Portal de Periódicos da CAPES

Cdc14B and APC/C Tackle DNA Damage

2008; Cell Press; Volume: 134; Issue: 2 Linguagem: Inglês

10.1016/j.cell.2008.07.004

1097-4172

Peter De Wulf, Rosella Visintin,

Microtubule and mitosis dynamics

Mitotic exit in budding yeast is regulated by the proteins Cdc14, APC/CCdh1, and Plk1. In this issue, Bassermann et al., 2008Bassermann F. Frescas D. Guardavaccaro D. Busino L. Peschiaroli A. Pagano M. Cell. 2008; (this issue, ■■■–■■■)PubMed Google Scholar show that this network of proteins has been rewired in human cells to control the cell cycle in response to DNA damage. Mitotic exit in budding yeast is regulated by the proteins Cdc14, APC/CCdh1, and Plk1. In this issue, Bassermann et al., 2008Bassermann F. Frescas D. Guardavaccaro D. Busino L. Peschiaroli A. Pagano M. Cell. 2008; (this issue, ■■■–■■■)PubMed Google Scholar show that this network of proteins has been rewired in human cells to control the cell cycle in response to DNA damage. Molecular networks often adapt to perform different tasks in different organisms. For example, the small GTP-binding Ras proteins that are implicated in nutrient uptake in the budding yeast Saccharomyces cerevisiae have evolved into central regulators of growth and proliferation in metazoans. In this issue of Cell, Bassermann and coworkers describe a similarly striking example of "molecular bricolage," a clever adaptation of materials at hand for a new purpose. The authors show that the conserved regulatory network consisting of the phosphatase Cdc14, the ubiquitin ligase anaphase-promoting complex or cyclosome (APC/C), and the Polo-like kinase 1 (Plk1)—long implicated in controlling mitotic exit in budding yeast—is repurposed in human cells to mediate the DNA-damage-induced G2 cell-cycle block. The goal of cell division is to generate two daughter cells genetically identical to the mother cell. To minimize genetic instability, eukaryotes have developed surveillance mechanisms called checkpoints (Figure 1) consisting of highly conserved signal transduction pathways that monitor cell-cycle progression and delay the process when key events go wrong to allow for error correction. The G2 checkpoint is an important component of the DNA-damage response, which delays entry into mitosis in the presence of DNA lesions. During the G2 block, numerous proteins including the ATR kinase collaborate to activate the kinase Chk1, which maintains a cell-cycle arrest through inhibition of mitotic cyclin-dependent kinase (Cdk1) activity. Chk1 inactivates the Cdc25 protein to ensure that Cdk1 remains in an inactive state (Figure 1). Cell-cycle reentry following DNA repair is mediated by Plk1, whose kinase activity triggers the degradation of a series of targets, including Claspin, an adaptor protein required by ATR to maintain Chk1 activity (Branzei and Foiani, 2008Branzei D. Foiani M. Nat. Rev. Mol. Cell Biol. 2008; 9: 297-308Crossref PubMed Scopus (839) Google Scholar, Harper and Elledge, 2007Harper J.W. Elledge S.J. Mol. Cell. 2007; 28: 739-745Abstract Full Text Full Text PDF PubMed Scopus (1191) Google Scholar). Although many of the proteins mediating the G2 cell-cycle block and recovery from it are known, numerous open questions remain. For example, how is Plk1 activity restrained during the G2 cell-cycle block? Bassermann and colleagues set out to elucidate this question and implicated the APC/C ubiquitin ligase complex in Plk1 degradation during the DNA-damage response in cultured human cells (Figure 1, shaded box). The APC/C, a central regulator of the cell cycle, is a multisubunit ubiquitin ligase thought to primarily function during mitosis and in G1. Its activity depends on two distinct activator proteins named Cdc20 and Cdh1 (the resulting complexes with the APC/C are indicated as APC/CCdc20 and APC/CCdh1, respectively). Whereas Cdc20 mediates entry into anaphase, Cdh1 mediates events that are important for mitotic exit and maintenance of the G1 state. APC/CCdh1 activity is suppressed during the G2 phase of the cell cycle. However, the APC/CCdh1 is transiently activated in G2 upon DNA damage (Sudo et al., 2001Sudo T. Ota Y. Kotani S. Nakao M. Takami Y. Takeda S. Saya H. EMBO J. 2001; 20: 6499-6508Crossref PubMed Scopus (117) Google Scholar). Bassermann et al. now connect this transient activation of the APC/CCdh1 to the regulation of Plk1. Out of 15 known APC/CCdh1 substrates, the authors found that only Plk1 was degraded while others such as Claspin appeared to be protected from degradation despite being ubiquitinated. Indeed, DNA-damaged cells expressing a mutant form of Plk1 not recognized by the APC/CCdh1 (and thus not degraded) showed a greater tendency to escape from the DNA-damage checkpoint and enter mitosis. Intriguingly, though Cdh1 is a not an essential gene for mouse embryonic and somatic cells, mouse embryonic fibroblasts lacking Cdh1 showed increased genomic instability, a possible consequence of a faulty DNA-damage checkpoint (Garci-Higuera et al., 2008Garci-Higuera I. Manchado E. Dubus P. Canamero M. Mendez J. Moreno S. Malumbres M. Nat. Cell Biol. 2008; 10: 802-811Crossref PubMed Scopus (275) Google Scholar). If APC/CCdh1 activity is normally suppressed in G2, how is it reactivated to downregulate Plk1 protein levels upon DNA damage in human cells? Previous studies have shown that in order to interact with the APC/C, the Cdk-mediated phosphorylation of Cdh1 must be reversed. In budding yeast, this dephosphorylation is mediated by the conserved phosphatase Cdc14, a protein crucial for exit from mitosis (Stegmeier and Amon, 2004Stegmeier F. Amon A. Annu. Rev. Genet. 2004; 38: 203-232Crossref PubMed Scopus (347) Google Scholar). Cdc14 activity in budding yeast is modulated spatially: Cdc14 is kept inactive inside the nucleolus by its inhibitor Cfi1/Net1 until metaphase. During early anaphase, Cdc14 is released into the nucleus. In mid-to-late anaphase, it also moves into the cytoplasm to reach its myriad substrates, including Cdh1 (Stegmeier and Amon, 2004Stegmeier F. Amon A. Annu. Rev. Genet. 2004; 38: 203-232Crossref PubMed Scopus (347) Google Scholar). In humans, the two orthologs of Cdc14 (Cdc14A and Cdc14B) localize dynamically during the cell cycle. Cdc14A localizes to interphase centrosomes and regulates centrosome separation, whereas Cdc14B localizes to interphase nucleoli similarly to budding yeast Cdc14. During mitosis, both isoforms disperse throughout the cell (Krasinska et al., 2007Krasinska L. de Bettignies G. Fisher D. Abrieu A. Fesquet D. Morin N. Exp. Cell Res. 2007; 313: 1225-1239Crossref PubMed Scopus (27) Google Scholar), but the details of their activity following their release remain poorly understood. Guided by previous studies in budding yeast, Bassermann and colleagues examined if Cdc14A and Cdc14B change localization upon DNA damage. The authors found that following the activation of the DNA-damage response, only Cdc14B relocalized from the nucleolus into the nucleus. Furthermore, Cdc14B bound to Cdh1 and triggered APC/CCdh1 activity in a manner similar to that observed in budding yeast. Consistent with a role in regulating Plk1 levels via regulation of Cdh1, silencing Cdc14B led to a stabilization of the Plk1 protein similar to that observed when Cdh1 is silenced. Though the role of Cdc14 in mitotic exit is well established in budding yeast, clues to its function in higher eukaryotes have been scanty until now. This exciting demonstration of association between Cdc14 activity and the DNA-damage response opens the door to many new questions. For example, does an analogous G2 regulatory pathway exist in budding yeast or is this newly discovered activity of Cdc14 and its targets a reflection of nature's creative use of a conserved pathway to provide solutions for a specific need? This regulatory network uncovered by Bassermann et al. also opens up new areas of inquiry. One question concerns the mechanism by which the APC/CCdh1 is activated. Previous analyses have demonstrated that the APC/CCdh1 is kept inactive during G2 not only via phosphorylation but also by its inhibitor Emi1 (Di Fiore and Pines, 2008Di Fiore B. Pines J. Chromosoma. 2008; 117: 333-338Crossref PubMed Scopus (21) Google Scholar). But how does Emi integrate into this role of APC/CCdh1 during the DNA-damage response? Is its inactivation required for Plk1 degradation? It will be important in future experiments to examine the fate of Emi1 during the DNA-damage response. Another question pertains to how and via what signals Cdc14B becomes released from the nucleolus upon DNA damage. In budding yeast, the release of Cdc14 occurs through the sequential action of two regulatory networks, the FEAR (Cdcfourteen early anaphase release) network and the MEN (mitotic exit network) (Stegmeier and Amon, 2004Stegmeier F. Amon A. Annu. Rev. Genet. 2004; 38: 203-232Crossref PubMed Scopus (347) Google Scholar). Although the components of both pathways are not fully conserved in humans, the initiation and maintenance of released Cdc14 in budding yeast depend on the Plk1 homolog Cdc5. By activating APC/CCdh1, which in turn promotes the degradation of Cdc5, Cdc14 triggers its own inactivation (Visintin et al., 2008Visintin C. Tomson B.N. Rahal R. Paulson J. Cohen M. Taunton J. Amon A. Visintin R. Genes Dev. 2008; 22: 79-90Crossref PubMed Scopus (65) Google Scholar). It is possible that a similar mechanism underlies the Cdc14B-mediated DNA-damage response. If Plk1 proves to be important for the release of Cdc14B in the DNA-damage response, then the same circuitry could govern both the entry and exit from the G2 cell-cycle block as Cdc14 inactivation could then allow for the accumulation of Plk to promote cell-cycle reentry. The work by Bassermann et al. suggests broader roles for both Cdc14 and APC/CCdh1 in the regulation of the cell cycle, thus shedding light on an interesting new pathway in the cellular response to DNA damage. We thank F. d'Adda di Fagagna, M. Foiani, W. Harper, A. Musacchio, and J. Pines for comments and suggestions. The Cdc14B-Cdh1-Plk1 Axis Controls the G2 DNA-Damage-Response CheckpointBassermann et al.CellJuly 25, 2008In BriefIn response to DNA damage in G2, mammalian cells must avoid entry into mitosis and instead initiate DNA repair. Here, we show that, in response to genotoxic stress in G2, the phosphatase Cdc14B translocates from the nucleolus to the nucleoplasm and induces the activation of the ubiquitin ligase APC/CCdh1, with the consequent degradation of Plk1, a prominent mitotic kinase. This process induces the stabilization of Claspin, an activator of the DNA-damage checkpoint, and Wee1, an inhibitor of cell-cycle progression, and allows an efficient G2 checkpoint. Full-Text PDF Open Archive