- Email: [email protected]
Kidney Stem Cells Found in Adult Zebrafish
Cell Stem Cell
Previews pathways enhanced human ESC engraftment (into the testis of immunocompetent mice [Grinnemo et al., 2008]). These authors postulated that the tolerance involved T regulatory (Treg)cells; however, Pearl et al. (2011) found that Treg activity probably did not account for immunosuppressive effects in their model. These results provide promising proof of concept for the use of costimulatory blockade and short-term immunosuppression to enable engraftment in ESC or iPSC therapies. However, extensive preclinical and clinical experience with costimulatory blockade has shown that promising results in mouse models are not always borne out in studies in nonhuman primates or in human trials (Ford and Larsen, 2009). Three host factors can limit the efficacy and duration of costimulatory blockade regimens (Ford and Larsen, 2009). The first is the level of
T cell precursors present before therapy, since high levels can bypass the need for strong costimulation. The second factor is the presence of memory T cells with cross reactivity to graft antigens and which do not require strong stimulation for activation, again obviating the need for costimulation. Finally, interferon can influence both costimulation and graft survival with potentially opposing effects on outcomes. These considerations notwithstanding, costimulatory blockade is constantly undergoing refinement and improved reagents are becoming available. The studies of Pearl et al. (2011) provide a firm foundation for future translational studies of less toxic immunosuppressive regimens in stem cell therapies. REFERENCES Chidgey, A.P., and Boyd, R.L. (2008). Cell Stem Cell 3, 357–358.
Chidgey, A.P., Layton, D., Trounson, A., and Boyd, R.L. (2008). Nature 453, 330–337. Ford, M.L., and Larsen, C.P. (2009). Immunol. Rev. 229, 294–306. Grinnemo, K.H., Genead, R., Kumagai-Braesch, M., Andersson, A., Danielsson, C., Ma˚nsson-Broberg, A., Dellgren, G., Stro¨mberg, A.M., Ekberg, H., Hovatta, O., et al. (2008). Stem Cells 26, 1850–1857.
Okamura, R.M., Lebkowski, J., Au, M., Priest, C.A., Denham, J., and Majumdar, A.S. (2007). J. Neuroimmunol. 192, 134–144. Pearl, J.I., Lee, A.S., Leveson-Gower, D.B., Sun, N., Ghosh, Z., Lan, F., Ransohoff, J., Negrin, R.S., Davis, M.M., and Wu, J.C. (2011). Cell Stem Cell 8, this issue, 309–317. Swijnenburg, R.J., Schrepfer, S., Govaert, J.A., Cao, F., Ransohoff, K., Sheikh, A.Y., Haddad, M., Connolly, A.J., Davis, M.M., Robbins, R.C., and Wu, J.C. (2008). Proc. Natl. Acad. Sci. USA 105, 12991–12996.
Kidney Stem Cells Found in Adult Zebrafish Xiankun Zeng1 and Steven X. Hou1,* 1The Mouse Cancer Genetics Program, National Cancer Institute at Frederick, National Institutes of Health, Frederick, MD 21702, USA *Correspondence: [email protected] DOI 10.1016/j.stem.2011.02.008
Recently in Nature, Davidson and coworkers (Diep et al., 2011) identified nephron progenitors/stem cells located at the point of fusion with the pronephric tubules in adult zebrafish. Clumps of progenitors give rise to functional nephrons after serial transplantation, demonstrating the ability of tissue stem cells to regenerate damaged kidney structures. The adult mammalian kidney is developed through reciprocal inductive interactions between the metanephric mesenchyme (MM) and adjacent ureteric bud (UB) (Dressler, 2006). The UB will outgrow to form the collecting ducts, and the MM cells will give rise to nephrons (Figure 1), the functional units of the kidney. The principal function of the nephrons is to keep blood clean and chemically balanced. The nephron comprises a blood filter, called the glomerulus, and a renal tubule, which measures useful chemicals and releases them back to the blood for body use. The development and function of the kidney are evolutionarily conserved from
human to fish and even to fly (Figure 1; Weavers et al., 2009; Wingert and Davidson, 2008). Loss of nephron function underlies most kidney diseases. Mammals can only partly repair their damaged nephrons and are unable to generate new ones. In contrast, throughout their lifespan, fish are able to add new nephrons and regenerate nephrons de novo after injury. The question of what cell population(s) may be responsible for this enhanced degree of adult organ regeneration and repair was addressed recently by Davidson and colleagues (Diep et al., 2011). To identify the source of nephron regeneration in adult zebrafish, Davidson
and colleagues performed a series of transplantation experiments (Diep et al., 2011). The recipient fish were first immunocompromised by radiation to prevent graft rejection and then injected with gentamicin (an established nephrotoxin) to induce nephron damage. Unpurified whole-kidney marrow cells (WKM) that express fluorescent reporters in the distal nephron (approximately 5 3 105 cells) were prepared from donors and injected into the head region of the kidney of recipient fish. The authors demonstrated that the donor cells were fully capable of generating donor-derived nephrons in 100% of the recipients. The donor-derived
Cell Stem Cell 8, March 4, 2011 ª2011 Elsevier Inc. 247
Cell Stem Cell
Previews
(Kobayashi et al., 2008) and nephrons were connected A B adult Drosophila Malpighian with host renal tubules, inditubules (Singh et al., 2007). cating that the engrafted In the developing kidney of nephrons had successfully mouse embryo, six2-expressintegrated into the recipient’s ing cap mesenchymal cells renal system. These donorgive rise to all cell types of derived nephrons were also the main body of the nephron able to take up 40 kDa fluoduring all stages of nephrorescent dextran, further suggenesis and represent a multigesting that they had intepotent nephron progenitor grated into the recipient’s population (Kobayashi et al., blood supply and were 2008). In the adult Drosophila capable of blood filtration. kidney (Malpighian tubules), These results demonstrated Figure 1. Functions and Adult Stem Cell Locations of the Zebrafish multipotent stem cells have that kidney progenitors/stem Nephrons and the Drosophila Excretory System Are Similar been identified in the region cells are present in adult ze(A and B) Schematic drawings (modified from Weavers et al., 2009) of the zebrafish nephrons (A) and the Drosophila excretory system (B). In the glomerof the lower tubules and brafish and that they are able ulus of vertebrate kidneys, podocytes perform the blood filtration function ureters (Figure 1; Singh et al., to form new functional neph(marked with red arrows). The insect nephrocyte is anatomically, molecularly, 2007). In each case, the stem rons in a host organ after and functionally similar to the glomerular podocyte. Black arrows indicate the direction of flow of blood through the renal tubule, and blue arrows represent cells give rise to several differtransplantation. rebalancing of blood ions and water through further filtration in the renal entiated cell types in lineageThe authors next sought tubule. tracing experiments. The aumolecular markers that would tocrine JAK-STAT and the identify the stem cells in vivo. It is known that nephrogenesis begins are located in the distal tubule; and Ras-Raf signal transduction pathways with the formation of ‘‘pretubular aggre- wt1b+ cells are found in the glomerulus regulate kidney stem cell self-renewal in gates,’’ which undergo a mesenchymal- and proximal tubule. Further, laser abla- the fly (Singh et al., 2007; Zeng et al., to-epithelial transition into renal vesicles tion of single lhx1a:EGFP+ aggregates re- 2010). As mentioned above, in zebrafish, in mammals (Dressler, 2006). The aggre- sulted in aborted nephrogenesis in the multiple nephron progenitor aggregates gates express several transcription targeted region without affecting neigh- are needed to form a nephron (Diep factors, including Lhx1/Lim1 and Wt1 (Ko- boring nephrons, demonstrating a et al., 2011). It is tempting to speculate bayashi et al., 2005; Georgas et al., 2009). requirement for lhx1a:EGFP+ cells during that the progenitors may also be regulated by an autocrine signal, and thus, The authors therefore examined the cells nephrogenesis. marked by lhx1a and wt1b in Tg To directly demonstrate the stem cell the transfer of aggregates may facilitate (lhx1a:EGFP) and Tg(wt1b:mCherry) function of the lhx1a:EGFP+ aggregates, their mutual induction to form a nephron. The ultimate goal of kidney stem cell transgenic zebrafish lines. They found the authors performed further transplanthat kidneys from untreated Tg(lhx1a: tation experiments. Transplantation of research is to identify adult kidney stem EGFP) adults contain a number of homo- individual lhx1a:EGFP+ aggregates from cells in humans in the hopes of harnessing geneous aggregates of lhx1a:EGFP+ adult fish resulted in successful formation them to treat human kidney disease. mesenchymal cells, ranging from a few of donor-derived nephrons in 33% (n = 15) However, searching for adult renal stem to approximately 30 cells (approximately of transplanted zebrafish, while transplan- cells in mammals (including humans) has 100 aggregates per kidney), that were tation of single lhx1a:EGFP+ cells or so far met with limited success. In the highly reminiscent of pretubular aggre- lhx1a:EGFP+/wt1b:mCherry+ renal vesi- developing kidney of mouse embryo, gates in mammals. cles failed to engraft conditioned recipi- six2-expressing progenitors give rise to To determine whether the Tg ents. Taken together, these results all cell types of the main body of the (lhx1a:EGFP)-marked aggregates are demonstrate that lhx1a:EGFP+ aggre- nephron, but these multipotent cells nephron progenitors, the authors took gates contain nephron progenitors and disappear around birth (Dressler, 2006; advantage of the optical transparency of that multiple nephron progenitors are Kobayashi et al., 2008). It is possible that larval zebrafish and carefully visualized needed to form a nephron. Whether the the mammalian kidney stem cells, while the behavior of GFP-marked cells during multiple cell types that are present in undetectable after birth, are still present nephrogenesis in vivo. They observed the regenerated nephrons derive clonally but have become quiescent or dormant that the lhx1a:EGFP+ aggregates arose from a single cell or are the progeny of after this developmental stage. Indeed, from the coalescence of three or four multiple, more-restricted progenitors adult fly kidney stem cells are relatively lhx1a:EGFP+ cells that expanded to form present in the transplanted aggregates quiescent and only divide once in one a renal vesicle (which express wt1b: remains open to question. However, the week (Singh et al., 2007). However, they mCherry), and the renal vesicle sub- function of the transferred population is can become very active and even develop sequently elongated into a nephron. suggestive of the presence of at least stem cell tumors upon activating the JAKSTAT signal transduction pathway or exThe location of the lhx1a+ cells is a subset of functional kidney stem cells. restricted to the point of fusion with the Besides zebrafish, kidney stem cells pressing the activated form of the Ras pronephric tubules (Figure 1); pax8+ cells were also found in the developing mouse oncogene (Singh et al., 2007; Zeng et al., 248 Cell Stem Cell 8, March 4, 2011 ª2011 Elsevier Inc.
Cell Stem Cell
Previews 2010). Further study of adult kidney stem cells in the genetic systems of Drosophila and zebrafish may lead to the identification of adult human kidney stem cells and the discovery of ways to activate the potentially dormant stem cells for therapeutic applications. Although the study of kidney progenitors and stem cells is still in its infant stage, identification of adult kidney stem cells in genetic model organisms prove that such cells exist in nature, and the prospect of a stem cell-based therapy for kidney disease now looks brighter.
REFERENCES Diep, C.Q., Ma, D., Deo, R.C., Holm, T.M., Naylor, R.W., Arora, N., Wingert, R.A., Bollig, F., Djordjevic, G., Lichman, B., et al. (2011). Nature 470, 95–100. Dressler, G.R. (2006). Annu. Rev. Cell Dev. Biol. 22, 509–529. Georgas, K., Rumballe, B., Valerius, M.T., Chiu, H.S., Thiagarajan, R.D., Lesieur, E., Aronow, B.J., Brunskill, E.W., Combes, A.N., Tang, D., et al. (2009). Dev. Biol. 332, 273–286. Kobayashi, A., Kwan, K.M., Carroll, T.J., McMahon, A.P., Mendelsohn, C.L., and Behringer, R.R. (2005). Development 132, 2809–2823.
Kobayashi, A., Valerius, M.T., Mugford, J.W., Carroll, T.J., Self, M., Oliver, G., and McMahon, A.P. (2008). Cell Stem Cell 3, 169–181. Singh, S.R., Liu, W., and Hou, S.X. (2007). Cell Stem Cell 1, 191–203. Weavers, H., Prieto-Sa´nchez, S., Grawe, F., Garcia-Lo´pez, A., Artero, R., Wilsch-Bra¨uninger, M., Ruiz-Go´mez, M., Skaer, H., and Denholm, B. (2009). Nature 457, 322–326. Wingert, R.A., and Davidson, A.J. (2008). Kidney Int. 73, 1120–1127. Zeng, X., Singh, S.R., Hou, D., and Hou, S.X. (2010). J. Cell. Physiol. 224, 766–774.
Cell Stem Cell 8, March 4, 2011 ª2011 Elsevier Inc. 249
Previews pathways enhanced human ESC engraftment (into the testis of immunocompetent mice [Grinnemo et al., 2008]). These authors postulated that the tolerance involved T regulatory (Treg)cells; however, Pearl et al. (2011) found that Treg activity probably did not account for immunosuppressive effects in their model. These results provide promising proof of concept for the use of costimulatory blockade and short-term immunosuppression to enable engraftment in ESC or iPSC therapies. However, extensive preclinical and clinical experience with costimulatory blockade has shown that promising results in mouse models are not always borne out in studies in nonhuman primates or in human trials (Ford and Larsen, 2009). Three host factors can limit the efficacy and duration of costimulatory blockade regimens (Ford and Larsen, 2009). The first is the level of
T cell precursors present before therapy, since high levels can bypass the need for strong costimulation. The second factor is the presence of memory T cells with cross reactivity to graft antigens and which do not require strong stimulation for activation, again obviating the need for costimulation. Finally, interferon can influence both costimulation and graft survival with potentially opposing effects on outcomes. These considerations notwithstanding, costimulatory blockade is constantly undergoing refinement and improved reagents are becoming available. The studies of Pearl et al. (2011) provide a firm foundation for future translational studies of less toxic immunosuppressive regimens in stem cell therapies. REFERENCES Chidgey, A.P., and Boyd, R.L. (2008). Cell Stem Cell 3, 357–358.
Chidgey, A.P., Layton, D., Trounson, A., and Boyd, R.L. (2008). Nature 453, 330–337. Ford, M.L., and Larsen, C.P. (2009). Immunol. Rev. 229, 294–306. Grinnemo, K.H., Genead, R., Kumagai-Braesch, M., Andersson, A., Danielsson, C., Ma˚nsson-Broberg, A., Dellgren, G., Stro¨mberg, A.M., Ekberg, H., Hovatta, O., et al. (2008). Stem Cells 26, 1850–1857.
Okamura, R.M., Lebkowski, J., Au, M., Priest, C.A., Denham, J., and Majumdar, A.S. (2007). J. Neuroimmunol. 192, 134–144. Pearl, J.I., Lee, A.S., Leveson-Gower, D.B., Sun, N., Ghosh, Z., Lan, F., Ransohoff, J., Negrin, R.S., Davis, M.M., and Wu, J.C. (2011). Cell Stem Cell 8, this issue, 309–317. Swijnenburg, R.J., Schrepfer, S., Govaert, J.A., Cao, F., Ransohoff, K., Sheikh, A.Y., Haddad, M., Connolly, A.J., Davis, M.M., Robbins, R.C., and Wu, J.C. (2008). Proc. Natl. Acad. Sci. USA 105, 12991–12996.
Kidney Stem Cells Found in Adult Zebrafish Xiankun Zeng1 and Steven X. Hou1,* 1The Mouse Cancer Genetics Program, National Cancer Institute at Frederick, National Institutes of Health, Frederick, MD 21702, USA *Correspondence: [email protected] DOI 10.1016/j.stem.2011.02.008
Recently in Nature, Davidson and coworkers (Diep et al., 2011) identified nephron progenitors/stem cells located at the point of fusion with the pronephric tubules in adult zebrafish. Clumps of progenitors give rise to functional nephrons after serial transplantation, demonstrating the ability of tissue stem cells to regenerate damaged kidney structures. The adult mammalian kidney is developed through reciprocal inductive interactions between the metanephric mesenchyme (MM) and adjacent ureteric bud (UB) (Dressler, 2006). The UB will outgrow to form the collecting ducts, and the MM cells will give rise to nephrons (Figure 1), the functional units of the kidney. The principal function of the nephrons is to keep blood clean and chemically balanced. The nephron comprises a blood filter, called the glomerulus, and a renal tubule, which measures useful chemicals and releases them back to the blood for body use. The development and function of the kidney are evolutionarily conserved from
human to fish and even to fly (Figure 1; Weavers et al., 2009; Wingert and Davidson, 2008). Loss of nephron function underlies most kidney diseases. Mammals can only partly repair their damaged nephrons and are unable to generate new ones. In contrast, throughout their lifespan, fish are able to add new nephrons and regenerate nephrons de novo after injury. The question of what cell population(s) may be responsible for this enhanced degree of adult organ regeneration and repair was addressed recently by Davidson and colleagues (Diep et al., 2011). To identify the source of nephron regeneration in adult zebrafish, Davidson
and colleagues performed a series of transplantation experiments (Diep et al., 2011). The recipient fish were first immunocompromised by radiation to prevent graft rejection and then injected with gentamicin (an established nephrotoxin) to induce nephron damage. Unpurified whole-kidney marrow cells (WKM) that express fluorescent reporters in the distal nephron (approximately 5 3 105 cells) were prepared from donors and injected into the head region of the kidney of recipient fish. The authors demonstrated that the donor cells were fully capable of generating donor-derived nephrons in 100% of the recipients. The donor-derived
Cell Stem Cell 8, March 4, 2011 ª2011 Elsevier Inc. 247
Cell Stem Cell
Previews
(Kobayashi et al., 2008) and nephrons were connected A B adult Drosophila Malpighian with host renal tubules, inditubules (Singh et al., 2007). cating that the engrafted In the developing kidney of nephrons had successfully mouse embryo, six2-expressintegrated into the recipient’s ing cap mesenchymal cells renal system. These donorgive rise to all cell types of derived nephrons were also the main body of the nephron able to take up 40 kDa fluoduring all stages of nephrorescent dextran, further suggenesis and represent a multigesting that they had intepotent nephron progenitor grated into the recipient’s population (Kobayashi et al., blood supply and were 2008). In the adult Drosophila capable of blood filtration. kidney (Malpighian tubules), These results demonstrated Figure 1. Functions and Adult Stem Cell Locations of the Zebrafish multipotent stem cells have that kidney progenitors/stem Nephrons and the Drosophila Excretory System Are Similar been identified in the region cells are present in adult ze(A and B) Schematic drawings (modified from Weavers et al., 2009) of the zebrafish nephrons (A) and the Drosophila excretory system (B). In the glomerof the lower tubules and brafish and that they are able ulus of vertebrate kidneys, podocytes perform the blood filtration function ureters (Figure 1; Singh et al., to form new functional neph(marked with red arrows). The insect nephrocyte is anatomically, molecularly, 2007). In each case, the stem rons in a host organ after and functionally similar to the glomerular podocyte. Black arrows indicate the direction of flow of blood through the renal tubule, and blue arrows represent cells give rise to several differtransplantation. rebalancing of blood ions and water through further filtration in the renal entiated cell types in lineageThe authors next sought tubule. tracing experiments. The aumolecular markers that would tocrine JAK-STAT and the identify the stem cells in vivo. It is known that nephrogenesis begins are located in the distal tubule; and Ras-Raf signal transduction pathways with the formation of ‘‘pretubular aggre- wt1b+ cells are found in the glomerulus regulate kidney stem cell self-renewal in gates,’’ which undergo a mesenchymal- and proximal tubule. Further, laser abla- the fly (Singh et al., 2007; Zeng et al., to-epithelial transition into renal vesicles tion of single lhx1a:EGFP+ aggregates re- 2010). As mentioned above, in zebrafish, in mammals (Dressler, 2006). The aggre- sulted in aborted nephrogenesis in the multiple nephron progenitor aggregates gates express several transcription targeted region without affecting neigh- are needed to form a nephron (Diep factors, including Lhx1/Lim1 and Wt1 (Ko- boring nephrons, demonstrating a et al., 2011). It is tempting to speculate bayashi et al., 2005; Georgas et al., 2009). requirement for lhx1a:EGFP+ cells during that the progenitors may also be regulated by an autocrine signal, and thus, The authors therefore examined the cells nephrogenesis. marked by lhx1a and wt1b in Tg To directly demonstrate the stem cell the transfer of aggregates may facilitate (lhx1a:EGFP) and Tg(wt1b:mCherry) function of the lhx1a:EGFP+ aggregates, their mutual induction to form a nephron. The ultimate goal of kidney stem cell transgenic zebrafish lines. They found the authors performed further transplanthat kidneys from untreated Tg(lhx1a: tation experiments. Transplantation of research is to identify adult kidney stem EGFP) adults contain a number of homo- individual lhx1a:EGFP+ aggregates from cells in humans in the hopes of harnessing geneous aggregates of lhx1a:EGFP+ adult fish resulted in successful formation them to treat human kidney disease. mesenchymal cells, ranging from a few of donor-derived nephrons in 33% (n = 15) However, searching for adult renal stem to approximately 30 cells (approximately of transplanted zebrafish, while transplan- cells in mammals (including humans) has 100 aggregates per kidney), that were tation of single lhx1a:EGFP+ cells or so far met with limited success. In the highly reminiscent of pretubular aggre- lhx1a:EGFP+/wt1b:mCherry+ renal vesi- developing kidney of mouse embryo, gates in mammals. cles failed to engraft conditioned recipi- six2-expressing progenitors give rise to To determine whether the Tg ents. Taken together, these results all cell types of the main body of the (lhx1a:EGFP)-marked aggregates are demonstrate that lhx1a:EGFP+ aggre- nephron, but these multipotent cells nephron progenitors, the authors took gates contain nephron progenitors and disappear around birth (Dressler, 2006; advantage of the optical transparency of that multiple nephron progenitors are Kobayashi et al., 2008). It is possible that larval zebrafish and carefully visualized needed to form a nephron. Whether the the mammalian kidney stem cells, while the behavior of GFP-marked cells during multiple cell types that are present in undetectable after birth, are still present nephrogenesis in vivo. They observed the regenerated nephrons derive clonally but have become quiescent or dormant that the lhx1a:EGFP+ aggregates arose from a single cell or are the progeny of after this developmental stage. Indeed, from the coalescence of three or four multiple, more-restricted progenitors adult fly kidney stem cells are relatively lhx1a:EGFP+ cells that expanded to form present in the transplanted aggregates quiescent and only divide once in one a renal vesicle (which express wt1b: remains open to question. However, the week (Singh et al., 2007). However, they mCherry), and the renal vesicle sub- function of the transferred population is can become very active and even develop sequently elongated into a nephron. suggestive of the presence of at least stem cell tumors upon activating the JAKSTAT signal transduction pathway or exThe location of the lhx1a+ cells is a subset of functional kidney stem cells. restricted to the point of fusion with the Besides zebrafish, kidney stem cells pressing the activated form of the Ras pronephric tubules (Figure 1); pax8+ cells were also found in the developing mouse oncogene (Singh et al., 2007; Zeng et al., 248 Cell Stem Cell 8, March 4, 2011 ª2011 Elsevier Inc.
Cell Stem Cell
Previews 2010). Further study of adult kidney stem cells in the genetic systems of Drosophila and zebrafish may lead to the identification of adult human kidney stem cells and the discovery of ways to activate the potentially dormant stem cells for therapeutic applications. Although the study of kidney progenitors and stem cells is still in its infant stage, identification of adult kidney stem cells in genetic model organisms prove that such cells exist in nature, and the prospect of a stem cell-based therapy for kidney disease now looks brighter.
REFERENCES Diep, C.Q., Ma, D., Deo, R.C., Holm, T.M., Naylor, R.W., Arora, N., Wingert, R.A., Bollig, F., Djordjevic, G., Lichman, B., et al. (2011). Nature 470, 95–100. Dressler, G.R. (2006). Annu. Rev. Cell Dev. Biol. 22, 509–529. Georgas, K., Rumballe, B., Valerius, M.T., Chiu, H.S., Thiagarajan, R.D., Lesieur, E., Aronow, B.J., Brunskill, E.W., Combes, A.N., Tang, D., et al. (2009). Dev. Biol. 332, 273–286. Kobayashi, A., Kwan, K.M., Carroll, T.J., McMahon, A.P., Mendelsohn, C.L., and Behringer, R.R. (2005). Development 132, 2809–2823.
Kobayashi, A., Valerius, M.T., Mugford, J.W., Carroll, T.J., Self, M., Oliver, G., and McMahon, A.P. (2008). Cell Stem Cell 3, 169–181. Singh, S.R., Liu, W., and Hou, S.X. (2007). Cell Stem Cell 1, 191–203. Weavers, H., Prieto-Sa´nchez, S., Grawe, F., Garcia-Lo´pez, A., Artero, R., Wilsch-Bra¨uninger, M., Ruiz-Go´mez, M., Skaer, H., and Denholm, B. (2009). Nature 457, 322–326. Wingert, R.A., and Davidson, A.J. (2008). Kidney Int. 73, 1120–1127. Zeng, X., Singh, S.R., Hou, D., and Hou, S.X. (2010). J. Cell. Physiol. 224, 766–774.
Cell Stem Cell 8, March 4, 2011 ª2011 Elsevier Inc. 249