RESEARCH NEWS & VIEWS
a
b Nras-mutant HSC
HSC
Self-renewal
or
Self-renewal
Proliferation
and
Proliferation
Figure 1 | Bimodal behaviour. a, Division of a haematopoietic stem cell (HSC) typically results in either proliferation of more-differentiated cells or self-renewal of the HSC. b, HSCs harbouring an activating mutation in Nras, however, show both enhanced proliferation and self-renewal1, possibly because Nras activation has different effects on different HSC subsets.
self-renewal potential of one subset of HSCs, b but increased the division and reduced the selfProliferation renewal potential of another subset. These findings suggest that there is a bimodal response to Nras activation in HSCsand (Fig. 1). The authors then studied signalling pathways downstream of Nras in HSCs, and observed not only activation of the MEK–ERK kinase Self-renewal pathway as expected, but also activation of the STAT5 signalling pathway, which is not well known as an effector of activated Ras proteins. Remarkably, deletion of just one of the two copies of the gene encoding STAT5 in the Nrasmutant HSCs attenuated the increase in both proliferation and self-renewal, suggesting that STAT5 could be a therapeutic target that eradicates not only the rapidly proliferating subset of cells, but also those cells that have enhanced self-renewal and are therefore more quiescent. These findings are of particular interest because STAT5 has been previously implicated in Rasdriven haematopoietic malignancies3, and MEK inhibition alone had a variable effect on cycling HSCs in these studies. In general, MEK inactivation does not reliably eliminate mutant HSCs in mouse models of Ras-pathway activation4. But Li and colleagues’ findings indicate that mutant Nras induces aberrant signalling in HSCs that could be exploited therapeutically. There is an expanding body of literature suggesting heterogeneity and diversity of function in HSCs. Of the most primitive (least differentiated) HSCs, some are poised for proliferation and differentiation, whereas others are programmed for quiescence, and these cell populations exist in a dynamic equilibrium5. It is also known that individual HSCs do not have identical lineage potential6,7. The bimodal effect defined by Li et al. could be explained by this functional heterogeneity, with some cells responding to mutation by enhancing their self-renewal potential and quiescence, and others responding with enhanced proliferation
and differentiation. This leads to the question of whether such bimodal behaviour is the result of a stochastic response or is determined by definable subsets of HSC responding in a predictable fashion. In light of recent data from normal HSCs5–7, we would predict the latter, but further work is required to test this. Another intriguing question raised by this study is whether the bimodal effect of activating mutations is unique to Nras, or whether it is also a feature of other oncogenic mutations, such as the mutations in the Ras family member Kras that are seen in many solid tumours and in some haematopoietic cancers. In mouse models, both Kras8 and Nras9 mutations lead to excess production of cells from the bone marrow that seems to be initiated from primitive HSCs, although the type of leukaemia that arises differs (T-ALL and AML, respectively).
It is unclear whether Kras activation increases HSC self-renewal potential in a similar manner to Nras activation, and this prompts the question of whether HSC responses to different Ras mutations could determine the type of leukaemia that develops. It will be important to study not only the similarities and differences in how these and other oncogenic mutations affect cell signalling and gene expression, but also the cellular contexts required for such responses. It is becoming increasingly clear that cellular context can influence the response to an oncogenic mutation10,11. This suggests that it is no longer sufficient to know simply which mutations are present in a tumour; we must also consider the influence of where and when. Thus, as we learn more about the mutations that occur in tumour cells, we will need to assess these mutations in functional assays such as those described by Li et al., to obtain a more accurate picture of the effects of mutations in specific cell types and at specific points in development. Such experiments will further enhance our understanding of the complex cellular heterogeneity found in cancers. ■ S. Haihua Chu and Scott A. Armstrong are at the Memorial Sloan-Kettering Cancer Center, New York, New York 10065, USA. e-mail: [email protected] 1. Li, Q. et al. Nature 504, 143–147 (2013). 2. Wang, J. et al. Blood 121, 5203–5207 (2013). 3. Kotecha, N. et al. Cancer Cell 14, 335–343 (2008). 4. Chang, T. et al. J. Clin. Invest. 123, 335–339 (2013). 5. Wilson, A. et al. Cell 135, 1118–1129 (2008). 6. Dykstra, B. et al. Cell Stem Cell 1, 218–229 (2007). 7. Yamamoto, R. et al. Cell 154, 1112–1126 (2013). 8. Sabnis, A. J. et al. PLoS Biol. 7, e1000059 (2009). 9. Li, Q. et al. Blood 117, 2022–2032 (2011). 10. Wang, Y. et al. Science 327, 1650–1653 (2010). 11. Friedmann-Morvinski, D. et al. Science 338, 1080–1084 (2012). This article was published online on 27 November 2013.
ASTR O PH YSI CS
Magnetic fields in γ-ray bursts Observations of a high degree of polarization in the immediate optical afterglow of a γ-ray burst indicate that these powerful cosmic explosions carry large-scale, ordered magnetic fields. See Letter p.119 M A X I M LY U T I KO V
N
aturally occurring magnetic fields protect life on Earth from energetic cosmic rays but are relatively weak and barely noticeable. However, for astronomical objects, magnetic fields can have a dynamically important, and often dominant, role, especially
9 2 | NAT U R E | VO L 5 0 4 | 5 D E C E M B E R 2 0 1 3
© 2013 Macmillan Publishers Limited. All rights reserved
for gravitationally collapsed objects such as neutron stars and accreting black holes. On page 119 of this issue, Mundell et al.1 report the possible observation of an ordered magnetic field that plays a significant part in a cosmic explosion known as a γ-ray burst. As a black hole gravitationally pulls matter in, magnetic fields, which are frozen into the
NEWS & VIEWS RESEARCH accreting plasma owing to the plasma’s high conductivity, are compressed and thus amplified. Compressed magnetic fields can produce spectacular astrophysical phenomena because of a key ingredient: rotation. Collapsed objects, as well as the material that they accrete, typically rotate rapidly. The combination of rotation and compressed magnetic fields leads to the astronomical realization of the Faraday wheel — an electric generator of constant electrical polarity — that produces large currents and voltages2. This is how powerful astrophysical outflows such as γ-ray bursts (GRBs) are produced: material accreting onto a rotating black hole brings with it the magnetic field and sets up a mega-version of the Faraday wheel3,4. The resulting plasma outflow may reach extremely high (relativistic) velocities and, in the case of minutes-long GRBs, carry energy comparable to the total energy that the Sun will radiate over its multibillion-year lifetime5. In addition, hoop stresses produced by the large-scale helical magnetic field that permeates the plasma outflow can collimate the outflow into a narrow beam, with an opening angle of only a few degrees. As a GRB outflow interacts with the surrounding medium, two shocks are launched: a forward shock into the external medium and a reverse shock into the ejecta. Observations of the emission from the reverse shock, whose frequency typically falls in the optical range, can probe the properties of the ejecta and the Faraday-wheel model. Testing this model requires verification that the outflow carries a large-scale, ordered magnetic field (assuming that the field extends well into the outflow). In their study, Mundell et al. report the possible detection of just such a magnetic field, in a GRB dubbed GRB 120308A, through observations of polarized, early optical emission from a reverse shock in the GRB. Medium-sized optical telescopes, such as the 2-metre Liverpool Telescope used in the present study, can slew to the position of the burst within minutes of receiving the alert that the burst has occurred, and detect a typically faint optical afterglow. But in the case reported here, the optical telescope was also equipped with a purpose-built polarimeter, called RINGO2, that could detect the preferred orientation, or polarization, of the afterglow’s oscillating electric field. Polarization indicates that the process that produced the afterglow is sensitive to a particular direction in the emitting plasma outflow. Mundell and colleagues measured a linear polarization content — how much the electric field vibrates in a fixed plane — of 28% for the optical emission, with the angle of polarization remaining stable. This high value presumably comes from the reverse shock in the ejecta. It is close to the maximum degree of polarization that can be
produced by synchrotron-radiation-emitting electrons in a relativistically expanding outflow carrying a large-scale, ordered magnetic field. This result is likely to contribute to the heated debate on the nature of GRBs. Despite decades of intensive research, we are still not clear about the basic, high-energy emission mechanism in GRBs, with the competing “This result models being synis likely to chrotron emission contribute from electrons and to the heated Compton scattering of debate on photons by electrons. the nature of Previous claims 6 of γ-ray bursts.” high polarization values in GRBs, which were based on observations made in the γ-ray energy regime, were inconclusive7 because polarization measurements are difficult to perform at high energies and subject to large uncertainties. By contrast, optical polarization, such as that obtained by the authors, can be
measured with much higher certainty. Mundell and colleagues’ detection of a high degree of polarization in the optical afterglow both confirms the Faraday-wheel model of launching powerful astrophysical outflows and argues in favour of synchrotron radiation being the dominant high-energy emission process. ■ Maxim Lyutikov is in the Department of Physics, Purdue University, West Lafayette, Indiana 47907-2036, USA. e-mail: [email protected] 1. Mundell, C. G. et al. Nature 504, 119–121 (2013). 2. Blandford, R. D. & Znajek, R. L. Mon. Not. R. Astron. Soc. 179, 433–456 (1977). 3. Blandford, R. D. in Lighthouses of the Universe: The Most Luminous Celestial Objects and Their Use for Cosmology (eds Gilfanov, M., Sunyaev, R. & Churazov, E.) 381–404 (Springer, 2002). 4. Lyutikov, M. New J. Phys. 8, 119 (2006). 5. Gehrels, N., Ramirez-Ruiz, E. & Fox, D. B. Annu. Rev. Astron. Astrophys. 47, 567–617 (2009). 6. Coburn, W. & Boggs, S. E. Nature 423, 415–417 (2003). 7. Rutledge, R. E. & Fox, D. B. Mon. Not. R. Astron. Soc. 350, 1288–1300 (2004).
STR UC TUR A L B I OLOGY
Ion channel seen by electron microscopy Structures of the heat-sensitive TRPV1 ion channel have been solved using single-particle electron cryo-microscopy, representing a landmark in the use of this technique for structural biology. See Articles p.107 & p.113 RICHARD HENDERSON
M
embrane proteins known as transient receptor potential (TRP) ion channels occur in species ranging from yeast to humans. Members of this receptor family are involved in the perception of an enormous range of stimuli1, including vision (in invertebrates), taste, hot or cold temperatures, pH and physical forces. On page 107 of this issue, Liao et al.2 report the first highresolution structure of TRPV1, the ion channel responsible for sensing heat. And in a second paper from the same group, Cao et al.3 (page 113) describe the sites at which three ligand molecules bind to TRPV1, and how this binding triggers the opening of the channel. There are 27 members of the TRP receptor family in humans, each with their own functions and different tissue distribution. Most TRP channels, including TRPV1, are weakly selective for calcium ions. TRPV1 was first identified4 as the receptor for capsaicin — the compound that makes chilli peppers seem hot — in 1997. The channel has four identical subunits, and the modified version used in the
present studies has an overall molecular weight of about 300 kilodaltons (bigger than most ion channels). Not only is TRPV1 opened by capsaicin, it is also strongly activated by toxins, such as resiniferatoxin from Euphorbia plants, or ‘cysteine-knot’ toxins from tarantulas. These chemosensory stimuli are thought to have evolved as protective deterrents against predators, and elicit a burning sensation by usurping normal heat sensing through TRPV1 activation. To solve the structure of TRPV1, Liao et al. used single-particle electron cryo-microscopy (cryo-EM), with no help from any of the more established methods of structural biology. The authors made full use of several technical advances: they used a slightly truncated rat TRPV1 construct that is biochemically stable; they transferred purified ion channels into a polymeric ‘amphipol’ framework5 to maintain the channels’ stability and solubility in water; and, most importantly, they used a camera that detects electrons directly (minimizing noise and allowing any image blurring during an exposure to be compensated for6,7) and a state-of-the-art computer program that
5 D E C E M B E R 2 0 1 3 | VO L 5 0 4 | NAT U R E | 9 3
© 2013 Macmillan Publishers Limited. All rights reserved
a
b Nras-mutant HSC
HSC
Self-renewal
or
Self-renewal
Proliferation
and
Proliferation
Figure 1 | Bimodal behaviour. a, Division of a haematopoietic stem cell (HSC) typically results in either proliferation of more-differentiated cells or self-renewal of the HSC. b, HSCs harbouring an activating mutation in Nras, however, show both enhanced proliferation and self-renewal1, possibly because Nras activation has different effects on different HSC subsets.
self-renewal potential of one subset of HSCs, b but increased the division and reduced the selfProliferation renewal potential of another subset. These findings suggest that there is a bimodal response to Nras activation in HSCsand (Fig. 1). The authors then studied signalling pathways downstream of Nras in HSCs, and observed not only activation of the MEK–ERK kinase Self-renewal pathway as expected, but also activation of the STAT5 signalling pathway, which is not well known as an effector of activated Ras proteins. Remarkably, deletion of just one of the two copies of the gene encoding STAT5 in the Nrasmutant HSCs attenuated the increase in both proliferation and self-renewal, suggesting that STAT5 could be a therapeutic target that eradicates not only the rapidly proliferating subset of cells, but also those cells that have enhanced self-renewal and are therefore more quiescent. These findings are of particular interest because STAT5 has been previously implicated in Rasdriven haematopoietic malignancies3, and MEK inhibition alone had a variable effect on cycling HSCs in these studies. In general, MEK inactivation does not reliably eliminate mutant HSCs in mouse models of Ras-pathway activation4. But Li and colleagues’ findings indicate that mutant Nras induces aberrant signalling in HSCs that could be exploited therapeutically. There is an expanding body of literature suggesting heterogeneity and diversity of function in HSCs. Of the most primitive (least differentiated) HSCs, some are poised for proliferation and differentiation, whereas others are programmed for quiescence, and these cell populations exist in a dynamic equilibrium5. It is also known that individual HSCs do not have identical lineage potential6,7. The bimodal effect defined by Li et al. could be explained by this functional heterogeneity, with some cells responding to mutation by enhancing their self-renewal potential and quiescence, and others responding with enhanced proliferation
and differentiation. This leads to the question of whether such bimodal behaviour is the result of a stochastic response or is determined by definable subsets of HSC responding in a predictable fashion. In light of recent data from normal HSCs5–7, we would predict the latter, but further work is required to test this. Another intriguing question raised by this study is whether the bimodal effect of activating mutations is unique to Nras, or whether it is also a feature of other oncogenic mutations, such as the mutations in the Ras family member Kras that are seen in many solid tumours and in some haematopoietic cancers. In mouse models, both Kras8 and Nras9 mutations lead to excess production of cells from the bone marrow that seems to be initiated from primitive HSCs, although the type of leukaemia that arises differs (T-ALL and AML, respectively).
It is unclear whether Kras activation increases HSC self-renewal potential in a similar manner to Nras activation, and this prompts the question of whether HSC responses to different Ras mutations could determine the type of leukaemia that develops. It will be important to study not only the similarities and differences in how these and other oncogenic mutations affect cell signalling and gene expression, but also the cellular contexts required for such responses. It is becoming increasingly clear that cellular context can influence the response to an oncogenic mutation10,11. This suggests that it is no longer sufficient to know simply which mutations are present in a tumour; we must also consider the influence of where and when. Thus, as we learn more about the mutations that occur in tumour cells, we will need to assess these mutations in functional assays such as those described by Li et al., to obtain a more accurate picture of the effects of mutations in specific cell types and at specific points in development. Such experiments will further enhance our understanding of the complex cellular heterogeneity found in cancers. ■ S. Haihua Chu and Scott A. Armstrong are at the Memorial Sloan-Kettering Cancer Center, New York, New York 10065, USA. e-mail: [email protected] 1. Li, Q. et al. Nature 504, 143–147 (2013). 2. Wang, J. et al. Blood 121, 5203–5207 (2013). 3. Kotecha, N. et al. Cancer Cell 14, 335–343 (2008). 4. Chang, T. et al. J. Clin. Invest. 123, 335–339 (2013). 5. Wilson, A. et al. Cell 135, 1118–1129 (2008). 6. Dykstra, B. et al. Cell Stem Cell 1, 218–229 (2007). 7. Yamamoto, R. et al. Cell 154, 1112–1126 (2013). 8. Sabnis, A. J. et al. PLoS Biol. 7, e1000059 (2009). 9. Li, Q. et al. Blood 117, 2022–2032 (2011). 10. Wang, Y. et al. Science 327, 1650–1653 (2010). 11. Friedmann-Morvinski, D. et al. Science 338, 1080–1084 (2012). This article was published online on 27 November 2013.
ASTR O PH YSI CS
Magnetic fields in γ-ray bursts Observations of a high degree of polarization in the immediate optical afterglow of a γ-ray burst indicate that these powerful cosmic explosions carry large-scale, ordered magnetic fields. See Letter p.119 M A X I M LY U T I KO V
N
aturally occurring magnetic fields protect life on Earth from energetic cosmic rays but are relatively weak and barely noticeable. However, for astronomical objects, magnetic fields can have a dynamically important, and often dominant, role, especially
9 2 | NAT U R E | VO L 5 0 4 | 5 D E C E M B E R 2 0 1 3
© 2013 Macmillan Publishers Limited. All rights reserved
for gravitationally collapsed objects such as neutron stars and accreting black holes. On page 119 of this issue, Mundell et al.1 report the possible observation of an ordered magnetic field that plays a significant part in a cosmic explosion known as a γ-ray burst. As a black hole gravitationally pulls matter in, magnetic fields, which are frozen into the
NEWS & VIEWS RESEARCH accreting plasma owing to the plasma’s high conductivity, are compressed and thus amplified. Compressed magnetic fields can produce spectacular astrophysical phenomena because of a key ingredient: rotation. Collapsed objects, as well as the material that they accrete, typically rotate rapidly. The combination of rotation and compressed magnetic fields leads to the astronomical realization of the Faraday wheel — an electric generator of constant electrical polarity — that produces large currents and voltages2. This is how powerful astrophysical outflows such as γ-ray bursts (GRBs) are produced: material accreting onto a rotating black hole brings with it the magnetic field and sets up a mega-version of the Faraday wheel3,4. The resulting plasma outflow may reach extremely high (relativistic) velocities and, in the case of minutes-long GRBs, carry energy comparable to the total energy that the Sun will radiate over its multibillion-year lifetime5. In addition, hoop stresses produced by the large-scale helical magnetic field that permeates the plasma outflow can collimate the outflow into a narrow beam, with an opening angle of only a few degrees. As a GRB outflow interacts with the surrounding medium, two shocks are launched: a forward shock into the external medium and a reverse shock into the ejecta. Observations of the emission from the reverse shock, whose frequency typically falls in the optical range, can probe the properties of the ejecta and the Faraday-wheel model. Testing this model requires verification that the outflow carries a large-scale, ordered magnetic field (assuming that the field extends well into the outflow). In their study, Mundell et al. report the possible detection of just such a magnetic field, in a GRB dubbed GRB 120308A, through observations of polarized, early optical emission from a reverse shock in the GRB. Medium-sized optical telescopes, such as the 2-metre Liverpool Telescope used in the present study, can slew to the position of the burst within minutes of receiving the alert that the burst has occurred, and detect a typically faint optical afterglow. But in the case reported here, the optical telescope was also equipped with a purpose-built polarimeter, called RINGO2, that could detect the preferred orientation, or polarization, of the afterglow’s oscillating electric field. Polarization indicates that the process that produced the afterglow is sensitive to a particular direction in the emitting plasma outflow. Mundell and colleagues measured a linear polarization content — how much the electric field vibrates in a fixed plane — of 28% for the optical emission, with the angle of polarization remaining stable. This high value presumably comes from the reverse shock in the ejecta. It is close to the maximum degree of polarization that can be
produced by synchrotron-radiation-emitting electrons in a relativistically expanding outflow carrying a large-scale, ordered magnetic field. This result is likely to contribute to the heated debate on the nature of GRBs. Despite decades of intensive research, we are still not clear about the basic, high-energy emission mechanism in GRBs, with the competing “This result models being synis likely to chrotron emission contribute from electrons and to the heated Compton scattering of debate on photons by electrons. the nature of Previous claims 6 of γ-ray bursts.” high polarization values in GRBs, which were based on observations made in the γ-ray energy regime, were inconclusive7 because polarization measurements are difficult to perform at high energies and subject to large uncertainties. By contrast, optical polarization, such as that obtained by the authors, can be
measured with much higher certainty. Mundell and colleagues’ detection of a high degree of polarization in the optical afterglow both confirms the Faraday-wheel model of launching powerful astrophysical outflows and argues in favour of synchrotron radiation being the dominant high-energy emission process. ■ Maxim Lyutikov is in the Department of Physics, Purdue University, West Lafayette, Indiana 47907-2036, USA. e-mail: [email protected] 1. Mundell, C. G. et al. Nature 504, 119–121 (2013). 2. Blandford, R. D. & Znajek, R. L. Mon. Not. R. Astron. Soc. 179, 433–456 (1977). 3. Blandford, R. D. in Lighthouses of the Universe: The Most Luminous Celestial Objects and Their Use for Cosmology (eds Gilfanov, M., Sunyaev, R. & Churazov, E.) 381–404 (Springer, 2002). 4. Lyutikov, M. New J. Phys. 8, 119 (2006). 5. Gehrels, N., Ramirez-Ruiz, E. & Fox, D. B. Annu. Rev. Astron. Astrophys. 47, 567–617 (2009). 6. Coburn, W. & Boggs, S. E. Nature 423, 415–417 (2003). 7. Rutledge, R. E. & Fox, D. B. Mon. Not. R. Astron. Soc. 350, 1288–1300 (2004).
STR UC TUR A L B I OLOGY
Ion channel seen by electron microscopy Structures of the heat-sensitive TRPV1 ion channel have been solved using single-particle electron cryo-microscopy, representing a landmark in the use of this technique for structural biology. See Articles p.107 & p.113 RICHARD HENDERSON
M
embrane proteins known as transient receptor potential (TRP) ion channels occur in species ranging from yeast to humans. Members of this receptor family are involved in the perception of an enormous range of stimuli1, including vision (in invertebrates), taste, hot or cold temperatures, pH and physical forces. On page 107 of this issue, Liao et al.2 report the first highresolution structure of TRPV1, the ion channel responsible for sensing heat. And in a second paper from the same group, Cao et al.3 (page 113) describe the sites at which three ligand molecules bind to TRPV1, and how this binding triggers the opening of the channel. There are 27 members of the TRP receptor family in humans, each with their own functions and different tissue distribution. Most TRP channels, including TRPV1, are weakly selective for calcium ions. TRPV1 was first identified4 as the receptor for capsaicin — the compound that makes chilli peppers seem hot — in 1997. The channel has four identical subunits, and the modified version used in the
present studies has an overall molecular weight of about 300 kilodaltons (bigger than most ion channels). Not only is TRPV1 opened by capsaicin, it is also strongly activated by toxins, such as resiniferatoxin from Euphorbia plants, or ‘cysteine-knot’ toxins from tarantulas. These chemosensory stimuli are thought to have evolved as protective deterrents against predators, and elicit a burning sensation by usurping normal heat sensing through TRPV1 activation. To solve the structure of TRPV1, Liao et al. used single-particle electron cryo-microscopy (cryo-EM), with no help from any of the more established methods of structural biology. The authors made full use of several technical advances: they used a slightly truncated rat TRPV1 construct that is biochemically stable; they transferred purified ion channels into a polymeric ‘amphipol’ framework5 to maintain the channels’ stability and solubility in water; and, most importantly, they used a camera that detects electrons directly (minimizing noise and allowing any image blurring during an exposure to be compensated for6,7) and a state-of-the-art computer program that
5 D E C E M B E R 2 0 1 3 | VO L 5 0 4 | NAT U R E | 9 3
© 2013 Macmillan Publishers Limited. All rights reserved
