Dynamic Spatial Formation and Distribution of Intrinsically Disordered Protein Droplets in Macromolecularly Crowded Protocells

All-aqueous synthesis of alginate complexed with fibrillated protein microcapsules for membrane-bounded culture of tumor spheroids

Water-in-water (W/W) emulsions provide bio-compatible all-aqueous compartments for artificial patterning and assembly of living cells. Successful entrapment of cells within a W/W emulsion via the formation of semipermeable capsules is a prerequisite for regulating on the size, shape, and architecture of cell aggregates. However, the high permeability and instability of the W/W interface, restricting the assembly of stable capsules, pose a fundamental challenge for cell entrapment. The current study addresses this problem by synthesizing multi-armed protein fibrils and controlling their assembly at the W/W interface. The multi-armed protein fibrils, also known as 'fibril clusters', were prepared by cross-linking lysozyme fibrils with multi-arm polyethylene glycol (PEG) via click chemistry. Compared to linear-structured fibrils, fibril clusters are strongly adsorbed at the W/W interface, forming an interconnected meshwork that better stabilizes the W/W emulsion. Moreover, when fibril clusters are complexed with alginate, the hybrid microcapsules demonstrate excellent mechanical robustness, semi-permeability, cytocompatibility and biodegradability. These advantages enable the encapsulation, entrapment and long-term culture of tumor spheroids, with great promise for applications for anti-cancer drug screening, tumor disease modeling, and tissue repair engineering.

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.

Peptide-Based Biomimetic Condensates via Liquid-Liquid Phase Separation as Biomedical Delivery Vehicles

Biomolecular condensates are dynamic liquid droplets through intracellular liquid-liquid phase separation that function as membraneless organelles, which are highly involved in various complex cellular processes and functions. Artificial analogs formed via similar pathways that can be integrated with biological complexity and advanced functions have received tremendous research interest in the field of synthetic biology. The coacervate droplet-based compartments can partition and concentrate a wide range of solutes, which are regarded as attractive candidates for mimicking phase-separation behaviors and biophysical features of biomolecular condensates. The use of peptide-based materials as phase-separating components has advantages such as the diversity of amino acid residues and customized sequence design, which allows for programming their phase-separation behaviors and the physicochemical properties of the resulting compartments. In this Perspective, we highlight the recent advancements in the design and construction of biomimicry condensates from synthetic peptides relevant to intracellular phase-separating protein, with specific reference to their molecular design, self-assembly via phase separation, and biorelated applications, to envisage the use of peptide-based droplets as emerging biomedical delivery vehicles.

Advancing Artificial Cells with Functional Compartmentalized Polymeric Systems - In Honor of Wolfgang Meier

The fundamental building block of living organisms is the cell, which is the universal biological base of all living entities. This micrometric mass of cytoplasm and the membrane border have fascinated scientists due to the highly complex and multicompartmentalized structure. This specific organization enables numerous metabolic reactions to occur simultaneously and in segregated spaces, without disturbing each other, but with a promotion of inter- and intracellular communication of biomolecules. At present, artificial nano- and microcompartments, whether as single components or self-organized in multicompartment architectures, hold significant value in the study of life development and advanced functional materials and in the fabrication of molecular devices for medical applications. These artificial compartments also possess the properties to encapsulate, protect, and control the release of bio(macro)molecules through selective transport processes, and they are capable of embedding or being connected with other types of compartments. The self-assembly mechanism of specific synthetic compartments and thus the fabrication of a simulated organelle membrane are some of the major aspects to gain insight. Considerable efforts have now been devoted to design various nano- and microcompartments and understand their functionality for precise control over properties. Of particular interest is the use of polymeric vesicles for communication in synthetic cells and colloidal systems to reinitiate chemical and biological communication and thus close the gap toward biological functions. Multicompartment systems can now be effectively created with a high level of hierarchical control. In this way, these structures can not only be explored to deepen our understanding of the functional organization of living cells, but also pave the way for many more exciting developments in the biomedical field.

Elastin-like polypeptide coacervates as reversibly triggerable compartments for synthetic cells

Compartmentalization is a vital aspect of living cells to orchestrate intracellular processes. In a similar vein, constructing dynamic and responsive sub-compartments is key to synthetic cell engineering. In recent years, liquid-liquid phase separation via coacervation has offered an innovative avenue for creating membraneless organelles (MOs) within artificial cells. Here, we present a lab-on-a-chip system to reversibly trigger peptide-based coacervates within cell-mimicking confinements. We use double emulsion droplets (DEs) as our synthetic cell containers while pH-responsive elastin-like polypeptides (ELPs) act as the coacervate system. We first present a high-throughput microfluidic DE production enabling efficient encapsulation of the ELPs. The DEs are then harvested to perform multiple MO formation-dissolution cycles using pH as well as temperature variation. For controlled long-term visualization and modulation of the external environment, we developed an integrated microfluidic device for trapping and environmental stimulation of DEs, with negligible mechanical force, and demonstrated a proof-of-principle osmolyte-based triggering to induce multiple MO formation-dissolution cycles. In conclusion, our work showcases the use of DEs and ELPs in designing membraneless reversible compartmentalization within synthetic cells via physicochemical triggers. Additionally, presented on-chip platform can be applied over a wide range of phase separation and vesicle systems for applications in synthetic cells and beyond.

Thermo-responsive aqueous two-phase system for two-level compartmentalization

Hierarchical compartmentalization responding to changes in intracellular and extracellular environments is ubiquitous in living eukaryotic cells but remains a formidable task in synthetic systems. Here we report a two-level compartmentalization approach based on a thermo-responsive aqueous two-phase system (TR-ATPS) comprising poly(N-isopropylacrylamide) (PNIPAM) and dextran (DEX). Liquid membraneless compartments enriched in PNIPAM are phase-separated from the continuous DEX solution via liquid-liquid phase separation at 25 °C and shrink dramatically with small second-level compartments generated at the interface, resembling the structure of colloidosome, by increasing the temperature to 35 °C. The TR-ATPS can store biomolecules, program the spatial distribution of enzymes, and accelerate the overall biochemical reaction efficiency by nearly 7-fold. The TR-ATPS inspires on-demand, stimulus-triggered spatiotemporal enrichment of biomolecules via two-level compartmentalization, creating opportunities in synthetic biology and biochemical engineering.

Protocells Featuring Membrane-Bound and Dynamic Membraneless Organelles

Living cells, especially eukaryotic ones, use multicompartmentalization to regulate intra- and extracellular activities, featuring membrane-bound and membraneless organelles. These structures govern numerous biological and chemical processes spatially and temporally. Synthetic cell models, primarily utilizing lipidic and polymeric vesicles, have been developed to carry out cascade reactions within their compartments. However, these reconstructions often segregate membrane-bound and membraneless organelles, neglecting their collaborative role in cellular regulation. To address this, we propose a structural design incorporating microfluidic-produced liposomes housing synthetic membrane-bound organelles made from self-assembled poly(ethylene glycol)-block-poly(trimethylene carbonate) nanovesicles and synthetic membraneless organelles formed via temperature-sensitive elastin-like polypeptide phase separation. This architecture mirrors natural cellular organization, facilitating a detailed examination of the interactions for a comprehensive understanding of cellular dynamics.

Kinetic Study of the Esterase-like Activity of Albumin following Condensation by Macromolecular Crowding

The ability of bovine serum albumin (BSA) to form condensates in crowded environments has been discovered only recently. Effects of this condensed state on the secondary structure of the protein have already been unraveled as some aging aspects, but the pseudo-enzymatic behavior of condensed BSA has never been reported yet. This article investigates the kinetic profile of para-nitrophenol acetate hydrolysis by BSA in its condensed state with poly(ethylene) glycol (PEG) as the crowding agent. Furthermore, the initial BSA concentration was varied between 0.25 and 1 mM which allowed us to modify the size distribution, the volume fraction, and the partition coefficient (varying from 136 to 180). Hence, the amount of BSA originally added was a simple way to modulate the size and density of the condensates. Compared with dilute BSA, the initial velocity (vi) with condensates was dramatically reduced. From the Michaelis-Menten fits, the extracted Michaelis constant Km and the maximum velocity Vmax decreased in control samples without condensates when the BSA concentration increased, which was attributed to BSA self-oligomerization. In samples containing condensates, the observed vi was interpreted as an effect of diluted BSA remaining in the supernatants and from the condensates. In supernatants, the crowding effect of PEG increased the kcat and catalytic efficiency. Last, Vmax was proportional to the volume fraction of the condensates, which could be controlled by varying its initial concentration. Hence, the major significance of this article is the control of the size and volume fraction of albumin condensates, along with their kinetic profile using liquid-liquid phase separation.

Effect of Polymer Gel Elasticity on Complex Coacervate Phase Behavior

Gels are key materials in biological systems such as tissues and may control biocondensate formation and structure. To further understand the effects of elastic environments on biomacromolecular assembly, we have investigated the phase behavior and radii of complex coacervate droplets in polyacrylamide (PAM) networks as a function of gel modulus. Poly-l-lysine (PLL) and sodium hyaluronate (HA) complex coacervate phases were prepared in PAM gels with moduli varying from 0.035 to 15.0 kPa. The size of the complex coacervate droplets is reported from bright-field microscopy and confocal fluorescence microscopy. Overall, the complex coacervate droplet volume decreases inversely with the modulus. Fluorescence microscopy is also used to determine the phase behavior and concentration of fluorescently tagged HA in the complex coacervate phases as a function of ionic strength (100-270 mM). We find that the critical ionic strength and complex coacervate stability are nonmonotonic as a function of the network modulus and that the local gel concentration can be used to control phase behavior and complex coacervate droplet size scale. By understanding how elastic environments influence simple electrostatic assembly, we can further understand how biomacromolecules exist in complex, crowded, and elastic cellular environments.

Contribution of the ELRs to the development of advanced in vitro models

Developing in vitro models that accurately mimic the microenvironment of biological structures or processes holds substantial promise for gaining insights into specific biological functions. In the field of tissue engineering and regenerative medicine, in vitro models able to capture the precise structural, topographical, and functional complexity of living tissues, prove to be valuable tools for comprehending disease mechanisms, assessing drug responses, and serving as alternatives or complements to animal testing. The choice of the right biomaterial and fabrication technique for the development of these in vitro models plays an important role in their functionality. In this sense, elastin-like recombinamers (ELRs) have emerged as an important tool for the fabrication of in vitro models overcoming the challenges encountered in natural and synthetic materials due to their intrinsic properties, such as phase transition behavior, tunable biological properties, viscoelasticity, and easy processability. In this review article, we will delve into the use of ELRs for molecular models of intrinsically disordered proteins (IDPs), as well as for the development of in vitro 3D models for regenerative medicine. The easy processability of the ELRs and their rational design has allowed their use for the development of spheroids and organoids, or bioinks for 3D bioprinting. Thus, incorporating ELRs into the toolkit of biomaterials used for the fabrication of in vitro models, represents a transformative step forward in improving the accuracy, efficiency, and functionality of these models, and opening up a wide range of possibilities in combination with advanced biofabrication techniques that remains to be explored.

Ultrasensitive Detection of PSA Using Antibodies in Crowding Polyelectrolyte Multilayers on a Silicon Nanowire Field-Effect Transistor

Immunosensors based on field-effect transistors with nanowire channels (NWFETs) provide fast and real-time detection of a variety of biomarkers without the need for additional labels. The key feature of the developed immunosensor is the coating of silicon NWs with multilayers of polyelectrolytes (polyethylenimine (PEI) and polystyrene sulfonate (PSS)). By causing a macromolecular crowding effect, it ensures the "soft fixation" of the antibodies into the 3-D matrix of the oppositely charged layers. We investigated the interaction of prostate-specific antigen (PSA), a biomarker of prostate cancer, and antibodies adsorbed in the PEI and PSS matrix. In order to visualize the formation of immune complexes between polyelectrolyte layers using SEM and AFM techniques, we employed a second clone of antibodies labeled with gold nanoparticles. PSA was able to penetrate the matrix and concentrate close to the surface layer, which is crucial for its detection on the nanowires. Additionally, this provides the optimal orientation of the antibodies' active centers for interacting with the antigen and improves their mobility. NWFETs were fabricated from SOI material using high-resolution e-beam lithography, thin film vacuum deposition, and reactive-ion etching processes. The immunosensor was characterized by a high sensitivity to pH (71 mV/pH) and an ultra-low limit of detection (LOD) of 0.04 fg/mL for PSA. The response of the immunosensor takes less than a minute, and the measurement is carried out in real time. This approach seems promising for further investigation of its applicability for early screening of prostate cancer and POC systems.

Chirality-Selected Coacervate by Chiral Gemini Surfactant

Chiral surfactants present opportunities to self-assemble into supramolecules with a chiral trait; however, the effects of stereochemistry on the formation of simple coacervates remain unclear. Here, we investigate the chirality-selected phase behavior in mixtures of chiral gemini surfactant 1,4-bis(dodecyl-N,N-dimethylammonium bromide)-2,3-butanediol (12-4(OH)2-12) with an oppositely charged chiral mandelic acid (MA). It demonstrates that altering the chirality of surfactants yields a heightened ability to regulate the phase behavior, leading to the formation of three different network-like structures, i.e., wormlike micelle, coacervate, and hydrogel, in the racemate, enantiomer, and mesomer, respectively. The different aggregate structures arise from the intermolecular and intramolecular hydrogen-bond interactions of the two hydroxyl groups located at stereogenic centers. Intriguingly, although they contain similar microstructures, the solid-like hydrogel and liquid-like wormlike micelle show similar low hydration ability and have no encapsulation capability, whereas only coacervate formed by the enantiomers of 12-4(OH)2-12 displays liquid-like characteristics, strong capacity to sequester diverse solutes, and high affinity for tightly bound water simultaneously. These findings further highlight the unique and advantageous properties of coacervates as a promising model for exploring the biological process and understanding how chirality plays a crucial role in early life scenarios and cell evolution at the molecular level.

Coacervate-Assisted Polymerization-Induced Self-Assembly of Chiral Alternating Copolymers into Hierarchical Bishell Capsules with Sub-5 nm Ultrathin Lamellae

Hierarchical self-assembly of synthetic polymers in solution represents one of the sophisticated strategies to replicate the natural superstructures which lay the basis for their superb functions. However, it is still quite challenging to increase the degree of complexity of the as-prepared assemblies, especially in a large scale. Liquid-liquid phase separation (LLPS) widely exists in cells and is assumed to be responsible for the formation of many cellular organelles without membranes. Herein, through integrating LLPS with the polymerization-induced self-assembly (PISA), a coacervate-assisted PISA (CAPISA) methodology to realize the one-pot and scalable preparation of hierarchical bishell capsules (BCs) from nanosheets with ultrathin lamellae phase (sub-5 nm), microflakes, unishell capsules to final BCs in a bottom-up sequence is presented. Both the self-assembled structure and the dynamic formation process of BCs have been disclosed. Since CAPISA has combined the advantages of coacervates, click chemistry, interfacial reaction and PISA, it is believed that it will become a promising option to fabricate biomimetic polymer materials with higher structural complexity and more sophisticated functions.

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.

Coacervate Droplets for Synthetic Cells

The design and construction of synthetic cells - human-made microcompartments that mimic features of living cells - have experienced a real boom in the past decade. While many efforts have been geared toward assembling membrane-bounded compartments, coacervate droplets produced by liquid-liquid phase separation have emerged as an alternative membrane-free compartmentalization paradigm. Here, the dual role of coacervate droplets in synthetic cell research is discussed: encapsulated within membrane-enclosed compartments, coacervates act as surrogates of membraneless organelles ubiquitously found in living cells; alternatively, they can be viewed as crowded cytosol-like chassis for constructing integrated synthetic cells. After introducing key concepts of coacervation and illustrating the chemical diversity of coacervate systems, their physicochemical properties and resulting bioinspired functions are emphasized. Moving from suspensions of free floating coacervates, the two nascent roles of these droplets in synthetic cell research are highlighted: organelle-like modules and cytosol-like templates. Building the discussion on recent studies from the literature, the potential of coacervate droplets to assemble integrated synthetic cells capable of multiple life-inspired functions is showcased. Future challenges that are still to be tackled in the field are finally discussed.

Transcription-Factor-Induced Aggregation of Biomimetic Oligonucleotide-b-Protein Micelles

Polymeric micelles and especially those based on natural diblocks are of particular interest due to their advantageous properties in terms of molecular recognition, biocompatibility, and biodegradability. We herein report a facile and straightforward synthesis of thermoresponsive elastin-like polypeptide (ELP) and oligonucleotide (ON) diblock bioconjugates, ON-b-ELP, through copper-catalyzed azide-alkyne cycloaddition. The resulting thermosensitive diblock copolymer self-assembles above its critical micelle temperature (CMT ∼30 °C) to form colloidally stable micelles of ∼50 nm diameter. The ON-b-ELP micelles hybridize with an ON complementary strand and maintain their size and stability. Next, we describe the capacity of these micelles to bind proteins, creating more complex structures using the classic biotin-streptavidin pairing and the specific recognition between a transcription factor protein and the ON strand. In both instances, the micelles are intact, form larger structures, and retain their sensitivity to temperature.

The construction of elastin-like polypeptides and their applications in drug delivery system and tissue repair

Elastin-like polypeptides (ELPs) are thermally responsive biopolymers derived from natural elastin. These peptides have a low critical solution temperature phase behavior and can be used to prepare stimuli-responsive biomaterials. Through genetic engineering, biomaterials prepared from ELPs can have unique and customizable properties. By adjusting the amino acid sequence and length of ELPs, nanostructures, such as micelles and nanofibers, can be formed. Correspondingly, ELPs have been used for improving the stability and prolonging drug-release time. Furthermore, ELPs have widespread use in tissue repair due to their biocompatibility and biodegradability. Here, this review summarizes the basic property composition of ELPs and the methods for modulating their phase transition properties, discusses the application of drug delivery system and tissue repair and clarifies the current challenges and future directions of ELPs in applications.

Control of Enzyme Reactivity in Response to Osmotic Pressure Modulation Mimicking Dynamic Assembly of Intracellular Organelles

In response to variations in osmotic stress, in particular to hypertonicity associated with biological dysregulations, cells have developed complex mechanisms to release their excess water, thus avoiding their bursting and death. When water is expelled, cells shrink and concentrate their internal bio(macro)molecular content, inducing the formation of membraneless organelles following a liquid-liquid phase separation (LLPS) mechanism. To mimic this intrinsic property of cells, functional thermo-responsive elastin-like polypeptide (ELP) biomacromolecular conjugates are herein encapsulated into self-assembled lipid vesicles using a microfluidic system, together with polyethylene glycol (PEG) to mimic cells' interior crowded microenvironment. By inducing a hypertonic shock onto the vesicles, expelled water induces a local increase in concentration and a concomitant decrease in the cloud point temperature (Tcp ) of ELP bioconjugates that phase separate and form coacervates mimicking cellular stress-induced membraneless organelle assemblies. Horseradish peroxidase (HRP), as a model enzyme, is bioconjugated to ELPs and is locally confined in coacervates as a response to osmotic stress. This consequently increases local HRP and substrate concentrations and accelerates the kinetics of the enzymatic reaction. These results illustrate a unique way to fine-tune enzymatic reactions dynamically as a response to a physiological change in isothermal conditions.

Peroxidase-mimicking poly-L-lysine/alginate microspheres with PtS2 nanoparticles for image-based colorimetric assays

Morphologically controllable ALG@ε-PL water-in-water microspheres were successfully prepared using a two-step method through precise control of the two-phase flow rate. Through further interfacial coagulation, the ALG@ε-PL microspheres possess a dense surface structure and good permeability. The sensor based on PtS₂@ALG@ε-PL microspheres was constructed by encapsulating PtS₂ nanosheets with peroxidase-like properties in ALG@ε-PL water-in-water microspheres. PtS₂ nanosheets catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) by H₂O₂ to produce blue oxTMB. The strong reducing property of the model analyte dopamine (DA) can reduce oxTMB, thus causing the blue color to fade and successfully achieving colorimetric detection of DA. The linear range of the assay is 2.0-200 μM, and the detection limit is 0.22 μM. The recoveries of DA in serum samples were determined by the spik method, and the results were reproducible.

Biotic communities inspired proteinosome-based aggregation for enhancing utilization rate of enzyme

Compared with the individuals, the collective behavior of biotic communities could show certain superior characteristics. Inspired by this idea and based on the conjugation between phenylboronic acid-grafted mesoporous silica nanoparticles and the polysaccharide functionalized membrane of proteinosomes, a type of proteinosomes-based aggregations was constructed. We demonstrated the emergent characteristics of proteinosomes aggregations including accelerated settling velocity and population surviving by sacrificing outside members for the inside. Moreover, this kind of "hand in hand" architecture provided the proteinosomes aggregations with the characteristic of resistance to the negative pressure phagocytosis of micropipette, as well as enhancing utilization rate of the encapsulated enzymes. Overall, it is anticipated that the construction and application of proteinosomes aggregations could contribute to advance the functionality of life-like assembled biomaterial in another way.

Signal processing and generation of bioactive nitric oxide in a model prototissue

The design and construction of synthetic prototissues from integrated assemblies of artificial protocells is an important challenge for synthetic biology and bioengineering. Here we spatially segregate chemically communicating populations of enzyme-decorated phospholipid-enveloped polymer/DNA coacervate protocells in hydrogel modules to construct a tubular prototissue-like vessel capable of modulating the output of bioactive nitric oxide (NO). By decorating the protocells with glucose oxidase, horseradish peroxidase or catalase and arranging different modules concentrically, a glucose/hydroxyurea dual input leads to logic-gate signal processing under reaction-diffusion conditions, which results in a distinct NO output in the internal lumen of the model prototissue. The NO output is exploited to inhibit platelet activation and blood clot formation in samples of plasma and whole blood located in the internal channel of the device, thereby demonstrating proof-of-concept use of the prototissue-like vessel for anticoagulation applications. Our results highlight opportunities for the development of spatially organized synthetic prototissue modules from assemblages of artificial protocells and provide a step towards the organization of biochemical processes in integrated micro-compartmentalized media, micro-reactor technology and soft functional materials.

A challenge for synthetic biology is the design and construction of prototissue. Here, the authors spatially segregate layers of enzyme-decorated coacervate protocells as a model prototissue capable of chemical signal processing and modulating outputs of nitric oxide to inhibit blood clot formation.

Morphology and Dynamics of Coexisting Phases in Coacervate Solely Controlled by Crowded Environment

The membraneless organelles (MLOs) play a key role in the cell, yet it is unclear what controls the morphology and dynamics of MLOs in crowded cell medium. Using a biphasic coacervate droplet as a model of MLO, we online monitored the liquid-liquid phase separation process in crowded medium provided by poly(ethylene oxide) (PEO) or dextran. In PEO solution, which has an affinity with the inner phase, the spherical droplets evolve into clusters, networks, and completely phase inverted spheres in sequence with increasing PEO concentration, while in dextran solution, which has an affinity with the outer phase, the coacervates maintain the morphology but vary in phase ratio. Flower-like and even Janus structures are formed in the mixed PEO/dextran medium. Our work demonstrates that MLOs could be controlled solely by the crowded cell medium.

Spatial confinement toward creating artificial living systems

Lifeforms are regulated by many physicochemical factors, and these factors could be controlled to play a role in the construction of artificial living systems. Among these factors, spatial confinement is an important one, which mediates biological behaviors at multiscale levels and participates in the biomanufacturing processes accordingly. This review describes how spatial confinement, as a fundamental biological phenomenon, provides cues for the construction of artificial living systems. Current knowledge about the role of spatial confinement in mediating individual cell behavior, collective cellular behavior, and tissue-level behavior are categorized. Endeavors on the synthesis of biomacromolecules, artificial cells, engineered tissues, and organoids in spatially confined bioreactors are then emphasized. After that, we discuss the cutting-edge applications of spatially confined artificial living systems in biomedical fields. Finally, we conclude by assessing the remaining challenges and future trends in the context of fundamental science, technical improvement, and practical applications.

Adaptive Recombinant Nanoworms from Genetically Encodable Star Amphiphiles

Recombinant nanoworms are promising candidates for materials and biomedical applications ranging from the templated synthesis of nanomaterials to multivalent display of bioactive peptides and targeted delivery of theranostic agents. However, molecular design principles to synthesize these assemblies (which are thermodynamically favorable only in a narrow region of the phase diagram) remain unclear. To advance the identification of design principles for the programmable assembly of proteins into well-defined nanoworms and to broaden their stability regimes, we were inspired by the ability of topologically engineered synthetic macromolecules to acess rare mesophases. To test this design principle in biomacromolecular assemblies, we used post-translational modifications (PTMs) to generate lipidated proteins with precise topological and compositional asymmetry. Using an integrated experimental and computational approach, we show that the material properties (thermoresponse and nanoscale assembly) of these hybrid amphiphiles are modulated by their amphiphilic architecture. Importantly, we demonstrate that the judicious choice of amphiphilic architecture can be used to program the assembly of proteins into adaptive nanoworms, which undergo a morphological transition (sphere-to-nanoworms) in response to temperature stimuli.

Microfluidic Control of Coexisting Chemical Microenvironments within Multiphase Water-in-Fluorocarbon Droplets

The use of aqueous polymer-based phase separation within water-in-oil emulsion droplets provides a powerful platform for exploring the impact of compartmentalization and preferential partitioning on biologically relevant solutes. By forming an emulsion, a bulk solution is converted into a large number of chemically isolated microscale droplets. Microfluidic techniques provide an additional level of control over the formation of such systems. This enables the selective production of multiphase droplets with desired solution compositions and specific characteristics, such as solute partitioning. Here, we demonstrate control over the chemical microenvironment by adjusting the composition to increase tie line length for poly(ethylene glycol) (PEG)-dextran aqueous two-phase systems (ATPS) encapsulated within multiphase water-in-fluorocarbon oil emulsion droplets. Through rational adjustment of microfluidic parameters alone, ATPS droplets containing differing compositions could be produced during the course of a single experiment, with the produced droplets demonstrating a controllable range of tie line lengths. This provided control over partitioning behavior for biologically relevant macromolecules such that the difference in local protein concentration between adjacent phases could be rationally tuned. This work illustrates a broadly applicable technique to rationally create emulsified multiphase aqueous systems of desired compositions through the adjustment of microfluidic parameters alone, allowing for easy and rapid screening of various chemical microenvironments.

Dynamic Structural Changes and Thermodynamics in Phase Separation Processes of an Intrinsically Disordered-Ordered Protein Model

Elastin-like proteins (ELPs) are biologically important proteins and models for intrinsically disordered proteins (IDPs) and dynamic structural transitions associated with coacervates and liquid-liquid phase transitions. However, the conformational status below and above coacervation temperature and its role in the phase separation process is still elusive. Employing matrix least-squares global Boltzmann fitting of the circular dichroism spectra of the ELPs (VPGVG)20 , (VPGVG)40 , and (VPGVG)60 , we found that coacervation occurs sharply when a certain number of repeat units has acquired β-turn conformation (in our sequence setting a threshold of approx. 20 repeat units). The character of the differential scattering of the coacervate suspensions indicated that this fraction of β-turn structure is still retained after polypeptide assembly. Such conformational thresholds may also have a role in other protein assembly processes with implications for the design of protein-based smart materials.

Higher-order structure of DNA determines its positioning in cell-size droplets under crowded conditions

Background

It is becoming clearer that living cells use water/water (w/w) phase separation to form membraneless organelles that exhibit various important biological functions. Currently, it is believed that the specific localization of biomacromolecules, including DNA, RNA and proteins in w/w microdroplets is closely related to their bio-activity. Despite the importance of this possible role of micro segregation, our understanding of the underlying physico-chemical mechanism is still unrefined. Further research to unveil the underlying mechanism of the localization of macromolecules in relation to their steric conformation in w/w microdroplets is needed.

Principal findings

Single-DNA observation of genome-size DNA (T4 GT7 bacteriophage DNA; 166kbp) by fluorescence microscopy revealed that DNAs are spontaneously incorporated into w/w microdroplets generated in a binary aqueous polymer solution with polyethylene glycol (PEG) and dextran (DEX). Interestingly, DNAs with elongated coil and shrunken conformations exhibit Brownian fluctuation inside the droplet. On the other hand, tightly packed compact globules, as well as assemblies of multiple condensed DNAs, tend to be located near the interface in the droplet.

Conclusion and significance

The specific localization of DNA molecules depending on their higher-order structure occurs in w/w microdroplet phase-separation solution under a binary aqueous polymer solution. Such an aqueous solution with polymers mimics the crowded conditions in living cells, where aqueous macromolecules exist at a level of 30–40 weight %. The specific positioning of DNA depending on its higher-order structure in w/w microdroplets is expected to provide novel insights into the mechanism and function of membraneless organelles and micro-segregated particles in living cells.

Engineering non-covalently assembled protein nanoparticles for long-acting gouty arthritis therapy

As an incurable metabolic disease, gouty arthritis (GA) requires long-term treatment with frequent drug administration several times per day. Compared to non-specific small organic medications, interleukin-1β (IL-1β) blocking therapies, such as IL-1 receptor antagonist (IL-1Ra), show great therapeutic potential in clinical trials of GA. However, IL-1Ra application is starkly limited due to its short half-life and poor bioavailability. Herein, we demonstrate a new type of nanotherapeutic formulation via noncovalent assembly of an engineered IL-1Ra chimera protein. PEGylation was employed to induce such assembly by exploiting electrostatic complexation and hydrophobic interactions. The engineered protein nanoparticles had a combination of biocompatibility, improved bioavailability and therapeutic performance. It showed extraordinary long-term anti-inflammatory effect and robust bio-efficacy for GA therapy in acute GA rat models. Strikingly, this nanoprotein system possesses an ultralong half-life of 27 hours and a bioavailability 7 times higher than that of pristine IL-1Ra, thus extending the dosing interval from several hours to more than 3 days. Therefore, our noncovalent assembly strategy via an engineered chimeric protein empowers the construction of potent delivery nanosystems for efficient GA treatment, and this might be adapted for other therapeutics to form long-acting formulations.

Emergence of uniform linearly-arranged micro-droplets entrapping DNA and living cells through water/water phase-separation

Living cells maintain their lives through self-organization in an environment crowded with a rich variety of biological species. Recently, it was found that micro-droplets containing biomacromolecules, which vary widely in size, are generated accompanied by water/water phase-separation by simple mechanical mixing of an aqueous solution with binary polymers. Here, we report that cell-sized droplets of nearly the same size are generated as a linear array within a glass capillary upon the introduction of a binary polymer solution of polyethylene glycol (PEG) and dextran (DEX). Interestingly, when DNA molecules are added to the polymer solution, stable droplets entrapping DNA molecules are obtained. Similarly, living cells are entrapped spontaneously for the linearly-arranged cell-sized droplets. This simple method for generating micro-droplets entrapping DNA and also living cells is expected to stimulate further study on the self-construction of protocells and micro organoids.

Spatiotemporal Dynamic Assembly/Disassembly of Organelle-Mimics Based on Intrinsically Disordered Protein-Polymer Conjugates

Design of reversible organelle-like microcompartments formed by liquid-liquid phase separation in cell-mimicking entities has significantly advanced the bottom-up construction of artificial eukaryotic cells. However, organizing the formation of artificial organelle architectures in a spatiotemporal manner within complex primitive compartments remains scarcely explored. In this work, thermoresponsive hybrid polypeptide-polymer conjugates are rationally engineered and synthesized, resulting from the conjugation of an intrinsically disordered synthetic protein (IDP), namely elastin-like polypeptide, and synthetic polymers (poly(ethylene glycol) and dextran) that are widely used as macromolecular crowding agents. Cell-like constructs are built using droplet-based microfluidics that are filled with such bioconjugates and an artificial cytoplasm system that is composed of specific polymers conjugated to the IDP. The distinct spatial organizations of two polypeptide-polymer conjugates and the dynamic assembly and disassembly of polypeptide-polymer coacervate droplets in response to temperature are studied in the cytomimetic protocells. Furthermore, a monoblock IDP with longer length is concurrently included with bioconjugates individually inside cytomimetic compartments. Both bioconjugates exhibit an identical surfactant-like property, compartmentalizing the monoblock IDP coacervates via temperature control. These findings lay the foundation for developing hierarchically structured synthetic cells with interior organelle-like structures which could be designed to localize in desired phase-separated subcompartments.

Construction of Hybrid Bi-microcompartments with Exocytosis-Inspired Behavior toward Fast Temperature-Modulated Transportation of Living Organisms

Inspired by the unique characteristics of living cells, the creation of life-inspired functional ensembles is a rapidly expanding research topic, enabling transformative applications in various disciplines. Herein, we report a facile method for the fabrication of phospholipid and block copolymer hybrid bi-microcompartments via spontaneous asymmetric assembly at the water/tributyrin interface, whereby the temperature-mediated dewetting of the inner microcompartments allowed for exocytosis to occur in the constructed system. The exocytosis location and commencement time could be controlled by the buoyancy of the inner microcompartment and temperature, respectively. Furthermore, the constructed bi-microcompartments showed excellent biocompatibility and a universal loading capacity toward cargoes of widely ranging sizes; thus, the proliferation and temperature-programmed transportation of living organisms was achieved. Our results highlight opportunities for the development of complex mesoscale dynamic ensembles with life-inspired behaviors and provide a novel platform for on-demand transport of various living organisms.

Thermosensitive Vesicles from Chemically Encoded Lipid-Grafted Elastin-like Polypeptides

Biomimetic design to afford smart functional biomaterials with exquisite properties represents synthetic challenges and provides unique perspectives. In this context, elastin-like polypeptides (ELPs) recently became highly attractive building blocks in the development of lipoprotein-based membranes. In addition to the bioengineered post-translational modifications of genetically encoded recombinant ELPs developed so far, we report here a simple and versatile method to design biohybrid brush-like lipid-grafted-ELPs using chemical post-modification reactions. We have explored a combination of methionine alkylation and click chemistry to create a new class of hybrid lipoprotein mimics. Our design allowed the formation of biomimetic vesicles with controlled permeability, correlated to the temperature-responsiveness of ELPs.

Self-immobilization of coacervate droplets by enzyme-mediated hydrogelation

An artificial protocell model mimicking stimuli-triggered extracellular matrix formation is demonstrated based on the self-immobilization of coacervate microdroplets. Endogenous enzyme activity within the microdroplets results in the release of Ca2+ ions that trigger hydrogelation throughout the external environment, which in turn mechanically supports and chemically stabilizes the protocells.

Exploring New Horizons in Liquid Compartmentalization via Microfluidics

Spatial organization of cellular processes is crucial to efficiently regulate life's essential reactions. Nature does this by compartmentalization, either using membranes, such as the cell and nuclear membrane, or by liquid-like droplets formed by aqueous liquid-liquid phase separation. Aqueous liquid-liquid phase separation can be divided in two different phenomena, associative and segregative phase separation, of which both are studied for their membraneless compartmentalization abilities. For centuries, segregative phase separation has been used for the extraction and purification of biomolecules. With the emergence of microfluidic techniques, further exciting possibilities were explored because of their ability to fine-tune phase separation within emulsions of various compositions and morphologies and achieve one of the simplest forms of compartmentalization. Lately, interest in aqueous liquid-liquid phase separation has been revived due to the discovery of membraneless phases within the cell. In this Perspective we focus on segregative aqueous phase separation, discuss the theory of this interesting phenomenon, and give an overview of the evolution of aqueous phase separation in microfluidics.

Folding and self-assembly of short intrinsically disordered peptides and protein regions

Proteins and peptide fragments are highly relevant building blocks in self-assembly for nanostructures with plenty of applications. Intrinsically disordered proteins (IDPs) and protein regions (IDRs) are defined by the absence of a well-defined secondary structure, yet IDPs/IDRs show a significant biological activity. Experimental techniques and computational modelling procedures for the characterization of IDPs/IDRs are discussed. Directed self-assembly of IDPs/IDRs allows reaching a large variety of nanostructures. Hybrid materials based on the derivatives of IDPs/IDRs show a promising performance as alternative biocides and nanodrugs. Cell mimicking, in vivo compartmentalization, and bone regeneration are demonstrated for IDPs/IDRs in biotechnological applications. The exciting possibilities of IDPs/IDRs in nanotechnology with relevant biological applications are shown.

Self-programmed enzyme phase separation and multiphase coacervate droplet organization

Membraneless organelles are phase-separated droplets that are dynamically assembled and dissolved in response to biochemical reactions in cells. Complex coacervate droplets produced by associative liquid-liquid phase separation offer a promising approach to mimic such dynamic compartmentalization. Here, we present a model for membraneless organelles based on enzyme/polyelectrolyte complex coacervates able to induce their own condensation and dissolution. We show that glucose oxidase forms coacervate droplets with a cationic polysaccharide on a narrow pH range, so that enzyme-driven monotonic pH changes regulate the emergence, growth, decay and dissolution of the droplets depending on the substrate concentration. Significantly, we demonstrate that time-programmed coacervate assembly and dissolution can be achieved in a single-enzyme system. We further exploit this self-driven enzyme phase separation to produce multiphase droplets via dynamic polyion self-sorting in the presence of a secondary coacervate phase. Taken together, our results open perspectives for the realization of programmable synthetic membraneless organelles based on self-regulated enzyme/polyelectrolyte complex coacervation.

Dynamic spatial and structural organization in artificial cells regulates signal processing by protein scaffolding

Structural and spatial organization are fundamental properties of biological systems that allow cells to regulate a wide range of biochemical processes. This organization is often transient and governed by external cues that initiate dynamic self-assembly processes. The construction of synthetic cell-like materials with similar properties requires the hierarchical and reversible organization of selected functional components on molecular scaffolds to dynamically regulate signaling pathways. The realization of such transient molecular programs in synthetic cells, however, remains underexplored due to the associated complexity of such hierarchical platforms. In this contribution, we effectuate dynamic spatial organization of effector protein subunits in a synthetic biomimetic compartment, a giant unilamellar vesicle (GUV), by associating in a reversible manner two fragments of a split luciferase to the membrane. This induces their structural dimerization, which consequently leads to the activation of enzymatic signaling. Importantly, such organization and activation are dynamic processes, and can be autonomously regulated - thus opening up avenues toward continuous spatiotemporal control over supramolecular organization and signaling in an artificial cell.

Dual Self-Assembled Nanostructures from Intrinsically Disordered Protein Polymers with LCST Behavior and Antimicrobial Peptides

Antimicrobial peptides (AMPs) have attracted great interest as they constitute one of the most promising alternatives against drug-resistant infections. Their amphipathic nature not only provides them antimicrobial and immunomodulatory properties but also the ability to self-assemble into supramolecular nanostructures. Here, we propose their use as self-assembling domains to drive hierarchical organization of intrinsically disordered protein polymers (IDPPs). Using a modular approach, hybrid protein-engineered polymers were recombinantly produced, thus combining designer AMPs and a thermoresponsive IDPP, an elastin-like recombinamer (ELR). We exploited the ability of these AMPs and ELRs to self-assemble to develop supramolecular nanomaterials by way of a dual-assembly process. First, the AMPs trigger the formation of nanofibers; then, the thermoresponsiveness of the ELRs enables assembly into fibrillar aggregates. The interplay between the assembly of AMPs and ELRs provides an innovative molecular tool in the development of self-assembling nanosystems with potential use for biotechnological and biomedical applications.