The life of the human kidney before birth: its secrets unfold

Expression of stem cell markers in the human fetal kidney

In the human fetal kidney (HFK) self-renewing stem cells residing in the metanephric mesenchyme (MM)/blastema are induced to form all cell types of the nephron till 34^th week of gestation. Definition of useful markers is crucial for the identification of HFK stem cells. Because wilms' tumor, a pediatric renal cancer, initiates from retention of renal stem cells, we hypothesized that surface antigens previously up-regulated in microarrays of both HFK and blastema-enriched stem-like wilms' tumor xenografts (NCAM, ACVRIIB, DLK1/PREF, GPR39, FZD7, FZD2, NTRK2) are likely to be relevant markers. Comprehensive profiling of these putative and of additional stem cell markers (CD34, CD133, c-Kit, CD90, CD105, CD24) in mid-gestation HFK was performed using immunostaining and FACS in conjunction with EpCAM, an epithelial surface marker that is absent from the MM and increases along nephron differentiation and hence can be separated into negative, dim or bright fractions. No marker was specifically localized to the MM. Nevertheless, FZD7 and NTRK2 were preferentially localized to the MM and emerging tubules (<10% of HFK cells) and were mostly present within the EpCAM^neg and EpCAM^dim fractions, indicating putative stem/progenitor markers. In contrast, single markers such as CD24 and CD133 as well as double-positive CD24⁺CD133⁺ cells comprise >50% of HFK cells and predominantly co-express EpCAM^bright, indicating they are mostly markers of differentiation. Furthermore, localization of NCAM exclusively in the MM and in its nephron progenitor derivatives but also in stroma and the expression pattern of significantly elevated renal stem/progenitor genes Six2, Wt1, Cited1, and Sall1 in NCAM⁺EpCAM^- and to a lesser extent in NCAM⁺EpCAM⁺ fractions confirmed regional identity of cells and assisted us in pinpointing the presence of subpopulations that are putative MM-derived progenitor cells (NCAM⁺EpCAM⁺FZD7⁺), MM stem cells (NCAM⁺EpCAM^-FZD7⁺) or both (NCAM⁺FZD7⁺). These results and concepts provide a framework for developing cell selection strategies for human renal cell-based therapies.

Development of kidney glomerular endothelial cells and their role in basement membrane assembly

Data showing that the embryonic day 12 (E12) mouse kidney contains its own pool of endothelial progenitor cells is presented. Mechanisms that regulate metanephric endothelial recruitment and differentiation, including the hypoxia-inducible transcription factors and vascular endothelial growth factor/vascular endothelial growth factor receptor signaling system, are also discussed. Finally, evidence that glomerular endothelial cells contribute importantly to assembly of the glomerular basement membrane (GBM), especially the laminin component, is reviewed. Together, this forum offers insights on blood vessel development in general, and formation of the glomerular capillary in particular, which inarguably is among the most unique vascular structures in the body.

Accumulation of malignant renal stem cells is associated with epigenetic changes in normal renal progenitor genes

Recent studies indicate a dual epigenetic role of the Polycomb group (PcG) proteins in self-renewal of stem cells and oncogenesis. Their elevation in our previous human kidney microarray screen led us examine whether they participate in processes involving normal and malignant renal progenitors. We therefore analyzed the expression of the PcG genes (EZH2, BMI-1, EED, SUZ12) in relation to that of the nephric-progenitor genes (WT1, PAX2, SALL1, SIX2, CITED1) using real-time polymerase chain reaction and methylation assays during renal development, regeneration, and tumorigenesis. Although all of the nephric-progenitor genes were shown to be developmentally regulated, analysis of polycomb gene expression during murine nephrogenesis and in an in vitro induction model of the nephrogenic mesenchyme indicated dynamic regulation only for EZH2 in the normal renal progenitor population. In contrast, induction of adult kidney regeneration by ischemia/reperfusion injury resulted primarily in rapid elevation of BMI-1, whereas EZH2 was silenced. Analysis of renal tumorigenesis in stem cell-like tumor xenografts established by serial passage of Wilms' tumor (WT) in immunodeficient mice showed cooperative upregulation of all PcG genes. This was accompanied by upregulation of WT1, PAX2, and SALL1 but downregulation of SIX2. Accordingly, methylation-specific quantitative polymerase chain reaction demonstrated promoter hypomethylation of WT1, PAX2, and SIX2 in primary WT and fetal kidneys, whereas progressive WT xenografts showed hypermethylation of SIX2, possibly leading to loss of renal differentiation. PcG genes vary in expression during renal development, regeneration, and tumorigenesis. We suggest a link between polycomb activation and epigenetic alterations of the renal progenitor population in initiation and progression of renal cancer.

Multiple imprinted and stemness genes provide a link between normal and tumor progenitor cells of the developing human kidney

Wilms' tumor (WT), the embryonic kidney malignancy, is suggested to evolve from a progenitor cell population of uninduced metanephric blastema, which typically gives rise to nephrons. However, apart from blastema, WT specimens frequently contain cells that have differentiated into renal tubular or stromal phenotypes, complicating their analysis. We aimed to define tumor-progenitor genes that function in normal kidney development using WT xenografts (WISH-WT), in which the blastema accumulates with serial passages at the expense of differentiated cells. Herein, we did transcriptional profiling using oligonucleotide microarrays of WISH-WT, WT source, human fetal and adult kidneys, and primary and metastatic renal cell carcinoma. Among the most significantly up-regulated genes in WISH-WT, we identified a surprising number of paternally expressed genes (PEG1/MEST, PEG3, PEG5/NNAT, PEG10, IGF2, and DLK1), as well as Meis homeobox genes [myeloid ecotropic viral integration site 1 homologue 1 (MEIS1) and MEIS2], which suppress cell differentiation and maintain self-renewal. A comparison between independent WISH-WT and WT samples by real-time PCR showed most of these genes to be highly overexpressed in the xenografts. Concomitantly, they were significantly induced in human fetal kidneys, strictly developmentally regulated throughout mouse nephrogenesis and overexpressed in the normal rat metanephric blastema. Furthermore, in vitro differentiation of the uninduced blastema leads to rapid down-regulation of PEG3, DLK1, and MEIS1. Interestingly, ischemic/reperfusion injury to adult mouse kidneys reinduced the expression of PEG3, PEG10, DLK1, and MEIS1, hence simulating embryogenesis. Thus, multiple imprinted and stemness genes that function to expand the renal progenitor cell population may lead to evolution and maintenance of WT.

Congenital obstructive uropathy: its origin and contribution to end-stage renal disease in children

Most children with end-stage renal failure are born with abnormal kidneys, with fetal obstructive uropathy accounting for about half these cases. Posterior urethral valves (PUV) is overwhelmingly the most common, specific diagnosis, and is confined to boys. In general, the condition is sporadic, although familial cases have been reported, perhaps suggesting that genetic factors play a role in pathogenesis. PUV can present antenatally, detected on routine fetal ultrasonography, or postnatally, when it is associated with renal failure and bladder dysfunction. Advances in postnatal surgery and medical management have reduced the mortality in PUV, but it is uncertain whether antenatal diagnosis or treatment (eg, with vesicoamniotic shunts) improves long-term renal outcome. Other disorders associated with human fetal obstructive uropathy are urethral atresia, the prune belly syndrome, and multicystic dysplastic kidney. The nephropathy associated with fetal obstructive uropathy is renal dysplasia and comprises undifferentiated and metaplastic tissues. Several studies in animals and in humans suggest that fetal urine flow impairment causes deregulation of renal precursor cell turnover and expression of growth factor/survival and transcription factor genes. Furthermore, some of these morphologic and molecular changes can be ameliorated experimentally by fetal surgical decompression or administration of specific growth factors.