Bioengineered Bacterial Membrane Vesicles with Multifunctional Nanoparticles as a Versatile Platform for Cancer Immunotherapy

Inducing immunogenic cell death (ICD) is a critical strategy for enhancing cancer immunotherapy. However, inefficient and risky ICD inducers along with a tumor hypoxia microenvironment seriously limit the immunotherapy efficacy. Non-specific delivery is also responsible for this inefficiency. In this work, we report a drug-free bacteria-derived outer membrane vesicle (OMV)-functionalized Fe₃O₄-MnO₂ (FMO) nanoplatform that realized neutrophil-mediated targeted delivery and photothermally enhanced cancer immunotherapy. In this system, modification of OMVs derived from Escherichia coli enhanced the accumulation of FMO NPs at the tumor tissue through neutrophil-mediated targeted delivery. The FMO NPs underwent reactive decomposition in the tumor site, generating manganese and iron ions that induced ICD and O₂ that regulated the tumor hypoxia environment. Moreover, OMVs are rich in pathogen-associated pattern molecules that can overcome the tumor immunosuppressive microenvironment and effectively activate immune cells, thereby enhancing specific immune responses. Photothermal therapy (PTT) caused by MnO₂ and Fe₃O₄ can not only indirectly stimulate systemic immunity by directly destroying tumor cells but also promote the enrichment of neutrophil-equipped nanoparticles by enhancing the inflammatory response at the tumor site. Finally, the proposed multi-modal treatment system with targeted delivery capability realized effective tumor immunotherapy to prevent tumor growth and recurrence.

Outer Membrane Vesicles as Molecular Biomarkers for Gram-negative Sepsis: Taking Advantage of Nature's Perfect Packages

Sepsis is an often life-threatening response to infection, occurring when host pro-inflammatory immune responses become abnormally elevated and dysregulated. To diagnose sepsis, the patient must have a confirmed or predicted infection, as well as other symptoms associated with the pathophysiology of sepsis. However, a recent study found that a specific causal organism could not be determined in the majority (70.1%) of sepsis cases, likely due to aggressive antibiotics or localized infections. The timing of a patient's sepsis diagnosis is often predictive of their clinical outcome, underlining the need for a more definitive molecular diagnostic test. Here, we outline the advantages and challenges to using bacterial outer membrane vesicles (OMVs), nanoscale spherical buds derived from the outer membrane of Gram-negative bacteria, as a diagnostic biomarker for Gram-negative sepsis. Advantages include OMV abundance, their robustness in the presence of antibiotics, and their unique features derived from their parent cell that could allow for differentiation between bacterial species. Challenges include the rigorous purification methods required to isolate OMVs from complex biofluids and the additional need to separate OMVs from similarly-sized extracellular vesicles, which can share physical properties with OMVs.

Lipidomics Analysis of Outer Membrane Vesicles and Elucidation of the Inositol Phosphoceramide Biosynthetic Pathway in Bacteroides thetaiotaomicron

Approximately one-third of the human colonic microbiome is formed by bacteria from the genus Bacteroides. These bacteria produce a large amount of uniformly sized outer membrane vesicles (OMVs), which are equipped with hydrolytic enzymes that play a role in the degradation of diet- and host-derived glycans. In this work, we characterize the lipid composition of membranes and OMVs from Bacteroides thetaiotaomicron VPI-5482. Liquid chromatography-mass spectrometry (LC-MS) analysis indicated that OMVs carry sphingolipids, glycerophospholipids, and serine-dipeptide lipids. Sphingolipid species represent more than 50% of the total lipid content of OMVs. The most abundant sphingolipids in OMVs are ethanolamine phosphoceramide (EPC) and inositol phosphoceramide (IPC). Bioinformatics analysis allowed the identification of the BT1522-1526 operon putatively involved in IPC synthesis. Mutagenesis studies revealed that BT1522-1526 is essential for the synthesis of phosphatidylinositol (PI) and IPC, confirming the role of this operon in the biosynthesis of IPC. BT1522-1526 mutant strains lacking IPC produced OMVs that were indistinguishable from the wild-type strain, indicating that IPC sphingolipid species are not involved in OMV biogenesis. Given the known role of sphingolipids in immunomodulation, we suggest that OMVs may act as long-distance vehicles for the delivery of sphingolipids in the human gut. IMPORTANCE Sphingolipids are essential membrane lipid components found in eukaryotes that are also involved in cell signaling processes. Although rare in bacteria, sphingolipids are produced by members of the phylum Bacteroidetes, human gut commensals. Here, we determined that OMVs carry sphingolipids and other lipids of known signaling function. Our results demonstrate that the BT1522-1526 operon is required for IPC biosynthesis in B. thetaiotaomicron.

The Eukaryotic Linear Motif resource: 2022 release

Almost twenty years after its initial release, the Eukaryotic Linear Motif (ELM) resource remains an invaluable source of information for the study of motif-mediated protein-protein interactions. ELM provides a comprehensive, regularly updated and well-organised repository of manually curated, experimentally validated short linear motifs (SLiMs). An increasing number of SLiM-mediated interactions are discovered each year and keeping the resource up-to-date continues to be a great challenge. In the current update, 30 novel motif classes have been added and five existing classes have undergone major revisions. The update includes 411 new motif instances mostly focused on cell-cycle regulation, control of the actin cytoskeleton, membrane remodelling and vesicle trafficking pathways, liquid-liquid phase separation and integrin signalling. Many of the newly annotated motif-mediated interactions are targets of pathogenic motif mimicry by viral, bacterial or eukaryotic pathogens, providing invaluable insights into the molecular mechanisms underlying infectious diseases. The current ELM release includes 317 motif classes incorporating 3934 individual motif instances manually curated from 3867 scientific publications. ELM is available at: http://elm.eu.org.

Visualization of Cytosolic Galectin Accumulation Around Damaged Vesicles and Organelles

Galectins are animal lectins that recognize β-galactoside and bind glycans. Recent studies have indicated that cytosolic galectins recognize cytosolically exposed glycans and accumulate around endocytic vesicles or organelles damaged by various disruptive substances. Accumulated galectins engage other cytosolic proteins toward damaged vesicles, leading to cellular responses, such as autophagy. Disruptive substances include bacteria, viruses, particulate matters, and protein aggregates; thus, this process is implicated in the pathogenesis of various diseases. In this chapter, we describe methods for studying three disruptive substances: photosensitizers, Listeria monocytogenes, and Helicobacter pylori. We summarize the tools used for the detection of cytosolic galectin accumulation around damaged vesicles.

Pore-Spanning Plasma Membranes Derived from Giant Plasma Membrane Vesicles

Giant plasma membrane vesicles (GPMVs) are a highly promising model system for the eukaryotic plasma membrane. The unresolved challenge, however, is a path to surface-based structures that allows accessibility to both sides of the plasma membrane through high-resolution techniques. Such an approach would pave the way to advanced chip-based technologies for the analysis of complex cell surfaces to study the roles of membrane proteins, host-pathogen interactions, and many other bioanalytical and sensing applications. This study reports the generation of planar supported plasma membranes and for the first-time pore-spanning plasma membranes (PSPMs) derived from pure GPMVs that are spread on activated solid and highly ordered porous silicon substrates. GPMVs were produced by two different vesiculation agents and were first investigated with respect to their growth behavior and phase separation. Second, these GPMVs were spread onto silicon substrates to form planar supported plasma membrane patches. PSPMs were obtained by spreading of pure GPMVs on oxygen-plasma activated porous substrates with pore diameters of 3.5 μm. Fluorescence micrographs unambiguously showed that the PSPMs partially phase separate in a mobile ordered phase surrounded by a disordered phase, which was supported by cholesterol extraction using methyl-β-cyclodextrin.

Polymer-Lipid Hybrid Vesicles and Their Interaction with HepG2 Cells

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

Intracellular Vesicle Fusion Requires a Membrane-Destabilizing Peptide Located at the Juxtamembrane Region of the v-SNARE

Intracellular vesicle fusion is mediated by soluble N-ethylmaleimide sensitive factor attachment protein receptors (SNAREs) and Sec1/Munc18 (SM) proteins. It is generally accepted that membrane fusion occurs when the vesicle and target membranes are brought into close proximity by SNAREs and SM proteins. In this work, we demonstrate that, for fusion to occur, membrane bilayers must be destabilized by a conserved membrane-embedded motif located at the juxtamembrane region of the vesicle-anchored v-SNARE. Comprised of basic and hydrophobic residues, the juxtamembrane motif perturbs the lipid bilayer structure and promotes SNARE-SM-mediated membrane fusion. The juxtamembrane motif can be functionally substituted with an unrelated membrane-disrupting peptide in the membrane fusion reaction. These findings establish the juxtamembrane motif of the v-SNARE as a membrane-destabilizing peptide. Requirement of membrane-destabilizing peptides is likely a common feature of biological membrane fusion.

Mechanical properties of plasma membrane vesicles correlate with lipid order, viscosity and cell density

Regulation of plasma membrane curvature and composition governs essential cellular processes. The material property of bending rigidity describes the energetic cost of membrane deformations and depends on the plasma membrane molecular composition. Because of compositional fluctuations and active processes, it is challenging to measure it in intact cells. Here, we study the plasma membrane using giant plasma membrane vesicles (GPMVs), which largely preserve the plasma membrane lipidome and proteome. We show that the bending rigidity of plasma membranes under varied conditions is correlated to readout from environment-sensitive dyes, which are indicative of membrane order and microviscosity. This correlation holds across different cell lines, upon cholesterol depletion or enrichment of the plasma membrane, and variations in cell density. Thus, polarity- and viscosity-sensitive probes represent a promising indicator of membrane mechanical properties. Additionally, our results allow for identifying synthetic membranes with a few well defined lipids as optimal plasma membrane mimetics.

Role of the endolysosomal system in Parkinson's disease

Parkinson's disease (PD) is one of the most common neurodegenerative disorders, affecting 1-1.5% of the total population. While progress has been made in understanding the neurodegenerative mechanisms that lead to cell death in late stages of PD, mechanisms for early, causal pathogenic events are still elusive. Recent developments in PD genetics increasingly point at endolysosomal (E-L) system dysfunction as the early pathomechanism and key pathway affected in PD. Clathrin-mediated synaptic endocytosis, an integral part of the neuronal E-L system, is probably the main early target as evident in auxilin, RME-8, and synaptojanin-1 mutations that cause PD. Autophagy, another important pathway in the E-L system, is crucial in maintaining proteostasis and a healthy mitochondrial pool, especially in neurons considering their inability to divide and requirement to function an entire life-time. PINK1 and Parkin mutations severely perturb autophagy of dysfunctional mitochondria (mitophagy), both in the cell body and synaptic terminals of dopaminergic neurons, leading to PD. Endolysosomal sorting and trafficking is also crucial, which is complex in multi-compartmentalized neurons. VPS35 and VPS13C mutations noted in PD target these mechanisms. Mutations in GBA comprise the most common risk factor for PD and initiate pathology by compromising lysosomal function. This is also the case for ATP13A2 mutations. Interestingly, α-synuclein and LRRK2, key proteins involved in PD, function in different steps of the E-L pathway and target their components to induce disease pathogenesis. In this review, we discuss these E-L system genes that are linked to PD and how their dysfunction results in PD pathogenesis. This article is part of the Special Issue "Synuclein".

Dynamics of Growth and Form in Prebiotic Vesicles

The growth, form, and division of prebiotic vesicles, membraneous bags of fluid of varying components and shapes is hypothesized to have served as the substrate for the origin of life. The dynamics of these out-of-equilibrium structures is controlled by physicochemical processes that include the intercalation of amphiphiles into the membrane, fluid flow across the membrane, and elastic deformations of the membrane. To understand prebiotic vesicular forms and their dynamics, we construct a minimal model that couples membrane growth, deformation, and fluid permeation, ultimately couched in terms of two dimensionless parameters that characterize the relative rate of membrane growth and the membrane permeability. Numerical simulations show that our model captures the morphological diversity seen in extant precursor mimics of cellular life, and might provide simple guidelines for the synthesis of these complex shapes from simple ingredients.

A novel fluorescence microscopic approach to quantitatively analyse protein-induced membrane remodelling

Membrane remodelling or the bending and rupture of the lipid bilayer occurs during diverse cellular processes such as cell division, synaptic transmission, vesicular transport, organelle biogenesis and sporulation. These activities are brought about by the localized change in membrane curvature, which in turn causes lipid-packing stress, of a planar lipid bilayer by proteins. For instance, vesicular transport processes are typically characterized by the cooperative recruitment of proteins that induce budding of a planar membrane and catalyse fission of the necks of membrane buds to release vesicles. The analysis of such membrane remodelling reactions has traditionally been restricted to electron microscopy-based approaches or force spectroscopic analysis of membrane tethers pulled from liposome-based model membrane systems. Our recent work has demonstrated the facile creation of tubular model membrane systems of supported membrane tubes (SMrTs), which mimic late-stage intermediates of typical vesicular transport reactions. This review addresses the nature of such an assay system and a fluorescence-intensity-based analysis of changes in tube dimensions that is indicative of the membrane remodelling capacity of proteins.

A Carbon Nanotube Optical Reporter Maps Endolysosomal Lipid Flux

Lipid accumulation within the lumen of endolysosomal vesicles is observed in various pathologies including atherosclerosis, liver disease, neurological disorders, lysosomal storage disorders, and cancer. Current methods cannot measure lipid flux specifically within the lysosomal lumen of live cells. We developed an optical reporter, composed of a photoluminescent carbon nanotube of a single chirality, that responds to lipid accumulation via modulation of the nanotube's optical band gap. The engineered nanomaterial, composed of short, single-stranded DNA and a single nanotube chirality, localizes exclusively to the lumen of endolysosomal organelles without adversely affecting cell viability or proliferation or organelle morphology, integrity, or function. The emission wavelength of the reporter can be spatially resolved from within the endolysosomal lumen to generate quantitative maps of lipid content in live cells. Endolysosomal lipid accumulation in cell lines, an example of drug-induced phospholipidosis, was observed for multiple drugs in macrophages, and measurements of patient-derived Niemann-Pick type C fibroblasts identified lipid accumulation and phenotypic reversal of this lysosomal storage disease. Single-cell measurements using the reporter discerned subcellular differences in equilibrium lipid content, illuminating significant intracellular heterogeneity among endolysosomal organelles of differentiating bone-marrow-derived monocytes. Single-cell kinetics of lipoprotein-derived cholesterol accumulation within macrophages revealed rates that differed among cells by an order of magnitude. This carbon nanotube optical reporter of endolysosomal lipid content in live cells confers additional capabilities for drug development processes and the investigation of lipid-linked diseases.

Impaired AQP2 trafficking in Fxyd1 knockout mice: A role for FXYD1 in regulated vesicular transport

The final adjustment of urine volume occurs in the inner medullary collecting duct (IMCD), chiefly mediated by the water channel aquaporin 2 (AQP2). With vasopressin stimulation, AQP2 accumulation in the apical plasma membrane of principal cells allows water reabsorption from the lumen. We report that FXYD1 (phospholemman), better known as a regulator of Na,K-ATPase, has a role in AQP2 trafficking. Daytime urine of Fxyd1 knockout mice was more dilute than WT despite similar serum vasopressin, but both genotypes could concentrate urine during water deprivation. FXYD1 was found in IMCD. In WT mice, phosphorylated FXYD1 was detected intracellularly, and vasopressin induced its dephosphorylation. We tested the hypothesis that the dilute urine in knockouts was caused by alteration of AQP2 trafficking. In WT mice at baseline, FXYD1 and AQP2 were not strongly co-localized, but elevation of vasopressin produced translocation of both FXYD1 and AQP2 to the apical plasma membrane. In kidney slices, baseline AQP2 distribution was more scattered in the Fxyd1 knockout than in WT. Apical recruitment of AQP2 occurred in vasopressin-treated Fxyd1 knockout slices, but upon vasopressin washout, there was more rapid reversal of apical AQP2 localization and more heterogeneous cytoplasmic distribution of AQP2. Notably, in sucrose gradients, AQP2 was present in a detergent-resistant membrane domain that had lower sedimentation density in the knockout than in WT, and vasopressin treatment normalized its density. We propose that FXYD1 plays a role in regulating AQP2 retention in apical membrane, and that this involves transfers between raft-like membrane domains in endosomes and plasma membranes.

Characterization of Type Three Secretion System Translocator Interactions with Phospholipid Membranes

In vitro characterization of type III secretion system (T3SS) translocator proteins has proven challenging due to complex purification schemes and their hydrophobic nature that often requires detergents to provide protein solubility and stability. Here, we provide experimental details for several techniques that overcome these hurdles, allowing for the direct characterization of the Shigella translocator protein IpaB with respect to phospholipid membrane interaction. The techniques specifically discussed in this chapter include membrane interaction/liposome flotation, liposome sensitive fluorescence quenching, and protein-mediated liposome disruption assays. These assays have provided valuable insight into the role of IpaB in T3SS-mediated phospholipid membrane interactions by Shigella and should readily extend to other members of this important class of proteins.

Controlled deformation of vesicles by flexible structured media

Liquid crystalline (LC) materials, such as actin or tubulin networks, are known to be capable of deforming the shape of cells. Here, elements of that behavior are reproduced in a synthetic system, namely, a giant vesicle suspended in a LC, which we view as a first step toward the preparation of active, anisotropic hybrid systems that mimic some of the functionality encountered in biological systems. To that end, we rely on a coupled particle-continuum representation of deformable networks in a nematic LC represented at the level of a Landau-de Gennes free energy functional. Our results indicate that, depending on its elastic properties, the LC is indeed able to deform the vesicle until it reaches an equilibrium, anisotropic shape. The magnitude of the deformation is determined by a balance of elastic and surface forces. For perpendicular anchoring at the vesicle, a Saturn ring defect forms along the equatorial plane, and the vesicle adopts a pancake-like, oblate shape. For degenerate planar anchoring at the vesicle, two boojum defects are formed at the poles of the vesicle, which adopts an elongated, spheroidal shape. During the deformation, the volume of the topological defects in the LC shrinks considerably as the curvature of the vesicle increases. These predictions are confirmed by our experimental observations of spindle-like shapes in experiments with giant unilamellar vesicles with planar anchoring. We find that the tension of the vesicle suppresses vesicle deformation, whereas anchoring strength and large elastic constants promote shape anisotropy.

microRNA Profiling of Amniotic Fluid: Evidence of Synergy of microRNAs in Fetal Development

Amniotic fluid (AF) continuously exchanges molecules with the fetus, playing critical roles in fetal development especially via its complex components. Among these components, microRNAs are thought to be transferred between cells loaded in microvesicles. However, the functions of AF microRNAs remain unknown. To date, few studies have examined microRNAs in amniotic fluid. In this study, we employed miRCURY Locked Nucleotide Acid arrays to profile the dynamic expression of microRNAs in AF from mice on embryonic days E13, E15, and E17. At these times, 233 microRNAs were differentially expressed (p< 0.01), accounting for 23% of the total Mus musculus microRNAs. These differentially-expressed microRNAs were divided into two distinct groups based on their expression patterns. Gene ontology analysis showed that the intersectional target genes of these differentially-expressed microRNAs were mainly distributed in synapse, synaptosome, cell projection, and cytoskeleton. Pathway analysis revealed that the target genes of the two groups of microRNAs were synergistically enriched in axon guidance, focal adhesion, and MAPK signaling pathways. MicroRNA-mRNA network analysis and gene- mapping showed that these microRNAs synergistically regulated cell motility, cell proliferation and differentiation, and especially the axon guidance process. Cancer pathways associated with growth and proliferation were also enriched in AF. Taken together, the results of this study are the first to show the functions of microRNAs in AF during fetal development, providing novel insights into interpreting the roles of AF microRNAs in fetal development.

Development of montelukast sodium loaded niosomal carriers by film hydration technique

The aim of this study was to develop and characterize montelukast sodium loaded niosomal drug carrier systems. The vesicles were prepared by film hydration technique using different surfactants. The optimized formulation was selected on the basis of results obtained from drug entrapment, morphology and in vitro drug release studies, and further evaluated for possible drug-excipient interaction, thermal behavior and drug physical state, before and after formulation using Fourier transform infrared spectroscopy, differential scanning calorimetry and X-ray diffraction analysis methods, respectively. The morphological characterization of vesicles was done using Transmission electron microscopy. Energy-dispersive X-ray spectroscopy system was used for elemental and dimensional analysis of developed vesicles. The vesicle surface charge was determined using zeta potential measurements. The results suggested that the optimized formulation had small size (103±6.01 nm) and high drug entrapment (72.20±2.10%). No chemical interaction was observed between the drug and excipients. The study revealed that Span 60 is a good nonionic surfactant for vesicle formulation. After 3 months storage at 2-8°C, the optimized formulation preserved stability in terms of formulation colour, drug amount and percent drug release. After 3 months, flocculation occured and hard cake was not formed on the settlement of vesicles. The preliminary results of this study suggest that the designed vesicles could enhance drug entrapment, reduce the initial burst release of drug and modulate the drug release.

Aminosilane/oleic acid vesicles as model membranes of protocells

Oleic acid vesicles represent good models of membrane protocells that could have existed in prebiotic times. Here, we report the formation, growth polymorphism, and dynamics of oleic acid spherical vesicles (1-10 μm), stable elongated vesicles (>50 μm length; 1-3 μm diameter), and chains of vesicles (pearl necklaces, >50 μm length; 1-3 μm diameter) in the presence of aminopropyl triethoxysilane and guanidine hydrochloride. These vesicles exhibit a remarkable behavior with temperature: spherical vesicles only are observed when keeping the sample at 4 °C for 2 h, and self-aggregated spherical vesicles occur upon freezing/unfreezing (-20/20 °C) samples. Rather homogeneous elongated vesicles are reformed upon heating samples at 80 °C. The phenomenon is reversible through cycles of freezing/heating or cooling/heating of the same sample. Deuterium NMR evidences a chain packing rigidity similar to that of phospholipid bilayers in cellular biomembranes. We expect these bilayered vesicles to be surrounded by a layer of aminosilane oligomers, offering a variant model for membrane protocells.

Polymersome magneto-valves for reversible capture and release of nanoparticles

Stomatocytes are polymersomes with an infolded bowl-shaped architecture. This internal cavity is connected to the outside environment via a small 'mouth' region. Stomatocytes are assembled from diamagnetic amphiphilic block-copolymers with a highly anisotropic magnetic susceptibility, which permits to magnetically align and deform the polymeric self-assemblies. Here we show the reversible opening and closing of the mouth region of stomatocytes in homogeneous magnetic fields. The control over the size of the opening yields magneto-responsive supramolecular valves that are able to reversibly capture and release cargo. Furthermore, the increase in the size of the opening is gradual and starts at fields below 10 T, which opens the possibility of using these structures for delivery and nanoreactor applications.

Protein selection and export via outer membrane vesicles

Outer membrane vesicles (OMVs) are constitutively produced by all Gram-negative bacteria. OMVs form when buds from the outer membrane (OM) of cells encapsulate periplasmic material and pinch off from the OM to form spheroid particles approximately 10 to 300nm in diameter. OMVs accomplish a diversity of functional roles yet the OMV's utility is ultimately determined by its unique composition. Inclusion into OMVs may impart a variety of benefits to the protein cargo, including: protection from proteolytic degradation, enhancement of long-distance delivery, specificity in host-cell targeting, modulation of the immune response, coordinated secretion with other bacterial effectors, and/or exposure to a unique function-promoting environment. Many enriched OMV-associated components are virulence factors, aiding in host cell destruction, immune system evasion, host cell invasion, or antibiotic resistance. Although the mechanistic details of how proteins become enriched as OMV cargo remain elusive, recent data on OM biogenesis and relationships between LPS structure and OMV-cargo inclusion rates shed light on potential models for OM organization and consequent OMV budding. In this review, mechanisms based on pre-existing OM microdomains are proposed to explain how cargo may experience differing levels of enrichment in OMVs and degrees of association with OMVs during extracellular export. This article is part of a Special Issue entitled: Protein trafficking and secretion in bacteria. Guest Editors: Anastassios Economou and Ross Dalbey.

Ultrastructural analysis of healthy synovial fluids in three mammalian species

A better knowledge of synovial fluid (SF) ultrastructure is required to further understand normal joint lubrication and metabolism. The aim of the present study was to elucidate SF structural features in healthy joints from three mammalian species of different size compared with features in biomimetic SF. High-resolution structural analysis was performed using transmission electron microscopy (TEM) and scanning electron microscopy (SEM) and environmental SEM/wet scanning transmission electron microscopy mode complemented by TEM and SEM cryogenic methods. Laser-scanning confocal microscopy (LCM) was used to locate the main components of SF with respect to its ultrastructural organization. The present study showed that the ultrastructure of healthy SF is built from a network of vesicles with a size range from 100 to a few hundred nanometers. A multilayered organization of the vesicle membranes was observed with a thickness of about 5 nm. LCM study of biological SF compared with synthetic SF showed that the microvesicles consist of a lipid-based membrane enveloping a glycoprotein gel. Thus, healthy SF has a discontinuous ultrastructure based on a complex network of microvesicles. This finding offers novel perspectives for the diagnosis and treatment of synovial joint diseases.

Anomalous diffusion due to hindering by mobile obstacles undergoing Brownian motion or Orstein-Ulhenbeck processes

In vivo measurements of the passive movements of biomolecules or vesicles in cells consistently report "anomalous diffusion," where mean-squared displacements scale as a power law of time with exponent α<1 (subdiffusion). While the detailed mechanisms causing such behaviors are not always elucidated, movement hindrance by obstacles is often invoked. However, our understanding of how hindered diffusion leads to subdiffusion is based on diffusion amidst randomly located immobile obstacles. Here, we have used Monte Carlo simulations to investigate transient subdiffusion due to mobile obstacles with various modes of mobility. Our simulations confirm that the anomalous regimes rapidly disappear when the obstacles move by Brownian motion. By contrast, mobile obstacles with more confined displacements, e.g., Orstein-Ulhenbeck motion, are shown to preserve subdiffusive regimes. The mean-squared displacement of tracked protein displays convincing power laws with anomalous exponent α that varies with the density of Orstein-Ulhenbeck (OU) obstacles or the relaxation time scale of the OU process. In particular, some of the values we observed are significantly below the universal value predicted for immobile obstacles in two dimensions. Therefore, our results show that subdiffusion due to mobile obstacles with OU type of motion may account for the large variation range exhibited by experimental measurements in living cells and may explain that some experimental estimates are below the universal value predicted for immobile obstacles.

Quantification of nanoparticle dose and vesicular inheritance in proliferating cells

Assessing dose in nanoparticle-cell interactions is inherently difficult due to a complex multiplicity of possible mechanisms and metrics controlling particle uptake. The fundamental unit of nanoparticle dose is the number of particles internalized per cell; we show that this can be obtained for large cell populations that internalize fluorescent nanoparticles by endocytosis, through calibration of cytometry measurements to transmission electron microscopy data. Low-throughput, high-resolution electron imaging of quantum dots in U-2 OS cells is quantified and correlated with high-throughput, low-resolution optical imaging of the nanoparticle-loaded cells. From the correlated data, we obtain probability distribution functions of vesicles per cell and nanoparticles per vesicle. Sampling of these distributions and comparison to fluorescence intensity histograms from flow cytometry provide the calibration factor required to transform the cytometry metric to total particle dose per cell, the mean value of which is 2.4 million. Use of the probability distribution functions to analyze particle partitioning during cell division indicates that, while vesicle inheritance is near symmetric, highly variable vesicle loading leads to a highly asymmetric particle dose within the daughter cells.

Presenilin controls kinesin-1 and dynein function during APP-vesicle transport in vivo

Neurons and other cells require intracellular transport of essential components for viability and function. Previous work has shown that while net amyloid precursor protein (APP) transport is generally anterograde, individual vesicles containing APP move bi-directionally. This discrepancy highlights our poor understanding of the in vivo regulation of APP-vesicle transport. Here, we show that reduction of presenilin (PS) or suppression of gamma-secretase activity substantially increases anterograde and retrograde velocities for APP vesicles. Strikingly, PS deficiency has no effect on an unrelated cargo vesicle class containing synaptotagmin, which is powered by a different kinesin motor. Increased velocities caused by PS or gamma-secretase reduction require functional kinesin-1 and dynein motors. Together, our findings suggest that a normal function of PS is to repress kinesin-1 and dynein motor activity during axonal transport of APP vesicles. Furthermore, our data suggest that axonal transport defects induced by loss of PS-mediated regulatory effects on APP-vesicle motility could be a major cause of neuronal and synaptic defects observed in Alzheimer's Disease (AD) pathogenesis. Thus, perturbations of APP/PS transport could contribute to early neuropathology observed in AD, and highlight a potential novel therapeutic pathway for early intervention, prior to neuronal loss and clinical manifestation of disease.

Dynamic modes of quasispherical vesicles: exact analytical solutions

In this paper we introduce a simple mathematical analysis to reexamine vesicle dynamics in the quasispherical limit (small deformation) under a shear flow. In this context, a recent paper [Misbah, Phys. Rev. Lett. 96, 028104 (2006)] revealed a dynamic referred to as the vacillating-breathing (VB) mode where the vesicle main axis oscillates about the flow direction and the shape undergoes a breathinglike motion, as well as the tank-treading and tumbling (TB) regimes. Our goal here is to identify these three modes by obtaining explicit analytical expressions of the vesicle inclination angle and the shape deformation. In particular, the VB regime is put in evidence and the transition dynamics is discussed. Not surprisingly, our finding confirms the Keller-Skalak solutions (for rigid particles) and shows that the VB and TB modes coexist, and whether one prevails over the other depends on the initial conditions. An interesting additional element in the discussion is the prediction of the TB and VB modes as functions of a control parameter Γ, which can be identified as a TB-VB parameter.

Effect of thermal noise on vesicles and capsules in shear flow

We add thermal noise consistently to reduced models of undeformable vesicles and capsules in shear flow and derive analytically the corresponding stochastic equations of motion. We calculate the steady-state probability distribution function and construct the corresponding phase diagrams for the different dynamical regimes. For fluid vesicles, we predict that at small shear rates thermal fluctuations induce a tumbling motion for any viscosity contrast. For elastic capsules, due to thermal mixing, an intermittent regime appears in regions where deterministic models predict only pure tank treading or tumbling.

Hydrodynamic interaction between two vesicles in a linear shear flow: asymptotic study

Interactions between two vesicles in an imposed linear shear flow are studied theoretically, in the limit of almost spherical vesicles, with a large intervesicle distance, in a strong flow, with a large inner to outer viscosity ratio. This allows to derive a system of ordinary equations describing the dynamics of the two vesicles. We provide an analytic expression for the interaction law. We find that when the vesicles are in the same shear plane, the hydrodynamic interaction leads to a repulsion. When they are not, the interaction may turn into attraction instead. The interaction law is discussed and analyzed as a function of relevant parameters.

Vesicle dynamics under weak flows: application to large excess area

Dynamics of a vesicle under simple shear flow is studied in the limit of small capillary number. A perturbative approach is used to derive the equation of vesicle dynamics. The expansions are shown to converge for significantly deflated vesicles (with excess area from the sphere as high as 2). In particular, we provide an explicit analytical expression for the tank-treading to tumbling bifurcation point. This expression is valid for excess areas up to 2.5. The results are compared with full 3D numerical simulations. The proposed method can be used for analytical or numerical solution of vesicle dynamics under weak flow of general form.

Pulling or compressing a vesicle by force: solution to the bending energy model

The shape transformation induced by pulling or compressing a vesicle along the symmetric axis is discussed in light of the numerical solution to the bending energy model with fixed enclosing volume and surface area. A complete phase diagram in terms of the external force and reduced vesicle volume is investigated. Some experimental observations are explained by the solution.

Phosphoinositides in the mammalian endo-lysosomal network

The endo-lysosomal system is an interconnected tubulo-vesicular network that acts as a sorting station to process and distribute internalised cargo. This network accepts cargoes from both the plasma membrane and the biosynthetic pathway, and directs these cargos either towards the lysosome for degradation, the peri-nuclear recycling endosome for return to the cell surface, or to the trans-Golgi network. These intracellular membranes are variously enriched in different phosphoinositides that help to shape compartmental identity. These lipids act to localise a number of phosphoinositide-binding proteins that function as sorting machineries to regulate endosomal cargo sorting. Herein we discuss regulation of these machineries by phosphoinositides and explore how phosphoinositide-switching contributes toward sorting decisions made at this platform.

Cellular uptake of coumarin-6 as a model drug loaded in solid lipid nanoparticles

The aim of present work was to elucidate the interaction of solid lipid nanoparticles (SLNs) with cellular plasma-membrane to gain insight of intracellular drug delivery. To this aim we followed the uptake of coumarin-6 (a drug model) either free in the extracellular medium or loaded on SLN (c-SLN). Alveolar epithelial cells were exposed to a biocompatible concentration of c-SLN (0.01 mg/ml of tripalmitin) prepared by warm microemulsion whose lipid matrix was constituted by low melting point molecules (fatty acids, triglycerides). Intracellular fluorescence and preferential accumulation in the perinuclear region were increased by 54.8% on comparing c-SLN to the same amount of free coumarin-6 in the medium. Lowering temperature from 37 ° to 4 °C decreased the intracellular signal intensity by about 48% equally for the free as well as for loaded drug, thus suggesting the inhibition of a similar non-endocytotic entrance pathway. No specific co-localization of the fluorescence with intracellular organelles was found. The c-SLN calorimetric profile obtained with differential scanning calorimetry (DSC), revealing transition within the range 58-62 °C, altered remarkably upon incubation with cells, suggesting a change in SLN structure after association with cells membranes. We propose that the uptake of the model drug loaded on SLN is only partly related to the endocytotic pathway; it occurs despite the loss of integrity of the original SLN structure and it appears to be more efficient when the drug is vehicled rather than being free in the culture medium.

Lattice-gas model for active vesicle transport by molecular motors with opposite polarities

We introduce a multispecies lattice-gas model for motor protein driven collective cargo transport on cellular filaments. We use this model to describe and analyze the collective motion of interacting vesicle cargos being carried by oppositely directed molecular motors, moving on a single biofilament. Building on a totally asymmetric exclusion process to characterize the motion of the interacting cargos, we allow for mass exchange with the environment, input, and output at filament boundaries and focus on the role of interconversion rates and how they affect the directionality of the net cargo transport. We quantify the effect of the various different competing processes in terms of nonequilibrium phase diagrams. The interplay of interconversion rates, which allow for flux reversal and evaporation-deposition processes, introduces qualitatively unique features in the phase diagrams. We observe regimes of three-phase coexistence, the possibility of phase re-entrance, and a significant flexibility in how the different phase boundaries shift in response to changes in control parameters. The moving steady-state solutions of this model allows for different possibilities for the spatial distribution of cargo vesicles, ranging from homogeneous distribution of vesicles to polarized distributions, characterized by inhomogeneities or shocks. Current reversals due to internal regulation emerge naturally within the framework of this model. We believe that this minimal model will clarify the understanding of many features of collective vesicle transport, apart from serving as the basis for building more exact quantitative models for vesicle transport relevant to various in vivo situations.

Non-spherical shapes of capsules within a fourth-order curvature model

We minimize a discrete version of the fourth-order curvature-based Landau free energy by extending Brakke's Surface Evolver. This model predicts spherical as well as non-spherical shapes with dimples, bumps and ridges to be the energy minimizers. Our results suggest that the buckling and faceting transitions, usually associated with crystalline matter, can also be an intrinsic property of non-crystalline membranes.

Atomic-resolution simulations predict a transition state for vesicle fusion defined by contact of a few lipid tails

Membrane fusion is essential to both cellular vesicle trafficking and infection by enveloped viruses. While the fusion protein assemblies that catalyze fusion are readily identifiable, the specific activities of the proteins involved and nature of the membrane changes they induce remain unknown. Here, we use many atomic-resolution simulations of vesicle fusion to examine the molecular mechanisms for fusion in detail. We employ committor analysis for these million-atom vesicle fusion simulations to identify a transition state for fusion stalk formation. In our simulations, this transition state occurs when the bulk properties of each lipid bilayer remain in a lamellar state but a few hydrophobic tails bulge into the hydrophilic interface layer and make contact to nucleate a stalk. Additional simulations of influenza fusion peptides in lipid bilayers show that the peptides promote similar local protrusion of lipid tails. Comparing these two sets of simulations, we obtain a common set of structural changes between the transition state for stalk formation and the local environment of peptides known to catalyze fusion. Our results thus suggest that the specific molecular properties of individual lipids are highly important to vesicle fusion and yield an explicit structural model that could help explain the mechanism of catalysis by fusion proteins.

Membrane fusion is a common underlying process critical to neurotransmitter release, cellular trafficking, and infection by many viruses. Proteins have been identified that catalyze fusion, and mutations to these proteins have yielded important information on how fusion occurs. However, the precise mechanism by which membrane fusion begins is the subject of active investigation. We have used atomic-resolution simulations to model the process of vesicle fusion and to identify a transition state for the formation of an initial fusion stalk. Doing so required substantial technical advances in combining high-performance simulation and distributed computing to analyze the transition state of a complex reaction in a large system. The transition state we identify in our simulations involves specific structural changes by a few lipid molecules. We also simulate fusion peptides from influenza hemagglutinin and show that they promote the same structural changes as are required for fusion in our model. We therefore hypothesize that these changes to individual lipid molecules may explain a portion of the catalytic activity of fusion proteins such as influenza hemagglutinin.

Mechanochemical crosstalk during endocytic vesicle formation

Membrane curvature has emerged as a key regulatory factor in endocytic vesicle formation. From a theoretical perspective, we summarize recent progress in understanding how membrane curvature and biochemical pathways are coupled and orchestrated during the coherent process of endocytic vesicle formation. We mainly focus on clathrin-mediated and actin-mediated endocytosis in yeast and in mammalian cells. We further speculate on how mechanochemical feedback could modulate other membrane-remodeling processes.

Rate of permeabilization of giant vesicles by amphiphilic polyacrylates compared to the adsorption of these polymers onto large vesicles and tethered lipid bilayers

We examined by fluorescence microscopy the permeabilization of giant vesicles by hydrophobically modified polyacrylates (called amphipols). Amphipols trigger permeabilization to FITC-dextran of egg-PC/DPPA vesicles with no breakage of the lipid bilayers. The polyanionic amphipols were passing through bilayers as shown by permeabilization of multilamellar vesicles. Remarkably, the vesicles were not simultaneously permeable but became leaky one after the other. Altogether, our observations suggest a random formation of pores having diameters above a few nanometers. Decreasing pH and increasing ionic strength and polymer concentration were increasing the rate of permeabilization. The rate and efficiency of permeabilization was compared to the rate and density of adsorption of amphipols onto lipid membranes (as estimated by titration calorimetry onto large unilamellar vesicles and neutron reflectivity measurements on tethered bilayers). The polymer adsorption layer is built up in a few minutes. We conclude that the rate-limiting step for permeabilization is not the adsorption from the bulk solution but relates to slow intramembrane reorganizations.

Gold nanoparticles prepared using cape aloe active components

A novel use of two components of Cape aloe, aloin A and aloesin, acting as stabilizers in the preparation of gold and silver nanoparticles, is reported. Stable water-soluble particles of different size and shape are prepared by varying the reaction conditions, temperature, reaction time, and reducing agents. Characterization of the obtained particles is performed using UV-visible, attenuated total reflection Fourier transform infrared (ATR-FTIR), and 1H NMR spectroscopies and transmission electron microscopy (TEM). The efficient cellular uptake of 50 nm sized aloin A and aloesin stabilized gold particles into macrophages and HeLa cells was investigated, proposing these particles as nanovehicles.

Monte Carlo simulations of small vesicles under osmotic pressure

A simple model for amphiphilic molecules is sufficient to predict self-assembly into spherical vesicles. The model is also useful to study the influence of osmotic pressure on the shape of vesicles, which serve as carriers in biological cells. The stability of small vesicles subjected to relatively high osmotic pressures is demonstrated. Fluctuations of small vesicles under osmotic stress are found to be in semiquantitative agreement with a macroscopic description. Small deviations between simulated results for vesicle fluctuations and macroscopic elasticity theory could result from the relative large membrane thickness in comparison to the vesicle radius. The simulations demonstrate that an osmotic pressure exerted by solute molecules outside a spherical vesicle can cause a shape transition, in agreement with results based on the elasticity theory of membranes.

A Myo6 mutation destroys coordination between the myosin heads, revealing new functions of myosin VI in the stereocilia of mammalian inner ear hair cells

Myosin VI, found in organisms from Caenorhabditis elegans to humans, is essential for auditory and vestibular function in mammals, since genetic mutations lead to hearing impairment and vestibular dysfunction in both humans and mice. Here, we show that a missense mutation in this molecular motor in an ENU-generated mouse model, Tailchaser, disrupts myosin VI function. Structural changes in the Tailchaser hair bundles include mislocalization of the kinocilia and branching of stereocilia. Transfection of GFP-labeled myosin VI into epithelial cells and delivery of endocytic vesicles to the early endosome revealed that the mutant phenotype displays disrupted motor function. The actin-activated ATPase rates measured for the D179Y mutation are decreased, and indicate loss of coordination of the myosin VI heads or ‘gating’ in the dimer form. Proper coordination is required for walking processively along, or anchoring to, actin filaments, and is apparently destroyed by the proximity of the mutation to the nucleotide-binding pocket. This loss of myosin VI function may not allow myosin VI to transport its cargoes appropriately at the base and within the stereocilia, or to anchor the membrane of stereocilia to actin filaments via its cargos, both of which lead to structural changes in the stereocilia of myosin VI–impaired hair cells, and ultimately leading to deafness.

Human deafness is extremely heterogeneous, with mutations in over 50 genes known to be associated with this common form of sensory loss. Among them, mutations in five myosins are associated with human hereditary hearing impairment, demonstrating that this family of proteins is essential for the proper function of the inner ear. Myosins, motor proteins found in eukaryotic cells, are responsible for actin-based motility. Composed of a motor domain and a tail, the former binds filamentous actin and uses ATP hydrolysis to generate force and move along the filaments, while the latter binds to cargos in the cell. Myosin VI is unique among myosins due to its movement along actin towards the minus or pointed end, rather than the positive or barbed end. Mutations in this myosin are associated with human deafness. Much of our information regarding myosin VI comes from studies in cell culture or mouse mutants with mutations leading to deafness. Here, we describe a deaf mouse mutant, Tailchaser, with a mutation in myosin VI. Our data describe new functions for myosin VI in the hair cells of the inner ear, showing how alterations in this motor can lead to a human sensory disorder.

2D-ELDOR using full S(c-) fitting and absorption lineshapes

Recent progress in developing 2D-ELDOR (2D electron-electron double resonance) techniques to better capture molecular dynamics in complex fluids, particularly in model and biological membranes, is reported. The new "full S(c-) method", which corrects the spectral analysis for the phase distortion effects present in the experiments, is demonstrated to enhance the sensitivity of 2D-ELDOR in reporting on molecular dynamics in complex membrane environments. That is, instead of performing spectral fitting in the magnitude mode, our new method enables simultaneous fitting of both the real and imaginary components of the S(c-) signal. The full S(c-) fitting not only corrects the phase distortions in the experimental data but also more accurately determines instrumental dead times. The phase corrections applied to the S(c-) spectrum enable the extraction of the pure absorption-mode spectrum, which is characterized by much better resolution than the magnitude-mode spectrum. In the absorption mode, the variation of homogeneous broadening, which reports on the dynamics of the spin probe, can even be observed by visual inspection. This new method is illustrated with results from model membranes of dipalmitoyl-sn-glycero-phosphatidylcholine (DPPC)-cholesterol binary mixtures, as well as with results from plasma membrane vesicles of mast cells. In addition to the dynamic parameters, which provide quantitative descriptions for membranes at the molecular level, the high-resolution absorption spectra themselves may be used as a "fingerprint" to characterize membrane phases and distinguish coexisting components in biomembranes. Thus we find that 2D-ELDOR is greatly improved with the new "full S(c-) method" especially for exploring the complexity of model and biological membranes.

Dynamic processes in endocytic transformation of a raft-exhibiting giant liposome

The dynamic response of a raft-exhibiting giant liposome to external stimuli, such as the addition of Triton X-100 or osmotic stress, was studied. We observed that daughter vesicles are generated inside of the liposome through endocytic budding. It was found that the budding to generate daughter vesicles is classified into two different routes, simple budding through the invagination of a whole raft and budding from the boundary of a raft accompanied by waving motion. Smaller rafts show a preference for simple budding, whereas large rafts mainly adopt the other process. We discuss the mechanism of this difference in terms of the kinetic pathway of internalization by considering the line energy and bending energy of the membrane.

Analysis of the selective advantage conferred by a C-E1 fusion protein synthesized by rubella virus DI RNAs

During serial passaging of rubella virus (RUB) in cell culture, the dominant species of defective-interfering RNA (DI) generated contains an in-frame deletion between the capsid protein (C) gene and E1 glycoprotein gene resulting in production of a C-E1 fusion protein that is necessary for the maintenance of the DI [Tzeng, W.P., Frey, T.K. (2006). C-E1 fusion protein synthesized by rubella virus DI RNAs maintained during serial passage. Virology 356 198-207.]. A BHK cell line stably expressing the RUB structural proteins was established which was used to package DIs into virus particles following transfection with in vitro transcripts from DI infectious cDNA constructs. Packaging of a DI encoding an in-frame C-GFP-E1 reporter fusion protein corresponding to the C-E1 fusion protein expressed in a native DI was only marginally more efficient than packaging of a DI encoding GFP, indicating that the C-E1 fusion protein did not function by enhancing packaging. However, infection with the DI encoding the C-GFP-E1 fusion protein (in the absence of wt RUB helper virus) resulted in formation of clusters of GFP-positive cells and the percentage of GFP-positive cells in the culture following infection remained relatively constant. In contrast, a DI encoding GFP did not form GFP-positive clusters and the percentage of GFP-positive cells declined by roughly half from 2 to 4 days post-infection. Cluster formation and sustaining the percentage of infected (GFP-positive) cells required the C part of the fusion protein, including the downstream but not the upstream of two arginine clusters (both of which are associated with RNA binding and association with mitochondrial p32 protein) and the E1 part through the transmembrane sequence, but not the C-terminal cytoplasmic tail. Among a collection of mutant DI constructs, cluster formation and sustaining infected cell percentage correlated with maintenance during serial passage with wt RUB. We hypothesize that cluster formation and sustaining infected cell percentage increase the likelihood of co-infection by a DI and wt RUB during serial passage thus enhancing maintenance of the DI. Cluster formation and sustaining infected cell percentage were found to be due to a combination of attenuated cytopathogenicity of DIs that express the C-E1 fusion protein and cell-to-cell movement of the DI. In infected cells, the C-GFP-E1 fusion protein was localized to potentially novel vesicular structures that appear to originate from ER-Golgi transport vacuoles. This species of DI expressing a C-E1 fusion protein that exhibits attenuated cytopathogenicity and the ability to increase the number of infected cells through cell-to-cell movement could be the basis for development of an attractive vaccine vector.

Primed vesicles can be distinguished from docked vesicles by analyzing their mobility

Neurotransmitters are released from nerve terminals and neuroendocrine cells by calcium-dependent exocytosis of vesicles. Before fusion, vesicles are docked to the plasma membrane and rendered release competent through a process called priming. Electrophysiological methods such as membrane capacitance measurements and carbon fiber amperometry accurately measure the fusion step of exocytosis with high time resolution but provide only indirect information about priming and docking. Total internal reflection fluorescence microscopy (TIRFM) enables the real-time visualization of vesicles, near the plasma membrane, as they undergo changes from one molecular state to the other. We devised a new method to analyze the mobility of vesicles, which not only allowed us to classify the movement of vesicles in three different categories but also to monitor dynamic changes in the mobility of vesicles over time. We selectively enhanced priming by treating bovine chromaffin cells with phorbol myristate acetate (PMA) or by overexpressing Munc13-1 (mammalian Unc) and analyzed the mobility of large dense-core vesicles. We demonstrate that nearly immobile vesicles represent primed vesicles because the pool of vesicles displaying this type of mobility was significantly increased after PMA treatment and Munc13-1 overexpression and decreased during tetanus toxin expression. Moreover, we showed that the movement of docked but unprimed vesicles is restricted to a confined region of approximately 220 nm diameter. Finally, a small third population of undocked vesicles showed a directed and probably active type of mobility. For the first time, we can thus distinguish the molecular state of vesicles in TIRFM by their mobility.

Multifunctional cargo systems for biotechnology

One of the challenges in the field of bio-nanotechnology is the development of nano-sized delivery systems comprising different functionalities. These systems should carry bioactive substances to predefined site and unload them in controlled manner. Capsules assembled layer-by-layer have been intensively studied in the past few years owing to their ability to be modified, their capacity to encapsulate a wide range of chemicals, their responsiveness to different factors, and the variety of functionalities with which they can be enhanced. Current research focuses on the development of carriers with remote guiding and activation (optical, magnetic or ultrasound), thereby addressing unique in vivo applications with multifunctional biomaterials. Submicron-sized capsules are good models to mimic biochemical processes in a confined geometry that imitates cell organelles. Moreover, the cellular and tissue-targeted delivery of the capsules might serve as an intracellular reporter or enzymatic reactor. However, several obstacles still have to be overcome before capsule technology can be implemented. This article discusses possible solutions as well as promising applications.

Large-scale fluid/fluid phase separation of proteins and lipids in giant plasma membrane vesicles

The membrane raft hypothesis postulates the existence of lipid bilayer membrane heterogeneities, or domains, supposed to be important for cellular function, including lateral sorting, signaling, and trafficking. Characterization of membrane lipid heterogeneities in live cells has been challenging in part because inhomogeneity has not usually been definable by optical microscopy. Model membrane systems, including giant unilamellar vesicles, allow optical fluorescence discrimination of coexisting lipid phase types, but thus far have focused on coexisting optically resolvable fluid phases in simple lipid mixtures. Here we demonstrate that giant plasma membrane vesicles (GPMVs) or blebs formed from the plasma membranes of cultured mammalian cells can also segregate into micrometer-scale fluid phase domains. Phase segregation temperatures are widely spread, with the vast majority of GPMVs found to form optically resolvable domains only at temperatures below approximately 25 degrees C. At 37 degrees C, these GPMV membranes are almost exclusively optically homogenous. At room temperature, we find diagnostic lipid phase fluorophore partitioning preferences in GPMVs analogous to the partitioning behavior now established in model membrane systems with liquid-ordered and liquid-disordered fluid phase coexistence. We image these GPMVs for direct visual characterization of protein partitioning between coexisting liquid-ordered-like and liquid-disordered-like membrane phases in the absence of detergent perturbation. For example, we find that the transmembrane IgE receptor FcepsilonRI preferentially segregates into liquid-disordered-like phases, and we report the partitioning of additional well known membrane associated proteins. Thus, GPMVs now provide an effective approach to characterize biological membrane heterogeneities.

Longin-like folds identified in CHiPS and DUF254 proteins: vesicle trafficking complexes conserved in eukaryotic evolution

Eukaryotic protein trafficking pathways require specific transfer of cargo vesicles to different target organelles. A number of vesicle trafficking and membrane fusion components participate in this process, including various tethering factor complexes that interact with small GTPases prior to SNARE-mediated vesicle fusion. In Saccharomyces cerevisiae a protein complex of Mon1 and Ccz1 functions with the small GTPase Ypt7 to mediate vesicle trafficking to the vacuole. Mon1 belongs to DUF254 found in a diverse range of eukaryotic genomes, while Ccz1 includes a CHiPS domain that is also present in a known human protein trafficking disorder gene (HPS-4). The present work identifies the CHiPS domain and a sequence region from another trafficking disorder gene (HPS-1) as homologs of an N-terminal domain from DUF254. This link establishes the evolutionary conservation of a protein complex (HPS-1/HPS-4) that functions similarly to Mon1/Ccz1 in vesicle trafficking to lysosome-related organelles of diverse eukaryotic species. Furthermore, the newly identified DUF254 domain is a distant homolog of the mu-adaptin longin domain found in clathrin adapter protein (AP) complexes of known structure that function to localize cargo protein to specific organelles. In support of this fold assignment, known longin domains such as the AP complex sigma-adaptin, the synaptobrevin N-terminal domains sec22 and Ykt6, and the srx domain of the signal recognition particle receptor also regulate vesicle trafficking pathways by mediating SNARE fusion, recognizing specialized compartments, and interacting with small GTPases that resemble Ypt7.

Direct binding to Rsp5 mediates ubiquitin-independent sorting of Sna3 via the multivesicular body pathway

The sorting of most integral membrane proteins into the lumenal vesicles of multivesicular bodies (MVBs) is dependent on the attachment of ubiquitin (Ub) to their cytosolic domains. However, Ub is not required for sorting of Sna3, an MVB vesicle cargo protein in yeast. We show that Sna3 circumvents Ub-mediated recognition by interacting directly with Rsp5, an E3 Ub ligase that catalyzes monoubiquitination of MVB vesicle cargoes. The PPAY motif in the C-terminal cytosolic domain of Sna3 binds the WW domains in Rsp5, and Sna3 is polyubiquitinated as a consequence of this association. However, Ub does not appear to be required for transport of Sna3 via the MVB pathway because its sorting occurs under conditions in which its ubiquitination is impaired. Consistent with Ub-independent function of the MVB pathway, we show by electron microscopy that the formation of MVB vesicles does not require Rsp5 E3 ligase activity. However, cells expressing a catalytically disabled form of Rsp5 have a greater frequency of smaller MVB vesicles compared with the relatively broad distribution of vesicles seen in MVBs of wild-type cells, suggesting that the formation of MVB vesicles is influenced by Rsp5-mediated ubiquitination.