Abstract
During cell migration, forces applied to a cell from its environment influence the motion. When the cell is placed on a substrate, such a force is provided by the cell-substrate adhesion. Modulation of adhesivity, often performed by the modulation of the substrate stiffness, tends to cause common responses for cell spreading, cell speed, persistence, and random motility coefficient. Although the reasons for the response of cell spreading and cell speed have been suggested, other responses are not well understood. In this study, we develop a simple toy model for cell migration driven by the relation of two forces: the adhesive force and the plasma membrane tension. The simplicity of the model allows us to perform the calculation not only numerically but also analytically, and the analysis provides formulas directly relating the adhesivity to cell spreading, persistence, and the random motility coefficient. Accordingly, the results offer a unified picture on the causal relations between those multiple cellular responses. In addition, cellular properties that would influence the migratory behavior are suggested.
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Acknowledgments
The author would like to thank Dr. Shunji Hattori at Nippi Research Institute of Biomatrix for fruitful discussions.
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This work was financially supported by JSPS KAKENHI Grant Number 26800208.
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Appendices
Appendix: A
Theoretical studies referenced in the main text are those focusing on the influence of adhesivity and/or substrate stiffness on the morphology or migration behavior. To compare those studies with the present study, they are briefly reviewed in the below particularly from the viewpoint of model’s characteristics (dimension of the space, continuous or discrete, the number of parameters etc.), calculation procedure (numerical or analytical etc.), and the scope of the study (what is explored among the cell size, speed, persistence and random motility coefficient etc.).
Compared with those studies, the characteristics of the present study can be summarized as follows: The present study aims to investigate, with a single model, as many as three cellular responses, namely the cell length, persistence length, and random motility coefficient, as well as durotaxis and the response to shear stress. In the model, the cell migrates in a 1D space and is represented by its boundaries stochastically changing their positions. The number of parameters, five (Fave, r, K, L0, σ), is the least compared with those studies. Analytical calculation is fully performed to obtain the results represented by some formulae. Thanks to the formulae, we explicitly find the influence of the adhesivity (Fave) and the cellular properties (r, K, L0, σ) on the cellular responses (\(\bar {L}\), ξ, μ) occurring in migration, which lead to the understanding of the mechanism underlying those responses.
The following provides the brief review.
The study [36] is a pioneering work that explores the effect of the adhesivity. The cell is described by coupled springs and dashpots moving in a 1D space. The model includes 13 parameters. The result is numerically obtained. The cell speed is explored.
In [37], the cell motion is described by three points moving in a 2D space. The dynamics is given by stochastic differential equations (SDEs). The model includes 16 parameters. The result is numerically obtained. The scope is the most similar to the present study. The cell speed, persistence and random motility coefficient are explored.
In [38], the cell is described by a continuous finite object in a 1D space. The dynamics is given by partial differential equations (PDEs). The model includes 11 parameters. The result is numerically obtained. The cell size and speed are explored.
In [39], the cell is described by a point moving in a 3D space. Its motion is provided by ordinary differential equations (ODEs). The model includes 9 parameters. The result is numerically obtained. The cell speed is explored.
In [40], the cell is described by a continuous finite object in a 3D space. The dynamics is given by PDEs. Since the PDEs are fully solved with the finite element method, any quantities would be obtained although none of them is explicitly shown. The numbers of parameters is not clearly written in the paper but should be numerous due to the model’s characteristics.
In [41], the cell is represented by the vertical cross section of the cell. The dynamics is given by PDEs. The model includes 10 parameters. While the calculation is proceeded analytically in part, the final results are obtained numerically by solving a system of algebraic equations. The cell speed is explored.
In [42], the cell is described by 95 nodes connected by springs and dashpots placed in a 2D space. Its motion is given by ODEs. The model includes 11 parameters. The results are numerically obtained. The cell speed and cell size are explored in addition to durotaxis.
In [43], the cell is described by an elastic element moving in a 3D space. Its motion is given by ODEs. The model includes 20 parameters. The results are numerically obtained. The effects of substrate stiffness on the traction force and cell speed are explored.
In [44], the cell and substrate are described by continuous substance in a 2D space. The finite element method is used to obtain the result. The model includes 11 parameters. The effects of substrate stiffness on the traction force and cell speed are explored.
In [45], the cell is described by a point in a 1D space. The intracellular processes are provided in detail to determine the motion. The dynamics is given by ODEs. The model includes 25 parameters. The results are numerically obtained. The cell speed is explored.
In [46], the cell is described by a continuous viscoelastic substance in a 1D space. The dynamics is given by PDEs. The model includes 8 parameters. The results are numerically obtained. The effects of the substrate stiffness on the traction force and cell speed are explored.
In [47], the cell is described by a phase field in a 2D space. The dynamics is given by PDEs. The model includes 24 parameters. The results are numerically obtained. The cell morphology is explored.
In [48], the cell is described by a point moving on a 2D space where matrix fibers are distributed. The dynamics is given by ODEs. The model includes 7 parameters. The results are numerically obtained. The effects of the substrate stiffness on the cell speed and persistence are explored as well as durotaxis.
In [49], the cell is described by a continuous active gel in a 1D space. The dynamics is given by PDEs. The model includes 7 parameters. While analytical calculation is performed to understand whether spontaneous movement will occur or not, the cell speed is obtained numerically.
In [50], the boundary of the cell is considered in a 2D space. Time evolution of the cell’s position is not calculated. Instead, the relations between the substrate stiffness, the cell shape and the anisotropy of the adhesive force are explored for the fixed cell. The model includes 14 parameters. The results are numerically obtained.
In [51], the cell is described by a continuous substance in a 2D space. The dynamics is given by PDEs. The model includes 23 parameters. The effect of the substrate stiffness on the cell spreading is explored.
In [52], the cell is described by two points representing the left edge and right edge in a 1D space, which is similar to the present model. The two points are connected with a spring, a dashpot and an active actuator. Periodic motion induced by the periodic attachment and detachment of the adhesion are assumed. The model includes 8 parameters. The result is numerically obtained. The influence of the phase differences of the periodic behavior on the cell velocity is explored.
In [53], 2D cross-section of the cell is considered and represented by a phase field. The dynamics is given by PDEs. The model includes 12 parameters. Results are obtained numerically except for some specific cell forms. The cell speed is explored.
Appendix: B
In the model, for simplicity, the cell is assumed to always switch the migration direction when adhesive force ff is less than membrane tension T, i.e., ff < T. Here, we consider a case where the switching does not always occur. In this case, we should analyze a random walk in which three actions are possible at each time step: 1) the cell keeps the migration direction and moves forward by unit length with probability p, 2) the cell is stationary, i.e., neither moves forward nor switches the migration direction with probability q, and 3) the cell switches the migration direction with probability r. Then the probability of migration forward without switching the migration direction is given by \(p+qp+q^{2}p+\cdots =\frac {p}{1-q}\). Inversely, the probability of switching the migration direction without migrating forward is given by \(r+qr+q^{2}r+\cdots =\frac {r}{1-q}\). Since p + q + r = 1 is satisfied, the ratio of the probability of migrating forward to the probability of switching the migration direction is equal to the ratio that does not involve the probability of the stationary state. Thus, the persistence length calculated in the main text does not change if we assume that switching of the migration direction does not always occur under the condition ff < T.
Appendix: C
Figure 8 shows representative cell trajectories obtained by numerical simulations for three values of mean adhesive force Fm. The cell tends to migrate persistently when it adheres strongly. This figure also shows the time evolution of cell length L(t). After a short period, the cell length reaches an equilibrium state in all cases.
Appendix: D
The analytical results derived in the main text and simulation results are compared under a specific combination of parameter values. Figure 9 shows the cell length L. The analytical results well describe the numerical results. Figure 10 shows the characteristic persistence length (CPL) ξ. Analytical results well describe numerical results. Comparison of the random motility coefficients (RMC) μ is not shown since it is proportional to CPL. We can notice a tendency that CPLs obtained numerically are a little smaller than the analytical one. The reason is, since the number of the samples of the simulation is finite, the probability distributions of the persistence length have to be truncated, and therefore the CPL estimated by simulation is smaller than that calculated from the formula obtained by the analysis.
Appendix: E
Figure 11 shows the probability distribution of persistence length Δx, the distance by which the cell moves between direction switches, constructed from the trajectories obtained in numerical simulations. Typical results for three values of adhesive force Fm are shown. All of them are described well by exponential functions, \(\exp (-{\Delta } x/\xi )\), where ξ a parameter referred to as characteristic persistence length.
Appendix: F
Let us consider a random walk in which the probability of keeping the migration direction is p and the probability of switching the the migration direction is 1 − p. Let N be the number of steps immediately after which the probability of keeping the same migration direction becomes 1/2. Then N satisfies pN = 1/2, which reads \(N=\frac {\log (1/2)}{\log p}\). This means that the probability of keeping the migration direction for sequential N times is 1/2. As a result of this keeping the direction of N times, the distance by which the walker moves is Nξ where ξ denotes the step size of this random walk. Thus, as far as, we focus on the long-time behavior, this asymmetric random walk is equivalent to the symmetric random walk whose unit length \(\tilde {\xi }\) is given by \(\tilde {\xi }=N\xi =\frac {\log (1/2)}{\log p} \xi \).
Appendix: G
Although the random motility coefficient shows a biphasic or monotonically decreasing dependence on increasing adhesive force in the main text, it also manifests an increasing dependence even when we slightly change the relation between adhesive force and cell speed. For example, when we use \(S[\mu \text {m/min}]=20\exp (-F_{m}[\text {Pa}]/200)\) given in Fig. 12 instead of S[μ m/min] = − 0.025Fm[Pa] + 12.5 used in the main text, a monotonically increasing dependence is observed when r is large, K is small, and σ is small (Fig. 13).
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Iwasa, M. A mechanical toy model linking cell-substrate adhesion to multiple cellular migratory responses. J Biol Phys 45, 401–421 (2019). https://doi.org/10.1007/s10867-019-09536-2
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DOI: https://doi.org/10.1007/s10867-019-09536-2