Molecular events during the interaction of envelopes of myxo- and RNA-tumor viruses with cell membranes. A 270 MHz 1H nuclear magnetic resonance study

The nuclear magnetic resonance (NMR) spectra of chick embryo cells have been analyzed after exposure to Newcastle disease virus (NDV). Virions that contained the envelope glycoproteins in the cleaved form and, thus, had full biological activity have been compared to virions that had reduced infectivity due to the presence of uncleaved glycoprotein F. After exposure to infectious virus, drastic changes occurred in the signals assigned to choline and the hydrocarbon chains of fatty acids. These observations are interpreted to demonstrate alteration of the fluid lipid bilayer structure of the cell membranes. This is compatible with the concept of membrane fusion as a penetration mechanism for NDV. Virus containing uncleaved F glycoprotein did not alter the NMR spectra. This indicates that infection is blocked at the stage of penetration.Similar, though less pronounced, differences have been observed when the effects of highly infectious influenza virus containing the hemagglutinin in the cleaved form were compared to the effects of virus which had a lower infectivity due to the presence of uncleaved hemagglutinin. Thus, it appears that the hemagglutinin of influenza virus is involved in penetration and that cleavage is necessary for this function.Alterations of the NMR spectra of the membrane lipids have also been observed when susceptible chick embryo cells (C/E) were infected with Rous sarcoma virus of subgroup B. Such alterations did not occur when nonsusceptible cells (C/B) were used. Thus, infection appears to be blocked again at the stage of penetration.     

Detergent-resistant membrane fractions contribute to the total 1H NMR-visible lipid signal in cells.

Leukocytes and other cells show an enhanced intensity of mobile lipid in their 1H NMR spectra under a variety of conditions. Such conditions include stimulation, which has recently been shown to involve detergent-resistant, plasma membrane domains (DRMs) often called lipid rafts. As there is much speculation surrounding the origin of cellular NMR-visible lipid, we analysed subcellular fractions, including DRMs, by NMR spectroscopy. We demonstrated that DRMs isolated by density gradient centrifugation from lymphoid (CEM-T4, stimulated Jurkat cells), and monocytoid (THP-1) cells produced NMR-visible, lipid signals. Large scale subfractionation of THP-1 cells determined that while cytoplasmic lipid droplets constituted much of the total NMR-visible lipid, the contribution of DRMs was significant. Qualitative and quantitative lipid analyses revealed that DRMs and lipid droplets differed in their lipid composition. DRMs were enriched in cholesterol and ganglioside GM1, and contained relatively unsaturated fatty acids compared with the lipid droplets. Both lipid droplets and DRMs contained neutral lipids (triacylgycerols, cholesterol ester, fatty acids in THP-1 cells) that could, in addition to phospholipids, contribute to the NMR-visible lipid. The lipid droplets also exhibited different protein profiles and contained 500-fold less protein than DRMs, confirming that DRMs and droplets were fractionated as separate entities. The NMR-visible lipid in DRMs is therefore unlikely to be a contaminant from lipid droplets. We propose a micropartitioning of the NMR-visible mobile lipid of whole cells between intracellular lipid droplets, where most of this lipid resides, and detergent-resistant plasma membrane domains.     

High-resolution mono- and multidimensional magic angle spinning 1H nuclear magnetic resonance of membrane peptides in nondeuterated lipid membranes and H2O.

High-speed (14 kHz) solid-state magic angle spinning (MAS) 1H NMR has been applied to several membrane peptides incorporated into nondeuterated dilauroyl or dimyristoylphosphatidylcholine membranes suspended in H2O. It is shown that solvent suppression methods derived from solution NMR, such as presaturation or jump-return, can be used to reduce water resonance, even at relatively high water content. In addition, regioselective excitation of 1H peptide resonances promotes an efficient suppression of lipid resonances, even in cases where these are initially two orders of magnitude more intense. As a consequence, 1H MAS spectra of the peptide low-field region are obtained without interference from water and lipid signals. These display resonances from amide and other exchangeable 1H as well as from aromatic nonexchangeable 1H. The spectral resolution depends on the specific types of resonance and membrane peptide. For small amphiphilic or hydrophobic oligopeptides, resolution of most individual amide resonance is achieved, whereas for the transmembrane peptide gramicidin A, an unresolved amide spectrum is obtained. Partial resolution of aromatic 1H occurs in all cases. Multidimensional 1H-MAS spectra of membrane peptides can also be obtained by using water suppression and regioselective excitation. For gramicidin A, F2-regioselective 2D nuclear Overhauser effect spectroscopy (NOESY) spectra are dominated by intermolecular through-space connectivities between peptide aromatic or formyl 1H and lipid 1H. These appear to be compatible with the known structure and topography of the gramicidin pore. On the other hand, for the amphiphilic peptide leucine-enkephalin, F2-regioselective NOESY spectra mostly display cross-peaks originating from though-space proximities of amide or aromatic 1H with themselves and with aliphatic 1H. F3-regioselective 3D NOESY-NOESY spectra can be used to obtain through-space correlations within aliphatic 1H. Such intrapeptide proximities should allow determination of the conformation of the peptide in membranes. It is suggested that high-speed MAS multidimensional 1H NMR of peptides in nondeuterated membranes and in H2O can be used for studies of both peptide structure and lipid-peptide interactions.     

Infection of monocytic cells by HIV1: combined role of FcR and CD4

Human immunodeficiency virus (HIV) complexed with human anti-HIV IgG can attach to Fcγ receptors (Fch) of mononuclear phagocytes. To determine whether the FcR-mediated infection that results also requires interaction between HIV gp 120 and cell membrane CD4, monocytic cells of the U937 line were transiently treated with phorbol 12, 13-dibutyrate (PDB) so that they temporarily presented a CD4⁻FcR⁺ phenotype at the time of HIV infection. HIV production was not abolished, but only significantly delayed after infection of these cells with free virus. Leu3a monoclonal antibody or soluble recombinant CD4 completely blocked this delayed infection. This indicates that enough CD4 still remained at the membrane to allow infection of a reduced cell number. Infection of PDB-treated cells with virus preincubated with high anti-HIV IgG concentrations was inhibited, contrasting with what was observed with control cells infected under the same conditions. Inhibition of infection was also observed when HIV became attached to untreated U937 cells through the binding of CD4-IgG hybrid molecules to FcR. Thus, the binding of IgG-coated virus to FcR is not sufficient in itself to elicit productive infection of monocytic cells, which still requires the interaction of viral gp120 and membrane CD4.     

Cell surface involvement in cancer metastasis: an NMR study

NMR spectroscopy is one of the few techniques which has the sensitivity to detect subtle changes to the surface chemistry of cells. It has previously been demonstrated that high resolution ¹H NMR methods can distinguish tumour cells with the capacity to metastasise and this information appears to arise from a type of proteolipid in or attached to the plasma membrane. Here we report that the ¹H NMR signal, which we have used to identify metastatic cells in rat tumours, is significantly reduced in intensity after cultured cells are treated with trypsin/EDTA. The long T₂ relaxation value (⪢ 350 ms) observed in metastatic cells is absent after enzyme treatment. 2D scalar correlated NMR (COSY) spectra of these treated cells show that a cross peak normally associated with malignancy and metastatic disease is markedly reduced. These findings indicate that the plasma membrane lipid particle which generates the high resolution spectrum is directly affected by trypsin/EDTA. Alterations to the cell surface properties were also demonstrated in vivo since reduced numbers of metastases were observed in animals injected with enzyme-treated cells. The correlation between the absence of a long T₂ relaxation value and the diminished numbers of metastases in animals suggests that the plasma membrane particle is involved in the metastatic process.     

Impact of exogenous lysolipids on sensitive and multidrug resistant K562 cells: 1H NMR studies

The ability of lysolipids to enter into a membrane bi-layer and disturb the membrane structure was used to study the behavior of K562 erythroleukemic cells, K562 wild type (K562wt) as well as the multidrug resistant cells K562adr. Both types of cells, when analyzed by proton NMR spectroscopy exhibit the high resolution signals assigned to so-called "mobile lipid" signals, which, in most cases, are located outside the lipid bi-layer as lipid droplets. In order to perform these studies, the K562wt and K562adr cells were treated for 48h with lysophosphatidylcholine oleoyl (LPC18), lysophosphatidylcholine palmitoyl (LPC16) and L-alpha-lysophosphatidyslerine (LPS). After evaluating toxicity of lysolipids, proton NMR of whole treated cells was used to analyze the mobile lipid content. Nile red staining and fluorescence microscopy were used to detect the presence of intracellular lipid droplets. Membrane lipid asymmetry perturbation was estimated by annexin V staining with use of flow cytometry. Using fluorescence spectroscopy the functioning of P-glycoprotein (P-gp) responsible for multidrug resistance was also evaluated after the treatment with lysolipids. Lysolipids were found to be more toxic for K562wt than for K562adr cells. LPS and LPC16 produced an increased of a mobile lipid NMR signal and amount of lipid droplets in K562wt cells only. LPC18, with the lowest toxicity, has shown more intense effects on NMR spectra with a large increase of lipid NMR signal without changes in lipid droplet staining. The functioning of the P-gp pump and membrane asymmetry were not modified by any of the lysolipids used.     

1H NMR visible lipids in the life and death of cells

A functionally and metabolically interesting class of cell lipid can be observed by 1H nuclear magnetic resonance (NMR) spectroscopy in situ. These prominent resonances are not only associated with malignancy and cell death, but also act as heralds of benign processes, such as cell activation and proliferation. Originally, these NMR observations were explained with a membrane lipid microdomain model. However, recent studies have identified intracellular droplets, so called lipid bodies, as important contributors to these resonances. This finding bears novel implications for our understanding and assessment of lipid biochemistry in the life and death of cells.     

Differences among human immunodeficiency virus strains in their capacities to induce cytolysis or persistent infection of a lymphoblastoid cell line immortalized by Epstein-Barr virus.

Four strains of human immunodeficiency virus (HIV) manifest consistent differences in biologic behavior after infection of the X50-7 line of human umbilical cord lymphocytes immortalized by Epstein-Barr virus (EBV). Some dilutions of the first strain examined, human T-cell lymphotropic virus type III B, which is derived from a pool of patient isolates propagated in H9 cells, caused transient cytopathic effects (CPE) followed by recovery of a subpopulation of X50-7 cells which became virus carrier cultures. Other dilutions of the same virus stock completely lysed X50-7 cells. Two other strains, RF2 and YW, both from individual patients with acquired immune deficiency syndrome, always induced complete cytolysis of X50-7 cells at all dilutions which infected the cells. However, RF2 did establish persistent infection of H9 cells. A fourth strain, PH1-MN, from a child with acquired immune deficiency syndrome-related complex, induced only transient CPE in X50-7 and H9 cells, which thereafter always recovered to form carrier cultures. For all four strains, the dilutions of HIV stocks which caused CPE corresponded to dilutions which resulted in the detection of HIV polypeptides by immunoblot. Cytolysis in HIV-infected X50-7 cells was accompanied by a decrease in the amount of EBV nuclear antigen; however, HIV infection did not induce EBV replication. Thus CPE in X50-7 cells is due to replication of HIV per se and not to activation of EBV. The observations indicate that there are differences in the cytolytic properties of HIVs and that these differences are influenced by the target cell.     

Evidence for budding of human immunodeficiency virus type 1 selectively from glycolipid-enriched membrane lipid rafts

A number of recent studies have demonstrated the significance of detergent-insoluble, glycolipid-enriched membrane domains or lipid rafts, especially in regard to activation and signaling in T lymphocytes. These domains can be viewed as floating rafts composed of sphingolipids and cholesterol which sequester glycosylphosphatidylinositol (GPI)-linked proteins, such as Thy-1 and CD59. CD45, a 200-kDa transmembrane phosphatase protein, is excluded from these domains. We have found that human immunodeficiency virus type 1 (HIV-1) particles produced by infected T-cell lines acquire the GPI-linked proteins Thy-1 and CD59, as well as the ganglioside GM1, which is known to partition preferentially into lipid rafts. In contrast, despite its high expression on the cell surface, CD45 was poorly incorporated into virus particles. Confocal fluorescence microscopy revealed that HIV-1 proteins colocalized with Thy-1, CD59, GM1, and a lipid raft-specific fluorescent lipid, DiIC(16)(3), in uropods of infected Jurkat cells. CD45 did not colocalize with HIV-1 proteins and was excluded from uropods. Dot immunoassay of Triton X-100-extracted membrane fractions revealed that HIV-1 p17 matrix protein and gp41 were present in the detergent-resistant fractions and that [(3)H]myristic acid-labeled HIV Gag showed a nine-to-one enrichment in lipid rafts. We propose a model for the budding of HIV virions through lipid rafts whereby host cell cholesterol, sphingolipids, and GPI-linked proteins within these domains are incorporated into the viral envelope, perhaps as a result of preferential sorting of HIV Gag to lipid rafts.     

Use of a novel human immunodeficiency virus type 1 reporter virus expressing human placental alkaline phosphatase to detect an alternative viral receptor.

We report here on the construction and use of a novel human immunodeficiency virus (HIV) type 1 reporter vector, HIV-AP, that encodes human placental alkaline phosphatase. Upon staining with chromogenic alkaline phosphatase substrates 24 to 36 h postinfection, cells infected with HIV-AP develop an intense purple color and can then be counted under a dissecting microscope. Alternatively, HIV-AP infectivity can be quantitated and infected cells can be sorted by a fluorescence-activated cell sorter after staining with a fluorescent alkaline phosphatase substrate. The assay is rapid and accurate, has very low background in a variety of cell lines and primary cells, and is not restricted to use in human cells. Infectious HIV-AP can be pseudotyped by various HIV or murine leukemia virus envelope glycoproteins. Using this virus, we have addressed the long-standing question of CD4-independent infection of cells by HIV. Our results confirm the presence on a human osteosarcoma cell line of an alternative receptor for HIV infection that functions with an efficiency approximately 1/20 that of CD4.     

Proton NMR study of the heme environment in bacterial quinol oxidases

The heme environment and ligand binding properties of two relatively large membrane proteins containing multiple paramagnetic metal centers, cytochrome bo3 and bd quinol oxidases, have been studied by high field proton nuclear magnetic resonance (NMR) spectroscopy. The oxidized bo3 enzyme displays well-resolved hyperfine-shifted 1H NMR resonance assignable to the low-spin heme b center. The observed spectral changes induced by addition of cyanide to the protein were attributed to the structural perturbations on the low-spin heme (heme b) center by cyanide ligation to the nearby high-spin heme (heme o) of the protein. The oxidized hd oxidase shows extremely broad signals in the spectral region where protons near high-spin heme centers resonate. Addition of cyanide to the oxidized bd enzyme induced no detectable perturbations on the observed hyperfine signals, indicating the insensitive nature of this heme center toward cyanide. The proton signals near the low-spin heme b558 center are only observed in the presence of 20% formamide, consistent with a critical role of viscosity in detecting NMR signals of large membrane proteins. The reduced bd protein also displays hyperfine-shifted 1H NMR signals, indicating that the high-spin heme centers (hemes b595 and d) remain high-spin upon chemical reduction. The results presented here demonstrate that structural changes of one metal center can significantly influence the structural properties of other nearby metal center(s) in large membrane paramagnetic metalloproteins.     

Human immunodeficiency virus type 1 induces 1-beta-D-arabinofuranosylcytosine resistance in human H9 cell line.

We have found that chronically HIV-1(IIIB)-infected H9 cells showed 21-fold resistance to 1-beta-D-arabinofuranosylcytosine (ARA-C) compared with uninfected H9 cells. In the infected H9 cells, a 37% increase of dCTP pool and a 34% increase of dATP were observed, and no alteration of dTTP and dGTP was observed, compared with the uninfected H9 cells. A marked decrease of ARA-CTP generation was observed in the infected H9 cells after 3-h incubation with 0.1-10 microM ARA-C. The level of deoxycytidine kinase activity with ARA-C as substrate was similar in both the infected and the uninfected cells; however, a 37-fold increase of cytidine deaminase activity was observed in the infected H9 cells. These results indicate that the induction of cytidine deaminase activity by HIV-1(IIIB) infection conferred ARA-C resistance to H9 cells. This conclusion was supported by the observation that a marked reversal of ARA-C resistance in the infected H9 cells occurred after treatment with the inhibitor of cytidine deaminase, 3,4,5,6-tetrahydrouridine. The understanding of these cellular alterations in drug sensitivity may facilitate the development of effective therapeutic strategies against HIV-1-infected cells.     

HIV-1 tat protein modulates the generation of cytotoxic T cell epitopes by modifying proteasome composition and enzymatic activity

Tat, the trans activation protein of HIV, is produced early upon infection to promote and expand HIV replication and transmission. However, Tat appears to also have effects on target cells, which may affect Ag recognition both during infection and after vaccination. In particular, Tat targets dendritic cells and induces their maturation and Ag-presenting functions, increasing Th1 T cell responses. We show in this work that Tat modifies the catalytic subunit composition of immunoproteasomes in B and T cells either expressing Tat or treated with exogenous biological active Tat protein. In particular, Tat up-regulates latent membrane protein 7 and multicatalytic endopeptidase complex like-1 subunits and down-modulates the latent membrane protein 2 subunit. These changes correlate with the increase of all three major proteolytic activities of the proteasome and result in a more efficient generation and presentation of subdominant MHC-I-binding CTL epitopes of heterologous Ags. Thus, Tat modifies the Ag processing and modulates the generation of CTL epitopes. This may have an impact on both the control of virally infected cells during HIV-1 infection and the use of Tat for vaccination strategies.     

Electrorotation studies of baby hamster kidney fibroblasts infected with herpes simplex virus type 1

The dielectric properties of baby hamster kidney fibroblast (BHK(C-13)) cells have been measured using electrorotation before and after infection with herpes simplex virus type 1 (HSV-1). The dielectric properties and morphology of the cells were investigated as a function of time after infection. The mean specific capacitance of the uninfected cells was 2.0 microF/cm2, reducing to a value of 1. 5 microF/cm2 at 12 h after infection. This change was interpreted as arising from changes in the cell membrane morphology coupled with alterations in the composition of the cell membrane as infection progressed. The measured changes in the cell capacitance were correlated with alterations in cellular morphology determined from scanning electron microscope (SEM) images. Between 9 and 12 h after infection the internal permittivity of the cell exhibited a rapid change, reducing in value from 75epsilono to 58epsilono, which can be correlated with the generation of large numbers of Golgi-derived membrane vesicles and enveloped viral capsids. The data are discussed in relation to the known life cycle of HSV-1 and indicate that electrorotation can be used to observe dynamic changes in both the dielectric and morphological properties of virus-infected cells. Calculations of the dielectrophoretic spectrum of uninfected and infected cells have been performed, and the results show that cells in the two states could be separated using appropriate frequencies and electrode arrays.     

Human macrophages infected with a high burden of ESAT-6-expressing M. tuberculosis undergo caspase-1- and cathepsin B-independent necrosis

Mycobacterium tuberculosis (Mtb) infects lung macrophages, which instead of killing the pathogen can be manipulated by the bacilli, creating an environment suitable for intracellular replication and spread to adjacent cells. The role of host cell death during Mtb infection is debated because the bacilli have been shown to be both anti-apoptotic, keeping the host cell alive to avoid the antimicrobial effects of apoptosis, and pro-necrotic, killing the host macrophage to allow infection of neighboring cells. Since mycobacteria activate the NLRP3 inflammasome in macrophages, we investigated whether Mtb could induce one of the recently described inflammasome-linked cell death modes pyroptosis and pyronecrosis. These are mediated through caspase-1 and cathepsin-B, respectively. Human monocyte-derived macrophages were infected with virulent (H37Rv) Mtb at a multiplicity of infection (MOI) of 1 or 10. The higher MOI resulted in strongly enhanced release of IL-1β, while a low MOI gave no IL-1β response. The infected macrophages were collected and cell viability in terms of the integrity of DNA, mitochondria and the plasma membrane was determined. We found that infection with H37Rv at MOI 10, but not MOI 1, over two days led to extensive DNA fragmentation, loss of mitochondrial membrane potential, loss of plasma membrane integrity, and HMGB1 release. Although we observed plasma membrane permeabilization and IL-1β release from infected cells, the cell death induced by Mtb was not dependent on caspase-1 or cathepsin B. It was, however, dependent on mycobacterial expression of ESAT-6. We conclude that as virulent Mtb reaches a threshold number of bacilli inside the human macrophage, ESAT-6-dependent necrosis occurs, activating caspase-1 in the process.     

Inhibition of bfl-1/A1 by siRNA inhibits mycobacterial growth in THP-1 cells by enhancing phagosomal acidification

Virulent tubercle bacilli inhibit apoptosis to establish a safe environment within the host cells. Here, we report that NF-kappaB dependent antiapoptotic protein bfl-1/A1 plays an important role in this process. Both virulent and avirulent mycobacteria bearing THP-1 cells expressed considerable amount of bfl-1/A1 after 4 h of infection. However, after 48 h of infection, bfl-1/A1 expression was evident only in Mycobacterium tuberculosis H37Rv but not in M. tuberculosis H37Ra infected cells. When parallel experiments were performed with Human monocyte-derived macrophages (MDMs), differential expression of bfl-1/A1 mRNA was observed in case of M. tuberculosis H37Rv and M. tuberculosis H37Ra infection. siRNA mediated inhibition of bfl-1/A1 induced apoptosis in M. tuberculosis H37Rv infected THP-1 and MDMs. Reduction in intracellular mycobacterial growth was observed in bfl-1/A1 siRNA transfected, M. tuberculosis H37Rv infected THP-1 cells. Enhancement of phagosome-lysosome fusion was observed in bfl-1/A1 siRNA treated and M. tuberculosis H37Rv infected THP-1 cells. These results clearly indicated that differential expression of bfl-1/A1 in M. tuberculosis H37Rv and M. tuberculosis H37Ra infected THP-1 cells probably account for the difference in infection outcome.