Directing Transition of Synthetic Protocell Models via Physicochemical Cues-Triggered Interfacial Dynamic Covalent Chemistry

Osmotic Pressure Induced Morphological Transformation of Membranized Coacervates

The stimulus-response behavior of protocells under environmental osmotic pressure changes has long been a subject of scientific inquiry. Herein, we demonstrate a way to membranized coacervate microdroplets based on cholesterol anchoring of phospholipids, which provides enhanced stability, enabling morphological transformations instead of dissociation during subsequent osmotic pressure changes. In hypotonic environments, these membranized coacervates equilibrate osmotic pressure through transient internal vacuole formation, concomitant with a transmembrane substrate influx that triggers enzymatic reaction acceleration. By contrast, in a hypertonic environment, the membranized coacervate responds with bursting-like deformation that can then quickly recover due to the anchoring effect of cholesterol on phospholipids. Notably, it is found that such bursting-like deformation could even successfully induce endocytosis of Staphylococcus aureus by the membranized coacervates. Furthermore, through the integration of Coa@DMPC's osmotic responsiveness, internal actin polymerization activated by the endocytic S. aureus is achieved. Not only our proposed method of phospholipid membranization of the coacervate could contribute a new model to mimic more complex bionic structures, but also the revealed morphological response behavior of the membranized coacervate under various osmotic pressure changes is expected to help explain the stress behaviors and emerging unique properties of cells in similar environments.

Multiphase coacervates: mimicking complex cellular structures through liquid-liquid phase separation

Coacervate microdroplets, arising from liquid-liquid phase separation, have emerged as promising models for primary cells, demonstrating the ability to regulate biomolecular enrichment, create chemical gradients, accelerate confined reactions, and even express proteins. Notably, multiphase coacervation provides a robust framework to replicate hierarchically complex cellular structures, offering valuable insights into cellular organization and function. In this review, we explore the recent advancements in the study of multiphase coacervates, focusing on design strategies, underlying mechanisms, structural control, and their applications in biomimetics. These developments highlight the potential of multiphase coacervates as powerful tools in the field of synthetic biology and material science.

Optical Control over Liquid–Liquid Phase Separation

Liquid-liquid phase separation (LLPS) is responsible for the emergence of intracellular membrane-less organelles and the development of coacervate protocells. Benefitting from the advantages of simplicity, precision, programmability, and noninvasiveness, light has become an effective tool to regulate the assembly dynamics of LLPS, and mediate various biochemical processes associated with LLPS. In this review, recent advances in optically controlling membrane-less organelles within living organisms are summarized, thereby modulating a series of biological processes including irreversible protein aggregation pathologies, transcription activation, metabolic flux, genomic rearrangements, and enzymatic reactions. Among these, the intracellular systems (i.e., optoDroplet, Corelet, PixELL, CasDrop, and other optogenetic systems) that enable the photo-mediated control over biomolecular condensation are highlighted. The design of photoactive complex coacervate protocells in laboratory settings by utilizing photochromic molecules such as azobenzene and diarylethene is further discussed. This review is expected to provide in-depth insights into phase separation-associated biochemical processes, bio-metabolism, and diseases.

Self-assembly of stabilized droplets from liquid-liquid phase separation for higher-order structures and functions

Dynamic microscale droplets produced by liquid-liquid phase separation (LLPS) have emerged as appealing biomaterials due to their remarkable features. However, the instability of droplets limits the construction of population-level structures with collective behaviors. Here we first provide a brief background of droplets in the context of materials properties. Subsequently, we discuss current strategies for stabilizing droplets including physical separation and chemical modulation. We also discuss the recent development of LLPS droplets for various applications such as synthetic cells and biomedical materials. Finally, we give insights on how stabilized droplets can self-assemble into higher-order structures displaying coordinated functions to fully exploit their potentials in bottom-up synthetic biology and biomedical applications.

Synthetic-Cell-Based Multi-Compartmentalized Hierarchical Systems

In the extant lifeforms, the self-sustaining behaviors refer to various well-organized biochemical reactions in spatial confinement, which rely on compartmentalization to integrate and coordinate the molecularly crowded intracellular environment and complicated reaction networks in living/synthetic cells. Therefore, the biological phenomenon of compartmentalization has become an essential theme in the field of synthetic cell engineering. Recent progress in the state-of-the-art of synthetic cells has indicated that multi-compartmentalized synthetic cells should be developed to obtain more advanced structures and functions. Herein, two ways of developing multi-compartmentalized hierarchical systems, namely interior compartmentalization of synthetic cells (organelles) and integration of synthetic cell communities (synthetic tissues), are summarized. Examples are provided for different construction strategies employed in the above-mentioned engineering ways, including spontaneous compartmentalization in vesicles, host-guest nesting, phase separation mediated multiphase, adhesion-mediated assembly, programmed arrays, and 3D printing. Apart from exhibiting advanced structures and functions, synthetic cells are also applied as biomimetic materials. Finally, key challenges and future directions regarding the development of multi-compartmentalized hierarchical systems are summarized; these are expected to lay the foundation for the creation of a "living" synthetic cell as well as provide a larger platform for developing new biomimetic materials in the future.

Signal Transduction in Artificial Cells

In recent years, significant progress has been made in the emerging field of constructing biomimetic soft compartments with life-like behaviors. Given that biological activities occur under a flux of energy and matter exchange, the implementation of rudimentary signaling pathways in artificial cells (protocells) is a prerequisite for the development of adaptive sense-response phenotypes in cytomimetic models. Herein, recent approaches to the integration of signal transduction modules in model protocells prepared by bottom-up construction are discussed. The approaches are classified into two categories involving invasive biochemical signals or non-invasive physical stimuli. In the former mechanism, transducers with intrinsic recognition capability respond with high specificity, while in the latter, artificial cells respond through intra-protocellular energy transduction. Although major challenges remain in the pursuit of a sophisticated artificial signaling network for the orchestration of higher-order cytomimetic models, significant advances have been made in establishing rudimentary protocell communication networks, providing novel organizational models for the development of life-like microsystems and new avenues in protoliving technologies.

Plant Cell-Inspired Membranization of Coacervate Protocells with a Structured Polysaccharide Layer

The design of compartmentalized colloids that exhibit biomimetic properties is providing model systems for developing synthetic cell-like entities (protocells). Inspired by the cell walls in plant cells, we developed a method to prepare membranized coacervates as protocell models by coating membraneless liquid-like microdroplets with a protective layer of rigid polysaccharides. Membranization not only endowed colloidal stability and prevented aggregation and coalescence but also facilitated selective biomolecule sequestration and chemical exchange across the membrane. The polysaccharide wall surrounding coacervate protocells acted as a stimuli-responsive structural barrier that enabled enzyme-triggered membrane lysis to initiate internalization and killing of Escherichia coli. The membranized coacervates were capable of spatial organization into structured tissue-like protocell assemblages, offering a means to mimic metabolism and cell-to-cell communication. We envision that surface engineering of protocells as developed in this work generates a platform for constructing advanced synthetic cell mimetics and sophisticated cell-like behaviors.

A Synthetic Protocell-Based Heparin Scavenger

Heparin is a commonly applied blood anticoagulant agent in clinical use. After treatment, excess heparin needs to be removed to circumvent side effects and recover the blood-clotting cascade. Most existing heparin antidotes rely on direct heparin binding and complexation, yet selective compartmentalization and sequestration of heparin would be beneficial for safety and efficiency. However, such systems have remained elusive. Herein, a semipermeable protein-based microcompartment (proteinosome) is loaded with a highly positively charged chitosan derivative, which can induce electrostatics-driven internalization of anionic guest molecules inside the compartment. Chitosan-loaded proteinosomes are subsequently employed to capture heparin, and an excellent heparin-scavenging performance is demonstrated under physiologically relevant conditions. Both the highly positive scavenger and the polyelectrolyte complex are confined and shielded by the protein compartment in a time-dependent manner. Moreover, selective heparin-scavenging behavior over serum albumin is realized through adjusting the localized scavenger or surrounding salt concentrations at application-relevant circumstances. In vitro studies reveal that the cytotoxicity of the cationic scavenger and the produced polyelectrolyte complex is reduced by protocell shielding. Therefore, the proteinosome-based systems may present a novel polyelectrolyte-scavenging method for biomedical applications.

Modulating liquid-liquid phase separation of FUS: mechanisms and strategies

Liquid-liquid phase separation (LLPS) of biomolecules inspires the construction of protocells and drives the formation of cellular membraneless organelles. The resulting biomolecular condensates featuring dynamic assembly, disassembly, and phase transition play significant roles in a series of biological processes, including RNA metabolism, DNA damage response, signal transduction and neurodegenerative disease. Intensive investigations have been conducted for understanding and manipulating intracellular phase-separated disease-related proteins (e.g., FUS, tau and TDP-43). Herein, we review current studies on the regulation strategies of intracellular LLPS focusing on FUS, which are categorized into physical stimuli, biochemical modulators, and protein structural modifications, with summarized molecular mechanisms. This review is expected to provide a sketch of the modulation of FUS LLPS with its pros and cons, and an outlook for the potential clinical treatments of neurodegenerative diseases.

Anisotropic Compartmentalization of the Liquid-Liquid Interface using Dynamic Imine Chemistry

The liquid-liquid interface offers a fascinating avenue for generating hierarchical compartments. Herein, the dynamic imine chemistry is employed at the oil-water interface to investigate the effect of dynamic covalent bonds for modulating the droplet shape. The imine bond formation between oil-soluble aromatic aldehydes and water-soluble polyethyleneimine greatly stabilized the oil-water interface by substantially lowering the interfacial tension. The successful jamming of imine-mediated assemblies was observed when a compressive force was applied to the droplet. Thus, the anisotropic compartmentalization of the liquid-liquid interface was created, and it was later altered by changing the pH of the surrounding environment. Finally, a proof-of-concept demonstration of a pH-triggered cargo release across the interfacial membrane confirmed the feasibility of stimuli-responsive behavior of dynamic imine assemblies.

A three-tiered colloidosomal microreactor for continuous flow catalysis

Integrative colloidosomes with hierarchical structure and advanced function may serve as biomimetic microreactors to carry out catalytic reactions by compartmentalizing biological species within semipermeable membranes. Despite of recent progress in colloidosome design, integration of biological and inorganic components into tiered structures to tackle the remaining challenges of biocatalysis is highly demanded. Here, we report a rational design of three-tiered colloidosomes via the Pickering emulsion process. The microreactor consists of crosslinked amphiphilic silica-polymer hybrid nanoparticles as the semipermeable shell, an enzyme-incorporated catalytic sub-layer, and a partially-silicified adsorptive lumen. By leveraging confinement and enrichment effect, we demonstrate the acceleration of lipase-catalyzed ester hydrolysis within the microcompartment of organic-inorganic hybrid colloidosomes. The catalytic colloidosomes are further assembled into a closely packed column for enzymatic reactions in a continuous flow format with enhanced reaction rates. The three-tiered colloidosomes provide a reliable platform to integrate functional building blocks into a biomimetic compartmentalized microreactor with spatially controlled organization and high-performance functions.