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A quantitative systems view of the spindle assembly checkpoint.

Ciliberto A, Shah JV - EMBO J. (2009)

Bottom Line: Here, we propose a systems view of the spindle assembly checkpoint to focus attention on the key regulators of the dynamics of this pathway.These regulators in turn have been the subject of detailed cellular measurements and computational modelling to connect molecular function to the dynamics of spindle assembly checkpoint signalling.A review of these efforts reveals the insights provided by such approaches and underscores the need for further interdisciplinary studies to reveal in full the quantitative underpinnings of this cellular control pathway.

View Article: PubMed Central - PubMed

Affiliation: IFOM-Firc Institute of Molecular Oncology, Milan, Italy. andrea.ciliberto@ifom-ieo-campus.it

ABSTRACT
The idle assembly checkpoint acts to delay chromosome segregation until all duplicated sister chromatids are captured by the mitotic spindle. This pathway ensures that each daughter cell receives a complete copy of the genome. The high fidelity and robustness of this process have made it a subject of intense study in both the experimental and computational realms. A significant number of checkpoint proteins have been identified but how they orchestrate the communication between local spindle attachment and global cytoplasmic signalling to delay segregation is not yet understood. Here, we propose a systems view of the spindle assembly checkpoint to focus attention on the key regulators of the dynamics of this pathway. These regulators in turn have been the subject of detailed cellular measurements and computational modelling to connect molecular function to the dynamics of spindle assembly checkpoint signalling. A review of these efforts reveals the insights provided by such approaches and underscores the need for further interdisciplinary studies to reveal in full the quantitative underpinnings of this cellular control pathway.

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Schematic view of spindle assembly checkpoint signalling. (A) Cells enter mitosis with unattached kinetochores that actively generate inhibitory signals (strong red alarm signal) to prevent APC/C activation. This stabilizes the high levels of cyclin B and securin that prevent anaphase onset. (B) Attachment of spindle microtubules to unattached kinetochores locally turns off kinetochore-mediated inhibition, but cytoplasmic inhibition, potentially diminished, is still supported by other unattached kinetochores (weaker red signal). The progressive attachment of microtubules generates a weak signal in the cytoplasm that promotes the disengagement of the checkpoint (weak green alarm signal) (C) Capture of all chromosomes results in the complete loss of signal generation from kinetochores (weakest red signal), permitting the greater relief of inhibition on the APC/C in the cytoplasm (stronger green alarm). Activation of the APC/C promotes the destabilization of cyclin B and securin. (D) Sufficient loss of substrates (cyclin B and securin) promotes the activation of separase and cleavage of the cohesins permitting the onset of anaphase and segregation of the sister chromatids.
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f1: Schematic view of spindle assembly checkpoint signalling. (A) Cells enter mitosis with unattached kinetochores that actively generate inhibitory signals (strong red alarm signal) to prevent APC/C activation. This stabilizes the high levels of cyclin B and securin that prevent anaphase onset. (B) Attachment of spindle microtubules to unattached kinetochores locally turns off kinetochore-mediated inhibition, but cytoplasmic inhibition, potentially diminished, is still supported by other unattached kinetochores (weaker red signal). The progressive attachment of microtubules generates a weak signal in the cytoplasm that promotes the disengagement of the checkpoint (weak green alarm signal) (C) Capture of all chromosomes results in the complete loss of signal generation from kinetochores (weakest red signal), permitting the greater relief of inhibition on the APC/C in the cytoplasm (stronger green alarm). Activation of the APC/C promotes the destabilization of cyclin B and securin. (D) Sufficient loss of substrates (cyclin B and securin) promotes the activation of separase and cleavage of the cohesins permitting the onset of anaphase and segregation of the sister chromatids.

Mentions: The basic schema of the spindle assembly checkpoint is a balance between an inhibitory signal to prevent anaphase and the activity of the anaphase-promoting machinery (Figure 1). The key site in the production of the inhibitory signal is the kinetochore, a protein complex that assembles at the centromere of mitotic chromosomes (reviewed in an accompanying contribution from Santaguida and Musacchio). The unattached kinetochore acts as a catalytic scaffold for inhibitor production. As cells enter mitosis, all kinetochores are unattached and generate a signal that acts to prevent the onset of anaphase through direct inhibition of the anaphase promoting machinery (Figure 1A). The capture of chromosomes at both sister kinetochores, by microtubules of the mitotic spindle, silences the production of this signal (Figure 1B and C). The stoppage in inhibitor production leads to the activation of anaphase-promoting activity. The origin of the anaphase-promoting activity is an E3 ubiquitin ligase, aptly named the anaphase-promoting complex or APC/C (King et al, 1995; Sudakin et al, 1995). To promote anaphase onset the APC/C, activated by its cofactor Cdc20, ubiquitinates (Fang et al, 1998a), and thereby targets for destruction by the proteasome, cyclin B and securin (Glotzer et al, 1991). Loss of cyclin B begins the program of mitotic exit through the reduction of cyclin-dependent kinase (Cdk1) activity. Loss of securin releases the activity of a protease known as separase that cleaves the ‘molecular glue', or cohesin complexes, which bind replicated chromatids together (Figure 1D). This transition to anaphase promotes both the segregation of the genetic material, and exit into the subsequent cell cycle for both progeny cells. The spindle assembly checkpoint delays APC/C activation until all kinetochores are properly attached to microtubules.


A quantitative systems view of the spindle assembly checkpoint.

Ciliberto A, Shah JV - EMBO J. (2009)

Schematic view of spindle assembly checkpoint signalling. (A) Cells enter mitosis with unattached kinetochores that actively generate inhibitory signals (strong red alarm signal) to prevent APC/C activation. This stabilizes the high levels of cyclin B and securin that prevent anaphase onset. (B) Attachment of spindle microtubules to unattached kinetochores locally turns off kinetochore-mediated inhibition, but cytoplasmic inhibition, potentially diminished, is still supported by other unattached kinetochores (weaker red signal). The progressive attachment of microtubules generates a weak signal in the cytoplasm that promotes the disengagement of the checkpoint (weak green alarm signal) (C) Capture of all chromosomes results in the complete loss of signal generation from kinetochores (weakest red signal), permitting the greater relief of inhibition on the APC/C in the cytoplasm (stronger green alarm). Activation of the APC/C promotes the destabilization of cyclin B and securin. (D) Sufficient loss of substrates (cyclin B and securin) promotes the activation of separase and cleavage of the cohesins permitting the onset of anaphase and segregation of the sister chromatids.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2722251&req=5

f1: Schematic view of spindle assembly checkpoint signalling. (A) Cells enter mitosis with unattached kinetochores that actively generate inhibitory signals (strong red alarm signal) to prevent APC/C activation. This stabilizes the high levels of cyclin B and securin that prevent anaphase onset. (B) Attachment of spindle microtubules to unattached kinetochores locally turns off kinetochore-mediated inhibition, but cytoplasmic inhibition, potentially diminished, is still supported by other unattached kinetochores (weaker red signal). The progressive attachment of microtubules generates a weak signal in the cytoplasm that promotes the disengagement of the checkpoint (weak green alarm signal) (C) Capture of all chromosomes results in the complete loss of signal generation from kinetochores (weakest red signal), permitting the greater relief of inhibition on the APC/C in the cytoplasm (stronger green alarm). Activation of the APC/C promotes the destabilization of cyclin B and securin. (D) Sufficient loss of substrates (cyclin B and securin) promotes the activation of separase and cleavage of the cohesins permitting the onset of anaphase and segregation of the sister chromatids.
Mentions: The basic schema of the spindle assembly checkpoint is a balance between an inhibitory signal to prevent anaphase and the activity of the anaphase-promoting machinery (Figure 1). The key site in the production of the inhibitory signal is the kinetochore, a protein complex that assembles at the centromere of mitotic chromosomes (reviewed in an accompanying contribution from Santaguida and Musacchio). The unattached kinetochore acts as a catalytic scaffold for inhibitor production. As cells enter mitosis, all kinetochores are unattached and generate a signal that acts to prevent the onset of anaphase through direct inhibition of the anaphase promoting machinery (Figure 1A). The capture of chromosomes at both sister kinetochores, by microtubules of the mitotic spindle, silences the production of this signal (Figure 1B and C). The stoppage in inhibitor production leads to the activation of anaphase-promoting activity. The origin of the anaphase-promoting activity is an E3 ubiquitin ligase, aptly named the anaphase-promoting complex or APC/C (King et al, 1995; Sudakin et al, 1995). To promote anaphase onset the APC/C, activated by its cofactor Cdc20, ubiquitinates (Fang et al, 1998a), and thereby targets for destruction by the proteasome, cyclin B and securin (Glotzer et al, 1991). Loss of cyclin B begins the program of mitotic exit through the reduction of cyclin-dependent kinase (Cdk1) activity. Loss of securin releases the activity of a protease known as separase that cleaves the ‘molecular glue', or cohesin complexes, which bind replicated chromatids together (Figure 1D). This transition to anaphase promotes both the segregation of the genetic material, and exit into the subsequent cell cycle for both progeny cells. The spindle assembly checkpoint delays APC/C activation until all kinetochores are properly attached to microtubules.

Bottom Line: Here, we propose a systems view of the spindle assembly checkpoint to focus attention on the key regulators of the dynamics of this pathway.These regulators in turn have been the subject of detailed cellular measurements and computational modelling to connect molecular function to the dynamics of spindle assembly checkpoint signalling.A review of these efforts reveals the insights provided by such approaches and underscores the need for further interdisciplinary studies to reveal in full the quantitative underpinnings of this cellular control pathway.

View Article: PubMed Central - PubMed

Affiliation: IFOM-Firc Institute of Molecular Oncology, Milan, Italy. andrea.ciliberto@ifom-ieo-campus.it

ABSTRACT
The idle assembly checkpoint acts to delay chromosome segregation until all duplicated sister chromatids are captured by the mitotic spindle. This pathway ensures that each daughter cell receives a complete copy of the genome. The high fidelity and robustness of this process have made it a subject of intense study in both the experimental and computational realms. A significant number of checkpoint proteins have been identified but how they orchestrate the communication between local spindle attachment and global cytoplasmic signalling to delay segregation is not yet understood. Here, we propose a systems view of the spindle assembly checkpoint to focus attention on the key regulators of the dynamics of this pathway. These regulators in turn have been the subject of detailed cellular measurements and computational modelling to connect molecular function to the dynamics of spindle assembly checkpoint signalling. A review of these efforts reveals the insights provided by such approaches and underscores the need for further interdisciplinary studies to reveal in full the quantitative underpinnings of this cellular control pathway.

Show MeSH