Intracellular Microrheology in the Presence of Myosin-Generated Forces in Living Cells

Abstract

The mechanics of cells are governed by a network of cytoskeletal filaments and molecular motor forming a dynamic mechanical entity. A recent experimental study by Mizuno showed local shear modulus of a synthesized cytoskeletal network could increase in the presence of myosin-generated internal stresses. It was speculated whether similar behaviors could also take place in cells. To examine this, we used an experimental method similar to Mizuno's that combined active and passive microrheology to measure myosin-generated fluctuating force and the local intracellular stiffness. Our data obtained from measurements in 15 HeLa cells showed a linear relationship between the magnitude of the fluctuating forces and the time-averaged shear modulus. Moreover, when myosin activities were inhibited by ML7, we saw a dramatic decrease in the intracellular forces, but surprisingly no significant changes were observed in the time-averaged intracellular stiffness by ML7. While we did not see a direct correlation between the mean intracellular shear modulus and the motor-generated fluctuating force, we did observe an increase in the fluctuation of the shear modulus with increasing motor forces.While it is not clear why results from the intracellular study were so different from the actin construct, some differences between the two systems may be attributed to the difference. First, the intracellular fluctuating forces were found to be weaker than that in the actin construct, thus perhaps not strong enough to modulate the cell stiffness. Secondly, the steady-state internal tension, which could dictate mechanical proprieties of a network, may be different between the two systems. Unfortunately, the experimental method we used could only measure fluctuating forces but not the steady-state tension in the system. Further studies that measure both fluctuating and steady-state forces at the same time may be required to address this problem.