A bending rigidity parameter for stress granule condensates

Core-shell model of the clusters of CPEB4 isoforms preceding liquid-liquid phase separation

Protein solutions can undergo liquid-liquid phase separation (LLPS), where a dispersed phase with a low protein concentration coexists with coacervates with a high protein concentration. We focus on the low complexity N-terminal domain of cytoplasmic polyadenylation element binding-4 protein, CPEB4NTD, and its isoform depleted of the Exon4, CPEB4Δ4NTD. They both exhibit LLPS, but in contrast to most systems undergoing LLPS, the single-phase regime preceding LLPS consists mainly of soluble protein clusters. We combine experimental and theoretical approaches to resolve the internal structure of the clusters and the basis for their formation. Dynamic light scattering and atomic force microscopy show that both isoforms exhibit clusters with diameters ranging from 35 to 80 nm. Electron paramagnetic resonance spectroscopy of spin-labeled CPEB4NTD and CPEB4Δ4NTD revealed that these proteins have two distinct dynamical properties in both the clusters and coacervates. Based on the experimental results, we propose a core-shell structure for the clusters, which is supported by the agreement of the dynamic light scattering data on cluster size distribution with a statistical model developed to describe the structure of clusters. This model treats clusters as swollen micelles (microemulsions) where the core and the shell regions comprise different protein conformations, in agreement with the electron paramagnetic resonance detection of two protein populations. The effects of ionic strength and the addition of 1,6-hexanediol were used to probe the interactions responsible for cluster formation. While both CPEB4NTD and CPEB4Δ4NTD showed phase separation with increasing temperature and formed clusters, differences were found in the properties of the clusters and the coacervates. The data also suggested that the coacervates may consist of aggregates of clusters.

Advantages of integrating Brillouin microscopy in multimodal mechanical mapping of cells and tissues

Recent research has highlighted the growing significance of the mechanical properties of cells and tissues in the proper execution of physiological functions within an organism; alterations to these properties can potentially result in various diseases. These mechanical properties can be assessed using various techniques that vary in spatial and temporal resolutions as well as applications. Due to the wide range of mechanical behaviors exhibited by cells and tissues, a singular mapping technique may be insufficient in capturing their complexity and nuance. Consequently, by utilizing a combination of methods-multimodal mechanical mapping-researchers can achieve a more comprehensive characterization of mechanical properties, encompassing factors such as stiffness, modulus, viscoelasticity, and forces. Furthermore, different mapping techniques can provide complementary information and enable the exploration of spatial and temporal variations to enhance our understanding of cellular dynamics and tissue mechanics. By capitalizing on the unique strengths of each method while mitigating their respective limitations, a more precise and holistic understanding of cellular and tissue mechanics can be obtained. Here, we spotlight Brillouin microscopy (BM) as a noncontact, noninvasive, and label-free mechanical mapping modality to be coutilized alongside established mechanical probing methods. This review summarizes some of the most widely adopted individual mechanical mapping techniques and highlights several recent multimodal approaches demonstrating their utility. We envision that future studies aim to adopt multimodal techniques to drive advancements in the broader realm of mechanobiology.

Fluctuations and Shape Dependence of Microphase Separation in Systems with Long-Range Interactions

The combination of phase separation and long-ranged, effective, Coulomb interactions results in microphase separation. We predict the sizes and shapes of such microdomains and uniquely their dependence on the macroscopic sample shape which also affects the effective interfacial tension of fluctuations of the lamellar phase. These are applied to equilibrium salt solutions and block copolymers. Nonequilibrium phase separation in the presence of chemical reactions (e.g., cellular condensates) is mapped to the Coulomb theory to which our predictions apply. In some cases, the effective interfacial tension can be ultralow.

Surface tension measurement and calculation of model biomolecular condensates

The surface tension of liquid-like protein-rich biomolecular condensates is an emerging physical principle governing the mesoscopic interior organisation of biological cells. In this study, we present a method to evaluate the surface tension of model biomolecular condensates, through straighforward sessile drop measurements of capillary lengths and condensate densities. Our approach bypasses the need for characterizing condensate viscosities, which was required in previously reported techniques. We demonstrate this method using model condensates comprising two mutants of the intrinsically disordered protein Ddx4N. Notably, we uncover a detrimental impact of increased protein net charge on the surface tension of Ddx4N condensates. Furthermore, we explore the application of Scheutjens-Fleer theory, calculating condensate surface tensions through a self-consistent mean-field framework using Flory-Huggins interaction parameters. This relatively simple theory provides semi-quantitative accuracy in predicting Ddx4N condensate surface tensions and enables the evaluation of molecular organisation at condensate surfaces. Our findings shed light on the molecular details of fluid-fluid interfaces in biomolecular condensates.