Hybrid vesicles as intracellular reactive oxygen species and nitric oxide generators

Advances in block copolymer-phospholipid hybrid vesicles: from physical-chemical properties to applications

Hybrid vesicles, made of lipids and amphiphilic block copolymers, have become increasingly popular thanks to their versatile properties that enable the construction of intricate membranes mimicking cellular structures. This tutorial review gives an overview over the different hybrid vesicle designs, and provides a detailed analysis of their properties, including their composition, membrane fluidity, membrane homogeneity, permeability, stability. The review puts emphasis on the application of these hybrid vesicles in bottom-up synthetic biology and aims to offer an overview of design guidelines, particularly focusing on composition, to eventually realize the intended applications of these hybrid vesicles.

Poly(Sitosterol)-Based Hydrophobic Blocks in Amphiphilic Block Copolymers for the Assembly of Hybrid Vesicles

Amphiphilic block copolymer and lipids can be assembled into hybrid vesicles (HVs), which are an alternative to liposomes and polymersomes. Block copolymers that have either poly(sitostryl methacrylate) or statistical copolymers of sitosteryl methacrylate and butyl methacrylate as the hydrophobic part and a poly(carboxyethyl acrylate) hydrophilic segment are synthesized and characterized. These block copolymers assemble into small HVs with soybean L-α-phosphatidylcholine (soyPC), confirmed by electron microscopy and small-angle X-ray scattering. The membrane's hybrid nature is illustrated by fluorescence resonance energy transfer between labeled building blocks. The membrane packing, derived from spectra when using Laurdan as an environmentally sensitive fluorescent probe, is comparable between small HVs and the corresponding liposomes with molecular sitosterol, although the former show indications of transmembrane asymmetry. Giant HVs with homogenous distribution of the block copolymers and soyPC in their membranes are assembled using the electroformation method. The lateral diffusion of both building blocks is slowed down in giant HVs with higher block copolymer content, but their permeability toward (6)-carboxy-X-rhodamine is higher compared to giant vesicles made of soyPC and molecular sitosterol. This fundamental effort contributes to the rapidly expanding understanding of the integration of natural membrane constituents with designed synthetic compounds to form hybrid membranes.

Advancing Biomimetic Functions of Synthetic Cells through Compartmentalized Cell-Free Protein Synthesis

Synthetic cells are artificial constructs that mimic the structures and functions of living cells. They are attractive for studying diverse biochemical processes and elucidating the origins of life. While creating a living synthetic cell remains a grand challenge, researchers have successfully synthesized hundreds of unique synthetic cell platforms. One promising approach to developing more sophisticated synthetic cells is to integrate cell-free protein synthesis (CFPS) mechanisms into vesicle platforms. This makes it possible to create synthetic cells with complex biomimetic functions such as genetic circuits, autonomous membrane modifications, sensing and communication, and artificial organelles. This Review explores recent advances in the use of CFPS to impart advanced biomimetic structures and functions to bottom-up synthetic cell platforms. We also discuss the potential applications of synthetic cells in biomedicine as well as the future directions of synthetic cell research.

Compartmentalized Intracellular Click Chemistry with Biodegradable Polymersomes

Polymersome nanoreactors that can be employed as artificial organelles have gained much interest over the past decades. Such systems often include biological catalysts (i.e., enzymes) so that they can undertake chemical reactions in cellulo. Examples of nanoreactor artificial organelles that acquire metal catalysts in their structure are limited, and their application in living cells remains fairly restricted. In part, this shortfall is due to difficulties associated with constructing systems that maintain their stability in vitro, let alone the toxicity they impose on cells. This study demonstrates a biodegradable and biocompatible polymersome nanoreactor platform, which can be applied as an artificial organelle in living cells. The ability of the artificial organelles to covalently and non-covalently incorporate tris(triazolylmethyl)amine-Cu(I) complexes in their membrane is shown. Such artificial organelles are capable of effectively catalyzing a copper-catalyzed azide-alkyne cycloaddition intracellularly, without compromising the cells' integrity. The platform represents a step forward in the application of polymersome-based nanoreactors as artificial organelles.

Recent Advances in Functional Nanomaterials for Catalytic Generation of Nitric Oxide: A Mini Review

As a gaseous second messenger, nitric oxide (NO) plays an important role in a series of signal pathways. Research on the NO regulation for various disease treatments has aroused wide concern. However, the lack of accurate, controllable, and persistent release of NO has significantly limited the application of NO therapy. Profiting from the booming development of advanced nanotechnology, a mass of nanomaterials with the properties of controllable release have been developed to seek new and effective NO nano-delivery approaches. Nano-delivery systems that generate NO through catalytic reactions exhibit unique superiority in terms of precise and persistent release of NO. Although certain achievements have been made in the catalytically active NO delivery nanomaterials, some basic but critical issues, such as the concept of design, are of low attention. Herein, an overview of the generation of NO through catalytic reactions and the design principles of related nanomaterials are summarized. Then, the nanomaterials that generate NO through catalytic reactions are classified. Finally, the bottlenecks and perspectives are also discussed in depth for the future development of catalytical NO generation nanomaterials.

Polymersome-based protein drug delivery - quo vadis?

Protein-based therapeutics are an attractive alternative to established therapeutic approaches and represent one of the fastest growing families of drugs. While many of these proteins can be delivered using established formulations, the intrinsic sensitivity of proteins to denaturation sometimes calls for a protective carrier to allow administration. Historically, lipid-based self-assembled structures, notably liposomes, have performed this function. After the discovery of polymersome-based targeted drug-delivery systems, which offer manifold advantages over lipid-based structures, the scientific community expected that such systems would take the therapeutic world by storm. However, no polymersome formulations have been commercialised. In this review article, we discuss key obstacles for the sluggish translation of polymersome-based protein nanocarriers into approved pharmaceuticals, which include limitations imparted by the use of non-degradable polymers, the intricacies of polymersome production methods, and the complexity of the in vivo journey of polymersomes across various biological barriers. Considering this complex subject from a polymer chemist's point of view, we highlight key areas that are worthy to explore in order to advance polymersomes to a level at which clinical trials become worthwhile and translation into pharmaceutical and nanomedical applications is realistic.

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.

Electroanalysis at a Single Giant Vesicle Generating Enzymatically a Reactive Oxygen Species

In the framework of artificial or synthetic cell development, giant liposomes are common basic structures. Their enclosed membrane allows encapsulating proteins, DNA, reactants, etc., while its phospholipid nature allows some exchanges with the surrounding medium. Biochemical reactions induced inside giant liposomes or vesicles are often monitored or imaged by fluorescence microscopy techniques. Here, we show that electrochemistry performed with ultramicroelectrodes is perfectly suitable to monitor an enzymatic reaction occurring in a single giant unilamellar vesicle. Glucose oxidase (GOx) was microinjected inside individual vesicles containing 1 mM glucose. H₂O₂ was thus generated in the vesicle and progressively diffused across the membrane toward the surrounding environment. An ultramicroelectrode sensitive to H₂O₂ (black platinum-modified carbon surface) was placed next to the membrane and provided a direct detection of the hydrogen peroxide flux generated by the enzyme activity. Electrochemistry offered a highly sensitive (in situ detection), selective (potential applied at the electrode), time-resolved analysis (chronoamperometry) of the GOx activity over an hour duration, without modifying the internal giant unilamellar vesicles (GUV) medium. These results demonstrate that electroanalysis with microsensors is well adapted and complementary to fluorescence microscopy to sense enzymatic activities, for instance, generating reactive oxygen species, at single vesicles further used to develop artificial cells.

Evaluation of Hybrid Vesicles in an Intestinal Cell Model Based on Structured Paper Chips

Cell culture-based intestinal models are important to evaluate nanoformulations intended for oral drug delivery. We report the use of a floating structured paper chip as a scaffold for Caco-2 cells and HT29-MTX-E12 cells that are two established cell types used in intestinal cell models. The formation of cell monolayers for both mono- and cocultures in the paper chip are confirmed and the level of formed cell-cell junctions is evaluated. Further, cocultures show first mucus formation between 6-10 days with the mucus becoming more pronounced after 19 days. Hybrid vesicles (HVs) made from phospholipids and the amphiphilic block copolymer poly(cholesteryl methacrylate)-block-poly(2-carboxyethyl acrylate) in different ratios are used as a representative soft nanoparticle to assess their mucopenetration ability in paper chip-based cell cultures. The HV assembly is characterized, and it is illustrated that these HVs cross the mucus layer and are found intracellularly within 3 h when the cells are grown in the paper chips. Taken together, the moist three-dimensional cellulose environment of structured paper chips offers an interesting cell culture-based intestinal model that can be further integrated with fluidic systems or online read-out opportunities.

Large hybrid Polymer/Lipid Unilamellar vesicle (LHUV) at the nanoscale: An insight into the lipid distribution in the membrane and permeability control

Membrane structuration of Large Hybrid Unilamellar Polymer/Lipid Vesicle (LHUV) is an important parameter on the optimization of their properties and thus their valuation in various fields. However, this kind of information is hardly accessible. In this work, we will focus on the development of LHUV obtained from the self-assembly of diblock poly(dimethylsiloxane)-b-poly(ethylene oxide) (PDMS-b-PEO) of different molar masses combined with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) at 15% and 25% w/w content. The hybrid character of the resulting vesicles as well as their membrane structure are characterized by the mean of different techniques such as small-angle neutron scattering (SANS) and cryo-transmission electron microscopy (cryo-TEM). We show that hybrid vesicles with homogeneous membrane structure are obtained whatever the molar mass of the block copolymer (from 2500 to 4000 g/mol), with of a small number of tubular structures observed with the higher molar mass. We also demonstrate that the permeability of the LHUV, evaluated through controlled release experiments of fluorescein loaded in LHUV, is essentially controlled by the lipid/polymer composition.

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.

Membrane reinforcement in giant hybrid polymer lipid vesicles achieved by controlling the polymer architecture

The physical properties of membranes of hybrid polymer lipid vesicles are so far relatively unknown. Since their discovery a decade ago, many studies have aimed to show their great potential in many fields of application, but so far, few systematic studies have been carried out to decipher the relationship between the molecular characteristics of the components (molar mass, chemical nature, and architecture of the copolymer), the membrane structure and its properties. In this work, we study the association of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and poly(dimethylsiloxane)-b-poly(ethylene oxide) (PDMS-b-PEO) diblock copolymers of different molar masses in giant hybrid vesicles and establish a complete phase diagram of the membrane structure. We also measured the mechanical properties of the giant hybrid unilamellar vesicle (GHUV) through micropipette aspiration at different lipid/polymer compositions. Thanks to a previous work using triblock PEO-b-PDMS-b-PEO copolymers, we were able to reveal the effect of the architecture of the block copolymer on membrane structure and properties. Besides, the association of diblock copolymers PDMS-b-PEO and POPC leads to the formation of hybrid vesicles with unprecedented membrane toughness.

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.

Multistimuli Sensing Adhesion Unit for the Self-Positioning of Minimal Synthetic Cells

Cells have the ability to sense different environmental signals and position themselves accordingly in order to support their survival. Introducing analogous capabilities to the bottom-up assembled minimal synthetic cells is an important step for their autonomy. Here, a minimal synthetic cell which combines a multistimuli sensitive adhesion unit with an energy conversion module is reported, such that it can adhere to places that have the right environmental parameters for ATP production. The multistimuli sensitive adhesion unit senses light, pH, oxidative stress, and the presence of metal ions and can regulate the adhesion of synthetic cells to substrates in response to these stimuli following a chemically coded logic. The adhesion unit is composed of the light and redox responsive protein interaction of iLID and Nano and the pH sensitive and metal ion mediated binding of protein His-tags to Ni²⁺ -NTA complexes. Integration of the adhesion unit with a light to ATP conversion module into one synthetic cell allows it to adhere to places under blue light illumination, non-oxidative conditions, at neutral pH and in the presence of metal ions, which are the right conditions to synthesize ATP. Thus, the multistimuli responsive adhesion unit allows synthetic cells to self-position and execute their functions.

Polymer-Lipid Hybrid Vesicles and Their Interaction with HepG2 Cells

Polymer-lipid hybrid vesicles are an emerging type of nano-assemblies that show potential as artificial organelles among others. Phospholipids and poly(cholesteryl methacrylate)-block-poly(methionine methacryloyloxyethyl ester (METMA)-random-2-carboxyethyl acrylate (CEA)) labeled with a Förster resonance energy transfer (FRET) reporter pair are used for the assembly of small and giant hybrid vesicles with homogenous distribution of both building blocks in the membrane as confirmed by the FRET effect. These hybrid vesicles have no inherent cytotoxicity when incubated with HepG2 cells up to 1.1 × 10¹¹ hybrid vesicles per mL, and they are internalized by the cells. In contrast to the fluorescent signal originating from the block copolymer, the fluorescent signal coming from the lipids is barely detectable in cells incubated with hybrid vesicles for 6 h followed by 24 h in cell media, suggesting that the two building blocks have a different intracellular fate. These findings provide important insight into the design criteria of artificial organelles with potential structural integrity.

The rise of bio-inspired polymer compartments responding to pathology-related signals

Self-organized nano- and microscale polymer compartments such as polymersomes, giant unilamellar vesicles (GUVs), polyion complex vesicles (PICsomes) and layer-by-layer (LbL) capsules have increasing potential in many sensing applications. Besides modifying the physicochemical properties of the corresponding polymer building blocks, the versatility of these compartments can be markedly expanded by biomolecules that endow the nanomaterials with specific molecular and cellular functions. In this review, we focus on polymer-based compartments that preserve their structure, and highlight the key role they play in the field of medical diagnostics: first, the self-assembling abilities that result in preferred architectures are presented for a broad range of polymers. In the following, we describe different strategies for sensing disease-related signals (pH-change, reductive conditions, and presence of ions or biomolecules) by polymer compartments that exhibit stimuli-responsiveness. In particular, we distinguish between the stimulus-sensitivity contributed by the polymer itself or by additional compounds embedded in the compartments in different sensing systems. We then address necessary properties of sensing polymeric compartments, such as the enhancement of their stability and biocompatibility, or the targeting ability, that open up new perspectives for diagnostic applications.

Light-triggered nitric oxide (NO) release from photoresponsive polymersomes for corneal wound healing

Polymersomes have been extensively used in the delivery of both small and macromolecular payloads. However, the controlled delivery of gaseous therapeutics (e.g., nitric oxide, NO) remains a grand challenge due to its difficulty in loading of gaseous payloads into polymersomes without premature leakage. Herein, NO-releasing vesicles could be fabricated via the self-assembly of NO-releasing amphiphiles, which were synthesized by the direct polymerization of photoresponsive NO monomers (abbreviated as oNBN, pNBN, and BN). These monomers were rationally designed through the integration of the photoresponsive behavior of N-nitrosoamine moieties and the self-immolative chemistry of 4-aminobenzyl alcohol derivatives, which outperformed conventional NO donors such as diazeniumdiolates (NONOates) and S-nitrosothiols (SNOs) in terms of ease of preparation, stability of storage, and controllability of NO release. The unique design made it possible to selectively release NO by a light stimulus and to regulate the NO release rates. Importantly, the photo-mediated NO release could be manipulated in living cells and showed promising applications in the treatment of corneal wounds. In addition to delivering NO, the current design enabled the synergistic delivery of NO and other therapeutic payloads by taking advantage of NO release-mediated traceless crosslinking of the vesicles.