The molecular control of renal branching morphogenesis: current knowledge and emerging insights

Gen1 mutation caused kidney hypoplasia and defective ureter-bladder connections in mice

Limited genetic factors were uncovered for the development of congenital anomalies of the kidney and urinary tract (CAKUT). We previously reported that a Holliday junction resolvase Gen1 was essential for early metanephric development in mice. This comprehensive follow-up study focused on the roles of Gen1 in late metanephric development. We found that Gen1 mutation impaired the late development of both kidney and urinary tract. In vivo and ex-vivo kidney primordia culture confirmed decreased ureteric bud branching in Gen1 mutants, which consequently caused hypoplasia. We also observed abnormal urinary tract development. Programmed apoptosis at the end of nephric duct disappeared in Gen1 mutants, which caused abnormal ureter-bladder connections, leading to vesicoureteral reflux (VUR) or ureterovesical junction obstruction (UVJO). Mechanistically, RNA-seq analysis proved that Gen1 mutation impaired the expression of multiple regulatory genes for the metanephric development, including Six2. Taken together, our study provides more insight into the roles of Gen1 in the development of the kidney and urinary tract, which may have potential clinical significance in the treatment and/or prevention of CAKUT.

Cellular and Molecular Mechanisms of Kidney Development: From the Embryo to the Kidney Organoid

Development of the metanephric kidney is strongly dependent on complex signaling pathways and cell-cell communication between at least four major progenitor cell populations (ureteric bud, nephron, stromal, and endothelial progenitors) in the nephrogenic zone. In recent years, the improvement of human-PSC-derived kidney organoids has opened new avenues of research on kidney development, physiology, and diseases. Moreover, the kidney organoids provide a three-dimensional (3D) in vitro model for the study of cell-cell and cell-matrix interactions in the developing kidney. In vitro re-creation of a higher-order and vascularized kidney with all of its complexity is a challenging issue; however, some progress has been made in the past decade. This review focuses on major signaling pathways and transcription factors that have been identified which coordinate cell fate determination required for kidney development. We discuss how an extensive knowledge of these complex biological mechanisms translated into the dish, thus allowed the establishment of 3D human-PSC-derived kidney organoids.

Transcription Factor 21 Is Required for Branching Morphogenesis and Regulates the Gdnf-Axis in Kidney Development

BACKGROUND

The mammalian kidney develops through reciprocal inductive signals between the metanephric mesenchyme and ureteric bud. Transcription factor 21 (Tcf21) is highly expressed in the metanephric mesenchyme, including Six2-expressing cap mesenchyme and Foxd1-expressing stromal mesenchyme. Tcf21 knockout mice die in the perinatal period from severe renal hypodysplasia. In humans, Tcf21 mRNA levels are reduced in renal tissue from human fetuses with renal dysplasia. The molecular mechanisms underlying these renal defects are not yet known.

METHODS

Using a variety of techniques to assess kidney development and gene expression, we compared the phenotypes of wild-type mice, mice with germline deletion of the Tcf21 gene, mice with stromal mesenchyme-specific Tcf21 deletion, and mice with cap mesenchyme-specific Tcf21 deletion.

RESULTS

Germline deletion of Tcf21 leads to impaired ureteric bud branching and is accompanied by downregulated expression of Gdnf-Ret-Wnt11, a key pathway required for branching morphogenesis. Selective removal of Tcf21 from the renal stroma is also associated with attenuation of the Gdnf signaling axis and leads to a defect in ureteric bud branching, a paucity of collecting ducts, and a defect in urine concentration capacity. In contrast, deletion of Tcf21 from the cap mesenchyme leads to abnormal glomerulogenesis and massive proteinuria, but no downregulation of Gdnf-Ret-Wnt11 or obvious defect in branching.

CONCLUSIONS

Our findings indicate that Tcf21 has distinct roles in the cap mesenchyme and stromal mesenchyme compartments during kidney development and suggest that Tcf21 regulates key molecular pathways required for branching morphogenesis.

Branching morphogenesis of the urinary collecting system in the human embryonic metanephros

An elaborate system of ducts collects urine from all nephrons, and this structure is known as the urinary collecting system (UCS). This study focused on how the UCS is formed during human embryogenesis. Fifty human embryos between the Carnegie stage (CS) 14 and CS23 were selected from the Kyoto Collection at the Congenital Anomaly Research Center of Kyoto University, Japan. Metanephroses, including the UCS, were segmented on serial digital virtual histological sections. Three-dimensional images were computationally reconstructed for morphological and quantitative analyses. A CS timeline was plotted. It consisted of the 3-D structural morphogenesis of UCS and quantification of the total amount of end-branching, average and maximum numbers of generations, deviation in the metanephros, differentiation of the urothelial epithelium in the renal pelvis, and timing of the rapid expansion of the renal pelvis. The first UCS branching generation occurred by CS16. The average branching generation reached a maximum of 8.74 ± 1.60 and was already the twelfth in CS23. The total end-branching number squared between the start and the end of the embryonic period. UCS would reach the fifteenth branching generation soon after CS23. The number of nephrons per UCS end-branch was low (0.21 ± 0.14 at CS19, 1.34 ± 0.49 at CS23), indicating that the bifid branching occurred rapidly and that the formation of nephrons followed after. The renal pelvis expanded mainly in CS23, which was earlier than that reported in a previous study. The number of nephrons connected to the UCS in the expanded group (246.0 ± 13.2) was significantly larger than that of the pre-expanded group (130.8 ± 80.1) (P < 0.05). The urothelial epithelium differentiated from the zeroth to the third generations at CS23. Differentiation may have continued up until the tenth generation to allow for renal pelvis expansion. The branching speed was not uniform. There were significantly more branching generations in the polar- than in the interpolar regions (P < 0.05). Branching speed reflects the growth orientation required to form the metanephros. Further study will be necessary to understand the renal pelvis expansion mechanism in CS23. Our CS-based timeline enabled us to map UCS formation and predict functional renal capacity after differentiation and growth.

A Gene Implicated in Activation of Retinoic Acid Receptor Targets Is a Novel Renal Agenesis Gene in Humans

Renal agenesis (RA) is one of the more extreme examples of congenital anomalies of the kidney and urinary tract (CAKUT). Bilateral renal agenesis is almost invariably fatal at birth, and unilateral renal agenesis can lead to future health issues including end-stage renal disease. Genetic investigations have identified several gene variants that cause RA, including EYA1, LHX1, and WT1 However, whereas compound null mutations of genes encoding α and γ retinoic acid receptors (RARs) cause RA in mice, to date there have been no reports of variants in RAR genes causing RA in humans. In this study, we carried out whole exome sequence analysis of two families showing inheritance of an RA phenotype, and in both identified a single candidate gene, GREB1L Analysis of a zebrafish greb1l loss-of-function mutant revealed defects in the pronephric kidney just prior to death, and F0 CRISPR/Cas9 mutagenesis of Greb1l in the mouse revealed kidney agenesis phenotypes, implicating Greb1l in this disorder. GREB1L resides in a chromatin complex with RAR members, and our data implicate GREB1L as a coactivator for RARs. This study is the first to associate a component of the RAR pathway with renal agenesis in humans.

Sonic hedgehog protein regulates fibroblast growth factor 8 expression in metanephric explant culture from BALB/c mice: Possible mechanisms associated with renal morphogenesis

The sonic hedgehog (SHH) morphogen regulates cell differentiation and controls a number of genes during renal morphogenesis. To date, the effects of SHH on fibroblast growth factors (Fgfs) in embryonic kidney development remain unclear. In the present study, explants of BALB/c mouse embryonic kidney tissues were used to investigate the role of exogenous SHH on Fgf8 and Fgf10 expression levels ex vivo. Ureteric bud branches and epithelial metanephric derivatives were used to determine the renal morphogenesis with Dolichos biflorus agglutinin or hematoxylin‑eosin staining. mRNA expression levels were determined using reverse transcription‑quantitative polymerase chain reaction, while the protein expression levels were examined using immunohistochemistry and western blot analysis. During the initial stages of metanephric development, low levels of SHH, Fgf8, and Fgf10 expression were observed, which were found to increase significantly during more advanced stages of metanephric development. In addition, exogenous SHH protein treatment increased the number of ureteric bud branches and enhanced the formation of nephrons. Exogenous SHH reduced the Fgf8 mRNA and protein expression levels, whereas cyclopamine (an SHH‑smoothened receptor inhibitor) interfered with SHH‑mediated downregulation of Fgf8 expression. By contrast, exogenous SHH protein was not found to modulate Fgf10 mRNA and protein expression levels. In conclusion, these results indicate that the modulatory effects of SHH on BALB/c mouse metanephric explant cultures may involve the regulation of Fgf8 expression but not Fgf10 expression, which provides evidence for the functional role of Fgf proteins in renal morphogenesis.

MicroRNAs: potential regulators of renal development genes that contribute to CAKUT

Congenital anomalies of the kidney and urinary tract (CAKUT) are the leading cause of childhood chronic kidney disease (CKD). While mutations in several renal development genes have been identified as causes for CAKUT, most cases have not yet been linked to known mutations. Furthermore, the genotype-phenotype correlation is variable, suggesting that there might be additional factors that have an impact on the severity of CAKUT. MicroRNAs (miRNAs) are small non-coding RNAs that regulate gene expression at the post-transcriptional level, and are involved in many developmental processes. Although little is known about the function of specific miRNAs in kidney development, several have recently been shown to regulate the expression of, and/or are regulated by, crucial renal development genes present in other organ systems. In this review, we discuss how miRNA regulation of common developmental signaling pathways may be applicable to renal development. We focus on genes that are known to contribute to CAKUT in humans, for which miRNA interactions in other contexts have been identified, with miRNAs that are present in the kidney. We hypothesize that miRNA-mediated processes might play a role in kidney development through similar mechanisms, and speculate that genotypic variations in these small RNAs or their targets could be associated with CAKUT.

Congenital anomalies of kidney and hand: a review

‘Acro-renal syndrome’ refers to co-occurrence of congenital renal and limb anomalies. The term acro-renal syndrome was coined by Curran et al. in 1972 though Dieker and Opitz were the first to report this phenomenon in three male patients in 1969. The common limb defects include oligodactyly, ectrodactyly, syndactyly or brachydactyly anomalies of the carpal and tarsal bones and the common renal anomalies observed are unilateral renal agenesis (URA), bilateral renal hypoplasia, ureteric hypoplasia, hydroureteronephrosis and duplication abnormalities. The acro-renal syndrome as originally described is rare, reported only in ∼20 patients in the international literature. We report a 23-year-old male patient with renal anomalies in the form of absent right kidney, left-sided vesicoureteric reflux (VUR) and skeletal anomalies viz short radius, absent first metacarpal ray in left hand and left undescended testis, consistent with Dieker's type acro-renal syndrome. Apart from the classical acro-renal syndrome, several anomalies of acro-renal patterns and the abnormal gene loci involved are described in the literature. This article is a comprehensive review of the development of kidneys, types of acro-renal syndromes, congenital anomalies of the kidney and urinary tract (CAKUT), syndromes associated with combined limb and renal anomalies, and anomalies associated with URA.

Midkine promotes selective expansion of the nephrogenic mesenchyme during kidney organogenesis

During kidney development, the growth and development of the stromal and nephrogenic mesenchyme cell populations and the ureteric bud epithelium is tightly coupled through intricate reciprocal signaling mechanisms between these three tissue compartments. Midkine, a target gene activated by retinoid signaling in the metanephros, encodes a secreted polypeptide with mitogenic and anti-apoptotic activities in a wide variety of cell types. Using immmunohistochemical methods we demonstrated that Midkine is found in the uninduced mesenchyme at the earliest stages of metanephric kidney development and only subsequently concentrated in the ureteric bud epithelium and basement membrane. The biological effects of purified recombinant Midkine were analyzed in metanephric organ culture experiments carried out in serum-free defined media. These studies revealed that Midkine selectively promoted the overgrowth of the Pax-2 and N-CAM positive nephrogenic mesenchymal cells, failed to stimulate expansion of the stromal compartment and suppressed branching morphogenesis of the ureteric bud. Midkine suppressed apoptosis and stimulated cellular proliferation of the nephrogenic mesenchymal cells, and was capable of maintaining the viability of isolated mesenchymes cultured in the absence of the ureteric bud. These results suggest that Midkine may regulate the balance of epithelial and stromal progenitor cell populations of the metanephric mesenchyme during renal organogenesis.

Novel genetic aspects of congenital anomalies of kidney and urinary tract

PURPOSE OF REVIEW

Congenital anomalies of the kidney and urinary tract (CAKUT) are among the most frequent organ malformations. They are a relevant cause of chronic renal failure in children. Apart from isolated forms of CAKUT, more than 500 syndromes have been described that are characterized by combined defects of the kidney and other organ systems. Familial aggregation of renal malformations in approximately 10% of patients suggests that genetic events might be involved. Modifying effects due to missense mutations in additional developmental genes seem to enhance the phenotypic variability in affected families. In these families, genetic counseling can be difficult. In contrast, in patients with defined autosomal dominant disease, genetic counseling is of high clinical relevance, also with respect to additional extrarenal symptoms.

RECENT FINDINGS

Due to the development of numerous genetic knock-out mouse models, the identification of specific renal developmental genes and the application of novel sequencing techniques of the human genome, our understanding of kidney organogenesis has largely improved during very recent years.

SUMMARY

This review will focus on important genetic factors that influence nephrogenesis and highlight important human disorders that are associated with anomalies of kidneys, proximal and distal urinary tract.

High glucose promotes nascent nephron apoptosis via NF-kappaB and p53 pathways

A hyperglycemic environment in utero reduces kidney size and nephron number due to nascent nephron apoptosis. However, the underlying mechanisms are incompletely understood. The present study investigated whether the nascent nephron apoptosis promoted by high glucose is mediated via the transcription factor NF-κB and p53 signaling pathways. Neonatal mouse kidneys from the offspring of nondiabetic, diabetic, and insulin-treated diabetic dams were used for in vivo studies, and MK4 cells, an embryonic metanephric mesenchymal (MM) cell line, were used for in vitro studies. Neonatal kidneys of the offspring of diabetic mothers exhibited an increased number of apoptotic cells and reactive oxygen species (ROS) generation, enhanced NF-κB activation, and nuclear translocation of its subunits (p50 and p65 subunits) as well as phosphorylation (Ser 15) of p53 compared with kidneys of offspring of nondiabetic mothers. Insulin treatment of diabetic dams normalized these parameters in the offspring. In vitro, high-glucose (25 mM) induced ROS generation and significantly increased MK4 cell apoptosis and caspase-3 activity via activation of NF-κB pathway, with p53 phosphorylation and nuclear translocation compared with normal glucose (5 mM). These changes in a high-glucose milieu were prevented by transient transfection of small interfering RNAs for dominant negative IκBα or IKK or p53. Our data demonstrate that high glucose-induced nascent nephron apoptosis is mediated, at least in part, via ROS generation and the activation of NF-κB and p53 pathways.

Functional analysis of BMP4 mutations identified in pediatric CAKUT patients

Human congenital anomalies of the kidney and urinary tract (CAKUT) represent the major causes of chronic renal failure (CRF) in children. This set of disorders comprises renal agenesis, hypoplasia, dysplastic or double kidneys, and/or malformations of the ureter. It has recently been shown that mutations in several genes, among them BMP4, are associated with hereditary renal developmental diseases. In BMP4, we formerly identified three missense mutations (S91C, T116S, N150K) in five pediatric CAKUT patients. These BMP4 mutations were subsequently studied in a cellular expression system, and here we present functional data demonstrating a lower level of messenger RNA (mRNA) abundance in Bmp4 mutants that indicates a possible negative feedback of the mutants on their own mRNA expression and/or stability. Furthermore, we describe the formation of alternative protein complexes induced by the S91C-BMP4 mutation, which results in perinuclear endoplasmic reticulum (ER) accumulation and enhanced lysosomal degradation of Bmp4. This work further supports the role of mutations in BMP4 for abnormalities of human kidney development.

Deletion of Frs2alpha from the ureteric epithelium causes renal hypoplasia

Fibroblast growth factor receptor 2 (Fgfr2) signaling is critical in maintaining ureteric branching architecture and mesenchymal stromal morphogenesis in the kidney. Fibroblast growth factor receptor substrate 2alpha (Frs2alpha) is a major docking protein for Fgfr2 with downstream targets including Ets variant (Etv) 4 and Etv5 in other systems. Furthermore, global deletion of Frs2alpha causes early embryonic lethality. The purpose of the study was to determine the role of Frs2alpha in mediating Fgfr2 signaling in the ureteric epithelium. To that end, we generated mice with conditional deletion of Frs2alpha in the ureteric epithelium (Frs2alpha(UB-/-)) and mice with point mutations in the Frs2alpha binding site of Fgfr2 (Fgfr2(LR/LR)). Frs2alpha(UB-/-) mice developed mild renal hypoplasia characterized by decreased ureteric branching morphogenesis but maintained normal overall branching architecture and had normal mesenchymal stromal development. Reduced nephron endowment in postnatal mutant mice was observed, corresponding with the reduction in branching morphogenesis. Furthermore, there were no apparent renal abnormalities in Fgfr2(LR/LR) mice. Interestingly, Etv4 and Etv5 expression was unaltered in Frs2alpha(UB-/-) mice, as was Sprouty1, an antagonist of Frs2alpha signaling. However, Ret and Wnt11 (molecules critical for ureteric branching morphogenesis) mRNA levels were lower in mutants vs. controls. Taken together, these findings suggest that Fgfr2 signals through adapter molecules other than Frs2alpha in the ureteric epithelium. Furthermore, Frs2alpha may transmit signals through other receptor kinases present in ureteric epithelium. Finally, the renal hypoplasia observed in Frs2alpha(UB-/-) mice is likely secondary to decreased Ret and Wnt11 expression.

Stimulatory and inhibitory signaling molecules that regulate renal branching morphogenesis

Branching morphogenesis, defined as the growth and branching of epithelial tubules, is a fundamental developmental process involved in the formation of a variety of mammalian tissues, including the kidney. Defective renal branching may result in a number of clinically relevant abnormalities, including renal agenesis, renal dysplasia, multiplex kidneys, and hypertension. In this review we describe the morphological events that generate the characteristic tree-like structure of the mammalian collecting system. We also highlight new knowledge related to both established and novel signaling systems that are important for stimulating and inhibiting branching morphogenesis.

[Recent perspectives on the development of the central nervous system and the genetic background of neural tube defects]

Neural tube defects are rare and mostly lethal malformations. The pattern of inheritance of these malformations is multifactorial, rendering the identification of the underlying causes. Numerous studies have been conducted to elucidate the genetic basis of the development of the central nervous system. Essential signaling pathways of the development of the central nervous system include the planar cell polarity pathway, which is important for the initiation of neural tube closure as well as sonic hedgehog pathway, which regulates the neural plate bending. Genes and their mutations influencing the different stages of neurulation have been investigated for their eventual role in the development of these malformations. Among the environmental factors, folic acid seems to be the most important modifier of the risk of human neural tube defects. Genes of the folate metabolism pathways have also been investigated to identify mutations resulting in increased risk of NTDs. In this review the author has attempted to summarize the knowledge on neural tube defects, with special regard to genetic factors of the etiology.

Recent perspectives on the genetic background of neural tube defects with special regard to iniencephaly

Iniencephaly is a rare and mostly lethal type of neural tube defect. The pattern of inheritance of this group of malformations is multifactorial, rendering the identification of the underlying causes. Numerous studies have been conducted to elucidate the genetic basis of human neurulation. Essential signaling pathways of the development of the CNS include the planar cell polarity pathway, which is important for the initiation of neural tube closure, as well as the sonic hedgehog pathway, which regulates the neural plate bending. Genes influencing the different stages of neurulation have been investigated for their eventual role in the development of these malformations. Among the environmental factors, folic acid seems to be the most important modifier of the risk of human neural tube defects. Genes of the folate metabolism pathways have also been investigated to identify mutations resulting in increased risk of neural tube defects. In this review we have attempted to summarize the knowledge on iniencephaly and neural tube defects, with special regard to genetic factors of the etiology.

Immuno-localization of CD44 and osteopontin in developing human kidney

CD44 is observed in ureteric bud structures and is implicated in branching morphogenesis during early mouse renal development. Healthy adult kidney demonstrates minimal CD44, but CD44 is up-regulated in renal diseases. CD44 may mediate binding of calcium oxalate crystals to tubular epithelia via the ligands osteopontin (OPN) and hyaluronan. Because 15% of premature infants develop nephrocalcinosis, developmental tubular CD44 expression might promote nephrocalcinosis. We studied CD44 and OPN immuno-localization in developing human kidney by immunohistochemical analysis. Human renal tissue between 18 and 40 wk of gestation showed CD44 immuno-localization in ureteric buds, with staining decreasing with increasing gestational age; CD44 was rarely observed in developing renal tubules. OPN was diffusely observed in proximal tubules, rarely observed in distal tubules, ureteric buds or metanephric structures. These data support the role of CD44 in early human nephron formation and branching morphogenesis. Rare CD44 staining in developing tubular epithelium suggests no role for CD44 in promoting calcium oxalate adherence to tubular epithelia in premature infants. Immuno-localization of OPN in tubules supports its role in tubular differentiation, but OPN does not seem to be necessary during early nephron formation.

The use of Endo-Porter to deliver morpholinos in kidney organ culture

Cellular interactions in development of the kidney are used as a model of reciprocal inductive events between epithelium and mesenchyme. Time- and labor-intensive methods have been developed to study this phenomenon. For example, in mice, the targeted disruption of genes in vivo has been used to modify the genetic program directing kidney development. However, gene targeting is a resource-intensive approach and alternative strategies for gene and protein modification in the kidney need to be developed. Herein, we have developed an efficient system for the delivery of antisense morpholino to alter normal protein expression. We describe the use of Endo-Porter to effectively deliver morpholinos to all parts and regions of the kidney explant. Also, we definitively show via confocal microscopy and Western blot analysis that the use of Endo-Porter in delivering antisense morpholinos is robust throughout the entire kidney explant, providing efficient suppression of protein expression. This method saves time and cost when compared with targeted disruption and is an improvement upon previous kidney organ culture methods.

Developmental biology of the human kidney

The mammalian kidney consists of highly specialised cells that function in an integrated manner to maintain homeostasis of body fluids, electrolytes and nutrients. Formation of multicellular structures and differentiation of kidney cells are tightly regulated processes that begin during the fifth week and end by approximately 34 weeks of human gestation. This review focuses on the morphological, cellular and molecular steps required for kidney formation. Although some of this information is derived from studies of the human kidney, much has arisen from the study of genetic models of mammalian kidney morphogenesis. These models reveal mechanisms by which cell lineages are established in the embryonic kidney and the genetic pathways that are involved in their establishment and maintenance.

Vesicoureteral reflux

Vesicoureteral reflux (VUR), the retrograde flow of urine from the bladder toward the kidney, is common in young children. About 30% of children with urinary tract infections will be diagnosed with VUR after a voiding cystourethrogram. For most, VUR will resolve spontaneously; 20% to 30% will have further infections, but few will experience long-term renal sequelae. Developmentally, VUR arises from disruption of complex signaling pathways and cellular differentiation. These mechanisms are probably genetically programmed but may be influenced by environmental exposures. Phenotypic expression of VUR is variable, ranging from asymptomatic forms to severe renal parenchymal disease and end-stage disease. VUR is often familial but is genetically heterogeneous with variability in mode of inheritance and in which gene, or the number of genes, that are involved. Numerous genetic studies that explore associations with VUR are available. The relative utility of these for understanding the genetics of VUR is often limited because of small sample size, poor methodology, and a diverse spectrum of patients. Much, if not all, of the renal parenchymal damage associated with end-stage disease is likely to be congenital, which limits the opportunity for intervention to familial cases where risk prediction may be available. Management of children with VUR remains controversial because there is no strong supportive evidence that prophylactic antibiotics or surgical intervention improve outcomes. Furthermore, well-designed genetic epidemiological studies focusing on the severe end of the VUR phenotype may help define the causal pathway and identify modifiable or disease predictive factors.

Maternal diabetes modulates renal morphogenesis in offspring

Maternal diabetes leads to an adverse in utero environment, but whether maternal diabetes impairs nephrogenesis is unknown. Diabetes was induced with streptozotocin in pregnant Hoxb7-green fluorescence protein mice at embryonic day 13, and the offspring were examined at several time points after birth. Compared with offspring of nondiabetic controls, offspring of diabetic mice had lower body weight, body size, kidney weight, and nephron number. The observed renal dysmorphogenesis may be the result of increased apoptosis, because immunohistochemical analysis revealed significantly more apoptotic podocytes as well as increased active caspase-3 immunostaining in the renal tubules compared with control mice. Regarding potential mediators of these differences, offspring of diabetic mice had increased expression of intrarenal angiotensinogen and renin mRNA, upregulation of NF-kappaB isoforms p50 and p65, and activation of the NF-kappaB pathway. In conclusion, maternal diabetes impairs nephrogenesis, possibly via enhanced intrarenal activation of the renin-angiotensin system and NF-kappaB signaling.

SIX2 and BMP4 mutations associate with anomalous kidney development

Renal hypodysplasia (RHD) is characterized by reduced kidney size and/or maldevelopment of the renal tissue following abnormal organogenesis. Mutations in renal developmental genes have been identified in a subset of affected individuals. Here, we report the first mutations in BMP4 and SIX2 identified in patients with RHD. We detected 3 BMP4 mutations in 5 RHD patients, and 3 SIX2 mutations in 5 different RHD patients. Overexpression assays in zebrafish demonstrated that these mutations affect the function of Bmp4 and Six2 in vivo. Overexpression of zebrafish six2.1 and bmp4 resulted in dorsalization and ventralization, respectively, suggesting opposing roles in mesendoderm formation. When mutant constructs containing the identified human mutations were overexpressed instead, these effects were attenuated. Morpholino knockdown of bmp4 and six2.1 affected glomerulogenesis, suggesting specific roles for these genes in the formation of the pronephros. In summary, these studies implicate conserved roles for Six2 and Bmp4 in the development of the renal system. Defects in these proteins could affect kidney development at multiple stages, leading to the congenital anomalies observed in patients with RHD.

Expression profiles of congenital renal dysplasia reveal new insights into renal development and disease

Congenital renal dysplasia (RD) is a major cause of renal failure in the pediatric population. Although molecular and genetic aspects of RD have been studied in animal models, limited studies have been done in human RD primarily due to lack of available material. To identify novel genes that are associated with RD and normal kidney development, we performed microarray analysis on total RNA extracted from age-matched fetal kidneys of normal and RD patients. In midgestational RD kidneys, we found 180 upregulated and 104 downregulated transcripts compared with normal kidneys. Among the increased transcripts in the dysplastic kidneys were matrix-degrading enzymes (MMP7, MMP19, TIMP1), inflammation- and immunity-related genes, and growth factors. Expression of genes known to be essential for normal kidney development, such as WT1, BMP7, renin, angiotensin receptor 2 (AGTR2), SAL-like 1 (SALL1) and glypican 3 (GPC3), were decreased in dysplastic kidneys. Expression of selected gene products (BMP7, renin, and MMP7) was further confirmed in parallel sections and in several normal and human dysplastic kidneys, supporting the role of these genes as putative RD biomarkers. These results are among the first to reveal disrupted expression profiles during gestation in human RD patients.

Pax-2 and N-myc regulate epithelial cell proliferation and apoptosis in a positive autocrine feedback loop

Both paired homeo box-2 (Pax-2) and N-myc genes play pivotal roles in renal morphogenesis via their effects on cell proliferation and differentiation, but whether and how they interact have not been addressed. In the present study, we investigated such a potential interaction using embryonic renal cells in vitro. Mouse embryonic mesenchymal (MK4) cells stably transfected with Pax-2 cDNA in sense (+) or antisense (-) orientation were used for experiments. Pax-2 promoter activity was monitored by luciferase assay. Reactive oxygen species (ROS) generation, cell proliferation, and cell apoptosis were evaluated. We found that Pax-2 and N-myc gene expression were upregulated and downregulated in Pax-2 (+) and Pax-2 (-) stable transformants, respectively. ROS generation and apoptosis were significantly reduced both in Pax-2 (+) transformants compared with Pax-2 (-) transformants and in naïve MK4 cells cultured in either normal- (5 mM) or high-glucose (25 mM) medium. Transient transfection of N-myc cDNA into Pax-2 (-) stable transformants restored Pax-2 gene expression and prevented ROS generation induced by high glucose. Our data demonstrate that Pax-2 gene overexpression prevents hyperglycemia-induced apoptosis, and N-myc appears to provide a positive autocrine feedback on Pax-2 gene expression in embryonic mesenchymal cells.

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.

Expression of p53/HGF/c-met/STAT3 signal in fetuses with neural tube defects

Neural tube defects (NTD) are morphogenetic alterations due to a defective closure of neural tube. Hepatocyte growth factor (HGF)/c-met system plays a role in morphogenesis of nervous system, lung, and kidney. HGF/c-met morphogenetic effects are mediated by signal transducers and activators of transcription (STAT)3 and both HGF and c-met genes are regulated from p53. The aim of our study was to analyze mRNA and protein expressions of p53, HGF, c-met, and STAT3 in fetuses with NTD. By reverse transcriptase-polymerase chain reaction and immunohistochemistry, we analyzed neural tissues from four NTD fetuses and the corresponding non-malformed lungs, kidneys and placentas. We found a reduced mRNA expression of HGF/c-met/STAT3 pathway, in the malformed nervous systems and placentas. The reduced expression of this pathway correlated with the absence of p53 in all these samples. On the contrary, detectable expression levels of p53, HGF, c-met, and STAT3 were observed in non-malformed lungs and kidneys obtained from the same fetuses. Comparable results were obtained by immunohistochemistry, with the exception of p53, which was undetected in all fetal tissues. In conclusion, in NTD fetuses, both the defective neural tube tissue and the placenta have a reduction in all components of the p53/HGF/c-met/STAT3 cascade. This raises the possibility of using the suppression of these genes for early diagnosis of NTD especially on chorionic villus sampling.

Embryology and genetics of primary vesico-ureteric reflux and associated renal dysplasia

Congenital anomalies of the kidney and urinary tract, as well as primary vesico-ureteric reflux (VUR) and associated renal dysplasia, are the most relevant causes of end-stage renal failure in the pediatric population. In vivo and in vitro experimental studies have allowed the identification of several genes involved both in ureteric bud branching, ureteric elongation and insertion into the bladder, and in nephrogenesis. It has been proposed that both renal and ureteral abnormalities, as well as the associated renal hypo-dysplasia, may derive from a common mechanism as the result of a dysregulation of the normal developmental program. The large homologies between mice and the human genome suggest that the same genes could be involved both in rodent and human VUR. Furthermore, epidemiological observations suggest that not only syndromic but also isolated VUR is an inherited trait. Linkage analysis for homologous mouse genes in humans, genome-wide linkage studies in multigenerational families and association studies by polymorphisms support the hypothesis that VUR is genetically heterogeneous and is caused by a number of different genes acting with random environmental effects. The present teaching paper is an overview of the embryology and genetics of primary VUR and associated congenital reflux nephropathy.

Branching morphogenesis of the ureteric epithelium during kidney development is coordinated by the opposing functions of GDNF and Sprouty1

Branching of ureteric bud-derived epithelial tubes is a key morphogenetic process that shapes development of the kidney. Glial cell line-derived neurotrophic factor (GDNF) initiates ureteric bud formation and promotes subsequent branching morphogenesis. Exactly how GDNF coordinates branching morphogenesis is unclear. Here we show that the absence of the receptor tyrosine kinase antagonist Sprouty1 (Spry1) results in irregular branching morphogenesis characterized by both increased number and size of ureteric bud tips. Deletion of Spry1 specifically in the epithelium is associated with increased epithelial Wnt11 expression as well as increased mesenchymal Gdnf expression. We propose that Spry1 regulates a Gdnf/Ret/Wnt11-positive feedback loop that coordinates mesenchymal-epithelial dialogue during branching morphogenesis. Genetic experiments indicate that the positive (GDNF) and inhibitory (Sprouty1) signals have to be finely balanced throughout renal development to prevent hypoplasia or cystic hyperplasia. Epithelial cysts develop in Spry1-deficient kidneys that share several molecular characteristics with those observed in human disease, suggesting that Spry1 null mice may be useful animal models for cystic hyperplasia.

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.

The pluripotent cytokine pleiotrophin is induced by wounding in human mesangial cells

Mesangial re-modeling and mesangial cell (MC) migration are features of several glomerular diseases including mesangiocapillary glomerulonephritis. In vitro investigations have recently identified ADAM-15, a multidomain adamalysin, as central to the migration of MC. The current study used array technology to investigate the expression of other genes in migrating cells and identified pleiotrophin (PTN), platelet-derived growth factor alpha polypeptide chain, colony stimulating factor, and four members of the tumor necrosis factor-alpha superfamily as major genes that were upregulated. Transcriptional induction of PTN was confirmed by reverse transcription-polymerase chain reaction and Northern blotting and induction of the protein by Western blotting and immunohistochemical localization. PTN was observed associated with mesangial 'hillocks' in confluent MC cultures. In contrast, in models of migration, migrating cells had the highest expression of cell-associated PTN. PTN protein was less evident, however, in the conditioned medium of MCs. Treatment of MC with heparanase removed PTN from the cells suggesting that its localization was owing to an association with heparan sulfates on the cell surface or in the extracellular matrix. This is the first description of the expression of PTN by human MCs and the data suggest that it is rapidly induced in cells that are triggered to migrate. The result of this induction is currently under investigation.

Reactive oxygen species and nuclear factor-kappa B pathway mediate high glucose-induced Pax-2 gene expression in mouse embryonic mesenchymal epithelial cells and kidney explants

Diabetic mellitus confers a major risk of congenital malformations, and is associated with diabetic embryopathy, affecting multiple organs including the kidney. The DNA paired box-2 (Pax-2) gene is essential in nephrogenesis. We investigated whether high glucose alters Pax-2 gene expression and aimed to delineate its underlying mechanism(s) of action using both in vitro (mouse embryonic mesenchymal epithelial cells (MK4) and ex vivo (kidney explant from Hoxb7-green florescent protein (GFP) mice) approaches. Pax-2 gene expression was determined by reverse transcriptase-polymerase chain reaction, Western blotting, and immunofluorescent staining. A fusion gene containing the full-length 5'-flanking region of the human Pax-2 promoter linked to a luciferase reporter gene, pGL-2/hPax-2, was transfected into MK4 cells with or without dominant negative IkappaBalpha (DN IkappaBalpha) cotransfection. Fusion gene expression level was quantified by cellular luciferase activity. Reactive oxygen species (ROS) generation was measured by lucigenin assay. Embryonic kidneys from Hoxb7-GFP mice were cultured ex vivo. High D(+) glucose (25 mM), compared to normal glucose (5 mM), specifically induced Pax-2 gene expression in MK4 cells and kidney explants. High glucose-induced Pax-2 gene expression is mediated, at least in part, via ROS generation and activation of the nuclear factor kappa B signaling pathway, but not via protein kinase C, p38 mitogen-activated protein kinase (MAPK), and p44/42 MAPK signaling.

Prevalence of mutations in renal developmental genes in children with renal hypodysplasia: results of the ESCAPE study

Renal hypodysplasia (RHD) is characterized by a reduced nephron number, small kidney size, and disorganized renal tissue. A hereditary basis has been established for a subset of affected patients, suggesting a major role of developmental genes that are involved in early kidney organogenesis. Gene mutations that have dominant inheritance and cause RHD, urinary tract anomalies, and defined extrarenal symptoms have been identified in TCF2 (renal cysts and diabetes syndrome), PAX2 (renal-coloboma syndrome), EYA1 and SIX1 (branchio-oto-renal syndrome), and SALL1 (Townes-Brocks syndrome). For estimation of the prevalence of these events, an unselected cohort of 99 unrelated patients with RHD that was associated with chronic renal insufficiency were screened for mutations in TCF2, PAX2, EYA1, SIX1, and SALL1. Mutations or variants in the genes of interest were detected in 17 (17%) unrelated families: One mutation, two variants, and four deletions of TCF2 in eight unrelated patients; four different PAX2 mutations in six families; one EYA1 mutation and one deletion in two patients with branchio-oto-renal syndrome; and one SALL1 mutation in a patient with isolated RHD. Of a total of 27 patients with renal cysts, six (22%) carried a mutation in TCF2. It is interesting that a SIX1 sequence variant was identified in two siblings with renal-coloboma syndrome as a result of a PAX2 mutation, suggesting an oligogenic inheritance. Careful clinical reevaluation that focused on discrete extrarenal symptoms and thorough family analysis revealed syndrome-specific features in nine of the 17 patients. In conclusion, 15% of patients with RHD show mutations in TCF2 or PAX2, whereas abnormalities in EYA1, SALL1, and SIX1 are less frequent.

CD 95 mediated apoptosis in embryogenesis: implication in tooth development

INTRODUCTION

Understanding of apoptotic mechanisms involved in tissue shaping is of particular interest because of possible targeted modulation of the development of organ structures such as teeth. Research of CD 95 mediated apoptosis has been focused particularly on cell death in the immune system and related disorders. However, CD 95 mediated apoptosis is also involved in embryogenesis of many organs as the kidney, the lung, the intestine and tissue networks such as the nervous system.

DESIGN

Narrative review.

RESULTS

This review briefly summarizes the current knowledge of CD 95 mediated apoptosis in embryogenesis with possible implication in tooth development. CD 95 receptor and CD 95 ligand are found at early stages of tooth development. The data suggest some positive correlations with dental apoptosis distribution, particularly in the primary enamel knot where apoptosis occurs during elimination of this structure. CD 95 deficient (lpr) adult mouse tooth phenotype, however, did not show any alterations in final tooth pattern and morphology.

CONCLUSION

To date studies of apoptotic machinery during tooth development show spatial localization of many of the components together with precise and localized timing of cell death. There is still much to be learned about the regulation and importance of apoptosis in tooth development. Nevertheless, the involvement of apoptotic regulatory mechanisms interplaying with other molecules participates to the cellular cross-talk in developing tissues, which opens possible targeted modulations as suggested, e.g. for future molecular dentistry.

Role of transcriptional networks in coordinating early events during kidney development

Many of the signaling pathways that regulate tissue specification and coordinate cellular differentiation during embryogenesis have been identified over the last decade. These pathways are integrated at the transcriptional level, enabling activation of specific developmental programs in a temporally and spatially restricted fashion. Such developmental events are usually thought of in terms of hierarchical relationships, in which the expression of upstream factors leads to the sequential activation of a linear cascade of downstream genes. Whereas these models provide a simplistic approach to understand complex cellular events, genetic and biochemical studies in mice and other model organisms provide ample evidence that many of these factors interact at multiple levels in vivo and emphasize the importance of considering these linear events in context. The purpose of this review is to emphasize the complexity of these regulatory networks during the early phases of mammalian kidney development, outlining some of the limitations and alternative approaches that are being used to explore the complex nature of these networks in vivo. Before describing these networks in detail, we will provide a brief overview of the main structural changes and tissue interactions involved in mammalian kidney development, and go on to describe some of the limitations of our current approaches to evaluate the role of these developmental pathways in vivo.

H-Ras, R-Ras, and TC21 differentially regulate ureteric bud cell branching morphogenesis

The collecting system of the kidney, derived from the ureteric bud (UB), undergoes repetitive bifid branching events during early development followed by a phase of tubular growth and elongation. Although members of the Ras GTPase family control cell growth, differentiation, proliferation, and migration, their role in development of the collecting system of the kidney is unexplored. In this study, we demonstrate that members of the R-Ras family of proteins, R-Ras and TC21, are expressed in the murine collecting system at E13.5, whereas H-Ras is only detected at day E17.5. Using murine UB cells expressing activated H-Ras, R-Ras, and TC21, we demonstrate that R-Ras-expressing cells show increased branching morphogenesis and cell growth, TC21-expressing cells branch excessively but lose their ability to migrate, whereas H-Ras-expressing cells migrated the most and formed long unbranched tubules. These differences in branching morphogenesis are mediated by differential regulation/activation of the Rho family of GTPases and mitogen-activated protein kinases. Because most branching of the UB occurs early in development, it is conceivable that R-Ras and TC-21 play a role in facilitating branching and growth in early UB development, whereas H-Ras might favor cell migration and elongation of tubules, events that occur later in development.

Developmental Changes in the Expression of Tight Junction Protein Claudins in Murine Metanephroi and Embryonic Kidneys

Claudins are the major constituents of tight junction (TJ) strands and participate in the cell-cell adhesion and permeability barrier in epithelial cell layers. To investigate the suitability of metanephroi for analysis of the function of the TJ protein claudins in renal tubular formation, mouse metanephroi from embryos at day 12 of gestation were cultured and expression of claudins was compared with that in embryonic kidneys. During in vitro culture for 8 days, the metanephroi showed expression patterns very similar to those observed in embryonic kidneys in reverse transcription-polymerase chain reaction for the claudins examined, including claudins 1-4, 8, 10, 11, and 16, and the TJ proteins occludin and ZO-1. Immunofluorescence microscopy for claudins 1-4, 8, 10, and 16 showed localization of these claudins at the TJ with occludin and ZO-1 in some restricted tubular segments. These findings indicate that the metanephroi show developmental changes in the expression of the TJ protein claudins, representing those in embryonic kidneys, and thus suggest that the mouse metanephros is suitable to examine the functions of specific claudins in the kidney.

Disruption of polycystin-1 function interferes with branching morphogenesis of the ureteric bud in developing mouse kidneys

The polycystic kidney disease (PKD1) gene-encoded protein, polycystin-1, is developmentally regulated, with highest expression levels seen in normal developing kidneys, where it is distributed in a punctate pattern at the basal surface of ureteric bud epithelia. Overexpression in ureteric epithelial cell membranes of an inhibitory pMyr-GFP-PKD1 fusion protein via a retroviral (VVC) delivery system and microinjection into the ureteric bud lumen of embryonic day 11 mouse metanephric kidneys resulted in disrupted branching morphogenesis. Using confocal quantitative analysis, significant reductions were measured in the numbers of ureteric bud branch points and tips, as well as in the total ureteric bud length, volume and area, while significant increases were seen as dilations of the terminal branches, where significant increases in outer diameter and volumes were measured. Microinjection of an activating 5TM-GFP-PKD1 fusion protein had an opposite effect and showed significant increases in ureteric bud length and area. These are the first studies to experimentally manipulate polycystin-1 expression by transduction in the embryonic mouse kidney and suggest that polycystin-1 plays a critical role in the regulation of epithelial morphogenesis during renal development.

Watching tubules glow and branch

Branching morphogenesis is an important mechanism of animal development yet, until recently, most details about this highly dynamic process have had to be inferred from fixed tissues. Several groups have now developed transgenic animals in which branching tubules express fluorescent proteins, enabling their morphogenesis to be studied dynamically using time-lapse microscopy. The results have shown that branch emergence is highly variable, that sprouting tracheae and blood vessels guide themselves by filopodial projections, that branching morphogenesis can involve highly ordered cell rearrangements, and that branches are subject to intense remodelling. Though they are very new, these fluorescent systems have already expanded our knowledge of branching morphogenesis; future work, in which fluorescence might be used to report processes in addition to anatomy, promises an even greater advance.

PLAC1 expression increases during trophoblast differentiation: evidence for regulatory interactions with the fibroblast growth factor-7 (FGF-7) axis

PLAC1 is a recently described, trophoblast-specific gene that localizes to a region of the X-chromosome important in placental development. Immunohistochemical analysis demonstrated that PLAC1 polypeptide localizes to the differentiated syncytiotrophoblast throughout gestation (8-41 weeks) as well as a small population of villous cytotrophoblasts. Consistent with these observations, quantitative RT-PCR demonstrated that PLAC1 mRNA increases more than 300-fold during cytotrophoblast differentiation in culture to form syncytiotrophoblasts. Agents known to be relevant to trophoblast differentiation were then tested for the ability to influence PLAC1 expression. Fibroblast growth factor-7 (FGF-7), also known as keratinocyte growth factor (KGF), stimulated PLAC1 mRNA expression approximately two-fold in the BeWo(b30) trophoblast cell line. FGF-7 stimulation was significantly inhibited by PD-98059 and wortmannin suggesting mediation via MAP kinase and PI-3 kinase-dependent signaling pathways. Interestingly, epidermal growth factor (EGF) treatment of trophoblasts had no effect on PLAC1 expression alone, but potentiated the effect of FGF-7, suggesting the presence of a regulatory interaction of the two growth factors. FGF-7 and its receptor, FGFR-2b, exhibited spatial overlap with PLAC1 suggesting these regulatory interactions are physiologically relevant during gestation. These data demonstrate PLAC1 expression is upregulated during trophoblast differentiation, localizing primarily to the differentiated syncytiotrophoblast. Furthermore PLAC1 expression is specifically regulated by peptide growth factors relevant to trophoblast differentiation.

The role of GDNF in patterning the excretory system

Mesenchymal-epithelial interactions are an important source of information for pattern formation during organogenesis. In the developing excretory system, one of the secreted mesenchymal factors thought to play a critical role in patterning the growth and branching of the epithelial ureteric bud is GDNF. We have tested the requirement for GDNF as a paracrine chemoattractive factor by altering its site of expression during excretory system development. Normally, GDNF is secreted by the metanephric mesenchyme and acts via receptors on the Wolffian duct and ureteric bud epithelium. Misexpression of GDNF in the Wolffian duct and ureteric buds resulted in formation of multiple, ectopic buds, which branched independently of the metanephric mesenchyme. This confirmed the ability of GDNF to induce ureter outgrowth and epithelial branching in vivo. However, in mutant mice lacking endogenous GDNF, kidney development was rescued to a substantial degree by GDNF supplied only by the Wolffian duct and ureteric bud. These results indicate that mesenchymal GDNF is not required as a chemoattractive factor to pattern the growth of the ureteric bud within the developing kidney, and that any positional information provided by the mesenchymal expression of GDNF may provide for renal branching morphogenesis is redundant with other signals.

Midkine, a newly discovered regulator of the renin–angiotensin pathway in mouse aorta: Significance of the pleiotrophin/midkine developmental gene family in angiotensin II signaling

We previously demonstrated that pleiotrophin (PTN the protein, Ptn the gene) highly regulates the levels of expression of the genes encoding the proteins of the renin-angiotensin pathway in mouse aorta. We now demonstrate that the levels of expression of these same genes are significantly regulated in mouse aorta by the PTN family member midkine (MK the protein, Mk the gene); a 3-fold increase in expression of renin, an 82-fold increase in angiotensinogen, a 6-fold decrease in the angiotensin converting enzyme, and a 6.5-fold increase in the angiotensin II type 1 and a 9-fold increase in the angiotensin II type 2 receptor mRNAs were found in Mk-/- mouse aorta in comparison with the wild type (WT, +/+). The results in Mk-/- mice are remarkably similar to those previously reported in Ptn-/- mouse aorta, with the single exception of that the levels of the angiotensinogen gene expression in Ptn-/- mice are equal to those in WT+/+ mouse aorta, and thus, in contrast to Mk gene expression unaffected by levels of Ptn gene expression. The data indicate that MK and PTN share striking but not complete functional redundancy. These data support potentially high levels importance of MK and the MK/PTN developmental gene family in downstream signals initiated by angiotensin II either in development or in the many pathological conditions in which MK expression levels are increased, such as atherosclerosis and many human neoplasms that acquire constitutive endogenous Mk gene expression by mutation during tumor progression and potentially provide a target through the renin-angiotensin pathway to treat advanced malignancies.

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.

Implications of gene networks for understanding resilience and vulnerability in the kidney branching program

Branching morphogenesis in the kidney is tightly regulated. Whereas disruption of certain pathways produces catastrophic effects, numerous instances exist in which mutation of ostensibly key molecules has minimal apparent phenotypic consequence. We suggest how the network structure of gene interactions in the branching program might explain these findings as well as apparant discrepancies between in vivo and in vitro studies. Emerging genetic, cell-biological, and microarray data should help test and/or clarify these ideas.

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.