Protein Folding, Aggregation, and Liquid-Liquid Phase Separation.

Editor's evaluation: Fixation can change the appearance of phase separation in living cells

Decision letter: Fixation can change the appearance of phase separation in living cells

Biomolecular condensates play a key role in organizing cellular fluids such as the cytoplasm and nucleoplasm. Most of these non-membranous organelles show liquid-like properties both in cells and when studied in vitro through liquid–liquid phase separation (LLPS) of purified proteins. In general, LLPS of proteins is known to be sensitive to variations in pH, temperature and ionic strength, but the role of crowding remains underappreciated. Several decades of research have shown that macromolecular crowding can have profound effects on protein interactions, folding and aggregation, and it must, by extension, also impact LLPS. However, the precise role of crowding in LLPS is far from trivial, as most condensate components have a disordered nature and exhibit multiple weak attractive interactions. Here, we discuss which factors determine the scope of LLPS in crowded environments, and we review the evidence for the impact of macromolecular crowding on phase boundaries, partitioning behavior and condensate properties. Based on a comparison of both in vivo and in vitro LLPS studies, we propose that phase separation in cells does not solely rely on attractive interactions, but shows important similarities to segregative phase separation.

Understanding how protein rich condensates formed upon liquid-liquid phase separation (LLPS) evolve into solid aggregates is of fundamental importance for several medical applications, since these are suspected to be hot-spots for many neurotoxic diseases. This requires developing experimental approaches to observe in real-time both LLPS and liquid-solid phase separation (LSPS), and to unravel the delicate balance of protein and water interactions dictating the free energy differences between the two. We present a vibrational THz spectroscopy approach that allows doing so from the point of view of hydration water. We focus on a cellular prion protein of high medical relevance, which we can drive to undergo either LLPS or LSPS with few mutations. We find that it is a subtle balance of hydrophobic and hydrophilic solvation contributions that allows tuning between LLPS and LSPS. Hydrophobic hydration provides an entropic driving force to phase separation, through the release of hydration water into the bulk. Water hydrating hydrophilic groups provides an enthalpic driving force to keep the condensates in a liquid state. As a result, when we modify the protein by a few mutations to be less hydrophilic, we shift from LLPS to LSPS. This molecular understanding paves the way for a rational design of proteins.

Protein aggregation and liquid‒liquid phase separation (LLPS), as key physicochemical processes, orchestrate protein behavior and function, and engineering a protein surface charge offers a robust approach to modulate protein‒protein interactions and, consequently, aggregation and phase separation. Among protein surface engineering methods, supercharging leads to a drastic increase in the protein net charge by replacing surface residues with charged amino acid residues. Previous studies have reported that some physicochemical properties of proteins are improved by supercharging, and changing the surface charge is considered to affect intermolecular interactions. In this study, we designed a new supercharged antigen-binding fragment (Fab) antibody mutant and investigated its aggregation behavior. Upon examination of the physicochemical properties of the designed supercharged antibody, the thermal stability, structure, and ligand binding affinity of the antibody were retained despite having the same charge pairing of both the antibody and the antigen. Furthermore, we revealed that the antibody exhibited reversible ligand- and salt concentration-dependent aggregation. Our study demonstrated how supercharging can potentially modulate protein aggregation and LLPS. It is expected that this approach can be extended to other proteins, through which its applicability in various biological and biotechnological fields can be explored.

The 2021 FASEB Virtual Science Research Conference on Protein Aggregation: Function, Dysfunction, and Disease, June 23-25, 2021.

Networking and Dynamic Switches in Biological Condensates

Liquid-liquid phase separation (LLPS) of proteins and other biomolecules play a critical role in the organization of extracellular materials and membrane-less compartmentalization of intra-organismal spaces through the formation of condensates. Structural properties of such mesoscopic droplet-like states were studied by spectroscopy, microscopy, and other biophysical techniques. The temperature dependence of biomolecular LLPS has been studied extensively, indicating that phase-separated condensed states of proteins can be stabilized or destabilized by increasing temperature. In contrast, the physical and biological significance of hydrostatic pressure on LLPS is less appreciated. Summarized here are recent investigations of protein LLPS under pressures up to the kbar-regime. Strikingly, for the cases studied thus far, LLPSs of both globular proteins and intrinsically disordered proteins/regions are typically more sensitive to pressure than the folding of proteins, suggesting that organisms inhabiting the deep sea and sub-seafloor sediments, under pressures up to 1 kbar and beyond, have to mitigate this pressure-sensitivity to avoid unwanted destabilization of their functional biomolecular condensates. Interestingly, we found that trimethylamine-N-oxide (TMAO), an osmolyte upregulated in deep-sea fish, can significantly stabilize protein droplets under pressure, pointing to another adaptive advantage for increased TMAO concentrations in deep-sea organisms besides the osmolyte's stabilizing effect against protein unfolding. As life on Earth might have originated in the deep sea, pressure-dependent LLPS is pertinent to questions regarding prebiotic proto-cells. Herein, we offer a conceptual framework for rationalizing the recent experimental findings and present an outline of the basic thermodynamics of temperature-, pressure-, and osmolyte-dependent LLPS as well as a molecular-level statistical mechanics picture in terms of solvent-mediated interactions and void volumes.

Targeting of biomolecular condensates to the autophagy pathway.

Liquid and solid phase separation during melt spinning and annealing in melt-spun Cu–Cr ribbons

Abnormal protein aggregation is a hallmark of neurodegenerative diseases, disrupting cellular homeostasis. Glucose-regulated protein 78 (GRP78), a key endoplasmic reticulum (ER) chaperone, plays a crucial role in protein folding and the ER stress response. Recent studies suggest that GRP78 undergoes liquid-liquid phase separation (LLPS) to form dynamic condensates; however, its functional implications under pathological conditions remain unclear. In this study, we designed and synthesized two fluorescent probes (ER-Pro and Agg-Pro) for specifically labeling GRP78 and monitoring microenvironmental polarity changes during protein phase transition. By integrating fluorescence lifetime imaging microscopy and confocal microscopy, we demonstrated that GRP78 undergoes LLPS under ER stress and recruits the amyotrophic lateral sclerosis-associated mutant protein SOD1(A4V), influencing its aggregation dynamics. Further investigations revealed that SOD1(A4V) aggregation is accompanied by local polarity changes, highlighting a potential role for GRP78 LLPS in protein quality control. Our findings provide new insights into ER homeostasis regulation and the pathogenesis of neurodegenerative diseases, offering potential strategies for early diagnosis and therapeutic intervention.

Aerosol Optical Tweezers Constrain the Morphology Evolution of Liquid-Liquid Phase-Separated Atmospheric Particles

Liquid-liquid phase separation (LLPS) of proteins can be considered an intermediate solubility regime between disperse solutions and solid fibers. While LLPS has been described for several pathogenic amyloids, recent evidence suggests that it is similarly relevant for functional amyloids. Here, we review the evidence that links spider silk proteins (spidroins) and LLPS and its role in the spinning process. Major ampullate spidroins undergo LLPS mediated by stickers and spacers in their repeat regions. During spinning, the spidroins droplets shift from liquid to crystalline states. Shear force, altered ion composition, and pH changes cause micelle-like spidroin assemblies to form an increasingly ordered liquid-crystalline phase. Interactions between polyalanine regions in the repeat regions ultimately yield the characteristic β-crystalline structure of mature dragline silk fibers. Based on these findings, we hypothesize that liquid-liquid crystalline phase separation (LLCPS) can describe the molecular and macroscopic features of the phase transitions of major ampullate spidroins during spinning and speculate whether other silk types may use a similar mechanism to convert from liquid dope to solid fiber.

Liquid condensates are membraneless organelles that form via phase separation in living cells. These condensates provide unique heterogeneous environments that have much potential in regulating a range of biochemical processes from gene expression to filamentous protein aggregation---a process linked to Alzheimer's and Parkinson's diseases. Here we theoretically study the physical interplay between protein aggregation, its inhibition, and liquid-liquid phase separation. Our key finding is that the action of protein aggregation inhibitors can be strongly enhanced by liquid condensates. The physical mechanism of this enhancement relies on the partitioning and colocalization of inhibitors with their targets inside the liquid condensate. Our theory uncovers how the physicochemical properties of condensates can be used to modulate inhibitor potency, and we provide experimentally testable conditions under which drug potency is maximal. Our findings suggest design principles for protein aggregation inhibitors with respect to their phase-separation properties.

The misfolding and aggregation of amyloid proteins are closely associated with a range of neurodegenerative diseases. Liquid-liquid phase separation (LLPS) can initiate the aggregation of proteins, indicating that LLPS may serve as an alternative pathway for the pathological aggregation of amyloid proteins. The co-occurrence of two or more amyloid pathologies has been observed in extensive pathophysiological studies and is linked to faster disease progression. The co- LLPS (also known as co-condensation) and co-aggregation of different disease-related proteins have been proposed as a potential molecular mechanism for combined neuropathology. Here, we reviewed the current state of knowledge regarding the co-aggregation and co-condensation of various amyloid proteins, including Aβ, tau, α-synuclein, TDP-43, FUS, and hnRNPA/B protein family, C9orf72 dipeptide repeats and prion protein. We briefly introduced the epidemiological correlation among different neurodegenerative diseases and specifically presented recent experimental findings about co-aggregation and co-condensation of two different amyloid proteins. Additionally, we discussed computational studies focusing on the molecular interactions between amyloid proteins to offer mechanistic insights into the co-LLPS and co-aggregation processes. This review provides an overview of the synergistic interactions between different disease-related proteins, which is helpful for understanding the mechanisms of combined neuropathology and developing targeted therapeutic strategies.