Rafting through traffic: Membrane domains in cellular logistics

Partitioning to ordered membrane domains regulates the kinetics of secretory traffic

The organelles of eukaryotic cells maintain distinct protein and lipid compositions required for their specific functions. The mechanisms by which many of these components are sorted to their specific locations remain unknown. While some motifs mediating subcellular protein localization have been identified, many membrane proteins and most membrane lipids lack known sorting determinants. A putative mechanism for sorting of membrane components is based on membrane domains known as lipid rafts, which are laterally segregated nanoscopic assemblies of specific lipids and proteins. To assess the role of such domains in the secretory pathway, we applied a robust tool for synchronized secretory protein traffic (RUSH, Retention Using Selective Hooks) to protein constructs with defined affinity for raft phases. These constructs consist solely of single-pass transmembrane domains (TMDs) and, lacking other sorting determinants, constitute probes for membrane domain-mediated trafficking. We find that while raft affinity can be sufficient for steady-state PM localization, it is not sufficient for rapid exit from the endoplasmic reticulum (ER), which is instead mediated by a short cytosolic peptide motif. In contrast, we find that Golgi exit kinetics are highly dependent on raft affinity, with raft preferring probes exiting the Golgi ~2.5-fold faster than probes with minimal raft affinity. We rationalize these observations with a kinetic model of secretory trafficking, wherein Golgi export can be facilitated by protein association with raft domains. These observations support a role for raft-like membrane domains in the secretory pathway and establish an experimental paradigm for dissecting its underlying machinery.

Dysregulation of cellular membrane homeostasis as a crucial modulator of cancer risk

Cellular membranes serve as an epicentre combining extracellular and cytosolic components with membranous effectors, which together support numerous fundamental cellular signalling pathways that mediate biological responses. To execute their functions, membrane proteins, lipids and carbohydrates arrange, in a highly coordinated manner, into well-defined assemblies displaying diverse biological and biophysical characteristics that modulate several signalling events. The loss of membrane homeostasis can trigger oncogenic signalling. More recently, it has been documented that select membrane active dietaries (MADs) can reshape biological membranes and subsequently decrease cancer risk. In this review, we emphasize the significance of membrane domain structure, organization and their signalling functionalities as well as how loss of membrane homeostasis can steer aberrant signalling. Moreover, we describe in detail the complexities associated with the examination of these membrane domains and their association with cancer. Finally, we summarize the current literature on MADs and their effects on cellular membranes, including various mechanisms of dietary chemoprevention/interception and the functional links between nutritional bioactives, membrane homeostasis and cancer biology.

Safari with an Electron Gun: Visualization of Protein and Membrane Interactions in Mitochondria in Natural Environment

This paper presents new structural data about mitochondria using correlative light and electron microscopy (CLEM) and cryo-electron tomography. These state-of-the-art structural biology methods allow studying biological objects at nanometer scales under natural conditions. Non-invasiveness of these methods makes them comparable to observing animals in their natural environment on a safari. The paper highlights two areas of research that can only be accomplished using these methods. The study visualized location of the Aβ42 amyloid aggregates in relation to mitochondria to test a hypothesis of development of mitochondrial dysfunction in Alzheimer's disease. The results showed that the Aβ42 aggregates do not interact with mitochondria, although some of them are closely located. Therefore, the study demonstrated that mitochondrial dysfunction is not directly associated with the effects of aggregates on mitochondrial structure. Other processes should be considered as sources of mitochondrial dysfunction. Second unique area presented in this work is high-resolution visualization of the mitochondrial membranes and proteins in them. Analysis of the cryo-ET data reveals toroidal holes in the lamellar structures of cardiac mitochondrial cristae, where ATP synthases are located. The study proposes a new mechanism for sorting and clustering protein complexes in the membrane based on topology. According to this suggestion, position of the OXPHOS system proteins in the membrane is determined by its curvature. High-resolution tomography expands and complements existing ideas about the structural and functional organization of mitochondria. This makes it possible to study the previously inaccessible structural interactions of proteins with each other and with membranes in vivo.

Partitioning to ordered membrane domains regulates the kinetics of secretory traffic

The organelles of eukaryotic cells maintain distinct protein and lipid compositions required for their specific functions. The mechanisms by which many of these components are sorted to their specific locations remain unknown. While some motifs mediating subcellular protein localization have been identified, many membrane proteins and most membrane lipids lack known sorting determinants. A putative mechanism for sorting of membrane components is based on membrane domains known as lipid rafts, which are laterally segregated nanoscopic assemblies of specific lipids and proteins. To assess the role of such domains in the secretory pathway, we applied a robust tool for synchronized secretory protein traffic (RUSH, Retention Using Selective Hooks) to protein constructs with defined affinity for raft phases. These constructs consist solely of single-pass transmembrane domains (TMDs) and, lacking other sorting determinants, constitute probes for membrane domain-mediated trafficking. We find that while raft affinity can be sufficient for steady-state PM localization, it is not sufficient for rapid exit from the endoplasmic reticulum (ER), which is instead mediated by a short cytosolic peptide motif. In contrast, we find that Golgi exit kinetics are highly dependent on raft affinity, with raft preferring probes exiting Golgi ~2.5-fold faster than probes with minimal raft affinity. We rationalize these observations with a kinetic model of secretory trafficking, wherein Golgi export can be facilitated by protein association with raft domains. These observations support a role for raft-like membrane domains in the secretory pathway and establish an experimental paradigm for dissecting its underlying machinery.

Efficient calculation of the free energy for protein partitioning using restraining potentials

An approach for the efficient simulation of phase-separated lipid bilayers, for use in the calculation of equilibrium free energies of partitioning between lipid domains, is proposed. The methodology exploits restraint potentials and rectangular aspect ratios that enforce lipid phase separation, allowing for the simulation of smaller systems that approximately reproduce bulk behavior. The utility of this approach is demonstrated through the calculation of potentials of mean force for the translation of a transmembrane protein between lipid domains. The impact of the imposed restraints on lipid tail ordering and lipid packing are explored, providing insight into how restraints can best be employed to compute accurate free-energy surfaces. This approach should be useful in the accurate calculation of equilibrium partition coefficients for transmembrane protein partitioning in heterogeneous membranes, providing insight into the thermodynamic driving forces that control this fundamental biophysical phenomenon.

Remodeling of yeast vacuole membrane lipidomes from the log (one phase) to stationary stage (two phases)

Upon nutrient limitation, budding yeast of Saccharomyces cerevisiae shift from fast growth (the log stage) to quiescence (the stationary stage). This shift is accompanied by liquid-liquid phase separation in the membrane of the vacuole, an endosomal organelle. Recent work indicates that the resulting micrometer-scale domains in vacuole membranes enable yeast to survive periods of stress. An outstanding question is which molecular changes might cause this membrane phase separation. Here, we conduct lipidomics of vacuole membranes in both the log and stationary stages. Isolation of pure vacuole membranes is challenging in the stationary stage, when lipid droplets are in close contact with vacuoles. Immuno-isolation has previously been shown to successfully purify log-stage vacuole membranes with high organelle specificity, but it was not previously possible to immuno-isolate stationary-stage vacuole membranes. Here, we develop Mam3 as a bait protein for vacuole immuno-isolation, and demonstrate low contamination by non-vacuolar membranes. We find that stationary-stage vacuole membranes contain surprisingly high fractions of phosphatidylcholine lipids (∼40%), roughly twice as much as log-stage membranes. Moreover, in the stationary stage, these lipids have higher melting temperatures, due to longer and more saturated acyl chains. Another surprise is that no significant change in sterol content is observed. These lipidomic changes, which are largely reflected on the whole-cell level, fit within the predominant view that phase separation in membranes requires at least three types of molecules to be present: lipids with high melting temperatures, lipids with low melting temperatures, and sterols.

Regulation of membrane protein structure and function by their lipid nano-environment

Transmembrane proteins comprise ~30% of the mammalian proteome, mediating metabolism, signalling, transport and many other functions required for cellular life. The microenvironment of integral membrane proteins (IMPs) is intrinsically different from that of cytoplasmic proteins, with IMPs solvated by a compositionally and biophysically complex lipid matrix. These solvating lipids affect protein structure and function in a variety of ways, from stereospecific, high-affinity protein-lipid interactions to modulation by bulk membrane properties. Specific examples of functional modulation of IMPs by their solvating membranes have been reported for various transporters, channels and signal receptors; however, generalizable mechanistic principles governing IMP regulation by lipid environments are neither widely appreciated nor completely understood. Here, we review recent insights into the inter-relationships between complex lipidomes of mammalian membranes, the membrane physicochemical properties resulting from such lipid collectives, and the regulation of IMPs by either or both. The recent proliferation of high-resolution methods to study such lipid-protein interactions has led to generalizable insights, which we synthesize into a general framework termed the 'functional paralipidome' to understand the mutual regulation between membrane proteins and their surrounding lipid microenvironments.

Diversity of Extracellular Vesicles in Human Follicular Fluid: Morphological Analysis and Quantification

The oocyte microenvironment constituted by the follicular fluid (FF) is a key for the optimal development of female gametes. Its composition reflects the physiological state of the ovarian follicle. The particularity of FF is to contain a huge diversity of extracellular vesicles specific to women, in the same way as seminal plasma in men. Here, we described and compared morphological aspects of broad subcategories of human FF-related Extracellular Vesicles (EVs). EVs participate in physiological and pathological processes and have potential applications in diagnostics or therapeutics. EVs isolated from FF are involved in different biological functions related to follicular growth, oocyte maturation, and embryo development. However, knowledge on the morphology of FF-derived EVs is limited, mainly due to their sub-micrometer size and to intrinsic limitations in methods applied for their characterization. The aim of this study was to provide a comprehensive morphological description of EVs from FF of healthy subjects and quantification. EVs separation was realized by centrifugation, with comparison of the EV yield obtained from differential centrifugation and one-step ultracentrifugation. Cryo-Transmission Electron Microscopy was used to reveal the morphology, size, and phenotype of EVs. Dynamic Light Scattering (DLS) and Nanoparticle Tracking Analysis (NTA) were used to quantify and analyze the size distribution for each centrifugation step. We performed a comprehensive inventory of human follicular fluid EVs. We show that human FF contains a huge diversity of EVs. This study brings novel insights on EVs from normal FF and provides a reference for further studies of EVs in ovarian diseases.

Direct quantification of ligand-induced lipid and protein microdomains with distinctive signaling properties

Lipid rafts are ordered lipid domains that are enriched in saturated lipids, such as the ganglioside GM1. While lipid rafts are believed to exist in cells and to serve as signaling platforms through their enrichment in signaling components, they have not been directly observed in the plasma membrane without treatments that artificially cluster GM1 into large lattices. Here, we report that microscopic GM1-enriched domains can form, in the plasma membrane of live mammalian cells expressing the EphA2 receptor tyrosine kinase in response to its ligand ephrinA1-Fc. The GM1-enriched microdomains form concomitantly with EphA2-enriched microdomains. To gain insight into how plasma membrane heterogeneity controls signaling, we quantify the degree of EphA2 segregation and study initial EphA2 signaling steps in both EphA2-enriched and EphA2-depleted domains. By measuring dissociation constants, we demonstrate that the propensity of EphA2 to oligomerize is similar in EphA2-enriched and -depleted domains. However, surprisingly, EphA2 interacts preferentially with its downstream effector SRC in EphA2-depleted domains. The ability to induce microscopic GM1-enriched domains in live cells using a ligand for a transmembrane receptor will give us unprecedented opportunities to study the biophysical chemistry of lipid rafts.

Fifty Years of the Fluid–Mosaic Model of Biomembrane Structure and Organization and Its Importance in Biomedicine with Particular Emphasis on Membrane Lipid Replacement

The Fluid–Mosaic Model has been the accepted general or basic model for biomembrane structure and organization for the last 50 years. In order to establish a basic model for biomembranes, some general principles had to be established, such as thermodynamic assumptions, various molecular interactions, component dynamics, macromolecular organization and other features. Previous researchers placed most membrane proteins on the exterior and interior surfaces of lipid bilayers to form trimolecular structures or as lipoprotein units arranged as modular sheets. Such membrane models were structurally and thermodynamically unsound and did not allow independent lipid and protein lateral movements. The Fluid–Mosaic Membrane Model was the only model that accounted for these and other characteristics, such as membrane asymmetry, variable lateral movements of membrane components, cis- and transmembrane linkages and dynamic associations of membrane components into multimolecular complexes. The original version of the Fluid–Mosaic Membrane Model was never proposed as the ultimate molecular description of all biomembranes, but it did provide a basic framework for nanometer-scale biomembrane organization and dynamics. Because this model was based on available 1960s-era data, it could not explain all of the properties of various biomembranes discovered in subsequent years. However, the fundamental organizational and dynamic aspects of this model remain relevant to this day. After the first generation of this model was published, additional data on various structures associated with membranes were included, resulting in the addition of membrane-associated cytoskeletal, extracellular matrix and other structures, specialized lipid–lipid and lipid–protein domains, and other configurations that can affect membrane dynamics. The presence of such specialized membrane domains has significantly reduced the extent of the fluid lipid membrane matrix as first proposed, and biomembranes are now considered to be less fluid and more mosaic with some fluid areas, rather than a fluid matrix with predominantly mobile components. However, the fluid–lipid matrix regions remain very important in biomembranes, especially those involved in the binding and release of membrane lipid vesicles and the uptake of various nutrients. Membrane phospholipids can associate spontaneously to form lipid structures and vesicles that can fuse with various cellular membranes to transport lipids and other nutrients into cells and organelles and expel damaged lipids and toxic hydrophobic molecules from cells and tissues. This process and the clinical use of membrane phospholipid supplements has important implications for chronic illnesses and the support of healthy mitochondria, plasma membranes and other cellular membrane structures.

Uncovering the Potential of Lipid Drugs: A Focus on Transient Membrane Microdomain-Targeted Lipid Therapeutics

Membrane lipids are generally viewed as inert physical barriers, but many vital cellular processes greatly rely on the interaction with these structures, as expressed by the membrane hypothesis that explain the genesis of schizophrenia, Alzheimer's and autoimmune diseases, chronic fatigue or cancer, among others. The concept that the cell membrane displays transient membrane microdomains with distinct lipid composition provide the basis for the development of selective lipid-targeted therapies, the membrane-lipid therapies (MLTs). In this concern, medicinal chemists may design therapeutically valuable compounds 1) with a higher affinity for the lipids in these microdomains to restore the normal physiological conditions, 2) that can directly or 3) indirectly (via enzyme inhibition/activation) replace damaged lipids or restore the regular lipid levels in the whole membrane or microdomain, 4) that alter the expression of genes related to lipid genesis/metabolism or 5) that modulate the pathways related to the membrane binding affinity of lipid-anchored proteins. In this context, this mini-review aims to explore the structural diversity and clinical applications of some of the main membrane and microdomain-targeted lipid drugs.

Ganglioside binding domains in proteins: Physiological and pathological mechanisms

Gangliosides are anionic lipids that form condensed membrane clusters (lipid rafts) and exert major regulatory functions on a wide range of proteins. In this review, we propose a new view of the structural features of gangliosides with special emphasis on emerging properties associated with protein binding modes. We analyze the different possibilities of molecular associations of gangliosides in lipid rafts and the role of cholesterol in this organization. We are particularly interested in amide groups of N-acetylated sugars which make it possible to neutralize the negative charge of the carboxylate group of sialic acids. We refer to this effect as "NH trick" and we demonstrate that it is operative in GM1, GD1a, GD1b and GT1b gangliosides. The NH trick is key to understand the different topologies adopted by gangliosides (chalice-like at the edge of lipid rafts, condensed clusters in central areas) and their impact on protein binding. We define three major types of ganglioside-binding domains (GBDs): α-helical, loop shaped, and large flat surface. We describe the mode of interaction of each GBD with typical reference proteins: synaptotagmin, 5HT1A receptor, cholera and botulinum toxins, HIV-1 surface envelope glycoprotein gp120, SARS-CoV-2 spike protein, cellular prion protein, Alzheimer's β-amyloid peptide and Parkinson's disease associated α-synuclein. We discuss the common mechanisms and peculiarities of protein binding to gangliosides in the light of physiological and pathological conditions. We anticipate that innovative ganglioside-based therapies will soon show an exponential growth for the treatment of cancer, microbial infections, and neurodegenerative diseases.

Quantitative Measurements of Membrane Lipid Order in Yeast and Fungi

Membrane lateral heterogeneity, historically referred to as the lipid raft hypothesis, has been extensively investigated through physiochemical experiments on model membranes. Currently, the basic principles are well understood; however, the physiological relevance of these structures in living organisms is still not clear. Thus, studying membrane organization in vivo is extremely important and elucidates the role of such structures in various membrane-associated processes. This is particularly true when a whole single-celled organism can be studied rather than an isolated cell. The ordered and disordered membrane phases are characterized by the degree of acyl chain packing in the lipid bilayer. Polar water molecules can penetrate into the low-density lipid packing of the disordered phase, but are more excluded from the tightly packed ordered phase. Here, polarity-sensitive probes, embedded in the lipid bilayer, are used to report on membrane organization and to quantitate this parameter via 2-channel fluorescence microscopy. Coupling genetic approaches, which are easily accessible in yeast model organisms, with the imaging approach described here provides a great opportunity to investigate how membrane heterogeneity impacts physiology.

A Brief Introduction to Some Aspects of the Fluid-Mosaic Model of Cell Membrane Structure and Its Importance in Membrane Lipid Replacement

Early cell membrane models placed most proteins external to lipid bilayers in trimolecular structures or as modular lipoprotein units. These thermodynamically untenable structures did not allow lipid lateral movements independent of membrane proteins. The Fluid-Mosaic Membrane Model accounted for these and other properties, such as membrane asymmetry, variable lateral mobilities of membrane components and their associations with dynamic complexes. Integral membrane proteins can transform into globular structures that are intercalated to various degrees into a heterogeneous lipid bilayer matrix. This simplified version of cell membrane structure was never proposed as the ultimate biomembrane description, but it provided a basic nanometer scale framework for membrane organization. Subsequently, the structures associated with membranes were considered, including peripheral membrane proteins, and cytoskeletal and extracellular matrix components that restricted lateral mobility. In addition, lipid-lipid and lipid-protein membrane domains, essential for cellular signaling, were proposed and eventually discovered. The presence of specialized membrane domains significantly reduced the extent of the fluid lipid matrix, so membranes have become more mosaic with some fluid areas over time. However, the fluid regions of membranes are very important in lipid transport and exchange. Various lipid globules, droplets, vesicles and other membranes can fuse to incorporate new lipids or expel damaged lipids from membranes, or they can be internalized in endosomes that eventually fuse with other internal vesicles and membranes. They can also be externalized in a reverse process and released as extracellular vesicles and exosomes. In this Special Issue, the use of membrane phospholipids to modify cellular membranes in order to modulate clinically relevant host properties is considered.

Messages from Mutant Desmosomes

Single gene disorders are ideally suited to establish robust genotype‒phenotype correlations and provide excellent opportunities to understand molecular pathomechanisms with relevance to complex disorders. The observation that patients diagnosed with the same causative mutation can present with phenotypic disease variability illustrates the significant role of disease modifiers and warns against oversimplification. In a new article in the Journal of Investigative Dermatology, Zimmer et al. (2021) analyze two mutations located in the desmoglein (DSG) 1 transmembrane domain (TMD) and find that both mutants fail to assemble into desmosomes owing to reduced membrane trafficking and lipid raft targeting. One mutation maintained normal protein expression levels and turnover relative to those of wild-type (WT) DSG1, and behaved as a dominant negative. The second mutant showed reduced stability and increased turnover compared with WT DSG1 as well as reduced desmosome size and abundance. A full understanding of the TMD of DSG1 requires cell biological approaches, underscoring the value of cell biology in biomedical research in general.

Role of RNA Motifs in RNA Interaction with Membrane Lipid Rafts: Implications for Therapeutic Applications of Exosomal RNAs

RNA motifs may promote interactions with exosomes (EXO-motifs) and lipid rafts (RAFT-motifs) that are enriched in exosomal membranes. These interactions can promote selective RNA loading into exosomes. We quantified the affinity between RNA aptamers containing various EXO- and RAFT-motifs and membrane lipid rafts in a liposome model of exosomes by determining the dissociation constants. Analysis of the secondary structure of RNA molecules provided data about the possible location of EXO- and RAFT-motifs within the RNA structure. The affinity of RNAs containing RAFT-motifs (UUGU, UCCC, CUCC, CCCU) and some EXO-motifs (CCCU, UCCU) to rafted liposomes is higher in comparison to aptamers without these motifs, suggesting direct RNA-exosome interaction. We have confirmed these results through the determination of the dissociation constant values of exosome-RNA aptamer complexes. RNAs containing EXO-motifs GGAG or UGAG have substantially lower affinity to lipid rafts, suggesting indirect RNA-exosome interaction via RNA binding proteins. Bioinformatics analysis revealed RNA aptamers containing both raft- and miRNA-binding motifs and involvement of raft-binding motifs UCCCU and CUCCC. A strategy is proposed for using functional RNA aptamers (fRNAa) containing both RAFT-motif and a therapeutic motif (e.g., miRNA inhibitor) to selectively introduce RNAs into exosomes for fRNAa delivery to target cells for personalized therapy.

Liquid-liquid phase separation as a common organizing principle of intracellular space and biomembranes providing dynamic adaptive responses

This work is devoted to the phenomenon of liquid-liquid phase separation (LLPS), which has come to be recognized as fundamental organizing principle of living cells. We distinguish separation processes with different dimensions. Well-known 3D-condensation occurs in aqueous solution and leads to membraneless organelle (MLOs) formation. 2D-films may be formed near membrane surfaces and lateral phase separation (membrane rafts) occurs within the membranes themselves. LLPS may also occur on 1D structures like DNA and the cyto- and nucleoskeleton. Phase separation provides efficient transport and sorting of proteins and metabolites, accelerates the assembly of metabolic and signaling complexes, and mediates stress responses. In this work, we propose a model in which the processes of polymerization (1D structures), phase separation in membranes (2D structures), and LLPS in the volume (3D structures) influence each other. Disordered proteins and whole condensates may provide membrane raft separation or polymerization of specific proteins. On the other hand, 1D and 2D structures with special composition or embedded IDRs can nucleate condensates. We hypothesized that environmental change may trigger a LLPS which can propagate within the cell interior moving along the cytoskeleton or as an autowave. New phase propagation quickly and using a low amount of energy adjusts cell signaling and metabolic systems to new demands. Cumulatively, the interconnected phase separation phenomena in different dimensions represent a previously unexplored system of intracellular communication and regulation which cannot be ignored when considering both physiological and pathological cell processes.

Regulation of Dynamic Protein S-Acylation

Protein S-acylation is the reversible addition of fatty acids to the cysteine residues of target proteins. It regulates multiple aspects of protein function, including the localization to membranes, intracellular trafficking, protein interactions, protein stability, and protein conformation. This process is regulated by palmitoyl acyltransferases that have the conserved amino acid sequence DHHC at their active site. Although they have conserved catalytic cores, DHHC enzymes vary in their protein substrate selection, lipid substrate preference, and regulatory mechanisms. Alterations in DHHC enzyme function are associated with many human diseases, including cancers and neurological conditions. The removal of fatty acids from acylated cysteine residues is catalyzed by acyl protein thioesterases. Notably, S-acylation is now known to be a highly dynamic process, and plays crucial roles in signaling transduction in various cell types. In this review, we will explore the recent findings on protein S-acylation, the enzymatic regulation of this process, and discuss examples of dynamic S-acylation.

Lipidomic atlas of mammalian cell membranes reveals hierarchical variation induced by culture conditions, subcellular membranes, and cell lineages

Lipid membranes are ubiquitous biological organizers, required for structural and functional compartmentalization of the cell and sub-cellular organelles. Membranes in living cells are compositionally complex, comprising hundreds of dynamically regulated, distinct lipid species. Cellular physiology requires tight regulation of these lipidomic profiles to achieve proper membrane functionality. While some general features of tissue- and organelle-specific lipid complements have been identified, less is known about detailed lipidomic variations caused by cell-intrinsic or extrinsic factors. Here, we use shotgun lipidomics to report detailed, comprehensive lipidomes of a variety of cultured and primary mammalian membrane preparations to identify trends and sources of variation. Unbiased principle component analysis (PCA) shows clear separation between cultured and primary cells, with primary erythrocytes, synaptic membranes, and other mammalian tissue lipidomes sharply diverging from all cultured cell lines and also from one other. Most broadly, cultured cell membrane preparations were distinguished by their paucity of polyunsaturated lipids. Cultured mammalian cell lines were comparatively similar to one another, although we detected clear, highly reproducible lipidomic signatures of individual cell lines and plasma membrane (PM) isolations thereof. These measurements begin to establish a comprehensive lipidomic atlas of mammalian cells and tissues, identifying some major sources of variation. These observations will allow investigation of the regulation and functional significance of mammalian lipidomes in various contexts.

Disturbances in PP2A methylation and one-carbon metabolism compromise Fyn distribution, neuritogenesis, and APP regulation

The nonreceptor protein tyrosine kinase Fyn and protein Ser/Thr phosphatase 2A (PP2A) are major multifunctional signaling molecules. Deregulation of Fyn and altered PP2A methylation are implicated in cancer and Alzheimer's disease (AD). Here, we tested the hypothesis that the methylation state of PP2A catalytic subunit, which influences PP2A subunit composition and substrate specificity, can affect Fyn regulation and function. Using Neuro-2a (N2a) neuroblastoma cell models, we first show that methylated PP2A holoenzymes containing the Bα subunit coimmunoprecipitate and copurify with Fyn in membrane rafts. PP2A methylation status regulates Fyn distribution and Fyn-dependent neuritogenesis, likely in part by affecting actin dynamics. A methylation-incompetent PP2A mutant fails to interact with Fyn. It perturbs the normal partitioning of Fyn and amyloid precursor protein (APP) in membrane microdomains, which governs Fyn function and APP processing. This correlates with enhanced amyloidogenic cleavage of APP, a hallmark of AD pathogenesis. Conversely, enhanced PP2A methylation promotes the nonamyloidogenic cleavage of APP in a Fyn-dependent manner. Disturbances in one-carbon metabolic pathways that control cellular methylation are associated with AD and cancer. Notably, they induce a parallel loss of membrane-associated methylated PP2A and Fyn enzymes in N2a cells and acute mouse brain slices. One-carbon metabolism also modulates Fyn-dependent process outgrowth in N2a cells. Thus, our findings identify a novel methylation-dependent PP2A/Fyn signaling module. They highlight the underestimated importance of cross talks between essential metabolic pathways and signaling scaffolds that are involved in normal cell homeostasis and currently being targeted for cancer and AD treatment.

Endocytosis of the non-catalytic ADAM23: Recycling and long half-life properties

A Disintegrin And Metalloprotease 23 (ADAM23) is a member of the ADAMs family of transmembrane proteins, mostly expressed in nervous system, and involved in traffic and stabilization of Kv1-potassium channels, synaptic transmission, neurite outgrowth, neuronal morphology and cell adhesion. Also, ADAM23 has been linked to human pathological conditions, such as epilepsy, cancer metastasis and cardiomyopathy. ADAM23 functionality depends on the molecule presence at the cell surface and along the secretory pathway, as expected for a cell surface receptor. Because endocytosis is an important functional regulatory mechanism of plasma membrane receptors and no information is available about the traffic or turnover of non-catalytic ADAMs, we investigated ADAM23 internalization, recycling and half-life properties. Here, we show that ADAM23 undergoes constitutive internalization from the plasma membrane, a process that depends on lipid raft integrity, and is redistributed to intracellular vesicles, especially early and recycling endosomes. Furthermore, we observed that ADAM23 is recycled from intracellular compartments back to the plasma membrane and thus has longer half-life and higher cell surface stability compared with other ADAMs. Our findings suggest that regulation of ADAM23 endocytosis/stability could be exploited therapeutically in diseases in which ADAM23 is directly involved, such as epilepsy, cancer progression and cardiac hypertrophy.

Membrane Heterogeneity Beyond the Plasma Membrane

The structure and organization of cellular membranes have received intense interest, particularly in investigations of the raft hypothesis. The vast majority of these investigations have focused on the plasma membrane of mammalian cells, yielding significant progress in understanding membrane heterogeneity in terms of lipid composition, molecular structure, dynamic regulation, and functional relevance. In contrast, investigations on lipid organization in other membrane systems have been comparatively scarce, despite the likely relevance of membrane domains in these contexts. In this review, we summarize recent observations on lipid organization in organellar membranes, including endoplasmic reticulum, Golgi, endo-lysosomes, lipid droplets, and secreted membranes like lung surfactant, milk fat globule membranes, and viral membranes. Across these non-plasma membrane systems, it seems that the biophysical principles underlying lipid self-organization contribute to lateral domains.

Structure and Dynamics of GPCRs in Lipid Membranes: Physical Principles and Experimental Approaches

Over the past decade, the vast amount of information generated through structural and biophysical studies of GPCRs has provided unprecedented mechanistic insight into the complex signalling behaviour of these receptors. With this recent information surge, it has also become increasingly apparent that in order to reproduce the various effects that lipids and membranes exert on the biological function for these allosteric receptors, in vitro studies of GPCRs need to be conducted under conditions that adequately approximate the native lipid bilayer environment. In the first part of this review, we assess some of the more general effects that a membrane environment exerts on lipid bilayer-embedded proteins such as GPCRs. This is then followed by the consideration of more specific effects, including stoichiometric interactions with specific lipid subtypes. In the final section, we survey a range of different membrane mimetics that are currently used for in vitro studies, with a focus on NMR applications.

Axon guidance receptors: Endocytosis, trafficking and downstream signaling from endosomes

During the development of the nervous system, axons extend through complex environments. Growth cones at the axon tip allow axons to find and innervate their appropriate targets and form functional synapses. Axon pathfinding requires axons to respond to guidance signals and these cues need to be detected by specialized receptors followed by intracellular signal integration and translation. Several downstream signaling pathways have been identified for axon guidance receptors and it has become evident that these pathways are often initiated from intracellular vesicles called endosomes. Endosomes allow receptors to traffic intracellularly, re-locating receptors from one cellular region to another. The localization of axon guidance receptors to endosomal compartments is crucial for their function, signaling output and expression levels. For example, active receptors within endosomes can recruit downstream proteins to the endosomal membrane and facilitate signaling. Also, endosomal trafficking can re-locate receptors back to the plasma membrane to allow re-activation or mediate downregulation of receptor signaling via degradation. Accumulating evidence suggests that axon guidance receptors do not follow a pre-set default trafficking route but may change their localization within endosomes. This re-routing appears to be spatially and temporally regulated, either by expression of adaptor proteins or co-receptors. These findings shed light on how signaling in axon guidance is regulated and diversified - a mechanism which explains how a limited set of guidance cues can help to establish billions of neuronal connections. In this review, we summarize and discuss our current knowledge of axon guidance receptor trafficking and provide directions for future research.

Exploring the mechanistic and temporal regulation of LRP6 endocytosis in canonical WNT signaling

Endocytosis plays a pivotal regulatory role in canonical WNT signaling. Internalization of the low-density lipoprotein receptor-related protein 6 (LRP6) receptor complex can either promote or attenuate canonical WNT signaling, depending on the employed internalization pathway. Detailed analysis of the mechanism of LRP6 internalization and its temporal regulation is crucial for understanding the different cellular responses to WNT stimulation under varying conditions and in various cell types. Here, we elucidate the mechanisms involved in the internalization of LRP6 and re-evaluate existing, partly contradicting, theories on the regulation of LRP6 receptor internalization. We utilize a computational approach that aims at finding a set of mechanisms that accounts for the temporal dynamics of LRP6 receptor internalization upon WNT stimulation. Starting with a simple simulation model, we successively extend and probe the model's behavior based on quantitative measurements. The final model confirms that LRP6 internalization is clathrin independent in vertebrates, is not restricted to microdomains, and that signalosome formation delays LRP6 internalization within the microdomains. These findings partly revise the current understanding of LRP6 internalization in vertebrates.

Macropinocytosis and Clathrin-Dependent Endocytosis Play Pivotal Roles for the Infectious Entry of Puumala Virus

Viruses from the family Hantaviridae are encountered as emerging pathogens causing two life-threatening human zoonoses: hemorrhagic fever with renal syndrome (HFRS) and hantavirus cardiopulmonary syndrome (HCPS), with case fatality rates of up to 50%. Here, we comprehensively investigated entry of the Old World hantavirus Puumala virus (PUUV) into mammalian cells, showing that upon treatment with pharmacological inhibitors of macropinocytosis and clathrin-mediated endocytosis, PUUV infections are greatly reduced. We demonstrate that the inhibitors did not interfere with viral replication and that RNA interference, targeting cellular mediators of macropinocytosis, decreases PUUV infection levels significantly. Moreover, we established lipophilic tracer staining of PUUV particles and show colocalization of stained virions and markers of macropinosomes. Finally, we report a significant increase in the fluid-phase uptake of cells infected with PUUV, indicative of a virus-triggered promotion of macropinocytosis.IMPORTANCE The family Hantaviridae comprises a diverse group of virus species and is considered an emerging global public health threat. Individual hantavirus species differ considerably in terms of their pathogenicity but also in their cell biology and host-pathogen interactions. In this study, we focused on the most prevalent pathogenic hantavirus in Europe, Puumala virus (PUUV), and investigated the entry and internalization of PUUV into mammalian cells. We show that both clathrin-mediated endocytosis and macropinocytosis are cellular pathways exploited by the virus to establish productive infections and demonstrate that pharmacological inhibition of macropinocytosis or a targeted knockdown using RNA interference significantly reduced viral infections. We also found indications of an increase of macropinocytic uptake upon PUUV infection, suggesting that the virus triggers specific cellular mechanisms in order to stimulate its own internalization, thus facilitating infection.

Pisum sativum Defensin 1 Eradicates Mouse Metastatic Lung Nodules from B16F10 Melanoma Cells

Psd1 is a pea plant defensin which can be actively expressed in Pichia pastoris and shows broad antifungal activity. This activity is dependent on fungal membrane glucosylceramide (GlcCer), which is also important for its internalization, nuclear localization, and endoreduplication. Certain cancer cells present a lipid metabolism imbalance resulting in the overexpression of GlcCer in their membrane. In this work, in vitroassays using B16F10 cells showed that labeled fluorescein isothiocyanate FITC-Psd1 internalized into live cultured cells and targeted the nucleus, which underwent fragmentation, exhibiting approximately 60% of cells in the sub-G0/G1 stage. This phenomenon was dependent on GlcCer, and the participation of cyclin-F was suggested. In a murine lung metastatic melanoma model, intravenous injection of Psd1 together with B16F10 cells drastically reduced the number of nodules at concentrations above 0.5 mg/kg. Additionally, the administration of 1 mg/kg Psd1 decreased the number of lung inflammatory cells to near zero without weight loss, unlike animals that received melanoma cells only. It is worth noting that 1 mg/kg Psd1 alone did not provoke inflammation in lung tissue or weight or vital signal losses over 21 days, inferring no whole animal cytotoxicity. These results suggest that Psd1 could be a promising prototype for human lung anti-metastatic melanoma therapy.

Lipid Rafts: Controversies Resolved, Mysteries Remain

The lipid raft hypothesis postulates that lipid-lipid interactions can laterally organize biological membranes into domains of distinct structures, compositions, and functions. This proposal has in equal measure exhilarated and frustrated membrane research for decades. While the physicochemical principles underlying lipid-driven domains has been explored and is well understood, the existence and relevance of such domains in cells remains elusive, despite decades of research. Here, we review the conceptual underpinnings of the raft hypothesis and critically discuss the supporting and contradicting evidence in cells, focusing on why controversies about the composition, properties, and even the very existence of lipid rafts remain unresolved. Finally, we highlight several recent breakthroughs that may resolve existing controversies and suggest general approaches for moving beyond questions of the existence of rafts and towards understanding their physiological significance.

Agonist-induced membrane nanodomain clustering drives GLP-1 receptor responses in pancreatic beta cells

The glucagon-like peptide-1 receptor (GLP-1R), a key pharmacological target in type 2 diabetes (T2D) and obesity, undergoes rapid endocytosis after stimulation by endogenous and therapeutic agonists. We have previously highlighted the relevance of this process in fine-tuning GLP-1R responses in pancreatic beta cells to control insulin secretion. In the present study, we demonstrate an important role for the translocation of active GLP-1Rs into liquid-ordered plasma membrane nanodomains, which act as hotspots for optimal coordination of intracellular signaling and clathrin-mediated endocytosis. This process is dynamically regulated by agonist binding through palmitoylation of the GLP-1R at its carboxyl-terminal tail. Biased GLP-1R agonists and small molecule allosteric modulation both influence GLP-1R palmitoylation, clustering, nanodomain signaling, and internalization. Downstream effects on insulin secretion from pancreatic beta cells indicate that these processes are relevant to GLP-1R physiological actions and might be therapeutically targetable.

Nanodomain segregation and clustering of the glucagon-like peptide-1 receptor, a key target for type 2 diabetes therapy, is regulated by agonist binding, leading to compartmentalization of downstream signaling and clathrin-dependent internalization and impacting pancreatic beta cell responses.

Proteomic Analysis of Lipid Rafts from RBL-2H3 Mast Cells

Lipid rafts are highly ordered membrane microdomains enriched in cholesterol, glycosphingolipids, and certain proteins. They are involved in the regulation of cellular processes in diverse cell types, including mast cells (MCs). The MC lipid raft protein composition was assessed using qualitative mass spectrometric characterization of the proteome from detergent-resistant membrane fractions from RBL-2H3 MCs. Using two different post-isolation treatment methods, a total of 949 lipid raft associated proteins were identified. The majority of these MC lipid raft proteins had already been described in the RaftProtV2 database and are among highest cited/experimentally validated lipid raft proteins. Additionally, more than half of the identified proteins had lipid modifications and/or transmembrane domains. Classification of identified proteins into functional categories showed that the proteins were associated with cellular membrane compartments, and with some biological and molecular functions, such as regulation, localization, binding, catalytic activity, and response to stimulus. Furthermore, functional enrichment analysis demonstrated an intimate involvement of identified proteins with various aspects of MC biological processes, especially those related to regulated secretion, organization/stabilization of macromolecules complexes, and signal transduction. This study represents the first comprehensive proteomic profile of MC lipid rafts and provides additional information to elucidate immunoregulatory functions coordinated by raft proteins in MCs.

Membrane Lipid Composition: Effect on Membrane and Organelle Structure, Function and Compartmentalization and Therapeutic Avenues

Biological membranes are key elements for the maintenance of cell architecture and physiology. Beyond a pure barrier separating the inner space of the cell from the outer, the plasma membrane is a scaffold and player in cell-to-cell communication and the initiation of intracellular signals among other functions. Critical to this function is the plasma membrane compartmentalization in lipid microdomains that control the localization and productive interactions of proteins involved in cell signal propagation. In addition, cells are divided into compartments limited by other membranes whose integrity and homeostasis are finely controlled, and which determine the identity and function of the different organelles. Here, we review current knowledge on membrane lipid composition in the plasma membrane and endomembrane compartments, emphasizing its role in sustaining organelle structure and function. The correct composition and structure of cell membranes define key pathophysiological aspects of cells. Therefore, we explore the therapeutic potential of manipulating membrane lipid composition with approaches like membrane lipid therapy, aiming to normalize cell functions through the modification of membrane lipid bilayers.

The desmosome is a mesoscale lipid raft-like membrane domain

Desmogleins (Dsgs) are cadherin family adhesion molecules essential for epidermal integrity. Previous studies have shown that desmogleins associate with lipid rafts, but the significance of this association was not clear. Here, we report that the desmoglein transmembrane domain (TMD) is the primary determinant of raft association. Further, we identify a novel mutation in the DSG1 TMD (G562R) that causes severe dermatitis, multiple allergies, and metabolic wasting syndrome. Molecular modeling predicts that this G-to-R mutation shortens the DSG1 TMD, and experiments directly demonstrate that this mutation compromises both lipid raft association and desmosome incorporation. Finally, cryo-electron tomography indicates that the lipid bilayer within the desmosome is ∼10% thicker than adjacent regions of the plasma membrane. These findings suggest that differences in bilayer thickness influence the organization of adhesion molecules within the epithelial plasma membrane, with cadherin TMDs recruited to the desmosome via the establishment of a specialized mesoscale lipid raft-like membrane domain.

Label-free tracking of nanosized graphene oxide cellular uptake by confocal Raman microscopy

Graphene oxide (GO), a partially oxidized two-dimensional allotrope of carbon, is an attractive nanocarrier for cancer diagnostics and therapy. The nanometer-sized GO is known to permeate cell membranes. Herein we studied the cellular uptake pathways of GO nanoflakes by cancer and non-cancerous cell lines. By employing confocal Raman imaging, we were able to track the GO cellular uptake in living cells (C33 and MDCK) without any additional fluorescent or plasmonic labels. This specific progress in label-free Raman imaging of GO facilitates the monitoring of nanoflakes at the cellular level.

Sphingolipids and lipid rafts: Novel concepts and methods of analysis

About twenty years ago, the functional lipid raft model of the plasma membrane was published. It took into account decades of research showing that cellular membranes are not just homogenous mixtures of lipids and proteins. Lateral anisotropy leads to assembly of membrane domains with specific lipid and protein composition regulating vesicular traffic, cell polarity, and cell signaling pathways in a plethora of biological processes. However, what appeared to be a clearly defined entity of clustered raft lipids and proteins became increasingly fluid over the years, and many of the fundamental questions about biogenesis and structure of lipid rafts remained unanswered. Experimental obstacles in visualizing lipids and their interactions hampered progress in understanding just how big rafts are, where and when they are formed, and with which proteins raft lipids interact. In recent years, we have begun to answer some of these questions and sphingolipids may take center stage in re-defining the meaning and functional significance of lipid rafts. In addition to the archetypical cholesterol-sphingomyelin raft with liquid ordered (Lo) phase and the liquid-disordered (Ld) non-raft regions of cellular membranes, a third type of microdomains termed ceramide-rich platforms (CRPs) with gel-like structure has been identified. CRPs are "ceramide rafts" that may offer some fresh view on the membrane mesostructure and answer several critical questions for our understanding of lipid rafts.

Lipidomimetic Compounds Act as HIV-1 Entry Inhibitors by Altering Viral Membrane Structure

The envelope of Human Immunodeficiency Virus type 1 (HIV-1) consists of a liquid-ordered membrane enriched in raft lipids and containing the viral glycoproteins. Previous studies demonstrated that changes in viral membrane lipid composition affecting membrane structure or curvature can impair infectivity. Here, we describe novel antiviral compounds that were identified by screening compound libraries based on raft lipid-like scaffolds. Three distinct molecular structures were chosen for mode-of-action studies, a sterol derivative (J391B), a sphingosine derivative (J582C) and a long aliphatic chain derivative (IBS70). All three target the viral membrane and inhibit virus infectivity at the stage of fusion without perturbing virus stability or affecting virion-associated envelope glycoproteins. Their effect did not depend on the expressed envelope glycoproteins or a specific entry route, being equally strong in HIV pseudotypes carrying VSV-G or MLV-Env glycoproteins. Labeling with laurdan, a reporter of membrane order, revealed different membrane structure alterations upon compound treatment of HIV-1, which correlated with loss of infectivity. J582C and IBS70 decreased membrane order in distinctive ways, whereas J391B increased membrane order. The compounds' effects on membrane order were reproduced in liposomes generated from extracted HIV lipids and thus independent both of virion proteins and of membrane leaflet asymmetry. Remarkably, increase of membrane order by J391B required phosphatidylserine, a lipid enriched in the HIV envelope. Counterintuitively, mixtures of two compounds with opposite effects on membrane order, J582C and J391B, did not neutralize each other but synergistically inhibited HIV infection. Thus, altering membrane order, which can occur by different mechanisms, constitutes a novel antiviral mode of action that may be of general relevance for enveloped viruses and difficult to overcome by resistance development.

Cellular mechanisms of physicochemical membrane homeostasis

Biological membranes are vital, active contributors to cell function. In addition to specific interactions of individual lipid molecules and lateral organization produced by membrane domains, the bulk physicochemical properties of biological membranes broadly regulate protein structure and function. Therefore, these properties must be homeostatically maintained within a narrow range that is compatible with cellular physiology. Although such adaptiveness has been known for decades, recent observations have dramatically expanded its scope by showing the breadth of membrane properties that must be maintained, and revealing the remarkable diversity of biological membranes, both within and between cell types. Cells have developed a broad palette of sense-and-respond machineries to mediate physicochemical membrane homeostasis, and the molecular mechanisms of these are being discovered through combinations of cell biology, biophysical approaches, and computational modeling.

Surfactant Interactions and Organization at the Gas-Water Interface (CTAB with Added Salt)

We have studied adsorbed layers of cetyltrimethylammonium bromide (CTAB) at air-water interfaces in the presence of added electrolyte. Fast bubble compression/expansion measurements were used to obtain the surface equation of state, i.e., the surface tension vs CTAB surface concentration dependence. We show that while a simple model where the surfactant molecules are assumed to be noninteracting is insufficient to describe the measured response of the surfactant layer, a modified Frumkin equation where the local interactions between the molecular components depend on their surface concentration captures the response. The variation of the effective interactions in the surfactant layer in the model shows that the interactions in the surfactant layer change from effectively repulsive to attractive with increasing surface concentration. Molecular dynamics simulations are performed to probe the origins of the change in the interactions. The simulations indicate that already at low surface concentrations the surfactants aggregate as highly dynamic rafts with surfactant orientation parallel to the interface. Increasing the concentration leads to a change in the assembly morphology at the interface: the surfactant layer thickens and the surfactants sample a range of tilted orientations with respect to the interfacial plane. The change from transient raftlike assemblies to dynamical aggregates at the interface involves a clear increase in the degree of counterion binding: we speculate that the flip of the effective interaction parameter in the model used to interpret the experimental results could result from this. The work here presents basic steps toward a proper understanding of the molecular organization and interactions of surfactants at an air-water interface. This is crucially important in understanding macroscopic properties of surfactant-stabilized systems such as foams, emulsions, and colloidal dispersions.

Uptake and intracellular fate of cholera toxin subunit b-modified mesoporous silica nanoparticle-supported lipid bilayers (aka protocells) in motoneurons

Cholera toxin B (CTB) modified mesoporous silica nanoparticle supported lipid bilayers (CTB-protocells) are a promising, customizable approach for targeting therapeutic cargo to motoneurons. In the present study, the endocytic mechanism and intracellular fate of CTB-protocells in motoneurons were examined to provide information for the development of therapeutic application and cargo delivery. Pharmacological inhibitors elucidated CTB-protocells endocytosis to be dependent on the integrity of lipid rafts and macropinocytosis. Using immunofluorescence techniques, live confocal and transmission electron microscopy, CTB-protocells were primarily found in the cytosol, membrane lipid domains and Golgi. There was no difference in the amount of motoneuron activity dependent uptake of CTB-protocells in neuromuscular junctions, consistent with clathrin activation at the axon terminals during low frequency activity. In conclusion, CTB-protocells uptake is mediated principally by lipid rafts and macropinocytosis. Once internalized, CTB-protocells escape lysosomal degradation, and engage biological pathways that are not readily accessible by untargeted delivery methods.

ω-3 polyunsaturated fatty acids direct differentiation of the membrane phenotype in mesenchymal stem cells to potentiate osteogenesis

Mammalian cells produce hundreds of dynamically regulated lipid species that are actively turned over and trafficked to produce functional membranes. These lipid repertoires are susceptible to perturbations from dietary sources, with potentially profound physiological consequences. However, neither the lipid repertoires of various cellular membranes, their modulation by dietary fats, nor their effects on cellular phenotypes have been widely explored. We report that differentiation of human mesenchymal stem cells (MSCs) into osteoblasts or adipocytes results in extensive remodeling of the plasma membrane (PM), producing cell-specific membrane compositions and biophysical properties. The distinct features of osteoblast PMs enabled rational engineering of membrane phenotypes to modulate differentiation in MSCs. Specifically, supplementation with docosahexaenoic acid (DHA), a lipid component characteristic of osteoblast membranes, induced broad lipidomic remodeling in MSCs that reproduced compositional and structural aspects of the osteoblastic PM phenotype. The PM changes induced by DHA supplementation potentiated osteogenic differentiation of MSCs concurrent with enhanced Akt activation at the PM. These observations prompt a model wherein the DHA-induced lipidome leads to more stable membrane microdomains, which serve to increase Akt activity and thereby enhance osteogenic differentiation. More broadly, our investigations suggest a general mechanism by which dietary fats affect cellular physiology through remodeling of membrane lipidomes, biophysical properties, and signaling.

Structural determinants and functional consequences of protein affinity for membrane rafts

Eukaryotic plasma membranes are compartmentalized into functional lateral domains, including lipid-driven membrane rafts. Rafts are involved in most plasma membrane functions by selective recruitment and retention of specific proteins. However, the structural determinants of transmembrane protein partitioning to raft domains are not fully understood. Hypothesizing that protein transmembrane domains (TMDs) determine raft association, here we directly quantify raft affinity for dozens of TMDs. We identify three physical features that independently affect raft partitioning, namely TMD surface area, length, and palmitoylation. We rationalize these findings into a mechanistic, physical model that predicts raft affinity from the protein sequence. Application of these concepts to the human proteome reveals that plasma membrane proteins have higher raft affinity than those of intracellular membranes, consistent with raft-mediated plasma membrane sorting. Overall, our experimental observations and physical model establish general rules for raft partitioning of TMDs and support the central role of rafts in membrane traffic.

Membrane Transport across Polarized Epithelia

Polarized epithelial cells line diverse surfaces throughout the body forming selective barriers between the external environment and the internal milieu. To cross these epithelial barriers, large solutes and other cargoes must undergo transcytosis, an endocytic pathway unique to polarized cell types, and significant for the development of cell polarity, uptake of viral and bacterial pathogens, transepithelial signaling, and immunoglobulin transport. Here, we review recent advances in our knowledge of the transcytotic pathway for proteins and lipids. We also discuss briefly the promise of harnessing the molecules that undergo transcytosis as vehicles for clinical applications in drug delivery.

Lipid composition of microdomains is altered in neuronopathic Gaucher disease sheep brain and spleen

Gaucher disease is a lysosomal storage disorder caused by a deficiency in glucocerebrosidase activity that leads to accumulation of glucosylceramide and glucosylsphingosine. Membrane raft microdomains are discrete, highly organized microdomains with a unique lipid composition that provide the necessary environment for specific protein-lipid and protein-protein interactions to take place. In this study we purified detergent resistant membranes (DRM; membrane rafts) from the occipital cortex and spleen from sheep affected with acute neuronopathic Gaucher disease and wild-type controls. We observed significant increases in the concentrations of glucosylceramide, hexosylsphingosine, BMP and gangliosides and decreases in the percentage of cholesterol and phosphatidylcholine leading to an altered DRM composition. Altered sphingolipid/cholesterol homeostasis would dramatically disrupt DRM architecture making them less ordered and more fluid. In addition, significant changes in the length and degree of lipid saturation within the DRM microdomains in the Gaucher brain were also observed. As these DRM microdomains are involved in many cellular events, an imbalance or disruption of the cell membrane homeostasis may impair normal cell function. This disruption of membrane raft microdomains and imbalance within the environment of cellular membranes of neuronal cells may be a key factor in initiating a cascade process leading to neurodegeneration.

The effect of sterol structure upon clathrin-mediated and clathrin-independent endocytosis

Ordered lipid domains (rafts) in plasma membranes have been hypothesized to participate in endocytosis based on inhibition of endocytosis by removal or sequestration of cholesterol. To more carefully investigate the role of the sterol in endocytosis, we used a substitution strategy to replace cholesterol with sterols that show various raft-forming abilities and chemical structures. Both clathrin-mediated endocytosis of transferrin and clathrin-independent endocytosis of clustered placental alkaline phosphatase were measured. A subset of sterols reversibly inhibited both clathrin-dependent and clathrin-independent endocytosis. The ability of a sterol to support lipid raft formation was necessary for endocytosis. However, it was not sufficient, because a sterol lacking a 3β-OH group did not support endocytosis even though it had the ability to support ordered domain formation. Double bonds in the sterol rings and an aliphatic tail structure identical to that of cholesterol were neither necessary nor sufficient to support endocytosis. This study shows that substitution using a large number of sterols can define the role of sterol structure in cellular functions. Hypotheses for how sterol structure can similarly alter clathrin-dependent and clathrin-independent endocytosis are discussed.

The Potential of Cyclodextrins as Novel Active Pharmaceutical Ingredients: A Short Overview

Cyclodextrins (CDs) are cyclic oligosaccharides of natural origin that were discovered more than 100 years ago. The peculiar cone-like conformation of the sugar ring, expressing a lipophilic cavity and a hydrophilic external surface, allows these substances to spontaneously complex poorly soluble compounds in an aqueous environment. For more than 50 years, these substances have found applicability in the pharmaceutical and food industries as solubilizing agents for poorly soluble chemical entities. Nowadays, several research groups all over the world are investigating their potential as active pharmaceutical ingredients (APIs) for the treatment of several illnesses (e.g., hypercholesterolemia, cancer, Niemann-Pick Type C disease). The aim of this review is to briefly retrace cyclodextrins’ legacy as complexing agents and describe the current and future prospects of this class of chemical entities in pharmaceutics as new APIs.

Domain Stability in Biomimetic Membranes Driven by Lipid Polyunsaturation

Biological membranes contain a broad variety of lipid species whose individual physicochemical properties and collective interactions ultimately determine membrane organization. A key aspect of the organization of cellular membranes is their lateral subdivision into domains of distinct structure and composition. The most widely studied membrane domains are lipid rafts, which are the biological manifestations of liquid-ordered phases that form in sterol-containing membranes. Detailed studies of biomimetic membrane mixtures have yielded wide-ranging insights into the physical principles behind lipid rafts; however, these simplified models do not fully capture the diversity and complexity of the mammalian lipidome, most notably in their exclusion of polyunsaturated lipids. Here, we assess the role of lipid acyl chain unsaturation as a driving force for phase separation using coarse-grained molecular dynamics (CGMD) simulations validated by model membrane experiments. The clear trends in our observations and good qualitative agreements between simulations and experiments support the conclusions that highly unsaturated lipids promote liquid-liquid domain stability by enhancing the differences in cholesterol content and lipid chain order between the coexisting domains. These observations reveal the important role of noncanonical biological lipids in the physical properties of membranes, showing that lipid polyunsaturation is a driving force for liquid-liquid phase separation.

Lipid rafts of mouse liver contain nonextended and extended acetylcholinesterase variants along with M3 muscarinic receptors

The observation of acetylcholinesterase (AChE) type H (AChE_H), which is the predominant AChE variant in visceral organs and immune cells, in lipid rafts of muscle supports functional reasons for the raft targeting of glypiated AChE_H The search for these reasons revealed that liver AChE activity is mostly confined to rafts and that the liver is able to make N-extended AChE variants and target them to rafts. These results prompted us to test whether AChE and muscarinic receptors existed in the same raft. Isolation of flotillin-2-rich raft fractions by their buoyancy in sucrose gradients, followed by immunoadsorption and matrix-assisted laser desorption ionization-time of flight-mass spectrometry application, gave the following results: 1) most hepatic AChE activity emanates from AChE-H mRNA, and its product, glypiated AChE_H, accumulates in rafts; 2) N-extended N-AChE readthrough variant, nonglypiated N-AChE_H, and N-AChE tailed variant were all identified in liver rafts; and 3) M3 AChRs were observed in rafts, and coprecipitation of raft-confined N-AChE and M3 receptors by using anti-M3 antibodies showed that enzyme and receptor reside in the same raft unit. A raft domain that harbors tightly packed muscarinic receptor and AChE may represent a molecular device that, by means of which, the intensity and duration of cholinergic inputs are regulated.-Montenegro, M. F., Cabezas-Herrera, J., Campoy, F. J., Muñoz-Delgado, E., Vidal, C. J. Lipid rafts of mouse liver contain nonextended and extended acetylcholinesterase variants along with M3 muscarinic receptors.