Global Quantification of Tissue Dynamics in the Developing Mouse Kidney

Application of 3D MAPs pipeline identifies the morphological sequence chondrocytes undergo and the regulatory role of GDF5 in this process

The activity of epiphyseal growth plates, which drives long bone elongation, depends on extensive changes in chondrocyte size and shape during differentiation. Here, we develop a pipeline called 3D Morphometric Analysis for Phenotypic significance (3D MAPs), which combines light-sheet microscopy, segmentation algorithms and 3D morphometric analysis to characterize morphogenetic cellular behaviors while maintaining the spatial context of the growth plate. Using 3D MAPs, we create a 3D image database of hundreds of thousands of chondrocytes. Analysis reveals broad repertoire of morphological changes, growth strategies and cell organizations during differentiation. Moreover, identifying a reduction in Smad 1/5/9 activity together with multiple abnormalities in cell growth, shape and organization provides an explanation for the shortening of Gdf5 KO tibias. Overall, our findings provide insight into the morphological sequence that chondrocytes undergo during differentiation and highlight the ability of 3D MAPs to uncover cellular mechanisms that may regulate this process.

Slit2-Robo Signaling Promotes Glomerular Vascularization and Nephron Development

BACKGROUND

Kidney function requires continuous blood filtration by glomerular capillaries. Disruption of glomerular vascular development or maintenance contributes to the pathogenesis of kidney diseases, but the signaling events regulating renal endothelium development remain incompletely understood. Here, we discovered a novel role of Slit2-Robo signaling in glomerular vascularization. Slit2 is a secreted polypeptide that binds to transmembrane Robo receptors and regulates axon guidance as well as ureteric bud branching and angiogenesis.

METHODS

We performed Slit2-alkaline phosphatase binding to kidney cryosections from mice with or without tamoxifen-inducible Slit2 or Robo1 and -2 deletions, and we characterized the phenotypes using immunohistochemistry, electron microscopy, and functional intravenous dye perfusion analysis.

RESULTS

Only the glomerular endothelium, but no other renal endothelial compartment, responded to Slit2 in the developing kidney vasculature. Induced Slit2 gene deletion or Slit2 ligand trap at birth affected nephrogenesis and inhibited vascularization of developing glomeruli by reducing endothelial proliferation and migration, leading to defective cortical glomerular perfusion and abnormal podocyte differentiation. Global and endothelial-specific Robo deletion showed that both endothelial and epithelial Robo receptors contributed to glomerular vascularization.

CONCLUSIONS

Our study provides new insights into the signaling pathways involved in glomerular vascular development and identifies Slit2 as a potential tool to enhance glomerular angiogenesis.

Study of cellular heterogeneity and differential dynamics of autophagy in human embryonic kidney development by single-cell RNA sequencing

Background

Autophagy is believed to participate in embryonic development, but whether the expression of autophagy-associated genes undergoes changes during the development of human embryonic kidneys remains unknown.

Methods

In this work, we identified 36,151 human renal cells from embryonic kidneys of 9–18 gestational weeks in 16 major clusters by single-cell RNA sequencing (scRNA-seq), and detected 1350 autophagy-related genes in all fetal renal cells. The abundance of each cell cluster in Wilms tumor samples from scRNA-seq and GDC TARGET WT datasets was detected by CIBERSORTx. R package Monocle 3 was used to determine differentiation trajectories. Cyclone tool of R package scran was applied to calculate the cell cycle scores. R package SCENIC was used to investigate the transcriptional regulons. The FindMarkers tool from Seurat was used to calculate DEGs. GSVA was used to perform gene set enrichment analyses. CellphoneDB was utilized to analyze intercellular communication.

Results

It was found that cells in the 13th gestational week showed the lowest transcriptional level in each cluster in all stages. Nephron progenitors could be divided into four subgroups with diverse levels of autophagy corresponding to different SIX2 expressions. SSBpod (podocyte precursors) could differentiate into four types of podocytes (Pod), and autophagy-related regulation was involved in this process. Pseudotime analysis showed that interstitial progenitor cells (IPCs) potentially possessed two primitive directions of differentiation to interstitial cells with different expressions of autophagy. It was found that NPCs, pretubular aggregates and interstitial cell clusters had high abundance in Wilms tumor as compared with para-tumor samples with active intercellular communication.

Conclusions

All these findings suggest that autophagy may be involved in the development and cellular heterogeneity of early human fetal kidneys. In addition, part of Wilms tumor cancer cells possess the characteristics of some fetal renal cell clusters.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12935-021-02154-w.

Spatial transcriptional mapping of the human nephrogenic program

Congenital abnormalities of the kidney and urinary tract are among the most common birth defects, affecting 3% of newborns. The human kidney forms around a million nephrons from a pool of nephron progenitors over a 30-week period of development. To establish a framework for human nephrogenesis, we spatially resolved a stereotypical process by which equipotent nephron progenitors generate a nephron anlage, then applied data-driven approaches to construct three-dimensional protein maps on anatomical models of the nephrogenic program. Single-cell RNA sequencing identified progenitor states, which were spatially mapped to the nephron anatomy, enabling the generation of functional gene networks predicting interactions within and between nephron cell types. Network mining identified known developmental disease genes and predicted targets of interest. The spatially resolved nephrogenic program made available through the Human Nephrogenesis Atlas (https://sckidney.flatironinstitute.org/) will facilitate an understanding of kidney development and disease and enhance efforts to generate new kidney structures.

The Impact of Functional and Structural Maturation of the Kidney on Susceptibility to Drug and Chemical Toxicity in Neonatal Rodents

Drug responses are often unpredictable in juvenile animal toxicity studies; hence, optimizing dosages is challenging. Renal functional differences based on age of development will often result in vastly different toxicologic responses. Developmental changes in renal function can alter plasma clearance of compounds with extensive renal elimination. Absorption, distribution, metabolism, and excretion of drugs vary depending on animal age and kidney maturation. Toxicity can result in malformations or renal degeneration. Although renal morphologic development in humans generally occurs in utero, maximal levels of tubular secretion, acid-base equilibrium, concentrating ability, or glomerular filtration rate (GFR) are reached postnatally in humans and animals and subject to drug effects. Maturation of renal metabolism and transporters occurs postnatally and plays a critical role in detoxification and excretion. Maturation times must be considered when designing juvenile toxicity studies and may require cohorts of animals of specific ages to achieve optimal dosing schemes and toxicokinetics. In recent years, critical end points and windows of susceptibility have been established comparatively between species to better model pharmacokinetics and understand pediatric nephrotoxicity. There are examples of agents where toxicity is enhanced in neonates, others where it is diminished, and others where rat nephrotoxicity is expressed as juvenile toxicity, but in humans as gestational toxicity.

Metabolic programming of nephron progenitor cell fate

Metabolic pathways are one of the first responses at the cellular level to maternal/fetal interface stressors. Studies have revealed the previously unrecognized contributions of intermediary metabolism to developmental programs. Here, we provide an overview of cellular metabolic pathways and the cues that modulate metabolic states. We discuss the developmental and physiological implications of metabolic reprogramming and the key role of metabolites in epigenetic and epiproteomic modifications during embryonic development and with respect to kidney development and nephrogenesis.

Increasing mTORC1 Pathway Activity or Methionine Supplementation during Pregnancy Reverses the Negative Effect of Maternal Malnutrition on the Developing Kidney

BACKGROUND

Low nephron number at birth is associated with a high risk of CKD in adulthood because nephrogenesis is completed in utero. Poor intrauterine environment impairs nephron endowment via an undefined molecular mechanism. A calorie-restricted diet (CRD) mouse model examined the effect of malnutrition during pregnancy on nephron progenitor cells (NPCs).

METHODS

Daily caloric intake was reduced by 30% during pregnancy. mRNA expression, the cell cycle, and metabolic activity were evaluated in sorted Six2 NPCs. The results were validated using transgenic mice, oral nutrient supplementation, and organ cultures.

RESULTS

Maternal CRD is associated with low nephron number in offspring, compromising kidney function at an older age. RNA-seq identified cell cycle regulators and the mTORC1 pathway, among other pathways, that maternal malnutrition in NPCs modifies. Metabolomics analysis of NPCs singled out the methionine pathway as crucial for NPC proliferation and maintenance. Methionine deprivation reduced NPC proliferation and lowered NPC number per tip in embryonic kidney cultures, with rescue from methionine metabolite supplementation. Importantly, in vivo, the negative effect of caloric restriction on nephrogenesis was prevented by adding methionine to the otherwise restricted diet during pregnancy or by removing one Tsc1 allele in NPCs.

CONCLUSIONS

These findings show that mTORC1 signaling and methionine metabolism are central to the cellular and metabolic effects of malnutrition during pregnancy on NPCs, contributing to nephrogenesis and later, to kidney health in adulthood.

Organ-Specific Branching Morphogenesis

A common developmental process, called branching morphogenesis, generates the epithelial trees in a variety of organs, including the lungs, kidneys, and glands. How branching morphogenesis can create epithelial architectures of very different shapes and functions remains elusive. In this review, we compare branching morphogenesis and its regulation in lungs and kidneys and discuss the role of signaling pathways, the mesenchyme, the extracellular matrix, and the cytoskeleton as potential organ-specific determinants of branch position, orientation, and shape. Identifying the determinants of branch and organ shape and their adaptation in different organs may reveal how a highly conserved developmental process can be adapted to different structural and functional frameworks and should provide important insights into epithelial morphogenesis and developmental disorders.

Clearly imaging and quantifying the kidney in 3D

For decades, measurements of kidney microanatomy using 2-dimensional sections has provided us with a detailed knowledge of kidney morphology under physiological and pathological conditions. However, the rapid development of tissue clearing methods in recent years, in combination with the development of novel 3-dimensional imaging modalities have provided new insights into kidney structure and function. This review article describes a range of novel insights into kidney development and disease obtained recently using these new methodological approaches. For example, in the developing kidney these approaches have provided new understandings of ureteric branching morphogenesis, nephron progenitor cell proliferation and commitment, interactions between ureteric tip cells and nephron progenitor cells, and the establishment of nephron segmentation. In whole adult mouse kidneys, tissue clearing combined with light sheet microscopy can image and quantify the total number of glomeruli, a major breakthrough in the field. Similar approaches have provided new insights into the structure of the renal vasculature and innervation, tubulointerstitial remodeling, podocyte loss and hypertrophy, cyst formation, the evolution of cellular crescents, and the structure of the glomerular filtration barrier. Many more advances in the understanding of kidney biology and pathology can be expected as additional clearing and imaging techniques are developed and adopted by more investigators.

Returning to kidney development to deliver synthetic kidneys

There is no doubt that the development of transplantable synthetic kidneys could improve the outcome for the many millions of people worldwide suffering from chronic kidney disease. Substantial progress has been made in the last 6 years in the generation of kidney tissue from stem cells. However, the limited scale, incomplete cellular complexity and functional immaturity of such structures suggests we are some way from this goal. While developmental biology has successfully guided advances to date, these human kidney models are limited in their capacity for ongoing nephrogenesis and lack corticomedullary definition, a unified vasculature and a coordinated exit path for urinary filtrate. This review will reassess our developmental understanding of how the mammalian embryo manages to create kidneys, how this has informed our progress to date and how both engineering and developmental biology can continue to guide us towards a synthetic kidney.

Postnatal prolongation of mammalian nephrogenesis by excess fetal GDNF

Nephron endowment, defined during the fetal period, dictates renal and related cardiovascular health throughout life. We show here that, despite its negative effects on kidney growth, genetic increase of GDNF prolongs the nephrogenic program beyond its normal cessation. Multi-stage mechanistic analysis revealed that excess GDNF maintains nephron progenitors and nephrogenesis through increased expression of its secreted targets and augmented WNT signaling, leading to a two-part effect on nephron progenitor maintenance. Abnormally high GDNF in embryonic kidneys upregulates its known targets but also Wnt9b and Axin2, with concomitant deceleration of nephron progenitor proliferation. Decline of GDNF levels in postnatal kidneys normalizes the ureteric bud and creates a permissive environment for continuation of the nephrogenic program, as demonstrated by morphologically and molecularly normal postnatal nephron progenitor self-renewal and differentiation. These results establish that excess GDNF has a bi-phasic effect on nephron progenitors in mice, which can faithfully respond to GDNF dosage manipulation during the fetal and postnatal period. Our results suggest that sensing the signaling activity level is an important mechanism through which GDNF and other molecules contribute to nephron progenitor lifespan specification.

Summary: Dosage of neurotropic factor GDNF regulates nephron progenitors and in utero growth factor augmentation can extend postnatal lifespan and differentiation of nephron progenitors.

Ret signaling in ureteric bud epithelial cells controls cell movements, cell clustering and bud formation

Ret signaling promotes branching morphogenesis during kidney development, but the underlying cellular mechanisms remain unclear. While Ret-expressing progenitor cells proliferate at the ureteric bud tips, some of these cells exit the tips to generate the elongating collecting ducts, and in the process turn off Ret. Genetic ablation of Ret in tip cells promotes their exit, suggesting that Ret is required for cell rearrangements that maintain the tip compartments. Here, we examine the behaviors of ureteric bud cells that are genetically forced to maintain Ret expression. These cells move to the nascent tips, and remain there during many cycles of branching; this tip-seeking behavior may require positional signals from the mesenchyme, as it occurs in whole kidneys but not in epithelial ureteric bud organoids. In organoids, cells forced to express Ret display a striking self-organizing behavior, attracting each other to form dense clusters within the epithelium, which then evaginate to form new buds. The ability of forced Ret expression to promote these events suggests that similar Ret-dependent cell behaviors play an important role in normal branching morphogenesis.

Kidney organoids: Research in developmental biology and emerging applications

Kidney organoids generated from human pluripotent stem cells (hPSCs) have drastically changed the field of stem cell research on human kidneys within a few years. They are self-organizing multicellular structures that contain nephron components such as glomeruli and renal tubules in most cases, but hPSC-derived ureteric buds, the progenitors of collecting ducts and ureters, can also form three-dimensional organoids. Today's challenges facing human kidney organoids are further maturation and anatomical integrity in order to achieve a complete model of the developing kidneys and ultimately a complete adult organ. Since chronic kidney disease (CKD) and impaired kidney function are an increasing burden on public health worldwide, there is an urgent need to develop effective treatments for various renal conditions. In this regard, hPSC-derived kidney organoids may impact medicine by providing new translational approaches. The unique ability of kidney organoids derived from disease-specific hPSCs to reproduce human diseases caused by genetic alterations may help provide the next generation of kidney disease models. Recent advances in the field of kidney organoid research have been generally accompanied by progress in developmental biology and other technological breakthroughs. In this review, we consider the current trends in kidney organoid technology, especially focusing on the relationship to the study of human kidney development, and discuss the remaining hurdles and prospects in regenerating human kidney structures beyond organoids.

Development of the metanephric kidney

The kidney plays an integral role in filtering the blood-removing metabolic by-products from the body and regulating blood pressure. This requires the establishment of large numbers of efficient and specialized blood filtering units (nephrons) that incorporate a system for vascular exchange and nutrient reabsorption as well as a collecting duct system to remove waste (urine) from the body. Kidney development is a dynamic process which generates these structures through a delicately balanced program of self-renewal and commitment of nephron progenitor cells that inhabit a constantly evolving cellular niche at the tips of a branching ureteric "tree." The former cells build the nephrons and the latter the collecting duct system. Maintaining these processes across fetal development is critical for establishing the normal "endowment" of nephrons in the kidney and perturbations to this process are associated both with mutations in integral genes and with alterations to the fetal environment.

The Biological Significance and Implications of Planar Cell Polarity for Nephrology

The orientation of cells in two-dimensional and three-dimensional space underpins how the kidney develops and responds to disease. The process by which cells orientate themselves within the plane of a tissue is termed planar cell polarity. In this Review, we discuss how planar cell polarity and the proteins that underpin it govern kidney organogenesis and pathology. The importance of planar cell polarity and its constituent proteins in multiple facets of kidney development is emphasised, including ureteric bud branching, tubular morphogenesis and nephron maturation. An overview is given of the relevance of planar cell polarity and its proteins for inherited human renal diseases, including congenital malformations with unknown aetiology and polycystic kidney disease. Finally, recent work is described outlining the influence of planar cell polarity proteins on glomerular diseases and highlight how this fundamental pathway could yield a new treatment paradigm for nephrology.

Multi-scale simulation of early kidney branching morphogenesis

An important feature of the branch morphogenesis during kidney development is the termination of the tips on the outer surface of a kidney. This feature requires the avoidance of the intersection between the tips and existing ducts inside the kidney. Here, we started from a continuous model and implemented the coarse grained rules into a fast and discrete simulations. The ligand-receptor-based Turing mechanism suggests a repulsion that decreases exponentially with distance between interacting branches, preventing the intersection between neighboring branches. We considered this repulsive effect in numerical simulations and successfully reproduce the key features of the experimentally observed branch morphology for an E15.5 kidney. We examine the similarity of several geometrical parameters between the simulation results and experimental observations. The good agreement between the simulations and experiments suggests that the concentration decay caused by the absorption of glial cell line derived neurotrophic factor might be the key factor to affect the geometry in early kidney development.

Embryonic Kidney Development, Stem Cells and the Origin of Wilms Tumor

The adult mammalian kidney is a poorly regenerating organ that lacks the stem cells that could replenish functional homeostasis similarly to, e.g., skin or the hematopoietic system. Unlike a mature kidney, the embryonic kidney hosts at least three types of lineage-specific stem cells that give rise to (a) a ureter and collecting duct system, (b) nephrons, and (c) mesangial cells together with connective tissue of the stroma. Extensive interest has been raised towards these embryonic progenitor cells, which are normally lost before birth in humans but remain part of the undifferentiated nephrogenic rests in the pediatric renal cancer Wilms tumor. Here, we discuss the current understanding of kidney-specific embryonic progenitor regulation in the innate environment of the developing kidney and the types of disruptions in their balanced regulation that lead to the formation of Wilms tumor.

A β-catenin-driven switch in TCF/LEF transcription factor binding to DNA target sites promotes commitment of mammalian nephron progenitor cells

The canonical Wnt pathway transcriptional co-activator β-catenin regulates self-renewal and differentiation of mammalian nephron progenitor cells (NPCs). We modulated β-catenin levels in NPC cultures using the GSK3 inhibitor CHIR99021 (CHIR) to examine opposing developmental actions of β-catenin. Low CHIR-mediated maintenance and expansion of NPCs are independent of direct engagement of TCF/LEF/β-catenin transcriptional complexes at low CHIR-dependent cell-cycle targets. In contrast, in high CHIR, TCF7/LEF1/β-catenin complexes replaced TCF7L1/TCF7L2 binding on enhancers of differentiation-promoting target genes. Chromosome confirmation studies showed pre-established promoter–enhancer connections to these target genes in NPCs. High CHIR-associated de novo looping was observed in positive transcriptional feedback regulation to the canonical Wnt pathway. Thus, β-catenin’s direct transcriptional role is restricted to the induction of NPCs, where rising β-catenin levels switch inhibitory TCF7L1/TCF7L2 complexes to activating LEF1/TCF7 complexes at primed gene targets poised for rapid initiation of a nephrogenic program.

Wnt signaling in kidney: the initiator or terminator?

The kidney is a key organ in the human body that excretes toxins and sustains the water-electrolyte balance. During embryonic development and disease progression, the kidney undergoes enormous changes in macrostructure, accompanied by a variety of microstructural histological changes, such as glomerular formation and sclerosis, tubule elongation and atrophy, interstitial establishment, and fibrosis progression. All of these rely on the frequent occurrence of cell death and growth. Notably, to overcome disease, some cells regenerate through self-repair or progenitor cell differentiation. However, the signaling mechanisms underlying kidney development and regeneration have not been elucidated. Recently, Wnt signaling has been noted to play an important role. Although it is a well-known developmental signal, the role of Wnt signaling in kidney development and regeneration is not well recognized. In this review, we review the role of Wnt signaling in kidney embryonic development, tissue repair, cell division, and progenitor cell differentiation after injury. Moreover, we briefly highlight advances in our understanding of the pathogenic mechanisms of Wnt signaling in mediating cellular senescence in kidney parenchymal and stem cells, an irreversible arrest of cell proliferation blocking tissue repair and regeneration. We also highlight the therapeutic targets of Wnt signaling in kidney diseases and provide important clues for clinical strategies.

Exacerbation of Neonatal Hemolysis and Impaired Renal Iron Handling in Heme Oxygenase 1-Deficient Mice

In most mammals, neonatal intravascular hemolysis is a benign and moderate disorder that usually does not lead to anemia. During the neonatal period, kidneys play a key role in detoxification and recirculation of iron species released from red blood cells (RBC) and filtered out by glomeruli to the primary urine. Activity of heme oxygenase 1 (HO1), a heme-degrading enzyme localized in epithelial cells of proximal tubules, seems to be of critical importance for both processes. We show that, in HO1 knockout mouse newborns, hemolysis was prolonged despite a transient state and exacerbated, which led to temporal deterioration of RBC status. In neonates lacking HO1, functioning of renal molecular machinery responsible for iron reabsorption from the primary urine (megalin/cubilin complex) and its transfer to the blood (ferroportin) was either shifted in time or impaired, respectively. Those abnormalities resulted in iron loss from the body (excreted in urine) and in iron retention in the renal epithelium. We postulate that, as a consequence of these abnormalities, a tight systemic iron balance of HO1 knockout neonates may be temporarily affected.

Simple Rules Determine Distinct Patterns of Branching Morphogenesis

Many metazoan organs are comprised of branching trees of epithelial tubes; how patterning occurs in these trees is a fundamental problem of development. Commonly, branch tips fill the volume of the organ approximately uniformly, e.g., in mammalian lung, airway branch tips are dispersed roughly uniformly throughout the volume of the lung. In contrast, in the developing metanephric kidney, the tips of the ureteric bud tree are located close to the outer surface of the kidney rather than filling the kidney. Here, we describe a simple alteration in the branching rules that accounts for the difference between the kidney pattern that leads to tips near the organ surface versus previously known patterns that lead to the branch tips being dispersed throughout the organ. We further use a simple toy model to deduce from first principles how this rule change accounts for the differences in the two types of trees.

Role of Claudins in Renal Branching Morphogenesis

Claudins are a family of tight junction proteins that are expressed during mouse kidney development. They regulate paracellular transport of solutes along the nephron and contribute to the final composition of the urinary filtrate. To understand their roles during development, we used a protein reagent, a truncated version of the Clostridium perfringens enterotoxin (C-CPE), to specifically remove a subset of claudin family members from mouse embryonic kidney explants at embryonic day 12. We observed that treatment with C-CPE decreased the number and the complexity of ureteric bud tips that formed: there were more single and less bifid ureteric bud tips when compared to control-treated explants. In addition, C-CPE-treated explants exhibited ureteric bud tips with larger lumens when compared to control explants (p < .05). Immunofluorescent analysis revealed decreased expression and localization of Claudin-3, -4, -6, and -8 to tight junctions of ureteric bud tips following treatment with C-CPE. Interestingly, Claudin-7 showed higher expression in the basolateral membrane of the ureteric bud lineage and poor localization to the tight junctions of the ureteric bud lineage both in controls and in C-CPE-treated explants. Taken together, it appears that claudin proteins may play a role in ureteric bud branching morphogenesis through changes in lumen formation that may affect the efficiency by which ureteric buds emerge and branch.

Branching morphogenesis as a driver of renal development

Branching morphogenesis is an integral developmental mechanism central to the formation of a range of organs including the kidney, lung, pancreas and mammary gland. The ramified networks of epithelial tubules it establishes are critical for the processes of secretion, excretion and exchange mediated by these tissues. In the kidney, branching serves to establish the collecting duct system that transports urine from the nephrons into the renal pelvis, ureter and finally the bladder. Generally speaking, the formation of these networks in different organs begins with the specification and differentiation of simple bud-like organ anlage, which then undergo a process of elaboration, typically by bifurcation. This process is often governed by the interaction of progenitor cells at the tips of the epithelia with neighboring mesenchymal cell populations which direct the branching process and which often themselves differentiate to form part of the adult organ. In the kidney, the tips of ureteric bud elaborate through a dynamic cell signaling relationship with overlying nephron progenitor cell populations. These cells sequentially commit to differentiation and the resulting nephrons reintegrate with the ureteric epithelium as development progresses. This review will describe recent advances in understanding the how the elaboration of the ureteric bud is patterned and consider the extent to which this process is shared with other organs.

Autocrine inhibition of cell motility can drive epithelial branching morphogenesis in the absence of growth

Epithelial branching morphogenesis drives the development of organs such as the lung, salivary gland, kidney and the mammary gland. It involves cell proliferation, cell differentiation and cell migration. An elaborate network of chemical and mechanical signals between the epithelium and the surrounding mesenchymal tissues regulates the formation and growth of branching organs. Surprisingly, when cultured in isolation from mesenchymal tissues, many epithelial tissues retain the ability to exhibit branching morphogenesis even in the absence of proliferation. In this work, we propose a simple, experimentally plausible mechanism that can drive branching morphogenesis in the absence of proliferation and cross-talk with the surrounding mesenchymal tissue. The assumptions of our mathematical model derive from in vitro observations of the behaviour of mammary epithelial cells. These data show that autocrine secretion of the growth factor TGF[Formula: see text]1 inhibits the formation of cell protrusions, leading to curvature-dependent inhibition of sprouting. Our hybrid cellular Potts and partial-differential equation model correctly reproduces the experimentally observed tissue-geometry-dependent determination of the sites of branching, and it suffices for the formation of self-avoiding branching structures in the absence and also in the presence of cell proliferation. This article is part of the theme issue 'Multi-scale analysis and modelling of collective migration in biological systems'.

The polycomb proteins EZH1 and EZH2 co-regulate chromatin accessibility and nephron progenitor cell lifespan in mice

SIX2 (SIX homeobox 2)-positive nephron progenitor cells (NPCs) give rise to all epithelial cell types of the nephron, the filtering unit of the kidney. NPCs have a limited lifespan and are depleted near the time of birth. Epigenetic factors are implicated in the maintenance of organ-restricted progenitors such as NPCs, but the chromatin-based mechanisms are incompletely understood. Here, using a combination of gene targeting, chromatin profiling, and single-cell RNA analysis, we examined the role of the murine histone 3 Lys-27 (H3K27) methyltransferases EZH1 (enhancer of zeste 1) and EZH2 in NPC maintenance. We found that EZH2 expression correlates with NPC growth potential and that EZH2 is the dominant H3K27 methyltransferase in NPCs and epithelial descendants. Surprisingly, NPCs lacking H3K27 trimethylation maintained their progenitor state but cycled slowly, leading to a smaller NPC pool and formation of fewer nephrons. Unlike Ezh2 loss of function, dual inactivation of Ezh1 and Ezh2 triggered overexpression of the transcriptional repressor Hes-related family BHLH transcription factor with YRPW motif 1 (Hey1), down-regulation of Six2, and unscheduled activation of Wnt4-driven differentiation, resulting in early termination of nephrogenesis and severe renal dysgenesis. Double-mutant NPCs also overexpressed the SIX family member Six1 However, in this context, SIX1 failed to maintain NPC stemness. At the chromatin level, EZH1 and EZH2 restricted accessibility to AP-1-binding motifs, and their absence promoted a regulatory landscape akin to differentiated and nonlineage cells. We conclude that EZH2 is required for NPC renewal potential and that tempering of the differentiation program requires cooperation of both EZH1 and EZH2.

Branching morphogenesis

Over the past 5 years, several studies have begun to uncover the links between the classical signal transduction pathways and the physical mechanisms that are used to sculpt branched tissues. These advances have been made, in part, thanks to innovations in live imaging and reporter animals. With modern research tools, our conceptual models of branching morphogenesis are rapidly evolving, and the differences in branching mechanisms between each organ are becoming increasingly apparent. Here, we highlight four branched epithelia that develop at different spatial scales, within different surrounding tissues and via divergent physical mechanisms. Each of these organs has evolved to employ unique branching strategies to achieve a specialized final architecture.

Generating Kidney from Stem Cells

Human kidney tissue can now be generated via the directed differentiation of human pluripotent stem cells. This advance is anticipated to facilitate the modeling of human kidney diseases, provide platforms for nephrotoxicity screening, enable cellular therapy, and potentially generate tissue for renal replacement. All such applications will rely upon the accuracy and reliability of the model and the capacity for stem cell-derived kidney tissue to recapitulate both normal and diseased states. In this review, we discuss the models available, how well they recapitulate the human kidney, and how far we are from application of these cells for use in cellular therapies.

Ontogeny and Cross-species Comparison of Pathways Involved in Drug Absorption, Distribution, Metabolism, and Excretion in Neonates (Review): Kidney

The kidneys play an important role in many processes, including urine formation, water conservation, acid-base equilibrium, and elimination of waste. The anatomic and functional development of the kidney has different maturation time points in humans versus animals, with critical differences between species in maturation before and after birth. Absorption, distribution, metabolism, and excretion (ADME) of drugs vary depending on age and maturation, which will lead to differences in toxicity and efficacy. When neonate/juvenile laboratory animal studies are designed, a thorough knowledge of the differences in kidney development between newborns/children and laboratory animals is essential. The human and laboratory animal data must be combined to obtain a more complete picture of the development in the kidneys around the neonatal period and the complexity of ADME in newborns and children. This review examines the ontogeny and cross-species differences in ADME processes in the developing kidney in preterm and term laboratory animals and children. It provides an overview of insights into ADME functionality in the kidney by identifying what is currently known and which gaps still exist. Currently important renal function properties such as glomerular filtration rate, renal blood flow, and ability to concentrate are generally well known, while detailed knowledge about transporter and metabolism maturation is growing but is still lacking. Preclinical data in those properties is limited to rodents and generally covers only the expression levels of transporter or enzyme-encoding genes. More knowledge on a functional level is needed to predict the kinetics and toxicity in neonate/juvenile toxicity and efficacy studies. SIGNIFICANCE STATEMENT: This review provides insight in cross-species developmental differences of absorption, distribution, metabolism, and excretion properties in the kidney, which should be considered in neonate/juvenile study interpretation, hypotheses generation, and experimental design.

Beyond a Passive Conduit: Implications of Lymphatic Biology for Kidney Diseases

The kidney contains a network of lymphatic vessels that clear fluid, small molecules, and cells from the renal interstitium. Through modulating immune responses and via crosstalk with surrounding renal cells, lymphatic vessels have been implicated in the progression and maintenance of kidney disease. In this Review, we provide an overview of the development, structure, and function of lymphatic vessels in the healthy adult kidney. We then highlight the contributions of lymphatic vessels to multiple forms of renal pathology, emphasizing CKD, transplant rejection, and polycystic kidney disease and discuss strategies to target renal lymphatics using genetic and pharmacologic approaches. Overall, we argue the case for lymphatics playing a fundamental role in renal physiology and pathology and treatments modulating these vessels having therapeutic potential across the spectrum of kidney disease.

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.

Genetic manipulation of ureteric bud tip progenitors in the mammalian kidney through an Adamts18 enhancer driven tet-on inducible system

The ureteric epithelial progenitor (UEP) population within the embryonic kidney generates the arborized epithelial network of the kidney's collecting system and plays a critical role in the expansion and induction of the surrounding nephron progenitor pool. Adamts18 shows UEP- restricted expression in the kidney and progenitor tip-restricted expression in several other organs undergoing branching epithelial growth. Adamts18 is encoded by 23 exons. Genetic removal of genomic sequence spanning exons 1 to 3 led to a specific loss of Adamts18 expression in UEPs, suggesting this region may encode a UEP-specific enhancer. Intron 2 (3 ​kb) was shown to have enhancer activity driving expression of the doxycycline inducible tet-on transcriptional regulator (rtTA) in an Adamts18en-rtTA transgenic mouse strain. Crossing Adamts18en-rtTA mice to a doxycycline dependent GFP reporter mouse enabled the live imaging of embryonic kidney explants. This facilitated the analysis of ureteric epithelial branching events at the cellular level. Ablation of UEPs at the initiation of ureteric bud outgrowth through the doxycycline-mediated induction of Diphtheria Toxin A (DTA) generated a range of phenotypes from complete kidneys agenesis, to duplex kidneys with double ureters. The latter outcome points to the potential of regulative processes to restore UEPs. In contrast, overexpression of YAP prior to ureteric bud outgrowth led to a complete failure of kidney development. Elevating YAP levels at later stages retarded branching growth. A similar phenotype was observed with the overexpression of MYC within the branch-tip localized UEP population. These experiments showcase the utility of the Adamts18en-rtTA transgenic model to the investigation of cellular and molecular events specific to branch tip progenitors within the mammalian kidney complementing existing CRE-dependent genetic tools. Further, the illustrative examples point to areas where new insight may be gained into the regulation of UEP programs.

Functional roles of Grainyhead-like transcription factors in renal development and disease

Proper renal function relies on the tightly regulated development of nephrons and collecting ducts. This process, known as tubulogenesis, involves dynamic cellular and molecular changes that instruct cells to form highly organized tubes of epithelial cells which compartmentalize the renal interstitium and tubular lumen via assembly of a selective barrier. The integrity and diversity of the various renal epithelia is achieved via formation of intercellular protein complexes along the apical-basal axis of the epithelial cells. In recent years, the evolutionarily conserved family of Grainyhead-like (GRHL) transcription factors which encompasses three mammalian family members (Grainyhead-like 1, 2, 3) has emerged as a group of critical regulators for organ development, epithelial differentiation, and barrier formation. Evidence from transgenic animal models supports the presence of Grainyhead-like-dependent transcriptional mechanisms that promote formation and maintenance of epithelial barriers in the kidney. In this review, we highlight different Grhl-dependent mechanisms that modulate epithelial differentiation in the kidney. Additionally, we discuss how disruptions in these mechanisms result in impaired renal function later in life.

Modulation of polycystic kidney disease by non-coding RNAs

PURPOSE OF REVIEW

microRNAs (miRNAs) are a class of small, evolutionarily conserved, non-coding RNAs (ncRNAs) that function as inhibitors of post-transcriptional mRNA expression. They are implicated in the pathogenesis of numerous diseases, including many common kidney conditions. In this review, we focus on how miRNAs impact autosomal dominant polycystic kidney disease (ADPKD) progression. We also discuss the feasibility of the emerging novel antisense oligonucleotides (ASOs) drug class, which includes anti-miRNA drugs, for the treatment of ADPKD.

RECENT FINDINGS

Aberrant miRNA expression is observed in multiple PKD murine models and human ADPKD samples. Gain and loss-of-function studies have directly linked dysregulated miRNA activity to kidney cyst growth. The most comprehensively studied miRNA in PKD is the miR-17 family, which promotes PKD progression through the rewiring of cyst metabolism and by directly inhibiting PKD1 and PKD2 expression. This discovery has led to the development of an anti-miR-17 drug for ADPKD treatment. Other miRNAs such as miR-21, miR-193, and miR-214 are also known to regulate cyst growth by modulating cyst epithelial apoptosis, proliferation, and interstitial inflammation.

SUMMARY

miRNAs have emerged as novel pathogenic regulators of ADPKD progression. Anti-miR-based drugs represent a new therapeutic modality to treat ADPKD patients.

Duplex kidney formation: developmental mechanisms and genetic predisposition

Congenital abnormalities of the kidney and urinary tract (CAKUT) are a highly diverse group of diseases that together belong to the most common abnormalities detected in the new-born child. Consistent with this diversity, CAKUT are caused by mutations in a large number of genes and present a wide spectrum of phenotypes. In this review, we will focus on duplex kidneys, a relatively frequent form of CAKUT that is often asymptomatic but predisposes to vesicoureteral reflux and hydronephrosis. We will summarise the molecular programs responsible for ureter induction, review the genes that have been identified as risk factors in duplex kidney formation and discuss molecular and cellular mechanisms that may lead to this malformation.

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.

Spatiotemporal dynamics and heterogeneity of renal lymphatics in mammalian development and cystic kidney disease

Heterogeneity of lymphatic vessels during embryogenesis is critical for organ-specific lymphatic function. Little is known about lymphatics in the developing kidney, despite their established roles in pathology of the mature organ. We performed three-dimensional imaging to characterize lymphatic vessel formation in the mammalian embryonic kidney at single-cell resolution. In mouse, we visually and quantitatively assessed the development of kidney lymphatic vessels, remodeling from a ring-like anastomosis under the nascent renal pelvis; a site of VEGF-C expression, to form a patent vascular plexus. We identified a heterogenous population of lymphatic endothelial cell clusters in mouse and human embryonic kidneys. Exogenous VEGF-C expanded the lymphatic population in explanted mouse embryonic kidneys. Finally, we characterized complex kidney lymphatic abnormalities in a genetic mouse model of polycystic kidney disease. Our study provides novel insights into the development of kidney lymphatic vasculature; a system which likely has fundamental roles in renal development, physiology and disease.

In most organs in the body, fluid tends to build up in the spaces between cells, especially if the organs become inflamed. Each organ has a ‘waste disposal system’; a set of specialized tubes called lymphatic vessels, to clear away this excess fluid and keep a check on inflammation. Defects in these tubes have been linked to a wide range of diseases including heart attacks, obesity, dementia and cancer.

The kidneys are responsible for filtering blood and balancing many of the body’s chemical processes. Polycystic kidney disease (PKD) is the most common genetic kidney disorder and it results in cysts filled with fluid building up in the kidney. The growth of cysts in PKD may be due to a problem with the lymphatic vessels. However, compared to other organs, how lymphatic vessels first form within the kidney and what they do is not well understood.

Now, Jafree et al. have used three-dimensional imaging to study how lymphatic vessels form in the kidneys of mice and humans. The experiments showed that lymphatic vessels first appear when mouse kidneys are about half developed, and start to grow rapidly when the kidneys are thought to begin filtering blood. Clusters of cells that may help lymphatic vessels to grow were also found hidden deep within the kidneys of mouse embryos. Treating the kidneys with a factor that stimulates the growth of lymphatic vessels increased the numbers of these clusters. Jafree et al. found similar clusters of cells in human kidneys, suggesting that lymphatic vessels in the kidneys of different mammals may develop in the same way.

Further experiments showed that the lymphatic vessels of kidneys in mice with PKD become distorted early on in the disease, when cysts are still small and before the mice develop symptoms. In the future, identifying drugs that target kidney lymphatic vessels may lead to more effective treatments for patients with PKD and other kidney diseases.

Design Principles of Branching Morphogenesis in Filamentous Organisms

The radiation of life on Earth was accompanied by the diversification of multicellular body plans in the eukaryotic kingdoms Animalia, Plantae, Fungi and Chromista. Branching forms are ubiquitous in nature and evolved repeatedly in the above lineages. The developmental and genetic basis of branch formation is well studied in the three-dimensional shoot and root systems of land plants, and in animal organs such as the lung, kidney, mammary gland, vasculature, etc. Notably, recent thought-provoking studies combining experimental analysis and computational modeling of branching patterns in whole animal organs have identified global patterning rules and proposed unifying principles of branching morphogenesis. Filamentous branching forms represent one of the simplest expressions of the multicellular body plan and constitute a key step in the evolution of morphological complexity. Similarities between simple and complex branching forms distantly related in evolution are compelling, raising the question whether shared mechanisms underlie their development. Here, we focus on filamentous branching organisms that represent major study models from three distinct eukaryotic kingdoms, including the moss Physcomitrella patens (Plantae), the brown alga Ectocarpus sp. (Chromista), and the ascomycetes Neurospora crassa and Aspergillus nidulans (Fungi), and bring to light developmental regulatory mechanisms and design principles common to these lineages. Throughout the review we explore how the regulatory mechanisms of branching morphogenesis identified in other models, and in particular animal organs, may inform our thinking on filamentous systems and thereby advance our understanding of the diverse strategies deployed across the eukaryotic tree of life to evolve similar forms.

In utero exposure to maternal diabetes impairs nephron progenitor differentiation

The incidence of diabetes mellitus has significantly increased among women of childbearing age, and it has been shown that prenatal exposure to maternal diabetes increases the risk of associated congenital anomalies of the kidney. Congenital anomalies of the kidney are among the leading causes of chronic kidney disease in children. To better understand the effect of maternal diabetes on kidney development, we analyzed wild-type offspring (DM_Exp) of diabetic Ins2+/C96Y mice (Akita mice). DM_Exp mice at postnatal day 34 have a reduction of ~20% in the total nephron number compared with controls, using the gold standard physical dissector/fractionator method. At the molecular level, the expression of the nephron progenitor markers sine oculis homeobox homolog 2 and Cited1 was increased in DM_Exp kidneys at postnatal day 2. Conversely, the number of early developing nephrons was diminished in DM_Exp kidneys. This was associated with decreased expression of the intracellular domain of Notch1 and the canonical Wnt target lymphoid enhancer binding factor 1. Together, these data suggest that the diabetic intrauterine environment impairs the differentiation of nephron progenitors into nephrons, possibly by perturbing the Notch and Wnt/β-catenin signaling pathways.

Kidney organoids: accurate models or fortunate accidents

There are now many reports of human kidney organoids generated via the directed differentiation of human pluripotent stem cells (PSCs) based on an existing understanding of mammalian kidney organogenesis. Such kidney organoids potentially represent tractable tools for the study of normal human development and disease with improvements in scale, structure, and functional maturation potentially providing future options for renal regeneration. The utility of such organotypic models, however, will ultimately be determined by their developmental accuracy. While initially inferred from mouse models, recent transcriptional analyses of human fetal kidney have provided greater insight into nephrogenesis. In this review, we discuss how well human kidney organoids model the human fetal kidney and how the remaining differences challenge their utility.

Histology Atlas of the Developing Mouse Urinary System With Emphasis on Prenatal Days E10.5-E18.5

Congenital abnormalities of the urinary tract are some of the most common human developmental abnormalities. Several genetically engineered mouse models have been developed to mimic these abnormalities and aim to better understand the molecular mechanisms of disease. This atlas has been developed as an aid to pathologists and other biomedical scientists for identification of abnormalities in the developing murine urinary tract by cataloguing normal structures at each stage of development. Hematoxylin and eosin- and immunohistochemical-stained sections are provided, with a focus on E10.5-E18.5, as well as a brief discussion of postnatal events in urinary tract development. A section on abnormalities in the development of the urinary tract is also provided, and molecular mechanisms are presented as supplementary material. Additionally, overviews of the 2 key processes of kidney development, branching morphogenesis and nephrogenesis, are provided to aid in the understanding of the complex organogenesis of the kidney. One of the key findings of this atlas is the histological identification of the ureteric bud at E10.5, as previous literature has provided conflicting reports on the initial point of budding. Furthermore, attention is paid to points where murine development is significantly distinct from human development, namely, in the cessation of nephrogenesis.

Inductive signals in branching morphogenesis - lessons from mammary and salivary glands

Branching morphogenesis is a fundamental developmental program that generates large epithelial surfaces in a limited three-dimensional space. It is regulated by inductive tissue interactions whose effects are mediated by soluble signaling molecules, and cell-cell and cell-extracellular matrix interactions. Here, we will review recent studies on inductive signaling interactions governing branching morphogenesis in light of phenotypes of mouse mutants and ex vivo organ culture studies with emphasis on developing mammary and salivary glands. We will highlight advances in understanding how cell fate decisions are intimately linked with branching morphogenesis. We will also discuss novel insights into the molecular control of cellular mechanisms driving the formation of these arborized ductal structures and reflect upon how distinct spatial patterns are generated.

New insights into the role of HNF-1β in kidney (patho)physiology

Hepatocyte nuclear factor-1β (HNF-1β) is an essential transcription factor that regulates the development and function of epithelia in the kidney, liver, pancreas, and genitourinary tract. Humans who carry HNF1B mutations develop heterogeneous renal abnormalities, including multicystic dysplastic kidneys, glomerulocystic kidney disease, renal agenesis, renal hypoplasia, and renal interstitial fibrosis. In the embryonic kidney, HNF-1β is required for ureteric bud branching, initiation of nephrogenesis, and nephron segmentation. Ablation of mouse Hnf1b in nephron progenitors causes defective tubulogenesis, whereas later inactivation in elongating tubules leads to cyst formation due to downregulation of cystic disease genes, including Umod, Pkhd1, and Pkd2. In the adult kidney, HNF-1β controls the expression of genes required for intrarenal metabolism and solute transport by tubular epithelial cells. Tubular abnormalities observed in HNF-1β nephropathy include hyperuricemia with or without gout, hypokalemia, hypomagnesemia, and polyuria. Recent studies have identified novel post-transcriptional and post-translational regulatory mechanisms that control HNF-1β expression and activity, including the miRNA cluster miR17 ∼ 92 and the interacting proteins PCBD1 and zyxin. Further understanding of the molecular mechanisms upstream and downstream of HNF-1β may lead to the development of new therapeutic approaches in cystic kidney disease and other HNF1B-related renal diseases.

Kidney-in-a-lymph node: A novel organogenesis assay to model human renal development and test nephron progenitor cell fates

Stem cell-derived organoids are emerging as sophisticated models for studying development and disease and as potential sources for developing organ substitutes. Unfortunately, although organoids containing renal structures have been generated from mouse and human pluripotent stem cells, there are still critical unanswered questions that are difficult to attain via in vitro systems, including whether these nonvascularized organoids have a stable and physiologically relevant phenotype or whether a suitable transplantation site for long-term in vivo studies can be identified. Even orthotopic engraftment of organoid cultures in the adult does not provide an environment conducive to vascularization and functional differentiation. Previously, we showed that the lymph node offers an alternative transplantation site where mouse metanephroi can differentiate into mature renal structures with excretory, homeostatic, and endocrine functions. Here, we show that the lymph node lends itself well as a niche to also grow human primary kidney rudiments and can additionally be viewed as a platform to interrogate emerging renal organoid cultures. Our study has a wide-ranging impact for tissue engineering approaches to rebuild functional tissues in vivo including-but not limited to-the kidney.

Single cell analysis of the developing mouse kidney provides deeper insight into marker gene expression and ligand-receptor crosstalk

Recent advances in the generation of kidney organoids and the culture of primary nephron progenitors from mouse and human have been based on knowledge of the molecular basis of kidney development in mice. Although gene expression during kidney development has been intensely investigated, single cell profiling provides new opportunities to further subsect component cell types and the signalling networks at play. Here, we describe the generation and analysis of 6732 single cell transcriptomes from the fetal mouse kidney [embryonic day (E)18.5] and 7853 sorted nephron progenitor cells (E14.5). These datasets provide improved resolution of cell types and specific markers, including subdivision of the renal stroma and heterogeneity within the nephron progenitor population. Ligand-receptor interaction and pathway analysis reveals novel crosstalk between cellular compartments and associates new pathways with differentiation of nephron and ureteric epithelium cell types. We identify transcriptional congruence between the distal nephron and ureteric epithelium, showing that most markers previously used to identify ureteric epithelium are not specific. Together, this work improves our understanding of metanephric kidney development and provides a template to guide the regeneration of renal tissue.

Defining the dynamic chromatin landscape of mouse nephron progenitors

Six2⁺ cap mesenchyme cells, also called nephron progenitor cells (NPC), are precursors of all epithelial cell types of the nephron, the filtering unit of the kidney. Current evidence indicates that perinatal ‘old’ NPC have a greater tendency to exit the progenitor niche and differentiate into nascent nephrons than their embryonic ‘young’ counterpart. Understanding the underpinnings of NPC development may offer insights to rejuvenate old NPC and expand the progenitor pool. Here, we compared the chromatin landscape of young and old NPC and found common features reflecting their shared lineage but also intrinsic differences in chromatin accessibility and enhancer landscape supporting the view that old NPC are epigenetically poised for differentiation. Annotation of open chromatin regions and active enhancers uncovered the transcription factor Bach2 as a potential link between the pro-renewal MAPK/AP1 and pro-differentiation Six2/b-catenin pathways that might be of critical importance in regulation of NPC fate. Our data provide the first glimpse of the dynamic chromatin landscape of NPC and serve as a platform for future studies of the impact of genetic or environmental perturbations on the epigenome of NPC.

Summary: An investigation of the chromatin landscape of mouse nephron progenitors across their life span supports the view that old nephron progenitors are epigenetically poised for differentiation.

MAPK/ERK Signaling in Regulation of Renal Differentiation

Congenital anomalies of the kidney and urinary tract (CAKUT) are common birth defects derived from abnormalities in renal differentiation during embryogenesis. CAKUT is the major cause of end-stage renal disease and chronic kidney diseases in children, but its genetic causes remain largely unresolved. Here we discuss advances in the understanding of how mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) activity contributes to the regulation of ureteric bud branching morphogenesis, which dictates the final size, shape, and nephron number of the kidney. Recent studies also demonstrate that the MAPK/ERK pathway is directly involved in nephrogenesis, regulating both the maintenance and differentiation of the nephrogenic mesenchyme. Interestingly, aberrant MAPK/ERK signaling is linked to many cancers, and recent studies suggest it also plays a role in the most common pediatric renal cancer, Wilms' tumor.

Modeling congenital kidney diseases in Xenopus laevis

Congenital anomalies of the kidney and urinary tract (CAKUT) occur in ∼1/500 live births and are a leading cause of pediatric kidney failure. With an average wait time of 3-5 years for a kidney transplant, the need is high for the development of new strategies aimed at reducing the incidence of CAKUT and preserving renal function. Next-generation sequencing has uncovered a significant number of putative causal genes, but a simple and efficient model system to examine the function of CAKUT genes is needed. Xenopus laevis (frog) embryos are well-suited to model congenital kidney diseases and to explore the mechanisms that cause these developmental defects. Xenopus has many advantages for studying the kidney: the embryos develop externally and are easily manipulated with microinjections, they have a functional kidney in ∼2 days, and 79% of identified human disease genes have a verified ortholog in Xenopus. This facilitates high-throughput screening of candidate CAKUT-causing genes. In this Review, we present the similarities between Xenopus and mammalian kidneys, highlight studies of CAKUT-causing genes in Xenopus and describe how common kidney diseases have been modeled successfully in this model organism. Additionally, we discuss several molecular pathways associated with kidney disease that have been studied in Xenopus and demonstrate why it is a useful model for studying human kidney diseases.

Summary: Understanding how congenital kidney diseases arise is imperative to their treatment. Using Xenopus as a model will aid in elucidating kidney development and congenital kidney diseases.