Abstract
BackgroundLiving cells are subjected to external and internal mechanical stresses. The effects of these stresses on the deformation and subsequent biological response of the cells remains unclear. This study tested the hypothesis that the rate at which pressure (or stress) is applied influence the viscoelastic properties of the cell associated with differences in the dynamics of the actin cytoskeleton.Principal FindingMicropipette aspiration was used to determine the instantaneous and equilibrium moduli and the viscosity of isolated chondrocytes based on the standard linear solid (SLS) model and a variation of this incorporating Boltzmann superposition. Cells were visualised for 180 seconds following aspiration to 7 cmH2O at 0.35, 0.70 and 5.48 cmH2O/sec. Cell recovery was then examined for a further 180 seconds once the pressure had been removed. Reducing the rate of application of pressure reduced the levels of cell deformation and recovery associated with a significant increase in modulus and viscosity. Using GFP transfection and confocal microscopy, we show that chondrocyte deformation involves distortion, disassembly and subsequent reassembly of the cortical actin cytoskeleton. At faster pressure rates, cell deformation produced an increase in cell volume associated with membrane bleb formation. GFP-actin transfection inhibited the pressure rate dependent variation in cell mechanics indicating that this behaviour is regulated by GFP-sensitive actin dynamics.ConclusionWe suggest that slower rates of aspiration pressure enable greater levels of cortical actin distortion. This is partially inhibited by GFP or faster aspiration rates leading to membrane bleb formation and an increase in cell volume. Thus the rate of application of pressure regulates the viscoelastic mechanical properties of living cells through pressure rate sensitive differences in actin dynamics. Therefore cells appear softer when aspirated at a faster rate in contrast to what is expected of a normal viscoelastic material.
Highlights
Living cells in a wide variety of tissues are subjected to a complex mechanical loading environment comprising of both externally applied and internally generated mechanical forces
We suggest that slower rates of aspiration pressure enable greater levels of cortical actin distortion
This is partially inhibited by GFP or faster aspiration rates leading to membrane bleb formation and an increase in cell volume
Summary
Living cells in a wide variety of tissues are subjected to a complex mechanical loading environment comprising of both externally applied and internally generated mechanical forces. The actin cytoskeleton exists in a constant state of turnover between polymerised filamentous Factin and globular G-actin This facilitates the remodelling of structures such as lamelipodia, filopodia and stress fibres and is important in many aspects of cell function including motility, morphogenesis, cell cycle progression and differentiation. Recent studies have shown that purified actin experiences strain hardening and softening in response to mechanical perturbation [11,12] It is unclear whether such phenomena exist within the cell. Living cells are subjected to external and internal mechanical stresses The effects of these stresses on the deformation and subsequent biological response of the cells remains unclear. This study tested the hypothesis that the rate at which pressure (or stress) is applied influence the viscoelastic properties of the cell associated with differences in the dynamics of the actin cytoskeleton. GFP-actin transfection inhibited the pressure rate dependent variation in cell mechanics indicating that this behaviour is regulated by GFP-sensitive actin dynamics
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