- Email: [email protected]
Cell Cycle: Deconstructing Tension
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References 1. Alexander, R.D. (1974). The evolution of social behavior. Annu. Rev. Ecol. Syst. 5, 324–383. 2. Van Schaik, C.P. (1983). Why are diurnal primates living in groups. Behaviour 87, 120–144. 3. Wrangham, R.W. (1980). An ecological model of female-bonded primate groups. Behaviour 75, 262–300. 4. Silk, J.B., Alberts, S.C., and Altmann, J. (2003). Social bonds of female baboons enhance infant survival. Science 302, 1231–1234. 5. Silk, J.B., Beehner, J.C., Bergman, T.J., Crockford, C., Engh, A.L., Moscovice, L.R., Wittig, R.M., Seyfarth, R.M., and Cheney, D.L. (2009). The benefits of social capital: Close social bonds among female baboons enhance offspring survival. Proc. R. Soc. Lond. B. Biol. Sci. 276, 3099–3104. 6. Silk, J., Beehner, J., Bergman, T., Crockford, C., Engh, A., Moscovice, L., Wittig, R., Seyfarth, R., and Cheney, D. (2010). Strong and consistent social bonds enhance the longevity of female baboons. Curr. Biol. 20, 1359–1361.
7. Barth, J., Schneider, S., and Von Kanel, R. (2010). Lack of social support in the etiology and the prognosis of coronary heart disease: A systematic review and meta-analysis. Psychosom. Med. 72, 229–238. 8. Cacioppo, J.T., and Hawkley, L.C. (2003). Social isolation and health, with an emphasis on underlying mechanisms. Perspect. Biol. Med. 46, S39–S52. 9. Seeman, T.E. (2000). Health promoting effects of friends and family on health outcomes in older adults. Am. J. Health Promotion 14, 362–370. 10. Uchino, B.N., Cacioppo, J.T., and Kiecoltglaser, J.K. (1996). The relationship between social support and physiological processes: A review with emphasis on underlying mechanisms and implications for health. Psychol. Bull. 119, 488–531. 11. House, J.S., Landis, K.R., and Umberson, D. (1988). Social relationships and health. Science 241, 540–545. 12. Taylor, S.E., Klein, L.C., Lewis, B.P., Gruenewald, T.L., Gurung, R.A.R., and Updegraff, J.A. (2000). Biobehavioral responses to stress in females: Tend-and-befriend, not fight-or-flight. Psychol. Rev. 107, 411–429.
Cell Cycle: Deconstructing Tension Prior to anaphase, sister chromatids must be attached to microtubules and under tension, a condition that satisfies the spindle checkpoint. Removal of sister chromatid cohesion is predicted to cause a fall in tension. Two studies shed light on how cells avoid re-activation of the spindle checkpoint when cohesion is lost. Andrea Musacchio The early life of sister chromatids, in the aftermath of DNA replication, is spent in the reassuring embrace of cohesion. Being prevented from loosing sight of each other, the sisters align as a pair on the mitotic spindle (metaphase). At this point, cohesion is removed and the sisters are abruptly parted to opposite spindle poles. As shocking as it may be, the separation of sisters at the metaphase–anaphase transition is for good. Failing to part sisters creates imbalances in chromosome numbers that derange cell physiology and put the rest of the family in jeopardy. Thus, when it comes to separating sisters, cells are quite inflexible and want to do it properly. Chromosomes attach to the mitotic spindle at kinetochores. These large protein scaffolds, built on centromeric DNA, promote the formation of load-bearing attachments to spindle microtubules [1]. They also regulate feedback control mechanisms required for errorless sister chromatid separation. The first mechanism, error correction, repairs erroneous connections of kinetochores with
spindle poles, such as syntelic (both sisters bound to the same pole) or merotelic (one sister bound to both poles) attachment. Likely, correction implies severing the incorrect connections, thus transiently generating unattached kinetochores. This, in turn, provides chromosomes with a new chance to bi-orient, i.e., reaching the correct configuration in which the sisters are bound to opposite spindle poles [2]. The second mechanism, the spindle assembly checkpoint, acts to synchronize mitotic exit to the achievement of bi-orientation of chromosomes on the mitotic spindle. Under normal conditions, the checkpoint becomes satisfied when all chromosomes are bound to spindle microtubules and bi-oriented. Once cells have transited through this obligatory step, sister chromatid cohesion can be removed [2]. The relationship between tension-dependent error correction and the spindle checkpoint is conceptually challenging and controversial. Based on pioneering studies by Nicklas on meiosis I spindles (reviewed in [2]), it was realized that tension stabilizes
13. Eriksson, B.G., Hessler, R.M., Sundh, J., and Steen, B. (1999). Cross-cultural analysis of longevity among swedish and american elders: The role of social networks in the gothenburg and missouri longitudinal studies compared. Arch. Gerontol. Geriatr. 28, 131–148. 14. Pope, T.R. (2000). Reproductive success increases with degree of kinship in cooperative coalitions of female red howler monkeys (alouatta seniculus). Behav. Ecol. Sociobiol. 48, 253–267. 15. Clutton-Brock, T., and Sheldon, B.C. (2010). The seven ages of pan. Science 327, 1207–1208. 16. Cheney, D.L., and Seyfath, R.M. (2007). Baboon Metaphysics: The Evolution of a Social Mind (Chicago: University of Chicago Press).
Department of Biology, Duke University, Box 90338, Durham NC 27708, USA. E-mail: [email protected] DOI: 10.1016/j.cub.2010.06.012
kinetochore–microtubule attachments, and that lack of tension favors error correction. Thus, a fundamental distinction between correct and incorrect attachments is that the former generate tension in the kinetochore and centromere region and are selectively stabilized, whereas the latter fail to do so and will eventually fall off. Understanding the molecular basis of this process is one of the current challenges in kinetochore biology. It has also largely become accepted that lack of microtubule attachment activates the checkpoint. The dispute concerns the role of tension (or lack thereof) in the spindle checkpoint. Three main models are crossing horns (Figure 1A). In Model 1, lack of tension acts indirectly on the checkpoint by promoting an error correction activity that ultimately generates unattached kinetochores (i.e., kinetochores that are devoid of microtubules). The latter, in turn, signal to the checkpoint. This model pictures the checkpoint and error correction as completely distinct but interconnected devices, purely sensing attachment and tension, respectively [3]. In Model 2, lack of tension acts directly on the checkpoint and on error correction regardless of whether unattached kinetochores are present. The checkpoint is imagined as consisting of two pathways, one sensing tension and one sensing attachment, and both possibly converging on the creation of the same effector complex. Finally, Model 3 makes the same assumptions
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as Model 2, but it neglects attachment, and imagines a single pathway having tension as the only relevant parameter. Tension may be completely missing in the absence of microtubules, and may be reaching a maximum when chromosomes are bi-oriented. In this model, error correction and the checkpoint are completely co-regulated, at least at the level of the sensory apparatus [1,2]. Two papers in this issue of Current Biology, by Mirchenko and Uhlmann [4] and by Vazquez-Novelle and Petronczki [5], and a recent paper by Oliveira and co-workers [6] in Nature Cell Biology, promise to ignite further discussion. All three papers start from the assumption, valid under all three models, that if sister chromatid cohesion balances spindle forces acting to part the sisters, its removal should decrease tension and re-activate error correction and the spindle checkpoint at anaphase, when the sisters are parted (Figure 1B). Re-activation of error correction due to a fall in tension, in turn, predicts that chromosomes should let go of microtubules. In a normal anaphase, however, this is never observed and it is interesting to ask why. Previous studies proposed that degradation of essential checkpoint components, such as Mps1, is crucial to prevent checkpoint re-activation at anaphase [7]. However, as noted by Mirchenko and Uhlmann [4], Mps1 degradation is a relatively late event during mitotic exit, not unlike the degradation of other checkpoint kinases, and it is therefore unlikely to be crucial for suppressing checkpoint re-activation at the metaphase–anaphase transition. The new papers, therefore, attempted to explore alternative hypotheses for why error correction and checkpoint signaling may be unable to shoot at anaphase. As cells satisfy the checkpoint, they start degrading Cyclin B and Securin (Pds1 in Saccharomyces cerevisiae), respectively triggering reduced Cdk1 activity and activation of separase (Esp1), a protease that cleaves cohesin. Cdk1 inactivation, or separase activation, may limit checkpoint re-activation and error correction at anaphase. The Mirchenko and Uhlmann and Vazquez-Novelle and Petronczki papers concentrated on the behavior of a four-subunit, evolutionarily conserved assembly named the chromosomal-passenger complex
A Model 1 Distinct error correction and checkpoint machinery
Model 2 Two branches
Proper or improper attachment Checkpoint satisfied
A. Unattached kinetochore Attachment-sensitive sensory machinery generates checkpoint signal
Improper attachment Lack of tension
B. Lack of tension (with improper attachment) Tension-sensitive sensory machinery turns on checkpoint and error correction
Lack of tension Error correction Model 3 Single pathway
Error correction Unattached kinetochore
Lack of tension (with or without attachment) Tension-sensitive sensory machinery generates checkpoint signal
Unattached kinetochore Checkpoint on
B Tension
Lack of tension
KT
Lack of tension
Centromere Kinetochore KT MT
Cleaved cohesin
Intact cohesin Prometaphase Error correction ON Checkpoint ON
Metaphase Error correction OFF Checkpoint OFF
Anaphase Error correction OFF? Checkpoint OFF?
Current Biology
Figure 1. Tension, attachment, and the spindle checkpoint. (A) Three hypothetical models for the relationship between error correction and the spindle assembly checkpoint are illustrated. Red and blue types indicate causes and consequences, respectively. In Model 1, there is different molecular machinery for error correction and for the checkpoint. Only attachment, whether correct or not, satisfies the checkpoint. However, as described in the inset, incorrect attachments failing to generate tension, such as syntelic or merotelic attachments [1], are intrinsically unstable and will be corrected. Likely, this requires the creation of an unattached kinetochore. The latter, in turn, signals to the checkpoint. Thus, lack of tension activates the checkpoint, but only indirectly through the creation of unattached kinetochores as an intermediate of error correction. In Model 2, the attachment- and tensionsensitive machinery is also distinct. However, tensionless attachments activate the checkpoint (as well as error correction) directly, rather than indirectly as in Model 1. An advantage of this model is that it does not require unattached kinetochores as an obligatory intermediate of correction, i.e. the checkpoint can be maintained because tension is missing even though all chromosomes are attached. This also applies to Model 3, which postulates the existence of a single pathway, which exclusively senses tension. No hypothesis is made concerning the possibility of sensing attachment because tension is the only relevant criterion to distinguish between proper attachments and improper or missing attachments. (B) Kinetochores devoid of microtubules are not under tension. Under these conditions, error correction and the spindle checkpoint are active. At metaphase, there is no error correction or spindle checkpoint activity because all the chromosomes are bi-oriented and under tension. Tension introduced by bound microtubules is resisted by cohesion in the centromere region. At anaphase, a decrease in tension is expected after cohesin is cleaved by separase (KT, kinetochore; MT, microtubule).
(CPC). Early work by Biggins, Tanaka, and co-workers [8,9] pointed to the CPC as a crucial component of the tension-sensing machinery required for error correction. At anaphase, the CPC relocates from centromeres to the
spindle midzone (from which it controls cytokinesis). Vazquez-Novelle and Petronczki [5] asked if interfering with the relocation of the CPC at anaphase in human cells is a sufficient condition to trigger error correction and checkpoint
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activation in anaphase. To prevent CPC relocation, they depleted Mklp2, a protein required for evicting the CPC from centromeres at anaphase [10,11], or mutated the CPC in a crucial residue required for the interaction with Mklp2. The Aurora B (Ipl1 in S. cerevisiae) kinase and its activator INCENP (Sli15) are two prominent subunits of the CPC, and Aurora B activity is crucial for error correction [12]. Despite clear evidence of Aurora B localization and activity at centromeres and kinetochores, Mklp2-depleted cells were unable to re-activate error correction at anaphase, and retained robust kinetochore fibers [5]. Thus, retention of active Aurora B at the centromere is per se insufficient to promote error correction. At first sight, these results are at odds with previous studies in Drosophila melanogaster demonstrating that loss of sister chromatid cohesion in the presence of non-degradable Cyclin B or through activation of an ectopic cohesin-cleaving protease causes the destabilization of kinetochoremicrotubule attachment at anaphase [6,13]. Also in this case, Aurora B retained its centromere localization because its relocation is inhibited in the presence of high Cdk activity [11] (see below). How can the discrepancy be explained? A sensible hypothesis is that the state of Cdk1 activity is relevant. Mklp2-depleted cells progress normally in the cell cycle and degrade Cyclin B with normal timing, so that at anaphase these cells experience low Cdk activity. Thus, a possible conclusion from the new analysis is that Cdk1 activity may be required for some aspect of error correction downstream of Aurora B, a testable hypothesis. Being subject to the same uncertainty whether sensing tension is directly or only indirectly affecting the checkpoint response, the role of the CPC in the checkpoint is highly controversial [2]. Although the checkpoint did not appear to be re-activated at anaphase when the CPC is retained at the centromere in Mklp2-depleted cells, three checkpoint kinases, Bub1, BubR1 and Mps1, were re-recruited to kinetochores in an Aurora B-dependent (and presumably Cdk1-independent) manner, despite the absence of error correction and of unattached kinetochores [5]. These results agree with previous observations on metaphase chromosomes experiencing a sudden
decrease of tension [14,15]. Changes in kinetochore behavior leading to Bub1, BubR1 and Mps1 re-recruitment confirm that kinetochores experience reduced tension at anaphase, and that as a minimum the removal of Aurora B from centromeres serves the purpose of preventing Bub1, BubR1 and Mps1 re-recruitment (and, possibly, re-activation). Overall, these observations are consistent with Models 2 and 3, and reinforce additional evidence for a direct role of Aurora B in the checkpoint, the most prominent of which is its requirement for the checkpoint response to unattached (and therefore tensionless) kinetochores in two model organisms such as S. pombe and X. laevis (reviewed in [2]). But why cannot the checkpoint be fully re-activated in Mklp2-depleted cells despite the effects on Bub1, BubR1 and Mps1? The authors show that two additional checkpoint proteins, Mad1 and Mad2, cannot be re-recruited to kinetochores [5]. A crucial question for the future is therefore why are not these proteins recruited back to kinetochores like Bub1, BubR1 and Mps1. A trivial answer is that these proteins cannot be recruited or retained at kinetochores when microtubules are bound there, in agreement with repeated observations that kinetochore localization of these proteins is incompatible with the presence of microtubules. However, Mad2 was not recruited to kinetochores even upon treating Mklp2-depleted anaphase cells with spindle-depolymerizing agents [5]. This result suggests that Cdk1 activity, which is low in Mklp2-depleted anaphase cells, regulates the localization at kinetochores of one or more of these proteins. Indeed, Cdk1 has been proposed to predispose the checkpoint target Cdc20 to inhibition by the checkpoint [16,17]. Whether Cdk1 controls additional steps of the checkpoint response is unclear, but suggested by the observations described above. Mirchenko and Uhlmann [4] and Oliveira and coworkers [6] applied conceptually similar approaches in different model systems, S. cerevisiae or D. melanogaster embryos, respectively. Anaphase was elicited ectopically through the expression of TEV protease and of a TEV-cleavable form of cohesin, and therefore under high CDK activity and in the absence of active Separase. Mirchenko and
Uhlmann [4] observed Ipl1-dependent re-recruitment of Bub1 to kinetochores as well as Mad1 phosphorylation, hallmarks of checkpoint activation. Oliveira and coworkers observed kinetochore detachment and re-recruitment of BubR1 to kinetochores, with both these effects becoming suppressed by the CDK-Cyclin inhibitor p27 [6]. While in other organisms the degradation of Cyclin B at the metaphase–anaphase transition is rapid, in S. cerevisiae the degradation of mitotic Cyclins is slow and extends well beyond the metaphase–anaphase transition. Thus, reduced Cdk activity is unlikely to be important for preventing checkpoint re-activation at anaphase in this organism, as substantial Cdk activity is normally present at this stage in the cell cycle. Thus, Mirchenko and Uhlmann looked at the rapid activation of Separase as an alternative hypothesis for irreversible inactivation of checkpoint and error correction at the metaphase–anaphase transition. Separase activation brings about additional effects other than Cohesin cleavage. Crucial for mitotic exit is the activation of the Cdc14 phosphatase, which reverses Cdk-mediated phosphorylation in this organism [18]. Inability to activate Cdc14 in cells where anaphase is triggered ectopically with TEV is the likely reason for checkpoint re-activation. Cdc14 has been shown to be important for the eviction of the Ipl1–Sli15 complex from centromeres [19]. In humans, the interaction of the CPC with Mklp2 is inhibited by Cdk1-dependent phosphorylation of INCENP [11]. Reduced Cdk1 activity at anaphase may result in INCENP dephosphorylation and removal of the CPC from the centromere, a mechanism that in its outline resembles that described for S. cerevisiae. If overall these results lend additional credit to Models 2 and 3, a mechanistic understanding for how tension may regulate the checkpoint is missing. Intra-kinetochore stretch has been recently shown to correlate with the state of checkpoint activation [2], suggesting that the answer to this question is to be sought in structural and chemical changes occurring dynamically within the kinetochore. In the future, it will be interesting to characterize the amount of intra-kinetochore stretch at anaphase. Models based on the ability of the CPC
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complex to ‘measure’ intra-kinetochore stretch have been proposed [1,9,20]. The new studies discussed here place additional emphasis on the role of Cdk inactivation during mitotic exit. A possible interpretation of the new observations is that Cdk activity may be required both for error correction and for the spindle checkpoint. Under the banner of Models 2 and 3, it is tempting to speculate that Cdk1 controls a specific step that is relevant both for error correction and the spindle checkpoint within a single pathway. Stay tuned on radio kinetochore for the next available updates.
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References 1. Santaguida, S., and Musacchio, A. (2009). The life and miracles of kinetochores. EMBO J. 28, 2511–2531. 2. Maresca, T.J., and Salmon, E.D. (2010). Welcome to a new kind of tension: translating kinetochore mechanics into a wait-anaphase signal. J. Cell Sci. 123, 825–835. 3. Khodjakov, A., and Rieder, C.L. (2009). The nature of cell-cycle checkpoints: facts and fallacies. J. Biol. 8, 88. 4. Mirchenko, L., and Uhlmann, F. (2010). Sli15 (INCENP) dephosphorylation prevents mitotic checkpoint re-engagement due to loss of tension at anaphase onset. Curr. Biol. 20, 1396–1401. 5. Vazquez-Novelle, M.D., and Petronczki, M. (2010). Relocation of the chromosomal passenger complex prevents mitotic
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checkpoint engagement at anaphase. Curr. Biol. 20, 1402–1407. Oliveira, R.A., Hamilton, R.S., Pauli, A., Davis, I., and Nasmyth, K. (2010). Cohesin cleavage and Cdk inhibition trigger formation of daughter nuclei. Nat. Cell Biol. 12, 185–192. Palframan, W.J., Meehl, J.B., Jaspersen, S.L., Winey, M., and Murray, A.W. (2006). Anaphase inactivation of the spindle checkpoint. Science 313, 680–684. Biggins, S., and Murray, A.W. (2001). The budding yeast protein kinase Ipl1/Aurora allows the absence of tension to activate the spindle checkpoint. Genes Dev. 15, 3118–3129. Tanaka, T.U., Rachidi, N., Janke, C., Pereira, G., Galova, M., Schiebel, E., Stark, M.J., and Nasmyth, K. (2002). Evidence that the Ipl1-Sli15 (Aurora kinase-INCENP) complex promotes chromosome bi-orientation by altering kinetochore-spindle pole connections. Cell 108, 317–329. Gruneberg, U., Neef, R., Honda, R., Nigg, E.A., and Barr, F.A. (2004). Relocation of Aurora B from centromeres to the central spindle at the metaphase to anaphase transition requires MKlp2. J. Cell Biol. 166, 167–172. Hummer, S., and Mayer, T.U. (2009). Cdk1 negatively regulates midzone localization of the mitotic kinesin Mklp2 and the chromosomal passenger complex. Curr. Biol. 19, 607–612. Carmena, M., Ruchaud, S., and Earnshaw, W.C. (2009). Making the Auroras glow: regulation of Aurora A and B kinase function by interacting proteins. Curr. Opin. Cell Biol. 21, 796–805. Parry, D.H., Hickson, G.R., and O’Farrell, P.H. (2003). Cyclin B destruction triggers changes in kinetochore behavior essential for successful anaphase. Curr. Biol. 13, 647–653. Famulski, J.K., and Chan, G.K. (2007). Aurora B kinase-dependent recruitment of
Evolutionary Genetics: Desperate Times Call for More Sex A new study has found that strains of the fungus Aspergillus nidulans produce more of their spores sexually in environments where they are less fit, resembling a hypothesized transitional stage in the evolution of sex. Clifford Zeyl Genetic recombination can amplify selection and accelerate adaptation by grouping adaptive mutations into fortunate genomes, and low-fitness alleles into genetic scapegoats that can be purged by selection, but that same randomization of allele combinations is also liable to break up a successful one. One way a parent might resolve this dilemma would be to clone itself if it is doing well, but to shuffle alleles if it is struggling. The mold Aspergillus nidulans is more reproductively flexible yet, varying the proportions of spores that it produces sexually and asexually. Schoustra et al. report in this issue [1] that strains of this fungus spend more
of their reproductive effort on sexual offspring in environments where they are faring poorly, improving the odds that recombination will yield offspring that will turn out to be better suited to the local conditions. Such opportunism may explain how alleles that increase sexuality incrementally could have spread through previously asexual populations in the first place, using recombination as a strategy to escape doomed genomes like metaphorical rats abandoning a sinking ship. The potential of a reshuffled genome to better suit changing conditions than either parent is the oldest and perhaps the most intuitively appealing of the many hypotheses for the evolutionary success of sex. But one reason why
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hZW10 and hROD to tensionless kinetochores. Curr. Biol. 17, 2143–2149. Skoufias, D.A., Andreassen, P.R., Lacroix, F.B., Wilson, L., and Margolis, R.L. (2001). Mammalian mad2 and bub1/bubR1 recognize distinct spindle-attachment and kinetochore-tension checkpoints. Proc. Natl. Acad. Sci. USA 98, 4492–4497. D’Angiolella, V., Mari, C., Nocera, D., Rametti, L., and Grieco, D. (2003). The spindle checkpoint requires cyclin-dependent kinase activity. Genes Dev. 17, 2520–2525. Chung, E., and Chen, R.H. (2003). Phosphorylation of Cdc20 is required for its inhibition by the spindle checkpoint. Nat. Cell Biol. 5, 748–753. Stegmeier, F., Visintin, R., and Amon, A. (2002). Separase, polo kinase, the kinetochore protein Slk19, and Spo12 function in a network that controls Cdc14 localization during early anaphase. Cell 108, 207–220. Pereira, G., and Schiebel, E. (2003). Separase regulates INCENP-Aurora B anaphase spindle function through Cdc14. Science 302, 2120–2124. Liu, D., Vader, G., Vromans, M.J., Lampson, M.A., and Lens, S.M. (2009). Sensing chromosome bi-orientation by spatial separation of aurora B kinase from kinetochore substrates. Science 323, 1350–1353.
Department of Experimental Oncology, European Institute of Oncology Via Adamello 16, I-20139 Milan, Italy. E-mail: andrea.musacchio@ ifom-ieo-campus.it
DOI: 10.1016/j.cub.2010.06.014
that success is still mystifying is that recombination also risks breaking up currently successful genotypes [2]. In addition to this genetic cost of sex, many facultatively sexual organisms pay an additional price for inserting sex into their fast-paced life cycles: sexual offspring take longer to produce than asexual ones. In taxa as diverse as crustaceans, algae and fungi, the products of either meiosis or syngamy can survive desiccation and other abuses as dormant spores or eggs, and are often the stage at which most dispersal occurs. But in the time it takes these structures to develop and mature, a competitor that remained asexual would typically be able to produce a greater number of cloned offspring. However, in facultatively sexual species that follow this pattern, the switch from faster and more efficient asexual reproduction to sex is triggered by stressful conditions such as overcrowding or starvation — circumstances that would make quick reproduction impossible anyway. These life cycles
References 1. Alexander, R.D. (1974). The evolution of social behavior. Annu. Rev. Ecol. Syst. 5, 324–383. 2. Van Schaik, C.P. (1983). Why are diurnal primates living in groups. Behaviour 87, 120–144. 3. Wrangham, R.W. (1980). An ecological model of female-bonded primate groups. Behaviour 75, 262–300. 4. Silk, J.B., Alberts, S.C., and Altmann, J. (2003). Social bonds of female baboons enhance infant survival. Science 302, 1231–1234. 5. Silk, J.B., Beehner, J.C., Bergman, T.J., Crockford, C., Engh, A.L., Moscovice, L.R., Wittig, R.M., Seyfarth, R.M., and Cheney, D.L. (2009). The benefits of social capital: Close social bonds among female baboons enhance offspring survival. Proc. R. Soc. Lond. B. Biol. Sci. 276, 3099–3104. 6. Silk, J., Beehner, J., Bergman, T., Crockford, C., Engh, A., Moscovice, L., Wittig, R., Seyfarth, R., and Cheney, D. (2010). Strong and consistent social bonds enhance the longevity of female baboons. Curr. Biol. 20, 1359–1361.
7. Barth, J., Schneider, S., and Von Kanel, R. (2010). Lack of social support in the etiology and the prognosis of coronary heart disease: A systematic review and meta-analysis. Psychosom. Med. 72, 229–238. 8. Cacioppo, J.T., and Hawkley, L.C. (2003). Social isolation and health, with an emphasis on underlying mechanisms. Perspect. Biol. Med. 46, S39–S52. 9. Seeman, T.E. (2000). Health promoting effects of friends and family on health outcomes in older adults. Am. J. Health Promotion 14, 362–370. 10. Uchino, B.N., Cacioppo, J.T., and Kiecoltglaser, J.K. (1996). The relationship between social support and physiological processes: A review with emphasis on underlying mechanisms and implications for health. Psychol. Bull. 119, 488–531. 11. House, J.S., Landis, K.R., and Umberson, D. (1988). Social relationships and health. Science 241, 540–545. 12. Taylor, S.E., Klein, L.C., Lewis, B.P., Gruenewald, T.L., Gurung, R.A.R., and Updegraff, J.A. (2000). Biobehavioral responses to stress in females: Tend-and-befriend, not fight-or-flight. Psychol. Rev. 107, 411–429.
Cell Cycle: Deconstructing Tension Prior to anaphase, sister chromatids must be attached to microtubules and under tension, a condition that satisfies the spindle checkpoint. Removal of sister chromatid cohesion is predicted to cause a fall in tension. Two studies shed light on how cells avoid re-activation of the spindle checkpoint when cohesion is lost. Andrea Musacchio The early life of sister chromatids, in the aftermath of DNA replication, is spent in the reassuring embrace of cohesion. Being prevented from loosing sight of each other, the sisters align as a pair on the mitotic spindle (metaphase). At this point, cohesion is removed and the sisters are abruptly parted to opposite spindle poles. As shocking as it may be, the separation of sisters at the metaphase–anaphase transition is for good. Failing to part sisters creates imbalances in chromosome numbers that derange cell physiology and put the rest of the family in jeopardy. Thus, when it comes to separating sisters, cells are quite inflexible and want to do it properly. Chromosomes attach to the mitotic spindle at kinetochores. These large protein scaffolds, built on centromeric DNA, promote the formation of load-bearing attachments to spindle microtubules [1]. They also regulate feedback control mechanisms required for errorless sister chromatid separation. The first mechanism, error correction, repairs erroneous connections of kinetochores with
spindle poles, such as syntelic (both sisters bound to the same pole) or merotelic (one sister bound to both poles) attachment. Likely, correction implies severing the incorrect connections, thus transiently generating unattached kinetochores. This, in turn, provides chromosomes with a new chance to bi-orient, i.e., reaching the correct configuration in which the sisters are bound to opposite spindle poles [2]. The second mechanism, the spindle assembly checkpoint, acts to synchronize mitotic exit to the achievement of bi-orientation of chromosomes on the mitotic spindle. Under normal conditions, the checkpoint becomes satisfied when all chromosomes are bound to spindle microtubules and bi-oriented. Once cells have transited through this obligatory step, sister chromatid cohesion can be removed [2]. The relationship between tension-dependent error correction and the spindle checkpoint is conceptually challenging and controversial. Based on pioneering studies by Nicklas on meiosis I spindles (reviewed in [2]), it was realized that tension stabilizes
13. Eriksson, B.G., Hessler, R.M., Sundh, J., and Steen, B. (1999). Cross-cultural analysis of longevity among swedish and american elders: The role of social networks in the gothenburg and missouri longitudinal studies compared. Arch. Gerontol. Geriatr. 28, 131–148. 14. Pope, T.R. (2000). Reproductive success increases with degree of kinship in cooperative coalitions of female red howler monkeys (alouatta seniculus). Behav. Ecol. Sociobiol. 48, 253–267. 15. Clutton-Brock, T., and Sheldon, B.C. (2010). The seven ages of pan. Science 327, 1207–1208. 16. Cheney, D.L., and Seyfath, R.M. (2007). Baboon Metaphysics: The Evolution of a Social Mind (Chicago: University of Chicago Press).
Department of Biology, Duke University, Box 90338, Durham NC 27708, USA. E-mail: [email protected] DOI: 10.1016/j.cub.2010.06.012
kinetochore–microtubule attachments, and that lack of tension favors error correction. Thus, a fundamental distinction between correct and incorrect attachments is that the former generate tension in the kinetochore and centromere region and are selectively stabilized, whereas the latter fail to do so and will eventually fall off. Understanding the molecular basis of this process is one of the current challenges in kinetochore biology. It has also largely become accepted that lack of microtubule attachment activates the checkpoint. The dispute concerns the role of tension (or lack thereof) in the spindle checkpoint. Three main models are crossing horns (Figure 1A). In Model 1, lack of tension acts indirectly on the checkpoint by promoting an error correction activity that ultimately generates unattached kinetochores (i.e., kinetochores that are devoid of microtubules). The latter, in turn, signal to the checkpoint. This model pictures the checkpoint and error correction as completely distinct but interconnected devices, purely sensing attachment and tension, respectively [3]. In Model 2, lack of tension acts directly on the checkpoint and on error correction regardless of whether unattached kinetochores are present. The checkpoint is imagined as consisting of two pathways, one sensing tension and one sensing attachment, and both possibly converging on the creation of the same effector complex. Finally, Model 3 makes the same assumptions
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as Model 2, but it neglects attachment, and imagines a single pathway having tension as the only relevant parameter. Tension may be completely missing in the absence of microtubules, and may be reaching a maximum when chromosomes are bi-oriented. In this model, error correction and the checkpoint are completely co-regulated, at least at the level of the sensory apparatus [1,2]. Two papers in this issue of Current Biology, by Mirchenko and Uhlmann [4] and by Vazquez-Novelle and Petronczki [5], and a recent paper by Oliveira and co-workers [6] in Nature Cell Biology, promise to ignite further discussion. All three papers start from the assumption, valid under all three models, that if sister chromatid cohesion balances spindle forces acting to part the sisters, its removal should decrease tension and re-activate error correction and the spindle checkpoint at anaphase, when the sisters are parted (Figure 1B). Re-activation of error correction due to a fall in tension, in turn, predicts that chromosomes should let go of microtubules. In a normal anaphase, however, this is never observed and it is interesting to ask why. Previous studies proposed that degradation of essential checkpoint components, such as Mps1, is crucial to prevent checkpoint re-activation at anaphase [7]. However, as noted by Mirchenko and Uhlmann [4], Mps1 degradation is a relatively late event during mitotic exit, not unlike the degradation of other checkpoint kinases, and it is therefore unlikely to be crucial for suppressing checkpoint re-activation at the metaphase–anaphase transition. The new papers, therefore, attempted to explore alternative hypotheses for why error correction and checkpoint signaling may be unable to shoot at anaphase. As cells satisfy the checkpoint, they start degrading Cyclin B and Securin (Pds1 in Saccharomyces cerevisiae), respectively triggering reduced Cdk1 activity and activation of separase (Esp1), a protease that cleaves cohesin. Cdk1 inactivation, or separase activation, may limit checkpoint re-activation and error correction at anaphase. The Mirchenko and Uhlmann and Vazquez-Novelle and Petronczki papers concentrated on the behavior of a four-subunit, evolutionarily conserved assembly named the chromosomal-passenger complex
A Model 1 Distinct error correction and checkpoint machinery
Model 2 Two branches
Proper or improper attachment Checkpoint satisfied
A. Unattached kinetochore Attachment-sensitive sensory machinery generates checkpoint signal
Improper attachment Lack of tension
B. Lack of tension (with improper attachment) Tension-sensitive sensory machinery turns on checkpoint and error correction
Lack of tension Error correction Model 3 Single pathway
Error correction Unattached kinetochore
Lack of tension (with or without attachment) Tension-sensitive sensory machinery generates checkpoint signal
Unattached kinetochore Checkpoint on
B Tension
Lack of tension
KT
Lack of tension
Centromere Kinetochore KT MT
Cleaved cohesin
Intact cohesin Prometaphase Error correction ON Checkpoint ON
Metaphase Error correction OFF Checkpoint OFF
Anaphase Error correction OFF? Checkpoint OFF?
Current Biology
Figure 1. Tension, attachment, and the spindle checkpoint. (A) Three hypothetical models for the relationship between error correction and the spindle assembly checkpoint are illustrated. Red and blue types indicate causes and consequences, respectively. In Model 1, there is different molecular machinery for error correction and for the checkpoint. Only attachment, whether correct or not, satisfies the checkpoint. However, as described in the inset, incorrect attachments failing to generate tension, such as syntelic or merotelic attachments [1], are intrinsically unstable and will be corrected. Likely, this requires the creation of an unattached kinetochore. The latter, in turn, signals to the checkpoint. Thus, lack of tension activates the checkpoint, but only indirectly through the creation of unattached kinetochores as an intermediate of error correction. In Model 2, the attachment- and tensionsensitive machinery is also distinct. However, tensionless attachments activate the checkpoint (as well as error correction) directly, rather than indirectly as in Model 1. An advantage of this model is that it does not require unattached kinetochores as an obligatory intermediate of correction, i.e. the checkpoint can be maintained because tension is missing even though all chromosomes are attached. This also applies to Model 3, which postulates the existence of a single pathway, which exclusively senses tension. No hypothesis is made concerning the possibility of sensing attachment because tension is the only relevant criterion to distinguish between proper attachments and improper or missing attachments. (B) Kinetochores devoid of microtubules are not under tension. Under these conditions, error correction and the spindle checkpoint are active. At metaphase, there is no error correction or spindle checkpoint activity because all the chromosomes are bi-oriented and under tension. Tension introduced by bound microtubules is resisted by cohesion in the centromere region. At anaphase, a decrease in tension is expected after cohesin is cleaved by separase (KT, kinetochore; MT, microtubule).
(CPC). Early work by Biggins, Tanaka, and co-workers [8,9] pointed to the CPC as a crucial component of the tension-sensing machinery required for error correction. At anaphase, the CPC relocates from centromeres to the
spindle midzone (from which it controls cytokinesis). Vazquez-Novelle and Petronczki [5] asked if interfering with the relocation of the CPC at anaphase in human cells is a sufficient condition to trigger error correction and checkpoint
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activation in anaphase. To prevent CPC relocation, they depleted Mklp2, a protein required for evicting the CPC from centromeres at anaphase [10,11], or mutated the CPC in a crucial residue required for the interaction with Mklp2. The Aurora B (Ipl1 in S. cerevisiae) kinase and its activator INCENP (Sli15) are two prominent subunits of the CPC, and Aurora B activity is crucial for error correction [12]. Despite clear evidence of Aurora B localization and activity at centromeres and kinetochores, Mklp2-depleted cells were unable to re-activate error correction at anaphase, and retained robust kinetochore fibers [5]. Thus, retention of active Aurora B at the centromere is per se insufficient to promote error correction. At first sight, these results are at odds with previous studies in Drosophila melanogaster demonstrating that loss of sister chromatid cohesion in the presence of non-degradable Cyclin B or through activation of an ectopic cohesin-cleaving protease causes the destabilization of kinetochoremicrotubule attachment at anaphase [6,13]. Also in this case, Aurora B retained its centromere localization because its relocation is inhibited in the presence of high Cdk activity [11] (see below). How can the discrepancy be explained? A sensible hypothesis is that the state of Cdk1 activity is relevant. Mklp2-depleted cells progress normally in the cell cycle and degrade Cyclin B with normal timing, so that at anaphase these cells experience low Cdk activity. Thus, a possible conclusion from the new analysis is that Cdk1 activity may be required for some aspect of error correction downstream of Aurora B, a testable hypothesis. Being subject to the same uncertainty whether sensing tension is directly or only indirectly affecting the checkpoint response, the role of the CPC in the checkpoint is highly controversial [2]. Although the checkpoint did not appear to be re-activated at anaphase when the CPC is retained at the centromere in Mklp2-depleted cells, three checkpoint kinases, Bub1, BubR1 and Mps1, were re-recruited to kinetochores in an Aurora B-dependent (and presumably Cdk1-independent) manner, despite the absence of error correction and of unattached kinetochores [5]. These results agree with previous observations on metaphase chromosomes experiencing a sudden
decrease of tension [14,15]. Changes in kinetochore behavior leading to Bub1, BubR1 and Mps1 re-recruitment confirm that kinetochores experience reduced tension at anaphase, and that as a minimum the removal of Aurora B from centromeres serves the purpose of preventing Bub1, BubR1 and Mps1 re-recruitment (and, possibly, re-activation). Overall, these observations are consistent with Models 2 and 3, and reinforce additional evidence for a direct role of Aurora B in the checkpoint, the most prominent of which is its requirement for the checkpoint response to unattached (and therefore tensionless) kinetochores in two model organisms such as S. pombe and X. laevis (reviewed in [2]). But why cannot the checkpoint be fully re-activated in Mklp2-depleted cells despite the effects on Bub1, BubR1 and Mps1? The authors show that two additional checkpoint proteins, Mad1 and Mad2, cannot be re-recruited to kinetochores [5]. A crucial question for the future is therefore why are not these proteins recruited back to kinetochores like Bub1, BubR1 and Mps1. A trivial answer is that these proteins cannot be recruited or retained at kinetochores when microtubules are bound there, in agreement with repeated observations that kinetochore localization of these proteins is incompatible with the presence of microtubules. However, Mad2 was not recruited to kinetochores even upon treating Mklp2-depleted anaphase cells with spindle-depolymerizing agents [5]. This result suggests that Cdk1 activity, which is low in Mklp2-depleted anaphase cells, regulates the localization at kinetochores of one or more of these proteins. Indeed, Cdk1 has been proposed to predispose the checkpoint target Cdc20 to inhibition by the checkpoint [16,17]. Whether Cdk1 controls additional steps of the checkpoint response is unclear, but suggested by the observations described above. Mirchenko and Uhlmann [4] and Oliveira and coworkers [6] applied conceptually similar approaches in different model systems, S. cerevisiae or D. melanogaster embryos, respectively. Anaphase was elicited ectopically through the expression of TEV protease and of a TEV-cleavable form of cohesin, and therefore under high CDK activity and in the absence of active Separase. Mirchenko and
Uhlmann [4] observed Ipl1-dependent re-recruitment of Bub1 to kinetochores as well as Mad1 phosphorylation, hallmarks of checkpoint activation. Oliveira and coworkers observed kinetochore detachment and re-recruitment of BubR1 to kinetochores, with both these effects becoming suppressed by the CDK-Cyclin inhibitor p27 [6]. While in other organisms the degradation of Cyclin B at the metaphase–anaphase transition is rapid, in S. cerevisiae the degradation of mitotic Cyclins is slow and extends well beyond the metaphase–anaphase transition. Thus, reduced Cdk activity is unlikely to be important for preventing checkpoint re-activation at anaphase in this organism, as substantial Cdk activity is normally present at this stage in the cell cycle. Thus, Mirchenko and Uhlmann looked at the rapid activation of Separase as an alternative hypothesis for irreversible inactivation of checkpoint and error correction at the metaphase–anaphase transition. Separase activation brings about additional effects other than Cohesin cleavage. Crucial for mitotic exit is the activation of the Cdc14 phosphatase, which reverses Cdk-mediated phosphorylation in this organism [18]. Inability to activate Cdc14 in cells where anaphase is triggered ectopically with TEV is the likely reason for checkpoint re-activation. Cdc14 has been shown to be important for the eviction of the Ipl1–Sli15 complex from centromeres [19]. In humans, the interaction of the CPC with Mklp2 is inhibited by Cdk1-dependent phosphorylation of INCENP [11]. Reduced Cdk1 activity at anaphase may result in INCENP dephosphorylation and removal of the CPC from the centromere, a mechanism that in its outline resembles that described for S. cerevisiae. If overall these results lend additional credit to Models 2 and 3, a mechanistic understanding for how tension may regulate the checkpoint is missing. Intra-kinetochore stretch has been recently shown to correlate with the state of checkpoint activation [2], suggesting that the answer to this question is to be sought in structural and chemical changes occurring dynamically within the kinetochore. In the future, it will be interesting to characterize the amount of intra-kinetochore stretch at anaphase. Models based on the ability of the CPC
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complex to ‘measure’ intra-kinetochore stretch have been proposed [1,9,20]. The new studies discussed here place additional emphasis on the role of Cdk inactivation during mitotic exit. A possible interpretation of the new observations is that Cdk activity may be required both for error correction and for the spindle checkpoint. Under the banner of Models 2 and 3, it is tempting to speculate that Cdk1 controls a specific step that is relevant both for error correction and the spindle checkpoint within a single pathway. Stay tuned on radio kinetochore for the next available updates.
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References 1. Santaguida, S., and Musacchio, A. (2009). The life and miracles of kinetochores. EMBO J. 28, 2511–2531. 2. Maresca, T.J., and Salmon, E.D. (2010). Welcome to a new kind of tension: translating kinetochore mechanics into a wait-anaphase signal. J. Cell Sci. 123, 825–835. 3. Khodjakov, A., and Rieder, C.L. (2009). The nature of cell-cycle checkpoints: facts and fallacies. J. Biol. 8, 88. 4. Mirchenko, L., and Uhlmann, F. (2010). Sli15 (INCENP) dephosphorylation prevents mitotic checkpoint re-engagement due to loss of tension at anaphase onset. Curr. Biol. 20, 1396–1401. 5. Vazquez-Novelle, M.D., and Petronczki, M. (2010). Relocation of the chromosomal passenger complex prevents mitotic
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Evolutionary Genetics: Desperate Times Call for More Sex A new study has found that strains of the fungus Aspergillus nidulans produce more of their spores sexually in environments where they are less fit, resembling a hypothesized transitional stage in the evolution of sex. Clifford Zeyl Genetic recombination can amplify selection and accelerate adaptation by grouping adaptive mutations into fortunate genomes, and low-fitness alleles into genetic scapegoats that can be purged by selection, but that same randomization of allele combinations is also liable to break up a successful one. One way a parent might resolve this dilemma would be to clone itself if it is doing well, but to shuffle alleles if it is struggling. The mold Aspergillus nidulans is more reproductively flexible yet, varying the proportions of spores that it produces sexually and asexually. Schoustra et al. report in this issue [1] that strains of this fungus spend more
of their reproductive effort on sexual offspring in environments where they are faring poorly, improving the odds that recombination will yield offspring that will turn out to be better suited to the local conditions. Such opportunism may explain how alleles that increase sexuality incrementally could have spread through previously asexual populations in the first place, using recombination as a strategy to escape doomed genomes like metaphorical rats abandoning a sinking ship. The potential of a reshuffled genome to better suit changing conditions than either parent is the oldest and perhaps the most intuitively appealing of the many hypotheses for the evolutionary success of sex. But one reason why
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hZW10 and hROD to tensionless kinetochores. Curr. Biol. 17, 2143–2149. Skoufias, D.A., Andreassen, P.R., Lacroix, F.B., Wilson, L., and Margolis, R.L. (2001). Mammalian mad2 and bub1/bubR1 recognize distinct spindle-attachment and kinetochore-tension checkpoints. Proc. Natl. Acad. Sci. USA 98, 4492–4497. D’Angiolella, V., Mari, C., Nocera, D., Rametti, L., and Grieco, D. (2003). The spindle checkpoint requires cyclin-dependent kinase activity. Genes Dev. 17, 2520–2525. Chung, E., and Chen, R.H. (2003). Phosphorylation of Cdc20 is required for its inhibition by the spindle checkpoint. Nat. Cell Biol. 5, 748–753. Stegmeier, F., Visintin, R., and Amon, A. (2002). Separase, polo kinase, the kinetochore protein Slk19, and Spo12 function in a network that controls Cdc14 localization during early anaphase. Cell 108, 207–220. Pereira, G., and Schiebel, E. (2003). Separase regulates INCENP-Aurora B anaphase spindle function through Cdc14. Science 302, 2120–2124. Liu, D., Vader, G., Vromans, M.J., Lampson, M.A., and Lens, S.M. (2009). Sensing chromosome bi-orientation by spatial separation of aurora B kinase from kinetochore substrates. Science 323, 1350–1353.
Department of Experimental Oncology, European Institute of Oncology Via Adamello 16, I-20139 Milan, Italy. E-mail: andrea.musacchio@ ifom-ieo-campus.it
DOI: 10.1016/j.cub.2010.06.014
that success is still mystifying is that recombination also risks breaking up currently successful genotypes [2]. In addition to this genetic cost of sex, many facultatively sexual organisms pay an additional price for inserting sex into their fast-paced life cycles: sexual offspring take longer to produce than asexual ones. In taxa as diverse as crustaceans, algae and fungi, the products of either meiosis or syngamy can survive desiccation and other abuses as dormant spores or eggs, and are often the stage at which most dispersal occurs. But in the time it takes these structures to develop and mature, a competitor that remained asexual would typically be able to produce a greater number of cloned offspring. However, in facultatively sexual species that follow this pattern, the switch from faster and more efficient asexual reproduction to sex is triggered by stressful conditions such as overcrowding or starvation — circumstances that would make quick reproduction impossible anyway. These life cycles