Human immunodeficiency virus type 1 uses lipid raft-colocalized CD4 and chemokine receptors for productive entry into CD4(+) T cells

Translational aspects of sphingolipid metabolism

Sphingolipids, a major class of lipids in cell membranes, play diverse roles in biological processes. As bioactive and structural molecules, they have signaling activities and biophysical properties that are essential for regulating various cellular, tissue and systemic functions. Moreover, sphingolipids are receiving increasing attention as contributors to the pathogenesis of several human disorders, including, cancer, inflammation and neurological, immune and metabolic disorders. Small-molecule inhibitors and monoclonal antibodies that target sphingolipid metabolism recently enabled giant strides toward treatment of malignant and autoimmune disorders. Here, we review the emerging roles of sphingolipids in disease pathogenesis and the attendant possibilities for sphingolipid-based therapeutics.

Filamin-A regulates actin-dependent clustering of HIV receptors

Human immunodeficiency virus (HIV)-1 infection requires envelope (Env) glycoprotein gp120-induced clustering of CD4 and coreceptors (CCR5 or CXCR4) on the cell surface; this enables Env gp41 activation and formation of a complex that mediates fusion between Env-containing and target-cell membranes. Kinetic studies show that viral receptors are actively transported to the Env-receptor interface in a process that depends on plasma membrane composition and the actin cytoskeleton. The mechanisms by which HIV-1 induces F-actin rearrangement in the target cell remain largely unknown. Here, we show that CD4 and the coreceptors interact with the actin-binding protein filamin-A, whose binding to HIV-1 receptors regulates their clustering on the cell surface. We found that gp120 binding to cell receptors induces transient cofilin-phosphorylation inactivation through a RhoA-ROCK-dependent mechanism. Blockade of filamin-A interaction with CD4 and/or coreceptors inhibits gp120-induced RhoA activation and cofilin inactivation. Our results thus identify filamin-A as an adaptor protein that links HIV-1 receptors to the actin cytoskeleton remodelling machinery, which may facilitate virus infection.

Lipid rafts in health and disease

Lipid rafts are sphingolipid- and cholesterol-rich domains of the plasma membrane which contain a variety of signalling and transport proteins. Different subtypes of lipid rafts can be distinguished according to their protein and lipid composition. Caveolae are types of rafts that are rich in proteins of the caveolin family (caveolin-1, -2 and -3) which present a distinct signalling platform. The importance of lipid raft signalling in the pathogenesis of a variety of conditions, such as Alzheimer's, Parkinson's, cardiovascular and prion diseases, systemic lupus erythematosus and HIV, has been elucidated over recent years and makes these specific membrane domains an interesting target for pharmacological approaches in the cure and prevention of these diseases. This Review analyses the importance of lipid raft proteins and lipids in health and disease, with a focus on the current state of knowledge.

Novel delivery system enhances efficacy of antiretroviral therapy in animal model for HIV-1 encephalitis

Most potent antiretroviral drugs (e.g., HIV-1 protease inhibitors) poorly penetrate the blood-brain barrier. Brain distribution can be limited by the efflux transporter, P-glycoprotein (P-gp). The ability of a novel drug delivery system (block co-polymer P85) that inhibits P-gp, to increase the efficacy of antiretroviral drugs in brain was examined using a severe combined immunodeficiency (SCID) mouse model of HIV-1 encephalitis (HIVE). Severe combined immunodeficiency mice inoculated with HIV-1 infected human monocyte-derived macrophages (MDM) into the basal ganglia were treated with P85, antiretroviral therapy (ART) (zidovudine, lamivudine and nelfinavir (NEL)), or P85 and ART. Mice were killed on days 7 and 14, and brains were evaluated for levels of viral infection. Antiviral effects of NEL, P85, or their combination were evaluated in vitro using HIV-1 infected MDM and showed antiretroviral effects of P85 alone. In SCID mice injected with virus-infected MDM, the combination of ART-P85 and ART alone showed a significant decrease of HIV-1 p24 expressing MDM (25% and 33% of controls, respectively) at day 7 while P85 alone group was not different from control. At day 14, all treatment groups showed a significant decrease in percentage of HIV-1 infected MDM as compared with control. P85 alone and combined ART-P85 groups showed the most significant reduction in percentage of HIV-1 p24 expressing MDM (8% to 22% of control) that were superior to the ART alone group (38% of control). Our findings indicate major antiretroviral effects of P85 and enhanced in vivo efficacy of antiretroviral drugs when combined with P85 in a SCID mouse model of HIVE.

A TRIM5alpha-independent post-entry restriction to HIV-1 infection of macaque cells that is dependent on the path of entry

The replication of human immunodeficiency type-1 (HIV-1) is restricted in macaque cells, in part due to host factors that provide intrinsic immunity after entry. Here we show that a rhesus macaque epithelial cell line engineered to express human CD4, sMAGI cells, has at least two post-entry restrictions to HIV-1 replication: one that is dependent on a previously described post-entry restriction factor of macaque cells, TRIM5alpha, and another that is primarily TRIM5alpha-independent. The TRIM5alpha restriction, which was observed with particles that had an HIV-1 core pseudotyped with VSV-G envelope, is saturable and can be completely abrogated by introducing TRIM5alpha-specific siRNA into the cells. A similar TRIM5alpha-dependent restriction was observed when sMAGI cells expressing human CCR5 were infected with an R5-HIV-1. In contrast, even when viruses enter sMAGI cells using CD4 and an endogenous rhesus coreceptor at levels sufficient to saturate TRIM5alpha, they do not productively infect the sMAGI cells. Nor does treatment of sMAGI cells with TRIM5alpha-specific siRNA relieve this post-entry restriction; this was true whether the HIV-1 core was pseudotyped with SIV envelope or an R5-HIV-1 envelope. Together these data suggest that there is an alternate restriction to replication, here called Lv3, that is encountered by viruses that enter via interaction with CD4 and an endogenous rhesus coreceptor. Thus, these findings suggest that post-entry events are dependent upon the mechanism by which HIV-1 enters the cell.

Myosin IIA is involved in the endocytosis of CXCR4 induced by SDF-1alpha

Endocytosis of chemokine receptors regulates signal transduction initiated by chemokines, but the molecular mechanisms underlying this process are not fully defined. In this work, we assessed the involvement of the motor protein nonmuscle myosin heavy chain IIA (MIIA) in the endocytosis of CXCR4 induced by SDF-1alpha (also known as CXCL12) in T lymphocytes. Overexpression of the C-terminal half of MIIA inhibited the ligand-induced endocytosis of CXCR4, but not that of transferrin receptor. Targeting MIIA either by silencing its expression with small interfering RNA (siRNA) or by blebbistatin treatment also inhibited endocytosis of CXCR4. Inhibition of endocytosis of CXCR4 by targeting endogenous MIIA resulted in an increased migration of T cells induced by SDF-1alpha, and in the inhibition of the HIV-1-Env antifusogenic activity of this chemokine. Coimmunoprecipitation and protein-protein binding studies demonstrated that MIIA interacts with both the cytoplasmic tail of CXCR4 and beta-arrestin. Moreover, SDF-1alpha promotes a rapid MIIA-beta-arrestin dissociation. Our data reveal a novel role for MIIA in CXCR4 endocytosis, which involves its dynamic association with beta-arrestin and highlights the role of endogenous MIIA as a regulator of CXCR4 internalization and, therefore, the onset of SDF-1alpha signaling.

Modulation of neuronal CXCR4 by the micro-opioid agonist DAMGO

The chemokine receptor CXCR4 regulates neuronal survival and differentiation and is involved in a number of pathologies, including cancer and human immunodeficiency virus (HIV). Recent data suggest that chemokines act in concert with neurotransmitters and neuropeptides, such as opioids. This study aimed to determine whether mu-opioid agonists alter the effect of CXCL12 (the specific CXCR4 ligand) on central neurons. Neuronal expression of CXCR4 and micro-opioid receptors (MORs) was analyzed by Western blot, immunostaining, and flow cytometry. Single-cell studies showed that all CXCR4-positive neurons coexpress MORs. Treatment of neuronal cultures with the selective MOR agonist DAMGO or the endogenous peptide endomorphin-1 inhibited intracellular signaling pathways (ERK1/2 and Akt) activated by CXCL12. Furthermore, DAMGO abolished the neuroprotective effect of CXCL12 in N-methyl-d-aspartate (NMDA) neurotoxicity studies. The effects of DAMGO and endomorphin-1 were inhibited by a general or a micro-specific opioid receptor antagonist, and not caused by changes in neuronal CXCR4 levels. DAMGO did not affect CXCL12-induced internalization of CXCR4. The authors propose that interactions between MOR and CXCR4 signaling can modulate the action of CXCL12 on neuronal survival-which may have important implications to neuroAIDS as well as other neuroinflammatory disorders.

The role of cohort studies in drug development: clinical evidence of antiviral activity of serotonin reuptake inhibitors and HMG-CoA reductase inhibitors in the central nervous system

BACKGROUND

Effective antiretroviral therapy (ART) has reduced the incidence of HIV-associated neurocognitive impairment (HNCI) but its prevalence remains high. Clinical trials have yet to identify a consistently effective treatment for HNCI, other than ART, but in vitro data support that some drugs approved by the Food and Drug Administration (FDA) for other indications might benefit individuals with HNCI. Some of these drugs, such as serotonin reuptake inhibitors (SRIs) and HMG-CoA reductase inhibitors (statins), may do so by reducing HIV replication in the CNS and are already widely used by HIV-infected individuals.

METHODS

Six-hundred fifty-eight HIV-infected participants of the CHARTER cohort had a baseline assessment, which included comprehensive neuropsychological (NP) testing and HIV RNA measurements in plasma and cerebrospinal fluid (CSF). Four-hundred sixty-seven (71%) subjects used ART, 195 (30%) used SRIs, and 63 (10%) used statins.

RESULTS

SRI users were less likely to have HIV RNA levels in CSF above 50 copies (c)/mL (29 vs. 37% in non-SRI users, OR 0.69, p = 0.05). This association was most evident for three of the seven SRIs (citalopram, sertraline, and trazodone, or "antiviral" SRIs, combined 25 vs. 38% in non-SRI users, OR 0.56, p = 0.01) and was strongest in those not taking concomitant ART (61 vs. 83%, OR 0.31, p = 0.01). "Antiviral" SRI users also performed better on NP tests (median global deficit score 0.37 vs. 0.47, p = 0.04). Statin users were also less likely to have HIV RNA levels in CSF above 50 c/mL (16 vs. 37%, p < 0.001) but, in contrast to SRIs, the association was strongest in those taking ART (2 vs. 18%, p < 0.001). Statin use was not associated with better NP performance. Multivariate analyses indicated that the use of "antiviral" SRIs-but not statins-was associated with undetectable HIV RNA levels in CSF and better NP performance.

CONCLUSIONS

SRIs may reduce HIV replication in CSF and improve NP performance. This was particularly true for three SRIs-supporting differences in antiviral efficacy between drugs-in individuals who were not taking ART. In contrast, statins were not associated with lower HIV replication in CSF in multivariate analyses and were not associated with better NP performance. These analyses support the value of large observational cohort studies in identifying FDA-approved drugs that may be worth further investigation.

The role of cholesterol and sphingolipids in chemokine receptor function and HIV-1 envelope glycoprotein-mediated fusion

Background

HIV-1 entry into cells is a multifaceted process involving target cell CD4 and the chemokine receptors, CXCR4 or CCR5. The lipid composition of the host cell plays a significant role in the HIV fusion process as it orchestrates the appropriate disposition of CD4 and co-receptors required for HIV-1 envelope glycoprotein (Env)-mediated fusion. The cell membrane is primarily composed of sphingolipids and cholesterol. The effects of lipid modulation on CD4 disposition in the membrane and their role in HIV-1 entry have extensively been studied. To focus on the role of lipid composition on chemokine receptor function, we have by-passed the CD4 requirement for HIV-1 Env-mediated fusion by using a CD4-independent strain of HIV-1 Env.

Results

Cell fusion mediated by a CD4-independent strain of HIV-1 Env was monitored by observing dye transfer between Env-expressing cells and NIH3T3 cells bearing CXCR4 or CCR5 in the presence or absence of CD4. Chemokine receptor signaling was assessed by monitoring changes in intracellular [Ca²⁺] mobilization induced by CCR5 or CXCR4 ligand. To modulate target membrane cholesterol or sphingolipids we used Methyl-β-cyclodextrin (MβCD) or 1-phenyl-2-hexadecanoylamino-3-morpholino-1-propanol (PPMP), respectively. Treatment of the target cells with these agents did not change the levels of CD4 or CXCR4, but reduced levels of CCR5 on the cell surface. Chemokine receptor signalling was inhibited by cholesterol removal but not by treatment with PPMP. HIV-1 Env mediated fusion was inhibited by >50% by cholesterol removal. Overall, PPMP treatment appeared to slow down the rates of CD4-independent HIV-1 Env-mediated Fusion. However, in the case of CXCR4-dependent fusion, the differences between untreated and PPMP-treated cells did not appear to be significant.

Conclusion

Although modulation of cholesterol and sphingolipids has similar effects on CD4 -dependent HIV-1 Env-mediated fusion, sphingolipid modulation had little effect on CD4-independent HIV-1 Env-mediated fusion. Chemokine receptor function remained intact following treatment of cells with PPMP. Therefore such treatment may be considered a more suitable agent to inhibit CD4 dependent HIV-1 infection.

Lipid rafts play an important role in the early stage of severe acute respiratory syndrome-coronavirus life cycle

Lipid rafts are involved in the life cycle of many viruses. In this study, we showed that lipid rafts also play an important role in the life cycle of severe acute respiratory syndrome (SARS)-coronavirus (CoV). Cholesterol depletion by pretreatment of Vero E6 cells with methyl-β-cyclodextrin (MβCD) inhibited the production of SARS-CoV particles released from the infected cells. This inhibition was prevented by addition of cholesterol to the culture medium, indicating that the reduction of virus particle release was caused by the loss of cholesterol in the cell membrane. In contrast, cholesterol depletion at the post-entry stage (3 h post-infection) caused only a limited effect on virus particle release. Northern blot analysis revealed that the levels of viral mRNAs were significantly affected by pretreatment with MβCD, but not by treatment at 3 h post-infection. Interestingly, no apparent evidence for colocalization of angiotensin converting enzyme 2 with lipid rafts in the membrane of Vero E6 cells was obtained. These results suggest that lipid rafts could contribute to SARS-CoV infection in the early replication process in Vero E6 cells.

A novel nonnucleoside analogue that inhibits human immunodeficiency virus type 1 isolates resistant to current nonnucleoside reverse transcriptase inhibitors

Nonnucleoside reverse transcriptase (RT) inhibitors (NNRTIs) are important components of current combination therapies for human immunodeficiency virus type 1 (HIV-1) infection. However, their low genetic barriers against resistance development, cross-resistance, and serious side effects can compromise the benefits of the two current drugs in this class (efavirenz and nevirapine). In this study, we report a novel and potent NNRTI, VRX-480773, that inhibits viruses from efavirenz-resistant molecular clones and most NNRTI-resistant clinical HIV-1 isolates tested. In vitro mutation selection experiments revealed that longer times were required for viruses to develop resistance to VRX-480773 than to efavirenz. RT mutations selected by VRX-480773 after 3 months of cell culture in the presence of 1 nM VRX-480773 carried the Y181C mutation, resulting in a less-than-twofold increase in resistance to the compound. A virus containing the double mutation V106I-Y181C emerged after 4 months, causing a sixfold increase in resistance. Viruses containing additional mutations of D123G, F227L, and T369I emerged when the cultures were incubated with increasing concentrations of VRX-480773. Most of the resistant viruses selected by VRX-480773 are susceptible to efavirenz. Oral administration of VRX-480773 to dogs resulted in plasma concentrations that were significantly higher than those required for the inhibition of wild-type and mutant viruses. These results warrant further clinical development of VRX-480773 for the treatment of HIV infection in both NNRTI-naive and -experienced patients.

Vaccinia virus interactions with the cell membrane studied by new chromatic vesicle and cell sensor assays

The potential danger of cross-species viral infection points to the significance of understanding the contributions of nonspecific membrane interactions with the viral envelope compared to receptor-mediated uptake as a factor in virus internalization and infection. We present a detailed investigation of the interactions of vaccinia virus particles with lipid bilayers and with epithelial cell membranes using newly developed chromatic biomimetic membrane assays. This analytical platform comprises vesicular particles containing lipids interspersed within reporter polymer units that emit intense fluorescence following viral interactions with the lipid domains. The chromatic vesicles were employed as membrane models in cell-free solutions and were also incorporated into the membranes of epithelial cells, thereby functioning as localized membrane sensors on the cell surface. These experiments provide important insight into membrane interactions with and fusion of virions and the kinetic profiles of these processes. In particular, the data emphasize the significance of cholesterol/sphingomyelin domains (lipid rafts) as a crucial factor promoting bilayer insertion of the viral particles. Our analysis of virus interactions with polymer-labeled living cells exposed the significant role of the epidermal growth factor receptor in vaccinia virus infectivity; however, the data also demonstrated the existence of additional non-receptor-mediated mechanisms contributing to attachment of the virus to the cell surface and its internalization.

Novel nonnucleoside inhibitors that select nucleoside inhibitor resistance mutations in human immunodeficiency virus type 1 reverse transcriptase

Mutations in and around the catalytic site of the reverse transcriptase (RT) of human immunodeficiency virus type 1 (HIV-1) are associated with resistance to nucleoside RT inhibitors (NRTIs), whereas changes in the hydrophobic pocket of the RT are attributed to nonnucleoside RT inhibitor (NNRTI) resistance. In this study, we report a novel series of nonnucleoside inhibitors of HIV-1, exemplified by VRX-329747 and VRX-413638, which inhibit both NNRTI- and NRTI-resistant HIV-1 isolates. Enzymatic studies indicated that these compounds are HIV-1 RT inhibitors. Surprisingly, however, following prolonged (6 months) tissue culture selection, this series of nonnucleoside inhibitors did not select NNRTI-resistant mutations in HIV-1 RT. Rather, four mutations (M41L, A62T/V, V118I, and M184V) known to cause resistance to NRTIs and two additional novel mutations (S68N and G112S) adjacent to the catalytic site of the enzyme were selected. Although the M184V mutation appears to be the initial mutation to establish resistance, this mutation alone confers only a two- to fourfold decrease in susceptibility to VRX-329747 and VRX-413638. At least two additional mutations must accumulate for significant resistance. Moreover, while VRX-329747-selected viruses are resistant to lamivudine and emtricitabine due to the M184V mutation, they remain susceptible to zidovudine, stavudine, dideoxyinosine, abacavir, tenofovir, and efavirenz. These results directly demonstrate that VRX-329747 and VRX-413638 are novel nonnucleoside inhibitors of HIV-1 RT with the potential to augment current therapies.

Inhibition of vesicular stomatitis virus infection in epithelial cells by alpha interferon-induced soluble secreted proteins

Interferons (IFNs) are potent antiviral cytokines that inhibit infection by a wide spectrum of viruses by activating the Janus kinase/signal transducers and activators of transcription (JAK/STAT) pathway. Several IFN-induced antiviral proteins including 2',5'-oligoadenylate synthetase, dsRNA-activated protein kinase and Mx play a critical role in conferring the antiviral properties of IFN. However, studies have shown that additional antiviral factors are involved in addition to these proteins during IFN-mediated antiviral action. In an effort to characterize these novel antiviral factors, the antiviral mechanism of alpha IFN (IFN-alpha) against vesicular stomatitis virus (VSV) was investigated in human lung epithelial A549 cells. These studies demonstrated that soluble secreted antiviral proteins as the constituents of conditioned medium prepared from IFN-alpha-treated cells reduced VSV infectivity by more than 2 logs, compared with a 4 log inhibition observed following treatment of cells with IFN-alpha. The antiviral mechanism of these secreted proteins appeared to act at the level of cellular entry of VSV. Interestingly, the IFN-alpha-induced antiviral proteins were secreted independently of STAT1 (an essential component of the JAK/STAT pathway), demonstrating that the release of such extracellular soluble antiviral proteins from cells may represent an alternative mechanism of the antiviral defence strategy of IFN towards VSV infection.

HIV gp120-induced interaction between CD4 and CCR5 requires cholesterol-rich microenvironments revealed by live cell fluorescence resonance energy transfer imaging

Binding of the human immunodeficiency virus (HIV) envelope gp120 glycoprotein to CD4 and CCR5 receptors on the plasma membrane initiates the viral entry process. Although plasma membrane cholesterol plays an important role in HIV entry, its modulating effect on the viral entry process is unclear. Using fluorescence resonance energy transfer imaging, we have provided evidence here that CD4 and CCR5 localize in different microenvironments on the surface of resting cells. Binding of the third variable region V3-containing gp120 core to CD4 and CCR5 induced association between these receptors, which could be directly monitored by fluorescence resonance energy transfer on the plasma membrane of live cells. Depletion of cholesterol from the plasma membrane abolished the gp120 core-induced associations between CD4 and CCR5, and reloading cholesterol restored the associations in live cells. Our studies suggest that, during the first step of the HIV entry process, gp120 binding alters the microenvironments of unbound CD4 and CCR5, with plasma membrane cholesterol required for the formation of the HIV entry complex.

Disruption of lipid rafts enhances activity of botulinum neurotoxin serotype A

Botulinum neurotoxin serotype A (BoNT/A), one of seven serotypes of botulinum neurotoxin, is taken up by neurons of the peripheral nervous system. Within the neurons it catalyzes cleavage of the synaptosomal-associated protein having a mass of 25kDa, SNAP-25, thereby blocking neurotransmission. BoNT/A has been shown to interact with SV2, as well as gangliosides that are often found in lipid rafts. Lipid rafts are microdomains that can be found on the outer leaflet of the plasma membrane and are enriched in cholesterol and glycosphingolipids. To determine whether lipid rafts are needed for BoNT/A activity, those associated with the plasma membranes of murine N2a neuroblastoma cells were disrupted using either methyl-beta-cyclodextrin or filipin. Disruption of cholesterol-containing lipid rafts by either reagent did not prevent the action of BoNT/A on N2a cells, in fact activity was enhanced. While our results indicate that disruption of lipid rafts enhances BoNT/A activity, disruption of clathrin-dependent endocytosis appeared to be inhibitory.

Direct stimulation of T lymphocytes by immunosomes: virus-like particles decorated with T cell receptor/CD3 ligands plus costimulatory molecules

Many infectious viruses coevolved with the vertebrate immune system. During the assembly of enveloped viruses, lipid ordered domains of the host cell plasma membrane, called lipid rafts, frequently function as a natural meeting point for viral proteins. The role of lipid rafts in the organization of complex combinations of immune receptors during antigen presentation and T cell signaling is widely recognized. In our studies, we determined whether lipid rafts, virus budding, and molecular interactions during T cell activation could be brought into a novel context to create artificial antigen-presenting particles. We show here that cell-free virus-like particles (VLP) expressing a surrogate TCR/CD3 ligand (OKT3scFv) and the costimulator CD80 polyclonally activate human T cells independently of accessory cells. VLP expressing the glycoprotein epitope 33-41 of the lymphocytic choriomeningitis virus in the context of H-2D(b) activate and expand naïve, antigen-specific CD8(+) T lymphocytes and differentiate them into cytotoxic effector cells. Efficient targeting of T cell ligands to lipid rafts and ultimately to VLP is achieved by C-terminal introduction of glycosyl phosphatidyl inositol acceptor sequences, replacing transmembrane and intracellular domains. In this work, basic functions of immunostimulatory molecules meet virus biology and translate into a reductionist antigen-specific T lymphocyte-stimulating vehicle, which we refer to as immunosomes. A large variety of agonistic and antagonistic accessory molecules on genuine antigen-presenting cells may complicate the predictable manipulation of T cells as well as the analysis of selected receptor combinations, making immunosomes potentially useful reagents for such purposes in the future.

Membrane rafts in host-pathogen interactions

Central elements in the infection of mammalian cells with viral, bacterial and parasitic pathogens include the adhesion of the pathogen to surface receptors of the cell, recruitment of additional receptor proteins to the infection-site, a re-organization of the membrane and, in particular, the intracellular signalosome. Internalization of the pathogen results in the formation of a phagosome that is supposed to fuse with lysosomes to form phagolysosomes, which serve the degradation of the pathogen, an event actively prevented by some pathogens. In summary, these changes in the infected cell permit pathogens to trigger apoptosis (for instance of macrophages paralysing the initial immune response), to invade the cell and/or to survive in the cell, but they also serve the mammalian cell to defeat the infection, for instance by activation of transcription factors and the release of cytokines. Distinct membrane domains in the plasma membrane and intracellular vesicles that are mainly composed of sphingolipids and cholesterol or enriched with the sphingolipid ceramide, are critically involved in all of these events occurring during the infection. These membrane structures are therefore very attractive targets for novel drugs to interfere with bacterial, viral and parasitic infections.

Host cell targets for antiviral therapy: an update

The discovery of antiviral therpeutic targets has traditionally focussed on conserved viral gene products. Since viruses are obligate molecular parasites requiring and manipulating host cell functions, the identification of host proteins essential to viruses but not cellular survival represent attractive and underutilized targets for antiviral therapies. Numerous studies have identified host cell functions, pathways and proteins that are essential for viral propagation, from entry to replication, assembly and egress. Importantly, several studies demonstrate virus inhibition through nonlethal impairment of host cell function both in vitro and in vivo. In addition, high-throughput experimental platforms allowing global or near-global surveillance of host gene products in the context of virus replication are now being used to systematically identify host cell targets. Recent breakthrough studies that have employed such methods successfully to identify host proteins and pathways crucial for virus growth are discussed in this review.

Tri-substituted triazoles as potent non-nucleoside inhibitors of the HIV-1 reverse transcriptase

A new series of 1,2,4-triazoles was synthesized and tested against several NNRTI-resistant HIV-1 isolates. Several of these compounds exhibited potent antiviral activities against efavirenz- and nevirapine-resistant viruses, containing K103N and/or Y181C mutations or Y188L mutation. Triazoles were first synthesized from commercially available substituted phenylthiosemicarbazides, then from isothiocyanates, and later by condensing the desired substituted anilines with thiosemicarbazones.

The temperature arrested intermediate of virus-cell fusion is a functional step in HIV infection

HIV entry occurs via membrane-mediated fusion of virus and target cells. Interactions between gp120 and cellular co-receptors lead to both the formation of fusion pores and release of the HIV genome into target cells. Studies using cell-cell fusion assays have demonstrated that a temperature-arrested state (TAS) can generate a stable intermediate in fusion related events. Other studies with MLV pseudotyped with HIV envelope also found that a temperature sensitive intermediate could be generated as revealed by the loss of a fluorescently labeled membrane. However, such an intermediate has never been analyzed in the context of virus infection. Therefore, we used virus-cell infection with replication competent HIV to gain insights into virus-cell fusion. We find that the TAS is an intermediate in the process culminating in the HIV infection of a target cell. In the virion-cell TAS, CD4 has been engaged, the heptad repeats of gp41 are exposed and the complex is kinetically predisposed to interact with coreceptor to complete the fusion event leading to infection.

Strategies to improve non-viral vectors – potential applications in clinical transplantation

Prevention of acute rejection has been well controlled with immunosuppressive drugs. However, the long-term control of rejection is less satisfactory and the side effects of chronic usage of these drugs are far from acceptable. Thus, more imaginative options for therapy need to be explored. Gene therapy has potential promise in preserving allografts, preventing rejection and inducing tolerance. Despite this initial promise in many animal models, the translation of gene therapy to the clinical arena has been slow. This may be related in part to the deficiencies in vector development. Existing viral vectors are efficient at transducing allografts, but they induce inflammatory and pathogenic effects. Although the alternative non-viral systems are relatively innocuous, they are less efficient at gene delivery. This review systematically analyses the limitations of non-viral vector technology and the strategies that have been developed to overcome these limitations. Future development of non-viral vectors may have potential application in clinical transplantation.

Murine leukemia virus particles activate Rac1 in HeLa cells

A number of viruses, when they bind to cells, activate intracellular signals that facilitate post-binding steps of infection. To determine if retroviruses activate intracellular signaling, we transduced HeLa cells with amphotropic retroviruses produced by TelCeB6 cells and examined cell lysates for activated Rac1. We found that retroviruses activate Rac1. Rac1 activation was blocked when cells were depleted of cholesterol, cultured in suspension, or incubated with an anti-beta(1) integrin antibody, and when viruses were treated with heparinase III. Retrovirus activation of Rac1 did not require the amphotropic envelope protein. Gene transfer was reduced 2.4-fold when viruses were treated with heparinase III, but did not change when cells were transduced in the presence of function-blocking anti-beta(1) integrin antibodies. The implications of these findings with respect to retrovirus-cell interactions are discussed.

Dynamic tradeoffs in the raft-mediated entry of human immunodeficiency virus type 1 into cells

To initiate an infection human immunodeficiency virus type 1 (HIV-1) particles must first bind to receptors on the surface of their host cells, a process that eventually leads to fusion of viral and cellular membranes and release of the viral genome into the cytoplasm. Understanding the molecular mechanisms of these processes may enable the development of new anti-HIV strategies. Disagreement currently prevails on the role in virus entry of microdomains within the cellular plasma membrane known as lipid rafts. Experiments have suggested that lipid rafts, in their interactions with cellular receptors and viral particles, either promote or have minimal effect on viral entry. Here we develop a dynamic model for HIV-1 entry that enables us to identify and quantitatively assess tradeoffs that can arise from the clustering of receptors in rafts. Specifically, receptor clustering can be detrimental to the initiation of viral infection by reducing the probability that a virus particle finds its primary receptor, CD4. However, receptor clustering can also enable a virus particle, once bound, to rapidly form multivalent interactions with receptors and co-receptors that are required for virus-cell membrane fusion. We show how the resolution of such tradeoffs hinges on the level and spatial distribution of receptors and co-receptors on the cell surface, and we discuss implications of these effects for the design of therapeutics that inhibit HIV-1 entry.

Cholesterol and sphingolipids as lipid organizers of the immune cells’ plasma membrane: Their impact on the functions of MHC molecules, effector T-lymphocytes and T-cell death

The possible regulatory mechanisms by which glycosphingolipid- and cholesterol-rich membrane microdomains, caveolar and non-caveolar lipid rafts, control the immune response are continuously expanding. In the present overview we will focus on how these membrane-organizing lipids are involved, in collaboration with tetraspanin proteins, in the formation of distinct MHC-I and MHC-II microdomains at the cell surface and will analyze the possible roles of MHC compartmentation in the processes of antigen presentation and regulation of various stages of the cellular immune response. Some basic, lipid raft- and tetraspan mediated mechanisms involved in the formation and function of immunological synapses between various APCs and T-cells will also be discussed. Finally, a new aspect of immune regulation by sphingolipids will be briefly described, namely how can the death or stress signals, leading to ceramide accumulation, result in raft-associated regulatory platforms controlling cell death or antigen-induced, TCRmediated signaling of T-lymphocytes. The influence of these signals and their cross-talk on the fate (death or survival) of T-cells and the outcome of T-cell response will also be discussed.

Role of non-raft cholesterol in lymphocytic choriomeningitis virus infection via alpha-dystroglycan

Dystroglycan (DG) is an extracellular matrix receptor necessary for the development of metazoans from flies to humans and is also an entry route for various pathogens. Lymphocytic choriomeningitis virus (LCMV), a member of the family Arenaviridae, infects by binding to alpha-DG. Here, the role of cholesterol lipid rafts in infection by LCMV via alpha-DG was investigated. The cholesterol-sequestering drugs methyl-beta-cyclodextrin (MbetaCD), filipin and nystatin inhibited the infectivity of LCMV selectively, but did not affect infection by vesicular stomatitis virus. Cholesterol loading after depletion with MbetaCD restored infectivity to control levels. DG was not found in lipid rafts identified with the raft marker ganglioside GM1. Treatment with MbetaCD, however, enhanced the solubility of DG. This may reflect the association of DG with cholesterol outside lipid rafts and suggests that association of DG with non-raft cholesterol is critical for infection by LCMV through alpha-DG.

[Viral fusion mechanisms]

The majority of viral fusion proteins can be divided into two classes. The influenza hemagglutinin (HA) belongs to the class I fusion proteins and undergoes a series of conformational changes at acidic pH, leading to membrane fusion. The crystal structures of the prefusion and the postfusion forms of HA have been revealed in 1981 and 1994, respectively. On the basis of these structures, a model for the mechanism of membrane fusion mediated by the conformational changes of HA has been proposed. The flavivirus E and alphavirus E1 proteins belong to the class II fusion proteins and mediate membrane fusion at acidic pH. Their prefusion structures are distinct from that of HA. Last year, however, it has become evident that the postfusion structures of these class I and class II fusion proteins are similar. The paramyxovirus F protein belongs to the class I fusion proteins. In contrast to HA, an interaction between F and its homologous attachment protein is required for F to undergo the conformational changes. Since F mediates fusion at neutral pH, the infected cells can fuse with neighboring uninfected cells. The crystal structures of F and the attachment protein HN have recently been clarified, which will facilitate studies of the molecular mechanism of F-mediated membrane fusion.

Cholesterol and the interaction of proteins with membrane domains

Cholesterol is not uniformly distributed in biological membranes. One of the factors influencing the formation of cholesterol-rich domains in membranes is the unequal lateral distribution of proteins in membranes. Certain proteins are found in cholesterol-rich domains. In some of these cases, it is as a consequence of the proteins interacting directly with cholesterol. There are several structural features of a protein that result in the protein preferentially associating with cholesterol-rich domains. One of the best documented of these is certain types of lipidations. In addition, however, there are segments of a protein that can preferentially sequester cholesterol. We discuss two examples of these cholesterol-recognition elements: the cholesterol recognition/interaction amino acid consensus (CRAC) domain and the sterol-sensing domain (SSD). The requirements for a CRAC motif are quite flexible and predict that a large number of sequences could recognize cholesterol. There are, however, certain proteins that are known to interact with cholesterol-rich domains of cell membranes that have CRAC motifs, and synthetic peptides corresponding to these segments also promote the formation of cholesterol-rich domains. Modeling studies have provided a rationale for certain requirements of the CRAC motif. The SSD is a larger protein segment comprising five transmembrane domains. The amino acid sequence YIYF is found in several SSD and in certain other proteins for which there is evidence that they interact with cholesterol-rich domains. The CRAC sequences as well as YIYF are generally found adjacent to a transmembrane helical segment. These regions appear to have a strong influence of the localization of certain proteins into domains in biological membranes. In addition to the SSD, there is also a domain found in soluble proteins, the START domain, that binds lipids. Certain proteins with START domains specifically bind cholesterol and are believed to function in intracellular cholesterol transport. One of these proteins is StAR-D1, that also has a mitochondrial targeting sequence and plays an important role in delivering cholesterol to the mitochondria of steroidogenic cells.

Proteomic Analysis of Human Immunodeficiency Virus Using Liquid Chromatography/Tandem Mass Spectrometry Effectively Distinguishes Specific Incorporated Host Proteins

A major challenge to studying virus-incorporated host proteins is the fact that they are not encoded by the viral genome. We used Liquid Chromatography/Tandem Mass Spectrometry (LC-MS/MS) on whole virions to obtain a snapshot of the HIV-1 proteome. We identified known viral and host-cellular proteins and also identified novel components of HIV-1 and confirm these by traditional biochemical methods. Our comparison of wild-type and mutant viruses demonstrates that LC-MS/MS has the specificity to distinguish the presence/absence of a single host protein in intact virions.

Human immunodeficiency virus fusion to dendritic cells declines as cells mature

The maturation of dendritic cells (DCs) is associated with a diminished ability to support human immunodeficiency virus (HIV) replication; however, the precise step in the HIV life cycle impaired by DC maturation remains uncertain. Using an HIV virion-based fusion assay, we now show that HIV fusion to monocyte-derived DCs (MDDCs) both decreases and kinetically slows when DCs are induced to mature with poly(I:C) and tumor necrosis factor alpha. Specifically, laboratory-adapted CCR5-tropic 81A virions fused with markedly lower efficiency to mature MDDCs than immature DCs. In contrast, fusion of NL4-3, the isogenic CXCR4-tropic counterpart of 81A, was low in both immature and mature MDDCs. Fusion mediated by primary HIV envelopes, including seven CCR5- and four CXCR4-tropic envelopes, also decreased with DC maturation. The kinetics of virion fusion were also altered by both the state of DC maturation and the coreceptor utilized. Fusion of 81A and NL4-3 virions was delayed in mature compared to immature MDDCs, and NL4-3 fused more slowly than 81A in both mature and immature MDDCs. Surprisingly, primary envelopes with CXCR4 tropism mediated fusion to immature MDDCs with efficiencies similar to those of primary CCR5-tropic envelopes. This result contrasted with the marked preferential fusion of the laboratory-adapted 81A over NL4-3 in immature MDDCs and in ex vivo Langerhans cells, indicating that these laboratory-adapted HIV strains do not fully recapitulate all of the properties of primary HIV isolates. In conclusion, our results demonstrate that the defect in HIV replication observed in mature MDDCs stems at least in part from a decline in viral fusion.

A novel soluble mimic of the glycolipid, globotriaosyl ceramide inhibits HIV infection

OBJECTIVE

To determine the effect of a gp120 binding, non-cytotoxic soluble analogue of the glycosphingolipid (GSL), globotriaosyl ceramide (Gb3) on HIV infection in vitro.

DESIGN

HIV-1(IIIB) (X4 virus) infection in Jurkat and phytohaemagglutinin (PHA)/interleukin-2 (IL2) activated, peripheral blood mononuclear cells (PBMC), and HIV-1(Ba-L) (R5 virus) infection of PHA activated PBMC in vitro were assessed. We monitored cell surface markers, cell viability, and viral/host cell morphology to eliminate pleiotropic effects. Viral-host cell fusion was measured to further address any inhibitory mechanism.

METHODS

HIV infection was monitored by p24(gag) ELISA. CD4, CCR5, CXCR4 and apoptosis were determined by fluorescent antibody cell sorting. A model fusion system comprising a cell line transfected with either CD4 and CXCR4 or CCR5, cocultured with a cell line expressing gp120 from either X4-, R5-tropic HIV-1 or HIV-2 virions, was used. PHA/IL2 activated PBMC GSL synthesis was monitored by metabolic radiolabelling.

RESULTS

AdamantylGb3 blocked X4 and R5 virus infection with a 50% inhibitory concentration of approximately 150 microM. A reverse transcriptase and a protease-resistant X4 HIV-1 strain retained adamantylGb3 sensitivity. AdamantylGb3 had minimal effect on cell viability. Treated Jurkat cells showed a small increase in CCR5/CXCR4 expression and a slight, transient CD4 down-regulation, which was probably not related to the mechanism of inhibition. Electron microscopy showed normal viral and host cell morphology following adamantylGb3 treatment, and viral entry was blocked. AdamantylGb3 was able to prevent virus-host cell fusion irrespective of HIV strain or chemokine receptor preference.

CONCLUSIONS

These results suggest that adamantylGb3 may provide a new basis for blocking HIV infections, irrespective of HIV envelope/chemokine co-receptor preference or resistance to other therapeutics.

Prolonged exposure to the candidate microbicide C31G differentially reduces cellular sensitivity to agent re-exposure

Comparative assays of in vitro cytotoxicity using nonoxynol-9 (N-9) and the candidate microbicides C31G and sodium dodecyl sulfate (SDS) demonstrated that these agents, which are, respectively, characterized as nonionic, amphoteric, and anionic surfactants, differed in their concentration-dependent effects on cell viability, especially after prolonged exposure. We hypothesized that differences in cellular sensitivity may have been due, in part, to cellular changes induced by long-term exposure to each agent. To examine this possibility, HeLa cells were exposed to N-9, C31G, or SDS for extended periods of time and subsequently reassessed for sensitivity to each of these agents. Following 10 continuous days of C31G exposure, HeLa cells were less sensitive to a subsequent C31G exposure compared to cells that had not undergone long-term C31G treatment. Interestingly, long-term C31G exposure also changed subsequent sensitivity to N-9 but not SDS. In contrast, prolonged exposure to either N-9 or SDS did not reduce sensitivity to re-exposure. The effect of long-term C31G exposure was both concentration-dependent and transient, as treated cells reverted to pre-exposure sensitivity in a time-dependent manner following the cessation of C31G exposure. Lipid analyses of cells exposed to C31G for extended durations revealed altered phospholipid profiles relative to C31G-naïve cells. Experiments examining the individual components of C31G demonstrated the involvement of the amine oxide moiety in reductions in cellular sensitivity. These studies, which provide new information concerning the cytotoxicity of surfactant microbicides, suggest that cervicovaginal epithelial cells may have greater in vivo tolerance for products containing C31G through unique interactions between C31G and components of the cellular membranes.

Single-molecule analysis of human immunodeficiency virus type 1 gp120-receptor interactions in living cells

A quantitative description of the binding interactions between human immunodeficiency virus (HIV) type 1 envelope glycoproteins and their host cell surface receptors remains incomplete. Here, we introduce a single-molecule analysis that directly probes the binding interactions between an individual viral subunit gp120 and a single receptor CD4 and/or chemokine coreceptor CCR5 in living cells. This analysis differentiates single-molecule binding from multimolecule avidity and shows that, while the presence of CD4 is required for gp120 binding to CCR5, the force required to rupture a single gp120-coreceptor bond is significantly higher and its lifetime is much longer than those of a single gp120-receptor bond. The lifetimes of these bonds are themselves shorter than those of the P-selectin/PSGL-1 bond involved in leukocyte attachment to the endothelium bonds during an inflammation response. These results suggest an amended model of HIV entry in which, immediately after the association of gp120 to its receptor, gp120 seeks its coreceptor to rapidly form a new bond. This "bond transfer" occurs only if CCR5 is in close proximity to CD4 and CD4 is still attached to gp120. The analysis presented here may serve as a general framework to study mechanisms of receptor-mediated interactions between viral envelope proteins and host cell receptors at the single-molecule level in living cells.

Cholesterol-enriched membrane microdomains are required for inducing host cell cytoskeleton rearrangements in response to attaching-effacing Escherichia coli

The diarrheal pathogens enterohemorrhagic Escherichia coli (EHEC) O157:H7 strain CL56 and enteropathogenic Escherichia coli (EPEC) O127:H6 strain E2348/69 adhere intimately to epithelial cells through attaching-effacing lesions, which are characterized by rearrangements of the host cytoskeleton, intimate adherence, and destruction of microvilli. These cytoskeletal responses require activation of host signal transduction pathways. Lipid rafts are signaling microdomains enriched in sphingolipid and cholesterol in the plasma membrane. The effect of perturbing plasma membrane cholesterol on bacterial intimate adherence was assessed. Infection of both HEp-2 cells and primary skin fibroblasts with strains CL56 and E2348/69 caused characteristic rearrangements of the cytoskeleton at sites of bacterial adhesion. CL56- and E2348/69-induced cytoskeletal rearrangements were inhibited following cholesterol depletion. Addition of exogenous cholesterol to depleted HEp-2 cells restored cholesterol levels and rescued bacterially induced alpha-actinin mobilization. Quantitative bacterial adherence assays showed that EPEC adherence to HEp-2 cells was dramatically reduced following cholesterol depletion, whereas the adherence of EHEC remained high. Cytoskeletal rearrangements on skin fibroblasts obtained from children with Niemann-Pick type C disease were markedly reduced. These findings indicate that host membrane cholesterol contained in lipid rafts is necessary for the cytoskeletal rearrangements following infection with attaching-effacing Escherichia coli. Differences in initial adherence indicate divergent roles for host membrane cholesterol in the pathogenesis of EHEC and EPEC infections.

The role of glycosphingolipids in HIV signaling, entry and pathogenesis

Although HIV uses CD4 and coreceptors (CCR5 and CXCR4) for productive infection of T cells, glycosphingolipids (GSL) may play ancillary roles in lymphoid and non-lymphoid cells. Interactions of the HIV Envelope Glycoprotein (Env) with GSL may help HIV in various steps of its pathogenesis. Physical-chemical aspects of the interactions between HIV Env and GSL leading to CD4-dependent entry into lymphocytes, the role of GSL in HIV transcytosis, and CD4-independent entry into non-lymphoid cells are reviewed. An overview of signaling properties of HIV receptors is provided with some speculation on how GSL may play a role in these events by virtue of being in membrane rafts. Finally, we summarize how interactions between HIV and coreceptors leading to signaling and/or fusion can be analyzed by the use of various tyrosine kinase and cytoskeletal inhibitors.

The effect of HIV-1 Gag myristoylation on membrane binding

During the viral life cycle, an HIV protein, Gag, assembles at the host membrane, specifically at lipid raft regions, at very high concentrations leading to viral particle budding. Gag is post-translationally modified with an N-terminal myristate group which is thought to target Gag to lipid rafts thus aiding in assembly. Here we have analyzed the membrane binding of myristoylated HIV-1 Gag and a non-myristoylated form of HIV-1 Gag to various membrane models. After assessing the extent of myristoylation by HPLC and radiometric assays, we compared membrane binding using fluorescence methods. We found that myristoylated Gag shows a greater than twofold increase in binding affinity to model rafts. A structural model to explain these results is presented.

The significance of lipid composition for membrane activity: New concepts and ways of assessing function

In the last decade or so, it has been realised that membranes do not just have a lipid-bilayer structure in which proteins are embedded or with which they associate. Structures are dynamic and contain areas of heterogeneity which are vital for their formation. In this review, we discuss some of the ways in which these dynamic and heterogeneous structures have implications during stress and in relation to certain human diseases. A particular stress is that of temperature which may instigate adaptation in poikilotherms or appropriate defensive responses during fever in mammals. Recent data emphasise the role of membranes in sensing temperature changes and in controlling a regulatory loop with chaperone proteins. This loop seems to need the existence of specific membrane microdomains and also includes association of chaperone (heat stress) proteins with the membrane. The role of microdomains is then discussed further in relation to various human pathologies such as cardiovascular disease, cancer and neurodegenerative diseases. The concept of modifying membrane lipids (lipid therapy) as a means for treating such pathologies is then introduced. Examples are given when such methods have been shown to have benefit. In order to study membrane microheterogeneity in detail and to elucidate possible molecular mechanisms that account for alteration in membrane function, new methods are needed. In the second part of the review, we discuss ultra-sensitive and ultra-resolution imaging techniques. These include atomic force microscopy, single particle tracking, single particle tracing and various modern fluorescence methods. Finally, we deal with computing simulation of membrane systems. Such methods include coarse-grain techniques and Monte Carlo which offer further advances into molecular dynamics. As computational methods advance they will have more application by revealing the very subtle interactions that take place between the lipid and protein components of membranes - and which are so essential to their function.

Statins Could Be Used to Control Replication of Some Viruses, Including HIV-1

Statins are mainly known for their plasma cholesterol-lowering properties and are widely used for the prevention of cardiovascular diseases. They however also exert pleiotropic effects through a variety of mechanisms, among which several immunosuppressive effects that are unrelated to their cholesterol-lowering activity. Interestingly, there has been recent evidence of antiviral effects, including preliminary studies on the efficacy of statins against HIV-1. This paper more particularly focuses on the specific inhibition of the binding of leukocyte function-associated antigen-1 (LFA-1) to intercellular adhesion molecule (ICAM-1) by statins, independently of the inhibition of HMGCoA reductase. Targeting the statin-binding site within LFA-1 or regulating LFA-1 affinity by inhibiting prenylation of the small GTPases could prove useful to treat inflammatory, autoimmune diseases and possibly viral infections, including HIV-1.

Wild-type-like viral replication potential of human immunodeficiency virus type 1 envelope mutants lacking palmitoylation signals

Palmitoylation of the cytoplasmic domain of the human immunodeficiency type virus type 1 (HIV-1) envelope (Env) transmembrane protein, gp41, has been implicated in Env targeting to detergent-resistant lipid rafts, Env incorporation into the virus, and viral infectivity. In contrast, we provide evidence here to show that HIV-1 infectivity, Env targeting to lipid rafts, and Env incorporation into the virus are independent of cytoplasmic tail palmitoylation. The T-cell (T)-tropic HXB2-based virus, which utilizes CXCR4 as the entry coreceptor, carrying a Cys-to-Ser mutation at residue 764 or 837 or at both replicated with wild-type (WT) virus replication kinetics in CD4+ T cells. The properties of Env expression, precursor processing, cell surface expression, and Env incorporation of these three mutant viruses were normal compared to those of the WT virus. These three mutant Env proteins all effectively mediated one-cycle virus infection. When the Cys residues were replaced by Ala residues, all single and double mutants still retained the phenotypes of infectivity, Env incorporation, and lipid raft localization of the WT Env. When Cys-to-Ala substitutions were introduced into the macrophage (M)-tropic ConB virus, which utilizes CCR5 as the coreceptor, these mutations did not affect the replication potential, Env phenotypes, lipid raft targeting, or Env assembly into the virus of the WT Env. These T- and M-tropic mutants also productively replicated in human primary CD4+ T cells. Moreover, mutations at both Cys residues significantly reduced the level of palmitoylation of the Env. Our results together support the notion that palmitoylation of the cytoplasmic tail of the HIV-1 Env is not essential for the HIV-1 virus life cycle.

An envelope-determined, pH-independent endocytic route of viral entry determines the susceptibility of human immunodeficiency virus type 1 (HIV-1) and HIV-2 to Lv2 restriction

We identified a postentry restriction, termed Lv2, which determines the cellular tropism of two related human immunodeficiency virus type 2 (HIV-2) isolates and is dependent on the sequence of the capsid (CA) and envelope (Env) proteins. To explain the reliance on both CA and Env, we proposed that restrictive Envs deliver susceptible capsids to a compartment where Lv2 is active whereas nonrestrictive Envs deliver capsids into a compartment where Lv2 is either absent or less active. To test this model, we used compounds that affect endocytic pathways (ammonium chloride, bafilomycin A1, hypertonic sucrose) or lipid rafts (methyl-beta-cyclodextrin) to treat restrictive cells and show that restricted virus can be rescued from Lv2 if a lipid-raft-dependent, pH-independent endocytic pathway is inhibited. Furthermore, viral entry into HeLa/CD4 cells containing a tailless CD4 receptor, located outside lipid rafts, was fully permissive. Finally, we show that a variety of primary HIV-1 and HIV-2 viruses are susceptible to Lv2. Thus, we show that the route of entry, determined by the viral envelope, can influence cellular tropism by avoiding intracellular blocks to infection.

Murine coronavirus requires lipid rafts for virus entry and cell-cell fusion but not for virus release

Thorp and Gallagher first reported that depletion of cholesterol inhibited virus entry and cell-cell fusion of mouse hepatitis virus (MHV), suggesting the importance of lipid rafts in MHV replication (E. B. Thorp and T. M. Gallagher, J. Virol. 78:2682-2692, 2004). However, the MHV receptor is not present in lipid rafts, and anchoring of the MHV receptor to lipid rafts did not enhance MHV infection; thus, the mechanism of lipid rafts involvement is not clear. In this study, we defined the mechanism and extent of lipid raft involvement in MHV replication. We showed that cholesterol depletion by methyl beta-cyclodextrin or filipin did not affect virus binding but reduced virus entry. Furthermore, MHV spike protein bound to nonraftraft membrane at 4 degrees C but shifted to lipid rafts at 37 degrees C, indicating a redistribution of membrane following virus binding. Thus, the lipid raft involvement in MHV entry occurs at a step following virus binding. We also found that the viral spike protein in the plasma membrane of the infected cells was associated with lipid rafts, whereas that in the Golgi membrane, where MHV matures, was not. Moreover, the buoyant density of the virion was not changed when MHV was produced from the cholesterol-depleted cells, suggesting that MHV does not incorporate lipid rafts into the virion. These results indicate that MHV release does not involve lipid rafts. However, MHV spike protein has an inherent ability to associate with lipid rafts. Correspondingly, cell-cell fusion induced by MHV was retarded by cholesterol depletion, consistent with the association of the spike protein with lipid rafts in the plasma membrane. These findings suggest that MHV entry requires specific interactions between the spike protein and lipid rafts, probably during the virus internalization step.

HIV Type 1 Glycoprotein 120 Inhibits Human B Cell Chemotaxis to CXC Chemokine Ligand (CXCL) 12, CC Chemokine Ligand (CCL)20, and CCL21

We analyzed the modulation of human B cell chemotaxis by the gp120 proteins of various HIV-1 strains. X4 and X4/R5 gp120 inhibited B cell chemotaxis toward CXCL12, CCL20, and CCL21 by 40-50%, whereas R5 gp120 decreased inhibition by 20%. This gp120-induced inhibition was strictly dependent on CXCR4 or CCR5 and lipid rafts but not on CD4 or V(H)3-expressing BCR. Inhibition did not impair the expression or ligand-induced internalization of CCR6 and CCR7. Our data suggest that gp120/CXCR4 and gp120/CCR5 interactions lead to the cross-desensitization of CCR6 and CCR7 because gp120 does not bind CCR6 and CCR7. Unlike CXCL12, gp120 did not induce the activation of phospholipase Cbeta3 and PI3K downstream from CXCR4, whereas p38 MAPK activation was observed. Similar results were obtained if gp120-treated cells were triggered by CCL21 and CCL20. Our results are consistent with a blockade restricted to signaling pathways using phosphatidylinositol-4,5-bisphosphate as a substrate. X4 and X4/R5 gp120 induced the cleavage of CD62 ligand by a mechanism dependent on matrix metalloproteinase 1 and 3, CD4, CXCR4, Galpha(i), and p38 MAPK, whereas R5 gp120 did not. X4 and X4/R5 gp120 also induced the relocalization of cytoplasmic CD95 to the membrane and a 23% increase in CD95-mediated apoptosis. No such effects were observed with R5 gp120. The gp120-induced decrease in B cell chemotaxis and CD62 ligand expression, and increase in CD95-mediated B cell apoptosis probably have major deleterious effects on B cell responsiveness during HIV infection and in vaccination trials.

Potent inhibition of HIV-1 entry by (s4dU)35

We have previously reported the potent in vitro HIV-1 anti-reverse transcriptase activity of a 35-mer of 4-thio-deoxyuridylate [(s(4)dU)(35)]. In efforts to define its activity in a more physiological system, studies were carried out to determine the stage of viral infection that this compound mediates its anti-viral effect. Results of the studies reported herein show that (s(4)dU)(35) is nontoxic and is capable of inhibiting both single and multi-drug resistant HIV strains (IC(50): 0.8-25.4 microg/ml) in vitro. Besides its previously reported anti-RT activity, (s(4)dU)(35) mediated its antiviral action by preventing virus attachment (IC(50): 0.002-0.003 microg/ml), and was stable in vitro and slowly degraded by DNAses. Competition studies and fluorescence resonance energy transfer (FRET) experiments indicated that (s(4)dU)(35) preferentially binds to CD4 receptors, but not to CD48. Confocal laser scanning microscopy (CLSM) studies showed that (s(4)dU)(35) did not penetrate into the cells and colocalized with cell surface thioredoxin. Our studies identify (s(4)dU)(35) as a potential novel HIV entry inhibitor that may have utility as either a systemic antiretroviral or as a preventing agent for HIV transmission.

Pseudotypes of vesicular stomatitis virus with CD4 formed by clustering of membrane microdomains during budding

Many plasma membrane components are organized into detergent-resistant membrane microdomains referred to as lipid rafts. However, there is much less information about the organization of membrane components into microdomains outside of lipid rafts. Furthermore, there are few approaches to determine whether different membrane components are colocalized in microdomains as small as lipid rafts. We have previously described a new method of determining the extent of organization of proteins into membrane microdomains by analyzing the distribution of pairwise distances between immunogold particles in immunoelectron micrographs. We used this method to analyze the microdomains involved in the incorporation of the T-cell antigen CD4 into the envelope of vesicular stomatitis virus (VSV). In cells infected with a recombinant virus that expresses CD4 from the viral genome, both CD4 and the VSV envelope glycoprotein (G protein) were found in detergent-soluble (nonraft) membrane fractions. However, analysis of the distribution of CD4 and G protein in plasma membranes by immunoelectron microscopy showed that both were organized into membrane microdomains of similar sizes, approximately 100 to 150 nm. In regions of plasma membrane outside of virus budding sites, CD4 and G protein were present in separate membrane microdomains, as shown by double-label immunoelectron microscopy data. However, virus budding occurred from membrane microdomains that contained both G protein and CD4, and extended to approximately 300 nm, indicating that VSV pseudotype formation with CD4 occurs by clustering of G protein- and CD4-containing microdomains.

Chemokine receptor CCR5: insights into structure, function, and regulation

CC chemokine receptor 5 (CCR5) is a seven-transmembrane, G protein-coupled receptor (GPCR) which regulates trafficking and effector functions of memory/effector T-lymphocytes, macrophages, and immature dendritic cells. It also serves as the main coreceptor for the entry of R5 strains of human immunodeficiency virus (HIV-1, HIV-2). Chemokine binding to CCR5 leads to cellular activation through pertussis toxin-sensitive heterotrimeric G proteins as well as G protein-independent signalling pathways. Like many other GPCR, CCR5 is regulated by agonist-dependent processes which involve G protein coupled receptor kinase (GRK)-dependent phosphorylation, beta-arrestin-mediated desensitization and internalization. This review discusses recent advances in the elucidation of the structure and function of CCR5, as well as the complex mechanisms that regulate CCR5 signalling and cell surface expression.

Inhibition of certain strains of HIV-1 by cell surface polyanions in the form of cholesterol-labeled oligonucleotides

Cholesterol-labeled oligonucleotides were found several years ago to inhibit HIV-1 in tissue culture at nanomolar concentrations. We present evidence that this is mainly due to an electrostatic interaction between polyanionic oligonucleotide concentrated at the cell surface and a positively charged region in the V3 loop of the HIV-1 envelope protein. When added to tissue culture, cholesterol-labeled oligonucleotides became concentrated at the plasma membrane and potently inhibited virus entry and cell fusion mediated by the envelope protein of some X4 strains of HIV-1, but had little effect on fusion mediated by R5 strains of HIV-1, amphotropic MLV envelope protein, or VSV-G protein. Noncholesterol-labeled oligonucleotides did not bind to the cell surface or inhibit fusion. The pattern of susceptibility to cholesterol-labeled oligonucleotides among HIV-1 strains was the same as reported for nonmembrane-associating polyanions such as dextran sulfate, but the cholesterol-labeled oligonucleotides were effective at lower concentrations. Substitution of a basic 33 amino acid V3 loop sequence from the envelope protein of a resistant strain into a susceptible strain made the envelope protein resistant to inhibition. Inhibition by cholesterol-labeled oligonucleotides was abrogated by the polycation DEAE-dextran. Cholesterol-labeled oligonucleotides bound to nonraft regions of the plasma membrane and did not inhibit HIV virus binding to cells. Many infectious agents first associate with target cells via relatively nonspecific charge interactions; our data suggest that molecules that combine a membrane-targeting motif with multiple negative charges might be useful to modify these interactions.

Vaccinia virus penetration requires cholesterol and results in specific viral envelope proteins associated with lipid rafts

Vaccinia virus infects a wide variety of mammalian cells from different hosts, but the mechanism of virus entry is not clearly defined. The mature intracellular vaccinia virus contains several envelope proteins mediating virion adsorption to cell surface glycosaminoglycans; however, it is not known how the bound virions initiate virion penetration into cells. For this study, we investigated the importance of plasma membrane lipid rafts in the mature intracellular vaccinia virus infection process by using biochemical and fluorescence imaging techniques. A raft-disrupting drug, methyl-beta-cyclodextrin, inhibited vaccinia virus uncoating without affecting virion attachment, indicating that cholesterol-containing lipid rafts are essential for virion penetration into mammalian cells. To provide direct evidence of a virus and lipid raft association, we isolated detergent-insoluble glycolipid-enriched membranes from cells immediately after virus infection and demonstrated that several viral envelope proteins, A14, A17L, and D8L, were present in the cell membrane lipid raft fractions, whereas the envelope H3L protein was not. Such an association did not occur after virions attached to cells at 4 degrees C and was only observed when virion penetration occurred at 37 degrees C. Immunofluorescence microscopy also revealed that cell surface staining of viral envelope proteins was colocalized with GM1, a lipid raft marker on the plasma membrane, consistent with biochemical analyses. Finally, mutant viruses lacking the H3L, D8L, or A27L protein remained associated with lipid rafts, indicating that the initial attachment of vaccinia virions through glycosaminoglycans is not required for lipid raft formation.