Guidance of cell migration by substrate dimension

Abstract

There is increasing evidence to suggest that physical parameters, including substrate rigidity, topography, and cell geometry, play an important role in cell migration. As there are significant differences in cell behavior when cultured in 1D, 2D, or 3D environments, we hypothesize that migrating cells are also able to sense the dimension of the environment as a guidance cue. NIH 3T3 fibroblasts were cultured on micropatterned substrates where the path of migration alternates between 1D lines and 2D rectangles. We found that 3T3 cells had a clear preference to stay on 2D rather than 1D substrates. Cells on 2D surfaces generated stronger traction stress than did those on 1D surfaces, but inhibition of myosin II caused cells to lose their sensitivity to substrate dimension, suggesting that myosin-II-dependent traction forces are the determining factor for dimension sensing. Furthermore, oncogene-transformed fibroblasts are defective in mechanosensing while generating similar traction forces on 1D and 2D surfaces. Dimension sensing may be involved in guiding cell migration for both physiological functions and tissue engineering, and for maintaining normal cells in their home tissue.

Figure 1

Micropattern with alternating 1D lines and 2D rectangles. The surface of polyacrylamide hydrogels is conjugated with gelatin in a defined micropattern (A), which is easily detected due to the concentration of fluorescent beads (B). Scale bar, 100μm.

Figure 2

Different responses to the 1D-2D interface between normal and blebbistatin-treated cells. (A) An NIH 3T3 cell migrating along a 1D line enters a 2D area, then moves deeply into the 1D exit on the opposite side of the rectangle before turning around. (B) In contrast, a cell treated with 10 μM blebbistatin for 30 min enters the 2D area from a 1D line and exits through the 1D line on the opposite side. Red dotted lines indicate the borders of micropatterning. Numbers indicate time in hours. Scale bar, 50 μm. (C, left) As a result of their preferential localization on 2D areas, an increasing percentage of NIH 3T3 cells becomes localized on 2D areas over a period of 24 h after seeding. N = 684, 701, and 685 cells at 3, 14, and 24 h, respectively (chi-square test, ^∗ p < 0.0001). (C, right) Cells treated with blebbistatin to inihibit myosin II show no significant accumulation on 2D areas. N = 226, 202, and 184 blebbistatin-treated cells at 3, 14, and 24 h, respectively. The experiment was performed with cells treated with mitomycin.

Figure 3

Traction stress measurements of cells migrating along 1D lines or on 2D rectangles. (A) The distribution of traction stress is shown as both vectors (small arrows) and heat maps (color-coded regions). (B) The bar graph shows the top 5% traction stress under different conditions. A significant difference between 2D and 1D is seen for control cells but not for blebbistatin-treated cells or PAP2 cells. N = 18, 14, and 18 for control, blebbistatin, and PAP2 cells, respectively, on 2D rectangles. N = 19, 15, and 19 for the corresponding measurements along 1D lines. Error bars represent the mean ± SE (t-test, ^∗p = 0.008). (C and D) Normalized traction stress of four cells (the time immediately before the cell reaches the 1D-2D interface is set as 0, and the corresponding traction stress is set as 1) and traction-stress heat maps (D) show an increase as cells migrate from 1D to 2D.

Figure 4

Size and number of focal adhesions along 1D lines and in 2D regions. (A) Immunofluorescence images of vinculin for NIH 3T3 cells show a larger number and/or size of focal adhesions in a 2D region than along a 1D line and after treatment with 10 μM blebbistatin. Arrows indicate elongated focal adhesions; arrowheads show small punctate adhesions in the cell treated with blebbistatin. Scale bar, 10 μm. (B) Bar graphs show average number of focal adhesions, focal adhesion size, and total focal adhesion area. Error bars represent the mean ± SE.

Figure 5

Stress fiber formation and myosin II activity differ between 1D and 2D. (A) Fluorescence images show the distribution of actin filaments and phosphorylated MRLC of NIH 3T3 cells in a 2D region, along a 1D line, and after treatment with 10 μM blebbistatin. Arrowheads indicate colocalization between phosphorylated MRLC and actin fibers. Arrow shows phosphorylated MRLC enrichment around the nucleus. Stress fibers are prominent in 2D regions, whereas cortical actin bundles are prominent in 1D. Treatment with blebbistatin causes disassembly of both forms of actin bundles. Scale bar, 10 μm. (B) Bar graph shows that MRLC is phosphorylated at a significantly higher level for cells spread on 2D surfaces compared to cells along 1D lines. Intensity is given in arbitrary units.

Figure 6

Lack of preference of ras-transformed NIH 3T3 fibroblasts (PAP2 cells) for localization in a 2D region. (A) A cell enters a 2D area from a 1D line but turns around to reenter the 1D line. Dotted lines indicate the border of the micropattern. Numbers indicate time in hours. Scale bar, 50 μm. (B) Consistent with a lack of dimensional preference, PAP2 cells show no significant accumulation on 2D areas over time. N = 333, 351, and 386 cells at 3, 14, and 24 h, respectively.