Deficient outgrowth of the ureteric bud underlies the renal agenesis phenotype in mice manifesting the limb deformity (ld) mutation

Mouse Models of Congenital Kidney Anomalies

Congenital anomalies of the kidney and urinary tract (CAKUT) are common birth defects, which cause the majority of chronic kidney diseases in children. CAKUT covers a wide range of malformations that derive from deficiencies in embryonic kidney and lower urinary tract development, including renal aplasia, hypodysplasia, hypoplasia, ectopia, and different forms of ureter abnormalities. The majority of the genetic causes of CAKUT remain unknown. Research on mutant mice has identified multiple genes that critically regulate renal differentiation. The data generated from this research have served as an excellent resource to identify the genetic bases of human kidney defects and have led to significantly improved diagnostics. Furthermore, genetic data from human CAKUT studies have also revealed novel genes regulating kidney differentiation.

Spontaneous one-kidney rats are more susceptible to develop hypertension by DOCA-NaCl and subsequent kidney injury compared with uninephrectomized rats

There is little clinical data of how hypertension may influence individuals with nephron deficiency in the context of being born with a single kidney. We recently developed a new rat model (the heterogeneous stock-derived model of unilateral renal agenesis rat) that is born with a single kidney and exhibits progressive kidney injury and decline in kidney function with age. We hypothesized that DOCA-salt would induce a greater increase in blood pressure and therefore accelerate the progression of kidney injury in rats born with a solitary kidney compared with rats that have undergone unilateral nephrectomy. Time course evaluation of blood pressure, kidney injury, and renal hemodynamics was performed in the following six groups of animals from weeks 13 to 18: 1) DOCA-treated rats with a solitary kidney (DOCA+S group), 2) placebo-treated rats with a solitary kidney, 3) DOCA-treated control rats with two kidneys (DOCA+C group), 4) placebo-treated control rats with two kidneys, 5) DOCA-treated rats with two kidneys that underwent uninephrectomy (DOCA+UNX8 group), and 6) placebo-treated rats with two kidneys that underwent uninephrectomy. DOCA+S rats demonstrated a significant rise (P < 0.05) in blood pressure (192 ± 4 mmHg), proteinuria (205 ± 31 mg/24 h), and a decline in glomerular filtration rate (600 ± 42 μl·min(-1)·g kidney weight(-1)) relative to the DOCA+UNX8 (173 ± 3 mmHg, 76 ± 26 mg/24 h, and 963 ± 36 μl·min(-1)·g kidney weight(-1)) and DOCA+C (154 ± 2 mmHg, 7 ± 1 mg/24 h, and 1,484 ± 121 μl·min(-1)·g kidney weight(-1)) groups. Placebo-treated groups showed no significant change among the three groups. An assessment of renal injury markers via real-time PCR/Western blot analysis and histological analysis was concordant with the measured physiological parameters. In summary, congenital solitary kidney rats are highly susceptible to the induction of hypertension compared with uninephrectomized rats, suggesting that low nephron endowment is an important driver of elevated blood pressure, hastening nephron injury through the transmission of elevated systemic blood pressure and thereby accelerating decline in kidney function.

Nephron Deficiency and Predisposition to Renal Injury in a Novel One-Kidney Genetic Model

Some studies have reported up to 40% of patients born with a single kidney develop hypertension, proteinuria, and in some cases renal failure. The increased susceptibility to renal injury may be due, in part, to reduced nephron numbers. Notably, children who undergo nephrectomy or adults who serve as kidney donors exhibit little difference in renal function compared with persons who have two kidneys. However, the difference in risk between being born with a single kidney versus being born with two kidneys and then undergoing nephrectomy are unclear. Animal models used previously to investigate this question are not ideal because they require invasive methods to model congenital solitary kidney. In this study, we describe a new genetic animal model, the heterogeneous stock-derived model of unilateral renal agenesis (HSRA) rat, which demonstrates 50%-75% spontaneous incidence of a single kidney. The HSRA model is characterized by reduced nephron number (more than would be expected by loss of one kidney), early kidney/glomerular hypertrophy, and progressive renal injury, which culminates in reduced renal function. Long-term studies of temporal relationships among BP, renal hemodynamics, and renal function demonstrate that spontaneous single-kidney HSRA rats are more likely than uninephrectomized normal littermates to exhibit renal impairment because of the combination of reduced nephron numbers and prolonged exposure to renal compensatory mechanisms (i.e., hyperfiltration). Future studies with this novel animal model may provide additional insight into the genetic contributions to kidney development and agenesis and the factors influencing susceptibility to renal injury in individuals with congenital solitary kidney.

Hypothesis: a new role for the Renin-Angiotensin system in ureteric bud branching

Branching morphogenesis in the developing mammalian kidney involves growth and branching of the ureteric bud (UB), leading to formation of its daughter collecting ducts, calyces, pelvis and ureters. Even subtle defects in the efficiency and/or accuracy of this process have profound effects on the ultimate development of the kidney and result in congenital abnormalities of the kidney and urinary tract. This review summarizes current knowledge regarding a number of genes known to regulate UB development and emphasizes an emerging role for the renin-angiotensin system (RAS) in renal branching morphogenesis. Mutations in the genes encoding components of the RAS in mice cause renal papillary hypoplasia, hydronephrosis, and urinary concentrating defect. These findings imply that UB-derived epithelia are targets for angiotensin (ANG) II actions during metanephric kidney development. Here, it is proposed that papillary hypoplasia in RAS-deficient mice is secondary to an intrinsic defect in the development of the renal medulla. This hypothesis is based on the following observations: (a) UB and surrounding stroma express angiotensinogen (AGT) and ANG II AT(1) receptors in vivo; (b) ANG II stimulates UB cell process extension, branching and cord formation in collagen gel cultures in vitro; and (c) AT(1) blockade inhibits ANG II-induced UB cell branching. It is further postulated that ANG II is a novel stroma-derived factor involved in stroma/UB cross-talk which regulates UB branching morphogenesis.

Lrp4 regulates initiation of ureteric budding and is crucial for kidney formation--a mouse model for Cenani-Lenz syndrome

Background

Development of the kidney is initiated when the ureteric bud (UB) branches from the Wolffian duct and invades the overlying metanephric mesenchyme (MM) triggering the mesenchymal/epithelial interactions that are the basis of organ formation. Multiple signaling pathways must be integrated to ensure proper timing and location of the ureteric bud formation.

Methods and Principal Findings

We have used gene targeting to create an Lrp4 null mouse line. The mutation results in early embryonic lethality with a subpenetrant phenotype of kidney agenesis. Ureteric budding is delayed with a failure to stimulate the metanephric mesenchyme in a timely manner, resulting in failure of cellular differentiation and resulting absence of kidney formation in the mouse as well as comparable malformations in humans with Cenani-Lenz syndrome.

Conclusion

Lrp4 is a multi-functional receptor implicated in the regulation of several molecular pathways, including Wnt and Bmp signaling. Lrp4^−/− mice show a delay in ureteric bud formation that results in unilateral or bilateral kidney agenesis. These data indicate that Lrp4 is a critical regulator of UB branching and lack of Lrp4 results in congenital kidney malformations in humans and mice.

NFIA haploinsufficiency is associated with a CNS malformation syndrome and urinary tract defects

Complex central nervous system (CNS) malformations frequently coexist with other developmental abnormalities, but whether the associated defects share a common genetic basis is often unclear. We describe five individuals who share phenotypically related CNS malformations and in some cases urinary tract defects, and also haploinsufficiency for the NFIA transcription factor gene due to chromosomal translocation or deletion. Two individuals have balanced translocations that disrupt NFIA. A third individual and two half-siblings in an unrelated family have interstitial microdeletions that include NFIA. All five individuals exhibit similar CNS malformations consisting of a thin, hypoplastic, or absent corpus callosum, and hydrocephalus or ventriculomegaly. The majority of these individuals also exhibit Chiari type I malformation, tethered spinal cord, and urinary tract defects that include vesicoureteral reflux. Other genes are also broken or deleted in all five individuals, and may contribute to the phenotype. However, the only common genetic defect is NFIA haploinsufficiency. In addition, previous analyses of Nfia^−/− knockout mice indicate that Nfia deficiency also results in hydrocephalus and agenesis of the corpus callosum. Further investigation of the mouse Nfia^+/− and Nfia^−/− phenotypes now reveals that, at reduced penetrance, Nfia is also required in a dosage-sensitive manner for ureteral and renal development. Nfia is expressed in the developing ureter and metanephric mesenchyme, and Nfia^+/− and Nfia^−/− mice exhibit abnormalities of the ureteropelvic and ureterovesical junctions, as well as bifid and megaureter. Collectively, the mouse Nfia mutant phenotype and the common features among these five human cases indicate that NFIA haploinsufficiency contributes to a novel human CNS malformation syndrome that can also include ureteral and renal defects.

Central nervous system (CNS) and urinary tract abnormalities are common human malformations, but their variability and genetic complexity make it difficult to identify the responsible genes. Analysis of human chromosomal abnormalities associated with such disorders offers one approach to this problem. In five individuals described herein, a novel human syndrome that involves both CNS and urinary tract defects is associated with chromosomal disruption or deletion of NFIA, encoding a member of the Nuclear Factor I (NFI) family of transcription factors. This syndrome includes brain abnormalities (abnormal corpus callosum, hydrocephalus, ventriculomegaly, and Chiari type I malformation), spinal abnormalities (tethered spinal cord), and urinary tract abnormalities (vesicoureteral reflux). Nfia disruption in mice was already known to cause hydrocephalus and abnormal corpus callosum, and is now shown to exhibit renal defects and disturbed ureteral development. Other genes besides NFIA are also disrupted or deleted and may contribute to the observed phenotype. However, loss of one copy of NFIA is the only genetic defect common to all five patients. The authors thus provide evidence that genetic loss of NFIA contributes to a distinct CNS malformation syndrome with urinary tract defects of variable penetrance.

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

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

Cenani–Lenz syndactyly in a patient with features of Kabuki syndrome

We report a 5 9/12-year-old male with Cenani-Lenz syndactyly (CLS). He presented with three malformed digits on each hand and bilateral 2,3 partial cutaneous toe syndactyly. The expression of this presumed autosomal recessive disorder is variable and has been confused with split hand foot malformation (SHFM). Not all CLS patients show the typical 'spoon hand' configuration originally described. Affected CLS patients show a disorganized appearance of the phalanges with some degree of syndactyly/oligodactyly, in addition to metacarpal and carpal fusions. Radioulnar synostosis may be present. Cases lacking radioulnar synostosis may show proximal dislocation of the radial heads. The distal limb anomalies seen in our patient resemble other reported CLS patients. Radiographic evaluation showed a disorganized appearance of the phalanges, a 'kissing delta phalanx' of one proximal phalanx, metacarpal fusions, carpal coalitions and posterior subluxation of the radial heads. Clinical examination revealed features suggestive of Kabuki syndrome including large, prominent ears, sparse, medially flared and arched eyebrows, wide palpebral fissures, long eyelashes, and oligodontia with flat head 'screwdriver-shaped' incisors, in addition to persistent finger and thumb-tip pads. The faces of CLS patients are rarely shown or described. However, some patients in published reports have large, prominent ears. A recent report suggested midfacial dysmorphism in two patients with CLS (). Although the findings of CLS with features of Kabuki syndrome in our patient may be coincidental, the faces of CLS patients need to be carefully evaluated and described to determine if there is a distinctive accompanying facial phenotype.

Mouse limb deformity mutations disrupt a global control region within the large regulatory landscape required for Gremlin expression

The mouse limb deformity (ld) mutations cause limb malformations by disrupting epithelial-mesenchymal signaling between the polarizing region and the apical ectodermal ridge. Formin was proposed as the relevant gene because three of the five ld alleles disrupt its C-terminal domain. In contrast, our studies establish that the two other ld alleles directly disrupt the neighboring Gremlin gene, corroborating the requirement of this BMP antagonist for limb morphogenesis. Further doubts concerning an involvement of Formin in the ld limb phenotype are cast, as a targeted mutation removing the C-terminal Formin domain by frame shift does not affect embryogenesis. In contrast, the deletion of the corresponding genomic region reproduces the ld limb phenotype and is allelic to mutations in Gremlin. We resolve these conflicting results by identifying a cis-regulatory region within the deletion that is required for Gremlin activation in the limb bud mesenchyme. This distant cis-regulatory region within Formin is also altered by three of the ld mutations. Therefore, the ld limb bud patterning defects are not caused by disruption of Formin, but by alteration of a global control region (GCR) required for Gremlin transcription. Our studies reveal the large genomic landscape harboring this GCR, which is required for tissue-specific coexpression of two structurally and functionally unrelated genes.

Molecular mechanism of ureteric bud development

The urinary collecting system is derived from an epithelial protrusion arising from the Wolffian duct called the ureteric bud (UB) by the signal from its inductive tissue, metanephric mesenchyme (MM). Targeted gene mutation studies have shown that several transcription factors and MM-secreted glial cell line-derived neurotrophic factor (GDNF) are critical for initiation of the UB. After initiation, the UB undergoes branching morphogenesis. Results obtained from in vitro culture systems, including an isolated UB culture, together with gene mutation studies suggest that interplay of multiple positive and negative soluble factors as well as extracellular matrix (ECM) and matrix-degrading proteinases regulate branching morphogenesis.

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.

Toward an etiological classification of developmental disorders of the kidney and upper urinary tract

Toward an etiological classification of developmental disorders of the kidney and upper urinary tract. There are a large number of developmental disorders and syndromes that affect the kidney and upper urinary tract. These have generally been classified according to morphological criteria established decades ago. Although these classifications have been useful, they are incomplete, including some developmental disorders while excluding others. Here, basic cellular and molecular biology studies of kidney and upper urinary tract development in both rodents and humans are utilized to suggest the basis of a new etiologic, if still tentative, classification scheme. This classification may help to identify candidate genes for human diseases by correlating morphology with pathogenetic mechanisms.

Failure of ureteric bud invasion: a new model of renal agenesis in mice

FUBI (failure of ureteric bud invasion) is a highly inbred strain of mouse with a high spontaneous incidence of uni- or bilateral renal agenesis (60%). Bilateral renal agenesis is lethal within 2 days after birth. The primary defect of FUBI is failure of the ureteric bud to penetrate into the metanephric mesenchyme at around embryonic day 11, resulting in apoptosis of metanephric cells and leading to renal agenesis on the affected side. The metanephros seemed to be normal because co-culturing of the FUBI metanephros with homologous spinal cord induced differentiation of the rudiment, but co-culturing with the homologous ureteric bud frequently did not. Genetic analysis revealed that more than two genes were involved in this malformation and we mapped one of the modifier loci, fubi1, on chromosome 2, at approximately 65 cM from the centromere. In this region, there are two possible candidate genes, Wilms' tumor 1 and formin, that play important roles in kidney development. Some of formin mutants shared a similar phenotype with FUBI; however, there was no difference in the expression of formin in embryonic kidneys between FUBI and control NFS/N mice. Studies of fubi1 congenic mice indicated that interaction of two or more loci is essential for the FUBI phenotype.

Intracellular and extracellular regulation of ureteric bud morphogenesis

The urinary collecting duct system of the permanent kidney develops by growth and branching of an initially unbranched epithelial tubule, the ureteric bud. Formation of the ureteric bud as an outgrowth of the wolffian duct is induced by signalling molecules (such as GDNF) that emanate from the adjacent metanephrogenic mesenchyme. Once it has invaded the mesenchyme, growth and branching of the bud is controlled by a variety of molecules, such as the growth factors GDNF, HGF, TGFbeta, activin, BMP-2, BMP-7, and matrix molecules such as heparan sulphate proteoglycans and laminins. These various influences are integrated by signal transduction systems inside ureteric bud cells, with the MAP kinase, protein kinase A and protein kinase C pathways appearing to play major roles. The mechanisms of morphogenetic change that produce branching remain largely obscure, but matrix metalloproteinases are known to be necessary for the process, and there is preliminary evidence for the involvement of the actin/myosin contractile cytoskeleton in creating branch points.

Branching morphogenesis during kidney development

Epithelial tissues such as kidney, lung, and breast arise through branching morphogenesis of a pre-existing epithelial structure. They share common morphological stages and a need for regulation of a similar set of developmental decisions--where to start; when, where, and in which direction to branch; and how many times to branch--decisions requiring regulation of cell proliferation, apoptosis, invasiveness, and cell motility. It is likely that similar molecular mechanisms exist for the epithelial branching program. Here we focus on the development of the collecting system of the kidney, where, from recent data using embryonic organ culture, cell culture models of branching morphogenesis, and targeted gene deletion experiments, the outlines of a working model for branching morphogenesis begin to emerge. Key branching morphogenetic molecules in this model include growth factors, transcription factors, distal effector molecules (such as extracellular matrix proteins, integrins, proteinases and their inhibitors), and genes regulating apoptosis and cell proliferation.

Perturbation of RET signaling in the embryonic kidney

We have used a RET-Ig fusion protein to disrupt signaling in the rat embryonic kidney development pathway. Treatment of embryonic kidney organ cultures with RET-Ig results in a decrease in branching of the ureteric bud and a down regulation in expression of the Wnt-11, Wnt-4, and ld genes. These data suggest that Wnt-11, Wnt-4, and ld function downstream of RET signaling in normal development. Expression of BMP-7, shh, and ptc were uneffected by RET-Ig treatment, implying that these genes are regulated independently of ret. We have also performed immunohistochemistry with a GFR alpha-1 specific polyclonal antisera to localize GFR alpha-1 protein expression in the developing kidney.

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 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.

Embryonic renal epithelia: induction, nephrogenesis, and cell differentiation

Embryonic metanephroi, differentiating into the adult kidney, have come to be a generally accepted model system for organogenesis. Nephrogenesis implies a highly controlled series of morphogenetic and differentiation events that starts with reciprocal inductive interactions between two different primordial tissues and leads, in one of two mainstream processes, to the formation of mesenchymal condensations and aggregates. These go through the intricate process of mesenchyme-to-epithelium transition by which epithelial cell polarization is initiated, and they continue to differentiate into the highly specialized epithelial cell populations of the nephron. Each step along the developmental metanephrogenic pathway is initiated and organized by signaling molecules that are locally secreted polypeptides encoded by different gene families and regulated by transcription factors. Nephrogenesis proceeds from the deep to the outer cortex, and it is directed by a second, entirely different developmental process, the ductal branching of the ureteric bud-derived collecting tubule. Both systems, the nephrogenic (mesenchymal) and the ductogenic (ureteric), undergo a repeat series of inductive signaling that serves to organize the architecture and differentiated cell functions in a cascade of developmental gene programs. The aim of this review is to present a coherent picture of principles and mechanisms in embryonic renal epithelia.

3D reconstruction of the mouse's mesonephros

The present work reports on the three-dimensional reconstruction of the segmented mesonephros during the embryonic development of the mouse. With a light microscope and an automatic reconstruction of surfaces, aspects of the mesonephros are described. These surfaces are obtained by using digitized contour lines. A new interpolation method called DSI (Discrete Smooth Interpolation) enables correction of the distortion induced by microtomy in paraffin sections. After a triangulation step, this method uses a smoothing algorithm, which implies a spatial redistribution of the vertices of the triangles to correct the rotational and translational misalignment. The use of this 3D program improves the understanding of the development patterns and helps us to appreciate changes in the rebuilt mesonephros. By 10.5 embryonic days, tubules emerge from the Wolffian body and begin their formation, then between 11.5 and 13.5 embryonic days, tortuous mesonephric tubules bound to the Wolffian duct form small curls, which grow and finally unwind. At the same time, mesonephric tubules unbound to the Wolffian duct appear, and on 13.5 embryonic days, the Müllerian duct is visible. After 14.5 embryonic days, the segmented mesonephros keeps its general aspect but decreases in size. At this time, each gonad is provided with both Wolffian and Müllerian ducts. Later, the Wolffian duct differentiates into the definitive male duct system, whilst the Müllerian duct regresses. Conversely, the paramesonephric duct differentiates into the definitive female duct system, whilst the mesonephric duct in turn degenerates. By this time degeneration has begun in the cranial portion of the mesonephros and this process progresses caudally. The spatial organization of the mesonephric tubules and the precise organization of all connections between these elements and the ducts may be well defined. Such approach can allow for a high definition of the normal pattern of mesonephros differentiation.

The pathophysiology of UPJ obstruction. Current concepts

It took more than half of a century for urologists to recognize that hydronephrosis is not necessarily equivalent to obstruction. Keeping this important truism in mind, particularly when dealing with antenatal hydronephrosis, one must also remember that hydronephrosis is not a normal condition. It is conceivable that although the initial intrinsic stenosis or ureterovascular obstruction may not be clinically significant in terms of renal functional damage, as compensatory renal pelvic dilatation develops, secondary obstructive elements may be recruited to create an insertional anomaly and peripelvic fibrosis. The individual types of UPJ obstruction that are seen in diagnostic studies or on the operating table may represent isolated "snapshots" of evolving pathophysiologic processes. If this is true, patients with asymptomatic congenital hydronephrosis, although lacking obvious renal function loss, require long-term follow-up.

The development of the kidney

This chapter describes the earlier stages of development of the vertebrate metanephric kidney. It focuses on the mouse and descriptive morphology is used for considering both molecular mechanisms, underpinning kidney morphogenesis and differentiation, and the ways in which these processes can go awry and lead to congenital kidney disorders—particularly in humans. The mature kidney is a fairly complex organ attached to an arterial input vessel and two output vessels, the vein and the ureter. Inside, the artery and vein are connected by a complex network of capillaries that invade a large number of glomeruli, the proximal entrance to nephrons, which are filtration units that link to an arborized collecting-duct system that drains into the ureter. The ability of the kidney and isolated metanephrogenic mesenchyme, to develop in culture means that the developing tissues can be subjected to a wide variety of experimental procedures designed to investigate their molecular and cellular properties and to test hypotheses about developmental mechanisms.

Model systems for the study of kidney development: use of the pronephros in the analysis of organ induction and patterning

Most vertebrate organs, once formed, continue to perform the function for which they were generated until the death of the organism. The kidney is a notable exception to this rule. Vertebrates, even those that do not undergo metamorphosis, utilize a progression of more complex kidneys as they grow and develop. This is presumably due to the changing conditions to which the organism must respond to retain what Homer Smith referred to as our physiological freedom. To quote, "Recognizing that we have the kind of blood we have because we have the kind of kidneys we have, we must acknowledge that our kidneys constitute the major foundation of our physiological freedom. Only because they work the way they do has it become possible for us to have bones, muscles, glands, and brains. Superficially, it might be said that the function of the kidneys is to make urine; but in a more considered view one can say that the kidneys make the stuff of philosophy itself" ("From Fish to Philosopher," Little, Brown and Co., Boston, 1953). Different kidneys are used to make the stuff of philosophy at different stages of development depending on the age and needs of the organism, rather than the usual approach of simply making embryonic organs larger as the animal grows. Although evolution has provided the higher vertebrates with complex adult kidneys, they continue to utilize simple kidneys in embryogenesis. In lower vertebrates with simple adult kidneys, even more simple versions are used during early developmental stages. In this review the anatomy, development, and gene expression patterns of the embryonic kidney, the pronephros, will be described and compared to the more complex kidney forms. Despite some differences in anatomy, similar developmental pathways seem to be responsible for the induction and the response to induction in both evanescent and permanent kidney forms. Gene expression patterns can, therefore, be added to the morphological and functional data indicating that all forms of the kidney are closely related structures. Given the similarities between the development of simple and complex kidneys, the embryonic kidneys may be an ideal model system in which to investigate the genesis of multicomponent organ systems.

The molecular basis of embryonic kidney development

The development of the mature mammalian kidney begins with the invasion of metanephric mesenchyme by ureteric bud. Mesenchymal cells near the bud become induced and convert to an epithelium which goes on to generate the functional filtering unit of the kidney, the nephron. The collecting duct system is elaborated by the branching ureter, the growth of which is dependent upon signals from the metanephric mesenchyme. The process of reciprocal induction between ureter and mesenchyme is repeated many times over during development and is the key step in generating the overall architecture of the kidney. Genetic studies in mice have allowed researchers to begin to unravel the molecular signals that govern these early events. These experiments have revealed that a number of essential gene products are required for distinct steps in kidney organogenesis. Here we review and summarize the developmental role played by some of these molecules, especially certain transcription factors and growth factors and their receptors. Although the factors involved are far from completely known a rough framework of a molecular cascade which governs embryonic kidney development is beginning to emerge.

Molecular interaction between limb deformity proteins (formins) and Src family kinases

Ld proteins (formins) are encoded by the limb deformity (ld) gene and define a family of related gene products regulating establishment of embryonic polarity. In this study we establish that chicken and murine Ld proteins interact directly with Src family kinases (c-Src and c-Fyn). Specific binding is mediated by the proline-rich domain present in Ld proteins and the ligand binding surface of the Src SH3 domain. Co-immunoprecipitation of Ld and c-Src proteins from transfected cells shows that these proteins associate in vivo. Immunolocalization and biochemical fractionation of fibroblasts confirms the predominant nuclear localization of Ld proteins, but unexpectedly identifies a population of Ld proteins associated to cellular membranes. This population of Ld proteins co-localizes with membrane-associated c-Src proteins at both plasma and perinuclear membranes. These studies indicate that the morphoregulatory Ld proteins interact with signal transduction cascades by association to membrane-bound Src family kinases.

Gene targeting in kidney development

Cancer and development are conceptually related because tumor formation in many cases results from the aberrant expression of a developmental program. This is certainly true of Wilms' tumors, which display a range of phenotypes resembling various stages of kidney development. WT1 has been identified as a tumor suppressor gene involved in a subset of Wilms' tumors. Gene targeting of the WT1 gene demonstrated the requirement for this gene product during early urogenital development. Several other genes, including Wnt-4, c-ret, ld and lim 1, have been shown by gene targeting to also be involved in early kidney development. This review discusses gene targeting as an approach to the study of development and reviews the phenotypes of these and other genes involved in kidney organogenesis.

Genetic evidence that formins function within the nucleus

The murine limb deformity (ld) locus encodes a set of proteins, termed formins, that are required for embryonic limb and kidney development. Previous studies had indicated that these proteins are located in the nucleus and cytoplasm and have biochemical properties consistent with an action within the nucleus. To test the notion that nuclear localization is crucial for formin function, we carried out molecular and biochemical studies on three ld alleles. We show that two transgene-induced alleles, ldTgHd and ldTgBri, generate similar COOH-truncated formins that lack the terminal 110 amino acids, while a third allele, ldIn2, generates a less extensively truncated formin that lacks the terminal 42 amino acids. Using subcellular fractionation analysis, we find that wild-type formin is detected in both nuclear and cytosolic fractions; in contrast, the truncated formins encoded by ldTgHd and ldTgBri are strictly cytosolic. The less extensively truncated ldIn2 formin shows a similar, but less complete, localization defect. Consistent with this weaker cellular phenotype, hind limbs from ldIn2 mice have milder skeletal defects than those of ldTgBri mice. These observations define a small region in the carboxyl terminus that is required for nuclear localization and suggest that nuclear localization plays a role in formin action.

Polydactyly in the Strong's luxoid mouse is suppressed by limb deformity alleles

The study of limb development has provided insight into pattern formation during vertebrate embryogenesis. Genetic approaches offer powerful ways to identify the critical molecules and their pathways of action required to execute a complex morphogenetic program. We have applied genetic analysis to the process of limb development by studying two mouse mutants, limb deformity (ld) and Strong's luxoid (lst). These mutations confer contrasting phenotypic alterations to the anteroposterior limb pattern. The six mutant ld alleles are fully recessive and result in oligosyndactyly of all four limbs. By contrast, the two mutant lst alleles result in a mirror-image polydactylous limb phenotype inherited in a semidominant fashion. Morphological and molecular analysis of embryonic limbs has shown that the ld and lst alleles affect the extent and distribution of two key signaling centers differentially: the apical ectodermal ridge and the zone of polarizing activity. Molecular characterization of the ld gene has defined a new family of evolutionarily conserved proteins termed the formins. The underlying molecular defect in the lst mutation has not been identified; however, both loci are tightly linked on mouse chromosome 2, suggesting the possibility that they may be allelic. In this study, we have used genetic analysis to examine the epistatic and allelic relationships of ld and lst. We observed that in + ld/lst + double heterozygotes, a single mutant ld allele is able to suppress the semi-dominant polydactylous lst limb phenotype. By segregating the lst and ld loci in a backcross, we observed that these loci recombine and are separated by a genetic distance of approximately 6 cM. Therefore, while our observations demonstrate a genetic interaction between ld and lst, it is probable that ld and lst are not allelic. Instead, lst and ld may be operating either in a linear or in a parallel (bypass) genetic pathway to affect the limb signaling centers.

Formin isoforms are differentially expressed in the mouse embryo and are required for normal expression of fgf-4 and shh in the limb bud

Mice homozygous for the recessive limb deformity (ld) mutation display both limb and renal defects. The limb defects, oligodactyly and syndactyly, have been traced to improper differentiation of the apical ectodermal ridge (AER) and shortening of the anteroposterior limb axis. The renal defects, usually aplasia, are thought to result from failure of ureteric bud outgrowth. Since the ld locus gives rise to multiple RNA isoforms encoding several different proteins (termed formins), we wished to understand their role in the formation of these organs. Therefore, we first examined the embryonic expression patterns of the four major ld mRNA isoforms. Isoforms I, II and III (all containing a basic amino terminus) are expressed in dorsal root ganglia, cranial ganglia and the developing kidney including the ureteric bud. Isoform IV (containing an acidic amino terminus) is expressed in the notochord, the somites, the apical ectodermal ridge (AER) of the limb bud and the developing kidney including the ureteric bud. Using a lacZ reporter assay in transgenic mice, we show that this differential expression of isoform IV results from distinct regulatory sequences upstream of its first exon. These expression patterns suggest that all four isoforms may be involved in ureteric bud outgrowth, while isoform IV may be involved in AER differentiation. To define further the developmental consequences of the ld limb defect, we analyzed the expression of a number of genes thought to play a role in limb development. Most significantly, we find that although the AERs of ld limb buds express several AER markers, they do not express detectable levels of fibroblast growth factor 4 (fgf-4), which has been proposed to be the AER signal to the mesoderm. Thus we conclude that one or more formins are necessary to initiate and/or maintain fgf-4 production in the distal limb. Since ld limbs form distal structures such as digits, we further conclude that while fgf-4 is capable of supporting distal limb outgrowth in manipulated limbs, it is not essential for distal outgrowth in normal limb development. In addition, ld limbs show a severe decrease in the expression of several mesodermal markers, including sonic hedgehog (shh), a marker for the polarizing region and Hoxd-12, a marker for posterior mesoderm. We propose that incomplete differentiation of the AER in ld limb buds leads to reduction of polarizing activity and defects along the anteroposterior axis.

Limb deformity proteins during avian neurulation and sense organ development

The nuclear Limb deformity (Ld) proteins (formins) are expressed during the avian primitive streak stages. Initially, they are detected predominantly in cells of the forming notochord, scattered mesodermal precursors and the induced neural plate. No expression is detected in endodermal cells. The subsequent graded distribution of Ld positive cells along the anterior-posterior axis of the neural tube follows the antero-posterior progression of its differentiation. The Ld proteins are also differentially expressed during induction and development of both the inner ear and eye. An unequal distribution of Ld proteins along the dorso-ventral axis of the otic vesicle is observed during its initial patterning. In the eye, the Ld proteins are expressed by the optic vesicle during secondary induction of the lens placode. Following induction, the proteins are also expressed by the newly formed lens placode, a process which is reminiscent of homeogenetic induction. During differentiation of the retina and lens, the Ld domains seem to demarcate territories, giving rise to specific eye structures. A comparative analysis of the Ld distribution and BrdU incorporation in the two sense organs indicates that the proteins are predominantly expressed by committed and/or differentiating (post-mitotic) cells. In general, expression of Ld proteins is induced during determination and remains during differentiation of particular cell-types. This study implies that the nuclear Ld proteins are involved in morphogenesis of both neuro-ectodermal and mesodermal structures.

The metanephric blastema differentiates into collecting system and nephron epithelia in vitro

The kidney forms from two tissue populations derived from intermediate mesoderm, the ureteric bud and metanephric mesenchyme. It is currently accepted that metanephric mesenchyme is committed to differentiating into nephrons while the ureteric bud is restricted to forming the renal collecting system. To test this hypothesis, we transferred lacZ into pure metanephric mesenchyme isolated from gestation day 13 rat embryos. The fate of tagged mesenchymal cells and their progeny was characterized after co-culture with isolated ureteric buds. When induced to differentiate by the native inducer of kidney morphogenesis, lineage-tagged mesenchymal cells exhibit the potential to differentiate into collecting system epithelia, in addition to nephrons. The fate of cells deriving from isolated ureteric buds was also examined and results of these lacZ gene transfer experiments indicate that the majority of ureteric bud cells differentiate into the renal collecting system. These cell fate studies combined with in situ morphological observations raise the possibility that collecting system morphogenesis in vivo occurs by growth of the ureteric bud and recruitment of mesenchymal cells from the metanephric blastema. Thus, metanephric mesenchyme may be a pluripotent renal stem population.