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Multipotent renal progenitor cells (MRPC) were isolated from adult rat kidneys. When MRPC were injected into the kidneys of rats with ischemia-reperfusion injury, they became incorporated into renal tubules in the cortex and outer medulla and expressed the proximal tubule marker, Phaseolus vulgaris erythroagglutinin, and the distal tubule marker, peanut agglutinin, as well as

the epithelial cell marker, zona occludens-1 (ZO-1). In addition, when injected under the capsule of rat kidneys, undifferentiated MRPC were shown to form nodules and cyst-like structures, and integrated into the renal tubules and formed multiple tubular-like structures. It has also been attempted to differentiate MRPC toward a renal cell lineage using a combination of fibroblast growth factor 2, TGF-β and leukaemia inhibitory factor. Under these conditions, the cells started to grow in aggregates and express cytokeratin and ZO-1 (Gupta et al. 2006). Another renal progenitor population that was demonstrated to engraft into damaged kidneys was derived from the adult mouse. These cells were shown to express the stem cell antigen-1 (Sca-1) and lacked CD45 expression. Also, the Sca-1+CD45- population was demonstrated to incorporate into renal tubules in mice in the ischemia-reperfusion injury model (Dekel et al. 2006). Direct injection of other murine progenitors, such as mouse kidney progenitor cells (MKPC), was demonstrated to protect mice with ischemia-reperfusion renal injury from the renal function deterioration and to improve renal structure following the injury. In consequence, the mice with renal injury that received MKPC survived longer then untreated mice. Ultimately MKPC after injection into the medulla of normal mice were found incorporated into vessels and capillaries as well as into distal tubules and Henle’s loop expressing Tamm-Horsfall glycoprotein (THP) (Lee et al. 2010).

Several groups have demonstrated the significance of a renal progenitor population expressing the CD133 antigen in the amelioration of acute kidney injury and in vitro differentiation (Bussolati et al. 2005; Sagrinati et al. 2006; Lazzeri et al. 2007; Ronconi et al. 2009). Accordingly, human CD133-positive renal progenitors derived from the cortex of normal human kidney were injected into mice with glycerol-induced acute kidney injury. The cells were found in the proximal and distal tubules of injured kidneys where they expressed the epithelial marker,

cytokeratin. In addition, the CD133-positive cells were shown to undergo in vitro epithelial differentiation upon stimulation with hepatocyte growth factor (HGF) and fibroblast growth factor-4, as the cells started to express cytokeratin, E-cadherin and ZO-1, as well as renal markers such as alkaline phosphatase, amino peptidase A, normally found in proximal tubular epithelial cells, and the thiazide-sensitive NaCl co-transporter present in distal tubular epithelial cells. When injected into severe combined immunodeficiency (SCID) mice, they also showed spontaneous in vivo differentiation towards tubular epithelia, characterized by the formation of tubular-like structures with a lumen harbouring cells that display short microvilli and tight junctions, and accompanied by the expression of cytokeratin, thiazide-sensitive NaCl co- transporter and alkaline phosphatase (Bussolati et al. 2005). Sagrinati et al. demonstrated that CD24+133+ human adult stem cells derived from the Bowman’s capsule can ameliorate glycerol- induced kidney injury and subsequently engraft into both proximal and distal tubules following injury. Differentiation towards tubular cells was accompanied by the acquisition of the expression of alkaline phosphatase and THP as well as the up-regulation of other renal markers like aminopeptidase A, AQP1, AQP3, and the thiazide-sensitive Na/Cl transporter. Finally, the tubular-like cells also acquired the ability to respond to angiotensin II with intracellular calcium influx (Sagrinati et al. 2006). Correspondingly, the behaviour of adult human CD133+CD24+ cells derived from the Bowman’s capsule was assessed in a model of adriamycin-induced renal injury in mouse. The administration of the cells led to enhanced functional and structural recovery of the injured kidneys. Accordingly, the cells were present in glomerular structures, expressing the podocyte-specific markers synaptopodin, WT1, nephrin, and podocin, as well tubules expressing binding sites for Lotus tetragonolobus agglutinin (LTA), a marker for proximal tubules. Furthermore, CD133+CD24+ cells were differentiated in vitro towards the tubular lineage using

HGF, and towards the podocyte lineage using vitamin D3 and retinoic-acid. Accordingly, tubular differentiation resulted in the acquisition of binding of LTA as well as up-regulation of the expression proximal tubule-specific genes, including aminopeptidase A, aquaporin-1, aquaporin- 3, and thiazide-sensitive Na/Cl transporter. At the same time, following podocyte differentiation, the cells started to express podocyte markers like nephrin, WT1, synaptopodin and podocin (Ronconi et al. 2009). Finally, a similar population of cells was derived from human foetal kidneys. In the glycerol-induced kidney injury model in SCID mice, the cells incorporated into tubules stained with the proximal tubule marker, LTA, and the collecting duct marker, Dolichos biflorus agglutinin. The cells improved the function as well as the structural recovery of the kidneys following the treatment with glycerol. Finally, following in vitro stimulation, the cells were shown to up-regulate expression of some important kidney genes, such as aminopepetidase A, aquaporin 1 and 3, thiazide-sensitive Na/Cl, megalin or THP (Lazzeri et al. 2007).

Ultimately, kidney-derived progenitors have been demonstrated to harbour the potential to contribute to the development of different compartments during nephrogenesis when injected into metanephroi (Challen et al. 2006; Maeshima et al. 2006; Ward et al. 2011) (Table 1.1). Among several types of renal progenitor populations, the label-retaining tubular cells (LRTC), or kidney side population (SP), showed renogenic potential. On the 5th day following injection into E15 rat kidney rudiment, the LRTC were found in and around the ureteric buds, as well as in tubules positive for LTA (Maeshima et al. 2006). Similarly, the SP cells (isolated from adult mouse kidneys using their ability to efflux the Hoechst dye), were shown to have the potential to contribute to kidney development. After 3 days from the injection of the cells into mouse E12.5 metanephroi, SP cells were found in UBs stained with calbindin and Pax2, as well as in MM

stained with Wt1 and Pax2. Importantly, it was shown that the kidney main population cells (i.e., the cells that were unable to efflux the Hoechst dye) were also able to engraft into UB and MM, albeit at a much lower percentage than the SP cells (Challen et al. 2006). Finally, human CD133/1+ kidney progenitors isolated from both the papilla and the cortex were injected into E12.5 mouse kidney rudiments. Accordingly, the cells demonstrated an ability to engraft into the tubular compartment of the metanephric kidney after 3 days of culture (Ward et al. 2011).