Mechanobiology: how mechanics regulate life

Selected Questions:

1. Why do cells retain memory of their exposure to viscosity? How can cells remember physical conditions? Does this memory persist through multiple cell divisions or following big changes in the extracellular environment (for example, a switch to low-viscosity environments like blood)? For how long could the effect persist? Could viscous memory be erased or reprogrammed?

2. On Figure 2g and h they're plotting the lifetimes, but how can they state the membrane tension is increasing? How do the calcium spikes relate to the increased membrane tension, and how do the number of spikes shown in Figure 2i inform about membrane tension? 

3. Would the increased actin-filament production induced by higher extracellular viscosity, which enhances cytoskeletal rigidity and facilitates cell detachment from the primary tumor, ultimately support or hinder subsequent metastatic steps such as intravasation and extravasation, where cellular softness and deformability seems critical?

4. Cell dissociation time is reduced when the viscosity is increased (Figure 1e). Does this result imply that viscosity acts not only on the motility of the cells but it also accelerates the start of migration?

5. The article discusses the effects of viscosity on hydraulic resistance at the leading edge of cells. How does this influence the water uptake and overall cell volume during migration?

6. Why did the authors do experiments with hypotonic solutions in the section ‘interplay of NHE1, membrane tension and TRPV4’? For migration studies 0.6% methylcellulose was incorporated into cell medium to obtain the media with viscosity of 8 cP, but in the test of osmolarity (Extended Data Figure. 1b) only 0.015% and 0.3% methylcellulose were used. Why?

8. Why is the difference in mobility between normal viscosity and elevated viscosity is not significant among non-cancerous cells, especially compared with cancer cells (Figure 1b)? For hAOSMCs, even 9.5 cP was applied, but the difference is still minor.

9. Authors state that “cells at 8cP exhibited a more intense ARP3 signal at their leading edge and, specifically, at the tips of protrusive filaments, as opposed to the relatively uniform distribution detected at baseline viscosity in cells on 2D surfaces and inside confinement”. Why does 8 cP show a uniform distribution but 0.77 cP show a spike at the leading-edge (Figure 1m)?

10. In the introduction, it is mentioned that the viscosity of interstitial fluid can vary up to 3.5 cP and may be further augmented by macromolecules, such as mucins, secreted by epithelial or tumor cells. However, the experiments in the article primarily compare viscosities at 0.77 cP and 8 cP. What is the typical range of interstitial fluid viscosity observed in vivo, particularly in different tissue types or under pathological conditions such as cancer? Under what conditions do cells decide to secrete macromolecules to alter the viscosity of their microenvironment, and what molecular signals might regulate this response? Could altered viscosity affect the diffusion of nutrients, oxygen or cellular interactions with the other cells potentially inducing bad effects?

11. Figure 3h: Why did the authors do a double knockdown when they had already concluded that Myosin IIA is the “primary effector of actomyosin contractility at 8 cP”? What is the difference between Myosin IIA and Myosin IIB? 

12. Higher fluid viscosity makes cells move faster instead of slowing them down. Does this contradict with basic fluid mechanics knowledge that viscosity is resistance to motion?