The mode of mechanical integrin stressing controls intracellular signaling in osteoblasts.

Following the idea that integrin receptors function as mechanotransducers, we applied defined physical forces to integrins in osteoblastic cells using a magnetic drag force device to show how cells sense different modes of physical forces. Application of mechanical stress to the beta1-integrin subunit revealed that cyclic forces of 1 Hz were more effective to stimulate the cellular calcium response than continuous load. Cyclic forces also induced an enhanced cytoskeletal anchorage of tyrosine-phosphorylated proteins and increased activation of the focal adhesion kinase (FAK) and mitogen activated protein (MAP) kinase. These events were dependent on an intact cytoskeleton and the presence of intracellular calcium. Analyses of the intracellular spatial organization of the calcium responses revealed that calcium signals originate in a restricted region in the vicinity of the stressed receptors, which indicates that cells are able to sense locally applied stress on the cell surface via integrins. The calcium signals can spread throughout the cell including the nucleus, which shows that calcium also is a candidate to transmit mechanically induced information into different cellular compartments.