Dynamic Docking and Undocking Processes Addressing Selectively the Outside and Inside of Polymersomes

Stretched or wrinkled? Looking into the polymer conformation within polymersome membranes

Self-assembly of amphiphilic block-copolymers into polymersomes is a well-established concept. In this membrane, the hydrophilic part is considered to be loosely assembled towards the solvent, and the hydrophobic part on the inside of the membrane is considered to be more densely packed. Within the membrane, this hydrophobic part could now have a stretched conformation or be a random coil, depending on the available space and also on the chemical nature of the polymer. We now analysed the literature for works on polymersomes that determined the membrane thickness via cryo-TEM and analysed the hydrophobic part of their polymers for their conformation. Over all available block-copolymers, a variety of trends became obvious: the longer a hydrophobic block, the more coiled the conformation and the bulkier the side chains, the more stretched the polymer became. Polymers with less conformational freedom like semi-crystalline ones were present in a more stretched conformation. Both trends could be exemplified on various occasions in this cross-literature meta-study. This overview hence provides additional insight into the physical chemistry of block-copolymer membranes.

Crosslinked and Multi-Responsive Polymeric Vesicles as a Platform to Study Enzyme-Mediated Undocking Behavior: Toward Future Artificial Organelle Communication

Various cellular functions are successfully mimicked, opening the door to the next generation of therapeutic approaches and systems biology. Herein, the first steps are taken toward the construction of artificial organelles for mimicking cell communication by docking and undocking of cargo in the membrane of swollen artificial organelles. Stimuli-responsive and crosslinked polymeric vesicles are used to allow docking processes at acidic pH at which ferrocene units in the swollen membrane state can undergo desired specific host-guest interaction using β-cyclodextrin as model cargo. The release of the cargo mediated by two different enzymes, glucose oxidase and α-amylase, is investigated, triggered by distinct enzymatic undocking mechanisms. Different release times for a useful transport are shown that can be adapted to different communication pathways. In addition, Förster resonance energy transfer (FRET) experiments further support the hypotheses of host-guest inclusion complexation formation and their time-dependent breakdown. This work paves a way to a platform based on polymeric vesicles for synthetic biology, cell functions mimicking, and the construction of multifunctional cargo delivery system.

Overcoming the Dilemma of Permeability and Stability of Polymersomes through Traceless Cross-Linking

In nature, cells are highly compartmentalized into many organelles that are well separated from the rest of the cellular space by unique membrane structures, which are of crucial importance to allow cells to perform various physiological functions in such a small and crowded space. Learning from the ubiquitous membrane structures of cells and organelles has continuously inspired the development of artificial self-assembled nanostructures, with lipid vesicles (liposomes) and polymer vesicles (polymersomes) being the most representative examples. Similar to the membrane-bound structures of cells and organelles, both liposomes and polymersomes contain an aqueous interior enclosed by a bilayer membrane. Therefore, liposomes and polymersomes have been extensively investigated to mimic the fundamental structures and functions of living cells. For example, liposomes and polymersomes have been successfully engineered as nanocarriers, smart nanoreactors, artificial organelles, and so on. Notably, living cells can exchange both energy and materials with surrounding environments, benefiting from the selective permeability of lipid membranes. The permselectivity of cell membranes is thus an essential attribute of living organisms. Compared to liposomes, polymersomes have increased structural stability but low membrane permeability. Indeed, polymersomes are almost impermeable to small molecules, ions, and even water molecules. To improve the permeability of polymersomes, much effort has been devoted to the incorporation of channel proteins, the coassembly of oppositely charged block copolymers (BCPs), the development of stimuli-responsive BCPs, and so on. Despite great achievements, these approaches generally lead to decreased stability of polymersomes and, sometimes, polymersome disintegration. In this Account, we discuss our recent efforts to reconcile the stability and permeability of polymersomes via a traceless cross-linking approach. Although cross-linking reactions within bilayer membranes generally lead to decreased permeability, the traceless cross-linking approach can concurrently improve the stability and permeability of polymersomes. Specifically, stimuli-responsive polymersomes undergo either covalent cross-linking or noncovalent cross-linking reactions under specific stimuli to increase bilayer stability, while the cross-linking processes can concurrently permeabilize polymersome bilayers through cross-linking-driven hydrophobic-to-hydrophilic transitions. Notably, unlike conventional cross-linking processes requiring additional cross-linkers, the traceless cross-linking process does not involve extra cross-linking agents but takes full advantage of the in situ generated active moieties. By taking advantage of the simultaneous modulation of the stability and permeability of polymersomes via traceless cross-linking, these polymersomes can be further engineered as smart nanocarriers and nanoreactors. The robustness and generality of this approach have been validated by both extracellular and intracellular stimuli such as light irradiation, glutathione, and hydrogen peroxide. Moreover, many functional groups such as fluorescent dyes and contrast agents can be integrated into this versatile platform as well, enabling the construction of theranostic nanovectors capable of responding to pathological microenvironments. This Account provides a new approach to regulating the permeability of polymersomes while maintaining their structural stability.

Redox- and pH-Responsive Polymersomes with Ferrocene Moieties Exhibiting Peroxidase-like, Chemoenzymatic Activity and H2O2-Responsive Release Behavior

The development of compartments for the design of cascade reactions in a local space requires a selective spatiotemporal control. The combination of enzyme-loaded polymersomes with enzymelike units shows a great potential in further refining the diffusion barrier and the type of reactions in nanoreactors. Herein, pH-responsive and ferrocene-containing block copolymers were synthesized to realize pH-stable and multiresponsive polymersomes. Permeable membrane, peroxidase-like behavior induced by the redox-responsive ferrocene moieties and release properties were validated using cyclovoltammetry, dye TMB assay, and rupture of host-guest interactions with β-cyclodextrin, respectively. Due to the incorporation of different block copolymers, the membrane permeability of glucose oxidase-loaded polymersomes was changed by increasing extracellular glucose concentration and in TMB assay, allowing for the chemoenzymatic cascade reaction. This study presents a potent synthetic, multiresponsive nanoreactor platform with tunable (e.g., redox-responsive) membrane properties for potential application in therapeutics.

Probing Crowdedness of Artificial Organelles by Clustering Polymersomes for Spatially Controlled and pH-Triggered Enzymatic Reactions

Most sophisticated biological functions and features of cells are based on self-organization, and the coordination and connection between their cell organelles determines their key functions. Therefore, spatially ordered and controllable self-assembly of polymersomes to construct clusters to simulate complex intracellular biological functions has attracted widespread attention. Here, we present a simple one-step copper-free click strategy to cross-link nanoscale pH-responsive and photo-cross-linked polymersomes (less than 100 nm) to micron-level clusters (more than 90% in 0.5-2 μm range). Various influencing factors in the clustering process and subsequent purification methods were studied to obtain optimal clustered polymeric vesicles. Even when polymeric vesicles separately loaded with different enzymes (glucose oxidase and myoglobin) are coclustered, the overall permeability of the clusters can still be regulated through tuning the pH values on demand. Compared with simple blending of those enzyme-loaded polymersomes, the rate of enzymatic cascade reaction increased significantly due to the interconnected complex microstructure established. The connection of catalytic nanocompartments into clusters confining different enzymes of a cascade reaction provides an excellent platform for the development of artificial systems mimicking natural organelles or cells.

Targeted delivery of miR-218 via decorated hyperbranched polyamidoamine for liver cancer regression

Hepatocellular carcinoma (HCC) is one of most common causes of cancer death worldwide. MicroRNA (miRNA) replacement gene therapy is a novel approach for HCC management. MiR-218 is a promising tumor suppressor miRNA that is down-regulated in HCC. Here, our aim was the targeted delivery of miR-218 expressing DNA plasmid (pmiR-218) to suppress HCC in vitro and in vivo. Hyperbranched polyamidoamine was synthesized via simple and economically one-pot reaction followed by decoration with lactobionic acid (LA-PAMAM) to selectively deliver and restore miR-218 expression in HCC. In vitro cytotoxicity investigations revealed the high biocompatibility of LA-PAMAM. Furthermore, decoration of hyperbranched polymer with LA moieties enabled LA-PAMAM to deliver pmiR-218 more efficiently to HepG2 cells compared to both PMAMA and naked pmiR-218. Such efficient delivery of miR-218 resulted in suppression of HepG2 proliferation and down-regulation of its oncogenic HOXA1 target. In vivo, LA-PAMAM/pmiR-218 treatment of HCC induced by DEN and CCl₄ in mice leads to an obvious decrease in the number and size of HCC nodules. In addition, LA-PAMAM/pmiR-218 significantly improved the liver histological features, as well as down-regulated the HOXA1 in liver tissue. In conclusion, this study showed the potential of LA-PAMAM carrier for the targeted delivery of tumor suppressor miR-218 as a therapeutic candidate for HCC.

Multivalent Protein-Loaded pH-Stable Polymersomes: First Step toward Protein Targeted Therapeutics

Synthetic platforms for mimicking artificial organelles or for designing multivalent protein therapeutics for targeting cell surface, extracellular matrix, and tissues are in the focus of this study. Furthermore, the availability of a multi-functionalized and stimuli-responsive carrier system is required that can be used for sequential in situ and/or post loading of different proteins combined with post-functionalization steps. Until now, polymersomes exhibit excellent key characteristics to fulfill those requirements, which allow specific transport of proteins and the integration of proteins in different locations of polymeric vesicles. Herein, different approaches to fabricate multivalent protein-loaded, pH-responsive, and pH-stable polymersomes are shown, where a combination of therapeutic action and targeting can be achieved, by first choosing two model proteins such as human serum albumin and avidin. Validation of the molecular parameters of the multivalent biohybrids is performed by dynamic light scattering, cryo-TEM, fluorescence spectroscopy, and asymmetrical flow-field flow fractionation combined with light scattering techniques. To demonstrate targeting functions of protein-loaded polymersomes, avidin post-functionalized polymersomes are used for the molecular recognition of biotinylated cell surface receptors. These versatile protein-loaded polymersomes present new opportunities for designing sophisticated biomolecular nanoobjects in the field of (extracellular matrix) protein therapeutics.

Artificial Organelles with Orthogonal-Responsive Membranes for Protocell Systems: Probing the Intrinsic and Sequential Docking and Diffusion of Cargo into Two Coexisting Avidin-Polymersomes

The challenge of effective integration and use of artificial organelles with orthogonal-responsive membranes and their communication in eukaryotic protocells is to understand the intrinsic membrane characteristics. Here, a novel photo-crosslinked and pH-responsive polymersome (Psome B) with 2-(N,N'-diisopropylamino)ethyl units in the membrane and its respective Avidin-Psome B hybrids, are reported as good candidates for artificial organelles. Biotinylated (macro)molecules are able to dock and diffuse into Avidin-Psome B to carry out biological activity in a pH- and size-dependent manner. Combined with another polymersome (Psome A) with 2-(N,N'-diethylamino)ethyl units in the membrane, two different pH-responsive polymersomes for mimicking different organelles in one protocell system are reported. The different intrinsic docking and diffusion processes of cargo (macro)molecules through the membranes of coexisting Psome A and B are pH-dependent as confirmed using pH titration-dynamic light scattering (DLS). Psome A and B show separated "open", "closing/opening", and "closed" states at various pH ranges with different membrane permeability. The results pave the way for the construction of multicompartmentalized protocells with controlled communications between different artificial organelles.

Construction of Eukaryotic Cell Biomimetics: Hierarchical Polymersomes-in-Proteinosome Multicompartment with Enzymatic Reactions Modulated Protein Transportation

The eukaryotic cell is a smart compartment containing an outer permeable membrane, a cytoskeleton, and functional organelles, presenting part structures for life. The integration of membrane-containing artificial organelles (=polymersomes) into a large microcompartment is a key step towards the establishment of exquisite cellular biomimetics with different membrane properties. Herein, an efficient way to construct a hierarchical multicompartment composed of a hydrogel-filled proteinosome hybrid structure with an outer homogeneous membrane, a smart cytoskeleton-like scaffold, and polymersomes is designed. Specially, this hybrid structure creates a micro-environment for pH-responsive polymersomes to execute a desired substance transport upon response to biological stimuli. Within the dynamic pH-stable skeleton of the protein hydrogels, polymersomes with loaded PEGylated insulin biomacromolecules demonstrate a pH-responsive reversible swelling-deswelling and a desirable, on-demand cargo release which is induced by the enzymatic oxidation of glucose to gluconic acid. This stimulus responsive behavior is realized by tunable on/off states through protonation of the polymersomes membrane under the enzymatic reaction of glucose oxidase, integrated in the skeleton of protein hydrogels. The integration of polymersomes-based hybrid structure into the proteinosome compartment and the stimuli-response on enzyme reactions fulfills the requirements of eukaryotic cell biomimetics in complex architectures and allows mimicking cellular transportation processes.

Avidin Localizations in pH-Responsive Polymersomes for Probing the Docking of Biotinylated (Macro)molecules in the Membrane and Lumen

To mimic organelles and cells and to construct next-generation therapeutics, asymmetric functionalization and location of proteins for artificial vesicles is thoroughly needed to emphasize the complex interplay of biological units and systems through spatially separated and spatiotemporal controlled actions, release, and communications. For the challenge of vesicle (= polymersome) construction, the membrane permeability and the location of the cargo are important key characteristics that determine their potential applications. Herein, an in situ and post loading process of avidin in pH-responsive and photo-cross-linked polymersomes is developed and characterized. First, loading efficiency, main location (inside, lumen, outside), and release of avidin under different conditions have been validated, including the pH-stable presence of avidin in polymersomes' membrane outside and inside. This advantageous approach allows us to selectively functionalize the outer and inner membranes as well as the lumen with several bio(macro)molecules, generally suited for the construction of asymmetrically functionalized artificial organelles. In addition, a fluorescence resonance energy transfer (FRET) effect was used to study the permeability or uptake of the polymersome membrane against a broad range of biotinylated (macro)molecules (different typology, sizes, and shapes) under different conditions.

The chemistry of cross-linked polymeric vesicles and their functionalization towards biocatalytic nanoreactors

Self-assembly of amphiphilic block copolymers into polymersomes continues to be a hot topic in modern research on biomimetics. Their well-known and valued mechanical strength can be increased even further if they are cross-linked. These additional bonds prevent a collapse or disassembly of the polymersomes and open the way towards smart nanoreactors. A variety of chemistries have been applied to obtain the desired cross-linked polymersomes, and therefore, the chemical approaches performed over time will be highlighted in this mini-review. Due to the large number of studies, a selected set of photo-cross-linked and pH-sensitive polymersomes will be specifically highlighted. This system has proven to be a very potent candidate for the formation of nanoreactors and drug delivery systems, and even for the formation of functional multicompartment cell mimics.

Reconstitution properties of biologically active polymersomes after cryogenic freezing and a freeze-drying process

Reconstitution of biologically active polymersomes from the frozen or solid state into any fluid state is still a challenging issue for the design of new biological experiments and for the formulation of therapeutic agents. To gain knowledge about the reconstitution of pH-responsive and photo-crosslinked polymersomes, surface-functionalized and enzyme-containing polymersomers were cryogenically frozen (−20 °C) or freeze-dried with inulin as the lyoprotectant (0.1% w/v) and stored for a defined time period. Reconstituting those polymersomes in solution by thawing or a re-dispersing process revealed their original physical properties as well as their function as a pH-switchable enzymatic nanoreactor.

Polymersomes can retain their physico-chemical properties and membrane permeability for enzymatic reactions after lyophilization or cryogenic freezing and storage.

Liposomes and polymersomes: a comparative review towards cell mimicking

Minimal cells: we compare and contrast liposomes and polymersomes for a bettera priorichoice and design of vesicles and try to understand the advantages and shortcomings associated with using one or the other in many different aspects (properties, synthesis, self-assembly, applications).