Glial cell line-derived neurotrophic factor stimulates ureteric bud outgrowth and enhances survival of ureteric bud cells in vitro

Organ-Specific Branching Morphogenesis

A common developmental process, called branching morphogenesis, generates the epithelial trees in a variety of organs, including the lungs, kidneys, and glands. How branching morphogenesis can create epithelial architectures of very different shapes and functions remains elusive. In this review, we compare branching morphogenesis and its regulation in lungs and kidneys and discuss the role of signaling pathways, the mesenchyme, the extracellular matrix, and the cytoskeleton as potential organ-specific determinants of branch position, orientation, and shape. Identifying the determinants of branch and organ shape and their adaptation in different organs may reveal how a highly conserved developmental process can be adapted to different structural and functional frameworks and should provide important insights into epithelial morphogenesis and developmental disorders.

Development of the metanephric kidney

The kidney plays an integral role in filtering the blood-removing metabolic by-products from the body and regulating blood pressure. This requires the establishment of large numbers of efficient and specialized blood filtering units (nephrons) that incorporate a system for vascular exchange and nutrient reabsorption as well as a collecting duct system to remove waste (urine) from the body. Kidney development is a dynamic process which generates these structures through a delicately balanced program of self-renewal and commitment of nephron progenitor cells that inhabit a constantly evolving cellular niche at the tips of a branching ureteric "tree." The former cells build the nephrons and the latter the collecting duct system. Maintaining these processes across fetal development is critical for establishing the normal "endowment" of nephrons in the kidney and perturbations to this process are associated both with mutations in integral genes and with alterations to the fetal environment.

Neurotrophic factors and their receptors in lung development and implications in lung diseases

Although lung innervation has been described by many studies in humans and rodents, the regulation of the respiratory system induced by neurotrophins is not fully understood. Here, we review current knowledge on the role of neurotrophins and the expression and function of their receptors in neurogenesis, vasculogenesis and during the embryonic development of the respiratory tree and highlight key implications relevant to respiratory diseases.

Identification of Betamethasone-Regulated Target Genes and Cell Pathways in Fetal Rat Lung Mesenchymal Fibroblasts

Preterm birth is characterized by severe lung immaturity that is frequently treated antenatally or postnatally with the synthetic steroid betamethasone. The underlying cellular targets and pathways stimulated by betamethasone in the fetal lung are poorly defined. In this study, betamethasone was compared with corticosterone in steroid-treated primary cultures of fetal rat lung fibroblasts stimulated for 6 hours and analyzed by whole-cell transcriptome sequencing and glucocorticoid (GC) receptor (GR) chromatin immunoprecipitation sequencing (ChIP-Seq) analysis. Strikingly, betamethasone stimulated a much stronger transcriptional response compared with corticosterone for both induced and repressed genes. A total of 483 genes were significantly stimulated by betamethasone or corticosterone, with 476 stimulated by both steroids, indicating a strong overlap in regulation. Changes in mRNA levels were confirmed by quantitative PCR for eight induced and repressed target genes. Pathway analysis identified cell proliferation and cytoskeletal/cell matrix remodeling pathways as key processes regulated by both steroids. One target, transglutaminase 2 (Tgm2), was localized to fetal lung mesenchymal cells. Tgm2 mRNA and protein levels were strongly increased in fibroblasts by both steroids. Whole-genome GR ChIP-Seq analysis with betamethasone identified GC response element-binding sites close to the previously characterized GR target genes Per1, Dusp1, Fkbp5, and Sgk1 and near the genes identified by transcriptome sequencing encoding Crispld2, Tgm2, Hif3α, and Kdr, defining direct genomic induction of expression in fetal lung fibroblasts via the GR. These results demonstrate that betamethasone stimulates specific genes and cellular pathways controlling cell proliferation and extracellular matrix remodeling in lung mesenchymal fibroblasts, providing a basis for betamethasone's treatment efficacy in preterm birth.

Congenital Disorders of the Human Urinary Tract: Recent Insights From Genetic and Molecular Studies

The urinary tract comprises the renal pelvis, the ureter, the urinary bladder, and the urethra. The tract acts as a functional unit, first propelling urine from the kidney to the bladder, then storing it at low pressure inside the bladder which intermittently and completely voids urine through the urethra. Congenital diseases of these structures can lead to a range of diseases sometimes associated with fetal losses or kidney failure in childhood and later in life. In some of these disorders, parts of the urinary tract are severely malformed. In other cases, the organs appear grossly intact yet they have functional deficits that compromise health. Human studies are beginning to indicate monogenic causes for some of these diseases. Here, the implicated genes can encode smooth muscle, neural or urothelial molecules, or transcription factors that regulate their expression. Furthermore, certain animal models are informative about how such molecules control the development and functional differentiation of the urinary tract. In future, novel therapies, including those based on gene transfer and stem cell technologies, may be used to treat these diseases to complement conventional pharmacological and surgical clinical therapies.

Mathematical Approaches of Branching Morphogenesis

Many organs require a high surface to volume ratio to properly function. Lungs and kidneys, for example, achieve this by creating highly branched tubular structures during a developmental process called branching morphogenesis. The genes that control lung and kidney branching share a similar network structure that is based on ligand-receptor reciprocal signalling interactions between the epithelium and the surrounding mesenchyme. Nevertheless, the temporal and spatial development of the branched epithelial trees differs, resulting in organs of distinct shape and size. In the embryonic lung, branching morphogenesis highly depends on FGF10 signalling, whereas GDNF is the driving morphogen in the kidney. Knockout of Fgf10 and Gdnf leads to lung and kidney agenesis, respectively. However, FGF10 plays a significant role during kidney branching and both the FGF10 and GDNF pathway converge on the transcription factors ETV4/5. Although the involved signalling proteins have been defined, the underlying mechanism that controls lung and kidney branching morphogenesis is still elusive. A wide range of modelling approaches exists that differ not only in the mathematical framework (e.g., stochastic or deterministic) but also in the spatial scale (e.g., cell or tissue level). Due to advancing imaging techniques, image-based modelling approaches have proven to be a valuable method for investigating the control of branching events with respect to organ-specific properties. Here, we review several mathematical models on lung and kidney branching morphogenesis and suggest that a ligand-receptor-based Turing model represents a potential candidate for a general but also adaptive mechanism to control branching morphogenesis during development.

Adhesion GPCRs in Kidney Development and Disease

Chronic kidney disease (CKD) represents the fastest growing pathology worldwide with a prevalence of >10% in many countries. In addition, kidney cancer represents 5% of all new diagnosed cancers. As currently no effective therapies exist to restore kidney function after CKD- as well as cancer-induced renal damage, it is important to elucidate new regulators of kidney development and disease as new therapeutic targets. G protein-coupled receptors (GPCRs) represent the most successful class of pharmaceutical targets. In recent years adhesion GPCRs (aGPCRs), the second largest GPCR family, gained significant attention as they are present on almost all mammalian cells, are associated to a plethora of diseases and regulate important cellular processes. aGPCRs regulate for example cell polarity, mitotic spindle orientation, cell migration, and cell aggregation; all processes that play important roles in kidney development and/or disease. Moreover, polycystin-1, a major regulator of kidney development and disease, contains a GAIN domain, which is otherwise only found in aGPCRs. In this review, we assess the potential of aGPCRs as therapeutic targets for kidney disease. For this purpose we have summarized the available literature and analyzed data from the databases The Human Protein Atlas, EURExpress, Nephroseq, FireBrowse, cBioPortal for Cancer Genomics and the National Cancer Institute Genomic Data Commons data portal (NCIGDC). Our data indicate that most aGPCRs are expressed in different spatio-temporal patterns during kidney development and that altered aGPCR expression is associated with a variety of kidney diseases including CKD, diabetic nephropathy, lupus nephritis as well as renal cell carcinoma. We conclude that aGPCRs present a promising new class of therapeutic targets and/or might be useful as diagnostic markers in kidney disease.

Identification of direct negative cross-talk between the SLIT2 and bone morphogenetic protein-Gremlin signaling pathways

Slit guidance ligand 2 (SLIT2) is a large, secreted protein that binds roundabout (ROBO) receptors on multiple cell types, including neurons and kidney podocytes. SLIT2-ROBO-mediated signaling regulates neuronal migration and ureteric bud (UB) outgrowth during kidney development as well as glomerular filtration in adult kidneys. Additionally, SLIT2 binds Gremlin, an antagonist of bone morphogenetic proteins (BMPs), and BMP-Gremlin signaling also regulates UB formation. However, direct cross-talk between the ROBO2-SLIT2 and BMP-Gremlin signaling pathways has not been established. Here, we report the discovery of negative feedback between the SLIT2 and BMP-Gremlin signaling pathways. We found that the SLIT2-Gremlin interaction inhibited both SLIT2-ROBO2 signaling in neurons and Gremlin antagonism of BMP activity in myoblasts and fibroblasts. Furthermore, BMP2 down-regulated SLIT2 expression and promoter activity through canonical BMP signaling. Gremlin treatment, BMP receptor inhibition, and SMAD family member 4 (SMAD4) knockdown rescued BMP-mediated repression of SLIT2. BMP2 treatment of nephron progenitor cells derived from human embryonic stem cells decreased SLIT2 expression, further suggesting an interaction between the BMP2-Gremlin and SLIT2 pathways in human kidney cells. In conclusion, our study has revealed direct negative cross-talk between two pathways, previously thought to be unassociated, that may regulate both kidney development and adult tissue maintenance.

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

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

Planar cell polarity genes Celsr1 and Vangl2 are necessary for kidney growth, differentiation, and rostrocaudal patterning

The mammalian kidney contains nephrons comprising glomeruli and tubules joined to ureteric bud-derived collecting ducts. It has a characteristic bean-like shape, with near-complete rostrocaudal symmetry around the hilum. Here we show that Celsr1, a planar cell polarity (PCP) gene implicated in neural tube morphogenesis, is required for ureteric tree growth in early development and later in gestation prevents tubule overgrowth. We also found an interaction between Celsr1 and Vangl2 (another PCP gene) in ureteric tree growth, most marked in the caudal compartment of the kidneys from compound heterozygous mutant mice with a stunted rump. Furthermore, these genes together are required for the maturation of glomeruli. Interestingly, we demonstrated patients with CELSR1 mutations and spina bifida can have significant renal malformations. Thus, PCP genes are important in mammalian kidney development and have an unexpected role in rostrocaudal patterning during organogenesis.

Innervation of Pulmonary Neuroendocrine Cells and Neuroepithelial Bodies in Developing Rabbit Lung

We investigated the development of innervation of the pulmonary neuroendocrine cell (PNEC) system composed of single cells and organoid cell clusters, neuroepithelial bodies (NEB) in rabbit fetal and neonatal lungs. To visualize the nerve fibers and their contacts with PNECs/NEBs, we used confocal microscopy and multilabel immunohistochemistry (IHC) with pan-neural marker, synaptic vesicle protein 2 (SV2), and serotonin (5-HT) as markers for PNECs/NEBs, and smooth muscle actin or cytokeratin to identify airway landmarks. The numbers and distribution of PNEC/NEB at different stages of lung development (E16, 18, 21, 26, and P2) and the density of innervation were quantified. First PNECs immunoreactive for 5-HT were identified in primitive airway epithelium at E18 as single cells or as small cell clusters with or without early nerve contacts. At E21 a significant increase in the number of PNECs with formation of early innervated NEB corpuscules was observed. The overall numbers of PNECs/NEBs and the density of mucosal, submucosal, and intercorpuscular innervation increased with progressing gestation and peaked postnataly (P2). At term, the majority of NEBs and single PNECs within airway mucosa possessed neural contacts. Such an extensive and complex innervation of the PNEC system indicates a multifunctional role in developing lung and during neonatal adaptation.

The Effects of Early Postnatal Diuretics Treatment on Kidney Development and Long-Term Kidney Function in Wistar Rats

BACKGROUND

Diuretics are administered to neonates to control fluid balance. We studied whether clinical doses affected kidney development and function and whether extrauterine growth retardation (EUGR) could be a modulator.

METHODS

Wistar rats were cross-fostered in normal food or food restricted litters at postnatal day (PND) 2 and treated daily with 0.9% NaCl, 5 mg/kg furosemide or 5 mg/kg hydrochlorothiazide (HCTZ) up to PND 8. Kidneys were evaluated on proliferation, apoptosis and a set of mRNA target genes at PND 8, glomerular- and glomerular generation count at PND 35, clinical pathology parameters at 3- and 9 months, neutrophil gelatinase-associated lipocalin at PND 8, 3 and 6 months, monthly blood pressure from 3 months onward and histopathology at study end.

RESULTS

Treatment with furosemide or HCTZ did not have relevant effects on measured parameters. EUGR resulted in lower body weight from day 3 onwards (-29% at weaning; p < 0.001, -10% at necropsy; p < 0.001), less glomerular generations (4.4 ± 0.32 vs. 5.0 ± 0.423; p = 0.025, males only), decreased glomerular numbers (27,861 ± 3,468 vs. 30,527 ± 4,096; p = 0.026), higher creatinine clearance (0.84 ± 0.1 vs. 0.77 ± 0.09 ml/min/kg; p = 0.047) at 3 months and lower plasma creatinine (25.7 ± 1.8 vs. 27.5 ± 2.8 µmol/l; p = 0.043) at 9 months.

CONCLUSION

Furosemide and HCTZ did not influence kidney development or function when administered in a clinically relevant dose to rat pups at a stage of ongoing nephrogenesis. EUGR led to impaired kidney development but did not modify furosemide or HCTZ findings.

To bud or not to bud: the RET perspective in CAKUT

Congenital anomalies of the kidneys or lower urinary tract (CAKUT) encompass a spectrum of anomalies that result from aberrations in spatio-temporal regulation of genetic, epigenetic, environmental, and molecular signals at key stages of urinary tract development. The Rearranged in Transfection (RET) tyrosine kinase signaling system is a major pathway required for normal development of the kidneys, ureters, peripheral and enteric nervous systems. In the kidneys, RET is activated by interaction with the ligand glial cell line-derived neurotrophic factor (GDNF) and coreceptor GFRα1. This activated complex regulates a number of downstream signaling cascades (PLCγ, MAPK, and PI3K) that control proliferation, migration, renewal, and apoptosis. Disruption of these events is thought to underlie diseases arising from aberrant RET signaling. RET mutations are found in 5-30 % of CAKUT patients and a number of Ret mouse mutants show a spectrum of kidney and lower urinary tract defects reminiscent of CAKUT in humans. The remarkable similarities between mouse and human kidney development and in defects due to RET mutations has led to using RET signaling as a paradigm for determining the fundamental principles in patterning of the upper and lower urinary tract and for understanding CAKUT pathogenesis. In this review, we provide an overview of studies in vivo that delineate expression and the functional importance of RET signaling complex during different stages of development of the upper and lower urinary tracts. We discuss how RET signaling balances activating and inhibitory signals emanating from its docking tyrosines and its interaction with upstream and downstream regulators to precisely modulate different aspects of Wolffian duct patterning and branching morphogenesis. We outline the diversity of cellular mechanisms regulated by RET, disruption of which causes malformations ranging from renal agenesis to multicystic dysplastic kidneys in the upper tract and vesicoureteral reflux or ureteropelvic junction obstruction in the lower tract.

Autonomic neural control of intrathoracic airways

Autonomic neural control of the intrathoracic airways aids in optimizing air flow and gas exchange. In addition, and perhaps more importantly, the autonomic nervous system contributes to host defense of the respiratory tract. These functions are accomplished by tightly regulating airway caliber, blood flow, and secretions. Although both the sympathetic and parasympathetic branches of the autonomic nervous system innervate the airways, it is the later that dominates, especially with respect to control of airway smooth muscle and secretions. Parasympathetic tone in the airways is regulated by reflex activity often initiated by activation of airway stretch receptors and polymodal nociceptors. This review discusses the preganglionic, ganglionic, and postganglionic mechanisms of airway autonomic innervation. Additionally, it provides a brief overview of how dysregulation of the airway autonomic nervous system may contribute to respiratory diseases.

Stage specific requirement of Gfrα1 in the ureteric epithelium during kidney development

Glial cell line-derived neurotrophic factor (GDNF) binds a coreceptor GDNF family receptor α1 (GFRα1) and forms a signaling complex with the receptor tyrosine kinase RET. GDNF-GFRα1-RET signaling activates cellular pathways that are required for normal induction of the ureteric bud (UB) from the Wolffian duct (WD). Failure of UB formation results in bilateral renal agenesis and perinatal lethality. Gfrα1 is expressed in both the epithelial and mesenchymal compartments of the developing kidney while Ret expression is specific to the epithelium. The biological importance of Gfrα1's wider tissue expression and its role in later kidney development are unclear. We discovered that conditional loss of Gfrα1 in the WD epithelium prior to UB branching is sufficient to cause renal agenesis. This finding indicates that Gfrα1 expressed in the nonepithelial structures cannot compensate for this loss. To determine Gfrα1's role in branching morphogenesis after UB induction we used an inducible Gfrα1-specific Cre-deletor strain and deleted Gfrα1 from the majority of UB tip cells post UB induction in vivo and in explant kidney cultures. We report that Gfrα1 excision from the epithelia compartment after UB induction caused a modest reduction in branching morphogenesis. The loss of Gfrα1 from UB-tip cells resulted in reduced cell proliferation and decreased activated ERK (pERK). Further, cells without Gfrα1 expression are able to populate the branching UB tips. These findings delineate previously unclear biological roles of Gfrα1 in the urinary tract and demonstrate its cell-type and stage-specific requirements in kidney development.

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.

Sprouty1 haploinsufficiency prevents renal agenesis in a model of Fraser syndrome

Deficiency of the extracellular matrix molecule FRAS1, normally expressed by the ureteric bud, leads to bilateral renal agenesis in humans with Fraser syndrome and blebbed (Fras1(bl/bl)) mice. The metanephric mesenchyme of these mutants fails to express sufficient Gdnf, which activates receptor tyrosine kinase (RTK) signalling, contributing to the phenotype. To determine whether modulating RTK signalling may overcome the abnormal nephrogenesis characteristic of Fraser syndrome, we introduced a single null Sprouty1 allele into Fras1(bl/bl) mice, thereby reducing the ureteric bud's expression of this anti-branching molecule and antagonist of RTK signalling. This prevented renal agenesis in Fras1(bl/bl) mice, permitting kidney development and postnatal survival. We found that fibroblast growth factor (FGF) signalling contributed to this genetic rescue, and exogenous FGF10 rescued defects in Fras1(bl/bl) rudiments in vitro. Whereas wild-type metanephroi expressed FRAS1 and the related proteins FREM1 and FREM2, FRAS1 was absent and the other proteins were downregulated in rescued kidneys, consistent with a reciprocally stabilized FRAS1/FREM1/FREM2 complex. In addition to contributing to knowledge regarding events during nephrogenesis, the demonstrated rescue of renal agenesis in a model of a human genetic disease raises the possibility that enhancing growth factor signaling might be a therapeutic approach to ameliorate this devastating malformation.

Protein kinase A regulates GDNF/RET-dependent but not GDNF/Ret-independent ureteric bud outgrowth from the Wolffian duct

Embryonic kidney development begins with the outgrowth of the ureteric bud (UB) from the Wolffian duct (WD) into the adjacent metanephric mesenchyme (MM). Both a GDNF-dependent and GDNF-independent (Maeshima et al., 2007) pathway have been identified. In vivo and in vitro, the GDNF-dependent pathway is inhibited by BMPs, one of the factors invoked to explain the limitation of UB formation in the unbudded regions of the WD surrounding the UB. However, the exact mechanism remains unknown. Here a previously described in vitro system that models UB budding from the WD was utilized to study this process. Because Protein kinase A (PKA) activation has been shown to prevent migration, morphogenesis and tubulogenesis of epithelial cells (Santos et al., 1993), its activity in budded and non-budded portions of the GDNF-induced WD was analyzed. The level of PKA activity was 15-fold higher in the unbudded portions of the WD compared to budded portions, suggesting that PKA activity plays a key role in controlling the site of UB emergence. Using well-characterized PKA agonists and antagonists, we demonstrated that at various levels of the PKA-signaling hierarchy, PKA regulates UB outgrowth from the WD by suppressing budding events. This process appeared to be PKA-2 isoform specific, and mediated by changes in the duct rather than the surrounding mesenchyme. In addition, it was not due to changes in either the sorting of junctional proteins, cell death, or cell proliferation. Furthermore, the suppressive effect of cAMP on budding did not appear to be mediated by spread to adjacent cells via gap junctions. Conversely, antagonism of PKA activity stimulated UB outgrowth from the WD and resulted in both an increase in the number of buds per unit length of WD as well as a larger surface area per bud. Using microarrays, analysis of gene expression in GDNF-treated WDs in which the PKA pathway had been activated revealed a nearly 14-fold decrease in Ret, a receptor for GDNF. A smaller decrease in GFRα1. a co-receptor for GDNF, was also observed. Using Ret-null WDs, we were able to demonstrate that PKA regulated GDNF-dependent budding but not GDNF-independent pathway for WD budding. We also found that BMP2 was higher in unbudded regions of the GDNF-stimulated WD. Treatment of isolated WDs with BMP2 suppressed budding and resulted in a 3-fold increase in PKA activity. The data suggests that the suppression of budding by BMPs and possibly other factors in non-budded zones of the WD may be regulated in part by increased PKA activity, probably partially through downregulation of Ret/GFRα1 coreceptor expression.

A novel, low-volume method for organ culture of embryonic kidneys that allows development of cortico-medullary anatomical organization

Here, we present a novel method for culturing kidneys in low volumes of medium that offers more organotypic development compared to conventional methods. Organ culture is a powerful technique for studying renal development. It recapitulates many aspects of early development very well, but the established techniques have some disadvantages: in particular, they require relatively large volumes (1–3 mls) of culture medium, which can make high-throughput screens expensive, they require porous (filter) substrates which are difficult to modify chemically, and the organs produced do not achieve good cortico-medullary zonation. Here, we present a technique of growing kidney rudiments in very low volumes of medium–around 85 microliters–using silicone chambers. In this system, kidneys grow directly on glass, grow larger than in conventional culture and develop a clear anatomical cortico-medullary zonation with extended loops of Henle.

p53 regulates metanephric development

p53 is best known as a tumor suppressor that regulates cell-cycle, differentiation, and apoptosis pathways, but its potential role in embryonic development and organogenesis remains controversial. Here, p53(-/-) embryos bred on C57Bl6 background exhibited a spectrum of congenital abnormalities of the kidney and urinary tract, including ureteric bud (UB) ectopia, double ureters/collecting systems, delayed primary branching of the UB, and hypoplastic metanephroi. We observed ectopic UB outgrowth from the Wolffian duct (WD) in one third of p53(-/-) embryos. The prevalence of duplex was higher in embryos than in neonates, and ex vivo organ culture suggested that ectopic ureters can regress over time, leaving behind a dysplastic pole ("segmental dysgenesis"). Transgenic expression of dominant negative p53 or conditional inactivation of p53 in the UB but not in the metanephric mesenchyme lineage recapitulated the duplex phenotype. Mechanistically, p53 inactivation in the WD associated with enhanced sensitivity to glial cell line-derived neurotrophic factor (GDNF)-induced ectopic budding and potentiated phosphatidylinositol-3 kinase activation by GDNF in UB cells. Unlike several other models of UB ectopia, hypersensitivity of p53(-/-) WD to GDNF is not accompanied by reduced Sprouty-1 or anterior expansion of the GDNF domain. In summary, our data lend support for a restrictive role for p53 activity in UB outgrowth from the WD.

Fras1, a basement membrane-associated protein mutated in Fraser syndrome, mediates both the initiation of the mammalian kidney and the integrity of renal glomeruli

FRAS1 is mutated in some individuals with Fraser syndrome (FS) and the encoded protein is expressed in embryonic epidermal cells, localizing in their basement membrane (BM). Syndactyly and cryptophthalmos in FS are sequelae of skin fragility but the bases for associated kidney malformations are unclear. We demonstrate that Fras1 is expressed in the branching ureteric bud (UB), and that renal agenesis occurs in homozygous Fras1 null mutant blebbed (bl) mice on a C57BL6J background. In vivo, the bl/bl bud fails to invade metanephric mesenchyme which undergoes involution, events replicated in organ culture. The expression of glial cell line-derived neurotrophic factor and growth-differentiation factor 11 was defective in bl/bl renal primordia in vivo, whereas, in culture, the addition of either growth factor restored bud invasion into the mesenchyme. Mutant primordia also showed deficient expression of Hoxd11 and Six2 transcription factors, whereas the activity of bone morphogenetic protein 4, an anti-branching molecule, was upregulated. In wild types, Fras1 was also expressed by nascent nephrons. Foetal glomerular podocytes expressed Fras1 transcripts and Fras1 immunolocalized in a glomerular BM-like pattern. On a mixed background, bl mutants, and also compound mutants for bl and my, another bleb strain, sometimes survive into adulthood. These mice have two kidneys, which contain subsets of glomeruli with perturbed nephrin, podocin, integrin α3 and fibronectin expression. Thus, Fras1 protein coats branching UB epithelia and is strikingly upregulated in the nephron lineage after mesenchymal/epithelial transition. Fras1 deficiency causes defective interactions between the bud and mesenchyme, correlating with disturbed expression of key nephrogenic molecules. Furthermore, Fras1 may also be required for the formation of normal glomeruli.

Semaphorin3a inhibits ureteric bud branching morphogenesis

Class 3 semaphorins are guidance proteins involved in axon pathfinding, vascular patterning and lung branching morphogenesis in the developing mouse embryo. Semaphorin3a (Sema3a) is expressed in renal epithelia throughout kidney development, including podocytes and ureteric bud cells. However, the role of Sema3a in ureteric bud branching is unknown. Here we demonstrate that Sema3a plays a role in patterning the ureteric bud tree in both metanephric organ cultures and Sema3a mutant mice. In vitro ureteric bud injection with Sema3a antisense morpholino resulted in increased branching, whereas recombinant SEMA3A inhibited ureteric bud branching and decreased the number of developing glomeruli. Additional studies revealed that SEMA3A effects on ureteric bud branching involve downregulation of glial cell-line derived neurotrophic factor (GDNF) signaling, competition with vascular endothelial growth factor A (VEGF-A) and decreased activity of Akt survival pathways. Deletion of Sema3a in mice is associated with increased ureteric bud branching, confirming its inhibitory role in vivo. Collectively, these data suggest that Sema3a is an endogenous antagonist of ureteric bud branching and hence, plays a role in patterning the renal collecting system as a negative regulator.

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

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

Reduction of BMP4 activity by gremlin 1 enables ureteric bud outgrowth and GDNF/WNT11 feedback signalling during kidney branching morphogenesis

Antagonists act to restrict and negatively modulate the activity of secreted signals during progression of embryogenesis. In mouse embryos lacking the extra-cellular BMP antagonist gremlin 1 (Grem1), metanephric development is disrupted at the stage of initiating ureteric bud outgrowth. Treatment of mutant kidney rudiments in culture with recombinant gremlin 1 protein induces additional epithelial buds and restores outgrowth and branching. All epithelial buds express Wnt11, and Gdnf is significantly upregulated in the surrounding mesenchyme, indicating that epithelial-mesenchymal (e-m) feedback signalling is restored. In the wild type, Bmp4 is expressed by the mesenchyme enveloping the Wolffian duct and ureteric bud and Grem1 is upregulated in the mesenchyme around the nascent ureteric bud prior to initiation of its outgrowth. In agreement, BMP activity is reduced locally as revealed by lower levels of nuclear pSMAD protein in the mesenchyme. By contrast, in Grem1-deficient kidney rudiments, pSMAD proteins are detected in many cell nuclei in the metanephric mesenchyme, indicative of excessive BMP signal transduction. Indeed, genetic lowering of BMP4 levels in Grem1-deficient mouse embryos completely restores ureteric bud outgrowth and branching morphogenesis. The reduction of BMP4 levels in Grem1 mutant embryos enables normal progression of renal development and restores adult kidney morphology and functions. This study establishes that initiation of metanephric kidney development requires the reduction of BMP4 activity by the antagonist gremlin 1 in the mesenchyme, which in turn enables ureteric bud outgrowth and establishment of autoregulatory GDNF/WNT11 feedback signalling.

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.

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

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

Suppression of ureteric bud apoptosis rescues nephron endowment and adult renal function in Pax2 mutant mice

The molecular mechanisms that set congenital nephron number are unknown. However, humans with modest suboptimal nephron number may be at increased risk for essential hypertension, and those with more severe nephron deficits at birth may develop progressive renal insufficiency. A model of branching morphogenesis during fetal kidney development in which the extent of ureteric bud arborization is dependent on suppression of programmed cell death has been proposed. This study shows that the increased apoptosis and reduced ureteric bud branching of heterozygous Pax2 mutant mice is associated with 40% decrease in nephron number at birth. This leads to postnatal glomerular hypertrophy and long-term renal insufficiency in the absence of glomerulosclerosis. To determine whether restoration of antiapoptotic factors alone is sufficient to rescue the nephron deficit in these mice, a BCL2 transgene that is under the control of the PAX2 promoter was targeted to the ureteric bud. The transgene suppressed programmed cell death in the ureteric bud lineage, increased nephron number to 90% of that of wild-type littermates at birth, and normalized renal function at 1 yr. These observations lend strong support to the hypothesis that factors that control ureteric bud apoptosis are powerful determinants of congenital nephron endowment.

GDNF/Ret signaling and the development of the kidney

Signaling by GDNF through the Ret receptor is required for normal growth of the ureteric bud during kidney development. However, the precise role of GDNF/Ret signaling in renal branching morphogenesis and the specific responses of ureteric bud cells to GDNF remain unclear. Recent studies have provided new insight into these issues. The localized expression of GDNF by the metanephric mesenchyme, together with several types of negative regulation, is important to elicit and correctly position the initial budding event from the Wolffian duct. GDNF also promotes the continued branching of the ureteric bud. However, it does not provide the positional information required to specify the pattern of ureteric bud growth and branching, as its site of synthesis can be drastically altered with minimal effects on kidney development. Cells that lack Ret are unable to contribute to the tip of the ureteric bud, apparently because GDNF-driven proliferation is required for the formation and growth of this specialized epithelial domain.

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.

The role of GDNF/Ret signaling in ureteric bud cell fate and branching morphogenesis

While GDNF signaling through the Ret receptor is critical for kidney development, its specific role in branching morphogenesis of the epithelial ureteric bud (UB) is unclear. Ret expression defines a population of UB "tip cells" distinct from cells of the tubular "trunks," but how these cells contribute to UB growth is unknown. We have used time-lapse mosaic analysis to investigate normal cell fates within the growing UB and the developmental potential of cells lacking Ret. We found that normal tip cells are bipotential, contributing to both tips and trunks. Cells lacking Ret are specifically excluded from the tips, although they contribute to the trunks, revealing that the tips form and expand by GDNF-driven cell proliferation. Surprisingly, the mutant cells assumed an asymmetric distribution in the UB trunks, suggesting a model of branching in which the epithelium of the tip and the adjacent trunk is remodeled to form new branches.

Guidance cues involved in the development of the peripheral autonomic nervous system

All peripheral autonomic neurons arise from neural crest cells that migrate away from the neural tube and navigate to the location where ganglia will form. After differentiating into neurons, their axons then navigate to a variety of targets. During the development of the enteric nervous system, GDNF appears to play a role in inducing vagal neural crest cells to enter the gut, in retaining neural crest cells within the gut and in promoting the migration of neural crest cells along the gut. Sema3A regulates the entry of extrinsic axons into the distal hindgut, netrin-DCC signaling is responsible for the centripetal migration of cells to form the submucosal ganglia within the gut, Slit-Robo signaling prevents trunk level neural crest cells from entering the gut, and neurturin plays a role in the innervation of the circular muscle layer. During the development of the sympathetic nervous system, the migration of trunk neural crest cells through the somites is influenced by ephrin-Bs, Sema3A and F-spondin. The migration of neural crest cells ventrally beyond the somites requires neuregulin signaling and the clumping of cells into columns adjacent to the dorsal aorta is regulated by Sema3A. The rostral migration of cells to form the superior cervical ganglion (SCG) and the extension of axons along blood vessels involves artemin signaling through Ret and GFRalpha3, and the entry of sympathetic axons into target tissues involves neurotrophins and GDNF. Relatively little is known about the development of parasympathetic ganglia, but GDNF appears to play a role in the migration of some cranial ganglion precursors to their correct location, and both GDNF and neurturin are involved in the growth of parasympathetic axons into particular targets.

Gremlin-mediated BMP antagonism induces the epithelial-mesenchymal feedback signaling controlling metanephric kidney and limb organogenesis

Epithelial-mesenchymal feedback signaling is the key to diverse organogenetic processes such as limb bud development and branching morphogenesis in kidney and lung rudiments. This study establishes that the BMP antagonist gremlin (Grem1) is essential to initiate these epithelial-mesenchymal signaling interactions during limb and metanephric kidney organogenesis. A Grem1 null mutation in the mouse generated by gene targeting causes neonatal lethality because of the lack of kidneys and lung septation defects. In early limb buds, mesenchymal Grem1 is required to establish a functional apical ectodermal ridge and the epithelial-mesenchymal feedback signaling that propagates the sonic hedgehog morphogen. Furthermore, Grem1-mediated BMP antagonism is essential to induce metanephric kidney development as initiation of ureter growth,branching and establishment of RET/GDNF feedback signaling are disrupted in Grem1-deficient embryos. As a consequence, the metanephric mesenchyme is eliminated by apoptosis, in the same way as the core mesenchymal cells of the limb bud.

Pattern and regulation of cell proliferation during murine ureteric bud development

Branched epithelia determine the anatomy of many mammalian organs; understanding how they develop is therefore an important element of understanding organogenesis as a whole. In recent years, much progress has been made in identifying paracrine factors that regulate branching morphogenesis in many organs, but comparatively little attention has been paid to the mechanisms of morphogenesis that translate these signals into anatomical change. Localized cell proliferation is a potentially powerful mechanism for directing the growth of a developing system to produce a specific final morphology. We have examined the pattern of cell proliferation in the ureteric bud system of the embryonic murine metanephric kidneys developing in culture. We detect a zone of high proliferation at the site of the presumptive ureteric bud even before it emerges from the Wolffian duct and later, as ureteric bud morphogenesis continues, proliferation is localized mainly in the very tips of the branching epithelium. Blocking cell cycling using methotrexate inhibits ureteric bud emergence. The proliferative zone is present at ureteric bud tips only when they are undergoing active morphogenesis; if branching is inhibited either by treatment with natural negative regulators (TGF-beta) or with antagonists of natural positive regulators (GDNF, glycosaminoglycans) then proliferation at the tips falls back to levels characteristic of the stalks behind them. Our results suggest that localized proliferation is an important morphogenetic mechanism in kidney development.

Cloning and characterization of a novel gene promoting ureteric bud branching in the metanephros

BACKGROUND

The ureteric buds and metanephric mesenchymal cells reciprocally induce each other's maturation during kidney development, and implicated transcription factors, secreted growth factors, and cell surface signaling peptides are critical regulators of renal branching morphogenesis. Protein kinase C (PKC) is a key enzyme in the signal transduction mechanisms in various biologic processes, including development, because it regulates growth and differentiation. Inhibition of PKC by the sphingolipid product ceramide interferes with nephron formation in the developing kidney, but the molecule that controls ureteric bud branching downstream of PKC is still unknown.

METHODS

Differential display polymerase chain reaction (PCR) of metanephroi cultured with a PKC activator and inhibitor was performed. We also examined the role of a novel gene in kidney development with organ culture system.

RESULTS

A novel gene encoding a 759 bp mRNA was identified, and we named it metanephros-derived tubulogenic factor (MTF)/L47. Inhibition of MTF with antisense oligonucleotide impaired ureteric bud branching by cultured metanephroi, and addition of recombinant MTF protein promoted ureteric bud branching in cultured metanephroi and increased cell proliferation.

CONCLUSION

We identified a novel molecule in developing kidney that is capable of modulating ureteric bud branching and kidney differentiation.

Ureteric Bud Apoptosis and Renal Hypoplasia in Transgenic PAX2-Bax Fetal Mice Mimics the Renal-Coloboma Syndrome

ABSTRACT. In humans, PAX2 haploinsufficiency causes renal-coloboma syndrome (RCS) involving eye abnormalities, renal hypoplasia, and renal failure in early life. The authors previously showed that heterozygous mutant Pax2 mice have smaller kidneys with fewer nephrons, associated with elevated apoptosis in the ureteric bud (UB). However, PAX2 may have a variety of developmental functions such as effects on cell fate and differentiation. To determine whether apoptosis alone is sufficient to cause a UB branching deficit, the authors targeted a pro-apoptotic gene (Baxα) to the embryonic kidney under the control of human PAX2 regulatory elements. The exogenous PAX2 promoter directed Baxα gene expression specifically to the developing kidney UB, eye, and mid/hindbrain. At E15.5 PAX2Promoter-Baxα fetal mice exhibited renal hypoplasia, elevated UB apoptosis, and retinal defects, mimicking the phenotype observed in RCS. The kidneys of E15.5 PAX2Promoter-Baxα fetal mice were 55% smaller than those of wild-type fetal mice, and they contained 70% of the normal level of UB branching. The data indicate that loss of Pax2 anti-apoptotic activity is sufficient to account for the reduced UB branching observed in RCS and suggest that elevated UB apoptosis may be a key process responsible for renal hypoplasia. The authors propose a morphogenic unit model in which cell survival influences the rate of UB branching and determines final nephron endowment. E-mail: meccles@otago.ac.nz

Ontogeny of airway smooth muscle: structure, innervation, myogenesis and function in the fetal lung

Airway smooth muscle (ASM) is an integral component of the primordial lung. It differentiates from the mesenchyme as a ring of cells around the base of the epithelial bud that express smooth muscle-specific proteins. These rapidly form into interlocking bundles that progressively become wider and more compact along the bronchial tree to the trachea. Their orientation is perpendicular to the long axis of the airway. The ASM exhibits rhythmic contractility (i.e. it is a phasic-type smooth muscle) soon after formation, and the spontaneous airway narrowing shifts the lung liquid distally causing expansion of the tubule walls. This stretching is the mechanical stimulus to smooth muscle (SM) myogenesis and lung growth. Neural tissue, i.e. precursor ganglia interconnected by nerve trunks and smaller bundles, forms a sheath over the ASM layer with varicose fibres descending to the muscle. These are guided by glial-derived neurotrophic factor (GDNF) that appears to be produced by ASM. Maturation of neural tissue is slower than the ASM; functional cholinergic innervation is manifest by the early canalicular stage when most neurotransmitters appear.

Regulation of metanephric kidney development by growth/differentiation factor 11

Growth/differentiation factor 11 (Gdf11) is a transforming growth factor beta family member previously shown to control anterior/posterior patterning of the axial skeleton. We now report that Gdf11 also regulates kidney organogenesis. Mice carrying a targeted deletion of Gdf11 possess a spectrum of renal abnormalities with the majority of mutant animals lacking both kidneys. Histological analysis revealed a failure in ureteric bud formation at the initial stage of metanephric development in most Gdf11 mutant embryos examined. The metanephric mesenchyme of mutant embryos lacking a ureteric bud was found to be defective in the expression of glial cell line-derived neurotrophic factor (Gdnf), a gene known to direct ureteric bud outgrowth. The addition of Gdnf protein to urogenital tracts taken from Gdf11 null embryos induced ectopic ureteric bud formation along the Wolffian duct. Our studies suggest that Gdf11 may be important in directing the initial outgrowth of the ureteric bud from the Wolffian duct by controlling the expression of Gdnf in the metanephric mesenchyme.

OFD1, the gene mutated in oral-facial-digital syndrome type 1, is expressed in the metanephros and in human embryonic renal mesenchymal cells

Oral-facial-digital syndrome type 1 (OFD1) causes polycystic kidney disease (PKD) and malformations of the mouth, face and digits. Recently, a gene on Xp22, OFD1, was reported to be mutated in a limited set of OFD1 patients. This study describes mutation analysis in six further OFD1 families. Additionally, gene expression was sought in human development. In two OFD1 kindreds affected by PKD, a frameshift mutation and a splice-site mutation were detected. In four apparently sporadic cases, three frameshift and a missense mutation were found. Using RT-PCR of RNA from first-trimester normal human embryos, both alternative splice forms of mRNA (OFD1a and OFD1b) were found to be widely expressed in organogenesis. Northern blot detected OFD1 mRNA in metanephros, brain, tongue, and limb, all organs affected in the syndrome. A polyclonal antibody directed to a C-terminal OFD1a epitope detected a 120-kD protein in the metanephros and in human renal mesenchymal cell lines. In normal human embryos, OFD1a immunolocalized to the metanephric mesenchyme, oral mucosa, nasal and cranial cartilage, and brain. Moreover, using normal human renal mesenchymal cell lines, the immunoreactive protein colocalized with gamma-tubulin, suggesting that OFD1 is associated with the centrosome. First, it is concluded that OFD1 mutations would generally be predicted to result in unstable transcripts or nonfunctional proteins. Second, OFD1 is expressed in human organogenesis; on the basis of the metanephric expression pattern, the results suggest that OFD1 plays a role in differentiation of metanephric precursor cells.

Nephron number, renal function, and arterial pressure in aged GDNF heterozygous mice

The loss of one allele for glial cell line-derived neurotrophic factor (GDNF) results in approximately 30% fewer but normal sized glomeruli in young mice. Low nephron number, inherited or acquired, has been linked to increased risk of development of hypertension and renal failure. This study examines whether GDNF heterozygous mice, with an inherent reduction in nephron number, demonstrate a deterioration in renal structure and function and rise in arterial pressure in later life. Fourteen-month-old male GDNF heterozygous (n=7) and wild-type (n=6) mice were anesthetized and prepared for measurement of mean arterial pressure, glomerular filtration rate (GFR), and renal blood flow. After measurement of renal function, kidneys were fixed for stereological determination of total glomerular number and mean glomerular volume. Mean arterial pressure was, on average, 18 mm Hg higher in GDNF heterozygous (98+/-4 mm Hg) than wild-type mice (80+/-2 mm Hg; P<0.01). However, GFR (0.656+/-0.054 versus 0.688+/-0.076 mL/min per g kidney wt) and renal blood flow (5.29+/-0.42 versus 4.70+/-0.34 mL/min per g kidney wt) were not different between groups. Fourteen-month-old GDNF heterozygous mice had approximately 30% fewer glomeruli than wild-type mice (9206+/-934 versus 13440+/-1275; P<0.01) and significantly larger glomeruli (4.51+/-0.39 versus 3.72+/-0.63x10(-4)mm(3); P<0.01). Thus, aged GDNF heterozygous mice maintained a normal GFR and renal blood flow despite reduced nephron numbers. The elevated arterial pressure, glomerular hypertrophy, and hyperfiltration demonstrated in the GDNF heterozygous mice at this age may indicate a compensatory mechanism whereby GFR is maintained in the presence of a reduced nephron endowment.

Glial cell line derived neurotrophic factor is expressed by epithelia of human renal dysplasia

PURPOSE

Differentiation of the metanephros is abnormal in cases of renal dysplasia, resulting in abnormal kidney organization. In vitro and in vivo studies indicate that glial cell line derived neurotrophic factor (GDNF) is a major regulator of kidney development and ureteral arborization. Therefore, we investigated the pattern of GDNF gene expression in human dysplastic kidneys.

MATERIALS AND METHODS

Specimens of whole tissues of human normal and dysplastic kidneys associated with obstructive uropathy were analyzed for GDNF mRNA by reverse transcriptase-polymerase chain reaction (RT-PCR). Immunohistochemistry with GDNF antibody and laser capture microdissection plus RT-PCR were done to identify cells producing GDNF. Apoptosis, BCL-2 and Ki67 were also studied.

RESULTS

There were few if any GDNF transcripts in normal kidneys, whereas GDNF was over expressed in renal dysplasia specimens. Strong GDNF expression was found in the dysplastic tubules of dysplastic kidneys, whereas peritubular mesenchyma expressed no GDNF protein. Laser capture microdissection/RT-PCR detected GDNF mRNA in epithelial cells isolated from dysplastic tubules but not in cells from the surrounding mesenchyma, which was confirmed by sequence analysis. GDNF expression by epithelial cells was associated with high proliferation, BCL-2 expression and rare apoptosis.

CONCLUSIONS

GDNF gene expression is restricted to the tubular epithelium of dysplastic human kidneys. Our results strongly suggest that GDNF not only influences kidney morphogenesis, but is also implicated in abnormal kidney development.

SBA-positive fibers between the CD ampulla, mesenchyme, and renal capsule

During kidney development, the CD shows two peculiarities. First, the tip of the CD ampulla is always found at a specific distance from the organ capsule. Second, the CD growth occurs as a perfectly straight elongation. It is unknown whether the CD-specific growth is dependent on hormonal action or on structural elements. Histochemical experiments on neonatal rabbit kidney yielded new insight into the interface of the CD ampulla and the surrounding nephrogenic mesenchyme. Incubation of tissue sections with soybean lectin (SBA) showed the existence of fibers extending in a radial course from the ampullar tip through the mesenchyme toward the organ capsule. SBA labeling did not colocalize with collagen type I, III, IV, V, and VI, laminin, fibronectin, and tenascin. It is assumed that while the kidney increases in volume the structural fixation of the ampullar tip by the SBA-positive fibers causes CD ampullae to maintain a constant distance from the organ capsule. The connection would explain the linear extension of the CD in relation to the organ capsule. In addition, the presented data suggest that the SBA-positive fibers between ampullar tip and organ capsule create a structural microcompartmentation of the nephrogenic zone.

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

Mammalian kidney development requires the formation of a patterned, branched network of collecting ducts, a process termed renal branching morphogenesis. Disruption of renal branching morphogenesis during human kidney development results in renal dysplasia, the major cause of renal failure in young children. Genetic evidence, combined with in vitro data, have implicated transcription factors, secreted growth factors, and cell surface signaling peptides as critical regulators of renal branching morphogenesis. This review discusses the current knowledge regarding the regulation of renal branching morphogenesis in vivo provided by the analysis of genetic mutations in mice and humans which disrupt collecting duct system development. In addition, in vivo and in vitro evidence regarding the functions of several other gene families are considered, rendering new insight into emerging regulatory roles for these molecules in renal branching morphogenesis.

Glial cell-derived neurotrophic factor expression in normal human lung and congenital cystic adenomatoid malformation

CONTEXT

It has been recently suggested that dysregulation of developmental factors and disruption of cell turnover could play a role in the pathogenesis of congenital cystic adenomatoid malformation of the lung (CCAM). The glial cell-derived neurotrophic factor (GDNF) is a growth factor involved in organogenesis, and the temporal pattern of GDNF expression suggests that this factor may play a role in lung development.

DESIGN

We studied GDNF expression by immunohistochemistry in postnatally resected CCAM of the lung (n = 10), normal fetal lung (n = 5), and normal postnatal lung (n = 5). We also studied the association between GDNF expression and both cell proliferation and apoptosis.

RESULTS

GDNF was expressed in both epithelial and endothelial compartments of normal fetal lung, whereas no expression was found in normal postnatal lung. In contrast, in CCAM tissue, there was strong GDNF immunostaining that was restricted to epithelial cells. The percentage of proliferating epithelial cells was higher in CCAM tissue than in normal postnatal lung (6.3% vs 1.7%, P <.005). Apoptotic bodies were found in the mesenchyme of both normal fetal lung and CCAM tissue, whereas virtually no apoptotic bodies were detected in normal postnatal lung.

CONCLUSIONS

Abnormal GDNF expression in CCAM suggests a dysregulation of the GDNF signaling pathway and argues in favor of a focal arrest in maturation during development. GDNF expression in lung tissue seems to be correlated with cell proliferation, suggesting that this factor could play a role in the growth of both fetal lung and CCAM.

Development of neural tissue and airway smooth muscle in fetal mouse lung explants: a role for glial-derived neurotrophic factor in lung innervation

We have characterized the distribution of neural tissue and its primary target tissue, airway smooth muscle (ASM), in an in vitro mouse model of early lung development comprising left lung lobes at embryonic Day 12, cultured for 2 or 5 d. Neural tissue was detected with antibodies to protein gene product 9.5 (PGP 9.5), synapsin, and p75NTR (the low-affinity neurotrophin receptor), and smooth muscle with an antibody to alpha-actin. Imaging by confocal microscopy revealed few PGP 9.5-positive neurons at the start of culture; after 2 d clusters of neurons and nerve fibers had appeared along the lobar bronchus and after 5 d along the secondary and tertiary branches. Neural tissue did not just follow the smooth muscle-covered tubules, as seen in vivo, but also grew outside the lobes onto a wide layer of alpha-actin-positive cells, suggesting that smooth muscle may express a trophic factor that attracts nerves. Explants cultured with glial-derived neurotrophic factor (GDNF) exhibited a striking increase in the amount of p75NTR- and PGP 9.5-positive tissue outside the lobes, whereas GDNF-impregnated beads attracted neuronal precursors and influenced the direction of neurite extension. We show that the mouse lung explant is suitable for investigating trophic signals involved in pulmonary innervation and that GDNF may have a role in the early innervation of the developing airways.

Nephron endowment in glial cell line-derived neurotrophic factor (GDNF) heterozygous mice

BACKGROUND

The exact molecular mechanisms that regulate ureteric branching morphogenesis in the developing metanephros have not been fully elucidated. However, in vivo and in vitro evidence indicates that glial cell line-derived neurotrophic factor (GDNF) is a key regulator of the initiation of ureteric branching. GDNF knockout mice show renal agenesis or severe dysgenesis and die 24 hours after birth from renal failure. Inhibition of GDNF activity in metanephric organ culture inhibits ureteric branching. Since nephron initiation only occurs at the tips of ureteric branches, the aim of the present study was to determine whether nephron number in GDNF heterozygous mice is reduced.

METHODS

Male GDNF heterozygous mice of hybrid 129/Sv and C57/BL genetic background were mated with C57BL/6 females. Offspring were genotyped at postnatal day 30 (PN30) by polymerase chain reaction. Left kidneys were used for estimating kidney volume and total nephron number. We also estimated absolute and relative volumes of ureteric duct epithelium. Unbiased stereological methods were used throughout (Cavalieri method, physical disector/fractionator combination).

RESULTS

GDNF wild-type and heterozygous mice had similar body weights at PN30. However, heterozygous kidneys were 25% smaller than wild-type kidneys (wild-type, 114.75 +/- 16.46 mm3; heterozygous, 87.11 +/- 21.84 mm3, P < 0.001) and contained approximately 30% fewer nephrons (wild-type, 11886 +/- 1277; heterozygous, 8573 +/- 2240, P < 0.01). In addition, the absolute ureteric duct volume was significantly reduced in heterozygous mice (P < 0.001).

CONCLUSIONS

: These results indicate that the loss of one GDNF allele results in reduced nephron endowment in the adult kidney, presumably as the result of reduced branching morphogenesis of the ureteric bud.

GDNF expression in Wilms tumor

PURPOSE

Wilms tumor or nephroblastoma is a developmental tumor of the kidney and one of the most frequent solid tumors in childhood. It derives from metanephrotic blastema and mimics nephrogenesis in a disorganized manner, offering an adequate model for study of human nephrogenesis. GDNF (glial cell line derived neurotrophic factor), a potent proliferation and survival factor of dopaminergic neurons, has recently been shown to have an early and major role in nephrogenesis. We studied the expression of GDNF in Wilms tumor.

MATERIALS AND METHODS

The study included 20 patients with nephroblastoma whose age at surgery ranged from 2 months to 13 years. Expression of GDNF protein was analyzed by an immunohistochemical technique using anti-GDNF antibody. Presence of GDNF-messenger (m)RNA and receptors GFRalpha1 and GFRalpha2-mRNA was analyzed by reverse transcription polymerase chain reaction. Specimens were also studied to evaluate apoptosis, proliferation index and Bcl-2 expression.

RESULTS

GDNF expression was mainly found in the epithelial cells of the most differentiated tubes, GDNF and co-receptors mRNA were found in specimens and proliferative activity was found on the same tubes as GDNF expression. Bcl-2 was expressed strongly in epithelial cells, in an intermediate fashion in the blastema and faintly in mesenchyma. Apoptosis was of low frequency in structures strongly expressing GDNF.

CONCLUSIONS

We have shown that GDNF is expressed by nephroblastoma tissue of human kidneys. This expression is mainly in the differentiated epithelial component of the nephroblastoma. We have also shown that tissue strongly expressing GDNF is positively proliferative and has less apoptotic activity. We speculate that the role of GDNF may not be limited to normal nephrogenesis but may interact with other factors in the process of proliferation and apoptosis involved in nephroblastoma tumorigenesis.