Rotational magnetic endosome microrheology: viscoelastic architecture inside living cells.

The previously developed technique of magnetic rotational microrheology [Phys. Rev. E 67, 011504 (2003)] is proposed to investigate the rheological properties of the cell interior. An endogeneous magnetic probe is obtained inside living cells by labeling intracellular compartments with magnetic nanoparticles, following the endocytosis mechanism, the most general pathway used by eucaryotic cells to internalize substances from an extracellular medium. Primarily adsorbed on the plasma membrane, the magnetic nanoparticles are first internalized within submicronic membrane vesicles (100 nm diameter) to finally concentrate inside endocytotic intracellular compartments (0.6 microm diameter). These magnetic endosomes attract each other and form chains within the living cell when submitted to an external magnetic field. Here we demonstrate that these chains of magnetic endosomes are valuable tools to probe the intracellular dynamics at very local scales. The viscoelasticity of the chain microenvironment is quantified in terms of a viscosity eta and a relaxation time tau by analyzing the rotational dynamics of each tested chain in response to a rotation of the external magnetic field. The viscosity eta governs the long time flow of the medium surrounding the chains and the relaxation time tau reflects the proportion of solidlike versus liquidlike behavior (tau=eta/G, where G is the high-frequency shear modulus). Measurements in HeLa cells show that the cell interior is a highly heterogeneous structure, with regions where chains are embedded inside a dense viscoelastic matrix and other domains where chains are surrounded by a less rigid viscoelastic material. When one compound of the cell cytoskeleton is disrupted (microfilaments or microtubules), the intracellular viscoelasticity becomes less heterogeneous and more fluidlike, in the sense of both a lower viscosity and a lower relaxation time.