Mechanobiology: how mechanics regulate life

Selected Questions:

1. According to Figures 2b and 2c, the migration velocity of control cells (black dots) appears to be related to the cell count in the cell trains. The velocity of cells at n=1 seems lower than that at n=4. Did the authors demonstrate the relationship between cell count and migration velocity?

2. In Equation 3, they defined an active traction profile to see the asymmetries in the drive motion. Why did they use this tension definition? It looks like it comes from nowhere.

3. In Figure 2b, why does the photoactivation seem to have a larger impact on the dual train rather than on the single cell? Intuitively, the photoactivation should have a bigger impact on the single cell, as it is "lighted" only on one.

4. The sum of the pulls exerted by the cells on the substrate does not indicate the direction of the speed of the cells. Why?

5. In Figure 2e, we observe that the percentage of trains undergoing collective migration in the direction of photoactivation is around 30% when Nc =< 2, which is pretty low, even if larger than for Nc > 3. Why is this percentage so low? Can we still say that photoactivation is an effective method to generate moving cells?

6. Figure 3b/c show how traction forces vary across different points of the cell train. Why do tractions tend to concentrate at the edges of single-cell clusters but spread across the cluster in multi-cell configurations? The force locations do not seem to correspond to the individual cell outlines. Figure 3d shows the average profiles of the longitudinal component of the traction forces. Can you discuss the shape of the profile for the cells with directed motion (in red)? Why is there a peak traction away from the trailing edge?

7. Authors mentioned that “no single cells are moving opposite to photoactivation”. Yet, in Figure 2b and 2c, when Nc = 1, some photoactivated cells have negative velocity. If that comes from contraction, what is the reason for that? Can you explain the phenomenon that anti-directed motion increases with increasing Nc based on the fact that non-photoactivated cell has no preferred direction and no photoactivated single cell moving opposite to photoactivation.

8. Does the illumination region move in coordination with the cell's migration direction to maintain continuous illumination? Given that cell migration velocity seems to vary, how can they ensure that the illumination region remains the same throughout the entire observation period, and how might the movement speed of the illumination region affect cell migration speed?

9. The authors define the traction quadrupole as being the “second moment of the traction field along the train axis with respect to the center of mass”. This definition is not very clear to me. Could you explain in your own words what a traction quadrupole is? And how does this theoretically relate to other values such as velocity, for example? Could you also clarify why the green and blue lines cross one another in Figure 5q? In Figure 3d, normalized traction quadrupole of train movements are displayed. Why do the trains exhibiting coherent movement show more variance than other trains?

10. How did the researchers use their model to extract traction parameters from experimental data?

11. In collective cell migration, is there a critical size beyond which, due to insufficient tension the rear cells cannot be effectively pulled to maintain coordinated movement of the group? If so, what factors are associated with this critical size?