Cell Movement Is Guided by the Rigidity of the Substrate
Chun-Min Lo, Hong-Bei Wang, Micah Dembo, and Yu-li Wang
Biophys J. 2000 Jul;79(1):144-52.

In creating any tissue construct, it is important to recognize that any artificially engineered tissue or synthetic biomaterial must not only provide function and immunogenic compatibility, but must often encourage proliferation and cell migration from outside of the construct as well. To that end, it is critical for tissue engineers to understand the basic mechanisms which underlie cellular migration. In this paper, a group from the University of Massachusetts Medical School looked at how changing the rigidity of the substrate might affect the movement of the cells cultured on top of it. NIH 3T3 fibroblasts were cultured on sheets of flexible polyacrylamide that were coated with collagen 1. By varying the concentration of a bis-acrylamide crosslinker in the collagen, the researchers were able to create a substrate with two varying zones of stiffness (a softer side and a stiffer side), and examine how cells behaved as they moved across the boundary.

By placing the cell sheets under a Zeiss IM-35 microscope equipped with a 40x lens, the researchers were able to record both phase-contrast and fluorescence images at five minute increments. Traction forces were inferred by watching the movement of fluorescent beads which were embedded near the substrate surface. An image processing algorithm was used to record the movements of individual beads and convert those into a map of displacement vectors. By knowing the position of the cell boundary and the Young’s modulus, as well as the Poisson ratio of the gel, they were able to calculate the traction stresses generated by the cell. Similarly, migration speeds were calculated by looking at time-lapse phase images recorded over a period of one hour, and noting the change of the position of the center of the nucleus at 15 minute intervals.

They found that cells migrated preferentially from the softer side of the collagen gel towards the stiffer side, a tendency that they termed “durotaxis”. That is to say, cells that approached the boundary from the softer side had no problems migrating across it. When the cells moved across this boundary, they also showed an increase in the spreading area of the cell as well as the traction forces that they were able to generate. On the other hand, the cells that were moving in the opposite direction (from the stiff side to the softer one) did not want to cross the boundary. Those cells displayed a smaller spreading area or changed directions entirely. They suggest that by changing the strains that are experienced in varying regions of the substrate (i.e. in front of or behind a polarized cell), one may be able to redirect the movement of the cell. They also found that the 3T3 cells that were placed on the stiffer substrate were able to generate stronger traction forces than those placed on a softer one.

This information is useful because it shows that cell movement is directed not only by gradients of chemical signals, but by purely physical interactions at the cell-substrate interface as well. The authors propose that this may work through a sensory feedback mechanism involving the lamellipodia, in which expansion and retraction may affect some ion channel regulation or receptor-ligand interactions that lead to some downstream biochemical changes at the cellular level. This was one of the most awesome papers ever, and a fun thing to read if you have some free time on Friday night.