Dynamic Change in Morphology and Traction Forces at Focal Adhesions in Cultured Vascular Smooth Muscle Cells During Contraction

Abstract

In order to study how cells change their traction forces at focal adhesions (FAs) following cell contraction, we observed the dynamic changes in traction forces at FAs, the morphology of FAs, and actin stress fibers (SFs) anchoring FAs in cultured porcine aortic smooth muscle cells (SMCs) during cell contraction. SMCs were cultured on polydimethylsiloxane-based elastic micropillar array substrates, and their traction forces at individual FAs were measured by the deflection of the pillars during cell contraction induced with 10−4 M serotonin. The traction forces started to increase immediately after the administration of serotonin, especially at the cell periphery, and their direction converged gradually to that of the cell major axis. The directional change of force reached a plateau in the early stages of the contraction. The time constants were significantly smaller for changes in direction (mean ± standard deviation: 8.0 ± 4.5 min; 116 pillars of 10 cells) compared to those concerning the magnitude of the force (16.6 ± 6.0 min). Surface reflective interference contrast microscopy revealed that the morphological changes in FAs showed a trend similar to that of their forces: FAs grew and aligned in the direction of the cell major axis in the early stage of the contraction. Some FAs then merged and continuously elongated along SFs. The number of FAs and SFs in each cell decreased similarly by 15–20% 60 min after the administration of serotonin, suggesting that contractile activation induced fusion of FAs and of SFs. Total FA area per cell more than doubled in 60 min. These results indicate that FAs may remodel themselves actively during cell contraction depending on the direction and strength of contractile forces of SFs. The fusion of FAs and of SFs may have the directions of the traction forces more coherent, and thus increase the net contraction force generated by each SMC. The concomitant increase in the FA area may make adhesion sites strong enough to transmit the increased force to the extracellular matrix.