Temporal correlation between oscillating force dipoles drives single cell migration in 3D

Background

While cells moving in 2D or in micro-channels often display clear spatial polarisation, characterised by F-actin flowing from the front to the back of the cell, many cells, especially when moving in 3D matrices, do not show such polarisation, but instead display sequences of protrusion/retraction. The mechanism by which time-reversal symmetry is broken under these conditions remains unknown, and altogether, basic principles governing 3D cellular motion remain elusive.
Using a combination of 3D live cell imaging, traction force microscopy and a minimal model with multipolar expansion-  a cell derived matrix (CDM), Godeau et al explore motility in 3D and show that the existence of a phase shift between the two dipoles, mediated mainly by the microtubule network, is involved in directed cell motion. They reveal that the cell controls its motility by synchronising dipolar forces distributed at front and back.

Key findings and developments

For their work, the authors generated a cell-derived matrix, which behaves as a soft and elastic material and can be easily deformed as cells move within it. They also used cells with key molecular players tagged, namely components of the cell cytoskeleton, acto-myosin and microtubules. This allowed on one hand, quantification of the associated matrix deformation, and on the other, observation of the cellular proteins responsible for force generation.

They observed that some wild-type cells showed persistent motion, while others did not move. Regardless of motility, all types of cells exhibited local zones of contraction-extension, equivalent of two force dipoles on either side of the nucleus. These deformations were found to be cyclic, with similar periods of oscillation between motile and non-motile cells. During contraction, local myosin clusters were formed, and there was protrusive activity at one end of the cells however, this spatial polarization was not enough for directed motion.

Investigation of the temporal correlation to contraction-extension at the two force dipoles showed that non-motile cells have no clear phase shift between the two ends, while migrating cells show a time lag. The authors relate this lag to two distinct sub-cellular contraction-extension dipoles in migrating cells, and the forces driving them.  Further investigation led the authors to conclude that absence of migration is not due to the lack of traction forces. Analysis of matrix deformation and tracking showed that migrating cells display phase shift between contraction-extension cycles at the two ends, and a time reversal symmetry breaking. To determine if there was a correlation between the oscillations at the two cell ends, and molecular actors, microtubules were de-polymerized with nocodazole. In nocodazole-treated cells oscillations at both cell ends were anti-correlated, and the cells did not show directed motion, supporting the idea that coupling between oscillations is needed for directed motion. Also, this shows the importance of the microtubule network in the correlation and time delay between oscillations across the cells, and promoting locomotion.

The authors then went on to study the geometry and dynamics of traction force distribution. In their model, the simplest self-propelled object is a micro- swimmer embedded in a fluid matrix and migrating due to hydrodynamic interactions. However, the micro-swimmer does not reproduce cell crawling, which depends on the dynamics of cell attachment and detachment from the surrounding matrix. In their work, the authors suggest that dipole contraction is associated with active contraction of acto-myosin clusters, while dipole extension is related to the elastic relaxation of the CDM following local cell detachment by the loss of focal contacts. The authors went on to explore additional dynamic parameters including crawling velocity and velocity oscillations- which they use as a proxy of the dynamics of internal force generation.

They hypothesize that the temporal coupling between spatially distributed force dipoles along the cell promote cell locomotion, and to prove this, used laser ablation to trigger localized cellular force dipoles by triggering contractions at either sides of the cell. Localized laser ablation led to the recruitment of actin cytoskeleton and localized cell contraction. This was sufficient to promote directed cell motion. Altogether, the authors suggest that while the microtubular network iskey for motility, other cytokeletal elements and their interplay with adhesion dynamics are likely to play a role as well.

 

What I like about this preprint

This preprint is consistent with interdisciplinary science, and addresses an interesting question for various fields of research, which is the biophysics and molecular biology underlying cell locomotion. I like that it is a concise and robust piece exploring various aspects of cell motility, with a clear discussion of adapted and novel methods and models. I found it an interesting piece altogether.

 

Open questions

1. In your work, the CDM has very specific properties. If you wish to model cell motility in a manner that reflects in vivo motility across tissues, how would you modify the CDM?
2. In your work you describe some differences between a swimmer and a crawler. Do you expect the parameters to vary if the CDM or substrate in which either a swimmer or a crawler are placed, varies?
3. Is it possible to modify the CDM to add other parameters present in vivo, and possibly influencing cell motility, such as the presence of lipids, collagen, platelets, etc.? How do you expect this will influence cell crawling as you explored in your work?
4. You discuss in your work, velocity oscillations, and go on to explore the role of the microtubules and acto-myosin network in cell crawling. In an in vivo situation, how do you integrate external factors in influencing the velocity of cell crawling? For instance signalling promoting cytokinesis, or exogenous stimuli such as the presence of immune cells, cell wounding, etc.?
5. Within a tissue/closed system, how does one cell’s motility and dynamics influence that of its neighbours?

 

References

1. Godeau A et al , Temporal correlation between oscillating force dipoles drives single cell migration in 3D, bioRxiv, 2020.

 

Posted on: 11 June 2020

doi: https://doi.org/10.1242/prelights.21837

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