Out-of-Equilibrium Microrheology inside Living Cells

Abstract

Both forced and spontaneous motions of magnetic microbeads engulfed by Dictyostelium cells have served as experimental probes of intracellular dynamics. The complex shear modulus G*(omega), determined from active oscillatory measurements, has a power-law dynamics and increases with the probe size, reflecting intracellular structural complexity. The combined use of passive microrheology allows one to derive the power spectrum of active forces acting on intracellular phagosomes and to test the validity of the fluctuation-dissipation theorem inside living cells.

Transition to superdiffusive behavior in intracellular actin-based transport mediated by molecular motors

“…The violation of the fluctuation dissipation theorem (FDT) has been observed in an in vitro system consisting of a cross-linked actin network with embedded myosin motors [7] and also for beads attached to the cell membrane of human airway smooth muscle (HASM) cells [8]. Likewise, direct evidence of the deviation from equilibrium inside the cell has been recently obtained [11,12]. In Ref.…”

“…Typically, the transition time between these two regimes is on the order of 1 s. While some authors attribute the subdiffusive behavior to elastic trapping [20], obstruction [22], crowding [23] or stalling [8], others propose that apparent subdiffusion can arise from noise inherent to single particle tracking (SPT) experiments due to slight errors on the determination of the actual particle position [24]. Although there is not a general consensus of the causes of subdiffusion yet, it is well accepted that superdiffusion has its origin in the collective behavior of molecular motors [8,11,12].…”

“…Moreover, in recent experiments it was observed that the forces power spectrum exhibits a power law behavior [8,11,12,14]. Then, we assume that the autocorrelation function…”

“…The subdiffusive behavior is characterized by an exponent ranging from 0.2 [19] to 1 [20,21] while the superdiffusive behav-ior presents exponents close to 1.5 [8,12]. Typically, the transition time between these two regimes is on the order of 1 s. While some authors attribute the subdiffusive behavior to elastic trapping [20], obstruction [22], crowding [23] or stalling [8], others propose that apparent subdiffusion can arise from noise inherent to single particle tracking (SPT) experiments due to slight errors on the determination of the actual particle position [24].…”

“…In Ref. [12] both forced and spontaneous motions of magnetic microbeads engulfed by Dyctyostelium cells are analyzed to derive the power spectrum of forces acting on intracellular phagosomes. However, the theoretical model proposed to describe their data did not distinguish between active and passive contribution to the transport.…”