news and views
Rejuvenating aged muscle stem cells C Florian Bentzinger & Michael A Rudnicki
npg
© 2014 Nature America, Inc. All rights reserved.
Decreased muscle stem cell function in aging has long been shown to depend on altered environmental cues, whereas the contribution of intrinsic mechanisms remained less clear. Two new studies now reveal that cell-autonomous changes in the p38 mitogen-activated protein kinase pathway are major phenotype determinants of aged muscles stem cells (pages 255–271). Aging is accompanied by a decline in neuromuscular performance, skeletal muscle mass and stem cell function1,2. Weakened muscles substantially increase the chance of injury, such as by falling. Because of the reduced regenerative capacity of aged tissues, recovery from such incidents and from subsequent surgical procedures is slow and possibly incomplete. Moreover, the inactivity associated with bed rest further accelerates muscle catabolism. These problems can lead to a vicious cycle of muscle loss, injury and inefficient repair, causing elderly patients to become increasingly sedentary over time. Thus, therapeutic strategies boosting muscle mass and regeneration in the aged are currently sought after. Satellite cells, the quintessential stem cells of skeletal muscle, have the capability to self-renew through asymmetric division, maintaining an undifferentiated mother cell and providing committed progeny for differentiation3. Owing to these mechanisms, healthy skeletal muscle can go through several rounds of injury and repair while maintaining a viable satellite cell pool. In contrast, it has been shown that, as a consequence of aging, the proliferative capacity and the myogenic differentiation potential of satellite cells is impaired2–4. Consequently, aged muscle tissue fails to regenerate successfully after injury. Evidence suggests that aging leads
C. Florian Bentzinger and Michael A. Rudnicki are in the Regenerative Medicine Program, Ottawa Hospital Research Institute, Ottawa, Ontario, Canada, and in the Faculty of Medicine, Department of Cellular and Molecular Medicine, University of Ottawa, Ottawa, Ontario, Canada. Michael A. Rudnicki is also at the Sprott Center for Stem Cell Research, Ottawa Hospital Research Institute, Ottawa, Ontario, Canada. e-mail: [email protected]
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to changes in the stem cell niche that negatively influence satellite cells2–4, including alterations in Wnt, Notch, transforming growth factor-β and fibroblast growth factor-2 (FGF-2) signals. Moreover, classic parabiosis experiments have shown that circulating systemic factors from young mice can restore muscle regeneration in aged animals5,6. In this issue of Nature Medicine, Bernet et al.7 and Cosgrove et al.8 show that a cell-autonomous increase in the activity of the p38 mitogenactivated protein kinase pathway leads to an impairment of the self-renewal capacity of aged muscle stem cells that cannot be overcome by a young environment, as even after transplantation into young hosts, the maintenance of aged satellite cells was reduced. Notably, transient pharmacologic inhibition of or knockdown of p38α and p38β (p38α/β) by siRNA was able to restore the maintenance of aged satellite cells after transplantation. To date, evidence for intrinsic age-associated changes in satellite cells is sparse. Shortening of telomeres, DNA damage and changes in Notch signal transduction are cell-autonomous mechanisms that have been proposed to underlie impaired satellite cell function during aging2–4. Recently, signals in the aged niche were shown to cause permanent defects in the ability of satellite cells to return to quiescence9, ultimately also impairing the maintenance of self-renewing satellite cells. However, whether aging is accompanied by defects in intrinsic molecular processes that are directly involved in satellite cell self-renewal remained unclear. Previous work by the Olwin group showed that p38 regulates asymmetric self-renewal of satellite cells10. During asymmetric satellite cell division, the Par complex controls the localization of p38α/β in only one daughter cell, inducing commitment to myogenic differentiation. In their current study, Bernet et al.7
showed that the increased activation of p38 in aged satellite cells leads to a loss of its asymmetric localization (Fig. 1), which results in an increased number of committed daughter cells at the expense of self-renewal. Pharmacologic reduction of p38α/β signaling rescues its asymmetric localization and restores self-renewal in aged satellite cells. Interestingly, FGF-2, a key regulatory niche signal promoting satellite cell expansion, has previously been shown to activate p38α/β (ref. 11). Using phosphospecific antibodies, Bernet et al.7 found that FGF-2 fails to activate the FGF receptor-1 (FGFR1) and downstream p38 signaling in aged satellite cells. Notably, reactivation of FGF signaling by expression of a constitutively active FGFR1 restored asymmetric distribution of p38 and enhanced self-renewal of aged satellite cells. Therefore, desensitization of the FGFR1 may be an important upstream mechanism involved in the deregulation of p38 signaling in aged satellite cells and provides an additional potential therapeutic target. Pharmacologic inhibition of the p38 cascade has been explored as a therapeutic strategy, mostly because of its role in regulating inflammatory responses12. Inhibitors targeting the p38 pathway are being evaluated in clinical trials for rheumatoid arthritis, chronic obstructive pulmonary disease, cardiovascular disease, pain, psoriasis, Crohn’s disease, atherosclerosis and cancer. Most direct p38 inhibitors investigated as therapeutics for these conditions target primarily p38α; however, Bernet et al.7 and Cosgrove et al.8 used agents, namely SB203580 and SB202190, that potently inhibit both p38α and p38β (ref. 13). Systemic administration of inhibitors for both p38α and p38β will increase the chance for side effects in tissues other than the skeletal muscle. This might be particularly problematic because adverse drug reactions are
volume 20 | number 3 | march 2014 nature medicine
news and views a
Return to quiescence
Young satellite cell FGFR1
Injury
Self-renewed satellite cell
FGF-2 ↑p38α/β
Polarization
Young muscle fiber
Differentiation
Committed satellite cells and myoblasts New muscle fibers
b
Aged satellite cell ↑p38α/β ↑FGF-2
Injury
Differentiation ↑p38α/β
npg
© 2014 Nature America, Inc. All rights reserved.
Aged muscle fiber
Loss of polarity
Inefficient muscle repair
Figure 1 Increased p38 signaling impairs self-renewal of aged satellite cells. (a) After injury, young satellite cells begin to proliferate. During asymmetric self-renewal, polarized activation of the FGFR1 leads to an induction of p38 signaling and to commitment to differentiation of the daughter cell. While the self-renewed mother cell becomes quiescent, the committed satellite cell transiently amplifies and generates myoblasts that can differentiate and fuse into new muscle fibers. (b) Aged satellite cells are characterized by intrinsically elevated p38 signaling. Higher levels of FGF-2 in the aged niche go along with a desensitization of the FGFR1 in aged satellite cells. After injury, the desensitized FGFR1 fails to establish polarity and restrict p38 signaling to one cell. As a consequence, asymmetric self-renewal is impaired and an increased number of cells become committed to differentiation. Owing to the decreased proliferation and differentiation potential of aged myogenic progenitors, muscle repair is slow and possibly incomplete. The loss of self-renewed satellite cells further reduces the regenerative capacity of the aged muscle tissue.
more common in elderly patients, the target population for a potential future treatment. Circumventing the possible problems associated with systemic application of p38α/β inhibitors, the Blau laboratory developed a cell therapy–oriented strategy using an ex vivo treatment of satellite cells before transplantation into aged muscles8. The authors took advantage of a previously established soft hydrogel system that simulates the physical properties of the muscle stem cell niche and reduces the premature commitment of satellite cells observed in conventional culture14. As demonstrated by serial transplantation, culture of isolated aged satellite cells on soft hydrogels in conjunction with pharmacological p38 inhibition profoundly enhanced the self-renewal capacity of satellite cells. Strikingly, transplantation of aged satellite cells that had undergone this treatment significantly improved the force generation of aged muscle. This approach thus provides an innovative paradigm for autologous stem cell therapy in the elderly. Stem cell aging depends on a combination of extrinsic and an intrinsic mechanisms2. It is conceivable that a purely intrinsic ‘cellular clock’ of aging is coupled to mitosis and metabolic activity; however, in tissues such as
skeletal muscle, stem cells are thought to be in a quiescent state for most of the organism’s lifetime2,3. This raises the question of how intrinsic aging is triggered in these stem cell types. It is also important to keep in mind that altered environmental factors in the aging niche may cause permanent changes in stem cells that manifest as ‘intrinsic’ defects. Therefore, only antiaging strategies taking both factors—the stem cell niche and the stem cells per se—into consideration may ultimately be successful. Accordingly, the desensitization of the FGFR1 observed by Bernet et al.7 in aged satellite cells is probably connected to the upregulation of FGF-2 that has been reported in aged skeletal muscle tissue9. Moreover, Cosgrove et al.8 showed synergistic positive effects on aged satellite cells when they simulated the environmental conditions of the niche using soft hydrogels in combination with inhibiting the intrinsic upregulation of p38 signaling. Thus, both studies provide examples of how explorations into the nature of intrinsic stem cell aging along with an advanced understanding of the stem cell niche can lead to the discovery of new therapeutic approaches. With human longevity steadily increasing, strategies for the maintenance of muscle
nature medicine volume 20 | number 3 | march 2014
mass and its regenerative potential have great relevance for society and economy. The p38 pathway has already been proven to be well suited for pharmacological targeting12, and it is hoped that this approach can ultimately be exploited for enhancing satellite cell function in the elderly population. COMPETING FINANCIAL INTERESTS The authors declare no competing financial interests. 1. Power, G.A., Dalton, B.H. & Rice, C.L. J. Sport Health Sci. 2, 215–226 (2013). 2. Liu, L. & Rando, T.A. J. Cell Biol. 193, 257–266 (2011). 3. Yin, H., Price, F. & Rudnicki, M.A. Physiol. Rev. 93, 23–67 (2013). 4. García-Prat, L., Sousa-Victor, P. & Munoz-Canoves, P. FEBS J. 280, 4051–4062 (2013). 5. Conboy, I.M. et al. Nature 433, 760–764 (2005). 6. Brack, A.S. et al. Science 317, 807–810 (2007). 7. Bernet, J.D. et al. Nat. Med. 20, 265–271 (2014). 8. Cosgrove, B.D. et al. Nat. Med. 20, 255–264 (2014). 9. Chakkalakal, J.V., Jones, K.M., Basson, M.A. & Brack, A.S. Nature 490, 355–360 (2012). 10. Troy, A. et al. Cell Stem Cell 11, 541–553 (2012). 11. Jones, N.C. et al. J. Cell Biol. 169, 105–116 (2005). 12. Arthur, J.S. & Ley, S.C. Nat. Rev. Immunol. 13, 679–692 (2013). 13. Bain, J. et al. Biochem. J. 408, 297–315 (2007). 14. Gilbert, P.M. et al. Science 329, 1078–1081 (2010).
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Rejuvenating aged muscle stem cells C Florian Bentzinger & Michael A Rudnicki
npg
© 2014 Nature America, Inc. All rights reserved.
Decreased muscle stem cell function in aging has long been shown to depend on altered environmental cues, whereas the contribution of intrinsic mechanisms remained less clear. Two new studies now reveal that cell-autonomous changes in the p38 mitogen-activated protein kinase pathway are major phenotype determinants of aged muscles stem cells (pages 255–271). Aging is accompanied by a decline in neuromuscular performance, skeletal muscle mass and stem cell function1,2. Weakened muscles substantially increase the chance of injury, such as by falling. Because of the reduced regenerative capacity of aged tissues, recovery from such incidents and from subsequent surgical procedures is slow and possibly incomplete. Moreover, the inactivity associated with bed rest further accelerates muscle catabolism. These problems can lead to a vicious cycle of muscle loss, injury and inefficient repair, causing elderly patients to become increasingly sedentary over time. Thus, therapeutic strategies boosting muscle mass and regeneration in the aged are currently sought after. Satellite cells, the quintessential stem cells of skeletal muscle, have the capability to self-renew through asymmetric division, maintaining an undifferentiated mother cell and providing committed progeny for differentiation3. Owing to these mechanisms, healthy skeletal muscle can go through several rounds of injury and repair while maintaining a viable satellite cell pool. In contrast, it has been shown that, as a consequence of aging, the proliferative capacity and the myogenic differentiation potential of satellite cells is impaired2–4. Consequently, aged muscle tissue fails to regenerate successfully after injury. Evidence suggests that aging leads
C. Florian Bentzinger and Michael A. Rudnicki are in the Regenerative Medicine Program, Ottawa Hospital Research Institute, Ottawa, Ontario, Canada, and in the Faculty of Medicine, Department of Cellular and Molecular Medicine, University of Ottawa, Ottawa, Ontario, Canada. Michael A. Rudnicki is also at the Sprott Center for Stem Cell Research, Ottawa Hospital Research Institute, Ottawa, Ontario, Canada. e-mail: [email protected]
234
to changes in the stem cell niche that negatively influence satellite cells2–4, including alterations in Wnt, Notch, transforming growth factor-β and fibroblast growth factor-2 (FGF-2) signals. Moreover, classic parabiosis experiments have shown that circulating systemic factors from young mice can restore muscle regeneration in aged animals5,6. In this issue of Nature Medicine, Bernet et al.7 and Cosgrove et al.8 show that a cell-autonomous increase in the activity of the p38 mitogenactivated protein kinase pathway leads to an impairment of the self-renewal capacity of aged muscle stem cells that cannot be overcome by a young environment, as even after transplantation into young hosts, the maintenance of aged satellite cells was reduced. Notably, transient pharmacologic inhibition of or knockdown of p38α and p38β (p38α/β) by siRNA was able to restore the maintenance of aged satellite cells after transplantation. To date, evidence for intrinsic age-associated changes in satellite cells is sparse. Shortening of telomeres, DNA damage and changes in Notch signal transduction are cell-autonomous mechanisms that have been proposed to underlie impaired satellite cell function during aging2–4. Recently, signals in the aged niche were shown to cause permanent defects in the ability of satellite cells to return to quiescence9, ultimately also impairing the maintenance of self-renewing satellite cells. However, whether aging is accompanied by defects in intrinsic molecular processes that are directly involved in satellite cell self-renewal remained unclear. Previous work by the Olwin group showed that p38 regulates asymmetric self-renewal of satellite cells10. During asymmetric satellite cell division, the Par complex controls the localization of p38α/β in only one daughter cell, inducing commitment to myogenic differentiation. In their current study, Bernet et al.7
showed that the increased activation of p38 in aged satellite cells leads to a loss of its asymmetric localization (Fig. 1), which results in an increased number of committed daughter cells at the expense of self-renewal. Pharmacologic reduction of p38α/β signaling rescues its asymmetric localization and restores self-renewal in aged satellite cells. Interestingly, FGF-2, a key regulatory niche signal promoting satellite cell expansion, has previously been shown to activate p38α/β (ref. 11). Using phosphospecific antibodies, Bernet et al.7 found that FGF-2 fails to activate the FGF receptor-1 (FGFR1) and downstream p38 signaling in aged satellite cells. Notably, reactivation of FGF signaling by expression of a constitutively active FGFR1 restored asymmetric distribution of p38 and enhanced self-renewal of aged satellite cells. Therefore, desensitization of the FGFR1 may be an important upstream mechanism involved in the deregulation of p38 signaling in aged satellite cells and provides an additional potential therapeutic target. Pharmacologic inhibition of the p38 cascade has been explored as a therapeutic strategy, mostly because of its role in regulating inflammatory responses12. Inhibitors targeting the p38 pathway are being evaluated in clinical trials for rheumatoid arthritis, chronic obstructive pulmonary disease, cardiovascular disease, pain, psoriasis, Crohn’s disease, atherosclerosis and cancer. Most direct p38 inhibitors investigated as therapeutics for these conditions target primarily p38α; however, Bernet et al.7 and Cosgrove et al.8 used agents, namely SB203580 and SB202190, that potently inhibit both p38α and p38β (ref. 13). Systemic administration of inhibitors for both p38α and p38β will increase the chance for side effects in tissues other than the skeletal muscle. This might be particularly problematic because adverse drug reactions are
volume 20 | number 3 | march 2014 nature medicine
news and views a
Return to quiescence
Young satellite cell FGFR1
Injury
Self-renewed satellite cell
FGF-2 ↑p38α/β
Polarization
Young muscle fiber
Differentiation
Committed satellite cells and myoblasts New muscle fibers
b
Aged satellite cell ↑p38α/β ↑FGF-2
Injury
Differentiation ↑p38α/β
npg
© 2014 Nature America, Inc. All rights reserved.
Aged muscle fiber
Loss of polarity
Inefficient muscle repair
Figure 1 Increased p38 signaling impairs self-renewal of aged satellite cells. (a) After injury, young satellite cells begin to proliferate. During asymmetric self-renewal, polarized activation of the FGFR1 leads to an induction of p38 signaling and to commitment to differentiation of the daughter cell. While the self-renewed mother cell becomes quiescent, the committed satellite cell transiently amplifies and generates myoblasts that can differentiate and fuse into new muscle fibers. (b) Aged satellite cells are characterized by intrinsically elevated p38 signaling. Higher levels of FGF-2 in the aged niche go along with a desensitization of the FGFR1 in aged satellite cells. After injury, the desensitized FGFR1 fails to establish polarity and restrict p38 signaling to one cell. As a consequence, asymmetric self-renewal is impaired and an increased number of cells become committed to differentiation. Owing to the decreased proliferation and differentiation potential of aged myogenic progenitors, muscle repair is slow and possibly incomplete. The loss of self-renewed satellite cells further reduces the regenerative capacity of the aged muscle tissue.
more common in elderly patients, the target population for a potential future treatment. Circumventing the possible problems associated with systemic application of p38α/β inhibitors, the Blau laboratory developed a cell therapy–oriented strategy using an ex vivo treatment of satellite cells before transplantation into aged muscles8. The authors took advantage of a previously established soft hydrogel system that simulates the physical properties of the muscle stem cell niche and reduces the premature commitment of satellite cells observed in conventional culture14. As demonstrated by serial transplantation, culture of isolated aged satellite cells on soft hydrogels in conjunction with pharmacological p38 inhibition profoundly enhanced the self-renewal capacity of satellite cells. Strikingly, transplantation of aged satellite cells that had undergone this treatment significantly improved the force generation of aged muscle. This approach thus provides an innovative paradigm for autologous stem cell therapy in the elderly. Stem cell aging depends on a combination of extrinsic and an intrinsic mechanisms2. It is conceivable that a purely intrinsic ‘cellular clock’ of aging is coupled to mitosis and metabolic activity; however, in tissues such as
skeletal muscle, stem cells are thought to be in a quiescent state for most of the organism’s lifetime2,3. This raises the question of how intrinsic aging is triggered in these stem cell types. It is also important to keep in mind that altered environmental factors in the aging niche may cause permanent changes in stem cells that manifest as ‘intrinsic’ defects. Therefore, only antiaging strategies taking both factors—the stem cell niche and the stem cells per se—into consideration may ultimately be successful. Accordingly, the desensitization of the FGFR1 observed by Bernet et al.7 in aged satellite cells is probably connected to the upregulation of FGF-2 that has been reported in aged skeletal muscle tissue9. Moreover, Cosgrove et al.8 showed synergistic positive effects on aged satellite cells when they simulated the environmental conditions of the niche using soft hydrogels in combination with inhibiting the intrinsic upregulation of p38 signaling. Thus, both studies provide examples of how explorations into the nature of intrinsic stem cell aging along with an advanced understanding of the stem cell niche can lead to the discovery of new therapeutic approaches. With human longevity steadily increasing, strategies for the maintenance of muscle
nature medicine volume 20 | number 3 | march 2014
mass and its regenerative potential have great relevance for society and economy. The p38 pathway has already been proven to be well suited for pharmacological targeting12, and it is hoped that this approach can ultimately be exploited for enhancing satellite cell function in the elderly population. COMPETING FINANCIAL INTERESTS The authors declare no competing financial interests. 1. Power, G.A., Dalton, B.H. & Rice, C.L. J. Sport Health Sci. 2, 215–226 (2013). 2. Liu, L. & Rando, T.A. J. Cell Biol. 193, 257–266 (2011). 3. Yin, H., Price, F. & Rudnicki, M.A. Physiol. Rev. 93, 23–67 (2013). 4. García-Prat, L., Sousa-Victor, P. & Munoz-Canoves, P. FEBS J. 280, 4051–4062 (2013). 5. Conboy, I.M. et al. Nature 433, 760–764 (2005). 6. Brack, A.S. et al. Science 317, 807–810 (2007). 7. Bernet, J.D. et al. Nat. Med. 20, 265–271 (2014). 8. Cosgrove, B.D. et al. Nat. Med. 20, 255–264 (2014). 9. Chakkalakal, J.V., Jones, K.M., Basson, M.A. & Brack, A.S. Nature 490, 355–360 (2012). 10. Troy, A. et al. Cell Stem Cell 11, 541–553 (2012). 11. Jones, N.C. et al. J. Cell Biol. 169, 105–116 (2005). 12. Arthur, J.S. & Ley, S.C. Nat. Rev. Immunol. 13, 679–692 (2013). 13. Bain, J. et al. Biochem. J. 408, 297–315 (2007). 14. Gilbert, P.M. et al. Science 329, 1078–1081 (2010).
235
