Cytotoxic activity of the amino-terminal region of HIV type 1 Nef protein

Contribution of yeast models to virus research

Abstract

Time and again, yeast has proven to be a vital model system to understand various crucial basic biology questions. Studies related to viruses are no exception to this. This simple eukaryotic organism is an invaluable model for studying fundamental cellular processes altered in the host cell due to viral infection or expression of viral proteins. Mechanisms of infection of several RNA and relatively few DNA viruses have been studied in yeast to date. Yeast is used for studying several aspects related to the replication of a virus, such as localization of viral proteins, interaction with host proteins, cellular effects on the host, etc. The development of novel techniques based on high-throughput analysis of libraries, availability of toolboxes for genetic manipulation, and a compact genome makes yeast a good choice for such studies. In this review, we provide an overview of the studies that have used yeast as a model system and have advanced our understanding of several important viruses.

Key points

• Yeast, a simple eukaryote, is an important model organism for studies related to viruses.

• Several aspects of both DNA and RNA viruses of plants and animals are investigated using the yeast model.

• Apart from the insights obtained on virus biology, yeast is also extensively used for antiviral development.

Inhibition of human immunodeficiency virus type 1 by triciribine involves the accessory protein nef

Triciribine (TCN) is a tricyclic nucleoside that inhibits human immunodeficiency virus type 1 (HIV-1) replication by a unique mechanism not involving the inhibition of enzymes directly involved in viral replication. This activity requires the phosphorylation of TCN to its 5' monophosphate by intracellular adenosine kinase. New testing with a panel of HIV and simian immunodeficiency virus isolates, including low-passage-number clinical isolates and selected subgroups of HIV-1, multidrug resistant HIV-1, and HIV-2, has demonstrated that TCN has broad antiretroviral activity. It was active in cell lines chronically infected with HIV-1 in which the provirus was integrated into chromosomal DNA, thereby indicating that TCN inhibits a late process in virus replication. The selection of TCN-resistant HIV-1 isolates resulted in up to a 750-fold increase in the level of resistance to the drug. DNA sequence analysis of highly resistant isolate HIV-1(H10) found five point mutations in the HIV-1 gene nef, resulting in five different amino acid changes. DNA sequencing of the other TCN-resistant isolates identified at least one and up to three of the same mutations observed in isolate HIV-1(H10). Transfer of the mutations from TCN-resistant isolate HIV-1(H10) to wild-type virus and subsequent viral growth experiments with increasing concentrations of TCN demonstrated resistance to the drug. We conclude that TCN is a late-phase inhibitor of HIV-1 replication and that mutations in nef are necessary and sufficient for TCN resistance.

Alterations in intracellular potassium concentration by HIV-1 and SIV Nef

Background

HIV-1 mediated perturbation of the plasma membrane can produce an alteration in the transmembrane gradients of cations and other small molecules leading to cell death. Several HIV-1 proteins have been shown to perturb membrane permeability and ion transport. Xenopus laevis oocytes have few functional endogenous ion channels, and have proven useful as a system to examine direct effects of exogenously added proteins on ion transport.

Results

HIV-1 Nef induces alterations in the intracellular potassium concentration in CD4+ T-lymphoblastoid cells, but not intracellular pH. Two electrode voltage-clamp recording was used to determine that Nef did not form ion channel-like pores in Xenopus oocytes.

Conclusion

These results suggest that HIV-1 Nef regulates intracellular ion concentrations indirectly, and may interact with membrane proteins such as ion channels to modify their electrical properties.

Human Immunodeficiency virus type 1 Nef potently induces apoptosis in primary human brain microvascular endothelial cells via the activation of caspases

The lentiviral protein Nef plays a major role in the pathogenesis of human immunodeficiency virus type I (HIV-1) infection. Although the exact mechanisms of its actions are not fully understood, Nef has been shown to be essential for the maintenance of high-titer viral replication and disease pathogenesis in in vivo models of simian immunodeficiency virus infection of monkeys. Nef has also been suggested to play a pivotal role in the depletion of T cells by promoting apoptosis in bystander cells. In this context, we investigated the ability of extracellular and endogenously expressed HIV-1 Nef to induce apoptosis in primary human brain microvascular endothelial cells (MVECs). Human brain MVECs were exposed to baculovirus-expressed HIV-1 Nef protein, an HIV-1-based vector expressing Nef, spleen necrosis virus (SNV)-Nef virus (i.e., SNV vector expressing HIV-1 Nef as a transgene), and the HIV-1 strain ADA and its Nef deletion mutant, ADADeltaNef. We observed that ADA Nef, the HIV-1 vector expressing Nef, and SNV-Nef were able to induce apoptosis in a dose-dependent manner. The mutant virus with a deletion in Nef was able to induce apoptosis in MVECs to modest levels, but the effects were not as pronounced as with the wild-type HIV-1 strain, ADA, the HIV-1-based vector expressing Nef, or SNV-Nef viruses. We also demonstrated that relatively high concentrations of exogenous HIV-1 Nef protein were able to induce apoptosis in MVECs. Gene microarray analyses showed increases in the expression of several specific proapoptotic genes. Western blot analyses revealed that the various caspases involved with Nef-induced apoptosis are processed into cleavage products, which occur only during programmed cell death. The results of this study demonstrate that Nef likely contributes to the neuroinvasion and neuropathogenesis of HIV-1, through its effects on select cellular processes, including various apoptotic cascades.

Positive and negative aspects of the human immunodeficiency virus protease: development of inhibitors versus its role in AIDS pathogenesis

In this review we summarize multiple aspects of the human immunodeficiency virus (HIV) protease from both structural and functional viewpoints. After an introductory overview, we provide an up-to-date status report on protease inhibitors (PI). This proceeds from a discussion of PI structural design, to how PI are optimally utilized in highly active antiretroviral triple therapy (one PI along with two reverse transcriptase inhibitors), the emergence of PI resistance, and the natural role of secretory leukocyte PI. Then we switch to another focus: the interaction of HIV protease with other genes in acute and persistent infection, which in turn may have an effect on AIDS pathogenesis. We conclude with a discussion on future directions in HIV treatment, involving multiple-target anti-HIV therapy, vaccine development, and novel reactivation-inhibitory reagents.

Expression of HIV-1 nef in yeast causes membrane perturbation and release of the myristylated Nef protein

The human immunodeficiency virus type 1 (HIV-1) Nef protein is essential for AIDS pathogenesis, but its function remains highly controversial. During stresses such as growth in the presence of copper or at elevated temperature, myristylated Nef is released from yeast cells and, after extended culture in stationary phase, it accumulates in the supernatant as a dense membranous material that can be centrifuged into a discrete layer above the cell pellet. This material is unique to Nef-producing cells and represents a convenient source of Nef that may have application in further biological studies. Within the yeast cell, electron microscopic examination shows that Nef localises in novel, membrane-bound bodies. These data support the evidence for a role of Nef in membrane perturbation and suggest that there may be a similar localisation for myristylated Nef in HIV-1 infected cells.

Could Nef and Vpr proteins contribute to disease progression by promoting depletion of bystander cells and prolonged survival of HIV-infected cells?

A growing body of literature suggests that the HIV accessory proteins Nef and Vpr could be involved in depletion of CD4(+) and non-CD4(+) cells and tissue atrophy, and in delaying the death of HIV-infected cells. Cell depletion is likely to be predominantly a bystander effect because the number of cells dying far outnumbers HIV-infected cells and is not confined to CD4(+) cells. The myristylated N-terminal region of Nef has severe membrane disordering properties, and when present in the extracellular medium causes rapid lysis in vitro of a wide range of CD4(+) and non-CD4(+) cells, suggesting a role for extracellular Nef in the depletion of bystander cells. A direct role for HIV-1 Nef in cytopathicity is supported by studies in HIV-infected Hu Liv/Thy SCID mice, in transgenic mice expressing nef gene alone, and in rhesus macaques infected with SIV/HIV chimeric virus containing HIV-1 nef. The N-terminal region of Nef has been directly implicated in development of simian AIDS. Extracellular Vpr and C-terminal fragments of Vpr cause membrane permeabilization and apoptosis of a wide range of CD4(+) and non-CD4(+) cells, and could also contribute to depletion of bystander cells. A direct in vivo role for Vpr in thymocyte depletion, thymic atrophy, and nephropathy is suggested in studies with vpr transgenic mice. Intracellular Nef and Vpr could help HIV-infected cells evade cell death by inhibiting apoptosis of infected cells and by avoiding virus-specific CTL response. Nef and Vpr are potential targets for therapeutic intervention and vaccine development, and strategies that prevent the death of bystander cells while promoting the early death of HIV-infected cells could arrest or retard progression to AIDS.

Nef protein induces differential effects in CD8+ cells from HIV-1-infected patients

BACKGROUND

The Nef protein of HIV-1 is suspected to play a role in the depletion of uninfected CD4+ lymphocytes that leads to AIDS. By contrast its effect on CD8+ cells, whose functions are also deregulated during HIV-1 infection, is presently unclear. Here we describe a number of derangements induced in vitro by Nef in CD8+ cells from HIV-1-infected patients.

DESIGN

Peripheral lymphocytes from 16 HIV-1+ subjects and 9 uninfected individuals were cultivated on a Nef-transfected mouse fibroblast layer exposing the carboxyl-terminal region of the viral protein on cell membrane. The cultures were then measured for both apoptosis and proliferation by subdiploid DNA content and Ki67 expression, respectively, whereas the molecular analysis of purified CD8+ cells investigated the Fas-L mRNA levels in Nef-treated CTLs. In addition, we evaluated the Nef-induced variation in the extent of CD8+/HLA-DR+ subset, which includes non cytotoxic cells secreting T-cell antiviral factor (CAF) and a soluble factor inhibiting the HIV-1 replication.

RESULTS

The viral protein induced in peripheral blood lymphocytes (PBL) a moderate tendency to proliferate, as measured by the increment of Ki67 antigen, particularly on the CD8+ subset of HIV-1 infected individuals (P < 0.05). This profile was particularly evident in cultures from patients with severe CD4+ lymphopenia and paralleled an apparent expansion of the CD8+/CD57+ suppressor cell subset. Molecular analysis of purified CD8+ cells revealed a defective expression of Fas-L mRNA in Nef-cultured CTLs, whereas the viral protein exerted a down modulatory effect on the CD8+/HLA-DR+ subset (P < 0.05), thus suggesting a potential inhibition of CAF.

CONCLUSIONS

These results support a potential role of Nef in the progression of HIV-1 infection as a number of cellular functions are affected in the CD8+ subset. In particular, the defective functions of CD8+ cells induced by the viral protein could contribute, at least partly, to the escape of HIV-1 from the immune control of these cells.

Structural requirements for the cytotoxicity of the N-terminal region of HIV type 1 Nef

We have found that the hemolytic and cytotoxic activities of myristoylated Nef N-terminal peptides require a net positive charge in the first seven amino residues of the sequence. The activities are considerably less dependent on the secondary structure of the peptides. Film balance studies showed that both active and inactive peptides interacted with neutral phospholipid monolayers, suggesting that binding to neutral lipids was not a sufficient condition for lytic activity. It was also found that nonmyristoylated N-terminal peptide did not interact to the same extent with the monolayer, indicating that myristoylation was essential for lipid interaction. It is considered that the positively charged residues of the proximate N terminus of Nef interact with acidic lipids of biological membranes, reinforcing the weak membrane-targeting properties of the myristyl chain. Parallels are drawn between this mode of interaction with membranes and that of members of the Src family of proteins, which are also myristoylated and have positively charged residues in their proximate N termini. In particular, these proteins and Nef also have serine residues in their proximal N-terminal regions, which when phosphorylated could neutralize the positive charge and thus provide a mechanism for modulating membrane interaction.