Eukaryotic cells rely on a surveillance mechanism known as the spindle checkpoint to ensure accurate chromosome segregation. The spindle checkpoint prevents sister chromatids from separating until all kinetochores achieve bipolar attachments to the mitotic spindle. Checkpoint proteins tightly inhibit the anaphase-promoting complex (APC), a ubiquitin ligase required for chromosome segregation and progression to anaphase. Unattached kinetochores promote the binding of checkpoint proteins Mad2 and BubR1 to the APC-activator Cdc20, rendering it unable to activate APC. Once all kinetochores are properly attached, however, cells inactivate the checkpoint within minutes, allowing for the rapid and synchronous segregation of chromosomes. How cells switch from strong APC inhibition before kinetochore attachment to rapid APC activation once attachment is complete remains a mystery. Here we show that checkpoint inactivation is an energy-consuming process involving APC-dependent multi-ubiquitination. Multi-ubiquitination by APC leads to the dissociation of Mad2 and BubR1 from Cdc20, a process that is reversed by a Cdc20-directed de-ubiquitinating enzyme. The mutual regulation between checkpoint proteins and APC leaves the cell poised for rapid checkpoint inactivation and ensures that chromosome segregation promptly follows the completion of kinetochore attachment. In addition, our results suggest a mechanistic basis for how cancer cells can have a compromised spindle checkpoint without corresponding mutations in checkpoint genes.
"Tetraploid cells have been shown to promote tumorigenesis in mice (Fujiwara et al. 2005) and can have multiple centrosomes, which increases the chance of forming merotelic attachments and developing aneuploidy (see below). Additionally , genes involved in SAC inactivation and mitotic progression, such as UBCH10 and CUEDC2, can lead to aneuploidy when overexpressed in cell lines and/or mouse models and are up-regulated in some human tumors (Reddy et al. 2007; van Ree et al. 2010; Gao et al. 2011; Xie et al. 2014). Taking into account these observations, it is likely that we are underestimating the frequency of alterations that impinge on the SAC in human tumors. "
[Show abstract][Hide abstract] ABSTRACT: Aneuploidy, defined as an abnormal number of chromosomes, is a hallmark of cancer. Paradoxically, aneuploidy generally has a negative impact on cell growth and fitness in nontransformed cells. In this work, we review recent progress in identifying how aneuploidy leads to genomic and chromosomal instability, how cells can adapt to the deleterious effects of aneuploidy, and how aneuploidy contributes to tumorigenesis in different genetic contexts. Finally, we also discuss how aneuploidy might be a target for anticancer therapies.
Cold Spring Harbor perspectives in biology 09/2014; 6(11). DOI:10.1101/cshperspect.a015842 · 8.68 Impact Factor
"The Mad2 inhibitor p31 comet is required for cyclin B1 degradation and mitotic adaptation p31 comet prevents the conformational activation of Mad2 and promotes MCC disassembly and APC/C Cdc20 activation during checkpoint silencing (Habu et al, 2002; Xia et al, 2004; Mapelli et al, 2006; Reddy et al, 2007; Yang et al, 2007; Jia et al, 2011; Teichner et al, 2011; Westhorpe et al, 2011). Depletion of p31 comet sensitizes cancer cells to mitotic arrest and killing by spindle poisons (Xia et al, 2004; Ma et al, 2012). "
[Show abstract][Hide abstract] ABSTRACT: The antimitotic anti-cancer drugs, including taxol, perturb spindle dynamics, and induce prolonged, spindle checkpoint-dependent mitotic arrest in cancer cells. These cells then either undergo apoptosis triggered by the intrinsic mitochondrial pathway or exit mitosis without proper cell division in an adaptation pathway. Using a genome-wide small interfering RNA (siRNA) screen in taxol-treated HeLa cells, we systematically identify components of the mitotic apoptosis and adaptation pathways. We show that the Mad2 inhibitor p31comet actively promotes mitotic adaptation through cyclin B1 degradation and has a minor separate function in suppressing apoptosis. Conversely, the pro-apoptotic Bcl2 family member, Noxa, is a critical initiator of mitotic cell death. Unexpectedly, the upstream components of the mitochondrial apoptosis pathway and the mitochondrial fission protein Drp1 contribute to mitotic adaption. Our results reveal crosstalk between the apoptosis and adaptation pathways during mitotic arrest.
The EMBO Journal 07/2014; 33(17). DOI:10.15252/embj.201487826 · 10.43 Impact Factor
"While attachment of kinetochores leads to progressive weakening of SAC signaling (Collin et al. 2013), full SAC silencing awaits stable biorientation of all chromosomes. In addition to removal of MAD1 from kinetochores, such silencing requires disassembly of MCC and release of APC/C activity, followed by degradation of cyclin B1 and securin (Reddy et al. 2007; Westhorpe et al. 2011; Varetti et al. 2011; Teichner et al. 2011; Mansfeld et al. 2011; Foster and Morgan 2012; Uzunova et al. 2012). SAC silencing is, however, reversible. "
[Show abstract][Hide abstract] ABSTRACT: Fidelity of chromosome segregation is monitored by the spindle assembly checkpoint (SAC). Key components of the SAC include MAD1, MAD2, BUB1, BUB3, BUBR1, and MPS1. These proteins accumulate on kinetochores in early prometaphase but are displaced when chromosomes attach to microtubules and/or biorient on the mitotic spindle. As a result, stable attachment of the final chromosome satisfies the SAC, permitting activation of the anaphase promoting complex/cyclosome (APC/C) and subsequent anaphase onset. SAC satisfaction is reversible, however, as addition of taxol during metaphase stops cyclin B1 degradation by the APC/C. We now show that targeting MAD1 to kinetochores during metaphase is sufficient to reestablish SAC activity after initial silencing. Using rapamycin-induced heterodimerization of FKBP-MAD1 to FRB-MIS12 and live monitoring of cyclin B1 degradation, we show that timed relocalization of MAD1 during metaphase can stop cyclin B1 degradation without affecting chromosome-spindle attachments. APC/C inhibition represented true SAC reactivation, as FKBP-MAD1 required an intact MAD2-interaction motif and MPS1 activity to accomplish this. Our data show that MAD1 kinetochore localization dictates SAC activity and imply that SAC regulatory mechanisms downstream of MAD1 remain functional in metaphase.
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