Force propagation between epithelial cells depends on active coupling and mechano-structural polarization
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Curated by eLife
eLife assessment
Using surface micropatterning, optical activation, and theoretical analysis, the authors provide compelling evidence that adjacent cells actively propagate mechanical stress in epithelial tissues. The response of the receiver cell is active and enhanced when the principal stress direction is perpendicular to the orientation of actin fibers. This work is important and a must-read for everybody wanting to understand tissue mechanics.
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Abstract
Cell-generated forces play a major role in coordinating the large-scale behavior of cell assemblies, in particular during development, wound healing, and cancer. Mechanical signals propagate faster than biochemical signals, but can have similar effects, especially in epithelial tissues with strong cell–cell adhesion. However, a quantitative description of the transmission chain from force generation in a sender cell, force propagation across cell–cell boundaries, and the concomitant response of receiver cells is missing. For a quantitative analysis of this important situation, here we propose a minimal model system of two epithelial cells on an H-pattern (‘cell doublet’). After optogenetically activating RhoA, a major regulator of cell contractility, in the sender cell, we measure the mechanical response of the receiver cell by traction force and monolayer stress microscopies. In general, we find that the receiver cells show an active response so that the cell doublet forms a coherent unit. However, force propagation and response of the receiver cell also strongly depend on the mechano-structural polarization in the cell assembly, which is controlled by cell–matrix adhesion to the adhesive micropattern. We find that the response of the receiver cell is stronger when the mechano-structural polarization axis is oriented perpendicular to the direction of force propagation, reminiscent of the Poisson effect in passive materials. We finally show that the same effects are at work in small tissues. Our work demonstrates that cellular organization and active mechanical response of a tissue are key to maintain signal strength and lead to the emergence of elasticity, which means that signals are not dissipated like in a viscous system, but can propagate over large distances.
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Author Response
Reviewer #1 (Public Review):
The authors study single and pairs of MDCK cells adherent to an H-shaped geometry on a flat surface. In this pattern, the cells form strong peripheral stress fibers. To a lesser extent, these cells also exhibit stress fibers in the cell interior, which otherwise has a rather homogenous actin distribution. Using a combination of traction force microscopy, from which they infer the stress distribution by monolayer stress microscopy, and "contour analysis" the authors quantify the 'bulk' and the 'surface' stress in these cells. This analysis shows that single cells are mechanically polarized whereas pairs are not.
The authors then go on to optogenetically activate the actomyosin contractility of either one half of a single cell or one cell of a pair. Combining their stress measurements in …
Author Response
Reviewer #1 (Public Review):
The authors study single and pairs of MDCK cells adherent to an H-shaped geometry on a flat surface. In this pattern, the cells form strong peripheral stress fibers. To a lesser extent, these cells also exhibit stress fibers in the cell interior, which otherwise has a rather homogenous actin distribution. Using a combination of traction force microscopy, from which they infer the stress distribution by monolayer stress microscopy, and "contour analysis" the authors quantify the 'bulk' and the 'surface' stress in these cells. This analysis shows that single cells are mechanically polarized whereas pairs are not.
The authors then go on to optogenetically activate the actomyosin contractility of either one half of a single cell or one cell of a pair. Combining their stress measurements in these situations and using a finite element mechanical model, the authors convincingly show that the mechanical response in the non-activated part is active. By varying the aspect ratio of the adhesion patterns, they also find that the efficacy of active stress propagation depends on the mechanical and structural polarity of the cell. Furthermore, they provide evidence that their results on cell pairs generalize to tissues.
Strengths:
This study uses a nice combination of physical tools to address an important question in tissue mechanics. The data is compelling and fully supports the authors' conclusions.
Weaknesses:
There are no major weaknesses.
In summary, although the fact that mechanical stress propagation in tissues is an active process might not come as a surprise, the study makes substantial contributions to a quantitative contribution of this process. As such it is of fundamental significance in the field. It will be interesting to explore the consequences of this mechanism for mechanical stress propagation in the context of developmental processes. It will be also of great interest to study how this local process can be accounted for in large-scale theories.
We thank reviewer #1 for this very positive assessment. We agree that in the future, our results should be used on the theory side to upscale them to tissue level. One way to do this would be the discontinuous Galerkin method, but it will take time to work this out. We also note that we would have loved to experimentally study intermediate cases between two and many cells, but it turned out to be very difficult to position few cells on micropattern and to repeat the force propagation analysis which we present here for two cells and for small tissues. In fact, it might be more rewarding to use optogenetics early in a developmental process with clearly defined cell positioning. In the revised manuscript, we now have added a comment on the challenge to work with three or four cells with the micropatterning approach, and that therefore we turned to small monolayers.
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eLife assessment
Using surface micropatterning, optical activation, and theoretical analysis, the authors provide compelling evidence that adjacent cells actively propagate mechanical stress in epithelial tissues. The response of the receiver cell is active and enhanced when the principal stress direction is perpendicular to the orientation of actin fibers. This work is important and a must-read for everybody wanting to understand tissue mechanics.
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Reviewer #1 (Public Review):
The authors study single and pairs of MDCK cells adherent to an H-shaped geometry on a flat surface. In this pattern, the cells form strong peripheral stress fibers. To a lesser extent, these cells also exhibit stress fibers in the cell interior, which otherwise has a rather homogenous actin distribution. Using a combination of traction force microscopy, from which they infer the stress distribution by monolayer stress microscopy, and "contour analysis" the authors quantify the 'bulk' and the 'surface' stress in these cells. This analysis shows that single cells are mechanically polarized whereas pairs are not.
The authors then go on to optogenetically activate the actomyosin contractility of either one half of a single cell or one cell of a pair. Combining their stress measurements in these situations and …
Reviewer #1 (Public Review):
The authors study single and pairs of MDCK cells adherent to an H-shaped geometry on a flat surface. In this pattern, the cells form strong peripheral stress fibers. To a lesser extent, these cells also exhibit stress fibers in the cell interior, which otherwise has a rather homogenous actin distribution. Using a combination of traction force microscopy, from which they infer the stress distribution by monolayer stress microscopy, and "contour analysis" the authors quantify the 'bulk' and the 'surface' stress in these cells. This analysis shows that single cells are mechanically polarized whereas pairs are not.
The authors then go on to optogenetically activate the actomyosin contractility of either one half of a single cell or one cell of a pair. Combining their stress measurements in these situations and using a finite element mechanical model, the authors convincingly show that the mechanical response in the non-activated part is active. By varying the aspect ratio of the adhesion patterns, they also find that the efficacy of active stress propagation depends on the mechanical and structural polarity of the cell. Furthermore, they provide evidence that their results on cell pairs generalize to tissues.
Strengths:
This study uses a nice combination of physical tools to address an important question in tissue mechanics. The data is compelling and fully supports the authors' conclusions.
Weaknesses:
There are no major weaknesses.
In summary, although the fact that mechanical stress propagation in tissues is an active process might not come as a surprise, the study makes substantial contributions to a quantitative contribution of this process. As such it is of fundamental significance in the field. It will be interesting to explore the consequences of this mechanism for mechanical stress propagation in the context of developmental processes. It will be also of great interest to study how this local process can be accounted for in large-scale theories.
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Reviewer #2 (Public Review):
In A. Ruppel, et al, the authors study the mechanics of one cell, two cells, and cell monolayers upon a transient local activation of contractility. First, the authors characterize the tractions and stress maps (measured via Traction Force Microscopy and Monolayer Stress Microscopy, resp.) for one and two cells in the absence of contractility activation, and found a correlation between the principal stress direction and actin fiber orientation. Next, the authors use the theory of foams to infer, combining traction force data and cell geometry data, the mechanical parameters of cells like the line tension or the force of adherent fibers. Next, the authors activate contractility by means of optogenetic tools on one half of the system and quantify the response on both halves, concluding that the receiver half …
Reviewer #2 (Public Review):
In A. Ruppel, et al, the authors study the mechanics of one cell, two cells, and cell monolayers upon a transient local activation of contractility. First, the authors characterize the tractions and stress maps (measured via Traction Force Microscopy and Monolayer Stress Microscopy, resp.) for one and two cells in the absence of contractility activation, and found a correlation between the principal stress direction and actin fiber orientation. Next, the authors use the theory of foams to infer, combining traction force data and cell geometry data, the mechanical parameters of cells like the line tension or the force of adherent fibers. Next, the authors activate contractility by means of optogenetic tools on one half of the system and quantify the response on both halves, concluding that the receiver half response is driven by active processes, increasing contractility for two cells, while fluidizing for one cell. Next, the authors estimate the level of active response in cell doublets by comparing the stress maps to numerical simulations of a thin elastic medium with anisotropic contractility. By varying aspect ratios of the H pattern, the authors find a correlation between the principal stress direction and the orientation of stress fibers and find that the previous active response is in general enhanced when the principal stress direction is perpendicular to the orientation of the fibers. Finally, these features are also found in a cell monolayer for a fixed confinement aspect ratio.
Overall, the manuscript contains a broad characterization of the steady state mechanics and the dynamical response to the activation of contractility for one cell, two cells, and cell monolayers.
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