Cholesterol modulates glycolipid conformation and receptor activity

Sialyllactose in viral membrane gangliosides is a novel molecular recognition pattern for mature dendritic cell capture of HIV-1

HIV-1 is internalized into mature dendritic cells (mDCs) via an as yet undefined mechanism with subsequent transfer of stored, infectious virus to CD4+ T lymphocytes. Thus, HIV-1 subverts a DC antigen capture mechanism to promote viral spread. Here, we show that gangliosides in the HIV-1 membrane are the key molecules for mDC uptake. HIV-1 virus-like particles and liposomes mimicking the HIV-1 lipid composition were shown to use a common internalization pathway and the same trafficking route within mDCs. Hence, these results demonstrate that gangliosides can act as viral attachment factors, in addition to their well known function as cellular receptors for certain viruses. Furthermore, the sialyllactose molecule present in specific gangliosides was identified as the determinant moiety for mDC HIV-1 uptake. Thus, sialyllactose represents a novel molecular recognition pattern for mDC capture, and may be crucial both for antigen presentation leading to immunity against pathogens and for succumbing to subversion by HIV-1.

Mechanisms underlying the confined diffusion of cholera toxin B-subunit in intact cell membranes

Multivalent glycolipid binding toxins such as cholera toxin have the capacity to cluster glycolipids, a process thought to be important for their functional uptake into cells. In contrast to the highly dynamic properties of lipid probes and many lipid-anchored proteins, the B-subunit of cholera toxin (CTxB) diffuses extremely slowly when bound to its glycolipid receptor GM(1) in the plasma membrane of living cells. In the current study, we used confocal FRAP to examine the origins of this slow diffusion of the CTxB/GM(1) complex at the cell surface, relative to the behavior of a representative GPI-anchored protein, transmembrane protein, and fluorescent lipid analog. We show that the diffusion of CTxB is impeded by actin- and ATP-dependent processes, but is unaffected by caveolae. At physiological temperature, the diffusion of several cell surface markers is unchanged in the presence of CTxB, suggesting that binding of CTxB to membranes does not alter the organization of the plasma membrane in a way that influences the diffusion of other molecules. Furthermore, diffusion of the B-subunit of another glycolipid-binding toxin, Shiga toxin, is significantly faster than that of CTxB, indicating that the confined diffusion of CTxB is not a simple function of its ability to cluster glycolipids. By identifying underlying mechanisms that control CTxB dynamics at the cell surface, these findings help to delineate the fundamental properties of toxin-receptor complexes in intact cell membranes.

Structure-dependent pseudoreceptor intracellular traffic of adamantyl globotriaosyl ceramide mimics

The verotoxin (VT) (Shiga toxin) receptor globotriaosyl ceramide (Gb(3)), mediates VT1/VT2 retrograde transport to the endoplasmic reticulum (ER) for cytosolic A subunit access to inhibit protein synthesis. Adamantyl Gb(3) is an amphipathic competitive inhibitor of VT1/VT2 Gb(3) binding. However, Gb(3)-negative VT-resistant CHO/Jurkat cells incorporate adaGb(3) to become VT1/VT2-sensitive. CarboxyadaGb(3), urea-adaGb(3), and hydroxyethyl adaGb(3), preferentially bound by VT2, also mediate VT1/VT2 cytotoxicity. VT1/VT2 internalize to early endosomes but not to Golgi/ER. AdabisGb(3) (two deacyl Gb(3)s linked to adamantane) protects against VT1/VT2 more effectively than adaGb(3) without incorporating into Gb(3)-negative cells. AdaGb(3) (but not hydroxyethyl adaGb(3)) incorporation into Gb(3)-positive Vero cells rendered punctate cell surface VT1/VT2 binding uniform and subverted subsequent Gb(3)-dependent retrograde transport to Golgi/ER to render cytotoxicity (reduced for VT1 but not VT2) brefeldin A-resistant. VT2-induced vacuolation was maintained in adaGb(3)-treated Vero cells, but vacuolar membrane VT2 was lost. AdaGb(3) destabilized membrane cholesterol and reduced Gb(3) cholesterol stabilization in phospholipid liposomes. Cholera toxin GM1-mediated Golgi/ER targeting was unaffected by adaGb(3). We demonstrate the novel, lipid-dependent, pseudoreceptor function of Gb(3) mimics and their structure-dependent modulation of endogenous intracellular Gb(3) vesicular traffic.

Cell membranes: the lipid perspective

Although cell membranes are packed with proteins mingling with lipids, remarkably little is known about how proteins interact with lipids to carry out their function. Novel analytical tools are revealing the astounding diversity of lipids in membranes. The issue is now to understand the cellular functions of this complexity. In this Perspective, we focus on the interface of integral transmembrane proteins and membrane lipids in eukaryotic cells. Clarifying how proteins and lipids interact with each other will be important for unraveling membrane protein structure and function. Progress toward this goal will be promoted by increasing overlap between different fields that have so far operated without much crosstalk.

Shotgun Lipidomics by Sequential Precursor Ion Fragmentation on a Hybrid Quadrupole Time-of-Flight Mass Spectrometer

Shotgun lipidomics has evolved into a myriad of multi-dimensional strategies for molecular lipid characterization, including bioinformatics tools for mass spectrum interpretation and quantitative measurements to study systems-lipidomics in complex biological extracts. Taking advantage of spectral mass accuracy, scan speed and sensitivity of improved quadrupole linked time-of-flight mass analyzers, we developed a bias-free global lipid profiling acquisition technique of sequential precursor ion fragmentation called MS/MS^ALL. This generic information-independent tandem mass spectrometry (MS) technique consists of a Q1 stepped mass isolation window through a set mass range in small increments, fragmenting and recording all product ions and neutral losses. Through the accurate MS and MS/MS information, the molecular lipid species are resolved, including distinction of isobaric and isomeric species, and composed into more precise lipidomic outputs. The method demonstrates good reproducibility and at least 3 orders of dynamic quantification range for isomeric ceramides in human plasma. More than 400 molecular lipids in human plasma were uncovered and quantified in less than 12 min, including acquisitions in both positive and negative polarity modes. We anticipate that the performance of sequential precursor ion fragmentation both in quality and throughput will lead to the uncovering of new avenues throughout the biomedical research community, enhance biomarker discovery and provide novel information target discovery programs as it will prospectively shed new insight into affected metabolic and signaling pathways.

Shiga toxin binding to glycolipids and glycans

BACKGROUND

Immunologically distinct forms of Shiga toxin (Stx1 and Stx2) display different potencies and disease outcomes, likely due to differences in host cell binding. The glycolipid globotriaosylceramide (Gb3) has been reported to be the receptor for both toxins. While there is considerable data to suggest that Gb3 can bind Stx1, binding of Stx2 to Gb3 is variable.

METHODOLOGY

We used isothermal titration calorimetry (ITC) and enzyme-linked immunosorbent assay (ELISA) to examine binding of Stx1 and Stx2 to various glycans, glycosphingolipids, and glycosphingolipid mixtures in the presence or absence of membrane components, phosphatidylcholine, and cholesterol. We have also assessed the ability of glycolipids mixtures to neutralize Stx-mediated inhibition of protein synthesis in Vero kidney cells.

RESULTS

By ITC, Stx1 bound both Pk (the trisaccharide on Gb3) and P (the tetrasaccharide on globotetraosylceramide, Gb4), while Stx2 did not bind to either glycan. Binding to neutral glycolipids individually and in combination was assessed by ELISA. Stx1 bound to glycolipids Gb3 and Gb4, and Gb3 mixed with other neural glycolipids, while Stx2 only bound to Gb3 mixtures. In the presence of phosphatidylcholine and cholesterol, both Stx1 and Stx2 bound well to Gb3 or Gb4 alone or mixed with other neutral glycolipids. Pre-incubation with Gb3 in the presence of phosphatidylcholine and cholesterol neutralized Stx1, but not Stx2 toxicity to Vero cells.

CONCLUSIONS

Stx1 binds primarily to the glycan, but Stx2 binding is influenced by residues in the ceramide portion of Gb3 and the lipid environment. Nanomolar affinities were obtained for both toxins to immobilized glycolipids mixtures, while the effective dose for 50% inhibition (ED(50)) of protein synthesis was about 10(-11) M. The failure of preincubation with Gb3 to protect cells from Stx2 suggests that in addition to glycolipid expression, other cellular components contribute to toxin potency.

Nanoscale membrane organization: where biochemistry meets advanced microscopy

Understanding the molecular mechanisms that shape an effective cellular response is a fundamental question in biology. Biochemical measurements have revealed critical information about the order of protein-protein interactions along signaling cascades but lack the resolution to determine kinetics and localization of interactions on the plasma membrane. Furthermore, the local membrane environment influences membrane receptor distributions and dynamics, which in turn influences signal transduction. To measure dynamic protein interactions and elucidate the consequences of membrane architecture interplay, direct measurements at high spatiotemporal resolution are needed. In this review, we discuss the biochemical principles regulating membrane nanodomain formation and protein function, ranging from the lipid nanoenvironment to the cortical cytoskeleton. We also discuss recent advances in fluorescence microscopy that are making it possible to quantify protein organization and biochemical events at the nanoscale in the living cell membrane.

Molecular dynamics simulations of membranes composed of glycolipids and phospholipids

Lipid membranes composed of 1,2-di-(9Z,12Z,15Z)-octadecatrienoyl-3-O-β-D-galactosyl-sn-glycerol or monogalactosyldiacylglycerol (MGDG) and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) were studied by means of molecular dynamics (MD) computer simulations. Three lipid compositions were considered: 0%, 20%, and 45% MGDG (by mole) denoted as MG-0, MG-20, and MG-45, respectively. The article is focused on the calculation of NMR dipolar interactions, which were confronted with previously reported experimental couplings. Dynamical processes and orientational distributions relevant for the averaging of dipolar interactions were evaluated. Furthermore, several parameters important for characterization of the bilayer structure, molecular organization, and dynamics were investigated. In general, only a minor change in DMPC properties was observed upon the increased MGDG/DMPC ratio, whereas properties related to MGDG undergo a more pronounced change. This effect was ascribed to the fact that DMPC is a bilayer (L(α)) forming lipid, whereas MGDG prefers a reverse hexagonal (H(II)) arrangement.

Impact of the nature and size of the polymeric backbone on the ability of heterobifunctional ligands to mediate shiga toxin and serum amyloid p component ternary complex formation

Inhibition of AB₅-type bacterial toxins can be achieved by heterobifunctional ligands (BAITs) that mediate assembly of supramolecular complexes involving the toxin’s pentameric cell membrane-binding subunit and an endogenous protein, serum amyloid P component, of the innate immune system. Effective in vivo protection from Shiga toxin Type 1 (Stx1) is achieved by polymer-bound, heterobifunctional inhibitors-adaptors (PolyBAITs), which exhibit prolonged half-life in circulation and by mediating formation of face-to-face SAP-AB₅ complexes, block receptor recognition sites and redirect toxins to the spleen and liver for degradation. Direct correlation between solid-phase activity and protective dose of PolyBAITs both in the cytotoxicity assay and in vivo indicate that the mechanism of protection from intoxication is inhibition of toxin binding to the host cell membrane. The polymeric scaffold influences the activity not only by clustering active binding fragments but also by sterically interfering with the supramolecular complex assembly. Thus, inhibitors based on N-(2-hydroxypropyl) methacrylamide (HPMA) show significantly lower activity than polyacrylamide-based analogs. The detrimental steric effect can partially be alleviated by extending the length of the spacer, which separates pendant ligand from the backbone, as well as extending the spacer, which spans the distance between binding moieties within each heterobifunctional ligand. Herein we report that polymer size and payload of the active ligand had moderate effects on the inhibitor’s activity.

The fusogenic tilted peptide (67-78) of α-synuclein is a cholesterol binding domain

Parkinson's disease-associated α-synuclein is an amyloidogenic protein not only expressed in the cytoplasm of neurons, but also secreted in the extracellular space and internalized into glial cells through a lipid raft-dependent process. We previously showed that α-synuclein interacts with raft glycosphingolipids through a structural motif common to various viral and amyloidogenic proteins. Here we report that α-synuclein also interacts with cholesterol, as assessed by surface pressure measurements of cholesterol-containing monolayers. Using a panel of recombinant fragments and synthetic peptides, we identified two distinct cholesterol-binding domains in α-synuclein. One of these domains, which corresponds to the tilted peptide of α-synuclein (67-78), bound cholesterol with high affinity and was toxic for cultured astrocytes. Molecular modeling suggested that cholesterol binds to this peptide with a tilt angle of 46°. α-synuclein also contains a cholesterol recognition consensus motif, which had a lower affinity for cholesterol and was devoid of toxicity. This motif is encased in the glycosphingolipid-binding domain (34-45) of α-synuclein. In raft-like model membranes containing both cholesterol and glycosphingolipids, the head groups of glycosphingolipids prevented the accessibility of cholesterol to exogenous ligands. Nevertheless, cholesterol appeared to 'signal' its presence by tuning glycosphingolipid conformation, thereby facilitating α-synuclein binding to raft-like membranes. We propose that the association of α-synuclein with lipid rafts involves both the binding of α-synuclein (34-45) to glycosphingolipids, and the interaction of the fusogenic tilted peptide (67-78) with cholesterol. Coincidentally, a similar mechanism is used by viruses (HIV-1, HTLV-I, Ebola) which display a tilted peptide and fuse with host cell membranes through a sphingolipid/cholesterol-dependent process.

Glycosphingolipid functions

The combination of carbohydrate and lipid generates unusual molecules in which the two distinctive halves of the glycoconjugate influence the function of each other. Membrane glycolipids can act as primary receptors for carbohydrate binding proteins to mediate transmembrane signaling despite restriction to the outer bilayer leaflet. The extensive heterogeneity of the lipid moiety plays a significant, but still largely unknown, role in glycosphingolipid function. Potential interplay between glycolipids and their fatty acid isoforms, together with their preferential interaction with cholesterol, generates a complex mechanism for the regulation of their function in cellular physiology.

Cholesterol- and sphingolipid-rich microdomains are essential for microtubule-based membrane protrusions induced by Clostridium difficile transferase (CDT)

Clostridium difficile toxin (CDT) is a binary actin-ADP-ribosylating toxin that causes depolymerization of the actin cytoskeleton and formation of microtubule-based membrane protrusions, which are suggested to be involved in enhanced bacterial adhesion and colonization of hypervirulent C. difficile strains. Here, we studied the involvement of membrane lipid components of human colon adenocarcinoma (Caco-2) cells in formation of membrane protrusions. Depletion of cholesterol by methyl-β-cyclodextrin inhibited protrusion formation in a concentration-dependent manner but had no major effect on the toxin-catalyzed modification of actin in target cells. Repletion of cholesterol reconstituted formation of protrusions and increased velocity and total amount of protrusion formation. Methyl-β-cyclodextrin had no effect on the CDT-induced changes in the dynamics of microtubules. Formation of membrane protrusions was also inhibited by the cholesterol-binding polyene antibiotic nystatin. Degradation or inhibition of synthesis of sphingolipids by sphingomyelinase and myriocin, respectively, blocked CDT-induced protrusion formation. Benzyl alcohol, which increases membrane fluidity, prevented protrusion formation. CDT-induced membrane protrusions were stained by flotillin-2 and by the fluorescent-labeled lipid raft marker cholera toxin subunit B, which selectively interacts with GM1 ganglioside mainly located in lipid microdomains. The data suggest that formation and especially the initiation of CDT-induced microtubule-based membrane protrusions depend on cholesterol- and sphingolipid-rich lipid microdomains.

Comparison of detection methods for cell surface globotriaosylceramide

The cell surface-expressed glycosphingolipid (GSL), globotriaosylceramide (Gb(3)), is becoming increasingly important and is widely studied in the areas of verotoxin (VT)-mediated cytotoxicity, human immunodeficiency virus (HIV) infection, immunology and cancer. However, despite its diverse roles and implications, an optimized detection method for cell surface Gb(3) has not been determined. GSLs are differentially organized in the plasma membrane which can affect their availability for protein binding. To examine various detection methods for cell surface Gb(3), we compared four reagents for use in flow cytometry analysis. A natural ligand (VT1B) and three different monoclonal antibodies (mAbs) were optimized and tested on various human cell lines for Gb(3) detection. A differential detection pattern of cell surface Gb(3) expression, which was influenced by the choice of reagent, was observed. Two mAb were found to be suboptimal. However, two other methods were found to be useful as defined by their high percentage of positivity and mean fluorescence intensity (MFI) values. Rat IgM anti-Gb(3) mAb (clone 38-13) using phycoerythrin-conjugated secondary antibody was found to be the most specific detection method while the use of VT1B conjugated to Alexa488 fluorochrome was found to be the most sensitive; showing a rare crossreactivity only when Gb(4) expression was highly elevated. The findings of this study demonstrate the variability in detection of Gb(3) depending on the reagent and cell target used and emphasize the importance of selecting an optimal methodology in studies for the detection of cell surface expression of Gb(3).

Adamantyl glycosphingolipids provide a new approach to the selective regulation of cellular glycosphingolipid metabolism

Mammalian glycosphingolipid (GSL) precursor monohexosylceramides are either glucosyl- or galactosylceramide (GlcCer or GalCer). Most GSLs derive from GlcCer. Substitution of the GSL fatty acid with adamantane generates amphipathic mimics of increased water solubility, retaining receptor function. We have synthesized adamantyl GlcCer (adaGlcCer) and adamantyl GalCer (adaGalCer). AdaGlcCer and adaGalCer partition into cells to alter GSL metabolism. At low dose, adaGlcCer increased cellular GSLs by inhibition of glucocerebrosidase (GCC). Recombinant GCC was inhibited at pH 7 but not pH 5. In contrast, adaGalCer stimulated GCC at pH 5 but not pH 7 and, like adaGlcCer, corrected N370S mutant GCC traffic from the endoplasmic reticulum to lysosomes. AdaGalCer reduced GlcCer levels in normal and lysosomal storage disease (LSD) cells. At 40 μM adaGlcCer, lactosylceramide (LacCer) synthase inhibition depleted LacCer (and more complex GSLs), such that only GlcCer remained. In Vero cell microsomes, 40 μM adaGlcCer was converted to adaLacCer, and LacCer synthesis was inhibited. AdaGlcCer is the first cell LacCer synthase inhibitor. At 40 μM adaGalCer, cell synthesis of only Gb(3) and Gb(4) was significantly reduced, and a novel product, adamantyl digalactosylceramide (adaGb(2)), was generated, indicating substrate competition for Gb(3) synthase. AdaGalCer also inhibited cell sulfatide synthesis. Microsomal Gb(3) synthesis was inhibited by adaGalCer. Metabolic labeling of Gb(3) in Fabry LSD cells was selectively reduced by adaGalCer, and adaGb(2) was produced. AdaGb(2) in cells was 10-fold more effectively shed into the medium than the more polar Gb(3), providing an easily eliminated "safety valve" alternative to Gb(3) accumulation. Adamantyl monohexosyl ceramides thus provide new tools to selectively manipulate normal cellular GSL metabolism and reduce GSL accumulation in cells from LSD patients.