Microrheology in mechanosensitive bone cells

Abstract

To understand how the mechanosensing by bone cells might relate to cellular metabolism and mechanical properties, a physical portrait of cell viscoelasticity is needed. Thus, we developed a novel application of two-particle microrheology using fibronectin-coated spherical probes to characterize the viscoelasticity, mechanically stimulate, and probe the mechano-activity of various cell types. We found that the elastic modulus of MLO-Y4 osteocytes was below 500 Pa, as well as for MC3T3-E1 osteoblasts, and primary osteocytes and osteoblasts. Interestingly, the nitric oxide released by MLO-Y4 cells increased after a mechanical stimulation at 5 pN, with cell-attached integrin-bound probes. This suggests that bone cells respond to forces at a similar range as for deforming integrins. MLO-Y4 cells interacted with the integrin-bound probes by changing their shape from spherical to being polar at 37oC. However, the same shape change at 22oC was acquired by stimulation with 5-20 pN forces. This suggests that temperature has a significant effect on cell morphology. To probe the mechano-activity of bone cells, we measured the fluctuation of force induced by the cells on the integrin-bound probes. induced by MLO-Y4 cells were proportional to ω-2 (where frequency = ω/(2π)) at frequencies < 3 Hz at 22oC. However, the force fluctuation was proportional to ω-2 for frequencies < 10 Hz at 37oC. Thus, temperature change possibly induces a sharp non-linear metabolic increase underlining the significant effect of the mechanical environment to cellular metabolism. Compared to MLO-Y4 cells, CCL-224 fibroblasts had a higher magnitude at 37oC, as might be expected considering the motility of fibroblastic cells. The linear relation of with ω-2 is a signature expected for continuums with slowly evolving internal processes. Hence, microrheology is a useful tool for understanding the varied set of observations on mechanosensing by bone cells and its implicatioimplications on the osteogenic response of bone to mechanical loading.