Measurement of Local Viscoelasticity and Forces in Living Cells by Magnetic Tweezers

Abstract

We measured the viscoelastic properties of the cytoplasm of J774 macrophages with a recently developed microrheometer. Ferromagnetic beads (1.3 microm in diameter) were used to determine the local viscoelastic moduli. Step-force pulses were applied to the magnetic beads and the displacement was observed by single particle tracking. By analyzing the creep response curves in terms of a triphasic mechanical equivalent circuit, we measured the shear elastic modulus, the effective viscosities, and the strain relaxation time. The values of the shear modulus vary by more than an order of magnitude within the cell population (range, 20-735 Pa; average, 343 Pa) and by a factor of 2 within single cells. The effective viscosity of the cytoplasm exhibits a relatively sharp distribution about an average of eta = 210 Pa s (+/- 143 Pa s). We measured the displacement field generated by the local forces by observing the induced motion of nonmagnetic beads. Even at distances of the order of 1 microm, no induced motion was seen, suggesting that the cytoplasm is composed of clusters of densely packed and cross-linked filaments separated by soft regions. In another series of experiments we analyzed the magnetophoretic motion of the ferromagnetic beads at a constant magnetic force. Measuring the bead velocity parallel and perpendicular to the applied force showed that local active forces on the beads varied from 50 to 900 pN.

Active mechanical stabilization of the viscoplastic intracellular space of Dictyostelia cells by microtubule–actin crosstalk

“…The experimental setup was described previously [25,32]. The inhomogeneous field is generated with a soft iron core exhibiting a sharp wedge-shaped edge.…”

“…A variety of colloidal probe microrheometry techniques have been developed over the last few years based on the analysis of Brownian motion [10,23,24] or of the viscoelastic responses evoked by local forces applied through optical traps [10,23] and magnetic tweezers. With magnetic tweezers, viscoelastic responses can be evoked by linear [25] or by torsional [26,27] excitations. The viscoelastic response to force pulses yield relaxation moduli G(t) and relaxation times, while oscillatory force scenarios yield complex viscoelastic impedances.…”

“…Microrheometric studies of the cytoplasm of mammalian cells by magnetic tweezer microrheometry [25] and force free laser tracking [10] have been interpreted in terms of the models of viscoelastic bodies exhibiting shear moduli of the order of 100 Pa. However, shear field mapping experiments did not provide any evidence for elastic coupling between intracellular compartments over distances of 0.5 lm [25]. More recent microrheological studies of Dictyostelia cells immobilized by agar overlays provided evidence that the cytoplasmic space behaves rather as a viscoplastic body [29].…”

Contribution of cellular contractility to spatial and temporal variations in cellular stiffness

“…The mechanical properties of the cellular cortex have been measured by several techniques such as cell poking [12,13], micropipette aspiration [14,15], optical tweezers [16][17][18] and magnetic beads twisting [19][20][21]. However, the temporal and/or spatial resolutions of these techniques do not suffice for a thorough understanding of the variations in mechanical properties caused by cellular functions.…”

On the Mechanical Interplay Between Intra- and Inter-Synchronization During Collective Cell Migration: A Numerical Investigation

“…Therefore, in these cases the Young modulus of the cells is described as a function of their state with respect to time. Furthermore, the Young modulus E ECM and the viscosity μ ECM of the ECM have been fixed to 10 Pa and 3 × 10 5 Pa s (Bausch et al 1999;Drury and Dembo 2001), respectively, and the density of the cells is equal to 1000 kg/m 3 (Fukui et al 2000). Finally, the intensity of the cyclic active strain e a0 and the duration of a migration cycle T have been set to 0.5 (Allena 2013; Allena and Aubry 2012) and 60 s (Allena 2013;Allena and Aubry 2012;Dong et al 2002) respectively (except for the fifth mode of migration as described in Sect.…”