Transient cell stiffening triggered by magnetic nanoparticle exposure

Abstract

Background The interactions between nanoparticles and the biological environment have long been studied, with toxicological assays being the most common experimental route. In parallel, recent growing evidence has brought into light the important role that cell mechanics play in numerous cell biological processes. However, despite the prevalence of nanotechnology applications in biology, and in particular the increased use of magnetic nanoparticles for cell therapy and imaging, the impact of nanoparticles on the cells’ mechanical properties remains poorly understood. Results Here, we used a parallel plate rheometer to measure the impact of magnetic nanoparticles on the viscoelastic modulus G*(f) of individual cells. We show how the active uptake of nanoparticles translates into cell stiffening in a short time scale (< 30 min), at the single cell level. The cell stiffening effect is however less marked at the cell population level, when the cells are pre-labeled under a longer incubation time (2 h) with nanoparticles. 24 h later, the stiffening effect is no more present. Imaging of the nanoparticle uptake reveals almost immediate (within minutes) nanoparticle aggregation at the cell membrane, triggering early endocytosis, whereas nanoparticles are almost all confined in late or lysosomal endosomes after 2 h of uptake. Remarkably, this correlates well with the imaging of the actin cytoskeleton, with actin bundling being highly prevalent at early time points into the exposure to the nanoparticles, an effect that renormalizes after longer periods. Conclusions Overall, this work evidences that magnetic nanoparticle internalization, coupled to cytoskeleton remodeling, contributes to a change in the cell mechanical properties within minutes of their initial contact, leading to an increase in cell rigidity. This effect appears to be transient, reduced after hours and disappearing 24 h after the internalization has taken place.

A microfluidic platform to investigate the role of mechanical constraints on tissue reorganization

“…This relaxation time value is of same order of magnitude than the one we found for MDCK cells in a 2D migration experiment of cells around an obstacle: τ ≈ 70min (28). Assuming that the effective viscosity due to rearrangements is η T1 ≈ G tissue τ relax , if we take at aggregate scale η T1 ≈ 10 5 Pa.s as we found previously (5,8), we find G ≈ 100 Pa, which is compatible with single cells rheological measurements performed on F9 cells (29).…”

A microfluidic platform to investigate the role of mechanical constraints on tissue reorganization

“…It is also expected to be close to the modulus of an isolated cell outside of an aggregate. In fact, the isolated F9 cell modulus is 100 Pa too (31).…”

In Vivo Cellular Magnetic Imaging: Labeled versus Unlabeled Cells

“…What may cause such limited migration? Elegant studies on the viscoelasticity of labeled and unlabeled cells revealed that SPIO‐labeling leads to cell “stiffening” within the first hours of uptake, returning to normal levels at 24 h. [ 47 ] This early time frame of stiffening correlated well with the actin cytoskeleton and their role in intracellular transport of endosomes. Since cell motility is initiated by an actin‐dependent protrusion of the cell's leading edge, which is composed of arm‐like structures called lamellipodia and filopodia, actin cytoskeleton reorganization with actin bundling may interfere with cell motility.…”