- Email: [email protected]
Thrombopoietin, HSCs, and the Osteoblast Niche: Holding On Loosely, but Not Letting G0
Cell Stem Cell
Previews Thrombopoietin, HSCs, and the Osteoblast Niche: Holding On Loosely, but Not Letting G0 Stephen G. Emerson1,* 1Haverford College, Haverford, PA 19041, USA *Correspondence: [email protected] DOI 10.1016/j.stem.2007.11.010
New findings in this issue of Cell Stem Cell by Qian et al. (2007) and Yoshihara et al. (2007) reveal that thrombopoietin modulates hematopoietic stem cell (HSC) cell-cycle progression at the osteoblast surface, linking a single cytokine with a specific postnatal niche cell. These observations indicate that simultaneous stimulation and suspension in a G0 state are critical for maintenance of the HSC pool. Clinical observations over the past 40 years in patients undergoing hematopoietic stem cell (HSC) transplants have suggested a close relationship between thrombopoiesis and stem cell activity. Delayed or inadequate platelet recoveries are the most sensitive indicators of inadequate numbers of transplanted stem cells, and thrombocytopenia is the first sign of graft failure (Ninan et al., 2007). When thrombopoietin (THPO) was first cloned by its binding to the Mpl receptor and characterized by its essential role in stimulating thrombopoiesis, it was thus no surprise that THPO was also found to have a key role in HSC maintenance and expansion (Sitnicka et al., 1996). Purified recombinant THPO itself was found to synergize with other proliferative cytokines to support HSC proliferation in vitro. Because THPO, like other cytokines such as SCF and IL-3, activates MAPK, AKT, and STAT pathways, such interactions might suggest overlapping, redundant functions (Dorsch et al., 1995). However, the finding that Mpl/ mice are stem cell deficient suggested that THPO might have an important and unique role not shared by other cytokines (Alexander et al., 1996). In this issue of Cell Stem Cell, Qian et al. (2007) begin by measuring HSC numbers both before and throughout life in Thpo/ versus normal mice. They find that postnatal HSCs are normal, both in number (as measured by cell surface phenotype) and in potency following stem cell transplantation. However, HSC numbers begin to decline within a few weeks after birth
and continue to do so throughout life, resulting in a 1503 reduction. They go on to show that Thpo/ HSCs show increased cycling, and reduced levels of the CDKIs p57kip2 and p19ink4D. These data indicate that postnatal HSCs, unlike fetal HSCs, are THPO dependent for their survival and maintenance. Again, this effect was observed both at baseline, as measured by cell surface phenotype, and functionally following HSC transplantation. In addition, they found that this THPO effect on HSC survival and maintenance is not Bcl-2 dependent. Although the authors do not speculate on what distinguishes prenatal from postnatal HSC THPO dependence, one clear difference between hematopoiesis before and after birth is the transition of HSCs to the bone marrow microenvironment. One might therefore hypothesize that if THPO has a unique postnatal role in the life of HSCs, then this role would be expressed directly in the topography of the bone marrow microenvironment, through interaction with the Mpl receptor on unique BM niche cells. Yoshihara et al. (2007) now present data supporting precisely this notion, that the THPO-Mpl axis is anatomically and functionally expressed between HSCs and the osteoblast surface in the bone marrow. In their precise analysis, they first show that long-term repopulating hematopoietic stem cells (LT-HSCs) that express Mpl are largely in G0 and are located in apposition to THPO+ osteoblasts. They then show that THPO treatment increases HSC expression of CDKIs and increases the entry of HSCs into G0 prior to their
proliferation. In contrast, treatment with neutralizing anti-Mpl antibody blocks restraint of in vivo HSCs in G0, and functionally releases them from their microniches, allowing their displacement by transplanted HSCs. Their results therefore support a key role for THPO in supporting HSC quiescence and further allude to key roles for osteoblasts in providing these signals in vivo. These two findings provide tentative answers to several important questions in HSC biology but leave other key issues still open to exploration. On the plus side of the ledger, these results provide the first molecular answer to what is the key HSC signal provided in the BM microenvironment, and where. Similarly, if THPO is the key HSC retentive signal provided by osteoblasts, this might explain why deletion of osteoblasts in vivo does not result in immediate loss of HSCs, but rather HSC redistribution out of the marrow, with HSC loss only occurring slowly. Still mysterious, however, is that efforts to date to culture and expand HSCs in vivo without loss of HSC potency, even with THPO, have met with limited success. Could there be other signals produced by osteoblasts, or even other cells within the BM niche, that are also required for HSC maintenance and proliferation? One surprising finding is that adherence to the niche seems to be multifactorial, or at least extremely delicate. Czechowicz et al. have recently found that HSCs can be functionally displaced from their niches by in vivo treatment with anti-c-kit antibody (Czechowicz et al., 2007). Osteopontin
Cell Stem Cell 1, December 2007 ª2007 Elsevier Inc. 599
Cell Stem Cell
Previews also appears to play a key role in HSC homing and adhesion to the niche (Stier et al., 2005), while blockade of CD44 appears to block adhesion of leukemic stem cells to the bone marrow microenvironment (Jin et al., 2006). Given the complex function of the bone marrow stem cell niche, could these critical adhesive and functional signals from the niche cells be induced only by the retrograde secretion or local presentation of specific molecules from HSCs? I.e., might HSCs condition their niche (Taichman et al., 2007)? Similarly, given these data, why are prenatal HSCs THPO independent? Are homologous signals, which activate similar survival pathways in HSCs, secreted by embryonic and fetal liver fibroblasts, or are fetal HSCs fundamentally different from postnatal HSCs? Similarly, one must ask whether similar paradigms apply to stem cells for other tissues, in other organ-specific niches? And of course, at a molecular level, why is maintenance of a G0 pool required for longterm support of HSC numbers? Finally, it is always worthwhile to attempt to draw tentative hypotheses
from elegant studies such as these into the world of clinical hematology. Would the provision of the proper pharmacologic concentration of THPO actually accelerate or quantitatively enhance HSC engraftment after transplantation? Conversely, would provision of anti-THPO antibody to patients with acute leukemia release their leukemic HSCs from the protection of G0 retention, thus rendering them more susceptible to anti-leukemic ablative therapies? Such queries could be directly tested in current immunodeficient mouse models, with the potential for decisive clinical application. In summary, Qian et al. and Yoshihara et al. present elegant and compelling new data demonstrating a key role for THPO in the maintenance of the long-term repopulating, quiescent HSC pool and furthermore suggest that provision of THPO is at least one of the unique contributions of osteoblasts after the transition of hematopoiesis from fetal liver to the bone marrow. Clearly osteoblasts have paid close attention to their musical roots and ‘‘hold on loosely, but don’t let G0.’’
REFERENCES Alexander, W.S., Roberts, A.W., Nicola, N.A., Li, R., and Metcalf, D. (1996). Blood 87, 2162–2170. Czechowicz, A., Kraft, D., Weissman, I.L., and Bhattacharya, D. (2007). Science 318, 1296– 1299. Dorsch, M., Fan, P.D., Bogenberger, J., and Goff, S.P. (1995). Biochem. Biophys. Res. Commun. 214, 424–431. Jin, L., Hope, K.J., Zhai, Q., Smadja-Joffe, F., and Dick, J.E. (2006). Nat. Med. 12, 1167–1174. Ninan, M.J., Flowers, C.R., Roback, J.D., Arellano, M.L., and Waller, E.K. (2007). Biol. Blood Marrow Transplant. 13, 895–904. Qian, H., Buza-Vidas, N., Hyland, C.D., Jensen, C.T., Antonchuk, J., Ma˚nsson, R., Thoren, L.A., Ekblom, M., Alexander, W.S., and Jacobsen, S.E.W. (2007). Cell Stem Cell 1, this issue, 671–684. Sitnicka, E., Lin, N., Priestley, G.V., Fox, N., Broudy, V.C., Wolf, N.S., and Kaushansky, K. (1996). Blood 87, 4998–5005. Stier, S., Ko, Y., Forkert, R., Lutz, C., Neuhaus, T., Gru¨newald, E., Cheng, T., Dombkowski, D., Calvi, L.M., Rittling, S.R., and Scadden, D.T. (2005). J. Exp. Med. 201, 1781–1791. Taichman, R.S., Reilly, M.J., Verma, R.S., and Emerson, S.G. (2007). Blood 89, 1165–1172. Yoshihara, H., Arai, F., Hosokawa, K., Hagiwara, T., Takubo, K., Nakamura, Y., Gomei, Y., Iwasaki, H., Matsuoka, S., Miyamoto, K., et al. (2007). Cell Stem Cell 1, this issue, 685– 697.
New Tools for Genome Modification in Human Embryonic Stem Cells Leon M. Ptaszek1,2,3,* and Chad A. Cowan2,3 1Cardiology
Division, Massachusetts General Hospital, 55 Fruit Street, Boston, MA 02114, USA Stem Cell Institute, 42 Church Street, Cambridge, MA 02138, USA 3Stowers Medical Institute, Center for Regenerative Medicine, Cardiovascular Research Center, Massachusetts General Hospital, 185 Cambridge Street, CPZN-4234, Boston, MA 02114, USA *Correspondence: [email protected] DOI 10.1016/j.stem.2007.11.004 2Harvard
Two recent papers outline improvements in gene editing technology that may facilitate the analysis of signaling networks important for development. The systems developed by both Thyagarajan and coworkers (Thyagarajan et al., 2007) in Nature Biotechnology, and Lombardo and colleagues (Lombardo et al., 2007) in Stem Cells, have the potential to advance our understanding of human embryonic stem cells. The introduction of human embryonic stem cell technology into the scientific mainstream in 1998 brought with it the possibility of cell-based therapy for
conditions not treatable with available pharmaceutical agents (Thomson et al., 1998). Yet, 10 years on, the stem cell field does not appear poised
600 Cell Stem Cell 1, December 2007 ª2007 Elsevier Inc.
to produce a therapeutic agent. How is it that we, as a field, are still so far from our mark? One explanation may be our incomplete knowledge of the forces
Previews Thrombopoietin, HSCs, and the Osteoblast Niche: Holding On Loosely, but Not Letting G0 Stephen G. Emerson1,* 1Haverford College, Haverford, PA 19041, USA *Correspondence: [email protected] DOI 10.1016/j.stem.2007.11.010
New findings in this issue of Cell Stem Cell by Qian et al. (2007) and Yoshihara et al. (2007) reveal that thrombopoietin modulates hematopoietic stem cell (HSC) cell-cycle progression at the osteoblast surface, linking a single cytokine with a specific postnatal niche cell. These observations indicate that simultaneous stimulation and suspension in a G0 state are critical for maintenance of the HSC pool. Clinical observations over the past 40 years in patients undergoing hematopoietic stem cell (HSC) transplants have suggested a close relationship between thrombopoiesis and stem cell activity. Delayed or inadequate platelet recoveries are the most sensitive indicators of inadequate numbers of transplanted stem cells, and thrombocytopenia is the first sign of graft failure (Ninan et al., 2007). When thrombopoietin (THPO) was first cloned by its binding to the Mpl receptor and characterized by its essential role in stimulating thrombopoiesis, it was thus no surprise that THPO was also found to have a key role in HSC maintenance and expansion (Sitnicka et al., 1996). Purified recombinant THPO itself was found to synergize with other proliferative cytokines to support HSC proliferation in vitro. Because THPO, like other cytokines such as SCF and IL-3, activates MAPK, AKT, and STAT pathways, such interactions might suggest overlapping, redundant functions (Dorsch et al., 1995). However, the finding that Mpl/ mice are stem cell deficient suggested that THPO might have an important and unique role not shared by other cytokines (Alexander et al., 1996). In this issue of Cell Stem Cell, Qian et al. (2007) begin by measuring HSC numbers both before and throughout life in Thpo/ versus normal mice. They find that postnatal HSCs are normal, both in number (as measured by cell surface phenotype) and in potency following stem cell transplantation. However, HSC numbers begin to decline within a few weeks after birth
and continue to do so throughout life, resulting in a 1503 reduction. They go on to show that Thpo/ HSCs show increased cycling, and reduced levels of the CDKIs p57kip2 and p19ink4D. These data indicate that postnatal HSCs, unlike fetal HSCs, are THPO dependent for their survival and maintenance. Again, this effect was observed both at baseline, as measured by cell surface phenotype, and functionally following HSC transplantation. In addition, they found that this THPO effect on HSC survival and maintenance is not Bcl-2 dependent. Although the authors do not speculate on what distinguishes prenatal from postnatal HSC THPO dependence, one clear difference between hematopoiesis before and after birth is the transition of HSCs to the bone marrow microenvironment. One might therefore hypothesize that if THPO has a unique postnatal role in the life of HSCs, then this role would be expressed directly in the topography of the bone marrow microenvironment, through interaction with the Mpl receptor on unique BM niche cells. Yoshihara et al. (2007) now present data supporting precisely this notion, that the THPO-Mpl axis is anatomically and functionally expressed between HSCs and the osteoblast surface in the bone marrow. In their precise analysis, they first show that long-term repopulating hematopoietic stem cells (LT-HSCs) that express Mpl are largely in G0 and are located in apposition to THPO+ osteoblasts. They then show that THPO treatment increases HSC expression of CDKIs and increases the entry of HSCs into G0 prior to their
proliferation. In contrast, treatment with neutralizing anti-Mpl antibody blocks restraint of in vivo HSCs in G0, and functionally releases them from their microniches, allowing their displacement by transplanted HSCs. Their results therefore support a key role for THPO in supporting HSC quiescence and further allude to key roles for osteoblasts in providing these signals in vivo. These two findings provide tentative answers to several important questions in HSC biology but leave other key issues still open to exploration. On the plus side of the ledger, these results provide the first molecular answer to what is the key HSC signal provided in the BM microenvironment, and where. Similarly, if THPO is the key HSC retentive signal provided by osteoblasts, this might explain why deletion of osteoblasts in vivo does not result in immediate loss of HSCs, but rather HSC redistribution out of the marrow, with HSC loss only occurring slowly. Still mysterious, however, is that efforts to date to culture and expand HSCs in vivo without loss of HSC potency, even with THPO, have met with limited success. Could there be other signals produced by osteoblasts, or even other cells within the BM niche, that are also required for HSC maintenance and proliferation? One surprising finding is that adherence to the niche seems to be multifactorial, or at least extremely delicate. Czechowicz et al. have recently found that HSCs can be functionally displaced from their niches by in vivo treatment with anti-c-kit antibody (Czechowicz et al., 2007). Osteopontin
Cell Stem Cell 1, December 2007 ª2007 Elsevier Inc. 599
Cell Stem Cell
Previews also appears to play a key role in HSC homing and adhesion to the niche (Stier et al., 2005), while blockade of CD44 appears to block adhesion of leukemic stem cells to the bone marrow microenvironment (Jin et al., 2006). Given the complex function of the bone marrow stem cell niche, could these critical adhesive and functional signals from the niche cells be induced only by the retrograde secretion or local presentation of specific molecules from HSCs? I.e., might HSCs condition their niche (Taichman et al., 2007)? Similarly, given these data, why are prenatal HSCs THPO independent? Are homologous signals, which activate similar survival pathways in HSCs, secreted by embryonic and fetal liver fibroblasts, or are fetal HSCs fundamentally different from postnatal HSCs? Similarly, one must ask whether similar paradigms apply to stem cells for other tissues, in other organ-specific niches? And of course, at a molecular level, why is maintenance of a G0 pool required for longterm support of HSC numbers? Finally, it is always worthwhile to attempt to draw tentative hypotheses
from elegant studies such as these into the world of clinical hematology. Would the provision of the proper pharmacologic concentration of THPO actually accelerate or quantitatively enhance HSC engraftment after transplantation? Conversely, would provision of anti-THPO antibody to patients with acute leukemia release their leukemic HSCs from the protection of G0 retention, thus rendering them more susceptible to anti-leukemic ablative therapies? Such queries could be directly tested in current immunodeficient mouse models, with the potential for decisive clinical application. In summary, Qian et al. and Yoshihara et al. present elegant and compelling new data demonstrating a key role for THPO in the maintenance of the long-term repopulating, quiescent HSC pool and furthermore suggest that provision of THPO is at least one of the unique contributions of osteoblasts after the transition of hematopoiesis from fetal liver to the bone marrow. Clearly osteoblasts have paid close attention to their musical roots and ‘‘hold on loosely, but don’t let G0.’’
REFERENCES Alexander, W.S., Roberts, A.W., Nicola, N.A., Li, R., and Metcalf, D. (1996). Blood 87, 2162–2170. Czechowicz, A., Kraft, D., Weissman, I.L., and Bhattacharya, D. (2007). Science 318, 1296– 1299. Dorsch, M., Fan, P.D., Bogenberger, J., and Goff, S.P. (1995). Biochem. Biophys. Res. Commun. 214, 424–431. Jin, L., Hope, K.J., Zhai, Q., Smadja-Joffe, F., and Dick, J.E. (2006). Nat. Med. 12, 1167–1174. Ninan, M.J., Flowers, C.R., Roback, J.D., Arellano, M.L., and Waller, E.K. (2007). Biol. Blood Marrow Transplant. 13, 895–904. Qian, H., Buza-Vidas, N., Hyland, C.D., Jensen, C.T., Antonchuk, J., Ma˚nsson, R., Thoren, L.A., Ekblom, M., Alexander, W.S., and Jacobsen, S.E.W. (2007). Cell Stem Cell 1, this issue, 671–684. Sitnicka, E., Lin, N., Priestley, G.V., Fox, N., Broudy, V.C., Wolf, N.S., and Kaushansky, K. (1996). Blood 87, 4998–5005. Stier, S., Ko, Y., Forkert, R., Lutz, C., Neuhaus, T., Gru¨newald, E., Cheng, T., Dombkowski, D., Calvi, L.M., Rittling, S.R., and Scadden, D.T. (2005). J. Exp. Med. 201, 1781–1791. Taichman, R.S., Reilly, M.J., Verma, R.S., and Emerson, S.G. (2007). Blood 89, 1165–1172. Yoshihara, H., Arai, F., Hosokawa, K., Hagiwara, T., Takubo, K., Nakamura, Y., Gomei, Y., Iwasaki, H., Matsuoka, S., Miyamoto, K., et al. (2007). Cell Stem Cell 1, this issue, 685– 697.
New Tools for Genome Modification in Human Embryonic Stem Cells Leon M. Ptaszek1,2,3,* and Chad A. Cowan2,3 1Cardiology
Division, Massachusetts General Hospital, 55 Fruit Street, Boston, MA 02114, USA Stem Cell Institute, 42 Church Street, Cambridge, MA 02138, USA 3Stowers Medical Institute, Center for Regenerative Medicine, Cardiovascular Research Center, Massachusetts General Hospital, 185 Cambridge Street, CPZN-4234, Boston, MA 02114, USA *Correspondence: [email protected] DOI 10.1016/j.stem.2007.11.004 2Harvard
Two recent papers outline improvements in gene editing technology that may facilitate the analysis of signaling networks important for development. The systems developed by both Thyagarajan and coworkers (Thyagarajan et al., 2007) in Nature Biotechnology, and Lombardo and colleagues (Lombardo et al., 2007) in Stem Cells, have the potential to advance our understanding of human embryonic stem cells. The introduction of human embryonic stem cell technology into the scientific mainstream in 1998 brought with it the possibility of cell-based therapy for
conditions not treatable with available pharmaceutical agents (Thomson et al., 1998). Yet, 10 years on, the stem cell field does not appear poised
600 Cell Stem Cell 1, December 2007 ª2007 Elsevier Inc.
to produce a therapeutic agent. How is it that we, as a field, are still so far from our mark? One explanation may be our incomplete knowledge of the forces