Cell Mechanics and Imaging

With the advent of newer, more precise technologies for exerting mechanical forces on cells (such as magnetic traps, optical tweezers, and atomic force microscopes), a greater need exists for monitoring the physical responses of the receptors. Since some receptors are thought to be directly involved in mechanical signaling, it is likely that understanding how the receptors physically respond will be crucial to further developing a model for the signaling pathway. To study this, we apply magnetic trapping with a recent innovation in polarization characterization of quantum dots. These dots (with typically nm diameters) can attach by a single molecule (such as an antibody) to a specific receptor. By using a custom polarization set-up, the orientation of the dot can be determined. Thus, it is possible to track the response of the receptor to an externally applied force in each of the six degrees of freedom (three translation, and three rotation), although in practice the z translation and rotation are more difficult to extract.

Single-pole magnetic trapping was used in conjunction with antibody-labeled magnetic beads in order to apply stresses to different cell types on specific receptors. The purpose of this study was to determine the variability in receptor and cell-type responses to physical stresses. It was determined that there is in fact large variabilities in responses, with the receptor-linkage to the cytoskeleton being relatively independent of the actual measured stiffness. Additionally, cell type is important- in cells with a presumed higher activity receptor level (not necessarily higher expression levels, such as for dystrogylcans), the receptor has higher apparent stiffness. Finally, despite the non-linkage to the cytoskeleton, transferrin receptors were found to exhibit moderate levels of stiffness exceeding values expected for a free-floating receptor. As a result, we conclude that magnetic trapping experiments need to account for cell variabilities and possible contributions of cell subnetworks, including those that segment the membrane into rafts.