Pattern and regulation of cell proliferation during murine ureteric bud development

Branching morphogenesis: from cells to organs and back

Many animal organs, such as the lung, the kidney, the mammary gland, and the vasculature, consist of branched tubular structures that arise through a process known as "branching morphogenesis" that results from the remodeling of epithelial or endothelial sheaths into multicellular tubular networks. In recent years, the combination of molecular biology, forward and reverse genetic approaches, and their complementation by live imaging has started to unravel rules and mechanisms controlling branching processes in animals. Common patterns of branch formation spanning diverse model systems are beginning to emerge that might reflect unifying principles of tubular organ formation.

Analysis of migration in primary ureteric bud epithelial cells

Kidney development has been widely used as a model system to study molecular control of inductive tissue interactions and mechanisms through which branching organs form. Due to lacking or poor methods, less focus has been in understanding details of cellular events that accomplish example ureteric bud (UB) branching. In order to form a branch point, cells need to proliferate, move in relation to each other, and change their shape as well as adhesive properties. In this chapter, detailed description is given how to set up primary UB epithelial cell cultures and study cell motility in these cells.

Sculpting organs: mechanical regulation of tissue development

The ramified architectures of organs such as the mammary gland and lung are generated via branching morphogenesis, a developmental process through which individual cells bud and pinch off of pre-existing epithelial sheets. Although specified by signaling programs, organ development requires integration of all aspects of the microenvironment. We describe the essential role of endogenous cellular contractility in the formation of branching tubes. We also highlight the role of exogenous forces in normal and aberrant branching.

TROP2 expressed in the trunk of the ureteric duct regulates branching morphogenesis during kidney development

TROP2, a cell surface protein structurally related to EpCAM, is expressed in various carcinomas, though its function remains largely unknown. We examined the expression of TROP2 and EpCAM in fetal mouse tissues, and found distinct patterns in the ureteric bud of the fetal kidney, which forms a tree-like structure. The tip cells in the ureteric bud proliferate to form branches, whereas the trunk cells differentiate to form a polarized ductal structure. EpCAM was expressed throughout the ureteric bud, whereas TROP2 expression was strongest at the trunk but diminished towards the tips, indicating the distinct cell populations in the ureteric bud. The cells highly expressing TROP2 (TROP2(high)) were negative for Ki67, a proliferating cell marker, and TROP2 and collagen-I were co-localized to the basal membrane of the trunk cells. TROP2(high) cells isolated from the fetal kidney failed to attach and spread on collagen-coated plates. Using MDCK cells, a well-established model for studying the branching morphogenesis of the ureteric bud, TROP2 was shown to inhibit cell spreading and motility on collagen-coated plates, and also branching in collagen-gel cultures, which mimic the ureteric bud's microenvironment. These results together suggest that TROP2 modulates the interaction between the cells and matrix and regulates the formation of the ureteric duct by suppressing branching from the trunk during kidney development.

More than nervous: the emerging roles of plexins

Plexins are the receptors for semaphorins, a large family of axon guidance cues. Accordingly, the role of plexins in the development of the nervous system was the first to be acknowledged. However, the expression of plexins is not restricted to neuronal cells, and recent research has been increasingly focused on the roles of plexin-semaphorin signalling outside of the nervous system. During embryogenesis, plexins regulate the development of many organs, including the cardiovascular system, skeleton and kidney. They have also been shown to be involved in immune system functions and tumour progression. Analyses of the plexin signalling in different tissues and cell types have provided new insight to the versatility of plexin interactions with semaphorins and other cell-surface receptors. In this review we try to summarise the current understanding of the roles of plexins in non-neural development and immunity.

Renin-angiotensin system in ureteric bud branching morphogenesis: insights into the mechanisms

Branching morphogenesis of the ureteric bud (UB) is a key developmental process that controls organogenesis of the entire metanephros. Notably, aberrant UB branching may result in a spectrum of congenital anomalies of the kidney and urinary tract (CAKUT). Genetic, biochemical and physiological studies have demonstrated that the renin-angiotensin system (RAS), a key regulator of the blood pressure and fluid/electrolyte homeostasis, also plays a critical role in kidney development. All the components of the RAS are expressed in the metanephros. Moreover, mutations in the genes encoding components of the RAS in mice or humans cause diverse types of CAKUT which include renal papillary hypoplasia, hydronephrosis, duplicated collecting system, renal tubular dysgenesis, renal vascular abnormalities, abnormal glomerulogenesis and urinary concentrating defect. Despite widely accepted role of the RAS in metanephric kidney and renal collecting system (ureter, pelvis, calyces and collecting ducts) development, the mechanisms by which an intact RAS exerts its morphogenetic actions are incompletely defined. Emerging evidence indicates that defects in UB branching morphogenesis may be causally linked to the pathogenesis of renal collecting system anomalies observed under conditions of aberrant RAS signaling. This review describes the role of the RAS in UB branching morphogenesis and highlights emerging insights into the cellular and molecular mechanisms whereby RAS regulates this critical morphogenetic process.

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

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

Receptor tyrosine kinases in kidney development

The kidney plays a fundamental role in the regulation of arterial blood pressure and fluid/electrolyte homeostasis. As congenital anomalies of the kidney and urinary tract (CAKUT) constitute one of the most common human birth defects, improved understanding of the cellular and molecular mechanisms that lead to CAKUT is critical. Accumulating evidence indicates that aberrant signaling via receptor tyrosine kinases (RTKs) is causally linked to CAKUT. Upon activation by their ligands, RTKs dimerize, undergo autophosphorylation on specific tyrosine residues, and interact with adaptor proteins to activate intracellular signal transduction pathways that regulate diverse cell behaviours such as cell proliferation, survival, and movement. Here, we review the current understanding of role of RTKs and their downstream signaling pathways in the pathogenesis of CAKUT.

Differential migration and proliferation of geometrical ensembles of cell clusters

Differential cell migration and growth drives the organization of specific tissue forms and plays a critical role in embryonic development, tissue morphogenesis, and tumor invasion. Localized gradients of soluble factors and extracellular matrix have been shown to modulate cell migration and proliferation. Here we show that in addition to these factors, initial tissue geometry can feedback to generate differential proliferation, cell polarity, and migration patterns. We apply layer by layer polyelectrolyte assembly to confine multicellular organization and subsequently release cells to demonstrate the spatial patterns of cell migration and growth. The cell shapes, spreading areas, and cell-cell contacts are influenced strongly by the confining geometry. Cells within geometric ensembles are morphologically polarized. Symmetry breaking was observed for cells on the circular pattern and cells migrate toward the corners and in the direction parallel to the longest dimension of the geometric shapes. This migration pattern is disrupted when actomyosin based tension was inhibited. Cells near the edge or corner of geometric shapes proliferate while cells within do not. Regions of higher rate of cell migration corresponded to regions of concentrated growth. These findings demonstrate that multicellular organization can result in spatial patterns of migration and proliferation.

Actin depolymerizing factors cofilin1 and destrin are required for ureteric bud branching morphogenesis

The actin depolymerizing factors (ADFs) play important roles in several cellular processes that require cytoskeletal rearrangements, such as cell migration, but little is known about the in vivo functions of ADFs in developmental events like branching morphogenesis. While the molecular control of ureteric bud (UB) branching during kidney development has been extensively studied, the detailed cellular events underlying this process remain poorly understood. To gain insight into the role of actin cytoskeletal dynamics during renal branching morphogenesis, we studied the functional requirements for the closely related ADFs cofilin1 (Cfl1) and destrin (Dstn) during mouse development. Either deletion of Cfl1 in UB epithelium or an inactivating mutation in Dstn has no effect on renal morphogenesis, but simultaneous lack of both genes arrests branching morphogenesis at an early stage, revealing considerable functional overlap between cofilin1 and destrin. Lack of Cfl1 and Dstn in the UB causes accumulation of filamentous actin, disruption of normal epithelial organization, and defects in cell migration. Animals with less severe combinations of mutant Cfl1 and Dstn alleles, which retain one wild-type Cfl1 or Dstn allele, display abnormalities including ureter duplication, renal hypoplasia, and abnormal kidney shape. The results indicate that ADF activity, provided by either cofilin1 or destrin, is essential in UB epithelial cells for normal growth and branching.

Development of the ureter and collecting ducts of the kidney requires extensive growth and branching of an epithelial tube, the ureteric bud. While many genes that control this process are known, the cellular events that underlie renal morphogenesis remain poorly understood. Many cellular changes that might contribute to ureteric bud morphogenesis, such as migration and changes in shape, involve the actin cytoskeleton. Actin depolymerizing factors (ADFs) are important for changes in the organization of the cytoskeleton in cultured cells, but the roles of the ADF genes in vivo remain to be fully elucidated. Here, we examine the importance of the ADFs cofilin1 and destrin in ureteric bud branching and find that lack of both genes arrests this process at an early stage, while lesser reductions in ADF gene dosage cause more subtle defects in kidney development. This finding may help us to understand the origins of certain congenital malformations in humans.

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.

Sall1-dependent signals affect Wnt signaling and ureter tip fate to initiate kidney development

Development of the metanephric kidney depends on precise control of branching of the ureteric bud. Branching events represent terminal bifurcations that are thought to depend on unique patterns of gene expression in the tip compared with the stalk and are influenced by mesenchymal signals. The metanephric mesenchyme-derived signals that control gene expression at the ureteric bud tip are not well understood. In mouse Sall1 mutants, the ureteric bud grows out and invades the metanephric mesenchyme, but it fails to initiate branching despite tip-specific expression of Ret and Wnt11. The stalk-specific marker Wnt9b and the beta-catenin downstream target Axin2 are ectopically expressed in the mutant ureteric bud tips, suggesting that upregulated canonical Wnt signaling disrupts ureter branching in this mutant. In support of this hypothesis, ureter arrest is rescued by lowering beta-catenin levels in the Sall1 mutant and is phenocopied by ectopic expression of a stabilized beta-catenin in the ureteric bud. Furthermore, transgenic overexpression of Wnt9b in the ureteric bud causes reduced branching in multiple founder lines. These studies indicate that Sall1-dependent signals from the metanephric mesenchyme are required to modulate ureteric bud tip Wnt patterning in order to initiate branching.

Patterning a complex organ: branching morphogenesis and nephron segmentation in kidney development

The two major components of the kidney, the collecting system and the nephron, have different developmental histories. The collecting system arises by the reiterated branching of a simple epithelial tube, while the nephron forms from a cloud of mesenchymal cells that coalesce into epithelial vesicles. Each develops into a morphologically complex and highly differentiated structure, and together they provide essential filtration and resorption functions. In this review, we will consider their embryological origin and the genes controlling their morphogenesis, patterning, and differentiation, with a focus on recent advances in several areas.

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.

Kidney development in the absence of Gdnf and Spry1 requires Fgf10

GDNF signaling through the Ret receptor tyrosine kinase (RTK) is required for ureteric bud (UB) branching morphogenesis during kidney development in mice and humans. Furthermore, many other mutant genes that cause renal agenesis exert their effects via the GDNF/RET pathway. Therefore, RET signaling is believed to play a central role in renal organogenesis. Here, we re-examine the extent to which the functions of Gdnf and Ret are unique, by seeking conditions in which a kidney can develop in their absence. We find that in the absence of the negative regulator Spry1, Gdnf, and Ret are no longer required for extensive kidney development. Gdnf−/−;Spry1−/− or Ret−/−;Spry1−/− double mutants develop large kidneys with normal ureters, highly branched collecting ducts, extensive nephrogenesis, and normal histoarchitecture. However, despite extensive branching, the UB displays alterations in branch spacing, angle, and frequency. UB branching in the absence of Gdnf and Spry1 requires Fgf10 (which normally plays a minor role), as removal of even one copy of Fgf10 in Gdnf−/−;Spry1−/− mutants causes a complete failure of ureter and kidney development. In contrast to Gdnf or Ret mutations, renal agenesis caused by concomitant lack of the transcription factors ETV4 and ETV5 is not rescued by removing Spry1, consistent with their role downstream of both RET and FGFRs. This shows that, for many aspects of renal development, the balance between positive signaling by RTKs and negative regulation of this signaling by SPRY1 is more critical than the specific role of GDNF. Other signals, including FGF10, can perform many of the functions of GDNF, when SPRY1 is absent. But GDNF/RET signaling has an apparently unique function in determining normal branching pattern. In contrast to GDNF or FGF10, Etv4 and Etv5 represent a critical node in the RTK signaling network that cannot by bypassed by reducing the negative regulation of upstream signals.

Kidney development requires the secreted protein GDNF, which signals via its cellular receptor RET to promote growth and branching of the ureteric bud, the progenitor of the collecting duct system. The transcription factors ETV4 and ETV5 regulate gene expression in response to GDNF. We report that deleting Spry1, a feedback inhibitor downstream of RET, largely rescues kidney development in mice lacking GDNF or RET, although not in those lacking ETV4 and ETV5. Thus, GDNF and RET become dispensable in the absence of SPRY1, when their roles can be largely assumed by other signals and receptors, while ETV4 and ETV5 remain indispensible. We identify FGF10 as the signal responsible for kidney development in the combined absence of GDNF/RET signaling and SPRY1 negative regulation. But while the ureteric bud branches extensively in Gdnf−/−;Spry1−/− and Ret−/−;Spry1−/− kidneys, its pattern of branching is severely perturbed. This points to a unique function of GDNF in ureteric bud patterning.

vHNF1 functions in distinct regulatory circuits to control ureteric bud branching and early nephrogenesis

Mouse metanephric kidney development begins with the induction of the ureteric bud (UB) from the caudal portion of the Wolffian duct by metanephric mesenchymal signals. While the UB undergoes branching morphogenesis to generate the entire urinary collecting system and the ureter, factors secreted by the UB tips induce surrounding mesenchymal cells to convert into epithelia and form the nephrons, the functional units of the kidney. Epithelial branching morphogenesis and nephrogenesis are therefore tightly orchestrated; defects in either of these processes lead to severe kidney phenotypes ranging from hypoplasia to complete aplasia. However, the underlying regulatory networks have been only partially elucidated. Here, we identify the transcription factor vHNF1 (HNF1beta) as a crucial regulator of these early developmental events. Initially involved in timing outgrowth of the UB and subsequent branching, vHNF1 is also required for nephric duct epithelial maintenance, Müllerian duct formation and early nephrogenesis. Mosaic analyses further suggest a cell-autonomous requirement for vHNF1 in the acquisition of a specialized tip domain and branching morphogenesis. vHNF1 exerts these intricate functions at least in part through the direct control of key regulatory molecules involved in different aspects of early kidney development. Notably, vHNF1 acting directly upstream of Wnt9b appears to orchestrate Wnt signaling action in the mesenchymal-epithelial transitions underlying the initiation of nephrogenesis. These results demonstrate that vHNF1 is an essential transcriptional regulator that, in addition to the known later functions in normal duct morphogenesis, plays a crucial role during the earliest stages of urogenital development and provide novel insights into the regulatory circuits controlling events.

Angiotensin II AT2 receptor regulates ureteric bud morphogenesis

ANG II AT2 receptor (AT2R)-deficient mice exhibit abnormal ureteric bud (UB) budding, increased incidence of double ureters, and vesicoureteral reflux. However, the role of the AT2R during UB morphogenesis and the mechanisms by which aberrant AT2R signaling disrupts renal collecting system development have not been fully defined. In this study, we mapped the expression of the AT2R during mouse metanephric development, examined the impact of disrupted AT2R signaling on UB branching, cell proliferation, and survival, and investigated the cross talk of the AT2R with the glial-derived neurotrophic factor (GDNF)/c-Ret/Wnt11 signaling pathway. Embryonic mouse kidneys express AT2R in the branching UB and the mesenchyme. Treatment of embryonic day 12.5 (E12.5) metanephroi with the AT2R antagonist PD123319 or genetic inactivation of the AT2R in mice inhibits UB branching, decreasing the number of UB tips compared with control (34 +/- 1.0 vs. 43 +/- 0.6, P < 0.01; 36 +/- 1.8 vs. 48 +/- 1.3, P < 0.01, respectively). In contrast, treatment of metanephroi with the AT2R agonist CGP42112 increases the number of UB tips compared with control (48 +/- 1.8 vs. 39 +/- 12.3, P < 0.05). Using real-time quantitative RT-PCR and whole mount in situ hybridization, we demonstrate that PD123319 downregulates the expression of GDNF, c-Ret, Wnt11, and Spry1 mRNA levels in E12.5 metanephroi grown ex vivo. AT(2)R blockade or genetic inactivation of AT2R stimulates apoptosis and inhibits proliferation of the UB cells in vivo. We conclude that AT2R performs essential functions during UB branching morphogenesis via control of the GDNF/c-Ret/Wnt11 signaling pathway, UB cell proliferation, and survival.

Morphogenesis of the thyroid gland

Congenital hypothyroidism is mainly due to structural defects of the thyroid gland, collectively known as thyroid dysgenesis. The two most prevalent forms of this condition are abnormal localization of differentiated thyroid tissue (thyroid ectopia) and total absence of the gland (athyreosis). The clinical picture of thyroid dysgenesis suggests that impaired specification, proliferation and survival of thyroid precursor cells and loss of concerted movement of these cells in a distinct spatiotemporal pattern are major causes of malformation. In normal development the thyroid primordium is first distinguished as a thickening of the anterior foregut endoderm at the base of the prospective tongue. Subsequently, this group of progenitors detaches from the endoderm, moves caudally and ultimately differentiates into hormone-producing units, the thyroid follicles, at a distant location from the site of specification. In higher vertebrates later stages of thyroid morphogenesis are characterized by shape remodeling into a bilobed organ and the integration of a second type of progenitors derived from the caudal-most pharyngeal pouches that will differentiate into C-cells. The present knowledge of thyroid developmental dynamics has emerged from embryonic studies mainly in chicken, mouse and more recently also in zebrafish. This review will highlight the key morphogenetic steps of thyroid organogenesis and pinpoint which crucial regulatory mechanisms are yet to be uncovered. Considering the co-incidence of thyroid dysgenesis and congenital heart malformations the possible interactions between thyroid and cardiovascular development will also be discussed.

Etv4 and Etv5 are required downstream of GDNF and Ret for kidney branching morphogenesis

Glial cell line-derived neurotrophic factor signaling through the Ret receptor tyrosine kinase is crucial for ureteric bud branching morphogenesis during kidney development, yet few of the downstream genes are known. Here we show that the ETS transcription factors Etv4 and Etv5 are positively regulated by Ret signaling in the ureteric bud tips. Mice lacking both Etv4 alleles and one Etv5 allele show either renal agenesis or severe hypodysplasia, whereas kidney development fails completely in double homozygotes. We identified several genes whose expression in the ureteric bud depends on Etv4 and Etv5, including Cxcr4, Myb, Met and Mmp14. Thus, Etv4 and Etv5 are key components of a gene network downstream of Ret that promotes and controls renal branching morphogenesis.

Ret-dependent cell rearrangements in the Wolffian duct epithelium initiate ureteric bud morphogenesis

While the genetic control of renal branching morphogenesis has been extensively described, the cellular basis of this process remains obscure. GDNF/RET signaling is required for ureter and kidney development, and cells lacking Ret are excluded from the tips of the branching ureteric bud in chimeric kidneys. Here, we find that this exclusion results from earlier Ret-dependent cell rearrangements in the caudal Wolffian duct, which generate a specialized epithelial domain that later emerges as the tip of the primary ureteric bud. By juxtaposing cells with elevated or reduced RET activity, we find that Wolffian duct cells compete, based on RET signaling levels, to contribute to this domain. At the same time, the caudal Wolffian duct transiently converts from a simple to a pseudostratified epithelium, a process that does not require Ret. Thus, both Ret-dependent cell movements and Ret-independent changes in the Wolffian duct epithelium contribute to ureteric bud formation.

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.

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.

Stimulatory and inhibitory signaling molecules that regulate renal branching morphogenesis

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

Scara5 is a ferritin receptor mediating non-transferrin iron delivery

Developing organs require iron for a myriad of functions, but embryos deleted of the major adult transport proteins, transferrin or its receptor transferrin receptor1 (TfR1(-/-)), still initiate organogenesis, suggesting that non-transferrin pathways are important. To examine these pathways, we developed chimeras composed of fluorescence-tagged TfR1(-/-) cells and untagged wild-type cells. In the kidney, TfR1(-/-) cells populated capsule and stroma, mesenchyme and nephron, but were underrepresented in ureteric bud tips. Consistently, TfR1 provided transferrin to the ureteric bud, but not to the capsule or the stroma. Instead of transferrin, we found that the capsule internalized ferritin. Since the capsule expressed a novel receptor called Scara5, we tested its role in ferritin uptake and found that Scara5 bound serum ferritin and then stimulated its endocytosis from the cell surface with consequent iron delivery. These data implicate cell type-specific mechanisms of iron traffic in organogenesis, which alternatively utilize transferrin or non-transferrin iron delivery pathways.

Geometric control of tissue morphogenesis

Morphogenesis is the dynamic and regulated change in tissue form that leads to creation of the body plan and development of mature organs. Research over the past several decades has uncovered a multitude of genetic factors required for morphogenesis in animals. The behaviors of individual cells within a developing tissue are determined by combining these genetic signals with information from the surrounding microenvironment. At any point in time, the local microenvironment is influenced by macroscale tissue geometry, which sculpts long range signals by affecting gradients of morphogens and mechanical stresses. The geometry of a tissue thus acts as both a template and instructive cue for further morphogenesis.

Branching morphogenesis

Tubular structures are a fundamental anatomic theme recurring in a wide range of animal species. In mammals, tubulogenesis underscores the development of several systems and organs, including the vascular system, the lungs, and the kidneys. All tubular systems are hierarchical, branching into segments of gradually diminishing diameter. There are only 2 cell types that form the lumen of tubular systems: either endothelial cells in the vascular system or epithelial cells in all other organs. The most important feature in determining the morphology of the tubular systems is the frequency and geometry of branching. Hence, deciphering the molecular mechanisms underlying the sprouting of new branches from preexisting ones is the key to understanding the formation of tubular systems. The morphological similarity between the various tubular systems is underscored by similarities between the signaling pathways which control their branching. A prominent feature common to these pathways is their duality--an agonist counterbalanced by an inhibitor. The formation of the tracheal system in Drosophila melanogaster is driven by fibroblast growth factor and inhibited by Sprouty/Notch. In vertebrates, the analogous pathways are fibroblast growth factor and transforming growth factor-beta in epithelial tubular systems or vascular endothelial growth factor and Notch in the vascular system.

Emphasis on the role of intestinal nervous system in the pathogenesis of inflammatory bowel disease

The etiology of inflammatory bowel disease, Crohn's disease and ulcerative colitis remains unknown. In a great many studies about the pathogenesis of IBD, great attention was paid to the immune dysfunction, genetic susceptibility, and various environmental factors, whereas the effects of enteric nervous system (ENS) were neglected. In fact, increasing evidence now indicates that ENS is involved in the pathogenesis of IBD. In this paper, we review the abnormal regulation of enteric nervous system in IBD.

Developmental plasticity and regenerative capacity in the renal ureteric bud/collecting duct system

Branching morphogenesis of epithelia is an important mechanism in animal development, being responsible for the characteristic architectures of glandular organs such as kidney, lung, prostate and salivary gland. In these systems, new branches usually arise at the tips of existing branches. Recent studies, particularly in kidney, have shown that tip cells express a set of genes distinct from those in the stalks. Tip cells also undergo most cell proliferation, daughter cells either remaining in the tip or being left behind as the tips advance, to differentiate and contribute to new stalk. Published time-lapse observations have suggested, though, that new branches may be able to arise from stalks. This happens so rarely, however, that it is not clear whether this reflects true plasticity and reversal of differentiation, or whether it is just an occasional instance of groups of tip cells being `left behind' by error in a mainly stalk zone. To determine whether cells that have differentiated into stalks really do retain the ability to make new tips, we have removed existing tips from stalks, verified that the stalks are free of tip cells, and assessed the ability of tip-free stalks to initiate new branches. We find stalks to be fully capable of regenerating tips that express typical tip markers, with these tips going on to form epithelial trees, at high frequency. The transition from tip to stalk is therefore reversible, at least for early stages of development. This observation has major implications for models of pattern formation in branching trees, and may also be important for tissue engineering and regenerative medicine.

Downregulation of Spry-1, an inhibitor of GDNF/Ret, causes angiotensin II-induced ureteric bud branching

Mutations of genes in the renin-angiotensin system are associated with congenital abnormalities of the kidney and urinary tract. The major signaling pathway for branching morphogenesis during kidney development is the c-Ret receptor tyrosine kinase whose ligand is GDNF and whose downstream target is Wnt11. We determined whether angiotensin II, an inducer of ureteric bud branching in vitro, influences the GDNF/c-Ret/Wnt11 pathway. Mouse metanephroi were grown in the presence or absence of angiotensin II or an angiotensin type 1 receptor (AT1R) antagonist and gene expression was measured by whole mount in situ hybridization. Angiotensin II induced the expression of c-Ret and Wnt11 in ureteric bud tip cells. GDNF, a Wnt11-regulated gene expressed in the mesenchyme, was also upregulated by angiotensin II but this downregulated Spry1, an endogenous inhibitor of Ret tyrosine kinase activity in an AT1R-dependent manner. Angiotensin II also decreased Spry1 mRNA levels in cultured ureteric bud cells. Exogenous angiotensin II preferentially stimulated ureteric bud tip cell proliferation in vivo while AT1R blockade increased cell apoptosis. Our findings suggest AT1R-mediated inhibition of the Spry1 gene increases c-Ret tyrosine kinase activity leading to upregulation of its downstream target Wnt11. Enhanced Wnt11 expression induces GDNF in adjacent mesenchyme causing focal bursts of ureteric bud tip cell proliferation, decreased tip cell apoptosis and branching.

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.

Association of transforming growth factor-beta1 gene polymorphism with familial vesicoureteral reflux

PURPOSE

Familial clustering of vesicoureteral reflux suggests that genetic factors have an important role in the pathogenesis of vesicoureteral reflux. Transforming growth factor-beta1 is a multifunctional peptide that controls proliferation and differentiation in many cell types. Recently an association between the transforming growth factor-beta1 -509 and +869 gene polymorphism, and renal parenchymal scarring was reported. We investigated the genetic contribution of transforming growth factor-beta1 in familial vesicoureteral reflux by examining the genotype frequencies of transforming growth factor-beta1 polymorphic variants.

MATERIALS AND METHODS

The study included 141 families in which 1 or more siblings had primary vesicoureteral reflux. Renal parenchymal scarring was assessed using dimercapto-succinic acid scans. Genotyping was performed in 280 patients with vesicoureteral reflux, including 133 index patients and 147 siblings, and in 74 controls for the position -509 and the coding region at position 10 (+869) of the transforming growth factor-beta1 gene polymorphism by polymerase chain reaction, gel analysis and appropriate restriction digest.

RESULTS

The genotype frequency of -509CC was significantly increased in the familial vesicoureteral reflux group compared to controls (58% vs 33%, p <0.01), whereas -509TT genotype frequency was significantly lower in the familial vesicoureteral reflux group compared to controls (7.5% vs 28%, p <0.01). Similarly there was a significant increase in the +869TT genotype (52% vs 32%, p <0.05), while the +869CC genotype was significantly lower in patients with familial vesicoureteral reflux compared to controls (11% vs 24%, p <0.01). There were no significant differences in transforming growth factor-beta1 genotype distribution between patients with vesicoureteral reflux with and without renal parenchymal scarring.

CONCLUSIONS

To our knowledge this study demonstrates for the first time the association of the cytokine transforming growth factor-beta1 gene polymorphism in patients with familial vesicoureteral reflux. Individuals with the transforming growth factor-beta1 -509CC and 869TT genotype may have increased susceptibility to vesicoureteral reflux.

The lectin Dolichos biflorus agglutinin is a sensitive indicator of branching morphogenetic activity in the developing mouse metanephric collecting duct system

The urine collecting duct system of the metanephric kidney develops by growth and branching morphogenesis of an unbranched progenitor tubule, the ureteric bud. Bud branching is mainly dichotomous and new branches form from existing branch tips, which are also the main sites of cell proliferation in the system. This behaviour, and the fact that some genes (e.g. Wnt11, Sox9) are expressed only in tips, suggests that tip cells are in a specific state of differentiation. In this report, we show that the lectin Dolichos biflorus agglutinin (DBA), hitherto regarded and used as a general marker of developing renal collecting ducts, binds to most of the duct system but does not bind to the very tips of growing branches. The zone avoided by DBA corresponds to the zone that expresses Wnt11, and the zone that shows enhanced cell proliferation. If branching of the ureteric bud of cultured embryonic kidneys is inhibited in organ culture, by blocking the kidney's endogenous glial cell-derived neurothrophic factor (GDNF)-based branch-promoting signals, the DBA-binding zone extends to the very end of the tip but is lost from there when branching is re-activated. Similarly, if excess GDNF is provided to growing kidneys, the DBA-free zone expands. DBA-staining status therefore appears to be a sensitive indicator of the morphogenetic activity of the collecting duct system.

Canonical WNT signaling during kidney development

The canonical WNT signaling pathway plays a crucial role in patterning of the embryo during development, but little is known about the specific developmental events which are under WNT control. To understand more about how the WNT pathway orchestrates mammalian organogenesis, we studied the canonical beta-catenin-mediated WNT signaling pathway in kidneys of mice bearing a beta-catenin-responsive TCF/betaGal reporter transgene. In metanephric kidney, intense canonical WNT signaling was evident in epithelia of the branching ureteric bud and in nephrogenic mesenchyme during its transition into renal tubules. WNT signaling activity is rapidly downregulated in maturing nephrons and becomes undetectable in postnatal kidney. Sites of TCF/betaGal activity are in proximity to the known sites of renal WNT2b and WNT4 expression, and these WNTs stimulate TCF reporter activity in kidney cell lines derived from ureteric bud and metanephric mesenchyme lineages. When fetal kidney explants from HoxB7/GFP mice were exposed to the canonical WNT signaling pathway inhibitor, Dickkopf-1, arborization of the ureteric bud was significantly reduced. We conclude that restricted zones of intense canonical WNT signaling drive branching nephrogenesis in fetal kidney.

Effects of growth factors on testicular morphogenesis

Since the discovery of the sex-determining gene Sry in 1990, research effort has focused on the events downstream of its expression. A range of different experimental approaches including gene expression, knocking-out and knocking-in genes of interest, and cell and tissue culture techniques have been applied, highlighting the importance of growth factors at all stages of testicular morphogenesis. Migration of primordial germ cells and the mesonephric precursors of peritubular myoid cells and endothelial cells to the gonad is under growth factor control. Proliferation of both germ cells and somatic cells within the gonadal primordium is also controlled by cytokines as is the interaction of Sertoli cells (with each other and with the extracellular matrix) to form testicular cords. Several growth factors/growth factor families (e.g., platelet-derived growth factor, fibroblast growth factor family, TGFbeta family, and neurotrophins) have emerged as key players, exerting an influence at different time points and steps in organogenesis. Although most evidence has emerged in the mouse, comparative studies are important in elucidating the variety, potential, and evolution of control mechanisms.

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.

A method for cold storage and transport of viable embryonic kidney rudiments

Organ culture of mouse embryonic kidneys is a powerful system for studying normal renal development and for researching the developmental effects of experimental perturbations (drugs, antibodies, interfering RNA, and so on). In standard protocols, embryonic kidneys are isolated by delicate micro-dissection and placed in organ culture as soon as possible after the death of the donor mouse, before there is time to genotype them or to transport them elsewhere. This report demonstrates that fully viable embryonic kidneys can be isolated and cultured from crudely dissected caudal portions of embryos that have been stored on ice or at 4 degrees C for several days. This very simple technique can save considerable resources in laboratories that culture kidneys of transgenic mice: (i) cold storage allows embryos to be genotyped before their kidneys are cultured, and (ii) cold transport allows kidney research laboratories to study kidneys of transgenic mice raised elsewhere without the need for expensive importing of the mouse line itself. It will therefore substantially augment the ability of kidney research labs to perform pilot experiments on large numbers of different transgenic animals and to collaborate in new ways.

Activin A is an endogenous inhibitor of ureteric bud outgrowth from the Wolffian duct

Development of metanephric kidney begins with ureteric bud outgrowth from the Wolffian duct (WD). GDNF is believed to be a crucial positive signal in the budding process, but the negative regulation of this process remains unclear. Here, we examined the role of activin A, a member of TGF-beta family, in bud formation using an in vitro WD culture system. When cultured with the surrounding mesonephros, WDs formed many ectopic buds in response to GDNF. While the activin signaling pathway is normally active along the non-budding WD (as measured by expression of activin A and phospho-Smad2/3), activin A was absent and phospho-Smad2/3 was undetectable in the ectopic buds induced by GDNF. To examine the role of activin A in bud formation, we attempted to inactivate activin action. Interestingly, the addition of neutralizing anti-activin A antibody potentiated GDNF action. To further clarify the role of activin A, we also tested the effect of activin blockade on the WD cultured in the absence of mesonephros. WDs without mesonephros did not form ectopic buds even in the presence of GDNF. In contrast, blockade of activin action with a variety of agents acting through different mechanisms (natural antagonist, neutralizing antibodies, siRNA) enabled GDNF to induce ectopic buds. Inhibition of GDNF-induced bud formation by activin A was accompanied by inhibition of cell proliferation, reduced expression of Pax-2, and decreased phosphorylation of PI3-kinase and MAP kinase in the WD. Our data suggest that activin A is an endogenous inhibitor of bud formation and that cancellation of activin A autocrine action may be critical for the initiation of this process.

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.

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

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

The developing mouse thyroid: Embryonic vessel contacts and parenchymal growth pattern during specification, budding, migration, and lobulation

Normal mouse thyroid development has been revised to identify critical morphogenetic events. The early thyroid primordium associates with the aortic sac endothelium at the time of specification and budding. The vascular contact is lost after the thyroid buds from the pharyngeal endoderm, but is resumed before the gland divides to form two lobes. Lateral expansion of parenchyma takes place along the course of the third pharyngeal arch arteries. Thyroid precursor cells expressing Titf1/Nkx2.1 do not proliferate until the migration stage, implicating that progenitors likely are recruited from outside the thyroid placode. Early lobulation involves engulfment of the entire ultimobranchial bodies by the growing midline thyroid. At the same time, proliferation of the ultimobranchial body epithelium is silenced preceding the differentiation of C cells. Before folliculogenesis, thyroid lobe enlargement is reminiscent of a budding-branching-like growth pattern. It is suggested that thyroid inductive signals arise from embryonic vessels, and that this provides ideas to conceptually new pathogenetic mechanisms of thyroid dysgenesis.

A role for microfilament-based contraction in branching morphogenesis of the ureteric bud

BACKGROUND

Branching morphogenesis of the ureteric bud/collecting duct epithelium is an important feature of kidney development. Recent work has identified many transcription factors and paracrine signaling molecules that regulate branching, but the physical mechanisms by which these signals act remain largely unknown. The actin cytoskeleton is a common component of mechanisms of morphogenesis. We have therefore studied the expression of, and requirement for actin filaments in the ureteric bud, a branching epithelium of the mammalian kidney.

METHODS

Embryonic kidney rudiments were grown in organ culture. Actin expression in kidneys growing normally and those in which branching was inhibited was examined using labeled phalloidin. The morphogenetic effects of inhibiting actin organization and tension using cytochalasin D, butanedione monoxime, and Rho kinase ROCK inhibitors were assessed using immunofluorescence.

RESULTS

F-actin is expressed particularly strongly in the apical domains of cells at the tips of branching ureteric bud, but this expression depends on the bud actively growing and branching. Blocking the polymerization of actin using cytochalasin D inhibits ureteric bud branching reversibly, as does blocking myosin function using butadiene monoxime. Inhibiting the activation of ROCK, a known activator of myosin, with the drugs Y27632 or with H1152 inhibits the expression of strong actin bundles in the ureteric bud tips and inhibits ureteric bud branching without inhibiting other aspects of renal development.

CONCLUSION

The formation of tension-bearing actin-myosin complexes is essential for branching morphogenesis in the developing kidney.

Heregulin induces glial cell line-derived neurotrophic growth factor-independent, non-branching growth and differentiation of ureteric bud epithelia

We have purified a protein present in a conditioned medium derived from the metanephric mesenchyme that supports non-branching growth and epithelial differentiation of the isolated ureteric bud (UB) independent of glial cell line-derived neurotrophic growth factor (GDNF). By sequential liquid chromatography, together with protein microsequencing, the protein was identified as heregulin (HRG)alpha. The addition of recombinant HRG to the isolated UB grown in three-dimensional culture confirmed the proliferative activity of HRG. In branching UBs induced by whole metanephric mesenchyme cell-conditioned medium, proliferating cells were localized at ampullae, where a binding receptor for GDNF, GFRalpha1, was found. In HRG-induced UBs, however, the expression of GFRalpha1 was down-regulated, and proliferating cells were distributed throughout the structure. Electron microscopic examination of the HRG-induced UB revealed the presence of structurally mature and polarized epithelial cells reminiscent of the epithelial cells found in the stalk portion of the branching UB. cDNA array analysis further revealed that genes ontologically classified as developmental were down-regulated by HRG, whereas those involved in transport were up-regulated. For example, the mRNA for the GDNF receptors, GFRalpha1 and ret9, was down-regulated, whereas the mRNA for collecting duct transporters, such as urea transporter2, aquaporin3, and sodium-hydrogen exchanger2 was up-regulated in HRG-treated UBs compared with UBs grown in the presence of branch-promoting factors. Moreover, HRG promoted growth of UBs cultured in the absence of GDNF. Taken together, the data suggest that HRG supports UB epithelial cell differentiation and non-GDNF-dependent growth, raising the possibility that this kind of activity plays a role in the growth and differentiation of the stalk portion of the branching epithelial UB.

Emergent patterns of growth controlled by multicellular form and mechanics

Spatial patterns of cellular growth generate mechanical stresses that help to push, fold, expand, and deform tissues into their specific forms. Genetic factors are thought to specify patterns of growth and other behaviors to drive morphogenesis. Here, we show that tissue form itself can feed back to regulate patterns of proliferation. Using micro-fabrication to control the organization of sheets of cells, we demonstrated the emergence of stable patterns of proliferative foci. Regions of concentrated growth corresponded to regions of high tractional stress generated within the sheet, as predicted by a finite-element model of multicellular mechanics and measured directly by using a micromechanical force sensor array. Inhibiting actomyosin-based tension or cadherin-mediated connections between cells disrupted the spatial pattern of proliferation. These findings demonstrate the existence of patterns of mechanical forces that originate from the contraction of cells, emerge from their multicellular organization, and result in patterns of growth. Thus, tissue form is not only a consequence but also an active regulator of tissue growth.

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.