Molecular insights into segmentation along the proximal-distal axis of the nephron

The pathogenic role of Notch activation in podocytes

Podocytes play a key role in the maintenance of the glomerular filtration barrier. Depletion or dysregulative mechanisms of podocytes can lead to the development of glomerulosclerosis. Signaling pathways that control these processes in podocytes are not fully understood. Recent studies from our and other laboratories found that genes that belong to the Notch pathway are regulated in patients and in animal models of renal disease. Genetic studies performed on mice with conditional expression of active Notch1 protein showed massive albuminuria, glomerulosclerosis, and ultimately renal failure and death of the animals. gamma-Secretase inhibitors and genetic deletion of Notch transcriptional binding partner (Rbpj) protected animals from nephrotic syndrome. Further studies are needed to define whether the activation of Notch pathway in podocytes represents a common pathomechanism in glomerular injury, and its potential to be a therapeutic target for the treatment of chronic kidney disease.

The zebrafish pronephros: a model to study nephron segmentation

Nephrons possess a segmental organization where each segment is specialized for the secretion and reabsorption of particular solutes. The developmental control of nephron segment patterning remains one of the enigmas within the field of renal biology. Achieving an understanding of the mechanisms that direct nephron segmentation has the potential to shed light on the causes of kidney birth defects and renal diseases in humans. Researchers studying embryonic kidney development in zebrafish and Xenopus have recently demonstrated that the pronephric nephrons in these vertebrates are segmented in a similar fashion as their mammalian counterparts. Further, it has been shown that retinoic acid signaling establishes proximodistal segment identities in the zebrafish pronephros by modulating the expression of renal transcription factors and components of signaling pathways that are known to direct segment fates during mammalian nephrogenesis. These findings present the zebrafish model as an excellent genetic system in which to interrogate the conserved developmental pathways that control nephron segmentation in both lower vertebrates and mammals.

Transcriptional control of terminal nephron differentiation

Terminal differentiation of epithelial cells into more specialized cell types is a critical step in organogenesis. Throughout the process of terminal differentiation, epithelial progenitors acquire or upregulate expression of renal function genes and cease to proliferate, while expression of embryonic genes is repressed. This exquisite coordination of gene expression is accomplished by signaling networks and transcription factors which couple the external environment with the new functional demands of the cell. While there has been much progress in understanding the early steps involved in renal epithelial cell differentiation, a major gap remains in our knowledge of the factors that control the steps of terminal differentiation. A number of signaling molecules and transcription factors have been recently implicated in determining segmental nephron identity and functional differentiation. While some of these factors (the p53 gene family, hepatocyte nuclear factor-1beta) promote the terminal epithelial differentiation fate, others (Notch, Brn-1, IRX, KLF4, and Foxi1) tend to regulate differentiation of specific nephron segments and individual cell types. This review summarizes current knowledge related to these transcription factors and discusses how diverse cellular signals are integrated to generate a transcriptional output during the process of terminal differentiation. Since these transcriptional processes are accompanied by profound changes in nuclear chromatin structure involving the genes responsible for creating and maintaining the differentiated cell phenotype, future studies should focus on identifying the nature of these epigenetic events and factors, how they are regulated temporally and spatially, and the chromatin environment they eventually reside in.

Lessons learned about adult kidney stem cells from the malpighian tubules of Drosophila

All multicellular organisms have a specialized organ to concentrate and excrete wastes from the body. The kidneys in vertebrates and the malpighian tubules in Drosophila accomplish these functions. Mammals and Drosophila share some similar features during renal tubular development. Vertebrate kidneys are derived through the mutual induction of the ureteric bud and metanephric mesoderm, whereas the malpighian tubules of Drosophila develop from the hindgut primordium and visceral mesoderm. The vertebrate kidney also has the capacity to recover and regenerate following episodes of acute injury. Previous studies suggest that stem cells and progenitor cells may be involved in the repair and regeneration of injured renal tissue. However, studies differ as to the source of the regenerating renal cells. Recently, multipotent stem cells in Drosophila malpighian tubules were identified, and it was demonstrated that several differentiated cells in the malpighian tubules arise from these stem cells. In this article, the current understanding of kidney development and stem cell fate in mammal and Drosophila is compared. Furthermore, the potential application of the adult renal stem cells in kidney repair and the treatment of kidney cancers are discussed.

Intravital multi-photon microscopy reveals several levels of heterogeneity in endocytic uptake by mouse renal proximal tubules

Understanding renal function requires one to integrate the structural complexity of kidney nephrons and the dynamic nature of their cellular processes. Multi-photon fluorescence microscopy is a state-of-the-art imaging technique for in vivo analysis of kidney tubules structure and function in real time. This study presents visual evidence for several levels of heterogeneity of proximal tubular endocytic uptake in the superficial renal mouse cortex and illustrates the potential of multi-photon microscopy for providing a comprehensive and dynamic portrayal of renal function.

Fate mapping using Cited1-CreERT2 mice demonstrates that the cap mesenchyme contains self-renewing progenitor cells and gives rise exclusively to nephronic epithelia

Classic tissue recombination and in vitro lineage tracing studies suggest that condensed metanephric mesenchyme (MM) gives rise to nephronic epithelium of the adult kidney. However, these studies do not distinguish between cap mesenchyme and pre-tubular aggregates comprising the condensed MM, nor do they establish whether these cells have self-renewing capacity. To address these questions, we generated Cited1-CreER(T2) BAC transgenic mice, which express tamoxifen-regulated Cre recombinase exclusively in the cap mesenchyme. Fate mapping was performed by crossing these mice with the Rosa26R(LacZ) reporter line and evaluating the location and cellular characteristics of LacZ positive cells at different time points following tamoxifen injection. These studies confirmed expected results from previous in vitro analysis of MM cell fate, and provide in vivo evidence that the cap mesenchyme does not contribute to collecting duct epithelium in the adult. Furthermore, by exploiting the temporally regulated Cre recombinase, these studies show that nephronic epithelium arising at different stages of nephrogenesis has distinct spatial distribution in the adult kidney, and demonstrate for the first time that the cap mesenchyme includes a population of self-renewing epithelial progenitor cells.