Cell Migration Driven by Cooperative Substrate Deformation Patterns

Entry by Helen Wu, AP225 Fall 2010

Reference

T. E. Angelini, E. Hannezo, X. Trepat, J. J. Fredberg, and D. A. Weitz, Physical Review Letters, 104, 168104(4) (2010).

Overview

Mechanical properties of the extracellular environment are known to play an important role in cell elasticity, migration, shape, and adhesion. These effects have been studied extensively with respect to single cells. This paper presents research into collective cellular behavior, which is more complicated and not well-understood. For example, neighboring cells can mechanically couple through the mutual substrate and how these interactions scale up to the tissue level is unknown.

The researchers used a confluent layer of epithelial cells on a deformable substrate to study substrate mediated cell-cell interactions as seen in migration patterns and found that collective cell migration is guided by multicellular mechanical cooperativity, demonstrated by long-distance substrate deformation fields.

Experiment

Substrate: polyacrylamide (PA) functionalized with Collagen I

Cells were seeded in a drop at the center of the substrate and cultured in media. Density and migration were observed through images in one-minute intervals. Substrate-displacement fields were generated using the traction force microscopy method and comparing images to an undeformed gel by cross-correlating pixels. Velocity fields were generated using a particle image velocimetry (PIV) -like method.

Figure 1. (a) Cells at low density. (b) Cells at high density. (c) Graph of cell area over time.

Results and discussion

Figure 2. Substrate-displacement fields with (a) cells at low density, (b) cells at high density. (c) Spatial correlation function graph. (d) Characteristic deformation correlation length increases with respect to time (cell density).
Figure 3. Velocity fields with (a) cells at low density, (b) cells at high density. (c) Spatial correlation function graph. (d) Swirl correlation length increases with respect to time (cell density).

Figure 1 shows that cell sizes were large at low densities and smaller at higher density.

Figure 2 shows large-scale patterns in the substrate-displacement fields.

A characteristic length scale was quantified using a spatial correlation function and found to decrease exponentially over the area. The characteristic deformation correlation length $\xi_d$ was calculated from the displacement field and shown to increase with respect to cell density (Figure 2d).

Figure 3 shows large-scale patterns in the velocity fields. The patterns are a measure of the collective cell motion domain size in the layer.

Domain size was quatified using the spatial correlation function also. In this case, it decreased over short distances and was negative at larger ones. The velocity fluctuation patterns were swirls and the migration correlation length (swirl size), $\xi_s$ was calculated. Cell shapes fluctuated at low densities but generally stayed round at higher ones, meaning small, round cells coordinate motion over greater distances and numbers than larger cells.

Deformability of the substrate affects the number of cells that move collectively. A linear correlation was found between $\xi_d$ and $\xi_s$, but on PA gels, $\xi_s$ was inversely proportional to cell projected area, while on glass, $\xi_s$ was directly proportional to it. Thus, at high densities, cells on rigid substrates move in small domains and cells on soft substrates move in large domains.

Migration-velocity fluctuations were demonstrated to always come after substrate-displacement fluctuations.

These results suggest that mechanical communication is a large influence on signals released during collective migration. Also, groups of cells can collectively deform a substrate in one direction, which requires force balancing on the collective scale as well.