A multifunctional domain in human CRM1 (exportin 1) mediates RanBP3 binding and multimerization of human T-cell leukemia virus type 1 Rex protein

Retroviral hijacking of host transport pathways for genome nuclear export

Recent advances in the study of virus-cell interactions have improved our understanding of how viruses that replicate their genomes in the nucleus (e.g., retroviruses, hepadnaviruses, herpesviruses, and a subset of RNA viruses) hijack cellular pathways to export these genomes to the cytoplasm where they access virion egress pathways. These findings shed light on novel aspects of viral life cycles relevant to the development of new antiviral strategies and can yield new tractable, virus-based tools for exposing additional secrets of the cell. The goal of this review is to summarize defined and emerging modes of virus-host interactions that drive the transit of viral genomes out of the nucleus across the nuclear envelope barrier, with an emphasis on retroviruses that are most extensively studied. In this context, we prioritize discussion of recent progress in understanding the trafficking and function of the human immunodeficiency virus type 1 Rev protein, exemplifying a relatively refined example of stepwise, cooperativity-driven viral subversion of multi-subunit host transport receptor complexes.

Viral Subversion of the Chromosome Region Maintenance 1 Export Pathway and Its Consequences for the Cell Host

In eukaryotic cells, the spatial distribution between cytoplasm and nucleus is essential for cell homeostasis. This dynamic distribution is selectively regulated by the nuclear pore complex (NPC), which allows the passive or energy-dependent transport of proteins between these two compartments. Viruses possess many strategies to hijack nucleocytoplasmic shuttling for the benefit of their viral replication. Here, we review how viruses interfere with the karyopherin CRM1 that controls the nuclear export of protein cargoes. We analyze the fact that the viral hijacking of CRM1 provokes are-localization of numerous cellular factors in a suitable place for specific steps of viral replication. While CRM1 emerges as a critical partner for viruses, it also takes part in antiviral and inflammatory response regulation. This review also addresses how CRM1 hijacking affects it and the benefits of CRM1 inhibitors as antiviral treatments.

Human Retrovirus Genomic RNA Packaging

Two non-covalently linked copies of the retrovirus genome are specifically recruited to the site of virus particle assembly and packaged into released particles. Retroviral RNA packaging requires RNA export of the unspliced genomic RNA from the nucleus, translocation of the genome to virus assembly sites, and specific interaction with Gag, the main viral structural protein. While some aspects of the RNA packaging process are understood, many others remain poorly understood. In this review, we provide an update on recent advancements in understanding the mechanism of RNA packaging for retroviruses that cause disease in humans, i.e., HIV-1, HIV-2, and HTLV-1, as well as advances in the understanding of the details of genomic RNA nuclear export, genome translocation to virus assembly sites, and genomic RNA dimerization.

Nucleocytoplasmic Transport: Regulatory Mechanisms and the Implications in Neurodegeneration

Nucleocytoplasmic transport (NCT) across the nuclear envelope is precisely regulated in eukaryotic cells, and it plays critical roles in maintenance of cellular homeostasis. Accumulating evidence has demonstrated that dysregulations of NCT are implicated in aging and age-related neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD), Alzheimer's disease (AD), and Huntington disease (HD). This is an emerging research field. The molecular mechanisms underlying impaired NCT and the pathogenesis leading to neurodegeneration are not clear. In this review, we comprehensively described the components of NCT machinery, including nuclear envelope (NE), nuclear pore complex (NPC), importins and exportins, RanGTPase and its regulators, and the regulatory mechanisms of nuclear transport of both protein and transcript cargos. Additionally, we discussed the possible molecular mechanisms of impaired NCT underlying aging and neurodegenerative diseases, such as ALS/FTD, HD, and AD.

Peripheral blood mononuclear cell microRNA profiles in syphilitic patients with serofast status

Syphilis is a chronic sexually transmitted disease caused by infection with Treponema pallidum, which can invade various system organs, leading to clinical manifestations such as neurosyphilis, ocular syphilis, and cardiovascular syphilis and seriously endangering human health. Serofast status is a common outcome after syphilis treatment that presents an important clinical problem. At present, the etiology of serofast status remains unknown. A systematic investigation of the microRNA (miRNA) expression profiles in peripheral blood mononuclear cells (PBMCs) of patients with serofast status or secondary syphilis and of healthy control subjects was conducted using small RNA-seq. The expression of miRNAs was further confirmed by real-time fluorescence quantitative PCR (qPCR) assays. The data reveal a specific miRNA expression profile that was displayed in cells from patients with serofast status. Known and novel predicted (np)-miRNAs were also identified and verified, such as miR-338-5p, np-miR-163, np-miR-128, np-miR-244, and np-miR-5, which together may be used as indicators for treatment evaluation. The functions of genes targeted by the miRNAs differentially expressed in serofast status patients were further analyzed; these genes were found to be involved in various biological functions, such as T-cell receptor signaling pathways, metabolism, and growth. Our study presents the first systematic landscape of miRNAs in PBMCs from patients with serofast status and proposes specific miRNAs linked with serofast status. Our results provide further evidence that serofast status is closely related to host immune function. Additionally, the miRNA expression profile in PBMCs of patients with serofast status generated by this work offers insight into the complex immune network in humans. We hope our results can provide new insights into the pathogenesis of serofast status.

Detection of Differentially Methylated Regions Using Bayes Factor for Ordinal Group Responses

Researchers in genomics are increasingly interested in epigenetic factors such as DNA methylation, because they play an important role in regulating gene expression without changes in the DNA sequence. There have been significant advances in developing statistical methods to detect differentially methylated regions (DMRs) associated with binary disease status. Most of these methods are being developed for detecting differential methylation rates between cases and controls. We consider multiple severity levels of disease, and develop a Bayesian statistical method to detect the region with increasing (or decreasing) methylation rates as the disease severity increases. Patients are classified into more than two groups, based on the disease severity (e.g., stages of cancer), and DMRs are detected by using moving windows along the genome. Within each window, the Bayes factor is calculated to test the hypothesis of monotonic increase in methylation rates corresponding to severity of the disease versus no difference. A mixed-effect model is used to incorporate the correlation of methylation rates of nearby CpG sites in the region. Results from extensive simulation indicate that our proposed method is statistically valid and reasonably powerful. We demonstrate our approach on a bisulfite sequencing dataset from a chronic lymphocytic leukemia (CLL) study.

The Jembrana disease virus Rev protein: Identification of nuclear and novel lentiviral nucleolar localization and nuclear export signals

The lentiviral Rev protein, which is a regulatory protein essential for virus replication, has been first studied in the human immunodeficiency virus type 1 (HIV-1). The main function of Rev is to mediate the nuclear exportation of viral RNAs. To fulfill its function, Rev shuttles between the cytoplasm and the nucleus. The Jembrana disease virus (JDV), a lentivirus, is the etiologic agent of the Jembrana disease which was first described in Bali cattle in Indonesia in 1964. Despite the high mortality rate associated with JDV, this virus remains poorly studied. Herein the subcellular distribution of JDV Rev, the nuclear and nucleolar localization signals (NLS and NoLS, respectively) and the nuclear export signal (NES) of the protein were examined. JDV Rev fused to the enhanced green fluorescent protein (EGFP) predominantly localized to the cytoplasm and nucleolus of transfected cells, as determined by fluorescence microscopy analyses. Through transfection of a series of deletion mutants of JDV Rev, it was possible to localize the NLS/NoLS region between amino acids (aa) 74 to 105. By substituting basic residues with alanine within this sequence, we demonstrated that the JDV Rev NLS encompasses aa 76 to 86, and is exclusively composed of arginine residues, whereas a bipartite NoLS was observed for the first time in any retroviral Rev/Rev-like proteins. Finally, a NES was identified downstream of the NLS/NoLS and encompasses aa 116 to 128 of the JDV Rev protein. The JDV Rev NES was found to be of the protein kinase A inhibitor (PKI) class instead of the HIV-1 Rev class. It also corresponds to the most optimal consensus sequence of PKI NES and, as such, is novel among lentiviral Rev NES.

The Interplay of Viral and Host Factors in Chikungunya Virus Infection: Targets for Antiviral Strategies

Chikungunya virus (CHIKV) has re-emerged as one of the many medically important arboviruses that have spread rampantly across the world in the past decade. Infected patients come down with acute fever and rashes, and a portion of them suffer from both acute and chronic arthralgia. Currently, there are no targeted therapeutics against this debilitating virus. One approach to develop potential therapeutics is by understanding the viral-host interactions. However, to date, there has been limited research undertaken in this area. In this review, we attempt to briefly describe and update the functions of the different CHIKV proteins and their respective interacting host partners. In addition, we also survey the literature for other reported host factors and pathways involved during CHIKV infection. There is a pressing need for an in-depth understanding of the interaction between the host environment and CHIKV in order to generate potential therapeutics.

Ancient hybridization and strong adaptation to viruses across African vervet monkey populations

Vervet monkeys are among the most widely distributed nonhuman primates, show considerable phenotypic diversity, and have long been an important biomedical model for a variety of human diseases and in vaccine research. Using whole-genome sequencing data from 163 vervets sampled from across Africa and the Caribbean, we find high diversity within and between taxa and clear evidence that taxonomic divergence was reticulate rather than following a simple branching pattern. A scan for diversifying selection across taxa identifies strong and highly polygenic selection signals affecting viral processes. Furthermore, selection scores are elevated in genes whose human orthologs interact with HIV and in genes that show a response to experimental simian immunodeficiency virus (SIV) infection in vervet monkeys but not in rhesus macaques, suggesting that part of the signal reflects taxon-specific adaptation to SIV.

Is unphosphorylated Rex, as multifunctional protein of HTLV-1, a fully intrinsically disordered protein? An in silico study

Intracellularlocation of a viral unspliced mRNA in host cell is a crucial factor for normal life of the virus. Rex is a neucleo-cytoplasmic shuffling protein of Human T-cell Leukemia Virus-1(HTLV-1)which has important role in active transport of cargo-containing RNA from nucleus to cytoplasm. Therefore, it plays a crucial role in the disease development by the virus. In spite of its importance, the 3d-structurephosphorylated and unphosphorylated of this protein has not been determined. In this study, first we predicted whether Rex protein is an ordered or disordered protein. In second step protein 3Dstructure of Rex was obtained. The content of disorder-promoting amino acids, flexibility, hydrophobicity, short linear motifs (SLiMs) and protein binding regions and probability of Rex crystallization were calculated by various In Silico methods. The3D models of Rex protein were obtained by various In Silico methods, such as homology modeling, threading and ab initio, including; I-TASSER, LOMETS, SPARSKS, ROBBETA and QUARK servers. By comparing and analyzing Qmean, z-scores and energy levels of selected models, the best structures with highest favored region in Ramachandran plot (higher than 90%) was refined with MODREFINER software. In silico analysis of Rex physicochemical properties and also predicted SLiMs and binding regions sites confirms that unphosphorylated Rex protein in HTLV-1 as Rev protin in HIV is wholly disordered protein belongs to the class of intrinsically disordered proteins with extended disorder (native coils, native pre-molten globules).

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Physico-chemical properties of Rex protein were confirmed unphosphorilated Rex protein is a wholly intrinsically disordered protein.

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The 3d-structure model of Rex protein was determined.

The export receptor Crm1 forms a dimer to promote nuclear export of HIV RNA

The HIV Rev protein routes viral RNAs containing the Rev Response Element (RRE) through the Crm1 nuclear export pathway to the cytoplasm where viral proteins are expressed and genomic RNA is delivered to assembling virions. The RRE assembles a Rev oligomer that displays nuclear export sequences (NESs) for recognition by the Crm1-Ran(GTP) nuclear receptor complex. Here we provide the first view of an assembled HIV-host nuclear export complex using single-particle electron microscopy. Unexpectedly, Crm1 forms a dimer with an extensive interface that enhances association with Rev-RRE and poises NES binding sites to interact with a Rev oligomer. The interface between Crm1 monomers explains differences between Crm1 orthologs that alter nuclear export and determine cellular tropism for viral replication. The arrangement of the export complex identifies a novel binding surface to possibly target an HIV inhibitor and may point to a broader role for Crm1 dimerization in regulating host gene expression.

Cooperativity among Rev-associated nuclear export signals regulates HIV-1 gene expression and is a determinant of virus species tropism

Murine cells exhibit a profound block to HIV-1 virion production that was recently mapped to a species-specific structural attribute of the murine version of the chromosomal region maintenance 1 (mCRM1) nuclear export receptor and rescued by the expression of human CRM1 (hCRM1). In human cells, the HIV-1 Rev protein recruits hCRM1 to intron-containing viral mRNAs encoding the Rev response element (RRE), thereby facilitating viral late gene expression. Here we exploited murine 3T3 fibroblasts as a gain-of-function system to study hCRM1's species-specific role in regulating Rev's effector functions. We show that Rev is rapidly exported from the nucleus by mCRM1 despite only weak contributions to HIV-1's posttranscriptional stages. Indeed, Rev preferentially accumulates in the cytoplasm of murine 3T3 cells with or without hCRM1 expression, in contrast to human HeLa cells, where Rev exhibits striking en masse transitions between the nuclear and cytoplasmic compartments. Efforts to bias Rev's trafficking either into or out of the nucleus revealed that Rev encoding a second CRM1 binding domain (Rev-2xNES) or Rev-dependent viral gag-pol mRNAs bearing tandem RREs (GP-2xRRE), rescue virus particle production in murine cells even in the absence of hCRM1. Combined, these results suggest a model wherein Rev-associated nuclear export signals cooperate to regulate the number or quality of CRM1's interactions with viral Rev/RRE ribonucleoprotein complexes in the nucleus. This mechanism regulates CRM1-dependent viral gene expression and is a determinant of HIV-1's capacity to produce virions in nonhuman cell types.

IMPORTANCE

Cells derived from mice and other nonhuman species exhibit profound blocks to HIV-1 replication. Here we elucidate a block to HIV-1 gene expression attributable to the murine version of the CRM1 (mCRM1) nuclear export receptor. In human cells, hCRM1 regulates the nuclear export of viral intron-containing mRNAs through the activity of the viral Rev adapter protein that forms a multimeric complex on these mRNAs prior to recruiting hCRM1. We demonstrate that Rev-dependent gene expression is poor in murine cells despite the finding that, surprisingly, the bulk of Rev interacts efficiently with mCRM1 and is rapidly exported from the nucleus. Instead, we map the mCRM1 defect to the apparent inability of this factor to engage Rev multimers in the context of large viral Rev/RNA ribonucleoprotein complexes. These findings shed new light on HIV-1 gene regulation and could inform the development of novel antiviral strategies that target viral gene expression.

Role of Nucleocytoplasmic RNA Transport during the Life Cycle of Retroviruses

Retroviruses have evolved mechanisms for transporting their intron-containing RNAs (including genomic and messenger RNAs, which encode virion components) from the nucleus to the cytoplasm of the infected cell. Human retroviruses, such as human immunodeficiency virus (HIV) and human T cell leukemia virus type 1 (HTLV-1), encode the regulatory proteins Rev and Rex, which form a bridge between the viral RNA and the export receptor CRM1. Recent studies show that these transport systems are not only involved in RNA export, but also in the encapsidation of genomic RNA; furthermore, they influence subsequent events in the cytoplasm, including the translation of the cognate mRNA, transport of Gag proteins to the plasma membrane, and the formation of virus particles. Moreover, the mode of interaction between the viral and cellular RNA transport machinery underlies the species-specific propagation of HIV-1 and HTLV-1, forming the basis for constructing animal models of infection. This review article discusses recent progress regarding these issues.

Human T lymphotropic virus type 1 regulatory and accessory gene transcript expression and export are not rex dependent

Human T lymphotropic virus type 1 (HTLV-1) requires regulated gene expression from unspliced and alternatively spliced transcripts for efficient replication and persistence. HTLV-1 Rex is known to facilitate cytoplasmic export of unspliced, gag/pol and incompletely spliced env mRNAs, but its contribution to the expression of other viral transcripts has not been experimentally assessed. In this study, we utilized HTLV-1 proviral clones, cellular fractionation, and real-time reverse transcriptase PCR to determine the role of Rex on the expression and export of all viral mRNAs. Our results indicate that the steady-state levels of the different viral mRNAs are modulated by Rex, which we attribute to a redistribution of completely spliced mRNAs toward incompletely spliced mRNAs. Furthermore, we confirmed the positive effect of Rex on the unspliced gag/pol mRNA and singly spliced env mRNA, resulting in increased cytoplasmic expression. However, the cytoplasmic export of the alternatively spliced HTLV-1 mRNAs encoding the accessory proteins and the antisense Hbz mRNA are independent of direct Rex regulation. This is consistent with the conclusion that viral mRNAs that contain the cis-acting repressive sequence (CRS) and/or a fully functional splice donor site require a Rex/RxRE interaction for efficient cytoplasmic expression.

Nuclear export and expression of human T-cell leukemia virus type 1 tax/rex mRNA are RxRE/Rex dependent

Human T-cell leukemia virus type 1 (HTLV-1) is a complex retrovirus associated with the lymphoproliferative disease adult T-cell leukemia/lymphoma (ATL) and the neurodegenerative disorder tropical spastic paraparesis/HTLV-1-associated myelopathy (TSP/HAM). Replication of HTLV-1 is under the control of two major trans-acting proteins, Tax and Rex. Previous studies suggested that Tax activates transcription from the viral long terminal repeat (LTR) through recruitment of cellular CREB and transcriptional coactivators. Other studies reported that Rex acts posttranscriptionally and allows the cytoplasmic export of unspliced or incompletely spliced viral mRNAs carrying gag/pol and env only. As opposed to HIV's Rev-responsive element (RRE), the Rex-responsive element (RxRE) is present in all viral mRNAs in HTLV-1. However, based on indirect observations, it is believed that nuclear export and expression of the doubly spliced tax/rex RNA are Rex independent. In this study, we demonstrate that Rex does stimulate Tax expression, through nuclear-cytoplasmic export of the tax/rex RNA, even though a Rex-independent basal export mechanism exists. This effect was dependent upon the RxRE element and the RNA-binding activity of Rex. In addition, Rex-mediated export of tax/rex RNA was CRM1 dependent and inhibited by leptomycin B treatment. RNA immunoprecipitation (RNA-IP) experiments confirmed Rex binding to the tax/rex RNA in both transfected cells with HTLV-1 molecular clones and HTLV-1-infected T cells. Since both Rex and p30 interact with the tax/rex RNA and with one another, this may offer a temporal and dynamic regulation of HTLV-1 replication. Our results shed light on HTLV-1 replication and reveal a more complex regulatory network than previously anticipated.

Comparison of the Genetic Organization, Expression Strategies and Oncogenic Potential of HTLV-1 and HTLV-2

Human T cell leukemia virus types 1 and 2 (HTLV-1 and HTLV-2) are genetically related complex retroviruses that are capable of immortalizing human T-cells in vitro and establish life-long persistent infections in vivo. In spite of these apparent similarities, HTLV-1 and HTLV-2 exhibit a significantly different pathogenic potential. HTLV-1 is recognized as the causative agent of adult T-cell leukemia/lymphoma (ATLL) and tropical spastic paraparesis/HTLV-1-associated myelopathy (TSP/HAM). In contrast, HTLV-2 has not been causally linked to human malignancy, although it may increase the risk of developing inflammatory neuropathies and infectious diseases. The present paper is focused on the studies aimed at defining the viral genetic determinants of the pathobiology of HTLV-1 and HTLV-2 through a comparison of the expression strategies and functional properties of the different gene products of the two viruses.

Evolution of a species-specific determinant within human CRM1 that regulates the post-transcriptional phases of HIV-1 replication

The human immunodeficiency virus type-1 (HIV-1) Rev protein regulates the nuclear export of intron-containing viral RNAs by recruiting the CRM1 nuclear export receptor. Here, we employed a combination of functional and phylogenetic analyses to identify and characterize a species-specific determinant within human CRM1 (hCRM1) that largely overcomes established defects in murine cells to the post-transcriptional stages of the HIV-1 life cycle. hCRM1 expression in murine cells promotes the cytoplasmic accumulation of intron-containing viral RNAs, resulting in a substantial stimulation of the net production of infectious HIV-1 particles. These stimulatory effects require a novel surface-exposed element within HEAT repeats 9A and 10A, discrete from the binding cleft previously shown to engage Rev's leucine-rich nuclear export signal. Moreover, we show that this element is a unique feature of higher primate CRM1 proteins, and discuss how this sequence has evolved from a non-functional, ancestral sequence.

Nuclear and cytoplasmic effects of human CRM1 on HIV-1 production in rat cells

The human immunodeficiency virus type 1 (HIV-1) regulatory protein, Rev, mediates the nuclear export of unspliced gag and singly spliced env mRNAs by bridging viral RNA and the export receptor, CRM1. Recently, rat CRM1 was found to be less efficient than human CRM1 in supporting Rev function in rats. In this study, to understand the role of CRM1 in HIV propagation, the mechanism underlying the function of human and rat CRM1 in HIV-1 replication was investigated in rat cells. The production of viral particles, represented by the p24 Gag protein, was greatly enhanced by hCRM1 expression in rat cells; however, this effect was not simply because of the enhanced export of gag mRNA. The translation initiation rate of gag mRNA was not increased, nor was the Gag protein stabilized in the presence of hCRM1. However, the processing of the p55 Gag precursor and the release of viral particles were facilitated. These results indicated that hCRM1 exports gag mRNA to the cytoplasm, not only more efficiently than rCRM1 but also correctly, leading to efficient processing of Gag proteins and particle formation.

Interaction of human T-cell lymphotropic virus type I Rex protein with Dicer suppresses RNAi silencing

Double-stranded RNAs suppress the expression of homologous genes through an evolutionarily conserved process called RNA interference (RNAi) or post-transcriptional gene silencing. A bidentate nuclease called Dicer has been implicated as the protein responsible for the production of short interfering RNAs (siRNAs). In our experiments, Rex overexpression reduced the efficiency of short hairpin RNA (shRNA)-mediated RNAi. The interaction of Dicer with Rex inhibited the conversion of shRNA to siRNA. These results suggest that the interaction of Dicer with HTLV-I Rex inhibits Dicer activity and thereby reduces the efficiency of the conversion of shRNA to siRNA.

Functional analysis of Foxp3 and CTLA-4 expressing HTLV-1-infected cells in a rat model

Human T-cell leukemia virus type 1 (HTLV-1) is the etiologic agent of adult T-cell leukemia (ATL). Some ATL cells express Foxp3, which is known as regulatory T cell (Treg cell) specific transcription factor. It is suggested that Treg cell like suppressive activity of Foxp3 expressing ATL cells is associated to ATL development and related immunodeficiency. To develop an HTLV-1 model system that enables to investigate the association of Treg function in ATL progression, we examined the expression of Foxp3 and CTLA-4, Treg cell-associated factor, in established HTLV-1-infected rat cell lines and their regulatory function. We found the expression of Foxp3 in 10 of 22 and CTLA-4 in 10 of 19 HTLV-1-infected rat cell lines. Moreover, some of the Foxp3 and/or CTLA-4 expressing cell lines suppressed proliferation of naïve T cells that were stimulated with anti-CD3 antibody. Particularly all Foxp3(+) CTLA-4(+) cells showed the suppressive activity. Our data suggest the usefulness of our rat model systems for further analysis of the role of Treg cell-associated factors on the development of ATL and related immunodeficiency in vivo.

The respiratory syncytial virus matrix protein possesses a Crm1-mediated nuclear export mechanism

The respiratory syncytial virus (RSV) matrix (M) protein is localized in the nucleus of infected cells early in infection but is mostly cytoplasmic late in infection. We have previously shown that M localizes in the nucleus through the action of the importin beta1 nuclear import receptor. Here, we establish for the first time that M's ability to shuttle to the cytoplasm is due to the action of the nuclear export receptor Crm1, as shown in infected cells, and in cells transfected to express green fluorescent protein (GFP)-M fusion proteins. Specific inhibition of Crm1-mediated nuclear export by leptomycin B increased M nuclear accumulation. Analysis of truncated and point-mutated M derivatives indicated that Crm1-dependent nuclear export of M is attributable to a nuclear export signal (NES) within residues 194 to 206. Importantly, inhibition of M nuclear export resulted in reduced virus production, and a recombinant RSV carrying a mutated NES could not be rescued by reverse genetics. That this is likely to be due to the inability of a nuclear export deficient M to localize to regions of virus assembly is indicated by the fact that a nuclear-export-deficient GFP-M fails to localize to regions of virus assembly when expressed in cells infected with wild-type RSV. Together, our data suggest that Crm1-dependent nuclear export of M is central to RSV infection, representing the first report of such a mechanism for a paramyxovirus M protein and with important implications for related paramyxoviruses.

HTLV-1 Yin and Yang: Rex and p30 master regulators of viral mRNA trafficking

Human retroviruses are associated with a variety of malignancies including Kaposi's sarcoma and Epstein-Barr virus-associated lymphoma in HIV infection, T-cell leukemia/lymphoma and a neurologic disorder in human T-cell lymphotropic virus type 1 (HTLV-1) infection. Both HIV and human T-cell lymphotropic virus type 1 have evolved a complex genetic organization for optimal use of their limited genome and production of all necessary structural and regulatory proteins. Use of alternative splicing is essential for balanced expression of multiple viral regulators from one genomic polycistronic RNA. In addition, nuclear export of incompletely spliced RNA is required for production of structural and enzymatic proteins and virus particles. Decisions controlling these events are largely guarded by viral proteins. In human T-cell lymphotropic virus type 1, Rex and p30 are both nuclear/nucleolar RNA binding regulatory proteins. Rex interacts with a Rex-responsive element to stimulate nuclear export of incompletely spliced RNA and increase production of virus particles. In contrast, human T-cell lymphotropic virus type 1 p30 is involved in the nuclear retention of the tax/rex mRNA leading to inhibition of virus expression and establishment of viral latency. How these two proteins, with apparently opposite functions, orchestrate virus replication and ensure vigilant control of viral gene expression is discussed.

Enhanced replication of human T-cell leukemia virus type 1 in T cells from transgenic rats expressing human CRM1 that is regulated in a natural manner

Human T-cell leukemia virus type 1 (HTLV-1) is the etiologic agent of adult T-cell leukemia (ATL). To develop a better animal model for the investigation of HTLV-1 infection, we established a transgenic (Tg) rat carrying the human CRM1 (hCRM1) gene, which encodes a viral RNA transporter that is a species-specific restriction factor. At first we found that CRM1 expression is elaborately regulated through a pathway involving protein kinase C during lymphocyte activation, initially by posttranscriptional and subsequently by transcriptional mechanisms. This fact led us to use an hCRM1-containing bacterial artificial chromosome clone, which would harbor the entire regulatory and coding regions of the CRM1 gene. The Tg rats expressed hCRM1 protein in a manner similar to expression of intrinsic rat CRM1 in various organs. HTLV-1-infected T-cell lines derived from these Tg rats produced 100- to 10,000-fold more HTLV-1 than did T cells from wild-type rats, and the absolute levels of HTLV-1 were similar to those produced by human T cells. We also observed enhancement of the dissemination of HTLV-1 to the thymus in the Tg rats after intraperitoneal inoculation, although the proviral loads were low in both wild-type and Tg rats. These results support the essential role of hCRM1 in proper HTLV-1 replication and suggest the importance of this Tg rat as an animal model for HTLV-1.

CRM1-mediated nuclear export: to the pore and beyond

CRM1 (chromosome region maintenance 1; also referred to as exportin1 or Xpo1) is a member of the importin beta superfamily of nuclear transport receptors, recognizing proteins bearing a leucine-rich nuclear export sequence. CRM1 is the major receptor for the export of proteins out of the nucleus and is also required for transport of many RNAs. Besides its established role in nuclear export, CRM1 is also implicated in various steps during mitosis, widening its functional spectrum within the cell.

Tristetraprolin inhibits HIV-1 production by binding to genomic RNA

HIV-1 genome has an AU-rich sequence and requires rapid nuclear export by Rev activity to prevent multiple splicing. HIV-1 infection occurs in activated CD4(+) T cells where the decay of mRNAs of cytokines and chemokines is regulated by the binding of AU-rich elements to the mRNA-destabilizing protein tristetraprolin. We here investigated the influence of tristetraprolin on the replication of HIV-1. Treatment of siRNA against tristetraprolin in a latently HIV-1 infected cell line increases HIV-1 production following stimulation. A chloramphenicol acetyltransferase and luciferase assay revealed that exogenous tristetraprolin reduced HIV-1 virion production and in contrast increased the multiply spliced products. Furthermore, quantitative RT-PCR analysis showed tristetraprolin increases the ratio of multiple-spliced RNAs to un-, single-spliced RNA. Moreover, an electrophoretic mobility shift assay showed that tristetraprolin binds to synthesized HIV-1 RNA with AU-rich sequence but not to RNA with less AU sequence. These results suggest that tristetraprolin is a regulator of HIV-1 replication and enhances splicing by direct binding to AU-rich sequence of HIV-1 RNAs.

CRM1, an RNA transporter, is a major species-specific restriction factor of human T cell leukemia virus type 1 (HTLV-1) in rat cells

Rat ortholog of human CRM1 has been found to be responsible for the poor activity of viral Rex protein, which is essential for RNA export of human T cell leukemia virus type 1 (HTLV-1). Here, we examined the species-specific barrier of HTLV-1 by establishing rat cell lines, including both adherent and CD4(+) T cells, which express human CRM1 at physiological levels. We demonstrated that expression of human CRM1 in rat cells is not harmful to cell growth and is sufficient to restore the synthesis of the viral structural proteins, Gag and Env, at levels similar to those in human cells. Gag precursor proteins were efficiently processed to the mature forms in rat cells and released into the culture medium as sedimentable viral particles. An HTLV-1 pseudovirus infection system suggested that the released virus particles are fully infectious. Our newly developed reporter cell system revealed that Env proteins produced in rat cells are fully fusogenic, which is the basis for cell-cell HTLV-1 infection. Moreover, we show that the early steps in infection, from post-entry uncoating to integration into the host chromosomes, occur efficiently in rat cells. These results, in conjunction with reports describing efficient entry of HTLV-1 into rat cells, may indicate that HTLV-1 is unique in that its major species-specific barrier is determined by CRM1 at a viral RNA export step. These observations will enable us to construct a transgenic rat model expressing human CRM1 that is sensitive to HTLV-1 infection.

The human T-cell leukemia virus Rex protein

A critical step in the life cycle of complex retroviruses, including HTLV-1 and HTLV-2 is the ability of these viruses to adopt a mechanism by which the genome-length unspliced mRNA as well as the partially spliced mRNAs are exported from the nucleus instead of being subjected to splicing or degradation. In HTLV, this is accomplished through the expression of the viral Rex, which recognizes a specific response element on the incompletely spliced mRNAs, stabilizes them, inhibits their splicing, and utilizes the CRM1-dependent cellular pathway for transporting them from the nucleus to the cytoplasm. Rex itself is regulated by phosphorylation, which implies that proper activation of the protein in response to certain cellular cues is an important tool for the virus to ensure that specific viral gene expression is allowed only when the host cell can provide the best conditions for virion production. Having such a critical role in HTLV life cycle, Rex is indispensable for efficient viral replication, infection and spread. Indeed, Rex is considered to regulate the switch between the latent and productive phases of the HTLV life cycle. Without a functional Rex, the virus would still produce regulatory and some accessory gene products; however, structural and enzymatic post-transcriptional gene expression would be severely repressed, essentially leading to non-productive viral replication. More detailed understanding of the exact molecular mechanism of action of Rex will thus allow for better design of therapeutic drugs against Rex function and ultimately HTLV replication. Herein we summarize the progress made towards understanding Rex function and its role in the HTLV life cycle.

Architecture of CRM1/Exportin1 suggests how cooperativity is achieved during formation of a nuclear export complex

CRM1/Exportin1 mediates the nuclear export of proteins bearing a leucine-rich nuclear export signal (NES) by forming a cooperative ternary complex with the NES-bearing substrate and the small GTPase Ran. We present a structural model of human CRM1 based on a combination of X-ray crystallography, homology modeling, and electron microscopy. The architecture of CRM1 resembles that of the import receptor transportin1, with 19 HEAT repeats and a large loop implicated in Ran binding. Residues critical for NES recognition are identified adjacent to the cysteine residue targeted by leptomycin B (LMB), a specific CRM1 inhibitor. We present evidence that a conformational change of the Ran binding loop accounts for the cooperativity of Ran- and substrate binding and for the selective enhancement of CRM1-mediated export by the cofactor RanBP3. Our findings indicate that a single architectural and mechanistic framework can explain the divergent effects of RanGTP on substrate binding by many import and export receptors.