Applications of BODIPY-sphingolipid analogs to study lipid traffic and metabolism in cells

BODIPY-Based Analogue of the TREM2-Binding Molecular Adjuvant Sulfavant A, a Chemical Tool for Imaging and Tracking Biological Systems

Recently, we described synthetic sulfolipids named Sulfavants as a novel class of molecular adjuvants based on the sulfoquinovosyl-diacylglycerol skeleton. The members of this family, Sulfavant A (1), Sulfavant R (2), and Sulfavant S (3), showed important effects on triggering receptor expressed on myeloid cells 2 (TREM2)-induced differentiation and maturation of human dendritic cells (hDC), through a novel cell mechanism underlying the regulation of the immune response. As these molecules are involved in biological TREM2-mediated processes crucial for cell survival, here, we report the synthesis and application of a fluorescent analogue of Sulfavant A bearing the 4,4-difluoro-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene moiety (Me4-BODIPY). The fluorescent derivative, named PB-SULF A (4), preserving the biological activity of Sulfavants, opens the way to chemical biology and cell biology experiments to better understand the interactions with cellular and in vivo organ targets and to improve our comprehension of complex molecular mechanisms underlying the not fully understood ligand-induced TREM2 activity.

Bringing Color to Sugars: The Chemical Assembly of Carbohydrates to BODIPY Dyes

The combination of carbohydrates with BODIPY fluorophores gives rise to a family of BODIPY-carbohydrate hybrids or glyco-BODIPYs, which mutually benefit from the encounter. Thus, from the carbohydrates standpoint, glyco-BODIPYs can be regarded as fluorescent glycoconjugate derivatives with application in imaging techniques, whereas from the fluorophore view the BODIPY-carbohydrate hybrids benefit from the biocompatibility, water-solubility, and reduced toxicity, among others, brought about by the sugar moiety. In this Account we have intended to present the collection of available methods for the synthesis of BODIPY-carbohydrate hybrids, with a focus on the chemical transformations on the BODIPY core.

Pore-forming toxins: Properties, diversity, and uses as tools to image sphingomyelin and ceramide phosphoethanolamine

Pore-forming toxins (PFTs) represent a unique class of highly specific lipid-binding proteins. The cytotoxicity of these compounds has been overcome through crystallographic structure and mutation studies, facilitating the development of non-toxic lipid probes. As a consequence, non-toxic PFTs have been utilized as highly specific probes to visualize the diversity and dynamics of lipid nanostructures in living and fixed cells. This review is focused on the application of PFTs and their non-toxic analogs as tools to visualize sphingomyelin and ceramide phosphoethanolamine, two major phosphosphingolipids in mammalian and insect cells, respectively. This article is part of a Special Issue entitled: Pore-Forming Toxins edited by Mauro Dalla Serra and Franco Gambale.

Interaction of fluorescent phospholipids with cyclodextrins

Fluorescent analogs of phospholipids are often employed to investigate the structure and dynamics of lipids in membranes. Some of those studies have used cyclodextrins e.g., to modulate the lipid phase. However, the role of the fluorescence moiety of analogs for the interaction between cyclodextrins and fluorescent lipids has not been investigated so far in detail. Therefore, in the present study the interaction of various fluorescent phospholipid analogs with methylated α-, β- and γ- cyclodextrins was investigated. The analogs differed in their structure, in the length of the fatty acyl chain, in the position of the fluorescence group, and in the attached fluorescence moiety (7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) or dipyrrometheneboron difluoride (BODIPY)). In aqueous buffer, cyclodextrins bind fluorescent lipids disturbing the organization of the analogs. When incorporated into lipid vesicles, analogs are selectively extracted from the membrane upon addition of cyclodextrins. The results show that the interaction of cyclodextrins with fluorescent phospholipids depends on the cyclodextrin species, the fluorescence moiety and the phospholipid structure. The presented data should be of interest for studies using fluorescent phospholipids and cyclodextrins, since the interaction between the fluorescence group and the cyclodextrin may interfere with the process(es) under study.

A new assay for determining ganglioside sialyltransferase activities lactosylceramide-2,3-sialyltransferase (SAT I) and monosialylganglioside-2,3-sialyltransferase (SAT IV)

A new assay for the determination of lactosylceramide-2,3-sialyltransferase (SAT I, EC 2.4.99.9) and monosialoganglioside sialyltransferase (SAT IV, EC 2.4.99.2) is described. The assay utilised the commercially available fluorophore labelled sphingolipids, boron dipyrromethene difluoride (BODIPY) lactosylceramide (LacCer), and BODIPY-monosialotetrahexosylganglioside (GM1) as the acceptor substrates, for SAT I and SAT IV, respectively. HPLC coupled with fluorescence detection was used to analyse product formation. The analysis was performed in a quick and automated fashion. The assay showed good linearity for both BODIPY sphingolipids with a quantitative detection limit of 0.05 pmol. The high sensitivity enabled the detection of SAT I and SAT IV activities as low as 0.001 μU, at least 200 fold lower than that of most radiometric assays. This new assay was applied to the screening of SAT I and SAT IV activities in ovine and bovine organs (liver, heart, kidney, and spleen). The results provided evidence that young animals, such as calves, start to produce ganglioside sialyltransferases as early as 7 days after parturition and that levels change during maturation. Among the organs tested from a bovine source, spleen had the highest specific ganglioside sialyltransferase activity. Due to the organ size, the greatest total ganglioside sialyltransferase activities (SAT I and SAT IV) were detected in the liver of both bovine and ovine origin.

Single cell metabolic profiling of tumor mimics

Chemical cytometry employs modern analytical methods to study the differences in composition between single cells to better understand development, cellular differentiation, and disease. Metabolic cytometry is a form of chemical cytometry wherein cells are incubated with and allowed to metabolize fluorescently labeled small molecules. Capillary electrophoresis with laser-induced fluorescence detection is then used to characterize the extent of metabolism at the single cell level. To date, all metabolic cytometry experiments have used conventional two-dimensional cell cultures. HCT 116 spheroids are a three-dimensional cell culture system, morphologically and phenotypically similar to tumors. Here, intact HCT 116 multicellular spheroids were simultaneously incubated with three fluorescently labeled glycosphingolipid substrates, GM3-BODIPY-FL, GM1-BODIPY-TMR, and lactosylceramide-BODIPY-650/665. These substrates are spectrally distinct, and their use allows the simultaneous probing of metabolism at three different points in the glycolipid metabolic cascade. Beginning with intact spheroids, a serial trypsinization and trituration procedure was used to isolate single cells from spatially distinct regions of the spheroid. Cells from the distinct regions showed unique metabolic patterns. Treatment with the lysosomal inhibitor and potential chemotherapeutic chloroquine consistently decreased catabolism for all substrates. Nearly 200 cells were taken for analysis. Principal component analysis with a multivariate measure of precision was used to quantify cell-to-cell variability in glycosphingolipid metabolism as a function of cellular localization and chloroquine treatment. While cells from different regions exhibited differences in metabolism, the heterogeneity in metabolism did not differ significantly across the experimental conditions.

Manipulating ionic strength to improve single cell electrophoretic separations

A capillary electrophoresis system with ultrasensitive two-color laser-induced fluorescence detection was used to probe the effect of ionic strength on single cell separations of glycosphingolipids. Differentiated PC12 cells were incubated with two ganglioside substrates tagged with different fluorophores within the BODIPY family such that two distinct metabolic patterns could be simultaneously monitored. Aspiration of single differentiated PC12 cells suspended in a phosphate-buffered saline solution showed excessive peak dispersion, poor resolution, and peak efficiencies below 100,000 theoretical plates. Aspiration of single differentiated PC12 cells suspended in deionized water corrected peak dispersion. Average peak efficiencies ranged between 400,000 and 600,000 theoretical plates. Improved performance was due to the dilution of the high salt concentrations inside of single neuronal-like cells to produce field amplified sample stacking. Single cell separations showed the highest resolution when aspiration of single differentiated PC12 cells suspended in deionized water were separated using a running buffer of high ionic strength. The improvement in resolution allowed for the identification of analytes not previously detected in single cell metabolism studies.

Capillary electrophoresis with three-color fluorescence detection for the analysis of glycosphingolipid metabolism

A capillary electrophoresis system with an ultrasensitive three-color laser-induced fluorescence detector was constructed for the simultaneous measurement of glycosphingolipids conjugated with a family of BODIPY fluorophores. The compounds were separated by capillary electrophoresis and detected by laser-induced fluorescence excited within a sheath-flow cuvette. Diode-pumped solid-state lasers operating at 473 nm and 532 nm, and a diode laser operating at 633 nm were used to excite glycosphingolipids tagged with BODIPY-FL, BODIPY-TMR, and BODIPY-650/665 fluorophores. Detection limits were 34 ± 1 molecules, 67 ± 7 molecules, and 36 ± 13 molecules of BODIPY-FL, BODIPY-TMR, and BODIPY-650/665 labeled glycosphingolipids. Separation efficiencies were typically one million theoretical plates. To test the ability of the system to analyze cellular contents in an in vitro biological model, differentiated PC12 cells were co-incubated with BODIPY-FL, BODIPY-TMR, and BODIPY-650/665 labeled lactosylceramide substrates. Cells were homogenized. The metabolic products originating from the glycosphingolipid substrates were simultaneously analyzed using the system.

Preparation and electrophoretic separation of Bodipy-Fl-labeled glycosphingolipids

Several glycosphingolipids were labeled with the fluorphore Bodipy-Fl and analyzed using capillary electrophoresis with laser-induced fluorescence detection. GM1-, LacCer-, and Cer-Bodipy-Fl were prepared through acylation using the N-hydroxysuccinimide ester of Bodipy-Fl. Several other glycosphingolipids including GT1a-, GD1a-, GM2-, GM3-, GD3-, and GlcCer-Bodipy-Fl were enzymatically synthesized. Micellar electrokinetic capillary chromatography with a TRIS/CHES/SDS/α-cyclodextrin buffer produced better separation than an established borate/deoxycholate/methyl-β-cyclodextrin buffer. The nine Bodipy-Fl-labeled glycosphingolipid standards were separated in under 5 min, theoretical plate counts were between 640,000 and 740,000, and the limit of detection was approximately 3 pM or 240 ymol analyte injected onto the capillary.

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.

Assessment of the roles of ordered lipid microdomains in post-endocytic trafficking of glycosyl-phosphatidylinositol-anchored proteins in mammalian fibroblasts

We have used artificial phosphatidylethanolamine-polyethylene glycol (PE-PEG)-anchored proteins, incorporated into living mammalian cells, to evaluate previously proposed roles for ordered lipid 'raft' domains in the post-endocytic trafficking of glycosylphosphatidylinositol (GPI)-anchored proteins in CHO and BHK cells. In CHO cells, endocytosed PE-PEG protein conjugates colocalized strongly with the internalized GPI-anchored folate receptor, concentrating in the endosomal recycling compartment, regardless of the structure of the hydrocarbon chains of the PE-PEG 'anchor'. However, internalized PE-PEG protein conjugates with long-chain saturated anchors recycled to the plasma membrane at a slow rate comparable to that measured for the GPI-anchored folate receptor, whereas conjugates with short-chain or unsaturated anchors recycled at a faster rate similar to that observed for the transferrin receptor. These findings support the proposal (Mayor et al. Cholesterol-dependent retention of GPI-anchored proteins in endosomes. EMBO J 1998;17:4628-4638) that the slow recycling of GPI proteins in CHO cells rests on their affinity for ordered lipid domains. In BHK cells, internalized PE-PEG protein conjugates with either saturated or unsaturated 'anchors' colocalized strongly with simultaneously endocytosed folate receptor and, like the folate receptor, gradually accumulated in late endosomes/lysosomes. These latter findings do not support previous suggestions that the sorting of GPI proteins to late endosomes in BHK cells depends on their association with lipid rafts.

Imaging lipid membrane domains with lipid-specific probes

Imaging membrane lipid domains to characterize their organization and function has been hindered by the lack of reliable lipid-specific probes. In this paper, we provide detailed methods to investigate, mainly by confocal microscopy, the distribution and dynamics of two components of the "lipid rafts," sphingomyelin (SM) and cholesterol, using two specific lipid probes that have been extensively studied in the laboratory: lysenin, a SM-binding toxin and the fluorescent esters of poly(ethylene glycol) cholesteryl ether (PEG-Chol) that label cholesterol-rich domains. The production of nontoxic forms of lysenin as well as its specific binding behavior have allowed monitoring the distribution and the dynamics of SM-rich domains in living cell membranes. Because of its water-solubility and low toxicity, the fluorescent PEG-Chol can be used to follow the reorganization of cell surface cholesterol-rich domains as well as intracellular cholesterol dynamics in living cells. These probes can thus provide important informations on lipid distribution and traffic in living cell membranes.

Steric and not structure-specific factors dictate the endocytic mechanism of glycosylphosphatidylinositol-anchored proteins

Diverse glycosylphosphatidylinositol (GPI)-anchored proteins enter mammalian cells via the clathrin- and dynamin-independent, Arf1-regulated GPI-enriched early endosomal compartment/clathrin-independent carrier endocytic pathway. To characterize the determinants of GPI protein targeting to this pathway, we have used fluorescence microscopic analyses to compare the internalization of artificial lipid-anchored proteins, endogenous membrane proteins, and membrane lipid markers in Chinese hamster ovary cells. Soluble proteins, anchored to cell-inserted saturated or unsaturated phosphatidylethanolamine (PE)-polyethyleneglycols (PEGs), closely resemble the GPI-anchored folate receptor but differ markedly from the transferrin receptor, membrane lipid markers, and even protein-free PE-PEGs, both in their distribution in peripheral endocytic vesicles and in the manner in which their endocytic uptake responds to manipulations of cellular Arf1 or dynamin activity. These findings suggest that the distinctive endocytic targeting of GPI proteins requires neither biospecific recognition of their GPI anchors nor affinity for ordered-lipid microdomains but is determined by a more fundamental property, the steric bulk of the lipid-anchored protein.

Clustering and lateral concentration of raft lipids by the MAL protein

MAL, a compact hydrophobic, four-transmembrane-domain apical protein that copurifies with detergent-resistant membranes is obligatory for the machinery that sorts glycophosphatidylinositol (GPI)-anchored proteins and others to the apical membrane in epithelia. The mechanism of MAL function in lipid-raft-mediated apical sorting is unknown. We report that MAL clusters formed by two independent procedures-spontaneous clustering of MAL tagged with the tandem dimer DiHcRED (DiHcRED-MAL) in the plasma membrane of COS7 cells and antibody-mediated cross-linking of FLAG-tagged MAL-laterally concentrate markers of sphingolipid rafts and exclude a fluorescent analogue of phosphatidylethanolamine. Site-directed mutagenesis and bimolecular fluorescence complementation analysis demonstrate that MAL forms oligomers via xx intramembrane protein-protein binding motifs. Furthermore, results from membrane modulation by using exogenously added cholesterol or ceramides support the hypothesis that MAL-mediated association with raft lipids is driven at least in part by positive hydrophobic mismatch between the lengths of the transmembrane helices of MAL and membrane lipids. These data place MAL as a key component in the organization of membrane domains that could potentially serve as membrane sorting platforms.

Composition-driven surface domain structuring mediated by sphingolipids and membrane-active proteins. Above the nano- but under the micro-scale: mesoscopic biochemical/structural cross-talk in biomembranes

Biomembranes contain a wide variety of lipids and proteins within an essentially two-dimensional structure. The coexistence of such a large number of molecular species causes local tensions that frequently relax into a phase or compositional immiscibility along the lateral and transverse planes of the interface. As a consequence, a substantial microheterogeneity of the surface topography develops and that depends not only on the lipid-protein composition, but also on the lateral and transverse tensions generated as a consequence of molecular interactions. The presence of proteins, and immiscibility among lipids, constitute major perturbing factors for the membrane sculpturing both in terms of its surface topography and dynamics. In this work, we will summarize some recent evidences for the involvement of membrane-associated, both extrinsic and amphitropic, proteins as well as membrane-active phosphohydrolytic enzymes and sphingolipids in driving lateral segregation of phase domains thus determining long-range surface topography.

Transbilayer movement of ceramide in the plasma membrane of live cells

Ceramide (Cer) is the precursor for sphingolipids and functions as a second messenger in a variety of cellular processes including apoptosis. However, no direct target of Cer leading to apoptosis has been identified. Understanding the movement and trafficking of Cer is important for fully understanding Cer signaling. In this study, we identified, for the first time, the transbilayer movement of Cer in the plasma membrane (PM) of living cells. We developed a new method to monitor transbilayer Cer movement using ceramide kinase activity. To produce Cer on the extracellular leaflet of the PM, bacterial sphingomyelinase (SMase) was added to rat basophilic leukemia cells. Interestingly, the dramatic elevation of ceramide 1-phosphate (C1P), the product of CerK, was observed following the increase of Cer induced by SMase treatment. Since we determined that both the protein and catalytic activity of CerK exists in the intracellular compartment, the all conversion of Cer to C1P by CerK should be occur intracellularly. This result indicates the rapid transbilayer movement of Cer from the outer leaflet to the inner leaflet of the PM of living cells. Furthermore, protease digestion of membrane proteins, inhibition of ABC transporters (by glibencramide) and of cation channels (by carbonyl cyanide m-chlorophenylhydrozone), and modification of cholesterol content did not affect the transbilayer movement of Cer. Thus, this movement might occur spontaneously. Our findings indicate not only Cer movement in the PM, but also identify an intrinsic property of Cer enabling Cer signaling.

Synthesis of reference standards to enable single cell metabolomic studies of tetramethylrhodamine-labeled ganglioside GM1

Ganglioside GM1 and its seven potential catabolic products: asialo-GM1, GM2, asialo-GM2, GM3, Lac-Cer, Glc-Cer and Cer, were labeled with tetramethylrhodamine (TMR) to permit ultra-sensitive analysis using laser-induced fluorescence (LIF) detection. The preparation involved acylation of the homogenous C(18)lyso-forms of GM1, Lac-Cer, Glc-Cer and Cer with the N-hydroxysuccinimide ester of a beta-alanine-tethered 6-TMR derivative, followed by conversion of these labeled products using galactosidase, sialidase, and sialyltransferase enzymes. The TMR-glycolipid analogs produced are detectable on TLC down to the 1 ng level by the naked eye. All eight compounds could be separated within 4 min in capillary electrophoresis where they could be detected at the zeptomole (ca. 1000 molecule) level using LIF.

Synthesis of fluorescent lactosylceramide stereoisomers

The intracellular distribution of synthetic glycosphingolipids (GSLs) bearing a fluorophore can be monitored in living cells by fluorescence microscopy. We reported previously that variation in the length of the long-chain base and in the structure of the carbohydrate-containing polar head group of (2S,3R) (or D-erythro-)-beta-lactosylceramide (LacCer) did not alter the mechanism of endocytic uptake from the plasma membrane of various mammalian cell types [Singh, R.D., Puri, V., Valiyaveettil, J.T., Marks, D.L., Bittman, R., Pagano, R.E., 2003. Selective caveolin-1-dependent endocytosis of glycosphingolipids. Mol. Biol. Cell 14, 3254-3265]. To extend our examination of the molecular features in LacCer that are responsible for its uptake by the caveolar-requiring endocytic pathway, we have synthesized the three unnatural stereoisomers [(2R,3R)-, (2S,3S)-, and (2R,3S)] of dipyrromethene difluoride (BODIPY)-LacCer. These analogues will be used to probe the role of stereochemistry in the long-chain base of LacCer in the mechanism of endocytic uptake.

Endocytic recycling compartments altered in cisplatin-resistant cancer cells

The clinical utility of cisplatin to treat human malignancies is often limited by the development of drug resistance. We have previously shown that cisplatin-resistant human KB adenocarcinoma cells that are cross-resistant to methotrexate and heavy metals have altered endocytic recycling. In this work, we tracked lipids in the endocytic recycling compartment (ERC) and found that the distribution of the ERC is altered in KB-CP.5 cells compared with parental KB-3-1 cells. A tightly clustered ERC is located near the nucleus in parental KB-3-1 cells but it appears loosely arranged and widely dispersed throughout the cytoplasm in KB-CP.5 cells. The altered distribution of the ERC in KB-CP.5 cells is related to the amount and distribution of stable detyrosinated microtubules (Glu-alpha-tubulin), as previously shown in Chinese hamster ovary B104-5 cells that carry a temperature-sensitive Glu-alpha-tubulin allele. In addition, B104-5 cells with a dispersed ERC under nonpermissive conditions were more resistant to cisplatin compared with B104-5 cells with a clustered ERC under permissive conditions. We conclude that resistance to cisplatin might be due, in part, to reduced uptake of cisplatin resulting from an endocytic defect reflecting defective formation of the ERC, possibly related to a shift in the relative amounts and distributions of stable microtubules.

Sterol carrier protein-2 expression alters sphingolipid metabolism in transfected mouse L-cell fibroblasts

The influence of sterol carrier protein-2 (SCP-2) on the cellular metabolism of sphingolipids was examined in control mouse L-cells and stably transfected clones expressing the protein SCP-2. Three approaches were used to examine for differences; (1) compositional analysis of endogenous sphingolipid classes, (2) metabolism of NBD-ceramide, and (3) live cell labelling via endocytic uptake of BODIPY-sphingomyelin. SCP-2 over expression significantly altered the endogenous levels of both neutral and acidic sphingolipid classes. Among the neutral sphingolipids, expression of SCP-2 induced a 1.7-fold increase in the level of lactosylceramide (LacCer, p < 0.05) and a similar fold decrease in the level of the higher-order neutral glycosylceramides (p < 0.05). Among the acidic sphingolipids, SCP-2 resulted in a 5.2-fold decrease in the endogenous plasma membrane level of ganglioside GM1 (p < 0.03). Incubation of both control and transfected cell lines with NBD-ceramide resulted in the rapid establishment of a steady-state distribution of NBD-labelled sphingomyelin (NBD-SM) and glucosylceramide (NBD-GlcCer). In the SCP-2 expressing clones the conversion of NBD-Cer to NBD-GlcCer was 30% lower during incubation periods between 5 and 30 min (p < 0.025). Inspection of the cells by fluorescence microscopy after incubation with BODIPY labelled sphingomyelin (BODIPY-SM) revealed similar punctuated patterns with no distinguishable differences between the cell types. These results imply that SCP-2 plays a role in modulating enzymatic steps involved in metabolism of sphingolipid homeostasis.

Use of a fluorescent membrane probe to identify zooxanthellae in hospite among dissociated endoderm cell culture from coral

Preparation of homogeneous endoderm cells and culture is a prerequisite to understanding the cellular and molecular mechanism of endosymbiosis in the cnidarian-dinoflagellate association. During the cell isolation from the stony coral Euphyllia glabrescens, various amounts of symbiotic endoderm cells were found to release their symbionts (Symbiodinium spp., or zooxanthellae in generic usage) into the culture. Due to the bulky occupation by zooxanthellae inside the endoderm cell, the symbiotic endoderm cells, or zooxanthellae in hospite, are difficult to be distinguished from released zooxanthellae by microscopic examination. We now report a method for this identification using a fluorescent analogue of sphingomyelin, N-[5-(5,7-dimethyl boron dipyrromethene difluoride)-1-pentanoyl]-D-erythro-sphingosylphosphorylcholine (C(5)-DMB-SM). Incubation of symbiotic endoderm cells with C(5)-DMB-SM-defatted bovine serum albumin (DF-BSA) complex results in bright fluorescent membrane staining. Nevertheless, the membrane staining of free-living or released zooxanthellae by this complex is significantly decreased or even diminished. This method has provided a fast and reliable assay to identify symbiotic endoderm cells and will greatly accelerate the progress of endosymbiosis research.

Macrophage internalization of fungal beta-glucans is not necessary for initiation of related inflammatory responses

Cell wall beta-glucans are highly conserved structural components of fungi that potently trigger inflammatory responses in an infected host. Identification of molecular mechanisms responsible for internalization and signaling of fungal beta-glucans should enhance our understanding of innate immune responses to fungi. In this study, we demonstrated that internalization of fungal beta-glucan particles requires actin polymerization but not participation of components of caveolar uptake mechanisms. Using fluorescence microscopy, we observed that uptake of 5-([4,6-dichlorotriazin-2-yl] amino)-fluorescein hydrochloride-Celite complex-labeled Saccharomyces cerevisiae beta-glucan by RAW macrophages was substantially reduced in the presence of cytochalasin D, which antagonizes actin-mediated internalization pathways, but not by treatment with nystatin, which blocks caveolar uptake. Interestingly, beta-glucan-induced NF-kappaB translocation, which is necessary for inflammatory activation, and tumor necrosis factor alpha production were both normal in the presence of cytochalasin D, despite defective internalization of beta-glucan particles following actin disruption. Dectin-1, a major beta-glucan receptor on macrophages, colocalized to phagocytic cups on macrophages and exhibited tyrosine phosphorylation after challenge with beta-glucan particles. Dectin-1 localization and other membrane markers were not affected by treatment with cytochalasin D. Furthermore, dectin-1 receptors rather than Toll-like receptor 2 receptors were shown to be necessary for both efficient internalization of beta-glucan particles and cytokine release in response to the fungal cell wall component.

Cellular stress failure in ventilator-injured lungs

The clinical and experimental literature has unequivocally established that mechanical ventilation with large tidal volumes is injurious to the lung. However, uncertainty about the micromechanics of injured lungs and the numerous degrees of freedom in ventilator settings leave many unanswered questions about the biophysical determinants of lung injury. In this review we focus on experimental evidence for lung cells as injury targets and the relevance of these studies for human ventilator-associated lung injury. In vitro, the stress-induced mechanical interactions between matrix and adherent cells are important for cellular remodeling as a means for preventing compromise of cell structure and ultimately cell injury or death. In vivo, these same principles apply. Large tidal volume mechanical ventilation results in physical breaks in alveolar epithelial and endothelial plasma membrane integrity and subsequent triggering of proinflammatory signaling cascades resulting in the cytokine milieu and pathologic and physiologic findings of ventilator-associated lung injury. Importantly, though, alveolar cells possess cellular repair and remodeling mechanisms that in addition to protecting the stressed cell provide potential molecular targets for the prevention and treatment of ventilator-associated lung injury in the future.

Ceramidase regulates synaptic vesicle exocytosis and trafficking

A screen for Drosophila synaptic dysfunction mutants identified slug-a-bed (slab). The slab gene encodes ceramidase, a central enzyme in sphingolipid metabolism and regulation. Sphingolipids are major constituents of lipid rafts, membrane domains with roles in vesicle trafficking, and signaling pathways. Null slab mutants arrest as fully developed embryos with severely reduced movement. The SLAB protein is widely expressed in different tissues but enriched in neurons at all stages of development. Targeted neuronal expression of slab rescues mutant lethality, demonstrating the essential neuronal function of the protein. C(5)-ceramide applied to living preparations is rapidly accumulated at neuromuscular junction (NMJ) synapses dependent on the SLAB expression level, indicating that synaptic sphingolipid trafficking and distribution is regulated by SLAB function. Evoked synaptic currents at slab mutant NMJs are reduced by 50-70%, whereas postsynaptic glutamate-gated currents are normal, demonstrating a specific presynaptic impairment. Hypertonic saline-evoked synaptic vesicle fusion is similarly impaired by 50-70%, demonstrating a loss of readily releasable vesicles. In addition, FM1-43 dye uptake is reduced in slab mutant presynaptic terminals, indicating a smaller cycling vesicle pool. Ultrastructural analyses of mutants reveal a normal vesicle distribution clustered and docked at active zones, but fewer vesicles in reserve regions, and a twofold to threefold increased incidence of vesicles linked together and tethered at the plasma membrane. These results indicate that SLAB ceramidase function controls presynaptic terminal sphingolipid composition to regulate vesicle fusion and trafficking, and thus the strength and reliability of synaptic transmission.

The 2003 ASBMB-Avanti Award in Lipids Address: Applications of novel synthetic lipids to biological problems

This paper is an overview of the 2003 Avanti Award in Lipids address that was presented by Robert Bittman at the American Society for Biochemistry and Molecular Biology (ASBMB) Annual Meeting held in San Diego, CA in conjunction with meetings of five other FASEB Societies, April 15, 2003. The theme of the lecture is: "How can the chemical synthesis of unnatural lipids provide insights into problems ranging from cell biology to biophysics?" The following examples are presented: (1) novel ceramide analogs as experimental anticancer agents, (2) photoactivatable sphingosine 1-phosphate analogs as probes of protein targets of this bioactive lipid, (3) a 13C-enriched cerebroside as a quantitative probe of glycosphingolipid (GSL) transbilayer distribution in bilayers with and without sphingomyelin, (4) cis and trans unsaturated sphingomyelin analogs as modulators of the existence of cholesterol-enriched microdomains (rafts) that may facilitate fusion of alphaviruses with target membranes, (5) ceramide as an indirect enhancer of the permeabilization of membranes induced by cholesterol-specific cytolysins, (6) fluorescent GSL analogs of widely disparate structure as probes of the molecular features responsible for the selective internalization of GSLs in caveolae of living mammalian cells, (7) enantiomeric lysophosphatidic acid (LPA) analogs as probes of receptor subtypes that mediate LPA signaling, and (8) phosphonocholine analogs of the antitumor ether lipid ET-18-OCH3 as tools for discerning the primary targets that are critical for cytotoxic activity in tumor cells.

Plasma Membrane Surface Increases with Tonic Stretch of Alveolar Epithelial Cells

Cyclic stretch stimulates numerous responses in alveolar epithelial cells--some beneficial, some injurious--often through mechanosensitive membrane-associated proteins such as stretch-activated ion channels. Tonic stretch, in contrast, stimulates only some of these responses. In this study, we hypothesized that the plasma membranes of alveolar epithelial cells expand during tonic stretch, not only through cell surface unfolding, but also through recruitment of additional phospholipids. Such plasma membrane expansion would reduce membrane tension and decrease stimulation of mechanosensitive membrane proteins. Primary rat alveolar epithelial cells were isolated, cultured for 48 h, and stretched between 3 and 40% change in basal membrane surface area. Gross changes in total cell surface area were obtained from stacks of thin fluorescent confocal micrographs; fine changes in plasma membrane area were measured via whole cell capacitance. A 1:1 correspondence linked changes in basal and total cell surface area, implying that cell surface area change is dominated by stretch of the attached basal surface. We also found that plasma membrane increased proportionally with surface area within 5 min of tonic stretch, showing that, given time to occur, plasma membrane expansion via lipid recruitment preponderates the changes in cell surface shape and size demanded by stretching the cell. Similarly, in cells tonically stretched 10 min to allow lipid insertion and then returned to an unstretched state, reabsorption of excess lipid occurred within 5 min. Finally, we found that lipid insertion induced by tonic stretch was unaffected by F-actin disassembly, ATP depletion, and calcium deprivation.

Uptake and rapid transfer of fluorescent ceramide analogues to acidosomes (late endosomes) in Paramecium

The ciliated protozoan Paramecium incorporates sphingolipids into its cell membranes. However, it is still unclear if these sphingolipids are metabolically synthesized in the cell or if their precursors are taken up from exogenous materials. Here we studied the route of uptake of fluorescence-labeled analogues of ceramide. Fluorescent ceramide was taken up rapidly independent of phagosome formation. Cold treatment caused a decrease in uptake, while reduction in the amount of cytosolic ATP induced by NaN(3) and deoxyglucose resulted in accumulation without internalization of fluorescence at the plasma membrane. These results suggest that uptake of fluorescent ceramide occurs at the plasma membrane, that it is an ATP-dependent process, and that it is not a result of simple diffusion. At first intracellular fluorescence appeared principally in the posterior half of the cell and then spread throughout the cytosol. In particular, a high accumulation of fluorescence occurred in association with acidosomes (late endosome or multivesicular body-like vesicles) that bind to the surface of nascent and young phagosomes. Therefore, in the Paramecium cell a significant proportion of ceramide apparently enters the cell by endocytosis and is quickly relayed to acidosomes along the endocytic pathway before becoming part of the digestive vacuole (phagoacidosome) membrane.

The role of de novo ceramide synthesis in the mechanism of action of the tricyclic xanthate D609

The cytotoxic effects of several chemotherapeutic drugs have been linked to elevated de novo ceramide biosynthesis. However, the relationship between the intracellular site(s) of ceramide accumulation and cytotoxicity is poorly understood. Here we examined the relationship between the site of ceramide deposition and inhibition of protein translation and induction of apoptosis by the antitumor/antiviral xanthate, D609. In Chinese hamster ovary (CHO)-K1, HEK-293, and NIH-3T3 cells, D609 caused rapid (1-5 min) and sustained eukaryotic initiation factor 2alpha (eIF2alpha) phosphorylation followed by apoptosis after 24 h. Concurrently, D609 stimulated de novo ceramide synthesis and increased ceramide mass 2-fold by 2 h in CHO-K1 cells. In D609-treated CHO-K1 cells, sphingomyelin synthesis was stimulated by brefeldin A, and C5-DMB-ceramide transport to the Golgi apparatus was blocked, indicating ceramide accumulation in the endoplasmic reticulum (ER). However, D609-mediated eIF2alpha phosphorylation, inhibition of protein synthesis, and apoptosis in CHO-K1 cells were not attenuated by fumonisin B1 or l-cycloserine. Interestingly, short-chain ceramide promoted eIF2alpha phosphorylation and inhibited protein synthesis in CHO-K1 cells, indicating that the effectiveness of endogenous ceramide could be limited by access to signaling pathways. Thus, expansion of the ER ceramide pool by D609 was not implicated in early (eIF2alpha phosphorylation) or late (apoptotic) cytotoxic events.

Pneumocystis carinii cell wall beta-glucans initiate macrophage inflammatory responses through NF-kappaB activation

beta-Glucans are major structural components of fungi. We have recently reported that the pathogenic fungus Pneumocystis carinii assembles a beta-glucan-rich cell wall that potently activates alveolar macrophages to release pro-inflammatory cytokines and chemokines. Purified P. carinii beta-glucans predictably induce both cytokine generation and associated neutrophilic lung inflammation. Herein, we demonstrate that P. carinii beta-glucan-induced macrophage stimulation results from activation of NF-kappaB. Although analogous to macrophage activation induced by bacterial lipopolysaccharide (LPS), P. carinii beta-glucan-induced macrophage NF-kappaB activation exhibits distinctly different kinetics, with slower induction and longer duration compared with LPS stimulation. Macrophage activation in response to P. carinii beta-glucan was also substantially inhibited with the NF-kappaB antagonist pyrrolidine dithiocarbamate. In addition to different kinetics of NF-kappaB activation, P. carinii beta-glucan and LPS also utilize different receptor systems to induce macrophage activation. Macrophages from Toll-like receptor 4-deficient and wild type mice produced equivalent amounts of tumor necrosis factor alpha when stimulated with P. carinii beta-glucan. However, Toll-like receptor 4-deficient macrophages were refractory to stimulation with LPS. In contrast, MyD88-deficient macrophages exhibited a significant (though partial) blunted response to P. carinii beta-glucan. These data demonstrate that P. carinii beta-glucan acts as potent inducer of macrophage activation through NF-kappaB utilizing cellular receptors and signaling pathways distinct from LPS.

Sphingolipid storage induces accumulation of intracellular cholesterol by stimulating SREBP-1 cleavage

We showed previously that the intracellular transport of sphingolipids (SLs) is altered in SL storage disease fibroblasts, due in part to the secondary accumulation of free cholesterol. In the present study we examined the mechanism of cholesterol elevation in normal human skin fibroblasts induced by treatment with SLs. When cells were incubated with various natural SLs for 44 h, cholesterol levels increased 25-35%, and cholesterol esterification was reduced. Catabolism of the exogenous SLs was not required for elevation of cholesterol because (i) a non-hydrolyzable and a degradable SL analog elevated cellular cholesterol to similar extents, and (ii) incubation of cells with various SL catabolites, including ceramide, had no effect on cholesterol levels. Elevated cholesterol was derived primarily from low density lipoproteins (LDL) and resulted from up-regulation of LDL receptors induced by cleavage of the sterol regulatory element-binding protein-1. Upon SL treatment, cholesterol accumulated with exogenous SLs in late endosomes and lysosomes. These results suggest a model in which excess SLs present in endocytic compartments serve as a "molecular trap" for cholesterol, leading to a reduction in cholesterol at the endoplasmic reticulum, induction of sterol regulatory element-binding protein-1 cleavage, and up-regulation of LDL receptors.

Future perspectives for glycolipid research in medicine

Medical interest in glycolipids has been mainly directed to the rare and complex glycosphingolipid storage disorders that are principally caused by unitary deficiencies of lysosomal acid hydrolases. However, glycolipids are critical components of cell membranes and occur within newly described membrane domains known as lipid rafts. Glycolipids are components of important antigen systems and membrane receptors; they participate in intracellular signalling mechanisms and may be presented to the immune system in the context of the novel CD1 molecules present on T lymphocytes. A knowledge of their mechanism of action in the control of cell growth and survival as well as developmental pathways is likely to shed light on the pathogenesis of the glycosphingolipid storage disorders as well as the role of lipid second messengers in controlling cell mobility and in the mobilization of intracellular calcium stores (a biological role widely postulated particularly for the lysosphingolipid metabolite sphingosine 1-phosphate). Other sphingolipid metabolites such as ceramide 1-phosphate may be involved in apoptotic responses and in phagocytosis and synaptic vesicle formation. The extraordinary pharmaceutical success of enzymatic complementation for Gaucher's disease using macrophage-targeted human glucocerebrosidase has focused further commercial interest in other glycolipid storage diseases: the cost of targeted enzyme therapy and its failure to restore lysosomal enzymatic deficiencies in the brain has also stimulated interest in the concept of substrate reduction therapy using diffusible inhibitory molecules. Successful clinical trials of the iminosugar N-butyldeoxynojirimycin in type 1 Gaucher's disease prove the principle of substrate reduction therapy and have attracted attention to this therapeutic method. They will also foster important further experiments into the use of glycolipid synthesis inhibitors for the severe neuronopathic glycosphingolipidoses, for which no definitive treatment is otherwise available. Future glycolipid research in medicine will be directed to experiments that shed light on the role of sphingolipids in signalling pathways, and in the comprehensive characterization and their secretory products in relation to the molecular pathogenesis of the storage disorders; experiments of use to improve the efficiency of complementing enzymatic delivery to the lysosomal compartment of storage cells are also needed. Further systematic screening for inhibitory compounds with specific actions in the pathways of glycosphingolipid biosynthesis will undoubtedly lead to clinical trials in the neuronopathic storage disorders and to wider applications in the fields of immunity and cancer biology.

Substrate reduction therapy: miglustat as a remedy for symptomatic patients with Gaucher disease type 1

Gaucher disease (GD) is an inborn error of glycosphingolipid metabolism, associated with multisystemic manifestations resulting from the lysosomal accumulation of an incompletely degraded material (glucosylceramide) within cells of monocyte/macrophage lineage. In the majority of GD patients with non-neurological involvement (type 1), the clinical features are dominated by haematological, visceral (hepatic and splenic) and skeletal complications. Infrequently, cardiopulmonary involvement may develop and lead to major morbidity. Investigations of substrate turnover within cellular compartments, such as the lysosome, indicate that progressive tissue storage occurs when the activity of the responsible hydrolytic enzyme declines. Thus, the provision of functional enzyme to deficient cells is a straightforward means of achieving metabolic correction. Conversely, the concentration of stored substrate within cells bearing particular affected enzymes (i.e., expressing residual activity) may be controlled by a reduction of the 'load'. This means that metabolic homeostasis can also be restored by restricting the amount of substrate presented to disease cells to a level that can be hydrolysed by the existing enzyme activity. Proof-of-concept for both approaches has been demonstrated through enzyme replacement therapy using alglucerase/imiglucerase and more recently, substrate reduction therapy by miglustat. Enzyme replacement therapy is proven to be safe and effective in the treatment of GD type 1, establishing imiglucerase as the current standard of care. The experience with substrate reduction therapy, specifically miglustat, is limited but encouraging. This review is an attempt to examine the potential role of this latter approach in light of current patient management. The consideration of miglustat as a therapeutic option requires the appropriate selection of patients (amongst those unwilling or unsuitable to receive enzyme replacement therapy), a definition of the therapeutic objectives and monitoring not only for response but potential adverse effects.

Clathrin-dependent and -independent internalization of plasma membrane sphingolipids initiates two Golgi targeting pathways

Sphingolipids (SLs) are plasma membrane constituents in eukaryotic cells which play important roles in a wide variety of cellular functions. However, little is known about the mechanisms of their internalization from the plasma membrane or subsequent intracellular targeting. We have begun to study these issues in human skin fibroblasts using fluorescent SL analogues. Using selective endocytic inhibitors and dominant negative constructs of dynamin and epidermal growth factor receptor pathway substrate clone 15, we found that analogues of lactosylceramide and globoside were internalized almost exclusively by a clathrin-independent ("caveolar-like") mechanism, whereas an analogue of sphingomyelin was taken up approximately equally by clathrin-dependent and -independent pathways. We also showed that the Golgi targeting of SL analogues internalized via the caveolar-like pathway was selectively perturbed by elevated intracellular cholesterol, demonstrating the existence of two discrete Golgi targeting pathways. Studies using SL-binding toxins internalized via clathrin-dependent or -independent mechanisms confirmed that endogenous SLs follow the same two pathways. These findings (a) provide a direct demonstration of differential SLs sorting into early endosomes in living cells, (b) provide a "vital marker" for endosomes derived from caveolar-like endocytosis, and (c) identify two independent pathways for lipid transport from the plasma membrane to the Golgi apparatus in human skin fibroblasts.

Niemann-Pick C variant detection by altered sphingolipid trafficking and correlation with mutations within a specific domain of NPC1

Niemann-Pick disease type C (NPC) is a fatal, autosomal recessive lipidosis characterized by lysosomal accumulation of unesterified cholesterol and multiple neurological symptoms, such as vertical supranuclear ophthalmoplegia, progressive ataxia, and dementia. More than 90% of cases of NPC are due to a defect in Niemann-Pick C1 (NPC1), a late endosomal, integral membrane protein that plays a role in cholesterol transport or homeostasis. Biochemical diagnosis of NPC has relied on the use of patient skin fibroblasts in an assay to demonstrate delayed low-density lipoprotein (LDL)-derived cholesterol esterification and a cytological technique-filipin staining-to demonstrate the intracellular accumulation of cholesterol. A small percentage of patients, referred to as "NPC variants," present with clinical symptoms of NPC but show near-normal results of these biochemical tests, making laboratory confirmation of NPC disease problematic. Here, we demonstrate that NPC-variant fibroblast samples can be detected as sphingolipid storage disease cells, using a fluorescent sphingolipid analog, BODIPY-lactosylceramide. This lipid accumulated in endosomes/lysosomes in variant cells preincubated with LDL cholesterol but targeted to the Golgi complex in normal cells under these conditions. The reproducibility of this technique was validated in a blinded study. In addition, we performed mutation analysis of the NPC1 gene in NPC variant and "classical" NPC cell samples and found a high incidence of specific mutations within the cysteine-rich region of NPC1 in variants. We also found that 5 of the 12 variant cell samples had no apparent defect in NPC1 but were otherwise indistinguishable from other variant cells. This is a surprising result, since, in general, approximately 90% of patients with NPC possess defects in NPC1. Our findings should be useful for the detection of NPC variants and also may provide significant new insight regarding NPC1 genotype/phenotype correlations.

Membrane traffic in sphingolipid storage diseases

In this review, we summarize our studies of membrane lipid transport in sphingolipid storage disease (SLSD) fibroblasts. We recently showed that several fluorescent SL analogs were internalized from the plasma membrane predominantly to the Golgi complex of normal cells, while in ten different SLSD cell types, these lipids accumulated in endosomes and lysosomes (The Lancet 1999;354: 901-905). Additional studies showed that cholesterol homeostasis is perturbed in multiple SLSDs secondary to SL accumulation and that mistargeting of SL analogs was regulated by cholesterol (Nature Cell Biol 1999;1: 386-388). Based on these findings, we hypothesize that endogenous sphingolipids, which accumulate in SLSD cells due to primary defects in lipid catabolism, result in an altered intracellular distribution of cholesterol, and that this alteration in membrane composition then results in defective sorting and transport of SLs. The importance of SL/cholesterol interactions and potential mechanisms underlying the regulation of lipid transport and targeting are also discussed. These studies suggest a new paradigm for regulation of membrane lipid traffic along the endocytic pathway and could have important implications for future studies of protein trafficking as well as lipid transport. This work may also lead to important future clinical developments (e.g. screening tests for SLSD, new methodology for screening drugs which abrogate lipid storage, and possible therapeutic approaches to SLSD).

Internalization and sorting of plasma membrane sphingolipid analogues in differentiating oligodendrocytes

We studied the formation of early endosomes in differentiating oligodendrocytes and type-2 astrocytes, which are derived from common precursor cells in rat neonates, using fluorescent analogues of lactosylceramide (LacCer) and sulfatide labeled with 4,4-difluoro-5,7-dimethyl-4-bora-3a,4a-diaza-s-indacene++ +-3-pentanoic acid (BODIPY FL C5). These sphingolipid analogues exhibit a concentration-dependent shift in their fluorescence emission maximum from green to red wavelengths that can be used to estimate the relative concentration of an analogue in the intracellular membranes of living cells by quantitative fluorescence microscopy. When oligodendrocytes at various stages of differentiation were incubated with 1 microM BODIPY-sphingolipid at 10 degrees C and washed, yellow/green plasma membrane fluorescence was observed. Quantitative studies confirmed that the amount of BODIPY-LacCer or -sulfatide incorporated into the plasma membrane of a given cell type was identical. When these cells were subsequently warmed to 37 degrees C for 2-10 min to allow internalization to occur, the BODIPY-sphingolipid analogues were distributed in a punctate pattern throughout the cytoplasm. Within individual cells labeled with BODIPY-sulfatide, some endosomes exhibited green fluorescence, whereas others emitted red/orange fluorescence. In contrast, when BODIPY-LacCer was used, only green endosomes were observed. Although this phenomenon could be observed at earlier stages of differentiation, it was most obvious in mature oligodendrocytes, where quantitative measurements of the red/green ratio of individual endosomes suggested about a threefold difference between the concentration of the LacCer and sulfatide analogues in endosomes. These results suggest that "lipid sorting" takes place during endocytosis in mature oligodendrocytes, resulting in selective exclusion of certain lipid species during the internalization process. This sorting event may result in the net addition of lipids to the differentiated oligodendrocyte plasma membrane.