The differential miscibility of lipids as the basis for the formation of functional membrane rafts

Phospholipid Membranes as Chemically and Functionally Tunable Materials

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

Impacts of Different Prenatal Supplementation Strategies on the Plasma Metabolome of Bulls in the Rearing and Finishing Phase

This study investigated the effects of maternal nutrition on the plasma metabolome of Nellore bulls in the rearing and finishing phases, and metabolic differences between these phases. For this study, three nutritional approaches were used in 126 cows during pregnancy: NP-(control) mineral supplementation; PP-protein-energy supplementation in the final third; and FP-protein-energy supplementation during the entire pregnancy. We collected blood samples from male offspring in the rearing (450 ± 28 days old) and finishing phases (660 ± 28 days old). The blood was processed, and from plasma samples, we performed the targeted metabolome analysis (AbsoluteIDQ® p180 Kit). Multiple linear regression, principal component analysis (PCA), repeated measures analysis over time, and an enrichment analysis were performed. PCA showed an overlap of treatments and time clusters in the analyses. We identified significant metabolites among the treatments (rearing phase = six metabolites; finishing phase = three metabolites) and over time (21 metabolites). No significant metabolic pathways were found in the finishing phase, however, we found significant pathways in the rearing phase (Arginine biosynthesis and Histidine metabolism). Thus, prenatal nutrition impacted on plasma metabolome of bulls during the rearing and finishing phase and the different production stages showed an effect on the metabolic levels of bulls.

Insights into Asymmetric Liposomes as a Potential Intervention for Drug Delivery Including Pulmonary Nanotherapeutics

Liposome-based drug delivery systems are nanosized spherical lipid bilayer carriers that can encapsulate a broad range of small drug molecules (hydrophilic and hydrophobic drugs) and large drug molecules (peptides, proteins, and nucleic acids). They have unique characteristics, such as a self-assembling bilayer vesicular structure. There are several FDA-approved liposomal-based medicines for treatment of cancer, bacterial, and viral infections. Most of the FDA-approved liposomal-based therapies are in the form of conventional "symmetric" liposomes and they are administered mainly by injection. Arikace^® is the first and only FDA-approved liposomal-based inhalable therapy (amikacin liposome inhalation suspension) to treat only adults with difficult-to-treat Mycobacterium avium complex (MAC) lung disease as a combinational antibacterial treatment. To date, no "asymmetric liposomes" are yet to be approved, although asymmetric liposomes have many advantages due to the asymmetric distribution of lipids through the liposome's membrane (which is similar to the biological membranes). There are many challenges for the formulation and stability of asymmetric liposomes. This review will focus on asymmetric liposomes in contrast to conventional liposomes as a potential clinical intervention drug delivery system as well as the formulation techniques available for symmetric and asymmetric liposomes. The review aims to renew the research in liposomal nanovesicle delivery systems with particular emphasis on asymmetric liposomes as future potential carriers for enhancing drug delivery including pulmonary nanotherapeutics.

Nucleation of Surfactant-Alkane Mixed Solid Monolayer and Bilayer Domains at the Air-Water Interface

We investigated the wetting transitions of tetradecane and hexadecane droplets in dodecyltrimethylammonium bromide (C12TAB), tetradecyltrimethylammonium bromide (C14TAB), and hexadecyltrimethylammonium bromide (C16TAB) aqueous solutions. By varying the surfactant concentration, the formation of mixed monolayers of a surfactant and an alkane was observed at the air–water interface. Depending on the combination of surfactant and alkane, these wetting monolayers underwent another thermal phase transition upon cooling either to a frozen mixed monolayer (S1) or a bilayer structure composed of a solid monolayer of a pure alkane rested on a liquid-like mixed monolayer (S2). Based on the phase diagrams determined by phase modulation ellipsometry, the difference in the morphology of the nucleated S1 and S2 phase domains was also investigated using Brewster angle microscopy. Domains of the S1 phase were relatively small and highly branched, whereas those of the S2 phase were large and circular. The difference in domain morphology was explained by the competition of the domain line tension and electrostatic dipole interactions between surfactant molecules in the domains.

Sphingolipid Metabolism and Signaling in Endothelial Cell Functions

The endothelium, inner layer of blood vessels, constitutes a metabolically active paracrine, endocrine, and autocrine organ, able to sense the neighboring environment and exert a variety of biological functions important to preserve the health of vasculature, tissues, and organs. Sphingolipids are both fundamental structural components of the eukaryotic membranes and signaling molecules regulating a variety of biological functions. Ceramide and sphingosine-1-phosphate (S1P), bioactive sphingolipids, have emerged as important regulators of cardiovascular functions in health and disease. In this review we discuss recent insights into the role of ceramide and S1P biosynthesis and signaling in regulating endothelial cell functions, in health and diseases. We also highlight advances into the mechanisms regulating serine palmitoyltransferase, the first and rate-limiting enzyme of de novo sphingolipid biosynthesis, with an emphasis on its inhibitors, ORMDL and NOGO-B. Understanding the molecular mechanisms regulating the sphingolipid de novo biosynthesis may provide the foundation for therapeutic modulation of this pathway in a variety of conditions, including cardiovascular diseases, associated with derangement of this pathway.

Long-lived humans have a unique plasma sphingolipidome

A species-specific lipidome profile is an inherent feature linked to longevity in the animal kingdom. However, there is a lack of lipidomic studies on human longevity. Here, we use mass spectrometry-based lipidomics to detect and quantify 151 sphingolipid molecular species and use these to define a phenotype of healthy humans with exceptional life span. Our results demonstrate that this profile specifically comprises a higher content of complex glycosphingolipids (hexosylceramides and gangliosides), and lower levels of ceramide species from the de novo pathway, sphingomyelin and sulfatide; while for ceramide-derived signaling compounds, their content remains unchanged. Our findings suggest that structural glycosphingolipids may be more relevant to achieve the centenarian condition than signaling sphingolipids.

Influence of amphotericin B on the DPPC/DOPC/sterols mixed monolayer in the presence of calcium ions

Amphotericin B, an acquainted antifungal drug, has reattracted the attention of most scholars due to its one important advantage of making the fungus less resistant. Amphotericin B's antifungal properties are derived from its ability to interact with ergosterols on the fungal cells' membrane to form pores. However, the cholesterol in the human cell membranes is similar in structure to ergosterol, which cause the drug to produce certain toxicity and make the clinical use of amphotericin B limited. The study of the interaction between amphotericin B and lipid monolayer in the presence of cholesterol or ergosterol is crucial to understanding the mechanism of effect of the drug on cell membranes. Langmuir monolayer as a model for half of cell membranes can precisely control the proportion of components and the solution environment, which has been used to do a lot of research about the interaction of amphotericin B with lipids. It is noteworthy that some ions associated with life activities play an important role in it, such as calcium ions. In this work, the surface pressure-mean molecular area isotherms, elastic modulus and the surface pressure-time curves of DPPC/DOPC/sterol mixed monolayer with or without amphotericin B were studied in the different concentration of calcium ions. The morphology of the Langmuir-Blodgett films transferred on the mica were observed by atomic force microscopy. The results shown that AmB changed the elastic modulus and surface morphology of DPPC/DOPC/sterol mxied monolayer, which was significantly different with different types of sterols. Calcium ions can regulate the effect of this drug, which was clearly different due to different types of sterols. This work provides useful information to further understand the influence mechanism of calcium ions on the interaction between AmB and phospholipid/sterol monolayer, which is helpful to find out the effect mechanism of calcium ion on the interaction between AmB and phospholipid monolayer containing ergosterol or cholesterol and to understand the mechanism of AmB influencing on the membrane of fungal or human cells.

Compartmentalization of phosphatidylinositol 4,5-bisphosphate metabolism into plasma membrane liquid-ordered/raft domains

Possible segregation of plasma membrane (PM) phosphoinositide metabolism in membrane lipid domains is not fully understood. We exploited two differently lipidated peptide sequences, L10 and S15, to mark liquid-ordered, cholesterol-rich (L_o) and liquid-disordered, cholesterol-poor (L_d) domains of the PM, often called raft and nonraft domains, respectively. Imaging of the fluorescent labels verified that L10 segregated into cholesterol-rich L_o phases of cooled giant plasma-membrane vesicles (GPMVs), whereas S15 and the dye FAST DiI cosegregated into cholesterol-poor L_d phases. The fluorescent protein markers were used as Förster resonance energy transfer (FRET) pairs in intact cells. An increase of homologous FRET between L10 probes showed that depleting membrane cholesterol shrank L_o domains and enlarged L_d domains, whereas a decrease of L10 FRET showed that adding more cholesterol enlarged L_o and shrank L_d Heterologous FRET signals between the lipid domain probes and phosphoinositide marker proteins suggested that phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P ₂] and phosphatidylinositol 4-phosphate (PtdIns4P) are present in both L_o and L_d domains. In kinetic analysis, muscarinic-receptor-activated phospholipase C (PLC) depleted PtdIns(4,5)P ₂ and PtdIns4P more rapidly and produced diacylglycerol (DAG) more rapidly in L_o than in L_d Further, PtdIns(4,5)P ₂ was restored more rapidly in L_o than in L_d Thus destruction and restoration of PtdIns(4,5)P ₂ are faster in L_o than in L_d This suggests that L_o is enriched with both the receptor G protein/PLC pathway and the PtdIns/PI4-kinase/PtdIns4P pathway. The significant kinetic differences of lipid depletion and restoration also mean that exchange of lipids between these domains is much slower than free diffusion predicts.

Lipid alterations in human frontal cortex in ALS-FTLD-TDP43 proteinopathy spectrum are partly related to peroxisome impairment

AIM

Peroxisomes play a key role in lipid metabolism, and peroxisome defects have been associated with neurodegenerative diseases such as X-adrenoleukodystrophy and Alzheimer's disease. This study aims to elucidate the contribution of peroxisomes in lipid alterations of area 8 of the frontal cortex in the spectrum of TDP43-proteinopathies. Cases of frontotemporal lobar degeneration-TDP43 (FTLD-TDP), manifested as sporadic (sFTLD-TDP) or linked to mutations in various genes including expansions of the non-coding region of C9ORF72 (c9FTLD), and of sporadic amyotrophic lateral sclerosis (sALS) as the most common TDP43 proteinopathies, were analysed.

METHODS

We used transcriptomics and lipidomics methods to define the steady-state levels of gene expression and lipid profiles.

RESULTS

Our results show alterations in gene expression of some components of peroxisomes and related lipid pathways in frontal cortex area 8 in sALS, sFTLD-TDP and c9FTLD. Additionally, we identify a lipidomic pattern associated with the ALS-FTLD-TDP43 proteinopathy spectrum, notably characterised by down-regulation of ether lipids and acylcarnitine among other lipid species, as well as alterations in the lipidome of each phenotype of TDP43 proteinopathy, which reveals commonalities and disease-dependent differences in lipid composition.

CONCLUSION

Globally, lipid alterations in the human frontal cortex of the ALS-FTLD-TDP43 proteinopathy spectrum, which involve cell membrane composition and signalling, vulnerability against cellular stress and possible glucose metabolism, are partly related to peroxisome impairment.

Naringenin suppresses Toll-like receptor 2-mediated inflammatory responses through inhibition of receptor clustering on lipid rafts

Toll-like receptors (TLRs) are important innate immune receptors that sometimes cause excessive inflammatory responses and a perpetuated inflammatory loop that can be involved in inflammatory and autoimmune diseases. TLR2 recognizes bacterial lipoproteins in association with TLR1 or TLR6, and triggers inflammatory responses through activation of the transcription factor NF-κB. Naringenin, a type of citrus flavonoid, has been shown to possess anti-inflammatory properties, but its detailed action against TLR2 remains to be fully elucidated. The present study was designed to determine whether naringenin affects the inflammatory responses triggered by TLR2. Naringenin inhibited proinflammatory cytokine production and attenuated NF-κB activation in cells stimulated with a synthetic triacylated-type lipopeptide known as a TLR2/TLR1 ligand, as well as a synthetic diacylated-type lipopeptide known as a TLR2/TLR6 ligand. Moreover, a similar inhibitory effect was observed in cells stimulated with a crude lipophilic fraction extracted from Staphylococcus aureus cell walls and in cells stimulated with S. aureus cells. Furthermore, we showed that such an effect is caused by inhibition of TLR2 clustering in lipid rafts on the cell membrane. These results suggest that naringenin suppresses the inflammatory responses induced by TLR2 signal transduction. Our findings indicate a novel anti-inflammatory property of naringenin, mediated through the regulation of cell surface TLR2 functioning.

Prebiological Membranes and Their Role in the Emergence of Early Cellular Life

Membrane compartmentalization is a fundamental feature of contemporary cellular life. Given this, it is rational to assume that at some stage in the early origins of life, membrane compartments would have potentially emerged to form a dynamic semipermeable barrier in primitive cells (protocells), protecting them from their surrounding environment. It is thought that such prebiological membranes would likely have played a crucial role in the emergence and evolution of life on the early Earth. Extant biological membranes are highly organized and complex, which is a consequence of a protracted evolutionary history. On the other hand, prebiotic membrane assemblies, which are thought to have preceded sophisticated contemporary membranes, are hypothesized to have been relatively simple and composed of single chain amphiphiles. Recent studies indicate that the evolution of prebiotic membranes potentially resulted from interactions between the membrane and its physicochemical environment. These studies have also speculated on the origin, composition, function and influence of environmental conditions on protocellular membranes as the niche parameters would have directly influenced their composition and biophysical properties. Nonetheless, the evolutionary pathways involved in the transition from prebiological membranes to contemporary membranes are largely unknown. This review critically evaluates existing research on prebiotic membranes in terms of their probable origin, composition, energetics, function and evolution. Notably, we outline new approaches that can further our understanding about how prebiotic membranes might have evolved in response to relevant physicochemical parameters that would have acted as pertinent selection pressures on the early Earth.

The Laminin-Induced Phosphorylation of PKCδ Regulates AQP4 Distribution and Water Permeability in Rat Astrocytes

In astrocytes, the water-permeable channel aquaporin-4 (AQP4) is concentrated at the endfeet that abut the blood vessels of the brain. The asymmetric distribution of this channel is dependent on the function of dystroglycan (DG), a co-expressed laminin receptor, and its associated protein complex. We have demonstrated that the addition of laminin to astrocytes in culture causes the clustering of AQP4, DG, and lipid rafts. The last, in particular, have been associated with the initiation of cell signaling. As laminin binding to DG in muscle cells can induce the tyrosine phosphorylation of syntrophin and laminin requires tyrosine kinases for acetylcholine receptor clustering in myotubes, we asked if signal transduction might also be involved in AQP4 clustering in astrocytes. We analyzed the timecourse of AQP4, DG, and monosialotetrahexosylganglioside (GM1) clustering in primary cultures of rat astrocytes following the addition of laminin, and determined that the clustering of DG precedes that of AQP4 and GM1. We also showed that laminin induces the formation of phosphotyrosine-rich clusters and that the tyrosine kinase inhibitor, genistein, disrupts the laminin-induced clustering of both β-DG and AQP4. Using the Kinexus antibody microarray chip, we then identified protein-serine kinase C delta (PKCδ) as one of the main proteins exhibiting high levels of tyrosine phosphorylation upon laminin treatment. Selective inhibitors of PKC and siRNA against PKCδ disrupted β-DG and AQP4 clustering, and also caused water transport to increase in astrocytes treated with laminin. Our results demonstrate that the effects of laminin on AQP4 localization and function are relayed, at least in part, through PKC signaling.

Rendezvous at Plasma Membrane: Cellular Lipids and tRNA Set up Sites of HIV-1 Particle Assembly and Incorporation of Host Transmembrane Proteins

The HIV-1 structural polyprotein Gag drives the virus particle assembly specifically at the plasma membrane (PM). During this process, the nascent virion incorporates specific subsets of cellular lipids and host membrane proteins, in addition to viral glycoproteins and viral genomic RNA. Gag binding to the PM is regulated by cellular factors, including PM-specific phospholipid PI(4,5)P2 and tRNAs, both of which bind the highly basic region in the matrix domain of Gag. In this article, we review our current understanding of the roles played by cellular lipids and tRNAs in specific localization of HIV-1 Gag to the PM. Furthermore, we examine the effects of PM-bound Gag on the organization of the PM bilayer and discuss how the reorganization of the PM at the virus assembly site potentially contributes to the enrichment of host transmembrane proteins in the HIV-1 particle. Since some of these host transmembrane proteins alter release, attachment, or infectivity of the nascent virions, the mechanism of Gag targeting to the PM and the nature of virus assembly sites have major implications in virus spread.

Differential impact of synthetic antitumor lipid drugs on the membrane organization of phosphatidic acid and diacylglycerol monolayers

Anti-tumour lipids are synthetic analogues of lysophosphatidylcholine. These drugs are both cytotoxic and cytostatic, and more interestingly, exert these effects preferentially in tumour cells. While the exact mechanism of action isn't fully elucidated, these drugs appear to preferentially partition into rigid lipid domains in cell membranes. Upon insertion, the compounds alter membrane domain organization, disrupt normal signal transduction, and cause cell death. Recently, it has been reported that these drugs induce accumulation of diacylglycerol in yeast cells which in turn sensitizes cells to the drugs. Conversely, phosphatidic acid accumulation appears to protect cells against the drugs. In the current work, the aim was to compare the biophysical effects of the drugs edelfosine, miltefosine and perifosine on monolayers of dimyristoyl phosphatidic acid, dimyristoyl glycerol and an equimolar mixture, to understand how these lipids modulate the mode of action. Surface pressure - area isotherms, compression moduli and Brewster angle microscopy were used to compare drug effects on lipid packing, monolayer compressibility and lateral domain organization of these films. Results suggest that edelfosine and miltefosine have stabilizing effects on all of the monolayers, while perifosine destabilizes dimyristoyl glycerol and the equimolar mixture. Additionally, all three drugs change the morphology of the domains observed. Based on these results the stabilization of diacylgylcerol by edelfosine and miltefosine may contribute to the mode of action as diacylglycerol is a known disruptor of bilayers. Perifosine however does not stabilize diacylglycerol, and therefore cell death may occur through a more direct inhibition of specific signal transduction. These results suggest that perifosine may illicit cytotoxicity through a different mechanism compared to the other antitumor lipid drugs.

Lipids and Membrane Microdomains: The Glycerolipid and Alkylphosphocholine Class of Cancer Chemotherapeutic Drugs

Synthetic antitumor lipids are metabolically stable lysophosphatidylcholine derivatives, encompassing a class of non-mutagenic drugs that selectively target cancerous cells. In this chapter we review the literature as relates to the clinical efficacy of these antitumor lipid drugs and how our understanding of their mode of action has evolved alongside key advances in our knowledge of membrane structure, organization, and function. First, the history of the development of this class of drugs is described, providing a summary of clinical outcomes of key members including edelfosine, miltefosine, perifosine, erufosine, and erucylphosphocholine. A detailed description of the biophysical properties of these drugs and specific drug-lipid interactions which may contribute to the selectivity of the antitumor lipids for cancer cells follows. An updated model on the mode of action of these lipid drugs as membrane disorganizing agents is presented. Membrane domain organization as opposed to targeting specific proteins on membranes is discussed. By altering membranes, these antitumor lipids inhibit many survival pathways while activating pro-apoptotic signals leading to cell demise.

FRET Detects the Size of Nanodomains for Coexisting Liquid-Disordered and Liquid-Ordered Phases

Biomembranes with as few as three lipid components can form coexisting liquid-disordered (Ld) and liquid-ordered (Lo) phases. In the coexistence region of Ld and Lo phases, the lipid mixtures 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC)/1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)/chol or brain sphingomyelin (bSM)/DOPC/chol form micron-scale domains that are easily visualized with light microscopy. Although large domains are not observed in the mixtures DSPC/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)/chol and bSM/POPC/chol, lateral heterogeneity is nevertheless detected using techniques with nanometer-scale spatial resolution. We propose a simple and accessible method to measure domain sizes below optical resolution (∼200 nm). We measured nanodomain size for the latter two mixtures by combining experimental Förster resonance energy transfer data with a Monte-Carlo-based analysis. We found a domain radius of 7.5-10 nm for DSPC/POPC/chol, similar to values obtained previously by neutron scattering, and ∼5 nm for bSM/POPC/chol, slightly smaller than measurable by neutron scattering. These analyses also detect the domain-size transition that is observed by fluorescence microscopy in the four-component lipid mixture bSM/DOPC/POPC/chol. Accurate measurements of fluorescent-probe partition coefficients are especially important for the analysis; therefore, we exploit three different methods to measure the partition coefficient of fluorescent molecules between Ld and Lo phases.

Folding and Misfolding of Human Membrane Proteins in Health and Disease: From Single Molecules to Cellular Proteostasis

Advances over the past 25 years have revealed much about how the structural properties of membranes and associated proteins are linked to the thermodynamics and kinetics of membrane protein (MP) folding. At the same time biochemical progress has outlined how cellular proteostasis networks mediate MP folding and manage misfolding in the cell. When combined with results from genomic sequencing, these studies have established paradigms for how MP folding and misfolding are linked to the molecular etiologies of a variety of diseases. This emerging framework has paved the way for the development of a new class of small molecule "pharmacological chaperones" that bind to and stabilize misfolded MP variants, some of which are now in clinical use. In this review, we comprehensively outline current perspectives on the folding and misfolding of integral MPs as well as the mechanisms of cellular MP quality control. Based on these perspectives, we highlight new opportunities for innovations that bridge our molecular understanding of the energetics of MP folding with the nuanced complexity of biological systems. Given the many linkages between MP misfolding and human disease, we also examine some of the exciting opportunities to leverage these advances to address emerging challenges in the development of therapeutics and precision medicine.

Role of Sphingomyelin in Alphaherpesvirus Entry

Bovine herpesvirus 1 (BoHV-1) is an alphaherpesvirus that causes disease in cattle populations worldwide. Sphingomyelin (SM) is the most abundant sphingolipid in the mammalian cell membrane, where it preferentially associates with cholesterol to form lipid raft domains. SM is a substrate for the lysosome-resident enzyme acid sphingomyelinase, which plays a role in cell membrane repair following injury. Treatment of cells with noncytotoxic concentrations of Staphylococcus aureus-derived sphingomyelinase successfully reduced cell surface-exposed sphingomyelin but did not significantly inhibit BoHV-1 entry and infection, as measured by the beta-galactosidase reporter assay. Interestingly, entry of the porcine alphaherpesvirus pseudorabies virus (PRV) was inhibited by sphingomyelin-depletion of cells. Treatment of BoHV-1 particles with sphingomyelinase inhibited viral entry activity, suggesting that viral SM plays a role in BoHV-1 entry, while cellular SM does not. Treatment of cells with noncytotoxic concentrations of the functional inhibitors of host acid sphingomyelinase, imipramine and amitriptyline, which induce degradation of the cellular enzyme, did not significantly inhibit BoHV-1 entry. In contrast, inhibition of cellular acid sphingomyelinase inhibited PRV entry. Entry of the human alphaherpesvirus herpes simplex virus 1 (HSV-1) was independent of both host SM and acid sphingomyelinase, in a manner similar to BoHV-1. Together, the results suggest that among the alphaherpesviruses, there is variability in entry requirements for cellular sphingomyelin and acid sphingomyelinase activity.IMPORTANCE Bovine herpesvirus 1 (BoHV-1) is an ubiquitous pathogen affecting cattle populations worldwide. Infection can result in complicated, polymicrobial infections due to the immunosuppressive properties of the virus. Available vaccines limit disease severity and spread but do not prevent infection. The financial and animal welfare ramifications of BoHV-1 are significant. In order to develop more effective prevention and treatment regimens, a more complete understanding of the initial steps in viral infection is necessary. We recently identified a low pH endocytosis pathway for BoHV-1. Here, we examine the role of cellular factors responsible for membrane integrity and repair in alphaherpesviral entry. This study allows comparisons of the BoHV-1 entry pathway with those of other alphaherpesviruses (pseudorabies virus [PRV] and herpes simplex virus 1 [HSV-1]). Lastly, this is the first report of sphingomyelin and lysosomal sphingomyelinase playing a role in the entry of a herpesvirus. The results may lead to the development of more effective prevention and treatment regimens.

Interaction of two antitumor peptides with membrane lipids - Influence of phosphatidylserine and cholesterol on specificity for melanoma cells

R-DIM-P-LF11-322 and DIM-LF11-318, derived from the cationic human host defense peptide lactoferricin show antitumor activity against human melanoma. While R-DIM-P-LF11-322 interacts specifically with cancer cells, the non-specific DIM-LF11-318 exhibits as well activity against non-neoplastic cells. Recently we have shown that cancer cells expose the negatively charged lipid phosphatidylserine (PS) in the outer leaflet of the plasma membrane, while non-cancer cells just expose zwitterionic or neutral lipids, such as phosphatidylcholine (PC) or cholesterol. Calorimetric and zeta potential studies with R-DIM-P-LF11-322 and cancer-mimetic liposomes composed of PS, PC and cholesterol indicate that the cancer-specific peptide interacts specifically with PS. Cholesterol, however, reduces the effectiveness of the peptide. The non-specific DIM-LF11-318 interacts with PC and PS. Cholesterol does not affect its interaction. The dependence of activity of R-DIM-P-LF11-322 on the presence of exposed PS was also confirmed in vitro upon PS depletion of the outer leaflet of cancer cells by the enzyme PS-decarboxylase. Further corresponding to model studies, cholesterol depleted melanoma plasma membranes showed increased sensitivity to R-DIM-P-LF11-322, whereas activity of DIM-LF11-318 was unaffected. Microscopic studies using giant unilamellar vesicles and melanoma cells revealed strong changes in lateral distribution and domain formation of lipids upon addition of both peptides. Whereas R-DIM-P-LF11-322 enters the cancer cell specifically via PS and reaches an intracellular organelle, the Golgi, inducing mitochondrial swelling and apoptosis, DIM-LF11-318 kills rapidly and non-specifically by lysis of the plasma membrane. In conclusion, the specific interaction of R-DIM-P-LF11-322 with PS and sensitivity to cholesterol seem to modulate its specificity for cancer membranes.

Platelet membrane lipid rafts protein composition varies following GPVI and CLEC-2 receptors activation

Lipid rafts are membrane microdomains that have been proposed to play an important role in several platelet-signalling cascades, including those mediated by the receptors Glycoprotein VI (GPVI), and C-type lectin domain family 1 member B (CLEC-2), both involved in thrombus formation. We have performed a LC-MS/MS proteomic analysis of lipid rafts isolated from platelets activated through GPVI and CLEC-2 as well as from resting platelets. Our aim was to determine the magnitude of changes in lipid rafts protein composition and to elucidate the relevance of these alterations in platelet function. A number of relevant signalling proteins were found enriched in lipid rafts following platelet activation (such as the tyrosine protein kinases Fyn, Lyn and Yes; the G proteins G(i) and G(z); and cAMP protein kinase). Interestingly, our results indicate that the relative enrichment of lipid rafts in these signalling proteins may not be a consequence of protein translocation to these domains upon platelet stimulation, but the result of a massive loss in cytoskeletal proteins after platelet activation. Thus, this study may help to better understand the effects of platelet activation in the reorganization of lipid rafts and set the basis for further proteomic studies of these membrane microdomains in platelets. SIGNIFICANCE: We performed the first proteomic comparative analysis of lipid rafts- protein composition in platelets activated through GPVI and CLEC-2 receptors and in resting state. We identified a number of signalling proteins essential for platelet activation relatively enriched in platelets activated through both receptors, and we show that lipid rafts reorganization upon platelet activation leads to a loss in cytoskeletal proteins, highly associated to these domains in resting platelets.

DNA Oligonucleotide-Functionalized Liposomes: Bioconjugate Chemistry, Biointerfaces, and Applications

Interfacing DNA with liposomes has produced a diverse range of programmable soft materials, devices, and drug delivery vehicles. By simply controlling liposomal composition, bilayer fluidity, lipid domain formation, and surface charge can be systematically varied. Recent development in DNA research has produced not only sophisticated nanostructures but also new functions including ligand binding and catalysis. For noncationic liposomes, a DNA is typically covalently linked to a hydrophobic or lipid moiety that can be inserted into lipid membranes. In this article, we discuss fundamental biointerfaces formed between DNA and noncationic liposomes. The methods to prepare such conjugates and the interactions at the membrane interfaces are also discussed. The effect of DNA lateral diffusion on fluid bilayer membranes and the effect of membrane on DNA assembly are emphasized. DNA hybridization can be programmed to promote fusion of lipid membranes. Representative applications of this conjugate for drug delivery, biosensor development, and directed assembly of materials are briefly described toward the end. Some future research directions are also proposed to further understand this biointerface.

The crosstalk between glycosphingolipids and neural stem cells

Until a few years ago, the majority of cell functions were envisioned as the result of protein and DNA activity. The cell membranes were considered as a mere structure of support and/or separation. In the last years, the function of cell membranes has, however, received more attention and their components of lipid nature have also been depicted as important cell mediators and the membrane organization was described as an important determinant for membrane-anchored proteins activity. In particular, because of their high diversity, glycosphingolipids offer a wide possibility of regulation. Specifically, the role of glycosphingolipids, in the fine-tuning of neuron activity, has recently received deep attention. For their pivotal role in vertebrate and mammals neural development, neural stem cells regulation is of main interest especially concerning their further functions in neurological pathology progression and treatment. Glycosphingolipids expression present a developmental regulation. In this view, glycosphingolipids can hold an important role in neural stem cells features because of their heterogeneity and their consequent capacity for eclectic interaction with other cell components.

Plant lipids: Key players of plasma membrane organization and function

The plasma membrane (PM) is the biological membrane that separates the interior of all cells from the outside. The PM is constituted of a huge diversity of proteins and lipids. In this review, we will update the diversity of molecular species of lipids found in plant PM. We will further discuss how lipids govern global properties of the plant PM, explaining that plant lipids are unevenly distributed and are able to organize PM in domains. From that observation, it emerges a complex picture showing a spatial and multiscale segregation of PM components. Finally, we will discuss how lipids are key players in the function of PM in plants, with a particular focus on plant-microbe interaction, transport and hormone signaling, abiotic stress responses, plasmodesmata function. The last chapter is dedicated to the methods that the plant membrane biology community needs to develop to get a comprehensive understanding of membrane organization in plants.

Complex formation of sphingomyelin synthase 1 with glucosylceramide synthase increases sphingomyelin and decreases glucosylceramide levels

Sphingolipids, including sphingomyelin (SM) and glucosylceramide (GlcCer), are generated by the addition of a polar head group to ceramide (Cer). Sphingomyelin synthase 1 (SMS1) and glucosylceramide synthase (GCS) are key enzymes that catalyze the conversion of Cer to SM and GlcCer, respectively. GlcCer synthesis has been postulated to occur mainly in cis-Golgi, and SM synthesis is thought to occur in medial/trans-Golgi; however, SMS1 and GCS are known to partially co-localize in cisternae, especially in medial/trans-Golgi. Here, we report that SMS1 and GCS can form a heteromeric complex, in which the N terminus of SMS1 and the C terminus of GCS are in close proximity. Deletion of the N-terminal sterile α-motif of SMS1 reduced the stability of the SMS1-GCS complex, resulting in a significant reduction in SM synthesis in vivo In contrast, chemical-induced heterodimerization augmented SMS1 activity, depending on an increase in the amount and stability of the complex. Fusion of the SMS1 N terminus to the GCS C terminus via linkers of different lengths increased SM synthesis and decreased GlcCer synthesis in vivo These results suggest that formation of the SMS1-GCS heteromeric complex increases SM synthesis and decreases GlcCer synthesis. Importantly, this regulation of relative Cer levels by the SMS1-GCS complex was confirmed by CRISPR/Cas9-mediated knockout of SMS1 or GCS combined with pharmacological inhibition of Cer transport protein in HEK293T cells. Our findings suggest that complex formation between SMS1 and GCS is part of a critical mechanism controlling the metabolic fate of Cer in the Golgi.

Small-Molecule Modulation of Lipid-Dependent Cellular Processes against Cancer: Fats on the Gunpoint

Lipid cell membrane composed of various distinct lipids and proteins act as a platform to assemble various signaling complexes regulating innumerous cellular processes which are strongly downregulated or altered in cancer cells emphasizing the still-underestimated critical function of lipid biomolecules in cancer initiation and progression. In this review, we outline the current understanding of how membrane lipids act as signaling hot spots by generating distinct membrane microdomains called rafts to initiate various cellular processes and their modulation in cancer phenotypes. We elucidate tangible drug targets and pathways all amenable to small-molecule perturbation. Ranging from targeting membrane rafts organization/reorganization to rewiring lipid metabolism and lipid sorting in cancer, the work summarized here represents critical intervention points being attempted for lipid-based anticancer therapy and future directions.

Unique Lipid Signatures of Extracellular Vesicles from the Airways of Asthmatics

Asthma is a chronic inflammatory disease process involving the conductive airways of the human lung. The dysregulated inflammatory response in this disease process may involve multiple cell-cell interactions mediated by signaling molecules, including lipid mediators. Extracellular vesicles (EVs) are lipid membrane particles that are now recognized as critical mediators of cell-cell communication. Here, we compared the lipid composition and presence of specific lipid mediators in airway EVs purified from the bronchoalveolar lavage (BAL) fluid of healthy controls and asthmatic subjects with and without second-hand smoke (SHS) exposure. Airway exosome concentrations were increased in asthmatics, and correlated with blood eosinophilia and serum IgE levels. Frequencies of HLA-DR⁺ and CD54⁺ exosomes were also significantly higher in asthmatics. Lipidomics analysis revealed that phosphatidylglycerol, ceramide-phosphates, and ceramides were significantly reduced in exosomes from asthmatics compared to the non-exposed control groups. Sphingomyelin 34:1 was more abundant in exosomes of SHS-exposed asthmatics compared to healthy controls. Our results suggest that chronic airway inflammation may be driven by alterations in the composition of lipid mediators within airway EVs of human subjects with asthma.

Role of sphingolipids in senescence: implication in aging and age-related diseases

Aging is defined as the progressive deterioration of physiological function with age. Incidence of many pathologies increases with age, including neurological and cardiovascular diseases and cancer. Aging tissues become less adaptable and renewable, and cells undergo senescence, a process by which they "irreversibly" stop dividing. Senescence has been shown to serve as a tumor suppression mechanism with clear desirable effects. However, senescence also has deleterious consequences, especially for cardiovascular, metabolic, and immune systems. Sphingolipids are a major class of lipids that regulate cell biology, owing to their structural and bioactive properties and diversity. Their involvement in the regulation of aging and senescence has been demonstrated and studied in multiple organisms and cell types, especially that of ceramide and sphingosine-1-phosphate; ceramide induces cellular senescence and sphingosine-1-phosphate delays it. These discoveries could be very useful in the future to understand aging mechanisms and improve therapeutic interventions.

Effects of phosphatidylcholine membrane fluidity on the conformation and aggregation of N-terminally acetylated α-synuclein

Membrane association of α-synuclein (α-syn), a neuronal protein associated with Parkinson's disease (PD), is involved in α-syn function and pathology. Most previous studies on α-syn-membrane interactions have not used the physiologically relevant N-terminally acetylated (N-acetyl) α-syn form nor the most naturally abundant cellular lipid, i.e. phosphatidylcholine (PC). Here, we report on how PC membrane fluidity affects the conformation and aggregation propensity of N-acetyl α-syn. It is well established that upon membrane binding, α-syn adopts an α-helical structure. Using CD spectroscopy, we show that N-acetyl α-syn transitions from α-helical to disordered at the lipid melting temperature (T_m ). We found that this fluidity sensing is a robust characteristic, unaffected by acyl chain length (T_m = 34-55 °C) and preserved in its homologs β- and γ-syn. Interestingly, both N-acetyl α-syn membrane binding and amyloid formation trended with lipid order (1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) > 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)/sphingomyelin/cholesterol (2:2:1) ≥ DOPC), with gel-phase vesicles shortening aggregation kinetics and promoting fibril formation compared to fluid membranes. Furthermore, we found that acetylation enhances binding to PC micelles and small unilamellar vesicles with high curvature (r ∼16-20 nm) and that DPPC binding is reduced in the presence of cholesterol. These results confirmed that the exposure of hydrocarbon chains (i.e. packing defects) is essential for binding to zwitterionic gel membranes. Collectively, our in vitro results suggest that N-acetyl α-syn localizes to highly curved, ordered membranes inside a cell. We propose that age-related changes in membrane fluidity can promote the formation of amyloid fibrils, insoluble materials associated with PD.

Gestational diabetes mellitus modulates cholesterol homeostasis in human fetoplacental endothelium

Gestational diabetes mellitus (GDM) is associated with excessive oxidative stress which may affect placental vascular function. Cholesterol homeostasis is crucial for maintaining fetoplacental endothelial function. We aimed to investigate whether and how GDM affects cholesterol metabolism in human fetoplacental endothelial cells (HPEC). HPEC were isolated from fetal term placental arterial vessels of GDM or control subjects. Cellular reactive oxygen species (ROS) were detected by H₂DCFDA fluorescent dye. Oxysterols were quantified by gas chromatography-mass spectrometry analysis. Genes and proteins involved in cholesterol homeostasis were detected by real-time PCR and immunoblotting, respectively. Cholesterol efflux was determined from [³H]-cholesterol labeled HPEC and [¹⁴C]-acetate was used as cholesterol precursor to measure cholesterol biosynthesis and esterification. We detected enhanced formation of ROS and of specific, ROS-derived oxysterols in HPEC isolated from GDM versus control pregnancies. ROS-generated oxysterols were simultaneously elevated in cord blood of GDM neonates. Liver-X receptor activation in control HPEC by synthetic agonist TO901319, 7-ketocholesterol, or 7β-hydroxycholesterol upregulated ATP-binding cassette transporters (ABC)A1 and ABCG1 expression, accompanied by increased cellular cholesterol efflux. Upregulation of ABCA1 and ABCG1 and increased cholesterol release to apoA-I and HDL₃ (78 ± 17%, 40 ± 9%, respectively) were also observed in GDM versus control HPEC. The LXR antagonist GGPP reversed ABCA1 and ABCG1 upregulation and reduced the increased cholesterol efflux in GDM HPEC. Similar total cellular cholesterol levels were detected in control and GDM HPEC, while GDM enhanced cholesterol biosynthesis along with upregulated 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR) and sterol O-acyltransferase 1 (SOAT1) mRNA and protein levels. Our results suggest that in GDM cellular cholesterol homeostasis in the fetoplacental endothelium is modulated via LXR activation and helps to maintain its proper functionality.

Cholesterol-rich lipid rafts play a critical role in bovine parainfluenza virus type 3 (BPIV3) infection

Lipid rafts are specialized lipid domains enriched in cholesterol and sphingolipid, which can be utilized in the lifecycle of numerous enveloped viruses. Bovine parainfluenza virustype3 (BPIV3) entry to cell is mediated by receptor binding and membrane fusion, but how lipid rafts in host cell membrane and BPIV3 envelope affect virus infection remains unclear. In this study, we investigated the role of lipid rafts in the different stages of BPIV3 infection. The MDBK cells were treated by methyl-β-cyclodextrin (MβCD) to disrupt cellular lipid raft, and the virus infection was determined. The results showed that MβCD significantly inhibited BPIV3 infection in a dose-dependent manner, but didn't block the binding of virus to the cell membrane. Whereas, the MDBK cells treated by MβCD after virus-entry had no effects on the virus infection, to suggest that BPIV3 infection was associated with lipid rafts in cell membrane during viral entry stage. To further confirm lipid rafts in viral envelope also affected BPIV3 infection, we treated BPIV3 with MβCD to determine the virus titer. We found that disruption of the viral lipid raft caused a significant reduction of viral yield. Cholesterol reconstitution experiment showed that BPIV3 infection was successfully restored by cholesterol supplementation both in cellular membrane and viral envelope, which demonstrated that cholesterol-rich lipid rafts played a critical role in BPIV3 infection. These findings provide insights on our understanding of the mechanism of BPIV3 infection and imply that lipid raft might be a good potential therapeutic target to prevent virus infection.

Effect of cholesterol on the molecular structure and transitions in a clinical-grade lung surfactant extract

The lipid-protein film covering the interface of the lung alveolar in mammals is vital for proper lung function and its deficiency is related to a range of diseases. Here we present a molecular-level characterization of a clinical-grade porcine lung surfactant extract using a multitechnique approach consisting of [Formula: see text]-[Formula: see text] solid-state nuclear magnetic spectroscopy, small- and wide-angle X-ray scattering, and mass spectrometry. The detailed characterization presented for reconstituted membranes of a lung extract demonstrates that the molecular structure of lung surfactant strongly depends on the concentration of cholesterol. If cholesterol makes up about 11% of the total dry weight of lung surfactant, the surfactant extract adopts a single liquid-ordered lamellar phase, [Formula: see text], at physiological temperatures. This [Formula: see text] phase gradually changes into a liquid-disordered lamellar phase, [Formula: see text], when the temperature is increased by a few degrees. In the absence of cholesterol the system segregates into one lamellar gel phase and one [Formula: see text] phase. Remarkably, it was possible to measure a large set of order parameter magnitudes [Formula: see text] from the liquid-disordered and -ordered lamellar phases and assign them to specific C-H bonds of the phospholipids in the biological extract with no use of isotopic labeling. These findings with molecular details on lung surfactant mixtures together with the presented NMR methodology may guide further development of pulmonary surfactant pharmaceuticals that better mimic the physiological self-assembly compositions for treatment of pathological states such as respiratory distress syndrome.

Sphingolipid Organization in the Plasma Membrane and the Mechanisms That Influence It

Sphingolipids are structural components in the plasma membranes of eukaryotic cells. Their metabolism produces bioactive signaling molecules that modulate fundamental cellular processes. The segregation of sphingolipids into distinct membrane domains is likely essential for cellular function. This review presents the early studies of sphingolipid distribution in the plasma membranes of mammalian cells that shaped the most popular current model of plasma membrane organization. The results of traditional imaging studies of sphingolipid distribution in stimulated and resting cells are described. These data are compared with recent results obtained with advanced imaging techniques, including super-resolution fluorescence detection and high-resolution secondary ion mass spectrometry (SIMS). Emphasis is placed on the new insight into the sphingolipid organization within the plasma membrane that has resulted from the direct imaging of stable isotope-labeled lipids in actual cell membranes with high-resolution SIMS. Super-resolution fluorescence techniques have recently revealed the biophysical behaviors of sphingolipids and the unhindered diffusion of cholesterol analogs in the membranes of living cells are ultimately in contrast to the prevailing hypothetical model of plasma membrane organization. High-resolution SIMS studies also conflicted with the prevailing hypothesis, showing sphingolipids are concentrated in micrometer-scale membrane domains, but cholesterol is evenly distributed within the plasma membrane. Reductions in cellular cholesterol decreased the number of sphingolipid domains in the plasma membrane, whereas disruption of the cytoskeleton eliminated them. In addition, hemagglutinin, a transmembrane protein that is thought to be a putative raft marker, did not cluster within sphingolipid-enriched regions in the plasma membrane. Thus, sphingolipid distribution in the plasma membrane is dependent on the cytoskeleton, but not on favorable interactions with cholesterol or hemagglutinin. The alternate views of plasma membrane organization suggested by these findings are discussed.

Mapping out the intricate relationship of the HIV envelope protein and the membrane environment

The HIV gp160 envelope fusion protein is situated in the viral membrane and mediates virus entry into its host cell. Increasing evidence suggests that virtually all parts of the HIV envelope are structurally and functionally dependent on membranes. Protein-lipid interactions and membrane properties influence the dynamics of a manifold of gp160 biological activities such as membrane fusion, immune suppression and gp160 incorporation into virions during HIV budding and assembly. In the following we will summarize our current understanding of this interdependence between membrane interaction, structural conformation and functionality of the different gp160 domains. This article is part of a Special Issue entitled: Lipid order/lipid defects and lipid-control of protein activity edited by Dirk Schneider.

Monosialoganglioside-GM1 triggers binding of the amyloid-protein salmon calcitonin to a Langmuir membrane model mimicking the occurrence of lipid-rafts

GM1 ganglioside is known to be involved in the amyloid-associated diseases and it is a crucial factor for the assembly of amyloid proteins on lipid-rafts, which are lipid structures located on the synaptic plasma membranes. Due to its slow aggregation rate, we employed salmon calcitonin (sCT) as a suitable probe representative of amyloid proteins, to study the interaction between this class of proteins and a membrane model. Here, we prepared a neuronal membrane model by depositing onto mica two Langmuir-Blodgett films in liquid-condensed phase: the outer monolayer was characterized by high content of GM1 (50%) and minority parts of cholesterol and POPC (25-25%), while the inner one by plain POPC. To deeply investigate the interaction of sCT with this model and the role-played by GM1, we prepared the outer leaflet adding sCT at a concentration such that the number of proteins equals that of GM1. Atomic Force Microscopy revealed the occurrence of two distinct kinds of flat surfaces, with globular aggregates localized exclusively on top of the highest one. To unravel the nature of the interaction, we studied by ζ-potential technique liposomes composed as the outer leaflet of the model. Results demonstrated that an electrostatic interaction sCT-GM1 occurred. Finally, to investigate the interaction thermodynamics between sCT and the outer leaflet, Langmuir films as the outer monolayer and containing increasing content of sCT were studied by compression isotherms and Brewster Angle Microscopy experiments. Based on the all body of results we propose an interaction model where GM1 plays a pivotal role.

Automatic detection of diffusion modes within biological membranes using back-propagation neural network

Background

Single particle tracking (SPT) is nowadays one of the most popular technique to probe spatio-temporal dynamics of proteins diffusing within the plasma membrane. Indeed membrane components of eukaryotic cells are very dynamic molecules and can diffuse according to different motion modes. Trajectories are often reconstructed frame-by-frame and dynamic properties often evaluated using mean square displacement (MSD) analysis. However, to get statistically significant results in tracking experiments, analysis of a large number of trajectories is required and new methods facilitating this analysis are still needed.

Results

In this study we developed a new algorithm based on back-propagation neural network (BPNN) and MSD analysis using a sliding window. The neural network was trained and cross validated with short synthetic trajectories. For simulated and experimental data, the algorithm was shown to accurately discriminate between Brownian, confined and directed diffusion modes within one trajectory, the 3 main of diffusion encountered for proteins diffusing within biological membranes. It does not require a minimum number of observed particle displacements within the trajectory to infer the presence of multiple motion states. The size of the sliding window was small enough to measure local behavior and to detect switches between different diffusion modes for segments as short as 20 frames. It also provides quantitative information from each segment of these trajectories. Besides its ability to detect switches between 3 modes of diffusion, this algorithm is able to analyze simultaneously hundreds of trajectories with a short computational time.

Conclusion

This new algorithm, implemented in powerful and handy software, provides a new conceptual and versatile tool, to accurately analyze the dynamic behavior of membrane components.

Electronic supplementary material

The online version of this article (doi:10.1186/s12859-016-1064-z) contains supplementary material, which is available to authorized users.

Accurate quantification of inter-domain partition coefficients in GUVs exhibiting lipid phase coexistence

Giant unilamellar vesicles (GUVs) with phase coexistence allow for the recovery of inter-domain partition coefficients (K_p) of fluorescent molecules through comparison of fluorescence intensities in each phase.

Proteomic Analysis of a Poplar Cell Suspension Culture Suggests a Major Role of Protein S-Acylation in Diverse Cellular Processes

S-acylation is a reversible post-translational modification of proteins known to be involved in membrane targeting, subcellular trafficking, and the determination of a great variety of functional properties of proteins. The aim of this work was to identify S-acylated proteins in poplar. The use of an acyl-biotin exchange method and mass spectrometry allowed the identification of around 450 S-acylated proteins, which were subdivided into three major groups of proteins involved in transport, signal transduction, and response to stress, respectively. The largest group of S-acylated proteins was the protein kinase superfamily. Soluble N-ethylmaleimide-sensitive factor-activating protein receptors, band 7 family proteins and tetraspanins, all primarily related to intracellular trafficking, were also identified. In addition, cell wall related proteins, including cellulose synthases and other glucan synthases, were found to be S-acylated. Twenty four of the identified S-acylated proteins were also enriched in detergent-resistant membrane microdomains, suggesting S-acylation plays a key role in the localization of proteins to specialized plasma membrane subdomains. This dataset promises to enhance our current understanding of the various functions of S-acylated proteins in plants.

Taming the sphinx: Mechanisms of cellular sphingolipid homeostasis

Sphingolipids are important structural membrane components of eukaryotic cells, and potent signaling molecules. As such, their levels must be maintained to optimize cellular functions in different cellular membranes. Here, we review the current knowledge of homeostatic sphingolipid regulation. We describe recent studies in Saccharomyces cerevisiae that have provided insights into how cells sense changes in sphingolipid levels in the plasma membrane and acutely regulate sphingolipid biosynthesis by altering signaling pathways. We also discuss how cellular trafficking has emerged as an important determinant of sphingolipid homeostasis. Finally, we highlight areas where work is still needed to elucidate the mechanisms of sphingolipid regulation and the physiological functions of such regulatory networks, especially in mammalian cells. This article is part of a Special Issue entitled: The cellular lipid landscape edited by Tim P. Levine and Anant K. Menon.

Cholesterol and Sphingomyelin-Containing Model Condensed Lipid Monolayers: Heterogeneities Involving Ordered Microdomains Assessed by Two Cholesterol Derivatives

Lipid monolayers are often considered as model membranes, but they are also the physiologic lipid part of the peripheral envelope of lipoproteins and cytosolic lipid bodies. However, their structural organization is still rather elusive, in particular when both cholesterol and sphingomyelin are present. To investigate such structural organization of hemimembranes, we measured, using alternative current voltammetry, the differential capacitance of condensed phosphatidylcholine-based monolayers as a function of applied potential, which is sensitive to their lipid composition and molecular arrangement. Especially, monolayers containing both sphingomyelin and cholesterol, at 15% w/w, presented specific characteristics of the differential capacitance versus potential curves recorded, which was indicative of specific interactions between these two lipid components. We then compared the behavior of two cholesterol derivatives (at 15% w/w), 21-methylpyrenyl-cholesterol (Pyr-met-Chol) and 22-nitrobenzoxadiazole-cholesterol (NBD-Chol), with that of cholesterol when present in model monolayers. Indeed, these two probes were chosen because of previous findings reporting opposite behaviors within bilayer membranes regarding their interaction with ordered lipids, with only Pyr-met-Chol mimicking cholesterol well. Remarkably, in monolayers containing sphingomyelin or not, Pyr-met-Chol and NBD-Chol presented contrasting behaviors, and Pyr-met-Chol mimicked cholesterol only in the presence of sphingomyelin. These two observations (i.e., optimal amounts of sphingomyelin and cholesterol, and the ability to discriminate between Pyr-met-Chol and NBD-Chol) can be interpreted by the existence of heterogeneities including ordered patches in sphingomyelin- and cholesterol-containing monolayers. Since such monolayer lipid arrangement shares some properties with the raft-type lipid microdomains well-described in sphingomyelin- and cholesterol-containing bilayer membranes, our data thus strongly suggest the existence of compact and ordered microdomains in model lipid monolayers.

Functions of cholera toxin B-subunit as a raft cross-linker

Lipid rafts are putative complexes of lipids and proteins in cellular membranes that are proposed to function in trafficking and signalling events. CTxB (cholera toxin B-subunit) has emerged as one of the most studied examples of a raft-associated protein. Consisting of the membrane-binding domain of cholera toxin, CTxB binds up to five copies of its lipid receptor on the plasma membrane of the host cell. This multivalency of binding gives the toxin the ability to reorganize underlying membrane structure by cross-linking otherwise small and transient lipid rafts. CTxB thus serves as a useful model for understanding the properties and functions of protein-stabilized domains. In the present chapter, we summarize current evidence that CTxB associates with and cross-links lipid rafts, discuss how CTxB binding modulates the architecture and dynamics of membrane domains, and describe the functional consequences of this cross-linking behaviour on toxin uptake into cells via endocytosis.

Disruption of lipid domain organization in monolayers of complex yeast lipid extracts induced by the lysophosphatidylcholine analogue edelfosine in vivo

The lysophosphatidylcholine analogue edelfosine is a potent antitumor and antiparasitic drug that targets cell membranes. Previous studies have shown that edelfosine alters membrane domain organization inducing internalization of sterols and endocytosis of plasma membrane transporters. These early events affect signaling pathways that result in cell death. It has been shown that edelfosine preferentially partitions into more rigid lipid domains in mammalian as well as in yeast cells. In this work we aimed at investigating the effect of edelfosine on membrane domain organization using monolayers prepared from whole cell lipid extracts of cells treated with edelfosine compared to control conditions. In Langmuir monolayers we were able to detect important differences to the lipid packing of the membrane monofilm. Domain formation visualized by means of Brewster angle microscopy also showed major morphological changes between edelfosine treated versus control samples. Importantly, edelfosine resistant cells defective in drug uptake did not display the same differences. In addition, co-spread samples of control lipid extracts with edelfosine added post extraction did not fully mimic the results obtained with lipid extracts from treated cells. Altogether these results indicate that edelfosine induces changes in membrane domain organization and that these changes depend on drug uptake. Our work also validates the use of monolayers derived from complex cell lipid extracts combined with Brewster angle microscopy, as a sensitive approach to distinguish between conditions associated with susceptibility or resistance to lysophosphatidylcholine analogues.

Ultrastructure and lipid composition of detergent-resistant membranes derived from mammalian sperm and two types of epithelial cells

Lipid rafts are micro-domains of ordered lipids (L_o phase) in biological membranes. The L_o phase of cellular membranes can be isolated from disordered lipids (L_d phase) after treatment with 1 % Triton  X-100 at 4 °C in which the L_o phase forms the detergent-resistant membrane (DRM) fraction. The lipid composition of DRM derived from Madin-Darby canine kidney (MDCK) cells, McArdle cells and porcine sperm is compared with that of the whole cell. Remarkably, the unsaturation and chain length degree of aliphatic chains attached to phospholipids is virtually the same between DRM and whole cells. Cholesterol and sphingomyelin were enriched in DRMs but to a cell-specific molar ratio. Sulfatides (sphingolipids from MDCK cells) were enriched in the DRM while a seminolipid (an alkylacylglycerolipid from sperm) was depleted from the DRM. Treatment with <5 mM methyl-ß-cyclodextrin (MBCD) caused cholesterol removal from the DRM without affecting the composition and amount of the phospholipid while higher levels disrupted the DRM. The substantial amount of (poly)unsaturated phospholipids in DRMs as well as a low stoichiometric amount of cholesterol suggest that lipid rafts in biological membranes are more fluid and dynamic than previously anticipated. Using negative staining, ultrastructural features of DRM were monitored and in all three cell types the DRMs appeared as multi-lamellar vesicular structures with a similar morphology. The detergent resistance is a result of protein–cholesterol and sphingolipid interactions allowing a relatively passive attraction of phospholipids to maintain the L_o phase. For this special issue, the relevance of our findings is discussed in a sperm physiological context.

Interfacing living cells and spherically supported bilayer lipid membranes

Spherically supported bilayer lipid membranes (SS-BLMs) exhibiting co-existing membrane microdomains were created on spherical silica substrates. These 5 μm SiO2-core SS-BLMs are shown to interact dynamically when interfaced with living cells in culture, while keeping the membrane structure and lipid domains on the SS-BLM surface intact. Interactions between the SS-BLMs and cellular components are examined via correlating fluorescently labeled co-existing microdomains on the SS-BLMs, their chemical composition and biophysical properties with the consequent organization of cell membrane lipids, proteins, and other cellular components. This approach is demonstrated in a proof-of-concept experiment involving the dynamic organization of cellular cytoskeleton, monitored as a function of the lipid domains of the SS-BLMs. The compositional versatility of SS-BLMs provides a means to address the relationship between the phenomenon of lipid phase separation and the other contributors to cell membrane lateral heterogeneity.