Generation of functional chimeric kidney containing exogenous progenitor-derived stroma and nephron via a conditional empty niche

Generation of new kidneys can be useful in various research fields, including organ transplantation. However, generating renal stroma, an important component tissue for structural support, endocrine function, and kidney development, remains difficult. Organ generation using an animal developmental niche can provide an appropriate in vivo environment for renal stroma differentiation. Here, we generate rat renal stroma with endocrine capacity by removing mouse stromal progenitor cells (SPCs) from the host developmental niche and transplanting rat SPCs. Furthermore, we develop a method to replace both nephron progenitor cells (NPCs) and SPCs, called the interspecies dual replacement of the progenitor (i-DROP) system, and successfully generate functional chimeric kidneys containing rat nephrons and stroma. This method can generate renal tissue from progenitors and reduce xenotransplant rejection. Moreover, it is a safe method, as donor cells do not stray into nontarget organs, thus accelerating research on stem cells, chimeras, and xenotransplantation.

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

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

Identification of candidate PAX2-regulated genes implicated in human kidney development

PAX2 is a transcription factor essential for kidney development and the main causative gene for renal coloboma syndrome (RCS). The mechanisms of PAX2 action during kidney development have been evaluated in mice but not in humans. This is a critical gap in knowledge since important differences have been reported in kidney development in the two species. In the present study, we hypothesized that key human PAX2-dependent kidney development genes are differentially expressed in nephron progenitor cells from induced pluripotent stem cells (iPSCs) in patients with RCS relative to healthy individuals. Cap analysis of gene expression revealed 189 candidate promoters and 71 candidate enhancers that were differentially activated by PAX2 in this system in three patients with RCS with PAX2 mutations. By comparing this list with the list of candidate Pax2-regulated mouse kidney development genes obtained from the Functional Annotation of the Mouse/Mammalian (FANTOM) database, we prioritized 17 genes. Furthermore, we ranked three genes-PBX1, POSTN, and ITGA9-as the top candidates based on closely aligned expression kinetics with PAX2 in the iPSC culture system and susceptibility to suppression by a Pax2 inhibitor in cultured mouse embryonic kidney explants. Identification of these genes may provide important information to clarify the pathogenesis of RCS, human kidney development, and kidney regeneration.

Single cell regulatory landscape of the mouse kidney highlights cellular differentiation programs and disease targets

Determining the epigenetic program that generates unique cell types in the kidney is critical for understanding cell-type heterogeneity during tissue homeostasis and injury response. Here, we profile open chromatin and gene expression in developing and adult mouse kidneys at single cell resolution. We show critical reliance of gene expression on distal regulatory elements (enhancers). We reveal key cell type-specific transcription factors and major gene-regulatory circuits for kidney cells. Dynamic chromatin and expression changes during nephron progenitor differentiation demonstrates that podocyte commitment occurs early and is associated with sustained Foxl1 expression. Renal tubule cells follow a more complex differentiation, where Hfn4a is associated with proximal and Tfap2b with distal fate. Mapping single nucleotide variants associated with human kidney disease implicates critical cell types, developmental stages, genes, and regulatory mechanisms. The single cell multi-omics atlas reveals key chromatin remodeling events and gene expression dynamics associated with kidney development.

New imaging tools to measure nephron number in vivo: opportunities for developmental nephrology

The mammalian kidney is a complex organ, requiring the concerted function of up to millions of nephrons. The number of nephrons is constant after nephrogenesis during development, and nephron loss over a life span can lead to susceptibility to acute or chronic kidney disease. New technologies are under development to count individual nephrons in the kidney in vivo. This review outlines these technologies and highlights their relevance to studies of human renal development and disease.

Knockdown of Nuclear lncRNAs by Locked Nucleic Acid (LNA) Gapmers in Nephron Progenitor Cells

Despite recent advance in our understanding on the role of long noncoding RNAs (lncRNAs), the function of the vast majority of lncRNAs remains poorly understood. To characterize the function of lncRNAs, knockdown studies are essential. However, the conventional silencing methods for mRNA, such as RNA interference (RNAi), may not be as efficient against lncRNAs, partly due to the mismatch of the localization of lncRNAs and RNAi machinery. To circumvent such limitation, a new technique has recently been developed, i.e., locked nucleic acid (LNA) gapmers. This system utilizes RNase H that distributes evenly in both nucleus and cytoplasm and is expected to knock down lncRNAs of interest more consistently regardless of their localization in the cell. In this chapter, we describe the procedure with tips to silence lncRNAs by LNA gapmers, by using mouse nephron progenitor cells as an example.

Targeting the Renin-Angiotensin-Aldosterone System to Prevent Hypertension and Kidney Disease of Developmental Origins

The renin-angiotensin-aldosterone system (RAAS) is implicated in hypertension and kidney disease. The developing kidney can be programmed by various early-life insults by so-called renal programming, resulting in hypertension and kidney disease in adulthood. This theory is known as developmental origins of health and disease (DOHaD). Conversely, early RAAS-based interventions could reverse program processes to prevent a disease from occurring by so-called reprogramming. In the current review, we mainly summarize (1) the current knowledge on the RAAS implicated in renal programming; (2) current evidence supporting the connections between the aberrant RAAS and other mechanisms behind renal programming, such as oxidative stress, nitric oxide deficiency, epigenetic regulation, and gut microbiota dysbiosis; and (3) an overview of how RAAS-based reprogramming interventions may prevent hypertension and kidney disease of developmental origins. To accelerate the transition of RAAS-based interventions for prevention of hypertension and kidney disease, an extended comprehension of the RAAS implicated in renal programming is needed, as well as a greater focus on further clinical translation.

Birth Weight and Susceptibility to Chronic Kidney Disease

The worldwide prevalence of noncommunicable diseases (NCDs) is projected to increase substantially over the next few decades. Chronic kidney disease (CKD) is a key determinant of poor health outcomes for major NCD. Genetic predisposition and environmental exposures are contributory factors, but increasingly, it is being recognized that fetal development is also an important modulator of the NCD risk. Low birth weight (LBW) and CKD affect more disadvantaged populations and ethnic minorities and, therefore, causes a disproportionate burden on the poor. Human nephron number is highly variable and may range from under half a million to almost over two million. Significant variability is already present at birth, highlighting the importance of early nephrogenesis. Nearly 60% of nephrons are developed in the third-trimester of pregnancy. Nephron numbers increase in proportion to birth weight and gestational age. This wide-variability probably contributes to individual susceptibility to develop CKD where individuals with nephron numbers on the lower side of the spectrum are those at higher risk of developing kidney dysfunction at higher rate and progress more toward end-stage CKD. This article aims at discussing LBW and the susceptibility to CKD. Furthermore, in postnatal environment, the weight gain or change at adult life increases the metabolic demand and determines the phenotypic expression of disease along with the spectrum of nephron number. Hence, a cycle of hyperfiltration mechanism of these nephrons leads to proteinuria, glomerulo- sclerosis, and progressive development of larger glomeruli, a greater risk of proteinuria and progressive CKD. Therefore, LBW offspring are at risk of developing CKD (defined as albuminuria, a reduced glomerular filtration rate, or renal failure) in later life. Furthermore, the impact of prenatal programming is expected to be compounded with age, and the association of LBW with the risk of CKD seen in younger adults may become greater with age. It would be prudent, to adopt policies of intensified life-long surveillance of LBW people, anticipating this risk.

Multivariate patterning of human pluripotent cells under perfusion reveals critical roles of induced paracrine factors in kidney organoid development

Creating complex multicellular kidney organoids from pluripotent stem cells shows great promise. Further improvements in differentiation outcomes, patterning, and maturation of specific cell types are, however, intrinsically limited by standard tissue culture approaches. We describe a novel full factorial microbioreactor array-based methodology to achieve rapid interrogation and optimization of this complex multicellular differentiation process in a facile manner. We successfully recapitulate early kidney tissue patterning events, exploring more than 1000 unique conditions in an unbiased and quantitative manner, and define new media combinations that achieve near-pure renal cell type specification. Single-cell resolution identification of distinct renal cell types within multilayered kidney organoids, coupled with multivariate analysis, defined the definitive roles of Wnt, fibroblast growth factor, and bone morphogenetic protein signaling in their specification, exposed retinoic acid as a minimal effector of nephron patterning, and highlighted critical contributions of induced paracrine factors on cell specification and patterning.

Von Hippel-Lindau Acts as a Metabolic Switch Controlling Nephron Progenitor Differentiation

BACKGROUND

Nephron progenitors, the cell population that give rise to the functional unit of the kidney, are metabolically active and self-renew under glycolytic conditions. A switch from glycolysis to mitochondrial respiration drives these cells toward differentiation, but the mechanisms that control this switch are poorly defined. Studies have demonstrated that kidney formation is highly dependent on oxygen concentration, which is largely regulated by von Hippel-Lindau (VHL; a protein component of a ubiquitin ligase complex) and hypoxia-inducible factors (a family of transcription factors activated by hypoxia).

METHODS

To explore VHL as a regulator defining nephron progenitor self-renewal versus differentiation, we bred Six2-TGCtg mice with VHLlox/lox mice to generate mice with a conditional deletion of VHL from Six2+ nephron progenitors. We used histologic, immunofluorescence, RNA sequencing, and metabolic assays to characterize kidneys from these mice and controls during development and up to postnatal day 21.

RESULTS

By embryonic day 15.5, kidneys of nephron progenitor cell-specific VHL knockout mice begin to exhibit reduced maturation of nephron progenitors. Compared with controls, VHL knockout kidneys are smaller and developmentally delayed by postnatal day 1, and have about half the number of glomeruli at postnatal day 21. VHL knockout nephron progenitors also exhibit persistent Six2 and Wt1 expression, as well as decreased mitochondrial respiration and prolonged reliance on glycolysis.

CONCLUSIONS

Our findings identify a novel role for VHL in mediating nephron progenitor differentiation through metabolic regulation, and suggest that VHL is required for normal kidney development.

Evolution, kidney development, and chronic kidney disease

There is a global epidemic of chronic kidney disease (CKD) characterized by a progressive loss of nephrons, ascribed in large part to a rising incidence of hypertension, metabolic syndrome, and type 2 diabetes mellitus. There is a ten-fold variation in nephron number at birth in the general population, and a 50% overall decrease in nephron number in the last decades of life. The vicious cycle of nephron loss stimulating hypertrophy by remaining nephrons and resulting in glomerulosclerosis has been regarded as maladaptive, and only partially responsive to angiotensin inhibition. Advances over the past century in kidney physiology, genetics, and development have elucidated many aspects of nephron formation, structure and function. Parallel advances have been achieved in evolutionary biology, with the emergence of evolutionary medicine, a discipline that promises to provide new insight into the treatment of chronic disease. This review provides a framework for understanding the origins of contemporary developmental nephrology, and recent progress in evolutionary biology. The establishment of evolutionary developmental biology (evo-devo), ecological developmental biology (eco-devo), and developmental origins of health and disease (DOHaD) followed the discovery of the hox gene family, the recognition of the contribution of cumulative environmental stressors to the changing phenotype over the life cycle, and mechanisms of epigenetic regulation. The maturation of evolutionary medicine has contributed to new investigative approaches to cardiovascular disease, cancer, and infectious disease, and promises the same for CKD. By incorporating these principles, developmental nephrology is ideally positioned to answer important questions regarding the fate of nephrons from embryo through senescence.

DNA Methyltransferase 1 Controls Nephron Progenitor Cell Renewal and Differentiation

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Generation of Three-Dimensional Nephrons from Mouse and Human Pluripotent Stem Cells

Nephrons, the functional units of the kidney, are derived from nephron progenitor cells (NPCs). Here, we describe methods to reconstruct nephron tissue via induction of NPCs from mouse and human pluripotent stem cells, which mimic multistep developmental signals in vivo. Induced NPCs differentiate into three-dimensional nephron structures, including glomerular podocytes and nephric tubules, which are useful for studying early stages of kidney specification and morphogenetic processes in the context of normal development or disease.

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

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

Regenerative potential of induced pluripotent stem cells derived from patients undergoing haemodialysis in kidney regeneration

Kidney regeneration from pluripotent stem cells is receiving a lot of attention because limited treatments are currently available for chronic kidney disease (CKD). It has been shown that uremic state in CKD is toxic to somatic stem/progenitor cells, such as endothelial progenitor and mesenchymal stem cells, affecting their differentiation and angiogenic potential. Recent studies reported that specific abnormalities caused by the non-inherited disease are often retained in induced pluripotent stem cell (iPSC)-derived products obtained from patients. Thus, it is indispensable to first assess whether iPSCs derived from patients with CKD due to non-inherited disease (CKD-iPSCs) have the ability to generate kidneys. In this study, we generated iPSCs from patients undergoing haemodialysis due to diabetes nephropathy and glomerulonephritis (HD-iPSCs) as representatives of CKD-iPSCs or from healthy controls (HC-iPSCs). HD-iPSCs differentiated into nephron progenitor cells (NPCs) with similar efficiency to HC-iPSCs. Additionally, HD-iPSC-derived NPCs expressed comparable levels of NPC markers and differentiated into vascularised glomeruli upon transplantation into mice, as HC-iPSC-derived NPCs. Our results indicate the potential of HD-iPSCs as a feasible cell source for kidney regeneration. This is the first study paving the way for CKD patient-stem cell-derived kidney regeneration, emphasising the potential of CKD-iPSCs.

Epigenetic regulation of renal development

Developmental changes in cell fate are tightly regulated by cell-type specific transcription factors. Chromatin reorganization during organismal development ensures dynamic access of developmental regulators to their cognate DNA sequences. Thus, understanding the epigenomic states of promoters and enhancers is of key importance. Recent years have witnessed significant advances in our knowledge of the transcriptional mechanisms of kidney development. Emerging evidence suggests that histone deacetylation by class I HDACs and H3 methylation on lysines 4, 27 and 79 play important roles in regulation of early and late gene expression in the developing kidney. Equally exciting is the realization that nephrogenesis genes in mesenchymal nephron progenitors harbor bivalent chromatin domains which resolve upon differentiation implicating chromatin bivalency in developmental control of gene expression. Here, we review current knowledge of the epigenomic states of nephric cells and current techniques used to study the dynamic chromatin states. These technological advances will provide an unprecedented view of the enhancer landscape during cell fate commitment and help in defining the complex transcriptional networks governing kidney development and disease.

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.

Growing Kidney Tissue from Stem Cells: How Far from "Party Trick" to Medical Application?

The successful generation of kidney-like structures from human pluripotent stem cells, although slower to come than other tissue types, brings the hope of new therapies. While the demand for alternative treatments for kidney failure is acute, huge challenges remain to move these exciting but preliminary results toward clinical use.

3D Culture Supports Long-Term Expansion of Mouse and Human Nephrogenic Progenitors

Transit-amplifying nephron progenitor cells (NPCs) generate all of the nephrons of the mammalian kidney during development. Their limited numbers, poor in vitro expansion, and difficult accessibility in humans have slowed basic and translational research into renal development and diseases. Here, we show that with appropriate 3D culture conditions, it is possible to support long-term expansion of primary mouse and human fetal NPCs as well as NPCs derived from human induced pluripotent stem cells (iPSCs). Expanded NPCs maintain genomic stability, molecular homogeneity, and nephrogenic potential in vitro, ex vivo, and in vivo. Cultured NPCs are amenable to gene targeting and can form nephron organoids that engraft in vivo, functionally couple to the host's circulatory system, and produce urine-like metabolites via filtration. Together, these findings provide a technological platform for studying human nephrogenesis, modeling and diagnosing renal diseases, and drug discovery.

Kindling the Kidney

Investigators have generated kidney-like organoids - complete with nephrons, collecting ducts, stroma, and vasculature - from induced pluripotent stem 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.

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.

Effects of exercise on the nephron of Goto-Kakizaki rats: morphological, and advanced glycation end-products and inducible nitric oxide synthase immunohistochemical analyses

The current study aimed to examine how exercise affects morphology of the nephron, and localization of advanced glycation end-products (AGEs) and inducible nitric oxide synthase (iNOS) immunoreactivity in diabetic Goto-Kakizaki rats. Four groups of male rats were studied. WIS SED (Wistar rats; sedentary) group served as a control. Other groups were WIS EX (Wistar rats; exercise), GK SED (Goto-Kakizaki diabetic rats; sedentary) and GK EX (Goto-Kakizaki diabetic rats; exercise) groups. The rats in EX groups were subjected to 15weeks of treadmill running at a speed of 15m/min for a total of 30minutes, three times a week. Changes in the structure of renal corpuscles and in the distribution of AGEs- and iNOS-immunoreactive cells of the uriniferous tubules were evaluated. Every parameter of GK EX was significantly different from that of GK SED (area of Bowman's capsules: p<0.001, area of glomeruli: p<0.05 and the occupancy of a glomerulus: p<0.05). These findings suggest that exercise may ameliorate glomerular filtration rate (GFR). The localizations of AGEs and iNOS immunostaining in the uriniferous tubules were similar in each group. Immunohistochemical assays revealed that the number of the AGEs and iNOS immunopositive cells of the proximal tubule of cortico-deep layer in EX groups were markedly greater than those in SED groups and that iNOS expression in GK EX was significantly higher than GK SED (p<0.05). Exercise seems to normalize the GFR and glomerular filtrate absorption from the uriniferous tubules in Goto-Kakizaki diabetic rats with the recovered shape of renal corpuscles and may be involved in the absorption and catabolization of AGEs with iNOS-related reactions for reabsorption.

Prolonging nephrogenesis in preterm infants: a new approach for prevention of kidney disease in adulthood?

Chronic kidney disease represents a dramatic worldwide resource-consuming problem. This problem is of increasing importance even in preterm infants, since nephrogenesis may go on only for a few weeks (4 to 6 weeks) after birth. Recent literature focusing on traditional regenerative medicine does not take into account the presence of a high number of active endogenous stem cells in the preterm kidney, which represents a unique opportunity for starting regenerative medicine in the perinatal period. Pluripotent cells of the blue strip have the capacity to generate new nephrons, improving kidney function in neonates and potentially protecting them from developing chronic kidney disease and end-stage renal disease in adulthood. There is a marked interindividual neonatal variability of nephron numbers. Moreover, the renal stem/progenitor cells appear as densely-packed small cells with scant cytoplasm, giving rise to a blue-appearing strip in hematoxylin-eosin-stained kidney sections ("the blue strip"). There are questions concerning renal regenerative medicine: among preliminary data, the simultaneous expression of Wilms tumor 1 and thymosin β4 in stem/progenitor cells of the neonatal kidney may bring new prospects for renal regeneration applied to renal stem cells that reside in the kidney itself. A potential approach could be to prolong the 6 weeks of postnatal renal growth of nephrons or to accelerate the growth of nephrons during the 6 weeks or both. Considering what we know today about perinatal programming, this could be an important step for the future to reduce the incidence and global health impact of chronic kidney disease.

Integrated β-catenin, BMP, PTEN, and Notch signalling patterns the nephron

The different segments of the nephron and glomerulus in the kidney balance the processes of water homeostasis, solute recovery, blood filtration, and metabolite excretion. When segment function is disrupted, a range of pathological features are presented. Little is known about nephron patterning during embryogenesis. In this study, we demonstrate that the early nephron is patterned by a gradient in β-catenin activity along the axis of the nephron tubule. By modifying β-catenin activity, we force cells within nephrons to differentiate according to the imposed β-catenin activity level, thereby causing spatial shifts in nephron segments. The β-catenin signalling gradient interacts with the BMP pathway which, through PTEN/PI3K/AKT signalling, antagonises β-catenin activity and promotes segment identities associated with low β-catenin activity. β-catenin activity and PI3K signalling also integrate with Notch signalling to control segmentation: modulating β-catenin activity or PI3K rescues segment identities normally lost by inhibition of Notch. Our data therefore identifies a molecular network for nephron patterning.

Concise review: different mesenchymal stromal/stem cell populations reside in the adult kidney

During fetal life, mesenchymal stromal/stem cells (MSCs) surround glomeruli and tubules and contribute to the development of the renal interstitium by secretion of growth factors that drive nephron differentiation. In the adult, an MSC-like population has been demonstrated in different compartments of human and murine nephrons. After injury, these cells might provide support for kidney regeneration by recapitulating the role they have in embryonic life. In this short review, we discuss the evidence of an MSC presence within the adult kidney and their potential contribution to the turnover of renal cells and injury repair.

Renal blood flow and oxygenation drive nephron progenitor differentiation

During kidney development, the vasculature develops via both angiogenesis (branching from major vessels) and vasculogenesis (de novo vessel formation). The formation and perfusion of renal blood vessels are vastly understudied. In the present study, we investigated the regulatory role of renal blood flow and O2 concentration on nephron progenitor differentiation during ontogeny. To elucidate the presence of blood flow, ultrasound-guided intracardiac microinjection was performed, and FITC-tagged tomato lectin was perfused through the embryo. Kidneys were costained for the vasculature, ureteric epithelium, nephron progenitors, and nephron structures. We also analyzed nephron differentiation in normoxia compared with hypoxia. At embryonic day 13.5 (E13.5), the major vascular branches were perfused; however, smaller-caliber peripheral vessels remained unperfused. By E15.5, peripheral vessels started to be perfused as well as glomeruli. While the interior kidney vessels were perfused, the peripheral vessels (nephrogenic zone) remained unperfused. Directly adjacent and internal to the nephrogenic zone, we found differentiated nephron structures surrounded and infiltrated by perfused vessels. Furthermore, we determined that at low O2 concentration, little nephron progenitor differentiation was observed; at higher O2 concentrations, more differentiation of the nephron progenitors was induced. The formation of the developing renal vessels occurs before the onset of blood flow. Furthermore, renal blood flow and oxygenation are critical for nephron progenitor differentiation.

Differentiation of human pluripotent stem cells into nephron progenitor cells in a serum and feeder free system

Objectives

Kidney disease is emerging as a critical medical problem worldwide. Because of limited treatment options for the damaged kidney, stem cell treatment is becoming an alternative therapeutic approach. Of many possible human stem cell sources, pluripotent stem cells are most attractive due to their self-renewal and pluripotent capacity. However, little is known about the derivation of renal lineage cells from human pluripotent stem cells (hPSCs). In this study, we developed a novel protocol for differentiation of nephron progenitor cells (NPCs) from hPSCs in a serum- and feeder-free system.

Materials and Methods

We designed step-wise protocols for differentiation of human pluripotent stem cells toward primitive streak, intermediate mesoderm and NPCs by recapitulating normal nephrogenesis. Expression of key marker genes was examined by RT-PCR, real time RT-PCR and immunocytochemistry. Each experiment was independently performed three times to confirm its reproducibility.

Results

After modification of culture period and concentration of exogenous factors, hPSCs can differentiate into NPCs that markedly express specific marker genes such as SIX2, GDNF, HOXD11, WT1 and CITED1 in addition to OSR1, PAX2, SALL1 and EYA1. Moreover, NPCs possess the potential of bidirectional differentiation into both renal tubular epithelial cells and glomerular podocytes in defined culture conditions. In particular, approximately 70% of SYN-positive cells were obtained from hPSC-derived NPCs after podocytes induction. NPCs can also form in vitro tubule-like structures in three dimensional culture systems.

Conclusions

Our novel protocol for hPSCs differentiation into NPCs can be useful for producing alternative sources of cell replacement therapy and disease modeling for human kidney diseases.

Ablation of the renal stroma defines its critical role in nephron progenitor and vasculature patterning

The renal stroma is an embryonic cell population located in the cortex that provides a structural framework as well as a source of endothelial progenitors for the developing kidney. The exact role of the renal stroma in normal kidney development hasn't been clearly defined. However, previous studies have shown that the genetic deletion of Foxd1, a renal stroma specific gene, leads to severe kidney malformations confirming the importance of stroma in normal kidney development. This study further investigates the role of renal stroma by ablating Foxd1-derived stroma cells themselves and observing the response of the remaining cell populations. A Foxd1cre (renal stroma specific) mouse was crossed with a diphtheria toxin mouse (DTA) to specifically induce apoptosis in stromal cells. Histological examination of kidneys at embryonic day 13.5–18.5 showed a lack of stromal tissue, mispatterning of renal structures, and dysplastic and/or fused horseshoe kidneys. Immunofluorescence staining of nephron progenitors, vasculature, ureteric epithelium, differentiated nephron progenitors, and vascular supportive cells revealed that mutants had thickened nephron progenitor caps, cortical regions devoid of nephron progenitors, aberrant vessel patterning and thickening, ureteric branching defects and migration of differentiated nephron structures into the medulla. The similarities between the renal deformities caused by Foxd1 genetic knockout and Foxd1DTA mouse models reveal the importance of Foxd1 in mediating and maintaining the functional integrity of the renal stroma.

Identification of human nephron progenitors capable of generation of kidney structures and functional repair of chronic renal disease

Identification of tissue-specific renal stem/progenitor cells with nephrogenic potential is a critical step in developing cell-based therapies for renal disease. In the human kidney, stem/progenitor cells are induced into the nephrogenic pathway to form nephrons until the 34 week of gestation, and no equivalent cell types can be traced in the adult kidney. Human nephron progenitor cells (hNPCs) have yet to be isolated. Here we show that growth of human foetal kidneys in serum-free defined conditions and prospective isolation of NCAM1(+) cells selects for nephron lineage that includes the SIX2-positive cap mesenchyme cells identifying a mitotically active population with in vitro clonogenic and stem/progenitor properties. After transplantation in the chick embryo, these cells-but not differentiated counterparts-efficiently formed various nephron tubule types. hNPCs engrafted and integrated in diseased murine kidneys and treatment of renal failure in the 5/6 nephrectomy kidney injury model had beneficial effects on renal function halting disease progression. These findings constitute the first definition of an intrinsic nephron precursor population, with major potential for cell-based therapeutic strategies and modelling of kidney disease.

Bmp7 maintains undifferentiated kidney progenitor population and determines nephron numbers at birth

The number of nephrons, the functional units of the kidney, varies among individuals. A low nephron number at birth is associated with a risk of hypertension and the progression of renal insufficiency. The molecular mechanisms determining nephron number during embryogenesis have not yet been clarified. Germline knockout of bone morphogenetic protein 7 (Bmp7) results in massive apoptosis of the kidney progenitor cells and defects in early stages of nephrogenesis. This phenotype has precluded analysis of Bmp7 function in the later stage of nephrogenesis. In this study, utilization of conditional null allele of Bmp7 in combination with systemic inducible Cre deleter mice enabled us to analyze Bmp7 function at desired time points during kidney development, and to discover the novel function of Bmp7 to inhibit the precocious differentiation of the progenitor cells to nephron. Systemic knockout of Bmp7 in vivo after the initiation of kidney development results in the precocious differentiation of the kidney progenitor cells to nephron, in addition to the prominent apoptosis of progenitor cells. We also confirmed that in vitro knockout of Bmp7 in kidney explant culture results in the accelerated differentiation of progenitor population. Finally we utilized colony-forming assays and demonstrated that Bmp7 inhibits epithelialization and differentiation of the kidney progenitor cells. These results indicate that the function of Bmp7 to inhibit the precocious differentiation of the progenitor cells together with its anti-apoptotic effect on progenitor cells coordinately maintains renal progenitor pool in undifferentiated status, and determines the nephron number at birth.

Superficial nephrons in BALB/c and C57BL/6 mice facilitate in vivo multiphoton microscopy of the kidney

Multiphoton microscopy (MPM) offers a unique approach for addressing both the function and structure of an organ in near-real time in the live animal. The method however is limited by the tissue-specific penetration depth of the excitation laser. In the kidney, structures in the range of 100 µm from the surface are accessible for MPM. This limitation of MPM aggravates the investigation of the function of structures located deeper in the renal cortex, like the glomerulus and the juxtaglomerular apparatus. In view of the relevance of gene-targeted mice for investigating the function of these structures, we aimed to identify a mouse strain with a high percentage of superficially located glomeruli. The mean distance of the 30 most superficial glomeruli from the kidney surface was determined in 10 commonly used mouse strains. The mean depth of glomeruli was 118.4±3.4, 123.0±2.7, 133.7±3.0, 132.3±2.6, 141.0±4.0, 145.3±4.3, 148.9±4.2, 151.6±2.7, 167.7±3.9, and 207.8±3.2 µm in kidney sections from 4-week-old C3H/HeN, BALB/cAnN, SJL/J, C57BL/6N, DBA/2N, CD1 (CRI), 129S2/SvPas, CB6F1, FVB/N and NMRI (Han) mice, respectively (n = 5 animals from each strain). The mean distance from the kidney surface of the most superficial glomeruli was significantly lower in the strains C3H/HeN Crl, BALB/cAnN, DBA/2NCrl, and C57BL/6N when compared to a peer group consisting of all the other strains (p<.0001). In 10-week-old mice, the most superficial glomeruli were located deeper in the cortex when compared to 4-week-old animals, with BALB/cAnN and C57BL/6N being the strains with the highest percentage of superficial glomeruli (25% percentile 116.7 and 121.9 µm, respectively). In summary, due to significantly more superficial glomeruli compared to other commonly used strains, BALB/cAnN and C57BL/6N mice appear to be particularly suitable for the investigation of glomerular function using MPM.

[Peculiarities of pre- and postnatal kidney development in vasopressin-deficient brattleboro rats]

The objective of this study was to examine pre- and postnatal development of the kidney in vasopressin-deficient Brattleboro rats in comparison as compared to that in Wistar rats. Histological, histochemical and morphometric methods at light microscopic level were used. The study included 50 fetuses at gestational days 16 and 18, and 46 rat pups at postnatal days 5, 10, 20, and 30. It was found that nephrogenesis sequence in both rat strains was similar, however, Brattleboro embryos and infant rats were characterized by an accelerated growth of renal corpuscles and renal tubules. The results suggest that vasopressin has no direct effect on the formation of nephron structural elements, however it may participate in the regulation of hyaluronan biosynthesis in the renal medullary interstitial tissue involved in the mechanism of urine osmotic concentration.

Six2 and Wnt regulate self-renewal and commitment of nephron progenitors through shared gene regulatory networks

A balance between Six2-dependent self-renewal and canonical Wnt signaling-directed commitment regulates mammalian nephrogenesis. Intersectional studies using chromatin immunoprecipitation and transcriptional profiling identified direct target genes shared by each pathway within nephron progenitors. Wnt4 and Fgf8 are essential for progenitor commitment; cis-regulatory modules flanking each gene are cobound by Six2 and β-catenin and are dependent on conserved Lef/Tcf binding sites for activity. In vitro and in vivo analyses suggest that Six2 and Lef/Tcf factors form a regulatory complex that promotes progenitor maintenance while entry of β-catenin into this complex promotes nephrogenesis. Alternative transcriptional responses associated with Six2 and β-catenin cobinding events occur through non-Lef/Tcf DNA binding mechanisms, highlighting the regulatory complexity downstream of Wnt signaling in the developing mammalian kidney.

[Cellular factors of ischemia/reperfusion injury pathogenesis in the renal transplantation]

A review of literature reveales the current conception of Russian and foreign authors on the cellular and humoral pathogenetic mechanisms of ischemic and reperfusion injury of kidney transplant.

RNA-Seq defines novel genes, RNA processing patterns and enhancer maps for the early stages of nephrogenesis: Hox supergenes

During kidney development the cap mesenchyme progenitor cells both self renew and differentiate into nephrons. The balance between renewal and differentiation determines the final nephron count, which is of considerable medical importance. An important goal is to create a precise genetic definition of the early differentiation of cap mesenchyme progenitors. We used RNA-Seq to transcriptional profile the cap mesenchyme progenitors and their first epithelial derivative, the renal vesicles. The results provide a global view of the changing gene expression program during this key period, defining expression levels for all transcription factors, growth factors, and receptors. The RNA-Seq was performed using two different biochemistries, with one examining only polyadenylated RNA and the other total RNA. This allowed the analysis of noncanonical transcripts, which for many genes were more abundant than standard exonic RNAs. Since a large fraction of enhancers are now known to be transcribed the results also provide global maps of potential enhancers. Further, the RNA-Seq data defined hundreds of novel splice patterns and large numbers of new genes. Particularly striking was the extensive sense/antisense transcription and changing RNA processing complexities of the Hox clusters.

An immunofluorescence method to analyze the proliferation status of individual nephron segments in the Xenopus pronephric kidney

Organ development requires the coordination of proliferation and differentiation of various cell types. This is particularly challenging in the kidney, where up to 26 different cell types with highly specialized functions are present. Moreover, even though the nephron initially develops from a common progenitor pool, the individual nephron segments are ultimately quite different in respect to cell numbers. This suggests that some cells in the nephron have a higher proliferative index (i.e., cell cycle length) than others. Here, we describe two different immunofluorescence-based approaches to accurately quantify such growth rates in the pronephric kidney of Xenopus laevis. Rapidly dividing cells were identified with the mitosis marker phospho-Histone H3, while slowly cycling cells were labeled using the thymidine analogue EdU. In addition, individual nephron segments were marked using cell type-specific antibodies. To accurately assess the number of positively stained cells, embryos were then serially sectioned and analyzed by immunofluorescence microscopy. Growth rates were established by counting the mitosis or S-phase events in relation to the overall cells present in the nephron segment of interest. This experimental design is very reproducible and can easily be modified to fit other animal models and organ systems.

Estimating nephron number in the developing kidney using the physical disector/fractionator combination

Design-based stereology is considered the gold-standard method for estimating the total number of glomeruli, and thereby nephrons, in the adult kidney. However, until recently, a design-based method for estimating nephron number in the developing kidney was not available. For such a method to provide accurate and precise estimates, unambiguous identification of developing nephrons is essential. Here, we describe a combined histochemical/stereological technique for estimating total nephron number in the developing mouse and rat kidney. The method can be modified for use in other species.

Wnt4 induces nephronic tubules in metanephric mesenchyme by a non-canonical mechanism

Wnt4 and β-catenin are both required for nephrogenesis, but studies using TCF-reporter mice suggest that canonical Wnt signaling is not activated in metanephric mesenchyme (MM) during its conversion to the epithelia of the nephron. To better define the role of Wnt signaling, we treated rat metanephric mesenchymal progenitors directly with recombinant Wnt proteins. These studies revealed that Wnt4 protein, which is required for nephron formation, induces tubule formation and differentiation markers Lim1 and E-cadherin in MM cells, but does not activate a TCF reporter or up regulate expression of canonical Wnt target gene Axin-2 and has little effect on the stabilization of β-catenin or phosphorylation of disheveled-2. Furthermore, Wnt4 causes membrane localization of ZO-1 and occludin in tight junctions. To directly examine the role of β-catenin/TCF-dependent transcription, we developed synthetic cell-permeable analogs of β-catenin's helix C, which is required for transcriptional activation, in efforts to specifically inhibit canonical Wnt signaling. One inhibitor blocked TCF-dependent transcription and induced degradation of β-catenin but did not affect tubule formation and stimulated the expression of Lim1 and E-cadherin. Since a canonical mechanism appears not to be operative in tubule formation, we assessed the involvement of the non-canonical Ca(2+)-dependent pathway. Treatment of MM cells with Wnt4 induced an influx of Ca(2+) and caused phosphorylation of CaMKII. Moreover, Ionomycin, a Ca(2+)-dependent pathway activator, stimulated tubule formation. These results demonstrate that the canonical Wnt pathway is not responsible for mesenchymal-epithelial transition (MET) in nephron formation and suggest that the non-canonical calcium/Wnt pathway mediates Wnt4-induced tubulogenesis in the kidney.

Gene expression using the PALM system

The PALM Robot MicroBeam laser microdissection system can isolate specified cells from complex tissues section, in a rapid and precise manner. Combined with other methods, PALM may be used for gene expression elucidating the role of specialized cell type in physiological and pathological activity. This chapter describes the application of the PALM MicroBeam system to isolate RNA from cells in a complex tissue for subsequent gene expression analysis. Protocols show the steps from preparation of tissue samples to the final quantitative results. The process is articulated in several steps, each of which requires optimal choices in order to obtain reliable data from a limited number of cells (500-10,000 cells). Furthermore, the notes regarding tissue preparation, microdissection of the interested cells, are also emphasized.

[The sequence of nephron tubule differentiation in the definitive human kidney during the fetal period]

The regularities of the formation of the undifferentiated renal tubules in the definitive kidney were studied in 174 human embryos and fetuses at 4.5 to 40 gestational weeks. On the basis of the study of morphological and morphometrical parameters of these tubules, it was shown that the increase of the average tubular crossectional area was associated with the consecutive formation of the nephrons of the definitive kidney and the differentiation of the segments of their tubular portions. The increase of the proportion of the epithelium in the structure of undifferentiated tubule is determined by the segregation of the proximal and distal tubules within the forming nephrons. The sequence of the initial stages of the renal tubule differentiation is dictated by the ergontic correlations; it developes from the proximal tubule to the distal one and is different from the succession of their formation in the process of the early nephronogenesis.

A novel, low-volume method for organ culture of embryonic kidneys that allows development of cortico-medullary anatomical organization

Here, we present a novel method for culturing kidneys in low volumes of medium that offers more organotypic development compared to conventional methods. Organ culture is a powerful technique for studying renal development. It recapitulates many aspects of early development very well, but the established techniques have some disadvantages: in particular, they require relatively large volumes (1–3 mls) of culture medium, which can make high-throughput screens expensive, they require porous (filter) substrates which are difficult to modify chemically, and the organs produced do not achieve good cortico-medullary zonation. Here, we present a technique of growing kidney rudiments in very low volumes of medium–around 85 microliters–using silicone chambers. In this system, kidneys grow directly on glass, grow larger than in conventional culture and develop a clear anatomical cortico-medullary zonation with extended loops of Henle.

Collective cell migration drives morphogenesis of the kidney nephron

Tissue organization in epithelial organs is achieved during development by the combined processes of cell differentiation and morphogenetic cell movements. In the kidney, the nephron is the functional organ unit. Each nephron is an epithelial tubule that is subdivided into discrete segments with specific transport functions. Little is known about how nephron segments are defined or how segments acquire their distinctive morphology and cell shape. Using live, in vivo cell imaging of the forming zebrafish pronephric nephron, we found that the migration of fully differentiated epithelial cells accounts for both the final position of nephron segment boundaries and the characteristic convolution of the proximal tubule. Pronephric cells maintain adherens junctions and polarized apical brush border membranes while they migrate collectively. Individual tubule cells exhibit basal membrane protrusions in the direction of movement and appear to establish transient, phosphorylated Focal Adhesion Kinase–positive adhesions to the basement membrane. Cell migration continued in the presence of camptothecin, indicating that cell division does not drive migration. Lengthening of the nephron was, however, accompanied by an increase in tubule cell number, specifically in the most distal, ret1-positive nephron segment. The initiation of cell migration coincided with the onset of fluid flow in the pronephros. Complete blockade of pronephric fluid flow prevented cell migration and proximal nephron convolution. Selective blockade of proximal, filtration-driven fluid flow shifted the position of tubule convolution distally and revealed a role for cilia-driven fluid flow in persistent migration of distal nephron cells. We conclude that nephron morphogenesis is driven by fluid flow–dependent, collective epithelial cell migration within the confines of the tubule basement membrane. Our results establish intimate links between nephron function, fluid flow, and morphogenesis.

The kidney's job is to maintain blood ion and metabolite concentrations in a narrow range that supports the function of all other organs. Blood is filtered and essential solutes are recovered in a structure called the nephron. Human kidneys have one million nephrons, while simpler kidneys like the zebrafish larval kidney have only two. Nephrons are segmented epithelial tubules; each segment takes on a particular shape (such as convoluted, straight, or U-shaped) and plays a specific role in recovering filtered solutes. How the nephron is proportioned into segments and how some tubule segments become convoluted is not known. This work takes advantage of the simple zebrafish kidney to image living cells during nephron formation. Unexpectedly, we found that nephron cells are actively migrating “upstream” toward the filtering end of the nephron. The cells remain connected to each other and migrate as an intact tube. This is similar to a process called “collective cell migration.” We find that collective cell migration establishes the final position of nephron segment boundaries and drives convolution of the tubule. We also find that cell migration is dependent on fluid flow in the tubules, supporting the idea that organ function is important in defining its final form.

Epithelial cell shape, tubule convolution, and segment boundary position along the kidney nephron unexpectedly involve the migration of fully differentiated epithelial cells against the flow of lumenal fluid.

IL-18 is expressed in the intercalated cell of human kidney

We determined the cellular location of interleukin-18 (IL-18) and caspase-1 and the purinergic receptor P2X7, two proteins necessary for its activation and secretion. The mRNA and protein of IL-18 were detectable in normal human kidney by means of polymerase chain reaction (PCR), in situ hybridization, and Western blot. Immunohistochemistry located IL-18 to nephron segments containing calbinbin-D28k or aquaporin-2 that suggest location in the distal convoluted and the connecting tubule and to parts of the collecting duct. IL-18 was not detected in the thick ascending limb of Henle. Confocal microscopy showed that IL-18 was expressed in cells negative for calbindin-D28k and for aquaporin-2 but positive for the vacuolar H(+)-ATPase. This demonstrates that the intercalated cells produce IL-18. These segments were also positive for caspase-1 and P2X7 that are essential for IL-18 secretion. Our results show that IL-18 is constitutively expressed by intercalated cells of the late distal convoluted tubule, the connecting tubule, and the collecting duct of the healthy human kidney. Since IL-18 is an early component of the inflammatory cytokine cascade, its location suggests that renal intercalated cells may contribute to immediate immune response of the kidney.

Aldosterone-induced increases in superoxide production counters nitric oxide inhibition of epithelial Na channel activity in A6 distal nephron cells

Oxygen radicals play an important role in signal transduction and have been shown to influence epithelial sodium channel (ENaC) activity. We show that aldosterone, the principal hormone regulating renal ENaC activity, increases superoxide (O2*) production in A6 distal nephron cells. Aldosterone (50 nM to 1.5 microM) induced increases in dihydroethidium fluorescence in a dose-dependent manner in confluent A6 epithelial cells. Using single-channel measurements, we showed that sequestering endogenous O2* (with the O2* scavenger 2,2,6,6-tetramethylpiperidine 1-oxyl) significantly decreased ENaC open probability from 0.10 +/- 0.03 to 0.03 +/- 0.01. We also found that increasing endogenous O2* in A6 cells, by applying a superoxide dismutase inhibitor, prevented nitric oxide (NO) inhibition of ENaC activity. ENaC open probability values did not significantly change from control values (0.23 +/- 0.05) after superoxide dismutase and 1.5 microM NO coincubation (0.21 +/- 0.04). We report that xanthine oxidase and hypoxanthine compounds increase local concentrations of O2* by approximately 30%; with this mix, an increase in ENaC number of channels times the open probability (from 0.1 to 0.3) can be achieved in a cell-attached patch. Our data also suggest that O2* alters NO activity in a cGMP-independent mechanism, since pretreating A6 cells with ODQ compound (a selective inhibitor of NO-sensitive guanylyl cyclase) failed to block 2,2,6,6-tetramethylpiperidine 1-oxyl inhibition of ENaC activity.

A comparative study of renal function in the desert-adapted spiny mouse and the laboratory-adapted C57BL/6 mouse: response to dietary salt load

The desert-adapted spiny mouse has a significantly lower glomerular number, increased glomerular size, and a more densely packed renal papillae compared with the similar-sized laboratory-adapted C57BL/6 mouse. In the present study we examined the functional consequences of these structural differences in young adult male spiny and C57BL/6 mice and detailed the impact of 1 wk of a high-salt (10% wt/wt NaCl) diet. Basal food and water intake, urine and feces production, and urinary electrolyte concentrations were not different between species, although urinary urea concentrations were higher in spiny mice (P < 0.05). On normal salt, MAP of the anesthetized spiny mouse was approximately 18 mmHg lower, effective renal plasma flow (ERPF) was 40% lower (P < 0.001), and glomerular filtration rate (GFR) tended to be lower than in the C57BL/6 mouse. On the high-salt diet, both species had similar 24-h NaCl excretions; but C57BL/6 mice required a significantly increased amount of water (lower urine NaCl concentration) than the spiny mice. Filtration fraction was greater in both species on the high-salt diet. Spiny mice had greater GFR and ERPF after the high-salt diet, whereas the C57BL/6 mouse showed little change in GFR. The ability of the spiny mouse to tolerate a significantly higher plasma osmolality after salt, measured by a decreased drinking response, and the ability to increase ERPF at a lower MAP are features that allow this species to conserve water more efficiently than can be done in the C57BL/6 mouse. These features are important, particularly since the desert mouse has a smaller kidney, with fewer nephrons.

Renal and extrarenal regulation of potassium

The ISN Forefronts in Nephrology Symposium took place 8-11 September 2005 in Kartause Ittingen, Switzerland. It was dedicated to the memory of Robert W. Berliner, who died at age 86 on 5 February 2002. Dr Berliner contributed in a major way to our understanding of potassium transport in the kidney. Starting in the late 1940s, without knowledge of how potassium was transported across specific nephron segments and depending only on renal clearance methods, he and his able associates provided a still-valid blueprint of the basic transport properties of potassium handling by the kidney. They firmly established that potassium was simultaneously reabsorbed and secreted along the nephron; that variations in secretion in the distal nephron segments play a major role in regulating potassium excretion; and that such secretion is modulated by sodium, acid-base factors, hormones, and diuretics. These conclusions were presented in a memorable Harvey Lecture some forty years ago, and they have remained valid ever since. The concepts have also provided the foundation and stimulation for later work on single nephrons, tubule cells, and transport proteins involved in potassium transport.