Investigating viscoelastic effects on cell adhesion using Atomic Force Microscopy

We report a new quantitative approach to investigate the effects of viscoelastic deformation on the adhesive behavior of soft biological cells. Atomic Force Microscopy Force Spectroscopy (AFM-FS) was applied to characterize adhesion and mechanics of HK2 kidney and pancreatic MIN6 beta cells. Both types of cells were treated using specific chemical agents, targeting to down-regulate adhesion of the former and up-regulate adhesion of the latter. Functionalized AFM tipless cantilevers were used for attaching single cells in suspension and bringing them into contact with adherent substrate cells. Retraction force-displacement curves provide important information regarding the adhesion between two coupled cells, such as maximum unbinding force (Fmax) and adhesion energy (Wa). AFM-FS indentation experiments were performed by indenting single cells with a spherical microbead attached to the cantilever for determining their Young?s modulus (E). The viscoelastic effects were examined by performing experiments at various retraction and indentation speeds. Following treatment of HK2 cells Fmax decreased by 20%, whereas Wa decreased significantly (53%) corresponding to a 42% increase in E. This evidence suggests that the increase in cell rigidity, which consequently reduced the displacement separation between two adherent cells, contributed significantly in the higher increase of Wa, compared with that of Fmax. Following treatment of MIN6 cells Fmax increased by 29%, while Wa increased by 37% in response to a 34% decrease in (E). This implies that the increase in cell-cell adhesion energy was not only due to higher ligand-receptor binding, but also due to an increase in cell elasticity. The results of both HK2 and MIN6 cells clearly demonstrate the influence of viscoelastic deformation on the adhesion energy between two adherent cells. Conclusively, cytomechanical influences such as viscoelasticity are important underlying factors contributing to cell-cell adhesion energy.