The number of fetal nephron progenitor cells limits ureteric branching and adult nephron endowment

Bayesian inference of agent-based models: a tool for studying kidney branching morphogenesis

The adult mammalian kidney has a complex, highly-branched collecting duct epithelium that arises as a ureteric bud sidebranch from an epithelial tube known as the nephric duct. Subsequent branching of the ureteric bud to form the collecting duct tree is regulated by subcellular interactions between the epithelium and a population of mesenchymal cells that surround the tips of outgrowing branches. The mesenchymal cells produce glial cell-line derived neurotrophic factor (GDNF), that binds with RET receptors on the surface of the epithelial cells to stimulate several subcellular pathways in the epithelium. Such interactions are known to be a prerequisite for normal branching development, although competing theories exist for their role in morphogenesis. Here we introduce the first agent-based model of ex vivo kidney uretic branching. Through comparison with experimental data, we show that growth factor-regulated growth mechanisms can explain early epithelial cell branching, but only if epithelial cell division depends in a switch-like way on the local growth factor concentration; cell division occurring only if the driving growth factor level exceeds a threshold. We also show how a recently-developed method, "Approximate Approximate Bayesian Computation", can be used to infer key model parameters, and reveal the dependency between the parameters controlling a growth factor-dependent growth switch. These results are consistent with a requirement for signals controlling proliferation and chemotaxis, both of which are previously identified roles for GDNF.

Uteroplacental insufficiency in rats induces renal apoptosis and delays nephrogenesis completion

AIM

Uteroplacental insufficiency in rats reduces nephron endowment, leptin concentrations and programmes cardiorenal disease in offspring. Cross-fostering growth-restricted (Restricted) offspring onto a mother with normal lactation restores leptin concentrations and nephron endowment. This study aimed to determine whether the reduced nephron endowment in Restricted offspring is due to delayed glomerular formation and dysregulation of renal genes regulating branching morphogenesis, apoptosis or leptin signalling. Furthermore, we aimed to investigate whether cross-fostering Restricted offspring onto Control mothers could improve glomerular maturation and restore renal gene abundance.

METHODS

Uteroplacental insufficiency was induced by bilateral uterine vessel ligation (Restricted) or sham (Control) surgery on gestation day 18 (E18). Kidneys were collected at E20, postnatal day 1 (PN1) and PN7. An additional cohort was cross-fostered onto separate mothers at birth and kidneys collected at PN7.

RESULTS

Kidneys were lighter in the Restricted group, but weight was restored with cross-fostering. At E20, abundance of Bax, Flt1 and Vegfa was increased in Restricted offspring, while Ret and Bcl2 transcripts were increased only in Restricted females. At PN7, abundance of Gdnf and Ret was higher in Restricted offspring, as was Casp3. Restricted offspring had a wider nephrogenic zone with more immature glomeruli suggesting a delayed or extended nephrogenic period. Cross-fostering had subtle effects on gene abundance and glomerular maturity.

CONCLUSION

Uteroplacental insufficiency induced apoptosis in the developing kidney and delayed and extended nephrogenesis. Cross-fostering Restricted offspring onto Control mothers had beneficial effects on kidney growth and renal maturity, which may contribute to the restoration of nephron endowment.

Optimal route of diphtheria toxin administration to eliminate native nephron progenitor cells in vivo for kidney regeneration

To address the lack of organs for transplantation, we previously developed a method for organ regeneration in which nephron progenitor cell (NPC) replacement is performed via the diphtheria toxin receptor (DTR) system. In transgenic mice with NPC-specific expression of DTR, NPCs were eliminated by DT and replaced with NPCs lacking the DTR with the ability to differentiate into nephrons. However, this method has only been verified in vitro. For applications to natural models, such as animal fetuses, it is necessary to determine the optimal administration route and dose of DT. In this study, two DT administration routes (intra-peritoneal and intra-amniotic injection) were evaluated in fetal mice. The fetus was delivered by caesarean section at E18.5, and the fetal mouse kidney and RNA expression were evaluated. Additionally, the effect of the DT dose (25, 5, 0.5, and 0.05 ng/fetus-body) was studied. Intra-amniotic injection of DT led to a reduction in kidney volume, loss of glomeruli, and decreased differentiation marker expression. The intra-peritoneal route was not sufficient for NPC elimination. By establishing that intra-amniotic injection is the optimal administration route for DT, these results will facilitate studies of kidney regeneration in vivo. In addition, this method might be useful for analysis of kidney development at various time points by deleting NPCs during development.

Nephron progenitor cell death elicits a limited compensatory response associated with interstitial expansion in the neonatal kidney

The final nephron number in an adult kidney is regulated by nephron progenitor cell availability and collecting duct branching in the fetal period. Fetal environmental perturbations that cause reductions in cell numbers in these two compartments result in low nephron endowment. Previous work has shown that maternal dietary factors influence nephron progenitor cell availability, with both caloric restriction and protein deprivation leading to reduced cell numbers through apoptosis. In this study, we evaluate the consequences of inducing nephron progenitor cell death on progenitor niche dynamics and on nephron endowment. Depletion of approximately 40% of nephron progenitor cells by expression of diphtheria toxin A at embryonic day 15 in the mouse results in 10-20% nephron reduction in the neonatal period. Analysis of cell numbers within the progenitor cell pool following induction of apoptosis reveals a compensatory response in which surviving progenitor cells increase their proliferation and replenish the niche. The proliferative response is temporally associated with infiltration of macrophages into the nephrogenic zone. Colony stimulating factor 1 (CSF1) has a mitogenic effect on nephron progenitor cells, providing a potential explanation for the compensatory proliferation. However, CSF1 also promotes interstitial cell proliferation, and the compensatory response is associated with interstitial expansion in recovering kidneys which can be pharmacologically inhibited by treatment with clodronate liposomes. Our findings suggest that the fetal kidney employs a macrophage-dependent compensatory regenerative mechanism to respond to acute injury caused by death of nephron progenitor cells, but that this regenerative response is associated with neonatal interstitial expansion.

Editor's choice: Formation of the kidney relies on maintaining progenitor cells throughout development. The authors find that apoptotic loss of nephron progenitor cells provokes compensatory proliferation mediated by trophic factors released by phagocytes.

Tracing the destiny of mesenchymal stem cells from embryo to adult bone marrow and white adipose tissue via Pdgfrα expression

Mesenchymal stem cells (MSCs) are somatic stem cells that can be derived from adult bone marrow (BM) and white adipose tissue (WAT), and that display multipotency and self-renewal capacity. Although MSCs are essential for tissue formation and have already been used in clinical therapy, the origins and markers of these cells remain unknown. In this study, we first investigated the developmental process of MSCs in mouse embryos using the gene encoding platelet-derived growth factor receptor α (Pdgfra) as a marker. We then traced cells expressing Pdgfra and other genes (brachyury, Sox1 and Pmx1) in various mutant mouse embryos until the adult stage. This tracing of MSC origins and destinies indicates that embryonic MSCs emerge in waves and that almost all adult BM MSCs and WAT MSCs originate from mesoderm and embryonic Pdgfrα-positive cells. Furthermore, we demonstrate that adult Pdgfrα-positive cells are involved in some pathological conditions.

Roles of angiotensin II type 2 receptor in mice with fetal growth restriction

Our previous report indicated that vascular injury enhances vascular remodeling in fetal growth restriction (FGR) mice. The angiotensin II type 2 receptor (AT₂R) is relatively highly expressed in fetal mice. Therefore, we investigated the roles of AT₂R in FGR-induced cardiovascular disease using AT₂R knockout (AT₂KO) mice. Dams (wild-type and AT₂KO mice) were fed an isocaloric diet containing 20% protein (NP) or 8% protein (LP) until delivery. Arterial blood pressure, body weight, and histological changes in organs were investigated in offspring. The birth weight of offspring from dams fed an LP diet (LPO) was significantly lower than that of offspring from dams fed an NP diet. The heart/body and kidney/body weight ratios in AT₂KO-LPO at 12 weeks of age were significantly higher than those in the other groups. Greater thickness of the left ventricular wall, larger cardiomyocyte size and enhancement of perivascular fibrosis were observed in AT₂KO-LPO. Interestingly, mRNA expression of collagen I and inflammatory cytokines was markedly higher in the AT₂KO-LPO heart at 6 weeks of age but not at 12 weeks of age. AT₂R signaling may be involved in cardiovascular disorders of adult offspring with FGR. Regulation of AT₂R could contribute to preventing future cardiovascular disease in FGR offspring.

Biphasic functions for the GDNF-Ret signaling pathway in chemosensory neuron development and diversification

While knowledge of signaling mechanisms orchestrating the development and diversification of peripheral somatosensory neurons is extensive, our understanding of the mechanisms controlling chemosensory neuron specification remains rudimentary. Lingually projecting sensory neurons of the geniculate ganglion are receptive to the five taste qualities, as well as temperature and tactile stimuli, but the mechanisms responsible for the diversification of the unique subpopulations that respond to one, or several, of these stimuli remain unknown. Here we demonstrate that the GDNF-Ret signaling pathway exerts a unique, dual function in peripheral taste system development and postnatal function. Ret acts embryonically to regulate the expression of the chemosensory master regulator Phox2b, thus inducing chemosensory differentiation, while postnatally acting to specify a molecularly unique subpopulation of lingual mechanoreceptors.

The development of the taste system relies on the coordinated regulation of cues that direct the simultaneous development of both peripheral taste organs and innervating sensory ganglia, but the underlying mechanisms remain poorly understood. In this study, we describe a novel, biphasic function for glial cell line-derived neurotrophic factor (GDNF) in the development and subsequent diversification of chemosensory neurons within the geniculate ganglion (GG). GDNF, acting through the receptor tyrosine kinase Ret, regulates the expression of the chemosensory fate determinant Phox2b early in GG development. Ret^−/− mice, but not Ret^fx/fx; Phox2b-Cre mice, display a profound loss of Phox2b expression with subsequent chemosensory innervation deficits, indicating that Ret is required for the initial amplification of Phox2b expression but not its maintenance. Ret expression is extinguished perinatally but reemerges postnatally in a subpopulation of large-diameter GG neurons expressing the mechanoreceptor marker NF200 and the GDNF coreceptor GFRα1. Intriguingly, we observed that ablation of these neurons in adult Ret-Cre/ER^T2; Rosa26^LSL-DTA mice caused a specific loss of tactile, but not chemical or thermal, electrophysiological responses. Overall, the GDNF-Ret pathway exerts two critical and distinct functions in the peripheral taste system: embryonic chemosensory cell fate determination and the specification of lingual mechanoreceptors.

Haploinsufficiency for the Six2 gene increases nephron progenitor proliferation promoting branching and nephron number

The regulation of final nephron number in the kidney is poorly understood. Cessation of nephron formation occurs when the self-renewing nephron progenitor population commits to differentiation. Transcription factors within this progenitor population, such as SIX2, are assumed to control expression of genes promoting self-renewal such that homozygous Six2 deletion results in premature commitment and an early halt to kidney development. In contrast, Six2 heterozygotes were assumed to be unaffected. Using quantitative morphometry, we found a paradoxical 18% increase in ureteric branching and final nephron number in Six2 heterozygotes, despite evidence for reduced levels of SIX2 protein and transcript. This was accompanied by a clear shift in nephron progenitor identity with a distinct subset of downregulated progenitor genes such as Cited1 and Meox1 while other genes were unaffected. The net result was an increase in nephron progenitor proliferation, as assessed by elevated EdU (5-ethynyl-2'-deoxyuridine) labeling, an increase in MYC protein, and transcriptional upregulation of MYC target genes. Heterozygosity for Six2 on an Fgf20-/- background resulted in premature differentiation of the progenitor population, confirming that progenitor regulation is compromised in Six2 heterozygotes. Overall, our studies reveal a unique dose response of nephron progenitors to the level of SIX2 protein in which the role of SIX2 in progenitor proliferation versus self-renewal is separable.

Generation of interspecies limited chimeric nephrons using a conditional nephron progenitor cell replacement system

Animal fetuses and embryos may have applications in the generation of human organs. Progenitor cells may be an appropriate cell source for regenerative organs because of their safety and availability. However, regenerative organs derived from exogenous lineage progenitors in developing animal fetuses have not yet been obtained. Here, we established a combination system through which donor cells could be precisely injected into the nephrogenic zone and native nephron progenitor cells (NPCs) could be eliminated in a time- and tissue-specific manner. We successfully achieved removal of Six2+ NPCs within the nephrogenic niche and complete replacement of transplanted NPCs with donor cells. These NPCs developed into mature glomeruli and renal tubules, and blood flow was observed following transplantation in vivo. Furthermore, this artificial nephron could be obtained using NPCs from different species. Thus, this technique enables in vivo differentiation from progenitor cells into nephrons, providing insights into nephrogenesis and organ regeneration.

The transplantation of tissue-specific progenitor cells may be an approach in organ regeneration. Here the authors show that the nephron progenitor population of a developing mouse kidney, when ablated, can be replaced by exogenously supplied rat nephron progenitors, generating interspecies nephrons.

Branching morphogenesis in the developing kidney is governed by rules that pattern the ureteric tree

Metanephric kidney development is orchestrated by the iterative branching morphogenesis of the ureteric bud. We describe an underlying patterning associated with the ramification of this structure and show that this pattern is conserved between developing kidneys, in different parts of the organ and across developmental time. This regularity is associated with a highly reproducible branching asymmetry that is consistent with locally operative growth mechanisms. We then develop a class of tip state models to represent elaboration of the ureteric tree and describe rules for 'half-delay' branching morphogenesis that describe almost perfectly the patterning of this structure. Spatial analysis suggests that the observed asymmetry may arise from mutual suppression of bifurcation, but not extension, between the growing ureteric tips, and demonstrates that disruption of patterning occurs in mouse mutants in which the distribution of tips on the surface of the kidney is altered. These findings demonstrate that kidney development occurs by way of a highly conserved reiterative pattern of asymmetric bifurcation that is governed by intrinsic and locally operative mechanisms.

Refuting the hypothesis that semaphorin-3f/neuropilin-2 exclude blood vessels from the cap mesenchyme in the developing kidney

BACKGROUND

During murine kidney development, new cortical blood vessels form and pattern in cycles that coincide with cycles of collecting duct branching and the accompanying splitting of the cap mesenchyme (nephron progenitor cell populations that "cap" collecting duct ends). At no point in the patterning cycle do blood vessels enter the cap mesenchyme. We hypothesized that the exclusion of blood vessels from the cap mesenchyme may be controlled, at least in part, by an anti-angiogenic signal expressed by the cap mesenchyme cells.

RESULTS

We show that semaphorin-3f (Sema3f), a known anti-angiogenic factor, is expressed in cap mesenchymal cells and its receptor, neuropilin-2 (Nrp2), is expressed by newly forming blood vessels in the cortex of the developing kidney. We hypothesized that Sema3f/Nrp2 signaling excludes vessels from the cap mesenchyme. Genetic ablation of Sema3f and of Nrp2, however, failed to result in vessels invading the cap mesenchyme.

CONCLUSIONS

Despite complementary expression patterns, our data suggest that Sema3f and Nrp2 are dispensable for the exclusion of vessels from the cap mesenchyme during kidney development. These results should provoke additional experiments to ascertain the biological significance of Sema3f/Nrp2 expression in the developing kidney. Developmental Dynamics 246:1047-1056, 2017. © 2017 Wiley Periodicals, Inc.

A Unifying Theory of Branching Morphogenesis

The morphogenesis of branched organs remains a subject of abiding interest. Although much is known about the underlying signaling pathways, it remains unclear how macroscopic features of branched organs, including their size, network topology, and spatial patterning, are encoded. Here, we show that, in mouse mammary gland, kidney, and human prostate, these features can be explained quantitatively within a single unifying framework of branching and annihilating random walks. Based on quantitative analyses of large-scale organ reconstructions and proliferation kinetics measurements, we propose that morphogenesis follows from the proliferative activity of equipotent tips that stochastically branch and randomly explore their environment but compete neutrally for space, becoming proliferatively inactive when in proximity with neighboring ducts. These results show that complex branched epithelial structures develop as a self-organized process, reliant upon a strikingly simple but generic rule, without recourse to a rigid and deterministic sequence of genetically programmed events.

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Branching morphogenesis follows conserved statistical rules in multiple organs

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Ductal tips grow and branch as default state and stop dividing in high-density regions

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Model reproduces quantitatively organ properties in a parameter-free manner

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Shows that complex organ formation proceeds in a stochastic, self-organized manner

Bim gene dosage is critical in modulating nephron progenitor survival in the absence of microRNAs during kidney development

Low nephron endowment at birth has been associated with an increased risk for developing hypertension and chronic kidney disease. We demonstrated in an earlier study that conditional deletion of the microRNA (miRNA)-processing enzyme Dicer from nephron progenitors results in premature depletion of the progenitors and increased expression of the proapoptotic protein Bim (also known as Bcl-2L11). In this study, we generated a compound mouse model with conditional deletion of both Dicer and Bim, to determine the biologic significance of increased Bim expression in Dicer-deficient nephron progenitors. The loss of Bim partially restored the number of nephron progenitors and improved nephron formation. The number of progenitors undergoing apoptosis was significantly reduced in kidneys with loss of a single allele, or both alleles, of Bim compared to mutant kidneys. Furthermore, 2 miRNAs expressed in nephron progenitors (miR-17 and miR-106b) regulated Bim levels in vitro and in vivo Together, these data suggest that miRNA-mediated regulation of Bim controls nephron progenitor survival during nephrogenesis, as one potential means of regulating nephron endowment.-Cerqueira, D. M., Bodnar, A. J., Phua, Y. L., Freer, R., Hemker, S. L., Walensky, L. D., Hukriede, N. A., Ho, J. Bim gene dosage is critical in modulating nephron progenitor survival in the absence of microRNAs during kidney development.

Low birth weight is associated with impaired murine kidney development and function

BackgroundLow birth weight (LBW) neonates have impaired kidney development that leaves them susceptible to kidney disease and hypertension during adulthood. The study here identifies events that blunt nephrogenesis and kidney development in the murine LBW neonate.MethodsWe examined survival, kidney development, GFR, gene expression, and cyto-/chemokines in the LBW offspring of malnourished (caloric and protein-restricted) pregnant mice.ResultsMalnourished pregnant mothers gave birth to LBW neonates that had 40% reduced body weight and 54% decreased survival. Renal blood perfusion was reduced by 37%, whereas kidney volume and GFR were diminished in the LBW neonate. During gestation, the LBW neonatal kidney had 2.2-fold increased apoptosis, 76% decreased SIX2+ progenitor cells, downregulation of mesenchymal-to-epithelial signaling factors Wnt9b and Fgf8, 64% less renal vesicle formation, and 32% fewer nephrons than controls. At birth, increased plasma levels of IL-1β, IL-6, IL-12(p70), and granulocyte-macrophage colony-stimulating factor in the LBW neonate reduced SIX2+ progenitor cells.ConclusionIncreased pro-inflammatory cytokines in the LBW neonate decrease SIX2+ stem cells in the developing kidney. Reduced renal stem cells (along with the decreased mesenchymal-to-epithelial signaling) blunt renal vesicle generation, nephron formation, and kidney development. Subsequently, the mouse LBW neonate has reduced glomeruli volume, renal perfusion, and GFR.

Repair after nephron ablation reveals limitations of neonatal neonephrogenesis

The neonatal mouse kidney retains nephron progenitor cells in a nephrogenic zone for 3 days after birth. We evaluated whether de novo nephrogenesis can be induced postnatally beyond 3 days. Given the long-term implications of nephron number for kidney health, it would be useful to enhance nephrogenesis in the neonate. We induced nephron reduction by cryoinjury with or without contralateral nephrectomy during the neonatal period or after 1 week of age. There was no detectable compensatory de novo nephrogenesis, as determined by glomerular counting and lineage tracing. Contralateral nephrectomy resulted in additional adaptive healing, with little or no fibrosis, but did not also stimulate de novo nephrogenesis. In contrast, injury initiated at 1 week of age led to healing with fibrosis. Thus, despite the presence of progenitor cells and ongoing nephron maturation in the newborn mouse kidney, de novo nephrogenesis is not inducible by acute nephron reduction. This indicates that additional nephron progenitors cannot be recruited after birth despite partial renal ablation providing a reparative stimulus and suggests that nephron number in the mouse is predetermined at birth.

Role of hypoxia during nephrogenesis

Mammals develop in a physiologically hypoxic state, and the oxygen tension of different tissues in the embryo is precisely controlled. Deviation from normal oxygenation, such as what occurs in placental insufficiency, can disrupt fetal development. Several studies demonstrate that intrauterine hypoxia has a negative effect on kidney development. As nascent nephrons are forming from nephron progenitors in the nephrogenic zone, they are exposed to varying oxygen tension by virtue of the development of the renal vasculature. Thus, nephrogenesis may be linked to oxygen tension. However, the mechanism(s) by which this occurs remains unclear. This review focuses on what is known about molecular mechanisms active in physiological and pathological hypoxia and their effects on kidney development.

Understanding kidney morphogenesis to guide renal tissue regeneration

The treatment of renal failure has seen little change in the past 70 years. Patients with end-stage renal disease (ESRD) are treated with renal replacement therapy, including dialysis or organ transplantation. The growing imbalance between the availability of donor organs and prevalence of ESRD is pushing an increasing number of patients to undergo dialysis. Although the prospect of new treatment options for patients through regenerative medicine has long been suggested, advances in the generation of human kidney cell types through the directed differentiation of human pluripotent stem cells over the past 2 years have brought this prospect closer to delivery. These advances are the result of careful research into mammalian embryogenesis. By understanding the decision points made within the embryo to pattern the kidney, it is now possible to recreate self-organizing kidney tissues in vitro. In this Review, we describe the key decision points in kidney development and how these decisions have been mimicked experimentally. Recreation of human nephrons from human pluripotent stem cells opens the door to patient-derived disease models and personalized drug and toxicity screening. In the long term, we hope that these efforts will also result in the generation of bioengineered organs for the treatment of kidney disease.

Gata3 Hypomorphic Mutant Mice Rescued with a Yeast Artificial Chromosome Transgene Suffer a Glomerular Mesangial Cell Defect

GATA3 is a zinc finger transcription factor that plays a crucial role in embryonic kidney development, while its precise functions in the adult kidney remain largely unexplored. Here, we demonstrate that GATA3 is specifically expressed in glomerular mesangial cells and plays a critical role in the maintenance of renal glomerular function. Newly generated Gata3 hypomorphic mutant mice exhibited neonatal lethality associated with severe renal hypoplasia. Normal kidney size was restored by breeding the hypomorphic mutant with a rescuing transgenic mouse line bearing a 662-kb Gata3 yeast artificial chromosome (YAC), and these animals (termed G3YR mice) survived to adulthood. However, most of the G3YR mice showed degenerative changes in glomerular mesangial cells, which deteriorated progressively during postnatal development. Consequently, the G3YR adult mice suffered severe renal failure. We found that the 662-kb Gata3 YAC transgene recapitulated Gata3 expression in the renal tubules but failed to direct sufficient GATA3 activity to mesangial cells. Renal glomeruli of the G3YR mice had significantly reduced amounts of platelet-derived growth factor receptor (PDGFR), which is known to participate in the development and maintenance of glomerular mesangial cells. These results demonstrate a critical role for GATA3 in the maintenance of mesangial cells and its absolute requirement for prevention of glomerular disease.

Osr1 Interacts Synergistically with Wt1 to Regulate Kidney Organogenesis

Renal hypoplasia is a common cause of pediatric renal failure and several adult-onset diseases. Recent studies have associated a variant of the OSR1 gene with reduction of newborn kidney size and function in heterozygotes and neonatal lethality with kidney defects in homozygotes. How OSR1 regulates kidney development and nephron endowment is not well understood, however. In this study, by using the recently developed CRISPR genome editing technology, we genetically labeled the endogenous Osr1 protein and show that Osr1 interacts with Wt1 in the developing kidney. Whereas mice heterozygous for either an Osr1 or Wt1 null allele have normal kidneys at birth, most mice heterozygous for both Osr1 and Wt1 exhibit defects in metanephric kidney development, including unilateral or bilateral kidney agenesis or hypoplasia. The developmental defects in the Osr1+/-Wt1+/- mouse embryos were detected as early as E10.5, during specification of the metanephric mesenchyme, with the Osr1+/-Wt1+/- mouse embryos exhibiting significantly reduced Pax2-positive and Six2-positive nephron progenitor cells. Moreover, expression of Gdnf, the major nephrogenic signal for inducing ureteric bud outgrowth, was significantly reduced in the metanephric mesenchyme in Osr1+/-Wt1+/- embryos in comparison with the Osr1+/- or Wt1+/- littermates. By E11.5, as the ureteric buds invade the metanephric mesenchyme and initiate branching morphogenesis, kidney morphogenesis was significantly impaired in the Osr1+/-Wt1+/- embryos in comparison with the Osr1+/- or Wt1+/- embryos. These results indicate that Osr1 and Wt1 act synergistically to regulate nephron endowment by controlling metanephric mesenchyme specification during early nephrogenesis.

Renal dysplasia in the neonate

PURPOSE OF REVIEW

Renal dysplasia is classically described as a developmental disorder whereby the kidneys fail to undergo appropriate differentiation, resulting in the presence of malformed renal tissue elements. It is the commonest cause of chronic kidney disease and renal failure in the neonate. Although several genes have been identified in association with renal dysplasia, the underlying molecular mechanisms are often complex and heterogeneous in nature, and remain poorly understood.

RECENT FINDINGS

In this review, we describe new insights into the fundamental process of normal kidney development, and how the renal cortex and medulla are patterned appropriately during gestation. We review the key genes that are indispensable for this process, and discuss how patterning of the kidney is perturbed in the absence of these signaling pathways. The recent use of whole exome sequencing has identified genetic mutations in patients with renal dysplasia, and the results of these studies have increased our understanding of the pathophysiology of renal dysplasia.

SUMMARY

At present, there are no specific treatments available for patients with renal dysplasia. Understanding the molecular mechanisms of normal kidney development and the pathogenesis of renal dysplasia may allow for improved therapeutic options for these patients.

Ret and Etv4 Promote Directed Movements of Progenitor Cells during Renal Branching Morphogenesis

Branching morphogenesis of the epithelial ureteric bud forms the renal collecting duct system and is critical for normal nephron number, while low nephron number is implicated in hypertension and renal disease. Ureteric bud growth and branching requires GDNF signaling from the surrounding mesenchyme to cells at the ureteric bud tips, via the Ret receptor tyrosine kinase and coreceptor Gfrα1; Ret signaling up-regulates transcription factors Etv4 and Etv5, which are also critical for branching. Despite extensive knowledge of the genetic control of these events, it is not understood, at the cellular level, how renal branching morphogenesis is achieved or how Ret signaling influences epithelial cell behaviors to promote this process. Analysis of chimeric embryos previously suggested a role for Ret signaling in promoting cell rearrangements in the nephric duct, but this method was unsuited to study individual cell behaviors during ureteric bud branching. Here, we use Mosaic Analysis with Double Markers (MADM), combined with organ culture and time-lapse imaging, to trace the movements and divisions of individual ureteric bud tip cells. We first examine wild-type clones and then Ret or Etv4 mutant/wild-type clones in which the mutant and wild-type sister cells are differentially and heritably marked by green and red fluorescent proteins. We find that, in normal kidneys, most individual tip cells behave as self-renewing progenitors, some of whose progeny remain at the tips while others populate the growing UB trunks. In Ret or Etv4 MADM clones, the wild-type cells generated at a UB tip are much more likely to remain at, or move to, the new tips during branching and elongation, while their Ret-/- or Etv4-/- sister cells tend to lag behind and contribute only to the trunks. By tracking successive mitoses in a cell lineage, we find that Ret signaling has little effect on proliferation, in contrast to its effects on cell movement. Our results show that Ret/Etv4 signaling promotes directed cell movements in the ureteric bud tips, and suggest a model in which these cell movements mediate branching morphogenesis.

Development of the Mammalian Kidney

The basic unit of kidney function is the nephron. In the mouse, around 14,000 nephrons form in a 10-day period extending into early neonatal life, while the human fetus forms the adult complement of nephrons in a 32-week period completed prior to birth. This review discusses our current understanding of mammalian nephrogenesis: the contributing cell types and the regulatory processes at play. A conceptual developmental framework has emerged for the mouse kidney. This framework is now guiding studies of human kidney development enabled in part by in vitro systems of pluripotent stem cell-seeded nephrogenesis. A near future goal will be to translate our developmental knowledge-base to the productive engineering of new kidney structures for regenerative medicine.

Preeclampsia As Modulator of Offspring Health

A balanced intrauterine homeostasis during pregnancy is crucial for optimal growth and development of the fetus. The intrauterine environment is extremely vulnerable to multisystem pregnancy disorders such as preeclampsia, which can be triggered by various pathophysiological factors, such as angiogenic imbalance, immune responses, and inflammation. The fetus adapts to these conditions by a mechanism known as developmental programming that can lead to increased risk of chronic noncommunicable diseases in later life. This is shown in a substantial number of epidemiological studies that associate preeclampsia with increased onset of cardiovascular and metabolic diseases in the later life of the offspring. Furthermore, animal models based predominantly on one of the pathophysiological mechanism of preeclampsia, for example, angiogenic imbalance, immune response, or inflammation, do address the susceptibility of the preeclamptic offspring to increased maternal blood pressure and disrupted metabolic homeostasis. Accordingly, we extensively reviewed the latest research on the role of preeclampsia on the offspring's metabolism and cardiovascular phenotype. We conclude that future research on the pathophysiological changes during preeclampsia and methods to intervene in the harsh intrauterine environment will be essential for effective therapies.

The Life Cycle of the Nephron Progenitor

Although we know that mesenchymal progenitors give rise to nephrons in the kidney, how they balance self-renewal versus differentiation is still unclear. In this issue of Developmental Cell, Chen et al. (2015) show that nephron progenitors age, but not necessarily irreversibly: old progenitors can be "rejuvenated" by a young crowd.

Intrinsic Age-Dependent Changes and Cell-Cell Contacts Regulate Nephron Progenitor Lifespan

During fetal development, nephrons of the metanephric kidney form from a mesenchymal progenitor population that differentiates en masse before or shortly after birth. We explored intrinsic and extrinsic mechanisms controlling progenitor lifespan in a transplantation assay that allowed us to compare engraftment of old and young progenitors into the same young niche. The progenitors displayed an age-dependent decrease in proliferation and concomitant increase in niche exit rates. Single-cell transcriptome profiling revealed progressive age-dependent changes, with heterogeneity increasing in older populations. Age-dependent elevation in mTor and reduction in Fgf20 could contribute to increased exit rates. Importantly, 30% of old progenitors remained in the niche for up to 1 week post engraftment, a net gain of 50% to their lifespan, but only if surrounded by young neighbors. We provide evidence in support of a model in which intrinsic age-dependent changes affect inter-progenitor interactions that drive cessation of nephrogenesis.

Towards a quantitative model of kidney morphogenesis

Kidney growth is dependent on functional interactions between mesenchymal nephron progenitors, the ureteric epithelium and surrounding stroma, which together make up the nephrogenic niche. Signalling between these populations regulates nephron progenitor maintenance, branching morphogenesis and nephron induction. Nephron endowment is sensitive to changes in the size of the nephron progenitor pool and to decreases in factors that promote branching morphogenesis. However, determining the morphogenic consequences of these disruptions in vivo has been challenging as quantitating kidney morphogenesis is hampered by the size, opacity and three-dimensional complexity of the tissue. The recent application of whole mount immunofluorescence and tissue clearing, coupled with multiscale imaging and quantitative analysis, has begun to give insights into the dynamics of kidney formation. This review focuses on how the quantitative nature of this approach has enabled mathematical modelling of cell cycle lengths, growth rates, cell number and branching rates and is advancing our understanding of kidney organogenesis.

Recent advances in understanding of chronic kidney disease

Chronic kidney disease (CKD) is defined as any condition that causes reduced kidney function over a period of time. Fibrosis, tubular atrophy and interstitial inflammation are the hallmark of pathological features in CKD. Regardless of initial insult, CKD has some common pathways leading CKD to end-stage kidney disease, including hypoxia in the tubulointerstitium and proteinuria. Recent advances in genome editing technologies and stem cell research give great insights to understand the pathogenesis of CKD, including identifications of the origins of renal myofibroblasts and tubular epithelial cells upon injury. Environmental factors such as hypoxia, oxidative stress, and epigenetic factors in relation to CKD are also discussed.

Maternal nutrient restriction inhibits ureteric bud branching but does not affect the duration of nephrogenesis in rats

BACKGROUND

Maternal nutrient restriction produces offspring with fewer nephrons. We studied whether the reduced nephron number is due to the inhibition of ureteric branching or early cessation of nephrogenesis in rats. Signaling pathways involved in kidney development were also examined.

METHODS

The offspring of dams given food ad libitum (control (CON)) and those subjected to 50% food restriction (nutrient restriceted (NR)) were examined.

RESULTS

At embryonic day 13 (E13), there was no difference between NR and CON in body weight or kidney size. Ureteric buds branched once in both NR and CON. At E14 and E15, body and kidney size were significantly reduced in NR. Ureteric bud tip numbers were also reduced to 50% of CON. On the other hand, the disappearance of nephrogenic zone and a nephron progenitor marker Cited1 was not different between CON and NR. The final glomerular number of NR was 80% of CON. Activated extracellular signal-regulated kinase (ERK), p38, PI3K, Akt, and mammallian target of rapamycin (mTOR), and protein expression of β-catenin were downregulated at E15.

CONCLUSION

Ureteric branching is inhibited and developmentally regulated signaling pathways are downregulated at an early stage by maternal nutrient restriction. These changes, not early cessation of nephrogenesis, may be a mechanism for the inhibited kidney growth and nephrogenesis.

Improving our resolution of kidney morphogenesis across time and space

As with many mammalian organs, size and cellular complexity represent considerable challenges to the comprehensive analysis of kidney organogenesis. Traditional analyses in the mouse have revealed early patterning events and spatial cellular relationships. However, an understanding of later events is lacking. The generation of a comprehensive temporospatial atlas of gene expression during kidney development has facilitated advances in lineage definition, as well as selective compartment ablation. Advances in quantitative and dynamic imaging have allowed comprehensive analyses at the level of organ, component tissue and cell across kidney organogenesis. Such approaches will enhance our understanding of the links between kidney development and final postnatal organ function. The final frontier will be translating this understanding to outcomes for renal disease in humans.

Understanding the role of maternal diet on kidney development; an opportunity to improve cardiovascular and renal health for future generations

The leading causes of mortality and morbidity worldwide are cardiovascular disease (high blood pressure, high cholesterol and renal disease), cancer and diabetes. It is increasingly obvious that the development of these diseases encompasses complex interactions between adult lifestyle and genetic predisposition. Maternal malnutrition can influence the fetal and early life environment and pose a risk factor for the future development of adult diseases, most likely due to impaired organogenesis in the developing offspring. This then predisposes these offspring to cardiovascular disease and renal dysfunction in adulthood. Studies in experimental animals have further illustrated the significant impact maternal diet has on offspring health. Many studies report changes in kidney structure (a reduction in the number of nephrons in the kidney) in offspring of protein-deprived dams. Although the early studies suggested that increased blood pressure was also present in offspring of protein-restricted dams, this is not a universal finding and requires clarification. Importantly, to date, the literature offers little to no understanding of when in development these changes in kidney development occur, nor are the cellular and molecular mechanisms that drive these changes well characterised. Moreover, the mechanisms linking maternal nutrition and a suboptimal renal phenotype in offspring are yet to be discerned—one potential mechanism involves epigenetics. This review will focus on recent information on potential mechanisms by which maternal nutrition (focusing on malnutrition due to protein restriction, micronutrient restriction and excessive fat intake) influences kidney development and thereby function in later life.

Cell-cell interactions driving kidney morphogenesis

The mammalian kidney forms via cell-cell interactions between an epithelial outgrowth of the nephric duct and the surrounding nephrogenic mesenchyme. Initial morphogenetic events include ureteric bud branching to form the collecting duct (CD) tree and mesenchymal-to-epithelial transitions to form the nephrons, requiring reciprocal induction between adjacent mesenchyme and epithelial cells. Within the tips of the branching ureteric epithelium, cells respond to mesenchyme-derived trophic factors by proliferation, migration, and mitosis-associated cell dispersal. Self-inhibition signals from one tip to another play a role in branch patterning. The position, survival, and fate of the nephrogenic mesenchyme are regulated by ECM and secreted signals from adjacent tip and stroma. Signals from the ureteric tip promote mesenchyme self-renewal and trigger nephron formation. Subsequent fusion to the CDs, nephron segmentation and maturation, and formation of a patent glomerular basement membrane also require specialized cell-cell interactions. Differential cadherin, laminin, nectin, and integrin expression, as well as intracellular kinesin and actin-mediated regulation of cell shape and adhesion, underlies these cell-cell interactions. Indeed, the capacity for the kidney to form via self-organization has now been established both via the recapitulation of expected morphogenetic interactions after complete dissociation and reassociation of cellular components during development as well as the in vitro formation of 3D kidney organoids from human pluripotent stem cells. As we understand more about how the many cell-cell interactions required for kidney formation operate, this enables the prospect of bioengineering replacement structures based on these self-organizing properties.

The regulation of apoptosis in kidney development: implications for nephron number and pattern?

Apoptosis is essential to remodel developing structures and eliminate superfluous cells in a controlled manner during normal development, and continues to be an important component of tissue remodeling and regeneration during an organism's lifespan, or as a response to injury. This mini review will discuss recent studies that have provided insights into the roles of apoptosis in the determination of nephron number and pattern, during normal and abnormal kidney development. The regulation of congenital nephron endowment has implications for risk of chronic kidney disease in later life, whereas abnormalities in nephron pattern are associated with congenital anomalies of the kidney and urinary tract (the leading cause of renal disease in children). Tight regulation of apoptosis is required in normal renal morphogenesis, although many questions remain regarding the regulation of apoptosis by genetic, epigenetic, and environmental factors, in addition to the functional requirement of different components of the apoptotic pathway.