The extracellular glycosphingolipid-binding motif of Fas defines its internalization route, mode and outcome of signals upon activation by ligand.

Selective compartmentalization and internalization have been shown as a means for regulating specific signals of cell surface receptors to correspond to cellular requirements and conditions. Here, we present a conserved extracellular glycosphingolipid-binding motif of Fas as one of the regulatory elements in the selection of its internalization route and consequently the signals transmitted upon ligand binding. This motif is required for clathrin-mediated internalization of Fas, which allows the transduction of its cell death signal. The loss of function of the motif drives the activated receptor to an alternative internalization route that is independent of clathrin and cholesterol-dependent rafts but dependent on ezrin, and thereby extinguishing its cell death signal while promoting its non-death functions. Through biochemical, biophysical, and genetic approaches, we present a protein/lipid-based mechanism as a key to the versatility of the signal transduction by the multifunctional Fas receptor-ligand system.

How sphingolipids bind and shape proteins: molecular basis of lipid-protein interactions in lipid shells, rafts and related biomembrane domains.

Understanding the molecular mechanisms controlling the association of proteins with lipid rafts is a central issue in cell biology and medicine. A structurally conserved motif (the 'sphingolipid binding domain') has been characterized in unrelated cellular and microbial proteins targeted to lipid rafts. I propose that the structuration of a sphingolipid shell around the sphingolipid binding domain not only extracts the protein from the liquid-disordered phase of the plasma membrane, and ensures its delivery to lipid rafts, but also influences its conformation. The chaperone activity of sphingolipids in shells and rafts may play an important role in infectious and conformational diseases(human immunodeficiency virus-1, prions, Alzheimer).

Characterization of galactosyl glycerolipids in the HT29 human colon carcinoma cell line.

Glycoglycerolipids constitute a family of glycolipids with apparently very restricted expression in human tissues. They have previously been detected only in the testis and the nervous system. In the present study, two glycoglycerolipids were isolated from the HT29 human colon carcinoma cell line. The glycoglycerolipids were structurally characterized as a monogalactosylglycerolipid (1-O-alkyl-2-O-acyl-3-O-(beta-galactosyl)-sn-glycerol) and a digalactosylglycerolipid (1-O-alkyl-2-O-acyl-3-O-(beta-galactosyl(1-4)alpha-galactosyl)-sn-glycerol) using NMR and mass spectrometry. This digalactosylglycerolipid has not previously been structurally characterized. When HT29 cells were allowed to differentiate into more enterocyte-like cells by culture in glucose-free medium, expression of both of these glycoglycerolipids was greatly diminished. The presence of glycoglycerolipids in a human colon carcinoma cell line indicates that expression of this family of glycolipids may not be as restricted as previously thought. Instead this class of glycolipids may serve as differentiation antigens in various normal tissues and in tumor development. The Galalpha1-4Gal epitope was previously identified as a receptor for bacterial adhesins and toxins. The finding that this epitope is also linked to a glycerolipid moiety opens up new possible roles for this carbohydrate receptor in intracellular signaling.

Gp120-induced Bob/GPR15 activation: a possible cause of human immunodeficiency virus enteropathy.

Human immunodeficiency virus (HIV)-infected patients often develop malabsorption and increased intestinal permeability with diarrhea, called HIV enteropathy, even without enteric opportunistic infections. HIV gp120-induced calcium signaling, microtubule loss, and physiological changes resembling HIV enteropathy were previously found in the HT-29 intestinal cell line. How gp120 caused these changes was unclear. We show that the HIV co-receptor Bob/GPR15, unlike CCR5 and CXCR4, is abundant at the basal surface of small intestinal epithelium. The gp120-induced effects on HT-29 cells were inhibited by anti-Bob neutralizing antibodies, the selective G protein inhibitor pertussis toxin, and the phospholipase inhibitor U73122, but not neutralizing antibodies to CXCR4. Gp120 strains that induced signaling in HT-29 cells also induced calcium fluxes in Bob-transfected Ghost (3) cells, whereas gp120 strains not activating HT-29 cells also did not activate Bob-transfected cells. Bob is the first HIV co-receptor shown to be abundantly expressed on the basolateral surface of intestinal epithelium. Although Bob is an inefficient infection-inducing co-receptor, it mediates viral strain-specific gp120-induced calcium signaling at low, physiologically reasonable gp120 concentrations, up to 10,000-fold lower gp120 concentrations than the principal co-receptors. Gp120-induced Bob activation is a plausible cause of HIV enteropathy.

The mycotoxin ochratoxin A alters intestinal barrier and absorption functions but has no effect on chloride secretion.

Ochratoxin A (OTA) is a mycotoxin that contaminates cereals and animal feed and causes nephropathy to a variety of animal species. OTA is also known as a potent immunotoxic, teratogenic, and carcinogenic mycotoxin. In addition, OTA ingestion induces intestinal injuries, including inflammation and diarrhea. With the aim to study the cellular mechanisms associated with the intestinal toxicity of OTA, two human epithelial intestinal cell lines (HT-29-D4 and Caco-2-14 cells), widely used as in vitro models for the intestinal epithelium, were incubated with OTA. The main effects of the mycotoxin were an inhibition of cellular growth and a dramatic decrease of transepithelial resistance in both cell lines. Since transepithelial resistance reflects the organization of tight junctions over the cell monolayer, these data may suggest that OTA could potentiate its own absorption through paracellular pathways. OTA induced a 60% decrease of sodium-dependent glucose absorption but increased the absorption of fructose and L-serine in HT-29-D4 cells. Moreover, the mycotoxin did not inhibit the cAMP-dependent chloride secretion through the cystic fibrosis transmembrane conductance regulator channel. The inhibitory effect of OTA on active glucose transport was partially antagonized by L-phenylalanine, but not by alpha-tocopherol, suggesting that the toxicity of OTA could result from an inhibition of protein synthesis, rather than an induction of lipid peroxidation. In particular, OTA affected the protein content of plasma membrane microdomains, which are known to regulate tight junction assembly and intestinal transport activity. Taken together, these data showed that OTA alters both barrier and absorption functions of the intestinal epithelium.

Amphiphilic anionic analogues of galactosylceramide: synthesis, anti-HIV-1 activity, and gp120 binding.

We describe the synthesis together with the results of anti-HIV-1 activity and gp120-monolayer binding experiments of new galactosyl amphiphiles, analogues of galactosylceramide, an alternative receptor used by HIV to infect CD4 negative cells. These compounds consist of single- and double-chain amphiphiles containing one or two galactose residues. To favor their clustering into galactosyl-rich microdomains, their molecular structure contains also an amino group or several hydroxyls or anionic groups, such as carboxylate, sulfate, sulfonate, and phosphate. Among the 12 new galactosylated compounds reported, a specific anti-HIV activity, although moderate (IC(50) from 10 to 50 microM), was detected only for three of them, i.e., I-GalSer[CO2Na][C14], II-GalSer[C14][C7SO3Na], and II-GalSer[C2SO4Na][C14], which contain an anionic group. The marked increase of surface pressure which was observed upon addition of gp120 into the aqueous subphase underneath the monolayers containing these galactolipids indicated gp120 insertion into the monolayers, suggesting that binding of these three derivatives to HIV-1 gp120 may be responsible for their anti-HIV activity.

Comparison of two commercial assays for the detection of insertion mutations of HIV-1 reverse transcriptase.

Insertions in the beta3-beta4 fingers subdomain of HIV-1 reverse transcriptase (RT) confer cross-resistance to various nucleoside analogs. The detection of these rearrangements in the region of codons 67-70 of RT is of primary importance for adapting and optimizing combination treatment regimen. Recent reports suggest that some genotyping techniques based on the hybridization of oligonucleotide probes may fail to detect insertion mutants of HIV-1 RT. In the present study, we have evaluated the efficiency of two commercial kits TruGene (based on Dye Primer sequencing) and Viroseq (Big Dye Terminator technique) for the detection of insertion mutations. The data were compared with an in-house dRhodamine sequencing method. Overall, all these cycle sequencing techniques were operative in the detection of insertion mutants. The best peak homogeneity in the electrophoregrams was observed with the Dye primer technique. However, specific compression artifacts were frequently encountered with this technique, rendering ambiguous the interpretation of the electrophoregrams in several regions of the sequence. This shortcoming did not occur with dRhodamine Dye terminator or Bigdye terminator cycle sequencing. In any case, a manual inspection of the electrophoregrams is highly recommended, for all types of cycle sequencing techniques, especially for detecting new mutational patterns of the RT and protease genes. Finally, some specific problems were encountered with the softwares provided with both Trugene and Viroseq kits.

Use of drug resistance sequence data for the systematic detection of non-B human immunodeficiency virus type 1 (HIV-1) subtypes: how to create a sentinel site for monitoring the genetic diversity of HIV-1 at a country scale.

To assess the molecular epidemiology of human immunodeficiency virus type 1 (HIV-1), a screening method was developed for identification of non-B subtypes from sequence data obtained for resistance testing. The method is based on the evaluation of the percentage of divergence of a given sequence from the reference B subtype HXB2. Analysis of 1720 reverse-transcriptase (RT) and 1824 protease sequences stored in a database allowed for the determination of a threshold level of divergence from HXB2 above which a non-B subtype could be unambiguously characterized regardless of the pattern of resistance mutations (>8.6% for RT; >10.8% for protease). This conclusion was validated by phylogenetic analysis of RT, protease, and env genes. Overall, 72 (4.2%) and 73 (4.0%) non-B sequences were identified in the RT and protease coding regions, respectively. This method allows for the rapid detection of non-B subtypes among retrospective, recent, and future RT and/or protease sequence databases.

Genetic analysis of hiv type 1 strains in bujumbura (burundi): predominance of subtype c variant.

In order to characterize the HIV-1 strains circulating in Burundi, 18 blood samples from nontreated patients were collected in Bujumbura and viral DNA and RNA were sequenced in the env and pol genes, respectively. The phylogenetic analysis of the V3 coding region of HIV-1 gp120 revealed that 83% (15/18) of the isolates belonged to the C subtype. The RT and protease coding regions of the pol gene also clustered with subtype C. A potential A/C recombinant between the protease (subtype A) and the RT and V3 coding regions (both subtype C) was identified. Drug resistance mutations were not detected in the RT gene. However, mutation M36I, associated with resistance to ritonavir and nelfinavir, was found in 17 of 18 Burundi isolates. In conclusion, this first characterization of HIV-1 strains circulating in Burundi confirms the dramatic emergence of subtype C in East Africa.

Total synthesis of mololipids: a new series of anti-HIV Moloka'iamine derivatives.

A new family of bioactive bromotyrosine derivatives, termed mololipids, was recently isolated from a Hawaiian sponge, but could not be resolved into individual components by chromatography. To complete their structural characterization and better understand structure-activity relationships, the first pure samples of dimyristoyl, distearoyl, dioleoyl, and stearoyl/oleoyl mololipids have now been prepared by total synthesis, and their anti-HIV activity investigated.

Mutation L210W of HIV-1 reverse transcriptase in patients receiving combination therapy. Incidence, association with other mutations, and effects on the structure of mutated reverse transcriptase.

Mutation L210W of HIV-1 reverse transcriptase (RT) is one of the six main mutations that confer in vivo resistance to zidovudine. Surprisingly, this mutation has received scant appraisal and its contribution to the genotypic resistance to nucleoside analogs is not well understood. The aim of this study was: (1) to study the frequency of mutation L210W in a large collection of HIV-1 sequences (2,049 samples, including 395 DNA and 1,654 RNA sequences) from patients receiving combination therapy, and (2) to analyze its association with the other mutations that confer resistance to zidovudine. A mutation at codon 210 (mainly L210W) was found in 647 (32%) of the 2,049 sequences analyzed. Only 43 (<7%) of these 647 genomes were also mutated at codon 70 (p < 10(-5)). In contrast, 98% of these 647 sequences were also mutated at codon 215 (essentially T215Y/F), and 94% at codon 41 (mainly M41L). These data showing a close association between L210W, T215Y/F, and M41L, and a mutual exclusion between K70R and L210W, were confirmed by analyzing the sequences stored in the HIV-1 sequences available through the Stanford HIV RT and Protease Database. Follow-up studies demonstrated that L210W appeared always after T215Y/F. This observation is consistent with crystallographic studies which suggested that the aromatic side chain of Trp 210 could stabilize the interaction of Phe/Tyr215 with the dNTP-binding pocket. This molecular cross-talk between amino acid chains occurs nearby the conserved Asp113 residue. Since the lateral chain of Arg70 may also interact with Asp113, this is likely to create a sterical hindrance around this residue. Thus, the R-->K reversion of codon 70 may represent a compensatory mechanism allowing a functional rearrangement of the dNTP-binding pocket in the mutated RT.

Synthesis of single- and double-chain fluorocarbon and hydrocarbon galactosyl amphiphiles and their anti-HIV-1 activity.

Galactosylceramide (GalCer) is an alternative receptor allowing HIV-1 entry into CD4(-)/GalCer(+) cells. This glycosphingolipid recognizes the V3 loop of HIV gp120, which plays a key role in the fusion of the HIV envelope and cellular membrane. To inhibit HIV uptake and infection, we designed and synthesized analogs of GalCer. These amphiphiles and bolaamphiphiles consist of single and double hydrocarbon and/or fluorocarbon chain beta-linked to galactose and galactosamine. They derive from serine (GalSer), cysteine (GalCys), and ethanolamine (GalAE). The anti-HIV activity and cytotoxicity of these galactolipids were evaluated in vitro on CEM-SS (a CD4(+) cell line), HT-29, a CD4(-) cell line expressing high levels of GalCer receptor, and/or HT29 genetically modified to express CD4. GalSer and GalAE derivatives, tested in aqueous medium or as part of liposome preparation, showed moderate anti-HIV-1 activities (IC50 in the 20-220 microM range), whereas none of the GalCys derivatives was found to be active. Moreover, only some of these anti-HIV active analogs inhibited the binding of [3H]suramin (a polysulfonyl compound which displays a high affinity for the V3 loop) to SPC3, a synthetic peptide which contains the conserved GPGRAF region of the V3 loop. Our results most likely indicate that the neutralization of the virion through masking of this conserved V3 loop region is not the only mechanism involved in the HIV-1 antiviral activity of our GalCer analogs.

Synthesis of glycolipid analogues that disrupt binding of HIV-1 gp120 to galactosylceramide.

HIV-1 has been shown to infect CD4 negative cells by the binding of HIV gp120 to the glycolipid galactosylceramide (1) (GalCer). Several analogues of 1 were prepared to investigate the specific orientation of 1 in the membrane bilayer that is involved in gp120 binding. Interestingly, N-stearyl-1-deoxynojirimycin (8) displayed potent and specific affinity for gp120 equal to that of 1, a finding that may shed light on the antiviral activity of N-butyl-1-deoxynojirimycin.

Multidrug resistance genotypes (insertions in the beta3-beta4 finger subdomain and MDR mutations) of HIV-1 reverse transcriptase from extensively treated patients: incidence and association with other resistance mutations.

Multiple nucleoside resistance involves specific mutational patterns of the HIV-1 pol gene that are independent of the classic mutations conferring resistance to individual dideoxynucleosides. These include a cluster of five mutations in the reverse-transcriptase (RT) coding region (A62V, V75I, F77L, F116Y, and Q151M) generally referred to as multidrug resistance (MDR) mutations, and insertions of one or several amino acid residues between codons 67 and 70 of RT, a flexible region joining two antiparrallel beta sheets (beta3-beta4 insertions). The objectives of this study were (i) to determine the prevalence of multidrug resistance genotypes (MDR mutations and beta3-beta4 insertions) in a cohort of 632 patients who were extensively pretreated with anti-HIV drugs and not responding to their current antiretroviral therapy, and (ii) to analyze the association of multidrug resistance genotypes with other resistance mutations in the RT and protease genes. Among viruses sequenced from these patients, 15 (2.4%) of them contained an insertion and 2 (0.3%) contained a deletion in the beta3-beta4 finger subdomain of RT. In 9 cases, the insertion was associated with a D67S, G, or E mutation. In addition, we identified 13 (2.1%) viruses harboring specific MDR mutations (mainly Q151M and/or A62V, V75I, F116Y). Interestingly, the A62V mutation was found in 6 of the 15 strains with an insertion, whereas the other MDR mutations were not observed in insertion mutant strains. Especially high levels of resistance to zidovudine were observed for viruses with a beta3-beta4 insertion in the background of A62V, L210W, and T215Y. Otherwise, MDR mutations and beta3-beta4 insertions were found in association with the classic mutations conferring resistance to zidovudine, lamivudine, nonnucleoside RT inhibitors, and protease inhibitors, according to treatment history. Finally, we observed a genome with a deletion of codon 70 associated with a Q151M MDR mutation. These data suggest that the emergence of HIV-1 multidrug resistance, which may occur in various genetic contexts, poses a challenging problem in formulating treatment strategies.

Role of glycosphingolipid microdomains in CD4-dependent HIV-1 fusion.

The fusion of HIV-1 with the plasma membrane of CD4+ cells is triggered by the interaction of HIV-1 surface envelope glycoprotein gp120 with the CD4 receptor, and requires coreceptors (CCR5 and CXCR4). Recent advances in the study of HIV-1 entry into CD4+ cells suggest that glycosphingolipids (GSL) may also participate in the fusion process. GSL are organized in functional microdomains which are associated with specific membrane proteins such as CD4. GSL-enriched microdomains were purified from human lymphocytes and reconstituted as a monomolecular film at the air-water interface of a Langmuir film balance. Surface pressure measurements allowed to characterize the sequential interaction of GSL with CD4 and with gp120. Using this approach, we identified globotriaosylceramide (Gb3) and ganglioside GM3 as the main lymphocyte GSL recognized by gp120. In both cases, the interaction was saturable and dramatically increased by CD4. We propose that GSL microdomains behave as moving platforms allowing the recruitment of HIV-1 coreceptors after the initial interaction between the viral particle and CD4. According to this model, the GSL microdomain may: i) stabilize the attachment of the virus with the cell surface through multiple low affinity interactions between the V3 domain of gp120 and the carbohydrate moiety of GSL, and ii) convey the virus to an appropriate coreceptor by moving freely in the outer leaflet of the plasma membrane. This model can be extrapolated to all envelope viruses (e.g. influenza virus) that use cell surface GSL of the host cells as receptors or coreceptors.

Glycosphingolipid (GSL) microdomains as attachment platforms for host pathogens and their toxins on intestinal epithelial cells: activation of signal transduction pathways and perturbations of intestinal absorption and secretion.

Glycosphingolipid (GSL)-enriched microdomains are used as cellular binding sites for various pathogens including viruses and bacteria. These attachment platforms are specifically associated with transducer molecules, so that the binding of host pathogens (or their toxins) to the cell surface may result in the activation of signal transduction pathways. In the intestinal epithelium, such pathogen-induced dysregulations of signal transduction can elicit a severe impairment of enterocytic functions. In this study, we demonstrate that the interaction of a bacterial toxin (cholera toxin) and a viral envelope glycoprotein (HIV-1 gp120) with the apical plasma membrane of intestinal cells is mediated by GSL-enriched microdomains that are associated with G regulatory proteins. These microbial proteins induce a GSL-dependent increase of intestinal fluid secretion by two mechanisms: activation of chloride secretion and inhibition of Na+ -dependent glucose absorption. Taken together, these data support the view that GSL-enriched microdomains in the apical plasma membrane of enterocytes are involved in the regulation of intestinal functions.