NEWS AND VIEWS
Lateral junction dynamics lead the way out Martin Behrndt and Carl-Philipp Heisenberg Epithelial cell layers need to be tightly regulated to maintain their integrity and correct function. Cell integration into epithelial sheets is now shown to depend on the N-WASP-regulated stabilization of cortical F-actin, which generates distinct patterns of apical–lateral contractility at E-cadherin-based cell–cell junctions. The formation and maintenance of epithelial cell sheets are key processes in embryogenesis, organogenesis and disease1. Epithelial sheets are highly polarized structures that serve as chemical and mechanical barriers and contribute to shaping the body structure of higher organisms. Consequently, their integrity has to be tightly controlled. The establishment and maintenance of epithelial cell topology depends on the generation and transmission of forces within the epithelium, which can be mediated by adherens junctions between epithelial cells2. At adherens junctions, the apical actomyosin cortex of one cell couples to the apical cortex of neighbouring cells through a belt of cadherin-based adhesion complexes. The mechanical coupling strength of adherens junctions depends on the trans-binding affinity of cadherins, and on the anchorage of cadherins to the actomyosin cortex 3. Disruption of this mechanical coupling leads to epithelial integrity defects2, whereas repositioning of the adherens belt along the apical–basal axis is involved in epithelial cell invagination4. Although analyses of the apical actomyosin cortex of epithelial cells have implicated adherens junctions and the apical cell cortex as central structures controlling epithelial maintenance and morphogenesis3, several observations indicate that contractile lateral junction dynamics might also be involved. For instance, endoderm cell invagination during ascidian gastrulation depends on basolateral myosin activation, shortening these cells along their apical–basal axis5. Furthermore, cadherins were shown to localize with F-actin and myosin II at lateral junctions and to display Martin Behrndt and Carl-Philipp Heisenberg are at the Institute of Science and Technology Austria, Am Campus 1, 3400 Klosterneuburg, Austria. e-mail: [email protected]
pronounced apical-to-basal flows in cultured epithelial cells6. However, the extent of lateral junction contractility, and its contribution to epithelial maintenance and morphogenesis, remained elusive. In this issue, Wu et al.7 report that regulation of lateral junction contractility is critical for the maintenance of epithelial integrity. Using cultured epithelial cells, the authors showed that the balance of tension at apical versus lateral junctions is regulated by differential stabilization of junctional F-actin. Modulating this tension pattern led to epithelial cell extrusion, demonstrating that a proper junctional tension balance along the apical–basal axis of epithelial cells is critical for epithelial integrity (Fig. 1). Imaging of epithelial cell contacts identified a network of condensed actin cables located basally to the apical adherens junctions. These actin cables were decorated with non-muscle myosin II and were connected to trans-interacting E-cadherin molecules that probably represented adhesion clusters. Reducing E-cadherin expression led to a concomitant reduction of both the actin-nucleating Arp2/3 complex that localized at E-cadherin clusters, and the accompanying lateral F-actin network, suggesting that E-cadherin promotes F-actin network assembly at lateral junctions by recruiting Arp2/3, similarly to the situation at adherens junctions2,8. In contrast to apical junctions, however, E-cadherin clusters at lateral junctions were dynamically coalescing and dispersing. These cadherin cluster oscillations were mediated by the contractile dynamics of the associated actomyosin network, as F-actin disruption or downregulation of myosin activity reduced their amplitude, and lateral E-cadherin clusters moved towards accumulations of actin and apart from each other when actin cables
NATURE CELL BIOLOGY VOLUME 16 | NUMBER 2 | FEBRUARY 2014 © 2014 Macmillan Publishers Limited. All rights reserved
dissipated. Collectively, these findings suggest that actomyosin contractility at lateral junctions leads to actin cable condensation and E-cadherin clustering, whereas local disassembly of actin cables leads to E-cadherin cluster dispersal. As a result, cadherin clusters at lateral junctions oscillate with a periodicity and amplitude that depends on the timescale of myosinmediated actin cable assembly and disassembly9. Oscillatory actomyosin network pulsations were previously observed at the apex of epithelial cells during, for example, Drosophila melanogaster mesoderm invagination and dorsal closure10,11, and were thought to drive the periodic shrinkage of the apical cell surface of epithelial cells. Interestingly, Wu et al.7 did not observe net constriction of lateral domains in response to actomyosin-mediated cadherin cluster oscillations. Moreover, actomyosin network tension estimated by laser ablation was much lower in lateral than in apical junctions. The extent to which myosin-generated cortical contraction translates into network tension depends on anchorage of the network to cell junctions12 or external frictional substrates13. Myosin-generated cortical tension can also depend on intrinsic properties of the actin meshwork such as fibre orientation14, crosslinking or turnover 9. The finding of Wu et al. that F-actin lifetime differed considerably in apical versus lateral junctions, and that actin network stabilization at lateral junctions reduced lateral E-cadherin oscillations and increased actomyosin network tension, indicates that the stability of the actin network itself determines the translation from local oscillatory actomyosin contractions to the effective build-up of network tension. To understand how actin stability and tension are regulated at apical versus lateral 127
NEWS AND VIEWS a Epithelial maintenance
b Cell extrusion
Apical Redistribution of apical-lateral
N-WASP Apical
N-WASP N-WASP
F-actin network stabilization E-cadherin cluster oscillation Network tension
N-WASP
F-actin/E-cad cluster condensation Lateral
F-actin
F-actin network stabilization
Lateral
F-actin disassembly
E-cadherin cluster Myosin II
N-WASP N-WASP N-WASP
F-actin network stabilization E-cadherin cluster oscillation Network tension
N-WASP
F-actin network stabilization E-cadherin cluster oscillation
F-actin network stabilization E-cadherin cluster oscillation
Network tension
Network tension
E-cad cluster dispersal
Figure 1 Model schematic of how differential stabilization of the actin cortex at apical versus lateral junctions controls epithelial integrity. (a) At adherens junctions, the apical actin belts of neighbouring cells connect through cadherin-based adhesion complexes. Actin is stabilized through N-WASP at the apical belt, and junctional tension is strongly elevated at apical compared to lateral junctions. As a result of reduced actin cortex stability at lateral junctions, local contractile myosin activity does not translate into effective network tension there. Instead, myosin activity at lateral junctions predominantly promotes oscillatory movements (small black arrows) of E-cadherin (E-cad) clusters by controlling the dynamic assembly and disassembly of actin cables to which the adhesive clusters are associated. (b) Redistribution of N-WASP from apical to lateral junctions stabilizes the lateral actin network, attenuating E-cadherin cluster oscillations and increasing tension at this site. Conversely, the reduction of N-WASP at the apical belt destabilizes actin organization, leading to increased E-cadherin cluster oscillations (small black double-ended arrows) and decreased myosin-generated tension there. Changing the apical-to-lateral tension pattern within the cellular junction promotes cell extrusion and loss of epithelial integrity. The colour of the cells denotes lower (pale pink) and higher (dark pink) F-actin network stabilization. Higher N-WASP abundance is indicated by bright yellow. The back-to-back bottom panels represent successive time points to illustrate the junctional dynamics.
junctions, the authors investigated the role of N-WASP (Wiskott-Aldrich syndrome protein; also known as WASL), which is enriched at the adherens junction and stabilizes F-actin8. Reducing N-WASP expression nearly equalized tension at lateral and apical junctions, and promoted oscillatory E-cadherin movements at apical junctions. Conversely, ectopic N-WASP localization to lateral junctions reduced lateral E-cadherin oscillations and increased lateral tension. These results implicate that differential N-WASP localization at apical versus lateral junctions represents an effective mechanism for regulating actin cortex stability and tension at these sites independently of alterating myosin activity. To investigate the physiological relevance of differential tension at lateral versus apical junctions, the authors performed experiments challenging the integrity of epithelial cell sheets through mosaic expression of oncogenic H-RasV12, which is known to cause cell 128
extrusion15. Interestingly, they observed redistribution of N-WASP from apical domains to lateral junctions at the interface between untransformed and transformed H-RasV12expressing cells. Consistent with a function of N-WASP in stabilizing actin, this redistribution attenuated E-cadherin movements and increased lateral junctional tension, whereas tension at apical junctions was diminished. Moreover, reducing tension at lateral junctions by decreasing N-WASP expression in transformed cells impaired cell extrusion. Together, these findings suggest that decreasing the ratio of apical to lateral junctional tension through the redistribution of N-WASP is a critical step in epithelial cell extrusion. To test whether changing the ratio of apical to lateral tension would also trigger epithelial cell extrusion, the authors ectopically targeted N-WASP to lateral junctions of individual epithelial cells. Consistent with a function of N-WASP in elevating junctional tension, the
tension at lateral junctions between ectopically expressing and unperturbed neighbouring cells was increased. However, cell extrusion was only triggered when endogenous expression of N-WASP was eliminated in all cells. Although the underlying cause for this observation is not entirely clear, N-WASP targeting to lateral junctions was more effective in changing the ratio of apical to lateral junctional tension when endogenous N-WASP expression was diminished, suggesting that a specific junctional tension balance needs to be reached before cells can be extruded. The study by Wu et al.7 provides important insight into the regulation of epithelial integrity. However, questions remain as to the precise function of junctional tension in cell extrusion. In particular, the role of the neighbouring cells surrounding the oncogenic transformed cells is not entirely resolved. Whereas cell extrusion of apoptotic cells or during crowding-induced live-cell delamination is driven by actomyosin rings formed in the neighbouring cell16, this
NATURE CELL BIOLOGY VOLUME 16 | NUMBER 2 | FEBRUARY 2014 © 2014 Macmillan Publishers Limited. All rights reserved
NEWS AND VIEWS was not observed for the extrusion of H-RasV12transformed cells15. Although the contractility of the actomyosin networks at lateral and apical junctions between the transformed and surrounding un-transformed cells are likely to influence each other — given that these networks are mechanically linked through adhesive complexes — how this crosstalk feeds back into the regulation of actin dynamics and contractile junctional activity is still unclear. Furthermore, cell extrusion only occurs when both apical tension is reduced and lateral tension is augmented, but the quantitative limits for this fine balance of intra-junctional tension remain unknown. Understanding these limits could bring valuable insight into the robustness of epithelial integrity and the requirements for fine-tuning these limits. A theoretical model describing the process at the appropriate spatiotemporal scales and reflecting the 3D architecture of the epithelium could provide the appropriate framework for a more quantitative understanding of extrusion mechanics. Such a physical model could be readily challenged by the quantitative data on mechanical tension as well as actin and cadherin
cluster dynamics that the study already provides. It would also have to be supplemented by additional experimental data, such as an analysis of apical–basal constriction dynamics during extrusion, to arrive at robust quantitative conclusions on three-dimensional epithelial mechanics and their transition from homeostasis. Moreover, it would be exciting to explore the extent to which the description of lateral junction dynamics is generalizable to other biological systems. Mechanical models and experimental studies of Drosophila epithelial morphogenesis, for instance, have mainly focused on apical force generation3,10,11. The present study now challenges this view, as it demonstrates that regulation of contractile activity at lateral junctions also critically contributes to epithelial tissue mechanics. Given the diverse epithelial architectures observed in invertebrates and vertebrates, exploration of junctional dynamics beyond the apical belt is likely to hold more surprises. The work of Wu et al. considerably extends our understanding of the maintenance of threedimensional epithelial architecture. Moreover, the findings on differential regulation of actin network stability as a myosin-independent
determinant of junctional tension are likely to have broad implications for actomyosin force generation at cellular interfaces as well as other contexts. COMPETING FINANCIAL INTERESTS The authors declare no competing financial interests. 1. Schöck, F. & Perrimon, N. Annu. Rev. Cell Dev. Biol. 18, 463–493 (2002). 2. Harris, T. J. C. & Tepass, U. Nat. Rev. Mol. Cell Biol. 11, 502–514 (2010). 3. Heisenberg, C-P. & Bellaïche, Y. Cell 153, 948–962 (2013). 4. Wang, Y-C., Khan, Z., Kaschube, M. & Wieschaus, E. F. Nature 484, 390–393 (2012). 5. Sherrard, K., Robin, F., Lemaire, P. & Munro, E. Curr. Biol. 20, 1499–1510 (2010). 6. Kametani, Y. & Takeichi, M. Nat. Cell Biol. 9, 92–98 (2006). 7. Wu, S. K. et al. Nat. Cell Biol. 16, 167–178 (2014). 8. Brieher, W. M. & Yap, A. S. Curr. Opin. Cell Biol. 25, 39–46 (2013). 9. Salbreux, G., Charras, G. & Paluch, E. Trends Cell Biol. 22, 536–545 (2012). 10. Martin, A. C., Kaschube, M. & Wieschaus, E. F. Nature 457, 495–499 (2009). 11. Solon, J., Kaya-Copur, A., Colombelli, J. & Brunner, D. Cell 137, 1331–1342 (2009). 12. Roh-Johnson, M. et al. Science 335, 1232–1235 (2012). 13. Behrndt, M. et al. Science 338, 257–260 (2012). 14. Reymann, A. C. et al. Science 336, 1310–1314 (2012). 15. Hogan, C. et al. Nat. Cell Biol. 11, 460–467 (2009). 16. Gu, Y. & Rosenblatt, J. Curr. Opin. Cell Biol. 24, 865– 870 (2012).
Methyltransferases modulate RNA stability in embryonic stem cells Shuibin Lin and Richard I. Gregory Emerging data support that RNA methylation plays important roles in RNA processing and metabolism. The methyltransferases Mettl3 and Mettl14 are shown to catalyse N6-methyladenosine (m6A) RNA modification in embryonic stem cells (ESCs). This m6A modification controls RNA metabolism and functions to destabilize mRNAs encoding developmental regulators to help sustain ESC self-renewal. More than 100 types of RNA modification have been identified in eukaryotic RNAs1. The N6-methyladenosine (m6A) modification is one of the most common that occurs in messenger RNA (mRNA), and other types Shuibin Lin and Richard I. Gregory are at the Stem Cell Program, Boston Children’s Hospital, Massachusetts 02115, USA; the Department of Biological Chemistry and Molecular Pharmacology, and Department of Pediatrics, Harvard Medical School, Boston, Massachusetts 02115, USA; and the Harvard Stem Cell Institute, Boston, Massachusetts 02115, USA. e-mail: [email protected]
of RNAs such as transfer RNA (tRNA), ribosomal RNA (rRNA), small nuclear RNA (snRNA) and long non-coding RNAs (lncRNA)2. Although the m6A modification was first identified in the 1970s3, its function and biological significance have remained mysterious for decades. Recently, the transcriptome-wide mapping of m6A sites and the identification of m6A demethylases and methyltrasferases provide important insights into the function of m6A modification in the regulation of RNA metabolism, apoptosis, the circadian clock and meiosis4–10. In this
NATURE CELL BIOLOGY VOLUME 16 | NUMBER 2 | FEBRUARY 2014 © 2014 Macmillan Publishers Limited. All rights reserved
issue, Zhao and colleagues identify two RNA methyltransferases (MTases) that catalyse the m6A modification in mouse ESCs. They find that the m6A modification participates in maintaining ESC self-renewal by modulating mRNA stability to suppress the expression of genes involved in cell differentiation11. m6A is an internal RNA modification that occurs at the N6 position of adenosine. Mapping of m6A sites using next-generation sequencing in human, mouse and yeast cells has revealed that thousands of genes harbour m6A modifications5‑7,9,11. Interestingly, the m6A 129
Lateral junction dynamics lead the way out Martin Behrndt and Carl-Philipp Heisenberg Epithelial cell layers need to be tightly regulated to maintain their integrity and correct function. Cell integration into epithelial sheets is now shown to depend on the N-WASP-regulated stabilization of cortical F-actin, which generates distinct patterns of apical–lateral contractility at E-cadherin-based cell–cell junctions. The formation and maintenance of epithelial cell sheets are key processes in embryogenesis, organogenesis and disease1. Epithelial sheets are highly polarized structures that serve as chemical and mechanical barriers and contribute to shaping the body structure of higher organisms. Consequently, their integrity has to be tightly controlled. The establishment and maintenance of epithelial cell topology depends on the generation and transmission of forces within the epithelium, which can be mediated by adherens junctions between epithelial cells2. At adherens junctions, the apical actomyosin cortex of one cell couples to the apical cortex of neighbouring cells through a belt of cadherin-based adhesion complexes. The mechanical coupling strength of adherens junctions depends on the trans-binding affinity of cadherins, and on the anchorage of cadherins to the actomyosin cortex 3. Disruption of this mechanical coupling leads to epithelial integrity defects2, whereas repositioning of the adherens belt along the apical–basal axis is involved in epithelial cell invagination4. Although analyses of the apical actomyosin cortex of epithelial cells have implicated adherens junctions and the apical cell cortex as central structures controlling epithelial maintenance and morphogenesis3, several observations indicate that contractile lateral junction dynamics might also be involved. For instance, endoderm cell invagination during ascidian gastrulation depends on basolateral myosin activation, shortening these cells along their apical–basal axis5. Furthermore, cadherins were shown to localize with F-actin and myosin II at lateral junctions and to display Martin Behrndt and Carl-Philipp Heisenberg are at the Institute of Science and Technology Austria, Am Campus 1, 3400 Klosterneuburg, Austria. e-mail: [email protected]
pronounced apical-to-basal flows in cultured epithelial cells6. However, the extent of lateral junction contractility, and its contribution to epithelial maintenance and morphogenesis, remained elusive. In this issue, Wu et al.7 report that regulation of lateral junction contractility is critical for the maintenance of epithelial integrity. Using cultured epithelial cells, the authors showed that the balance of tension at apical versus lateral junctions is regulated by differential stabilization of junctional F-actin. Modulating this tension pattern led to epithelial cell extrusion, demonstrating that a proper junctional tension balance along the apical–basal axis of epithelial cells is critical for epithelial integrity (Fig. 1). Imaging of epithelial cell contacts identified a network of condensed actin cables located basally to the apical adherens junctions. These actin cables were decorated with non-muscle myosin II and were connected to trans-interacting E-cadherin molecules that probably represented adhesion clusters. Reducing E-cadherin expression led to a concomitant reduction of both the actin-nucleating Arp2/3 complex that localized at E-cadherin clusters, and the accompanying lateral F-actin network, suggesting that E-cadherin promotes F-actin network assembly at lateral junctions by recruiting Arp2/3, similarly to the situation at adherens junctions2,8. In contrast to apical junctions, however, E-cadherin clusters at lateral junctions were dynamically coalescing and dispersing. These cadherin cluster oscillations were mediated by the contractile dynamics of the associated actomyosin network, as F-actin disruption or downregulation of myosin activity reduced their amplitude, and lateral E-cadherin clusters moved towards accumulations of actin and apart from each other when actin cables
NATURE CELL BIOLOGY VOLUME 16 | NUMBER 2 | FEBRUARY 2014 © 2014 Macmillan Publishers Limited. All rights reserved
dissipated. Collectively, these findings suggest that actomyosin contractility at lateral junctions leads to actin cable condensation and E-cadherin clustering, whereas local disassembly of actin cables leads to E-cadherin cluster dispersal. As a result, cadherin clusters at lateral junctions oscillate with a periodicity and amplitude that depends on the timescale of myosinmediated actin cable assembly and disassembly9. Oscillatory actomyosin network pulsations were previously observed at the apex of epithelial cells during, for example, Drosophila melanogaster mesoderm invagination and dorsal closure10,11, and were thought to drive the periodic shrinkage of the apical cell surface of epithelial cells. Interestingly, Wu et al.7 did not observe net constriction of lateral domains in response to actomyosin-mediated cadherin cluster oscillations. Moreover, actomyosin network tension estimated by laser ablation was much lower in lateral than in apical junctions. The extent to which myosin-generated cortical contraction translates into network tension depends on anchorage of the network to cell junctions12 or external frictional substrates13. Myosin-generated cortical tension can also depend on intrinsic properties of the actin meshwork such as fibre orientation14, crosslinking or turnover 9. The finding of Wu et al. that F-actin lifetime differed considerably in apical versus lateral junctions, and that actin network stabilization at lateral junctions reduced lateral E-cadherin oscillations and increased actomyosin network tension, indicates that the stability of the actin network itself determines the translation from local oscillatory actomyosin contractions to the effective build-up of network tension. To understand how actin stability and tension are regulated at apical versus lateral 127
NEWS AND VIEWS a Epithelial maintenance
b Cell extrusion
Apical Redistribution of apical-lateral
N-WASP Apical
N-WASP N-WASP
F-actin network stabilization E-cadherin cluster oscillation Network tension
N-WASP
F-actin/E-cad cluster condensation Lateral
F-actin
F-actin network stabilization
Lateral
F-actin disassembly
E-cadherin cluster Myosin II
N-WASP N-WASP N-WASP
F-actin network stabilization E-cadherin cluster oscillation Network tension
N-WASP
F-actin network stabilization E-cadherin cluster oscillation
F-actin network stabilization E-cadherin cluster oscillation
Network tension
Network tension
E-cad cluster dispersal
Figure 1 Model schematic of how differential stabilization of the actin cortex at apical versus lateral junctions controls epithelial integrity. (a) At adherens junctions, the apical actin belts of neighbouring cells connect through cadherin-based adhesion complexes. Actin is stabilized through N-WASP at the apical belt, and junctional tension is strongly elevated at apical compared to lateral junctions. As a result of reduced actin cortex stability at lateral junctions, local contractile myosin activity does not translate into effective network tension there. Instead, myosin activity at lateral junctions predominantly promotes oscillatory movements (small black arrows) of E-cadherin (E-cad) clusters by controlling the dynamic assembly and disassembly of actin cables to which the adhesive clusters are associated. (b) Redistribution of N-WASP from apical to lateral junctions stabilizes the lateral actin network, attenuating E-cadherin cluster oscillations and increasing tension at this site. Conversely, the reduction of N-WASP at the apical belt destabilizes actin organization, leading to increased E-cadherin cluster oscillations (small black double-ended arrows) and decreased myosin-generated tension there. Changing the apical-to-lateral tension pattern within the cellular junction promotes cell extrusion and loss of epithelial integrity. The colour of the cells denotes lower (pale pink) and higher (dark pink) F-actin network stabilization. Higher N-WASP abundance is indicated by bright yellow. The back-to-back bottom panels represent successive time points to illustrate the junctional dynamics.
junctions, the authors investigated the role of N-WASP (Wiskott-Aldrich syndrome protein; also known as WASL), which is enriched at the adherens junction and stabilizes F-actin8. Reducing N-WASP expression nearly equalized tension at lateral and apical junctions, and promoted oscillatory E-cadherin movements at apical junctions. Conversely, ectopic N-WASP localization to lateral junctions reduced lateral E-cadherin oscillations and increased lateral tension. These results implicate that differential N-WASP localization at apical versus lateral junctions represents an effective mechanism for regulating actin cortex stability and tension at these sites independently of alterating myosin activity. To investigate the physiological relevance of differential tension at lateral versus apical junctions, the authors performed experiments challenging the integrity of epithelial cell sheets through mosaic expression of oncogenic H-RasV12, which is known to cause cell 128
extrusion15. Interestingly, they observed redistribution of N-WASP from apical domains to lateral junctions at the interface between untransformed and transformed H-RasV12expressing cells. Consistent with a function of N-WASP in stabilizing actin, this redistribution attenuated E-cadherin movements and increased lateral junctional tension, whereas tension at apical junctions was diminished. Moreover, reducing tension at lateral junctions by decreasing N-WASP expression in transformed cells impaired cell extrusion. Together, these findings suggest that decreasing the ratio of apical to lateral junctional tension through the redistribution of N-WASP is a critical step in epithelial cell extrusion. To test whether changing the ratio of apical to lateral tension would also trigger epithelial cell extrusion, the authors ectopically targeted N-WASP to lateral junctions of individual epithelial cells. Consistent with a function of N-WASP in elevating junctional tension, the
tension at lateral junctions between ectopically expressing and unperturbed neighbouring cells was increased. However, cell extrusion was only triggered when endogenous expression of N-WASP was eliminated in all cells. Although the underlying cause for this observation is not entirely clear, N-WASP targeting to lateral junctions was more effective in changing the ratio of apical to lateral junctional tension when endogenous N-WASP expression was diminished, suggesting that a specific junctional tension balance needs to be reached before cells can be extruded. The study by Wu et al.7 provides important insight into the regulation of epithelial integrity. However, questions remain as to the precise function of junctional tension in cell extrusion. In particular, the role of the neighbouring cells surrounding the oncogenic transformed cells is not entirely resolved. Whereas cell extrusion of apoptotic cells or during crowding-induced live-cell delamination is driven by actomyosin rings formed in the neighbouring cell16, this
NATURE CELL BIOLOGY VOLUME 16 | NUMBER 2 | FEBRUARY 2014 © 2014 Macmillan Publishers Limited. All rights reserved
NEWS AND VIEWS was not observed for the extrusion of H-RasV12transformed cells15. Although the contractility of the actomyosin networks at lateral and apical junctions between the transformed and surrounding un-transformed cells are likely to influence each other — given that these networks are mechanically linked through adhesive complexes — how this crosstalk feeds back into the regulation of actin dynamics and contractile junctional activity is still unclear. Furthermore, cell extrusion only occurs when both apical tension is reduced and lateral tension is augmented, but the quantitative limits for this fine balance of intra-junctional tension remain unknown. Understanding these limits could bring valuable insight into the robustness of epithelial integrity and the requirements for fine-tuning these limits. A theoretical model describing the process at the appropriate spatiotemporal scales and reflecting the 3D architecture of the epithelium could provide the appropriate framework for a more quantitative understanding of extrusion mechanics. Such a physical model could be readily challenged by the quantitative data on mechanical tension as well as actin and cadherin
cluster dynamics that the study already provides. It would also have to be supplemented by additional experimental data, such as an analysis of apical–basal constriction dynamics during extrusion, to arrive at robust quantitative conclusions on three-dimensional epithelial mechanics and their transition from homeostasis. Moreover, it would be exciting to explore the extent to which the description of lateral junction dynamics is generalizable to other biological systems. Mechanical models and experimental studies of Drosophila epithelial morphogenesis, for instance, have mainly focused on apical force generation3,10,11. The present study now challenges this view, as it demonstrates that regulation of contractile activity at lateral junctions also critically contributes to epithelial tissue mechanics. Given the diverse epithelial architectures observed in invertebrates and vertebrates, exploration of junctional dynamics beyond the apical belt is likely to hold more surprises. The work of Wu et al. considerably extends our understanding of the maintenance of threedimensional epithelial architecture. Moreover, the findings on differential regulation of actin network stability as a myosin-independent
determinant of junctional tension are likely to have broad implications for actomyosin force generation at cellular interfaces as well as other contexts. COMPETING FINANCIAL INTERESTS The authors declare no competing financial interests. 1. Schöck, F. & Perrimon, N. Annu. Rev. Cell Dev. Biol. 18, 463–493 (2002). 2. Harris, T. J. C. & Tepass, U. Nat. Rev. Mol. Cell Biol. 11, 502–514 (2010). 3. Heisenberg, C-P. & Bellaïche, Y. Cell 153, 948–962 (2013). 4. Wang, Y-C., Khan, Z., Kaschube, M. & Wieschaus, E. F. Nature 484, 390–393 (2012). 5. Sherrard, K., Robin, F., Lemaire, P. & Munro, E. Curr. Biol. 20, 1499–1510 (2010). 6. Kametani, Y. & Takeichi, M. Nat. Cell Biol. 9, 92–98 (2006). 7. Wu, S. K. et al. Nat. Cell Biol. 16, 167–178 (2014). 8. Brieher, W. M. & Yap, A. S. Curr. Opin. Cell Biol. 25, 39–46 (2013). 9. Salbreux, G., Charras, G. & Paluch, E. Trends Cell Biol. 22, 536–545 (2012). 10. Martin, A. C., Kaschube, M. & Wieschaus, E. F. Nature 457, 495–499 (2009). 11. Solon, J., Kaya-Copur, A., Colombelli, J. & Brunner, D. Cell 137, 1331–1342 (2009). 12. Roh-Johnson, M. et al. Science 335, 1232–1235 (2012). 13. Behrndt, M. et al. Science 338, 257–260 (2012). 14. Reymann, A. C. et al. Science 336, 1310–1314 (2012). 15. Hogan, C. et al. Nat. Cell Biol. 11, 460–467 (2009). 16. Gu, Y. & Rosenblatt, J. Curr. Opin. Cell Biol. 24, 865– 870 (2012).
Methyltransferases modulate RNA stability in embryonic stem cells Shuibin Lin and Richard I. Gregory Emerging data support that RNA methylation plays important roles in RNA processing and metabolism. The methyltransferases Mettl3 and Mettl14 are shown to catalyse N6-methyladenosine (m6A) RNA modification in embryonic stem cells (ESCs). This m6A modification controls RNA metabolism and functions to destabilize mRNAs encoding developmental regulators to help sustain ESC self-renewal. More than 100 types of RNA modification have been identified in eukaryotic RNAs1. The N6-methyladenosine (m6A) modification is one of the most common that occurs in messenger RNA (mRNA), and other types Shuibin Lin and Richard I. Gregory are at the Stem Cell Program, Boston Children’s Hospital, Massachusetts 02115, USA; the Department of Biological Chemistry and Molecular Pharmacology, and Department of Pediatrics, Harvard Medical School, Boston, Massachusetts 02115, USA; and the Harvard Stem Cell Institute, Boston, Massachusetts 02115, USA. e-mail: [email protected]
of RNAs such as transfer RNA (tRNA), ribosomal RNA (rRNA), small nuclear RNA (snRNA) and long non-coding RNAs (lncRNA)2. Although the m6A modification was first identified in the 1970s3, its function and biological significance have remained mysterious for decades. Recently, the transcriptome-wide mapping of m6A sites and the identification of m6A demethylases and methyltrasferases provide important insights into the function of m6A modification in the regulation of RNA metabolism, apoptosis, the circadian clock and meiosis4–10. In this
NATURE CELL BIOLOGY VOLUME 16 | NUMBER 2 | FEBRUARY 2014 © 2014 Macmillan Publishers Limited. All rights reserved
issue, Zhao and colleagues identify two RNA methyltransferases (MTases) that catalyse the m6A modification in mouse ESCs. They find that the m6A modification participates in maintaining ESC self-renewal by modulating mRNA stability to suppress the expression of genes involved in cell differentiation11. m6A is an internal RNA modification that occurs at the N6 position of adenosine. Mapping of m6A sites using next-generation sequencing in human, mouse and yeast cells has revealed that thousands of genes harbour m6A modifications5‑7,9,11. Interestingly, the m6A 129
