Separation and characterization of late endosomal membrane domains

New insights into the mechanisms of macroautophagy in mammalian cells

Macroautophagy is a self-digesting pathway responsible for the removal of long-lived proteins and organelles by the lysosomal compartment. Parts of the cytoplasm are first segregated in double-membrane-bound autophagosomes, which then undergo a multistep maturation process including fusion with endosomes and lysosomes. The segregated cytoplasm is then degraded by the lysosomal hydrolases. The discovery of ATG genes has greatly enhanced our understanding of the mechanisms of this pathway. Two novel ubiquitin-like protein conjugation systems were shown to function during autophagosome formation. Autophagy has been shown to play a role in a wide variety of physiological processes including energy metabolism, organelle turnover, growth regulation, and aging. Impaired autophagy can lead to diseases such as cardiomyopathy and cancer. This review summarizes current knowledge about the formation and maturation of autophagosomes, the role of macroautophagy in various physiological and pathological conditions, and the signaling pathways that regulate this process in mammalian cells.

Molecular imaging of membrane interfaces reveals mode of beta-glucosidase activation by saposin C

Acid beta-glucosidase (GCase) is a soluble lysosomal enzyme responsible for the hydrolysis of glucose from glucosylceramide and requires activation by the small nonenzymatic protein saposin C (sapC) to gain access to the membrane-embedded glycosphingolipid substrate. We have used in situ atomic force microscopy (AFM) with simultaneous confocal and epifluorescence microscopies to investigate the interactions of GCase and sapC with lipid bilayers. GCase binds to sites on membranes transformed by sapC, and enzyme activity occurs at loci containing both GCase and sapC. Using FRET, we establish the presence of GCase/sapC and GCase/product contacts in the bilayer. These data support a mechanism in which sapC locally alters regions of bilayer for subsequent attack by the enzyme in stably bound protein complexes.

Selective incorporation of docosahexaenoic acid into lysobisphosphatidic acid in cultured THP-1 macrophages

Lysobisphosphatidic acid (LBPA) is highly accumulated in specific domains of the late endosome and is involved in the biogenesis and function of this organelle. Little is known about the biosynthesis and metabolism of this lipid. We examined its FA composition and the incorporation of exogenous FA into LBPA in the human monocytic leukemia cell line THP-1. The LBPA FA composition in THP-1 cells exhibits an elevated amount of oleic acid (18:1n-9) and enrichment of PUFA, especially DHA (22:6n-3). DHA supplemented to the medium was efficiently incorporated into LBPA. In contrast, arachidonic acid (20:4n-6) was hardly esterified to LBPA under the same experimental conditions. The turnover of DHA in LBPA was similar to that in other phospholipids. Specific incorporation of DHA into LBPA was also observed in baby hamster kidney fibroblasts, although LBPA in these cells contains very low endogenous levels of DHA in normal growth conditions. Our resuIts, together with published observations, suggest that the specific incorporation of DHA into LBPA is a common phenomenon in mammalian cells. The physiological significance of DHA-enriched LBPA is discussed.

Late endosomal cholesterol accumulation leads to impaired intra-endosomal trafficking

Background

Pathological accumulation of cholesterol in late endosomes is observed in lysosomal storage diseases such as Niemann-Pick type C. We here analyzed the effects of cholesterol accumulation in NPC cells, or as phenocopied by the drug U18666A, on late endosomes membrane organization and dynamics.

Methodology/Principal Findings

Cholesterol accumulation did not lead to an increase in the raft to non-raft membrane ratio as anticipated. Strikingly, we observed a 2–3 fold increase in the size of the compartment. Most importantly, properties and dynamics of late endosomal intralumenal vesicles were altered as revealed by reduced late endosomal vacuolation induced by the mutant pore-forming toxin ASSP, reduced intoxication by the anthrax lethal toxin and inhibition of infection by the Vesicular Stomatitis Virus.

Conclusions/Significance

These results suggest that back fusion of intralumenal vesicles with the limiting membrane of late endosomes is dramatically perturbed upon cholesterol accumulation.

A novel role of the Batten disease gene CLN3: association with BMP synthesis

Juvenile Neuronal Ceroid Lipofuscinosis (JNCL) results from a deficiency of CLN3, a protein recently identified within detergent-resistant membranes (DRMs). To study the function of CLN3 within these domains we isolated DRMs from control and JNCL-brain and noted that JNCL-derived DRMs are less buoyant than control. Analysis of DRM phospholipids derived from JNCL-brain revealed a reduction of bis(monoacylglycerol)phosphate. Metabolic labeling of JNCL-fibroblasts demonstrated a reduction in the synthesis of bis(monoacylglycerol)phosphate which was restored following complementation with wild-type-CLN3, substantiating our initial observation in brain. Metabolic labeling of cell lines overexpressing wild-type-CLN3 resulted in increased bis(monoacylglycerol)phosphate synthesis, while overexpression of mutant CLN3-L170P decreased bis(monoacylglycerol)phosphate synthesis. These data illustrate a new finding, a strong correlation between CLN3 protein expression and synthesis of bis(monoacylglycerol)phosphate.

The lipofuscin fluorophore A2E perturbs cholesterol metabolism in retinal pigment epithelial cells

Proteins involved in cholesterol trafficking are known to contribute to the pathogenesis of atherosclerosis and Alzheimer's disease. Allelic variants in the cholesterol transporters apolipoprotein E and ATP-binding cassette protein A1 (ABCA1) have recently been associated with susceptibility to age-related macular degeneration (AMD). Histopathological analyses of eyes with AMD demonstrate the presence of cholesterol and cholesteryl ester deposits beneath the retinal pigment epithelium (RPE), implicating abnormal cholesterol trafficking in disease progression. Here, we show that A2E, a quaternary amine and retinoid by-product of the visual cycle, causes the accumulation of free and esterified cholesterol in RPE cells. The mechanism involves neither generalized alterations in late endosomal/lysosomal pH nor a direct inhibition of acid lipase activity. Rather, A2E prevents cholesterol efflux from these organelles, which in turn indirectly inhibits acid lipase, leading to a subsequent accumulation of cholesteryl esters. Transcriptional activation of the ABCA1 cholesterol transporter by agonists of the liver X receptor/peroxisome proliferator-activated receptor pathway relieves the A2E-induced block on cholesterol efflux and restores cholesterol homeostasis in RPE cells. Our data also demonstrate that A2E, which is a cone-shaped lipid, increases the chemical activity and displacement of cholesterol from model membranes, providing a biophysical mechanism for cholesterol sequestration in A2E-loaded cells. Although endogenously produced A2E in the RPE has been associated with macular degeneration, the precise mechanisms are unclear. Our results provide direct evidence that A2E causes aberrant cholesterol metabolism in RPE cells which could likely contribute to AMD progression.

De novo biosynthesis of the late endosome lipid, bis(monoacylglycero)phosphate

Bis(monoacylglycero)phosphate (BMP) is a unique lipid enriched in the late endosomes participating in the trafficking of lipids and proteins through this organelle. The de novo biosynthesis of BMP has not been clearly demonstrated. We investigated whether phosphatidylglycerol (PG) and cardiolipin (CL) could serve as precursors of de novo BMP synthesis using two different cellular models: CHO cells deficient in phosphatidylglycerophosphate (PGP) synthase, the enzyme responsible for the first step of PG synthesis; and human lymphoblasts from patients with Barth syndrome (BTHS), characterized by mutations in tafazzin, an enzyme implicated in the deacylation-reacylation cycle of CL. The biosynthesis of both PG and BMP was reduced significantly in the PGP synthase-deficient CHO mutants. Furthermore, overexpression of PGP synthase in the deficient mutants induced an increase of BMP biosynthesis. In contrast to CHO mutants, BMP biosynthesis and its fatty acid composition were not altered in BTHS lymphoblasts. Our results thus suggest that in mammalian cells, PG, but not CL, is a precursor of the de novo biosynthesis of BMP. Despite the decrease of de novo synthesis, the cellular content of BMP remained unchanged in CHO mutants, suggesting that other pathway(s) than de novo biosynthesis are also used for BMP synthesis.

Hepatitis B virus maturation is sensitive to functional inhibition of ESCRT-III, Vps4, and gamma 2-adaptin

Hepatitis B virus (HBV) is an enveloped DNA virus that presumably buds at intracellular membranes of infected cells. HBV budding involves two endocytic host proteins, the ubiquitin-interacting adaptor gamma 2-adaptin and the Nedd4 ubiquitin ligase. Here, we demonstrate that HBV release also requires the cellular machinery that generates internal vesicles of multivesicular bodies (MVBs). In order to perturb the MVB machinery in HBV-replicating liver cells, we used ectopic expression of dominant-negative mutants of different MVB components, like the ESCRT-III complex-forming CHMP proteins and the Vps4 ATPases. Upon coexpression of mutated CHMP3, CHMP4B, or CHMP4C forms, as well as of ATPase-defective Vps4A or Vps4B mutants, HBV assembly and egress were potently blocked. Each of the MVB inhibitors arrested virus particle maturation by entrapping the viral core and large and small envelope proteins in detergent-insoluble membrane structures that closely resembled aberrant endosomal class E compartments. In contrast, HBV subvirus particle release was not affected by MVB inhibitors, hinting at different export routes used by viral and subviral particles. To further define the role gamma 2-adaptin plays in HBV formation, we examined the effects of its overexpression in virus-replicating cells. Intriguingly, excess gamma 2-adaptin blocked HBV production in a manner similar to the actions of CHMP and Vps4 mutants. Moreover, overexpressed gamma 2-adaptin perturbed the endosomal morphology and diminished the budding of a retroviral Gag protein, implying that it may act as a principal inhibitor of the MVB sorting pathway. Together, these results demonstrate that HBV exploits the MVB machinery with the aid of gamma 2-adaptin.

The multiple personalities of Alix

Alix is a cytosolic protein in mammalian cells that was originally identified on the basis of its association with pro-apoptotic signaling. More recent evidence has established that Alix has a hand in regulating other cellular mechanisms, including endocytic membrane trafficking and cell adhesion. Although Alix appears to participate directly in these various activities, the role it plays in each process has largely been inferred from the functions of proteins with which it interacts. For example, recruitment of Alix to endosomes is mediated by its N-terminal Bro1 domain, the structure of which was recently solved for its yeast orthologue, Bro1. The diversity of Alix functions is due to its proline-rich C-terminus, which provides multiple protein-binding sites. With this blueprint in hand, we can now ask whether Alix acts simply as an adaptor that links different proteins into networks or, instead, contributes a specific function to distinct molecular machineries.

The vaccinia virus F13L YPPL motif is required for efficient release of extracellular enveloped virus

The Tyr-X-X-Leu (YxxL) motif of the vaccinia virus F13L protein was examined for late (L) domain activity. The ability of an F13L deletion virus to form plaques was restored by PCR products containing single alanine substitutions within the motif and a YAAL construct but not by constructs lacking both the Y and L residues. Recombinant viruses possessing alanine substitutions in place of the tyrosine or the leucine residue in the YxxL motif demonstrated small, asymmetrical plaques. RNA interference-dependent depletion of Alix and TSG101 (host proteins involved in L domain-dependent protein trafficking) diminished extracellular enveloped virion production to various degrees, suggesting that the YxxL motif is a genuine L domain.

Diversity of raft-like domains in late endosomes

BACKGROUND

Late endosomes, the last sorting station in the endocytic pathway before lysosomes, are pleiomorphic organelles composed of tubular elements as well as vesicular regions with a characteristic multivesicular appearance, which play a crucial role in intracellular trafficking. Here, we have investigated whether, in addition to these morphologically distinguishable regions, late endosomal membranes are additionally sub-compartmentalized into membrane microdomains.

METHODOLOGY/PRINCIPAL FINDINGS

Using sub-organellar fractionation techniques, both with and without detergents, combined with electron microscopy, we found that both the limiting membrane of the organel and the intraluminal vesicles contain raft-type membrane domains. Interestingly, these differentially localized domains vary in protein composition and physico-chemical properties.

CONCLUSIONS/SIGNIFICANCE

In addition to the multivesicular organization, we find that late endosomes contain cholesterol rich microdomains both on their limiting membrane and their intraluminal vesicles that differ in composition and properties. Implications of these findings for late endosomal functions are discussed.

Tetraspanins as regulators of protein trafficking

Small transmembrane proteins of the tetraspanin superfamily are believed to function as the main structural blocks of specialized membrane microdomains (referred to as tetraspanin-enriched microdomains, TERM or TEM). Through a multitude of homotypic and heterotypic interactions, tetraspanins regulate lateral clustering and, consequently, signalling involving adhesion and growth factor receptors as well as costimulatory proteins. The presence of major histocompatibility complex (MHC) I and MHCII molecules in TERM led to suggestion of tetraspanins' involvement in antigen presentation. In addition, certain tetraspanins function as viral co-receptors and may be important for viral egress from infected cells. It has recently become apparent that in addition to their purely structural function as organizers of TERM, tetraspanins also regulate various aspects of trafficking and biosynthetic processing of associated receptors. Here, we review recent studies, which specifically focus on this issue.

Biogenesis and function of multivesicular bodies

The two major cellular sites for membrane protein degradation are the proteasome and the lysosome. Ubiquitin attachment is a sorting signal for both degradation routes. For lysosomal degradation, ubiquitination triggers the sorting of cargo proteins into the lumen of late endosomal multivesicular bodies (MVBs)/endosomes. MVB formation occurs when a portion of the limiting membrane of an endosome invaginates and buds into its own lumen. Intralumenal vesicles are degraded when MVBs fuse to lysosomes. The proper delivery of proteins to the MVB interior relies on specific ubiquitination of cargo, recognition and sorting of ubiquitinated cargo to endosomal subdomains, and the formation and scission of cargo-filled intralumenal vesicles. Over the past five years, a number of proteins that may directly participate in these aspects of MVB function and biogenesis have been identified. However, major questions remain as to exactly what these proteins do at the molecular level and how they may accomplish these tasks.

Inefficient targeting to the endoplasmic reticulum by the signal recognition particle elicits selective defects in post-ER membrane trafficking

The signal recognition particle (SRP) is required for protein translocation into the endoplasmic reticulum (ER). With RNA interference we reduced its level about ten-fold in mammalian cells to study its cellular functions. Such low levels proved insufficient for efficient ER-targeting, since the accumulation of several proteins in the secretory pathway was specifically diminished. Although the cells looked unaffected, they displayed noticeable and selective defects in post-ER membrane trafficking. Specifically, the anterograde transport of VSV-G and the retrograde transport of the Shiga toxin B-subunit were stalled at the level of the Golgi whereas the endocytosed transferrin receptor failed to recycle to the plasma membrane. Endocytic membrane trafficking from the plasma membrane to lysosomes or Golgi was undisturbed and major morphological changes in the ER and the Golgi were undetectable at low resolution. Selective membrane trafficking defects were specifically suppressed under conditions when low levels of SRP became sufficient for efficient ER-targeting and are therefore a direct consequence of the lower targeting capacity of cells with reduced SRP levels. Selective post-ER membrane trafficking defects occur at SRP levels sufficient for survival suggesting that changes in SRP levels and their effects on post-ER membrane trafficking might serve as a mechanism to alter temporarily the localization of selected proteins.

Exosome lipidomics unravels lipid sorting at the level of multivesicular bodies

Exosomes are part of the family of "bioactive vesicles" and appear to be involved in distal communications between cells. They vehiculate bioactive lipids and lipolytic enzymes and their biogenesis require specific lipids and a membrane reorganisation. Their biogenesis pathway could be a way to secrete enzymes involved in lipid signalling and to generate "particulate agonists". However, this pathway seems also to be used by pathogens such as HIV. This review will consider several aspects of lipidomics studies which might help to understand the fate and role of these fascinating vesicles.

Cell entry and cAMP imaging of anthrax edema toxin

The entry and enzymatic activity of the anthrax edema factor (EF) in different cell types was studied by monitoring EF-induced changes in intracellular cAMP with biochemical and microscopic methods. cAMP was imaged in live cells, transfected with a fluorescence resonance energy transfer biosensor based on the protein kinase A regulatory and catalytic subunits fused to CFP and YFP, respectively. The cAMP biosensor was located either in the cytosol or was membrane-bound owing to the addition of a tag determining its myristoylation/palmitoylation. Real-time imaging of cells expressing the cAMP biosensors provided the time course of EF catalytic activity and an indication of its subcellular localization. Bafilomycin A1, an inhibitor of the vacuolar ATPase proton pump, completely prevented EF activity, even when added long after the toxin. The time course of appearance of the adenylate cyclase activity and of bafilomycin A1 action suggests that EF enters the cytosol from late endosomes. EF remains associated to these compartments and its activity shows a perinuclear localization generating intracellular cAMP concentration gradients from the cell centre to the periphery.

pH-dependent formation of membranous cytoplasmic body-like structure of ganglioside G(M1)/bis(monoacylglycero)phosphate mixed membranes

Membrane structures of the mixtures of ganglioside G(M1) and endosome specific lipid, bis (monoacylglycero) phosphate (BMP, also known as lysobisphosphatidic acid) were examined at various pH conditions by freeze-fracture electron microscopy and small-angle x-ray scattering. At pH 8.5-6.5, a G(M1)/BMP (1:1 mol/mol) mixture formed small vesicular aggregates, whereas the mixture formed closely packed lamellar structures under acidic conditions (pH 5.5, 4.6) with the lamellar repeat distance of 8.06 nm. Since BMP alone exhibits a diffuse lamellar structure at a broad range of pH values and G(M1) forms a micelle, the results indicate that both G(M1) and BMP are required to produce closely stacked multilamellar vesicles. These vesicles resemble membranous cytoplasmic bodies in cells derived from patients suffering from G(M1) gangliosidosis. Similar to G(M1) gangliosidosis, cholesterol was trapped in BMP vesicles in G(M1)- and in a low pH-dependent manner. Studies employing different gangliosides and a G(M1) analog suggest the importance of sugar chains and a sialic acid of G(M1) in the pH-dependent structural change of G(M1)/BMP membranes.

Lipid rafts: new tools and a new component

Lipid rafts are liquid ordered membrane domains enriched with sphingolipids and cholesterol. After 20 years since the proposal of the original concept, the structure and function of lipid rafts are still obscure. Recently new tools to study lipid rafts have been developed. Lysenin is a sphingomyelin binding protein that specifically recognizes the lipid clusters. Poly(ethyleneglycol)-derivatized cholesterol ether (PEG-Chol) is a non-toxic cholesterol probe. These probes have revealed the heterogeneity of lipid rafts. The heterogeneity of lipid rafts is further supported by the discovery of a new lipid component, phosphatidylglucoside. Metabolic inhibitors are another useful tool. Sulfamisterin is a new addition to the serine palmitoyltransferase inhibitors. Recent findings have uncovered a previously unrecognized activity of a glycosphingolipid synthesis inhibitor, D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (D-PDMP).

Intra-endosomal membrane traffic

Following endocytosis, ubiquitinated signaling receptors are incorporated within intraluminal vesicles of forming multivesicular endosomes. These vesicles then follow the pathway from early to late endosomes, remaining within the endosomal lumen, and are eventually delivered to lysosomes, where they are degraded together with their protein cargo. However, intraluminal vesicles do not always end up in lysosomes for degradation; they can also fuse back with the limiting membrane of late endosomes. This route, which might be regulated by lyso-bisphosphatidic acid and its putative effector Alix, can be hijacked by the anthrax toxin and vesicular stomatitis virus and is presumably exploited by proteins and lipids that transit through intraluminal vesicles. Alternatively, these vesicles can be released extracellularly, like HIV in macrophages, upon fusion of endosomes or lysosomes with the plasma membrane.

A new role of diacylglycerol kinase alpha on the secretion of lethal exosomes bearing Fas ligand during activation-induced cell death of T lymphocytes

Exosomes are small membrane vesicles that intracellularly accumulate into late or multivesicular endosomes (multivesicular bodies, MVB). Exosomes have a particular lipid and protein content, reflecting their origin as intraluminal vesicles of late endosomes. The stimulation of several hematopoietic cells induces the fusion of the limiting membrane of the MVB with the plasma membrane, leading to the release of exosomes towards the extracellular environment. In T lymphocytes, stimulation of the T cell receptor (TCR) induces the fusion of the MVBs with the plasma membrane and exosomes carrying several bio-active proteins are secreted. Among these proteins, the pro-apoptotic protein Fas ligand (FasL) is released as a non-proteolysed form (mFasL), associated to the exosomes. These mFasL-bearing exosomes may trigger the apoptosis of T lymphocytes. Here, we present evidences supporting a role of diacylglycerol kinase alpha (DGKalpha), a diacylglycerol (DAG)-consuming enzyme, on the secretion of exosomes carrying mFasL, and the subsequent activation-induced cell death (AICD) on a T cell line and primary T lymphoblasts.

Localized PtdIns 3,5-P2synthesis to regulate early endosome dynamics and fusion

Perturbations in the intracellular PtdIns 3,5-P2pool or the downstream transmission of PtdIns 3,5-P2signals often result in a gradual development of gross morphological changes in the pleiomorphic multivesicular endosomes, culminating with the appearance of cytoplasmic vacuoles. To identify the onset of PtdIns 3,5-P2functional requirements along the endocytic system, in this study we characterized the morphological changes associated with early expression of the dominant-negative kinase-deficient form (K1831E) of the PtdIns 3,5-P2-producing kinase PIKfyve, before the formation of cytoplasmic vacuoles in transfected COS cells. Enlarged PIKfyveK1831E-positive vesicles co-localizing with dilated EEA1- and Rab5aWT-positive perinuclear endosomes were observed (WT, wild type). This was dependent on the presence of active forms of Rab5 and the generation of PtdIns 3-P-enriched platforms on early endosomess. Because PIKfyveWTdid not substantially colocalize with EEA1- or Rab5-positive endosomes in COS cells, the dynamic PIKfyve-catalyzed PtdIns 3-to-PtdIns 3,5-P2switch was suggested to drive away PIKfyveWTfrom early endosomes toward later compartments. Late endosomes/lysosomes marked by LAMP1 or Rab7 were dislocated from their typical perinuclear position upon PIKfyveK1831Eearly expression. Cytosols derived from cells stably expressing PIKfyveK1831Estimulated endosome fusion in vitro, whereas PIKfyveWT-enriched cytosols had the opposite effect, consistent with PtdIns 3,5-P2production negatively regulating the endosome fusion. Together, our data indicate that PtdIns 3,5-P2defines specific endosome platforms at the onset of the degradation pathway to regulate the complex process of membrane remodeling and dynamics.

Concise synthesis of ether analogues of lysobisphosphatidic acid

We describe a versatile, efficient method for the preparation of ether analogues of (S,S)-lysobisphosphatidic acid (LBPA) and its enantiomer from (S)-solketal. Phosphorylation of a protected sn-2-O-octadecenyl glyceryl ether with 2-cyanoethyl bis-N,N-diisopropylamino phosphine and subsequent deprotection generated the bisether LBPA analogues. By simply changing the sequence of deprotection steps, we obtained the (R,R)- and (S,S)-enantiomers of 2,2'-bisether LBPA. An ELISA assay with anti-LBPA monoclonal antibodies showed that the bisether LBPAs were recognized with the same affinity as the natural 2,2'-bisoleolyl LBPA. [reaction: see text]

Molecular mechanisms of late endosome morphology, identity and sorting

Recent studies using electron microscopy, protein crystallography, classic biochemistry and novel live-cell imaging have provided numerous insights into the endocytic pathway, describing a dynamic system in which compartment morphology, molecular identity and the mechanics of cargo sorting are intimately connected. Current evidence supports a model of maturation in which the lipids, cargo proteins and Rab population at the endosome determine its competence to perform the functions of late endosomes, including the sorting of cargoes into lumenal vesicles and fusion with lysosomes.

Mechanisms of pathogen entry through the endosomal compartments

Several pathogens - bacteria, viruses and parasites - must enter mammalian cells for survival, replication and immune-system evasion. These pathogens generally make use of existing cellular pathways that are designed for nutrient uptake, receptor downregulation and signalling. Because most of these pathways end in lysosomes, an organelle that is capable of killing microorganisms, pathogens have developed remarkable means to avoid interactions with this lytic organelle.

Roles of endogenously synthesized sterols in the endocytic pathway

The effect(s) of endogenously synthesized cholesterol (endo-CHOL) on the endosomal system in mammalian cells has not been examined. Here we treated Chinese hamster ovary cell lines with lovastatin (a hydroxymethylglutaryl-CoA reductase inhibitor) and mevalonate (a precursor for isoprenoids) to block endo-CHOL synthesis and then examined its effects on the fate of cholesterol liberated from low density lipoprotein (LDL-CHOL). The results showed that blocking endo-CHOL synthesis for 2 h or longer does not impair the hydrolysis of cholesteryl esters but partially impairs the transport of LDL-CHOL to the plasma membrane. Blocking endo-CHOL synthesis for 2 h or longer also alters the localization patterns of the late endosomes/lysosomes and retards their motility, as monitored by time-lapse microscopy. LDL-CHOL overcomes the effect of blocking endo-CHOL synthesis on endosomal localization patterns and on endosomal motility. Overexpressing Rab9, a key late endosomal small GTPase, relieves the endosomal cholesterol accumulation in Niemann-Pick type C1 cells but does not revert the reduced endosomal motility caused by blocking endo-CHOL synthesis. Our results suggested that endo-CHOL contributes to the cholesterol content of late endosomes and controls its motility, in a manner independent of NPC1. These results also supported the concept that endosomal motility plays an important role in controlling cholesterol trafficking activities.

Trafficking from CD63-positive late endocytic multivesicular bodies is essential for intracellular development of Chlamydia trachomatis

Chlamydiae are obligate intracellular bacterial pathogens that replicate solely within the confines of a membrane-bound vacuole termed an inclusion. Within this protected organelle, chlamydiae acquire host-cell-derived biosynthetic precursors necessary for intracellular subsistence, yet the mechanisms and pathways responsible for this acquisition remain elusive. The present study identifies an interaction between the chlamydial inclusion and multivesicular bodies, complex organelles pivotal in protein and lipid transport that are positioned along the endosome-lysosome pathway, and intersect the exocytic pathway in various cell types. Resident protein and lipid constituents of multivesicular bodies colocalized with intracellular chlamydiae, with direct delivery of the resident protein CD63 to the chlamydial inclusion. Interruption of trafficking from multivesicular bodies by pharmacological inhibitors and exogenous antibodies subsequently disrupted sphingolipid delivery to the maturing chlamydial inclusion and intracellular bacterial growth. This study identifies a trafficking pathway from CD63-positive multivesicular bodies to the bacterial inclusion, a novel interaction that provides essential lipids necessary for maintenance of a productive intracellular infection.

Venezuelan equine encephalitis virus entry mechanism requires late endosome formation and resists cell membrane cholesterol depletion

Virus envelope proteins determine receptor utilization and host range. The choice of receptor not only permits specific targeting of cells that express it, but also directs the virus into specific endosomal trafficking pathways. Disrupting trafficking can result in loss of virus infectivity due to redirection of virions to non-productive pathways. Identification of the pathway or pathways used by a virus is, thus, important in understanding virus pathogenesis mechanisms and for developing new treatment strategies. Most of our understanding of alphavirus entry has focused on the Old World alphaviruses, such as Sindbis and Semliki Forest virus. In comparison, very little is known about the entry route taken by more pathogenic New World alphaviruses. Here, we use a novel contents mixing assay to identify the cellular requirements for entry of a New World alphavirus, Venezuelan equine encephalitis virus (VEEV). Expression of dominant negative forms of key endosomal trafficking genes shows that VEEV must access clathrin-dependent endocytic vesicles for membrane fusion to occur. Unexpectedly, the exit point is different from Old World alphaviruses that leave from early endosomes. Instead, VEEV also requires functional late endosomes. Furthermore, unlike the Old World viruses, VEEV entry is insensitive to cholesterol sequestration from cell membranes and may reflect a need to access an endocytic compartment that lacks cholesterol. This indicates fundamental differences in the entry route taken by VEEV compared to Old World alphaviruses.

Human autoantibodies to diacyl-phosphatidylethanolamine recognize a specific set of discrete cytoplasmic domains

The aim of this study was to characterize a novel human autoantibody-autoantigen system represented as cytoplasmic discrete speckles (CDS) in indirect immunofluorescence (IIF). A distinct CDS IIF pattern represented by 3-20 discrete speckles dispersed throughout the cytoplasm was identified among other cytoplasmic speckled IIF patterns. The cytoplasmic domains labelled by human anti-CDS-1 antibodies did not co-localize with endosome/lysosome markers EEA1 and LAMP-2, but showed partial co-localization with glycine-tryptophan bodies (GWB). CDS-1 sera did not react with several cellular extracts in immunoblotting and did not immunoprecipitate recombinant GW182 or EEA1 proteins. The typical CDS-1 IIF labelling pattern was abolished after delipidation of HEp-2 cells. Moreover, CDS-1 sera reacted strongly with a lipid component co-migrating with phosphatidylethanolamine (PE) in high performance thin-layer chromatography (HPTLC)-immunostaining of HEp-2 cell total lipid extracts. The CDS-1 major molecular targets were established by electrospray ionization-mass spectrometry (ESI-MS), HPTLC-immunostaining and chemiluminescent enzyme-linked immunosorbent assay as diacyl-PE species, containing preferentially a cis-C18 : 1 fatty acid chain at C-2 of the glycerol moiety, namely 1,2-cis-C18 : 1-PE and 1-C16 : 0-2-cis-C18 : 1-PE. The clinical association of CDS-1 sera included a variety of systemic and organ-specific autoimmune diseases but they were also observed in patients with no evidence of autoimmune disease.

Saposin B binds and transfers phospholipids

Saposin B (Sap B) is a member of a family of four small glycoproteins, Sap A, B, C, and D. Like the other three saposins, Sap B plays a physiological role in the lysosomal degradation of sphingolipids (SLs). Although the interaction of Sap B with SLs has been investigated extensively, that with the main membrane lipid components, namely phospholipids and cholesterol (Chol), is scarcely known. Using large unilamellar vesicles (LUVs) as membrane models, we have now found that Sap B simultaneously extracts from the lipid surface neutral [phosphatidylcholine (PC)] and anionic [phosphatidylinositol (PI)] phospholipids, fewer SLs [ganglioside GM1 (GM1) or cerebroside sulfate (CS)], and no Chol. More PI than SL (GM1 or CS) was solubilized from LUVs containing equal amounts of PI and SLs. An increase in PI level had a poor effect on the Sap B-induced solubilization of GM1 or CS but strongly inhibited that of PC. Sap B was able not only to bind, but also to transfer phospholipids between lipid surfaces. Both the phospholipid binding and transfer activities were optimal at low pH values. These results represent the first biochemical analysis of the Sap B interaction with phospholipids. The capacity of Sap B to bind and transfer phospholipids occurs under conditions mimicking the interior of the late endosomal/lysosomal compartment and thus might have physiological relevance.

Role of lipids in the MAPK signaling pathway

The mitogen-activated protein kinase (MAPK) signaling pathway is activated in response to a variety of extracellular stimuli such as growth factor stimulation. The best-characterized MAPK pathway involves the sequential activation of Raf, MEK and ERK proteins, capable of regulating the gene expression required for cell proliferation. Binding to specific lipids can regulate both the subcellular localization of these MAPK signaling proteins as well as their kinase activities. More recently it has become increasingly clear that the majority of MAPK signaling takes place intracellularly on endosomes and that the perturbation of endocytic pathways has dramatic effects on the MAPK pathway. This review highlights the direct effects of lipids on the localization and regulation of MAPK pathway proteins. In addition, the indirect effects lipids have on MAPK signaling via their regulation of endocytosis and the biophysical properties of different membrane lipids as a result of growth factor stimulation are discussed. The ability of a protein to bind to both lipids and proteins at the same time may act like a "ZIP code" to target that protein to a highly specific microlocation and could also allow a protein to be "handed off" to maintain tight control over its binding partners and location.

NPC1 late endosomes contain elevated levels of non-esterified ('free') fatty acids and an abnormally glycosylated form of the NPC2 protein

NPC (Niemann-Pick type C) disease is a rare lipidosis characterized by the accumulation of LDL (low-density lipoprotein)-derived non-esterified cholesterol in the E/L (endosomal/lysosomal) system. The gene products that are responsible for the two NPC complementation groups are distinct and dissimilar, yet their cellular and disease phenotypes are virtually indistinguishable. To investigate the relationship between NPC1 and NPC2 and their potential role in NPC disease pathogenesis, we have developed a method for the rapid and efficient isolation of late endocytic vesicles from mouse liver by magnetic chromatography. Late endosomes from Wt (wild-type) and NPC1 mice were found to differ not only in their cholesterol and sphingomyelin content, as expected, but also in their non-esterified ('free') fatty acid content, with NPC1 vesicles showing an approx. 7-fold increase in non-esterified fatty acid levels compared with Wt vesicles. Furthermore, we show that the NPC2 protein is in an incompletely deglycosylated form in NPC1 late endosomes by a mechanism that is specific to the NPC2 protein and not a global aberration of protein glycosylation/deglycosylation or trafficking, since NPC2 secreted from NPC1 cells is indistinguishable from that secreted from Wt cells. Also, a greater proportion of the normally soluble cellular NPC2 protein partitions with detergent-insoluble late endosomal internal membrane domains in NPC1 vesicles. In addition, we show that, although a small amount of the NPC2 protein associates with these membranes in Wt vesicles, this localization becomes much more pronounced in NPC1 vesicles. These results suggest that the function of the NPC2 protein may be compromised as well in NPC1 endosomes, which might explain the paradoxical phenotypic similarities of the two NPC disease complementation groups.

New metabolically stabilized analogues of lysophosphatidic acid: agonists, antagonists and enzyme inhibitors

Lysophosphatidic acid (LPA) is a metabolically labile natural phospholipid with a bewildering array of physiological effects. We describe herein a variety of long-lived receptor-specific agonists and antagonists for LPA receptors. Several LPA and PA (phosphatidic acid) analogues also inhibit LPP (lipid phosphate phosphatase). The sn-1 or sn-2 hydroxy groups have been replaced by fluorine, difluoromethyl, difluoroethyl, O-methyl or O-hydroxyethoxy groups to give non-migrating LPA analogues that resist acyltransferases. Alkyl ether replacement of acyl esters produced lipase and acyltransferase-resistant analogues. Replacement of the bridging oxygen in the monophosphate by an alpha-monofluoromethylene-, alpha-bromomethylene- or alpha,alpha-difluoromethylenephosphonate gave phosphatase-resistant analogues. Phosphorothioate analogues with O-acyl and O-alkyl chains are potent, long-lived agonists for LPA1 and LPA3 receptors. Most recently, we have (i) prepared stabilized O-alkyl analogues of lysobisphosphatidic acid, (ii) explored the structure-activity relationship of stabilized cyclic LPA analogues and (iii) synthesized neutral head group trifluoromethylsulphonamide analogues of LPA. Through collaborative studies, we have collected data for these stabilized analogues as selective LPA receptor (ant)agonists, LPP inhibitors, TREK (transmembrane calcium channel) K+ channel agonists, activators of the nuclear transcription factor PPAR-gamma (peroxisome-proliferator-activated receptor-gamma), promoters of cell motility and survival, and radioprotectants for human B-cells.

The N370S (Asn370-->Ser) mutation affects the capacity of glucosylceramidase to interact with anionic phospholipid-containing membranes and saposin C

The properties of the endolysosomal enzyme GCase (glucosylceramidase), carrying the most prevalent mutation observed in Gaucher patients, namely substitution of an asparagine residue with a serine at amino acid position 370 [N370S (Asn370-->Ser) GCase], were investigated in the present study. We previously demonstrated that Sap (saposin) C, the physiological GCase activator, promotes the association of GCase with anionic phospholipid-containing membranes, reconstituting in this way the enzyme activity. In the present study, we show that, in the presence of Sap C and membranes containing high levels of anionic phospholipids, both normal and N370S GCases are able to associate with the lipid surface and to express their activity. Conversely, when the amount of anionic phospholipids in the membrane is reduced (approximately 20% of total lipids), Sap C is still able to promote binding and activation of the normal enzyme, but not of N370S GCase. The altered interaction of the mutated enzyme with anionic phospholipid-containing membranes and Sap C was further demonstrated in Gaucher fibroblasts by confocal microscopy, which revealed poor co-localization of N370S GCase with Sap C and lysobisphosphatidic acid, the most abundant anionic phospholipid in endolysosomes. Moreover, we found that N370S Gaucher fibroblasts accumulate endolysosomal free cholesterol, a lipid that might further interfere with the interaction of the enzyme with Sap C and lysobisphosphatidic acid-containing membranes. In summary, our results show that the N370S mutation primarily affects the interaction of GCase with its physiological activators, namely Sap C and anionic phospholipid-containing membranes. We thus propose that the poor contact between N370S GCase and its activators may be responsible for the low activity of the mutant enzyme in vivo.

Cardiolipin and its metabolites move from mitochondria to other cellular membranes during death receptor-mediated apoptosis

We previously reported that during death receptor-mediated apoptosis, cardiolipin (CL) relocates to the cell surface, where it reacts with autoantibodies from antiphospholipid syndrome sera. Here, we analysed the intracellular distribution of CL and its metabolites during the early phase of cell death signalling triggered by Fas stimulation in U937 cells and mouse liver. We found a redistribution of mitochondrial CL to the cell surface by using confocal microscopy and flow cytometry. Mass spectrometry revealed that CL and its metabolites relocated from mitochondria to other intracellular organelles during apoptosis, with a conversion into non-mitochondrial lipids. Concomitantly, cytosolic Bid relocated to the light membranes comprised in fraction P100, including the plasma membrane and associated vesicular systems. A direct Bid-CL interaction was demonstrated by the observation that CL and monolysoCL coimmunoprecipitated with Bid especially after Fas stimulation, suggesting a dynamic interaction of the protein with CL and its metabolites.

Guilty until proven innocent: the case of NPC1 and cholesterol

Cholesterol accumulation in the endosomes and lysosomes of Niemann-Pick C (NPC) cells is considered to be the hallmark of this disorder, so the main focus of NPC research has revolved around cholesterol and its role in disease pathogenesis. However, recent data indicate that cholesterol is not the primary culprit in this human lipidosis. I propose a new hypothesis for the potential action or function of the NPC1 protein in the endosome. In this context, the relationship of NPC2 and NPC1 is also discussed.

Endosome-to-cytosol transport of viral nucleocapsids

During viral infection, fusion of the viral envelope with endosomal membranes and nucleocapsid release were thought to be concomitant events. We show here that for the vesicular stomatitis virus they occur sequentially, at two successive steps of the endocytic pathway. Fusion already occurs in transport intermediates between early and late endosomes, presumably releasing the nucleocapsid within the lumen of intra-endosomal vesicles, where it remains hidden. Transport to late endosomes is then required for the nucleocapsid to be delivered to the cytoplasm. This last step, which initiates infection, depends on the late endosomal lipid lysobisphosphatidic acid (LBPA) and its putative effector Alix/AIP1, and is regulated by phosphatidylinositol-3-phosphate (PtdIns3P) signalling via the PtdIns3P-binding protein Snx16. We conclude that the nucleocapsid is exported into the cytoplasm after the back-fusion of internal vesicles with the limiting membrane of late endosomes, and that this process is controlled by the phospholipids LBPA and PtdIns3P and their effectors.

MLN64 is involved in actin-mediated dynamics of late endocytic organelles

MLN64 is a late endosomal cholesterol-binding membrane protein of an unknown function. Here, we show that MLN64 depletion results in the dispersion of late endocytic organelles to the cell periphery similarly as upon pharmacological actin disruption. The dispersed organelles in MLN64 knockdown cells exhibited decreased association with actin and the Arp2/3 complex subunit p34-Arc. MLN64 depletion was accompanied by impaired fusion of late endocytic organelles and delayed cargo degradation. MLN64 overexpression increased the number of actin and p34-Arc-positive patches on late endosomes, enhanced the fusion of late endocytic organelles in an actin-dependent manner, and stimulated the deposition of sterol in late endosomes harboring the protein. Overexpression of wild-type MLN64 was capable of rescuing the endosome dispersion in MLN64-depleted cells, whereas mutants of MLN64 defective in cholesterol binding were not, suggesting a functional connection between MLN64-mediated sterol transfer and actin-dependent late endosome dynamics. We propose that local sterol enrichment by MLN64 in the late endosomal membranes facilitates their association with actin, thereby governing actin-dependent fusion and degradative activity of late endocytic organelles.

Spatial and Functional Heterogeneity of Sphingolipid-rich Membrane Domains

Little is known about the organization of lipids in biomembranes. Lipid rafts are defined as sphingolipid- and cholesterol-rich clusters in the membrane. Details of the lipid distribution of lipid rafts are not well characterized mainly because of a lack of appropriate probes. Ganglioside GM1-specific protein, cholera toxin, has long been the only lipid probe of lipid rafts. Recently it was shown that earthworm toxin, lysenin, specifically recognizes sphingomyelin-rich membrane domains. Binding of lysenin to sphingomyelin is accompanied by the oligomerization of the toxin that leads to pore formation in the target membrane. In this study, we generated a truncated lysenin mutant that does not oligomerize and thus is non-toxic. Using this mutant lysenin, we showed that plasma membrane sphingomyelin-rich domains are spatially distinct from ganglioside GM1-rich membrane domains in Jurkat T cells. Like T cell receptor activation and cross-linking of GM1, cross-linking of sphingomyelin induced calcium influx and ERK phosphorylation in the cell. However, unlike CD3 or GM1, cross-linking of sphingomyelin did not induce significant protein tyrosine phosphorylation. Combination of lysenin and sphingomyelinase treatment suggested the involvement of G-protein-coupled receptor in sphingomyelin-mediated signal transduction. These results thus suggest that the sphingomyelin-rich domain provides a functional signal cascade platform that is distinct from those provided by T cell receptor or GM1. Our study therefore elucidates the spatial and functional heterogeneity of lipid rafts.

Diacylglycerol kinase alpha regulates the secretion of lethal exosomes bearing Fas ligand during activation-induced cell death of T lymphocytes

Fas ligand (FasL) mediates both apoptotic and inflammatory responses in the immune system. FasL function critically depends on the different forms of FasL; soluble Fas ligand lacking the transmembrane and cytoplasmic domains is a poor mediator of apoptosis, whereas full-length, membrane-associated FasL (mFasL) is pro-apoptotic. mFasL can be released from T lymphocytes, via the secretion of mFasL-bearing exosomes. mFasL in exosomes retains its activity in triggering Fas-dependent apoptosis, providing an alternative mechanism of cell death that does not necessarily imply cell-to-cell contact. Diacylglycerol kinase alpha (DGKalpha), a diacylglycerol (DAG)-consuming enzyme, is involved in the attenuation of DAG-derived responses initiated at the plasma membrane that lead to T lymphocyte activation. Here we studied the role of DGKalpha on activation-induced cell death on a T cell line and primary T lymphoblasts. The inhibition of DGKalpha increases the secretion of lethal exosomes bearing mFas ligand and subsequent apoptosis. On the contrary, the overactivation of the DGKalpha pathway inhibits exosome secretion and subsequent apoptosis. DGKalpha was found associated with the trans-Golgi network and late endosomal compartments. Our results support the hypothesis that the DGKalpha effect on apoptosis occurs via the regulation of the release of lethal exosomes by the exocytic pathway, and point out that the spatial orchestration of the different pools of DAG (plasma membrane and Golgi membranes) by DGKalpha is crucial for the control of cell activation and also for the regulation of the secretion of lethal exosomes, which in turn controls cell death.

Interaction of the Macrolide Antibiotic Azithromycin with Lipid Bilayers: Effect on Membrane Organization, Fluidity, and Permeability

PURPOSE

To investigate the effect of a macrolide antibiotic, azithromycin, on the molecular organization of DPPC:DOPC, DPPE:DOPC, SM:DOPC, and SM:Chol:DOPC lipid vesicles as well as the effect of azithromycin on membrane fluidity and permeability.

METHODS

The molecular organization of model membranes was characterized by atomic force microscopy (AFM), and the amount of azithromycin bound to lipid membranes was determined by equilibrium dialysis. The membrane fluidity and permeability were analyzed using fluorescence polarization studies and release of calcein-entrapped liposomes, respectively.

RESULTS

In situ AFM images revealed that azithromycin leads to the erosion and disappearance of DPPC and DPPE gel domains, whereas no effect was noted on SM and SM:cholesterol domains. Although azithromycin did not alter the permeability of DPPC:DOPC, DPPE:DOPC, SM:DOPC, and SM:Chol:DOPC lipid vesicles, it increased the fluidity at the hydrophilic/hydrophobic interface in DPPC:DOPC and DPPE:DOPC models. This effect may be responsible for the ability of azithromycin to erode the DPPC and DPPE gel domains, as observed by AFM.

CONCLUSIONS

This study shows the interest of both AFM and biophysical methods to characterize the drug-membrane interactions.

Saposin C-LBPA interaction in late-endosomes/lysosomes

Acidic phospholipids and saposins associations are involved in the degradation process of glycosphingolipids/sphingolipids in late endosomes/lysosomes. In this report, we showed the colocalization of saposin C and lysobisphosphatidic acid (LBPA) in human fibroblasts by using cytoimmunofluorescence analysis. This colocalization pattern was not seen with other saposins. Large numbers of saposins A, B, and D illustrated the staining patterns that differ from LBPA. In addition, ingested anti-LBPA antibody altered the location of saposin C in human wild-type fibroblasts. In vitro assays demonstrated that saposin C at nM concentrations induced membrane fusion of LBPA containing phospholipid vesicles. Under the same condition, other saposins had no fusion induction on these vesicles. These results suggested a specific interaction between saposin C and LBPA. Total saposin-deficient fibroblasts showed a massive accumulation of multivesicular bodies (MVBs) by electron microscopic analysis. No significant increase of MVBs was found in saposins A and B deficient cells. Interestingly, the accumulated MVBs were significantly reduced by loading saposin C alone into the total saposin-deficient cells. Therefore, we propose that saposin C-LBPA interaction plays a role in the regulation of MVB formation in cells.

New effects in polynucleotide release from cationic lipid carriers revealed by confocal imaging, fluorescence cross-correlation spectroscopy and single particle tracking

We report on new insights into the mechanisms of short single and double stranded oligonucleotide release from cationic lipid complexes (lipoplexes), used in gene therapy. Specifically, we modeled endosomal membranes using giant unilamellar vesicles and investigated the roles of various individual cellular phospholipids in interaction with lipoplexes. Our approach uses a combination of confocal imaging, fluorescence cross-correlation spectroscopy and single particle tracking, revealing several new aspects of the release: (a) phosphatidylserine and phosphatidylethanolamine are equally active in disassembling lipoplexes, while phosphatidylcholine and sphingomyelin are inert; (b) in contrast to earlier findings, phosphatidylethanolamine alone, in the absence of anionic phosphatidylserine triggers extensive release; (c) a double-stranded DNA structure remains well preserved after release; (d) lipoplexes exhibited preferential binding to transient lipid domains, which appear at the onset of lipoplex attachment to originally uniform membranes and vanish after initiation of polynucleotide release. The latter effect is likely related to phosphatidyleserine redistribution in membranes due to lipoplex binding. Real time tracking of single DOTAP/DOPE and DOTAP/DOPC lipoplexes showed that both particles remained compact and associated with membranes up to 1-2 min before fusion, indicating that a more complex mechanism, different from suggested earlier rapid fusion, promotes more efficient transfection by DOTAP/DOPE complexes.

Intermolecular interactions of lysobisphosphatidic acid with phosphatidylcholine in mixed bilayers

Lysobisphosphatidic acid (LBPA) can be regarded to represent a unique derivative of phosphatidylglycerol. This lipid is highly enriched in late endosomes where it can comprise up to 10-15 mol% of all lipids and in these membranes, LBPA appears to be segregated into microdomains. We studied the thermotropic behavior of pure dioleoyl-LBPA mono- and bilayers using Langmuir-lipid monolayers, electron microscopy, differential scanning calorimetry (DSC), and fluorescence spectroscopy. LBPA formed metastable, liquid-expanded monolayers at an air/buffer interface, and its compression isotherms lacked any indication for structural phase transitions. Neat LBPA formed multilamellar vesicles with no structural transitions or phase transitions between 10 and 80 degrees C at a pH range of 3.0-7.4. We then proceeded to study mixed LBPA/dipalmitoylphosphatidylcholine (DPPC) bilayers by DSC and fluorescence spectroscopy. Incorporating increasing amounts of LBPA (up to X(LBPA) (molar fraction)=0.10) decreased the co-operativity of the main transition for DPPC, and a decrease in the main phase transition as well as pretransition temperature of DPPC was observed yet with no effect on the enthalpy of this transition. In keeping with the DSC data for DPPC, 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC)/LBPA mixed bilayers were more fluid, and no evidence for lateral phase segregation was observed. These results were confirmed using fluorescence microscopy of Langmuir-lipid films composed of POPC and LBPA up to X(LBPA)=0.50 with no evidence for lateral phase separation. As late endosomes are eminently acidic, we examined the effect of lowering pH on lateral organization of mixed PC/LBPA bilayers by DSC and fluorescence spectroscopy. Even at pH 3.0, we find no evidence of LBPA-induced microdomain formation at LBPA contents found in cellular organelles.

Domain formation in phosphatidylinositol monophosphate/phosphatidylcholine mixed vesicles

Phosphoinositides have been shown to control membrane trafficking events by targeting proteins to specific cellular sites, which requires a tight regulation of phosphoinositide generation and turnover as well as a high degree of compartmentalization. To shed light on the processes that lead to the formation of phosphoinositide-enriched microdomains, phosphatidylcholine/phosphatidylinositol monophosphate (phosphatidylinositol-3-phosphate (PI-3P), -4-phosphate (PI-4P), or -5-phosphate (PI-5P)) mixed vesicles were investigated by calorimetric (DSC) Fourier transform infrared spectroscopic (FTIR), and fluorescence resonance energy transfer (FRET) measurements. The experiments furnished results consistent with a pH-dependent formation of phosphatidylinositol monophosphate-enriched microdomains. The domain formation was most pronounced between pH approximately 7 and approximately 9.5, whereas slightly acidic pH values (pH 4) resulted in the disintegration of the domains. This pH-dependent phosphatidylcholine/phosphatidylinositol monophosphate demixing was observed for the gel phase (FTIR experiments) as well as for the fluid lipid phase (FRET measurements). The observed microdomains are presumably stabilized by hydroxyl/hydroxyl as well as hydroxyl/phosphomonoester and phosphodiester interactions. While the pH dependence of the mutual phosphatidylinositol monophosphate interaction was largely the same for all investigated phosphatidylinositol monophosphates, it turned out that the relative stability of phosphatidylinositol monophosphate-enriched microdomains (pH 7-9.5) was governed by the position of the phosphomonoester group at the inositol ring (PI-4P > PI-5P > PI-3P). Demixing was also observed for phosphatidylcholine/phosphatidylinositol mixed vesicles; however, in this case the microdomain formation was only slightly affected by pH changes.

Cell surface delivery of the measles virus nucleoprotein: a viral strategy to induce immunosuppression

Although only a few blood cells are infected during measles, this infectious disease is followed by acute immunosuppression, associated with high infant mortality. Measles virus nucleoprotein has been suggested to contribute to virus-induced inhibition of the immune response. However, it has been difficult to understand how this cytosolic viral protein could leave an infected cell and then perturb the immune response. Here we demonstrate that intracellularly synthesized nucleoprotein enters the late endocytic compartment, where it recruits its cellular ligand, the Fcgamma receptor. Nucleoprotein is then expressed at the surfaces of infected leukocytes associated with the Fcgamma receptor and is secreted into the extracellular compartment, allowing its interaction with uninfected cells. Finally, cell-derived nucleoprotein inhibits the secretion of interleukin-12 and the generation of the inflammatory reaction, both shown to be impaired during measles. These results reveal nucleoprotein egress from infected cells as a novel strategy in measles-induced immunosuppression.

Characterization of interaction between cationic lipid-oligonucleotide complexes and cellular membrane lipids using confocal imaging and fluorescence correlation spectroscopy

Complexes formed by cationic liposomes and single-strand oligodeoxynucleotides (CL-ODN) are promising delivery systems for antisense therapy. ODN release from the complexes is an essential step for inhibiting activity of antisense drugs. We applied fluorescence correlation spectroscopy and confocal laser scanning microscopy to monitor CL-ODN complex interaction with membrane lipids leading to ODN release. To model cellular membranes we used giant unilamellar vesicles and investigated the transport of Cy-5-labeled ODNs across DiO-labeled membranes. For the first time, we directly observed that ODN molecules are transferred across the lipid bilayers and are kept inside the giant unilamellar vesicles after release from the carriers. ODN dissociation from the carrier was assessed by comparing diffusion constants of CL-ODN complexes and ODNs before complexation and after release. Freely diffusing Cy-5-labeled ODN (16-nt) has diffusion constant D(ODN) = 1.3 +/- 0.1 x 10(-6) cm2/s. Fluorescence correlation spectroscopy curves for CL-ODN complexes were fitted with two components, which both have significantly slower diffusion in the range of D(CL-ODN) = approximately 1.5 x 10(-8) cm2/s. Released ODN has the mean diffusion constant D = 1.1 +/- 0.2 x 10(-6) cm2/s, which signifies that ODN is dissociated from cationic lipids. In contrast to earlier studies, we report that phosphatidylethanolamine can trigger ODN release from the carrier in the full absence of anionic phosphatidylserine in the target membrane and that phosphatidylethanolamine-mediated release is as extensive as in the case of phosphatidylserine. The presented methodology provides an effective tool for probing a delivery potential of newly created lipid formulations of CL-ODN complexes for optimal design of carriers.

Mast cell- and dendritic cell-derived exosomes display a specific lipid composition and an unusual membrane organization

Exosomes are small vesicles secreted from multivesicular bodies, which are able to stimulate the immune system leading to tumour cell eradication. We have analysed lipids of exosomes secreted either upon stimulation from rat mast cells (RBL-2H3 cells), or constitutively from human dendritic cells. As compared with parent cells, exosomes displayed an enrichment in sphingomyelin, but not in cholesterol. Phosphatidylcholine content was decreased, but an enrichment was noted in disaturated molecular species as in phosphatidylethanolamines. Lyso(bis)phosphatidic acid was not enriched in exosomes as compared with cells. Fluorescence anisotropy demonstrated an increase in exosome-membrane rigidity from pH 5 to 7, suggesting their membrane reorganization between the acidic multivesicular body compartment and the neutral outer cell medium. NMR analysis established a bilayer organization of exosome membrane, and ESR studies using 16-doxyl stearic acid demonstrated a higher flip-flop of lipids between the two leaflets as compared with plasma membrane. In addition, the exosome membrane exhibited no asymmetrical distribution of phosphatidylethanolamines. Therefore exosome membrane displays a similar content of the major phospholipids and cholesterol, and is organized as a lipid bilayer with a random distribution of phosphatidylethanolamines. In addition, we observed tight lipid packing at neutral pH and a rapid flip-flop between the two leaflets of exosome membranes. These parameters could be used as a hallmark of exosomes.

HIV-1 egress is gated through late endosomal membranes

HIV-1 buds from the surface of activated T lymphocytes. In macrophages, however, newly formed HIV-1 particles amass in the lumen of an intracellular compartment. Here, we demonstrate by live-cell imaging techniques, by immunocytochemistry and by immuno-electron microscopy that HIV-1 structural proteins, particularly the internal structural protein Gag, accumulate at membranes of the late endocytic compartment in a variety of cell types and not just in monocyte/macrophage-derived cells. Recent biochemical and genetic studies have implicated components of the mammalian vacuolar protein sorting pathway in retroviral budding. Together with those observations, our study suggests that HIV-1 morphogenesis is thoroughly rooted in the endosomal system.