Spherical and Cubic Magnetic Colloids to Decipher the Mechanics of the Cytoskeleton

Abstract

We developed a new method of mechanical measurement, based on the dipolar attraction between magnetic colloids. A spherical gel formed around magnetic colloids is progressively deformed between two surfaces as the dipolar attraction between the colloids increase. Applied force is controlled by the external magnetic field, while the gel deformation is monitored by video-microscopy. Elastic modulus of the gel material is extracted by the fit of the force-deformation curve. This method was used on in vitro dense branched networks formed by the Arp2/3-complex nucleator, which are crucial for cell migration. The high throughput of this method allowed the probing of thousands of gels, orders of magnitude more than previous experiments done with demanding techniques such as atomic force microscopy. By changing the concentration of proteins used during the growth of the gel, we changed the proportion of branches and the length of the filaments, and conducted the first systematic study linking the architecture of the actin networks to their architecture. A limitation of this technique is the requirement of sphericity to extract the elastic modulus from force-deformation curves. We solved this issue by developing new magnetic colloids of arbitrary shapes (cubes, cylinders, disks, etc.). This allows the deformation of objects between two flat surfaces while retaining the high throughput of this method. The technique now permits the measurement of non-linear elasticity, viscous modulus and speed of polymerization versus applied force for dense branched actin gels. Micro-objects of different shapes can be probed, and development is underway to use this technique for the measurement of adherent cells' mechanical properties. This technique could then be used as a diagnosis tool by probing metastatic cancer cells with a great throughput.