RESEARCH NEWS & VIEWS opposite charge, in accordance with a profound theoretical result from multi-particle quantum physics called the CPT theorem. The essence of this theorem is that performing a charge conjugation (C, which interchanges particles and antiparticles), a parity inversion (P, which involves a mirror reflection and a rotation) and a time reversal (T, which changes the direction of the flow of time) leaves physical laws unchanged, and so the combined CPT transformation represents a symmetry of nature. The theorem holds for realistic multi-particle quantum theories only if Einstein’s relativity is exactly valid, so tiny deviations from CPT symmetry would be accompanied by tiny violations in the laws of relativity6. One consequence of the theorem is that the proton and antiproton magnetic moments must be equal in magnitude, so comparing these two quantities experimentally offers a sharp test of CPT symmetry, and therefore an opportunity to search for the tiny relativity violations predicted in some theories of nature. In practice, any difference in the observed proton and antiproton magnetic moments would emerge from shifts in their quantum energies and would involve two factors, one constant and the other varying with time owing to the motion of the laboratory as Earth rotates on its axis and revolves around the Sun7. Disentangling these two effects requires a large data set, and a detailed experimental study with protons and antiprotons has not yet been published. Pioneering experiments of this type have used electrons and positrons (antielectrons) in Penning traps to constrain both constant8 and time-varying9 energy shifts violating the CPT theorem to about 2 parts in 1021 of the electron’s rest energy (0.511 megaelectronvolts). Mooser and colleagues’ new techniques offer one promising route by which to extend these tests to protons and antiprotons. The prospects for future improvements in the measurement precision of the proton magnetic moment are excellent. For the double Penning trap, further reducing the field inhomogeneity and sharpening the experimental procedure are expected to increase precision by a factor of ten1. A different and ambitious scheme now under development involves a precision array with two Penning traps, one containing a proton or antiproton, and the other an atomic ion10. The ion would improve the control of the magnetic field and the measurement procedure, and the timescale required for a measurement would be reduced from about 2 hours to approximately a second. These and other future experiments on protons and antiprotons will stringently test and enhance our understanding of the fundamental laws of nature. ■ V. Alan Kostelecký is in the Physics Department and the Center for Spacetime Symmetries, Indiana University, Bloomington, Indiana 47405, USA. e-mail: [email protected]
1. Mooser, A. et al. Nature 509, 596–599 (2014). 2. Winkler, P. F., Kleppner, D., Myint, T. & Walther, F. G. Phys. Rev. A 5, 83–114 (1972). 3. Hanneke, D., Fogwell, S. & Gabrielse, G. Phys. Rev. Lett. 100, 120801 (2008). 4. Rodegheri, C. C. et al. New J. Phys. 14, 063011 (2012). 5. DiSciacca, J. & Gabrielse, G. Phys. Rev. Lett. 108, 153001 (2012).
6. Greenberg, O. W. Phys. Rev. Lett. 89, 231602 (2002). 7. Bluhm, R., Kostelecký, V. A. & Russell, N. Phys. Rev. D 57, 3932–3943 (1998). 8. Dehmelt, H., Mittleman, R., Van Dyck, R. S. & Schwinberg, P. Phys. Rev. Lett. 83, 4694–4696 (1999). 9. Mittleman, R. K., Ioannou, I. I., Dehmelt, H. G. & Russell, N. Phys. Rev. Lett. 83, 2116–2119 (1999). 10. Ospelkaus, C. et al. Nature 476, 181–184 (2011).
CA R D I OVASC U L A R B I OLOGY
Switched at birth Exposure to atmospheric oxygen in the days after a mammal’s birth causes its heart muscle cells to stop proliferating. The finding may explain why zebrafish, which live in a hypoxic environment, can regenerate their hearts as adults. K AT H E R I N E E . Y U T Z E Y
C
ardiac muscle cells in the mature mammalian heart are notorious for their inability to proliferate in sufficient numbers to repair the heart after injury, or to prevent heart failure during the course of chronic disease1. But zebrafish and some amphibians can regenerate large parts of their hearts as adults, providing hope that regenerative mechanisms might be active in mammals under certain circumstances2. In fact, the hearts of newborn mice can regenerate after injury if it occurs within a few days of birth, but they lose this ability soon thereafter3. Writing in Cell, Puente et al.4 show that the transition to an oxygen-rich environment after birth is what induces cardiac muscle cells to stop proliferating in mammals, but that this transition does not occur in zebrafish. These insights may have implications for efforts devoted to repairing, or ultimately regenerating, adult mammalian hearts. The authors compared the proliferation and metabolism of heart muscle cells (cardiomyocytes) from neonatal mice and zebrafish.
Adult zebrafish
They found that exposure of newborn mice to a mildly hypoxic environment (15% oxygen, compared with the standard atmospheric level of 21%) delayed the withdrawal from the cell cycle that normally occurs in mammalian cardiomyocytes soon after birth. By contrast, hyperoxia (100% oxygen) accelerated this process, supporting the hypothesis that oxygen exposure at birth stops cardiomyocyte proliferation. On the basis of these results, the authors suggest that the hypoxic environments of the mouse fetus and adult zebrafish enhance cardiomyocyte proliferation and regenerative capacity, but that these characteristics are lost in mammals with exposure to atmospheric oxygen at birth. In many ways — including its hypoxic environment, use of glucose as a source of energy and capacity of its cardiomyocytes to proliferate — the mouse fetus is more like a fish than the adult of its own species (Fig. 1). The neonatal period in mammals is a time of major trauma. All of an animal’s organs receive multiple stimuli as a result of both exposure to atmospheric oxygen and the need to feed, move independently, secrete waste and
Fetal/newborn mouse
Hypoxic environment Glycolytic metabolism Cardiomyocyte proliferation Regenerative heart
Adult mouse
Atmospheric-oxygen environment Oxidative metabolism Non-cycling cardiomyocytes Non-regenerative heart
Figure 1 | Fish-like beginnings. The mammalian heart during fetal development and in the neonatal period has similarities to that of an adult zebrafish. Both exist in low-oxygen environments, and the heart muscle cells (cardiomyocytes) use non-oxidative, glucose-based (glycolytic) metabolism and are proliferative, such that they have the potential to regenerate a large part of their hearts after injury. By contrast, the adult mammalian heart exists in an oxygen-rich environment and generates energy by oxidative metabolism. Puente et al.4 show that, in mice, the transition to oxygenated surroundings after birth leads to the withdrawal of cardiomyocytes from the cell cycle and the inability of the heart to regenerate.
5 7 2 | NAT U R E | VO L 5 0 9 | 2 9 M AY 2 0 1 4
© 2014 Macmillan Publishers Limited. All rights reserved
NEWS & VIEWS RESEARCH regulate body temperature. Although Puente and colleagues single out changes in atmospheric oxygen as the cause of cardiomyocyte cell-cycle withdrawal, other factors — such as metabolic stress resulting from periods of starvation between birth and the initiation of nursing or intermittent feeding — could also contribute to cardiomyocyte maturation. However, the finding that oxygen-level manipulation is sufficient to affect the timing of cellcycle withdrawal demonstrates that oxygen is a powerful stimulus in the developing heart. Much of what is known of neonatal development is based on research in mice, which have a gestation period of 19–21 days. Humans are born with more-mature organs than mice, after a nine-month gestation period. The observation that neonatal mice can regenerate part of the heart after injury3 has led to much speculation over potential treatment of human congenital heart defects. However, little is known about how human cardiomyocytes proliferate and mature during late fetal or neonatal development, and it is not known to what extent the development of the newborn mouse heart corresponds to that of a human heart. It is possible that exposure to atmospheric oxygen at birth has a comparable effect on human cardiomyocytes and their potential to regenerate, but studies of large animals with longer gestation periods than mice, or of nonhuman primates, will be necessary to determine whether there is likely to be comparable flexibility in heart growth or regeneration in the first weeks of human life. Oxidative stress at birth leads to the production of chemically reactive molecules called reactive oxygen species (ROS) and induction of a DNA-damage response. Puente et al. identify these processes as the stimuli for cardiomyo cyte cell-cycle withdrawal in a high-oxygen environment in newborn mice, and show that inhibition of oxidative stress through treatment with a ROS scavenger (N-acetylcysteine) prolongs the ability of the mouse heart to regenerate to 21 days after birth4, compared with the normal 7-day window3. But even this window of proliferation closed after this point, indicating that inhibition of oxidative stress can delay, but not prevent, cardiomyocyte cell-cycle withdrawal. Delayed withdrawal for 1–2 weeks after birth has been observed in several mouse models, but in most cases the cells eventually stop dividing2. The overriding mechanism that ultimately prevents cardiomyocytes from proliferating is not fully understood and remains an area of intense research interest. Although Puente and colleagues show that ROS scavenging can prolong the heart’s regenerative capacity in newborn mice, they do not test whether this treatment promotes regeneration of the adult mouse heart after injury. Antioxidant therapy using the same ROS scavenger has been tested in humans as a means of reducing muscle damage after a heart attack5, but limited benefit was observed, and
this therapeutic approach is not in wide use. Because adult heart muscle is distinct from that of neonates in terms of metabolism and cell-cycle regulation, it seems unlikely that, under conditions of oxidative stress, adult cardiomyocytes will show an increased regenerative response to ROS scavenging. However, this is certainly an area worthy of future investigation. It is interesting to speculate that the evolutionary transition to terrestrial life in atmospheric oxygen led to the inability of mammals to regenerate heart muscle. Zebrafish inhabit a relatively hypoxic environment and have a greater capacity to regenerate their hearts and limbs than mammals. Although it may not be possible to test the effects of atmospheric levels of oxygen or hyperoxia in fish, manipulation of oxygen levels in amphibians, which can also regenerate their hearts, could be used to test this hypothesis3. The definitive test might be to examine heart regeneration in terrestrial lizards, which can regenerate their tails (although the new tail is not exactly the same as the original)6.
In mammals, it seems that exposure to atmospheric oxygen at birth leads to development of the more powerful muscle cells needed in a closed circulatory system, with the tradeoff being a relative inability to regenerate heart muscle after injury or disease in adulthood. But further understanding of the transitions that occur in newborn mammals, such as that provided by Puente et al., could reveal approaches by which to rejuvenate or repair diseased hearts. ■ Katherine E. Yutzey is in the Division of Molecular Cardiovascular Biology, Cincinnati Children’s Medical Center, Cincinnati, Ohio 45229, USA. e-mail: [email protected] 1. Xin, M., Olson, E. N. & Bassel-Duby, R. Nature Rev. Mol. Cell Biol. 14, 529–541 (2013). 2. Kikuchi, K. & Poss, K. D. Annu. Rev. Cell Dev. Biol. 28, 719–741 (2012). 3. Porrello, E. R. et al. Science 331, 1078–1080 (2011). 4. Puente, B. N. et al. Cell 157, 565–579 (2014). 5. Sochman, J. J. Am. Coll. Cardiol. 39, 1422–1428 (2002). 6. Ritzman, T. B. et al. Anat. Rec. 295, 1596–1608 (2012).
I M M U N O LO GY
To affinity and beyond Tracking B cells in germinal centres — hotspots of B-cell proliferation and mutation during an immune response — reveals that those cells presenting the most antigen on their surface are programmed to dominate. See Letter p.637 D AV I D M . TA R L I N T O N
O
n page 637 of this issue, Gitlin et al.1 provide compelling insight into the processes driving the extraordinary increases that can occur in the affinity of antibodies for their target antigen molecules during vertebrate immune responses. The study resolves the question of how the rare B cells that produce high-affinity antibodies, which arise at random in a sea of average-affinity B cells, can dominate the response numerically within days. The answer is proliferation, but not through increased frequency or speed in the cells’ basic response cycle of ‘proliferate, mutate and select’. Instead, they are programmed by T cells, on the basis of the amount of antigen presented on the B-cell surface, to divide more times than lesser-affinity B cells in between rounds of affinity-based selection. During an immune response, B cells undergo random mutations in their immunoglobulin variable-region (IgV) genes, which encode the antigen-binding region of antibody proteins. B cells in which this somatic (non-germline) mutation results in enhanced antibody affinity are selected for further rounds of proliferation2. This process of affinity maturation occurs in
germinal centres (GCs) — regions of high cell density in secondary lymphoid organs, such as the lymph nodes and spleen, that form in the early stages of an immune response. The characterization of an enzyme known as activation-induced cytidine deaminase3 provided a biochemical mechanism for the somatic mutation of IgV genes. Subsequently, experiments in many systems4,5 suggested that affinity-based selection is probably driven by B-cell competition in the GC for interactions with follicular helper T cells (TFH cells), which stimulate multiple signalling pathways6, including those involved in survival, division, differentiation and migration. But how changes in the affinity of a B cell for antigen are translated into changes in relative representation of that cell in the immune response remained unclear. Germinal centres form spatially distinct light and dark zones. B cells in the light zone are considered to be neither dividing nor undergoing IgV-gene mutation, but to be receiving signals from TFH cells and thus being subjected to affinity-based selection. Darkzone B cells, by contrast, are thought of as the survivors of previous rounds of selection, and to be actively proliferating and mutating. Affinity maturation occurs by B cells cycling 2 9 M AY 2 0 1 4 | VO L 5 0 9 | NAT U R E | 5 7 3
© 2014 Macmillan Publishers Limited. All rights reserved
1. Mooser, A. et al. Nature 509, 596–599 (2014). 2. Winkler, P. F., Kleppner, D., Myint, T. & Walther, F. G. Phys. Rev. A 5, 83–114 (1972). 3. Hanneke, D., Fogwell, S. & Gabrielse, G. Phys. Rev. Lett. 100, 120801 (2008). 4. Rodegheri, C. C. et al. New J. Phys. 14, 063011 (2012). 5. DiSciacca, J. & Gabrielse, G. Phys. Rev. Lett. 108, 153001 (2012).
6. Greenberg, O. W. Phys. Rev. Lett. 89, 231602 (2002). 7. Bluhm, R., Kostelecký, V. A. & Russell, N. Phys. Rev. D 57, 3932–3943 (1998). 8. Dehmelt, H., Mittleman, R., Van Dyck, R. S. & Schwinberg, P. Phys. Rev. Lett. 83, 4694–4696 (1999). 9. Mittleman, R. K., Ioannou, I. I., Dehmelt, H. G. & Russell, N. Phys. Rev. Lett. 83, 2116–2119 (1999). 10. Ospelkaus, C. et al. Nature 476, 181–184 (2011).
CA R D I OVASC U L A R B I OLOGY
Switched at birth Exposure to atmospheric oxygen in the days after a mammal’s birth causes its heart muscle cells to stop proliferating. The finding may explain why zebrafish, which live in a hypoxic environment, can regenerate their hearts as adults. K AT H E R I N E E . Y U T Z E Y
C
ardiac muscle cells in the mature mammalian heart are notorious for their inability to proliferate in sufficient numbers to repair the heart after injury, or to prevent heart failure during the course of chronic disease1. But zebrafish and some amphibians can regenerate large parts of their hearts as adults, providing hope that regenerative mechanisms might be active in mammals under certain circumstances2. In fact, the hearts of newborn mice can regenerate after injury if it occurs within a few days of birth, but they lose this ability soon thereafter3. Writing in Cell, Puente et al.4 show that the transition to an oxygen-rich environment after birth is what induces cardiac muscle cells to stop proliferating in mammals, but that this transition does not occur in zebrafish. These insights may have implications for efforts devoted to repairing, or ultimately regenerating, adult mammalian hearts. The authors compared the proliferation and metabolism of heart muscle cells (cardiomyocytes) from neonatal mice and zebrafish.
Adult zebrafish
They found that exposure of newborn mice to a mildly hypoxic environment (15% oxygen, compared with the standard atmospheric level of 21%) delayed the withdrawal from the cell cycle that normally occurs in mammalian cardiomyocytes soon after birth. By contrast, hyperoxia (100% oxygen) accelerated this process, supporting the hypothesis that oxygen exposure at birth stops cardiomyocyte proliferation. On the basis of these results, the authors suggest that the hypoxic environments of the mouse fetus and adult zebrafish enhance cardiomyocyte proliferation and regenerative capacity, but that these characteristics are lost in mammals with exposure to atmospheric oxygen at birth. In many ways — including its hypoxic environment, use of glucose as a source of energy and capacity of its cardiomyocytes to proliferate — the mouse fetus is more like a fish than the adult of its own species (Fig. 1). The neonatal period in mammals is a time of major trauma. All of an animal’s organs receive multiple stimuli as a result of both exposure to atmospheric oxygen and the need to feed, move independently, secrete waste and
Fetal/newborn mouse
Hypoxic environment Glycolytic metabolism Cardiomyocyte proliferation Regenerative heart
Adult mouse
Atmospheric-oxygen environment Oxidative metabolism Non-cycling cardiomyocytes Non-regenerative heart
Figure 1 | Fish-like beginnings. The mammalian heart during fetal development and in the neonatal period has similarities to that of an adult zebrafish. Both exist in low-oxygen environments, and the heart muscle cells (cardiomyocytes) use non-oxidative, glucose-based (glycolytic) metabolism and are proliferative, such that they have the potential to regenerate a large part of their hearts after injury. By contrast, the adult mammalian heart exists in an oxygen-rich environment and generates energy by oxidative metabolism. Puente et al.4 show that, in mice, the transition to oxygenated surroundings after birth leads to the withdrawal of cardiomyocytes from the cell cycle and the inability of the heart to regenerate.
5 7 2 | NAT U R E | VO L 5 0 9 | 2 9 M AY 2 0 1 4
© 2014 Macmillan Publishers Limited. All rights reserved
NEWS & VIEWS RESEARCH regulate body temperature. Although Puente and colleagues single out changes in atmospheric oxygen as the cause of cardiomyocyte cell-cycle withdrawal, other factors — such as metabolic stress resulting from periods of starvation between birth and the initiation of nursing or intermittent feeding — could also contribute to cardiomyocyte maturation. However, the finding that oxygen-level manipulation is sufficient to affect the timing of cellcycle withdrawal demonstrates that oxygen is a powerful stimulus in the developing heart. Much of what is known of neonatal development is based on research in mice, which have a gestation period of 19–21 days. Humans are born with more-mature organs than mice, after a nine-month gestation period. The observation that neonatal mice can regenerate part of the heart after injury3 has led to much speculation over potential treatment of human congenital heart defects. However, little is known about how human cardiomyocytes proliferate and mature during late fetal or neonatal development, and it is not known to what extent the development of the newborn mouse heart corresponds to that of a human heart. It is possible that exposure to atmospheric oxygen at birth has a comparable effect on human cardiomyocytes and their potential to regenerate, but studies of large animals with longer gestation periods than mice, or of nonhuman primates, will be necessary to determine whether there is likely to be comparable flexibility in heart growth or regeneration in the first weeks of human life. Oxidative stress at birth leads to the production of chemically reactive molecules called reactive oxygen species (ROS) and induction of a DNA-damage response. Puente et al. identify these processes as the stimuli for cardiomyo cyte cell-cycle withdrawal in a high-oxygen environment in newborn mice, and show that inhibition of oxidative stress through treatment with a ROS scavenger (N-acetylcysteine) prolongs the ability of the mouse heart to regenerate to 21 days after birth4, compared with the normal 7-day window3. But even this window of proliferation closed after this point, indicating that inhibition of oxidative stress can delay, but not prevent, cardiomyocyte cell-cycle withdrawal. Delayed withdrawal for 1–2 weeks after birth has been observed in several mouse models, but in most cases the cells eventually stop dividing2. The overriding mechanism that ultimately prevents cardiomyocytes from proliferating is not fully understood and remains an area of intense research interest. Although Puente and colleagues show that ROS scavenging can prolong the heart’s regenerative capacity in newborn mice, they do not test whether this treatment promotes regeneration of the adult mouse heart after injury. Antioxidant therapy using the same ROS scavenger has been tested in humans as a means of reducing muscle damage after a heart attack5, but limited benefit was observed, and
this therapeutic approach is not in wide use. Because adult heart muscle is distinct from that of neonates in terms of metabolism and cell-cycle regulation, it seems unlikely that, under conditions of oxidative stress, adult cardiomyocytes will show an increased regenerative response to ROS scavenging. However, this is certainly an area worthy of future investigation. It is interesting to speculate that the evolutionary transition to terrestrial life in atmospheric oxygen led to the inability of mammals to regenerate heart muscle. Zebrafish inhabit a relatively hypoxic environment and have a greater capacity to regenerate their hearts and limbs than mammals. Although it may not be possible to test the effects of atmospheric levels of oxygen or hyperoxia in fish, manipulation of oxygen levels in amphibians, which can also regenerate their hearts, could be used to test this hypothesis3. The definitive test might be to examine heart regeneration in terrestrial lizards, which can regenerate their tails (although the new tail is not exactly the same as the original)6.
In mammals, it seems that exposure to atmospheric oxygen at birth leads to development of the more powerful muscle cells needed in a closed circulatory system, with the tradeoff being a relative inability to regenerate heart muscle after injury or disease in adulthood. But further understanding of the transitions that occur in newborn mammals, such as that provided by Puente et al., could reveal approaches by which to rejuvenate or repair diseased hearts. ■ Katherine E. Yutzey is in the Division of Molecular Cardiovascular Biology, Cincinnati Children’s Medical Center, Cincinnati, Ohio 45229, USA. e-mail: [email protected] 1. Xin, M., Olson, E. N. & Bassel-Duby, R. Nature Rev. Mol. Cell Biol. 14, 529–541 (2013). 2. Kikuchi, K. & Poss, K. D. Annu. Rev. Cell Dev. Biol. 28, 719–741 (2012). 3. Porrello, E. R. et al. Science 331, 1078–1080 (2011). 4. Puente, B. N. et al. Cell 157, 565–579 (2014). 5. Sochman, J. J. Am. Coll. Cardiol. 39, 1422–1428 (2002). 6. Ritzman, T. B. et al. Anat. Rec. 295, 1596–1608 (2012).
I M M U N O LO GY
To affinity and beyond Tracking B cells in germinal centres — hotspots of B-cell proliferation and mutation during an immune response — reveals that those cells presenting the most antigen on their surface are programmed to dominate. See Letter p.637 D AV I D M . TA R L I N T O N
O
n page 637 of this issue, Gitlin et al.1 provide compelling insight into the processes driving the extraordinary increases that can occur in the affinity of antibodies for their target antigen molecules during vertebrate immune responses. The study resolves the question of how the rare B cells that produce high-affinity antibodies, which arise at random in a sea of average-affinity B cells, can dominate the response numerically within days. The answer is proliferation, but not through increased frequency or speed in the cells’ basic response cycle of ‘proliferate, mutate and select’. Instead, they are programmed by T cells, on the basis of the amount of antigen presented on the B-cell surface, to divide more times than lesser-affinity B cells in between rounds of affinity-based selection. During an immune response, B cells undergo random mutations in their immunoglobulin variable-region (IgV) genes, which encode the antigen-binding region of antibody proteins. B cells in which this somatic (non-germline) mutation results in enhanced antibody affinity are selected for further rounds of proliferation2. This process of affinity maturation occurs in
germinal centres (GCs) — regions of high cell density in secondary lymphoid organs, such as the lymph nodes and spleen, that form in the early stages of an immune response. The characterization of an enzyme known as activation-induced cytidine deaminase3 provided a biochemical mechanism for the somatic mutation of IgV genes. Subsequently, experiments in many systems4,5 suggested that affinity-based selection is probably driven by B-cell competition in the GC for interactions with follicular helper T cells (TFH cells), which stimulate multiple signalling pathways6, including those involved in survival, division, differentiation and migration. But how changes in the affinity of a B cell for antigen are translated into changes in relative representation of that cell in the immune response remained unclear. Germinal centres form spatially distinct light and dark zones. B cells in the light zone are considered to be neither dividing nor undergoing IgV-gene mutation, but to be receiving signals from TFH cells and thus being subjected to affinity-based selection. Darkzone B cells, by contrast, are thought of as the survivors of previous rounds of selection, and to be actively proliferating and mutating. Affinity maturation occurs by B cells cycling 2 9 M AY 2 0 1 4 | VO L 5 0 9 | NAT U R E | 5 7 3
© 2014 Macmillan Publishers Limited. All rights reserved
