IL-18 is expressed in the intercalated cell of human kidney

We determined the cellular location of interleukin-18 (IL-18) and caspase-1 and the purinergic receptor P2X7, two proteins necessary for its activation and secretion. The mRNA and protein of IL-18 were detectable in normal human kidney by means of polymerase chain reaction (PCR), in situ hybridization, and Western blot. Immunohistochemistry located IL-18 to nephron segments containing calbinbin-D28k or aquaporin-2 that suggest location in the distal convoluted and the connecting tubule and to parts of the collecting duct. IL-18 was not detected in the thick ascending limb of Henle. Confocal microscopy showed that IL-18 was expressed in cells negative for calbindin-D28k and for aquaporin-2 but positive for the vacuolar H(+)-ATPase. This demonstrates that the intercalated cells produce IL-18. These segments were also positive for caspase-1 and P2X7 that are essential for IL-18 secretion. Our results show that IL-18 is constitutively expressed by intercalated cells of the late distal convoluted tubule, the connecting tubule, and the collecting duct of the healthy human kidney. Since IL-18 is an early component of the inflammatory cytokine cascade, its location suggests that renal intercalated cells may contribute to immediate immune response of the kidney.

Aldosterone-induced increases in superoxide production counters nitric oxide inhibition of epithelial Na channel activity in A6 distal nephron cells

Oxygen radicals play an important role in signal transduction and have been shown to influence epithelial sodium channel (ENaC) activity. We show that aldosterone, the principal hormone regulating renal ENaC activity, increases superoxide (O2*) production in A6 distal nephron cells. Aldosterone (50 nM to 1.5 microM) induced increases in dihydroethidium fluorescence in a dose-dependent manner in confluent A6 epithelial cells. Using single-channel measurements, we showed that sequestering endogenous O2* (with the O2* scavenger 2,2,6,6-tetramethylpiperidine 1-oxyl) significantly decreased ENaC open probability from 0.10 +/- 0.03 to 0.03 +/- 0.01. We also found that increasing endogenous O2* in A6 cells, by applying a superoxide dismutase inhibitor, prevented nitric oxide (NO) inhibition of ENaC activity. ENaC open probability values did not significantly change from control values (0.23 +/- 0.05) after superoxide dismutase and 1.5 microM NO coincubation (0.21 +/- 0.04). We report that xanthine oxidase and hypoxanthine compounds increase local concentrations of O2* by approximately 30%; with this mix, an increase in ENaC number of channels times the open probability (from 0.1 to 0.3) can be achieved in a cell-attached patch. Our data also suggest that O2* alters NO activity in a cGMP-independent mechanism, since pretreating A6 cells with ODQ compound (a selective inhibitor of NO-sensitive guanylyl cyclase) failed to block 2,2,6,6-tetramethylpiperidine 1-oxyl inhibition of ENaC activity.

A comparative study of renal function in the desert-adapted spiny mouse and the laboratory-adapted C57BL/6 mouse: response to dietary salt load

The desert-adapted spiny mouse has a significantly lower glomerular number, increased glomerular size, and a more densely packed renal papillae compared with the similar-sized laboratory-adapted C57BL/6 mouse. In the present study we examined the functional consequences of these structural differences in young adult male spiny and C57BL/6 mice and detailed the impact of 1 wk of a high-salt (10% wt/wt NaCl) diet. Basal food and water intake, urine and feces production, and urinary electrolyte concentrations were not different between species, although urinary urea concentrations were higher in spiny mice (P < 0.05). On normal salt, MAP of the anesthetized spiny mouse was approximately 18 mmHg lower, effective renal plasma flow (ERPF) was 40% lower (P < 0.001), and glomerular filtration rate (GFR) tended to be lower than in the C57BL/6 mouse. On the high-salt diet, both species had similar 24-h NaCl excretions; but C57BL/6 mice required a significantly increased amount of water (lower urine NaCl concentration) than the spiny mice. Filtration fraction was greater in both species on the high-salt diet. Spiny mice had greater GFR and ERPF after the high-salt diet, whereas the C57BL/6 mouse showed little change in GFR. The ability of the spiny mouse to tolerate a significantly higher plasma osmolality after salt, measured by a decreased drinking response, and the ability to increase ERPF at a lower MAP are features that allow this species to conserve water more efficiently than can be done in the C57BL/6 mouse. These features are important, particularly since the desert mouse has a smaller kidney, with fewer nephrons.

Renal and extrarenal regulation of potassium

The ISN Forefronts in Nephrology Symposium took place 8-11 September 2005 in Kartause Ittingen, Switzerland. It was dedicated to the memory of Robert W. Berliner, who died at age 86 on 5 February 2002. Dr Berliner contributed in a major way to our understanding of potassium transport in the kidney. Starting in the late 1940s, without knowledge of how potassium was transported across specific nephron segments and depending only on renal clearance methods, he and his able associates provided a still-valid blueprint of the basic transport properties of potassium handling by the kidney. They firmly established that potassium was simultaneously reabsorbed and secreted along the nephron; that variations in secretion in the distal nephron segments play a major role in regulating potassium excretion; and that such secretion is modulated by sodium, acid-base factors, hormones, and diuretics. These conclusions were presented in a memorable Harvey Lecture some forty years ago, and they have remained valid ever since. The concepts have also provided the foundation and stimulation for later work on single nephrons, tubule cells, and transport proteins involved in potassium transport.

Molecular insights into segmentation along the proximal-distal axis of the nephron

The structure of a mammalian kidney is parsed into large collections of polarized nephrons, and each segment is home to a diverse community of cells that specialize in renal endocrine and excretory functions. Early developmental lengthening and diversification of nephron segments along a proximal--distal axis initiate all subsequent facets of tubular growth and function. Morphogenic cues and biochemical interactions that are critical to this process are starting to emerge. The underlying principles of regional cell signaling and transcriptional control organizing early segmentation are the subject of this review.

Mouse embryonic stem cell-derived embryoid bodies generate progenitors that integrate long term into renal proximal tubules in vivo

The metanephric kidney is a mesodermal organ that develops as a result of reciprocal interactions between the ureteric bud and the blastema. The generation of embryonic stem (ES) cell-derived progenitors offers potential for regenerative therapies but is often limited by development of tumor formation. Because brachyury (T) denotes mesoderm specification, a mouse ES cell line with green fluorescence protein (GFP) knocked into the functional T locus as well as lacZ in the ROSA26 locus (LacZ/T/GFP) was used in cell selection and lineage tracing. In the absence of leukemia inhibitory factor, mouse ES cells give rise to embryoid bodies that can differentiate into mesoderm. Culture conditions were optimized (4 d, 10 ng/ml Activin-A) to generate maximal numbers of renal progenitor populations identified by expression of the specific combination of renal markers cadherin-11, WT-1, Pax-2, and Wnt-4. LacZ/T/GFP+ cells were further enriched by FACS selection. Five days after injection of LacZ/T/GFP+ cells into embryonic kidney explants in organ culture, beta-galactosidase immunohistochemistry showed incorporation into blastemal cells of the nephrogenic zone. After a single injection into developing live newborn mouse kidneys, co-localization studies showed that the LacZ/T/GFP+ cells were stably integrated into proximal tubules with normal morphology and normal polarization of alkaline phosphatase and aquaporin-1 for 7 mo, without teratoma formation. It is concluded that defined differentiation of ES cells into embryoid bodies with Activin-A and selection for T expression provides a means to isolate and purify renal proximal tubular progenitor cells with the potential for safe use in regenerative therapies.

Normal Lactational Environment Restores Nephron Endowment and Prevents Hypertension after Placental Restriction in the Rat

Uteroplacental insufficiency in the rat restricts fetal growth, impairs mammary development, compromising postnatal growth; and increases adult BP. The roles of prenatal and postnatal nutritional restraint on later BP and nephron endowment in offspring from mothers that underwent bilateral uterine vessel ligation (restricted) on day 18 of pregnancy were examined. Sham surgery (control) and a group of rats with reduced litter size (reduced; litter size reduced at birth to five, equivalent to restricted group) were used as controls. Offspring (control, reduced, and restricted) were cross-fostered on postnatal day 1 onto a control (normal lactation) or restricted (impaired lactation) mother. BP in male offspring was determined by tail cuff at 8, 12, and 20 wk of age, with glomerular number and volume (Cavalieri/Physical Dissector method) and renal angiotensin II type 1 receptor (AT(1)R) mRNA expression (real-time PCR) determined at 6 mo. Restricted-on-restricted male offspring developed hypertension (+16 mmHg) by 20 wk together with a nephron deficit (-26%) and glomerular hypertrophy (P < 0.05). In contrast, providing a normal lactational environment to restricted offspring improved postnatal growth and prevented the nephron deficit and hypertension. Reduced-on-restricted pups that were born of normal weight but with impaired growth during lactation subsequently grew faster, developed hypertension (+16 mmHg), had increased AT(1A)R and AT(1B)R mRNA expression (P < 0.05), but had no nephron deficit. Our study identifies the prenatal and postnatal nutritional environments in the programming of adult hypertension, associated with distinct renal changes. It is shown for the first time that a prenatally induced nephron deficit can be restored by correcting growth restriction during lactation.

Prenatal corticosterone exposure results in altered AT1/AT2, nephron deficit and hypertension in the rat offspring

Maternal treatment with the synthetic glucocorticoid, dexamethasone has been reported to result in a nephron deficit and development of hypertension in the offspring of rats. However, it is not known whether elevated maternal corticosterone (CORT), the natural glucocorticoid, has similar effects on blood pressure and nephron endowment. The present study investigated the effects of CORT (0.8 mg kg(-1) day(-1)) administration on embryonic day 14 (E14) and E15 of pregnancy on: (1) nephron number at postnatal day 30 (PN30); (2) blood pressure at PN120; and (3) receptors of the renal renin-angiotensin system (RRAS) (AT(1)Ra, AT(1)Rb and AT(2)Ra) during both embryonic (E16, E20) and adolescent (PN30) life. Plasma CORT concentrations were approximately doubled 30 min after injection. Unbiased stereological analysis revealed that maternal CORT treatment resulted in a nephron deficit of 21 and 19% in male and female offspring, respectively. Mean arterial pressures were significantly elevated in offspring of both sexes from the CORT group. Real-time PCR revealed that CORT treatment increased expression of AT(1)Ra and AT(2)R at E16, and at PN30. Expression of AT(1)Rb was downregulated in embryonic life but upregulated at PN30. We believe that these results are the first to demonstrate that maternal CORT treatment results in a nephron deficit and development of hypertension in the rat offspring. Changes in the RRAS may be contributing to these phenotypes. Critically, this study suggests that increased but physiological levels of the natural glucocorticoid can programme similar changes to those seen with pharmacological doses of the synthetic glucocorticoid. This may have important implications for women experiencing significant stress during pregnancy.

Notch signaling controls the differentiation of transporting epithelia and multiciliated cells in the zebrafish pronephros

Epithelial tubules consist of multiple cell types that are specialized for specific aspects of organ function. In the zebrafish pronephros, multiciliated cells (MCCs) are specialized for fluid propulsion, whereas transporting epithelial cells recover filtered-blood solutes. These cell types are distributed in a ;salt-and-pepper' fashion in the pronephros, suggesting that a lateral inhibition mechanism may play a role in their differentiation. We find that the Notch ligand Jagged 2 is expressed in MCCs and that notch3 is expressed in pronephric epithelial cells. Morpholino knockdown of either jagged 2 or notch3, or mutation in mind bomb (in which Notch signaling is impaired), dramatically expands ciliogenic gene expression, whereas ion transporter expression is lost, indicating that pronephric cells are transfated to MCCs. Conversely, ectopic expression of the Notch1a intracellular domain represses MCC differentiation. Gamma-secretase inhibition using DAPT demonstrated a requirement for Notch signaling early in pronephric development, before the pattern of MCC differentiation is apparent. Strikingly, we find that jagged 2 knockdown generates extra cilia and is sufficient to rescue the kidney cilia mutant double bubble. Our results indicate that Jagged 2/Notch signaling modulates the number of multiciliated versus transporting epithelial cells in the pronephros by way of a genetic pathway involving repression of rfx2, a key transcriptional regulator of the ciliogenesis program.

Catecholamine production along the nephron

The present work proposes an extra neural site of catecholamine production along the nephron. LLC-PK(1), MDCK, and mIMCD-3 (proximal and distal tubules and inner medullary collecting duct, respectively) presented the following amine concentrations in the cell homogenates: Norepinephrine = 275+/-34, 56+/-16 and 255+/-21; Epinephrine = 161+/-20, 83+/-17 and 53+/-7; and Dopamine = 63+/-15, 39+/-6 and 36+/-7 pg/mg cell protein (Means +/- SEM), respectively. The culture medium showed Norepinephrine = 168+/-25, 22+/-3 and 135+/-8; Epinephrine = 32+/-6, 152+/-17 and 39+/-5; and Dopamine = 27+/-9, 241+/-34 and 26+/-5 pg/mg cell protein, respectively. The synthesis enzymes as tyrosine hydroxylase, dopa decarboxylase and dopamine beta-hydroxylase were detected by Western blotting. Biopterin, the enzymatic cofactor of tyrosine hydroxylase, was quantified in the intracellular and medium of mIMCD-3 cells (17+/-4 and 24+/-3 nmol/mg cell protein, respectively) and in the medium of MDCK cells (19+/-4 nmol/mg cell protein). The data confirmed that the proximal tubule is an important source of dopa decarboxilase and Dopamine and epithelial cell along the nephron express the biochemical pathway for catecholamine production.

Epithelial Na+ channels are fully activated by furin- and prostasin-dependent release of an inhibitory peptide from the gamma-subunit

Epithelial sodium channels (ENaC) are expressed in the apical membrane of high resistance Na(+) transporting epithelia and have a key role in regulating extracellular fluid volume and the volume of airway surface liquids. Maturation and activation of ENaC subunits involves furin-dependent cleavage of the ectodomain at two sites in the alpha subunit and at a single site within the gamma subunit. We now report that the serine protease prostasin further activates ENaC by inducing cleavage of the gamma subunit at a site distal to the furin cleavage site. Dual cleavage of the gamma subunit is predicted to release a 43-amino acid peptide. Channels with a gamma subunit lacking this 43-residue tract have increased activity due to a high open probability. A synthetic peptide corresponding to the fragment cleaved from the gamma subunit is a reversible inhibitor of endogenous ENaCs in mouse cortical-collecting duct cells and in primary cultures of human airway epithelial cells. Our results suggest that multiple proteases cleave ENaC gamma subunits to fully activate the channel.

The cellular basis of kidney development

Mammalian kidney development has helped elucidate the general concepts of mesenchymal-epithelial interactions, inductive signaling, epithelial cell polarization, and branching morphogenesis. Through the use of genetically engineered mouse models, the manipulation of Xenopus and chick embryos, and the identification of human renal disease genes, the molecular bases for many of the early events in the developing kidney are becoming increasingly clear. Early patterning of the kidney region depends on interactions between Pax/Eya/Six genes, with essential roles for lim1 and Odd1. Ureteric bud outgrowth and branching morphogenesis are controlled by the Ret/Gdnf pathway, which is subject to positive and negative regulation by a variety of factors. A clear role for Wnt proteins in induction of the kidney mesenchyme is now well established and complements the classic literature nicely. Patterning along the proximal distal axis as the nephron develops is now being investigated and must involve aspects of Notch signaling. The development of a glomerulus requires interactions between epithelial cells and infiltrating endothelial cells to generate a unique basement membrane. The integrity of the glomerular filter depends in large part on the proteins of the nephrin complex, localized to the slit diaphragm. Despite the kidney's architectural complexity, with the advent of genomics and expression arrays, it is becoming one of the best-characterized organ systems in developmental biology.

Six2 is required for suppression of nephrogenesis and progenitor renewal in the developing kidney

During kidney development and in response to inductive signals, the metanephric mesenchyme aggregates, becomes polarized, and generates much of the epithelia of the nephron. As such, the metanephric mesenchyme is a renal progenitor cell population that must be replenished as epithelial derivatives are continuously generated. The molecular mechanisms that maintain the undifferentiated state of the metanephric mesenchymal precursor cells have not yet been identified. In this paper, we report that functional inactivation of the homeobox gene Six2 results in premature and ectopic differentiation of mesenchymal cells into epithelia and depletion of the progenitor cell population within the metanephric mesenchyme. Failure to renew the mesenchymal cells results in severe renal hypoplasia. Gain of Six2 function in cortical metanephric mesenchymal cells was sufficient to prevent their epithelial differentiation in an organ culture assay. We propose that in the developing kidney, Six2 activity is required for maintaining the mesenchymal progenitor population in an undifferentiated state by opposing the inductive signals emanating from the ureteric bud.

High-conductance K channels in intercalated cells of the rat distal nephron

High-conductance (BK or maxi) K(+) channels were observed in cell-attached patches of the apical membrane of the isolated split-open rat connecting tubule (CNT). These channels were quite rare in cells identified visually as principal cells (PCs; 5/162 patches) but common in intercalated cells (ICs; 24/26 patches). The BK-expressing intercalated cells in the CNT and cortical collecting duct (CCD) were characterized by a low membrane potential (-36 mV) under short-circuit conditions, measured from the reversal potential of the channel currents with similar K(+) concentrations on both sides of the membrane. Under whole-cell clamp conditions with low intracellular Ca(2+), ICs had a very low K(+) conductance. When cell Ca(2+) was increased to 200 nM, a voltage-dependent, tetraethylammonium (TEA)-sensitive outward conductance was activated with a limiting value of 90 and 140 nS/cell in the CNT and CCD, respectively. Feeding animals a high-K diet for 1 wk did not increase these currents. TEA-sensitive currents were much smaller in PCs and usually below detection limits. To examine the possibility that the ICs participate in transepithelial K(+) secretion, we measured Na/K pump activity as a ouabain-sensitive current. Although these currents were easily observed in PCs, averaging 79 +/- 14 and 250 +/- 50 pA/cell in the CCD and CNT, respectively, they were below the level of detection in the ICs. We conclude that ICs have BK channel densities that are sufficient to support renal secretion of K(+) if cell Ca(2+) is elevated. However. a pathway for K(+) entry into these cells has not been identified.

Obesity, proteinuria and progression of renal failure

PURPOSE OF REVIEW

Recent studies have reported an alarming increase in the incidence of obesity-related glomerulopathy, in a context of a worldwide spread of obesity.

RECENT FINDINGS

Several epidemiological investigations have confirmed that obesity is a significant risk factor for the appearance of proteinuria and end-stage renal disease in a normal population. Obesity-induced hemodynamic changes and glomerular deposition of lipids (partly mediated by sterol regulatory element-binding proteins) play an important role in the pathogenesis of obesity-related renal disease. In addition, the renin-angiotensin-aldosterone system is markedly activated in obesity, adipocytes being an important source of these hormones. Weight loss induces a marked reduction in all renin-angiotensin-aldosterone system components. Patients with reduced renal mass of any origin appeared to be particularly susceptible to the detrimental influence of obesity: body mass index was the most important risk factor for the development of proteinuria and renal insufficiency in patients with unilateral renal agenesis, unilateral nephrectomy and remnant kidneys. Weight loss induces a very important reduction in proteinuria in chronic proteinuric nephropathies of different etiologies.

SUMMARY

Prevention and treatment of obesity should be a first-line objective in the therapeutic approach of patients with diabetic and nondiabetic chronic renal diseases.

Role of transcriptional networks in coordinating early events during kidney development

Many of the signaling pathways that regulate tissue specification and coordinate cellular differentiation during embryogenesis have been identified over the last decade. These pathways are integrated at the transcriptional level, enabling activation of specific developmental programs in a temporally and spatially restricted fashion. Such developmental events are usually thought of in terms of hierarchical relationships, in which the expression of upstream factors leads to the sequential activation of a linear cascade of downstream genes. Whereas these models provide a simplistic approach to understand complex cellular events, genetic and biochemical studies in mice and other model organisms provide ample evidence that many of these factors interact at multiple levels in vivo and emphasize the importance of considering these linear events in context. The purpose of this review is to emphasize the complexity of these regulatory networks during the early phases of mammalian kidney development, outlining some of the limitations and alternative approaches that are being used to explore the complex nature of these networks in vivo. Before describing these networks in detail, we will provide a brief overview of the main structural changes and tissue interactions involved in mammalian kidney development, and go on to describe some of the limitations of our current approaches to evaluate the role of these developmental pathways in vivo.

Retinoic acid signalling is required for specification of pronephric cell fate

The mechanisms by which a subset of mesodermal cells are committed to a nephrogenic fate are largely unknown. In this study, we have investigated the role of retinoic acid (RA) signalling in this process using Xenopus laevis as a model system and Raldh2 knockout mice. Pronephros formation in Xenopus embryo is severely impaired when RA signalling is inhibited either through expression of a dominant-negative RA receptor, or by expressing the RA-catabolizing enzyme XCyp26 or through treatment with chemical inhibitors. Conversely, ectopic RA signalling expands the size of the pronephros. Using a transplantation assay that inhibits RA signalling specifically in pronephric precursors, we demonstrate that this signalling is required within this cell population. Timed antagonist treatments show that RA signalling is required during gastrulation for expression of Xlim-1 and XPax-8 in pronephric precursors. Moreover, experiments conducted with a protein synthesis inhibitor indicate that RA may directly regulate Xlim-1. Raldh2 knockout mouse embryos fail to initiate the expression of early kidney-specific genes, suggesting that implication of RA signalling in the early steps of kidney formation is evolutionary conserved in vertebrates.

NHE3 inhibits PKA-dependent functional expression of CFTR by NHERF2 PDZ interactions

It has been shown that when CFTR and NHE3 are co-expressed on the apical membrane of the A6-NHE3 cell monolayers, the two transporters interact via a shared regulatory complex composed of NHERF2, ezrin, and PKA. We observe here that co-expression of NHE3 reduced both PKA-dependent apical CFTR expression and its activation once in place by approximately 50%. To analyze the role of NHERF2 in this process, we transfected NHE3 expressing and non-expressing A6 monolayers with NHERF2 cDNA in which its binding domains had been deleted. When only CFTR is expressed on the apical membrane, deletion of any of the NHERF2 binding domains inhibited both PKA-dependent apical CFTR expression and its activation, while when NHE3 was co-expressed with CFTR PDZ2 deletion was without effect on CFTR sorting and activity. This suggests that when the PDZ2 domain is "sequestered" by interacting with NHE3 it can no longer participate in CFTR functional expression.

Dissecting stages of mesenchymal-to-epithelial conversion during kidney development

During embryonic development, the structures of the nephron from the glomerulus to distal tubule derive from the metanephric mesenchyme. The mesenchymal cells change their cell type and produce highly organized epithelia under the influence of signals from the ureteric bud. The morphological sequence of this conversion includes the formation of a corona of mesenchymal cells surrounding the tips of the ureteric bud, followed by the development of a pre-tubular aggregate, which evolves into preliminary forms of the segmented nephron. Currently, these stages are largely based on histomorphologic criteria and expression of marker molecules. However, to dissect the effects of inductive signals from the ureteric bud in more detail, a sophisticated readout of stages in the conversion process is required, based on the onset of epithelial polarity and the occurrence of vectorial transport. In this review, we discuss some of the new approaches in establishing the staging of the conversion process.

Establishment and characterization of renal progenitor like cells from S3 segment of nephron in rat adult kidney

Kidney is thought to be a regenerative organ in terms of repair from acute tubular injury. It is unknown whether cell population contributes to repair disordered kidney. We attempted to identify and isolate highly proliferative cells from a single cell. We dissected a single nephron from adult rat kidney. Isolated nephrons were separated into segments and cultured. Outgrowing cells were replated after limiting dilution so that each well contained a single cell. One of cell line which was the most potent to grow was designated as rKS56. rKS56 cells showed cobblestone appearance and expressed immature cell markers relating to kidney development and mature tubular cell markers. rKS56 cells grew exponentially and could be maintained for 300 days without transformation. In different culture conditions, rKS56 cells differentiated into mature tubular cells defined by aquaporin-1, 2 expression, and responsiveness to parathyroid hormone or vasopressin. Engrafted to kidney in rat ischemic reperfusion model, rKS56 cells replaced in injured tubules in part after implantation and improved renal function. These results suggest rKS56 cells possess character such as self-renewal, multi-plasticity and capability of tissue repair. rKS56 may possibly contribute to the future development of cell therapy for renal regeneration.

Large-scale identification of genes implicated in kidney glomerulus development and function

To advance our understanding of development, function and diseases in the kidney glomerulus, we have established and large-scale sequenced cDNA libraries from mouse glomeruli at different stages of development, resulting in a catalogue of 6053 different genes. The glomerular cDNA clones were arrayed and hybridized against a series of labeled targets from isolated glomeruli, non-glomerular kidney tissue, FACS-sorted podocytes and brain capillaries, which identified over 300 glomerular cell-enriched transcripts, some of which were further sublocalized to podocytes, mesangial cells and juxtaglomerular cells by in situ hybridization. For the earliest podocyte marker identified, Foxc2, knockout mice were used to analyze the role of this protein during glomerular development. We show that Foxc2 controls the expression of a distinct set of podocyte genes involved in podocyte differentiation and glomerular basement membrane maturation. The primary podocyte defects also cause abnormal differentiation and organization of the glomerular vascular cells. We surmise that studies on the other novel glomerulus-enriched transcripts identified in this study will provide new insight into glomerular development and pathomechanisms of disease.

Mutagenesis of the epithelial polarity gene, discs large 1, perturbs nephrogenesis in the developing mouse kidney

BACKGROUND

During development of the permanent mammalian kidney (metanephros) several key epithelial events occur such as ureteric branching morphogenesis and nephrogenesis. One of the first stages of nephrogenesis involves the conversion of mesenchymal cells to epithelial cells, and thus the metanephros provides an excellent model to study epithelial polarization. The aim of this study was to investigate the role of the epithelial polarity gene, discs large 1 (dlg1), during development of the mouse kidney.

METHODS

We utilized mice with a gene trap vector insertion within dlg1 (dlg(gt)) resulting in a truncated Dlg1 protein, lacking the SH3, protein 4.1 and guanylate kinase-like (GUK) domains, fused to a LacZ reporter. These mice were used to analyze the expression of Dlg1 during kidney development, the subcellular localization of Dlg1 in epithelial cells, and the ability of Dlg1 to bind to calmodulin-associated serine/threonine kinase (CASK). Metanephric organ culture was used to study branching morphogenesis and nephrogenesis in wild-type and dlg(gt) mutant mice.

RESULTS

Dlg1 was expressed in ureteric and mesenchyme-derived epithelial cells during kidney development. Truncation of Dlg1 altered the normal basolateral localization of Dlg1 restricting it to the adherens junction. Due to the loss of the SH3 domain the binding capacity of Dlg1 to CASK was reduced. Nephrogenesis was altered in dlg(gt)/dlg(gt) metanephroi with a 30% decrease in nephron number.

CONCLUSION

Our results indicate that the loss of the SH3, protein 4.1 and/or GUK domains of Dlg1 disrupt epithelial polarity and perturb nephrogenesis either as a secondary consequence to a defect in ureteric branching morphogenesis and/or delay in mesenchyme-to- epithelial transition.

Apoptosis induced by vitamin A signaling is crucial for connecting the ureters to the bladder

Removal of toxic substances from the blood depends on patent connections between the kidney, ureters and bladder that are established when the ureter is transposed from its original insertion site in the male genital tract to the bladder. This transposition is thought to occur as the trigone forms from the common nephric duct and incorporates into the bladder. Here we re-examine this model in the context of normal and abnormal development. We show that the common nephric duct does not differentiate into the trigone but instead undergoes apoptosis, a crucial step for ureter transposition controlled by vitamin A-induced signals from the primitive bladder. Ureter abnormalities occur in 1-2% of the human population and can cause obstruction and end-stage renal disease. These studies provide an explanation for ureter defects underlying some forms of obstruction in humans and redefine the current model of ureter maturation.

FGF8 is required for cell survival at distinct stages of nephrogenesis and for regulation of gene expression in nascent nephrons

During kidney morphogenesis, the formation of nephrons begins when mesenchymal nephron progenitor cells aggregate and transform into epithelial vesicles that elongate and assume an S-shape. Cells in different regions of the S-shaped body subsequently differentiate into the morphologically and functionally distinct segments of the mature nephron. Here, we have used an allelic series of mutations to determine the role of the secreted signaling molecule FGF8 in nephrogenesis. In the absence of FGF8 signaling, nephron formation is initiated, but the nascent nephrons do not express Wnt4or Lim1, and nephrogenesis does not progress to the S-shaped body stage. Furthermore, the nephron progenitor cells that reside in the peripheral zone, the outermost region of the developing kidney, are progressively lost. When FGF8 signaling is severely reduced rather than eliminated, mesenchymal cells differentiate into S-shaped bodies. However, the cells within these structures that normally differentiate into the tubular segments of the mature nephron undergo apoptosis, resulting in the formation of kidneys with severely truncated nephrons consisting of renal corpuscles connected to collecting ducts by an abnormally short tubular segment. Thus, unlike other FGF family members, which regulate growth and branching morphogenesis of the collecting duct system, Fgf8 encodes a factor essential for gene regulation and cell survival at distinct steps in nephrogenesis.