Mechanobiology: how mechanics regulate life

Selected Questions:

1. Could nuclear jamming be reversed when it occurs? If yes, under what conditions and what would be the mechanism? Do cells actively control the nuclear volume fraction and anisotropy or these parameters are specific for each cell type?

2. The authors state that the specific value of the nuclear jamming volume fraction is independent of the magnitude of tension fluctuations at cell-cell contacts (Extended Data Figure 1). How did the authors come up with this conclusion? What are the implications of this decoupling in terms of mechanical stability?

3. Please explain the strain and stress measurements performed using magnetically responsive oil microdroplets in Figure 4. What is the strain value being reported here? How did the authors ensure that differences in droplet size would not interfere with the results?

4. In Figure 4c, the authors compare the strain upon magnetic activation in control and lamina A overexpressing cells. Why is it interesting to compare these two cell types? Is this comparison relevant in terms of mechanics? I think it would make more sense to apply the same stress rather than the same magnetic activation. How did the authors measure the effective stiffness of the cells?

5. The conclusion from the simulation to predict nuclear jamming is “high nuclear volume fractions for round nuclei lead to nuclear jamming, which overtakes cellular jamming”. However, it is also discussed that the stiffness of the nucleus has an impact in nuclear jamming. Does the conclusion still stand for soft nuclei? What happens if the nuclear volume fraction is high but the nucleus is elongated instead of round?

6. Where does the restoring force mentioned in the article come from and what real physical process does this force model? The researchers mention that this restoring force helps provide realistic results. Do they have any quantitative evidence of this?

7. Is it clear what drives the rounding of nuclei throughout the development of studied cells? Do we have evidence to say that higher nuclear volume fraction lead to more uniform load distribution on the nucleus and thus rounding?

8. How might the simplifications in the computational modeling—specifically, the assumptions of fixed nuclear properties and the omission of factors such as nuclear shape deformation and friction between the nucleus and cell boundary—affect the accuracy of the predictions regarding nuclear jamming? 

9. The simulations suggest that nuclear jamming happens when the nuclear volume fraction exceeds 0.6. In the experiments, the authors conclude that nuclear jamming occurs between 55 and 72 hpf (Figure 3n and 3o). However, when looking at Figure 3i, the nuclear jamming transition based on the nuclear volume fraction (0.6) occurs at around 36 hpf. Does this invalidate the simulations? How would you explain this difference between simulation results and experimental values?

10. How did the authors measure the values of the following model parameters: shape factor, defect density, bond length, and angle variation?