news and views
Mst-1 switches between cardiac cell life and death Rimpy Dhingra & Lorrie A Kirshenbaum
Arguably, one of the most intriguing and compelling aspects of contemporary biology is the concept of autophagy, which translates from the Greek words auto (self) and phagein (to eat), meaning self-eating1,2. Autophagy is an ancient catabolic process by which cells recycle macromolecular proteins and damaged organelles (such as mitochondria) by an elaborate autophagolysosomal pathway. Autophagy is rapidly induced during nutrient deprivation and ischemia, presumably as an initial survival mechanism to subvert the accumulation of lethal protein aggregates and damaged organelles. However, autophagy beyond a certain threshold is maladaptive and incompatible with life. The fact that autophagy is regulated and potentially amenable to genetic manipulations holds promise for treating human diseases in which autophagy is the primary underlying defect. Indeed, autophagy has been associated with several human pathologies including cancer, neurodegenerative diseases and, more recently, heart disease3–5. Despite an increasing awareness of autophagy in different human diseases, the exact role of autophagy as an adaptive or maladaptive process has not been resolved, and even less is known about the mechanisms that regulate this process under normal or disease conditions. Hence, the differential outcomes of autophagy with regards to cell life or death may reflect the temporal activation of downstream effectors in a cell- and context-specific manner. For example, in the Rimpy Dhingra is at The Institute of Cardiovascular Sciences, St. Boniface Hospital Research Centre, Department of Physiology, University of Manitoba, Winnipeg, Manitoba, Canada. Lorrie A. Kirshenbaum is at The Institute of Cardiovascular Sciences, St. Boniface Hospital Research Centre, Departments of Physiology and Pharmacology and Therapeutics, Faculty of Medicine, University of Manitoba, Winnipeg, Manitoba, Canada. e-mail: [email protected]
context of the heart, defects in autophagy that result in the accumulation of cytotoxic protein aggregates have been postulated as an underlying cause of ventricular remodeling and decline in ventricular performance in patients with heart failure6. Furthermore, during ischemiareperfusion injury, activation of autophagy upon early ischemia is protective, whereas late or delayed activation of autophagy during reperfusion is detrimental7. This paradox of too little or too much autophagy as beneficial or detrimental is analogous in many ways to the fairy tale “Goldilocks and the Three Bears,” in which Goldilocks tries in succession each of the three bears’ porridge, chairs and beds until she finds one of each that is “just right.” Applying the principles of this fable to autophagy highlights the intricacies of its regulation and finding the right balance. At present the mechanisms that underlie autophagy in the heart remain unknown. It is also undetermined whether autophagy uses cellular effectors that are distinct or common Normal
to other cellular processes that regulate cell fate such as programmed cell death or apoptosis. Apoptosis is another ancient, highly conserved cellular process that is crucial for many aspects of normal life, including embryonic development and tissue homeostasis8. Apoptosis is activated by an extrinsic pathway involving cytokines and death receptors as well as an intrinsic pathway involving the BCL-2 family of proteins and mitochondrion. BCL-2, which was first discovered through a translocation breakpoint mutation in human B cell lymphomas, is the archetypical member of a large family of multidomain proteins that promote or prevent cell death on the basis of their BCL-2 homology (BH) domain structure. The ability of a given BCL-2 protein to avert or promote apoptosis is attributed to its capacity to form homo- or heterotypic interactions with other BCL-2 members or cellular proteins, respectively. Proapoptotic proteins such as BAX and BAK provoke apoptosis by inducing mitochondrial perturbations
Stress
Bcl-2
Beclin1 Atg14L
Bax
p150 Vps34
Bax
Mst-1
P
Beclin1 Atg14L
Bcl-2
Beclin1
P
p150 Vps34
Cyt c Caspase activation
Apoptosis
Autophagy
Apoptosis
Aggresome Katie Vicari
npg
© 2013 Nature America, Inc. All rights reserved.
Autophagy and apoptosis are ancient processes that regulate cell fate under normal and disease conditions. A new study in mice identifies mammalian Ste20-like kinase-1 (Mst-1) as a missing link that interfaces between these pathways for cell survival and death during cardiac stress (pages 1478–1488).
Autophagy
Figure 1 Dual regulation of autophagy and apoptosis by Mst-1. Maejima et al.11 now provide evidence that Mst-1 acts as a switch between autophagy and apoptosis. Left, under basal conditions Beclin1 forms complexes with Atg14L, Vps34 and p150 to induce autophagy, whereas Bcl-2 binds and sequesters Bax, preventing activation of the intrinsic mitochondrial death pathway and apoptosis. Right, during cellular stress, Mst-1 becomes activated and phosphorylates Beclin1 at Thr108; this causes Beclin1 to dissociate from the Atg14L–Vps34 complex and bind Bcl-2, which then displaces Bax from Bcl-2. Formation of Bcl-2–Beclin1 complexes suppresses autophagy, resulting in the accumulation of protein aggregates. Unbound Bax translocates to mitochondria, and this triggers mitochondrial perturbations, leading to cytochrome c (cyt c) release, caspase activation and apoptosis.
nature medicine volume 19 | number 11 | NOVEMBER 2013
1367
npg
© 2013 Nature America, Inc. All rights reserved.
news and views associated with the intrinsic death pathway. In contrast, the antiapoptotic factors, including BCL-2 and the related protein BCL-xL, avert apoptosis by sequestering BAX and BAK, ostensibly preventing mitochondrial injury and cell death9. Several lines of investigation suggest that, in addition to regulating apoptosis, certain BCL-2 proteins, including Beclin1, also regulate vital cellular processes such as autophagy10. Although unproven, an appealing hypothesis has been that autophagy and apoptosis may be facets of the same signaling pathway, linked by a common cellular factor. In this issue of Nature Medicine, Maejima et al.11 provide compelling evidence that the cellular kinase Mst-1 acts as a molecular switch that selectively drives autophagy or apoptosis by preferentially altering the formation of Bcl-2–Beclin1 complexes. Thus, the authors suggest that Mst-1 is the missing link that bridges the elusive connection between apoptosis and autophagy. In their new work Maejima et al.11 showed that Mst-1 subverts autophagy and promotes apoptosis in the heart by a two-step process, involving the differential assembly of cellular complexes composed of Bcl-2–Beclin1 or Beclin1–Atg14L– Vps34 to drive autophagy or apoptosis, respectively. Using a combination of elegantly designed gain- and loss-of-function in vivo and in vitro experiments, the authors systematically dissected the molecular signaling pathways downstream of Mst-1. First, they demonstrated that activation of Mst-1 during ischemic stress in mouse hearts resulted in a marked reduction in autophagy and a corresponding increase in apoptosis. The authors then showed that Mst-1 strongly interacted with and phosphorylated Beclin1 at Thr108, which lies within the key BH3 domain of Beclin1 required for inducing autophagy. Phosphorylation of Thr108 substantially
impaired the ability of Beclin1 to interact with Atg14L–Vps34 and induce autophagy. Hence, this lack of Beclin1–Atg14L complexes in cardiac cells expressing Mst-1 provides the first direct evidence to explain how Mst-1 impairs autophagy in the heart during ischemic stress. Under basal nonapoptotic conditions, BCL-2 binds and sequesters BAX. Maejima et al.11 hypothesized that the increased cardiomyocyte apoptosis associated with Mst-1 activation may result from the loss of Bcl-2–Bax inhibitory complexes. Consistent with this theory, the authors found that phosphorylation of Beclin1 by Mst-1 at Thr108, the same residue that impaired autophagy, dramatically increased the binding affinity of Beclin1 for Bcl-2. Thus, Bax was displaced from Bcl-2, increasing the amounts of free cellular Bax and leading to increased apoptosis during ischemia (Fig. 1). Although displacement models have previously been proposed to explain the regulation of apoptosis and autophagy by BCL-2 proteins12, none to date have demonstrated the coupling of these pathways by a common signaling factor. The novelty of the work by Maejima et al.11 is the identification of Mst-1 as a factor that preferentially phosphorylates Beclin1 during cellular stress, so that Mst-1 acts as a switch to dually regulate apoptosis and autophagy. Loss of Beclin1–Atg14L–Vps34 complexes impairs autophagy, resulting in protein aggregation, whereas loss of Bcl-2–Bax complexes promotes apoptosis (Fig. 1). Although this study provides answers to several previously posed questions, it raises a number of other issues that should be carefully considered. First, despite the impairment of autophagy and increase in apoptosis induced by Mst-1 during ischemic injury, it remains unknown whether this signaling pathway is restricted to the heart or whether it also
applies to other human diseases associated with altered autophagy flux and apoptosis, such as neurodegenerative diseases and cancer. Second, it is unknown how Mst-1 activation occurs during cellular stress and whether it drives other forms of programmed cell death such as necrosis. For example, the authors showed that cells genetically ablated for Bax or Bak still die in the presence of Mst-1, suggesting that other factors may be involved in inducing apoptosis. Third, it is unknown whether Mst-1 alters the binding affinity of other BH3-domain-like proteins, such as Bak, Bim, PUMA, NOXA and Bnip3, to regulate apoptosis. Despite these unresolved questions, the findings of the study by Maejima et al.11 may provide an explanation for how cells mechanistically integrate autophagy and apoptosis during stress. In addition, this study highlights Mst-1 as a potential therapeutic target, and the selective modulation of Mst-1 activity may prove beneficial in balancing autophagy and apoptosis during cardiac injury. COMPETING FINANCIAL INTERESTS The authors declare no competing financial interests. 1. Mizushima, N., Levine, B., Cuervo, A.M. & Klionsky, D.J. Nature 451, 1069–1075 (2008). 2. Kaushik, S. & Cuervo, A.M. Pharmacol. Res. 66, 484–493 (2012). 3. Nixon, R.A. Nat. Med. 19, 983–997 (2013). 4. Levine, B., Mizushima, N. & Virgin, H.W. Nature 469, 323–335 (2011). 5. Rothermel, B.A. & Hill, J.A. Circ. Res. 103, 1363–1369 (2008). 6. Zhu, H. et al. J. Clin. Invest. 117, 1782–1793 (2007). 7. Sciarretta, S., Hariharan, N., Monden, Y., Zablocki, D. & Sadoshima, J. Pediatr. Cardiol. 32, 275–281 (2011). 8. Wyllie, A.H. Curr. Opin. Genet. Dev. 5, 97–104 (1995). 9. Korsmeyer, S.J., Shutter, J.R., Veis, D.J., Merry, D.E. & Oltvai, Z.N. Semin. Cancer Biol. 4, 327–332 (1993). 10. Levine, B., Sinha, S. & Kroemer, G. Autophagy 4, 600–606 (2008). 11. Maejima, Y. et al. Nat. Med. 19, 1478–1488 (2013). 12. He, C. et al. Nature 481, 511–515 (2012).
An endogenous factor mediates shock-induced injury Peter A Ward An endogenous molecule secreted by macrophages during stress is a new mediator for the wave of inflammation triggered during hemorrhagic and septic shock. This finding suggests a potential drug target to reduce organ injury and death after shock (pages 1489–1495). Release of tissue-damaging factors from infectious agents (such as bacteria, viruses or fungi) or from cells (for example, endothelial or Peter A. Ward is at the Department of Pathology, University of Michigan Medical School, Ann Arbor, Michigan, USA. e-mail: [email protected]
1368
epithelial) after hypoxia, hemorrhagic and septic shock, polytrauma, and other conditions induces release of proinflammatory mediators, the outcome of which is often tissue and organ damage1. Molecules released from infectious agents have been described as pathogenassociated molecular patterns (PAMPs), whereas danger-associated molecular patterns
(DAMPs) are those released from damaged tissues in the absence of infectious pathogens, also called ‘sterile inflammation’2–5. PAMPs and DAMPs seem to function via interactions with Toll-like receptor 2 (TLR2) and TLR4 (refs. 3,5), as well as with NOD-like receptors. To date, the only therapies available to patients with hemorrhagic or septic shock are supportive and
volume 19 | number 11 | NOVEMBER 2013 nature medicine
Mst-1 switches between cardiac cell life and death Rimpy Dhingra & Lorrie A Kirshenbaum
Arguably, one of the most intriguing and compelling aspects of contemporary biology is the concept of autophagy, which translates from the Greek words auto (self) and phagein (to eat), meaning self-eating1,2. Autophagy is an ancient catabolic process by which cells recycle macromolecular proteins and damaged organelles (such as mitochondria) by an elaborate autophagolysosomal pathway. Autophagy is rapidly induced during nutrient deprivation and ischemia, presumably as an initial survival mechanism to subvert the accumulation of lethal protein aggregates and damaged organelles. However, autophagy beyond a certain threshold is maladaptive and incompatible with life. The fact that autophagy is regulated and potentially amenable to genetic manipulations holds promise for treating human diseases in which autophagy is the primary underlying defect. Indeed, autophagy has been associated with several human pathologies including cancer, neurodegenerative diseases and, more recently, heart disease3–5. Despite an increasing awareness of autophagy in different human diseases, the exact role of autophagy as an adaptive or maladaptive process has not been resolved, and even less is known about the mechanisms that regulate this process under normal or disease conditions. Hence, the differential outcomes of autophagy with regards to cell life or death may reflect the temporal activation of downstream effectors in a cell- and context-specific manner. For example, in the Rimpy Dhingra is at The Institute of Cardiovascular Sciences, St. Boniface Hospital Research Centre, Department of Physiology, University of Manitoba, Winnipeg, Manitoba, Canada. Lorrie A. Kirshenbaum is at The Institute of Cardiovascular Sciences, St. Boniface Hospital Research Centre, Departments of Physiology and Pharmacology and Therapeutics, Faculty of Medicine, University of Manitoba, Winnipeg, Manitoba, Canada. e-mail: [email protected]
context of the heart, defects in autophagy that result in the accumulation of cytotoxic protein aggregates have been postulated as an underlying cause of ventricular remodeling and decline in ventricular performance in patients with heart failure6. Furthermore, during ischemiareperfusion injury, activation of autophagy upon early ischemia is protective, whereas late or delayed activation of autophagy during reperfusion is detrimental7. This paradox of too little or too much autophagy as beneficial or detrimental is analogous in many ways to the fairy tale “Goldilocks and the Three Bears,” in which Goldilocks tries in succession each of the three bears’ porridge, chairs and beds until she finds one of each that is “just right.” Applying the principles of this fable to autophagy highlights the intricacies of its regulation and finding the right balance. At present the mechanisms that underlie autophagy in the heart remain unknown. It is also undetermined whether autophagy uses cellular effectors that are distinct or common Normal
to other cellular processes that regulate cell fate such as programmed cell death or apoptosis. Apoptosis is another ancient, highly conserved cellular process that is crucial for many aspects of normal life, including embryonic development and tissue homeostasis8. Apoptosis is activated by an extrinsic pathway involving cytokines and death receptors as well as an intrinsic pathway involving the BCL-2 family of proteins and mitochondrion. BCL-2, which was first discovered through a translocation breakpoint mutation in human B cell lymphomas, is the archetypical member of a large family of multidomain proteins that promote or prevent cell death on the basis of their BCL-2 homology (BH) domain structure. The ability of a given BCL-2 protein to avert or promote apoptosis is attributed to its capacity to form homo- or heterotypic interactions with other BCL-2 members or cellular proteins, respectively. Proapoptotic proteins such as BAX and BAK provoke apoptosis by inducing mitochondrial perturbations
Stress
Bcl-2
Beclin1 Atg14L
Bax
p150 Vps34
Bax
Mst-1
P
Beclin1 Atg14L
Bcl-2
Beclin1
P
p150 Vps34
Cyt c Caspase activation
Apoptosis
Autophagy
Apoptosis
Aggresome Katie Vicari
npg
© 2013 Nature America, Inc. All rights reserved.
Autophagy and apoptosis are ancient processes that regulate cell fate under normal and disease conditions. A new study in mice identifies mammalian Ste20-like kinase-1 (Mst-1) as a missing link that interfaces between these pathways for cell survival and death during cardiac stress (pages 1478–1488).
Autophagy
Figure 1 Dual regulation of autophagy and apoptosis by Mst-1. Maejima et al.11 now provide evidence that Mst-1 acts as a switch between autophagy and apoptosis. Left, under basal conditions Beclin1 forms complexes with Atg14L, Vps34 and p150 to induce autophagy, whereas Bcl-2 binds and sequesters Bax, preventing activation of the intrinsic mitochondrial death pathway and apoptosis. Right, during cellular stress, Mst-1 becomes activated and phosphorylates Beclin1 at Thr108; this causes Beclin1 to dissociate from the Atg14L–Vps34 complex and bind Bcl-2, which then displaces Bax from Bcl-2. Formation of Bcl-2–Beclin1 complexes suppresses autophagy, resulting in the accumulation of protein aggregates. Unbound Bax translocates to mitochondria, and this triggers mitochondrial perturbations, leading to cytochrome c (cyt c) release, caspase activation and apoptosis.
nature medicine volume 19 | number 11 | NOVEMBER 2013
1367
npg
© 2013 Nature America, Inc. All rights reserved.
news and views associated with the intrinsic death pathway. In contrast, the antiapoptotic factors, including BCL-2 and the related protein BCL-xL, avert apoptosis by sequestering BAX and BAK, ostensibly preventing mitochondrial injury and cell death9. Several lines of investigation suggest that, in addition to regulating apoptosis, certain BCL-2 proteins, including Beclin1, also regulate vital cellular processes such as autophagy10. Although unproven, an appealing hypothesis has been that autophagy and apoptosis may be facets of the same signaling pathway, linked by a common cellular factor. In this issue of Nature Medicine, Maejima et al.11 provide compelling evidence that the cellular kinase Mst-1 acts as a molecular switch that selectively drives autophagy or apoptosis by preferentially altering the formation of Bcl-2–Beclin1 complexes. Thus, the authors suggest that Mst-1 is the missing link that bridges the elusive connection between apoptosis and autophagy. In their new work Maejima et al.11 showed that Mst-1 subverts autophagy and promotes apoptosis in the heart by a two-step process, involving the differential assembly of cellular complexes composed of Bcl-2–Beclin1 or Beclin1–Atg14L– Vps34 to drive autophagy or apoptosis, respectively. Using a combination of elegantly designed gain- and loss-of-function in vivo and in vitro experiments, the authors systematically dissected the molecular signaling pathways downstream of Mst-1. First, they demonstrated that activation of Mst-1 during ischemic stress in mouse hearts resulted in a marked reduction in autophagy and a corresponding increase in apoptosis. The authors then showed that Mst-1 strongly interacted with and phosphorylated Beclin1 at Thr108, which lies within the key BH3 domain of Beclin1 required for inducing autophagy. Phosphorylation of Thr108 substantially
impaired the ability of Beclin1 to interact with Atg14L–Vps34 and induce autophagy. Hence, this lack of Beclin1–Atg14L complexes in cardiac cells expressing Mst-1 provides the first direct evidence to explain how Mst-1 impairs autophagy in the heart during ischemic stress. Under basal nonapoptotic conditions, BCL-2 binds and sequesters BAX. Maejima et al.11 hypothesized that the increased cardiomyocyte apoptosis associated with Mst-1 activation may result from the loss of Bcl-2–Bax inhibitory complexes. Consistent with this theory, the authors found that phosphorylation of Beclin1 by Mst-1 at Thr108, the same residue that impaired autophagy, dramatically increased the binding affinity of Beclin1 for Bcl-2. Thus, Bax was displaced from Bcl-2, increasing the amounts of free cellular Bax and leading to increased apoptosis during ischemia (Fig. 1). Although displacement models have previously been proposed to explain the regulation of apoptosis and autophagy by BCL-2 proteins12, none to date have demonstrated the coupling of these pathways by a common signaling factor. The novelty of the work by Maejima et al.11 is the identification of Mst-1 as a factor that preferentially phosphorylates Beclin1 during cellular stress, so that Mst-1 acts as a switch to dually regulate apoptosis and autophagy. Loss of Beclin1–Atg14L–Vps34 complexes impairs autophagy, resulting in protein aggregation, whereas loss of Bcl-2–Bax complexes promotes apoptosis (Fig. 1). Although this study provides answers to several previously posed questions, it raises a number of other issues that should be carefully considered. First, despite the impairment of autophagy and increase in apoptosis induced by Mst-1 during ischemic injury, it remains unknown whether this signaling pathway is restricted to the heart or whether it also
applies to other human diseases associated with altered autophagy flux and apoptosis, such as neurodegenerative diseases and cancer. Second, it is unknown how Mst-1 activation occurs during cellular stress and whether it drives other forms of programmed cell death such as necrosis. For example, the authors showed that cells genetically ablated for Bax or Bak still die in the presence of Mst-1, suggesting that other factors may be involved in inducing apoptosis. Third, it is unknown whether Mst-1 alters the binding affinity of other BH3-domain-like proteins, such as Bak, Bim, PUMA, NOXA and Bnip3, to regulate apoptosis. Despite these unresolved questions, the findings of the study by Maejima et al.11 may provide an explanation for how cells mechanistically integrate autophagy and apoptosis during stress. In addition, this study highlights Mst-1 as a potential therapeutic target, and the selective modulation of Mst-1 activity may prove beneficial in balancing autophagy and apoptosis during cardiac injury. COMPETING FINANCIAL INTERESTS The authors declare no competing financial interests. 1. Mizushima, N., Levine, B., Cuervo, A.M. & Klionsky, D.J. Nature 451, 1069–1075 (2008). 2. Kaushik, S. & Cuervo, A.M. Pharmacol. Res. 66, 484–493 (2012). 3. Nixon, R.A. Nat. Med. 19, 983–997 (2013). 4. Levine, B., Mizushima, N. & Virgin, H.W. Nature 469, 323–335 (2011). 5. Rothermel, B.A. & Hill, J.A. Circ. Res. 103, 1363–1369 (2008). 6. Zhu, H. et al. J. Clin. Invest. 117, 1782–1793 (2007). 7. Sciarretta, S., Hariharan, N., Monden, Y., Zablocki, D. & Sadoshima, J. Pediatr. Cardiol. 32, 275–281 (2011). 8. Wyllie, A.H. Curr. Opin. Genet. Dev. 5, 97–104 (1995). 9. Korsmeyer, S.J., Shutter, J.R., Veis, D.J., Merry, D.E. & Oltvai, Z.N. Semin. Cancer Biol. 4, 327–332 (1993). 10. Levine, B., Sinha, S. & Kroemer, G. Autophagy 4, 600–606 (2008). 11. Maejima, Y. et al. Nat. Med. 19, 1478–1488 (2013). 12. He, C. et al. Nature 481, 511–515 (2012).
An endogenous factor mediates shock-induced injury Peter A Ward An endogenous molecule secreted by macrophages during stress is a new mediator for the wave of inflammation triggered during hemorrhagic and septic shock. This finding suggests a potential drug target to reduce organ injury and death after shock (pages 1489–1495). Release of tissue-damaging factors from infectious agents (such as bacteria, viruses or fungi) or from cells (for example, endothelial or Peter A. Ward is at the Department of Pathology, University of Michigan Medical School, Ann Arbor, Michigan, USA. e-mail: [email protected]
1368
epithelial) after hypoxia, hemorrhagic and septic shock, polytrauma, and other conditions induces release of proinflammatory mediators, the outcome of which is often tissue and organ damage1. Molecules released from infectious agents have been described as pathogenassociated molecular patterns (PAMPs), whereas danger-associated molecular patterns
(DAMPs) are those released from damaged tissues in the absence of infectious pathogens, also called ‘sterile inflammation’2–5. PAMPs and DAMPs seem to function via interactions with Toll-like receptor 2 (TLR2) and TLR4 (refs. 3,5), as well as with NOD-like receptors. To date, the only therapies available to patients with hemorrhagic or septic shock are supportive and
volume 19 | number 11 | NOVEMBER 2013 nature medicine
