Multicompartment Artificial Organelles Conducting Enzymatic Cascade Reactions inside Cells

Synthetic Cells Revisited: Artificial Cells Construction Using Polymeric Building Blocks

The exponential growth of research on artificial cells and organelles underscores their potential as tools to advance the understanding of fundamental biological processes. The bottom-up construction from a variety of building blocks at the micro- and nanoscale, in combination with biomolecules is key to developing artificial cells. In this review, artificial cells are focused upon based on compartments where polymers are the main constituent of the assembly. Polymers are of particular interest due to their incredible chemical variety and the advantage of tuning the properties and functionality of their assemblies. First, the architectures of micro- and nanoscale polymer assemblies are introduced and then their usage as building blocks is elaborated upon. Different membrane-bound and membrane-less compartments and supramolecular structures and how they combine into advanced synthetic cells are presented. Then, the functional aspects are explored, addressing how artificial organelles in giant compartments mimic cellular processes. Finally, how artificial cells communicate with their surrounding and each other such as to adapt to an ever-changing environment and achieve collective behavior as a steppingstone toward artificial tissues, is taken a look at. Engineering artificial cells with highly controllable and programmable features open new avenues for the development of sophisticated multifunctional systems.

On-Chip Fabrication of Colloidal Suprastructures by Assembly and Supramolecular Interlinking of Microgels

In this report, a versatile method is demonstrated to create colloidal suprastructures by assembly and supramolecular interlinking of microgels using droplet-based microfluidics. The behavior of the microgels is systematically investigated to evaluate the influence of their concentration on their distribution between the continuous, the droplet phase, and the interface. At low concentrations, microgels are mainly localized at the water-oil interface whereas an excess of microgels results, following the complete coverage of the water-oil interface, in their distribution in the continuous phase. To stabilize the colloidal suprastructure, on-chip gelation is introduced by adding natural polyphenol tannic acid (TA) in the water phase. TA forms interparticle linking between the poly(N-vinylcaprolactam) (PVCL) microgels by supramolecular interactions. The combination of supramolecular interlinking with the variation of the microgel concentration in microfluidic droplets enables on-chip fabrication of defined colloidal suprastructures with morphologies ranging from colloidosomes to colloidal supraballs. The obtained supracolloidal structures exhibit a pH-responsive behavior with a disintegration at alkaline conditions within a scale of seconds. The destabilization process results from the deprotonation of phenolic groups and destruction of hydrogen bonds with PVCL chains at higher pH.

Liposomal Enzyme Nanoreactors Based on Nanoconfinement for Efficient Antitumor Therapy

Enzymatic reactions can consume endogenous nutrients of tumors and produce cytotoxic species and are therefore promising tools for treating malignant tumors. Inspired by nature where enzymes are compartmentalized in membranes to achieve high reaction efficiency and separate biological processes with the environment, we develop liposomal nanoreactors that can perform enzymatic cascade reactions in the aqueous nanoconfinement of liposomes. The nanoreactors effectively inhibited tumor growth in vivo by consuming tumor nutrients (glucose and oxygen) and producing highly cytotoxic hydroxyl radicals (⋅OH). Co-compartmentalization of glucose oxidase (GOx) and horseradish peroxidase (HRP) in liposomes could increase local concentration of the intermediate product hydrogen peroxide (H2 O2 ) as well as the acidity due to the generation of gluconic acid by GOx. Both H2 O2 and acidity accelerate the second-step reaction by HRP, hence improving the overall efficiency of the cascade reaction. The biomimetic compartmentalization of enzymatic tandem reactions in biocompatible liposomes provides a promising direction for developing catalytic nanomedicines in antitumor therapy.

Multicompartment colloid systems with lipid and polymer membranes for biomedical applications

Multicompartment structures have the potential for biomedical applications because they can act as multifunctional systems and provide simultaneous delivery of drugs and diagnostics agents of different types. Moreover, some of them mimic biological cells to some extent with organelles as separate sub-compartments. This article analyses multicompartment colloidal structures with smaller sub-units covered with lipid or polymer membranes that provide additional protection for the encapsulated substances. Vesosomes with small vesicles encapsulated in the inner pools of larger liposomes are the most studied systems to date. Dendrimer molecules are enclosed by a lipid bilayer shell in dendrosomes. Capsosomes, polymersomes-in-polymer capsules, and cubosomes-in-polymer capsules are composed of sub-compartments encapsulated within closed multilayer polymer membranes. Janus or Cerberus emulsions contain droplets composed of two or three phases: immiscible oils in O/W emulsions and aqueous polymer or salt solutions that are separated into two or three phases and form connected droplets in W/O emulsions. In more cases, the external surface of engulfed droplets in Janus or Cerberus emulsions is covered with a lipid or polymer monolayer. eLiposomes with emulsion droplets encapsulated into a bilayer shell have been given little attention so far, but they have very great prospects. In addition to nanoemulsion droplets, solid lipid nanoparticles, nanostructured lipid carriers and inorganic nanoparticles can be loaded into eLiposomes. Molecular engineering of the external membrane allows the creation of ligand-targeted and stimuli-responsive multifunctional systems. As a result, the efficacy of drug delivery can be significantly enhanced.

Self-Organization of Iron Sulfide Nanoparticles into Complex Multicompartment Supraparticles

Self-assembled compartments from nanoscale components are found in all life forms. Their characteristic dimensions are in 50-1000 nm scale, typically assembled from a variety of bioorganic "building blocks". Among the various functions that these mesoscale compartments carry out, protection of the content from the environment is central. Finding synthetic pathways to similarly complex and functional particles from technologically friendly inorganic nanoparticles (NPs) is needed for a multitude of biomedical, biochemical, and biotechnological processes. Here, it is shown that FeS2 NPs stabilized by l-cysteine self-assemble into multicompartment supraparticles (mSPs). The NPs initially produce ≈55 nm concave assemblies that reconfigure into ≈75 nm closed mSPs with ≈340 interconnected compartments with an average size of ≈5 nm. The intercompartmental partitions and mSP surface are formed primarily from FeS2 and Fe2 O3 NPs, respectively. The intermediate formation of cup-like particles enables encapsulation of biological cargo. This capability is demonstrated by loading mSPs with DNA and subsequent transfection of mammalian cells. Also it is found that the temperature stability of the DNA cargo is enhanced compared to the traditional delivery vehicles. These findings demonstrate that biomimetic compartmentalized particles can be used to successfully encapsulate and enhance temperature stability of the nucleic acid cargo for a variety of bioapplications.

Designing Intracellular Compartments for Efficient Engineered Microbial Cell Factories

With the rapid development of synthetic biology, various kinds of microbial cell factories (MCFs) have been successfully constructed to produce high-value-added compounds. However, the complexity of metabolic regulation and pathway crosstalk always cause issues such as intermediate metabolite accumulation, byproduct generation, and metabolic burden in MCFs, resulting in low efficiencies and low yields of industrial biomanufacturing. Such issues could be solved by spatially rearranging the pathways using intracellular compartments. In this review, design strategies are summarized and discussed based on the types and characteristics of natural and artificial subcellular compartments. This review systematically presents information for the construction of efficient MCFs with intracellular compartments in terms of four aspects of design strategy goals: (1) improving local reactant concentration; (2) intercepting and isolating competing pathways; (3) providing specific reaction substances and environments; and (4) storing and accumulating products.

Assembly of biomimetic microreactors using caged-coacervate droplets

Complex coacervates are liquid-like droplets that can be used to create adaptive cell-like compartments. These compartments offer a versatile platform for the construction of bioreactors inspired by living cells. However, the lack of a membrane significantly reduces the colloidal stability of coacervates in terms of fusion and surface wetting, which limits their suitability as compartments. Here, we describe the formation of caged-coacervates surrounded by a semipermeable shell of silica nanocapsules. We demonstrate that the silica nanocapsules create a protective shell that also regulates the molecular transport of water-soluble compounds as a function of nanocapasule size. The adjustable semipermeability and intrinsic affinity of enzymes for the interior of the caged-coacervates allowed us to assemble biomimetic microreactors with enhanced colloidal stability.

DNA-assisted selective electrofusion (DASE) of Escherichia coli and giant lipid vesicles

Synthetic biology and cellular engineering require chemical and physical alterations, which are typically achieved by fusing target cells with each other or with payload-carrying vectors. On one hand, electrofusion can efficiently induce the merging of biological cells and/or synthetic analogues via the application of intense DC pulses, but it lacks selectivity and often leads to uncontrolled fusion. On the other hand, synthetic DNA-based constructs, inspired by natural fusogenic proteins, have been shown to induce a selective fusion between membranes, albeit with low efficiency. Here we introduce DNA-assisted selective electrofusion (DASE) which relies on membrane-anchored DNA constructs to bring together the objects one seeks to merge, and applying an electric impulse to trigger their fusion. The DASE process combines the efficiency of standard electrofusion and the selectivity of fusogenic nanostructures, as we demonstrate by inducing and characterizing the fusion of spheroplasts derived from Escherichia coli bacteria with cargo-carrying giant lipid vesicles.

Nanozyme-Based Artificial Organelles: An Emerging Direction for Artificial Organelles

Artificial organelles are compartmentalized nanoreactors, in which enzymes or enzyme-mimic catalysts exhibit cascade catalytic activities to mimic the functions of natural organelles. Importantly, research on artificial organelles paves the way for the bottom-up design of synthetic cells. Due to the separation effect of microcompartments, the catalytic reactions of enzymes are performed without the influence of the surrounding medium. The current techniques for synthesizing artificial organelles rely on the strategies of encapsulating enzymes into vesicle-structured materials or reconstituting enzymes onto the microcompartment materials. However, there are still some problems including limited functions, unregulated activities, and difficulty in targeting delivery that hamper the applications of artificial organelles. The emergence of nanozymes (nanomaterials with enzyme-like activities) provides novel ideas for the fabrication of artificial organelles. Compared with natural enzymes, nanozymes are featured with multiple enzymatic activities, higher stability, easier to synthesize, lower cost, and excellent recyclability. Herein, the most recent advances in nanozyme-based artificial organelles are summarized. Moreover, the benefits of compartmental structures for the applications of nanozymes, as well as the functional requirements of microcompartment materials are also introduced. Finally, the potential applications of nanozyme-based artificial organelles in biomedicine and the related challenges are discussed.

Multi-compartment supracapsules made from nano-containers towards programmable release

The assembly of nanomaterials into suprastructures offers the possibility to fabricate larger scale functional materials, whose inner structure strongly influences their functionality for a vast range of applications. In spite of the many current strategies, achieving multi-compartment structures in a targeted and versatile way remains highly challenging. Here, we describe a controllable and straightforward route to create uniform suprastructured materials with a multi-compartmentalized architecture by confining primary nanocapsules into droplets using a cross-junction microfluidic device. Following solvent evaporation from the droplets, the nanocapsules spontaneously assemble into precisely sized multi-compartment particles, which we term supracapsules. Thanks to the process, each spatially separated nanocapsule unit retains its cargo and functionalities within the resulting supracapsules. However, new collective properties emerge, and, particularly, programmable release profiles that are distinct from those of single-compartment capsules. Finally, the suprastructures can be disassembled into single-compartment units by applying ultra-sonication, switching their release to a burst-release mode. These findings open up exciting opportunities to fabricate multi-compartment suprastructures incorporating diverse functionalities for materials with emerging properties.

Synthetic cells in biomedical applications

Synthetic cells are engineered vesicles that can mimic one or more salient features of life. These features include directed localization, sense-and-respond behavior, gene expression, metabolism, and high stability. In nanomedicine, many of these features are desirable capabilities of drug delivery vehicles but are difficult to engineer. In this focus article, we discuss where synthetic cells offer unique advantages over nanoparticle and living cell therapies. We review progress in the engineering of the above life-like behaviors and how they are deployed in nanomedicine. Finally, we assess key challenges synthetic cells face before being deployed as drugs and suggest ways to overcome these challenges. This article is categorized under: Therapeutic Approaches and Drug Discovery > Emerging Technologies Biology-Inspired Nanomaterials > Lipid-Based Structures.

Enzyme-active liquid coacervate microdroplets as artificial membraneless organelles for intracellular ROS scavenging

Artificial organelles are microcompartments capable of performing catalytic reactions in living cells to replace absent or lost cellular functions. Coacervate microdroplets, formed via liquid-liquid phase separation, have been developed as...

Arrested Coalescence of Ionic Liquid Droplets: A Facile Strategy for Spatially Organized Multicompartment Assemblies

Multicompartment assemblies attract much attention for their wide applications. However, the fabrication of multicompartment assemblies usually requires elaborately designed building blocks and careful controlling. The emergence of droplet networks has provided a facile way to construct multiple droplet architectures, which can further be converted to multicompartment assemblies. Herein, the bind motif-free building blocks are presented, which consist of the hydrophobic Tf₂ N^- -based ionic liquid (IL) dissolving LiTf₂ N salt, that can conjugate via arrested coalescence in confined-space templates to form IL droplet networks. Subsequent ultraviolent polymerization generates robust free-standing multicompartment assemblies. The conjugation of building blocks relies not on the peripheral bind motif but on the interfacial instability-induced arrested coalescence, avoiding tedious surface modification and assembly process. By tuning structures of templates and building blocks, multicompartment assemblies with 0D, 1D, 2D, and 3D structures are prepared in a facile and high-throughput way. Importantly, the bottom-up construction enables modular control over the compositions and spatial positions of individual building blocks. Combining with the excellent solvency of ILs, this system can serve as a general platform towards versatile multicompartment architectures. As demonstrations, by tailoring the chambers the multicompartment assemblies can spatiotemporally sense and report the chemical cues and perform various modes of motion.

Directed Signaling Cascades in Monodisperse Artificial Eukaryotic Cells

The bottom-up assembly of multicompartment artificial cells that are able to direct biochemical reactions along a specific spatial pathway remains a considerable engineering challenge. In this work, we address this with a microfluidic platform that is able to produce monodisperse multivesicular vesicles (MVVs) to serve as synthetic eukaryotic cells. Using a two-inlet polydimethylsiloxane channel design to co-encapsulate different populations of liposomes we are able to produce lipid-based MVVs in a high-throughput manner and with three separate inner compartments, each containing a different enzyme: α-glucosidase, glucose oxidase, and horseradish peroxidase. We demonstrate the ability of these MVVs to carry out directed chemical communication between the compartments via the reconstitution of size-selective membrane pores. Therefore, the signal transduction, which is triggered externally, follows a specific spatial pathway between the compartments. We use this platform to study the effects of enzyme cascade compartmentalization by direct analytical comparison between bulk, one-, two-, and three-compartment systems. This microfluidic strategy to construct complex hierarchical structures is not only suitable to study compartmentalization effects on biochemical reactions but is also applicable for developing advanced drug delivery systems as well as minimal cells in the field of bottom-up synthetic biology.

Multicore Assemblies from Three-Component Linear Homo-Copolymer Systems: A Coarse-Grained Modeling Study

Multicore polymer micelles and aggregates are assemblies that contain several cores. The dual-length-scale compartmentalized solvophobic–solvophilic molecular environment makes them useful for, e.g., advanced drug delivery, high-precision synthesis platforms, confined catalysis, and sensor device applications. However, designing and regulating polymer systems that self-assemble to such morphologies remains a challenge. Using dissipative particle dynamics (DPD) simulations, we demonstrate how simple, three-component linear polymer systems consisting of free solvophilic and solvophobic homopolymers, and di-block copolymers, can self-assemble in solution to form well-defined multicore assemblies. We examine the polymer property range over which multicore assemblies can be expected and how the assemblies can be tuned both in terms of their morphology and structure. For a fixed degree of polymerization, a certain level of hydrophobicity is required for the solvophobic component to lead to formation of multicore assemblies. Additionally, the transition from single-core to multicore requires a relatively high solvophobicity difference between the solvophilic and solvophobic polymer components. Furthermore, if the solvophilic polymer is replaced by a solvophobic species, well-defined multicore–multicompartment aggregates can be obtained. The findings provide guidelines for multicore assemblies’ formation from simple three-component systems and how to control polymer particle morphology and structure.

Artificial Organelles: Towards Adding or Restoring Intracellular Activity

Compartmentalization is one of the main characteristics that define living systems. Creating a physically separated microenvironment allows nature a better control over biological processes, as is clearly specified by the role of organelles in living cells. Inspired by this phenomenon, researchers have developed a range of different approaches to create artificial organelles: compartments with catalytic activity that add new function to living cells. In this review we will discuss three complementary lines of investigation. First, orthogonal chemistry approaches are discussed, which are based on the incorporation of catalytically active transition metal-containing nanoparticles in living cells. The second approach involves the use of premade hybrid nanoreactors, which show transient function when taken up by living cells. The third approach utilizes mostly genetic engineering methods to create bio-based structures that can be ultimately integrated with the cell's genome to make them constitutively active. The current state of the art and the scope and limitations of the field will be highlighted with selected examples from the three approaches.

Polymeric Nanoreactors as Emerging Nanoplatforms for Cancer Precise Nanomedicine

How to precisely detect and effectively cure cancer which is defined as precise nanomedicine has drawn great attention worldwide. Polymeric nanoreactors which can in situ catalyze inert species into activated ones, can greatly increase imaging quality and enhance therapeutic effects along with decreased background interference and reduced serious side effects. After a brief introduction, the design and preparation of polymeric nanoreactors are discussed from the following aspects, that is, solvent-switch, pH-tuning, film rehydration, hard template, electrostatic interaction, and polymerization-induced self-assembly (PISA). Subsequently, the biomedical applications of these nanoreactors in the fields of cancer imaging, cancer therapy, and cancer theranostics are highlighted. The last but not least, conclusions and future perspectives about polymeric nanoreactors are given. It is believed that polymeric nanoreactors can bring a great opportunity for future fabrication and clinical translation of precise nanomedicine.

Interfacing Living and Synthetic Cells as an Emerging Frontier in Synthetic Biology

The construction of artificial cells from inanimate molecular building blocks is one of the grand challenges of our time. In addition to being used as simplified cell models to decipher the rules of life, artificial cells have the potential to be designed as micromachines deployed in a host of clinical and industrial applications. The attractions of engineering artificial cells from scratch, as opposed to re-engineering living biological cells, are varied. However, it is clear that artificial cells cannot currently match the power and behavioural sophistication of their biological counterparts. Given this, many in the synthetic biology community have started to ask: is it possible to interface biological and artificial cells together to create hybrid living/synthetic systems that leverage the advantages of both? This article will discuss the motivation behind this cellular bionics approach, in which the boundaries between living and non-living matter are blurred by bridging top-down and bottom-up synthetic biology. It details the state of play of this nascent field and introduces three generalised hybridisation modes that have emerged.

Interfacing Living and Synthetic Cells as an Emerging Frontier in Synthetic Biology

The construction of artificial cells from inanimate molecular building blocks is one of the grand challenges of our time. In addition to being used as simplified cell models to decipher the rules of life, artificial cells have the potential to be designed as micromachines deployed in a host of clinical and industrial applications. The attractions of engineering artificial cells from scratch, as opposed to re-engineering living biological cells, are varied. However, it is clear that artificial cells cannot currently match the power and behavioural sophistication of their biological counterparts. Given this, many in the synthetic biology community have started to ask: is it possible to interface biological and artificial cells together to create hybrid living/synthetic systems that leverage the advantages of both? This article will discuss the motivation behind this cellular bionics approach, in which the boundaries between living and non-living matter are blurred by bridging top-down and bottom-up synthetic biology. It details the state of play of this nascent field and introduces three generalised hybridisation modes that have emerged.

In situ formation of a biomimetic lipid membrane triggered by an aggregation-enhanced photoligation chemistry

Nature or synthetic systems that can self-assemble into biomimetic membranes and form compartments in aqueous solution have received extensive attention. However, these systems often have the problems of requiring complex processes or lacking of control in simulating lipid synthesis and membrane formation of cells. This paper demonstrates a conceptually new strategy that uses a photoligation chemistry to convert nonmembrane molecules to yield liposomes. Lysosphingomyelin (Lyso) and 2-nitrobenzyl alcohol derivatives (NBs) are used as precursors and the amphiphilic character of Lyso promotes the formation of mixed aggregates with NBs, bringing the lipid precursors into close proximity. Light irradiation triggers the conversion of NBs into reactive aldehyde intermediates, and the preassembly facilitates the efficient and specific ligation between aldehyde and Lyso amine over other biomolecules, thereby accelerating the synthesis of phospholipids and forming membrane compartments similar to natural lipids. The light-controllable transformation represents the use of an external energy stimulus to form a biomimetic phospholipid membrane, which has a wide range of applications in medicinal chemistry, synthetic biological and abiogenesis.

Compartmentalized biosynthesis of fungal natural products

Fungal natural products (NPs) with diverse chemical structures and biological activities are rich resources of both drugs and toxins, thus causing Janus-like effects on human beings. Significant progress has been made in discovery and mining of novel fungal NPs in the past decades. Unlike prokaryotic organisms, eukaryotic cells of fungi have discrete organelles to form compartmentalized assembly lines for the highly ordered and hence efficient biosynthesis of fungal NPs. In this review, we summarize a limited but growing number of studies on compartmentalized biosynthesis of fungal NPs. The emerging strategies and efforts for engineering of subcellular localization of relevant biosynthetic enzymes are also discussed. We expect to provide some new insights and perspectives on the more complex NP biogenesis in higher microorganisms.

Multifunctional Integrated Compartment Systems for Incompatible Cascade Reactions Based on Onion-Like Photonic Spheres

One of the central aims of synthetic biology and metabolic engineering is to mimic the integrality of eukaryotic cells to construct a multifunctional compartment system to perform multistep incompatible cascade reactions in a one-pot, controlled, and selective fashion. The key challenge is how to address the coexistence of antagonistic reagents and to incorporate these functionalities into an integrated system in a smart and efficient way. A novel strategy called "iterative etching-grafting" is proposed here based on monodispersed photonic spheres (PSs) prepared by microfluidics, which constructs a universal platform for incompatible cascade reactions. As a proof of concept, we spatiotemporally regulated the degree of etching of PSs, then grafted precursory groups of acid and base onto PSs, and incorporated a photocleavage method, which were capable of compartmentalizing the acid and base inside PSs. Utilizing the band-gap offsets of PSs could track the progress of cascade reactions in situ, and grafting various charged polymers on the surface of the pores by surface-initiated atom transfer radical polymerization (SI-ATRP) achieved the selectivity of the substrates, which flexibly constructed a multifunctional and integrated acid-base photonic multicompartment system (PMCS). The created PMCS shows excellent catalytic performance, convenient monitoring, and efficient substrate selectivity in the deacetalization-Knoevenagel cascade reaction. Furthermore, two types of electrophile/nucleophile PMCSs have also been accessibly constructed, demonstrating the facile generation of other incompatible systems with the versatility as well as the advancement and extensibility of the developed strategy.

Cell mimicry as a bottom-up strategy for hierarchical engineering of nature-inspired entities

Artificial biology is an emerging concept that aims to design and engineer the structure and function of natural cells, organelles, or biomolecules with a combination of biological and abiotic building blocks. Cell mimicry focuses on concepts that have the potential to be integrated with mammalian cells and tissue. In this feature article, we will emphasize the advancements in the past 3-4 years (2017-present) that are dedicated to artificial enzymes, artificial organelles, and artificial mammalian cells. Each aspect will be briefly introduced, followed by highlighting efforts that considered key properties of the different mimics. Finally, the current challenges and opportunities will be outlined. This article is categorized under: Nanotechnology Approaches to Biology > Nanoscale Systems in Biology.

Oriented immobilization of enzyme-DNA conjugates on magnetic Janus particles for constructing a multicompartment multienzyme system with high activity and stability

Various organelles (e.g., mitochondria and chloroplasts) have a multicompartment structure, providing superior function of material transformation, selective segregation and energy conversion. Enlightened by the elegant evolution of nature, intended isolation of the biochemical process by cooperative multicompartments in cells has become an appealing blueprint to construct bioreactors. In this study, we develop a "soft separation" way to establish a delicate multicompartment multienzyme system (MMS) with polyphenol-encapsulated enzyme-DNA conjugates, which are anchored on magnetic Janus particles, providing a biomimetic catalysis network with the model cascade reactions in confinement. The well-designed MMS exhibits preferable bioactivity benefitting from the dependable DNA bridges and the oriented immobilization of enzymes, while the polyphenol shell further protects the anchored enzymes from exterior attacks, such as heat and enzymatic degradation. Moreover, by applying the MMS as nanomotors, the asymmetrical distribution of enzymes on Janus particles is found to improve mutual elevation between the self-driven locomotion and enzyme-mediated reactions, delivering enhanced dispersal ability and bioactivity. Owing to the excellent enzymatic activity, promoted stability and satisfying biocompatibility, the assembled MMS is proved to be promising for the in vitro and intracellular sensing of glucose, showing significant potential for biochemical analysis applications.

Multicompartment Polymeric Nanocarriers for Biomedical Applications

Multicompartment polymeric nanocarriers which mimic the compartmentalized architecture of living cells have received considerable research attention in the biomedical field. The advancement of synthetic polymeric chemistry has allowed multicompartment polymeric nanocarriers to be tailored for biomedical applications such as drug delivery, encapsulated catalysis, and artificial cellular mimics. In this review, polymer-based multicompartment nanocarriers (multicompartment micelles, multicompartment polymersomes, and capsosomes) have been discussed. This review focuses on multicompartment systems applied to biomedical applications over the last ten years. The synthetic procedures and structural properties that impact the specific application are also highlighted.

Ferritin Nanocages for Protein Delivery to Tumor Cells

The delivery of therapeutic proteins is one of the greatest challenges in the treatment of human diseases. In this frame, ferritins occupy a very special place. Thanks to their hollow spherical structure, they are used as modular nanocages for the delivery of anticancer drugs. More recently, the possibility of encapsulating even small proteins with enzymatic or cytotoxic activity is emerging. Among all ferritins, particular interest is paid to the Archaeoglobus fulgidus one, due to its peculiar ability to associate/dissociate in physiological conditions. This protein has also been engineered to allow recognition of human receptors and used in vitro for the delivery of cytotoxic proteins with extremely promising results.

Biomolecule–polymer hybrid compartments: combining the best of both worlds

Recent advances in bio/polymer hybrid compartments in the quest to obtain artificial cells, biosensors and catalytic compartments.

Toward long-lasting artificial cells that better mimic natural living cells

Chemical communication is ubiquitous in biology, and so efforts in building convincing cellular mimics must consider how cells behave on a population level. Simple model systems have been built in the laboratory that show communication between different artificial cells and artificial cells with natural, living cells. Examples include artificial cells that depend on purely abiological components and artificial cells built from biological components and are driven by biological mechanisms. However, an artificial cell solely built to communicate chemically without carrying the machinery needed for self-preservation cannot remain active for long periods of time. What is needed is to begin integrating the pathways required for chemical communication with metabolic-like chemistry so that robust artificial systems can be built that better inform biology and aid in the generation of new technologies.

A dual-component carrier with both non-enzymatic and enzymatic antioxidant activity towards ROS depletion

While ROS display crucial functions in many physiological processes, elevated ROS levels are also related to the initiation and progression of many severe diseases such as cancer, cardiovascular conditions or neurologic disorders. Research approaches to diminish ROS levels during disease progression are currently being focused on the therapeutic administration of antioxidant enzymes. However, enzyme administration suffers from several limitations including their fast elimination from blood upon administration, thus making crucial the development of enzyme encapsulating platforms. We have recently reported a multicompartment architecture constituted by two inherently different types of materials, i.e., polymeric microgels and liposomes. Poly(N-isopropylacrylamide-co-acrylic acid) microgels decorated with liposomes and subsequently coated by a protective poly(dopamine) shell (PDA) combine the benefits of both systems while minimizing some of their drawbacks. Herein, we exploit this dual-component platform as a microreactor for ROS depletion. We combine the intrinsic PDA's antioxidant properties with the encapsulation of the catalase enzyme within the liposomal compartments. The surface of the carrier is further functionalised with a poly(ethylene glycol) layer and the low fouling properties are demonstrated in terms of reduction of protein adsorption and cellular uptake. The potential of the carrier as an antioxidant microreactor is shown by its ability to deplete superoxide radicals and hydrogen peroxide, which can also take place in the presence of the two relevant cell lines.

Synthetic organelles

One approach towards the creation of bottom-up synthetic biological systems of higher complexity relies on the subcompartmentalization of synthetic cell structures using artificially generated organelles — roughly mimicking the architecture of eukaryotic cells. Organelles create dedicated chemical environments for specific synthesis tasks — they separate incompatible processes from each other and help to create or maintain chemical gradients that drive other chemical processes. Artificial organelles have been used to compartmentalize enzyme reactions, to generate chemical fuels via photosynthesis and oxidative phosphorylation, and they have been utilized to spatially organize cell-free gene expression reactions. In this short review article, we provide an overview of recent developments in this field, which involve a wide variety of compartmentalization strategies ranging from lipid and polymer membrane systems to membraneless compartmentalization via coacervation.

Mimicking Cellular Compartmentalization in a Hierarchical Protocell through Spontaneous Spatial Organization

A systemic feature of eukaryotic cells is the spatial organization of functional components through compartmentalization. Developing protocells with compartmentalized synthetic organelles is, therefore, a critical milestone toward emulating one of the core characteristics of cellular life. Here we demonstrate the bottom-up, multistep, noncovalent, assembly of rudimentary subcompartmentalized protocells through the spontaneous encapsulation of semipermeable, polymersome proto-organelles inside cell-sized coacervates. The coacervate microdroplets are membranized using tailor-made terpolymers, to complete the hierarchical self-assembly of protocells, a system that mimics both the condensed cytosol and the structure of a cell membrane. In this way, the spatial organization of enzymes can be finely tuned, leading to an enhancement of functionality. Moreover, incompatible components can be sequestered in the same microenvironments without detrimental effect. The robust stability of the subcompartmentalized coacervate protocells in biocompatible milieu, such as in PBS or cell culture media, makes it a versatile platform to be extended toward studies in vitro, and perhaps, in vivo.

Tyrosinase-Loaded Multicompartment Microreactor toward Melanoma Depletion

Melanoma is malignant skin cancer occurring with increasing prevalence with no effective treatment. A unique feature of melanoma cells is that they require higher concentrations of ltyrosine (l-tyr) for expansion than normal cells. As such, it has been demonstrated that dietary l-tyr restriction lowers systemic l-tyr and suppresses melanoma advancement in mice. Unfortunately, this diet is not well tolerated by humans. An alternative approach to impede melanoma progression will be to administer the enzyme tyrosinase (TYR), which converts l-tyr into melanin. Herein, a multicompartment carrier consisting of a polymer shell entrapping thousands of liposomes is employed to act as a microreactor depleting l-tyr in the presence of melanoma cells. It is shown that the TYR enzyme can be incorporated within the liposomal subunits with preserved catalytic activity. Aiming to mimic the dynamic environment at the tumor site, l-tyr conversion is conducted by co-culturing melanoma cells and microreactors in a microfluidic setup with applied intratumor shear stress. It is demonstrated that the microreactors are concurrently depleting l-tyr, which translates into inhibited melanoma cell growth. Thus, the first microreactor where the depletion of a substrate translates into antitumor properties in vitro is reported.

PEO-b-PPO star-shaped polymers enhance the structural stability of electrostatically coupled liposome/polyelectrolyte complexes

We propose a strategy to counteract the salt-driven disassembly of multiliposomal complexes made by electrostatic co-assembly of anionic small unilamellar liposomes and cationic star-shaped polyelectrolytes (made of quaternized poly(dimethylaminoethyl methacrylate) (qPDMAEMA100)3.1). The combined action of (qPDMAEMA100)3.1 and a nonionic star-shaped polymer (PEO12-b-PPO45)4, which comprises diblock copolymer arms uniting a poly(ethylene oxide) PEO inner block and a poly(propylene oxide) PPO terminal block, leads to a stabilization of these complexes against disintegration in saline solutions. Hereby, the anchoring of the PPO terminal blocks to the lipid bilayer and the bridging between several liposomes are at the origin of the promoted structural stability. Two-focus fluorescence correlation spectroscopy verifies the formation of multiliposomal complexes with (PEO12-b-PPO45)4. The polyelectrolyte and the amphiphilic polymer work synergistically, as the joint action still assures some membrane integrity, which is not seen for the mere (PEO12-b-PPO45)4-liposome interaction alone.

Artificial Gel-Based Organelles for Spatial Organization of Cell-Free Gene Expression Reactions

Gel-based artificial organelles have been developed that enable sequence-specific and programmable localization of cell-free transcription and translation reactions inside an artificial cellular system. To this end, we utilize agarose microgels covalently modified with DNA templates coding for various functions and encapsulate them into emulsion droplets. We show that RNA signals transcribed from transcription organelles can be specifically targeted to capture organelles via hybridization to the corresponding DNA addresses. We also demonstrate that mRNA molecules, produced from transcription organelles and controlled by toehold switch riboregulators, are only translated in translation organelles containing their cognate DNA triggers. Spatial confinement of transcription and translation in separate organelles is thus superficially similar to gene expression in eukaryotic cells. Combining communicating gel spheres with specialized functions opens up new possibilities for programming artificial cellular systems at the organelle level.

Polymer Capsules for Enzymatic Catalysis in Confined Environments

Catalysis is at the base of a series of biological and technological application processes. In recent years, the tendency has developed to carry out catalyzed reactions within confined structures, thus forming systems called micro or nanoreactors. Compartmentalized structures are cavities delimited by a wall where specific functions are introduced with a defined concentration and in the desired sites. These containers are generally referred to as nano or microcapsules, assuming the function of reactors in the presence of chemical reactions. Among the various types of existing structures, one of the most interesting is represented by systems made with polymers. This review aims to highlight some of the current advances in the use of functionalized structures that are useful for catalysis reactions, paying particular attention to polymer capsules and enzymes. The built-up methods used for the production of polymer capsules, as well as the aspects that influence membrane permeability and reactivity to environmental conditions, are discussed. Recent advances on biocatalysis confined in polymeric capsules are illustrated, and the strengths and weaknesses of the principal nanoreactors are considered.

Biotransporting Biocatalytic Reactors toward Therapeutic Nanofactories

Drug-delivery systems (DDSs), in which drug encapsulation in nanoparticles enables targeted delivery of therapeutic agents and their release at specific disease sites, are important because they improve drug efficacy and help to decrease side effects. Although significant progress has been made in the development of DDSs for the treatment of a wide range of diseases, new approaches that increase the scope and effectiveness of such systems are still needed. Concepts such as nanoreactors and nanofactories are therefore attracting much attention. Nanoreactors, which basically consist of vesicle-encapsulated enzymes, provide prodrug conversion to therapeutic agents rather than simple drug delivery. Nanofactories are an extension of this concept and combine the features of nanoreactors and delivery carriers. Here, the required features of nanofactories are discussed and an overview of current strategies for the design and fabrication of different types of nanoreactors, i.e., systems based on lipid or polymer vesicles, capsules, mesoporous silica, viral capsids, and hydrogels, and their respective advantages and shortcomings, is provided. In vivo applications of biocatalytic reactors in the treatment of cancer, glaucoma, neuropathic pain, and alcohol intoxication are also discussed. Finally, the prospects for further progress in this important and promising field are outlined.

Biomimetic Cascade Network between Interactive Multicompartments Organized by Enzyme-Loaded Silica Nanoreactors

Physical separation of reactions by interactive multicompartments in biological cells is an attractive motif to design efficient microreactors that create biomimetic cascade reactions. We present an aqueous compartment with three different subcompartments that comprise of silica nanoreactors with encapsulated enzymes, namely, β-glucosidase, glucose oxidase, and peroxidase, providing a model cascade reaction in confinement. The encapsulated enzymes retain their activity as the substrate can reach the active site and the silica shell further protects the enzymes from external stresses, such as heat and proteolytic degradation. We demonstrate the biomimetic cascade reaction in between the compartments ("organelles") inside of an additional microconfinement (water-in-oil emulsion). This strategy will allow us to design efficient multicompartmentalized reactors for further biological and organic reactions.

Shear stress regulated uptake of liposome-decorated microgels coated with a poly(dopamine) shell

Advanced multicompartment drug delivery platforms ensure the co-localization of several drugs within the same carrier, thus making it possible to achieve a more effective and safe therapeutic outcome. Herein, we report a novel multicompartment architecture by combining two intrinsically different systems, i.e., polymeric microgels and liposomes, with the aim to achieve different release kinetics for model compounds. We assemble poly(N-isopropylacrylamide-co-acrylic acid) microgels decorated with liposomes which are subsequently coated with a protective poly(dopamine) shell and a poly(ethylene glycol) (PEG) layer. Since any intravenous administered drug delivery vehicle will get in contact with the dynamics of the blood flow, we evaluate the stealth properties of this novel multicompartment carrier towards protein adsorption and cellular uptake by three relevant cell lines (macrophages, endothelial and cancer cells) under physiological shear stress conditions. Our results demonstrate less protein adsorption for the PEGylated carriers and differences in the extent of internalized carriers depending on the presence of a PEG coating, the studied cell line and the intensity of the applied shear stress. Additionally, we demonstrate that, for all three tested cell lines, shear stress results in the activation of different cell entry pathways as compared to static conditions. All in all, we report a thorough study about the effect of shear stress on the cell association/uptake with a novel multicompartment carrier.

Biomimetic enzyme cascade reaction system in microfluidic electrospray microcapsules

Mimicking subcellular compartments containing enzymes in organisms is considered a promising approach to substitute for missing or lost cellular functions. Inspired by the multicompartment structures of cellular architectures, we present a novel multienzyme system based on hollow hydrogel microcapsules with flexible enzymatic inverse opal particles. Benefiting from the precise operation capability of the microfluidic electrospray and the remarkable structural color marks in the inverse opal particles, we developed a multienzyme system with controllable number, type, and spatial arrangement of the encapsulated enzymes. The hydrogel shells also could improve enzyme stability against proteolysis in the system. The multienzyme system containing alcohol oxidase and catalase could act as a cascade biocatalyst and reduce alcohol levels in media, providing an alternative antidote and prophylactic for alcohol intoxication. These features indicated that our strategy provides an ideal enzyme cascade reaction system for complex biocatalysis and biomimetic functions of some organelles or organs.