Conditioned medium from a rat ureteric bud cell line in combination with bFGF induces complete differentiation of isolated metanephric mesenchyme

Effects of amnion derived mesenchymal stem cells on fibrosis in a 5/6 nephrectomy model in rats

Chronic kidney disease (CKD) is characterized by disruption of the glomerulus, tubule and vascular structures by renal fibrosis. Mesenchymal stem cells (MSC) ameliorate CKD. We investigated the effects of human amnion derived MSC (hAMSC) on fibrosis using expression of transforming growth factor beta (TGF-β), collagen type I (COL-1) and bone morphogenetic protein (BMP-7). We also investigated levels of urinary creatinine and nitrogen in CKD. We used a 5/6 nephrectomy (5/6 Nx) induced CKD model. We used 36 rats in six groups of six animals: sham group, 5/6 Nx group, 15 days after 5/6 Nx (5/6 Nx + 15) group, 30 days after 5/6 Nx (5/6 Nx + 30) group, transfer of hAMSC 15 days after 5/6 Nx (5/6 Nx + hAMSC + 15) group and transfer of hAMSC 30 days after 5/6 Nx (5/6 Nx + hAMSC + 30) group. We isolated 10⁶ hAMSC from the amnion and transplanted them via the rat tail vein into the 5/6 Nx + hAMSC + 15 and 5/6 Nx + hAMSC + 30 groups. We measured the expression of BMP-7, COL-1 and TGF-β using western blot and immunohistochemistry, and their gene expressions were analyzed by quantitative real time PCR. TGF-β and COL-1 protein, and gene expressions were increased in the 5/6 Nx +30 group compared to the 5/6 Nx + hAMSC + 30 group. Conversely, both protein and gene expression of BMP-7 was increased in 5/6 Nx + hAMSC + 30 group compared to the 5/6 Nx groups. Increased TGF-β together with decreased BMP-7 expression may cause fibrosis by epithelial-mesenchymal transition due to chronic renal injury. Increased COL-1 levels cause accumulation of extracellular matrix in CKD. Levels of urea, creatinine and nitrogen were increased significantly in 5/6 Nx + 15 and 5/6 Nx + 30 groups compared to the hAMSC groups. We found that hAMSC ameliorate CKD.

Kidney Nephron Determination

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

Potency of Human Urine-Derived Stem Cells for Renal Lineage Differentiation

Kidney is one of the most difficult organs for regeneration. Several attempts have been performed to regenerate renal tissue using stem cells, the results were not satisfactory. Urine is major product of kidney and contains cells from renal components. Moreover, urine-derived stem cells (USCs) can be easily obtained without any health risks throughout a patient's entire life. Here, we evaluated the utility of USCs for renal tissue regeneration. In this study, the ability of USCs to differentiate into renal lineage cells was compared with that of adipose tissue-derived stem cells (ADSCs) and amniotic fluid-derived stem cells (AFSCs), with respect to surface antigen expression, morphology, immunocytochemistry, renal lineage gene expression, secreted factors, immunomodulatory marker expression, in vivo safety, and renal differentiation potency. Undifferentiated USCs were positive for CD44 and CD73, negative for CD34 and CD45, and formed aggregates after 3 weeks of renal differentiation. Undifferentiated USCs showed high SSEA4 expression, while renal-differentiated cells expressed PAX2, WT1, and CADHERIN 6. In the stem/renal lineage-associated gene analysis, OCT4, SSEA4, and CD117 were significantly downregulated over time, while PAX2, LIM1, PDGFRA, E-CADHERIN, CD24, ACTB, AQP1, OCLN, and NPHS1 were gradually upregulated. In the in vivo safety evaluation, renal-differentiated USCs did not show abnormal histology. These findings demonstrated that USCs have a similar MSC potency, renal lineage-differentiation ability, immunomodulatory effects, and in vivo safety as ADSCs and AFSCs, and showed higher levels of growth factor secretion for paracrine effects. Therefore, urine and USCs can be one of good cell sources for kidney regeneration.

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

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

SIGNIFICANCE

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

Basic fibroblast growth factor reduces functional and structural damage in chronic kidney disease

Chronic kidney disease (CKD) is characterized by loss of renal function. The pathological processes involved in the progression of this condition are already known, but the molecular mechanisms have not been completely explained. Recent reports have shown the intrinsic capacity of the kidney to undergo repair after acute injury through the reexpression of repairing proteins (Villanueva S, Cespedes C, Vio CP. Am J Physiol Regul Integr Comp Physiol 290: R861-R870, 2006). Stimulation with basic fibroblast growth factor (bFGF) could accelerate this process. However, it is not known whether bFGF can induce this phenomenon in kidney cells affected by CKD. Our aim was to study the evolution of renal damage in animals with CKD treated with bFGF and to relate the amount of repairing proteins with renal damage progression. Male Sprague-Dawley rats were subjected to 5/6 nephrectomy (NPX) and treated with bFGF (30 μg/kg, NPX+bFGF); a control NPX group was treated with saline (NPX+S). Animals were euthanized 35 days after bFGF administration. Functional effects were assessed based on serum creatinine levels; morphological damage was assessed by the presence of macrophages (ED-1), interstitial α-smooth muscle actin (α-SMA), and interstitial collagen through Sirius red staining. The angiogenic factors VEGF and Tie-2 and the epithelial/tubular factors Ncam, bFGF, Pax-2, bone morphogenic protein-7, Noggin, Lim-1, Wnt-4, and Smads were analyzed. Renal stem cells were evaluated by Oct-4. We observed a significant reduction in serum creatinine levels, ED-1, α-SMA, and Sirius red as well as an important induction of Oct-4, angiogenic factors, and repairing proteins in NPX+bFGF animals compared with NPX+S animals. These results open new perspectives toward reducing damage progression in CKD.

Human mesenchymal stem cells derived from adipose tissue reduce functional and tissue damage in a rat model of chronic renal failure

Therapeutic approaches for CKD (chronic kidney disease) have been able to reduce proteinuria, but not diminish the disease progression. We have demonstrated beneficial effects by injection of BM (bone marrow)-derived MSCs (mesenchymal stem cells) from healthy donors in a rat model with CKD. However, it has recently been reported that BM-MSCs derived from uraemic patients failed to confer functional protection in a similar model. This suggests that autologous BM-MSCs are not suitable for the treatment of CKD. In the present study, we have explored the potential of MSCs derived from adipose tissue (AD-MSCs) as an alternative source of MSCs for the treatment of CKD. We have isolated AD-MSCs and evaluated their effect on the progression of CKD. Adult male SD (Sprague-Dawley) rats subjected to 5/6 NPX (nephrectomy) received a single intravenous infusion of 0.5×10(6) AD-MSCs or MSC culture medium alone. The therapeutic effect was evaluated by plasma creatinine measurement, structural analysis and angiogenic/epitheliogenic protein expression. AD-MSCs were detected in kidney tissues from NPX animals. This group had a significant reduction in plasma creatinine levels and a lower expression of damage markers ED-1 and α-SMA (α-smooth muscle actin) (P<0.05). In addition, treated rats exhibited a higher level of epitheliogenic [Pax-2 and BMP-7 (bone morphogenetic protein 7)] and angiogenic [VEGF (vascular endothelial growth factor)] proteins. The expression of these biomarkers of regeneration was significantly related to the improvement in renal function. Although many aspects of the cell therapy for CKD remain to be investigated, we provide evidence that AD-MSCs, a less invasive and highly available source of MSCs, exert an important therapeutic effect in this pathology.

Reciprocal induction of simple organogenesis by mouse kidney progenitor cells in three-dimensional co-culture

Kidney development is regulated by a coordinated reciprocal induction of metanephric mesenchymal (MM) and ureteric bud (UB) cells. Here, established MM and UB progenitor cell lines were recombined in three-dimensional Matrigel implants in SCID mice. Differentiation potential was examined for changes in phenotype, organization, and the presence of specialized proteins using immunofluorescence and bright-field and electron microscopy. Both cell types, when grown alone, did not develop into specialized structures. When combined, the cells organized into simple organoid structures of polarized epithelia with lumens surrounded by capillary-like structures. Tracker experiments indicated the UB cells formed the tubuloid structures, and the MM cells were the source of the capillary-like cells. The epithelial cells stained positive for pancytokeratin, the junctional complex protein ZO-1, collagen type IV, as well as UB and collecting duct markers, rearranged during transfection (RET), Dolichos biflorus lectin, EndoA cytokeratin, and aquaporin 2. The surrounding cells expressed α-smooth muscle actin, vimentin, platelet endothelial cell adhesion molecule 1 (PECAM), and aquaporin 1, a marker of vasculogenesis. The epithelium exhibited apical vacuoles, microvilli, junctional complexes, and linear basement membranes. Capillary-like structures showed endothelial features with occasional pericytes. UB cell epithelialization was augmented in the presence of MM cell-derived conditioned medium, glial-derived neurotrophic factor (GDNF), hepatocyte growth factor (HGF), or fibronectin. MM cells grown in the presence of UB-derived conditioned medium failed to undergo differentiation. However, UB cell-derived conditioned medium induced MM cell migration. These studies indicate that tubulogenesis and vasculogenesis can be partially recapitulated by recombining individual MM and UB cell lineages, providing a new model system to study organogenesis ex vivo.

FGF/EGF signaling regulates the renewal of early nephron progenitors during embryonic development

Recent studies indicate that nephron progenitor cells of the embryonic kidney are arranged in a series of compartments of an increasing state of differentiation. The earliest progenitor compartment, distinguished by expression of CITED1, possesses greater capacity for renewal and differentiation than later compartments. Signaling events governing progression of nephron progenitor cells through stages of increasing differentiation are poorly understood, and their elucidation will provide key insights into normal and dysregulated nephrogenesis, as well as into regenerative processes that follow kidney injury. In this study, we found that the mouse CITED1(+) progenitor compartment is maintained in response to receptor tyrosine kinase (RTK) ligands that activate both FGF and EGF receptors. This RTK signaling function is dependent on RAS and PI3K signaling but not ERK. In vivo, RAS inactivation by expression of sprouty 1 (Spry1) in CITED1(+) nephron progenitors results in loss of characteristic molecular marker expression and in increased death of progenitor cells. Lineage tracing shows that surviving Spry1-expressing progenitor cells are impaired in their subsequent epithelial differentiation, infrequently contributing to epithelial structures. These findings demonstrate that the survival and developmental potential of cells in the earliest embryonic nephron progenitor cell compartment are dependent on FGF/EGF signaling through RAS.

Mesenchymal stem cell injection ameliorates chronic renal failure in a rat model

CKD (chronic kidney disease) has become a public health problem. The therapeutic approaches have been able to reduce proteinuria, but have not been successful in limiting disease progression. In this setting, cell therapies associated with regenerative effects are attracting increasing interest. We evaluated the effect of MSC (mesenchymal stem cells) on the progression of CKD and the expression of molecular biomarkers associated with regenerative effects. Adult male Sprague-Dawley rats subjected to 5/6 NPX (nephrectomy) received a single intravenous infusion of 0.5×106 MSC or culture medium. A sham group subjected to the same injection was used as the control. Rats were killed 5 weeks after MSC infusion. Dye tracking of MSC was followed by immunofluorescence analysis. Kidney function was evaluated using plasma creatinine. Structural damage was evaluated by H&E (haematoxylin and eosin) staining, ED-1 abundance (macrophages) and interstitial α-SMA (α-smooth muscle actin). Repairing processes were evaluated by functional and structural analyses and angiogenic/epitheliogenic protein expression. MSC could be detected in kidney tissues from NPX animals treated with intravenous cell infusion. This group presented a marked reduction in plasma creatinine levels and damage markers ED-1 and α-SMA (P<0.05). In addition, treated rats exhibited a significant induction in epitheliogenic [Pax-2, bFGF (basic fibroblast growth factor) and BMP-7 (bone morphogenetic protein-7)] and angiogenic [VEGF (vascular endothelial growth factor) and Tie-2] proteins. The expression of these biomarkers of regeneration was significantly related to the increase in renal function. Many aspects of the cell therapy in CKD remain to be investigated in more detail: for example, its safety, low cost and the possible need for repeated cell injections over time. Beyond the undeniable importance of these issues, what still needs to be clarified is whether MSC administration has a real effect on the treatment of this pathology. It is precisely to this point that the present study aims to contribute.

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.

STAT1 activation regulates proliferation and differentiation of renal progenitors

We have shown previously that activation of STAT1 contributes to the pathogenesis of Wilms tumor. This neoplasm caricatures metanephric development and is believed to originate from embryonic renal mesenchymal progenitors that lose their ability to undergo mesenchymal-epithelial transition (MET). Therefore, we hypothesized that STAT1 is also activated and functional during metanephric development. Here we have demonstrated that both STAT1 and STAT3 are activated during normal development of the embryonic kidney. Furthermore, activation of STAT1 stimulated the proliferation of metanephric mesenchymal cells, but it prevented MET and tubulogenesis induced by leukemia inhibitory factor, which preferentially activates STAT3. Consistent with its negative regulation of metanephric mesenchymal differentiation, inhibition of STAT1 activation with protein kinase CK2 inhibitor TBB or RNAi-mediated knockdown of STAT1 promoted differentiation of metanephric progenitors and abolished the effect of cytokine-induced STAT1 activation in these cells. Additionally, a cell-permeable peptide that inhibits STAT1-mediated transactivation by targeting the STAT1 N-domain also blocked cytokine-induced STAT1-dependent proliferation in metanephric progenitors and promoted LIF-induced MET and tubulogenesis. Finally, the STAT1 peptide inhibitor caused the down regulation of survival/anti-apoptotic factors, Mcl-1 and Hsp-27, and induced apoptosis in renal tumor cells with constitutively active STAT1, indicating that STAT1 is required for these cells to survive. These findings show that both metanephric progenitors and renal tumor cells utilize a STAT1-dependent mechanism for growth or survival.

In vitro differentiation of MSC into cells with a renal tubular epithelial-like phenotype

Bone marrow mesenchymal stem (stromal) cells (MSCs) are shown to differentiate into different renal lineages in in vivo injury models. Nevertheless, the in vitro differentiation of MSCs into a renal tubular epithelial lineage has not been investigated. We hypothesize that the injured renal epithelial cells express renotypic factors that may influence the differentiation of MSCs into a renal tubular epithelial lineage. MSCs were cocultured for up to seven days with injured or uninjured murine cortical tubular renal epithelial cells (MCTs), which are separated by a physical barrier; following the coculture, MSCs were examined for the expression of two renal tubular epithelial-specific markers, kidney-specific cadherin (Ksp-cadherin) and aquaporin-1 (AQP1). MSCs differentiated into a tubular epithelial-like phenotype, as shown by the appearance of Ksp-cadherin and AQP1 by day 7 when cocultured with injured MCTs. Further, MSCs showed tubulogenic characteristics when cocultured in a three-dimensional matrix. Nonetheless, MSCs cultured with the conditioned medium from injured MCTs, cocultured with ureteric bud cells, or treated with nephrogenic factors did not differentiate into renal epithelial cells. Based on our findings, we conclude that MSCs can differentiate into a renal epithelial lineage independent of cell fusion when cocultured with injured renal cells.

Conditioned medium from renal tubular epithelial cells initiates differentiation of human mesenchymal stem cells

OBJECTIVES

Mesenchymal-epithelial interactions play a pivotal role in tubular morphogenesis and in maintaining the integrity of the kidney. During renal repair, similar mechanisms may regulate cellular reorganization and differentiation. We have hypothesized that soluble factors from proximal tubular epithelial cells (PTC) induce differentiation of adipose-derived adult mesenchymal stem cells (ASC). This hypothesis has been tested using cultured ASC and PTC.

MATERIAL AND METHODS

Conditioned medium was prepared from injured PTC and transferred to ASC cultures. ASC proliferation was analysed by a fluorometric and photometric assay. Signal transduction was analysed by phosphorylation of extracellular signal-regulated kinase 1 and 2 (ERK1/ERK2). Grade of ASC differentiation was assessed by morphological analysis and cell expression of characteristic markers.

RESULTS

Conditioned medium significantly induced proliferation and phosphorylation of ERK1/ERK2 of ASC. After 12 days of incubation, cell morphology changed to an epithelial-like monolayer. Expression of cytokeratin 18 was induced by conditioned medium, while alpha-smooth muscle actin, CD49a and CD90 expression decreased. These alterations strongly indicate onset of the differentiation process to the epithelial lineage. In summary, soluble factors from PTC induce signal transduction and differentiation of ASC.

CONCLUSIONS

Our study shows that conditioned medium from renal tubular epithelial cells provides a convenient source of inductive signals to initiate differentiation of ASC towards epithelial lineage. We deduce that these interactions may play an important role during renal repair mechanisms.

Inheritance of susceptibility to induction of nephroblastomas in the Noble rat

Noble (Nb) strain rats are susceptible to nephroblastoma induction with transplacental exposure to direct-acting alkylating agent N-nitrosoethylurea (ENU), while F344 strain rats are highly resistant. To study the inheritance of susceptibility to induction of these embryonal renal tumors, fetal Nb and F344 rats and F1, F2 and reciprocal backcross hybrids were exposed transplacentally to ENU once on day 18 of gestation. Nephroblastomas developed in 53% of Nb offspring with no apparent gender difference, while no nephroblastomas developed in inbred F344 offspring. F1 and F2 hybrid offspring had intermediate responses, 28% and 30%, respectively. Nephroblastoma incidence in the offspring of F1 hybrids backcrossed to the susceptible strain Nb was 46%, while that in F1 hybrids backcrossed to resistant strain F344 was much lower (16%). Carcinogenic susceptibility is therefore consistent with the involvement of one major autosomal locus; the operation of a gene dosage effect; and a lack of simple Mendelian dominance for either susceptibility or resistance. Since established Wilms tumor-associated suppressor genes, Wt1 and Wtx, were not mutated in normal or neoplastic tissues, genomic profiling was performed on isolated Nb and F344 metanephric progenitors to identify possible predisposing factors to nephroblastoma induction. Genes preferentially elevated in expression in Nb rat progenitors included Wnt target genes Epidermal growth factor receptor, Inhibitor of DNA binding 2, and Jagged1, which were further increased in nephroblastomas. These studies demonstrate the value of this model for genetic analysis of nephroblastoma development and implicate both the Wnt and Notch pathways in its pathogenesis.

Urothelium patterns bladder smooth muscle location

Smooth muscle differentiation is induced in the embryonic bladder by the centrally located urothelium in the undifferentiated mesenchyme in the periphery adjacent to the serosa. We hypothesize that under the appropriate signal the entire undifferentiated bladder mesenchyme is capable of smooth muscle differentiation and that the urothelium patterns fibromuscular development. Embryonic bladders of wild-type and Green Fluorescent Protein mice were separated into urothelial and mesenchymal components before smooth muscle differentiation (E12.5-E13). The urothelial layer green fluorescent protein was recombined and grafted with the mesenchyme (wild-type) in an orthotopic position, heterotopic position and ectopic position. In all cases, a zone of smooth muscle inhibition was observed adjacent to the epithelium whether the urothelium was in an orthotopic or heterotypic position. Bladder mesenchyme and bladder epithelium grafted alone did not grow. In conclusion, the full thickness of bladder mesenchyme is capable of smooth muscle differentiation dependent on the location of urothelium. These experiments support the hypothesis that urothelium secretes a diffusible factor that at high concentrations inhibits smooth muscle and at low concentrations induces smooth muscle, thus patterning mesenchymal cell differentiation across the full thickness of the fibromuscular bladder wall.

Inhibition of bFGF-receptor type 2 increases kidney damage and suppresses nephrogenic protein expression after ischemic acute renal failure

Recovery from acute renal failure (ARF) requires the replacement of injured cells by new cells that are able to restore tubule epithelial integrity. We have recently described the expression of nephrogenic proteins [Vimentin, neural cell adhesion molecule, basic fibroblast growth factor (bFGF), Pax-2, bone morphogen protein-7, Noggin, Smad 1-5-8, p-Smad, hypoxia-inducible factor-1alpha, vascular endothelial growth factor], in a time frame similar to that observed in kidney development, after ischemic ARF induced in an ischemia-reperfusion (I/R) model. Furthermore, we show that bFGF, a morphogen involved in mesenchyme/epithelial transition in kidney development, induces a reexpression of morphogenic proteins in an earlier time frame and accelerates the recovery process after renal damage. Herein, we confirm that renal morphogenes are modulated by bFGF and hypothesized that a decrease in bFGF receptor 2 (bFGFR2) levels by the use of antisense oligonucleotides diminishes the expression of morphogenes. Male Sprague-Dawley rats submitted to ischemic injury were injected with 112 microg/kg bFGFR2 antisense oligonucleotide (bFGFR2-ASO) followed by reperfusion. Rats were killed, and the expression of nephrogenic proteins and renal marker damage was analyzed by immunohistochemistry and immunoblot. Animals subjected to I/R treated with bFGFR2-ASO showed a significant reduction in morphogen levels (P < 0.05). In addition, we observed an increase in markers of renal damage: macrophages (ED-1) and interstitial alpha-smooth muscle actin. These results confirm that bFGF participates in the recovery process and that treatment with bFGFR2-ASO induces an altered expression of morphogen proteins.

Ex vivo whole embryonic kidney culture: a novel method for research in development, regeneration and transplantation

PURPOSE

Whole metanephric organ culture represents a novel investigatory approach with potential application in many aspects of research in kidney regeneration and transplantation. We report the current status of embryonic kidney culture, discussing issues such as the appropriate culture conditions and methods, histological results, values of and limitations to the different techniques used today. To optimize this system in vitro for the benefit of future studies we focused our efforts on evaluating and developing a new durable 3-dimensional organ culture system using a uniquely modified approach.

MATERIALS AND METHODS

Metanephric kidneys were microdissected from the embryos of timed pregnant WT C57BL/C6 mice on days 12 to 16 of gestation (embryonic days 12 to 16). Novel perfusion channels were created in the harvested embryonic kidneys before placing them in culture. Embryonic kidneys were placed on a 0.4 microm pore size Transwell membrane, cultured in base medium at a medium gas interphase and incubated at 37C with fully humidified 5% CO2. Histological and immunocytochemical analysis was performed to evaluate for signs of necrosis, and the structural integrity and functionality of organs during culture.

RESULTS

We confirmed histologically that our organ culture system was capable of maintaining normal kidney structures significantly longer (mean 10 days) than previously reported standard protocols. Condensation and aggregation of the metanephric mesenchyma at the tips of the ureteral bud were observed, including the formation of well developed nephrons and glomeruli without evidence of necrosis. Organ maturation occurred in a developmentally appropriate centrifugal pattern and the expression of key regulatory factors was demonstrated.

CONCLUSIONS

Our in vitro model replicates closely the in vivo processes involved in normal kidney development. We also present what is to our knowledge the first demonstration of a durable 3-dimensional kidney culture system reported in the literature. This system may represent an uncomplicated method for in vitro kidney culture that we hope will serve as an effective adjunct to research focused on signaling pathways, development and regeneration as applied to the kidney.

ELR+-CXC chemokines and their receptors in early metanephric development

Although originally identified as mediators of inflammation, it is now apparent that chemokines play a fundamental role in tissue development. In this study, ELR(+)-CXC chemokine family members CXCL2 and CXCL7, along with their preferred receptor CXCR2, were expressed at the earliest stages of metanephric development in the rat, and signaling through this receptor was required for the survival and maintenance of the undifferentiated metanephric mesenchyme (MM). A specific antagonist of the CXCR2 receptor SB225002 induced apoptosis in this population but did not affect more mature structures or cells in the ureteric bud. CXCL7 treatment of isolated MM elicited an angiogenic response by upregulation of matrix metalloprotease 9 and endothelial and mesangial markers (platelet-endothelial cell adhesion molecule, Megsin, Thy-1, PDGF receptor alpha, and vascular alpha-actin) and induced SB225002-sensitive cell invasion through a matrix. Because Wilms' tumor cells may similarly depend on CXCR2 signaling for survival, primary tumor samples were analyzed, and 15 of 16 Wilms' tumors were found to be CXCR2 positive, whereas grossly normal kidney tissues from tumor patients or renal cell carcinomas were CXCR2 negative. Furthermore, cell lines derived from Wilms' tumors but not those from renal cell carcinomas were sensitive to SB225002-induced apoptosis. These data provide evidence for a prosurvival and proangiogenic role of ELR(+)-CXC chemokines and their receptor CXCR2 during metanephric development and suggest a novel mechanism for chemotherapeutic intervention in Wilms' tumor.

Cellular mechanisms of Müllerian duct formation in the mouse

Regardless of their sex chromosome karyotype, amniotes develop two pairs of genital ducts, the Wolffian and Müllerian ducts. As the Müllerian duct forms, its growing tip is intimately associated with the Wolffian duct as it elongates to the urogenital sinus. Previous studies have shown that the presence of the Wolffian duct is required for the development and maintenance of the Müllerian duct. The Müllerian duct is known to form by invagination of the coelomic epithelium, but the mechanism for its elongation to the urogenital sinus remains to be defined. Using genetic fate mapping, we demonstrate that the Wolffian duct does not contribute cells to the Müllerian duct. Experimental embryological manipulations and molecular studies show that precursor cells at the caudal tip of the Müllerian duct proliferate to deposit a cord of cells along the length of the urogenital ridge. Furthermore, immunohistochemical analysis reveals that the cells of the developing Müllerian duct are mesoepithelial when deposited, and subsequently differentiate into an epithelial tube and eventually the female reproductive tract. Our studies define cellular and molecular mechanisms for Müllerian duct formation.

The cellular basis of kidney development

Mammalian kidney development has helped elucidate the general concepts of mesenchymal-epithelial interactions, inductive signaling, epithelial cell polarization, and branching morphogenesis. Through the use of genetically engineered mouse models, the manipulation of Xenopus and chick embryos, and the identification of human renal disease genes, the molecular bases for many of the early events in the developing kidney are becoming increasingly clear. Early patterning of the kidney region depends on interactions between Pax/Eya/Six genes, with essential roles for lim1 and Odd1. Ureteric bud outgrowth and branching morphogenesis are controlled by the Ret/Gdnf pathway, which is subject to positive and negative regulation by a variety of factors. A clear role for Wnt proteins in induction of the kidney mesenchyme is now well established and complements the classic literature nicely. Patterning along the proximal distal axis as the nephron develops is now being investigated and must involve aspects of Notch signaling. The development of a glomerulus requires interactions between epithelial cells and infiltrating endothelial cells to generate a unique basement membrane. The integrity of the glomerular filter depends in large part on the proteins of the nephrin complex, localized to the slit diaphragm. Despite the kidney's architectural complexity, with the advent of genomics and expression arrays, it is becoming one of the best-characterized organ systems in developmental biology.

bFGF induces an earlier expression of nephrogenic proteins after ischemic acute renal failure

Recovery from acute renal failure (ARF) requires the replacement of injured cells with new cells that restore tubule epithelial integrity. We described recently the expression of a wide range of nephrogenic proteins in tubular cells after ARF induced by ischemia-reperfusion (I/R) (Villanueva S, Cespedes C, and Vio CP. Am J Physiol Regul Integr Comp Physiol 290: R861-R870, 2006). These markers, namely, Vimentin, neural cell adhesion molecules (Ncam), basic fibroblast growth factor (bFGF), paired homeobox-2 (Pax-2), bone morphogene protein-7 (BMP-7), Noggin, Lim-1, Engrailed, Smad, phospho-Smad, hypoxia-induced factor-1alpha (HIF-1alpha), VEGF, and Tie-2, are expressed in a time frame similar to that observed in normal kidney development. bFGF participates in early kidney development as a morphogen involved in mesenchyme/epithelial transition, and it is reexpressed in the recovery phase of ARF. To test the hypothesis that bFGF can accelerate the regeneration after renal damage, we used recombinant bFGF and studied the expression pattern of the above described morphogens in ARF. Male Sprague-Dawley rats were subjected to 30 min of renal ischemic injury and were injected with bFGF 30 microg/kg followed by reperfusion. Rats were killed and the expression of nephrogenic proteins were analyzed by immunohistochemistry and Western blot analysis. In the animals subjected to I/R treated with bFGF, we observed a 12- to 24-h earlier and more abundant reexpression of the proteins Ncam, bFGF, Pax-2, BMP-7, Noggin, Lim-1, Engrailed, VEGF, and Tie-2 than the I/R untreated rats. In addition, we observed a reduction in renal damage markers ED-1 and alpha-smooth muscle actin. These results indicate that bFGF can participate in the regeneration process and suggest that the treatment with bFGF can induce an earlier regeneration process after ischemic acute renal failure.

Cells differentiated from mouse embryonic stem cells via embryoid bodies express renal marker molecules

Differentiation of mouse embryonic stem (ES) cells via embryoid bodies (EB) is established as a suitable model to study cellular processes of development in vitro. ES cells are known to be pluripotent because of their capability to differentiate into cell types of all three germ layers including germ cells. Here, we show that ES cells differentiate into renal cell types in vitro. We found that genes were expressed during EB cultivation, which have been previously described to be involved in renal development. Marker molecules characteristic for terminally differentiated renal cell types were found to be expressed predominantly during late stages of EB cultivation, while marker molecules involved in the initiation of nephrogenesis were already expressed during early steps of EB development. On the cellular level--using immunostaining--we detected cells expressing podocin, nephrin and wt-1, characteristic for differentiated podocytes and other cells, which expressed Tamm-Horsfall protein, a marker for distal tubule epithelial cells of kidney tissue. Furthermore, the proximal tubule marker molecules renal-specific oxido reductase, kidney androgen-related protein and 25-hydroxyvitamin D3alpha-hydroxylase were found to be expressed in EBs. In particular, we could demonstrate that cells expressing podocyte marker molecules assemble to distinct ring-like structures within the EBs. Because the differentiation efficiency into these cell types is still relatively low, application of fibroblast growth factor (FGF)-2 in combination with leukaemia inhibitory factor was tested for induction, but did not enhance ES cell-derived renal differentiation in vitro.

Mesenchymal cells from adult kidney support angiogenesis and differentiate into multiple interstitial cell types including erythropoietin-producing fibroblasts

Mesenchymal cells have been isolated from embryos and multiple adult organs where they may differentiate into various connective tissue cell types and provide paracrine support for surrounding cells. With the use of a technique for culturing multipotent mesenchymal cells from adult tissues, a fibroblast-like cell clone (4E) was isolated from adult mouse kidney. 4E cells were able to differentiate along multiple mesodermal lineages including cell types located in the renal interstitium such as fibroblasts and pericytes. Coculture of 4E cells with ureteric bud and epithelial cell lines and analysis of resulting changes in gene expression revealed that these cells support angiogenesis and tubulogenesis and expressed genes characteristic of embryonic renal stromal cells. Following subcapsular injection after unilateral ischemia-reperfusion in adult mice, 4E cells migrated to a peritubular interstitial location and expressed interstitial cell markers, whereas cells injected in control kidneys remained stationary. Incubation in hypoxic or anoxic conditions resulted in erythropoietin expression in a small subset of ecto-5'-nucleotidase-positive cells and resulted in increased vascular endothelial growth factor expression in the same cell population. Our findings suggest that the adult kidney may contain interstitial mesenchymal cell progenitors with embryonic stromal cell characteristics that are able to provide paracrine support for surrounding vessels and tubular epithelial cells and differentiate into erythropoietin producing fibroblasts.

Ischemic acute renal failure induces the expression of a wide range of nephrogenic proteins

Ischemia-induced acute renal failure (ARF) is a disorder with high morbidity and mortality. ARF is characterized by a regeneration phase, yet its molecular basis is still under study. Changes in gene expression have been reported in ARF, and some of these genes are specific for nephrogenic processes. We tested the hypothesis that the regeneration process developed after ischemia-induced ARF can be characterized by the reexpression of important regulatory proteins of kidney development. The distribution pattern and levels of nephrogenic proteins in rat kidneys after ischemia were studied by immunohistochemistry and immunoblot analysis. Ischemic damage was assessed by conventional morphology, serum creatinine, and the apoptotic markers terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) and caspase 3. The hypoxia levels induced by ischemia were assessed by specific markers: hypoxia induced factor (HIF)-1alpha and 2-pimonidazole. In kidneys with ARF, an important initial damage was observed through periodic acid Schiff staining, by the induction of damage markers alpha-smooth muscle actin (alpha-SMA) and macrophages (ED-1) and by apoptosis induction. In agreement with diminishing renal damage at the initial reparation phase, the expression of the mesenchymal proteins vimentin, neural cell adhesion molecules (Ncam), and the epithelial markers, Pax-2, Noggin, and basic fibroblast growth factor was observed; after, in a second phase, the tubular markers bone morphogen protein 7, Engrailed, and Lim-1, as well as the transcription factors Smad and p-Smad, were observed. Additionally, the endothelial markers VEGF and Tie-2 were induced at the initial and middle stages of regeneration phase, respectively. The expression of these proteins was restricted in time and space, as well as spatially and temporally. Because all of these proteins are important in maintaining a functional kidney, these results suggest that during the regeneration process after induced hypoxia, these nephrogenic proteins can be reexpressed in a similar fashion to that observed during development, thus restoring mature kidney function.

Temporal and spatial transcriptional programs in murine kidney development

We have performed a systematic temporal and spatial expression profiling of the developing mouse kidney using Compugen long-oligonucleotide microarrays. The activity of 18,000 genes was monitored at 24-h intervals from 10.5-day-postcoitum (dpc) metanephric mesenchyme (MM) through to neonatal kidney, and a cohort of 3,600 dynamically expressed genes was identified. Early metanephric development was further surveyed by directly comparing RNA from 10.5 vs. 11.5 vs. 13.5dpc kidneys. These data showed high concordance with the previously published dynamic profile of rat kidney development (Stuart RO, Bush KT, and Nigam SK. Proc Natl Acad Sci USA 98: 5649-5654, 2001) and our own temporal data. Cluster analyses were used to identify gene ontological terms, functional annotations, and pathways associated with temporal expression profiles. Genetic network analysis was also used to identify biological networks that have maximal transcriptional activity during early metanephric development, highlighting the involvement of proliferation and differentiation. Differential gene expression was validated using whole mount and section in situ hybridization of staged embryonic kidneys. Two spatial profiling experiments were also undertaken. MM (10.5dpc) was compared with adjacent intermediate mesenchyme to further define metanephric commitment. To define the genes involved in branching and in the induction of nephrogenesis, expression profiling was performed on ureteric bud (GFP+) FACS sorted from HoxB7-GFP transgenic mice at 15.5dpc vs. the GFP- mesenchymal derivatives. Comparisons between temporal and spatial data enhanced the ability to predict function for genes and networks. This study provides the most comprehensive temporal and spatial survey of kidney development to date, and the compilation of these transcriptional surveys provides important insights into metanephric development that can now be functionally tested.

Cited1 Is a Bifunctional Transcriptional Cofactor That Regulates Early Nephronic Patterning

In a screen to identify factors that regulate the conversion of mesenchyme to epithelium during the early stages of nephrogenesis, it was found that the Smad4-interacting transcriptional cofactor, Cited1, is expressed in the condensed cap mesenchyme surrounding the tip of the ureteric bud (UB), is downregulated after differentiation into epithelia, and has the capacity to block UB branching and epithelial morphogenesis in cultured metanephroi. Cited1 represses Wnt/beta-catenin but activates Smad4-dependent transcription involved in TGF-beta and Bmp signaling. By modifying these pathways, Cited1 may coordinate cellular differentiation and survival signals that regulate nephronic patterning in the metanephros.

Wnt-4 activates the canonical beta-catenin-mediated Wnt pathway and binds Frizzled-6 CRD: functional implications of Wnt/beta-catenin activity in kidney epithelial cells

The Wnt signaling pathway is central to the development of all animals and to cancer progression, yet largely unknown are the pairings of secreted Wnt ligands to their respective Frizzled transmembrane receptors or, in many cases, the relative contributions of canonical (beta-catenin/LEF/TCF) versus noncanonical Wnt signals. Specifically, in the kidney where Wnt-4 is essential for the mesenchymal to epithelial transition that generates the tissue's collecting tubules, the corresponding Frizzled receptor(s) and downstream signaling mechanism(s) are unclear. In this report, we addressed these issues using Madin-Darby Canine Kidney (MDCK) cells, which are competent to form tubules in vitro. Employing established reporter constructs of canonical Wnt/beta-catenin pathway activity, we have determined that MDCK cells are highly responsive to Wnt-4, -1, and -3A, but not to Wnt-5A and control conditions, precisely reflecting functional findings from Wnt-4 null kidney mesenchyme ex vivo rescue studies. We have confirmed that Wnt-4's canonical signaling activity in MDCK cells is mediated by downstream effectors of the Wnt/beta-catenin pathway using beta-Engrailed and dnTCF-4 constructs that suppress this pathway. We have further found that MDCK cells express the Frizzled-6 receptor and that Wnt-4 forms a biochemical complex with the Frizzled-6 CRD. Since Frizzled-6 did not appear to transduce Wnt-4's canonical signal, data supported recently by Golan et al., there presumably exists another as yet unknown Frizzled receptor(s) mediating Wnt-4 activation of beta-catenin/LEF/TCF. Finally, we report that canonical Wnt/beta-catenin signals cells help maintain cell growth and survival in MDCK cells but do not contribute to standard HGF-induced (nonphysiologic) tubule formation. Our results in combination with work from Xenopus laevis (not shown) lead us to believe that Wnt-4 binds both canonical and noncanonical Frizzled receptors, thereby activating Wnt signaling pathways that may each contribute to kidney tubulogenesis.

Gene expression profiling of advanced ovarian cancer: characterization of a molecular signature involving fibroblast growth factor 2

Epithelial ovarian cancer (EOC) is the gynecological disease with the highest death rate. We applied an automatic class discovery procedure based on gene expression profiling to stages III-IV tumors to search for molecular signatures associated with the biological properties and progression of EOC. Using a complementary DNA microarray containing 4451 cancer-related, sequence-verified features, we identified a subset of EOC characterized by the expression of numerous genes related to the extracellular matrix (ECM) and its remodeling, along with elements of the fibroblast growth factor 2 (FGF2) signaling pathway. A total of 10 genes were validated by quantitative real-time polymerase chain reaction, and coexpression of FGF2 and fibroblast growth factor receptor 4 in tumor cells was revealed by immunohistochemistry, confirming the reliability of gene expression by cDNA microarray. Since the functional relationships among these genes clearly suggested involvement of the identified molecular signature in processes related to epithelial-stromal interactions and/or epithelial-mesenchymal cellular plasticity, we applied supervised learning analysis on ovarian-derived cell lines showing distinct cellular phenotypes in culture. This procedure enabled construction of a gene classifier able to discriminate mesenchymal-like from epithelial-like cells. Genes overexpressed in mesenchymal-like cells proved to match the FGF2 signaling and ECM molecular signature, as identified by unsupervised class discovery on advanced tumor samples. In vitro functional analysis of the cell plasticity classifier was carried out using two isogenic and immortalized cell lines derived from ovarian surface epithelium and displaying mesenchymal and epithelial morphology, respectively. The results indicated the autocrine, but not intracrine stimulation of mesenchymal conversion and cohort/scatter migration of cells by FGF2, suggesting a central role for FGF2 signaling in the maintenance of cellular plasticity of ovary-derived cells throughout the carcinogenesis process. These findings raise mechanistic hypotheses on EOC pathogenesis and progression that might provide a rational underpinning for new therapeutic modalities.

Update of extracellular matrix, its receptors, and cell adhesion molecules in mammalian nephrogenesis

One of the hallmarks of mammalian nephrogenesis includes a mesenchymal-epithelial transition that is accomplished by intercalation of the ureteric bud, an epithelium-lined tubelike structure, into an undifferentiated mesenchyme, and the latter then undergoes an inductive transformation and differentiates into an epithelial phenotype. At the same time, the differentiating mesenchyme reciprocates by inducing branching morphogenesis of the ureteric bud, which forms a treelike structure with dichotomous iterations. These reciprocal inductive interactions lead to the development of a functioning nephron unit made up of a glomerulus and proximal and distal tubules. The inductive interactions and differentiation events are modulated by a number of transcription factors, protooncogenes, and growth factors and their receptors, which regulate the expression of target morphogenetic modulators including the ECM, integrin receptors, and cell adhesion molecules. These target macromolecules exhibit spatiotemporal and stage-specific developmental regulation in the metanephros. The ECM molecules expressed at the epithelial-mesenchymal interface are perhaps the most relevant and conducive to the paracrine-juxtacrine interactions in a scenario where the ligand is expressed in the mesenchyme while the receptor is located in the ureteric bud epithelium or vice versa. In addition, expression of the target ECM macromolecules is regulated by matrix metalloproteinases and their inhibitors to generate a concentration gradient at the interface to further propel epithelial-mesenchymal interactions so that nephrogenesis can proceed seamlessly. In this review, we discuss and update our current understanding of the role of the ECM and related macromolecules with respect to metanephric development.

Matrix metalloproteinases in renal development

Matrix metalloproteinases (MMPs) are enzymes with metal ion-dependent activity that degrade extracellular matrix (ECM) glycoproteins. MMPs play a vital role in various biological processes, such as embryogenesis, tissue remodeling, angiogenesis, and wound healing, and in certain disease processes, for example, metastasis of cancer cells. Following their activation, MMPs are believed to modulate both cell-cell and cell-matrix interactions, which in turn regulate cellular differentiation, migration, proliferation, and cell survival. Being involved in pericellular proteolysis, they maintain a gradient of ECM proteins by balancing ECM synthesis and degradation. Such a balance is critical for various mammalian developmental processes during embryonic life and also for the homeostasis of various organs and reparative processes in later life. During the past two decades the role of MMPs in the morphogenesis of various organs, including that of the metanephros, has been investigated extensively. Mammalian nephrogenesis comprises a series of intricate events characterized by a sustained remodeling and turnover of ECM, suggesting a potential role of MMPs in renal development. Conceivably, reciprocal inductive epithelial-mesenchymal interactions that take place at the very commencement of nephrogenesis are modulated by a number of ECM proteins. Their expression, especially at the epithelial-mesenchymal interface, are critical for metanephric development, and such a strategic expression is likely to be modified by a number of different macromolecules that exhibit spatiotemporal and stage-specific expression. Among them the most suitable candidate that could exert such a control would be MMPs. This review addresses the current status of our understanding of the functions and the role of MMPs in renal development.

Ureteric bud controls multiple steps in the conversion of mesenchyme to epithelia

Conversion of renal mesenchyme into epithelia depends on the ureteric bud, but its specific actions are not established. From conditioned media of ureteric bud cells, we have identified molecules that mimic the growth and epithelialization of mesenchyme in vivo. LIF targets late epithelial progenitors surrounding the ureteric bud, and in combination with survival factors, converts them into nephrons. In contrast, 24p3/Ngal targets early progenitors at the kidney's periphery through an iron-mediated, but a transferrin-independent mechanism. Hence, the ureteric bud controls many steps of cell conversion. A genome wide search for ureteric bud-specific molecules will identify additional pathways that induce morphogenesis.

Renal development: perspectives on a Wnt-dependent process

Specification of embryonic progenitors to generate the branched collecting duct system and tubular epithelia of the nephron in the metanephros is mediated by families of soluble factors that cooperate to regulate morphogenesis. These include multiple members of the FGF, TGF-beta, and Wnt families; however, the complexity of interactions through cell-cell and extracellular matrix-mediated contacts, the redundancy of factors involved, and multiplicity of cooperative signaling mechanisms limit our understanding of events responsible for this development. With available in vitro and targeted mutagenesis models, we are now beginning to comprehend how the secreted inductive proteins and associated transcription factors direct competent cells to produce a functional filtering tubular epithelium and its tightly integrated vascular network.

The Secreted Frizzled Related Protein 2 (SFRP2) Gene Is a Target of the Pax2 Transcription Factor

Despite their essential role in vertebrate development, the function of Pax proteins in gene regulation is not well understood. To identify potential genes regulated by the Pax2 protein, we screened embryonic kidney cells transformed with Pax2-expressing retroviruses for genes activated in response to Pax2 expression. In this system, the gene encoding the secreted frizzled related protein, Sfrp2, was strongly activated in all Pax2b-expressing cells. This activation of Sfrp2 expression correlated with changes in chromatin structure at the Sfrp2 locus, particularly in and around regions of Pax2 binding. Although the amount of Pax2-dependent transactivation was low in transient assays, the data suggests that local alterations of chromatin structure by Pax proteins can greatly enhance expression when presented in the right cellular context.

Iron, lipocalin, and kidney epithelia

Brilliant new discoveries in the field of iron metabolism have revealed novel transmembrane iron transporters, novel hormones that regulate iron traffic, and iron's control of gene expression. An important role for iron in the embryonic kidney was first identified by Ekblom, who studied transferrin (Landschulz W and Ekblom P. J Biol Chem 260: 15580-15584, 1985; Landschulz W, Thesleff I, and Ekblom P. J Cell Biol 98: 596-601, 1984; Thesleff I, Partanen AM, Landschulz W, Trowbridge IS, and Ekblom P. Differentiation 30: 152- 158, 1985). Nevertheless, how iron traffics to developing organs remains obscure. This review discusses a member of the lipocalin superfamily, 24p3 or neutrophil gelatinase-associated lipocalcin (NGAL), which induces the formation of kidney epithelia. We review the data showing that lipocalins transport low-molecular-weight chemical signals and data indicating that 24p3/NGAL transports iron. We compare 24p3/NGAL to transferrin and a variety of other iron trafficking pathways and suggest specific roles for each in iron transport.

Activin a produced by ureteric bud is a differentiation factor for metanephric mesenchyme

The present study was conducted to investigate the role of the activin-follistatin system in the development of metanephros. Organ culture system and cultured metanephric mesenchymal cells were used to address this issue. Activin A was localized in ureteric bud. Activin type II receptor was localized in ureteric bud as well as metanephric mesenchyme. In an organ culture system, exogenous activin A reduced the size of cultured metanephroi, delayed ureteric bud branching, and enlarged the tips of ureteric bud. Follistatin, an antagonist of activin A was used to clarify the role of endogenous activin A. Exogenous follistatin enlarged the size of cultured metanephroi, increased ureteric bud branching, and promoted cell growth in ureteric bud. Blockade of activin signaling by adenoviral transfection of dominantly negative activin mutant receptor mimics the effect of follistatin. In cultured metanephric mesenchymal cells, activin A promoted cell growth; conversely, follistatin induced apoptosis. Furthermore, activin A induced the expressions of epithelial differentiation markers in these cells. These results suggest that activin A produced by ureteric bud is not only an important regulator of ureteric bud branching, but also a differentiation factor for metanephric mesenchyme during kidney development.

Conditionally immortalized cell line of inducible metanephric mesenchyme

BACKGROUND

The mesenchymal-epithelial conversion of metanephric mesenchyme (MM) in the formation of nephronic tubules has long served as a paradigm for inductive signaling in morphogenesis. However, the mechanisms underlying this differentiation have remained an enigma due to insufficient numbers of primary mesenchymal cells that must be isolated manually from animal embryos. To overcome this problem, we have established a conditionally immortalized cell line, the rat-inducible metanephric mesenchyme (RIMM-18) by transfection of primary mesenchymal cells with a vector, encoding an estradiol-dependent E1A-ER fusion protein.

METHODS

Reverse transcription-polymerase chain reaction (RT-PCR), luciferase reporter assay, electrophoretic mobility shift assay, immunocytochemical, and immunohistochemical stainings were used to characterize the established cell line.

RESULTS

We demonstrate that in the presence of estradiol, the RIMM-18 cell line proliferates continuously, maintaining mesenchymal characteristics for over 40 passages. These cells are vimentin-positive and cytokeratin-negative. Under inductive conditions in the absence of estradiol, they are responsive to a number of cytokines, which are established inducers of mesenchymal cells in vivo and in vitro [i.e., fibroblast growth factor 2 (FGF2), leukemia inhibitory factor (LIF), and transforming growth factor-beta 2 (TGF-beta 2)]. We show the presence in RIMM-18 cells of specific protein markers and functionally active signaling pathways required for induction of tubule formation in MM. Furthermore, induced RIMM-18 cells change morphology, acquiring epithelial-like features, and begin to express epithelial markers (e.g., E-cadherin, cytokeratin, gamma-glutamyl-transpeptidase, and secreted frizzled-related protein 2 (sFRP2).

CONCLUSION

This preliminary characterization of the RIMM-18 cell line suggests that it will be useful in the study of biochemical and molecular mechanisms of nephronic development and, possibly, of some types of renal cancer such as Wilms' tumor, which caricatures the normal process of kidney development.

An iron delivery pathway mediated by a lipocalin

Despite the critical need for iron in many cellular reactions, deletion of the transferrin pathway does not block organogenesis, suggesting the presence of alternative methods to deliver iron. We show that a member of the lipocalin superfamily (24p3/Ngal) delivers iron to the cytoplasm where it activates or represses iron-responsive genes. Iron unloading depends on the cycling of 24p3/Ngal through acidic endosomes, but its pH sensitivity and its subcellular targeting differed from transferrin. Indeed, during the conversion of mesenchyme into epithelia (where we discovered the protein), 24p3/Ngal and transferrin were endocytosed by different cells that characterize different stages of development, and they triggered unique responses. These studies identify an iron delivery pathway active in development and cell physiology.

An epithelial precursor is regulated by the ureteric bud and by the renal stroma

Kidney epithelia develop from the metanephric mesenchyme after receiving inductive signals from the ureteric bud and from the renal stroma. However, it is not clear how these signals induce the different types of epithelia that make up the nephron. To investigate inductive signaling, we have isolated clusters of epithelial progenitors from the metanephric mesenchyme, thereby separating them from the renal stroma. When the isolated progenitors were treated with the ureteric bud factor LIF, they expressed epithelial proteins (ZO-1, E-cadherin, laminin alpha(5)) and produced nephrons (36 glomeruli with 58 tubules), indicating that they are the target of inductive signaling from the ureteric bud, and that renal stroma is not absolutely required for epithelial development in vitro. In fact, stroma-depleted epithelial progenitors produced sevenfold more glomeruli than did intact metanephric mesenchyme (5 glomeruli, 127 tubules). Conversely, when epithelial progenitors were treated with both LIF and proteins secreted from a renal stromal cell line, glomerulogenesis was abolished but tubular epithelia were expanded (0 glomeruli, 47 tubules). Hence, by isolating epithelial progenitors from the metanephric mesenchyme, we show that they are targeted by factors from the ureteric bud and from the renal stroma, and that epithelial diversification is stimulated by the ureteric bud and limited by renal stroma.

ERK and p38 MAP kinase are required for rat renal development

BACKGROUND

We previously demonstrated that extracellular signal-regulated protein kinase (ERK) and p38 mitogen-activated protein (MAP) kinase (p38) are strongly expressed in the embryonic kidney. In the present study, we investigated the role of ERK and p38 during kidney development.

METHODS

Rat metanephroi were cultured from 15-day-old embryos, and exposed to inhibitors of MEK, an activator of ERK, PD98059 (300 micromol/L), U0126 (10 micromol/L), or a p38 inhibitor SB203580 (30 micromol/L) 24 to 120 hours after the start of culture. Growth of metanephroi was measured by surface area and thymidine incorporation. Ureteric buds and glomeruli were identified by labeling with Dolichos biflorus lectin and peanut agglutinin, respectively. PCNA staining and TUNEL assay were performed on kidney sections. The level of apoptosis was evaluated by examining DNA ladder formation.

RESULTS

Growth of metanephroi was significantly inhibited by SB203580 but not by PD98059 or U0126. Ureteric bud branching was not affected by SB203580 or MEK inhibitors. Glomerular number was markedly reduced by SB203580 and to a lesser extent by U0126 (14 +/- 2 and 48 +/- 10% of controls, respectively). On histological examination, the number of tubuloglomerular structures was reduced in MEK inhibitor-treated metanephroi compared to controls. Very few mesenchymal condensates were observed in kidneys incubated with SB203580. PCNA-positive cells were reduced in SB203580-treated metanephroi compared to control and PD98059-treated kidneys. Apoptosis was increased in SB203580-treated kidneys and to a lesser extent in PD98059-treated cultures.

CONCLUSIONS

Both ERK and p38 are required for renal development. ERK appears to play a role in nephrogenesis and p38 for kidney growth and nephrogenesis.

Protein restriction in pregnancy is associated with increased apoptosis of mesenchymal cells at the start of rat metanephrogenesis

BACKGROUND

In rats, offspring born to mothers supplied low protein diets during pregnancy have fewer glomeruli than normal. We hypothesized that such nephron deficits are associated with altered cell turnover in the metanephros, the embryonic precursor of the adult kidney.

METHODS

Wistar rats were supplied with one of three isocaloric diets from day 0 of pregnancy: control (18% protein) or low protein (9% or 6%) diets. All had a normal chow after birth. Groups were compared by multilevel statistical modeling.

RESULTS

At two weeks postnatally, when nephrogenesis has finished, controls had 16.8 x 103 +/- 0.7 x 10(3) (mean +/- SEM) glomeruli/kidney, whereas offspring exposed to 9% diet had 5.1 x 10(3) +/- 1.2 x 10(3) fewer and those exposed to 6% diet had 6.9 x 10(3) +/- 1.7 x 10(3) fewer glomeruli/kidney (P < 0.001, both diets). At embryonic day 13 (E13), when the metanephros has just formed, control metanephroi contained 2.35 x 10(4) +/- 0.15 x 10(4) cells, with no significant differences in low protein groups. At E15, when mesenchyme begins forming primitive nephrons but glomeruli are still absent, controls had 2.00 x 10(6) +/- 0.13 x 10(6) cells. E15 embryos exposed to 9% protein had 1.09 x 10(6) +/- 0.36 x 10(6) fewer cells/metanephros than controls, while those exposed to 6% diet had 1.45 x 10(6) +/- 0.37 x 10(6) fewer (P < 0.01, both diets). Apoptotic cells were detected by molecular (in-situ end-labeling) and morphological (propidium iodide staining) techniques. In all diets, apoptosis was noted in condensing mesenchyme (nephron precursors) and loose mesenchyme (interstitial precursors). Control E13 metanephroi had 63 +/- 7 apoptotic cells/mm2, whereas those exposed to 9% diet had an increase of 77 +/- 26 cells/mm2 (P < 0.01) and those exposed to 6% diet had an increase of 55 +/- 26 cells/mm2 (P < 0.05). By E15, apoptosis was similar in all groups but metanephric mitosis was significantly increased in the 6% protein diet group. No change was found in the level of apoptosis in E13 mesonephroi.

CONCLUSIONS

Maternal low protein diets reduce final numbers of glomeruli in association with enhanced deletion of mesenchymal cells at the start of kidney development. Whether aberrant nephrogenesis is a direct effect from deletion of nephron precursors, or an indirect effect from loss of supportive interstitial precursors, requires further investigation.

Characterization of Glis2, a novel gene encoding a Gli-related, Krüppel-like transcription factor with transactivation and repressor functions. Roles in kidney development and neurogenesis

In this study, we describe the characterization of a gene encoding a novel Krüppel-like protein, named Glis2. Glis2 encodes a relatively proline-rich, basic 55.8-kDa protein. Its five tandem Cys(2)-His(2) zinc finger motifs exhibit the highest homology to those of members of the Gli and Zic subfamilies of Krüppel-like proteins. Confocal microscopic analysis demonstrated that Glis2 localizes to the nucleus. Analysis of the genomic structure of the Glis2 gene showed that it is composed of 6 exons separated by 5 introns spanning a genomic region of more than 7.5 kb. Fluorescence in situ hybridization mapped the mouse Glis2 gene to chromosome 16A3-B1. Northern blot analysis showed that the Glis2 gene encodes a 3.8-kb transcript that is most abundant in adult mouse kidney. By in situ hybridization, expression was localized to somites and neural tube, and during metanephric development predominantly to the ureteric bud, precursor of the collecting duct, and inductor of nephronic tubule formation. One-hybrid analysis using Glis2 deletion mutants identified a novel activation function (AF) at the N terminus. The activation of transcription through this AF domain was totally suppressed by two repressor functions just downstream from the AF. One of the repressor functions is contained within the first zinc finger motif. The level of transcriptional activation and repression varied with the cell line tested, which might be due to differences in cell type-specific expression of co-activators and co-repressors. Our results suggest that Glis2 behaves as a bifunctional transcriptional regulator. Both the activation and repressor functions may play an important role in the regulation of gene expression during embryonic development.

Nephron induction revisited: from caps to condensates

Conversion of mesenchyme to epithelium in the metanephric kidney is clearly a multimolecular, multistep and partly redundant process. The present short review focuses on a neglected morphological aspect of kidney differentiation: the development of two transitory mesenchymal condensations that precede epithelial differentiation of nephrons. The first appearing condensate covers the tips of the collecting ducts and is termed a cap condensate. In the early kidney rudiment this structure has been referred to as a primary or early condensate. A few cells of the cap condensate (maybe only four to six cells), situated at the lateral edge of the cap, start proliferating rapidly and form a pretubular aggregate (or pretubular condensate), which converts to secretory nephron epithelia and finally segregates to different tubule segments. Throughout nephrogenesis, the cap condensates and pretubular aggregates are clearly distinguishable structures that show only partly overlapping gene expression profiles. Apart from being the source for the pretubular aggregates, the role of the cap condensate is unknown. It is now proposed that the cap regulates ureteric branching morphogenesis.

Genes and proteins involved in mesenchymal to epithelial transition

It has been known for many years that the epithelia of the urogenital system derive from mesenchyme. Essential regulators of this conversion have recently been discovered, and cellular changes have been described. However, we do not have a coherent view of how these dramatic changes are integrated, nor do we know the source or identity of extracellular signals that must regulate epithelialization of mesenchymal precursors. The metanephric kidney, Wolffian duct, and the Drosophila midgut are the leading model systems to describe how epithelia derive from mesenchyme.

TGF beta 2, LIF and FGF2 cooperate to induce nephrogenesis

The metanephric kidney develops from interactions between the epithelial ureteric bud and adjacent metanephric mesenchyme, which is induced by the bud to form the epithelia of the nephron. We have found that leukemia inhibitory factor (LIF) and transforming growth factor beta 2 (TGF beta 2) are secreted by inductive rat bud cells and cooperate to enhance and accelerate renal tubule formation in uninduced rat metanephric mesenchymal explants. LIF alone or TGF beta 2 with fibroblast growth factor 2 induced numerous tubules in isolated mesenchymes over an 8 day period, while (in combination) all three caused abundant tubule formation in 72 hours. Furthermore, neutralization of Wnt ligands with antagonist-secreted Frizzled-related protein 1 abrogated these responses and combinatorial cytokine/growth factor stimulation of explants augmented nuclear activation of Tcf1/Lef1, suggesting that LIF and TGF beta 2/FGF2 cooperate to regulate nephrogenesis through a common Wnt-dependent mechanism.

Secreted Frizzled-related proteins can regulate metanephric development

Wnt-4 signaling plays a critical role in kidney development and is associated with the epithelial conversion of the metanephric mesenchyme. Furthermore, secreted Frizzled-related proteins (sFRPs) that can bind Wnts are normally expressed in the developing metanephros, and function in other systems as modulators of Wnt signaling. sfrp-1 is distributed throughout the medullary and cortical stroma in the metanephros, but is absent from condensed mesenchyme and primitive tubular epithelia of the developing nephron where wnt-4 is highly expressed. In contrast, sfrp-2 is expressed in primitive tubules. To determine their role in kidney development, recombinant sFRP-1, sFRP-2 or combinations of both were applied to cultures of 13-dpc rat metanephroi. Both tubule formation and bud branching were markedly inhibited by sFRP-1, but concurrent sFRP-2 treatment restored some tubular differentiation and bud branching. sFRP-2 itself showed no effect on cultures of metanephroi. In cultures of isolated, induced rat metanephric mesenchymes, sFRP-1 blocked events associated with epithelial conversion (tubulogenesis and expression of lim-1, sfrp-2 and E-cadherin); however, it had no demonstrable effect on early events (compaction of mesenchyme and expression of wt1). As shown herein, sFRP-1 binds Wnt-4 with considerable avidity and inhibits the DNA-binding activity of TCF, an effector of Wnt signaling, while sFRP-2 had no effect on TCF activation. These observations suggest that sFRP-1 and sFRP-2 compete locally to regulate Wnt signaling during renal organogenesis. The antagonistic effect of sFRP-1 may be important either in preventing inappropriate development within differentiated areas of the medulla or in maintaining a population of cortical blastemal cells to facilitate further renal expansion. On the other hand, sFRP-2 might promote tubule formation by permitting Wnt-4 signaling in the presence of sFRP-1.