Abstract
There are many intracellular signalling pathways where the spatial distribution of the molecular species cannot be neglected. These pathways often contain negative feedback loops and can exhibit oscillatory dynamics in space and time. Two such pathways are those involving Hes1 and p53–Mdm2, both of which are implicated in cancer.
In this paper we further develop the partial differential equation (PDE) models of Sturrock et al. (J. Theor. Biol., 273:15–31, 2011) which were used to study these dynamics. We extend these PDE models by including a nuclear membrane and active transport, assuming that proteins are convected in the cytoplasm towards the nucleus in order to model transport along microtubules. We also account for Mdm2 inhibition of p53 transcriptional activity.
Through numerical simulations we find ranges of values for the model parameters such that sustained oscillatory dynamics occur, consistent with available experimental measurements. We also find that our model extensions act to broaden the parameter ranges that yield oscillations. Hence oscillatory behaviour is made more robust by the inclusion of both the nuclear membrane and active transport. In order to bridge the gap between in vivo and in silico experiments, we investigate more realistic cell geometries by using an imported image of a real cell as our computational domain. For the extended p53–Mdm2 model, we consider the effect of microtubule-disrupting drugs and proteasome inhibitor drugs, obtaining results that are in agreement with experimental studies.
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Acknowledgements
The authors gratefully acknowledge the support of the ERC Advanced Investigator Grant 227619, “M5CGS—From Mutations to Metastases: Multiscale Mathematical Modelling of Cancer Growth and Spread”.
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Appendix
Appendix
1.1 A.1 Non-dimensionalisation of Hes1 Model
We summarise our non-dimensionalisation of the extended Hes1 model (described in Sect. 2.2). The original Hes1 model (described in Sect. 2.1) is non-dimensionalised in a similar way; for details, see Sturrock et al. (2011).
To non-dimensionalise the extended Hes1 model given by (1)–(4) and (15), subject to the conditions in (8)–(14), we first define re-scaled variables by dividing each variable by a reference value. Re-scaled variables are given overlines to distinguish them from variables that are not re-scaled. Thus we can write:
where the right-hand side of each equation is a dimensional variable divided by its reference value. From (47), we can write variables in terms of re-scaled variables and then substitute these expressions into (1)–(4) and (15), and into the conditions in (8)–(14). This gives a model defined in terms of re-scaled variables which has the same form as the dimensional model, but now the parameters are all non-dimensional. Denoting the non-dimensional parameters with an asterisk, they are related to dimensional parameters as follows:
We solve the non-dimensional model using the method described in Sect. 2.1. We simulate the model in COMSOL 3.5a, finding non-dimensional parameter values that yield oscillatory dynamics. We chose the same values as in Eq. (25) in Sturrock et al. (2011) except for those parameters which were new because of our extension to the model. These latter values were chosen as follows: \(D^{*}_{m} = D^{*}_{i_{j}}/5\), \(D^{*}_{p} = D^{*}_{i_{j}}/15\), d ∗=0.01, a ∗=0.03, l ∗=0.63.
Finally, we calculated the dimensional parameter values. To do this, we needed to estimate the reference values. Since Her1 in zebrafish and Hes1 in mice are both pathways connected with somitogenesis, we used the reference concentrations for Her1 protein and her1 mRNA in Terry et al. (2011) as our reference concentrations for Hes1 protein and hes1 mRNA. Thus, we chose [m 0]=1.5×10−9 M and [p 0]=10−9 M. We assumed a cell to be of width 30 μm. But from Figs. 2 and 4, the cell width is equal to three non-dimensional spatial units or 3L-dimensional units (using (47)). Hence we set 3L=30 μm, so that L=10 μm. The experimentally observed period of oscillations of Hes1 is approximately 2 hours (Hirata et al. 2002). Our simulations of the non-dimensionalised model gave oscillations with a period of approximately 300 non-dimensional time units or 300 τ-dimensional units (using (47)). Hence we set 300τ=2 h=7200 s, so that τ=24 s. Using our references values and non-dimensional parameter values, we found dimensional parameter values from (48).
Note that we chose our reference time τ=24 s based on simulations of the extended Hes1 model since this was our most realistic Hes1 model. For the original Hes1 model and for all special cases of the Hes1 model (for example, setting active transport rates to zero), we retained the reference time τ=24 s.
1.2 A.2 Non-dimensionalisation of p53–Mdm2 Model
We non-dimensionalised the p53–Mdm2 model defined in Sect. 3.1, and the extended p53–Mdm2 model defined in Sect. 3.2, using the technique described above for non-dimensionalising the extended Hes1 model. We give brief details for our non-dimensionalisation of the extended p53–Mdm2 model.
To non-dimensionalise the extended p53–Mdm2 model given by (19)–(26) and (44)–(45), subject to conditions (27) and (32)–(43), we define re-scaled variables (denoted by overlines) by dividing each variable by a reference value:
Substituting the scaling in (49) into the extended p53–Mdm2 model gives a non-dimensionalised model with non-dimensional parameters (which we denote with asterisks) that are related to dimensional parameters as follows:
We solve the non-dimensional model using COMSOL 3.5a, finding non-dimensional parameter values that yield oscillatory dynamics. We chose the same values as in Eq. (60) in Sturrock et al. (2011) except for those parameters which were new because of our extension to the model. These latter values were chosen as follows: θ ∗=1, ζ ∗=0.35, \(D^{*}_{m} = D^{*}_{i_{j}}/5\), \(D^{*}_{p} =D^{*}_{i_{j}}/15\), d ∗=0.01, a ∗=0.03, l ∗=0.63.
Finally, we calculated the dimensional parameter values. To do this, we had to estimate the reference values. As in Sturrock et al. (2011), we chose the following reference concentrations: [p530]=0.5 μM, [Mdm2m 0]=0.05 μM, [Mdm20]=2 μM. In addition, we chose [p53m 0]=0.025 μM (in keeping with relative concentrations of mRNA and protein revealed by simulation of the non-dimensionalised model). As with the Hes1 model, we assumed a cell to be of width 30 μm, which again leads to the reference length L=10 μm. Our simulations of the non-dimensionalised model gave oscillations with a period of approximately 360 non-dimensional time units or 360 τ-dimensional units (using (50)), and the experimentally observed period is approximately 3 hours (Monk 2003). Hence we set 360τ=3 h=10800 s, so that τ=30 s. The reference time τ=30 s was based on simulations of the extended p53–Mdm2 model since this was our most realistic p53–Mdm2 model. For all variants of this model (for example, setting active transport rates to zero), we retained the reference time τ=30 s for ease of comparison of the numerical results. Using our references values and non-dimensional parameter values, we found dimensional parameter values from (50).
1.3 A.3 Influence of Cell Shape
As discussed in Sect. 2.2.5, we carried out simulations for the extended Hes1 model on a variety of cell geometries. Results from these simulations are presented in Figs. 24 and 25. It is clear from these figures that sustained oscillatory dynamics are strongly robust to changes in cell shape. Such robustness is reassuring since the shape of eukaryotic cells is highly variable (Baserga 2007; Pincus and Theriot 2007).
Only one of the geometries in Figs. 24 or 25 shows significant damping after the initial peaks in Hes1 protein and hes1 mRNA total concentrations. This occurs in the second row in Fig. 25, where the MTOC surrounding the nucleus is significantly increased in size. The increased size of the MTOC reduces the size of the region in which active transport may occur. Hence the results in the second row in Fig. 25 are similar to those presented in Sect. 2.2.4 in which the active transport rate is set to zero.
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Sturrock, M., Terry, A.J., Xirodimas, D.P. et al. Influence of the Nuclear Membrane, Active Transport, and Cell Shape on the Hes1 and p53–Mdm2 Pathways: Insights from Spatio-temporal Modelling. Bull Math Biol 74, 1531–1579 (2012). https://doi.org/10.1007/s11538-012-9725-1
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DOI: https://doi.org/10.1007/s11538-012-9725-1