Novel dynamic rheological behavior of individual focal adhesions measured within single cells using electromagnetic pulling cytometry.

The rheology of cells and sub-cellular structures, such as focal adhesions, are important for cell form and function. Here we describe electromagnetic pulling cytometry (EPC), a technique to analyze cell rheology by applying dynamic tensional forces to ligand-coated magnetic microbeads bound to cell surface integrin receptors. EPC utilizes an electromagnetic microneedle that is integrated with a computerized control and image acquisition system and an inverted microscope and CCD camera to monitor bead displacement. Arbitrary force regimens may be defined over a wide range of frequency (DC to 10 Hz) and force (100 pN to 10 nN). With EPC, the viscoelastic creep response of individual focal adhesions was measured over three decades in time using RGD-coated magnetic microbeads bound to integrins that induce local focal adhesion assembly and coupling to the internal cytoskeleton. These data were compared to the power-law-like predictions from the soft glassy model of cell rheology proposed by Fabry et al. Although power-law-like behavior was observed in some focal adhesions, 52% of these structures did not exhibit power-law-like behavior, but instead exhibited either a multi-phase response characterized by abrupt changes in slope or experienced a retraction in the opposite direction to the applied force, especially in response to prolonged force application. These data suggest that while the soft glassy model may provide reasonable estimates for aggregate mechanical behavior of living cells, the rheological behavior of individual focal adhesions may be more heterogeneous and complex than suggested by the soft glassy model. These results are considered in context with the hierarchical nature of cytoskeletal architecture.