Formation of geometrically complex lipid nanotube-vesicle networks of higher-order topologies

Phospholipid Membranes as Chemically and Functionally Tunable Materials

The sheet-like lipid bilayer is the fundamental structural component of all cell membranes. Its building blocks are phospholipids and cholesterol. Their amphiphilic structure spontaneously leads to the formation of a bilayer in aqueous environment. Lipids are not just structural elements. Individual lipid species, the lipid membrane structure, and lipid dynamics influence and regulate membrane protein function. An exciting field is emerging where the membrane-associated material properties of different bilayer systems are used in designing innovative solutions for widespread applications across various fields, such as the food industry, cosmetics, nano- and biomedicine, drug storage and delivery, biotechnology, nano- and biosensors, and computing. Here, the authors summarize what is known about how lipids determine the properties and functions of biological membranes and how this has been or can be translated into innovative applications. Based on recent progress in the understanding of membrane structure, dynamics, and physical properties, a perspective is provided on how membrane-controlled regulation of protein functions can extend current applications and even offer new applications.

Artificial Cells for Dissecting Exocytosis

The fusion of vesicles and exocytosis release of neurotransmitters into the extracellular space for detection and chemical signal decoding by neighboring cells is the key process in neuronal communication. It is important to understand what regulates exocytosis because the amount of neurotransmitters released into the synaptic cleft has a direct impact on brain function such as cognition learning and memory as well as on brain malfunctions. Much success in molecular biology can be credited for the existence of simplified model systems. Therefore, for gaining deeper insights into the details of exocytosis and what controls vesicle-mediated neurotransmission, functional artificial cells for exocytosis have been developed that can be used for studying various biophysical aspects and roles of molecules affecting exocytosis, which is difficult to study in living cells. Here, we describe the design and fabrication of specific artificial cell models and how chemical measurements at these cells can be implemented for probing dynamics of the exocytosis fusion pore and its effect on the regulation of neurochemical release. We introduce bottom-up synthetic methods for constructing model cells using protein-free giant unilamellar vesicles (GUV) as starting material, which allows further tuning of molecular complexity in a manner that is not possible in living cells and therefore can be used for dissecting the role of essential molecular components affecting the exocytosis process. The experimental setup uses microscopy video recording, micromanipulation and microelectroinjection techniques, and amperometry detection to study neurotransmitter release from these cells mimicking exocytosis.

Topological Connection between Vesicles and Nanotubes in Single-Molecule Lipid Membranes Driven by Head-Tail Interactions

Lipid nanotube-vesicle networks are important channels for intercellular communication and transport of matter. Experimentally observed in neighboring mammalian cells but also reproduced in model membrane systems, a broad consensus exists on their formation and stability. Lipid membranes must be composed of at least two molecular components, each stabilizing low (generally a phospholipid) and high curvatures. Strong anisotropy or enhanced conical shape of the second amphiphile is crucial for the formation of nanotunnels. Anisotropic driving forces generally favor nanotube protrusions from vesicles. In this work, we report the unique case of topologically connected nanotubes-vesicles obtained in the absence of directional forces, in single-molecule membranes, composed of an anisotropic bolaform glucolipid, above its melting temperature, T_m. Cryo-TEM and fluorescence confocal microscopy show the interconnection between vesicles and nanotubes in a single-phase region, between 60 and 90 °C under diluted conditions. Solid-state NMR demonstrates that the glucolipid can assume two distinct configurations, head-head and head-tail. These arrangements, seemingly of comparable energy above the T_m, could explain the existence and stability of the topologically connected vesicles and nanotubes, which are generally not observed for classical single-molecule phospholipid-based membranes above their T_m.

Reversible Deformation of Artificial Cell Colonies Triggered by Actin Polymerization for Muscle Behavior Mimicry

The use of artificial cells to mimic living tissues is beneficial for understanding the mechanism of interaction among cells. Artificial cells hold immense potential in the field of tissue engineering. Self-powered artificial cells capable of reversible deformation are developed by encapsulating living mitochondria, actins, and methylcellulose. Upon addition of pyruvate molecules, the mitochondria produce adenosine triphosphate (ATP), which acts as an energy source to trigger actin polymerization. The reversible deformation of artificial cells occurs with a spindle shape resulting from the polymerization of actins to form filaments adjacent to the lipid bilayer that subsequently returns to a spherical shape resulting from the depolymerization of actin filaments upon laser irradiation. The linear colonies composed of these artificial cells exhibit collective contraction and relaxation to mimic muscle tissues. At maximum contraction, the long axis of each giant unilamellar vesicle (GUV) is parallel to each other. All the colonies are synchronized in the contraction phase. The deformation of each GUV in the colonies is influenced by its adjacent GUVs. The muscle-like artificial cell colonies described here pave the way to develop sustainably self-powered artificial tissues.

Lipid nanotubes: Formation and applications

Lipids, the fundamental components of cell membrane, play important roles in the whole cycle of cell life, thus attracting worldwide attention, owing to their physicochemical property and extensive use in the applications based on lipid assemblies. Compared with liposomes, lipid nanotubes (LNTs) usually possess unique properties, such as highly ordered structure, precise molecular recognition, and the possibility of substance transport, thus providing more potential applications in different research fields. However, until now, there are still quite rare cases of LNTs successfully employed in practical applications. Bearing this in mind and based on our own experience in this field, we summarized and discussed the recent progress of the fabrication approaches and representative applications of the LNTs in the past decade, which would potentially provide basic understanding and guidance towards their future development.

Fabrication of Lipid Nanotubules by Ultrasonic Drag Force

This work reports a new method of fabricating lipid nanotubules using ultrasonic Stokes drag force in theory and experiment. Ultrasonic Stokes drag force generated using a planar piezoelectric ultrasonic transducer in a remotely controllable way is introduced. When ultrasonic Stokes drag force is applied on lipid vesicles, the lipid nanotubules attached can be dragged out from the lipid film. In order to demonstrate the formation mechanism of the lipid nanotubules produced by ultrasonic drag force clearly, a theoretical kinetic model is developed. In the experiments, the lipid nanotubules can be rapidly and efficiently fabricated using this ultrasonic transducer both in deionized water and NaCl solutions with different concentrations. The stretching speed of the lipid nanotubules can reach 33 μm/s, approximately 10 times faster than that of the existing methods. The formed lipid nanotubules have a diameter of 600 ± 100 nm (>80%). The length can reach the millimeter level. This work provided a remotely controllable, highly efficient, high-velocity, and solution environment-independent approach for fabricating lipid nanotubules.

Subcompartmentalization and Pseudo-Division of Model Protocells

Membrane enclosed intracellular compartments have been exclusively associated with the eukaryotes, represented by the highly compartmentalized last eukaryotic common ancestor. Recent evidence showing the presence of membranous compartments with specific functions in archaea and bacteria makes it conceivable that the last universal common ancestor and its hypothetical precursor, the protocell, may have exhibited compartmentalization. To the authors' knowledge, there are no experimental studies yet that have tested this hypothesis. They report on an autonomous subcompartmentalization mechanism for protocells which results in the transformation of initial subcompartments to daughter protocells. The process is solely determined by the fundamental materials properties and interfacial events, and does not require biological machinery or chemical energy supply. In the light of the authors' findings, it is proposed that similar events may have taken place under early Earth conditions, leading to the development of compartmentalized cells and potentially, primitive division.

Biological lipid nanotubes and their potential role in evolution

The membrane of cells and organelles are highly deformable fluid interfaces, and can take on a multitude of shapes. One distinctive and particularly interesting property of biological membranes is their ability to from long and uniform nanotubes. These nanoconduits are surprisingly omnipresent in all domains of life, from archaea, bacteria, to plants and mammals. Some of these tubes have been known for a century, while others were only recently discovered. Their designations are different in different branches of biology, e.g. they are called stromule in plants and tunneling nanotubes in mammals. The mechanical transformation of flat membranes to tubes involves typically a combination of membrane anchoring and external forces, leading to a pulling action that results in very rapid membrane nanotube formation - micrometer long tubes can form in a matter of seconds. Their radius is set by a mechanical balance of tension and bending forces. There also exists a large class of membrane nanotubes that form due to curvature inducing molecules. It seems plausible that nanotube formation and functionality in plants and animals may have been inherited from their bacterial ancestors during endosymbiotic evolution. Here we attempt to connect observations of nanotubes in different branches of biology, and outline their similarities and differences with the aim of providing a perspective on their joint functions and evolutionary origin.

Dissipative self-assembly, competition and inhibition in a self-reproducing protocell model

The bottom-up synthesis of artificial, life-like systems promises to enable the study of emergent properties distinctive to life. Here, we report protocell systems generated from phase-separated building blocks. Vesicle protocells self-reproduce through a phase-transfer mechanism, catalysing their own formation. Dissipative self-assembly by the protocells is achieved when a hydrolysis step to destroy the surfactant is introduced. Competition between micelle and vesicle based replicators for a common feedstock shows that environmental conditions can control what species predominates: under basic conditions vesicles predominate, but in a neutral medium micelles are selected for via a mechanism which inhibits vesicle formation. Finally, the protocells enable orthogonal reactivity by catalysing in situ formation of an amphiphilic organocatalyst, which after incorporation into the vesicle bilayer enantioselectively forms a secondary product.

Microfluidic chip with pillar arrays for controlled production and observation of lipid membrane nanotubes

Lipid membrane nanotubes (NTs) are a widespread template for in vitro studies of cellular processes happening at high membrane curvature. Traditionally NTs are manufactured one by one, using sophisticated membrane micromanipulations, while simplified methods for controlled batch production of NTs are in growing demand. Here we propose a lab-on-a-chip (LOC) approach to the simultaneous formation of multiple NTs with length and radius controlled by the chip design. The NTs form upon rolling silica microbeads covered by lipid lamellas over the pillars of a polymer micropillar array. The array's design and surface chemistry set the geometry of the resulting free-standing NTs. The integration of the array inside a microfluidic chamber further enables fast and turbulence-free addition of components, such as proteins, to multiple preformed NTs. This LOC approach to NT production is compatible with the use of high power objectives of a fluorescence microscope, making real-time quantification of the different modes of the protein activity in a single experiment possible.

Bacterial Quorum Sensing Signals Self-Assemble in Aqueous Media to Form Micelles and Vesicles: An Integrated Experimental and Molecular Dynamics Study

Many species of common bacteria communicate and coordinate group behaviors, including toxin production and surface fouling, through a process known as quorum sensing (QS). In Gram-negative bacteria, QS is regulated by N-acyl-l-homoserine lactones (AHLs) that possess a polar homoserine lactone headgroup and a nonpolar aliphatic tail. Past studies demonstrate that AHLs can aggregate in water or adsorb at interfaces, suggesting that molecular self-assembly could play a role in processes that govern bacterial communication. We used a combination of biophysical characterization and atomistic molecular dynamics (MD) simulations to characterize the self-assembly behaviors of 12 structurally related AHLs. We used static light scattering and measurements of surface tension to characterize the assembly of four naturally occurring AHLs (3-oxo-C8-AHL, 3-oxo-C12-AHL, C12-AHL, and C16-AHL) in aqueous media and determine their critical aggregation concentrations (CACs). MD simulations and alchemical free energy calculations were used to predict thermodynamically preferred aggregate structures for each AHL. Those calculations predicted that AHLs with 10 or 12 tail carbon atoms should form spherical micelles and that AHLs with 14 or 16 tail carbon atoms should form vesicles in solution. Characterization of solutions of AHLs using negative stain transmission electron microscopy (TEM) and dynamic light scattering (DLS) revealed aggregates with sizes consistent with spherical micelles or small unilamellar vesicles for 3-oxo-C12-AHL and C12-AHL and the formation of large vesicles (∼250 nm) in solutions of C16-AHL. These experimental findings are in general agreement with our simulation predictions. Overall, our results provide insight into processes of self-assembly that can occur in this class of bacterial amphiphiles and, more broadly, provide a potential basis for understanding how AHL structure could influence processes that bacteria use to drive important group behaviors.

Multicomponent and Multiphase Lipid Nanotubes Formed by Gliding Microtubule-Kinesin Motility and Phase-Separated Giant Unilamellar Vesicles

Cytoskeletal filaments and motor proteins are critical components in the transport and reorganization of membrane-based organelles in eukaryotic cells. Previous studies have recapitulated the microtubule-kinesin transport system in vitro to dynamically assemble large-scale nanotube networks from multilamellar liposomes and polymersomes. Moving toward more biologically relevant systems, the present work examines whether lipid nanotube (LNT) networks can be generated from giant unilamellar vesicles (GUVs) and subsequently characterizes how the lipid composition may be tuned to alter the dynamics, structure, and fluidity of networks. Here, we describe a two-step process in which microtubule motility (i) drives the transport and aggregation of GUVs to form structures with a decreased energy barrier for LNT formation and (ii) extrudes LNTs without destroying parent GUVs, allowing for the formation of large LNT networks. We further show that the lipid composition of the GUV influences formation and morphology of the extruded LNTs and associated networks. For example, LNTs formed from phase-separated GUVs (e.g., liquid-solid phase-separated and coexisting liquid-ordered and liquid-disordered phase-separated) display morphologies related to the specific phase behavior reflective of the parent GUVs. Overall, the ability to form nanotubes from compositionally complex vesicles opens the door to generating lipid networks that more closely mimic the structure and function of those found in cellular systems.

Physicochemical considerations for bottom-up synthetic biology

The bottom-up construction of synthetic cells from molecular components is arguably one of the most challenging areas of research in the life sciences. We review the impact of confining biological systems in synthetic vesicles. Complex cell-like systems require control of the internal pH, ionic strength, (macro)molecular crowding, redox state and metabolic energy conservation. These physicochemical parameters influence protein activity and need to be maintained within limits to ensure the system remains in steady-state. We present the physicochemical considerations for building synthetic cells with dimensions ranging from the smallest prokaryotes to eukaryotic cells.

Synthetic tissues

While significant advances have been achieved with non-living synthetic cells built from the bottom-up, less progress has been made with the fabrication of synthetic tissues built from such cells. Synthetic tissues comprise patterned three-dimensional (3D) collections of communicating compartments. They can include both biological and synthetic parts and may incorporate features that do more than merely mimic nature. 3D-printed materials based on droplet-interface bilayers are the basis of the most advanced synthetic tissues and are being developed for several applications, including the controlled release of therapeutic agents and the repair of damaged organs. Current goals include the ability to manipulate synthetic tissues by remote signaling and the formation of hybrid structures with fabricated or natural living tissues.

Combining patch-clamping and fluorescence microscopy for quantitative reconstitution of cellular membrane processes with Giant Suspended Bilayers

In vitro reconstitution and microscopic visualization of membrane processes is an indispensable source of information about a cellular function. Here we describe a conceptionally novel free-standing membrane template that facilitates such quantitative reconstitution of membrane remodelling at different scales. The Giant Suspended Bilayers (GSBs) spontaneously swell from lipid lamella reservoir deposited on microspheres. GSBs attached to the reservoir can be prepared from virtually any lipid composition following a fast procedure. Giant unilamellar vesicles can be further obtained by GSB detachment from the microspheres. The reservoir stabilizes GSB during deformations, mechanical micromanipulations, and fluorescence microscopy observations, while GSB-reservoir boundary enables the exchange of small solutes with GSB interior. These unique properties allow studying macro- and nano-scale membrane deformations, adding membrane-active compounds to both sides of GSB membrane and applying patch-clamp based approaches, thus making GSB a versatile tool for reconstitution and quantification of cellular membrane trafficking events.

Mechanics of the Formation, Interaction, and Evolution of Membrane Tubular Structures

Membrane nanotubes, also known as membrane tethers, play important functional roles in many cellular processes, such as trafficking and signaling. Although considerable progresses have been made in understanding the physics regulating the mechanical behaviors of individual membrane nanotubes, relatively little is known about the formation of multiple membrane nanotubes due to the rapid occurring process involving strong cooperative effects and complex configurational transitions. By exerting a pair of external extraction upon two separate membrane regions, here, we combine molecular dynamics simulations and theoretical analysis to investigate how the membrane nanotube formation and pulling behaviors are regulated by the separation between the pulling forces and how the membrane protrusions interact with each other. As the force separation increases, different membrane configurations are observed, including an individual tubular protrusion, a relatively less deformed protrusion with two nanotubes on its top forming a V shape, a Y-shaped configuration through nanotube coalescence via a zipper-like mechanism, and two weakly interacting tubular protrusions. The energy profile as a function of the separation is determined. Moreover, the directional flow of lipid molecules accompanying the membrane shape transition is analyzed. Our results provide new, to our knowledge, insights at a molecular level into the interaction between membrane protrusions and help in understanding the formation and evolution of intra- and intercellular membrane tubular networks involved in numerous cell activities.

Sculpting and fusing biomimetic vesicle networks using optical tweezers

Constructing higher-order vesicle assemblies has discipline-spanning potential from responsive soft-matter materials to artificial cell networks in synthetic biology. This potential is ultimately derived from the ability to compartmentalise and order chemical species in space. To unlock such applications, spatial organisation of vesicles in relation to one another must be controlled, and techniques to deliver cargo to compartments developed. Herein, we use optical tweezers to assemble, reconfigure and dismantle networks of cell-sized vesicles that, in different experimental scenarios, we engineer to exhibit several interesting properties. Vesicles are connected through double-bilayer junctions formed via electrostatically controlled adhesion. Chemically distinct vesicles are linked across length scales, from several nanometres to hundreds of micrometres, by axon-like tethers. In the former regime, patterning membranes with proteins and nanoparticles facilitates material exchange between compartments and enables laser-triggered vesicle merging. This allows us to mix and dilute content, and to initiate protein expression by delivering biomolecular reaction components.

Assembly of higher-order artificial vesicles can unlock new applications. Here, the authors use optical tweezers to construct user-defined 2D and 3D architectures of chemically distinct vesicles and demonstrate inter-vesicle communication and light-enabled compartment merging.

Electroformation of phospholipid giant unilamellar vesicles in physiological phosphate buffer

We report a protocol to prepare phospholipid Giant Unilamellar Vesicles (GUVs) by electroformation in PBS physiological buffer.

Superstructures with diverse morphologies and highly ordered fullerene C60 arrays from 1 : 1 and 2 : 1 adamantane-C60 hybrid molecules

Superstructures from fullerene C₆₀-containing compounds, especially those tethered to rigid functional groups with defined shapes, remain largely unexplored. Being the smallest diamondoid, adamantane (Ad) can be viewed as a promising building block for the construction of well-defined superstructures. Here, we report the syntheses of 1 : 1 (4a) and 2 : 1 (4b) Ad-C₆₀ hybrid molecules, which were then used to construct superstructures in binary solvent mixtures via a modified liquid/liquid interfacial precipitation (LLIP) method using CHCl₃ as a good solvent. Typically in the combination of DMSO/CHCl₃ with a final concentration (c_f) of 1.0 mmol L^-1, 4a successively forms spheres, plates, nanoflowers and plicated particles with increasing content of DMSO while 4b forms cuboid blocks and microparticles with hierarchically organized surfaces. Changing from DMSO to other poor solvents including acetone, MeOH and EtOAc leads to variations of the morphology of the superstructures for both 4a and 4b. At the nanometer length scale, 4a and 4b adopt different organizations within the superstructures. While 4a tends to self-organize into lamellae with highly ordered C₆₀ layers, the hexagonal phase is dominant in the superstructures formed by 4b. Wettability tests indicate that films formed by the superstructures of 4a and 4b show anti-wetting properties. Besides the solvent effect, the morphology of the superstructures can be also tuned by concentration. For example, when c_f is lowered to 0.5 mmol L^-1, a new form of superstructure, i.e., fibers, was detected for 4a. Our results also indicate that besides the solvent-induced aggregate transition, gravity-induced sedimentation and subsequent structure ripening can have a significant influence on the final morphology of the superstructures and the aggregate transition pathways.

Lipid vesicles in pulsed electric fields: Fundamental principles of the membrane response and its biomedical applications

The present review focuses on the effects of pulsed electric fields on lipid vesicles ranging from giant unilamellar vesicles (GUVs) to small unilamellar vesicles (SUVs), from both fundamental and applicative perspectives. Lipid vesicles are the most popular model membrane systems for studying biophysical and biological processes in living cells. Furthermore, as vesicles are made from biocompatible and biodegradable materials, they provide a strategy to create safe and functionalized drug delivery systems in health-care applications. Exposure of lipid vesicles to pulsed electric fields is a common physical method to transiently increase the permeability of the lipid membrane. This method, termed electroporation, has shown many advantages for delivering exogenous molecules including drugs and genetic material into vesicles and living cells. In addition, electroporation can be applied to induce fusion between vesicles and/or cells. First, we discuss in detail how research on cell-size GUVs as model cell systems has provided novel insight into the basic mechanisms of cell electroporation and associated phenomena. Afterwards, we continue with a thorough overview how electroporation and electrofusion have been used as versatile methods to manipulate vesicles of all sizes in different biomedical applications. We conclude by summarizing the open questions in the field of electroporation and possible future directions for vesicles in the biomedical field.

Engineering Compartmentalized Biomimetic Micro- and Nanocontainers

Compartmentalization of biological content and function is a key architectural feature in biology, where membrane bound micro- and nanocompartments are used for performing a host of highly specialized and tightly regulated biological functions. The benefit of compartmentalization as a design principle is behind its ubiquity in cells and has led to it being a central engineering theme in construction of artificial cell-like systems. In this review, we discuss the attractions of designing compartmentalized membrane-bound constructs and review a range of biomimetic membrane architectures that span length scales, focusing on lipid-based structures but also addressing polymer-based and hybrid approaches. These include nested vesicles, multicompartment vesicles, large-scale vesicle networks, as well as droplet interface bilayers, and double-emulsion multiphase systems (multisomes). We outline key examples of how such structures have been functionalized with biological and synthetic machinery, for example, to manufacture and deliver drugs and metabolic compounds, to replicate intracellular signaling cascades, and to demonstrate collective behaviors as minimal tissue constructs. Particular emphasis is placed on the applications of these architectures and the state-of-the-art microfluidic engineering required to fabricate, functionalize, and precisely assemble them. Finally, we outline the future directions of these technologies and highlight how they could be applied to engineer the next generation of cell models, therapeutic agents, and microreactors, together with the diverse applications in the emerging field of bottom-up synthetic biology.

Stationary patterns in star networks of bistable units: Theory and application to chemical reactions

We present theoretical and experimental studies on pattern formation with bistable dynamical units coupled in a star network configuration. By applying a localized perturbation to the central or the peripheral elements, we demonstrate the subsequent spreading, pinning, or retraction of the activations; such analysis enables the characterization of the formation of stationary patterns of localized activity. The results are interpreted with a theoretical analysis of a simplified bistable reaction-diffusion model. Weak coupling results in trivial pinned states where the activation cannot propagate. At strong coupling, a uniform state is expected with active or inactive elements at small or large degree networks, respectively. A nontrivial stationary spatial pattern, corresponding to an activation pinning, is predicted to occur at an intermediate number of peripheral elements and at intermediate coupling strengths, where the central activation of the network is pinned, but the peripheral activation propagates toward the center. The results are confirmed in experiments with star networks of bistable electrochemical reactions. The experiments confirm the existence of the stationary spatial patterns and the dependence of coupling strength on the number of peripheral elements for transitions between pinned and retreating or spreading fronts in forced network configurations (where the central or periphery elements are forced to maintain their states).

Sphere-To-Tube Transition toward Nanotube Formation: A Universal Route by Inverse Plateau-Rayleigh Instability

Nanotube formation in low-temperature solution has attracted intense interest since the 1990s. How to disclose the in-depth physicochemical nature of nanotubes and pursue new available chemical strategies is still highly desirable but remains a challenge. Here, we report that sphere-to-tube transition triggered by inverse Plateau-Rayleigh instability can be a chemical route for scalable production of nanotubes. As a proof of concept, formation of a phosphorus nitride (PN) nanotube and various hierarchical nanotube architectures by coalescence of the PN hollow spheres is achieved under systematic solvothermal reaction. The combination of theoretical analysis and dynamic simulation elucidates that the inverse Plateau-Rayleigh instability driven by the competition between curvature elasticity and surface energy is responsible for the PN nanotube formation observed in experiments. We anticipate that the sphere-to-tube transition provides a paradigm for nanotube synthesis for practical applications.

Spontaneous formation of nanometer scale tubular vesicles in aqueous mixtures of lipid and block copolymer amphiphiles

Many common amphiphiles self-assemble in water to produce heterogeneous populations of discrete and symmetric but polydisperse and multilamellar vesicles isolating the encapsulated aqueous core from the surrounding bulk. But when mixtures of amphiphiles of vastly different elastic properties co-assemble, their non-uniform molecular organization can stabilize lower symmetries and produce novel shapes. Here, using high resolution electron cryomicroscopy and tomography, we identify the spontaneous formation of a membrane morphology consisting of unilamellar tubular vesicles in dilute aqueous solutions of binary mixtures of two different amphiphiles of vastly different origins. Our results show that aqueous phase mixtures of a fluid-phase phospholipid and an amphiphilic block copolymer spontaneously assume a bimodal polymorphic character in a composition dependent manner: over a broad range of compositions (15-85 mol% polymer component), a tubular morphology co-exists with spherical vesicles. Strikingly, in the vicinity of equimolar compositions, an exclusively tubular morphology (L_t; diameter, ∼15 nm; length, >1 μm; core, ∼2.0 nm; wall, ∼5-6 nm) emerges in an apparent steady state. Theory suggests that the spontaneous stabilization of cylindrical vesicles, unaided by extraneous forces, requires a significant spontaneous bilayer curvature, which in turn necessitates a strongly asymmetric membrane composition. We confirm that such dramatic compositional asymmetry is indeed produced spontaneously in aqueous mixtures of a lipid and polymer through two independent biochemical assays - (1) reduction in the quenching of fluorophore-labeled lipids and (2) inhibition in the activity of externally added lipid-hydrolyzing phospholipase A2, resulting in a significant enrichment of the polymer component in the outer leaflet. Taken together, these results illustrate the coupling of the membrane shape with local composition through spontaneous curvature generation under conditions of asymmetric distribution of mixtures of disparate amphiphiles.

Dynamic assembly of polymer nanotube networks via kinesin powered microtubule filaments

We describe for the first time how biological nanomotors may be used to actively self-assemble mesoscale networks composed of diblock copolymer nanotubes. The collective force generated by multiple kinesin nanomotors acting on a microtubule filament is large enough to overcome the energy barrier required to extract nanotubes from polymer vesicles comprised of poly(ethylene oxide-b-butadiene) in spite of the higher force requirements relative to extracting nanotubes from lipid vesicles. Nevertheless, large-scale polymer networks were dynamically assembled by the motors. These networks displayed enhanced robustness, persisting more than 24 h post-assembly (compared to 4-5 h for corresponding lipid networks). The transport of materials in and on the polymer membranes differs substantially from the transport on analogous lipid networks. Specifically, our data suggest that polymer mobility in nanotubular structures is considerably different from planar or 3D structures, and is stunted by 1D confinement of the polymer subunits. Moreover, quantum dots adsorbed onto polymer nanotubes are completely immobile, which is related to this 1D confinement effect and is in stark contrast to the highly fluid transport observed on lipid tubules.

Pattern formation in multiplex networks

The advances in understanding complex networks have generated increasing interest in dynamical processes occurring on them. Pattern formation in activator-inhibitor systems has been studied in networks, revealing differences from the classical continuous media. Here we study pattern formation in a new framework, namely multiplex networks. These are systems where activator and inhibitor species occupy separate nodes in different layers. Species react across layers but diffuse only within their own layer of distinct network topology. This multiplicity generates heterogeneous patterns with significant differences from those observed in single-layer networks. Remarkably, diffusion-induced instability can occur even if the two species have the same mobility rates; condition which can never destabilize single-layer networks. The instability condition is revealed using perturbation theory and expressed by a combination of degrees in the different layers. Our theory demonstrates that the existence of such topology-driven instabilities is generic in multiplex networks, providing a new mechanism of pattern formation.

The engineering of artificial cellular nanosystems using synthetic biology approaches

Artificial cellular systems are minimal systems that mimic certain properties of natural cells, including signaling pathways, membranes, and metabolic pathways. These artificial cells (or protocells) can be constructed following a synthetic biology approach by assembling biomembranes, synthetic gene circuits, and cell-free expression systems. As artificial cells are built from bottom-up using minimal and a defined number of components, they are more amenable to predictive mathematical modeling and engineered controls when compared with natural cells. Indeed, artificial cells have been implemented as drug delivery machineries and in situ protein expression systems. Furthermore, artificial cells have been used as biomimetic systems to unveil new insights into functions of natural cells, which are otherwise difficult to investigate owing to their inherent complexity. It is our vision that the development of artificial cells would bring forth parallel advancements in synthetic biology, cell-free systems, and in vitro systems biology. For further resources related to this article, please visit the WIREs website. Conflict of interests: The authors declare that they have no competing financial interests.

Nano-pipette directed transport of nanotube transmembrane channels and hybrid vesicles

Using computational modeling, we simulate the interactions between a nanopipette and transmembrane, end-functionalized nanotubes that are localized within flat bilayers or nanoscopic vesicles. The functional groups (hairs) provide a "handle" for the moving pipette to controllably pick up and move the nanotubes to specific locations in the flat membrane, or the hybrid vesicle to specified regions on a surface. The ability to localize these hybrid vesicles on surfaces paves the way for creating nanoreactor arrays in fluidic devices.

Sedimentation-induced tether on a settling vesicle

Destabilization of soft interfaces into thin cylindrical filaments under external stresses is ubiquitous and is generally the first step toward breakup. We show that such filaments, called tethers, emerge from a vesicle subjected to gravity. Contrary to the pendant drop experiment, we demonstrate that the bending rigidity, a specific membrane property of vesicles, ensures the tethers reach a stationary state. Moreover, unlike point-like force experiments, we show that the family of shapes is continuous.

A continuous network of lipid nanotubes fabricated from the gliding motility of kinesin powered microtubule filaments

Synthetic interconnected lipid nanotube networks were fabricated on the millimeter scale based on the simple, cooperative interaction between phospholipid vesicles and kinesin-microtubule (MT) transport systems. More specifically, taxol-stabilized MTs, in constant 2D motion via surface absorbed kinesin, extracted and extended lipid nanotube networks from large Lα phase multilamellar liposomes (5-25 μm). Based on the properties of the inverted motility geometry, the total size of these nanofluidic networks was limited by MT surface density, molecular motor energy source (ATP), and total amount and physical properties of lipid source material. Interactions between MTs and extended lipid nanotubes resulted in bifurcation of the nanotubes and ultimately the generation of highly branched networks of fluidically connected nanotubes. The network bifurcation was easily tuned by changing the density of microtubules on the surface to increase or decrease the frequency of branching. The ability of these networks to capture nanomaterials at the membrane surface with high fidelity was subsequently demonstrated using quantum dots as a model system. The diffusive transport of quantum dots was also characterized with respect to using these nanotube networks for mass transport applications.

Cell-cell nanotubes: Tunneling through several types of synapses

Nanotube can be generally seen as a nanoscale cylindrical structure. Membrane (or tunneling) nanotube (TNT) is a cytoplasmic tunnel between two cells. Such direct cell-cell channel is used for a physical transport of biochemical cargo, whereas nanotubular networks between cells may be a novel principle of communicative and integrative biology. Recently, TNTs and their networks were discovered in plant cells and then they were reported also in animal cells. Just the reverse, a notion of plant synapse has been also proposed only recently, long after the corresponding notion of neuronal synapse in animals. However, both TNTs and synapses seem to be closely related and evolutionary conserved structures through different types of cells. Accordingly, this mini-review aims to demonstrate that TNTs may represent one of the deep functional similarities between neuronal, immune, viral and plant synapses.

Generation of phospholipid vesicle-nanotube networks and transport of molecules therein

We describe micromanipulation and microinjection procedures for the fabrication of soft-matter networks consisting of lipid bilayer nanotubes and surface-immobilized vesicles. These biomimetic membrane systems feature unique structural flexibility and expandability and, unlike solid-state microfluidic and nanofluidic devices prepared by top-down fabrication, they allow network designs with dynamic control over individual containers and interconnecting conduits. The fabrication is founded on self-assembly of phospholipid molecules, followed by micromanipulation operations, such as membrane electroporation and microinjection, to effect shape transformations of the membrane and create a series of interconnected compartments. Size and geometry of the network can be chosen according to its desired function. Membrane composition is controlled mainly during the self-assembly step, whereas the interior contents of individual containers is defined through a sequence of microneedle injections. Networks cannot be fabricated with other currently available methods of giant unilamellar vesicle preparation (large unilamellar vesicle fusion or electroformation). Described in detail are also three transport modes, which are suitable for moving water-soluble or membrane-bound small molecules, polymers, DNA, proteins and nanoparticles within the networks. The fabrication protocol requires ∼90 min, provided all necessary preparations are made in advance. The transport studies require an additional 60-120 min, depending on the transport regime.

Liposome and lipid bilayer arrays towards biosensing applications

Sensitive and selective biosensors for high-throughput screening are having an increasing impact in modern medical care. The establishment of robust protein biosensing platforms however remains challenging, especially when membrane proteins are involved. Although this type of proteins is of enormous relevance since they are considered in >60% of the pharmaceutical drug targets, their fragile nature (i.e., the requirement to preserve their natural lipid environment to avoid denaturation and loss of function) puts strong additional prerequisites onto a successful biochip. In this review, the leading approaches to create lipid membrane-based arrays towards the creation of membrane protein biosensing platforms are described. Liposomes assembled in micro- and nanoarrays and the successful set-ups containing functional membrane proteins, as well as the use of liposomes in networks, are discussed in the first part. Then, the complementary approaches to create cell-mimicking supported membrane patches on a substrate in an array format will be addressed. Finally, the progress in assembling free-standing (functional) lipid bilayers over nanopore arrays for ion channel sensing will be reported. This review illustrates the rapid pace by which advances are being made towards the creation of a heterogeneous biochip for the high-throughput screening of membrane proteins for diagnostics, drug screening, or drug discovery purposes.

Protein-membrane interactions: the virtue of minimal systems in systems biology

The plasma membrane of cells can be viewed as a highly dynamic, regulated, heterogeneous environment with multiple functions. It constitutes the boundary of the cell, encapsulating all its components. Proteins interact with the membrane in many ways to accommodate essential processes, such as membrane trafficking, membrane protrusions, cytokinesis, signaling, and cell-cell communication. A vast amount of literature has already fostered our current understanding of membrane-protein interactions. However, many phenomena still remain to be understood, e.g., the exact mechanisms of how certain proteins cause or assist membrane transformations. Systems biology aims to predict biological processes on the basis of the set of molecules involved. Many key processes arise from interactions with the lipid membrane. Protein interactome maps do not consider such specific interactions, and thus cannot predict precise outcomes of the interactions of the involved proteins. These can only be inferred from experimental approaches. We describe examples of how an emergent behavior of protein-membrane interactions has been demonstrated by the use of minimal systems. These studies contribute to a deeper understanding of protein interactomes involving membranes and complement other approaches of systems biology.

Intercellular nanotubes: insights from imaging studies and beyond

Cell-cell communication is critical to the development, maintenance, and function of multicellular organisms. Classical mechanisms for intercellular communication include secretion of molecules into the extracellular space and transport of small molecules through gap junctions. Recent reports suggest that cells also can communicate over long distances via a network of transient intercellular nanotubes. Such nanotubes have been shown to mediate intercellular transfer of organelles as well as membrane components and cytoplasmic molecules. Moreover, intercellular nanotubes have been observed in vivo and have been shown to enhance the transmission of pathogens such as human immunodeficiency virus (HIV)-1 and prions in vitro. These studies indicate that intercellular nanotubes may play a role both in normal physiology and in disease.

A method for heat-stimulated compression of poly(N-isopropyl acrylamide) hydrogels inside single giant unilamellar vesicles

In this letter, a hydrogel compression method for the facile generation of high polymer concentration within single giant unilamelar vesicles (GUV) is presented. A GUV with an internalized poly(N-isopropyl acrylamide) solution is heated above the lower critical solution temperature (LCST) until compartmentalization by collapsed hydrogel microaggregates occurs. Subsequently, the volume of the vesicle is decreased by reducing the area of the enclosing membrane. An increase in concentration by a factor of up to 11 is reached while the membrane remains intact, bringing the model system close to the living cell situation with its high concentration of proteins in the cytoplasm. The polymer-filled vesicles are also in a suitable density range to significantly improve the flow control performance of hydrogel valves embedded in soft matter nanofluidic devices.

Lipid-based nanotherapeutics for siRNA delivery

RNA interference (RNAi) is a specific gene-silencing mechanism triggered by small interfering RNA (siRNA). The application of RNAi in the clinic requires the development of safe and effective delivery systems. Inspired by progress with lipid-based systems in drug delivery, efforts have been dedicated to the development of liposomal siRNA delivery systems. Many of the lipid-based delivery vehicles self-assemble with siRNA through electrostatic interactions with charged amines, generating multi-lamellar lipoplexes with positively charged lipid bilayers separated from one another by sheets of negatively charged siRNA strands. Internalization of lipid-based siRNA delivery systems into cells typically occurs through endocytosis; accordingly, delivery requires materials that can facilitate endosomal escape. The size of the carrier is important as carriers <100 nm in diameter have been reported to have higher accumulation levels in tumours, hepatocytes and inflamed tissue, whereas larger particles tend to be taken up by Kupffer cells or other components of the reticuloendothelial system (RES). To reduce RES uptake and increase circulation time, carriers have been modified on the surface with hydrophilic materials, such as polyethyleneglycol. Herein, we review the molecular and structural parameters of lipid-based siRNA delivery systems.

Coupled growth and division of model protocell membranes

The generation of synthetic forms of cellular life requires solutions to the problem of how biological processes such as cyclic growth and division could emerge from purely physical and chemical systems. Small unilamellar fatty acid vesicles grow when fed with fatty acid micelles and can be forced to divide by extrusion, but this artificial division process results in significant loss of protocell contents during each division cycle. Here we describe a simple and efficient pathway for model protocell membrane growth and division. The growth of large multilamellar fatty acid vesicles fed with fatty acid micelles, in a solution where solute permeation across the membranes is slow, results in the transformation of initially spherical vesicles into long thread-like vesicles, a process driven by the transient imbalance between surface area and volume growth. Modest shear forces are then sufficient to cause the thread-like vesicles to divide into multiple daughter vesicles without loss of internal contents. In an environment of gentle shear, protocell growth and division are thus coupled processes. We show that model protocells can proceed through multiple cycles of reproduction. Encapsulated RNA molecules, representing a primitive genome, are distributed to the daughter vesicles. Our observations bring us closer to the laboratory synthesis of a complete protocell consisting of a self-replicating genome and a self-replicating membrane compartment. In addition, the robustness and simplicity of this pathway suggests that similar processes might have occurred under the prebiotic conditions of the early Earth.

Relaxation dynamics of fluid membranes

We study the effect of membrane viscosity in the dynamics of liquid membranes-possibly with free or internal boundaries-driven by conservative forces (curvature elasticity and line tension) and dragged by the bulk dissipation of the ambient fluid and the friction occurring when the amphiphilic molecules move relative to each other. To this end, we formulate a continuum model which includes a form of the governing equations for a two-dimensional viscous fluid moving on a curved, time-evolving surface. The effect of membrane viscosity has received very limited attention in previous continuum studies of the dynamics of fluid membranes, although recent coarse-grained discrete simulations suggest its importance. By applying our model to the study of vesiculation and membrane fusion in a simplified geometry, we conclude that membrane viscosity plays a dominant role in the relaxation dynamics of fluid membranes of sizes comparable to those found in eukaryotic cells, and is not negligible in many large synthetic systems of current interest.

Shape optimization in lipid nanotube networks

Starting from a high surface free-energy state, lipid nanotube networks are capable to self-organize into tree-like structures with particular geometrical features. In this work we analyze the process of self-organization in such networks, and report a strong similarity to the Euclidian Steiner Tree Problem (ESTP). ESTP is a well-known NP-hard optimization problem of finding a network connecting a given set of terminal points on a plane, allowing addition of auxiliary points, with the overall objective to minimize the total network length. The present study shows that aggregate lipid structures self-organize into geometries that correspond to locally optimal solutions to such problems.

Injection and transport of bacteria in nanotube-vesicle networks

The microinjection of bacteria (the MG1655 strain of E. coli) into unilamellar lipid vesicles contained in surface-immobilized nanotube-vesicle networks is demonstrated. The density of bacteria can be controlled from a single bacterium up to several thousands of bacteria per injected vesicle. The bacteria retain flagellar motion and propulsion. The bacteria (approximately 2 × 0.8 μm) cannot escape from one vesicle to another as the size of the nanotubes is too small (∼200 nm in diameter) to allow for entry. Bacteria can, however, be moved from one vesicle to another in a nanotube-vesicle network by using Marangoni flows. Thus, single or several species can be transferred to a neighboring vesicle at will. The technique offers new possibilities for live matter functionalization into synthetic host networks, and may provide means for studying the effect of compartmentalization and perfusion of chemical species on a single bacterium. Furthermore, it may serve as an experimental model to study how vesicle-encapsulated bacteria evade destruction in macrophages or how bacteria surf along thin membrane nanotubes toward connected macrophage cell bodies.