Connecting active and passive microrheology in living cells

TL;DR

This paper develops a fractional Langevin equation model with external noise to unify active and passive microrheology in living cells, revealing how nonequilibrium noise influences intracellular transport and linking experimental regimes through a generalized Stokes-Einstein relation.

Contribution

It introduces a novel model connecting active and passive microrheology in living cells, accounting for nonequilibrium external noise effects.

Findings

The model reproduces subdiffusive and superdiffusive behaviors observed in experiments.

External noise in cells is nonstationary with a time-dependent spectral density.

A generalized Stokes-Einstein relation links active and passive microrheology results.

Abstract

We use a model based on the fractional Langevin equation with external noise to describe the anomalous dynamics observed in microrheology experiments in living cells. This model reproduces both the subdiffusive short-time and the superdiffusive long-time behavior. We show that the former reflects the equilibrium properties of the cell, while the latter is due to the nonequilibrium external noise. This allows to infer the transport properties of the system under active measurements from the transient behavior obtained from passive measurements, extending the connection between active and passive microrheology to the nonequilibrium regime. The active and passive results can be linked via a generalized Stokes-Einstein relation based on an effective time-dependent temperature, which can be determined from the transient passive behavior. In order to reproduce experimental data, we further find that the external noise describing the active components of the cell has to be nonstationary. We establish that the latter leads to a time-dependent noise spectral density.