Interplay Between Intracellular Transport Dynamics and Liquid‒Liquid Phase Separation.

Liquid‒liquid phase separation (LLPS) is a ubiquitous process in which proteins, RNA, and biomolecules assemble into membrane-less compartments, playing important roles in many biological functions and diseases. The current knowledge on the biophysical and biochemical principles of LLPS is largely from in vitro studies; however, the physiological environment in living cells is complex and not at equilibrium. The characteristics of intracellular dynamics and their roles in physiological LLPS remain to be resolved. Here, by using single-particle tracking of quantum dots and dynamic monitoring of the formation of stress granules (SGs) in single cells, the spatiotemporal dynamics of intracellular transport in cells undergoing LLPS are quantified. It is shown that intracellular diffusion and active transport are both reduced. Furthermore, the formation of SG droplets contributes to increased spatial heterogeneity within the cell. More importantly, the study demonstrated that the LLPS of SGs can be regulated by intracellular dynamics in two stages: the reduced intracellular diffusion promotes SG assembly and the microtubule-associated transport facilitates SG coalescences. The work on intracellular dynamics not only improves the understanding of the mechanism of physiology phase separations occurring in nonequilibrium environments but also reveals an interplay between intracellular dynamics and LLPS.

Intracellular transport dynamics revealed by single-particle tracking.

Intracellular transport is the basis for the transfer of matter, energy, and information in cells and is critical to many cellular functions. Within the nonequilibrium environment of living cells, the transport behaviours are far from the traditional motion in liquid but are more complex and active. With the advantage of high spatial and temporal resolution, the single-particle tracking (SPT) method is widely utilized and has achieved great advances in revealing intracellular transport dynamics. This review describes intracellular transport from a physical perspective and classifies it into two modes: diffusive motion and directed motion. The biological functions and physical mechanisms for these two transport modes are introduced. Next, we review the principle of SPT and its advances in two aspects of intracellular transport. Finally, we discuss the prospect of near infrared SPT in exploring the in vivo intracellular transport dynamics.