Abstract
Although its importance is widely accepted recently, force measurement has been difficult in living biological systems, mainly due to the lack of the versatile non-invasive force measurement methods. The fluctuation theorem, which represents the thermodynamic properties of small fluctuating non-equilibrium systems, has been applied to the analysis of the thermodynamic properties of motor proteins in vitro. Here, we extend it to the axonal transport of endosomes. The fluctuation of their displacement distributed with several distinct peaks at multiples of a unit value, which the fluctuation theorem can convert into the drag force exerted on the endosomes. The results demonstrated that a single cargo vesicle is often conveyed by two to three force producing units. Furthermore, this method enabled us to compare the force-velocity relations in vitro and in vivo. These results showed the versatility of this approach for general non-invasive force measurements in vivo.