Cubic membrane structure in amoeba (Chaos carolinensis) mitochondria determined by electron microscopic tomography

The involvement of the mitochondrial membrane in drug delivery

Treatment effectiveness and biosafety are critical for disease therapy. Bio-membrane modification facilitates the homologous targeting of drugs in vivo by exploiting unique antibodies or antigens, thereby enhancing therapeutic efficacy while ensuring biosafety. To further enhance the precision of disease treatment, future research should shift focus from targeted cellular delivery to targeted subcellular delivery. As the cellular powerhouses, mitochondria play an indispensable role in cell growth and regulation and are closely involved in many diseases (e.g., cancer, cardiovascular, and neurodegenerative diseases). The double-layer membrane wrapped on the surface of mitochondria not only maintains the stability of their internal environment but also plays a crucial role in fundamental biological processes, such as energy generation, metabolite transport, and information communication. A growing body of evidence suggests that various diseases are tightly related to mitochondrial imbalance. Moreover, mitochondria-targeted strategies hold great potential to decrease therapeutic threshold dosage, minimize side effects, and promote the development of precision medicine. Herein, we introduce the structure and function of mitochondrial membranes, summarize and discuss the important role of mitochondrial membrane-targeting materials in disease diagnosis/treatment, and expound the advantages of mitochondrial membrane-assisted drug delivery for disease diagnosis, treatment, and biosafety. This review helps readers understand mitochondria-targeted therapies and promotes the application of mitochondrial membranes in drug delivery. STATEMENT OF SIGNIFICANCE: Bio-membrane modification facilitates the homologous targeting of drugs in vivo by exploiting unique antibodies or antigens, thereby enhancing therapeutic efficacy while ensuring biosafety. Compared to cell-targeted treatment, targeting of mitochondria for drug delivery offers higher efficiency and improved biosafety and will promote the development of precision medicine. As a natural material, the mitochondrial membrane exhibits excellent biocompatibility and can serve as a carrier for mitochondria-targeted delivery. This review provides an overview of the structure and function of mitochondrial membranes and explores the potential benefits of utilizing mitochondrial membrane-assisted drug delivery for disease treatment and biosafety. The aim of this review is to enhance readers' comprehension of mitochondrial targeted therapy and to advance the utilization of mitochondrial membrane in drug delivery.

Assembly-driven protection from hydrolysis as key selective force during chemical evolution

The origins of biopolymers pose fascinating questions in prebiotic chemistry. The marvelous assembly proficiencies of biopolymers suggest they are winners of a competitive evolutionary process. Sophisticated molecular assembly is ubiquitous in life where it is often emergent upon polymerization. We focus on the influence of molecular assembly on hydrolysis rates in aqueous media and suggest that assembly was crucial for biopolymer selection. In this model, incremental enrichment of some molecular species during chemical evolution was partially driven by the interplay of kinetics of synthesis and hydrolysis. We document a general attenuation of hydrolysis by assembly (i.e., recalcitrance) for all universal biopolymers and highlight the likely role of assembly in the survival of the 'fittest' molecules during chemical evolution.

Prismatic Block Copolymer Hexosomes

Cubosomes and hexosomes are recent solution morphologies with an ordered porous structure and are observed for lipids and amphiphilic block copolymers (BCPs) with high hydrophobic fractions. Whereas lipid hexosomes typically exhibit a prismatic shape, BCP hexosomes have so far only been observed as closed microspheres where inner channels are not connected to the surrounding medium. Here, we describe the formation of flat, prismatic BCP hexosomes with pronounced faceting and a highly ordered lattice of hexagonally packed channels. We assemble polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP or SV) into the hexosome framework using polystyrene-block-poly(4-vinylpyridine)-block-poly(tert-butyl methacrylate) (PS-b-P4VP-b-PT or SVT) as a macromolecular surfactant in low-χ solvents. During solvent exchange, SV-rich domains form through liquid-liquid phase separation, followed by solidification and confined assembly within these domains. Since the final solvent (acetone) has a very low χ parameter toward PS and P4VP (equaling low interfacial tension), solidification of the hexosome occurs under confinement conditions that we term "supersoft". The low interfacial tension allows the stabilization of the hexagonal-prismatic shape, which originates from the hexagonal lattice of channels. Increasing the interfacial tension with polar cosolvents at some point dominates the particle shape, resulting in deformation of prismatic BCP hexosomes into spinning-top structures. The use of low-χ solvents for confined assembly of BCPs may allow the formation of unusual particle shapes simply by tuning the polymer-solvent interaction.

SPIRE-a software tool for bicontinuous phase recognition: application for plastid cubic membranes

Bicontinuous membranes in cell organelles epitomize nature's ability to create complex functional nanostructures. Like their synthetic counterparts, these membranes are characterized by continuous membrane sheets draped onto topologically complex saddle-shaped surfaces with a periodic network-like structure. Their structure sizes, (around 50-500 nm), and fluid nature make transmission electron microscopy (TEM) the analysis method of choice to decipher their nanostructural features. Here we present a tool, Surface Projection Image Recognition Environment (SPIRE), to identify bicontinuous structures from TEM sections through interactive identification by comparison to mathematical "nodal surface" models. The prolamellar body (PLB) of plant etioplasts is a bicontinuous membrane structure with a key physiological role in chloroplast biogenesis. However, the determination of its spatial structural features has been held back by the lack of tools enabling the identification and quantitative analysis of symmetric membrane conformations. Using our SPIRE tool, we achieved a robust identification of the bicontinuous diamond surface as the dominant PLB geometry in angiosperm etioplasts in contrast to earlier long-standing assertions in the literature. Our data also provide insights into membrane storage capacities of PLBs with different volume proportions and hint at the limited role of a plastid ribosome localization directly inside the PLB grid for its proper functioning. This represents an important step in understanding their as yet elusive structure-function relationship.

Returning to the Fold for Lessons in Mitochondrial Crista Diversity and Evolution

Cristae are infoldings of the mitochondrial inner membrane jutting into the organelle's innermost compartment from narrow stems at their base called crista junctions. They are emblematic of aerobic mitochondria, being the fabric for the molecular machinery driving cellular respiration. Electron microscopy revealed that diverse eukaryotes possess cristae of different shapes. Yet, crista diversity has not been systematically examined in light of our current knowledge about eukaryotic evolution. Since crista form and function are intricately linked, we take a holistic view of factors that may underlie both crista diversity and the adherence of cristae to a recognizable form. Based on electron micrographs of 226 species from all major lineages, we propose a rational crista classification system that postulates cristae as variations of two general morphotypes: flat and tubulo-vesicular. The latter is most prevalent and likely ancestral, but both morphotypes are found interspersed throughout the eukaryotic tree. In contrast, crista junctions are remarkably conserved, supporting their proposed role as diffusion barriers that sequester cristae contents. Since cardiolipin, ATP synthase dimers, the MICOS complex, and dynamin-like Opa1/Mgm1 are known to be involved in shaping cristae, we examined their variation in the context of crista diversity. Moreover, we have identified both commonalities and differences that may collectively be manifested as diverse variations of crista form and function.

Bioimaging Applications of Non-Lamellar Liquid Crystalline Nanoparticles

Self-assembling processes of amphiphilic lipids in water give rise to complex architectures known as lyotropic liquid crystalline (LLC) phases. Particularly, bicontinuous cubic and hexagonal LLC phases can be dispersed in water forming colloidal nanoparticles respectively known as cubosomes and hexosomes. These non-lamellar LLC dispersions are of particular interest for pharmaceutical and biomedical applications as they are potentially non-toxic, chemically stable, and biocompatible, also allowing encapsulation of large amounts of drugs. Furthermore, conjugation of specific moieties enables their targeting, increasing therapeutic efficacies and reducing side effects by avoiding exposure of healthy tissues. In addition, as they can be easy loaded or functionalized with both hydrophobic and hydrophilic imaging probes, cubosomes and hexosomes can be used for the engineering of multifunctional/theranostic nanoplatforms. This review outlines recent advances in the applications of cubosomes and hexosomes for in vitro and in vivo bioimaging.

Effect of receiver shape and volume on the Alzheimer disease for molecular communication via diffusion

Nano-devices are featured to communicate via molecular interaction, the so-called molecular communication (MC). In MC systems, the information is carried by molecules where the amount of molecules constitutes the level of the signal. In this study, an MC-based system was analysed with different receiver topology and related parameters, such as size, shape, and orientation of receptors on the receiver. Also in the concept of nano-medicine, the effect of amyloid-beta ([inline-formula removed]), which is believed as the main cause of Alzheimer disease, on the successful reception ratio of molecules with the proposed receiver models was investigated. It was demonstrated that the cubic receiver model is superior to sphere one in terms of the correct reception ratio of the molecular signal. A cubic model where its edge (not rotated around the centre) is placed across the transmitter demonstrated a better performance in reducing the effect of [inline-formula removed] as compared to the sphere model while a cubic model where its corner (rotated around the centre) is placed across the transmitter demonstrated a worse performance than the spherical model. From this expression, it may be concluded that with the adjustment of topological system parameters the probability of successful reception ratio in MC may be possible.

Consequences of Folding the Mitochondrial Inner Membrane

A fundamental first step in the evolution of eukaryotes was infolding of the chemiosmotic membrane of the endosymbiont. This allowed the proto-eukaryote to amplify ATP generation while constraining the volume dedicated to energy production. In mitochondria, folding of the inner membrane has evolved into a highly regulated process that creates specialized compartments (cristae) tuned to optimize function. Internalizing the inner membrane also presents complications in terms of generating the folds and maintaining mitochondrial integrity in response to stresses. This review describes mechanisms that have evolved to regulate inner membrane topology and either preserve or (when appropriate) rupture the outer membrane.

A unique insight into the defect structures of bicontinuous mesophases in liquid crystals and hybrid materials

Han et al. [(2020), IUCrJ, 7, 228-237] using advanced electron microscopy and crystallographic modelling rationalise the microstructure of twinning defects in order to visualize mesophase transitions and surface properties of G and D bicontinuous cubic mesostructured silica. This work furthers our understanding of how these phases originate in many natural and synthetic systems.

Bicontinuous cubic phases in biological and artificial self-assembled systems

Nature has created innumerable life forms with miraculous hierarchical structures and morphologies that are optimized for different life events through evolution over billions of years. Bicontinuous cubic structures, which are often described by triply periodic minimal surfaces (TPMSs) and their constant mean curvature (CMC)/parallel surface companions, are of special interest to various research fields because of their complex form with unique physical functionalities. This has prompted the scientific community to fully understand the formation, structure, and properties of these materials. In this review, we summarize and discuss the formation mechanism and relationships of the relevant biological structures and the artificial self-assembly systems. These structures can be formed through biological processes with amazing regulation across a great length scales; nevertheless, artificial construction normally produces the structure corresponding to the molecular size and shape. Notably, the block copolymeric system is considered to be an applicable and attractive model system for the study of biological systems due to their versatile design and rich phase behavior. Some of the phenomena found in these two systems are compared and discussed, and this information may provide new ideas for a comprehensive understanding of the relationship between molecular shape and resulting interface curvature and the self-assembly process in living organisms. We argue that the co-polymeric system may serve as a model to understand these biological systems and could encourage additional studies of artificial self-assembly and the creation of new functional materials.

Cubosomes: The Next Generation of Smart Lipid Nanoparticles?

Cubosomes are highly stable nanoparticles formed from the lipid cubic phase and stabilized by a polymer based outer corona. Bicontinuous lipid cubic phases consist of a single lipid bilayer that forms a continuous periodic membrane lattice structure with pores formed by two interwoven water channels. Cubosome composition can be tuned to engineer pore sizes or include bioactive lipids, the polymer outer corona can be used for targeting and they are highly stable under physiological conditions. Compared to liposomes, the structure provides a significantly higher membrane surface area for loading of membrane proteins and small drug molecules. Owing to recent advances, they can be engineered in vitro in both bulk and nanoparticle formats with applications including drug delivery, membrane bioreactors, artificial cells, and biosensors. This review outlines recent advances in cubosome technology enabling their application and provides guidelines for the rational design of new systems for biomedical applications.

The stabilization of primitive bicontinuous cubic phases with tunable swelling over a wide composition range

In this paper we investigate the pseudo-ternary phase diagram of glycerol monooleate (GMO), a cationic lipid (DOTAP - 1,2-dioleoyl-3-trimethylammonium propane), and a "PEGylated" lipid (DOPE-PEG - 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethylene glycol)-2000 kDa]) in excess water. We use small angle X-ray scattering (SAXS) and cryogenic transmission electron microscopy (Cryo-EM) to map out a phase diagram in a regime of low DOPE-PEG content (1-5 mol%), which is pertinent for the application of lipid systems as carriers of biomolecular cargo to cells. Pure GMO is known to self-assemble into bicontinuous cubic phases of the gyroid type at low water content and of the diamond type in excess water. These complex structures have numerous advantages reaching beyond drug delivery, e.g. as protein crystallization matrices, but their formulation is challenging as very small contents of guest molecules can shift the phase behavior towards other geometries such as the lamellar phase. In this work, we show that the ternary GMO/DOTAP/DOPE-PEG system allows the stabilization of bicontinuous cubic phases in excess water over a wide composition range. The symmetry of the phase can be tuned by varying the amount of PEGylated lipid, with the primitive type dominating at low DOPE-PEG content (1-3 mol%) and the diamond phase arising at 5 mol% DOPE-PEG. In addition, we found that the diamond phase is virtually non-responsive to electrostatic swelling. In contrast, primitive bicontinuous cubic lattice dimensions swell up in equilibrium to 650 Å with increased cationic lipid content.

Cubic membrane formation supports cell survival of amoeba Chaos under starvation-induced stress

Cubic membranes (CM) are highly organized membrane structures found in biological systems. They are mathematically well defined and reveal a three-dimensional nano-periodic structure with cubic symmetry. These membrane arrangements are frequently induced in cells under stress, disease conditions, or upon viral infection. In this study, we investigated CM formation in the mitochondria of amoeba Chaos carolinense and observed a striking correlation between the organism's ability to generate CM and the cell survival under starvation. Since starvation also induces autophagy, rapamycin was used to pharmacologically induce autophagy, and interestingly, CM formation was observed in parallel. Conversely, inhibition of autophagy reverted the cubic mitochondrial inner membrane morphology to tubular structure. In starved Chaos cells, mitochondria and autophagosomes did not co-localize and ATP production was sustained. CM transition in the mitochondria during starvation or upon induction of autophagy might prevent their sequestration by autophagosomes, thus slowing their rate of degradation. Such sustained mitochondrial activity may allow amoeba Chaos cells to survive for a longer period upon starvation.

Evaluation of radical scavenging system in amoeba Chaos carolinense during nutrient deprivation

The frequent appearance of non-lamellar membrane arrangements such as cubic membranes (CMs) in cells under stressed or pathological conditions points to an intrinsic cellular response mechanism. CM represents highly curved, three-dimensional nano-periodic structures that correspond to mathematically well-defined triply periodic minimal surfaces. Specifically, cellular membrane may transform into CM organization in response to pathological, inflammatory and oxidative stress conditions. CM organization, thus, may provide an advantage to cope with various types of stress. The identification of inducible membrane systems, such as in the mitochondrial inner membranes to cubic morphology upon starvation, opens new avenues for understanding the molecular mechanisms of cellular responses to oxidative stress. In this study, we compared the cellular responses of starved and fed amoeba Chaos carolinense to oxidative stress. Food deprivation from C. carolinense induces a significant increase in prooxidants such as superoxide and hydrogen peroxide. Surprisingly, we observed a significant lower rate of biomolecular damage in starved cells (with higher free radicals generation) when compared with fed cells. Specifically, lipid and RNA damages were significantly less in starved cells compared with fed cells. This observation was not due to the upregulation of intracellular antioxidants, as starved amoeba show reduced antioxidant enzymatic activities; however, it could be attributed to CM formation. CM could uptake and retain short segments of nucleic acids (resembles cellular RNA) in vivo and in vitro. Previous results showed that nucleic acids retained within CM sustain a minimal oxidative damage in vitro upon exposure to high level of superoxide. We thus propose that CM may act as a 'protective' shelter to minimize the oxidation of biologically essential macromolecules such as RNA. In summary, we examined enzymatic antioxidant activities as well as oxidative damage biomarkers in starved amoeba C. carolinense in correlation with the potential role of CM as an optimal intracellular membrane organization for the protection of biological macromolecules against oxidative damage.

Diffusion of Polymers through Periodic Networks of Lipid-Based Nanochannels

We present an experimental investigation of the diffusion of unfolded polymers in the triply-periodic water-channel network of inverse bicontinuous cubic phases. Depending on the chain size, our results indicate the presence of two different dynamical regimes corresponding to Zimm and Rouse diffusion. We support our findings by scaling arguments based on a combination of blob and effective-medium theories and suggest the presence of a third regime where dynamics is driven by reptation. Our experimental results also show an increasing behavior of the partition coefficient as a function of the polymer molecular weight, indicative of a reduction in the conformational degrees of freedom induced by the confinement.

Function of CYP11A1 in the mitochondria

Steroids are synthesized from the adrenal glands and gonads by enzymes of the cytochromes P450 and hydroxysteroid dehydrogenase in nature. These enzymes are located in the membrane of endoplasmic reticulum and mitochondria to catalyze redox reactions using electrons transported from the membrane. In the mitochondria, steroidogenic enzymes are inserted into the inner membrane with the bulk of the protein facing the matrix. They are not only important for steroid biosynthesis, their presence also affects mitochondrial morphology. Mitochondria undergo constant fission and fusion; they play important roles in energy production, apoptosis, and metabolism. Their defects often lead to human diseases. Mitochondrial cristae are usually lamellar in shape, but can also assume different shapes. Cristae in the mitochondria of steroidogenic cells are tubular-vesicular in shape. This cristae shape is also related to the degree of steroidogenic cell differentiation. Steroidogenic enzymes in the mitochondria appear to have a dual role in shaping the morphology of mitochondria and in steroid production.

Dynamics of different steps of the photopyrolytic cycle of an eminent anticancer drug topotecan inside biocompatible lyotropic liquid crystalline systems

Dynamics of different steps of photopyrolytic processes of an eminent anticancer drug topotecan have been investigated inside different lyotropic liquid crystalline systems.

Phase Separation on Bicontinuous Cubic Membranes: Symmetry Breaking, Reentrant, and Domain Faceting

We study the phase separation of binary lipid mixtures that form bicontinuous cubic phases. The competition between the nonuniform Gaussian membrane curvature and line tension leads to a very rich phase diagram, where we observe symmetry breaking of the membrane morphologies and reentrant phenomena due to the formation of bridges between segregated domains. Upon increasing the line tension contribution, we also find faceting of lipid domains that we explain using a simple argument based on the symmetry of the underlying surface and topology.

Topological Influence of Lyotropic Liquid Crystalline Systems on Excited-State Proton Transfer Dynamics

In the present work, we have investigated the excited-state proton transfer (ESPT) dynamics inside lipid-based reverse hexagonal (HII), gyroid Ia3d, and diamond Pn3m LLC phases. Polarized light microscopy (PLM) and small-angle X-ray scattering (SAXS) techniques have been employed for the characterization of LLC systems. Time-resolved fluorescence results reveal the retarded ESPT dynamics inside liquid crystalline systems compared to bulk water, and it follows the order HII < Ia3d < Pn3m < H2O. The slower solvation, hampered "Grotthuss" proton transfer process, and most importantly, topological influence, of the LLC systems are believed to be mainly responsible for the slower and different extent of ESPT dynamics. Interestingly, recombination dynamics is found to be faster with respect to bulk water and it follows the order H2O < Pn3m < Ia3d < HII. Faster recombination dynamics arises due to lower dielectric constant and different channel diameters of these LLC systems. However, the dissociation dynamics is found to be slower than bulk water and it follows the order HII < Ia3d < Pn3m < H2O. Differences in critical packing parameter of LLC systems are believed to be the governing factors for the slower dissociation dynamics in these liquid crystalline systems.

Oxidation of Membrane Curvature-Regulating Phosphatidylethanolamine Lipid Results in Formation of Bilayer and Cubic Structures

Oxidation is associated with conditions related to chronic inflammations and aging. Cubic structures have been observed in the smooth endoplasmic reticulum and mitochondrial membranes of cells under oxidative stress (e.g., tumor cells and virus-infected cells). It has been previously suspected that oxidation can result in the rearrangement of lipids from a fluid lamellar phase to a cubic structure in organelles containing membranes enriched with amphiphiles that have nonzero intrinsic curvature, such as phosphatidylethanolamine (PE) and cardiolipin. This study focuses on the oxidation of 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE), a lipid that natively forms an inverted hexagonal phase at physiological conditions. The oxidized samples contain an approximately 3:2 molar ratio of nonoxidized to oxidized DOPE. Optical microscopy images collected during the hydration of this mixture from a dried film suggest that the system evolves into a coexistence of a stable fluid lamellar phase and transient square lattice structures with unit cell sizes of 500-600 nm. Small-angle X-ray scattering of the same lipid mixture yielded a body-centered Im3m cubic phase with the lattice parameter of 14.04 nm. On average, the effective packing parameter of the oxidized DOPE species was estimated to be 0.657 ± 0.069 (standard deviation). This suggests that the oxidation of PE leads to a group of species with inverted molecular intrinsic curvature. Oxidation can create amphiphilic subpopulations that potently impact the integrity of the membrane, since negative Gaussian curvature intrinsic to cubic phases can enable membrane destabilization processes.

Supramolecular Organization of Respiratory Complexes

Since the discovery of the existence of superassemblies between mitochondrial respiratory complexes, such superassemblies have been the object of a passionate debate. It is accepted that respiratory supercomplexes are structures that occur in vivo, although which superstructures are naturally occurring and what could be their functional role remain open questions. The main difficulty is to make compatible the existence of superassemblies with the corpus of data that drove the field to abandon the early understanding of the physical arrangement of the mitochondrial respiratory chain as a compact physical entity (the solid model). This review provides a nonexhaustive overview of the evolution of our understanding of the structural organization of the electron transport chain from the original idea of a compact organization to a view of freely moving complexes connected by electron carriers. Today supercomplexes are viewed not as a revival of the old solid model but rather as a refined revision of the fluid model, which incorporates a new layer of structural and functional complexity.

Vermistella arctica n. sp. Nominates the Genus Vermistella as a Candidate for Taxon with Bipolar Distribution

A new amoebozoan species, Vermistella arctica n. sp., is described from marine habitats in the central part of Svalbard archipelago. This is the first report on Arctic amoebae belonging to the genus Vermistella Moran and Anderson, 2007, the type species of which was described from the opposite pole of the planet. Psychrophily proved in the new strains qualifies the genus Vermistella as a bipolar taxon. Molecular phylogenetic analyses based on 18S rDNA and actin sequences did not show any affinity of the genus Vermistella to Stygamoeba regulata ATCC(®) 50892(™) strain. A close phylogenetic relationship was found between Vermistella spp. and a sequence originating from an environmental sample from Cariaco basin, the largest marine permanently anoxic system in the world. Possible mechanisms of bipolar distribution are discussed.

Equilibrium shapes of planar elastic membranes

Using a rod theory formulation, we derive equations of state for a thin elastic membrane subjected to several different boundary conditions-clamped, simply supported, and periodic. The former is applicable to membranes supported on a softer substrate and subjected to uniaxial compression. We show that a wider family of quasistatic equilibrium shapes exist beyond the previously obtained analytical solutions. In the latter case of periodic membranes, we were able to derive exact solutions in terms of elliptic functions. These equilibria are verified by considering a fluid-structure interaction problem of a periodic, length-preserving bilipid membrane modeled by the Helfrich energy immersed in a viscous fluid. Starting from an arbitrary shape, the membrane dynamics to equilibrium are simulated using a boundary integral method.

Structural Diversity of Arthropod Biophotonic Nanostructures Spans Amphiphilic Phase-Space

Many organisms, especially arthropods, produce vivid interference colors using diverse mesoscopic (100-350 nm) integumentary biophotonic nanostructures that are increasingly being investigated for technological applications. Despite a century of interest, precise structural knowledge of many biophotonic nanostructures and the mechanisms controlling their development remain tentative, when such knowledge can open novel biomimetic routes to facilely self-assemble tunable, multifunctional materials. Here, we use synchrotron small-angle X-ray scattering and electron microscopy to characterize the photonic nanostructure of 140 integumentary scales and setae from ∼127 species of terrestrial arthropods in 85 genera from 5 orders. We report a rich nanostructural diversity, including triply periodic bicontinuous networks, close-packed spheres, inverse columnar, perforated lamellar, and disordered spongelike morphologies, commonly observed as stable phases of amphiphilic surfactants, block copolymer, and lyotropic lipid-water systems. Diverse arthropod lineages appear to have independently evolved to utilize the self-assembly of infolding lipid-bilayer membranes to develop biophotonic nanostructures that span the phase-space of amphiphilic morphologies, but at optical length scales.

Ancient homology of the mitochondrial contact site and cristae organizing system points to an endosymbiotic origin of mitochondrial cristae

Mitochondria are eukaryotic organelles that originated from an endosymbiotic α-proteobacterium. As an adaptation to maximize ATP production through oxidative phosphorylation, mitochondria contain inner membrane invaginations called cristae. Recent work has characterized a multi-protein complex in yeast and animal mitochondria called MICOS (mitochondrial contact site and cristae organizing system), responsible for the determination and maintenance of cristae [1-4]. However, the origin and evolution of these characteristic mitochondrial features remain obscure. We therefore conducted a comprehensive search for MICOS components across the major groups that encompass eukaryotic diversity to determine the extent of conservation of this complex. We detected homologs for the majority of MICOS components among opisthokonts (the group containing animals and fungi), but only Mic60 and Mic10 were consistently identified outside this group. The conservation of Mic60 and Mic10 in eukaryotes is consistent with their central role in MICOS function [5-7], indicating that the basic mechanism for cristae determination arose early in evolution and has remained relatively unchanged. We found that eukaryotes with ultrastructurally simplified anaerobic mitochondria that lack cristae have also lost MICOS. We then searched for a prokaryotic MICOS and identified a homolog of Mic60 present only in α-proteobacteria, providing evidence for the endosymbiotic origin of mitochondrial cristae. Our study clarifies the origins of mitochondrial cristae and their subsequent evolutionary history, provides evidence for a general mechanism of cristae formation and maintenance in eukaryotes, and points to a new potential factor involved in membrane differentiation in prokaryotes.

Electrostatic swelling of bicontinuous cubic lipid phases

Lipid bicontinuous cubic phases have attracted enormous interest as bio-compatible scaffolds for use in a wide range of applications including membrane protein crystallisation, drug delivery and biosensing. One of the major bottlenecks that has hindered exploitation of these structures is an inability to create targeted highly swollen bicontinuous cubic structures with large and tunable pore sizes. In contrast, cubic structures found in vivo have periodicities approaching the micron scale. We have been able to engineer and control highly swollen bicontinuous cubic phases of spacegroup Im3m containing only lipids by (a) increasing the bilayer stiffness by adding cholesterol and (b) inducing electrostatic repulsion across the water channels by addition of anionic lipids to monoolein. By controlling the composition of the ternary mixtures we have been able to achieve lattice parameters up to 470 Å, which is 5 times that observed in pure monoolein and nearly twice the size of any lipidic cubic phase reported previously. These lattice parameters significantly exceed the predicted maximum swelling for bicontinuous cubic lipid structures, which suggest that thermal fluctuations should destroy such phases for lattice parameters larger than 300 Å.

Isolation of mitochondria with cubic membrane morphology reveals specific ionic requirements for the preservation of membrane structure

Biological membranes with cubic symmetry are a hallmark of virus-infected or diseased cells. The mechanisms of formation and specific cellular functions of cubic membranes, however, are unclear. The best-documented cubic membrane formation occurs in the free-living giant amoeba Chaos carolinense. In that system, mitochondrial inner membranes undergo a reversible structural change from tubular to cubic membrane organization upon starvation of the organism. As a prerequisite to further analyze the structural and functional features of cubic membranes, we adapted protocols for the isolation of mitochondria from starved amoeba and have identified buffer conditions that preserve cubic membrane morphology in vitro. The requirement for high concentration of ion-chelating agents in the isolation media supports the importance of a balanced ion milieu in establishing and maintaining cubic membranes in vivo.

Temperature and pressure tuneable swollen bicontinuous cubic phases approaching nature's length scales

Bicontinuous cubic structures offer enormous potential in applications ranging from protein crystallisation to drug delivery systems and have been observed in cellular membrane structures. One of the current bottlenecks in understanding and exploiting these structures is that cubic scaffolds produced in vitro are considerably smaller in size than those observed in biological systems, differing by almost an order of magnitude in some cases. We have addressed this technological bottleneck and developed a methodology capable of manufacturing highly swollen bicontinuous cubic membranes with length scales approaching those seen in vivo. Crucially, these cubic systems do not require the presence of proteins. We have generated highly swollen Im3m symmetry bicontinuous cubic phases with lattice parameters of up to 480 Å, composed of ternary mixtures of monoolein, cholesterol and negatively charged lipid (DOPS or DOPG) and we have been able to tune their lattice parameters. The swollen cubic phases are highly sensitive to both temperature and pressure; these structural changes are likely to be controlled by a fine balance between lipid headgroup repulsions and lateral pressure in the hydrocarbon chain region.

Lytic and non-lytic permeabilization of cardiolipin-containing lipid bilayers induced by cytochrome C

The release of cytochrome c (cyt c) from mitochondria is an important early step during cellular apoptosis, however the precise mechanism by which the outer mitochondrial membrane becomes permeable to these proteins is as yet unclear. Inspired by our previous observation of cyt c crossing the membrane barrier of giant unilamellar vesicle model systems, we investigate the interaction of cyt c with cardiolipin (CL)-containing membranes using the innovative droplet bilayer system that permits electrochemical measurements with simultaneous microscopy observation. We find that cyt c can permeabilize CL-containing membranes by induction of lipid pores in a dose-dependent manner, with membrane lysis eventually observed at relatively high (µM) cyt c concentrations due to widespread pore formation in the membrane destabilizing its bilayer structure. Surprisingly, as cyt c concentration is further increased, we find a regime with exceptionally high permeability where a stable membrane barrier is still maintained between droplet compartments. This unusual non-lytic state has a long lifetime (>20 h) and can be reversibly formed by mechanically separating the droplets before reforming the contact area between them. The transitions between behavioural regimes are electrostatically driven, demonstrated by their suppression with increasing ionic concentrations and their dependence on CL composition. While membrane permeability could also be induced by cationic PAMAM dendrimers, the non-lytic, highly permeable membrane state could not be reproduced using these synthetic polymers, indicating that details in the structure of cyt c beyond simply possessing a cationic net charge are important for the emergence of this unconventional membrane state. These unexpected findings may hold significance for the mechanism by which cyt c escapes into the cytosol of cells during apoptosis.

Myotubularin and PtdIns3P remodel the sarcoplasmic reticulum in muscle in vivo

The sarcoplasmic reticulum (SR) is a specialized form of endoplasmic reticulum (ER) in skeletal muscle and is essential for calcium homeostasis. The mechanisms involved in SR remodeling and maintenance of SR subdomains are elusive. In this study, we identified myotubularin (MTM1), a phosphoinositide phosphatase mutated in X-linked centronuclear myopathy (XLCNM, or myotubular myopathy), as a key regulator of phosphatidylinositol 3-monophosphate (PtdIns3P) levels at the SR. MTM1 is predominantly located at the SR cisternae of the muscle triads, and Mtm1-deficient mouse muscles and myoblasts from XLCNM patients exhibit abnormal SR/ER networks. In vivo modulation of MTM1 enzymatic activity in skeletal muscle using ectopic expression of wild-type or a dead-phosphatase MTM1 protein leads to differential SR remodeling. Active MTM1 is associated with flat membrane stacks, whereas dead-phosphatase MTM1 mutant promotes highly curved cubic membranes originating from the SR and enriched in PtdIns3P. Overexpression of a tandem FYVE domain with high affinity for PtdIns3P alters the shape of the SR cisternae at the triad. Our findings, supported by the parallel analysis of the Mtm1-null mouse and an in vivo study, reveal a direct function of MTM1 enzymatic activity in SR remodeling and a key role for PtdIns3P in promoting SR membrane curvature in skeletal muscle. We propose that alteration in SR remodeling is a primary cause of X-linked centronuclear myopathy. The tight regulation of PtdIns3P on specific membrane subdomains may be a general mechanism to control membrane curvature.

The three dimensionality of cell membranes: lamellar to cubic membrane transition as investigated by electron microscopy

Biological membranes are generally perceived as phospholipid bilayer structures that delineate in a lamellar form the cell surface and intracellular organelles. However, much more complex and highly convoluted membrane organizations are ubiquitously present in many cell types under certain types of stress, states of disease, or in the course of viral infections. Their occurrence under pathological conditions make such three-dimensionally (3D) folded and highly ordered membranes attractive biomarkers. They have also stimulated great biomedical interest in understanding the molecular basis of their formation. Currently, the analysis of such membrane arrangements, which include tubulo-reticular structures (TRS) or cubic membranes of various subtypes, is restricted to electron microscopic methods, including tomography. Preservation of membrane structures during sample preparation is the key to understand their true 3D nature. This chapter discusses methods for appropriate sample preparations to successfully examine and analyze well-preserved highly ordered membranes by electron microscopy. Processing methods and analysis conditions for green algae (Zygnema sp.) and amoeba (Chaos carolinense), mammalian cells in culture and primary tissue cells are described. We also discuss methods to identify cubic membranes by transmission electron microscopy (TEM) with the aid of a direct template matching method and by computer simulation. A 3D analysis of cubic cell membrane topology by electron tomography is described as well as scanning electron microscopy (SEM) to investigate surface contours of isolated mitochondria with cubic membrane arrangement.

A look through 'lens' cubic mitochondria

Cell membranes may fold up into three-dimensional nanoperiodic cubic structures in biological systems. Similar geometries are well studied in other disciplines such as mathematics, physics and polymer chemistry. The fundamental function of cubic membranes in biological systems has not been uncovered yet; however, their appearance in specialized cell types indicates a role as structural templates or perhaps direct physical entities with specialized biophysical properties. The mitochondria located at the inner segment of the retinal cones of tree shrew (Tupaia glis and Tupaia belangeri) contain unique patterns of concentric cristae with a highly ordered membrane arrangement in three dimensions similar to the photonic nanostructures observed in butterfly wing scales. Using a direct template matching method, we show that the inner mitochondrial membrane folds into multi-layered (8 to 12 layers) gyroid cubic membrane arrangements in the photoreceptor cells. Three-dimensional simulation data demonstrate that such multi-layer gyroid membrane arrangements in the retinal cones of a tree shrew's eye can potentially function as: (i) multi-focal lens; (ii) angle-independent interference filters to block UV light; and (iii) a waveguide photonic crystal. These theoretical results highlight for the first time the significance of multi-layer cubic membrane arrangements to achieve near-quasi-photonic crystal properties through the simple and reversible biological process of continuous membrane folding.

Synthesis, toxicology and potential of ordered mesoporous materials in nanomedicine

Although ordered mesoporous silica materials have been studied for almost 20 years, their utilization within life science applications is relatively new and unexplored. An increasing number of researchers are transcending their respective fields in order to bridge the knowledge gap between materials chemistry and biotechnology, and to exploit the potential of mesoporous materials. Their intricate porosity with order in the nanoscale translates into high surface areas above 1000 m(2)/g, high selectivity for the encapsulation of biorelevant molecules as well as controlled surface chemistry. Their uses in pharmaceutics to improve drug formulation, drug bioavailability, mitigate drug toxicity and in cellular targeting, through controlled drug delivery strategies, have been shown. The incorporation of a high concentration of fluorescent and nuclear markers within their pores, whilst retaining good diffusion through their porous matrix, has shown them to be ideal candidates for sensing devices, in immunoassays such as flow cytometry and for their use in novel theranostic applications. This article aims to bring to the forefront some of the most important properties of mesoporous materials, which prove advantageous for their use in nanomedical applications and to highlight some of the potential areas into which the field may now emerge.

Membrane morphology induced by anisotropic proteins

There are a great many proteins that localize to and collectively generate curvature in biological fluid membranes. We study changes in the topology of fluid membranes due to the presence of highly anisotropic, curvature-inducing proteins. Generically, we find a surprisingly rich phase diagram with phases of both positive and negative Gaussian curvature. As a concrete example modeled on experiments, we find that a lamellar phase in a negative Gaussian curvature regime exhibits a propensity to form screw dislocations of definite Burgers scalar but of both chiralities.

Tensile forces and shape entropy explain observed crista structure in mitochondria

We present a model from which the observed morphology of the inner mitochondrial membrane can be inferred as minimizing the system's free energy. In addition to the usual energetic terms for bending, surface area, and pressure difference, our free energy includes terms for tension that we hypothesize to be exerted by proteins and for an entropic contribution due to many dimensions worth of shapes available at a given energy. We also present measurements of the structural features of mitochondria in HeLa cells and mouse embryonic fibroblasts using three-dimensional electron tomography. Such tomograms reveal that the inner membrane self-assembles into a complex structure that contains both tubular and flat lamellar crista components. This structure, which contains one matrix compartment, is believed to be essential to the proper functioning of mitochondria as the powerhouse of the cell. Interpreting the measurements in terms of the model, we find that tensile forces of ∼20 pN would stabilize a stress-induced coexistence of tubular and flat lamellar cristae phases. The model also predicts a pressure difference of -0.036 ± 0.004 atm (pressure higher in the matrix) and a surface tension equal to 0.09 ± 0.04 pN/nm.

The Cubic "Faces" of Biomembranes

Biomembranes are traditionally viewed as flat phospholipid-bilayer sheets delineating the cell boundaries and dividing the cell into multiple subcellular organelles with specialized functions. However, biological membranes may also fold up into three-dimensional nanoperiodic arrangements, termed cubic membranes. This type of geometry is mathematically well described and extensively studied in lipidic cubic phase systems. This chapter will (1) summarize similarities and dissimilarities between cubic membranes and cubic phases; (2) provide an update on the experimental data describing the role of lipids, proteins and electrostatic charges on the biogenesis of cubic membranes; and (3) discuss their potential function in intracellular macromolecular transport and as optical filters, as well as potential practical applications such as gene delivery vehicles.

Topical delivery of interferon alpha by biphasic vesicles: evidence for a novel nanopathway across the stratum corneum

Noninvasive delivery of macromolecules across intact skin is challenging but would allow for needle-free administration of many pharmaceuticals. Biphasic vesicles, a novel lipid-based topical delivery system, have been shown to deliver macromolecules into the skin. Investigation of the delivery mechanism of interferon alpha (IFN alpha), as a model protein, by biphasic vesicles could improve understanding of molecular transport through the stratum corneum and allow for the design of more effective delivery systems. The interaction of biphasic vesicles with human skin and isolated stratum corneum membrane was investigated by confocal microscopy, differential scanning calorimetry (DSC) and small- and wide-angle X-ray scattering (SAXS and WAXS). Confocal microscopy revealed that biphasic vesicles delivered IFN alpha intercellularly, to a depth of 70 microm, well below the stratum corneum and into the viable epidermis. DSC and SAXS/WAXS data suggest that the interaction of biphasic vesicles with SC lipids resulted in the formation of a three-dimensional cubic Pn3m polymorphic phase by the molecular rearrangement of intercellular lipids. This cubic phase could be an intercellular permeation nanopathway that may explain the increased delivery of IFN alpha by biphasic vesicles. Liposomes and submicrometer emulsion (the individual building blocks of biphasic vesicles) separately and methylcellulose gel, an alternative topical vehicle, did not induce a cubic phase and delivered low amounts of IFN alpha below the stratum corneum. Molecular modeling of the cubic Pn3m phase and lamellar-to-cubic phase transitions provides a plausible mechanism for transport of IFN alpha. It is hypothesized that induction of a Pn3m cubic phase in stratum corneum lipids could make dermal and transdermal delivery of other macromolecules also possible.

Influence of the stabilizer concentration on the internal liquid crystalline order and the size of oil-loaded monolinolein-based dispersions

The internal phase of monolinolein-based dispersions loaded with tetradecane or (R)-(+)-limonene was investigated as a function of the stabilizer content by small-angle X-ray scattering. Phase transitions at the colloidal scale were found in some of nanostructured aqueous dispersions by increasing the stabilizer content. For particles containing a bicontinuous cubic phase, a large increase of the stabilizer concentration promoted a liquid crystalline phase transition from the Pn3m to the Im3m cubic symmetry. The coexistence of both phases is observed in an intermediate stabilizer concentration range. For particles with an internal micellar cubic Fd3m symmetry, the internal structure changes in the isotropic fluid L(2) phase. In case of particles with an internal hexagonal phase (H(2) symmetry), the increasing amount of stabilizer did not alter the lattice parameter but decreased the size of the nanostructured domain. Moreover, we showed for hexagonal and emulsified micellar phase particles that the increase of the stabilizer content induced a strong decrease of the mean hydrodynamic size of the particles, allowing producing nanostructured lipid-based liquid crystalline particles down to a radius of 70 nm at the same energy input.

Do viruses subvert cholesterol homeostasis to induce host cubic membranes?

Biological membranes with cubic morphology are a hallmark of stressed or diseased cellular conditions; both protein–protein interactions and lipid alterations appear to contribute to their biogenesis, yet their specific cellular functions are unknown. The occurrence of cubic membranes strikingly correlates with viral infections; notably, virus entry, proliferation, and release are processes closely linked to cellular cholesterol metabolism, and dys-regulation of cholesterol synthesis at the level of HMG-CoA reductase also induces cubic membrane formation, in the absence of viral infection. We propose that virus-induced cubic membranes could result from viral interference of cellular cholesterol homeostasis, generating a protective membrane environment to facilitate virus assembly and proliferation. Preventing cubic membrane formation might thus disrupt the ‘virus factory’ and offer new avenues to combat viral infections.