Probing Local Responses to Substrate Rigidity Using a Photolabile Hydrogel

Frey and Wang, Soft Matter. 5:1918-1924 (2009)

To determine the responses of different regions of migrating cells to changes in substrate rigidity, we have developed a modulatable hydrogel by reacting linear polyacrylamide with a photosensitive crosslinker. This material allows UV-mediated softening by 20–30% upon irradiation at a dose tolerated by live cells. We found that localized softening of the posterior substrate of polarized cells causes no apparent effect, while softening of the anterior substratum elicits pronounced retraction, indicating that rigidity sensing is localized to the frontal region.

Response of NIH 3T3 Cells to Global Softening of the Substrate.

The recording shows cellular behavior from 2 h before to 2 h after UV exposure.  The irradiation, which takes place during the blank frame, induces a decrease of spread area.

Immediate Response of an NIH 3T3 Cell upon Softening of the Substrate Underneath the Whole Cell.

The two alternating images, acquired just before and after irradiation respectively, show recoiling of a traction-loaded cell in response to softening of the supporting substrate.  Beads embedded in the substratum indicated a concomitant increase in strain.

Immediate Recoiling of Beads within the Substrate upon Substrate Softening.

The first frame was taken just before irradiation of the substrate and the subsequent frames taken immediately after in 2 min intervals.  The beads (arrows) show rapid inward displacements upon initial illumination, which reflect the increase in strain caused by softening.  The strain is then released, indicating a decrease in traction forces and possible release of focal adhesions.

Lack of Polarity Reversal of an NIH 3T3 cell upon Substrate Softening in the Posterior Region.

The recording shows cellular behavior from 20 min before to 2 h after UV exposure.  Posterior softening produces no apparent response. The region of UV irradiation is indicated by a circle.

Reversal of Polarity of an NIH 3T3 cell upon Substrate Softening in the Anterior Region.

The recording shows cellular behavior from 20 min before to 2 h after UV exposure.  Softening of the anterior substratum elicits retraction of the leading edge followed by the reversal of polarity for about half of the cells studied. The region of UV irradiation is indicated by a circle.

Lack of Response of an NIH 3T3 cell on Control Substrate upon UV Irradiation in the Anterior Region.

The recording shows cellular behavior from 20 min before to 2 h after UV exposure.  The cell shows no apparent response to the localized irradiation.