- Email: [email protected]
Epicardial Origin of Cardiac CFU-Fs
Cell Stem Cell
Previews Epicardial Origin of Cardiac CFU-Fs Igor I. Slukvin1,2,* 1National
Primate Research Center, University of Wisconsin Graduate School, 1220 Capitol Court, Madison, WI 53715, USA
2Department of Pathology and Laboratory Medicine, University of Wisconsin Medical School, 600 Highland Avenue, Madison, WI 53792, USA
*Correspondence: [email protected] DOI 10.1016/j.stem.2011.11.010
The epicardium has been recognized as a source of cardiovascular progenitors during embryogenesis and postnatal life. In this issue of Cell Stem Cell, Chong et al. (2011) identify cardiac CFU-Fs as cardiac-resident cells of epicardial origin with broad multilineage differentiation potential. Bone marrow cells with potential to form colonies composed of fibroblastoid cells (colony-forming-unit fibroblasts; CFU-Fs) were identified over 40 years ago (Friedenstein et al., 1970). Because these cells were capable of long-term expansion in vitro and differentiation to multiple connective cell types, they were defined as mesenchymal stem cells (MSCs) (Caplan, 1991). Subsequently, MSCs have been isolated from almost all types of tissues, including adipose tissue and umbilical cord blood (Kern et al., 2006). Mesenchymal cells are one of the major components of a variety of tissues and are critical for establishing tissue organization and response to injury. We know that MSCs isolated from different organs and tissues display similar morphology and phenotype, share an ability to proliferate substantially in vitro, and differentiate to multiple cell types. However, we still know little about the origin and the distinct functional properties of tissuespecific mesenchymal cells and the hierarchy of mesenchymal progenitors. In this current issue, Chong et al. (2011) chose to ask whether the heart contains CFU-Fs and, if so, where they originate. Using a noncardiomyocyte population of cells isolated from mouse hearts and standard conditions for CFU-F detection, they demonstrated that embryonic and adult hearts contain CFU-F cells with long-term in vitro expansion potential. These cardiac-derived MSCs exhibited multipotency in vitro and in vivo for mesodermal lineages, including smooth muscle, bone, cartilage, adipose, endothelial, and a-actinin+ striated cardiactype muscle cells. These so-called cardiac CFU-F (cCFU-F) cells were capable of contributing to nonmesodermal lineage cells within teratomas when cotransplanted with ESCs. Using
Pdgfra/GFP knockin mice, immunostaining, and cell sorting, the authors demonstrated that cCFU-Fs expressed PDGFRa MSC marker and SCA1 stem cell marker but essentially lacked expression of another stem cell marker, cKIT, as well as endothelial (CD31, FLK1), hematopoietic (CD45), cardiac (NKX2-5), and ESCspecific (Sox2, Rex1, Nanog, and Klf4) molecules. Because two major germ layers (mesoderm and neural crest/ectoderm) contribute to mesenchymal cells during embryonic development, Chong et al. (2011) used Mesp1 (mesodermal) and Wnt1 (neural crest) Cre-recombinase lineage tracing to define the source of cCFU-Fs. These experiments revealed that cCFU-Fs originate from mesoderm but not from neural crest. Additional lineage tracing experiments using Wt1Cre and Gata5-Cre epicardial lineage and Myl2-Cre and Nkx2-5-Cre cardiomyocyte lineage tags found that cCFU-Fs arise from proepicardium and epicardium but not from cardiomyocytes. Despite similar phenotype and molecular signature, cCFU-Fs displayed a CRE lineage signature distinct from bone marrow CFU-Fs, indicating that organ-specific MSCs have diverse postgastrulation origins. Recently, the epicardium has been recognized as a source of cardiovascular progenitors during embryogenesis and postnatal life. A subset of epicardial mesothelial cells expressing Wt1 undergo endothelial-mesenchymal transition (EMT) and contribute to interstitial fibroblasts, vasculature, and cardiomyocytes (Smart et al., 2011; Zhou et al., 2008). These cells are defined as epicardiumderived cells (EPDCs) (Gittenberger-de Groot et al., 1998). However, the distinct steps in EMT transition and hierarchy of
492 Cell Stem Cell 9, December 2, 2011 ª2011 Elsevier Inc.
progenitors formed during transition remain unclear. Chong et al. (2011) found that at least some cells undergoing EMT acquire CFU-F potential and contribute predominately to cellular elements of the cardiac interstitium and vasculature. In addition, Limana et al. (2007) demonstrated that epicardium contains cKIT+ progenitors, which participate in myocardial regeneration and are capable of generating mesenchymal cells after culture in vitro. These progenitors seem different form cCFUs because they express Nkx2-5 cardiac marker and cKIT. However, the origin of epicardial cKIT+ cells is unclear. It has been suggested that these cells represent the earliest step of EMT or interstitial Cajallike cells. The epicardium is not the sole source of mesenchymal cells in the heart; the endocardium and neural crest also contribute to the mesenchymal population within the developing heart. Similar to epicardial mesothelium, endocardial endothelial cells undergo endothelialmesenchymal transition and contribute to endocardial cushions, the primary atrial septum, and leaflets of the atrio-ventricular valves in the postnatal heart (de Lange et al., 2004). Neural-crest-derived mesenchymal cells are essential for development of aortico-pulmonary septum and truncal ridges (Hutson and Kirby, 2007). Thus, complete characterization of the mesenchymal cell populations in the heart would require an analysis of the contribution of endocardium and neural crest to CFU-Fs in the heart and major vessels. In the present study, Chong et al. (2011) have reported that the aorta contains CFU-Fs originating from the neural crest. However, endocardial cushions and adult valve leaflets yielded few small CFU-Fs with limited expansion potential. Although
Cell Stem Cell
Previews these data indicate that endocardium may not contribute significantly to cardiac CFU-Fs, the possibility remains that CFU-Fs of endocardial origin may be functionally different from epicardial CFU-Fs and require a different cell culture medium for optimal detection. Understanding the developmental biology of the heart is instrumental in developing novel technologies for heart regeneration and cellular therapies. It is critical to identify the type and origin of cells capable of reconstituting a heart. To determine whether a cardiac mesenchymal cell compartment could be reconstituted by bone marrow cells after injury, Chong et al. (2011) performed CFU-F evaluation in the heart after transplantation of bone marrow cells following myocardial infarction, irradiation, and hematopoietic stem cell immobilization. While the authors found that bone marrow cells could reconstitute the marrow CFU-F compartment, they failed to detect their contribution to cardiac CFU-Fs. These findings support the view that bone marrow cells do not reconstitute cardiac cells after cardiac injury. In this context, the activation of cardiac resident progenitors could be considered as a reasonable approach to promote cardiac tissue regeneration and vessel growth. Recent
work by Smart et al. (2011) found that expression of embryonic epicardial progenitor marker Wt1 can be activated in the adult epicardium by stimulation with thymosin b4 prior to myocardial infarction. Subsequently, these cells migrate into the heart and contribute to smooth and cardiac muscles and interstitial fibroblasts. Further work will be required to determine the relationship between epicardial progenitors described by Smart and colleagues (Smart et al., 2011) and cCFU-Fs identified by Chong and colleagues (Chong et al., 2011). Do they represent similar or hierarchically related progenitors? Are there any stimuli that can activate cCFU-Fs and facilitate cardiac repair and vascularization after myocardial infarction? Since repair consists of a combination of regeneration and scar formation, it is important to determine the contribution of cCFU-Fs to postinfarct scar formation and whether improvement in restoration of cardiac function can be achieved by modulation of sclerogenic versus tissue-forming response of cCFU-Fs. Certainly, the findings presented by Chong et al. (2011) introduce several interesting and novel avenues of investigation, particularly in the context of cardiac regenerative therapies.
REFERENCES Caplan, A.I. (1991). J. Orthop. Res. 9, 641–650. Chong, J.J.H., Chandrakanthan, V., Xaymardan, M., Asli, N.S., Li, J., Ahmed, I., Heffernan, C., Menon, M.K., Scarlett, C.J., Rashidianfar, A., et al. (2011). Cell Stem Cell 9, this issue, 527–540. de Lange, F.J., Moorman, A.F., Anderson, R.H., Ma¨nner, J., Soufan, A.T., de Gier-de Vries, C., Schneider, M.D., Webb, S., van den Hoff, M.J., and Christoffels, V.M. (2004). Circ. Res. 95, 645–654. Friedenstein, A.J., Chailakhjan, R.K., and Lalykina, K.S. (1970). Cell Tissue Kinet. 3, 393–403. Gittenberger-de Groot, A.C., Vrancken Peeters, M.P., Mentink, M.M., Gourdie, R.G., and Poelmann, R.E. (1998). Circ. Res. 82, 1043–1052. Hutson, M.R., and Kirby, M.L. (2007). Semin. Cell Dev. Biol. 18, 101–110. Kern, S., Eichler, H., Stoeve, J., Klu¨ter, H., and Bieback, K. (2006). Stem Cells 24, 1294–1301. Limana, F., Zacheo, A., Mocini, D., Mangoni, A., Borsellino, G., Diamantini, A., De Mori, R., Battistini, L., Vigna, E., Santini, M., et al. (2007). Circ. Res. 101, 1255–1265. Smart, N., Bollini, S., Dube´, K.N., Vieira, J.M., Zhou, B., Davidson, S., Yellon, D., Riegler, J., Price, A.N., Lythgoe, M.F., et al. (2011). Nature 474, 640–644. Zhou, B., Ma, Q., Rajagopal, S., Wu, S.M., Domian, I., Rivera-Feliciano, J., Jiang, D., von Gise, A., Ikeda, S., Chien, K.R., and Pu, W.T. (2008). Nature 454, 109–113.
Blood Cells Need Glia, Too: A New Role for the Nervous System in the Bone Marrow Niche Katja Bru¨ckner1,* 1Department of Cell and Tissue Biology, Department of Anatomy, Eli and Edythe Broad Center of Regeneration Medicine and Stem Cell Research, University of California, San Francisco, 35 Medical Center Way, San Francisco, CA 94143-0669, USA *Correspondence: [email protected] DOI 10.1016/j.stem.2011.11.016
Regulation of hematopoietic stem cell (HSC) dormancy by specific cell types in the hematopoietic niche remains poorly understood. Yamazaki et al. (2011) now report that nerve-associated nonmyelinating Schwann cells activate TGF-b to maintain dormant HSCs, suggesting that glia are novel players in the bone marrow niche. Hematopoietic stem cells (HSCs) reside in specialized microenvironments, or niches, that ensure their localization, survival, controlled proliferation, and differentiation.
Components of the bone marrow (BM) hematopoietic niche include mesenchymal stem cells (MSCs), which secrete HSC homing and maintenance factors; osteo-
blasts, which derive from MSCs and line the surface of the endosteum on the inside of the bone cavity; and sympathetic nerve fibers, which coordinate hematopoietic
Cell Stem Cell 9, December 2, 2011 ª2011 Elsevier Inc. 493
Previews Epicardial Origin of Cardiac CFU-Fs Igor I. Slukvin1,2,* 1National
Primate Research Center, University of Wisconsin Graduate School, 1220 Capitol Court, Madison, WI 53715, USA
2Department of Pathology and Laboratory Medicine, University of Wisconsin Medical School, 600 Highland Avenue, Madison, WI 53792, USA
*Correspondence: [email protected] DOI 10.1016/j.stem.2011.11.010
The epicardium has been recognized as a source of cardiovascular progenitors during embryogenesis and postnatal life. In this issue of Cell Stem Cell, Chong et al. (2011) identify cardiac CFU-Fs as cardiac-resident cells of epicardial origin with broad multilineage differentiation potential. Bone marrow cells with potential to form colonies composed of fibroblastoid cells (colony-forming-unit fibroblasts; CFU-Fs) were identified over 40 years ago (Friedenstein et al., 1970). Because these cells were capable of long-term expansion in vitro and differentiation to multiple connective cell types, they were defined as mesenchymal stem cells (MSCs) (Caplan, 1991). Subsequently, MSCs have been isolated from almost all types of tissues, including adipose tissue and umbilical cord blood (Kern et al., 2006). Mesenchymal cells are one of the major components of a variety of tissues and are critical for establishing tissue organization and response to injury. We know that MSCs isolated from different organs and tissues display similar morphology and phenotype, share an ability to proliferate substantially in vitro, and differentiate to multiple cell types. However, we still know little about the origin and the distinct functional properties of tissuespecific mesenchymal cells and the hierarchy of mesenchymal progenitors. In this current issue, Chong et al. (2011) chose to ask whether the heart contains CFU-Fs and, if so, where they originate. Using a noncardiomyocyte population of cells isolated from mouse hearts and standard conditions for CFU-F detection, they demonstrated that embryonic and adult hearts contain CFU-F cells with long-term in vitro expansion potential. These cardiac-derived MSCs exhibited multipotency in vitro and in vivo for mesodermal lineages, including smooth muscle, bone, cartilage, adipose, endothelial, and a-actinin+ striated cardiactype muscle cells. These so-called cardiac CFU-F (cCFU-F) cells were capable of contributing to nonmesodermal lineage cells within teratomas when cotransplanted with ESCs. Using
Pdgfra/GFP knockin mice, immunostaining, and cell sorting, the authors demonstrated that cCFU-Fs expressed PDGFRa MSC marker and SCA1 stem cell marker but essentially lacked expression of another stem cell marker, cKIT, as well as endothelial (CD31, FLK1), hematopoietic (CD45), cardiac (NKX2-5), and ESCspecific (Sox2, Rex1, Nanog, and Klf4) molecules. Because two major germ layers (mesoderm and neural crest/ectoderm) contribute to mesenchymal cells during embryonic development, Chong et al. (2011) used Mesp1 (mesodermal) and Wnt1 (neural crest) Cre-recombinase lineage tracing to define the source of cCFU-Fs. These experiments revealed that cCFU-Fs originate from mesoderm but not from neural crest. Additional lineage tracing experiments using Wt1Cre and Gata5-Cre epicardial lineage and Myl2-Cre and Nkx2-5-Cre cardiomyocyte lineage tags found that cCFU-Fs arise from proepicardium and epicardium but not from cardiomyocytes. Despite similar phenotype and molecular signature, cCFU-Fs displayed a CRE lineage signature distinct from bone marrow CFU-Fs, indicating that organ-specific MSCs have diverse postgastrulation origins. Recently, the epicardium has been recognized as a source of cardiovascular progenitors during embryogenesis and postnatal life. A subset of epicardial mesothelial cells expressing Wt1 undergo endothelial-mesenchymal transition (EMT) and contribute to interstitial fibroblasts, vasculature, and cardiomyocytes (Smart et al., 2011; Zhou et al., 2008). These cells are defined as epicardiumderived cells (EPDCs) (Gittenberger-de Groot et al., 1998). However, the distinct steps in EMT transition and hierarchy of
492 Cell Stem Cell 9, December 2, 2011 ª2011 Elsevier Inc.
progenitors formed during transition remain unclear. Chong et al. (2011) found that at least some cells undergoing EMT acquire CFU-F potential and contribute predominately to cellular elements of the cardiac interstitium and vasculature. In addition, Limana et al. (2007) demonstrated that epicardium contains cKIT+ progenitors, which participate in myocardial regeneration and are capable of generating mesenchymal cells after culture in vitro. These progenitors seem different form cCFUs because they express Nkx2-5 cardiac marker and cKIT. However, the origin of epicardial cKIT+ cells is unclear. It has been suggested that these cells represent the earliest step of EMT or interstitial Cajallike cells. The epicardium is not the sole source of mesenchymal cells in the heart; the endocardium and neural crest also contribute to the mesenchymal population within the developing heart. Similar to epicardial mesothelium, endocardial endothelial cells undergo endothelialmesenchymal transition and contribute to endocardial cushions, the primary atrial septum, and leaflets of the atrio-ventricular valves in the postnatal heart (de Lange et al., 2004). Neural-crest-derived mesenchymal cells are essential for development of aortico-pulmonary septum and truncal ridges (Hutson and Kirby, 2007). Thus, complete characterization of the mesenchymal cell populations in the heart would require an analysis of the contribution of endocardium and neural crest to CFU-Fs in the heart and major vessels. In the present study, Chong et al. (2011) have reported that the aorta contains CFU-Fs originating from the neural crest. However, endocardial cushions and adult valve leaflets yielded few small CFU-Fs with limited expansion potential. Although
Cell Stem Cell
Previews these data indicate that endocardium may not contribute significantly to cardiac CFU-Fs, the possibility remains that CFU-Fs of endocardial origin may be functionally different from epicardial CFU-Fs and require a different cell culture medium for optimal detection. Understanding the developmental biology of the heart is instrumental in developing novel technologies for heart regeneration and cellular therapies. It is critical to identify the type and origin of cells capable of reconstituting a heart. To determine whether a cardiac mesenchymal cell compartment could be reconstituted by bone marrow cells after injury, Chong et al. (2011) performed CFU-F evaluation in the heart after transplantation of bone marrow cells following myocardial infarction, irradiation, and hematopoietic stem cell immobilization. While the authors found that bone marrow cells could reconstitute the marrow CFU-F compartment, they failed to detect their contribution to cardiac CFU-Fs. These findings support the view that bone marrow cells do not reconstitute cardiac cells after cardiac injury. In this context, the activation of cardiac resident progenitors could be considered as a reasonable approach to promote cardiac tissue regeneration and vessel growth. Recent
work by Smart et al. (2011) found that expression of embryonic epicardial progenitor marker Wt1 can be activated in the adult epicardium by stimulation with thymosin b4 prior to myocardial infarction. Subsequently, these cells migrate into the heart and contribute to smooth and cardiac muscles and interstitial fibroblasts. Further work will be required to determine the relationship between epicardial progenitors described by Smart and colleagues (Smart et al., 2011) and cCFU-Fs identified by Chong and colleagues (Chong et al., 2011). Do they represent similar or hierarchically related progenitors? Are there any stimuli that can activate cCFU-Fs and facilitate cardiac repair and vascularization after myocardial infarction? Since repair consists of a combination of regeneration and scar formation, it is important to determine the contribution of cCFU-Fs to postinfarct scar formation and whether improvement in restoration of cardiac function can be achieved by modulation of sclerogenic versus tissue-forming response of cCFU-Fs. Certainly, the findings presented by Chong et al. (2011) introduce several interesting and novel avenues of investigation, particularly in the context of cardiac regenerative therapies.
REFERENCES Caplan, A.I. (1991). J. Orthop. Res. 9, 641–650. Chong, J.J.H., Chandrakanthan, V., Xaymardan, M., Asli, N.S., Li, J., Ahmed, I., Heffernan, C., Menon, M.K., Scarlett, C.J., Rashidianfar, A., et al. (2011). Cell Stem Cell 9, this issue, 527–540. de Lange, F.J., Moorman, A.F., Anderson, R.H., Ma¨nner, J., Soufan, A.T., de Gier-de Vries, C., Schneider, M.D., Webb, S., van den Hoff, M.J., and Christoffels, V.M. (2004). Circ. Res. 95, 645–654. Friedenstein, A.J., Chailakhjan, R.K., and Lalykina, K.S. (1970). Cell Tissue Kinet. 3, 393–403. Gittenberger-de Groot, A.C., Vrancken Peeters, M.P., Mentink, M.M., Gourdie, R.G., and Poelmann, R.E. (1998). Circ. Res. 82, 1043–1052. Hutson, M.R., and Kirby, M.L. (2007). Semin. Cell Dev. Biol. 18, 101–110. Kern, S., Eichler, H., Stoeve, J., Klu¨ter, H., and Bieback, K. (2006). Stem Cells 24, 1294–1301. Limana, F., Zacheo, A., Mocini, D., Mangoni, A., Borsellino, G., Diamantini, A., De Mori, R., Battistini, L., Vigna, E., Santini, M., et al. (2007). Circ. Res. 101, 1255–1265. Smart, N., Bollini, S., Dube´, K.N., Vieira, J.M., Zhou, B., Davidson, S., Yellon, D., Riegler, J., Price, A.N., Lythgoe, M.F., et al. (2011). Nature 474, 640–644. Zhou, B., Ma, Q., Rajagopal, S., Wu, S.M., Domian, I., Rivera-Feliciano, J., Jiang, D., von Gise, A., Ikeda, S., Chien, K.R., and Pu, W.T. (2008). Nature 454, 109–113.
Blood Cells Need Glia, Too: A New Role for the Nervous System in the Bone Marrow Niche Katja Bru¨ckner1,* 1Department of Cell and Tissue Biology, Department of Anatomy, Eli and Edythe Broad Center of Regeneration Medicine and Stem Cell Research, University of California, San Francisco, 35 Medical Center Way, San Francisco, CA 94143-0669, USA *Correspondence: [email protected] DOI 10.1016/j.stem.2011.11.016
Regulation of hematopoietic stem cell (HSC) dormancy by specific cell types in the hematopoietic niche remains poorly understood. Yamazaki et al. (2011) now report that nerve-associated nonmyelinating Schwann cells activate TGF-b to maintain dormant HSCs, suggesting that glia are novel players in the bone marrow niche. Hematopoietic stem cells (HSCs) reside in specialized microenvironments, or niches, that ensure their localization, survival, controlled proliferation, and differentiation.
Components of the bone marrow (BM) hematopoietic niche include mesenchymal stem cells (MSCs), which secrete HSC homing and maintenance factors; osteo-
blasts, which derive from MSCs and line the surface of the endosteum on the inside of the bone cavity; and sympathetic nerve fibers, which coordinate hematopoietic
Cell Stem Cell 9, December 2, 2011 ª2011 Elsevier Inc. 493