Nephron formation adopts a novel spatial topology at cessation of nephrogenesis

Jamming of nephron-forming niches in the developing mouse kidney creates cyclical mechanical stresses

Urinary collecting tubules form during kidney embryogenesis through the branching of the ureteric bud epithelium. A travelling mesenchyme niche of nephron progenitor cells caps each branching ureteric bud tip. These 'tip domain' niches pack more closely over developmental time and their number relates to nephron endowment at birth. Yet, how the crowded tissue environment impacts niche number and cell decision-making remains unclear. Here, through experiments and mathematical modelling, we show that niche packing conforms to physical limitations imposed by kidney curvature. We relate packing geometries to rigidity theory to predict a stiffening transition starting at embryonic day 15 in the mouse, validated by micromechanical analysis. Using a method to estimate tip domain 'ages' relative to their most recent branch events, we find that new niches overcome mechanical resistance as they branch and displace neighbours. This creates rhythmic mechanical stress in the niche. These findings expand our understanding of kidney development and inform engineering strategies for synthetic regenerative tissues.

Identification of countercurrent tubule-vessel arrangements in the early development of mouse kidney based on immunohistochemistry and computer-assisted 3D visualization

Nephron loop-vessel countercurrent arrangement in the medulla provides the structural basis for the formation of concentrated urine. To date, the morphogenesis of it and relevant water and solutes transportation has not been fully elucidated. In this study, with immunohistochemistry for aquaporins (AQP) and Na-K-2Cl co-transporter (NKCC2), as well as 3D visualization, we noticed in embryonic day 14.5 kidneys that the countercurrent arrangement of two pairs of loop-vessel was established as soon as the loop and vessel both extended into the medulla. One pair happened between descending limb and ascending vasa recta, the other occurred between thick ascending limb and descending vasa recta. Meanwhile, the immunohistochemical results showed that the limb and vessel expressing AQP-1 such as descending thick and thin limb and descending vasa recta was always accompanied with AQP-1 negative ascending vasa recta or capillaries and thick ascending limb, respectively. Moreover, the thick ascending limb expressing NKCC2 closely contacted with descending vasa recta without expressing NKCC2. As kidney developed, an increasing number of loop-vessels in countercurrent arrangement extended into the interstitium of the medulla. In addition, we observed that the AQP-2 positive ureteric bud and their branches were separated from those pairs of tubule-vessels by a relatively large and thin-walled veins or capillaries. Thus, the present study reveals that the loop-vessel countercurrent arrangement is formed at the early stage of nephrogenesis, which facilitates the efficient transportation of water and electrolytes to maintain the medullary osmolality and to form a concentrated urine.

A single-cell time-lapse of mouse prenatal development from gastrula to birth

The house mouse (Mus musculus) is an exceptional model system, combining genetic tractability with close evolutionary affinity to humans1,2. Mouse gestation lasts only 3 weeks, during which the genome orchestrates the astonishing transformation of a single-cell zygote into a free-living pup composed of more than 500 million cells. Here, to establish a global framework for exploring mammalian development, we applied optimized single-cell combinatorial indexing3 to profile the transcriptional states of 12.4 million nuclei from 83 embryos, precisely staged at 2- to 6-hour intervals spanning late gastrulation (embryonic day 8) to birth (postnatal day 0). From these data, we annotate hundreds of cell types and explore the ontogenesis of the posterior embryo during somitogenesis and of kidney, mesenchyme, retina and early neurons. We leverage the temporal resolution and sampling depth of these whole-embryo snapshots, together with published data4-8 from earlier timepoints, to construct a rooted tree of cell-type relationships that spans the entirety of prenatal development, from zygote to birth. Throughout this tree, we systematically nominate genes encoding transcription factors and other proteins as candidate drivers of the in vivo differentiation of hundreds of cell types. Remarkably, the most marked temporal shifts in cell states are observed within one hour of birth and presumably underlie the massive physiological adaptations that must accompany the successful transition of a mammalian fetus to life outside the womb.

Geometric effects position renal vesicles during kidney development

During kidney development, reciprocal signaling between the epithelium and the mesenchyme coordinates nephrogenesis with branching morphogenesis of the collecting ducts. The mechanism that positions the renal vesicles, and thus the nephrons, relative to the branching ureteric buds has remained elusive. By combining computational modeling and experiments, we show that geometric effects concentrate the key regulator, WNT9b, at the junctions between parent and daughter branches where renal vesicles emerge, even when uniformly expressed in the ureteric epithelium. This curvature effect might be a general paradigm to create non-uniform signaling in development.

Characterizing post-branching nephrogenesis in the neonatal rabbit

Human nephrogenesis ends prior to birth in term infants (34-36 week gestation), with most (60%) nephrons forming in late gestation in two post-branching nephrogenesis (PBN) periods: arcading and lateral branch nephrogenesis. Preterm infants, however, must execute PBN postnatally. Extreme prematurity is associated with low nephron counts. Identifying additional model(s) that undergo PBN postnatally will help support postnatal PBN in preterm infants. The rabbit exhibits longer postnatal nephrogenesis than the mouse but whether it forms nephrons through PBN has not been determined. We performed morphologic and immunohistological assessments of rabbit nephrogenesis from birth (post-conceptual day 31 or 32) to PC49 using H&E and antibodies against SIX1, SIX2, WT1, ZO-1, and JAG1 in the postnatal period. We performed 3D rendering of the nephrogenic niche to assess for PBN, and supplemented the staining with RNAScope to map the expression of Six1, Six2 (nephron progenitors, NPC), and Ret (ureteric bud tip) transcripts to determine the nephrogenic niche postnatal lifespan. Unlike the mouse, rabbit SIX2 disappeared from NPC before SIX1, resembling the human niche. Active nephrogenesis as defined by the presence of SIX1 + naïve NPC/tip population persisted only until PC35-36 (3-5 postnatal days). 3D morphologic assessments of the cortical nephrons identified an elongated tubule with attached glomeruli extending below the UB tip, consistent with PBN arcades, but not with lateral branch nephrogenesis. We conclude that the rabbit shows morphologic and molecular evidence of PBN arcades continuing postnatally for a shorter period than previously thought. The rabbit is the first non-primate expressing SIX1 in the progenitor population. Our findings suggest that studies of arcading in postnatal nephrogenic niche should be performed within the first 5 days of life in the rabbit.

A single-cell transcriptional timelapse of mouse embryonic development, from gastrula to pup

The house mouse, Mus musculus, is an exceptional model system, combining genetic tractability with close homology to human biology. Gestation in mouse development lasts just under three weeks, a period during which its genome orchestrates the astonishing transformation of a single cell zygote into a free-living pup composed of >500 million cells. Towards a global framework for exploring mammalian development, we applied single cell combinatorial indexing (sci-*) to profile the transcriptional states of 12.4 million nuclei from 83 precisely staged embryos spanning late gastrulation (embryonic day 8 or E8) to birth (postnatal day 0 or P0), with 2-hr temporal resolution during somitogenesis, 6-hr resolution through to birth, and 20-min resolution during the immediate postpartum period. From these data (E8 to P0), we annotate dozens of trajectories and hundreds of cell types and perform deeper analyses of the unfolding of the posterior embryo during somitogenesis as well as the ontogenesis of the kidney, mesenchyme, retina, and early neurons. Finally, we leverage the depth and temporal resolution of these whole embryo snapshots, together with other published data, to construct and curate a rooted tree of cell type relationships that spans mouse development from zygote to pup. Throughout this tree, we systematically nominate sets of transcription factors (TFs) and other genes as candidate drivers of the in vivo differentiation of hundreds of mammalian cell types. Remarkably, the most dramatic shifts in transcriptional state are observed in a restricted set of cell types in the hours immediately following birth, and presumably underlie the massive changes in physiology that must accompany the successful transition of a placental mammal to extrauterine life.

The developing murine kidney actively negotiates geometric packing conflicts to avoid defects

The physiological functions of several organs rely on branched epithelial tubule networks bearing specialized structures for secretion, gas exchange, or filtration. Little is known about conflicts in development between building enough tubules for adequate function and geometric constraints imposed by organ size. We show that the mouse embryonic kidney epithelium negotiates a physical packing conflict between increasing tubule tip numbers through branching and limited organ surface area. Through imaging of whole kidney explants, combined with computational and soft material modeling of tubule families, we identify six possible geometric packing phases, including two defective ones. Experiments in explants show that a radially oriented tension on tubule families is necessary and sufficient for them to switch to a vertical packing arrangement that increases surface tip density while avoiding defects. These results reveal developmental contingencies in response to physical limitations and create a framework for classifying congenital kidney defects.

Regulation of nephron progenitor cell lifespan and nephron endowment

Low nephron number - resulting, for example, from prematurity or developmental anomalies - is a risk factor for the development of hypertension, chronic kidney disease and kidney failure. Considerable interest therefore exists in the mechanisms that regulate nephron endowment and contribute to the premature cessation of nephrogenesis following preterm birth. The cessation of nephrogenesis in utero or shortly after birth is synchronized across multiple niches in all mammals, and is coupled with the exhaustion of nephron progenitor cells. Consequently, no nephrons are formed after the cessation of developmental nephrogenesis, and lifelong renal function therefore depends on the complement of nephrons generated during gestation. In humans, a tenfold variation in nephron endowment between individuals contributes to differences in susceptibility to kidney disease; however, the mechanisms underlying this variation are not yet clear. Salient advances in our understanding of environmental inputs, and of intrinsic molecular mechanisms that contribute to the regulation of cessation timing or nephron progenitor cell exhaustion, have the potential to inform interventions to enhance nephron endowment and improve lifelong kidney health for susceptible individuals.

Omics profiling identifies the regulatory functions of the MAPK/ERK pathway in nephron progenitor metabolism

Nephron endowment is defined by fetal kidney growth and crucially dictates renal health in adults. Defects in the molecular regulation of nephron progenitors contribute to only a fraction of reduced nephron mass cases, suggesting alternative causative mechanisms. The importance of MAPK/ERK activation in nephron progenitor maintenance has been previously demonstrated, and here, we characterized the metabolic consequences of MAPK/ERK deficiency. Liquid chromatography/mass spectrometry-based metabolomics profiling identified 42 reduced metabolites, of which 26 were supported by in vivo transcriptional changes in MAPK/ERK-deficient nephron progenitors. Among these, mitochondria, ribosome and amino acid metabolism, together with diminished pyruvate and proline metabolism, were the most affected pathways. In vitro cultures of mouse kidneys demonstrated a dosage-specific function for pyruvate in controlling the shape of the ureteric bud tip, a regulatory niche for nephron progenitors. In vivo disruption of proline metabolism caused premature nephron progenitor exhaustion through their accelerated differentiation in pyrroline-5-carboxylate reductases 1 (Pycr1) and 2 (Pycr2) double-knockout kidneys. Pycr1/Pycr2-deficient progenitors showed normal cell survival, indicating no changes in cellular stress. Our results suggest that MAPK/ERK-dependent metabolism functionally participates in nephron progenitor maintenance by monitoring pyruvate and proline biogenesis in developing kidneys.

Principles of human and mouse nephron development

The mechanisms underlying kidney development in mice and humans is an area of intense study. Insights into kidney organogenesis have the potential to guide our understanding of the origin of congenital anomalies and enable the assembly of genetic diagnostic tools. A number of studies have delineated signalling nodes that regulate positional identities and cell fates of nephron progenitor and precursor cells, whereas cross-species comparisons have markedly enhanced our understanding of conserved and divergent features of mammalian kidney organogenesis. Greater insights into the complex cellular movements that occur as the proximal-distal axis is established have challenged our understanding of nephron patterning and provided important clues to the elaborate developmental context in which human kidney diseases can arise. Studies of kidney development in vivo have also facilitated efforts to recapitulate nephrogenesis in kidney organoids in vitro, by providing a detailed blueprint of signalling events, cell movements and patterning mechanisms that are required for the formation of correctly patterned nephrons and maturation of physiologically functional apparatus that are responsible for maintaining human health.

Comparative whole-genome transcriptome analysis in renal cell populations reveals high tissue specificity of MAPK/ERK targets in embryonic kidney

Background

MAPK/ERK signaling is a well-known mediator of extracellular stimuli controlling intracellular responses to growth factors and mechanical cues. The critical requirement of MAPK/ERK signaling for embryonic stem cell maintenance is demonstrated, but specific functions in progenitor regulation during embryonic development, and in particular kidney development remain largely unexplored. We previously demonstrated MAPK/ERK signaling as a key regulator of kidney growth through branching morphogenesis and normal nephrogenesis where it also regulates progenitor expansion. Here, we performed RNA sequencing-based whole-genome expression analysis to identify transcriptional MAPK/ERK targets in two distinct renal populations: the ureteric bud epithelium and the nephron progenitors.

Results

Our analysis revealed a large number (5053) of differentially expressed genes (DEGs) in nephron progenitors and significantly less (1004) in ureteric bud epithelium, reflecting likely heterogenicity of cell types. The data analysis identified high tissue-specificity, as only a fraction (362) of MAPK/ERK targets are shared between the two tissues. Tissue-specific MAPK/ERK targets participate in the regulation of mitochondrial energy metabolism in nephron progenitors, which fail to maintain normal mitochondria numbers in the MAPK/ERK-deficient tissue. In the ureteric bud epithelium, a dramatic decline in progenitor-specific gene expression was detected with a simultaneous increase in differentiation-associated genes, which was not observed in nephron progenitors. Our experiments in the genetic model of MAPK/ERK deficiency provide evidence that MAPK/ERK signaling in the ureteric bud maintains epithelial cells in an undifferentiated state. Interestingly, the transcriptional targets shared between the two tissues studied are over-represented by histone genes, suggesting that MAPK/ERK signaling regulates cell cycle progression and stem cell maintenance through chromosome condensation and nucleosome assembly.

Conclusions

Using tissue-specific MAPK/ERK inactivation and RNA sequencing in combination with experimentation in embryonic kidneys, we demonstrate here that MAPK/ERK signaling maintains ureteric bud tip cells, suggesting a regulatory role in collecting duct progenitors. We additionally deliver new mechanistic information on how MAPK/ERK signaling regulates progenitor maintenance through its effects on chromatin accessibility and energy metabolism.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01309-z.

Growth control of the kidney

The functional mass of kidney tissue in an adult is an important determinant of human health. Kidney formation during development is an essential determinant of the final nephron endowment of the adult organ, and no evidence has been reported that mice or humans are able to generate new nephrons after the developmental period. Mechanisms controlling organ growth after development are essential to establish the final adult organ size. The potential for organ growth is maintained in adult life and the size of one kidney may be significantly increased by loss of the contralateral kidney. The mouse has provided a model system for investigators to critically explore genetic, cell biological, and hormonal control of developmental and juvenile kidney growth. This article reviews three basic aspects of kidney size regulation: (1) Mechanisms that control nephron formation and how these are altered by the cessation of nephrogenesis at the end of the developmental period. (2) Applicability of the general model for growth hormone-insulin like growth factor control to kidney growth both pre- and postnatally. (3) Commonalities between mechanisms of juvenile kidney growth and the compensatory growth that is stimulated in adult life by reduction of kidney mass. Understanding the mechanisms that determine set-points for cell numbers and size in the kidney may inform ongoing efforts to generate kidney tissue from stem cells.

DevKidCC allows for robust classification and direct comparisons of kidney organoid datasets

BACKGROUND

While single-cell transcriptional profiling has greatly increased our capacity to interrogate biology, accurate cell classification within and between datasets is a key challenge. This is particularly so in pluripotent stem cell-derived organoids which represent a model of a developmental system. Here, clustering algorithms and selected marker genes can fail to accurately classify cellular identity while variation in analyses makes it difficult to meaningfully compare datasets. Kidney organoids provide a valuable resource to understand kidney development and disease. However, direct comparison of relative cellular composition between protocols has proved challenging. Hence, an unbiased approach for classifying cell identity is required.

METHODS

The R package, scPred, was trained on multiple single cell RNA-seq datasets of human fetal kidney. A hierarchical model classified cellular subtypes into nephron, stroma and ureteric epithelial elements. This model, provided in the R package DevKidCC ( github.com/KidneyRegeneration/DevKidCC ), was then used to predict relative cell identity within published kidney organoid datasets generated using distinct cell lines and differentiation protocols, interrogating the impact of such variations. The package contains custom functions for the display of differential gene expression within cellular subtypes.

RESULTS

DevKidCC was used to directly compare between distinct kidney organoid protocols, identifying differences in relative proportions of cell types at all hierarchical levels of the model and highlighting variations in stromal and unassigned cell types, nephron progenitor prevalence and relative maturation of individual epithelial segments. Of note, DevKidCC was able to distinguish distal nephron from ureteric epithelium, cell types with overlapping profiles that have previously confounded analyses. When applied to a variation in protocol via the addition of retinoic acid, DevKidCC identified a consequential depletion of nephron progenitors.

CONCLUSIONS

The application of DevKidCC to kidney organoids reproducibly classifies component cellular identity within distinct single-cell datasets. The application of the tool is summarised in an interactive Shiny application, as are examples of the utility of in-built functions for data presentation. This tool will enable the consistent and rapid comparison of kidney organoid protocols, driving improvements in patterning to kidney endpoints and validating new approaches.

DevKidCC allows for robust classification and direct comparisons of kidney organoid datasets

Background

While single-cell transcriptional profiling has greatly increased our capacity to interrogate biology, accurate cell classification within and between datasets is a key challenge. This is particularly so in pluripotent stem cell-derived organoids which represent a model of a developmental system. Here, clustering algorithms and selected marker genes can fail to accurately classify cellular identity while variation in analyses makes it difficult to meaningfully compare datasets. Kidney organoids provide a valuable resource to understand kidney development and disease. However, direct comparison of relative cellular composition between protocols has proved challenging. Hence, an unbiased approach for classifying cell identity is required.

Methods

The R package, scPred, was trained on multiple single cell RNA-seq datasets of human fetal kidney. A hierarchical model classified cellular subtypes into nephron, stroma and ureteric epithelial elements. This model, provided in the R package DevKidCC (github.com/KidneyRegeneration/DevKidCC), was then used to predict relative cell identity within published kidney organoid datasets generated using distinct cell lines and differentiation protocols, interrogating the impact of such variations. The package contains custom functions for the display of differential gene expression within cellular subtypes.

Results

DevKidCC was used to directly compare between distinct kidney organoid protocols, identifying differences in relative proportions of cell types at all hierarchical levels of the model and highlighting variations in stromal and unassigned cell types, nephron progenitor prevalence and relative maturation of individual epithelial segments. Of note, DevKidCC was able to distinguish distal nephron from ureteric epithelium, cell types with overlapping profiles that have previously confounded analyses. When applied to a variation in protocol via the addition of retinoic acid, DevKidCC identified a consequential depletion of nephron progenitors.

Conclusions

The application of DevKidCC to kidney organoids reproducibly classifies component cellular identity within distinct single-cell datasets. The application of the tool is summarised in an interactive Shiny application, as are examples of the utility of in-built functions for data presentation. This tool will enable the consistent and rapid comparison of kidney organoid protocols, driving improvements in patterning to kidney endpoints and validating new approaches.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13073-022-01023-z.

Chromatin accessibility and microRNA expression in nephron progenitor cells during kidney development

Mammalian nephrons originate from a population of nephron progenitor cells, and changes in these cells' transcriptomes contribute to the cessation of nephrogenesis, an important determinant of nephron number. To characterize microRNA (miRNA) expression and identify putative cis-regulatory regions, we collected nephron progenitor cells from mouse kidneys at embryonic day 14.5 and postnatal day zero and assayed small RNA expression and transposase-accessible chromatin. We detect expression of 1104 miRNA (114 with expression changes), and 46,374 chromatin accessible regions (2103 with changes in accessibility). Genome-wide, our data highlight processes like cellular differentiation, cell migration, extracellular matrix interactions, and developmental signaling pathways. Furthermore, they identify new candidate cis-regulatory elements for Eya1 and Pax8, both genes with a role in nephron progenitor cell differentiation. Finally, we associate expression-changing miRNAs, including let-7-5p, miR-125b-5p, miR-181a-2-3p, and miR-9-3p, with candidate cis-regulatory elements and target genes. These analyses highlight new putative cis-regulatory loci for miRNA in nephron progenitors.

Regrow or Repair: An Update on Potential Regenerative Therapies for the Kidney

Fifteen years ago, this journal published a review outlining future options for regenerating the kidney. At that time, stem cell populations were being identified in multiple tissues, the concept of stem cell recruitment to a site of injury was of great interest, and the possibility of postnatal renal stem cells was growing in momentum. Since that time, we have seen the advent of human induced pluripotent stem cells, substantial advances in our capacity to both sequence and edit the genome, global and spatial transcriptional analysis down to the single-cell level, and a pandemic that has challenged our delivery of health care to all. This article will look back over this period of time to see how our view of kidney development, disease, repair, and regeneration has changed and envision a future for kidney regeneration and repair over the next 15 years.

Post-translational modifications by SIRT3 de-2-hydroxyisobutyrylase activity regulate glycolysis and enable nephrogenesis

Abnormal kidney development leads to lower nephron number, predisposing to renal diseases in adulthood. In embryonic kidneys, nephron endowment is dictated by the availability of nephron progenitors, whose self-renewal and differentiation require a relatively repressed chromatin state. More recently, NAD⁺-dependent deacetylase sirtuins (SIRTs) have emerged as possible regulators that link epigenetic processes to the metabolism. Here, we discovered a novel role for the NAD⁺-dependent deacylase SIRT3 in kidney development. In the embryonic kidney, SIRT3 was highly expressed only as a short isoform, with nuclear and extra-nuclear localisation. The nuclear SIRT3 did not act as deacetylase but exerted de-2-hydroxyisobutyrylase activity on lysine residues of histone proteins. Extra-nuclear SIRT3 regulated lysine 2-hydroxyisobutyrylation (Khib) levels of phosphofructokinase (PFK) and Sirt3 deficiency increased PFK Khib levels, inducing a glycolysis boost. This altered Khib landscape in Sirt3^−/− metanephroi was associated with decreased nephron progenitors, impaired nephrogenesis and a reduced number of nephrons. These data describe an unprecedented role of SIRT3 in controlling early renal development through the regulation of epigenetics and metabolic processes.

Progenitor translatome changes coordinated by Tsc1 increase perception of Wnt signals to end nephrogenesis

Mammalian nephron endowment is determined by the coordinated cessation of nephrogenesis in independent niches. Here we report that translatome analysis in Tsc1+/- nephron progenitor cells from mice with elevated nephron numbers reveals how differential translation of Wnt antagonists over agonists tips the balance between self-renewal and differentiation. Wnt agonists are poorly translated in young niches, resulting in an environment with low R-spondin and high Fgf20 promoting self-renewal. In older niches we find increased translation of Wnt agonists, including R-spondin and the signalosome-promoting Tmem59, and low Fgf20, promoting differentiation. This suggests that the tipping point for nephron progenitor exit from the niche is controlled by the gradual increase in stability and possibly clustering of Wnt/Fzd complexes in individual cells, enhancing the response to ureteric bud-derived Wnt9b inputs and driving synchronized differentiation. As predicted by these findings, removing one Rspo3 allele in nephron progenitors delays cessation and increases nephron numbers in vivo.

Regulation of Solute Carriers OCT2 and OAT1/3 in the Kidney: A Phylogenetic, Ontogenetic and Cell Dynamic Perspective

Over the course of more than 500 million years, the kidneys have undergone a remarkable evolution from primitive nephric tubes to intricate filtration-reabsorption systems that maintain homeostasis and remove metabolic end products from the body. The evolutionarily conserved solute carriers Organic Cation Transporter 2 (OCT2), and Organic Anion Transporters 1 and 3 (OAT1/3) coordinate the active secretion of a broad range of endogenous and exogenous substances, many of which accumulate in the blood of patients with kidney failure despite dialysis. Harnessing OCT2 and OAT1/3 through functional preservation or regeneration could alleviate the progression of kidney disease. Additionally, it would improve current in vitro test models that lose their expression in culture. With this review, we explore OCT2 and OAT1/3 regulation using different perspectives: phylogenetic, ontogenetic and cell dynamic. Our aim is to identify possible molecular targets to both help prevent or compensate for the loss of transport activity in patients with kidney disease, and to enable endogenous OCT2 and OAT1/3 induction in vitro in order to develop better models for drug development.

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.

Renal pathology in a mouse model of severe Spinal Muscular Atrophy is associated with downregulation of Glial Cell-Line Derived Neurotrophic Factor (GDNF)

Spinal muscular atrophy (SMA) occurs as a result of cell-ubiquitous depletion of the essential survival motor neuron (SMN) protein. Characteristic disease pathology is driven by a particular vulnerability of the ventral motor neurons of the spinal cord to decreased SMN. Perhaps not surprisingly, many other organ systems are also impacted by SMN depletion. The normal kidney expresses very high levels of SMN protein, equivalent to those found in the nervous system and liver, and levels are dramatically lowered by ~90-95% in mouse models of SMA. Taken together, these data suggest that renal pathology may be present in SMA. We have addressed this using an established mouse model of severe SMA. Nephron number, as assessed by gold standard stereological techniques, was significantly reduced. In addition, morphological assessment showed decreased renal vasculature, particularly of the glomerular capillary knot, dysregulation of nephrin and collagen IV, and ultrastructural changes in the trilaminar filtration layers of the nephron. To explore the molecular drivers underpinning this process, we correlated these findings with quantitative PCR measurements and protein analyses of glial cell-line-derived neurotrophic factor, a crucial factor in ureteric bud branching and subsequent nephron development. Glial cell-line-derived neurotrophic factor levels were significantly reduced at early stages of disease in SMA mice. Collectively, these findings reveal significant renal pathology in a mouse model of severe SMA, further reinforcing the need to develop and administer systemic therapies for this neuromuscular disease.

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.

The Rhesus Macaque Serves As a Model for Human Lateral Branch Nephrogenesis

BACKGROUND

Most nephrons are added in late gestation. Truncated extrauterine nephrogenesis in premature infants results in fewer nephrons and significantly increased risk for CKD in adulthood. To overcome the ethical and technical difficulties associated with studies of late-gestation human fetal kidney development, third-trimester rhesus macaques served as a model to understand lateral branch nephrogenesis (LBN) at the molecular level.

METHODS

Immunostaining and 3D rendering assessed morphology. Single-cell (sc) and single-nucleus (sn) RNA-Seq were performed on four cortically enriched fetal rhesus kidneys of 129-131 days gestational age (GA). An integrative bioinformatics strategy was applied across single-cell modalities, species, and time. RNAScope validation studies were performed on human archival tissue.

RESULTS

Third-trimester rhesus kidney undergoes human-like LBN. scRNA-Seq of 23,608 cells revealed 37 transcriptionally distinct cell populations, including naïve nephron progenitor cells (NPCs), with the prior noted marker genes CITED1, MEOX1, and EYA1 (c25). These same populations and markers were reflected in snRNA-Seq of 5972 nuclei. Late-gestation rhesus NPC markers resembled late-gestation murine NPC, whereas early second-trimester human NPC markers aligned to midgestation murine NPCs. New, age-specific rhesus NPCs (SHISA8) and ureteric buds (POU3F4 and TWIST) predicted markers were verified in late-gestation human archival samples.

CONCLUSIONS

Rhesus macaque is the first model of bona fide LBN, enabling molecular studies of late gestation, human-like nephrogenesis. These molecular findings support the hypothesis that aging nephron progenitors have a distinct molecular signature and align to their earlier human counterparts, with unique markers highlighting LBN-specific progenitor maturation.

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.

Estrogens and development of the rete testis, efferent ductules, epididymis and vas deferens

Estrogen has always been considered the female hormone and testosterone the male hormone. However, estrogen's presence in the testis and deleterious effects of estrogen treatment during development have been known for nearly 90 years, long before estrogen receptors (ESRs) were discovered. Eventually it was learned that testes actually synthesize high levels of estradiol (E2) and sequester high concentrations in the reproductive tract lumen, which seems contradictory to the overwhelming number of studies showing reproductive pathology following exogenous estrogen exposures. For too long, the developmental pathology of estrogen has dominated our thinking, even resulting in the "estrogen hypothesis" as related to the testicular dysgenesis syndrome. However, these early studies and the development of an Esr1 knockout mouse led to a deluge of research into estrogen's potential role in and disruption of development and function of the male reproductive system. What is new is that estrogen action in the male cannot be divorced from that of androgen. This paper presents what is known about components of the estrogen pathway, including its synthesis and target receptors, and the need to achieve a balance between androgen- and estrogen-action in male reproductive tract differentiation and adult functions. The review focuses on what is known regarding development of the male reproductive tract, from the rete testis to the vas deferens, and examines the expression of estrogen receptors and presence of aromatase in the male reproductive system, traces the evidence provided by estrogen-associated knockout and transgenic animal models and discusses the effects of fetal and postnatal exposures to estrogens. Hopefully, there will be enough here to stimulate discussions and new investigations of the androgen:estrogen balance that seems to be essential for development of the male reproductive tract.

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.

Molecular detection of maturation stages in the developing kidney

Recent advances in stem cell biology have enabled the generation of kidney organoids in vitro, and further maturation of these organoids is observed after experimental transplantation. However, the current organoids remain immature and their precise maturation stages are difficult to determine because of limited information on developmental stage-dependent gene expressions in the kidney in vivo. To establish relevant molecular coordinates, we performed single-cell RNA sequencing (scRNA-seq) on developing kidneys at different stages in the mouse. By selecting genes that exhibited upregulation at birth compared with embryonic day 15.5 as well as cell lineage-specific expression, we generated gene lists correlated with developmental stages in individual cell lineages. Application of these lists to transplanted embryonic kidneys revealed that most cell types, other than the collecting ducts, exhibited similar maturation to kidneys at the neonatal stage in vivo, revealing non-synchronous maturation across the cell lineages. Thus, our scRNA-seq data can serve as useful molecular coordinates to assess the maturation of developing kidneys and eventually of kidney organoids.

AP-2β/KCTD1 Control Distal Nephron Differentiation and Protect against Renal Fibrosis

The developmental mechanisms that orchestrate differentiation of specific nephron segments are incompletely understood, and the factors that maintain their terminal differentiation after nephrogenesis remain largely unknown. Here, the transcription factor AP-2β is shown to be required for the differentiation of distal tubule precursors into early stage distal convoluted tubules (DCTs) during nephrogenesis. In contrast, its downstream target KCTD1 is essential for terminal differentiation of early stage DCTs into mature DCTs, and impairment of their terminal differentiation owing to lack of KCTD1 leads to a severe salt-losing tubulopathy. Moreover, sustained KCTD1 activity in the adult maintains mature DCTs in this terminally differentiated state and prevents renal fibrosis by repressing β-catenin activity, whereas KCTD1 deficiency leads to severe renal fibrosis. Thus, the AP-2β/KCTD1 axis links a developmental pathway in the nephron to the induction and maintenance of terminal differentiation of DCTs that actively prevents their de-differentiation in the adult and protects against renal fibrosis.

Distinct Functional Requirements for Podocalyxin in Immature and Mature Podocytes Reveal Mechanisms of Human Kidney Disease

Dominant and recessive mutations in podocalyxin (PODXL) are associated with human kidney disease. Interestingly, some PODXL mutations manifest as anuria while others are associated with proteinuric kidney disease. PODXL heterozygosity is associated with adult-onset kidney disease and podocalyxin shedding into the urine is a common biomarker of a variety nephrotic syndromes. It is unknown, however, how various lesions in PODXL contribute to these disparate disease pathologies. Here we generated two mouse stains: one that deletes Podxl in developmentally mature podocytes (Podxl^∆Pod) and a second that is heterozygous for podocalyxin in all tissues (Podxl^+/-). We used histologic and ultrastructural analyses, as well as clinical chemistry assays to evaluate kidney development and function in these strains. In contrast to null knockout mice (Podxl^-/-), which die shortly after birth from anuria and hypertension, Podxl^∆Pod mice develop an acute congenital nephrotic syndrome characterized by focal segmental glomerulosclerosis (FSGS) and proteinuria. Podxl^+/- mice, in contrast, have a normal lifespan, and fail to develop kidney disease under normal conditions. Intriguingly, although wild-type C57Bl/6 mice are resistant to puromycin aminonucleoside (PA)-induced nephrosis (PAN), Podxl^+/- mice are highly sensitive and PA induces severe proteinuria and collapsing FSGS. In summary, we find that the developmental timepoint at which podocalyxin is ablated (immature vs. mature podocytes) has a profound effect on the urinary phenotype due to its critical roles in both the formation and the maintenance of podocyte ultrastructure. In addition, Podxl^∆Pod and Podxl^+/- mice offer powerful new mouse models to evaluate early biomarkers of proteinuric kidney disease and to test novel therapeutics.

The core SWI/SNF catalytic subunit Brg1 regulates nephron progenitor cell proliferation and differentiation

Chromatin-remodeling complexes play critical roles in establishing gene expression patterns in response to developmental signals. How these epigenetic regulators determine the fate of progenitor cells during development of specific organs is not well understood. We found that genetic deletion of Brg1 (Smarca4), the core enzymatic protein in SWI/SNF, in nephron progenitor cells leads to severe renal hypoplasia. Nephron progenitor cells were depleted in Six2-Cre, Brg1^flx/flx mice due to reduced cell proliferation. This defect in self-renewal, together with impaired differentiation resulted in a profound nephron deficit in Brg1 mutant kidneys. Sall1, a transcription factor that is required for expansion and maintenance of nephron progenitors, associates with SWI/SNF. Brg1 and Sall1 bind promoters of many progenitor cell genes and regulate expression of key targets that promote their proliferation.

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.

Shaping of the nephron - a complex, vulnerable, and poorly explored backdrop for noxae impairing nephrogenesis in the fetal human kidney

Background

The impairment of nephrogenesis is caused by noxae, all of which are significantly different in molecular composition. These can cause an early termination of nephron development in preterm and low birth weight babies resulting in oligonephropathy. For the fetal human kidney, there was no negative effect reported on the early stages of nephron anlage such as the niche, pretubular aggregate, renal vesicle, or comma-shaped body. In contrast, pathological alterations were identified on subsequently developing S-shaped bodies and glomeruli. While the atypical glomeruli were closely analyzed, the S-shaped bodies and the pre-stages received little attention even though passing the process of nephron shaping. Since micrographs and an explanation about this substantial developmental period were missing, the shaping of the nephron in the fetal human kidney during the phase of late gestation was recorded from a microanatomical point of view.

Results

The nephron shaping starts with the primitive renal vesicle, which is still part of the pretubular aggregate at this point. Then, during extension of the renal vesicle, a complex separation is observed. The medial part of its distal pole is fixed on the collecting duct ampulla, while the lateral part remains connected with the pretubular aggregate via a progenitor cell strand. A final separation occurs, when the extended renal vesicle develops into the comma-shaped body. Henceforth, internal epithelial folding generates the tubule and glomerulus anlagen. Arising clefts at the medial and lateral aspect indicate an asymmetrical expansion of the S-shaped body. This leads to development of the glomerulus at the proximal pole, whereas in the center and at the distal pole, it results in elongation of the tubule segments.

Conclusions

The present investigation deals with the shaping of the nephron in the fetal human kidney. In this important developmental phase, the positioning, orientation, and folding of the nephron occur. The demonstration of previously unknown morphological details supports the search for traces left by the impairment of nephrogenesis, enables to refine the assessment in molecular pathology, and provides input for the design of therapeutic concepts prolonging nephrogenesis.

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.

In Search of Imprints Left by the Impairment of Nephrogenesis

Clinical aspects dealing with the impairment of nephrogenesis in preterm and low birth weight babies were intensely researched. In this context it was shown that quite different noxae can harm nephron formation, and that the morphological damage in the fetal kidney is rather complex. Some pathological findings show that the impairment leads to changes in developing glomeruli that are restricted to the maturation zone of the outer cortex in the fetal human kidney. Other data show also imprints on the stages of nephron anlage including the niche, the pretubular aggregate, the renal vesicle, and comma- and S-shaped bodies located in the overlying nephrogenic zone of the rodent and human kidneys. During our investigations it was noticed that the stages of nephron anlage in the fetal human kidney during the phase of late gestation have not been described in detail. To contribute, these stages were recorded along with corresponding images. The initial nephron formation in the rodent kidney served as a reference. Finally, the known imprints left by the impairment in both specimens were listed and discussed. In sum, the relatively paucity of data on nephron formation in the fetal human kidney during the late phase of gestation is a call to start with intense research so that concepts for a therapeutic prolongation of nephrogenesis can be designed.

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.

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.

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.

Dynamics of Organic Anion Transporter-Mediated Tubular Secretion during Postnatal Human Kidney Development and Maturation

BACKGROUND AND OBJECTIVES

The neonatal and juvenile human kidney can be exposed to a variety of potentially toxic drugs (e.g., nonsteroidal anti-inflammatory drugs, antibiotics, antivirals, diuretics), many of which are substrates of the kidney organic anion transporters, OAT1 (SLC22A6, originally NKT) and OAT3 (SLC22A8). Despite the immense concern about the consequences of drug toxicity in this vulnerable population, the developmental regulation of OATs in the immature postnatal kidney is poorly understood.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Recognizing that today it is difficult to obtain rich data on neonatal kidney handling of OAT probes due to technical, logistic, and ethical considerations, multiple older physiologic studies that used the prototypical organic anion substrate para-aminohippurate (PAH) were reanalyzed in order to provide a quantitative description of OAT-mediated tubular secretion across the pediatric age continuum. Parametric and semiparametric models were evaluated for kidney function outcome variables of interest (maximum tubular secretory capacity of PAH [Tm_PAH], effective renal plasma flow [ERPF], and GFR).

RESULTS

Data from 119 neonates, infants, and children ranging in age from 1 day to 11.8 years were used to fit Tm_PAH, ERPF, and GFR as functions of postnatal age. Tm_PAH is low in the immediate postnatal period and increases markedly after birth, reaching 50% of the adult value (80 mg/min) at 8.3 years of age. During the first 2 years of life, Tm_PAH is lower than that of GFR when viewed as the fraction of the adult value.

CONCLUSIONS

During postnatal human kidney development, proximal tubule secretory function-as measured using PAH, a surrogate for OAT-mediated secretion of organic anion drugs, metabolites, and toxins-is low initially but increases rapidly. Despite developmental differences between species, this overall pattern is roughly consistent with animal studies. The human data raise the possibility that the acquisition of tubular secretory function may not closely parallel glomerular filtration.

Reporter-based fate mapping in human kidney organoids confirms nephron lineage relationships and reveals synchronous nephron formation

Nephron formation continues throughout kidney morphogenesis in both mice and humans. Lineage tracing studies in mice identified a self-renewing Six2-expressing nephron progenitor population able to give rise to the full complement of nephrons throughout kidney morphogenesis. To investigate the origin of nephrons within human pluripotent stem cell-derived kidney organoids, we performed a similar fate-mapping analysis of the SIX2-expressing lineage in induced pluripotent stem cell (iPSC)-derived kidney organoids to explore the feasibility of investigating lineage relationships in differentiating iPSCs in vitro Using CRISPR/Cas9 gene-edited lineage reporter lines, we show that SIX2-expressing cells give rise to nephron epithelial cell types but not to presumptive ureteric epithelium. The use of an inducible (CreERT2) line revealed a declining capacity for SIX2⁺ cells to contribute to nephron formation over time, but retention of nephron-forming capacity if provided an exogenous WNT signal. Hence, while human iPSC-derived kidney tissue appears to maintain lineage relationships previously identified in developing mouse kidney, unlike the developing kidney in vivo, kidney organoids lack a nephron progenitor niche capable of both self-renewal and ongoing nephrogenesis.

Direct reprogramming to human nephron progenitor-like cells using inducible piggyBac transposon expression of SNAI2-EYA1-SIX1

All nephrons in the mammalian kidney arise from a transient nephron progenitor population that is lost close to the time of birth. The generation of new nephron progenitors and their maintenance in culture are central to the success of kidney regenerative strategies. Using a lentiviral screening approach, we previously generated a human induced nephron progenitor-like state in vitro using a pool of six transcription factors. Here, we sought to develop a more efficient approach for direct reprogramming of human cells that could be applied in vivo. PiggyBac transposons are a non-viral integrating gene delivery system that is suitable for in vivo use and allows for simultaneous delivery of multiple genes. Using an inducible piggyBac transposon system, we optimized a protocol for the direct reprogramming of HK2 cells to induced nephron progenitor-like cells with expression of only 3 transcription factors (SNAI2, EYA1, and SIX1). Culture in conditions supportive of the nephron progenitor state further increased the expression of nephron progenitor genes. The refined protocol was then applied to primary human renal epithelial cells, which integrated into developing nephron structures in vitro and in vivo. Such inducible reprogramming to nephron progenitor-like cells could facilitate direct cellular reprogramming for kidney regeneration.

Nephron progenitor commitment is a stochastic process influenced by cell migration

Progenitor self-renewal and differentiation is often regulated by spatially restricted cues within a tissue microenvironment. Here, we examine how progenitor cell migration impacts regionally induced commitment within the nephrogenic niche in mice. We identify a subset of cells that express Wnt4, an early marker of nephron commitment, but migrate back into the progenitor population where they accumulate over time. Single cell RNA-seq and computational modelling of returning cells reveals that nephron progenitors can traverse the transcriptional hierarchy between self-renewal and commitment in either direction. This plasticity may enable robust regulation of nephrogenesis as niches remodel and grow during organogenesis.

DNA Methyltransferase 1 Controls Nephron Progenitor Cell Renewal and Differentiation

BACKGROUND

Nephron number is a major determinant of long-term renal function and cardiovascular risk. Observational studies suggest that maternal nutritional and metabolic factors during gestation contribute to the high variability of nephron endowment. However, the underlying molecular mechanisms have been unclear.

METHODS

We used mouse models, including DNA methyltransferase (Dnmt1, Dnmt3a, and Dnmt3b) knockout mice, optical projection tomography, three-dimensional reconstructions of the nephrogenic niche, and transcriptome and DNA methylation analysis to characterize the role of DNA methylation for kidney development.

RESULTS

We demonstrate that DNA hypomethylation is a key feature of nutritional kidney growth restriction in vitro and in vivo, and that DNA methyltransferases Dnmt1 and Dnmt3a are highly enriched in the nephrogenic zone of the developing kidneys. Deletion of Dnmt1 in nephron progenitor cells (in contrast to deletion of Dnmt3a or Dnm3b) mimics nutritional models of kidney growth restriction and results in a substantial reduction of nephron number as well as renal hypoplasia at birth. In Dnmt1-deficient mice, optical projection tomography and three-dimensional reconstructions uncovered a significant reduction of stem cell niches and progenitor cells. RNA sequencing analysis revealed that global DNA hypomethylation interferes in the progenitor cell regulatory network, leading to downregulation of genes crucial for initiation of nephrogenesis, Wt1 and its target Wnt4. Derepression of germline genes, protocadherins, Rhox genes, and endogenous retroviral elements resulted in the upregulation of IFN targets and inhibitors of cell cycle progression.

CONCLUSIONS

These findings establish DNA methylation as a key regulatory event of prenatal renal programming, which possibly represents a fundamental link between maternal nutritional factors during gestation and reduced nephron number.

Wnt11 directs nephron progenitor polarity and motile behavior ultimately determining nephron endowment

A normal endowment of nephrons in the mammalian kidney requires a balance of nephron progenitor self-renewal and differentiation throughout development. Here, we provide evidence for a novel action of ureteric branch tip-derived Wnt11 in progenitor cell organization and interactions within the nephrogenic niche, ultimately determining nephron endowment. In Wnt11 mutants, nephron progenitors dispersed from their restricted niche, intermixing with interstitial progenitors. Nephron progenitor differentiation was accelerated, kidneys were significantly smaller, and the nephron progenitor pool was prematurely exhausted, halving the final nephron count. Interestingly, RNA-seq revealed no significant differences in gene expression. Live imaging of nephron progenitors showed that in the absence of Wnt11 they lose stable attachments to the ureteric branch tips, continuously detaching and reattaching. Further, the polarized distribution of several markers within nephron progenitors is disrupted. Together these data highlight the importance of Wnt11 signaling in directing nephron progenitor behavior which determines a normal nephrogenic program.

Changes in cell fate determine the regenerative and functional capacity of the developing kidney before and after release of obstruction

Congenital obstructive nephropathy is a major cause of chronic kidney disease (CKD) in children. The contribution of changes in the identity of renal cells to the pathology of obstructive nephropathy is poorly understood. Using a partial unilateral ureteral obstruction (pUUO) model in genetically modified neonatal mice, we traced the fate of cells derived from the renal stroma, cap mesenchyme, ureteric bud (UB) epithelium, and podocytes using Foxd1Cre, Six2Cre, HoxB7Cre, and Podocyte.Cre mice respectively, crossed with double fluorescent reporter (membrane-targetted tandem dimer Tomato (mT)/membrane-targetted GFP (mG)) mice. Persistent obstruction leads to a significant loss of tubular epithelium, rarefaction of the renal vasculature, and decreased renal blood flow (RBF). In addition, Forkhead Box D1 (Foxd1)-derived pericytes significantly expanded in the interstitial space, acquiring a myofibroblast phenotype. Degeneration of Sine Oculis Homeobox Homolog 2 (Six2) and HoxB7-derived cells resulted in significant loss of glomeruli, nephron tubules, and collecting ducts. Surgical release of obstruction resulted in striking regeneration of tubules, arterioles, interstitium accompanied by an increase in blood flow to the level of sham animals. Contralateral kidneys with remarkable compensatory response to kidney injury showed an increase in density of arteriolar branches. Deciphering the mechanisms involved in kidney repair and regeneration post relief of obstruction has potential therapeutic implications for infants and children and the growing number of adults suffering from CKD.

Regulation of Renal Differentiation by Trophic Factors

Classically, trophic factors are considered as proteins which support neurons in their growth, survival, and differentiation. However, most neurotrophic factors also have important functions outside of the nervous system. Especially essential renal growth and differentiation regulators are glial cell line-derived neurotrophic factor (GDNF), bone morphogenetic proteins (BMPs), and fibroblast growth factors (FGFs). Here we discuss how trophic factor-induced signaling contributes to the control of ureteric bud (UB) branching morphogenesis and to maintenance and differentiation of nephrogenic mesenchyme in embryonic kidney. The review includes recent advances in trophic factor functions during the guidance of branching morphogenesis and self-renewal versus differentiation decisions, both of which dictate the control of kidney size and nephron number. Creative utilization of current information may help better recapitulate renal differentiation in vitro, but it is obvious that significantly more basic knowledge is needed for development of regeneration-based renal therapies.

Single-cell analysis of progenitor cell dynamics and lineage specification in the human fetal kidney

The mammalian kidney develops through reciprocal interactions between the ureteric bud and the metanephric mesenchyme to give rise to the entire collecting system and the nephrons. Most of our knowledge of the developmental regulators driving this process arises from the study of gene expression and functional genetics in mice and other animal models. In order to shed light on human kidney development, we have used single-cell transcriptomics to characterize gene expression in different cell populations, and to study individual cell dynamics and lineage trajectories during development. Single-cell transcriptome analyses of 6414 cells from five individual specimens identified 11 initial clusters of specific renal cell types as defined by their gene expression profile. Further subclustering identifies progenitors, and mature and intermediate stages of differentiation for several renal lineages. Other lineages identified include mesangium, stroma, endothelial and immune cells. Novel markers for these cell types were revealed in the analysis, as were components of key signaling pathways driving renal development in animal models. Altogether, we provide a comprehensive and dynamic gene expression profile of the developing human kidney at the single-cell level.

Summary: New markers for specific cell types in the developing human kidney are identified and computational approaches infer developmental trajectories and interrogate the complex network of signaling pathways and cellular transitions.

Kidney Nephron Determination

The nephron is a multifunctional filtration device equipped with an array of sophisticated sensors. For appropriate physiological function in the human and mouse, nephrons must be stereotypically arrayed in large numbers, and this essential structural property that defines the kidney is determined during its fetal development. This review explores the process of nephron determination in the fetal kidney, providing an overview of the foundational literature in the field as well as exploring new developments in this dynamic research area. Mechanisms that ensure that a large number of nephrons can be formed from a small initial number of progenitor cells are central to this process, and the question of how the nephron progenitor cell population balances epithelial differentiation with renewal in the progenitor state is a subject of particular interest. Key growth factor signaling pathways and transcription factor networks are discussed.

Nephron progenitor cell commitment: Striking the right balance

The filtering component of the kidney, the nephron, arises from a single progenitor population. These nephron progenitor cells (NPCs) both self-renew and differentiate throughout the course of kidney development ensuring sufficient nephron endowment. An appropriate balance of these processes must be struck as deficiencies in nephron numbers are associated with hypertension and kidney disease. This review will discuss the mechanisms and molecules supporting NPC maintenance and differentiation. A focus on recent work will highlight new molecular insights into NPC regulation and their dynamic behavior in both space and time.