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There is an increasing amount of evidence that biomolecular condensates are linked to neurodegenerative diseases associated with protein aggregation, such as Alzheimer's disease and amyotrophic lateral sclerosis, although the mechanisms underlying this link remain elusive. In this Review, we summarize the possible connections between condensates and protein aggregation. We consider both liquid-to-solid transitions of phase-separated proteins and the partitioning of proteins into host condensates. We distinguish five key factors by which the physical and chemical environment of a condensate can influence protein aggregation, and we discuss their relevance in studies of protein aggregation in the presence of biomolecular condensates: increasing the local concentration of proteins, providing a distinct chemical microenvironment, introducing an interface wherein proteins can localize, changing the energy landscape of aggregation pathways, and the presence of chaperones in condensates. Analysing the role of biomolecular condensates in protein aggregation may be essential for a full understanding of amyloid formation and offers a new perspective that can help in developing new therapeutic strategies for the prevention and treatment of neurodegenerative diseases., (© 2024. Springer Nature Limited.)

Biochemical reactions occurring in highly crowded cellular environments require different means of control to ensure productivity and specificity. Compartmentalization of reagents by liquid-liquid phase separation is one of these means. However, extremely high local protein concentrations of up to 400 mg/ml can result in pathological aggregation into fibrillar amyloid structures, a phenomenon that has been linked to various neurodegenerative diseases. Despite its relevance, the process of liquid-to-solid transition inside condensates is still not well understood at the molecular level. We thus herein use small peptide derivatives that can undergo both liquid-liquid and subsequent liquid-to-solid phase transition as model systems to study both processes. Using solid-state nuclear magnetic resonance (NMR) and transmission electron microscopy (TEM), we compare the structure of condensed states of leucine, tryptophan and phenylalanine containing derivatives, distinguishing between liquid-like condensates, amorphous aggregates and fibrils, respectively. A structural model for the fibrils formed by the phenylalanine derivative was obtained by an NMR-based structure calculation. The fibrils are stabilised by hydrogen bonds and side-chain π-π interactions, which are likely much less pronounced or absent in the liquid and amorphous state. Such noncovalent interactions are equally important for the liquid-to-solid transition of proteins, particularly those related to neurodegenerative diseases., (© 2023 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

Biomolecular condensates present in cells can fundamentally affect the aggregation of amyloidogenic proteins and play a role in the regulation of this process. While liquid-liquid phase separation of amyloidogenic proteins by themselves can act as an alternative nucleation pathway, interaction of partly disordered aggregation-prone proteins with preexisting condensates that act as localization centers could be a far more general mechanism of altering their aggregation behavior. Here, we show that so-called host biomolecular condensates can both accelerate and slow down amyloid formation. We study the amyloidogenic protein α-synuclein and two truncated α-synuclein variants in the presence of three types of condensates composed of nonaggregating peptides, RNA, or ATP. Our results demonstrate that condensates can markedly speed up amyloid formation when proteins localize to their interface. However, condensates can also significantly suppress aggregation by sequestering and stabilizing amyloidogenic proteins, thereby providing living cells with a possible protection mechanism against amyloid formation.

Liquid-liquid phase separation of disordered proteins has emerged as a ubiquitous route to membraneless compartments in living cells, and similar coacervates may have played a role when the first cells formed. However, existing coacervates are typically made of multiple macromolecular components, and designing short peptide analogues capable of self-coacervation has proven difficult. Here we present a short peptide synthon for phase separation, made of only two dipeptide stickers linked via a flexible, hydrophilic spacer. These small-molecule compounds self-coacervate into micrometre-sized liquid droplets at sub-millimolar concentrations, which retain up to 75 wt% water. The design is general and we derive guidelines for the required sticker hydrophobicity and spacer polarity. To illustrate their potential as protocells, we create a disulfide-linked derivative that undergoes reversible compartmentalization controlled by redox chemistry. The resulting coacervates sequester and melt nucleic acids, and act as microreactors that catalyse two different anabolic reactions yielding molecules of increasing complexity. This provides a stepping stone for new coacervate-based protocells made of single peptide species., (© 2021. The Author(s), under exclusive licence to Springer Nature Limited.)

Coacervates are condensed liquid-like droplets formed by liquid-liquid phase separation of molecules through multiple weak associative interactions. In recent years it has emerged that not only long polymers, but also short peptides are capable of forming simple and complex coacervates. The coacervate droplets they form act as compartments that sequester and concentrate a wide range of solutes, and their spontaneous formation make coacervates attractive protocell models. The main advantage of peptides as building blocks lies in the functional diversity of the amino acid residues, which allows for tailoring of the peptide's phase separation propensity, their selectivity in guest molecule uptake and the physicochemical and catalytic properties of the compartments. The aim of this tutorial review is to illustrate the recent developments in the field of peptide-based coacervates in a systematic way and to deduce the basic requirements for both simple and complex coacervation of peptides. We review a selection of peptide coacervates that illustrates the essentials of phase separation, the limitations, and the properties that make peptide coacervates biomimetic protocells. Finally, we provide some perspectives of this novel research field in the direction of active droplets, moving away from thermodynamic equilibrium.