Focal Contact Clustering in Osteoblastic Cells under Mechanical Stresses: Microgravity and Cyclic Deformation

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Abstract
We quantitatively compared vinculin-related adhesion parameters in osteoblastic cells submitted to two opposing mechanical stresses: low deformation and frequency strain regimens (stretch conditions) and microgravity exposure (relaxed conditions). In both ROS 17/2.8 cells and rat primary osteoblastic cells, 1% cyclic deformations at 0.05 Hz for 10 min per day for seven days stimulated cell growth compared to static culture conditions, while relaxed ROS cells proliferated in a similar way to static cultures (BC). We studied the short-term (up to 24 h) adaptation of focal contact reorganization under these two conditions. Cyclic deformation induced a biphasic response comprising the formation of new focal contacts followed by clustering of these focal contacts in both ROS cells and primary osteoblasts. Microgravity exposure induced a reduction in focal contact number and clustering in ROS cells. To evaluate whether the proliferation (stretch) or survival (relaxed) status of ROS cells influences focal contact organization, we inhibited the ERK proliferative-dependent pathway. Inhibition of proliferation by PD98059 was partially reversed, but not fully restored by stretch. Stretch-induced clustering of vinculin-positive contacts also persisted in the presence of PD98059, whereas the increase in focal contact number was abolished. In conclusion, we show that focal contacts are mechanoeffectors, and we suggest that their morphologic organization might serve as a discriminant functional parameter between survival and proliferation status in ROS 17/2.8 osteoblastic cells.