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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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Molecular interpretation of wiring diagrams proposed in biophysical models. (A) In the simplest model, ‘direct inhibition model' in Doncic et al (2005), the checkpoint (here Cdc20—active) is inhibited (Cdc20i—inactive) by direct contact with the kinetochore (black dot in the figure). In red, inhibited species, in green, the active ones. (B) According to the ‘indirect inhibition model' (Doncic et al, 2005), kinetochores produces an active species (Mad2*) that inhibits Cdc20 through a complex that is dissociated in the cytoplasm. (C) The ‘Mad2 template model' postulates that the role of the kinetochore in (B) can be played by the complex Mad2:Cdc20 in the cytoplasm. In this description, we include the activated species Mad2*, which is missing in the original formulation of the template model (De Antoni et al, 2005). The combination of (B, C) gives rise to the full template model. Of note is the autocatalytic loop that is present in (C).
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f3: Molecular interpretation of wiring diagrams proposed in biophysical models. (A) In the simplest model, ‘direct inhibition model' in Doncic et al (2005), the checkpoint (here Cdc20—active) is inhibited (Cdc20i—inactive) by direct contact with the kinetochore (black dot in the figure). In red, inhibited species, in green, the active ones. (B) According to the ‘indirect inhibition model' (Doncic et al, 2005), kinetochores produces an active species (Mad2*) that inhibits Cdc20 through a complex that is dissociated in the cytoplasm. (C) The ‘Mad2 template model' postulates that the role of the kinetochore in (B) can be played by the complex Mad2:Cdc20 in the cytoplasm. In this description, we include the activated species Mad2*, which is missing in the original formulation of the template model (De Antoni et al, 2005). The combination of (B, C) gives rise to the full template model. Of note is the autocatalytic loop that is present in (C).

Mentions: Doncic and colleagues argued, as above, that any model of the spindle assembly checkpoint had to recapitulate two properties: the capability of the spindle assembly checkpoint to robustly halt cell cycle progression, and its quick disengagement once all kinetochores are attached. Using observations from the closed mitosis of budding yeast, these requirements meant that successful molecular mechanisms were asked to have at least 95% of the cellular Cdc20 sequestered (1000 molecules in a spherical nuclear volume 1 μm in radius, or ∼130 nM). The calculations were done assuming one unattached kinetochore (10 nm in radius) placed at the centre of a simple spherical geometry and simple diffusion (diffusion coefficient ∼1 μm2/s). Moreover, they required that >90% of Cdc20 (or equivalently the APC/C) would be re-activated 3 mins after the last kinetochore was attached. First, they tested the simplest possible model for the spindle assembly checkpoint, called ‘direct inhibition' (Figure 3A) whereby Cdc20 molecules are inhibited by recruitment to the unattached kinetochore (Acquaviva et al, 2004) and activated constitutively in the cytoplasm. Making the assumption that all Cdc20 molecules passing by the kinetochore are inhibited, they show that direct inhibition cannot maintain an anaphase delay because of the disparity between Cdc20 visitation rate and cytoplasmic reactivation rate—molecules get reactivated quicker than they can visit the kinetochore. A second possibility tested by Doncic et al is ‘cytoplasmic amplification', a model in which inhibited molecules of Cdc20 in the cytoplasm induce the further inhibition of other Cdc20 molecules. Such a possibility, reminiscent of models proposed by De Antoni et al (2005) (but see later for a more thorough comparison), displays tight inhibition. However, in this formulation of the autocatalysis, the checkpoint cannot be turned off as even after the kinetochore is silenced the cytoplasmic inhibitory activity remains potent. Finally, they explore a model by which a stoichiometric inhibitor can be generated at the kinetochore (Figure 3B). The inhibitor binds to and inhibits Cdc20 and the resulting complex undergoes dissociation at some fixed rate. In this case, the kinetochore can ‘overproduce' inhibitor to buffer any free Cdc20 that may form in the cytoplasm. Once the kinetochore is silenced by microtubule attachment, the dissociation activity rapidly reactivates Cdc20 to permit checkpoint exit. This ‘indirect inhibition' model matches all the requirements laid out by Doncic and colleagues for an efficient spindle assembly checkpoint. Of note is that this scheme is similar, in principle, to the production of MCC, a stoichiometric inhibitor, and its binding to and inhibition of the APC/C.


A quantitative systems view of the spindle assembly checkpoint.

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

Molecular interpretation of wiring diagrams proposed in biophysical models. (A) In the simplest model, ‘direct inhibition model' in Doncic et al (2005), the checkpoint (here Cdc20—active) is inhibited (Cdc20i—inactive) by direct contact with the kinetochore (black dot in the figure). In red, inhibited species, in green, the active ones. (B) According to the ‘indirect inhibition model' (Doncic et al, 2005), kinetochores produces an active species (Mad2*) that inhibits Cdc20 through a complex that is dissociated in the cytoplasm. (C) The ‘Mad2 template model' postulates that the role of the kinetochore in (B) can be played by the complex Mad2:Cdc20 in the cytoplasm. In this description, we include the activated species Mad2*, which is missing in the original formulation of the template model (De Antoni et al, 2005). The combination of (B, C) gives rise to the full template model. Of note is the autocatalytic loop that is present in (C).
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f3: Molecular interpretation of wiring diagrams proposed in biophysical models. (A) In the simplest model, ‘direct inhibition model' in Doncic et al (2005), the checkpoint (here Cdc20—active) is inhibited (Cdc20i—inactive) by direct contact with the kinetochore (black dot in the figure). In red, inhibited species, in green, the active ones. (B) According to the ‘indirect inhibition model' (Doncic et al, 2005), kinetochores produces an active species (Mad2*) that inhibits Cdc20 through a complex that is dissociated in the cytoplasm. (C) The ‘Mad2 template model' postulates that the role of the kinetochore in (B) can be played by the complex Mad2:Cdc20 in the cytoplasm. In this description, we include the activated species Mad2*, which is missing in the original formulation of the template model (De Antoni et al, 2005). The combination of (B, C) gives rise to the full template model. Of note is the autocatalytic loop that is present in (C).
Mentions: Doncic and colleagues argued, as above, that any model of the spindle assembly checkpoint had to recapitulate two properties: the capability of the spindle assembly checkpoint to robustly halt cell cycle progression, and its quick disengagement once all kinetochores are attached. Using observations from the closed mitosis of budding yeast, these requirements meant that successful molecular mechanisms were asked to have at least 95% of the cellular Cdc20 sequestered (1000 molecules in a spherical nuclear volume 1 μm in radius, or ∼130 nM). The calculations were done assuming one unattached kinetochore (10 nm in radius) placed at the centre of a simple spherical geometry and simple diffusion (diffusion coefficient ∼1 μm2/s). Moreover, they required that >90% of Cdc20 (or equivalently the APC/C) would be re-activated 3 mins after the last kinetochore was attached. First, they tested the simplest possible model for the spindle assembly checkpoint, called ‘direct inhibition' (Figure 3A) whereby Cdc20 molecules are inhibited by recruitment to the unattached kinetochore (Acquaviva et al, 2004) and activated constitutively in the cytoplasm. Making the assumption that all Cdc20 molecules passing by the kinetochore are inhibited, they show that direct inhibition cannot maintain an anaphase delay because of the disparity between Cdc20 visitation rate and cytoplasmic reactivation rate—molecules get reactivated quicker than they can visit the kinetochore. A second possibility tested by Doncic et al is ‘cytoplasmic amplification', a model in which inhibited molecules of Cdc20 in the cytoplasm induce the further inhibition of other Cdc20 molecules. Such a possibility, reminiscent of models proposed by De Antoni et al (2005) (but see later for a more thorough comparison), displays tight inhibition. However, in this formulation of the autocatalysis, the checkpoint cannot be turned off as even after the kinetochore is silenced the cytoplasmic inhibitory activity remains potent. Finally, they explore a model by which a stoichiometric inhibitor can be generated at the kinetochore (Figure 3B). The inhibitor binds to and inhibits Cdc20 and the resulting complex undergoes dissociation at some fixed rate. In this case, the kinetochore can ‘overproduce' inhibitor to buffer any free Cdc20 that may form in the cytoplasm. Once the kinetochore is silenced by microtubule attachment, the dissociation activity rapidly reactivates Cdc20 to permit checkpoint exit. This ‘indirect inhibition' model matches all the requirements laid out by Doncic and colleagues for an efficient spindle assembly checkpoint. Of note is that this scheme is similar, in principle, to the production of MCC, a stoichiometric inhibitor, and its binding to and inhibition of the APC/C.

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