CELL CYCLE 2017, VOL. 16, NO. 3, 233–234 http://dx.doi.org/10.1080/15384101.2016.1214044
EDITORIALS: CELL CYCLE FEATURES
DNA replication stress: NoCut to the rescue Nuno Amarala,b,#, Nicola Brownlowa,b, and Manuel Mendozaa,b a Centre for Genomic Regulation (CRG), The Barcelona Institute of Science and Technology, Barcelona, Spain; bUniversitat Pompeu Fabra (UPF), Barcelona, Spain
ARTICLE HISTORY Received 18 June 2016; Accepted 26 June 2016 KEYWORDS aurora B; abscission; chromatin bridges; cytokinesis; NoCut; replication stress
It is well known that checkpoint systems operate in the cell to inhibit progression through mitosis until errors detected in previous stages have been properly completed. Less is known, however, about mechanisms protecting cells against errors in chromosome segregation after anaphase has begun. Yet late segregation errors, such as lagging chromosomes and anaphase bridges, are not rare in normal conditions, and are frequent in tumors. Late-segregating DNA is thus exposed to potential damage by the cell division machinery. How do cells react to such a threat? We previously proposed that in budding yeast, an Aurora-Bdependent monitoring system, termed the NoCut checkpoint, responds to late DNA segregation errors by inhibiting completion of cytokinesis until chromatin is cleared from the cleavage plane.1-2 An Aurora-B-dependent abscission checkpoint homologous to NoCut was subsequently identified in human cells.3 However, it remained unclear under what physiological conditions NoCut is activated, and equally importantly, under what conditions the checkpoint prevents DNA bridge breakage. In a recent study,4 we determined how budding yeast cells respond to various types of chromatin bridges. Using time-lapse fluorescence microscopy and electron tomography, we found that the Aurora-B-dependent NoCut checkpoint delays abscission in response to chromatin bridges caused by inactivation of condensin and Topoisomerase II function. However, DNA breaks eventually occur in these 2 cases, probably linked to the fact that bridges cannot be resolved in the absence of condensin or Topoisomerase II, which are essential for chromosome segregation. This raised the question of whether the NoCut checkpoint can prevent DNA damage during cytokinesis. We found that in the presence of bridges caused by DNA replication stress after exposure to hydroxyurea, wild type cells delay abscission in a manner dependent on Aurora B, and that in this case NoCut does prevent DNA damage and ensures cell survival. We identified 2 essential requirements for abscission inhibition in response to DNA bridges. Firstly, Aurora-B kinase must be active and associated with microtubules of the spindle midzone during cytokinesis.2 These findings suggest that Aurora B
acts as a sensor monitoring the presence of chromatin around the spindle midzone. Secondly, anaphase spindles must be stabilized during cytokinesis to allow NoCut function.4 Indeed, under normal conditions anaphase spindles disassemble before completion of cytokinesis, but we observed a delay in spindle disassembly in cells with chromatin bridges. Specifically, spindle factors that are normally degraded at the end of mitosis by the Anaphase Promoting Complex (APC) are stabilized in cells with catenated, decondensed, and replication stress-induced anaphase bridges. Spindle stabilization is essential for NoCut, because treatment of anaphase cells with the microtubule depolymerizing drug nocodazole allows completion of cytokinesis in the presence of catenated DNA bridges. These findings suggest that after DNA replication stress and/or defects in chromatin condensation and decatenation, degradation of spindle-stabilizing factors is delayed, thereby stabilizing the spindle during cytokinesis and allowing for detection of lagging chromatin by Aurora B at the spindle midzone. This in turn delays abscission and prolongs the lifetime of the chromatin bridge, allowing time for its final resolution (Fig. 1). Strikingly, a particular type of chromatin bridge failed to inhibit abscission. We found that activation of a conditionally dicentric chromosome led to dicentric bridges across the cell division plane, yet abscission proceeded normally.4 What is the basis for this differential response to chromatin bridges? Notably, spindle stabilization during cytokinesis did not occur in cells with dicentric bridges, offering a potential explanation for their lack of abscission defect. Indeed, impairment of the APC activator Cdh1 delayed spindle disassembly and restored NoCut function in the presence of dicentric bridges. These data suggest that inhibition of APC-dependent spindle disassembly might be required for efficient detection of chromatin bridges by Aurora B. We hypothesize that a chromatin-based damage signal is essential for the NoCut response upstream of Aurora B. The signal might be common to replication stress, and to condensation and decatenation defects; and be absent from the dicentric bridge due to its normal replication, condensation and decatenation. Whatever its nature, this signal would
CONTACT Manuel Mendoza [email protected] Centre de Regulacio Genomica (CRG), Cell and Developmental Biology Programme, Dr. Aiguader 88, 08003 Barcelona, Spain. Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/kccy. # Current address: University of Southern California, Molecular and Computational Biology Department, Los Angeles, CA 90089, USA Feature to: Amaral N, et al. The Aurora-B-dependent NoCut checkpoint prevents damage of anaphase bridges after DNA replication stress. Nat Cell Biol 2016; 18(5):516-26; PMID: 27111841; http://dx.doi.org/10.1038/ncb3343 © 2017 Taylor & Francis
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Figure 1. Tomogram of a cell with defective DNA segregation, visible as a nuclear bridge with spindle microtubules (green lines) at the cytokinesis site. A chromatinbased signal inhibits APC-Cdh1, stabilizing the spindle and allowing bridge detection by Aurora B. Abscission is delayed, preventing bridge damage. NE, nuclear envelope; PM, plasma membrane. Scale bar, 0.5 mm. Tomogram by Nuno Amaral and Charlotta Funaya.
determine spindle stabilization and maintain Aurora B in the spindle midzone to allow detection of unsegregated DNA (Fig. 1). Our study shows that in budding yeast, a key physiological role of NoCut is to protect damage of DNA bridges after replication stress, and suggests that a similar surveillance system might help maintain genomic stability in animal cells after DNA replication stress. In human cells, abscission is delayed or inhibited in the presence of spontaneous chromatin bridges, and this can prevent their damage.5 In contrast to this, lagging chromosomes and dicentric chromosome bridges are also known to brake after passage through cytokinesis.6-7 Our study raises the intriguing possibility that these apparently contradictory outcomes are due, at least in part, to the different molecular origin of chromatin bridges examined. Systematic analysis of cytokinesis and DNA damage in the presence of various types of DNA bridges could shed light into how human cells respond to late segregation errors. This may add to our understanding of genome stability maintenance and how this is lost in pathogenic situations such as cancer.
Disclosure of potential conflicts of interest No potential conflicts of interest were disclosed.
References [1] Norden C, Mendoza M, Dobbelaere J, Kotwaliwale CV, Biggins S, Barral Y. The NoCut pathway links completion of cytokinesis to spindle midzone function to prevent chromosome breakage. Cell 2006; 125:85-98; PMID:16615892; http://dx.doi.org/10.1016/j.cell. 2006.01.045 [2] Mendoza M, Norden C, Durrer K, Rauter H, Uhlmann F, Barral Y. A mechanism for chromosome segregation sensing by the NoCut checkpoint. Nat Cell Biol 2009; 11:477-83; PMID:19270692; http://dx.doi. org/10.1038/ncb1855 [3] Steigemann P, Wurzenberger C, Schmitz MHA, Held M, Guizetti J, Maar S, Gerlich DW. Aurora B-mediated abscission checkpoint protects against tetraploidization. Cell 2009; 136:473-84; PMID: 19203582; http://dx.doi.org/10.1016/j.cell.2008.12.020 [4] Amaral N, Vendrell A, Funaya C, Idrissi F-Z, Maier M, Kumar A, Neurohr G, Colomina N, Torres-Rosell J, et al. The Aurora-B-dependent NoCut checkpoint prevents damage of anaphase bridges after DNA replication stress. Nature Cell Biology 2016; 18:516-26; PMID:27111841; http://dx.doi.org/10.1038/ncb3343 [5] Carlton JG, Caballe A, Agromayor M, Kloc M, Martin-Serrano J. ESCRT-III governs the Aurora B–mediated abscission checkpoint through CHMP4C. Science 2012; 336:220-5; PMID:22422861; http:// dx.doi.org/10.1126/science.1217180 [6] Janssen A, van der Burg M, Szuhai K, Kops GJPL, Medema RH. Chromosome segregation errors as a cause of DNA damage and structural chromosome aberrations. Science 2011; 333:1895-8; PMID:21960636; http://dx.doi.org/10.1126/science.1210214 [7] Maciejowski J, Li Y, Bosco N, Campbell PJ, de Lange T. Chromothripsis and kataegis induced by telomere crisis. Cell 2015; 163:1641-54; PMID:26687355; http://dx.doi.org/10.1016/j.cell.2015.11.054
EDITORIALS: CELL CYCLE FEATURES
DNA replication stress: NoCut to the rescue Nuno Amarala,b,#, Nicola Brownlowa,b, and Manuel Mendozaa,b a Centre for Genomic Regulation (CRG), The Barcelona Institute of Science and Technology, Barcelona, Spain; bUniversitat Pompeu Fabra (UPF), Barcelona, Spain
ARTICLE HISTORY Received 18 June 2016; Accepted 26 June 2016 KEYWORDS aurora B; abscission; chromatin bridges; cytokinesis; NoCut; replication stress
It is well known that checkpoint systems operate in the cell to inhibit progression through mitosis until errors detected in previous stages have been properly completed. Less is known, however, about mechanisms protecting cells against errors in chromosome segregation after anaphase has begun. Yet late segregation errors, such as lagging chromosomes and anaphase bridges, are not rare in normal conditions, and are frequent in tumors. Late-segregating DNA is thus exposed to potential damage by the cell division machinery. How do cells react to such a threat? We previously proposed that in budding yeast, an Aurora-Bdependent monitoring system, termed the NoCut checkpoint, responds to late DNA segregation errors by inhibiting completion of cytokinesis until chromatin is cleared from the cleavage plane.1-2 An Aurora-B-dependent abscission checkpoint homologous to NoCut was subsequently identified in human cells.3 However, it remained unclear under what physiological conditions NoCut is activated, and equally importantly, under what conditions the checkpoint prevents DNA bridge breakage. In a recent study,4 we determined how budding yeast cells respond to various types of chromatin bridges. Using time-lapse fluorescence microscopy and electron tomography, we found that the Aurora-B-dependent NoCut checkpoint delays abscission in response to chromatin bridges caused by inactivation of condensin and Topoisomerase II function. However, DNA breaks eventually occur in these 2 cases, probably linked to the fact that bridges cannot be resolved in the absence of condensin or Topoisomerase II, which are essential for chromosome segregation. This raised the question of whether the NoCut checkpoint can prevent DNA damage during cytokinesis. We found that in the presence of bridges caused by DNA replication stress after exposure to hydroxyurea, wild type cells delay abscission in a manner dependent on Aurora B, and that in this case NoCut does prevent DNA damage and ensures cell survival. We identified 2 essential requirements for abscission inhibition in response to DNA bridges. Firstly, Aurora-B kinase must be active and associated with microtubules of the spindle midzone during cytokinesis.2 These findings suggest that Aurora B
acts as a sensor monitoring the presence of chromatin around the spindle midzone. Secondly, anaphase spindles must be stabilized during cytokinesis to allow NoCut function.4 Indeed, under normal conditions anaphase spindles disassemble before completion of cytokinesis, but we observed a delay in spindle disassembly in cells with chromatin bridges. Specifically, spindle factors that are normally degraded at the end of mitosis by the Anaphase Promoting Complex (APC) are stabilized in cells with catenated, decondensed, and replication stress-induced anaphase bridges. Spindle stabilization is essential for NoCut, because treatment of anaphase cells with the microtubule depolymerizing drug nocodazole allows completion of cytokinesis in the presence of catenated DNA bridges. These findings suggest that after DNA replication stress and/or defects in chromatin condensation and decatenation, degradation of spindle-stabilizing factors is delayed, thereby stabilizing the spindle during cytokinesis and allowing for detection of lagging chromatin by Aurora B at the spindle midzone. This in turn delays abscission and prolongs the lifetime of the chromatin bridge, allowing time for its final resolution (Fig. 1). Strikingly, a particular type of chromatin bridge failed to inhibit abscission. We found that activation of a conditionally dicentric chromosome led to dicentric bridges across the cell division plane, yet abscission proceeded normally.4 What is the basis for this differential response to chromatin bridges? Notably, spindle stabilization during cytokinesis did not occur in cells with dicentric bridges, offering a potential explanation for their lack of abscission defect. Indeed, impairment of the APC activator Cdh1 delayed spindle disassembly and restored NoCut function in the presence of dicentric bridges. These data suggest that inhibition of APC-dependent spindle disassembly might be required for efficient detection of chromatin bridges by Aurora B. We hypothesize that a chromatin-based damage signal is essential for the NoCut response upstream of Aurora B. The signal might be common to replication stress, and to condensation and decatenation defects; and be absent from the dicentric bridge due to its normal replication, condensation and decatenation. Whatever its nature, this signal would
CONTACT Manuel Mendoza [email protected] Centre de Regulacio Genomica (CRG), Cell and Developmental Biology Programme, Dr. Aiguader 88, 08003 Barcelona, Spain. Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/kccy. # Current address: University of Southern California, Molecular and Computational Biology Department, Los Angeles, CA 90089, USA Feature to: Amaral N, et al. The Aurora-B-dependent NoCut checkpoint prevents damage of anaphase bridges after DNA replication stress. Nat Cell Biol 2016; 18(5):516-26; PMID: 27111841; http://dx.doi.org/10.1038/ncb3343 © 2017 Taylor & Francis
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Figure 1. Tomogram of a cell with defective DNA segregation, visible as a nuclear bridge with spindle microtubules (green lines) at the cytokinesis site. A chromatinbased signal inhibits APC-Cdh1, stabilizing the spindle and allowing bridge detection by Aurora B. Abscission is delayed, preventing bridge damage. NE, nuclear envelope; PM, plasma membrane. Scale bar, 0.5 mm. Tomogram by Nuno Amaral and Charlotta Funaya.
determine spindle stabilization and maintain Aurora B in the spindle midzone to allow detection of unsegregated DNA (Fig. 1). Our study shows that in budding yeast, a key physiological role of NoCut is to protect damage of DNA bridges after replication stress, and suggests that a similar surveillance system might help maintain genomic stability in animal cells after DNA replication stress. In human cells, abscission is delayed or inhibited in the presence of spontaneous chromatin bridges, and this can prevent their damage.5 In contrast to this, lagging chromosomes and dicentric chromosome bridges are also known to brake after passage through cytokinesis.6-7 Our study raises the intriguing possibility that these apparently contradictory outcomes are due, at least in part, to the different molecular origin of chromatin bridges examined. Systematic analysis of cytokinesis and DNA damage in the presence of various types of DNA bridges could shed light into how human cells respond to late segregation errors. This may add to our understanding of genome stability maintenance and how this is lost in pathogenic situations such as cancer.
Disclosure of potential conflicts of interest No potential conflicts of interest were disclosed.
References [1] Norden C, Mendoza M, Dobbelaere J, Kotwaliwale CV, Biggins S, Barral Y. The NoCut pathway links completion of cytokinesis to spindle midzone function to prevent chromosome breakage. Cell 2006; 125:85-98; PMID:16615892; http://dx.doi.org/10.1016/j.cell. 2006.01.045 [2] Mendoza M, Norden C, Durrer K, Rauter H, Uhlmann F, Barral Y. A mechanism for chromosome segregation sensing by the NoCut checkpoint. Nat Cell Biol 2009; 11:477-83; PMID:19270692; http://dx.doi. org/10.1038/ncb1855 [3] Steigemann P, Wurzenberger C, Schmitz MHA, Held M, Guizetti J, Maar S, Gerlich DW. Aurora B-mediated abscission checkpoint protects against tetraploidization. Cell 2009; 136:473-84; PMID: 19203582; http://dx.doi.org/10.1016/j.cell.2008.12.020 [4] Amaral N, Vendrell A, Funaya C, Idrissi F-Z, Maier M, Kumar A, Neurohr G, Colomina N, Torres-Rosell J, et al. The Aurora-B-dependent NoCut checkpoint prevents damage of anaphase bridges after DNA replication stress. Nature Cell Biology 2016; 18:516-26; PMID:27111841; http://dx.doi.org/10.1038/ncb3343 [5] Carlton JG, Caballe A, Agromayor M, Kloc M, Martin-Serrano J. ESCRT-III governs the Aurora B–mediated abscission checkpoint through CHMP4C. Science 2012; 336:220-5; PMID:22422861; http:// dx.doi.org/10.1126/science.1217180 [6] Janssen A, van der Burg M, Szuhai K, Kops GJPL, Medema RH. Chromosome segregation errors as a cause of DNA damage and structural chromosome aberrations. Science 2011; 333:1895-8; PMID:21960636; http://dx.doi.org/10.1126/science.1210214 [7] Maciejowski J, Li Y, Bosco N, Campbell PJ, de Lange T. Chromothripsis and kataegis induced by telomere crisis. Cell 2015; 163:1641-54; PMID:26687355; http://dx.doi.org/10.1016/j.cell.2015.11.054
