Tailbud-derived mesenchyme promotes urinary tract segmentation via BMP4 signaling

Urothelial progenitors in development and repair

Urothelium is a specialized multilayer epithelium that lines the urinary tract from the proximal urethra to the kidney. In addition to proliferation and differentiation during development, urothelial injury postnatally triggers a robust regenerative capacity to restore the protective barrier between the urine and tissue. Mounting evidence supports the existence of dedicated progenitor cell populations that give rise to urothelium during development and in response to injury. Understanding the cellular and molecular basis for urothelial patterning and repair will inform tissue regeneration therapies designed to ameliorate a number of structural and functional defects of the urinary tract. Here, we review the current understanding of urothelial progenitors and the signaling pathways that govern urothelial development and repair. While most published studies have focused on bladder urothelium, we also discuss literature on upper tract urothelial progenitors. Furthermore, we discuss evidence supporting existence of context-specific progenitors. This knowledge is fundamental to the development of strategies to regenerate or engineer damaged or diseased urothelium.

Renal engineering: strategies to address the problem of the ureter

Current techniques for making renal organoids generate tissues that show function when transplanted into a host, but they have no ureter through which urine can drain. There are at least 4 possible strategies for adding a ureter: connecting to ta host ureter; inducing an engineered kidney to make a ureter; making a stem-cell derived ureter; and replacement of only damaged cortex and outer medulla, using remaining host calyces, pelvis and ureter. Here we review progress: local BMP4 can induce a collecting duct tubule to become a ureter; a urothelial tube can be produced directly from pluripotent cells, and connect to the collecting duct system of a renal organoid; it is possible to graft ES cell-derived ureters into host kidney rudiments and see connection, smooth muscle development and spontaneous contraction, but this has not yet been achieved with all components being derived from ES cells. Remaining problems are discussed.

Osr1 Is Required for Mesenchymal Derivatives That Produce Collagen in the Bladder

The extracellular matrix of the bladder consists mostly of type I and III collagen, which are required during loading. During bladder injury, there is an accumulation of collagen that impairs bladder function. Little is known about the genes that regulate production of collagens in the bladder. We demonstrate that the transcription factor Odd-skipped related 1 (Osr1) is expressed in the bladder mesenchyme and epithelium at the onset of development. As development proceeds, Osr1 is mainly expressed in mesenchymal progenitors and their derivatives. We hypothesized that Osr1 regulates mesenchymal cell differentiation and production of collagens in the bladder. To test this hypothesis, we examined newborn and adult mice heterozygous for Osr1, Osr1^+/−. The bladders of newborn Osr1^+/− mice had a decrease in collagen I by western blot analysis and a global decrease in collagens using Sirius red staining. There was also a decrease in the cellularity of the lamina propria, where most collagen is synthesized. This was not due to decreased proliferation or increased apoptosis in this cell population. Surprisingly, the bladders of adult Osr1^+/− mice had an increase in collagen that was associated with abnormal bladder function; they also had a decrease in bladder capacity and voided more frequently. The results suggest that Osr1 is important for the differentiation of mesenchymal cells that give rise to collagen-producing cells.

A global Slc7a7 knockout mouse model demonstrates characteristic phenotypes of human lysinuric protein intolerance

Lysinuric protein intolerance (LPI) is an inborn error of cationic amino acid (arginine, lysine, ornithine) transport caused by biallelic pathogenic variants in SLC7A7, which encodes the light subunit of the y+LAT1 transporter. Treatments for the complications of LPI, including growth failure, renal disease, pulmonary alveolar proteinosis, autoimmune disorders and osteoporosis, are limited. Given the early lethality of the only published global Slc7a7 knockout mouse model, a viable animal model to investigate global SLC7A7 deficiency is needed. Hence, we generated two mouse models with global Slc7a7 deficiency (Slc7a7em1Lbu/em1Lbu; Slc7a7Lbu/Lbu and Slc7a7em1(IMPC)Bay/em1(IMPC)Bay; Slc7a7Bay/Bay) using CRISPR/Cas9 technology by introducing a deletion of exons 3 and 4. Perinatal lethality was observed in Slc7a7Lbu/Lbu and Slc7a7Bay/Bay mice on the C57BL/6 and C57BL/6NJ inbred genetic backgrounds, respectively. We noted improved survival of Slc7a7Lbu/Lbu mice on the 129 Sv/Ev × C57BL/6 F2 background, but postnatal growth failure occurred. Consistent with human LPI, these Slc7a7Lbu/Lbu mice exhibited reduced plasma and increased urinary concentrations of the cationic amino acids. Histopathological assessment revealed loss of brush border and lipid vacuolation in the renal cortex of Slc7a7Lbu/Lbu mice, which combined with aminoaciduria suggests proximal tubular dysfunction. Micro-computed tomography of L4 vertebrae and skeletal radiographs showed delayed skeletal development and suggested decreased mineralization in Slc7a7Lbu/Lbu mice, respectively. In addition to delayed skeletal development and delayed development in the kidneys, the lungs and liver were observed based on histopathological assessment. Overall, our Slc7a7Lbu/Lbu mouse model on the F2 mixed background recapitulates multiple human LPI phenotypes and may be useful for future studies of LPI pathology.

Hedgehog Signaling for Urogenital Organogenesis and Prostate Cancer: An Implication for the Epithelial-Mesenchyme Interaction (EMI)

Hedgehog (Hh) signaling is an essential growth factor signaling pathway especially in the regulation of epithelial-mesenchymal interactions (EMI) during the development of the urogenital organs such as the bladder and the external genitalia (EXG). The Hh ligands are often expressed in the epithelia, affecting the surrounding mesenchyme, and thus constituting a form of paracrine signaling. The development of the urogenital organ, therefore, provides an intriguing opportunity to study EMI and its relationship with other pathways, such as hormonal signaling. Cellular interactions of prostate cancer (PCa) with its neighboring tissue is also noteworthy. The local microenvironment, including the bone metastatic site, can release cellular signals which can affect the malignant tumors, and vice versa. Thus, it is necessary to compare possible similarities and divergences in Hh signaling functions and its interaction with other local growth factors, such as BMP (bone morphogenetic protein) between organogenesis and tumorigenesis. Additionally, this review will discuss two pertinent research aspects of Hh signaling: (1) the potential signaling crosstalk between Hh and androgen signaling; and (2) the effect of signaling between the epithelia and the mesenchyme on the status of the basement membrane with extracellular matrix structures located on the epithelial-mesenchymal interface.

Asymmetric paralog evolution between the "cryptic" gene Bmp16 and its well-studied sister genes Bmp2 and Bmp4

The vertebrate gene repertoire is characterized by “cryptic” genes whose identification has been hampered by their absence from the genomes of well-studied species. One example is the Bmp16 gene, a paralog of the developmental key genes Bmp2 and -4. We focus on the Bmp2/4/16 group of genes to study the evolutionary dynamics following gen(om)e duplications with special emphasis on the poorly studied Bmp16 gene. We reveal the presence of Bmp16 in chondrichthyans in addition to previously reported teleost fishes and reptiles. Using comprehensive, vertebrate-wide gene sampling, our phylogenetic analysis complemented with synteny analyses suggests that Bmp2, -4 and -16 are remnants of a gene quartet that originated during the two rounds of whole-genome duplication (2R-WGD) early in vertebrate evolution. We confirm that Bmp16 genes were lost independently in at least three lineages (mammals, archelosaurs and amphibians) and report that they have elevated rates of sequence evolution. This finding agrees with their more “flexible” deployment during development; while Bmp16 has limited embryonic expression domains in the cloudy catshark, it is broadly expressed in the green anole lizard. Our study illustrates the dynamics of gene family evolution by integrating insights from sequence diversification, gene repertoire changes, and shuffling of expression domains.

The molecular biology of pelvi-ureteric junction obstruction

Over recent years routine ultrasound scanning has identified increasing numbers of neonates as having hydronephrosis and pelvi-ureteric junction obstruction (PUJO). This patient group presents a diagnostic and management challenge for paediatric nephrologists and urologists. In this review we consider the known molecular mechanisms underpinning PUJO and review the potential of utilising this information to develop novel therapeutics and diagnostic biomarkers to improve the care of children with this disorder.

Asymmetric BMP4 signalling improves the realism of kidney organoids

We present a strategy for increasing the anatomical realism of organoids by applying asymmetric cues to mimic spatial information that is present in natural embryonic development, and demonstrate it using mouse kidney organoids. Existing methods for making kidney organoids in mice yield developing nephrons arranged around a symmetrical collecting duct tree that has no ureter. We use transplant experiments to demonstrate plasticity in the fate choice between collecting duct and ureter, and show that an environment rich in BMP4 promotes differentiation of early collecting ducts into uroplakin-positive, unbranched, ureter-like epithelial tubules. Further, we show that application of BMP4-releasing beads in one place in an organoid can break the symmetry of the system, causing a nearby collecting duct to develop into a uroplakin-positive, broad, unbranched, ureter-like ‘trunk’ from one end of which true collecting duct branches radiate and induce nephron development in an arrangement similar to natural kidneys. The idea of using local symmetry-breaking cues to improve the realism of organoids may have applications to organoid systems other than the kidney.

Coexistent duplication of urethra and a refluxing ectopic ureter presenting as recurrent epididymo-orchitis in a child

Congenital anomalies of the kidney and urinary tract (CAKUTs) occur in 3-6 per 1000 live births, accounting for most cases of paediatric end-stage kidney disease.1 However, the molecular basis of CAKUT and anomalies of the external genitalia is poorly understood. We, herein, describe a case with left recurrent epididymo-orchitis with a coexistent urethral duplication and an ectopic ureter with an ipsilateral non-functioning kidney, which is, to the best of our knowledge, the first reported case of its kind. This case may bring about a paradigm shift in our comprehension of the development of the two entities. Understanding the pathogenesis may help develop preventive and renal preservation strategies. The Sonic hedgehog gene and bone morphogenetic protein 4 play crucial roles in preventing anomalies of the ureters and the external genitalia. In this article, we look at possible molecular pathways that could explain the synchronicity of this rare entity.

A SHH-FOXF1-BMP4 signaling axis regulating growth and differentiation of epithelial and mesenchymal tissues in ureter development

The differentiated cell types of the epithelial and mesenchymal tissue compartments of the mature ureter of the mouse arise in a precise temporal and spatial sequence from uncommitted precursor cells of the distal ureteric bud epithelium and its surrounding mesenchyme. Previous genetic efforts identified a member of the Hedgehog (HH) family of secreted proteins, Sonic hedgehog (SHH) as a crucial epithelial signal for growth and differentiation of the ureteric mesenchyme. Here, we used conditional loss- and gain-of-function experiments of the unique HH signal transducer Smoothened (SMO) to further characterize the cellular functions and unravel the effector genes of HH signaling in ureter development. We showed that HH signaling is not only required for proliferation and SMC differentiation of cells of the inner mesenchymal region but also for survival of cells of the outer mesenchymal region, and for epithelial proliferation and differentiation. We identified the Forkhead transcription factor gene Foxf1 as a target of HH signaling in the ureteric mesenchyme. Expression of a repressor version of FOXF1 in this tissue completely recapitulated the mesenchymal and epithelial proliferation and differentiation defects associated with loss of HH signaling while re-expression of a wildtype version of FOXF1 in the inner mesenchymal layer restored these cellular programs when HH signaling was inhibited. We further showed that expression of Bmp4 in the ureteric mesenchyme depends on HH signaling and Foxf1, and that exogenous BMP4 rescued cell proliferation and epithelial differentiation in ureters with abrogated HH signaling or FOXF1 function. We conclude that SHH uses a FOXF1-BMP4 module to coordinate the cellular programs for ureter elongation and differentiation, and suggest that deregulation of this signaling axis occurs in human congenital anomalies of the kidney and urinary tract (CAKUT).

The mammalian ureter is a simple tube with a specialized multi-layered epithelium, the urothelium, and a surrounding coat of fibroblasts and peristaltically active smooth muscle cells. Besides its important function in urinary drainage, the ureter represents a simple model system to study epithelial and mesenchymal tissue interactions in organ development. The differentiated cell types of the ureter coordinately arise from precursor cells of the distal ureteric bud and its surrounding mesenchyme. How their survival, growth and differentiation is regulated and coordinated within and between the epithelial and mesenchymal tissue compartments is largely unknown. Previous work identified Sonic hedgehog (SHH) as a crucial epithelial signal for growth and differentiation of the ureteric mesenchyme, but the entirety of the cellular functions and the molecular mediators of its mesenchymal signaling pathway have remained obscure. Here we showed that epithelial SHH acts in a paracrine fashion onto the ureteric mesenchyme to activate a FOXF1-BMP4 regulatory module that directs growth and differentiation of both ureteric tissue compartments. HH signaling additionally acts in outer mesenchymal cells as a survival factor. Thus, SHH is an epithelial signal that coordinates various cellular programs in early ureter development.

Fibroblast growth factor receptor signaling in kidney and lower urinary tract development

Fibroblast growth factor receptors (FGFRs) and FGF ligands are highly expressed in the developing kidney and lower urinary tract. Several classic studies showed many effects of exogenous FGF ligands on embryonic renal tissues in vitro and in vivo. Another older landmark publication showed that mice with a dominant negative Fgfr fragment had severe renal dysplasia. Together, these studies revealed the importance of FGFR signaling in kidney and lower urinary tract development. With the advent of modern gene targeting techniques, including conditional knockout approaches, several publications have revealed critical roles for FGFR signaling in many lineages of the kidney and lower urinary tract at different stages of development. FGFR signaling has been shown to be critical for early metanephric mesenchymal patterning, Wolffian duct patterning including induction of the ureteric bud, ureteric bud branching morphogenesis, nephron progenitor survival and nephrogenesis, and bladder mesenchyme patterning. FGFRs pattern these tissues by interacting with many other growth factor signaling pathways. Moreover, the many genetic Fgfr and Fgf animal models have structural defects mimicking numerous congenital anomalies of the kidney and urinary tract seen in humans. Finally, many studies have shown how FGFR signaling is critical for kidney and lower urinary tract patterning in humans.

Identification of a multipotent self-renewing stromal progenitor population during mammalian kidney organogenesis

The mammalian kidney is a complex organ consisting of multiple cell types. We previously showed that the Six2-expressing cap mesenchyme is a multipotent self-renewing progenitor population for the main body of the nephron, the basic functional unit of the kidney. However, the cellular mechanisms establishing stromal tissues are less clear. We demonstrate that the Foxd1-expressing cortical stroma represents a distinct multipotent self-renewing progenitor population that gives rise to stromal tissues of the interstitium, mesangium, and pericytes throughout kidney organogenesis. Fate map analysis of Foxd1-expressing cells demonstrates that a small subset of these cells contributes to Six2-expressing cells at the early stage of kidney outgrowth. Thereafter, there appears to be a strict nephron and stromal lineage boundary derived from Six2-expressing and Foxd1-expressing cell types, respectively. Taken together, our observations suggest that distinct multipotent self-renewing progenitor populations coordinate cellular differentiation of the nephron epithelium and renal stroma during mammalian kidney organogenesis.

Smad4 regulates ureteral smooth muscle cell differentiation during mouse embryogenesis

Proper formation of ureteral smooth muscle cells (SMCs) during embryogenesis is essential for ureter peristalsis that propels urine from the kidney to the bladder in mammals. Currently the molecular factors that regulate differentiation of ureteral mesenchymal cells into SMCs are incompletely understood. A recent study has reported that Smad4 deficiency reduces the number of ureteral SMCs. However, its precise role in the ureteral smooth muscle development remains largely unknown. Here, we used Tbx18:Cre knock-in mouse line to delete Smad4 to examine its requirement in the development of ureteral mesenchyme and SMC differentiation. We found that mice with specific deletion of Smad4 in Tbx18-expressing ureteral mesenchyme exhibited hydroureter and hydronephrosis at embryonic day (E) 16.5, and the mutant mesenchymal cells failed to differentiate into SMCs with increased apoptosis and decreased proliferation. Molecular markers for SMCs including alpha smooth muscle actin (α-SMA) and smooth muscle myosin heavy chain (SM-MHC) were absent in the mutant ureters. Moreover, disruption of Smad4 significantly reduced the expression of genes, including Sox9, Tbx18 and Myocardin associated with SMC differentiation. These findings suggest that Smad4 is essential for initiating the SMC differentiation program during ureter development.

Renal branching morphogenesis: morphogenetic and signaling mechanisms

The human kidney is composed of an arborized network of collecting ducts, calyces and urinary pelvis that facilitate urine excretion and regulate urine composition. The renal collecting system is formed in utero, completed by the 34th week of gestation in humans, and dictates final nephron complement. The renal collecting system arises from the ureteric bud, a derivative of the intermediate-mesoderm derived nephric duct that responds to inductive signals from adjacent tissues via a process termed ureteric induction. The ureteric bud subsequently undergoes a series of iterative branching and remodeling events in a process called renal branching morphogenesis. Altered signaling that disrupts patterning of the nephric duct, ureteric induction, or renal branching morphogenesis leads to varied malformations of the renal collecting system collectively known as congenital anomalies of the kidney and urinary tract (CAKUT) and is the most frequently detected congenital renal aberration in infants. Here, we describe critical morphogenetic and cellular events that govern nephric duct specification, ureteric bud induction, renal branching morphogenesis, and cessation of renal branching morphogenesis. We also highlight salient molecular signaling pathways that govern these processes, and the investigative techniques used to interrogate them.

Vesicoureteric reflux and reflux nephropathy: from mouse models to childhood disease

Vesicoureteric reflux (VUR) is a common congenital urinary tract defect that predisposes children to recurrent kidney infections. Kidney infections can result in renal scarring or reflux nephropathy defined by the presence of chronic tubulo-interstitial inflammation and fibrosis that is a frequent cause of end-stage renal failure. The discovery of mouse models with VUR and with reflux nephropathy has provided new opportunities to understand the pathogenesis of these conditions and may provide insight on the genes and the associated phenotypes that need to be examined in human studies.

Genetic controls and cellular behaviors in branching morphogenesis of the renal collecting system

The mammalian kidney, which at maturity contains thousands of nephrons joined to a highly branched collecting duct (CD) system, is an important model system for studying the development of a complex organ. Furthermore, congenital anomalies of the kidney and urinary tract, often resulting from defects in ureteric bud branching morphogenesis, are relatively common human birth defects. Kidney development is initiated by interactions between the nephric duct and the metanephric mesenchyme, leading to the outgrowth and repeated branching of the ureteric bud epithelium, which gives rise to the entire renal CD system. Meanwhile, signals from the ureteric bud induce the mesenchyme cells to form the nephron epithelia. This review focuses on development of the CD system, with emphasis on the mouse as an experimental system. The major topics covered include the origin and development of the nephric duct, formation of the ureteric bud, branching morphogenesis of the ureteric bud, and elongation of the CDs. The signals, receptors, transcription factors, and other regulatory molecules implicated in these processes are discussed. In addition, our current knowledge of cellular behaviors that are controlled by these genes and underlie development of the collecting system is reviewed.

Follistatin-like 1 in vertebrate development

Follistatin-like 1 (Fstl1) is a member of the secreted protein acidic rich in cysteins (SPARC) family and has been implicated in many different signaling pathways, including bone morphogenetic protein (BMP) signaling. In many different developmental processes like, dorso-ventral axis establishment, skeletal, lung and ureter development, loss of function experiments have unveiled an important role for Fstl1. Fstl1 largely functions through inhibiting interactions with the BMP signaling pathway, although, in various disease models, different signaling pathways, like activation of pAKT, pAMPK, Na/K-ATPase, or innate immune responses, are linked to Fstl1. How Fstl1 inhibits BMP signaling remains unclear, although it is known that Fstl1 does not function through a scavenging mechanism, like the other known extracellular BMP inhibitors such as noggin. It has been proposed that Fstl1 interferes with BMP receptor complex formation and as such inhibits propagation of the BMP signal into the cell. Future challenges will encompass the identification of the factors that determine the mechanisms that underlie the fact that Fstl1 acts by interfering with BMP signaling during development, but through other signaling pathways during disease.

Directing human embryonic stem cell differentiation towards a renal lineage generates a self-organizing kidney

With the prevalence of end-stage renal disease rising 8% per annum globally, there is an urgent need for renal regenerative strategies. The kidney is a mesodermal organ that differentiates from the intermediate mesoderm (IM) through the formation of a ureteric bud (UB) and the interaction between this bud and the adjacent IM-derived metanephric mesenchyme (MM). The nephrons arise from a nephron progenitor population derived from the MM (ref. ). The IM itself is derived from the posterior primitive streak. Although the developmental origin of the kidney is well understood, nephron formation in the human kidney is completed before birth. Hence, there is no postnatal stem cell able to replace lost nephrons. In this study, we have successfully directed the differentiation of human embryonic stem cells (hESCs) through posterior primitive streak and IM under fully chemically defined monolayer culture conditions using growth factors used during normal embryogenesis. This differentiation protocol results in the synchronous induction of UB and MM that forms a self-organizing structure, including nephron formation, in vitro. Such hESC-derived components show broad renal potential ex vivo, illustrating the potential for pluripotent-stem-cell-based renal regeneration.

The EYA-SO/SIX complex in development and disease

Eyes absent (EYA) and Sine oculis (SO/SIX) proteins function as transcriptional activation complexes and play essential roles in organogenesis during embryonic development in regulating cell proliferation and survival and coordination of particular differentiation programs. Mutations of the Eya and So/Six genes cause profound developmental defects in organisms as diverse as flies, frogs, fish, mice, and humans. EYA proteins also possess an intrinsic phosphatase activity, which is essential for normal development. Here, we review crucial roles of EYA and SO/SIX in development and disease in mice and humans.

Tbx18 expression demarcates multipotent precursor populations in the developing urogenital system but is exclusively required within the ureteric mesenchymal lineage to suppress a renal stromal fate

The mammalian urogenital system derives from multipotent progenitor cells of different germinal tissues. The contribution of individual sub-populations to specific components of the mature system, and the spatiotemporal restriction of the respective lineages have remained poorly characterized. Here, we use comparative expression analysis to delineate sub-regions within the developing urogenital system that express the T-box transcription factor gene Tbx18. We show that Tbx18 is transiently expressed in the epithelial lining and the subjacent mesenchyme of the urogenital ridge. At the onset of metanephric development Tbx18 expression occurs in a band of mesenchyme in between the metanephros and the Wolffian duct but is subsequently restricted to the mesenchyme surrounding the distal ureter stalk. Genetic lineage tracing reveals that former Tbx18(+) cells of the urogenital ridge and the metanephric field contribute substantially to the adrenal glands and gonads, to the kidney stroma, the ureteric and the bladder mesenchyme. Loss of Tbx18 does not affect differentiation of the adrenal gland, the gonad, the bladder and the kidney. However, ureter differentiation is severely disturbed as the mesenchymal lineage adopts a stromal rather than a ureteric smooth muscle fate. DiI labeling and tissue recombination experiments show that the restriction of Tbx18 expression to the prospective ureteric mesenchyme does not reflect an active condensation process but is due to a specific loss of Tbx18 expression in the mesenchyme out of range of signals from the ureteric epithelium. These cells either contribute to the renal stroma or undergo apoptosis aiding in severing the ureter from its surrounding tissues. We show that Tbx18-deficient cells do not respond to epithelial signals suggesting that Tbx18 is required to prepattern the ureteric mesenchyme. Our study provides new insights into the molecular diversity of urogenital progenitor cells and helps to understand the specification of the ureteric mesenchymal sub-lineage.

TSHZ3 and SOX9 regulate the timing of smooth muscle cell differentiation in the ureter by reducing myocardin activity

Smooth muscle cells are of key importance for the proper functioning of different visceral organs including those of the urogenital system. In the mouse ureter, the two transcriptional regulators TSHZ3 and SOX9 are independently required for initiation of smooth muscle differentiation from uncommitted mesenchymal precursor cells. However, it has remained unclear whether TSHZ3 and SOX9 act independently or as part of a larger regulatory network. Here, we set out to characterize the molecular function of TSHZ3 in the differentiation of the ureteric mesenchyme. Using a yeast-two-hybrid screen, we identified SOX9 as an interacting protein. We show that TSHZ3 also binds to the master regulator of the smooth muscle program, MYOCD, and displaces it from the coregulator SRF, thereby disrupting the activation of smooth muscle specific genes. We found that the initiation of the expression of smooth muscle specific genes in MYOCD-positive ureteric mesenchyme coincides with the down regulation of Sox9 expression, identifying SOX9 as a possible negative regulator of smooth muscle cell differentiation. To test this hypothesis, we prolonged the expression of Sox9 in the ureteric mesenchyme in vivo. We found that Sox9 does not affect Myocd expression but significantly reduces the expression of MYOCD/SRF-dependent smooth muscle genes, suggesting that down-regulation of Sox9 is a prerequisite for MYOCD activity. We propose that the dynamic expression of Sox9 and the interaction between TSHZ3, SOX9 and MYOCD provide a mechanism that regulates the pace of progression of the myogenic program in the ureter.

Lower urinary tract development and disease

Congenital anomalies of the lower urinary tract (CALUT) are a family of birth defects of the ureter, the bladder, and the urethra. CALUT includes ureteral anomaliesc such as congenital abnormalities of the ureteropelvic junction (UPJ) and ureterovesical junction (UVJ), and birth defects of the bladder and the urethra such as bladder-exstrophy-epispadias complex (BEEC), prune belly syndrome (PBS), and posterior urethral valves (PUVs). CALUT is one of the most common birth defects and is often associated with antenatal hydronephrosis, vesicoureteral reflux (VUR), urinary tract obstruction, urinary tract infections (UTI), chronic kidney disease, and renal failure in children. Here, we discuss the current genetic and molecular knowledge about lower urinary tract development and genetic basis of CALUT in both human and mouse models. We provide an overview of the developmental processes leading to the formation of the ureter, the bladder, and the urethra, and different genes and signaling pathways controlling these developmental processes. Human genetic disorders that affect the ureter, the bladder and the urethra and associated gene mutations are also presented. As we are entering the postgenomic era of personalized medicine, information in this article may provide useful interpretation for the genetic and genomic test results collected from patients with lower urinary tract birth defects. With evidence-based interpretations, clinicians may provide more effective personalized therapies to patients and genetic counseling for their families.

Analysis of the Sonic Hedgehog signaling pathway in normal and abnormal bladder development

In this study, we examined the expression of Sonic Hedgehog, Patched, Gli1, Gli2, Gli3 and Myocardin in the developing bladders of male and female normal and megabladder (mgb-/-) mutant mice at embryonic days 12 through 16 by in situ hybridization. This analysis indicated that each member of the Sonic Hedgehog signaling pathway as well as Myocardin displayed distinct temporal and spatial patterns of expression during normal bladder development. In contrast, mgb-/- bladders showed both temporal and spatial changes in the expression of Patched, Gli1 and Gli3 as well as a complete lack of Myocardin expression. These changes occurred primarily in the outer mesenchyme of developing mgb-/- bladders consistent with the development of an amuscular bladder phenotype in these animals. These results provide the first comprehensive analysis of the Sonic Hedgehog signaling pathway during normal bladder development and provide strong evidence that this key signaling cascade is critical in establishing radial patterning in the developing bladder. In addition, the lack of detrusor smooth muscle development observed in mgb-/- mice is associated with bladder-specific temporospatial changes in Sonic Hedgehog signaling coupled with a lack of Myocardin expression that appears to result in altered patterning of the outer mesenchyme and poor initiation and differentiation of smooth muscle cells within this region of the developing bladder.

Cell biology of ureter development

The mammalian ureter contains two main cell types: a multilayered water-tight epithelium called the urothelium, surrounded by smooth muscle layers that, by generating proximal to distal peristaltic waves, pump urine from the renal pelvis toward the urinary bladder. Here, we review the cellular mechanisms involved in the development of these tissues, and the molecules that control the process. We consider the relevance of these biologic findings for understanding the pathogenesis of human ureter malformations.

Plumbing the depths of urinary tract obstruction by using murine models

Urinary tract obstruction leads to obstructive nephropathy, which in turn, frequently results in renal failure. Congenital urinary tract obstruction can be traced back to errors during the organogenesis of the urinary system. A fundamental understanding of the causes of urinary tract obstruction and the developmental processes involved are critical for improving the diagnostic and therapeutic strategies for this disease. A number of laboratories, including ours, have been using genetically engineered and spontaneously occurring mouse models to study the primary causes and the pathogenesis of urinary tract obstruction. These studies have shown that urinary tract obstruction is a very heterogeneous disease that can be caused by a diverse set of factors targeting multiple levels of the urinary system. Accumulating evidence also indicates that the development of the urinary tract requires the integration of progenitor cells of diverse embryonic origins, leading to the formation of multiple junctions prone to developmental errors. In addition, the high sensitivity of the pyeloureteral peristaltic machinery to disturbance affecting the structural or functional integrity of its components also contributes to the high incidence rate of urinary tract obstruction.

Canonical Wnt signaling regulates smooth muscle precursor development in the mouse ureter

Smooth muscle cells (SMCs) are a key component of many visceral organs, including the ureter, yet the molecular pathways that regulate their development from mesenchymal precursors are insufficiently understood. Here, we identified epithelial Wnt7b and Wnt9b as possible ligands of Fzd1-mediated β-catenin (Ctnnb1)-dependent (canonical) Wnt signaling in the adjacent undifferentiated ureteric mesenchyme. Mice with a conditional deletion of Ctnnb1 in the ureteric mesenchyme exhibited hydroureter and hydronephrosis at newborn stages due to functional obstruction of the ureter. Histological analysis revealed that the layer of undifferentiated mesenchymal cells directly adjacent to the ureteric epithelium did not undergo characteristic cell shape changes, exhibited reduced proliferation and failed to differentiate into SMCs. Molecular markers for prospective SMCs were lost, whereas markers of the outer layer of the ureteric mesenchyme fated to become adventitial fibroblasts were expanded to the inner layer. Conditional misexpression of a stabilized form of Ctnnb1 in the prospective ureteric mesenchyme resulted in the formation of a large domain of cells that exhibited histological and molecular features of prospective SMCs and differentiated along this lineage. Our analysis suggests that Wnt signals from the ureteric epithelium pattern the ureteric mesenchyme in a radial fashion by suppressing adventitial fibroblast differentiation and initiating smooth muscle precursor development in the innermost layer of mesenchymal cells.

Mammalian kidney development: principles, progress, and projections

The mammalian kidney is a vital organ with considerable cellular complexity and functional diversity. Kidney development is notable for requiring distinct but coincident tubulogenic processes involving reciprocal inductive signals between mesenchymal and epithelial progenitor compartments. Key molecular pathways mediating these interactions have been identified. Further, advances in the analysis of gene expression and gene activity, coupled with a detailed knowledge of cell origins, are enhancing our understanding of kidney morphogenesis and unraveling the normal processes of postnatal repair and identifying disease-causing mechanisms. This article focuses on recent insights into central regulatory processes governing organ assembly and renal disease, and predicts future directions for the field.

Mutation screening of BMP4 and Id2 genes in Chinese patients with congenital ureteropelvic junction obstruction

Ureteropelvic junction obstruction (UPJO) is the most common congenital anomaly of the urinary tract. Evidence has shown that BMP4 and Id2 play crucial roles in nephrogenesis, alterations of which may cause ureteral developmental anomalies. Here, we directly sequenced the coding sequences in BMP4 and Id2 genes of 108 unrelated Chinese patients with ureteropelvic junction stenosis. One missense mutation c.485G> A (p.R162Q) in BMP4 and two synonymous mutations (c.1167T> C in BMP4 and c.108A> G in Id2) were detected in three cases. None of these variations were present in the 150 normal controls. Comparative amino acid sequence alignments of BMP4 in humans and other vertebrate orthologs show that p.R162 located to a highly conserved amino acid residue. Moreover, computational analysis predicted that R162Q probably infect the function of BMP4 protein.

CONCLUSION

The mutation c.485G> A in BMP4 might be one of the causes of human UPJO. Further functional studies are required to validate the association between this variation and UPJO. Otherwise, Id2 mutations do not seem to be involved in this disease.

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.

Six1 and Eya1 are critical regulators of peri-cloacal mesenchymal progenitors during genitourinary tract development

The evolutionarily conserved Six1-Eya1 transcription complex is central to mammalian organogenesis, and deletion of these genes in mice results in developmental anomalies of multiple organs that recapitulate human branchio-oto-renal (BOR) and DiGeorge syndromes. Here, we report that both Six1 and Eya1 are strongly expressed in the peri-cloacal mesenchyme (PCM) surrounding the cloaca, the terminal end of hindgut dilation. Six1 and Eya1 are absent from the intra-cloacal mesenchyme (ICM), a cell mass that divides the cloaca into dorsal hindgut and ventral urogenital sinus. Deletion of either or both Six1 and Eya1 genes results in a spectrum of genitourinary tract defects including persistent cloaca - hypoplastic perineum tissue between external urogenital and anorectal tracts; hypospadias - ectopic ventral positioning of the urethral orifice; and hypoplastic genitalia. Analyses of critical signaling molecules indicate normal expression of Shh in the cloaca and cloaca-derived endodermal epithelia. Using a Cre/loxP genetic fate mapping strategy, we demonstrate that Six1-positive PCM progenitors give rise to the most caudal structures of the body plan including the urogenital and anorectal complex, and the perineum region. Thus, Six1 and Eya1 are key regulators of both upper and lower urinary tract morphogenesis. Results from this study uncover essential roles of the PCM progenitors during genitourinary tract formation.

BMP signaling in the nephron progenitor niche

Bone morphogenic proteins (BMPs) play diverse roles in embryonic kidney development, regulating essential aspects of both ureteric bud and nephron development. In this review, we provide an overview of reported expression patterns and functions of BMP signaling components within the nephrogenic zone or nephron progenitor niche of the developing kidney. Reported in situ hybridization results are relatively challenging to interpret and sometimes conflicting. Comparing these with high-resolution microarray gene expression data available in Gudmap, we propose a consensus gene expression pattern indicating that essential components of both the Smad-mediated pathway and the Smad-independent MAPK pathways are expressed in the nephron progenitor cell compartment and may be activated by BMPs, but that cortical interstitium may only be able to respond to BMPs through mitogen activated protein kinase (MAPK) signaling. Localization of phosphorylated Smad transcription factors and studies of a BMP reporter mouse strain however indicate limited transcriptional responsiveness to Smad-mediated signaling in cap mesenchyme. An overview of genetic inactivation, organ culture, and primary cell studies indicates that BMP signaling may elicit two important biological outcomes in the nephrogenic zone: survival of the cap mesenchyme, and the physical segregation of interstitial and progenitor cell compartments. Ongoing studies using a novel primary cell system that establishes the nephrogenic zone ex vivo are pursuing the concept that the balance between Smad-mediated and Smad-independent responses to BMP ligand may underlie these distinct outcomes.

Expression patterns and roles of periostin during kidney and ureter development

PURPOSE

Periostin is a secreted extracellular matrix protein that is differentially expressed in the developing kidney. We analyzed the temporal-spatial expression of periostin in the developing kidney and ureter as well as its roles in ureter branching morphogenesis, nephrogenesis and ureter development.

MATERIALS AND METHODS

RNA in situ hybridization and immunofluorescence histochemistry were used to investigate the expression of periostin, αv integrin and α-smooth muscle actin during mouse renal and ureteral development. Metanephric explants were cultured in the presence of recombinant periostin, and ureteral branch points/tips and the glomerular number were quantified. Explants were also cultured in the presence of exogenous bone morphogenetic protein 4 and the effect on periostin mRNA levels was determined by quantitative real-time polymerase chain reaction.

RESULTS

Periostin expression was observed in the mesenchyme surrounding the kidney and ureter, renal stroma, metanephric mesenchyme, ureter epithelium and developing nephrons. At embryonic day 15.5 periostin and αv integrin, a common subunit of periostin receptors, were co-expressed in smooth muscle cells of the ureter, renal artery and intrarenal arteries. Bone morphogenetic protein 4 up-regulated periostin mRNA expression and exogenous periostin inhibited branching morphogenesis and glomerular number.

CONCLUSIONS

Bone morphogenetic protein 4 which inhibits ureteral branching morphogenesis and promotes smooth muscle cell migration in the ureter up-regulated periostin mRNA expression in the developing kidney. Ureteral smooth muscle cells express periostin and αv integrin. Periostin inhibited ureteral branching morphogenesis and glomerular number. Together these results suggest that periostin and bone morphogenetic protein 4 may have a role in branching morphogenesis, nephrogenesis and possibly smooth muscle cell migration.

Hox10 genes function in kidney development in the differentiation and integration of the cortical stroma

Organogenesis requires the differentiation and integration of distinct populations of cells to form a functional organ. In the kidney, reciprocal interactions between the ureter and the nephrogenic mesenchyme are required for organ formation. Additionally, the differentiation and integration of stromal cells are also necessary for the proper development of this organ. Much remains to be understood regarding the origin of cortical stromal cells and the pathways involved in their formation and function. By generating triple mutants in the Hox10 paralogous group genes, we demonstrate that Hox10 genes play a critical role in the developing kidney. Careful examination of control kidneys show that Foxd1-expressing stromal precursor cells are first observed in a cap-like pattern anterior to the metanephric mesenchyme and these cells subsequently integrate posteriorly into the kidney periphery as development proceeds. While the initial cap-like pattern of Foxd1-expressing cortical stromal cells is unaffected in Hox10 mutants, these cells fail to become properly integrated into the kidney, and do not differentiate to form the kidney capsule. Consistent with loss of cortical stromal cell function, Hox10 mutant kidneys display reduced and aberrant ureter branching, decreased nephrogenesis. These data therefore provide critical novel insights into the cellular and genetic mechanisms governing cortical cell development during kidney organogenesis. These results, combined with previous evidence demonstrating that Hox11 genes are necessary for patterning the metanephric mesenchyme, support a model whereby distinct populations in the nephrogenic cord are regulated by unique Hox codes, and that differential Hox function along the AP axis of the nephrogenic cord is critical for the differentiation and integration of these cell types during kidney organogenesis.

Genetic analysis reveals an unexpected role of BMP7 in initiation of ureteric bud outgrowth in mouse embryos

Background

Genetic analysis in the mouse revealed that GREMLIN1 (GREM1)-mediated antagonism of BMP4 is essential for ureteric epithelial branching as the disruption of ureteric bud outgrowth and renal agenesis in Grem1-deficient embryos is restored by additional inactivation of one Bmp4 allele. Another BMP ligand, BMP7, was shown to control the proliferative expansion of nephrogenic progenitors and its requirement for nephrogenesis can be genetically substituted by Bmp4. Therefore, we investigated whether BMP7 in turn also participates in inhibiting ureteric bud outgrowth during the initiation of metanephric kidney development.

Methodology/Principal Findings

Genetic inactivation of one Bmp7 allele in Grem1-deficient mouse embryos does not alleviate the bilateral renal agenesis, while complete inactivation of Bmp7 restores ureteric bud outgrowth and branching. In mouse embryos lacking both Grem1 and Bmp7, GDNF/WNT11 feedback signaling and the expression of the Etv4 target gene, which regulates formation of the invading ureteric bud tip, are restored. In contrast to the restoration of ureteric bud outgrowth and branching, nephrogenesis remains aberrant as revealed by the premature loss of Six2 expressing nephrogenic progenitor cells. Therefore, very few nephrons develop in kidneys lacking both Grem1 and Bmp7 and the resulting dysplastic phenotype is indistinguishable from the one of Bmp7-deficient mouse embryos.

Conclusions/Significance

Our study reveals an unexpected inhibitory role of BMP7 during the onset of ureteric bud outgrowth. As BMP4, BMP7 and GREM1 are expressed in distinct mesenchymal and epithelial domains, the localized antagonistic interactions of GREM1 with BMPs could restrict and guide ureteric bud outgrowth and branching. The robustness and likely significant redundancy of the underlying signaling system is evidenced by the fact that global reduction of Bmp4 or inactivation of Bmp7 are both able to restore ureteric bud outgrowth and epithelial branching in Grem1-deficient mouse embryos.

Six1 regulates Grem1 expression in the metanephric mesenchyme to initiate branching morphogenesis

Urinary tract morphogenesis requires subdivision of the ureteric bud (UB) into the intra-renal collecting system and the extra-renal ureter, by responding to signals in its surrounding mesenchyme. BMP4 is a mesenchymal regulator promoting ureter development, while GREM1 is necessary to negatively regulate BMP4 activity to induce UB branching. However, the mechanisms that regulate the GREM1-BMP4 signaling are unknown. Previous studies have shown that Six1-deficient mice lack kidneys, but form ureters. Here, we show that the tip cells of Six1(-/-) UB fail to form an ampulla for branching. Instead, the UB elongates within Tbx18- and Bmp4-expressing mesenchyme. We find that the expression of Grem1 in the metanephric mesenchyme (MM) is Six1-dependent. Treatment of Six1(-/-) kidney rudiments with GREM1 protein restores ampulla formation and branching morphogenesis. Furthermore, we demonstrate that genetic reduction of BMP4 levels in Six1(-/-) (Six1(-/-); Bmp4(+/-)) embryos restores urinary tract morphogenesis and kidney formation. This study uncovers an essential function for Six1 in the MM as an upstream regulator of Grem1 in initiating branching morphogenesis.

Comprehensive genetic analysis of OEIS complex reveals no evidence for a recurrent microdeletion or duplication

Omphalocele-exstrophy of the bladder-imperforate anus-spinal defects (OEIS) complex, or cloacal exstrophy (EC), is a rare constellation of malformations in humans involving the urogenital, gastrointestinal, and skeletal systems, and less commonly the central nervous system. Although OEIS complex is well-recognized in the clinical setting, there remains a significant lack of understanding of this condition at both the developmental and the genetic level. While most cases are sporadic, familial cases have been reported, suggesting that one or more specific genes may play a significant role in this condition. Several developmental mechanisms have been proposed to explain the etiology of OEIS complex, and it is generally considered to be a defect early in caudal mesoderm development and ventral body wall closure. The goal of this study was to identify genetic aberrations in 13 patients with OEIS/EC using a combination of candidate gene analysis and microarray studies. Analysis of 14 candidate genes in combination with either high resolution SNP or oligonucleotide microarray did not reveal any disease-causing mutations, although novel variants were identified in five patients. To our knowledge, this is the most comprehensive genetic analysis of patients with OEIS complex to date. We conclude that OEIS is a complex disorder from an etiological perspective, likely involving a combination of genetic and environmental predispositions. Based on our data, OEIS complex is unlikely to be caused by a recurrent chromosomal aberration.

Hydroureternephrosis due to loss of Sox9-regulated smooth muscle cell differentiation of the ureteric mesenchyme

Congenital ureter anomalies, including hydroureter, affect up to 1% of the newborn children. Despite the prevalence of these developmental abnormalities in young children, the underlying molecular causes are only poorly understood. Here, we show that the high mobility group domain transcription factor Sox9 plays an important role in ureter development in the mouse. Transient Sox9 expression was detected in the undifferentiated ureteric mesenchyme and inactivation of Sox9 in this domain resulted in strong proximal hydroureter formation due to functional obstruction. Loss of Sox9 did not affect condensation, proliferation and apoptosis of the undifferentiated mesenchyme, but perturbed cyto-differentiation into smooth muscle cells (SMCs). Expression of genes encoding extracellular matrix (ECM) components was strongly reduced, suggesting that deficiency in ECM composition and/or signaling may underlie the observed defects. Prolonged expression of Sox9 in the ureteric mesenchyme led to increased deposition of ECM components and SMC dispersal. Furthermore, Sox9 genetically interacts with the T-box transcription factor 18 gene (Tbx18) during ureter development at two levels--as a downstream mediator of Tbx18 function and in a converging pathway. Together, our results argue that obstructive uropathies in campomelic dysplasia patients that are heterozygous for mutations in and around SOX9 arise from a primary requirement of Sox9 in the development of the ureteric mesenchyme.

Smooth muscle differentiation and patterning in the urinary bladder

Smooth muscle differentiation and patterning is a fundamental process in urinary bladder development that involves a complex array of local environmental factors, epithelial-mesenchymal interaction, and signaling pathways. An epithelial signal is necessary to induce smooth muscle differentiation in the adjacent bladder mesenchyme. The bladder epithelium (urothelium) also influences the spatial organization of the bladder wall. Sonic hedgehog (Shh), which is expressed by the urothelium, promotes mesenchymal proliferation and induces differentiation of smooth muscle from embryonic bladder mesenchyme. Shh, whose signal is mediated through various transcription factors including Gli2 and BMP4, is likely also important in the patterning of bladder smooth muscle. However, it is not known to what extent early mediators of mesenchymal migration, other Shh-associated transcription factors, and crosstalk between the Shh signaling cascade and other pathways are involved in the patterning of bladder smooth muscle. Here we review the role of epithelial-mesenchymal interaction and Shh signaling in smooth muscle differentiation and patterning in the bladder. We also discuss emerging signaling molecules, transcription factors, and mesenchyme properties that might be fruitful areas of future research in the process of smooth muscle formation in the bladder.

SIX1 acts synergistically with TBX18 in mediating ureteral smooth muscle formation

Dysfunction of the ureter often leads to urine flow impairment from the kidney to the bladder, causing dilation of the ureter and/or renal pelvis. Six1 is a crucial regulator of renal development: mutations in human SIX1 cause branchio-oto-renal (BOR) syndrome and Six1(-/-) mice exhibit renal agenesis, although the ureter is present. It remains unclear whether Six1 plays a role in regulating ureter morphogenesis. We demonstrate here that Six1 is differentially expressed during ureter morphogenesis. It was expressed in undifferentiated smooth muscle (SM) progenitors, but was downregulated in differentiating SM cells (SMCs) and had disappeared by E18.5. In Six1(-/-) mice, the ureteral mesenchymal precursors failed to condense and differentiate into normal SMCs and showed increased cell death, indicating that Six1 is required for the maintenance and normal differentiation of SM progenitors. A delay in SMC differentiation was observed in Six1(-/-) ureters. A lack of Six1 in the ureter led to hydroureter and hydronephrosis without anatomical obstruction when kidney formation was rescued in Six1(-/-) embryos by specifically expressing Six1 in the metanephric mesenchyme, but not the ureter, under control of the Eya1 promoter. We show that Six1 and Tbx18 genetically interact to synergistically regulate SMC development and ureter function and that their gene products form a complex in cultured cells and in the developing ureter. Two missense mutations in SIX1 from BOR patients reduced or abolished SIX1-TBX18 complex formation. These findings uncover an essential role for Six1 in establishing a functionally normal ureter and provide new insights into the molecular basis of urinary tract malformations in BOR patients.

Ureter myogenesis: putting Teashirt into context

After the basic shape of the mammalian ureter is established, its epithelia mature and a coat of smooth muscle cells differentiate around nascent urothelia. The ureter actively propels tubular fluid from the renal pelvis to the bladder, and this peristalsis, which starts in the fetal period, requires coordinated smooth muscle contraction. Teashirt-3 (Tshz3) is expressed in smooth muscle cell precursors that form the wall of the forming mammalian ureter. The Teashirt gene family was first identified in Drosophila where Teashirt (Tsh) protein acts as a transcription factor directing embryonic anterior-posterior patterning and leg and eye development. In fly embryonic renal tubules, Tsh is expressed in mesodermally derived stellate cells intercalating between principal cells, and a paralogue, tiptop, is expressed in forming tubules. Teashirt is a component of several gene networks in flies and it is notable that similar networks control mammalian renal tract development. Null mutation of Tshz3 in mice leads to failure of functional muscularization in the top of the ureter and this is followed by congenital hydronephrosis. A signaling pathway can be envisaged, starting with sonic hedgehog secreted by the nascent ureteric urothelium and ending with ureteric smooth muscle cell differentiation, with Tshz3 downstream of bone morphogenetic protein 4 and upstream of myocardin and smooth muscle cell contractile protein synthesis. The phenotype of Tshz3 mutant mice resembles that of human congenital pelviureteric junction obstruction, and we suggest these individuals may have mutations of genes encoding molecules in the differentiation pathway mediated by Tshz3.

Kidney development: from ureteric bud formation to branching morphogenesis

Epithelial branching morphogenesis is critical to the formation of various organs such as the vasculature, mammary glands, lungs, and kidneys in vertebrate embryos. One fascinating aspect of branching morphogenesis is to understand how a simple epithelial tube grows by reiterative branching to form a complex epithelial tree structure. Recent studies combining mouse genetics and chimeric analysis with live imaging have uncovered the molecular networks and interactions that govern kidney branching morphogenesis. This review focuses on ureteric bud (UB) formation and epithelial branching during kidney development. The invasion of the metanephric mesenchyme by the UB is a fundamental step toward establishing the cyto-architecture of the kidney and determining the number of nephrons, which form the filtration units of the adult kidney.

Cre/lox recombination in the lower urinary tract

Tbx18 is a T-Box transcription factor that has specific expression and indispensible function in the lower urinary tract. Here, we report the generation and characterization of a bacterial artificial chromosome (BAC) transgene expressing Cre under the control of Tbx18 regulatory elements. When crossed to the ROSA26R-lacZ reporter mice, the Tbx18-Cre transgene mediates loxP recombination in the mesenchymal derivatives in the lower urinary tract, especially in the smooth muscle cells (SMCs) and the stromal cells. There is no expression of this transgene in the urothelium or in the kidney. This Tbx18-Cre transgene recapitulates the endogenous Tbx18 expression in the urinary system and can be used for the study of the development, physiology, and diseases in the urinary tract. Its additional expression in the epicardium, limb, vibrissae, and other structures would be useful for studies in the relevant fields.

New horizons at the caudal embryos: coordinated urogenital/reproductive organ formation by growth factor signaling

The cloaca/urogenital sinus and its adjacent region differentiate into the urogenital/reproductive organs. Caudal regression syndrome (CRS; including mermaid syndrome), a type of severe cloacal malformation displays hindlimb fusion and urogenital organ defects, thus suggesting that such defects are caused by several morphogenetic alterations during early development. The attenuation of bone morphogenetic protein (Bmp) signaling at the posterior primitive streak of embryos leads to the caudal dysmorphogenesis including the cloaca and fusion of both hindlimbs. Genetic tissue lineage studies indicate the presence of coordinated organogenesis. Hedgehog (HH)-responding cells derived from peri-cloacal mesenchyme (PCM) contribute to the urogenital/reproductive organs. These findings indicate the existence of developmental programs for the coordinated organogenesis of urogenital/reproductive tissues based on growth factor function and crosstalk.

Genetic and developmental basis for urinary tract obstruction

Urinary tract obstruction results in obstructive nephropathy and uropathy. It is the most frequent cause of renal failure in infants and children. In the past two decades studies of transgenic models and humans have greatly enhanced our understanding of the genetic factors and developmental processes important in urinary tract obstruction. The emerging picture is that development of the urinary tract requires precise integration of a variety of progenitor cell populations of different embryonic origins. Such integration is controlled by an intricate signaling network that undergoes dynamic changes as the embryo develops. Most congenital forms of urinary tract obstruction result from the disruption of diverse factors and genetic pathways involved in these processes, especially in the morphogenesis of the urinary conduit or the functional aspects of the pyeloureteral peristaltic machinery.