The Direction of Cyclic Stretch-Induced Stress Fiber Orientation Depends on Matrix Rigidity

Mechanical properties of the cellular environment such as elastic rigidity have been shown to play an important role in the regulation of important cellular processes such as proliferation, differentiation and apoptosis (1–3). Intracellular tension decreases with decreasing matrix rigidity (1). Actin stress fibers (SFs), the major structural element in cells bearing tension, are also less prevalent on soft vs. rigid matrices (4). We have developed a theoretical model of stretch-induced SFs that predicts SFs reorient perpendicular to the direction of cyclic stretch in order to maintain SF tension at a homeostatic level (5). A theoretical model developed by the Safran group (6) predicts that cells will also align perpendicular to cyclic stretch on soft substrates. To test these predictions, we subjected cells to cyclic uniaxial stretch on soft collagen hydrogels. Interestingly, the cells and their SFs aligned parallel to the direction of stretch without co-alignment of collagen fibrils, indicating the need for a new model to describe the effects of cyclic stretch on SF reorganization on soft matrices.