Functional role of human immunodeficiency virus type 1 vpu

Genetic regulation of human immunodeficiency virus

Human immunodeficiency virus (HIV) has a complex life cycle in which both cellular and virus-encoded factors participate to determine the level of virus production. Two of the viral genes, tat and rev, are essential for virus replication and encode novel trans-activators that interact specifically with their cognate RNA target elements. Elucidation of their mechanisms of action is likely to expand our knowledge of gene regulation at transcriptional and posttranscriptional levels in the eukaryotic cell. Several viral genes (vif, vpu, and vpr) facilitate virus infection and/or release and may play a role in target cell tropism and infection in vivo. The functions of yet other viral genes (nef, vpt) remain unclear. Recent data also suggest that the tat gene product may have a role in HIV pathogenesis that goes beyond trans-activating virus expression. It can potentially impact on uninfected cells as a diffusible molecule and alter the growth of different cell types.

Effect of mutations affecting the p6 gag protein on human immunodeficiency virus particle release

Mutations in sequences at the C terminus of the capsid precursor protein of human immunodeficiency virus type 1 that affect the viral p6 protein prevent release of budded virus particles from the cell surface. The experiments reported here define an important step in the life cycle of the virus, the release of the budded particle from a tether that binds the assembled particle to the cell surface. Inhibition of the release of the viral capsid proteins by interferon alpha indicates that this step of virus maturation may be sensitive to inhibition by antiviral drugs.

Structural characteristics of the M2 protein of influenza A viruses: evidence that it forms a tetrameric channel

The evidence presented shows that the M2 protein of influenza A viruses exists in infected cells as a homotetramer composed of two disulfide-linked dimers held together by noncovalent interactions. The amphiphilic nature of the transmembrane alpha-helical domain is consistent with the protein forming a transmembrane channel with which amantadine, the specific anti-influenza A drug, interacts. Together these features provide a structural basis for the hypothesis that M2 has a proton translocation function capable of regulating the pH of vesicles of the trans-Golgi network, a role important in promoting the correct maturation of the hemagglutinin glycoprotein.

Inhibition of human immunodeficiency virus type 1 morphogenesis in T cells by alpha interferon

Some murine retroviruses exhibit altered release of virus when cells are treated with alpha interferon (IFN-alpha), resulting in the accumulation of intracellular virions in cytoplasmic vacuoles. In studies of the inhibitory effect of IFN-alpha (Wellferon) on acute human immunodeficiency virus type 1 infection of human T-cell lines, we found that in C3 cells, the 50% effective concentration was 9 U/ml and the 90% effective concentration was 310 U/ml. There was no apparent accumulation of intracellular particles detected by p24 antigen levels or by processing the cells for electron microscopy. Extracellular reverse transcriptase activity and p24 levels decreased in parallel with increasing IFN, whereas the intracellular viral proteins decreased only slightly. By electron microscopy, cells treated with higher concentrations of IFN (512 U/ml) disclosed very few particles budding into extracellular spaces; no intracellular particles could be seen, despite nearly normal levels of intracellular viral protein detected by the p24 antigen assay and correct processing detected by Western blot (immunoblot) analysis. Thus in human immunodeficiency virus-infected cells, the major block produced by IFN-alpha appeared to be late in the viral cycle at the morphogenesis stage of virion production. Chronically infected Jurkat cells treated with IFN appeared to be inhibited in growth rate, as virus production decreased proportionally with cell number.

The complete nucleotide sequence of a pathogenic molecular clone of simian immunodeficiency virus

The complete nucleotide sequence of an infectious clone of simian immunodeficiency virus of macaques, SIVmac239, has been determined. Virus produced from this molecular clone causes AIDS in rhesus monkeys in a time frame suitable for laboratory investigation. The proviral genome including both long terminal repeats is 10,279 base pairs in length and contains open reading frames for gag, pol, vif, vpr, vpx, tat, rev, and env. The nef gene contains an in-frame premature stop after the 92nd codon. At the nucleotide level, SIVmac239 is closely related to SIVmac251 (98%) and SIVmac142 (96%). It will not be possible to test which features of the viral sequence are critical molecular determinants for the pathogenesis of AIDS.

Env and Vpu proteins of human immunodeficiency virus type 1 are produced from multiple bicistronic mRNAs

Three size classes of human immunodeficiency virus type 1 (HIV-1) mRNAs are produced in infected cells: full-length, intermediate, and small. Here we report that the intermediate-size class of viral mRNAs is heterogeneous, consisting of at least 12 differentially spliced species. This group contains nine bicistronic mRNAs producing Env and Vpu and three mRNAs expressing only the first exon of tat. In the latter mRNAs, Env and Vpu expression is blocked by the presence of the upstream tat open reading frame. We conclude that internal initiation of translation is not the mechanism for generation of the bicistronic env mRNAs. Translation of HIV-1 mRNAs is consistent with the scanning mechanism in which Env is produced by leaky scanning from mRNAs that contain env as the second or third reading frame. Env and Vpu proteins are expressed from the same mRNAs and are coordinately regulated by Rev. This arrangement may reflect a requirement for coordinate expression of Vpu and Env.

Intracellular distribution of the envelope glycoprotein of human immunodeficiency virus and its role in the production of cytopathic effect in CD4+ and CD4- human cell lines

Human CD4+ and CD4- monocytoid cell lines were transfected with a constructed plasmid that has the envelope gene of human immunodeficiency virus under the transcriptional control of human metallothionein IIA promoter; the transfected cells were then cloned. These CD4+ and CD4- transfectant cell clones, both of which expressed almost the same amount of gp160 after induction with metal ions, were used for ultrastructural analysis of the distribution of the envelope glycoprotein in the cytoplasm. Immunofluorescence microscopy with an anti-envelope glycoprotein monoclonal antibody showed localized distribution of gp160 in the CD4+ cell clone and diffuse distribution of gp160 in the CD4- cell clone. These observations were substantiated by immunoelectron microscopy, in which the aggregated form of gp160 was observed in the cytoplasm of CD4+ cells but was scarce in that of CD4- cells. A notable finding was that the sites corresponding to the nuclear pores were occupied with aggregates of gp160 in CD4+ cells, exhibiting cytopathic effects. Both freeze-fracture and transmission electron microscopy also showed abnormal morphology around the nuclear pores and perinuclear space. These results support the possibility that such gp160 complexes accumulated around the nuclear pores primarily disturb the transportation of many molecules between the nucleus and the cytoplasm, resulting in a cytopathic effect in the CD4+ cell clone.

Molecular targets for AIDS therapy

The development of antiretroviral therapy against acquired immunodeficiency syndrome (AIDS) has been an intense research effort since the discovery of the causative agent, human immunodeficiency virus (HIV). A large array of drugs and biologic substances can inhibit HIV replication in vitro. Nucleoside analogs--particularly those belonging to the dideoxynucleoside family--can inhibit reverse transcriptase after anabolic phosphorylation. 3'-Azido-2',3'-dideoxythymidine (AZT) was the first such drug tested in individuals with AIDS, and considerable knowledge of structure-activity relations has emerged for this class of drugs. However, virtually every step in the replication of HIV could serve as a target for a new therapeutic intervention. In the future, non-nucleoside-type drugs will likely become more important in the experimental therapy of AIDS, and antiretroviral therapy will exert major effects against the morbidity and mortality caused by HIV.

Viral determinants of human immunodeficiency virus type 1 T-cell or macrophage tropism, cytopathogenicity, and CD4 antigen modulation

The genome of the human immunodeficiency virus type 1 (HIV-1) is highly heterogeneous. Some of this genomic variability is reflected in the biologic and serologic differences observed among various strains of HIV-1. To map the viral determinants that correlate with pathogenicity of the virus, recombinant viruses were generated between biologically active molecular clones of HIV-1 strains that show differences in T-cell or macrophage tropism, cytopathogenicity, CD4 antigen modulation, and susceptibility to serum neutralization. The results of these studies indicate that the envelope region contains the major determinants of these viral features. Further studies with sequence exchanges within this region should help identify specific domains that contribute to HIV pathogenesis.

The antibody response to the HIV-1 specific "out" (vpu) protein: identification of an immunodominant epitope and correlation of antibody detectability to clinical stages

Overlapping decapeptides based on the sequences of two human immunodeficiency virus type 1 (HIV-1) strains (HXB2 and ELI) were used to identify an immunodominant epitope of the nonstructural protein "out" (vpu) of the human immunodeficiency virus type 1. Of 29 HIV-1 antibody-positive sera, 6 reacted with decapeptides corresponding to the C-terminal amino acid sequence VEMGVEMGHHAPWDVDDL of the "out" (vpu) protein. This oligopeptide was synthesized by the solid phase method and used to develop an enzyme-linked immunosorbent assay (ELISA) for screening of 243 HIV-1-seropositive and 75 HIV-1-seronegative sera. It was found that 26% of the HIV-1 antibody-positive sera were reactive in the "out" (vpu) peptide ELISA, whereas none of the HIV-1-negative sera reacted with the oligopeptide. Correlation of reactivity of sera with the Walter Reed (WR) staging classification demonstrated that individuals classified WR 1 (36%) and WR 2 (42%) were more often reactive than patients classified WR 3-6 (11%).

Human immunodeficiency virus type 1 Pr55gag and Pr160gag-pol expressed from a simian virus 40 late replacement vector are efficiently processed and assembled into viruslike particles

Human immunodeficiency virus type 1 (HIV-1) gag and pol genes were expressed by using fragments of the BH10 clone of HIV inserted into a simian virus 40 late replacement vector. An initial construct containing the entire coding regions of gag, pol, and vif produced only minute amounts of the gag precursor, Pr55gag. However, high-level expression was obtained when an additional sequence from the env gene (the rev-responsive element) was inserted 3' of vif in the correct orientation, and rev was provided in trans from a second vector. Western immunoblot analysis of transfected cells showed the presence of large amounts of both Pr55gag and Pr160gag-pol as well as all of the expected cleavage products. Electron microscopy of thin sections of transfected cells showed a multitude of viruslike particles. Both immature particles in the process of budding and particles containing the condensed core characteristic of HIV were observed. Analysis of the released viruslike particles showed the presence of active reverse transcriptase. Sucrose gradient analysis of particles produced from [3H]uridine-labeled cells indicated a peak of radioactivity which cosedimented with a peak of p24, suggesting that the particles contained RNA.

Biology of retroviruses: detection, molecular biology, and treatment of retroviral infection

The general physical characteristics and replication of retroviruses are considered, along with assays for viral products. The specific agent for acquired immunodeficiency syndrome, the human immunodeficiency virus (HIV), is characterized as a lentivirus causing persistent, lifelong infection. While human immunodeficiency virus retroviruses share many of the same properties as other replication-competent viruses, genetic variability occurs among HIV isolates, and this variability may have a considerable effect on the virus' virulence, cell type specificity, viral susceptibility to antiviral compounds, clinical presentation, and disease progression. The most notable difference between HIV replication and other retroviruses is the intricate control of HIV gene expression by viral and cellular factors. Possible mechanisms by which HIV kills infected cells include the formulation of multinucleate syncytia; cytopathic components within the virions themselves; and interaction between viral envelope proteins and the CD4 molecule on the cell surface. Agents shown to inhibit viral replication at the level of the reverse transcriptase are phosphonoformate, sulfated polysaccharides, rifabutin, and nucleoside analogs, as well as purine and pyrimidine analogs. To date, only one nucleoside analog, zidovudine, has demonstrated clear clinical benefit and anti-HIV activity.

Human immunodeficiency virus vpr product is a virion-associated regulatory protein

The vpr product of the human immunodeficiency virus type 1 (HIV-1) acts in trans to accelerate virus replication and cytopathic effect in T cells. Here it is shown that the HIV-1 viral particle contains multiple copies of the vpr protein. The vpr product is the first regulatory protein of HIV-1 to be found in the virus particle. This observation raises the possibility that vpr acts to facilitate the early steps of infection before de novo viral protein synthesis occurs.

Role of capsid precursor processing and myristoylation in morphogenesis and infectivity of human immunodeficiency virus type 1

The effects on human immunodeficiency virus type 1 virion morphogenesis and on virus replication of mutations that affect posttranslational processing of the capsid precursor protein are described. A change in the glycine residue at position two from the N terminus abolishes the myristoylation of the precursor proteins and also prevents virus particle release. Mutations in the viral protease gene abolish proteolytic cleavage of the capsid precursor but do not prevent the formation and budding of virion particles of immature appearance. Mutations that alter the sequence of the sites normally used for cleavage of the major capsid protein p24 from the capsid precursor alter virion morphogenesis and prevent virus replication.