Effects of the Initial Orientation of Actin Fibers on Global Tensile Properties of Cells

Abstract

Tensile tests of a single cell were simulated in order to understand the effects of the initial orientation of actin fibers (AFs) on global tensile properties. The properties examined included cell deformation, stiffness, and AF behavior. In the model used, the mechanical properties of cellular components, including the cell membrane with an associated actin network, nuclear envelope, and AFs, are expressed as a result of springs that generate force as a function of their extension. Cell shape during the tensile test was determined by a quasi-static approach couched in the framework of the minimum energy concept. Cells with various initial AF orientations were prepared; in particular, AFs in four different initial orientations, namely random (mean ± SD of the initial orientation angle, 46.4 ± 26.9°), parallel to the stretched direction (3.8 ± 3.5°), perpendicular (85.9 ± 2.6°), and diagonally oriented (44.5 ± 3.6°) were examined. The results show a significant drop in initial stiffness with an increase in mean initial AF orientation angles of 0 to 45°. The initial stiffness of the cell with parallel-oriented AFs was much larger than that with perpendicularly oriented AFs. The results also demonstrate that cell elongation induces a passive reorientation of AFs in a stretched direction, thereby causing an increase in cell stiffness. When comparing the rate of change for cell stiffness of the diagonally oriented model with that of the randomly oriented model, our data reveal that the rate of change of cell stiffness is characterized not only by the mean of the initial AF orientation angle, but also by the variation of their distribution.