Role of membrane organization and membrane domains in endocytic lipid trafficking

Atomistic Insight into the Lipid Nanodomains of Synaptic Vesicles

Membrane curvature, once regarded as a passive consequence of membrane composition and cellular architecture, has been shown to actively modulate various properties of the cellular membrane. These changes could also lead to segregation of the constituents of the membrane, generating nanodomains with precise biological properties. Proteins often linked with neurodegeneration (e.g., tau, alpha-synuclein) exhibit an unintuitive affinity for synaptic vesicles in neurons, which are reported to lack distinct, ordered nanodomains based on their composition. In this study, all-atom molecular dynamics simulations are used to study a full-scale synaptic vesicle of realistic Gaussian curvature and its effect on the membrane dynamics and lipid nanodomain organization. Compelling indicators of nanodomain formation, from the perspective of composition, surface areas per lipid, order parameter, and domain lifetime, are identified in the vesicle membrane, which are absent in a flat bilayer of the same lipid composition. Therefore, our study supports the idea that curvature may induce phase separation in an otherwise fluid, disordered membrane.

Concentration-dependent mechanism of the binding behavior of ibuprofen to the cell membrane: A molecular dynamic simulation study

Ibuprofen is a commonly used drug for treating headaches, pain, and fever. The lipid bilayer is the primary and most important interface for drugs to interact with biological systems. However, the molecular interactions between ibuprofen and the cell membrane are not well understood. Our findings suggest that the interactions between ibuprofen and the bilayer involve multiple steps and depend on the concentration of the drug. At low concentrations of ibuprofen, it can bind to the surface of the lipid bilayer. The electrostatic and vdW energies of IBU-lipid at 0 ns of the simulation were -22.5 ± 3.2 and -5.9 ± 1.2 kj.mol-1 Fig. 2. In the following, the vdW energy of the IBU-lipid was increased by around -134.6 ± 3.7 kj.mol-1 whereas the electrostatic energy of the IBU-lipid was significantly decreased. This binding is facilitated by electrostatic and vdW interactions between ibuprofen and the head group of lipids. In the second step, ibuprofen is inserted into the lipid bilayer and positioned at the interface between the bilayer and the aqueous phase. In high concentrations of ibuprofen, it moved to the central region of the lipid bilayer. At this concentration, the physical and structural properties of the cell membrane change significantly. Results from the radial distribution function analysis indicate that at low concentrations, ibuprofen molecules are situated close to the head groups of phosphate groups. However, at high concentrations of ibuprofen, these molecules move to the inner side of the lipid bilayer. In addition, our findings indicate that at low concentrations of ibuprofen, these molecules did not significantly alter the physical properties of the cell membrane. In contrast, at high concentrations of ibuprofen, the physical parameters of the hydrocarbon tails, such as thickness, fluidity, and order, changed dramatically. APL parameter for POPC membrane increased slightly to 0.60 and 0.63 nm2 in the presence of low and high concentrations of ibuprofen molecules. The three-step interaction between ibuprofen and the lipid bilayer involves several events, such as the movement of ibuprofen molecules towards the central region of the lipid bilayer and the deformation and alteration of the structural and stability properties of the cell membrane. These effects are observed only at high concentrations of ibuprofen. It appears that the side effects of ibuprofen overdose are related to changes in the properties of the cell membrane and, subsequently, the function of membrane-anchored target proteins.

Nanoscale membrane curvature sorts lipid phases and alters lipid diffusion

The precise spatiotemporal control of nanoscale membrane shape and composition is the result of a complex interplay of individual and collective molecular behaviors. Here, we employed single-molecule localization microscopy and computational simulations to observe single-lipid diffusion and sorting in model membranes with varying compositions, phases, temperatures, and curvatures. Supported lipid bilayers were created over 50-nm-radius nanoparticles to mimic the size of naturally occurring membrane buds, such as endocytic pits and the formation of viral envelopes. The curved membranes recruited liquid-disordered lipid phases while altering the diffusion and sorting of tracer lipids. Disorder-preferring fluorescent lipids sorted to and experienced faster diffusion on the nanoscale curvature only when embedded in a membrane capable of sustaining lipid phase separation at low temperatures. The curvature-induced sorting and faster diffusion even occurred when the sample temperature was above the miscibility temperature of the planar membrane, implying that the nanoscale curvature could induce phase separation in otherwise homogeneous membranes. Further confirmation and understanding of these results are provided by continuum and coarse-grained molecular dynamics simulations with explicit and spontaneous curvature-phase coupling, respectively. The curvature-induced membrane compositional heterogeneity and altered dynamics were achieved only with a coupling of the curvature with a lipid phase separation. These cross-validating results demonstrate the complex interplay of lipid phases, molecular diffusion, and nanoscale membrane curvature that are critical for membrane functionality.

The manipulation of cell signaling and host cell biology by cholera toxin

Vibrio cholerae colonizes the small intestine and releases cholera toxin into the extracellular space. The toxin binds to the apical surface of the epithelium, is internalized into the host endomembrane system, and escapes into the cytosol where it activates the stimulatory alpha subunit of the heterotrimeric G protein by ADP-ribosylation. This initiates a cAMP-dependent signaling pathway that stimulates chloride efflux into the gut, with diarrhea resulting from the accompanying osmotic movement of water into the intestinal lumen. G protein signaling is not the only host system manipulated by cholera toxin, however. Other cellular mechanisms and signaling pathways active in the intoxication process include endocytosis through lipid rafts, retrograde transport to the endoplasmic reticulum, the endoplasmic reticulum-associated degradation system for protein delivery to the cytosol, the unfolded protein response, and G protein de-activation through degradation or the function of ADP-ribosyl hydrolases. Although toxin-induced chloride efflux is thought to be an irreversible event, alterations to these processes could facilitate cellular recovery from intoxication. This review will highlight how cholera toxin exploits signaling pathways and other cell biology events to elicit a diarrheal response from the host.

Recombination between 13C and 2H to Form Acetylide (13C22H-) Probes Nanoscale Interactions in Lipid Bilayers via Dynamic Secondary Ion Mass Spectrometry: Cholesterol and GM1 Clustering

Although it is thought that there is lateral heterogeneity of lipid and protein components within biological membranes, probing this heterogeneity has proven challenging. The difficulty in such experiments is due to both the small length scale over which such heterogeneity can occur, and the significant perturbation resulting from fluorescent or spin labeling on the delicate interactions within bilayers. Atomic recombination during dynamic nanoscale secondary ion imaging mass spectrometry (NanoSIMS) is a non-perturbative method for examining nanoscale bilayer interactions. Atomic recombination is a variation on conventional NanoSIMS imaging, whereby an isotope on one molecule combines with a different isotope on another molecule during the ionization process, forming an isotopically enriched polyatomic ion in a distance-dependent manner. We show that the recombinant ion, ¹³C₂²H^-, is formed in high yield from ¹³C- and ²H-labeled lipids. The low natural abundance of triply labeled acetylide also makes it an ideal ion to probe GM₁ clusters in model membranes and the effects of cholesterol on lipid-lipid interactions. We find evidence supporting the cholesterol condensation effect as well as the presence of nanoscale GM₁ clusters in model membranes.

ROOT HAIR DEFECTIVE 2 vesicular delivery to the apical plasma membrane domain during Arabidopsis root hair development

Arabidopsis (Arabidopsis thaliana) root hairs develop as long tubular extensions from the rootward pole of trichoblasts and exert polarized tip growth. The establishment and maintenance of root hair polarity is a complex process involving the local apical production of reactive oxygen species generated by A. thaliana nicotinamide adenine dinucleotide phosphate (NADPH) oxidase respiratory burst oxidase homolog protein C/ROOT HAIR-DEFECTIVE 2 (AtRBOHC/RHD2). Loss-of-function root hair defective 2 (rhd2) mutants have short root hairs that are unable to elongate by tip growth, and this phenotype is fully complemented by GREEN FLUORESCENT PROTEIN (GFP)-RHD2 expressed under the RHD2 promoter. However, the spatiotemporal mechanism of AtRBOHC/RHD2 subcellular redistribution and delivery to the plasma membrane (PM) during root hair initiation and tip growth are still unclear. Here, we used advanced microscopy for detailed qualitative and quantitative analysis of vesicular compartments containing GFP-RHD2 and characterization of their movements in developing bulges and growing root hairs. These compartments, identified by an independent molecular marker mCherry-VTI12 as the trans-Golgi network (TGN), deliver GFP-RHD2 to the apical PM domain, the extent of which corresponds with the stage of root hair formation. Movements of TGN/early endosomes, but not late endosomes, were affected in the bulging domains of the rhd2-1 mutant. Finally, we revealed that structural sterols might be involved in the accumulation, docking, and incorporation of TGN compartments containing GFP-RHD2 to the apical PM of root hairs. These results help in clarifying the mechanism of polarized AtRBOHC/RHD2 targeting, maintenance, and recycling at the apical PM domain, coordinated with different developmental stages of root hair initiation and growth.

Plasma Membrane Lipids: An Important Binding Site for All Lipoprotein Classes

Cholesterol is one of the main constituents of plasma membranes; thus, its supply is of utmost importance. This review covers the known mechanisms of cholesterol transfer from circulating lipoprotein particles to the plasma membrane, and vice versa. To achieve homeostasis, the human body utilizes cellular de novo synthesis and extracellular transport particles for supply of cholesterol and other lipids via the blood stream. These lipoprotein particles can be classified according to their density: chylomicrons, very low, low, and high-density lipoprotein (VLDL, LDL, and HDL, respectively). They deliver and receive their lipid loads, most importantly cholesterol, to and from cells by several redundant routes. Defects in one of these pathways (e.g., due to mutations in receptors) usually are not immediately fatal. Several redundant pathways, at least temporarily, compensate for the loss of one or more of them, but the defects trigger systemic diseases, such as atherosclerosis later on. Recently, intracellular membrane–membrane contact sites were shown to be involved in intracellular cholesterol transfer and the plasma membrane itself has been proposed to act as a binding site for lipoprotein-mediated cargo unloading.

Pattern formation in reaction-diffusion system on membrane with mechanochemical feedback

Shapes of biological membranes are dynamically regulated in living cells. Although membrane shape deformation by proteins at thermal equilibrium has been extensively studied, nonequilibrium dynamics have been much less explored. Recently, chemical reaction propagation has been experimentally observed in plasma membranes. Thus, it is important to understand how the reaction-diffusion dynamics are modified on deformable curved membranes. Here, we investigated nonequilibrium pattern formation on vesicles induced by mechanochemical feedback between membrane deformation and chemical reactions, using dynamically triangulated membrane simulations combined with the Brusselator model. We found that membrane deformation changes stable patterns relative to those that occur on a non-deformable curved surface, as determined by linear stability analysis. We further found that budding and multi-spindle shapes are induced by Turing patterns, and we also observed the transition from oscillation patterns to stable spot patterns. Our results demonstrate the importance of mechanochemical feedback in pattern formation on deforming membranes.

Lipoproteins-Nanocarriers as a Promising Approach for Targeting Liver Cancer: Present Status and Application Prospects

The prevalence of liver cancer is increasing over the years and it is the fifth leading cause of mortality worldwide. The intrusive features and burden of low survival rate make it a global health issue in both developing and developed countries. The recommended chemotherapy drugs for patients in the intermediate and advanced stages of various liver cancers yield a low response rate due to the nonspecific nature of drug delivery, thus warranting the search for new therapeutic strategies and potential drug delivery carriers. There are several new drug delivery methods available to ferry the targeted molecules to the specific biological environment. In recent years, the nano assembly of lipoprotein moieties (lipidic nanoparticles) has emerged as a promising and efficiently tailored drug delivery system in liver cancer treatment. This increased precision of nano lipoproteins conjugates in chemotherapeutic targeting offers new avenues for the treatment of liver cancer with high specificity and efficiency. This present review is focused on concisely outlining the knowledge of liver cancer diagnosis, existing treatment strategies, lipoproteins, their preparation, mechanism and their potential application in the treatment of liver cancer.

DDHD1, but Not DDHD2, Suppresses Neurite Outgrowth in SH-SY5Y and PC12 Cells by Regulating Protein Transport From Recycling Endosomes

DDHD1 and DDHD2 are both intracellular phospholipases A₁ and hydrolyze phosphatidic acid in vitro. Given that phosphatidic acid participates in neurite outgrowth, we examined whether DDHD1 and DDHD2 regulate neurite outgrowth. Depletion of DDHD1 from SH-SY5Y and PC12 cells caused elongation of neurites, whereas DDHD2 depletion prevented neurite elongation. Rescue experiments demonstrated that the enzymatic activity of DDHD1 is necessary for the prevention of neurite elongation. Depletion of DDHD1 caused enlargement of early endosomes and stimulated tubulation of recycling endosomes positive for phosphatidic acid-binding proteins syndapin2 and MICAL-L1. Knockout of DDHD1 enhanced transferrin recycling from recycling endosomes to the cell surface. Our results suggest that DDHD1 negatively controls the formation of a local phosphatidic acid-rich domain in recycling endosomes that serves as a membrane source for neurite outgrowth.

Phase Separation in Membrane Biology: The Interplay between Membrane-Bound Organelles and Membraneless Condensates

In eukaryotic cells, various membrane-bound organelles compartmentalize diverse cellular activities in a spatially and temporally controlled manner. Numerous membraneless organelles assembled via liquid-liquid phase separation (LLPS), known as condensates, also facilitate compartmentalization of cellular functions. Emerging evidence shows that these two organelle types interact in many biological processes. Membranes modulate the biogenesis and dynamics of phase-separated condensates by serving as assembly platforms or by forming direct contacts. Phase separation of membrane-associated proteins participates in various trafficking events, such as clustering of vesicles for temporally controlled fusion and storage, and transport of membraneless condensates on membrane-bound organelles. Phase separation also acts in cargo trafficking pathways by sorting and docking cargos for translocon-mediated transport across membranes, by shuttling cargos through the nuclear pore complex, and by triggering the formation of surrounding autophagosomes for delivery to lysosomes. The coordinated actions of membrane-bound and membraneless organelles ensure spatiotemporal control of various cellular functions.

Raman Spectroscopy Study of Curvature-Mediated Lipid Packing and Sorting in Single Lipid Vesicles

Cellular plasma membrane deformability and stability is important in a range of biological processes. Changes in local curvature of the membrane affect the lateral movement of lipids, affecting the biophysical properties of the membrane. An integrated holographic optical tweezers and Raman microscope was used to investigate the effect of curvature gradients induced by optically stretching individual giant unilamellar vesicles (GUVs) on lipid packing and lateral segregation of cholesterol in the bilayer. The spatially resolved Raman analysis enabled detection of induced phase separation and changes in lipid ordering in individual GUVs. Using deuterated cholesterol, the changes in lipid ordering and phase separation were linked to lateral sorting of cholesterol in the stretched GUVs. Stretching the GUVs in the range of elongation factors 1-1.3 led to an overall decrease in cholesterol concentration at the edges compared to the center of stretched GUVs. The Raman spectroscopy results were consistent with a model of the bilayer accounting for cholesterol sorting in both bilayer leaflets, with a compositional asymmetry of 0.63 ± 0.04 in favor of the outer leaflet. The results demonstrate the potential of the integrated holographic optical tweezers-Raman technique to induce deformations to individual lipid vesicles and to simultaneously provide quantitative and spatially resolved molecular information. Future studies can extend to include more realistic models of cell membranes and potentially live cells.

Modeling Membrane Curvature Generation due to Membrane⁻Protein Interactions

To alter and adjust the shape of the plasma membrane, cells harness various mechanisms of curvature generation. Many of these curvature generation mechanisms rely on the interactions between peripheral membrane proteins, integral membrane proteins, and lipids in the bilayer membrane. Mathematical and computational modeling of membrane curvature generation has provided great insights into the physics underlying these processes. However, one of the challenges in modeling these processes is identifying the suitable constitutive relationships that describe the membrane free energy including protein distribution and curvature generation capability. Here, we review some of the commonly used continuum elastic membrane models that have been developed for this purpose and discuss their applications. Finally, we address some fundamental challenges that future theoretical methods need to overcome to push the boundaries of current model applications.

Reorganization of Ternary Lipid Mixtures of Nonphosphorylated Phosphatidylinositol Interacting with Angiomotin

Phosphatidylinositol (PI) lipids are necessary for many cellular signaling pathways of membrane associated proteins, such as angiomotin (Amot). The Amot family regulates cellular polarity, growth, and migration. Given the low concentration of PI lipids in these membranes, it is likely that such protein-membrane interactions are stabilized by lipid domains or small lipid clusters. By small-angle X-ray scattering, we show that nonphosphorylated PI lipids induce lipid demixing in ternary mixtures of phosphatidylcholine (PC) and phosphatidylethanolamine (PE), likely because of preferential interactions between the head groups of PE and PI. These results were obtained in the presence of buffer containing tris(hydroxymethyl)aminomethane, 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid, NaCl, ethylenediaminetetraacetic acid, dithiothreitol, and benzamidine at pH 8.0 that in previous work showed an ability to cause PC to phase separate but are necessary to stabilize Amot for in vitro experimentation. Collectively, this provided a framework for determining the effect of Amot on lipid organization. Using fluorescence spectroscopy, we were able to show that the association of Amot with this lipid platform causes significant reorganization of the lipid into a more homogenous structure. This reorganization mechanism could be the basis for Amot membrane association and fusogenic activity previously described in the literature and should be taken into consideration in future protein-membrane interaction studies.

Entropic forces drive clustering and spatial localization of influenza A M2 during viral budding

The influenza A matrix 2 (M2) transmembrane protein facilitates virion release from the infected host cell. In particular, M2 plays a role in the induction of membrane curvature and/or in the scission process whereby the envelope is cut upon virion release. Here we show using coarse-grained computer simulations that various M2 assembly geometries emerge due to an entropic driving force, resulting in compact clusters or linearly extended aggregates as a direct consequence of the lateral membrane stresses. Conditions under which these protein assemblies will cause the lipid membrane to curve are explored, and we predict that a critical cluster size is required for this to happen. We go on to demonstrate that under the stress conditions taking place in the cellular membrane as it undergoes large-scale membrane remodeling, the M2 protein will, in principle, be able to both contribute to curvature induction and sense curvature to line up in manifolds where local membrane line tension is high. M2 is found to exhibit linactant behavior in liquid-disordered-liquid-ordered phase-separated lipid mixtures and to be excluded from the liquid-ordered phase, in near-quantitative agreement with experimental observations. Our findings support a role for M2 in membrane remodeling during influenza viral budding both as an inducer and a sensor of membrane curvature, and they suggest a mechanism by which localization of M2 can occur as the virion assembles and releases from the host cell, independent of how the membrane curvature is produced.

The role of traction in membrane curvature generation

Curvature of biological membranes can be generated by a variety of molecular mechanisms including protein scaffolding, compositional heterogeneity, and cytoskeletal forces. These mechanisms have the net effect of generating tractions (force per unit length) on the bilayer that are translated into distinct shapes of the membrane. Here, we demonstrate how the local shape of the membrane can be used to infer the traction acting locally on the membrane. We show that buds and tubes, two common membrane deformations studied in trafficking processes, have different traction distributions along the membrane and that these tractions are specific to the molecular mechanism used to generate these shapes. Furthermore, we show that the magnitude of an axial force applied to the membrane as well as that of an effective line tension can be calculated from these tractions. Finally, we consider the sensitivity of these quantities with respect to uncertainties in material properties and follow with a discussion on sources of uncertainty in membrane shape.

The glycolipid GM1 reshapes asymmetric biomembranes and giant vesicles by curvature generation

The ganglioside GM1 is present in neuronal membranes at elevated concentrations with an asymmetric spatial distribution. It is known to generate curvature and can be expected to strongly influence the neuron morphology. To elucidate these effects, we prepared giant vesicles with GM1 predominantly present in one leaflet of the membrane, mimicking the asymmetric GM1 distribution in neuronal membranes. Based on pulling inward and outward tubes, we developed a technique that allowed the direct measurement of the membrane spontaneous curvature. Using vesicle electroporation and fluorescence intensity analysis, we were able to quantify the GM1 asymmetry across the membrane and to subsequently estimate the local curvature generated by the molecule in the bilayer. Molecular-dynamics simulations confirm the experimentally determined dependence of the membrane spontaneous curvature as a function of GM1 asymmetry. GM1 plays a crucial role in connection with receptor proteins. Our results on curvature generation of GM1 point to an additional important role of this ganglioside, namely in shaping neuronal membranes.

Exploring Molecular-Biomembrane Interactions with Surface Plasmon Resonance and Dual Polarization Interferometry Technology: Expanding the Spotlight onto Biomembrane Structure

The molecular analysis of biomolecular-membrane interactions is central to understanding most cellular systems but has emerged as a complex technical challenge given the complexities of membrane structure and composition across all living cells. We present a review of the application of surface plasmon resonance and dual polarization interferometry-based biosensors to the study of biomembrane-based systems using both planar mono- or bilayers or liposomes. We first describe the optical principals and instrumentation of surface plasmon resonance, including both linear and extraordinary transmission modes and dual polarization interferometry. We then describe the wide range of model membrane systems that have been developed for deposition on the chips surfaces that include planar, polymer cushioned, tethered bilayers, and liposomes. This is followed by a description of the different chemical immobilization or physisorption techniques. The application of this broad range of engineered membrane surfaces to biomolecular-membrane interactions is then overviewed and how the information obtained using these techniques enhance our molecular understanding of membrane-mediated peptide and protein function. We first discuss experiments where SPR alone has been used to characterize membrane binding and describe how these studies yielded novel insight into the molecular events associated with membrane interactions and how they provided a significant impetus to more recent studies that focus on coincident membrane structure changes during binding of peptides and proteins. We then discuss the emerging limitations of not monitoring the effects on membrane structure and how SPR data can be combined with DPI to provide significant new information on how a membrane responds to the binding of peptides and proteins.

Interaction of Soybean 7S Globulin Peptide with Cell Membrane Model via Isothermal Titration Calorimetry, Quartz Crystal Microbalance with Dissipation, and Langmuir Monolayer Study

To understand the underlying molecular mechanism of the cholesterol-lowering effect of soybean 7S globulins, the interactions of their pepsin-released peptides (7S-peptides) with cell membrane models consisting of dipalmitoylphosphatidylcholine (DPPC), dioleoylphosphatidylcholine (DOPC), and cholesterol (CHOL) were systematically studied. The results showed that 7S-peptides were bound to DPPC/DOPC/CHOL liposomes mainly through van der Waals forces and hydrogen bonds, and the presence of higher CHOL concentrations enhanced the binding affinity (e.g., DPPC/DOPC/CHOL = 1:1:0, binding ratio = 0.114; DPPC/DOPC/CHOL = 1:1:1, binding ratio = 2.02). Compression isotherms indicated that the incorporation of 7S-peptides increased the DPPC/DOPC/CHOL monolayer fluidity and the lipid raft size. The presence of CHOL accelerated the 7S-peptide accumulation on lipid rafts, which could serve as platforms for peptides to develop into β-sheet rich structures. These results allow us to hypothesize that 7S-peptides may indirectly influence membrane protein functions via altering the membrane organization in the enterocytes.

Ganglioside Metabolism and Its Inherited Diseases

Gangliosides are sialic acid containing glycosphingolipids, which are abundant in mammalian brain tissue. Several fatal human diseases are caused by defects in glycolipid metabolism. Defects in their degradation lead to an accumulation of metabolites upstream of the defective reactions, whereas defects in their biosynthesis lead to diverse problems in a large number of organs.Gangliosides are primarily positioned with their ceramide anchor in the neuronal plasma membrane and the glycan head group exposed on the cell surface. Their biosynthesis starts in the endoplasmic reticulum with the formation of the ceramide anchor, followed by sequential glycosylation reactions, mainly at the luminal surface of Golgi and TGN membranes, a combinatorial process, which is catalyzed by often promiscuous membrane-bound glycosyltransferases.Thereafter, the gangliosides are transported to the plasma membrane by exocytotic membrane flow. After endocytosis, they are degraded within the endolysosomal compartments by a complex machinery of degrading enzymes, lipid-binding activator proteins, and negatively charged lipids.

Electroformation of phospholipid giant unilamellar vesicles in physiological phosphate buffer

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

The Lipids of the Early Endosomes: Making Multimodality Work

Early endosomes are dynamic intracellular compartments that fuse with incoming endocytic carrier vesicles and associated cargoes from the plasma membrane. It has been long known that the chemical structures of lipids confer striking properties and rich biochemistry on bilayers. Although the organisational principles of the plasma membrane are relatively better understood, understanding endosomal membranes has been challenging. It has become increasingly apparent that endosomal membranes, because of their lipid compositions and interactions, use distinct lipid chemistries. We discuss the biochemical and biophysical phenomena in play at the early endosomal membrane. We focus on cholesterol, phosphoinositides, and phosphatidylserine and their clear roles in endosome functions. We discuss the various principles and mechanisms underpinning how these lipids are implicated at the functional level in the working of endosomes, and we summarise early endosomes as a multimodal organelle employing distinct lipid-specific mechanisms.

Membrane localization and dynamics of geranylgeranylated Rab5 hypervariable region

The small GTPase Rab5 is a key regulator of endosomal trafficking processes and a marker for the early endosome. The C-terminal hypervariable region (HVR) of Rab5 is post-translationally modified at residues Cys²¹² and Cys²¹³ to accommodate two geranylgeranyl anchors (C20 carbon chain length) in order to associate Rab5 with the membrane. The structural role of the HVR regarding protein-early endosome membrane recruitment is not resolved due to its high degree of flexibility and lack of crystallographic information. Here, full-atomistic and coarse-grained molecular dynamics simulations of the truncated Rab5 HVR^206-215 in three model membranes of increasing complexity (pure phospholipid bilayer, ternary membrane with cholesterol, six-component early endosome) were performed. Specific electrostatic interactions between the HVR^206-215 Arg²⁰⁹ residue and the phosphate group of the inositol ring of PI(3)P were detected. This shows that PI(3)P acts as a first contact site of protein recruitment to the early endosome. The free energy change of HVR^206-215 extraction from the bilayer was largest for the physiological negatively charged membrane. 5μs coarse-grained simulations revealed an active recruitment of PI(3)P to the HVR^206-215 supporting the formation of Rab5- and PI(3)P enriched signaling platforms.

Analysis of diffusion in curved surfaces and its application to tubular membranes

Diffusion of particles in curved surfaces is inherently complex compared with diffusion in a flat membrane, owing to the nonplanarity of the surface. The consequence of such nonplanar geometry on diffusion is poorly understood but is highly relevant in the case of cell membranes, which often adopt complex geometries. To address this question, we developed a new finite element approach to model diffusion on curved membrane surfaces based on solutions to Fick's law of diffusion and used this to study the effects of geometry on the entry of surface-bound particles into tubules by diffusion. We show that variations in tubule radius and length can distinctly alter diffusion gradients in tubules over biologically relevant timescales. In addition, we show that tubular structures tend to retain concentration gradients for a longer time compared with a comparable flat surface. These findings indicate that sorting of particles along the surfaces of tubules can arise simply as a geometric consequence of the curvature without any specific contribution from the membrane environment. Our studies provide a framework for modeling diffusion in curved surfaces and suggest that biological regulation can emerge purely from membrane geometry.

Plant phosphoinositide signaling - dynamics on demand

Eukaryotic membranes contain small amounts of lipids with regulatory roles. An important class of such regulatory lipids are phosphoinositides (PIs). Within membranes, PIs serve as recruitment signals, as regulators of membrane protein function or as precursors for second messenger production, thereby influencing a multitude of cellular processes with key importance for plant function and development. Plant PIs occur locally and transiently within membrane microdomains, and their abundance is strictly controlled. To understand the functions of the plant PI-network it is important to understand not only downstream PI-effects, but also to identify and characterize factors contributing to dynamic PI formation. This article is part of a Special Issue entitled: Plant Lipid Biology edited by Kent D. Chapman and Ivo Feussner.

Hydrogen Exchange Mass Spectrometry of Proteins at Langmuir Monolayers

Hydrogen exchange (HX) mass spectrometry (MS) is valuable for providing conformational information for proteins/peptides that are very difficult to analyze with other methods such as peripheral membrane proteins and peptides that interact with membranes. We developed a new type of HX MS measurement that integrates Langmuir monolayers. A lipid monolayer was generated, a peptide or protein associated with it, and then the monolayer-associated peptide or protein was exposed to deuterium. The deuterated species was recovered from the monolayer, digested, and deuterium incorporation monitored by MS. Test peptides showed that deuterium recovery in an optimized protocol was equivalent to deuterium recovery in conventional solution HX MS. The reproducibility of the measurements was high, despite the requirement of generating a new monolayer for each deuterium labeling time. We validated that known conformational changes in the presence of a monolayer/membrane could be observed with the peptide melittin and the myristoylated protein Arf-1. Results in an accompanying paper show that the method can reveal details of conformational changes in a protein (HIV-1 Nef), which adopts a different conformation, depending on whether or not it is able to insert into the lipid layer. Overall, the HX MS Langmuir monolayer method provided new and meaningful conformational information for proteins that associate with lipid layers. The combination of HX MS results with neutron or X-ray reflection of the same proteins in Langmuir monolayers can be more informative than the isolated use of either method.

Phase separation behavior of mixed lipid systems at neutral and low pH: coarse-grained simulations with DMD/LIME

We extend LIME, an intermediate resolution, implicit solvent model for phospholipids previously used in discontinuous molecular dynamics simulations of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) bilayer formation at 325 K, to the description of the geometry and energetics of 1,2-distearoyl-sn-glycero-3-phospho-L-serine (DSPS) and 1,2-dihenarachidoyl-sn-glycero-3-phosphocholine (21PC) and mixtures thereof at both neutral and low pH at 310 K. A multiscale modeling approach is used to calculate the LIME parameters from atomistic simulation data on a mixed DPPC/DSPS system at different pH values. In the model, 17 coarse-grained sites represent DSPS and 18 coarse-grained sites represent 21PC. Each of these coarse-grained sites is classified as 1 of 9 types. LIME/DMD simulations of equimolar bilayers show the following: (1) 21PC/DSPS bilayers with and without surface area restrictions separate faster at low pH than at neutral pH, (2) 21PC/DSPS systems separate at approximately the same rate regardless of whether they are subjected to surface area restrictions, and (3) bilayers with a molar ratio of 9:1 (21PC:DSPS) phase separate to form heterogeneous domains faster at low pH than at neutral pH. Our results are consistent with experimental findings of Sofou and co-workers (Bandekar et al. Mol. Pharmaceutics, 2013, 10, 152-160; Karve et al. Biomaterials, 2010, 31, 4409-4416) that more doxorubicin is released from 21PC/DSPS liposomes at low pH than at neutral pH, presumably because greater phase separation is achieved at low pH than at neutral pH. These are the first molecular-level simulations of the phase separation in mixed lipid bilayers induced by a change in pH.

Role of lipid polymorphism in acoustically sensitive liposomes

Ultrasound (US) triggered drug release is a promising drug delivery method that allows ex vivo modulation of treatment intensity and duration. This method relies on the synergistic interaction between the rupture of sonosensitive particles and enhanced plasma membrane permeability. Conventional liposomal systems where the drug passively diffuses through the membrane show virtually no response to acoustic energy. One method to activate drug transport is to induce a topological restructuring of the lipid membrane (zero intrinsic curvature, H = 0) by puncturing pores (H < 0) through which the drug can readily leak out from the interior of the liposomes. In this work we demonstrate strategies to lower the energy cost of creating such membrane defects by introducing lipid molecules with molecular shapes prone to self-assemble into non-lamellar (negative intrinsic curvature, H < 0) structures. All formulations investigated comprise the relevant components typically required for delivery applications such as stealth moieties, cholesterol, and phospholipids. Small angle X-ray scattering studies of a number of lipid systems at increasing amounts of phosphatidylethanolamine (PE) phospholipids reveal that membranes without PE respond to ultrasound by thinning ca. 10 Å, which concomitantly lowers the bending rigidity quadratically in addition to increasing the passive drug permeability. However, at the appropriate PE content the lipid systems display a classic lamellar structure (H = 0) that undergoes a topological transformation after ultrasound exposure into lipid tubes of the reversed type (H < 0) packed in a 2D hexagonal array. At the dilute regime, Fluorescence Microscopy of giant unilamellar vesicles (GUVs) comprising DOPE also experience ultrasound induced restructuring that can be modulated by DOPE content. In general, smaller vesicles of diverse shape connect and form into a "pearl-necklace" configuration. We argue that the inclusion of DOPE within the GUV membrane may result in curvature-driven lipid sorting, providing the system with local membrane instabilities that drive vesicle pearling when exposed to ultrasound.

The physical chemistry of the stratum corneum lipids

This article summarizes the current knowledge of the composition, self-assembly, and molecular organization of the stratum corneum (SC) lipids, reviews the evidence connecting these parameters and the barrier properties of human skin, and outlines the immediate issues in the field of SC lipid research.

Rab proteins: the key regulators of intracellular vesicle transport

Vesicular/membrane trafficking essentially regulates the compartmentalization and abundance of proteins within the cells and contributes in many signalling pathways. This membrane transport in eukaryotic cells is a complex process regulated by a large and diverse array of proteins. A large group of monomeric small GTPases; the Rabs are essential components of this membrane trafficking route. Most of the Rabs are ubiquitously expressed proteins and have been implicated in vesicle formation, vesicle motility/delivery along cytoskeleton elements and docking/fusion at target membranes through the recruitment of effectors. Functional impairments of Rabs affecting transport pathways manifest different diseases. Rab functions are accompanied by cyclical activation and inactivation of GTP-bound and GDP-bound forms between the cytosol and membranes which is regulated by upstream regulators. Rab proteins are characterized by their distinct sub-cellular localization and regulate a wide variety of endocytic, transcytic and exocytic transport pathways. Mutations of Rabs affect cell growth, motility and other biological processes.

Designer lipids for drug delivery: from heads to tails

For four decades, liposomes composed of both naturally occurring and synthetic lipids have been investigated as delivery vehicles for low molecular weight and macromolecular drugs. These studies paved the way for the clinical and commercial success of a number of liposomal drugs, each of which required a tailored formulation; one liposome size does not fit all drugs! Instead, the physicochemical properties of the liposome must be matched to the pharmacology of the drug. An extensive biophysical literature demonstrates that varying lipid composition can influence the size, membrane stability, in vivo interactions, and drug release properties of a liposome. In this review we focus on recently described synthetic lipid headgroups, linkers and hydrophobic domains that can provide control over the intermolecular forces, phase preference, and macroscopic behavior of liposomes. These synthetic lipids further our understanding of lipid biophysics, promote targeted drug delivery and improve liposome stability. We further highlight the immune reactivity of novel synthetic headgroups as a key design consideration. For instance it was originally thought that synthetic PEGylated lipids were immunologically inert; however, it's been observed that under certain conditions PEGylated lipids induce humoral immunity. Such immune activation may be a limitation to the use of other engineered lipid headgroups for drug delivery. In addition to the potential immunogenicity of engineered lipids, future investigations on liposome drugs in vivo should pay particular attention to the location and dynamics of payload release.

Absorption, distribution and pathological injury in mice due to ricin poisoning via the alimentary pathway

The aim of this work was to investigate the potential interactions between intestinal absorbance and ricin poisoning. The Caco-2 cell monolayer and everted intestinal sac (VEIS) models were used. The distribution of ricin in CD-1 mice intoxicated with 0.1 mg/kg of ricin intragastrically was determined by immunohistochemistry. The results showed that ricin could not transfer across the healthy Caco-2 cell monolayer within three hours after poisoning. However, it could pass through the everted rat intestinal wall after 0.5 h of incubation. The toxin in the liver, spleen, lungs and kidneys of mice could be detected as early as 1 h after intoxication. The pathological results were in accordance with the cytotoxicities of ricin in Caco-2, HepG 2, H1299 and MDCK cells, indicating that though no significant symptom in mice could be observed within 3 h after ricin intoxication, important tissues, especially the kidneys, were being injured by the toxin and that the injuries were progressing.

Biophysics of cell membrane lipids in cancer drug resistance: Implications for drug transport and drug delivery with nanoparticles

In this review, we focus on the biophysics of cell membrane lipids, particularly when cancers develop acquired drug resistance, and how biophysical changes in resistant cell membrane influence drug transport and nanoparticle-mediated drug delivery. Recent advances in membrane lipid research show the varied roles of lipids in regulating membrane P-glycoprotein function, membrane trafficking, apoptotic pathways, drug transport, and endocytic functions, particularly endocytosis, the primary mechanism of cellular uptake of nanoparticle-based drug delivery systems. Since acquired drug resistance alters lipid biosynthesis, understanding the role of lipids in cell membrane biophysics and its effect on drug transport is critical for developing effective therapeutic and drug delivery approaches to overcome drug resistance. Here we discuss novel strategies for (a) modulating the biophysical properties of membrane lipids of resistant cells to facilitate drug transport and regain endocytic function and (b) developing effective nanoparticles based on their biophysical interactions with membrane lipids to enhance drug delivery and overcome drug resistance.

Myelin recovery in multiple sclerosis: the challenge of remyelination

Multiple sclerosis (MS) is the most common demyelinating and an autoimmune disease of the central nervous system characterized by immune-mediated myelin and axonal damage, and chronic axonal loss attributable to the absence of myelin sheaths. T cell subsets (Th1, Th2, Th17, CD8⁺, NKT, CD4⁺CD25⁺ T regulatory cells) and B cells are involved in this disorder, thus new MS therapies seek damage prevention by resetting multiple components of the immune system. The currently approved therapies are immunoregulatory and reduce the number and rate of lesion formation but are only partially effective. This review summarizes current understanding of the processes at issue: myelination, demyelination and remyelination—with emphasis upon myelin composition/architecture and oligodendrocyte maturation and differentiation. The translational options target oligodendrocyte protection and myelin repair in animal models and assess their relevance in human. Remyelination may be enhanced by signals that promote myelin formation and repair. The crucial question of why remyelination fails is approached is several ways by examining the role in remyelination of available MS medications and avenues being actively pursued to promote remyelination including: (i) cytokine-based immune-intervention (targeting calpain inhibition), (ii) antigen-based immunomodulation (targeting glycolipid-reactive iNKT cells and sphingoid mediated inflammation) and (iii) recombinant monoclonal antibodies-induced remyelination.

Multivalent immune complexes divert FcRn to lysosomes by exclusion from recycling sorting tubules

Study of receptor sorting between recycling and degradative pathways shows that sorting into the recycling pathway depends not only on recognition of sorting motifs by cytosolic adaptors, but also on the physical properties of the endosomal luminal complexes, as shown by the neonatal receptor for IgG FcRn.

The neonatal receptor for immunoglobulin G (IgG; FcRn) prevents IgG degradation by efficiently sorting IgG into recycling endosomes and away from lysosomes. When bound to IgG-opsonized antigen complexes, however, FcRn traffics cargo into lysosomes, where antigen processing can occur. Here we address the mechanism of sorting when FcRn is bound to multivalent IgG-opsonized antigens. We find that only the unbound receptor or FcRn bound to monomeric IgG is sorted into recycling tubules emerging from early endosomes. Cross-linked FcRn is never visualized in tubules containing the unbound receptor. Similar results are found for transferrin receptor, suggesting a general mechanism of action. Deletion or replacement of the FcRn cytoplasmic tail does not prevent diversion of trafficking to lysosomes upon cross-linking. Thus physical properties of the lumenal ligand–receptor complex appear to act as key determinants for sorting between the recycling and lysosomal pathways by regulating FcRn entry into recycling tubules.

Dynamic sorting of lipids and proteins in multicomponent membranes

Dynamic sorting of lipids and proteins in cellular membranes plays a critical role in establishing and maintaining distinct compositions in various organelles. Recent experiments found that the lipid sorting in a membrane tube highly depends on the pulling speed at the tip. However, the mechanism of this velocity dependence has not yet been revealed. In this Letter, we found that when a membrane is deformed rapidly, the lipid flow induced by fast membrane shape change will significantly affect the sorting results. The competition between the curvature-driven lipid sorting and the pulling-induced lipid flow leads to novel behaviors. When a membrane tube is pulled out from a liquid ordered (L(o)) domain at a constant speed, slow pulling leads to the formation of a liquid disordered (L(d)) tube, while fast pulling results in a L(o) tube. Interestingly, in a membrane tube pulled at an intermediate speed, alternate L(d) and L(o) domains appear in the tube. The sorting dynamics and the corresponding pulling force were systematically studied. The results of this study could lead to a better understanding of the dynamic sorting and traffic of lipids and proteins in living cells.

Towards understanding the function of stress-inducible PtdIns(4,5)P(2) in plants

The phosphoinositide (PI) system has been conserved in evolution between all eukaryotic kingdoms. Studies on mammalian and yeast cells indicate that cellular functions regulated by PIs include the production of soluble inositolpolyphosphates with signaling functions as well as recruitment of proteins required for endo- or exocytosis. In contrast to other models, knowledge on PI functions in plants is limited and, despite of reports of transient PI-increases upon stress-treatments, plant cellular processes involving changes in PI-levels have remained unclear. In previous studies various groups have proposed that PI-increases upon hyperosmotic stress support the generation of soluble second messengers with possible roles in stress adaptation. Based on a combination of biochemical analysis and imaging of fluorescent reporters we have now demonstrated that intact phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P(2)) associates with clathrin-coated vesicles (CCVs) in a stress-inducible manner in plant cells. In analogy to previous studies on other models, association with CCVs suggests a role for PtdIns(4,5)P(2) in the recruitment of vesicle coat proteins and in membrane internalization that is alternative to functions in second messenger production. The determination of subcellular sites of PtdIns(4,5)P(2) increases, thus, opens new avenues of investigation in the plant PI-field and allows development of testable hypotheses to delineate PI-functions in plants.

Dynamic of Ion Channel Expression at the Plasma Membrane of Cardiomyocytes

Cardiac myocytes are characterized by distinct structural and functional entities involved in the generation and transmission of the action potential and the excitation-contraction coupling process. Key to their function is the specific organization of ion channels and transporters to and within distinct membrane domains, which supports the anisotropic propagation of the depolarization wave. This review addresses the current knowledge on the molecular actors regulating the distinct trafficking and targeting mechanisms of ion channels in the highly polarized cardiac myocyte. In addition to ubiquitous mechanisms shared by other excitable cells, cardiac myocytes show unique specialization, illustrated by the molecular organization of myocyte-myocyte contacts, e.g., the intercalated disc and the gap junction. Many factors contribute to the specialization of the cardiac sarcolemma and the functional expression of cardiac ion channels, including various anchoring proteins, motors, small GTPases, membrane lipids, and cholesterol. The discovery of genetic defects in some of these actors, leading to complex cardiac disorders, emphasizes the importance of trafficking and targeting of ion channels to cardiac function. A major challenge in the field is to understand how these and other actors work together in intact myocytes to fine-tune ion channel expression and control cardiac excitability.

Annexins as organizers of cholesterol- and sphingomyelin-enriched membrane microdomains in Niemann-Pick type C disease

Growing evidence suggests that membrane microdomains enriched in cholesterol and sphingomyelin are sites for numerous cellular processes, including signaling, vesicular transport, interaction with pathogens, and viral infection, etc. Recently some members of the annexin family of conserved calcium and membrane-binding proteins have been recognized as cholesterol-interacting molecules and suggested to play a role in the formation, stabilization, and dynamics of membrane microdomains to affect membrane lateral organization and to attract other proteins and signaling molecules onto their territory. Furthermore, annexins were implicated in the interactions between cytosolic and membrane molecules, in the turnover and storage of cholesterol and in various signaling pathways. In this review, we focus on the mechanisms of interaction of annexins with lipid microdomains and the role of annexins in membrane microdomains dynamics including possible participation of the domain-associated forms of annexins in the etiology of human lysosomal storage disease called Niemann-Pick type C disease, related to the abnormal storage of cholesterol in the lysosome-like intracellular compartment. The involvement of annexins and cholesterol/sphingomyelin-enriched membrane microdomains in other pathologies including cardiac dysfunctions, neurodegenerative diseases, obesity, diabetes mellitus, and cancer is likely, but is not supported by substantial experimental observations, and therefore awaits further clarification.

The fusion of early endosomes induces molecular-motor-driven tubule formation and fission

Organelles in the endocytic pathway interact and communicate through the crucial mechanisms of fusion and fission. However, any specific link between fusion and fission has not yet been determined. To study the endosomal interactions with high spatial and temporal resolution, we enlarged the endosomes by two mechanistically different methods: by expression of the MHC-class-II-associated chaperone invariant chain (Ii; or CD74) or Rab5, both of which increased the fusion rate of early endosomes and resulted in enlarged endosomes. Fast homotypic fusions were studied, and immediately after the fusion a highly active and specific tubule formation and fission was observed. These explosive tubule formations following fusion seemed to be a direct effect of fusion. The tubule formations were dependent on microtubule interactions, and specifically controlled by Kif16b and dynein. Our results show that fusion of endosomes is a rapid process that destabilizes the membrane and instantly induces molecular-motor-driven tubule formation and fission.

Dynamics within tetraspanin pairs affect MHC class II expression

Late endosomal multivesicular bodies (MVBs) are complicated organelles with various subdomains located at the limiting membrane and the internal vesicles (ILVs). ILVs accumulate tetraspanins such as CD63 and CD82 that might form protein assemblies, including major histocompatibility complex class II (MHC-II) and its chaperone human leukocyte antigen (HLA)-DM. Here, we studied the effect of four late endosomal tetraspanin proteins on MHC-II expression. Silencing CD9, CD63 and CD81 enhanced MHC-II expression whereas silencing CD82 did not. No effect on peptide loading was observed. Using confocal FRET technology, we measured the dynamics of CD63 and CD82 interaction with MHC-II and its chaperone HLA-DM. CD63-CD82 interactions remained unaltered in the two MVB subdomains whereas the interactions between CD63 or CD82 homologous pairs changed. CD63 stably associated with MHC-II, and CD82 with HLA-DM, on both MVB subdomains whereas the CD82-MHC-II and CD63-HLA-DM interactions changed. These data visualize for the first time the protein dynamics of tetraspanin assemblies in MVB subdomains. CD63, unlike CD82, stably interacts with MHC-II at both MVB subdomains and controls MHC-II expression.

Role of phospholipase A(2) in retrograde transport of ricin

Ricin is a protein toxin classified as a bioterror agent, for which there are no known treatment options available after intoxication. It is composed of an enzymatically active A-chain connected by a disulfide bond to a cell binding B-chain. After internalization by endocytosis, ricin is transported retrogradely to the Golgi and ER, from where the ricin A-chain is translocated to the cytosol where it inhibits protein synthesis and thus induces cell death. We have identified cytoplasmic phospholipase A₂ (PLA₂) as an important factor in ricin retrograde transport. Inhibition of PLA₂ protects against ricin challenge, however the toxin can still be endocytosed and transported to the Golgi. Interestingly, ricin transport from the Golgi to the ER is strongly impaired in response to PLA₂ inhibition. Confocal microscopy analysis shows that ricin is still colocalized with the trans-Golgi marker TGN46 in the presence of PLA₂ inhibitor, but less is colocalized with the cis-Golgi marker GM130. We propose that PLA₂ inhibition results in impaired ricin transport through the Golgi stack, thus preventing it from reaching the ER. Consequently, ricin cannot be translocated to the cytosol to exert its toxic action.

Characterization of the EFC/F-BAR domain protein, FCHO2

We have previously shown that SGIP1α is an endocytic protein specifically expressed in neural tissues. SGIP1α has a lipid-binding domain called the MP domain, which shows no significant homology to any other domains. In this study, we characterized FCHO2, a protein with a high level of homology to SGIP1α. FCHO2 lacks the MP domain but has another lipid-binding domain, the EFC/F-BAR domain. FCHO2 was ubiquitously expressed. The FCHO2 EFC domain bound to phosphatidylserine and phosphoinositides and deformed the plasma membrane and liposomes into narrow tubes. FCHO2 localized to clathrin-coated pits at the plasma membrane and bound to Eps15, an important adaptor protein in clathrin-mediated endocytosis. FCHO2 knockdown reduced transferrin endocytosis. These results suggest that FCHO2 regulates clathrin-mediated endocytosis through its interactions with membranes and Eps15. These properties of FCHO2 are similar to those of SGIP1α. FCHO2 is likely to be a ubiquitous homologue of SGIP1α. We furthermore found that FCHO2 was subjected to monoubiquitination, and gel filtration analysis showed that FCHO2 formed an oligomer. These new properties might also contribute to the role of FCHO2 in clathrin-mediated endocytosis.