Fate mapping using Cited1-CreERT2 mice demonstrates that the cap mesenchyme contains self-renewing progenitor cells and gives rise exclusively to nephronic epithelia

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.

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.

Disabling of nephrogenesis in porcine embryos via CRISPR/Cas9-mediated SIX1 and SIX4 gene targeting

SIX1 and SIX4 genes play critical roles in kidney development. We evaluated the effect of these genes on pig kidney development by generating SIX1^-/- and SIX1^-/- /SIX4^-/- pig foetuses using CRISPR/Cas9 and somatic cell nuclear transfer. We obtained 3 SIX1^-/- foetuses and 16 SIX1^-/- /SIX4^-/- foetuses at different developmental stages. The SIX1^-/- foetuses showed a migration block of the left kidney and a smaller size for both kidneys. The ureteric bud failed to form the normal branching and collecting system. Abnormal expressions of kidney development-related genes (downregulation of PAX2, PAX8, and BMP4 and upregulation of EYA1 and SALL1) were also observed in SIX1^-/- foetal kidneys and confirmed in vitro in porcine kidney epithelial cells (PK15) following SIX1 gene deletion. The SIX1^-/- /SIX4^-/- foetuses exhibited more severe phenotypes, with most foetuses showing retarded development at early stages of gestation. The kidney developed only to the initial stage of metanephros formation. These results demonstrated that SIX1 and SIX4 are key genes for porcine metanephros development. The creation of kidney-deficient porcine foetuses provides a platform for generating human kidneys inside pigs using blastocyst complementation.

Long-Term Culture of Nephron Progenitor Cells Ex Vivo

Nephrons differentiate from the cap mesenchyme of the fetal kidney. Nephron progenitor cells that populate the cap mesenchyme efficiently balance self-renewal and epithelial differentiation to enable repeated rounds of nephron formation during development. Here we describe a method to isolate and propagate these cells from the embryonic mouse kidney. Using this method, nephron progenitor cells from a single litter of mice can be propagated to hundreds of millions of cells that express appropriate markers of the undifferentiated state and retain epithelial differentiation capacity in vitro.

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.

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.

Loss of Dis3l2 partially phenocopies Perlman syndrome in mice and results in up-regulation of Igf2 in nephron progenitor cells

Loss of function of the DIS3L2 exoribonuclease is associated with Wilms tumor and the Perlman congenital overgrowth syndrome. Here, Hunter et al.’s analysis of Dis3l2-null nephron progenitor cells reveals up-regulation of Igf2, a growth-promoting gene strongly associated with Wilms tumorigenesis.

Loss of function of the DIS3L2 exoribonuclease is associated with Wilms tumor and the Perlman congenital overgrowth syndrome. LIN28, a Wilms tumor oncoprotein, triggers the DIS3L2-mediated degradation of the precursor of let-7, a microRNA that inhibits Wilms tumor development. These observations have led to speculation that DIS3L2-mediated tumor suppression is attributable to let-7 regulation. Here we examine new DIS3L2-deficient cell lines and mouse models, demonstrating that DIS3L2 loss has no effect on mature let-7 levels. Rather, analysis of Dis3l2-null nephron progenitor cells, a potential cell of origin of Wilms tumors, reveals up-regulation of Igf2, a growth-promoting gene strongly associated with Wilms tumorigenesis. These findings nominate a new potential mechanism underlying the pathology associated with DIS3L2 deficiency.

Highly tamoxifen-inducible principal cell-specific Cre mice with complete fidelity in cell specificity and no leakiness

An ideal inducible system should be cell specific and have absolutely no background recombination without induction (i.e., no leakiness), a high recombination rate after induction, and complete fidelity in cell specificity (i.e., restricted recombination exclusively in cells where the driver gene is expressed). However, such an ideal mouse model remains unavailable for collecting duct research. Here, we report a mouse model that meets these criteria. In this model, a cassette expressing ERT2CreERT2 ( ECE) is inserted at the ATG of the endogenous Aqp2 locus to disrupt Aqp2 function and to express ECE under the control of the Aqp2 promoter. The resulting allele is named Aqp2ECE. There was no indication of a significant impact of disruption of a copy of Aqp2 on renal function and blood pressure control in adult Aqp2ECE/+ heterozygotes. Without tamoxifen, Aqp2ECE did not activate a Cre-dependent red fluorescence protein (RFP) reporter in adult kidneys. A single injection of tamoxifen (2 mg) to adult mice enabled Aqp2ECE to induce robust RFP expression in the whole kidney 24 h postinjection, with the highest recombination efficiency of 95% in the inner medulla. All RFP-labeled cells expressed principal cell markers (Aqp2 and Aqp3), but not intercalated cell markers (V-ATPase B1B2, and carbonic anhydrase II). Hence, Aqp2ECE confers principal cell-specific tamoxifen-inducible recombination with absolutely no leakiness, high inducibility, and complete fidelity in cell specificity, which should be an important tool for temporospatial control of target genes in the principal cells and for Aqp2+ lineage tracing in adult mice.

Reduced Abd-B Hox function during kidney development results in lineage infidelity

Hox genes can function as key drivers of segment identity, with Hox mutations in Drosophila often resulting in dramatic homeotic transformations. In addition, however, they can serve other essential functions. In mammals, the study of Hox gene roles in development is complicated by the presence of four Hox clusters with a total of 39 genes showing extensive functional overlap. In this study, in order to better understand shared core Hox functions, we examined kidney development in mice with frameshift mutations of multiple Abd-B type Hox genes. The resulting phenotypes included dramatically reduced branching morphogenesis of the ureteric bud, premature depletion of nephron progenitors and abnormal development of the stromal compartment. Most unexpected, however, we also observed a cellular level lineage infidelity in nephron segments. Scattered cells within the proximal tubules, for example, expressed genes normally expressed only in collecting ducts. Multiple combinations of inappropriate nephron segment specific marker expression were found. In some cases, cells within a tubule showed incorrect identity, while in other cases cells showed ambiguous character, with simultaneous expression of genes associated with more than one nephron segment. These results give evidence that Hox genes have an overlapping core function at the cellular level in driving and/or maintaining correct differentiation decisions.

Conserved and Divergent Molecular and Anatomic Features of Human and Mouse Nephron Patterning

The nephron is the functional unit of the kidney, but the mechanism of nephron formation during human development is unclear. We conducted a detailed analysis of nephron development in humans and mice by immunolabeling, and we compared human and mouse nephron patterning to describe conserved and divergent features. We created protein localization maps that highlight the emerging patterns along the proximal-distal axis of the developing nephron and benchmark expectations for localization of functionally important transcription factors, which revealed unanticipated cellular diversity. Moreover, we identified a novel nephron subdomain marked by Wnt4 expression that we fate-mapped to the proximal mature nephron. Significant conservation was observed between human and mouse patterning. We also determined the time at which markers for mature nephron cell types first emerge-critical data for the renal organoid field. These findings have conceptual implications for the evolutionary processes driving the diversity of mammalian organ systems. Furthermore, these findings provide practical insights beyond those gained with mouse and rat models that will guide in vitro efforts to harness the developmental programs necessary to build human kidney structures.

Conserved and Divergent Features of Mesenchymal Progenitor Cell Types within the Cortical Nephrogenic Niche of the Human and Mouse Kidney

Cellular interactions among nephron, interstitial, and collecting duct progenitors drive mammalian kidney development. In mice, Six2⁺ nephron progenitor cells (NPCs) and Foxd1⁺ interstitial progenitor cells (IPCs) form largely distinct lineage compartments at the onset of metanephric kidney development. Here, we used the method for analyzing RNA following intracellular sorting (MARIS) approach, single-cell transcriptional profiling, in situ hybridization, and immunolabeling to characterize the presumptive NPC and IPC compartments of the developing human kidney. As in mice, each progenitor population adopts a stereotypical arrangement in the human nephron-forming niche: NPCs capped outgrowing ureteric branch tips, whereas IPCs were sandwiched between the NPCs and the renal capsule. Unlike mouse NPCs, human NPCs displayed a transcriptional profile that overlapped substantially with the IPC transcriptional profile, and key IPC determinants, including FOXD1, were readily detected within SIX2⁺ NPCs. Comparative gene expression profiling in human and mouse Six2/SIX2⁺ NPCs showed broad agreement between the species but also identified species-biased expression of some genes. Notably, some human NPC-enriched genes, including DAPL1 and COL9A2, are linked to human renal disease. We further explored the cellular diversity of mesenchymal cell types in the human nephrogenic niche through single-cell transcriptional profiling. Data analysis stratified NPCs into two main subpopulations and identified a third group of differentiating cells. These findings were confirmed by section in situ hybridization with novel human NPC markers predicted through the single-cell studies. This study provides a benchmark for the mesenchymal progenitors in the human nephrogenic niche and highlights species-variability in kidney developmental programs.

Conserved and Divergent Molecular and Anatomic Features of Human and Mouse Nephron Patterning

The nephron is the functional unit of the kidney, but the mechanism of nephron formation during human development is unclear. We conducted a detailed analysis of nephron development in humans and mice by immunolabeling, and we compared human and mouse nephron patterning to describe conserved and divergent features. We created protein localization maps that highlight the emerging patterns along the proximal-distal axis of the developing nephron and benchmark expectations for localization of functionally important transcription factors, which revealed unanticipated cellular diversity. Moreover, we identified a novel nephron subdomain marked by Wnt4 expression that we fate-mapped to the proximal mature nephron. Significant conservation was observed between human and mouse patterning. We also determined the time at which markers for mature nephron cell types first emerge-critical data for the renal organoid field. These findings have conceptual implications for the evolutionary processes driving the diversity of mammalian organ systems. Furthermore, these findings provide practical insights beyond those gained with mouse and rat models that will guide in vitro efforts to harness the developmental programs necessary to build human kidney structures.

Conserved and Divergent Features of Human and Mouse Kidney Organogenesis

Human kidney function is underpinned by approximately 1,000,000 nephrons, although the number varies substantially, and low nephron number is linked to disease. Human kidney development initiates around 4 weeks of gestation and ends around 34-37 weeks of gestation. Over this period, a reiterative inductive process establishes the nephron complement. Studies have provided insightful anatomic descriptions of human kidney development, but the limited histologic views are not readily accessible to a broad audience. In this first paper in a series providing comprehensive insight into human kidney formation, we examined human kidney development in 135 anonymously donated human kidney specimens. We documented kidney development at a macroscopic and cellular level through histologic analysis, RNA in situ hybridization, immunofluorescence studies, and transcriptional profiling, contrasting human development (4-23 weeks) with mouse development at selected stages (embryonic day 15.5 and postnatal day 2). The high-resolution histologic interactive atlas of human kidney organogenesis generated can be viewed at the GUDMAP database (www.gudmap.org) together with three-dimensional reconstructions of key components of the data herein. At the anatomic level, human and mouse kidney development differ in timing, scale, and global features such as lobe formation and progenitor niche organization. The data also highlight differences in molecular and cellular features, including the expression and cellular distribution of anchor gene markers used to identify key cell types in mouse kidney studies. These data will facilitate and inform in vitro efforts to generate human kidney structures and comparative functional analyses across mammalian species.

Conserved and Divergent Features of Mesenchymal Progenitor Cell Types within the Cortical Nephrogenic Niche of the Human and Mouse Kidney

Cellular interactions among nephron, interstitial, and collecting duct progenitors drive mammalian kidney development. In mice, Six2⁺ nephron progenitor cells (NPCs) and Foxd1⁺ interstitial progenitor cells (IPCs) form largely distinct lineage compartments at the onset of metanephric kidney development. Here, we used the method for analyzing RNA following intracellular sorting (MARIS) approach, single-cell transcriptional profiling, in situ hybridization, and immunolabeling to characterize the presumptive NPC and IPC compartments of the developing human kidney. As in mice, each progenitor population adopts a stereotypical arrangement in the human nephron-forming niche: NPCs capped outgrowing ureteric branch tips, whereas IPCs were sandwiched between the NPCs and the renal capsule. Unlike mouse NPCs, human NPCs displayed a transcriptional profile that overlapped substantially with the IPC transcriptional profile, and key IPC determinants, including FOXD1, were readily detected within SIX2⁺ NPCs. Comparative gene expression profiling in human and mouse Six2/SIX2⁺ NPCs showed broad agreement between the species but also identified species-biased expression of some genes. Notably, some human NPC-enriched genes, including DAPL1 and COL9A2, are linked to human renal disease. We further explored the cellular diversity of mesenchymal cell types in the human nephrogenic niche through single-cell transcriptional profiling. Data analysis stratified NPCs into two main subpopulations and identified a third group of differentiating cells. These findings were confirmed by section in situ hybridization with novel human NPC markers predicted through the single-cell studies. This study provides a benchmark for the mesenchymal progenitors in the human nephrogenic niche and highlights species-variability in kidney developmental programs.

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.

A Pdgf-cCreERT2 knock-in mouse model for tracing PDGF-C cell lineages during development

PDGF-C, a member of the platelet-derived growth factor (PDGF) family, plays important roles in the development of craniofacial structures, the neural system, the vascular system, and tumors. PDGF-C could also be required for the regulation of certain types of stem or progenitor cells as suggested by its expression in the regions where these cells are located. To further characterize the role of PDGF-C in development, we generated a Pdgf-c^CreERT2 mouse strain, in which a tamoxifen-inducible Cre (CreERT2) cDNA was specifically targeted into the Pdgf-c genomic locus and controlled by the endogenous Pdgf-c regulatory elements. We also showed that Cre activity in this mouse strain could be specifically induced by tamoxifen, which allowed the fate of PDGF-C-expressing cells to be traced at various stages of development. Using this model system, we demonstrated for the first time that PDGF-C-expressing cells could be multipotent, generating multiple cell lineages required for the formation of the cerebellum. Therefore, the Pdgf-c^CreERT2 mouse strain generated in this study will be a valuable transgenic tool for exploring the function of PDGF-C in development and stem cell biology.

Myc cooperates with β-catenin to drive gene expression in nephron progenitor cells

For organs to achieve their proper size, the processes of stem cell renewal and differentiation must be tightly regulated. We previously showed that in the developing kidney, Wnt9b regulates distinct β-catenin-dependent transcriptional programs in the renewing and differentiating populations of the nephron progenitor cells. How β-catenin stimulated these two distinct programs was unclear. Here, we show that β-catenin cooperates with the transcription factor Myc to activate the progenitor renewal program. Although in multiple contexts Myc is a target of β-catenin, our characterization of a cell type-specific enhancer for the Wnt9b/β-catenin target gene Fam19a5 shows that Myc and β-catenin cooperate to activate gene expression controlled by this element. This appears to be a more general phenomenon as we find that Myc is required for the expression of every Wnt9b/β-catenin progenitor renewal target assessed as well as for proper nephron endowment in vivo This study suggests that, within the developing kidney, tissue-specific β-catenin activity is regulated by cooperation with cell type-specific transcription factors. This finding not only provides insight into the regulation of β-catenin target genes in the developing kidney, but will also advance our understanding of progenitor cell renewal in other cell types/organ systems in which Myc and β-catenin are co-expressed.

WT1 Is Necessary for the Proliferation and Migration of Cells of Renin Lineage Following Kidney Podocyte Depletion

Wilms' tumor suppressor 1 (WT1) plays an important role in cell proliferation and mesenchymal-epithelial balance in normal development and disease. Here, we show that following podocyte depletion in three experimental models, and in patients with focal segmental glomerulosclerosis (FSGS) and membranous nephropathy, WT1 increased significantly in cells of renin lineage (CoRL). In an animal model of FSGS in RenWt1^fl/fl reporter mice with inducible deletion of WT1 in CoRL, CoRL proliferation and migration to the glomerulus was reduced, and glomerular disease was worse compared with wild-type mice. To become podocytes, CoRL undergo mesenchymal-to-epithelial transformation (MET), typified by reduced staining for mesenchymal markers (MYH11, SM22, αSMA) and de novo expression of epithelial markers (E-cadherin and cytokeratin18). Evidence for changes in MET markers was barely detected in RenWt1^fl/fl mice. Our results show that following podocyte depletion, WT1 plays essential roles in CoRL proliferation and migration toward an adult podocyte fate.

Repression of Interstitial Identity in Nephron Progenitor Cells by Pax2 Establishes the Nephron-Interstitium Boundary during Kidney Development

The kidney contains the functional units, the nephrons, surrounded by the renal interstitium. Previously we discovered that, once Six2-expressing nephron progenitor cells and Foxd1-expressing renal interstitial progenitor cells form at the onset of kidney development, descendant cells from these populations contribute exclusively to the main body of nephrons and renal interstitial tissues, respectively, indicating a lineage boundary between the nephron and renal interstitial compartments. Currently it is unclear how lineages are regulated during kidney organogenesis. We demonstrate that nephron progenitor cells lacking Pax2 fail to differentiate into nephron cells but can switch fates into renal interstitium-like cell types. These data suggest that Pax2 function maintains nephron progenitor cells by repressing a renal interstitial cell program. Thus, the lineage boundary between the nephron and renal interstitial compartments is maintained by the Pax2 activity in nephron progenitor cells during kidney organogenesis.

A Sall1-NuRD interaction regulates multipotent nephron progenitors and is required for loop of Henle formation

The formation of the proper number of nephrons requires a tightly regulated balance between renal progenitor cell self-renewal and differentiation. The molecular pathways that regulate the transition from renal progenitor to renal vesicle are not well understood. Here, we show that Sall1interacts with the nucleosome remodeling and deacetylase complex (NuRD) to inhibit premature differentiation of nephron progenitor cells. Disruption of Sall1-NuRD in vivo in knock-in mice (ΔSRM) resulted in accelerated differentiation of nephron progenitors and bilateral renal hypoplasia. Transcriptional profiling of mutant kidneys revealed a striking pattern in which genes of the glomerular and proximal tubule lineages were either unchanged or upregulated, and those in the loop of Henle and distal tubule lineages were downregulated. These global changes in gene expression were accompanied by a significant decrease in THP-, NKCC2- and AQP1-positive loop of Henle nephron segments in mutant ΔSRM kidneys. These findings highlight an important function of Sall1-NuRD interaction in the regulation of Six2-positive multipotent renal progenitor cells and formation of the loop of Henle.

Regulation of Nephron Progenitor Cell Self-Renewal by Intermediary Metabolism

Nephron progenitor cells (NPCs) show an age-dependent capacity to balance self-renewal with differentiation. Older NPCs (postnatal day 0) exit the progenitor niche at a higher rate than younger (embryonic day 13.5) NPCs do. This behavior is reflected in the transcript profiles of young and old NPCs. Bioenergetic pathways have emerged as important regulators of stem cell fate. Here, we investigated the mechanisms underlying this regulation in murine NPCs. Upon isolation and culture in NPC renewal medium, younger NPCs displayed a higher glycolysis rate than older NPCs. Inhibition of glycolysis enhanced nephrogenesis in cultured embryonic kidneys, without increasing ureteric tree branching, and promoted mesenchymal-to-epithelial transition in cultured isolated metanephric mesenchyme. Cotreatment with a canonical Wnt signaling inhibitor attenuated but did not entirely block the increase in nephrogenesis observed after glycolysis inhibition. Furthermore, inhibition of the phosphatidylinositol 3-kinase/Akt self-renewal signaling pathway or stimulation of differentiation pathways in the NPC decreased glycolytic flux. Our findings suggest that glycolysis is a pivotal, cell-intrinsic determinant of NPC fate, with a high glycolytic flux supporting self-renewal and inhibition of glycolysis stimulating differentiation.

Tissue-Specific Functions of p53 During Kidney Development

p53 is best identified as a tumor suppressor for its transcriptional control of genes involved in cell cycle progression and apoptosis. Beyond its irrefutable involvement in restraining unchecked cell proliferation, research over the past several years has indicated a requirement for p53 function in sustaining normal development. Here I summarize the role of p53 in embryonic development, with a focus on knowledge gained from p53 loss and overexpression during kidney development. In contrast to its classical role in suppressing proliferative pathways, p53 positively regulates nephron progenitor cell (NPC) renewal. Emerging evidence suggests p53 may control cell fate decisions by preserving energy metabolism homeostasis of progenitors in the nephrogenic niche. Maintaining a critical level of p53 function appears to be a prerequisite for optimal nephron endowment. Defining the molecular networks targeted by p53 in the NPC may well provide new targets not only for regenerative medicine but also for cancer treatment.

Development and Diseases of the Collecting Duct System

The collecting duct of the mammalian kidney is important for the regulation of extracellular volume, osmolarity, and pH. There are two major structurally and functionally distinct cell types: principal cells and intercalated cells. The former regulates Na⁺ and water homeostasis, while the latter participates in acid-base homeostasis. In vivo lineage tracing using Cre recombinase or its derivatives such as CreGFP and CreER^T2 is a powerful new technique to identify stem/progenitor cells in their native environment and to decipher the origins of the tissue that they give rise to. Recent studies using this technique in mice have revealed multiple renal progenitor cell populations that differentiate into various nephron segments and collecting duct. In particular, emerging evidence suggests that like principal cells, most of intercalated cells originate from the progenitor cells expressing water channel Aquaporin 2. Mutations or malfunctions of the channels, pumps, and transporters expressed in the collecting duct system cause various human diseases. For example, gain-of-function mutations in ENaC cause Liddle's syndrome, while loss-of-function mutations in ENaC lead to Pseudohypoaldosteronism type 1. Mutations in either AE1 or V-ATPase B1 result in distal renal tubular acidosis. Patients with disrupted AQP2 or AVPR2 develop nephrogenic diabetes insipidus. A better understanding of the function and development of the collecting duct system may facilitate the discovery of new therapeutic strategies for treating kidney disease.

Evidence of In Vitro Preservation of Human Nephrogenesis at the Single-Cell Level

During nephrogenesis, stem/progenitor cells differentiate and give rise to early nephron structures that segment to proximal and distal nephron cell types. Previously, we prospectively isolated progenitors from human fetal kidney (hFK) utilizing a combination of surface markers. However, upon culture nephron progenitors differentiated and could not be robustly maintained in vitro. Here, by culturing hFK in a modified medium used for in vitro growth of mouse nephron progenitors, and by dissection of NCAM⁺/CD133⁻ progenitor cells according to EpCAM expression (NCAM⁺/CD133⁻/EpCAM⁻, NCAM⁺/CD133⁻/EpCAM^dim, NCAM⁺/CD133⁻/EpCAM^bright), we show at single-cell resolution a preservation of uninduced and induced cap mesenchyme as well as a transitioning mesenchymal-epithelial state. Concomitantly, differentiating and differentiated epithelial lineages are also maintained. In vitro expansion of discrete stages of early human nephrogenesis in nephron stem cell cultures may be used for drug screening on a full repertoire of developing kidney cells and for prospective isolation of mesenchymal or epithelial renal lineages for regenerative medicine.

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mNPEM enables in vitro preservation of human renal embryonic CM and epithelia

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EpCAM allows further dissection of expanded NCAM⁺CD133⁻ early nephric population

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Single-cell analysis unveils a continuous lineage hierarchy in nephrogenesis and WT

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Splice isoform switching confirms a unified MET hierarchy in nephrogenesis and WT

Inhibition of Pax2 Transcription Activation with a Small Molecule that Targets the DNA Binding Domain

The Pax gene family encodes DNA binding transcription factors that control critical steps in embryonic development and differentiation of specific cell lineages. Often, Pax proteins are re-expressed or ectopically expressed in cancer and other diseases of abnormal proliferation, making them attractive targets for tissue specific inhibition by small molecules. In this report, we used a homology model of the Pax2 paired domain and a virtual screen to identify small molecules that can inhibit binding of the paired domain to DNA and Pax2 mediated transcription activation. Candidates from the virtual screen were then confirmed in a cell based Pax2 transactivation assay. Subsequently, we tested analogs of these hits to identify a single compound that effectively blocked Pax2 activity and DNA binding with a K_d of 1.35-1.5 μM. The compound, termed EG1, was used to inhibit embryonic kidney development, a process directly dependent on Pax2 activity. Furthermore, we show that EG1 can inhibit proliferation of Pax2 positive renal and ovarian cancer cell lines but has little effect on Pax2 negative cancer cells. These data confirm that small molecules targeting the DNA binding paired domain can be identified and may be good lead compounds for developing tissue and cell-type specific anticancer therapies.

Self-organisation after embryonic kidney dissociation is driven via selective adhesion of ureteric epithelial cells

Human pluripotent stem cells, after directed differentiation in vitro, can spontaneously generate complex tissues via self-organisation of the component cells. Self-organisation can also reform embryonic organ structure after tissue disruption. It has previously been demonstrated that dissociated embryonic kidneys can recreate component epithelial and mesenchymal relationships sufficient to allow continued kidney morphogenesis. Here, we investigate the timing and underlying mechanisms driving self-organisation after dissociation of the embryonic kidney using time-lapse imaging, high-resolution confocal analyses and mathematical modelling. Organotypic self-organisation sufficient for nephron initiation was observed within a 24 h period. This involved cell movement, with structure emerging after the clustering of ureteric epithelial cells, a process consistent with models of random cell movement with preferential cell adhesion. Ureteric epithelialisation rapidly followed the formation of ureteric cell clusters with the reformation of nephron-forming niches representing a later event. Disruption of P-cadherin interactions was seen to impair this ureteric epithelial cell clustering without affecting epithelial maturation. This understanding could facilitate improved regulation of patterning within organoids and facilitate kidney engineering approaches guided by cell-cell self-organisation.

Nephron number, hypertension, and CKD: physiological and genetic insight from humans and animal models

The kidneys play a vital role in the excretion of waste products and the regulation of electrolytes, maintenance of acid-base balance, regulation of blood pressure, and production of several hormones. Any alteration in the structure of the nephron (basic functional unit of the kidney) can have a major impact on the kidney's ability to work efficiently. Progressive decline in kidney function can lead to serious illness and ultimately death if not treated by dialysis or transplantation. While there have been numerous studies that implicate lower nephron numbers as being an important factor in influencing susceptibility to developing hypertension and chronic kidney disease, a direct association has been difficult to establish because of three main limitations: 1) the large variation in nephron number observed in the human population; 2) no established reliable noninvasive methods to determine nephron complement; and 3) to date, nephron measurements have been done after death, which doesn't adequately account for potential loss of nephrons with age or disease. In this review, we will provide an overview of kidney structure/function, discuss the current literature for both humans and other species linking nephron deficiency and cardio-renal complications, as well as describe the major molecular signaling factors involved in nephrogenesis that modulate variation in nephron number. As more detailed knowledge about the molecular determinants of nephron development and the role of nephron endowment in the cardio-renal system is obtained, it will hopefully provide clinicians the ability to accurately identify people at risk to develop CKD/hypertension and lead to a shift in patient care from disease treatment to prevention.

Imaging, Analysing and Interpreting Branching Morphogenesis in the Developing Kidney

The kidney develops as an outgrowth of the epithelial nephric duct known as the ureteric bud, in a position specified by a range of rostral and caudal factors which serve to ensure two kidneys form in the appropriate positions in the body. At its simplest level, kidney development can be viewed as the process by which this single bud then undergoes a process of arborisation to form a complex connected network of ducts which will serve to drain urine from the nephrons in the adult organ. The process of bud elaboration is dictated by factors expressed by both the bud itself and by surrounding cells of the metanephric mesenchyme which control cell division and bifurcation. These cells play two critical roles. Firstly, they potentiate the ongoing elaboration of the ureteric tree: remove them and branching ceases. Secondly, they harbour progenitor cells which are fated to undergo their own process of tubulogenesis to form the nephrons of the adult organ. In this chapter, we will discuss how the ureteric bud arises in the developing embryo, how it undergoes branching, how we can measure and study this process and finally the likely relevance that this process has for our understanding of congenital and acquired kidney disease.

Growth Factor Regulation in the Nephrogenic Zone of the Developing Kidney

New nephrons are induced by the interaction between mesenchymal progenitor cells and collecting duct tips, both of which are located at the outer edge of the kidney. This leading edge of active nephron induction is known as the nephrogenic zone. Cell populations found within this zone include collecting duct tips, cap mesenchyme cells, pretubular aggregates, nephrogenic zone interstitium, hemoendothelial progenitor cells, and macrophages. The close association of these dynamic progenitor cell compartments enables the intricate and synchronized patterning of the epithelial and the vascular components of the nephron. Understanding signaling interactions between the distinct progenitor cells of the nephrogenic zone are essential to determining the basis for new nephron formation, an important goal in regenerative medicine. A variety of technologies have been applied to define essential signaling pathways, including organ culture, mouse genetics, and primary cell culture. This chapter provides an overview of essential signaling pathways and discusses how these may be integrated.

Nephron Progenitor But Not Stromal Progenitor Cells Give Rise to Wilms Tumors in Mouse Models with β-Catenin Activation or Wt1 Ablation and Igf2 Upregulation

Wilms tumor, a common childhood tumor of the kidney, is thought to arise from undifferentiated renal mesenchyme. Variable tumor histology and the identification of tumor subsets displaying different gene expression profiles suggest that tumors may arise at different stages of mesenchyme differentiation and that this ontogenic variability impacts tumor pathology, biology, and clinical outcome. To test the tumorigenic potential of different cell types in the developing kidney, we used kidney progenitor-specific Cre recombinase alleles to introduce Wt1 and Ctnnb1 mutations, two alterations observed in Wilms tumor, into embryonic mouse kidney, with and without biallelic Igf2 expression, another alteration that is observed in a majority of tumors. Use of a Cre allele that targets nephron progenitors to introduce a Ctnnb1 mutation that stabilizes β-catenin resulted in the development of tumors with a predominant epithelial histology and a gene expression profile in which genes characteristic of early renal mesenchyme were not expressed. Nephron progenitors with Wt1 ablation and Igf2 biallelic expression were also tumorigenic but displayed a more triphasic histology and expressed early metanephric mesenchyme genes. In contrast, the targeting of these genetic alterations to stromal progenitors did not result in tumors. These data demonstrate that committed nephron progenitors can give rise to Wilms tumors and that committed stromal progenitors are less tumorigenic, suggesting that human Wilms tumors that display a predominantly stromal histology arise from mesenchyme before commitment to a stromal lineage.

Making new kidneys: On the road from science fiction to science fact

PURPOSE OF REVIEW

Allogenic kidney transplantation use is limited because of a shortage of kidney organ donors and the risks associated with a long-term immunosuppression. An emerging treatment prospect is autologous transplants of ex vivo produced human kidneys. Here we will review the research advances in this area.

RECENT FINDINGS

The creation of human induced pluripotent cells (iPSCs) from somatic cells and the emergence of several differentiation protocols that are able to convert iPSCs cells into self-organizing kidney organoids are two large steps toward assembling a human kidney in vitro. Several groups have successfully generated urine-producing kidney organoids upon transplantation in a mouse host. Additional advances in culturing nephron progenitors in vitro may provide another source for kidney engineering, and the emergence of genome editing technology will facilitate correction of congenital mutations.

SUMMARY

Basic research into the development of metanephric kidneys and iPSC differentiation protocols, the therapeutic use of iPSCs, along with emergence of new technologies such as CRISPR/Cas9 genome editing have accelerated a trend that may prove transformative in the treatment of ESRD and congenital kidney disorders.

Renal progenitors: Roles in kidney disease and regeneration

Kidney disease is a devastating condition that affects millions of people worldwide, and its prevalence is predicted to significantly increase. The kidney is a complex organ encompassing many diverse cell types organized in a elaborate tissue architecture, making regeneration a challenging feat. In recent years, there has been a surge in the field of stem cell research to develop regenerative therapies for various organ systems. Here, we review some recent progressions in characterizing the role of renal progenitors in development, regeneration, and kidney disease in mammals. We also discuss how the zebrafish provides a unique experimental animal model that can provide a greater molecular and genetic understanding of renal progenitors, which may contribute to the development of potential regenerative therapies for human renal afflictions.

Lats1/2 Regulate Yap/Taz to Control Nephron Progenitor Epithelialization and Inhibit Myofibroblast Formation

In the kidney, formation of the functional filtration units, the nephrons, is essential for postnatal life. During development, mesenchymal progenitors tightly regulate the balance between self-renewal and differentiation to give rise to all nephron epithelia. Here, we investigated the functions of the Hippo pathway serine/threonine-protein kinases Lats1 and Lats2, which phosphorylate and inhibit the transcriptional coactivators Yap and Taz, in nephron progenitor cells. Genetic deletion of Lats1 and Lats2 in nephron progenitors of mice led to disruption of nephrogenesis, with an accumulation of spindle-shaped cells in both cortical and medullary regions of the kidney. Lineage-tracing experiments revealed that the cells that accumulated in the interstitium derived from nephron progenitor cells and expressed E-cadherin as well as vimentin, a myofibroblastic marker not usually detected after mesenchymal-to-epithelial transition. The accumulation of these interstitial cells associated with collagen deposition and ectopic expression of the myofibroblastic markers vimentin and α-smooth-muscle actin in developing kidneys. Although these myofibroblastic cells had high Yap and Taz accumulation in the nucleus concomitant with a loss of phosphorylated Yap, reduction of Yap and/or Taz expression levels completely rescued the Lats1/2 phenotype. Taken together, our results demonstrate that Lats1/2 kinases restrict Yap/Taz activities to promote nephron progenitor cell differentiation in the mammalian kidney. Notably, our data also show that myofibroblastic cells can differentiate from nephron progenitors.

Direct Isolation and Characterization of Human Nephron Progenitors

Mature nephrons originate from a small population of uninduced nephrogenic progenitor cells (NPs) within the cap mesenchyme. These cells are characterized by the coexpression of SIX2 and CITED1. Many studies on mouse models as well as on human pluripotent stem cells have advanced our knowledge of NPs, but very little is known about this population in humans, since it is exhausted before birth and strategies for its direct isolation are still limited. Here we report an efficient protocol for direct isolation of human NPs without genetic manipulation or stepwise induction procedures. With the use of RNA‐labeling probes, we isolated SIX2⁺CITED1⁺ cells from human fetal kidney for the first time. We confirmed their nephrogenic state by gene profiling and evaluated their nephrogenic capabilities in giving rise to mature renal cells. We also evaluated the ability to culture these cells without complete loss of SIX2 and CITED1 expression over time. In addition to defining the gene profile of human NPs, this in vitro system facilitates studies of human renal development and provides a novel tool for renal regeneration and bioengineering purposes. Stem Cells Translational Medicine2017;6:419–433

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.

Differential regulation of mouse and human nephron progenitors by the Six family of transcriptional regulators

Nephron endowment is determined by the self-renewal and induction of a nephron progenitor pool established at the onset of kidney development. In the mouse, the related transcriptional regulators Six1 and Six2 play non-overlapping roles in nephron progenitors. Transient Six1 activity prefigures, and is essential for, active nephrogenesis. By contrast, Six2 maintains later progenitor self-renewal from the onset of nephrogenesis. We compared the regulatory actions of Six2 in mouse and human nephron progenitors by chromatin immunoprecipitation followed by DNA sequencing (ChIP-seq). Surprisingly, SIX1 was identified as a SIX2 target unique to the human nephron progenitors. Furthermore, RNA-seq and immunostaining revealed overlapping SIX1 and SIX2 activity in 16 week human fetal nephron progenitors. Comparative bioinformatic analysis of human SIX1 and SIX2 ChIP-seq showed each factor targeted a similar set of cis-regulatory modules binding an identical target recognition motif. In contrast to the mouse where Six2 binds its own enhancers but does not interact with DNA around Six1, both human SIX1 and SIX2 bind homologous SIX2 enhancers and putative enhancers positioned around SIX1. Transgenic analysis of a putative human SIX1 enhancer in the mouse revealed a transient, mouse-like, pre-nephrogenic, Six1 regulatory pattern. Together, these data demonstrate a divergence in SIX-factor regulation between mouse and human nephron progenitors. In the human, an auto/cross-regulatory loop drives continued SIX1 and SIX2 expression during active nephrogenesis. By contrast, the mouse establishes only an auto-regulatory Six2 loop. These data suggest differential SIX-factor regulation might have contributed to species differences in nephron progenitor programs such as the duration of nephrogenesis and the final nephron count.

Nephron organoids derived from human pluripotent stem cells model kidney development and injury

Kidney cells and tissues derived from human pluripotent stem cells (hPSCs) would enable organ regeneration, disease modeling, and drug screening in vitro. We established an efficient, chemically defined protocol for differentiating hPSCs into multipotent nephron progenitor cells (NPCs) that can form nephron-like structures. By recapitulating metanephric kidney development in vitro, we generate SIX2+SALL1+WT1+PAX2+ NPCs with 90% efficiency within 9 days of differentiation. The NPCs possess the developmental potential of their in vivo counterparts and form PAX8+LHX1+ renal vesicles that self-pattern into nephron structures. In both 2D and 3D culture, NPCs form kidney organoids containing epithelial nephron-like structures expressing markers of podocytes, proximal tubules, loops of Henle, and distal tubules in an organized, continuous arrangement that resembles the nephron in vivo. We also show that this organoid culture system can be used to study mechanisms of human kidney development and toxicity.

Urine of Preterm Neonates as a Novel Source of Kidney Progenitor Cells

In humans, nephrogenesis is completed prenatally, with nephrons formed until 34 weeks of gestational age. We hypothesized that urine of preterm neonates born before the completion of nephrogenesis is a noninvasive source of highly potent stem/progenitor cells. To test this hypothesis, we collected freshly voided urine at day 1 after birth from neonates born at 31-36 weeks of gestational age and characterized isolated cells using a single-cell RT-PCR strategy for gene expression analysis and flow cytometry and immunofluorescence for protein expression analysis. Neonatal stem/progenitor cells expressed markers of nephron progenitors but also, stromal progenitors, with many single cells coexpressing these markers. Furthermore, these cells presented mesenchymal stem cell features and protected cocultured tubule cells from cisplatin-induced apoptosis. Podocytes differentiated from the neonatal stem/progenitor cells showed upregulation of podocyte-specific genes and proteins, albumin endocytosis, and calcium influx via podocyte-specific transient receptor potential cation channel, subfamily C, member 6. Differentiated proximal tubule cells showed upregulation of specific genes and significantly elevated p-glycoprotein activity. We conclude that urine of preterm neonates is a novel noninvasive source of kidney progenitors that are capable of differentiation into mature kidney cells and have high potential for regenerative kidney repair.

p53 Enables metabolic fitness and self-renewal of nephron progenitor cells

Contrary to its classic role in restraining cell proliferation, we demonstrate here a divergent function of p53 in the maintenance of self-renewal of the nephron progenitor pool in the embryonic mouse kidney. Nephron endowment is regulated by progenitor availability and differentiation potential. Conditional deletion of p53 in nephron progenitor cells (Six2Cre(+);p53(fl/fl)) induces progressive depletion of Cited1(+)/Six2(+) self-renewing progenitors and loss of cap mesenchyme (CM) integrity. The Six2(p53-null) CM is disorganized, with interspersed stromal cells and an absence of a distinct CM-epithelia and CM-stroma interface. Impaired cell adhesion and epithelialization are indicated by decreased E-cadherin and NCAM expression and by ineffective differentiation in response to Wnt induction. The Six2Cre(+);p53(fl/fl) cap has 30% fewer Six2(GFP(+)) cells. Apoptotic index is unchanged, whereas proliferation index is significantly reduced in accordance with cell cycle analysis showing disproportionately fewer Six2Cre(+);p53(fl/fl) cells in the S and G2/M phases compared with Six2Cre(+);p53(+/+) cells. Mutant kidneys are hypoplastic with fewer generations of nascent nephrons. A significant increase in mean arterial pressure is observed in early adulthood in both germline and conditional Six2(p53-null) mice, linking p53-mediated defects in kidney development to hypertension. RNA-Seq analyses of FACS-isolated wild-type and Six2(GFP(+)) CM cells revealed that the top downregulated genes in Six2Cre(+);p53(fl/fl) CM belong to glucose metabolism and adhesion and/or migration pathways. Mutant cells exhibit a ∼ 50% decrease in ATP levels and a 30% decrease in levels of reactive oxygen species, indicating energy metabolism dysfunction. In summary, our data indicate a novel role for p53 in enabling the metabolic fitness and self-renewal of nephron progenitors.

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.

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.

Concurrent BMP7 and FGF9 signalling governs AP-1 function to promote self-renewal of nephron progenitor cells

Self-renewal of nephron progenitor cells (NPCs) is governed by BMP, FGF and WNT signalling. Mechanisms underlying cross-talk between these pathways at the molecular level are largely unknown. Here we delineate the pathway through which the proliferative BMP7 signal is transduced in NPCs in the mouse. BMP7 activates the MAPKs TAK1 and JNK to phosphorylate the transcription factor JUN, which in turn governs transcription of AP-1-element containing G1-phase cell cycle regulators such as Myc and Ccnd1 to promote NPC proliferation. Conditional inactivation of Tak1 or Jun in cap mesenchyme causes identical phenotypes characterized by premature depletion of NPCs. While JUN is regulated by BMP7, we find that its partner FOS is regulated by FGF9. We demonstrate that BMP7 and FGF9 coordinately regulate AP-1 transcription to promote G1-S cell cycle progression and NPC proliferation. Our findings identify a molecular mechanism explaining the important cooperation between two major NPC self-renewal pathways.

The growth factors BMP and FGF both stimulate the self-renewal of nephron progenitor cells (NPCs), but how these signals overlap is unclear. Here in the mouse, Muthukrishnan et al. find BMP7 and FGF9 coordinately regulate AP-1 transcriptional activity, promoting G1-S cell cycle progression and NPC proliferation.

Concise Review: Understanding the Renal Progenitor Cell Niche In Vivo to Recapitulate Nephrogenesis In Vitro

Chronic kidney disease (CKD), defined as progressive kidney damage and a reduction of the glomerular filtration rate, can progress to end-stage renal failure (CKD5), in which kidney function is completely lost. CKD5 requires dialysis or kidney transplantation, which is limited by the shortage of donor organs. The incidence of CKD5 is increasing annually in the Western world, stimulating an urgent need for new therapies to repair injured kidneys. Many efforts are directed toward regenerative medicine, in particular using stem cells to replace nephrons lost during progression to CKD5. In the present review, we provide an overview of the native nephrogenic niche, describing the complex signals that allow survival and maintenance of undifferentiated renal stem/progenitor cells and the stimuli that promote differentiation. Recapitulating in vitro what normally happens in vivo will be beneficial to guide amplification and direct differentiation of stem cells toward functional renal cells for nephron regeneration.

SIGNIFICANCE

Kidneys perform a plethora of functions essential for life. When their main effector, the nephron, is irreversibly compromised, the only therapeutic choices available are artificial replacement (dialysis) or renal transplantation. Research focusing on alternative treatments includes the use of stem cells. These are immature cells with the potential to mature into renal cells, which could be used to regenerate the kidney. To achieve this aim, many problems must be overcome, such as where to take these cells from, how to obtain enough cells to deliver to patients, and, finally, how to mature stem cells into the cell types normally present in the kidney. In the present report, these questions are discussed. By knowing the factors directing the proliferation and differentiation of renal stem cells normally present in developing kidney, this knowledge can applied to other types of stem cells in the laboratory and use them in the clinic as therapy for the kidney.

Each niche has an actor: multiple stem cell niches in the preterm kidney

The preterm kidney cannot be simply considered as a kidney small in size: as compared to the adult kidney, the developing organ of the preterm infant is characterized by marked differences regarding the architecture and cell components. At macroscopy, fine linear demarcations indenting the renal surface characterize the fetal and preterm kidney. At microscopy, multiple major architectural changes differentiate the developing kidney from the adult one: a large capsule with a high cellularity; the branching ureteric bud, extending from the hilum towards the renal capsule; striking morphological differences among superficial (just born) and deep (more mature) glomeruli; persistence of remnants of the metanephric mesenchyme in the hylum; incomplete differentiation of developing proximal and distal tubules. At cellular level, kidneys of preterm infants are characterized by huge amounts of stem/precursor cells showing different degrees of differentiation, admixed with mature cell types. The most striking difference between the preterm and adult kidney is represented by the abundance of stem/progenitor cells in the former. Multiple stem cell niches may be identified in the preterm kidney, including the capsule, the sub-capsular nephrogenic zone, the cap mesenchyme embracing the ureteric bud tips, the cortical and medullary interstitium, and the hilar zone in proximity of the ureteric origin. The sub-capsular area represents the major stem cell niche in the prenatal kidney. It has been defined “blue strip”, due to the scarcity of cytoplasm of the undifferentiated stem/progenitors, which appear as small cells arranged in a solid pattern. All these data taken together, the morphological approach to the analysis of the preterm kidney appears completely different from that typically utilized in kidney biopsies from adult subjects. Such a different structure should be taken into account when evaluating renal function in a preterm infant in clinical practice. Moreover, a better knowledge of molecular biology of the blue strip stem/progenitor cells could be at the basis of a new “endogenous” regenerative medicine, finalized to maintain and protect the nephrogenic potential of preterm infants till the 36th week of post-conceptional age, allowing them to escape oligonephronia and chronic kidney disease later in life.

Renal stromal miRNAs are required for normal nephrogenesis and glomerular mesangial survival

MicroRNAs are small noncoding RNAs that post-transcriptionally regulate mRNA levels. While previous studies have demonstrated that miRNAs are indispensable in the nephron progenitor and ureteric bud lineage, little is understood about stromal miRNAs during kidney development. The renal stroma (marked by expression of FoxD1) gives rise to the renal interstitium, a subset of peritubular capillaries, and multiple supportive vascular cell types including pericytes and the glomerular mesangium. In this study, we generated FoxD1^GC;Dicer^fl/fl transgenic mice that lack miRNA biogenesis in the FoxD1 lineage. Loss of Dicer activity resulted in multifaceted renal anomalies including perturbed nephrogenesis, expansion of nephron progenitors, decreased renin-expressing cells, fewer smooth muscle afferent arterioles, and progressive mesangial cell loss in mature glomeruli. Although the initial lineage specification of FoxD1⁺ stroma was not perturbed, both the glomerular mesangium and renal interstitium exhibited ectopic apoptosis, which was associated with increased expression of Bcl2l11 (Bim) and p53 effector genes (Bax, Trp53inp1, Jun, Cdkn1a, Mmp2, and Arid3a). Using a combination of high-throughput miRNA profiling of the FoxD1⁺-derived cells and mRNA profiling of differentially expressed transcripts in FoxD1^GC;Dicer^fl/fl kidneys, at least 72 miRNA:mRNA target interactions were identified to be suppressive of the apoptotic program. Together, the results support an indispensable role for stromal miRNAs in the regulation of apoptosis during kidney development.

Kidney regeneration from human induced pluripotent stem cells

PURPOSE OF REVIEW

Human induced pluripotent stem cells (iPSCs) and embryonic stem cells (ESCs) are potential unlimited cell sources for renal cells in regenerative medicine. This review highlights recent advance in the directed differentiation of human iPSCs into kidney lineages and discusses the remaining challenges to generate functional or mature renal cells from human iPSCs.

RECENT FINDINGS

Recently, directed differentiation methods from human iPSCs/ESCs into embryonic renal progenitor cells, such as those included in metanephric mesenchyme and ureteric bud, that mimic embryonic development have been reported. These studies show the developmental potential of progenitor cells by forming renal tubule-like or glomerulus-like structures in vitro. However, it has not been verified whether the physiological functions of the induced progenitors are equivalent to their in-vivo counterparts. The establishment of definitive marker genes for kidney lineages and functional assay systems is essential for the verification. Such achievement is needed before kidney regeneration can provide cell replacement therapy, reliable disease models and elucidation of the mechanisms of kidney development.

SUMMARY

In conclusion, this review outlines milestones in directed differentiation methods for functional renal cell types from human iPSCs toward clinical application and practical use.

Therapeutic use of human renal progenitor cells for kidney regeneration

The ability of the human kidney to repair itself is limited. Consequently, repeated injury can trigger a maladaptive response that is characterized by fibrosis and loss of renal function. The transcription patterns that characterize nephrogenesis in fetal renal progenitor cells (RPCs) are only partially activated during renal repair in adults. Nevertheless, evidence suggests that segment-restricted progenitor resident cells support renal healing in adults. In this Review, we discuss the evidence for the existence of functional human RPCs in adults and their role in renal repair, and consider the controversial issue of whether RPCs are a fixed population or arise through phenotypical plasticity of tubular cells that is mediated by the microenvironment. We also discuss the strategies for generating renal progenitor cells from pluripotent stem cells or differentiated cells and their use in therapy. Finally, we examine preclinical data on the therapeutic use of human fetal cells, adult progenitor cells and adult renal cells.

A synthetic niche for nephron progenitor cells

FGF, BMP, and WNT balance embryonic nephron progenitor cell (NPC) renewal and differentiation. By modulating these pathways, we have created an in vitro niche in which NPCs from embryonic kidneys or derived from human embryonic stem cells can be propagated. NPC cultures expanded up to one billion-fold in this environment can be induced to form tubules expressing nephron differentiation markers. Single-cell culture reveals phenotypic variability within the early CITED1-expressing NPC compartment, indicating that it is a mixture of cells with varying progenitor potential. Furthermore, we find that the developmental age of NPCs does not correlate with propagation capacity, indicating that cessation of nephrogenesis is related to factors other than an intrinsic clock. This in vitro nephron progenitor niche will have important applications for expansion of cells for engraftment and will facilitate investigation of mechanisms that determine the balance between renewal and differentiation in these cells.