Viscoelastic response of cells and the role of actin cytoskeletal remodelling.
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The mechanical properties of living cells provide useful information on cellular structure and function. In the present study a micropipette aspiration technique was developed to investigate the viscoelastic parameters of isolated articular chondrocytes. The Standard Linear Solid (SLS) and the Boltzmann Standard Linear Solid (BSLS) models were used to compute the instantaneous and equilibrium moduli and viscosity based on the response to an aspiration pressure of 7 cm of water. The modulus and viscosity of the chondrocytes increased with decreasing pressure rate. For example, the median equilibrium moduli obtained using the BSLS model increased from 0.19 kPa at 5.48 cmH2O/s to 0.62 kPa at 0.35 cmH2O/s. Cell deformation during micropipette aspiration was associated with an increase in cell volume and remodelling of the cortical actin visualised using GFP-actin. Interestingly, GFP-actin transfection inhibited the increase in cell moduli observed at the slower aspiration rate. Thus actin remodelling appears to be necessary for the pressure rate-dependent behaviour. A hypothesis is proposed explaining the role of actin remodelling and interaction with the membrane in regulating cell mechanics. Further studies investigated a mechanical injury model of cartilage explants which resulted in significant increases in all three viscoelastic parameters. Treatment with IL-1β also increased the instantaneous moduli of cells treated in explants but there was no difference in equilibrium moduli or viscosity. IL-1β treatment in monolayer had no effect on cell mechanics suggesting that previously reported changes in actin associated with IL-1β may be lost during cell isolation or trypsinisation. Separate studies demonstrated increases in chondrocyte moduli and viscosity during passage indicating changes in cell structure-function associated with de-differentiation in monolayer. In conclusion, this study has developed an optimised micropipette aspiration technique which was successfully used to quantify chondrocyte viscoelastic behaviour and to elucidate the underlying role of actin dynamics and response to pathological stimuli and in vitro culture.
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