are enriched with specific segments of these two epithelial structures. Here, we provide an overview of the major molecular and cellular processes underlying the morphogenesis and patterning of the ureteric bud epithelium and its roles in the cortico-medullary patterning of the metanephric kidney.

β-Catenin overexpression in the metanephric mesenchyme leads to renal dysplasia genesis via cell-autonomous and non-cell-autonomous mechanisms

Renal dysplasia, a developmental disorder characterized by defective ureteric branching morphogenesis and nephrogenesis, ranks as one of the major causes of renal failure among the pediatric population. Herein, we demonstrate that the levels of activated β-catenin are elevated in the nuclei of ureteric, stromal, and mesenchymal cells within dysplastic human kidney tissue. By using a conditional mouse model of mesenchymal β-catenin overexpression, we identify two novel signaling pathways mediated by β-catenin in the development of renal dysplasia. First, the overexpression of β-catenin within the metanephric mesenchyme leads to ectopic and disorganized branching morphogenesis caused by β-catenin directly binding Tcf/lef consensus binding sites in the Gdnf promoter and up-regulating Gdnf transcription. Second, β-catenin overexpression in the metanephric mesenchyme leads to elevated levels of transcriptionally active β-catenin in the ureteric epithelium. Interestingly, this increase of β-catenin-mediated transcription results from a novel Ret/β-catenin signaling pathway. Consistent with these findings, analysis of human dysplastic renal tissue demonstrates that undifferentiated mesenchymal cells expressing high levels of β-catenin also express increased GDNF. Furthermore, dysplastic ureteric tubules that were surrounded by high levels of GDNF also exhibited increased levels of activated β-catenin. Together, these data support a model in which the elevation of β-catenin in the metanephric mesenchyme results in cell-autonomous and non-cell-autonomous events that lead to the genesis of renal dysplasia.

Kidney branching morphogenesis under the control of a ligand-receptor-based Turing mechanism

The main signalling proteins that control early kidney branching have been defined. Yet the underlying mechanism is still elusive. We have previously shown that a Schnakenberg-type Turing mechanism can recapitulate the branching and protein expression patterns in wild-type and mutant lungs, but it is unclear whether this mechanism would extend to other branched organs that are regulated by other proteins. Here, we show that the glial cell line-derived neurotrophic factor-RET regulatory interaction gives rise to a Schnakenberg-type Turing model that reproduces the observed budding of the ureteric bud from the Wolffian duct, its invasion into the mesenchyme and the observed branching pattern. The model also recapitulates all relevant protein expression patterns in wild-type and mutant mice. The lung and kidney models are both based on a particular receptor-ligand interaction and require (1) cooperative binding of ligand and receptor, (2) a lower diffusion coefficient for the receptor than for the ligand and (3) an increase in the receptor concentration in response to receptor-ligand binding (by enhanced transcription, more recycling or similar). These conditions are met also by other receptor-ligand systems. We propose that ligand-receptor-based Turing patterns represent a general mechanism to control branching morphogenesis and other developmental processes.

Association of BMPR1A polymorphism, but not BMP4, with kidney size in full-term newborns

BACKGROUND

A correlation between renal mass and nephron number in newborns allows the use of total kidney volume at birth as a surrogate for congenital nephron number. As the bone morphogenetic protein type 4 (BMP4), and its receptor type 1A (BMPR1A, ALK3), play an important role in renal development, we hypothesized that common, functional polymorphisms in their genes might be responsible for variation in kidney size among healthy individuals.

METHODS

We recruited 179 healthy full-term newborns born to healthy women. Kidney volume was measured sonographically. Total kidney volume (TKV) was calculated as the sum of left and right kidneys, and normalized for body surface area (TKV/BSA). Genomic DNA was extracted from umbilical cord blood leukocytes, and c.455T > C (rs17563) BMP4 and c.67 + 5659A > T (rs7922846) BMPR1A genotypes were identified by PCR-RFLP.

RESULTS

TKV/BSA in newborns carrying at least one A BMPR1A allele (AA + AT) was significantly reduced by approximately 13 % as compared with TT homozygous newborns (106.7 ± 21.5 ml/m(2) vs. 122.7 ± 43.8 ml/m(2), p < 0.02). No significant differences in TKV/BSA were found among newborns with different BMP4 genotypes.

CONCLUSIONS

Results suggest that rs7922846 BMPR1A polymorphism may account for subtle variation in kidney size at birth, reflecting congenital nephron endowment.

Wnt signaling in kidney development and disease

The Wnt gene family, which encodes secreted growth and differentiation factors, has been implicated in kidney organogenesis. The Wnts control both ureteric bud development and signaling, but they also serve as inductive factors to regulate nephrogenesis in the mesenchcymal cells. Several of the Wnt genes are expressed in the developing kidney, and gene knock-out studies have revealed specific developmental functions for these. Consistent with this, changes in Wnt ligands and pathway components are associated with many kidney diseases, including kidney cancers, renal fibrosis, cystic kidney diseases, acute renal failure, diabetic nephropathy and ischaemic injury. It is these associations of the Wnt signaling system with kidney development and kidney diseases that form to topic of this review.

Absence of canonical Smad signaling in ureteral and bladder mesenchyme causes ureteropelvic junction obstruction

Obstruction of the ureteropelvic junction (UPJ) is a common congenital anomaly frequently associated with ureteral defects. To study the molecular mechanisms that modulate ureteral development, we inactivated Smad4, the common Smad critical for transcriptional responses to TGF-β and Bmp signaling, in the ureteral and bladder mesenchyme during embryogenesis. Loss of canonical Smad signaling in these tissues caused bilateral UPJ obstruction and severe hydronephrosis beginning at embryonic day 17.5. Despite a reduction in quantity of ureteral smooth muscle, differentiation proceeded without Smad4, producing a less severe phenotype than Bmp4 mutants; this finding suggests that at least some Bmp4 functions in ureteral smooth muscle may be Smad-independent. The absence of canonical Smad signaling in the ureteral mesenchyme, but not in the urothelium itself, led to urothelial disorganization, highlighting the importance of mesenchymal support for epithelial development. Transcript profiling revealed altered expression in known Bmp targets, smooth muscle-specific genes, and extracellular matrix-related genes in mutant ureters before the onset of hydronephrosis. Expression of the Bmp target Id2 was significantly lower in Smad4 mutants, consistent with the observation that Id2 mutants develop UPJ obstruction. In summary, Smad4 deficiency reduces the number and contractility of ureteral smooth muscle cells, leading to abnormal pyeloureteral peristalsis and functional obstruction. The subsequent bending and luminal constriction of the ureter at the UPJ marks the transition from a functional obstruction to a more intractable physical obstruction, suggesting that early intervention for this disease may prevent more irreversible damage to the urinary tract.

A secreted BMP antagonist, Cer1, fine tunes the spatial organization of the ureteric bud tree during mouse kidney development

The epithelial ureteric bud is critical for mammalian kidney development as it generates the ureter and the collecting duct system that induces nephrogenesis in dicrete locations in the kidney mesenchyme during its emergence. We show that a secreted Bmp antagonist Cerberus homologue (Cer1) fine tunes the organization of the ureteric tree during organogenesis in the mouse embryo. Both enhanced ureteric expression of Cer1 and Cer1 knock out enlarge kidney size, and these changes are associated with an altered three-dimensional structure of the ureteric tree as revealed by optical projection tomography. Enhanced Cer1 expression changes the ureteric bud branching programme so that more trifid and lateral branches rather than bifid ones develop, as seen in time-lapse organ culture. These changes may be the reasons for the modified spatial arrangement of the ureteric tree in the kidneys of Cer1+ embryos. Cer1 gain of function is associated with moderately elevated expression of Gdnf and Wnt11, which is also induced in the case of Cer1 deficiency, where Bmp4 expression is reduced, indicating the dependence of Bmp expression on Cer1. Cer1 binds at least Bmp2/4 and antagonizes Bmp signalling in cell culture. In line with this, supplementation of Bmp4 restored the ureteric bud tip number, which was reduced by Cer1+ to bring it closer to the normal, consistent with models suggesting that Bmp signalling inhibits ureteric bud development. Genetic reduction of Wnt11 inhibited the Cer1-stimulated kidney development, but Cer1 did not influence Wnt11 signalling in cell culture, although it did inhibit the Wnt3a-induced canonical Top Flash reporter to some extent. We conclude that Cer1 fine tunes the spatial organization of the ureteric tree by coordinating the activities of the growth-promoting ureteric bud signals Gndf and Wnt11 via Bmp-mediated antagonism and to some degree via the canonical Wnt signalling involved in branching.

Epithelial Bmp (Bone morphogenetic protein) signaling for bulbourethral gland development: a mouse model for congenital cystic dilation

The bulbourethral gland (BUG) is a male-specific organ, which secretes part of the semen fluid. As the BUG is located in the deep pelvic floor, its developmental process is still unclear. Bone morphogenetic protein (Bmp) signaling plays pivotal roles in various organs. However, the function of Bmp signaling for BUG development is still unclear. The present study aimed to elucidate the role of Bmp signaling in the development of the BUG. We observed the prominent nuclear accumulation of phosphorylated (p) SMAD1/5/8, the downstream molecules of Bmp signaling, during BUG epithelial development. These results suggest that Bmp signaling contributes to BUG development. Bmp receptor1a (Bmpr1a) is known as the major type 1 signal transducer in some organogeneses. To analyze the Bmp signaling function for BUG development, we examined epithelial cell-specific Bmpr1a gene conditional mutant mice utilizing the tamoxifen-inducible Cre recombinase system. We observed cystic dilation and epithelial hyperplasia of the BUG in the Bmpr1a conditional knockout mice. The mutant cystic BUG specimens also showed inflammatory lesions. These BUG abnormalities resembled some of the BUG malformations observed in human congenital syndromes. The current study suggests that Bmp signaling possesses an essential role in BUG development and homeostasis. This would be the first report showing that the mutation of the Bmpr1a gene in the BUG epithelia phenocopied some abnormalities of human congenital syndromes affecting the BUG duct.

Alk3 controls nephron number and androgen production via lineage-specific effects in intermediate mesoderm

The mammalian kidney and male reproductive system are both derived from the intermediate mesoderm. The spatial and temporal expression of bone morphogenetic protein (BMP) 2 and BMP4 and their cognate receptor, activin like kinase 3 (ALK3), suggests a functional role for BMP-ALK3 signaling during formation of intermediate mesoderm-derivative organs. Here, we define cell autonomous functions for Alk3 in the kidney and male gonad in mice with CRE-mediated Alk3 inactivation targeted to intermediate mesoderm progenitors (Alk3(IMP null)). Alk3-deficient mice exhibit simple renal hypoplasia characterized by decreases in both kidney size and nephron number but normal tissue architecture. These defects are preceded by a decreased contribution of Alk3-deleted cells to the metanephric blastema and reduced expression of Osr1 and SIX2, which mark nephron progenitor cells. Mutant mice are also characterized by defects in intermediate mesoderm-derived genital tissues with fewer mesonephric tubules and testicular Leydig cells, epithelial vacuolization in the postnatal corpus epididymis, and decreased serum testosterone levels and reduced fertility. Analysis of ALK3-dependent signaling effectors revealed lineage-specific reduction of phospho-p38 MAPK in metanephric mesenchyme and phospho-SMAD1/5/8 in the testis. Together, these results demonstrate a requirement for Alk3 in distinct progenitor cell populations derived from the intermediate mesoderm.

β-catenin causes renal dysplasia via upregulation of Tgfβ2 and Dkk1

Renal dysplasia, defined by defective ureteric branching morphogenesis and nephrogenesis, is the major cause of renal failure in infants and children. Here, we define a pathogenic role for a β-catenin-activated genetic pathway in murine renal dysplasia. Stabilization of β-catenin in the ureteric cell lineage before the onset of kidney development increased β-catenin levels and caused renal aplasia or severe hypodysplasia. Analysis of gene expression in the dysplastic tissue identified downregulation of genes required for ureteric branching and upregulation of Tgfβ2 and Dkk1. Treatment of wild-type kidney explants with TGFβ2 or DKK1 generated morphogenetic phenotypes strikingly similar to those observed in mutant kidney tissue. Stabilization of β-catenin after the onset of kidney development also caused dysplasia and upregulation of Tgfβ2 and Dkk1 in the epithelium. Together, these results demonstrate that elevation of β-catenin levels during kidney development causes dysplasia.

Elevated TGFbeta-Smad signalling in experimental Pkd1 models and human patients with polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is a common inherited renal disease characterized by many fluid-filled cysts and interstitial fibrosis in the kidneys, leading to chronic renal failure. During cystogenesis the renal tubules undergo extensive structural alterations that are accompanied by altered cellular signalling, directly and/or indirectly regulated by the PKD1 and PKD2 proteins. Since transforming growth factor (TGF)-beta signalling modulates cell proliferation, differentiation, apoptosis, adhesion and migration of various cell types, we studied the activation of this signalling pathway in Pkd1-mutant mouse models at different stages of the disease. Therefore, we analysed expression of the TGFbeta-Smad signalling pathway and its target genes in different Pkd1 mutant mouse models in various stages of polycystic disease. Nuclear accumulation of P-Smad2 in cyst lining epithelial cells was not observed in the initiation phase but was observed at mild and more advanced stages of PKD. This coincides with mild fibrosis and increased mRNA levels of TGFbeta target genes, such as fibronectin, collagen type I, plasminogen activator inhibitor 1 and matrix metalloproteinase-2. At this stage many interstitial fibroblasts were found around cysts, which also showed nuclear localization for P-Smad2. However, bone morphogenetic protein (BMP) signalling, which can antagonize TGFbeta signalling, is not affected, since nuclear expression of P-Smad1/5/8 and expression of the BMP target gene, inhibitor of DNA binding/differential-1 (ID-1) is not altered compared to wild-type controls. Also, human kidneys with progressive ADPKD showed increased nuclear localization of P-Smad2, while in general expression of P-Smad1/5/8 was weak. These results exclude TGFbeta signalling at the initiation of cystogenesis, but indicate an important role during cyst progression and in fibrogenesis of progressive ADPKD.

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.

BMP-9-induced osteogenic differentiation of mesenchymal progenitors requires functional canonical Wnt/beta-catenin signalling

Bone morphogenetic protein 9 (BMP-9) is a member of the transforming growth factor (TGF)-beta/BMP superfamily, and we have demonstrated that it is one of the most potent BMPs to induce osteoblast differentiation of mesenchymal stem cells (MSCs). Here, we sought to investigate if canonical Wnt/beta-catenin signalling plays an important role in BMP-9-induced osteogenic differentiation of MSCs. Wnt3A and BMP-9 enhanced each other's ability to induce alkaline phosphatase (ALP) in MSCs and mouse embryonic fibroblasts (MEFs). Wnt antagonist FrzB was shown to inhibit BMP-9-induced ALP activity more effectively than Dkk1, whereas a secreted form of LPR-5 or low-density lipoprotein receptor-related protein (LRP)-6 exerted no inhibitory effect on BMP-9-induced ALP activity. beta-Catenin knockdown in MSCs and MEFs diminished BMP-9-induced ALP activity, and led to a decrease in BMP-9-induced osteocalcin reporter activity and BMP-9-induced expression of late osteogenic markers. Furthermore, beta-catenin knockdown or FrzB overexpression inhibited BMP-9-induced mineralization in vitro and ectopic bone formation in vivo, resulting in immature osteogenesis and the formation of chondrogenic matrix. Chromatin immunoprecipitation (ChIP) analysis indicated that BMP-9 induced recruitment of both Runx2 and beta-catenin to the osteocalcin promoter. Thus, we have demonstrated that canonical Wnt signalling, possibly through interactions between beta-catenin and Runx2, plays an important role in BMP-9-induced osteogenic differentiation of MSCs.

Distinct roles and regulations for HoxD genes in metanephric kidney development

Hox genes encode homeodomain-containing proteins that control embryonic development in multiple contexts. Up to 30 Hox genes, distributed among all four clusters, are expressed during mammalian kidney morphogenesis, but functional redundancy between them has made a detailed functional account difficult to achieve. We have investigated the role of the HoxD cluster through comparative molecular embryological analysis of a set of mouse strains carrying targeted genomic rearrangements such as deletions, duplications, and inversions. This analysis allowed us to uncover and genetically dissect the complex role of the HoxD cluster. Regulation of metanephric mesenchyme-ureteric bud interactions and maintenance of structural integrity of tubular epithelia are differentially controlled by some Hoxd genes during renal development, consistent with their specific expression profiles. We also provide evidence for a kidney-specific form of colinearity that underlies the differential expression of two distinct sets of genes located on both sides and overlapping at the Hoxd9 locus. These insights further our knowledge of the genetic control of kidney morphogenesis and may contribute to understanding certain congenital kidney malformations, including polycystic kidney disease and renal hypoplasia.

Hox genes encode proteins that control embryonic development along the head-to-tail axis and in multiple organs. Here, we show that several members of this gene family are necessary for the normal development of the mammalian kidneys. These genes are clustered in one site on the chromosome and their respective positions within the group determine which component of the kidneys they will contribute to. Using a large collection of engineered mutations in this system, we show that these genes are required both for the growth of the kidneys and for their proper organization, such that mutations in some genes reduce the size of the organs, whereas mutations in others induce polycystic kidneys. Our set of genetic rearrangements also allowed us to localize the position of regulatory sequences, which control the expression of these genes during kidney development.

Bone morphogenetic protein signaling in the developing kidney: present and future

Bone morphogenetic proteins (BMPs) are members of the transforming growth factor-beta superfamily. A critical role for BMP signaling in the development of the metanephric kidney is supported by a growing number of studies using in vitro assays and in vivo animal models. Here we review current knowledge of BMPs, BMP receptors and regulators of the BMP signaling pathway in the developing kidney. We highlight major gaps in our knowledge of the roles of BMP signaling in the development of the normal and abnormal kidney and identify areas and techniques likely to improve our understanding.

Reduced BMP4 abundance in Gata2 hypomorphic mutant mice result in uropathies resembling human CAKUT

Constitutive loss of transcription factor GATA-2 leads to embryonic lethality from primitive erythropoietic failure. We serendipitously discovered an essential contribution of GATA-2 to urogenital development when the hematopoietic deficiency of Gata2 null mutant animals was complemented by a Gata2 yeast artificial chromosome (YAC) transgene; these mice died from a perinatal lethal urogenital abnormality. Here, we report the generation and analysis of Gata2 hypomorphic mutant (Gata2(fGN)/(/fGN)) mice, which suffered from hydronephrosis and megaureter, as do the YAC-rescued Gata2 null mutants. Gata2(fGN)/(/fGN) mutants exhibit anteriorly displaced ureteric budding from the Wolffian duct as well as reduced BMP4 expression in the intermediate mesoderm derivatives in a manner that is temporally coincident with ureteric bud emergence. In Bmp4 mutant heterozygotes, rostral displacement of the initial bud site on the Wolffian duct results in abnormal urogenital development. We show here that Bmp4 mRNA is reduced approximately twofold in Gata2(fGN)/(/fGN) mice (as in Bmp4 null heterozygotes), and that GATA-2 trans-activates a Bmp4 first intron element-directed reporter plasmid in co-transfection assays. These experiments taken together implicate GATA-2 as a direct regulator of Bmp4 transcription. The pathophysiology described in Gata2 hypomorphic mutant animals resembles human congenital anomalies of the kidney and urinary tract.

BMP receptor ALK3 controls collecting system development

The molecular signals that regulate growth and branching of the ureteric bud during formation of the renal collecting system are largely undefined. Members of the bone morphogenetic protein (BMP) family signal through the type I BMP receptor ALK3 to inhibit ureteric bud and collecting duct cell morphogenesis in vitro. We investigated the function of the BMP signaling pathway in vivo by generating a murine model of ALK3 deficiency restricted to the ureteric bud lineage (Alk3(UB-/-) mice). At the onset of branching morphogenesis, Alk3(UB-/-) kidneys are characterized by an abnormal primary (1 degrees ) ureteric bud branch pattern and an increased number of ureteric bud branches. However, during later stages of renal development, Alk3(UB-/-) kidneys have fewer ureteric bud branches and collecting ducts than wild-type kidneys. Postnatal Alk3(UB-/-) mice exhibit a dysplastic renal phenotype characterized by hypoplasia of the renal medulla, a decreased number of medullary collecting ducts, and abnormal expression of beta-catenin and c-MYC in medullary tubules. In summary, normal kidney development requires ALK3-dependent BMP signaling, which controls ureteric bud branching.

Loss of a quiescent niche but not follicle stem cells in the absence of bone morphogenetic protein signaling

During the hair cycle, follicle stem cells (SCs) residing in a specialized niche called the "bulge" undergo bouts of quiescence and activation to cyclically regenerate new hairs. Developmental studies have long implicated the canonical bone morphogenetic protein (BMP) pathway in hair follicle (HF) determination and differentiation, but how BMP signaling functions in the hair follicle SC niche remains unknown. Here, we use loss and gain of function studies to manipulate BMP signaling in the SC niche. We show that when the Bmpr1a gene is conditionally ablated, otherwise quiescent SCs are activated to proliferate, causing an expansion of the niche and loss of slow-cycling cells. Surprisingly, follicle SCs are not lost, however, but rather, they generate long-lived, tumor-like branches that express Sox4, Lhx2, and Sonic Hedgehog but fail to terminally differentiate to make hair. A key component of BMPR1A-deficient SCs is their elevated levels of both Lef1 and beta-catenin, which form a bipartite transcription complex required for initiation of the hair cycle. Although beta-catenin can be stabilized by Wnt signaling, we show that BMPR1A deficiency enhances beta-catenin stabilization in the niche through a pathway involving PTEN inhibition and PI3K/AKT activation. Conversely, sustained BMP signaling in the SC niche blocks activation and promotes premature hair follicle differentiation. Together, these studies reveal the importance of balancing BMP signaling in the SC niche.

Expression and potential role of dentin phosphophoryn (DPP) in mouse embryonic tissues involved in epithelial-mesenchymal interactions and branching morphogenesis

Dentin sialophosphoprotein (DSPP) is synthesized in both mesenchyme and epithelium at varying stages of tooth development. At the tooth cap stage, corresponding to embryonic day (E) 13.5 of mouse embryonic life, the phosphophoryn (DPP) portion of DSPP was immunohistochemically localized to the enamel organ with intense staining of oral ectoderm but no expression in dental follicle mesenchyme. Surprisingly, DPP was also expressed in ureteric bud branches of embryonic metanephric kidney and alveolar epithelial buds of developing lung. Reverse transcriptase-polymerase chain reaction analysis verified the presence of DSPP mRNA with identical sequences in the tooth, lung, and kidney. The DSPP(-/-) mouse with ablated DPP expression in the teeth, also exhibited aberrant organogenesis in kidney and lung. In the kidney, malformed metanephric S-shaped bodies and increased mesenchymal apoptosis were observed. Inclusion of anti-DPP antibodies in organ culture of metanephroi, harvested from E13.5 wild-type mice, likewise resulted in altered ureteric bud morphogenesis, suggesting a role for DPP in epithelial-mesenchymal interactions in meristic tissues during embryonic development.

Wnt/BMP signal integration regulates the balance between proliferation and differentiation of neuroepithelial cells in the dorsal spinal cord

Multiple signaling pathways regulate proliferation and differentiation of neural progenitor cells during early development of the central nervous system (CNS). In the spinal cord, dorsal signaling by bone morphogenic protein (BMP) acts primarily as a patterning signal, while canonical Wnt signaling promotes cell cycle progression in stem and progenitor cells. However, overexpression of Wnt factors or, as shown here, stabilization of the Wnt signaling component beta-catenin has a more prominent effect in the ventral than in the dorsal spinal cord, revealing local differences in signal interpretation. Intriguingly, Wnt signaling is associated with BMP signal activation in the dorsal spinal cord. This points to a spatially restricted interaction between these pathways. Indeed, BMP counteracts proliferation promoted by Wnt in spinal cord neuroepithelial cells. Conversely, Wnt antagonizes BMP-dependent neuronal differentiation. Thus, a mutually inhibitory crosstalk between Wnt and BMP signaling controls the balance between proliferation and differentiation. A model emerges in which dorsal Wnt/BMP signal integration links growth and patterning, thereby maintaining undifferentiated and slow-cycling neural progenitors that form the dorsal confines of the developing spinal cord.

Smad4 cooperates with lymphoid enhancer-binding factor 1/T cell-specific factor to increase c-myc expression in the absence of TGF-beta signaling

The c-myc protooncogene is a key regulator of cell proliferation whose expression is reduced in normal epithelial cells in response to the growth inhibitory cytokine TGF-beta. Smad4 mediates this inhibitory effect of TGF-beta by forming a complex with Smad3, E2F4/5, and p107 at the TGF-beta inhibitory element (TIE) element on the c-myc promoter. In contrast, cell proliferation and c-myc expression are increased in response to Wnt ligands; this effect is mediated through the lymphoid enhancer-binding factor 1/T cell-specific factor (LEF/TCF) family of transcription factors on the c-myc promoter LEF/TCF-binding elements (TBE1 and TBE2). We report that a peptide aptamer designed to inhibit the binding between Smad4 and LEF/TCF reduced c-myc expression and the growth rate of HepG2 cells. Further analysis demonstrated that, in the absence of TGF-beta, Smad4 was bound to the positive regulatory element TBE1 from the c-myc promoter and activated c-myc promoter activity. Smad4 binding to the positive TBE1 c-myc element was reduced by TGF-beta, consistent with Smad4's inhibitory role on c-myc expression in response to TGF-beta. Reduction of Smad4 levels by RNAi knockdown also reduced c-myc expression levels and sensitized hepatocytes to cell death by serum deprivation. Two tumor-derived mutant Smad4 proteins that fail to mediate TGF-beta responses were still competent to cooperate with LEF1 to activate the c-myc promoter. These results support a previously unreported TGF-beta-independent function for Smad4 in cooperating with LEF/TCF to activate c-myc expression.

Renal branching morphogenesis: concepts, questions, and recent advances

The ureteric bud (UB) is an outgrowth of the Wolffian duct, which undergoes a complex process of growth, branching, and remodeling, to eventually give rise to the entire urinary collecting system during kidney development. Understanding the mechanisms that control this process is a fascinating problem in basic developmental biology, and also has considerable medical significance. Over the past decade, there has been significant progress in our understanding of renal branching morphogenesis and its regulation, and this review focuses on several areas in which there have been recent advances. The first section deals with the normal process of UB branching morphogenesis, and methods that have been developed to better observe and describe it. The next section discusses a number of experimental methodologies, both established and novel, that make kidney development in the mouse a powerful and attractive experimental system. The third section discusses some of the cellular processes that are likely to underlie UB branching morphogenesis, as well as recent data on cell lineages within the growing UB. The fourth section summarizes our understanding of the roles of two groups of growth factors that appear to be particularly important for the regulation of UB outgrowth and branching: GDNF and FGFs, which stimulate this process via tyrosine kinase receptors, and members of the TGFbeta family, including BMP4 and Activin A, which generally inhibit UB formation and branching.

Wnt/beta-catenin signaling modulates survival of high glucose-stressed mesangial cells

Glomerulosclerosis and diabetic nephropathy are attributable to high glucose induction of mesangial cell apoptosis. Whereas Wnt signaling has been found to regulate renal morphogenesis and pathogenesis, the biologic role of Wnt/beta-catenin signaling in controlling high glucose-induced mesangial cell apoptosis is not well defined. Herein is reported that Wnt/beta-catenin signaling is required for protecting glomerular mesangial cells from high glucose-mediated cell apoptosis. High glucose downregulated Wnt4 and Wnt5a expression and the subsequent nuclear translocation of beta-catenin, whereas it increased glycogen synthase kinase-3beta (GSK-3beta) and caspase-3 activities and apoptosis of glomerular mesangial cells. Suppression of GSK-3beta activation or increase in nuclear beta-catenin by transfection of Wnt4 or Wnt5a or stable beta-catenin (S33Y) reversed Akt activation and reduced the high glucose-mediated caspase-3 cleavage and cell apoptosis. Pharmacologic inhibition of GSK-3beta by recombinant Wnt5a or bromoindirubin-3'-oxime or LiCl increased Akt phosphorylation and beta-catenin translocation and abrogated high glucose-mediated proapoptotic activities. Exogenous bromoindirubin-3'-oxime treatment reduced phospho-Ser(9)-GSK-3beta and beta-catenin expression and apoptosis of cells adjacent to glomeruli in diabetic kidneys and attenuated urinary protein secretion in diabetic rats. Taken together, mesangial cells responded to high glucose by impairing that canonical Wnt pathway to increase proapoptotic activities. Sustaining Wnt/beta-catenin signaling is beneficial for promoting survival of mesangial cells that are exposed to high glucose stress.

GLI3-dependent transcriptional repression of Gli1, Gli2 and kidney patterning genes disrupts renal morphogenesis

Truncating mutations in Gli3, an intracellular effector in the SHH-SMO-GLI signaling pathway, cause renal aplasia/dysplasia in humans and mice. Yet, the pathogenic mechanisms are undefined. Here, we report the effect of decreased SHH-SMO signaling on renal morphogenesis, the expression of SHH target genes and GLI binding to Shh target genes. Shh deficiency or cyclopamine-mediated SMO inhibition disrupted renal organogenesis, decreased expression of GLI1 and GLI2 proteins, but increased expression of GLI3 repressor relative to GLI3 activator. Shh deficiency decreased expression of kidney patterning genes (Pax2 and Sall1) and cell cycle regulators (cyclin D1 and MYCN). Elimination of Gli3 in Shh(-/-) mice rescued kidney malformation and restored expression of Pax2, Sall1, cyclin D1, MYCN, Gli1 and Gli2. To define mechanisms by which SHH-SMO signaling controls gene expression, we determined the binding of GLI proteins to 5' flanking regions containing GLI consensus binding sequences in Shh target genes using chromatin immunoprecipitation. In normal embryonic kidney tissue, GLI1 and/or GLI2 were bound to each target gene. By contrast, treatment of embryonic kidney explants with cyclopamine decreased GLI1 and/or GLI2 binding, and induced binding of GLI3. However, cyclopamine failed to decrease Gli1 and Gli2 expression and branching morphogenesis in Gli3-deficient embryonic kidney tissue. Together, these results demonstrate that SHH-SMO signaling controls renal morphogenesis via transcriptional control of Gli, renal patterning and cell cycle regulator genes in a manner that is opposed by GLI3.

DNA-binding domain mutations in SMAD genes yield dominant-negative proteins or a neomorphic protein that can activate WG target genes in Drosophila

Mutations in SMAD tumor suppressor genes are involved in approximately 140,000 new cancers in the USA each year. At this time, how the absence of a functional SMAD protein leads to a tumor is unknown. However, clinical and biochemical studies suggest that all SMAD mutations are loss-of-function mutations. One prediction of this hypothesis is that all SMAD mutations cause tumors via a single mechanism. To test this hypothesis, we expressed five tumor-derived alleles of human SMAD genes and five mutant alleles of Drosophila SMAD genes in flies. We found that all of the DNA-binding domain mutations conferred gain-of-function activity, thereby falsifying the hypothesis. Furthermore, two types of gain-of-function mutation were identified - dominant negative and neomorphic. In numerous assays, the neomorphic allele SMAD4(100T) appears to be capable of activating the expression of WG target genes. These results imply that SMAD4(100T) may induce tumor formation by a fundamentally different mechanism from other SMAD mutations, perhaps via the ectopic expression of WNT target genes - an oncogenic mechanism associated with mutations in Adenomatous Polyposis Coli. Our results are likely to have clinical implications, because gain-of-function mutations may cause tumors when heterozygous, and the life expectancy of individuals with SMAD4(100T) is likely to be different from those with other SMAD mutations. From a larger perspective, our study shows that the genetic characterization of missense mutations, particularly in modular proteins, requires experimental verification.

Selective inhibition of TGF-beta responsive genes by Smad-interacting peptide aptamers from FoxH1, Lef1 and CBP

Transforming growth factor beta (TGF-beta) stimulation results in the assembly of Smad-containing protein complexes that mediate activation or repression of TGF-beta responsive genes. To determine if disruption of specific Smad protein-protein interactions would selectively inhibit responses to TGF-beta or generally interfere with Smad-dependent signaling, we developed three Smad-binding peptide aptamers by introducing Smad interaction motifs from Smad-binding proteins CBP, FoxH1 and Lef1 into the scaffold protein E. coli thioredoxin A (Trx). All three classes of aptamers bound to Smads by GST pulldown assays and co-immunoprecipitation from mammalian cells. Expression of the aptamers in HepG2 cells did not generally inhibit Smad-dependent signaling as evaluated using seven TGF-beta responsive luciferase reporter genes. The Trx-xFoxH1b aptamer inhibited TGF-beta-induced expression from a reporter dependent on the Smad-FoxH1 interaction, A3-lux, by 50%. Trx-xFoxH1b also partially inhibited two reporters not dependent on a Smad-FoxH1 interaction, 3TP-lux and Twntop, and endogenous PAI-1 expression. Trx-Lef1 aptamer only inhibited expression of the Smad-Lef1 responsive reporter gene TwnTop. The Trx-CBP aptamer had no significant effect on reporter gene expression. The results suggest that Smad-binding peptide aptamers can be developed to selectively inhibit TGF-beta-induced gene expression.

Integrin-linked kinase mediates bone morphogenetic protein 7-dependent renal epithelial cell morphogenesis

Bone morphogenetic protein 7 (BMP7) stimulates renal branching morphogenesis via p38 mitogen-activated protein kinase (p38(MAPK)) and activating transcription factor 2 (ATF-2) (M. C. Hu, D. Wasserman, S. Hartwig, and N. D. Rosenblum, J. Biol. Chem. 279:12051-12059, 2004). Here, we demonstrate a novel role for integrin-linked kinase (ILK) in mediating renal epithelial cell morphogenesis in embryonic kidney explants and identify p38(MAPK) as a target of ILK signaling in a cell culture model of renal epithelial morphogenesis. The spatial and temporal expression of ILK in embryonic mouse kidney cells suggested a role in branching morphogenesis. Adenovirus-mediated expression of ILK stimulated and expression of a dominant negative ILK mutant inhibited ureteric bud branching in embryonic mouse kidney explants. BMP7 increased ILK kinase activity in inner medullary collecting duct 3 (IMCD-3) cells, and adenovirus-mediated expression of ILK increased IMCD-3 cell morphogenesis in a three-dimensional culture model. In contrast, treatment with a small molecule ILK inhibitor or expression of a dominant negative-acting ILK (ILK(E359K)) inhibited epithelial cell morphogenesis. Further, expression of ILK(E359K) abrogated BMP7-dependent stimulation. To investigate the role of ILK in BMP7 signaling, we showed that ILK overexpression increased basal and BMP7-induced levels of phospho-p38(MAPK) and phospho-ATF-2. Consistent with its inhibitory effects on IMCD-3 cell morphogenesis, expression of ILK(E359K) blocked BMP7-dependent increases in phospho-p38(MAPK) and phospho-ATF-2. Inhibition of p38(MAPK) activity with the specific inhibitor, SB203580, failed to inhibit BMP7-dependent stimulation of ILK activity, suggesting that ILK functions upstream of p38(MAPK) during BMP7 signaling. We conclude that ILK functions in a BMP7/p38(MAPK)/ATF-2 signaling pathway and stimulates epithelial cell morphogenesis.

Cross-talk between the TGFβ and Wnt signaling pathways in murine embryonic maxillary mesenchymal cells

The transforming growth factor beta (TGFbeta) and Wnt signaling pathways play central roles regulating embryogenesis and maintaining adult tissue homeostasis. TGFbeta mediates its cellular effects through types I and II cell surface receptors coupled to the nucleocytoplasmic Smad proteins. Wnt signals via binding to a cell surface receptor, Frizzled, which in turn activates intracellular Dishevelled, ultimately leading to stabilization and nuclear translocation of beta-catenin. Previous studies have demonstrated several points of cross-talk between the TGFbeta and Wnt signaling pathways. In yeast two-hybrid and GST-pull down assays, Dishevelled-1 and Smad 3 have been shown to physically interact through the C-terminal one-half of Dishevelled-1 and the MH2 domain of Smad 3. The current study demonstrates that co-treatment of murine embryonic maxillary mesenchyme (MEMM) cells with Wnt-3a and TGFbeta leads to enhanced reporter activity from TOPflash, a Wnt-responsive reporter plasmid. Transcriptional cooperation between TGFbeta and Wnt did not require the presence of a Smad binding element, nor did it occur when a TGFbeta-responsive reporter plasmid (p3TP-lux) was transfected. Overexpression of Smad 3 further enhanced the cooperation between Wnt and TGFbeta while overexpression of dominant-negative Smads 2 and 3 inhibited this effect. Co-stimulation with TGFbeta led to greater nuclear translocation of beta-catenin, providing explanation for the effect of TGFbeta on Wnt-3a reporter activity. Wnt-3a exerted antiproliferative activity in MEMM cells, similar to that exerted by TGFbeta. In addition, Wnt-3a and TGFbeta in combination led to synergistic decreases in MEMM cell proliferation. These data demonstrate a functional interaction between the TGFbeta and Wnt signaling pathways and suggest that Wnt activation of the canonical pathway is an important mediator of MEMM cell growth.

SOX7 is an immediate-early target of VegT and regulates Nodal-related gene expression in Xenopus

In zebrafish, the divergent F-type SOX casanova acts downstream of Nodal signaling to specify endoderm. While no casanova orthologs have been identified in tetrapods, the F-type SOX, SOX7, is supplied maternally in Xenopus (Fawcett and Klymkowsky, 2004. GER 4, 29). Subsequent RT-PCR and section-based in situ hybridization analyses indicate that SOX7 mRNA is localized to the vegetal region of the blastula-stage embryo. Overexpression and maternal depletion studies reveal that the T-box transcription factor VegT, which initiates mesoendodermal differentiation, directly regulates SOX7 expression. SOX7, but not SOX17 (another F-type SOX), binds to sites within the Xnr5 promoter and SOX7, but not SOX17, induces expression of the Nodal-related genes Xnr1, Xnr2, Xnr4, Xnr5, and Xnr6, the homeodomain transcription factor Mixer, and the endodermal marker SOX17beta; both SOX7 and SOX17 induce expression of the pan-endodermal marker endodermin. SOX7's induction of Xnr expression in animal caps is independent of Mixer and Nodal signaling. In animal caps, VegT's ability to induce Mixer and Edd appears to depend upon SOX7 activity. Whole embryo experiments suggests that vegetal factors partially compensate for the absence of SOX7. Based on the antagonistic effects of SOX7 and SOX3 (Zhang et al., 2004. Dev. Biol. 273, 23) and their common binding sites in the Xnr5 promoter, we propose a model in which competitive interactions between these two proteins are involved in refining the domain of endodermal differentiation.

The homeobox gene Gax inhibits angiogenesis through inhibition of nuclear factor-kappaB-dependent endothelial cell gene expression

The growth and metastasis of tumors are heavily dependent on angiogenesis, but much of the transcriptional regulation of vascular endothelial cell gene expression responsible for angiogenesis remains to be elucidated. The homeobox gene Gax is expressed in vascular endothelial cells and inhibits proliferation and tube formation in vitro. We hypothesized that Gax is a negative transcriptional regulator of the endothelial cell angiogenic phenotype and studied its regulation and activity in vascular endothelial cells. Several proangiogenic factors caused a rapid down-regulation of Gax mRNA in human vascular endothelial cells, as did conditioned media from breast cancer cell lines. In addition, Gax expression using a replication-deficient adenoviral vector inhibited human umbilical vein endothelial cell migration toward proangiogenic factors in vitro and inhibited angiogenesis in vivo in Matrigel plugs. To identify putative downstream targets of Gax, we examined changes in global gene expression in endothelial cells due to Gax activity. Gax expression resulted in changes in global gene expression consistent with a quiescent, nonangiogenic phenotype, with increased expression of cyclin kinase inhibitors and decreased expression of genes implicated in endothelial cell activation and angiogenesis. Further analysis revealed that Gax down-regulated numerous nuclear factor-kappaB (NF-kappaB) target genes and decreased the binding of NF-kappaB to its target sequence in electrophoretic mobility shift assays. To our knowledge, Gax is the first homeobox gene described that inhibits NF-kappaB activity in vascular endothelial cells. Because NF-kappaB has been implicated in endothelial cell activation and angiogenesis, the down-regulation of NF-kappaB-dependent genes by Gax suggests one potential mechanism by which Gax inhibits the angiogenic phenotype.

TGF-? induces novel Lef-1 splice variants through a Smad-independent signaling pathway

The lymphoid enhancer-binding factor (Lef-1) transcription factor is best known for the ability to transduce Wnt signals during development and to mediate excessive Wnt signaling in certain types of cancer. We recently identified and characterized a novel Wnt-like effect of transforming growth factor-beta (TGF-beta) on beta-catenin, the binding partner of Lef-1. Therefore, we sought to determine the effect of TGF-beta on expression of the Lef/T-cell-specific transcription factor (TCF) components of the Wnt pathway. We found that TGF-beta markedly induced Lef-1 mRNA expression in cell lines originating from fetal lung (Mv1Lu) and newborn skin (Balb/MK), tissues that normally express Lef-1 during development. Lef-1 induction was temporally related to but independent of TGF-beta-induced G1 cell cycle arrest. Furthermore, the induction of Lef-1 was independent of both new protein synthesis and Smad-mediated signaling. Using TGF-beta-treated Mv1Lu cells, we identified multiple splice forms of Lef-1, including novel variants that lack both exons 2 and 3. We conclude that the induction of Lef-1 has permissive effects on the well-characterized TGF-beta signal that inhibits c-myc expression and induces a G1 arrest.

Smad1, β-catenin and Tcf4 associate in a molecular complex with the Myc promoter in dysplastic renal tissue and cooperate to control Myc transcription

Renal dysplasia, the major cause of childhood renal failure in humans,arises from perturbed renal morphogenesis and molecular signaling during embryogenesis. Recently, we discovered induction of molecular crosstalk between Smad1 and β-catenin in the TgAlk3QD mouse model of renal medullary cystic dysplasia. Our finding that Myc, a Smad andβ-catenin transcriptional target and effector of renal epithelial dedifferentiation, is misexpressed in dedifferentiated epithelial tubules provided a basis for investigating coordinate transcriptional control by Smad1 and β-catenin in disease. Here, we report enhanced interactions between a molecular complex consisting of Smad1, β-catenin and Tcf4 and adjacent Tcf- and Smad-binding regions located within the Myc promoter in TgAlk3QD dysplastic renal tissue, and Bmp-dependent cooperative control of Myc transcription by Smad1, β-catenin and Tcf4. Analysis of nuclear extracts derived from TgAlk3QD and wild-type renal tissue revealed increased levels of Smad1/β-catenin molecular complexes, and de novo formation of chromatin-associated Tcf4/Smad1 molecular complexes in TgAlk3QD tissues. Analysis of a 476 nucleotide segment of the 1490 nucleotide Myc genomic region upstream of the transcription start site demonstrated interactions between Tcf4 and the Smad consensus binding region and associations of Smad1, β-catenin and Tcf4 with oligo-duplexes that encode the adjacent Tcf- and Smad-binding elements only in TgAlk3QD tissues. In collecting duct cells that express luciferase under the control of the 1490 nucleotide Myc genomic region, Bmp2-dependent stimulation of Myc transcription was dependent on contributions by each of Tcf4, β-catenin and Smad1. These results provide novel insights into mechanisms by which interacting signaling pathways control transcription during the genesis of renal dysplasia.

TGFβ superfamily signals are required for morphogenesis of the kidney mesenchyme progenitor population

The TGFβ superfamily plays diverse and essential roles in kidney development. Gdf11 and Bmp4 are essential for outgrowth and positioning of the ureteric bud, the inducer of metanephric mesenchyme. During nephrogenesis, Bmp7 is required for renewal of the mesenchyme progenitor population. Additionally, in vitro studies demonstrate inhibitory effects of BMPs and TGFβs on collecting duct branching and growth. Here,we explore the predicted models of TGFβ superfamily function by cell-specific inactivation of Smad4, a key mediator of TGFβsignaling. Using a HoxB7cre transgene expressed in ureteric bud and collecting duct, we find that development of the collecting duct is Smad4 independent. By contrast, removal of Smad4 in nephrogenic mesenchyme using the Bmp7cre/+ allele leads to disorganization of the nephrogenic mesenchyme and impairment of mesenchyme induction. Smad4-deficient metanephric mesenchyme does not display defects in inducibility in LiCl or spinal cord induction assays. However, in situ hybridization and lineage analysis of Smad4 null mesenchyme cells at E11.5 show that the nephrogenic mesenchyme does not aggregate tightly around the ureteric bud tips, but remains loosely associated, embedded within a population of cells expressing markers of both nephrogenic mesenchyme and peripheral stroma. We conclude that the failure of recruitment of nephrogenic mesenchyme leaves a primitive population of mesenchyme at the periphery of the kidney. This population is gradually depleted, and by E16.5 the periphery is composed of cells of stromal phenotype. This study uncovers a novel role for TGFβ superfamily signaling in the recruitment and/or organization of the nephrogenic mesenchyme at early time-points of kidney development. Additionally, we present conclusive genetic lineage mapping of the collecting duct and nephrogenic mesenchyme.