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.

Aldosterone regulates microRNAs in the cortical collecting duct to alter sodium transport

A role for microRNAs (miRs) in the physiologic regulation of sodium transport in the kidney has not been established. In this study, we investigated the potential of aldosterone to alter miR expression in mouse cortical collecting duct (mCCD) epithelial cells. Microarray studies demonstrated the regulation of miR expression by aldosterone in both cultured mCCD and isolated primary distal nephron principal cells. Aldosterone regulation of the most significantly downregulated miRs, mmu-miR-335-3p, mmu-miR-290-5p, and mmu-miR-1983 was confirmed by quantitative RT-PCR. Reducing the expression of these miRs separately or in combination increased epithelial sodium channel (ENaC)-mediated sodium transport in mCCD cells, without mineralocorticoid supplementation. Artificially increasing the expression of these miRs by transfection with plasmid precursors or miR mimic constructs blunted aldosterone stimulation of ENaC transport. Using a newly developed computational approach, termed ComiR, we predicted potential gene targets for the aldosterone-regulated miRs and confirmed ankyrin 3 (Ank3) as a novel aldosterone and miR-regulated protein. A dual-luciferase assay demonstrated direct binding of the miRs with the Ank3-3' untranslated region. Overexpression of Ank3 increased and depletion of Ank3 decreased ENaC-mediated sodium transport in mCCD cells. These findings implicate miRs as intermediaries in aldosterone signaling in principal cells of the distal kidney nephron.

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.

Lack of Na(+),K (+)-ATPase expression in intercalated cells may be compensated by Na(+)-ATPase: a study on MDCK - C11 cells

The lack of Na(+),K(+)-ATPase expression in intercalated cells (IC) is an intriguing condition due to its fundamental role in cellular homeostasis. In order to better understand this question we compared the activities of Na(+),K(+)-ATPase and Na(+)-ATPase in two MDCK cell clones: the C11, with IC characteristics, and the C7, with principal cells (PC) characteristics. The Na(+),K(+)-ATPase activity found in C11 cells is far lower than in C7 cells and the expression of its beta-subunit is similar in both cells. On the other hand, a subset of C11 without alpha-subunit expression has been found. In C11 cells the Na(+)-ATPase activity is higher than that of the Na(+),K(+)-ATPase, and it is increased by medium alkalinization, suggesting that it could account for the cellular Na(+)-homeostasis. Although further studies are necessary for a better understanding of these findings, the presence of Na(+)-ATPase may explain the adequate survival of cells that lack Na(+),K(+)-ATPase.