The relationship between nephron number, kidney size and body weight in two inbred mouse strains

The molecular and cellular anatomy of a fetal programming defect - the impact of low protein diet on the developing kidney

Low nephron number correlates with the development of hypertension and chronic kidney disease later in life. While intrauterine growth restriction caused by maternal low protein diet (LPD) is thought to be a significant cause of reduced nephron endowment in impoverished communities, its influence on the cellular and molecular processes which drive nephron formation are poorly understood. We conducted a comprehensive characterization of the impact of LPD on kidney development using tomographic and confocal imaging to quantify changes in branching morphogenesis and the cellular and morphological features of nephrogenic niches across development. These analyses were paired with single-cell RNA sequencing to dissect the transcriptional changes that LPD imposes during renal development. Differences in the expression of genes involved in metabolism were identified in most cell types we analyzed, yielding imbalances and shifts in cellular energy production. We further demonstrate that LPD impedes branching morphogenesis and significantly reduces the number of pretubular aggregates - the initial precursors to nephron formation. The most striking observation was that LPD changes the developmental trajectory of nephron progenitor cells, driving the formation of a partially committed cell population which likely reflects a failure of cells to commit to nephron formation and which ultimately reduces endowment. This unique profile of a fetal programming defect demonstrates that low nephron endowment arises from the pleiotropic impact of changes in branching morphogenesis and nephron progenitor cell commitment, the latter of which highlights a critical role for nutrition in regulating the cell fate decisions underpinning nephron endowment.

Significance Statement

While a mother's diet and behavior can negatively impact the number of nephrons in the kidneys of her offspring, the root cellular and molecular drivers of these deficits have not been rigorously explored. In this study we use advanced imaging and gene expression analysis in mouse models to define how a maternal low protein diet, analogous to that of impoverished communities, results in reduced nephron endowment. We find that low protein diet has pleiotropic effects on metabolism and the normal programs of gene expression. These profoundly impact the process of branching morphogenesis necessary to establish niches for nephron generation and change cell behaviors which regulate how and when nephron progenitor cells commit to differentiation.

FastCellpose: A Fast and Accurate Deep-Learning Framework for Segmentation of All Glomeruli in Mouse Whole-Kidney Microscopic Optical Images

Automated evaluation of all glomeruli throughout the whole kidney is essential for the comprehensive study of kidney function as well as understanding the mechanisms of kidney disease and development. The emerging large-volume microscopic optical imaging techniques allow for the acquisition of mouse whole-kidney 3D datasets at a high resolution. However, fast and accurate analysis of massive imaging data remains a challenge. Here, we propose a deep learning-based segmentation method called FastCellpose to efficiently segment all glomeruli in whole mouse kidneys. Our framework is based on Cellpose, with comprehensive optimization in network architecture and the mask reconstruction process. By means of visual and quantitative analysis, we demonstrate that FastCellpose can achieve superior segmentation performance compared to other state-of-the-art cellular segmentation methods, and the processing speed was 12-fold higher than before. Based on this high-performance framework, we quantitatively analyzed the development changes of mouse glomeruli from birth to maturity, which is promising in terms of providing new insights for research on kidney development and function.

Sensing ultrasound localization microscopy for the visualization of glomeruli in living rats and humans

BACKGROUND

Estimation of glomerular function is necessary to diagnose kidney diseases. However, the study of glomeruli in the clinic is currently done indirectly through urine and blood tests. A recent imaging technique called Ultrasound Localization Microscopy (ULM) has appeared. It is based on the ability to record continuous movements of individual microbubbles in the bloodstream. Although ULM improved the resolution of vascular imaging up to tenfold, the imaging of the smallest vessels had yet to be reported.

METHODS

We acquired ultrasound sequences from living humans and rats and then applied filters to divide the data set into slow-moving and fast-moving microbubbles. We performed a double tracking to highlight and characterize populations of microbubbles with singular behaviors. We decided to call this technique "sensing ULM" (sULM). We used post-mortem micro-CT for side-by-side confirmation in rats.

FINDINGS

In this study, we report the observation of microbubbles flowing in the glomeruli in living humans and rats. We present a set of analysis tools to extract quantitative information from individual microbubbles, such as remanence time or normalized distance.

INTERPRETATION

As glomeruli play a key role in kidney function, it would be possible that their observation yields a deeper understanding of the kidney. It could also be a tool to diagnose kidney diseases in patients. More generally, it will bring imaging capabilities closer to the functional units of organs, which is a key to understand most diseases, such as cancer, diabetes, or kidney failures.

FUNDING

This study was funded by the European Research Council under the European Union Horizon H2020 program (ERC Consolidator grant agreement No 772786-ResolveStroke).

Genetic Variants in ARHGEF6 Cause Congenital Anomalies of the Kidneys and Urinary Tract in Humans, Mice, and Frogs

BACKGROUND

About 40 disease genes have been described to date for isolated CAKUT, the most common cause of childhood CKD. However, these genes account for only 20% of cases. ARHGEF6, a guanine nucleotide exchange factor that is implicated in biologic processes such as cell migration and focal adhesion, acts downstream of integrin-linked kinase (ILK) and parvin proteins. A genetic variant of ILK that causes murine renal agenesis abrogates the interaction of ILK with a murine focal adhesion protein encoded by Parva , leading to CAKUT in mice with this variant.

METHODS

To identify novel genes that, when mutated, result in CAKUT, we performed exome sequencing in an international cohort of 1265 families with CAKUT. We also assessed the effects in vitro of wild-type and mutant ARHGEF6 proteins, and the effects of Arhgef6 deficiency in mouse and frog models.

RESULTS

We detected six different hemizygous variants in the gene ARHGEF6 (which is located on the X chromosome in humans) in eight individuals from six families with CAKUT. In kidney cells, overexpression of wild-type ARHGEF6 -but not proband-derived mutant ARHGEF6 -increased active levels of CDC42/RAC1, induced lamellipodia formation, and stimulated PARVA-dependent cell spreading. ARHGEF6-mutant proteins showed loss of interaction with PARVA. Three-dimensional Madin-Darby canine kidney cell cultures expressing ARHGEF6-mutant proteins exhibited reduced lumen formation and polarity defects. Arhgef6 deficiency in mouse and frog models recapitulated features of human CAKUT.

CONCLUSIONS

Deleterious variants in ARHGEF6 may cause dysregulation of integrin-parvin-RAC1/CDC42 signaling, thereby leading to X-linked CAKUT.

Too bright for 2 dimensions: recent progress in advanced 3-dimensional microscopy of the kidney

The kidney is a structurally and functionally complex organ responsible for the control of water, ion, and other solute homeostasis. Moreover, the kidneys excrete metabolic waste products and produce hormones, such as renin and erythropoietin. The functional unit of the kidney is the nephron, which is composed by a serial arrangement of a filter unit called the renal corpuscle and several tubular segments that modulate the filtered fluid by reabsorption and secretion. Within each kidney, thousands of nephrons are closely intermingled and surrounded by an intricate network of blood vessels and various interstitial cell types, including fibroblasts and immune cells. This complex tissue architecture is essential for proper kidney function. In fact, kidney disease is often reflected or even caused by a derangement of the histologic structures. Frequently, kidney histology is studied using microscopic analysis of 2-dimensional tissue sections, which, however, misses important 3-dimensional spatial information. Reconstruction of serial sections tries to overcome this limitation, but is technically challenging, time-consuming, and often inherently linked to sectioning artifacts. In recent years, advances in tissue preparation (e.g., optical clearing) and new light- and electron-microscopic methods have provided novel avenues for 3-dimensional kidney imaging. Combined with novel machine-learning algorithms, these approaches offer unprecedented options for large-scale and automated analysis of kidney structure and function. This review provides a brief overview of these emerging imaging technologies and presents key examples of how these approaches are already used to study the normal and the diseased kidney.

Cryo-Fluorescence Micro-Optical Sectioning Tomography for Volumetric Imaging of Various Whole Organs with Subcellular Resolution

Optical visualization of complex microstructures in the entire organ is essential for biomedical research. However, the existing methods fail to accurately acquire the detailed microstructures of whole organs with good morphological and biochemical preservation. This study proposes a cryo-fluorescence micro-optical sectioning tomography (cryo-fMOST) to image whole organs in three dimensions (3D) with submicron resolution. The system comprises a line-illumination microscope module, cryo-microtome, three-stage refrigeration module, and heat insulation device. To demonstrate the imaging capacity and wide applicability of the system, we imaged and reconstructed various organs of mice in 3D, including the healthy tongue, kidney, and brain, as well as the infarcted heart. More importantly, imaged brain slices were performed sugar phosphates determination and fluorescence in situ hybridization imaging to verify the compatibility of multi-omics measurements. The results demonstrated that cryo-fMOST is capable of acquiring high-resolution morphological details of various whole organs and may be potentially useful for spatial multi-omics.

•

Cryo-fluorescence micro-optical sectioning tomography (Cryo-fMOST) was achieved

•

3D imaging of whole mouse tongue, kidney, heart, and brain at submicron resolution

•

Frozen state well preserves tissues’ original morphology and biochemical information

•

Cryo-fMOST is compatible with sugar phosphates determination and FISH measurement

Expression of collagen type IV in human kidney during prenatal development

Background/Aim. Type IV collagen belongs to the group of nonfibrillar collagens and is an important component of the basement membranes, where it accounts for approximately 50% of its structural elements. The aim of the study was to describe the expression and distribution of collagen type IV in the embryonic and fetal metanephric kidney and to determine the volume density of collagen type IV in kidney tissue in each trimester of development. Methods. The material consisted of 19 human embryos/fetuses, in the gestational age from 8th to 37th week. Kidney tissue specimens were routinely processed to paraffin molds, stained immunohistochemically using polyclonal anti-collagen IV antibody and counterstained with Mayer hematoxylin and eosin. Stained slides were examined using a light microscope, and images of the selected areas under different lens magnification were captured with a digital camera. Volume density of collagen type IV was determined using ImageJ 1.48v and a plugin of the software, which inserted a grid system with 336 points. For the data comparison, the One-Way Analysis of Variance (ANOVA) was used. Results. Strong collagen IV immunopositivity was seen in all specimens, with a distribution in the basement membranes of urinary bud, parietal leaf of Bowman?s capsule, glomerular basement membrane, basement membrane of interstitial blood vessels, and basement membranes of nephron tubules and collecting ducts. No statistically significant difference in the volume density of type IV collagen was found among the different trimesters of the embryonic and fetal development. Conclusion. The synthesis and secretion of collagen type IV simultaneously follow the development of nephron structures, collecting system and blood vessels. The volume density of collagen type IV remains constant throughout all the trimesters of metanephric kidney development, indicating that it plays a crucial role in the normal development of nephron and collecting system structures, as well as in maintaining the normal kidney function.

Sex Differences in Kidney Function and Metabolism Assessed Using Hyperpolarized [1-13C]Pyruvate Interleaved Spectroscopy and Nonspecific Imaging

Metabolic sex differences have recently been shown to be particularly important in tailoring treatment strategies. Sex has a major effect on fat turnover rates and plasma lipid delivery in the body. Differences in kidney structure and transporters between male and female animals have been found. Here we investigated sex-specific renal pyruvate metabolic flux and whole-kidney functional status in age-matched healthy Wistar rats. Blood oxygenation level–dependent and dynamic contrast-enhanced (DCE) magnetic resonance imaging (MRI) were used to assess functional status. Hyperpolarized [1-¹³C]pyruvate was used to assess the metabolic differences between male and female rats. Female rats had a 41% ± 3% and 41% ± 5% lower absolute body and kidney weight, respectively, than age-matched male rats. No difference was seen between age-matched male and female rats in the kidney-to-body weight ratio. A 56% ± 11% lower lactate production per mL/100 mL/min was found in female rats than in age-matched male rats measured by hyperpolarized magnetic resonance and DCE MRI. Female rats had a 33% ± 11% higher glomerular filtration rate than age-matched male rats measured by DCE MRI. A similar renal oxygen tension (T2*) was found between age-matched male and female rats as shown by blood oxygenation level–dependent MRI. The results were largely independent of the pyruvate volume and the difference in body weight. This study shows an existing metabolic difference between kidneys in age-matched male and female rats, which indicates that sex differences need to be considered when performing animal experiments.

Kidney-intrinsic factors determine the severity of ischemia/reperfusion injury in a mouse model of delayed graft function

Delayed graft function due to transplant ischemia/reperfusion injury adversely affects up to 50% of deceased-donor kidney transplant recipients. However, key factors contributing to the severity of ischemia/reperfusion injury remain unclear. Here, using a clinically relevant mouse model of delayed graft function, we demonstrated that donor genetic background and kidney-intrinsic MyD88/Trif-dependent innate immunity were key determinants of delayed graft function. Functional deterioration of kidney grafts directly corresponded with the duration of cold ischemia time. The graft dysfunction became irreversible after cold ischemia time exceeded six hours. When cold ischemia time reached four hours, kidney grafts displayed histological features reflective of delayed graft function seen in clinical kidney transplantation. Notably, kidneys of B6 mice exhibited significantly more severe histological and functional impairment than kidneys of C3H or BALB/c mice, regardless of recipient strains or alloreactivities. Furthermore, allografts of B6 mice also showed an upregulation of IL-6, neutrophil gelatinase-associated lipocalin, and endoplasmic reticulum stress genes, as well as an increased influx of host neutrophils and memory CD8 T-cells. In contrast, donor MyD88/Trif deficiency inhibited neutrophil influx and decreased the expression of IL-6 and endoplasmic reticulum stress genes, along with improved graft function and prolonged allograft survival. Thus, kidney-intrinsic factors involving genetic characteristics and innate immunity serve as critical determinants of the severity of delayed graft function. This preclinical murine model allows for further investigations of the mechanisms underlying delayed graft function.

A Stereological Study of the Toxic Effects of Cerium Oxide during Pregnancy on Kidney Tissues in Neonatal NMRI Mice

Background

Both antioxidant and prooxidant activities have been previously reported for cerium oxide (CeO₂). The aim of this study was to investigate the effects of CeO₂ at different doses on changes in kidney tissues and markers in neonatal mice.

Methods

We randomly divided 30 pregnant NMRI mice into five groups (n = 6 per group)-a control group and four groups treated with intraperitoneal (i.p.) administration of different doses of CeO₂ (10, 25, 80, or 250 mg/kg body weight (bw)) on gestation days (GD) 7 and GD14. At the end of the treatment period, we analyzed the kidney tissues and serum samples. The levels of two serum redox markers, malondialdehyde (MDA) and ferric reducing/antioxidant power (FRAP), were determined. Data were analyzed using one-way ANOVA and Tukey's test, and a P value of <0.05 was considered significant.

Results

The mean total volumes of the renal corpuscle, glomeruli, and Bowman's capsule membranes significantly increased, and there was a significant decrease in the mean total volume of Bowman's space in the high-dose CeO₂ group compared to that in the control group. No statistically significant differences existed in the serum levels of MDA and FRAP in the treated and control groups.

Conclusion

Our results suggest that high doses of CeO₂ impair fetal renal development in pregnant mice, which results in kidney damage. Therefore, CeO₂ administration during pregnancy could have dose-dependent adverse effects on the developing kidneys in neonates.

Deletion of hypoxia-responsive microRNA-210 results in a sex-specific decrease in nephron number

Low nephron number results in an increased risk of developing hypertension and chronic kidney disease. Intrauterine growth restriction is associated with a nephron deficit in humans, and is commonly caused by placental insufficiency, which results in fetal hypoxia. The underlying mechanisms by which hypoxia impacts kidney development are poorly understood. microRNA-210 is the most consistently induced microRNA in hypoxia and is known to promote cell survival in a hypoxic environment. In this study, the role of microRNA-210 in kidney development was evaluated using a global microRNA-210 knockout mouse. A male-specific 35% nephron deficit in microRNA-210 knockout mice was observed. Wnt/β-catenin signaling, a pathway crucial for nephron differentiation, was misregulated in male kidneys with increased expression of the canonical Wnt target lymphoid enhancer binding factor 1. This coincided with increased expression of caspase-8-associated protein 2, a known microRNA-210 target and apoptosis signal transducer. Together, these data are consistent with a sex-specific requirement for microRNA-210 in kidney development.

Representation and relative abundance of cell-type selective markers in whole-kidney RNA-Seq data

Bulk-tissue RNA-Seq is increasingly being used in the study of physiological and pathophysiological processes in the kidney; however, the presence of multiple cell types in kidney tissue complicates data interpretation. We addressed the question of which cell types are represented in whole-kidney RNA-Seq data in order to identify circumstances in which bulk-kidney RNA-Seq can be successfully interpreted. We carried out RNA-Seq in mouse whole kidneys and in microdissected renal tubule segments. To aid in the interpretation of the data, we compiled a database of cell-type selective protein markers for 43 cell types believed to be present in kidney tissue. The whole-kidney RNA-Seq analysis identified transcripts corresponding to 17,742 genes, distributed over 5 orders of magnitude of expression level. Markers for all 43 curated cell types were detectable. Analysis of the cellular makeup of mouse and rat kidney, calculated from published literature, suggests that proximal tubule cells account for more than half of the mRNA in a kidney. Comparison of RNA-Seq data from microdissected proximal tubules with data from whole kidney supports this view. RNA-Seq data for cell-type selective markers in bulk-kidney samples provide a valid means to identify changes in minority-cell abundances in kidney tissue. Because proximal tubules make up a substantial fraction of whole-kidney samples, changes in proximal tubule gene expression can be assessed presumptively by bulk-kidney RNA-Seq, although results could potentially be complicated by the presence of mRNA from other cell types.

Inhibition of heparanase protects against chronic kidney dysfunction following ischemia/reperfusion injury

Renal ischemia/reperfusion (I/R) injury occurs in patients undergoing renal transplantation and with acute kidney injury and is responsible for the development of chronic allograft dysfunction as characterized by parenchymal alteration and fibrosis. Heparanase (HPSE), an endoglycosidase that regulates EMT and macrophage polarization, is an active player in the biological response triggered by ischemia/reperfusion (I/R) injury. I/R was induced in vivo by clamping left renal artery for 30 min in wt C57BL/6J mice. Animals were daily treated and untreated with Roneparstat (an inhibitor of HPSE) and sacrificed after 8 weeks. HPSE, fibrosis, EMT-markers, inflammation and oxidative stress were evaluated by biomolecular and histological methodologies together with the evaluation of renal histology and measurement of renal function parameters. 8 weeks after I/R HPSE was upregulated both in renal parenchyma and plasma and tissue specimens showed clear evidence of renal injury and fibrosis. The inhibition of HPSE with Roneparstat-restored histology and fibrosis level comparable with that of control. I/R-injured mice showed a significant increase of EMT, inflammation and oxidative stress markers but they were significantly reduced by treatment with Roneparstat. Finally, the inhibition of HPSE in vivo almost restored renal function as measured by BUN, plasma creatinine and albuminuria. The present study points out that HPSE is actively involved in the mechanisms that regulate the development of renal fibrosis arising in the transplanted organ as a consequence of ischemia/reperfusion damage. HPSE inhibition would therefore constitute a new pharmacological strategy to reduce acute kidney injury and to prevent the chronic pro-fibrotic damage induced by I/R.

Micro-CT imaging and structural analysis of glomeruli in a model of Adriamycin-induced nephropathy

Glomeruli number and size are important for determining the pathogenesis of glomerular disease, chronic kidney disease, and hypertension. Moreover, renal injury can occur in specific cortical layers and alter glomerular spatial distribution. In this study, we present a comprehensive structural analysis of glomeruli in a model of Adriamycin (doxorubicin) nephropathy. Glomeruli are imaged (micro-CT at 10 × 10 × 10 μm³) in kidney specimens from C57Bl/6 mouse cohorts: control treated with saline ( n = 9) and Adriamycin treated with 20 mg/kg Adriamycin ( n = 7). Several indices were examined, including glomerular number, glomerular volume, glomerular volume heterogeneity, and spatial density at each glomerulus and in each cortical layer (superficial, midcortical, and juxtamedullary). In the Adriamycin-treated animals, glomerular number decreased significantly in the left kidney [control: 8,298 ± 221, Adriamycin: 6,781 ± 630 (mean ± SE)] and right kidney (control: 7,317 ± 367, Adriamycin: 5,522 ± 508), and glomerular volume heterogeneity increased significantly in the left kidney (control: 0.642 ± 0.015, Adriamycin: 0.786 ± 0.018) and right kidney (control: 0.739 ± 0.016, Adriamycin: 0.937 ± 0.023). Glomerular spatial density was not affected. Glomerular volume heterogeneity increased significantly in the superficial and midcortical layers of the Adriamycin cohort. Adriamycin did not affect glomerular volume or density metrics in the juxtamedullary region, suggesting a compensatory mechanism of juxtamedullary glomeruli to injury in the outer cortical layers. Left/right asymmetry was observed in kidney size and various glomeruli metrics. The methods presented here can be used to evaluate renal disease models with subtle changes in glomerular endowment locally or across the entire kidney, and they provide an imaging tool to investigate diverse interventions and therapeutic drugs.

Histopathological Evaluation of Contrast-Induced Acute Kidney Injury Rodent Models

Contrast-induced acute kidney injury (CI-AKI) can occur in 3–25% of patients receiving radiocontrast material (RCM) despite appropriate preventive measures. Often patients with an atherosclerotic vasculature have to receive large doses of RCM. Thus, animal studies to uncover the exact pathomechanism of CI-AKI are needed. Sensitive and specific histologic end-points are lacking; thus in the present review we summarize the histologic appearance of different rodent models of CI-AKI. Single injection of RCM causes overt renal damage only in rabbits. Rats and mice need an additional insult to the kidney to establish a clinically manifest CI-AKI. In this review we demonstrate that the concentrating ability of the kidney may be responsible for species differences in sensitivity to CI-AKI. The most commonly held theory about the pathomechanism of CI-AKI is tubular cell injury due to medullary hypoxia. Thus, the most common additional insult in rats and mice is some kind of ischemia. The histologic appearance is tubular epithelial cell (TEC) damage; however severe TEC damage is only seen if RCM is combined by additional ischemia. TEC vacuolization is the first sign of CI-AKI, as it is a consequence of RCM pinocytosis and lysosomal fusion; however it is not sensitive as it does not correlate with renal function and is not specific as other forms of TEC damage also cause vacuolization. In conclusion, histopathology alone is insufficient and functional parameters and molecular biomarkers are needed to closely monitor CI-AKI in rodent experiments.

Fully Automated Evaluation of Total Glomerular Number and Capillary Tuft Size in Nephritic Kidneys Using Lightsheet Microscopy

The total number of glomeruli is a fundamental parameter of kidney function but very difficult to determine using standard methodology. Here, we counted all individual glomeruli in murine kidneys and sized the capillary tufts by combining in vivo fluorescence labeling of endothelial cells, a novel tissue-clearing technique, lightsheet microscopy, and automated registration by image analysis. Total hands-on time per organ was <1 hour, and automated counting/sizing was finished in <3 hours. We also investigated the novel use of ethyl-3-phenylprop-2-enoate (ethyl cinnamate) as a nontoxic solvent-based clearing reagent that can be handled without specific safety measures. Ethyl cinnamate rapidly cleared all tested organs, including calcified bone, but the fluorescence of proteins and immunohistochemical labels was maintained over weeks. Using ethyl cinnamate-cleared kidneys, we also quantified the average creatinine clearance rate per glomerulus. This parameter decreased in the first week of experimental nephrotoxic nephritis, whereas reduction in glomerular numbers occurred much later. Our approach delivers fundamental parameters of renal function, and because of its ease of use and speed, it is suitable for high-throughput analysis and could greatly facilitate studies of the effect of kidney diseases on whole-organ physiology.

Missense Mutation of POU Domain Class 3 Transcription Factor 3 in Pou3f3L423P Mice Causes Reduced Nephron Number and Impaired Development of the Thick Ascending Limb of the Loop of Henle

During nephrogenesis, POU domain class 3 transcription factor 3 (POU3F3 aka BRN1) is critically involved in development of distinct nephron segments, including the thick ascending limb of the loop of Henle (TAL). Deficiency of POU3F3 in knock-out mice leads to underdevelopment of the TAL, lack of differentiation of TAL cells, and perinatal death due to renal failure. Pou3f3L423P mutant mice, which were established in the Munich ENU Mouse Mutagenesis Project, carry a recessive point mutation in the homeobox domain of POU3F3. Homozygous Pou3f3L423P mutants are viable and fertile. The present study used functional, as well as qualitative and quantitative morphological analyses to characterize the renal phenotype of juvenile (12 days) and aged (60 weeks) homo- and heterozygous Pou3f3L423P mutant mice and age-matched wild-type controls. In both age groups, homozygous mutants vs. control mice displayed significantly smaller kidney volumes, decreased nephron numbers and mean glomerular volumes, smaller TAL volumes, as well as lower volume densities of the TAL in the kidney. No histological or ultrastructural lesions of TAL cells or glomerular cells were observed in homozygous mutant mice. Aged homozygous mutants displayed increased serum urea concentrations and reduced specific urine gravity, but no evidence of glomerular dysfunction. These results confirm the role of POU3F3 in development and function of the TAL and provide new evidence for its involvement in regulation of the nephron number in the kidney. Therefore, Pou3f3L423P mutant mice represent a valuable research model for further analyses of POU3F3 functions, or for nephrological studies examining the role of congenital low nephron numbers.

The Role of Endothelin System in Renal Structure and Function during the Postnatal Development of the Rat Kidney

Renal development in rodents, unlike in humans, continues during early postnatal period. We aimed to evaluate whether the pharmacological inhibition of Endothelin system during this period affects renal development, both at structural and functional level in male and female rats. Newborn rats were treated orally from postnatal day 1 to 20 with vehicle or bosentan (Actelion, 20 mg/kg/day), a dual endothelin receptor antagonist (ERA). The animals were divided in 4 groups: control males, control females, ERA males and ERA females. At day 21, we evaluated renal function, determined the glomerular number by a maceration method and by morphometric analysis and evaluated possible structural renal alterations by three methods: 〈alpha〉-Smooth muscle actin (α-SMA) immunohistochemistry, Masson's trichrome and Sirius red staining. The pharmacological inhibition of Endothelin system with a dual ERA during the early postnatal period of the rat did not leads to renal damage in the kidneys of male and female rats. However, ERA administration decreased the number of glomeruli, the juxtamedullary filtration surface area and the glomerular filtration rate and increased the proteinuria. These effects could predispose to hypertension or renal diseases in the adulthood. On the other hand, these effects were more pronounced in male rats, suggesting that there are sex differences that could be greater later in life. These results provide evidence that Endothelin has an important role in rat renal postnatal development. However these results do not imply that the same could happen in humans, since human renal development is complete at birth.

Claudin-7, -16, and -19 during mouse kidney development

Members of the claudin family of tight junction proteins are critical for establishing epithelial barriers and for the regulation of paracellular transport. To understand their roles during kidney development, we first performed RT-PCR analyses and determined that 23 claudin family members were expressed in embryonic day (E) 13.5 mouse kidneys. Based on their developmental expression and phenotypes in mouse models, we hypothesized that 3 claudin members could affect nephron formation during kidney development. Using whole mount in situ hybridization and immunohistochemistry, we demonstrated that Claudin-7 (Cldn7) was expressed in the nephric duct, the emerging ureteric bud, and in tubules derived from ureteric bud branching morphogenesis. In contrast, Claudin-16 (Cldn16) and Claudin-19 (Cldn19) were expressed at later stages of kidney development in immature renal tubules that become the Loop of Henle. To determine if a loss of these claudins would perturb kidney development, we examined newborn kidneys from mutant mouse models lacking Cldn7 or Cldn16. In both models, we noted no evidence for any congenital renal malformation and quantification of nephron number did not reveal a decrease in nephron number when compared to wildtype littermates. In summary, Cldn7, Cldn16, and Cldn19 are expressed in different epithelial lineages during kidney development. Mice lacking Cldn7 or Cldn16 do not have defects in de novo nephron formation, and this suggests that these claudins primarily function to regulate paracellular transport in the mature nephron.

The impact of maternal cigarette smoke exposure in a rodent model on renal development in the offspring

This study aimed to investigate whether maternal cigarette smoke exposure can disrupt fetal kidney development by changing the expression of growth and transcription factors essential for renal development, and thereafter predispose the offspring to chronic kidney disease later in life. Female Balb/c mice (6 weeks) were exposed either to cigarette smoke or air under identical conditions, 6 weeks prior to mating, during gestation and during lactation. Male offspring were sacrificed at three time points, postnatal day (P)1, P20 (weaning age), and 13 weeks (mature age). Blood, urine, and kidneys were collected for analysis. At P1, the developmental genes fibroblast growth factor 2, glial cell-line derived neurotrophic factor and paired box 2 were upregulated at mRNA and protein levels; whilst fibroblast growth factor (FGF) 7 and FGF10 were downregulated. At P20, mRNA expression of FGF2, FGF10 and Wingless-type 4 was upregulated by maternal smoke exposure. These changes were normalised in adulthood. Nephron development was delayed, with fewer nephron numbers from P1 persisted to adulthood; while glomerular volume was increased at P20 but reduced in adulthood. Pro-inflammatory marker monocyte chemoatractant protein 1 (MCP1) was increased in the kidney by maternal smoke exposure. These changes were accompanied by an increased albumin/creatinine ratio in adulthood, suggesting reduced renal dysfunction. In conclusion maternal cigarette smoke exposure prior to and during pregnancy, as well as lactation leads to significant renal underdevelopment and functional abnormalities in adulthood. This study confirms the hypothesis that maternal smoking predisposes offspring to chronic kidney disorders.

Hyperoxia exposure impairs nephrogenesis in the neonatal rat: role of HIF-1α

Preterm neonates are exposed at birth to high oxygen concentrations relative to the intrauterine environment. We have previously shown in a rat model that a hyperoxic insult results in a reduced nephron number in adulthood. Therefore, the aim of this study was to determine the effects of transient neonatal hyperoxia exposure on nephrogenesis. Sprague-Dawley rat pups were raised in 80% O₂ or room air from P3 to P10. Pups (n = 12/group, 6 males and 6 females) were sacrificed at P5 (during active nephrogenesis) and at P10 (after the completion of nephrogenesis). Hyperoxia exposure resulted in a significant reduction in both nephrogenic zone width and glomerular diameter at P5, and a significantly increased apoptotic cell count; however, nephron number at P10 was not affected. HIF-1α expression in the developing kidney was significantly reduced following hyperoxia exposure. Systemic administration of the HIF-1α stabilizer dimethyloxalylglycine (DMOG) resulted in enhanced expression of HIF-1α and improved nephrogenesis: kidneys from hyperoxia-exposed pups treated with DMOG exhibited a nephrogenic zone width and glomerular diameter similar to room-air controls. These findings demonstrate that neonatal hyperoxia exposure results in impaired nephrogenesis, which may be at least in part HIF-1α-mediated. Although nephron number was not significantly reduced at the completion of nephrogenesis, early indicators of maldevelopment suggest the potential for accelerated nephron loss in adulthood. Overall, this study supports the premise that prematurely born neonates exposed to high oxygen levels after birth are vulnerable to impaired renal development.

Differentiated kidney epithelial cells repair injured proximal tubule

Whether kidney proximal tubule harbors a scattered population of epithelial stem cells is a major unsolved question. Lineage-tracing studies, histologic characterization, and ex vivo functional analysis results conflict. To address this controversy, we analyzed the lineage and clonal behavior of fully differentiated proximal tubule epithelial cells after injury. A CreER(T2) cassette was knocked into the sodium-dependent inorganic phosphate transporter SLC34a1 locus, which is expressed only in differentiated proximal tubule. Tamoxifen-dependent recombination was absolutely specific to proximal tubule. Clonal analysis after injury and repair showed that the bulk of labeled cells proliferate after injury with increased clone size after severe compared with mild injury. Injury to labeled proximal tubule epithelia induced expression of CD24, CD133, vimentin, and kidney-injury molecule-1, markers of putative epithelial stem cells in the human kidney. Similar results were observed in cultured proximal tubules, in which labeled clones proliferated and expressed dedifferentiation and injury markers. When mice with completely labeled kidneys were subject to injury and repair there was no dilution of fate marker despite substantial proliferation, indicating that unlabeled progenitors do not contribute to kidney repair. During nephrogenesis and early kidney growth, single proximal tubule clones expanded, suggesting that differentiated cells also contribute to tubule elongation. These findings provide no evidence for an intratubular stem-cell population, but rather indicate that terminally differentiated epithelia reexpress apparent stem-cell markers during injury-induced dedifferentiation and repair.

Proximal nephron

The kidney plays a fundamental role in maintaining body salt and fluid balance and blood pressure homeostasis through the actions of its proximal and distal tubular segments of nephrons. However, proximal tubules are well recognized to exert a more prominent role than distal counterparts. Proximal tubules are responsible for reabsorbing approximately 65% of filtered load and most, if not all, of filtered amino acids, glucose, solutes, and low molecular weight proteins. Proximal tubules also play a key role in regulating acid-base balance by reabsorbing approximately 80% of filtered bicarbonate. The purpose of this review article is to provide a comprehensive overview of new insights and perspectives into current understanding of proximal tubules of nephrons, with an emphasis on the ultrastructure, molecular biology, cellular and integrative physiology, and the underlying signaling transduction mechanisms. The review is divided into three closely related sections. The first section focuses on the classification of nephrons and recent perspectives on the potential role of nephron numbers in human health and diseases. The second section reviews recent research on the structural and biochemical basis of proximal tubular function. The final section provides a comprehensive overview of new insights and perspectives in the physiological regulation of proximal tubular transport by vasoactive hormones. In the latter section, attention is particularly paid to new insights and perspectives learnt from recent cloning of transporters, development of transgenic animals with knockout or knockin of a particular gene of interest, and mapping of signaling pathways using microarrays and/or physiological proteomic approaches.

Neonatal hyperoxia: effects on nephrogenesis and long-term glomerular structure

Preterm neonates are born while nephrogenesis is ongoing and are commonly exposed to factors in the extrauterine environment that may impair renal development. Supplemental oxygen therapy exposes the preterm infant to a hyperoxic environment that may induce oxidative stress. Our aim was to determine the immediate and long-term effects of exposure to hyperoxia, during the period of postnatal nephrogenesis, on renal development. Newborn mice (C57BL/6J) were kept in a normoxic (room air, 21% oxygen) or a controlled hyperoxic (65% oxygen) environment from birth to postnatal day 7 (P7d). From P7d, animals were maintained in room air until early adulthood at postnatal day 56 (P56d) or middle age (10 mo; P10mo). Pups were assessed for glomerular maturity and renal corpuscle cross-sectional area at P7d (control n = 14; hyperoxic n = 14). Nephron number and renal corpuscle size were determined stereologically at P56d (control n = 14; hyperoxic n = 14) and P10mo (control n = 10; hyperoxic n = 10). At P7d, there was no effect of hyperoxia on glomerular size or maturity. In early adulthood (P56d), body weights, relative kidney weights and volumes, and nephron number were not different between groups, but the renal corpuscles were significantly enlarged. This was no longer evident at P10mo, with relative kidney weights and volumes, nephron number, and renal corpuscle size not different between groups. Furthermore, hyperoxia exposure did not significantly accelerate glomerulosclerosis in middle age. Hence, our findings show no overt long-term deleterious effects of early life hyperoxia on glomerular structure.

Visualizing the mouse podocyte with multiphoton microscopy

The podocyte is a highly specialized kidney glomerular epithelial cell that plays an essential role in glomerular filtration and is believed to be the target of numerous glomerular diseases leading to proteinuria. Despite the leaps in our understanding of podocyte biology, new methodologies are needed to facilitate research into the cell. Multiphoton microscopy (MPM) was used to image the nephrin knockout/green fluorescent protein (GFP) knock-in heterozygote (Nphs1(tm1Rkl)/J) mouse. The nephrin promoter restricts GFP expression to the podocytes that fluoresce green under excitation. From the exterior of an intact kidney, MPM can peer into the renal parenchyma and visualize the podocytes that outline the globular shape of the glomeruli. Details as fine as the podocyte's secondary processes can be resolved. In contrast, podocytes exhibit no fluorescence in the wildtype mouse and are invisible to MPM. Phenotypically, there are no significant differences between wildtype and Nphs1(tm1Rkl)/J mice in body weight, urinary albumin excretion, creatinine clearance, or glomerular depth. Interestingly, the glomeruli are closer to the kidney capsule in female mice, making the gender the preferred choice for MPM. For the first time, green fluorescent podocytes in a mouse model free of confounding phenotypes can be visualized unequivocally and in the "positive" by MPM, facilitating intravital studies of the podocyte.

Developmental cigarette smoke exposure: kidney proteome profile alterations in low birth weight pups

The Brenner hypothesis states that a congenital reduction in nephron number predisposes to adult-onset hypertension and renal failure. The reduction in nephron number induced by proportionally smaller kidney mass may predispose offspring to glomerular hyperfiltration with maturity onset obesity. Developmental cigarette smoke exposure (CSE) results in intrauterine growth retardation with a predisposition to obesity and cardiovascular disease at maturity. Utilizing a mouse model of 'active' developmental CSE (gestational day [GD] 1-postnatal day [PD] 21; cotinine>50 ng/mL) characterized by persistently smaller offspring with proportionally decreased kidney mass, the present study examined the impact of developmental CSE on the abundance of proteins associated with cellular metabolism in the kidney. Following cessation of CSE on PD21, kidney tissue was collected from CSE and Sham exposed pups for 2D-SDS-PAGE based proteome profiling with statistical analysis by partial least squares-discriminant analysis (PLS-DA) with affected molecular pathways identified by ingenuity pathway analysis. Proteins whose expression in the kidney were affected by developmental CSE belonged to the inflammatory disease, cell to cell signaling/interaction, lipid metabolism, small molecule biochemistry, cell cycle, respiratory disease, nucleic acid and carbohydrate metabolism networks. The present findings indicate that developmental CSE alters the kidney proteome. The companion paper details the liver proteome alterations in the same offspring.