Cell membrane deformations under magnetic force modulation characterized by optical tracking and non-interferometric widefield profilometry

We measured cell membrane deformations under the modulation of piconewton magnetic force by using optical tracking and noninterferometric widefield optical profilometry. The magnetic force was applied to fibronectin-coated paramagnetic beads that bound to transmembrane protein integrins. At an image-acquisition rate of 20 frame/min, optical tracking provided positioning accuracy better than 70 nm for bead displacements on cell membranes, and optical profilometry obtained membrane topography with 20 nm depth resolution. We elucidated the correlation between the bead movements and membrane deformations. When the magnetic force dominated the bead movements, the membrane arose in front of the bead and the height increased with the bead velocity. On the other hand, when the bead was mainly driven by the cytoskeletons, the membrane profiles showed no relevance to the motion of the bead. In this case, the bead moved faster on smooth membranes. A model based on the dynamics of actin cytoskeletons is proposed to explain these observation results.