Stromal cells mediate retinoid-dependent functions essential for renal development

Bibliometric analysis of research on retinoic acid in the field of kidney disorders

Retinoic acid is an active metabolite with significant physiological functions in human development, immunity, vision, and skin health. In recent years, research on retinoic acid in the field of kidney disorders has been increasing gradually. Yet, there is a lack of systematic bibliometric analysis of retinoic acid research in the kidney domain. This study included 1,368 articles published between 1998 and 2023 on treating kidney diseases with retinoic acid. Using the bibliometric analysis software VOSviewer and CiteSpace, we analyzed data on publication trends, contributing countries and institutions, journals and cocited journals, authors and cocited authors, cocited references, research hotspots, and frontiers. On the basis of the results of the bibliometric analysis, we identified the research efforts and their developmental trends, providing the groundwork for future research on retinoic acid.

Retinoic acid receptor α activity in proximal tubules prevents kidney injury and fibrosis

Chronic kidney disease (CKD) is characterized by a gradual loss of kidney function and affects ~13.4% of the global population. Progressive tubulointerstitial fibrosis, driven in part by proximal tubule (PT) damage, is a hallmark of late stages of CKD and contributes to the development of kidney failure, for which there are limited treatment options. Normal kidney development requires signaling by vitamin A (retinol), which is metabolized to retinoic acid (RA), an endogenous agonist for the RA receptors (RARα, β, γ). RARα levels are decreased in a mouse model of diabetic nephropathy and restored with RA administration; additionally, RA treatment reduced fibrosis. We developed a mouse model in which a spatiotemporal (tamoxifen-inducible) deletion of RARα in kidney PT cells of adult mice causes mitochondrial dysfunction, massive PT injury, and apoptosis without the use of additional nephrotoxic substances. Long-term effects (3 to 4.5 mo) of RARα deletion include increased PT secretion of transforming growth factor β1, inflammation, interstitial fibrosis, and decreased kidney function, all of which are major features of human CKD. Therefore, RARα's actions in PTs are crucial for PT homeostasis, and loss of RARα causes injury and a key CKD phenotype.

Stromal netrin 1 coordinates renal arteriogenesis and mural cell differentiation

The kidney vasculature has a complex architecture that is essential for renal function. The molecular mechanisms that direct development of kidney blood vessels are poorly characterized. We identified a regionally restricted, stroma-derived signaling molecule, netrin 1 (Ntn1), as a regulator of renal vascular patterning in mice. Stromal progenitor (SP)-specific ablation of Ntn1 (Ntn1SPKO) resulted in smaller kidneys with fewer glomeruli, as well as profound defects of the renal artery and transient blood flow disruption. Notably, Ntn1 ablation resulted in loss of arterial vascular smooth muscle cell (vSMC) coverage and in ectopic SMC deposition at the kidney surface. This was accompanied by dramatic reduction of arterial tree branching that perdured postnatally. Transcriptomic analysis of Ntn1SPKO kidneys revealed dysregulation of vSMC differentiation, including downregulation of Klf4, which we find expressed in a subset of SPs. Stromal Klf4 deletion similarly resulted in decreased smooth muscle coverage and arterial branching without, however, the disruption of renal artery patterning and perfusion seen in Ntn1SPKO. These data suggest a stromal Ntn1-Klf4 axis that regulates stromal differentiation and reinforces stromal-derived smooth muscle as a key regulator of renal blood vessel formation.

Sex difference and risk factors in burden of urogenital congenital anomalies from 1990 to 2019

Urogenital congenital anomalies (UCAs) is defined as "any live-birth with a urinary or genital condition" and affects millions of men and women worldwide. However, sex differences and related environmental risk factors in UCAs burden on a global scale have not been assessed. Using data from the Global Burden of Diseases, Injuries, and Risk Factors Study (GBD) 2019, we estimated prevalence, incidence, mortality and disability-adjusted life years (DALYs) of UCAs from 1990 to 2019 by sex, region, and socio-demographic Index (SDI) in 204 countries and territories. The disease burden of UCAs was also estimated attributable to each risk factor were estimated according to risk exposure. In 2019, UCAs caused 10,200 all-ages deaths (95% UI 7550-13,400). The combined global incidence rate was 8.38 per 1000 (95% UI 5.88-12.0) live births. The ASIR increased slightly, while the ASDR decreased from 1990 to 2019.The UCAs burden varies greatly depending on the development level and geographical location. The UCAs burden was significantly higher in men than in women, and the sex differences showed an enlarging trend. Health risks and issues, including pollution, child and maternal malnutrition, diet habits, unsafe sanitation and water source, were detected to be positively related to UCAs burden. Albeit the age-standardised prevalence, mortality, incidence, and DALYs of UCAs have decreased, they still cause a public health challenge worldwide. The high deaths and DALYs rates in low and low-middle SDI countries highlight the urgent need for improved preventive, diagnostic, and therapeutic measures. Global strategies for enhancing water safety, reducing pollution, and healthy diets are crucial steps in reducing the burden of UCAs.

Vitamin A and retinoid signaling in the kidneys

Vitamin A (VA, retinol) and its metabolites (commonly called retinoids) are required for the proper development of the kidney during embryogenesis, but retinoids also play key roles in the function and repair of the kidney in adults. Kidneys filter 180-200 liters of blood per day and each kidney contains approximately 1 million nephrons, which are often referred to as the 'functional units' of the kidney. Each nephron consists of a glomerulus and a series of tubules (proximal tubule, loop of Henle, distal tubule, and collecting duct) surrounded by a network of capillaries. VA is stored in the liver and converted to active metabolites, most notably retinoic acid (RA), which acts as an agonist for the retinoic acid receptors ((RARs α, β, and γ) to regulate gene transcription. In this review we discuss some of the actions of retinoids in the kidney after injury. For example, in an ischemia-reperfusion model in mice, injury-associated loss of proximal tubule (PT) differentiation markers occurs, followed by re-expression of these differentiation markers during PT repair. Notably, healthy proximal tubules express ALDH1a2, the enzyme that metabolizes retinaldehyde to RA, but transiently lose ALDH1a2 expression after injury, while nearby myofibroblasts transiently acquire RA-producing capabilities after injury. These results indicate that RA is important for renal tubular injury repair and that compensatory mechanisms exist for the generation of endogenous RA by other cell types upon proximal tubule injury. ALDH1a2 levels also increase in podocytes, epithelial cells of the glomeruli, after injury, and RA promotes podocyte differentiation. We also review the ability of exogenous, pharmacological doses of RA and receptor selective retinoids to treat numerous kidney diseases, including kidney cancer and diabetic kidney disease, and the emerging genetic evidence for the importance of retinoids and their receptors in maintaining or restoring kidney function after injury. In general, RA has a protective effect on the kidney after various types of injuries (eg. ischemia, cytotoxic actions of chemicals, hyperglycemia related to diabetes). As more research into the actions of each of the three RARs in the kidney is carried out, a greater understanding of the actions of vitamin A is likely to lead to new insights into the pathology of kidney disorders and the development of new therapies for kidney diseases.

Retinoic acid signaling pathway perturbation impacts mesodermal-tissue development in the zebrafish embryo: Biomarker candidate identification using transcriptomics

The zebrafish embryo (ZE) model provides a developmental model well conserved throughout vertebrate embryogenesis, with relevance for early human embryo development. It was employed to search for gene expression biomarkers of compound-induced disruption of mesodermal development. We were particularly interested in the expression of genes related to the retinoic acid signaling pathway (RA-SP), as a major morphogenetic regulating mechanism. We exposed ZE to teratogenic concentrations of valproic acid (VPA) and all-trans retinoic acid (ATRA), using folic acid (FA) as a non-teratogenic control compound shortly after fertilization for 4 h, and performed gene expression analysis by RNA sequencing. We identified 248 genes specifically regulated by both teratogens but not by FA. Further analysis of this gene set revealed 54 GO-terms related to the development of mesodermal tissues, distributed along the paraxial, intermediate, and lateral plate sections of the mesoderm. Gene expression regulation was specific to tissues and was observed for somites, striated muscle, bone, kidney, circulatory system, and blood. Stitch analysis revealed 47 regulated genes related to the RA-SP, which were differentially expressed in the various mesodermal tissues. These genes provide potential molecular biomarkers of mesodermal tissue and organ (mal)formation in the early vertebrate embryo.

Transcription Factors YAP/TAZ and SRF Cooperate To Specify Renal Myofibroblasts in the Developing Mouse Kidney

BACKGROUND

The embryonic renal stroma consists of multiple molecularly distinct cell subpopulations, the functional significance of which is largely unknown. Previous work has demonstrated that the transcription factors YAP and TAZ play roles in the development and morphogenesis of the nephrons, collecting ducts, and nephron progenitor cells.

METHODS

In embryonic mouse kidneys, we identified a subpopulation of stromal cells with enriched activity in YAP and TAZ. To evaluate the function of these cell types, we genetically ablated both Yap and Taz from the stromal progenitor population and examined how gene activity and development of YAP/TAZ mutant kidneys are affected over a developmental time course.

RESULTS

We found that YAP and TAZ are active in a subset of renal interstitium and that stromal-specific coablation of YAP/TAZ disrupts cortical fibroblast, pericyte, and myofibroblast development, with secondary effects on peritubular capillary differentiation. We also demonstrated that the transcription factor SRF cooperates with YAP/TAZ to drive expression of at least a subset of renal myofibroblast target genes and to specify myofibroblasts but not cortical fibroblasts or pericytes.

CONCLUSIONS

These findings reveal a critical role for YAP/TAZ in specific embryonic stromal cells and suggest that interaction with cofactors, such as SRF, influence the expression of cell type-specific target genes, thus driving stromal heterogeneity. Further, this work reveals functional roles for renal stroma heterogeneity in creating unique microenvironments that influence the differentiation and maintenance of the renal parenchyma.

Disease mechanisms of monogenic congenital anomalies of the kidney and urinary tract American Journal of Medical Genetics Part C

Congenital Anomalies of the Kidney and Urinary Tract (CAKUT) is a developmental disorder of the kidney and/or genito-urinary tract that results in end stage kidney disease (ESKD) in up to 50% of children. Despite the congenital nature of the disease, CAKUT accounts for almost 10% of adult onset ESKD. Multiple lines of evidence suggest that CAKUT is a Mendelian disorder, including the observation of familial clustering of CAKUT. Pathogenesis in CAKUT is embryonic in origin, with disturbances of kidney and urinary tract development resulting in a heterogeneous range of disease phenotypes. Despite polygenic and environmental factors being implicated, a significant proportion of CAKUT is monogenic in origin, with studies demonstrating single gene defects in 10%-20% of patients with CAKUT. Here, we review monogenic disease causation with emphasis on the etiological role of gene developmental pathways in CAKUT.

Building kidney organoids from pluripotent stem cells

PURPOSE OF REVIEW

During embryogenesis, the kidney is mainly generated from three progenitor cells; nephron progenitors, ureteric bud progenitors and stromal progenitors. Mutual interactions of the all three progenitor populations are essential to form a functional kidney with the higher-order structure. Pluripotent stem cells have potential to differentiate into all cell types of the animal body, including the kidney. In this review, we will summarize recent advances in reconstructing kidney organoids from pluripotent stem cells.

RECENT FINDINGS

In the past years, major advances were reported to induce nephron and ureteric bud progenitors from pluripotent stem cells in mice and humans, and to create kidney organoids of nephron and/or ureteric bud-derived collecting duct tissues in vitro. These kidney organoid technologies were applied to high-throughput genetic screenings and small chemical screenings to identify key factors for kidney development and disease. Furthermore, a novel method was established to induce stromal progenitors from pluripotent stem cells, leading to creation of kidney organoids with the higher-order structures completely derived from pluripotent stem cells.

SUMMARY

These advances in kidney organoids from pluripotent stem cells should lay a foundation to establish a novel therapy for kidney disease, which ultimately eliminate the need of dialysis and kidney transplantation for patients with kidney disease in the future.

Overlap of vitamin A and vitamin D target genes with CAKUT-related processes

Congenital Anomalies of the Kidney and Urinary Tract (CAKUT) are a group of abnormalities affecting the kidneys and their outflow tracts. CAKUT patients display a large clinical variability as well as a complex aetiology. Only 5% to 20% of the cases have a monogenic origin. It is thereby suspected that interactions of both genetic and environmental factors contribute to the disease. Vitamins are among the environmental factors that are considered for CAKUT aetiology. In this study, we aimed to investigate whether vitamin A or vitamin D could have a role in CAKUT aetiology. For this purpose we collected vitamin A and vitamin D target genes and computed their overlap with CAKUT-related gene sets. We observed limited overlap between vitamin D targets and CAKUT-related gene sets. We however observed that vitamin A target genes significantly overlap with multiple CAKUT-related gene sets, including CAKUT causal and differentially expressed genes, and genes involved in renal system development. Overall, these results indicate that an excess or deficiency of vitamin A might be relevant to a broad range of urogenital abnormalities.

The origin and role of the renal stroma

The postnatal kidney is predominantly composed of nephron epithelia with the interstitial components representing a small proportion of the final organ, except in the diseased state. This is in stark contrast to the developing organ, which arises from the mesoderm and comprises an expansive stromal population with distinct regional gene expression. In many organs, the identity and ultimate function of an epithelium is tightly regulated by the surrounding stroma during development. However, although the presence of a renal stromal stem cell population has been demonstrated, the focus has been on understanding the process of nephrogenesis whereas the role of distinct stromal components during kidney morphogenesis is less clear. In this Review, we consider what is known about the role of the stroma of the developing kidney in nephrogenesis, where these cells come from as well as their heterogeneity, and reflect on how this information may improve human kidney organoid models.

Synthetic Retinoids Beyond Cancer Therapy

While the uses of retinoids for cancer treatment continue to evolve, this review focuses on other therapeutic areas in which retinoids [retinol (vitamin A), all-trans retinoic acid (RA), and synthetic retinoic acid receptor (RAR)α-, β-, and γ-selective agonists] are being used and on promising new research that suggests additional uses for retinoids for the treatment of disorders of the kidneys, skeletal muscles, heart, pancreas, liver, nervous system, skin, and other organs. The most mature area, in terms of US Food and Drug Administration-approved, RAR-selective agonists, is for treatment of various skin diseases. Synthetic retinoid agonists have major advantages over endogenous RAR agonists such as RA. Because they act through a specific RAR, side effects may be minimized, and synthetic retinoids often have better pharmaceutical properties than does RA. Based on our increasing knowledge of the multiple roles of retinoids in development, epigenetic regulation, and tissue repair, other exciting therapeutic areas are emerging. Expected final online publication date for the Annual Review of Pharmacology and Toxicology, Volume 62 is January 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Pathogenesis of Anorectal Malformations in Retinoic Acid Receptor Knockout Mice Studied by HREM

Anorectal malformations (ARMs) are relatively common congenital abnormalities, but their pathogenesis is poorly understood. Previous gene knockout studies indicated that the signalling pathway mediated by the retinoic acid receptors (RAR) is instrumental to the formation of the anorectal canal and of various urogenital structures. Here, we show that simultaneous ablation of the three RARs in the mouse embryo results in a spectrum of malformations of the pelvic organs in which anorectal and urinary bladder ageneses are consistently associated. We found that these ageneses could be accounted for by defects in the processes of growth and migration of the cloaca, the embryonic structure from which the anorectal canal and urinary bladder originate. We further show that these defects are preceded by a failure of the lateral shift of the umbilical arteries and propose vascular abnormalities as a possible cause of ARM. Through the comparisons of these phenotypes with those of other mutant mice and of human patients, we would like to suggest that morphological data may provide a solid base to test molecular as well as clinical hypotheses.

Generation of patterned kidney organoids that recapitulate the adult kidney collecting duct system from expandable ureteric bud progenitors

Current kidney organoids model development and diseases of the nephron but not the contiguous epithelial network of the kidney’s collecting duct (CD) system. Here, we report the generation of an expandable, 3D branching ureteric bud (UB) organoid culture model that can be derived from primary UB progenitors from mouse and human fetal kidneys, or generated de novo from human pluripotent stem cells. In chemically-defined culture conditions, UB organoids generate CD organoids, with differentiated principal and intercalated cells adopting spatial assemblies reflective of the adult kidney’s collecting system. Aggregating 3D-cultured nephron progenitor cells with UB organoids in vitro results in a reiterative process of branching morphogenesis and nephron induction, similar to kidney development. Applying an efficient gene editing strategy to remove RET activity, we demonstrate genetically modified UB organoids can model congenital anomalies of kidney and urinary tract. Taken together, these platforms will facilitate an enhanced understanding of development, regeneration and diseases of the mammalian collecting duct system.

Here, the authors model the collecting duct system in kidneys by taking ureteric bud (UB) progenitor cells from both mouse and human primary tissues, as well as from hESC and hiPSC to generate organoids, which can model congenital anomalies of the kidney and urinary tract.

Overlap of vitamin A and vitamin D target genes with CAKUT-related processes

Congenital Anomalies of the Kidney and Urinary Tract (CAKUT) are a group of abnormalities affecting the kidneys and their outflow tracts. CAKUT patients display a large clinical variability as well as a complex aetiology. Only 5% to 20% of the cases have a monogenic origin. It is thereby suspected that interactions of both genetic and environmental factors contribute to the disease. Vitamins are among the environmental factors that are considered for CAKUT aetiology. In this study, we aimed to investigate whether vitamin A or vitamin D could have a role in CAKUT aetiology. For this purpose we collected vitamin A and vitamin D target genes and computed their overlap with CAKUT-related gene sets. We observed limited overlap between vitamin D targets and CAKUT-related gene sets. We however observed that vitamin A target genes significantly overlap with multiple CAKUT-related gene sets, including CAKUT causal and differentially expressed genes, and genes involved in renal system development. Overall, these results indicate that an excess or deficiency of vitamin A might be relevant to a broad range of urogenital abnormalities.

Fibroblast heterogeneity and tertiary lymphoid tissues in the kidney

Fibroblasts reside in various organs and support tissue structure and homeostasis under physiological conditions. Phenotypic alterations of fibroblasts underlie the development of diverse pathological conditions, including organ fibrosis. Recent advances in single‐cell biology have revealed that fibroblasts comprise heterogeneous subpopulations with distinct phenotypes, which exert both beneficial and detrimental effects on the host organs in a context‐dependent manner. In the kidney, phenotypic alterations of resident fibroblasts provoke common pathological conditions of chronic kidney disease (CKD), such as renal anemia and peritubular capillary loss. Additionally, in aged injured kidneys, fibroblasts provide functional and structural supports for tertiary lymphoid tissues (TLTs), which serve as the ectopic site of acquired immune reactions in various clinical contexts. TLTs are closely associated with aging and CKD progression, and the developmental stages of TLTs reflect the severity of renal injury. In this review, we describe the current understanding of fibroblast heterogeneity both under physiological and pathological conditions, with special emphasis on fibroblast contribution to TLT formation in the kidney. Dissecting the heterogeneous characteristics of fibroblasts will provide a promising therapeutic option for fibroblast‐related pathological conditions, including TLT formation.

Recapitulating kidney development in vitro by priming and differentiating mouse embryonic stem cells in monolayers

In order to harness the potential of pluripotent stem cells, we need to understand how to differentiate them to our target cell types. Here, we developed a protocol to differentiate mouse embryonic stem cells (ESCs) to renal progenitors in a step-wise manner. Microarrays were used to track the transcriptional changes at each stage of differentiation and we observed that genes associated with metanephros, ureteric bud, and blood vessel development were significantly upregulated as the cells differentiated towards renal progenitors. Priming the ESCs and optimizing seeding cell density and growth factor concentrations helped improve differentiation efficiency. Organoids were used to determine the developmental potential of the renal progenitor cells. Aggregated renal progenitors gave rise to organoids consisting of LTL+/E-cadherin+ proximal tubules, cytokeratin+ ureteric bud-derived tubules, and extracellular matrix proteins secreted by the cells themselves. Over-expression of key kidney developmental genes, Pax2, Six1, Eya1, and Hox11 paralogs, during differentiation did not improve differentiation efficiency. Altogether, we developed a protocol to differentiate mouse ESCs in a manner that recapitulates embryonic kidney development and showed that precise gene regulation is essential for proper differentiation to occur.

Sperm capacitation is associated with phosphorylation of the testis-specific radial spoke protein Rsph6a†

Mammalian sperm undergo a series of biochemical and physiological changes collectively known as capacitation in order to acquire the ability to fertilize. Although the increase in phosphorylation associated with mouse sperm capacitation is well established, the identity of the proteins involved in this signaling cascade remains largely unknown. Tandem mass spectrometry (MS/MS) has been used to identify the exact sites of phosphorylation and to compare the relative extent of phosphorylation at these sites. In the present work, we find that a novel site of phosphorylation on a peptide derived from the radial spoke protein Rsph6a is more phosphorylated in capacitated mouse sperm. The Rsph6a gene has six exons, five of which are conserved during evolution in flagellated cells. The exon containing the capacitation-induced phosphorylation site was found exclusively in eutherian mammals. Transcript analyses revealed at least two different testis-specific splicing variants for Rsph6a.Rsph6a mRNA expression was restricted to spermatocytes. Using antibodies generated against the Rsph6a N-terminal domain, western blotting and immunofluorescence analyses indicated that the protein remains in mature sperm and localizes to the sperm flagellum. Consistent with its role in the axoneme, solubility analyses revealed that Rsph6 is attached to cytoskeletal structures. Based on previous studies in Chlamydomonas reinhardtii, we predict that Rsph6 participates in the interaction between the central pair of microtubules and the surrounding pairs. The findings that Rsph6a is more phosphorylated during capacitation and is predicted to function in axonemal localization make Rsph6a a candidate protein mediating signaling processes in the sperm flagellum.

Myofibroblasts acquire retinoic acid-producing ability during fibroblast-to-myofibroblast transition following kidney injury

Tubular injury and interstitial fibrosis are the hallmarks of chronic kidney disease. While recent studies have verified that proximal tubular injury triggers interstitial fibrosis, the impact of fibrosis on tubular injury and regeneration remains poorly understood. We generated a novel mouse model expressing diphtheria toxin receptor on renal fibroblasts to allow for the selective disruption of renal fibroblast function. Administration of diphtheria toxin induced upregulation of the tubular injury marker Ngal and caused tubular proliferation in healthy kidneys, whereas administration of diphtheria toxin attenuated tubular regeneration in fibrotic kidneys. Microarray analysis revealed down-regulation of the retinol biosynthesis pathway in diphtheria toxin-treated kidneys. Healthy proximal tubules expressed retinaldehyde dehydrogenase 2 (RALDH2), a rate-limiting enzyme in retinoic acid biosynthesis. After injury, proximal tubules lost RALDH2 expression, whereas renal fibroblasts acquired strong expression of RALDH2 during the transition to myofibroblasts in several models of kidney injury. The retinoic acid receptor (RAR) RARγ was expressed in proximal tubules both with and without injury, and αB-crystallin, the product of an RAR target gene, was strongly expressed in proximal tubules after injury. Furthermore, BMS493, an inverse agonist of RARs, significantly attenuated tubular proliferation in vitro. In human biopsy tissue from patients with IgA nephropathy, detection of RALDH2 in the interstitium correlated with older age and lower kidney function. These results suggest a role of retinoic acid signaling and cross-talk between fibroblasts and tubular epithelial cells during tubular injury and regeneration, and may suggest a beneficial effect of fibrosis in the early response to injury.

Gen1 Modulates Metanephric Morphology Through Retinoic Acid Signaling

Congenital anomalies of the kidney and urinary tract (CAKUT) are the leading cause of end-stage renal disease in children. Our group has discovered that Holliday Junction resolvase gene Gen1 is a potential candidate gene for CAKUT. Gen1 mutant mice showed CAKUT phenotypes similar to those observed in retinoic acid (RA)-deficient models. The expression of Raldh2, which encodes the key enzyme in RA synthesis, was reduced in Gen1 mutant metanephros through RNA sequencing. By real-time reverse transcription-PCR, the expression of both Raldh2 and downstream Ret was reduced in embryonic day (E) 11.5 Gen1 mutant ureters and E13.5 kidneys, and expression of RA receptor alpha was decreased in E13.5 Gen1 mutant ureters and kidneys. Further studies showed that all-trans retinoic acid (ATRA) rescued solitary kidney phenotype and improved ureteric branching; ATRA should be administered after ureteric budding to avoid increasing the incidence of ectopic budding in Gen1 mutants. Luciferase intensity of RA response element was lower in CHO-K1 cells transfected with Gen1 siRNA than in those transfected with scrambled RNA, and this inhibitory effect could be reversed by ATRA. These findings indicate that Gen1 mutation can result in renal malformation through RA signaling and Gen1-loss-induced CAKUT can be partly rescued by ATRA.

Amelioration of Diabetic Nephropathy Using a Retinoic Acid Receptor β2 Agonist

Vitamin A (VA) and its derivatives, known as retinoids, play critical roles in renal development through retinoic acid receptor β2 (RARβ2). Disruptions in VA signaling pathways are associated with the onset of diabetic nephropathy (DN). Despite the known role of RARβ2 in renal development, the effects of selective agonists for RARβ2 in a high-fat diet (HFD) model of DN are unknown. Here we examined whether AC261066 (AC261), a highly selective agonist for RARβ2, exhibited therapeutic effects in a HFD model of DN in C57BL/6 mice. Twelve weeks of AC261 administration to HFD-fed mice was well tolerated with no observable side effects. Compared with HFD-fed mice, HFD + AC261-treated mice had improved glycemic control and reductions in proteinuria and urine albumin-to-creatinine ratio. Several cellular hallmarks of DN were mitigated in HFD + AC261-treated mice, including reductions in tubule lipid droplets, podocyte (POD) effacement, endothelial cell collapse, mesangial expansion, and glomerular basement membrane thickening. Mesangial and tubule interstitial expression of the myofibroblast markers α-smooth muscle actin (α-SMA) and type IV collagen (Col-IV) was lower in HFD + AC261-treated mice compared with HFD alone. Ultrastructural and immunohistochemistry analyses showed that, compared with HFD-fed mice, HFD + AC261-treated mice showed preservation of POD foot process and slit-diaphragm morphology, an increase in the levels of slit-diagram protein podocin, and the transcription factor Wilms tumor-suppressor gene 1 in PODs. Given the need for novel DN therapies, our results warrant further studies of the therapeutic properties of AC261 in DN.

Hedgehog-GLI signaling in Foxd1-positive stromal cells promotes murine nephrogenesis via TGFβ signaling

Normal kidney function depends on the proper development of the nephron: the functional unit of the kidney. Reciprocal signaling interactions between the stroma and nephron progenitor compartment have been proposed to control nephron development. Here, we show that removal of hedgehog intracellular effector smoothened (Smo-deficient mutants) in the cortical stroma results in an abnormal renal capsule, and an expanded nephron progenitor domain with an accompanying decrease in nephron number via a block in epithelialization. We show that stromal-hedgehog-Smo signaling acts through a GLI3 repressor. Whole-kidney RNA sequencing and analysis of FACS-isolated stromal cells identified impaired TGFβ2 signaling in Smo-deficient mutants. We show that neutralization and knockdown of TGFβ2 in explants inhibited nephrogenesis. In addition, we demonstrate that concurrent deletion of Tgfbr2 in stromal and nephrogenic cells in vivo results in decreased nephron formation and an expanded nephrogenic precursor domain similar to that observed in Smo-deficient mutant mice. Together, our data suggest a mechanism whereby a stromal hedgehog-TGFβ2 signaling axis acts to control nephrogenesis.

Protective effect of retinoic acid receptor α on hypoxia-induced epithelial to mesenchymal transition of renal tubular epithelial cells associated with TGF-β/MMP-9 pathway

Retinoic acid receptor α (RARα), a member of family of the nuclear retinoic acid receptors (RARs), plays an essential role in various chronic kidney diseases (CKD). Renal tubular epithelial to mesenchymal transition (EMT) is a common mechanism of progression of renal interstitial fibrosis (RIF). Hypoxia has been extensively considered as one of major inducers of renal tubular EMT. However, the effects of RARα on hypoxia-induced EMT have not yet been described so far. The aim of the present study was to explore the roles and potential mechanisms of RARα in hypoxia-induced EMT of renal tubular epithelial cells (RTECs). Our results showed that expression of RARα in RTECs subjected to hypoxia significantly was reduced, accompanied by decreased expression level of the epithelial marker E-cadherin, and increased expression levels of the mesenchymal markers α-smooth muscle actin (α-SMA) and vimentin, in accord with EMT. Meanwhile, hypoxia could cause RTECs to obviously express TGF-β and matrix metalloproteinase-9 (MMP-9). Furthermore, using lentivirus-based delivery vectors to overexpress RARα in RTECs, we demonstrated that RARα alleviated hypoxia-induced EMT of RTECs and downregulated the expression levels of TGF-β and MMP-9. In a word, RARα protects RTECs against EMT induced by hypoxia associated with TGF-β/MMP-9 pathway.

Constitutive metanephric mesenchyme-specific expression of interferon-gamma causes renal dysplasia by regulating Sall1 expression

Transplacental viral and parasitic infections have been shown to initiate an innate response in the mammalian embryo by increasing the expression of pro-inflammatory cytokines such as interferon-gamma (Ifng). However, the developmental consequences of an activated innate immunity and, in particular, the effects of induction of Ifng expression independent of infection have been largely overlooked. Here, we demonstrate in vivo that the conditional overexpression of Ifng in metanephric mesenchymal (MM) progenitors results in renal agenesis or hypoplasia. Cell death was observed in and around the MM region of E10.5-11.5 mutants where Ifng was constitutively expressed during early kidney development and resulted in a retardation of branching morphogenesis. Furthermore, isolated mutant or normal Ifng-treated metanephroi replicated this phenotype in culture, demonstrating the inherent nature of the aberrant morphogenesis. The expression of renal progenitor marker Sall1 was significantly decreased in the MM of mutant kidneys, suggesting that a reduction in Sall1 may be the cause of cell death in the MM during early kidney development and that, in turn, retards UB branching in the mutants. Therefore, the aberrant induction of Ifng expression, as part of an innate immune response, may contribute to renal agenesis or hypoplasia during early metanephric development by regulating the MM progenitor population.

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.

Foxd1 is an upstream regulator of the renin-angiotensin system during metanephric kidney development

BackgroundWe tested the hypothesis that Foxd1, a transcription factor essential for normal kidney development, is an upstream regulator of the renin-angiotensin system (RAS) during ureteric bud (UB)-branching morphogenesis.MethodsUB branching, RAS gene, and protein expression were studied in embryonic mouse kidneys. RAS mRNA expression was studied in mesenchymal MK4 cells.ResultsThe number of UB tips was reduced in Foxd1^-/- compared with that in Foxd1^+/+ metanephroi on embryonic day E12.5 (14±2.1 vs. 28±1.3, P<0.05). Quantitative real-time reverse-transcription polymerase chain reaction (qRT-PCR) demonstrated that renin, angiotensin I-converting enzyme (ACE), and angiotensin (Ang) II receptor type 1 (AT₁R) mRNA levels were decreased in Foxd1^-/- compared with those in Foxd1^+/+ E14.5 metanephroi. Western blot analysis and immunohistochemistry showed decreased expression of AGT and renin proteins in Foxd1^-/- metanephroi compared with that in Foxd1^+/+ metanephroi. Foxd1 overexpression in mesenchymal MK4 cells in vitro increased renin, AGT, ACE, and AT₁R mRNA levels. Exogenous Ang II stimulated UB branching equally in whole intact E12.5 Foxd1^-/- and Foxd1^+/+ metanephroi grown ex vivo (+364±21% vs. +336±18%, P=0.42).ConclusionWe conclude that Foxd1 is an upstream positive regulator of RAS during early metanephric development and propose that the cross-talk between Foxd1 and RAS is essential in UB-branching morphogenesis.

Novel Insights into the Pathogenesis of Monogenic Congenital Anomalies of the Kidney and Urinary Tract

Congenital anomalies of the kidneys and urinary tract (CAKUT) comprise a large spectrum of congenital malformations ranging from severe manifestations, such as renal agenesis, to potentially milder conditions, such as vesicoureteral reflux. CAKUT causes approximately 40% of ESRD that manifests within the first three decades of life. Several lines of evidence indicate that CAKUT is often caused by recessive or dominant mutations in single (monogenic) genes. To date, approximately 40 monogenic genes are known to cause CAKUT if mutated, explaining 5%-20% of patients. However, hundreds of different monogenic CAKUT genes probably exist. The discovery of novel CAKUT-causing genes remains challenging because of this pronounced heterogeneity, variable expressivity, and incomplete penetrance. We here give an overview of known genetic causes for human CAKUT and shed light on distinct renal morphogenetic pathways that were identified as relevant for CAKUT in mice and humans.

Regulation of retinoic acid synthetic enzymes by WT1 and HDAC inhibitors in 293 cells

All-trans retinoic acid (atRA), which is mainly generated endogenously via two steps of oxidation from vitamin A (retinol), plays an indispensible role in the development of the kidney and many other organs. Enzymes that catalyze the oxidation of retinol to generate atRA, including aldehyde dehydrogenase 1 family (ALDH1)A1, ALDH1A2 and ALDH1A3, exhibit complex expression patterns at different stages of renal development. However, molecular triggers that control these differential expression levels are poorly understood. In this study, we provide in vitro evidence to demonstrate that Wilms' tumor 1 (WT1) negatively regulates the expression of the atRA synthetic enzymes, ALDH1A1, ALDH1A2 and ALDH1A3, in the 293 cell line, leading to significant blockage of atRA production. Furthermore, we demonstrate that the suppression of ALDH1A1 by WT1 can be markedly attenuated by histone deacetylase inhibitors (HDACis). Taken together, we provide evidence to indicate that WT1 and HDACs are strong regulators of endogenous retinoic acid synthetic enzymes in 293 cells, indicating that they may be involved in the regulation of atRA synthesis.

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.

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.

Origin and Function of the Renal Stroma in Health and Disease

The renal stroma is defined as a heterogeneous population of cells that serve both as a supportive framework and as a source of specialized cells in the renal capsule, glomerulus, vasculature, and interstitium. In this chapter, we review published evidence defining what, where, and why stromal cells are important. We describe the functions of the renal stroma andhow stromal derivatives are crucial for normal kidney function. Next, we review the specification of stromal cells from the Osr1+ intermediate mesoderm and T+ presomitic mesoderm during embryogenesis and stromal cell differentiation. We focus on stromal signaling mechanisms that act in both a cell and non-cell autonomous manner in communication with the nephron progenitor and ureteric lineages. To conclude, stromal cells and the contribution of stromal cells to renal fibrosis and chronic kidney disease are described.

Enzymatic Metabolism of Vitamin A in Developing Vertebrate Embryos

Embryonic development is orchestrated by a small number of signaling pathways, one of which is the retinoic acid (RA) signaling pathway. Vitamin A is essential for vertebrate embryonic development because it is the molecular precursor of the essential signaling molecule RA. The level and distribution of RA signaling within a developing embryo must be tightly regulated; too much, or too little, or abnormal distribution, all disrupt embryonic development. Precise regulation of RA signaling during embryogenesis is achieved by proteins involved in vitamin A metabolism, retinoid transport, nuclear signaling, and RA catabolism. The reversible first step in conversion of the precursor vitamin A to the active retinoid RA is mediated by retinol dehydrogenase 10 (RDH10) and dehydrogenase/reductase (SDR family) member 3 (DHRS3), two related membrane-bound proteins that functionally activate each other to mediate the interconversion of retinol and retinal. Alcohol dehydrogenase (ADH) enzymes do not contribute to RA production under normal conditions during embryogenesis. Genes involved in vitamin A metabolism and RA catabolism are expressed in tissue-specific patterns and are subject to feedback regulation. Mutations in genes encoding these proteins disrupt morphogenesis of many systems in a developing embryo. Together these observations demonstrate the importance of vitamin A metabolism in regulating RA signaling during embryonic development in vertebrates.

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.

Interstitial stromal progenitors during kidney development: here, there and everywhere

In recent years, the renal interstitium has been identified as the site of multiple cell types, giving rise to multiple contiguous cellular networks with multiple fundamental structural and functional roles. Few studies have been carried out on the morphological and functional properties of the stromal/interstitial renal cells during the intrauterine life. This work was aimed at reviewing the peculiar features of renal interstitial stem/progenitor cells involved in kidney development. The origin of the renal interstitial progenitor cells remains unknown. During kidney development, besides the Six2 + cells of the cap mesenchyme, a self-renewing progenitor population, characterized by the expression of Foxd1, represents the first actor of the non-nephrogenic lineage. Foxd1 + interstitial progenitors originate the cortical and the renal medullary interstitial progenitors. Here, the most important stromal/interstitial compartments present in the developing human kidney will be analyzed: capsular stromal cells, cortical interstitial cells, medullary interstitial cells, the interstitium inside the renal stem cell niche, Hilar interstitial cells and Ureteric interstitial cells. Data reported here indicate that the different interstitial compartments of the developing kidney are formed by different cell types that characterize the different renal areas. Further studies are needed to better characterize the different pools of renal interstitial progenitors and their role in human nephrogenesis.

All-trans retinoic acid potentiates cisplatin-induced kidney injury in rats: impact of retinoic acid signaling pathway

Cisplatin (cis-diammine dichloroplatinum (II), CDDP) is a widely used drug for treatment of various types of cancers. However, CDDP-induced nephrotoxicity remains the main dose-limiting side effect. Retinoids are a group of vitamin A-related compounds that exert their effects through retinoid receptors activation. In this study, we investigated the effect of CDDP treatment on retinoic acid receptor-α (RAR-α) and retinoid X receptor-α (RXR-α) expression. In addition, we investigated the possible modulatory effects of RAR agonist, all-trans retinoic acid (ATRA), on CDDP-induced nephrotoxicity. Rats were treated with saline, DMSO, CDDP, ATRA, or CDDP/ATRA. Twenty-four hours after the last ATRA injection, rats were killed; blood samples were collected; kidneys were dissected; and biochemical, immunohistochemical, and histological examinations were performed. Our results revealed that CDDP treatment significantly increased serum levels of creatinine and urea, with concomitant decrease in serum albumin. Moreover, reduced glutathione (GSH) content as well as superoxide dismutase (SOD) and catalase (CAT) activities were significantly reduced with concurrent increase in kidney malondialdehyde (MDA) content following CDDP treatment. Furthermore, CDDP markedly upregulated tubular RAR-α, RXR-α, fibrin, and inducible nitric oxide synthase (iNOS) protein expression. Although administration of ATRA to control rats did not produce marked alterations in kidney function parameters, administration of ATRA to CDDP-treated rats significantly exacerbated CDDP-induced nephrotoxicity. In addition, CDDP/ATRA co-treatment significantly increased RAR-α, RXR-α, fibrin, and iNOS protein expression compared to CDDP alone. In conclusion, we report, for the first time, the crucial role of retinoid receptors in CDDP-induced nephrotoxicity. Moreover, our findings indicate that co-administration of ATRA with CDDP, although beneficial on the therapeutic effects, their deleterious effects on the kidney may limit their clinical use.

Mesenchymal Hox6 function is required for mouse pancreatic endocrine cell differentiation

Despite significant advances in our understanding of pancreatic endocrine cell development, the function of the pancreatic mesodermal niche in this process is poorly understood. Here we report a novel role for mouse Hox6 genes in pancreatic organogenesis. Hox6 genes are expressed exclusively in the mesoderm of the developing pancreas. Genetic loss of all three Hox6 paralogs (Hoxa6, Hoxb6 and Hoxc6) leads to a dramatic loss of endoderm-derived endocrine cells, including insulin-secreting β-cells, and to mild delays and disruptions in pancreatic branching and exocrine differentiation. Ngn3-expressing pan-endocrine progenitor cells are specified normally in Hox6 mutant pancreata, but fail to mature into hormone-producing cells. Reduced expression of Wnt5a is observed in mutant pancreatic mesenchyme, leading to subsequent loss of expression of the crucial Wnt inhibitors Sfrp3 and Dkk1 in endocrine progenitor cells. These results reveal a key role for Hox6 genes in establishing Wnt mesenchymal-epithelial crosstalk in pancreatic development.

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.

Stromal Fat4 acts non-autonomously with Dchs1/2 to restrict the nephron progenitor pool

Regulation of the balance between progenitor self-renewal and differentiation is crucial to development. In the mammalian kidney, reciprocal signalling between three lineages (stromal, mesenchymal and ureteric) ensures correct nephron progenitor self-renewal and differentiation. Loss of either the atypical cadherin FAT4 or its ligand Dachsous 1 (DCHS1) results in expansion of the mesenchymal nephron progenitor pool, called the condensing mesenchyme (CM). This has been proposed to be due to misregulation of the Hippo kinase pathway transcriptional co-activator YAP. Here, we use tissue-specific deletions to prove that FAT4 acts non-autonomously in the renal stroma to control nephron progenitors. We show that loss of Yap from the CM in Fat4-null mice does not reduce the expanded CM, indicating that FAT4 regulates the CM independently of YAP. Analysis of Six2(-/-);Fat4(-/-) double mutants demonstrates that excess progenitors in Fat4 mutants are dependent on Six2, a crucial regulator of nephron progenitor self-renewal. Electron microscopy reveals that cell organisation is disrupted in Fat4 mutants. Gene expression analysis demonstrates that the expression of Notch and FGF pathway components are altered in Fat4 mutants. Finally, we show that Dchs1, and its paralogue Dchs2, function in a partially redundant fashion to regulate the number of nephron progenitors. Our data support a model in which FAT4 in the stroma binds to DCHS1/2 in the mouse CM to restrict progenitor self-renewal.

Maintenance of Mouse Nephron Progenitor Cells in Aggregates with Gamma-Secretase Inhibitor

Knowledge on how to maintain and expand nephron progenitor cells (NPC) in vitro is important to provide a potentially valuable source for kidney replacement therapies. In our present study, we examined the possibility of optimizing NPC maintenance in the "re-aggregate" system. We found that Six2-expressing (Six2(+))-NPC could be maintained in aggregates reconstituted with dispersed cells from E12.5 mouse embryonic kidneys for at least up to 21 days in culture. The maintenance of Six2(+)-NPC required the presence of ureteric bud cells. The number of Six2(+)-NPC increased by more than 20-fold at day 21, but plateaued after day 14. In an attempt to further sustain NPC proliferation by passage subculture, we found that the new (P1) aggregates reconstituted from the original (P0) aggregates failed to maintain NPC. However, based on the similarity between P1 aggregates and aggregates derived from E15.5 embryonic kidneys, we suspected that the differentiated NPC in P1 aggregates may interfere with NPC maintenance. In support of this notion, we found that preventing NPC differentiation by DAPT, a γ-secretase inhibitor that inhibits Notch signaling pathway, was effective to maintain and expand Six2(+)-NPC in P1 aggregates by up to 65-fold. The Six2(+)-NPC in P1 aggregates retained their potential to epithelialize upon exposure to Wnt signal. In conclusion, we demonstrated in our present study that the "re-aggregation" system can be useful for in vitro maintenance of NPC when combined with γ-secretase inhibitor.

The beneficial role of retinoids in glomerular disease

The primary etiology of CKD is a direct consequence of initial dysfunction and injury of the glomerulus, the main filtration system. Podocytes are terminally differentiated epithelial cells in the glomerulus, whose major function is the maintenance of this renal filtration barrier. Podocyte injury is implicated in many glomerular diseases including focal segmental glomerular sclerosis and HIV-associated nephropathy. In many of these diseased conditions, the podocyte can either undergo dedifferentiation and proliferation, apoptosis, or cell detachment. Regardless of the initial type of injury, the podocyte ultimately loses its functional capacity to maintain the glomerular filtration barrier. Significant injury resulting in a loss of the podocytes and failure to maintain the renal filtration barrier contributes to progressive kidney disease. Consequently, therapies that prevent podocyte injury and promote their regeneration will have a major clinical impact on glomerular disease. Retinoic acid (RA), which is a derivative of vitamin A, has many cellular functions including induction of cell differentiation, regulation of apoptosis, and inhibition of inflammation and proliferation. RA is required for kidney development and is essential for cellular differentiation in the setting of podocyte injury. The mechanism by which RA directs its beneficial effects is multifactorial, ranging from its anti-inflammatory and anti-fibrotic effects to a direct effect of upregulating podocyte differentiation markers in the podocyte. The focus of this review is to provide an overview of RA in kidney development and glomerular disease. We also highlight the key mechanism(s) by which RA restores podocyte differentiation markers and ameliorates glomerular disease.

The ureteric bud epithelium: morphogenesis and roles in metanephric kidney patterning

The mammalian metanephric kidney is composed of two epithelial components, the collecting duct system and the nephron epithelium, that differentiate from two different tissues -the ureteric bud epithelium and the nephron progenitors, respectively-of intermediate mesoderm origin. The collecting duct system is generated through reiterative ureteric bud branching morphogenesis, whereas the nephron epithelium is formed in a process termed nephrogenesis, which is initiated with the mesenchymal-epithelial transition of the nephron progenitors. Ureteric bud branching morphogenesis is regulated by nephron progenitors, and in return, the ureteric bud epithelium regulates nephrogenesis. The metanephric kidney is physiologically divided along the corticomedullary axis into subcompartments that are enriched with specific segments of these two epithelial structures. Here, we provide an overview of the major molecular and cellular processes underlying the morphogenesis and patterning of the ureteric bud epithelium and its roles in the cortico-medullary patterning of the metanephric kidney.

Nephron reconstitution from pluripotent stem cells

It has been a challenge in developmental biology and regenerative medicine to generate nephron progenitors that reconstitute the three-dimensional (3D) nephron structure in vitro. Many studies have tried to induce nephron progenitors from pluripotent stem cells by mimicking the developmental processes in vivo. However, the current developmental model does not precisely address the spatiotemporal origin of nephron progenitors, hampering our understanding of cell fate decisions in the kidney. Here, we present a revised model of early-stage kidney specification, suggesting distinct origins of the two major kidney components: the ureteric bud and metanephric mesenchyme. This model enables the induction of metanephric nephron progenitors from both mouse and human pluripotent stem cells. The induced cells self-organize in the presence of Wnt signaling and reconstitute 3D nephron structures including both nephric tubules with a clear lumina and glomeruli with podocytes. The engrafted kidney tissue develops vascularized glomeruli and nephric tubules, but it does not produce urine, suggesting the requirement for further maturation. Nevertheless, the generation of nephron components from human-induced pluripotent stem cells will be useful for future application in regenerative therapy and modeling of congenital kidney diseases in vitro. This review discusses the possibility of de novo organogenesis of a functional kidney from pluripotent stem cells and the future direction toward clinical applications.

Identification of a novel isoform of the leukemia-associated MLLT1 (ENL/LTG19) protein

Genome wide transcriptional profiles offer abundant information regarding mRNA levels in specific tissues, organs or developmental stages. Although these data sets do not offer spatial or cell type-specific information, they can be extremely useful for gene discovery when analyzed by the appropriate techniques. Previously, we proposed and validated the use of combinatorial dataset analysis techniques to identify novel genes required during pre-implantation development. Now we build upon this work to identify genes that have dynamic expression during gametogenesis. Here we present detailed analysis of the expression pattern of leukemia-associated, myeloid/lymphoid or mixed-lineage leukemia; translocated to 1 (Mllt1) gene. We document a novel splice isoform of Mllt1 and confirm that both Mllt1 mRNA isoforms are translated. We provide data supporting that MLLT1 protein isoforms display distinct stage-specific expression during spermiogenesis and adult tissues. Finally, we evaluated genes neighboring the Mllt1 locus, and show dynamic stage specific expression patterns of other genes Catsperd, Prr22, Rfx2 and Slc25a41. We document testes expressed alternative isoforms of Prr22 and Rfx2. These results indicate that transcriptome data mining, combined with specific expression analysis provides a wealth of novel gene expression information.

Vitamin a deficiency and alterations in the extracellular matrix

Vitamin A or retinol which is the natural precursor of several biologically active metabolites can be considered the most multifunctional vitamin in mammals. Its deficiency is currently, along with protein malnutrition, the most serious and common nutritional disorder worldwide. It is necessary for normal embryonic development and postnatal tissue homeostasis, and exerts important effects on cell proliferation, differentiation and apoptosis. These actions are produced mainly by regulating the expression of a variety of proteins through transcriptional and non-transcriptional mechanisms. Extracellular matrix proteins are among those whose synthesis is known to be modulated by vitamin A. Retinoic acid, the main biologically active form of vitamin A, influences the expression of collagens, laminins, entactin, fibronectin, elastin and proteoglycans, which are the major components of the extracellular matrix. Consequently, the structure and macromolecular composition of this extracellular compartment is profoundly altered as a result of vitamin A deficiency. As cell behavior, differentiation and apoptosis, and tissue mechanics are influenced by the extracellular matrix, its modifications potentially compromise organ function and may lead to disease. This review focuses on the effects of lack of vitamin A in the extracellular matrix of several organs and discusses possible molecular mechanisms and pathologic implications.

Induction and patterning of the metanephric nephron

The functional unit of the mammalian metanephric kidney is the nephron: a complex tubular structure dedicated to blood filtration and maintenance of several important physiological functions. Nephrons are assembled from a nephron-restricted pool of mesenchymal progenitors over an extensive developmental period that is completed prior to (human), or shortly after (mouse), birth. An appropriate balance in the expansion and commitment of nephron progenitors to nephron formation is essential for normal kidney function. Too few nephrons increase risk of kidney disease later in life while the failure of normal progenitor differentiation in Wilm's tumor patients leads to massive growth of a nephroblast population often necessitating surgical removal of the kidney. An inductive process within the metanephric mesenchyme leads to the formation of a pretubular aggregate which transitions into an epithelial renal vesicle: the precursor for nephron assembly. Growth, morphogenesis and patterning transform this simple cyst-like structure into a highly elongated mature nephron with distinct cell types positioned along a proximal (glomerular) to distal (connecting segment) axis of functional organization. This review discusses our current understanding of the specification, maintenance and commitment of nephron progenitors, and the regulatory processes that transform the renal vesicle into a nephron.

Renal branching morphogenesis: morphogenetic and signaling mechanisms

The human kidney is composed of an arborized network of collecting ducts, calyces and urinary pelvis that facilitate urine excretion and regulate urine composition. The renal collecting system is formed in utero, completed by the 34th week of gestation in humans, and dictates final nephron complement. The renal collecting system arises from the ureteric bud, a derivative of the intermediate-mesoderm derived nephric duct that responds to inductive signals from adjacent tissues via a process termed ureteric induction. The ureteric bud subsequently undergoes a series of iterative branching and remodeling events in a process called renal branching morphogenesis. Altered signaling that disrupts patterning of the nephric duct, ureteric induction, or renal branching morphogenesis leads to varied malformations of the renal collecting system collectively known as congenital anomalies of the kidney and urinary tract (CAKUT) and is the most frequently detected congenital renal aberration in infants. Here, we describe critical morphogenetic and cellular events that govern nephric duct specification, ureteric bud induction, renal branching morphogenesis, and cessation of renal branching morphogenesis. We also highlight salient molecular signaling pathways that govern these processes, and the investigative techniques used to interrogate them.

Wnt signaling inhibits adrenal steroidogenesis by cell-autonomous and non-cell-autonomous mechanisms

Wnt/β-catenin (βcat) signaling is critical for adrenal homeostasis. To elucidate how Wnt/βcat signaling elicits homeostatic maintenance of the adrenal cortex, we characterized the identity of the adrenocortical Wnt-responsive population. We find that Wnt-responsive cells consist of sonic hedgehog (Shh)-producing adrenocortical progenitors and differentiated, steroidogenic cells of the zona glomerulosa, but not the zona fasciculata and rarely cells that are actively proliferating. To determine potential direct inhibitory effects of βcat signaling on zona fasciculata-associated steroidogenesis, we used the mouse ATCL7 adrenocortical cell line that serves as a model system of glucocorticoid-producing fasciculata cells. Stimulation of βcat signaling caused decreased corticosterone release consistent with the observed reduced transcription of steroidogenic genes Cyp11a1, Cyp11b1, Star, and Mc2r. Decreased steroidogenic gene expression was correlated with diminished steroidogenic factor 1 (Sf1; Nr5a1) expression and occupancy on steroidogenic promoters. Additionally, βcat signaling suppressed the ability of Sf1 to transactivate steroidogenic promoters independent of changes in Sf1 expression level. To investigate Sf1-independent effects of βcat on steroidogenesis, we used Affymetrix gene expression profiling of Wnt-responsive cells in vivo and in vitro. One candidate gene identified, Ccdc80, encodes a secreted protein with unknown signaling mechanisms. We report that Ccdc80 is a novel βcat-regulated gene in adrenocortical cells. Treatment of adrenocortical cells with media containing secreted Ccdc80 partially phenocopies βcat-induced suppression of steroidogenesis, albeit through an Sf1-independent mechanism. This study reveals multiple mechanisms of βcat-mediated suppression of steroidogenesis and suggests that Wnt/βcat signaling may regulate adrenal homeostasis by inhibiting fasciculata differentiation and promoting the undifferentiated state of progenitor cells.