Growth dynamics and phase transitions of aqueous phase-separated condensates

Abstract

Biomolecular condensates are membrane-less organelles that play crucial roles in cellular organization and functions, including RNA transcription [1, 2], stress response [3, 4], and noise buffering [5, 6]. Recent studies have revealed that these condensates form through the liquid-liquid phase separation (LLPS) of biomolecules such as proteins and RNAs, driven by multivalent interactions [7, 8]. In laboratory settings, synthetic systems such as complex coacervates and aqueous two-phase systems (ATPS) are generated through the associative or segregative LLPS [9, 10]. These synthetic systems also lack physical barriers and exhibit physicochemical properties similar to biological condensates, including viscoelasticity, interfacial tension, and surface charges [9, 11]. Consequently, they have been widely used to not only uncover the mechanistic principles that enable biological condensates to concentrate biomolecules and modulate biochemical reactions [9, 12], but also construct artificial cells from basic molecules [13, 14]. Over the past decade, studies have been primarily focused on identifying and explaining how condensates form and function across different disciplines [15, 16]. However, advances in understanding the dynamics and phase transitions of condensates or coacervates have been limited. For example, while many condensates behave as liquid-like droplets, their growth dynamics often deviate from predictions by classic theories that describe droplet growth via Brownian motion-induced coalescence or Ostwald ripening. In particular, many studies have shown that the growth of biomolecular condensates is significantly suppressed in cells or reconstituted systems [17, 18]. In addition, the material properties of condensates vary in response to environmental stimuli or during the aging process, leading to phase transitions between liquid-like droplets and solid-like aggregates [19-21]. Notably, the liquid-to-solid phase transition (LSPT) of biomolecular condensates is implicated in the pathogenesis of neurodegenerative disease and cancers [22, 23]. The lack of comprehensive understanding of the dynamics and phase transitions of condensates limits our ability to fully appreciate their unique roles in cellular organization and function, as well as hinders the development of condensate-based biomaterials for relevant industrial and biomedical applications. Given these unresolved questions, we utilize coacervates and ATPS as model systems to study the growth dynamics and phase transitions of condensates, using a combination of experiments, simulations, and theoretical analysis. Chapter 1 provides an overview of phase separation and biomolecular condensates, along with fundamental physical concepts and theoretical models. In Chapters 2 and 3, we study the non-equilibrium coarsening of complex coacervates with varying material properties. Chapter 4 explores how condensates, upon reaching equilibrium, scale their sizes in relation to their surrounding enclosed environments. The chapter 5 focuses on revealing molecular principles that govern phase transitions of condensates, particularly those affected by alcohol molecules. In Chapter 6, I present advancements in creating heterogeneous hydrogels using ATPS systems to mimic the representative features of biological architectures at different length scales. Finally, the concluding chapter summarizes key findings and outlines directions for future research.