The leucine domain of the visna virus Tat protein mediates targeting to an AP-1 site in the viral long terminal repeat

Comparative Analysis of Tat-Dependent and Tat-Deficient Natural Lentiviruses

The emergence of human immunodeficiency virus (HIV) causing acquired immunodeficiency syndrome (AIDS) in infected humans has resulted in a global pandemic that has killed millions. HIV-1 and HIV-2 belong to the lentivirus genus of the Retroviridae family. This genus also includes viruses that infect other vertebrate animals, among them caprine arthritis-encephalitis virus (CAEV) and Maedi-Visna virus (MVV), the prototypes of a heterogeneous group of viruses known as small ruminant lentiviruses (SRLVs), affecting both goat and sheep worldwide. Despite their long host-SRLV natural history, SRLVs were never found to be responsible for immunodeficiency in contrast to primate lentiviruses. SRLVs only replicate productively in monocytes/macrophages in infected animals but not in CD4+ T cells. The focus of this review is to examine and compare the biological and pathological properties of SRLVs as prototypic Tat-independent lentiviruses with HIV-1 as prototypic Tat-dependent lentiviruses. Results from this analysis will help to improve the understanding of why and how these two prototypic lentiviruses evolved in opposite directions in term of virulence and pathogenicity. Results may also help develop new strategies based on the attenuation of SRLVs to control the highly pathogenic HIV-1 in humans.

FIV as a Model for HIV: An Overview

Animal models for human immunodeficiency virus (HIV) infection play a key role in understanding the pathogenesis of AIDS and the development of therapeutic agents and vaccines. As the only lentivirus that causes an immunodeficiency resembling that of HIV infection, in its natural host, feline immunodeficiency virus (FIV) has been a unique and powerful model for AIDS research. FIV was first described in 1987 by Niels Pedersen and co-workers as the causative agent for a fatal immunodeficiency syndrome observed in cats housed in a cattery in Petaluma, California. Since this landmark observation, multiple studies have shown that natural and experimental infection of cats with biological isolates of FIV produces an AIDS syndrome very similar in pathogenesis to that observed for human AIDS. FIV infection induces an acute viremia associated with Tcell alterations including depressed CD4 :CD8 T-cell ratios and CD4 T-cell depletion, peripheral lymphadenopathy, and neutropenia. In later stages of FIV infection, the host suffers from chronic persistent infections that are typically self-limiting in an immunocompetent host, as well as opportunistic infections, chronic diarrhea and wasting, blood dyscracias, significant CD4 T-cell depletion, neurologic disorders, and B-cell lymphomas. Importantly, chronic FIV infection induces a progressive lymphoid and CD4 T-cell depletion in the infected cat. The primary mode of natural FIV transmission appears to be blood-borne facilitated by fighting and biting. However, experimental infection through transmucosal routes (rectal and vaginal mucosa and perinatal) have been well documented for specific FIV isolates. Accordingly, FIV disease pathogenesis exhibits striking similarities to that described for HIV-1 infection.

The molecular biology of bovine immunodeficiency virus: a comparison with other lentiviruses

Bovine immunodeficiency virus (BIV) was first isolated in 1969 from a cow, R-29, with a wasting syndrome. The virus isolated induced the formation of syncytia in cell cultures and was structurally similar to maedi-visna virus. Twenty years later, it was demonstrated that the bovine R-29 isolate was indeed a lentivirus with striking similarity to the human immunodeficiency virus. Like other lentiviruses, BIV has a complex genomic structure characterized by the presence of several regulatory/accessory genes that encode proteins, some of which are involved in the regulation of virus gene expression. This manuscript aims to review biological and, more particularly, molecular aspects of BIV, with emphasis on regulatory/accessory viral genes/proteins, in comparison with those of other lentiviruses.

Genomic characterization of a slow/low maedi visna virus

The complete genomic sequence of a sheep lentivirus isolate that presents a slow/low phenotype in vitro has been determined. The virus, designated P1OLV, was isolated from lung cells of a naturally infected sheep in Portugal. Three overlapping DNA fragments amplified by PCR, and encompassing the entire viral genome were cloned and sequenced. This isolate has an overall similarity of approximately 80% with the K1514 Maedi Visna virus (MVV) and approximately 70% with the caprine arthritis encephalitis virus (CAEV) Co strain. Phylogenetic analysis based on SU and RT nucleotide sequences grouped P1OLV with previously reported ovine MVV. To determine the virus replication rate, sheep choroid plexus (SCP) and lung cells, macrophages (MØ), and goat synovial membrane (GSM) cells were inoculated with either P1OLV or with the lytic North American strain WLC-1. Viral RNA in culture supernatants was measured by one-tube real time quantitative RT-PCR. Significant differences were observed between the replication rates of the two viruses, with WLC-1 growing rapidly and to high levels in all the cells tested, while P1OLV replicated more slowly and to lower levels inducing persistent infections in lung and SCP cells. The U3 region of the LTR of P1OLV lacks the sequence repeats that are present in the LTRs of WLC-1 and MVV prototype K1514 and that contain additional binding sites for the AML(vis) transcriptional factor. To evaluate the contribution of LTR in the virus replication rate in vitro, we measured the basal activity of the promoter from P1OLV and WLC-1 in a luciferase-driven gene expression assay and lower levels of expression were achieved for P1OLV. The genetic and biological properties of P1OLV will be useful for the study of virus transcriptional factors and genes that may be responsible for the slow/low phenotype.

Lack of trans-activation function for Maedi Visna virus and Caprine arthritis encephalitis virus Tat proteins

All lentiviruses contain an open reading frame located shortly upstream or inside of the env gene and encoding a small protein which has been designated Tat. This designation was mainly with respect to the positional analogy with the first exon of the trans-activator protein of the well studied human immunodeficiency virus type 1 (HIV-1). In this work we comparatively studied the trans- activation activity induced by Tat proteins of the small ruminant Maedi Visna virus (MVV) of sheep and Caprine arthritis encephalitis virus (CAEV) of goats on MVV and CAEV LTRs with that induced by the human lentivirus HIV-1 on its own LTR. The HIV-1 LTR alone weakly expresses the reporter GFP gene except when the HIV-1 Tat protein is coexpressed, the GFP expression is increased 60-fold. In similar conditions only minimal trans-activation increasing two- to three-fold the MVV and CAEV LTR activity was found with MVV Tat protein, and no trans-activation activity was detected in any used cell type or with any virus strain when CAEV Tat was tested. These results indicate that the small ruminant lentiviruses (SRLV) differ from the primate lentiviruses in their control of expression from the viral LTRs and put into question the biological role of the encoded protein named "Tat."

Visna virus-induced activation of MAPK is required for virus replication and correlates with virus-induced neuropathology

It is well accepted that viruses require access to specific intracellular environments in order to proliferate or, minimally, to secure future proliferative potential as latent reservoirs. Hence, identification of essential virus-cell interactions should both refine current models of virus replication and proffer alternative targets for therapeutic intervention. In the present study, we examined the activation states of mitogen-activated protein kinases (MAPKs), ERK-1/2, in primary cells susceptible to visna virus and report that virus infection induces and sustains activation of the ERK/MAPK pathway. Treatment of infected cells with PD98059, a specific inhibitor of the ERK/MAPK pathway, abolishes visna virus replication, as evidenced by extremely low levels of Gag protein expression and reverse transcriptase activity in culture supernatants. In addition, although visna virus-induced activation of MAPK is detectable within 15 min, early events of viral replication (i.e., reverse transcription, integration, and transcription) are largely unaffected by PD98059. Interestingly, further examination demonstrated that treatment with PD98059 results in decreased cytoplasmic expression of gag and env, but not rev, mRNA, highly suggestive of an ERK/MAPK-dependent defect in Rev function. In vivo analysis of ERK-1/2 activation in brains derived from visna virus-infected sheep demonstrates a strong correlation between ERK/MAPK activation and virus-associated encephalitis. Moreover, double-labeling experiments revealed that activation of MAPK occurs not only in cells classically infected by visna virus (i.e., macrophages and microglia), but also in astrocytes, cells not considered to be major targets of visna virus replication, suggesting that activation of the ERK/MAPK pathway may contribute to the virus-induced processes leading to neurodegenerative pathology.

Construction and in vitro characterization of attenuated feline immunodeficiency virus long terminal repeat mutant viruses

AP-1- and ATF-binding sites are cis-acting transcriptional elements within the U3 domain of the feline immunodeficiency virus (FIV) long terminal repeat (LTR) that serve as targets for cellular activation pathways and may regulate virus replication. We report that FIV LTR mutant proviruses encoding U3 deletions of the ATF-binding sequence exhibited restricted virus expression and replication in both feline lymphocytes and macrophages. In contrast, deletion of the AP-1 site had negligible effects on virus expression and replication. FIV LTR mutant proviruses encoding deletions of both the AP-1 and ATF sites or a 72-bp deletion encompassing the AP-1 site, duplicated C/EBP sites, and ATF sites were severely restricted for virus expression. These results demonstrate that deletion of either the ATF-binding site or multiple cis-acting transcriptional elements attenuates FIV. These attenuated FIV mutants provide opportunities to characterize the role of cis-acting elements in virus replication in vivo and to test LTR mutants as attenuated virus vaccines.

Targeting of the visna virus tat protein to AP-1 sites: interactions with the bZIP domains of fos and jun in vitro and in vivo

The visna virus Tat protein is required for efficient viral transcription from the visna virus long terminal repeat (LTR). AP-1 sites within the visna virus LTR, which can be bound by the cellular transcription factors Fos and Jun, are also necessary for Tat-mediated transcriptional activation. A potential mechanism by which the visna virus Tat protein could target the viral promoter is by protein-protein interactions with Fos and/or Jun bound to AP-1 sites in the visna virus LTR. Once targeted to the visna virus promoter, the Tat protein could then interact with basal transcription factors to activate transcription. To examine protein-protein interactions with cellular proteins at the visna virus promoter, we used an in vitro protein affinity chromatography assay and electrophoretic mobility shift assay, in addition to an in vivo two-hybrid assay, to show that the visna virus Tat protein specifically interacts with the cellular transcription factors Fos and Jun and the basal transcription factor TBP (TATA binding protein). The Tat domain responsible for interactions with Fos and Jun was localized to an alpha-helical domain within amino acids 34 to 69 of the protein. The TBP binding domain was localized to amino acids 1 to 38 of Tat, a region previously described by our laboratory as the visna virus Tat activation domain. The bZIP domains of Fos and Jun were found to be important for the interactions with Tat. Mutations within the basic domains of Fos and Jun abrogated binding to Tat in the in vitro assays. The visna virus Tat protein was also able to interact with covalently cross-linked Fos and Jun dimers. Thus, the visna virus Tat protein appears to target AP-1 sites in the viral promoter in a mechanism similar to the interaction of human T-cell leukemia virus type 1 Tax with the cellular transcription factor CREB, by binding the basic domains of an intact bZIP dimer. The association between Tat, Fos, and Jun would position Tat proximal to the viral TATA box, where the visna virus Tat activation domain could contact TBP to activate viral transcription.

Equine infectious anemia virus transactivator is a homeodomain-type protein

Lentiviral transactivator (Tat) proteins are essential for viral replication. Tat proteins of human immunodeficiency virus type 1 and bovine immunodeficiency virus form complexes with their respective RNA targets (Tat responsive element, TAR), and specific binding of the equine anemia virus (EIAV) Tat protein to a target TAR RNA is suggested by mutational analysis of the TAR RNA. Structural data on equine infectious anemia virus Tat protein reveal a helix-loop-helix-turn-helix limit structure very similar to homeobox domains that are known to bind specifically to DNA. Here we report results of gel-shift and footprinting analysis as well as fluorescence and nuclear magnetic resonance spectroscopy experiments that clearly show that EIAV Tat protein binds to DNA specifically at the long terminal repeat Pu.1 (GTTCCTGTTTT) and AP-1 (TGACGCG) sites, and thus suggest a common mechanism for the action of some of the known lentiviral Tat proteins via the AP-1 initiator site. Complex formation with DNA induces specific shifts of the proton NMR resonances originating from amino acids in the core and basic domains of the protein.

Caprine arthritis encephalitis virus dysregulates the expression of cytokines in macrophages

Caprine arthritis encephalitis virus (CAEV) is a lentivirus of goats that leads to chronic mononuclear infiltration of various tissues, in particular, the radiocarpal joints. Cells of the monocyte/macrophage lineage are the major host cells of CAEV in vivo. We have shown that infection of cultured goat macrophages with CAEV results in an alteration of cytokine expression in vitro. Constitutive expression of interleukin 8 (IL-8) and monocyte chemoattractant protein 1 (MCP-1) was increased in infected macrophages, whereas transforming growth factor beta1 (TGF-beta1) mRNA was down-regulated. When macrophages were infected with a CAEV clone lacking the trans-acting nuclear regulatory gene tat, IL-8 and MCP-1 were also increased. No significant differences from cells infected with the wild-type clone were observed, suggesting that Tat is not required for the increased expression of IL-8 and MCP-1 in infected macrophages. Furthermore, infection with CAEV led to an altered pattern of cytokine expression in response to lipopolysaccharide (LPS), heat-killed Listeria monocytogenes plus gamma interferon, or fixed cells of Staphylococcus aureus Cowan I. In infected macrophages, tumor necrosis factor alpha, IL-1beta, IL-6, and IL-12 p40 mRNA expression was reduced in response to all stimuli tested whereas changes in expression of granulocyte-macrophage colony-stimulating factor depended on the stimulating agent. Electrophoretic mobility shift assays demonstrated that, in contrast to effects of human immunodeficiency virus infection of macrophages, CAEV infection had no effect on the level of constitutive nuclear factor-kappaB (NF-kappaB) activity or on the level of LPS-stimulated NF-kappaB activity, suggesting that NF-kappaB is not involved in altered regulation of cytokine expression in CAEV-infected cells. In contrast, activator protein 1 (AP-1) binding activity was decreased in infected macrophages. These data show that CAEV infection may result in a dysregulation of expression of cytokines in macrophages. This finding suggests that CAEV may modulate the accessory functions of infected macrophages and the antiviral immune response in vivo.