Attenuation of HIV-1 infectivity by an inhibitor of oligosaccharide processing

1-Deoxynojirimycin and its Derivatives: A Mini Review of the Literature

1-Deoxynojirimycin (1-DNJ) is a naturally occurring sugar analogue with unique bioactivities. It is found in mulberry leaves and silkworms, as well as in the metabolites of certain microorganisms, including Streptomyces and Bacillus. 1-DNJ is a potent α-glucosidase inhibitor and it possesses anti-hyperglycemic, anti-obese, anti-viral and anti-tumor properties. Some derivatives of 1-DNJ, like miglitol, miglustat and migalastat, were applied clinically to treat diseases such as diabetes and lysosomal storage disorders. The present review focused on the extraction, determination, pharmacokinetics and bioactivity of 1-DNJ, as well as the clinical application of 1-DNJ derivatives.

Structure of human endo-α-1,2-mannosidase (MANEA), an antiviral host-glycosylation target

Mammalian protein N-linked glycosylation is critical for glycoprotein folding, quality control, trafficking, recognition, and function. N-linked glycans are synthesized from Glc₃Man₉GlcNAc₂ precursors that are trimmed and modified in the endoplasmic reticulum (ER) and Golgi apparatus by glycoside hydrolases and glycosyltransferases. Endo-α-1,2-mannosidase (MANEA) is the sole endo-acting glycoside hydrolase involved in N-glycan trimming and is located within the Golgi, where it allows ER-escaped glycoproteins to bypass the classical N-glycosylation trimming pathway involving ER glucosidases I and II. There is considerable interest in the use of small molecules that disrupt N-linked glycosylation as therapeutic agents for diseases such as cancer and viral infection. Here we report the structure of the catalytic domain of human MANEA and complexes with substrate-derived inhibitors, which provide insight into dynamic loop movements that occur on substrate binding. We reveal structural features of the human enzyme that explain its substrate preference and the mechanistic basis for catalysis. These structures have inspired the development of new inhibitors that disrupt host protein N-glycan processing of viral glycans and reduce the infectivity of bovine viral diarrhea and dengue viruses in cellular models. These results may contribute to efforts aimed at developing broad-spectrum antiviral agents and help provide a more in-depth understanding of the biology of mammalian glycosylation.

Mechanisms of Antiviral Activity of Iminosugars Against Dengue Virus

The antiviral mechanism of action of iminosugars against many enveloped viruses, including dengue virus (DENV), HIV, influenza and hepatitis C virus, is believed to be mediated by inducing misfolding of viral N-linked glycoproteins through inhibition of host endoplasmic reticulum-resident α-glucosidase enzymes. This leads to reduced secretion and/or infectivity of virions and hence lower viral titres, both in vitro and in vivo. Free oligosaccharide analysis from iminosugar-treated cells shows that antiviral activity correlates with production of mono- and tri-glucosylated sugars, indicative of inhibition of ER α-glucosidases. We demonstrate that glucose-mimicking iminosugars inhibit isolated glycoprotein and glycolipid processing enzymes and that this inhibition also occurs in primary cells treated with these drugs. Galactose-mimicking iminosugars that have been tested do not inhibit glycoprotein processing but do inhibit glycolipid processing, and are not antiviral against DENV. By comparison, the antiviral activity of glucose-mimetic iminosugars that inhibit endoplasmic reticulum-resident α-glucosidases, but not glycolipid processing, demonstrates that inhibition of α-glucosidases is responsible for iminosugar antiviral activity against DENV. This monograph will review the investigations of many researchers into the mechanisms of action of iminosugars and the contribution of our current understanding of these mechanisms for optimising clinical delivery of iminosugars. The effects of iminosugars on enzymes other than glucosidases, the induction of ER stress and viral receptors will be also put into context. Data suggest that inhibition of α-glucosidases results in inhibited release of virus and is the primary antiviral mechanism of action of iminosugars against DENV.

1-Deoxynojirimycin: Occurrence, Extraction, Chemistry, Oral Pharmacokinetics, Biological Activities and In Silico Target Fishing

1-Deoxynojirimycin (DNJ, C₆H13NO₄, 163.17 g/mol), an alkaloid azasugar or iminosugar, is a biologically active natural compound that exists in mulberry leaves and Commelina communis (dayflower) as well as from several bacterial strains such as Bacillus and Streptomyces species. Deoxynojirimycin possesses antihyperglycemic, anti-obesity, and antiviral features. Therefore, the aim of this detailed review article is to summarize the existing knowledge on occurrence, extraction, purification, determination, chemistry, and bioactivities of DNJ, so that researchers may use it to explore future perspectives of research on DNJ. Moreover, possible molecular targets of DNJ will also be investigated using suitable in silico approach.

Synthesis and glycosidase inhibitory activities of 2-(aminoalkyl)pyrrolidine-3,4-diol derivatives

Several 2-(aminomethyl)-and 2-(2-aminoethyl)-pyrrolidine-3,4-diol derivatives have been assayed for their inhibitory activities towards glycosidases. Good inhibitors of alpha-mannosidases must have the (2R,3R,4S) configuration and possess 2-(benzylamino)methyl substituents. Stereomers with the (2S,3R,4S) configuration are also competitive inhibitors of alpha-mannosidases, but less potent as they share the configuration of C(1), C(2), C(3) of beta-D-mannosides rather than that of alpha-D-mannosides. Interestingly, (2S,3R,4S)-2-[2-[(4-phenyl)phenylamino]ethyl]pyrrolidine-3,4-diol (12g) inhibits several enzymes, for instance alpha-L-fucosidase from bovine epididymis (K(i)=6.5microM, competitive), alpha-galactosidase from bovine liver (K(i)=5microM, mixed) and alpha-mannosidase from jack bean (K(i)=102microM, mixed). Diamines such as (2R,3S,4R)-2-[2-(phenylamino) or 2-(benzylamino)ethyl]pyrrolidine-3,4-diol (ent-12a, ent-12b) inhibit beta-glucosidase from almonds (K(i)=13-40microM, competitive).

Glycosylation inhibitors and neuraminidase enhance human immunodeficiency virus type 1 binding and neutralization by mannose-binding lectin

Mannose-binding lectin (MBL), a C-type lectin component of the human innate immune system, binds to the gp120 envelope glycoprotein of human immunodeficiency virus type 1 (HIV-1). The objective of this study was to assess the effects of inhibitors of endoplasmic reticulum glucosidases and Golgi mannosidase as well as neuraminidase (NA) on the interaction between HIV and MBL. Production of HIV in the presence of the mannosidase I inhibitor deoxymannojirimycin (dMM) significantly enhanced binding of HIV to MBL and increased MBL neutralization of an M-tropic HIV primary isolate. In contrast, culturing HIV in the presence of alpha-glucosidase I and II inhibitors castanospermine and deoxynojirimycin only slightly affected virus binding and neutralization by MBL. Removal of sialic acid from HIV by NA also significantly enhanced virus binding and neutralization by MBL. Treatment of virus grown in the presence of dMM with endoglycosidase F1 substantially reduced binding to MBL, indicating that dMM increased MBL binding by increasing high-mannose carbohydrates on the virus. In contrast, endoglycosidase F1 did not decrease the MBL interaction with NA-treated virus, suggesting that NA exposed novel MBL binding sites. Treatment with dMM increased the immunocapture of HIV by monoclonal antibodies 2F5 and 2G12, indicating that altering the glycosylation of viral glycoproteins increases the accessibility or reactivity of some epitopes. This study shows that specific alterations of the N-linked carbohydrates on HIV gp120/gp41 can enhance MBL-mediated neutralization of virus by strengthening the interaction of HIV-1 with MBL.

The alpha-glucosidase inhibitor 1-deoxynojirimycin blocks human immunodeficiency virus envelope glycoprotein-mediated membrane fusion at the CXCR4 binding step

1-Deoxynojirimycin (DNM) is a saccharide decoy that inhibits cellular alpha-glucosidase I-II activity. Treatment by DNM of human immunodeficiency virus (HIV)-infected lymphocyte cultures inhibits virus spread. The functional properties of the membrane-associated Env glycoprotein (Env) modified in the presence of DNM remain unclear because previous reports on this subject have essentially used recombinant soluble Envs whose properties differ notably from those of Env anchored on the surface of the virus. To model virus-associated Env synthesized in the presence of DNM, native Env was expressed at the surface of mammalian cells treated with DNM. As expected, its glycosylation pattern was altered in the presence of the inhibitor. Env was found able to bind CD4, whereas its ability to induce membrane fusion was abolished. The immunoreactivity of regions involved in interactions of Env with CXCR4 (V1, V2, C2, and V3) was modified and Env displayed altered interaction with this coreceptor. These results are consistent with the inhibition by DNM of virus entry at the Env/coreceptor interaction step. Finally, preliminary data indicate that suboptimal concentrations of DNM and natural or synthetic CXCR4 ligands used in combination potently inhibit the Env-mediated membrane fusion process. Altogether, our results suggest that DNM and its analogs deserve further investigation as anti-HIV agents in combination with experimental compounds targeting CXCR4 to inhibit each partner of this crucial step of HIV entry.

N-linked glycosylation sites adjacent to and within the V1/V2 and the V3 loops of dualtropic human immunodeficiency virus type 1 isolate DH12 gp120 affect coreceptor usage and cellular tropism

The envelope glycoprotein of human immunodeficiency virus type 1 (HIV-1) is extensively glycosylated, containing approximately 23 asparagine (N)-linked glycosylation sites on its gp120 subunit. In this study, specific glycosylation sites on gp120 of a dualtropic primary HIV-1 isolate, DH12, were eliminated by site-directed mutagenesis and the properties of the resulting mutant envelopes were evaluated using a recombinant vaccinia virus-based cell-to-cell fusion assay alone or in the context of viral infections. Of the glycosylation sites that were evaluated, those proximal to the V1/V2 loops (N135, N141, N156, N160) and the V3 loops (N301) of gp120 were functionally critical. The glycosylation site mutations near the V1/V2 loop compromised the use of CCR5 and CXCR4 equally. In contrast, a mutation within the V3 loop preferentially inhibited the usage of CCR5; although this mutant protein completely lost its CCR5-dependent fusion activity, it retained 50% of the wild-type fusion activity with CXCR4. The replication of a virus containing this mutation was severely compromised in peripheral blood mononuclear cells, MT-4 cells, and primary monocyte-derived macrophages. A revertant virus, which acquired second site changes in the V3 loop that resulted in an increase in net positive charge, was isolated. The revertant virus fully recovered the usage of CXCR4 but not of CCR5, thereby altering the tropism of the parental virus from dualtropic to T-tropic. These results suggest that carbohydrate moieties near the V1/V2 and the V3 loops play critical roles in maintaining proper conformation of the variable loops for optimal interaction with receptors. Our results, combined with those of previously reported studies, further demonstrate that the function of individual glycans may be virus isolate dependent.

Effect of changes in the glycosylation of the human immunodeficiency virus type 1 envelope on the immunoreactivity and sensitivity to thrombin of its third variable domain

The influence of HIV Env glycosylation on the conformation of the third variable domain (V3) of Env was studied by both deglycosylation of mature Env and the use of Env produced by recombinant systems in which alpha-glucosidase activity was inhibited by either deoxynojirimycin (DNM) or mutation. Selective deglycosylation affected anti-V3 antibody binding. The immunoreactivity and sensitivity to thrombin cleavage of V3 presented on Env produced in baby hamster kidney cells were changed by DNM treatment. In contrast, Env expressed in alpha-glucosidase I-deficient Chinese hamster ovary cells or in their parental cells treated by DNM fully retained these V3 properties. These results are discussed in relation to the inconsistent data obtained on V3 property changes resulting from Env glycosylation changes.

Recombinant HIV envelope expressed in an alpha-glucosidase I-deficient CHO cell line and its parental cell line in the presence of 1-deoxynojirimycin is functional

alpha-Glucosidase inhibitors-e.g., 1-deoxynojirimycin (DNM)-interfere with HIV infectivity in CD4+ cell cultures but have proven unsuccessful in clinical trials. In vitro, several HIV Env properties, including the cleavage of the Env precursor gp 160, the immunoreactivity of the third variable domain (V3) of Env, the binding to the CD4 receptor, and the induction of the membrane fusion between the virus and the host cell, have been reported to be altered by such inhibitors. We have studied these properties for Env expressed via a recombinant vaccinia virus in two Chinese hamster ovary cell lines, an alpha-glucosidase I-deficient cell line and its parental cell line, treated with DNM under conditions that have been reported to alter Env properties. The glycosylation of Env, but not the quantity produced, varied in accordance with the experimental conditions. However, irrespective of these conditions, Env cleavage, V3 immunoreactivity, CD4 binding, membrane expression, and ability to induce syncytium formation were similar. Thus, neither the alpha-glucosidase I deficiency nor DNM treatment had a significant effect on the properties of Env produced here. Cellular mechanisms that may allow the normal expression of Env are discussed and may offer an explanation for the many discrepant results obtained to date on the effects of DNM on HIV Env.

N-butyldeoxynojirimycin-mediated inhibition of human immunodeficiency virus entry correlates with changes in antibody recognition of the V1/V2 region of gp120

The alpha-glucosidase inhibitor N-butyldeoxynojirimycin (NB-DNJ) is an inhibitor of human immunodeficiency virus (HIV) replication and HIV-induced syncytium formation in vitro. Although NB-DNJ appears to inhibit HIV entry at the level of post-CD4 binding (P.B. Fischer, M. Collin, G.B. Karlsson, W. James, T.D. Butters, S.J. Davis, S. Gordon, R.A. Dwek, and F.M. Platt, J. Virol. 69:5791-5797, 1995), the exact mechanism of action remains to be established. In this study we have examined the effect of NB-DNJ on the structure of recombinant gp120 (rgpl20), expressed in CHO cells, by using a panel of 40 monoclonal antibodies. The levels of binding of antibodies to rgp120 produced in the presence [rgpl20(+)] and absence [rgpl20(-)] of NB-DNJ were compared by enzyme-linked immunosorbent assay and surface plasmon resonance (BIAcore; Pharmacia). The results showed an increase in the binding to rgp120(+) of antibodies directed against the C1 and C2 regions and a decrease in the binding of antibodies directed against the V1/V2 loops compared with antibody binding to rgpl20(-). A decrease in the binding to rgpl20(+) of antibodies directed against discontinuous epitopes was also observed. No differences were seen in the binding of antibodies directed against the crown of the V3 loop and the C4 region of gp120. Treatment of rgpl20 with alpha-glucosidases I and II had no effect on the differential binding observed, whereas treatment with sialidase abolished the differences seen in the binding of antibodies directed against the C1 and C2 regions of gp120. In addition to these findings, rgpl20(+) showed increased sensitivity to proteases released by CHO cells during expression, as well as to exogenous thrombin. Taken together, the data presented in this paper suggest that production of gp120 in the presence of NB-DNJ affects the conformation of the Vl/V2 loops of gpl20, as well as the overall charge of the C1 and C2 regions. These effects may play a role in the previously described NB-DNJ-mediated inhibition of HIV entry at the level of post-CD4 binding.

N-butyldeoxynojirimycin-mediated inhibition of human immunodeficiency virus entry correlates with impaired gp120 shedding and gp41 exposure

The alpha-glucosidase inhibitor N-butyldeoxynojirimycin (NB-DNJ) is an inhibitor of human immunodeficiency virus (HIV) replication and HIV-induced syncytium formation in vitro. Although an NB-DNJ-mediated change in viral envelope N-glycan composition inhibits HIV entry at the level of post-CD4 binding, the exact mechanism of inhibition remains to be established. In this study we have examined the effects of NB-DNJ on virion envelope composition and CD4-induced gp120 shedding and gp41 exposure. Virion composition analysis revealed an NB-DNJ-mediated reduction of 15% in overall virion envelope glycoprotein content and a reduction of 26% in the proteolytic maturation of virion gp160. Taken together, these two effects resulted in a reduction of approximately 40% in virion gp120 content. CD4-induced shedding of gp120 from the surfaces of envelope-transfected Cos cells was undetectable when gp120 was expressed in the presence of NB-DNJ. Similarly, the shedding of virion-associated gp120 was reduced 7.4-fold. CD4-induced exposure of cryptic gp41 epitopes on the surfaces of HIV-expressing ACH-2 cells was also greatly impaired, and the exposure of virion-associated gp41 epitopes was reduced 4.0-fold. Finally, CD4-induced increases in the binding of antibodies to the V3 loop of ACH-2-cell-expressed envelope glycoproteins were reduced 25-fold when the glycoproteins were expressed in the presence of NB-DNJ. These results suggest that the NB-DNJ-mediated retention of glycosylated N-glycans inhibits HIV entry by a combined effect of a reduction in virion gp120 content and a qualitative defect within the remaining gp120, preventing it from undergoing conformational changes after CD4 binding.

Structural comparison of a 15 residue peptide from the V3 loop of HIV-1IIIb and an O-glycosylated analogue

As part of a program to study the effect of glycosylation on the three-dimensional structures of HIV-1IIIB V3 peptide constructs, we have examined the solution structures of a 15 residue peptide (RIQRGPGRAFVTIGK, P18IIIB)- originally mapped as an epitope recognized by CD8+ Dd class I MHC-restricted murine cytotoxic T-lymphocytes (CTL), and an analogue (P18IIIB-g), O-glycosylated with an alpha-galactosamine on Thr-12, using NMR, circular dichroism and molecular modeling methods. Our studies show that the peptides sample mainly random conformations in aqueous solution near 25 degrees C and become more ordered by the addition of trifluoroethanol. Upon decreasing the temperature to 5 degrees C, a reverse turn is formed around the immunodominant tip (G5-R8). Glycosylation on T12 'tightens' the turn slightly as suggested by NOE and CD analysis. In addition, the sugar has a defined conformation with respect to the peptide backbone and influences the local peptide conformation. These data suggest that simple glycosylation may influence the conformational equilibrium of a V3 peptide which contains a domain critical for antibody recognition and virus neutralization. We also show that the ability of cytotoxic T-lymphocytes (CTL) to lyse tumor cells presenting P18IIIB was completely abrogated by threonine glycosylation.

The alpha-glucosidase inhibitor N-butyldeoxynojirimycin inhibits human immunodeficiency virus entry at the level of post-CD4 binding

The alpha-glucosidase inhibitor N-butyldeoxynojirimycin (NB-DNJ) is a potent inhibitor of human immunodeficiency virus (HIV) replication and syncytium formation in vitro. However, the exact mechanism of action of NB-DNJ remains to be determined. In this study we have examined the impairment of HIV infectivity mediated by NB-DNJ. By two independent HIV entry assays [PCR-based HIV entry assay and entry of Cocal(HIV) pseudotypes], the reduction in infectivity was found to be due to an impairment of viral entry. No effect of NB-DNJ treatment was seen on the kinetics of the interaction between gp120 and CD4 (surface plasmon resonance; BIAcore) or on the binding of virus particles to H9 cells (using radiolabeled virions). We therefore conclude that a major mechanism of action of NB-DNJ as an inhibitor of HIV replication is the impairment of viral entry at the level of post-CD4 binding, due to an effect on viral envelope components.

Antiviral therapy for human immunodeficiency virus infections

Depending on the stage of their intervention with the viral replicative cycle, human immunodeficiency virus inhibitors could be divided into the following groups: (i) adsorption inhibitors (i.e., CD4 constructs, polysulfates, polysulfonates, polycarboxylates, and polyoxometalates), (ii) fusion inhibitors (i.e., plant lectins, succinylated or aconitylated albumins, and betulinic acid derivatives), (iii) uncoating inhibitors (i.e., bicyclams), (iv) reverse transcription inhibitors acting either competitively with the substrate binding site (i.e., dideoxynucleoside analogs and acyclic nucleoside phosphonates) or allosterically with a nonsubstrate binding site (i.e., non-nucleoside reverse transcriptase inhibitors), (v) integration inhibitors, (vi) DNA replication inhibitors, (vii) transcription inhibitors (i.e., antisense oligodeoxynucleotides and Tat antagonists), (viii) translation inhibitors (i.e., antisense oligodeoxynucleotides and ribozymes), (ix) maturation inhibitors (i.e., protease inhibitors, myristoylation inhibitors, and glycosylation inhibitors), and finally, (x) budding (assembly/release) inhibitors. Current knowledge, including the therapeutic potential, of these various inhibitors is discussed. In view of their potential clinical the utility, the problem of virus-drug resistance and possible strategies to circumvent this problem are also addressed.

Enhancement of HIV type 1 infectivity in vitro by capsular polysaccharide of Cryptococcus neoformans and Haemophilus influenzae

High concentrations of the cryptococcal capsular polysaccharide (CCP) are present in the serum, cerebrospinal fluid or both in the majority of AIDS patients infected with Cryptococcus neoformans. Because the prognosis of AIDS patients infected with cryptococcus is poor, we investigated whether the presence of CCP enhanced HIV-1 infection. The presence of CCP markedly increased the infectivity of HIV-1-infected H9 cells and subsequent production of infectious HIV-1 and formation of syncytia. In addition to enhancing the infectivity of H9 cells infected with laboratory isolates of HIV-1, the presence of CCP also increased the infectivity of peripheral blood mononuclear cells (PBMCs) infected with primary field strains of HIV-1. The in vitro infectivity of PBMCs from 20 of 44 HIV-1-infected individuals was significantly increased when cultured with CCP. Furthermore, HIV-1 was isolated from the PBMCs of three of these individuals only when cultured in the presence of CCP. CCP increased the binding of HIV-1 and recombinant gp120 to H9 cells and recombinant CD4, respectively. Thus, it is possible that the enhancement of HIV-1 infectivity by CCP is due to its capacity to increase the adherence of HIV-1 to target cells. Whereas the capsular polysaccharide of Haemophilus influenzae also markedly enhanced the infectivity of HIV-1, the capsular polysaccharides of C. freundii or S. flexneri had minimal effects on the infectivity of HIV-1. This indicated that the capacity to enhance HIV-1 infectivity was a property of polysaccharides from some pathogens and not others.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of alpha-glucosidase I of the glycoprotein-processing enzymes by 6-O-butanoyl castanospermine (MDL 28,574) and its consequences in human immunodeficiency virus-infected T cells

The 6-O-butanoyl derivative of castanospermine (MDL 28,574) was previously shown to be approximately 30-fold more potent than the naturally occurring molecule at inhibiting the replication of human immunodeficiency virus (HIV) (D. L. Taylor, P. S. Sunkara, P. S. Liu, M. S. Kang, T. L. Bowlin, and A. S. Tyms, AIDS 5:693-698, 1991). We now report that consistent with its improved anti-HIV activity, MDL 28,574 is more effective (50% inhibitory concentration [IC50], 20 microM) than the parent molecule (IC50, 254 microM) at causing the accumulation of glucosylated oligosaccharides in HIV-infected cells by inhibition of glycoprotein processing. These were predominantly of the glucose 3 type, as determined by P4 Bio-Gel analysis after digestion with purified alpha-glucosidase I, indicating that, intracellularly, this enzyme is the major target for inhibition. MDL 28,574, however, was less active (IC50, 1.27 microM) than castanospermine (IC50, 0.12 microM) against the mutual target enzyme, cellular alpha-glucosidase I, in a cell-free assay system. The increased effects of MDL 28,574 against alpha-glucosidase I in cell culture were attributed to the improved cellular uptake of the more lipophilic derivative. Inhibition of this enzyme activity in HIV-infected H9 cells impaired viral glycoprotein processing and resulted in the expression of abnormally configured gp120. This did not affect virus production, but the virions had decreased infectivity which was partially related to a reduced ability to bind to CD4+ T cells.

Alkaloidal glycosidase inhibitors (AGIs) as the cause of sporadic scrapie, and the potential treatment of both transmissible spongiform encephalopathies (TSEs) and human immunodeficiency virus (HIV) infection

AGIs are produced by plants and microorgansims in the environment. They are absorbed from the gut, distributed throughout the body and are concentrated inside cells. AGIs alter the glycan chains of cellular glycoproteins (CGP) during their formation so that the same CGP produced by different clones of cells (and hence with different glycan chains) becomes structurally the same. Prion protein (PrP), a CGP, is rendered indestructable to cellular mechanisms (as PrPi) by the TSE infective process; it is suggested that AGIs could both cause and prevent this by altering the primary structure of PrP. HIV envelope protein, gp120, carries glycan chains that are decided by the clone of the cells by which it is produced. Each cellular clone would be expected to add a specific group of glycan chains, making the gp120 antigenically separate. As HIV infection progresses, infected clone numbers rise, the antigenic diversity of gp120 may rise as would antibody production, trying to keep pace. Antigenically stimulated CD4+ cells carrying HIV genes, increase HIV production with gp120 antigenically different from its stimulant. AGIs prevent the glycan diversity and may prevent the extension of HIV infection.

Functions of HIV envelope glycans

The unusually highly glycosylated state of the major envelope glycoprotein (gp160) of the human immunodeficiency virus has offered a challenge to both glycobiologists and virologists. What is the functional significance of such a mass of glycans and how might they be manipulated to disadvantage virus pathogenesis? Some answers to each of these questions have already been obtained: N-linked glycans are necessary for the creation, but not the maintenance, of a bioactive conformation, and drug-induced alteration of the glycosylation pattern can lead to impaired virus infectivity. As a model for studying glycan function and as a target for antiviral therapy, gp160 represents a unique candidate.

In vitro modification of human immunodeficiency virus type 1 (HIV-1) infectivity by the U937 cells

The effect of host cell factors on infectivity of human immunodeficiency virus type 1 (HIV-1) was studied by infecting a monoblastoid cell line (U937) or a T-cell line (MOLT-4) with a highly infective single clone of HIV-1 and comparing the infectivity of the produced viruses to different cell lines. Chronically infected U937 cells consistently produced viruses with minimal infectivity. This phenotypic change was host-dependent as the back-passage of the U937-produced low infective viruses into MOLT-4 cells resulted in regaining their original high infectivity. Southern and Northern blot analyses of the HIV-1 grown in U937 cells did not reveal any genomic difference between it and the virus grown it MOLT-4 cells. The radioimmunoprecipitation analysis of viral proteins showed that the HIV-1-infected U937 cells had a different pattern of envelope glycoproteins and core proteins, which well correlated with the low infectivity of the produced viruses. This experimental system using MOLT-4 and U937 cell lines would be useful to further explore host cell factor(s) which play an important role in the regulation of HIV-1 infectivity.

Mechanism of action of N-butyl deoxynojirimycin in inhibiting HIV-1 infection and activity in combination with nucleoside analogs

The effects on HIV-1 infection of a glucosidase inhibitor, N-butyl deoxynojirimycin (N-buDNJ), were examined. The combinations of N-buDNJ and nucleoside analogs dideoxyinosine (DDI), dideoxycytidine (DDC), or azidothymidine (AZT) were examined in an acute infection assay. The combination of N-buDNJ and nucleoside analog reduced the yield of reverse transcriptase activity more than did either agent alone, and the effects on the number of infectious virus particles were additive or synergistic. In studies of the mechanism whereby N-buDNJ alters HIV-1 envelope fusion activity, no effects on CD4 binding were detected. However, cleavage within the V3 loop of gp120 was reduced by N-buDNJ treatment, possibly reflecting an altered conformation of this region of the envelope protein.

Clinical aspects of glycoprotein biosynthesis

Glycoproteins are widely distributed among species in soluble and membrane-bound forms, associated with many different functions. The heterogenous sugar moieties of glycoproteins are assembled in the endoplasmic reticulum and in the Golgi and are implicated in many roles that require further elucidation. Glycoprotein-bound oligosaccharides show significant changes in their structures and relative occurrences during growth, development, and differentiation. Diverse alterations of these carbohydrate chains occur in diseases such as cancer, metastasis, leukemia, inflammatory, and other diseases. Structural alterations may correlate with activities of glycosyltransferases that assemble glycans, but often the biochemical origin of these changes remains unclear. This suggests a multitude of biosynthetic control mechanisms that are functional in vivo but have not yet been unraveled by in vitro studies. The multitude of carbohydrate alterations observed in disease states may not be the primary cause but may reflect the growth and biochemical activity of the affected cell. However, knowledge of the control mechanisms in the biosynthesis of glycoprotein glycans may be helpful in understanding, diagnosing, and treating disease.

Loss of infectivity by progeny virus from alpha interferon-treated human immunodeficiency virus type 1-infected T cells is associated with defective assembly of envelope gp120

Levels of human immunodeficiency virus (HIV) DNA, RNA, or p24 antigen and reverse transcriptase activity in T-cell cultures treated with 500 IU of recombinant alpha interferon (rIFN alpha) per ml were comparable to those in control cultures. Radioimmunoprecipitation analysis of proteins in lysates of IFN-treated T cells documented a marked accumulation of HIV proteins. Localization of gp120 by immunofluorescence showed a diffuse pattern in IFN-treated cells quite distinct from the ring pattern in untreated control cells. That large quantities of gp120 in aberrant cell compartments might affect HIV morphogenesis was confirmed in infectivity studies: virions from IFN-treated cells were 100- to 1,000-fold less infectious than an equal number of virions from control cells. Direct examination of IFN-treated and control HIV-infected cells by transmission electron microscopy showed little difference in the number or distribution of viral particles. However, quantitation of gp120 by immunogold particle analysis revealed a marked depletion of envelope glycoprotein in virions released from IFN-treated cells. This defect in gp120 assembly onto mature viral particles provides a molecular basis for this loss of infectivity.

Modulation of cell-surface transferrin receptor by the imino sugar N-butyldeoxynojirimycin

The imino sugar, N-butyldeoxynojirimycin, is an inhibitor of the glycoprotein-processing enzyme glucosidase I and exhibits anti-(human immunodeficiency virus) activity in vitro. We have investigated the effect(s) of this compound on cell-surface glycoproteins by flow cytometry. We observed selective modulation of the transferrin receptor in response to treatment with 0.5 mM N-butyldeoxynojirimycin resulting in reduced cell-surface transferrin-receptor expression. The receptor modulation was dose dependent, resulted in reduced 59Fe uptake by treated cells and was fully reversible within 24 h of culture in the absence of the compound. Pulse/chase analysis in conjunction with endoglycosidase-H digestion demonstrated that transferrin-receptor glycosylation was altered following N-butyldeoxynojirimycin treatment, which is compatible with glucosidase inhibition. In addition, modulation of transferrin receptor in response to N-butyldeoxynojirimycin was not confined to a single cell line, but was also observed with certain human lymphoid and myeloid cell lines. Mechanism(s) of action of the imino sugar resulting in reduced cell-surface transferrin-receptor expression are discussed.

Role of asparagine-linked glycosylation in human immunodeficiency virus type 1 transmembrane envelope function

Transmembrane envelope protein (TM) residues 100, 105, and 128 of human immunodeficiency virus type 1 (HIV-1) strain HXB2 are potential sites for asparagine-linked oligosaccharide additions which are conserved among HIV-1 isolates, and all other lentivirus TM proteins. Site-specific mutants of each of the asparagine residues did not eliminate the ability of the virus to infect and replicate in CD4+ cells, but infectivity was reduced with all of these mutants, and syncytia induction was attenuated with two of these mutants. Studies of envelope expression of the mutant with the most severe defect demonstrated no significant effects on envelope protein synthesis, conformation, processing, multimerization, or release into the culture medium, suggesting that N-linked oligosaccharides are important in the specific fusion activity of TM.

Glucosidase inhibitors for treatment of HIV-1 infection

The HIV-1 envelope protein is a glycoprotein composed of 120 kD and 41 kD subunits. It contains 30-38 potential asparagine-linked glycosylation sites which have been shown to play a role in CD4 binding, virus uptake, and cytopathogenicity. Several inhibitors of oligosaccharide attachment or modification have been tested. An agent which inhibits glucosidases, N-butyl deoxynojirimycin was found to inhibit HIV-1 and SIVmac infectivity, and is currently in clinical trials.

Conserved cysteine residues in the human immunodeficiency virus type 1 transmembrane envelope protein are essential for precursor envelope cleavage

The transmembrane (TM) protein of human immunodeficiency virus type 1 has been demonstrated to be involved in viral infectivity and syncytium formation. Two highly conserved cysteine residues in the extracellular region of the TM protein are shown to be essential for processing the 160-kDa envelope precursor into the active 120- and 41-kDa mature forms.

Demonstration of two distinct cytopathic effects with syncytium formation-defective human immunodeficiency virus type 1 mutants

The mechanism of human immunodeficiency virus type 1 (HIV-1) cytopathicity is poorly understood and might involve formation of multinucleated giant cells (syncytia), single-cell lysis, or both. In order to determine the contributions of the fusion domain to syncytium formation, single-cell lysis, and viral infectivity and to clarify the molecular details of these events, insertion mutations were made in the portion of env encoding this sequence in the functional HIV-1 proviral clone HXB2. Viruses produced from these mutant clones were found to have a partial (F3) or complete (F6) loss of syncytium-forming ability in acutely infected CEM, Sup T1, and MT4 T-cell lines. During the early stage of acute infection by F6 virus, there was a loss of the syncytial cytopathic effect, which resulted in increased cell viability, and a 1.9- to 2.6-fold increase in virus yield in the cell lines tested. In the late stage of acute infection, the single-cell cytopathic effect of F6 virus was similar to that of the parental HXB2 virus. The F3 and F6 viruses were also found to have a 1.7- to 43-fold reduction in infectivity compared with the HXB2 virus. The mutant F3 and F6 and parental HXB2 envelope proteins were expressed in vaccinia virus, and the mutant envelope proteins were observed to be defective in their ability to form syncytia. BSC-40 cells infected with vaccinia virus recombinants revealed no differences in kinetics of cleavage, cell surface expression, or CD4 binding capacity of the mutant and parental envelope proteins. These results demonstrate that a loss of syncytium formation results in an attenuation of infectivity and a loss of the syncytial cytopathic effect without a loss of single-cell lysis. These mutants may reflect in tissue culture the changes observed in the HIV isolates in vivo during disease progression, which exhibit marked differences in syncytium production.

Inhibition of HIV and SIV infectivity by blockade of alpha-glucosidase activity

Processing of HIV and SIV envelope oligosaccharides is critical for proper intracellular trafficking and function. An inhibitor of alpha-glucosidases I and II, N-butyl deoxynojirimycin (N-BuDNJ), retards HIV-1 and SIVmac spread in lymphocytes and monocytes by diminishing virus infectivity, and also causes a reduction in syncytia formation between infected cells and uninfected lymphocytes. N-BuDNJ retards envelope processing from the precursor form to the mature surface (SU) and transmembrane proteins in HIV-1- and SIVmac-infected cells, as well as in cells infected with vaccinia-HIV-1 envelope recombinant virus. However, no significant reduction is seen in the amount of SU in released virus particles, though the virus particle-associated SU from N-BuDNJ-treated cells has an altered electrophoretic mobility. In contrast, N-BuDNJ had no effect on GAG protein synthesis and processing. These findings demonstrate a critical requirement for oligosaccharide processing by alpha-glucosidases I and II for HIV-1 and SIVmac envelope processing and fusogenicity.