Thiol/disulfide exchange is a prerequisite for CXCR4-tropic HIV-1 envelope-mediated T-cell fusion during viral entry

A Role for Sperm Surface Protein Disulfide Isomerase Activity in Gamete Fusion: Evidence for the Participation of ERp57

In mammals, sperm-egg interaction is based on molecular events either unique to gametes or also present in somatic cells. In gamete fusion, it is unknown which features are gamete specific and which are shared with other systems. Conformational changes mediated by thiol-disulfide exchange are involved in the activation of some virus membrane fusion proteins. Here we asked whether that mechanism is also operative in sperm-egg fusion. Different inhibitors of protein disulfide isomerase (PDI) activity were able to inhibit sperm-egg fusion in vitro. While pretreatment of oocytes had no effect, pretreatment of sperm reduced their fusion ability. Some members of the PDI family were detected on the sperm head, and use of specific antibodies and substrates suggested that the oxidoreductase ERp57 has a role in gamete fusion. The results support the idea that thiol-disulfide exchange is a mechanism that may act in gamete fusion to produce conformational changes in fusion-active proteins.

Proteins pattern alteration in AZT-treated K562 cells detected by two-dimensional gel electrophoresis and peptide mass fingerprinting

In this study we report the effect of AZT on the whole protein expression profile both in the control and the AZT-treated K562 cells, evidenced by two-dimensional gel electrophoresis and peptide mass fingerprinting analysis. Two-dimensional gels computer digital image analysis showed two spots that appeared up-regulated in AZT-treated cells and one spot present only in the drug exposed samples. Upon extraction and analysis by peptide mass fingerprinting, the first two spots were identified as PDI-A3 and stathmin, while the third one was proved to be NDPK-A. Conversely, two protein spots were present only in the untreated K562 cells, and were identified as SOD1 and HSP-60, respectively.

Extracellular disulfide exchange and the regulation of cellular function

An emerging concept is that disulfide bonds can act as a dynamic scaffold to present mature proteins in different conformational and functional states on the cell surface. Two examples are the conversion of the receptor, integrin alphaIIbbeta3, from a low affinity to a high affinity state, and the interaction of CD4 receptor with the HIV-1 envelope glycoprotein gp120 to promote virus-cell fusion. In both of these cases there is a remodeling of the protein disulfide bonding pattern. The formation and rearrangement of disulfide bonds is modulated by a family of enzymes known as the thiol isomerases, which include protein disulfide isomerase (PDI), ERp5, ERp57, and ERp72. While these enzymes were reported originally to be restricted in location to the endoplasmic reticulum, in some cells thiol isomerases are found on the cell surface. This may indicate a wider role for these enzymes in cell function. In platelets it has been shown that reagents that react with cell surface sulfhydryl groups are capable of blocking a number of functional responses, including integrin-mediated aggregation, adhesion, and granule secretion. Furthermore, the use of function blocking antibodies to either PDI or ERp5 causes inhibition of these functional responses. This review summarizes current knowledge of the extracellular regulation of disulfide exchange and the implications of this in the regulation of cell function.

Role of protein disulfide isomerase and other thiol-reactive proteins in HIV-1 envelope protein-mediated fusion

Cell-surface protein disulfide isomerase (PDI) has been proposed to promote disulfide bond rearrangements in HIV-1 envelope protein (Env) that accompany Env-mediated fusion. We evaluated the role of PDI in ways that have not been previously tested by downregulating PDI with siRNA and by overexpressing wild-type or variant forms of PDI in transiently and stably transfected cells. These manipulations, as well as treatment with anti-PDI antibodies, had only small effects on infection or cell fusion mediated by NL4-3 or AD8 strains of HIV-1. However, the cell-surface thiol-reactive reagent 5, 5'-dithiobis(2-nitrobenzoic acid) (DTNB) had a much stronger inhibitory effect in our system, suggesting that cell-surface thiol-containing molecules other than PDI, acting alone or in concert, have a greater effect than PDI on HIV-1 Env-mediated fusion. We evaluated one such candidate, thioredoxin, a PDI family member reported to reduce a labile disulfide bond in CD4. We found that the ability of thioredoxin to reduce the disulfide bond in CD4 is enhanced in the presence of HIV-1 Env gp120 and that thioredoxin also reduces disulfide bonds in gp120 directly in the absence of CD4. We discuss the implications of these observations for identification of molecules involved in disulfide rearrangements in Env during fusion.

Significant redox insensitivity of the functions of the SARS-CoV spike glycoprotein: comparison with HIV envelope

The capacity of the surface glycoproteins of enveloped viruses to mediate virus/cell binding and membrane fusion requires a proper thiol/disulfide balance. Chemical manipulation of their redox state using reducing agents or free sulfhydryl reagents affects virus/cell interaction. Conversely, natural thiol/disulfide rearrangements often occur during the cell interaction to trigger fusogenicity, hence the virus entry. We examined the relationship between the redox state of the 20 cysteine residues of the SARS-CoV (severe acute respiratory syndrome coronavirus) Spike glycoprotein S1 subdomain and its functional properties. Mature S1 exhibited ∼4 unpaired cysteines, and chemically reduced S1 displaying up to ∼6 additional unpaired cysteines still bound ACE2 and enabled fusion. In addition, virus/cell membrane fusion occurred in the presence of sulfhydryl-blocking reagents and oxidoreductase inhibitors. Thus, in contrast to various viruses including HIV (human immunodeficiency virus) examined in parallel, the functions of the SARS-CoV Spike glycoprotein exhibit a significant and surprising independence of redox state, which may contribute to the wide host range of the virus. These data suggest clues for molecularly engineering vaccine immunogens.

Time-resolved imaging of HIV-1 Env-mediated lipid and content mixing between a single virion and cell membrane

A method has been developed to follow fusion of individual pseudotyped virus expressing HIV-1 Env to cells by time-resolved fluorescence microscopy. Viral envelopes were labeled with a fluorescent lipid dye (DiD) and virus content was rendered visible by incorporating a Gag-GFP chimera. The Gag-GFP is naturally cleaved to the much smaller NC-GFP fragment in the mature virions. NC-GFP was readily released upon permeabilization of the viral envelope, whereas the capsid was retained. The NC-GFP thus provides a relatively small and mobile aqueous marker to follow viral content transfer. In fusion experiments, virions were bound to cells at low temperature, and fusion was synchronously triggered by a temperature jump. DiD transferred from virions to cells without a significant lag after the temperature jump. Some virions released DiD but retained NC-GFP. Surprisingly, the fraction of lipid mixing events yielding NC-GFP transfer was dependent on the type of target cell: of three infectable cell lines, only one permitted NC-GFP transfer within minutes of raising temperature. NC-GFP release did not correlate with the level of CD4 or coreceptor expression in the target cells. The data indicate that fusion pores formed by HIV-1 Env can remain small for a relatively long time before they enlarge.

Rationale for the treatment of cancer with sodium selenite

Epidemiological studies conducted during several decades of the last century have demonstrated the importance of sufficient nutritional supply of selenium (Se) for human health. More importantly, low blood Se levels were found to be associated with an increased incidence and mortality from various types of cancers. Recently, attention of researchers was drawn to the relationship between free radical generation, known otherwise as oxidative stress, and carcinogenesis. It was therefore thought that antioxidants should be beneficial for prevention and inhibition of different malignancies. However, there appeared to be a paradox, because tumor growth is associated with tissue hypoxia that is accompanied by the formation of reductive rather than oxidative free radicals. Various organic and inorganic Se compounds, generally considered to be antioxidants, produced mixed results when tested in animal models and human subjects. Amongst them, sodium selenite has been shown to be the most effective in an in vitro and in vivo carcinogenesis studies. As recently demonstrated, selenite is not an antioxidant, but possesses oxidizing properties in the presence of specific substrates. Thus selenite is capable of oxidizing polythiols to corresponding disulfides, but does not react with monothiols. Such polythiols associated with cancer membrane-bound proteins appear under the reducing conditions of hypoxic tumor tissue. These thiol groups can, in turn, initiate a disulfide exchange reaction with plasma proteins, predominantly with fibrinogen, to form an insoluble and protease-resistant fibrin-like polymer. As the result, tumor cells become surrounded by a coat which masks specific tumor antigens thus allowing cancer cells to escape immune recognition and elimination by natural killer (NK) cells. Selenite by virtue of oxidizing cell membrane thiols, can prevent the formation of the coat and consequently makes cancer cells vulnerable to the immune surveillance and destruction. In addition, selenite may directly activate NK cells, as well as inhibit angiogenesis without undesirable decrease in the oxidative potential of cellular environment. It is, therefore, postulated that sodium selenite, in view of its relative low toxicity, might become a drug of choice for many types of cancer including leukemia.

Keynote review: Progress in targeting HIV-1 entry

Current HIV entry inhibitors target the binding of the viral envelope glycoprotein gp120 to cellular CD4 and co-receptors, or block a late stage of the fusogenic activation of adjacent gp41. New targets are suggested by the role of cell surface protein disulfide isomerase (PDI), which attaches to the primary receptor CD4 close to the gp120-binding site. This could enable PDI to reduce gp120 disulfide bonds, which triggers the major conformational changes in gp120 and gp41 required for virus entry. Inhibiting cell surface PDI prevents HIV-1 entry. The new potential targets outlined are PDI activity as well as the sites of PDI-CD4 and PDI-gp120 interaction.

Functional chimeras of human immunodeficiency virus type 1 Gp120 and influenza A virus (H3) hemagglutinin

In an attempt to produce a protein that will allow determination of the native human immunodeficiency virus type 1 (HIV-1) gp120 (Env) structure in its trimeric state, we fused the globular head of gp120 to the stalk region of influenza virus A (X31) hemagglutinin (HA). The chimeric protein (EnvHA) has been expressed by using a recombinant vaccinia virus system, and its functional characteristics were determined. EnvHA is expressed as a 120- to 150-kDa protein that can oligomerize to form dimers and trimers. It retains the low-pH (5.2 to 5.4) requirement of X31-HA to trigger membrane fusion but, unlike X31-HA, it is not absolutely dependent on exogenously added trypsin for protein processing to release the HA2 fusion peptide. In terms of receptor binding the chimeric protein retains specificity for human CD4 but, in relation to the membrane fusion event, it appears to lose the Env coreceptor specificity of the parental HIV-1 strains: NL43 for CXCR4 and JRFL for CCR5. These properties suggest that stable, functional EnvHAs are being produced and that they may be exploited in terms of structural studies. Further, the potential of introducing the envHA genes into influenza viruses, by use of reverse genetics, and their use as a therapeutic vaccine for HIV are discussed.

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

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

Inter-subunit disulfide bonds in soluble HIV-1 envelope glycoprotein trimers

Soluble forms of the trimeric human immunodeficiency virus (HIV-1) envelope glycoproteins are important tools for structural studies and in the construction of improved immunogens. We found that a substantial fraction of soluble envelope glycoprotein trimers contain inter-subunit disulfide bonds (inter-S-S bonds) that render the trimers resistant to heat and denaturing agents. These inter-S-S bonds can be reduced without disrupting the trimers by treatment with a low concentration of beta-mercaptoethanol or DTT. Antibody mapping studies suggest that the soluble HIV-1 envelope glycoprotein trimers lacking the inter-S-S bonds exhibit a conformation closer to that of the native HIV-1 envelope glycoprotein complex. However, reducing these inter-S-S bonds had only modest effects on the inefficient elicitation of neutralizing antibodies by the soluble trimers. These studies provide guidance in improving the resemblance of tractable, soluble forms of the HIV-1 envelope glycoproteins to the native virion spikes.

Glycosaminoglycans and protein disulfide isomerase-mediated reduction of HIV Env

Conformational changes within the human immunodeficiency virus-1 (HIV-1) surface glycoprotein gp120 result from binding to the lymphocyte surface receptors and trigger gp41-mediated virus/cell membrane fusion. The triggering of fusion requires cleavage of two of the nine disulfide bonds of gp120 by a cell-surface protein disulfide-isomerase (PDI). Soluble glycosaminoglycans such as heparin and heparan sulfate bind gp120 via V3 and, possibly, a CD4-induced domain. They exert anti-HIV activity by interfering with the HIV envelope glycoprotein (Env)/cell-surface interaction. Env also binds cell-surface glycosaminoglycans. Here, using surface plasmon resonance, we observed an inverse relationship between heparin binding by gp120 and its thiol content. In vitro, and in conditions in which gp120 could bind CD4, heparin and heparan sulfate reduced PDI-mediated gp120 reduction by approximately 80%. Interaction of Env with the surface of lymphocytes treated using sodium chlorate, an inhibitor of glycosaminoglycan synthesis, led to gp120 reduction. We conclude that besides their capacity to block Env/cell interaction, soluble glycosaminoglycans can effect anti-HIV activity via interference with PDI-mediated gp120 reduction. In contrast, their presence at the cell surface is dispensable for Env reduction during the course of interaction with the lymphocyte surface. This work suggests that the reduction of exofacial proteins in various diseases can be inhibited by compounds targeting the substrates (not by targeting PDI, as is usually done), and that glycosaminoglycans that primarily protect proteins by preserving them from proteolysis also have a role in preventing reduction.

A role for the thiol isomerase protein ERP5 in platelet function

Formation and rearrangement of disulfide bonds during the correct folding of nascent proteins is modulated by a family of enzymes known as thiol isomerases, which include protein disulfide isomerase (PDI), endoplasmic reticulum protein 5 (ERP5), and ERP57. Recent evidence supports an alternative role for this family of proteins on the surface of cells, where they are involved in receptor remodeling and recognition. In platelets, blocking PDI with inhibitory antibodies inhibits a number of platelet activation pathways, including aggregation, secretion, and fibrinogen binding. Analysis of human platelet membrane fractions identified the presence of the thiol isomerase protein ERP5. Further study showed that ERP5 is resident mainly on platelet intracellular membranes, although it is rapidly recruited to the cell surface in response to a range of platelet agonists. Blocking cell-surface ERP5 using inhibitory antibodies leads to a decrease in platelet aggregation in response to agonists, and a decrease in fibrinogen binding and P-selectin exposure. It is possible that this is based on the disruption of integrin function, as we observed that ERP5 becomes physically associated with the integrin beta(3) subunit during platelet stimulation. These results provide new insights into the involvement of thiol isomerases and regulation of platelet activation.