Cell-cycle control Chapter 7 of Aguda & Friedman

Amanda GalanteCancer Dynamics RITSeptember 25, 2009

Cell-cycle checkpoints

• Restriction point– Regulate initiation of

DNA replication

• G2-M checkpoint– Checks DNA damage

• Spindle checkpoint– Checks chromosome

alignment

Restriction Point

• ‘Commitment point’ for DNA replication• R point – time after which cell will enter S

phase, even in absence of growth factors• Many cancers involve malfunctions of this

checkpoint

G2 DNA Damage Checkpoint (G2DDC)

• Need to understand how coupled PD (phosphorylation-dephosphorylation) cycles work

• Need to establish bistability

PD cycle simple example

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Transcritical Bifurcation Point

Two PD cycles

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Applied to G2DDC

Results of G2DDC model

• Made up rate parameters (not experimentally available)

• Was able to show transcritical bistability as

25

x

Cdctotal

MPFtotalp

• Note that MPF and Cdc25 become active at same time – ‘hallmark of transcritical bifurcation in positively coupled cyclic reactions’

PD cycle conclusions

• Established existence of transcritical bifurcation point for two coupled PD cycles– Allows system to ‘check whether all components

are ready for the next cell cycle event’• MPF, Cdc25, Wee1 coupled PD cycles can be

shown to generate bistability when including other reactions

• Also applicable to R point for cyclin E/CDK2 and Cdc25a

Mitotic spindle checkpoint

Wikipedia - Kinetochore

Model Assumptions

• Final kinetochore attachment• Cell-cycle progression triggered by a protein c*

• c diffuses throughout nucleus• ρ = kinetochore radius (0.01 µm)• R = nucleus radius (1 µm)• D = diffusivity of c (1 µm2/s)

Model Objectives1.After final kinetochore attachment, a protein c

which was previously in an inhibited state c* , becomes sufficiently activated at time Tb < 3 minutes

2.At steady state, c is predominantly in an inhibited state (want at least 90% inhibited, or Ac<0.1). In this way, the system resets itself.

Model framework

Doncic 2005

Summary:Tb = 1.5 min – good!A = 0.4 (i.e. 40% of c molecules are inhibited)-- too high

Direct Inhibition Model

Summary:Tb = not happening…A < 0.1

“Self-Propagating Inhibition” Model

Summary:Tb = 2.5 minA = 0.05 (i.e. the system resets itself)

“Emitted Inhibition” Model

Varying parameters

Conclusions of Mitotic Spindle Checkpoint Model

• Single unattached kinetochore activating protein is a matter of speculation

• Illustrates the impact of temporal & spatial constraints

• Were able to develop a model which met the objectives – ‘Emitted Inhibition Model’

ReferencesAguda, BD & A Friedman. Models of Cellular Regulation. Oxford

University Press, 2008.Aguda, BD. (1999) ‘Instabilities in phosphorylation-dephosphorylation

cascades and cell cycle checkpoints,’ Oncogene 18, 2846-2851.Aguda, BD. (1999) ‘A quantitative analysis of the kinetics of the G2

DNA damage checkpoint system,’ PNAS 96, 11352-11357. Doncic, A, Ben-Jacob, E and N Barkai. (2006) ‘Evaluating putative

mechanisms of the mitotic spindle checkpoint,’ PNAS 102, 6332-6337.

Other picture references• Wikipedia• http://www.mun.ca/biology/desmid/brian/BIOL2060/BIOL2060-19

/CB19.html