Dominant effects of RET receptor misexpression and ligand-independent RET signaling on ureteric bud development

Morphogenesis during mouse embryonic kidney explant culture

BACKGROUND

Renal organogenesis is routinely studied using cultured murine embryonic kidneys, but the application of this model has not yet been subjected to rigorous standards.

METHODS

We measured ex vivo growth and morphogenesis of day 13 murine kidneys and evaluated the importance of culture conditions and biological variables.

RESULTS

Kidney size was measured in two dimensions as planar surface area and was shown to correlate highly with volume (R2 = 0.60, P < 0.005). The final surface area of kidneys was directly dependent on the initial starting size (R2 = 0.61, P < 0.05), suggesting that the final surface area is not a valid outcome measurement unless starting size is equal among treatments. Relative growth rate, defined as (final surface area - initial surface area)/initial surface area, was a good measure of growth and independent of size and anatomical position (P> 0.05). Significant differences in size and growth rates were observed among litters (P < 0.05), implying that kidneys from a given litter must be randomized to avoid confounding results. Planar surface area of each explant increased in proportion to ureteric bud branching (R2 = 0.6854, P < 0.05). In a comparison of a variety of base media and supplements, kidney explants were observed to grow best in Dulbecco's modified Eagle's medium (DMEM)/F12 with 5% fetal bovine serum and to sustain growth for up to 96 hours, despite decreased proliferation and increased apoptosis at this time point.

CONCLUSIONS

These results represent an important step in establishing standardized procedures for the use of cultured embryonic kidneys and will improve our ability to apply the model to better understand kidney morphogenesis.

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.

Distal ureter morphogenesis depends on epithelial cell remodeling mediated by vitamin A and Ret

Almost 1% of human infants are born with urogenital abnormalities, many of which are linked to irregular connections between the distal ureters and the bladder. During development, ureters migrate by an unknown mechanism from their initial integration site in the Wolffian ducts up to the base of the bladder in a process that we call ureter maturation. Rara(-/-) Rarb2(-/-) mice display impaired vitamin A signaling and develop syndromic urogenital malformations similar to those that occur in humans, including renal hypoplasia, hydronephrosis and mega-ureter, abnormalities also seen in mice with mutations in the proto-oncogene Ret. Here we show that ureter maturation depends on formation of the 'trigonal wedge', a newly identified epithelial outgrowth from the base of the Wolffian ducts, and that the distal ureter abnormalities seen in Rara(-/-) Rarb2(-/-) and Ret(-/-) mutant mice are probably caused by a failure of this process. Our studies indicate that formation of the trigonal wedge may be essential for correct insertion of the distal ureters into the bladder, and that these events are mediated by the vitamin A and Ret signaling pathways.

RET receptor tyrosine kinase isoforms in kidney function and disease

The RET proto-oncogene encodes two major isoforms, RET9 and RET51, which differ at the carboxyl-terminal. Loss-of-function mutations in RET result in gut aganglionosis while gain of function mutations result in cancer syndromes. From studies on transgenic mice, RET9 is important for early development of the kidney and the enteric nervous system. Little is known about the function of RET isoforms in later life. Here we report the expression of RET isoforms and its signalling complex, GDNF and GFRalpha1, in foetal and adult human kidneys. We found their expression in both the developing and the adult renal collecting system. We further show that only RET51 but not RET9 could promote the survival and tubulogenesis of mIMCD3 (mouse inner medullary collecting duct) cells in collagen gel. Our results agree with the hypothesis that RET51 signalling is related to differentiation events in later kidney organogenesis. In addition, it may also have a function in the adult kidney. We further extend our study by showing increased RET and GDNF expression in collecting duct cysts of polycystic kidney patients. This suggests that GDNF/RET signalling may contribute to proliferation of the collecting duct epithelium in an autocrine/paracrine manner.

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.

Enteric nervous system: development and developmental disturbances--part 2

This review, which is presented in two parts, summarizes and synthesizes current views on the genetic, molecular, and cell biological underpinnings of the early embryonic phases of enteric nervous system (ENS) formation and its defects. Accurate descriptions of the phenotype of ENS dysplasias, and knowledge of genes which, when mutated, give rise to the disorders (see Part 1 in the previous issue of this journal), are not sufficient to give a real understanding of how these abnormalities arise. The often indirect link between genotype and phenotype must be sought in the early embryonic development of the ENS. Therefore, in this, the second part, we provide a description of the development of the ENS, concentrating mainly on the origin of the ENS precursor cells and on the cell migration by which they become distributed throughout the gastrointestinal tract. This section also includes experimental evidence on the controls of ENS formation derived from classic embryological, cell culture, and molecular genetic approaches. In addition, for reasons of completeness, we also briefly describe the origins of the interstitial cells of Cajal, a cell population closely related anatomically and functionally to the ENS. Finally, a brief sketch is presented of current notions on the developmental processes between the genes and the morphogenesis of the ENS, and of the means by which the known genetic abnormalities might result in the ENS phenotype observed in Hirschsprung's disease.

Deregulated expression of the homeobox gene Cux-1 in transgenic mice results in downregulation of p27(kip1) expression during nephrogenesis, glomerular abnormalities, and multiorgan hyperplasia

Cux-1 is a murine homeobox gene that is highly expressed in the developing kidney with expression restricted to the nephrogenic zone. Cux-1 is highly expressed in cyst epithelium of polycystic kidneys from C57BL/6J-cpk/cpk mice, but not in kidneys isolated from age-matched phenotypically normal littermates. To further elucidate the role of Cux-1 in renal development, we generated transgenic mice expressing Cux-1 under the control of the CMV immediate early gene promoter. Mice constitutively expressing Cux-1 developed multiorgan hyperplasia and organomegaly, but not an overall increase in body size. Transgenic kidneys were enlarged 50% by 6 weeks of age, with the increased growth primarily restricted to the cortex. Proliferating cells were found in proximal and distal tubule epithelium throughout the cortex, and the squamous epithelium that normally lines Bowman's capsule was replaced with proximal tubule epithelium. However, the total number of nephrons was not increased. In the developing kidneys of transgenic mice, Cux-1 was ectopically expressed in more highly differentiated tubules and glomeruli, and this was associated with reduced expression of the cyclin kinase inhibitor, p27. Transient transfection experiments revealed that Cux-1 is an inhibitor of p27 promoter activity. These results suggest that Cux-1 regulates cell proliferation during early nephrogenesis by inhibiting expression of p27.

Tetracycline-inducible gene expression in cultured rat renal CD cells and in intact CD from transgenic mice

The renal collecting duct (CD) plays a key role in the control of ion and fluid homeostasis. Several genetic diseases that involve mutations in genes encoding for ion transporters or hormone receptors specifically expressed in CD have been described. Suitable cellular or transgenic animal models expressing such mutated genes in an inducible manner should represent attractive systems for structure-function relationship analyses and the generation of appropriate physiopathological models of related diseases. Our first goal was to develop a CD cell line that allows inducible gene expression using the tetracycline-inducible system (Tet-On). We designed several strategies aimed at the development of a tight and highly inducible system in RCCD1 cells, a rat cortical collecting duct (CCD) cell line exhibiting several properties of the native CCD. Analysis of reporter gene expression demonstrated that the Tet-On system is suitable for inducible gene expression in these cells. In a second step, we have tested whether transgenic Tet-On mice expressing the tetracycline transactivator under the control of the human cytomegalovirus promoter were suitable for inducible gene expression in tubule epithelial cells. The results indicate that, in vivo, the inducible expression of the lacZ reporter gene appeared to be restricted to the CD. This particular strain of transgenic mice may therefore be useful for the expression of genes of interest in an inducible manner in the collecting duct.

The RET receptor: function in development and dysfunction in congenital malformation

Germline mutations in the RET proto-oncogene are responsible for two unrelated neural crest disorders: Hirschsprung disease, a congenital absence of the enteric nervous system in the hindgut, and multiple endocrine neoplasia type 2, a dominantly inherited cancer syndrome. Moreover, somatic rearrangements of RET are causally involved in the genesis of papillary thyroid carcinoma. The receptor tyrosine kinase encoded by the RET gene acts as the subunit of a multimolecular complex that binds four distinct ligands and activates a signalling network crucial for neural and kidney development. Over the past few years, a clearer picture of the mode of RET activation and of its multifaceted role during development has started to emerge. These findings, which provide new clues to the molecular mechanisms underlying RET signalling dysfunction in Hirschsprung disease, are summarized in this review.

Differential activities of the RET tyrosine kinase receptor isoforms during mammalian embryogenesis

The RET receptor tyrosine kinase has a critical role in kidney organogenesis and the development of the enteric nervous system. Two major isoforms, RET9 and RET51, differ in the amino acid sequence of the C-terminal tail as a result of alternative splicing. To determine the roles of these isoforms in vivo, we used targeted mutagenesis to generate mice that express either RET9 or RET51. Monoisoformic RET9 mice, which lack RET51, are viable and appear normal. In contrast, monoisoformic RET51 animals, which lack RET9, have kidney hypodysplasia and lack enteric ganglia from the colon. To study the differential activities of the two RET isoforms further, we generated transgenic mice expressing ligand-dependent and constitutively active forms of RET9 or RET51 under the control of the Hoxb7 regulatory sequences. Such RET9 transgenes are capable of rescuing the kidney agenesis in RET-deficient mice or causing kidney hypodysplasia in wild-type animals. In contrast, similar RET51 transgenes fail to rescue the kidney agenesis or cause hypodysplasia. Our findings show that RET9 and RET51 have different signaling properties in vivo and define specific temporal and spatial requirements of c-Ret function during renal development and histogenesis of the enteric nervous system.

Multiple endocrine neoplasia type 2 syndromes may be associated with renal malformations

OBJECTIVE

The RET proto-oncogene is known to be the susceptibility gene for various disease phenotypes, including multiple endocrine neoplasia type 2 (MEN 2). Recent studies have also suggested an involvement of RET in the development of the mammalian kidney. Although kidney agenesis or dysgenesis has been observed in mice lacking functional ret, no clinically relevant kidney abnormalities have been reported in individuals with known RET mutations and familial medullary thyroid carcinoma (FMTC). We have studied a family with five members affected with isolated FMTC. DNA analysis was performed and the involved RET mutation was identified. Amongst these patients were a woman and her son.

DESIGN

Case report.

SETTING

University department.

PATIENTS

A 32-year-old woman and her son with FMTC and unilateral renal agenesis.

RESULTS

The woman's abdominal ultrasound findings demonstrated unilateral renal absence of the left kidney. Her son, when only a few months old, had undergone surgical treatment for Hirschsprung's disease. Abdominal ultrasonography was performed recently, and left-side renal absence was diagnosed. Intravenous pyelography confirmed the agenesis of his left kidney, whilst the contralateral kidney displayed compensatory hypertrophy.

CONCLUSIONS

The involvement of the RET proto-oncogene in the early growth and differentiation of the human kidney is now generally accepted. We believe that at least a proportion of patients with MEN 2 may have undiagnosed renal malformations. We suggest therefore that noninvasive imaging techniques, such as ultrasonography, should be used to explore the presence of renal abnormalities in subjects with demonstrated RET mutations.

Galectin-3 modulates ureteric bud branching in organ culture of the developing mouse kidney

Galectin-3 is a mammalian beta-galactoside-specific lectin with functions in cell growth, adhesion, and neoplastic transformation. On the basis of expression patterns in humans, it is proposed that galectin-3 modulates fetal collecting duct growth. This article provides evidence that galectin-3 can modulate branching morphogenesis of the mouse ureteric bud/collecting duct lineage. With the use of immunohistochemistry, galectin-3 was not detected in early metanephrogenesis but was upregulated later in fetal kidney maturation when the protein was prominent in basal domains of medullary collecting ducts. Addition of galectin-3 to embryonic days 11 and 12 whole metanephric cultures inhibited ureteric bud branching, whereas galectin-1 did not perturb morphogenesis, nor did a galectin-3 mutant lacking wild-type high-affinity binding to extended oligosaccharides. Exogenous galectin-3 retarded conversion of renal mesenchyme to nephrons in whole metanephric explants but did not affect nephron induction by spinal cord in isolated renal mesenchymes. Finally, addition of a blocking antiserum to galectin-3 caused dilation and distortion of developing epithelia in embryonic day 12 metanephroi cultured for 1 wk. The upregulation of galectin-3 protein during kidney maturation, predominantly at sites where it could mediate cell/matrix interactions, seems to modulate growth of the ureteric tree.

GDNF is a chemoattractant for enteric neural cells

In situ hybridization revealed that GDNF mRNA in the mid- and hindgut mesenchyme of embryonic mice was minimal at E10.5 but was rapidly elevated at all gut regions after E11, but with a slight delay (0.5 days) in the hindgut. GDNF mRNA expression was minimal in the mesentery and in the pharyngeal and pelvic mesenchyme adjacent to the gut. To examine the effect of GDNF on enteric neural crest-derived cells, segments of E11.5 mouse hindgut containing crest-derived cells only at the rostral ends were attached to filter paper supports and grown in catenary organ culture. With GDNF (100 ng/ml) in the culture medium, threefold fewer neurons developed in the gut explants and fivefold more neurons were present on the filter paper outside the gut explants, compared to controls. Thus, in controls, crest-derived cells colonized the entire explant and differentiated into neurons, whereas in the presence of exogenous GDNF, most crest-derived cells migrated out of the gut explant. This is consistent with GDNF acting as a chemoattractant. To test this idea, explants of esophagus, midgut, superior cervical ganglia, paravertebral sympathetic chain ganglia, or dorsal root ganglia from E11.5-E12.5 mice were grown on collagen gels with a GDNF-impregnated agarose bead on one side and a control bead on the opposite side. Migrating neural cells and neurites from the esophagus and midgut accumulated around the GDNF-impregnated beads, but neural cells in other tissues showed little or no chemotactic response to GDNF, although all showed GDNF-receptor (Ret and GFRalpha1) immunoreactivity. We conclude that GDNF may promote the migration of crest cells throughout the gastrointestinal tract, prevent them from straying out of the gut (into the mesentery and pharyngeal and pelvic tissues), and promote directed axon outgrowth.

BMP7 controls collecting tubule cell proliferation and apoptosis via Smad1-dependent and -independent pathways

Bone morphogenetic protein-7 (BMP7) controls ureteric bud and collecting duct morphogenesis in a dose-dependent manner (Piscione TD, Yager TD, Gupta IR, Grinfeld B, Pei Y, Attisono L, Wrana JL, and Rosenblum ND. Am J Physiol Renal Physiol 273: F961-F975, 1997). We defined cellular and molecular mechanisms underlying these effects in embryonic kidney explants and in the mIMCD-3 cell model of collecting tubule morphogenesis. Low-dose (0.25 nM) BMP7 significantly increased tubule number and cell proliferation. Similar to BMP2, high-dose (10 nM) BMP7 inhibited cell proliferation and stimulated apoptosis. To define molecular mechanisms, we identified signaling events downstream of BMP7. High-dose BMP7, but not low-dose BMP7, activated Smad1 in mIMCD-3 cells. Moreover, the inhibitory effects of high-dose BMP7 and BMP2, but not the stimulatory effects of low-dose BMP7, on tubulogenesis and cell proliferation were significantly reduced in mIMCD-3 cells stably expressing Smad1(Delta458), a dominant negative mutant form of Smad1, but not in cells stably expressing wild-type Smad1. We conclude that BMP7 exerts dose-dependent effects on ureteric bud or collecting duct cell proliferation and apoptosis by signaling via Smad1-dependent and Smad1-independent pathways.

Vitamin A controls epithelial/mesenchymal interactions through Ret expression

Mutations or rearrangements in the gene encoding the receptor tyrosine kinase RET result in Hirschsprung disease, cancer and renal malformations. The standard model of renal development involves reciprocal signaling between the ureteric bud epithelium, inducing metanephric mesenchyme to differentiate into nephrons, and metanephric mesenchyme, inducing the ureteric bud to grow and branch. RET and GDNF (a RET ligand) are essential mediators of these epithelial-mesenchymal interactions. Vitamin A deficiency has been associated with widespread embryonic abnormalities, including renal malformations. The vitamin A signal is transduced by nuclear retinoic acid receptors (RARs). We previously showed that two RAR genes, Rara and Rarb2, were colocalized in stromal mesenchyme, a third renal cell type, where their deletion led to altered stromal cell patterning, impaired ureteric bud growth and downregulation of Ret in the ureteric bud. Here we demonstrate that forced expression of Ret in mice deficient for both Rara and Rarb2 (Rara(-/-)Rarb2(-/-)) genetically rescues renal development, restoring ureteric bud growth and stromal cell patterning. Our studies indicate the presence of a new reciprocal signaling loop between the ureteric bud epithelium and the stromal mesenchyme, dependent on Ret and vitamin A. In the first part of the loop, vitamin-A-dependent signals secreted by stromal cells control Ret expression in the ureteric bud. In the second part of the loop, ureteric bud signals dependent on Ret control stromal cell patterning.

Stanniocalcin gene expression during mouse urogenital development: a possible role in mesenchymal-epithelial signalling

Stanniocalcin (STC) is a polypeptide hormone first discovered in fish and more recently in mammals. In mammals, the STC gene is widely expressed and the hormone is involved in a variety of functions, but STC does not normally circulate in the blood. In both kidney and gut, STC regulates phosphate fluxes across the transporting epithelia, whereas in brain it protects neurons against cerebral ischemia and promotes neuronal cell differentiation. However, the gene is most highly expressed in ovary and expression is dramatically up-regulated by both pregnancy and nursing. STC mRNA levels are also high in the developing mouse embryo, but literally nothing is known of the tissue pattern of gene expression. Therefore, the aim of this study was to map the temporal and spatial patterns of gene expression during mouse embryologic development, starting with the urogenital system where the gene is so highly expressed in adults. STC mRNA was evident as early as E10.5 in both the mesonephros and genital ridge. Between E10.5 and 14.5 in developing kidney, STC was produced in undifferentiated mesenchyme cells and sequestered by ureteric bud epithelial cells that did not express the gene but nonetheless contained high levels of STC protein. Thereafter, the distribution pattern resembled that in adults such that gene expression predominated in collecting duct cells, whereas protein was present in most nephron segments. The pattern of gene expression during gonadal development was sexually dimorphic. In males, expression was first evident on E12.5 in interstitial mesenchyme cells surrounding the developing sex cords, whereas the protein accumulated in developing gonocytes within the sex cords that did not express the gene. This pattern became more pronounced over the course of gestation. In contrast, ovarian gene expression was only weakly evident during development. Collectively, the evidence suggests that in addition to its regulatory effects in adults, STC has novel and distinctive roles in the mesenchymal-epithelial interactions that are vital to normal organogenesis.

GDNF and GFRalpha-1 are components of the axolotl pronephric duct guidance system

In mammals, secretion of GDNF by the metanephrogenic mesenchyme is essential for branching morphogenesis of the ureteric bud and, thus, metanephric development. However, the expression pattern of GDNF and its receptor complex-the GPI-linked ligand-binding protein, GFRalpha-1, and the Ret tyrosine kinase signaling protein-indicates that it could operate at early steps in kidney development as well. Furthermore, the developing nephric systems of fish and amphibian embryos express components of the GDNF signaling system even though they do not make a metanephros. We provide evidence that GDNF signaling through GFRalpha-1 is sufficient to direct pathfinding of migrating pronephric duct cells in axolotl embryos by: (1) demonstrating that application of soluble GFRalpha-1 to an embryo lacking all GPI-linked proteins rescues PND migration in a dose-dependent fashion, (2) showing that application of excess soluble GFRalpha-1 to a normal embryo inhibits migration and that inhibition is dependent upon GDNF-binding activity, and (3) showing that the PND will migrate toward a GDNF-soaked bead in vivo, but will fail to migrate when GDNF is applied uniformly to the flank. These data suggest that PND pathfinding is accomplished by migration up a gradient of GDNF.

Cross-talk in kidney development

As in most organs, the emerging theme in kidney development is the importance of cross-talk between several tissues and cell lineages to allow morphogenesis to proceed in a complex but highly regulated way. Over the past few years, knock-out and transgenic analyses in mice and evolutionary comparison with non-mammalian species have been particularly instrumental in identifying molecules with crucial functions for tissue-tissue interactions. The transcription factors Wt1 and Eya1, the signalling molecules Gdnf and LIF and the receptors c-Ret and GdnfRalpha have been demonstrated to fulfil fundamental roles in the first step of metanephric induction, the outgrowth of the ureter. Signalling by members of the Wnt, BMP and FGF families, regulated by transcription factors such as Pax2, mediates nephrogenesis by adjusting the balance between the ureteric bud epithelium, stromal and nephrogenic tissues. The stromal tissue, neglected for many years, has been shown to serve important functions in regulating the growth of nephrons. Finally, we have also begun to gain insight into the molecular events underlying patterning of the nephron into distinct functional units including glomerulus, proximal and distal tubule.

Bone morphogenetic protein 4 regulates the budding site and elongation of the mouse ureter

In the normal mouse embryo, Bmp4 is expressed in mesenchymal cells surrounding the Wolffian duct (WD) and ureter stalk, whereas bone morphogenetic protein (BMP) type I receptor genes are transcribed either ubiquitously (Alk3) or exclusively in the WD and ureter epithelium (Alk6). Bmp4 heterozygous null mutant mice display, with high penetrance, abnormalities that mimic human congenital anomalies of the kidney and urinary tract (CAKUT), including hypo/dysplastic kidneys, hydroureter, ectopic ureterovesical (UV) junction, and double collecting system. Analysis of mutant embryos suggests that the kidney hypo/dysplasia results from reduced branching of the ureter, whereas the ectopic UV junction and double collecting system are due to ectopic ureteral budding from the WD and accessory budding from the main ureter, respectively. In the cultured metanephros deprived of sulfated glycosaminoglycans (S-GAGs), BMP4-loaded beads partially rescue growth and elongation of the ureter. By contrast, when S-GAGs synthesis is not inhibited, BMP4 beads inhibit ureter branching and expression of Wnt 11, a target of glial cell-derived neurotrophic factor signaling. Thus, Bmp4 has 2 functions in the early morphogenesis of the kidney and urinary tract. One is to inhibit ectopic budding from the WD or the ureter stalk by antagonizing inductive signals from the metanephric mesenchyme to the illegitimate sites on the WD. The other is to promote the elongation of the branching ureter within the metanephros, thereby promoting kidney morphogenesis.

Kidney morphogenesis: cellular and molecular regulation

Development of an organ is directed by cell and tissue interactions and these also occur during the formation of functional kidney. During vertebrate development inductive signalling between mesenchyme and epithelium controls the organogenesis of all three kinds of kidneys: pronephros, mesonephros and metanephros. In higher animals the metanephros differentiates into the permanent kidney and in this review we will mainly concentrate on its development. Molecular interactions currently known to function during nephrogenesis have primarily been based on the use of knockout techniques. These studies have highlighted the role for transcription factors, signalling molecules, growth factors and their receptors and also for extracellular matrix components in kidney development. Finally in this review we will represent our own model for kidney development according to the knowledge of the genes involved in the development of the functional excretory organ, kidney.

The basic-helix-loop-helix protein pod1 is critically important for kidney and lung organogenesis

Epithelial-mesenchymal interactions are required for the development of all solid organs but few molecular mechanisms that underlie these interactions have been identified. Pod1 is a basic-helix-loop-helix (bHLH) transcription factor that is highly expressed in the mesenchyme of developing organs that include the lung, kidney, gut and heart and in glomerular visceral epithelial cells (podocytes). To determine the function of Pod1 in vivo, we have generated a lacZ-expressing null Pod1 allele. Null mutant mice are born but die in the perinatal period with severely hypoplastic lungs and kidneys that lack alveoli and mature glomeruli. Although Pod1 is exclusively expressed in the mesenchyme and podocytes, major defects are observed in the adjacent epithelia and include abnormalities in epithelial differentiation and branching morphogenesis. Pod1 therefore appears to be essential for regulating properties of the mesenchyme that are critically important for lung and kidney morphogenesis. Defects specific to later specialized cell types where Pod1 is expressed, such as the podocytes, were also observed, suggesting that this transcription factor may play multiple roles in kidney morphogenesis.

Sympathoadrenal hyperplasia causes renal malformations in Ret(MEN2B)-transgenic mice

The tyrosine kinase receptor Ret is expressed in the ureteric bud and is required for normal renal development. Constitutive loss of Ret, its co-receptor gfralpha-1, or the ligand glial cell line-derived neurotrophic factor results in renal agenesis. Transgenic embryos that express a constitutively active form of Ret (Ret(MEN2B)) under the control of the dopamine-beta-hydroxylase (DbetaH) promoter develop profound neuroglial hyperplasia of their sympathetic ganglia and adrenal medullae. Embryos from two independent DbetaH-Ret(MEN2B)-transgenic lines exhibit renal malformations. In contrast with ret-/- embryos, renal maldevelopment in DbetaH-Ret(MEN2B)-transgenic embryos results from primary changes in sympathoadrenal organs extrinsic to the kidney. The ureteric bud invades the metanephric mesenchyme normally, but subsequent bud branching and nephrogenesis are retarded, resulting in severe renal hypoplasia. Ablation of sympathoadrenal precursors restores normal renal growth in vivo and in vitro. We postulate that disruption of renal development results because Ret(MEN2B) derived from the hyperplastic nervous tissue competes with endogenous renal Ret for gfralpha-1 or other signaling components. This hypothesis is supported by the observation that renal malformations, which do not normally occur in a transgenic line with low levels of DbetaH-Ret(MEN2B) expression, arise in a gdnf+/- background. However, renal maldevelopment was not recapitulated in kidneys that were co-cultured with explanted transgenic ganglia in vitro. Our observations illustrate a novel pathogenic mechanism for renal dysgenesis that may explain how putative activating mutations of the RET gene can produce a phenotype usually associated with RET deficiency.

Cell adhesion molecules and extracellular-matrix constituents in kidney development and disease

Functional analyses of cell-matrix interactions during kidney organogenesis have provided compelling evidence that extracellular-matrix glycoproteins and their receptors play instructive roles during kidney development. Two concepts are worthy of emphasis. First, matrix molecules appear to regulate signal transduction pathways, either by activating cell-surface receptors such as integrins directly or by modulating the activity of signaling molecules such as WNTs. Second, basement membranes are highly organized structures and have distinct molecular compositions, which are optimized for their diverse functions. The importance of these findings is highlighted by the fact that mutations affecting basement-membrane components lead to inherited forms of kidney disease.

The malformed kidney: disruption of glomerular and tubular development

Renal malformations are the major cause of renal failure during early childhood. They are found in approximately 100 genetic syndromes. We review the embryologic development of the kidney and its molecular control. Important new information has been derived from mutational analysis in humans and mice. We describe how mutations in nine transcription factors, 12 signaling molecules and nine gene products involved in a variety of other cellular functions disrupt renal morphogenesis. The information presented provides a template for integrating new discoveries on the molecular basis of renal development, for classifying renal malformations observed in the clinical setting, and for identifying defective genes in affected patients.