Stem cell options for kidney disease

Stem cell marker TRA-1-60 is expressed in foetal and adult kidney and upregulated in tubulo-interstitial disease

The kidney has an intrinsic ability to repair itself when injured. Epithelial cells of distal tubules may participate in regeneration. Stem cell marker, TRA-1-60 is linked to pluripotency in human embryonic stem cells and is lost upon differentiation. TRA-1-60 expression was mapped and quantified in serial sections of human foetal, adult and diseased kidneys. In 8- to 10-week human foetal kidney, the epitope was abundantly expressed on ureteric bud and structures derived therefrom including collecting duct epithelium. In adult kidney inner medulla/papilla, comparisons with reactivity to epithelial membrane antigen, aquaporin-2 and Tamm-Horsfall protein, confirmed extensive expression of TRA-1-60 in cells lining collecting ducts and thin limb of the loop of Henle, which may be significant since the papillae were proposed to harbour slow cycling cells involved in kidney homeostasis and repair. In the outer medulla and cortex there was rare, sporadic expression in tubular cells of the collecting ducts and nephron, with positive cells confined to the thin limb and thick ascending limb and distal convoluted tubules. Remarkably, in cortex displaying tubulo-interstitial injury, there was a dramatic increase in number of TRA-1-60 expressing individual cells and in small groups of cells in distal tubules. Dual staining showed that TRA-1-60 positive cells co-expressed Pax-2 and Ki-67, markers of tubular regeneration. Given the localization in foetal kidney and the distribution patterns in adults, it is tempting to speculate that TRA-1-60 may identify a population of cells contributing to repair of distal tubules in adult kidney.

Renal fibrosis: novel insights into mechanisms and therapeutic targets

Renal fibrosis is the common end point of virtually all progressive kidney diseases. Renal fibrosis should not be viewed as a simple and uniform 'scar', but rather as a dynamic system that involves extracellular matrix components and many, if not all, renal and infiltrating cell types. The involved cells exhibit enormous plasticity or phenotypic variability-a fact that we are only beginning to appreciate. Only a detailed understanding of the underlying mechanisms of renal fibrosis can facilitate the development of effective treatments. In this Review, we discuss the most recent advances in renal, or more specifically, tubulointerstitial fibrosis. Novel mechanisms as well as potential treatment targets based on different cell types are described. Problems that continue to plague the field are also discussed, including specific therapeutic targeting of the kidney, the development of improved diagnostic methods to assess renal fibrosis and the shortcomings of available animal models.

Cell therapy for cystinosis

In the September 2009 issue of Blood, Syres et al. [1] report on syngeneic bone marrow cell (BMC) and haematopoietic stem cell (HSC) therapy as a successful treatment in a mouse model of cystinosis, an autosomal recessive metabolic disease caused by a defect in the transport of cystine across the lysosomal membrane. The accumulation of cystine crystals in lysosomes leads to a multi-organ dysfunction including proximal tubulopathy and renal failure, corneal deposits, myopathy and central nervous system defects. By using Ctns knock-out (Ctns(-/-)) mice as a model for cystinosis, Syres et al. show that BMC transplantation leads to a major reduction of cystine content in all tissues tested, reflected by a significant attenuation of the development and progression of kidney injury and reduction in the number of mice with corneal cystine crystals. These changes were correlated with the engraftment of donor BMC producing a functional cystine transporter in the tissues tested. The transplantation of mouse HSC had the same therapeutic effect than whole BMC in this model, which is important as such HSC can readily be isolated from peripheral blood in humans. This work suggests that BMC or HSC transplantation is a potential treatment for cystinosis and other renal tubular disorders.

Kidney regeneration

Neonephrogenesis, the capacity to regenerate renal tissue, is a distinctive feature of fish but not usually of mammals. However, evidence exists for kidney repair in response to insulting agents for animals and human beings. Studies have therefore been designed in the past few years to clarify the cellular and molecular basis of renal repair, with the aim to investigate the potential regenerative capacity of animal and human kidneys. Three main questions are being addressed by this research: whether terminally differentiated cells in adult animal kidneys have regenerative capacity; whether multipotent progenitor cells exist in kidneys; and whether renal repair can be favoured or accelerated by cells of extrarenal origin migrating to the kidney in response to injury. In this Review, we describe evidence of cellular and molecular pathways related to renal repair and regeneration, and review data from animal and human studies that show that the kidney might have regenerative capacity.

Foreign body-induced granulation tissue is a source of adult stem cells

In the current study, we have cultured and propagated the cells obtained from the granulation tissue that forms around perforated polyvinyl tubes placed in the subcutaneous space of normal rats. We found that these cells (called granulation tissue-derived stem cells [GTSCs]) expressed markers of embryonic pluripotent cells (Oct-4 and Nanog) and of adult stem cells (CXCR4 and Thy1.1) as well as produced high levels of vascular endothelial growth factor (VEGF) for up to 10 passages. By fluorescence-activated cell-sorting (FACS) analysis, GTSCs were positive for stem-cell surface markers CD90, CD59, and CD44 and were negative for CD45, which suggests that they were of mesenchymal origin and not of hematopoietic lineage. When incubated in specific differentiation medium, these cells transformed into adipogenic, osteogenic, and chondrogenic lineages, which shows that they were multipotent. Furthermore, after systemic injection, these cells were found in the vicinity of an injured site created in the liver but not in normal areas of the liver, which indicates their propensity to seek and engraft to an injured area in the body. We conclude that granulation tissue induced by a large foreign body is a convenient source of adult stem cells that can be maintained in culture and can be used to repair and regenerate injured tissue.

Isolation of clonogenic, long-term self renewing embryonic renal stem cells

A tissue stem cell should exhibit long-term self-renewal, clonogenicity and a capacity to differentiate into the tissue of origin. Such a postnatal renal stem cell has not been formally identified. The metanephric mesenchyme (MM) of the developing kidney gives rise to both the renal interstitium and the nephrons and is regarded as the progenitor population of the developing kidney. However, isolated MM does not self renew and requires immortalization for survival in culture. Here we report the isolation and sustained culture of long-term repopulating, clonal progenitors from the embryonic kidney as free floating nephrospheres. Such cells displayed clonal self renewal for in excess of twenty passages when cultured with bFGF and thrombin, showed broad mesodermal multipotentiality, but retained expression of key renal transcription factors (Wt1, Sall1, Eya1, Six1, Six2, Osr1 and Hoxa11). While these cells did display limited capacity to contribute to developing embryonic kidney explants, nephrospheres did not display in vitro renal epithelial capacity. Nephrospheres could be cultured from both Sall1(+) and Sall1(-) fractions of embryonic kidney, suggesting that they were derived from the MM as a whole and not specifically the MM-derived cap mesenchyme committed to nephron formation. This embryonic renal stem cell population was not able to be isolated from postnatal kidney confirming that while the embryonic MM represents a mulitpotent stem cell population, this does not persist after birth.

The interface between generating renal tubules and a polyester fleece in comparison to the interstitium of the developing kidney

An increasing number of investigations is dealing with the repair of acute and chronic renal failure by the application of stem/progenitor cells. However, accurate data concerning the cell biological mechanisms controlling the process of regeneration are scarce. For that reason new implantation techniques, advanced biomaterials and morphogens supporting regeneration of renal parenchyma are under research. Special focus is directed to structural and functional features of the interface between generating tubules and the surrounding interstitial space. The aim of the present experiments was to investigate structural features of the interstitium during generation of tubules. Stem/progenitor cells were isolated from neonatal rabbit kidney and mounted between layers of a polyester fleece to create an artificial interstitium. Perfusion culture was performed for 13 days in chemically defined Iscove's Modified Dulbecco's Medium containing aldosterone (1 x 10(-7) M) as tubulogenic factor. Recordings of the artificial interstitium in comparison to the developing kidney were performed by morphometric analysis, scanning and transmission electron microscopy. The degree of differentiation was registered by immunohistochemistry. The data reveal that generated tubules are embedded in a complex network of fibers consisting of newly synthesized extracellular matrix proteins. Morphometric analysis further shows that the majority of tubules within the artificial interstitium develops in a surprisingly close distance between 5 and 25 mum to each other. The abundance of synthesized extracellular matrix acts obviously as a spacer keeping generated tubules in distance. For comparison, the same principle of construction is found in the developing parenchyma of the neonatal kidney. Most astonishingly, scanning electron microscopy reveals that the composition of interstitial matrix is not homogeneous but differs along a cortico-medullary axis of proceeding tubule development.

Treatment strategies and clinical trial design in ADPKD

More frequent utilization and continuous improvement of imaging techniques has enhanced appreciation of the high phenotypic variability of autosomal dominant polycystic kidney disease, improved understanding of its natural history, and facilitated the observation of its structural progression. At the same time, identification of the PKD1 and PKD2 genes has provided clues to how the disease develops when they (genetic mechanisms) and their encoded proteins (molecular mechanisms) are disrupted. Interventions designed to rectify downstream effects of these disruptions have been examined in animal models, and some are currently tested in clinical trials. Efforts are underway to determine whether interventions capable to slow down, stop, or reverse structural progression of the disease will also prevent decline of renal function and improve clinically significant outcomes.

Biology and applications of mesenchymal stem cells

Undifferentiated adult stem cells are responsible for cell replacement in adult organisms. Initially isolated from the bone marrow, they are now known to be distributed throughout the organism as a whole, with a perivascular location. They are defined by properties which include proliferation as adherent cells, a defined immunophenotype, and the capacity to differentiate in vitro into osteoblasts, adipocytes and chondroblasts. Mesenchymal stem cells (MSCs) are considered as one of the most promising cell types for therapeutic applications. Mechanisms responsible for this therapeutic role are not well understood, and may involve diferentiation or, as most evidences point out, paracrine activity. The ability to modulate the immune system opens a wide range of applications, mainly for autoimmune diseases and graft-versus-host disease. Preclinical and clinical studies show promising results, but controversial results are still reported, indicating the need for further basic and preclinical investigation on their therapeutic potential. This review will focus on recent advances in understanding MSC biology and applications in cell therapy.

Review article: Potential cellular therapies for renal disease: can we translate results from animal studies to the human condition?

The incidence of chronic kidney disease is increasing worldwide, prompting considerable research into potential regenerative therapies. These have included studies to determine whether an endogenous renal stem cell exists in the postnatal kidney and whether non-renal adult stem cells, such as mesenchymal stem cell, can ameliorate renal damage. Such stem cells will either need to be recruited to the damaged kidney to repair the damage in situ or be differentiated into the desired cell type and delivered into the damaged kidney to subsequently elicit repair without maldifferentiation. To date, these studies have largely been performed using experimental and genetic models of renal damage in rodents. The translation of such research into a therapy applicable to human disease faces many challenges. In this review, we examine which animal models have been used to evaluate potential cellular therapies and how valid these are to human chronic kidney disease.

Renal progenitor cells contribute to hyperplastic lesions of podocytopathies and crescentic glomerulonephritis

Glomerular injury can involve excessive proliferation of glomerular epithelial cells, resulting in crescent formation and obliteration of Bowman's space. The origin of these hyperplastic epithelial cells in different glomerular disorders is controversial. Renal progenitors localized to the inner surface of Bowman's capsule can regenerate podocytes, but whether dysregulated proliferation of these progenitors contributes to crescent formation is unknown. In this study, we used confocal microscopy, laser capture microdissection, and real-time quantitative reverse transcriptase-PCR to demonstrate that hypercellular lesions of different podocytopathies and crescentic glomerulonephritis consist of three distinct populations: CD133(+)CD24(+)podocalyxin (PDX)(-)nestin(-) renal progenitors, CD133(+)CD24(+)PDX(+)nestin(+) transitional cells, and CD133(-)CD24(-)PDX(+)nestin(+) differentiated podocytes. In addition, TGF-beta induced CD133(+)CD24(+) progenitors to produce extracellular matrix, and these were the only cells to express the proliferation marker Ki67. Taken together, these results suggest that glomerular hyperplastic lesions derive from the proliferation of renal progenitors at different stages of their differentiation toward mature podocytes, providing an explanation for the pathogenesis of hyperplastic lesions in podocytopathies and crescentic glomerulonephritis.

Is there such a thing as a renal stem cell?

Increasing interest in the potential of adult stem cells in regenerative medicine has led to numerous studies focused on the identification of endogenous renal stem cells within the mature mammalian kidney. A variety of approaches have been taken to identify such cells, including physical location, cell surface marker expression, and functional properties. Proof of clonogenicity or renal potential remains questionable, and few such populations have been characterized in humans; however, recent evidence that even podocytes, a cell type with limited proliferative capacity under normal conditions, are constantly regenerated from a population within the Bowman's capsule has breathed new life into the quest for a renal stem cell. Here we examine whether current evidence is sufficient to conclude such a population does indeed exist or whether the jury is still out. We also ask which properties we would wish such a cell to possess to allow for repair of the diseased kidney.

Stem cells in pathobiology and regenerative medicine

This issue of the Journal of Pathology contains 16 articles largely dealing with the role of tissue-specific adult stem cells in the pathogenesis of disease, notably cancer. These authoritative reviews begin by describing the current knowledge regarding the identity and molecular regulation of normal tissue-specific stem cells, before itemizing their role in the aetiology and progression of disease. Fundamental concepts regarding the stem cell niche have been gleaned from studies of germ line stem cells in Drosophila and Caenorhabditis elegans, and these are described in detail in this issue. Somatic cell reprogramming, a process underlying not only therapeutic cloning but also the production of induced pluripotent stem (iPS) cells, is further discussed. Much attention is given to embryonic stem (ES) and iPS cells within the scientific community; this issue of the Journal of Pathology redresses this imbalance by illustrating the pivotal role of adult stem cells in much of human disease.

Attributes of adult stem cells

While cultured embryonic stem (ES) cells can be harvested in abundance and appear to be the most versatile of cells for regenerative medicine, adult stem cells also hold promise, but the identity and subsequent isolation of these comparatively rare cells remains problematic in most tissues, perhaps with the notable exception of the bone marrow. The ability to continuously self-renew and produce the differentiated progeny of the tissue of their location are their defining properties. Identifying surface molecules (markers) that would aid in stem cell isolation is a major goal. Considerable overlap exists between different putative organ-specific stem cells in their repertoire of gene expression, often related to self-renewal, cell survival and cell adhesion. More robust tests of 'stemness' are now being employed, using lineage-specific genetic marking and tracking to show production of long-lived clones and multipotentiality in vivo. Moreover, the characterization of normal stem cells in specific tissues may provide a dividend for the treatment of cancer. The successful treatment of neoplastic disease may well require the specific targeting of neoplastic stem cells, cells that may well have many of the characteristics of their normal counterparts.