Na+ and K+ concentrations and the regulation of protein synthesis in Sindbis virus-infected chick cells

Synthetic Naphthoquinone Inhibits Herpes Simplex Virus Type-1 Replication Targeting Na+, K+ ATPase

Since 1970 acyclovir (ACV) has been the reference drug in treating herpes simplex virus (HSV) infections. However, resistant herpes simplex virus type 1 (HSV-1) strains have emerged, narrowing the treatment efficacy. The antiviral activity of classical Na+, K+ ATPase enzyme (NKA) inhibitors linked the viral replication to the NKA's activity. Herein, we evaluated the anti-HSV-1 activity of synthetic naphthoquinones, correlating their antiviral activity with NKA inhibition. We tested seven synthetic naphthoquinones initially at 50 μM on HSV-1-infected African green monkey kidney cells (VERO cells). Only one compound, 2-hydroxy-3-(2-thienyl)-1,4-naphthoquinone (AN-06), exhibited higher antiviral activity with a low cytotoxicity. AN-06 reduced the viral titer of 9 (log10) to 1.32 (log10) and decreased the steps of attachment and penetration. The addition of AN-06 up to 20 h postinfection (hpi) interfered with the viral cycle. The viral infection alone increases NKA activity 3 h postinfection (hpi), scaling up to 6 hpi. The addition of AN-06 in a culture infected with HSV-1 decreased NKA activity, suggesting that its antiviral action is linked to NKA inhibition. Also, docking results showed that this compound binds at the same site of NKA in which adenosine triphosphate (ATP) binds. AN-06 exhibited promising pharmacokinetic and toxicology properties. Thus, we postulate that AN-06 may be a good candidate for antiviral compounds with a mechanism of action targeting NKA activity.

Interplay between H1N1 influenza a virus infection, extracellular and intracellular respiratory tract pH, and host responses in a mouse model

During influenza A virus (IAV) entry, the hemagglutinin (HA) protein is triggered by endosomal low pH to undergo irreversible structural changes that mediate membrane fusion. HA proteins from different isolates vary in the pH at which they become activated in endosomes or become irreversible inactivated if exposed to extracellular acid. Little is known about extracellular pH in the upper respiratory tracts of mammals, how pH may shift during IAV infection, and its impact on replication of viruses that vary in HA activation pH. Here, we inoculated DBA/2J mice intranasally with A/TN/1-560/2009 (H1N1) (activation pH 5.5) or a mutant containing the destabilizing mutation HA1-Y17H (pH 6.0). We measured the kinetics of extracellular pH during infection using an optical pH-sensitive microsensor probe placed in the naris, nasal sinus, soft palate, and trachea. We also measured intracellular pH of single-cell suspensions of live, primary lung epithelial cells with various wavelength pH-sensitive dyes localized to cell membranes, cytosol, endosomes, secretory vesicles, microtubules, and lysosomes. Infection with either virus decreased extracellular pH and increased intracellular pH. Peak host immune responses were observed at 2 days post infection (DPI) and peak pH changes at 5 DPI. Extracellular and intracellular pH returned to baseline by 7 DPI in mice infected with HA1-Y17H and was restored later in wildtype-infected. Overall, IAV infection altered respiratory tract pH, which in turn modulated replication efficiency. This suggests a virus-host pH feedback loop that may select for IAV strains containing HA proteins of optimal pH stability, which may be approximately pH 5.5 in mice but may differ in other species.

Alphavirus Replication: The Role of Cardiac Glycosides and Ion Concentration in Host Cells

Alphaviruses are arthropod-borne viruses that can cause fever, rash, arthralgias, and encephalitis. The mosquito species Aedes aegypti and Aedes albopictus are the most frequent transmitters of alphaviruses. There are no effective vaccines or specific antivirals available for the treatment of alphavirus-related infections. Interestingly, changes in ion concentration in host cells have been characterized as critical regulators of the alphavirus life cycle, including fusion with the host cell, glycoprotein trafficking, genome translation, and viral budding. Cardiac glycosides, which are classical inhibitors of the Na⁺ K⁺ ATPase (NKA), can inhibit alphavirus replication although their mechanisms of action are poorly understood. Nonetheless, results from multiple studies suggest that inhibition of NKA may be a suitable strategy for the development of alphavirus-specific antiviral treatments. This review is aimed at exploring the role of changes in ion concentration during alphavirus replication and at considering the possibility of NKA as a potential therapeutic target for antiviral drugs.

G protein-coupled and ATP-sensitive inwardly rectifying potassium ion channels are essential for HIV entry

The high genetic diversity of Human Immunodeficiency virus (HIV), has hindered the development of effective vaccines or antiviral drugs against it. Hence, there is a continuous need for identification of new antiviral targets. HIV exploits specific host proteins also known as HIV-dependency factors during its replication inside the cell. Potassium channels play a crucial role in the life cycle of several viruses by modulating ion homeostasis, cell signaling, cell cycle, and cell death. In this study, using pharmacological tools, we have identified that HIV utilizes distinct cellular potassium channels at various steps in its life cycle. Members of inwardly rectifying potassium (Kir) channel family, G protein-coupled (GIRK), and ATP-sensitive (KATP) are involved in HIV entry. Blocking these channels using specific inhibitors reduces HIV entry. Another member, Kir 1.1 plays a role post entry as inhibiting this channel inhibits virus production and release. These inhibitors are not toxic to the cells at the concentration used in the study. We have further identified the possible mechanism through which these potassium channels regulate HIV entry by using a slow-response potential-sensitive probe DIBAC4(3) and have observed that blocking these potassium channels inhibits membrane depolarization which then inhibits HIV entry and virus release as well. These results demonstrate for the first time, the important role of Kir channel members in HIV-1 infection and suggest that these K+ channels could serve as a safe therapeutic target for treatment of HIV/AIDS.

The Regulation of Translation in Alphavirus-Infected Cells

Sindbis virus (SINV) contains an RNA genome of positive polarity with two open reading frames (ORFs). The first ORF is translated from the genomic RNA (gRNA), rendering the viral non-structural proteins, whereas the second ORF is translated from a subgenomic mRNA (sgRNA), which directs the synthesis of viral structural proteins. SINV infection strongly inhibits host cell translation through a variety of different mechanisms, including the phosphorylation of the eukaryotic initiation factor eIF2α and the redistribution of cellular proteins from the nucleus to the cytoplasm. A number of motifs have been identified in SINV sgRNA, including a hairpin downstream of the AUG initiation codon, which is involved in the translatability of the viral sgRNA when eIF2 is inactivated. Moreover, a 3′-UTR motif containing three stem-loop structures is involved in the enhancement of translation in insect cells, but not in mammalian cells. Accordingly, SINV sgRNA has evolved several structures to efficiently compete for the cellular translational machinery. Mechanistically, sgRNA translation involves scanning of the 5′-UTR following a non-canonical mode and without the requirement for several initiation factors. Indeed, sgRNA-directed polypeptide synthesis occurs even after eIF4G cleavage or inactivation of eIF4A by selective inhibitors. Remarkably, eIF2α phosphorylation does not hamper sgRNA translation during the late phase of SINV infection. SINV sgRNA thus constitutes a unique model of a capped viral mRNA that is efficiently translated in the absence of several canonical initiation factors. The present review will mainly focus in the non-canonical mechanism of translation of SINV sgRNA.

Inhibition of host protein synthesis by Sindbis virus: correlation with viral RNA replication and release of nuclear proteins to the cytoplasm

Infection of mammalian cells by Sindbis virus (SINV) profoundly blocks cellular mRNA translation. Experimental evidence points to viral non-structural proteins (nsPs), in particular nsP2, as the mediator of this inhibition. However, individual expression of nsP1, nsP2, nsP3 or nsP1-4 does not block cellular protein synthesis in BHK cells. Trans-complementation of a defective SINV replicon lacking most of the coding region for nsPs by the co-expression of nsP1-4 propitiates viral RNA replication at low levels, and inhibition of cellular translation is not observed. Exit of nuclear proteins including T-cell intracellular antigen and polypyrimidine tract-binding protein is clearly detected in SINV-infected cells, but not upon the expression of nsPs, even when the defective replicon was complemented. Analysis of a SINV variant with a point mutation in nsP2, exhibiting defects in the shut-off of host protein synthesis, indicates that both viral RNA replication and the release of nuclear proteins to the cytoplasm are greatly inhibited. Furthermore, nucleoside analogues that inhibit cellular and viral RNA synthesis impede the blockade of host mRNA translation, in addition to the release of nuclear proteins. Prevention of the shut-off of host mRNA translation by nucleoside analogues is not due to the inhibition of eIF2α phosphorylation, as this prevention is also observed in PKR(-/-) mouse embryonic fibroblasts that do not phosphorylate eIF2α after SINV infection. Collectively, our observations are consistent with the concept that for the inhibition of cellular protein synthesis to occur, viral RNA replication must take place at control levels, leading to the release of nuclear proteins to the cytoplasm.

Requirements for eIF4A and eIF2 during translation of Sindbis virus subgenomic mRNA in vertebrate and invertebrate host cells

We have examined the requirements for the initiation factors (eIFs) eIF4A and eIF2 to translate Sindbis virus (SV) subgenomic mRNA (sgmRNA) in the natural hosts of SV: vertebrate and arthropod cells. Notably, this viral mRNA does not utilize eIF4A in SV-infected mammalian cells. However, eIF4A is required to translate this mRNA in transfected cells. Therefore, SV sgmRNA exhibits a dual mechanism for translation with respect to the use of eIF4A. Interestingly, SV genomic mRNA requires eIF4A for translation during the early phase of infection. In sharp contrast to what is observed in mammalian cells, active eIF2 is necessary to translate SV sgmRNA in mosquito cells. However, eIF4A is not necessary for SV sgmRNA translation in this cell line. In the SV sgmRNA coding region, proximal to the initiation codon is a hairpin structure that confers eIF2 independence only in mammalian cells infected by SV. Strikingly, this structure does not provide independence for eIF4A neither in mammalian nor in mosquito cells. These findings provide the first evidence of different eIF requirements for translation of SV sgmRNA in vertebrate and invertebrate cells. These observations can help to understand the interaction of SV with its host cells.

Cytopathic mechanisms of HIV-1

The human immunodeficiency virus type 1 (HIV-1) has been intensely investigated since its discovery in 1983 as the cause of acquired immune deficiency syndrome (AIDS). With relatively few proteins made by the virus, it is able to accomplish many tasks, with each protein serving multiple functions. The Envelope glycoprotein, composed of the two noncovalently linked subunits, SU (surface glycoprotein) and TM (transmembrane glycoprotein) is largely responsible for host cell recognition and entry respectively. While the roles of the N-terminal residues of TM is well established as a fusion pore and anchor for Env into cell membranes, the role of the C-terminus of the protein is not well understood and is fiercely debated. This review gathers information on TM in an attempt to shed some light on the functional regions of this protein.

Viral translation is coupled to transcription in Sindbis virus-infected cells

During the late phase of Sindbis virus infection, the viral subgenomic mRNA is translated efficiently in BHK cells, whereas host protein synthesis is inhibited. However, transfection of in vitro-generated Sindbis virus subgenomic mRNA leads to efficient translation in uninfected BHK cells, whereas it is a poor substrate in infected cells. Therefore, the structure of the subgenomic mRNA itself is not sufficient to confer its translatability in infected cells. In this regard, translation of the subgenomic mRNA requires synthesis from the viral transcription machinery. The lack of translation of transfected viral mRNAs in infected cells is not due to their degradation nor is it a consequence of competition between viral transcripts and transfected mRNAs, because a replicon that cannot produce subgenomic mRNA also interferes with exogenous mRNA translation. Interestingly, subgenomic mRNA is translated more efficiently when it is transfected into uninfected cells than when it is transcribed from a transfected replicon. Finally, a similar behavior was observed for other RNA viruses, such as vesicular stomatitis virus and encephalomyocarditis virus. These findings support the notion that translation is coupled to transcription in cells infected with different animal viruses.

Effects on the immune system

Marijuana and other exogenous cannabinoids alter immune function and decrease host resistance to microbial infections in experimental animal models and in vitro. Two modes of action by which delta9-tetrahydrocannabinol (THC) and other cannabinoids affect immune responses have been proposed. First, cannabinoids may signal through the cannabinoid receptors CB1 and CB2. Second, at sites of direct exposure to high concentrations of cannabinoids, such as the lung, membrane perturbation may be involved. In addition, endogenous cannabinoids or endocannabinoids have been identified and have been proposed as native modulators of immune functions through cannabinoid receptors. Exogenously introduced cannabinoids may disturb this homoeostatic immune balance. A mode by which cannabinoids may affect immune responses and host resistance maybe by perturbing the balance of T helper (Th)1 pro-inflammatory versus Th2 anti-inflammatory cytokines. While marijuana and various cannabinoids have been documented to alter immune functions in vitro and in experimental animals, no controlled longitudinal epidemiological studies have yet definitively correlated immunosuppressive effects with increased incidence of infections or immune disorders in humans. However, cannabinoids by virtue of their immunomodulatory properties have the potential to serve as therapeutic agents for ablation of untoward immune responses.

Mayaro virus infection alters glucose metabolism in cultured cells through activation of the enzyme 6-phosphofructo 1-kinase

Although it is well established that cellular transformation with tumor virus leads to changes on glucose metabolism, the effects of cell infection by non-transforming virus are far to be completely elucidated. In this study, we report the first evidence that cultured Vero cells infected with the alphavirus Mayaro show several alterations on glucose metabolism. Infected cells presented a two fold increase on glucose consumption, accompanied by an increment in lactate production. This increase in glycolytic flux was also demonstrated by a significant increase on the activity of 6-phosphofructo 1-kinase, one of the regulatory enzymes of glycolysis. Analysis of the kinetic parameters revealed that the regulation of 6-phosphofructo 1-kinase is altered in infected cells, presenting an increase in Vmax along with a decrease in Km for fructose-6-phosphate. Another fact contributing to an increase in enzyme activity was the decrease in ATP levels observed in infected cells. Additionally, the levels of fructose 2,6-bisphosphate, a potent activator of this enzyme, was significantly reduced in infected cells. These observations suggest that the increase in PFK activity may be a compensatory cellular response to the viral-induced metabolic alterations that could lead to an impairment of the glycolytic flux and energy production.

Membrane-permeabilizing motif in Semliki forest virus E1 glycoprotein

Cell infection by alphaviruses is accompanied by membrane permeability changes. New predictive approaches, including the computation of interfacial affinity and corresponding hydrophobic moments, suggest a segmented amphipathic-at-interface domain in the stem region of Semliki Forest virus fusion protein E1. Expression of E1 sequences in Escherichia coli cells confirmed that the membrane proximal plus transmembrane (TM) domain unit permeabilizes cells as efficiently as the 6K viroporin. Both our predictive and experimental data support the involvement of the E1 stem-TM region in membrane insertion and permeabilization. We propose to combine Wimley-White hydrophobicity analysis with expression-coupled permeability assays in order to identify viral products implied in breaching cell membrane barriers during infection.

Extensive immune-mediated hippocampal damage in mice surviving infection with neuroadapted Sindbis virus

Viral infections of the central nervous system and immune responses to these infections cause a variety of neurological diseases. Infection of weanling mice with Sindbis virus causes acute nonfatal encephalomyelitis followed by clearance of infectious virus, but persistence of viral RNA. Infection with a neuroadapted strain of Sindbis virus (NSV) causes fatal encephalomyelitis, but passive transfer of immune serum after infection protects from fatal disease and infectious virus is cleared. To determine whether persistent NSV RNA is associated with neurological damage, we examined the brains of recovered mice and found progressive loss of the hippocampal gyrus, adjacent white matter, and deep cerebral cortex associated with mononuclear cell infiltration. Mice deficient in CD4(+) T cells showed less tissue loss, while mice lacking CD8(+) T cells showed lesions comparable to those in immunocompetent mice. Mice deficient in both CD4(+) and CD8(+) T cells developed severe tissue loss similar to immunocompetent mice and this was associated with extensive infiltration of macrophages. The number of CD4(+) cells and macrophage/microglial cells, but not CD8(+) cells, infiltrating the hippocampal gyrus was correlated with the number of terminal deoxynucleotidyltransferase-mediated dUTP nick end-labeling positive pyramidal neurons. These results suggest that CD4(+) T cells can promote progressive neuronal death and tissue injury, despite clearance of infectious virus.

Translational control of viral gene expression in eukaryotes

As obligate intracellular parasites, viruses rely exclusively on the translational machinery of the host cell for the synthesis of viral proteins. This relationship has imposed numerous challenges on both the infecting virus and the host cell. Importantly, viruses must compete with the endogenous transcripts of the host cell for the translation of viral mRNA. Eukaryotic viruses have thus evolved diverse mechanisms to ensure translational efficiency of viral mRNA above and beyond that of cellular mRNA. Mechanisms that facilitate the efficient and selective translation of viral mRNA may be inherent in the structure of the viral nucleic acid itself and can involve the recruitment and/or modification of specific host factors. These processes serve to redirect the translation apparatus to favor viral transcripts, and they often come at the expense of the host cell. Accordingly, eukaryotic cells have developed antiviral countermeasures to target the translational machinery and disrupt protein synthesis during the course of virus infection. Not to be outdone, many viruses have answered these countermeasures with their own mechanisms to disrupt cellular antiviral pathways, thereby ensuring the uncompromised translation of virion proteins. Here we review the varied and complex translational programs employed by eukaryotic viruses. We discuss how these translational strategies have been incorporated into the virus life cycle and examine how such programming contributes to the pathogenesis of the host cell.

Role of potassium in human immunodeficiency virus production and cytopathic effects

Acute infection of CD4+ lymphoid cells by human immunodeficiency virus type 1 (HIV-1) induces an increase in the intracellular concentration of potassium (K+). Media containing reduced or elevated concentrations of K+ were used to investigate the role of this ion in HIV-1 production and cytopathology. Incubation of CD4+ lymphoblastoid cells acutely infected by HIV-1 (strain LAI) in low K+ medium resulted in an approximately 50% decrease in HIV-1 production and markedly diminished HIV-1 induced cytopathic effects (CPE) relative to cells incubated in medium containing a normal K+ concentration (approximately 5 mM). Incubation of HIV-1 infected cells in media containing elevated concentrations of K+ medium. Cells mM) increased HIV-1 production by two- to fivefold over the amount produced in cells incubated in normal K+ medium. Cells incubated in high K+ media also displayed enhanced HIV-1-induced cytopathology. The decrease in HIV-1 production by low K+ medium and increase by high K+ media could be a accounted for by effects on HIV-1 reverse transcription. However, low K+ medium inhibited HIV-1 protein synthesis and high K+ media increased HIV-1 protein synthesis. These results suggest that the HIV-1-induced increase in intracellular is required for efficient viral replication and to induce cytopathology.

Inhibition of HIV type 1 production by hygromycin B

HIV infection alters the cellular uptake of ions and other small molecules. This study was designed to determine whether hygromycin B, a low molecular weight (MW 527) aminoglycoside protein synthesis inhibitor that is normally impermeable to mammalian cells at micromolar concentrations, can selectively inhibit HIV expression and cytopathology. CD4+ T lymphoblastoid cells (H9) and peripheral blood mononuclear cells (PBMCs) were infected with HIV-1, then incubated in medium containing various concentrations of hygromycin B. HIV-1-induced formation of multinucleated giant cells and single cell killing were dramatically reduced in the presence of micromolar concentrations of hygromycin B. Hygromycin B also inhibited HIV-1 production in a dose-dependent manner during acute infection. G418, a larger and more hydrophobic aminoglycoside (MW 692), did not display the same selective inhibition of HIV-1 production as hygromycin B. Relative to mock-infected cells, protein synthesis in acutely infected H9 cells was selectively inhibited by hygromycin B. Hygromycin B also reduced HIV production in PBMCs and in H9 cells persistently infected with HIV. PCR analysis demonstrated that hygromycin B did not inhibit HIV-1 reverse transcription. These results demonstrate that HIV-1 infection renders cells more sensitive to hygromycin B than uninfected cells, and provides support for the hypothesis that HIV-1 induces an alteration of plasma membrane permeability. The HIV-modified cell membrane may be a potential target for antiviral intervention and chemotherapy.

High-level expression of the human CB2 cannabinoid receptor using a baculovirus system

A human CB2 recombinant baculovirus (AcNPV-hCB2) was generated by site-specific transposition and employed to express the human CB2 cannabinoid receptor. Northern analysis of total RNA from Spodoptera frugiperda (Sf9) insect cells infected with AcNPV-hCB2 revealed novel expression of a unique 2.3 kb transcript when probed with hCB2 cDNA. This transcript corresponded to the size expected for hCB2 generated from the recombinant virus construct. Western immunoblot analysis of whole cell homogenates of recombinant baculovirus-infected Sf9 cells, using affinity-purified antibody to a human CB2 carboxy terminal domain (anti-hCB2.CV), revealed the presence of novel immunoreactive protein. In addition, when anti-hCB2.CV was employed in immunofluorescence staining, an intense signal was observed within AcNPV-hCB2-infected cells but not within uninfected cells or cells infected with a control beta-galactosidase recombinant baculovirus. The pattern of immunofluorescence at early periods post-infection was in a perinuclear arrangement with a "signet-ring" appearance, suggestive of glycosylation of the expressed recombinant protein. Transmission electron microscopy revealed regions of intranuclear recombinant virus assembly and the presence of numerous intracytoplasmic proteinaceous vesicular inclusions consistent with hyperproduction of hCB2. Scatchard-Rosenthal analysis of [3H]-(-)3-[2-hydroxy-4-(1,1-dimethylheptyl)phenyl]-4-[3-hydroxypro pyl]cyclohexan-1-ol ([3H]CP 55,940) receptor binding indicated a Kd of 2.24 nM and a Bmax equal to 5.24 pmol/mg of protein. The lack of [3H]CP 55,940 displacement with N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-met hyl-1H-pyrazole-3-carboxamidehydrochloride (SR 141716A), the CB1-selective antagonist, confirmed the identity of the receptor as CB2. These data indicate that AcNPV-hCB2 expresses high levels of the human CB2, which retains properties of the native receptor. Thus, this recombinant virus may prove suitable for hyperproduction of receptor for basic biochemical and biophysical characterization studies.

The papillomavirus minor capsid protein, L2, induces localization of the major capsid protein, L1, and the viral transcription/replication protein, E2, to PML oncogenic domains

We have used immunofluorescent staining and confocal microscopy to examine the subcellular localization of structural and nonstructural bovine papillomavirus (BPV) proteins in cultured cells that produce infectious virions. When expressed separately, L1, the major capsid protein, showed a diffuse nuclear distribution while L2, the minor capsid protein, was found to localize to punctate nuclear regions identified as promonocytic leukemia protein (PML) oncogenic domains (PODs). Coexpression of L1 and L2 induced a relocation of L1 into the PODs, leading to the colocalization of L1 and L2. The effect of L2 expression on the distribution of the nonstructural viral proteins E1 and E2, which are required for maintenance of the genome and viral DNA synthesis, was also examined. The localization of the E1 protein was unaffected by L2 expression. However, the pattern of anti-E2 staining was dramatically altered in L2-expressing cells. Similar to L1, E2 was shifted from a dispersed nuclear locality into the PODs and colocalized with L2. The recruitment of full-length E2 by L2 occurred in the absence of other viral components. L2 was shown previously to be essential for the generation of infectious BPV. Our present results provide evidence for a role for L2 in the organization of virion components by recruiting them to a distinct nuclear domain. This L2-dependent colocalization probably serves as a mechanism to promote the assembly of papillomaviruses either by increasing the local concentration of virion constituents or by providing the physical architecture necessary for efficient packaging and assembly. The data also suggest a role for a nonstructural viral protein, E2, in virion assembly, specifically the recruitment of the viral genome to the sites of assembly, through its high-affinity interaction with specific sequences in the viral DNA.

Regulators of apoptosis on the road to persistent alphavirus infection

Alphavirus infection can trigger the host cell to activate its genetically programmed cell death pathway, leading to the morphological features of apoptosis. The ability to activate this death pathway is dependent on both viral and cellular determinants. The more virulent strains of alphavirus induce apoptosis with increased efficiency both in animal models and in some cultured cells. Although the immune system clearly plays a central role in clearing virus, the importance of other cellular factors in determining the outcome of virus infections are evident from the observation that mature neurons are better able to resist alphavirus-induced apoptosis than immature neurons are, both in culture and in mouse brains. These findings are consistent with the age-dependent susceptibility to disease seen in animals. Cellular genes that are known to regulate the cell death pathway can modulate the outcome of alphavirus infection in cultured cells and perhaps in animals. The cellular bax and bak genes, which are known to accelerate cell death, also accelerate virus-induced apoptosis. In contrast, inhibitors of apoptotic cell death such as bcl-2 suppress virus-induced apoptosis, which can facilitate a persistent virus infection. Thus, the balance of cellular factors that regulate cell death may be critical in virus infections. Additional viral factors also contribute to this balance. The more virulent strains of alphavirus have acquired the ability to induce apoptosis in mature neurons, while mature neurons are resistant to cell death upon infection with less virulent strains. Here we discuss a variety of cellular and viral factors that modulate the outcome of virus infection.

The role of antibody in recovery from alphavirus encephalitis

Alphaviruses infect neurons in the brain and spinal cord and cause acute encephalomyelitis in a variety of mammals. The outcome of infection is determined by whether the neurons survive infection and this, in turn, is determined by the virulence of the virus and the age of the host at the time of infection. We have been studying Sindbis virus (SV) infection of mice as a model system for alphavirus-induced encephalomyelitis. Investigation of intracerebral infection of weanling mice with two different strains of SV has allowed us to analyze the role of the immune response in protection from fatal disease (virulent NSV strain) and in clearance of virus from the nervous system during non-fatal disease (less virulent SV AR339 strain). Neutralizing and non-neutralizing antibodies to the E1 and E2 surface glycoproteins can protect mice from fatal NSV infection when given before or after infection, while T cells are not protective. The mechanism of antibody-mediated protection is not known, but it is likely that more than one mechanism is involved and that different mechanisms are involved in pre-infection and post-infection treatment protection. Clearance of infectious virus from the nervous system of mice during recovery from non-fatal disease is accomplished by antibodies to the E2 glycoprotein. The process does not involve damage to the infected neurons and is independent of complement and mononuclear cells. Bivalent antibody is required and binds to the surface of the infected cell. Initially, release of virus by budding from the cell surface is prevented and, subsequently, intracellular virus replication is inhibited possibly through antiviral mechanisms induced in co-operation with interferon. This non-lytic mechanism for control of virus infection results in the prolonged presence of viral RNA in tissue and the need for prolonged intrathecal synthesis of antiviral antibody by B cells within the central nervous system.

Identification of mutations in a Sindbis virus variant able to establish persistent infection in BHK cells: the importance of a mutation in the nsP2 gene

Sindbis virus is a positive strand RNA virus that has provided a valuable model for studying virus structure and replication. It is also being developed as a vector for the expression of heterologous proteins. Many studies with this virus are carried out in cultured BHK cells where infection is usually highly cytopathic and within 1 or 2 days after infection all of the cells are dead. Weiss et al. had established a persistently infected culture of BHK cells by infecting the cells with a virus preparation highly enriched in defective interfering (DI) particles and had isolated an attenuated virus, SIN-1 virus, from the culture [Weiss et al. (1980) J. Virol. 33, 463-474]. SIN-1 virus, free of DI particles, was able to establish a persistent infection in BHK cells. We initiated studies to determine what changes in the genome of the virus were responsible for this phenotype. We describe here the cDNA cloning and sequencing of the 5' terminus and the four nonstructural protein genes from SIN-1 virus. A single coding mutation in the nsP2 gene (a predicted change of Pro-726 --> Ser) produced a virus that was able to establish persistent infection in BHK cells. Additional mutations in the other genes were required to decrease the synthesis of viral RNA to a level similar to that found in cells infected with SIN-1 virus. Incorporation of the nsP2 mutation into a Sindbis virus expression vector led to a higher level of synthesis of the reporter protein, beta-galactosidase, than that obtained with the original Sindbis virus replicon.

Molecular mechanisms of virus-mediated cytopathology

Lytic virus infections of animal cells usually lead to a variety of morphological and biochemical lesions that include inhibition of cellular macromolecular syntheses. These cytopathic effects vary in intensity for different virus-cell combinations and probably involve several overlapping mechanisms. Inhibition may be mediated by components of parental virions or require viral gene expression. In many infected cell systems the initiation of host protein synthesis is selectively blocked. This shut-off phenomenon can result from changes in membrane permeability that alter the intracellular ionic environment in favour of viral expression, successful competition of viral mRNAs for limited translational components, or a decrease in the level of cell mRNAs by inhibition of synthesis or nucleocytoplasmic transport. However, the early onset and rapidity of virus-induced inhibition, sometimes under non-permissive conditions, implies more direct mechanisms of translational inactivation. These include enhanced degradation of cellular mRNAs or specific modification of the translation apparatus in infected cells. A dramatic example of the latter occurs in poliovirus-infected HeLa cells in which intact, functional cellular mRNA persists but host protein synthesis is almost completely inhibited. The virus-induced defect is apparently related to inactivation of a protein factor that binds to the 5' end of m7G-capped mRNAs and is required for translation of host (capped) mRNAs but not for the expression of poliovirus RNA, which is not capped. This process and other possible molecular mechanisms of virus-mediated cytopathology are discussed.

Human immunodeficiency virus infection of T-lymphoblastoid cells reduces intracellular pH

Alterations in plasma membrane function are induced by many cytopathic viruses, including human immunodeficiency virus type 1 (HIV-1). These alterations can result in changes in the intracellular content of ions and other small molecules and can contribute to cytolysis and death of the infected cell. The pH-sensitive fluorescent probe 2',7'-bis(2-carboxyethyl)-5,6-carboxyfluorescein-acetoxymethyl ester was used to quantitate intracellular pH (pHi) in HIV-1-infected T cells. Infection of cells from the CD4+ T-lymphoblastoid line HUT-78 (RH9 subclone) with HIV-1 strain LAI resulted in a significant decrease of pHi, from approximately 7.2 in mock-infected cells to below 6.7 by day 4 after infection, when cells were undergoing acute cytopathic effects. The pHi in persistently infected cells that survived the acute cytopathic effects of HIV-1 was approximately 6.8 to 7.0. Studies with amiloride, an inhibitor of the Na+/H+ exchange system, suggest that HIV-1-induced intracellular acidification in lymphocytes is due, in part, to dysfunction of this plasma membrane ion transport system. The alterations in pHi may mediate certain cytopathic effects of HIV-1, thereby contributing to depletion of CD4+ T lymphocytes in patients with AIDS.

Alteration of intracellular potassium and sodium concentrations correlates with induction of cytopathic effects by human immunodeficiency virus

Increases in intracellular concentrations of potassium ([K+]i) and sodium ([Na+]i) occur concomitantly with cytopathic effects induced in a CD4+ T-lymphoblastoid cell line acutely infected by human immunodeficiency virus (HIV). This [K+]i increase was greater in cells infected by cytopathic HIV strains than in cells infected by less cytopathic strains. T cells persistently infected by HIV had an increased [K+]i but displayed an [Na+]i similar to that of mock-infected cells. HIV induced increases in [K+]i and [Na+]i after cytopathic infection of human peripheral blood mononuclear cells, but the magnitude of the Na+ changes did not correlate with the extent of the cytopathic effect. Enhanced movement of cations may osmotically drive water entry, resulting in balloon degeneration and lysis of HIV-infected cells. These observations offer potential approaches for antiviral therapies.

Effects of anti-E2 monoclonal antibody on sindbis virus replication in AT3 cells expressing bcl-2

Antibodies directed to Sindbis virus (SV) envelope protein E2 are able to control virus replication in vivo and in persistently infected cultures of neurons in vitro. We investigated the mechanisms by which anti-E2 monoclonal antibody (MAb) alters virus replication by using AT3 rat prostatic carcinoma cells expressing the inhibitor of apoptosis bcl-2. Treatment of SV-infected AT3-bcl-2 cells with anti-E2 MAb G5 for 2 h decreased the rate of virus release for 6 to 8 h after removal of the antibody. Electron microscopic analysis of MAb-treated cells revealed that failure of virus release was linked to a defect in the budding process. The decrease in extracellular virus particles occurred despite continued formation of nucleocapsids and synthesis of envelope glycoproteins. MAb treatment delayed the inhibition of K+ influx and shutoff of host cell protein synthesis by SV infection in a dose-dependent manner. Synthesis of host cell factors and of nonstructural polyprotein precursors required for the formation of initial replication complexes was also prolonged, causing a slower shutdown of overall viral RNA synthesis. We conclude that one mechanism by which anti-E2 MAb treatment down-regulates SV replication is by reestablishing certain critical host cell functions in infected cells.

Role of amino acid-induced changes in ion fluxes in the regulation of hepatic protein synthesis

Alanine is a powerful stimulator of hepatic protein synthesis whose mechanism of action has not yet been ascertained. The present work aimed to elucidate whether rate changes in ion fluxes accompanying the transport of this amino acid could play a role in the stimulation of protein synthesis. In perfused livers, the utilization of alanine produced a net uptake of K+ of 1.5 mumol/min/liver, a progressively increasing efflux of Ca2+ to reach a maximum of 0.9 mumol/min/liver, and alkalization of the extracellular medium. Inhibition of Na+/K+ exchange by ouabain reversed only the uptake of K+, indicating that this is the main way for the efflux of Na+ cotransported with alanine. In isolated hepatocytes, the uptake of alanine increased the intracellular content of K+ and the cell volume. The following observations suggest that these changes, and not an increased intracellular concentration of Na+, are associated with the stimulation of protein synthesis: 1) Ouabain inhibited the alanine stimulation of L-[3H]-valine incorporation into protein without altering the basal rate of protein labeling; 2) ouabain had no effects on alanine uptake indicating that Na+ influx is not involved in the alanine stimulation of protein synthesis; 3) disruption of Na+ gradient across the plasma membrane by specific ionophores, monensin and gramicidin D, inhibited both basal and alanine-stimulated protein synthesis, but substitution of extracellular Na+ by K+ did not prevent the stimulatory action of alanine. The observation that hypotonic buffer enhanced protein synthesis to the same degree than alanine in liver cells indicates that alanine-induced cell swelling could be sufficient to stimulate protein synthesis.

The effect of a mesogenic and a lentogenic Newcastle disease virus strain on Burkitt lymphoma Daudi cells

The destructive effect of Newcastle disease virus (NDV) strains on Burkitt lymphoma Daudi cells was investigated. Interaction of an active and UV-inactivated mesogenic strain (Roakin), as well as an active attenuated lentogenic strain (B1), grown in the allantoic sac of embryonated eggs, at high multiplicity, caused inhibition in cellular DNA synthesis and arrest in cell multiplication, eventually killing of the cells. The lentogenic strain cultivated in chicken fibroblasts exhibited only a moderate activity. The mechanism of the cytolytic effect is presumably linked to the increase in cell membrane permeability indicated by the elevation in 51Cr release. Thus it appears that the massive adsorption and/or penetration of viral particles, active or UV-inactivated (or possibly a toxic component that resides in the virion), damages the plasma membrane and may be responsible for the killing of the cells.

Modification of membrane permeability by animal viruses

Animal viruses permeabilize cells at two well-defined moments during infection: (1) early, when the virus gains access to the cytoplasm, and (2) during the expression of the virus genome. The molecular mechanisms underlying both events are clearly different; early membrane permeability is induced by isolated virus particles, whereas late membrane leakiness is produced by newly synthesized virus protein(s) that possess activities resembling ionophores or membrane-active toxins. Detailed knowledge of the mechanisms, by which animal viruses permeabilize cells, adds to our understanding of the steps involved in virus replication. Studies on early membrane permeabilization give clues about the processes underlying entry of animal viruses into cells; understanding gained on the modification by viral proteins of membrane permeability during virus replication indicates that membrane leakiness is required for efficient virus release from infected cells or virus budding, in the case of enveloped viruses. In addition, the activity of these membrane-active virus proteins may be related to virus interference with host cell metabolism and with the cytopathic effect that develops after virus infection.

The alphaviruses: gene expression, replication, and evolution

The alphaviruses are a genus of 26 enveloped viruses that cause disease in humans and domestic animals. Mosquitoes or other hematophagous arthropods serve as vectors for these viruses. The complete sequences of the +/- 11.7-kb plus-strand RNA genomes of eight alphaviruses have been determined, and partial sequences are known for several others; this has made possible evolutionary comparisons between different alphaviruses as well as comparisons of this group of viruses with other animal and plant viruses. Full-length cDNA clones from which infectious RNA can be recovered have been constructed for four alphaviruses; these clones have facilitated many molecular genetic studies as well as the development of these viruses as expression vectors. From these and studies involving biochemical approaches, many details of the replication cycle of the alphaviruses are known. The interactions of the viruses with host cells and host organisms have been exclusively studied, and the molecular basis of virulence and recovery from viral infection have been addressed in a large number of recent papers. The structure of the viruses has been determined to about 2.5 nm, making them the best-characterized enveloped virus to date. Because of the wealth of data that has appeared, these viruses represent a well-characterized system that tell us much about the evolution of RNA viruses, their replication, and their interactions with their hosts. This review summarizes our current knowledge of this group of viruses.

Inhibition of macrophage inducible protein expression by delta-9-tetrahydrocannabinol

Macrophages have been shown to undergo a sequential process to full activation in response to priming and triggering signals such as gamma interferon (IFN gamma) and bacterial lipopolysaccharide (LPS). These cells also may be driven directly to full activation by exposure to relatively high concentrations of LPS. Each of the stages to activation is associated with differential protein expression suggesting that newly synthesized proteins are associated with the functional activities attributable to that activation state. These observations indicate that protein profiles may serve as a barometer of the macrophage activation state. Delta-9-tetrahydrocannabinol (THC), the major psychoactive component in marijuana, was shown to inhibit inducible protein expression in response to the priming agents Concanavalin A (Con A) supernatant and IFN gamma. THC also suppressed protein expression in response to LPS. P388D1 and RAW264.7 macrophage-like cells, treated with Con A supernatant or IFN gamma, exhibited restructuring of protein profiles based on iso-Dalt two-dimensional gel electrophoresis. Protein profile restructuring, distinctive from that elicited in response to priming agents, was seen for macrophages treated with LPS. Treatment of macrophages with Con A supernatant, IFN gamma, or LPS in concert with THC (10(-7) M to 10(-5) M), resulted in the generation of protein profiles whose patterns reverted approximately to those of unprimed or unactivated macrophages. THC was shown to alter the expression of select proteins whose induction is associated with macrophage priming or activation. The expression of P388D1 macrophage class II Ia molecules of the major histocompatibility complex (MHC), in response to Con A supernatant and IFN gamma, was inhibited. THC also altered the expression of tumor necrosis factor alpha (TNF alpha) elicited by RAW264.7 cells in response to LPS. These results suggest that THC alters macrophage functional activities, at least in part, by suppressing their capacity to express effector molecules elicited in response to priming and activating signals.

Picornavirus inhibitors

Picornaviruses are among the best understood animal viruses in molecular terms. A number of important human and animal pathogens are members of the Picornaviridae family. The genome organization, the different steps of picornavirus growth and numerous compounds that have been reported as inhibitors of picornavirus functions are reviewed. The picornavirus particles and several agents that interact with them have been solved at atomic resolution, leading to computer-assisted drug design. Picornavirus inhibitors are useful in aiding a better understanding of picornavirus biology. In addition, some of them are promising therapeutic agents. Clinical efficacy of agents that bind to picornavirus particles has already been demonstrated.

Lipid composition and fluidity of the human immunodeficiency virus envelope and host cell plasma membranes

Previous studies have indicated that human immunodeficiency virus (HIV) is enclosed with a lipid envelope similar in composition to cell plasma membranes and to other viruses. Further, the fluidity, as measured by spin resonance spectroscopy, is low and the viral envelope is among the most highly ordered membranes analyzed. However, the relationship between viral envelope lipids and those of the host cell is not known. Here we demonstrate that the phospholipids within the envelopes of HIV-1RF and HIV-2-L are similar to each other but significantly different from their respective host cell surface membranes. Further, we demonstrate that the cholesterol-to-phospholipid molar ratio of the viral envelope is approximately 2.5 times that of the host cell surface membranes. Consistent with the elevated cholesterol-to-phospholipid molar ratio, the viral envelopes of HIV-1RF and HIV-2-L were shown to be 7.5% and 10.5% more ordered than the plasma membranes of their respective host cells. These data demonstrate that HIV-1 and HIV-2-L select specific lipid domains within the surface membrane of their host cells through which to emerge during viral maturation.

Changes in membrane permeability during Semliki Forest virus induced cell fusion

The infection of Aedes albopictus cells by Semliki Forest virus (SFV) is a non lytic event. Exposure of infected cells to mildly acidic pH (less than 6.2) leads to syncytium formation. This polykaryon formation is accompanied by an influx of protons into the cells (Kempf et al. Biosci. Rep. 7, 761-769, 1987). We have further investigated this permeability change using various fluorescent or radiolabeled compounds. A significant, pH dependent increase of the membrane permeability to low molecular weight compounds (M(r) less than 1000) was observed when infected cells were exposed to a pH less than 6.2. The pH dependence of the permeability change was very similar to the pH dependence of cell-cell fusion. The permeability change was sensitive to divalent cations, protons and anionic antiviral drugs such as trypan blue. The nature of this virus induced, pH dependent permeability change is discussed.

Killing of Burkitt-lymphoma-derived Daudi cells by ultraviolet-inactivated vaccinia virus

Interaction of active and UV-inactivated vaccinia virus at high multiplicity caused cytological changes and inhibition in cellular protein and DNA synthesis, thus arresting the multiplication of Burkitt-lymphoma-derived Daudi cells and eventually killing the cells. Adsorption to the cells but the lack of penetration was evident by immunofluorescence, electron microscopy and [3H]thymidine-labeled virus incorporation. Viral DNA synthesis or virus replication was not demonstrated. Thus, it appears that the massive adsorption of viral particles, active or UV-inactivated, or possibly a "toxic" component that resides in the virion, damages the plasma membrane and may be responsible for killing the cells by a mechanism of lysis from without.

Chronic marijuana smoke alters alveolar macrophage morphology and protein expression

Male rhesus monkeys were subjected to chronic exposure to marijuana smoke. High dose animals (HI) were exposed 7 days/week to 1 MJ cigarette/day; low dose animals (LO) were exposed on 2 consecutive weekend days to 1 MJ cigarette/day; placebo animals (EM) were exposed to 1 ethanol-extracted MJ cigarette/day for 7 days/week; sham animals (SH) were exposed to sham smoking conditions 7 days/week. This regimen was maintained for 1 year and was followed by a 7 month rest period. Alveolar macrophages of animals exposed to the LO and HI dose smoking regimens exhibited irregular cell surface morphology, increased vacuolization, and a spherical conformation upon adherence to plastic. Gel protein profiles of purified macrophages from HI and LO animals showed marked differences in both constitutive and bacterial lipopolysaccharide-elicited protein expression when compared with those of macrophages from the EM or SH animals. These results indicate that chronic THC exposure alters macrophage morphology and protein expression to external stimuli even after a 7 month rest period.

The role of monovalent cation transport in Sindbis virus maturation and release

Alterations in intracellular monovalent cation concentrations in Sindbis virus-infected avian cells result, in part, from a reduction in Na+/K+ ATPase (Na+ pump) activity. Inhibition of Na+ pump activity was shown previously to temporally correlate with the appearance of viral envelope proteins on the cell surface and the release of virus particles. Cells infected with envelope-defective temperature-sensitive mutants exhibited reduced Na+ pump activity at the nonpermissive temperature, where viral particles are not released. By contrast, Na+ pump activity was not inhibited in Sindbis virus-infected cells treated with tunicamycin or with antiviral serum, which block virus maturation and release. Diuretic-sensitive transport of 86Rb+, aK+ tracer, was stimulated in cells which express virus envelope proteins, but fail to release virus particles. In these cells, the furosemide-sensitive 86Rb+ influx exhibited an increase in Vmax and was responsive to changes in the extracellular concentration of NaCl. Furosemide inhibited the rapid release of virus from low salt-inhibited cells after shift to isotonic conditions. Alterations in ion transport during alphavirus infection may, therefore, facilitate the efficient release of progeny virus particles.

Control of protein synthesis by extracellular Na+ in cultured fibroblasts

In chick embryo fibroblasts (CEFs), a partial substitution of extracellular Na+ with other cations or carbohydrates decreased the intracellular Na+ content without altering the K+ level. Concomitantly, a significant decrease in the serum-dependent rate of protein synthesis occurred. This phenomenon appeared to be quickly reversible upon reconstitution of the correct extracellular Na+ concentration in the culture medium. The presence of a transcriptional inhibitor such as actinomycin D during the treatment did not inhibit the reversibility of the phenomenon. The presence in the culture medium of K+ in such excess as to dissipate the membrane potential did not alter the observed relationship between the protein synthesis rate and the internal Na+ content. Analysis of the amino acid pool indicated that the observed inhibition of the rate of protein synthesis in CEFs incubated in low Na+ medium was not caused by an unbalanced availability of intracellular amino acids. In addition, intracellular pH, as estimated by the measurement of the equilibrium distribution of benzoic acid, did not show any significant alteration in cells incubated in the presence of bicarbonate buffer and in low extracellular Na+. Moreover, the relationship between the rate of protein synthesis and the internal Na+ content was still observed in CEFs cultured in bicarbonate-containing media, but at lower or higher than physiological pH. Analysis by two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) of the proteins synthesized by CEFs cultured at a reduced extracellular Na+ concentration showed that specific alterations of gene expression occurred.

Alteration of intracellular monovalent cation concentrations by a poliovirus mutant which encodes a defective 2A protease

Poliovirus mutant 2A-1, which encodes a defective protease 2A, fails to inhibit translation of capped mRNAs selectively. Despite the failure of 2A-1 to inactivate cap-dependent translation, a reduction in the overall rate of protein synthesis, both virus and cell-specified, does occur after 2A-1 infection. This global reduction in protein synthesis is temporally correlated with an increase in [Na+]i and a decrease in [K+]i. The extensive global shutoff of protein synthesis is not observed in 2A-1 infected cells incubated in low NaCl medium or medium containing an elevated concentration of KCl which compensate for the virally-induced alterations in intracellular monovalent cation concentrations. Furthermore, 2A-1-specified protein synthesis is only partly resistant to hypertonic NaCl media which increase [Na+]i, in contrast to protein synthesis specified by wild-type poliovirus. These results suggest that shutoff of host and viral protein synthesis during infection by poliovirus mutant 2A-1 is a consequence of the virus-induced changes in intracellular monovalent cation concentrations.

Sensitivity of Burkitt lymphoma Daudi cells to inactive influenza virus

Interaction of UV-inactivated influenza A/X47 virus at high multiplicity caused a rapid inhibition in cellular protein and DNA synthesis, thus arresting Burkitt-lymphoma-derived Daudi cell multiplication, and eventually killing the cells. The mechanism of the cytolytic effect is presumably, linked to the increase in cell membrane permeability indicated by elevation in 51Cr release. This might be the consequence of the mass adsorption and/or penetration of viral particles.

Modification of membrane permeability by animal viruses

Animal viruses modify membrane permeability during lytic infection. There is a co-entry of macromolecules and virion particules during virus penetration and a drastic change in transport and membrane permeability at the late stages of the lytic cycle. Both events are of importance to understand different molecular aspects of viral infection, as virus entry into the cell and the interference of virus infection with cellular metabolism. Other methods of cell permeabilization of potential relevance to understand the mechanism of viral damage of the membrane are also discussed.

Sindbis virus infection decreases intracellular pH: alkaline medium inhibits processing of Sindbis virus polyproteins

Infection of baby hamster kidney cells by Sindbis virus, an alphavirus, resulted in a decrease in the intracellular pH of approximately 0.5 units within the first 1-2 hr after infection as measured either by equilibrium labeling with [14C]benzoic acid or by use of a pH-sensitive fluorescent probe, 2,7-bis-carboxyethyl-5,6-carboxyfluorescein-acetooxymethyl ester. In contrast, intralysosomal pH, as measured using an endocytized pH-sensitive probe, fluorescein isothiocyanate-labeled dextran, was not altered by Sindbis virus infection. Production of Sindbis virus was reduced by more than 90% and post-translational processing of Sindbis virus envelope precursors was inhibited in infected cells incubated in alkaline medium.

Autogenous growth factor production by reticuloendotheliosis virus-transformed hematopoietic cells

Reticuloendotheliosis virus strain T (REV-T)-transformed cells gave rise spontaneously to variants which secrete a factor that forms a distinct visible ring of precipitation (halo) surrounding colonies grown in soft agar. An Mr 15,000 protein was produced at higher levels by halo variants than by nonhalo-producing cells. An assay designed to detect the formation of precipitates enabled purification of an Mr 15,000 protein, p15, from serum-free medium conditioned by the growth of REV-T-transformed hematopoietic cells. Fractions enriched in p15 permitted the growth of REV-T-transformed cells under conditions where they normally failed to proliferate.

Cell surface effects of human immunodeficiency virus

Cell killing by human immunodeficiency virus (HIV) is thought to contribute to many of the defects of the acquired immunodeficiency syndrome (AIDS). Two types of cytopathology are observed in HIV-infected cultured cells: cell-cell fusion and killing of single cells. Both killing processes appear to involve cell surface effects of HIV. A model is proposed for the HIV-mediated cell surface processes which could result in cell-cell fusion and single cell killing. The purpose of this model is to define the potential roles of individual viral envelope and cell surface molecules in cell killing processes and to identify alternative routes to the establishment of persistently-infected cells. Elucidation of HIV-induced cell surface effects may provide the basis for a rational approach to the design of antiviral agents which are selective for HIV-infected cells.

Selective inhibition of protein synthesis by hypertonic medium in Marituba (Bunyaviridae) virus-infected L-A9 cells

Elevation of the NaCl concentration in the growth medium of L-A9 cells caused an inhibition of the protein synthesis accompanied by a complete breakdown of polyribosomes. However, a complete recovery of the rate of protein synthesis was observed when isotonicity was restored. In Marituba virus infected cells, protein synthesis became resistant to hypertonic treatment. Under hypertonic conditions cellular protein synthesis was selectively suppressed and an enhancement of virus proteins was observed. Analysis of the virus specific proteins by polyacrylamide gel electrophoresis revealed that the synthesis of G1 was unalterable, and N was stimulated.