Hierarchical Heterogeneities in Spatio-Temporal Dynamics of the Cytoplasm

Understanding of the dynamics inherent to biological matter is crucial for illuminating the physical mechanisms underlying cellular processes. In this study, we employ bright-field differential dynamic microscopy (DDM) to investigate density fluctuations inherent in a cell-free model of eukaryotic cytoplasm. Our measurements reveal subdiffusive fractional Brownian motion and non-Gaussian displacement distributions, highlighting cytoplasmic heterogeneity. We introduce an empirical model that combines fractional Brownian motion with an inverse Gaussian distribution of diffusivities to describe the observed non-Gaussianity. Validated through Monte Carlo simulations, this model allows us to estimate the fractional diffusivity and exponent effectively. By altering macromolecular composition, the addition of energy, and assembly of a cytoskeleton, we identify three independent mechanisms that result in similar fractional exponents yet distinct diffusivities. We find that energy addition leads to non-stationary dynamics, in contrast to the stationary behavior observed under passive conditions. Presence of microtubules introduces a secondary dynamical timescale, which we describe using a two-state fractional Brownian motion model to differentiate between cytosolic and microtubule network associated contributions. Our findings demonstrate the effectiveness of DDM as a label-free tool for quantifying viscoelastic and heterogeneous properties of the cytoplasm and provide insights into how physical and biochemical factors, including cytoskeletal organization, govern subcellular dynamics.