Implementing an optical tweezers setup as a microrheology technique to study the mechanical properties of the cytoplasm

Abstract

Cells in our body are constantly sensing and generating forces through the cytoskeleton. The cytoskeleton is a network of protein filaments that mechanically connects the cells to their substrate and to their nucleus, thus allowing the transmission of mechanical stimuli from the membrane of the cell to the nucleus. More importantly, mechanical forces acting on the nucleus can trigger changes in gene expression and regulate cell behaviour. A recent finding indicates that fibrillar adhesions, a type of cellsubstrate adhesions located under the nucleus, alter the transmission of mechanical stimuli from the membrane of the cell to the nucleus, making the nucleus less susceptible to mechanical perturbations. Here, we hypothesise that they do it by tuning the mechanical properties of the cytoplasm. In order to test this hypothesis, we first develop a protocol to acquire microrheological measurements of the cytoplasm with an optical tweezers setup by following three steps: (1) defining a protocol to allow cells to internalise optical tweezers beads in their cytoplasm; (2) establishing a method to acquire microrheological measurements of the cytoplasm with an optical tweezers setup and, furthermore, proving the correct functioning of the system by adding a control that tunes its mechanical properties; (3) implementing a MATLAB program to automatically display and statistically test the data obtained. Once the system is optimised, we measure the mechanical properties of the cytoplasm with and without fibrillar adhesions and we find that cells with fibrillar adhesions exhibit a softer cytoplasm, suggesting a mechanism by which fibrillar adhesions can shield the nucleus from external mechanical perturbations.