Cell-scale Contractile Forces Emerge from Non-mechnosensitive Active Displacement

Background: Contraction of the extracellular matrix (ECM) is a fundamental process that cells use for testing mechanical properties of their environment (‘mechanosensing’). Cells form adhesions that connect the actomyosin cytoskeleton with the ECM, allowing direct transmission of forces. Well-documented observation that increase in ECM stiffness drives generation of larger forces, led to the conventional approach that the process is mechanosensitive. Yet, this model has traditionally disregarded time-dependence of the forces, and myosin-based deformations. We designed a model of cellular contractility predicting that the ratio of the forces and the ECM stiffness is intrinsic, cell-specific and time-dependent, and thus non-mechanosensitive. Differences in actin cytoskeleton organization and F-actin to myosin ratio can provide key information on this process. Concentrating on the long-term forces, we are aiming to shed light on the dynamic mechanisms regulating their formation.

Methods: Measurements of displacement were performed in different conditions, using micron-scale pillar arrays and live-cell imaging. F-actin and myosin in WT MEFs were imaged with high-resolution confocal microscopy and a novel super-resolution processing algorithm SRRF, to provide resolution of ~50 nm. ImageJ Trainable Weka Segmentation was used to calculate the correlation.

Results: For each cell line, the displacement over time was constant for all rigidities. Counting F-actin branches and phospho-myosin heads, we reached correlation of 0.6. Comparison of the local F-actin intensity over time with pillar displacement over time in showed similar trend.

Conclusion: Force loading is dynamic and does not depend on the stiffness, but rather is cell-line specific. Correlation between F-actin and myosin exists.