- Email: [email protected]
Modeling the Glomerular Filtration Barrier: Are You Kidney-ing Me?
Cell Stem Cell
Previews histone marks including H3K4me1/2 and H3K27ac, this association is less apparent in the intestine. A previous study from the Shivdasani group detected a striking similarity in distributions of active histone marks between secretory and enterocyte progenitors (Kim et al., 2014). In contrast, the current study reports that they clearly exhibit distinct open chromatin profiles. It is tempting to speculate that the extraordinary plasticity of the intestinal epithelium is directly related to this fact. It seems that no semi-permanent epigenetic modifications are installed to specify cell fate during intestinal epithelial cell differentiation. Instead, due to the strict spatial organization of the tissue that presents ample opportunities for regulation of transcription factor activity by direct cell-cell signaling, such elements are not required, and modification of chromatin accessibility alone may be sufficient. One important question arising from these findings is whether there is a maturation point at which cells definitively lose the potential to revert to a more primitive ISC state. It is likely that given the absence of epigenetic marks and the ease at which the chromatin barrier is overcome during de-differentiation, such restriction might have only a physical ba-
sis, with cells being physically separated from the niche, rather than having a cellular foundation. These novel insights also have important consequences for tumor formation. In the normal intestine, ISCs are considered as the cell of origin for malignant transformation. However, inflammatory signals from the micro-environment, mirroring signals driving regeneration, also allow differentiated cells to transform (Schwitalla et al., 2013; van der Heijden et al., 2016). The absence of clear barricades between cell fates in the gut renders it unlikely that the cell of origin is a major contributor to interpatient cancer heterogeneity in this organ. The resemblance of colon cancer subtypes to different intestinal cell types therefore most likely results from aberrantly activated cell programs due to oncogenic mutations and extracellular signals and not from cell-of-origin-specific epigenetic profiles such as in other malignancies. REFERENCES Barker, N., van Es, J.H., Kuipers, J., Kujala, P., van den Born, M., Cozijnsen, M., Haegebarth, A., Korving, J., Begthel, H., Peters, P.J., and Clevers, H. (2007). Nature 449, 1003–1007.
Buczacki, S.J., Zecchini, H.I., Nicholson, A.M., Russell, R., Vermeulen, L., Kemp, R., and Winton, D.J. (2013). Nature 495, 65–69. Jadhav, U., Saxena, M., O’Neill, N.K., Saadatpour, A., Yuan, G.-C., Herbert, Z., Murata, K., and Shivdasani, R.A. (2017). Cell Stem Cell 21, this issue, 65–77. Kim, T.H., Li, F., Ferreiro-Neira, I., Ho, L.L., Luyten, A., Nalapareddy, K., Long, H., Verzi, M., and Shivdasani, R.A. (2014). Nature 506, 511–515. Schwitalla, S., Fingerle, A.A., Cammareri, P., Nebelsiek, T., Go¨ktuna, S.I., Ziegler, P.K., Canli, O., Heijmans, J., Huels, D.J., Moreaux, G., et al. (2013). Cell 152, 25–38. Tetteh, P.W., Basak, O., Farin, H.F., Wiebrands, K., Kretzschmar, K., Begthel, H., van den Born, M., Korving, J., de Sauvage, F., van Es, J.H., et al. (2016). Cell Stem Cell 18, 203–213. Tian, H., Biehs, B., Warming, S., Leong, K.G., Rangell, L., Klein, O.D., and de Sauvage, F.J. (2011). Nature 478, 255–259. van der Heijden, M., Zimberlin, C.D., Nicholson, A.M., Colak, S., Kemp, R., Meijer, S.L., Medema, J.P., Greten, F.R., Jansen, M., Winton, D.J., and Vermeulen, L. (2016). Nat. Commun. 7, 10916. van Es, J.H., Sato, T., van de Wetering, M., Lyubimova, A., Nee, A.N., Gregorieff, A., Sasaki, N., Zeinstra, L., van den Born, M., Korving, J., et al. (2012). Nat. Cell Biol. 14, 1099–1104. Yan, K.S., Gevaert, O., Probert, C., Zheng, G., Larkin, K., Davies, P., Cheng, Z., Kaddis, J., Wilhelmy, J., Grimeset, S., et al. (2017). Cell Stem Cell 21, this issue, 78–90.
Modeling the Glomerular Filtration Barrier: Are You Kidney-ing Me? Paola Romagnani1,2,* and Laura Lasagni1 1Excellence Centre for Research, Transfer and High Education for the development of DE NOVO Therapies (DENOTHE) and Department of Biomedical, Experimental and Clinical Sciences ‘Mario Serio’, University of Florence, 50121 Florence, Italy 2Pediatric Nephrology Unit, Meyer Children’s Hospital, University of Florence, 50139 Florence, Italy *Correspondence: [email protected] http://dx.doi.org/10.1016/j.stem.2017.06.008
Podocyte depletion drives kidney disease and kidney failure progression, but podocyte complexity at the glomerular filtration barrier is difficult to model in vitro. In Nature Biomedical Engineering, Musah et al. (2017) developed a multifluidic device with iPS-derived podocytes mimicking a functional glomerular filtration barrier that elevates standards for modeling glomerular diseases. The glomerular filtration barrier (GFB) is a highly specialized interface responsible for blood filtration, displaying a high conductance to small and midsized
solutes in plasma but retaining relative impermeability to macromolecules. GFB integrity is maintained by a constant physicochemical interaction among its
three principal constituents: the glomerular endothelial cell, the basement membrane, and the visceral epithelial cell (podocyte) (Figure 1). Podocyte means
Cell Stem Cell 21, July 6, 2017 ª 2017 Elsevier Inc. 7
Cell Stem Cell
Previews
Figure 1. How to Model the Glomerular Filtration Barrier? The glomerular filtration barrier consists of the capillary endothelial cell (EC), the glomerular basement membrane (GBM), and the podocytes (left). In the multifunctional microfluidic device, iPSCs-derived podocytes and endothelial cells are co-cultured to recapitulate the podocyte-GBM-endothelial interface (right).
‘‘cell with the foot,’’ as each podocyte has many elongated processes called foot processes, or pedicels, that interdigitate with those of adjacent podocytes and leave gaps or thin filtration slits between them, the slit-diaphragms (Reiser and Sever, 2013). The podocyte has no capacity to divide, so it can only be replaced by a progenitor cell localized within the Bowman’s capsule, but this regeneration is limited (Lasagni et al., 2015). For all these reasons, the podocyte is the Achilles’ heel of the kidney and a severe podocyte loss secondary to any type of injury leads to chronic kidney disease (CKD) (Reiser and Sever, 2013). Rebuilding a fully differentiated podocyte is thus essential for modeling the GFB and studying kidney disorders. In their study, Musah et al. (2017) propose a new protocol to efficiently differentiate induced pluripotent stem cells (iPSC) into podocytes in culture. Up to now, various studies have reported obtaining in vitro podocytes from several strategies and different cell sources. All these studies obtained cells with high levels of nephrin, podocin, and other slit diaphragm protein expression that specifically characterize a podocyte (Musah et al., 2017; Lasagni et al., 2015; Lazzeri et al., 2015; Xinaris et al., 2016; Freedman et al., 2015; Ciampi et al., 2016). However, in their study, Musah et al. pushed the attempt one step forward, putting iPSC-derived po8 Cell Stem Cell 21, July 6, 2017
docytes in the context of an engineered GFB. To achieve this goal, they built a multifunctional microfluidic device that recapitulates the structural, functional, and mechanical properties of a three-dimensional cross-section of the human glomerular capillary wall (Musah et al., 2017). The microfluidic device was composed of a flexible poly(dimethylsiloxane) (PDMS) elastomer that contains two closely opposed, parallel microchannels separated by a laminin-511-coated, porous flexible PDMS membrane. To model the dynamic mechanical strain observed in living glomeruli due to the cyclic pulsations of renal blood flow, Musah et al. also incorporated two hollow chambers on either side of the central microfluidic channels and applied cyclic suction to produce cyclic stretching and relaxation of the flexible membrane and adherent cell layers. Then, human iPSC-derived podocytes were cultured on the top of the laminin-coated membrane and primary human glomerular endothelial cells were cultured on the opposite side of the same membrane to recapitulate the podocyte–GBM–endothelial interface. By exposing the cells to differentiation medium, they induced the expression of podocyte markers including proteins of the slit diaphragm, such as nephrin or podocin, with long interdigitating (foot) processes connecting with each other and with endothelial cells. Human iPSC-derived podocytes differen-
tiated in the microfluidic devices produced glomerular basement-membrane collagen and secreted vascular endothelial growth factor (VEGF)-A. More importantly, this ‘‘glomerulus in a chip’’ mimicked the GFB in restricting permeability to large molecules such as albumin, but freely filtering exogenous small molecules, such as inulin, from plasma (Musah et al., 2017). In addition, after treatment with adriamycin, podocytes underwent dosedependent delamination, mirroring podocyte detachment from the GFB and loss in urinary flow that characterize the adriamycin nephropathy model in vivo (Lasagni et al., 2015) as well as the related human disease focal segmental glomerulosclerosis (FSGS; Reiser and Sever 2013). In vivo, podocyte detachment also results in significant loss of albumin from the vascular channel and increased entry into the urinary compartment, i.e., ‘‘albuminuria’’ (Reiser and Sever 2013; Lazzeri et al., 2015). Far from being only a biomarker of GFB damage widely used in the clinics, albuminuria is also directly harmful and favors CKD progression by impairing renal progenitor differentiation into podocytes and their regeneration after injury (Peired et al., 2013). In this in vitro model, albumin also nonselectively leaked through the GFB, resulting in increased uptake by the human iPSC-derived podocytes lining the urinary compartment of the microfluidic device (Musah et al., 2017), efficiently modeling GFB damage. Musah et al. (2017) propose that this device may be an innovative way for modeling certain kidney disorders, particularly podocytopathies induced by toxic agents. However, many other conditions may be modeled with this strategy; for example, pathogenic mutations in more than 40 genes involved in maintaining podocyte structure and function cause proteinuric forms of CKD (Vivante and Hildebrandt, 2016). The high genetic heterogeneity of podocytopathies requires extended DNA sequencing and renders diagnosis frequently challenging because of the identification of variants of unknown functional significance (Vivante and Hildebrandt, 2016). Musah et al.’s microfluidic device would allow one to test the functional significance of genetic mutations in podocytes in the context of the high pressure conditions that challenge them in vivo. This could help to identify novel genes, to provide information on the
Cell Stem Cell
Previews pathogenic mechanisms that link each gene to disease, and also to support diagnosis in affected patients. Furthermore, persistent glomerular hypertension is a critical driver of CKD progression (Reiser and Sever, 2013). As both foot process maintenance as well as VEGF-A secretion by podocytes are required for an efficient GFB in vivo, these results suggest that perturbing glomerular filtration pressure may translate into podocyte malfunction that characterizes many renal disorders (Reiser and Sever, 2013). In addition, the lack of these critical features in vitro explains why culturing podocytes has been difficult and typically associated with a poorly differentiated phenotype that may not often correctly mirror podocyte biology in vivo (reviewed in Shankland et al., 2007). In conclusion, Musah et al. (2017) demonstrate that for in vitro modeling of post-mitotic, highly specialized cells that are difficult to amplify in culture, like podo-
cytes, differentiation from stem cells in culture may represent a more suitable option to fully achieve their phenotypic properties. In addition, this study demonstrates that in order to model podocyte complexity in vivo, it is essential to consider the 3D interactions, physical forces, and chemical stimuli that continuously control the cells’ shape, function, and differentiation status, setting a new standard for modeling glomerular disease. REFERENCES Ciampi, O., Iacone, R., Longaretti, L., Benedetti, V., Graf, M., Magnone, M.C., Patsch, C., Xinaris, C., Remuzzi, G., Benigni, A., and Tomasoni, S. (2016). Stem Cell Res. (Amst.) 17, 130–139.
Lazzeri, E., Ronconi, E., Angelotti, M.L., Peired, A., Mazzinghi, B., Becherucci, F., Conti, S., Sansavini, G., Sisti, A., Ravaglia, F., et al. (2015). J. Am. Soc. Nephrol. 26, 1961–1974. Musah, S., Mammoto, A., Ferrante, T.C., Jeanty, S.S.F., Hirano-Kobayashi, M., Mammoto, T., Roberts, K., Chung, S., Novak, R., Ingram, M., et al. (2017). Nat. Biomed. Eng. Published online May 10, 2017. http://dx.doi.org/10.1038/s41551017-0069. Peired, A., Angelotti, M.L., Ronconi, E., la Marca, G., Mazzinghi, B., Sisti, A., Lombardi, D., Giocaliere, E., Della Bona, M., Villanelli, F., et al. (2013). J. Am. Soc. Nephrol. 24, 1756–1768. Reiser, J., and Sever, S. (2013). Annu. Rev. Med. 64, 357–366. Shankland, S.J., Pippin, J.W., Reiser, J., and Mundel, P. (2007). Kidney Int. 72, 26–36.
Freedman, B.S., Brooks, C.R., Lam, A.Q., Fu, H., Morizane, R., Agrawal, V., Saad, A.F., Li, M.K., Hughes, M.R., Werff, R.V., et al. (2015). Nat. Commun. 6, 8715.
Vivante, A., and Hildebrandt, F. (2016). Nat. Rev. Nephrol. 12, 133–146.
Lasagni, L., Angelotti, M.L., Ronconi, E., Lombardi, D., Nardi, S., Peired, A., Becherucci, F., Mazzinghi, B., Sisti, A., Romoli, S., et al. (2015). Stem Cell Reports 5, 248–263.
Xinaris, C., Benedetti, V., Novelli, R., Abbate, M., Rizzo, P., Conti, S., Tomasoni, S., Corna, D., Pozzobon, M., Cavallotti, D., et al. (2016). J. Am. Soc. Nephrol. 27, 1400–1411.
Retinoic Acid Puts Hematopoietic Stem Cells Back To Sleep Alexandra Rundberg Nilsson1,2 and Cornelis Jan Pronk1,2,3,* 1Medical
Faculty, Division of Molecular Hematology, Institution for Laboratory Medicine, Lund University, 221 84 Lund, Sweden Faculty, Lund Stem Cell Center, Lund University, 221 84 Lund, Sweden 3Department of Pediatric Oncology/Hematology, Ska ˚ ne University Hospital, 221 85 Lund, Sweden *Correspondence: [email protected] http://dx.doi.org/10.1016/j.stem.2017.06.002 2Medical
Dormant hematopoietic stem cells (dHSCs) display superior serial reconstitution capacity compared to active HSCs, although their role in normal hematopoiesis has not been thoroughly investigated. Recently in Cell, Cabezas-Wallscheid et al. (2017) demonstrate involvement of retinoic acid signaling in murine dHSCs for preservation of the HSC pool. Hematopoietic stem cells (HSCs) maintain continuous hematopoiesis throughout life. This ability has been ascertained primarily based on experimental findings during stressed hematopoiesis, as the role of HSCs in maintaining hematopoiesis during steady state is more controversial (reviewed in Busch and Rodewald, 2016). Nevertheless, it is likely that the hematopoietic system is recurrently challenged during a lifetime (by for instance infec-
tions), thus presumably relying on active input from HSCs for upholding normal lifelong hematopoiesis. To fulfill the demand of continuous hematopoiesis, HSCs must balance mature blood cell production versus HSC pool maintenance. Most HSCs reside in a quiescent state and only occasionally enter the cell cycle—a characteristic that supposedly protects HSCs from exhaustion and cell-cycleassociated mutational damage. A fraction
of HSCs has shown exceptionally slow turnover rates during unchallenged hematopoiesis, only dividing about five times during a lifetime. Such HSCs are named dormant HSCs (dHSCs) (Wilson et al., 2008) and have been studied mainly using label-retention assays. The exact role for dHSCs in regulation of hematopoiesis and contribution to mature blood cell production during steady state and stress hematopoiesis remains to be
Cell Stem Cell 21, July 6, 2017 ª 2017 Elsevier Inc. 9
Previews histone marks including H3K4me1/2 and H3K27ac, this association is less apparent in the intestine. A previous study from the Shivdasani group detected a striking similarity in distributions of active histone marks between secretory and enterocyte progenitors (Kim et al., 2014). In contrast, the current study reports that they clearly exhibit distinct open chromatin profiles. It is tempting to speculate that the extraordinary plasticity of the intestinal epithelium is directly related to this fact. It seems that no semi-permanent epigenetic modifications are installed to specify cell fate during intestinal epithelial cell differentiation. Instead, due to the strict spatial organization of the tissue that presents ample opportunities for regulation of transcription factor activity by direct cell-cell signaling, such elements are not required, and modification of chromatin accessibility alone may be sufficient. One important question arising from these findings is whether there is a maturation point at which cells definitively lose the potential to revert to a more primitive ISC state. It is likely that given the absence of epigenetic marks and the ease at which the chromatin barrier is overcome during de-differentiation, such restriction might have only a physical ba-
sis, with cells being physically separated from the niche, rather than having a cellular foundation. These novel insights also have important consequences for tumor formation. In the normal intestine, ISCs are considered as the cell of origin for malignant transformation. However, inflammatory signals from the micro-environment, mirroring signals driving regeneration, also allow differentiated cells to transform (Schwitalla et al., 2013; van der Heijden et al., 2016). The absence of clear barricades between cell fates in the gut renders it unlikely that the cell of origin is a major contributor to interpatient cancer heterogeneity in this organ. The resemblance of colon cancer subtypes to different intestinal cell types therefore most likely results from aberrantly activated cell programs due to oncogenic mutations and extracellular signals and not from cell-of-origin-specific epigenetic profiles such as in other malignancies. REFERENCES Barker, N., van Es, J.H., Kuipers, J., Kujala, P., van den Born, M., Cozijnsen, M., Haegebarth, A., Korving, J., Begthel, H., Peters, P.J., and Clevers, H. (2007). Nature 449, 1003–1007.
Buczacki, S.J., Zecchini, H.I., Nicholson, A.M., Russell, R., Vermeulen, L., Kemp, R., and Winton, D.J. (2013). Nature 495, 65–69. Jadhav, U., Saxena, M., O’Neill, N.K., Saadatpour, A., Yuan, G.-C., Herbert, Z., Murata, K., and Shivdasani, R.A. (2017). Cell Stem Cell 21, this issue, 65–77. Kim, T.H., Li, F., Ferreiro-Neira, I., Ho, L.L., Luyten, A., Nalapareddy, K., Long, H., Verzi, M., and Shivdasani, R.A. (2014). Nature 506, 511–515. Schwitalla, S., Fingerle, A.A., Cammareri, P., Nebelsiek, T., Go¨ktuna, S.I., Ziegler, P.K., Canli, O., Heijmans, J., Huels, D.J., Moreaux, G., et al. (2013). Cell 152, 25–38. Tetteh, P.W., Basak, O., Farin, H.F., Wiebrands, K., Kretzschmar, K., Begthel, H., van den Born, M., Korving, J., de Sauvage, F., van Es, J.H., et al. (2016). Cell Stem Cell 18, 203–213. Tian, H., Biehs, B., Warming, S., Leong, K.G., Rangell, L., Klein, O.D., and de Sauvage, F.J. (2011). Nature 478, 255–259. van der Heijden, M., Zimberlin, C.D., Nicholson, A.M., Colak, S., Kemp, R., Meijer, S.L., Medema, J.P., Greten, F.R., Jansen, M., Winton, D.J., and Vermeulen, L. (2016). Nat. Commun. 7, 10916. van Es, J.H., Sato, T., van de Wetering, M., Lyubimova, A., Nee, A.N., Gregorieff, A., Sasaki, N., Zeinstra, L., van den Born, M., Korving, J., et al. (2012). Nat. Cell Biol. 14, 1099–1104. Yan, K.S., Gevaert, O., Probert, C., Zheng, G., Larkin, K., Davies, P., Cheng, Z., Kaddis, J., Wilhelmy, J., Grimeset, S., et al. (2017). Cell Stem Cell 21, this issue, 78–90.
Modeling the Glomerular Filtration Barrier: Are You Kidney-ing Me? Paola Romagnani1,2,* and Laura Lasagni1 1Excellence Centre for Research, Transfer and High Education for the development of DE NOVO Therapies (DENOTHE) and Department of Biomedical, Experimental and Clinical Sciences ‘Mario Serio’, University of Florence, 50121 Florence, Italy 2Pediatric Nephrology Unit, Meyer Children’s Hospital, University of Florence, 50139 Florence, Italy *Correspondence: [email protected] http://dx.doi.org/10.1016/j.stem.2017.06.008
Podocyte depletion drives kidney disease and kidney failure progression, but podocyte complexity at the glomerular filtration barrier is difficult to model in vitro. In Nature Biomedical Engineering, Musah et al. (2017) developed a multifluidic device with iPS-derived podocytes mimicking a functional glomerular filtration barrier that elevates standards for modeling glomerular diseases. The glomerular filtration barrier (GFB) is a highly specialized interface responsible for blood filtration, displaying a high conductance to small and midsized
solutes in plasma but retaining relative impermeability to macromolecules. GFB integrity is maintained by a constant physicochemical interaction among its
three principal constituents: the glomerular endothelial cell, the basement membrane, and the visceral epithelial cell (podocyte) (Figure 1). Podocyte means
Cell Stem Cell 21, July 6, 2017 ª 2017 Elsevier Inc. 7
Cell Stem Cell
Previews
Figure 1. How to Model the Glomerular Filtration Barrier? The glomerular filtration barrier consists of the capillary endothelial cell (EC), the glomerular basement membrane (GBM), and the podocytes (left). In the multifunctional microfluidic device, iPSCs-derived podocytes and endothelial cells are co-cultured to recapitulate the podocyte-GBM-endothelial interface (right).
‘‘cell with the foot,’’ as each podocyte has many elongated processes called foot processes, or pedicels, that interdigitate with those of adjacent podocytes and leave gaps or thin filtration slits between them, the slit-diaphragms (Reiser and Sever, 2013). The podocyte has no capacity to divide, so it can only be replaced by a progenitor cell localized within the Bowman’s capsule, but this regeneration is limited (Lasagni et al., 2015). For all these reasons, the podocyte is the Achilles’ heel of the kidney and a severe podocyte loss secondary to any type of injury leads to chronic kidney disease (CKD) (Reiser and Sever, 2013). Rebuilding a fully differentiated podocyte is thus essential for modeling the GFB and studying kidney disorders. In their study, Musah et al. (2017) propose a new protocol to efficiently differentiate induced pluripotent stem cells (iPSC) into podocytes in culture. Up to now, various studies have reported obtaining in vitro podocytes from several strategies and different cell sources. All these studies obtained cells with high levels of nephrin, podocin, and other slit diaphragm protein expression that specifically characterize a podocyte (Musah et al., 2017; Lasagni et al., 2015; Lazzeri et al., 2015; Xinaris et al., 2016; Freedman et al., 2015; Ciampi et al., 2016). However, in their study, Musah et al. pushed the attempt one step forward, putting iPSC-derived po8 Cell Stem Cell 21, July 6, 2017
docytes in the context of an engineered GFB. To achieve this goal, they built a multifunctional microfluidic device that recapitulates the structural, functional, and mechanical properties of a three-dimensional cross-section of the human glomerular capillary wall (Musah et al., 2017). The microfluidic device was composed of a flexible poly(dimethylsiloxane) (PDMS) elastomer that contains two closely opposed, parallel microchannels separated by a laminin-511-coated, porous flexible PDMS membrane. To model the dynamic mechanical strain observed in living glomeruli due to the cyclic pulsations of renal blood flow, Musah et al. also incorporated two hollow chambers on either side of the central microfluidic channels and applied cyclic suction to produce cyclic stretching and relaxation of the flexible membrane and adherent cell layers. Then, human iPSC-derived podocytes were cultured on the top of the laminin-coated membrane and primary human glomerular endothelial cells were cultured on the opposite side of the same membrane to recapitulate the podocyte–GBM–endothelial interface. By exposing the cells to differentiation medium, they induced the expression of podocyte markers including proteins of the slit diaphragm, such as nephrin or podocin, with long interdigitating (foot) processes connecting with each other and with endothelial cells. Human iPSC-derived podocytes differen-
tiated in the microfluidic devices produced glomerular basement-membrane collagen and secreted vascular endothelial growth factor (VEGF)-A. More importantly, this ‘‘glomerulus in a chip’’ mimicked the GFB in restricting permeability to large molecules such as albumin, but freely filtering exogenous small molecules, such as inulin, from plasma (Musah et al., 2017). In addition, after treatment with adriamycin, podocytes underwent dosedependent delamination, mirroring podocyte detachment from the GFB and loss in urinary flow that characterize the adriamycin nephropathy model in vivo (Lasagni et al., 2015) as well as the related human disease focal segmental glomerulosclerosis (FSGS; Reiser and Sever 2013). In vivo, podocyte detachment also results in significant loss of albumin from the vascular channel and increased entry into the urinary compartment, i.e., ‘‘albuminuria’’ (Reiser and Sever 2013; Lazzeri et al., 2015). Far from being only a biomarker of GFB damage widely used in the clinics, albuminuria is also directly harmful and favors CKD progression by impairing renal progenitor differentiation into podocytes and their regeneration after injury (Peired et al., 2013). In this in vitro model, albumin also nonselectively leaked through the GFB, resulting in increased uptake by the human iPSC-derived podocytes lining the urinary compartment of the microfluidic device (Musah et al., 2017), efficiently modeling GFB damage. Musah et al. (2017) propose that this device may be an innovative way for modeling certain kidney disorders, particularly podocytopathies induced by toxic agents. However, many other conditions may be modeled with this strategy; for example, pathogenic mutations in more than 40 genes involved in maintaining podocyte structure and function cause proteinuric forms of CKD (Vivante and Hildebrandt, 2016). The high genetic heterogeneity of podocytopathies requires extended DNA sequencing and renders diagnosis frequently challenging because of the identification of variants of unknown functional significance (Vivante and Hildebrandt, 2016). Musah et al.’s microfluidic device would allow one to test the functional significance of genetic mutations in podocytes in the context of the high pressure conditions that challenge them in vivo. This could help to identify novel genes, to provide information on the
Cell Stem Cell
Previews pathogenic mechanisms that link each gene to disease, and also to support diagnosis in affected patients. Furthermore, persistent glomerular hypertension is a critical driver of CKD progression (Reiser and Sever, 2013). As both foot process maintenance as well as VEGF-A secretion by podocytes are required for an efficient GFB in vivo, these results suggest that perturbing glomerular filtration pressure may translate into podocyte malfunction that characterizes many renal disorders (Reiser and Sever, 2013). In addition, the lack of these critical features in vitro explains why culturing podocytes has been difficult and typically associated with a poorly differentiated phenotype that may not often correctly mirror podocyte biology in vivo (reviewed in Shankland et al., 2007). In conclusion, Musah et al. (2017) demonstrate that for in vitro modeling of post-mitotic, highly specialized cells that are difficult to amplify in culture, like podo-
cytes, differentiation from stem cells in culture may represent a more suitable option to fully achieve their phenotypic properties. In addition, this study demonstrates that in order to model podocyte complexity in vivo, it is essential to consider the 3D interactions, physical forces, and chemical stimuli that continuously control the cells’ shape, function, and differentiation status, setting a new standard for modeling glomerular disease. REFERENCES Ciampi, O., Iacone, R., Longaretti, L., Benedetti, V., Graf, M., Magnone, M.C., Patsch, C., Xinaris, C., Remuzzi, G., Benigni, A., and Tomasoni, S. (2016). Stem Cell Res. (Amst.) 17, 130–139.
Lazzeri, E., Ronconi, E., Angelotti, M.L., Peired, A., Mazzinghi, B., Becherucci, F., Conti, S., Sansavini, G., Sisti, A., Ravaglia, F., et al. (2015). J. Am. Soc. Nephrol. 26, 1961–1974. Musah, S., Mammoto, A., Ferrante, T.C., Jeanty, S.S.F., Hirano-Kobayashi, M., Mammoto, T., Roberts, K., Chung, S., Novak, R., Ingram, M., et al. (2017). Nat. Biomed. Eng. Published online May 10, 2017. http://dx.doi.org/10.1038/s41551017-0069. Peired, A., Angelotti, M.L., Ronconi, E., la Marca, G., Mazzinghi, B., Sisti, A., Lombardi, D., Giocaliere, E., Della Bona, M., Villanelli, F., et al. (2013). J. Am. Soc. Nephrol. 24, 1756–1768. Reiser, J., and Sever, S. (2013). Annu. Rev. Med. 64, 357–366. Shankland, S.J., Pippin, J.W., Reiser, J., and Mundel, P. (2007). Kidney Int. 72, 26–36.
Freedman, B.S., Brooks, C.R., Lam, A.Q., Fu, H., Morizane, R., Agrawal, V., Saad, A.F., Li, M.K., Hughes, M.R., Werff, R.V., et al. (2015). Nat. Commun. 6, 8715.
Vivante, A., and Hildebrandt, F. (2016). Nat. Rev. Nephrol. 12, 133–146.
Lasagni, L., Angelotti, M.L., Ronconi, E., Lombardi, D., Nardi, S., Peired, A., Becherucci, F., Mazzinghi, B., Sisti, A., Romoli, S., et al. (2015). Stem Cell Reports 5, 248–263.
Xinaris, C., Benedetti, V., Novelli, R., Abbate, M., Rizzo, P., Conti, S., Tomasoni, S., Corna, D., Pozzobon, M., Cavallotti, D., et al. (2016). J. Am. Soc. Nephrol. 27, 1400–1411.
Retinoic Acid Puts Hematopoietic Stem Cells Back To Sleep Alexandra Rundberg Nilsson1,2 and Cornelis Jan Pronk1,2,3,* 1Medical
Faculty, Division of Molecular Hematology, Institution for Laboratory Medicine, Lund University, 221 84 Lund, Sweden Faculty, Lund Stem Cell Center, Lund University, 221 84 Lund, Sweden 3Department of Pediatric Oncology/Hematology, Ska ˚ ne University Hospital, 221 85 Lund, Sweden *Correspondence: [email protected] http://dx.doi.org/10.1016/j.stem.2017.06.002 2Medical
Dormant hematopoietic stem cells (dHSCs) display superior serial reconstitution capacity compared to active HSCs, although their role in normal hematopoiesis has not been thoroughly investigated. Recently in Cell, Cabezas-Wallscheid et al. (2017) demonstrate involvement of retinoic acid signaling in murine dHSCs for preservation of the HSC pool. Hematopoietic stem cells (HSCs) maintain continuous hematopoiesis throughout life. This ability has been ascertained primarily based on experimental findings during stressed hematopoiesis, as the role of HSCs in maintaining hematopoiesis during steady state is more controversial (reviewed in Busch and Rodewald, 2016). Nevertheless, it is likely that the hematopoietic system is recurrently challenged during a lifetime (by for instance infec-
tions), thus presumably relying on active input from HSCs for upholding normal lifelong hematopoiesis. To fulfill the demand of continuous hematopoiesis, HSCs must balance mature blood cell production versus HSC pool maintenance. Most HSCs reside in a quiescent state and only occasionally enter the cell cycle—a characteristic that supposedly protects HSCs from exhaustion and cell-cycleassociated mutational damage. A fraction
of HSCs has shown exceptionally slow turnover rates during unchallenged hematopoiesis, only dividing about five times during a lifetime. Such HSCs are named dormant HSCs (dHSCs) (Wilson et al., 2008) and have been studied mainly using label-retention assays. The exact role for dHSCs in regulation of hematopoiesis and contribution to mature blood cell production during steady state and stress hematopoiesis remains to be
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