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

Artificial Receptor in Synthetic Cells Performs Transmembrane Activation of Proteolysis

The design of artificial, synthetic cells is a fundamentally important and fast-developing field of science. Of the diverse attributes of cellular life, artificial transmembrane signaling across the biomolecular barriers remains a high challenge with only a few documented successes. Herein, the study achieves signaling across lipid bilayers and connects an exofacial enzymatic receptor activation to an intracellular biochemical catalytic response using an artificial receptor. The mechanism of signal transduction for the artificial receptor relies on the triggered decomposition of a self-immolative linker. Receptor activation ensues its head-to-tail decomposition and the release of a secondary messenger molecule into the internal volume of the synthetic cell. Transmembrane signaling is demonstrated in synthetic cells based on liposomes and mammalian cell-sized giant unilamellar vesicles and illustrates receptor performance in cell mimics with a diverse size and composition of the lipid bilayer. In giant unilamellar vesicles, transmembrane signaling connects exofacial receptor activation with intracellular activation of proteolysis. Taken together, the results of this study take a step toward engineering receptor-mediated, responsive behavior in synthetic cells.

Antioxidant Microgels Support Peroxide-Challenged Hepatic Cells

Access to therapeutic strategies that counter cellular stress induced by reactive oxygen species (ROS) is an important, long-standing challenge. Here, the assembly of antioxidant artificial cells is based on alginate hydrogels equipped with non-native catalysts, namely platinum nanoparticles and an EUK compound. These artificial cells are able to preserve the viability and lower the intracellular ROS levels of challenged hepatic cells by removing peroxides from the extracellular environment. Conceptually, this strategy illustrates the potential use of artificial cells with a synthetic catalyst toward long-term support of hepatic cells and potentially other mammalian cells.

Organismal Function Enhancement through Biomaterial Intervention

Living organisms in nature, such as magnetotactic bacteria and eggs, generate various organic-inorganic hybrid materials, providing unique functionalities. Inspired by such natural hybrid materials, researchers can reasonably integrate biomaterials with living organisms either internally or externally to enhance their inherent capabilities and generate new functionalities. Currently, the approaches to enhancing organismal function through biomaterial intervention have undergone rapid development, progressing from the cellular level to the subcellular or multicellular level. In this review, we will concentrate on three key strategies related to biomaterial-guided bioenhancement, including biointerface engineering, artificial organelles, and 3D multicellular immune niches. For biointerface engineering, excess of amino acid residues on the surfaces of cells or viruses enables the assembly of materials to form versatile artificial shells, facilitating vaccine engineering and biological camouflage. Artificial organelles refer to artificial subcellular reactors made of biomaterials that persist in the cytoplasm, which imparts cells with on-demand regulatory ability. Moreover, macroscale biomaterials with spatiotemporal regulation characters enable the local recruitment and aggregation of cells, denoting multicellular niche to enhance crosstalk between cells and antigens. Collectively, harnessing the programmable chemical and biological attributes of biomaterials for organismal function enhancement shows significant potential in forthcoming biomedical applications.

Cell-Like Capsules with "Smart" Compartments

Eukaryotic cells have inner compartments (organelles), each with distinct properties and functions. One mimic of this architecture, based on biopolymers, is the multicompartment capsule (MCC). Here, MCCs in which the inner compartments are chemically unique and "smart," i.e., responsive to distinct stimuli in an orthogonal manner are created. Specifically, one compartment alone is induced to degrade when the MCC is contacted with an enzyme while other compartments remain unaffected. Similarly, just one compartment gets degraded upon contact with reactive oxygen species generated from hydrogen peroxide (H₂ O₂ ). And thirdly, one compartment alone is degraded by an external, physical stimulus, namely, by irradiating the MCC with ultraviolet (UV) light. All these specific responses are achieved without resorting to complicated chemistry to create the compartments: the multivalent cation used to crosslink the biopolymer alginate (Alg) is simply altered. Compartments of Alg crosslinked by Ca²⁺ are shown to be sensitive to enzymes (alginate lyases) but not to H₂ O₂ or UV, whereas the reverse is the case with Alg/Fe³⁺ compartments. These results imply the ability to selectively burst open a compartment in an MCC "on-demand" (i.e., as and when needed) and using biologically relevant stimuli. The results are then extended to a sequential degradation, where compartments in an MCC are degraded one after another, leaving behind an empty MCC lumen. Collectively, this work advances the MCC as a platform that not only emulates key features of cellular architecture, but can also begin to capture rudimentary cell-like behaviors.

Artificial Organelles with Digesting Characteristics: Imitating Simplified Lysosome- and Macrophage-Like Functions by Trypsin-Loaded Polymersomes

Defects in cellular protein/enzyme encoding or even in organelles are responsible for many diseases. For instance, dysfunctional lysosome or macrophage activity results in the unwanted accumulation of biomolecules and pathogens implicated in autoimmune, neurodegenerative, and metabolic disorders. Enzyme replacement therapy (ERT) is a medical treatment that replaces an enzyme that is deficient or absent in the body but suffers from short lifetime of the enzymes. Here, this work proposes the fabrication of two different pH-responsive and crosslinked trypsin-loaded polymersomes as protecting enzyme carriers mimicking artificial organelles (AOs). They allow the enzymatic degradation of biomolecules to mimic simplified lysosomal function at acidic pH and macrophage functions at physiological pH. For optimal working of digesting AOs in different environments, pH and salt composition are considered the key parameters, since they define the permeability of the membrane of the polymersomes and the access of model pathogens to the loaded trypsin. Thus, this work demonstrates environmentally controlled biomolecule digestion by trypsin-loaded polymersomes also under simulated physiological fluids, allowing a prolonged therapeutic window due to protection of the enzyme in the AOs. This enables the application of AOs in the fields of biomimetic therapeutics, specifically in ERT for dysfunctional lysosomal diseases.

Complex Coacervate Materials as Artificial Cells

Cells have evolved to be self-sustaining compartmentalized systems that consist of many thousands of biomolecules and metabolites interacting in complex cycles and reaction networks. Numerous subtle intricacies of these self-assembled structures are still largely unknown. The importance of liquid-liquid phase separation (both membraneless and membrane bound) is, however, recognized as playing an important role in achieving biological function that is controlled in time and space. Reconstituting biochemical reactions in vitro has been a success of the last decades, for example, establishment of the minimal set of enzymes and nutrients able to replicate cellular activities like the in vitro transcription translation of genes to proteins. Further than this though, artificial cell research has the aim of combining synthetic materials and nonliving macromolecules into ordered assemblies with the ability to carry out more complex and ambitious cell-like functions. These activities can provide insights into fundamental cell processes in simplified and idealized systems but could also have an applied impact in synthetic biology and biotechnology in the future. To date, strategies for the bottom-up fabrication of micrometer scale life-like artificial cells have included stabilized water-in-oil droplets, giant unilamellar vesicles (GUV's), hydrogels, and complex coacervates. Water-in-oil droplets are a valuable and easy to produce model system for studying cell-like processes; however, the lack of a crowded interior can limit these artificial cells in mimicking life more closely. Similarly membrane stabilized vesicles, such as GUV's, have the additional membrane feature of cells but still lack a macromolecularly crowded cytoplasm. Hydrogel-based artificial cells have a macromolecularly dense interior (although cross-linked) that better mimics cells, in addition to mechanical properties more similar to the viscoelasticity seen in cells but could be seen as being not dynamic in nature and limiting to the diffusion of biomolecules. On the other hand, liquid-liquid phase separated complex coacervates are an ideal platform for artificial cells as they can most accurately mimic the crowded, viscous, highly charged nature of the eukaryotic cytoplasm. Other important key features that researchers in the field target include stabilizing semipermeable membranes, compartmentalization, information transfer/communication, motility, and metabolism/growth. In this Account, we will briefly cover aspects of coacervation theory and then outline key cases of synthetic coacervate materials used as artificial cells (ranging from polypeptides, modified polysaccharides, polyacrylates, and polymethacrylates, and allyl polymers), finishing with envisioned opportunities and potential applications for coacervate artificial cells moving forward.

Polymer Micelles vs Polymer-Lipid Hybrid Vesicles: A Comparison Using RAW 264.7 Cells

Bottom-up synthetic biology aims to integrate artificial moieties with living cells and tissues. Here, two types of structural scaffolds for artificial organelles were compared in terms of their ability to interact with macrophage-like murine RAW 264.7 cells. The amphiphilic block copolymer poly(cholesteryl methacrylate)-block-poly(2-carboxyethyl acrylate) was used to assemble micelles and polymer-lipid hybrid vesicles together with 1,2-dioleoyl-sn-glycero-3-phosphocholine or 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) lipids in the latter case. In addition, the pH-sensitive fusogenic peptide GALA was conjugated to the carriers to improve their lysosomal escape ability. All assemblies had low short-term toxicity toward macrophage-like murine RAW 264.7 cells, and the cells internalized both the micelles and hybrid vesicles within 24 h. Assemblies containing DOPE lipids or GALA in their building blocks could escape the lysosomes. However, the intracellular retention of the building blocks was only a few hours in all the cases. Taken together, the provided comparison between two types of potential scaffolds for artificial organelles lays out the fundamental understanding required to advance soft material-based assemblies as intracellular nanoreactors.

Recent Advances in Nucleic Acid Modulation for Functional Nanozyme

Nanozymes have the potential to replace natural enzymes, so they are widely used in energy conversion technologies such as biosensors and signal transduction (converting biological signals of a target into optical, electrical, or metabolic signals). The participation of nucleic acids leads nanozymes to produce richer interface effects and gives energy conversion events more attractive characteristics, creating what are called “functional nanozymes”. Since different nanozymes have different internal structures and external morphological characteristics, functional modulation needs to be compatible with these properties, and attention needs to be paid to the influence of nucleic acids on nanozyme activity. In this review, “functional nanozymes” are divided into three categories, (nanozyme precursor ion)/ (nucleic acid) self-assembly, nanozyme-nucleic acid irreversible binding, and nanozyme-nucleic acid reversible binding, and the effects of nucleic acids on modulation principles are summarized. Then, the latest developments of nucleic acid-modulated nanozymes are reviewed in terms of their use in energy conversion technology, and their conversion mechanisms are critically discussed. Finally, we outline the advantages and limitations of “functional nanozymes” and discuss the future development prospects and challenges in this field.