Efficient Fusion of Liposomes by Nucleobase Quadruple-Anchored DNA

Programmable RNA Loading of Extracellular Vesicles with Toehold-Release Purification

Synthetic nanoparticles as lipid nanoparticles (LNPs) are widely used as drug delivery vesicles. However, they hold several drawbacks, including low biocompatibility and unfavorable immune responses. Naturally occurring extracellular vesicles (EVs) hold the potential as native, safe, and multifunctional nanovesicle carriers. However, loading of EVs with large biomolecules remains a challenge. Here, we present a controlled loading methodology using DNA-mediated and programmed fusion between EVs and messenger RNA (mRNA)-loaded liposomes. The fusion efficiency is characterized at the single-particle level by real-time microscopy through EV surface immobilization via lipidated biotin-DNA handles. Subsequently, fused EV-liposome particles (EVLs) can be collected by employing a DNA strand-replacement reaction. Transferring the fusion reaction to magnetic beads enables us to scale up the production of EVLs one million times. Finally, we demonstrated encapsulation of mCherry mRNA, transfection, and improved translation using the EVLs compared to liposomes or LNPs in HEK293-H cells. We envision this as an important tool for the EV-mediated delivery of RNA therapeutics.

DNA-Programmed Lipid Nanoreactors for Synthesis of Carbohydrate Mimetics by Fusion of Aqueous Sub-attoliter Compartments

Lipid nanoreactors are biomimetic reaction vessels (nanoreactors) that can host aqueous or membrane-associated chemical and enzymatic reactions. Nanoreactors provide ultra-miniaturization from atto- to zeptoliter volumes per reaction vessel with the major challenge of encoding and spatio-temporal control over reactions at the individual nanoreactor or population level, thereby controlling volumes several orders of magnitude below advanced microfluidic devices. We present DNA-programmed lipid nanoreactors (PLNs) functionalized with lipidated oligonucleotides (LiNAs) that allow programming and encoding of nanoreactor interactions by controlled membrane fusion, exemplified for a set of carbohydrate mimetics with mono- to hexasaccharide azide building blocks connected by click-chemistry. Programmed reactions are initiated by fusion of distinct populations of nanoreactors with individually encapsulated building blocks. A focused library of triazole-linked carbohydrate-Cy5 conjugates formed by strain-promoted azide-alkyne cycloadditions demonstrated LiNA-programmed chemistry, including two-step reaction schemes. The PLN method is developed toward a robust platform for synthesis in confined space employing fully programmable nanoreactors, applicable to multistep synthesis for the generation of combinatorial libraries with subsequent analysis of the molecules formed, based on the addressability of the lipid nanoreactors.

Nucleic acid amphiphiles: Synthesis, properties, and applications

Nucleic acid amphiphiles, referring to nucleic acids modified with large hydrophobic groups, have been widely used in programmable bioengineering. Since nucleic acids are intrinsically hydrophilic, the hydrophobic groups endow nucleic acid amphiphiles with unique properties, such as self-assembling, interactions with artificial or biological membranes, and transmembrane transport. Importantly, the hybridization or target binding capability of oligonucleotide itself supplies nucleic acid amphiphiles with excellent programmability. As a result, this type of molecule has attracted considerable attention in academic studies and has enormous potential for further applications. For a comprehensive understanding of nucleic acid amphiphiles, we review the reported research on nucleic acid amphiphiles from their molecular design to final applications, in which we summarize the synthetic strategies for nucleic acid amphiphiles and draw much attention to their unique properties in different contexts. Finally, a summary of the applications of nucleic acid amphiphiles in drug development, bioengineering, and bioanalysis are critically discussed.

Label-free observation of DNA-encoded liposome fusion by surface plasmon resonance

Assembly and fusion between different populations of lipid nanoparticles was mediated by membrane-anchored lipidated nucleic acid (LiNA) strands and observed using surface plasmon resonance (SPR) as a label-free real-time assay. Irreversible membrane fusion was distinguished from reversible assembly by enzymatical cleavage of dsDNA tethers in situ. The assay enables user-friendly monitoring and application of membrane fusion in the context of liposomal drug delivery or synthetic biology.

Dynamic Fusion of Nucleic Acid Functionalized Nano-/Micro-Cell-Like Containments: From Basic Concepts to Applications

Membrane fusion processes play key roles in biological transformations, such as endocytosis/exocytosis, signal transduction, neurotransmission, or viral infections, and substantial research efforts have been directed to emulate these functions by artificial means. The recognition and dynamic reconfiguration properties of nucleic acids provide a versatile means to induce membrane fusion. Here we address recent advances in the functionalization of liposomes or membranes with structurally engineered lipidated nucleic acids guiding the fusion of cell-like containments, and the biophysical and chemical parameters controlling the fusion of the liposomes will be discussed. Intermembrane bridging by duplex or triplex nucleic acids and light-induced activation of membrane-associated nucleic acid constituents provide the means for spatiotemporal fusion of liposomes or nucleic acid modified liposome fusion with native cell membranes. The membrane fusion processes lead to exchange of loads in the fused containments and are a means to integrate functional assemblies. This is exemplified with the operation of biocatalytic cascades and dynamic DNA polymerization/nicking or transcription machineries in fused protocell systems. Membrane fusion processes of protocell assemblies are found to have important drug-delivery, therapeutic, sensing, and biocatalytic applications. The future challenges and perspectives of DNA-guided fused containments and membranes are addressed.

Lipid-linked nucleoside triphosphates for enzymatic synthesis of hydrophobic oligonucleotides with enhanced membrane anchoring efficiency

We designed and synthesized a series of 2'-deoxyribonucleoside triphosphates (dNTPs) bearing various lipid moieties. Fatty acid- and cholesterol-modified dNTPs proved to be substrates for KOD XL DNA polymerase in primer extension reactions. They were also mutually compatible for simultaneous multiple incorporations into the DNA strand. The methodology of enzymatic synthesis opened a pathway to diverse structurally unique lipid-ON probes containing one or more lipid units. We studied interactions of such probes with the plasma membranes of live cells. Employing a rational design, we found a series of lipid-ONs with enhanced membrane anchoring efficiency. The in-membrane stability of multiply modified ONs was superior to that of commonly studied ON analogues, in which a single cholesterol molecule is typically tethered to the thread end. Notably, some of the probes were detected at the cell surface even after 24 h upon removal of the probe solution. Such an effect was general to several studied cell lines.

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.

DNA-induced fusion between lipid domains of peptide-lipid hybrid vesicles

Peptide-lipid hybrid vesicles were prepared with complementary DNA strands in their lipid domains. Hybridization of the complementary DNA strands induced the controlled fusion of the vesicles during repeated heating and cooling cycles. Vesicle fusion was indicated by a decrease in the efficiency of Förster resonance energy transfer between lipid-localized probes (from 72 to 42%) and transmission electron microscopy analysis. We suggest that this approach is a general strategy for the creation of polymersomes with membrane-fusion functionality.

Modulating membrane fusion through the design of fusogenic DNA circuits and bilayer composition

Membrane fusion is a ubiquitous phenomenon linked to many biological processes, and represents a crucial step in liposome-based drug delivery strategies. The ability to control, ever more precisely, membrane fusion pathways would thus be highly valuable for next generation nano-medical solutions and, more generally, the design of advanced biomimetic systems such as synthetic cells. In this article, we present fusogenic nanostructures constructed from synthetic DNA which, different from previous solutions, unlock routes for modulating the rate of fusion and making it conditional to the presence of soluble DNA molecules, thus demonstrating how membrane fusion can be controlled through simple DNA-based molecular circuits. We then systematically explore the relationship between lipid-membrane composition, its biophysical properties, and measured fusion efficiency, linking our observations to the stability of transition states in the fusion pathway. Finally, we observe that specific lipid compositions lead to the emergence of complex bilayer architectures in the fusion products, such as nested morphologies, which are accompanied by alterations in biophysical behaviour. Our findings provide multiple, orthogonal strategies to program lipid-membrane fusion, which leverage the design of either the fusogenic DNA constructs or the physico/chemical properties of the membranes, and could thus be valuable in applications where some design parameters are constrained by other factors such as material cost and biocompatibility, as it is often the case in biotechnological applications.

Ionic Combisomes: A New Class of Biomimetic Vesicles to Fuse with Life

The construction of biomembranes that faithfully capture the properties and dynamic functions of cell membranes remains a challenge in the development of synthetic cells and their application. Here a new concept for synthetic cell membranes based on the self-assembly of amphiphilic comb polymers into vesicles, termed ionic combisomes (i-combisomes) is introduced. These combs consist of a polyzwitterionic backbone to which hydrophobic tails are linked by electrostatic interactions. Using a range of microscopies and molecular simulations, the self-assembly of a library of combs in water is screened. It is discovered that the hydrophobic tails form the membrane's core and force the backbone into a rod conformation with nematic-like ordering confined to the interface with water. This particular organization resulted in membranes that combine the stability of classic polymersomes with the biomimetic thickness, flexibility, and lateral mobility of liposomes. Such unparalleled matching of biophysical properties and the ability to locally reconfigure the molecular topology of its constituents enable the harboring of functional components of natural membranes and fusion with living bacteria to "hijack" their periphery. This provides an almost inexhaustible palette to design the chemical and biological makeup of the i-combisomes membrane resulting in a powerful platform for fundamental studies and technological applications.

Liposome fusion with orthogonal coiled coil peptides as fusogens: the efficacy of roleplaying peptides

Biological membrane fusion is a highly specific and coordinated process as a multitude of vesicular fusion events proceed simultaneously in a complex environment with minimal off-target delivery. In this study, we develop a liposomal fusion model system with specific recognition using lipidated derivatives of a set of four de novo designed heterodimeric coiled coil (CC) peptide pairs. Content mixing was only obtained between liposomes functionalized with complementary peptides, demonstrating both fusogenic activity of CC peptides and the specificity of this model system. The diverse peptide fusogens revealed important relationships between the fusogenic efficacy and the peptide characteristics. The fusion efficiency increased from 20% to 70% as affinity between complementary peptides decreased, (from K_F ≈ 10⁸ to 10⁴ M⁻¹), and fusion efficiency also increased due to more pronounced asymmetric role-playing of membrane interacting ‘K’ peptides and homodimer-forming ‘E’ peptides. Furthermore, a new and highly fusogenic CC pair (E₃/P1_K) was discovered, providing an orthogonal peptide triad with the fusogenic CC pairs P2_E/P2_K and P3_E/P3_K. This E₃/P1_k pair was revealed, via molecular dynamics simulations, to have a shifted heptad repeat that can accommodate mismatched asparagine residues. These results will have broad implications not only for the fundamental understanding of CC design and how asparagine residues can be accommodated within the hydrophobic core, but also for drug delivery systems by revealing the necessary interplay of efficient peptide fusogens and enabling the targeted delivery of different carrier vesicles at various peptide-functionalized locations.

We developed a liposomal fusion model system with specific recognition using a set of heterodimeric coiled coil peptide pairs. This study unravels important structure–fusogenic efficacy relationships of peptide fusogens.

Modulation of cell membrane functionalization with aggregates of oligodeoxynucleotides containing alkyl chain-modified uridines

In this study, we prepared oligodeoxynucleotides (ODNs) containing the uridine base modified by an alkyl chain at the 5-position (AU) and characterized their aggregate formation, localization, and functions in cells. These experiments revealed that aggregates of these ODNs were readily transported into cells, but their localization was dependent upon the number of hydrophobic units. ODNs with one modified AU were transported in the cytosol, while ODNs with multiple AU modifications resulted in their accumulation at the cell membrane. We also examined the ability of the AU-modified ODNs to capture small molecules at the cell membrane and their cellular uptake. We positioned a thioflavin-T (ThT)-binding aptamer on the cell membrane by means of hybridization with ODNs with three AUs at the strand end. Treatment with ThT resulted in its efficient uptake into cells, due to the capture of the ThT by the aptamers on the cell membrane. Thus, we demonstrated the functionalization of cell membranes with modified ODNs and the efficient delivery of small molecules into the cells.

Recent Progress of DNA Nanostructures on Amphiphilic Membranes

Employing DNA nanostructures mimicking membrane proteins on artificial amphiphilic membranes have been widely developed to understand the structures and functions of the natural membrane systems. In this review, the recent developments in artificial systems constructed by amphiphilic membranes and DNA nanostructures are summarized. First, the preparations and properties of the amphipathic bilayer models are introduced. Second, the interactions are discussed between the membrane and the DNA nanostructures, as well as their coassembly behaviors. Next, the alternative systems related to membrane protein-mediated signal transmission, selective distribution, transmembrane channels, and membrane fusion are also introduced. Moreover, the constructions of membrane skeleton protein-mimicking DNA nanostructures are also highlighted.

Recent Advances of DNA Nanostructure-Based Cell Membrane Engineering

Materials that can regulate the composition and structure of the cell membrane to fabricate engineered cells with defined functions are in high demand. Compared with other biomolecules, DNA has unique advantages in cell membrane engineering due to its excellent programmability and biocompatibility. Especially, the near-atomic scale precision of DNA nanostructures facilitates the investigation of structure-property relations on the cell membrane. In this review, first the state of the art of functional DNA nanostructures is summarized, and then the overview of the use of DNA nanostructures to engineer the cell membrane is presented. Subsequently, applications of DNA nanostructures in modifying cell membrane morphology, controlling ions transport, and synthesizing high precise liposomes are highlighted. Finally, the challenges and outlook on using DNA nanostructures for cell membrane engineering are discussed.

Lipid-Modified Peptide Nucleic Acids: Synthesis and Application to Programmable Liposome Fusion

Peptide nucleic acids (PNAs) can be modified with aliphatic lipid chains and designed to be water soluble and able to spontaneously insert into phospholipid bilayers. Liposomes with 1.5% negatively charged POPG can be driven to fuse and mix their inner content volumes via functionalization with such lipidated peptide nucleic acids (LiPNAs). During fusion, only low amounts of leakage occur (<5%). We describe here the synthesis and purification of such LiPNAs using an automated peptide synthesizer and the preparation of LiPNA functionalized liposomes. Further, we describe the measurement of LiPNA-induced fusion using a fluorescence-based assay for the content mixing between a liposome population with an encapsulated self-quenching fluorescent dye (SRB) and a buffer-filled liposome population.

Near-infrared light-activated membrane fusion for cancer cell therapeutic applications

The spatiotemporal stimulation of liposome-liposome or liposome-membrane fusion processes attracts growing interest as a means to mimic cell-cell interactions in nature and for using these processes for biomedical applications. We report the use of o-nitrobenzyl phosphate functionalized-cholesterol tethered nucleic acid-modified liposomes as functional photoresponsive units for inducing, by NIR-irradiation, spatiotemporal liposome-liposome or liposome-membrane fusion processes. The liposomes are loaded with upconversion nanoparticles (UCNPs) and their NIR irradiation (λ = 980 nm) yields luminescence at λ = 365 nm, providing a localized light-source to deprotect the o-nitrobenzyl phosphate groups and resulting in the fragmentation of the nucleic acid structures. In one system, the NIR-triggered fusion of two liposomes, L1 and L2, is exemplified. Liposome L1 is loaded with UCNPs and Tb3+ ions, and the liposome boundary is functionalized with a cholesterol-tethered, o-nitrobenzyl phosphate caged hairpin nucleic acid structure. Liposome L2 is loaded with 2,6-pyridinedicarboxylic acid, DPA, and its boundary is modified with a cholesterol-tethered nucleic acid, complementary to a part of the caged hairpin, associated with L1. NIR-irradiation of the L1/L2 mixture resulted in the photocleavage of the hairpin structure, associated with L1, and the resulting fragmented nucleic acid associated with L1 hybridized with the nucleic acid linked to L2, leading to the fusion of the two liposomes. The fusion process was followed by dynamic light scattering, and by monitoring the fluorescence of the Tb3+-DPA complex generated upon the fusion of the liposomes and their exchange of contents (fusion efficiency 30%). In a second system, the fusion of the liposomes L1, loaded with UCNPs and doxorubicin (DOX), with HeLa cancer cells functionalized with nucleic acid tethers, complementary to the hairpin units associated with the boundary of L1, and linked to the MUC-1 receptor sites associated with the HeLa cells, through a MUC-1 aptamer unit is exemplified. The effect of DOX-loaded L1/HeLa cell fusion on the cytotoxicity towards HeLa cells is addressed. The NIR UCNP-stimulated cleavage of the o-nitrobenzyl phosphate caged hairpin units associated with L1 leads to the fragmentation of the hairpin units and the resulting nucleic acid tethers hybridize with the nucleic acid-modified HeLa cells, resulting in the liposome-HeLa cell fusion and the release of DOX into the HeLa cells. Selective spatiotemporal cytotoxicity towards HeLa cells is demonstrated (ca. 40% cell killing within two days). The study presents a comprehensive stepwise set of experiments directed towards the development of NIR-driven liposome-liposome or liposome-membrane fusion processes.

Controlled Peptide-Mediated Vesicle Fusion Assessed by Simultaneous Dual-Colour Time-Lapsed Fluorescence Microscopy

We have employed a model system, inspired by SNARE proteins, to facilitate membrane fusion between Giant Unilamellar Vesicles (GUVs) and Large Unilamellar Vesicles (LUVs) under physiological conditions. In this system, two synthetic lipopeptide constructs comprising the coiled-coil heterodimer-forming peptides K₄, (KIAALKE)₄, or E₄, (EIAALEK)₄, a PEG spacer of variable length, and a cholesterol moiety to anchor the peptides into the liposome membrane replace the natural SNARE proteins. GUVs are functionalized with one of the lipopeptide constructs and the fusion process is triggered by adding LUVs bearing the complementary lipopeptide. Dual-colour time lapse fluorescence microscopy was used to visualize lipid- and content-mixing. Using conventional confocal microscopy, lipid mixing was observed on the lipid bilayer of individual GUVs. In addition to lipid-mixing, content-mixing assays showed a low efficiency due to clustering of K₄-functionalized LUVs on the GUVs target membranes. We showed that, through the use of the non-ionic surfactant Tween 20, content-mixing between GUVs and LUVs could be improved, meaning this system has the potential to be employed for drug delivery in biological systems.

DNA-Mediated Liposome Fusion Observed by Fluorescence Spectrometry

DNA-programmed and controlled fusion of lipid membranes have recently been optimized to reliably mix the contents between two populations of liposomes, each functionalized with complementary lipidated DNA (LiNA) oligomer. In this chapter we describe a procedure for DNA-controlled fusion of liposomes mediated by LiNAs that are designed to force bilayers into close proximity. Using a self-quenching fluorescent dye (Sulforhodamine B) to monitor both the mixing of the internal volumes and leakage of the dye into the outer volume we measure the efficiency of content mixing in the bulk population, allowing for direct comparison between different LiNA designs. By generating samples for calibration corresponding to different amounts of content mixing, the average number of fusion events per labeled liposome can be estimated.

Lipidated Polyaza Crown Ethers as Membrane Anchors for DNA-Controlled Content Mixing between Liposomes

The ability to manipulate and fuse nano-compartmentalized volumes addresses a demand for spatiotemporal control in the field of synthetic biology, for example in the bottom-up construction of (bio)chemical nanoreactors and for the interrogation of enzymatic reactions in confined space. Herein, we mix entrapped sub-attoliter volumes of liposomes (~135 nm diameter) via lipid bilayer fusion, facilitated by the hybridization of membrane-anchored lipidated oligonucleotides. We report on an improved synthesis of the membrane-anchor phosphoramidites that allows for a flexible choice of lipophilic moiety. Lipid-nucleic acid conjugates (LiNAs) with and without triethylene glycol spacers between anchor and the 17 nt binding sequence were synthesized and their fusogenic potential evaluated. A fluorescence-based content mixing assay was employed for kinetic monitoring of fusion of the bulk liposome populations at different temperatures. Data obtained at 50 °C indicated a quantitative conversion of the limiting liposome population into fused liposomes and an unprecedently high initial fusion rate was observed. For most conditions and designs only low leakage during fusion was observed. These results consolidate LiNA-mediated membrane fusion as a robust platform for programming compartmentalized chemical and enzymatic reactions.

Fast, Efficient, and Targeted Liposome Delivery Mediated by DNA Hybridization

Safety and efficacy, two significant parameters in drug administration, can be improved by site-specific delivery approaches. Here a fast, efficient, and targeted liposome delivery system steered by a DNA hybridization recognition mechanism is presented. For this purpose, lipid-terminated DNA is inserted in both liposome and cell membranes by simple mixing of the components. Cellular accumulation of cargo encapsulated in the liposomal core is substantially enhanced when the DNA sequence on the cell is complementary to that on the liposome. Additionally, in mixed cell populations, liposomes discriminate targets by their complementary DNA sequences. Exposure of cells to low temperature and endocytosis inhibitors suggests a caveolae-dependent endocytosis uptake pathway. Mechanistically, hybridization between DNA strands spatially traps liposomes and cell membranes in close proximity, consequently increases the local liposome concentration, and thereby enhances cellular uptake of liposomes and their payload. This programmable delivery system might contribute to new applications in molecular biology and drug delivery.

Interaction of SNARE Mimetic Peptides with Lipid bilayers: Effects of Secondary Structure, Bilayer Composition and Lipid Anchoring

The coiled-coil forming peptides 'K' enriched in lysine and 'E' enriched in glutamic acid have been used as a minimal SNARE mimetic system for membrane fusion. Here we describe atomistic molecular dynamics simulations to characterize the interactions of these peptides with lipid bilayers for two different compositions. For neutral phosphatidylcholine (PC)/phosphatidylethanolamine (PE) bilayers the peptides experience a strong repulsive barrier against adsorption, also observed in potential of mean force (PMF) profiles calculated with umbrella sampling. For peptide K, a minimum of -12 k_BT in the PMF provides an upper bound for the binding free energy whereas no stable membrane bound state could be observed for peptide E. In contrast, the electrostatic interactions with negatively charged phosphatidylglycerol (PG) lipids lead to fast adsorption of both peptides at the head-water interface. Experimental data using fluorescently labeled peptides confirm the stronger binding to PG containing bilayers. Lipid anchors have little effect on the peptide-bilayer interactions or peptide structure, when the peptide also binds to the bilayer in the absence of a lipid anchor. For peptide E, which does not bind to the PC bilayer without a lipid anchor, the presence of such an anchor strengthens the electrostatic interactions between the charged side chains and the zwitterionic head-groups and leads to a stabilization of the peptide's helical fold by the membrane.

DNA Nanotechnology Enters Cell Membranes

DNA is more than a carrier of genetic information: It is a highly versatile structural motif for the assembly of nanostructures, giving rise to a wide range of functionalities. In this regard, the structure programmability is the main advantage of DNA over peptides, proteins, and small molecules. DNA amphiphiles, in which DNA is covalently bound to synthetic hydrophobic moieties, allow interactions of DNA nanostructures with artificial lipid bilayers and cell membranes. These structures have seen rapid growth with great potential for medical applications. In this Review, the current state of the art of the synthesis of DNA amphiphiles and their assembly into nanostructures are first summarized. Next, an overview on the interaction of these DNA amphiphiles with membranes is provided, detailing on the driving forces and the stability of the interaction. Moreover, the interaction with cell surfaces in respect to therapeutics, biological sensing, and cell membrane engineering is highlighted. Finally, the challenges and an outlook on this promising class of DNA hybrid materials are discussed.

Influence of Membrane-Fusogen Distance on the Secondary Structure of Fusogenic Coiled Coil Peptides

Liposomal membrane fusion is an important tool to study complex biological fusion mechanisms. We use lipidated derivatives of the specific heterodimeric coiled coil pair E: (EIAALEK)3 and K: (KIAALKE)3 to study and control the fusion of liposomes. In this model system, peptides are tethered to their liposomes via a poly(ethylene glycol) (PEG) spacer and a lipid anchor. The efficiency of the fusion mechanism and function of the peptides is highly affected by the PEG-spacer length and the lipid anchor type. Here, the influence of membrane-fusogen distance on the peptide-membrane interactions and the peptide secondary structures is studied with Langmuir film balance and infrared reflection absorption spectroscopy. We found that the introduction of a spacer to monolayer-tethered peptide E changes its conformation from solvated random coils to homo-oligomers. In contrast, the described peptide-monolayer interaction of peptide K is not affected by the PEG-spacer length. Furthermore, the coexistence of different conformations when both lipopeptides E and K are present at the membrane surface is demonstrated empirically, which has many implications for the design of effective fusogenic recognition units and the field of artificial membrane fusion.

Performing DNA nanotechnology operations on a zebrafish

Nanoscale engineering of surfaces is becoming an indispensable technique to modify membranes and, thus cellular behaviour. Here, such membrane engineering related was explored on the surface of a living animal using DNA nanotechnology. We demonstrate the immobilization of oligonucleotides functionalized with a membrane anchor on 2 day old zebrafish. The protruding single-stranded DNA on the skin of zebrafish served as a handle for complementary DNAs, which allowed the attachment of small molecule cargo, liposomes and dynamic relabeling by DNA hybridization protocols. Robust anchoring of the oligonucleotides was proven as DNA-based amplification processes were successfully performed on the outer membrane of the zebrafish enabling the multiplication of surface functionalities from a single DNA-anchoring unit and the dramatic improvement of fluorescent labeling of these animals. As zebrafish are becoming an alternative to animal models in drug development, toxicology and nanoparticles characterization, we believe the platform presented here allows amalgamation of DNA nanotechnology tools with live animals and this opens up yet unexplored avenues like efficient bio-barcoding as well as in vivo tracking.

Amphiphilic DNA Organic Hybrids: Functional Materials in Nanoscience and Potential Application in Biomedicine

Due to the addressability and programmability, DNA has been applied not merely in constructing static elegant nanostructures such as two dimensional and three dimensional DNA nanostructures but also in designing dynamic nanodevices. Moreover, DNA could combine with hydrophobic organic molecules to be a new amphiphilic building block and then self-assemble into nanomaterials. Of particular note, a recent state-of-the-art research has turned our attention to the amphiphilic DNA organic hybrids including small molecule modified DNA (lipid-DNA, fluorescent molecule-DNA, etc.), DNA block copolymers, and DNA-dendron hybrids. This review focuses mainly on the development of their self-assembly behavior and their potential application in nanomaterial and biomedicine. The potential challenges regarding of the amphiphilic DNA organic hybrids are also briefly discussed, aiming to advance their practical applications in nanoscience and biomedicine.

Guiding protein delivery into live cells using DNA-programmed membrane fusion

Intracellular delivery of proteins provides a direct means to manipulate cell function and probe the intracellular environment. However, direct cytoplasmic delivery of proteins suffers from limited availability of efficient toolsets, and thus remains challenging in research and therapeutic applications. Natural biological cargo delivery processes, like SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) complex mediated membrane fusion and other vesicle fusion in live cells, enable targeted delivery with high efficiency. A surrogate of SNARE machinery represents a new direction in intracellular protein delivery. Here, we report a DNA-programmed membrane fusion strategy for guiding the efficient intracellular delivery of proteins into live cells. The inherent programmability of DNA hybridization provides spatiotemporal control of the fusion between protein-encapsulated liposomes and cell membranes, enabling rapid release of proteins directly into the cytoplasm, while still remaining functional due to the bypassing of the endosomal trap. We further demonstrate that delivered exogenous Cytochrome c effectively regulates the cell fate. Hence, this DNA-mediated fusion strategy holds great potential for protein drug delivery, regenerative medicine and gene editing.

Programmable fusion of liposomes mediated by lipidated PNA

We recently reported a DNA-programmed fusion cascade enabling the use of liposomes as nanoreactors for compartmentalized chemical reactions. This communication reports an alternative and robust strategy based on lipidated peptide nucleic acids (LiPs). LiPs enabled fusion of liposomes with remarkable 31% efficiency at 50 °C with low leakage (5%).

Peptide-Mediated Liposome Fusion: The Effect of Anchor Positioning

A minimal model system for membrane fusion, comprising two complementary peptides dubbed "E" and "K" joined to a cholesterol anchor via a polyethyleneglycol spacer, has previously been developed in our group. This system promotes the fusion of large unilamellar vesicles and facilitates liposome-cell fusion both in vitro and in vivo. Whilst several aspects of the system have previously been investigated to provide an insight as to how fusion is facilitated, anchor positioning has not yet been considered. In this study, the effects of placing the anchor at either the N-terminus or in the center of the peptide are investigated using a combination of circular dichroism spectroscopy, dynamic light scattering, and fluorescence assays. It was discovered that anchoring the "K" peptide in the center of the sequence had no effect on its structure, its ability to interact with membranes, or its ability to promote fusion, whereas anchoring the 'E' peptide in the middle of the sequence dramatically decreases fusion efficiency. We postulate that anchoring the 'E' peptide in the middle of the sequence disrupts its ability to form homodimers with peptides on the same membrane, leading to aggregation and content leakage.

Membrane-Fusogen Distance Is Critical for Efficient Coiled-Coil-Peptide-Mediated Liposome Fusion

We have developed a model system for membrane fusion that utilizes lipidated derivatives of a heterodimeric coiled-coil pair dubbed E3 (EIAALEK)3 and K3 (KIAALKE)3. In this system, peptides are conjugated to a lipid anchor via a poly(ethylene glycol) (PEG) spacer, and this contribution studies the influence of the PEG spacer length, coupled with the type of lipid anchor, on liposome-liposome fusion. The effects of these modifications on peptide secondary structure, their interactions with liposomes, and their ability to mediate fusion were studied using a variety of different content mixing experiments and CD spectroscopy. Our results demonstrate the asymmetric role of the peptides in the fusion process because alterations to the PEG spacer length affect E3 and K3 differently. We conclude that negatively charged E3 acts as a "handle" for positively charged K3 and facilitates liposome docking, the first stage of the fusion process, through coiled-coil formation. The efficacy of this E3 handle is enhanced by longer spacer lengths. K3 directs the fusion process via peptide-membrane interactions, but the length of the PEG spacer plays two competing roles: a PEG4/PEG8 spacer length is optimal for membrane destabilization; however, a PEG12 spacer increases the fusion efficiency over time by improving the peptide accessibility for successive fusion events. Both the anchor type and spacer length affect the peptide structure; a cholesterol anchor appears to enhance K3-membrane interactions and thus mediates fusion more efficiently.

Efficient liposome fusion mediated by lipid–nucleic acid conjugates

Highly efficient fusion and content mixing of liposomes encoded by lipidated oligonucleotides (LiNAs). “Hot fusion of biomembranes” – a low leakage process at elevated temperature.