Separation with no boundaries

Every living cell has a highly complex internal organization. NCCR Bio-Inspired Materials researchers at Zurich’s Federal Institute of Technology (ETHZ) have been investigating the physical phenomena that determine intracellular compartmentalization.  

Phase separation is a central concept of materials physics – think of the three distinct phases of water as ice, a liquid and vapor. However, as a universal physical phenomenon, it still some distance from being entirely understood. Biological phase separation, for example, relates often to intracellular compartmentalization without membranes, featuring complex compositions and elasticity. Cells need to organize a variety of biochemical reactions within their confines. Part of this conundrum is solved by compartments called organelles, which contain their own environment, have their own boundary – usually a membrane – and allow chemical reactions to take place inside. One example are mitochondria, often called the cell’s power plant. Other compartments do not have a membrane though, such as the nucleoli, which produce the ribosomes that perform protein synthesis, inside the cell nucleus. How these different compartments coexist without membranes is still not entirely clear. One potential explanation is so-called liquid-liquid phase separation.

Researchers led by NCCR Principal Investigator Prof. Eric Dufresne (ETHZ) chose to study the impact of elasticity on phase separation in synthetic polymer networks. They were inspired by previous work carried out by Dufresne on non-iridescent structural color in bird feathers. Results then suggested that phase separation was responsible for the nanostructures that generated vivid hues, but how the process began and ended was unclear. Understanding this biomimetic phase separation could provide a novel route for the assembly of useful photonic structures. Past experiments have shown that elasticity can markedly impact liquid–liquid phase separation in swollen synthetic polymer networks. In a homogeneous network, droplets grow to a fixed size, controlled by the network stiffness. When the network has an anisotropic state of stress, droplets grow with scale-independent ellipsoidal shapes. Other researchers have demonstrated that in the nucleus of living cells, phase-separating domains were found for example to form preferentially in chromatin-poor regions. Chromatin’s main task is to package long DNA molecules into more compact, denser structures. After drops were triggered to grow in chromatin-rich regions, they migrated toward chromatin-poor regions.

For their study, Dufresne and his colleagues used a synthetic polymer system, where mechanical properties and chemical solubility were tuned independently to reveal the impact of network mechanics on droplet nucleation and ripening. They found that network elasticity can suppress droplet nucleation deep inside the thermodynamically immiscible region, mechanically stabilizing supersaturated mixtures. The long-term stability of droplets is strongly affected by network elasticity. They also demonstrated that in a mechanically heterogeneous network the solute moves from stiff to soft regions by diffusive transport through the dilute phase.

These phenomena suggest that living cells might regulate the localization of organelles without membranes through gradients of their mechanical properties. “We were able to show that compressive stresses in a polymer network can suppress phase separation of the solvent that swells it, stabilizing mixtures well beyond the liquid–liquid phase-separation boundary,” explains Dufresne. Network stresses also drive a new form of ripening, driven by transport of solute down stiffness gradients. This elastic ripening can be much faster than conventional Ostwald ripening driven by surface tension, in which small particles shrink by releasing molecules that coalesce on larger ones.

According to Dufresne, further research is now required to quantify the heterogeneous mechanical properties of the cell interior, and the size distribution and surface tension of droplets. This will enable scientists to assess the relative contributions of surface tension and elasticity in a specific cellular context.

Reference: Rosowski, K. A.; Sai, T.; Vidal-Henriquez, E.; Zwicker, D.; Style, R. W.; Dufresne, E. R. Elastic ripening and inhibition of liquid–liquid phase separation, Nat. Phys., 2020, 16, 422–425.