Effect of cluster shape, traction distribution and dynamics on the tensional homeostasis in multi-cellular clusters
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Various types of mammalian cells exhibit the remarkable ability to adapt to external applied mechanical stresses and strains. This ability allows cells to maintain a stable endogenous mechanical tension at a preferred (homeostatic) level, which is of great importance for normal physiological function of cells and tissues, and for a protection from various diseases, including atherosclerosis and cancer. Previous studies have shown that the cell ability to maintain tensional homeostasis is cell type-dependent. For example, isolated endothelial cell cannot maintain tensional homeostasis, whereas clusters of endothelial cells can, more so the greater the size of the cluster is. On the other hand, cell clustering does not affect tensional homeostasis of fibroblasts and vascular smooth muscle cells. Underlying mechanisms for these behaviors of different cell types are largely unknown. In this study, we combined theoretical analysis and mathematical modeling to investigate several biophysical factors, including cluster shape and size, magnitude and dynamics of cellular traction forces, and applied shear forces that may influence tensional homeostasis in cells and clusters. We developed two-dimensional models of cells clusters of different shapes and sizes. To simulate temporal fluctuations of cell-extracellular matrix traction forces, we used a Monte Carlo approach. We also applied physical forces obtained from previous experimental measurements to the models. Results of the analysis and modeling revealed that cluster size, magnitude and dynamics of focal adhesion traction forces have a major influence on traction field variability, whereas the influence of cluster shape appears to be minor. The dynamics of traction forces seems to be related to cell types and it can explain why in certain cell types, such as endothelial cells, cell clustering promotes tensional homeostasis, whereas in other cell types, such as fibroblasts, clustering has virtually no effect on homeostasis. To further investigate mechanisms that may affect tensional homeostasis, we investigated the effect of applied steady shear stress on the traction field dynamics of endothelial cells and clusters. We applied steady shear stress to our two-dimensional model of cell clusters and then computed ensuing changes in the traction force variability. These simulations mimicked the effect of flow-induced shear stress on tensional homeostasis of endothelial cells and clusters. We found that under steady shear stress, temporal fluctuations of the traction field of endothelial cells became attenuated. This result agrees with the viewpoint that steady shear flow promotes tensional homeostasis in the endothelium. Together, results of this study advance our understanding of biophysical mechanisms that contribute to the cell ability to maintain tensional homeostasis. Furthermore, these results will help us to modify our current experimental procedures, as well as to design new experiments for our investigation of tensional homeostasis.
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